Green Synthesis of NanoMaterials for BioSensing

Abstract

From the beginning of the Industrial Revolution in Great Britain back on the eighteenth century, and mainly from the enormous technological development in several fields (electronics, computing, and robotics) occurred on the second half of the twentieth century that affected deeply industrial processing and manufacturing, most part of society, and even scientific community in some occasions, have usually associated the concept of industry in general, but specifically of Chemical Industry, to several rather negative concerns, such as: exploitation of natural resources; contamination, in general; air, water, and/or soil pollution; emission of tons of wastes; and, more recently, the menace of global warming and emerging pollutants. The main reason relies on the involvement of one or more chemical processes, with the subsequent spending of natural feedstocks and energy and the generation of by-products, in the manufacturing of most products that make our life easier. In fact, it is not possible to imagine how our life would be like without the enormous amount of goods generated by Chemical Industry. Nevertheless, from the establishment of the term “Green Chemistry” two decades ago, both academia and chemical industry are devoting their efforts to remove those old prejudices and to redirect the modern chemical industry to a greener, more efficient, and more sustainable development and production. One of the keystones of the Green Industry is the use of efficient synthetic routes (green synthesis) for producing compounds or (nano)materials in general that may be used directly or as part of more complex manufactures or devices, such as sensors and biosensors, widely used for analytical purposes. In this sense, this chapter intends to provide an overview of the most important green synthesis, mainly based on the clean ultrasound and microwave technologies or biosynthesis routes, and how they can be used to produce nanomaterials or materials for building (bio)sensing devices, whose production, in some cases, can be scaled up or industrially done, and even employed in the industry itself for detection and/or determination purposes and/or for quality control, among other applications. Finally, a critical discussion about the abovementioned topics and their relationships with sustainable development of chemical industry is reported.

1 Introduction

“Chemophobia ” (Mckinnon 1981; Kauffman 1991; Chalupa and Nesměrák 2018) (or chemphobia or chemonoia) (Ropeik 2015), an irrational panic or preconception against chemical compounds or chemistry, is a relatively very new term quite widespread, with rather repercussions in food chemistry mainly in both the Western world and Asia (Gribble 2013) but founded in conceptions and apprehensions that have been coming for centuries. This mistrust, fed in part by news media’s one-sided stories, natural and technological disasters of different magnitude (Cherry et al. 2018; Liu and Wang 2019) and current environmental problems of anthropogenic origin, such as extreme plastics pollution (Eriksen et al. 2014; Bergmann et al. 2017; Belontz et al. 2019), the greenhouse effect (Stephens et al. 2016; Büntgen et al. 2019), global warming (Trenberth et al. 2014), the erosion of ozone layer (Kuttippurath et al. 2018), and the proliferation of emerging pollutants and wastes (Gavrilescu et al. 2015; Calvo-Flores et al. 2018), among many others, has spread to industry, in general, and to chemical industry, in particular.

Arriving at this point, someone may wonders, would it be possible to imagine a life without chemicals or without things manufactured with them? Would it be possible to imagine our life without the enormous amount of goods generated by Chemical Industry? The answers to these questions are very simple: absolutely not and there would be no life at all, at least as we know it now, respectively. This is due to most manufactured goods involve at least one and in most cases many chemical processes. Chemistry and chemical industry have always had a central role in the provision of food and energy, materials, and medicines. Hence, they form part of our daily life (Fortineau 2004; Roy 2016). Nevertheless, chemical industry must follow a pathway that allows it to recuperate the lost confidence, to claim itself as an essential part of the engine that makes the world go round. The sustainability of these industries and hence of our planet depends on strategic choices made by governments. These entities intend to speed up the development and growth of industries to the transition towards a low-carbon economy by implementing green and strategic industrial policies (Cosbey 2013; Schmitz et al. 2015). That is why during the past few decades, scientific community and industry have been cooperating to make it possible, transmitting a new conception based on concerns that are more positive: that of a much greener, ecological, and sustainable chemical industry that generates environmentally benign products, which would be rather more appreciated by Governments, ONGs, and society in general.

For this purpose, the philosophy of “Green Chemistry” saw the light in the 1980s (Clark 2005). Anastas and Warner defined this term at the end of 1990s as “the design of chemical products and processes that reduce or eliminate the use and generation of hazardous substances” (Anastas and Warner 1998). This term was reformulated and completed by Manley et al. 10 years later as “the design, development, and implementation of chemical products and processes to reduce or eliminate the use and generation of toxic compounds to human health and the environment” (Manley et al. 2008). The Green Chemistry idea was accepted and worldwide extended as an essential development of chemistry and chemical industry. Despite the general acceptance of this philosophy for the sustainable development , its application, not only in developed countries but also in India and China, was fragmented at the beginning, representing only a small part of the chemistry done in the past decades. Currently this concept has influenced education, research, and industrial practice in such a manner that an increasing and significant part of the most environmentally aware society daily demands greener and sustainable products, processes, and methodologies. In fact, Green Chemistry is understood now as a group of actions and attitudes rather than being constrained to chemical analyses based on nontoxic solvents; in other words, it must be considered as multidimensional (Kogawa and Salgado 2015). It is thinking about the process as a whole and reducing steps, energy, reagents, and costs to the minimum (de Marco et al. 2019). This sustainability even reaches Academia, since it is responsible for teaching new university students to gain knowledge on how to safeguard the earth for future generations (Płotka-Wasylka et al. 2018). However, as these authors state, there exist several false “greenness” in chemical literature and teaching practices, what suggests that this proper knowledge must be carefully obtained through what we call a “sustainable and green education,” “environmental education” as United States Environmental Protection Agency reports (U.S. Environmental Protection Agency 1992), or “education for sustainable development” as UNESCO calls (Leicht et al. 2018).

As it is well known, the increasing industrialization process was a keystone for world economic evolution. During the second half of the twentieth century, social movements promoted a revolution in Green Chemistry and provoked changes that affected all the industry and its processes, trying making them more sustainable. As a consequence of this, environmental impact and companies and population awareness increased. That is why Paul Anastas and John Warner in the 1990s (Anastas and Warner 1998; Anastas and Eghbali 2010) listed the 12 Green Chemistry principles (Table 1), mainly based on the reduction or rejection of toxic solvents and the non-generation of residues in analyses and chemical processes.

Table 1 The 12 principles of green chemistry (Anastas and Warner 1998; Anastas and Eghbali 2010)

In fact, the core of Green Chemistry can be understood as a set of reduction processes (Fig. 1). The reductions shown in the figure lead to many economic, environmental, and social benefits (ENDS 2003; European Commission 2003). Costs can be saved by reducing by-products and energy use, as well as increasing the efficiency of the whole process due to decreasing materials consumption. The abovementioned reductions also drive to environmental profit in terms of both feedstock depletion and end-of-life disposal. Moreover, the growing employment of renewable resources will make the manufacturing industry more sustainable (ENDS 2004). The minimization of hazardous events and the handling of dangerous substances offers additional social benefits to plant operators and local communities (Clark 2005).

Fig. 1

“Reducing” as the core of green chemistry

Figure 2 summarizes the main important aspects about Green Chemistry and the role that all actors play for reaching global sustainabili ty.

Fig. 2

Concepts related to green chemistry: philosophical aspects

One of the most important documents regarding industrial sustainability is the Green Industry Initiative , where sustainable industrial development was tried to be placed in the context of new global sustainable development challenges through Green Industry (UNIDO 2011). In fact, in this document the main points in which this initiative is based are highlighted.

In this declaration, firstly the need for Green Industry is discussed; later, Green Industry as a tool for implementing sustainable development is exposed; then, the benefits of Green Industry are described: economic, social, and environmental; in fourth place, the opportunities that Green Industry entails regarding mitigation of climate change and chemical pollution are presented. Finally, the existing hindrances towards the evolution of Green Industry in developing countries are outlined.

As previously affirmed, most goods and products that industry, especially Chemical Industry, manufactures and people use involve one or more chemical processes. Our life without them would not be the same for sure. However, Green Chemistry helps industry to become more ecological and sustainable, generating in this way environmentally benign products. Green Chemistry is considered as a multidisciplinary field that encompasses many areas like catalysis, solvents, synthesis, raw materials, products, and efficient processes (Song and Han 2015). Green synthesis constitutes one of the pillars of Green Chemistry and that is why the search for efficient synthetic routes is of vital relevancy to achieve the sustainable industrial production, like in healthcare industry (Morgon 2015). Generation of chemists, mainly organic chemists, have been trained to formulate synthetic reactions in order to maximize yield and purity. Nevertheless, many chemical production processes lack efficiency in the employment of feedstocks and generate large amount of side products. A crucial point to reduce both is increasing atom economy, premise that would satisfy 4 out of the 12 Green Chemistry principles , particularly, principles of Prevention, Atom Economy, Less Hazardous Chemical Synthesis, and Use of Renewable Feedstocks (see Principles 1, 2, 3, and 7 in Table 1). Ideally, all the atoms in reactants should be turned into the desired products. The perfect synthesis, according to Green Chemistry, can be represented in Fig. 3.

Fig. 3

Scheme and features of the perfect synthesis

Despite that in all industrial chemical processes 100% of atom economy is an utopia. Another form of minimizing the formation of secondary products is the integration of different reactions and processes, being the by-product in a certain reaction the feedstock of another (Song and Han 2015). Of course, yield, product isolation easiness, and purity needs, among other factors, should not be replaced by the concept of atom economy when implementing a chemical synthesis; in fact, it should be considered as an additional aspect (Lancaster 2002). It is noteworthy to mention that there are some reaction types that, due to their nature, would probably minimize waste because of being inherently atom efficient. A3 coupling (Alkyne, Aldehyde, and Amine) (Wei and Li 2002; Wei et al. 2004) and mainly Diels-Alder reaction are two typical and excellent examples of atom-economical reactions (Trost 1991, 1995) (Fig. 4).

Fig. 4

Examples of two atom-economical reactions

Other either well-known or new efficient synthetic tools can be found in Anastas and Eghbali’s work (Anastas and Eghbali 2010): cycloadditions, rearrangements, cascade or tandem reactions, multicomponent coupling reactions, metathesis, C-H activation, and enzymatic reactions, for citing some examples. Hence, these reaction types are necessary to be taken into account when implementing a synthetic strategy. However, other elements must obviously be considered as responsibles of the most competitive, efficient, and eco-friendly route: cost and feedstocks availability; toxicity/hazardous nature of feedstocks; yield; product isolation and purification easiness; energy, solvent, and cost-effective equipment exigencies; process times; and waste materials nature (Lancaster 2002).

On the other hand, green synthesis cannot be only circumscribed to organic chemistry, as it is formerly suggested, but to other disciplines as well, such as inorganic chemistry, materials science, or even analytical chemistry. In the latter area, it is also possible to point out Green Analytical Chemistry (De la Guardia and Garrigues 2012; Koel and Kaljurand 2019). This recent discipline focuses on the elaboration of new, green, and sustainable analytical procedures for organic and inorganic compounds determination in different kinds of samples characterized by complex matrices composition (Płotka-Wasylka and Namieśnik 2019). In fact, according to experts, the attention should be focused on making sample-pretreatment and analytical methods much greener thanks to the development of new strategies and tools (Armenta et al. 2008). To these ones, other complementary practices can be added, such as minimization of wastes, recovery of reagents, on-line decontamination of wastes, and the use of reagent-free methodologies. Hence, it is mandatory to fix a group of clear and concise reccomendations constituting the principles of Green Analytical Chemistry , susceptible to be applied in laboratory practices. In this way, the 12 principles of Green Analytical Chemistry were proposed in 2013 (Gałuszka et al. 2013) (see Table 2). They supposed a reformulation of the existing principles of Green Chemistry and Green Engineering since they did not fully fulfill the needs of analytical chemistry. From the 12 principles of Green Chemistry, only four can be directly made suitable for analytical chemistry : (1) residues prevention (principle 1); (2) environmentally friendly solvents and reagents (principle 5); (3) energetically efficient designs (principle 6); and (4) minimization of derivatization (principle 8). These four principles constitute the core of synthesis in some areas of analytical chemistry. Besides, two more key goals should be considered to achieve sustainability in analytical synthesis : (5) removing or reducing chemical substances whatever the purpose they are used for and (6) increased safety for the operator. As it can be seen, most of these topics demand reductions (Clark 2005) (see Fig. 1). However, the necessity of reaching an agreement between the performance parameters and Green Analytical Chemistry exigencies constitutes one of the disadvantages of green laboratory practices. Most of the strategies for analytical chemists projected in the 12 principles collected in Table 2 may worsen some quality analytical parameters : sensitivity, precision, accuracy, selectivity, detectability, or representativeness (Gałuszka et al. 2013). Regardless, the fast technological advance and knowledge about current problems will lead to an enhancement of green analytical methods.

Table 2 The principles of green analytical chemistry expressed as the mnemonic significance

According to Gałuszka et al. (2013), many green alternatives existing in different fields of analytical chemistry versus conventional methods imply the use of sensors and biosensors , i.e., bisphenol A or 17β-estradiol determination in urban wastewater (instead of gas chromatography/mass spectrometry); folic acid determination in medicines, blood glucose or atrazine in water (instead of spectrophotometry); and lead in water (instead of graphite furnace/atomic absorption spectrometry). (Bio)sensors show several advantages versus other commonly used analytical techniques like chromatography or mass spectrometry, including no or simple sample treatment, non-complex instrumentation, low-cost, high specificity, sensitivity, fast response, relatively compact size, multiparameter analysis, in situ determination, and ease of implementation to detect biomolecules (Bahadir and Sezgintürk 2015; Amine et al. 2016; García-Guzmán et al. 2019; Vogiazi et al. 2019). All these advantages are in agreement with the principles of Green Chemistry and Green Analytical Chemistry (Arduini et al. 2019). Hence, (bio)sensors are a good and serious alternative to determine different kinds of analytes from biomedical, agrifood, or environmental interest versus other typical analytical techniques, which require expensive instrumentations, laboratory setup, skilled personnel, and usually the employment of organic solvents, producing hazardous waste. Due to the aforementioned advantages, (bio)sensing devices are being recently considered of great importance for being used in industry (Siontorou 2019), mainly in food processing industry (Murugaboopathi et al. 2013; Thakur and Ragavan 2013; Mehrotra 2016; Mustafa and Andreescu 2018; Neethirajan et al. 2018a), and also in pharmaceutical (Macdonald 2019), environmental (Patil et al. 2019), and biomedical industries (Cifric et al. 2020).

Nevertheless, most times these (bio)sensors are employed in combination with different kind of materials and nanomaterials in order to enhance analytical quality parameters , like sensitivity, limit of detection, and selectivity, among others, and, thus, their analytical performance (Attar et al. 2015; Bernardo-Boongaling et al. 2019; Shafiei-Irannejad et al. 2019), which make them quite competitive versus other analytical techniques. Among the materials and nanomaterials employed in (bio)sensing, metal (Cubillana-Aguilera et al. 2011; Franco-Romano et al. 2014; Zarzuela et al. 2018; Shukla and Iravani 2019) and metal oxide nanoparticles (González-Álvarez et al. 2016; Henam et al. 2019; Sundaresan et al. 2019), nanowires (Liu et al. 2012; Luo et al. 2019), nanocarbon-based materials (Roh et al. 2019), graphene (Chang et al. 2019; Dong et al. 2019; Hafeez et al. 2019) and magnetic nanostructured molecularly imprinted polymers (Lahcen et al. 2019), among others (Kumar 2007; Merkoçi 2009), as well as new electrode materials stand out (Cordero-Rando et al. 2002; Hidalgo-Hidalgo de Cisneros et al. 2003; Cubillana-Aguilera et al. 2006; López-Iglesias et al. 2016, 2018; Palacios-Santander et al. 2017). There are many different technologies for preparing them, but in this chapter special attention will be paid to two of the most commonly used environmentally friendly technologies, from the energy cost savings point of view: microwave and ultrasound (Strauss 2002; Timothy and Cintas 2002; Chatel 2016). Both techniques are based on the use of focused radiation that reduces reaction times, increases product yields, and also makes reactions more selective (Clark 2005); that is why ultrasound and microwave can be considered the base of the most powerful, ecological, and interesting technologies developed for green analytical synthesis (Lodeiro and Capelo-Martínez 2009). Their advantages versus other synthetic routes can be summarized as follows: (1) environmentally friendliness, (2) very low energy requirements, (3) drastically reduced time of synthesis: from days/hours to few minutes, and (4) simple and (5) low cost instrumentation compared to other technologies.

Moreover, synthesis routes of functional materials and nanomaterials through biomineralization and biotemplating, typically known as biosynthetic routes , by using biopolymers (Khomand and Afsharpour 2019), plant extracts (Agarwal et al. 2019) and other biomolecular structures (bacteria and fungi, among many others) (Gahlawat and Choudhury 2019) play also an important role in Green Analytical Chemistry and attract tremendous amount of interest. The reason for this attention is the promise to achieve enhanced control over positioning and linking different functional nanostructures to give place to complex nanodevices (Padalkar et al. 2010). In the last decade, the exploitation of natural biopolymers fibers , like cellulose, and plant extracts (olive, geranium, Aloe vera, and a much wide etcetera) for synthesizing inorganic nanoparticles, nanoparticle chains, and nanowires has increased a lot. The advantages of using these biostructures in synthetic routes of (nano)materials are evident. For example, in the case of cellulose or other biopolymers-based materials, they are relatively inexpensive, renewable, abundant in many different forms, and have hydroxyl and other functional groups that are accessible for chemical modification (Azizi Samir et al. 2005). With respect to plant extracts (Makarov et al. 2014; Shah et al. 2015), the enormous diversity of reactive chemical compounds possessing different functional groups (alcohols, ketones, esters, etc.) may also offer, in some cases, other advantages, such as abundancy, long lifetime, low or zero-cost requirements for their culture, the easiness of extract preparation, antioxidant activity, antimicrobial properties, and bacterial growth inhibition of some compounds present in the extracts of most plants (Franco-Romano et al. 2014). That is why biosynthesis routes for obtaining (nano)materials will also be paid special attention in this work.

This chapter intends to provide a summary of the most relevant green synthesis or biosynthesis routes, mainly based on the clean ultrasound and microwave technologies, or even in hybrid techniques (Sect. 2), and how they can be employed to synthesize nanomaterials or materials for building (bio)sensing devices (Sect. 3). Currently some of these materials are directly or can be susceptible of being produced by industry, due to scaling-up processes and employed in the industry itself for detection and/or determination purposes and/or for quality control, among other applications in different kinds of industrial companies: food, environmental, biopharmaceutical, and biomedical industries (Sects. 4 and 5). Finally, a critical discussion about the abovementioned topics and their relationships with sustainable development of chemical industry is reported.

2 Green Synthesis Routes

Synthesis routes followed by Green Chemistry principles were given particularly relevance in the last decades. Many research efforts are focused on their development for many reasons, such as short reaction times, low cost, easy workup, and low energy requirements.

Three main synthetic routes can be distinguished: microwave-assisted, ultrasound-assisted, and biosynthesis. A schematic representation is shown in Fig. 5.

Fig. 5

Schematic representation of the major green synthetic routes

In addition to their importance in Green Chemistry, a brief overview for each one will be discussed.

2.1 Ultrasound-Assisted Method

Nowadays, sonochemistry constitutes a broad research field with a growing interest, especially for synthesis purposes. Thus, the employment of ultrasound has been developed as an emergent powerful tool for obtaining an extensive set of organic and inorganic compounds.

2.1.1 Basic Aspects of Ultrasound: Cavitation Process

Sonochemical reactions involves the ultrasonic cavitation , phenomenon based on the formation, growth, and implosion of air bubbles in the liquid phase (Cravotto and Cintas, 2006). Several theories concerning the cavitation phenomena have been proposed: electrical, plasma discharge, supercritical, and hot spots. According to the last one mentioned, high local pressures and temperatures are produced inside the air bubbles and at their interfaces after collapsing, reaching values around 5000 K (Leong et al. 2011). Under these extreme conditions, short-live species from solvent and/or substrate molecules pyrolysis are produced. Hydrogen peroxide and oxygen are generated as by-products by coupling of radical species (H^• and OH^•). This process, known as sonolysis (Torres-Palma and Serna-Galvis 2018), is represented as follows.

Decomposition of water molecules by ultrasound

$$ {\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}^{\bullet }+{\mathrm{OH}}^{\bullet } $$

Formation of secondary radicals

$$ {\mathrm{H}}^{\bullet }+{\mathrm{O}}_2\to {{\mathrm{H}\mathrm{O}}_2}^{\bullet } $$

Generation of hydrogen peroxide by radical coupling

$$ {{\mathrm{H}\mathrm{O}}_2}^{\bullet }+{{\mathrm{H}\mathrm{O}}_2}^{\bullet}\to {\mathrm{H}}_2{\mathrm{O}}_2+{\mathrm{O}}_2 $$

$$ {\mathrm{O}\mathrm{H}}^{\bullet }+{\mathrm{O}\mathrm{H}}^{\bullet}\to {\mathrm{H}}_2{\mathrm{O}}_2 $$

The radical species derived from ultrasonic waves can react with other reagents, following a single-electron transfer pathway (SET) . A representative example is the alkylation of 4-nitrobenzylbromide, which led to a different product under sonication with respect to the one obtained under silent conditions (Fig. 6a). This result can be explained with the involvement of radicals species from reagents’ cleavage under sonication (sonochemical switching), as shown in Fig. 6b (Dickens and Luche 1991).

Fig. 6

(a) Products obtained by alkylation of benzylbromide under sonication and silent conditions; (b) Kornblum-Russell reaction mechanism under ultrasound

The ultrasound approach has been extensively applied to assist the synthesis of inorganic and organic compounds. In Sects. 2.1.2 and 2.1.3, numerous examples are exposed.

2.1.2 Ultrasound-Assisted Synthesis of Organic Compounds

There are several reviews in the bibliography related to the synthesis of organic compounds assisted by ultrasound, providing a detailed discussion including the reaction pathways proposed for each case (Baig and Varma 2012; Banerjee 2017). Ando’s reaction is an illustrative example of organic synthesis in which different product was obtained under ultrasound. The reaction between benzylbromide, potassium cyanide, and alumina in toluene gave the benzylcyanide under sonication, instead of the diphenylmethane derivative obtained under mechanical stirring conditions. The authors proposed the inactivation of the acidic sites on the alumina surface by the strong adsorption of potassium cyanide under ultrasound conditions (Ando and Kimura 1990).

Not all the ultrasound-assisted organic reactions involve the sonochemical switching phenomena. Despite this fact, two important advantages are ascribed to the use of ultrasound: high yields and low reaction times, among others. All of them lead to several features in terms of Green Chemistry, summarized in Gregory Chatel’s work (Chatel 2018): reduction of waste products, easy synthesis workup, and minimization of energy requirements.

Table 3 shows the reaction time and yield obtained for different organic products obtained synthesized under silent conditions and assisted by ultrasound.

Table 3 Reaction times and yields of different organic compounds obtained under silent conditions and assisted by ultrasound

As shown in the previous table, shorter reaction times, as well as higher yields were achieved under ultrasound conditions. This demonstrates the benefits of ultrasound in organic synthesis.

Other important aspect in some conventional organic syntheses is the use of toxic reagents, harmful for the environmental and human health. By using ultrasound, organic hazardous solvents can be replaced by greener ones or totally removed (solvent-free conditions). The selective oxidation of sulfides to sulfoxides can be carried out by using hydrogen peroxide as solvent (Mahamuni et al. 2006), instead of methanol. Polymerization can also be mediated by ultrasound, taking place by polymer or solvent-derived radical processes. Several examples of these types of reactions are reported by McKenzie and coworkers (McKenzie et al. 2019).

2.1.3 Ultrasound-Assisted Synthesis of Inorganic Compounds

Inorganic synthesis can also be ultrasound-assisted , leading to several advantages with respect to conventional routes. Suslick and coworkers summarized the influence of chemical and physical effects of ultrasound in the synthesis of nanostructured materials (Bang and Suslick 2010; Xu et al. 2013). Among all of them, metallic and metallic-oxide nanoparticles have received wide attention in the past few decades due to their optical properties, as well as their unique reactivity, very useful in many applications (Christian et al. 2008).

For noble metals, radical species generated by sonolysis of the water (H^• and OH^•) can act as reducing agents, avoiding the use of any additional reducing compound in the synthesis. However, secondary species can be added to produce secondary radical species, promoting the rate of the process.

The overall process was detailed in Suslick and coworkers’ work (Xu et al. 2013) and can be summarized as follows (recombination of radicals derived from the sonolysis of water was not included).

Decomposition of water molecules by ultrasound

$$ {\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}^{\bullet }+{\mathrm{OH}}^{\bullet } $$

Formation of secondary radicals (R ^• )

$$ \mathrm{RH}+{\mathrm{OH}}^{\bullet}\to {\mathrm{R}}^{\bullet }+{\mathrm{H}}_2\mathrm{O} $$

$$ \mathrm{RH}+{\mathrm{H}}^{\bullet}\to {\mathrm{R}}^{\bullet }+{\mathrm{H}}_2 $$

Chemical reduction of noble metallic salt (M)

$$ {\mathrm{M}}^{\mathrm{n}+}+{\mathrm{n}\mathrm{H}}^{\bullet}\to {\mathrm{M}}^0 $$

$$ {\mathrm{M}}^{\mathrm{n}+}+{\mathrm{n}\mathrm{R}}^{\bullet}\to {\mathrm{M}}^0 $$

Formation of noble metallic salt nanoparticles (M _m )

$$ {\mathrm{m}\mathrm{M}}^0\to {\mathrm{M}}_{\mathrm{m}} $$

By using this approach, gold ( Okitsu et al. 2007), platinum (Mizukoshi et al. 1999), silver (Elsupikhe et al. 2015), and palladium (Nemamcha et al. 2006) nanoparticles were synthesized.

The sonochemical method provides mainly spherical nanoparticles, although other nanomaterial shapes can be obtained under ultrasound. Different gold nanoparticles morphologies were formed from the reduction of the gold salt under ultrasound by setting the sodium dodecyl sulfate concentration and the ultrasonic intensity. According to this research work (Park et al. 2006), sodium dodecyl sulfate concentration values lower than critical micelle concentration drove to the obtention of nonspherical nanoparticles. With respect to the influence of the ultrasonic power, low ultrasonic irradiation produces lesser radical species in comparison with those produced at higher values, and hence the generation of nanodisks, nanoprisms, and nanorods, among other structural shapes, is promoted due to the slow reduction rate of the gold salt. Other examples about the obtaining of nonspherical gold nanoparticles from the chemical reduction of the gold salt by ultrasound can be stated. Gold nanorods were obtained when using cetyltimethylammonium bromide as stabilizing agent and ascorbic acid. In this report, the pH of the solution plays a significant role in the aspect ratio of nanorods (length/width ratio): the average aspect ratio decreases as pH value increases (Okitsu et al. 2009). Gold nanobelts were formed in the presence of α-d glucose by using a green, nonhazardous, and rapid synthesis method. The effect of the concentration of α-d glucose was discussed in this research work. Furthermore, the use of ultrasound was proposed to accelerate the process, as well as to enhance the reorganization of the molecules of glucose on gold crystals (Zhang et al. 2006).

Nonspherical nanoparticles constituted by other metals were synthesized following the sonochemical route. Cubic silver nanoparticles were formed by self-arrangement of dodecylbenzenesulfonic acid sodium salt as surfactant. The nanoparticle morphology was also influenced by the concentration of poly(vinylpyrrolidinone), the ripening time, and the ultrasound (Moghimi-Rad et al. 2011). The formation of copper nanowhiskers was reported in Min Xu and coworkers’ report (Xu et al. 2015). According to this work, ultrasound plays an essential role in the synthesis of nanowhiskers: low ultrasonic power favors the growth of the particles in one dimension, while higher values lead to irregular particles.

Gold nanoparticles can also be synthesized using sodium citrate as reductant under high-power ultrasound conditions (Cubillana-Aguilera et al. 2011). Their characteristic features, optical properties and size, can be easily characterized by analytical routine techniques, dynamic light scattering, and UV–vis spectrophotometry. The characterization of nanoparticles’ size by dynamic light scattering, considered as a no complex and low-time consuming methodology, was also performed in the silicon oxide nanoparticles synthesis by ultrasound (González-Álvarez et al. 2016), as an alternative versus the classical Stöber method, and successfully correlated to the results obtained with transmission electron microscopy, the most used technique for evidencing particle shape and size. Thus, the utility of routine analysis in the monitoring of optical properties and sizes of metal or metal oxide nanoparticles is demonstrated.

Therefore, the development of ultrasound-assisted syntheses as an alternative to conventional methods is demonstrated. The production of radical species by ultrasonic irradiation enhances the reaction rate, as well as reduces the formation of by-products in some cases by sonochemical switching phenomena. Thus, shorter reaction times, higher yields, and high selectivity were ascribed to the employment of ultrasound, leading to several improvements in terms of Green Chemistry. Furthermore, the role of ultrasound in the formation of shape-controlled nanomaterials can also be stated, allowing tailor-made structures to reach unique and useful properties for sensing and catalysis, among other applications.

2.2 Microwave-Assisted Synthesis

In Sect. 2.1, ultrasound has been exposed as an interesting approach towards Green Chemistry, although it is not the only one. In this section, microwaves assistance will be discussed as an excellent alternative to conventional synthesis routes.

2.2.1 Microwave Heating: Fundamentals and Mechanisms

Microwaves is a kind of electromagnetic radiation comprised between radio waves and infrared, with a wavelength range from 1 mm to 1 m (0.3–300 GHz). The interaction between this radiation and the matter has different physical effects, such as mobilizing the electric charges in liquids or conducting ions in solids. The electromagnetic energy produced by microwave irradiation is transformed into heat, promoting the conversion of reagents to products. As an example, a schematic representation of a reaction assisted by microwaves radiation appears in Fig. 7. The heating mechanism of this phenomena will be explained deeply later (Sekhon, 2010).

Fig. 7

Example of reaction promoted by microwave

Regarding the interaction between microwave and the compound, three different situations can be established:

• 1. Microwaves have no effects in the system, e.g., sulfur. The radiation travels through the species without altering it.

• 2. Microwave is reflected by the system, e.g., metals such as copper. The radiation cannot travel through it and it is reflected.

• 3. Microwaves are absorbed by the system, e.g., water. The absorption leads to an increase of the temperature.

Based on the previous classification, compounds which absorb the microwaves will be considered in this section due to their predominance in microwave chemistry. However, the other materials have interesting application as well; some of them will be explained later.

There are three main mechanisms about the role of the microwave radiation in the heating process : dipolar polarization, conduction mechanism, and interfacial polarization. All of them will be described in this section.

2.2.1.1 Dipolar Polarization

A polar molecule , which is affected by a varying electric field (microwave radiation), try to reorientate itself. The molecular friction effect causes a release of heat to the system. Thus, it is necessary to possess a dipole to cause heat with this mechanism. In this sense, it is possible to take advantage of this phenomenon by using polar solvent, such as ethanol, methanol, and water, among others, or polar solutes such as ammonia or formic acid. The most notorious factor is the application of a nonconstant field with a suitable frequency, which facilitates the interaction of the particle . This frequency must not be too high because the molecule will not be fast enough to follow the changes in the field, leading to a premature stop of the molecule. In another way, if the frequency is too low, the molecule will have enough time to realign itself with no effective interaction between molecules. By using the microwave radiation (frequency 0.3–30 GHz) it is possible to provoke an effective particle interaction (Lidström et al. 2001). An example of dipolar polarization can be observed in Fig. 8a.

Fig. 8

Heating mechanism of microwave radiation: (a) dipolar polarization and (b) conduction mechanism

2.2.1.2 Conduction Mechanism

Electrons moved throughout a resistance provide heat to the system. This principle is the basis of the conduction mechanism . A conductor submitted into an electric field generates a flow of electron, or ions, which is against the internal resistance, leading to the heating of the conductor. Therefore, charge carriers (electrons, ions, etc.) in a sample can be moved using an electric field. The inner resistance will heat the sample due to the induced currents formed. It is important to clarify that high conductive materials will not be heated in this way due to the reflective properties of these ones (Wathey et al. 2002). A scheme of the conduction mechanism is presented in Fig. 8b.

2.2.1.3 Interfacial Polarization

The combination of the previous mechanisms gives, as a result, a third heating mechanism . It is relevant when a conducting material is dispersed in a non-conductive medium such as metal powder in sulfur. Even if the sulfur does not absorb the radiation and the metal reflects it, the whole system is an appropriate microwave absorbing material. This heating mechanism is based on the dipolar polarization but it is slightly different. In this case, the metal powder limits the movement of the ions by forces that are similar to the cases of polar solvents . The restriction of these ions results in a random motion and, afterwards, the heating of the sample (Gabriel et al. 1998).

2.2.2 The Use of Microwaves as a Green Approach in Organic and Inorganic Syntheses

Classically, the heating methods used in organic and inorganic reactions were based on convection, which involves complex workup, such as oil baths, Bunsen burner, or furnace, among others. Other drawbacks in relation with green terms can be ascribed to conventional synthesis methods. They involved petrochemical ingredients , several catalyst and separation and purification processes. Furthermore, organic synthesis implied many health issues and risk for workers, long reaction times, high cost, and inefficiency to heat the system. The last one is based on the convection mechanism, leading to energy losses due to the heat dissipation phenomena. Microwave assistance is proposed as a suitable alternative to conventional routes. The microwave radiation affects only target molecules or solvent molecules, involving less energy. Thus, the heating procedure is more focused and effective.

Besides, the microwave provides other advantages like higher reaction rates, fast and easy optimization, and more reproducible syntheses . In addition to the more efficient heating mechanism, the microwave also reaches higher temperatures, decreasing even more the time needed for certain syntheses. For example, the fluorescein synthesis lasts 10 h with conventional procedures; otherwise, it can be performed in 35 min using a microwave approach (Charde et al. 2012). Moreover, in this kind of synthesis higher final yields of the desired product were reached, minimizing the formation of side products. Thus, the purification procedure is easier and takes shorter times.

In Table 4, the synthesis parameters obtained for synthesis procedures assisted by microwaves and by conventional heating are exposed.

Table 4 Reaction promoted by microwaves and their time and yield in comparison with the ones obtained by using conventional heating

In the previous table the reduction in the time reaction and the general increase in yields are demonstrated. It is also established that the microwave approach is highly versatile in organic synthesis. On the other hand, as it has been mentioned previously, there are several reactions where the organic solvents are minimized or completely removed (solvent free). A mineral support is used instead of the non-eco-friendly solvent and the microwave is adjusted to focus only on the sample (Mordini and Faigl 2005; Algul et al. 2008; Gaba and Dhingra 2011). These reactions will be much greener than their conventional analogous process due to saving in toxic solvents. Other aspect that can be stated is the possibility to carry out several reactions at the same time by using a multimode microwave device.

Microwave approach is also used in polymer syntheses . The energy saving in these procedures and the heating efficiency make this technique an economic and suitable option. In addition, the employment of microwave radiation in the curing process has greatly shortened the reaction time. It has been established that this process is strongly dependent on how the pulse is applied; on the contrary, the power applied is not so important. A higher volume of product can be produced by using large reactors and a controlled solvent-free synthesis with microwave heating. For instance, it has been reported that the peptides production leads high yields and purity. This process implies the assembly of peptide chains of about 30 amino acids carried out only in one night with an automated microwave system (Nayak et al. 2016).

Despite its wide use in organic syntheses , there is a growing increase in the use of microwave in inorganic compound syntheses (Darvishi et al. 2017; Zhao et al. 2017; Li et al. 2019; Rossini et al. 2019; Xu et al. 2019), especially in the nanomaterial field. In these cases, the microwave radiation provides advantages, such as narrow particle size distribution, energetic efficiency, particle size controllability, and fast crystallization rate, among others. The use of microwave was proposed for generating some inorganic compounds by hydrothermal and solvothermal method, leading to some improvements in terms of yields and reaction times, in comparison with the conventional method (Gaikwad and Han 2019). Zhou et al. (2014) proposed a microwave-assisted hydrothermal method to obtain CuO spheres, which are wrapped and linked by graphene nanosheets. Furthermore, it is possible to combine a redox system with a microwave-assisted hydrothermal method. Chen et al. (2013) obtained polymorphic MnO₂, being able to grow the material with different crystallographic phases (α, β and γ) modifying the synthesis parameters. Zhang et al. (2019) have obtained CoFe₂O₄ nanoparticles, by a microwave-assisted solvothermal method, which even possess better properties than the nanoparticles resulting from a conventional solvothermal method. Palma-Goyes et al. (2018) provide a facile and rapid microwave-assisted solvothermal method to synthesize RuO₂ nanoparticles employing citric acid in ethylene glycol as stabilizing agent, H₂O₂ as oxidizing agent, and the metallic precursor.

Moreover, it is also possible to use the microwave radiation to assist a sol-gel process . A great number of studies have been carried out in this direction. Ghule et al. (2011) informed the synthesis of zinc oxide nanorods taking advantage of the microwave radiation employing zinc, nitrate, ethylene glycol, and sodium hydroxide as precursors. Shrike et al. (2011) synthesized pure anatase TiO₂ nanoparticles with a particular porous structure by employing a sol-gel microwave-assisted method. Garadkar et al. (2013) developed an easy procedure to synthesize ZnWO₄ nanoparticles controlling successfully their size by using a microwave-assisted procedure. This approach can also be used to synthesize magnetic nanoparticles as well. Obaidullah et al. (2019) reported an easy and fast method to make Fe₂O₃ nanoparticles coated by a shell of SiO₂ retaining their magnetic properties even at high temperatures. Thus, the microwave radiation has greatly supported conventional materials syntheses methods enlarging the possibilities in this field. Consequently, the number of green process has been considerably increased over the last few years.

Therefore, it is noteworthy to mention some aspects regarding the use of microwave as green approach in synthesis . Unlike the monotonous conventional heating, which is very time consuming, microwave chemistry opens a wide range of new possibilities for the development of new methods of synthesis. The time saving, replacing the toxic solvents to greener ones, greater selectivity, enhancement of reaction yields, and easier setup are also highlighted. For all these reasons, microwaves assistance is widely considered as a very effective and promising tool in Green Chemistry (Ravichandran and Karthikeyan 2011).

2.3 Biosynthesis

The use of biological compounds is currently considered by the scientific community as an attractive way to synthesize complex molecules by one-pot method (Shamaila et al. 2016). In this manner, biosynthesis constitutes an environmental-friendly, economic, and low-time consuming synthesis route. In this chapter, the main groups used in biosynthesis are presented, providing some information about their role in syntheses.

2.3.1 Plant Extracts

Plants are a rich source of chemical compounds, commonly known as phytochemicals , such as polyphenols, terpenoids, alkaloids, and proteins, for instance. They can be easily extracted by using different solvents, such as water, methanol, ethanol, and dimethylformamide, among others (Altemimi et al. 2017). To follow the green rules, water-based and low-toxicity solvents are required for extraction purposes. Concerning the extraction method, microwave and ultrasound constitutes economic, low cost, low time-consuming, and viable tools in the extraction of phytochemicals (Tiwari 2015; Dhanani et al. 2017), and hence their use is encouraged in Green Chemistry.

The phytochemical compounds from plant extracts contain functionalized groups able to reduce, as well as to stabilize, the final product. Based on this dual role, plant extracts are excellent candidates to synthesize metallic nanoparticles, allowing controlling many properties, e.g., size and morphology. The influence of several parameters to obtain tailor-made nanoparticles, such as temperature, pH, and type of extract, will be commented in Sect. 3.4.1 of this chapter.

Plant extracts are very complex matrices; therefore, a detailed study about the bioreduction mechanism is required. Several research groups investigated the reducing role of flavonoids, proteins, and amino acids available in plant extracts (Makarov et al. 2014). In the case of flavonoids, the reactive hydrogen was released by keto-enol tautomerism, leading to the keto-form. As an example, A. K. Singh and coworkers summarizes a possible mechanism for the synthesis of metallic nanoparticles mediated by the keto-enol tautomeric process of eugenol, shown in Fig. 9 (Singh et al. 2010).

Fig. 9

Schematic representation of the reducing role of eugenol in the bioreduction process

The participation of amino acids and proteins in the reduction process was also reported owing to the existence of functionalized amino groups. Based on the above, the overall process of metal nanoparticles formation implies reducing the metal precursor, followed by the nucleation and growth processes.

2.3.2 Biopolymers

Biopolymers can be obtained via microbial synthesis from biological resources as starting products (Ahmad et al. 2015). Among all of them, polysaccharide biopolymers have some excellent properties, enabling the development of advance functionalized materials for several applications (Wróblewska-Krepsztul et al. 2019). Figure 10 shows some representative polysaccharide biopolymeric structures.

Fig. 10

Representative polysaccharide biopolymers

The high reducing properties of metallic salts , together with their ability to coordinate metal ions, make them promising compounds for the biosynthesis of metallic nanoparticles. The reducing/stabilizing dual role enables the formation of metal nanoparticle with specific features, such as tailored sizes, biocompatibility, and nontoxicity, among others (Wang et al. 2017).

As happened with plant extracts, the influence of some synthesis parameters on the size and morphology of the generated nanoparticles, as well as more details about the synthesis approaches, will be given in Sect. 3.4.2.

2.3.3 Microorganisms

Living microscopic organisms are involved in several synthesis routes, usually performed at mild conditions (Wang et al. 2016). The use of several microorganisms, such as algae, bacteria, and fungi, should be highlighted for biosynthetic purposes (Schmid et al. 2015; Dahoumane et al. 2017; Skellam 2019).

The metabolic engineering constitutes a powerful tool in biosynthetic routes using microorganisms, by enhancing endogenous metabolic pathways or by introducing exogenous pathways (Grunwald 2012). R. Kumar and S. Prasad summarized the fundamentals of metabolic engineering of bacteria, including three steps required in the metabolic process: understanding the metabolic pathway, use of a computational approach, and its application at experimental level using different engineering approaches (Kumar and Prasad 2011).

Thus, biosynthesis of several compounds mediated by engineered microorganisms was recently reported. Biodiesel was produced using oleaginous microorganisms from organic wastes by cost-effective approaches (Cho and Park 2018). The production of biofuels and biofuel feedstocks was reported using engineered microorganisms, such as yeasts, cyanobacteria, bacteria, and microalgae (Majidian et al. 2018). The synthesis of 2-phenylethanol, an important aromatic compound, was performed by metabolic engineering in yeast and bacteria, reducing the formation of toxic by-products, usually obtained by chemical synthesis route (Wang et al. 2019b). Lycopene was synthesized from cytosolic isoprenoid precursors using a viral vector (Majer et al. 2017). Biosynthesis of polyhydroxyalkanoate (PHA) seems to be improved by engineered microorganisms (Chen and Jiang 2018). Competing pathways of PHA were minimized, channeling the resources to the PHA biosynthesis pathways.

Microorganisms are also entailed in the synthesis of metal nanoparticles (Ovais et al. 2018). However, the biosynthesis of metal nanoparticles using plants and biopolymers will be presented in Sects. 3.4.1 and 3.4.2.

2.4 Hybrid Green Synthetic Routes

In Sects. 2.1, 2.2, and 2.3, several green approaches have been overviewed individually. However, a new generation of assisted green methods is recently arising based on the combination of some of the advance methods previously discussed. Therefore, new tandems have been proposed as green synthesis advances, such as microwave-ultrasound, microwave-biosynthesis, and ultrasound-biosynthesis assisted methods. These new methodologies take advantages of the synergistic effect involved using different techniques. In this section, they will be briefly presented, discussing some of the most notorious and promising results already available, and exposing their high potential applicability in the green synthesis.

2.4.1 Microwave-Ultrasound-Assisted Methods

The use of ultrasound and microwave radiation has been extensively commented in Sects. 2.1 and 2.2, respectively. Regarding ultrasound, the cavitation phenomena are the core of this approach, which implies physical-chemical process and the generation, growth, and collapse of bubbles. With respect to the microwave radiation, the high effective transference of heat (creation of “hot spots”) leads to higher yields and faster processes than those obtained in conventional heating. However, the limitations of this approach with respect to the use of nonpolar compounds should be mentioned (Martina et al., 2016).

Despite microwaves and ultrasounds are based on two different phenomena, their combination leads to better results than the ones provided by the individual use of each technique. In this way, synergistic effect should be remarkable by using the hybrid approach. This synergistic effect is mainly explained by two different contributions: the highly effective heating of the system and the efficient temperature stimulation supplied by the cavitation process. Besides, the microwave heating prevents the loss of energy and the thermal pollution of the environment. In addition, ultrasounds allow the suitable mixing of substrates, even if they are found in different phases, avoiding the need of any additives, such as surfactants (Pawełczyk et al. 2018).

Many researchers have tried to exploit this binomial in order to develop even more efficient and fast synthesis of organic and inorganic compounds (Cravotto et al. 2015). According to the organic synthesis reactions, several kinds of processes have been improved by this approach. Firstly, the transesterification reactions, Martinez-Guerra et al. reported a protocol to convert waste vegetable oil into biodiesel with a yield of 98% in only 82 min (Martinez-Guerra and Gude 2014). The Heck reaction has also been improved with the simultaneous microwave/ultrasound irradiation. Saaco et al. have performed one of the most recent advances: the creation of carbon-carbon bonds by olefin metathesis under microwaves/ultrasound with 86% of yield (Sacco et al. 2015). The microwave/ultrasound irradiation approach has also provided very good alternatives in the C-Heteroatom Bond formation reactions, such as the ethers synthesis and aromatic azo compounds, among others. For instance, the hydrazinolysis of methyl salicylate has greatly improved investing 40 s instead of 9 h with a yield of 84% (Wu et al. 2008). On the other hand, the inorganic synthesis has been a matter of interest from the point of view of microwave/ultrasound irradiation approach, especially in the case of nanoparticles and nanomaterials (Cravotto and Boffa 2014). It is noteworthy to mention that in the case of metallic nanoparticles the application of microwave irradiation has no dangerous implication due to excessive metal dilution in the solution. Following the microwave/ultrasound irradiation methodology, copper nanoparticles has been successfully generated reducing Cu(OAc)₂ with hydrazine in ethylene glycol. Transmission electron microscopy and X-ray diffraction were used to characterize the nanoparticles, resulting in a highly pure and spherical nanomaterial. The yields and reaction time were 97% and 4.5 min, respectively, in comparison with those obtained by using the conventional method: 52% and 12 h, respectively (Feng et al. 2014). Cherkasov et al. obtained a solid supported Pd catalyst. In this case, the irradiation was applied sequentially. Palladium was firstly reduced by ultrasound radiation, and after then, it was deposited on the solid surface by microwave irradiation; the clusters obtained were around 100 nm. One of the most notorious milestone of this method is the absence of any surfactants, which make the process much greener than the previously made (Wu et al. 2015). The surface morphology of the nanoparticles can be selected altering the conditions of the synthesis. Mesoporous hydroxyapatite nanoparticles were obtained by Liang et al. through microwave/ultrasound irradiation. However, the most relevant results were the change of the porous structure given for different synthesis conditions. A flake-like non-mesoporous structure was generated at low temperature (10–50 °C) and microwave power. On the other hand, a clear mesoporous derivative was obtained by using higher temperatures (50–90 °C). Besides, if the microwave is increased until 200 W, a more mesoporous structure is found (Liang et al. 2013). Thus, due to all the reasons previously mentioned, the microwave/ultrasound irradiation approach can be established as an alternative route in order to obtain greener and more efficient procedures.

2.4.2 Microwave-Assisted Biosynthesis

The next approach to discuss is the employment of microwave radiation in a synthesis performed using biological extracts. Both methods have been widely discussed in Sects. 2.2 and 2.3, respectively. In this case, the synergistic effect is more evident than in the previous case. On the one hand, hazardous reagents were replaced to more ecofriendly reducing agents. On the other hand, the microwave radiation highly increases the efficiency and greatly reduces the time necessary for the synthesis process.

This approach has been employed to synthesize majorly metallic nanoparticles. Bhagavanth et al. described a gold nanoparticles synthesis employing an extract of Annosa Squamosal L. assisted with microwave radiation. In this work, the synthesis was carried out in 5 min, obtaining spherical-shaped forms and a distribution size of 11 ± 2 nm. In addition, the nanoparticles generated possess good stability due to the carbonyl and hydroxyl groups, which surround the nanoparticles (Reddy et al. 2018). Jahan et al. used an extract of Rosa Santana and the microwave radiation to generate silver nanoparticles. It should be noticed that the stability of the nanoparticles was outstanding, being able to maintain their characteristics during 9 months. The size distribution of the nanoparticles provided an average of 14.48 nm with a roughly spherical shape (Jahan et al. 2019). Other researchers have also reported their results in the silver nanoparticles synthesis employing this approach (Eshghi et al. 2018; Francis et al. 2018; Ukkund et al. 2019). Besides, metallic oxide nanoparticles can also be obtained by this hybrid technique. Chankaew et al. reported the synthesis of ZnO nanoparticles and their application in solar cell technology. They used a crude water extract of Dimocarpus longan as biological component. The nanoparticles were obtained in about 30 min using cycles of 1 min-off 1 min-on. The distribution size was about 10–100 nm and amorphous shape was observed. However, it is noteworthy to mention that the final nanoparticles possessed a pure hexagonal phase (Chankaew et al. 2019). SnO₂ quantum dots have been obtained using this approach as well. In this case, the extract of Parkia speciosa was employed with a microwave program consisting in 30 shots of 10 s each one. Highly pure crystalline and tetragonal rutile polycrystalline structure was observed. Besides, morphology of this nanoparticles was spherical with an average diameter of 1.9 nm (Begum and Ahmaruzzaman 2018). Biopolymers can be employed in the metallic nanoparticles synthesis assisted by microwave as well. Torabfam et al. described the silver nanoparticles generation by using chitosan and microwave radiation. Experimental design was carried out to study the best conditions for the chitosan solution. The final synthesis was performed in 100 s leading to an average size of 37 nm. In addition, a spherical shape was noted. The high zeta value obtained (+50 mV) indicated the high stability of the nanoparticles obtained (Torabfam and Jafarizadeh-Malmiri 2018). Naggar et al. performed similar process for the obtention of gold and bimetallic gold/silver nanoparticles. The biopolymer used in this work was curdlan, firstly reported for this purpose. The synthesis of both nanomaterials was done in 10 min with microwave assistance. AuNPs with 52 nm as average size were synthesized. Concerning the nanoparticles built as silver core and gold shell, lower size was appreciated, around 45 nm. Furthermore, despite the fact that no important differences were observed in Ag and Au X-ray diffraction patterns due to their close lattice constant, the structure face-centered cubic was confirmed (El-Naggar et al. 2016). Thus, as it can be noticed, this tandem seems to offer good stability , fast reaction times, and good distribution size of the nanomaterials obtained.

2.4.3 Ultrasound-Assisted Biosynthesis

The use of ultrasound as support in the biosynthesis will be briefly discussed in this section. Each individual component has been exposed in Sects. 2.1 and 2.3, respectively. As it has been commented in Sect. 2.4.1, the interest of these hybrid techniques lays in the synergistic effect gained through their simultaneous use. In this case, the cavitation phenomena creates nano-reactors and aids the chemical reaction, meanwhile the selected biological material will interact with the precursor, stabilizing afterwards the final product formed. This is translated into an easier formation of highly stable nanoparticles, much reduced reaction times, and higher yields. As in Sect. 2.4.2, this approach has been mainly exploited in the synthesis of nanoparticles. Manjamadha et al. have informed the synthesis of Ag nanoparticles using an extract of Lantana camara and the assistance of ultrasounds. The spherical AgNPs obtained had an average size of 33.8 nm. The process was done in 10 min obtaining pure crystalline phases. However, the nanoparticles obtained exhibit a wide size distribution (Manjamadha and Muthukumar 2016). The synthesis of gold nanoparticles was performed by Franco-Romano et al. They employed geranium extract (Pelargonium zonale) and a high-power ultrasound probe. The whole process was done in 3.5 min obtaining nanoparticles of 12 ± 3 nm. In this study, the synthesis conditions were optimized by means of an experimental design, demonstrating that higher volume of reducing agent and precursor led to better synthesis process. On the other hand, it was also exposed that higher volumes of the metallic precursor were related with nonspherical-shaped nanoparticles. The nanoparticles stability was also assessed, showing 8 weeks of lifetime (Franco-Romano et al. 2014). Gu et al. reported the ultrasound-assisted generation of CuO nanoparticles using an alga extract (Cystoseira trinodis). The process was performed in 90 min, obtaining a mean size of 9 nm, and a pure nanocrystalline phase was observed too. Nevertheless, several agglomeration were exposed in transmission electron microscopy analysis; these agglomeration increased the range of the size distribution (Gu et al. 2018). Bayrami et al. synthesized another metal oxide nanoparticle, ZnO₂, by using a leaf extract of Vaccinium arctostaphylos. The ultrasound application lasted 15 min, obtaining nonspherical nanoparticles with a size about 100 nm. A hexagonal wurtzite with a high grade of crystallinity was checked in X-ray diffraction assays. In addition, assays for the medical applications of these nanoparticles were done, exposing the improvement in their characteristics, antidiabetic and antibacterial, in comparison with the conventional chemical route of synthesis (Bayrami et al. 2019). Not only can nanoparticles be made employing this approach, but also biopolymers. Zhu et al. described the synthesis of chitosan employing a biological agent (Ganoderma lucidum spore powder) and the assistance of ultrasound. This synthesis was based on the ultrasound-assisted deacetylation approach, which reduce the temperatures required, diminish the time needed, and lesser severely the depolymerization risk. X-ray diffraction, thermogravimetric analysis, and Fourier transformed infrared spectroscopy characterization confirmed the successful generation of this polymer. Finally, the antibacterial properties were examined and the chitosan prepared via ultrasound-assisted deacetylation showed better antibacterial properties (Zhu et al. 2018a). Therefore, all the data previously exposed indicates that the ultrasound-biosynthesis binomial is an interesting approach which can offer several advantages over the traditional methods.

3 Trends in the Green Synthesis of (Nano)Materials

The development of (nano)materials with specific features has received much attention in the last decades. Many researchers have made great efforts on the modification of bare devices in order to improve their (bio)sensing properties following the Green Chemistry rules. In addition to the properties of the resulting material, other characteristics, such as low cost, easy setup, low toxicity, and high scalability, are desirable to obtain competitive materials with high applicability in several fields.

In this section, the obtention of diverse materials following green approaches will be summarized, while their application in (bio)sensing will be discussed in Sect. 4. Furthermore, the green synthesis of electrode materials used as bare electrochemical transducers is also reported.

3.1 Graphene-Based Materials

Graphene is a two-dimensional material constituted by a single or few layers of carbon atoms comprised in hexagonal rings. Its promising optical, magnetic, electric, and thermal properties, such as high electrical and thermal conductivity, tunable band-gap, and high tensile strength, are suitable for sensing purposes (Rao et al. 2009). According to all these remarkable reasons, the modification of bare electrodes by graphene layer deposition was performed to constitute electrochemical devices with excellent analytical features (Kim et al. 2010).

Bottom-up and top-down approaches can be used to obtain pristine graphene. It is noteworthy to mention that only liquid-phase exfoliation, chemical reduction of graphene oxide (top-down approaches), and chemical vapor deposition (bottom-up approach) will be considered due to their high scalability, allowing obtaining graphene layers at industrial scale (Backes et al. 2017; Zhu et al. 2018b).

3.1.1 Graphene Layer by Liquid-Phase Exfoliation

Graphene flakes were produced via liquid-phase exfoliation of graphite by sonication or shear forces mixing. Figure 11 shows a schematic representation of this pro cess.

Fig. 11

Liquid-phase exfoliation of graphite

Xu and coworkers summarized the main aspects regarding the exfoliation process , highlighting the use of solvents as dispersing agents of graphite and the subsequent graphene sheets (Xu et al. 2018). In their work, the selection of the solvent constitutes major relevance, since the dispersibility of the solid in the liquid strongly depends on their interfacial tension: high values lead to poor dispersibility, favoring the agglomeration of the graphene flakes. Organic solvents with surface tension values from 40 to 50 mJ m⁻² seem to be the best choices for obtaining stable graphene dispersions by liquid-phase exfoliation (Du et al. 2013). In Table 5 common organic solvents employed in exfoliation, together with their surfaces tension values reported in the literature, are summarized.

Table 5 Common organic solvents employed for liquid-phase exfoliation of graphite

Various research papers were devoted to the replacement of organic solvents to water-based systems . The use of a wide variety of surfactants in the liquid-phase exfoliation of graphite was tested, since surface tension of water could be reduced, making feasible the exfoliation of graphite (Narayan and Kim 2015). A recent work regarding the role of surfactant in liquid-phase exfoliation of graphite could be highlighted, demonstrating its current research interest (Sukumaran et al. 2019).

The employment of ionic liquids was reported as promising solvents for liquid exfoliation of graphite in some research works owing to their extraordinary thermal stability, low vapor pressure, and low flammability (Bari et al. 2014; Godoy et al. 2019). However, their toxicity constitutes a high controversy nowadays (Bystrzanowska et al. 2019).

The use of natural extracts was also proposed for the exfoliation of graphene. Chitosan and alginate were studied by Uysal and coworkers as alternatives to organic solvents in the exfoliation of graphite (Uysal Unalan et al. 2015). In their work, the stability of the graphene dispersion was higher when chitosan-assisted, which can be explained in terms of the affinity between the biopolymer and the graphene sheets. Regarding the use of chitosan, the nonpolar segments have a solid affinity with the graphene surface. Moreover, the ionic repulsion between the amine groups and the sheets prevents their agglomeration, leading to stable graphene dispersion. In the case of alginate, the compatibility with the graphene sheets is thermodynamically unfavorable, leading to the restacking and precipitation of the graphene sheets. Black tea was employed to produce graphene in one-step exfoliation method by using a kitchen mixer (Ismail et al. 2017). Other research work reported the use of instant coffee to produce few-layer graphene by ultrasound, proposing the chlorogenic acid as the chemical active component for the graphene functionalization (Abdullah et al. 2019).

The manufacturing of graphene nanoplatelets by high temperature vapor exfoliation of graphite should be mentioned. In this work, no chemicals or surfactants were required for the exfoliation or dispersion of graphene, reaching good dispersibility even at higher concentrations (Ding et al. 2018).

3.1.2 Graphene Layer By Chemical Reduction of Graphene Oxide

The reduction of graphene oxide (GO) constitutes another scalable top-down approach to get graphene flakes for mass production (Lavin-Lopez et al. 2017). The first step consists of the oxidation and exfoliation of graphite to graphene oxide, which forms a stable colloid dispersion in water due to the presence of hydroxyl groups. The reduction of graphene oxide leads to reduced-graphene oxide (r-GO) by removing these groups, restoring partially the π-π∗ conjugation (Chua and Pumera 2014) (Fig. 12).

Fig. 12

Chemical reduction of GO to generate reduced-graphene oxide

The graphene oxide has poor electrical conductivity due to the presence of high content of hydroxyl groups on the edge and basal planes. After the reduction process, some residual oxygen groups still remain in the structure, leading to lower electrical conductivity in comparison with the pristine graphene layer (Pei and Cheng 2012).

C/O ratio should be a critical parameter to evaluate the electrical conductivity, since lower content of oxygen should lead to graphene sheets with higher electrical conductivity. However, this factor is also affected by other parameters, such as sheet orientation or percolation effects (Guex et al. 2017). In spite of this fact, C/O ratio could be used to evaluate the efficient removal of oxygen in resulting graphene sheets, by comparing this value with the one obtained for the initial GO.

The chemical GO reduction involves the addition of a reductant, which plays a key factor in the obtention of graphene layers with the desirable properties for sensing purposes. Several research works were focused on the generation of reduced-graphene oxide by employing diverse reductants, evaluating their C/O ratio. In this sense, hydrazine, hydrazine hydrate, and sodium borohydride were employed for obtaining high-quality graphene layer (Luo et al. 2011; Guex et al. 2017). Due to their toxicity for the living organisms and the environment, their replacing to greener ones is well studied in the last years. With this purpose, less hazardous chemicals and plant extracts, among others, were proposed as alternative reducing agents (De Silva et al. 2017). In Table 6 some examples reported in literature are summarized.

Table 6 Compounds employed in the chemical reduction of graphene oxide

As observed in the previous table, the C/O ratio values of r-GO were higher than the one obtained for GO, indicating the removing of oxygen after reduction.

Chemical reduction of GO can also be assisted by microwave irradiation. Hassan and coworkers performed the chemical reduction of GO assisted by microwave employing hydrazine hydrate as reductant, obtaining r-GO in few minutes (Hassan et al. 2009), in comparison with the one required by conventional method, about 24 h (Stankovich et al. 2007). Hence, shorter reaction times seem to be endorsed to the application of microwave due to the effective transference of energy to the precursors (Hu et al. 2012a).

The sequential chemical reduction and microwave irradiation of GO was performed by Wen and coworkers (Wen et al. 2014). The electrical conductivity of the rGO after microwave treatment was higher than those obtained by using either single microwave-assisted or chemical reduction of GO. Besides, defects of the rGO obtained by chemical reduction seem to be repaired by microwave.

Microwave-assisted GO reduction was performed with ascorbic acid as reductant under N₂ atmosphere, leading to rGO in 3 min (Iskandar et al. 2017). The flow of nitrogen was found to be more effective, since gases formed as side products during reduction process were removed. Furthermore, the electrical conductivity of the resulting graphene flakes was increased after performing the annealing treatment. Another recent work shows novel approach to obtain graphene patterns onto graphene oxide film by using an rGO template assisted by microwave. This method provides shape-controlled graphene patterns with excellent electrical conductivity (Zhao and He 2019).

Therefore, microwave can be employed as a green powerful tool to obtain graphene flakes from the chemical reduction of GO at short reaction times. However, this technique is yet to be explored, since oxygen and some structural defects are still retained, disrupting the electronic conjugation and, hence, decreasing the electrical conductivity in comparison with pristine graphene (Xie et al. 2019).

3.1.3 Chemical Vapor Deposition

Chemical vapor deposition has attracted huge attention for the obtention of uniform and large area graphene layers with low defects. The first step involves the decomposition of the carbon precursor, carried in the vapor phase with an inert gas, e.g., argon. Afterwards, carbon was deposited and grown on a metal substrate surface, such as copper, nickel, palladium, or iridium, among others, which plays a catalytic role in the nucleation and growth process (Zhang et al. 2013a) (Fig. 13).

Fig. 13

Chemical vapor deposition of graphene

The first industrial chemical vapor deposition process of graphene deposited on copper foil was reported by Ruoff and coworkers using methane as carbon source. The graphene film can be moved to another substrate, like SiO₂/Si (Li et al. 2009).

The employment of high temperatures in the synthesis of graphene via chemical vapor deposition, around 1000 °C, is usually required (Zhang et al. 2013a). Other chemical vapor deposition derivatives, such as plasma enhanced (Li et al. 2016), inductively coupled plasma (Pekdemir et al. 2017), microwave plasma (Fang et al. 2016), and photo-induced, can be excellent alternatives to reduce the working temperature. Son and Ham summarized the recent progress in the employment of these techniques for synthesis of graphene, highlighting its application in electronic devices (Son and Ham 2017).

New alternative carbon sources were investigated as green precursors . The deposition of graphene on a copper substrate via microwave plasma-chemical vapor deposition using a camphor precursor was reported by Uchida and coworkers (Hideo Uchida et al. 2016). In their work, graphene sheets using camphor showed superior qualities in terms of lower sheet resistance than those obtained using methane. Other natural resources, such as palm oil (Salifairus et al. 2016) and tea tree extract (Jacob et al. 2015), were also employed as green carbon sources due to its high availability and low cost.

A recent work reported a new chemical vapor deposition method using a solid waste plastic as carbon solid precursor (You et al. 2017). A waste material was transformed into a functional material by following a green and sustainable approach at atmospheric pressure, since no chemicals and reagents were used. Therefore, this method seems to be a promising way to obtain high-quality monolayer graphene flakes at industrial scale.

3.2 Carbon and Graphene-Quantum Dots

Carbon-based dots are fluorescent particles with high applicability in analytical and biomedical fields. Cayuela and coworkers summarized the three main types, carbon nanodots, graphene-quantum dots, and carbon-quantum dots, defining their properties and providing a detailed discussion about their applications (Cayuela et al. 2016).

Carbon quantum dots are small carbon particles lower than 10 nm. Graphene quantum dots are constituted by graphene disks from 2 to 20 nm. Both materials have promising applications in sensing and bioimaging owing to their tunable fluorescent properties, small sizes, and low toxicity. Regarding the sensing applications, their deposition onto bare surfaces improves the electrochemical features of the resulting devices (Sun et al. 2013; Algarra et al. 2018).

Carbon-quantum dots can be doped or co-doped with heteroatoms to improve their fluorescence efficacy in terms of quantum yields. Hence, the doping with boron, nitrogen, sulfur, phosphorous, and fluorine was reported in literature (Kandasamy 2019). Regarding graphene-quantum dots, the doping with heteroatoms is also reported (Feng et al. 2018; Kaur et al. 2018).

Several ecofriendly methods have been recently developed for the obtaining of carbon-quantum dots and graphene-quantum dots and their doped derivatives by using different carbon precursors, including waste products and low-toxic chemical reagents (Das et al. 2018). In this sense, microwave and hydrothermal methods emerged in the production of carbon-quantum dots and graphene-quantum dots from green precursors. In Table 7 some recent examples found in the literature are summarized.

Table 7 Obtention of C-QDs and G-QDs assisted by microwave and hydrothermal methods using different precursors

Graphene-quantum dots can also be obtained from bulk carbon sources. They were synthesized using different carbon precursors by hydrothermal route (De Xie et al. 2007). In this work, hydrogen peroxide was used as oxidizing reagent, instead of harsh chemicals used in conventional hydrothermal route, such as nitric acid and sulfuric acid. Another hydrothermal method using graphene oxide as carbon source and hydrogen peroxide was reported (Tian et al. 2016). The acid-free synthesis of graphene-quantum dots by sonochemical method with intermittent microwave heating was performed using graphene oxide as carbon source. The resulting graphene-quantum dots exhibited a high quantum and product yields (Nair et al. 2017). Other acid-free approach by microwave was performed to obtain boron-doped graphene-quantum dots from graphene oxide (Hai et al. 2015).

3.3 Multi-Walled Carbon Nanotubes

Carbon nanotubes are carbon allotropes consisting of rolled-up sheets of carbon atoms. Their outstanding optical, mechanical, thermal, and electrical properties make them suitable candidates in gas sensing (Mao et al. 2014) and electrochemical biosensing (Du et al. 2017), among other applications.

Regarding the synthesis of carbon nanotubes, chemical vapor deposition, arc discharge, and laser ablation constitute the most popular methods for production of MWCNTs nanotubes (Rahman et al. 2019). Several drawbacks are ascribed to them, such as high temperatures required, low energy efficiency, and low yields. Furthermore, fossil carbon sources were employed, e.g., methane (Li et al. 2004) and ethylene (Weizhong et al. 2003). Several green attempts using biomass and other ecological products as raw precursors were carried out. In this section, chemical vapor deposition and microwave pyrolysis will be considered.

3.3.1 Carbon Nanotubes Production by Chemical Vapor Deposition from Ecological Sources

As previously commented, chemical vapor deposition raised as a well-established synthesis method of carbon nanomaterials at both laboratory and industrial scale (Pang et al. 2016).

The conventional carbon sources used in chemical vapor deposition were replaced to green precursors in the last decade. An environmental-friendly hydrocarbon, namely camphor, was reported in the obtention of carbon nanotubes via chemical vapor deposition (Pandey et al. 2013). This precursor was also used as carbon source using rice straws hydrothermally treated by a carbonization hydrothermal method with either ferrocene or ferrocene mixed with nickel nitrate (Fathy 2017). Neem oil was also employed in the synthesis of carbon nanotubes by spray pyrolysis-assisted chemical vapor deposition using ferrocene as catalyst (Kumar et al. 2011).

The use of catalysts, commonly transition metals, was required for the catalytic chemical vapor deposition of carbon nanotubes (Rashid et al. 2015). New alternative catalysts obtained from natural sources were proposed over metallic ones. The catalytic role of several plant leaves extracts, garden grass, rose, kaner, and walnut was proposed in the carbon nanotubes growth on silicon substrate (Tripathi et al. 2017). In this work, nontoxicity, high availability, low temperature, and low cost, among other advantages, are highlighted. Natural laterite powder was also recently employed as a catalyst source for the carbon nanotubes growth through chemical vapor deposition, proposing a growth mechanism involving the fragmentation of the pristine iron-containing material (Kumar et al. 2018). The minerals present in natural resources were reported to play a catalytic role in the carbon nanotubes grown process. The minerals obtained from bamboo charcoal, especially calcium silicate and magnesium metasilicate, were found to be responsible of the nucleation and growth of carbon nanotubes carried out by chemical vapor deposition (Zhu et al. 2012). Another work reports the participation of minerals present in the coconut shell in the growth of carbon nanotubes using radio frequency plasma-enhanced chemical vapor deposition (Araga and Sharma 2017).

Thus, new carbon sources and catalysts were investigated for carbon nanotubes production by means of a chemical vapor deposition process. The green methodologies proposed aimed to reduce the overall costs, energy requirements, reaction times, and the formation of toxic side products.

3.3.2 Carbon Nanotubes Production by Microwave Pyrolysis of Biomass

The carbon nanotubes production by microwave pyrolysis of the biomass has attracted huge attention nowadays. This method involves the decomposition of biomass by microwave heating, leading to higher yields of pyrolytic products in comparison with those obtained by conventional heating (Dhyani and Bhaskar 2018). The principles and mechanisms of pyrolysis by microwave were reported in the bibliography (Nomanbhay et al. 2017; Nizamuddin et al. 2018).

Gumwood was used as carbon source in the carbon nanotubes synthesis via microwave pyrolysis (Shi et al. 2014). The decomposition rate of the biomass by using microwave was found to be greater than the one obtained by means of conventional heating, leading to higher yields of pyrolytic products, as commented before. The microwave pyrolysis of bagasse was performed by using iron and cobalt as metal susceptors (Debalina et al. 2017). Another recent work reported the use of palm kerner shell as raw material, obtaining carbon nanotubes in mild conditions and without adding any catalyst (Omoriyekomwan et al. 2019). The role of cellulose in the synthesis and growth of carbon nanotubes was also reported.

Therefore, microwave heating of biomass constitutes a promising synthesis method in terms of Green Chemistry. However, the mechanism of formation and growth of carbon nanotubes are not fully understood, and hence further investigations are required.

3.3.3 Functionalization of Carbon Nanotubes

Although this subsection is focused on the obtention of carbon nanotubes by green methodologies, their functionalization is also relevant, since their sensing properties seem to be enhanced (Setaro 2017). Among all the functionalization approaches, the decoration of carbon nanotubes with metal nanoparticles constitutes one of the most important methods (Kharisov et al. 2016).

Several decoration procedures were reported in the bibliography. A cost-effective and ecofriendly method using focused solar irradiation over metal precursor and carbon nanotubes dispersion was reported. Different metal, metal oxide, and metal alloy nanoparticles, such as Au, Pt, Ag, NiO, ZnO, and Pt₃Co, were successfully attached on carbon nanotubes surface by using this approach (Baro et al. 2013). Palladium nanoparticles were deposited onto the multi-walled carbon nanotubes surface by drop casting of palladium salt precursor, followed by ultraviolet irradiation (Yoo et al. 2019). The decoration of carbon nanotubes with bimetallic nanoparticles was mediated by aqueous plant extracts (Mendoza-Cachú et al. 2018). Silver nanoparticles were deposited on carboxylated and hydroxylated carbon nanotubes by using a modified Tollens process, which involves the chemical reduction of [Ag(NH₃)₂]⁺ complex with sugars (Dinh et al. 2015). A one-pot microwave synthesis of copper oxide nanoparticle-carbon nanotubes was performed in a solvent-free system using copper acetate as precursor (Rudd et al. 2019). Figure 14 represents a scheme of each functionalization procedure described.

Fig. 14

Different decoration methods of carbon nanotubes with metal nanoparticles: (a) Deposition of metal/metal oxide NPs by solar radiation, (b) deposition of PdNPs by drop casting on MWCNT/Si substrate, (c) deposition of Ag-AuNPs mediated by plant extract, (d) deposition of AgNPs by using a modified Tollens method, and (e) deposition of CuONPs mediated by the sonication and subsequent microwave treatment of copper acetate

3.4 Metal and Metal Oxide Nanoparticles-Supported Materials

The one-pot synthesis of metallic nanoparticles mediated by biological compounds is currently considered by the scientific community (Shamaila et al. 2016). A brief overview of the metallic nanoparticles synthesis using natural extracts and biopolymers will be stated in this section.

3.4.1 Synthesis of Metal Nanoparticles Mediated by Plant Extracts

The employment of several organisms , such as fungi (Vágó et al. 2016; Gudikandula et al. 2017), bacteria (Ahmad et al. 2017), and plants (Makarov et al. 2014), for one-pot metal nanoparticles synthesis has been extensively studied. Among all of them, plant leaf extracts seems to be a good choice based on green terms: scalable process, lower time required, and eco-friendly waste products (Mittal et al. 2013; Yadi et al. 2018).

The last one is a crucial point to reduce the presence of pollutants, improving the human health. Furthermore, the synthesis procedure is very easy, allowing the generation of metal nanoparticles by direct mixing of the plant extract and the metal solution at room temperature (Fig. 15). Thus, high temperatures and additives are not required for the synthesis step.

Fig. 15

Synthesis of metal nanoparticles mediated by plant extracts

The synthesis mechanism of metal nanoparticles using plant extracts is very complex owing to the high amount of phytochemicals able to reduce the metal salt, as well as to avoid the aggregation of the resulting nanoparticles. These compounds, like flavonoids, sugars, and amino acids, among others, contain functionalized groups responsible for the bioreduction and the subsequent stabilization processes (Rai et al. 2013; Jain and Mehata 2017; Zarzuela et al. 2018).

According to the biochemical reduction mechanism proposed in several reports, the plant extract composition is a key factor in the shape and size of the metal nanoparticles obtained. Table 8 summarizes several examples reported in the bibliography in the last years.

Table 8 Size and shape of several metal and metal oxide nanoparticles obtained by using different plant extracts

As observed in Table 8, the morphology of AuNPs is strongly dependent on the composition of the extract. Particularly, Lee and coworkers carried out a detailed study about the reducing role of the phytochemicals contained in the extract for the gold nanoparticles synthesis (Lee et al. 2016). This research work revealed different morphologies depending on the fraction of the extract considered. The crude extract led a heterogeneous mixture of gold nanoparticles, while hexane fraction displayed spherical gold nanoparticles with a few proportion of anisotropic nanoparticles. Regarding the chloroform fraction, gold discs lower than 200 nm were mostly obtained. The aqueous and n-butanol extracts led to Au platelets and irregular nanoparticles, respectively. A similar study was performed for gold nanoparticles synthesized using Zostera noltii extract (an aquatic plant). The buthanolic extract led to mostly spherical nanoparticles with bimodal bell-shaped distribution, while the aqueous/dimethylsulfoxide flavone fraction of buthanolic extract led to nanoparticles with triangular, spherical, and polyhedral shapes (Zarzuela et al. 2018).

The concentration of the phytocompounds present in the extract can influence the shape and size of metal nanoparticles. Smitha and coworkers reported higher proportion of gold anisotropic particles at lower volumes, while spherical shape dominates at higher ones (Smitha et al. 2009). The change in the AuNPs shape by varying the concentration of the extract can be stated on the biosynthesis of gold using other sources. Gold nanotriangles were obtained at low concentration of Aloe vera, while spherical nanoparticles were obtained at higher values (Chandran et al. 2006). Anisotropic gold nanoparticles were also obtained using low concentration of Magnolia kobus (Song et al. 2009).

The nanoparticle’s morphology is influenced by other elements, like pH, reduction time, and temperature, all of them summarized in Baranwal and coworkers review (Baranwal et al. 2016).

3.4.2 Synthesis of Metal Nanoparticles Mediated by Polysaccharide-Based Biopolymers

The green synthesis of metallic nanoparticles also includes the use of biopolymers , mainly characterized by their biodegradability, biocompatibility, and nontoxicity (Hernández et al. 2014). Among all of them, polysaccharides are mainly used for biosynthesis (Wang et al. 2017).

As in the case of the plant extracts use, the metallic nanoparticles synthesis using biopolymers is of great interest from the Green Chemistry point of view. The role of the biopolymer as reducing and/or capping agent avoids the use of any additive, reducing the employment of hazardous reagents. Furthermore, the formation of metal nanoparticles can be reached at short times, minimizing the energy requirements (Balachandran et al. 2015). The main synthesis approach consists of the direct mixing of the biopolymer and the metal solution, obtaining the metal nanoparticles after stirring in one-step.

The gold nanoparticles synthesis using pectin was investigated by Ahmed and coworkers (Ahmed et al. 2016). In their report, the fragmented units of pectin obtained after alkali treatment reduce Au³⁺ to Au⁰ and promote the nucleation process. In addition, the binding between the reducing sugar units and the gold nanoparticles was proposed, preventing their agglomeration. Consequently, small nanoparticles distribution was obtained.

The chemical bonding metal-biopolymer was also proposed in other reports related to the synthesis of metal nanoparticles using polysaccharide biopolymers. Guibal and coworkers summarized the mechanisms involved in metal ion binding on chitosan, proposing the formation of different metallic chitosan complexes (Guibal et al. 2014). The employment of this biopolymer in the synthesis of copper nanoparticles is reported, leading to improvements in their stability (Muthukrishnan 2015). Gold and silver nanoparticles with high stability were also mediated by chitosan (Wei and Qian 2008). The chelating role of soluble starch was also reported by the linkage between the aldehyde terminal of amylose units and silver in the silver nanoparticles synthesis (Yakout and Mostafa 2015).

Table 9 shows some examples of silver nanoparticles’ size and morphology synthesized using biopolymers.

Table 9 Size and morphology of silver nanoparticles using different polysaccharide biopolymers

The synthesis of metallic nanoparticles can also be mediated by biopolymeric nanostructures using two approaches, according to several research works (Preiss et al. 2014): incorporation of metallic nanoparticles into a polymeric matrix (ex situ) and the synthesis of metallic nanoparticles in the biopolymeric matrix (in situ).

Boury and Plumejeau summarized the main aspects concerning the in situ synthesis of metal oxide nanocomposites (Boury and Plumejeau 2015). The combination between metal precursor and the biopolymer lead to biopolymer-metallic composite. Afterwards, the biotemplate was removed by thermal treatments, obtaining the metallic oxide nanocomposite. A general scheme is shown in Fig. 16.

Fig. 16

Synthesis of metallic nanocomposites assisted by biopolymer templating

The metal oxide nanoparticles can be formed and included into the polymeric scaffold using the sol-gel process, controlling their morphology. Catalytic effects in the sol-gel kinetics are ascribed to the bipolymer, as well as a strong chemical affinity with metals due to the presence of hydroxyl and amine groups (Plumejeau et al. 2015). Zlotski and Uglov reported the synthesis and immobilization of different oxide nanoparticles on cellulose fibers template by sol-gel process (Zlotski and Uglov 2017). A flake-like structure was observed after calcination with irregular pores, testifying that cellulose fibers provided seeding points of metal oxide nanoparticles nucleation and growth. Calcium alginate is also used as biotemplate for the synthesis of titanium oxide beads by sol-gel route (Kimling and Caruso 2012).

β-Cyclodextrin, chitosan , and starch were used for the synthesis of titanium oxide nanoparticles by Bao and coworkers using a simple mineralization process (Bao et al. 2013). In their work, the polysaccharide biotemplate plays a key role on the final nanostructure: small rods for β-cyclodextrin, chestnut-like for chitosan, and small nanoparticles for soluble starch. The influence of the polysaccharide on the crystalline structure is also discussed. The rutile phase was obtained mediated by chitosan, while anatase was obtained mediated by starch and β-cyclodextrin.

Therefore, the biosynthesis of metal and metal oxide nanoparticles was carried out by plant extracts and biopolymers, obtained from renewable and clean sources. Their reducing and capping agent role can be highlighted, reducing the waste products. Furthermore, the nanoparticles shape and size can be monitored by the synthesis conditions, such as phytocompounds composition and concentration of the extract and the nature of the polysaccharide, among others.

3.5 Polymers

Polymers are compounds constituted by several monomeric units linked by covalent chemical bondings. They can be employed as immobilization matrices of several enzymatic compounds, increasing the selectivity of the system with respect to a target analyte (García-Guzmán et al. 2018; Bilal and Iqbal 2019). Their use as bulk transducers in some electronic devices should also be highlighted, improving their electronic transference and, hence, their electrochemical performance (Gautam et al. 2018).

Polymers can be ordered into two groups: intrinsic conducting polymers and nonconducting polymers. Several synthesis routes to produce some of them will be shown in this subsection.

3.5.1 Intrinsic Conducting Polymers: Polythiophene Derivatives, Polyaniline, and Polypyrrole

Conducting polymers have emerged in the last decade due to their high electrical properties , among others. Their high conductivity, attributed to the π-conjugation and electronic doping, enables their use in electrochemical sensing (Kenry and Liu 2018).

Moreover, their redox properties make them suitable for the fabrication of electrochromic and sensing devices (Naveen et al. 2017). Figure 17 shows the chemical structure of the most representative intrinsic conducting polymers, poly-(3,4-ethylenedioxythiophene) (PEDOT) , polyaniline (PANI), and polypyrrole (Ppy).

Fig. 17

Chemical structure of PEDOT, PANI, and Ppy

The chemical polymerization is a high-scalable approach based on the chemical oxidation of the monomer. Many oxidizing agents and dopants were investigated to control the electrical properties of the resulting polymer and increase their oxidation degree (Nguyen and Yoon 2016). Several synthesis strategies were investigated as alternative polymerization routes.

The ultrasound-assisted synthesis led to high polymerization rates, as well as an improvement in the electrical features of the resulting polymer in comparison with conventional route (Ali Mohsin et al. 2016). The influence of frequency waves was studied in the acid polymerization of aniline with ammonium persulfate (Husin et al. 2014). PANI nanofibers’ diameter decreased when ultrasonic frequency takes higher values. Thus, the role of the ultrasound in the morphology of PANI was demonstrated. Uniform spherical PEDOT nanoparticles were produced under ultrasound, in contrast with irregular-shaped nanoparticles obtained by conventional stirring (Zhong et al. 2010). PEDOT microspheres with controlled morphology were obtained via ultrasonic spray polymerization by using different oxidants and solvents (Zhang and Suslick 2015). The microwave-assisted synthesis of conducting polymers was also proposed as an alternative to conventional synthesis, reducing significantly the reaction time and leading to improvements in product yields. Polyaniline was obtained under microwave in 5 min, with a similar yield value than the one obtained under conventional synthesis at 5 h, around 76%. In addition to the reaction time, the morphology was also affected by microwave heating. Nanofibrillar morphology was observed under microwave, while granular morphology was obtained under conventional conditions (Gizdavic-Nikolaidis et al. 2010). A more recent work shows the influence of the concentration of hydrochloric acid in the morphology of PANI, synthesized by oxidative microwave-assisted polymerization: low acid concentration values provided oligomeric chains with flat structure, while high acid concentration led to nanofibers (Qiu et al. 2017). It should be noted that longer oligomeric chains were observed in microwave-assisted polymerization with respect to those obtained under conventional synthesis at same reaction times, demonstrating the role of microwave heating in the improvement of reaction rates and yields.

Enzyme-catalyzed synthesis is another environmental friendly route to produce conducting polymers with desirable electrical and morphological features at mild conditions. The enzymatic polymerization of thiophene, aniline, and pyrrole with embedded glucose oxidase was performed (German et al. 2019). Several synthesis parameters, such as pH, monomer concentration, and ratio enzyme/substrate, were investigated by spectrophotometric assays. Laccase produced by Aspergillus oryzae was used as biocatalyst in the polymerization of aniline (De Salas et al. 2016). The polymerization of EDOT was achieved mediated by horseradish peroxidase with high efficiency, by using hydrogen peroxide as clean oxidant (Wang et al. 2014). Two Aspergillus niger strains were employed in the polymerization of pyrrole with hydrogen peroxide (Apetrei et al. 2018).

Therefore, biosynthetic routes provide several advantages with respect to the conventional polymerization process, like improvements in product yields, reduction of reaction times, tailored morphological features, and high electrical conductivities. However, some instrumental parameters required a specific control in order to obtain conducting polymers with the desired electrical properties and nanosized morphology.

3.5.2 Nonconducting Polymers: Polysaccharide Biopolymers

Polysaccharide biopolymers are obtained by natural resources. Several pieces of research were focused on their biosynthesis in order to understand the different mechanism pathways. The metabolic engineering offers some improvements in the green synthesis of tailored biopolymers, and thus different synthetic biologic strategies were reported in the literature (Anderson et al. 2018). The biosynthetic routes for three relevant polysaccharide biopolymers are listed in this subsection.

Chitosan and chitin are located in the shell of crustaceans, exoskeleton of mollusks, and fungi’s cell walls. For industrial applications, crustacean shells constitute the main source for the production of chitosan and chitin. The chemical extraction procedure to obtain the pure product, shown in a recent review (Abo Elsoud and El Kady 2019), involves five steps: demineralization, discoloration, deproteinization, acid reflux, and deacetylation. The fungal biosynthesis of chitosan was also proposed in the same work for industrial applications over crustacean sources due to several reasons: simple extraction procedure, low cost waste management, and high availability, among others.

Alginate was initially isolated from farmed brown seaweeds for commercial production. However, these algal alginates suffer from heterogeneity in composition and material properties. Other alternative biosynthetic route is the microbial synthesis, summarized in the following review (Hay et al. 2013).

Two types of bacteria, pseudomonas and azotobacter, were used in the alginate biosynthesis (Hay et al. 2013). In this way, the production of alginate was reported using Pseudomonas mandelii (Vásquez-Ponce et al. 2017), aeruginosa (McCaslin et al. 2015), and fluorescens (Maleki et al. 2017). Regarding azotobacter, Azotobacter vinelandii was widely employed for alginate biosynthesis (Saeed et al. 2016). Starch is mainly located in the endosperm of cereal grains. Its biosynthesis via ADP-glucose pathway involves the use of sucrose as carbon source, imported from leaves, and the participation of three primary enzymes, ADP-glucose pyrophosphorylase, starch synthase, and starch branching (Thitisaksakul et al. 2012). The starch biosynthesis was investigated in the last decade by using different cereals, such as rice (Fujita 2014), wheat (Chen et al. 2016), maize (Jiang et al. 2013), and grass (Tetlow and Emes 2017). The starch biosynthesis in plants, such as Cassava (Tappiban et al. 2019) and Arabidopsis thaliana (Malinova et al. 2018), was also investigated.

3.6 Bare Electrochemical Devices and Their Modifications: Carbon Ceramic Materials

Although the majority of reports related to chemical sensors were based on the deposition onto glassy carbon, metallic or ITO surfaces, the development of silicon and carbon ceramic materials by using sol-gel technology and assisted by high-power ultrasound was carried out by some research groups (Hidalgo-Hidalgo-de-Cisneros et al. 2001).

The sol-gel process has been extensively applied to obtain silicon oxide and silicon oxide-derived monolithic substrates at room temperature from silane alkoxides or metallic precursors (Kajihara 2013). The employment of organic solvents, like methanol or ethanol, is required due to the immiscibility of the silane/metallic precursors in aqueous solution. Their use can be suppressed by high-power ultrasound, leading to a new type of silica monolith with higher density matrix. This also implies the reduction of waste products. The structural and mechanical properties of these new materials, namely Sonogels, were extensively studied (Blanco et al. 1999).

The addition of carbon powder after the sonication process leads to a conducting material able to be used as electrochemical device. In the first formulations, Sonogel-Carbon, graphite powder was employed owing to its low cost and high availability, as well as its high electrical conductivity (Cordero-Rando et al. 2002; Cubillana-Aguilera et al. 2006). Figure 18 shows a schematic representation of the fabrication of the Sonogel-Carbon electrodes.

Fig. 18

Fabrication of Sonogel-Carbon electrodes

The synthesis process of Sonogel-Carbon is characterized by its high versatility, allowing the obtention of new bare electrode materials. The graphite powder added to the sonosol after sonication can be replaced totally by multi-walled carbon nanotubes and nanocarbon (Palacios-Santander et al. 2017) or partially by 1-furoylthiourea (Cubillana-Aguilera et al. 2010), β-cyclodextrin (Izaoumen et al. 2009), and L-cysteine (El Bouhouti et al. 2009), among other modifiers (Bellido-Milla et al. 2013). The substitution of the acid solution used as catalyst by gold nanoparticles colloidal solution obtained from a green route is under investigation (Franco-Romano et al. 2013). Based on their acid pH, they can catalyze the formation of the silicon oxide network from the silane precursor. Finally, the inclusion of a conducting polymer in the silicon oxide network by using high-power ultrasound was reported in other work (López-Iglesias et al. 2018). Figure 19 summarizes all the synthesis routes mentioned in this subsection.

Fig. 19

Different synthesis routes for the fabrication of Sonogel-Carbon derived composites

Therefore, the synthesis of several conducting ceramic materials assisted by high-power ultrasound in one-step was reported in the bibliography. The easy, low-cost and low-time consuming methods are highlighted, together with the use of high-power ultrasound, allowing decrease in the reaction time, as well as minimize the use of hazardous solvents. Moreover, the versatility and greenness of the synthesis scheme, as shown in Figs. 18 and 19, may increase, in the near future, the number of materials susceptible of being employed in the fabrication of ceramic-based electrode devices (López-Iglesias et al. 2016).

4 Application of Green (Nano)Materials in Analytical (Bio)Sensing

The applications of the different (nano)materials obtained by a green synthetic route for (bio)sensing are the main focus of this section. In order to clarify the structure that will be followed, several tables collecting the different materials by type have been included.

In Table 10, a representative collection of ionic liquids used in sensor systems are shown. Some of them correspond to enzymatic biosensors used either as amperometric sensors or in colorimetric tests. The half of them employs nanomaterials to enhance sensitivity, such as graphene and carbon nanotubes.

Table 10 Sensors and biosensor systems based on different ionic liquids and most relevant figures of merits

The amount of analytes that can be detected by these devices based on ionic liquids is quite diverse, including analytes of interest in food industry (like ascorbic and caffeic acid), medical and healthcare industry (i.e., cholesterol and neurotransmitters), and environmental applications (pesticides). They were determined in biological and agrifood matrices such as water, juices, human serum, and blood. The performance of ILs-based sensor systems can be evaluated by several quality analytical parameters, i.e., limit of detection and reproducibility. In general, the figures of merits are quite good in most cases for the linear ranges indicated.

The original interest and applications of ILs in (bio)sensing started decreasing when the scientific community realized the possible problematic related to the elimination of the ILs after their use from the environment (Bystrzanowska et al. 2019). More information concerning analytical application of ILs in sensing systems can be found in Yavir and coworkers’ review (Yavir et al. 2019).

The use of other kinds of green nanomaterials used for building sensing and biosensing devices, like graphene, quantum dots, carbon dots, and metal and metal oxide nanoparticles, principally, may also be assessed. Table 11 reports optical devices and Table 12 electrochemical ones for multiple sensing applications.

Table 11 Optical sensors and biosensor devices based on green nanomaterials and most relevant figures of merits

Table 12 Electrochemical sensors and biosensor devices based on green nanomaterials and most relevant figures of merits

With respect to the optical sensors described in Table 11, most of them are based on carbon or quantum dots, in some cases doped with lantanide metals to enhance the luminescence signal, and applied directly in buffer solutions. The variety of target analytes is much higher than in the previous case: nitrophenols, cations, anions, metals, amino acids, enzymes, guanine phosphorylated derivatives, and/or their nanocomposites. Some of them are considered as priority environmental pollutants and others have high applicability in biomedical or pharmaceutical fields. According to this classification, the type of samples measured are highly diverse, including cellular systems, such as MCF-7 and HeLa cells, soil samples, natural water samples, or serum samples. The limits of detection obtained for the analytes mentioned is quite low, most of them in the nanomolar range for more or less wide dynamic ranges.

On the other hand, Table 12 shows some examples of electrochemical sensing and (bio)sensing devices mainly based on carbon transducers modified with the aforementioned green nanomaterials. Their application is fundamentally focused on biomedical analysis, including some pharmaceuticals. That is why the real samples where the determination has been accomplished correspond to human samples (i.e., serum, blood, and urine). Only in two cases food and environmental samples are measured. As observed in the previous devices reported, limits of detection are at nanomolar level; besides, reproducibility is excellent (lower than 5–7%) and, when applicable, sensitivity is also quite good. Other examples can be seen in Table 12, but exclusively referred to real environmental, biomedical, and food industrial uses. Excellent figures of merits are reported in all cases.

Hence, most of these (bio)sensing devices can be or are susceptible of being produced and/or used directly in industry. As discussed in the previous paragraphs and tables, the performance of these analytical systems are more than adequate for fulfilling the industry requirements whatever the purpose they are used for: detection and/or determination, quality, and waste control, in different kinds of industrial companies, such as food, environmental, biopharmaceutical, and biomedical industries (Siontorou 2019).

5 Industrial Application of (Bio)Sensing Devices

As mentioned in Sect. 4, analytical sensing constitutes high interest in the chemical industry , attracting great attention nowadays. Thus, the employment of robust analytical systems with suitable performance in the in situ analytical detection/determination or quality control is being implemented in industry. In this section, some applications of (bio)sensing devices able to be used at industrial scale are reported.

Several recent reports concerning the application of sensing devices in several fields of interest such as food, clinical, and medicines at industrial scale are highlighted. A recent review is focused on the application of biosensors for whole-cell bacterial detection. The interest of portable stand-alone biosensors in the rapid detection and diagnosis of critical illnesses (meningitis, food-borne pathogens, sexually transmitted diseases, anthrax detection) is reported. Furthermore, the review discusses recent progress in the use of the biosensors without the need for sample processing compared with current methods of bacterial detection. A particular focus on electrochemical biosensors is made (Ahmed et al. 2014). Another review encompasses recent developments in the technological innovations concerning the detection of food allergens. Hypersensitivity towards this kind of allergens is increasing worldwide. Therefore, improving the methodology for food allergens detection will permit the identification of individual immune triggers, the prediction of the response severity, and the monitoring of allergen tolerance over time. Neethirajan and coworkers reported several biosensors as innovative analytical devices for enzymes, antibodies, aptamers, and single-stranded DNA detection. Optical, electromechanical, and electrochemical biosensors employed in the detection of food allergens are also reviewed (Neethirajan et al. 2018b).

A review about nanogenerators for self-powered gas sensing discusses their ability as technological and economical driver for global industries. Nanogenerators are applied as self-powered environmental sensors and the paper summarized 24 references of nanogenerators-based self-powered gas sensors (Wen et al. 2017).

Another paper reported the virtual sensing technology, known as soft-sensors in the area of chemical engineering. This technology is a key to estimate successfully product quality when online analyzers are not available. The applications are broad and extendable to fields such as petrochemical, steel, and pharmaceutical industries (Kano and Fujiwara 2013).

Therefore, with some effort made by all the societal sectors : public in general, academia and industries, and always thinking on as much global as possible sustainability, some of these (bio)sensing devices might reach the widespread usefulness and success of the currently most extended biosensor, the glucometer, which in many cases is also complemented with the determination of other related analytes, such as cholesterol and lactic acid, among others (Calabria et al. 2017; Pilas et al. 2017; Cunha-Silva and Arcos-Martinez 2018; Kotanen et al. 2018). In this way, not only would biomedical point-of-care diagnoses be possible, but also environmental, pharmaceutical, and food ones.

6 Conclusions

As it has been highlighted throughout this chapter, Chemical Industry, with the aim of Academia and increasingly with the growing support of Society, is currently looking for and reaching, in many cases, sustainability thanks to Green Chemistry. This multidimensional philosophy can offer many different alternatives and tools to redirect the modern industry to a more ecological and efficient manufacturing and production. The concept of “reducing” (costs, energy, wastes, and materials, among others), sustainable and green education, and government policies are considered important milestones to bring this matter to a successful conclusion.

Particularly, green synthesis is part of the foundations of Green Chemistry, since the search for the perfect synthesis with all its implications (safe, atom economy, high yield, etc.) is of vital importance to achieve sustainable industrial production. Moreover, efficient synthetic routes can be extended to different areas, especially to Analytical Chemistry, where the use of sensors and biosensors can represent a green choice with respect to conventional methodologies, as widely reported in literature during the last decade. Their many advantages among which stand out rapidity, analytical performance, on line and in situ measurements and cost-effectiveness, make them of great relevancy for being used in many kinds of industries as for analyte detection/determination as for quality control. That is why it is essential to know the different possibilities regarding the green synthesis of materials and nanomaterials that can be used for building sensors and biosensors.

In this chapter, a summary of the most relevant green materials synthesis or biosynthesis routes is provided. Special attention has been paid to ultrasound and microwave technologies. During the past few decades, both has been widely recognized as ecological and clean, as well as cost and time effective, having been demonstrated that their use fulfills most of the 12 Principles of Green (Analytical) Chemistry and is quite sustainable for synthesis purposes of both organic and inorganic materials. Moreover, the use of biological compounds to synthesize complex molecules or nanostructures, preferably by a one-pot method, is also emphasized. The focus is put on the employment of plant extracts, biopolymers, and microorganisms, such as algae, bacteria, and fungi. The possibility of combining microwave or ultrasound technologies and any of them with some biosynthetic route is also discussed through the most recent and representative achievements reported in literature within the field of green synthesis of (nano)materials. It is also necessary to mention that the most important and current trends in green (nano)materials production affects to graphene, carbon and graphene quantum dots, multi-walled carbon nanotubes, metal and metal oxide nanoparticles and polymers. A very wide set of examples concerning green synthesized materials is reported in Sect. 3, most of them already applied or with high potential applicability in industry. Furthermore, the possibility of replacing actual and more expensive metal, glassy carbon or ITO-based materials to new ceramic electrode materials, mainly for electrochemical devices, is also explored. The versatility and greenness of their scheme of synthesis and their excellent analytical performance makes them a strong alternative to typical and conventional electrode materials. It has been demonstrated as well that the applicability of the mentioned green (nano)materials for analytical (bio)sensing is widely extended even at industrial level. An extensive set of optical or electrochemical devices employed for that purpose are described on depth according to their most relevant quality analytical parameters: limit of detection, sensitivity, linear range, and reproducibility. As reported, the kind of analytes that can be determined with (bio)sensing devices includes the fields of food, environment and medicine and healthcare.

Despite all, the path of industry towards sustainability has just begun and there is still too much work to do in order to completely erase the prejudices and the fear to chemistry, in general; in other words to banish chemophobia . However, one thing is absolutely clear: our life without chemistry or chemicals would be unthinkable, since life itself is chemistry.