Synonyms

Nanomaterials application in soil and food testing

Definition

Nanomaterials: generally referring to materials with the size of 0.1–100 nm.

Carbon nanotube: allotropes of carbon with a cylindrical nanostructure.

Biosensor: a device for the detection of an analyte that combines a biological component with a physicochemical detector component.

Introduction

The Food and Agriculture Organization (FAO) is the main United Nations agency specializing in all aspects of food quality and safety, and in all the different stages of food production, harvest, postharvest handling, storage, transport, processing, and distribution. Food analysis is the discipline dealing with the development, application, and study of analytical procedures for characterizing the properties of foods (Nielsen, 2003). These analytical procedures are used to provide information about a wide variety of different characteristics of foods, including their composition, structure, physicochemical properties, and sensory attributes. This information is critical to our rational understanding of the factors that determine the properties of foods, as well as to our ability to economically produce foods that are consistently safe, nutritious, and desirable and for consumers to make informed choices about their diet. One of the most important reasons for analyzing foods from both the consumers and the manufacturers’ standpoint is to ensure that they are safe.

Precision farming has been a long-desired goal to maximize output (i.e., crop yields) while minimizing input (i.e., fertilizers, pesticides, and herbicides) through monitoring environmental variables and applying targeted action. A soil analysis is used to determine the level of nutrients found in a soil sample. Quality crops with high yields require a sufficient supply and maintenance of nutrient elements. As nutrients are utilized by one crop and not replaced for subsequent crop production, yields will decrease accordingly. Accurate monitoring of nutrient before and after crop production and soil analysis results will help the efficient management of fertilizer applications. Soil analysis can also help to reduce agricultural waste and thus keep environmental pollution to a minimum. Researchers are exploring to come up with sensors for detection of soil nutrients, pesticides, pollutants up to very minute fractions by exploiting novel properties of nanomaterials.

The definition of nanomaterial is based on the prefix “nano,” which is from the Greek word meaning “dwarf.” The word nanomaterials is generally used when referring to materials with the size of 0.1–100 nm; however, it is also inherent that these materials should display different properties from bulk (or micrometric and larger) materials as a result of their size (Rao et al., 2004). These differences include physical strength, chemical reactivity, electrical conductance, magnetism, and optical effects. The potential of nanomaterials to revolutionize the health care, textile, materials, information and communication technology, and energy sectors has been well publicized. In fact, several products enabled by nanomaterials are already in the market, such as antibacterial dressings, transparent sunscreen lotions, stain-resistant fabrics, scratch-free paints for cars, and self-cleaning windows.

Nanomaterials such as nanotubes (NTs), nanowires (NWs), and nanoparticles present new opportunities as sensing platforms for biological and environmental applications. Having micrometer-scale lengths and nanometer-scale diameters, NTs and NWs can be manipulated with current microfabrication, as well as self-assembly techniques to fabricate nanoscale devices and sensors (Rao et al., 2004). Examples of different nanomaterials-based analytical techniques for the detection of major families of environmental pollutants, i.e., organic contaminants, heavy metals, and air pollutants are reported. Application of the nanomaterials in the field of soil and food analysis is promising. This article covers the recent developments and issues in electrochemical biosensors for food analysis such as ease of preparation, robustness, sensitivity, and realizations of mass production of the detection strategies. This article also emphasizes the current development of electrochemical biosensors combined with nanotechnology.

The synthesis, characterization, and utilization of nanomaterials are part of an emerging and rapidly growing field. Nanomaterials may be grouped under nanoparticles (the building blocks), nano-intermediates, and nanocomposites. Nanostructured materials are synthesized by supramolecular chemistry yielding nanoassemblies (Rao et al., 2004). The nanoparticles serve as the building blocks of nanomaterials and devices. They include nanocrystalline materials such as ceramic, metal and metal oxide nanoparticles; fullerenes, nanotubes, nanorods, and related structures; nanofibers and wires, and precise organic as well as hybrid organic–inorganic nanoarchitechtures such as dendrimers and polyhedral silsesquioxanes, liposomes, or nanosomes, respectively.

Nanocrystalline materials

Included here are ceramics, metals, and metal oxide nanoparticles. These materials are assembled from nanometer-sized building blocks, mostly crystallites. The building blocks may differ in their atomic structure, crystallographic orientation, or chemical composition. In other words, materials assembled of nanometer-sized building blocks are microstructurally heterogeneous, consisting of the building blocks (e.g., crystallites) and the regions between adjacent building blocks (e.g., grain boundaries). One of the primary applications of metals in chemistry is their use as heterogeneous catalysts in a variety of reactions (Rao et al., 2004). Due to their vastly increased surface area over macroscale materials, nanometals and oxides are ultrahigh activity catalysts. Nanometals and oxides are also widely used in the formation of nanocomposites. Aside from their synthetic utility, they have many useful and unique magnetic, electric, and optical properties.

Carbon nanotubes

Carbon nanotubes (CNTs) are hollow cylinders of carbon atoms. Their appearance is that of rolled tubes of graphite such that their walls are hexagonal carbon rings and are often formed in large bundles. Generally speaking, there are two types of CNTs: single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) (Rao et al., 2004). As their names imply, SWCNTs consist of a single, cylindrical graphene layer, whereas MWCNTs consist of multiple graphene layers telescoped about one another. CNT-based nanodevices are a hot research area at the moment. Applications could include novel semiconducting devices, chemical sensors, and ultrasensitive electromechanical sensors (Wang, 2005).

Nanocomposites

Nanocomposites are materials with a nanoscale structure that improve the macroscopic properties of products. Typically, nanocomposites are clay, polymer or carbon, or a combination of these materials with nanoparticle building blocks. Nanocomposites, materials with nanoscale separation of phases can generally be divided into two types: multilayer structures and inorganic/organic composites. Multilayer structures are typically formed by gas phase deposition or from the self-assembly of monolayers. Inorganic/organic composites can be formed by sol–gel techniques, bridging between clusters (as in silsequioxanes), or by coating nanoparticles, in polymer layers for example.

Biosensors

Biosensors are molecular sensors that combine a biological recognition mechanism with a physical transduction technique. They provide a new class of inexpensive, portable instrument that permit sophisticated analytical measurements to be undertaken rapidly at decentralized locations. The sampling component of a biosensor contains a bio-sensitive layer that can either contain bioreceptors or be made of bioreceptors covalently attached to the transducer. The interaction of the analyte with the bioreceptor is designed to produce an effect measured by the transducer, which converts the information into a measurable effect, for example, an electrical signal. There are four major types of transducers: electrochemical (electrodes), mass (piezoelectric crystals or surface acoustic wave devices), optical (optrodes) and thermal (thermistors or heat-sensitive sensors). Among the various types of biosensors, the electrochemical biosensors are the most common as a result of numerous advances leading to their well-understood biointeraction and detection process (Eggins, 2002).

The state of the art of nanomaterials and nanotechnologies represents a new trend in the development of sensors and electronic chips that will have a big impact on the future of nanoscience. It is essential to distinguish between nanotechnology and nanomaterials, because in the first case nanotechnologies represent new possibilities for sensor construction and for the developing of novel methods. In the second case, nanomaterials have been widely used to immobilize enzymes, antigens, and nucleic acids on transducer surfaces, to promote the direct electron transfer reactions, and to amplify and orient the analytic signal of the bio-recognition events.

Applications

In chromatography

Separation science, based on chromatographic and electrophoretic techniques, has achieved many advances employing nanomaterials. Separation media and channels in the above two approaches have sizes and shapes comparable to those of nanomaterials, which makes the latter useful for specific applications in separation science on a micro- and nanometer scale. Nanomaterials have played various roles (e.g., modifier, stabilizer, and stationary phase) in chromatography. The effective pi–pi interactions between fullerenes and phenyl group have utilized to develop fullerene-based stationary phases for the separation of solutes with phenyl moieties in their structures. The conjugated pi- electron system on the surface of SWCNT as well as surface functionalization provides an opportunity to synthesize a stationary phase with good selectivity (Zhang et al., 2006). Nanoparticles, including silica nanoparticles, gold nanoparticles, titanium oxide nanoparticles, polymer nanoparticles, molecularly imprinted polymers, molecular micelles, and dendrimers, used as pseudostationary phases in CEC, have been reviewed by Nilsson et al. (2006).

In optical sensors

Nanomaterials-based optical sensors have been much interested to the trace detection of analytes of interest in the agriculture and food industry. The changes in the optical properties of nanomaterials such spectral absorbance, photoluminescence (PL), and chemiluminescence (CL) phenomena induced by the interaction between nanomaterials and various analytes is utilized to the determination of chemical and biochemical analytes (Shi et al., 2004). Quantum dots (QDs) are nanocrystals of inorganic semiconductors that are somewhat restricted to a spherical shape of around 2–8 nm diameter (Smith and Nie, 2004). Their fluorescent properties are size-dependent and therefore they can be tuned to emit at desired wavelengths (between 400 and 2,000 nm) if synthesized in different composition and size. In this way, QDs of different sizes can be excited with a single wavelength and emission controlled at different wavelengths, thus providing for simultaneous detection. These, together with their highly robust emission properties, make them more advantageous for labeling and optical detection than conventional organic dyes (Patolsky et al., 2006). Their high quantum yields and their narrow emission bands produce sharper colors, lead to higher sensitivity and the possibility of multiplexing of analysis (Tully et al., 2006). The unique optical properties of plasmonic nanoparticles have led to the development of label-free chemical and environmental sensor since the surface plasmon resonance (SPR) is sensitive to the local environment. Some research groups are exploring biosensors based on the SPR exhibited by metal nanoparticles (Haes and Van Duyne, 2002).

In electrochemical biosensors

One-dimensional (1-D) nanostructures, such as CNT and semiconductor- or conducting polymer nanowires, are particularly attractive materials for working electrode in biosensors. Nature of biosensing surface is very important, namely, the prolonged use of the sensor and an anticipated extended storage and working stability. High surface-to-volume ratio and electron transport properties of CNT opens the possibility of developing superior electrochemical sensing devices, ranging from amperometric enzyme electrodes to label-free DNA hybridization biosensors (Zhang et al., 2009). The possibility of direct electron-transfer between enzymes and electrode surfaces could pave the way for superior reagentless biosensing devices, as it obviates the need for co-substrates or mediators and allows efficient transduction of the bio-recognition event. “Trees” of aligned CNT in the nanoforest, prepared by self-assembly, can act as molecular wires to allow electrical communication between the underlying electrode and redox proteins covalently attached to the ends of the SWCNT (Gooding et al., 2003). Viswanathan et al. (2009) demonstrated that vertically aligned SWCNT on gold electrode for pesticides determination (Figure 1). Arrays of nanoscopic gold tubes or wires have been prepared by electroless deposition of the metal within the pores of polycarbonate particle track-etched membranes (Marc and Sophie, 2003). A sensitive and selective genomagnetic assay for the electrochemical detection of food pathogens based on in situ DNA amplification with magnetic primers reported by Lermo et al. (2007). Liposomes are microscopic, fluid-filled, pouches with endless walls that are made of layers of phospholipids identical to the phospholipids that make up cell membranes. Electroactive marker encapsulated immuno liposomes are typically used as signal amplifier for electrochemical immunoassays (Viswanathan et al., 2006). Chitosan (CS) is the second abundant polysaccharide and a cationic polyelectrolyte present in nature. Chitosan nanoparticles are promising biometarials for various analytical applications. Ferrocene-conjugated chitosan nanoparticles were used as the electroactive indicator of hybridization (Kerman et al., 2008).

Nanomaterials in Soil and Food Analysis, Figure 1
figure 131figure 131

Atomic force microscopic image of ssDNA-wrapped single-walled carbon nanotube (SWCNT) self-assembled monolayer on Au(111) surface (Viswanathan et al., 2009).

Full size image

Electronic tongue

Electronic tongue systems are hybrid micro or nanoarrays of electronic sensors that measure and compare tastes. E tongue is mainly based on potentiometric, voltammetric, ion-selective field-effect transistor (ISFET), piezoelectric, and optical sensors with pattern recognition tools for data processing. The information given by each sensor is complementary and the combination of all sensors results generates a unique fingerprint. Most of the detection thresholds of sensors are similar or better than those of human receptors. The electronic tongue appeared to be capable of distinguishing between different sorts of beverages: natural and artificial mineral waters, individual and commercial brands of coffee, flesh food, and commercial and experimental samples of soft drinks containing different sweeteners (Scampicchio et al., 2008). Ciosek and Wroblewski (2007) have reviewed about recent developments of multisensor array based electronic tongue for food and soil analysis.

Electronic nose

Electronic nose is a specific kind of semiconducting sensor arrays that can mimic the natural olfaction sense, according to the electronic response (e.g., voltage, resistance, conductivity) arising from the different gas sensors, usually metal-oxide chemosensors. After exposure of the volatile compounds to the sensor array, a signal pattern is collected and results are evaluated with multivariate analysis or processed by an artificial neural network. Arrays of these nanosensors are able to detect molecules on the order of one part per million, sniffing molecules out of the air or taste them in liquid, suggesting applications in foods and food industry. A novel hybrid chemical sensor array composed of individual In2O3 nanowires, SnO2 nanowires, ZnO nanowires, and single-walled carbon nanotubes with integrated micromachined hotplates for sensitive gas discrimination was demonstrated by Chen et al. (2009). Mycotoxins are secondary metabolites that mold produce naturally from some fungal species. Many researchers have reported efficient e-nose application such as mycotoxins analysis in grains (Falasconi et al., 2005), Salmonella typhimurium in stored beef (Zhang et al. 2008).

Mass-sensitive sensors

Researchers have taken advantage of the unique coupled semiconducting and piezoelectric properties of metal oxide nanowires to create a new class of electronic components and devices that could provide the foundation for a broad range of sensor applications. Plata et al. (2008) reported the microcantilever-based sensor for the determination of total carbonate in soil.

Conclusions

Soil and food analysis has become a very important and interesting area of research because of the rapid expansion of food trade and awareness of organic farming. Quality food is important both for consumer protection and also for the food industry. Nanomaterials such as nanoparticles, nanowires, and nanotubes open a new door as sensing platforms for sensor applications. They have allowed introducing novel strategies in sensors and biosensor technology. In particular, the development and application of nanomaterials in soil and food analysis are discussed, with focus on sensors, separation and extraction techniques, including the use of nanomaterials as transducer elements for sensors. Although not fully implemented yet, tiny sensors and monitoring systems enabled by nanotechnology will have a large impact on future precision farming methodologies. The prediction is that nanotechnology will transform the entire food industry, changing the way food is produced, processed, packaged, transported, and consumed.

Cross-references

Agrophysical Objects (Soils, Plants, Agricultural Products, and Foods)

Chemical Imaging in Agriculture

DNA in Soils: Mobility by Capillarity

Electrochemical Measurements in Soils

Enzymes in Soils

Leaching of Chemicals in Relation to Soil Structure

Oxidation–Reduction Reactions in the Environment

Physical Degradation of Soils, Risks and Threats

Precision Agriculture: Proximal Soil Sensing

Quality of Agricultural Products in Relation to Physical Conditions