1 Introduction

Over the years, carbon-based nanomaterials have found applications in a plethora of areas. The major application is in single and multi-walled carbon nanotubes (SWCNT, MWCNT), which can be classified into one-dimensional materials. Graphene and graphene oxide (GO), graphene quantum dots (GQDs), etc. come in the two-dimensional category, whereas fullerene, nanodiamonds, and carbon dots (CDs) can be termed as zero-dimensional [1,2,3]. Figure 1 shows the different forms of carbon nanoallotropes [2, 4]. Carbon nanodots (CDs) are carbon nanoparticles having a size in the dimensions of < 10 nm [5,6,7], are quasi-spherical, and are well dispersed [5, 8]. CDs, in general, comprise carbon skeleton besides other primary elements such as oxygen and hydrogen in varying ratios. CDs mostly exist in amorphous quasi-spherical shapes containing both sp2 and sp3 hybridized carbon atoms having a size < 10 nm [6].

Fig. 1
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Nanoallotropes of carbon [2, 4]

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In recent years, owing to their remarkable properties, such as low toxicity, high biocompatibility, nanoscale size, ease of chemical functionalization, and tunability of photoluminescence, CDs have been widely explored. The major applications are in catalysis, sensors, optical devices, and bioimaging [5, 9, 10]. CDs have been synthesized using both the top-down and the bottom-up methods depending upon the required application [6, 11]. The latter is the preferred one because of the large-scale and low-cost synthesis. Due to the stringent environmental regulation, nowadays, the greenness and sustainability of nanomaterials are major requirements. This is to avoid the discharge of any toxic/harmful effluent to the soil/aquatic life, leading to possible contamination/environmental pollution.

Several research articles have covered the chemical synthesis methodologies of CDs and their wide applicability. This article presents a brief account of the various synthesis strategies of CDs and their application. The important properties of the CDs are outlined and the biocompatible nature is highlighted. The focus is on the green and environmentally friendly synthesis of CDs with ease of preparation, scalability, and green nature. Accordingly, some of the modern synthetic techniques are reviewed for the preparation of CDs with desirable properties. Some of the greener precursors of CDs are outlined, and a brief account on the applications of the CDs is presented. The scope of further research in this area is outlined.

2 Green synthesis of CDs

2.1 Top-down and bottom-up approaches

Depending upon the requirement for the target application, various synthetic strategies have been explored for the preparation of CDs. Figure 2a shows the general mechanism for the preparation of CDs. The different preparation methods can be broadly categorized into the top-down and the bottom-up approaches, as shown in Fig. 2b. Chemical oxidation, mechanical grinding of activated carbon to get the nanoscale precursor (e.g., nanodiamond, carbon black, graphite, CNTs, ash), discharge, laser ablation, electrochemical preparation, etc., are the major top-down methods [3, 9, 10]. The bottom-up method describes the production of the nanomaterials using small molecules as precursors. These involve pyrolysis, microwave/ultrasonic irradiation, hydrothermal/solvothermal preparation, etc. The choice of the synthesis route stems from the following goals: (i) simplicity and reproducibility of the method, (ii) achieving high yield or scalability of the method, (iii) minimizing waste production, (iv) cost-effectiveness, and (v) optical properties. Figure 2c shows the major techniques used for the characterization of CDs. Some of the major properties and applications of the CDs are outlined in Fig. 2d.

Fig. 2
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a General mechanism for the preparation of CDs [12]; b top-down and bottom-up methods for the preparation of CDs; c major techniques used for the structural characterization of CDs; and d properties and applications of CDs

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2.2 Green synthetic methods of CDs

Green chemistry is the branch of science dealing with environmentally benign synthesis techniques, using safer solvents or solventless synthesis, minimizing waste production, use of enzymes, biocatalysts, etc. [13]. In this connection, herein, we present the microwave- (MW) and ultrasonication (US)-assisted methods for the preparation of CDs. Conventional organic synthesis methods require a long time for the organic synthesis to complete; therefore, these methods are considered inefficient and slower. Further, these methods offer poor selectivity and lower yields of the desired products. On the other hand, using the non-conventional methods of microwave and ultrasonic irradiations [12, 14, 15], the synthesis can be completed within a fraction of minutes and with a high product yield. These methods offer advantages of greater temperature homogeneity, uniform heating, reduced generation of waste, higher reaction rate and consquently low synthesis time, low operating cost, and high purity. Therefore, these techniques are considered environmentally benign and green methods. Almost all kinds of organic transformations have been reported using microwave and ultrasonic irradiations.

For the preparation of CDs, hydrothermal and solvothermal techniques are generally preferred. However, employing the MW and US methods, the synthesis time could be reduced from several hours to a few minutes [12, 16,17,18]. In addition, homogeneous heating during the MW irradiation method ensures the formation of uniform size CDs [19]. The use of template synthesis can provide good monodispersity and controlled size during CD preparation. The template synthesis methods can be facilitated using MW and US methods. Careful selection of MW/US parameters can provide a control over the size and morphology of nanoparticles even in the absence of any template [20]. Further, compared to the solvothermal method that employs non-aqueous solvents, the MW and US techniques can be carried out quite easily in aqueous media, thereby avoiding the use of toxic solvents. In addition, during the MW and US methods, visual changes and temperature profiles in the synthesis vessels can be easily followed and directly recorded, which is not possible for the hydrothermal method. Among MW and US techniques, the basic working principle is quite different and both techniques have their own merits as discussed below. Various kinds of catalytic reaction schemes can be further tuned by optimizing the MW and US parameters.

3 Significance of green synthetic methods for the preparation of CDs

3.1 MW irradiation

Conventional organic synthesis is slower, and the healing process can lead to the development of a temperature gradient within the sample. Besides, local overheating can lead to the decomposition of the desired product, substrate, and the used reagents. A domestic microwave (MW) is a common household appliance that is very commonly adopted for the cooking/re-heating of food items. This technique has also been reported successfully for numerous organic syntheses, preparation of nanomaterials, and solid-state science [21]. The MW region in the electromagnetic spectrum exists between the infrared and radio waves. Microwaves operate within wavelengths of 1 mm to 1 m, corresponding to frequencies between 0.3 and 300 GHz. MW radar and telecommunications systems occupy several band frequencies in this region. Thus, to avoid the disturbance from these, the MW apparatus for domestic and industrial purposes works at a frequency of 2.45 GHz. In the past few years, several reports have come up in the literature on the application of the MW method for the synthesis of nanomaterials [22,23,24]. The working principle of the MW reactors is based on dipolar polarization and ionic conduction [25, 26]. MW-induced synthesis offers the acceleration of organic reactions under selectivity of the resulting products by a careful selection of MW parameters [24, 27,28,29]. This offers considerable benefits over the conventional synthesis, such as quicker heating rates and greater thermal homogeneity.

The uniform heating of the reaction vessel in the MW method avoids the formation of heating gradients, which are common in the conventional hot plate and reflux methods [26, 30]. Thus, the MW method allows the development of uniform-sized nanomaterials [12]. Further, the choice of aqueous or suitable non-aqueous methods allows efficient hydrothermal/solvothermal synthesis. By modifying the conditions, viz., the MW power, synthesis time, and stirring speed (mechanical or magnetic), the desired shapes and sizes of nanomaterials can be obtained. Due to these benefits, the MW synthesis is a widely recognized green chemistry technique. Figure 3a–c shows the synthesis of CDs using xylan [31], l-cysteine [32], and xylose [33] as precursors following the MW technique. It has been observed that with an increase in MW reaction time, the UV–visible absorption peak at ~ 260 nm of the resulting CDs shifted to shorter wavelengths (4 min, 265.5 nm to 14 min, 253.5 nm) [34]. The authors have also noted that the QY of the CDs at first increased with an increase in the MW time and then decreased significantly [32]. In another report, the authors have noted that upon decreasing the MW reaction temperature, the QY is lowered. The FTIR spectra of the CDs prepared using xylan as discussed above using NH4OH and PEI as precursors are shown in Fig. 4. In the following section, we present some examples from the literature on the application of MW method for the preparation of CDs.

Fig. 3
figure 3

Schematic of microwave-assisted synthesis of CDs using precursors: a xylan, NH4OH, and PEI, to produce CDs having variable PL behavior [31]; b l-cysteine having N, S, doping, and variable sizes [32]; and c xylose [33]

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Fig. 4
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FTIR spectra of CDs prepared from xylan following MW irradiation using different N-doping agents [31]

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3.2 US irradiation

Ultrasound waves act as an alternative source of energy and have garnered significant interest in the area of green chemistry. The high-frequency ultrasonic (US) waves (20 kHz–10 MHz) induce a series of compression/rarefaction cycles through a solvent medium through which they are passed. This leads to the cavitation phenomenon in which bubbles form and collapse rapidly and provide energy to carry out the US-based reactions [21]. The creation of localized extremes of high temperature and pressure (hot spots) around/inside the cavitation bubbles not only destroys them, but also activates the reacting molecules of organic reagents [35]. Besides, micro jet streams and shock waves arising due to cavitation facilitate the mass transfer and dispersion of molecules in the medium [36]. Figure 5a–c shows the schematic of CDs synthesis employing the US method using gelatin hydrolysate [37], D-fructose [38], and crab shells [39] as precursors and its application in bioimaging, anti-counterfeiting, and heavy metal detection.

Fig. 5
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Schematic of ultrasound-assisted preparation of CDs using precursors: a gelatin hydrolysate [37] and application in bioimaging and anti-counterfeiting; b d-fructose [38] and application in methylmercury detection; c crab shells [39], and application in the imaging of cancer cells

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Ultrasonic (US) cleaner bath is the simplest ultrasonic processor commonly used for cleaning of artificial dentures, jewelry, and electrodes, and also for some organic synthesis in the laboratories. The US probe processor, on the other hand, is useful for dedicated application in organic chemistry, nanomaterial preparation, and other scientific purposes [14, 18, 36, 40, 41]. The probe processor comes with varying sizes of probes having radii generally varying from 3 to 10 mm. The major difference between the US cleaner and the US probe processor is that in the former, a reaction vessel is inserted into the water. In contrast, in the latter, the probe is directly inserted into the reaction vessel. The probe processor is more sophisticated and is of significantly greater power in comparison to the ultrasonic cleaner bath. This leads to more effective US action, and the probe instrument is dedicated to the specialized application. The synthesis procedure can be further modulated by adjusting the solvent, power, pulse period, energy, frequency, amplitude, sonication time, temperature, precursors, and catalysts. It has been noted that prolonged US treatment (6 h) results in the formation of heterogeneous nanoparticles [18]. In such conditions, the initially synthesized nanoparticles could behave as seeds that can promote the synthesis of greater-sized nanoparticles. However, the exact particle size will also be influenced by the size of the smaller-size particles and the concentration of the precursors. In general, for the preparation of CDs, a sonication time of 90–120 min is required. On the other hand, the sonication time also considerably influences the fluorescence intensity of the formed CDs. It has also been observed that the yield of CDs increases with sonication time and temperature [17]. In the following section, we have presented some examples from the literature on the application of the US technique for the preparation of CDs.

4 Importance of green precursors for CDs synthesis

In the above sections, we have outlined the application of MW and US techniques for the preparation of CDs using various types of precursor chemicals. In addition to the above, there are several precursors that can be categorized as green and environmentally benign, and their application in the synthesis of CDs is under the provisos of green chemistry principles of environmentally benign and low-toxicity reagents, renewable raw materials, and minimizing waste production (Table S1 in the Supporting Information) [13, 42]. The major categories of such kinds of precursors include the extracts derived from naturally occurring plants [43], carbohydrates [44, 45], amino acids [46], and proteins. It is understood that the above-mentioned categories are of natural origin and have obvious biocompatibility. Other categories include heterocyclic biomolecules [47], pharmaceutical products, and ionic liquids (Fig. 6) [47]. All of these types of chemicals are abundantly available, environmentally benign, and cost-effective. An additional benefit is that these molecules allow achieving the synthesis of CDs with scalable preparation. Another aspect is that these molecules are composed of chemicals having plenty of heteroatoms (N, S, O, P), which results in the doped form of the CDs, surface passivation, formation of functionalized CDs and no further requirement of post-modification.

Fig. 6
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Some of the green precursors used for the preparation of CDs

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5 Luminescence mechanism in CDs

The property of luminescence is an exciting feature of the CDs that has facilitated a myriad of applications of these nanomaterials. For this purpose, endeavor has been made to understand the underlying mechanisms thoroughly. However, the universally acceptable origin of the fluorescence of CDs is still a mystery, and the topic is debatable. CDs are produced by various precursors, employing different techniques, which results in complex components and structures of the resulting CDs. This means that CDs prepared following different synthesis methods, precursor molecules, and post-treatment procedures have varying optical performance, suggesting that the luminescence mechanism of CDs is quite intricate and it is difficult to formulate a unified theory [10]. At present, the most acceptable mechanisms are (i) surface state, (ii) quantum confinement, and (iii) molecular fluorescence.

The surface state effect is the most acceptable luminescence mechanism and includes the degree of surface oxidation and surface functional groups. The oxygen content on the surface of the CDs has a profound influence on the red-shifted emission of the CDs. The greater the degree of surface oxidation, the greater is the number of surface defects [50]. These defects can trap excitons, and the radiation from the recombination of trapped excitons causes the red-shift emission. A series of purified CDs with excitation-independent fluorescence from blue to red is shown in Fig. 7a. The extent of CDs surface oxidation gradually increased along with the red shift of fluorescence emission. In addition, the surface states are also correlated with the surface functional groups, also known as the molecular states, such as C=O and C=N that have proved to be related to CD’s fluorescence. Various surface functional groups can cause a diversity of the fluorophores or energy levels in the CDs. The quantum confinement effect is noticed when the size of a particle is too small to be comparable to the wavelength of the electron. The crystal boundary significantly influences the electron distribution when a semiconductor crystal is in the nanometer scale size, which displays some properties such as bandgap and size-dependent energy relaxation dynamics [51]. With an increase in the particle size of the CDs, the absorption peak energy decreases due to the quantum confinement effect (Fig. 7b). It has also been observed that the formation of fluorescent impurities during the bottom-up chemical synthesis (i.e., molecular fluorescence) contributes to the emission from CDs [52]. Molecular fluorophores that were attached to the CDs exert a great influence on the optical properties of the CDs.

Fig. 7
figure 7

a Mechanism for the fluorescence of CDs based on the degree of surface oxidation [48]; b change in fluorescence emission based on variation in the average size of the CDs (shown by the HRTEM images) [49]

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6 Application of green synthesized CDs

6.1 Light-emitting diodes

Nowadays, CDs-based LEDs (Fig. 8a) have been prepared by two general strategies: (i) CDs as phosphors and (ii) CDs as active emitters. The former is realized by using CDs as phosphors on a GaN-based UV or blue LED chip, which is frequently used to achieve CDs-based multicolor and white LEDs. Employing CDs as an emissive layer in EL LEDs is the most promising application for flat-panel displays. The CD-based active emission layers and some organic conjugate buffer layer materials are processed by solution processing [53]. Hence, the solubility of CDs in solvents is of significant importance in constructing LED devices. Several CDs have been applied in LEDs as discussed herein. Using citric acid as carbon source, 1-hexadecylamine-passivated CDs were prepared following pyrolysis [54]. Following the same method, solution-processed inverted white LEDs were prepared [55]. Using banana leaves as the precursor, in a single-step hydrothermal method, CDs were prepared and used as the electron transporting layer (ETL) in organic LED [56]. N-doped CDs were prepared following a single-step hydrothermal treatment using starch as a carbon source and ethylenediamine as N dopant [57]. White LEDs based on starch/CDs composite were prepared, and an ultraviolet LED chip was fabricated. The device emitted white light when operated at 3.0 V. Using ammonium citrate and urea, highly green emissive CDs were prepared following a solid-state reaction method [58]. The CDs depicted green emission with thermal stability from room temperature to 90 °C, and photochemical stability up to 3 h. Following a solvothermal treatment, CDs were prepared using citric acid as a precursor [59]. Bright PL emission from blue to red was observed, following which monochrome LEDs with blue, green, yellow, and red color were prepared. All the LEDs showed stable- and voltage-independent EL emissions. Table 1 provides a list of some CDs prepared using green precursors and their application in LED.

Fig. 8
figure 8

a A typical device structure of CDs-based electroluminescent LED [53]; b schematic of the charge transfer process in dye-sensitized solar cells [60]; schematic of c photocatalytic and d photoelectrochemical hydrogen evolution [61]

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Table 1 Structural properties of some green synthesized CDs and their application in LED
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6.2 Solar panels

The excellent electrical properties, strong light absorptions, bright photoluminescences, high chemical stabilities, and great electrical conductivities indicate great promise for applications of CDs to a broad range of solar cells. A schematic of the dye-sensitized solar cell (DSSC) is shown in Fig. 8b. Abundant functional groups (e.g., amino, carboxyl, amide, imine, carbonyl) on CD surfaces endow CDs with excellent electron-transfer abilities (i.e., good donor/acceptor characteristics), which enable CDs to be used as electron acceptors [62,63,64] or donors [65, 66] in solar cells. CDs have been used in different functional layers of solar cells, electron-transporting layers, active absorbing layers, hole-transporting layers, and as interlayer spacing employed to align and adjust the energy levels of other components. Three different CDs were prepared using chitin, chitosan, and glucose and employed for the preparation of solid-state nanostructured solar cells [67]. Layer-by-layer coating of two different types of CDs resulted in the highest efficiency. N-doped CDs were prepared using citric acid and ammonia, following a pyrolysis procedure for 200 °C for 3 h [68]. The N-doping was readily modified by the mass ratio of the reactants. The CDs were green emissive with power-conversion efficiency of 0.79%. Using citric acid and ethanediamine, CDs were in situ grown on TiO2 surface using the hydrothermal method [69]. A low-cost, environment-friendly CDs-sensitized solar cell was developed with a power conversion efficiency of 0.87%. N-doped CDs were prepared from hydrothermal treatment using strawberry powder as a precursor [70]. The CDs were co-sensitized using commercially available N719 dye to develop a high-performance solar cell with a power conversion efficiency of 9.29%. Using glucose as the precursor, CDs were prepared in a hydrothermal treatment followed by surface modification with PEG [71]. A solar cell prototype was developed co-sensitized with N719 dye and PEG-modified CDs. Table 2 provides a list of CDs prepared using green precursors and their application in solar cells.

Table 2 Structural properties of some green synthesized CDs and their application in solar cells
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6.3 Electrocatalysis

CDs possess excellent electrical transfer/transport capabilities and numerous surface/edge-active sites [62, 72, 73]. Therefore, combinations of CDs with various active metals and semiconductors were used to improve the hydrogen evolution reaction (HER) in electrocatalytic water splitting, and the incorporated CDs provided more interfacial reaction sites. CDs show more abundant heteroatom-doped surface chemistry than other carbon-based nanomaterials such as graphene, CNTs, and diamonds. Figure 8c, d shows the schematic of photocatalytic and photoelectrochemical water splitting. Abundant surface/edge sites and easily modifiable structures of individual CDs make them promising for application to efficient oxygen evolution reaction (OER) processes [74,75,76]. Further, individual CDs have shown great oxygen reduction reaction (ORR) activities in fuel cells. In addition, CDs are expected to be an important component of electrocatalytic carbon dioxide reduction reaction (CO2RR) [75, 77,78,79] and in other electrocatalytic reactions, including alcohol oxidation reactions (AOR) and nitrogen reduction reactions (NRR).

Several research reports are available mentioning the application of CDs in electrocatalysis. An anionic metal organic framework (MOF) was prepared, and following a high-temperature treatment, CDs were obtained [80]. The synthesized CDs showed electrocatalytic activity as a metal-free catalyst for the oxygen reduction reaction (ORR). The incorporation of Co nanoparticles improved the catalytic activity and stability. In a one-pot hydrothermal synthesis using citric acid and dicyandiamide, N, S-doped CDs were obtained [81]. The N, S-doped CDs showed efficient oxygen electrocatalytic activity comparable to noble metals. In another study, CDs were prepared following a hydrothermal treatment of citric acid and urea, followed by N-doping using pyrrole [82]. The obtained N-CDs were successfully applied as synergistic agents with tungsten nitride for electrocatalysis of ORR. CDs were hydrothermally synthesized using citric acid and ethylenediamine, and a composite with Co9S8 nanoparticles was prepared [83]. In the presence of this system, aniline was polymerized to afford effective catalysts for both OER and ORR in high-performance Zn–air batteries. CDs were prepared from glucose and dicyandiamine following a one-pot hydrothermal synthesis route [84]. A nitrogen-doped carbon-encapsulated cobalt nanoparticles (N–C@Co NPs) system was in situ constructed that acted as an efficient electrocatalyst for OER in water splitting. Following hydrothermal treatment of willow leaves, N-doped CDs were prepared and utilized for ORR [85]. The activity was comparable to commercially available Pt/C catalyst. Table 3 provides a list of some CDs prepared using green precursors and their application in electrocatalysis.

Table 3 Structural properties of some green synthesized CDs and their application in electrocatalysis
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6.4 Photocatalysis

Nanomaterials have found extensive application in photocatalysis in recent years [86, 87]. CDs have also been utilized as effective catalysts for the degradation of a number of organic dyes and in photo-splitting of water for hydrogen production [88, 89] (Fig. 9a). Ashes of the eggshell membrane were subjected to MW irradiation for 1 min to produce CDs having a size < 10 nm [90]. The synthesized CDs were highly fluorescent under UV irradiation. Under sunlight irradiation, the CDs successfully degraded methylene blue dye in the range of 26–43% at 5–1 mg L−1 of the dye. CDs were prepared via US treatment and further subjected to sol–gel treatment and spin coating to obtain a heterostructure of ZnO/CDs [91]. The material was successfully applied in the photocatalytic degradation of Rhodamine B. Following ultrasonication in molten Ga and PEG-400 as the precursor, Ga-doped CDs (Ga@CDs) were prepared [92]. A greater photosensitization was shown in doped CDs compared to the pristine CDs. Waste polyethylene terephthalate bottles were used as a precursor via air oxidation and sulfuric acid-assisted hydrothermal treatment [93]. The synthesized CDs were used in catalytic dehydration of fructose to 5-hydroxymethylfurfural (HMF) at low temperatures, leading to high yields (up to 97.4%) of HMF. MW synthesis of N-doped CDs was performed in the presence of d-glucose and 1-ethyl-3-methylimidazolium ethylsulfate in aqueous medium [94]. The synthesized CDs were used as a metal-free catalyst for the selection and production of H2O2. Table 4 provides a list of some CDs prepared using green precursors and their application in photocatalysis.

Fig. 9
figure 9

Principles of a photocatalysis by CDs; and b photodynamic therapy for cancer treatment

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Table 4 Structural properties of some green synthesized CDs and their application in photocatalysis
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6.5 Photodynamic therapy

Photodynamic therapy is an advanced biomedical technique wherein energy transfer can be applied for the destruction of damaged cells and tissues [95,96,97]. This method is helpful in cancer treatment because it efficiently targets the malignant tissue leading to its destruction without harming normal healthy tissue [98]. The photodynamic therapy procedure employs a photosensitizer (a photosensitive molecule that can be localized in the target biological environment) and an activator (which activates the photosensitizer by transmitting a suitable wavelength light), both of which should be non-cytotoxic [95, 99]. Reactive oxygen species [ROS, e.g., superoxide (O2·−), hydroxyl radical (·OH), and/or singlet oxygen (1O2)] are generated by the photosensitizer via the transfer of energy from photons to molecular oxygen. These ROS species cause cytotoxic effects, and their action is specific to the target cells/tissues exposed to light [95]. This photo-induced toxicity can kill malignant tumors or cancer cells [98] (Fig. 9b). For the purpose of antimicrobial activity, it is a combination of a non-toxic photosensitizer and light of a suitable wavelength that can interact with molecular oxygen to afford ROS that can selectively kill pathogenic microbial cells [23, 96]. Several reports are available in the literature on the antimicrobial activity of CDs synthesized via different procedures [100, 101]. The CDs are tested as antimicrobial agents in vitro and in vivo against Gram-positive and Gram-negative bacteria. Furthermore, CDs–drug conjugates and metal-doped CDs are reported to show enhancement in antimicrobial action. Table 5 provides a list of CDs prepared using green precursors and their application in photodynamic therapy.

Table 5 Structural properties of some green synthesized CDs and their application in photodynamic therapy
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Folic acid (FA)-functionalized CDs were developed as carriers for the PS zinc phthalocyanine (ZnPc) for simultaneous bioimaging and targeted photodynamic therapy in cancer cells [102]. CDs were surface passivated using PEG and loaded with the photosensitizer zinc phthalocyanine (ZnPc). CD-PEG-FA/ZnPc exhibited excellent imaging and therapy of cancer cells. N- and S-doped CDs were prepared using citric acid and thiourea following hydrothermal treatment and used for photothermal therapy (PTT) in mouse models [103]. Using Cannabis sativa leaf extract, CDs and Ag@CDs were prepared [104]. The structural characterization revealed functional groups from the aromatic, carboxylated, and hydroxylated moieties. The synthesized CDs showed remarkable antibacterial behavior against E. coli and S. aureus. Highly photoluminescent CDs were prepared from metronidazole, a widely used antibacterial drug, following a hydrothermal approach [105]. The synthesized CDs were successfully utilized in the growth inhibition of Porphyromonas gingivalis. Using wheat bran as green precursor, CDs were prepared having high QY of 33.23% [106]. The synthesized CDs were used for the preparation of conjugate with drug amoxicillin (AMX), and the CD–AMX conjugate was used for antibacterial activity against Gram-positive and Gram-negative bacteria. N- and S-doped CDs were produced via MW irradiation from amino acids arginine, lysine, histidine, cysteine, and methionine [107]. The synthesized CDs showed a minimum inhibitory concentration (MBC) of 1 mg L−1 against Gram-positive and Gram-negative bacteria. One-pot MW synthesis of CDs was carried out using citric acid as precursor and investigated in vitro and in vivo (in mice) against S. aureus bacteria [16]. Complete healing of the wounded tissue in mice was observed.

6.6 Bioimaging

Imaging of cells and tissues is an important aspect relevant to the diagnosis of various diseases, especially cancer. Several systems having considerable fluorescence have been reported for bioimaging purposes. These include organic and inorganic dyes and nanoparticles [108,109,110]. The incorporation of carbon nanomaterials in this application presents a primary advantage in the lack of metal ions that can adversely influence the in vivo environment via cytotoxicity. The suitable agent for bioimaging should possess biocompatibility, tunable emission spectra, and lack of cytotoxicity. The advent of nanomaterials has found application not only in bioimaging for diagnosis, but also in therapy. The theranostic applications are performed by functionalizing the molecules of the required chemical to the fluorescent nanomaterial via chemical functionalization. A number of quantum dots such as ZnO, GQDs, and CDs have been reported for the imaging application of living cells [111, 112]. The CDs produce attractive optical features, viz., up-conversion photoluminescence, piezo-chromic fluorescence, solid-state fluorescence, excitation-dependent emission, and phosphorescence. Due to their excellent photoluminescence behavior, CDs are reported as a potential agent for bioimaging of cells in both in vitro and in vivo conditions (Fig. 11a). Cao et al. first reported the application of CDs in bioimaging in vitro and in vivo [113]. Table 6 provides a list of some CDs prepared using green precursors and their application in bioimaging.

Table 6 Structural properties of some green synthesized CDs and their application in bioimaging
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A sonochemical approach was adopted to produce N-doped CDs from crab shells showing excellent water solubility and high quantum yield [39]. The synthesized CDs were applied as nanoprobes to target imaging of cancer cells. Gelatin hydrolysate was used as a green precursor via the US method within 30 min [37]. The developed CDs were employed in the application of cell imaging and anti-counterfeiting. Using citric acid, triethylenetetramine (TETA), tetraethylenepentamine (TEPA), polyene polyamine (PEPA), CDs having sizes of the order of 3 nm and green color PL emission with QY of 9–11% [114] were produced. Figure 10 shows the synthesized CDs samples' TEM, HRTEM, and XPS survey scan spectra. The TEM images indicate uniform particle size distribution without any aggregation, with the lattice planes showing spacing of 0.21 nm and 0.245 nm corresponding to (103) diffraction plane of diamond-like (sp3) carbon and (100) facet of graphitic carbon, respectively. The XPS spectra show considerable N-doping with an increase in the N content of the passivating agents. The synthesized CDs were employed for the in vivo bioimaging of HepG2 cells. Andrographis paniculata (Kalmegh), an Ayurvedic medicinal plant, was used as a precursor for the production of CDs [115]. The synthesized CDs were successfully utilized in cellular imaging of human breast carcinoma cells. In addition to the natural products, carbohydrates, and amino acids, CDs have also been synthesized using protein molecules as discussed above. In another study, CDs were prepared from gelatin protein and from algal biomass of Pectinodesmus sp. via the hydrothermal method [116]. The gelatin-synthesized CDs displayed activity in the imaging of plant and bacterial cells. On the other hand, the CDs from the algal source had anticancer applications. CDs were prepared following a single-step MW irradiation of carbohydrates without employing any surface passivating agents [34]. The synthesized CDs were biocompatible and cell permeable and were applied successfully in bio-labeling and bioimaging. Fluorescent N-doped CDs were prepared from milk that were well-dispersed in an aqueous medium [117]. The synthesized CDs were employed in the bioimaging of HeLa cells. Using sucrose as precursor and diethylene glycol as the reaction medium, CDs were prepared following the MW irradiation method [118]. The CDs were well dispersed in an aqueous medium with a bright green luminescence. The application was reported for the bioimaging of C6 glioma cells. Following MW heating of glutathione, near-infrared emissive CDs were prepared and utilized for deep tissue bioimaging and drug delivery [119].

Fig. 10
figure 10

TEM (a–e) and HRTEM (b–f) of CDs-TETA (a, b), CDs-TEPA (c, d) and CDs-PEPA (e, f). Inset: corresponding particle size distributions and fast Fourier transform images. XPS (g, j, m), XPS C1s (h, k, n), and XPS N1s (i, l, o) spectra of CDs-TETA, CDs-TEPA and CDs-PEPA, respectively [114]

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6.7 Chemical sensors

There is a growing research interest in the application of nanomaterials as chemical/biosensors [120,121,122]. A number of reports are available on the application of metal nanoparticles, carbon nanomaterials, and semiconductor nanostructures for sensor development. CDs have been extensively reported for sensor development [111, 122,123,124]. Employing the fluorescence activity and surface functionalization of CDs, several sensors for chemical and biological species have been developed using CDs. The CDs have interesting features such as bright and intense fluorescence emission in the visible range, fluorescence quenching by the target analyte, variable emission spectra, negligible cytotoxicity, and feasibility of preparation. The general working principle of the CDs-based sensor is shown in Fig. 11b. Table 7 provides a list of some CDs prepared using green precursors and their application in sensor development.

Fig. 11
figure 11

Principles of a bioimaging using CDs; b CDs-based sensors; and c CDs as corrosion inhibitors

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Table 7 Structural properties of some green synthesized CDs and their application in sensors
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Using citric acid, amino acid l-cysteine, and the polysaccharide dextrin, in a one-step MW method, highly fluorescent CDs having a size of 2.61 nm were obtained [125]. The CDs were N, and S doped and showed efficient performance in Cu2+ ion detection. Using lactose as precursor and the MW method, CDs were prepared and applied as a nanosensor platform for the detection of heterocyclic amines in the range of concentration 0.35–0.45 mg L−1 [126]. Amphibious yellow emitting CDs were prepared using a rapid MW method and applied as a colorimetric nanosensor for the discrimination of Cr(III) and Cr(VI) [127]. Using bovine serum albumin (BSA) and urea as the precursor, highly photoluminescent CDs were synthesized in one-step MW treatment [128]. A multifunctional fluorescent nanosensor for pH and temperature was developed using the CDs. This method suggested the development of CDs using other protein-based sources, e.g., lipase and trypsin, and application in metal ion detection. In other studies, xylose-derived CDs having N, and S doping were developed via MW treatment and application in anti-counterfeiting [129] and pH sensing was explored [33]. Figure 12 shows the diffraction patterns and rings of CDs samples synthesized using xylose as precursor employing MW at different conditions [33]. The HRTEM images display a clear lattice spacing of 0.34 nm consistent with the (002) lattice plane of graphite. Lemon extract was used as a green and biocompatible precursor to develop CDs following the US method [130]. The synthesized CDs were used to prepare MgFe2O4-CDs nanocomposite, which was utilized in the fluorescence detection of Ni (II), Cd (II), Hg (II) ions and Pseudomonas aeruginosa. D-fructose was used as a green precursor for the sonochemical synthesis of CDs having average size of 2.5 nm and a narrow size distribution [38]. The synthesized CDs were utilized for preparation of a fluorescent nanoassay for detection of methylmercury.

Fig. 12
figure 12

a–h TEM images of different CDs samples from 1-CDs to 8-CDs, with insets showing corresponding SAED patterns; i–l HRTEM of 1-CD, 2-CDs, 3-CDs and 4-CDs [33]

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Using citric acid and thiourea, in a hydrothermal treatment, CDs were prepared having faint yellow to bright blue emission colors [131]. The synthesized CDs were used as a selective and cost-effective nanoprobe for the detection of living cancer cells. Citric acid was subjected to hydrothermal treatment in the presence of a series of amino acids to develop amino acid-functionalized CDs [132]. High QY up to 62% was achieved with tunability of surface charge and hydrophobicity. The synthesized CDs were used in array-based protein sensing. In another study, CDs were modified using ethylenediamine and different amino acids and studied for sensing metal cations via the fluorescence quenching method [133]. The decrease in the fluorescence intensity was found to be in direct proportion with the increasing concentration of metallic ions. CDs were prepared via the hydrothermal method employing grape skin as precursor [5]. The synthesized CDs were employed for the picric acid detection via fluorescence quenching in real water samples. Fluorescent CDs were prepared from gardenia plant following a hydrothermal synthesis procedure [134]. The synthesized CDs were applied in the sensing of metronidazole based on fluorescence quenching in pharmaceuticals and rabbit plasma.

Fluorescent CDs were prepared from leaf extract of Azadirachta indica and used for peroxidase-mimetic activity for detection of H2O2 and ascorbic acid in fresh fruit samples [135]. The peroxidase-mimetic activity was studied toward the oxidation of peroxidase substrate 3,3′,5,5′-tetramethylbenzidine (TMB) with H2O2. Using citric acid and thiosemicarbazide, N, S-co-doped CDs were prepared using the hydrothermal method [136]. The synthesized CDs were used for selective detection of picric acid in an aqueous solution and in living cells. Using algal biomass of Dunaliella salina, N, P-doped CDs were prepared following the hydrothermal method [137]. The CDs exhibited blue color under UV light with a QY of 8%. The CDs were used for intracellular detection of Hg (II) and Cr (VI). N, S-doped CDs were prepared from histidine and thiourea with an excellent QY of 46% [138]. The synthesized CDs showed efficient performance for the detection of H2O2 and glutathione in human blood serum samples. CDs were prepared from Jatropha fruit following hydrothermal method of synthesis with a high QY of 13.7% [139]. The CDs were applied for the detection of pesticides in environmental and agricultural samples with a limit of detection 2 ng L−1. The latex of Euphorbia milii was successfully employed in the development of CDs via hydrothermal treatment [140]. The synthesized CDs were applied for the detection of glutathione in human blood serum. Mustard seeds were used to prepare CDs via hydrothermal technique and employed for a peroxidase-like mimetic activity for colorimetric detection of H2O2 and ascorbic acid in real samples [141].

6.8 Corrosion protection

Corrosion can be described as the natural degradation of metallic materials upon exposure to the environment to the ore forms from where the original metallic form was extracted and purified [21, 142, 143]. This process can be aggravated by the presence of extreme environments, e.g., seawater, alkaline conditions, acidic media, high temperatures, and flow conditions. Severe damage to the metallic structures takes place under such conditions. This results in a considerable detrimental effect to the surfaces of metals and can lead to the potential failure of structures, environmental pollution, economic losses, and even hazards to human life. Different forms of corrosion include pitting, uniform corrosion, crevice formation, galvanic corrosion, stress corrosion, intergranular corrosion, and erosion corrosion [144, 145]. The global loss of corrosion amounts to about USD $2.4 trillion, which is around 3.4% of the global GDP [146]. To address this issue, one of the commonly used methods is the application of corrosion inhibitors. A corrosion inhibitor, by definition, is a chemical additive, which, when added to a corrosive medium, retards the rate of corrosion. Generally, inorganic inhibitors such as nitrates, nitrites, chromates, and phosphonates are applied in the industries. The organic inhibitors commercially used include imidazolines, amides, acetylenic alcohols, etc. These categories are highly toxic and carcinogenic, and hence their usage is discouraged by the environmental regulatory bodies across the globe. Environmentally benign and green molecules are being put forward as corrosion inhibitors, such as heterocyclic biomolecules, ionic liquids, natural extracts, carbohydrates, amino acids, proteins, and pharmaceutical products. From the above sections, it can be noted that the same molecules also act as novel and environmentally benign precursors for the synthesis of CDs.

Conventionally, the nanomaterials such as metal oxides, graphene, and graphene oxide (GO) find application in the preparation of anti-corrosion coatings. This is owing to their hydrophobic behavior, impermeability, chemical stability, and barrier action [2, 147,148,149]. Extensive research is available on the application of graphene and GO-based coatings. In addition, other nanomaterials such as ZnO, TiO2, NiO, montmorillonite [150], and graphydine [151] have also been utilized in the development of coatings. The nanosized materials are used to cover the pores in the coatings, enhance the surface coverage of the coating on the underlying metallic substrate, and improve the hydrophobicity of the coatings. Conventionally reported corrosion inhibitors are based on heterocyclic molecules and polymers [24, 29, 152,153,154,155,156,157]. Considering the importance of nanomaterials, researchers have also explored the polymer composites of AgNp with biopolymers as corrosion inhibitors [158,159,160] and polymer–ZnO nanocomposites [161].

In the past few years, several reports have come in the literature focusing on the application of functionalized GO-based corrosion inhibitors [162,163,164,165,166,167,168,169,170,171,172]. The oxygen-containing polar functional groups, e.g., –C=O, –OH, –COOH, etc., present on the GO surface, allow GO structure's chemical modification, thereby affording efficient corrosion inhibitors. While the nanomaterial-based coatings are reported most for the neutral and saline media, the modified GO-based corrosion inhibitors have been reported for aggressive environments, such as 1 M HCl and 15% HCl, and even at elevated temperatures. Recently, CDs have also been explored as inhibitors for aqueous corrosive environments (Fig. 11c). The following section provides some of the literature reports available on green CDs-based corrosion inhibitors. Herein, we have chosen the examples of CDs-based inhibitors focusing on MW, US synthesized methods or using green precursors.

Table 8 provides a comparison of the inhibition performance of some green CDs-based inhibitors. 4-Aminosalicylic acid (ASA), an antibiotic primarily used for tuberculosis treatment, has been considerably reported for the synthesis of CDs following the solvothermal approach. Figure 13a shows the reaction schemes for the preparation of CDs using ASA as the precursor along with some amino acids. The synthesized CDs were N-doped and evaluated as corrosion inhibitors for copper in 0.5 M H2SO4 [173], Q235 carbon steel in 1 M HCl [174], and carbon steel in CO2 saturated 3.5% NaCl [175]. High efficiencies were obtained in a concentration range from 50 to 100 mg L−1. The adsorption of the CDs on the copper and Q235 steel substrates was in agreement with the Langmuir model with a mixed mode of physical and chemical adsorptions. For the carbon steel in a sweet corrosion environment, the authors compared ASA with the CDs. It was revealed that CDs showed better performance and made the metallic substrate more hydrophobic in comparison to that of ASA alone.

Table 8 Performance comparison of some green CDs-based corrosion inhibitors
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Fig. 13
figure 13

Schemes for the synthesis of CDs employing hydrothermal and solvothermal methods using a 4-aminosalicylic acid [174,175,176] and b citric acid [177,178,179,180] in the presence of some amino acids

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In another study, ASA was used along with L-histidine to afford N-doped CDs that were evaluated for Q235 steel surface in 1 M HCl solution [176]. Weight loss tests, electrochemical corrosion evaluation, and surface analysis supported strong adsorption of the N-CDs on the metal surface. Figure 13b shows the schematic of preparing CDs using citric acid and other precursors. Citric acid is often reported as a green precursor for the synthesis of CDs, alone and in combination with other reagents. Citric acid and L-histidine were used as precursors to obtain N-doped CDs [177]. The CDs showed effective corrosion inhibition with > 90% protection at 100 mg L−1. The studies on density functional theory (DFT) and molecular dynamics (MD) simulations evidenced a low energy gap and high binding energy of the inhibitor. L-Serine and citric acid were used in a solvothermal protocol to afford N-doped CDs [178]. The synthesized nanomaterial was used as an inhibitor against the corrosion of copper using electrochemical, surface, and spectroscopic analyses. High efficiency of 98.5% was afforded after an immersion time of 24 h. An imidazole-based ionic liquid was used to functionalize citric acid-derived CDs to obtain a green corrosion inhibitor for carbon steel surface in 1 M HCl and 3.5% NaCl [179]. The inhibitor showed adherence to Langmuir adsorption isotherm in both cases and followed the physicochemical mode of adsorption.

Using tryptophan as a precursor, a series of N-doped CDs were obtained (Fig. 14a) depending on varying hydrothermal reaction conditions [181]. The developed CDs were used as inhibitors for carbon steel corrosion in 1 M HCl, and high efficiency was obtained at 200 mg L−1. Both physical and chemical adsorption of the CDs was observed on the metallic substrate. Using folic acid and o-phenylenediamine, N-doped CDs were prepared (Fig. 14b) and used as an inhibitor for Q235 steel surface in 1 M HCl solution [182]. High efficiency of 95.4% was obtained in accordance with Langmuir isotherm with a mixed mode of physical and chemical adsorption. Heterocyclic molecules, especially triazole derivatives, have shown considerable adsorption and protection performance for copper metal. Considering this fact, CDs were prepared using glucose, ascorbic acid, and a triazole derivative (Fig. 14c) to afford triazole-modified CDs [183]. The CDs were employed as inhibitors for Cu substrate in 3.5% NaCl using electrochemical techniques and surface examination, wherein a high protection of 88% at 70 mg L−1 was obtained.

Fig. 14
figure 14

Schemes for the synthesis of CDs employing a tryptophan [181], b folic acid [182], c glucose, ascorbic acid [183], and d dopamine [184] as precursors

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N, and S-doped, blue-emitting CDs were obtained using citric acid, isoniazid, and thiourea as precursor molecules following a hydrothermal reaction [180]. The material showed efficient performance in 15% HCl on mild steel with > 98% protection after a 6 h immersion period. After prolonged exposure to corrosive media of 48 h, still, high efficiency of > 91% was achieved. CDs derived from Coffea caneophora and urea after the pyrolysis procedure was used for Cu corrosion inhibition in 1% NaCl and showed high protection performance [185]. Dopamine was used as a green precursor to afford N-doped CDs (Fig. 14d) that were utilized for the protection of Q235 steel surface in 1 M HCl solution [184]. Even after prolonged exposure of 60 h, a high extent of protection with > 92% efficiency was achieved, which remained almost unchanged even at the elevated temperature of 328 K. Blue-emitting CDs were obtained using durian juice as the precursor, which contained C=O, O–H, and S=O functional groups [186]. The CDs showed strong EIPL with moderate QY. When evaluated for copper in 1% NaCl solution, high protection of 86% was afforded at 800 mg L−1.

7 Conclusions and outlook

This review presents a brief introduction to the significance of CDs with attention focused on the sustainable sources and methodologies for their synthesis. Special emphasis is given to microwave (MW) and ultrasonic (US) irradiation as modern synthetic techniques in compliance with the green chemistry principles. The basic principles governing the organic transformations based on MW and US techniques are described in this report. In comparison to the conventional heating of the reaction mixture using hot plate stirring as well as solvent refluxing, the nanomaterials can be more efficiently produced using MW and US heating with significantly high yield and selectivity. The MW and US heating offer environmentally benign and quicker methods to develop uniform size CDs with scalable yield, and precise control over physicochemical properties. A brief review of literature on the MW- and US-based synthesis of CDs is provided while discussing the important properties such as size, photoluminescence behavior (excitation dependence/independence), emission color, and the quantum yield of the CDs. The major mechanisms of luminescence of CDs are briefly discussed. The significance of green precursors in the production of CDs is given. Some examples of the green precursor-derived CDs is highlighted, e.g., carbohydrates, amino acids, proteins, ionic liquids, and pharmaceutical products. It is shown that using these precursors, the obtained CDs can be doped with N, and S-heteroatoms, heterocycles, and ionic liquid moieties, with high hydrophilicity and biocompatibility.

Some of the major applications of the CDs are also highlighted in the present review. In general, most of the reports focus on LEDs, electrocatalysis, solar cells, antibacterial activity, anticancer activity, sensors, photocatalysis, etc. In addition to the above, in recent years, there have been several reports on the application of CDs as corrosion inhibitors. Generally, carbon-based nanomaterials such as graphene and graphene oxide have been applied as materials of choice for anti-corrosion coatings and corrosion inhibition. Due to high aqueous solubility, the CDs-based inhibitors can potentially be used as a synergistic component to develop commercial corrosion inhibitor formulations. The utilization of green CDs as corrosion inhibitors is likely to address the issues of toxicity and harmful discharges posed by conventional corrosion inhibitors. It is shown that the CDs display high inhibition performance at a considerably low inhibitor dose and at long exposure times.