2. GQDs from Eco-Friendly Raw Materials by Green Approaches

The synthesis methods of GQDs are generally classified into two groups, based on the reaction mechanism involved: top-down and bottom-up. The top-down approach consists of cutting down large graphene sheets, carbon nanotubes, carbon fibers, or graphite into small pieces of graphene sheet. Since top-down processes involve the conversion of macromolecules using physical forces into smaller ones, the main reaction mechanism involved is oxidative cleavage, although hydrothermal process is preferred, because it is simpler and faster than oxidative methods. Other top-down processes are electrochemical oxidation, microwave irradiation, and laser ablation. The strategy of the bottom–up method is the use of small molecules as starting materials for the production of the GQDs ^[22]. The bottom-up technique consists of the controllable synthesis of carbon sp² from organic polymers, or of pyrolysis/carbonization processes starting from organic molecules. Typically, polycyclic aromatic hydrocarbon molecules are the most reliable precursors to form high-quality GQDs ^[23]. The first method results in a complicated process but has the advantage of obtaining products that can be controlled in terms of size and morphology. Carbonization, on the other hand, is an ecological, easy method, but the structure and the morphology of the GQDs are not controllable and the yield is lower. However, both top-down and bottom-up methods involve the use of very expensive non-renewable raw materials, such as CNTs, graphene, graphene oxide, or other graphene-based precursors, which, if prepared from bulk graphite, require the use of toxic chemicals and strong acidic treatment for the disintegration of the strong and well-ordered structure of graphene into small-sized GQDs, as well as high pressure, high-temperature equipment, resulting in low yields and limited production scalability ^[24].

The use of green synthetic routes is an emerging area in the field of nanotechnology and offers economic and environmental benefits as an alternative to conventional methods. As is well known, the preparation of graphene quantum dots often needs strong acids or organic solvents, and their green production via sustainable strategies, involving non-toxic and biosafe reagents, still faces important challenges; therefore, eco-friendly synthetic approaches, with easy separation and without complicated post-processes, should be designed and developed.

The synthesis of carbonaceous nanomaterials and the choice of precursor materials can be considered equally relevant as, depending on the process and the feedstock, the product will have different features affecting its future applicability. Furthermore, yields that guarantee large-scale production also depend on them. As mentioned in the introduction, in recent years interest has been growing in exploiting adequate renewable resources to decrease dependence on non-renewable resources and increase energy security and environmental safety ^[25], leading to the development of several attempts to exploit different natural carbon sources for the production of GQDs, by combining both top-down and bottom-up processes. The real advantage of bio based GQDs is the possibility of using a wide range of precursors and several technological approaches ^[26]. Several biomaterials have been already proposed for a wide range of electrochemical applications to obtain biomass-derived GQDs, ranging from simple and natural molecules to complex compounds, including wheat straw, wood charcoal, rice husk, coffee ground, forestry processing residue, livestock and poultry manure, organic waste from food processing, and municipal solid waste (Figure 1) ^[9][27][28]. Among these, citric acid (CA), a weak organic acid, and glucose, a carbohydrate, both available in nature, are undoubtedly the most popular carbon precursors, because of their biocompatibility, low cost and ease of supply. Furthermore, carbonaceous materials obtained via green synthesis from CA and glucose, show both photoluminescence from blue to red regions ^[29] and extremely high QYs (more than 80%) ^[30].

Figure 1. Green synthesis of GQDs and their recent applications in electrochemical sensors ^{[9][27][28][31][32][33][34]}.

In the following paragraphs, a selection of the main innovative and interesting synthetic approaches proposed in the last years, as summarized in Table 1, involving the use of biomass waste or other natural starting materials, by green routes for the production of graphene quantum dots, focusing attention on their eco-compatibility and their perspectives for future development, will be discussed.

Table 1. GQDs synthesized from different natural sources through green approaches by several techniques and their applications.

2.1. Oxidative Method

Oxidative cleavage is one of the most versatile approaches frequently used for the synthesis of GQDs from larger graphitized carbon materials with relatively high yields. An interesting class of nearly uniform size (~5 nm) of graphene quantum dots (E-GQDs) was prepared via eco-friendly carbon electro-oxidation of wood charcoal by Nirala et al. ^[27]. Coal derived from wood has proved to be an excellent precursor for the synthesis of carbon nanomaterials as it guarantees an efficient electrochemical oxidative cleavage in multilayer graphene sheets. Electrochemical oxidation was performed in a two-electrode system by modulating the current intensity and the electrochemical cleavage was guaranteed both by the free radicals of the water and the peroxide of ammonium sulfate. Free radicals of water act like “scissors”, cutting carbonaceous macromolecules into multilayer sheets of graphene, and facilitate the easy insertion of SO₄·radicals to easily insert into the sheets and, finally, reduce them to GQDs with peculiar structural and optical features. One of the disadvantages of the oxidative method is that in many cases it requires the use of strong acids or dangerous and harmful strong oxidants, i.e., non-eco-compatible approaches. Recently, the use of hydrogen peroxide as an oxidant, suggested by Lu et al., represents a valid green approach to obtain GQDs with good stability, applicable as fluorescent probes for bioimaging. Indeed, this latter process can be applied to several raw materials such as biomass wastes. Likewise, Liu et al. ^[35] used hydrogen peroxide in an environmentally friendly process for the synthesis of quantum dots from waste of the coke industry. Coal tar previously suspended in H₂O₂ was treated at 100 °C under reflux for 2 h, to obtain a monodispersed solid fluorescent GQDs (~1.7 ± 0.4 nm) with a high yield of more than 80 wt%. Similarly, Halder et al. ^[36] used hydrogen peroxide as a green oxidant to synthesize GQDs, starting from graphene oxide (GO) mixed with H₂O₂ (2–6%) at 180 °C for 2 h. The quantum dots obtained can be applied as fluorescent nanoprobes with high features for bioimaging, diagnostics, and drug delivery ^[37], since they showed high photostability and substantial biocompatibility as evidenced by cell viability tests. The method can be considered green, despite the fact that the pre-synthesized GO was obtained by treating synthetic graphite flakes with H₂O₂, concentrated sulfuric acid, concentrated hydrochloric acid, phosphorus pentoxide, potassium persulfate, potassium permanganate, and quinine sulphate. Using the same precursor but a different approach, Su et al. ^[38] synthesized nitrogen doped GQDs (N-GQDs) as promising probes for bio-imaging, from graphene oxide (GO), ethylenediamine and hydrogen peroxide. First, GO was synthesized from expandable graphite by the modified Hummers method ^[39] (Figure 2a) and then treated in an autoclave at 200 °C for 3 h. An interesting electrochemical exfoliation approach for the large-scale production of GQDs was proposed by He et al. ^[40], using coke obtained by pyrolysis at 1000 °C as starting material. The electrochemical process was carried out in a two-electrode system, with a piece of advanced coke as the reference electrode and a platinum plate as the counter electrode, using as electrolyte (NH₄)₂S₂O₈ dissolved in a mixture of MeOH and H₂O. The authors found that the amount of water in the electrolyte solution and the applied current density affect the multi-color fluorescence and the size of the obtained GQDs, ranging from 3.02 to 4.61 nm with fluorescence emissions at 500, 530, and 560 nm, respectively. The reaction parameters also play an important role in the quantum yields allowing the production of high yields from 13.04to 42.86 wt% and 31.13 wt%, making possible eventual industrial scalability of the production process. Furthermore, GQDs were available for engineering of their solid-state GQDs/epoxy composites for advanced applications in multicolor light-emitting diode instruments. Finally, in a recent report of Duarte de-Menezes et al. ^[41], quantum dots were synthesized via a green electrochemical process based on the electrolysis of citric acid and sodium citrate for 24 h. Several tests of the mutagenicity in vivo of the nanoparticles performed on the obtained GQDs investigated their cytotoxicity and evidenced their possible use in human applications.

Figure 2. Four green approaches adopted to prepare GQDs. (a) Illustration of the preparative strategy for N-GQDs by electrochemical exfoliation and RGOQDs from GO with Hummers method. (Adapted with the permission of Ref. ^[42]). (b) Synthetic one-step process for the production of SL/GQDs used as sensing platform for Fe3+ detection by Xu et al. (Adapted with the permission of Ref. ^[43]). (c) The hydrothermal strategy for the functionalization of GQDs synthetized from glucose. (d) Schematic representation of the nucleation and growth of GQDs obtained from citric acid.

2.2. Laser Ablation

Chemical oxidation methods require the use of strong acids which can damage instruments in the long run and can be considered non-eco-friendly approaches. Therefore, simpler and more environmentally friendly alternatives have been developed such as pulsed laser ablation in the liquid phase. This technique was used by Narasimhan et al. ^[44] to set up fluorescent probes of GQDs for bio-imaging applications. GQDs were synthesized from a graphite plate immersed in an aqueous solution of polyethylene glycol submitted to laser ablation: a nanosecond pulsed laser source was optically directed in the direction of the substrate for 30 min causing the ablation of the graphite plate surface, obtaining GQDs in solution, subsequently separated by centrifugation and filtration, and larger graphene sheets settled to the bottom. The GQDs produced by this method showed good properties as fluorescent biomarkers. More recently, Kang et al. ^[45] employed pulsed laser ablation in the liquid phase, for the single-step synthesis of GQDs, from graphite flakes suspended in a solution of ethanol and 3-mercaptopropionic acid (MPA), pointing the pulsed laser source onto the suspension for 30 min at room temperature. The pulsed laser caused the decomposition of graphite flakes to form carbonaceous nanoparticles. C binds to S, derived from the decomposition of MPA, producing sulfur-doped graphene nanosheets. One of the main disadvantages of laser ablation is the high cost of the instruments, and until today only a few numbers of examples of GQD synthesized by laser ablation from eco-friendly raw materials have been reported.

2.3. Controllable Synthesis

The controllable synthesis process, over the years, has aroused less success because it includes complex chemical reactions in several phases, which require a long time ^[46]. The method starts from small molecules (derivatives of substituted benzene) to obtain quantum dots of colloidal graphene with the desired size and morphology ^[47]. A recent interesting report advanced by Lu et al. ^[42] consists of the controllable synthesis of GQDs with an average diameter of about 3.5 nm from glucose coupled with a hydrothermal treatment for 3 h at 200 °C. The main advantage of this technique is that, in addition to synthetic glucose, any natural source or biomass waste with high glucose content can easily be exploited as synthetic precursors. Previously, Naik et al. ^[48] produced GQDs from citric acid pyrolyzed for 25–30 min. The heating caused its decomposition and the formation of a hydronium ion acting as catalyst in the subsequent decomposition reactions, leading to the formation of GQDs from the nucleation of aromatic clusters. 1.5 M NaOH was added to the citric acid solution and the effects of pH on synthesis yield and UV-Vis absorption spectra were investigated.

2.4. Pyrolysis

Under severe reaction temperature and inert atmosphere, many organic molecules can be carbonized into graphitized materials for subsequent exfoliation of GQDs. Pyrolysis is one of the simplest carbonization processes and consists of exploiting high temperatures to convert various renewable starting materials into carbonaceous nanoparticles. A facile pyrolysis synthesis of monodisperse GQDs has been developed for the first time by Mahesh et al. ^[31], via emulsion-templated carbonization of carbohydrates from honey/water emulsion in the presence of butanol. The GQDs obtained from honey have been applied as transparent security ink and a component for white-light emission. Instead, Hassanzadeh et al. ^[49] used glucose directly for the synthesis of GQDs. They prepared a chemiluminescent biosensor to detect cholesterol using MoS₂ nanosheets and graphene quantum dots synthesized by pyrolysis of glucose at 180 °C, for a few minutes. The obtained GQDs have an average diameter of 14.5 ± 4.6 nm and a maximum emission at an excitation wavelength of 360 nm, which gave a quantum yield of 46%. Veeramani et al. ^[50] synthesized porous graphene sheet-like carbon nanoparticles (GPACs) via carbonization from Bougainvillea spectabilis flowers. The flowers were crushed, dried, and subsequently pyrolyzed for 6 h at 200 °C. The synthesized material was exploited for the preparation of electrodes for the detection of catechin. Earlier, Kalita et al. ^[51] used rice grains as biomass by using a similar synthetic approach. In particular, the raw material was fried in a pan at 200 °C for different times, then dispersed in deionized water, and finally subjected to filtration. The high temperature causes the hydrolysis of the glycosidic bonds of the rice starch to decompose into glucose monomers, that subsequently undergo nucleation. The synthesis yield of GQDs from rice grains was approximately 56% and the obtained GQDs showed excellent fluorescence properties with the possibility of exploitation for the preparation of bio-imaging probes. A large scale and controllable synthesis of GQDs, consisting of a combination of two approaches, was recently proposed by Wang et al. ^[28]: the first belongs to the bottom-up approaches and involves the conversion of rice husk, and the second is a top-down approach, consisting of a hydrothermally assisted method. The rice husk was dried, crushed, and carbonized in a tubular stove in N₂ atmosphere at 700 °C and reacted with NaOH at 900 °C for 2 h. The ash obtained was first suspended in an aqueous solution of sulfuric acid and subsequently solubilized by ultrasound for 5 h, then suspended again with the addition of nitric acid and ultrasonicated for 10 h. The solution was washed, filtered, and placed in an autoclave at 200 °C for 10 h. During this process, the silica contained in the rice husk was simultaneously used to synthesize mesoporous silica nanoparticles. This combined approach may open the door to new high-throughput syntheses of GQDs starting from new natural precursors.

2.5. Hydrothermal Method

As mentioned above, pyrolysis is the typical thermal method widely used for preparing nanoparticle carbonaceous materials starting from biomass. However, it has the disadvantage that the carbon source is progressively converted into dots via a series of heating, dehydration, degradation, and carbonization phenomena under drastic conditions requiring high temperatures and long reaction times. On the contrary, hydrothermal treatment, which consists of thermochemical degradation under mild conditions, by exploiting its high moisture content, could represent a valid top-down green technology for GQD synthesis from several natural feedstocks. In fact, the hydrothermal technique is today the most commonly used green technique, from simple precursors such as glucose, sucrose, citric acid and more complex materials, such as biomass wastes. A new interesting hydrothermal approach for converting biomass into GQDs was recently proposed by Wang et al. ^[52]. They synthesized GQDs, with an average size of ca. 3.9 nm with 2−3 graphene layers, by treating rice husk at 150 °C for 5 h in a Teflon-lined autoclave. As prepared, the RH-GQDs can be steadily dispersed in water, exhibiting intense photoluminescence and a highly selective quenching to Fe³⁺ ions, making them a promising material for Fe³⁺ ions sensing. Hydrazine hydrate assisted hydrothermal cutting, at 150/200 °C for 6–10 h, has been successfully applied by Wang et al. ^[9], for the synthesis of quantum dots starting from coffee grounds, resulting in blue luminescent GQDs. The obtained GQDs, after functionalization with polyethylenimine (PEI), showed enhanced fluorescent properties related to band-edge photoluminescence with single exponential decay, and their sensing and bioimaging applications were documented by Wang. An advancement in the hydrothermal approach was obtained by Wang et al. ^[53], who synthesized S-doped GQDs by hydrothermal carbonization of durian. The latter was crushed and dispersed in deionized water, then carbonized in an autoclave for 12 h at 150 °C, in the presence of platinum as a catalyst. It has been demonstrated that the low molecular weight saccharides contained in durian are the main sources of sp2 carbon. Furthermore, the high quantum yield and stable luminescence of the synthesized S-GQDs suggest that they can be successfully exploited in bio-imaging. Tade and Patil in a very recent study reported a simple approach for converting waste biomass of bamboo wood. The synthesis of GQDs was obtained by hydrothermal treatment at 180 °C for 8 h of cellulose nanocrystals (CNC), obtained from bamboo wood (Bf) and previously prepared ^[54]. The carbonaceous product was then subjected to ultrasound, subsequently filtered and finally lyophilized. The authors evidenced that the high temperatures and pressures required by hydrothermal treatment ensure the catalytic conversion of CNCs into one-pot GQD since the cleavage of the 1,4-glycosidic bond of cellulose, and the intramolecular polymerization leading to the formation of the graphite structure, occur almost simultaneously ^[55]. Indeed, the morphological and optical characterization of the synthesized Bf-GQDs encouraged their use as fluorescence sensors for the detection of curcumin. Ahmed et al. ^[56] used corn powder for the green synthesis of GQDs. The corn powder was dispersed in EtOH and stirred at 40 °C for 25 min and hydrothermally heated in an autoclave at 200 °C for 10 h. After one day of cooling, the product was solubilized in deionized water and then centrifuged seven times. The obtained multifunctional GQDs were efficiently used as an additive or as interlayers in perovskite solar cells (PSC) representing a new and effective way, in terms of environmental impact and economic feasibility, for PSC commercialization. Recently, Foong et al. ^[57] prepared several samples starting from sucrose, by varying the concentration of sucrose and solvent (mixture of EtOH and H₂O). The sample was heated in an autoclave up to 190 °C for 12 h. The product obtained was dried in an oven at 80 °C for 24 h and then purified. The characterization confirmed the success of the synthesis from sucrose. Furthermore, the results showed that the addition of additives such as FeCl₃ and oxalic acid significantly improve the quality of the final product. Bayat et al. ^[58] used glucose powder as a synthetic precursor of graphene quantum dots. Quantum dots of single-layer graphene were synthesized by hydrothermal treatment of the glucose powder in deionized water at 200 °C for 8 h. In a recent work of Chen et al. ^[59], polymeric cellulose was used as a synthetic precursor to obtain GQDs via an easy, eco-friendly, and one-pot hydrothermal approach. Previously, the same authors have synthesized GQDs from starch with a green method ^[60]. No acids or corrosive substances were used in this process, only water and cellulose. The cellulose is ultrasonicated for 30 min and the suspension obtained is subjected to HTC for 8 h at 180 °C involving a first step of hydroxylation followed by a phase of ring-closure condensation. The obtained GQDs show low cytotoxicity and good photoluminescence behavior with consequent high potential for in vitro cell imaging of human cervical carcinoma cells. Quantum dots of N-doped graphene modified with CuO/ZnO nanoarrays were synthesized by Safaei-Ghomi et al. ^[61] via a simple, green hydrothermal process for 9 h at 180 °C. Ethylenediamine and citric acid were dissolved in deionized water and mixed with the CuO/ZnO heterojunctions already prepared by a one-step hydrothermal process. Then, N-doped GQDs were formed by direct pyrolysis with 2-hydroxypropan-1,2,3-tricarboxylic acid and the hybrid composite generated with CuO/ZnO nanoarrays. In a recent approach, Zhu et al. ^[62] proposed the synthesis of GQDs by pyrolysis of citric acid, in a round bottom flask at 200 °C for 30 min. The GQDs were then functionalized with glycine, under alkaline conditions, at 120 °C (Gly-GQDs). The latter was characterized and the maximum fluorescence QY was obtained by optimizing the reaction at 120 °C for 90 min and pH 12. The Gly-GQDs were tested as highly effective fluorescence sensors for detection of Hg²⁺ in water because of the real quenching effect of metal ions by a non-radiative electron transfer. Again, by pyrolysis of citric acid, Hasanzadeh et al. ^[63] synthesized GQDs, used for the development of electrodes useful in detecting doxorubicin in the blood. Citric acid was pyrolyzed at 200 °C for 5 min until an orange liquid was obtained which was poured dropwise into an aqueous solution of NaOH. The solution thus obtained was characterized by showing a SEM graphitic morphological structure. The obtained sample was used for the preparation of electrodes, whose doxorubicin detection activity was studied by cyclic voltammetry. According to previous authors, Qu et al. ^[64] synthesized N-GQDs by HTC of citric acid solubilized in water and ammonia for 3 h at 200 °C. The product was then dialyzed for 8 h. The N-GQDs obtained were mixed with both tyrosinase (TYR) and acid phosphatase (ACP) and the relative fluorescence spectra were detected. The same composites were used in human blood samples, demonstrating the synthesis of a new fluorescence biosensor for detecting the activity of these enzymes. Xu et al. ^[43] used lignin sulfonates (SL), an uncommon biomass obtained from extraction of lignin from wood. Citric acid was firstly pyrolyzed by a green hydrothermal method at 200 °C for 15 min, transferred into a mixture of NaOH and lignin sulfonate (SL), and finally dialyzed in a bag for two days. A similar process was carried out but without the addition of SL dispersion and the obtained SL/GQD composites show excellent fluorescence revealing an emission intensity of the SL/GQDs four times higher than that of free GQDs. The peculiar features of the SL/GQDs have been exploited for the detection of Fe³⁺ (Figure 2b) and, to evaluate the specificity of the sensor, the PL intensities were measured in the presence of other interfering ions: the composite with Fe³⁺ showed the strongest fluorescence quenching efficiency. As emerged from the references reviewed, citric acid is a green precursor widely used in the synthesis of nanocarbon materials. Therefore, as previously mentioned for glucose, waste citrus fruits containing a high amount of citric acid could be exploited as interesting bio-precursors for the green synthesis of GQDs.