Introduction

The discovery of fluorescent carbon dots (CDs), also known as carbon quantum dots (CQDs), has attracted tremendous interest from many researchers because of their versatile applications in optoelectronics, biomedical applications, and chemical biosensors [1,2,3]. All nano-sized fluorescent carbon materials with one dimension less than 10 nm can be classified as CDs, and these can be derived from various carbon materials such as fullerenes, graphite, carbon nanotubes, and graphene [4,5,6]. CDs have several advantages compared to other conventional fluorescent sensors. For example, organic dyes are inexpensive and effective as fluorescent probes, but they are easily photobleached. In contrast, CDs are much more resistant to photobleaching [7,8,9]. Additionally, semiconductor quantum dots (QDs) are comparably as good as CDs in terms of photostability, quantum efficiency, and tunable fluorescence, but QDs cannot be used to trace a single molecule for long-term monitoring because of their intrinsic blinking [10,11,12,13,14,15]. Moreover, the main pitfall of QDs is their toxicity, which is due to their heavy metal content, including metals such as cadmium; this limits their biological and environmental applications [16,17,18,19]. Compared to other fluorescent raw materials, CDs are synthesized from inexpensive carbon sources that are abundant in nature and are, thus, bio-friendly. Furthermore, there are several simple methods to modify the surface state of CDs, which allow researchers to tune the solubility and quantum yields of CDs according to their experimental requirements [20,21,33]. Simply, the former process cleaves bulk carbonaceous materials into CDs via physical, chemical, or electrochemical methods, whereas the latter synthesizes CDs from appropriate precursors from various carbon sources. Surface modification can be applied after or during CD synthesis via surface passivation, do**, or functionalization. Because many synthetic procedures have been summarized elsewhere, here, we briefly describe the development and advances in CD research from the early years of their discovery.

Top-down Methods

In the top-down method, carbon macromolecules are cut into smaller pieces using physical forces such as arc-discharge, laser ablation, or electrochemical reactions. Subsequently, further surface modification is applied to enhance and tune their fluorescence [34]. CDs were discovered as a byproduct of the synthesis of single-walled nanotubes (SWNTs) prepared by the arc-discharge method [35]. Because of the impurities in the resultant suspension, further electrophoretic separation occurred, and a fluorescent and fast moving band was isolated; these were referred to as fluorescent nanoparticles. Since then, researchers have extended their study to other carbon allotropes, and various modification methods to create a range of fluorescent materials have been reported. Sun et al. reported photoluminescent CDs prepared using laser ablation (Fig. 2a) [32]. These CDs were prepared from the hot-pressing of cement and graphite and cut with laser in a hot vapor-filled chamber. Because the products were composed of various sizes of non-fluorescent particles, further polymeric passivation with diaminopolyethylene glycol (PEG1500N) or poly(propionylethyleneimine-co-ethyleneimine) (PPEI-EI) was applied to confer the dots with fluorescence. A subsequent study tested whether there was a relationship between the fluorescence of the CDs and the solvent type. Initial CDs were prepared from graphite irradiated with a laser in PEG200N/water [37]. Because the CDs prepared from PEG200N were fluorescent, the study concluded that solvents can be used for the functionalization of CDs.

Fig. 2
figure 2

Illustration of the formation of a fluorescent CDs via laser ablation with PEG attached to the surface [32] and b GCDs prepared by exfoliation in an ionic liquid [36]. The inset is a transmission electron microscopy (TEM) image of the fabricated GCDs and the solution illuminated by an UV lamp

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Another study reported another fluorescent CD prepared using multiwall carbon nanotubes (MWCNTs) via an electrochemical method [38]. The MWCNTs were placed between two electrodes in an electrolytic solution, and a voltage was applied at a constant rate. The voltage cycling recurrently led to the oxidization and reduction of the MWCNTs, and this broke down C-C bonds of the MWCNTs, widened defects to allow the incorporation of oxygen, and generated hydroxyl/carboxyl residues. As this reaction progressed, the solution changed from yellow to dark brown and emitted blue light under UV irradiation. The particles were uniformly spherical with a size of 2.8 nm in diameter. Similarly, other CDs have been synthesized from graphite using electrochemical exfoliation, where two graphite electrodes are placed in an alkali electrolyte solution (NaOH/ethanol), followed by the application of a current. The graphite rods are exfoliated into chips and generate fluorescent CDs with a size of 4 nm [39].

Subsequently, researchers tried to develop simpler and more efficient methods of CD synthesis. The selection of electrolytes provides another way to control the properties of the CDs. For example, an imidazole ionic liquid can be used as an electrolyte. This liquid performs two roles, acting as an electron acceptor at the anode and also penetrating the graphite sheet and accelerating the exfoliation process [36]. However, its application generated a range of particle sizes and morphologies, and its removal is complicated and time-consuming.

The generation of fluorescent graphene quantum dots (GQDs) from graphene requires more steps than other types of carbon macromolecules [40]; first, the graphene must be separated from a chunk of graphite by oxidation [41]; subsequently, the graphene oxide (GO) must be cut with various methods as mentioned above [42,43,44]. Pan’s group reported a simple hydrothermal approach for the cutting of graphene sheet into GQDs with bright blue photoluminescence [45]. In addition, Zhu et al. reported the creation of GQDs with a large-scale zigzag edge structure through acidic exfoliation and etching of pitch carbon fibers [4], and Le et al. prepared fluorescent CDs by the exfoliation of graphite in ionic liquids (Fig. 2b) [46].

Bottom-up Method

Bottom-up methods synthesize CDs from various small carbon molecules including citrates, carbohydrates, and other green materials. In this method, it is easy to control the surface state, as well as the size, of the CDs [47,48,49]. The whole synthetic procedure is briefly described. The process is initiated from the carbonization of carbon precursors, which occurs concurrently with dehydration via heat-treatment through hydrothermal, microwave, or pyrolysis methods in concentrated acids (Fig. 3) [50,51,52]. The microwave-assisted hydrothermal method is common, and it is easy to synthesize CDs from various organic sources, including sucrose, glucose, saccharides, amino acids, and proteins, which can all be used as building blocks for CDs [53,54,55,56]. Because of the diversity of precursor materials, various functional groups remain after synthesis, and these are beneficial for enhancing the fluorescence of the CDs. It is also possible to create CDs by refluxing candle soot in a strong acid, where the oxidation by the acid is important for the dissolution of the soot [57]. However, the fluorescent products derived from small carbon molecules limit the mass production of CDs and lack quality control because of the heterogeneity.

Fig. 3
figure 3

a Scheme showing bottom-up formation of CDs at different temperatures and b the relationship between different products [50]

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Physical Properties of Carbon Dots

Structures

Understanding the structures of CDs is critical to understanding their key features, including fluorescence. CDs mostly have a graphitic in-plane lattice spacing of 0.18–0.24 nm and graphitic interlayer spaces of 0.32 nm (Fig. 4a). Although the detailed structure of the CDs varies depending on the raw materials and synthetic method, it is generally accepted that CDs are composed of carbon crystalline cores similar to sp2 carbon and amorphous clusters (Fig. 4b) [33, 58, 59]. Generally, the degree of crystallinity of CDs is lower than that of GQDs, and some CDs contain diamond-like sp3 carbon [37]. Raman spectroscopy reinforces these observations, and two peaks around 1350 and 1600 cm−1 are typically observed, indicating disordered sp2 carbon and crystalline graphitic carbon, respectively [33, 59]. In addition to the core carbon framework, different functional groups are usually introduced into CDs via surface passivation or functionalization, and these protect the surface and enhance the fluorescence of the CDs.

Fig. 4
figure 4

a High-resolution TEM images showing measurement of the space between lattices of CDs and b schematic representation of CDs with a carbogenic core containing sp2 carbon [33]

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Fluorescence

It is generally accepted that the surface state of CDs is closely correlated with their fluorescence. However, because of structural complexity of CDs, the exact underlying mechanism of CD fluorescence is unclear and requires clarification. Pan et al. addressed this question with full-color and blue-color CDs (Fig. 5) [60]. The optical properties of CDs, even those prepared from the same materials (mixtures with the same ratio of citric acids and formamide), can differ depending on the temperature and duration of the heat applied in the microwave hydrothermal method. That is, two different CD samples can display different fluorescence spectra. The CDs prepared at high temperatures for long reaction times showed a full color spectrum, whereas those created in a short period at low temperatures showed a blue color when irradiated with the same wavelength. This could be attributed to the differences in size of the CDs, which affects the emission profiles of CDs because, like semiconductor QDs, their emission depends on quantum confinement effects; i.e., as the size of QDs decreases, the energy gap between the valence shell and conduction band widens, and the emission wavelength decreases. However, the differences can also arise because of the surface state of the CDs, and investigation revealed that the full-colored CDs had more functional groups, including C=N/C=O and C-N groups, on their surfaces than the other sample [59, 60]. Consistent with previous study, the evidence suggests that the fluorescence of CDs is not caused by a single factor but arises from a combination of several factors such as size, surface passivation, functional groups, and heteroatoms [61].

Fig. 5
figure 5

a Schematic of the preparation of the full-color-emission CDs. b Fluorescence spectra of the fluorescent CDs (F-CDs) under different excitation wavelengths. c UV-vis absorption spectra of the F-CDs. d Fluorescence emission photographs of the F-CDs recorded from 330 to 600 nm in 30 nm increments. All spectra and photographs were obtained in deionized H2O [60]

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Surface Passivation and Do**

Pristine CDs, also called undoped CDs, have exposed carbon and oxygen sites after the initial synthesis step [33]. Passivation protects the carbon and oxygen-containing groups on the surface from interacting with other organic molecules, thus preserving the optoelectronic properties of the CDs. Polymeric PEG1500N has been introduced onto CDs by acid treatment, and this has been shown to enhance the fluorescence of the CDs [32]. Surface passivation itself also contributes to the functionalization of CDs with no need for further modification. Many other materials have also been applied, such as different molecular weights of PEG, branched polyethyleneimine (b-PEI), and diamine-terminated oligomeric PEG, yielding polyamine-passivated CDs and CDs functionalized with free amines; this allows fluorescence tuning [62]. Different functional groups affect the energy levels of the CDs, which alter and enhance the light absorption and emissive spectrum of the probes. Additionally, surface modification also enables the modulation of the solubility of CDs in certain solvents. For example, the acid treatment of CDs generally results in the incorporation of carboxyl, carbonyl, and hydroxyl groups [32, 57].

Burlinos et al. demonstrated the functionalization of CDs by one-step pyrolysis, in which a mixture of citric acid and different amines underwent thermal decomposition. In this system, citrate provided the carbon core, whereas the amines were attached as functional groups on the CDs [63]. Yang et al. reported a method for the large-scale preparation of heavy metal-doped CDs with tunable photoluminescence [6). Moreover, these N- and S-doped CDs were very sensitive for the detection of Cu2+ and Hg2+, respectively [82]. Ju et al. reported that N-doped GQDs synthesized from citric acid and doped with hydrazine through a simple hydrothermal method that are sensitive to Fe3+, having a detection limit of 90 nM [83]. Thus, heteroatom do** can drastically change the electronic characteristics of GQDs, and the label-free sensitive and selective detection of Fe(III) ions could be performed in real water samples. Thus, this method provides a simple and low-cost route for the production of sensing platforms.

Nitrogen–sulfur co-doped CDs prepared from a single polymeric precursor as highly sensitive photoluminescent probes for mercury detection were developed by Mohapatra et al. The turn on–off fluorescence changed upon mercury addition, and this is attributed to the nonradiative electron transfer from the excited state to the d-orbital of the metal ion. The soft–soft and acid–base interactions between the sulfur part of the CD and Hg2+ make the fluorescent probe more specific and selective toward Hg2+, having a limit of detection of 0.05 nM for mercury ions [84]. In addition, Wang et al. reported the synthesis of boron-doped CDs (B-C-dots) by hydrothermal synthesis using ascorbic acid and boric acid as precursors. Due to the charge transfer between the chelate oxygen atoms on the CD surface, the strong fluoresce can be quenched by Cu (II) and Pb (II) ion [85].

Barman et al. reported highly blue fluorescent graphitic carbon nitride QDs (g-CNQDs) for the detection of mercuric and iodide ions. Mercury was chosen as a target because it causes a neurological syndrome called Minamata disease [86]. To synthesize the g-CNQDs, microwave-mediated synthesis was used with a formamide precursor. Because of their greater affinity toward nitrogen than carbon, their large radius, and their ability to form complexes with nitrogen, Hg2+ ions can affect the quenching of the fluorescence properties of g-CNQDs sensitively and selectively. The formation of the non-fluorescent cleating g-CNQD-(Hg2+)x complex resulted in a non-fluorescent “OFF” state, whereas the addition of I− ions changed this “OFF” state to an “ON” state, indicating that the formation of chelating Hg2+ complex had occurred (Fig. 8).

Fig. 8
figure 8

a Schematic of N-doped graphene-(Hg2+) complex and N-doped graphene on the addition of I-ions. b The change in the fluorescence emission of N-doped graphene (5 μg L−1) in water on the addition of Hg2+ ions. c Fluorescence quenching of Hg2+ ions compared to that of other metal ions [86]

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Biomolecules and Natural Materials

Biomolecules have great potential for the modification or synthesis of CDs when there are concerns regarding toxicity and biocompatibility. Various biochemical components produced in nature, including amino acids, oligosaccharides, and their macromolecules and derivatives, can be used. Liu et al. reported that lysine-coated CQDs modified with bovine serum albumin (CQDs-BSA-Lys) could be used for the detection of Cu2+ ions [87]. The synthesis of the pristine CDs was carried out using a mixture of glucose and PEG200 by microwave treatment. BSA was mixed with a coupling reagent with gentle stirring, resulting in carbodiimide formation. The subsequent addition of lysine greatly enhanced the fluorescence of the CQDs-BSA, probably because of the interactions between the carboxylic acids and amines from both BSA and lysine, as well as the formation of a coating layer, which likely reduced the surface defects on the CDs. The CDs were tested for their function as a copper-selective probe in the presence of various heavy metals, and the probe showed specificity for copper, detecting Cu2+ concentrations of 2 nmol (Fig. 9). The Cu2+ ions appear to form multiple coordination complexes around the carboxylic acids and amines of lysine in the CQDs and glycine on the partially uncoated CQDs [87].

Fig. 9
figure 9

a Schematic of the CD modification with BSA and Lys and Cu2+ detection. b The selectivity of the CDs-BSA-lysine fluorescent probe toward 2 nmol Cu2+ in the presence of other cations under optimum conditions [87]

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Valine-functionalized GQDs (Val-GQDs) were synthesized by simultaneous mixing with citric acid via thermal pyrolysis [93].

Metal Nanoparticles

Novel metal nanoparticles, such as those of Au, Ag, and Pt, exhibit distinctive surface plasmon resonance (SPR) peaks depending on their size and shape. Interestingly, composites of carbon-based nanomaterials and novel metal nanoparticles have been studied because of their characteristic optical properties. Noble metal clusters can be immobilized with great stability through hybridization between the sp2 dangling bonds at the defect sites of graphene sheets and the clusters. After immobilization, the fluorescence of the GQDs can be quenched by these metal nanoparticles or clusters of ions can form by charge transfer processes [94]. Inspired by these phenomena, Ran et al. synthesized Ag nanoparticles decorated with GQDs for the rapid, and sensitive detection of Ag+ and bithiols [95]. The formation of AgNPs on GQDs quenches the fluorescence of the GQDs, and the addition of bithiols causes a further turn-off phenomenon via their strong interactions through the formation of Ag–S bonds.

Ting et al. reported novel conjugates of cysteamine-capped gold nanoparticles (AuNPs) and GQDs, and these were used for the sensitive electrochemical detection of Hg2+ and Cu2+ with detection limits of 0.02 and 0.05 nM, respectively [96]. The Hg2+ ions are pre-concentrated onto the electrode by applying a negative voltage (− 0.2 V and 120 s), and the negatively charged hydroxyl and carboxyl groups interact with Hg2+ because of the formation of R-COO-(Hg2+)-OOC-R groups, as well as the initial binding of mercury onto AuNPs. In the case of Cu2+ ions, the anodic strip** voltage of copper occurs at 0 V, meaning that it is clearly separated from that of mercury ions and implying the possibility of the simultaneous detection of the two-ion species. In addition, Bourlinos et al. presented the synthesis of ultrafine sized Gd(III)-doped CDs with dual fluorescence/magnetic resonance imaging (MRI) character via the thermal decomposition of a precursor composed of an organic salt and a gadolinium(III) complex. The dots are water-dispersible, display bright fluorescence in the visible range upon light excitation, and show strong T1-weighted MRI contrast comparable to that of commercial Gadovist, as well as possess low cytotoxicity (Fig. 11) [97].

Fig. 11
figure 11

Synthesis of Gd-QCDs. The Gd(III) centers are immobilized in the carbonaceous matrix through coordination by residual O and N heteroatoms [97]. The inset shows the MRI positive contrast effects in T1-weighted images of the Gd-QCDs and the commercial Gd-based contrast agent-Gadovist

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Zhang et al. reported an efficient CQD-gold nanocluster (CQDs/AuNCs) nanohybrid prepared by a one-step hydrothermal treatment with alanine and histidine. The hybrid materials were used for ratiometric fluorescent probe for sensitive and selective sensing of CD (II) ions with a detection limit of 32.5 nM. Interestingly, the quenched fluorescence by Cd2+ can be gradually recovered upon the concentration of l-ascorbic acid (AA)with a detection limit of 105 nM and this fluorescent “on-off-on” system can be practically used for the excellent detection to Cd2+ and AA in lake water and in human serum, respectively [98].

Conclusion

Much research into carbon-based quantum dots has been reported in the last few decades, and a wide range of synthetic methods and characterization techniques have been used. In most cases, studies of these fluorescent materials have focused on their bioimaging applications. Although some heavy metals are essential in the human body, excess heavy metals cause disease, for example, Minamata disease and Itai-itai disease. Thus, recent progress in fluorescent CDs has opened the possibility of develo** portable detectors for dangerous heavy metals, and we have outlined recent studies related to surface materials that will enable the development of heavy metal sensors as a portable device [99]. Moreover, the progress in biocompatible fluorescent CDs enables harmless onsite detection as well as the color-mediated analysis provides easy interpretable readout even for non-professional persons. However, relatively low solubility of CDs in water remains challenges and low cost for fabricating devices is another requirement for the use of CDs in various fields, even though many synthetic methods have been developed. In addition, the exact mechanism for different photoluminescent which depends on the synthetic method and raw carbon sources should be more cleared. We hope that this review will inform researchers about the recent progress in carbon-based quantum dots for heavy metal sensing, leading to develop new eco-friend and cost-effective synthetic methods and practical use.