Carbon Dots as an Effective Fluorescent Sensing Platform for Metal Ion Detection
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Fluorescent carbon dots (CDs) including carbon quantum dots (CQDs) and graphene quantum dots (GQDs) have drawn great interest because of their low cost and low toxicity, and they represent a new class of carbon materials prepared by simple synthetic routes. In particular, the optical properties of CDs can be easily tuned by the surface passivation of the organic layer and functionalization of the CDs. Based on the advantages of these carbon materials, CQDs and GQDs have been applied in various fields as nanoplatforms for sensing, imaging, and delivery. In this review, we discuss several synthetic methods for preparing CQDs and GQDs, as well as their physical properties, and further discuss the progress in CD research with an emphasis on their application in heavy metal sensing.
KeywordsCarbon dots Graphene quantum dots Heavy metal ions Sensing
Carbon quantum dots
Lysine-coated CQDs modified with bovine serum albumin
Graphitic carbon nitride QDs
- g-GQDs and b-GQDs
Green and blue luminescent GQDs
Graphene quantum dots
Magnetic resonance imaging
Multiwall carbon nanotubes
Surface plasmon resonance
Transmission electron microscopy
Valine functionalized GQDs
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, 22, 23, 24, 25, 26, 27, 28, 29, 30].
Synthesis of Carbon Quantum Dots
In the past decades, numerous synthetic methods for the preparation of CDs have been explored. These methods can be largely categorized into two approaches: top-down and bottom-up . 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, doping, 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.
Another study reported another fluorescent CD prepared using multiwall carbon nanotubes (MWCNTs) via an electrochemical method . 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 .
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 . 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 ; first, the graphene must be separated from a chunk of graphite by oxidation ; 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 . 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 , and Le et al. prepared fluorescent CDs by the exfoliation of graphite in ionic liquids (Fig. 2b) .
Physical Properties of Carbon Dots
Surface Passivation and Doping
Pristine CDs, also called undoped CDs, have exposed carbon and oxygen sites after the initial synthesis step . 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 . 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 . 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].
Decoration of CDs for the Detection of Heavy Metals
Heavy metals are often necessary and are rarely harmful to human health at low concentrations, but their accumulation can lead to a wide spectrum of debilitating diseases. In addition, heavy metal pollution, which is predominantly caused by Hg2+, As3+, Pb2+, Cd2+, and Cu2+, is considered to be one of the most deleterious threats to the environment that could permanently undermine global sustainability . Therefore, the development of versatile systems to monitor trace heavy metals continuously is crucial in modern society.
CDs are desirable candidates for use in potable detectors because of their abundance, high stability, low toxicity, and inexpensive nature [68, 69, 70, 71]. Moreover, surface modification is facile and can be used to make the CDs soluble in water, as well as resulting in high fluorescence quantum yields, making them attractive candidates for biocompatible nanomaterials . The binding and interaction between the probes and heavy metals causes changes in physicochemical properties of the fluorophores, including the fluorescence intensity, lifetime, and anisotropy, and provides a meaningful signal than can selectively indicate analytes with high sensitivity as a result of quantum confinement. Here, we outline recent studies related to different types of surface materials that will facilitate the application of CDs in heavy metal detection [73, 74, 75, 76, 77].
The initially synthesized CDs exhibit no fluorescence and are poorly dispersed in polar solvents such as H2O and ethanol, which limits the utilization of fluorescent CDs as environmental probes or for biological applications for detecting heavy metals. Accordingly, numerous researchers have focused on the development of CDs to enhance their quantum yield and dispersibility in polar solvents. One easy way to achieve this is to incorporate various functional groups on the surface of the CDs. Zhu et al. reported a facile hydrothermal method using citric acid and ethylene diamine; interestingly, they investigated how changes in the ratio of the two precursors affected the quantum yield in response to Fe3+. They found that changing the ratio of the two components altered the number of incorporated hydroxyl and carboxyl residues. Thus, the final product showed different fluorescence intensities. Without amine groups, the quantum yield was less than 10%, and the maximum quantum yield was 60% in comparison to those of quinine sulfate. The fluorescence of the CDs was quenched in the presence of Fe3+, likely because of coordination between the hydroxyl groups of the CDs and Fe3+. The detection limit for Fe3+ was 1 ppm . This result clearly suggests that the tuning of the functional groups is important for achieving optimal probe fluorescence. Sun et al. also reported the preparation of amine-functionalized GQDs from ammonia by hydrothermal treatment, and this increased the quantum yield by eight times compared to that of the native GQDs. In addition, the GQDs showed high selectivity to copper ions . Dong et al. reported an effective method to detect trace amounts of Cu2+ ions using branched polyethyleneimine-functionalized CDs as fluorescent probes . An increase in the fluorescence intensity occurred on exposure to Cu2+. Furthermore, they tested this probe in real river water samples, and it showed a linear response from a Cu2+ concentration of 0 to 9 μM; this sensor was affected by the pH, only showing sensitivity at pH 4.0, however.
One method to tailor carbon-based nanomaterials is the introduction of other atoms such as nitrogen and sulfur, thus changing the electronic properties. The doping of graphene with nitrogen forms N-graphene, which has different properties compared to pristine graphene. The nitrogen dopants affect the distribution of the charge and spin densities of the carbon atoms, thereby activating the graphene surface [81, 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 . Thus, heteroatom doping 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 . 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 .
Biomolecules and Natural Materials
Valine-functionalized GQDs (Val-GQDs) were synthesized by simultaneous mixing with citric acid via thermal pyrolysis . The base GQDs were formed from pyrolyzed citric acid through dehydration and carbonization, and the incorporated valine led to changes in the fluorescence. The quantum yield of the Val-GQDs was increased fourfold compared to that of pristine GQDs. The increase in the quantum yield was caused by changes in the steric and electronic properties, likely induced by the increase of nitrogen moieties in pyridine and pyrrole groups formed after the functionalization with valine [88, 89]. Interestingly, the presence of valine moieties in the Val-GQDs resulted in a more sensitive fluorescent response to Hg2+, showing a detection limit of 0.4 nM (signal-to-noise ratio = 3) and a sensitivity 14-times greater that of the unmodified GQDs.
Chitosan is a natural material and is the main component of the outer shells of shellfish such as crabs. Its abundance and biosafety are advantageous for its use as a CD precursor, and studies have shown that it can be used to produce N-doped CDs in a simple process because it provides both carbon and nitrogen together . This method overcomes the general problems suffered by CDs derived from natural materials, which often have low quantum yields, and the CDs showed a 31.8% quantum yield. In addition to smartphone applications, these materials also have possible applications as portable detection probes for Hg2+, having a detection limit of 80 nM. The N-doped CDs showed strong fluorescence near 440 nm without Hg2+, whereas the fluorescence was greatly quenched in the presence of Hg2+. Its fluorescence decay was linear within a range of 80–300 μM Hg2+ .
Sahu et al. reported a green synthesis for the fabrication of highly fluorescent CDs from natural source, the leaves of Ocimum sanctum, in a single step. The eco-friendly prepared CDs have excellent selectivity toward Pb2+ ions with a detection limit of 0.59 nM and linear detection range of 0.01–1.0 μM and good cell-permeability and low cytotoxicity, thus effectively used for the fluorescence cell imaging .
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 . Inspired by these phenomena, Ran et al. synthesized Ag nanoparticles decorated with GQDs for the rapid, and sensitive detection of Ag+ and bithiols . 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.
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 .
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 developing 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 . 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.
MHP conceived the study and supervised the whole study. DY, YP, and BC drafted the manuscript including the design of the figures. These three authors contributed equally to this work. All authors read and approved the final manuscript.
The research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF-2016R1D1A1A02937456) and by the Commercialization Promotion Agency for R&D Outcomes (2018K000370).
The authors declare that they have no competing interest.
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