Review of Carbon and Graphene Quantum Dots for Sensing | ACS

Review Cite This: ACS Sens. XXXX, XXX, XXX−XXX

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Review of Carbon and Graphene Quantum Dots for Sensing Meixiu Li,†,§ Tao Chen,†,§ J. Justin Gooding,*,‡ and Jingquan Liu*,† †

College of Materials Science and Engineering, Institute for Graphene Applied Technology Innovation, State Key Laboratory of Bio-Fibers and Eco-Textiles, Collaborative Innovation Center for Marine Biomass Fibers Materials and Textiles of Shandong Province, Qingdao University, Qingdao 266071, China ‡ School of Chemistry, Australian Centre for NanoMedicine and ARC Centre of Excellence in Convergent Bio-Nano Science and Technology, The University of New South Wales, Sydney, New South Wales 2052, Australia Downloaded via NOTTINGHAM TRENT UNIV on July 17, 2019 at 07:19:46 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: Carbon and graphene quantum dots (CQDs and GQDs), known as zero-dimensional (0D) nanomaterials, have been attracting increasing attention in sensing and bioimaging. Their unique electronic, fluorescent, photoluminescent, chemiluminescent, and electrochemiluminescent properties are what gives them potential in sensing. In this Review, we summarize the basic knowledge on CQDs and GQDs before focusing on their application to sensing thus far followed by a discussion of future directions for research into CQDs- and GQD-based nanomaterials in sensing. With regard to the latter, the authors suggest that with the potential of these nanomaterials in sensing more research is needed on understanding their optical properties and why the synthetic methods influence their properties so much, into methods of surface functionalization that provide greater selectivity in sensing and into new sensing concepts that utilize the virtues of these nanomaterials to give us new or better sensors that could not be achieved in other ways. KEYWORDS: carbon quantum dots, graphene quantum dots, optical property, luminescence, sensing

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arbon quantum dots (CQDs),1 often just called carbon dots (CDs),2,3 are small fluorescent carbon nanoparticles with sizes less than 10 nm in diameter.4−6 They have attracted considerable interest over the past few years due to their properties of high chemical stability, good conductivity and luminescence, and broadband optical absorption.5 Graphene quantum dots (GQDs),7 are a subset of CQDs usually derived from graphene and/or graphene oxide.8 GQDs exhibit physical and chemical properties similar to those of graphene.9 From the point of dimension, GQDs are small graphene sheets with lateral dimensions less than 10 nm with less than 10 graphene layers forming the final particle.10−12 However, some of the properties of CQDs diverge from those of graphene because of the dominance of edge effects with CQDs and because of quantum confinement.13,14 In comparison with semiconducting quantum dots such as CdSe, WO3−x,15 and CdS quantum dots, CQDs and GQDs exhibit superior photostability against blinking and photobleaching, low toxicity, and hence potentially greater biocompatibility.16−18 From the first discovery of CQDs by Xu et al. in 2004,19 a great deal of effort has been devoted to exploring new methods for the preparation of CQDs and GQDs with the pursuit of simplicity, high yield, low cost, less pollution, better properties, large-scale production, and uniform sizes.20 The methods of synthesis of CQDs and GQDs have been well-reviewed elsewhere and will only be touched on here.4,12,21,22 The © XXXX American Chemical Society

properties of the CQDs and GQDs can vary with size, morphology, and doping type or amount.22 Because of their properties, and the ability to tune these properties, CQDs and GQDs have been widely explored for applications in fields such as biomedicine,23 catalyst,24 energy,25,26 and photoelectronic applications,27 as well as sensing.28 To date, a number of excellent review articles on utilizations of CQDs and GQDs have been published.4,5,8,22,29,30 However, the rapidly expanding literature recently on the use of CQDs and GQDs in sensing31−34 has yet to be reviewed. In this Review, we will briefly summarize the synthetic approaches for CQDs and GQDs since their discovery and the properties of the CQDs and GQDs that arise from the different synthetic methods. Then we primarily focused on their typical applications in sensing in the past five years that utilized CQDs and GQDs as a key component in different techniques and principles. Future directions and perspectives for the applications of CQDs- and GQD-based nanomaterials are discussed. We hope this review article can assist in providing guidance on the future utilization of GQDs for sensing. Received: March 15, 2019 Accepted: July 3, 2019 Published: July 3, 2019 A

DOI: 10.1021/acssensors.9b00514 ACS Sens. XXXX, XXX, XXX−XXX

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produced N-GQDs studied by Qu et al.96 N atoms were introduced into the GQDs using tetrabutylammonium perchlorate in acetonitrile. It was found with an N/C atomic ratio in the doped GQDs blue luminescence was emitted as distinct from the green luminescent before N doping. The blue shift with N doping can be attributed to the stronger electronwithdrawing ability of N atoms. Qu et al.103 synthesized NGQDs using urea as the N-containing bases, gaving a large increase in the photoluminescence quantum yield of the GQDs compared with the undoped GQDs. Lu and Zhou105 successfully prepared nitrogen-doped CQDs (N-CQDs) by a simple one-step method via ultrasonic treatment of dopamine using dimethylformamide as solvent. It was found that the asprepared N-CQDs showed good water dispersibility, low cytotoxicity, high photostability, and bright and stable fluorescence against variations of pH. Similarly, boron-doped CQDs (B-CQDs) were obtained through a one-pot solvothermal process by Li’s group.106 The as-prepared BCQDs showed an obvious red shift, higher longitudinal relaxivity, and increased fluorescence intensity compared to that of boron-free CQDs. Next, considering the surface functionality, naturally a large amount of carboxyl moieties can lead to water dispersibility and biocompatibility. The oxygen-containing groups on the CQDs surfaces are useful for subsequent surface modification via covalent bonding,107,108 the sol−gel technique,109 and using coordination chemistry.110 Therefore, many studies have investigated how to tune the properties of CQDs by chemical modification using a series of organic, polymeric, inorganic, and biological materials. With GQDs, chemical modification involving strongly electron-accepting or electron-donating molecules can also have an appreciable effect on the electronic properties of graphene.12 Electron-withdrawing groups decrease the lowest unoccupied molecular orbital levels, while electron-donating moieties raise the highest occupied molecular orbitals levels.111 As such there has been considerable effort focused on the modification of GQDs.112−115 For example Zhu et al.116 successfully changed green luminescent GQDs into blue luminescent GQDs via surface chemical modification with alkylamines (Figure 1A,B). After modifica-

PROPERTIES OF CQDS AND GQDS Morphology and Composition. Carbon-based QDs (CQDs here but also abbreviations such as C-dots or CDs are used) and GQDs are luminescent carbon nanoparticles. It should be noted that, compared with GQDs, CQDs usually show poorer crystallinity and much more defects owning to the content of less crystalline sp2 carbon.24,35 GQDs are often prepared by graphene-based starting materials or graphene-like polycyclic aromatic hydrocarbon molecules.10,36 As a consequence, GQDs usually possess graphene lattices, which are similar to the crystalline structure of a single or a few layered graphene.21 The in-plane lattice spacing of both CQDs and GQDs ranges from 0.18 to 0.25 nm, while the interlayer spacing of graphite is in the range of 0.32−0.34 nm or bigger when graphite is treated by oxidation for example.37−39 Both CQDs and GQDs usually have a plethora of oxygen-containing functional groups on their surfaces.37,40 In general, the size and height are the main aspects of the morphologies of CQDs. The average diameter of most CQDs reported are less than 10 nm.7,10,41−43 The synthesis of CQDs- and GQD-based nanomaterials can be achieved using “bottom-up” or “top-down” approaches.30,38,44 The bottom-up approaches include electrochemical carbonization,45 microwave irradiation synthesis,46−52 hydrothermal/solvothermal treatment,53−82 and thermal decomposition.83−85 Typically, rigorous reaction conditions such as high-grade carbon precursors, high temperature, concentrated alkali/acid treatments, and toxic organic solvents are required for these methods.44 The bottom-up approaches have the advantage that they provide good synthetic control and hence good size control. Even more important is that the origins of the carbon source can greatly influence the properties of the CQDs, including the sensing properties, where seemingly the same CQDs, made with the same method, but from different precursors, will have very different selectivity for different metal ions.54,57,71,72,86 The top-down methods44 include electrochemical oxidation,87,88 laser ablation,89−93 chemical ablation or oxidation,94 arc-discharge/ plasma treatment19 and ultrasonication.95 Cheap raw carbonaceous materials could be applied as the starting materials with these methods. Furthermore, the top-down methods are suitable for mass production of CQDs- and GQD-based nanomaterials because of the simple preparation steps.44 The resultant CQDs from any synthesis consist predominantly of carbon, hydrogen, and oxygen. The surface of CQDs can consist of a diverse range of oxygen-containing groups including carbonyl, hydroxyl, carboxylic acid, and epoxy/ether as a result of oxidation processes. Furthermore, other elements, especially N and S, can easily be doped into the CQDs. With regard to doping, the chemical composition of the precursor carbon source from which the CQDs are derived is one simple method of doping. Both the surface functionality and heteroatom doping can influence the properties of the CQDs. Turning to doping first, the heteroatoms can influence the intrinsic properties of carbon nanomaterials, including their electronic properties, optical properties, local chemistry, and surface reactivity.96−98 Of the myriad of possibilities, nitrogen doping is probably the most common dopant used, with many examples of N-doped graphene,99,100 N-doped carbon nanotubes,101 and N-doped GQDs (N-GQDs).102−104 The power of N-doping on the optical properties of GQDs is shown by electrochemically

Figure 1. (A) Scheme of preparation routes (suggested structures). (B) Scheme of bandgap changing of GQDs, modified GQDs, and reduced GQDs. Reprinted with permission from ref 116. Copyright 2012 John Wiley and Sons.

tion, the previously present epoxy and -COOH moieties of the GQDs are converted into -CNHR and -CONHR groups, respectively. In both cases these chemical conversions at the surface of the GQDs decreased nonradiative recombination and therefore converted the GQDs from a defect state into intrinsic emissive states.113 Furthermore, Zhang et al.114 found B


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attention. For instance, the linear optical absorption of diamond-shaped GQDs was systematically explored by Basak’s group.126 They presented theoretical calculations of the electronic structure and linear optical absorption spectra of GQDs with varying sizes employing a methodology based upon the Pariser−Parr−Pople model Hamiltonian. Not only were red-shifted absorption spectra suggested by their calculations but also the variation in the behavior of the linear absorption spectrum with increasing size of GQDs was predicted. These calculations suggested a significant difference to the predictions from the tight-binding model. Besides, the absorption spectrum of the hydrogen passivated counterpart of GQDs with 48 carbon atoms was also investigated theoretically, although such well-defined nanoparticles are yet to be synthesized. Once excited via the absorption of light, fluorescence is the most obvious and investigated property of CQDs and GQDs in the view of both fundamental studies and practical applications. The emission wavelength is sensitive to size, surface defects, surface states, and the degree of π conjugation.125 If the CQDs are embedded within a matrix, this can also influence the optical properties. For example, luminescent CQDs were embedded in a mesoporous silica/ polyacrylonitrile (mesoSiO2/PAN) nanofibrous membrane with hexadecyltrimethylammonium bromide as the directing agent by the Yuan group.127 Compared with the freely dispersed CQDs, the fluorescence excitation and emission peaks of the CQDs/mesoSiO2/PAN nanofibrous membrane showed red shifts of 15 and 40 nm, respectively. The excitation-dependent emission at longer wavelengths of the CQDs were no longer evident after immobilization of the CQD immobilization, suggesting that the photoluminescence of the CQDs originated from the edge defects or emissive traps on their surfaces instead of the quantum confinement of different-sized CQDs. The low quantum yield is restrictive to the application of CQDs in fluorescence sensing and as such enormous efforts have been devoted to improve the fluorescence properties of CQDs via surface functionalization and doping with other heteroatoms (such as N, S, B, and P). For example the detection of Ag+ and Al3+ was improved by using N-CQDs and B-GQDs, respectively, over undoped quantum dots.59,128 This is one of the first systems to start to systematically demonstrate how the synthesis of the CQDs may influence the selectivity of the final sensor. The PET mechanism is where fluorescence of an excited chromophore is quenched by electron transfer to an acceptor species.129,130 In the application of the PET with GQDs, there have been a number of interesting studies recently.131−134 For example, Ghosh et al.132 investigated the PET process using a series of aniline derivatives to GQDs to study the PET process. In their study, the solution-processable GQDs were synthesized first from graphene oxide and exhibited a strong luminescence at 510 nm upon photoexcitation at 440 nm, respectively. The aniline derivatives served as electron donors. The results showed that the aniline derivatives could interact with GQDs in the excited state, leading to an obvious luminescence quenching of the GQDs. PET is extensively used in the development of CQD- and GQD-based sensors. For example PET between graphene and GQDs was used to develop a fluorescence turn-on nanosensor by Qian and coworkers.134 The as-prepared sensor could monitor Pb2+ with a detection limit of 0.6 nM, with a broad linear range up to 400.0 nM, and with fast response time.

that when GQDs were reduced with hydrazine hydrate, the new hydrazide groups were found to be attached to the edges of the GQD surface through an esterification reaction with the peripheral carboxylic acid groups, which could result in the π−π* and n−π* transitions of modified hydrazide groups and GQDs. A strong yellow luminescence could be observed after modification. However, no yellow luminescence, but weak blue luminescence, could be observed when the hydrazine hydrate was changed to NaBH4. Similarly, CQDs modified with polyamidoamine (PAMAM) and (3-aminopropyl)triethoxysilane (APTES) were found to exhibit higher fluorescent intensity compared to that of CQDs modified with APTES or PAMAM alone.117 The increased emissions of comodified CQDs were attributed to the presence of multiple nitrous groups and oxygenated groups introduced by PAMAM and APTES. It was suggested that these moieties could conjugate with the functional groups on the surface of CQDs to simultaneously generate a nitrogen and silicon hybrid, leading to the formation of uncommon surface states. Then, the electrons caught by the formed surface states could facilitate a high yield of radiative recombination. In addition, the Hao group successfully achieved CQDs with tunable amphiphilicity by a noncovalent method.118 In their study, highly and positively charged CQDs were synthesized by onepot pyrolysis using citric acid as carbon source and imidazolium-based ionic liquid as capping agent. The amphiphilicity of the as-prepared CQDs could be easily tuned by surface modification through anion exchange, which could result in the phase transfer between oil and water phase. Surface modification has also been shown to be an effective way to change the properties of CQDs especially fluorescence and physical properties.107,108 There are also reported strategies using CQDs in composite and hybrid materials.24,58,60,119−123 The importance of the changes in the properties of the CQDs and GQDs with surface modification is clearly vital for sensing. This is because it does not just offer opportunities to tune the properties in a way that the end user requires but also means that in modifying these carbon nanomaterials to provide them with selectivity for sensing, one needs to be aware of how the surface modification will influence the desired property that will be used for the transduction. So with the term “quantum dots” in the name of these nanomaterials, what are the optical properties they possess that are suitable for sensing? Optical Properties. CQDs and GQDs with different structures usually possess quite similar optical properties in terms of their optical absorption, fluorescence, chemiluminescence (CL), electrochemiluminescence (ECL), phosphorescence, up-conversion photoluminescence (PL), and photoinduced electron transfer (PET) property. Zhu and his coworkers124 discussed four PL mechanisms: a quantum confinement effect, a surface state dependent on hybridization of the carbon backbone and the connected chemical groups, a molecule state determined solely by the fluorescent molecules connected on the surface or interior of the CQDs or GQDs, and cross-link-enhanced emission effect. Their optical properties will be discussed as follows. Due to electronic transitions from π to π* of the phenyl rings and CC bonds or from n to π* of CO bonds or related moieties, CQDs and GQDs exhibit absorption mainly in the near-ultraviolet region with lower absorption intensity in the visible and near-infrared (NIR) region.125 The optical absorptions of CQDs and GQDs have attracted widespread C


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GQD through kidney. Impressively, injections with GQD and graphene oxide revealed that GQDs had no obvious effect on mice. This is distinct from the graphene oxide which showed some toxicity, even causing death in mice due to the aggregation of graphene oxide in vivo. Yuan et al.147 studied the cytotoxicity of GQDs with different functional groups (including NH 2, COOH, and CO-N(CH3 ) 2 ) using a combination of trypan blue assay, flow cytometry analysis, and thiazoyl blue colorimetric (MTT) assay in human A549 lung carcinoma cells and human neural glioma C6 cells. Their results showed that all three types of GQDs showed good biocompatibility. In brief, the modification of GQDs with different functional groups appears to have little influence on their cytotoxicity and biocompatibility. However, recently, the Zhou group145 studied the influence of functional groups on the cytotoxicity of GQDs by comparing the reactive oxygen species (ROS) generation ability of different GQD derivatives as shown in Figure 3A,B. By selectively deactivating the

CQDs have also been shown to exhibit up-conversion photoluminescence.124 For instance, in 2015, Alam and coworkers135 studied the down- and up-conversion photoluminescence properties of CQDs, which were synthesized via a facile one-step green low-temperature carbonized method with cabbage as the natural source of carbon. Up-conversion photoluminescence emission at around 485 nm was observed when CQDs were excited between 600 and 800 nm. The attractiveness of up-conversion in sensing is apparent as it means that near-infrared light can be used to excite CQDs in biological media, which would absorb shorter wavelengths of light. We have already seen CQDs used for bioimaging in MCF-7 cancer cells where they were shown to accumulate in the nucleoli region in live cell imaging experiments.136−138 Fluorescence is not the only form of emission used for sensing with CQDs and GQDs. The phosphorescent properties of CQDs have been widely studied.139−142 Chemiluminescence of CQDs, with a range of chemical initiators (H2O2, K3Fe(CN)6, KMnO4, and NaIO4), has also been reported.143 Similarly, electrochemiluminescence was reported by Su and his co-workers,88 who suggested the electrochemiluminescence mechanism of the CQDs as follows: first, strongly oxidizing SO4•− radicals and CQD•− radicals were produced by electrochemical reduction of S2O82− and CQDs, respectively. The SO4•− radicals reacts with the CQDs•− via an electrontransfer annihilation, producing an excited state (CQDs*) that finally emitted light. In another study, Cai and co-workers143 found that the solution of CQDs has no electrochemiluminescence emission itself; S2O82− produces a very weak electrochemiluminescence signal between −1.2 and 0.5 V. However, the mixture of S2O82− and CQDs has an electrochemiluminescence emission intensity of 75 times with a maximum intensity at −1.2 V compared with S2O82− alone (Figure 2A). The result indicated that the electrochemilumi-

Figure 3. (A) Chemical titration processes for the (i) ketonic carbonyl, (ii) hydroxyl, and (iii) carboxylic groups on GQDs. (B) Different phototoxicities of four kinds of GQDs. Reprinted with permission from ref 145. Copyright 2017 Royal Society of Chemistry.

different groups of GQDs (ketonic carbonyl, carboxylic, or hydroxyl groups), the authors found that ketonic carbonyl groups had an obvious influence on the ROS generation ability of GQDs. More importantly, the removal of the oxygen functional groups on GQDs could enhance the photostability and decrease the cytotoxicity, which afforded molecular level evidence for the guidance of better design of biocompatible GQDs. On the basis of their good biocompatibility and low cytotoxicity, GQDs with a large number of oxygen-containing groups were successfully employed as effective nano-radiosensitizer for radiotherapy by the Ge group.149 They found that the GQDs could obviously enhance the sensitivity of colorectal carcinoma cells toward ionizing irradiation. Further study found that the GQDs in combination with ionizing radiation could apparently suppress cell proliferation, increase apoptosis, and enhance the G2/M stage arrest of cells. It was mainly due to the overproduction of ROS by GQDs in synergy with the ionizing radiation, which could activate the apoptosis-related regulation proteins and lead to tumor cell apoptosis. This study developed a kind of efficient, cheap, and safe nano-radiosensitizer for tumor radiotherapy, which broadened the potential applications of CQDs in vivo.

Figure 2. (A) Electrochemiluminescence−potential curves of (a) CQDs + 20 mM S2O82−, (b) 20 mM S2O82−, and (c) CQDs. (B) Schematic showing the electrochemiluminescence detection of pentachlorophenol with CQDs in S2O82− solution. Reprinted with permission from ref 143. Copyright 2013 Royal Society of Chemistry.

nescence comes from the reaction between carbon QDs and S2O82−. It is thought that the electron-transfer annihilation between CQDs•− and the electrogenerated SO4•− causes ECL as illustrated in Figure 2B. Biocompatibility, Nanotoxicology, and Cytotoxicity. Good biocompatibility is important for a number of biological applications of GQDs, in particular in bioimaging. There have been a few studies on GQDs that demonstrate the low toxicity of these materials in mammalian systems.144−148 Chong et al.144 studied GQDs in in vitro measurements using HeLa cells. Their results revealed the low cytotoxicity of GQDs, which was attributed to their high oxygen content and ultrasmall size. The subsequent in vivo biodistribution assay using mice showed no GQDs accumulation in the main organs and fast clearance of

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SENSING APPLICATIONS The variety of properties discussed above for CQDs and GQDs make them potential for applications in bioimaging and sensing.1,150,151 It is envisaged in particular that CQDs could be employed as fluorescent probes for the detection of various analytes due to their intrinsic fluorescent properties. In general, D


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promise in using GQDs and PET for the sensing of organic species but also shows that selectivity will need to be improved. In contrast to simply using simple CQDs with ill-defined surface states, Qian and co-workers134 did report a graphene oxide and GQDs hybrid sensor for Pb2+ with a well-controlled fluorescence turn-on process based on PET between the graphene oxide and the GQDs. The sensor could detect Pb2+ with a broad linear range of up to 400.0 nM, a fast response time, and low detection limit of 0.6 nM. Furthermore, this sensor could distinguish Pb2+ from other ions with good reproducibility and high sensitivity. Changes in fluorescence of CQDs have also been employed for the detection of a number of inorganic molecules.47 Wang et al. achieved the detection of NO2 gas using quantum dotfunctionalized aerogels as the sensing probes.262 In the system, the porous CQD-functionalized aerogels showed strong fluorescence activities in the solid state. The fluorescence was quenched by NO2 gas, while no quenching was observed when the CQD-functionalized aerogels were exposed to a pure nitrogen atmosphere. In another case, water dispersible CQDs functionalized with thiol groups at their surface were synthesized and applied for arsenite detection in environmental water by Nayak et al.47 They found that the fluorescence intensity of CQDs was enhanced with the addition of arsenite. On the basis of this mechanism, the asprepared CQDs probes were suggested to have selectivity for arsenite. Finally, organic molecules such as many organic thiols,34,55,76,251,263 aniline,132 ascorbic acid,65 glucosamine,264 and diniconazole265 have been successfully detected using CQDs as the sensing probes. The sensitivity of CQDs sensors for organics based on fluorescence quenching would be improved with the CQDs that had higher quantum yields. With this in mind, the MacFarlane group266 synthesized novel N and S co-doped CQDs (N,S-CQDs) using ionic liquids as a single source by a simple ultrasonic method. The results showed that the as-prepared CQDs had long photoluminescence lifetimes that enabled them to be used for sensitive detection of pesticides. Chemiluminescence Chemosensors. Chemiluminescence (CL) chemosensor, with the advantages of simple instrumentation, high sensitivity, wide linear range, and no interference from background scattering light, has seen its rise in popularity in sensing.267 CQDs-based sensor where the transduction was achieved using chemiluminescence for the detection of Cu2+ was reported by Amjadi et al.233 The CQDs were doped with sulfur and nitrogen (S,N-CQDs). The study found that, among a range of oxidants, KMnO4 generated a stronger chemiluminescence than the other oxidants. The influence of metal ions on the KMnO4−S,N-CQDs chemiluminescent system was then investigated, and it showed that the chemiluminescence intensity diminished upon exposure to Cu2+. The sensor was applied to the detection of Cu2+ in diluted human plasma and water samples. Chemiluminescence in combination with CQDs has also been used for sensing of organic species. An example is the monitoring of indomethacin using S,N-CQDs which were prepared by a hydrothermal method.58 In this study, the best chemilumnescent system for determination of indomethacin was investigated by reacting S,N-CQDs with a number of oxidants including K3Fe(CN)6, KMnO4, H2O2, and NaIO4 (Figure 5). The results showed that the S,N-CQD-KMnO4 reaction exhibited the highest sensi-

the sensors prepared thus far with CQDs and GQDs can be divided into luminescence sensors, electrochemical sensors, optical biosensors, electrochemical biosensors, and photoelectrochemical (PEC) biosensors. The features of different sensors using various QDs as the sensing probes are summarized in Table S1 (Supporting Information). Photoluminescence Chemosensors. CQDs and GQDs have been extensively explored as fluorescent probes for detection of pesticide and heavy metal ions.131,152−154 To date, metal ions have become the most widely explored analytes using CQDs as the sensing probes.125,155 Therein, sensors for a series of metal ions including Fe 3 + , 5 9 , 8 7 , 1 5 6 − 1 9 6 Hg2+,62,64,73,185,197−230 Zn2+ ,231,232 Cd2+, Cu2+ ,86,233−250 Au3+,251 Co2+,252 Ni2+,253 Pd2+,254,255 Pb2+,134 Mn2+,190 Bi3+,59 Al3+,128,256 K+,230,257 Be2+, Sn2+,258 Cr6+,259 and Ag+260 have been developed. As can be seen from Table S1, many of the studies are conceptually similar in that the metal ion of interest quenches the luminescence of the CQDs. In most cases the innovation comes from the material from which the CQDs are sourced or the method of generating the CQDs. Typically, the studies do not involve any specific surface modification, so how selectivity is derived appears to be an open question at present. An example is by Zhang and Chen,62 who found that Hg2+ quenched the fluorescence of N-doped CQDs which was attributed to the change of the surface states of N-CQDs influenced by the Hg2+. The data presented showed that the sensor was selective for Hg2+ in the existence of other metal ions in high concentrations. Similarly, a recent study with 4.0 nm CQDs explored the effect of a series of metal ions including Fe3+, Fe2+, Co2+, Ni2+, Ag+, Cd2+, Cu2+, Pb2+, Zn2+, Hg2+, Mg2+, Mn2+, and Al3+ on the PL intensity of the CQDs (Figure 4A).87 It was observed that the fluorescence

Figure 4. (A) Plot of the fractional change in PL intensity of the bright yellow CQDs after addition of different metal ions in acidic conditions. (B) PL intensity of bright yellow CQDs in the presence of Fe3+ ions at different concentrations from 0 to 200 μM. Reprinted with permission from ref 87. Copyright 2016 Royal Society of Chemistry.

intensities of CQDs were only significantly quenched in the presence of Fe3+ solution. As such, the CQDs were used to analyze Fe3+ sensing in tap water with results with an unknown sample comparable to that determined using atomic adsorption spectroscopy (Figure 4B). Again the question of how the selectivity is achieved remains open, and what is it about the of N-CQDs that makes them selective for Hg+ while CQDs are selective for Fe3+? To confuse the situation there is also one report by Raju and co-workers261 that reports the fluorescence of CQDs prepared by employing camphor as the precursors could be quenched by Fe2+ but not the more commonly reported Fe3+. This highlights the need to understand the mechanism of the transduction and how different preparations may give selectivity for one metal over another. These observations suggest that there is considerable E


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Figure 6. (A) Amperometric response of NGQDs@NC@Pd/GCE with successive step changes of H2O2 concentration. (B) Influence of (a) O2, (b) dopamine, (c) uric acid, and (d) ascorbic acid at 20 μM, and (e) O2, (f) dopamine, (g) uric acid, and (h) ascorbic acid at 50 μM on the amperometric response of equal amount of H2O2 in 0.1 M PBS. Reprinted with permission from ref 272. Copyright 2016 American Chemical Society.

Figure 5. Optimization of chemiluminescent reaction conditions. Effect of (A) type of acid, (B) amount of S,N-CQD, and (C) concentration of KMnO4. Reprinted with permission from ref 58. Copyright 2017 John Wiley and Sons.

Electrochemiluminescence Chemosensors. Because of the high sensitivity, low background, and good controllability, electrochemiluminescence has gradually become an important and powerful analytical tool in many fields, such as pharmaceutical analysis, environmental pollution determination, and immunoassay.268 By integration with other materials, CQDs-based composites have been reported for the detection of microRNA,46 sophoridine,60,74 chlorinated phenols,269 chlorpromazine,270 and pentachlorophenol143 using electrochemiluminescence. For example, CQDs have been used in the sensing of sophoridine by the Liu group,270 who fabricated an electrochemiluminescence sensing platform using a Ru(bpy)32+/CQDs-poly(vinyl alcohol) composite film as modifier. They obtained a detection limit (5.0 × 10−11 M). The roles of CQDs were to improve the conductivity, increase the electron transmission rate, and act as electron tunnel to allow more Ru(bpy)32+ involved in the reaction system. This group have also successfully developed a Ru(bpy)32+/CQDs/gelatin composite film as a electrochemiluminescence platform for sensing of chlorpromazine with the detection limit as low as 8.0 × 10−11 M.270 Electrochemical Chemosensors. Although the name quantum dot suggests the use of these materials in optical sensing, CQDs and GQDs have also been used in electrochemical sensors.271−276 For example, a hydrogen peroxide sensor was developed by Xi et al.272 using PdNPs-functionalized N-GQDs@N-doped carbon hollow nanospheres. In this system, the authors suggested the N-GQDs and PdNPs were applied as both signal-amplifying probes and effective electrocatalyst in electrochemical sensing. The sensor gave an insignificant response to other electroactive species including cathodic interference from oxygen (O2) and anodic interferences by dopamine, ascorbic acid, and uric acid (Figure 6), avoided interference signals, and showed satisfied performance such as high sensitivity, low detection limit, and short response time in real-time detection of hydrogen peroxide secreted from cancer cells. There are also a number of electrochemical studies that have explored the use of GQDs for the sensing of organic molecules.269,271,277−280 In general, GQDs act as multivalent redox species under differential pulse voltammetry and CV measurements in the electrochemical sensors.271−273 For example, the Liu group281 detected glucose using a uniform three-dimensional graphene nanodots-encaged porous gold electrode with a broad linear range from 0.05 to 100 mM and a detection limit of 30 μM. Compared with GQDs, CQDs have been more widely applied in the electrochemical analysis of organic molecules.61,67,282,283 In 2016, by modifying a GCE with CQDs, Nguyen et al.271 successfully designed a

electrochemical sensing system for the detection of etoposide by differential pulse voltammetry. The CQDs applied in their study were modified by carboxyl using poly(vinylpyrrolidone), to increase the sensitivity, photoreversibility, and stability of the CQDs. Biosensors. In many of the examples of chemical sensors listed above the as-prepared CQDs or GQDs have been employed without any further modification to allow detection of a variety of species.284 Many of these studies throw up questions pertaining to how selectivity is achieved. When employing CQDs and GQDs for biosensing those queries are obviated as such systems require modification with enzymes, DNA, RNA antibodies, antigens, microorganisms, cells, or tissues.281,285−295 There are a limited number of biosensors reported so far that employ CQDs or GQDs, but the number of contributions is expected to expand rapidly.296−303 Optical Biosensors. GQDs in combination with AuNPs were employed by Shi et al.123 to develop what the authors refer to as a fluorescence resonance energy transfer (FRET) biosensor for detecting a gene sequence specific to Staphylococcus aureus. The GQDs were modified with aminated ssDNA by activating surface carboxylic moieties on the GQDs with classical carbodiimide chemistry. The AuNPs were also modified with thiolated DNA probes. If the sequence from Staphylococcus aureus was present, it would bridge between the probe sequences on the GQDs and that on the AuNPs to bring the two sufficiently close together for energy transfer to occur. The fluorescence quenching efficiency of the fluorescence signals after addition of target oligo (100 nM) could reach about 87%. This study starts to show the power of well-defined modification of GQDs and CQDs in biosensing. Similarly, well-defined modification of GQDs was employed in a biosensor for detection where the GQDs were modified with a pyrene-functionalized molecular beacons (py-MBs)-based fluorescence biosensor.304 In this study, molecular beacons were first labeled with pyrene and fluorescent dye, and then the py-MBs were strongly immobilized on the GQDs surface via π−π interactions. The introduction of pyrene to molecular beacons allowed FRET between fluorescent dyes and GQDs. The resulting fluorescent intensity change generated a signal for target miRNA detection with satisfied discrimination abilities in a wide detection range from 0.1 to 200 nM, providing a new method for miRNA detection based on GQDs. CQDs and GQDs can be utilized in the fabrication of immunosensor.56,60,305−307 For example Zhang et al.60 prepared an immunosensor for 8-hydroxy-2′-deoxyguanosine F


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biotin labeled aptamer, this biosensor exhibited a linear range from 0.1 to 100 nM, low detection limit of 0.031 nM, and good selectivity after introduction of 6-benzylaminopurine, gibberellin, indole-3-acetic acid, indole-3-propionic acid, and indole3- butyric acid. In another example, the first self-powdered biosensor based on N-CQDs/TiO2 was constructed for detection of chlorpyrifos by Gao et al.292 It was found that the photocurrent response of the N-CQD/TiO2/ITO electrode exhibited about 42 times than that of the TiO2/ ITO under visible light.

(8-OHdG) where CQD coated Au/SiO2 core−shell NPs were immobilized onto a platinum electrode. The carboxylic acids groups on the CQDs were activated using carbodiimide chemistry to all anti-8-OHdG antibodies to be immobilized that were selective for 8-OHdG. The rest of the surface was passivated with bovine serum albumin to limit effects from nonspecific adsorption. Transduction in the immunosensor was achieved using electrochemiluminescence. The binding of antigen limited access of reagents to the underlying electrode and hence a decrease in electrochemiluminescence was observed with higher amounts of 8-OHdG. The novelty of this electrochemiluminescence immunosensor was based around the use of CQDs rather than the transduction principle. Similarly, a sandwiched immunosensor for carcinoembryonic antigen detection based on CQDs that uses electrochemiluminescence for transduction was also reported by the Wang group.56 In this device CQDs, PEI-graphene oxide, AgNPs, and AuNPs were combined to give an electrochemiluminescence immunosensor with good performance (a wide detection range of 5 pg mL−1 to 500 ng mL−1 and a low detection limit of 1.67 pg mL−1), for carcinoembryonic antigen. Electrochemical Biosensors. GQDs have been used as a platform for enzyme immobilization in the construction of biosensors.308 Recently, a biosensor used for epinephrine was fabricated by Baluta et al. on the basis of immobilizing laccase and GQDs on glassy carbon electrodes.309 The idea was to immobilize a monolayer of GQDs followed by immobilizing laccase. The resultant biosensor had a wide linear concentration range for epinephrine with a low detection (1−120 μM) and a detection limit of 83 nM. What the role is of the GQDs in this performance is however not clear as in many ways this study is reminiscent of other studies where enzymes’ electrodes are formed where the base electrode is modified by carbon nanomaterials.59 GQDs have also begun to be used in biosensors that employ or detect nucleic acids. Li et al.310 constructed GQDs-based electrochemical DNA sensor where pyrolytic graphite electrode was modified with GQDs. The electrode was applied to the sample with probe single-stranded DNA (ssDNA). If there is the complementary target sequence present, it will hybridize with the probe ssDNA to form a duplex. Since only ssDNA is adsorbed onto the GQDs, any DNA duplexes that form reduce the amount of ssDNA that adsorbs onto the electrode. Transduction is achieved using the redox species ferricyanide. Since the ssDNA electrostatically repels the ferricyanide, the more target DNA present, the less ssDNA that adsorbs and hence the higher the current observed. As a result, a GQDsbased biosensor was successfully established, although it is clear that the amounts of probe ssDNA must be carefully controlled and in excess of the target. The selectivity of the sensor in complex media, or when there are multiple other sequences of DNA present, is expected to be challenging. Photoelectrochemical Biosensors. The optical properties of CQDs and GQDs do provide opportunities for these nanomaterials to be used in photelectrochemical biosensor where electron transfer occurs at the electrode under photoirradiation only.311 Yin’s group 311 developed an aptamer-based photoelectrochemical biosensor for zeatin detection based on the GQDs/graphite-like C3N4(g-C3N4) nanocomposite system. As photoactivity improvement reagent, the role of GQDs was to improve photoelectric conversion efficiency of g-C3N4. In cooperation with AuNPs and DNA

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CONCLUSIONS AND PERSPECTIVES In this short review, we have highlighted the recent progress in CQDs and GQDs in terms of their synthesis, properties, and sensing applications. In the past 5 years, there have emerged many new routes to fabricate both pure or doped CQDs and GQDs with different morphologies and properties, which accelerate the constant optimization of analytical performance, bringing them closer to real world applications. Although various facile synthetic methods have been developed for CQDs fabrication, research into GQDs is still at a more nascent stage. For instance, the well-defined and atom-precise structures have not been reported yet, which restrict the deeper studies on the relationships between structure and properties, precise control of properties, and exploration of new methods and applications. Furthermore, a greater understanding of the optical properties of CQDs is required, which requires more theoretical studies and perhaps better defined materials. When it comes to sensing, we feel that the field is even less well developed. As Table S1 shows, there has been a large number of studies into the sensing of metal ions. Most of these involve the quenching of the fluorescence of the CQDs and GQDs, but how this quenching operates and how selectivity is achieved is far from well-understood. This is apparent when CQDs prepared from the same carbon source can be used for many different ions. For example, CQDs synthesized using citric have been used to develop sensors for Fe3+, Cu2+, Hg2+, Zn2+, and Pd2+ with some claims of selectivity particularly for Fe3+ over other cations162 but Hg2+ was not tested for while other studies show good selectivity for Hg2+ with no response for Fe3+ or a range of other cations.266 So clearly there is much to understand with regard to selectivity. Doping and surface states seem to be important, but it seems we are a long way from understanding such a complex system and how CQDs from a similar carbon source, made in a similar way, give such a different performance. This is a major opportunity for research in this field as the initial results are rather promising. It is also incredibly important from the perspective of a commercial device because if minor changes in materials or procedures alter the properties of the final sensor, a device cannot be reliably manufactured. With organic species and inorganic species, similar questions arise about transduction mechanisms and how selectivity is achieved also arises. We feel the reporting of further examples of similar sensing concepts’ materials from different sources is now less important. The direction we feel the field needs to go is to explore these twin issues of the transduction mechanism and achieving selectivity. The issue of selectivity is beginning to be addressed with surface modification and the development of biosensors based on CQDs and GQDs. We discuss a number of the biosensing studies where the CQDs or GQDs are platforms for immobilization in electrochemical biosensors, some optical biosensors, and PEC biosensors. G


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method to measure the absorption of light radiation of specific gaseous atoms; biocompatibility, ability of living tissue to react with inactive materials and generally refers to the compatibility between materials and host; ground state, stationary state in which the electrons move in the nearest orbit to the nucleus when the atom is in the lowest energy level; MCF-7 cancer cells, human breast cancer cells, a diseased cell; MTT colorimetric, method for detecting cell survival and growth

We feel that demonstrating that immobilization of biomolecules can facilitate selectivity is important. For the electrochemical systems we feel there has yet to be concepts demonstrated that have not been achieved with other carbon nanomaterials. We feel it is imperative if CQDs and GQDs are to develop into important materials for sensing that they are used to give sensors that have properties that could not be achieved with other materials. At present this seems much more likely to be achieved if the optical properties of these carbon nanomaterials are exploited as this is what differentiates CQDs and GQDs from many other carbon nanomaterials. To summarize, it is very early days for carbon and graphene quantum dots and their potential is incredibly exciting. We feel the research field needs to now move beyond the phase of developing synthetic methods and demonstrating sensing concepts alone. These foci will continue as important areas of research into CQDs and GQDs, but we feel research into sensing with these materials also needs to move toward a better understanding of how the materials’ synthesis influences the optical properties of these materials, understanding the mechanisms of transduction and the reported selectivity, developing methods of surface functionalization to provide greater selectivity, and exploring sensing concepts that could not be achieved, or as effectively achieved, with other materials.

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ABBREVIATIONS 0D,zero-dimensional; 8-OHdG,8-hydroxy-2′-deoxyguanosine; APTES,(3-aminopropyl) triethoxysilane; B-CQDs,borondoped CQDs; CDs,carbon dots; CL,chemiluminescence; CQDs,carbon quantum dots; CV,cyclic voltammetry; ECL,electrochemiluminescence; FRET,fluorescence resonance energy transfer; g-C3N4,graphite-like C3N4; GQDs,graphene quantum dots; mesoSiO2/PAN,mesoporous silica/polyacrylonitrile; MTT,thiazoyl blue colorimetric; N-CQDs,nitrogendoped CQDs; N-GQDs,N-doped GQDs; NIR,near-infrared; N,S-CQDs,sulfur co-doped CQDs; PAMAM,polyamidoamine; PEC,photoelectrochemical; PEI,polyethylenimine; PET,photoinduced electron transfer; PL,photoluminescence; ROS,reactive oxygen species; ssDNA,single-stranded DNA

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssensors.9b00514. Table S1 summarizing CQD- or GQD-based sensors (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.Q.L.). *E-mail: [email protected] (J.J.G.). ORCID

J. Justin Gooding: 0000-0002-5398-0597 Jingquan Liu: 0000-0001-6178-8661 Author Contributions §

M.L. and T.C. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China (Grant No. 51173087) and the Qingdao Innovation Leading Expert Program and Taishan Scholars Program. JJG acknowledges the generous financial support from the Australian Research Council Centre of Excellence in Convergent Bio-Nano Science and Technology (CE140100036) and an ARC Australian Laureate Fellowship (FL150100060).

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VOCABULARY atomic adsorption spectroscopy, analytical method to quantitatively measure the content of the measured elements based on the absorption intensity of the outer electrons of the ground state atoms in the gaseous state to the corresponding atomic resonance radiation in the ultraviolet and visible light, a H


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Review

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ACS Sensors

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Review

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