Electrochemical Methods to Study Photoluminescent Carbon

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Electrochemical Methods to Study Photoluminescent Carbon Nanodots: Preparation, Photoluminescence Mechanism and Sensing Bao-Ping Qi, Lei Bao, Zhi-Ling Zhang, and Dai-Wen Pang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11551 • Publication Date (Web): 24 Feb 2016 Downloaded from http://pubs.acs.org on February 26, 2016

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ACS Applied Materials & Interfaces

Electrochemical Methods to Study Photoluminescent Carbon Nanodots: Preparation, Photoluminescence Mechanism and Sensing Bao-Ping Qi,‡ Lei Bao,‡ Zhi-Ling Zhang and Dai-Wen Pang* Key Laboratory of Analytical Chemistry for Biology and Medicine (Ministry of Education), College of Chemistry and Molecular Sciences, State Key Laboratory of Virology, The Institute for Advanced Studies, and Wuhan Institute of Biotechnology, Wuhan University, Wuhan, 430072, P. R. China. KEYWORDS: carbon nanodot, electrochemical, preparation, photoluminescence mechanism, sensing

ABSTRACT: With unique and tunable photoluminescence (PL) properties, carbon nanodots (CNDs) as a new class of optical tags have been extensively studied. Due to their merits of controllability and sensitivity to the surface of nanomaterials, electrochemical methods have already been adopted to study the intrinsic electronic structures of CNDs. In this review, we mainly deal with the electrochemical researches of CNDs, including preparation, PL mechanism and biosensing.

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1. INTRODUCTION Carbon nanodots (CNDs) are emerging as superior and universal fluorophores due to many merits, including excellent photostability, small size, facile functionalization, low toxicity, favourable biocompatibility and inert chemical properties.1-7 As new carbonaceous photoluminescent nanodots, CNDs provide additional opportunities for bioimaging5-7 and related optical sensing.8-11 Since their initial discovery by arc-discharge methods,12 CNDs have been studied and fabricated by numerous research groups hoping to achieve better synthetic routes, glean a better understanding of the origins of their photophysical behaviors, and develop novel applications for the emergent nanomaterials.1 As a green means, electrochemical methods are powerful for the controllable preparation of CNDs and for the investigation of the structures of CNDs. On the one hand, electrochemical methods have provided a facile, inexpensive and moderate route for the fabrication of photoluminescent CNDs.13-16 By fine-tuning the applied potential, the oxidizing or reducing power can be continuously varied.17 Both the sizes and the surface oxidation degrees of CNDs, which have considerable impact on the photoluminescence (PL) properties of CNDs, could be tuned conveniently.13 On the other hand, electrochemical methods can also be applied to investigate the electronic structure and band gap of CNDs. They can be used to gain the information on the electronic structure, the stability of charged CNDs and the nanostructure stability upon charge transfer in CNDs. Therefore, these methods are crucial for studies on the origin of PL. For example, the electrochemiluminescence (ECL, also called electrogenerated chemiluminescence) technique is demonstrated to be more sensitive than PL related to surface states of nanoparticles,18-20 which makes it a powerful tool to study surface states of CNDs.21 Especially, the current signal and the light signal can be simultaneously obtained in the process


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of ECL, facilitating the investigation of PL mechanism of CNDs. The clear PL mechanism and explicit structures of CNDs would have an important significance for tuning the PL properties of CNDs. In brief, the electrochemical methods do have advantages in the fabrication and exploring the intrinsic properties of CNDs. The electrochemical studies of CNDs have been mentioned in the previous reviews.1-4 However, to our best knowledge, rare comprehensive review has appeared to summarize the achievements of CNDs-related electrochemistry. Herein, the latest works of the electrochemistry of CNDs have been reviewed, including the preparation, structure analysis, PL mechanism and sensing by electrochemistry. We also share our viewpoints on some critical issues in the study of CNDs. 2. PROPERTIES 2.1 Structures and compositions Typically, CNDs are less than 10 nm in size, composed of a carbon backbone with sp2 carbon and sp3 carbon. They are also abundant in oxygen-containing groups at the edge or base plane. CNDs usually include two 0D cousins, which are carbon dots (CDs) and graphene quantum dots (GQDs). CDs have a graphite and/or amorphous carbon framework and a surface with oxygencontaining groups, polymers, or other species (Figure 1a). CDs can be divided into carbon nanoparticles without crystal lattice or with partial lattice. The GQDs possess single or a few layers of graphene, with oxygenous functional groups at their edges. They are usually in the discal shape with lateral dimension larger than height (Figures 1b and 1c). However, it is still unclear whether the UV-visible absorption and PL spectra of GQDs are ascribed to the quantum confinement effect or not (Figure 1d), which are obviously different from those of the semiconductor QDs (Figure 1e).22-26 The knowledge of semiconductor QDs cannot be simply


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applied to the system of GQDs and carbon nanoparticles. Typically, the carbonaceous photoluminescent nanodots CDs and GQDs display many parallel performances, such as excitation-dependent PL behaviors (Figure 1f), resistance to photobleaching, low-toxicity, good stability, etc. Based on these similar structures and properties, herein both CDs and GQDs are categorized as CNDs. 2.2 Optical properties Typically, CNDs show π-π* transition of C=C bonds and n-π* transition of C=O bonds with strong optical absorption in the UV region (200 - 320 nm), with a tail extending into the visible range. Two shoulders at ca. 220 nm and 300 nm are ascribed to π-π* transition of aromatic rings and n-π* transition of C=O bonds, respectively (Figure 1d). The most fascinating feature of CNDs is their PL properties, exhibiting non-blinking PL, excellent photostability, excitation dependence and pH dependence. One of the most notable issues is whether the emerging carbonaceous photoluminescent CNDs are able to substitute commonly used semiconductor QDs containing toxic heavy metals and thus having unknown environmental and potential biological hazards. Those aspects have been well summarized in the previous reviews.1-4 As the work goes on, PL colour-tunable CNDs, like semiconductor QDs, have been already prepared.29,30 Due to their facile functionalization, CNDs have been widely studied in cell imaging, in vivo and dynamic tracking.5-7,11,31-33 At the early stage, most of CNDs are blue and green in PL emissions (between 420 nm and 520 nm). At present, the PL emissions of CNDs can cover the entire visible light region (between 380 nm and 700 nm).30 In most cases, the quantum yields of CNDs are less than 10%, while as high as ca. 80% quantum yield has been also reported.11 In addition, some CNDs were also characteristic of upconversion PL.34-39 Although CNDs have been studied for twelve years,12 the knowledge of the origins of their PL is still a matter of current debate


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requiring further clarification. The PL emissions of CNDs have been suggested to originate from surface states,13,40,41 quantum effect,42-44 conjugated structures,45-47 edge defects48-50 or free zigzag sites.51-53 Compared with semiconductor QDs,54,55 the PL spectra of CNDs are spectrally broad. This might be attributed to optical selection of different surface states on CNDs or different-sized nanodots or another unrevealed mechanism.30 To resolve these problems, it is particularly important to develop facile preparation approaches and efficient separation methods to obtain CNDs with unity of surface and uniform size. 3. CARBON NANODOTS PREPARED BY ELECTROCHEMICAL METHODS When purifying single-walled carbon nanotubes (SWCNDs) through the electrophoretic method,12 Xu et al. accidentally discovered photoluminescent CNDs. Thereafter, a variety of methods have been developed to prepare photoluminescent CNDs, including wet oxidation,56 laser ablation,57 pyrolysis,58 microwave-assisted fabrication,59 and electrochemical methods etc. Among the available methods, electrochemical approaches could provide a speedy and moderate route for the fabrication of photoluminescent CNDs. In particular, by controlling the applied potential and current density, the sizes, compositions and PL properties of CNDs could be easily controlled. The comprehensive reviews on those synthesis methods have been listed in the references [1-4]. For more information, please refer to these review articles. 3.1. Electrochemical etching As a top-down approach, the electrochemical etching method, where the CNDs are formed or “peeled off” from a larger carbon structure,1 is frequently used in the preparation of CNDs.1315,60-62

In the process of electrochemical etching, various conductive carbonaceous materials

(carbon fiber, and graphite rod, etc), characteristic of abundant resources in nature and low cost,


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are used as the sacrifice electrodes. The sizes and compositions of CNDs could be conveniently adjusted by changing a number of reaction parameters, including pH, concentration, composition of the electrolyte solution and mode of electrolysis, such as potentiostatic, galvanostatic, and cyclic voltammetry (CV), etc. 3.1.1. Formation mechanism of CNDs in the electrochemical process For the first time, Zhou et al. adopted CV to fabricate blue CNDs in the organic solution.15 Especially, the electrolyte tetrabutylammonium perchlorate (TBAP) was deemed to act as an intercalator to break the multi-walled carbon nanotubes (MWCNTs) electrode and facilitate the generation of photoluminescent CNDs. Electrochemical intercalation of some organic molecules into MWCNTs and graphite has been reported in the literature,63 but this is the first time that the breaking of MWCNTs to generate photoluminescent CNDs by cycling the applied potentials was observed. Zhao et al. prepared photoluminescent CNDs through the electro-oxidation of graphite rod in 0.1 mol/L NaH2PO4 aqueous solution.14 It is the first time that CNDs were obtained by electrochemical approach in aqueous solution. Two fractions of CNDs with sizes of 1.9 ± 0.3 nm and 3.2 ± 0.5 nm emitted blue- and yellow-coloured PL, respectively. Zheng et al. also adopted this method to prepare photoluminescent CNDs with a size of ~ 2 nm in pH 7.0 phosphate buffer solution (PBS) through CV between -3.0 and 3.0 V.60 Lu et al. developed a unified one-pot electrochemical method to prepare photoluminescent CNDs from the exfoliation graphite electrode in the mixture electrolyte of water with ionic liquid.64 It has been also suggested that the intercalation of BF4- anion from ionic liquid into the graphite plane plays an important role in the formation of CNDs. The graphene sheets functionalized with BF4- anion were testified by the X-ray photoelectron spectroscopy (XPS) survey scan. By using potassium-sodium tartrate (KNaC4H4O6·4H2O) as both an intercalator into graphite and a solvent in the solvothermal


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reaction (Figure 2), Song et al. developed an easy and mass-producible route to obtain highquality CNDs by controlled oxidation.49 In addition to the intercalation, another important factor in the formation of CNDs by electrochemical methods is deemed to be the hydroxyl radicals generated by the anodic oxidation of water. The hydroxyl and oxygen radicals derived from the anodic oxidation of water play the role of an electrochemical “scissors” in the oxidative cleavage reaction. Typically, by tuning the water content and type of counterions in the ionic liquid, the PL as well as the distribution of carbon nanomaterials can be tuned.64 The PL of as-prepared CNDs could be tuned from the ultraviolet to visible light region by increasing the water content of the electrolyte solution. Zhou et al. reported that the graphene oxide (GO) sheets could be converted into CNDs step by step through the photo Fenton reaction,65 demonstrating that hydroxyl radicals from the Fenton reagent can break C-C bonds. Alkali condition, where hydroxyl radicals is advantageously generated in the electrochemical oxidation process, is in favour of electrochemical preparation of CNDs. Using graphite rods as both anode and cathode, and NaOH/EtOH as electrolyte, Li et al. synthesized CNDs at a current density of 10-200 mA cm-2.66 In a series of control experiments using acids (e.g. H2SO4/EtOH) as electrolyte, no CNDs were formed, indicating that alkaline is the key factor and OH- group is essential for the formation of CNDs in the electrochemical oxidation process. As far as the mechanism of the exfoliation by electrochemical etching, it is still an open question. It may be due to a complex interplay of radicals from the anodic oxidation and the ionic intercalation from the electrolyte. 3.1.2. The properties of CNDs tuned by electrochemical methods


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In principle, kinetic control can be carried out by controlling the current passing through the cell, while thermodynamic control can be performed by choosing the applied potential. By fine-tuning the applied potential, the oxidizing or reducing power can be continuously varied and suitably selected.17 Such a method is moderate and does not require expensive chemicals. Bao et al. proposed a new strategy to controllably prepare photoluminescent CNDs by electrochemical etching of carbon fibers.13 Monodisperse photoluminescent CNDs with different sizes can be obtained controllably by only adjusting the applied potentials, without further separations or surface passivation. The higher the applied potential, the smaller the resulting CNDs. At an applied potential, once the CNDs were exfoliated from the carbon fibers into the solution, the size of the as-prepared CNDs will not change further. Moreover, the surface oxidation degree of CNDs could be tuned through the change of electro-oxidation conditions, which provides a chance to validate the effect of the surface oxidation degree on the PL properties of CNDs. This work has clearly demonstrated that both the sizes and components of CNDs can be effectively modulated by electrochemical methods. Additionally, the surface oxidation degree of CNDs could also be controllably adjusted by changing the compositions of the carbon paste electrodes (CPEs).62 Two home-made CPEs consisting of 64% and 73% (mass fraction) conductive carbon black were employed by Long et al. to prepare photoluminescent CNDs in 0.1 mol/L NaH2PO4 aqueous solution. It was revealed that a bigger oxidation current passed through the CPE containing less compact carbon black particles. That is, at the same oxidation potential, the surface oxidation degree of CNDs@64% was higher than that of CNDs@73%. Consequently, the shifting and non-shifting PLs at different excitation wavelengths were observed on CNDs@64% and CNDs@73%, respectively.


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Shinde et al. reported an electrochemical transformation of MWCNTs into CNDs by a two-step process in propylene carbonate-containing LiClO4 as supporting electrolyte solution.67 More specifically, photoluminescent CNDs with a uniform size of 3, 5, and 8.2 ( ± 0.3) nm in diameter were prepared by the sustained oxidation of MWCNTs at 1 V followed by reduction at - 1 V for 2 h. The PL of these CNDs can be tailored by size variation through a systematic change of key process parameters, such as diameter of carbon nanotube, electric field, concentration of supporting electrolyte, and temperature. 3.2. Bottom-up Electrochemical synthesis An electrochemical synthesis is achieved by passing an electric current between two or more electrodes separated by an electrolyte. By definition, the synthesis takes place close to the electrode within the electric double layer, which has a very high potential gradient of 105 V cm-1. Under these conditions, the products which are hardly obtained at room temperature by a wet chemical synthesis can be synthesized.17 The electrochemical synthesis system consists of inert electrodes and electrochemically active molecules in the electrolyte. However, the bottom-up approaches where CNDs are prepared from molecular precursors have been rarely reported.68,69 The formation mechanism of CNDs by electrochemical synthesis is speculated to include the following steps: decomposition of molecular precursors, polymerization of carbon substances, growth, and the oxidation of CNDs.68 Li et al. found that the amount of photoluminescent carbon product was larger than the loss amount of graphite rod after electrochemical treatment.68 They have testified that the extra carbon products came from ethanol by a one-step electrochemical treatment of ethanol under sodium hydroxide assistance. Deng et al. have found this method possessed the universality for low-molecular-weight alcohols and the sizes of the as-prepared CNDs could also be adjusted by the applied potential.69 With increasing applied potential, the


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size of the CNDs increased, which was contrary to the electrochemical etching of carbon fibers.12 The higher the applied potential, the higher amount of oxidized alcohol molecules and free radicals that might be produced to undergo crosslinking and dehydration to form larger CNDs. But in the top-down approaches, the increased number of free radicals would be in favour of the oxidative cleavage reaction, resulting in the smaller sizes of CNDs.64 4.

ELECTROCHEMICAL

METHODS

TO

STUDY

INTRINSIC

ELECTRONIC

STRUCTRUES AND PHOTOLUMINESCENCE MECHANISM OF CNDS 4.1. Voltammetry Both the abundant active sites at the edge of CNDs and large specific surface area of the base plane are in favour of the electron transfer of CNDs. Tian et al. discovered that CNDs prepared by refluxing the natural gas soot in nitric acid exhibited electrochemical activities (Figure 3a).70 In electrochemical measurements, two pairs of well-defined cyclic voltammetric waves were observed. The voltammetric behaviours of CNDs in aqueous media were similar to phenanthrenequinone, which underwent two-proton and two-electron redox reactions, leading to a cathodic shift of 59 mV with the increase of one pH unit at room temperature. Thus, it was speculated that the peripheral functional moieties analogous to phenanthrenequinone derivatives existed at the edge of CNDs. Qi et al. investigated by CV the behaviours of CNDs prepared by the oxidation of graphene in piranha solution.45 A pair of symmetrical redox peaks with a formal potential located at 0.29 V were detected (Figure 3b), which could be attributed to the orthoquinone structures at the edge of CNDs by contrast experiments. Based on the efficient acidcatalyzed condensation reaction of ortho-quinone (structures) at the edge of CNDs with 1,2diamine compounds, a facile route was developed to modulate PL emissions of CNDs. The high


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specificity of this approach makes the reaction proceed smoothly under mild conditions to produce stable aromatic pyrazine structures, which can provide the chance to investigate the effect of the expanded conjugate structures on the PL properties of CNDs. With the conjugate structures being expanded with 1,2-diamines, the PL emissions of CNDs would gradually red shift. Thus, changing the conjugate structures in CNDs can tune the band gap of CNDs. DFTbased calculation can be combined to understand the mechanism. The differential pulse voltammetry (DPV) technique can minimize the background charging current through the current difference before and after the pulse superimposed on a staircase potential.71 Zhou et al. elucidated the electronic structures of blue photoluminescent CNDs by DPV (Figure 3c).21 Discrete reduction steps were exhibited in the negative potential region, which corresponded to the injection of the single electron into CNDs. An irreversible oxidation peak at 1.5 V might reflect the position of the highest occupied molecular orbital (HOMO). The band gap of the CNDs could be determined to be larger than 2.95 eV from the PL peak at 420 nm. Therefore, the position of the lowest unoccupied molecular orbital (LUMO) must be more negative than - 1.4 V. Thus, the reduction peaks ( ~ 0.0 and - 1.2 V) more positive than - 1.4 V ought to be associated with surface states on CNDs. The ultra-microelectrode technique with low capacitance has the fine traits of a rapid response rate and high signal-to-noise ratio. The steady state of diffusion balance could be established in a very short time at such a small surface of electrodes. Thus, ultra-microelectrodes are extensively employed to study fast charge transfer or chemical reaction.72,73 With a Pt ultra-microelectrode, Shine et al. showed that CNDs with the sizes of 2.2 ± 0.3, 2.6 ± 0.2, and 3 ± 0.3 nm could act as multivalent redox species (Figure 3d).28 A typical DPV response for CNDs displayed the evenly spaced peaks, which were characteristic of charge injection into the carbon core. As the size of


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CNDs decreased, the average distance (∆E) between the spaced peaks of the DPV gradually increased, suggesting that ∆E was inversely proportional to the geometrical capacitance. These regularly spaced peaks were sharp, reversible, highly reproducible, and comparable to similar single-electron transfer processes of Si71 and CdTe74 QDs. This was the first report of such a remarkable resolution of individual redox peaks for CNDs at 273 K, which undoubtedly confirmed that CNDs were indeed multivalent species. It presented exciting opportunities for CNDs in a variety of applications including single-electron transistors, molecular switches, and resonant tunneling diodes. Besides the redox properties and charge transfer behaviours revealed by voltammetry, the electro-catalysis of CNDs has also been noticed. Such properties of CNDs are considered to be complementary for understanding the electronic structures of CNDs. Li et al. developed a solution chemistry approach to synthesize N-doped CNDs (N-CNDs).43 Unlike their N-free counterparts, N-CNDs could electrochemically catalyze the oxygen reduction reaction (ORR) with a size-dependent activity. The authors attributed the size-dependent properties to the high HOMO levels in large CNDs which could be easily oxidized. Li et al. found that N-CNDs with oxygen-rich functional groups still were electro-catalytically active.75 The electro-catalytic activity of N-CNDs with a N/C atomic ratio of ca. 4.3% for the ORR in an alkaline medium was comparable with that of a commercially available Pt/C catalyst. Notably, doping CNDs with heteroatoms to tune their intrinsic properties and exploit new functions is attractive. 4.2 Electrochemiluminescence ECL is the process whereby species generated at electrodes undergo high-energy electrontransfer reactions to form excited states that emit light.76 The smart combination of


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chemiluminescence with electrochemistry brings ECL many potential advantages. ECL is demonstrated to be more sensitive than PL to the surface state, which is a powerful tool to probe surface states of nanoparticles.18,19,71,74 Especially, the current signal and light signal can be obtained simultaneously, facilitating the investigation of PL mechanism of the luminophor, the investigation of electrochemical reactions by ECL, as well as simultaneous ECL and electrochemical detection.77 Zheng et al. observed ECL from the CNDs at a Pt disk working electrode by cycling between + 1.8 and - 1.5 V.60 The ECL mechanism of the CNDs was suggested to involve the formation of excited-state CNDs (R*) via electron-transfer annihilation of negatively charged (R• -) with positively charged (R• +) CNDs (Figure 4b). The intensity of cathodic ECL was larger than that of anodic ECL, indicating that R• + was more stable than R• (Figure 4a). The maximum ECL peak (535 nm) was substantially red-shifted compared with the PL maximum (455 nm), which was attributed to the fact that the energy splitting of the CND surface states (for ECL emission) was smaller than the band gap of the CND core (for PL emission). Zhou et al. studied the ECL of CNDs prepared by electro-oxidation of MWCNTs in the organic electrolyte in an air-tight electrochemical cell. The positively charged CNDs were also found to be more stable than the negatively charged CNDs.21 The ECL peak at 470 nm was red shifted about 50 nm from the PL peak. A shoulder at 420 nm (maximum PL emission) could be seen in the ECL spectrum. The red shifted ECL (relative to PL) suggested that the ECL emission from CNDs originated from surface states. However, Zhu et al. discovered that as for ECL of CNDs modified with poly (ethylene glycol),59 the anodic ECL intensity was larger than that at the cathode, indicating that R• - was more stable than R• +.60 Evidently, it is imperative that the stability of R• - and R• + of CNDs should be carefully dealt with. The complexity of electrochemical production of the intermediates might also affect the


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ECL process of CNDs,78 which has received little attention in the past studies. To solve this problem, it is necessary to design a suitable electrolytic cell to separate the working electrode from the counter electrode. Nevertheless, one notable common finding from the above studies was that, compared to PL peak from CNDs, there was a significant red shift of the ECL peak. In the past decade, semiconductor QDs have been extensively studied by ECL.55,71,79 Most results suggest the ECL from semiconductor QDs originates from surface states, where the ECL peaks were often significantly red shifted from the PL peaks by several hundred nanometers since these surface states were located in the band gap.18-20 However, the ECL peak from CNDs was only red shifted by tens nanometers compared to the PL peak. The ECL peaks of CNDs in aqueous solution60 and in dichloromethane solution21 were red shifted by about 80 nm and 50 nm from the PL peak, respectively. Both the research groups attributed the ECL origin to the surface states in CNDs. But the ECL of CNDs with a peak tinily red-shifted suggested negligible surface states of CNDs.80 Anyway, the relationship between ECL phenomena and the surface states of CNDs is still a pending question. In order to deeply elucidate the nature of the ECL phenomenon from CNDs, a series of CNDs prepared by the same material have been used in the ECL research. Bao et al. obtained CNDs with different sizes (< 3 kDa, 3-10 kDa, and 10-30 kDa) by ultrafiltration separation.30 Dramatically, each ECL peak was very close to its corresponding PL peak (Figures 4c and 4d), indicating that the PL of CNDs most likely emitted from the same surface states as the ECL of the CNDs. Dong et al. prepared a series of CNDs with the same oxidation degree by chemical oxidation, and PL spectra of the oxidized CNDs with different sizes (1-3, 3-5, 5-10, 10-30 kDa) were particle size-dependent.81 However, ECL peaks of CNDs were all 600 nm, independent of the size, only resulting from the surface states of CNDs.


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The ECL phenomenon of CNDs has also been attributed to the active sites at the edge of CNDs. The same chemiluminescence (CL) spectrum, PL spectrum and ECL spectrum of the hydrazidemodified CNDs implied that they resulted from similar excited states.82 Dong et al. attributed this phenomenon to the abundant luminol-like units at the edges of CNDs, which were a wellknown luminophore for PL, CL and ECL. Compared to CNDs with high oxygen content, the reduction of CNDs was more beneficial to acceleration of electron transport during ECL process. However, unlike the stronger PL emission from the reduced CNDs, the ECL intensity of the reduced CNDs was substantially weaker.16,80 This ECL phenomenon could be explained from the viewpoint of inactivation of active sites in the reduction process. In short, the ECL research on CNDs is at its early stage. Since it is surface-sensitive, the ECL method is playing an important role in the study of the surface state-related luminescence mechanism of CNDs. 5. ELECTROCHEMICAL SENSING BASED ON CARBON NONADOTS The PL, electronic and electrochemical properties of CNDs are sensitive to minute perturbations, rendering them of a great potential for sensing applications. Moreover, with a size comparable to that of biomolecules, CNDs can be used for biolabelling/biodetection. 5.1. Electrochemical Sensing CNDs show unique intrinsic properties, such as strong ability for electron transfer, abundant edge sites, high catalytic activity, and large specific surface area due to the small particle size, which make CNDs advantageous for electrochemical sensing. Due to the high peroxidase-like activity of CNDs, the covalently assembled CNDs/Au electrode exhibits good performance in


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H2O2 detection.83 The catalytic property of CNDs can be preserved properly after the crosslinking reaction. Based on the as-prepared CNDs/Au electrode, the dynamic H2O2 release in MCF-7 human breast cancer cells was monitored with the stability and reusability. Shao et al. found the high electro-catalytic activity of CNDs toward the stable chelate compound TPEACu2+, and realized electrochemical detection of cerebral Cu2+ in a rat brain.84 The results by this method were found to be comparable to those obtained by the conventional ICP-AES method. Based on the abundance of hydrophilic edges and hydrophobic plane in CNDs which can enhance the enzyme adsorption on the electrode surface, Razmi et al. introduced CNDs as a suitable substrate for glucose oxidase immobilization.85 Owing to their small sizes, CNDs could access the catalytic centers of enzymes to facilitate electron transfer, and a pair of well-defined quasi-reversible redox peaks were observed. This biosensor well loaded with the enzyme could sensitively detect the target glucose. Zhao et al. designed a simple but smart platform to fabricate a “signal-on” electrochemical biosensor based on the excellent conductivity of CNDs and the interaction between the nucleobases and graphene.86 Various electrochemical biosensors can be readily developed with this proposed platform87,88. Based on the conductivity, CNDs can be developed not only as an electrolyte-free flexible electrochromic device89 but also as a sensing platform for hydrogen peroxide detection through horseradish peroxidase (HRP) functionalized with CNDs.90 5.2. Electrochemiluminescence Sensing ECL, as a unique and sensitive analytical method, has been widely used for detection in many fields due to its good selectivity, low background and rapid sample analysis.76,91 As a new promising ECL luminophore, CNDs, characteristic of facile functionalization and low biological toxicity, provide the chance for fabrication of new ECL biosensors.


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In order to improve the ECL of the luminophor CNDs, various coreactants80,92,93 and nanomaterials94-99 have been adopted as signal amplification agents to enhance the performance of biosensors. Yang et al. fabricated an ECL sensor based on the CNDs/graphene hybrid for the supersensitive detection of pentachlorophenol (PCP) through multistage amplification of the ECL signal by graphene and coreactant S2O82- (Figure 5).100 Due to its good electrical conductivity and excellent electro-catalytic activity, graphene could efficiently amplify the ECL signal of CNDs. The resulting ECL sensor enabled the real-time detection of pentachlorophenol with unprecedented sensitivity of up to 1.0×10-12 mol/L in a wide linear range of 1.0×10-12 mol/L to 1.0 ×10-8 mol/L. Zhang et al. reported a dual-peak ECL system of CNDs, which could more efficiently distinguish metallic ions than single-peak ECL.101 ECL-1 was more sensitive to the environment than ECL-2, and thus was used for ECL detection of metallic ions. On the basis of the dual-peak ECL of CNDs, iron ion was detected by the internal standard method, showing a linear response of 5×10-6 mol/L to 8×10-5 mol/L with a detection limit of 7×10-7 mol/L. Due to their abundant edge sites, electrical property and high catalytic activity, CNDs could also improve the ECL signal from other luminophor such as luminol,102 CdS,103 CdTe,99 Ru(bpy)32+,104,105 and so on. Since the maximum emission wavelength of CdTe and CDs was close, Li et al. attributed 3.8-fold enhancement of the ECL intensity from CdTe to the energy transfer occurring efficiently between CdTe and CNDs in the nanocomposite.99 However, there are few reports on the role for CNDs to exactly play in amplification of the ECL signal. Long et al. adopted the surface-sensitive tool ECL to probe the unrevealed property of CNDs (Figure 6).105 It has been found that CNDs can act as the co-reactant for the anodic ECL of Ru(bpy)32+. During the anodic scan, the benzylic alcohol units on the surface of CNDs were oxidized “homogeneously” by electrogenerated Ru(bpy)33+ to form reductive radical intermediate, which


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would further reduce Ru(bpy)33+ into Ru(bpy)32+* that could produce a strong ECL signal. This work not only provides an insight into the ECL mechanism of the CNDs-involved system, but also facilitates the understanding of photocatalytic activity and redox properties of CNDs. Moreover, the ECL system of Ru(bpy)32+/CNDs exhibited a robust stability, based on which a model molecule dopamine was quantitatively determined in the range of 0.5 µmol/L to 20 µmol/L. Owing to the eco-friendly feature of CNDs, the ECL system of Ru(bpy)32+/CNDs is promising in bioanalysis. Xu et al. also demonstrated that CNDs prepared by ionic liquidassisted electrochemical exfoliation of graphite electrode could act as the ECL probe for detecting Ru(bpy)32+ in the solution based on the enhanced ECL of Ru(bpy)32+.104 In addition, the PL of CNDs could be quenched by Ru(bpy)32+, and the PL probe for Ru(bpy)32+ detection was also developed. The detection limit harnessing the PL and ECL of CNDs is 0.72 µmol/L and 0.43 µmol/L, respectively. 6. SUMMARY AND OUTLOOK In this review, the electrochemical study of photoluminescent CNDs has been highlighted and discussed. The electrochemical methods to prepare CNDs have advantages due to their low cost and easy manipulation. The PL properties of CNDs can also be efficiently tuned by these methods. Many significant results have been obtained by electrochemical study of the behaviours of CNDs. At the same time, CNDs show the potential in the fields of electrochemical sensing due to their strong ability for electron transfer, abundant edge sites, high catalytic activity and large specific surface area. Although electrochemical methods have been used for preparation of size- and compositioncontrollable CNDs, the advantages of electrochemical methods in the functionalization of CNDs


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have scarcely been exploited. At proper potentials, many compounds such as azides can produce free radicals, which have been successfully bonded to other carbon nanomaterials.106-111 In addition, the “one-step” method to combine preparation with functionalization of CNDs will not only save much time without tedious post-processing steps, but also realize the grafting of multiple functionalities onto the surface of CNDs in a single process.105 This will provide a variety of anticipated materials for the study and application of CNDs. Finally, the necessity is to develop strategies to make clear the structures and PL mechanism of CNDs. Only based on the understanding of PL mechanism of CNDs can we design applicable CNDs with an ultra-small size but long emission wavelengths and a high quantum yield to meet the needs for versatility, especially for in vivo imaging.


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Figure 1. (a) Schematic diagram of a CD structure.27 Reproduced with permission from ref 27. Copyright 2010 American Chemical Society. (b,c) Typical TEM and AFM images of GQDs.28 Reprinted in part with permission from ref 28. Copyright 2013 Wiley. (d) UV-vis absorption and PL spectrum of CNDs in aqueous solution. Inset: digital photo for the product, illuminated with a UV lamp.14 Reprinted in part with permission from ref 14. Copyright 2008 Royal Society of Chemistry. (e) UV/Vis absorption and PL spectra of CdS QDs with different sizes.26 Reproduced with permission from ref 26. Copyright 2002 Wiley. (f) PL spectra of CNDs obtained at 2.5 V with electrolyte containing trace water. The excitation wavelength ranged from 300 to 460 nm with 20 nm increments as indicated.13 Reprinted in part with permission from ref 13. Copyright 2011 Wiley.


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Figure 2. Illustration of the preparation strategy of CNDs by proper intercalation of compounds.49 Reprinted in part with permission from ref 49. Copyright 2014 Wiley.


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Figure 3. (a) CVs of a glassy carbon electrode (GCE) in an aqueous solution containing 1.5 mg/mL CNDs (solid curves) at varied potential sweep rates (as depicted in figure legends).70 Reproduced with permission from ref 70. Copyright 2009 American Chemical Society. (b) CVs of a GCE in 0.2 mol/L pH 7.2 PBS with (solid line) and without CNDs at 0.1 V s−1.45 Reprinted in part with permission from ref 45. Copyright 2015 Royal Society of Chemistry. (c) DPVs for CNDs in 0.1 mol/L TBAP dichloromethane solution.21 Reproduced with permission from ref 21. Copyright 2010 Elsevier. (d) DPV responses of CNDs in CH2Cl2 using a 20 mV pulse amplitude on a Pt ultra-microelectrode (20 µm) at a typical scan rate of 25 mVs-1 at 273 K.28 Reproduced with permission from ref 28. Copyright 2013 Wiley.


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Figure 4. (a) ECL responses with (red curve) and without (line curve) CNDs at a Pt electrode in 0.1 mol/L PBS (pH 7.0).60 (b) Schematic illustration of the ECL and PL mechanisms for CNDs. R•+, R•-, and R* represent negatively charged, positively charged, and excited-state CNDs, respectively.60 Reproduced with permission from ref 60. Copyright 2009 American Chemical Society. PL spectra (c) and ECL spectra (d) of CNDs (48h

Electrochemical Methods to Study Photoluminescent Carbon

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