Shi Ying
Lim
,
Wei
Shen
and
Zhiqiang
Gao
*
Department of Chemistry, National University of Singapore, Singapore 117543. E-mail: chmgaoz@nus.edu.sg; Fax: +65 6779-1691; Tel: +65 6516-3887
First published on 15th October 2014
Fluorescent carbon nanoparticles or carbon quantum dots (CQDs) are a new class of carbon nanomaterials that have emerged recently and have garnered much interest as potential competitors to conventional semiconductor quantum dots. In addition to their comparable optical properties, CQDs have the desired advantages of low toxicity, environmental friendliness low cost and simple synthetic routes. Moreover, surface passivation and functionalization of CQDs allow for the control of their physicochemical properties. Since their discovery, CQDs have found many applications in the fields of chemical sensing, biosensing, bioimaging, nanomedicine, photocatalysis and electrocatalysis. This article reviews the progress in the research and development of CQDs with an emphasis on their synthesis, functionalization and technical applications along with some discussion on challenges and perspectives in this exciting and promising field.
The accidental discovery of CQDs during the separation and purification of single-walled carbon nanotubes (SWCNTs) by Xu et al. in 2004 triggered subsequent studies to exploit the fluorescence properties of CQDs and create a new class of viable fluorescent nanomaterials.6 Fluorescent carbon nanoparticles received their name “carbon quantum dots” in 2006 from Sun et al.7 who proposed a synthetic route to produce CQDs with much enhanced fluorescence emissions via surface passivation. CQDs are synthesised by two routes, namely the top-down route7–9 and the bottom-up route.3,10–12 CQDs are typically quasi-spherical nanoparticles comprising amorphous to nanocrystalline cores with predominantly graphitic or turbostratic carbon (sp2 carbon) or graphene and graphene oxide sheets fused by diamond-like sp3 hybridised carbon insertions (Fig. 1).13–16 Oxidised CQDs contain considerable amounts of carboxyl moieties at their surface. Depending on the synthetic route, the oxygen content in the oxidised CQDs ranges from 5 to 50% (weight).14 As shown in Fig. 1, there are many carboxyl moieties on the CQD surface, which impart excellent water solubility and suitable chemically reactive groups for further functionalization and surface passivation with various organic, polymeric, inorganic or biological materials to CQDs. Upon surface passivation, the fluorescence properties of CQDs are enhanced. Surface functionalization also modifies their physical properties, like their solubility in aqueous and non-aqueous solvents.
Fig. 1 Chemical structure of CQDs. (Reproduced with permission from ref. 13.) |
As a group of newly emerged fluorescent nanomaterials, CQDs have shown tremendous potential as versatile nanomaterials for a wide range of applications, including chemical sensing, biosensing, bioimaging, drug delivery, photodynamic therapy, photocatalysis and electrocatalysis. Compared to conventional semiconductor quantum dots, the unique attributes of CQDs, for example their benign chemical composition, tunable fluorescence emissions, facile functionalization and excellent physicochemical and photochemical stability (non-photobleaching or non-photoblinking), render them very attractive for technical applications. Together with other advantages such as low cost and ease of synthesis,17 CQDs are in a favourable position for achieving unprecedented performance. On the other hand, complex procedures for their separation, purification and functionalization, their generally low quantum yields, and ambiguity in their geometry, composition and structure are some of the issues that need to be tackled before they can truly outperform their semiconductor quantum dot counterparts in areas like bioimaging, biosensing and nanomedicine. This article reviews the progress in the research and development of CQDs and their technical applications. We first examine the fluorescence properties of CQDs and their tunable emissions. Then, we discuss the various synthetic approaches for their production and possible surface passivation and functionalization routes to impart desired properties to CQDs. Finally, we discuss in great detail the applications of CQDs in chemical sensing, biosensing, bioimaging, nanomedicine, photocatalysis and electrocatalysis, especially the advantages they could bring to these fields. In view of several excellent review articles focusing on different aspects of CQDs, such as their synthesis and physicochemical properties,14,18 surface functionalization,19 bioimaging and biosensing19,20 and photocatalysis and optoelectronics,18 it is hoped that this article will provide a comprehensive overview of the current status of CQD research and open new perspectives toward the research and development of CQDs with much improved physicochemical properties.
In this type of bandgap transitions, single-layer graphene sheets have to be used to prevent interlayer quenching.24 The single-layer graphene sheets are used as precursors for electronically slicing into isolated π-conjugated domains, which resemble large aromatic molecules with extended π-conjugation of specific electronic energy bandgap for optical absorption and fluorescence emissions.25 Such electronic transitions display strong absorption in the ultraviolet (UV) region, but weak or no fluorescence emissions (Fig. 2A). The strong absorption is likely due to light absorption by a large amount of high density π-electrons in the sp2 hybridised islands, which form excitonic states while the weak emissions are possibly a result of quenching via radiationless relaxations to the ground state during exciton migration to energy traps.13
Fig. 2 (A) CQDs with strong absorption in the UV region and weak emissions and (B) CQDs with weak absorption in the near UV-vis region but strong multicolour emissions in the visible region. (Reproduced with permission from ref. 13.) |
Robertson and O'Reilly suggested that the optical properties of carbon nanomaterials which contain both sp2 and sp3 bonds are determined by the π-states of the sp2 sites.29 Thus, the bright surface defect-derived fluorescence of CQDs is due to the recombination of electron–hole pairs in the strongly localised π and π* electronic levels of the sp2 sites. These sites lie between the bandgap of the σ and σ* states of the sp3 matrix,30,31 leading to strong visible emissions. Such electronic transitions exhibit weak absorption in the near ultraviolet-visible (UV-vis) region but strong emissions in the visible region as shown in Fig. 2B. In addition, upon surface passivation or functionalization, the surface defects become more stable to facilitate more effective radiative recombination of surface-confined electrons and holes, thus achieving brighter fluorescence emissions.25
Upon surface passivation with organic or polymeric materials, such as poly(propionyl ethyleneimine-co-ethyleneimine) (PPEI-EI) attached to the CQD surface, surface defects are stabilised and strong fluorescence emissions both in solution-like suspension and in solid state were detected. The emissions of such passivated CQDs covered a broad range of the visible region and extended into the near-infrared (NIR) region as shown in Fig. 3.7
Fig. 3 (A) Aqueous solutions of PEG1500N-passivated CQDs (a) excited at 400 nm and (b) excited at the indicated wavelengths; (B) absorption and emission spectra of PPEI-EI passivated CQDs in water with increasing λex from 400 nm on the left with 20 nm increments. Inset: emission intensities normalized to quantum yields. (Reproduced with permission from ref. 7.) |
It should be noted that the surface passivation agents used were not emissive in the visible and NIR regions, thus any fluorescence emissions observed must have originated from the surface-passivated CQDs. The tunable emission property of CQDs is clearly demonstrated in Fig. 3. From the fluorescence spectra of PPEI-EI-passivated CQDs, it is evident that the emissions are broad and excitation wavelength-dependent.7,26 The tunable emissions of the surface-passivated CQDs could be a result of varied fluorescence characteristics of particles of different sizes of the CQDs and the distribution of different emissive sites on the surface of the CQDs. However, the exact mechanism accounting for the excitation wavelength-dependent emission remains to be established and the requirement for surface passivation in order to produce fluorescence emissions is poorly understood. Experimental observations are not helpful since controversial results are often observed. Moreover, the optical properties of CQDs are closely associated with the synthetic routes used in their preparation. For example, Sun and co-workers attributed the fluorescence emissions to the radiative recombination of excitons of surface energy traps of CQDs. Upon surface passivation, these energy traps are stabilised and therefore become emissive – a phenomenon that has been observed in semiconductor quantum dots.32 They postulated that there must be a quantum confinement effect of emissive energy traps on the surface of the CQDs. Nonetheless, such tunable fluorescence emission property of CQDs provides an added advantage in the selection of different emission wavelengths with different excitation wavelengths that can be applied to optical labelling and fluorescence imaging.
In addition to the excitation wavelength-dependent emission, several reports have indicated that the fluorescence emissions of CQDs are pH-dependent.10,33,34 Liu and colleagues noticed that the fluorescence intensity of their CQDs decreases when the pH of the solution is shifted from the optimal value of 7.0 regardless of whether pH is increased or decreased.10 In another report, it was observed that the fluorescence intensity of CQDs only shows a slight decrease of ∼3% when the pH of the solution is changed from 5 to 9.33 Jia et al. reported that CQDs prepared from direct heating of ascorbic acid solution show a practically linear dependence on the pH of the solution in the range of 4.0 to 8.0. The fluorescence intensity decreased by as much as 90% from pH 4.0 to 8.0. Varying the pH of the solution from 4.0 to 8.0, corresponding to the deprotonation of the carboxyl groups on the surface of the CQDs, might cause electrostatic doping/charging to the CQDs and shift the Fermi level.35 As seen above, the significantly large variation of fluorescence intensity with the pH of the solution and the synthetic routes used in the preparation of the CQDs adds additional complexity in finding out a widely accepted fluorescence emission mechanism.
This is, however, not without dispute. In view of the practically constant energy difference of ∼1.1 eV between the excitation light and emission light, Shen and colleagues argued that the multi-photon excitation is inadequate to account for the up-conversion fluorescence emission properties of CQDs.26 They postulated that the up-conversion fluorescence emission originates from the relaxation of electrons from a higher energy state of the π orbital (LUMO) to the σ orbital since some electrons would inevitably transit to the LUMO when a large number of low-energy photons excite the electrons in the π orbital. Of course, the electrons in the σ orbital can also be excited, but they only emit conventional down-conversion light.
On the other hand, Wen et al. believed that some of the apparent up-conversion fluorescence emissions are artefacts originating from the conventional down-conversion emissions which have been excited by the leaking component from the second diffraction in the monochromator of the spectrofluorometer.40 By simply inserting a long pass filter into the excitation pathway the leaking component can be removed. Thus, great care must be taken when interpreting the fluorescence emissions of CQDs and further clarification is necessary to explain up-conversion CQDs.
Fig. 4 Supported-synthesis of CQDs using modified silica spheres as carriers and resols as carbon precursors. (Reproduced with permission from ref. 3.) |
Cost is one of the important determinants of CQDs becoming viable competitors to semiconductor quantum dots. However, the above-described approaches require either costly precursors, complex instrumental set-ups or post-treatments to synthesise fluorescent CQDs. Recently, green synthetic approaches were introduced. With these green routes, CQDs were produced in one step without the need for expensive materials and elaborate experimental set-ups. Li's team was one of the first groups to introduce the concept of preparing fluorescent CQDs using a simple and green route.50 Their approach only involved a single electrochemical treatment step of ethanol with sodium hydroxide. This approach is less costly and easier to manage, and the CQDs produced were found to possess desirable fluorescence properties along with high water solubility, stability and sensitivity to pH. Thereafter, the use of other inexpensive and biocompatible starting materials, like ethanol,50 citrate,51 glucosamine,52 ascorbic acid,53,54 saccharides,49,55,56 candle soot,57 watermelon peels,58 pomelo peels,59 orange juice,60 strawberry juice,61 sugar cane juice,62 chicken eggs,63 chitosan,64,65 organogel66 and gelatine,67 were developed. Many of these “green” CQDs displayed excellent performance in cell imaging and chemical sensing applications. Table 1 summarises the representative examples of the green synthetic routes developed for the preparation of CQDs.
Precursor | Synthetic method | Quantum yield (%) | Application | Ref. |
---|---|---|---|---|
Phenol/formaldehyde resin, silica particle | Carbonisation at 900 °C, NaOH etching | 14.7 | Bioimaging | 3 |
Ascorbic acid | Heat treatment at 90 °C | 3.22 | pH sensing | 34 |
Citrate | Carbonisation in air at 300 °C or hydrothermal treatment at 300 °C | 3 | — | 46 |
Carbohydrate | H2SO4, HNO3 treatment, amine passivation | 13 | — | 47 |
Carbohydrate (glucose) | Alkali- or acid-assisted ultrasonic synthesis | 7 | — | 48 |
poly(ethylene glycol) and saccharide | Microwave treatment (500 W) | 3.1–6.3 | — | 49 |
Ethanol in NaOH solution | Electrochemical treatment (25–40 V) | 4 | — | 50 |
Citrate | Hydrothermal treatment at 180 °C | 68 | Hg2+ sensing | 51 |
Glucosamine–HCl | Hydrothermal treatment at 140 °C | — | — | 52 |
Ascorbic acid | Hydrothermal treatment at 140 °C | 5.7 | Bioimaging, pH sensing | 53 |
Ascorbic acid | Hydrothermal treatment at 180 °C | 6 | — | 54 |
Glucose | Hydrothermal treatment at 200 °C | 1.1–2.4 | Bioimaging | 55 |
Sucrose | Microwave oven at 100 W | — | Bioimaging | 56 |
Candle soot | HNO3 oxidation | 3 | Bioimaging | 57 |
Watermelon peels | Carbonisation at 220 °C | 7.1 | Bioimaging | 58 |
Pomelo peels | Hydrothermal treatment at 200 °C | 6.9 | Hg2+ sensing | 59 |
Orange juice | Hydrothermal treatment at 120 °C | 26 | Bioimaging | 60 |
Strawberry juice | Hydrothermal treatment at 120 °C | 6.3 | Hg2+ sensing | 61 |
Sugar cane juice | Hydrothermal treatment at 120 °C | 5.76 | Bioimaging | 62 |
Chicken egg | Plasma irradiation (50 V, 2.4 A) | 6.8 | Printing | 63 |
Chitosan | Hydrothermal treatment at 180 °C | 43 | Bioimaging | 64 |
Chitosan | Microwave oven | — | — | 65 |
Organogel | Topochemical polymerisation | — | — | 66 |
Gelatine | Hydrothermal treatment at 200 °C | 31.6 | Bioimaging | 67 |
Hair fibre | H2SO4 treatment | 11.1 | Bioimaging | 68 |
Ionic liquids | Microwave oven (700 W) | 1.65–5.14 | Quercetin sensing | 69 |
3-(3,4-Dihydroxyphenyl)-L-alanine, L-histidine, and L-arginine | Carbonisation at 300 °C | — | Bioimaging | 70 |
Citric acid and ethylenediamine | Hydrothermal treatment at 150–300 °C | 80 | Fe3+ sensing, printing | 71 |
Acetic acid | Carbonisation with P2O5 | — | Bioimaging | 72 |
Grass | Hydrothermal treatment at 150–200 °C | 2.5–6.2 | Cu2+ sensing | 73 |
In addition to acid-oxidative treatment and surface passivation, other methods have been explored in a bid to increase quantum yield and obtain CQDs with better fluorescence properties. One approach is to dope newly-produced and surface-passivated CQDs with inorganic compounds such as ZnS and ZnO.74 An aqueous suspension of CQDs and Zn(CH3COO)2 was hydrolysed with NaOH or precipitated with Na2S to obtain ZnO- and ZnS-doped CQDs, respectively. In the case of doping with ZnO, an additional thermal annealing step was required to transform Zn(OH)2 to ZnO. The respective quantum yields of the ZnS- and ZnO-doped CQDs in aqueous solutions were above 50% and around 45% with the former being very close to the quantum yield of commercially available semiconductor CdSe/ZnS quantum dots.55 It was speculated by the authors that doping might have reinforced the surface passivation effect or even served as an additional form of surface passivation mode together with the existing organic passivating agents. These doped CQDs were shown to have strong multi-photon fluorescence properties,55 giving them great potential in the field of one- and two-photon excitation imaging applications.
Anilkumar's group introduced the concept of crosslinking surface passivated CQDs for better optical performance.79 They showed that crosslinking of the passivating PEG1500N on the surface of CQDs results in the formation of fluorescent particles that contain multiple CQDs in covalently-bound clusters (Fig. 5). Interestingly, it was observed that the fluorescence properties of the CQDs in each particle are additive and up to seven CQDs exist in a single particle for maximum brightness when one particle is subjected to fluorescence imaging. It was suggested that crosslinking most likely results in the stabilisation of surface functionalization by reinforcing the structure of the soft shell of PEG1500N molecules that surround the hard fluorescent CQD cores, thus achieving improved fluorescence emissions.
Fig. 5 Crosslinking of PEG1500N-functionalised CQDs by reaction with dimethyl pimelimidate in a pH 8 phosphate buffer. (Reproduced with permission from ref. 79.) |
Functionalization of CQDs is important because the introduction of functional groups, such as amines and carboxyls, can impose different defects on the CQD surface. These defects work as excitation energy traps and lead to large variations in fluorescence emissions.63,64 In fact, oxidative treatment using strong acids is a simple and very effective means to introduce carbonyl and carboxyl groups on the surface of CQDs, imparting greater water solubility to the CQDs.14 It has been shown by Liu's group that an oxidant is a necessary component to produce visible fluorescence emissions.10 His group proposed that an additional function of this acid oxidative treatment is to possibly break down carbon aggregates into smaller nanoparticles. Therefore, an oxidative acid like nitric acid was often added to the reaction mixture in the synthesis of CQDs.
Very often, surface passivating agents act as functionalising agents as well, where the physical properties of the CQDs are modified together with their fluorescence properties. Thus, there is no need for additional modification steps in the later stage of synthesis. For example, Dong and colleagues used branched polyethylenimine (b-PEI) as both the surface passivating and functionalising agent, where the polyamines passivate the CQD surface and the free amine groups allow for the functionalization of the passivated CQDs.80 In another report, surface passivation was achieved using diamine-terminated oligomeric PEG, thereby achieving bright luminescence and surface functionalization of CQDs simultaneously.81
As a class of novel fluorescent nanoparticles with environmental and biological benign composition and high biocompatibility, major technical applications of CQDs would naturally be in the fields of bioimaging and biosensing. In order to rival their organic dye and semiconductor quantum dot counterparts, a high quantum yield of CQDs is vital for these applications. Consequently, extensive research efforts have been devoted to engineering CQDs to improve their quantum yields. A wide variety of synthetic routes for the preparation of CQDs have been developed. Although a quantum yield as high as ∼80% has been obtained,71 the majority of the CQDs synthesised so far have quantum yields below 10% (Table 1). In addition to surface-passivation, doping with heteroatoms and nitrogen in particular has shown great potential to significantly enhance the quantum yield of CQDs. As demonstrated by Zhu et al.,71 nitrogen-doped CQDs (N-CQDs) exhibit a quantum yield of 80%, comparable to most organic dyes and semiconductor quantum dots. Leveraging on the synergistic effect of N- and Mg-doping, a quantum yield of 83% was recently reported.82 Nonetheless, unlike organic dyes and semiconductor quantum dots of which both composition and structure are well-defined, considerable ambiguities in the composition and structure of CQDs are likely the cause of their low and often varied quantum yields. Therefore, in addition to significantly enhancing the quantum yields of CQDs, synthetic approaches with high reproducibility and scalability that are capable of producing geometrically, compositionally and structurally well-defined CQDs with high reproducibility and scalability are urgently needed.
One of the first attempts of utilising CQDs in chemical sensing is the selective detection of Hg2+ in aqueous solutions51,61,68–87 and live cells.87 Goncalves and colleagues demonstrated that the fluorescence emissions of both CQD solution and CQDs immobilized in sol–gel are sensitive to the presence of Hg2+,84,85 In their study, laser-ablated and NH2-PEG200 and N-acetyl-L-cysteine-passivated CQDs were used as fluorescent probes. It was observed that the fluorescence intensity of the CQDs is efficiently quenched by micromolars of Hg2+ with a Stern–Volmer constant of 1.3 × 105 M−1. Therefore, judging from the relatively large magnitude of the Stern–Volmer constant,88 the quenching provoked by Hg2+ is probably due to static quenching arising from the formation of a stable non-fluorescent complex between CQD and Hg2+. A substantial improvement in the sensitivity down to nanomolars was later realised by replacing the laser-ablated CQDs with N-CQDs. Again, static quenching is thought to be responsible for the quenching of fluorescence but with a much larger Stern–Volmer constant of 1.4 × 107 M−1, two orders of magnitude higher than that of the previous system.85 It was suggested that the presence of nitrogen element in the N-CQDs, most probably –CN groups on the N-CQD surface, is responsible for the much improved performance of Hg2+ sensing.
More recently, Yan and co-workers adopted the Hg2+–CQD system for selective detection of Hg2+ in aqueous solution as well as in live cells.87 The authors reported the synthesis of two types of CQDs using citric acid with 1,2-ethyldiamine (CQD-1) and N-(b-aminoethyl)-g-aminopropyl (CQD-2) that possess high quantum yields of 65.5 and 55.4%, respectively. They studied the effective and selective quenching of fluorescence emissions of CQD-1 and CQD-2 by Hg2+. Both CQDs acted as selective and sensitive fluorescent probes for the detection of traces of Hg2+ in both aqueous solutions and live cells. Upon the addition of 20 μM of Hg2+, the fluorescence intensity of CQD-1 was rapidly quenched by 80%, while that of CQD-2 was quenched by 55%, and both remained stable after 1 h of observation (Fig. 6). This substantiates the viability of using CQD-1 and CQD-2 as chemical sensing probes for Hg2+. The selectivity of CQD-1 and CQD-2 toward Hg2+ was then assessed by comparing the extent of fluorescence quenching of CQD-1 and CQD-2 by the addition of 20 μM of different metal ions. As shown in Fig. 6, among all metal ions tested, Hg2+ quenched both the fluorescence of CQD-1 and CQD-2 to the largest extent. Such quenched fluorescence was reversible and could be recovered by adding a strong chelating agent such as EDTA, making these CQDs reversible fluorescent probes. Furthermore, successful attempts were made in detecting Hg2+ in cultured cells.87
Fig. 6 UV-vis (black lines) and fluorescence spectra of (a) CQD-1 and (b) CQD-2 aqueous solutions in the absence (blue lines) and presence (red lines) of Hg2+; fluorescence responses of (c) CQD-1 and (d) CQD-2 aqueous solutions in the presence of 20 μM of different metal ions. (λex = 360 nm). (Reproduced with permission from ref. 87.) |
Other applications of CQDs in chemical sensing included the detections of Cu2+,89–96 Fe3+,97,98 Pb2+,99 Cr(VI)100 and Ag+.93 Similar to the Hg2+ sensing, most of the procedures proposed are based on the above-mentioned fluorescence quenching by the metal ions. For example, Liu et al. reported a procedure for selective detection of Cu2+ in aqueous samples such as tap water by CQDs modified with lysine and bovine serum albumin (CQDs-BSA-Lys).89 Highly sensitive detection of Cu2+ was achieved by making use of the coordination reaction of Cu2+ with both the –COOH and –NH2 groups of the CQDs-BSA-Lys. To improve the sensitivity of the assays, apart from coating CQDs with various polymeric materials, metal–organic frameworks (ZIF-8 – zinc imidazolate frameworks)94 and silica nanoparticles95 were employed in the configuration of more sensitive CQD-based fluorescent probes. For instance, Lin and co-workers showed that highly sensitive nanocomposite fluorescent probes can be readily prepared by encapsulating b-PEI-coated CQDs into ZIF-8. Leveraging on the synergistic effect of the strong fluorescence of the encapsulated CQDs and the selective accumulation effect of the ZIF-8 host, as low as 80 pM Cu2+ was detected by the CQD-ZIF-8 nanocomposite probes.74 It is envisioned that the same strategy can be extended to the preparation of other CQD–metal–organic framework probes for highly sensitive and selective detections of many other analytes.
Along with the development of sensitive metal ion assays, CQDs have also found niche applications in the detections of pH,93,101 C2O42−,96 PO43−,102 CN−,103 F−,104 S2−,105 ClO−,106 I−85,107 and NO2 gas.108 Contrary to the metal ion assays which leverage on the fluorescence quenching mechanism, many of the anion assays are based on the fluorescence enhancement (fluorescence recovery) of the already quenched CQD–metal complexes. For example, in the I− assay, the fluorescence of the CQDs was recovered due to the formation of more stable complexes between I− and the metal ions, displacing the CQDs in the CQD–metal complexes.107 Furthermore, assays for small organic compounds including ascorbic acid100via fluorescence enhancement; and 4-nitrophenol,109 quercetin,69 2,4-dinitrophenol and 2-amino-3,4,8-trimethyl-3H-imidazo[4,5-f]quinoxalin110 through fluorescence quenching have been reported.
Apart from utilising the fluorescence of CQDs as an analytical signal, recent studies have revealed that CQDs exhibit good chemiluminescence111 and electrochemiluminescence.112 Therefore, several groups have developed chemiluminescent assays for NO2−113 and Co2+;114 and electrochemiluminescent assays for traces of pentachlorophenol115 and Cu2+.116
So far, a wide variety of procedures and a large number of starting materials have been used in the preparation of CQDs for the assays, thus leading to substantial batch-to-batch and lab-to-lab variations. Such variations in the synthesis of CQDs have serious consequences since studies have suggested that the fluorescence characteristics are strongly dependent on the composition of CQDs and residue chemical groups on their surface; and different starting materials and procedures inevitably produce CQDs with rather different physicochemical properties and optical properties in particular. Standardisation is therefore urgently needed in the preparation of CQDs and the assessment of the performance of CQDs.
Fig. 7 Schematic illustration of NALFIA. (Reproduced with permission from ref. 117.) |
Employing the principles of Förster resonance energy transfer (FRET) and homogenous immunoassay, Bu and co-workers119 proposed an immunosensor for quick and specific detection of 4,4′-dibrominated biphenyl (PBB15) – a persistent organic pollutant that disturbs the endocrine system.120 This immunosensor comprised of gold nanoparticle (AuNP)-functionalised anti-PBB15 antibodies and CQD-labelled PBB15 antigens that function as the fluorescence acceptors and donors, respectively. As a result of FRET, the fluorescence of the CQDs was effectively quenched by the AuNPs. Upon the addition of PBB15 to the solution, the CQD-labelled antigens were released from the AuNP surface due to competitive immunoreaction and fluorescence recovery occurred. This immunosensor serves as a good example for future development of immunoassays to detect other analytes with suitable antibodies and antigens.
The concept of fluorescence quenching was also applied to the detection of nucleic acids, where the level of selectivity was so high that even a single-base mismatch could be identified.121 First, a single-stranded DNA (ss-DNA) labelled with a fluorescent dye adsorbed onto a CQD, effectively quenching the fluorescent dye. When the ss-DNA hybridised with its complementary target to form a double-stranded DNA (ds-DNA), the newly-formed ds-DNA desorbed from the CQD surface and the fluorescence recovered. This application showed great prospects in the detection of single-nucleotide polymorphisms based on the fluorescence intensity changes.
Another application of CQDs was based on the exploitation of aptamers, which are target binders selected from a large nucleic acid library.13 The aptamers are usually coupled with a conformational change induced by the target and could possibly result in a detectable change in radiometric response.122 Such an application was realised by Xu's group,123 who demonstrated that thrombin induces aptamer-functionalised CQDs to form a sandwich-structure with aptamer-functionalised silica nanoparticles through specific thrombin–aptamer interaction. This assay was highly specific to thrombin with a detection limit of 1.0 nM, making it one of the more sensitive fluorescent assays for thrombin. Stable fluorescent CQDs prepared using a single-step microwave pyrolysis method were used for the detection of proteins after they were separated by gel electrophoresis.124 Staining of proteins using these CQDs turned out to give comparable or even better sensitivity as compared to conventional staining agents such as Coomassie Brilliant Blue and Ag+. Water-soluble CQDs, prepared from thermal combustion of rice straws in a furnace under insufficient air flow, were used for rapid detection and counting of bacteria cells in sewage water.125 These CQDs selectively interacted with the receptors on the bacterial cell membrane only.
In addition to macrobiomolecules, CQDs have also shown promise as fluorescent probes in the detection of small bioanalytes like anti-bacterial drugs. One such example was shown by the experiments performed by Niu and Gao.126 First, fluorescent N-CQDs were produced from glutamic acid through a one-step pyrolysis method. These N-CQDs were subsequently used for the detection of amoxicillin. Amoxicillin is a common drug used to treat bacterial infections. The authors found that amoxicillin molecules effectively separate the N-CQDs from each other, thus lowering the frequency of non-radiative transitions that ultimately leads to a rise in fluorescence intensity. CQDs were also applied to the detections of other small bioanalytes such as dopamine,98,127,128 ascorbic acid100 and glucose.129 For instance, Qu's group synthesised highly fluorescent CQDs using dopamine as carbon source and applied them to label-free detection of dopamine. Similar to the anion sensing mentioned earlier, dopamine effectively recovered the fluorescence of the already quenched Fe3+–CQD complex. It was observed that the enhancement of the fluorescence is proportional to the dopamine concentration in the range of 0.1–10 μM with a detection limit of 68 nM.98 However, dopamine was believed to be an efficient quencher in another report.128 This controversy likely originates from the different carbon sources and synthetic routes used in the preparation of the CQDs. Therefore, caution must be taken when interpreting the data and standardisation in CQD synthesis is urgently needed.
It was revealed that organic dye-conjugated CQDs are effective fluorescent probes for H2S. A FRET process took place in the presence of trace amounts of H2S, turning the blue emission of the organic dye-conjugated CQDs to green.132 Previous work has proved that H2S can penetrate cell membrane by simple diffusion.133 Using a fluorescence microscope, the ability of the organic dye-conjugated CQDs to visualise changes in physiologically relevant levels of H2S in live cells was evaluated. Fig. 8 displays the fluorescence images of HeLa and L929 cells incubated with the organic dye-conjugated CQDs before and after being treated with H2S. As seen in Fig. 8, the intracellular fluorescence of the organic dye-conjugated CQD-stained cells exposed to H2S for 30 min at 37 °C turned green, clearly indicating that the organic dye-conjugated CQDs are promising fluorescent probes to track H2S level change in live cells.132 In another example, CQDs synthesised using N-(β-aminoethyl)-γ-aminopropyl methyl-dimethoxysilane as a carbon source selectively interacted with Cu2+ because of the residue ethylenediamine groups on their surface.95 Additionally, dual-emission probes for Cu2+ were prepared by coating such CQDs on the surface of Rhodamine B (RhB)-doped silica nanoparticles. The fluorescence of the CQDs was efficiently quenched by Cu2+, while that of RhB was negligibly affected. Utilising these probes, successful attempts were made in in vivo imaging of Cu2+ in live cells.95
Fig. 8 Fluorescence images of the organic dye-conjugated CQDs loaded live cells before (A, D) and after incubating with 30 (B, E) and 100 mM H2S (C, F). (Reproduced with permission from ref. 132.) |
Cell images obtained by Hsu et al. showed that CQDs are mostly localised in the cytoplasm and cell membrane.134 It was also shown that water-soluble CQDs passivated with PPEI-EI could label the cell membrane and cytoplasm of MCF-7 cells and they do not reach the nucleus.1 Furthermore, CQDs synthesised from activated carbon selectively labelled the cell membrane and cytoplasm of COS-7 cells.135 On the other hand, cells incubated with silica-encapsulated CQDs were found to only exhibit bright fluorescence in the cytoplasmic area.136 Fowley and colleagues prepared CQDs that were enclosed in an amphiphilic biocompatible polymer, which were subsequently found to travel across Chinese hamster ovary cell membrane and settle in the cytosol.137 Hu's team prepared b-PEI-coated CQDs with a quantum yield of 54.3% and these CQDs were observed to distribute uniformly throughout the cytoplasm.138 These examples clearly show that the localisation of CQD varies, depending on the choice of the surface passivating agents and the mode of surface passivation.
Before imaging, cells are usually incubated with CQDs so that the CQDs can be internalised by the cells. This ability of cells to take in CQDs was revealed to be dependent on temperature, where no CQDs were found to internalise into the cells at 48 °C.1 It was proposed that CQDs likely translocate into cells by endocytosis. Also, it was suggested that the uptake of CQDs may be enhanced by coupling CQDs with membrane translocation peptides, so as to facilitate this translocation procedure by overcoming the cell membrane barrier.139,140
As mentioned above, CQDs are able to exhibit multicolour emissions, which is a huge advantage that sets them apart from the majority of labelling agents. This allows researchers to control and choose the excitation and emission wavelengths.141 As illustrated in Fig. 9, the property of tunable emissions of CQDs was clearly visible,134 where light at different wavelengths was emitted upon excitation at different wavelengths. This property was also seen in HepG2 cells incubated with 4,7,10-trioxa-1,13-tridecanediamine-passivated CQDs, which portrayed multicolour emissions when excited at different wavelengths.142 “Green” CQDs prepared from sugar cane juice also exhibited multicolour emissions upon different excitation modes in bacteria and yeast cells.62 Notably, Ray et al. revealed that surface passivation might not be necessary to achieve a high level of fluorescence intensity required for cell imaging.57 CQDs prepared from the thermal combustion of soot and treated with acid were able to translocate into Ehrlich ascites carcinoma cells successfully even though they were not coated with any surface passivating agents.57
Fig. 9 (a) Emission spectra of CQDs at different excitation wavelengths; fluorescence images of MCF-10A cells treated with CQDs upon excitation with (b) UV, (c) blue and (d) green light. (Reproduced with permission from ref. 134.) |
If the excitation wavelength is red-shifted enough, CQDs could emit in the NIR region. Although the emission in the NIR region is relatively weak, CQDs have great potential for in vivo fluorescence tracking studies141 because the animal body is practically transparent in the NIR region.143 Yang's group was the first to explore the viability of CQDs as contrast agents in live mice.15 In their experiments, PEG1500N-passivated CQDs were injected subcutaneously into mice and bright fluorescence emissions were observed, only fading away 24 h after the injection. If the CQDs were injected intravenously, emissions were only observed in the bladder region, thereby suggesting that urine extraction is the main exiting route for intravenously introduced CQDs. The same group also successfully tracked the migration in lymph vessels using ZnS-doped CQDs (Fig. 10).15
Fig. 10 Tracking of migration through lymph vessels using ZnS-doped CQDs. (Reproduced with permission from ref. 15.) |
In another report, Cao et al. employed the ZnS-doped CQDs for in vivo imaging in mice, which showed comparable brightness with the well-established CdSe/ZnS quantum dots.144 However, after intradermal injections of the CQDs and CdSe/ZnS quantum dots into mice, it was observed that though both types of quantum dots moved to the axillary lymph nodes, the former moved at a much slower rate than the latter.36 This was ascribed to the PEG molecules on the CQD surface, which reduce the interaction between the CQDs and the lymph cells. Of particular interest was the observed competitive performance of the CQDs in vivo to that of the CdSe/ZnS quantum dots. The above results suggest that CQDs may be further developed into a new class of high-performance yet non-toxic agents for bioimaging.144
The exact mechanism of CQD uptake by cells remains to be elucidated, but increasing experimental evidence suggested that CQDs are likely internalised by the cell through endocytosis without significant infiltration to the cell nucleus.1,14,18,19 In addition, the interaction between protective/functional coatings on CQDs is believed to play an important role in selective cell targeting. In future, better targeting ability of CQDs to cells and perhaps even the nuclei may be achieved by conjugating CQDs with facilitating proteins or peptides, enabling the CQDs cross the cell membrane barrier more readily.
The work by Bechet and co-workers showed that CQDs can be used for photodynamic therapy. Photodynamic therapy is a clinical treatment mainly for superficial tumours.146 It involves the localisation and accumulation of photosensitizers in the tumour tissue, following which they are irradiated with a specific wavelength, triggering the formation of singlet oxygen species that result in cell death. It has been validated that CQDs have high inhibition effect on MCF-7 and MDA-MB-231 cancer cells.134 This phenomenon was attributed to CQDs being able to generate more reactive oxygen species, making them promising photosensitizers. It was also noted that the circulation and uptake of CQDs in the body is dependent on their surface coating and the route of administration.147 Huang et al. investigated the effect of the injection route on the distribution, clearance and tumour uptake of CQDs.81 It was learnt that CQDs are quickly and effectively excreted from the body when intravenous, intramuscular and subcutaneous injection routes are used. Additionally, the high tumour-to-background fluorescence contrast and low fluorescence levels in other tissues and organs demonstrated the suitability of CQDs to act as photosensitizers as they are able to localise selectively into tumours (Fig. 11).
Fig. 11 Fluorescence images of tumour-bearing mice. (Reproduced with permission from ref. 81.) |
Juzenas and colleagues also explored the use of CQDs as photosensitizers in photodynamic therapy to destroy cancer cells.148 Small CQDs functionalised with PPEI-EI (PPEI-EI-CQDs) were prepared. Upon irradiation with UV light, these CQDs displayed substantial photodynamic effect in Du145 and PC3 cells. It was proposed that photo-induced generation of singlet oxygen (Type II mechanism) and other reactive oxygen species and radicals (Type I mechanism) is responsible for the observed photodynamic effect. TiO2 is one of the most popular semiconductor photocatalysts, but it can only be excited with UV light due to the size of its bandgap.149 Unfortunately, UV radiation has a low penetration power and can only penetrate slightly into the skin tissue, thus resulting in poor photodynamic efficiency in deep tissues.150 Compared to TiO2, PPEI-EI-CQDs have the additional advantage of tunable bandgap, where their bandgap can be made smaller to allow excitation at longer wavelengths. This would allow for the destruction of buried tumours because light at longer wavelengths can penetrate deeper into tissues. By attaching a photosensitizer (chlorin e6) to CQDs, a synergistic photodynamic therapy platform was developed.151 It was shown that the CQDs can indirectly excite the photosensitizer by FRET mechanism. Moreover, the capability of up-conversion fluorescence emissions of CQDs is more attractive in photodynamic therapy. As demonstrated by Fowley et al.,152 the up-conversion property of CQDs is potentially useful in the treatment of deep-seated tumours in photodynamic therapy. In their study, a conventional photosensitizer (protoporphyrin IX) was first conjugated to CQDs. The photosensitizer was indirectly excited via FRET by the up-conversion fluorescence emission of the CQDs upon excitation at 800 nm. The excitation light of 800 nm is in the phototherapeutic window and can penetrate human tissue four times deeper than the 630 nm light used in clinical photodynamic therapy.
In addition to phototherapy, CQDs can be used for radiotherapy. As described in a report by Andrius et al.,153 PEG-CQDs coated with a silver shell (C-Ag-PEG CQDs) could be used as radiosensitizers in Du145 cells. When irradiated with low energy X-rays, electrons were ejected from the C-Ag-PEG CQDs, which in turn generates free radicals and damages the cancer cells surrounding the CQDs, reducing the damage of normal cells and increasing therapeutic selectivity.
In the field of nanomedicine, a fusion of nanotechnology and medicine, CQDs could function as nano-carriers for tracking and delivery of drugs or genes. In particular, CQDs with fluorescence in the red or NIR region would be the most desirable because background illumination from endogeneous fluorophores can be avoided during imaging.154 Demonstrated by Hu's team, bPEI-coated CQDs (bPEI-CQDs) displayed great potential in the application of gene delivery.138 These CQDs have a large number of amino groups on their surface, which could condense DNA to aid in their function as gene carriers. In order to check for the viability of these CQDs as gene carriers, the authors carried out additional transfection experiments using enhanced green fluorescent protein (EGFP) gene, which served as the reporter gene. As presented in Fig. 12, the overlay of the three images revealed that the pPEI-CQDs exhibit high fluorescence intensity and transfection efficiency comparable to EGFR. Evidently, the gene carried by the bPEI-CQDs was successfully brought into the cells. Because of its significant positive charge density and proton-sponge effect, bPEI spontaneously attracts and condenses gene (polyanionic DNA strands) to form toroidal complexes that are readily taken up by cells through endocytosis. The un-protonated amine moieties in the complexes are thought to buffer endolysosomal pH, thereby allowing cytoplasmic release of gene.155
Fig. 12 Fluorescence images of 293T cells transfected with (A) EGFR and (B) DNA–bPEI-CQD complexes; (C) the bright field image (D) the overlay of the three images. (Reproduced with permission from ref. 138.) |
Drug delivery systems (DDSs) based on nanotechnology are increasingly developed in recent years. The most widely investigated DDSs are based on AuNPs, but the issue of toxicity limits their applications in clinical therapy.156 AuNPs also require thiol groups for drug loading through Au–thiol interaction, which imposes a further limitation of drug choice.154 In addition, AuNPs are difficult to track in in vivo systems because they cause the quenching of fluorophores.157 Therefore, CQDs serve as good alternatives to AuNPs since different functionalizations could result in many possibilities for conjugation with drug molecules in combination with targeting agents, expanding the drug choices for delivery.154
Zheng and co-workers conjugated a platinum(VI)-based anti-cancer pro-drug – oxidised oxaliplatin (Oxa(VI)–COOH) on the surface of CQDs through chemical coupling.158 The pro-drug-conjugated CQDs were taken up by cancer cells through endocytosis and the drug was released upon the reduction of Oxa(IV)–COOH to oxaliplatin(II) because of the highly reducing environment in cancer cells. It was also demonstrated that the distribution of the pro-drug-conjugated CQDs can be closely tracked by monitoring the fluorescence signal of the CQDs, thereby offering great help in the customisation of the injection time and dosage of the medicine. To engineer a controlled-release mechanism, Karthik et al. introduced a photosensitive molecule (quinolone) to the CQD-based drug delivery system.159 The strong fluorescent properties of the CQDs serve as a conventional means to precisely track the distribution of the drug–CQD conjugates and the quinoline molecules on the surface of the CQDs serve as triggers for photo-regulated drug release. Other drug-releasing mechanism like pH-triggered drug release has also been tested.160,161 In an attempt to enhance the loading capacity of CQDs, Mewada and co-workers tested the drug carrying and folic acid-mediated delivering capacities of highly fluorescent swarming CQDs.162 Folic acid on the CQD surface was used as a navigational molecule thanks to its widespread association with most types of cancer cells.163 The drug loading capacity for an anti-cancer drug doxorubicin (DOX) was estimated to be ∼86% and the release of DOX from the DOX-loaded CQDs followed first order release kinetics at physiological pH – an ideal drug release profile. More interestingly, due to the better targeting ability of the folic acid molecule, the DOX-loaded CQDs showed a higher killing rate of cancer cells than free DOX and were less toxic to normal cells.162
Besides serving as drug carriers and fluorescent tracers, CQDs were found to be able to control drug release. An example was given by Lai and colleagues where CQDs loaded with DOX could control the release of DOX in HeLa cells.164 Any pre-release of DOX before its uptake by the target cells was reduced, thus drastically increasing the anti-cancer efficacy of this drug. Moreover, CQDs functionalised with PEG oligomers allowed for a longer circulation time in the physiological systems before they targeted the selected tissues to achieve localised therapy.165 However, thus far, there have not been any reports of CQDs that can specifically target a disease state,81 thus severely limiting their uses in therapeutics.
As one of the most popular photocatalysts, TiO2 has been used in the removal of organic pollutants and in the generation of H2 through water splitting.176 However, a major drawback in its photocatalytic efficiency resides in its ineffective utilisation of visible light as the irradiation source. Because the bandgap of bulk TiO2 lies in the UV region (3.0–3.2 eV), only less than 5% of sunlight is utilised by TiO2. Therefore, bandgap engineering by possible modification of TiO2-based materials is one of the plausible approaches to enhance the performance of TiO2 photocatalysts. In view of their attractive optical properties and up-conversion in particular, a nanocomposite of CQDs and TiO2 is expected to realise the efficient usage of the full spectrum of sunlight. Using methylene blue (MB) as a model compound, Li et al. showed that TiO2–CQD nanocomposites are able to completely degrade MB (50 mg mL−1) within 25 min under visible light irradiation, where only <5% of MB is degraded when pure TiO2 is used as the photocatalyst.36
Apart from harvesting visible light and converting it to shorter wavelength light through up-conversion (Fig. 13), which in turn excites TiO2 to form electron–hole pairs,36,177–179 it is believed that the CQDs in the nanocomposites facilitate the transfer of electrons from TiO2 and the electrons can be shuttled freely along the conducting paths of the CQDs, allowing charge separation, stabilisation and hindering recombination, thereby generating long-lived holes on the TiO2 surface.180 The longer-lived holes then account for the much enhanced photocatalytic activity of the TiO2–CQD nanocomposites. Likewise, similar behaviour was observed with CQD–TiO2 nanotube composites in the photocatalytic degradation of MB181 and CQD–TiO2 nanosheet nanocomposites in the photocatalytic degradation of RhB.182 Further to the above TiO2–CQD-based photocatalysts, other metal oxide nanoparticle–CQD nanocomposites like Fe2O3–CQD,183,184 ZnQ–CQD185 and Cu2O–CQD;186 and metal phosphate–CQD composite (Ag3PO4–CQD),187 were also used to harness the full spectrum of sunlight in their respective photocatalytic systems. Moreover, Kang and colleagues reported SiO2–CQD nanocomposites in the photocatalytic degradation of MB36 and selective hydrocarbon oxidation.188 Nonetheless, more work is needed to improve the lifespan of the above-mentioned photocatalysts before they can be employed in practical scenarios. Such work will be worthwhile because the photocatalytic activities of the nanocomposites are much greater than that of the well-known TiO2.
Fig. 13 Possible catalytic mechanism of theTiO2–CQD nanocomposites under visible light. (Reproduced with permission from ref. 36.) |
In recent years, researchers have succeeded in utilising TiO2–CQD nanocomposites as photocatalysts to generate H2 through water splitting. One of the first reports was authored by Zhang and co-workers.189 They noted that TiO2 nanotube arrays loaded with CQDs are capable of producing H2 from water through photocatalysis under visible light. As schematically depicted in Fig. 14, the TiO2 nanotubes function as active sites for the photochemical reduction of water into H2 and the CQDs as photosensitizers. The presence of the CQDs effectively extends the light harvesting range of the TiO2 nanotube arrays to the visible and NIR regions. As a result, a four-fold enhancement in photocurrent density and a hydrogen evolution rate of 14.1 mmol h−1 at 0 V (vs. Ag/AgCl) were obtained.189 To further improve the photocatalytic efficiency, the TiO2–CQD nanocomposites were modified by adding narrow bandgap semiconductor quantum dots (CdSe) to them to leverage on the up-conversion property of the CQDs and the synergistic effect between the CdSe quantum dots and the CQDs.190 Similar to the TiO2 nanotube array–CQD nanocomposites, photochemical production of H2 from water under irradiation from visible light was observed. The nanocomposites were stable in the catalytic reaction and the photocurrent density was dependent on the size of the CQDs. The highest current density of 0.9 mA cm−2 was obtained with 3.8 nm CQDs, which is four times higher than that of pristine TiO2.191
Fig. 14 Illustration of the sensitization mechanism of CQDs. (Reproduced with permission from ref. 189.) |
In addition to being used as photocatalysts, CQDs have been capturing the attention of researchers as potential photosensitizers in solar cells. For example, a CQD–RhB–TiO2 system showed that the CQDs effectively bridge the RhB molecules to the TiO2 substrate by acting as a one-way electron transfer intermediary. Comparing to the RhB–TiO2 system, the presence of CQDs significantly enhanced the photoelectric conversion efficiency by as much as seven times.192 In another report, a CQD/TiO2 electrode was employed in a solar cell. The photocurrent density was 2.7 times larger than that of pristine TiO2 electrode under visible light illumination.181 Enhanced performance of a solar cell was also obtained when N-CQDs were used as photosensitizers.193 Although the photo-to-electricity conversion efficiency of the above-mentioned solar cells is far from satisfactory, these findings definitely encourage more research in the application of CQDs in photovoltaic devices.
The ultra-small size of CQDs along with their high stability and good electrical conductivity makes them interesting contenders as electrocatalytic materials for ORR. Previous investigations on graphene have indicated that doped nitrogen atoms in carbon materials, especially in the form of pyridinium moieties, play a critical role in enhancing their electrocatalytic activities toward ORR.195 One of the pioneering reports on the use of CQDs as electrocatalysts for ORR was by Li and co-workers.196 They demonstrated that N-CQDs with oxygen-rich functional groups prepared via an electrochemical procedure are electrocatalytically active toward electrochemical reduction of oxygen. The onset potential of ORR was found to be −0.16 V (vs. Ag/AgCl), which is close to that of commercial platinum-based electrocatalysts (Fig. 15). Similar results were later obtained by Yan and co-workers197 and Liu et al.198 with N-CQDs synthesised by totally different procedures. A comparison between nitrogen-free CQDs and the N-CQDs suggested that the electrocatalytic activity of the N-CQDs is indeed closely associated with the N-doping effect. In addition, the N-CQDs exhibited excellent tolerance to a possible crossover effect from methanol. First-principles investigations of the N-CQDs suggested that pyridinic and graphitic nitrogen are responsible for the observed electrocatalytic activity.199 In another report, Zhu and colleagues investigated the electrocatalytic activity of CQDs prepared from natural biomass – soy milk.200 Similar to the N-CQDs, a much enhanced electrochemical reduction profile of oxygen was obtained.
Fig. 15 Cyclic voltammograms of (a) N-CQD/graphene and (b) commercial Pt/C on a GC electrode in N2-saturated 0.1 M KOH, O2-saturated 0.1 M KOH and O2-saturated 3 M CH3OH solutions. (Reproduced with permission from ref. 196.) |
Likewise, OER also suffers from sluggish kinetics and a high over-potential is required in order to drive OER at a reasonably high rate. Currently, the best electrocatalysts for OER are ruthenium- and iridium-based materials. Again, the formidably high cost of these materials has urged researchers to search for alternative electrocatalysts that can offer high efficiency in OER and yet readily available at low cost. Unfortunately, reasonably high electrocatalytic activity of CQDs toward OER has yet to be reported. On the other hand, CQD-based hybrid materials such as CQD–NiFe-layered double-hydroxide composites have shown some promise in OER.201 It was reported that the composites exhibit high electrocatalytic activity toward the oxidation of water at a relatively low over-potential of ∼235 mV in 1 M KOH at a current density of 10 mA cm−2, which is comparable to those of the most active perovskite-type electrocatalysts. It was suggested that the synergistic effect arising from strong association of the CQDs with the NiFe hydroxide greatly facilitates charge transport, thereby improving the catalytic activity of the NiFe hydroxide. It is likely that this kind of CQD composites will offer new opportunities for OER as well as many other electrocatalytic applications.
The advent of CQD research has heralded a new chapter in biomedicine. The proof-of-concept experiments mentioned above have confirmed that CQDs are able to contribute to the advances of biomedicine via applications in bioimaging and nanomedicine. The excellent chemical and photochemical stability of CQDs together with their chemically non-toxic composition give a clear advantage in the context of in vivo biomedical applications. With these attractive features, CQDs have clearly become a legitimate competitor to the conventional semiconductor quantum dots as bioimaging agents with comparable or even better performance due to their excellent biocompatibility, high optical performance without photoblinking and the ability to be functionalised with various moieties. More importantly, by conjugating targeting moieties and therapeutic components, CQDs will enable “theranostics” which holds the potential to address the challenges of cancer heterogeneity and adaptation. Therefore, CQDs have the technical capability that will enable the development of new diagnostics, therapeutics and preventives, which can cause a paradigm shift in the way we diagnose, treat and prevent cancer. Nonetheless, in drug delivery and controlled release, a more fundamental question that should be probed is why CQDs would be efficacious at all. What are the advantages of CQDs for drug delivery compared with monoclonal antibodies in conjugation with anticancer drugs,202 and nanospheres made of biodegradable polymers?203 The applications of CQDs in nanomedicine should then be to first determine the tasks for which CQDs are particularly valuable although preliminary results are encouraging. Further research is required in the development of CQDs with better targeting ability to target specific cell types and specific cell compartments. It is also essential that CQDs to be used in bioimaging use relatively low-energy excitation sources, preferably red or NIR sources, thus making them more effective in tissue penetration and reducing the interference from background fluorescence. Consequently, a major challenge in CQD research is the synthesis of CQDs with brighter fluorescence emissions excitable by red or NIR sources. Also, more research is desired on the synthesis of CQDs with controlled shape, size and functionalities. Although significant findings have been reported with regard to the applications of CQDs, their exact mechanism of cellular uptake and precise toxicological effect remain to be elucidated due to the fact that the pharmacokinetics and bio-distribution of CQDs are dependent upon multiple factors such as size, shape, surface chemistry and so on. Through rational design and optimisation of these factors, non-specific uptake of CQDs can be significantly reduced and blood circulation can be extended, thereby conferring CQDs enhanced abilities in targeting specific tissues in the human body. First, surface functionalization is the most important factor to alter and fine-tune the pharmacological properties of CQDs. For example, CQDs with oligomeric PEG coatings exhibited much lower toxicity, longer blood circulation half-life and better tumour targeting efficiency in vivo than CQDs without appropriate surface coatings.81 Moreover, the right shape and size of CQDs can result in reduced non-specific capture by macrophages, which can also affect their excretion route, circulation half-life, as well as how they will interact with different tissues in vivo. Therefore, there is a need for reliable techniques to produce CQDs with controlled and consistent properties. Considerably more research must be carried out before the viability of CQDs can be fully realised and the development of CQDs for potential biomedical uses should proceed in parallel with a thorough evaluation of the cytotoxic effect of CQDs.
The general strategy for the adoption of environmentally benign CQDs as photocatalysts in synthetic chemistry represents an attractive approach in the development of green chemistry, which may eventually lessen the burden of energy consumption, product clean-up and waste disposal. The immediate goal in this emerging area should be geared toward the discovery of photochemical solutions for increasingly ambitious synthetic goals. The long-term goals should be to improve efficiency and synthetic utility and to eventually perform chemical synthesis under sunlight.
Compared to other applications of CQDs, there have been fewer studies in the usability of CQDs as electrocatalysts for ORR and OER. In-depth theoretical and experimental studies are needed to delicately design CQD-based electrocatalysts with desirable electrocatalytic activity and long-term operation stability. The combination of CQD doping and CQD-based nanocomposites with other nanomaterials may open up new avenues to systematically study the effect of structural parameters and chemical compositions on the catalytic performance of the electrocatalysts, thus leading to fundamental insights and practical applications.
Although still in the midst of development, CQDs have already shown immense potential to play a big role in nanotechnology for the development of assays, sensors, bioimaging agents, drug carriers, phototherapy, photocatalysis and electrocatalysis. Despite the fact that many optical and electronic properties of CQDs are not well understood yet, there is no doubt that CQDs will play a huge role in bioimaging and biomedical research in the near future upon further development. Being highly versatile, CQDs have the propensity to be rationally modified according to different needs. Applications of CQDs in niche areas such as photocatalysis exemplify the versatility of CQDs in the most unexpected areas. It is heartening to witness the diversion of research interest in CQDs away from traditional fields such as bioimaging, and into more urgent and pressing needs such as green chemistry and clean energy production. The fact that the advantages of CQDs are being recognised by researchers with interest in areas as diverse as materials science, synthetic chemistry, drug delivery, nanomedicine and clean energy suggests that research on CQDs will continue to grow in synergistic relationship with intellectually adjacent fields. It seems clear that the future of CQDs remains promising.
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