Carbon nanodots- synthesis, properties and

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Materials Physical Organic Inorganic Nanoscience Food Biological Environmental Analytical Energy Catalysis Chemical Biology & Medicinal General Chemistry Themed issue: Nanomaterials for Energy Conversion and Storage ISSN 0959-9428

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FEATURE ARTICLE Zhenhui Kang, Yang Liu, Shuit-Tong Lee et al. Carbon nanodots: synthesis, properties and applications

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420 nm) irradiation. (Reprinted with permission from ref. 107. Copyright 2012 American Chemical Society.)

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Fig. 31 (a) Comparison of visible light-driven photocatalytic activity of pure TiO2, C-dots, TiO2/C-dots and P25 for photo-degradation of MB. (b) Transient photocurrents of the as-prepared TiO2/C-dots and P25 irradiated with the wavelength larger than 510 nm. (c) Proposed formation mechanism of dyadic structure with charge transfer-type orbital hybridizations at the surface of TiO2 and C-dots. (Reproduced from ref. 8b.)

Possible mechanisms to explain the better photocatalytic performance of TiO2/C-dots can be attributed to three features (Fig. 31c). Firstly, hybridization of C-dots with TiO2 extends the light response of TiO2 into the visible range of the solar spectrum, due to the electronic coupling between the pi states of graphite-essential C-dots and the conduction band states of TiO2. Such electronic coupling would induce a smaller nanohybrid band gap compared to pure TiO2, or new carbon energy levels in the TiO2 bandgap also leading to a smaller bandgap energy and a red shift. In addition, the up-conversion properties of the C-dots will transfer longer wavelength light into shorter wavelength emission. Consequently, light with a wider range of wavelengths, from UV to visible, can be used by the TiO2/C-dots. O2 adsorbed on the surface of the C-dots can accept e
and form O2
, and oxidize MB directly on the surface. Secondly, C-dots can combine with TiO2 efficiently to form TiO2/C-dots, which can generate a large amount of photoreactive species (O2
and _OH) under visible light. These photoreactive species can diffuse easily in reactants and products during the reaction because of the high surface area of the TiO2/C-dots. Thirdly, when the TiO2/ C-dots photocatalysts are excited, C-dots can act as an electron reservoir to trap electrons emitted from TiO2 under irradiation, and thus hinder electron–hole recombination. Water-soluble fluorescent N-doped C-dots (NCDs) were synthesized from a one-pot ultrasonic reaction between glucose and ammonium hydroxide. Besides strong luminescence in the visible-to-near infrared range, NCDs displayed clear UCPL properties. Moreover, the NCDs showed excellent photocatalytic properties in the photodegradation of methyl orange under visible light.91 Kang et al. prepared C-dots/Cu2O composite with protruding nanostructures on the surface by a facile one-step ultrasonic This journal is ª The Royal Society of Chemistry 2012

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treatment. They demonstrated that this photocatalytic system could harness (near) infrared (NIR) light to enhance its photocatalytic activity based on the collective effect of the superior light reflecting ability of Cu2O protruding nanostructures and the UCPL property of C-dots (Fig. 32a and b).94b The photocatalytic process of C-dots/Cu2O composite under (N)IR light is schematically illustrated in Fig. 32c. When the C-dots/Cu2O composite photocatalyst is illuminated (Fig. 32c, (1)), the protruding nanostructures allow multiple reflections of (N)IR light among the vacant spaces between these protruding particles, which can make better use of the source light and therefore offering an improved photocatalytic activity. Additionally, C-dots can absorb (N)IR light (>700 nm), and then emit shorter wavelength light (390–564 nm) as a result of up-conversion, which in turn further excites Cu2O to form electron/hole (e
/h+) pairs (Fig. 32c, (2)).8a The electron/hole pairs then react with the adsorbed oxidants/reducers (usually O2/OH
) to produce active oxygen radicals (e.g. _O2, _OH), which subsequently cause the degradation of organic dye (MB).92–94 Significantly, when C-dots are attached on the surface of Cu2O, the relative position of C-dots band edge permits the transfer of electrons from the Cu2O surface, allowing charge separation, stabilization, and then hindering of e
/h+ pairs’ recombination. The electrons can be shuttled freely along the conducting network of C-dots, and the longer-lived holes on the Cu2O then account for the higher activity of the composite photocatalyst. For energy saving and green environment issues, C-dots offer great potential for a broad range of applications, including biosensors, bioimaging, laser, and light-emitting diodes. They may also serve as a promising candidate for a new type of fluorescence marker, as well as high-efficiency catalyst design for applications in bioscience and energy technology.

transport mobility and good thermal/chemical stability.23b In particular, the high transport mobility and environmentally friendly properties of graphene meet the important requirements of optoelectronic devices.23c,d Apart from the conducting film and transparent anode developed previously,23e–g high mobility of graphene renders it a promising electrode and electron-acceptor material for photovoltaic device applications.23a However, the easy aggregation and the poor dispersion of 2D graphene sheets in common solvents limit its optoelectronic application. Solution-processable functionalized graphenes (SPFGs) has been developed to solve such problem,23h but their non-uniform size and shape (on a scale of several hundred nanometers and even micrometers) remains a daunting challenge for the fabrication of high-quantity photovoltaic devices, with active layer thicknesses in only nanometer scales. To facilitate the application of graphene in nanodevices and to effectively tune the bandgap of graphenes, a promising approach is to convert the 2D graphene sheets into 0D C-dots. Apart from unique electron transport properties,23i new phenomena associated with edge effects and quantum confinement are expected from C-dots.23j Additionally, the compatible surface chemistry, excellent photostability, good solubility in polar solvents and extensive optical absorption throughout the visible and nearinfrared wavelength regions render C-dots potentially useful sensitizers for photovoltaic applications.23a,108–110 Li et al. applied colloidal GQDs with green luminescence as the electron acceptor materials in conjugated polymer, poly(3hexylthiophene) (P3HT)-based thin film solar cells.23a Fig. 33a and b shows respectively the schematic and the energy level alignment of a device configuration of ITO/poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS)/ P3HT:GQDs/Al, in which GQDs (c ¼ 10 wt%) provided an

5. Optoelectronics Graphene has attracted tremendous research interest due to its large surface area, superior mechanical flexibility, high carrier

Fig. 32 (a) Up-converted PL spectra of C-dots; (b) energy distributions of up-converted emissions of C-dots located in the range from 390 to 564 nm (shown as shaded areas); (c) schematic photocatalytic mechanism for C-dots/Cu2O composite under (N)IR light irradiation. (Reproduced from ref. 94b.)

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Fig. 33 Schematic (a) and energy band (b) diagrams of the ITO/ PEDOT:PSS/P3HT:GQDs/Al device. (c) J–V characteristic curves for the ITO/PEDOT:PSS/P3HT/Al, ITO/PEDOT:PSS/P3HT:GQDs/Al and ITO/PEDOT:PSS/P3HT:GQDs/Al devices after annealing at 140  C for 10 min, single log scale. (Reprinted with permission from ref. 23a. Copyright 2011 John Wiley and Sons.)

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effective interface for charge separation and a pathway for electron transport, as clearly evidenced by the greatly increased photocurrent as compared to the device fabricated with P3HT only (Fig. 33c). The device performance was further improved after thermal annealing, with an overall power conversion efficiency (PCE) of 1.28%. Yan et al. found that GQDs have high optical absorptivity and nearly optimized absorption in the visible and near IR region.4,51 Additionally, the calculated energy levels in GQDs suggested the possibility of electron injection from GQDs to large bandgap semiconductors (e.g. TiO2) upon photoexcitation and regeneration of GQDs by accepting an electron from I
(Fig. 34a). Thus, GQDs provide excellent opportunities for the development of inexpensive, high-performance GQD-sensitized solar cells by substituting GQDs for metal organic dyes as photosensitizers. As a proof-of-concept example, Yan et al. exploited GQDs with 168 conjugated carbon atoms to sensitize a nanocrystalline TiO2 photoanode and obtained a photocurrent density of 200 mA cm
2 under AM (air mass, path-length through the atmosphere relative to the vertical thickness of the atmosphere) 1.5 illumination (Fig. 34b).51 The low performance was due largely to the low affinity of GQDs to TiO2 surface since no chemical binding was formed as in conventional dye-sensitized solar cells, in which ruthenium dyes are covalently bonded to the TiO2 surface for much higher optical absorption and faster exciton dissociation. Zhu et al. studied the photon-to-electron conversion capability of GQDs under different light sources (UV or near infrared light).111 The photocurrents generated by GQDs–PEG and GQDs on ITO electrodes were measured by a three-electrode system. The GQDs–PEG photoelectrode generated considerable photocurrent with both 365 nm UV light and 808 nm NIR laser, while the pure GQD photoelectrode showed lower photocurrent than the GQDs–PEG. So the authors thought GQDs could be a new solar cell dopant material and that the light of the photonto-electron conversion may be extended from UV to nearinfrared range.4 Gupta et al. demonstrated that GQDs blended with regioregular P3HT or poly (2-methoxy-5-(2-ethylhexyloxy)1,4 phenylenevinylene) (MEH-PPV) polymer resulted in a significant improvement in solar cell efficiency or organic light emitting diode (OLED) characteristics as compared to graphene nanosheets (GS) blended conjugated polymers.109 Mirtchev et al. fabricated a C-dots-sensitized nanocrystalline TiO2 solar cell.108 It is known that the most effective Ru-based sensitizers contain a number of carboxylate ligands to enable

Fig. 34 (a) Calculated energy level alignment. (b) J–V characteristics of colloidal GQD-sensitized TiO2 nanoparticle solar cells. (Reprinted with permission from ref. 51. Copyright 2010 American Chemical Society.)

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Fig. 35 (a) CQD-sensitized TiO2 nanoparticle and the proposed CQD–TiO2 bonds, (b) UV–Vis–NIR absorption spectrum of CQD thin film on quartz substrate, (c) Current–voltage characteristics of CQD-sensitized solar cells prepared from a MeOH CQD solution and from an aqueous solution of C-dots refluxed in nitric acid to ensure complete surface oxidation and (d) current–voltage characteristics of aqueous CQD-sensitized solar cell. (Reproduced from ref. 108.)

coordination of the dye to the TiO2 surface as shown in Fig. 35a, and the anchoring scheme of sensitizer to TiO2 can be mimicked by surface functionalized C-dots.112 The absorption spectrum of a thin film of C-dots is shown in Fig. 35b, displaying the broad absorption throughout the visible region. An electronic bandgap of approximately 1.1 eV for an indirect transition and 3.1 eV for a direct transition was calculated using Tauc relations113a with the experimental absorption onset at approximately 800 nm, see Fig. 35b. The current–voltage characteristics of the CQD-sensitized solar cell prepared from an aqueous CQD solution under simulated AM 1.5 irradiation are shown in Fig. 35d. A shortcircuit current density (Jsc) of 0.53 mA cm
2 and an open-circuit voltage (Voc) of 0.38 V were produced with a fill factor (FF) of 0.64 for a power conversion efficiency of 0.13%. Cells fabricated from MeOH CQD solutions and aqueous C-dots refluxed in HNO3 to ensure complete surface oxidation showed similar performance, whereas a non-sensitized nanocrystalline TiO2 gave an efficiency of 0.03%. Similar to the recent report of graphene dot-sensitized nanocrystalline TiO2 solar cells, the Jsc seems to the limiting factor on the efficiency of these cells, with both the Voc and FF values comparable to those of Ru-complex sensitized cells.51 Since C-dots exhibit relatively high QY, they may be a good light converter for use in light emitting diode (LED) applications.15b,113b,c Lau et al. investigated the light converting property of C-dots deriving from glucose.15b To demonstrate white light emission via down-converting blue light, C-dots were coated onto a commercially available blue LED. As shown in Fig. 36, the uncoated blue LED emitted blue light centered at 410 nm. After coating with C-dots, the intensity of the blue light weakens, but a broad band appears at 510 nm. The corresponding LED with and without applied voltage are shown in the left-hand side of the inset of Fig. 36. The applied voltage and current for the LED were 2.9 V and 10 mA, respectively. The Commission International d’Eclairage (CIE) chromaticity coordinates for the blue LEDs with and without C-dots coating are shown in the This journal is ª The Royal Society of Chemistry 2012

quantum dot. Density functional theory, calculations reveal that these additional peaks result from a splitting of the lowest unoccupied orbitals of the graphene into three orbitals with distinct energy levels (Fig. 37).

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6. Energy and charge transfer

Fig. 36 Color converter of the GQDs. Luminescence spectra of the blue LED with and without GQDs coating. Left inset: photographs of the GQD coated LED without (top) and with applied voltage (bottom). Right inset: the CIE chromaticity coordinates for the illuminating blue LED with and without GQD layer. (Reprinted with permission from ref. 15b. Copyright 2012 American Chemical Society.)

right-hand side of the inset of Fig. 36. Clearly, with a C-dot layer, the CIE chromaticity coordinates of the blue LED are shifted from (0.242, 0.156) to (0.282, 0.373), demonstrating that the Cdots are capable of converting blue light into white light. The water-soluble C-dots exhibit excellent light converting properties as compared to compound-semiconductor-based QDs such as multishell-structured CdSe/ZnS/CdSZnS and CdSe/CdS/ZnS/ CdSZnS QDs. Choi et al. develop a simple solution method to prepare emissive hybrid quantum dots consisting of a ZnO core wrapped in a shell of single-layer graphene (ZnO–graphene quasiquantum dots, ZnO–graphene QD).94c They then use these quantum dots to make a white LED with a brightness of 798 cd m
2. The strain introduced by curvature opens an electronic bandgap of 250 meV in the graphene, and two additional blue emission peaks are observed in the luminescence spectrum of the

Fig. 37 Photoluminescence and electroluminescence transition scheme for ZnO–graphene quasi-quantum dots. Transitions 1, 2 and 3 correspond to electron transitions from the conduction band (CB) of ZnO, LUMO + 2 and LUMO levels induced by G–Oepoxy to the valence band (VB) of ZnO, respectively. (Reprinted with permission from ref. 94c. Copyright 2012 Nature Publishing Group.)

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Carbon-based systems play a major role in industrial and technological advances.114,115a For fuel cell development or electrochemical energy storage, carbon with high surface area is instrumental in maximizing the performance of catalyst and energy systems.115b,c Increasing the surface area of carbon by fine particle dispersion or by using porous carbon has advanced the development of storage batteries and electrocatalysts for fuel cells.115c A lot of work has been devoted to the study of CNTs, graphene/GO/reduced-GO(RGO) and fullerene-based composite, especially on the energy and charge transfer research of these composites.115a,d–i For example, three specific approaches for designing nanohybrid assemblies are deposition of metal nanoparticles, anchoring semiconductor nanoparticles, and functionalization with photoactive molecules.115a,d–f The interactions between graphene-based carbon nanostructures and excited states of molecules/semiconductor nanoparticles often involve energy and/or electron transfer. The emission of the excited molecule (or semiconductor nanoparticle) serves as an excellent probe to monitor the interactions and thus establish a quenching pathway. Brus and co-workers demonstrated efficient energy transfer from individual CdSe/ZnS nanocrystals to single- and few-layer graphene.115g The fluorescence intensity of single nanocrystals was quenched by a factor of 70 on single-layer graphene, and the quenching efficiency increased with the layer number (Fig. 38). Zhang et al. reported that MWCNTs enhanced the separation of photoinduced electron–hole pairs produced in the ZnO-nanowires (NWs) because of the unique three-dimensional structure of the ZnO-NWs/ MWCNTs heterostructure.115h The electron transfer process in this heterojunction is similar to that in the semiconductor metal composite, as indicated in Fig. 39. Given the economical cost of graphene and the need to seek alternate materials for next-generation electronic devices, there is a significant drive within the scientific community to gain a greater understanding of the properties of C-dots and explore new applications. The potential of C-dot-based solar cells has been explored. However, photochemical and photovoltaic aspects of such nanocarbon structures are relatively less studied.

Fig. 38 Wide-field fluorescence image of individual CdSe/ZnS nanocrystals on quartz and on single-layer graphene. (Reprinted with permission from ref. 115a. Copyright 2011 American Chemical Society.)

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Fig. 40 Schematic illustration of the FRET model based on C-dots– graphene and the mechanism of K+ determination. (Reproduced from ref. 115k.) Fig. 39 Schematic diagram of charge separation and transportation at the ZnO-NWs/MWCNTs heterojunction. (Reprinted with permission from ref. 115h. Copyright 2009 American Chemical Society.)

They have the same core issue: energy and charge transfer in the related nanosystem. There have been several reports highlighting improvements in the photocatalysis of C-dots/semiconductor composites and the degradation of organic compounds.8,11–13 For example, the enhanced degradation rate seen with C-dots/TiO2 as compared to that with TiO2 alone in these systems highlights the beneficial role of C-dots.8 The origin of such beneficial effects is rather indirect and not initiated by the direct absorption of light by Cdots. In particular, the carbon nanostructure facilitates dispersion of catalyst particles, improved charge separation within the C-dots/TiO2 composite, and increased concentration of organic molecules near the catalyst surface. These arguments are consistent with earlier reports that showed TiO2 deposited on adsorbent supports, such as activated carbon and graphene, can have a significant role in concentrating the pollutants from low solution-phase concentration.114,115 In addition, C-dots loaded on the semiconductor surface and forming the composite structure would provide access to photo-induced charge transfer transitions under light irradiation. At the composite structure, the photo-induced electron is transferred to joint charge transfer states predominately located at the C-dots, while the separated hole stays electronically and structurally near the wide bandgap semiconductor. This process can effectively hinder the electron–hole recombination, and guarantee the high reactivity of photogenerated electron and hole excited by visible light. Then the oxygen radicals (_O2
) are formed by the combination of electrons in the conducting network of Cdots with O2 adsorbed on the surfaces of C-dots.8,13 Moreover, the C-dots with UCPL properties can convert the longer wavelength to shorter wavelength light, which can in turn excite wide bandgap semiconductors to form electron–hole pairs.8,11–13 Qu’s group reveal energy transfer from C-dots to graphene and consequently construct a fluorescence resonance energy transfer (FRET) sensor which could be used for measuring the concentration of potassium ions (K+) with high selectivity and tunable dynamic range (Fig. 40).115k They design a model in which the donor (C-dots) and the acceptor (graphene) are brought into FRET proximity through specific cation–ligand complexation (Fig. 40). C-dots are covalently aminated, and graphene is noncovalently functionalized with 18-crown-6 ether (18C6E). The known tight binding of primary alkyl-ammonium with 18C6E10 will bring C-dots and graphene into appropriate 24248 | J. Mater. Chem., 2012, 22, 24230–24253

proximity and hence induce energy transfer. Thereafter, the FRET process is inhibited because of competition between K+ and ammonium for 18C6E, which has high potassium selectivity. Notably, C-dot-based photovoltaic devices with enhanced performance have been successfully fabricated. C-dots provided an effective interface for charge separation and a pathway for electron transport, as clearly evidenced by the greatly increased photocurrent as compared to the device fabricated without Cdots.23a,51 In particular, a C-dot-decorated PEG photoelectrode can generate photocurrent with 808 nm NIR laser, and serve as a new solar cell dopant material, and the light of the photon-toelectron conversion may be extended from ultraviolet to near infrared.111 The calculated energy levels in C-dots suggested the possibility of electron injection from C-dots to wide bandgap semiconductors (e.g., TiO2) upon photoexcitation and regeneration of C-dots by accepting an electron.51 The MB–GQDs dispersed in the MEH-PPV polymer provided more electrical transport paths, which resulted in an enhancement of charge injection and hence an increase in carrier density, thus achieving a lower turn-on voltage and a much higher efficiency.109 The significant improvements provided by GQDs in solar cell efficiency or organic light emitting diode characteristics are higher than those provided by graphene nanosheets blended conjugated polymers. Recently, Guo et al. studied the property of electron transfer quenching of the fluorescence of C-dots by nitroxide radicals.115j The fluorescence of C-dots was found to be efficiently quenched by the paramagnetic nitroxide radical. A singly occupied molecular orbital (SOMO) facilitating electron transfer was proposed to account for the quenching of the fluorescence of Cdots in the C-dot@TEMPO (2,2,6,6-tetramethylpiperidine-Noxide free radical) conjugate, formed via electrostatic interaction between the negatively charged C-dots and the cationic 4-amino2,2,6,6-tetramethylpiperidine-N-oxide free radical (4-AT). The study of C-dot-based energy and charge transfer properties opens up new avenues in the design of optoelectronic and lightenergy conversion devices or systems.8,15b,23a,108–110 Indeed, careful design of hybrid assemblies is necessary to exploit the electronic properties of C-dots. Multifunctional C-dot-based materials with different semiconductor and metal nanoparticles can provide new pathways to design catalyst systems for light energy conversion.8,11–13,107 The abilities of C-dots in the storage and transport of electrons has yet to be exploited fully. Carefully designed C-dots composites have the potential to expand the capacity of nextgeneration energy storage, photovoltaic devices, and photocatalysts. This journal is ª The Royal Society of Chemistry 2012

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7. Bioimaging Traditional QDs, such as CdTe and related core–shell nanoparticles, have been used in various in vitro and in vivo optical imaging experiments.116–118 As the QDs contain toxic heavy metals, their application has raised health and environmental concerns.117 Due to their excellent PL properties and low toxicity, C-dots may be an attractive candidate for bioimaging applications.2,4,119–129 Sun et al. and other groups have studied the use of C-dots in bioimaging applications.7,17,18,58 They reported the potential of C-dots passivated with PPEI-EI for two-photon luminescence microscopy using human breast cancer MCF-7 cells.7 Zhu et al. reported the bioimaging application of GQDs by incubating a solution of MG-63 (human osteosarcoma) cells with suspensions of GQDs from the stock solution with Dulbecco’s phosphate buffer saline (DPBS).120 Chang et al. used pig kidney cell line (LLC-PK1 cells) to test the practicality of the as-prepared C-dots for cell imaging.127 C-dots were localized in the cell membrane and cytoplasm of LLC-PK1 cells. The C-dots were likely internalized into the LLC-PK1 cells through endocytosis.7,129 When excited at longer wavelengths (510–530 nm), red fluorescence was observed from LLC-PK1 cells that had been cultured in a medium containing 1.2 mg mL
1 C-dots for 24 h. There was no autofluorescence from cells when excited at the same wavelength. The strong PL intensity in the cells demonstrated the stability of the C-dots, confirming their strong and stable fluorescence in high ionic strength media. The cell viability was measured after cells had been treated with different amounts of C-dots (0–2.4 mg mL
1). The average cell viability was greater than 95% at a C-dots concentration of up to 1.8 mg mL
1. The results revealed that C-dots are more biocompatible than QDs.2,130 A549 human lung adenocarcinoma cells were used to evaluate the cytocompatibility of the amino-functionalized C-dots obtained by hydrothermal carbonization of chitosan.124 The cell viability of the C-dots was determined by a methylthiazolteterazolium (MTT) assay.131 MTT assays of cell viability studies suggested that the C-dots exhibited low cytotoxicity and posed insignificant toxic effects. These results suggested that C-dots can be used in high concentration for imaging or other biomedical applications. C-dots were introduced into A549 cells to show their bioimaging capabilities using a confocal microscopy test in vitro. The results showed that the photoluminescent spots were observed only in the cell membrane and cytoplasmic area of the cell, but were very weak in the central region corresponding to the nucleus, indicating that C-dots easily penetrated into the cell but did not enter the nuclei. The observation is consistent with previous studies on the interaction of living cells with nanomaterials, in which genetic disruption did not occur.132 The results further confirmed the low cytotoxicity of C-dots. In vivo optical imaging using C-dots produced by laser ablation has been demonstrated by Sun et al.17 Recently, C-dots made from MWNTs were chosen for in vivo fluorescence imaging studies by Liu and Kang et al.39b A nude mouse was subcutaneously administered with C-dots at three different locations. Various excitations including blue, green, yellow, orange, red, deep red, and NIR light with center wavelengths at 455, 523, 595, 605, 635, 661, and 704 nm, respectively, were applied during in vivo imaging of the mouse. After spectral unmixing to This journal is ª The Royal Society of Chemistry 2012

differentiate the background autofluorescence (green), the subcutaneously injected spots (red) on the mouse were seen in these fluorescence images under all of the different light excitations (Fig. 41). Compared to images acquired using blue (455 nm) and green (523 nm) light excitation, those taken under longer wavelength excitations (595 nm and beyond) showed much better signal-to-background separation. Although the fluorescence emission of C-dots is weaker at longer wavelengths, the tissue autofluorescence background decreased even more, resulting in an improved signal-to-noise ratio under red and NIR excitation. In vivo optical imaging at longer wavelengths is usually preferred owing to the improved photon tissue penetration and reduced background autofluorescence, especially in the NIR region. The capability of C-dots for NIR in vivo fluorescence imaging (excitation ¼ 704 nm, emission ¼ 770 nm) demonstrates the great potential of C-dots for use as optical nanoprobes in biomedical imaging.

8. Sensor and surface-enhanced Raman scattering Based on their excellent properties, C-dots can be designed for sensors,115k,133–139 such as biosensors for DNA,133,134 nitrite sensing,135 sensors for phosphate,136a glucose,136b a-fetoprotein136c or metal ions.137–139 For example, Li et al. reported the use of

Fig. 41 In vivo fluorescence imaging. (A) In vivo fluorescence images of a C-dots-injected mouse. The images were taken under various excitation wavelength at 455, 523, 595, 605, 635, 661, and 704 nm. Red and green represent the fluorescent signals of C-dots and the tissue autofluorescence, respectively. (B) Signal-to-background separation of the spectral image taken under the NIR (704 nm) excitation. The fluorescence of C-dots was well separated from the tissue autofluorescence background. (Reprinted with permission from ref. 39b. Copyright 2012 John Wiley and Sons.)

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unmodified C-dots as novel and environmentally friendly fluorescence probes for the sensing of Hg2+ and biothiols with high sensitivity and selectivity. The luminescence emission of C-dots arises from the radiative recombination of excitons, similarly to QDs.140 On the other hand, Hg2+ can quench the fluorescence (FL) of QDs due to facilitation of non-radiative electron/hole recombination through an effective electron transfer process.138 The strategy inspired by these phenomena and the approach are depicted in Fig. 42. Initially, the free C-dots showed strong FL in aqueous solution. However, the FL of C-dot was quenched significantly in the presence of Hg2+ through the charge transfer process. Meanwhile, due to the strong binding preference of biothiols toward Hg2+ by forming a Hg2+–S bond,141,142 Hg2+ was removed from the surface of C-dots and thus the FL of C-dots could be recovered. Therefore, by taking advantage of the observed FL change, a facile FL sensor can be fabricated to allow sensitive detection of Hg2+ and biothiols. Au nanoparticles have been widely used as efficient enhancers for Raman scattering and this phenomenon is known as the SERS effect.143 SERS effect is recognized to be due to the excitation of surface plasmon resonance (SPR) on metal surface, which greatly increases the local E-field near the surface. SERS has found important applications in chemistry, biology and material science as an ultra-sensitive detection technique capable of identifying traces of molecules.144–146 Conventional SERS substrates are constructed by placing metal nanoparticles on planar surfaces, which thereby have a limited surface area. The replacement of planar surfaces by spatially defined nanoporous surfaces with large surface areas can adsorb more molecules for SERS signals and further improve the Raman detection sensitivity. Indeed, Jiang and coworkers147 have demonstrated that the Raman enhancement effect can be dramatically increased by replacing the planar surface with a unique nanoporous superaligned carbon nanotube film with cross stacking. Fan et al. have assembled well-confined two-dimensional (2D) and 3D C-dos honeycomb structures148 by electrodeposition of oxygen-rich functional C-dots within the interstitial voids of assemblies of SiO2 nanospheres, followed by extraction of the SiO2 cores with HF treatment. Although made from quantum sized C-dots, the C-dot assemblies presented a solid porous framework (Fig. 43a and b), which can be used as a sacrificial template for the fabrication of new nanostructures made from other functional materials. Based on the unique honeycomb architecture of C-dots, which allows more efficient adsorption of molecules, the Au nanoparticles formed on C-dots honeycomb exhibit 8–11 times stronger SERS than the widely used Au nanoparticle SERS substrate for the sensitive detection of target molecules. This work provides a new

Fig. 42 Schematic illustration of detection mechanism of Hg2+ and biothiols using C-dots. (Reproduced from ref. 138.)

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Fig. 43 SEM images of Au deposited on Si (a) and on the C-dots honeycomb structure (b), and (c) Raman spectra of 4  10
6 M R6G drop-cast on to a Si wafer (,), adsorbed on Au-coated Si (>), the C-dots honeycomb on Si (O), the Au-deposited C-dots honeycomb on Si (B) and the Raman spectrum of the C-dots honeycomb on Si (*). The inset in (b) is a TEM image of Au nanoparticles deposited on the wall of the C-dots honeycomb. Scale bars in a and b are 100 nm. (Reproduced from ref. 148.)

approach for the design and fabrication of ultrasensitive SERS platforms for various detections. Shi et al. also performed similar work on C-dots or C-dots/metal composites for SERS application (Fig. 43c).149,150

9. Outlook In this review, we have described the recent advances in the research on C-dots, focusing on their synthesis, surface functionalization, PL properties, and applications in photocatalysis, energy and sensor issues. A variety of synthesis techniques already exist for producing C-dots of different characteristics. The PL and optical properties of C-dots are both interesting and intriguing, constituting a rich and hot research topic. The potential of C-dots in the storage and transport of electrons impacted by light has yet to be exploited fully. In the future, we expect the advent of more facile and robust synthetic routes and creative applications to better realize the potential of the increasingly important C-dot materials. C-dots stand to have a huge impact in biotechnological and environmental applications because of their potential as nontoxic alternates to traditional heavy-metal-based QDs. In addition, the unique photoinduced electron transfer ability, as well as excellent light harvesting capability, make C-dots an exceptional candidate for photocatalytic and photovoltaic applications. Carefully designed C-dots composites have the potential to expand the capabilities of next-generation energy-storage and photovoltaic devices, photocatalysts and sensors. By surface and band gap modification of C-dots via functionalization or semiconductors, we can expect to design novel catalysts from C-dots for green chemistry and energy issues. This journal is ª The Royal Society of Chemistry 2012

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Acknowledgements This work is supported by the National Basic Research Program of China (973 Program) (no. 2012CB825800), National Natural Science Foundation of China (NSFC) (no. 51132006, 51072126, 21073127, 21071104, 91027041), a Foundation for the Author of National Excellent Doctoral Dissertation of P R China (FANEDD) (no. 200929), a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project no. CityU102010), a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), a Suzhou Planning Project of Science and Technology (ZXG2012028) and a project supported by the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (Grant no. 11KJB150015). We also thank the Innovation Program of Graduate Students in Jiangsu Province (no: CXZZ11_0097).

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