There has been significant recent interest in the development of highly fluorescent nanomaterials as contrast agents for optical imaging in vivo. 1 The imaging agents should ideally be bright, nontoxic, biocompatible, and stable against photobleaching. Among the extensively studied are those based on semiconductor quantum dots (QDs) such as CdSe/ZnS. 2 The rationale for the use of QDs over conventional organic dyes is now generally accepted in the literature. 3 There are already successful in vivo imaging demonstrations of QDs on tumor vasculature, tumor-specific membrane antigens, sentinel lymph nodes, etc. 2,4 The semiconductor QDs containing cadmium or other heavy metals are unfortunately known for their significant toxicity even at relatively low concentrations, 5,6 which may prove prohibitive to any patient studies. Therefore, the search for benign alternatives has continued. Of particular interest and significance was the recent finding that small carbon nanoparticles could be surface-passivated by organic or bio-molecules to become strongly fluorescent. 7 These fluorescent carbon nanoparticles, 7,8 dubbed "carbon dots" (C-Dots, Scheme 1), were found to be physicochemically and photochemically stable and non-blinking. The carbon particle core could also be doped with an inorganic salt such as ZnS before the surface functionalization to significantly enhance the fluorescence brightness (C ZnS -Dots, Scheme 1). 9 These carbon dots have been successfully used for in vitro cell imaging with both one-and two-photon excitations. 7,9,10 Carbon is hardly considered as an intrinsically toxic element. Available results from the ongoing toxicity evaluation of the oligomeric PEG-functionalized C-Dots 7 in mice have suggested no meaningful toxic effects, 11 raising the prospect for in vivo biocompatibility and uses of carbon dots. Here we report the first study of carbon dots for optical imaging in vivo. The results suggest that the carbon dots are not only brightly fluorescent in solution, as reported previously, 7,9 but also well-behaved as contrast agents in live mice.The C-Dots and C ZnS -Dots with the PEG diamine, H 2 NCH 2 (CH 2 CH 2 O) n CH 2 CH 2 CH 2 NH 2 (n ∼ 35, PEG 1500N ), as the surface passivation agent were prepared and characterized as previously reported. 7,9,10 Shown in Figure 1 For subcutaneous injection, female DBA/1 mice (∼25 g) were shaved in the back area surrounding the injection point. Upon the injection of a C-Dots solution (30 µg carbon coreequivalent in 30 µL) or a C ZnS -Dots solution (65 µg in 30 µL), the mice were imagined in a Lumazone FA in vivo Imaging System (MAG Biosystems) with 470 nm (FWHM ∼ 40 nm) excitation and 525 nm (FWHM ∼ 47 nm) emission filters. As shown in Figure 2, the fluorescence images of the subcutaneously injected mice exhibited bright emissions from CDots and C ZnS -Dots. The relatively stronger fluorescence from the latter is consistent with the previously reported solution-phase results. 9 The injected carbon dots in mice diffused relatively slowly, with the ...

The photoluminescence in carbon dots (surface-passivated small carbon nanoparticles) could be quenched efficiently by electron acceptor or donor molecules in solution, namely that photo-excited carbon dots are both excellent electron donors and excellent electron acceptors, thus offering new opportunities for their potential uses in light energy conversion and related applications.Quantum-sized semiconductor nanoparticles (quantum dots) have emerged as an important class of photoactive nano-materials for a variety of purposes and applications. [1][2][3][4] For the utilization of semiconductor quantum dots in light energy conversion and related areas, there have been extensive investigations on their photoresponse and photoinduced charge separation and electron transfer processes. [5][6][7][8] Alternative to the traditional semiconductors, other quantum-sized nanoparticles have been explored and developed for similar photophysical and photochemical properties. Of particular interest and significance is the recent finding that small carbon nanoparticles could be surface-passivated by organic molecules or polymers to become highly photoactive, exhibiting strong photoluminescence in the visible and near-infrared spectral regions. 9-15 These photoluminescent carbon nano-particles, dubbed "carbon dots" (Scheme 1), were found to be physico-chemically and photochemically stable and nonblinking in their luminescent emissions. 9 Here we report that the photoluminescence from carbon dots could be quenched highly efficiently by either electron acceptor or electron donor molecules in solution, namely that the photo-excited carbon dots are excellent as both electron donors and electron acceptors. These interesting photoinduced electron transfer properties may offer new opportunities in potentially using carbon dots for light energy conversion and related applications, in addition to their being valuable to the effort on mechanistic elucidation.The carbon dots in this study were prepared by using the same procedures as those reported previously. 9 In the preparation, the small carbon nanoparticles (separated from the laser ablation-produced powdery sample) were refluxed in aqueous nitric acid solution for the purpose of oxidizing surface carbons into carboxylic acids, followed by thionyl chloride treatment and then amidation with the oligomeric ethylene glycol diamine H 2 NCH 2 (C 2 H 4 O) 35 C 2 H 4 CH 2 NH 2 (PEG 1500N ) to form the carbon dots with surface-attached PEGs (Scheme 1). The transmission electron microscopy (TEM) results ( Fig. 1) suggested that these dots were well-dispersed, with sizes averaging about 4.2 nm (based on statistical analyses of more than 300 dots), as also supported by the atomic force microscopy (AFM) results (Fig. 1).Photoluminescence spectra of the carbon dots in aqueous or organic solutions were generally broad ( NIH Public Access Author ManuscriptChem Commun (Camb) 3). Obviously 2,4-dinitrotoluene was a much more effective quencher than 4-nitrotoluene, consistent with its being a s...