We report that nanoscale carbon particles (carbon dots) upon simple surface passivation are strongly photoluminescent in both solution and the solid state. The luminescence emission of the carbon dots is stable against photobleaching, and there is no blinking effect. These strongly emissive carbon dots may find applications similar to or beyond those of their widely pursued silicon counterparts.

Two-photon fluorescence materials have attracted much recent attention for their many promising applications, especially in the growing field of biomedical imaging. 1-5 Among the best performing two-photon fluorescence materials are semiconductor quantum dots such as CdSe and related core-shell nanoparticles. 6-8 These quantum dots have been demonstrated in various optical imaging experiments in vitro and in vivo. 6,9,10 At the same time, however, heavy metals as the essential elements in available high-performance semiconductor quantum dots have prompted serious health and environmental concerns in the community and beyond. Therefore, the search for benign alternatives has become increasingly important and urgent. Recently, we found and reported 11 that nanosized pure carbon particles may be surfacepassivated to exhibit bright photoluminescence in the visible. These photoluminescent carbon dots (C-Dots, Figure 1a) are of two distinctive features: one is that the underlying carbon particles are very small (sub-10 nm); and the other is that the particle surface is passivated by organic or other molecules via either covalent linkages or chemical adsorption. 11 Mechanistically, the carbon-based photoluminescence has been attributed to passivated defects on the carbon particle surface acting as excitation energy traps. 11,12 Here we report that the C-Dots also exhibit strong luminescence with two-photon excitation in the near-infrared. The estimated two-photon absorption cross-sections of the C-Dots are comparable to those of available high-performance semiconductor quantum dots. In addition, the two-photon luminescence microscopy imaging of the C-Dots internalized in human cancer cells is demonstrated.

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 ...