There has been much discussion on the need to develop fluorescent quantum dots (QDs) as ultracompact probes, with overall size profiles comparable to those of the genetically encoded fluorescent tags. In the use of conventional semiconductor QDs for such a purpose, the beautifully displayed dependence of fluorescence color on the particle diameter becomes a limitation. More recently, carbon dots have emerged as a new platform of QD-like fluorescent nanomaterials. The optical absorption and fluorescence emissions in carbon dots are not bandgap in origin, different from those in conventional semiconductor QDs. The absence of any theoretically defined fluorescence color-dot size relationships in carbon dots may actually be exploited as a unique advantage in the size reduction toward having carbon dots serve as ultracompact QD-like fluorescence probes. Here we report on carbon dots of less than 5 nm in the overall dot diameter with the use of 2,2'-(ethylenedioxy)bis(ethylamine) (EDA) molecules for the carbon particle surface passivation. The EDA-carbon dots were found to be brightly fluorescent, especially over the spectral range of green fluorescent protein. These aqueous soluble smaller carbon dots also enabled more quantitative characterizations, including the use of solution-phase NMR techniques, and the results suggested that the dot structures were relatively simple and better-defined. The potential for these smaller carbon dots to serve as fluorescence probes of overall sizes comparable to those of fluorescent proteins is discussed.

Few-layer graphene materials or "carbon nanosheets" were covalently functionalized with poly(vinyl alcohol) via ester linkages, and the resulting functionalized sample became soluble, allowing solution-phase processing for various purposes such as the fabrication of polymer-carbon nanosheets composites containing no dispersion agents or any other foreign substances.

Carbon nanotubes (CNTs) have been shown to cross cell membranes and can mediate the internalization of macromolecules. These characteristics have constituted CNTs as an exciting new tool for drug delivery and biological sensing. While CNTs exhibit great potential in biomedical and pharmaceutical applications, neither the cell penetration mechanism of CNTs nor the intracellular fate of the internalized CNTs are fully understood. In this study, time-lapse fluorescence microscopy was used to investigate the intracellular distribution of FITC labeled PEGylated single-walled CNTs (FITC-PEG-SWCNTs) in living cells and shown that PEGylated SWCNTs entered the nucleus of several mammalian cell lines in an energy-dependent process. The presence of FITC-PEG-SWCNTs in the cell nucleus did not cause discernible changes in the nuclear organization and had no effect on the growth kinetics and cell cycle distribution for up to 5 days. Remarkably, upon removal of the FITC-PEG-SWCNTs from the culture medium, the internalized FITC-PEG-SWCNTs rapidly moved out of the nucleus and were released from the cells. Thus, the intracellular PEGylated SWCNTs were highly dynamic and the cell penetration of PEGylated SWCNTs appeared as bidirectional. These observations suggest SWCNTs may be used as an ideal nanovector in biomedical and pharmaceutical applications.

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Quantum dots (QDs), generally referring to semiconductor nanocrystals that display the quantum confinement effect, have been widely pursued for many energy conversion applications. More recently, carbon dots (CDots), which are small carbon nanoparticles with various surface passivation schemes, have been found to possess optical properties and photoinduced redox characteristics resembling those of conventional semiconductor QDs and thus are amenable to some of the same uses in energy conversions. Among the various carbon nanomaterials, fullerenes have been extensively investigated for their use as critical components in optoelectronic devices and systems. Carbon nanoparticles, representing a largely ignored nanoscale carbon allotrope, are in fact more effective in some of the same functions, which are materialized and much enhanced upon the surface passivation of the nanoparticles in CDots. In this perspective article on CDots for energy conversion applications, the optical properties and redox characteristics of CDots, including the related mechanistic framework and its relationship to the use of CDots as potent photocatalysts for the conversion of CO2 into small organic molecules, are highlighted. Also highlighted are results from representative studies using CDots in light-emitting diodes and various solar cells to demonstrate their excellent potential for a wide range of roles in optoelectronic devices and systems. Issues and opportunities in the further development of the CDots platform and related technologies are discussed.