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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.

Carbon dots, generally defined as small carbon nanoparticles with various surface passivation schemes, have emerged as a new class of quantum-dot-like nanomaterials, with their optical properties and photocatalytic functions resembling those typically found in conventional nanoscale semiconductors. In this work, carbon dots were evaluated for their photoinduced bactericidal functions, with the results suggesting that the dots were highly effective in bacteria-killing with visible-light illumination. In fact, the inhibition effect could be observed even simply under ambient room lighting conditions. Mechanistic implications of the results are discussed and so are opportunities in the further development of carbon dots into a new class of effective visible/natural light-responsible bactericidal agents for a variety of bacteria control applications.

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Carbon “quantum” dots (CDots), generally defined as small carbon nanoparticles with various surface passivation schemes, have emerged to represent a rapidly advancing and expanding research field. CDots are known for their bright and colorful fluorescence emissions, where the colorfulness is associated with the emissions being excitation wavelength dependent. In this work, CDots with 2,2′-(ethylenedioxy)­bis­(ethylamine) (EDA) for surface functionalization were studied systematically by using steady-state and time-resolved fluorescence methods. The observed fluorescence quantum yields are strongly excitation wavelength dependent, and the dependence apparently tracks closely the observed absorption profile of the EDA-CDots, whereas the excitation wavelength dependence of observed fluorescence lifetimes is much weaker, obviously decoupled from the quantum yields. Mechanistically, the presence of two sequential processes immediately following the photoexcitation of CDots leading to fluorescence is used to rationalize these effects, and the experimental results seem better explained by attributing one of the two processes to be primarily responsible for the characteristic excitation wavelength dependence. Significant implications of the mechanistic probing to the understanding of CDots as a new class of quantum dot-like fluorescent nanomaterials are discussed, and so are further challenges and opportunities.