Rsc_cc_c2cc33869f 1..3

Photoluminescent carbon nanodots (CDs) have attracted growing interest very recently. In comparison to fluorescent semiconductor quantum dots (QDs), which are usually composed of heavy metals as the essential elements to achieve high performance fluorescence, the carbon nanodots are advantageous both in their green synthesis and good biocompatibility for biomedical applications. So far, various methods have been explored to prepare the CDs, including laser ablation, electrochemical oxidation, chemical oxidation, thermolysis and microwave assisted methods. Among them, the microwave assisted pyrolysis of proper carbon source(s) is a low-cost, efficient and facile technique for large quantity production of CDs. Compared to other methods in conjunction with or without a sorting process, however, the microwave pyrolysis method still suffers from amain drawback as the resultant CDs often show a relatively low quantum yield (typically, QY o10%). In our previous work, we have demonstrated that the introduction of amine-group enriched molecules, such as 4,7,10-trioxa-1,13-tridecanediamine (TTDDA) and polyethylenimine (PEI), as the passivation agents during the microwave pyrolysis process can enhance the fluorescent performance of the resultant CDs with QY values up to 12.02% and 15.3%, respectively. However, the mechanism of fluorescent enhancement by the amine passivation agent is still unknown. If revealed, further fluorescent enhancement reaching to the levels of practical significance could be possible. Using citric acid (CA), containing carboxyl groups to facilitate the dehydration and carbonization, as the carbon source and 1,2-ethylenediamine (EDA) as the surface passivation agent in the present study, we produced, by microwave pyrolysis, CDs with an unprecedented photoluminescent (PL) QY value of 30.2% (Fig. S1w), which is beyond our expectation and comparable to that of common fluorescent semiconductor quantum dots. To gain a better understanding of the role of amine groups in the fabrication of these highly luminescent carbon nanodots by the microwave pyrolysis method, we also used various other amine molecules, including diethylamine (DEA), triethylamine (TEA) and 1,4-butanediamine (BDA), as the passivation agents and correlated their structures to the PL properties of the resultant CDs (Scheme 1). Herein, we report the use of this newly-developed microwave pyrolysis method for preparing the first CDs with drastically improved photoluminescent performance comparable to common fluorescent semiconductor quantum dots. Furthermore, the mechanism of fluorescent enhancement by the amine passivation agent is discussed while the potential use of the resultant CDs for biomedical imaging is demonstrated by laser scanning confocal microscopy imaging of L929 cells with and without the CD labeling. The UV-Vis spectrum of the as-prepared EDA-CDs given in Fig. 1a shows a peak at around 360 nm and another absorption band at 240 nm attributable to the p–p* transition. A bright blue color PL emission with a peak at around 460 nm was observed upon excitation of the aqueous solution of EDA-CDs at 360 nm (Fig. 1a). Fig. 1b shows a high-resolution transmission