Concentration-dependent color tunability of nitrogen-doped carbon dots and their application for iron(III) detection and multicolor bioimaging.

Nitrogen doping can effectly adjust the compositions and structures of carbon dots and hence enhance their fluorescence. In this work, we report a fast and low-cost route for synthesis of nitrogen-doped carbon dots (N-CDs) by microwave pyrolysis of citric acid and ammonium within 7 min. The as-prepared N-CDs contain plentiful oxygen and nitrogen functional groups, and dispaly intense fluorescence with high quantum yield of ca. 44.3% and have an average size of 1.8 nm. The obtained N-CDs exhibit highly stable against photobleaching, ionic strengths, and can be used for selective and sensitive detection of Fe(III). It is postulated that the Fe3+-mediated fluorescence quenching is attributed to the charge transfer between N-CDs and Fe3+. In particular, the emission peaks from blue to red region can be tuned by interparticle distance of N-CDs, simply by increasing the concentration of N-CDs in aqueous solution, which indicates its potential applications as a promising optical image probe in multicolor cellular imaging.