Red emissive carbon dots obtained from direct calcination of 1,2,4-triaminobenzene for dual-mode pH sensing in living cells

Real-time monitoring of intracellular pH is urgently required for studying cell metabolism and apoptosis. Carbon dot-based fluorescence probes are the most likely candidates to fulfil such an application, as they possess excellent chemical inertness, low cytotoxicity, high resistance to photobleaching, good biocompatibility, and tuneable emission wavelength. Our research program aims to develop tailor-designed red emissive carbon dots (r-CDs) using technology innovations in synthetic strategies. The r-CDs showing responses to pH were prepared through direct calcination of 1,2,4-triaminobenzene as the only carbon source. Measurements of fluorescent emission wavelength and quantum yield were carried out to optimize the carbonized temperature and heating time. The obtained r-CDs can sensitively respond to pH both in colorimetric and fluorometric modes, namely, with the increase of pH from 4.0 to 8.0, the color of the r-CD solution under ambient light changed from red to orange and then to yellow, and the corresponding fluorescence varied from purple to orange then to yellow under UV irradiation, which can be easily observed by the naked eye. Experimental characterization along with density functional theory (DFT) simulation study reveal that the pH response originates from the reversible transformation between azo and quinone structures during the protonation process. This solid-state preparation is not only simple in procedures and ready to scale up production, more significantly the resulting r-CDs exhibit a number of distinct advantages, and have great potential for biological sensing applications.