Mechanistic studies on the reversible photophysical properties of carbon nanodots at different pH.

The pH-dependent photoluminescence (PL) behavior of carbon nanodots (C-dots) and its mechanism has been exhaustively studied in this work. The PL and UV-vis absorption spectra are reversible in the pH between 3 and 13. We speculate that two kinds of reactions (fast and slow) occurring at the surface of C-dots may contribute to this pH-dependent PL behavior. When C-dots solutions are switched to acidic conditions, they will quickly self-assembled aggregate into larger particles and surface oxygen-related groups of C-dots would be slowly oxidized at room temperature. Moreover, it should be noted that this is the first direct observation of self-assembled aggregation of C-dots under acidic conditions. In addition, the optimal PL spectra of C-dots blue-shift while their sizes increase, so-called 'inverse PL shift' phenomenon is also observed. Meanwhile, as the solution is adjusted to alkaline conditions, a structural tautomerization of C-dots rapidly takes place and hydrogenation/deoxygenation reaction proceeds in a much slower rate. Furthermore, through distinct decay dynamics as well as the characterizations of C-dots at different pHs, the PL properties are proposed to be mainly related to the surface states of C-dots.