Surface passivated p-phenylenediamine carbon quantum dots (p-CQDs) as fluorescent turn-on probes for the detection of Li+ and L-arginine by two different mechanisms

Surface modification technique is currently acknowledged as a practical, efficient, and acceptable tool for producing large quantities of high-quality carbon quantum dots (CQDs), however the research on the quenching and recovery mechanism of the fluorescence of CQDs should receive more attention. Herein, the p-phenylenediamine carbon quantum dots (p-CQDs) exhibiting outstanding characteristics has been synthesized from erbium nitrate (Er(NO3)3) ethanol solution. It shows as a highly efficient "on-off-on" fluorescent probe for the detection of Fe3+, Li+ and l-arginine (L-Arg). The fluorescence of p-CQDs can be quenched by the appearance of Fe3+ (p-CQDs-Fe3+), then can be restored by adding either Li+ or L-Arg (p-CQDs-Fe3+-Li+ or p-CQDs-Fe3+-L-Arg). The zeta potentials were measured for the four fluorescent systems p-CQDs, p-CQDs-Fe3+, p-CQDs-Fe3+-Li+ and p-CQDs-Fe3+-L-Arg. The results show that the loss of charges on the surface of CQDs is the key point which can determine this probe to remain "on" or "off" mode. In addition, we found that the mechanism of fluorescence "turn-on" is different in p-CQDs-Fe3+-Li+ and p-CQDs-Fe3+-L-Arg system, the former is trying to open the transmission channel of intramolecular photons, which manifests as fluorescence recovery. The latter may strip Fe3+ from the surface of p-CQDs.