ε-Poly-L-lysine-protected TiC MXene quantum dots with high quantum yield for fluorometric determination of cytochrome c and trypsin.

Titanium carbide quantum dots functionalized with ε-poly-L-lysine (PLL) were synthesized by sonication cutting and hydrothermal synthesis. The deprotonated TiC MXene quantum dots (TiC MQDs) exhibit excitation wavelength-dependent blue photoluminescence with typical excitation/emission peaks at 330/415 nm and a quantum yield of 22% due to strong quantum confinement. The fluorescence of ε-poly-L-lysine protected TiC MQDs (PLL-protected TiC MQDs) is reduced via an inner filter effect after the addition of cytochrome c (cyt-c). Response to cyt-c is linear in the 0.2 to 40 μM concentration range and the detection limit is 20.5 nM. In the presence of trypsin, cyt-c is hydrolyzed to small peptides, and the Fe ion in cyt-c probably is reduced to Fe with the aid of the digestive enzyme. This results in the restoration of the blue fluorescence of the modified MQDs. Fluorescence increases linearly in the 0.5 to 80 μg mL trypsin concentration range with the detection limit of 0.1 μg mL. The method was successfully applied to the determination of cyt-c and trypsin in spiked serum samples. Graphical abstractSchematic of a method for the fluorometric "turn-off-on" determination of cytochrome c and trypsin based on ε-poly-L-lysine (PLL) protect MXene quantum dots (TiC MQDs).