ε-Poly-L-lysine-protected Ti3C2 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 Ti3C2 MXene quantum dots (Ti3C2 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 Ti3C2 MQDs (PLL-protected Ti3C2 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 Fe3+ ion in cyt-c probably is reduced to Fe2+ 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-1 trypsin concentration range with the detection limit of 0.1 µg mL-1. 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 (Ti3C2 MQDs).