Label-free and real-time monitoring of trypsin activity in living cells by quantum-dot-based fluorescent sensors

In this study, a quantum dots (QDs)-based label-free fluorescence assay has been developed for real-time monitoring of intracellular trypsin activity and screening the corresponding inhibitor. Negatively charged CdTe QDs and positively charged cytochrome c (cyt. c) first form a hybrid complex structure through an electrostatic attraction effect. The fluorescence of the QDs is well quenched because of electron transfer from QDs to cyt. c. Then, added trypsin breaks the hybrid complex structure because of the hydrolysis of cyt. c catalyzed by the trypsin. Thus, the electron transfer is switched off and a substantial fluorescence recovery is obtained. Under optimal conditions, the initial rate of the hydrolysis reaction is linearly proportional to the concentration of trypsin between 1.25 and 375 nM, and the detection limit is as low as 0.42 nM. The proposed method is effective, simple and cost-effective, demonstrating great potential for point-of-care diagnosis applications.