A simple “turn-on” detection platform for trypsin activity and inhibitor screening based on N-acetyl-l-cysteine capped CdTe Quantum Dots

In this work, we designed a simple, convenient and high sensitive “turn-on” fluorescence platform for trypsin assay based on N -acetyl- l -cysteine capped CdTe Quantum Dots (NAC-CdTe QDs). A series of NAC-CdTe QDs with different sizes were synthesized in aqueous via refluxing routes. The fluorescence of NAC-CdTe QDs could be effectively quenched by hemoglobin (Hb) via the electron transfer interactions from NAC-CdTe QDs to hemoglobin due to the electrostatic attraction of QDs/Hb complex. In the presence of trypsin, hemoglobin would be hydrolyzed to small peptides, releasing inactive hemin molecules.Hemin has slight effect on the fluorescent intensity of NAC-CdTe QDs, so the combination of hemoglobin and NAC-CdTe QDs would be dissociated, leading to the fluorescence recovery of NAC-CdTe QDs. Therefore, we could monitor the trypsin activity by utilizing the different fluoresence responses of NAC-CdTe QDs to hemoglobin and hemin. The recovered fluorescence intensity of NAC-CdTe QDs was proportional to the logarithm of trypsin concentration in the range of 0.05–10   mU   mL −1 and the detection limit for trypsin was 0.036   mU   mL −1 . A model for trypsin inhibition was further established to verify the feasibility of the system. The IC 50 value is 3.06   μg   mL −1 for the inhibitor from soybean. Thus, a label free real time fluorometric assay for trypsin and inhibitor screening has been developed. The established method showed a high selectivity and sensitivity for trypsin over other biological relevant enzyme, and it was applied to the determination of trypsin in human urine sample with satisfactory results.