Equipment-free determination of trypsin and its inhibitor based on the distance-readout via viscosity change on paper substrates

Herein, a simple, convenient, and cost-effective distance-readout paper-based sensor is developed for the detection of trypsin. Trypsin hydrolyzes the gelatin hydrogel via the gel-to-sol transition, resulting in a change of viscosity and leading to a change in diffusion behavior. The concentration of trypsin is thus quantitatively determined by measuring the difference in diffusion diameter of the spot on the paper. The linear range for trypsin detection is from 0.01 to 10.0 mg/mL with a limit of detection of 0.0019 mg/mL. It is further applied to the quantitative detection of trypsin in human serum and the standard inhibitor aprotinin. In addition, the inhibitors screening in 10 traditional Chinese medicines has been successfully achieved. It only requires the use of the gelatin hydrogel, MCE filter paper, and a Vernier caliper. Therefore, this convenient method is very promising in point-of-care testing (POCT) applications and the development of commercial products.