Detection of Sub-Nanomolar Concentration of Trypsin by Thickness-Shear Mode Acoustic Biosensor and Spectrophotometry

The determination of protease activity is very important for disease diagnosis, drug development, and quality and safety assurance for dairy products. Therefore, the development of low-cost and sensitive methods for assessing protease activity is crucial. We report two approaches for monitoring protease activity: in a volume and at surface, via colorimetric and acoustic wave-based biosensors operated in the thickness-shear mode (TSM), respectively. The TSM sensor was based on a beta-casein substrate immobilized on a piezoelectric quartz crystal transducer. After an enzymatic reaction with trypsin, it cleaved the surface-bound beta-casein, which increased the resonant frequency of the crystal. The limit of detection (LOD) was 0.48 +/- 0.08 nM. A label-free colorimetric assay for trypsin detection has also been performed using beta-casein and 6-mercaptohexanol (MCH) functionalized gold nanoparticles (AuNPs/MCH-beta-casein). Due to the trypsin cleavage of beta-casein, the gold nanoparticles lost shelter, and MCH increased the attractive force between the modified AuNPs. Consequently, AuNPs aggregated, and the red shift of the absorption spectra was observed. Spectrophotometric assay enabled an LOD of 0.42 +/- 0.03 nM. The Michaelis-Menten constant, K-M, for reverse enzyme reaction has also been estimated by both methods. This value for the colorimetric assay (0.56 +/- 0.10 nM) is lower in comparison with those for the TSM sensor (0.92 +/- 0.44 nM). This is likely due to the better access of the trypsin to the beta-casein substrate at the surface of AuNPs in comparison with those at the TSM transducer.

Automated summary

The study presents two methods for monitoring protease activity: one using a thickness-shear mode (TSM) sensor and the other utilizing a colorimetric assay with gold nanoparticles. Both methods demonstrate sensitivity and accuracy in detecting protease activity.