Highly sensitive, label-free colorimetric assay of trypsin using silver nanoparticles.

Herein, we report a simple, sensitive label-free colorimetric assay of trypsin based on silver nanoparticles (AgNPs) aggregation. Generally, a specially designed short peptide chain acts as both the stabilizer of AgNPs and the substrate of trypsin. In the presence of trypsin, the negatively charged part of peptides will be hydrolyzed, leaving the positively charged dipeptide capped on the surface of AgNPs. The electrostatic property alteration then leads to the AgNPs' aggregation in certain salt condition. The solution color may change correspondingly due to the localized surface plasmon resonance, which can be monitored by naked eye and UV-vis spectrophotometry. This novel AgNPs-based colorimetric method for quantitative determination of trypsin has a linear detection range from 2.5 to 200 ng mL(-1) and a rather low detection limit down to 2 ng mL(-1). The determination of trypsin can also be realized in complex biological fluids by the proposed method, demonstrating its great potential utility in the clinical applications in the future.