Tyndall-effect-amplified colorimetric detection of trypsin based on the analyte-regulated formation of Au nanoparticle network with no visual change in surface plasmon resonance response

The classical colorimetric assays with Au nanoparticles (AuNPs) are not suitable for analytical systems where the analyte could not induce a red-to-blue color change related to their surface plasmon resonance (SPR). To respond this challenge, herein this study utilizes Tyndall effect (TE) of AuNPs as amplified colorimetric signaling to design a universal visual assay for sensitive detection of trypsin (model analyte). The AuNPs are modified with biotinylated peptides. In the absence of trypsin, streptavidin binds biotin on these probes to form a network-like complex consisting of repeated units of “AuNP-biotin-streptavidin-biotin-AuNP” resulting in a mixture that shows an intense TE response as well a SPR-related red color. Upon trypsin incubation, however, the analyte can cleave the peptide chain to inhibit the streptavidin-mediated production of network-like complex; the resultant red reaction solution containing free AuNP probes would display a weak TE signal. The reduction degree in the TE positively relies on the trypsin level in sample. The results demonstrate that this new equipment-free method only requires a laser pointer pen (for the TE creation) and a smartphone (for portable TE imaging readout) to linearly detect trypsin in a concentration range of 3.9–1000 nM, with an estimated limit of ∼0.63 nM (3σ). The satisfactory recovery of trypsin in actual human serum samples is between 94.93–102.21 %. Such TE-enhanced strategy may significantly expand theoretical scope of widely used AuNP-based visual assays and their applications in various resources-limited settings like point-of-care testing of trypsin for medical diagnosis.