Sensitive electrochemical immunoassay based on polythionine-Au nanocomposites as enhanced sensing signal for selective detection of biomarker with high isoelectric point

Herein, polythionine-Au nanocomposites (PTh-Au) were prepared through one-pot reaction using HAuCl4 as co-oxidant in thionine solution. The prepared PTh-Au exhibited intrinsic electrochemical activity based on the dual participation of protons and electrons during electrochemical doping/dedoping process. Thus, a label-free electrochemical immunoassay to detect a high isoelectric point (pI) biomarker (using trypsin as a model) was developed based on co-participation of proton and electron in the PTh-Au's response. In a detecting buffer with pH values lower than trypsin's pI (=10.8), positively charged trypsin prevented proton and electron transfer simultaneously to reduce the current response of PTh-Au. The decreasing amplitude of the current can be used to quantify the concentration of trypsin, which can be utilized to determine the health condition of pancreas. The proposed immunoassay exhibited wide linear range, low limit of detection and high specificity in a detecting buffer (pH 5.5). The obtained immunosensor was successfully applied to detect trypsin in human serum accurately. Additionally, this sensing strategy may be used to detect other important biomarkers with relatively high pIs (>7).