Simple and fast Ag deposition method using a redox enzyme label and quinone substrate for the sensitive electrochemical detection of thyroid-stimulating hormone

Enzyme-induced seedless Ag deposition is useful for selective Ag deposition and subsequent electrochemical Ag oxidation; however, a washing step is required after the deposition and before the electrochemical oxidation as the enzyme substrate can be oxidized during the electrochemical oxidation. Here, we report a fast Ag deposition method using a redox enzyme and quinone substrate that does not require a washing step. We found that the quinone substrate is reduced by a redox enzyme label, which is later oxidized to its original form via the reduction of Ag+ to Ag. Moreover, the quinone substrate is not electrochemically oxidized during the electrochemical Ag oxidation. We selected one diaphorase and 1,4-naphthoquinone from among seven redox enzymes (four diaphorases and three glucose-oxidizing enzymes) and six quinones, respectively. We applied this Ag deposition method for the detection of thyroid-stimulating hormone (TSH) over a dynamic range from 100 fg/mL to 100 ng/mL and found that TSH could be detected at concentrations as low as approximately 100 fg/mL in artificial serum. Therefore, the Ag deposition strategy developed in this study exhibits promising potential for ultrasensitive clinical applications.