Ultrasensitive Detection of Parathyroid Hormone through Fast Silver Deposition Induced by Enzymatic Nitroso Reduction and Redox Cycling

Enzymatically induced silver deposition and subsequent electrochemical oxidation have been widely used in electrochemical biosensors. However, this method is ineffective for producing highly enhanced silver deposition for use in ultrasensitive detection. Herein, we report a fast silver deposition method that simultaneously uses three signal amplification processes: (i) enzymatic amplification, (ii) chemical-chemical (CC) redox cycling, and (iii) chemical-enzymatic (CN) redox cycling. DT-diaphorase (DT-D) is used for enzymatic amplification to convert a nitroso compound, a species incapable of directly reducing Ag+ to an amine compound, which can directly reduce Ag+. NADH acts as a reducing agent for the indirect reduction of Ag+ via the two redox cycling processes. 4-Nitroso-1-naphthol is converted to 4-amino-1-naphthol (NH2-N) in the presence of DT-D. NH2-N initiates two redox cycling processes: NH2-N, along with Ag+ and NADH, are involved in the CC redox cycling, whereas NH2-N, along with Ag+, DT-D, and NADH, are involved in the CN redox cycling. Finally, the deposited silver is electrochemically oxidized to produce a signal. When this triple signal amplification strategy for fast silver deposition is applied to an electrochemical immunosensor for detecting parathyroid hormone (PTH), a detection limit as low as, similar to 100 fg/mL is obtained. The concentrations of PTH in clinical serum determined using the developed immunosensor are found to agree with those measured using a commercial instrument. Thus, the use of this strategy for fast silver deposition is highly promising for ultrasensitive electrochemical detection and biosensing applications.