Phenolic Tyrosinase Substrate with a Formal Potential Lower than That of Phenol to Obtain a Sensitive Electrochemical Immunosensor

The high and selective catalytic activities of tyrosinase (Tyr) have frequently led to its application in sensitive biosensors. However, in affinity-based biosensors, the use of Tyr as a catalytic label is less common compared to horseradish peroxidase and alkaline phosphatase owing to the fact that phenolic Tyr substrates have yet to be investigated in detail. Herein, four phenolic compounds that have lower formal potentials than phenol were examined for their applicability as Tyr substrates, and three reducing agents were examined as potential strong reducing agents for electrochemical–chemical (EC) redox cycling involving an electrode, a Tyr product, and a reducing agent. The combination of 4-methoxyphenol (MP) and ammonia-borane (AB) allows for (i) a high electrochemical signal level owing to rapid EC redox cycling and (ii) a low electrochemical background level owing to the slow oxidation of AB at a low applied potential and no reaction between MP and AB. When this combination was applied to an electrochemical immunosensor for parathyroid hormone (PTH) detection, a detection limit of 2 pg/mL was obtained. This detection limit is significantly lower than that obtained when a combination of phenol and AB was employed (300 pg/mL). It was also found that the developed immunosensor works well in PTH detection in clinical serum samples. This new phenolic substrate could therefore pave the way for Tyr to be more commonly used as a catalytic label in affinity-based biosensors.