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

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