A simple, effective NADH sensor constructed with phenothiazine via AFM tip-induced oxidative polymerization

In this work, we demonstrate a simple, effective NADH sensor developed with phenothiazine compounds via atomic force microscope (AFM) tip (field)-induced local oxidation (ALO). When voltage pulses were applied with an AFM tip that was stationary above thionine (TH) on an indium-doped tin oxide (ITO) glass square, the monomer transformed into nanodomes immediately on the site. The resulting polymer (denoted poly(TH)) was highly symmetrical in shape and varied indifferently with the pulse width from 0.01 to 1 s, indicating that the field-induced polymerization was fast in kinetics, reaching completion in less than 0.01 s. Water was essential to the formation of poly(TH). However, hot electrons rather than oxyanions were the oxidants responsible for the polymerization. The ALO-induced polymerization showed potential for application in microlithography. In addition, when poly(TH) nanoline was positioned via ALO with a moving tip between a pair of source and drain electrodes prefabricated on ITO, separated by a 200 nm-wide microfluidic channel, the resulting device showed responses to NADH when NADH was injected through the channel. The sensitivity varied with the voltage applied to the drain (relative to the source), reaching the optimum condition near 0.5 V. Under this condition, the lowest detection limit for NADH reached a level around 1 μM. Toluindine blue and methylene blue also showed similar effects with NADH when substituted for TH. This simple device shows that ALO and phenothiazines are a promising approach for constructing NADH sensors.