Single tungsten nanowires as pH sensitive electrodes

The electrochemical potentials of tungsten nanowire samples, covered with their own oxide, were measured in dependence of the pH value. The samples were prepared by selective etching of a directionally solidified eutectic NiAl–W alloy. Directional solidification in a Bridgman-type crystal growth furnace yields nanostructured two-phase materials. Electrochemical processing allows selective etching of the phases exposing the nanoscale structures. In this work, pointed samples with a single wire 200 nm in diameter protruding from the tip were produced. Subsequently the tungsten oxide layer on these single nanowires was electrochemically modified to optimize their pH sensing capabilities. The method has a potential for further downsizing since the wire diameter and exposed length can be controlled by the process parameters during solidification and during electrochemical processing. The advantages of these nanowire pH probes along with possible applications such as the pH measurement in ultra small cavities and other small systems of interest such as corrosion pits and biological cells are discussed.