Vertically Aligned Antimony Nanowires as Solid‐State pH Sensors

Abstract

Antimony (Sb) has a remarkable linear response to the H concentration in solution and it is applied for commercial pH electrodes. Unlike conventional glass tube-based pH electrode, the solid-state Sb electrode represents a good candidate for an integrated pH sensor with a fabrication process that is compatible to the silicon-based complementary metal oxide semiconductor (CMOS) process. In addition, due to its high resistance to corrosion, the Sb-based pH electrode has been used in a wide range of applications. For instance, the Sb electrode withstands hydrofluoric (HF) acid and allows reliable determination of the proton concentration in HF-containing solution in which a glass electrode normally cannot be used. Moreover, the Sb electrode has demonstrated effectiveness in clinical operations. It has been utilized to determine the intramyocardial pH during open-heart surgery, the ambulatory esophageal pH, and dental plaque pH. In addition, by coating with suitable enzymes, the Sb electrode functions as a potentiometric enzyme electrode that can be used to determine the concentration of urea. Because of its versatile sensing applications, the Sb probe with a size scale that is compatible to cells has great potential for meeting the demands of biomedical research. The in vivo and in vitro intracellular real time monitoring of pH, Na , K—a key subject in cell biology and physiology—may require vertically oriented nanoscale electrodes in close contact with cells. Such nanoelectrodes can be implemented by vertically grown nanowires and they can be applied to penetrate smoothly and gently into a cell without causing cell apoptosis. Herein, we describe a method for fabrication of vertical Sb nanowire electrodes, present their property characterization and demonstrate their potential application for pH sensing. The synthesis of antimony nanowires has been previously reported using different approaches including pulsed electrodeposition, self-assembly of Sb nanoparticles, and microwave-assisted growth. Herein, antimony nanowires with a high aspect ratio were produced by pulsed electrodeposition in a template of a porous anodic aluminum oxide (AAO) membrane (see Experimental Section). The fabrication procedures of vertically oriented Sb nanowire arrays are illustrated in Figure 1. In the first step, a bilayer metal thin film, Ti/Au (20/ 300 nm), was patterned onto the AAO membrane as the