DOI: 10.1002/cphc.200600514 Vertically Aligned Antimony Nanowires as Solid-State pH Sensors Pai-Chun Chang, [a] Hsiang-Yu Chen, [a] Jian-Shan Ye, [b] Fwu-Shan Sheu,* [b] and Jia G. Lu* [a] Antimony (Sb) has a remarkable linear response to the H + con- centration in solution and it is applied for commercial pH elec- trodes. [1, 2] 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 compat- ible to the silicon-based complementary metal oxide semicon- ductor (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 with- stands 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, [3] the ambulatory esophageal pH, [4] and dental plaque pH. [5] In addition, by coating with suitable enzymes, the Sb electrode functions as a potentiometric enzyme electrode that can be used to determine the concen- tration of urea. [6] 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 re- search. [4, 7] The in vivo and in vitro intracellular real time moni- toring of pH, [8] Na + ,K + —a key subject in cell biology and physiology—may require vertically oriented nanoscale electro- des in close contact with cells. Such nanoelectrodes can be im- plemented by vertically grown nanowires and they can be ap- plied to penetrate smoothly and gently into a cell without causing cell apoptosis. Herein, we describe a method for fabri- cation of vertical Sb nanowire electrodes, present their proper- ty characterization and demonstrate their potential application for pH sensing. The synthesis of antimony nanowires has been previously re- ported using different approaches including pulsed electrode- position, [9] self-assembly of Sb nanoparticles, [10] and micro- wave-assisted growth. [11] Herein, antimony nanowires with a high aspect ratio were produced by pulsed electrodeposition in a template of a porous anodic aluminum oxide (AAO) mem- brane (see Experimental Section). [12, 13] The fabrication proce- dures 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 bottom electrode. Figure 1 a depicts a scanning electron micro- scopy (SEM) top-view image of unfilled AAO channels with pore diameters of around 80 nm and an interpore distance of about 240 nm. In the second step as shown in Figure 1 b, the AAO template was submersed into a three-electrode electrolyt- ic cell, with the bottom Ti/Au electrode serving as the working electrode. The SEM image in Figure 1 b demonstrates that the AAO nanochannels are filled with antimony. Subsequently, a wet etching process was carried out to partially remove the AAO template and to expose the Sb nanowire arrays. A thin layer of AAO was intentionally left to serve as an isolation layer at the interface between the liquid analyte and the bottom electrode contact. Figure 1 c shows a schematic diagram of a nanoelectrode array and a SEM image revealing vertically ori- ented Sb nanowires after wet etching. To further control the number of exposed nanowires, photolithography can be utiliz- ed to define the position of the nanowire electrodes, and [a] P.-C. Chang, H.-Y. Chen, Prof. J.G. Lu Department of Electrical Engineering and Department of Physics & Astronomy University of Southern California, Los Angeles, CA 90089 (USA) Fax:(+ 1)213-740-6653 E-mail:jialu@usc.edu [b] Dr. J.-S. Ye, Prof. F.-S. Sheu Department of Biological Sciences National University of Singapore 14 Science Drive 4, Singapore 117543 (Singapore) Fax:(+ 65)6779-2486 E-mail:dbssfs@nus.edu.sg Figure 1. SEM images and corresponding schematics of the fabrication steps. a) The AAO template with a Ti/Au bottom electrode was prepared on top of a glass substrate. The top view SEM image shows the highly ordered AAO template. b) Sb was electrodeposited into the porous channels. c) The vertically aligned Sb nanowire array is obtained after etching of AAO. d) An isolated nanowire electrode can be fabricated. ChemPhysChem 0000,00,1–5 # 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim &1& These are not the final page numbers! ÞÞ