DOI: 10.1002/cphc.200600514Vertically Aligned AntimonyNanowires 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 anintegrated 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 resistanceto corrosion, the Sb-based pH electrode has been used in awide range of applications. For instance, the Sb electrode with-stands hydrofluoric (HF) acid and allows reliable determinationof the proton concentration in HF-containing solution in whicha glass electrode normally cannot be used. Moreover, the Sbelectrode has demonstrated effectiveness in clinical operations.It has been utilized to determine the intramyocardial pHduring open-heart surgery,[3]the ambulatory esophageal pH,[4]and dental plaque pH.[5]In addition, by coating with suitableenzymes, the Sb electrode functions as a potentiometricenzyme 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 hasgreat 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 andphysiology—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 withoutcausing cell apoptosis. Herein, we describe a method for fabri-cation of vertical Sb nanowire electrodes, present their proper-ty characterization and demonstrate their potential applicationfor 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 ahigh aspect ratio were produced by pulsed electrodepositionin 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 inFigure 1. In the first step, a bilayer metal thin film, Ti/Au (20/300 nm), was patterned onto the AAO membrane as thebottom electrode. Figure 1a depicts a scanning electron micro-scopy (SEM) top-view image of unfilled AAO channels withpore diameters of around 80 nm and an interpore distance ofabout 240 nm. In the second step as shown in Figure 1b, theAAO template was submersed into a three-electrode electrolyt-ic cell, with the bottom Ti/Au electrode serving as the workingelectrode. The SEM image in Figure 1b demonstrates that theAAO nanochannels are filled with antimony. Subsequently, awet etching process was carried out to partially remove theAAO template and to expose the Sb nanowire arrays. A thinlayer of AAO was intentionally left to serve as an isolation layerat the interface between the liquid analyte and the bottomelectrode contact. Figure 1c shows a schematic diagram of ananoelectrode array and a SEM image revealing vertically ori-ented Sb nanowires after wet etching. To further control thenumber 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. LuDepartment of Electrical Engineering andDepartment of Physics & AstronomyUniversity of Southern California, Los Angeles, CA 90089 (USA)Fax: (+ 1) 213-740-6653E-mail: [email protected][b] Dr. J.-S. Ye, Prof. F.-S. SheuDepartment of Biological SciencesNational University of Singapore14 Science Drive 4, Singapore 117543 (Singapore)Fax: (+ 65) 6779-2486E-mail: [email protected] 1. SEM images and corresponding schematics of the fabricationsteps. a) The AAO template with a Ti/Au bottom electrode was prepared ontop of a glass substrate. The top view SEM image shows the highly orderedAAO template. b) Sb was electrodeposited into the porous channels. c) Thevertically aligned Sb nanowire array is obtained after etching of AAO. d) Anisolated nanowire electrode can be fabricated.ChemPhysChem 0000, 00, 1 – 5 B 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim&1&These are not the final page numbers!ÞÞtherefore to control the electrode density. Figure 1d illustratesan example of achieving a single vertically oriented Sb nano-wire electrode.Transmission electron microscopy (TEM) and energy disper-sive X-ray spectroscopy (EDXS) analyses were performed tocharacterize the as-synthesized Sb nanowires. A cross-sectionalview of the AAO template and the Sb metal electrodepositedin the nanochannels is displayed in Figure 2a. It is clearlyshown that the Sb nanowires are confined by AAO channels.The EDX spectrum in Figure 2b reveals the elemental composi-tion showing distinct Al, O and Sb peaks. For structure charac-terization, randomly distributed Sb nanowires (Figure 2c) areobtained after complete dissolution of AAO. They are found tobe polycrystalline and the most frequently observed crystalplane is (012) with an interplanar distance of around 0.31 nm(Figure 2d). The morphology of a single Sb nanowire illustrat-ed in Figure 2e demonstrates its high aspect ratio structureand a diameter of around 80 nm. The corresponding EDX spec-trum (Figure 2 f) confirms the high purity of the Sb nanowire.The acid and base properties of electrolytes in living cellsplay an important role in any biological process, as the pHvalue is the most critical parameter in chemical and biochemi-cal reactions. Conventionally, pH meters use glass tube-basedpH electrodes for measuring the H+concentration in testedsamples containing a sufficient amount of aqueous electrolyte.Those glass electrodes are inappropriate for use in miniatur-ized and real-time monitoring devices. With the increasingdemand of portable biomedical sensing devices, it is crucial tofind a solution to make nano/biointegrated sensors for precisepH determination. In the following, we demonstrate that theSb nanowire functions as a reliable solid-state pH sensor.Before performing pH measurements, a set of standard solu-tions was prepared with a