Delivered by Publishing Technology to: University of New South Wales IP: 149.171.37.165 On: Mon, 06 Jan 2014 07:42:43 Copyright: American Scientific Publishers RESEARCH ARTICLE Copyright © 2013 American Scientific Publishers All rights reserved Printed in the United States of America Journal of Nanoscience and Nanotechnology Vol. 13, 2885–2891, 2013 Transparent Anodic TiO 2 Nanotube Arrays on Plastic Substrates for Disposable Biosensors and Flexible Electronics Samira Farsinezhad 1 , Arash Mohammadpour 1 , Ashley N. Dalrymple 1 , Jared Geisinger 1 , Piyush Kar 1 , Michael J. Brett 1 2 , and Karthik Shankar 1 ∗ 1 Department of Electrical and Computer Engineering, University of Alberta, Edmonton, AB, T6G 2V4, Canada 2 National Institute for Nanotechnology, National Research Council, 11421 Saskatchewan Drive, Edmonton, AB, T6G 2M9, Canada Exploitation of anodically formed self-organized TiO 2 nanotube arrays in mass-manufactured, dis- posable biosensors, rollable electrochromic displays and flexible large-area solar cells would greatly benefit from integration with transparent and flexible polymeric substrates. Such integration requires the vacuum deposition of a thin film of titanium on the desired substrate, which is then anodized in suitable media to generate TiO 2 nanotube arrays. However the challenges associated with control of Ti film morphology, nanotube array synthesis conditions, and film adhesion and transparency, have necessitated the use of substrate heating during deposition to temperatures of at least 300  C and as high as 500  C to generate highly ordered open-pore nanotube arrays, thus preventing the use of polymeric substrates. We report on a film growth technique that exploits atomic peening to achieve high quality transparent TiO 2 nanotube arrays with lengths up to 5.1 m at room tem- perature on polyimide substrates without the need for substrate heating or substrate biasing or a Kauffman ion source. The superior optical quality and uniformity of the nanotube arrays was evi- denced by the high specular reflectivity and the smooth pattern of periodic interferometric fringes in the transmission spectra of the nanotube arrays, from which the wavelength-dependent effec- tive refractive index was extracted for the air-TiO 2 composite medium. A fluorescent immunoassay biosensor constructed using 5.1 m-long transparent titania nanotube arrays (TTNAs) grown on Kapton substrates detected human cardiac troponin I at a concentration of 0.1 g ml -1 . Keywords: Transparent, Metal Oxide, Anodization, Self-Organized, Thin Film Deposition, Polymer Substrates, Atomic Peening, Troponin Biomarker Assay. 1. INTRODUCTION The three major classes of porous nanomaterials that are curently the subject of intense research activity are: microporous metal-organic frameworks (MOFs), which primarily consist of pore sizes < 2 nm, 1 mesoporous materials with pore-sizes in the range 2–50 nm 2 3 and nanoporous materials which consist of pores 20–200 nm in size. Among nanoporous materials, anodically formed self-organized TiO 2 nanotube arrays have an extremely versatile application spectrum 4–6 ranging from biomedical devices through electrochromic and photovoltaic devices to catalysts, separation membranes and chemical sen- sors, enabled by the following properties: a high surface ∗ Author to whom correspondence should be addressed. area, ordered pore architecture, n-type semiconductive behavior when crystallized, well-defined percolation path- ways for charge carriers and tunability of the diameter, 7 8 tube-length, wall-thickness and effective refractive index over a wide size range, and high refractive index. Specific applications for titania nanotube arrays include stem-cell differentiators, 9 10 drug-eluting osteogenic impants, 11 ultra- sensitive immunoassays, 12 13 amperometric sensors, 14–16 biofiltration membranes, 17 electrochromic displays, 18 19 oxidative photocatalysts for the degradation of organic contaminants, 20 reductive photocatalysts for conversion of CO 2 into hydrocarbon fuels, 21 22 photoanodes for sunlight- driven water splitting, 23 24 supercapacitor electrodes in energy storage devices 25 and electron-collection scaffolds for dye-sensitized and ordered bulk heterojunction solar cells. 26–28 Several of the above mentioned applications rely J. Nanosci. Nanotechnol. 2013, Vol. 13, No. 4 1533-4880/2013/13/2885/007 doi:10.1166/jnn.2013.7409 2885