A Gradient Microarray Electronic Nose Based on Percolating SnO 2 Nanowire Sensing Elements Victor V. Sysoev,* ,² Joachim Goschnick,* ,‡ Thomas Schneider, ‡ Evghenii Strelcov, § and Andrei Kolmakov* ,§ Department of Physics, SaratoV State Technical UniVersity, SaratoV 410054, Russia, Forschungszentrum Karlsruhe, Hermann-Von-Helmholtz-Platz 1, Karlsruhe 76021, Germany, and Southern Illinois UniVersity, Carbondale, Illinois 62901-4401 Received July 25, 2007; Revised Manuscript Received September 3, 2007 ABSTRACT Fabrication, characterization, and tests of the practical gradient microarray electronic nose with SnO 2 nanowire gas-sensing elements are reported. This novel device has demonstrated an excellent performance as a gas sensor and e-nose system capable of promptly detecting and reliably discriminating between several reducing gases in air at a ppb level of concentration. It has been found that, in addition to the temperature gradient across the nanowire layer, the density and morphological inhomogeneities of nanowire mats define the discriminating power of the electronic nose. A key requirement in the development of gas-analytical sensor systems for industrial, medical, security, and even domestic applications is their ability to promptly and reliably “detect” and “recognize” a broad range of gases, often in low concentrations, 1 while working in a continuous mode. It is well-known that materials or chemical processes release characteristic complex gas ensembles (called odors if per- ceptible by the human nose) that can be used like a fingerprint for condition monitoring, an indispensable task of future intelligent systems and therefore an urgently needed key technology for tomorrow. A biology inspired but challenging approach to design appropriate sensor systems mimics the data acquisition principles of mammalian olfac- tory systems, 2,3 which allows one to discriminate single gases as well as odorlike gas ensembles by creating and processing a multidimensional pattern of many signals generated by a receptor (sensor) array. 4-7 Devices employing this pattern recognition concept are frequently called “electronic noses” (see terms, history, and references in the comprehensive monograph ref 8). Recent developments in micro- and nanotechnologies 9-30 have made available new material platforms, device fabrica- tion alternatives, and novel sensing concepts to improve sensitivity, reliability, energy consumption, and response time of the next generation sensors. In particular, the recently fabricated quasi-one-dimensional (1D) metal oxide nanostructures 14-16 have been found to be outstanding for these applications because a multitude of sensing properties are substantially improved compared to compact metal oxide gas detecting elements. Namely, high surface-to-bulk ratio allows very sensitive transduction of the gas/surface interactions (adsorption or catalytic oxidation) into a change of the electrical conductivity. 17-25 The radius of these nanostructures approaches the material’s Debye length, which makes nearly the whole nanowire a depletion (accumulation) zone of mobile charge carriers in response to surface redox process 26 and thus establishes an extreme sensitivity of the electron (hole) transport to charge transfer interactions of gas molecules at the surface. In addition, their ability to accept a variety of morphologies 14,27,28 and struc- tures in conjunction with their surface and bulk doping 29 offers wide possibilities to tune the gas-sensing properties. 30,31 An advantage over nanostructured oxide films, which hamper the gas diffusion with reduction of the grain size, the nanowire layers render the empty space between the nano- fibers no matter how small the diameter of the individual nanowires is. As a result, these structures are currently considered to be a prospective platform for the next genera- tion of electronic (nano)-noses. 30 However, despite encouraging demonstrations of an array of individual metal oxide nanowires, 32 there still exists a technological gap between the laboratory demonstrations and a practical e-nose microdevice suitable for up-to-date large- scale microfabrication and capable of operating in real-world * Corresponding authors. E-mail: vsysoev@sstu.ru (V.V.S.); Achim.Goschnick@imt.fzk.de (J.G.); akolmakov@physics.siu.edu (A.K.). ² Saratov State Technical University. ‡ Forschungszentrum Karlsruhe. § Southern Illinois University. NANO LETTERS 2007 Vol. 7, No. 10 3182-3188 10.1021/nl071815+ CCC: $37.00 © 2007 American Chemical Society Published on Web 10/10/2007