Published: August 25, 2011 r2011 American Chemical Society 18829 dx.doi.org/10.1021/jp206287f | J. Phys. Chem. C 2011, 115, 18829–18834 ARTICLE pubs.acs.org/JPCC Paper-Based Photoconductive Infrared Sensor Alejandro J. Gimenez, † J. M. Y  a~ nez-Lim on, † and Jorge M. Seminario* ,‡ † Centro de Investigaci on y Estudios Avanzados del Instituto Polit  ecnico Nacional Unidad Quer  etaro, Libramiento Norponiente No. 2000, Fracc. Real de Juriquilla CP 76230 Queretaro Qro., M exico ‡ Department of Chemical Engineering, Department of Electrical and Computer Engineering Materials Science and Engineering Program, Texas A&M University, College Station, Texas 77843, United States ABSTRACT: We report sensitivity to infrared radiation from a simple paper-made device, which increases its conductivity when exposed to hot objects. We propose that the conductivity of this device is due to ionic currents involving electrolyte salts dissolved in thin film of liquid dispersed over the paper surface, thus the current increases because of heating caused by absorption of infrared light from hot sources. The fast response to stimulus exposure of this sensor suggests that the heating effect is related to a radiative interaction rather than to another kind of heat transfer such as convection or conduction. 1. INTRODUCTION Many products used commonly in several applications are made of cellulose; this is because cellulose is an abundant, low cost, and environment friendly material. 1 Thus it is a good idea to employ it in several applications. It is been reported recently that paper and cellulose are used in sensing applications; our group reported the fabrication of UV photoconductive sensors mixing paper with ZnO crystals. 2 Other groups are fabricating chemical sensors on paper for medical diagnosis, taking advantage of the hydrophilic properties of cellulose. 3,4 In this work we analyze cellulose-based devices to sense electromagnetic radiation in the mid-infrared range. This is relevant because mid- and far-IR ranges are difficult to detect mainly because its wavelengths are not high enough energetically to generate electron transitions detectable in semiconductor devices and the wavelengths are too short to be processed by antennas and amplifiers as RF signals. However, sensing radiation in the mid- and far-infrared range is important because bodies close to room temperature emit electromagnetic radiation in this range as stated by the Planck’s law on blackbody radiation. 5 One direct use of this kind of sensor would be to detect people, animals, or any type of body having a temperature above the background. Another potential application of devices sensitive to mid-IR is to sense specific molecules by recognition of their IR signatures. The approach normally used to fabricate sensors in the mid- infrared part of the spectrum is to use pyroelectric materials. 6 Pyroelectricity in materials is related to the crystal structure and is present in crystals without a center of symmetry; some studies demonstrate that cellulose as other polymers feature piezoelectric 7 and pyroelectric 8 effects due to their asymmetry. In the case of our device, it is not likely that the cellulose pyroelectricity is the origin of its sensitivity because the pyroelectric effect has been observed only in specially aligned grown films, being sensitive in vacuum conditions; however our device uses paper as it is and works at room conditions. The experiments we report in this work suggest that the electrical conductivity of our IR sensor devices comes from an ionic current involving electrolyte salts dissolved in a thin liquid layer dispersed over the paper. It is well-know that electrolyte solution conductiv- ities are strongly dependent on temperature, 9 so it appears that the sensor paper substrate efficiently absorbs the IR radiation and turns it into heat that is translated to a higher conductivity of the device. Figure 1 shows a schematic representation that explains the con- ductivity and sensitivity observed in paper devices. Received: July 4, 2011 Revised: August 20, 2011