Sensors and Actuators B 173 (2012) 547–554 Contents lists available at SciVerse ScienceDirect Sensors and Actuators B: Chemical journa l h o mepage: www.elsevier.com/locate/snb Ratiometric info-chemical communication system based on polymer-coated surface acoustic wave microsensors J. Yang a , Z. Rácz b , J.W. Gardner b,∗ , M. Cole b , H. Chen a a School of Mechanical Engineering, Nanjing University of Science and Technology, No. 200 Xiaolingwei, Nanjing 210094, People’s Republic of China b Microsensors and Bioelectronics Laboratory, School of Engineering, University of Warwick, Coventry CV4 7AL, UK a r t i c l e i n f o Article history: Received 1 March 2012 Received in revised form 8 July 2012 Accepted 10 July 2012 Available online 24 July 2012 Keywords: Chemical microsensors SAW sensor array Binary mixtures Info-chemical communication Ratiometric coding a b s t r a c t A novel ratiometric info-chemical communication system has been developed that is based upon an array of four surface acoustic wave (SAW) resonator microsensors operating at a frequency of 262 MHz and under ambient conditions. The info-chemical mixtures were generated by a micro-evaporator and transported to the SAW sensors inside a flow chamber. Binary mixtures of 3-methylbutan-1-ol and ethyl acetate were used to demonstrate the principle of encoding and decoding different ratios of volatile info-chemicals in this system. The resonant frequencies of the four polymer-coated SAW sensors and the associated reference (uncoated) SAW sensors were used to determine differential responses cor- responding to the different ratiometric mixtures of the info-chemicals. The SAW sensors were spray coated with four different stationary phase polymer compounds, namely polycaprolactone, polyvinyl- carbazole, polystyrene-co-butadiene, and polyethylene-co-vinylacetate. Principal components analysis was performed on both steady-state and dynamic features extracted from the sensor responses. Our results show clear linear separability of the different chemical ratios as distinct clusters in multi-variate space. In conclusion, we believe that this is the first demonstration of the encoding, transmitting, and decoding ratiometric information using a system based upon an array of SAW-based microsensors. This novel info-chemical communication system, based upon fixed ratios, transmits chemical information over distances that would otherwise be impossible using absolute concentrations. © 2012 Elsevier B.V. All rights reserved. 1. Introduction The transmission of information by airborne chemicals is gen- erally referred to as “info-chemical communication” and exists widely in the animalia kingdom. Many species of insects, with their sophisticated olfactory systems, rely on volatiles (e.g. pheromones and plant compounds) as messengers to locate mates, organize communities, avoid dangers and identify food sources. This form of communication using chemicals alone has inspired development of a new technological field [1] for labelling, information transmis- sion and biochemical interfacing. Practical applications envisaged are in automated identification and data capture, product labelling, search and rescue missions, air silent communication, medical diagnosis/treatment, environmental monitoring and source local- ization. In the volatile-mediated forms of communication, volumetric ratios of the chemical components often carry information impor- tant to the animal. For instance, ratiometric encoding takes place either in the exocrine system of insects (via pheromone glands) ∗ Corresponding author. E-mail address: j.w.gardner@warwick.ac.uk (J.W. Gardner). or in host plants (via flowers or fruits) [2], and decoding takes place in the insects’ olfactory and corresponding nervous system [3]. Most insect pheromones consist of multi-component blends of geometric or optical isomers, and single component pheromones are rare [4]. The sex pheromone of the red-banded leafroller moth (Argyrotaenia velutinana) contains at least seven compounds two of which are responsible for a set of behavioural responses. The extent of activity of the male moth depends on the exact ratio of these two compounds, irrespective of the absolute amount of either material presented, and the optimal ratio is similar to the ratio found in the female pheromone gland [5]. Similarly, apple maggots (Rhagoletis pomonella) produce the same behavioural responses to both the synthetics of the pheromone compounds and the natu- ral pheromone extract, but full activity is not elicited for either the synthetics or the natural components when presented alone [6]. The biological inspiration for the work in this paper is a well- studied moth, Spodoptera littoralis, whose behaviour is based on a precisely controlled blend of info-chemicals (e.g. sex pheromones). Females produce and release minute quantities of a specific mix- ture of pheromone components from their pheromone gland into a gentle breeze which can be detected by the extremely sensitive antennal system of the males and processed by their antennal lobe. Based on this biological blue-print, an innovative modular system 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2012.07.043