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Passive chemiresistor sensors with integral wireless power and data links are in development, and their potential applicability for the detection of organic vapors is demonstrated. The chemiresistors comprise a planar gold electrode array onto which chemically sensitive thin films are cast using micro-fabrication techniques. The thin film sensors are interfaced to a proprietary radio-frequency smartcard designed for use as a distributed chemical or biological detector, and which is compatible with the ISO15693 radio-frequency identification (RFID) wireless protocol. The measurement of thin film sensor resistance is performed by a four-contact method, and is controlled with on-card electronics. Thin polymer films based on the organic semiconductors 3, 4-ethylenedioxythiophene-poly(styrenesulfonate) (PEDOT-PSS) and polyaniline (PANI) have been used as chemically sensitive thin films to demonstrate function of this platform for detection of gaseous analytes. Potential applications of smartcard chemiresistors include mobile and wearable sensors for chemical exposure and contamination monitoring and smart personal diagnostics. The standardization of short-range wireless radio protocols such as Bluetooth, RuBee, ZigBee, WiFi and RFID is creating new markets for distributed sensors and sensor networks, and the fusion of chemical and biosensor technologies with short-range, low-cost, wireless technologies will create new opportunities for chemical and biological sensor systems in healthcare, environmental monitoring, process and quality control, and chemical and biological threat detection [1]. We are developing high-value applications beyond typical cool-chain temperature tracking by designing novel sensors onto custom RFID labels compatible with the ISO15693 smartcard protocol. The RFID protocol is particularly suitable for short-range contactless sensor applications, since it supports both passive (batteryless) and semi-passive (powered) devices and has an anti-collision protocol that supports reading of up to 255 sensors in a single scan. This work is concerned with developing and integrating conductometric chemiresistor sensors onto a short-range wireless platform with which to address a subset of these emerging applications. RFID-based sensor chips are now commercially available for temperature logging [4], and investigational chemical devices have been reported for both gases [5] and vapors [6]. We have developed a custom RFID processor and antenna onto a credit-card sized device [fig.1] which includes a conductometric sensor interface [fig.2]. The smartcard interfaces with a low-cost disposable part on which the chemiresistor is fabricated [fig.3]. First results demonstrate a reversible response of the sensors on exposure to ethanol vapour [fig.4]. The smartcard chemiresistors described here could in future be suitable for use in distributed wireless data applications and chemical sensor networks where organic vapours are to be detected.

Conductometric sensors based on an organic semiconductor, poly(3, 4-ethylenedioxythiophene)/poly(styrenesulfonate), PEDOT-PSS, have been fabricated and electrically characterised. The sensors consist of planar gold electrodes on glass-fibre (FR4) substrates, coated with thin films of PEDOT-PSS cast using a spin coating technique. PEDOT-PSS was chosen as a model thin film sensor material in order to characterise four different electrode configurations. The thin film conductometric sensors have been investigated and characterised using both DC resistance and AC impedance measurement techniques. This work forms part of a wider project to develop different interfaces for wireless chemical sensors based on radio frequency tags. An electronic tag developed for use with conductometric sensors has been tested with the gold/PEDOT-PSS sensors, and its measurement performance evaluated and calibrated against a laboratory standard 4-point probe instrument. The tag is compatible with the International Standards Organisation ISO 15693 radio-frequency identification (RFID) wireless protocol. Conductometric measurements are controlled and sampled by the tags electronic circuits, and data acquired from the sensor can be sent over a wireless link to a reader. The results presented include the optimisation of the PEDOT-PSS cocktail, the spin conditions needed for fabrication of the conducting polymer thin films on gold electrodes, ageing effects of the films, characteristic DC resistance and AC impedance spectroscopy data, a comparison of 2-point and 4-point DC resistance measurement methods, and the calibration of the wireless tag. Finally, an example of using a conductometric sensor as a detector for ethanol vapour is given. The integration of chemical sensors with wireless technologies has many applications in healthcare, environmental monitoring, process and quality control, and chemical and biological threat detection. In the future, wireless tags with integrated conductometric sensors could be used as wearable sensors for chemical and biological exposure and contamination monitoring, smart personal diagnostics, or for domestic and industrial applications such as in-building environment monitoring.

The integration of chemical and biosensor technologies with wireless devices has application in healthcare, personal care, environmental monitoring, process and quality control, and chemical and biological threat detection. Our present work is aimed at developing and integrating conductometric type sensors onto a wireless platform that could be used for distributed sensing applications. We have developed CNTag, a wireless chemical sensor platform based on the ISO standard Radio-Frequency IDentification (RFID) protocol. It is the combination of a conductometric transducer with an RFID processor into a credit-card sized device [fig. 1]. CNTag could be deployed as a stand-alone chemical sensor, or as a node in a wireless sensor network (WSN). Figure 1. A conductometric transducer and radio-frequency identification (RFID) processor on a credit-card sized tag, known as CNTag Thin films of the organic semiconducting polymer, Poly(3, 4-ethylenedioxythiophene) poly(styrenesulfonate), or PEDOT-PSS, were coated by spin-coating onto gold electrode structures on planar substrates to produce various different conductometric transducer discs [fig. 2]. PEDOT/PSS is a p-type organic semiconductor that can be reversibly doped and de-doped by the addition or removal of cations. The change in cation doping level modulates the conductance of the polymer thin film over several orders of magnitude, and this effect may be exploited to make highly sensitive chemical sensors.

The standardisation of short-range wireless radio protocols such as Bluetooth, ZigBee, WiFi and radio frequency identification (RFID) is opening new markets for distributed sensors and sensor networks. The fusion of chemical and biosensor technologies with these various short-range, low-cost, wireless technologies is creating new opportunities for ubiquitous chemical and biological sensor systems in healthcare, environmental monitoring, process and quality control, and chemical and biological threat detection. In this work we present a radio frequency (RF) sensor tag for use as a distributed smart chemical and biological detector, based on the RFID platform. The tag has a planar 4-point gold electrode (conductometric) interface onto which a chemically or biologically sensitive film is immobilised using planar fabrication techniques. The conductometric measurement is controlled and sampled by on-tag electronics, which then communicates the (bio)chemical information wirelessly over the RF link. The low-cost sensor tag is suitable for use in distributed, ubiquitous and pervasive wireless data applications and sensor networks. We envisage a diverse range of deployment scenarios for the sensor tag, including short-term mobile applications, such as wearable sensors for chemical and biological exposure and contamination monitoring and smart personal diagnostics, to longer-term in-situ applications such as in-building environment monitoring and structural integrity maintenance.