Abstract
A battery-free, wireless optical chemical sensor with integral contactless power and data link is demonstrated. The chemical sensor and its optoelectronic interface form an integral part of a radio-frequency tag which we have developed specifically for use as a wireless chemical and biological sensor, and which is compatible with the International Standards Organisation ISO15693 radio-frequency identification (RFID) protocol. The chemical sensor, optoelectronic interface, RFID processor and antenna are designed and fabricated on a credit-card sized tag which is operated in a passive mode using wireless energy transfer to power the sensor and read its response. The optical chemical sensing capability of the RFID tag is demonstrated by measuring pH using a thin sol–gel film containing a pH-sensitive indicator dye, bromocresol green (BCG), as a model chemical sensor interface. The film exhibits a colourimetric response to pH in the range 5.0–8.5 pH units. The optoelectronic interface of the tag comprises a silicon photodiode and two light emitting diode (LED) sources measuring the optical absorption of the pH-induced colour change of the thin film at two discrete wavelengths. The pH response curve and the p K value of the immobilised indicator were determined using the RFID tag system and compared with the results obtained using a standard laboratory spectrophotometer. A good correlation between the measurements was achieved, demonstrating the viable integration of an optical absorption based chemical sensor onto a passive RFID tag. By combining the relative advantages of simple, tuneable, optical sol–gel sensing chemistries and integrating these onto the RFID wireless communication platform, it is possible to envisage in the near future a range of new wireless sensors for distributed industrial, scientific and medical applications, in particular for smart packaging, personal (wearable) diagnostics, and infrastructure-related applications.
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