Charge transfer complex-forming dyes incorporated in solid polymer electrolyte for optical humidity sensing Prakash R. Somani a,* , A.K. Viswanath a , R.C. Aiyer b , S. Radhakrishnan c a Photonics and Advanced Materials Laboratory, Centre for Materials for Electronics Technology C-MET), Panchawati, Off Pashan Road, Pune 411008, India b Physics Department, University of Poona, Pune 411007, India c National Chemical Laboratory, Pashan Road, Pune 411008, India Received 21 March 2001; received in revised form 26 June 2001; accepted 10 July 2001 Abstract Our experimental observations shows that certain dyes when incorporated in solid polymer electrolytes SPE) can serve as an excellent candidatesforopticalhumiditysensing.CrystalVioletCV)andMethyleneBlueMB)areincorporatedinPVA/H 3 PO 4 ,whichisknownto be a SPE and a good proton conductor. Films of PVA/H 3 PO 4 containing dyes are studied for use in optical humidity sensing and the interesting results are discussed in this paper. A mechanism is proposed for the changes observed in the optical properties of such ®lms by the variation of humidity. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Solid polymer electrolyte SPE); Polyvinyl alcohol) PVA); Crystal Violet CV); Methylene Blue MB); Relative humidity RH); Optical humidity sensing 1. Introduction Optodes optical electrodes) are based on analyte-depen- dent changes of optical properties such as absorbance, re¯ectance or ¯uorescence of their receptor part. Optical ®ber chemical sensors have attracted increasing interest in the recent years owing to their inherent characteristics such as immunity to electrical noise, ease of miniaturization and the possibility of real time monitoring and remote sensing. They have many advantages in comparison with electrical sensors. They have fast response time as compared to electrical sensors and do not need electrical contacts that could be damaged by the sensed substance. For the same reason, they are much safer in case of ¯ammable gas or vapors. These features make optodes a powerful tool for environmental and industrial process monitoring. The relative humidity RH) of air is an important para- meter which needs to be measured or monitored in various chemical processes, such as in the food, electronics, textile, chemicals and pharmaceutical industries and in the ambient environment. In these respects, different humidity sensors basedonmeasurementofelectricalandopticalpropertiesare described in the literature. Conductometric humidity sensor based on polyvinyl alcohol) PVA) and graphitized carbon black is studied by Barkauskas [1]. Humidity sensors based on organic polymeric ®lms are also constructed and studied [2]. Perez and Freyre employed the change in relative permittivity of polyethylene terephthalate) to measure RH [2]. Humidity sensors based on ceramic materials are also described frequently [3,4]. Inorganic substances are rarely appliedfortheconstructionofopticalchemo-sensitivemem- branes. The application of CoCl 2 ®lms for optical sensing of humidity can serve as an example [5±7]. The change in color from blue to pink that accompanies the hydration of cobalt chloride is the analytical signal of such optodes. It is well established that both organic and inorganic polymersconstituteimportantcomponentofopticalsensors. A variety of polymers are being used in optical sensors which include silicones, polyvinyl chloride) PVC), poly- tetra¯uoro ethylene) PTFE), na®on, nylon, agarose, sol± gels,etc.[8].Ingeneral,polymersprovidegoodmechanical properties to the sensor design. The chemical sensing and signalprocessingcapabilityofthechemo-opticalinterfaceis generally achieved by the incorporation of a reagent. The reagent layer usually consist of a suitable dye which rever- siblyreactwiththeanalytecontendedinthesample.Selec- tivecomplex-formingindicatordyeschromoionophores)or acid±base indicator dyes are frequently used. Otsuki et al. Sensors and Actuators B 80 2001) 141±148 * Corresponding author. Tel.: 91-20-589-9273/8390/8141; fax: 91-20-589-8180/8085. E-mail address: psomani_cmetp@yahoo.com P.R. Somani). 0925-4005/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII:S0925-400501)00907-8