An optical pH sensor based on excitation energy transfer in Na®on 1 ®lm Vinita Misra, H. Mishra * , H.C. Joshi 1 , T.C. Pant Photophysics Laboratory, DSB Campus, Kumaun University, Nainital 263002, India Received 25 January 2001; received in revised form 9 September 2001; accepted 31 October 2001 Abstract Excitation energy transfer, between a dye pair acri¯avine and rhodamine 6G) doped in an ion exchange polymer matrix Na®on 1 ), has been investigated from the viewpoint of fabrication of an optical pH sensor. The study is done by steady state as well as time domain ¯uorescence measurements. Using ®xed donor±acceptor concentration, excitation energy transfer in the pH range 2±12 has been found to follow Fo Èrster function for dipole±dipole interaction. Further, pH has been calibrated by the energy transfer parameters, e.g. overlap integral O DA , energy transfer ef®ciency Z T , rate of energy transfer K R , critical transfer distance R 0A and reduced concentration g 0A in the range 2±12 and is found to show an approximately linear dependence. These results in Na®on 1 matrix are similar to those found by us in solution [Sens. Actuators: B Chem. 63 2000) 18]. The system appears to be attractive for optical pH sensor with reversibility and sensitivity of 0.01 pH units. # 2002 Published by Elsevier Science B.V. Keywords: Energy transfer; pH Sensor; Na®on 1 ®lm 1. Introduction Recently, ¯uorescence sensors or optrodes have been found to gain considerable interest in the detection of various analytes [1±34]. Optrodes have advantages over conventional devices because of their small size micro sensors can be fabricated), freedom from electrical inter- ference, remote sensing, etc. Basically, an optrode consists of a ¯uorescent molecule embedded in a support-sensing matrix polymer, porous glass, etc.) deposited at the end of a bifurcated optical ®ber to carry the exciting and emitted radiation [2]. In the presence of the analyte, the ¯uorescence properties are affected and hence, the determination of the analyte can be done. For getting good response, the support matrix should be of such a nature that the ¯uorophore comes in contact with the analyte in a very short time. Optical measurement of pH has number of potential applications in analytical, bio-medical and clinical chem- istry [3,4]. Wolfbeis and co-workers [5±12] have investi- gated various types of pH optrodes having different pH ranges, sensitivity and response times in different support matrices, viz. cellulose, polystyrene, sol±gel, etc. using different immobilized dye molecules. Use of a light emitting diode for an optrode, for the range 6±9 pH has also been suggested by them [8]. Excited state proton transfer based pH sensors in the range 1±3 in a cellulose support have also been discussed [9]. Recently, Nguyen et al. [16] have shown optochemical sensing of pH by immobilizing carboxy-¯uor- escein-encapsulating liposomes in sol±gel thin ®lms having sensitivity of 0.01 pH units and range 6±7.5. However, at present most of the reported pH sensors are based on ¯uorescence measurement technique using wavelength ratiometric methods, which are less sensitive to optical attenuation. Such probes display shifts or enhancement in their absorption or emission spectra with pH variation [11± 15] and work at varied range and sensitivity. Intensity based optrodes suffer from the problem of scattering, concentra- tion of the ¯uorophores, inner ®lter effects, photobleaching, etc. Time-resolved studies, however, better suited for sen- sing due to ease of calibration and are usually independent of ¯uorophore concentration, intensity ¯uctuation, photodetec- tor response, photobleaching, etc. Non-radiative excitation energy transfer has already been considered for sensor applications [17±24]. Oxygen and sulfur dioxide sensors based on energy transfer have been investigated by Sharma and Wolfbeis [17,18]. The idea of distance variation with ionic strength has also been used in energy transfer based sensors for hydrogen sul®de, nitrous oxide, pH and alkali metal ions [19]. a-Amylase sensing based on energy transfer between ¯uorescein derivative Sensors and Actuators B 82 2002) 133±141 * Corresponding author. Tel.: 91-5942-37450 O), 31941; fax: 91-5942-35576. E-mail address: hirdyesh@yahoo.com H. Mishra). 1 Present address: Department of Analytical Department of Chemistry and Applied Spectroscopy, Faculty of Science, Vrije Universititeit, De Boelelaan 1083; 1081 HV, Amsterdam, The Netherlands. 0925-4005/02/$ ± see front matter # 2002 Published by Elsevier Science B.V. PII:S0925-400501)00998-4