Analytica Chimica Acta 553 (2005) 169–176 Highly selective single-use fluoride ion optical sensor based on aluminum(III)-salen complex in thin polymeric film Ibrahim H.A. Badr 1 , Mark E. Meyerhoff ∗ University of Michigan, Department of Chemistry, 930N. University, Ann Arbor, MI 48105-1055, USA Received 26 April 2005; received in revised form 4 August 2005; accepted 5 August 2005 Available online 19 September 2005 Abstract A highly selective optical sensor for fluoride ion based on the use of an aluminum(III)-salen complex as an ionophore within a thin polymeric film is described. The sensor is prepared by embedding the aluminum(III)-salen ionophore and a suitable lipophilic pH-sensitive indicator (ETH-7075) in a plasticized poly(vinyl chloride) (PVC) film. Optical response to fluoride occurs due to fluoride extraction into the polymer via formation of a strong complex with the aluminum(III)-salen species. Co-extraction of protons occurs simultaneously, with protonation of the indicator dye yielding the optical response at 529 nm. Films prepared using dioctylsebacate (DOS) are shown to exhibit better response (e.g., linear range, detection limit, and optical signal stability) compared to those prepared using ortho-nitrophenyloctyl ether (o-NPOE). Films formulated with aluminum(III)-salen and ETH-7075 indicator in 2 DOS:1 PVC, exhibit a significantly enhanced selectivity for fluoride over a wide range of lipophilic anions including salicylate, perchlorate, nitrate, and thiocyanate. The optimized films exhibit a sub-micromolar detection limit, using glycine-phosphate buffer, pH 3.00, as the test sample. The response times of the fluoride optical sensing films are in the range of 1–10 min depending on the fluoride ion concentration in the sample. The sensor exhibits very poor reversibility owing to a high co-extraction constant (log K = 8.5 ± 0.4), indicating that it can best be employed as a single-use transduction device. The utility of the aluminum(III)-salen based fluoride sensitive films as single-use sensors is demonstrated by casting polymeric films on the bottom of standard polypropylene microtiter plate wells (96 wells/plate). The modified microtiter plate optode format sensors exhibit response characteristics comparable to the classical optode films cast on quartz slides. The modified microtiter is utilized for the analysis of fluoride in diluted anti-cavity fluoride rinse samples and the results obtained are shown to correlate well with the analysis performed using the LaF 3 single crystal ion-selective electrode. © 2005 Elsevier B.V. All rights reserved. Keywords: Fluoride ion sensor; Aluminum(III)-salen complex; pH-chromoionophore; Single-use optical sensors; Microtiter plate format optode 1. Introduction Highly selective carrier-based ion-selective polymeric mem- brane electrodes and optodes have now been developed for a wide range of cations [1–4]. In contrast, analogous sensors with high selectivity for given anions are few in number due to the lack of lipophilic ionophore species that selectively interact with tar- get anions. Especially challenging has been the development of selective carrier-based polymeric membrane optodes/electrodes that respond selectively to hydrophilic anions (e.g., sulfate, phos- phate, and fluoride) due to their very negative Gibbs free energy of hydration. Such hydration energies make the partitioning of ∗ Corresponding author. Tel.: +1 734 763 5916; fax: +1 734 647 4865. E-mail address: mmeyerho@umich.edu (M.E. Meyerhoff). 1 On sabbatical leave from Department of Chemistry, Faculty of Science, Ain Shams University, Cairo, Egypt. these ions into the organic membrane phase of the sensors unfa- vorable [5]. Several strategies have been implemented to design selective carriers for given anions, including the use of spe- cific axial anion ligation to the metal ion centers of lipophilic metal–ligand complexes [6–8], multiple complementary inter- actions of organic ionophores with the guest anions through hydrogen bonding and electrostatic interactions [9,10], and use of a Lewis acid cavity within the carrier that fits a given anion based on shape/size [11]. The development of ionophore-based fluoride optical and electrochemical sensors is of potential practical importance for monitoring the level of fluoride ion in municipal drinking water, which must be carefully controlled (e.g., the level of fluoride in fluorinated water in the US is 0.7–1.2 ppm) [12]. Higher levels of fluoride can cause effects that range from dental fluo- rosis to serious bone disorders [13,14], while low levels lead to tooth decay [15]. Highly selective fluoride sensors would 0003-2670/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2005.08.037