Surface plasmon resonance study on the interaction of a dithiosquarylium dye with metal ions Sung-Hoon Kim a, *, Sun-Kyung Han a , Gab-Soo Jang b , Kwang-Nak Koh b , Shin-Won Kang b , Sam-Rok Keum c , Cheol-Min Yoon c a Department of Dyeing and Finishing, College of Engineering, Kyungpook National University, Taegu, 702-701, South Korea b Sensor Technology Research Center, Kyungpook National University, Taegu, 702-701, South Korea c Department of Chemistry, College of Science and Technology, Korea University, Jochiwon, Choong-nam, 339-700, South Korea Received 5 April 1999; accepted 23 August 1999 Abstract The detection of some metal ions has been investigated using an optical sensor based on surface plasmon resonance (SPR). A multilayer sensing chip was prepared with a dithiosquarylium (DTSQ) dye sensing layer coating on a gold layer. Using the DTSQ dye as a metal ion sensing molecule, a highly selective response for Ag + was observed. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Surface plasmon resonance (SPR); dithiosquarylium (DTSQ) dye; Metal ion sensor; Resonance angle 1. Introduction Surface plasmon resonance (SPR) is one of the promising optical techniques for chemical sensing, and SPR con®guration systems such as immu- noassay [1], liquid [2], gas [3] and thin ®lms [4] have been reported. A surface plasmon is a surface charge-density wave at a metal surface. It is well known that sur- face plasmons can be excited by the attenuated- total-re¯ectance (ATR) method reported by Kretschmann [5]. Since the resonance angle is very sensitive to variations in the refractive index of the medium just outside the metal thin ®lm, the refractive index of a ®lm, e.g. multilayer Lang- muir±Blodgett ®lm [6] or polymeric ®lm [7], can be obtained by the surface plasmon technique. The measurement of the refractive index of a bulk chemical sample can be used to determine the concentration of an analyte, assuming that the refractive index of a solution depends solely on analyte concentration. This can be performed by employing selective chemical sensing layers (e.g. selective membrane, chromoionophore, and speci®c binding layer). A chemical sensing layer consists of a thin ®lm having optical properties that vary with concentration of the analyte to be sensed. Squarylium dyes are 1,3-disubstituted com- pounds synthesized from squaric acid and two equivalents of various types of electron-donating carbocycles or heterocycles such as azulene [8], pyrroles [9] or heterocyclic methylene bases [10]. 0143-7208/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0143-7208(99)00093-5 Dyes and Pigments 44 (2000) 169±173 * Corresponding author. Tel.: +82-53-950-5641; fax:+82- 53-950-6617. E-mail address: shokim@bh.kyungpook.ac.kr (S.-H. Kim).