Bromophenol blue, a pH indicator, was entrapped in porous glass films prepared by the sol-gel method. The calorimetric response of the fihns to pH was assessed visibly and by attenuated total reflectance spectrometry. The entrapped indicator was nonleachable and responded to solution pH changes in a reversible manner. The immobilization of indicator molecules in porous sol-gel films appears to be a particularly promising approach to optical waveguide sensor development. ZGzywordx Attenuated total reflectance; Calorimetry; Bromophenol blue; Indicators; Porous glass films; Sol-gel method Optical sensing using immobilized indicators is a rapidly developing area in chemical and biochemical analysis [1,2]. Although the properties desired of an immobilized indicator phase will vary depending on the intended application, in general the ideal immobilization technique should produce a highly stable assembly of molecules that remain sterically accessible to dissolved ligands. Covalent attachment to a functionalized support [3] and physical entrapment, either in a porous polymer matrix [4] or behind a selectively porous membrane [51, are two commonly employed approaches. Entrapment is the technically simpler technique, but the response time of an entrapped indicator to a dissolved analyte is often relatively slow. Furthermore, the organic polymers typically used as entrapment matrices are labile and optically inferior compared to inorganic materials.
Recently, the entrapment of optical indicators in porous, optically clear glasses prepared by.solgel techniques has been reported.