Photodecomposition of a target compound detected using an optical fibre long period grating coated with a molecularly imprinted titania thin film

Abstract

ABSTRACT An optical fibre long period grating (LPG) coated with an inorganic molecularly imprinted thin film based on TiO 2 was used to measure in real time the photodecomposition of an organic target compound. The TiO 2 film was deposited onto the LPG using the aqueous liquid phase deposition method and its photo-catalytic properties were employed to decompose a porphyrin. The principle of operation is based on the measurements of the refractive index (RI) change of the TiO 2 thin film at the binding and removal of the imprinted organic compound. Keywords: long period grating (LPG), molecularly imprinte d polymers (MIPs), selective photodecomposition. 1. INTRODUCTION Optical techniques are considered widely to be powerful tools for the development of chemical and biological sensors, covering a wide range of applications [1]. Sensing techniques based upon the use of optical fibre long period grating (LPG) devices to probe the opti cal characteristics of nanomaterials that exhi bit changes in their optical properties upon exposure to targeted chemical species are particularly attractive, due to their potential high sensitivity, the ability to multiplex arrays of sensors, and the prospect for remote sensing [2]. Molecularly imprinted materials provide a universal platform for the creation of artificial receptors that can be well controlled and tailored to achieve precise molecular recognition [3]. The technique is very versatile as any compound with functional groups can in theory be imprinted in different porogens (either water or organics). The basic concept of molecular imprinting is based on the creation of imprints of the template (i.e. chemical molecules or biological species that need to be detected – the analyte) into a matrix. The matrix is most often polymeric, with molecular imprinting achieved by incorporation of the template during the polymerization step, which is subsequently removed to leave behind a cavity specific for the template material, as illustrated in figure 1. Inorganic and hybrid inorganic-organic molecularly imprinted materials have advantages over those prepared from organic materials. Such materials are highly stable and reproducible, which is esp ecially important in real world sensing applications. Inorganic MI has been achi eved successfully using various metal alkoxi des, most often using silica to form the inorganic matrix via a sol-gel process that incorporates the template molecule [3]. Other methods that have been employed for the preparation of the thin imprinted films include organically modi fied sol-gels [4], gas-phase surface sol-gel deposition [3], liquid phase deposition [5 ] and spin coating [6]. One of the disadvantages of using metal alkoxides is the need to use organic solvent which limits the ability to use water soluble templates in the MI technique. In this work we propose a different approach for the fabrication of molecularly imprinted thin inorganic films using water soluble compounds. The approach involves the deposition onto the LPG of the TiO