Fluorescent Optical pH Sensors Based on Organic-Inorganic Hybrid Materials

Optical sensors (optodes) have certain advantages, like not suffering from electrical interferences, being miniaturizable, and operatable with simple devices, such as only a light source, an optical response part, and a detector. In the case of fluorescence measurements, optodes are known to perform with high sensitivity. Organic-inorganic hybrid materials are attracting attention in many fields, such as chemistry, physics, biology, and material science. Their application contributes to innovations in a variety of technical areas: electronics, environment, medicine, and optics. Therefore, in this study, the author developed novel fluorescence-based pH optodes that rely on organic-inorganic hybrid materials. Two types of fluorescent optical sensors based on organic inorganic hybrid materials were developed, as described below. The first was a novel pH optode based on a silane coupling reagent modified H+-responsive fluoroionophore KBH-01 (KBH-01-Si) immobilized to a mesoporous silica thin film by covalent bonding via a sol-gel grafting method. The resulting pH optode allows for the determination of accurate pH values in aqueous sample solutions. The use of mesoporous silica as a sensor substrate, resulting in short diffusion times and easy access of protons, enables a rapid pH response, as well as real-time monitoring. The second was a novel double-layer ratiometric fluorescent pH optode using quantum dots (QDs) and pH-indicators. A first silica layer encapsulates two types of QDs emitting at different wavelengths. A second silica layer encapsulates light-absorbing pH-indicators. The resulting double-layer pH optodes showed a ratiometric fluorescence response with good repeatability, notable photostability and high storage stability. The dynamic response pH range could be optimized up to a range from pH 0 to 10 by changing the combination of absorbing pH-indicators and QDs. These results indicated that organic-inorganic materials have the potential being applied to pH optodes.