High performance optical ratiometric sol–gel-based pH sensor

We present an optical sol–gel-based pH sensor which compares well with the current state-of-the-art in terms of the combination of resolution, stability, response time and leaching characteristics. The pH-sensitive dye used is the fluorescent indicator 8-hydroxy-1,3,6-pyrene trisulfonic acid (HPTS) which has been ion-paired with hexadecyltrimethylammonium bromide (CTAB). The sol–gel matrix is a composite of the precursors 3-glycidoxypropyltrimethoxysilane (GPTMS) and ethyltriethoxysilane (ETEOS) in which the dye is completely physically entrapped with no leaching. A referenced excitation ratiometric sensor detection system is used which capitalises on the dual excitation bands of the dye. The sensor has a dynamic range from pH 5.0 to pH 8.0, a resolution of 0.02 pH units and exhibits excellent reproducibility, reversibility, temporal stability and a short response time of 12 s.