Development of a textile-optoelectronic pH meter based on hybrid xerogel doped with Methyl Red

A textile fabric treated with an organically modified silicate (ORMOSIL) was assembled together with a novel low-power color sensor in order to obtain a wearable and reliable pH sensor. The ORMOSIL matrix is a composite obtained via sol–gel by chemical reaction between the 3-glycidoxypropyltrimethoxysilane (GPTMS) precursor and Methyl Red (MR). To investigate the chemical structure of the organically modified silicate, a preliminary study by FTIR and UV–visible spectroscopy was performed on sensitive dye before and after the covalent bond formation with the opened epoxy ring of the precursor. It is found that the use of MR based on hybrid xerogels is successful in providing a pH sensing textile fabric with adequate operational stability. The developed color sensing device, based on a white LED and a photodiode whose spectral sensitivity curve is monotonic in the smart fabric color variation range, implements a light- to-frequency conversion. The miniaturized circuit results to be extremely low power with respect to other commercial solutions and it is thus suitable for portable applications aimed to log data even for many hours. The developed device was completely characterized using buffer solutions at different pH values. The pH sensor, composed by the smart fabric and its color sensing electronics, exhibits a dynamic range from pH 4.0 to pH 8.0 and an estimated resolution of 0.05 pH unit. Moreover, it shows excellent reproducibility, reversibility, temporal stability and a response time of 180 s when the fabric is dry and few seconds when it is wet. The developed system could find applications as a non-invasive, continuous sweat pH sensor for healthcare, fitness and wellness areas. Furthermore, it could be used as a wearable environmental sensor, for pollution monitoring or providing an additional sensing functionality in professional garments, for improving workers safety.