Development of a polymer optical fiber pH sensor for on-body monitoring application

Abstract A pH sensor based on polymethylmethacrylate (PMMA) optical fibers has been developed for on-body monitoring in biological fluids. Detection relies on evanescent wave absorption in a thin film deposited on the fiber core. The sensitive film was prepared using sol–gel technology from a mixture of tetraethyl orthosilicate (TEOS) and methyltriethoxysilane (MTES) or phenyltriethoxysilane (PTES), forming a porous hybrid organic–inorganic layer upon drying. The so-called ORMOSILs (organically modified silicate) were doped with bromophenol blue (BB) and the formulation was optimized to suppress dye leaching by varying the fraction of organic precursor, the sol aging time and the drying conditions. The molecular structure of the films was examined by Fourier-transformed infrared (FTIR) spectroscopy, and their optical responses characterized with UV–VIS absorption spectroscopy. Homogeneous crack-free films were obtained with an optimal MTES content of 50%. Leaching from these films was greatly reduced, with more than 80% of dyes remaining after 2 months in phosphate buffered saline (PBS) buffer. The entrapped indicator showed pronounced increase of apparent p K a and a broader response range compared to the free form in solution. The sensor demonstrated good sensitivity from pH 3 to 9, with an optical signal variation of 3.5 dB. It showed sensitivity to ionic strength variations, but not to temperature in the range between 20 and 50 °C. Reversible pH monitoring between pH 5 and 8 with a precision of 0.2 pH unit was demonstrated in human serum as a model biological fluid, without degradation of the sensing fiber over 24 h. This simple and low cost device will be particularly valuable to support diagnosis and the evaluation of therapies for wound healing.