Development of a biocompatible pH sensor is of importance in biomedical applications, particularly for in vivo measurement, providing necessary information for clinical diagnosis and treatment such as chronic wounds and foetal acidosis. Traditional pH-indicator based optical sensors have problems of dyeleaching and photobleaching that restrict their uses in long-term monitoring. In this work, a dye-free fibre optic pH sensor is proposed consisting of a U-shape multimode optical fibre coated with a hybrid organic-inorganic composite film. The film is formed by cross-linking ethyl cellulose with a silica matrix at an optimised ethyl cellulose/silica molar ratio of 0.0065 via weakly interacted hydrogen bonding. This bonding is affected by hydrogen concentration (i.e., pH) in a solution resulting in a morphological change of the polymer aggregation presented in the silica matrix leading to refractive index change of the film. The developed sensor shows a reversible response to pH from 4.5 to 12.5 and exhibits linear correlation between transmitted light power and pH with a limit of agreement (LoA) between the sensor and a commercial probe of ±0.2 pH. For a clinically important range of pH values between 6 and 8 the LoA is ±0.1 pH. The sensor has low cross-sensitivity to temperature as the maximum interpreted pH change attributed to the power change is 0.12 pH when the temperature changes from 21°C to 39°C. To demonstrate biomedical relevance, the sensor is used to monitor pH of human serum. An in-house cytotoxicity assay is conducted with mouse fibroblast cell revealing that the film is not cytotoxic.

Channel waveguides have been produced in LiNbO3, LiTaO3, and BaTiO3 ferroelectric crystals by depositing thick SiO2 films at an elevated temperature and patterning them by reactive ion etching. The static strain resulting from the large thermal expansion mismatch between the substrate and film causes a localized increase in the refractive index via the strain-optic effect. In addition, an electro-optic contribution to the index increase is believed to result from a surface charge distribution which compensates the electric field due to the piezoelectric effect. Single-mode waveguides at a wavelength of 0.633 μm for both polarizations have been produced in x-cut LiNbO3, with losses of 0.8 dB/cm for TE polarization and 0.9 dB/cm for TM polarization. The 11-μm-wide channel waveguides were formed by a z-axis strain induced by a 2.8-μm-thick SiO2 film deposited at 300 °C. Electro-optic modulation was also demonstrated in these waveguides. Guiding for both polarizations was also observed in LiTaO3 and BaTiO3 channel waveguides at 0.83 μm wavelength.

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