Utilization of white light interferometry in pH sensing applications by mean of the fabrication of nanostructured cavities

In this work it is proposed a new approach for optical pH sensing applications based on the interferometric response of nanofilms built onto standard optical fibers. The main strength of the suggested approach is that it enables the measurement of the interference pattern of a sensor tip using a low-coherence light source such as a halogen lamp, what means a simple and cost-effective experimental setup. The electrostatic layer-by-layer technique is used here to create thin polymeric films onto standard optical fiber substrates, which operate as Fabry–Perot interferometric cavities of a few hundreds of nanometers suitable to be excited with a halogen white light source (FWHM 300 nm). The basic structure of these polymeric films consists in the alternate deposition of the weak polyelectrolytes; poly(allylamine hydrochloride) as the cationic material, and the polymer poly(acrylic acid) as the anionic one. In this particular case, the sensor operation is based on the pH-dependence of the morphology of the weak polyelectrolyte chains, usually known as swelling. Furthermore, several measurements are performed with the fabricated devices showing their robustness against changes of the optical power and the high sensitivity achieved about 0.051 and 0.0181 pH units per wavelength in the worst and best cases respectively.