Miniaturized fiber optic probe based on colloid crystals of hydrogel coated nanoparticles for relative humidity measurements

This work describes a novel relative humidity sensing device based on photonic crystal colloids auto-assembled directly on the tip of a multi-mode optical fiber via a vertical dip-coating deposition technique. Specifically, the photonic crystal colloids are periodically-structured nano-spheres chemically formed by a polystyrene (PS) core incorporated into a hydrogel shell. The hydrogel shell swells after the water absorption depending on the relative humidity and the reflectance spectrum of the proposed devices changes in terms of wavelength shift and amplitude of the reflected resonant peak. Here spectral characterization and sensing analysis of the probe is presented and discussed. Maximum sensitivity around 6 nm/RH% is found.