Reversible pH sensing utilizing U-bent optical fibers coated with a layer-by-layer application of polymer multilayers

This study presents a U-shaped evanescent wave optical fiber with a layered nano-thin film of PAH and PAA, demonstrating its application as a pH sensor. The optical fiber demonstrated that decreased light transmittance intensified as the number of films increased, indicating a rise in the thin film's refractive index. The film was created in 5, 10, and 15 layers, and pH tests were conducted in aqueous solutions with varying pH values. When immersed in an aqueous solution with a pH of 3 to 9, the sensor exhibited excellent reproducibility and high durability. The light intensity increased when immersed in a pH higher than 3, and the sensitivity of the 5-layer film was highest in the elevated pH range. This may be due to changes in the electrostatic interactions between the layers of the PAH/PAA film, as H+ and OH– ions adsorb in the aqueous solution, causing the thin film to swell and shrink repeatedly.