Advanced hydrogel optical fiber sensors with triple-readout for real-time pH sensing.

Precise pH monitoring is essential in fields like scientific research, healthcare, and industrial processes. Herein, we have developed hydrogel-based optical fiber (OF) sensors for continuous and real-time pH sensing using a hydrogel matrix with the pH-sensitive dye Neutral Red to obtain a color-switching hydrogel matrix. Two sensor configurations, (i) attached to the glass slide (SA) and (ii) an OF tip sensor, were fabricated. Aztec diffractive nanostructures were imprinted during the polymerization of hydrogel matrix for both SA and OF sensors using a simple replication process. The sensors provide triple readout capabilities: colorimetric changes visible to the naked eye, shifting from red (pH 4) to yellow (pH 10); spectroscopic analysis in transmission and reflection modes, showing wavelength-specific dips; and diffractive power variations recorded using a 532 nm green laser. Reflection measurements confirmed the sensors' suitability for remote sensing applications, highlighting their potential for non-invasive detection scenarios. These sensors demonstrated high sensitivity, rapid response times, excellent repeatability, and robust performance across multiple cycles. A smartphone-based detection method further enhances practicality, enabling portable use. These hydrogel-based sensors offer a reliable, versatile solution for pH monitoring and hold significant promise for applications in the beverage, food, pharmaceutical, and point-of-care diagnostic industries.