# pH Measurements Using Leaky Waveguides with Synthetic Hydrogel Films

## TL;DR

This paper introduces a new pH sensor using synthetic hydrogel films that can measure pH changes with high resolution and minimal interference.

## Contribution

A novel pH sensor using a synthetic hydrogel-based leaky waveguide with high resolution and minimal interference is developed.

## Key findings

- The sensor has a pH resolution of ~0.004, better than the ~0.01 of a glass electrode.
- The sensor is largely insensitive to common interferents like sodium chloride and humic acid.
- Response times are 0.90 ± 0.14 min for low to high pH and 2.38 ± 0.22 min for high to low pH.

## Abstract

Leaky waveguides (LWs) are low-refractive-index films deposited on glass substrates. In these, light can travel in the film while leaking out at the film–substrate interface. The angle at which light can travel in the film is dependent on its refractive index and thickness, which can change with pH when the film is made of pH-responsive materials. Herein, we report an LW comprising a waveguide film made of a synthetic hydrogel containing the monomers acrylamide and N-[3-(dimethylamino)propyl]methacrylamide (DMA) and a bisacrylamide crosslinker for pH measurements between 4 and 8. The response of the LW pH sensor was reversible and the response times were 0.90 ± 0.14 and 2.38 ± 0.22 min when pH was changed from low to high and high to low, respectively. The reported LW pH sensor was largely insensitive to typical concentrations of common interferents, including sodium chloride, urea, aluminum sulfate, calcium chloride, and humic acid. Compared to a glass pH electrode, the measurement range is smaller but is close to the range required for monitoring the pH of drinking water. The pH resolution of the hydrogel sensor was ~0.004, compared to ~0.01 for the glass electrode.