Optical Hydrogel Detector for pH Measurements

Abstract

Measuring pH has become a major key for determining health conditions, and food safety. The traditional pH assessment approaches are costly and offer low sensitivity. Here, a novel pH sensor based on a pH-responsive hydrogel has been developed. A Fresnel lens pattern was replicated on the surface of the pH-responsive hydrogel using the replica mould method. The pH sensors were tested in a pH range of 4-7. Introducing various pH solutions to the pH sensor led to volumetric shifts as the hydrogel swelled with pH. Consequently, the dimensions of the replicated Fresnel lens changed, modifying the focal length and the focus efficiency of the optical sensor. As a result, the measured optical power at a fixed distance from the sensor changed with pH. The optical sensor showed the best performance in the acidic region when pH changed from 4.5 to 5.5, in which the recorded power increased by 13%. The sensor exhibited high sensitivity to pH changes with a short respond time in a reversible manner. The developed pH optical sensor may have applications in medical point-of-care diagnostics and wearable continuous pH detection devices.

Keywords: Fresnel lens; hydroxyethyl methacrylate; pH-responsive hydrogels; sensors.

Figure 1

(A) A schematic illustration of the fabrication process for replication of the Fresnel lens on the pH-sensitive hydrogel. (B,C) schematics for the setups used to interrogate the sensor. (D) a photo of the master Fresnel lens that was replicated on the pH-responsive hydrogel. (E) a photo shows the diffraction pattern for the laser beam passed through the Fresnel lens. (F) a microscopic image of the surface of the pH-responsive hydrogel imprinted with the Fresnel lens, and (G) a photo of the pH sensor illuminated by a white light beam.

Figure 2

(A) The spatial profile of the measured power intensity for the pH sensor immersed in different pH solutions. (B) the spatial profile for the laser beam passed through the pH sensor that was stored for 7 days before testing. (C) the response on daily basis for the pH sensor interrogated in pH 4.5 and 5.5, for the first 10 days while the sensor was stored in PBS buffer in the intervals between the tests. (D) the optical power versus the pH for the sensor while it was immersed in pH 4.5 and 5.5 for four cycles; the standard deviation error bars calculated for n = 3. (E) pH continual detection carried out over time in different pH solutions, and (F) the recorded power versus the pH levels while the sensor was tested in the continuous pH detection mode.

Figure 3

(A–F) Images taken by the optical microscope for the free-standing pH sensor immersed in different pH solutions, (G) diameter of the first ring of the imprinted Fresnel lens when the sensor was immersed in different pH solutions, and (H) the impact of the temperature on the pH sensors.