Self‐Reporting Hydrogel Sensors Based on Surface Instability‐Induced Optical Scattering

Abstract

Interest regarding the development of hydrogel sensors continues to grow due to the associated high sensitivity, fast response, low fabrication cost, and versatile application when responding to an analyte. The strategies for transducing these responses range from electrochemical to optical, often utilizing characterization equipment to measure the minute changes in the material. However, the costly nature of such equipment counteracts the original advantages of utilizing a hydrogel‐based sensor, rendering the overall system slow and inaccessible for many. Therefore, hydrogel sensors capable of self‐reporting values to the naked eye are needed. Here, a new light‐scattering‐based transduction method is explored—indiscriminately visible to the naked eye. By engineering the surface instability behavior of a hydrogel film, an exemplary design is developed using an anionic, pH‐responsive poly(acrylamide‐co‐acrylic acid) hydrogel to self‐report the pH via subsequent buckling (i.e., its scattering of light). By modeling the behavior based on three system parameters (critical concentration, critical swelling ratio, and modulus), a general design model is produced to guide practical implementations of this instability‐induced scattering (IIS) sensor system. The viability of the IIS design model is exemplified through a proof‐of‐concept application to sensing urea, illustrating the modular and adaptable design of the presented transduction method.