A multi-stimuli responsive polyoxometalate-guanosine monophosphate hybrid chromogenic smart hydrogel

Abstract

Chromic materials have attracted tremendous technological importance owing to their change in optical properties in response to various stimuli. Integrating multiple chromogenic responsiveness into one device could offer versatility and aid in resource conservation; however, most chromogenic materials developed respond to only one or two stimuli. Polyoxometalates owing to their redox properties are being explored as chromic materials; however, several drawbacks such as opacity, leaching, and inhomogeneous aggregation limit their use in bare form for practical applications. Herein, we report a chromogenic supramolecular hydrogel system developed via a simple self-assembly of Keggin-structured polyoxometalate, phosphomolybdic acid with guanosine monophosphate in an aqueous medium. The mechanically robust, flexible, and thermo-reversible hydrogel not only exhibited a stimuli-triggered visible change in its optical property, from yellow to blue upon exposure to multiple stimuli, such as visible light, stress, electricity, temperature, and metals, but also showed enhanced stimuli responsiveness as compared to its bare counterpart. The hydrogel also exhibits chemochromism towards biologically relevant thiols, such as cysteine and glutathione, and selectively towards iodide among all halides. Further, phosphomolybdic acid induced self-assembly of guanosine monophosphate molecules into a G-quartet structure makes the hydrogel conductive owing to the ion-conducting property of the supramolecular assembly. This enables the designing of a 2-layered electrochromic device (ECD) as a simpler alternative to a traditional 5-layered setup.