Interferometers Utilizing Reversible X-ray-induced Chemical Changes in Poly(N-isopropylacrylamide) Microgels

Abstract

Photonic materials, which react to light, have garnered interest due to their capability to exhibit adjustable structural colors. Typically, light targets the UV, visible, or near-IR spectrums. In this study, microgel-based photonic materials that are capable of reversibly responding to X-rays have been engineered. To accomplish this, azobenzene (Azo)-containing poly(N-isopropylacrylamide) (pNIPAm)-based microgels are synthesized. Subsequently, ZnS scintillator and Cr/Au are applied on each side of the poly(methyl methacrylate (PMMA) substrate. Subsequently, the Azo MG monolayer is deposited onto the Au surface, followed by the deposition of an additional layer of Cr/Au. This process generates ZnS/PMMA/Cr/Au/Azo MG/Cr/Au or ZnS/Au-Azo MG-Au structure. Functioning as a typical interferometer, ZnS/Au-Azo MG-Au demonstrates tunable colors based on the separation distance between the two Au layers. The ZnS scintillator can absorb and convert X-rays into UV light, initiating the transition of the Azo groups from a trans to a cis state. Consequently, this transition causes the Azo MG to swell. As Azo MG swells, the distance between the two Au layers increases, resulting in a red-shift of approximately 350 nm in the optical signal of the ZnS/Au-Azo MG-Au interferometer. Remarkably, this X-ray responsivity of the interferometer is reversible, as it returns to its initial state after being stored in the dark for 24 h. To demonstrate its capabilities, the ZnS/Au-Azo MG-Au interferometer successfully releases a drug when triggered by X-ray stimulation, thus validating its potential. The microgel-based interferometers hold significant promise for applications in chemoradiotherapy, radiobiology, and actuators in space.