Laser Interference Lithography for the Nanofabrication of Stimuli-Responsive Bragg Stacks

Dynamic structural coloration in Tmesisternus isabellae beetle elytra is a unique example of Bragg stack-based wavelength tuning in response to external stimuli. The underlying principles could guide the design of quantitative optical stimuli-responsive polymers. Existing nanofabrication techniques to create such materials are costly, time-consuming, and require high expertise. This study reports a nanofabrication method to produce slanted Bragg stack structures in poly(acrylamide-co-poly(ethylene glycol) diacrylate) hydrogel films by combining laser interference lithography and silver halide chemistry in a cost-effective and rapid process (≈10 min). The Bragg stacks consist of silver bromide nanocrystal multilayers having a lattice spacing of ≈200 nm. Upon broadband light illumination, the Bragg stacks diffract a narrow-band peak at 520 nm at ≈10° with respect to the normal incidence. The lattice spacing of the hydrogel films can be modulated by external stimuli to shift the Bragg peak for dynamic quantitative measurements. To demonstrate the utility of this method, the Bragg stacks are functionalized with phenylboronic acid molecules. Bragg peak shift analysis allows reversible glucose sensing within a physiological dynamic range (0.0-20.0 mmol L^-1) having a sensitivity of 0.2 mmol L^-1. The developed Bragg stacks may have application in portable, wearable, and implantable real-time medical diagnostics at point-of-care settings.