Polymer diffraction gratings on stimuli-responsive hydrogel surfaces: Soft-lithographic fabrication and optical sensing properties

In this work, a soft-lithographic contact printing approach was developed to fabricate polymer diffraction gratings on stimuli-responsive hydrogel surfaces. For the fabrication, azo polymer films with surface-relief-gratings (SRGs) on surfaces were used as masters, which were inscribed by irradiating solid films of BP-AZ-CA with interfering Ar+ laser beams. Elastomeric stamps were prepared by replica molding of poly(dimethylsiloxane) (PDMS) prepolymer against the masters and curing properly. One-dimensional (1D) and two-dimensional (2D) gratings were fabricated on the surfaces of pH-sensitive hydrogel plates through soft-lithographic contact printing. Results show that the swellings of the hydrogel in buffer solutions are sensitively reflected by the variation of the gratings’ diffraction parameter. The pH of the buffer solutions shows a significant influence on the rate and degree of the diffraction parameter variation. Through the hydrogel volume change in response to external stimuli, the diffraction gratings can optically sense the variation of solution pH, which can be extended to sense the ionic strength and other analytes in solutions. Such a transduction mechanism is useful for optically detecting the external stimuli or molecular recognition events. The sensitive grating can also be used as the smart optical filters and other optical devices.