Deswelling Kinetics of Color Tunable Poly(N-Isopropylacrylamide) Microgel-Based Etalons

Abstract

Poly(N-isopropylacrylamide) (pNIPAm) microgel-based etalons are optical materials fabricated by depositing a monolithic layer of microgels on a semitransparent Au film adhered to a glass coverslip, followed by the deposition of a second semitransparent Au layer over the microgel layer (overlayer). These materials exhibit characteristic colors and multipeak reflectance spectra, both of which depend on the distance between the Au surfaces (mediated by the microgel diameter) and the refractive index of the microgel layer. In this submission, the deswelling kinetics of pNIPAm microgel-based etalons are investigated by inducing microgel deswelling through exposure to a 30% methanol/H(2)O solution. Exposed to this solvent system, the transition temperature of the microgels is lowered to a temperature below the experimental temperature and the microgels comprising the etalon collapse. This collapse induces an etalon color change, which is observed as a blue shift in the reflectance spectrum. The kinetics of deswelling were shown to be strongly dependent on the thickness of the Au overlayer, e.g., thicker overlayers slow the solvent exchange and the resultant deswelling kinetics. Additionally, for thicker overlayers, the rate of deswelling increases with decreasing etalon size. Taken together, these results suggest that the kinetics depend strongly on the ability of the solvent to exchange from/to the microgel layer. For example, if the Au overlayer is thin, more solvent can exchange through the overlayer in a given amount of time compared to an etalon composed of a thick overlayer. Likewise, etalons of smaller dimensions have faster deswelling kinetics due to the shorter distance the solvent needs to travel laterally through the microgel layer to exchange. The results from this study are of fundamental importance but will be used to develop sensors with fast response times for point-of-care diagnostics.