Penetration of Polyelectrolytes into Charged Poly(N-isopropylacrylamide) Microgel Layers Confined between Two Surfaces

pH and temperature sensitive poly(N-isopropylacrylamide) (pNIPAm) microgel-based etalons were fabricated by sandwiching a monolithic microgel layer between two semitransparent, 15 nm Au layers. The devices exhibit visual color and multipeak reflectance spectra, both of which primarily depend on the distance between the Au surfaces (mediated by the microgel diameter). In this submission, we fabricate etalons from either pNIPAm-co-acrylic acid (pNIPAm-co-AAc) or pNIPAm-co-N-(3-aminopropyl)methacrylamide hydrochloride (pNIPAm-co-APMAH) microgels and investigate their response to the presence of polycations or polyanions. We show that when the etalon is at a pH that renders the microgels multiply charged, the microgel layer of the etalon deswells in the presence of the oppositely charged polyelectrolyte; it is unresponsive to the presence of the like charged polyelectrolyte. Furthermore, the etalon's response depended on the thickness of the Au overlayer. For example, low molecular weight (MW) polyelectrolyte could penetrate all Au overlayer thicknesses, while high MW polyelectrolytes could only penetrate the etalons fabricated from thin Au overlayers. We hypothesize that this is due to a decrease in the Au pore size with increasing thickness, which excludes the high MW polyelectrolytes from penetrating the microgel-based layer. These devices show promise as MW selective sensors and biosensors.