Probing the response of poly (N-isopropylacrylamide) microgels to solutions of various salts using etalons.

The Hofmeister series is a qualitative ordering of ions according to their ability to precipitate proteins in aqueous solution and is extremely important to consider when trying to understand materials and biomolecular structure and function. Herein, we utilized optical devices (etalons) composed of poly(N-isopropylacrylamide) (pNIPAm)-co-10% acrylic acid (AAc) or pNIPAm-based microgels to investigate how various salts in the Hofmeister series influenced the microgel hydration state. Etalons were exposed to a series of salts solutions at different concentrations and the position of the peaks in the reflectance spectra monitored using reflectance spectroscopy. As expected, pNIPAm-co-10%AAc microgel-based etalons responded to the presence of ions, although in this case the response to cations deviated from the Hofmeister series. However, when using etalons prepared with pNIPAm-based microgels, the responses followed the Hofmeister series for both cation and anions. Finally, we observed that the sensitivity of etalons prepared with pNIPAm microgels was significantly higher than the response obtained from etalons composed of pNIPAm-co-10%AAc microgels. This was explained by considering the charge on the pNIPAm-co-10%AAc microgels that influences how osmotic and Hofmeister effects impacts hydration state.