Temperature-responsive properties of poly(N-vinylcaprolactam) multilayer hydrogels in the presence of Hofmeister anions

We report on the effect of Hofmeister anions on the temperature-induced volume transitions and optical responses of ultrathin hydrogels of poly(N-vinylcaprolactam) (PVCL). The hydrogels were produced by glutaraldehyde-assisted cross-linking of hydrogen-bonded multilayers of poly(N-vinylcaprolactam)-co-(aminopropyl) methacrylamide) and poly(methacrylic acid). We found that swelling and temperature-induced shrinkage of PVCL hydrogels were suppressed in the order SO42- > H2PO4- > Cl-, following the Hofmeister series. In contrast, I- increased hydrogel swelling but suppressed thermal response. A layer of glutathione-stabilized gold nanoparticles was introduced within the PVCL hydrogel to initiate an optical response in the presence of anions. We found the signal intensity of (PVCL)(81)-Au hydrogels and the plasmon band position to be largely controlled by ion type and concentration when the temperature reversibly changed from 20 degrees C to 50 degrees C. The band consistently shifted to lower wavelengths with an increase in chloride concentration. In contrast, a red shift was observed in the iodide solutions with increasing salt concentration; an exception to this was for the 0.1 M solution which resulted in a blue shift. We believe that our findings provide new prospects for understanding the effect of Hofmeister anions on ultrathin non-ionic polymer networks. In addition, the (PVCL)(81)-Au hybrid hydrogels afford a clear and fast optical monitoring of hydrogel temperature-triggered response at varied ion concentrations.