Adsorption of pNIPAM Layers on Hydrophobic Gold Surfaces, Measured in Situ by QCM and SPR

Abstract

In situ surface plasmon resonance (SPR) and quartz crystal microbalance (QCM) measurements have been employed to measure the adsorption kinetics and absolute adsorbed amount of the poly( N -isopropyl acrylamide) (pNIPAM) from bulk aqueous solution onto a hydrophobized gold substrate. The adsorption was carried out at 31 °C, which is just below the lower critical solution temperature of pNIPAM in water. We find that the shift in the coupling angle of the surface plasmon (proportional to the “optical thickness”) and the shift in the resonance frequency of the quartz crystal (proportional to the “acoustic thickness”) increase in parallel for most of the adsorption. Also, the change of dissipation is proportional to the change in frequency. These observations suggest that the buildup of the polymer layer proceeds via growth in thickness rather than by densification of a layer with constant thickness. We interpret this finding in the sense that the dense high-temperature phase wets the hydrophobic gold surface. The wetting layer has a fixed density and grows in thickness. In addition, the QCM has been used to study the temperature-induced conformational change for pNIPAM around the critical temperature. It was found that the technique was able to monitor additional adsorption that occurs when crossing the critical point, which was due to bulk phase separation. Desorption was also noted when crossing the critical point from the opposite direction, and for the given system the process was entirely reversible.