Optical and acoustic studies of pH-dependent swelling in microgel thin films

We report investigations on the swelling behavior of poly(N-isopropylacrylamide-co-acrylic acid) (pNIPAm-co-AAc) microgel thin films as a function of pH using quartz crystal impedance (QCI) analysis and surface plasmon resonance (SPR) spectroscopy. In this work, pNIPAm-co-AAc microgels are assembled into thin films using a spin-coating layer-by-layer (scLbL) assembly approach by alternatively exposing 2-mercaptoethylamine-functionalized Au QCI and SPR substrates to polyanionic pNIPAm-co-AAc microgels and polycationic poly(allylamine hydrochloride). The scLbL process is followed by QCI analysis, which displays a linear decrease in resonant frequency, i.e., increase in mass, as a function of the microgel layer number. The swelling behavior of an assembled film in response to acidic (pH approximate to 3.0) and neutral (pH approximate to 6.5) solution conditions is monitored using QCI. Exposure of a microgel thin film to acidic solution results in a dramatic decrease in resonant frequency and an increase in motional resistance. When this same film is exposed to a neutral pH solution, the resonant frequency and resistance of the quartz again decrease and increase, respectively, but with the changes being more dramatic than those of the film in acidic solution. This behavior is believed to be due to the film transitioning from a highly solvent swollen state under acidic conditions to a less swollen state near neutral pH. To confirm this interpretation, SPR measurements were performed, which suggest that the film is less optically dense under acidic conditions, i.e., more solvent swollen, than the same film at pH approximate to 6.5. This differential solvation behavior is highly reversible, with the solvation/desolvation rates being dependent on the number of microgel layers in the film.