Dynamics of Swollen Gel Layers Anchored to Solid Surfaces

Thin responsive hydrogel films are currently under development for biosensor applications. Photocrosslinkable poly(N-isopropylacrylamide) (PNIPAAm) based chains are spin coated as thin films (about 1 micron) and UV irradiated with variable doses to control the crosslink density. The obtained anchored gel layers can swell in ethanol or water up to about 10 microns for low crosslinking densities. Dynamics of the swollen layers and diffusion of different tracers (as analyte mimicks) are studied by dynamic light scattering (PCS) and fluorescence correlation spectroscopy (FCS). PCS resolved fast and slow diffusions, attributed to cooperative diffusion and long range concentration heterogeneities. Higher crosslink densities give rise to faster cooperative diffusion, i.e. short dynamic mesh sizes. FCS revealed the importance of electrostatic interactions between probe and negatively charged network. While a negatively charged dye senses local dynamics with a moderate slow down, a positively charged dye exhibited substantially retarded diffusion. Larger tracers are used to assess the size dependent gel penetrability, whereas large particles, trapped into the network, expectedly follow the network dynamics.