Novel ultrathin poly(ethylene glycol) films as flexible platform for biological applications and plasmonics.

We present a novel approach to prepare ultrathin, biocompatible films based on cross-linking of multi-functionalized, star-branched poly(ethylene glycols) (STAR-PEGs) with tunable film thicknesses of 4-200 nm. A two-component mixture of amine- and epoxy-terminated four-arm STAR-PEGs (MN=2000 g/mol) was spin-coated on a flat substrate. Gentle heating induced an extensive chemical cross-linking of the macromonomers, resulting in a stable, hydrogel-like film with a density close to that of bulk PEG material. The cross-linking process could be monitored in situ, exhibiting the expected kinetics. The films revealed pronounced swelling behavior, which was fully reversible and could be precisely controlled. Additionally, they provided a high affinity to citrate-stabilized gold nanoparticles (AuNP) that could be adsorbed with high densities into the PEG matrix from an aqueous solution. These novel PEG/AuNP composite films offer interesting and potentially useful optical properties. The adsorption could also be performed in a lithographic fashion, resulting in AuNP patterns imbedded into the PEG matrix.