Single-molecule adhesion of a stimuli-responsive oligo(ethylene glycol) copolymer to gold

Adhesion of environmentally responsive polymers to biocompatible surfaces is an important issue that has been explored in several nanobiotechnology applications. Here, we prepared multi-responsive statistical copolymers of two oligo(ethylene glycol) methyl ether methacrylate macromonomers with differing ethylene glycol side chain lengths using RAFT polymerization. The lower critical solution temperature of the copolymers was characterized using visible light extinction, and the chemical composition and molecular weight were measured using NMR spectroscopy and size-exclusion chromatography, respectively. The characterization results demonstrated that the transition temperature could be controlled by varying the macromonomer feed ratios, and the molecular weight could be controlled by varying the amount of the RAFT chain transfer agent in the polymerization feed. Using AFM single-molecule force spectroscopy, we measured the adhesion characteristics of single copolymer molecules to a gold surface. We found that dehydration and collapse of the copolymer in a high ionic strength buffer resulted in dramatically reduced bridging length distributions that maintained their single-molecule bimodal character. In the collapsed state, the polymer exhibited a lower absolute desorption force while cooperativity effects were found to increase the desorption force per chain for multi-chain interactions. Our results confirmed that the polymer in a collapsed conformation exhibited a dramatically reduced volume occupancy above the gold surface. These results demonstrate at the single-molecule level how solvent-induced collapse of an environmentally responsive copolymer modulates surface adhesion forces and bridging length distributions in a controllable way.