Stimulus-responsive, surface confined poly(N-isopropylacrylamide) (pNIPAAM) brush nanopatterns were prepared on gold-coated silicon substrates in a "grafting-from" approach that combines "nanoshaving", a scanning probe lithography method, with surface-initiated polymerization using atom transfer radical polymerization (ATRP). The reversible, stimulus-responsive conformational height change of these nanopatterned polymer brushes was demonstrated by inverse transition cycling in water, and water−methanol mixtures (1:1, v:v). Our findings are consistent with the behavior of laterally confined and covalently attached polymer chains, where chain mobility is restricted largely to the out-of-plane direction. Our nanofabrication approach is generic and can likely be extended to a wide range of vinyl monomers.

In this paper we report the surface-initiated polymerization of poly(N-isopropylacrylamide) (pNIPAAM), a stimulus-responsive polymer, from monolayers of ω-mercaptoundecyl bromoisobutyrate on gold-coated surfaces. pNIPAAM was polymerized in aqueous solution at a low methanol concentration at room temperature to maintain the growing pNIPAAM chains in a hydrophilic and an extended conformational state. Under these conditions thick polymer brush layers (up to 500 nm in the swollen state) are produced after 1 h of polymerization. We present a new and simple strategy to fabricate stimulus-responsive, surface-confined pNIPAAM brush nanopatterns prepared in a “grafting-from” approach that combines “nanoshaving”, a scanning probe lithography method, with surface-initiated polymerization. The reversible, stimulus-responsive conformational height change of bulk and nanopatterned polymer brushes was demonstrated by repeated cycling in water and water/methanol mixtures (1:1, v/v). Our findings are consistent with the behavior of laterally confined and covalently attached polymer chains, where chain mobility is restricted largely to the out-of-plane direction. The present work is significant because the triggered control of interfacial properties on the nanometer scale holds significant promise for actuation in bio-nanotechnology applications where polymeric actuators may manipulate the transport, separation, and detection of biomolecules.

A protocol for preparation of polymer hydrogel spherical particles on a nanometer scale (nanogels) was developed. The protocol includes encapsulation of hydrogel-forming components into liposomes, UV-induced polymerization within the liposomes, solubilization of the lipid bilayer by detergent, removal of the phospholipid, detergent molecules, and their micelles by dialysis, and drying nanogels by evaporation in a temperature gradient. Dynamic light scattering technique was employed to characterize the size distribution of poly(acrylamide), poly(N-isopropylacrylamide), and poly(N-isopropylacrylamideco-1-vinylimidazole) hydrogel particles. Hydrophobic chains of N-octadecylacrylamide were immobilized onto the surface of the poly(N-isopropylacrylamide-co-1-vinylimidazole) hydrogel particles to use them as anchors for attaching the nanogels onto the lipid bilayers. The diameter of the nanogels prepared varied between 30 and 300 nm. The solvent, temperature, and pH sensitivities of the liposomes, nanogels, and their mixtures were studied. It was found that the phospholipid bilayer always coats the surface of both unanchored and anchored hydrogel particles upon mixing. Aggregation of the lipid bilayer-coated nanogels (lipobeads) was observed when the gel particles collapsed. The mechanism of aggregation differs for the lipobeads containing unanchored cores and those containing anchored hydrogel cores. The hydrogel-liposome structures of a nanometer size are of potential importance for applications such as biomimetic sensory systems, controlled release devices, and multivalent receptors.

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