Thermoresponsive Nanohydrogels Cross-Linked by Gold Nanoparticles

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Abstract
Thermoresponsive nanohydrogels cross-linked by gold nanoparticles (AuNPs) were prepared by 1,3-dipolar cycloaddition reactions and in situ reversible addition−fragmentation chain-transfer (RAFT) polymerization. In order to synthesize thermoresponsive nanohydrogels, AuNPs decorated with azide groups (AuNPs-N3) were prepared through ligand exchange. Click reactions between AuNPs-N3 and dialkynetrithiocarbonate yielded cross-linked AuNP aggregates. The size and cross-linking density of AuNP aggregates increased with the molar ratio of acetylene groups to azide groups. After click reactions, the absorption maximum of the plasmon band of AuNPs red-shifted to a long wavelength. Thermoresponsive nanohydrogels were prepared by in situ RAFT polymerization of N-isopropylacrylamide (NIPAM) using trithiocarbonate in the cross-linked AuNP aggregates as chain-transfer agents. The thermoresponsive nanohydrogels presented a low critical solution temperature at around 32 °C due to the “coil-to-globule” transition of connecting PNIPAM chains in the nanohydrogels. The size of the thermoresponsive nanohydrogels was determined by the molar ratio of acetylene groups to azide groups.

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