Temperature-induced flocculation of gold particles with an adsorbed thermoresponsive cationic copolymer

In this article, we report a study of the adsorption of a charged thermoresponsive poly(N-isopropylacrylamide)-block-poly(3-acrylamidopropyl) trimethylammonium chloride) [abbreviated as PNIPAAM24-b-PAMPTMA(+) 18] block copolymer onto gold nanoparticles by means of dynamic light scattering (DLS), zeta-potential measurements, cryo-TEM, rheology small-angle light scattering (rheo-SALS), and UV-visible absorbance spectroscopy. At low surface coverage, the cationic copolymer was found to mediate flocculation of the particles at elevated temperatures, whereas at higher polymer concentration, the gold particles were stabilized by the adsorbed copolymer layer. The DLS results showed that the adsorbed polymer was compressed prior to flocculation at elevated temperatures and that the flocs coexisted with single particles. The latter finding was confirmed by cryo-TEM. Rheo-SALS measurements revealed that shear flow disrupted the large flocs formed at higher temperatures. UV-visible absorbance measurements demonstrated that the temperature-induced flocculation process gives rise to a red shift of the localized surface plasmon resonance (LSPR) peak, together with the appearance of a second peak at higher wavelengths. © 2010 American Chemical Society.