Thermoresponsive nanoparticle agglomeration/aggregation in salt solutions: Dependence on graft density.

Gold nanoparticles with a graft density of 0.09, 0.30 and 0.40chains/nm of poly(N-isopropylacrylamide) were reproducibly synthesized by varying the ratio of disulfide terminated poly(N-isopropylacrylamide) to gold nanoparticle. The polymer coated nanoparticles were stable at room temperature in 50mM NaCl, yet agglomerated at 37°C. Previous studies have observed conflicting results as to the reversibility of this agglomeration. Particle agglomeration with three different graft densities was studied in 50mM NaCl by measurements of their localized surface plasmon resonance and hydrodynamic diameter, and imaging with electron microscopy. Agglomerates with a polymer graft density of 0.30 and 0.40chains/nm could be dispersed with sonication, while particles with a graft density of 0.09chains/nm irreversibly aggregated. The graft density dependence on whether agglomeration or aggregation occurred is due to changes in collapsed polymer steric effects. Localized surface plasmon resonance measurements of agglomerates were discordant with hydrodynamic diameter measurements in determining agglomeration reversibility, which shed light on reasons previous reports yielded different interpretations on the reversibility of this agglomeration. This work demonstrates how polymer graft density affects thermoresponsive nanoparticle stability in salt solutions and the need for use of complementary techniques when determining agglomeration.