Interaction of gold nanoparticles with thermoresponsive microgels: influence of the cross-linker density on optical properties

The interaction of spherical gold nanoparticles (Au-NPs) with microgels composed of chemically cross-linked poly-(N-isopropylacrylamide) is reported. Simple mixing of the two components leads to adsorption of the gold particles onto the microgels. Different loading densities can be achieved by varying the ratio of gold particles to microgel particles. The adsorption of gold nanoparticles is analysed by TEM, UV-Vis absorption spectroscopy and SAXS. The influence of the microgel mesh size on the adsorption of gold nanoparticles is investigated by using microgels with three different cross-linker densities. The results suggest a strong relationship between the nanoparticle penetration depth and the cross-linker density. This, in turn, directly influences the optical properties of the colloids due to plasmon resonance coupling. In addition, information about the mesh size distribution of the microgels is obtained. For the first time the change in optical properties by varying cross-linker density and temperature is directly related to the formation of dimers of gold particles, proven by SAXS.