Plasmonic and colloidal stability behaviours of Au-acrylic core–shell nanoparticles with thin pH-responsive shells

The localised surface plasmon resonance (LSPR) of Au nanoparticles (NPs) as well as its interaction with nearby entities provides a wealth of fundamental and practical information at the nanometre scale. A number of studies have investigated core–shell NPs with Au cores and polymer shells that are temperature-responsive. However, there are very few studies of pH-responsive Au-polymer NP shells. Precipitation polymerisation is a scalable method and here we establish such a method to synthesise pH-responsive Au-poly(methyl methacrylate) copolymer core–shell NPs without the need for pre-functionalisation. The comonomers used were methacrylic acid (MAA) or 2-carboxyethyl acrylate (CEA) and the shells were crosslinked with ethylene glycol dimethacrylate. A series of five core–shell systems with collapsed shell thicknesses less than 30 nm are studied. The shell-thicknesses for the CEA-based core–shell NPs are relatively thin (≤5 nm) compared to related Au-polymer core–shell NPs prepared using precipitation polymerisation. The LSPR properties of the core–shell NPs were dependent on the shell thickness and were successfully simulated using finite difference time domain (FDTD) calculations. Two systems are considered further as exemplars. The MAA-based core–shell system with the thickest shell exhibited enhanced colloidal stability to added electrolyte. The CEA-based core–shell dispersion with the thinnest shells displayed reversible pH-triggered aggregation and was cytocompatible for HeLa cells. Proof-of-concept data are presented that demonstrate intracellular pH reporting.