Au nanoparticles with diameters ranging between 15 and 170 nm have been synthesised in aqueous solution using a seed-mediated growth method, employing hydroxylamine hydrochloride as a reducing agent. Thiolated polyethylene glycol (mPEG-SH) polymers, with molecular weights ranging from 2100 to 51 000 g mol-1, were used as efficient particle stabilising ligands. Dynamic light scattering and zeta potential measurements confirmed that the overall mean diameter and zeta potential of the capped nanoparticles increased in a non-linear way with increasing molecular weight of the mPEG-SH ligand. Electron microscopy and thermal gravimetric analysis of the polymer-capped nanoparticles, with a mean gold core diameter of 15 nm, revealed that the grafting density of the mPEG-SH ligands decreased from 3.93 to 0.31 PEG nm22 as the molecular weight of the ligands increased from 2100 to 51 400 g mol-1 respectively, due to increased steric hindrance and polymer conformational entropy with increase in the PEG chain length. Additionally, the number of bound mPEG-SH ligands, with a molecular weight of 10 800
g mol-1, was found to increase in a non-linear way from 278 (s = 42) to approximately 12 960 PEG (s = 1227) when the mean Au core diameter increased from 15 to 115 nm respectively. However, the grafting density of mPEG10 000-SH ligands was higher on 15 nm Au nanoparticles and decreased slightly from 1.57 to 0.8 PEG nm22 when the diameter increased; this effect can be attributed to the fact that smaller particles offer higher surface curvature, therefore allowing increased polymer loading per nm2. Au nanoparticles were also shown to interact with CT-26 cells without causing noticeable toxicity.