Here we report the synthesis of well dispersed gold nanoparticles (Au NPs), with diameters ranging between 5 and 60 nm, in water and demonstrate their potential usefulness in catalysis and biological applications. Functionalised polyethylene glycol-based thiol polymers (mPEG-SH) were used to stabilise the pre-synthesised NPs. Successful PEGylation of the NPs was confirmed by Dynamic Light Scattering (DLS) and zeta potential measurements. PEG coating of the NPs was found to be key to their colloidal stability in high ionic strength media, compared to bare citrate-stabilised NPs. Our results show that PEG–Au NPs with diameters <30 nm were useful as catalysts in the homocoupling of arylboronic acids in water. Additionally, PEG–Au NPs were also shown to be stable in biological fluids, non-cytotoxic to B16.F10 and CT-26 cell lines and able to successfully deliver siRNA to CT-26 cells, achieving a significant reduction (p < 0.05) in the expression levels of luciferase protein; making these NPs attractive for further biological studies.