A Solution to the PEG Dilemma: Efficient Bioconjugation of Large Gold Nanoparticles for Biodiagnostic Applications using Mixed Layers

Gold nanoparticles are of high interest in the biodiagnostic and bioimaging field owing to their unique optical properties such as localized surface plasmon resonance (LSPR) and high Rayleigh scattering efficiency in the visible range. Although biofunctionalization is a prerequirement prior to their integration in diagnostic procedures, aggregation-free conjugation of biomacromolecules to large gold nanoparticle is not trivial. Here, a robust and simple method based on commercially available reactants is reported for the efficient biofunctionalization of gold nanoparticles with sizes ranging from 15 to 175 nm. It is demonstrated that mixed poly(ethylene glycol) (PEG) layers, prepared using specific ratios of low- and high-molecular-weight PEG chains, can be conjugated to proteins and monoclonal antibodies using standard carbodiimide chemistry without detectable aggregation. The properties of the mixed PEG interlayer modified gold nanoparticles were investigated using UV-vis spectrometer, dynamic light scattering, and X-ray photoelectron spectroscopy, which demonstrated the importance of controlling biointerfacial properties. Using the epithelial cell adhesion molecule (EpCAM) as a model target antigen, the benefit of the mixed PEG layers over coatings prepared using high-molecular-weight PEG chains only is demonstrated in vitro using bright field microscopy and reflectance confocal microscopy. Very high binding affinity to breast cancer cells was obtained for the mixed PEG layers. This robust procedure demonstrates that, under optimal conditions, a compromise can be achieved between the excellent steric protection provided by thick PEG adlayers and the high bioconjugation yields afforded by adlayers from low-molecular-weight tethers.