Disclosed is a method to prepare gold nanoparticles colloid with desirable properties for biomedical applications. The method is based on exposing the precursor solution to a synchronic X-ray source without any pre-added stabilizer or surfactant. The gold nanoparticles can be highly concentrated and possess sufficient stability and re-dispersion capability. In addition, the poly ethylene glycol (PEG) can be added before exposure, such that the surface of the gold nanoparticles is coated by the PEG to form a PEG-Au. Both colloidal gold have been successfully employed in the studies of interactions between biological species and nanoparticles.
