A method of synthesizing ligand-conjugated gold nanoparticles (AuNPs) is disclosed. The method comprises: a) providing an amine-modified silica particle; b) providing a solution comprising Au+3 ions; c) suspending the amine-modified silica particle in the solution comprising Au+3 ions; d) allowing the Au3+ ions to be adsorbed and/or immobilized onto the surface of the amine-modified silica particle; e) exposing the Au3+ ions immobilized onto the surface of the amine-modified silica particle to radiation to obtain bare gold nanoparticles (AuNPs) adsorbed and/or immobilized onto the surface of the amine-modified silica particle, wherein the bare AuNPs are without organic surface modifications; and f) reacting a ligand with the bare AuNPs adsorbed and/or immobilized onto the surface of the amine-modified SiNP and thereby obtain ligand-conjugated gold nanoparticles (AuNPs).