Dykman, L. and Khlebtsov, N. (2012) Gold Nanoparticles in Biomedical Applications: Recent Advances and Perspectives. Chemical Society Reviews, 41, 2256-2282. https://doi.org/10.1039/C1CS15166E
There are different methods, but I think using covalent binding is the best way. Three are free passive hydroxyl groups (OH) on surface of nanoparticles solution then you need to convert it to the active binding groups such as carboxyl group (COOH) first using chemicals such as citric acid and then latter by different types of covalent binding such as amide bond (EDC/NHS clic chemistry) make your attachment. Read below papers that explain the procedure in details. Good luck.