The zeta-potentials of silica, copper, platinum and gold particles have been measured as a function of pH. The isoelectric points were found to be at pH 3.0, 5.8, 3.0 and 3.5, respectively. In the pH range 3.0 to 5.8 copper and silica particles are oppositely charged and accordingly the coating of silica with copper particles could be demonstrated. In the case of gold and platinum the sign of the charge is such that direct adhesion to silica particles cannot be expected and this was also demonstrated in the case of platinum.
Lindberg, R., Sundholm, G., Sjöblom, J., Ahonen, P., & Kauppinen, E. I. (1999). Studies of adhesion of metal particles to silica particles based on zeta potential measurements. Journal of Dispersion Science and Technology, 20(1-2), 715-722. https://doi.org/10.1080/01932699908943816