We report the reaction of muonium (Mu = [μ+e-]), a light isotopic analog of hydrogen, with uncapped gold nanoparticles embedded in mesoporous silica. Using the radio-frequency muon spin rotation (RF-μSR) technique, we directly observe and characterize the resulting final state on the nanoparticle surface, showing conclusively its diamagnetic nature. The magnetic environment experienced by the reacted muons is only weakly perturbed compared to that of muons in a silica reference, consistent with the surface of the gold nanoparticles being metallic and non-magnetic. We demonstrate the potential of RF-μSR for the investigation of the surface properties of nanoparticles and show the feasibility of Knight shift measurements of muons on metal surfaces.