Spectral changes associated to physicochemical alterations in a colloidal system are key in nanosensor design for decentralized analyses. Citrate-capped spherical gold nanoparticles in basic medium interact with lead (II) ions to give rise to different products depending on the molar concentration ratio nanoparticles/lead (II). For low values of this ratio, lead (II) ions act as charge shield forming ionic associations. For intermediate values, lead (II) ions coordinate to carboxylate moieties from citrate, inducing aggregation of the nanosystem. Finally, for the higher values, a redox reaction is favoured leading to the deposition of PbO2 on the nanoparticle surface, thus causing changes in the UV/VIS/NIR spectra compatible with the formation of nanotriangles. Transmission Electron Microscopy confirms this shape change and the incidence of the incident rays in the vacuum chamber on the sample confirm the selective melting of the formed PbO2 layer.