Copper (Cu) can provide an alternative to gold (Au) for the development of efficient, low-cost and low-loss plasmonic nanoparticles (NPs), as well as selective nanocatalysts. Unlike Au, the surface oxidation of Cu NPs can be an issue restricting their applicability. Here, we selectively reduce the Cu NPs by low power laser illumination in vacuum and use dark-field scattering to reveal in real time the optical signatures of the reduction process and its influence on the Cu NP plasmonic resonance. We then study reactive processes at the single particle level, using individual Cu catalyst nanoparticles for the selective laser-induced chemical vapour deposition of germanium nanostructures.