Professor Paul Mulvaney is a pioneer in the field of nanoscience. He developed the technique of surface plasmon spectroscopy (SPS), which utilizes the unique colours of metal nanocrystals, to monitor surface chemical reactions. Using SPS, he was the first person to measure rates of electron transfer to metal nanocrystals and to follow the nucleation and growth of silver from the atom to the bulk. He showed that SPS enables the optical detection of electrons in metal nanostructures. This versatile optical tool opened up applications in single molecule sensing and biolabelling, and contributed to the development of the new multidisciplinary field of plasmonics.