The copper nanoparticles seeded onto the SiO 2 microparticle supports act as a template for the electroless deposition of a platinum film . H 2 PtCl 6 is added to the Cu-SiO 2 particle suspension and mixed in an N2 atmosphere. From the respective half-cell potentials of platinum and copper, a spontaneous galvanic replacement reaction between Cu 0 and Pt is predicted. After 30 minutes, formaldehyde is added as a weak reducing agent to assist the autocatalytic film growth of Pt from the Pt-Cu seeds on the SiO 2 microparticle support . The (yellow) mixture is left to react overnight in an N 2 atmosphere under magnetic stirring. The (now clear) suspension is then centrifuged with double-distilled water, and the resulting particles are black, indicating a successful exchange with Pt and subsequent film growth (Figures ). We note that by increasing the Pt concentrations and reaction times used in the film growth step, the functionalised particles possess thicker and more uniform Pt films. FIB-SEM images show the likely mechanism of film growth, as Pt islands gradually fuse before increasing in thickness (Figure ). We finally mention here the importance of following the colour changes of the particles and solutions as an indicator for the success of the various reaction steps.