Copper seeding onto SiO 2 microparticles SiO 2 microparticles were modified in water for reasons outlined in the Supporting Information (Text S3). Briefly, 125 mg of 4.16 µm SiO 2 microparticles (SD = 0.19 µm, microparticles GmbH) were dispersed in 49 mL double distilled water, and then 1mL of NH 4 OH was added under stirring (1000 RPM). After 5 minutes, 90 µL of APTES was added dropwise, and the suspension was left to stir for a further 1 hour. The APTES-modified particles were then washed 4 times by centrifugation (1300 RPM, 1 minute) with double-distilled water (50 mL). The suspension was then made up to 45 mL with doubledistilled water, and 5 mL of an aqueous 2mg/mL CuSO 4 solution was added dropwise under stirring (1000 RPM) and left for 1 hour. The Cu(II)-APTES SiO 2 microparticles were then washed 4 times with double-distilled water (50 mL) by centrifugation (1300 RPM, 1 minute).