Regrowth of SiO2 particles to 250 nm. The 30 nm silica seed particles were regrown to a larger size following a seeded Stöber approach. EtOH (910 mL) and ammonium hydroxide solution (70 mL) were mixed in a 2 L round-bottomed flask to which 1.6 mL of the 30 nm SiO2 nanoparticle suspension was added under stirring. Tetraethyl orthosilicate (100 mL) was then added by a syringe pump (KDScientific, 78-9100F) at a rate of 1.4 mL h -1 through a cannula (Braun, Venofix A). The solution was left for ∼24 h after the tetraethyl orthosilicate addition had completed. The ammonia was removed and the volume of the nanoparticle solution was reduced by rotary evaporation (ice-cooled condenser, 60 °C, 0.2 atm, 240 rpm). Once a suitable volume had been reached, the particles were cleaned by centrifugation (3000 g, 5 min) before resuspension in a fresh solvent (MeOH, EtOH, i-PrOH, or t-BuOH). The size of the particles was determined to be (250 ± 3) nm by TEM (Figure ). The concentration of the particles was determined by dry extraction to be 26.9 g L -1 (1.5 × 10 12 mL -1 ).