For the synthesis of copper oxide nanoparticles coated with mesoporous silica shells (CuO@SiO2), aqueous solutions of 37.5 mg of CTAB in 8.90 mL of water, 10 mg of L-arginine in 1 mL of water and 0.5 mL of CuO core nanoparticles in water (~1.99 × 10 13 particles, calculated based on 45 nm particle size, 12 mg/mL particle amount, and a CuO density of 6.31 g/cm 3 ) were added to 4.5 mL of Milli-Q water. After brief sonication (20 s), the resulting solution was heated to a temperature setpoint of 95 °C for the heating blocks (resulting in an internal temperature of ca. 80 °C) with stirring at 400 rpm for 30 minutes. Subsequently, 100 µL of TEOS were added at a rate of 5 mL/min, while the heating and stirring were continued for another 3 h. The particles were then cooled down, washed by three cycles of centrifugation (14k rcf, 15 mins, 25 °C) and ultrasonic redispersion (30 s), once with water (12 mL) and twice with ethanol (12 mL).