Synthesis of 27 nm Gold Nanoparticles (G27s). Freshly prepared aqueous solution of gold (III) chloride trihydrate (0.25 mM, 400 mL) was placed in a 500 mL two-necked round-bottomed flask, and NaOH (1 mM, 50 mL) was then added directly into the solution. The mixture was stirred for 30 mins and then heated to reflux in a 130 °C oil bath under magnetic stirring. After the solution started to reflux, trisodium citrate solution (166 mM, 6 mL) was then quickly added. The solution colour gradually changed from yellow to light red in 15 mins. The reaction was refluxed for another 1 hr to complete the synthesis. The solution was then taken out of the oil bath and kept stirring for 1 hr till it was cooled down to RT. This produced citrate stabilized GNPs with a mean diameter of ∼27 nm (G27) as confirmed by TEM images (see SI, Fig. ). The G27 stock was transferred to a clean glass container and stored at RT till use.