Synthesis of citrate-capped PtNPs. The synthesis of citrate-PtNPs was based on a previously reported
procedure that used a citrate reduction method37. First, all glassware and a magnetic stirrer bar were cleaned in
aqua regia (HCl/HNO3 3:1 v/v), then rinsed with ultrapure water several times and oven-dried overnight. 7.78 mg
of platinum chloride (H2PtCl6 · 6H2O), dissolved in deionized water (40 mL), was vigorously mixed with trisodium
citrate (0.4 mL, 100 mM). After stirring for 30 min at room temperature, freshly prepared aqueous sodium borohydride
(200 μ L, 50 mM) was added dropwise to the mixture. Next, the mixture was stirred slowly until the color
of the solution remained stable. Finally, the colloidal solution was stored at 4 °C in the dark. The stored solution of
PtNPs was still color-uniformity, almost no aggregations and precipitates can be found during half a year period.