*3.1. Reagents and Materials*

All the reagents were used as received, without further purification. All the solutions were prepared using deionized water (Milli-Q water, 18.2 MΩcm−<sup>1</sup> at 25 ◦C). Zn(CH3COO)<sup>2</sup> (99.99%, trace metal basis), HAuCl4•3H2O (ACS Reagent, 49.0+% Au basis), Ethanol (200 proof, anhydrous, ≥99.5%), NaBH<sup>4</sup> (99.99% trace metals basis), melamine (2,4,6-Triamino-1,3,5-triazine, 99%), reduced graphene oxide (rGO), ethylenediaminetetraacetic acid, disodium salt dihydrate (EDTA, ACS Reagent, 99.9+%), P-benzoquinone (Spect. Grade, 99.5+%), and methanol (HPLC Plus, 99.9+%) were provided by Sigma– Aldrich (Darmstadt, Germany). NaOH (98+%) and HCl (36% *w*/*w* aq. soln) were acquired from Alfa Aesar. Whatman® Puradisc 13 disposable syringe filters (Clifton, NJ, USA) were used for the aliquot filtration process.
