Overnight digestion was conducted on both samples using aqua regia (25 μL aqua regia was added to each sample to digest and oxidize the AuNPs and the dissolved organic matter, the samples were diluted with DI water the next day to measure Au concentration using iCAP 6000 Series ICP-OES Spectrometer (Thermo Fisher Scientific Inc, UK), calibrated using Gold Standard for ICP (Sigma Aldrich Co Ltd., UK) at concentrations of 0, 5, 10, 15, and 20 ppm.