2.2. Characterization of NPs

Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to characterize the morphology and pristine size of Ag NPs and HA-Au NPs. For SEM, 2 µL of Ag NP (2 g/L) or HA-Au NP (1 g/L) stock dispersions were pipetted on a conductive carbon adhesive tape. Excess water was removed with filter paper, the samples were air-dried and then imaged using HITACHI SU8010 SEM, Tokyo, Japan. For TEM, 2 µL of Ag NP or HA-Au NP stock dispersions were diluted to 0.2 g/L. Then, 2 µL of the NP dilution was pipetted on a carbon film supported by a copper grid, air-dried and imaged with HITACHI HT7700 TEM, Tokyo, Japan. NP size distributions were measured from the TEM and SEM images using ImageJ [20]. Approximately 100–200 particles were measured for each NP type. The mean hydrodynamic diameters (HDD) of Ag NPs and HA-Au NPs in water or medium were determined by dynamic light scattering (DLS) measurements using a Zetasizer Nano S90 instrument (Malvern, UK). The ζ potential of NPs was determined with nanoPartica SZ-100V2 (Horiba, Kyoto, Japan). For the measurements, NPs were diluted to 100 mg/L in the respective media and immediately measured in triplicates. Ultraviolet-visible (UV-Vis) absorption spectra were recorded on a Shimadzu UV-Vis spectrophotometer (UV-3600 Plus, Tokyo, Japan). Ag NP spectra were measured in the range of 300–700 nm and HA-Au NP spectra between 400–800 nm.