2.4. Characterization of Gold NPs

Transmission electron microscopy (TEM): To evaluate the size and the presence of aggregates, all particle species were imaged using a 100 kV Zeiss Libra 120 electron microscope (Carl Zeiss AG, Oberkochen, Germany) at a magnification of 80,000×. For analysis, samples were pipetted onto carbon-coated copper grids (300 mesh; Plano, Wetzlar, Germany) and incubated for 3 min. Excess NP dispersion was removed with a filter paper. All grids were air-dried and stored in a desiccator until TEM analysis.

Hydrodynamic diameter and zeta potential: The size and zeta potential of all NP species were measured with a Malvern Zetasizer Nano ZS (Malvern, Herrenberg, Germany). All samples were measured with a 633 nm He-Ne laser at an angle of 173° backward scatter (25 °C) in 10% Dulbecco’s Phosphate-Buffered Saline liquid (DPBS) using either polystyrene semi-microcuvettes (dynamic light scattering; Sarstedt, Nuembrecht, Germany) or folded capillary cells (zeta potential; Malvern, Herrenberg, Germany), respectively.

UV-Vis spectroscopy: UV-Vis spectroscopic measurements were performed with a FluoStar Omega (BMG Labtech, Ortenberg, Germany). UV-Vis spectra of all particle types were taken in DPBS (λ = 400 to 800 nm).