2.7 X-ray Photoelectron Spectroscopy (XPS).
XPS analyses were performed using a PHI VersaProbe II spectrometer equipped with a monochromatic Al Kα X-ray source (1486.6 eV) operated at a spot size of 100 μm corresponding to a power of 24.5 W. Survey (0-1400 eV) and high resolution (C 1s, O 1s, N 1s, Au 4f) spectra were recorded in FAT (fixed analyzer transmission) mode at pass energy of 117.4 and 46.95 eV, respectively. All spectra were acquired at a take-off angle of 45° with respect to the sample normal. Dual-beam charge neutralization was constantly applied during analysis. XPS analysis was repeated on three different spots of the sample. Special attention was devoted to verify that no change in the sample was induced by exposure to the X-ray beam and to the dual-beam used for charge neutralization. Detailed spectra processing was performed by commercial MultiPak software (Version 9.5.0.8, 30-10-2013, Ulvac-PHI, Inc.). Charge correction of the spectra was performed by taking the hydrocarbon (C-C/C-H) component of the C 1s spectrum as internal reference (binding energy, BE = 285.0 eV). The high-resolution spectra were fitted with mixed Gaussian-Lorentzian peaks after a Shirley background subtraction. A maximum relative standard deviation of 10% was estimated on the area percentages of the curve-fitting components, while the determined standard deviation of their position was ±0.2 eV.
2.8 Zeta potential and size measurements.
The average of the hydrodynamic diameters and the polydispersity index (PDI) of the AuNPs-SS were measured by dynamic light scattering and using a Zetasizer Nano ZS (Malvern Instruments Ltd., Worcestershire, UK). The surface charge of AuNPs-SS was measured as zeta potential by laser doppler velocimetry using the same instrument after dilution with 1 mM KCl.