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.