## 2 Materials and Methods
### Characterization of AgNPs-FITC NP and AgNPs-RITC  
UV-Vis spectra were recorded in aqueous solution using quartz cells with a Varian Cary 100 Scan spectrophotometer. The sizes, size distributions, and Z-potential of AgNPs-FITC and AgNPs-FITC in aqueous solution were investigated using the dynamic light scattering (DLS) technique using a Zetasizer instrument (Malvern) at a temperature (T) of 25.0 \(\pm\) 0.2 \({}^{\circ}\)C and NPs concentration of 0.02 mg/mL. Z-potential was measured by means of the laser microelectrophoresis technique in a thermostated cell (T = 25.0 \(\pm\) 0.2 \({}^{\circ}\)C). DLS data were acquired and fitted in concordance to our previous works [58].  
Synchrotron radiation (SR)-XPS measurements were carried out at the CNR beamline BACH (Beamline for Advanced DiCHroism) at the ELETTRA facility in Trieste (Italy). The BACH beam line exploits intense radiation emitted from an undulator front-end. XPS data were collected in angular mode (A21, pass energy = 50 eV), with the entrance and exit slits of the monochromator fixed at 30 and 20 \(\upmu\)m, respectively. A photon energy of 360 eV was used for C1s and S2p spectral regions, while for the N1s, Ag3d, and O1s spectral regions, a photon energy value of 596 eV was selected to maximize signal intensity. The total binding energy resolution for C1s and S2p was 0.17 eV, while that for N1s, Ag3d, and O1s was 0.22 eV. The energy scale was calibrated using as reference the C1s aromatic signal at 284.7 eV and the Ag3d\({}_{5/2}\) signal of AgNPs arising by the metallic silver core, always found at 367.80 eV.  
Curve-fitting analysis of the C1s, S2p, N1s, O1s, and Ag3d spectra was performed using Gaussian curves as fitting functions identified using a peak-fitting procedure (fityk 1.3.1, [[http://fityk.nieto.pl](http://fityk.nieto.pl)]([http://fityk.nieto.pl](http://fityk.nieto.pl)) accessed on 10 December 2020). S2p3/2,1/2 and Ag3d5/2,3/2 doublets were fitted using the same full width at half-maximum (FWHM) for each pair of components of the same core level, a spin-orbit splitting of 1.2 eV and 6.0 eV, and branching ratios of S2p3/2/S2p1/2 = 2/1 and Ag3d5/2/Ag3d3/2 = 3/2. When several different species were identified in a spectrum, the same FWHM value was used for all individual photoemission bands.  
The pump-probe experiments were performed using a laser system consisting of a 1 kHz chirped pulse amplifier (800 nm, 4 mJ, 35 fs) seeded by a Ti:Sa oscillator. The pump pulse was either the second harmonic of the amplifier (400 nm) or the output of a tunable optical parametric amplifier seeded by the amplifier (560 nm). The probe was a white light supercontinuum (360-780 nm) generated in a commercial transient absorbance spectrometer (FemtoFrame II, IB Photonics) by focusing about 3 \(\upmu\)J of the amplifier beam into a rotating 3 mm thick CaF\({}_{2}\) crystal. The pump pulse was loosely focused (circular spot of diameter = 700 \(\upmu\)m) onto the sample contained in a 1 mm quartz cell, with a power density of 180 \(\upmu\)J/cm\({}^{2}\). The spot diameter of the probe pulse was approximately 150 \(\upmu\)m, and its delay time with respect to the pump pulse was scanned in time by varying the length of its optical path. The transmitted probe spectrum was collected in a monochromator and recorded on a CCD camera. The transient absorbance (TA) spectra were measured in two polarization states: the first in which the pump and probe pulses were parallel, and the second with polarizations at the magic angle. The instrument response function was measured to be approximately 50 fs. More experimental details of the setup can be found in previous publications [59].