Gold nanoparticles (AuNPs) chemically grafted on substrates are widely used as sensors due to their plasmonic properties. The efficiency and robustness of such sensors strongly depend on the molecular sublayer structure, which influences the distribution of AuNPs, and therefore the plasmonic properties of the layer. Few spectroscopic tools are able to sense the grafting layer both before and after particle deposition. Here, we use sum-frequency generation (SFG) spectroscopy to deeply investigate both the grafting layer and the immobilized AuNPs. We combine SFG with reflectance UV−visible spectroscopy and scanning electron microscopy (SEM) for 14 nm diameter AuNPs, dispersed on modified silicon surfaces with either amine or mixed amine/thiol terminated layers. SFG spectra show the specific vibrational fingerprint of each supporting layer through the amplitudes of methylene and methyl vibration modes and prove the presence of unreacted ethoxy groups from (3-aminopropyl) triethoxysilane. We establish a linear evolution of the absorbance amplitudes with AuNP surface coverage, a relationship valid up to the aggregation limit of 10 11 AuNPs·cm −2. In the same way, SFG amplitudes follow a quadratic dependence with the UV−vis absorbance amplitudes, showing the close correlation between nonlinear and linear optical properties. In addition, the optical properties of the AuNP layers are stable for several months (plasmon position and damping) despite their storage in ambient air and long exposure to visible laser light.