The resonant wave modes in monomodal and multimodal planar Surface Plasmon Resonance (SPR) sensors and their response to a bidimensional array of gold nanoparticles (AuNPs) are analyzed both theoretically and experimentally, to investigate the parameters that rule the correct nanoparticle counting in the emerging metal nanoparticle-amplified surface plasmon resonance (PA-SPR) spectroscopy. With numerical simulations based on the Finite Element Method (FEM), we evaluate the error performed in the determination of the surface density of nanoparticles σ when the Maxwell-Garnett effective medium theory is used for fast data processing of the SPR reflectivity curves upon nanoparticle detection. The deviation increases directly with the manifestations of non-negligible scattering cross-section of the single nanoparticle, dipole-dipole interactions between adjacent AuNPs and dipolar interactions with the metal substrate. Near field simulations show clearly the set-up of dipolar interactions when the dielectric thickness is smaller than 10 nm and confirm that the anomalous dispersion usually observed experimentally is due to the failure of the effective medium theories. Using citrate stabilized AuNPs with a nominal diameter of about 15 nm, we demonstrate experimentally that Dielectric Loaded Waveguides (DLWGs) can be used as accurate nanocounters in the range of surface density between 20 and 200 NP/µm², opening the way to the use of PA-SPR spectroscopy on systems mimicking the physiological cell membranes on SiO₂ supports.

The strategies used to associate peptide arginylglycylaspartic acid (RGD) with calcium phosphate grafts to enhance cell–biomaterial interactions have been controversial in the literature. Several works have demonstrated that RGD-functionalized hydroxyapatite (HA) surfaces improve cell adhesion, whereas others claim that RGD-loaded HA has an inhibitory effect when serum is present in the biological medium. To investigate such contradictory results, we associated RGD with the HA surface using physical adsorption and chemical bonding methods and evaluated the cell adhesion and spreading in pre-osteoblasts culture with and without fetal bovine serum (FBS). The effect of functionalization methods on the physicochemical characteristics of both surfaces was analyzed using multiscale techniques. Adsorption assays of serum allowed us to estimate the impact of the association method on the HA surface’s reactivity. Physically adsorbed RGD did not increase the number of adhered cells due to the weak interactions between the peptide and the surface. Although chemical binding stabilizes RGD on the HA, the functionalization procedure covered the surface with molecules such as (3-aminopropyl)triethoxysilane (APTEs) and carbodiimide, changing the surface’s chemical activity. Serum protein adsorption decreased by 90%, revealing a significant reduction in the surface interactions with molecules of the biological medium. The present study’s findings showed that the RGD’s physical association with HA did not improve cell adhesion and that this phenomenon is highly dependent on the presence of serum proteins.