Figure 5 FEM simulated LSPR peak shift with the change of A) protein concentration in the solution and B) protein corona thickness around nanoparticles (for AuNS the PC is designed around the core only).
Based on the experimental (Figure 3 A–C ) and simulation results, the PC on spherical and anisotropic nanoparticles seems to form quite differently. The PC on spherical and ellipsoidal nanoparticles (AuNP50 and AuNP70) reaches a certain thickness (about ~ 6.5 nm see Supporting Information Table S1 ) with the lowest concentrations of BSA (which is more than enough to form a monolayer, considering the number of molecules per particle 156 to 1119, depending on BSA orientation), after which further increase of protein amount does not contribute to the hard BSA-layer formation.[38] However, the LSPR shift of AuNS is more pronounced and changes with the increase of protein concentration in the media, which indicates an increase of the PC thickness.
2.3 Protein interaction with spherical and anisotropic nanoparticles
Surface enhanced Raman spectroscopy (SERS) signal by GNPs requires close proximity (up to 5 nm) between the analyte and the particle surface,[39,40] preferably near “hot spots”. We, therefore, used SERS measurements to confirm the BSA adsorption onto the AuNP50 and AuNP70. As can be seen from Figure 6 A and B , there are multiple high intensity peaks in the SERS spectra of spherical particles incubated with BSA (see Supporting Information Table S2 for peaks assignment). By way of contrast, the SERS spectrum of AuNS incubated with BSA (Figure 6 C ) does not have any distinct peaks above the noise level. We know from the extinction spectroscopy and zeta potential analysis given above that the protein does adsorb on the AuNS particles, but it seems that nanoparticles “hot-spots” are protein-free.