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.