Figure 4 Zeta potential (surface charge) of GNPs as prepared
and of GNPs incubated with BSA after removal of unbound proteins and
resuspended in water.
To understand the differences in the LSPR peak shift and influence of
adsorbed and free BSA molecules on the extinction of GNPs we performed
FEM simulations of extinction spectra of AuNP50, AuNP70 and AuNS, with
different BSA concentrations (unbound proteins) and with different
thicknesses of PC around them (bound proteins) (see Supporting
Information for more details). The approximate thickness of PC was
calculated by evaluating the size of AuNP50 and AuNS70 before and after
incubation with BSA (estimated to be about ~ 6.5 nm seeSupporting Information Table S1 ). According to the simulation
results, the LSPR peak position is not affected much by the presence of
free protein in the solution, while the thickness of PC has a
significant impact, creating a larger red shift with the increase of its
thickness (Figure 5 ).