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 ).