Figure 7 A) The simulated enhanced electric field distribution near the tips (top row) and around the core (bottom row) of AuNS. E is the polarisation direction and K is the propagation direction. The wavelength of the incident light is 785 nm; B) The simulated LSPR peaks of AuNS@BSA with different thicknesses of protein layers around the core (PC – protein corona and its thickness).
To further study this phenomenon we performed MD simulations to reveal the potential link between gold surface curvature (shape of GNPs) and the adsorption of BSA protein. Four types of gold surface models were constructed (Figures 8 A–D ): plane (P), truncated cone large (TCL), truncated cone small (TCS), and spherical tip (ST). For other details regarding the simulation please see Supporting Information . Initially, the protein was placed away from the gold surface with a minimum distance of 0.8 nm. The protein was adsorbed on the gold surface within a 1 ns simulation. The energy of the system was averaged over a 1 ns simulation after adsorption under a canonical ensemble. The temperature was set to be 300 K. To quantify the adsorption strength between BSA and the gold surface, the adsorption energy \(E_{\text{ad}}\) was calculated as (Equation 1):
\(E_{\text{ad}}=\ -\left(E_{Gold@BSA}-E_{\text{BSA}}-E_{\text{Gold}}\right)\)(1)
where \(E_{Gold\&BSA},\ E_{\text{BSA}}\ \)and \(E_{\text{Gold}}\) are energies of Gold@BSA, BSA and gold surface structures respectively. A larger adsorption energy indicates a stronger interaction between BSA and the gold surface, so the BSA is more likely to remain adsorbed and less likely to desorb under the local thermal fluctuations.[41] For each gold surface model, the extreme and average adsorption energies along 10 directions are presented in Figure 8 E . The complete results including each orientation can be found in the Supporting Information Table S3 . When the curvature of the gold surface increases, the number of gold atoms within the force cut-off distance of BSA decreases, and the van der Waals force decreases. Thus, with the increase of the radius of curvature, the adsorption energy decreases significantly. Such results illustrate that BSA is less likely to be adsorbed on the surface with large curvatures, such as the spikes on the gold nano star surface, which supports the experimental finding that BSA is seldom found on the AuNS tips.