Electronic Structure and Bonding of Au on a ${\mathrm{S}\mathrm{i}\mathrm{O}}_2$ Cluster: A Nanobullet for Tumors

The binding of gold atoms to a small silica cluster has been studied by using density functional theory with generalized gradient approximation for exchange and correlation. It is shown that gold atoms bind to silicon atoms with dangling bonds and serve as seeds for the growth of Au islands. The large electron affinity of gold causes a significant change in the electronic structure of silica resulting in a substantial reduction in the highest occupied and the lowest unoccupied molecular orbital and the optical gap, thus allowing it to absorb near infrared radiation. This suggests that a small cluster can have a similar functionality in the treatment of cancer as the large size nanoshell, but for a different mechanism.

Figure 1

Geometry and charge distribution of $({\mathrm{S}\mathrm{i}\mathrm{O}}_2{)}_3$. The numbers in parentheses refer to charges, while other numbers refer to bond lengths in $\AA$. The balls with light color are for O atoms. The arrows specify the charges on Si atoms.

Figure 2

(a) Six initial structures of $\mathrm{A}\mathrm{u}({\mathrm{S}\mathrm{i}\mathrm{O}}_2{)}_3$ used for geometry optimization. (b) Final optimized geometry.

Figure 3

Geometry and charge distributions in ${\mathrm{A}\mathrm{u}}_2\mathrm{\text{−}}({\mathrm{S}\mathrm{i}\mathrm{O}}_2{)}_3$.

Figure 4

Three different initial configurations for ${\mathrm{A}\mathrm{u}}_3\mathrm{\text{−}}{\mathrm{S}\mathrm{i}}_3{\mathrm{O}}_6$.

Figure 5

Two low energy isomers of ${\mathrm{A}\mathrm{u}}_3\mathrm{\text{−}}{\mathrm{S}\mathrm{i}}_3{\mathrm{O}}_6$.

Figure 6

HOMO and LUMO orbitals for ${\mathrm{A}\mathrm{u}}_3\mathrm{\text{−}}(\mathrm{S}\mathrm{i}{\mathrm{O}}_2{)}_3$.

Figure 7

Energy eigenvalues for $({\mathrm{S}\mathrm{i}\mathrm{O}}_2{)}_3$ and ${\mathrm{A}\mathrm{u}}_3\mathrm{\text{−}}({\mathrm{S}\mathrm{i}\mathrm{O}}_2{)}_3$. The solid lines refer to occupied states, and the dotted lines for unoccupied levels. Because of the multiplicity of two for ${\mathrm{A}\mathrm{u}}_3\mathrm{\text{−}}({\mathrm{S}\mathrm{i}\mathrm{O}}_2{)}_3$, spin-up and spin-down states are drawn separately.