Helical gold nanorods as chiral recognition nanostructures: a relativistic density functional theory study.

We establish helical gold nanorods as the first examples of chiral recognition nanostructures by examining the simple chiral molecule CClHDT adsorbed on the helical Au40 nanorod. We calculate the vibrational circular dichroism (VCD) spectra of the R and S enantiomers of CClHDT adsorbed on the R (or S) enantiomer of Au40 using relativistic density functional theory. The highest adsorption energy is found when the Cl atom of CClHDT binds to a low-coordinated Au atom at the edge of Au40. There are three adsorption modes (essentially identical in energy) corresponding to three orientations of the HDT moiety. We show that, for each adsorption mode, the VCD spectra are distinctly different for the Au40(R)-ClHDT(R) and Au40(R)-CClHDT(S) complexes, and we give a qualitative explanation for this based on the principle of chirality transfer. For comparison with the results for Au40, we calculate the VCD spectra of the R and S enantiomers of CClHDT adsorbed on the achiral Au20 tetrahedral cluster. Again, there are three adsorption modes (essentially identical in energy) corresponding to three orientations of the HDT moiety. However, we show that, for each adsorption mode, the VCD spectra are mirror symmetric but otherwise essentially identical for the Au20-CClHDT(R) and Au20-CClHDT(S) complexes. Thus, the inherent chirality of the helical Au40 nanorod is essential for its chiral recognition functionality.