Spectral evidence on the effectiveness of the ligand exchange. (a) FTIR spectra of pure capping ligands including CTAB, PVP, DEA, and TSC. (b) FTIR spectra of AuNRs with different capping ligands obtained at different stages of the ligand exchange. (c–f) 1H NMR spectra of the AuNRs with the capping ligands of CTAB, PVP, DEA, and TSC, respectively. The spectra of pure chemicals are listed for comparison.