Raman spectroscopy of gold nanoparticle conjugates of cosmetic ingredient kinetin

We performed a feasibility test of the cosmetic ingredient kinetin (KT)-gold nanoparticle (AuNP) conjugates by means of vibrational Raman spectroscopy and quantum mechanical calculations. The adsorptions of KT on AuNP surfaces were examined by absorption spectra and surface-enhanced Raman scattering (SERS). The size of KT at the initial concentrations of 10−5 M with the AuNP composites was measured to be 22 nm. Density functional theory (DFT) calculations were performed to estimate the energetic stabilities of KT on an Au cluster atom. The two tautomeric forms of 9H-amino and 7H-amino in KT are predicted to have similar energies on Au. The N3-coordinating geometries in both 9H-amino and 7H-amino forms of KT are predicted to be most stable on an Au cluster. Vibrational analysis also suggested that the two tautomers of KT should coexist in the adsorbed state on Au. The concentration-dependent SERS spectra of KT indicated that 5 × 10−5 M exhibited the highest SERS signals.