Adsorption of bipyridine compounds on gold nanoparticle surfaces investigated by UV-vis absorbance spectroscopy and surface enhanced Raman scattering

Adsorption behaviors of 2,2'-bipyridine (2,2'-BiPy), 2,4'-bipyridine (2,4'-BiPy), and 4,4'-bipyridine (4,4'-BiPy) on gold nanoparticle surfaces have been comparatively investigated by means of UV-vis absorbance spectroscopy and surface-enhanced Raman scattering (SERS). The three bipyridine compounds are assumed to have a standing geometry on Au surfaces as indicated from several spectral features and relative vibrational intensity factors on the basis of the electromagnetic (EM) selection rule. 2,4'-BiPy appears to adsorb on Au surfaces via the 4-pyridyl nitrogen atom as suggested from the stronger enhancement of the vibrational bands ascribed to the 4-pyridyl ring. The SERS intensities for the three bipyridine compounds on Au could be ascribed to both the electromagnetic (EM) and charge transfer (CT) enhancement mechanism. The charge transfer is assumed to be dissimilar for 2,2'-BiPy, 2,4'-BiPy, and 4,4'-BiPy due to their different positions of the nitrogen atoms as indicated from the disparate nu(8a) band enhancements upon adsorption on surfaces.