Previous studies found that when 4MPy adsorbs onto silver NP and film surfaces, it assumes a roughly vertical orientation . The SER data on 4MPy adsorbed on AuAg NP OCB supports this finding. In the region 500-1800 cm -1 , the a1 and b2 modes of 4MPy are enhanced compared with the out-of-plane a2 and b1 vibrations (Figure ). For an upright pyridine orientation, the a1 and b2 modes are normal to the surface. This is consistent with the evidence and interpretations that SER-responsive modes perpendicular to the surface receive the largest enhancement . In contrast, the out-of-plane a2 and b1 vibrations that are roughly parallel to the surface receive only minor enhancement . The bimetallic AuAg NP SER spectra suggest that 4MPy is bound through the sulfur atom and primarily oriented vertically.