Evidence suggests that 4MPy coexists as the neutral and the protonated species when bound to AuAg on the OCB, but not bound to the metal through the nitrogen atom. Protonation of 4MPy is known to weaken and shift the 9a1 β(CH) 1191 cm -1 peak to a higher wavenumber in the range between ~1200 and ~1225 cm -1 (25 -27), making it difficult to distinguish from the 3b2 β(CH) band located at 1227 cm -1 . The 8a1 νCC band for the non-protonated nitrogen occurs around 1580 cm -1 , while the 8a1 νCC peak at 1617 cm -1 is signature for protonated nitrogen . In other words, SERS detects both protonated and non-protonated 4MPy on the OCB. Finally, the peak at 1285 cm -1 is assigned to β(CH)/δ(NH) , while minor peaks below 900 cm -1 are likely pyridine ring deformation and aromatic CH out-of-plane bending modes .