Ammonia modified graphene oxide - Gold nanoparticles composite as a substrate for surface enhanced Raman spectroscopy

Simple treatment of graphene oxide (GO) with ammonia solution at room temperature changes its chemical composition as revealed by infrared, Raman and X-ray photoelectron spectroscopy results. Both - pristine GO and GO modified with ammonia solution (NH3-GO) were combined with spherical gold nanoparticles (AuNPs) or gold nanourchins (AuNURs) and studied as possible substrates for surface enhanced Raman spectroscopy (SERS) using Rhodamine 6G (R6G) as probe. AuNPs or (AuNURs) were adsorbed on thin layers of either as-prepared GO or NH3-GO. The AuNPs/NH3-GO support enables easy identification of the characteristic bands of R6G at the 10(-7) M concentration, while for AuNPs/GO the R6G concentration must be at least ten times higher in order to obtain the same quality spectra. For AuNURs the difference between NH3-GO and GO is even more significant. The observed enhancement factors are higher for AuNURs too. The improved SERS intensity results mainly from more efficient adsorption of AuNPs on NH3-GO comparing to pristine GO, as suggested by SEM and EDS results. Another possible factor affecting the SERS activity may be partial reduction of GO by NH3 and introduction of nitrogen functionalities which in turn affects the chemical and dipole contribution to SERS enhancement.