Surface-enhanced raman spectra of both uranyl(VI) and 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol in silver colloids

Abstract

Resonance Raman (RR) and, in aqueous silver colloids, Surface Enhanced Raman (SER) spectra of both uranyl(VI) and 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol (Br-PADAP) have been studied. The neutral Br-PADAP molecule had previously been shown to appear mainly as a type Z tautomer (zwitterion with a N(C 2H 5) 2H + group) in polar media or types Q tautomers in non-polar solvents, with the proton on pyridylazo nitrogen atoms. The RR and SER spectra of metal–Br-PADAP complexes are also of type Q, with covalent bonds between the metal and, simultaneously, nitrogen atoms of the pyridylazo group and the phenolate oxygen. The SER spectrum of Br-PADAP, observed down to 10 −8 M, is essentially of type Z, suggesting that the molecule is not covalently bonded to silver and is rather brought close to the surface through electrostatic and hydrophobic forces. A peak at 828 cm −1 is observed for the symmetric stretching mode of uranyl a short time after adding a small uranyl(VI) concentration to a colloid containing about 10 −5 M Br-PADAP, which displays an overall type Z spectrum profile. The intensity of this band increases linearly with the uranyl(VI) concentration in the range 5×10 −9 M to 10 −7 M. When this concentration becomes similar to or larger than the Br-PADAP one, the SER spectrum of Br-PADAP reverts to type Q, while the uranyl(VI) band no longer increases. Similar results are observed in the pH range 5–8, despite a long term unstability of uranyl(VI)–Br-PADAP complexes at the high pH limit, provided spectra are recorded soon after introduction of uranyl(VI) in the sol.