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

Resonance Raman (RR) and, in aqueous silver colloids, Surface Enhanced Rama n (SER) spectra of both uranyl(VI) and 2-(5-bromo-2-pyridylazo)-5-diethylam inopenol (Br-PADAP) have been studied. The neutral Br-PADAP molecule had pr eviously been shown to appear mainly as a type Z tautomer (zwitterion with a N(C2H5)(2)H+ group) in polar media or types Q tautomers in non-polar solv ents, 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 covalentl y bonded to silver and is rather brought close to the surface through elect rostatic and hydrophobic forces. A peak at 828 cm(-1) is observed for the s ymmetric 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 d isplays an overall type Z spectrum profile. The intensity of this band incr eases linearly with the uranyl(VI) concentration in the range 5 x 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 ura nyl(VI) band no longer increases. Similar results are observed in the pH ra nge 5-8, despite a long term unstability of uranyl(VI)-Br-PADAP complexes a t the high pH limit, provided spectra are recorded soon after introduction of uranyl(VI) in the sol. (C) 1999 Elsevier Science B.V. All rights reserve d.