SERS Surfaces Modified with a 4-(2-Pyridylazo)resorcinol Disulfide Derivative: Detection of Copper, Lead, and Cadmium

We have developed a surface-enhanced Raman spectroscopic technique for the determination of Pb 2+ , Cd 2+ , and Cu 2+ concentration using a 4-(2-pyridylazo)resorcinol (PAR) coating modified with a disulfide. The disulfide provided a strong anchor to a roughened silver substrate. Atomic specificity was deponstrated by the distinct spectral changes that occurred through the interaction of Pb 2+ , Cd 2+ , and Cu 2+ ions with the indicator. The absorption of these metals by the coating was followed with SERS and could be fit to a Frumkin isotherm. Langmuirian behavior was not observed; this is most likely due to the electrostatic repulsions as the metal cations are absorbed. The detection limits at pH 6 for Pb 2+ , Cd 2+ , and Cu 2+ were 522, 50.3, and 1.49 ppb, respectively. Flow experiments using an optical fiber probe indicated instantaneous response to changes in metal concentration