We have developed a surface-enhanced Raman spectroscopic technique for the determination of Pb2+, Cd2+, and Cu2+ 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 demonstrated by the distinct spectral changes that occurred through the interaction of Pb2+, Cd2+, and Cu2+ 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 Pb2+, Cd2+, and Cu2+ were 522,50.3, and 1.49 ppb, respectively. How experiments using an optical fiber probe indicated instantaneous response to changes in metal concentration.We have recently demonstrated the ability to detect ionic species1-4 and organic compounds in aqueous solutions using chemically modified silver surfaces.* 12345•6 The approach we have developed combines surface-enhanced Raman spectroscopy (SERS), analyte-specific affinity coatings, and fiber-optic sensors. The coatings vary from simple alkanes to complex macrocycles with the common theme of containing a thiol or disulfide group to anchor the coating to a silver substrate. Long-chain alkyl disulfides and thiols have been shown to form self-assembled monolayers (SAM) at noble metal surfaces.7•8 These surfaces mimic reversed-phase HPLC coatings and provide excellent surfaces for the detection of hydrophobic organic analytes. The same types of intermolecular interactions responsible for selfassembly may not be present in some of the complex metal ion chelating and macrocyclic coatings that we have synthesized for metal ion detection. However, it has been found that these
Methyl red, cresol red, and 4-pyrldlnethlol were examined for thelr aMHy to detetmlne the pH at a surface. Methyl red and cresol red were coupled to cystamlne. Thls produced a dlsulflde derlvatlzed Indicator whlch formed robust monolayers on sllver substrates. The spectroscoplc technlque used to examlne these compounds was surfaceanhanced Raman spectroscopy. We determlned that all of these compounds possessed surface-enhanced Raman bands which were characterktlc of the molecular structure assoclated wtth the kwllcator and tts conjugate ackl. Methyl red represents a near resonance Indicator. It showed a hear log ( IlU,,/f ,,,15) vs pH relatlonshlp trom pH 2 to 4.5. Some Interference due to buffer anlons was observed. Cresol red was examlned as a resonance Raman Indkator. Its llnear range was pH 2-8 for the ratlo of Zlm/I 1990. An Inflection In the callbratlon curve was observed near pH 5. Cresol red was tested on an optkal flber for Its usefulness as a remote pH probe. It functloned very well and was used to follow a flowing, pH gradlent pr+ duced by adjustlng a reservolr pH wlth acetlc acld. 4-Pyrldlnethlol lo a normal Raman Indlcator.It possessed a hear log (IlOw/Zllm) vs pH relationship between pH 5 and pH Q. The callbratlon curves, lncludlng the lnflectlon In the case of cresol red, are explalned wIth Gouy-Chapman-Stem theory.
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