SERS detection of oxyanion contaminants (arsenite, arsenate and perchlorate) on silver nanofilms

Surface-enhanced Raman scattering (SERS) has gained a reputation as one of the most sensitive spectroscopic methods for chemical and biomolecular sensing. In this dissertation the SERS identification and quantification analyses of arsenite (As(III), arsenate (As(V)) and perchlorate have been investigated using two types of Ag nanofilms deposited on solid supports as robust SERS-active substrates (Ag/GL substrate and Ag/Cu substrate) prepared by two simple, controllable and inexpensive one-step electroless plating processes. The substrates have been optimized by varying preparation conditions and comparing SERS intensities of the target molecules, chemically modified with an organic compound, i.e. cysteamine hydrochloride for improved SERS performance (Cys-Ag/Cu or Cys-Ag/rCu substrates), and characterized fully by SEM, EDX elemental analyses, UV-Vis spectra and wettability measurements.;The identification and the quantitative detection of the target molecules on the optimized substrates have been demonstrated with high sensitivity and good reproducibility. The SERS spectra of As(V) on the Ag/GL substrates show that characteristic SERS bands of As(V) appear at ∼ 780 and ∼ 420 cm -1 for dried samples and ∼ 785 and ∼ 425 cm -1 for aqueous samples, while these two SERS bands of aqueous As(III) shift to ∼ 726 and ∼ 439 cm-1, respectively. The characteristic SERS band of perchlorate occurres at ∼ 930 cm-1 consistently for both dried samples on Ag/Cu substrates and aqueous samples on Cys modified substrates. By discerning these characteristic SERS bands, the As(III), As(V) and perchlorated can be identified and distinguished between As(III) and As(V). Various calibration curves with excellent linear relationships have been fabricated successfully in the absence and the presence of coexisting electrolytes. The lowest limit of detection (LOD) was determined to be ∼ 5 &mgr;g L -1 (ppb) for both As(V) and perchlorate. Furthermore, the concentrations of both As (III) and As(V) in a contaminated groundwater have been determined with relatively low measurement errors using the SERS technique coupled with a standard addition method. These findings have demonstrated the great potential of the quantitative SERS detections of these anions in the natural water samples.;In order to fully evaluate the feasibility of the SERS methods for analysis of natural water samples, the effects of coexisting solutes on the SERS spectra and quantitative detections of the target molecules have been examined as well as the selectivities of the substrates. The coexisting solutes were found to suppress SERS enhancements of both As(V) and perchlorate. However, the presence of low concentrations of the coexisting solutes (e.g. 20 &mgr;M total anions concentration) could greatly enhance SERS sensitivity of As(V) and depress the background SERS bands of the Ag/GL substrate. The experiments indicate that the Ag/GL substrate and the Cys-Ag/rCu substrate possess the highest selectivities to As(V) and perchlorate over other common anions, particularly oxyanions. The effects of the solution pH value as well as the temporary stability and reusability of the substrate have been tested for perchlorate SERS sensing on Cys modified substrates. The results indicated that the strongest SERS intensity was observed in the range of pH 6--7. It was found the substrate is fairly stable in the air and could be regenerated easily and reused for perchlorate SERS sensing.;Finally, the SERS enhancement mechanism of perchlorate by cysteamine has been investigated. A new concept referred to as molecular conformation controlled SERS enhancement mechanism has been proposed. It was found that the gauche conformer of Cys molecule contributed more to the perchlorate SERS than the trans conformer, which is a new evidence of the distance dependence of SERS electromagnetic mechanism.;In summary, the findings in this dissertation study have resulted in novel Ag nanofilms as robust SERS sensor platforms for the detection of oxyanions like As(III), As(V) and perchlorate ions, and have moved SERS technique forward for routine use in field assays coupled with commercially available portable Raman systems.