Qualitative and semi-quantitative trace analysis of acidic monoazo dyes by surface enhanced resonance Raman scattering

Abstract

Resonance Raman (RR) and surface enhanced resonance Raman scattering (SERRS) procedures are described for the analysis of acidic monoazo dyes. By the comparison of the RR spectra, discrimination is achieved between 20 acidic monoazo dyes, including structural isomers. However, difficulties are experienced due to fluorescence, to the narrow concentration range over which scattering is observed (10^–3–10^–4 mol l^–1) and to the relatively high detection limit (approximately 3–5 µg). These difficulties were overcome by the development of a robust, sensitive and selective SERRS procedure. Controlled aggregation of a citrate-reduced silver colloid and strong SERRS of the adsorbed dyes can be obtained if a 0.01% aqueous solution of poly(L-lysine) is added to an aliquot of colloid followed by aqueous solutions of the dye and ascorbic acid. The enhancement in scattering intensity compared to solution resonance is approximately 10⁵–10⁶ and strong SERRS is observed for sub-nanogram amounts of dye. In addition, the fluorescence background is quenched and a wide concentration range can be examined. Models are proposed for the bonding of poly(L-lysine) with o-hydroxy-, p-hydroxy- and o-dihydroxyarylazo dyes and a single model of interaction is proposed for the adsorption of the full set of dyes on the silver surface in the presence of ascorbic acid. The results of a blind trial confirm the usefulness of SERRS for qualitative analysis and highlight the importance of sample purity. Linearity in a plot of concentration versus scattering intensity was observed at low solution concentrations (<3 × 10^–6 mol l^–1), supporting the application of SERRS for both qualitative and semi-quantitative analysis of trace amounts (300–500 pg) of acidic monoazo dyes.