Semi‐quantitative analysis of alizarin and purpurin by surface‐enhanced resonance Raman spectroscopy (SERRS) using silver colloids

Abstract

Abstract Surface‐enhanced resonance Raman spectroscopy (SERRS) was used for the semi‐quantitative analysis of the anthraquinone dyes alizarin and purpurin, using aggregated citrate‐reduced silver colloids. Laser excitation wavelengths of 514.5 and 632.8 nm were employed. The limits of detection for the analysis of alizarin, by SERRS, were 4 ppm (632.8 nm excitation) using the vibrational band at 1265 cm −1 and 39 ppm (514.5 nm excitation) for the vibrational band at 1334 cm −1 and those for the analysis of purpurin, by SERRS, were 42 ppb (514.5 nm excitation) using the vibrational band at 1330 cm −1 and 8 ppm (632.8 nm excitation) for the vibrational band at 1070 cm −1 . For the first time, the ability to differentiate between purpurin and alizarin using SERRS is reported. Multiple molecular species of both dyes were detected, as evidenced by several isosbestic points in a SERRS pH study using both laser‐exciting wavelengths of 514.5 and 632.8 nm. Unusually, for dye molecules, equally intense SERRS signals can be obtained for alizarin at both acidic and alkaline pH values. Copyright © 2004 John Wiley & Sons, Ltd.