While surface-enhanced Raman scattering (SERS) can increase the Raman cross-section by 4-6 orders of magnitude, for SERS to be effective it is necessary for the analyte to be either chemically bonded or within close proximity to the metal surface used. Therefore most studies investigating the biochemical constituents of microorganisms have introduced an external supply of gold or silver nanoparticles. As a consequence, the study of bacteria by SERS has to date been focused almost exclusively on the extracellular analysis of the Gram-negative outer cell membrane. Bacterial cells typically measure as little as 0.5 by 1 mum, and it is difficult to introduce a nanometer sized colloidal metal particle into this tiny environment. However, dissimilatory metal-reducing bacteria, including Shewanella and Geobacter species, can reduce a wide range of high valence metal ions, often within the cell, and for Ag(I) and Au(III) this can result in the formation of colloidal zero-valent particles. Here we report, for the first time, SERS of the bacterium Geobacter sulfurreducens facilitated by colloidal gold particles precipitated within the cell. In addition, we show SERS from the same organism following reduction of ionic silver, which results in colloidal silver depositions on the cell surface.

Sudan-1 has been used for coloring food. However, recent alarms worldwide about the carcinogenic and mutagenic properties of azo-compounds have led to concerns over their human consumption. In the U.K. in 2005, over 570 products were found to be contaminated with the azo dye Sudan-1 and this and the health risks associated with this dye resulted in the subsequent international ban of this additive in all foodstuff, at all levels, relating to human consumption. These incidents have also necessitated the need for high throughput low cost reliable approaches for the detection and quantification of food contaminated by such azo compounds. While there are a small number of analytical techniques that can be considered portable, many lack sensitivity. By contrast, we show that employing a portable Raman spectrometer, using surface enhanced Raman scattering (SERS), can provide good sensitivity, such that Sudan-1 can be quantified in a complex food matrix reliably over the range of 10 -3 to 10 -4 mol L -1. We also demonstrate that a variety of multivariate approaches including principal components analysis (PCA), partial least-squares (PLS) regression, artificial neural networks (ANNs), and support vector regression (SVR) can be employed for the chemical analysis of this dye in a quantitative manner. Compared to the commonly used univariate approaches, where the area under a single band in assessed, the advantage of using multivariate approaches is that these algorithms can analyze the full spectra directly and the laborious task of selecting and integrating marker appropriate quantitative spectral bands can be avoided thus greatly simplifying and speeding up data analysis.

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.

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In a novel application SERRS has been used, by employing a silver sol, to monitor and analyse the conversion of indigo into the indigo carmine dye.