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The study of the biological role of metal ions has a long history in medicine, in pharmacology and in toxicology, but it is only recently that the extent and variety of metal ion involvement has been appreciated. For example, among the transition metals, the elements V, Cr, Mn, Fe, Co, Ni, Cu, Zn and Mo have been shown to be essential to life and the elements Au, Ag, Pt, Pd, Ir, Os, Ti, and others have either been used in therapy or claimed to be of therapeutic value. In recent years the toxic nature of the complexes of many transition metals and of main group metals, such as cadmium, lead, mercury and aluminium, has evoked much worry and stimulated research into the effect of these metals on terrestial and marine biochemical systems. The ecological and health effects of these elements are often exacerbated by environmental transformation of less hazardous forms to extremely toxic compounds. An example of the latter is the biosynthesis of methylmercury cations from inorganic mercury salts by, amongst others, sulphate-reducing bacteria in marine sediments (Campion and Barthia, 1985).

Surface-enhanced resonance-Raman scattering (SERRS) experiments using a citrate-coated silver colloid have been performed. Transmission electron microscopy showed that the colloid consists of nearly mondisperse hexagonal particles with a maximum dimension of 36 nm. For maximum sensitivity, SERRS requires the controlled aggregation of the colloid. The nature of the aggregate formed is dependent on the method of aggregation. The effect of poly-L-lysine on the aggregation procedure was characterised using transmission and scanning electron microscopy. With SERRS, sensitivities down to 6.87 × 10–18 mol dm–3 were obtained using rhodamine dye. This corresponds to less than 200 molecules in the beam at any one time and suggests a sensitivity equal to or greater than that of fluorescence. A comparison of the use of SERRS with surface-enhanced Raman scattering (SERS) to follow adsorbate reactivity on a surface at less than monolayer coverage was obtained by studying metal complex formation using a preresonant Schiff base (SERRS) and 2-hydroxy-1-naphthaldehyde excited off resonance (SERS). In the former case, complex formation on the surface at well below monolayer coverage was observed and differences between the surface and the bulk complex characterised. In the latter complex formation was again observed but the bands were broad and problems of contamination, photodecomposition and selectivity evident.