An Evaluation of Surface-Enhanced Raman Spectroscopy (SERS) as a Chemical Sensing Technique

The basic aim of this work was to explore ways of applying the technique of Surface-enhanced Raman spectroscopy (SERS) to the routine analysis of organic and inorganic compounds. Intensive study of the activity of a series of modified tris(2,2'-bipyridyl) ruthenium(II) complexes was designed to address the question of the nature of the surface/adsorbate molecule bond. The research was divided into three main areas. (1) The preparation of disubstituted analogues of the complex tris(2,2'-bipyridyl) ruthenium(II) diiodide, Ru(bipy)3l2. Four complexes, in addition to the parent complex, were prepared, each incorporating three 2,2'-bipyridyl ligands substituted in the C4 and C4-positions. These complexes were tris(4,4'-diethoxycarbonyl, -diamino, -diphenyl and -dimethyl-2,2'-bipyridine) ruthenium(II). The 4,4'-diethoxycarbonyl and 4,4'-diamino ligands were prepared from literature procedures and the 4,4'-diphenyl and 4,4'-dimethyl ligands were used as received. Each of the complexes was purified by column chromatography and characterised, primarily, by electronic absorption spectroscopy. (2) A study of SERS-active substrates. The analytical potential of several categories of surfaces was probed with organic molecules and Ru(bipy)3l2 and other complexes. The results showed that silver-coated microscope slides, prepared through the reduction of ammonical silver nitrate solution, exhibited appreciable activity with a range of adsorbate species and particularly strong enhancement with Ru(bipy)3I2. It was found that the molecules physisorbed on the metal surface. Vapour-deposited silver-coated polycarbonate substrates also displayed SERRS-activity with Ru(bripy)3I2. The resultant spectra were considerably less enhanced but showed evidence of chemisorption of the adsorbate. These findings are discussed in relation to current theories of SERS enhancement. (3) A comparitive study of the SERRS of [Ru(bipy)3]2+ and disubstituted analogues with silver slides and silver polycarbonate. The effect of substitution of the 2,2'-bipyridyl ligands incorporated into the complexes was studied. These results and the assignment of the SERR spectra showed that disubstitution does not affect the adsorption of the complex on silver slides, but does have a limited effect on the adsorption on silver polycabonate. All the findings of the work are summarised and discussed with regard to the analytical applications of the technique. It is proposed that SERS has only limited applicability in the area of chemical sensing and that there is no ideal SERS-active substrate due to the large number of variables operating in any SERS analysis