Surface-enhanced Raman spectroscopy (SERS) has been suggested as a powerful tool to identify bacteria, drawing from its high fingerprint (vibrational) information content, its extreme sensitivity (down to the single molecule level) and its obliviousness to the aqueous environment intrinsic to biological systems. We review here in a comparative manner the various studies that attempted to utilize SERS for this important goal in light of the work carried out by our own group over the past 10 years or so. We show that SERS has an additional major advantage, namely, it introduces a new dimension of selectivity, which, on the one hand, makes it even more suitable as an analytical tool, but on the other hand, it requires gaining control of the precise manner in which the SERS-active metal centers are produced and brought into contact with the micro-organism. Our emphasis in this review is on understanding the spectra in terms of the nature of the SERS-active centers and their placement within the bacterium. On the interpretation and assignment of the spectra, we constantly keep in mind the final goal of bacteria identification.

Nanosized hydrophobic, oleate stabilized silver organosols in various organic solvents are obtained using a solvent exchange method. The silver particles are initially prepared as a hydrosol in the presence of sodium oleate (surfactant). Then a transfer of the colloid to an organic phase is induced by a low concentration of several agents such as orthophosphoric acid, with a transfer efficiency of 50−70%. The hydrophobic colloid is stable and the particles retain their integrity even after the solvent is evaporated and the dried deposit is resuspended in a variety of other solvents. We present the preparation method in detail and characterize the hydrosol and organosol particles by electron microscopy, electrophoresis, and UV−visible extinction spectroscopy. On the basis of IR spectroscopy we discuss the conformation of the surfactant adsorbed on the silver cores and the changes in it as the particles transfer into the organic environment.

Treatment of bacteria with silver yields intense and highly specific surface-enhanced Raman spectroscopy (SERS) spectra from various cellular chemical components located in the vicinity of the silver colloids. In particular, we demonstrate an extreme sensitivity to flavin components associated with the cell envelope and to their state of oxidation. Different spectra, possibly associated with DNA, carboxylates, and perhaps phosphates, are obtained from the soluble interior fraction of the cell.

A synthesis of various metal sulfide nanoparticles at relatively low temperature with use of a single precursor under ambient conditions is described. Metal alkyl xanthates (as well as thiocarbamates and thiocarbonates) are used as the precursor. Lewis base alkylamine solvents promote the reaction at low temperatures (from below room temperature up to ∼150 °C). By this method we form crystalline particles which are size- and shape-tunable and are usually monodiserse. This tunability is achieved by controlling parameters such as the reaction temperature, the reaction time, the concentration of the precursor, and the alkyl chain length. Core/shell structures are synthesized with the same method, using the same precursors, applying either a single-step or a dual-step process. CdS spherical particles, for instance, exhibit a narrow (∼30 nm fwhm) tunable excitonic fluorescence, and a broad, long wavelength defect emission, which intensity can be adjusted in a controlled manner, and even totally eliminated. Quantum yields of these particles are ∼2% for the “bare” particles and ∼16% for the ZnS-shelled, annealed particles. A short-wavelength (390−420 nm) narrow (30−35 nm fwhm) excitonic emission is observed for ZnS/CdS structures.