SERS spectra of deoxyribonucleotides

For SERS spectroscopy of biomolecules [31], the use of small sample volumes is mandatory. For this purpose, two spectroelectrochemical cells were constructed in which the hquid sample is held in a thin cylindrical volume between a silver rod electrode and the optical access window. The design of the cells (from glass or teflon) and the oxidation-reduction cycle, used for the required roughening of the silver, have been described in detail by Zimmermaim [32]. 

The power of the technique is illustrated by the SERS spectrum of a mononucleotide, 5 -adenosine-monophosphate (5 -rAMP). The top spectrum in Fig. 20.12 (left) was recorded with a concentration of 1.5 mM 5 -rAMP in phosphate buffer, with the electrolyte concentration adjusted to 100 mM. In particular, attention is drawn to the excellent signal-to-noise ratio achieved at a millimolar concentration. 

The high salinity conditions that are often used to induce a transition into left-helical conformations in oligo or polynucloetides were critically considered for the envisaged application. 

Therefore, the SERS spectrum of 5 -rAMP was also recorded in the presence of NaCl (3.3 M). The obtained high quality spectrum (Fig. 20.12, lower trace) demonstrates that SERS spectroscopy is feasible in the media that are typically used to prepare and study the Z conformation. 

Corresponding experiments with 5 -cytidine-monophosphate (5 -rCMP) showed an optimum adsorption potential of-0.3 V vs. Ag/AgCl (Fig. 20.11, right). Because the silver surface is positively charged at this potential, chloride and phosphate ions as well as impurities from the solution are adsorbed much more strongly to the electrode surface as compared to the potential of-0.5 V where the 5 -rAMP adsorption had been found to be an optimum (left). Therefore, the sensitivity in the 5 -rCMP, recorded at a concentration of 2 mM, was considerably lower as compared to the 5 -rAMP spectrum [33]. 

---