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Surface-Enhanced Raman Spectroscopic Study of Calf Thymus DNA on Two Different Silver Colloids

Two different silver colloids were produced by the reduction of AgNO3 using sodium citrate (SCAg colloid) or hydroxylamine hydrochloride (HHAg colloid) as reductant. A comparative study was made on SERS spectra (λexc=514.5 nm) of the calf thymus DNA on the two silver colloids in solution of 0.06mol/L NaCl, pH7.0. With the SCAg colloid, the bands at 732, 915, 1340, and 1448 cm-1 for adenine, and 800, 1180, 1590, 1625 and 1651 cm-1 for thymidine are dramatically enhanced. It shows that the DNA molecules can be oriented relatively perpendicularly to the SCAg colloidal surface via the N6H2, N7 of adenine and C2=O of thymine residues. While with the HHAg colloid, most of the in-plane modes of the bases are poorly enhanced, except for some weak bands or shoulders assigned to cytosine and guanine. Thus the DNA molecules is supposed to be adsorbed on the HHAg colloid surface in a tilted mode.