The Short-Range Sensitivity of SERS

The natural geometry of biomolecules can be exploited to clarify the SERS sensitivity dependence on distance from the surface. This aspect of SERS spectroscopy is important for the interpretation of the SERS spectrum of a biopolymer. The electromagnetic model predicts very rapid decay of SERS with increasing distance Thus, in small molecules with a dimension of approximately 0.6 nm (benzene) all vibrations of the molecule can be enhanced. In large biomolecules with diameters of about 6 nm (hemoglobin protein) only groups which are attached directly to the surface will yield SERS. This important aspect is illustrated by SERS studies of three examples mono-, di-, and polynucleotides. 

The possible interfacial orientations of 3 -CMP on the electrode can be explained by the interaction of the electric field of the electrode with the charge and dipole 

Other base (C in ApC, U in ApU) is repelled from the surface therefore attaining a more solution-sided position. 

While the intensity of the adenine band at 734 cm decreases upon a cathodic shift of the potential from —0.1 to —0.75 V, the cytosine band at 792 cm in ApC and the uracil band at 796 cm in ApU increase in intensity. When the applied voltage is set to —0.6 V the ring-breathing mode signals of adenine/cytosine and adenine/uracil are approximately equal in intensity, indicating that at this potential the base residues populate the surface equally. 

At Es of —0.75 V, near the zero charge potential, the population of the surface is 

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