Charge-Transfer Enhancement Involved in the SERS of Adenine on Rh and Pd Demonstrated by Ultraviolet to Visible Laser Excitation

In an attempt to understand the single-molecule SERS of some small nonresonant molecules, such as adenine, it is inevitable to include the chemical enhancement mechanism to provide additional enhancement to the electromagnetic mechanism, although it may be much smaller than the electromagnetic field enhancement. We will report here the first experimental investigation of the charge-transfer (CT) enhancement of protonated adenine molecules on Rh and Pd by performing the potential-dependent SERS using one UV laser (325 nm) and two visible lasers (514.5 and 632.8 nm). A UV laser displays a significant role in the verification of the CT process due to its much larger photon energy and thus a much larger shift of potential of the maximum SERS intensity (E-max) than visible lasers. We find a well-discernible E-max and a linear relationship between E-max and the photon energy of the laser for adenine on both Rh and Pd surfaces. The E-max was found to shift positively with the increasing photon energy, which strongly indicates an electron transfer from the E-f of Rh and Pd to the lowest unoccupied orbital of adenine molecules. In addition, different contributions of CT enhancement to adenine Raman bands are also briefly discussed. By analyzing the wavelength-dependent intensity change and UV-vis absorption spectroscopy, we propose the contribution of preresonance Raman enhancement to the UV-SERS signal for the band at around 1330 cm(-1). The present study demonstrates that the use of a UV laser opens a promising way to understand the enhancement mechanism, especially the chemical enhancement mechanism.