The potential (E)-dependent spectral behavior of both C-O and Pt-CO stretching vibrations from a saturated CO(ad) layer at a Au core-Pt shell nanoparticle film electrode has been examined in a wide potential window by surface-enhanced Raman spectroscopy (SERS). The Stark slopes of C-O stretching for linear-(CO(L)) and bridge-(CO(B)) binding CO adsorbates are positive and increase toward more negative potentials, which is explained by a synergetic effect from the potential-induced site conversion, the consequent changes in dipole-dipole coupling interactions among the nearby CO oscillators in addition to the potential-dependent chemical bonding changes. The Stark slopes of the Pt-CO stretching of CO(L) and CO(B) are negative and roughly constant (ca. -5 and -20 cm(-1)/V) throughout the potential regime examined, which is mainly attributed to the potential-dependent changes in the Pt-CO chemical bonding. From the measured potential -dependent frequencies of the Pt-CO stretching, the potential-induced changes in Pt-CO bond length are estimated to be 0.005 (0.01) angstrom/V for CO(L) (CO(B)) and the changes in CO binding energies (Delta E(b)/Delta E) are ca. 0.20 eV/V (for CO(L)) and 0.37 eV/V (for CO(B)), respectively. The higher Delta E(b)/Delta E for CO(B) than that for CO(L) reveals that the chemical bonding of Pt-CO(B) is more sensitive to the changes in the interfacial electric field, as is consistent with theoretical predictions. From the DFT calculations using two different (2 x 2)-3CO slab models with Pt(111), we found that the overall trends of the potential-dependent frequencies of Pt-CO and C-O stretching predicted by the DFT slab model are in good agreement with the observation by SERS. However, great discrepancies in the Stark slopes between the calculations and experimental results exist; possible origins for such differences have been discussed. ; National Natural Science Foundation of China (NSFC) [20773116]; Chinese Academy of Sciences; Ministry of Science and Technology of China [2010CB923302]; State Key Laboratory of Physical Chemistry of Solid Surfaces of Xiamen University [200706]; Education