Impact of the outer-sphere and inner-sphere association in the surface enhanced Raman spectra of metal complexes and gold nanoparticles

Transition metal complexes, such as the low-spin bis (phenylterpyridine) (A) and bis (pyridylterpyrazine)iron (II) (B) complexes, provide didactic chromophore species for demonstrating the Raman, resonance Raman, and the surface-enhanced Raman scattering (SERS) behavior in coordination chemistry, as well as for elucidating the nature of inner-sphere and outer-sphere association with plasmonic nanoparticles. Their electrostatically stabilized ion pairs with citrate–gold nanoparticles have been studied in an aqueous solution, from the pronounced changes in the plasmonic band at 540 nm. Complex A, lacking any coordinating site, can only generate outer-sphere complexes with citrate–gold nanoparticles, but they are stable enough to give a strong SERS response, even at 10⁻⁸ M. At 10⁻⁶ M, agglomeration accompanies the decrease of the electrostatic repulsion, resulting in a sharp decay of the plasmon resonance band at 540 nm. This is followed by the rise of a plasmon coupling band above 700 nm. However, at 10⁻⁴ M, the excess of the complex in the adsorption layer produces a reverse effect, decreasing agglomeration. The observed Raman spectra are essentially similar for the several concentrations employed because the outer-sphere interaction implies a SERS electromagnetic mechanism. In contrast, complex B exhibits several pyridine and pyrazine N-atoms available to form inner-sphere-associated species. A selective enhancement of the SERS signals is observed at 10⁻⁸ M, clearly indicating a chemical mechanism, consistent with a bridging mode. At 10⁻⁶ M and above, the agglomeration leads to a plasmon coupling band at 800 nm, while the SERS response indicates a change in the binding modes dictated by the excess of the complexing molecules.