Raman spectroelectrochemical study of Meldola blue, adsorbed and electropolymerized at a gold electrode

Surface-enhanced Raman spectroscopy (SERS) was used to probe the structure of adsorbed and electropolymerized Meldola blue (MB) films on roughened gold surfaces in solutions with pH 1.0 and 7.0 by using 785 nm excitation wavelength. Spectral bands were assigned based on density functional theory (DFT) calculations at B3LYP/6-311+G(2d, p) level. The most characteristic band of the oxidized MB form was found to be the breathing vibration of the central ring containing heteroatoms at 596 cm−1. Based on a red shift of bands assigned to vibrations of double CN(C2H6) bonds and adjacent ring CC bonds in surface spectra as compared with solution 1 it was suggested that polymerization and interaction with an electrode surface proceed through these moieties. The presence of out-of-plane bands in SERS spectra was attributed to “flat” or slightly “tilted” orientation of aromatic rings at the interface. Potential-dependent spectral changes were followed by SERS spectroscopy. Raman spectra of the reduced MB form were obtained in both pH 1.0 and pH 7.0 solutions by analysis of the potential-difference SERS spectra. Reduced MB form can be recognized by characteristic bands near 1620, 1574, 1374, and 1234 cm−1. By comparing the intensities of 1637 cm−1 (oxidized MB form) and 1374 cm−1 (reduced MB form) bands in experimental spectra of polymerized MB in pH 1.0 solution, a reduction-induced decrease by factor of 7 was estimated. A similar tendency in intensity changes showed calculations indicating that this effect is associated with reduction-induced changes in the molecular structure of the dye.

Thin layers of adsorbed and electropolymerized redox dye Meldola blue were prepared at a gold electrode, and features of its structure were studied by surface-enhanced Raman spectroelectrochemistry.Figure optionsDownload high-quality image (41 K)Download as PowerPoint slideResearch highlights
► Electropolymerized layers of Meldola blue were prepared and studied by surface-enhanced Raman spectroscopy.
► Flat or slightly tilted orientation of aromatic rings of the dye with respect to electrode surface has been deduced from Raman spectra.
► Electrochemical redox processes of poly(Meldola blue) layers can be followed by characteristic Raman bands and their relative intensities.