A Fourier Transform Infrared Reflection−Absorption Spectroscopy Study of Redox Polyelectrolyte Films

Self-assembled polyelectrolyte multilayer films comprised of poly(allylamine) derivatized with an Os(bpy)2ClPyCH− complex (PAH−Os), and poly(vinylsulfonate), PVS, or poly(styrensulfonate), PSS, have been studied by Fourier transform infrared reflection−absorption spectroscopy. The infrared absorbances of the characteristic SO3-, CH2, NH3+, and aromatic bipyridine and pyridine groups have been characterized, and their intensity increases with the number of self-assembled layers and redox charge. The characteristic infrared signatures are the 1040 cm-1 band assigned to the aromatic ligands in the osmium complex (ν(Py)), PAH−Os, and the 1040 cm-1 (νs(SO3-)) and 1213 cm-1 (νa(SO3-)) bands for SO3 groups in PVS. The νs(SO3-) vibrational mode of PVS senses the local NH3+ environment of the cationic PAH−Os resulting in a band shift of 22 cm-1 for the first polyallylamine layer. Subtractively normalized Fourier transform infrared spectroscopy during the oxidation of the Os centers in the (PAH−Os)n(PVS)m multilayer reveals that different vibrational modes of bipyridine ligands in the osmium redox center of PAH−Os and the sulfonate groups of PVS are affected by charge−ligand electrostatic interaction and dipole reorganization in the multilayers

Methylene Blue Incorporation into Alkanethiol SAMs on Au(111): Effect of Hydrocarbon Chain Ordering

A detailed polarization modulation infrared reflection absorption spectroscopy, scanning tunneling microscopy, and electrochemical study on methylene blue (MB) incorporation into alkanethiolate self-assembled monolayers (SAMs) on Au(111) is reported. Results show that the amount of MB incorporated in the SAMs reaches a maximum for intermediate hydrocarbon chain lengths (C10−C12). Well-ordered SAMs of long alkanethiols (C > C12) hinder the incorporation of the MB molecules into the SAM. On the other hand, less ordered SAMs of short alkanethiols (C ≤ C6) are not efficient to retain the MB incorporated through the defects. For C12 the amount of incorporated MB increases as the SAM disorder is increased. This information is essential to the design of efficient thiol-based Au vectors for transport and delivery of molecules as well as thiol-based Au devices for molecular sensing

Biomimetics with a Self-Assembled Monolayer of Catalytically Active Tethered Isoalloxazine on Au

A new biomimetic nanostructured electrocatalyst comprised of a self-assembled monolayer (SAM) of flavin covalently attached to Au by reaction of methylformylisoalloxazine with chemisorbed cysteamine is introduced. Examinations by Fourier transform infrared spectroscopy and scanning tunneling microscopy (STM) show that the flavin molecules are oriented perpendicular to the surface with a 2 nm separation between flavin molecules. As a result of the contrast observed in the STM profiles between areas only covered by unreacted cysteamine and those covered by flavin−cysteamine moieties, it can be seen that the flavin molecules rise 0.7 nm above the chemisorbed cysteamines. The SAM flavin electrocatalyst undergoes fast electron transfer with the underlying Au and shows activity toward the oxidation of enzymatically active β-NADH at pH 7 and very low potential (−0.2 V vs Ag/AgCl), a requirement for use in an enzymatic biofuel cell, and a 100-fold increase in activity with respect to the collisional reaction in solution

Oxygen Reduction on Iron−Melanin Granular Surfaces

We report the catalytic activity of iron−eumelanin granular deposits supported on graphite for the oxygen reduction in neutral and alkaline solutions. These deposits contain quinone groups and iron−melanin complexes as revealed by XPS, XANES, EXAFS, and IR spectroscopy. Voltammetric data show that the iron−eumelanin system exhibits higher electrocatalytic activity than quinone/hydroquinone films (Q/QH) on the same substrate. In contrast to Q/QH deposits, the iron-containing eumelanin system is able to reduce oxygen with transfer of four electrons, thus allowing the formation of reactive hydroxyl species. Our results can explain the physical chemistry basis of the oxygen-radical induced lipid peroxidation and consequent neurodegeneration of the melanin-containing dopaminergic neurons observed by several authors