Aromatic and Aliphatic Thiol Self-Assembled Monolayers on Au: Anchoring and Delivering Copper Species

The immobilization of dinuclear copper(II) acetate complexes (Cu₂(CH₃COO)₄·2H₂O) on self-assembled monolayers (SAMs) of different aliphatic and aromatic thiols with oxygen and nitrogen donors at the end group is investigated on preferentially oriented (111) gold surfaces (Au(111)). The Cu species are immobilized at the outer plane of these terminal groups by the interaction of the terminal moiety of the functionalized SAMs. The highest electrochemical activity for the Cu(II)/Cu(I) redox couple is found for the metallic complexes immobilized on SAMs of short thiols, irrespective of their aliphatic or aromatic character, or the nature of the terminal group of the linking molecule, suggesting that direct tunneling is the main path for charge transfer to the Au substrate. Even though a progressive demetalation of the copper acetate complex immobilized on N-terminated SAMs by the release of Cu ions to the solution is induced by repeated potential scans, this process is negligible for Cu species immobilized on O-terminated thiols. The Cu(I)/Cu(0) reaction is not observable in the overall potential range where thiol SAMs are stable on the Au(111) surface. In contrast, this reaction is clearly visible by using nanostructured Au, a substrate that exhibits a wider potential window of SAM stability and larger capture areas than ordered smooth metal surfaces. Finally, spatial 2D and 3D confinement of the copper complex can be performed by SAMs of mixed thiols with different immobilization abilities and by building complex electrochemically active supramolecular structures. Our results are important to understand the behavior of Cu centers of enzymes, the electrochemical metallization of thin organic films with Cu, and the preparation of complex three-dimensional supramolecular Cu-containing structures with spatial order

Magneto-Optical Enhancement by Plasmon Excitations in Nanoparticle/Metal Structures

Coupling magnetic materials to plasmonic structures provides a pathway to dramatically increase the magneto-optical response of the resulting composite architecture. Although such optical enhancement has been demonstrated in a variety of systems, some basic aspects are scarcely known. In particular, reflectance/transmission modulations and electromagnetic field intensification, both triggered by plasmon excitations, can contribute to the magneto-optical enhancement. However, a quantitative evaluation of the impact of both factors on the magneto-optical response is lacking. To address this issue, we have measured magneto-optical Kerr spectra on corrugated gold/dielectric interfaces with magnetic (nickel and iron oxide) nanoparticles. We find that the magneto-optical activity is enhanced by up to an order of magnitude for wavelengths that are correlated to the excitation of propagating or localized surface plasmons. Our work sheds light on the fundamental principles for the observed optical response and demonstrates that the outstanding magneto-optical performance is originated by the increase of the polarization conversion efficiency, whereas the contribution of reflectance modulations is negligible