Optical Nanoantennas for Multiband Surface-Enhanced Infrared and Raman Spectroscopy

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Incident wavelength resolved resonant SERS on Au sphere segment void (SSV) arrays

Sphere segment void (SSV) arrays allow the reproducible engineering of plasmon-polariton modes from the near infrared to the ultraviolet through the tuning of the void height and diameter. The wavelength dependence of surface-enhanced Raman scattering (SERS) can then in principle be controlled by selecting these parameters. Using 4-mercaptopyridine as a covalently bonded nonresonant molecular probe, we report a detailed study of such wavelength dependence of SERS in Au SSV arrays as a function of void diameter and height. We conclude that the SERS mechanism on SSV arrays depends on the plasmonic properties of the substrates and also that additional effects contribute significantly to the observed enhancement including a chemical contribution related to the molecular probe and a nanostructuring induced surface plasmon localization existent for the smaller cavity dimensions

Quantitative electrochemical SERS of flavin at a structured silver surface

In situ electrochemical surface enhanced Raman spectra (SERS) for an immobilized monolayer of a flavin analogue (isoalloxazine) at nanostructured silver surfaces are reported. Unique in the present study, the flavin is not directly adsorbed at the Ag surface but is attached through a chemical reaction between cysteamine adsorbed on the Ag surface and methylformylisoalloxazine. Even though the flavin is held away from direct contact with the metal, strong surface enhancements are observed. The nanostructured silver surfaces are produced by electrodeposition through colloidal templates to produce thin (<1 ?m) films containing close-packed hexagonal arrays of uniform 900 nm sphere segment voids. The sphere segment void (SSV) structured silver surfaces are shown to be ideally suited to in situ electrochemical SERS studies at 633 nm, giving stable, reproducible surface enhancements at a range of electrode potentials, and we show that the SER spectra are sensitive to subfemtomole quantities of immobilized flavin. Studies of the SER spectra as a function of the electrode potential show clear evidence for the formation of the flavin semiquinone at the electrode surface at cathodic potential

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

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SERS in PAH-Os and gold nanoparticle self-assembled multilayers

We present a detailed structural and surface-enhanced Raman scattering (SERS) study of poly(allylamine) modified with Os (byp)2 ClPyCHO (PAH-Os) and gold nanoparticles self-assembled multilayers [PAH-Os+ (Au-nanoparticlesPAH-Os)n, n=1 and 5]. Atomic force microscopy and variable-angle spectroscopic ellipsometry measurements indicate that the first nanoparticle layer grows homogenously by partially covering the substrate without clustering. Analyzing the sample thickness and roughness we infer that the growth process advances thereafter by filling with nanoparticles the interstitial spaces between the previously adsorbed nanoparticles. After five immersion steps the multilayers reach a more compact structure. The interaction between plasmons of near-gold nanoparticles provides a new optical absorption around 650 nm which, in addition, allows a more effective SERS process in that spectral region than at the single-plasmon resonance (∼530 nm). We compare the electronic resonance Raman and SERS amplification mechanisms in these self-assembled multilayers analyzing Raman resonance scans and Raman intensity micromaps. As a function of nanoparticle coverage we observe large changes in the Raman intensity scans, with maxima that shift from the electronic transitions, to the plasmon resonance, and finally to the coupled-plasmon absorption. The Raman micromaps, on the other hand, evidence huge intensity inhomogeneities which we relate to "hot spots." Numerical discrete dipole approximation calculations including the interaction between gold nanoparticles are presented, providing a qualitative model for the coupled-plasmon absorption and redshifted Raman hot spots in these samples. © 2005 American Institute of Physics.Fil:Calvo, E. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Bonazzola, C. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

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

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

Abiotic Degradation of Glyphosate into Aminomethylphosphonic Acid in the Presence of Metals

Glyphosate [<i>N</i>-phosphono-methylglycine (PMG)] is the most used herbicide worldwide, particularly since the development of transgenic glyphosate-resistant (GR) crops. Aminomethylphosphonic acid (AMPA) is the main glyphosate metabolite, and it may be responsible for GR crop damage upon PMG application. PMG degradation into AMPA has hitherto been reckoned mainly as a biological process, produced by soil microorganisms (bacteria and fungi) and plants. In this work, we use density functional calculations to identify the vibrational bands of PMG and AMPA in surface-enhanced Raman spectroscopy (SERS) and attenuated total reflectance Fourier transform infrared (ATR–FTIR) spectra experiments. SERS shows the presence of AMPA after glyphosate is deposited from aqueous solution on different metallic surfaces. AMPA is also detected in ATR–FTIR experiments when PMG interacts with metallic ions in aqueous solution. These results reveal an abiotic degradation process of glyphosate into AMPA, where metals play a crucial role