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.

Electrochemistry of Os(2,2'-bpy)2ClPyCH2NHCOPh tethered to Au electrodes by S-Au and C-Au junctions.

Osmium pyridine-bipyridine redox centers have been tethered to Au electrodes by chemical modification through Au-S and Au-C bonds respectively. 4-Mercapto benzoic acid and the reduction product of the aryl diazonium salt of 4-amino benzoic acid were reacted on Au surfaces, with further post-functionalization by chemical reaction of the osmium complex amino-pyridine derivative with the surface carboxylates. The resulting modified Au surfaces were characterized by polarization modulated infrared reflection absorption spectroscopy (PM-IRRAS), scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), resonant raman spectroscopy and cyclic voltammetry.

Wired-enzyme core-shell Au nanoparticle biosensor.

We report a fully integrated core-shell nanoparticle system responsive to glucose. The system is comprised of self-assembled glucose oxidase and an osmium molecular wire on core-shell Au nanoparticles. Characterization of the functional nanoparticles by spectroscopy, quartz crystal microbalance and electrochemical techniques has shown that the catalytically active shell has a structure as designed and all components are active in the self-assembled multilayer shell. Furthermore, amperometric reagentless detection of glucose and contactless photonic biosensing by the Os(II) resonant Raman signal have been demonstrated. The enzymatic reduction of FAD by glucose and further reduction of the Raman silent Os(III) by FADH 2 yields a characteristic enzyme-substrate calibration curve in the millimolar range. Furthermore, coupling of electronic resonant Raman of the osmium complex with the SERS amplification by Au NPs plasmon resonance has been demonstrated which leads to an extra enhancement of the biosensor signal. We present a proof of concept extending the work done with planar surfaces to core-shell NPs as an advance in the design of glucose-responsive chemistry detected by SERS-like methods.

Abiotic degradation of glyphosate into aminomethylphosphonic acid in the presence of metals.

Glyphosate [N-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.

Restricted surface mobility of thiolate-covered metal surfaces: a simple strategy to produce high-area functionalized surfaces.

We have studied the self-assembly of thiol monolayers on high-area nanostructured gold surfaces. These surfaces are highly irregular with a fractal dimension close to 2.5. Auger electron spectroscopy and voltammetric data indicate that thiol self-assembly with a maximum surface coverage approximately 1/3 takes place, the same result as that found for smooth gold surfaces. Therefore, neither curvature effects, which would promote higher coverage, nor excluded volume effects, which would result in lower coverage, are present in these irregular surfaces. The high surface area of the bare electrodes exhibits a rapid surface decay in different liquid media that is hindered by alkanethiolate chemisorption. The presence of thiolate SAMs reduces markedly the mass transport surface diffusion of gold adatoms, hindering surface area decay and freezing the system in a metastable state for days. This effect cannot be explained by considering only hydrocarbon-hydrocarbon chain interactions, because it is also observed for ordered arrays of adsorbed S atoms. Therefore, interactions between ordered chemisorbed species at high coverage seem to be responsible for the observed behavior. The thiol-covered high-area metallic substrates can be used to efficiently anchor a large number of molecules, biomolecules, or nanostructures, improving the performance of SAM-based optical and electrochemical devices.

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)2ClPyCHO (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 (approximately 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.

Resonant Raman spectroscopy of PAH--Os self-assembled multilayers.

We present a resonant Raman scattering study of (PAH--Os/PVS)n and (PAH--Os/GOx)m self-assembled multilayers (n=1-11 and m=1-3). These Os polymer multilayers can be used in electrodes as efficient molecular wires for biomolecular recognition. The Raman intensity dependence on the number of self-assembly cycles provides information on the deposition process. The spectra are identical to that observed for PAH--Os in aqueous solution, indicating that the PAH--Os metal complex structure is conserved in the multilayers. We observe at approximately 500 nm incoming and outgoing Raman resonances of osmium and bipyridine vibrational modes. These resonances are associated to the metal-to-ligand charge transfer (MLCT) transition. We study the evolution of these Raman modes as a function of the Os oxidation state during in situ electrochemistry. During the oxidation process, Os(II)-->Os(III), the Raman resonance related to the MLCT disappears and the bipyridine related modes harden by approximately 10 cm(-1). These results are correlated with optical transmission measurements which show the disappearance of the visible region absorption when the Os complex is oxidized. We also find partial quenching of the Raman mode intensity after in situ voltamperometric cycles which demonstrates the existence of photo-electro-chemical processes.