Self-optimized metal coatings for fiber plasmonics by electroless deposition

We present a novel method to prepare optimized metal coatings for infrared Surface Plasmon Resonance (SPR) sensors by electroless plating. We show that Tilted Fiber Bragg grating sensors can be used to monitor in real-time the growth of gold nano-films up to 70 nm in thickness and to stop the deposition of the gold at a thickness that maximizes the SPR (near 55 nm for sensors operating in the near infrared at wavelengths around 1550 nm). The deposited films are highly uniform around the fiber circumference and in spite of some nanoscale roughness (RMS surface roughness of 5.17 nm) the underlying gratings show high quality SPR responses in water

Evaluating Molecular Mechanical Potentials for Helical Peptides and Proteins

Multiple variants of the AMBER all-atom force field were quantitatively evaluated with respect to their ability to accurately characterize helix-coil equilibria in explicit solvent simulations. Using a global distributed computing network, absolute conformational convergence was achieved for large ensembles of the capped A21 and Fs helical peptides. Further assessment of these AMBER variants was conducted via simulations of a flexible 164-residue five-helix-bundle protein, apolipophorin-III, on the 100 ns timescale. Of the contemporary potentials that had not been assessed previously, the AMBER-99SB force field showed significant helix-destabilizing tendencies, with beta bridge formation occurring in helical peptides, and unfolding of apolipophorin-III occurring on the tens of nanoseconds timescale. The AMBER-03 force field, while showing adequate helical propensities for both peptides and stabilizing apolipophorin-III, (i) predicts an unexpected decrease in helicity with ALA→ARG+ substitution, (ii) lacks experimentally observed 310 helical content, and (iii) deviates strongly from average apolipophorin-III NMR structural properties. As is observed for AMBER-99SB, AMBER-03 significantly overweighs the contribution of extended and polyproline backbone configurations to the conformational equilibrium. In contrast, the AMBER-99φ force field, which was previously shown to best reproduce experimental measurements of the helix-coil transition in model helical peptides, adequately stabilizes apolipophorin-III and yields both an average gyration radius and polar solvent exposed surface area that are in excellent agreement with the NMR ensemble

Improved refractive-index sensitivity of silver-nanocube monolayers on silicon films

Suitable substrates: An improvement in the refractive-index sensitivity is observed for the quadrupolar plasmonic mode of a monolayer of 60 nm silver nanocubes deposited on a thin silicon film. Dipolar plasmonic band splitting on this high-refractive-index substrate is stronger than on a glass substrate (see picture). As a result, the quadrupolar band is easier to identify, even in high-refractive-index liquids. The work demonstrates the importance of using high-refractive-index substrates. Copyrigh

Spatially inhomogeneous enhancement of fluorescence by a monolayer of silver nanoparticles

Near-field scanning optical microscopy (NSOM) was applied to study the effect of a two-dimensional array of silver nanoparticles on the spatial distribution and magnitude of fluorescence signal enhancement for a monolayer of Rhodamine 6G (Rh6G). Twenty polyelectrolyte monolayers were deposited between the nanoparticles and the dye by a layer-by-layer deposition technique resulting in a 15-20 nm separation cushion, necessary to minimize the fluorescence signal quenching. The fluorescence signal in NSOM images was found to be distributed inhomogeneously as small (100-200 nm in diameter) fluorescent clusters with typically 5-30 times higher fluorescence intensities than a sample without nanoparticles. The position and relative intensity of the clusters was found to be dependent on the excitation wavelength, suggesting that the enhancement originates from the nanoparticle surface plasmon resonance

Reflection and absorption spectra of silver nanocubes on a dielectric substrate: Anisotropy, angle, and polarization dependencies

Angle and polarization dependent UV-visible re fl ection and transmission spectra were measured for monolayers of weakly interacting silver nanocubes (of 40 and 80 nm edge lengths) supported by thin films of titanium oxide on glass utilizing both front and rear geometries with respect to the incident radiation. The supporting substrate mediates the hybridization of dipolar and quadrupolar plasmon resonances. From the spectra absolute extinction and absorption c

Plasmonic properties of silver nanocube monolayers deposited on thin metal films

The present work investigates the plasmonic properties and behaviour of silver nanocube monolayers deposited on thin gold films. Monolayers were deposited via the Langmuir-Blodgett metho

UV resonance raman spectroscopic detection of nitrate and nitrite in wastewater treatment processes

The 204- and 229-nm excited UV resonance Raman spectra of wastewater solutions containing sodium nitrite and nitrate were measured in the concentration range 7 μM to 3.5 mM (0.1-50 ppm nitrogen). The other chemical species present in wastewater do not interfere with Raman measurements of NO2-/NO3- bands. We observe detection limits of <14 μM (<200 ppb) for both NO2- and NO3-. UV resonance Raman spectroscopy appears to be an excellent tool for on-line monitoring of NO2-/NO3- in wastewater for the real-time control of water treatment plants

UV resonance Raman study of the spatial dependence of α-helix unfolding

We used ultraviolet resonance Raman (UVRR) spectra to examine the spatial dependence and the thermodynamics of α-helix melting of an isotopically labeled α-helical, 21-residue, mainly alanine peptide. The peptide was synthesized with six natural abundance amino acids at the center and mainly perdeuterated residues elsewhere. Cα deuteration of a peptide bond decouples Cα-H bending from N-H bending, which significantly shifts the random coil conformation amide III band; this shift clearly resolves it from the amide III band of the nondeuterated peptide bonds. Analysis of the isotopically spectrally resolved amide III bands from the external and central peptide amide bonds show that the six central amide bonds have a higher α-helix melting temperature (∼32°C) than that of the exterior amide bonds (∼5°C)

Assignments and conformational dependencies of the amide III peptide backbone UV resonance Raman bands

We investigated the assignments and the conformational dependencies of the UV resonance Raman bands of the 21-residue mainly alanine peptide (AP) and its isotopically substituted derivatives in both their α-helical and PPII states. We also examined smaller peptides to correlate conformation, hydrogen bonding, and structure. Our vibrational mode analysis confirms the complex nature of the amide III region, which contains many vibrational modes. We assign these bands by interpreting the isotopically induced frequency shifts and the conformational sensitivity of these bands and their temperature dependence. Our assignments of the amide bands in some cases agree, but in other cases challenge previous assignments by Lee and Krimm (Biopolymers 1998, 46, 283-317), Overman and Thomas (Biochemistry 1998, 37, 5654-5665), and Diem et al. (J. Phys. Chem. 1992, 96, 548-554). We see evidence for the partial dehydration of α-helices at elevated temperatures