Preparation of Graded Materials by Laterally Controlled Template Synthesis

A novel scheme is presented for the synthesis of graded materials by electrodeposition in porous insulating templates. Lateral control of copper electrodeposition in nanoporous alumina membranes is achieved by application of a lateral potential gradient on a thin Au film evaporated on the membrane, used as the cathode. Formation of metal gradients in the membranes is shown to occur under conditions where essentially no gradient is formed on similar bare electrodes. This is attributed to the permanent resistivity of the thin Au film between the pores, which does not disappear upon Cu deposition, allowing a potential gradient to be maintained. Formation of a copper gradient in porous alumina membranes by uniform deposition followed by gradient dissolution is also demonstrated. These results establish the feasibility of controlled electrodeposition and gradient formation in nanoporous insulating templates

Dislodgement Of Nasal Cannula In Hospitalized Veterans On Oxygen Therapy: The Nurse Perspective

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Widely-Applicable Gold Substrate for the Study of Ultrathin Overlayers

Ultrathin films on gold substrates have been the subject of enormous scientific and technological interest. Comprehensive study of such systems requires concomitant application of a variety of complementary characterization techniques. The reliability of the result is frequently hampered by the fact that different characterization methods impose different requirements on the Au substrate, resulting in the need to use different types of Au substrates for different measurements, possibly influencing the overlayer structure. This results in an average, rather than exact, structure determination. Here, we show that 15-nm-thick Au films evaporated at 0.5 Å/sec on silanized glass and annealed are semi-transparent, electrically conducting, and morphologically well-defined, showing a smooth, {111} textured surface. Such Au films provide a high-quality, widely applicable and relatively inexpensive platform for ultrathin overlayers, enabling characterization by a wide spectrum of experimental methods, applied to the same substrate. The exceptional qualities and analytical capabilities of such substrates are demonstrated with several different systems:  (i) Cu underpotential deposition (upd); (ii) alkanethiol self-assembly; (iii) formation of Au nanoparticle layers; (iv) binding of the chromophore protoporphyrin IX (PPIX) to a monolayer of 11-mercaptoundecanoic acid (MUA). In the latter case it is shown that the use of Cu2+ ions for binding between the carboxylate groups of PPIX and MUA promotes better organization of the porphyrin layer

Improved Sensitivity of Localized Surface Plasmon Resonance Transducers Using Reflection Measurements

The refractive index sensitivity (RIS) of a localized surface plasmon resonance (LSPR) transducer is one of the key parameters determining its effectiveness in sensing applications. LSPR spectra of nanoparticulate gold films, including Au island films prepared by evaporation on glass and annealing as well as immobilized Au nanoparticle (NP) films, were measured in the transmission and reflection modes. It is shown that the RIS, measured as the wavelength shift in solvents with varying refractive index (RI), is significantly higher in reflection measurements

Mass Thickness Analysis of Gold Thin Films Using Room Temperature Gas-Phase Chlorination

Chemical interaction of ultrathin gold films with gaseous chlorine at room temperature results in quantitative metal chlorination and formation of Au(III) chloride. Combination of gas-phase chlorination of Au nanostructures with dissolution of the chloride product and spectrophotometric determination of the chloroaurate concentration presents a simple procedure for quantitative determination of the mass thickness of Au nanostructures. The method is demonstrated by mass thickness analysis of the condensation coefficient of Au adatoms on solid substrates, the extinction coefficient of Au nanoparticles, as well as study of the integrity of Au island films upon interaction with solvents

A General Kinetic-Optical Model for Solid-State Reactions Involving the Nano Kirkendall Effect. The Case of Copper Nanoparticle Oxidation

Oxidation of copper nanoparticles (NPs) and other solid-state reactions have been shown recently to involve the nanoscale Kirkendall effect (NKE). The process, resulting in the formation of internal voids, has been rarely considered in most of the simple reaction kinetic models currently available. Here we present a general solid-state reaction kinetic model based on the assumption of steady-state diffusion profiles for rationalizing the evolution of the optical behavior observed in plasmonic Cu NPs undergoing solid-state oxidation to Cu₂O. While the model is applied here to an oxidation process, it is applicable in principle to any system showing Kirkendall voiding. An analytical expression of the rate law for spherical NPs is derived, and implications of the model are discussed. By combining the general kinetic model with Mie scattering solutions under the quasi-static approximation, the extinction cross section of Cu NPs as a function of the Cu-to-Cu₂O conversion fraction is modeled, for NPs of different initial sizes and for different degrees of shrinkage upon formation of a Cu₂O shell layer. The experimental observation of an initial increase in the surface plasmon (SP) band extinction intensity, followed by a decrease, is qualitatively reproduced using the combined model. This characteristic behavior of Cu nanoplasmonic systems is not expected theoretically in sufficiently small NPs; however, NPs of > ∼6 nm in diameter are expected to exhibit this type of behavior. Our results suggest that the NKE may be important in describing the optical behavior of plasmonic NPs subject to solid-state oxidation

Refractive Index Sensing Using Visible Electromagnetic Resonances of Supported Cu₂O Particles

Plasmonic metal nanostructures, in colloidal or surface-supported forms, have been extensively studied in the context of metamaterials design and applications, in particular as refractometric sensing platforms. Recently, high refractive index (high-<i>n</i>) dielectric subwavelength structures have been experimentally shown to support strong Mie scattering resonances, predicted to exhibit analogous refractive index sensing capabilities. Here we present the first experimental demonstration of the use of supported high-<i>n</i> dielectric nano/microparticle ensembles as refractive index sensing platforms, using cuprous oxide as a model high-<i>n</i> material. Single-crystalline Cu₂O particles were deposited on transparent substrates using a chemical deposition scheme, showing well-defined electric and magnetic dipolar resonances (EDR and MDR, respectively) in the visible range, which change in intensity and wavelength upon changing the medium refractive index (<i>n</i>_m). The significant modulation of the MDR intensity when <i>n</i>_m is modified appears to be the most valuable empirical sensing parameter. The Mie scattering properties of Cu₂O particles, particularly the spectral dependence of the MDR on <i>n</i>_m, are theoretically modeled to support the experimental observations. MDR extinction changes (i.e., refractive index sensitivity) per particle are >100 times higher compared to localized surface plasmon resonance (LSPR) changes in supported Au nanoislands, encouraging the evaluation of Cu₂O and other high-<i>n</i> dielectric particles and sensing modes in order to improve the sensitivity in optical (bio)­sensing applications

High-Resolution Lateral Differentiation Using a Macroscopic Probe: XPS of Organic Monolayers on Composite Au−SiO₂ Surfaces

X-ray photoelectron spectroscopy (XPS), an essentially macroscopic probe, is used to analyze mesoscopic systems at a lateral resolution given by the substrate structure. The method is based on controlled differential charging of multi-component surfaces, using a simple, commonly available XPS function, the electron flood gun. This new approach is applied here to a novel composite surface comprising SiO2 clusters on a {111} gold substrate, onto which different molecules are self-assembled to form a mixed organic monolayer. The method allows direct correlation of adsorbed molecules with surface sites, by analyzing XPS line shifts, which reflect local potential variations resulting from differential surface conductivity. This provides a powerful tool for resolving complex ultrathin films on heterogeneous substrates, on a length scale much smaller than the probe size

Sensitivity and Optimization of Localized Surface Plasmon Resonance Transducers

Gold nanoisland films displaying localized surface plasmon resonance optical response were constructed by evaporation on glass and annealing. The surface plasmon distance sensitivity and refractive index sensitivity (RIS) for island films of different nominal thicknesses and morphologies were investigated using layer-by-layer polyelectrolyte multilayer assembly. Since the polymer forms a conformal coating on the Au islands and the glass substrate between islands, the relative sensitivity of the optical response to adsorption on and between islands was evaluated. The RIS was also determined independently using a series of solvents. An apparent discrepancy between the behavior of the RIS for wavelength shift and intensity change is resolved by considering the different physical nature of the two quantities, leading to the use of a new variable, that is, RIS (for intensity change) normalized to the surface density of islands. In the present system the surface plasmon decay length and RIS are shown to be directly correlated; both parameters increase with increasing average island size. This result implies that a higher RIS is not always beneficial for sensing; maximizing the transducer optical response requires the interrelated RIS and decay length to be optimized with respect to the dimensions of the studied analyte-receptor system. It is shown that, as a rule, transducers comprising larger islands furnish better overall sensitivity for thicker adlayers, whereas thinner adlayers produce a larger response when sensed using transducers comprising smaller islands, despite the lower RIS of the latter

Raman Spectroelectrochemistry of Molecules within Individual Electromagnetic Hot Spots

The role of chemical enhancement in surface-enhanced Raman scattering (SERS) remains a contested subject. We study SERS spectra of 4-mercaptopyridine molecules excited far from the molecular resonance, which are collected from individual electromagnetic hot spots at concentrations close to the single-molecule limit. The hot spots are created by depositing Tollen’s silver island films on a transparent electrode incorporated within an electrochemical cell. Analysis of the intensity of the spectra relative to those obtained from individual rhodamine 6G molecules on the same surface provides a lower limit of ∼3 orders of magnitude for the chemical enhancement. This large enhancement is likely to be due to a charge transfer resonance involving the transfer of an electron from the metal to an adsorbed molecule. Excitation at three different wavelengths, as well as variation of electrode potential from 0 to −1.2 V, lead to significant changes in the relative intensities of bands in the spectrum. It is suggested that while the bulk of the enhancement is due to an Albrecht A-term resonance Raman effect (involving the charge transfer transition), vibronic coupling provides additional enhancement which is sensitive to electrode potential. The measurement of potential-dependent SERS spectra from individual hot spots opens the way to a thorough characterization of chemical enhancement, as well to studies of redox phenomena at the single-molecule level