Andrew C. Hillier

Nanoscale Imaging of Molecular Adsorption

In situ atomic force microscope observations were made of the adsorption of anions (1– or 2–) of the organic diacid 5-benzoyl-4-hydroxy-2-methoxybenzenesulfonic acid from aqueous solution onto the (0001) surface of hydrotalcite (HT), a layered clay. This adsorption process is believed to mimic the ion-exchange reactions that occur within the layers of HT and other layered clays. Atomic force microscope images of the (0001) surfaces of HT, acquired in aqueous solutions, reveal an ordered structure with respect to magnesium and aluminum atoms. In the presence of the anions, atomic force microscopy indicates pH-dependent adsorption onto the formally cationic HT surface. The anion coverage is governed by electroneutrality and steric interactions between the bulky anions within the adsorbed layer, whereas the orientation of the anions with respect to the HT surface is dictated by coulombic interactions and hydrogen bonding between the anions sulfonate moiety and clay hydroxyl triads. These observations reveal that the reversible adsorption of molecular species can be examined directly by in situ atomic force microscopy, providing details of surface stoichiometry and adlayer symmetry on the local, molecular level.

Surface Plasmon Resonance Enhanced Transmission of Light through Gold-Coated Diffraction Gratings

Narrow peaks are observed in the transmission spectra of p-polarized light passing through a thin gold film that is coated on the surface of a transparent diffraction grating. The spectral position and intensity of these peaks can be tuned over a wide range of wavelengths by simple rotation of the grating. The wavelengths where these transmission peaks are observed correspond to conditions where surface plasmon resonance occurs at the gold-air interface. Light diffracted by the grating couples with surface plasmons in the metal film to satisfy the resonant condition, resulting in enhanced light transmission through the film. Notably, this phenomenon is not observed at flat, gold-coated surfaces or uncoated gratings, where coupling to surface plasmons does not occur. The nature of the coupling and, thus, the details of light transmission are governed by the momentum matching conditions between the diffracted light and the surface plasmons. In the presence of bound analytes or surface films, the enhanced transmission peaks are red-shifted, making a simple, yet highly responsive sensing platform. The utility of this platform is demonstrated for ex situ sensing by analyzing thin films of various thicknesses and detecting a model immunoreaction between bovine serum albumin and anti-bovine serum albumin. This grating-based transmission surface plasmonic device represents a simple and sensitive platform, which can be readily tuned to enhance performance and be used in the study of a variety of surface adsorption processes or analysis of biomolecular interactions.

Superstructure control in the crystal growth and ordering of urea inclusion compounds

A template-directed mechanism of crystal growth is demonstrated for urea inclusion compounds (UICs). For UICs containing n-alkanone or α+1,ω−1-alkanedione guests, x-ray diffraction revealed superstructure relations between host and guest repeats along the channel axis for guests containing 8 to 14 carbons. For a favorable structural match between host and guest, UICs typically grow as flat hexagonal plates, and atomic force microscopy of {001} surfaces revealed molecularly smooth terraces differing in height by multiples of the guest repeat. If the match is poor, protrusion of guests from the {001} surfaces nucleates growth along the channel axis to form hexagonal needles.

Scanning electrochemical microscopy of hydrogen electro-oxidation. Rate constant measurements and carbon monoxide poisoning on platinum

Abstract We describe an application of the scanning electrochemical microscope that uses tip–sample feedback to characterize the electro-oxidation of hydrogen on a polycrystalline platinum electrode in sulfuric acid solutions in the presence and absence of adsorbed carbon monoxide. The hydrogen oxidation reaction is probed by reducing protons at a diffusion-limited rate at the microscopes tip electrode while it is positioned near a platinum substrate. A series of approach curves measured as a function of the substrate potential provides hydrogen oxidation rate constant values over a wide range of substrate conditions. In the absence of CO, the rate of hydrogen oxidation exceeds 1 cm s −1 at potentials within the hydrogen adsorption and double layer charging regions. A Tafel slope of ∼30 mV per decade is determined near the reversible potential. At increasingly positive substrate potentials, the hydrogen oxidation rate decreases exponentially with increasing potential as the surface is covered with an oxide layer. The adsorption of solution-phase carbon monoxide completely deactivates the platinum substrate towards steady-state hydrogen oxidation over a large range of substrate potentials. Approach curves indicate a near-zero rate constant for hydrogen oxidation on CO-covered platinum at potentials below oxide formation. An increase in the hydrogen oxidation rate is seen at potentials sufficiently positive that CO fails to adsorb and the platinum oxide forms. In comparison, dynamic tip–substrate voltammetry depicts a complex substrate response whereby the adsorbed carbon monoxide layer transforms from a weakly adsorbed state at low potentials to a strongly adsorbed state at high potentials. Although steady-state approach curve measurements depict the complete deactivation of catalytic activity at these potentials, a significant hydrogen oxidation current is observed during the potential-induced transformation between these weakly and strongly adsorbed CO states. The rate of hydrogen oxidation approaches that of a pristine platinum surface during this surface transformation before returning to the poisoned state.

Wavelength Tunable Surface Plasmon Resonance-Enhanced Optical Transmission Through a Chirped Diffraction Grating

We report the construction and testing of a chirped diffraction grating, which serves as a substrate for surface plasmon-enhanced optical transmission. This grating possesses a spatial variation in both pitch and amplitude along its surface. It was created by plasma oxidation of a curved poly(dimethoxysilane) sheet, which resulted in nonuniform buckling along the polymer surface. A gold-coated replica of this surface elicited an optical response that consisted of a series of narrow, enhanced transmission peaks spread over the visible spectrum. The location and magnitude of these transmission peaks varied along the surface of the grating and coincided with conditions where surface plasmons were excited in the gold film via coupling to one or more of the gratings diffracted orders. A series of measurements were carried out using optical diffraction, atomic force microscopy, and normal incidence optical transmission to compare the grating topology to the corresponding optical response. In addition, the impact of a thin dielectric coating on the transmission response was determined by depositing a thin silicon oxide film over the grating surface. After coating, wavelength shifts were observed in the transmission peaks, with the magnitude of the shifts being a function of the film thickness, the local grating structure, and the diffracted order associated with each peak. These results illustrate the ability of this surface to serve as an information-rich optical sensor whose properties can be tuned by control of the local grating topology.

Imaging the Reactivity of Electro‐oxidation Catalysts with the Scanning Electrochemical Microscope

We describe an application of the scanning electrochemical microscope that utilizes tip-sample feedback to characterize the reactivity of electro-oxidation catalysts. The kinetics of hydrogen oxidation are probed at a platinum electrode as a function of applied potential in aqueous sulfuric acid solution. Tip-sample feedback generates a tip response that reflects the rate of hydrogen oxidation at the substrate. The ability to directly measure the rate constant for hydrogen oxidation is, subsequently, used to perform reactivity mapping of heterogeneous electrodes consisting of catalytic and noncatalytic domains. Differences in reaction kinetics clearly distinguish surface composition variations based upon their reactivity toward hydrogen oxidation and illustrate the capability of this technique as a screening tool in the design of electrocatalysis. Further experiments investigating the oxidation of methanol illustrate the ability to characterize complex multistep reactions.

Measuring Electrocatalytic Activity on a Local Scale with Scanning Differential Electrochemical Mass Spectrometry

This paper describes a new technique entitled scanning differential electrochemical mass spectrometry ~SDEMS! that combines a quadrupole mass spectrometer with a membrane-covered capillary inlet and a high resolution positioning system that is designed to perform spatial mapping in solution near an electrode interface. Potential applications of this technique include the local characterization of anode catalysts for fuel cells as well as a range of analytical measurements and combinatorial screening studies. The capabilities of this technique are demonstrated by monitoring product evolution in several model electrocatalytic reactions, including the hydrogen evolution reaction, carbon monoxide oxidation, and the direct oxidation of methanol on platinum and platinum-ruthenium electrodes. The inlet of the SDEMS is based upon a small diameter capillary tube to which a nanoporous, hydrophobic membrane is attached. The capillary inlet is positioned near a substrate electrode using a three-dimensional positioning system. The effect of capillary substrate separation and substrate current on the sensitivity and time response of mass spectrometer’s ion current are illustrated during hydrogen evolution at a platinum substrate. The sensitivity is demonstrated further by detection of carbon dioxide evolution during the oxidation of a monolayer of carbon monoxide adsorbed on platinum. The ability to address more complex reactions involving complete and partial oxidation products is illustrated with methanol oxidation. In order to demonstrate the ability of this technique to perform spatial mapping, an eight-element band electrode was interrogated for hydrogen evolution and methanol oxidation. Detection of ion currents associated with complete and partial oxidation products of methanol on a set of platinum-ruthenium band electrodes illustrates the ability of this method to spatially discriminate between various reactive sites on a surface, which has potential utility in analytical characterization as well as application as a screening tool in combinatorial catalysis studies.

Diffraction-Based Tracking of Surface Plasmon Resonance Enhanced Transmission Through a Gold-Coated Grating

Surface plasmon resonance enhanced transmission through metal-coated nanostructures represents a highly sensitive yet simple method for quantitative measurement of surface processes and is particularly useful in the development of thin film and adsorption sensors. Diffraction-induced surface plasmon excitation can produce enhanced transmission at select regions of the visible spectrum, and wavelength shifts associated with these transmission peaks can be used to track adsorption processes and film formation. In this report, we describe a simple optical microscope-based method for monitoring the first-order diffracted peaks associated with enhanced transmission through a gold-coated diffraction grating. A Bertrand lens is used to focus the gratings diffraction image onto a CCD camera, and the spatial position of the diffracted peaks can be readily transformed into a spectral signature of the transmitted light without the use of a spectrometer. The surface plasmon peaks appear as a region of enhanced transmission when the sample is illuminated with p-polarized light, and the peak position reflects the local dielectric properties of the metal interface, including the presence of thin films. The ability to track the position of the plasmon peak and, thus, measure film thickness is demonstrated using the diffracted peaks for samples possessing thin films of silicon oxide. The experimental results are then compared with calculations of optical diffraction through a model, film-coated grating using the rigorously coupled wave analysis simulation method.

Electrochemically modulated transport through a conducting polymer membrane

Abstract The barrier properties of a poly(aniline) membrane may be tuned by electrochemical control of the polymer’s oxidation state. We have exploited this behavior to electrochemically modulate the permeation of two probe molecules of differing charge through a supported poly(aniline) membrane. The trans-membrane flux was measured for neutrally charged phenol and negatively charged 4-hydroxybezenesulfonate as a function of the polymer oxidation state, which was controlled by stepping the potential applied to a poly(aniline) film from a reducing potential to increasingly more oxidizing values. The fluxes of phenol and of 4-hydroxybezenesulfonate were enhanced when the polymer was poised in the half-oxidized, emeraldine state as compared to that of the reduced, leucoemeraldine state. Additional film oxidation to the pernigraniline state produced an enhanced flux of 4-hydroxybenzenesulfonate over phenol. The flux responses for these probe molecules are explained through a combination of oxidation-induced film swelling and electrostatic effects due to variable film charge. At high membrane potentials, film modulation was disrupted by poly(aniline) hydrolysis and decomposition. This work demonstrates that the permeation of a supported poly(aniline) membrane may be modified electrochemically to produce controllable barrier properties.

Resonance Quenching and Guided Modes Arising from the Coupling of Surface Plasmons with a Molecular Resonance

In this paper, we describe experimental and modeling results that illucidate the nature of coupling between surface plasmon polaritons in a thin silver film with the molecular resonance of a zinc phthalocyanine dye film. This coupling leads to several phenomena not generally observed when plasmons are coupled to transparent materials. The increased absorption coefficient near a molecular resonance leads to a discontinuity in the refractive index, which causes branching of the plasmon resonance condition and the appearance of two peaks in the p-polarized reflectance spectrum. A gap exists between these peaks in the region of the spectrum associated with the molecular resonance and reflects quenching of the plasmon wave due to violation of the resonance condition. A second observation is the appearance of a peak in the s-polarized reflection spectra. The initial position of this peak corresponds to where the refractive index of the adsorbate achieves its largest value, which occurs at wavelengths just slightly larger than the maximum in the molecular resonance. Although this peak initially appears to be nondispersive, both experimental data and optical modeling indicate that increasing the film thickness shifts the peak position to longer wavelengths, which implies that this peak is not associated with the molecular resonance but, rather, is dispersive in nature. Indeed, modeling shows that this peak is due to a guided mode in the film, which appears in these conditions due to the abnormally high refractive index of the film near the absorbance maximum. Results also show that, with increasing film thickness, numerous additional guided modes appear and move throughout the visible spectrum for both s- and p-polarized light. Notably, these guided modes are also quenched near the location of the molecular resonance. The quenching of both the plasmon resonance and the guided modes can be explained by a large decrease in the in-plane wave propagation length that occurs near the molecular resonance, which is a direct result of the films large absorption coefficient.