Ian J. Burgess

Electric Field-Driven Transformations of a Supported Model Biological Membrane—An Electrochemical and Neutron Reflectivity Study

A mixed bilayer of cholesterol and dimyristoylphosphatidylcholine has been formed on a gold-coated block of quartz by fusion of small unilamellar vesicles. The formation of this bilayer lipid membrane on a conductive surface allowed us to study the influence of the supports surface charge on the structure and hydration of the bilayer lipid membrane. We have employed electrochemical measurements and the specular reflection of neutrons to measure the thickness and water content in the bilayer lipid membrane as a function of the charge on the supports surface. When the surface charge density is close to zero, the lipid vesicles fuse directly on the surface to form a bilayer with a small number of defects and hence small water content. When the supports surface is negatively charged the film swells and incorporates water. When the charge density is more negative than -8 micro C cm(-2), the bilayer starts to detach from the metal surface. However, it remains in a close proximity to the metal electrode, being suspended on a thin cushion of the electrolyte. The field-driven transformations of the bilayer lead to significant changes in the film thicknesses. At charge densities more negative than -20 micro C cm(-2), the bilayer is approximately 37 A thick and this number is comparable to the thickness determined for hydrated multilayers of dimyristoylphosphatidylcholine from x-ray diffraction experiments. The thickness of the bilayer decreases at smaller charge densities to become equal to approximately 26 A at zero charge. This result indicates that the tilt of the acyl chains with respect to the bilayer normal changes from approximately 35 degrees to 59 degrees by moving from high negative charges (and potentials) to zero charge on the metal.

Quaternary ammonium bromide surfactant adsorption on low-index surfaces of gold. 2. Au(100) and the role of crystallographic-dependent adsorption in the formation of anisotropic nanoparticles.

A qualitative and quantitative description of the coadsorption of a quaternary ammonium bromide surfactant on Au(100) has been determined using electrochemical techniques. Cyclic voltammetry reveals that both the cationic surfactant ion and its halide counterion are adsorbed on the surface of unreconstructed Au(100) over a wide range of electrode potentials or charge densities. The relative Gibbs excesses of the cationic and anionic components of octyltrimethylammonium (OTA(+)) bromide have been determined using the thermodynamics of ideally polarized electrodes. Coadsorbed OTA(+) does not strongly affect the behavior of bromide layers on Au(100) with low-coverage films being replaced by commensurate overlayers at positive electrode charge densities. The presence of surface bromide allows for the stabilization of adsorbed OTA(+) at positive polarizations. Furthermore, charge-induced phase changes in the bromide layer lead to subtle but appreciable changes in the surface excesses of OTA(+) ions which is consistent with a hierarchical model of surfactant adsorbed upon a halide-modified Au(100) surface. A comparison of the OTA(+) adsorption isotherms on Au(100) and Au(111) reveals that the presence of coadsorbed bromide does not lead to preferential accumulation of cationic surfactant ions on a particular crystal facet. These results are inconsistent with explanations of anisotropic nanoparticle formation that invoke a thermodynamic argument of preferred surfactant adsorption on different crystal facets of an embryonic nanoparticle seed crystal.

Quaternary Ammonium Bromide Surfactant Adsorption on Low-Index Surfaces of Gold. 1. Au(111)

The coadsorption of the anionic and cationic components of a model quaternary ammonium bromide surfactant on Au(111) has been measured using the thermodynamics of an ideally polarized electrode. The results indicate that both bromide and trimethyloctylammonium (OTA(+)) ions are coadsorbed over a broad range of the electrical state of the gold surface. At negative polarizations, the Gibbs surface excess of the cationic surfactant is largely unperturbed by the presence of bromide ions in solution. However, when the Au(111) surface is weakly charged the existence of a low-coverage, gaslike phase of adsorbed halide induces an appreciable (~25%) enhancement of the interfacial concentration of the cationic surfactant ion. At more positive polarizations, the coadsorbed OTA(+)/Br(-) layer undergoes at least one phase transition which appears to be concomitant with the lifting of the Au(111) reconstruction and the formation of a densely packed bromide adlayer. In the absence of coadsorbed halide, the OTA(+) ions are completely desorbed from the Au(111) surface at the most positive electrode polarizations studied. However, with NaBr present in the electrolyte, a high surface excess of bromide species leads to the stabilization of adsorbed OTA(+) at such positive potentials (or equivalent charge densities).

Electrochemical Studies of Capping Agent Adsorption Provide Insight into the Formation of Anisotropic Gold Nanocrystals

The ability of the 4-dimethylaminopyridine (DMAP) to stabilize and control the formation of anisotropic gold nanocrystals produced via the borohydride reduction of gold(III) salts is reported here. Electrochemical measurements of DMAP electrosorption on different low-index single crystal and polycrystalline electrodes is provided and shows a propensity for DMAP to preferentially adsorb on {100} facets. Measuring the electrochemical potential during nanocrystal formation shows that experimental conditions can easily be manipulated so that the growth of nanoseeds occurs at potentials that support preferential DMAP adsorption on {100} surfaces giving rise to highly anisotropic nanocrystals (nanorods, bipyramids, and nanopods). Nanopods with nearly 50 nm arm lengths are shown to form and produce a surface plasmon mode that extends well into the near IR (λ(max) ≈ 1350 nm). Evidence is provided of the slow, partial reduction of tetrachloroaurate to a DMAP stabilized Au(I) species. Shape control is achieved simply by varying the length of time, τ, that DMAP is allowed to partially reduce the Au(III) ions prior to the addition of the strong reducing agent, NaBH(4). Thus the role of DMAP in producing anisotropic particle shapes is shown to be multifunctional. A mechanism accounting for the dependence of particle shape on τ is provided.

Surface enhanced infrared absorption spectroscopy studies of DMAP adsorption on gold surfaces.

Attenuated total reflectance surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) measurements have been employed to study the adsorption of dimethylaminopyridine (DMAP) and its conjugate acid (DMAPH+) on gold surfaces as a function of applied potential and solution pH. Based on our transmission measurements, we have been able to demonstrate that the acid/base forms of this pyridine derivative can be readily differentiated due to their distinct IR signals. When the solution pH is equal to the pKa of DMAPH+, we demonstrate that the adsorbing species is DMAP, oriented with its heterocyclic ring perpendicular to the electrode surface. In acidic electrolytes, our SEIRAS data provide direct spectroscopic evidence of DMAP monolayer formation even though the pH is 5 units below the pKa of the conjugate acid. Our data support a potential induced deprotonation of the endocyclic nitrogen and resulting coordination of the nitrogen lone pair to the gold surface. Both of these results confirm our existing model of DMAP adsorption previously based solely on electrochemical measurements. However, the present SEIRAS study also indicates that, at low pH, DMAPH+ can electrostatically coordinate to very negatively charged surfaces. This mode of adsorption was previously unobserved, illustrating the ability of in situ spectroscopic techniques to reveal new information that is not apparent from traditional electrochemical techniques such as differential capacity and chronocoulometry.

Step-scan IR spectroelectrochemistry with ultramicroelectrodes: nonsurface enhanced detection of near femtomole quantities using synchrotron radiation.

The result of interfacing step-scan spectroelectrochemistry with an IR microscope and synchrotron infrared (SIR) radiation is provided here. An external reflectance cell containing a 25 μm gold ultramicroelectrode is employed to achieve an electrochemical time constant less than one microsecond. The use of a prototypical electrochemical system, i.e., the mass-transport controlled reduction of ferricyanide, allows for a proof of principle evaluation of the viability of SIR for step-scan spectroelectrochemistry. An analysis of the importance of accounting for synchrotron source variation over the prolonged duration of a step-scan experiment is provided. Modeling of the material flux in the restricted diffusion space afforded by the external reflectance cell allows the quantitative IR results to be compared to theoretical predictions. The results indicate that only at very short times does linear diffusion within the cavity dominate the electrode response and the majority of the transient signal operates under conditions of quasi-hemispherical diffusion. The analytical information provided by the IR signal is found to be considerably less than that derived from the current response due the latters pronounced edge effects. The results provide a detection limit of 36 fmol for step-scan SIR measurements of ferrocyanide. Implications for future IR spectroelectrochemical studies in the microsecond domain are discussed.

Synchrotron Infrared Radiation for Electrochemical External Reflection Spectroscopy: A Case Study Using Ferrocyanide

Synchrotron infrared radiation has been successfully coupled through an infrared (IR) microscope to a thin-cavity external reflectance cell to study the diffusion controlled redox of a ferrocyanide solution. Excellent signal-to-noise ratios were achieved even at aperture settings close to the diffraction limit. Comparisons of noise levels as a function of aperture size demonstrate that this can be attributed to the high brilliance of synchrotron radiation relative to a conventional thermal source. Time resolved spectroscopic studies of diffusion controlled redox behavior have been measured and compared to purely electrochemical responses of the thin-cavity cell. Marked differences between the two measurements have been explained by analyzing diffusion in both the axial (linear) and radial dimensions. Whereas both terms contribute to the measured current and charge, only species that originate in the volume element above the electrode and diffuse in the direction perpendicular to the electrode surface are interrogated by IR radiation. Implications for the use of ultramicroelectrodes and synchrotron IR (SIR) to study electrochemical processes in the submillisecond time domain are discussed.

Neutron reflectivity studies of field driven transformations in a monolayer of 4-pentadecyl pyridine at Au electrode surfaces

Neutron reflectometry (NR) has been employed to study the structure and composition of thin films formed by 4-pentadecylpyridine (C15-4Py) at a gold electrode surface. The thickness and the water content of films of C15-4Py have been measured as a function of the potential applied to the electrode. At positive potentials, where condensed film is formed, the monolayer contains defects that are filled with water. At very negative potentials, the film is desorbed from the electrode surface. NR has demonstrated that, at these potentials, the amphiphilic molecules remain in close proximity to the gold surface as a thicker and water rich film. # 2003 Elsevier B.V. All rights reserved.

Electroanalytical chemistry with zeolites

Abstract In this paper we explore how solution phase molecules affect the electrochemistry of silver-cation exchanged zeolite-modified electrodes (ZMEs). Furthermore, we examine the potential utility of ZME response to quantify solution phase analytes in aqueous and non-aqueous solutions. We give several examples which show that ZMEs are useful in assessing detection efficiencies and analyte selectivities. However, flow systems are better if one desires incorporation into a device. A useful method is to place the zeolite in a conventional HPLC column coupled to a conventional thin-layer amperometric electrochemical detector. The detection method described in this paper is based upon suppressed electroactivity of intra-zeolite silver cations. Indirect analyte detection occurs where the analyte accelerates the departure of silver ions into the solution phase, whereupon they are electrochemically determined. We show examples where promotion of silver in the solution phase occurs, via both analyte–silver interactions, and interactions between the supporting electrolyte and the analyte. Amperometric determinations of alkali metal cations, benzene, trichloroethylene and water are described.

Surface Enhanced Infrared Studies of 4-Methoxypyridine Adsorption on Gold Film Electrodes

This work uses electrochemical surface sensitive vibrational spectroscopy to characterize the adsorption of a known metal nanoparticle stabilizer and growth director, 4-methoxypyridine (MOP). Surface enhanced infrared absorption spectroscopy (SEIRAS) is employed to study the adsorption of 4-methoxypyridine on gold films. Experiments are performed under electrochemical control and in different electrolyte acidities to identify both the extent of protonation of the adsorbed species as well as its orientation with respect to the electrode surface. No evidence of adsorbed conjugated acid is found even when the electrolyte pH is considerably lower than the pKa. Through an analysis of the transition dipole moments, determined from DFT calculations, the SEIRA spectra support an adsorption configuration through the ring nitrogen which is particularly dominant in neutral pH conditions. Adsorption is dependent on both the electrical state of the Au film electrode as well as the presence of ions in the electrolyte that compete for adsorption sites at positive potentials. Combined differential capacitance measurements and spectroscopic data demonstrate that both a horizontal adsorption geometry and a vertical adsorption phase can be induced, with the former being found on negatively charged surfaces in acidic media and the latter over a wide range of polarizations in neutral solutions.