Potential-dependent Reorientation Research Articles

Fluorescence Quenching Studies of Potential-Dependent DNA Reorientation Dynamics at Glassy Carbon Electrode Surfaces

The potential-dependent reorientation dynamics of double-stranded DNA (ds-DNA) attached to planar glassy carbon electrode (GCE) surfaces were investigated. The orientation state of surface-bound ds-DNA was followed by monitoring the fluorescence from a 6-carboxyfluorescein (FAM6) fluorophore covalently linked to the distal end of the DNA. Positive potentials (i.e., +0.2 V vs open circuit potential, OCP) caused the ds-DNA to align parallel to the electrode surface, resulting in strong dipole-electrode quenching of FAM6 fluorescence. Switching of the GCE potential to negative values (i.e., -0.2 V vs OCP) caused the ds-DNA to reorient perpendicular to the electrode surface, with a concomitant increase in FAM6 fluorescence. In addition to the very fast (submilliseconds) dynamics of the initial reorientation process, slow (0.1-0.9 s) relaxation of FAM6 fluorescence to intermediate levels was also observed after potential switching. These dynamics have not been previously described in the literature. They are too slow to be explained by double layer charging, and chronoamperometry data showed no evidence of such effects. Both the amplitude and rate of the dynamics were found to depend upon buffer concentration, and ds-DNA length, demonstrating a dependence on the double layer field. The dynamics are concluded to arise from previously undetected complexities in the mechanism of potential-dependent ds-DNA reorientation. The possible origins of these dynamics are discussed. A better understanding of these dynamics will lead to improved models for potential-dependent ds-DNA reorientation at electrode surfaces and will facilitate the development of advanced electrochemical devices for detection of target DNAs.

Potential-modulated fluorescence spectroscopy of zwitterionic and dicationic membrane-potential-sensitive dyes at the 1,2-dichloroethane/water interface

The previously introduced technique of potential-modulated fluorescence (PMF) spectroscopy was used to study the potential-induced fluorescence change of some different dyes at the polarized 1,2-dichloroethane (DCE)/water (W) interface. A zwitterionic dye (POLARIC 488PPS) showed a PMF response similar to that for the previously studied dye (di-4-ANEPPS) with the same ionic state, and the PMF response was likewise explained by the potential-dependent reorientation of the dye at the DCE/W interface. Though a monocationic dye (POLARIC 488PM) showed no distinct PMF signal, a dicationic dye (di-2-ANEPEQ) showed two relatively weak but detectable PMF signals at lower and higher potential. It has thus been found that the ionic state of a potential-sensitive dye strongly influences the potential-induced reorientation of the dye at the interface and consequently its PMF response. These results support the reorientation/solvatochromic mechanism proposed for "slow" dyes but do not necessarily exclude the electrochromic mechanism proposed for "fast" dyes. PMF spectroscopy would provide useful information on the design of slow dyes for the measurement of the resting potential of cell membranes.

In Situ Probing of Solid-Electrolyte Interfaces with Nonlinear Coherent Vibrational Spectroscopy

Vibrational sum-frequency generation spectroscopy (SFG) is used for in situ probing of molecular vibrations at interfaces associated with solid-electrolyte interphases (SEI) in systems relevant to lithium-ion batteries. SFG is interface-selective and can suppress nonresonant signals from metal electrodes. Two interfaces were observed: the electrode-SEI interface and the electrolyte-SEI interface. The SEI was formed on Au or Cu by potential cycling from 2.0 V–0.2 V (vs. Li/Li+) in ethylene carbonate (EC) and LiClO4 in tetrahydrofuran (THF). Li deposition occurs on Au but not on Cu. SFG of the electrolyte-SEI interface with Cu shows EC transitions whose intensities oscillate during potential cycling. The smaller oscillations are attributed to EC potential-dependent reorientation; the larger to an optical interference effect associated with the SEI layer thickness. The larger EC interference oscillations seen on Cu are absent on Au because the SEI on Au is thicker and more opaque. Lithium ethylene dicarbonate (LiEDC) and possibly ethylene oxide are observed at the electrode-SEI interface. This interfacial structure varies little after the first cycle of SEI formation. THF is also observed at the electrode interface with a degree of mobility that increases during the first few four potential cycles and then levels off.

Potential-Dependent Water Orientation on Pt(111), Pt(100), and Pt(110), As Inferred from Laser-Pulsed Experiments. Electrostatic and Chemical Effects

The laser-induced temperature jump method is used to characterize the net orientation of interfacial water on well-defined platinum surfaces, Pt(111), Pt(100), and Pt(110), as a function of the applied potential. A clear effect of the surface structure on the potential of water reorientation is observed, being 0.37 for Pt(111), 0.33 for Pt(100), and 0.14 V vs RHE for Pt(110) in 0.1 M HClO4 solution. The potential of water reorientation also exhibits a different pH dependency for the three basal planes, shifting 0.060 for Pt(111), 0.030 for Pt(100), and 0.015 V/dec for Pt(110). Comparison with charge density data provides a deeper understanding of these results. A quantitative analysis of the electrostatic and chemical effects governing the potential-dependent reorientation of the interfacial water network is addressed. It is concluded that water on Pt(111) exhibits a small net orientation in the absence of electric field at the interphase. On the other hand, the agreement between the relative position of values of the potential of water reorientation and work functions, for the three basal planes, suggests that the same situation holds for Pt(100) and Pt(110).

Voltammetric study of adsorption layers of various 4-pyridyl terminated surfactants on a Au(1 1 1) electrode: Effects of electronic property of pyridyl group and intermolecular hydrogen bonding upon potential-driven phase changes

The major factors determining the potential-driven phase changes of the films of 4-pyridyl terminated water-insoluble neutral surfactants on a Au(1 1 1) electrode were described using the results of dc and ac voltammetric measurements. In total six surfactants were used to elucidate the effects of electronic property of the 4-pyridyl head group, hydrogen bonding ability between amide groups, direction of the amide bond, and bulkiness around the head group upon the potential dependent reorientation and adsorption–desorption processes. Comparison of the behavior of pentadecyl 4-pyridyl ether (C15–O–Py) and 1-pyridin-4-yl-hexadecan-1-one ( C15–(C O)–Py), possessing respectively, the highest and lowest excess electronic charge on the pyridyl nitrogen among the six surfactants, revealed that the latter exhibits more distinct reorientation of pyridyl group corresponding to phase transition between condensed and loose-packed states and narrower potential region of the compact film formation. Introduction of amide group in the alkyl chains of the surfactants reduced the packing of the adsorbed film in the potential region of the compact film formation, as evidenced by an increase of the differential capacitance. It was found that the existence of amide group facilitates the potential-driven phase transition of closely packed film from a condensed state to a loosely packed state, whereas it suppresses the transition of loosely packed film. The concomitant steric effect was also discussed.

Structure of Water at the Electrified Platinum−Water Interface: A Study by Surface-Enhanced Infrared Absorption Spectroscopy

Surface-enhanced infrared absorption spectroscopy in the attenuated total reflection mode is used to examine the structure of water on a polycrystalline Pt electrode in H2SO4 and HClO4 as a function of applied potential. The electrode surface covered with CO is used as the reference in recording spectra, which enables us to obtain the absolute infrared spectrum of the interfacial water layer (monolayer or bilayer) in contact with the surface with negligible interference from the bulk water. The spectrum of the interfacial water is largely different from that of bulk water and changes around the potential of zero charge of the electrode. The spectral changes are ascribed to the potential-dependent reorientation of water molecules from a weakly hydrogen-bonded oxygen-up orientation at the negatively charged surface to a strongly hydrogen-bonded nearly flat orientation at the positively charged surface in agreement with theoretical simulations reported in the literature. Clear experimental evidence of the fo...

Potential-dependent reorientation of thiocyanate on Au electrodes.

Thiocyanate (SCN) adsorption on an Au electrode is examined using surface-enhanced Raman scattering (SERS) measurements, along with detailed density functional theory (DFT) calculations. Both the calculation and the spectroscopic measurements show that three different geometries are adopted by SCN adsorption in the potential region studied (0.0 V <or= E <or= 1.2 V vs NHE). At low potential both N-bound and S-bound forms are found, at intermediate potentials around the potential of zero charge (E(pzc)) the S-bound form dominates, and at high potentials SCN associates with the Au surface via a bridging geometry. The Stark slope observed for the C-N stretch in the N- and S-bound forms is positive, while the bridge form exhibits a negative Stark slope. DFT calculations show that the potential-dependent reorientation of SCN arises from overlap of specific SCN-based orbitals with the Fermi level of the Au surface; as the Fermi energy changes, different orbital overlaps are found. The calculations also provide an explanation for the different signs of the Stark slope observed for different geometries. For the end-on adsorption, the lowest unoccupied molecular orbital (LUMO) associated with the C-N bond is antibonding, while the corresponding LUMO in the bridge form is bonding.

Orientational Phase Transition in a Pyridine Adlayer on Gold(111) in Aqueous Solution Studied by in Situ Infrared Spectroscopy and Scanning Tunneling Microscopy

The orientation and structure of pyridine adsorbed on a highly ordered Au(111) surface from 0.1 M NaClO4 + x M (10-6 ≤ x ≤ 10-3) pyridine aqueous solutions have been investigated as a function of applied potential by in situ surface-enhanced infrared absorption spectroscopy (SEIRAS) and scanning tunneling microscopy (STM). Symmetric in-plane pyridine ring vibrations (A1 modes) were observed in the SEIRA spectra at potentials positive of about −0.3 V versus SCE, while asymmetric in-plane ring vibrations (B1 modes) were hardly detected. The symmetric ring-breathing mode showed a blue shift upon adsorption, indicating the adsorption via the N atom. The band intensities were found not to be proportional to the surface concentration (the relative Gibbs surface excess) reported in the literature. On the basis of the surface selection rule in SEIRAS, the results are explained in terms of the potential-dependent reorientation of pyridine. The molecule is flatly adsorbed on the surface at negative potentials, and its molecular plane rises up as the applied potential increases and the surface concentration increases. Flat-lying, tilted, and vertically standing pyridine molecules were observed at different potentials also by STM for the first time.

In Situ Infrared Study of Water−Sulfate Coadsorption on Gold(111) in Sulfuric Acid Solutions

Potential-dependent reorientation of a water molecule, adsorption of sulfate, and interactions between water and sulfate on a highly ordered Au(111) electrode surface in sulfuric acid solutions have been investigated in situ as a function of applied potential by means of surface-enhanced infrared absorption spectroscopy. The spectrum of the water layer at the interface changes in both intensity and frequency as the applied potential changes due to the reorientation of water molecules. The orientations deduced from infrared spectra are in good agreement with the predictions made by molecular dynamics simulations at potentials below and around the potential of zero charge (pzc) of the electrode where sulfate adsorption is negligible. At potentials above the pzc, sulfate anion is adsorbed at 3-fold hollow sites on the (111) surface via three oxygen atoms. When the potential is increased and the fractional coverage of sulfate reaches to about one-half of full coverage, adsorbed sulfate anions start to form sh...

The influence of heterocyclic thiols on the electrodeposition of Cu on Au(111)

We report the influence of two structurally related heterocyclic thiols, 2-mercaptobenzothiazole and 2-mercaptobenzimidazole, on the underpotential (upd) and bulk deposition of Cu on Au(111). X-ray photoelectron spectroscopy analysis of the organic adlayers reveal that after self-assembly from the liquid phase, the primary adsorbate species is consistent with a thiolate. The Au(111)-(√3 × 22) reconstruction is lifted to the (1 × 1) phase upon thiol self-assembly. At submonolayer thiol coverages, a new upd feature is observed, with an intensity that goes through a maximum at intermediate thiol coverages. This feature is consistent with Cu adsorption in the vicinity of chemisorbed thiol. A thiol coverage-dependent study of Cu upd suggests formation of small two-dimensional thiol islands. At saturation coverage, the adsorbates passivate the surface towards Cu upd and significantly hinder bulk processes. Investigation of a previously reported potential-dependent reorientation of 2-mercaptobenzothiazole disputes a structural mechanism of ring ‘opening’ and ‘closing’ of the thiol layer. Instead, we suggest that instability of the thiol during cycles of Au oxide formation and reduction results in irreversible desorption of the thiol and complete restoration of Cu upd.

Potential-Dependent Reorientation of Water Molecules at an Electrode/Electrolyte Interface Studied by Surface-Enhanced Infrared Absorption Spectroscopy

The structure and orientation of water molecules at a highly ordered Au(111) electrode surface in perchloric acid have been investigated in-situ as a function of applied potential by means of surface-enhanced infrared absorption spectroscopy. This newly developed infrared spectroscopy technique enables the observation of the electrode/electrolyte interface at a very high sensitivity without interference from the bulk solution. The spectrum of the interfacial water significantly differs from that of bulk water and drastically changes in peak frequencies and band widths around the potential of zero charge (pzc) of the electrode and at about 0.3 V positive from the pzc. The interfacial water molecules are weakly hydrogen-bonded at potentials below the pzc and form a strongly hydrogen-bonded ice-like structure at potentials slightly above the pzc. The ice-like structure is broken at more positive potentials due to the specific adsorption of perchlorate ion, where one OH moiety of water is non-hydrogen-bonded and the other OH moiety is hydrogen-bonded to another water molecule. The intensities of the fundamental modes of water are also a strong function of applied potential. They are very weak around the pzc and increase as the potential changes in both positive and negative directions. These results are explained in terms of the potential-dependent reorientation of water molecules from oxygen-up to oxygen-down as the surface charge changes from negative to positive. The adsorption of hydronium and perchlorate ions on gold is also discussed.

The potential-dependent reorientation of adsorbed thiopyridine at a gold electrode in phosphate buffer

This preliminary note reports an FTIR study of the dissociative adsorption of 4,4'-dithiopyridine at a gold electrode in aqueous solution.