Frank Marken

Water-induced accelerated ion diffusion: voltammetric studies in 1-methyl-3-[2,6-(S)-dimethylocten-2-yl]imidazolium tetrafluoroborate, 1-butyl-3-methylimidazolium tetrafluoroborate and hexafluorophosphate ionic liquids

The electrochemical properties of the room temperature ionic liquids 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM+BF4−), 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM+PF6−) and 1-methyl-3-[2,6-(S)-dimethylocten-2-yl]imidazolium tetrafluoroborate (MDIM+BF4−) as solvents have been studied using micro-samples, with a volume of 10 μL, of the ionic liquids under vacuum conditions and under conditions with controlled gas and moisture supplies. The impact of water—absorbed into the ionic liquid in a controlled manner from the gas phase—on the voltammetry of dissolved redox systems and on the accessible potential window of the ionic liquids was investigated. The diffusion coefficients for three representative redox systems, the oxidation of neutral N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD), the reduction of cationic methyl viologen (MV2+) and reduction of anionic hexacyanoferrate(III), Fe(CN)63−, have been determined as a function of the water content of the ionic liquids. Water is shown to have a much more dramatic acceleration effect on the diffusion of the ionic compounds compared to its effect on neutral species in ionic liquids. A model based on nanoscale structural features of wet ionic liquid materials is proposed. The novel methodology, which employs redox-active compounds dissolved or partitioned in microdroplets of ionic liquid, uses conditions suitable for the study of ionic liquids for applications in electrochemical gas phase reactors and gas sensor systems.

Plasmon Resonance Scattering Spectroscopy at the Single‐Nanoparticle Level: Real‐Time Monitoring of a Click Reaction

A method based on plasmon resonance Rayleigh scattering (PRRS) spectroscopy and dark-field microscopy (DFM) was established for the real-time monitoring of a click reaction at the single-nanoparticle level. Click reactions on the surface of single gold nanoparticles (GNPs) result in interparticle coupling, which leads to a red-shift of the λmax (Δλmax =43 nm) in the PRRS spectra and a color change of the single gold nanoparticles in DFM (from green to orange).

Electrochemical and X-ray diffraction study of the redox cycling of nanocrystals of 7,7,8,8-tetracyanoquinodimethane. Observation of a solid–solid phase transformation controlled by nucleation and growth

The redox cycling of nanocrystals of 7,7,8,8-tetracyanoquinodimethane (TCNQ) immobilized on the surface of a variety of electrodes has been carried out in aqueous solutions of 1:1 electrolytes containing Group 1 cations (Na+, K+, Rb+, Cs+). It is found that the overall process follows the general equation [graphic omitted] where M+ is a Group 1 cation. It is also found, by a combination of voltammetric and XRD techniques, that the overall process is rate-controlled by nucleation and growth of the solid phases. Simple intercalation is ruled out by observing that significant structural rearrangement of the solid phases accompanies the redox reactions, and that x and y are integers. In the voltammograms, unusual inert zones are observed between the reduction and re-oxidation peaks, unlike anything that is seen in the conventional redox cycling of solution species. Theoretical analysis reveals that these are caused by the need to expend energy to create the solid/solid interface between the reduced and neutral forms of TCNQ in the critical nuclei of the solid phases.

Generator-collector double electrode systems: A review

A variety of generator-collector systems are reviewed, from the original rotating ring-disc electrodes developed in the 1950s, to very recent developments using new geometries and microelectrodes. An overview of both theoretical and experimental aspects are given, and the power of these double electrode systems in analytical electrochemistry is illustrated with a range of applications.

Chemically surface-modified carbon nanoparticle carrier for phenolic pollutants: Extraction and electrochemical determination of benzophenone-3 and triclosan

Chemically surface-modified (tosyl-functionalized) carbon nanoparticles (Emperor 2000 from Cabot Corp.) are employed for the extraction and electrochemical determination of phenolic impurities such as benzophenone-3 (2-hydroxy-4-methoxybenzophenone) or triclosan (5-chloro-2-(2,4-dichlorophenoxy)phenol). The hydrophilic carbon nanoparticles are readily suspended and separated by centrifugation prior to deposition onto suitable electrode surfaces and voltammetric analysis. Voltammetric peaks provide concentration information over a 10-100microM range and an estimated limit of detection of ca. 10microM (or 2.3ppm) for benzophenone-3 and ca. 20microM (or 5.8ppm) for triclosan. Alternatively, analyte-free carbon nanoparticles immobilized at a graphite or glassy carbon electrode surface and directly immersed in analyte solution bind benzophenone-3 and triclosan (both with an estimated Langmuirian binding constants of K approximately 6000mol(-1)dm(3) at pH 9.5) and they also give characteristic voltammetric responses (anodic for triclosan and cathodic for benzophenone-3) with a linear range of ca. 1-120microM. The estimated limit of detection is improved to ca.5microM (or 1.2ppm) for benzophenone-3 and ca. 10microM (or 2.3ppm) for triclosan. Surface functionalization is discussed as the key to further improvements in extraction and detection efficiency.

The Development of Boronic Acids as Sensors and Separation Tools

Synthetic receptors for diols that incorporate boronic acid motifs have been developed as new sensors and separation tools. Utilizing the reversible interactions of diols with boronic acids to form boronic esters under new binding regimes has provided new hydrogel constructs that have found use as dye-displacement sensors and electrophoretic separation tools; similarly, molecular boronic-acid-containing chemosensors were constructed that offer applications in the sensing of diols. This review provides a somewhat-personal perspective of developments in boronic-acid-mediated sensing and separation, placed in the context of the seminal works of others in the area, as well as offering a concise summary of the contributions of the co-authors in the area.

Assembly of N-hexadecyl-pyridinium-4-boronic acid hexafluorophosphate monolayer films with catechol sensing selectivity

The highly water insoluble N-hexadecyl-pyridinium-4-boronic acid hexafluorophosphate is synthesised and investigated for sensor applications. This amphiphilic molecule is immobilised by evaporation of an acetonitrile solution at a basal plane pyrolytic graphite (HOPG) electrode surface and is shown to provide a monolayer film. By varying the amount of deposit partial or full coverage can be achieved. The N-hexadecyl-pyridinium-4-boronic acid hexafluorophosphate monolayer acts as an active receptor for 1,2-dihydroxy-benzene (catechol) derivatives in aqueous media. The ability to bind alizarin red S is investigated and the Langmuirian binding constant determined as a function of pH. It is shown that the immobilised boronic acid monolayer acts as sensor film for a wider range of catechols. A comparison of Langmuirian binding constants for alizarin red S (1.4 × 105 mol−1 dm3), catechol (8.4 × 104 mol−1 dm3), caffeic acid (7.5 × 104 mol−1 dm3), dopamine (1.0 × 104 mol−1 dm3), and L-dopa (8 × 103 mol−1 dm3) reveals that a combination of hydrophobicity and electrostatic interaction causes considerable selectivity effects.

Fluorescent Boron Bis(phenolate) with Association Response to Chloride and Dissociation Response to Fluoride

Addition of chloride ions to boron bis(phenolate) 5 in dichloromethane solution produces a selective fluorescence decrease. The fluorescence change is believed to be caused by associative hydrogen bonding between the chloride ion and two boronic acid groups. While addition of fluoride ions to bis(phenolate) 5 generates a purple colorimetric response, the colorimetric response is caused by fluoride induced B-O bond cleavage and air oxidation of the phenolate anion formed by this dissociation.

Metastable Ionic Diodes Derived from an Amine‐Based Polymer of Intrinsic Microporosity

A highly rigid amine-based polymer of intrinsic microporosity (PIM), prepared by a polymerization reaction involving the formation of Trögers base, is demonstrated to act as an ionic diode with electrolyte-dependent bistable switchable states.

Microwave activation of electrochemical processes: convection, thermal gradients and hot spot formation at the electrode|solution interface

Microwave activation of electrochemical processes is possible by self-focussing of intense microwave radiation at the electrode|solution (electrolyte) interface of an electrode immersed in a solution and placed in a microwave cavity. Considerable changes in voltammetric current responses are observed experimentally for the one-electron reduction of Ru(NH3)63+ in aqueous 0.1 M KCl and for the stepwise two-electron reduction of the methylviologen dication (MV2+) in aqueous 0.1 M NaCl. The formation and interconversion of two distinct forms of solid deposits, MVam0 and MVcryst0, on a mercury electrode surface is investigated, both in the presence of microwave activation and with conventional heating. It is shown that microwave activation achieves (i) high temperatures in the vicinity of the electrode, (ii) thermal desorption of deposits from the electrode surface and (iii) limiting currents an order of magnitude higher compared to those induced by conventional isothermal heating to the same electrode temperature.A simple physical model based on Joule heating of the aqueous solution phase is employed in a finite element simulation (FIDAPTM) procedure to explain the differences observed experimentally between conventional heating and microwave activation. Based on the comparison of simulation and experimental data, a considerable thermal gradient and ‘hot spot ’ region in the diffusion layer of the electrode, together with convective mass transport are proposed.