Designer electrode interfaces simultaneously comprising three different metal nanoparticle (Au, Ag, Pd)/carbon microsphere/carbon nanotube composites: progress towards combinatorial electrochemistry

In this report gold, silver and palladium metal nanoparticles are separately supported on glassy carbon microspheres (GCM) using bulk electroless deposition techniques to produce three different materials labelled as GCM-Au, GCM-Ag and GCM-Pd respectively. These three materials are then combined together into a composite film on a glassy carbon (GC) electrode surface using multiwalled carbon nanotubes (MWCNTs). The MWCNTs serve to not only mechanically support this composite film as a "binder" but they also help to "wire up" each modified GCM to the underlying substrate. The intelligently designed structure of this electrode interface allows this single modified electrode to simultaneously behave as if it were a macrodisc electrode constructed of gold, silver or palladium, whilst using only a fraction of the equivalent amount of these precious metals. Furthermore this unique structure allows the possibility of combinatorial electrochemistry to be realised using a relatively facile electrode construction which avoids the problems of alloy formation, co-deposition and the formation of bimetallic species. For instance a mixture of several different analytes, which can each only be detected on a different specific substrate, can simultaneously be determined using one electrode in a single voltammetric experiment! Alternatively a substrate could undergo electrocatalytic reactions on one substrate, whilst the products, and hence the progress of this reaction, can be studied at a different substrate simultaneously at the same electrode surface. Proof-of-concept examples are presented herein and the designer electrode interface is shown to produce analytical responses to model target analytes such as hydrazine, bromide and thallium(I) ions that are comparable, if not better, than those obtained at metal macrodisc electrodes and even at other state-of-the-art nanoparticle modified electrodes.     

Removal of Toxic Metal‐Ion Pollutants from Water by Using Chemically Modified Carbon Powders

The thermodynamics of and the kinetic parameters controlling the sequestration of the toxic heavy‐metal ion Cd^II from aqueous media by using a novel material consisting of glassy carbon microspheres (10–20 μm in diameter) chemically modified with L ‐cysteine methyl ester are presented. In an effort to reduce the cost and increase the efficiency of toxic‐metal‐ion removal, this modification strategy was expanded to attach L ‐cysteine methyl chemically ester to less‐expensive graphite powders (2–20 μm in diameter), and the thermodynamic and kinetic parameters of the sequestration of Cd^II, Cu^II, and As^III toxic metal ions are presented. It was found that the use of chemically modified graphite powder greatly increased both the rate and the amount of metal ions removed from aqueous media. This work has important potential applications to filtration of drinking water and environmental remediation.     

A New Mode of Chemical Reactivity for Metal‐Free Hydrogen Activation by Lewis Acidic Boranes

We herein explore whether tris(aryl)borane Lewis acids are capable of cleaving H₂ outside of the usual Lewis acid/base chemistry described by the concept of frustrated Lewis pairs (FLPs). Instead of a Lewis base we use a chemical reductant to generate stable radical anions of two highly hindered boranes: tris(3,5‐dinitromesityl)borane and tris(mesityl)borane. NMR spectroscopic characterization reveals that the corresponding borane radical anions activate (cleave) dihydrogen, whilst EPR spectroscopic characterization, supported by computational analysis, reveals the intermediates along the hydrogen activation pathway. This radical‐based, redox pathway involves the homolytic cleavage of H₂, in contrast to conventional models of FLP chemistry, which invoke a heterolytic cleavage pathway. This represents a new mode of chemical reactivity for hydrogen activation by borane Lewis acids.     

A sensitive reagentless pH probe with a ca. 120 mV/pH unit response

The use of a basal plane pyrolitic graphite electrode immobilised with two redox active species each capable of undergoing a two-electron, two-proton redox process has allowed the development of a sensitive pH probe over a wide pH and temperature range. When the values of the peak potentials of the two processes are combined they shift by ca. 120 mV/pH unit at 25 °C, as measured against two independent, defined reference electrodes.Dedicated to Zbigniew Galus on the occasion of his 70th birthday     

A Combined “Electrochemical–Frustrated Lewis Pair” Approach to Hydrogen Activation: Surface Catalytic Effects at Platinum Electrodes

Herein, we extend our “combined electrochemical–frustrated Lewis pair” approach to include Pt electrode surfaces for the first time. We found that the voltammetric response of an electrochemical–frustrated Lewis pair (FLP) system involving the B(C₆F₅)₃/[HB(C₆F₅)₃]^? redox couple exhibits a strong surface electrocatalytic effect at Pt electrodes. Using a combination of kinetic competition studies in the presence of a H atom scavenger, 6‐bromohexene, and by changing the steric bulk of the Lewis acid borane catalyst from B(C₆F₅)₃ to B(C₆Cl₅)₃, the mechanism of electrochemical–FLP reactions on Pt surfaces was shown to be dominated by hydrogen‐atom transfer (HAT) between Pt, [Pt?H] adatoms and transient [HB(C₆F₅)₃] ^? electrooxidation intermediates. These findings provide further insight into this new area of combining electrochemical and FLP reactions, and proffers additional avenues for exploration beyond energy generation, such as in electrosynthesis.     

Investigating the thermodynamic causes behind the anomalously large shifts in pKa values of benzoic acid-modified graphite and glassy carbon surfaces

The difference between the values of 4-carboxyphenyl groups, covalently attached to either graphite (BAcarbon) or glassy carbon (BA-GC) surfaces, and benzoic acid in solution is explored using potentiometric titration and cyclic voltammetry. In solution, benzoic acid has a pKa of 4.20 at 25 degrees C. However, the observed pKa value on the graphitic surfaces shows significant deviations, with BAcarbon exhibiting a large shift to higher pKa values (pKa = 6.45) in contrast to BA-GC, which is shifted to lower pKa values (pKa = 3.25). Potentiometric titrations at temperatures between 25 and 50 degrees C allowed us to determine the surface pKa of these materials at each temperature studied and hence to determine the enthalpy, entropy, and Gibbs' energy changes associated with the ionization of the carboxylic acid groups. It was found that the enthalpic contribution is negligible and that the changes in surface pKa values are entropically controlled. This suggests that solvent ordering/disordering around the interface strongly influences the observed pKa value, which then reflects the relative hydrophobicity/hydrophilicity of the different graphitic surfaces.     

The Formazanate Ligand as an Electron Reservoir: Bis(Formazanate) Zinc Complexes Isolated in Three Redox States

The synthesis of bis(formazanate) zinc complexes is described. These complexes have well‐behaved redox‐chemistry, with the ligands functioning as a reversible electron reservoir. This allows the synthesis of bis(formazanate) zinc compounds in three redox states in which the formazanate ligands are reduced to “metallaverdazyl” radicals. The stability of these ligand‐based radicals is a result of the delocalization of the unpaired electron over four nitrogen atoms in the ligand backbone. The neutral, anionic, and dianionic compounds (L₂Zn^0/?1/?2) were fully characterized by single‐crystal X‐ray crystallography, spectroscopic methods, and DFT calculations. In these complexes, the structural features of the formazanate ligands are very similar to well‐known β‐diketiminates, but the nitrogen‐rich (NNCNN) backbone of formazanates opens the door to redox‐chemistry that is otherwise not easily accessible.     

Oxygen reduction catalysis at anthraquinone centres molecularly wired via carbon nanotubes

Multi-walled carbon nanotubes (MWCNTs) have been chemically derivatised via the reduction of anthraquinone-1-diazonium chloride with hypophosphorous acid to attach 1-anthraquinonyl groups to the MWCNTs, most likely at edge plane like defects. The covalently attached quinone moiety attached to the nanotubes (‘molecular wire’) acts as an effective mediator for the electrocatalytic reduction of oxygen.     

Synthesis and characterization of carbon nanotubes covalently functionalized with amphiphilic polymer coated superparamagnetic nanocrystals

The kinetic investigations of oxidation of tris(1,10-phenanthroline)iron(II) by oxone have been studied spectrophotometrically in phosphate buffer medium of pH 6.8, temperature 308 K, and ionic strength 0.25 mol L(-1). The reactions were also carried out in presence of globular transport protein, bovine serum albumin (BSA) having isoelectric point 4.9, anionic surfactant sodium dodecyl sulfate (SDS), and their mixtures. The critical aggregation concentration (CAC) and critical micelle concentration (CMC) of SDS in presence of BSA have been determined using conductivity and kinetic measurement techniques. The secondary structure of BSA was examined by Circular Dichroism (CD) measurement at 308 K. The helix nature of BSA decreases with increase of SDS concentration. The effect of pH on rate in presence of BSA is opposite to its absence, and the effect of urea on rate in presence of BSA indicates the denaturation of BSA. The results depict that amphiphile SDS interacts with BSA and different molecular events, for example, specific binding, cooperative binding, protein unfolding, and micelle formation act. Activation parameters of the reaction in different environments have been determined.