Voltammetry at spatially heterogeneous electrodes

Recent advances are overviewed which enable simulation of the voltammetric behaviour of surfaces which respond in an electrochemically spatially heterogeneous fashion. By use of the concept of a “diffusion domain” computationally expensive three-dimensional simulations may be reduced to tractable two-dimensional equivalents. In this way the electrochemical response of partially blocked electrodes and microelectrode arrays may be predicted, and are found to be consistent with experimental data. It is, furthermore, possible to adapt the “blocked” electrode analysis to enable the voltammetric sizing of inert particles present on an electrode surface. Finally theory of this type predicts the voltammetric behaviour of electrochemically heterogeneous electrodes—for example composites whose different spatial zones display contrasting electrochemical behaviour toward the same redox couple.     

Voltammetric determination of mercury (II) in aqueous media using glassy carbon electrodes modified with novel calix[4]arene

A novel calix[4]arene derivative containing benzothiazole group was coated on glassy carbon electrode (GCE) and then applied to the recognition of mercury ion. Cyclic and square wave voltammetric results showed that the modified electrode selectively recognizes Hg²⁺ ion in aqueous media. A new anodic stripping peak at −0.3 V (vs. Ag/Ag⁺) can be obtained by scanning the potential from −0.6 to 0.6 V, and the peak currents are proportional to the Hg²⁺ concentration. The modified electrode in a 0.1 M H₂SO₄+0.01 M NaCl solution shows linear voltammetric response in the range of 25-300 μg l⁻¹ and detection limit of 5 μg l⁻¹ (ca. 2.5×10⁻⁸ M). This modified GCE does not present any significant interference from alkali, alkaline and transition metal ions except for Pb²⁺, Ag⁺ and Cu²⁺ ions. Only 500, 50 and 100-fold molar excess of Pb²⁺, Ag⁺ and Cu²⁺ ions, respectively, can lead to voltammetric response comparable with that of Hg²⁺. The proposed method was successfully applied to determine mercury in natural water.     

Enhancement of Anodic Response for DMSO at Ruthenium Oxide Film Electrodes as a Result of Doping with Iron(III)

The oxidation of dimethyl sulfoxide (DMSO) to dimethyl sulfone (DMSO₂) is representative of numerous anodic oxygen‐transfer reactions of organosulfur compounds that suffer from slow kinetics at noble metal electrodes. Anodic voltammetric data for DMSO are examined at various RuO₂‐film electrodes prepared by thermal deposition on titanium substrates. The response for DMSO is slightly larger at RuO₂ films prepared in a flame as compared with films prepared in a furnace; however, temperature is more easily controlled in the furnace. Doping of the RuO₂ films with Fe(III) further improves the sensitivity of anodic response for DMSO. Optimal response is obtained at an Fe(III)‐doped RuO₂‐film electrode prepared using a deposition solution of 50 mM RuCl₃ and 10 mM FeCl₃ in a 1 : 1 mixture of isopropanol and 12 M HCl at an annealing temperature of 450 °C. The Levich plot (i vs. ω^1/2) and Koutecky‐Levich plot (1/i vs. 1/ω^1/2) of amperometric data for the oxidation of DMSO at an Fe(III)‐doped RuO₂‐film electrode configured as a rotated disk are consistent with an anodic response controlled by mass‐transport processes at low rotational velocities. Flow injection data demonstrate that Fe(III)‐doped RuO₂‐film electrodes exhibit detection capability for methionine and cysteine in addition to DMSO. Detection limits for 100‐μL injections of the three compounds are ca. 3.2×10^?4 mM, i.e., ca. 32 pmol.     

Cyclopolymerization of dipropargyl isopropylidene malonate and characterization of the product

The polymerization of dipropargyl isopropylidene malonate (DPIPM) was polymerized by WCl₆ and MoCl₅ associated with various organometallic cocatalysts. MoCl₅ was found to be the most effective catalyst and Ph₄Sn was observed to have a high cocatalyst activity. Structure and physical properties of poly(DPIPM) were investigated. The spectral data indicated that poly(DPIPM) contains alternating double and single bonds along the polymer backbone and a cyclic recurring unit. The poly(DPIPM) was partially soluble in common organic solvents. The M?_n values of the polymer from soluble fraction were in the range of 5100–8000 relative to polystyrene standards by GPC. In addition, poly(DPIPM) possesses good stability to air oxidition. When poly(DPIPM) is exposed to iodine vapor, the electrical conductivity was increased from 4.5 × 10^?11 to 7 × 10^?2 S/cm. © 1993 John Wiley & Sons, Inc.     

An Investigation into Outer‐Surface Biofouling and Electrode Passivation Effects on Gold Electrodes Modified with Calix[4]resorcinarenetetrathiol and a PEG Derivative on Exposure to Whole Human Blood

An investigation into the hemo‐biocompatibility of calix[4]resorcinarenetetrathiol and calix‐PEG derivative coatings on gold electrodes is reported, with respect to the detection of the ascorbate ion in aqueous solution. The electrochemical behavior of the ascorbate ion at calix‐modified electrodes is also discussed. Findings show that a facilitated adsorption of ascorbate from solution is seen at calix‐modified gold electrodes. Following exposure to whole human blood, calix‐PEG derivative coatings were seen to offer some protection against both outer‐surface biofouling and electrode passivation effects in comparison to calix[4]resorcinarenetetrathiol counterparts. The individual effects of outer surface biofouling and electrode passivation on exposure to whole blood have been investigated. Our findings suggest that PEG moieties help prevent sorption of the biofouling film, thus possibly minimizing the release of a number of low molecular weight solutes which are thought to be involved with the passivation of electrode responses within clinical bio‐ and chemical sensors.     

Electrocatalytic oxidation of some biologically important compounds at conducting polymer electrodes modified by metal complexes

Conducting poly(3-methylthiophene) electrodes were electrochemically prepared. The resulting polymer films were modified with an inorganic complex, ferrocene. The incorporation of the ferrocene/ferrocenium moiety into the polymer film resulted in enhanced charge transfer towards the oxidation of some organic molecules of biological interest. The electrochemical response of the complex-containing polymer electrode was compared to that of the unmodified polymer electrode and that of the substrate. Apparent diffusion coefficients of the redox species were estimated from the cyclic voltammetric data for different biological molecules at the ferrocene-containing polymer electrode. Infra-red spectroscopic measurements for the “as-grown” films revealed the presence of the inorganic complex within the polymer. The modified polymer electrode showed noticeable enhancement for the charge transfer across the film interface and can be used as an electrochemical sensor for biological compounds. Received: 3 June 1997 / Accepted: 7 July 1997     

Perchlorate-selective membrane electrode based on a new complex of uranil

A potentiometric ion-selective electrode based on new compound, as a carrier, has been successfully developed for detection of perchlorate anion in aqueous solution. Within the perchlorate ion concentration range 1.0×10^–6 to 1.0 mol L^–1 the electrode had a linear response with a Nernstian slope of 60.6±1.0 mV per decade . The limit of detection as determined from the intersection of the extrapolated linear segments of the calibration plot was 8.0×10^–7 mol L^–1. The proposed electrode has fairly a good discriminating ability towards ClO₄^– ion in comparison to other anions. The sensor has a response time of 10 s and can be used for at least 2 months without substantial divergence in potential. It was successfully applied to direct determination of perchlorate in urine and water.     

A Disposable Biosensor for Organophosphorus Nerve Agents Based on Carbon Nanotubes Modified Thick Film Strip Electrode

A disposable biosensor based on acetylcholinesterase‐functionalized acid purified multi‐wall carbon nanotubes (CNTs) modified thick film strip electrode for organophosphorus (OP) insecticides was developed. The degree of inhibition of the enzyme acteylcholinesterase (AChE) by OP compounds was determined by measuring the electrooxidation current of the thiocholine generated by the AChE catalyzed hydrolysis of acteylthiocholine (ATCh). The large surface area and electro‐catalytic activity of carbon nanotubes lowered the overpotential for thiocholine oxidation to 200 mV (vs. Ag/AgCl) without the use of mediating redox species and enzyme immobilization by physical adsorption. The biosensor detected as low as 0.5 nM (0.145 ppb) of the model organophosphate nerve agent paraoxon with good precision, electrode to electrode reproducibility and stability. Analysis of real water sample using the sensor demonstrated the feasibility of the application of the sensor for on site monitoring of OP compounds.     

Preparation of Thallium Hexacyanoferrate Film and Mixed‐Film Modified Electrodes with Cobalt(II) Hexacyanoferrate

Thallium hexacyanoferrate films have been prepared from various aqueous electrolyte solutions using consecutive cyclic voltammetry. The cyclic voltammograms recorded the direct deposition of thallium hexacyanoferrate films from the mixing of Tl³⁺ and [Fe(CN)₆]^3? ions from solutions of seven cations: Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, H⁺, and Tl⁺. An electrochemical quartz crystal microbalance (EQCM) and cyclic voltammetry were used to study the in situ growth of the thallium hexacyanoferrate films. The thallium hexacyanoferrate film shows a single redox couple with a formal potential between +0.6 V and +1.2 V, and shows a cation effect (H⁺, Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, and Tl⁺). A mixed film and a two‐layered modified electrodes composed of a thallium hexacyanoferrate film with cobalt(II) hexacyanoferrate film were prepared.     

Na-Li-[V3O8] insertion electrodes: Structures and diffusion pathways

The potential insertion-electrode compounds Na_1.2[V₃O₈] (NaV) and Na_0.7Li_0.7[V₃O₈] (NaLiV) were synthesized from mixtures of Na₂CO₃, Li₂CO₃ and V₂O₅, which were melted at 750° and subsequently cooled to room temperature. The structures of NaV and LiV contain sheets of polymerized (VO_n) polyhedra, which are topologically identical to the sheet of polymerized polyhedra in Li_1.2[V₃O₈] (LiV). Vanadium occurs in three different coordination environments: [2+3] V(1), [2+2+2] V(2) and [1+4+1] V(3). Calculated bond-valence sums indicate that V⁴⁺ occurs preferentially at the V(3) site, which agrees with the general observation that [6]-coordinated V⁴⁺ prefers [1+4+1]-rather than [2+2+2]-coordination. The M-cations Na and Li occur at three distinct sites, M(1), M(2) and M(3) between the vanadate sheets. The M(1)-site is fully occupied and has octahedral coordination. The M(2) sites are partly occupied in NaV and NaLiV, in which they occur in [4]- and [6]-coordination, respectively. Li partly occupies the M(3) site in NaLiV, in which it occurs in [3]-coordination. The M(2) and M(3) sites in NaLiV occur closer to the vanadate sheets than the M(2) sites in NaV and LiV. The shift in these cation positions is a result of the larger distance between the vanadate sheets in NaLiV than in LiV, which forces interstitial Li to move toward one of the vanadate sheets to satisfy its coordination requirements. Bond-valence maps for the interstitial cations Na and Li are presented for NaV, NaLiV and LiV. These maps are used to determine other potential cation positions in the interlayer and to map the regions of the structure where the Na and Li have their bond-valence requirements satisfied. These regions are potential pathways for Na and Li diffusion in these structures, and are used to explain chemical diffusion properties of Na and Li in the Na-Li-[V₃O₈] compounds.