Direct electrochemistry of cytochrome c on a phosphonic acid terminated self-assembled monolayers

The direct electrochemistry of cytochrome c (cyt c) on a gold electrode modified with 3-mercaptopropylphosphonic acid [HS-(CH₂)₃-PO₃H₂, MPPA] self-assembled monolayers (SAMs) was for the first time investigated. Electrochemical measurements and surface-enhanced infrared absorption spectroscopic reveal that the adsorption kinetics of cyt c on the MPPA-SAMs is very fast (saturation adsorption is completed within 5 s) and the immobilized cyt c molecules retain their native secondary protein structure. The nature of interaction between cyt c and -PO₃H₂ groups is mainly the electrostatic interaction. The direct electrochemistry of the immobilized cyt c on the -PO₃H₂ terminated SAMs with short chain is nearly reversible. Its formal potential (E⁰′ = 18 ± 3 mV vs. SCE) is very close to that of cyt c in an aqueous solution (E⁰′ = 18-22 mV vs. SCE). In addition, the electron transfer rate of cyt c immobilized on -PO₃H₂ terminated SAMs is relatively slow as compared to -SO₃H and -COOH terminated SAMs, indicating excess negative charge density on the SAMs surface will decrease the electron transfer rate of cyt c.     

Voltammetric determination of quercetin at a multi-walled carbon nanotubes paste electrode

Quercetin can effectively accumulate at multi-walled carbon nanotubes-paraffin oil paste electrodes (CNTPE) and cause a sensitive anodic peak at around 0.32 V (vs. SCE) in a 0.10 M phosphate buffer solution (pH = 4.0). Under optimized conditions, the anodic peak current is linear to quercetin concentration in the ranges of 2.0 × 10^− 9−1.0 × 10^− 7 M and 1.0 × 10^− 7−2.0 × 10^− 5 M, and the regression equations are i_p (μA) = 0.0017 + 0.928c (μM, r = 0.999) and i_p (μA) = 0.183 + 0.0731c (μM, r = 0.995), respectively. This paste electrode can be regenerated by repetitively cycling in a blank solution for about 2 min. A 1.0 × 10^− 6 M quercetin solution is measured for 10 times using the same electrode regenerated after every determination, and the relative standard deviation of the peak current is 1.7%. The method has been applied to the determination of quercetin in hydrolysate product of rutin and the recovery is 99.2–102.6%. In comparison with graphite paste electrode, carbon nanotubes-nujol paste electrode and carbon nanotubes casting film modified glassy carbon electrode, the CNTPE gives higher ratio of signal to background current and better defined voltammetric peak.     

Voltammetric determination of N-acetylcysteine using a carbon paste electrode modified with copper(II) hexacyanoferrate(III)

The preparation and electrochemical characterization of a carbon paste electrode modified with copper(II) hexacyanoferrate(III) (CuHCF) as well as its behavior as electrocatalyst toward the oxidation of N-acetylcysteine were investigated. The electrochemical behavior of the modified electrode and the electrooxidation of N-acetylcysteine were explored using sweep linear voltammetry. The best voltammetric response was observed for a paste composition of 20% (w/w) copper(II) hexacyanoferrate(III) complex, acetate buffer solution at pH of 6.0 as the electrolyte and scan rate of 10 mV s^− 1. A linear voltammetric response for N-acetylcysteine was obtained in the concentration range from 1.2 × 10^− 4 to 8.3 × 10^− 4 mol L^− 1, with a detection limit of 6.3 × 10^− 5 mol L^− 1. The proposed electrode is useful for the quality control and routine analysis of N-acetylcysteine in pharmaceutical formulations.     

Palladized aluminum electrode covered by Prussian blue film as an effective transducer for electrocatalytic oxidation and hydrodynamic amperometry of N-acetyl-cysteine and glutathione

The mediated oxidation of N-acetyl cysteine (NAC) and glutathione (GL) at the palladized aluminum electrode modified by Prussian blue film (PB/Pd–Al) is described. The catalytic activity of PB/Pd–Al was explored in terms of Fe^III[Fe^III(CN)₆]/Fe^III[Fe^II(CN)₆]¹⁻ system by taking advantage of the metallic palladium layer inserted between PB film and Al, as an electron-transfer bridge. The best mediated oxidation of NAC and GL on the PB/Pd–Al electrode was achieved in 0.5 M KNO₃ + 0.2 M potassium acetate of pH 2. The mechanism and kinetics of the catalytic oxidation reactions of the both compounds were monitored by cyclic voltammetry and chronoamperometry. The charge transfer-rate limiting step as well as overall oxidation reaction of NAC or GL is found to be a one-electron abstraction. The values of transfer coefficients α, catalytic rate constant k and diffusion coefficient D are 0.5, 3.2 × 10² M⁻¹ s⁻¹ and 2.45 × 10⁻⁵ cm² s⁻¹ for NAC and 0.5, 2.1 × 10² M⁻¹ s⁻¹ and 3.7 × 10⁻⁵ cm² s⁻¹ for GL, respectively. The modifying layers on the Pd–Al substrate have reproducible behavior and a high level of stability in the electrolyte solutions. The modified electrode is exploited for hydrodynamic amperometry of NAC and GL. The amperometric calibration graph is linear in concentration ranges 2 × 10⁻⁶–40 × 10⁻⁶ for NAC and 5 × 10⁻⁷–18 × 10⁻⁶ M for GL and the detection limits are 5.4 × 10⁻⁷ and 4.6 × 10⁻⁷ M, respectively.     

Direct electrochemistry and spectroelectrochemistry of osmium substituted horseradish peroxidase

In this contribution the substitution of the central protoporphyrin IX iron complex of horseradish peroxidase by the respective osmium porphyrin complex is described. The direct electrochemical reduction of the Os containing horseradish peroxidase (OsHRP) was achieved at ITO and modified glassy carbon electrodes and in combination with spectroscopy revealed the three redox couples Os^IIIHRP/Os^IVHRP, Os^IVHRP/Os^VHRP and Os^VHRP/Os^VIHRP. The midpoint potentials differ dependent on the electrode material used with E_1/2 (Os^III/IV) of − 0.4 V (ITO) and − 0.25 V (GC), E_1/2 (Os^IV/^V) of − 0.16 V (ITO) and + 0.10 V (GC), and E_1/2 (Os^V/VI)of + 0.18 V (ITO), respectively. Moreover, with immobilised OsHRP the direct electrocatalytic reduction of hydrogen peroxide and tert-butyl hydroperoxide was observed. In comparison to electrodes modified with native HRP the sensitivity of the OsHRP-electrode for tert-butyl hydroperoxide is higher.     

Sub- and post-micellar catalytic and inhibitory effects of cetlytrimethylammonium bromide in the permanganate oxidation of phenylalanine

The kinetics of phenylalanine (phe) oxidation by permanganate has been investigated in absence and presence of cetlytrimethylammonium bromide (CTAB) using conventional spectrophotometric technique. The rate shows first- and fractional-order dependence on [MnO₄⁻] and [phe] in presence of CTAB. At lower values of [CTAB] (≤10.0 × 10⁻⁴ mol dm⁻³), the catalytic ability of CTAB aggregates are strong. In contrast, at higher values of [CTAB] (≥10.0 × 10⁻⁴ mol dm⁻³), the inhibitory effect was observed in absence of H₂SO₄. We find that anions (Br⁻, Cl⁻ and NO₃⁻) in the form of sodium salts are strong inhibitors for the CTAB catalyzed oxidation. Kinetic and spectrophotometric evidences for the formation of an intermediate complex and an ion-pair complex between phe and MnO₄⁻, CTAB and MnO₄⁻, respectively, are presented. A mechanism consistent with kinetic results has been discussed. Complex formation constant (K_c) and micellar binding constant (K_s) were calculated at 30 °C and found to be K_c = 319 mol⁻¹ dm⁻³ and K_s = 1127 mol⁻¹ dm⁻³, respectively.     

Use of a graphite–polyurethane composite electrode for electroanalytical determination of indole-3-acetic acid in soil samples

A new electroanalytical methodology was developed for the quantification of the phytohormone indole-3-acetic acid (IAA), using a graphite–polyurethane composite electrode (GPU) and the square wave voltammetry (SWV), in 0.1 mol L^− 1 phosphoric acid solution (pH 1.6). Analytical curves were constructed under optimized conditions (f = 100 s^− 1, a = 50 mV, E_i = 5 mV) and the reached detection and quantification limits were 26 μg L^− 1 and 0.2 mg L^− 1, respectively. The developed methodology is simple and accurate for the routine determination of IAA. In order to verify the application of the electroanalytical methodology in fortified soil samples without previous treatment, an IAA assay was performed without serious interferences of the soil constituents.     

Study on surface acid-base property of carboxylic acid-terminated self-assembled monolayers by cyclic voltammetry and electrochemical impedance spectroscopy

Cyclic voltammetry and electrochemical impedance spec-troscopy were used to study the surface acid-base property of carboxylic acid-terminated self-assembled monolayers(SAMs).A carboxylic acid-terminated thiol,such as thioctic acid(1,2-dithiolane-3-pentanoic acid),was self-assembled on gold electrodes.Electron transfer between the bulk solution and the SAM modified electrode was studied at different pH using Fe(CN)63-as a probe.The surface pka of thioctic acid was determined by cyclic voltammetry and electrochemical impedance spectroscopy to be 5.6 ±0.1 and 5.8±0.1,respectively.The method is compared with other methods of monolayer pKa measurement.     

Electrodeposition of Lu on W and Al electrodes: Electrochemical formation of Lu–Al alloys and oxoacidity reactions of Lu(III) in the eutectic LiCl–KCl

The electrodeposition of lutetium on inert electrodes and the formation of lutetium–aluminium alloys were investigated in the eutectic LiCl–KCl in the temperature range 673–823 K. On a tungsten electrode, the electroreduction of Lu(III) proceeds in a single step and electrocrystalization plays an important role. Experimental current–time transients are in good agreement with theoretical models based on either instantaneous or progressive nucleation with three dimensional growth of the nuclei, depending on the working temperature. The diffusion coefficient of Lu(III) was determined by chronopotentiometry by applying the Sand equation. The activation energy for diffusion was found to be 31.5 ± 1.3 kJ mol⁻¹. Al₃Lu and mixtures of Al₃Lu and Al₂Lu, characterized by XRD analysis and SEM, were obtained from the LiCl–KCl melt containing Lu(III) by potentiostatic electrolysis using an Al electrode. The activity of Lu and the standard Gibbs energies of formation for Al₃Lu were estimated from open-circuit chronopotentiometric measurements. The E–pO²⁻(potential–oxoacidity) diagram for Lu–O stable compounds in LiCl–KCl at 723 K has been constructed by combining theoretical and experimental data. In this way, the apparent standard potential for the Lu(III)/Lu system has been determined by potentiometry. Potentiometric titrations of Lu(III) solutions with oxide donors, using a yttria stabilized zirconia membrane electrode “YSZME” as a pO²⁻ indicator electrode, have shown the stability of LuOCl and Lu₂O₃ in the melt and their solubility products have been determined at 723 K.     

Formation of a novel electroactive film by electropolymerization of 5-amino-1-naphthol

The electrooxidation of 5-amino-1-naphthol in acetonitrile solutions led to the formation of a purplish gray, air-stable, polymeric film (poly(5-amino-1-naphthol), poly(5,1-ANT)), which presents a nonfibrillar morphology, on basal-plane pyrolytic graphite and In-Sn oxide conducting glass (ITO) electrodes. The film showed a reversible, well-defined oxidation-reduction response in both aqueous solutions of pH 1–13 and protic non-aqueous solutions, and was semiconducting. The formal redox potential (E^o') was 0.075 V vs. a sodium chloride saturated calomel electrode in 0.2 M NaClO₄ aqueous solution (pH 1.0). The pH dependence of (E^o') suggested that the electrode processes at pH 1-5 and pH 8–13 are 1e⁻ − 1H⁺and 1e^t- − 2 H⁺ reactions, respectively. The poly(5,1-ANT) film-coated ITO electrode displayed an electrochromic character: the oxidized form is purplish gray and the reduced one is pale brownish yellow. In addition, based on the electrochemical and IR absorption spectroscopic characterization of the film, some possible candidates for the structures of electroactive moieties of poly(5,1-ANT) are proposed.     

Nitric oxide and singlet oxygen photo-generation by light irradiation in the phototherapeutic window of a nitrosyl ruthenium conjugated with a phthalocyanine rare earth complex

The photochemical, photophysical and photobiological studies of a mixture containing cis-[Ru(H-dcbpy⁻)₂(Cl)(NO)] (H₂-dcbpy = 4,4′-dicarboxy-2,2′-bipyridine) and Na₄[Tb(TsPc)(acac)] (TsPc = tetrasulfonated phthalocyanines; acac = acetylacetone), a system capable of improving photodynamic therapy (PDT), were accomplished. cis-[Ru(H-dcbpy⁻)₂(Cl)(NO)] was obtained from cis-[Ru(H₂-dcbpy)₂Cl₂]·2H₂O, whereas Na₄[Tb(TsPc)(acac)] was obtained by reacting phthalocyanine with terbium acetylacetonate. The UV–Vis spectrum of cis-[Ru(H-dcbpy⁻)₂(Cl)(NO)] displays a band in the region of 305 nm (λ_max in 0.1 mol L⁻¹ HCl)(π–π*) and a shoulder at 323 nm (MLCT), while the UV–Vis spectrum of Na₄[Tb(TsPc)(acac)] presents the typical phthalocyanine bands at 342 nm (Soret λ_max in H₂O) and 642, 682 (Q bands). The cis-[Ru(H-dcbpy⁻)₂(Cl)(NO)] FTIR spectrum displays a band at 1932 cm⁻¹ (Ru–NO⁺). The cyclic voltammogram of the cis-[Ru(H-dcbpy⁻)₂(Cl)(NO)] complex in aqueous solution presented peaks at E = 0.10 V (NO^+/0) and E = −0.50 V (NO^0/−) versus Ag/AgCl. The NO concentration and ¹O₂ quantum yield for light irradiation in the λ > 550 nm region were measured as [NO] = 1.21 ± 0.14 μmol L⁻¹ and ø_OS = 0.41, respectively. The amount of released NO seems to be dependent on oxygen concentration, once the NO concentration measured in aerated condition was 1.51 ± 0.11 μmol L⁻¹ The photochemical pathway of the cis-[Ru(H-dcbpy⁻)₂(Cl)(NO)]/Na₄[Tb(TsPc)(acac)] mixture could be attributed to a photoinduced electron transfer process. The cytotoxic assays of cis-[Ru(H-dcbpy^-)₂(Cl)(NO)] and of the mixture carried out with B16F10 cells show a decrease in cell viability to 80% in the dark and to 20% under light irradiation. Our results document that the simultaneous production of NO and ¹O₂ could improve PDT and be useful in cancer treatment.     

Simultaneous voltammetric determination of ascorbic acid and dopamine at the surface of electrodes modified with self-assembled gold nanoparticle films

Gold nanoparticles (GNs) could be efficiently immobilized on binary mixed self-assembled monolayers (SAMs) on a gold surface composed of 1,6-hexanedithiol and 1-octanethiol (nano-Au/SAMs gold electrode). This GN chemically modified electrode was used for electrochemical determination of ascorbic acid (AA) and dopamine (DA) in aqueous media. The result showed that the GN-modified electrode could clearly resolve the oxidation peaks of AA and DA, with a peak-to-peak separation (∆E _p) of 110 mV enabling determination of AA and DA in the presence of each other. The linear analytical curves were obtained in the ranges of 0.3–1.4 mM for AA and 0.2–1.2 mM for DA concentrations using differential pulse voltammetry. The detection limits (3σ) were 9.0 × 10⁻⁵ M for AA and 9.0 × 10⁻⁵ M for DA.     

Synthesis, spectroscopic and electrochemical properties of manganese, nickel and iron octakis-(2-diethylaminoethanethiol)-phthalocyanine

The syntheses, spectroscopic and electrochemical properties of manganese (3), nickel (4) and iron (5) phthalocyanine complexes, octa-substituted at the peripheral positions with diethlyaminoethanethiol substituent, are reported. The electrochemistry of these complexes and the corresponding cobalt complex (6) are reported. Complex 3 showed two reversible reduction couples attributed to the Mn^IIIPc⁻²/Mn^IIPc⁻² (E_½ = −0.12 V versus Ag|AgCl) and Mn^IIPc⁻²/Mn^IIPc⁻³ (E_½ = −0.82 V versus Ag|AgCl) species. Two ring-based reduction couples were also observed for complex 4. Two reduction couples, assigned to the Fe^IIPc⁻²/Fe^IPc⁻² (E_½ = −0.35 V versus Ag|AgCl) and Fe^IPc⁻²/Fe^IPc⁻³ (E_½ = −0.96 V versus Ag|AgCl) species, and an oxidation couple, attributed to Fe^IIIPc⁻²/Fe^IIPc⁻² (E_½ = 0.26 V versus Ag|AgCl) species, were observed. For complex 6, two reductions and one oxidation were also observed with the potential range of 1.2 to −1.8 V versus Ag|AgCl Spectroelectrochemical studies were used to confirm some of the assigned processes.     

A new generation of cyanide ion-selective membranes for flow injection application: Part III. A simple approach to the determination of toxic metal-cyanide complexes without preliminary separation

A new flow injection approach to total weak acid-dissociable (WAD) metal–cyanide complexes is proposed, which eliminates the need of a separation step (such as gas diffusion or pervaporation) prior to the detection. The cornerstone of the new methodology is based on the highly selective flow-injection potentiometric detection (FIPD) system that makes use of thin-layer electroplated silver chalcogenide ion-selective membranes of non-trivial composition and surface morphology: Ag_2 + δSe_1 − xTe_x and Ag_2 + δSe. An inherent feature of the FIP-detectors is their specific response to the sum of simple CN⁻ + Zn(CN)₄²⁻ + Cd(CN)₄²⁻. For total WAD cyanide determination, ligand exchange (LE) and a newly developed electrochemical pre-treatment procedure for release of the bound cyanide were used. The LE pre-treatment ensures complete recovery only when the sample does not contain Hg(CN)₄²⁻. This limitation is overcome by implementing electrochemical pre-treatment which liberates completely the bound WAD cyanide through cathodic reduction of the complexed metal ions. A complete recovery of toxic WAD cyanide is achieved in the concentration range from 156 μg L⁻¹ up to 13 mg L⁻¹. A three-step protocol for individual and group WAD cyanide speciation is proposed for the first time. The speciation protocol comprises three successive measurements: (i) of non-treated, (ii) LE-exchange pre-treated; (iii) electrochemically pre-treated sample. In the presence of all WAD complexes this procedure provides complete recovery of the total bound cyanide along with its quantitative differentiation into the following groups: (1) Hg(CN)₄²⁻; (2) CN⁻ + Cd(CN)₄²⁻ + Zn(CN)₄²⁻; (3) Cu(CN)₄³⁻ + Ni(CN)₄²⁻ + Ag(CN)₂⁻. The presence of a 100-fold excess in total of the following ions: CO₃²⁻, SCN⁻, NH₄⁺, SO₄²⁻ and Cl⁻ does not interferes. Thus the proposed approach offers a step ahead to meeting the ever increasing demand for cyanide-species-specific methods. The equipment simplicity makes the procedure a good candidate for implementing in portable devices for in-field cyanide monitoring.     

Electroanalytical properties of cytochrome c by direct electrochemistry on multi-walled carbon nanotubes incorporated with DNA biocomposite film

A novel conductive biocomposite film (MWCNTs–DNA–cyt c) which contains multi-walled carbon nanotubes (MWCNTs) along with the incorporation of DNA and cytochrome c (cyt c) has been synthesized on glassy carbon electrode (GCE), gold (Au), indium tin oxide (ITO) and screen printed carbon electrode (SPCE) by potentiostatic methods. The presence of both MWCNTs and DNA in the biocomposite film enhances the surface coverage concentration (Γ), increases the electron transfer rate constant (K_s) up to 21% and decreases the degradation of cyt c during the cycling. The biocomposite film also exhibits a promising enhanced electrocatalytic activity towards the reduction of halogen oxyanions and oxidation of biochemical compounds such as ascorbic acid and l-cysteine. The cyclic voltammetry has been used for the measurement of electroanalytical properties of analytes by means of biocomposite film modified GCEs. The sensitivity of MWCNTs–DNA–cyt c modified GCE possess higher values than the values obtained for DNA–cyt c film modified GCE. Further, the reduction potentials of halogen oxyanions E_pc, clearly shows that the activity of the biocomposite is dependent on the electronegativity of halogen oxyanions. Electrochemical quartz crystal microbalance studies revealed the enhancements in the functional properties of MWCNTs, DNA and cyt c. We have studied the surface morphology of the biocomposite films using scanning electron microscopy and atomic force microscopy, which revealed that DNA and cyt c have been incorporated on MWCNTs. Finally, the flow injection analysis has been used for the amperometric detection of analytes at MWCNTs–DNA–cyt c film modified SPCE.     

Enhancing electrochemical preparation of polyaluminum chloride by a magnetic field

A novel effective method for preparing polyaluminum chloride (PAC) with high content of Al₁₃ polymer through conventional electrolysis coupled with rare earth Nd-Fe-B magnetic field was introduced. The content of Al₁₃ polymer in PAC synthesized by this method was highly influenced by the electrobath voltage (E), the magnetic flux density (B), the current density (i) and the distance between the two adjacent electrodes (d_adj). A total aluminum concentration (Al_T) in the PAC solution of 0.8 mol l⁻¹ was obtained when the E, B and i was 2.2 V, 0.4 T and 3.2 A dm⁻², respectively. The optimum d_adj and circulating flow (Q_f) were 20 mm and 23.7 l h⁻¹. With accelerated mass transfer rate by external magnetic field and high Q_f, this process had the advantages of forming more Al(OH)₄⁻ as the precursor of Al₁₃ polymer and inhibiting the concentration polarization more obviously than conventional electrolysis process. Under the optimum conditions, the amount of Al₁₃ polymer in PAC accounted for 82.3% of the Al_T (Al_T = 0.8 M, basicity = 2.2), which was higher than that of PAC prepared by other methods.     

Study on dynamic interfacial tension of 3-dodecyloxy-2-hydroxypropyl trimethyl ammonium bromide at liquid/liquid interface

Dynamic interfacial tension between aqueous solutions of 3-dodecyloxy-2-hydroxypropyl trimethyl ammonium bromide (R₁₂HTAB) and n-hexane were measured using the spinning drop method. The effects of the R₁₂HTAB concentration (the concentration below the CMC) and temperature on the dynamic interfacial tension have been investigated; the reason of the change of dynamic interfacial tension with time has been discussed. The effective diffusion coefficient, D_a, and the adsorption barrier, _a, have been obtained from the experimental data using the extended Word–Tordai equation. The results show that the dynamic interfacial tension becomes smaller while _a becomes higher with increasing R₁₂HTAB concentration in the bulk aqueous phase. D_a decreases from 5.56 × 10⁻¹² m⁻² s⁻¹ to 0.87 × 10⁻¹² m⁻² s⁻¹ while _a increases from 5.41 kJ mol⁻¹ to 7.74 kJ mol⁻¹ with the increase of concentration in the bulk solution of R₁₂HTAB from 0.5 × 10⁻³ mol dm⁻³ to 4 × 10⁻³ mol dm⁻³. Change of temperature affects the adsorption rate through altering D_a and _a. The value of D_a increases from 5.56 × 10⁻¹² m⁻² s⁻¹ to 13.98 × 10⁻¹² m⁻² s⁻¹ while that of _a decreases from 5.41 kJ mol⁻¹ to 5.07 kJ mol⁻¹ with temperature ascending from 303 K to 323 K. The adsorption of surfactant from the bulk phase into the interface follows a mixed diffusion–activation mechanism, which has been discussed in the light of interaction between surfactant molecules, diffusion and thermo-motion of molecules.     

C60 trianion-mediated electrocatalysis and amperometric sensing of hydrogen peroxide

Heterogeneous electrocatalytic reduction of hydrogen peroxide (H₂O₂) by C₆₀ is reported for the first time. C₆₀ is embedded in tetraoctylammonium bromide (TOAB) film and is characterized by scanning electron microscopy and cyclic voltammetry. Electrocatalytic studies show that the trianion of C₆₀ mediates the electrocatalytic reduction of H₂O₂ in aqueous solution containing 0.1 M KCl. Application of such film modified electrode as an amperometric sensor for H₂O₂ determination is also examined. The sensor shows a fast response within 1 s and a linear response is obtained (R = 0.9986) in the concentration range from 3.33 × 10⁻⁵ to 2.05 × 10⁻³ mol L⁻¹ for H₂O₂, with the detection limit of 2 × 10⁻⁵ mol L⁻¹ and the sensitivity of 1.65 μA mM⁻¹. A good repeatability and stability is shown for the sensor during the experiment.     

Functionalization of gold electrode surface with heterobifunctional poly(ethylene oxide)s having both mercapto and aldehyde groups

We synthesized heterobifunctional poly(ethylene oxide) (PEO) (α‐formyl‐ω‐mercapto‐PEO; CHO‐PEO₄₀₀‐SH, average molecular weight of PEO part being 400), which had both an aldehyde group as a binding site with amino group of protein and a mercapto group for gold electrode surface. The CHO‐PEO₄₀₀‐SH was adsorbed on a gold electrode surface and cytochrome c (cyt.c) was fixed on this modified electrode. The redox response of covalently immobilized cyt.c was observed on the cyclic voltammetry measurement, showing that CHO‐PEO₄₀₀‐SH can be used as a linker to fix cyt.c on an electrode. Another type of heterobifunctional PEO (α‐formyl‐ω‐(2‐pyridyldithio)‐PEO; CHO‐PEO₃₀₀‐SS‐Py), which had an aldehyde group and a 2‐pyridinethiol (2‐Py) through disulfide bond, was synthesized to form co‐adsorbed monolayer of PEO chain and 2‐Py on an electrode surface. It was expected, due to the spacer with shorter PEO chain and lower surface density, that better redox response of the fixed cyt.c was obtained. However, the redox response of fixed cyt.c was not detected on the CHO‐PEO₃₀₀‐SS‐Py modified gold electrode. Instead, this heterobifunctional PEO was found to function as a good promoter for cyt.c dissolved in phosphate buffer solution. Copyright © 2003 John Wiley & Sons, Ltd.     

Electrochemical Characterization of Pyrocatechol Sulfonephthalein-Modified Glassy Carbon Electrode and Separation of Electrocatalytic Responses for Ascorbic Acid and Dopamine Oxidation

A pyrocatechol sulfonephthalein- (PS-) modified glassy carbon (PS/GC) electrode has been prepared by adsorption of PS on a glassy carbon electrode surface. Cyclic voltammograms of the PS/GC electrode indicate the presence of a couple of well-defined redox peaks, and the formal potential shifts in the negative direction with increasing solution pH. The relation between formal potential,E^0′, and solution pH can be fit to the equationE^0′(mV) = −51.4 pH + 538.7. The PS/GC electrode shows high electrocatalytic activity toward ascorbic acid oxidation, with an overpotential ca. 380 mV less than that of the bare electrode and a drastic enhancement of the anodic currents. The electrocatalytic reaction rate constant (k), which was decreased with increasing concentration of H₂A, was determined using rotating disk electrode measurements. The values ofkwas also affected by the solution pH. The electrode can also separate the electrochemical responses of ascorbic acid and dopamine. The separation between the anodic peak potentials of ascorbic acid and dopamine is more than 50 mV by the differential pulse voltammetry.