Electrical Control of Alkanethiols Self-Assembly on a Gold Surface as an Approach for Preparation of Microelectrode Arrays

 It is shown by capacitive monitoring that the self-assembly of alkanethiols on gold electrodes and desorption of these self-assembled monolayers from the electrodes are controlled by the electrode potential. At neutral pH, chemical adsorption of alkanethiols was observed at an electrode potential of +300 mV vs SCE, but only physical adsorption was detected when the electrode potential was −1400 mV vs SCE. At electrode potentials between these values (−300 mV, −600 mV), chemical adsorption of alkanethiols occurred, but the alkanethiol monolayers were not stable in the absence of the alkanethiol in the bulk solution and were desorbed from the gold electrode. The desorption rate was higher at more negative electrode potentials. These results can be used in designing methods for electrically addressable immobilization of different receptors on (micro)electrode arrays. This has been demonstrated by deposition of two different types of alkanethiols onto a two-electrode array. Received June 24, 1998. Revision October 19, 1998.     

Triethylene glycol ether end-grafted carbosilane dendrimer: a potential ionophore for potassium ion recognition.

The performance of a newly synthesized carbosilane dendrimer bearing four triethylene glycol ether (TEG) units, Si(CH2CH2CH2Si(Me)2CH2CH2CH2(OCH2CH2)3Me)4 (1), as ionophores in ion-selective electrodes has been investigated. Optimization of the plasticized polyvinyl chloride membrane composition has produced electrodes that exhibit a Nernstian response for potassium ions. The best general characteristics exhibited by the electrodes were found when the membrane composition ratio of DPE:1:NaTPB:PVC 60:3:2:35 (wt%) was used. The response of the electrode was linear with a Nernstian slope of 58.3 mV/decade over the K+ ion concentration range of 1.9x10(-7) to 1.0x10(-1) M with a detection limit of 3.1x10(-7) M. The response time to achieve a 95% steady potential for the K+ concentration ranging from 1.0x10(-1) to 1.0x10(-8) M was less than 10 s, and it was found that the electrode is suitable for use within a pH range of 5.5-8.5. The selectivity coefficients (log KPotK+,Mn+)), which were determined by the fixed interference method, showed good selectivity for K+ against most of the interfering cations. The influence of this selective ion-binding behavior using electrospray ionization time-of-flight (ESI-TOF) mass spectrometric studies is discussed.     

Electrochemical reduction of NO by hemin adsorbed at pyrolitic graphite

The mechanism of the electrochemical reduction of nitric oxide (NO) by hemin adsorbed at pyrolitic graphite was investigated. The selectivity of NO reduction was probed by combining the rotating ring disk electrode (RRDE) technique with a newly developed technique called on-line electrochemical mass spectroscopy (OLEMS). These techniques show that NO reduction by adsorbed heme groups results in production of hydroxylamine (NH(2)OH) with almost 100% selectivity at low potentials. Small amounts of nitrous oxide (N(2)O) were only observed at higher potentials. The rate-determining step in NO reduction most likely consists of an electrochemical equilibrium involving a proton transfer, as can be derived from the Tafel slope value of 62 mV/dec and the pH dependence of -42 mV/pH. The almost 100% selectivity toward NH(2)OH distinguishes this system both from NO reduction on bare metal electrodes, which often yields NH(3), and from biological NO reduction in cytochrome P450nor, which yields N(2)O exclusively.     

The passivation phenomenon of rhodium in fused lithium chloride + potassium chloride eutectic

The passivation phenomenon of rhodium was investigated in fused lithium chloride + potassium chloride eutectic by means of the potential sweep method. The current-potential curve obtained showed a typical N-shaped negative impedance. An anodic current rise was observed at +0.3 V vs. Ag/AgCl (0.1). The current was controlled by mass transfer and was ascribed to the dissolution of rhodium into rhodium(III) ions. The observed Flade potential was +0.45 V at 400°C. The passivation was found to occur due to the precipitation of supersaturated Rh(III) chloride onto the electrode surface. The residual current for the passivation was fairly high and the current increased significantly as the temperature was made higher. The dissolution current of rhodium into Rh(III) ions decreased with increase in the concentration of oxide ions. The fact revealed that the rhodium was passivated preferentially due to the formation of Rh(III) oxide. The residual current of the oxide passivation film was low enough. The thickness of the film corresponded to 10–20 atomic layers of the parent metal. Rhodium showed another anomalous passivation due to the formation of oxide. The oxide was formed at ?0.3 V and was reduced at ?0.6 V. It was considered to be a low valence rhodium oxide, RhO or Rh₂O. Rhodium was found to have a low chlorine overpotential. However, it was redissolved at the chlorine evolution region, above +1.25 V. The dissolution was not prevented even in the oxide ion containing melts.     

Chronoamperometry as a tool for the evaluation of antioxidant properties of red wines

Cyclic voltammetry and chronoamperometry at two different potentials of +500 and +800 mV was applied to nine commercial dry red wines of five different vintages in order to study their capability in oxidation. A plateau in a certain potential range of +0.82 ± 0.04 V obtained for all investigated wine samples revealed the presence of oxidation mechanisms with successive oxidation steps. Chronoamperometric data at +800 mV were found to be consistent with data coming from the antioxidant protocol against 1,1-diphenyl-2-dipicrylhydrazyl (DPPH). Indeed, the results showed a positive correlation of chronoamperometric signal of wine samples at +800 mV with their reactivity towards DPPH, expressed as Trolox equivalents, followed by satisfactory statistics. Therefore, the chronoamperometric signal of wine samples can be considered as a friendly alternative for the rapid screening of antioxidant properties of wine samples.     

Electrolytic in situ STM investigation of h-BN-Nanomesh

Single sheet boron nitride layers on thin rhodium (1 1 1) films were formed upon thermal decomposition of borazine under ultra-high vacuum (UHV) conditions. They were transferred and investigated in an electrolytic environment. In 0.1 M HClO₄, the presence and stability of the so-called nanomesh super structure with a lattice constant of 3.2 nm is established with electrochemical impedance spectroscopy, cyclic voltammetry and subsequent imaging with in situ scanning tunneling microscopy (STM) under potential control. In the electrolyte, the BN nanomesh acts as a dielectric layer with an unusual behaviour of the impedance, where the capacitive component is larger than on a Rh(1 1 1) reference sample. It exhibits reversible hydrogen adsorption and desorption at a potential of −600 mV vs. a saturated mercury sulphate reference electrode (MSE). The unit cell of the nanomesh is imaged by STM and shows hexagonally arranged two-dimensional pores with a diameter of 2 nm. At a fixed potential, the nanomesh was stable for long time, but after repeated potential sweeps between +260 and −540 mV vs. MSE, STM indicates roughening, though a 12 × 12 superstructure was recovered after annealing in UHV.     

Synthesis and electrochemical studies of diiron complexes of 1,8-naphthyridine-based dinucleating ligands to model features of the active sites of non-heme diiron enzymes

A bis(mu-carboxylato)(mu-1,8-naphthyridine)diiron(II) complex, [Fe2(BPMAN)(mu-O2CPhCy)2](OTf)2 (1), was prepared by using the 1,8-naphthyridine-based dinucleating ligand BPMAN, where BPMAN = 2,7-bis[bis(2-pyridylmethyl)aminomethyl]-1,8-naphthyridine. The cyclic voltammogram (CV) of this complex in CH2Cl2 exhibited two reversible one-electron redox waves at +296 mV (DeltaE(p) = 80 mV) and +781 mV (DeltaE(p) = 74 mV) vs Cp2Fe+/Cp2Fe, corresponding to the FeIIIFeII/FeIIFeII and FeIIIFeIII/FeIIIFeII couples, respectively. This result is unprecedented for diiron complexes having no single atom bridge. Dinuclear complexes [Fe2(BPMAN)(mu-OH)(mu-O2CPhCy)](OTf)2 (2) and [Mn2(BPMAN)(mu-O2CPhCy)2](OTf)2 (3) were also synthesized and structurally characterized. The cyclic voltammogram of 2 in CH2Cl2 exhibited one reversible redox wave at -22 mV only when the potential was kept below +400 mV. The CV of 3 showed irreversible oxidation at potentials above +900 mV. Diiron(II) complexes [Fe2(BEAN)(mu-O2CPhCy)3](OTf) (4) and [Fe2(BBBAN)(mu-OAc)2(OTf)](OTf) (6) were also prepared and characterized, where BEAN = 2,7-bis(N,N-diethylaminomethyl)-1,8-naphthyridine and BBBAN = 2,7-bis[2-[2-(1-methyl)benzimidazolylethyl]-N-benzylaminomethyl]-1,8-naphthyridine. The cyclic voltammograms of these complexes were recorded. The M?ssbauer properties of the diiron compounds were studied.     

Photoelectrochemical deposition of thin tellurium films

Effect of polychromatic light on the electrochemical deposition of tellurium(IV) ions on a glass carbon electrode from acid solutions of 0.45 M Na₂SO₄ + 0.05 M H₂SO₄ with pH 2.2 was studied. It is shown that electrochemical reduction of tellurium(IV) is possible in two stages in the potentiodynamic mode at potentials in the range from 0 to ?1000 mV. Elementary tellurium is formed in the first stage (E = ?320 ± 20 mV) and is reduced to telluride ions in the second (E = ?700 ± 50 mV). It is demonstrated that, under potentiodynamic deposition conditions, visible light affects the generation of Te^2? ions at potentials more positive than the electrochemical potential. The chronoamperometric method revealed differences in the behavior of transient currents in the dark and under illumination. The elemental composition and the film surface morphology were studied by electron-probe analysis.     

Detection of Uric Acid in the Presence of Dopamine and High Concentration of Ascorbic Acid Using PDDA Modified Graphite Electrode

The properties of graphite electrode (Gr) modified with poly(diallyl dimethyl ammonium chloride) (PDDA) for the detection of uric acid (UA) in the presence of dopamine (DA) and high concentration of ascorbic acid (AA) have been investigated by cyclic voltammetry, differential pulse voltammetry and chronoamperometry. The polymer modified graphite electrode was prepared by a very simple method just by immersing the graphite electrode in PDDA solution for 20 minutes. The PDDA/Gr modified electrode displayed excellent electrocatalytic activity towards the oxidation of UA, DA and AA compared to that at the bare graphite electrode. The electrochemical oxidation signals of UA, DA and AA are well resolved into three distinct peaks with peak potential separations of 220 mV, 168 mV and 387 mV between AA‐DA, DA‐UA and AA‐UA respectively in cyclic voltammetry studies and the corresponding peak potential separations are 230 mV, 130 mV and 354 mV respectively in differential pulse voltammetry. The lowest detection limits obtained for UA, DA and AA were 1×10^?7 M, 2×10^?7 M and 800×10^?9 M respectively. The PDDA/Gr electrode efficiently eliminated the interference of DA and a high concentration of AA in the determination of UA with good selectivity, sensitivity and reproducibility. The modified electrode was also successfully applied for simultaneous determination of UA, DA and AA in their ternary mixture.     

Characterization of electrodeposited Co + Ni alloys by application of the ALSV technique Professor Aleksandar Despic to commemorate his 70th birthday

Anomalous codeposition of Co and Ni onto a gold RDE was investigated in a solution containing simple sulfate salts with the addition of sodium citrate. It was shown that the dependence of the percentage of Co in the deposit on the percentage of Co in the bath follows the shape found in the literature, with the percentage of Co in the deposit being slightly higher than in electrolytes containing pure simple salts. Alloy layers of different composition, electrodeposited at constant charge Q_dep = 1 C cm⁻² (thickness 0.34 μm) were submitted to anodic dissolution at a sweep rate of 1 mV s⁻¹ (ALSV technique) in a solution of 1 M NaCl, pH 2. All samples were found to dissolve through a single anodic peak, indicating that both constituents of the alloy dissolve simultaneously. Alloys with higher Ni content (above 40at.%) were found to dissolve at potentials more positive than the potential of pure Ni dissolution as a consequence of the Gibbs energy change of formation of electrodeposited solid solution type Co + Ni alloys. The composition of electrodeposited alloys was determined by the atomic absorption technique. An attempt was made to obtain a correlation between the peak potentials of anodic dissolution of alloy samples and the composition of alloys, to determine the composition of the alloy from the peak potential of its dissolution. It is found that such a correlation can be used only for strictly defined conditions of alloy deposition and dissolution, caused by the contribution of the Gibbs energy change of formation of electrodeposited alloys. Also, the presence of a CoNi₃ ordered structure in the system is not detected as a separate ALSV peak, but its existence could be the cause of the shape of the Gibbs energy change with composition of the alloy for alloys electrodeposited at low current density.     

Activated platinum electrodes as transducer for a glucose sensor using glucose oxidase in a photopolymer membrane

A planar platinum electrode was covered by a photopolymer membrane containing glucose oxidase (GOD) to construct an amperometric glucose sensor. The application of a photopolymer system in membrane formation gives the opportunity to manufacture cheap biosensors with good reproducibility by means of automated techniques and to miniaturise sensors using photolithography. The electrodes were pretreated mechanically and chemically resulting in a half-wave potential (E_1/2) of the H₂O₂ oxidation shifted towards more negative potentials. This shift allows the determination of glucose at a low working potential (300 mV vs. SCE) without addition of mediators. The important advantage of such applied potential decreasing lies in minimising the interference of oxidisable substances such as uric acid, bilirubin and paracetamol. The selectivity to ascorbic acid could also be proved without the application of additional protection layers. The glucose sensor developed has a high life-time, selectivity and sensitivity and a linear working concentration range from 0.05 up to 10 mmol/l of glucose. The sensor was used for the glucose determination in human serum samples with a very good correlation to a common photometric reference method. Received: 13 July 1996 / Revised: 11 September 1996 / Accepted: 14 September 1996     

铋(Ⅲ)-卤素络阴离子选择电极的研究

我们曾报导了PVC膜四碘络铋离子选择电极的研制。现又研制了铋(Ⅲ)-溴,铋(Ⅲ)-氯的络阴离子PVC膜选择电极。本文比较了三种铋(Ⅲ)-卤素络阴离子选择电极的主要性能,并对络阴离子电极配位体的选择条件及阳离子的干扰规律进行了讨论。     

Potentiometric membrane sensor based on 6-(4-nitrophenyl)-2,4-diphenyl-3,5-diaza-bicyclo[3.1.0]hex-2-ene for detection of Sn(II) in real samples

A Sn²⁺ ion-selective electrode which was prepared with a polymeric membrane based on 6-(4-nitrophenyl)-2,4-diphenyl-3,5-diaza-bicyclo[3.1.0]hex-2-ene (NDDBH) as a ionophore. Effects of experimental parameters such as membrane composition, nature and amount of plasticizer, the amount of additive and concentration of internal solution on the potential response of Sn²⁺ sensor were investigated. The electrode exhibited a Nernstian slope of 28.8 ± 1.1 mV/decade of Sn²⁺ over a concentration range of 1.0 × 10⁻⁵ to 1.0 × 10⁻¹ M of Sn²⁺ in an acidic solution (pH 1). The limit of detection was 4.0 × 10⁻⁶ M. The results show that this electrode can be used in ethanol media until 20% (v/v) concentration without interference. It can be used for more than 6 weeks without any considerable divergence in the potentials. The proposed membrane electrode revealed very good selectivity for Sn(II) ions over a wide variety of other cations and could be used in acidic media. The standard electrode potentials were determined at different temperatures and used to calculate the isothermal coefficient of the electrode. The stability constant (log K_s) of the Sn(II)-ionophore complex was determined at 25 °C by potentiometric titration in mixed aqueous solution. It was used as indicator electrode in potentiometric determination of Sn(II) ion in real samples.     

离子液体中铜纳米粒子膜的电沉积制备及表面增强拉曼光谱

采用恒电位方法研究了室温离子液体1-丁基-3-甲基咪唑氟硼酸盐([BMIM]BF₄)中氧化铟锡(ITO)电极表面Cu的沉积过程. 结果表明, Cu²⁺在ITO表面经中间产物Cu⁺还原为Cu. 采用XRD和SEM对不同电位下沉积层的组成和结构进行了考察. 结果表明, 表面组成和结构与沉积电位有关. 在较正电位下沉积所得为Cu/Cu₂O的混合纳米粒子; 在较负电位下为Cu纳米粒子. 随着沉积电位逐渐变负, 其形貌经历了花状、 棒状向球形纳米粒子转变的过程, 且所得沉积膜逐渐变得致密, 其中Cu纳米粒子膜表现出较强的表面增强拉曼散射(SERS)活性, 并且SERS信号均一.     

Surface charge and zeta-potential of metabolically active and dead cyanobacteria

Zeta potential and acid-base titrations of active, inactivated, and dead Planktothrix sp. and Synechococcus sp. cyanobacteria were performed to determine the degree to which cell surface electric potential and proton/hydroxyl adsorption are controlled by metabolism or cell membrane structure. Surface OH(-) excess from potentiometric data, showed differences in surface charge between active and dead cyanobacteria from pH 3 to 10. Average zero salt effect pH (pH(pzse)) of 5.8+/-0.1 and 6.3+/-0.1 were obtained for active Planktothrix sp. and Synechococcus sp., respectively. Similarly for dead cyanobacteria pH(pzse) values of 5.8+/-0.1 and 4.6+/-0.1 were obtained. Zeta potentials of active Planktothrix sp. and Synechococcus sp. were positive at alkaline conditions, with a maximum of +13.7+/-1.5 mV at a pH of 9.0+/-0.1 for both species. This positive potential diminished in the presence of 1 mM HCO(-)(3). The zeta potential of Planktothrix sp. and Synechococcus sp. cells was negative at alkaline pH following their exposure to NaN(3), a metabolic inhibitor. The zeta potential of dead cyanobacteria was negative for Planktothrix sp., from pH 2.5 to 10.5, at -30 to -20 mV. Dead Synechococcus sp. exposed to a pH 2.5 solution recorded negative potentials to a minimum of -30 mV at pH 8, but positive potentials were found at higher pH reaching a maximum of +10 mV at pH 9.1. Zeta potentials for dead, but non-acidified Synechococcus sp. remained negative at -30 mV from an initial pH of 5.6 to 10.5, reflecting differences in cell wall structure between these species. These results indicate that Planktothrix sp. and Synechococcus sp. may metabolically control their surface charge to electrostatically attract bicarbonate anions at alkaline pH, required for photosynthesis.     

An iron(III) ion-selective sensor based on a μ-bis(tridentate) ligand

A μ-bis(tridentate) ligand named 2-phenyl-1,3-bis[3′-aza-4′-(2′-hydroxyphenyl)-prop-4-en-1′-yl]-1,3-imidazolidine (I) has been synthesized and scrutinized to develop iron(III)-selective sensors. The addition of sodium tetraphenyl borate and various plasticizers, viz., chloronaphthalene, dioctylphthalate, o-nitrophenyl octyl ether and dibutylphthalate has been used to substantially improve the performance of the sensors. The membranes of various compositions of the ligand were investigated and it was found that the best performance was obtained for the membrane of composition (I) (10 mg):PVC (150 mg):chloronaphthalene (200 mg):sodium tetraphenyl borate (9 mg). The sensor showed a linear potential response to iron(III) over wide concentration range 6.3 × 10⁻⁶ to 1.0 × 10⁻¹ M (detection limit 5.0 × 10⁻⁶ M) with Nernstian slope (20.0 mV/decade of activity) between pH 3.5 and 5.5 with a quick response time of 15 s. The potentiometric selectivity coefficient values as determined by match potential method (MPM) indicate excellent selectivity for Fe³⁺ ions over interfering cations. The sensor exhibits adequate life of 2 months with good reproducibility. The sensor could be used in direct potentiometry.     

Electrochemical behavior of copper complexes with substituted polypyridinic ligands: an experimental and theoretical study

Experimental redox potentials of the couples [Cu(R-L ( n ))(CH 3CN)] (2+,+), where L (1) is bis-(pyridine-2-ylmethyl)-benzylamine, L (2) is (pyridine-2-ylethyl)(pyridine-2-ylmethyl)-benzylamine, and R is H, Me, or CF 3, were determined in dichloromethane solution. The compounds exhibited one simple quasi-reversible wave over the measured potential range of -500 to +1200 mV, and the E 1/2 values varied from +200 to +850 mV versus SCE. These experimental values were correlated with redox potentials calculated using density functional theory. The optimized geometries and the predicted redox potentials were obtained using the BP86 functional and a combination of the basis sets LACV3P** (for Cu) and cc-pVTZ(-f) (for light atoms). A distortion analysis of all of the optimized geometries for both oxidation states was performed using the generalized interconversion coordinate phi. A linear relation was obtained between this parameter and the redox potentials. However, the [Cu(CF 3-L (1))(CH 3CN)] (+) complex showed the largest deviation, which was explained by the more-rigid structure of the ligand.     

EC STM investigations of corrosion due to chloride solutions on thin CrN coatings

The focus of the investigations presented is to evaluate local alterations caused by chloride ions affecting thin, magnetron-sputtered CrN layers. Scanning-probe microscopy and analysis techniques are used for this estimation. Thin CrN layers were deposited by reactive magnetron sputtering. They were investigated in electrochemical scanning tunnelling microscopy (EC STM) by cyclic voltammetry in 1 mol L(-1) NaCl. Simultaneously, the surface topography changes were recorded with STM.Above 100 mV the anodic oxidation leads to formation of chromium(III) hydroxide and at sample potentials above 350 mV oxidation of Cr(OH)(2) and Cr(OH)(3) towards chromium(VI) as a soluble chromate starts. Transpassive dissolution of the coating takes place above 900 mV. Yellow colour of the electrolyte is a visible sign for the formation of chromium(VI). Changes of the surface topography indicate the formation of surface layers at anodic potentials. At cathodic potentials increase in current is measured due to the reduction of chromium(III) hydroxide to divalent chromium and metallic chromium. Roughness of surface topography increases.Follow-up explorations with scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), atomic-force microscopy (AFM), scanning tunnelling microscopy/scanning tunnelling spectroscopy (STM/STS) and X-ray photoelectron spectroscopy (XPS) not only evidence the formation of various chromium oxides, but also indicate the existence of chromium hydroxide.     

Electrochemical Properties of Hydroxyphenazine-Coated Electrodes

Glassy carbon and gold electrodes were coated with 1-hydroxyphenazine, and the electrochemical properties of these electrodes were tested using them as a rotating disc electrode to reduce Ru (bipy)₃³⁺, Fe³⁺, quinoxaline, O₂, and to oxidize Eu²⁺. The fixed redox couple can be reversibly reduced and oxidized, and acts as an intermediate medium for the electron transfer. For example the Ru(bipy)₃³⁺ (E_1/2= 1010 mV vs. SCE. (saturated calomel electrode) on a glassy carbon electrode in 1M H₂SO₄) is only reduced at 50 mV, whereas the oxidation of Eu²⁺ (E_1/2= ?460 mV vs. SCE. on a Hg-electrode in 1M HCl) takes place at ? 100 mV. The heterogeneous rate constant depends on the second order reaction between the attached coating and the redox couple in solution. Depending on this rate constant, selectivity of the electrode is observed.     

Studies on the application of Fe-Zr mixed hydrous oxide membranes in ion-selective potentiometry

A solid membrane electrode selective to chloride ions has been prepared from Fe-Zr mixed hydrous oxide (with iron as a major constituent) with polystyrene as binder. Although the response of the electrode is non-Nernstian it can be utilized to estimate chloride in the concentration range 10(-4)-10(-1)M. The potentials generated across the membrane are reproducible within +/-0.2 mV and the response time is a few seconds. The standard deviation of the potential measurements is 0.4 mV at the 0.1M level. The useful pH range is 4-7 and the electrode can also be used in partially non-aqueous systems. The electrode exhibits fairly good selectivity for chloride.