Single-component and mixed ferrocene-terminated alkyl monolayers covalently bound to Si(111) surfaces

Self-assembled ferrocene monolayers covalently bound to monocrystalline Si(111) surfaces have been prepared from the attachment of an amine-substituted ferrocene derivative to a pre-assembled acid-terminated alkyl monolayer using carbodiimide coupling. This derivatization strategy yielded nanometer-scale clean, densely packed monolayers, with the ferrocene units being more than 20 A from the semiconductor surface. The amount of immobilized electroactive units could be varied in the range 2 x 10(-11) to approximately 3.5 x 10(-10) mol cm(-2) by diluting the ferrocene-terminated chains by inert n-decyl chains. The highest coverage obtained for the single-component monolayer corresponded to 0.25-0.27 bound ferrocene per surface silicon atom. The electrochemical characteristics of the mixed n-decyl/ferrocene-terminated monolayers were found to not depend significantly on the surface coverage of ferrocene units. The reversible one-electron wave of the ferrocene/ferrocenium couple was observed at E degrees ' = 0.50 +/- 0.01 V vs SCE, and the rate constant of electron transfer kapp was about 50 s(-1).     

Electrochemical determination of electroinactive guests of b-cyclodextrin at a self-assembled monolayer interface

A novel determination method of electroinactive molecules by means of electrochemical technique is presented. A new self-assembled monolayer containing cyclodextrin(CD) is prepared with mono(6-o-p-tolylsulfonyl)-b-cyclodextrin. Although this derivatization process leads to a b-CD coverage of 10% of a full monolayer, this layer shows an effective host-guest response to ferrocene. The interfacial ferrocene complexation gives a response similar to that expected for a Langmuir adsorption isotherm yielding a stability constant of 4.2×104 mol-1@L and a maximum ferrocene coverage of 8.6′10-12 mol/cm2. The redox peak currents of the surface-confined ferrocene de-crease upon addition of competing b-CD guest species to the solution, such as m-toluic acid(mTA) and sodium dodecyl sulfonate(SDS). This principle has been used for the determination of the electroinactive molecules, mTA and SDS in the concentration ranges of 0.8-2.7 mmol/L and 5-100 nmol/L, respectively.     

Electrical contacting of flavoenzymes and NAD(P)+-dependent enzymes by reconstitution and affinity interactions on phenylboronic acid monolayers associated with Au-electrodes

The preparation of integrated, electrically contacted, flavoenzyme and NAD(P)(+)-dependent enzyme-electrodes is described. The reconstitution of apo-glucose oxidase, apo-GOx, on a FAD cofactor linked to a pyrroloquinoline quinone (PQQ) phenylboronic acid monolayer yields an electrically contacted enzyme monolayer (surface coverage 2.1 x 10(-)(12) mol cm(-)(2)) exhibiting a turnover rate of 700 s(-)(1) (at 22 +/- 2 degrees C). The system is characterized by microgravimetric quartz-crystal microbalance analyses, Faradaic impedance spectroscopy, rotating disk electrode experiments, and cyclic voltammetry. The performance of the enzyme-electrode for glucose sensing is described. Similarly, the electrically contacted enzyme-electrodes of NAD(P)(+)-dependent enzymes malate dehydrogenase, MalD, and lactate dehydrogenase, LDH, are prepared by the cross-linking of affinity complexes generated between the enzymes and the NADP(+) and NAD(+) cofactors linked to a pyrroloquinoline quinone phenylboronic acid monolayer, respectively. The MalD enzyme-electrode (surface coverage 1.2 x 10(-)(12) mol cm(-)(2)) exhibits a turnover rate of 190 s(-)(1), whereas the LDH enzyme-electrode (surface coverage 7.0 x 10(-)(12) mol cm(-)(2)) reveals a turnover rate of 2.5 s(-)(1). Chronoamperometric experiments reveal that the NAD(+) cofactor is linked to the PQQ-phenylboronic acid by two different binding modes. The integration of the LDH with the two NAD(+) cofactor configurations yields enzyme assemblies differing by 1 order of magnitude in their bioelectrocatalytic activities.     

A novel ferroceneylazobenzene self-assembled monolayer on an ITO electrode: photochemical and electrochemical behaviors

A novel ferroceneylazobenzene self-assembled monolayer (SAM) has been constructed on an indium-tin oxide (ITO) electrode via the covalent attachment of 4-(4'-11-ferrocenyl-undecanoxyphenylazo)benzoic acid ( FcAzCOOH) onto a silanized ITO substrate surface and verified by reflectance infrared spectroscopy and water contact angle. Atomic force microscopy (AFM) and cyclic voltammogram (CV) indicated that the FcAzCOOH formed a uniform and reproducible SAM on the ITO electrode with a surface coverage of ca. 1.9 x 10 (-10) mol/cm (2) (87 A (2)/molecule). The reversible photoisomerization behavior of the SAM was characterized by UV-vis spectra. The azo pi-pi* transition band intensity of the SAM gradually decreased with UV (365 nm) irradiation and was almost recovered again when subsequent exposure to ambient room light (400-800 nm). The increased tilt angle of the molecules on the ITO substrate after UV irradiation further confirmed the trans-to- cis isomerization of azobenzene moieties. The CV of the trans- FcAzCOOH modified ITO electrode showed a pair of waves due to redox of the ferrocene groups in the potential range of 0 to +800 mV (vs SCE), and the peak separation of the redox wave became larger after UV irradiation and almost returned to its original value after subsequent exposure to the visible light. Rate-dependent CV curves indicated that the charge transfer rate between the ferrocene species in the SAM and the ITO electrode was slowed down after UV irradiation due to the smaller porosity of the monolayer film and the more compact barrier layer between the redox species and the ITO electrode. It is the first time to directly observe the influence of photoisomerization of the azobenzene moiety on the redox behavior of redox species in the ferroceneylazobenzene-functionalized SAM. The present results provide profound insight into the role of redox microenvironment on electron transfer kinetics and also provide a simple and facile approach to the preparation of photocontrollable electrodes.     

Electrocatalytic response of dopamine at a metallothioneins self-assembled gold electrode.

The metallothioneins (MT) self-assembled monolayer modified gold electrode (MT/Au) is demonstrated to catalyze the electrochemical response of dopamine (DA) by cyclic voltammetry. A pair of well-defined redox waves was obtained and the calculated standard rate constant (k(s)) is 6.97 x 10(-3) cm s(-1) (20 degrees C) at the self-assembled electrode. The electrode reaction is a quasi-reversible process. The oxidation peak of DA can be used to determine the concentration of DA. The peak current and the concentration of DA follow a linear relationship in the range of 2.0 x 10(-5) M to 8.0 x 10(-4) M. The detection limit is 6.0 x 10-6 M. By ac impedance spectroscopy, the apparent electron transfer rate constant (k(app)) of Fe(CN)6(3-)/Fe(CN)6(4-) at the MT/Au electrode was obtained as 2.0 x 10(-5) cm s(-1). The MT/Au was characterized with grazing angle FT-IR spectroscopy and contact angle goniometry.     

Electrochemical response of dopamine at a penicillamine self-assembled gold electrode.

The penicillamine (Pen) self-assembled monolayer (SAM) modified gold electrode (Pen/Au) is demonstrated to catalyze the electrochemical response of dopamine (DA) by cyclic voltammetry. A pair of well-defined redox waves was obtained and the calculated standard rate constant (k(s)) is 3.88 x 10(-3) cm/s at the self-assembled electrode. The electrode reaction is a quasi-reversible process. The oxidation peak of DA can be used to determine the concentration of DA. The peak current and the concentration of DA are a linear relationship in the range of 2.0 x 10(-5) M to 8.0 x 10(-4) M. The detection limit is 4.0 x 10(-6) M. By ac impedance spectroscopy the apparent electron transfer rate constant (k(app)) of Fe(CN)(3-)/Fe(CN)(4-) at the Pen/Au electrode was obtained as 2.08 x 10(-5) cm/s. The Pen SAM was characterized with X-ray photoelectron spectroscopy (XPS), grazing angle FT-IR spectroscopy and contact angle goniometer.     

Covalent attachment of glucose oxidase to an Au electrode modified with gold nanoparticles for use as glucose biosensor

A feasible method to fabricate glucose biosensor was developed by covalent attachment of glucose oxidase (GOx) to a gold nanoparticle monolayer modified Au electrode. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) of ferrocyanide followed and confirmed the assemble process of biosensor, and indicated that the gold nanoparticles in the biosensing interface efficiently improved the electron transfer between analyte and electrode surface. CV performed in the presence of excess glucose and artificial redox mediator, ferrocenemethanol, allowed to quantify the surface concentration of electrically wired enzyme (Gamma(E)(0)) on the basis of kinetic models reported in literature. The Gamma(E)(0) on proposed electrode was high to 4.1 x 10(-12) mol.cm(-2), which was more than four times of that on electrode direct immobilization of enzyme by cystamine without intermediate layer of gold nanoparticles and 2.4 times of a saturated monolayer of GOx on electrode surface. The analytical performance of this biosensor was investigated by amperometry. The sensor provided a linear response to glucose over the concentration range of 2.0 x 10(-5)-5.7 x 10(-3) M with a sensitivity of 8.8 microA.mM(-1).cm(-2) and a detection limit of 8.2 microM. The apparent Michaelis-Menten constant (K(m)(app)) for the sensor was found to be 4.3 mM. In addition, the sensor has good reproducibility, and can remain stable over 30 days.     

Electrochemical characteristics of ferrocenecarboxylate-coupled aminoundecylthiol self-assembled monolayers

The electrochemical characteristics of the modified electrodes with ferrocenecarboxylate-coupled aminoundecylthiol monolayers prepared in two sequential steps were studied. The self-assembled monolayer (SAM) was prepared through the covalent attachment of ferrocenecarboxylate in an activation solution containing N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide coupling agent to aminoundecylthiol SAMs formed on a substrate. In the ferrocenecarboxylate-coupled aminoundecylthiol monolayers, the ferrocene moieties were expected to be packed regularly with enhanced ordering compared with those in the FcCOO(CH2)11SH monolayer. As the ferrocene coverage increases, the formal potential for the ferrocene-ferricenium (Fc/Fc+) couple shifts to the positive potential and the full width at half-maximum (deltaE(fwhm)) increases also. The maximum coverage is found to be about 3 x 10(-10) mol cm(-2), which is considered to be a value obtained from a well-ordered ferrocene-tethered SAM. As for the mass change, the increase in ferrocene coverage caused the enhancement in ion association between the ferricenium cations and perchlorate anions resulting in a mass increase upon oxidation; however, the mass change per mole electron decreases. The results obtained from the ferrocenecarboxylate-coupled aminoundecylthiol monolayers were explained to be due to the well-ordered packing with regular spacing compared with those of the FcCOO(CH2)11SH monolayer.     

Alkanethiol monolayers at reduced and oxidized zinc surfaces with corrosion protection: a sum frequency generation and electrochemistry investigation

In this work, octadecanethiol (ODT) was demonstrated to form ordered monolayers at either electrochemically reduced or oxidized Zn surfaces, by means of sum frequency generation (SFG) spectroscopy, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). The SFG spectra of ODT-modified Zn electrodes featured three methyl group resonances in the C-H vibrational region (2800-3100 cm(-1)). A significant decrease in interfacial capacitance and an increase in charge-transfer resistance were observed in EIS measurement after ODT modification. The alkane chain tilt angle of ODT within a monolayer at the Zn surface was estimated as 0 degrees with respect to the surface normal by interfacial capacitance measurement via EIS. CV and SFG investigation revealed that ODT monolayers undergo reductive desorption from the Zn electrode in 0.5 M NaOH at -1.66 V (vs SCE) and in 0.5 M NaClO4 at -1.62 V. The integrated charge consumed to the desorption of ODT is determined as 87 mC/cm2 from the reductive peak on CV curve, resulting in a coverage of 9.0 x 10(-10) mol/cm2 (5.4 x 10(14) molecules/cm2) if assuming the reduction follows a one-electron process. ODT monolayers show corrosion protection to underlying zinc at the early immersion stage in base, salt, and acid media. However, the protection efficiency was reduced with immersion time due to the presence of defects within the monolayers.     

The direct electrochemistry of myoglobin at a DL-homocysteine self-assembled gold electrode

A pair of well-defined redox waves of myoglobin at a DL-homocysteine self-assembled gold electrode was achieved. Myoglobin can strongly bind to the homocysteine self-assembled gold electrode, so the Mb-Hcy self-assembled gold electrode was prepared and the standard rate constant (ks) of Mb was calculated as 9.3 x 10(-1) s(-1). X-ray photoelectron spectroscopy (XPS) measurements demonstrated that Hcy monolayer was formed and Mb was bound to Hcy monolayer.     

Electrochemical determination of electroinactive guests of β-cyclodextrin at a self-assembled monolayer interface

A novel determination method of electroinactive molecules by means of electrochemical technique is presented. A new self-assembled monolayer containing cyclodextrin (CD) is prepared with mono(6-o-p-tolylsulfonyl)-β-cyclodextrin. Although this derivatization process leads to a β-CD coverage of 10% of a full monolayer, this layer shows an effective host-guest response to ferrocene. The interfacial ferrocene complexation gives a response similar to that expected for a Langmuir adsorption isotherm yielding a stability constant of 4.2 ×104 mol-1· L and a maximum ferrocene coverage of 8.6×10-12 mol/cm2. The redox peak currents of the surface-confined ferrocene decrease upon addition of competing β-CD guest species to the solution, such as m-toluic acid (mTA) and sodium dodecyl sulfonate (SDS). This principle has been used for the determination of the electroinactive molecules, mTA and SDS in the concentration ranges of 0.8-2.7 μmol/L and 5-100 nmol/L, respectively.     

Characterization of transparent conducting oxide surfaces using self-assembled electroactive monolayers

The electronic properties of various transparent conducting oxide (TCO) surfaces are probed electrochemically via self-assembled monolayers (SAMs). A novel graftable probe molecule having a tethered trichlorosilyl group and a redox-active ferrocenyl functionality (Fc(CH2) 4SiCl3) is synthesized for this purpose. This molecule can be self-assembled via covalent bonds to form monolayers on various TCO surfaces. On as-received ITO, saturation coverage of 6.6 x 10(-10) mol/cm2 by a close-packed monolayer and an electron-transfer rate of 6.65 s(-1) is achieved after 9 h of chemisorption, as determined by cyclic voltammetry (CV) and synchrotron X-ray reflectivity. With this molecular probe, it is found that O2 plasma-treated ITO has a significantly greater electroactive coverage of 7.9 x 10 (-10) mol/cm2 than as-received ITO. CV studies of this redox SAM on five different TCO surfaces reveal that MOCVD-derived CdO exhibits the greatest electroactive coverage (8.1 x 10(-10) mol/cm2) and MOCVD-derived ZITO (ZnIn2.0Sn1.5O) exhibits the highest electron transfer rate (7.12 s(-1)).     

Catalytic oxidation of ascorbic acid by some ferrocene derivative mediators at the glassy carbon electrode. Application to the voltammetric resolution of ascorbic acid and dopamine in the same sample

Direct-current cyclic voltammetry is used to investigate the suitability of some ferrocene derivatives such as ferrocenecarboxylic acid, ferroceneacetic acid and ferrocenemethanol as mediators for ascorbic acid oxidation in aqueous solutions with low pH. The ascorbic acid coupled catalytically to three ferrocene derivatives exhibiting homogeneous second-order rate constants k(s), in the range 7.36 x 10(5) - 1.23 x 10(7). The catalytic oxidation peak current was linearly dependent on the ascorbic acid concentration and the linearity range obtained in the presence of ferrocenecarboxilic acid, having the largest second-order rate constant, was 5 x 10(-5) - 1.5 x 10(-3) M. The catalytic effect of the ferrocene derivatives on the electrochemical oxidation of ascorbic acid reduced the oxidation potential of ascorbic acid, resulting in the separation of the overlapping voltammograms of ascorbic acid and dopamine at the glassy carbon electrode in a mixture. This allowed the determination of ascorbic acid in the presence of dopamine. The calibration graph obtained by linear sweep voltammetry for ascorbic acid in the presence of dopamine of fixed concentration is linear in the range 5 x 10(-5) - 1.5 x 10(-3) M. In a similar manner, dopamine is determined in the presence of a high concentration of ascorbic acid, up to 100 times that of dopamine, using ferroceneacetic acid as the most suitable mediator for this purpose.     

Sodium dodecyl sulfate-modified carbon paste electrodes for selective determination of dopamine in the presence of ascorbic acid

A carbon paste electrode (CPE) modified by a monolayer film of sodium dodecyl sulfate (SDS) was used for detection of dopamine (DA). Cyclic voltammetry demonstrated improved response of the DA sensor. This suggests the effectivity of surface modification of CPE by SDS. Impedance spectroscopy was used for the characterization of CPE surface properties. The effect of SDS concentration on the electrode quality also reveals that SDS formed a monolayer on CPE surface with a high density of negative-charged end directed outside the electrode. As a result, the carbon paste electrode modified with SDS (SDS/CPE) exerted discrimination against ascorbic acid in physiological circumstance. Thus, it can selectively determine dopamine even in the presence of 220-fold AA combined with differential pulse stripping voltammetry. In pH 7.40 phosphate buffer solution, the oxidation peak current on differential pulse voltammograms increases linearly with the concentration of DA in the range of 5.0 x 10(-7) to 8.0 x 10(-4) mol . L(-1) with a detection limit of 5.0 x 10(-8) mol . L(-1). Satisfying results are achieved when detecting the DA in injection and simulated biology sample.     

Electrochemical impedance spectroscopy study of a hydrogen electrode reaction at a Zn electrode in a molten LiCl-KCl-LiH system

The hydrogen electrode reaction involving hydride ion, H-, at a Zn electrode is investigated in a molten LiCl-KCl-LiH system at 673 K. The charge-transfer resistances were measured by electrochemical impedance spectroscopy in the overpotential region of 0.10 < or = eta < or = 0.35 V and over the H- concentrations of 1.5 x 10(-4) < or = C(H)- < or = 1.2 x 10(-3) mol cm(-3). The logarithm plot of the charge-transfer resistance against the overpotential at C(H)- = 3.0 x 10(-4) mol cm(-3) gives the symmetry factor, beta, of 0.50 and the exchange current density, j0, of 5.8 x 10(-3)A cm(-2), respectively. Analysis of the dependence of j0 on H- concentration independently gives a beta of 0.55. The reasonable beta values indicate that the H- <==> H(ad)(M) + e- step is rate-determining.     

Immobilization and electrochemical redox behavior of cytochrome c on fullerene film-modified electrodes

Two different fullerene film-modified electrodes were prepared and used for surface immobilization and electrochemical property investigation of horse heart cytochrome c (cyt c). Both a pristine fullerene film and fullerene-palladium (C(60)-Pd) polymer film-modified platinum, glassy carbon and indium-tin-oxide (ITO) electrodes were used. The immobilized cyt c was characterized by piezoelectric microgravimetry at a quartz crystal microbalance (QCM), UV-visible absorption, and X-ray photoelectron spectroscopy (XPS), as well as cyclic voltammetry (CV) techniques. The UV-visible spectral studies revealed a small blue shift of both the Soret and Q band of the heme moiety of cyt c, immobilized on the C(60)-Pd polymer film-modified ITO electrode, as compared to the bands of cyt c in solution suggesting that molecules of cyt c are densely packed onto the surface of the modified electrode. The CV studies revealed a quasi-reversible electrode behavior of the heme moiety indicating the occurrence of kinetically hindered electron transfer. A good agreement was found between the values of cyt c electrode surface coverage determined by piezoelectric microgravimetry and cyclic voltammetry. For piezoelectric microgravimetry, these values ranged from 0.5 x 10(-10) to 2.5 x 10(-10) mol cm(-2), depending upon the amount of cyt c present in solution and the time allowed for immobilization, which compared with a value of 3.6+/-0.4 x 10(-10) mol cm(-2) determined by CV. The possible mechanisms of cyt c immobilization on the C(60) film and C(60)-Pd film-modified electrodes are also discussed.     

Self-assembly of [60]fullerene-thiol derivatives on mercury surfaces

Herein we report the first self-assembly of fullerene-thiol conjugates (1 and 2) on thin mercury films (TMF) deposited on a glassy carbon electrode (GCE). The fullerene-containing SAMs were investigated by cyclic voltammetry and water contact angle measurements. Two reversible, surface-confined redox couples were obtained for the fullerene-containing SAMs on TMF/GCE in CH(2)Cl(2) solution. The surface coverage of both fullerene derivatives 1 and 2 on TMF/GCE was measured to be in the range of (1.7-1.8) x 10(-10) mol cm(-2). Both SAMs of 1 and 2 partially blocked the electron transfer across the electrode in aqueous solution. The contact angle measurements performed on TMFs clearly showed an enhancement of the surface hydrophobicity upon formation of the fullerene-containing monolayer.     

Preparation of polypyrrole-coated platinum modified electrode in chloroform in the presence of various supporting electrolytes and its use for the catalytic oxidation of hydroquinone in aqueous and chloroform solutions

Polypyrrole (PPy)-coated platinum modified electrode was prepared electrochemically in chloroform in the presence of tetrabutylammonium perchlorate, tosylate, tetrafluoroborate, periodate, and hydrogen sulphate. Evidence in favour of the occurrence of a redox reaction at the film/solution interface was given by ferrocene oxidation. The electrocatalytic effect of the PPy-coated electrode was revealed by oxidation of hydroquinone on it in water and chloroform. The influence of dopant anion, method of electrosynthesis and operation temperature were demonstrated. The calculated heterogeneous electron transfer rate constant (k(o)) at the PPy-modified Pt electrode at 25 degrees C was 1.4 x 10(-3) cm/sec which is 1000-fold greater than the value (< 10(-6) cm/sec) reported for bare electrode.     

Ferrocene branched chitosan for the construction of a reagentless amperometric hydrogen peroxide biosensor

Chitosan was chemically branched with ferrocene moieties and further used as a support for the immobilization of horseradish peroxidase on a glassy carbon electrode. The reagentless biosensor device showed a linear amperometric response toward hydrogen peroxide concentrations between 35 x 10(-6) M and 2.0 x 10(-3) M. The biosensor reached 95% of the steady-state current in about 20 s and its sensitivity was 28.4 x 10(-3) microA x M(-1). The enzyme electrode retained 94% of its initial activity after 2 weeks of storage at 4 degrees C in 50 x 10(-3) M sodium phosphate buffer at pH 7.0.     

Diffusion of metal complexes inside of silica-surfactant nanochannels within a porous alumina membrane

The present paper describes diffusivities of a series of metal complexes inside of silica-surfactant nanochannels (channel diameter = 3.4 nm), which were formed within a porous alumina membrane by a surfactant-templated method using cetyltrimethylammonium bromide (CTAB) as a template surfactant. The metal complexes used in this study were Fe(CN)6(3-), Ru(NH3)6(3-), ferrocenecarboxylic acid (Fc-COO-), (ferrocenylmethyl)-trimethylammonium (Fc-NMe3+), N,N-(dimethylamminomethyl)ferrocene (Fc-NMe2), and ferrocene methanol (Fc-OH). Apparent diffusion coefficients of these metal complexes were estimated by measuring their mass transports through the silica-surfactant nanochannels. The estimated apparent diffusion coefficients were on the order of 10(-11) cm2 s(-1) for Fe(CN)6(3-) and Ru(NH3)6(3-), and these values were five orders of magnitude smaller than those in a bulk aqueous solution. For the ferrocene derivatives, the apparent diffusion coefficients of charged ferrocene derivatives are almost the same (5.3 x 10(-11) cm2 s(-1) for Fc-COO- and 5.4 x 10(-11) cm2 s(-1) for Fc-NMe3+), whereas neutral ferrocene derivatives (Fc-NMe2 and Fc-OH) show faster diffusion than the charged species. In addition, the apparent diffusion coefficient of Fc-NMe2 (27 x 10(-11) cm2 s(-1)) was about three times larger than that of Fc-OH (10 x 10-11 cm2 s(-1)). The difference in these diffusion coefficients is discussed by considering the mesostructure of the silica-surfactant nanochannels, that is, an ionic interface with cationic head groups of CTA and their counteranions, a hydrophobic interior of the micellar phase, and a silica framework. As a result, it is inferred that the slow diffusivities of the charged metal complexes are due to the electrostatic interaction between the charged species and the ionic interface, whereas less interaction between neutral ferrocenes and the ionic interface causes distribution of metal complexes into the hydrophobic micellar phase, which is a less viscous medium compared to the ionic interface, resulting in the faster diffusivities of the neutral species.