Influence of silanization on voltammetry at electrodes modified with silica films of controlled porosity formed by electrochemically initiated sol-gel processing

Silica sol-gel (SG) films with templated pores were deposited on glassy carbon (GC) electrodes by an electrochemically initiated process. Generation-4 poly(amidoamine), PAMAM, dendrimer was included in the tetraethoxysilane precursor to facilitate pore formation. The PAMAM adsorbs to the GC, which blocks SG formation at those sites on the electrode. The pore size was 10?±?5?nm. After removal of the PAMAM, cyclic voltammetry of Fe(CN)₆ ^3? and Ru(NH₃)₆ ³⁺ at pH?6.2 showed that the residual negative charge on the silica attenuated the current for the former and increased the current for the latter, presumably by electrostatic repulsion and ion-exchange preconcentration, respectively. This premise was supported by repeating the measurements at the isoelectric point. Methylation of the silanol sites was used to eliminate the charge of the SG. At the end-capped SG, the voltammetry of Fe(CN)₆ ^3? and Ru(NH₃)₆ ³⁺ yielded currents that were independent of pH over the range 2.1 to 7.2. Circumventing the need for the silanization by using (3-glycidyloxypropyl)trimethoxysilane as the sol-gel precursor failed because the oxygen plasma treatment to remove the PAMAM attacked the organically modified sol-gel backbone. The resulting modified electrode mitigated the influence of proteins on the voltammetry of test species and stabilized functionalize nanoparticle catalysts under hydrodynamic conditions.     

Electrochemical Studies of Thiol Self‐Assembled Monolayers at a Heated Gold‐Wire Microelectrode

Mercaptoundecanoic acid (MUA) and glutathione (GSH) self‐assembled monolayers were prepared on gold‐ wire microelectrode. Cyclic voltammetry was used to investigate the influence of temperature on electrochemical behaviors of Fe(CN)₆^3?/4? and Ru(NH₃)₆^3+/2+ at these SAMs modified electrodes in aqueous solution. It is found that temperature shows great influence on electron transfer (ET) and mass transport (MT) for the two SAMs modified electrodes and the influence of temperature depends on the charge properties of the redox couples and terminal groups of SAMs and the structure of the monolayer on gold surface. The temperature can greatly increase MT rate of Fe(CN)₆^3?/4? at both MUA and GSH modified electrodes. However, the increased MT rate doesn't have any effect on the CV's for Fe(CN)₆^3?/4? /MUA system. For Ru(NH₃)₆^3+/2+ , temperature can greatly improve the electrochemical reaction in both MUA and GSH modified electrodes, which is ascribed to temperature‐induced diffusion and convection and the electrostatic interaction between Ru(NH₃)₆^3+/2+ and negatively charged carboxyl groups on the electrode surface.     

Electrochemistry of a carotenoid self-assembled monolayer

A carotenoid self-assembled monolayer was prepared by dipping a gold electrode into a solution of 4^′-thioxo-β,β-caroten-4-one in acetonitrile. Electrochemistry of the surface layer was investigated by cyclic voltammetry in an aqueous solution. No electrochemical reaction was detected in the potential region between 0.5 and −0.6 V vs. SCE. The anodic reaction of adsorbed carotenoid occurs at 0.8 V, whereas the irreversible anodic desorption proceeds at 1.4 V in 0.01 M HClO₄. Formation of the surface layer resulted in a decrease of the charging current as well as in a strong inhibition of the electron transfer reaction for species such as Fe(CN)₆³⁻, Ru(NH₃)₆³⁺, and dissolved oxygen. Prolonged voltage cycling in the O₂ reduction range induced some changes in the surface layer characteristics that were tentatively accounted for by the cross-linking of adsorbed molecules under the effect of transient oxygen radicals.     

Electrochemically assisted fabrication of size-exclusion films of organically modified silica and application to the voltammetry of phospholipids

Modification of electrodes with nanometer-scale organically modified silica films with pore diameters controlled at 10- and 50-nm is described. An oxidation catalyst, mixed-valence ruthenium oxide with cyano cross-links or gold nanoparticles protected by dirhodium-substituted phosphomolybdate (AuNP-Rh₂PMo₁₁), was immobilized in the pores. These systems comprise size-exclusion films at which the biological compounds, phosphatidylcholine and cardiolipin, were electrocatalytically oxidized without interference from surface-active concomitants such as bovine serum albumin. Ten-nanometer pores were obtained by adding generation-4 poly(amidoamine) dendrimer, G4-PAMAM, to a (CH₃)₃SiOCH₃ sol. Fifty-nanometer pores were obtained by modifying a glassy carbon electrode (GC) with a sub-monolayer film of aminopropyltriethoxylsilane, attaching 50-nm diameter poly(styrene sulfonate), PSS, spheres to the protonated amine, transferring this electrode to a (CH₃)₃SiOCH₃ sol, and electrochemically generating hydronium at uncoated GC sites, which catalyzed ormosil growth around the PSS. Voltammetry of Fe(CN)₆ ^3? and Ru(NH₃)₆ ³⁺ demonstrated the absence of residual charge after removal of the templating agents. With the 50-nm system, the pore structure was sufficiently defined to use layer-by-layer electrostatic assembly of AuNP-Rh₂PMo₁₁ therein. Flow injection amperometry of phosphatidylcholine and cardiolipin demonstrated analytical utility of these electrodes.     

Preparation and characterization on the carbon nanotube chemically modified electrode grown in situ

Carbon nanotube chemically modified electrode (CNT-CME) was prepared by growing carbon nanotube (CNT) in situ on the pretreated graphite electrode (GE) via the catalytic chemical vapor deposition. The pretreated GE was prepared by ultrasonic immersion method using Ni(NO₃)₂ as the catalyst. The CNT growing on the CNT-CME was characterized by transmission electron microscope and scanning electron microscope. The obtained electrode electrochemical performance was characterized by cyclic voltammetry with the Na₂SO₄ solution and [Fe(CN)₆]³⁻/[Fe(CN)₆]⁴⁻ solution. The results showed that the obtained electrode has good current responsive sensitivity and good testing result accuracy, indicating that the obtained electrode may have great application in electrochemical testing field.     

Low Temperature Synthesis of Zeolite Films on Glassy Carbon: Towards Designing Molecularly Selective Electrochemical Devices

Faujasite‐type zeolite films have been grown on glassy carbon electrode surfaces embedded in an epoxy resin matrix by a quasi ambient temperature hydrothermal synthesis. The resulting zeolite modified electrodes are mechanically stable and provide an advantageous combination of electrochemical detection with charge and size selectivities at the molecular level, as demonstrated with the aid of some model electroactive probes or analytes (Ru(NH₃)₆³⁺, Fe(CN)₆^3?, Ru(bpy)₃²⁺, dopamine, ascorbic acid). To act efficiently as a physical molecular sieve barrier between the electrode surface and the external solution, these films must have a thickness of about 0.7 μm. These results open the door for further development of molecular sieving electrochemistry.     

A layer-by-layer assembly label-free electrochemical immunosensor for the detection of microcystin-LR based on CHIT/PAMAM dendrimer/silver nanocubes

Utilising the affinity and high combination ability between silver nanocubes and amino group (–NH₂), a novel electrochemical immunosensor was constructed for the ultrasensitive detection of microcystin-LR (MC-LR) based on G4-polyamidoamine (PAMAM) dendrimer and Ag nanocubes as immobilised substrate of anti-MC-LR. G4-PAMAM dendrimers were covalently bound on the chitosan (CHIT) – modified electrode by glutaraldehyde (GA), providing abundant amino groups to absorb much more Ag nanocubes comparing without using PAMAM. Subsequently, antibodies of MC-LR were immobilised with highly dense through Ag-NH₂. K₃Fe(CN)₆/K₄Fe(CN)₆ was used as electroactive redox probe. Ag nanocubes/PAMAM can enhance the antibody loading amount, which would bind more MC-LR and hinder the electron transfer of K₃Fe(CN)₆/K₄Fe(CN)₆. Differential pulse voltammetry (DPV) was employed to evaluate the analytical performance of the fabricated signal-off immunosensor. The response current had negative correlation with the concentration of MC-LR. The linear range covered was from 0.05 ng/mL to 25 μg/mL with detection limit (DL) of 0.017 ng/mL at 3σ. The proposed approach showed high specificity for the detection of MC-LR, with acceptable reproducibility, stability and reliability. Compared with the enzyme-linked immunoassay (ELISA) method by analyzing real water samples from Dian Lake, this immunosensor revealed acceptable accuracy with a relative error of 12.7%, indicating a potential alternative method for MC-LR detection in water sample.     

Clay modified electrodes: Part VI. Aluminum and silicon pillared clay-modified electrodes

Metal (Ru, Os, Fe) bipyridyl complexes and Ru(NH₃)³⁺₆ were incorporated into several montmorillonite clays and their aluminum and silicon-pillared analogues. Film concentrations and apparent diffusion coefficients were measured and compared by using spectrophotometry, cyclic voltammetry and chronocoulometry. The catalytic role of isomorphously substituted iron was evaluated and implicated by its effects on Ru(bpy)³⁺₃ voltammetric behavior. A model is proposed for charge transport within the novel silicon-pillared clay.     

Direct cytochrome c electrochemistry at boron-doped diamond electrodes

Highly boron-doped diamond electrodes are characterized voltammetrically employing Ru(NH₃)₆^3+/2+, Fe(CN)₆^3−/4−, benzoquinone/hydroquinone, and cytochrome c redox systems. The diamond electrodes, which are polished to nanometer finish, are initially `activated' electrochemically and then pretreated by oxidation, reduction, or polishing. All electrodes give reversible cyclic voltammetric responses for the reduction of Ru(NH₃)₆³⁺ in aqueous solution.Redox systems other than Ru(NH₃)₆^3+/2+ show characteristic electrochemical behavior as a function of diamond surface pretreatment. In particular, the horse heart cytochrome c redox system is shown to give reversible voltammetric responses at Al₂O₃ polished boron-doped diamond electrodes. No voltammetric response for cytochrome c is detected at anodically pretreated diamond electrodes. The observations are attributed to preferential interaction of the polished diamond surface with the reactive region of the cytochrome c molecule and low interference due to a lack of protein electrode fouling.     

Carbon Nanotube Immobilized Electrode Using Amphiphilic Phospholipid Polymer with Anti‐fouling and Dispersion Property for Electrochemical Analysis

A carbon nanotube (CNT)‐modified electrode was fabricated by dropping a dispersion of multi‐walled CNTs in water‐soluble and amphiphilic phospholipid polymer with both dispersing ability and anti‐biofouling property onto a Au electrode. A poly(2‐methacryloyloxyethyl phosphorylcholine‐co‐n‐butyl methacrylate) (PMB) composed from 50 mol% of 2‐methacryloxylethyl phosphorylcholine and 50 mol% of n‐butyl methacrylate (PMB50) was used as dispersing reagent for CNTs. The dispersion of water‐insoluble material by PMB50 and its antifouling effects in electrochemical analysis were investigated. The CNT‐modified electrode showed an anodic peak potential that was shifted negatively and an increase in the current value for the electrolytic oxidation of nicotinamide adenine dinucleotide. In addition, the charge on PMB50 did not inhibit the electrochemical reaction of the redox compounds K₃[Fe(CN)₆], [Ru(NH₃)₆]Cl₃, and hydroxymethylferrocene. Cyclic voltammetry of K₃[Fe(CN)₆] in 4 % bovine serum albumin (BSA) using a bare Au electrode, the anodic peak current was reduced to 47 % of that without BSA. In contrast, the antifouling effect of the PMB50‐coated electrode meant that the current was only reduced to 70 % of that without BSA.     

Mixed Valence Chemistry of Pyrimidine Bridged Binuclear Complex of Pentacyanoferrate and Pentaammineruthenium

The pyrimidine bridged binuclear complex (CN)₅FepymRu(NH₃)₅- (I) was prepared in aqueous solution by mixing cquimolar of Fe(CN)₅OH₂^3? and Ru(NH₃)₅pym²⁺. Its mixed valence state molecule (CN)₅FepymRu(NH₃)₅(II) was obtained upon oxidation of I by one equivalent of peroxydisulfate ion. Both binuclear complexes and corresponding Fe(II) and Ru(II) mononuclear complexes displayed a metal-to-ligand charge transfer absorption in 400–450 nm region. Rate constants of formation and dissociation of I and II were measured, and the values of k_f (?10³M^?1s^?1) and k_d (?10^?3-10^?4 s^?1) were consistent with kinetic results expected for the substitution of Fe(CN)₅OH₂^3? with di- and trivalent ligands. Cyclic voltammetry of I exhibited two one-electron steps of oxidation corresponding to [III, L, II] + e → [II, L, II] and [III, L, III] + e → [III, L, II], respectively. The mixed valence binuclear complex II showed an intervalence band at 955 nm with a molar extinction coefficient 5.80 × 10² M^?1cm^?1 and a half-width 5100 cm^?l. The properties of the IT band conform to Hush's theory. Spectroscopic, electrochemical and kinetic results of II suggest that the mixed valence complex features a trapped - valence formulation with localized oxidation states of Fe(II) and Ru(III).     

Electrostatic bending and outer-sphere intervalence transfer in a flexible ligand-bridged ruthenium(III)-iron(II) complex

Abstract

In the mixed-valence complex [Ru^III(NH₃)₅(μ-dpypn)Fe^II(CN)₅] with the flexible bridging ligand 1,3-di(4-pyridyl)propane (dpypn), electrostatic interactions between the {Ru(NH₃)₅}³⁺ and {Fe(CN)₅}^3? moieties drive a strong bending of dpypn and approximation of the Ru^III and Fe^II centers, from which the enhanced electronic coupling between metal ions produces an intense intervalence-transfer absorption in the near-infrared region. Density functional theory calculations corroborate both the electrostatic bending in this heterobinuclear complex and a linear geometry in the homobinuclear counterparts [Ru(NH₃)₅(μ-dpypn)Ru(NH₃)₅]⁵⁺ and [Fe(CN)₅(μ-dpypn)Fe(CN)₅]^5?, for which no evidence of electronic coupling was found because of the separation between metal centers. Furthermore, the heterobinuclear species formed an inclusion complex with β-cyclodextrin where the imposed linear geometry prevents significant electronic coupling and intervalence charge transfer between the Ru^III and Fe^II centers.     

Supramolecular microrods can be prepared by mixing aqueous Ru(NH<Subscript>3</Subscript>)<Subscript>6</Subscript>Cl<Subscript>3</Subscript> and K<Subscript>3</Subscript>Fe(CN)<Subscript>6</Subscript> solutions at room temperature

This paper describes the ionic self-assembly method to fabricate supramolecular one-dimensional microrods in solution. Such microrods were formed in a one-step process through the mixing aqueous Ru(NH₃)₆Cl₃ and K₃Fe(CN)₃ solutions at room temperature. Chemical composition of the resulting structures, which are composed of from Fe(CN)₆⁴⁻ and Ru(NH₃)₆³⁺, was determined by energy-dispersed spectroscopy. The data show that the formation of microrods depends on the molar ratio and concentration of the reactants.     

Fabrication of Chitosan‐Multiwall Carbon Nanotube Nanocomposite Containing Ferri/Ferrocyanide: Application for Simultaneous Detection of D‐Penicillamine and Tryptophan

We report a simple and effective strategy for fabrication of the nanocomposite containing chitosan (CS) and multiwall carbon nanotube (MWNT) coated on a glassy carbon electrode (GCE). The characterization of the modified electrode (CS‐MWNT/GC) was carried out using scanning electron microscopy (SEM) and UV–vis absorption spectroscopy. The electrochemical behavior of CS‐MWNT/GC electrode was investigated and compared with the electrochemical behavior of chitosan modified GC (CS/GC), multiwalled carbon nanotube modified GC (MWNT/GC) and unmodified GC using cyclic voltammetry (CV) and electron impedance spectroscopy (EIS). The chitosan films are electrochemically inactive; similar background charging currents are observed at bare GC. The chitosan films are permeable to anionic Fe(CN)₆^3?/4? (FC) redox couple. Electrochemical parameters, including apparent diffusion coefficient for the Fe(CN)₆^3?/4? redox probe at FC/CS‐MWNT/GC electrode is comparable to values reported for cast chitosan films. This modified electrode also showed electrocatalytic effect for the simultaneous determination of D‐penicillamine (D‐PA) and tryptophan (Trp). The detection limit of 0.9 μM and 4.0 μM for D‐PA and Trp, respectively, makes this nanocomposite very suitable for determination of them with good sensitivity.     

Grafting of diaminoalkane on glassy carbon surface and its functionalization

Diaminoalkanes (NH₂(CH₂)_nNH₂, n=7,10,12) were grafted onto a glassy carbon electrode (GCE) surface by amino cation radical formed during electrooxidation of amino group. The presence of diamine grafted layer at the GCE is demonstrated by X-ray photoelectron spectroscopy. The effect of the grafted layer at the GCE surface on the redox responses of Ru(NH₃)₆³⁺ and Fe(CN)₆³⁻ redox probes has been investigated. Electrochemical impedance experiments indicate that the kinetics of electron transfer are slowed down when the scan rate taken to modify the GCE is low, and that diaminoalkane with longer alkyl-chain used has higher blocking characteristics. The amine-functionalized GCE is versatile not only to further covalently immobilize ferrocene acetic acid via carbodiimide coupling, but also as a charge-rich substrate to successfully adsorb heteropolyanion P₂W₁₈ in acidic solution by electrostatic interaction.     

The Effect of Ionic Liquid Covalent Bonding to Sol‐Gel Processed Film on Ion Accumulation and Transfer

Accumulation of electroactive anions into a silicate film with covalently bonded room temperature ionic liquid film deposited on an indium tin oxide electrode was studied and compared with an electrode modified with an unconfined room temperature ionic liquid. A thin film containing imidazolium cationic groups was obtained by sol‐gel processing of the ionic liquid precursor 1‐methyl‐3‐(3‐trimethoxysilylpropyl)imidazolium bis(trifluoromethylsulfonyl)imide together with tetramethylorthosilicate on the electrode surface. Profilometry shows that the obtained film is not smooth and its approximate thickness is above 1 μm. It is to some extent permeable for a neutral redox probe – 1,1′‐ferrocene dimethanol. However, it acts as a sponge for electroactive ions like Fe(CN)₆^3?, Fe(CN)₆^4? and IrCl₆^3?. This effect can be traced by cyclic voltammetry down to a concentration equal to 10^?7 mol dm^?3. Some accumulation of the redox active ions also occurs at the electrode modified with the ionic liquid precursor, but the voltammetric signal is significantly smaller compare with the bare electrode. The electrochemical oxidation of the redox liquid t‐butyloferrocene deposited on silicate confined ionic liquid film is followed by the expulsion of the electrogenerated cation into an aqueous solution. On the other hand, the voltammetry obtained with the electrode modified with t‐butyloferrocene solution in the ionic liquid precursor exhibits anion sensitive voltammetry. This is explained by anion insertion into the unconfined ionic liquid deposit following t‐butylferricinium cation formation.     

Kinetic Study of the Ru(II) Catalyzed Oxidation of Pentacyanoferrate(ll) Complexes by Peroxydisulfate

The oxidation reactions of Fe(CN)₅L^3? (L = 4-ampy, py, dpa) complexes by S₂O₈^2? were catalyzed upon the addition of a trace amount of Ru(NH₃)₅L′²⁺ (L′ = pz, py or dcb) complex, and the reaction becomes zero-order in Fe(II). The reaction time is ～10² fold faster than the simple Fe(CN)₅L^3?-S₂O₈^2? system. The mechanism of this Ru(II) catalyzed redox reaction is proposed as Ru(NH₃)₅L′²⁺ + 1/2 S₂O₈^2? → Ru(NH₃)₅L′³⁺ + SO₄^2? Ru(NH₃)₅L′³⁺ + Fe(CN)₅L^3? ? Ru(NH₃)₅L′²⁺ + Fe(CN)₅L^2?     

Ca2+ induced Fe(CN)63−/4− electron transfer at Pt supported BLM electrode

The effect of Ca²⁺ on the electron transfer of Fe(CN)₆^3−/4− couple at Pt supported lecithin (PC)/cholesterol (CH) bilayer lipid membrane (BLM) electrode has been studied by voltammetry and ac impedance spectroscopy. Experimental results suggest that the interaction of Ca²⁺ with the BLM can open some kind of channel for Fe(CN)₆^3−/4− ions, allowing increased access of the redox couple to the electrode surface. This phenomenon may be related to the Ca²⁺ regulation or activation effect of ion channel in some biomembranes.     

Nanocarbon Paste Electrodes

The electrochemical behaviour of carbon paste electrodes prepared using nanocarbon and mineral oil was investigated and the results contrasted with different carbon and carbon pastes electrodes. The composition of carbon paste was studied by performing cyclic voltammetry performed in 0.1 M KCl solution in the presence of 4.0 mM Ru(NH₃)₆Cl₃, a well‐characterized redox system commonly used to test the electrode behaviour. After optimisation of the paste composition, the sensors chosen were tested for the analysis and characterization of three different systems: Ru(NH₃)₆^3+/2+, FcCH₂OH/FcCH₂OH⁺ and acetaminophen. The ability to obtain high quality voltammetry from the nanocarbon electrode was demonstrated and simulation of the voltammetry allowed the extraction of electrode kinetic parameters with high precision.     

Electrochemical and thermodynamic properties of hexacyanoferrate(II)/(III) redox system on multi-walled carbon nanotubes

Novel films consist of multi-walled carbon nanotubes (MWCNT) were fabricated by means of catalytic chemical vapor deposition (CVD) technique with decomposition of either acetonitrile (ACN) or benzene (BZ) using ferrocene (FeCp₂) as catalyst. The electrochemical and thermodynamic behavior of the ferrocyanide/ferricyanide, [Fe(CN)₆]^3−/4− redox couple on synthesized MWCNT-based films was investigated by means of cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) techniques at T = (278.15, 283.15, 293.15, and 303.15) K. The redox couple [Fe(CN)₆]^3−/4− behaves quasi-reversibly on fabricated MWCNT-based films and its reversibility is enhanced upon increasing temperature. Namely, the findings establish that with the rise in temperature the barrier for interfacial electron transfer decreases, leading, consequently, to an enhancement of the kinetics of the charge transfer process. According to thermodynamics the equilibrium of the redox process is shifted towards the formation of [Fe(CN)₆]³⁻ at elevated temperatures.