Thin film pyrolytic carbon electrodes: A new class of carbon electrode for electroanalytical sensing applications

This communication describes the electrochemical properties of thin pyrolytic carbon (PyC) films created using a reliable, non-catalytic chemical vapour deposition (CVD) process. After deposition, the electron transfer characteristics of the films are optimised using a simple oxygen plasma treatment. The redox probes Ru(NH₃)₆^3+/2+, Fe(CN)₆^3?/4? and Fe^3+/2+ are employed to demonstrate that the resulting material is endowed with a large electrochemical surface area and outstanding electron transfer properties. Atomic force microscopy (AFM), Raman and X-ray photoelectron spectroscopy (XPS) are used to elucidate the morphology and chemical composition of the electrode surfaces. This material represents a new class of carbon electrode, and its large densities of edge-plane sites and oxygenated functionalities make it an ideal candidate for electrochemical sensor applications.     

Electrochemiluminescence from Ru(bpy)3 immobilized in poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate)-poly(vinyl alcohol) composite films

Electrochemical behavior and electrogenerated chemiluminescence (ECL) of tris(2,2′-bipyridyl)ruthenium(II) (Ru(bpy)₃²⁺) immobilized in poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate)-poly(vinyl alcohol) (PEDOT/PSS-PVA) composite films via ion-exchange have been investigated with tripropylamine (TPA) as the co-reactant at a glassy carbon electrode. The immobilized Ru(bpy)₃²⁺ performed a surface-controlled electrode reaction. The Ru(bpy)₃²⁺ modified electrode showed a fast ECL response to TPA, and was used for the ECL detection of TPA with high sensitivity. The ECL intensity was linearly related to concentrations of TPA over the range from 0.50 μmol L⁻¹ to 0.80 mmol L⁻¹, and the detection limit was 0.10 μmol L⁻¹ (S/N = 3). The as-prepared electrode exhibited good precision and long-term stability for TPA determination.     

Quartz crystal microbalance monitoring of density changes in mesoporous TiO2 phytate films during redox and ion exchange processes

Nanofilm deposits of TiO₂ nanoparticle phytates are formed on gold electrode surfaces by ‘directed assembly’ methods. Alternate exposure of a 3-mercapto-propionic acid modified gold surface to (i) a TiO₂ sol and (ii) an aqueous phytic acid solution (pH 3) results in layer-by-layer formation of a mesoporous film. Ru(NH₃)₆³⁺ is shown to strongly adsorb/accumulate into the mesoporous structure whilst remaining electrochemically active. Scanning the electrode potential into a sufficiently negative potential range allows the Ru(NH₃)₆³⁺ complex to be reduced to Ru(NH₃)₆²⁺ which undergoes immediate desorption. When applied to a gold coated quartz crystal microbalance (QCM) sensor, electrochemically driven adsorption and desorption processes in the mesoporous structure become directly detectable as a frequency response, which corresponds directly to a mass or density change in the membrane. The frequency response (at least for thin films) is proportional to the thickness of the mass-responsive film, which suggests good mechanical coupling between electrode and film. Based on this observation, a method for the amplified QCM detection of small mass/density changes is proposed by conducting measurements in rigid mesoporous structures.     

Study on Electrochemiluminesce of Ru(bpy3)2+ Immobilized in a Titania Sol‐Gel Membrane

The immobilization of tris(2,2′‐bipyridyl)ruthenium(II), Ru(bpy)₃²⁺, at a glassy carbon electrode was achieved by entrapping the Ru(bpy)₃²⁺ in a vapor deposited titania sol‐gel membrane. The electrogenerated chemiluminescence (ECL) of the immobilized Ru(bpy)₃²⁺ was studied. The Ru(bpy)₃²⁺ modified electrode showed a fast ECL response to both oxalate and proline. The ECL intensity was linearly related to concentrations of oxalate and proline over the ranges from 20 to 700 μmol L^?1 and 20 to 600 μmol L^?1, respectively. The detection limits for oxalate and proline at 3σ were 5.0 μmol L^?1 and 4.0 μmol L^?1, respectively. This electrode possessed good precision and stability for oxalate and proline determinations. The electrogenerated chemiluminescence mechanism of proline system was discussed. This work provided a new way for the immobilization of Ru(bpy)₃²⁺ and the application of titania sol‐gel membrane in electrogenerated chemiluminescence.     

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.     

Spectrocyclic Voltammetry of Ruthenium Purple Electrodeposited on a WO3/Tris(2,2′-bipyridine)-ruthenium(II)/Polymer Hybrid Film

Summary: Electrochemical reactions of Ruthenium purple, Feequation/tex2gif-stack-1.gif[Ru^II(CN)₆]₃ (RP; Fe^III-Ru^II) were studied using a spectrocyclic voltammetry (SCV) technique. The SCV measurement for an RP film coated on an ITO electrode showed a reversible redox between RP and Ruthenium white (RW; Fe^II-Ru^II) at 0.14 V vs saturated calomel reference electrode (SCE). An RP film was electrodeposited on a hybrid film of tungsten trioxide (WO₃)/tris(2,2′-bipyridine)ruthenium(II) ([Ru(bpy)₃]²⁺; bpy = 2,2′-bipyridine)/poly(sodium 4-styrenesulfonate) (PSS) (denoted as WRP film) from a colloidal solution containing 0.5 mM FeCl₃, 0.5 mM K₄[Ru(CN)₆] and 40 mM KCl using a potentiodynamic multi-sweep technique. In a cyclic voltammogram (CV) of a WRP/RP film, a redox response was observed at 0.61 V in addition to essential redox responses of WRP hybrid film (a [Ru(bpy)₃]²⁺/[Ru(bpy)₃]³⁺ redox at 1.03 V and a H_xWO₃/WO₃ redox below 0.09 V), but a redox response of RW/RP was not observed at 0.14 V. The SCV measurement for the WRP/RP film suggested that the redox response at 0.61 V is attributed to a redox of [Ru(bpy)₃]²⁺/[Ru(bpy)₃]³⁺ interacted electrostatically with RP. It also showed that RW is oxidized to RP via [Ru(bpy)₃]²⁺/[Ru(bpy)₃]³⁺ redox and RP is reversibly reduced to RW via H_xWO₃/WO₃ redox. This unique geared electrochemical reaction for the WRP/RP film leads to a hysteresis property of an RW/RP redox.     

A label-free electrochemical biosensor for trace uranium based on DNAzymes and gold nanoparticles

A novel and highly sensitive electrochemical DNAzymes biosensor was fabricated using Au nanoparticles (AuNPs) immobilized on the surface of Au electrode that had been previously modified with self-assembled monolayers of 1,6-hexanedithiol. Different modified electrodes were prepared and characterized by cyclic voltammetry and electrochemical impedance spectroscopy. The AuNPs were found to have a large surface area to anchor a large number of negatively charged phosphate backbones of DNAzymes, which further absorbed the electroactive indicator of hexaammineruthenium(III) ([Ru(NH₃)₆]³⁺) to amplify the electrochemical signal. In the presence of target molecules, a large amount of DNA partly associated with [Ru(NH₃)₆]³⁺ were removed from the electrode surface, leading to a significant decrease in peak current. Differential pulse voltammetry signals of [Ru(NH₃)₆]³⁺ provided quantitative measures of the concentrations of uranyl ion (UO₂ ²⁺), with linear calibration ranging from 13 pM to 0.15 nM and a detection limit of 5 pM. The presence of other metal ions did not affect the detection of UO₂ ²⁺, which indicated the high specificity of UO₂ ²⁺. Therefore, a new electrochemical DNAzymes sensor was designed with specific DNAzymes and AuNPs as immobilization platform and signal amplifier.     

Assessing a Spectroelectrochemical Sensor’s Performance for Detecting [Ru(bpy)3]2+ in Natural and Treated Water

A spectroelectrochemical sensor that combines three modes of selectivity in a single device was evaluated in natural and treated water samples using tris‐(2,2′‐bipyridyl) ruthenium(II) dichloride hexahydrate, [Ru(bpy)₃]²⁺, as a model analyte. The sensor was an optically transparent indium tin oxide (ITO) electrode coated with a thin film of partially sulfonated polystyrene‐block‐poly(ethylene‐ran‐butylene)‐block‐polystyrene (SSEBS). As the potential of the ITO electrode was cycled from +0.7 to +1.3 V, the analyte changed from the colored [Ru(bpy)₃]²⁺ complex to colorless [Ru(bpy)₃]³⁺ complex and the change in absorbance at 450 nm was used as the optical signal for quantification. Calibration curves were obtained for [Ru(bpy)₃]²⁺ in natural well water, river water and treated tap water with detection limits of 108, 139 and 264 nM, respectively. A standard addition method was developed to determine an ‘unknown’ spike addition concentration of [Ru(bpy)₃]²⁺ in well water. The spectroelectrochemical sensor determined the concentration of [Ru(bpy)₃]²⁺ spiked into a sample of Hanford well water to be 0.39±0.03 µM versus the actual concentration of 0.40 µM.     

Electrochemiluminescence at nitrogen doped diamond-like carbon film electrodes

In the given paper the results of investigation of nitrogen doped diamond-like carbon films (DLF) for the fabrication of electrodes of electrochemical and electrochemiluminescent (ECL) cells are presented. The surface nanomorphology of modified electrodes was studied using the atomic-force microscopy method. The mechanisms of ECL reactions with Ru(bpy) ₃ ¹⁺ /Ru(bpy) ₃ ³⁺ (recombination Ru(bpy) ₃ ³⁺ /OH^? (ECL without co-reactant) and Ru(bpy) ₃ ³⁺ /tripropylamine (ECL with co-reactant) were studied on DLF modified electrodes in the aqueous solution. The work shows advantages of DLF as an electrode material for further analytical applications.     

Redox properties of bis(1,10-phenanthroline)-(pyridine)ruthenium(II) complexes

The redox properties of a series of [Ru(phen)₂(py)X]ⁿ⁺ cations (X = pyridine, NH₃, Cl, Br, I, CN, SCN, N₃ and NO₂) have been investigated in acctonitrile. Two reversible reduction steps are seen at ? 1.35 and ? 1.6 V vs Ag/AgCl; the invariance of these processes with X-group is indicative of electron addition to molecular orbitals mainly of phenanthroline ligand π^* origin. Irreversible multi-electron reductions follow below ? 2.20 V. The Ru(II)/Ru(III) couple is seen as a reversible wave near + 0.8 V vs the normal hydrogen electrode, from calibration with ferrocene, except in the cases of the NO₂ and SCN complexes, where rapid reactions involving these ligands occur.     

Solid-state electrochemiluminescence sensor through the electrodeposition of Ru(bpy)3/AuNPs/chitosan composite film onto electrode

Tris(2,2′-bipyridyl)ruthenium(II) (Ru(bpy)₃²⁺) has been successfully immobilized onto electrode through the electrodeposition of Ru(bpy)₃²⁺/AuNPs/chitosan composite film. In the experiments, chitosan solution was first mixed with Au nanoparticles (AuNPs) and Ru(bpy)₃²⁺. Then, during chronopotentiometry experiments in this mixed solution, a porous 3D network structured film containing Ru(bpy)₃²⁺, AuNPs and chitosan has been electrodeposited onto cathode due to the deposition of chitosan when pH value is over its pK_a (6.3). The applied current density is crucial to the film thickness and the amount of the entrapped Ru(bpy)₃²⁺. Additionally, these doping Ru(bpy)₃²⁺ in the composite film maintained their intrinsic electrochemical and electrochemiluminescence activities. Consequently, this Ru(bpy)₃²⁺/AuNPs/chitosan modified electrode has been used in ECL to detect tripropylamine, and the detection limit was 5 × 10⁻¹⁰ M.     

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.     

Silver(I) ions-enhanced electrochemiluminescence of tris(2,2-bipyridyl)ruthenium(II)

In this paper, we demonstrate an electrochemiluminescence (ECL) enhancement of tris(2,2-bipyridyl)ruthenium(II) (Ru(bpy)₃²⁺) by the addition of silver(I) ions. The maximum enhancement factor of about 5 was obtained on a glassy carbon electrode in the absence of co-reactant. The enhancement of ECL intensity was possibly attributed to the unique catalytic activity of Ag⁺ for reactions between Ru(bpy)₃³⁺ with OH. The higher enhancement was observed in phosphate buffer solutions compared with that from borate buffer solutions. This resulted from the fact that formation of nanoparticles with large surface area in the phosphate buffer solution exhibited high catalytic activity. The amount of Ag⁺, solution pH and working electrode materials played important roles for the ECL enhancement. We also studied the effects of Ag⁺ on Ru(bpy)₃²⁺/tripropylamine and Ru(bpy)₃²⁺/C₂O₄²⁻ ECL systems.     

Effects of Divalent Metal Ions on Electrochemiluminescence Sensor with Ru(bpy)32+ Immobilized in Eastman‐AQ Membrane

Different effects of divalent metal ions on electrochemiluminescence (ECL) sensor with Ru(bpy)₃²⁺ immobilized in Eastman‐AQ membrane were investigated. Mg²⁺, Ca²⁺ and Fe²⁺ can elevate the ECL of Ru(bpy)₃²⁺/proline; while metal ions that underwent redox reactions on the electrode such as Mn²⁺ and Co²⁺ presented intensive quenching effects on Ru(bpy)₃²⁺ ECL. Also, the quenching effect of Mn²⁺ on the ECL sensor with Ru(bpy)₃²⁺ immobilized in Eastman‐AQ membrane enhanced to about 30‐folds compared with the case that Ru(bpy)₃²⁺ was dissolved in phosphate buffer, and the enhanced quenching effects of Mn²⁺ were studied.     

Titanium Carbide Thin‐Film Electrodes: Characterization and Evaluation as Working Electrodes

Titanium carbide (TiC) polycrystalline thin films, obtained by a hybrid chemical vapor deposition/powder flowing technique, were characterized and used to assemble working electrodes. The potential window and the electrochemistry of standard redox couples ([Ru(NH₃)₆]³⁺, [Fe(CN)₆]^3?) have been investigated in cyclic voltammetry, demonstrating a behavior similar to glassy carbon electrodes. The quinone (Q)/hydroquinone(H₂Q) redox couple presented an interesting quasireversible behavior(E_p=0.07 mV) , confirmed also for other quinones.     

A Zeolite‐Supported Molecular Ruthenium Complex with η6‐C6H6 Ligands: Chemistry Elucidated by Using Spectroscopy and Density Functional Theory

An essentially molecular ruthenium–benzene complex anchored at the aluminum sites of dealuminated zeolite Y was formed by treating a zeolite‐supported mononuclear ruthenium complex, [Ru(acac)(η²‐C₂H₄)₂]⁺ (acac=acetylacetonate, C₅H₇O₂^?), with ¹³C₆H₆ at 413 K. IR, ¹³C NMR, and extended X‐ray absorption fine structure (EXAFS) spectra of the sample reveal the replacement of two ethene ligands and one acac ligand in the original complex with one ¹³C₆H₆ ligand and the formation of adsorbed protonated acac (Hacac). The EXAFS results indicate that the supported [Ru(η⁶‐C₆H₆)]²⁺ incorporates an oxygen atom of the support to balance the charge, being bonded to the zeolite through three Ru? O bonds. The supported ruthenium–benzene complex is analogous to complexes with polyoxometalate ligands, consistent with the high structural uniformity of the zeolite‐supported species, which led to good agreement between the spectra and calculations at the density functional theory level. The calculations show that the interaction of the zeolite with the Hacac formed on treatment of the original complex with ¹³C₆H₆ drives the reaction to form the ruthenium–benzene complex.     

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.     

Ultrasensitive electrochemiluminescent aptasensor for ochratoxin A detection with the loop-mediated isothermal amplification

In this study, we for the first time presented an efficient, accurate, rapid, simple and ultrasensitive detection system for small molecule ochratoxin A (OTA) by using the integration of loop-mediated isothermal amplification (LAMP) technique and subsequently direct readout of LAMP amplicons with a signal-on electrochemiluminescent (ECL) system. Firstly, the dsDNA composed by OTA aptamer and its capture DNA were immobilized on the electrode. After the target recognition, the OTA aptamer bond with target OTA and subsequently left off the electrode, which effectively decreased the immobilization amount of OTA aptamer on electrode. Then, the remaining OTA aptamers on the electrode served as inner primer to initiate the LAMP reaction. Interestingly, the LAMP amplification was detected by monitoring the intercalation of DNA-binding Ru(phen)₃²⁺ ECL indictors into newly formed amplicons with a set of integrated electrodes. The ECL indictor Ru(phen)₃²⁺ binding to amplicons caused the reduction of the ECL intensity due to the slow diffusion of Ru(phen)₃²⁺–amplicons complex to the electrode surface. Therefore, the presence of more OTA was expected to lead to the release of more OTA aptamer, which meant less OTA aptamer remained on electrode for producing LAMP amplicons, resulting in less Ru(phen)₃²⁺ interlaced into the formed amplicons within a fixed Ru(phen)₃²⁺ amount with an obviously increased ECL signal input. As a result, a detection limit as low as 10 fM for OTA was achieved. The aptasensor also has good reproducibility and stability.     

Tris(2,2′-bipyridyl)ruthenium(II) electrogenerated chemiluminescence sensor based on carbon nantube dispersed in sol-gel-derived titania-Nafion composite films

A highly sensitive and stable tris(2,2′-bipyridyl)ruthenium(II) (Ru(bpy)₃²⁺) electrogenerated chemiluminescence (ECL) sensor was developed based on carbon nanotube (CNT) dispersed in mesoporous composite films of sol-gel titania and perfluorosulfonated ionomer (Nafion). Single-wall (SWCNT) and multi-wall carbon nanotubes (MWCNT) can be easily dispersed in the titania-Nafion composite solution. The hydrophobic CNT in the titania-Nafion composite films coated on a glassy carbon electrode certainly increased the amount of Ru(bpy)₃²⁺ immobilized in the ECL sensor by adsorption of Ru(bpy)₃²⁺ onto CNT surface, the electrocatalytic activity towards the oxidation of hydrophobic analytes, and the electronic conductivity of the composite films. Therefore, the present ECL sensor based on the CNT-titania-Nafion showed improved ECL sensitivity for tripropylamine (TPA) compared to the ECL sensors based on both titania-Nafion composite films without CNT and pure Nafion films. The present Ru(bpy)₃²⁺ ECL sensor based on the MWCNT-titania--Nafion composite gave a linear response (R² = 0.999) for TPA concentration from 50 nM to 1.0 mM with a remarkable detection limit (S/N = 3) of 10 nM while the ECL sensors based on titania-Nafion composite without MWCNT, pure Nafion films, and MWCNT-Nafion composite gave a detection limit of 0.1 μM, 1 μM, and 50 nM, respectively. The present ECL sensor showed outstanding long-term stability (no signal loss for 4 months).     

Fe(phen)32+‐Modified Zeolite Particles and Their Energetic Studies

Chemically modified zeolite Y (NaY) particles and their resulting modified electrodes were prepared with acridinium (AcH+), iron(II) and 1,10‐phenanthroline (phen) for energetic studies. According to diffuse reflectance absorption spectroscopy and cyclic voltammetry, AcH⁺ and Fe(phen)₃²⁺ were successfully entrapped in the zeolite particles. Transient emission spectra measurements showed that the life time of AcH+* in the zeolite particles (to 35 ns; λ_ex 365 nm; λ_em 500 nm) was greatly reduced upon incorporating Fe(phen)₃²⁺ and Fe²⁺. The fast de cay of AcH+*(NaY) suggested that a reductive quench was likely to take place in the zeolite particle. Probably due to a size‐exclusion effect, the bulky electron donor, N, N‐diethyl‐2‐methyl‐1,4‐phenylenediamine (DEPD), revealed a difficulty in reaching the photosensitizer, AcH⁺, in side the zeolite particle. As a consequence, the in significant photocurrent for the oxidation of DEPD was from the NaY|AcH⁺ electrode. However, if Fe²⁺ and Fe(phen)₃²⁺ were incorporated, the photocurrent would become more significant. Closer examinations, in addition, showed that the photooxidaton of DEPD occurred more rapidly on the NaY|AcH⁺|Fe(phen)₃²⁺ electrode, compared to the NaY|AcH⁺|Fe²⁺ electrode. This difference apparently results from a greater gap in energetics between DEPD and Fe(phen)₃³⁺_(NaY) than that between DEPD and Fe³⁺_(NaY). Due to this effect, a greater amount of indophenol blue, derived from the coupling reaction of the oxidized DEPD with 1‐naphthol, was formed and de posited on the NaY|AcH⁺|Fe(phen)₃²⁺modified electrode. Thanks to this photo‐induced charge‐transfer reaction, the NaY|AcH⁺|Fe(phen)₃²⁺ particle showed an application potential in image recording.