Oxide-covered Aluminum Electrodes Research Articles

Hot electron-induced electrochemiluminescence of calcein and calcein-Tb(III) complex at disposable oxide-covered aluminum and polyvinyl butyral-carbon black/metal composite electrodes in aqueous solutions

Hot electron-induced electrochemiluminescence (HECL) of calcein and calcein-Tb(III) complex was generated at oxide-covered aluminum electrode during cathodic pulse polarization. The excitation of calcein as a molecule or as a ligand is based on subsequent one-electron oxidation and reduction steps by oxidizing radicals and solvated electrons. During HECL excitation of calcein-Tb(III) solution a reaction product of the calcein is formed that also enables the photoexcitation of Tb(III) via the formed ligand derivative by ligand-sensitized mechanism. The determination of low concentrations of calcein with aluminum electrodes was complicated by a relatively strong background electroluminescence originating from the Al2O3-layer. Polyvinyl butyral-carbon black composite electrodes coated on brass were fabricated to solve this problem and a fifty-fold lower background emission was obtained for these novel composite electrodes in comparison to that of oxide-covered 99.9% pure aluminum electrodes. The obtained detection limits were 3.2·10−10 M for calcein and 6.4·10−9 M for calcein-Tb(III) at the present composite electrodes. These species could potentially be utilized as electrochemiluminescent labels in bioaffinity assays.

Heterogeneous time-resolved electrochemiluminoimmunoassay of thyroid stimulating hormone with magnetic beads at oxide-covered aluminum electrode

The heterogeneous immunoassay of thyroid stimulating hormone (TSH) was detected by the time-resolved cathodic electrochemiluminescence immunoassay (tr-CECLIA) method using magnetic beads as a mobile support. The magnetic beads coated with sandwich type TSH immunoassay were captured with a magnet for washing and detection processes. The time-resolved cathodic electrochemiluminescence (tr-CECL) signal of Tb(III) chelate label was generated by cathodic pulse polarization in the aluminum working electrode and platinum counter electrode system. The detection method causes injection of high energy electrons into the aqueous solution near the aluminum electrode and creates rigid simultaneous oxidative and reductive conditions that excitate the Tb(III) chelate used as a label luminophore in the heterogeneous sandwich type immunoassay of TSH in the surface of the magnetic beads. The limit of detection of the method was about 50mIUL−1. The precision of it was noticed to be good; the coefficient variation percentage was realized to be lower than 10 %. Unfortunately the limit of detection is not good enough for determination of analyte levels of very low concentrations for instance TSH in body fluids. The possible application areas of the method are in highly sophisticated micro or nano fluidic detection and sensor systems where the aspiration level of the analyte detection limit is not very high and the mobility and manageability and the large surface area of the magnetic beads can be utilized efficiently in separating, washing, moving, coating and detecting processes. In the case of tr-CECLIA the presented method makes possible to use multipurpose working electrodes instead of disposable ones.

Printable Hecl Electrode Materials for Disposable Bioaffinity Assay Cartridges

We have demonstrated earlier that hot electrons can be injected into aqueous electrolyte solution from thin insulating film coated cathodes [1, 2]. Insulating film have typically been fabricated from an insulator material with a band gap of ca. 5 eV or preferably higher, with the Fermi level of the insulator somewhere in the mid band gap region. The hot electrons are emitted to aqueous electrolyte solution by direct field-assisted tunneling at these devices [2, 3]. After injection into the conduction band of water the hot electrons are thermalized and solvated to form hydrated electrons which finally act as reducing mediators in the Hot Electron-induced Electrochemiluminescence (HECL) systems. HECL is utilized mainly in immunoanalysis and in DNA-probe assays in which electrochemiluminescent labels are utilized [ 2 - 5 ]. Best labels are certain aromatic lanthanide chelates which allow sensitive time-resolved HECL detection of the labels [3, 6]. Silicon electrodes or silicon chips on which the working electrode/electrodes and counter electrode have been integrated have been shown to be excellent functional components of disposable assay cartridges [5,7]. Recently, we have been able to develop printable HECL electrode materials that show almost equal performance and are based on conductor particles embedded in polystyrene or in some other organic insulating materials.

Electrochemiluminescence Study of Europium (III) Complex with Coumarin3-Carboxylic Acid

The europium (III) complex of coumarin-3-carboxylic acid (C3CA) has been prepared and characterized on the basis of elemental analysis, IR, and emission (photoluminescence and electrochemiluminescence) spectroscopy. The synthesised complex having a formula Eu was photophysically characterized in solution and in the solid state. Electrochemiluminescence, ECL, of the system containing the Eu(III)/C3CA complex was studied using an oxide-covered aluminium electrode. The goal of these studies was to show the possibility of the use of electrochemical excitation of the Eu(III) ion in aqueous solution for emission generation. The generated ECL emission was very weak, and therefore its measurements and spectral analysis were carried out with the use of cut-off filters method. The studies proved a predominate role of the ligand-to-metal energy transfer (LMET) in the generated ECL.

Electrogenerated luminescence of chosen lanthanide complexes at stationary oxide-covered aluminium electrode

The electrochemiluminescence (ECL) of aqueous solutions of Tb3+, Dy3+, and Eu3+ complexes having a variety of ligand groups was studied using an oxide-covered aluminium electrode. The ligand groups, under study, were the aromatic acids (salicylic, phthalic), the chelatic ligands (ethylenediamine dl(o-hydroxy-phenylacetic acid), EDDHA and ethylenediamine tetraacetic acid, EDTA), as well as Schiff bases: 1,10-disalicylidene-4,7-diaza-1,10-decyldiamine and 2-salicylideneamine-2-hydroxymethyl-1,3-propanediol. The results show that the generated emissions were mainly the result of energy transfer from the ligands to the metals. The best ECL properties were observed in the case of the complexes Tb(III)–EDDHA, Dy(III)–EDDHA, and Dy(III)–salicylic acid. In the ternary systems: Schiff base–Tb(III)–Eu(III) energy transfer to the emitting level of the Eu(III) ion was observed.

Electrochemiluminescence on Dy(III) and Tb(III)-doped Al/Al 2O 3 surface electrode

The electrochemiluminescence (ECL) of aqueous electrolyte solutions containing EDDHA and an oxide-covered aluminum electrode doped with Dy(III) or Tb(III) during the cathodic pulse-polarisation was investigated. Injection of tunnel-emitted hot electrons from the electrode into complexes of solutions containing S 2 O 8 2 - , induces ECL, as observed in the 5D 4 → 7F J transitions in Tb(III) and 4F 9/2 → 6H 15/2 and 4F 9/2 → 6H 13/2 in Dy(III) ions. Specific electrogenerated luminescence was obtained by producing highly oxidizing and reducing species as: hydrated electrons, hydroxyl and sulfate radicals. These strong red-ox reactants efficiently excite the complexed Tb(III) and Dy(III) ions by ligand → metal energy transfer.

Generation of free radicals and electrochemiluminescence from simple aromatic molecules in aqueous solutions

Oxide-covered aluminium electrodes were used to demonstrate that aromatic compounds, such as the simple derivatives of benzene, can be electrochemically excited at cathodically pulse-polarized conductor/insulator/electrolyte (C/I/E) tunnel junction electrodes (e.g. oxide-covered aluminium electrodes). The primary cathodic process at these electrodes was a tunnel emission of hot electrons into an aqueous electrolyte solution. Fluorescence (FL) and electrochemiluminescence (ECL) spectra were compared and the dependence of the electrochemiluminescence on the concentrations of benzene, toluene, phenol, p-cresol and aniline were measured and detailed mechanisms for the present electrochemiluminescence are proposed.

Time-Resolved Detection of Hot Electron-Induced Electrochemiluminescence of Fluorescein in Aqueous Solution

Strong electrogenerated chemiluminescence (ECL) of fluorescein is generated during cathodic pulse polarization of oxide-covered aluminum electrodes and the resulting decay of emission is so sluggish that time-resolved detection of fluorescein is feasible. The present ECL in aqueous solution is based on the tunnel emission of hot electrons into the aqueous electrolyte solution, which probably results in the generation of hydrated electrons and hydroxyl radicals acting as redox mediators. The successive one-electron redox steps with the primary radicals result in fluorescein in its lowest excited singlet state. The method allows the detection of fluorescein (or its derivatives containing usable linking groups to biomolecules) over several orders of magnitude of concentration with detection limits well below nanomolar concentration level. The detection limits can still be lowered, e.g., by addition of azide or bromide ions as coreactants. The results suggest that the derivatives of fluorescein, such as fluorescein isothiocyanate (FITC), can be detected by time-resolved measurements and thus be efficiently used as electrochemiluminescent labels in bioaffinity assays.

Hot electron-induced time-resolved electrogenerated chemiluminescence of a europium(III) label in fully aqueous solutions

Time-resolved electrogenerated chemiluminescence of multidentate phenolic Eu(III) chelates were studied in aqueous solution. 2,6-bis[ N, N-bis(carboxymethyl)-aminomethyl]-4-benzoylphenol forms a photoluminescent and electrochemiluminescent Eu(III) chelate, whereas 2,6-bis[ N, N-bis(carboxymethyl)-aminomethyl]-4-methyl phenol-chelated Eu(III) turned out to be not luminescent at all. The importance of the redox properties of both the ground and the excited states of the ligands and the central ion is shown. The former chelate shows relatively weak ECL at an oxide-covered aluminum electrode but the ECL intensity can be strongly enhanced by the addition of peroxodisulfate ions. In the presence of 1 mM peroxodisulfate ions the ECL lifetime of this chelate is 0.94 ms, thus easily allowing time-resolved detection of the chelate. This chelate can be conjugated to antibodies by thioureido linkage and used as an electrochemiluminescent label in immunoassays as a marker which displays long-lived luminescence in the red end of the optical spectrum. The present ECL is mainly based on the ligand sensitized redox excitation of the chelate by analogous pathways to those known from the studies of aromatic Tb(III) chelates but the energy transfer from the emission centers of the aluminum oxide film can also have minor contribution to the excitation of the label.

Hot electron-induced electrochemiluminescence of fluorescein in aqueous solution

Strong electrogenerated chemiluminescence (ECL) of fluorescein is generated in aqueous solution during cathodic pulse polarization of oxide-covered aluminum electrodes. The present method is based on the injection of hot electrons into the aqueous electrolyte solution, which probably results in the generation of hydrated electrons as reducing mediators. The successive one-electron redox reactions result in fluorescein in its lowest excited singlet state, i.e., the emission spectrum is similar to the corresponding fluorescence emission spectrum. This ECL can be enhanced by added coreactants, e.g., by peroxodisulfate, peroxodiphosphate, hydrogen peroxide and azide ions. The method can detect fluorescein over several orders of magnitude of concentration with detection limit below nanomolar concentration level in the presence of azide ions. The results suggest that the derivatives of fluorescein, such as fluorescein isothiocyanate, can be used as electrochemiluminescent labels in aqueous solution in bioaffinity assays carried out on the surface of thin aluminum oxide film-coated aluminum electrodes. This cathodic ECL is mainly based on the one-electron oxidation of fluorescein by the cathodically produced oxidizing radicals followed by reduction of the formed semioxidized fluorescein by hot or hydrated electrons back to the original oxidation state which finally emits light.

Cathodic electrogenerated chemiluminescence of Ru(bpy) 32+ chelate at oxide-coated heavily doped silicon electrodes

High amplitude cathodic pulse polarization of ultra thin oxide film-coated heavily doped silicon electrodes induces tunnel emission of hot electrons into aqueous electrolyte solution, which probably results in the generation of hydrated electrons in the vicinity of the electrode surface. The method allows the detection of tris (2,2′-bipyridine) ruthenium(II) chelate at subnanomolar concentration level. This paper shows that both n- and p-type heavily doped silicon electrodes can be used, illustrates the effect of oxide film thickness upon the silicon electrode on the intensity of ECL of tris (2,2′-bipyridine) ruthenium(II) and discusses the basic features of tris (2,2′-bipyridine) ruthenium(II) chelate-specific ECL at these electrodes. Thin oxide film-coated silicon electrodes provide a lower blank emission and a higher ECL intensity of the present ruthenium chelate than oxide-covered aluminium electrodes. This suggests that thin oxide film-coated silicon is a very promising working electrode material, especially in microanalytical systems made fully or partly of silicon.

Influence of Local Heat Development on Film Thickness for Anodizing Aluminum in Sulfuric Acid

Joule heating of the oxide-covered aluminum electrode is the major heat source during anodizing, and the extent of heat removal is dependent on the electrolyte flow i.e., convection. For various galvanostatic and potentiostatic anodizing conditions, at different bulk electrolyte temperatures it was found that when the convection heat transfer is not uniform along the electrode surface, i.e., in a wall-jet electrode reactor, a local temperature distribution is established. Due to this nonuniform temperature, the anodic film exhibits a thickness distribution. Further, at higher current densities or lower bulk electrolyte temperatures, anomalous anodizing behavior occurs under certain galvanostatic conditions, associated with spots of increased oxide thickness and high-temperature peaks. By associating the local film thickness to the local temperature distribution, it was found that the higher the local temperature, the greater the local film thickness, implying a higher local oxidation current. This relation is explained by considering thermal enhancement of the field-assisted oxide dissolution at the pore bases, which necessitates a local current density rise to maintain the local anodizing equilibrium. Using transmission electron microscopy, cyclic voltammetry, and capacitance measurements, the previous was confirmed from measurement of an unchanged barrier layer thickness on the anodized aluminum specimen. © 2003 The Electrochemical Society. All rights reserved.

Heterogeneous and homogeneous electrochemiluminoimmunoassays of hTSH at disposable oxide-covered aluminum electrodes

Heterogeneous and homogeneous immunoassays of human thyroid stimulating hormone (hTSH) were developed on immunometric basis using aromatic Tb(III) chelates as electrochemiluminescent labels and varied types of disposable oxide-covered aluminum electrodes as the solid phase of the immunoassays. The long luminescence lifetime of the present labels allows the use of time-resolved electrochemiluminescence detection and provide the low detection limits of these labels and, thus, sensitive immunoassays. The primary antibody of immunometric immunoassays was coated upon aluminum oxide surface by physical absorption. In homogeneous immunoassays using 66 μl cell and 15 min incubation time, a linear calibration range of 0.25–324 μU/ml was obtained by applying only a single cathodic excitation pulse in the detection step of the assay.

Hot electron-induced electrogenerated chemiluminescence of 1-aminonaphthalene-4-sulphonate at oxide-covered aluminium electrodes in aqueous solution

Electrogenerated chemiluminescence of 1-aminonaphthalene-4-sulphonate (ANS) provides a sensitive means for the detection of the ANS in aqueous solution when oxide-covered aluminium electrodes are used as tunnel emission electrodes and cathodic pulse polarisation is used for the excitation of the luminophore. During the pulse polarisation of insulating oxide film-coated aluminium cathodes, hot electrons are tunnel emitted from the aluminium to the aqueous electrolyte solution by direct field-assisted tunnelling (in the case of oxide films of thickness 2–6 nm) or by Fowler–Nordheim (FN) tunnelling in the cases of thicker oxide films. As a result of direct tunnel emission of these energetic electrons, the generation of hydrated electrons (e aq −) becomes possible. These electrochemically generated, extremely strong reductants (dry hot or hydrated electrons) make the efficient excitation of various types of luminophores at thin insulating film-covered electrodes possible and provide a means for sensitive immunoassays and DNA-probing assays when these luminophores are used as label molecules.

Time-resolved electrochemiluminescence of platinum(II) coproporphyrin

Cathodic pulse polarisation of oxide-covered aluminium electrodes can generate electrochemiluminescence (ECL) from metalloporphyrins. This is based on the tunnel emission of hot electrons into aqueous electrolyte solution, which probably results in the generation of hydrated electrons as reducing mediators. These tunnel emitted electrons allow the production of highly reactive radicals, such as sulfate radicals from peroxodisulfate ions, which can induce strong redox luminescence from various organic chemiluminophores including metalloporphyrins. The work presented here illustrates the generation of ECL from platinum(II) coproporphyrin (PtCP) and its bovine serum albumin (BSA) conjugate. This allows the detection of these molecules below nanomolar concentrations and over several orders of magnitude of concentration. The relatively long luminescence lifetime of PtCP allows discrimination from the background ECL signal using time resolved measurements, leading to higher sensitivity and the detection of PtCP-BSA indicates the potential use of metalloporphyrins as labels in ECL-based bioassays such as immunoassays and DNA-binding assays.

Study on the radiative [formula omitted] relaxation dynamics of Tb(III) in electrochemically excited self-assembled dimeric heterodinuclear Tb(III)–Ln(III)′ chelates

A study has been conducted to explore the inter-lanthanide(III) energy interactions in self-assembled dimeric heterodinuclear 1-Tb(III)–Ln(III)′- 1 chelates (Ln(III)′ = Dy(III), Nd(III), Y(III), Yb(III)) ( 1 = 2,6-bis[ N, N-bis(carboxymethyl)-aminomethyl]-4-benzoylphenol]) and their effects on the electrochemically initiated radiative 5 D 4→ 7 F j transitions of Tb(III) of these dinuclear chelates at a cathodically pulse-polarized oxide-covered aluminium electrode. From the selected heterodinuclear Tb(III)–Ln(III)′ edifices, (i) the Tb(III)–Dy(III) edifice initiates the exceptionally short-lived 5 D 4→ 7 F j transitions of Tb(III) with k i ≈ 9.0 ms −1 mainly due to the competitive inter-lanthanide(III) Tb(III)–Dy(III) 5 D 4→ 4 F 9/2 back energy transfer, (ii) likewise, the Tb(III)–Y(III) edifice initiates the 5 D 4→ 7 F j transitions of Tb(III) with k i ≈ 1.4 ms −1 caused by the absence of any Y(III)-specific energy level capable of acting as the acceptor level for the ligand-specific 3ππ * and/or Tb(III)-specific 5 D 4 relaxations and, finally, (iii) the Tb(III)–Nd(III) and Tb(III)–Yb(III) edifices do not generate these radiative Tb(III) specific transitions; in the case of Ln(III)′ = Nd(III), this inhibition originates from the preferential ligand-specific 3ππ * relaxation to the Nd(III)-specific acceptor levels while in the case of Ln(III)′ = Yb(III), this 3ππ * relaxation seems to occur directly to the excited 2 F 5/2 state of Yb(III) by the ET intermediate.

Hot Electron-Induced Electrogenerated Chemiluminescence of Ru(bpy)32+ Chelate at Oxide-Covered Aluminum Electrodes

Ruthenium(II) tris-(2,2‘-bipyridine) chelate exhibits strong electrogenerated chemiluminescence during cathodic pulse polarization of oxide-covered aluminum electrodes in aqueous solutions. The present method is based on a tunnel emission of hot electrons into an aqueous electrolyte solution. The method allows the detection of ruthenium(II) tris-(2,2‘-bipyridine) and its derivatives below nanomolar concentration levels and yields linear log−log calibration plots spanning several orders of magnitude of concentration. This method allows simultaneous excitation of derivatives of ruthenium(II) tris-(2,2‘-bipyridine) and Tb(III)-chelates. The former label compounds have a luminescence lifetime of the order of microseconds, while the latter compounds generally have a luminescence lifetime of around 2 ms. Thus, the combined use of these labels easily provides the basis for two-parameter bioaffinity assays by either using wavelength or time discrimination or their combination.

Hot electron-induced electrogenerated luminescence of Tl(I) at disposable oxide-covered aluminum electrodes

Cathodic pulse polarisation of oxide-covered aluminum electrodes in Tl(I) solutions induce strong Tl(I)-specific electrogenerated luminescence (EL). It is mainly regarded as electrochemiluminescence, induced by one-electron oxidation of cathodically produced thallium atoms in the close vicinity of the electrode surface, but solid state electroluminescence is also produced, especially, with thicker oxide films. Depending on the EL system applied, the oxidant in response of the excitation event is either a cathodically produced hydroxyl or sulfate radical, or an F +-centre of the thin oxide film, and the source of short-lived thallium atom colloids is a reduction of Tl(I) ions by tunnel-emitted hot electrons and/or cathodically generated hydrated electrons. The present method allows the detection of Tl(I) ions below nanomolar concentration level and provides linear log-log calibration graphs spanning several orders of magnitude of concentration of Tl(I).

Electrochemiluminescent labels for applications in fully aqueous solutions at oxide-covered aluminium electrodes

Oxide-covered aluminium electrodes as well as other tunnel emission electrodes allow various label molecules having very different redox and optical properties to be excited cathodically. Low detection limits are obtained and the linear calibration concentration range of the labels spans 5 or 6 orders of magnitude. The lowest detection limits are obtained with Tb(III) chelates which can be detected down to picomolar levels in aqueous solution using time-resolved measurement techniques. Luminophores, such as, 9-fluorenylmethyl chloroformate, derivatives of fluorescein and its analogues, aromatic lanthanide(III) chelates, various coumarins and porphyrins can be used as labels emitting in different spectral regions. The extraordinary analytical power of the tunnel emission electrodes lies in the possibility of simultaneously exciting several different labels emitting either in the UV, visible or NIR range and luminescence lifetimes varying from the ns to the ms range. Therefore, wavelength or time discrimination or their combination can be exploited in separation of the electrochemiluminescence signals from different labels.

Primary cathodic steps of electrogenerated chemiluminescence of lanthanide(III) chelates at oxide-covered aluminum electrodes in aqueous solution

The primary processes occurring at cathodically polarized oxide-covered aluminum electrode are discussed in detail. It is pointed out that more energetic cathodic processes can be induced in aqueous media at thin insulating film-coated electrodes than at any semiconductor or active metal electrode. It is proposed that tunnel emission of hot electrons with energies well above the level of the conduction band edge of water occur, and the thermalization and solvation of the emitted electrons can result in generation of hydrated electrons. The cathodically pulse-polarized oxide-covered aluminum also generates a strong oxidant (or oxidants) at the oxide/electrolyte interface, and it is proposed that this species is the hydroxyl radical which is generated either by cathodic high field-induced ejection of self-trapped holes as oxygen dianions (i.e. oxide radical ions) into the electrolyte solution, or by the action of anion vacancies and/or F +-centers as the primary oxidants capable of oxidizing hydroxide ions or the hydroxyl groups of the hydroxylated surface on the oxide film. These radicals, hydrated electrons/hydroxyl radicals, can act as mediating reductants/oxidants in reduction/oxidation of solutes. The formation of the primary species is monitored by electrochemiluminophores which cannot be cathodically excited at active metal electrodes in fully aqueous solutions, but which can be chemically excited in aqueous media in the simultaneous presence of highly reducing and highly oxidizing radicals.