T. Ala-Kleme

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

Abstract 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.

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 66l cell and 15 min incubation time, a linear calibration range of 0.25-324U/ml was obtained by applying only a single cathodic excitation pulse in the detection step of the assay.

Hot Electron Injection into Aqueous Electrolyte Solution from Thin Insulating Film-Coated Electrodes

Hot electron injection into aqueous electrolyte solution was studied with electrochemiluminescence and electron paramagnetic resonance (EPR) methods. Both methods provide further indirect support for the previously proposed hot electron emission mechanisms from thin insulating film-coated electrodes to aqueous electrolyte solution. The results do not rule out the possibility of hydrated electron being as a cathodic intermediate in the reduction reactions at cathodically pulse-polarized thin insulating film-coated electrodes. However, no direct evidence for electrochemical generation of hydrated electrons could be obtained with EPR, only spin-trapping experiments could give information about the primary cathodic steps.

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.

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.

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

Abstract 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).

Hydrated electron-induced chemiluminescence of thallium(I) ion

Abstract Dissolution of X-ray irradiated sodium chloride or additively coloured potassium chloride induces generation of hydrated electrons which produce thallium(I)-specific chemiluminescence in the presence of thallium(I) ions. Such a luminescence is also called extrinsic lyoluminescence (ELL) of X-ray irradiation coloured sodium chloride, or additively coloured potassium chloride. The present ELL is considered chemiluminescence induced by dissolution-produced strong reducing and oxidising agents, i.e. hydrated electrons, surface-bound and only partially hydrated hole centres, and hydroxyl or sulphate radicals (in the case of additively coloured potassium chloride dissolved in solution containing peroxodisulphate ions). Dichlorothallate(I) (Tl(I)Cl − 2 ) was the main emitter in the ELL processes at about 430xa0nm. The reduction-initiated excitation pathway of TlCl − 2 involves a colloidal thallium atom surrounded by chloride anions (Tl(O):2Cl − ) while in the less important oxidation-initiated excitation pathway the precursor of excited Tl(I)Cl − 2 is transiently existing Tl(II)Cl − 2 produced by dissolution-uncovered hole centres (or sulphate radicals). Both of the lyoluminescence procedures allow Tl(I) to be detected below nanomolar concentrations, yielding linear log–log calibration plots spanning several orders of magnitude of concentration.

Electrochemiluminescence of Lanthanides

Electrogenerated luminescence of lanthanides is reviewed with emphasis on the electrochemiluminescence (ECL) of lanthanide chelates. Main application area of lanthanide chelates in this field is their use as electrochemiluminescent labels in bioaffinity assays such as in immunoassays or DNA probe assays. With lanthanide chelates as labels, hot electron-induced ECL at thin insulating film-coated cathodes outperforms ECL based on traditional electrochemistry at active metal electrodes. ECL excitation of lanthanide(III) chelates occurs normally by ligand-sensitized mechanisms wherein the ligand is first excited by redox reactions followed by energy transfer from ligand to the central ion, which finally emits by f–f transitions. Luminescent lanthanide ions can be excited at oxide-coated metal electrodes when these ions act as luminescence centers in the oxide film and/or at the oxide/electrolyte interface or as solvated in the close vicinity of the electrode surface. These ions typically show high field-induced solid-state electroluminescence when embedded inside of the oxide films and ECL at the surface of the electrode or in solution close to the electrode surface. These lanthanide-doped oxide films can be fabricated either by anodization of certain lanthanide-doped valve metals or from pure valve metals by anodization in lanthanide ion-containing anodization bath preferably with AC voltages. Some lanthanide ions can be electrically excited also in strongly acidic sulfuric acid solutions at platinum electrodes with mechanisms not known with certainty.

Miniature Tunneloxide Electrodes on Silicon for Aqueous Hot Electron Electrochemistry and Electrochemiluminecscence Studies

The basic principles of cathodic hot electron-induced electrochemiluminescence (HECL) and hot electron (HE) injection into aqueous electrolyte solution are shortly discussed. The applicability of miniaturized oxide-coated silicon electrodes as working electrodes in detection of electrochemiluminescent labels by HECL is studied. In addition, the fabrication processes of these tunnel oxide electrodes are described, and an immunoassay is used as an example of a real bioaffinity assay carried out using oxide-coated silicon electrodes.