Timo Ala-Kleme

Energetic electrochemiluminescence of (9-fluorenyl)methanol induced by injection of hot electrons into aqueous electrolyte solution

Hot electron injection into aqueous electrolyte solutions from metal/insulator/metal/electrolyte and metal/insulator/electrolyte tunnel junctions is considered and the possibility of an electrochemical generation of hydrated electrons is discussed. The hot electron-induced UV electrochemiluminescence of (9-fluorenyl)methanol was used to demonstrate the presence of highly energetic transient species in aqueous solution at several metal/insulator/electrolyte hot electron tunnel emitters. These transient species cannot be produced electrochemically in fully aqueous solutions at any active metal electrodes. A detailed mechanism for the present electrochemiluminescence is suggested.

F-Centre luminescence from oxide-covered aluminium cathode induced by two-step reduction of peroxydisulfate anions

Light emission from aluminium oxide during cathodic pulse-polarisation of oxide-covered aluminium in aqueous solution was observed to be strongly enhanced in the presence of peroxydisulfate ions. The spectrum of the light emission had a broad maximum between 400 and 450 nm being attributed to F-centre luminescence of aluminium oxide. The mechanism of the luminescence is associated with the two-step reduction of peroxydisulfate anions near the oxide-covered cathode where the first one-electronic reduction step occurs either (i) by tunnel-emission generated hydrated electrons or (ii) by trickling down the surface states to the energy level of peroxydisulfate ion or (iii) by direct heterogeneous electron transfer from the bottom of the aluminium oxide conduction band to peroxydisulfate ions during strong downward band bending induced by cathodic pulse-polarisation. The second step occurs by electrons from F- or F + -centres at the oxide/electrolyte interface. Transitions of aluminium oxide conduction band electrons to fill the sulfate radical-emptied electron trapping sites (oxygen vacancies) produces analogous F- and F + -centre luminescence to that occurring during photoluminescence and thermoluminescence of aluminium oxide. No enhancement of light emission was observed in the presence of hydrogen peroxide which is also reduced in a two-step process at oxide-covered aluminium electrode. This can be explained by the fact that the energy level of hydroxyl radical under the present conditions lies ca. 1 eV above, whereas the energy level of sulfate radical lies somewhat below the colour-centre sub-band of aluminium oxide. Therefore, the sulfate radical is a sufficiently strong oxidant but the hydroxyl radical is too weak an oxidant to abstract electrons from F- and F + -centres.

Hot electron-induced electrogenerated chemiluminescence of rare earth(III) chelates at oxide-covered aluminum electrodes

Aromatic Gd(III) and Y(III) chelates produce ligand-centered emissions during cathodic pulse polarization of oxide-covered aluminum electrodes, while the corresponding Tb(III) chelates produce metal-centered5D4 →7Fj emissions. It was observed that a redox-inert paramagnetic heavy lanthanoid ion, Gd(III), seems to enhance strongly intersystem crossing in the excited ligand and direct the deexcitation toward a triplet-state emission, while a lighter diamagnetic Y(III) ion directs the photophysical processes toward a singlet-state emission of the ligand. The luminescence lifetime of Y(III) chelates was too short to be measured with our apparatus, but the luminescence lifetime of Gd(III) chelates was between 20 and 70 μs. The mechanisms of the ECL processes are discussed in detail.

Electrochemiluminoimmunoassay of hTSH at disposable oxide-coated n-silicon electrodes

Abstract Cathodic pulse polarisation of thin insulating film-coated electrodes enables the generation of electrochemiluminescence (ECL) by tunnel emission of hot electrons from the Fermi level of the conductor material of the conductor–insulator–aqueous electrolyte solution junction to the solutes at the vicinity of the electrode surface and probably also to the conduction band of water. The latter process can generate hydrated electrons as strongly reducing slightly longer-lived cathodic intermediate s , which are known to be able to induce chemiluminescence (CL) of various types of luminophores having very different photophysical and chemical properties. The generation of the above-mentioned cathodic primary species provides good possibilities to use many types of luminophores as label molecules in sensitive immuno and DNA-probing assays. This paper introduces an electrochemiluminoimmunoassay (ECLIA) for human thyroid stimulating hormone (hTSH) at oxide-coated n-silicon electrodes and demonstrates the suitability of silicon electrodes covered with thermally grown silicon dioxide film as disposable working electrodes (WEs) in sensitive time-resolved ECL (tr-ECL) measurements in aqueous solution. The label chelate can be detected almost down to picomolar level and the calibration curve of the chelate covers more than five orders of magnitude of chelate concentration. Also the calibration curve of the immunometric hTSH assay was found to be linear over a wide range of hTSH concentration, the detection limit of the hormone being below 1 mU l −1 (4 pmol l −1 ).

Time-resolved luminescence detection of europium(III) chelates in capillary electrophoresis

An instrumentally simple on-column time-resolved luminescence capillary electrophoresis (CE) detector for determining and characterizing migrated europium(III) chelates on the basis of their long-lived 5D0→7F-multiplet transitions has been constructed. In addition to the background fluorescence arising from the cell walls, this time-resolved luminescence method of measurement also allows an efficient discrimination of the analyte emission against the short-lived background fluorescence caused by possible fluorescent impurities in the electrophoresis buffer; this should provide a feasible means of constructing a CE detector for the trace analysis of europium(III) chelates and also of samples of biological interest labelled with europium(III) chelates. If supplied additionally with a device for determining the excited-state lifetimes of separated europium(III) chelates, this time-resolved luminescence detector should make possible the peak identification of migrated europium(III) chelates.

Near-infrared electrogenerated chemiluminescence of ytterbium(III) chelates in aqueous electrolytes

Abstract The near-infrared electrogenerated chemiluminescence (IR ECL) of Yb(III) chelates at a pulse-polarized oxide-covered aluminium cathode is described for the first time. The observed 2 F 5/2 → 2 F 7/2 radiative transition of Yb(III) is the lowest-energy ECL in aqueous solution reported to date. Also, a proposed explanation for the excitation and emission processes of this near-IR ECL of Yb(III) chelates has been given. The phenomenon offers many new application possibilities for analytical purposes, for example, a new point of view for wavelength discrimination.

Sonoluminescence of chelated terbium(III) in aqueous solution

TbIII chelates containing aromatic moieties show sonoluminescence in aqueous solutions during the sonolysis of water. The observed 5D4→7FJ transitions of TbIII are due to the excitation of ligand, followed by an intramolecular energy transfer from the ligand to the central ion, which finally emits. No sonoluminescence of hydrated or EDTA-chelated TbIII ion could be observed. Ligand excitation can be based either on an energy transfer from the intrinsic emission centres of the sonolysis of water to the aromatic ligand, or on redox reactions between the ligand and hydroxyl radicals and hydrogen atoms produced by the sonolysis of water. Experimental results give greater support to the latter, chemiluminescent, excitation pathway.

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.

Intrinsic and 1-aminonaphthalene-4-sulfonate-specific extrinsic lyoluminescences of X-ray irradiated sodium chloride

Abstract 1-Aminonaphthalene-4-sulfonate (ANS)-specific extrinsic lyoluminescence (LL) of X-ray irradiated sodium chloride is observed at 425 nm when the irradiated salt is dissolved in an aqueous solution of ANS. The paper discusses, in detail, the mechanism of the ANS-specific LL and its analytical applicability. Also, the intrinsic LL of X-ray irradiated sodium chloride is studied. Hydrated electron as well as hole scavenger experiments support the proposal that in the case of the intrinsic LL of X-ray irradiated sodium chloride, trapped electrons (mainly F-center electrons) are released and hydrated whereas trapped holes (V-centers) remain surface-bound and are only partially hydrated before recombination occurs. These hydrated electrons and dissolving solid surface-bound hole centers, which are probably only partially hydrated, are able to act as reducing and oxidizing agents, respectively, in the luminophore oxidation-initiated reductive excitation pathway of ANS. Solution additives (halides and pseudohalides) show that in the chemiluminescence processes in question, oxidizing agents will follow the Marcus theory of electron transfer reactions. The LL method described allows the determination of ANS in the concentration range ≈10 −11 − 10 −7 M. This suggests that aminonaphthalene derivatives can be used as label molecules in high sensitivity lyoluminobioaffinity assays.

Quenching of cathodic electrogenerated F-center luminescence of aluminium oxide by lanthanide cations at the electrode/electrolyte interface

Abstract Energy transfer between aluminium oxide F-center and lanthanide cations at an oxide-covered aluminium electrode during the cathodic pulse-polarization of the electrode is investigated by means of Stern-Volmer luminescence quenching kinetics. Terbium III -specific extrinsic luminescence is observed while some other lanthanides are observed to quench the F-center luminescence. Different quenching efficiencies of the lanthanides are discussed to be dependent on the different energy acceptor characteristics of the tri- or divalent lanthanides.