A. Kulmala

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.

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.

X-ray irradiated sodium chloride as an excitation source for the sensitized terbium(III)-specific chemiluminescence of aromatic Tb(III) chelates in aqueous solutions

Abstract The viability of X-ray irradiated sodium chloride as the excitation source to generate the radiative 5 D 4 → 7 F j transitions of chelated terbium(III) is demonstrated. The dissolution of X-ray irradiated sodium chloride in an aqueous solution produces a dynamic solid/solution interface containing hydrated electrons and dissolution-uncovered holes as strong one-electron reducing and oxidizing intermediates, respectively, which provides the energetic basis to initiate the sensitized Tb(UI)-specific extrinsic lyoluminescence of X-ray irradiated sodium chloride by the ligand-oxidation-initiated reductive and/or ligand-reduction-initiated oxidative excitation pathways. Redox luminescence mechanisms of Tb(III) chelates containing phenol, aniline or hydroxybenzophenone derivatives as their chemical energy-accepting aromatic moieties are discussed in detail.

Terbium(III)-“2,6-bis[N,N-bis(carboxymethyl)aminomethyl]-4-benzoylphenol” chelate enhanced lyoluminescence of X-ray irradiated sodium chloride

Abstract Present chelate produces a typical terbium(III) emission spectrum during dissolution of X-ray irradiated sodium chloride in aqueous chelate solutions. The observed emission is predominantly chemiluminescence produced by the dissolution generated free radicals. The excitation of chelate proceeds dominatingly by a ligand oxidation-initiated route where the chemically excited ligand transfers energy intramolecularly to the central ion which is finally radiatively deexcitated.

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.