Keijo Haapakka

The mechanism of the electrogenerated chemiluminescence of luminol in aqueous alkaline solution

Abstract The mechanism of the electrogenerated chemiluminescence of luminol in aqueous alkaline solution based on the rotating ring—disc electrode system is discussed. The disc electrode is maintained at a negative potential and the ring electrode at a symmetrically changing double-step potential. Hydrogen peroxide generated at the disc electrode by the reduction of oxygen is immediately transported to the ring electrode because of electrode rotation. Hydrogen peroxide and luminol are oxidized at the ring electrode during the positive pulse of the double-step potential. These oxidation processes generate a superoxide radical and a luminol radical as intermediates. The luminol radical reacts with the superoxide radical (or oxygen) emitting light.

Cathodically induced time-resolved lanthanide(III) electroluminescence at stationary aluminium disc electrodes

Abstract A time-resolved electroluminometer, which utilizes electrogenerated luminescence (EL) from disposable oxide-covered aluminium electrodes, is described in detail. The cathodic polarization of this disc electrode in aqueous electrolytes induces a faint background EL with peak emission at 570 nm and with a lifetime of 6 μs. This cell can be utilized to excite dysprosium(III), samarium(III) and terbium(III) by electrochemical means to their lowest excited singlet states and, thus, to produce lanthanide(III)-specific emissions. From the lanthanide(III) El systems tested so far, a cathodically induced EL of 2,6-bis[ N , N ,bis(carboxymethyl)aminomethyl]-4-benzoylphenol-chelated terbium(III) with a 2.1-ms lifetime is the longest lived and can be efficiently discriminated against the background EL on a time-resolved basis using instrumentally simple time-resolved detection; this time-resolved terbium(III) EL makes possible the determination of terbium(III) even below the picomole level in aqueous solutions.

Immunoassay by time-resolved electrogenerated luminescence

Abstract An immunoanalytical method based on electrogenerated luminescence was developed in which the labelling compound is the terbium chelate of 2,6-bis[ N,N -bis(carboxymethyl)aminomethyl]-4-benzoylphenol which is linked by a thioureido group to the antibody. The sandwich complex of the labelled antibody, antigen and antibody immobilized on the surface of an aluminium electrode is excited by an electrical pulse and, after a short delay from the end of the pulse, light emission is measured. The electroluminescence immunoassay is exemplified by the heterogeneous and homogeneous assays of human pancreatic phospholipase A 2 , for which both assay methods give a reasonably good linearity in the range 10–300 ng ml −1 .

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.

Application of the electrochemiluminescence of luminol to the determination of copper

Abstract The electrochemiluminescence of aqueous alkaline luminol solutions has been studied by using rapid alternate positive and negative electrical pulses. Traces of copper are shown to change the proportion of the light emitted during the positive and negative pulses, measurement of which gives a linear log—log calibration plot for 1 × 10 -7 to 6 × 10 -6 M copper in glycine buffer at pH 10.15. Out of ten common cations tested only Hg(II), Pb(II) and Mn(II) interfered.

Mechanism of electrogenerated luminescence of terbium(III)-“2,6-bis[N,N-bis(carboxymethyl)aminomethyl]-4-benzoylphenol” chelate at an oxide-covered aluminium electrode

Abstract Generation of hydrated electrons at an oxide-covered aluminium electrode during its cathodic pulse-polarization in an aqueous electrolyte may well be the primary step of the electrogenerated luminescence of aromatic Tb(III) chelates. In the presence of appropriate precursors, hydrated electrons produce secondary oxidizing radicals, thus resulting in an energetic electrode/electrolyte interface which initiates the radiative 5D4 → 7Fj transitions of chelated terbium(III); the aromatic moiety of a Tb(III) chelate is excited by its consecutive interactions with hydrated electron and the secondary oxidizing radical followed by an intramolecular transfer of this excitation energy to the 5D4 level of coordinated terbium(III).

Feasibility of low-voltage cathodic electroluminescence at oxide-covered aluminum electrodes for trace metal determinations in aqueous solutions

Abstract Metal-catalyzed electroluminescence is generated at an oxide-covered aluminum electrode during the reduction of oxygen, potassium peroxodisulfate, and especially hydrogen peroxide in aqueous solutions. The feasibility of this electroluminescence for the determination of copper (5 × 10−9 M) and thallium (> 10−10 M) is demonstrated.

Apparatus for mechanistic and analytical studies of the electrogenerated chemiluminescence of luminol

Abstract A new apparatus based on the rotating ring—disc electrode system is described. The symmetric double-step potential is connected to the ring electrode to oxidize luminol, while the disc electrode is maintained at a negative potential to reduce oxygen to hydrogen peroxide. Because of the electrode rotation, hydrogen peroxide is immediately transported to the ring electrode at which it reacts with luminol oxidation product to emit light. Preliminary electrogenerated chemiluminescence measurements indicate that the intensity of the chemiluminescence of luminol is highly dependent on the ring and disc electrode materials and that some metal ions have a catalytic or inhibitive effect on this luminescence reaction of luminol.

The acid-catalyzed hydrolysis of β-d-xylofuranosides

Abstract The rate constants for the hydrolysis of six alkyl and four aryl β- d -xylofuranosides in aqueous perchloric acid at various temperatures have been measured. The effects of varying the aglycon structure on the hydrolysis rate are interpreted in terms of two concurrent reactions. Either, the substrate, protonated on the glycosidic oxygen atom, undergoes a rate-limiting heterolysis to form a cyclic oxocarbonium ion, or, an initial rapid protonation of the ring oxygen is followed by a unimolecular cleavage of the five-membered ring, all subsequent reactions being fast. It is suggested that xylofuranosides having strongly electron-attracting aglycon groups react mainly by the former pathway, whereas the latter is more favourable for substrates having electron-repelling aglycon groups. The negative entropies of activation obtained with the latter compounds are attributed to the rate-limiting opening of the five-membered ring. The rate variations of the hydrolyses of alkyl β- d -xylofuranosides in aqueous perchloric acid-methyl sulfoxide mixtures are interpreted as lending further support for the suggested chance in mechanism.

Application of electrogenerated chemiluminescence of luminol to determination of traces of cobalt(II) in aqueous alkaline solution

Abstract A selective method for determination of traces of cobalt(II) in aqueous alkaline solution has been developed, based on the electrochemical generation of luminescence from luminol at a rotating ring—disc electrode. The detection limit is 10 -8 M, and the linear calibration range extends up to 3 × 10 -6 M; the r.s.d. for 2.0 × 10 -7 M cobalt is 6%. Of 21 metal ions, only chromium(III) and copper(II) interfere seriously; EDTA also interferes.