Electrochemiluminescence

Academic and commercial interest in electrochemiluminescence = ECL gave a great impetus to research in elechtrochemically generated radical ions. The simplified scheme for ECL is shown by Eq. (240)  

By annihilation of a radical cation and a radical anion neutral species are formed and light is emitted. Reduction and oxidation of the neutral compounds regenerates the radical ions for a new cycle. By fast repetition of this cycle, e.g., via electrolysis on an AC-line, continous conversion of current into light should be possible. 

The electron from the antibonding orbital of the radical anion is transferred to the antibonding orbital of the radical cation. The transition of this electron to thejrround state produces the fluorescence spectrum. 

Bard etaL 5S6 5571 and Visco etaL 558) have quantitatively analyzed the intensity of pulsed ECL of 9,10-diphenylanthracene, tetraphenylpyrene and rubrene. By computer simulation of the electrode process and the subsequent chemical reactions the rates for chemical decay of the radical ions could be determined. Weaker ECL with fluorescence emission 559 or electrophosphorescence S60) occurs if the radical anion R - reacts with a dissimilar radical cation R,+ of insufficient high oxidation potential to gain enough energy for fluorescence emission, that is, if ht fluorescence) 23.06 (Ej +. -Ej -.), e.g., in the annihilation of the anthracene radical anion with Wurster s blue. For these process the following schemes are assumed (Eq. (242) )  

The triplet emission can be totally suppressed by adding an efficient quencher, e.g., 1,3,5-hexatriene 560 . 

Further detector devices worked with heated modified carbon paste electrodes and made use of electrochemiluminescence [61, 62]. Details will be given in next paragraph. 

In 2006, first time a heated electrode was used to improve the efficiency of an electrochemiluminescent detection system [63]. The Ru(bpy)3 -ECL and Ru (bpy)3 -oxalate-ECL systems were used together with a directly heated 25 pm platinum wire electrode. The detection limit of oxalate was found to be decreased by two orders of magnitude if the temperature was increased from 22 °C to 80 °C. The authors emphasise the advantage that temperature is acting at the place where it is necessary, but leaves the dissolved sensitive constiments unaffected. An overview on the system is given in Fig. 6.15. The system was used also for electroluminescence studies with luminol [64] and with lucigenin [65]. In both applications. 

Pastes made of carbonaceous materials with ionic liquids acted also as heated electrochemiluminescence sensors. A mixture of multi-wall carbon nanotubes with an ionic liquid was used in connection with lucigenine to detect ascorbic acid which influences the luminescence of the latter [68]. Another example was a carbon/ionic liquid paste electrode where Ne-isopentenyl-adenine was detected using its enhancement action on the electrochemiluminescence of ruthenium bipyridyl [69]. 

In a series of papers, heated indium tin oxide (ITO) electrodes were proposed as carriers for electrochemiluminescent sensors of diverse analytes [70-74]. The system couples the advantages of a heated electrode with the optical transparency property of ITO glass. In [70], H2O2/MCLA and TPrA/Ru(bpy)3 have been used to test the arrangement which was constructed very similar to the scheme given in Fig. 6.15. In [71], TPrA and colchicine were detected in human serum by means of the ruthenium bipyridyle ECL system. Immobilised xanthine oxidase as a sensor system for hypoxanthine has been mentioned already. It has been used also with a heated ITO electrode [72]. Further examples for electrochemiluminescence detection with heated ITO electrodes were the analysis of Ns-methyladenosine in urine 

Whenever there is light emission we can register a spectrum, which is very useful in the identification of the light-emitting species. 

Gale (ed.), Spectroelectrochemistry, theory and practice, Plenum, New York, 1988. 

Compton and A. Hamnett (ed.), Comprehensive chemical kinetics, Elsevier, Amsterdam, 1989, Vol. 29. 

Gutierrez and C. Melendres (ed.), Spectroscopic and diffraction techniques in interfacial electrochemistry, Proceedings of NATO ASI 1988, Kluwer, Dordrecht, 1990. 

Varma and J. R. Selman (ed.), Techniques for characterization of electrodes and electrochemical processes, Wiley, New York, 1991. 

In addition to UV-vis absorption measurements, other spectroscopic techniques can be used for monitoring the dynamics of electrochemical events or the fate of electrogenerated species. Earticularly informative are the couplings of electrochemistry with electron spin resonance, nuclear magnetic resonance, and mass spectroscopy. A variety of specially designed cells have been con- 

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The mechanism of chemiluminescence is still being studied and most mechanistic interpretations should be regarded as tentative. Nevertheless, most chemiluminescent reactions can be classified into (/) peroxide decomposition, including biolurninescence and peroxyoxalate chemiluminescence (2) singlet oxygen chemiluminescence and (J) ion radical or electron-transfer chemiluminescence, which includes electrochemiluminescence. 

Under optimum conditions electron transfer can produce excited states efficiently. Triplet fluoranthrene was reported to be formed in nearly quantitative yield from reaction of fluoranthrene radical anion with the 10-phenylphenothia2ine radical cation (171), and an 80% triplet yield was indicated for electrochemiluminescence of fluoranthrene by measuring triplet sensiti2ed isomeri2ation of trans- to i j -stilbene (172). 

Electrochemiluminescence quantum yields of 8—10% from 9,10-diphenylanthracene and 14—20% from the 9,10-diphenylanthracene anion—thianthrene cation combination have been reported using the rotating ring disk electrode technique (157,173). 

In this paper the electtode anodic reactions of a number of dihydropyridine (DHP) derivatives, quantum-chemical calculations of reactions between DHP cation-radicals and electrochemiluminescers anion-radicals (aromatic compounds) and DHP indirect ECL assay were investigated. The actuality of this work and its analytical value follow from the fact that objects of investigation - DHP derivatives - have pronounced importance due to its phaiTnacology properties as high effective hypertensive medical product. 

ELECTROCHEMILUMINESCENT HYBRIDISATION CHIP USING ELECTRIC FIELD AIDED HYBRIDISATION AND IMMOBILISATION... 

MICRO ELECTRO-MECHANICAL SYSTEMS FOR THE DETECTION OF BIOANALYTES USING ELECTROCHEMILUMINESCENCE... 

The reduction of dimensions also reduces volumes which are accessible to the detector. Thus, detection principles related to geometric dimensions of the detector cell ai e not ideally suited for coupling to microsystems, whereas surface sensitive principles, such as electrochemical methods or optical methods utilizing the evanescent field of a waveguide, or methods which can be focussed on a small amount of liquid, such as electrochemiluminescence (ECE), ai e better suited. This is why electrochemiluminescence detectors ai e combined to microsystems. Moreover ECE has found wide applications in biochemistry because of its high sensitivity, relatively simplicity and feasibility under mild conditions. 

A study of the electrochemical oxidation and reduction of certain isoindoles (and isobenzofurans) has been made, using cyclic voltammetry. The reduction wave was found to be twice the height of the oxidation wave, and conventional polarography confirmed that reduction involved a two-electron transfer. Peak potential measurements and electrochemiluminescence intensities (see Section IV, E) are consistent vidth cation radicals as intermediates. The relatively long lifetime of these intermediates is attributed to steric shielding by the phenyl groups rather than electron delocalization (Table VIII). 

Emission spectra have been recorded for four aryl-substituted isoindoles rmder conditions of electrochemical stimulation. Electrochemiluminescence, which was easily visible in daylight, was measured at a concentration of 2-10 mM of emitter in V jV-dimethylformamide with platinum electrodes. Emission spectra due to electrochemi-luminescence and to fluorescence were found to be identical, and quantum yields for fluorescence were obtained by irradiation with a calibrated Hght source. Values are given in Table X. As with peak potentials determined by cyclic voltammetry, the results of luminescence studies are interpreted in terms of radical ion intermediates. ... 

Not included in this survey are limiting currents at porous electrodes, because they usually are not controlled exclusively by convective diffusion [for exceptions, see (HI lb)], limiting currents due to limited gas solubility at an electrode (N8b), or limiting currents recorded by electrochemiluminescence (H6c, C12b). 

Electrochemiluminescence of Organometallics and Other Transition Metal Complexes... 

A variety of transition metal complexes including organometallics was subjected to an ac electrolysis in a simple undivided electrochemical cell, containing only two current-carrying platinum electrodes. The compounds (A) are reduced and oxidized at the same electrode. If the excitation energy of these compounds is smaller than the potential difference of the reduced (A ) and oxidized (A ) forms, back electron transfer may regenerate the complexes in an electronically excited state (A+ + A A + A). Under favorable conditions an electrochemiluminescence (eel) is then observed (A A + hv). A weak eel appeared upon electrolysis o t]jie following complexes Ir(III)-(2-phenylpyridine-C, N ) [Cu(I)(pyridine)i],... 

A new cholesterol flow injection analysis biosensor has also been described as an application of the H2O2 ECL sensor56. In that work, the luminol electrochemiluminescence, previously studied in aqueous media, was implemented in Veronal buffer added of 0.3% triton X-100 (v/v), 0.3% PEG and 0.4% cholate to enable the solubilisation of the cholesterol and then its efficient oxidation catalyzed by the immobilized cholesterol oxidase. The ECL reaction occurred thus in a micellar medium and the performances of the H2O2 ECL sensor were investigated. 

The chemiluminescence and electrochemiluminescence of luminol have been also exploited for the development of enzyme, DNA and immuno-biochips. Different approaches were studied in our laboratory, in which... 

This new material was used to design biochips based on the electrochemiluminescence reaction of luminol in the presence of enzymatically... 

Figure 11. Electrochemiluminescent PDMS-graphite biochip formats (a) nucleic acid-based biochip (b) immunochip (competitive immunoassay).

Marquette C.A., Blum L.J., Luminol electrochemiluminescence-based fibre optic biosensors for flow injection analysis of glucose and lactate in natural samples, Anal. 

Tsafack V. C., Marquette C. A., Leca B., Blum L. J., An electrochemiluminescence-based fibre optic biosensor for choline flow injection analysis, Analyst 2000 125 151-155. 

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