Electrogenerated chemiluminescence ECL

Experimentally, radical ions can be produced electrochemically - the chemiluminescence occurring called electrogenerated chemiluminescence (ECL) 

The first examples of this significant new form of chemiluminescence were reported in 1964 [1, 2, 3, 4]. Its importance Ues in the precision with which the energy of the species involved can be measured and the simplicity of the electron transfer excitation step. In addition the generation of the reactive radical ions is easily controlled and followed by modern electrochemical techniques. In view of 

In its simplest and classical form a fluorescent aromatic hydrocarbon, such as 9,10-diphenyl anthracene (DPA) undergoes alternating oxidation and reduction at an electrode with an A. C. supply. 

A sequence such as that in the scheme occurs. In principle this luminescence cycle can be repeated indefinitely, but in practice loss of DPA by side reactions, and oxidation of the solvent and supporting electrolyte interfere. 

The formation of the radical anion/cation in the diffusion layer is followed by an annihilation reaction. If there is sufficient energy in this electron transfer to populate the excited state of the hydrocarbon, luminescence results. The energy available is determined by the redox potential of the ions. 

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Electrogenerated chemiluminescence (ECL) has proved to be useful for analytical applications including organic analysis, ECL-based immunosensors, DNA probe assays, and enzymatic biosensors. In the last few years, the electrochemistry and ECL of compound semiconductor nanocrystallites have attracted much attention due to their potential applications in analytical chemistry (ECL sensors). 

As it was indicated above, the main optical interrogation methods include absorbance, reflectance, fluorescence, chemiluminescence (CL), or electrogenerated chemiluminescence (ECL)9,41"42. 

Electrogenerated chemiluminescence (ECL) is the process whereby a chemiluminescence emission is produced directly, or indirectly, as a result of electrochemical reactions. It is also commonly known as electrochemiluminescence and electroluminescence. In general, electrically generated reactants diffuse from one or more electrodes, and undergo high-energy electron transfer reactions either with one another or with chemicals in the bulk solution. This process yields excited-state molecules, which produce a chemiluminescent emission in the vicinity of the electrode surface. 

Flow cells may also act as reactors. In BL, enzymes may be immobilized inside the cell either by chemical bonding on the inner surface or by entrapping the enzyme as a heterogeneous system by mechanical ways. This approach has the advantage of low consumption of expensive reagents and enhancement of their stability, which is usually low. Many bioluminescent reactions have utilized the benefit of this process. The flow cell is also used as a reactor in the case of electrogenerated chemiluminescence (ECL) when used with FI manifolds. Some of these applications are included in Table 4. 

A unique CL reagent, /n.v(2,2 -bipyridyl)rut.hcnium(II) [Ru(bpy)32+] for the postcolumn CL reaction, was applied to HPLC detection. The oxidative-reduction reaction scheme of CL from Ru(bpy)32+ is shown in Figure 17. When the production of light following an oxidation of Ru(bpy)32+ to Ru(bpy)33+ at an electrode surface is measured, this CL reaction is termed electrogenerated chemiluminescence (ECL). The CL intensity is directly proportional to the amount of the reduc-tant, that is, the analyte. 

Finally, in complexes of this type, where [Run(bipy)3]2+ is one of the most studied, the lowest spin-forbidden excited state [ Ruu(bipy)3]2+ is responsible for the chemiluminescence produced when solutions of [Rum(bipy)3]3+ are mixed with suitable reducing agents. This same excited state can be produced electrochemically35 in a process whereby electrical energy is converted into light —electrogenerated chemiluminescence (ECL). Thus if cyclic square waves are applied to a solution of [Run(bipy)3]2+ between the potentials of formation of [RuI(bipy)3]+ and [Rum(bipy)3]3+, then ECL is observed via the following mechanism ... 

Electrogenerated chemiluminescence (ECL) experiments [7] are difficult to perform in the dry box due to the need to exclude light from all external sources. Placing a black cloth over the dry box window is usually not sufficient unless the intensity of the ECL is very high. It may be necessary to use a monochromator in these experiments. This is especially unfortunate in the case of ECL at an RRDE, since the complexity of cell and motor assembly is greatly increased. Nevertheless, the dry box is very useful for performing initial ECL experiments, perhaps to establish whether or not ECL occurs. 

No discussion of digital simulation would be complete without some mention of the methods employed in treating electrogenerated chemiluminescence (ECL). In the most common statement of the problem, an aromatic hydrocarbon (A) is alternately oxidized and reduced at a single electrode to produce its radical anion and cation species (B and D). These species, in turn, react within the diffusion layer to regenerate the hydrocarbon. 

Since its discovery in 1964 electrogenerated chemiluminescence (ECL) has been an active area of chemical research O ). The light producing mechanism common to all ECL systems consists of an oxidation and reduction, followed by charge annihilation to produce the excited state of an emitting compound. 

The electrogenerated chemiluminescence (ECl) of five l-amino-3-anthryl-9-propane derivatives has been studied in tetrahydrofuran. Emission from intramolecular exciplexes in ECl spectra and weak emission from the locally excited anthracene moiety were observed. The influence of triplet state interaction in ECl emission is discussed. The chemiluminescent decomposition of three a-peroxy-lactones gives CO2 and the corresponding ketone in high yield. The chemiluminescent species produced has been investigated in some detail by measurements of lifetime, energy-transfer activation parameters, and photochemical reactions. 

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