Oxidative reduction, ECL

The first co-reactant discovered was oxalate in 1977. The introduction of the co-reactant in ECL exhibits distinct advantage in comparison with the annihilation reaction (1) it can overcome the limited potential window of solvent and the poor stability of radical anions or cations (2) the coreactant ECL can be beneficial for some fluorescent compounds that have only a electrochemical reduction or oxidation (3) the use of co-reactant can produce more intense ECL emission when the annihilation reaction between oxidized and reduced species is not efficient (4) it can eliminate the oxygen quenching effect frequently encountered in ion annihilation reaction and facilitate the ECL in the air. All commercially available ECL analytical instruments are based on this pathway. According to the generated intermediates and the polarity of the applied potential, the corresponding coreactant ECL can be classified as oxidative-reduction ECL and reductive-oxidation ECL, respectively. 

Oxidative-Reduction ECL. The ECL system involving the generation of a strong reducing intermediate species upon electrochemical oxidation was defined as oxidative-reduction ECL. The ECL of Ru(bpy)3 " "/oxalate (C204 ) system is a typical example. The corresponding mechanism was described in Eqn (7)-(13). 

This was the first account of coreactant ECL system reported in the literature by Bard s group in 1977 (16), and is a classical example of oxidative-reduction ECL. The ECL mechanism of this system was proposed to be as in Scheme 13.4 (17). 

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. 

Both reacting intermediates, TPrA and Ru(bipy)33 + species, are produced simultaneously during electrochemical oxidation Actual ECL mechanism, however, is somewhat more complicated than expressed by the above reaction pattern with ECL emission from Ru(bipy)32+/TPrA system depending on the applied electrode potential. Usually, the direct oxidation of TPrA at the electrode occurs at more negative potentials than characteristic for the Ru(bipy)32+/Ru(bipy)33 + redox couple. Generally, the ECL emission from Ru(bipy)32+/TPrA system as a function of applied potential consists of two emission waves (both associated with the emission from 3 Ru(bipy)32 + ) attributed to TPrA and Ru(bipy)32 + oxidation, respectively.154 First emission wave corresponds to annihilation of sufficiently stable TPrA + (with half-life of 0.2 ms) and Ru(bipy)3 + species with Ru(bipy)3 + intermediate formed from the reduction of Ru(bipy)32+ by TPrA free radical ... 

Another popular oxidative-reduction example is Ru(bpy)3 " /tripropyl-amine (TPrA) system. It is the basis of commercial systems for immunoassay and DNA analysis due to the high ECL efficiency. The mechanism of Ru(bpy)3 +/TPrA system is very complicated and has been elucidated in detail in the literature. Generally, the ECL emission spectra of this system... 

Higgins et al. (1999) developed a system for the detection of Staphylococcus enterotoxinB (SEB), ricin toxin. Yersinia pestisYl antigen and 5ac/7/MX anthracis PA antigen based on the principle of electrochemiluminescence (ECL). ECL is a process that involves the generation of light from a voltage-dependent, cyclic oxidation-reduction reaction of rathenium heavy metal chelate. The redox... 

Keywords Annihilation Pathway Electron-transfer reaction Gibbs free energy Energy sufficient reaction Reductive-oxidation co-reactant Oxidative-reduction co-reactant Hot-electron ECL... 

Leland JK, Powell MJ (1990) Electrogenerated chemiluminescence an oxidative-reduction type ECL reaction sequence using Tripropyl Amine. J Electrochem Soc 137(10) 3127-3131. doi 10.1149/1.2086171... 

Fig. 5.22 Asymmetric light-emitting electrochemical swimmer. Simultaneous reduction of H20 at the cathodic pole (bottom of the bead) and oxidation of ECL reagents at the anodic pole (top of the bead) induces both motion and light emission from the bead in a glass capillary. P corresponds to a side product of the TPA radicals formed during the ECL process. Rc ninted with permission from Ref. [138]. Copyright 2012 Wiley...

This was the first example of so called reductive-oxidation coreactant ECL system reported in the literature (124, 129). Because Ru(bpy)3+ is unstable in aqueous solutions and (NH4)2S20g has a low solubility in MeCN solutions, the MeCN-H20 mixed solutions were chosen to produce intense ECL emission (124). Scheme 13.5 summarizes the possible pathways for the production of Ru(bpy)3 when S20g is used as the coreactant, in which the strongly oxidizing intermediate S04 , generated during reduction of 820, has a redox potential of > 3.15 V vs. SCE (130). 

The mechanism of the second ECL wave follows the classic oxidative-reduction coreactant mechanism (Section 13.3.2.1), where oxidation of TPrA generates a strongly reducing species TPrA (EpiOTrA 1.7 V vs. SCE (112)). This oxidation can be via a catalytic route where electrogenerated Ru(bpy)3 -" reacts with TPrA as well as by direct reaction of TPrA at the electrode described by both Scheme 13.7 and Scheme 13.8 (137) ... 

The analytical usefulness of this reaction, stems mainly from that fact that the electrochemically generated Ru(bpy)33+ species can be reduced by a large number of potential analyte compounds, or their electrochemical derivatives, via high-energy electron transfer reactions, to produce the Ru(bpy)32+ excited species, without the need for an electrochemical reduction step. The converse is also true. The reduction of peroxodisulfate (S2082-) for example, in the presence of Ru(bpy)32+, produces the Ru(bpy)32+ excited species and an ECL emission, from the reaction of Ru(bpy)3+ and S04 [20], Although this latter system has been used for the determination of both Ru(bpy)32+ [21] and S2082- [22], the vast majority of analytical applications use the co-oxidation route. 

The mechanism of cathodic luminescence is distinctly different from other ECL systems. Light is emitted from oxide-covered, so-called valve metal, electrodes, namely aluminium and tantalum, during the reduction of peroxodisulfate, hydrogen peroxide, or oxygen, in aqueous solution, at relatively low potentials (<10 V). The mechanism involving persulfate, for example, is as follows. A conduc-... 

Haapakka and Kankare have studied this phenomenon and used it to determine various analytes that are active at the electrode surface [44-46], Some metal ions have been shown to catalyze ECL at oxide-covered aluminum electrodes during the reduction of hydrogen peroxide in particular. These include mercu-ry(I), mercury(II), copper(II), silver , and thallium , the latter determined to a detection limit of <10 10 M. The emission is enhanced by organic compounds that are themselves fluorescent or that form fluorescent chelates with the aluminum ion. Both salicylic acid and micelle solubilized polyaromatic hydrocarbons have been determined in this way to a limit of detection in the order of 10 8M. 

Based on the reaction of luminol and hydrogen peroxide, detection by electrogenerated CL (ECL) was also applied in CE [85], In this detection technique, which has been used until now in LC and in FIA, the production of light is followed by an oxidation or reduction reaction at an electrode that serves the... 

The most versatile technique for producing emission by the generation of ion radicals followed by their oxidation or reduction in situ is the electrochemical method. The emission produced by this technique has been termed electrochemiluminescence (ECL). Chemical oxidants and reductants have also been widely used, although their employment, especially in quantitative work, is more cumbersome. This review attempts to describe the current knowledge, both experimental and... 

Extensive investigations of ECL processes have established a general, sometime quite complicated, scheme for ECL emission. In the electrochemical reactions, an electron acceptor A is reduced to A at the reduction potential Ercd and an electron donor is oxidized to D + at the oxidation potential Eox as follows ... 

Light generation in ECL processes is realized in electron transfer reactions involving strong oxidant and reductant. Principally, both reactants can be prepared in the common chemical way if reactive intermediates are stable enough,49-51 but usage of the chemically produced oxidants and/or reductants seems to be quite cumbersome, especially in quantitative works. The electrochemical way appears to be much more practical (in the cases of relatively unstable intermediates) and advantageous (due to... 

Consequently, the excited 3 Ru(bipy)32+ state can be produced via three different routes (i) Ru(bipy)3+ oxidation by TPrA"+ cation radical, (ii) Ru(bipy)33+ reduction by TPrA" free radical, and (iii) the Ru(bipy)33 + and Ru(bipy)3 + annihilation reaction. The ECL intensity for the first and second waves was found to be proportional to the concentration of both Ru(bipy)32+ and TPrA species in a very large dynamic range with reported detection limits as low as 0.5 pM155 for Ru(bipy)32+ and 10 nM156 for TPrA. In addition to Ru(bipy)32+, many other metal chelates and aromatic compounds or their derivatives can produce ECL in the presence of TPrA as a coreactant upon electrochemical oxidation (cf. Chapter 4 in the Bard s ECL monograph.32). 

The above-presented examples clearly shown that application of coreactant does not require the direct electrochemical generation of both oxidized and reduced forms of a given luminophore. This can be a significant advantage because the use of a coreactant can make ECL possible even in solvents with a narrow potential window so that only a reduced or oxidized form of a luminophore can be produced. Additionally, it is still possible to generate ECL by using a coreactant for some fluorescent compounds that shown only a reversible electrochemical reduction or oxidation. Sometime, when the annihilation reaction between the oxidized and the reduced species is not efficient, the use of a coreactant may produce more intense ECL. 

Similarly as it is in the case of solution phase, most of reported studies have been devoted to immobilization of Ru(bipy)32 + due to universality of this chelate in ECL based analytical application. When electrochemical stimulation is applied to the system, Ru(bipy)32+ immobilized on the electrode is oxidized to Ru(bipy)33 + followed by further reduction of Ru(bipy)33 + with a coreactant (e.g., TPrA) present in the bulk solution. Then ECL signal is obtained from the produced excited 3 Ru(bipy)32+ state, similarly as in the case of the solution-phase excitation. There are several methods allowing the immobilization such as (i) self-organization of Ru(bipy)32+ derivatives by means of Langmuir-Blodgett (LB) or self-assembly... 

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