Coreactant ECL

The use of a coreactant can make ECL possible even for some fluorescent compounds that have only a reversible electrochemical reduction or oxidation. 

Even with solvents for ECL that have a narrow potential window so that only a reduced or oxidized form of a luminophore can be produced, e.g., tris(2,2 -bipyridine) ruthenium(ll), Ru(bpy 3 (bpy = 2,2 -bipyridine), in aqueous solutions, it is still possible to generate ECL by use of a coreactant. 

When the annihilation reaction between oxidized and reduced species is not efficient, the use of a coreactant may produce more intense ECL. 

The following six criteria are generally required for a good coreactant compound  

Solubility. The coreactant should be reasonably soluble in the reaction media, because the ECL intensity is generally proportional to the concentration of the coreactants. 

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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. 

Although there are a wide variety of molecules that exhibit ECL, the overwhelming majority of publications concerned with coreactant ECL and its analytical applications are based on chemistry involving Ru(bpy)3 +, or closely related analogs as the emitting species (8), because of their excellent chemical, electrochemical, and photochemical properties even in aqueons media and in the presence of oxygen (125). As a result, much of this section concerns Rn(bpy)3 +/coreactant ECL systems. 

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). 

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). 

For coreactant ECL studies, e.g., in aqueous Ru(bpy)3 /TPrA solution, degassing is often unneeded. Thus, Fisherbrand glass tooled-neck vials (www.fishersci.com) are frequently used as ECL cells. When ECL experiments are carried out at a wafer-type electrode, such as indium tin oxide (ITO), Au/Si (151) and highly oriented pyrolytic graphite (HP(Xj) (152), the effective area of the electrode can be controlled by using a cell similar to that shown in Figure 13.8. The electrode surface exposed to the electrolyte solution containing a coreactant should face the window of the photo-detector. 

Coreactant ECL of Rulbpylj /TPrA system in aqueous solutions... 

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). 

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... 

This complex has been widely used in sensing applications since both radical ions of the complex are relatively stable to decomposition reactions. Many systems using this chromophore exist in which ECL is produced at a single electrode via coreactant oxidation or reduction schemes as discussed in the first segment of this section [Eqs. (5) through (9)]. For example, the reduction product of the peroxydisulfate dianion, S2Og, can function as an oxidant in the ECL reaction by annihilation with the electrochemically generated Ru1+ to yield the MLCT excited state of the Ru(II) complex by the mechanism [24] ... 

In systems where ECL arises upon application of a single potential and reaction of a coreactant, as outlined in Eqs. (5) through (9), concentration distance profiles differ and depend on many more factors. In cases where a positive potential is applied and both the ECL chromophore and coreactant are oxidized at the electrode surface, concentrations of the two initial radical ion species will decay with increasing distance from the electrode, as will the concentration of the strong reducing agent formed upon decomposition of the coreactant. The zone where luminescence arises depends on relative rate constants for... 

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