Quenching of ECL

Abstract ECL quenching may play an important role in designing new methodologies for sensitive detection of analytes. Quenching proposes prospective advantages in the framework of ECL and has acquired considerable attention and is inextricably associated with the selectivity of luminophore and co-reactant. It can be used in diverse helds in the detection of many analytes, DNA detection, and hybridization, etc. Processes, reactions, and equations involving quenching are discussed as well. 

Keywords Quenching Forster transfer Stem-Volmer equation Electron-transfer-quenching path Cathodic ECL Energy scavenging process 

For example, if the ground state is reduced at —1.0 V and the excitation energy is 2.5 eV, the excited state would be reduced at +1.5 V. Similarly, oxidation of the excited state occurs at E° (D , D) —E. Thus, excited states can be quenched rather easily by an electron-transfer process, as shown in Fig. 6.1, so that the radical ions are effective quenchers. 

Inhibition mechanism was based on the formation of protein Ru(bpy)3 super molecule, which would prevent Ru(bpy)3 from reaching the working electrode surface to induce ECL quenching [17]. 

Quenching effect of ferrocene (Fc) is proved very practical for the development of ECL sensors. A reagentless signal-on ECL biosensor for DNA hybridization detection was developed (see Fig. 6.2). In the development of this biosensor, the quenching effect of Fc on intrinsic cathodic ECL at thin oxide-covered glassy carbon (C/C Oi-x) electrodes was employed. The main advantage of the present sensor lies in the fact that ECL is generated from the electrode itself and no luminophore or luminophore-labeled DNA probe is needed. The detection limit was ca. 5.0 pM (S/N = 3) [18]. 

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It is also possible to exploit quenching of ECL in the detection of various substances. Recently Richter and coworkers have shown that ECL from [(bpy)3Ru]2+, generated following oxidation in the presence of trialkylamines, is quenched by quinones and other aromatic hydrocarbons in nonaqueous solvents [60],... 

The emission should also be affected by bimolecular quenching of the triplet. In fluid media this is normally sufficient to quench all em-mission resulting from triplets generated by ordinary photolytic means. It can still be argued, however, that the redox process results in an exceptionally high local concentration of triplets. Even under these conditions, however, the emission should show sensitivity to added materials which are efficient triplet quenchers. To properly function in an ECL experiment, a triplet quencher must be electroinactive under the experimental conditions, and it must be chemically unreactive to all the species present in the emitting system. 

It is also possible for species that are created in ECL reactions to interact with each other in ways that interfere with the generation of ECL and partially, if not completely, quench the emission. For example, one difficulty in direct sensing of coreactants is that the coreactant may also quench the luminescence of the excited state generated in the annihilation process. This difficulty was recognized several years ago by Bard and coworkers in the examination of the [(bpy)3Ru]2+/S20g system [24], Luminescence arises upon reduction of the Ru(II) complex and reduction of S20g mediated by the Ru(I) complex formed. The intermediate SOJ ion formed is a powerful oxidant and annihilation with the Ru(I) complex will yield the excited state of Ru (II) complex [Eq. (13d)]. However, the persulfate ion is an effective quencher of the MLCT excited state of the Ru(II) complex. Figure 9 shows the observed ECL intensity for this system... 

A number of techniques have been applied to probe the details of the ECL processes. Triplet intermediates resulting from the redox reaction have been identified by interception with fnms-stilbene and determination of the extent of trans- to cis-isomerization (Freed and Faulkner, 1971) as well as by observing the effect of an external magnetic field on the emission intensity (Faulkner et al., 1972 Tachikawa and Bard, 1974). In the latter studies, it was shown that a magnetic field decreases the rate of the triplet-triplet annihilation reaction, (112) or (114), and also the rate of quenching of triplets by radical ions [eqn (116)]. Since the rate of the radical ion redox... 

Fig. 4.4. Aciylamide and CsCI quenching ecls on the ascorbate oxidase em ssion properties. Steady-state (open squares) and dynamic (Filed squares) quenching of ascoibale oxidase by aciylamide (A) and CsCI (B) upon excitation at 293 nm. Source Di Venerc, A, Mei, C.. Gibrdi, G.. Ros, N., Oe MatteE, F., McKs. R.. Cralton. E. and Finazzi Agro, A. I99S. Fur J. Bioclicm 257. 337-343. AudxHization of reprint accorded by Blackwell PuUisliing.

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