ECL Efficiency

Forbes et al. [14] introduced working electrode into the separation capillary to perform the ECL and ECL signal was measmed using a parabolic reflector to 

It is worth mentioning that a europium(III)-doped prussian blue analog (Eu-PB) film was modified chemically on the surface of a microdisk platinum working electrode to avoid the possible electrode fouling as well as to improve the ECL efficiency and detection sensitivity. After optimizing the conditions, the ECL intensity was in proportion to analyte concentration in the range from 0.01 to 

Simultaneous EC and ECL detection of amphetamines namely methamphet-amine, 3,4-methylenedioxyamphetamine, and 3,4-methylenedioxymethamphet-amine was performed employing CE. Eactors affecting the separation and detection performance, such as the detection potential, the pH value and concentration of the running buffer, the separation voltage, and the pH of the detection buffer, were explored. A liquid-liquid extraction with ethyl acetate procedure was 

Recently, CE coupled with Ru(bpy)3 ECL for highly sensitive detection of metformin hydrochloride (MH) derivatized with acetaldehyde is reported. The precolumn derivatization of MH with acetaldehyde was performed in phosphate buffer solution (0.3 mol L pH 7.5) at room temperature for 120 min. The factors affecting this method of analysis, e.g., acetaldehyde concentration, buffer pH, electrokinetic voltage and injection time were examined and under optimization of these conditions, the MH ECL detection sensitivity was more than 120 times that without derivatization. The detection of 0.3 ng mL with S/N = 3 was attained. The proposed method is simple, economical, and sensitive and is used for the determination of MH in urine [42]. A method combining CE with Ru(bpy)3 ECL detection that can be applied to amine-containing clinical species was developed, and the performance of CE-ECL as a quantitative method for the determination of sulpiride in human plasma or urine was evaluated [43]. 

In the following year, lincomycin was determined by the microchip CE-ECL system where ITO working electrode was fabricated by photolithographic method from an ITO-coated glass slide (chip substrate) located at the end of the separation channel (Fig. 5.10). The top layer made up of a poly(dimethylsiloxane) (PDMS) layer consisting of two channels, namely separation and injection channels. This microchip CE-ECL system can be successfully applied for the rapid analysis of 

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Wallace WL, Bard AJ (1979) Electrogenerated chemi-luminescence. 35. Temperature-dependence of the ECL efficiency of Ru(bpy)32+ in acetonitrile and evidence for very high excited-state yields from electron-transfer reactions. J Phys Chem 83 1350-1357... 

MLCT) state occurs with an efficiency < es approaching 100%. Consequently, the overall, actinometrically determined ECL efficiency ( ci expressed in emitted photons produced per annihilation event) is close to the luminescence quantum... 

Extremely high ECL efficiencies seem to be a common feature of the homoleptic-IrL3 as well as the heteroleptic-L2Ir(X) iridium(III) cyclometallated complexes. Extremely high ECL efficiencies (up to 0.55) were observed via ion annihilation between the electrochemically generated L2Ir(acac)+ or L2Ir(pico) + cations (where... 

ECL emission has been also observed in the mixed ECL systems involving PAHs with reaction partners like aromatic amines or ketones forming radical cations D + or radical anions A-, respectively.114127 Such approach solves the problems caused by the instability of ECL reactants but lowers distinctly the free energy available for the formation of an excited state. Usually, the energy released in electron transfer between A- + D + ions is insufficient to populate emissive 11A or D states directly and the annihilation of the radical ions usually generates only nonemissive3 A or 3 D triplets that produce light via triplet-triplet annihilation. Consequently ECL efficiencies in the mixed ECL systems are usually very low. Only in some cases, when radiative electron transfer between A + D+ species is operative, relatively intense [A D + ] exciplex emission can be observed. 

The observation of luminescence from laser dyes by ECL methods offers the possibility of using this approach to create dye lasers. A laser operating by ECL would not require an additional pump laser, and enhanced power, tunability, and wavelength selection are additional factors. While the pumping rate achieved by ECL previously has been two orders of magnitude lower than the optimal, Horiuchi et al. have reported a device structure designed to enhance the ECL efficiency and realize laser action driven by ECL [67], This experiment is illustrated in Fig. 14. A pair of sputter-deposited platinum film electrodes were positioned facing each other 2 to 7 microns apart. One electrode functioned... 

With the advent of rapid-scan and high-frequency pulse methods, more direct approaches for evaluating annihilation mechanisms and dynamics have been developed. Early work of van Duyne, using triple potential steps with very short step times, allowed estimation of the annihilation rate constant for DPA anion and cation radicals [29]. More recently, Wightman and coworkers have used multicycle generation of ECL at microelectrodes to determine annihilation rate constants and ECL efficiencies [41, 42]. Figure 7 shows the normalized ECL intensity from DPA at a 1-pm Pt disk as a function of time (t/tf) at different oscillation frequencies. The intensity increases rapidly after the potential is switched, and then decays as the reactants are depleted. As the oscillation frequency is increased, the annihilation occurs closer to the electrode surface, the intensity-time profile broadens and... 

This simplest route is the most important case, because the experimental values of the ECL efficiencies may be compared with those theoretically predicted. However, it should be noted that the case of only two reaction pathways, i.e., formation of the emissive excited state and the ground state, has been found only for the ECL systems of some coordination compounds of transition metals (e.g., [98,99]). 

Fig. 12. Rubrene ECL efficiency (i ed) as a function of the solvent polarity parameter (Pekar factor (1/n — le)) in ( ) single and (o) mixed 1 1 solvents. Solvent name abbreviations ACN. acetonitrile BL, 7-butyrolactone BN, butyronitrile DME, 1,2-dimethoxyethane DMF, N,N-dimethylformamide NMP, Al-methylpyr-rolidone THE, tetrahydrofuran benzene CH3 toluene f>CN, benzonitrile. Adapted from [123].

The more exergonic formation of the ground states is inhibited and it is expected to occur at the rate ko- k. In view of these considerations and taking into account the fact that the ECL efficiency does not exceed a few percent k < 3), Eq. (47) can be further simplified. Now efficiencies of the direct formation of the excited singlet are expressed as follows ... 

Equation (51) may be directly applied in the interpretation of ECL efficiency data for DPA. With the ECL efficiencies of this system [122] one can obtain Ao values in the range 0.13-0.22 eV. Similar values are obtained from the calculation according to Eq. (6). Assuming the effective radii of both DPA radicals n = T2 r = 0.48 nm (from the molar volume of DPA) and reaction distance ri2 = 0.55 nm, the following values can be obtained Ao = 0.15eV in 1,2-dimethoxyethane and 0.20 eV in acetonitrile solutions (Ao = 0.38 eV in N,N-dimethylformamide solutions has been found in [130] for electron exchange between anthracene and its radical anion). These calculations must be treated only as a semiquantitative approach until the systematic temperature study of eci efficiency has been done, especially as the DPA ECL system has been studied only in a limited number of solvents. 

Fig. 14. Ratio of the pre-exponential (statistical) factors in the kinetic description of jubrene ECL efficiency as a function of the separation rate (kscp) in 1,2-dimethoxyethane (DME), M -dimethylformamide (DMF), butyronitrile (BN) and 7-butytolactone (BL) solutions. The solid line represents the fit with triplet-triplet up-conversion rate kupc = 1 x 10 m" s . Adapted from [123].

The reaction mechanism described above also allows an explanation for the influence of the magnetic field on ECL efficiencies. In the case of DPA, the absence of magnetic field effects [104,114] in ECL supports the dominance of the S-route. Moreover, results reported in [104,112,133] for the rubrene ECL system are not in conflict with the reaction mechanism proposed it can be explained by assuming that... 

Appropriate values of these rates are not directly available, but a quantitative estimate is possible in view of the experimental values of ECL efficiencies that have... 

Fig. 25. ECL efficiencies (squares), luminescence quantum yields (circles) and efficiencies of formation of the excited state (diamonds) for Ru(bpy)3 in acetonitrile (open symbols) and Ru(dph)j in butyroni-trile solutions (solid symbols) as a function of the temperature (F). Data from [180] and [189].

The excellent solubility of MogCl tetra-n-butylammonium salt in nonaqueous solutions permits investigations in various organic solvents, with the most important conclusion that the observed ECL efficiency (4>es in the range 0.014-0.50) depends strongly on the reaction medium [199]. The es efficiency obtained is quite small (0.065) in ACN solutions but increases with lowering of the solvent polarity in a similar way to what is observed for organic ECL systems (up to 0.50 in dichloromethane or 1,2-dichloroethane solutions). 

The results obtained clearly demonstrate that the Marcus model for ECL processes may be used for qualitative as well as for quantitative descriptions of this kind of electron transfer reactions. The more sophisticated approach, taking into account the vibronic excitation in the reaction products (important in the inverted Marcus region), solvent molecular dynamics (important in the case of large values of the electronic coupling elements), as well as the changes in the electron transfer distance, should be used. The results indicate that the Marcus theory may also be used for predicting the ECL efficiency, provided that some conditions are fulfilled. Especially, during the ECL process, only the annihilation of ions should occur, without any competitive reactions. The necessary rate constants can be evaluated from pertinent electrochemical and spectroscopic data. 

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