Electrochemically generated luminescence

Much of the study of ECL reactions has centered on two areas electron transfer reactions between certain transition metal complexes, and radical ion-annihilation reactions between polyaromatic hydrocarbons. ECL also encompasses the electrochemical generation of conventional chemiluminescence (CL) reactions, such as the electrochemical oxidation of luminol. Cathodic luminescence from oxide-covered valve metal electrodes is also termed ECL in the literature, and has found applications in analytical chemistry. Hence this type of ECL will also be covered here. 

The speed of p- and n-type doping and that of p-n junction formation depend on the ionic conductivity of the solid electrolyte. Because of the generally nonpolar characteristics of luminescent polymers like PPV, and the polar characteristics of solid electrolytes, the two components within the electroactive layer will phase separate. Thus, the speed of the electrochemical doping and the local densities of electrochemically generated p- and n-type carriers will depend on the diffusion of the counterions from the electrolyte into the luminescent semiconducting polymer. As a result, the response time and the characteristic performance of the LEC device will highly depend on the ionic conductivity of the solid electrolyte and the morphology and microstructure of the composite. 

Figure 3.38 — Integrated flow-through sensors. (A) With electrochemical generation of the luminescent reagent. The flow stream path follows the line between the analyte inlet and the outlet to waste. (B) With immobilization of a phosphor (length, 3 cm internal diameter, 2 mm) 1 immobilized phosphor 2 CFG 3 quartz wool plug 4 KEL-F caps 5 hand-tightened screw 6 stainless steel capillaries. (C) Sensor based on reflectance measurements. The sensor membrane is fixed on a Plexiglas disc. Reflectance spectra are measured from the rear side. (Reproduced from [267] and [269] with permission of the American Chemical Society and Elsevier Science Publishers, respectively).

The occurrence and deactivation of excited states of the first type are schematically shown in Fig. 35. Let the minority carriers (holes) be injected into the semiconductor in the course of an electrode reaction (reduction of substance A). The holes recombine with the majority carriers (electrons). The energy, which is released in the direct band-to-band recombination, is equal to the energy gap, so that we have the relation ha> = Eg for the emitted light quantum (case I). More probable, however, is recombination through surface or bulk levels, lying in the forbidden band, which successively trap the electrons and holes. In this case the excess energy of recombined carriers is released in smaller amounts, so that hco < Eg (case II in Fig. 35). Both these types of recombination are revealed in luminescence spectra recorded with n-type semiconductor electrodes under electrochemical generation of holes (Fig. 

The observation of molecular luminescence at electrode solution interfaces results from high-energy annihilation reactions between electrochemically generated radical ions that result in the formation of an electronically excited species [6-16], The radical ions can be generated at two separate electrodes in close proximity to one another or at the same electrode by alternating between reductive and oxidative potentials. This is particularly useful when the radical ions are unstable since they can be produced in situ immediately prior to, or during, the reaction. The general mechanism of an ECL reaction is as follows. 

There is a variety of analytical applications that involve electrochemical generation of luminescence in systems for which the chemistry is not clearly understood. A few are mentioned here. 

The thrust of this chapter has been to provide a brief overview of this fascinating area, from the essential theoretical framework provided by electron-transfer theory to the array of potential novel device applications. From early studies of radiative charge recombination, experimental approaches developed greater sophistication and the range of chemical reaction types expanded. The ability to generate luminescence by electrochemical excitation presents a rich array of potential device applications and the future of research in this area is certainly bright. 

Kuwana T, Epstein B, Seo ET (1963) Electrochemical generation of solution luminescence. J Phy Chem 67 2243-2244... 

Oxidized and reduced states of DPA electrochemically generated on the anode and the cathode, respectively, collide with each other to generate the excited state of DPA. Consequently, luminescence is obtained from the excited state of DPA. 

When a solution of Pt2(pop)4 is subjected to alternating current electrolysis with variable frequency, the 514-nm luminescence is observed at one electrode. This observation of chemiluminescence has been explained by the generation of triplet Pt2(pop)4 from the electron transfer reaction between electrochemically generated Pt2(pop)4 and Pt2(pop)4 [Eq. 4.16)]. ... 

Mager, H. I. X., etal. (1990). Electrochemical superoxidation of flavins generation of active precursors in luminescent model systems. Photochem. Photobiol. 52 1049-1056. 

Some electrochemically active substances that can generate photons on an electrode surface are suitable labels for homogeneous immunoassays. A labelled antigen exhibits an electrochemical reactivity and produces luminescence, but when it is immunochemically complexed, the labelled antigen loses its electrochemiluminescent properties. One optical immunosensor for homogeneous immunoassays was assembled by spattering platinum on the end surface of an optical fibre. Spattered platinum maintains optical transparency and functions as an electrode. An optical electrode efficiently... 

Mediated electrochemical sensors aside, there are few sensors involving a reaction at the sensing microzone by which the analyte is not retained to some extent during the time the analytical response is generated. Such is the case with sensors based on luminescence quenching and a few others. Although many of the reactions on which the analytical measurement rests in sensors based on acid—base reactions involve retention of protons, the sensors in question are dealt with in this Section. 

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