Analytical Chemiluminescence/Electrochemiluminescence

D7. Electrochemiluminescence
Electrochemiluminescence is chemiluminescence arising as a result of electrochemical reactions. It includes electrochemical initiation of ordinary chemiluminescent reactions, electrochemical modification of an analyte enabling it to take part in a chemiluminescent reaction, or electron transfer reactions between radicals or ions generated at electrodes. Prominent in the work done on electrochemiluminescence are reactions involving polyaromatic hydrocarbons or transition metal complexes, especially those of ruthenium, palladium, osmium and platinum.

Applications have made use of the sensitivity, selectivity and wide working range of analytical chemiluminescence, but electrochemiluminescence offers additional advantages without adding much to the inexpensive instrumentation. Electrodes can be designed to achieve maximum detection of the light emitted and electrochemical measurements can be made simultaneously with the light output. Generation of chemiluminescence reagents at electrodes gives control over the course of light producing reactions, which can effectively be switched on and off by alteration of the applied potential; this is particularly useful when using unstable reagents or intermediates. Other possible benefits include generation of reagents from inactive precursors and regeneration of reagents, which permits the use of lower concentrations or immobilization of the reagents on the electrode. Analytes can also be regenerated, so that each analyte molecule can produce many photons, increasing sensitivity, or they can be modified to make them detectable by the chemiluminescence reaction in use. Electrochemiluminescence can be coupled with high performance liquid chromatography or with capillary electrophoresis.

The usefulness of tris-(2, 2/-bipyridyl)ruthenium(II) (discussed in chapter B9 ADD LINK) in electrochemiluminescence rests on its activity with very high efficiency at easily accessible potentials and ambient temperature in aqueous buffer solutions in the presence of dissolved oxygen and other impurities. The reaction sequence that leads to electrochemiluminescence is shown in equations D7.1 to D7.4:

(D7.1) Oxidation:                 [Ru(bipy)3]2+  ─ e― → [Ru(bipy)3]3+

(D7.2) Reduction by analyte:      [Ru(bipy)3]2+ + e― → [Ru(bipy)3]+

(D7.3) Electron transfer:         [Ru(bipy)3]3+ + [Ru(bipy)3]+ → [Ru(bipy)3]2+ + [Ru(bipy)3]2+*

(D7.4) Chemiluminescence:         [R(bipy)3]2+* → [Ru(bipy)3]2+ + light



Figure D7.1 – A flow injection manifold for measuring electrochemiluminescence.

The oxidation occurs electrochemically at the anode, whereas the reduction is brought about chemically by the analyte in the free solution. Electron transfer and subsequent chemiluminescence also occur in the free solution close to the anode, where the [Ru(bipy)3]3+ is concentrated. Other analytes, e.g. alkylamines, are oxidized at the anode to form a highly reducing radical intermediate that reacts with [Ru(bipy)3]3+ to form [Ru(bipy)3]2+*, which emits light. Oxalates, on the other hand, are oxidized by [Ru(bipy)3]3+ to radicals that then reduce more [Ru(bipy)3]3+ to give [Ru(bipy)3]2+* and chemiluminescence.

Instrumentation for electrochemiluminescence differs from that for other chemiluminescence only in having a flow cell provided with working, counter and reference electrodes, regulated by a potentiostat, which is in turn controlled by the computer that receives input from the photomultiplier or other transducer that receives the light signals. Figure D7.1 shows the usual flow injection manifold used for measuring electrochemiluminescence. The flow cell is in a light-tight box to exclude ambient light. A more portable alternative is a probe containing a set of electrodes and a fibre optic bundle to carry emitted light to a photomultiplier. Ambient light is excluded by means of baffles in the channels that admit the test solution. Because it can be electrochemically regenerated, it is useful to immobilize [Ru(bipy)3]3+ in a cation exchange resin deposited on the electrode to form a sensor that does not need a continual reagent supply.