Electrogenerated chemiluminescence (also called electrochemiluminescence and abbreviated ECL) is the process whereby species generated at electrodes undergo high-energy electron-transfer reactions to form excited states that emit light. The first detailed ECL studies were described by Hercules and Bard et al. in the mid-1960s, although reports concerning light emission during electrolysis date back to the 1920s by Harvey. After about 40 years study, ECL has now become a very powerful analytical technique and been widely used in the areas of, for example, immunoassay, food and water testing, and biowarfare agent detection. ECL has also been successfully exploited as a detector of flow injection analysis (FIA), high-performance liquid chromatography (HPLC), capillary electrophoresis, and micro total analysis (μTAS). Figure 1 illustrates a time line of various events in the development of ECL. A literature survey using SciFinder Scholar reveals that more than 2000 journal articles, book chapters, and patents on various topics of ECL have been published. The overall number of publications, as shown in Figure 2, has increased exponentially over the past 20 years, of which 40–50% were biorelated. Similar amounts of ECL papers could be also found from the Thomson ISI Web of Science as well as † Telephone (601) 266 4716; fax (601) 266 6075; e-mail wujian.miao@ usm.edu. Wujian Miao received his undergraduate diploma in chemistry from Nantong University (Nantong, China) in 1982, his M.Sc. degree in analytical chemistry from Zhongshan University (Guangzhou, China, with Jinyuan Mo) in 1991, and his Ph.D. degree in electrochemistry from Monash University (Melbourne, Australia, with Alan M. Bond) in 2000. He then served as a Research Scientist in CSIRO (Melbourne, Australia), followed by a postdoctoral fellowship at the University of Texas at Austin with Allen J. Bard in 2001. Since 2004 he has served as an assistant professor of chemistry at the University of Southern Mississippi.

Electrogenerated Chemiluminescence and Its Biorelated Applications

Electrochemiluminescence (ECL) is chemiluminescence triggered by electrochemical techniques. More than 150 ECL assays with remarkably high sensitivity and extremely wide dynamic range are currently available, and accounts for hundreds of millions of dollars in sales per year. The recent development of ECL is particularly rapid. After a brief introduction to ECL, this critical review presents the active and/or emerging areas of ECL research as well as new applications and phenomena of ECL, such as light-emitting electrochemical cell, wireless electrochemical microarray using ECL as photonic reporter, high throughput analysis, aptasensors, immunoassays and DNA analysis, ECL of nanoclusters and carbon nanomaterials, ECL imaging techniques, scanning ECL microscopy, colorimetric ECL sensor, surface plasmon-coupled ECL, electrostatic chemiluminescence, soliton-like ECL waves, ECL investigation of molecular interaction, and single molecule detection. Finally, some perspectives on this rapidly developing field are discussed (322 references).

Applications and trends in electrochemiluminescence.

The reaction occurring on electrooxidation of Ru(bpy)32+ (bpy = 2,2‘-bipyridine) and tri-n-propylamine (TPrA) leads to the production of Ru(bpy)32+* and light emission. The accepted mechanism of this widely used reaction involves the reaction of Ru(bpy)33+ and a reduced species derived from the free radical of the TPrA. However, this mechanism does not account for many of the observed features of this reaction. A new route involving the intermediacy of TPrA cation radicals (TPrA•+) in the generation of Ru(bpy)32+* was established, based on results of scanning electrochemical microscopy (SECM)-electrogenerated chemiluminescence (ECL) experiments, as well as cyclic voltammetry simulations. A half-life of ∼0.2 ms was estimated for TPrA•+ in neutral aqueous solution. Direct evidence for TPrA•+ in this medium was obtained via flow cell electron spin resonance (ESR) experiments at ∼20 °C. The ESR spectra of the TPrA•+ species consisted of a relatively intense and sharp septet with a splitting of ∼20 G and a g v...

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Electrogenerated Chemiluminescence 69: The Tris(2,2‘-bipyridine)ruthenium(II), (Ru(bpy)32+)/Tri-n-propylamine (TPrA) System RevisitedA New Route Involving TPrA•+ Cation Radicals

Recent applications of electrogenerated chemiluminescence in chemical analysis.

This work reported for the first time the anodic electrochemiluminescence (ECL) of CdTe quantum dots (QDs) in aqueous system and its analytical application based on the ECL energy transfer to analytes. The CdTe QDs were modified with mercaptopropionic acid to obtain water-soluble QDs and stable and intensive anodic ECL emission with a peak value at +1.17 V (vs Ag/AgCl) in pH 9.3 PBS at an indium tin oxide (ITO) electrode. The ECL emission was demonstrated to involve the participation of superoxide ion produced at the ITO surface, which could inject an electron into the 1Se quantum-confined orbital of CdTe to form QDs anions. The collision between these anions and the oxidation products of QDs led to the formation of the excited state of QDs and ECL emission. The ECL energy transfer from the excited CdTe QDs to quencher produced a novel methodology for detection of catechol derivatives. Using dopamine and l-adrenalin as model analytes, this ECL method showed wide linear ranges from 50 nM to 5 μM and 80 nM ...

Anodic electrochemiluminescence of CdTe quantum dots and its energy transfer for detection of catechol derivatives.

Efficient quenching of Ru(bpy)32+ (bpy = 2,2‘-bipyridine) electrogenerated chemiluminescence has been observed in the presence of phenols, catechols, hydroquinones, and benzoquinones. In most instances, quenching is observed with 100-fold excess of quencher over Ru(bpy)32+, with complete quenching observed between 1000- and 2000-fold excess. The mechanism of quenching is believed to involve energy transfer from the excited-state luminophore to benzoquinone. In the case of phenols, catechols, and hydroquinones, quenching is believed to occur via a benzoquinone derivative formed at the electrode surface. Photoluminescence and UV−visible experiments coupled with bulk electrolysis support the formation of benzoquinone products upon electrochemical oxidation.

Quenching of electrogenerated chemiluminescence by phenols, hydroquinones, catechols, and benzoquinones.

Efficient quenching of Ru(bpy)32+ (bpy = 
2,2′-bipyridine) electrogenerated chemiluminescence (ECL) was 
observed in the presence of phenol and substituted phenols (e.g., 
4-fluorophenol). Spectroscopic and electrochemical studies indicated that 
the mechanism of quenching involves energy transfer from the excited state 
luminophore to a benzoquinone derivative formed at the electrode. The 
efficiency of ECL quenching is directly related to the position of the 
substituent on the aromatic ring, with meta derivatives displaying 
the greatest magnitude of quenching. The degree of quenching does not 
appear to be related to the electron-donating or -withdrawing ability of 
the phenol substituent.

Phenol substituent effects on electrogenerated chemiluminescence quenching

The effects of electron withdrawing and electron donating groups on the electrochemiluminescent (ECL) properties of tris(2,2′-bipyridyl)ruthenium(II) (Ru(bpy)32+ where bpy = 2,2′-pyridine) are reported. The electrochemistry, photophysics and ECL of (bpy)2Ru(DC-bpy)2+, and (bpy)2Ru(DM-bpy)2+ (DC = 4,4′-dicarboxy-2,2′-bipyridine; DM = 4,4′-dimethyl-2,2′-bipyridine) have been studied relative to Ru(bpy)32+ in 50∶50 (v/v) acetonitrile(CH3CN)∶H2O (0.1 M KH2PO4), and aqueous solutions. Furthermore, the effects of Triton X-100 (polyethylene glycol tert-octylphenyl ether) on the electrochemical, spectroscopic and ECL properties of these compounds are reported. The anodic oxidation of Ru(bpy)32+, (bpy)2Ru(DC-bpy)2+, and (bpy)2Ru(DM-bpy
)2+ produces ECL in the presence of tri-n-propylamine (TPrA) in all solvent systems. ECL efficiencies (ϕecl, photons produced per redox event) of 0.73 and 0.84 for (bpy)2Ru(DC-bpy)2+, and (bpy)2Ru(DM-bpy)2+ were obtained in aqueous buffered solution, using Ru(bpy)32+ as a relative standard (ϕecl = 1.0). Addition of 0.4 mM Triton X-100 results in a greater than 2-fold increase in ECL efficiences (i.e., 3.8, 2.4 and 2.3 for Ru(bpy)32+, (bpy)2Ru(DC-bpy)2+, and (bpy)2Ru(DM-bpy)2+, respectively) using aqueous Ru(bpy)32+ containing no surfactant as standard (ϕecl = 1.0). ECL efficiencies of 27.4, 16.5 and 26.1 were found in 50∶50 (v/v) CH3CN∶H2O (0.1 M KH2PO4) for Ru(bpy)32+,
(bpy)2Ru(DC-bpy)2+, and (bpy)2Ru(DM-bpy)2+, respectively, using aqueous Ru(bpy)32+ containing no surfactant as standard (ϕecl = 1.0). Detailed studies support adsorption of surfactant on the electrode surface, thus facilitating TPrA and ruthenium oxidation.

Effects of electron withdrawing and donating groups on the efficiency of tris(2,2'-bipyridyl)ruthenium(II)/tri-n-propylamine electrochemiluminescence.

The electrochemical and electrochemiluminescence (ECL) properties of Cu[dmp]2+ (dmp = 2,9-dimethyl-1,10-phenanthroline) have been investigated. ECL has been observed for Cu(dmp)2+ in aqueous, nonaqueous, and mixed solvent solutions using tri-n-propylamine as an oxidative−reductive coreactant. The ECL intensity peaks at potential corresponding to oxidation of both the coreactant and Cu(dmp)2+. The peak potential corresponding to maximum ECL emission is ∼500 mV more anodic than corresponding oxidative peak potentials, indicating that the ECL emission may be due to the formation of either the *Cu(dmp)2+ metal-to-ligand charge-transfer excited state or an excited-state product of Cu(dmp)2+ oxidation. ECL efficiencies (φecl = photons generated per redox event) are solvent-dependent (φecl (CH3CN) > φecl (50:50 (v/v) CH3CN:H2O) > φecl (H2O)) and correspond fairly well with photoluminescence efficiencies. Increased ECL efficiencies (≥50-fold) are observed in the presence of the nonionic surfactant Triton X-100.

Electrochemiluminescence of copper(I) bis(2,9-dimethyl-1,10- phenanthroline).

An undergraduate instrumental analysis laboratory exercise is presented for the characterization of light emission generated using electrochemiluminescence (ECL). ECL involves the electrochemical generation of excited states and as such is a sensitive probe of electrochemical, electron-transfer and energy-transfer processes at electrified interfaces. An objective of this experiment is to have students develop an understanding of the mechanisms and factors affecting ECL. Also, this exercise gives students experience in coupling two powerful analytical techniques: electrochemistry and spectroscopy. With the recent development of ECL technology for use in clinical diagnostics applications, this exercise also facilitates discussions on the importance of basic research and the practical aspects of taking a technology from the bench top to commercial reality.

Jeff McCall

Papers

Quenching of electrogenerated chemiluminescence by phenols, hydroquinones, catechols, and benzoquinones.

Phenol substituent effects on electrogenerated chemiluminescence quenching

Effects of electron withdrawing and donating groups on the efficiency of tris(2,2'-bipyridyl)ruthenium(II)/tri-n-propylamine electrochemiluminescence.

Electrochemiluminescence of copper(I) bis(2,9-dimethyl-1,10- phenanthroline).

An Instrumental Analysis Laboratory Using Electrogenerated Chemiluminescence