Analytical applications of tris(2,2′-bipyridyl)ruthenium(III) as a chemiluminescent reagent

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Abstract

This paper reviews the analytical applications of the chemiluminescence reactions involving tris(2,2′-bipyridyl)ruthenium(III), from the earliest paper in 1978 to mid 1998. After an introduction which briefly describes historical perspectives, spectroscopic and mechanistic considerations, the review is divided into two major sections. The first section discusses the methods of generation of tris(2,2′-bipyridyl)ruthenium(III) reagent, including chemical/photochemical, electrochemical and in situ electrochemiluminescence methods. The second section describes the applications of this reagent to analysis under broad classifications according to the type of analyte determined. Entries for indirect methods, immunoassay and DNA probe assays have also been included.

Introduction

Luminescence, the emission of light without heat, and more specifically the generation of light from chemical reactions (chemiluminescence) have both enchanted and mystified observers for millennia. This long and fascinating history has been meticulously detailed in the excellent text by Harvey [1]. Chemiluminescence is no longer a laboratory curiosity with the number and variety of liquid phase applications increasing markedly in recent years. This trend is reflected in recent review articles by Worsfold and co-workers 2, 3, 4, Pringle [5], and Rongen et al. [6]. For a comprehensive treatment of the principles, instrumentation and analytical applications of various chemiluminescent reactions the reader is directed to the relevant sections of “The Encyclopedia of Analytical Science” and the respective bibliographies therein [7]. The present review deals primarily with the analytical utility of chemiluminescence resulting from the reactions of tris(2,2′-bipyridyl)ruthenium(III) {Ru(bipy)3+3} with various substrates. Further information on the chemistry of the polypyridyl chelates of ruthenium, can be found in the definitive text by Seddon and Seddon [8]. However, to fully appreciate the significance of the analytical applications of Ru(bipy)3+3, some background knowledge regarding the synthesis, redox properties, luminescence spectroscopy and chemiluminescence reaction mechanisms of this particular ruthenium complex cation are essential.

A publication by Burstall [9] on the synthesis and optical activity of several tris(2,2′-bipyridyl)ruthenium(II) salts {Ru(bipy)2+3} (see Fig. 1) appeared in 1936, in which he noted that the action of either concentrated nitric acid or warm concentrated sulphuric acid upon the “bright red leaflets” of tris(2,2′-bipyridyl)ruthenium(II) chloride hexahydrate, “furnished a deep green solution”. We have recently repeated the concentrated nitric acid experiment and can confirm Burstall’s observations [9]. Of more relevance to this review however, was that the neutralisation of the resultant blue/green solution with aqueous sodium hydroxide (approximately 5 M), as expected, generated intense orange chemiluminescence. It is interesting to speculate that if Burstall had performed a similar neutralisation in even a slightly darkened room, he may have reported the chemiluminescence from excited Ru(bipy)2+3 some three decades earlier than Hercules and Lytle [10]. While Burstall [9] offered no explanation for the marked change in colour of the complex with the action of concentrated nitric acid, by 1942 Steigman and co-workers [11] had evaluated the reaction, shown in the following scheme,Ru(bipy)2+3(orange/red)(oxidation)Ru(bipy)3+3(green)as a redox indicator and determined that the couple involved a potential of 1.33 V. When the oxidation potential was re-investigated by Brandt and Smith [12] in 1949 they arrived at a value of 1.25 V under similar conditions. These workers [12] also claimed the first publication of the molar absorptivity (or molecular extinction coefficient) for tris(2,2′-bipyridyl)ruthenium(II)Cl2.6H2O, which they calculated to be 14 500 at 453 nm. At the concentrations used for redox indication (approximately 10−4 M), the colour change was noted as yellow to colourless, the authors also reported that the indicator was particularly suitable in hot concentrated acid solutions [11]. Notwithstanding this utility, the study by Steigman et al. [11] was the only paper, discussing the analytical use of Ru(bipy)2+3, to be cited by Brandt et al. [13] in their extensive review of such chelates, some 12 years later.

The redox indicator characteristics of several periodic table group 8 bipyridyl and cyano-complexes including Ru(bipy)2+3, were published by Schilt [14] in 1963, interestingly without reference by the study of Steigman et al. [11]. Schilt [14] reacted eight reducing agents with each of the oxidised complexes so as to evaluate the efficacy of the proposed indicator systems. As one of the reducing agents evaluated was oxalic acid, (which was first shown to exhibit intense chemiluminescence with Ru(bipy)3+3 by Chang et al. in 1977 [15]), Schilt [14] was also tantalisingly close to the first published report of this type of chemiluminescence reaction. Likewise, Steigman et al. [11] employed sodium oxalate as a reductant to evaluate the redox indicator properties of Ru(bipy)2+3, however, neither of these studies 11, 14 would have produced visually observable chemiluminescence, under normal lighting, due to the low concentration of Ru(bipy)2+3 used (ca. 10−5 M).

Paris and Brandt [16] first observed the photoluminescent nature of Ru(bipy)2+3 in 1959. These workers used a solution of Ru(bipy)2+3 (10−4 M) at 77 K to record both absorption and emission spectra with the latter exhibiting a single narrow band centred at approximately 17 500 cm−1 (ca. 570 nm). By assignments of the major peaks in the absorption spectra to electronic transitions within the complex, Paris and Brandt [16] concluded that the excitation and emission were due to charge transfer transitions between a d-orbital on the ruthenium and a π* antibonding orbital on the ligand. Subsequently Veening and Brandt [17] evaluated 2,2′-bipyridine, 1,10-phenanthroline and their various methyl derivatives as ligands for the selective spectrofluorometric determination of ruthenium in the presence of other platinum group metals. They showed that Ru(bipy)2+3 in simple solution luminesced at around 580 nm after excitation at 460 nm but unfortunately these workers did not provide any analytical figures of merit for the Ru(bipy)2+3 complex [17]. The fluorescent nature of Ru(bipy)2+3 along with other ruthenium complexes was however, successfully utilised for the indication of the end points of oxidation–reduction titrations by Kratochvil and colleagues 18, 19.

As mentioned earlier, the first account of chemically induced luminescence from Ru(bipy)2+3, in the open literature, was presented by Hercules and Lytle [10] in 1966, however, these authors credit Jean P. Paris with observing chemiluminescence from a ruthenium chelate in 1962. Surprisingly, Paris does not mention these observations in the chapter entitled “Chemiluminescence and other Luminescence Processes” which was contributed to a textbook edited by Hercules [20], which is an expansion of eight invited papers presented at an American Chemical Society meeting in 1964. The preliminary chemiluminescence results [10] reappeared in summary form in 1969 [21] and were eventually published with a more complete discussion of reaction kinetics and proposed mechanisms in 1971 [22]. These workers 10, 11, 22 evaluated several ruthenium chelates of 2,2′-bipyridine and substituted 1,10-phenanthrolines for their ability to produce chemiluminescence using the following overall reaction (see scheme below) where `L' represents the various polypyridine ligands.Ru(L)2+3Lead dioxideRu(L)3+3Hydroxide or hydrazine[Ru(L)2+3]→Ru(L)2+3+hν

They reported that the only chelate that gave sufficient chemiluminescence intensity to record a spectrum was Ru(bipy)2+3[22], thus implying that the chemiluminescence quantum yields for the various phenanthroline complexes were extremely poor under the reaction conditions employed. The various ruthenium(II) chelates were made up in acidic solution (0.05 M) at the millimolar level and oxidised with solid lead dioxide and the resultant ruthenium(III) chelate solutions were then decanted after centrifugation 10, 21, 22. When the Ru(bipy)3+3 solution was reacted with either sodium hydroxide (9 M) or hydrazine (0.1 M) intense orange chemiluminescence resulted 10, 21, 22. They noted that the chemiluminescence spectra generated with either sodium hydroxide or hydrazine were identical (λ[em] approximately 600 nm) [22] and that these correlated very closely with the phosphorescence spectrum of Ru(bipy)2+3 produced in an earlier spectrophotometric investigation [23], thus conclusively identifying the emitting species as an excited state of Ru(bipy)2+3. Lytle and Hercules [22] also found that whilst many oxidants could be employed to produce Ru(bipy)3+3 (with the exception of hydrogen peroxide) common reductants, including titanium(III) chloride, sodium sulphite, tin(II) chloride and sodium thiosulphate did not yield chemiluminescence upon reaction with the oxidised complex. Using stopped flow methodology, Lytle and Hercules [22] recorded chemiluminescence intensity vs. time profiles for three different ratios of Ru(bipy)3+3 to hydrazine (200:1, 50:1 and 10:1). These kinetic plots revealed that there were significant differences in both emission intensity and duration at each of the ratios [22]. Their observations have important implications for the attainment of linear calibration functions when employing this type of chemiluminescence detection for either flow analysis or liquid chromatography (LC). Consequently the kinetics and the proposed reaction mechanisms for a variety of substrates will be discussed later on in this review.

The preliminary paper by Paris and Brandt [16] in 1959 postulated that the photoluminescence of Ru(bipy)2+3 could be attributed to a charge transfer transition involving both ligand and ruthenium orbitals. During the following decade several spectroscopic investigations were published which sought to debate the nature of this transition 20, 23, 24, 25, 26, 27, 28. Most of these reports have also discussed the photoluminescent properties of other ruthenium(II) complexes including those of 2,2′,2″-tripyridine and 1,10-phenanthroline. In 1965 Crosby et al. [24] presented a detailed account of their studies on the electronic absorption spectroscopy of Ru(bipy)2+3 and concluded that the bright orange photoluminescence resulted from a singlet–singlet d–d transition on the metal ion rather than a charge transfer from the ligand. Two years later Klassen and Crosby [25] reassigned the luminescence from Ru(bipy)2+3 as a charge transfer electronic transition with either singlet–singlet or triplet–singlet multiplicity being possible. It is especially noteworthy that this scientific “about face” was precipitated by two significant pieces of evidence. Firstly, a weak band in the absorption spectrum of Ru(bipy)2+3 originally assigned to a singlet–triplet (d–d) transition [24] was found in fact to result from the presence of an unknown impurity [25]. This mistake serves to emphasise the intrinsically poor performance of electronic molecular spectroscopy for qualitative analysis. Furthermore Klassen and Crosby [25] synthesised three ruthenium complexes each having two 2,2′-bipyridine and either one ethylene diamine, one oxalate or two chloride ligands, respectively. Since the latter three ligand types vary widely in their relative crystal field strengths, then if the luminescence was from a d–d transition the resultant emission from the three complexes should have increased markedly in wavelength from the ethylene diamine to the chloride: such a shift was not observed by Klassen and Crosby [25].

These authors then published a more detailed spectroscopic study [26] of nine ruthenium(II) complexes, (three of which had been reported earlier [25]) and using the crystal field argument arrived at the same conclusion as before that the luminescence was indeed a charge transfer process of unknown multiplicity. At almost the same time as the crystal field study [26] was produced, Demas and Crosby [27] submitted another paper in which they had measured the mean luminescent lifetimes of eight of the nine ruthenium(II) complexes studied earlier [26] and found that the values all fell within the range from 6×10−7 to 1.1×10−5 s. They noted [27] that these lifetimes were significantly longer than those normally associated with spin-allowed transitions [20]. Additionally, they pointed out that, ruthenium (a heavy atom), should enhance the inter-system crossing to the triplet state [20] and that each compound had exhibited only one emission band. Coupling this evidence with their estimations of high quantum yields, Demas and Crosby [27] concluded that the luminescence was indeed phosphorescence.

In 1969 Lytle and Hercules [23] confirmed the above conclusion by quantitatively determining the rate constants for various spectroscopic processes, including phosphorescence and inter-system crossing together with the temperature dependence of both the quantum yield and excited state lifetime. Demas and Crosby [28] subsequently reported quantum efficiencies and excited state lifetimes for a variety of ruthenium, osmium and iridium complexes including Ru(bipy)2+3. Their results for the latter complex (at a similar temperature) were vastly different to those obtained by Lytle and Hercules [23]. While the solvent systems used in each study 23, 28 were quite different, a more important experimental parameter was at variance. Lytle and Hercules had deoxygenated all solvents whereas Demas and Crosby [28] had air saturated all solvents, believing that oxygen quenching was not an issue at low temperature (77 K). This position is certainly at odds with the proven practice of oxygen removal in analytical phosphorimetry 7, 20, 29. Since that time numerous spectroscopic investigations have been undertaken in order to determine the nature of the lowest excited state of Ru(bipy)2+3 and this body of work has been extensively reviewed 30, 31, 32, 33, 34, 35, 36, 37, 38. Thus it is now generally agreed that the excited state is indeed a triplet which arises from a localised metal to ligand charge transfer process (as postulated by Paris and Brandt [16]) which has been shown to occur in either solutions, rigid glasses 39, 40, 41, 42, 43, 44 or crystalline materials 45, 46, 47, 48, 49, 50, 51. Therefore, the bright orange chemiluminescence observed from [Ru(bipy)2+3]*, (also credited to Paris [10]), would appear to be a rare example of chemically induced phosphorescence in simple solution.

Lytle and Hercules [22] proposed a 10 step mechanism for the chemiluminescence reaction between Ru(bipy)3+3 and hydrazine which was based upon the well documented mechanism for the oxidation of hydrazine by Fe(III) in aqueous acid 52, 53, 54, 55, 56. Their postulated pathway was used in conjunction with a kinetic modelling programme [57] to predict both the chemiluminescence intensity and Ru(bipy)2+3 absorbance vs. time profiles at various concentrations of the two reactants. The computer generated chemiluminescence and absorbance functions accurately matched the respective experimentally obtained curves for Ru(bipy)3+3 at 5×10−4 M and hydrazine at 2.5×10−2 M [22]. However, using other reactant concentrations the agreement between the predicted and actual data was only qualitative. As a consequence of these findings they concluded that their proposed chemiluminescence pathway was lacking certain important processes [22]. Martin and co-workers [58] suggested that high energy intermediates such as HO2 and N2H2, may be involved during the chemiluminescence reactions of Ru(bipy)3+3 with sodium hydroxide and hydrazine, respectively.

Several studies conducted by Bard’s group 15, 59, 60, 61, 62, 63, 64 have provided greater insight into the mechanistic aspects of this particular chemiluminescent system. It was Tokel and Bard [59] who in 1972 described an electrochemical method that resulted in the generation of the excited state [Ru(bipy)2+3]* and hence electrochemiluminescence from the interaction between Ru(bipy)3+3 and Ru(bipy)+3, (see scheme 3 below).Ru(bipy)2+3−e→Ru(bipy)3+3Ru(bipy)2+3→Ru(bipy)+3+eRu(bipy)+3+Ru(bipy)3+3→[Ru(bipy)2+3]+Ru(bipy)2+3[Ru(bipy)2+3]→Ru(bipy)2+3+hv

Some time later Chang et al. [15] published the first account of chemiluminescence emanating from the reaction of Ru(bipy)3+3 with tetra-n-butylammonium oxalate. In an attempt to identify the postulated intermediate 1,2-dioxetanedione from the so-called “peroxyoxalate” chemiluminescent reaction [65], these workers [15] studied the electrochemical oxidation of the oxalate ion in acetonitrile. It was observed that the oxidation of oxalate in the presence of certain fluorescent compounds produced light [15]. Of particular note was the observation that electro-oxidation of oxalate in the presence of Ru(bipy)2+3 did not yield chemiluminescence. However, the electro-oxidation of both oxalate and Ru(bipy)2+3 simultaneously at a platinum electrode yielded an emission of light [15], characteristic of that attributed to the luminescence seen from [Ru(bipy)2+3]*10, 16, 21. Subsequently, Rubinstein and Bard [62] proposed the following mechanism (scheme below) for the observed electrochemiluminescence.Ru(bipy)2+3−e→Ru(bipy)2+3Ru(bipy)3+3+C2O2−4→Ru(bipy)2+3+C2O2−⋅4C2O2−⋅4→CO−⋅2+CO2followed by eitherRu(bipy)3+3+CO−⋅2→[Ru(bipy)2+3]+CO2orRu(bipy)3+3+CO−⋅2→Ru(bipy)+3+CO2Ru(bipy)+3+Ru(bipy)3+3→[Ru(bipy)3+3]+Ru(bipy)2+3then[Ru(bipy)2+3]→Ru(bipy)2+3+hν.

The above mechanism in scheme (4) highlights the essential role of the high energy radical anion intermediate (CO−⋅2) in the production of the excited state species [Ru(bipy)2+3]* from which the luminescence is observed. Rubinstein and Bard [62] also noted that for the organic acid pyruvate, no luminescence was observed under identical conditions to those which produced an emission from oxalate. However, when Ru(bipy)2+3 was added to a system containing pyruvate and cerium(III), subsequent voltage pulsing to the cerium(III) oxidation potential generated an orange emission. These workers [62] proposed that cerium(IV), having a higher oxidation potential than Ru(bipy)2+3, oxidises the pyruvate to form an intermediate radical [62] of sufficient energy to react with Ru(bipy)3+3, as shown in the following scheme:Ru(bipy)2+3→Ru(bipy)3+3+eCe3+→Ce4++eCe4++CH3COCO2→Ce3++CH3COCO2CH3COCO2→CH3CO+CO2Ru(bipy)3+3+CH3CO+H2O→Ru(bipy)2+∗3+CH3CO2H+H+[Ru(bipy)2+3]→Ru(bipy)2+3+hν.

Similarly, light emission was observed when cerium(IV) was added to a solution of Ru(bipy)2+3 and reacted with organic acids such as malonic and lactic acids or pyruvate [62]. Thus, chemical generation of such luminescence served to confirm the observations made utilising electrochemically produced cerium(IV) [62]. White and Bard [63] described a reaction sequence for the generation of electrochemiluminescence from the reaction of peroxydisulphate (S2O2−8) with Ru(bipy)+3. The proposed reaction (see scheme below)Ru(bipy)2+3+e→Ru(bipy)+3Ru(bipy)+3+S2O2−8→Ru(bipy)2+3+S2O3−8S2O3−8→SO2−4+SO−⋅4Ru(bipy)2+3+SO−⋅4→Ru(bipy)3+3+SO2−4Ru(bipy)+3+Ru(bipy)3+3→Ru(bipy)2+3+[Ru(bipy)2+3][Ru(bipy)2+3]→Ru(bipy)2+3+hνwas based on the production of a strong reducing agent, electrochemically produced Ru(bipy)+3, which subsequently reduces S2O2−8 to generate a strong oxidising intermediate, SO−⋅4. As peroxydisulphate has been shown to be an effective quenching agent of [Ru(bipy)2+3]*[66], conditions for this system necessitated careful selection to reduce the quenching effect.

In 1987, Noffsinger and Danielson [67] proposed a mechanism for the chemiluminescence observed upon reaction of aliphatic amines with Ru(bipy)3+3 based on the electrochemical study conducted by Smith and Mann [68] and the postulated oxalate mechanism reported earlier by Rubinstein and Bard [62]. Smith and Mann [68] performed the anodic oxidation of tertiary aliphatic amines in a variety of solvents, and suggested a general reaction scheme, based on the oxidation of tri-n-propylamine to yield an amine radical cation that was probably short-lived and produced a neutral radical via the loss of a proton [68]. In the presence of some water, the reaction resulted in the dealkylation of the amine to produce the secondary amine, an aldehyde and the amine salt. Founded on the postulate that the neutral amine radical would have sufficient energy to react with either Ru(bipy)2+3 or Ru(bipy)3+3 to produce excited state [Ru(bipy)2+3]* and hence luminescence emission, Noffsinger and Danielson proposed a simplistic mechanism [67], which was later expounded by He et al. [69], (see scheme below)Ru(bipy)2+3→Ru(bipy)3+3−eRu(bipy)3+3+R2′NCH2R″→Ru(bipy)2+3+R2N⋅+CH2R″Ru(bipy)2+3+R2N⋅+CH2R″+H2O→2H++R2′NH+OCHR″+Ru(bipy)+3Ru(bipy)+3+Ru(bipy)3+3→Ru(bipy)2+3+[Ru(bipy)2+3][Ru(bipy)2+3]→Ru(bipy)2+3+hν

Although Noffsinger and Danielson [67] reported chemiluminescence from Ru(bipy)3+3 and amines, it was, Leland and Powell [70], however, who focussed solely on the electrochemical oxidation of tripropylamine to produce chemiluminescence with Ru(bipy)2+3 and discussed in greater detail a possible mechanism. These workers [70] demonstrated the dependence on the oxidation of the amine to produce a strong reducing intermediate with sufficient chemical energy to generate the excited state species, [Ru(bipy)2+3]*. The reducing intermediate was thought to be formed by a one-electron oxidation of tripropylamine, followed by deprotonation to form the neutral radical species 71, 72, 73. Similarly to other workers 67, 74, it was observed that the emission intensity was pH dependent, with maximum emission observed in slightly basic conditions. This appears to be consistent with the proposed radical cation deprotonation step, crucial for the formation of the postulated high energy reducing species, namely, the neutral amine radical. Although not entirely conclusive, the general reaction mechanism put forward by the above workers 69, 70 has been adapted by Brune and Bobbitt [75] who proposed a mechanism for amino acids that was founded on the oxidative action of alkaline hexacyanoferrate(III) upon amino acids as reported by Laloo and Mahanti [76]. Laloo and Mahanti [76] performed colorimetric and electron spin resonance spectroscopy on the reaction system to elucidate the oxidation mechanism which was thought to proceed via a radical intermediate yielded by the loss of hydrogen from the methylene group. Evidence for the existence of this radical was supported by the electron spin resonance spectroscopic data and the isolation and characterisation of the α-ketoacid reaction product. Brune and Bobbit [77] studied substitution, structure/activity effects on several amino acids, which indicated that chemiluminescence production was dependent upon the electron donating or withdrawing capabilities of the functional group attached to the α-carbon adjacent to the amine. Electron donating groups were observed to enhance the emission, possibly by stabilising the postulated radical intermediate, whereas electron withdrawing groups tended to decrease the emission. Interestingly, the reaction ratio of Ru(bipy)2+3 to amine was determined to have a stoichiometry of 2:1 [77]: this observation was congruous with the consumption of Ru(bipy)2+3 in their proposed mechanism [75].

More recently, Chen and Sato [78] have also suggested a mechanism, based on that of Rubinstein and Bard [62] which involved the generation of an ascorbate radical. Although analogous to the oxalate mechanism, (see scheme (4)), a detailed study of the reactive intermediate and reaction products was not conducted to confirm their postulation [78]. Chen et al. [79] also proposed a mechanism for the electrochemiluminescence reaction of various monohydric alcohols under alkaline conditions, with the production of an alkoxide radical ion as a key sequence in their reaction scheme. He and co-workers [80] utilised a cerium(IV)/Ru(bipy)2+3 system to investigate several organic acids and based on their kinetic data concluded that the organic acids involved in the reaction formed an activated complex with cerium(IV) prior to the production of a radical intermediate. They proposed that the rate of complex formation and hence radical intermediate production, depended upon the nature of the analyte substrate. This postulate facilitated the discrimination between chemiluminescence emissions due to different analyte reaction rates [80]. Greenway’s group 81, 82, 83, 84, 85 have measured the relative chemiluminescence from various analytes containing an amine functionality. Knight and Greenway [86] then reviewed their own findings 81, 82, 83, 84, 85 and noted, in agreement with Brune and Bobbit 75, 77 the effect of the α-hydrogen and α-carbon substituents on chemiluminescence activity. These workers [86] also postulated that mesomeric or resonance stabilisation in aromatic amines appeared to reduce the reactivity of the proposed radical intermediate and thus decrease Ru(bipy)2+3 emission. Interestingly, they noted that the configuration of a molecule after electro-oxidation affected the formation of the radical intermediate, for example, molecules such as 1,4-diazobicyclio[2.2.2]octane, quinuclidine, and quinine which are hindered in attaining a planar geometry on electro-oxidation resulted in orders of magnitude less response than tripropylamine which tends towards a trigonal planar geometry after oxidation resulting in an effective delocalisation of charge [86].

Section snippets

Methods for the generation of tris(2,2′-bipyridyl)ruthenium(III) [(bipy)33+]

Since the initial discovery of tris(2,2′-bipyridyl)ruthenium(II) chemiluminescence more than 30 years ago [10], its utility has only been exploited for a relatively limited number of analytical applications (see Table 1Table 2Table 3Table 4Table 5Table 6). In all reported cases however, these methodologies are in general, reliant upon the fundamental sequence as shown in the following scheme: Ru(bipy)2+3(oxidation)Ru(bipy)3+3(analyte)(reduction)[Ru(bipy)2+3]→Ru(bipy)2+3+hν

Common to all

Analytical Applications: Analytes Investigated

As described in the previous section, a number of methods have been developed to exploit the analytical utility of Ru(bipy)3+3 chemiluminescence. Various analytes have been determined and are tabled and discussed under broad classifications (see Table 1, Table 2, Table 3, Table 4, Table 5, Table 6 below).

Immunoassay and DNA probe assay

The application of chemiluminescence for determining analytes specific to human biological activity has received much attention [188]. Clinical applications [5] as well as immunoassay techniques based on chemiluminescence detection have become increasingly the focus of research over the past decade 6, 189. The use of chemiluminescent labels are clearly advantageous due to their non-radioactive nature, hence inherent safety, stability, cost and associated simple analytical instrumentation. The

Conclusions

Although the chemiluminescence behaviour of Ru(bipy)2+3 was first observed over 30 years ago, its analytical potential has only been realised in the last two decades. Within this time, some 90 papers have appeared in the open literature. It would appear that the majority (nearly 90%) of analytical applications published employ some form of electrochemical means to produce the Ru(bipy)3+3 reagent. Within this particular subset, over two thirds utilise in situ electrochemiluminescence. The

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