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Cathodic electrochemiluminescence of lucigenin at disposable oxide-coated aluminum electrodes
Abstract
Electrogenerated chemiluminescence (ECL) of lucigenin is induced at oxide-coated aluminum electrodes in aqueous solution by cathodic pulse polarization. This ECL can be enhanced by the presence of coreactants such as peroxodisulfalte. The present method is based on the injection of hot electrons into the aqueous electrolyte solution, which probably results in the generation of hydrated electrons as reducing mediators. The successive one-electron redox reactions result in the excited states of lucigenin or its fragmentation products. The method can detect lucigenin over several orders of magnitude of concentration with detection limit below nanomolar concentration level. In addition, the relatively long lifetime of the ECL of lucigenin makes time-resolved detection possible. This study suggests that the derivatives of lucigenin can be utilized as electrochemiluminescent labels in aqueous solution in bioaffinity assays at thin insulating film-coated cathodes. The cathodic ECL reaction mechanisms are discussed.
... As shown in Figure 10A, the degradation efficiency of RhB was slightly decreased when increasing the initial dye concentration from 1 × 10 −5 to 4 × 10 −5 M. This was mainly because of the increase of the dye molecules around the active sites leading to inhibiting the penetration of light to the surface of the catalyst [66]. Figure 9C,D). ...
... As shown in Figure 10A, the degradation efficiency of RhB was slightly decreased when increasing the initial dye concentration from 1 × 10 −5 to 4 × 10 −5 M. This was mainly because of the increase of the dye molecules around the active sites leading to inhibiting the penetration of light to the surface of the catalyst [66]. The effect of the initial pH on the degradation of RhB on the degradation of RhB over Ni/Fe-MOF/Light/H2O2 system was also investigated. ...
Mixed Ni/Fe-base metal-organic framework (Ni/Fe-MOF) with different molar ratios of Ni2+/Fe3+ have been successfully produced using an appropriate solvothermal router. Physicochemical properties of all samples were characterized using X-ray diffraction (XRD), Raman, field emission scanning electron microscopes (FE-SEM), fourier-transform infrared spectroscopy (FT-IR), N2 adsorption-desorption analysis, X-ray photoelectron spectroscopy (XPS), ultraviolet-visible diffuse reflectance spectra (UV-Vis DRS), and photoluminescence spectra (PL). The photocatalytic degradation performances of the photocatalysts were evaluated in the decomposition of rhodamine B (RhB) under a compact fluorescent daylight lamp. From XRD, IR, XPS, and Raman results, with the presence of mixed ion Fe3+ and Ni2+, MIL-88B (MIL standing for Materials of Institut Lavoisier) crystals based on the mixed metal Fe2NiO cluster were formed, while MIL-53(Fe) was formed with the presence of single ion Fe3+. From UV-Vis DRS results, Ni/Fe-MOF samples exhibited the absorption spectrum up to the visible region, and then they showed the high photocatalytic activity under visible light irradiation. A Ni/Fe-MOF sample with a Ni2+/Fe3+ molar ratio of 0.3 showed the highest photocatalytic degradation capacity of RhB, superior to that of the MIL-53(Fe) sample. The obtained result could be explained as a consequence of the large surface area with large pore volumes and pore size by the Ni2+ incorporating into the MOF’s structure. In addition, a mixed metal Fe/Ni-based framework consisted of mixed-metal cluster Fe2NiO with an electron transfer effect and may enhance the photocatalytic performance
... The photocatalytic activities of magnetic MIL-53(Fe) hybrids photocatalysts were investigated by the photocolordation of RhB dye. Fig. 8 For the influence of pH on the photocatalytic degradation capacity, the experiments were performed in the pH range of 5-9, and the RhB concentration was 3 × 10 -5 M. According to Fig. 9A, the highest photocatalytic degradation capacity was pH = 5, and the degradation rate of RhB decreased with the increase of pH from 5 to 9. According to the degradation results provided in Fig. 9B, the degradation efficiency of RhB was slightly reduced when increasing initial dye concentration from 1 × 10 -5 to 4 × 10 -5 M because of the increase of the dye molecules around the active sites which inhibits the penetration of light to the surface of the catalyst [26]. ...
As a novel 2-dimensional nanomaterials, black phosphorus quantum dots (BPQDs) have attracted many interests in recent years. In the present work, the potential application of BPQDs in lucigenin chemiluminescence (CL) was researched. Under mild alkaline condition, lucigenin CL was extremely weak, and was difficult for analytical application. Both BPQDs and Co²⁺ exhibited enhancing effect on lucigenin CL due to their catalytic effects. The synergic effect of BPQDs and Co2+ could generate the strongest CL emission in weak alkaline solution, which provided the possibility for higher sensitive detection of Co²⁺. Several impacting factors, such as the concentrations of lucigenin and H2O2, the pH value, the mixing sequence of reagents, and the diluted ratio of BPQDs were investigated. Under the optimal condition, Co²⁺ was sensitively detected in the range of 8.0 × 10⁻⁹ to 8.0 × 10⁻⁸ mol L ⁻ ¹, and the detection limit was determined as 3.0 × 10⁻⁹ mol L ⁻ ¹ (3σ). The proposed method exhibited higher sensitivity, wider linear range, and good selectivity compared with traditional CL method. Finally, the mechanism for BPQDs sensitized lucigenin CL was discussed.
Acridines play an important role in various fields of sciences. Besides their application for treatment of a number of infections, cancer, viral and prion diseases, they have been extensively used in chemiluminescence. Electrochemiluminescence (ECL) is a unique kind of chemiluminescence with the precise temporal and spatial control of light emitting reactions by utilizing electrochemistry techniques. These advantages result in increasing attention towards acridines ECL. This review summarizes acridines used in ECL, ECL mechanisms, strategies for increasing ECL intensities, and ECL analytical applications, as well as prospects the future development of acridine ECL.
Electrogenerated chemiluminescence (ECL) of lucignein often suffered from weak signal at positive potential in neutral condition. In the present study, strong anodic ECL was obtained in neutral lucigenin solution at a CdSe quantum dots modified glassy carbon electrode. Electrochemical results suggested that CdSe quantum dots can catalyze the oxidation of lucigenin and bromide, which can generate the anodic ECL. The fluorescence and the ECL spectra revealed that ECL resonance energy transfer (ECL-RET) can occur between lucigenin and CdSe quantum dots. The oxidation product of bromide can promote ECL-RET and increase the anodic ECL signal significantly. The mechanisms of the anodic ECL based on the ECL-RET were proposed. Cytochrome C exhibited apparent inhibiting effect on the anodic ECL emission, based on which a sensitive ECL sensor for the detection of Cytochrome C was established.
An amplified and stable electrochemiluminescent signal of lucigenin was observed on an electrochemically reduced graphene oxide-containing sensing platform. Herein, chitosan was used as an excellent dispersant, which favors the better dispersion of graphene oxide in solution and improved electrochemical reduction of graphene oxide. The electrochemically reduced graphene oxide showed outstanding conductivity which was propitious to the transfer of electrons, leading to the enhancement of the electrochemiluminescent signals of lucigenin. In addition, bisphenol A, as a classical food borne pollutant, was first detected due to its strong inhibiting action of the electrochemiluminescent response of lucigenin. Then, we studied the quenched electrochemiluminescent system and the possible mechanism of the platform in detail. Under optimum conditions, the proposed electrochemiluminescent sensor exhibited a linear response range from 1.0 x 10(-9) mol L-1 to 1.0 x 10(-4) mol L-1 with a low detection limit of 3.0 x 10(-10) mol L-1 for bisphenol A, which might find promising applications in developing a new type of biosensor.
A proof-of-concept for the production of simultaneous electroluminescence in a single electrochemical cell in a basic medium and using H2O2 as a co reactant is obtained: at the anode by means of the direct oxidation of luminol to yield an excited emitting species, and in the catholyte by an indirect mediated process involving the initial reduction of ClO2− to produce ClO−, which then reacts with luminol (also to produce an excited emitting species). Emission spectra and possible mechanistic pathways are discussed.
It was found that noble metal nanoparticles including Ag, Au, and Pt nanoparticles in the presence of adsorbates such as iodide ion, cysteine, mercaptoacetic acid, mercaptopropionic acid, and thiourea could reduce lucigenin (bis-N-methylacridinium) to produce chemiluminescence (CL). Lucigenin−Ag−KI system was chosen as a model to study the CL process. Absorption spectra and X-ray photoelectron spectra showed that when the Ag colloid was mixed with KI, Ag nanoparticles were covered by adsorbed iodide ions. X-ray diffraction patterns and fluorescence spectra indicated that Ag nanoparticles were oxidized to AgI and lucigenin was converted to N-methylacridone in the CL reaction. The addition of superoxide dismutase could inhibit the CL. According to Nernst's equation, the presence of iodide ions decreased the oxidation potential of Ag nanoparticles. As a result, lucigenin was rapidly reduced by Ag nanoparticle to a monocation radical, which reacted with oxygen to generate a superoxide anion; then the superoxide anion reacted with the monocation radical to produce CL. Other adsorbates such as cysteine, mercaptoacetic acid, mercaptopropionic acid, and thiourea that could decrease the oxidation potential of Ag nanoparticles could also induce the CL reaction.
The electrogenerated chemiluminescence (ECL) behavior of lucigenin in ethanol solution at a polycrystalline gold electrode was studied under conventional cyclic voltammetric conditions. Compared with the ECL of lucigenin in aqueous solution, one cathodic ECL peak (ECL-1 at −0.98 V versus SCE) with a shoulder (S1 at −0.42 V) and three new anodic ECL peaks (ECL-2 at −0.53 V, ECL-3 at 0.20 V, and ECL-4 at 0.51 V) were observed, respectively, on the curve of ECL intensity versus potential. The effects of initial potential scan direction, the presence of O2 or N2, potential scan ranges, supporting electrolyte and the concentration of lucigenin on these ECL peaks were examined. The electrochemistry of lucigenin in ethanol solution was also studied. The emitter of all ECL peaks was identified as N-methylacridone (NMA) by analyzing the ECL spectra. The mechanism for these ECL peaks is proposed to be due to the reactions of lucigenin and its redox products such as Luc and DBA with dissolved oxygen or O2 electrogenerated by the dissolved oxygen at different potentials. The formation of new anodic ECL peaks in ethanol solution is due to longer lifetime of superoxide ions and easier electro-oxidation of DBA in nonaqueous solution, revealing that the solvent plays an important role in the lucigenin ECL reactions.
In this chapter, we discuss the basics of cathodic hot electron-induced electrogenerated chemiluminescence (HECL). In the
applications of HECL, we discuss, e.g., the usable electrode materials and their advantages as well as the applicable solution
conditions in aqueous media. We also summarize the luminophore types excitable by this method and their usability as labels
in practical bioaffinity assay applications.
A new flow-injection chemiluminescence method (FI-CL) was developed for the determination of trace amounts of Pt(IV). The method is based on the quenching effect of the analyte on CL emission generated by lucigenin in alkaline solution. Application of a column filled with an algae Chlorella vulgaris immobilized on Cellex-T resin allowed to preconcentrate and separate the Pt(IV) ions from complex environmental samples, such as road dust. The developed method is simple and does not require sophisticated instrumentation. It is also characterized by a very low limit of detection (0.1 ng mL−1), good sensitivity and precision (RSD < 3%). The accuracy of the presented method was confirmed by analysis of a certified reference material of tunnel dust (BCR-723). The content of Pt in road dust samples collected in Białystok (Poland) in 2009 determined by the evaluated method was 351.8 ± 54.6 ng g−1 and was higher than in samples collected in years 2000 and 2003.
Online reviews provide valuable information about products and services to consumers. However, spammers are joining the community trying to mislead readers by writing fake reviews. Previous attempts for spammer detection used reviewers' behaviors, text similarity, linguistics features and rating patterns. Those studies are able to identify certain types of spammers, e.g., those who post many similar reviews about one target entity. However, in reality, there are other kinds of spammers who can manipulate their behaviors to act just like genuine reviewers, and thus cannot be detected by the available techniques. In this paper, we propose a novel concept of a heterogeneous review graph to capture the relationships among reviewers, reviews and stores that the reviewers have reviewed. We explore how interactions between nodes in this graph can reveal the cause of spam and propose an iterative model to identify suspicious reviewers. This is the first time such intricate relationships have been identified for review spam detection. We also develop an effective computation method to quantify the trustiness of reviewers, the honesty of reviews, and the reliability of stores. Different from existing approaches, we don't use review text information. Our model is thus complementary to existing approaches and able to find more difficult and subtle spamming activities, which are agreed upon by human judges after they evaluate our results.
Ruthenium(II) tris-(2,2′-bipyridine) chelate exhibits strong electrogenerated chemiluminescence during cathodic pulse polarization of oxide-covered aluminum electrodes in aqueous solutions. The present method is based on a tunnel emission of hot electrons into an aqueous electrolyte solution. The method allows the detection of ruthenium(II) tris-(2,2′-bipyridine) and its derivatives below nanomolar concentration levels and yields linear log-log calibration plots spanning several orders of magnitude of concentration. This method allows simultaneous excitation of derivatives of ruthenium(II) tris-(2,2′-bipyridine) and Tb(III)-chelates. The former label compounds have a luminescence lifetime of the order of microseconds, while the latter compounds generally have a luminescence lifetime of around 2 ms. Thus, the combined use of these labels easily provides the basis for two-parameter bioaffinity assays by either using wavelength or time discrimination or their combination.
This chapter discusses the reduction potentials involving inorganic free radicals in aqueous solution. Because free radicals are usually transients, knowing their thermodynamic properties is primarily useful in mechanistic studies. Thus, the useful redox couples associated with a given free radical correspond to plausible elementary steps in reaction mechanisms. All potentials are expressed against the normal hydrogen electrode (NHE). Apart from the NHE, the standard state for all solutes is the unit molar solution at 25 °C. This violates the usual convention for species, such as O2 that occur as gases, but because the rates of bimolecular reactions in solution are significant, the unit molar standard state is most convenient. The emphasis is on electron transfer reactions in which no bonds are formed or broken, electron transfer reactions in which concerted electron transfer and bond cleavage could occur, and certain atom transfer reactions. The chapter also presents a tabulation of ∆fG° values for all the radicals. A common approach in estimating the thermochemistry of aqueous free radicals is to use gas-phase data with approximations of solvation energies.
In recent years there has been an increasing interest in the electrochemical properties of metal sulfides. Our main sources of copper, lead, zinc, and nickel occur as sulfides and they are converted to the metal by pyrometallurgical processes which produce gaseous sulfur dioxide. The environmental requirements being imposed on sulfide smelters have led to a greater effort to develop hydrometallurgical routes for the treatment of sulfides in order to avoid the evolution of sulfur dioxide. These processes involve the oxidation of sulfides to sulfur or sulfate either using oxidants such as oxygen, ferric ion, or cupric ion1 or by direct anodic oxidation in an electrolyte.2 The oxidation of sulfides by an oxidant can be regarded as an electrochemical reaction with the cathodic reduction of the oxidant and the anodic oxidation of the sulfide,3 and it can consequently be studied by electrochemical techniques. Recent work4,5 has also shown that flotation, a key process in concentrating sulfide ores, can be electrochemical in nature.
Kinetic data for the radicals H⋅ and ⋅OH in aqueous solution,and the corresponding radical anions, ⋅O− and eaq−, have been critically pulse radiolysis, flash photolysis and other methods. Rate constants for over 3500 reaction are tabulated, including reaction with molecules, ions and other radicals derived from inorganic and organic solutes.
Lucigenin is shown to emit light during electrolysis in aqueous alkaline solutions at a platinum electrode. In the mechanism proposed, lucigenin is initially reduced at —0.30 V vs. Ag/AgCl and subsequently the reaction product reacts either with oxygen or a reduction product of oxygen. Previous evidence on this phenomenon is contradictory.
Light emission from aluminium oxide during cathodic pulse-polarisation of oxide-covered aluminium in aqueous solution was observed to be strongly enhanced in the presence of peroxydisulfate ions. The spectrum of the light emission had a broad maximum between 400 and 450 nm being attributed to F-centre luminescence of aluminium oxide. The mechanism of the luminescence is associated with the two-step reduction of peroxydisulfate anions near the oxide-covered cathode where the first one-electronic reduction step occurs either (i) by tunnel-emission generated hydrated electrons or (ii) by trickling down the surface states to the energy level of peroxydisulfate ion or (iii) by direct heterogeneous electron transfer from the bottom of the aluminium oxide conduction band to peroxydisulfate ions during strong downward band bending induced by cathodic pulse-polarisation. The second step occurs by electrons from F- or F
+
-centres at the oxide/electrolyte interface. Transitions of aluminium oxide conduction band electrons to fill the sulfate radical-emptied electron trapping sites (oxygen vacancies) produces analogous F- and F
+
-centre luminescence to that occurring during photoluminescence and thermoluminescence of aluminium oxide. No enhancement of light emission was observed in the presence of hydrogen peroxide which is also reduced in a two-step process at oxide-covered aluminium electrode. This can be explained by the fact that the energy level of hydroxyl radical under the present conditions lies ca. 1 eV above, whereas the energy level of sulfate radical lies somewhat below the colour-centre sub-band of aluminium oxide. Therefore, the sulfate radical is a sufficiently strong oxidant but the hydroxyl radical is too weak an oxidant to abstract electrons from F- and F
+
-centres.
Hot electron injection into aqueous electrolyte solutions
from metal/insulator/metal/electrolyte and
metal/insulator/electrolyte tunnel junctions is considered
and the possibility of an electrochemical generation of
hydrated electrons is discussed. The hot electron-induced UV
electrochemiluminescence of (9-fluorenyl)methanol was used
to demonstrate the presence of highly energetic transient
species in aqueous solution at several
metal/insulator/electrolyte hot electron tunnel emitters.
These transient species cannot be produced electrochemically
in fully aqueous solutions at any active metal electrodes. A
detailed mechanism for the present electrochemiluminescence
is suggested.
The crystal structure of ammonioleucite was refined by a Rietveld analysis using X-ray diffraction data of the type specimen. The refinement with a space group I4l/a was converged at Rwp=12.12, Rp=9.39, RF=4.86, RB=7.71, and Re=2.55% with lattice parameters of a=13.2106(6) and c=13.7210(7) Å. Ammonioleucite is isostructural with leucite. The (Si, Al)O4 tetrahedra form 4-, 6-, and 8-membered rings by sharing their apical oxygen atoms. The crystal structure consists of the three-dimensional framework of the rings of the tetrahedra. Ammonium ions are located in cavities in the framework. The (NH4)+ ions in these cavities are larger than the corresponding K+ ions in leucite, causing the cavities to be enlarged relative to those in leucite. Among the 4-, 6-, and 8-membered rings of (Si,Al)O4 tetrahedra, only the 8-membered rings have diameters large enough to allow the migration of exchangeable K+, (NH4)+ and Na+ ions.
Generation of hydrated electrons at an oxide-covered aluminium electrode during its cathodic pulse-polarization in an aqueous electrolyte may well be the primary step of the electrogenerated luminescence of aromatic Tb(III) chelates. In the presence of appropriate precursors, hydrated electrons produce secondary oxidizing radicals, thus resulting in an energetic electrode/electrolyte interface which initiates the radiative 5D4 → 7Fj transitions of chelated terbium(III); the aromatic moiety of a Tb(III) chelate is excited by its consecutive interactions with hydrated electron and the secondary oxidizing radical followed by an intramolecular transfer of this excitation energy to the 5D4 level of coordinated terbium(III).
The effect of halides and different buffer anions on the quenching of the fluorescence of the new probe 10,10′-bis(3-sulfopropyl)-9,9′-biacridine (SPBA) has been studied using fluorescence and decay time measurements. The linearity of the Stern-Volmer plot indicates that fluorescence quenching by halides can be described reasonably well by a single-exponential decay with a K of 4.06 x 106 M-1S-1 for chloride, 7.83 x 106 M-1 s-1 for bromide and 1.12 x 107 M-1 s-1 for iodide. We have found that SPBA is collisionally quenched also by the buffers 3-(N-morpholino)propanesulfonic acid (MOPS) and N-2-hydroxyethylpiperazine-N′-ethansulfonic acid (HEPES). The bimolecular rate constants are 1.67 x 106 M-1 s-1 for HEPES and 1.44 x 106 M-1 s-1 for MOPS. DOI: 10.1111/j.1751-1097.1999.tb08255.x
Electrogenerated chemiluminescent (ECL) reaction of an acridinium ester, methyl-9-(p-formylphenyl) acridinium carboxylate fluorosulfonate (MFPA), on the surface of a platinum electrode has been studied using a homemade ECL apparatus. The ECL intensity was affected by the presence of micro amounts of chloride and hydrogen peroxide in the solution. A positive square wave pulse was exerted on the reaction cell. The applied voltage was 2.5 V (vs SCE). This method allowed MFPA to be determined in the range 1.0 × 10−9–4.0 × 10−7mol L−1. A line calibration plot was constructed in order to evaluate the limit of detection, which was determined to be 2.1 × 10−10mol L−1. The application of this method to immunoassay was demonstrated by the determination of human chorionic gonadotropin using MFPA as label. Good correlation was obtained between the results of the proposed method and the other method. A reaction mechanism study showed that MFPA decomposed toN-methylacridone and 4-hydrobenzaldehyde in the alkaline aqueous solution which contained hydrogen peroxide.N-Methylacridone could be oxidized to form an excited state molecule at the surface of the electrode. As some of these excited state molecules went to their ground state, they released light in the visible wavelength range. The ECL intensity enhanced by addition of chloride may be due to the oxidation of the chloride ion to OCl−and the latter acts on MFPA and then gives out luminescence.
Electrochemical studies have shown that the reduction of persulfate and hydrogen peroxide is a two step mechanism, the first step occurs by electron transfer with the conduction band and the second step by hole injection with the valence band. It could be concluded from corresponding measurements performed with a semiconductor electrode that the electrochemical properties of these oxidizing agents have to be described by two instead of one redox (normal) potential. One normal potential is much lower and the other much larger than the theoretical value determined from thermodynamic data. These values are estimated as and .
Absolute rate constants have been measured for the reaction of the N ∴ N three-electron bonded 1,6-diazabicyclo[4.4.4]dodecane radical cation ([4.4.4]+˙) with various free radicals produced by means of pulse radiolysis. Reduction and oxidation reactions occur with rate constants generally somewhat below the diffusion limit. This is considered to reflect the inwardly oriented structure of the [4.4.4]+˙. High rate constants (ca. 109 mol–1 dm3 s–1) have been measured for hydrogen-atom abstraction from [4.4.4]+˙ by almost all radicals except eeq–. The most remarkable of these reactions appears to be H-atom abstraction by a thiyl radical [(CH3)3CS˙], which occurs with k 3.2 × 109 mol–1 dm3 s–1. This indicates highly labile C–H bonds in [4.4.4]+˙, which are considered to be those located on CH2 groups α to the nitrogen atoms. The fast radical–radical H-atom transfer is considered to be energetically assisted by favourable stereoelectronics and least heavy atom motion.
Luminol produces chemiluminescence during the dissolution of F-colored alkali halides in aqueous solutions. There seems to be an unknown excitation pathway which requires only the presence of luminol, hydrated electrons and alkaline conditions. This study was made using X-ray irradiated sodium chloride as a primary radical-generating material, which allows the simultaneous generation of hydrated electrons and highly oxidizing chlorine-containing radicals by dissolution of the irradiated solid in an aqueous solution. In alkaline solutions the reactions of chlorine-containing radicals with hydroxide ions can produce oxyradicals and hydrogen peroxide which suggest that chemiluminescence can be generated simultaneously by several pathways, and the emitting species can be either 3-aminophthalate in the case of chemiluminescence of luminol, or the luminol molecule itself, or both. However, it was clear that the dissolution-uncovered holes were the primary species responsible for initiation of most of the luminescence observed. The present method allows luminol detection below nanomolar level, and the linear logarithmic calibration range covers several orders of magnitude of concentration. Therefore, derivatives of luminol are proposed as labels in lyoluminescence bioaffinity assays.
Reduction of an electron acceptor (oxidant), A, or oxidation of an electron donor (reductant), A2−, is often achieved stepwise via one-electron processes involving the couples A/A⋅− or A⋅−/A2− (or corresponding prototropic conjugates such as A/AH⋅ or AH⋅/AH2). The intermediate A⋅−(AH⋅) is a free radical. The reduction potentials of such one-electron couples are of value in predicting the direction or feasibility, and in some instances the rate constants, of many free-radical reactions. Electrochemical methods have limited applicability in measuring these properties of frequently unstable species, but fast, kinetic spectrophotometry (especially pulse radiolysis) has widespread application in this area. Tables of ca. 1200 values of reduction potentials of ca. 700 one-electron couples in aqueous solution are presented. The majority of organic oxidants listed are quinones, nitroaryl and bipyridinium compounds. Reductants include phenols, aromatic amines, indoles and pyrimidines, thiols and phenothiazines. Inorganic couples largely involve compounds of oxygen, sulfur, nitrogen and the halogens. Proteins, enzymes and metals and their complexes are excluded.
It is demonstrated that electroluminescent porous silicon (PS) diodes operate as surface-emitting cold cathodes. The PS diode is composed of a semitransparent thin Au film, PS layer, n-type Si substrate and ohmic contact. When a sufficient positive bias voltage is applied to the Au contact in vacuum, the diode uniformly emits electrons as well as visible light. This cold emission is explained by the model that electrons are injected from the substrate, drifted by the field within the PS layer, and ejected as hot electrons through the thin Au film. This mode gives important insight for understanding the electroluminescence mechanism of PS, and shows the potential of PS devices not only for optoelectronic applications, but also for vacuum microelectronic ones.
In this review the authors have attempted to show the present state of knowledge concerning anodic oxide films on aluminum, complete from formation to dissolution. It is clear that the formation, and the kinetics of formation of barrier-type films, involving high-field ionic conduction, is not yet clearly understood, although several promising theories are available. The formation of a porous film from a preceding barrier-type film is not understood clearly, although the important characteristics of such a transformation are known. The importance of considering an external surface dissolution process during porous film formation has been explained and found to be consistent with several experimental observations. The structure of porous films has yet to be fully established, although a considerable weight of evidence supports the cylindrical or possibly the truncated cone pore model. Satisfactory correlation between this pore model and all experimental observations has yet to be achieved, although considerable progress in this direction has been made recently. Finally, dissolution of porous oxide films was outlined. The importance of such a study to the elucidation of the porous structure was mentioned. Several basic questions concerning dissolution were posed, the answers to which would do much to forward our knowledge of the porous anodic oxide films on aluminum. For further reading and study note ref 300-335.
The electrochemically generated chemiluminescence (ECL) of lucigenin has been studied in aqueous and nonaqueous systems. The electrochemical reduction of lucigenin has been shown to lead to dimethylbiacridene (DBA) as the product. The nonaqueous ECL has been shown to arise from reaction of superoxide with lucigenin. The spectral characteristics of the ECL, when compared with the fluorescence spectra and quantum efficiencies of NMA, DBA, and lucigenin, allow identification of the emitting species. NMA is the primary emitter formed in the excited state by the reaction of superoxide with lucigenin. In several systems a second longer wavelength component of emission was observed. This was the fluorescence of either DBA or lucigenin, depending on their respective concentrations and quantum efficiencies. Both DBA and lucigenin act as energy acceptors from excited NMA via singlet-singlet energy transfer. The experimental values of energy transfer rate constants (in the order of 1.5 × 1011 l./(mole sec)) correlate well with those derived from theoretical calculations.
The effect of an external magnetic field on the intensity of electrogenerated chemiluminescence (ecl) was determined for nine systems involving anthracene, 9,10-diphenylanthracene (DPA), rubrene, 1,3,6,8-tetraphenylpyrene (TPP), and fluoranthene as emitting species in N,N-dimethylformamide (DMF) solutions. Enhancements in emission intensity up to 27% with increasing field strength were noted for the energy-deficient oxidations of anthracene, DPA, rubrene, and TPP anion radicals by Wurster's Blue cation, for the energy-deficient oxidation of fiuoranthene anion radical by the cation radical of 10-methylphenothiazine, and for the energy-deficient reduction of the rubrene cation radical by the anion radical derived from p-benzoquinone. No field effect was seen on luminescence arising from the apparently energy-sufficient mutual annihilation of the anion and cation radicals derived from DPA. The field enhanced luminescence from the reaction between the rubrene anion and cation radicals, but it exerted no effect on the intensity of emission from the TPP anion-cation radical annihilation. These results have been interpreted as reflecting a dual mechanism for chemiluminescent electron-transfer processes which divides on or about the line of energy sufficiency. In particular it is suggested that the field effect accompanying luminescence from energy-deficient systems arises from an inhibition by the field of the rate of quenching of emitter triplets by radical ions. Thus, the results are consistent with the triplet mechanism for luminescence from energy-deficient systems. This interpretation also indicates that energy-sufficient systems yield luminescence without required triplet intermediates. For the two marginal systems involving rubrene and TPP alone, it is suggested that the rubrene anion-cation annihilation gives rise to luminescence predominantly via the triplet mechanism, while the TPP anion-cation reaction may be essentially energy sufficient, and that most luminescence from this process arises by direct population of the emitting state.
The reactions undergone by lucigenin (N,N′-dimethyl-9,9′-biacridinium dication, 1) in alkaline solutions in the presence of molecular oxygen are very complex, yielding as products N-methylacridone (2), N,N′-dimethyl-9,9′-biacrylidine (3), N,N′-dimethylacrylidene oxide (4), N,N′-dimethyl-9,9′-dihydroxybiacridan (5), N-methylacridanol (6), and N-methylacridan (7). A very minor reaction (quantum yield (ΦCL) = 1 × 10-4) provides excited N-methylacridone (2*). Under the pseudo-first-order conditions of the present investigation ([1] = 1.75 × 10-5 M,[O2] = 1.5 × 10-4 M, and at constant pHs) the minor chemiluminescent (CL) reaction and the major and non-CL reactions comprise competing parallel first-order paths for the disappearance of 1. The pseudo-first-order rate constants for disappearance of 1 must then (and do) equal the pseudofirst-order rate constants for the decay of intensity of photon emission. The rate law for the consumption of 1 (-d[1]/dt = {k1 [HO-] + k2[HO-]2}[1]) establishes that the critical transition states for the reactions leading to 2-7 are composed of one molecule of 1 and one or two HO- ions (dependent upon pH). The rate law for the formation of 2* via the CL reactions has been obtained from the concentration dependencies of the quantum yields (ΦCL) and the rate law for disappearance of 1. The critical transition states for the CL reactions are composed of two molecules of 1 and one or two HO- ions (dependent upon pH) [i.e., d[2*]/dt = {k[HO-] + k[HO-]2}[1]2]. Addition of one HO- to 1 is reversible (pKa = 12.43) while addition of a second HO- species is only partially reversible (i.e., 1 + HO- ⇄ 1(OH); 1(OH) + HO- → 1(OH)2). The major non-CL reactions and the minor CL reactions arise from the partitioning of 1(OH) and 1(OH)2. Thus, for the CL reactions d[2*]/dt = kx[1][1(OH)] + ky[1][1(OH)2] while for the non-CL reaction d[1]/dt = k1[1(OH)] + k2[1(OH)2]. That intermediate 1(OH) reacts with 1 to provide 2* is supported by the finding of a lag phase in the formation of 2* which can be shown (by way of the concentration dependence for the exponential increase in the intensity of photon emission) to be first order in [1] and [HO-]. Nucleophiles of sufficient basicity (CF3CH2O-, CN-, CH3CH2NH2) may substitute for HO- to provide chemiluminescence. In the reactions of 1 with HO- and CF3CH2O- the lag phase in photon emission is accompanied by a very similar lag phase in ESR signal intensity. Both the intensity of the ESR signal and the value of the integrated photon emission increase and become constant at comparable times. The mechanisms for the CL reactions are suggested to involve 1e- transfer from the nucleophilic adducts of 1 (i.e., when nucleophile = HO- the adducts are 1(OH) and 1(OH)2) to 1, converting the latter to 1· which then reacts with molecular oxygen to provide the dioxetane (1(OO)). Thermal cleavage of 1(OO) provides 2 + 2*. That 1e- transfer to 1 from dihydroacridine structures (as 1(OH) and 1(OH2)) provides chemiluminescence in the presence of molecular oxygen has been demonstrated by the intense and O2-dependent chemiluminescence seen when 1 reacts with the dihydrobiacridines 3 and 4.
The chemiluminescent reaction of lucigenin (N,N′-dimethyl-9,9′-biacridine, 1) at various pH values (solvent H2O, 30°C, μ = 1.0) with hydrogen peroxide in excess is first order to 6 half-lives. At constant pH the reaction rate was determined to be dependent upon the first power of the concentration of hydrogen peroxide and to be independent of concentrations of 1 and buffers. From the pH dependence of the pseudo-first-order rate constant (kobsd, [H2O2]T ≫ [1]) the rate of disappearance of 1 follows the general expression d[1]/dt = [1]{k1[HO2-] + k2[HO2-][HO-]}. Diminution in light emission with time was shown to follow the same rate law and the quantum yield was established to be independent of [1], total hydrogen peroxide concentration ([H2O2]T), and pH. These results establish that the chemiluminescent reaction of 1 with hydrogen peroxide follows the same rate law as does the overall disappearance of 1 from solution in the presence of varying concentrations of H2O2 and at all pH values. Replacement of H2O with t-BuO2H provides less light than obtained in the absence of any peroxide agent. Lucigenin reacts with H2O2 and t-BuO2H at various pH values to produce N-methylacridone (2) as the major fluorescent product. The chemiluminescent spectrum at any time could be shown to result from emission by excited 2 and subsequent absorption by 1. Competing dioxetane and linear peroxide mechanisms (with H2CO2 and t-BuO2H) are proposed and discussed. Evidence is presented which suggests that the 9,9′-dioxetane of 1 provides 2 + 2* by the two competing pathways of spontaneous fragmentation and le- transfer.
Ruthenium(II) tris-(2,2‘-bipyridine) chelate exhibits strong electrogenerated chemiluminescence during cathodic pulse polarization of oxide-covered aluminum electrodes in aqueous solutions. The present method is based on a tunnel emission of hot electrons into an aqueous electrolyte solution. The method allows the detection of ruthenium(II) tris-(2,2‘-bipyridine) and its derivatives below nanomolar concentration levels and yields linear log−log calibration plots spanning several orders of magnitude of concentration. This method allows simultaneous excitation of derivatives of ruthenium(II) tris-(2,2‘-bipyridine) and Tb(III)-chelates. The former label compounds have a luminescence lifetime of the order of microseconds, while the latter compounds generally have a luminescence lifetime of around 2 ms. Thus, the combined use of these labels easily provides the basis for two-parameter bioaffinity assays by either using wavelength or time discrimination or their combination.
Luminol exhibits strong electrogenerated chemiluminescence during cathodic pulse polarization of oxide-covered aluminum electrodes in aqueous solution. This electrogenerated chemiluminescence can be enhanced by the presence of dissolved oxygen or by the addition of other coreactants such as hydrogen peroxide, peroxydisulfate, or peroxydiphosphate ions. However, luminol detection is most sensitive in the presence of azide ions, which not only enhance the electrogenerated chemiluminescence intensity but also decrease the intrinsic electroluminescence of the thin aluminum oxide film on the electrodes mainly producing the blank emission. The present method is based on tunnel emission of hot electrons into an aqueous electrolyte solution and allows the detection of luminol, isoluminol, and its derivatives below nanomolar concentration levels. The linear logarithmic calibration range covers several orders of magnitude of concentration of luminol or N-(6-aminohexyl)-N-ethylisoluminol. Therefore, the above-mentioned labeling substances can be used as one of several available alternatives of simultaneous markers in multiparameter bioaffinity assays at disposable oxide-covered aluminum electrodes. The main advantage of the present electrochemiluminescence generation method is that luminescent compounds having very different photophysics and chemistry can be simultaneously excited, thus providing good possibilities for internal standardization and multiparameter bioaffinity assays.
Luminol shows strong chemiluminescence with an emission maximum at 430 nm in the presence of sulfate radicals. Sulfate radicals were produced by the dissolution of UV-irradiated potassium peroxodisulfate powder in aqueous luminol solutions. The UV irradiation at 6.7 eV produces a solid solution of sulfate radical in potassium peroxodisulfate by rupturing −O−O− bonds, as in solution, but now the solid solution is stable in a time scale of years in dryness. In the present system, luminol chemiluminescence is produced via several parallel pathways having a common triggering step, one-electron oxidation of luminol monoanion by sulfate or hydroxyl radical. Present chemiluminescence allows sensitive luminol detection from picomolar to micromolar level with a linear response over 5 orders of magnitude, after which luminescence is too strong for single-photon counting. The high sensitivity of luminol detection allows us to propose extrinsic lyoluminescence of potassium peroxodisulfate as a new and simple method for detection step of bioaffinity assays using luminol or isoluminol derivatives as label compounds.
Rate constants have been compiled for reactions of various inorganic radicals produced by radiolysis or photolysis, as well as by other chemical means in aqueous solutions. Data are included for the reactions of ⋅CO2−, ⋅CO3⋅-, O3, ⋅N3, ⋅NH2, ⋅NO2, NO3⋅, ⋅PO32-, PO4⋅2-, SO2⋅ -, ⋅SO3 -, SO4⋅-, ⋅SO5⋅-, ⋅SeO3⋅ -, ⋅(SCN)2⋅ -, ⋅CL2⋅-, ⋅Br2⋅ -, ⋅I2⋅ -, ⋅ClO2⋅, ⋅BrO2⋅, and miscellaneous related radicals, with inorganic and organic compounds. © 1988, American Institute of Physics for the National Institute of Standards and Technology. All rights reserved.
Addition of photo- or radiooxygenated amines or N,N-dialkylamides to aqueous lucigenin results in efficient chemiluminescence. Chemiluminescence quantum yields for amines are two orders of magnitude higher than those of dialkylamides for the same illumination or irradiation time and can become as high as 4×10−3 einstein/mol. The mechanisms of the photo- or radiooxygenation and chemiluminescence are discussed. The combined photolysis or radiolysis-chemiluminescence reactions constitute light storage systems and prospective radiation dosemeters.
One-electron reduction of oxygen, hydrogen peroxide, potassium peroxodisulphate and potassium peroxodiphosphate was studied during the dissolution of oxide-covered aluminum in alkaline aqueous solution. The production of free oxidizing radicals was monitored by luminol chemiluminescence (CL). It was observed superoxide, hydroxyl, sulphate and phosphate radicals can be generated by the present method. In addition, luminol can be detected below nanomolar level, the linear logarithmic calibration range covering several orders of magnitude of concentration. The metallic aluminum and low-valent aluminum ions are the primary reductants of the system. The electron transfer to the solution is proposed to occur by tunneling through a thin insulating aluminum oxide film at the solid/electrolyte interface in moderately alkaline solutions with simultaneous dissolution of the forming oxide film. In a highly alkaline solution, it is more probable that the oxidation of aluminum species occurs in direct contact of the metallic aluminum with the aqueous solution. In the latter case, short-lived solvated low-valent aluminum ions, hydrogen atom and its deprotonated form, the hydrated electron, can exist as reducing mediators in the chemical reactions in the close vicinity of the dissolving solid/electrolyte interface. Luminol was also observed to exhibit CL under purely reducing conditions produced by a presently unknown excitation pathway.
Cathodic pulse polarisation of oxide-covered aluminium electrodes can generate electrochemiluminescence (ECL) from metalloporphyrins. This is based on the tunnel emission of hot electrons into aqueous electrolyte solution, which probably results in the generation of hydrated electrons as reducing mediators. These tunnel emitted electrons allow the production of highly reactive radicals, such as sulfate radicals from peroxodisulfate ions, which can induce strong redox luminescence from various organic chemiluminophores including metalloporphyrins. The work presented here illustrates the generation of ECL from platinum(II) coproporphyrin (PtCP) and its bovine serum albumin (BSA) conjugate. This allows the detection of these molecules below nanomolar concentrations and over several orders of magnitude of concentration. The relatively long luminescence lifetime of PtCP allows discrimination from the background ECL signal using time resolved measurements, leading to higher sensitivity and the detection of PtCP-BSA indicates the potential use of metalloporphyrins as labels in ECL-based bioassays such as immunoassays and DNA-binding assays.
Two cathodic electrochemiluminescence (ECL) pathways were firstly resolved at glassy carbon electrodes for alkaline lucigenin system by using the potential-resolved ECL method. The light emitters for the ECL emission were identified as excited N-methylacridone and excited lucigenin molecules according to the in situ UV–visible absorption spectra and the fluorescence spectra. Two mechanistic schemes are proposed: (1) the pathway for the ECL pre-peak at about −0.37 (versus Ag) is due to the electroreduction of lucigenin forming its radical cation at the electrode surface, which then reacts with dissolved oxygen in the solution to yield light emission; (2) the pathway of the major ECL peak at −0.50 V is due to the reaction of lucigenin with the electrogenerated H2O2.
Electrogenerated chemiluminescence (ECL) of lucigenin (Luc2+·2NO−3, N,N′-dimethyl-9,9′-biacridinium dinitrate) in dioxygen-saturated alkaline aqueous solutions of pH 10 has been examined utilizing modifications of electrodes (i.e. self-assembled monolayer (SAM)-modified gold electrodes) and of solutions (i.e. micellar solutions containing a nonionic surfactant, Triton X-100) for the first time. In both cases of the modifications, enhanced ECL was observed, and the ECL was considered to be derived from the decomposition of a dioxetane-type intermediate formed by the radical–radical coupling reaction between an electrogenerated superoxide ion (O2−) and a one-electron reduced form of Luc2+ (i.e. a radical cation, Luc+). The surface modification of gold electrodes was achieved with dimercaptosuccinic acid (DMSA) and dl-thioctic acid (dl-TA) having carboxyl end groups. The amount of ECL at dl-TA-SAM-modified electrodes was about five times as great as that at the bare electrode. The enhancement of ECL at the SAM-modified electrodes would be due to the concentration effect of positively charged Luc2+ ions, the prevention of adsorption of the electrogenerated chemiluminescent product (i.e. N-methylacridone (NMA)) and the two-electron reduced form of Luc2+ (Luc0) on the electrode surfaces, and the effective generation of O2−. On the other hand, the amount of ECL in the micellar solution increased by about six times in comparison with that in the solution containing no surfactant. The enhanced ECL in micellar solutions would result mainly from the inhibition of adsorption of NMA and Luc0 insoluble in water on electrode surfaces.
New water-soluble chloride-sensitive indicator dyes based on a bisacridine backbone were synthesized. These quaternized bisacridinium derivatives and the commercial available lucigenin are compared with acridinium compounds in terms of absorbance, fluorescence excitation and emission spectra, their fluorescence decay time and Stern-Volmer constants (K SV) for quenching by a series of anions. All these dyes are quenched dynamically on exposure to chloride and obey the Stern-Volmer law. The effect of pH, buffer composition and ionic strength on the quenching mechanism was investigated. ©1999 Elsevier Science S.A. All rights reserved.
Ruthenium(II) tris(2,2 -bipyridine) chelate shows chemiluminescence (CL) both during dissolution of metallic aluminium in alkaline conditions, and during dissolution of magnesium metal in acidic conditions. The presence of peroxodisulfate ions strongly enhances the CL. Magnesium system provides considerably better detectability of the present chelate giving linear calibration plot spanning over many orders of magnitude of concentration down to subnanomolar concentration levels. The possible primary species generated and luminescence mechanisms are shortly discussed.
The effect of halides and different buffer anions on the quenching of the fluorescence of the new probe 10,10′-bis(3-sulfopropyl)-9,9′-biacridine (SPBA) has been studied using fluorescence and decay time measurements. The linearity of the Stern-Volmer plot indicates that fluorescence quenching by halides can be described reasonably well by a single-exponential decay with a K of 4.06 times 106M-1s-1for chloride, 7.83 times 106M-1s-1for bromide and 1.12 times 107M-1s-1for iodide. We have found that SPBA is collisionally quenched also by the buffers 3-(N-mor-pholino)propanesulfonic acid (MOPS) and N-2-hydroxy-ethylpiperazine-N′-ethansulfonic acid (HEPES). The bi-molecular rate constants are 1.67 × 106M-ls-1for HEPES and 1.44 times 106M-1s-1for MOPS.
Aromatic Gd(III) and Y(III) chelates produce ligand-centered emissions during cathodic pulse polarization of oxide-covered
aluminum electrodes, while the corresponding Tb(III) chelates produce metal-centered5D4 →7Fj emissions. It was observed that a redox-inert paramagnetic heavy lanthanoid ion, Gd(III), seems to enhance strongly intersystem
crossing in the excited ligand and direct the deexcitation toward a triplet-state emission, while a lighter diamagnetic Y(III)
ion directs the photophysical processes toward a singlet-state emission of the ligand. The luminescence lifetime of Y(III)
chelates was too short to be measured with our apparatus, but the luminescence lifetime of Gd(III) chelates was between 20
and 70 μs. The mechanisms of the ECL processes are discussed in detail.
Hot electron injection into aqueous electrolyte solution was studied with electrochemiluminescence and electron paramagnetic
resonance (EPR) methods. Both methods provide further indirect support for the previously proposed hot electron emission mechanisms
from thin insulating film-coated electrodes to aqueous electrolyte solution. The results do not rule out the possibility of
hydrated electron being as a cathodic intermediate in the reduction reactions at cathodically pulse-polarized thin insulating
film-coated electrodes. However, no direct evidence for electrochemical generation of hydrated electrons could be obtained
with EPR, only spin-trapping experiments could give information about the primary cathodic steps.
Results of studies of Al‐Al 2 O 3 ‐(Au or Al) thin‐film emission diodes are in agreement with a model based on electron injection by internal TF emission with subsequent energy loss in the insulating film, characterized by isotropic scattering with energy loss ΔE=0.1 eV and mean free path λ i =6 Å. The experimental results for Au overlayer films are found to be consistent with a modified ballistic transport model allowing weak elastic interactions, with a value for the electron‐electron scattering mean free path at 7.0‐V bias of λ e ∼47 Å. Low‐energy electron bombardment of the thin‐film sample is found to allow detection of overlayer‐film porosity, and a quantitative assessment of hot electron emission through holes in Au overlayer films is obtained. The results indicate that preferential electron emission through holes in pure Au films is not of significance over the entire range of Au film thicknesses studied; this conclusion is substantiated by results obtained with composite Al‐Au‐overlayer films.
The energy distribution of hot electrons in high‐field stressed amorphous silicon dioxide (SiO 2 ) films have been measured using a vacuum emission technique. Electrons having average energies ≳2 eV and an energy relaxation length of λ≊32 Å are observed at all fields studied (≳ 2 MV/cm). However, contrary to previous theoretical expectations, the majority of carriers in the distribution remains stable at all fields. The results are in agreement with other recent experiments (electroluminescence and carrier separation) which only measure the average energy of hot electrons in SiO 2 and with recent Monte Carlo transport calculations which include scattering by both optical and acoustic phonon modes. Results for varying SiO 2 thickness, metal gate thickness, oxide composition, and metal gate composition will be discussed.
Leakage currents introduced in the low‐field, direct‐tunneling regime of thin oxides during high‐field stress are related to defects produced by hot‐electron transport in the oxide layer. From these studies, it is concluded that the ‘‘generation’’ of neutral electron traps in thin oxides is the dominant cause of this phenomenon. Other mechanisms due to anode hole injection or oxide nonuniformities are shown to be unrealistic for producing these currents. Exposure of thin oxides to atomic hydrogen from a remote plasma is shown to cause leakage currents similar to those observed after high‐field stress, supporting the conclusion that these currents are related to hydrogen‐induced defects. © 1995 American Institute of Physics.
Cesiated silicon cold cathodes are capable of emitting high current densities (in excess of 1000 A/cm2) into a vacuum. The energy spread of the emitted electrons amounts to 1.2 eV. Fortunately, the aperture‐limiting diaphragm acts as a kind of energy selector which results in an energy spread of only ∼0.35 eV in the beam after the diaphragm. The intrinsic RC time of 1‐μm‐diam emitters is only ∼1 ps, which, after reduction of parasitic RC times, means that very fast switching of the emission current is possible, especially because a voltage swing of only ∼1 V is needed. Arrays and matrices of closely spaced emitters can be produced using currently available silicon technology. This means that, in principle at least, writing times in electron lithography systems can be reduced by a very large factor (∼1000), making direct slice writing with 0.1‐μm details economically feasible.
The primary processes occurring at cathodically polarized oxide-covered aluminum electrode are discussed in detail. It is pointed out that more energetic cathodic processes can be induced in aqueous media at thin insulating film-coated electrodes than at any semiconductor or active metal electrode. It is proposed that tunnel emission of hot electrons with energies well above the level of the conduction band edge of water occur, and the thermalization and solvation of the emitted electrons can result in generation of hydrated electrons. The cathodically pulse-polarized oxide-covered aluminum also generates a strong oxidant (or oxidants) at the oxide/electrolyte interface, and it is proposed that this species is the hydroxyl radical which is generated either by cathodic high field-induced ejection of self-trapped holes as oxygen dianions (i.e. oxide radical ions) into the electrolyte solution, or by the action of anion vacancies and/or F+-centers as the primary oxidants capable of oxidizing hydroxide ions or the hydroxyl groups of the hydroxylated surface on the oxide film. These radicals, hydrated electrons/hydroxyl radicals, can act as mediating reductants/oxidants in reduction/oxidation of solutes. The formation of the primary species is monitored by electrochemiluminophores which cannot be cathodically excited at active metal electrodes in fully aqueous solutions, but which can be chemically excited in aqueous media in the simultaneous presence of highly reducing and highly oxidizing radicals.
The energetics of reactions involving oxyradicals are reviewed. Thermodynamic data of radical species containing O and H are presented first, followed by those of metallocomplexes and metalloproteins, halogen containing species and organic radicals. Our approach is to calculate Gibb energy changes with the help of reduction potentials. If a particular potential is unknown, we estimate it by way of a thermodynamic cycle. The number of assumptions and estimates increases with the order of the topics covered. Thus, our calculations regarding reactions involving H and O containing species are quite rigorous, while conclusions related to organic radicals are more tentative. In particular reactions of hydrogen peroxide, metal ions in lower oxidation states, hydroquinones, hypohalide ions and ascorbic acid leading to short-lived species such as superoxide radical, hydroxyl radical and singlet oxygen are discussed. Appendices contain conventions regarding electrode potentials and solubility data of oxygen in water.
Heterogeneous and homogeneous immunoassays of human thyroid stimulating hormone (hTSH) were developed on immunometric basis using aromatic Tb(III) chelates as electrochemiluminescent labels and varied types of disposable oxide-covered aluminum electrodes as the solid phase of the immunoassays. The long luminescence lifetime of the present labels allows the use of time-resolved electrochemiluminescence detection and provide the low detection limits of these labels and, thus, sensitive immunoassays. The primary antibody of immunometric immunoassays was coated upon aluminum oxide surface by physical absorption. In homogeneous immunoassays using 66 μl cell and 15 min incubation time, a linear calibration range of 0.25–324 μU/ml was obtained by applying only a single cathodic excitation pulse in the detection step of the assay.
Injection of tunnel-emitted hot electrons from a pulse-polarised oxide-covered aluminium electrode into aqueous Tb(III) ion solution induces electrogenerated luminescence which comprises → transitions of Tb(III). The luminescence lifetime of a hydrated Tb(III) ion is ca. 500 μs while that of the main component of the cathodic intrinsic electrogenerated luminescence of the oxide film is only ca. 6 μs. This allows sensitive detection of Tb(III) ion on a time-resolved basis. Tb(III)-specific electrogenerated luminescence is enhanced by the presence of azide ions or coreactants like hydrogen peroxide and peroxodisulfate ions which produce highly oxidising radicals by one-electron reduction. The Tb(III)-specific electrogenerated luminescence is produced via several parallel mechanisms which usually involve oxidation state changes of Tb(III) that are difficult to produce in an aqueous medium. The cathodically produced primary species are tunnel-emitted hot electrons and F+ centres of the oxide film. These primary species are capable of hard reductions and oxidations not normally encountered in the electrochemistry of aqueous solutions, and can probably produce hydrated electrons and hydroxyl radicals which are able to mediate reductions and oxidations in aqueous solution beyond the tunnelling distance from the oxide/electrolyte interface. The present electrogenerated luminescence is formed as a sum of the luminescence emitted by Tb(III) ions adsorbed at the oxide/electrolyte interface, partially incorporated in the oxide, and located in the solution at the close vicinity of the oxide/electrolyte interface.
The electrogenerated chemiluminescent (ECL) behavior of hemin at a platinum electrode in the alkaline solution has been investigated in detail. Under the optimum conditions the linear response range of hemin is 1.0×10−5–1.0×10−8 g ml−1, the detection limit was 1.0×10−8 g ml−1, and the relative standard derivation for 1×10−7 g ml−1 hemin was 2.8%. It has been also found that hemin would catalyze the ECL of lucigenin at a platinum electrode in a neutral solution in the presence of hydrogen peroxide, the catalytic ECL intensity was linear with the concentration of hemin in the range of 1.0×10−14–1.0×10−10 g ml−1. IgG labeled with hemin was used to examine the ECL catalytic activity of hemin after conjugating to protein, and the results showed that hemin retained ECL catalytic activity when conjugated to protein.
Oxide-covered aluminium electrodes as well as other tunnel emission electrodes allow various label molecules having very different redox and optical properties to be excited cathodically. Low detection limits are obtained and the linear calibration concentration range of the labels spans 5 or 6 orders of magnitude. The lowest detection limits are obtained with Tb(III) chelates which can be detected down to picomolar levels in aqueous solution using time-resolved measurement techniques. Luminophores, such as, 9-fluorenylmethyl chloroformate, derivatives of fluorescein and its analogues, aromatic lanthanide(III) chelates, various coumarins and porphyrins can be used as labels emitting in different spectral regions. The extraordinary analytical power of the tunnel emission electrodes lies in the possibility of simultaneously exciting several different labels emitting either in the UV, visible or NIR range and luminescence lifetimes varying from the ns to the ms range. Therefore, wavelength or time discrimination or their combination can be exploited in separation of the electrochemiluminescence signals from different labels.
A new highly-sensitive assay for cholesterol in serum is described. This assay utilizes chemiluminescence (CL) generated by the reaction of lucigenin and hydrogen peroxide. The latter is produced by the cholesterol/cholesterol oxidase system. Since lucigenin reacts not only with hydrogen peroxide but also with many reductants in serum to give CL, means for suppressing such undesirable CL have been investigated. This was successfully achieved by the addition of copper(II) ions, guanidine hydrochloride and Triton X-100, and accurate adjustment of the pH to 11.75–11.9 with sodium phosphate buffer. Under the optimal conditions the detection limit (2σ) for cholesterol was 1 mg 1−1, and gave results that correlated linearly with a conventional assay up to 2.5 g 1−1.
A time-resolved electroluminometer, which utilizes electrogenerated luminescence (EL) from disposable oxide-covered aluminium electrodes, is described in detail. The cathodic polarization of this disc electrode in aqueous electrolytes induces a faint background EL with peak emission at 570 nm and with a lifetime of 6 μs. This cell can be utilized to excite dysprosium(III), samarium(III) and terbium(III) by electrochemical means to their lowest excited singlet states and, thus, to produce lanthanide(III)-specific emissions. From the lanthanide(III) El systems tested so far, a cathodically induced EL of 2,6-bis[N,N,bis(carboxymethyl)aminomethyl]-4-benzoylphenol-chelated terbium(III) with a 2.1-ms lifetime is the longest lived and can be efficiently discriminated against the background EL on a time-resolved basis using instrumentally simple time-resolved detection; this time-resolved terbium(III) EL makes possible the determination of terbium(III) even below the picomole level in aqueous solutions.