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Hot electron-induced cathodic electrochemiluminescence of rhodamine B at disposable oxide-coated aluminum electrodes
Abstract
Rhodamine B (RhB) exhibits strong cathodic electrogenerated chemiluminescence (ECL) in aqueous solutions during high-amplitude pulse polarization at thin oxide film-coated aluminum electrodes. This method allows the detection of RhB below nanomolar concentration level and provides linear calibration plots spanning over several orders of magnitude of concentration. In addition, a relatively long ECL lifetime of RhB provides a basis for time-resolved detection. Thus, widely used RhB-based labels can also be suggested to be usable as electrochemiluminescent labels in fully aqueous solutions in bioaffinity assays such as in immunoassays and DNA-probing assays. Support was obtained for the chemiluminescence generation mechanism to be essentially the same as that of radiochemiluminescence in aqueous solution.
... bioaffinity assays that are important in real-world point of care testing [12,15,16]. In these assays, the lowest determination limits are typically obtained by using aromatic Tb (III) chelates as labels, however many organic luminophores [5,[17][18][19] or Tris(bipyridine)ruthenium(II) [Ru(bpy) 3 ...
... ] -type labels [20,21] can also be used when lower assay sensitivity is sufficient. These labels are typically excited with sequential oneelectron reduction and oxidation steps either by red-ox, or ox-red routes depending on the (1) redox properties of the luminophores or ligands of the complexes, and (2) the stability of luminophore or ligand radicals in the aqueous solution [4,18,19,22,23]. ...
This paper presents a simple and inexpensive method to fabricate chemically and mechanically resistant hot electron-emitting composite electrodes on reusable substrates. In this study, the hot electron emitting composite electrodes were manufactured by doping a polymer, nylon 6,6, with few different brands of carbon particles (graphite, carbon black) and by coating metal substrates with the aforementioned composite ink layers with different carbon-polymer mass fractions. The optimal mass fractions in these composite layers allowed to fabricate composite electrodes that can inject hot electrons into aqueous electrolyte solutions and clearly generate hot electron-induced electrochemiluminescence (HECL). An aromatic terbium (III) chelate was used as a probe that is known not to be excited on the basis of traditional electrochemistry but to be efficiently electrically excited in the presence of hydrated electrons and during injection of hot electrons into aqueous solution. Thus, the presence of hot, pre-hydrated or hydrated electrons at the close vicinity of the composite electrode surface were monitored by HECL. The study shows that the extreme pH conditions could not damage the present composite electrodes. These low-cost, simplified and robust composite electrodes thus demonstrate that they can be used in HECL bioaffinity assays and other applications of hot electron electrochemistry.
... Hence, electrons were able to react with oxygen dissolved in water forming superoxide and hydroxyl radicals and holes can react with organic compounds as follows. Oxidation potentials recalculated to be against NHE were added into the diagram: 1.10 V for Ofloxacin, 1.9 V for Amoxicillin [82], and 1.2 V for RhB [83]. These oxidation potentials explain the lower photocatalytic degradation efficiency of Amoxicillin because it was degraded by superoxide radicals only. ...
Graphitic carbon nitride (g-C3N4) was synthesised from melamine at 550 °C in the air for a period of 4 h. As such prepared g-C3N4 was dispersed in PCl3 and POCl3 with and without pyridine at an ambient temperature for us to dope g-C3N4 with phosphorus. The bulk structural properties of g-C3N4 examined by X-ray diffraction (XRD) and Fourier transformed infrared (FTIR) spectroscopy were not changed. On the contrary, a surface modification in terms of pore size distribution studied using physisorption of nitrogen and electron microscopy was observed. Using PCl3 (in the presence of pyridine), nitrogen vacancies were filled with phosphorus and phosphoramidate groups were formed (with and without pyridine). When POCl3 was used nitrogen vacancies were removed and the surface structure was rearranged, but no phosphorus was doped in g-C3N4. The band gap energies varied from 2.69 to 2.73 eV and specific surface areas varied from 8 to 11 m² g⁻¹.
The g-C3N4 surface structure rearrangement was associated with altered electronic properties which led to higher photocatalytic activity observed by the degradation of Ofloxacin, Amoxicillin and Rhodamine B (RhB) under LED irradiation of 420 nm. A degradation efficiency decreased in the order: Ofloxacin > RhB > Amoxicillin. Superoxide radicals were found to be able to react with all the organic compounds, but holes could react only with Ofloxacin and RhB. All the modified materials were more active than the pristine g-C3N4 and the best photocatalyst was prepared through the reaction with PCl3 in the presence of pyridine.
... However, when the concentration of BPA increased from 10 to 30 ppm, the mineralization efficiency of TOC decreased from 62.7% to 48.2%. This might be due to the excessive molar extinction coefficient of BPA, resulting in lower efficiencies of light utilization [64]. ...
Heterostructured composites with an excellent photocatalytic activity have attracted increasing attention because of their great application in environmental remediation. Herein, a MIL-101(Fe)/g-C3N4 heterojunction was synthesized via in-situ growth of MIL-101(Fe) onto g-C3N4 surface. The heterojunctions were applied as a bifunctional photocatalyst for simultaneous reduction of Cr(VI) and degradation of bisphenol-A (BPA) under visible light and exhibited an obvious enhancement in photocatalytic performance compared with MIL-101(Fe) or g-C3N4. The improved activity could be attributed to the enhanced light absorption and efficient charge carrier separation by forming a direct Z-scheme heterojunction with appropriate band alignment between MIL-101(Fe) and g-C3N4. The radical trapping and electron spin resonance showed that photo-generated electrons are responsible for the reduction of Cr(VI) and BPA degradation, following an oxygen-induced pathway. This work provides new insight into the construction of metal-free semiconductor/MOFs heterojunctions as a bifunctional visible-light-driven photocatalyst for efficient and simultaneous treatment of multiple toxic pollutants in water.
... However, when the concentration of MG is increased from 20 to 30 ppm, the degradation rate of MG and reduction rate of Cr(VI) decreased, respectively. It is reasonable because a large molar extinction coefficient of MG leading to a lower efficiencies of light utilization [44]. Furthermore, the similar activity trend is also observed in Cr(VI)/RhB systems (see Fig. S2(b)). ...
Background
The hybrid nanocomposites of graphitic carbon nitride (g-C3N4) and char were synthesised from melamine and hydroxyethyl cellulose/glucose to obtain metal-free photocatalysts active under visible irradiation. The nanocomposites were tested for the degradation of phenol and ofloxacin.
Methods
The nanocomposites were characterized by elemental and thermal analysis. Their electronic properties were studied by UV-Vis and photoluminescence spectroscopy. Texture and morphology were studied by physisorption of nitrogen, X-ray diffraction, infrared spectroscopy, X-ray photoelectron spectroscopy, and by transmission electron microscopy. The photocatalytic experiments were performed under the LED irradiation of 420 nm.
Significant findings
Hydroxyethyl cellulose and glucose formed char which was incorporated in the g-C3N4 structure. The char acted as an electron collector enabling the separation of photoinduced electrons and holes. In this way, the photocatalytic activity of g-C3N4 increased and 96 % of ofloxacin was degraded during after 120 min. In addition, in this work the total combustion of graphitic carbon nitride up to 800 oC was investigated for the first time.
For hot electron-induced electrochemiluminescence (HECL), developing facile and controllable insulating layer on the electrode surface is key to the effective electron tunneling. Herein, a polystyrene modified glassy carbon electrode based the spin-coating was proposed for HECL. The constructed interface was characterized by atomic force microscopy (AFM). And HECL performance was investigated in details using the system of Ru(bpy)3²⁺ and S2O8²⁻. The as-prepared film shows the stable and reproducible HECL performance and the HECL mechanism was discussed using ECL spectra and hydrated electron quenchers. Finally, on the basis of the inhibition of SO4•-, the present polystyrene film was applied to the detection of rutin in the range of 0.1-1.3 μM with a detection limit of 90 nM (S/N = 3).
Hot electron-induced electrochemiluminescence (HECL) of calcein and calcein-Tb(III) complex was generated at oxide-covered aluminum electrode during cathodic pulse polarization. The excitation of calcein as a molecule or as a ligand is based on subsequent one-electron oxidation and reduction steps by oxidizing radicals and solvated electrons. During HECL excitation of calcein-Tb(III) solution a reaction product of the calcein is formed that also enables the photoexcitation of Tb(III) via the formed ligand derivative by ligand-sensitized mechanism. The determination of low concentrations of calcein with aluminum electrodes was complicated by a relatively strong background electroluminescence originating from the Al2O3-layer. Polyvinyl butyral-carbon black composite electrodes coated on brass were fabricated to solve this problem and a fifty-fold lower background emission was obtained for these novel composite electrodes in comparison to that of oxide-covered 99.9% pure aluminum electrodes. The obtained detection limits were 3.2·10⁻¹⁰ M for calcein and 6.4·10⁻⁹ M for calcein-Tb(III) at the present composite electrodes. These species could potentially be utilized as electrochemiluminescent labels in bioaffinity assays.
Magnetic solid phase extraction coupled with spectrofluorometry was proposed to determine the presence of trace rhodamine B in chili powder and tap water. The characterization results showed that Fe3O4 covered the surface of the 3D reduced graphene oxide (3D-RGO) composite. The adsorption behaviour and adsorption capacity of the 3D-RGO composite were investigated. Several significant factors that affect the adsorption and desorption performance were optimized. The proposed method gave a wide linear range, from 0.025 to 150 μg L-1 with R2=0.9991; low limit of detection (3 Sb), and quantification (10 Sb) of 0.0074 μg L-1 and 0.025 μg L-1 (n=3), respectively. Furthermore, this method was successfully applied to determine the amount of rhodamine B in real samples, and the recoveries at three different spiked levels (0.03, 10, and 20 μg L-1) were in the range of 95.66-99.98%, with a relative standard deviation of less than 2.01% (n=3).
Electrophoresis on plastic microchips is an economic way for rapid separation and determination of multiple analytes, but its application is frequently limited by the unstable electroosmotic flow (EOF) and non-specific adsorption of certain analytes and sample matrices. In this work, cationic polymer poly (diallyldimethylammonium chloride) (PDDA) was used as the background electrolyte (BGE) to eliminate the non-specific adsorption, control the EOF, regulate the viscosity and improve the separation efficiency on cyclic olefin copolymer microchips with 75 μm id microchannels. Three rhodamine dyes that are easily adsorbed on plastics were separated in the presence of organic solvents. Influences of the experimental conditions were systematically investigated and the performance was compared with anionic polymer polyacrylic acid or neutral polymer hydroxypropyl cellulose. The effectiveness of ionic polymers for the modulation of EOF was demonstrated. With PDDA as BGE, three rhodamine dyes were well separated and baseline separation could be achieved in 20 s with a voltage as low as 400 V using a microchannel of 1 cm long and 75 μm id. The determination of rhodamine dyes in foods and lipsticks was performed to verify the robustness and applicability of the proposed methods Separation of peptides and proteins labeled by rhodamine B isothiocyanate was also performed with the proposed running medium.
A novel composite, CdS/MIL-53(Fe), was successfully fabricated via a facile solvothermal method and characterized with XRD, SEM, TEM, XPS, FT-IR and UV–vis DRS. The results showed that the fabrication was able to result in a good dispersion of CdS nanoparticles onto MIL-53(Fe). The photocatalytic activities of the as-synthesized composite were investigated through the degradation of Rhodamine B (RhB) in water under visible light irradiation. It was found that the composite prepared at the mass ratio of CdS/MIL-53(Fe) = 1.5 displayed the highest photocatalytic activity. An approximately 92.5% of photocatalytic degradation of RhB was achieved at 0.5 g/L of 1.5-CdS/MIL dosage, 10 mg/L of initial RhB concentration and 23 °C of reaction temperature under visible light irradiation. The RhB photocatalytic degradation followed well the first-order kinetics equation and the increased catalyst dosage and optimal initial RhB concentration were responsible for the enhanced photocatalytic degradation. Quenching tests revealed that the predominant free radicals in the CdS/MIL-(53)-RhBaq-visible light system was O2⁻; nevertheless, h⁺ and OH also contributed to a certain degree. The enhanced photocatalytic performance was ascribed to the formation of heterojunction structure between CdS and MIL-53(Fe) which significantly suppressed the recombination of photogenerated electron-hole pairs. Moreover, the reusability of 1.5-CdS/MIL composite was also studied.
Introducing molecular dopants in carbon nitride has been shown to dramatically modify its electronic structure, resulting in efficient charge separation and improved photocatalytic efficiency. Herein, we have studied the effect of doping carbon nitride with 2-aminothiophene-3-carbonitrile. A fundamental photoelectrochemical characterization has been performed comparing the behavior of the resulting material (ATCN) with carbon nitride (CN). On the one hand, it is shown that the photocurrent onset shifts with the pH up to a value of 8 for both materials, as expected theoretically. On the other, ATCN, which benefits from additional light absorption, shows an improved photoactivity toward hydrogen evolution. In addition, it displays intriguing photoluminescence properties that can be additionally engineered by modulating the potential. In a more general vein, this study illustrates how to shed light on the effects of introducing molecular dopants in the CN matrix.
Due to awfully harmful to the human health, the qualitative and quantitative determinations of rhodamine B (RhB) are of great significance. In this paper, per-6-thio-β-cyclodextrin functionalized nanogold/hollow carbon nanospheres (β-CD-AuNPs/HCNS) nanohybrids were prepared and then applied successfully in highly sensitive electrochemical detection of RhB. Due to the synergetic effects from HCNS (excellent electrochemical properties and large surface area), β-CD (high host-guest recognition and water-solubility) and AuNPs (excellent electrocatalytic activity), the oxidation peak currents of RhB on the β-CD-AuNPs/HCNS modified glassy carbon electrode (GCE) are much higher than those on the AuNPs/HCNS/GCE and bare GCE. The β-CD-AuNPs/HCNS/GCE has a linear response range of 4.79-958.00 μg L-1 with a detection limit of 0.96 μg L-1 for RhB.
Efficient removal of organic pollutants from wastewater has become a hot research topic due to its ecological and environmental importance. Traditional water treatment methods such as adsorption, coagulation, and membrane separation suffer from high operating costs, and even generate secondary pollutants. Photocatalysis on semiconductor catalysts (TiO2, ZnO, Fe2O3, CdS, GaP, and ZnS) has demonstrated efficiency in degrading a wide range of organic pollutants into biodegradable or less toxic organic compounds, as well as inorganic CO2, H2O, NO3−, PO43−, and halide ions. However, the difficult post-separation, easy agglomeration, and low solar energy conversion efficiency of these inorganic catalysts limit their large scale applications. Exploitation of new catalysts has been attracting great attention in the related research communities. In the past two decades, a class of newly-developed inorganic–organic hybrid porous materials, namely metal–organic frameworks (MOFs) has generated rapid development due to their versatile applications such as in catalysis and separation. Recent research has showed that these materials, acting as catalysts, are quite effective in the photocatalytic degradation of organic pollutants. This review highlights research progress in the application of MOFs in this area. The reported examples are collected and analyzed; and the reaction mechanism, the influence of various factors on the catalytic performance, the involved challenges, and the prospect are discussed and estimated. It is clear that MOFs have a bright future in photocatalysis for pollutant degradation.
Metal–organic frameworks (MOFs), a new class of porous crystalline materials, have attracted great interest as a promising candidate for sustainable energy and environmental remediation. In this study, we demonstrate that an iron terephthalate metal–organic framework MIL-53(Fe) synthesized by a facile solvothermal reaction was capable of activating hydrogen peroxide (H2O2) to achieve high efficiency in photocatalytic process. It could completely decompose the 10 mg L−1 Rhodamine B (RhB) in the presence of a certain amount of H2O2 under visible light irradiation within 50 min. The catalytic activities were found to be strongly affected by the various operating parameters, such as solution pH, initial dye concentration, and H2O2 dosage. The activation effects of MIL-53(Fe) were investigated through the detection of hydroxyl radicals (OH) and transient photocurrent responses, which revealed that the H2O2 behaved in two ways during the catalytic process: (i) it could be catalytically decomposed by MIL(Fe)-53 to produce OH radicals through the Fenton-like reaction; (ii) it could capture the photogenerated electrons in the conduction band of excited MIL-53(Fe) to form OH radicals under visible light irradiation. The ability of such iron-based MOFs to activate H2O2 may enable rational design of advanced MOF-based catalysts for environmental remediation.
This paper reported an innovative simple voltammetric approach for determination of rhodamine B based on a glassy carbon electrode. Cyclic voltammetry and differential pulse voltammetry were used to investigate the electrochemical behavior of rhodamine B. After optimizing the experimental conditions, the anodic peak current of rhodamine B was linear to its concentration in the range of 4.78–956.1 μg L−1, and the limit of detection was 2.93 μg L−1 in pH 4.0 buffer solution. The electrode showed good repeatability and acceptable selectivity. This method was successfully applied to the detection of rhodamine B in preserved fruit and fruit juice samples, which has shown good reliability.
In the present work, a novel sensor for luminol electrochemiluminescence (ECL) was constructed on the base of a C-doped titanium oxide amorphous semiconductor electrode. The morphology, structural and electrochemical properties of the electrode was characterized by X-Ray diffraction, X-Ray photoelectron spectroscopy and electrochemical methods. The ECL behavior of luminol excited by hot electrons injected from C-doped oxide film-covered electrodes in aqueous medium has been investigated in B–R buffer solution (pH=9) when linear sweep cyclic voltammetry (CV) was applied. Two ECL peaks were observed at −1.0V (vs. Ag/AgCl, reduction process) and −0.75V (vs. Ag/AgCl, oxidation process). The possible mechanism was discussed. The C-doped Ti oxide electrode shows excellent properties for sensitive determination of luminol with good reproducibility and stability. The linear response of luminol was in the range of 1×10−8 to 9×10−8mol/L with the detection limit of 3×10−9mol/L (S/N=3). Since luminol is one of the most useful ECL probe, many bioactive compounds which can be labeled by luminol are able to be detected by using the proposed method.
In this paper the aqueous electrochemiluminescence (ECL) from polymethylmethacryl ate Langmuir-Blodgett films incorporating water insoluble luminophors as rubrene (RUB) and 9,10-diphenylanthracene (DPA) at the electrode was obtained. ECL was excited using reaction with coreactant tripropylamine in aqueous solution in the anodic potential range using scanning voltammetry method. Dependence of ECL response of RUB films versus their thickness and coreactant concentration was also studied. (c) 2008 Elsevier Ltd. All rights reserved.
In this paper, a new niobate semiconductor photocatalyst Sr(0.4)H(1.2)Nb(2)O(6)·H(2)O (HSN) nanoparticle was applied to investigate the cathodic electrochemiluminescent (ECL) behavior of luminol for the first time. The results presented here demonstrated that there were two ECL peaks of luminol at the cathodic potential attributed to immobilization of HSN on the electrode surface. It is implied that HSN can be electrically excited and injected electrons into aqueous electrolytes from this electrode under a quite low potential that only excites luminol. A mechanism for this luminol-ECL system on HSN/GCE has been proposed. Additionally, this HSN/GCE has lots of advantages, such as high stability, good anti-interference ability, simple instrumentation, rapid procedure and ultrasensitive ECL response. It is envisioned that this HSN/GCE has further applications in biosensors.
All autonomous machines share the same requirement-namely, they need some form of energy to perform their operations and nanovalves are no exception. Supramolecular nanovalves constructed from [2]pseudorotaxanes-behaving as dissociatable complexes attached to mesoporous silica which acts as a supporting platform and reservoir-rely on donor-acceptor and hydrogen bonding interactions between the ring component and the linear component to control the ON and OFF states. The method of operation of these supramolecular nanovalves involves primarily the weakening of these interactions. The [2]pseudorotaxane [BHEEEN subset of CBPQT](4+) [BHEEEN 1,5-bis[2-(2-(2-hydroxyethoxy)ethoxy)ethoxy]naphthalene and CBPQT(4+) cyclobis(paraquat-p-phenylene)], when this 1:1 complex is tethered on the surface of the mesoporous silica, constitutes the supramolecular nanovalves. The mesoporous silica is charged against a concentration gradient with luminescence probe molecules, e.g., tris(2,2'-phenylpyridyl)iridium(III), Ir(ppy)(3) (ppy=2,2'-phenylpyridyl), followed by addition of CBPQT center dot 4Cl to form the tethered [2]pseudorotaxanes. This situation corresponds to the OFF state of the supramolecular nanovalves. Their ON state can be initiated by reducing the CBPQT(4+) ring with NaCNBH3, thus weakening the complexation and causing dissociation of the CBPQT(4+) ring away from the BHEEEN stalks on the mesoporous silica particles MCM-41 to bring about ultimately the controlled release of the luminescence probe molecules from the mesoporous silica particles with an average diameter of 600 nm. This kind of functioning supramolecular system can be reconfigured further with built-in photosensitizers, such as tethered 9-anthracenecarboxylic acid and tethered [Ru(bPY)(2)(bpy(CH2OH)(2))](2+) (bpy = 2,2'-bipyridine). Upon irradiation with laser light of an appropriate wavelength, the excited photosensitizers; transfer electrons to the near-by CBPQT(4+) rings, reducing them so that they dissociate away from the BHEEEN stalks on the surface of the mesoporous silica particles, leading subsequently to a controlled release of the luminescent probe molecules. This control can be expressed in both a regional and temporal manner by the use of light as the ON/OFF stimulus for the supramolecular nanovalves.
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.
Electrogenerated luminescence of lanthanides is reviewed with emphasis on the electrochemiluminescence (ECL) of lanthanide chelates. Main application area of lanthanide chelates in this field is their use as electrochemiluminescent labels in bioaffinity assays such as in immunoassays or DNA probe assays. With lanthanide chelates as labels, hot electron-induced ECL at thin insulating film- coated cathodes outperforms ECL based on traditional electrochemistry at active metal electrodes. ECL excitation of lanthanide(III) chelates occurs normally by ligand-sensitized mechanisms wherein the ligand is first excited by redox reactions followed by energy transfer from ligand to the central ion, which finally emits by f–f transitions. Luminescent lanthanide ions can be excited at oxide-coated metal electrodes when these ions act as luminescence centers in the oxide film and/or at the oxide/electrolyte interface or as solvated in the close vicinity of the electrode surface. These ions typically show high field-induced solid-state electroluminescence when embedded inside of the oxide films and ECL at the surface of the electrode or in solution close to the electrode surface. These lanthanide-doped oxide films can be fabricated either by anodization of certain lanthanide-doped valve metals or from pure valve metals by anodization in lanthanide ion-containing anodization bath preferably with AC voltages. Some lanthanide ions can be electrically excited also in strongly acidic sulfuric acid solutions at platinum electrodes with mechanisms not known with certainty.
This paper describes a heterogeneous oligonucleotide-hybridization assay based on hot electron-induced electrochemiluminescence (HECL) of a rhodamine label. Thin oxide-film coated aluminum and silicon electrodes were modified with an aminosilane layer and derivatized with short, 15-mer oligonucleotides via diisothiocyanate coupling. Target oligonucleotides were conjugated with tetramethylrhodamine (TAMRA) dye at their amino modified 5′ end and hybridization was detected using HECL of TAMRA. Preliminary results indicate sensitivity down to picomolar level and low nonspecific adsorption. The sensitivity was better on oxide-coated silicon compared to oxide-coated aluminum electrodes and two-base pair mismatched hybrids were successfully discriminated. The experimental results presented here might be useful for the design of disposable electrochemiluminescent DNA biosensors.
C-reactive protein (CRP) was determined in the concentration range 0.01-10 mg L(-1) using hot electron induced electrochemiluminescence (HECL) with devices combining both working and counter electrodes and sample confinement on a single chip. The sample area on the electrodes was defined by a hydrophobic ring, which enabled dispensing the reagents and the analyte directly on the electrode. Immunoassay of CRP by HECL using integrated electrodes is a good candidate for a high-sensitivity point-of-care CRP-test, because the concentration range is suitable, miniaturisation of the measurement system has been demonstrated and the assay method with integrated electrodes is easy to use. High-sensitivity CRP tests can be used to monitor the current state of cardiovascular disease and also to predict future cardiovascular problems in apparently healthy people.
Thin antimony oxide covered AuSb alloy electrode was firstly found to be an excellent cold cathode for generating hot electrons during cathodic pulse polarization. Owing to the injection of hot electrons and the subsequent generation of hydrated electrons, fluorescein iso-thiocyanate (FITC) that cannot be excited in common ECL was cathodically excited at the alloy electrode. Self-assembled thiol monolayers were formed on the electrode surface due to the presence of Au in the alloy, to which strepavidin was covalently bound, and then biotinylated antibody was immobilized through the strepavidin-biotin interaction. As a simple model, an immunosensor for the detection of human IgG (hIgG) using FITC as labeling agent was fabricated. ECL signals were responsive to the amount of hIgG bounded to the immunosensor. The ECL intensity was linearly changed with the logarithm of hIgG concentration in the range of 1.0-1000 ng mL(-1), and the detection limit was ca. 0.3 ng mL(-1) (S/N=3). The proposed immunosensor showed a broad linear range (three magnitudes), good reproducibility and stability, which is promising in detecting FITC-based labels in various types of bioaffinity assays.
Cathodic electrochemiluminescence (ECL) is firstly observed at a carbon oxide covered glassy carbon (C/C(x)O(1-x)) electrode as a large cathodic pulse polarization is applied. This insulating carbon oxide (C(x)O(1-x)) film is constructed on a glassy carbon (GC) substrate by electrochemical oxidization in basic media. The film properties, such as the composition of carbon and oxygen, and the thickness as well, can be controlled by the potential and the duration in the oxidizing process. X-Ray photoelectron spectroscopy (XPS) studies show that carbonyl and carboxyl dominate at the oxidized surface, to which antibodies can be covalently bound. The specific immunoreaction between antigen (Ag) and antibody (Ab) resulted in a decrease in the ECL intensity, thus creating an interesting basis for the development of a label-free cathodic ECL immunosensor. As an example, human IgG (hIgG) was sensitively determined in the concentration range of 0.01-100 ng mL(-1), and the detection limit was ca. 1.0 pg mL (-1) (S/N = 3). In addition, the content of hIgG in human serum has been assayed by the developed immunosensor and a commercially available immune turbidimetry method, respectively, and consistent results were obtained. The prepared immunosensor provides a promising approach for the clinical determination of IgG levels in human serum, because it is simple, rapid, highly sensitive, specific, and without the need of tedious labeling operations.
Rhodamine B base is employed as a photoactive redox system in a liquid | liquid | electrode triple phase boundary electrochemical experiment. Microdroplets of the water-immiscible 3-(4-phenylpropyl)-pyridine (PPP) containing rhodamine B are deposited onto a basal plane pyrolytic graphite electrode surface which is then immersed into an aqueous solution containing 0.5 M phosphate buffer solution (pH 12). In cyclic voltammetry experiments, it is shown that dark reduction of rhodamine B to leuco-rhodamine B in PPP occurs in two steps--both one electron processes--and the re-oxidation occurs in a single two-electron process. Voltammetric signals are consistent with Na(+) transfer (for the first reduction step) and proton transfer (for the second reduction step) coupled to the electron transfer. A proton-driven disproportionation reaction (with k(dis)=3 mol(-1) dm(3) s(-1) at pH 12) is observed for the one-electron reduced intermediate. In the presence of light, a new photocatalysed oxidation process for the two-electron conversion of leuco-rhodamine B to rhodamine B is observed. Photoexcitation of rhodamine B is shown to trigger an effective photo-comproportionation mechanism. Quantitative insights into the dark and photomechanisms are obtained by comparison of rhodamine B and rhodamine B octadecylester and by applying an approximate numerical simulation procedure based on DigiSim.
Die durch Röntgen-Strahlen erregte Fluoreszenz wäßriger Farbstofflösungen zeigt bei Zusatz von löschenden, selbst nicht fluoreszierenden Substanzen ein gänzlich anderes Verhalten als ihre durch UV erregte Fluoreszenz. Es ist wahrscheinlich, daß ein wesentlicher Teil der Fluoreszenz durch Vereinigung von hydratisierten Elektronen und positiven Farbstoffionen zustande kommt.
This paper describes the development of an assay for 2,4-dinitrophenylhydrazine suitable for application to the determination of carbonyl compounds. Complete factorial design was used to investigate the effects of solvents and carrier on the oxidative chemiluminescence of phenylhydrazines in flow injection analysis. 2,4-Dinitrophenylhydrazine gave a better analyte:blank signal ratio than other phenylhydrazines. Aqueous propan-2-ol was found to be the preferred solvent and aqueous formic acid containing no sensitiser the preferred carrier solution. Rhodamine B sensitiser enhanced all signals but, by its effect on blank signals, reduced the signal to blank ratio. Simplex optimisation was carried out on six variables. The criterion was (A – B)/B, where A and B are the analyte and blank signals, respectively; conditions giving analyte signals of relative standard deviation >10% were eliminated whatever the value of (A – B)/B. The optimum conditions were fine-tuned by univariate exploration and emerged as 22.0% propan-2-ol in the sample, 0.75 M formic acid carrier, 5.0 × 10–5M permanganate and 0.041 M sulfuric acid as oxidant, 3.25 ml min–1 total flow-rate and the water-bath at 40 °C. These conditions can be explained in terms of a combination of flow-rate and factors known to increase the rate of oxidation of 2,4-dinitrophenylhydrazine. The effects of formic acid carrier and propan-2-ol solvent are best understood in terms of their effects of enhancing the signal by energy transfer and diminishing it by competition for permanganate. The log–log calibration for 2,4-dinitrophenylhydrazine in optimum conditions was linear (r = 0.9972, n = 10) from 1 × 10–7 to 2 × 10–5M; the detectivity calculated at three standard deviations above blank was 1.1 × 10–7M (1.1 pmol analyte). The method was found to be subject to interference from common metal ions from V to Zn in the periodic table, especially Fe2+, though EDTA largely corrected this latter effect.
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.
The techniques of pulse radiolysis and laser flash photolysis have been used to obtain both novel and improved data on the yields, lifetimes and absorption spectra of the triplet state, radical anion and radical cation of a number of rhodamine dyes. These parameters have also been measured for the widely-studied rhodamine 6G and are compared with previous data obtained using different techniques. In view of the importance of rhodamine dyes in laser action, in photobiology and in other biological processes, the results of this study provide key data on the photophysical and free radical properties of this important class of dyes.
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.
The electrogenerated chemiluminescence (e.c.l.) and electrochemistry of the laser dyes, coumarin-2, coumarin-30, rhodamine-6G (perchlorate), rhodamine-B (perchlorate), oxazine-1 (perchlorate), and Nile Blue (perchlorate) were studied in acetonitrile using 0.1 M tetra-n-butylammonium perchlorate (TBAP) as a supporting electrolyte. Rather low intensity e.c.l. was obtained for all dyes except Nile Blue. A study of the electrochemical oxidation and reduction of coumarin-30, oxazine-1 and rhodamine-6G using cyclic voltammetry and controlled potential coulometry demonstrated that chemical side reactions of the electrogenerated reactants are responsible for the low e.c.l. efficiency. In several cases the one-electron transfer reaction at the electrode is followed by a dimerization reaction. The neutral free radical formed on reduction of oxazine-1 was investigated by electron spin resonance spectroscopy and coupling constants for it are reported. Some experiments in which the e.c.l. of mixtures of the dyes with rubrene or 9,10-diphenylanthracene were determined are also described.
1-Naphthol, 2-naphthol, 2-naphthol-6-sulphonic acid, 1-amino-2-naphthol-4-sulphonic acid and 1-amino-2-naphthol hydrochloride are determined by the chemiluminescence produced by acidic permanganate oxidation in a flow system. Rhodamine B is used as sensitizer. The limits of detection are ca. 5×10−7 M in a 20-μ sample.
Luminol-specific extrinsic lyoluminescence of UV-irradiated potassium peroxodiphosphate and potassium nitrate has been studied. UV irradiation of solid peroxodiphosphate can induce rupture of the peroxide bond and produce a stable solid solution of phosphate radicals in potassium peroxodiphosphate. The dissolution of the irradiated salt can liberate phosphate radicals which initiate a luminol chemiluminescence-generating pathway in alkaline solution. UV irradiation of solid potassium nitrate induces partial photoisomerisation to potassium peroxonitrite, which has been previously reported to generate hydroxyl radicals during dissolution in aqueous solution. Support for hydroxyl radical generation was obtained and the hydroxyl radical was observed to be a key intermediate for strong chemiluminescence of luminol during dissolution of irradiated KNO3 in luminol sample solutions. Both lyoluminescence systems allow the detection of luminol down to sub-nM levels with linear calibration curves spanning five orders of magnitude of luminol concentration, suggesting that they can be used as tools in bioaffinity assays utilising luminol and isoluminol derivatives as labels.
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.
Hydrated Tb(III) can be excited in aqueous solutions by various pathways during X-ray irradiation. The dominating pathway is probably due to chemical excitation, induced by oxidation of Tb(III) by ionized water molecules, followed by immediate reduction of the resulting Tb(IV) to Tb(III) by hydrated electrons or atomic hydrogen. The high enthalpy of this reduction reaction produces Tb(III) in its highly excited states resulting in typical 5D4 → 7Fi multiplet peak emissions. Other possible excitation pathways are direct excitation of Tb(III) by an impact of X-ray photons or photoelectrons, photoluminescent UV excitation due to the solid state emission of the cell materials, and/or an energy transfer from the excited water molecules.The same excitation mechanisms are also valid for chelated Tb(III), but the chelates with aromatic moieties produce luminescence predominantly by a sensitized mechanism where the aromatic moiety is first chemically excited by radical reactions, followed by an energy transfer to the central ion. Radiative relaxation of chelated Tb(III) results in an emission spectrum similar to that of hydrated Tb(III), but with a radioluminescence intensity considerably stronger than that of hydrated Tb(III).
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 fusogenic properties of Rz1, the proline-rich lipoprotein that is the bacteriophage λRz1 gene product, were studied. Light scattering was used to monitor Rz1-induced aggregation of artificial neutral (dipalmitoylphosphatidylcholine/cholesterol) and negatively charged (dipalmitoylphosphatidylcholine/cholesterol/dioleoylphosphatidylserine) liposomes. Fluorescence assays [the resonance energy transfer between N-(7-nitro-2,1,3-benzoxadiazol-4-yl)phosphatidylethanolamine and N-(lissamine rhodamine B sulfonyl)dihexadecanol-sn-glycero-3-phosphoethanolamine lipid fluorescent probes, as well as fluorescent complex formation between terbium ions and dipicolinic acid encapsulated in two liposome populations and calcein fluorescence] were used to monitor Rz1-induced lipid mixing, contents mixing and leakage of neutral and negatively charged liposomes. The results demonstrated that Rz1 caused adhesion of neutral and negatively charged liposomes with concomitant lipid mixing; membrane distortion, leading to the fusion of liposomes and hence their internal content mixing; and local destruction of the membrane accompanied by leakage of the liposome contents. The use of artificial membranes showed that Rz1 induced the fusion of membranes devoid of any proteins. This might mean that the proline stretch of Rz1 allowed interaction with membrane lipids. It is suggested that Rz1-induced liposome fusion was mediated primarily by the generation of local perturbation in the bilayer lipid membrane and to a lesser extent by electrostatic forces.
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.
Chemiluminescence signals were obtained when aqueous propan-2-ol solutions of phenylhydrazine, 2-nitrophenylhydrazine, 4-nitrophenylhydrazine, 2,4-dinitrophenylhydrazine and hexanal oxime or aqueous solutions of hydroxylamine and dimethylglyoxime were injected into a carrier stream of formic acid which merged with acidified potassium permanganate solution. Rhodamine-B was present in the carrier stream as a sensitiser. The chemiluminescence was deduced to originate from the oxidation of the hydrazine or amine functional groups. For phenylhydrazine oxidation, dinitrogen was proposed as the emitting molecule; in the oxidation of hydroxylamine, a nitrogen oxide emitter was proposed. The role of the formic acid was as a source of formate ions, which are oxidised to carbon dioxide in a relatively slow reaction. It is further proposed that excited carbon dioxide molecules, as well as producing a feeble emission in what seems to be an autocatalytic reaction, transfer energy to the emitting products of analyte oxidation, thus enhancing their chemiluminescence. Injection of formate ions, or their massive production by injections of strong alkali, cause chemiluminescence signals which would interfere with those from the analyte. Iron salts, which catalyse the permanganate–formic acid reaction, have a similar effect but at much lower concentrations.
Tunnel-emitted hot electrons induce the triplet state emission of the ligand of an aromatic Gd(III) chelate while the corresponding Tb(III) chelate shows a metal-centred emission at cathodically pulse–polarised oxide-covered aluminum electrodes in fully aqueous solutions at room temperature.
The chemiluminescent characteristics of the oxidation of some xanthone dyes were studied in detail. The chemiluminescence spectra of these dyes and the absorption and fluorescence spectra of some products of the chemiluminescent reactions were investigated; all these dyes were studied by the Hückel molecular orbital method. On the basis of these investigations, an initial explanation for the relationship between the structure of the dye molecules and their chemiluminescent behaviour and a possible mechanism for this chemiluminescent reaction were proposed. The effects of various types of surfactants on the chemiluminescent reaction were also studied. The mechanism of the enchancement of chemiluminescence by surfactants is discussed. The catalysis of some metal ions was also examined, and it was found that trace Co(II) would catalyse the chemiluminescent reaction in the presence of the cationic surfactant cetyltrimethylammonium bromide. The detection limit was 4–10 pg Co ml−1, depending on the dye.
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.
THE fluorescence of aqueous dye solutions induced by ultra-violet and X-irradiation was the subject of an earlier investigation1. The emission induced by X-rays was postulated to involve the reactions of the primary products formed in an irradiated aqueous solution2. It seemed useful to carry out additional investigations of these systems by the pulse technique, and as a result an unexpected delayed emission was observed when dilute aqueous solutions of several dyes were pulse radiolysed.
Electronic absorption spectra of rhodamine B have been examined in 16 protic and 14 aprotic solvents. Rhodamine B exists in solution as a highly colored zwitterion, a colorless lactone and an intensely colored cation. Protic solvents stabilize the zwitterion: the position of the lactone-zwitterion equilibrium depends upon solvent-dye hydrogen bonding and solvent dielectric/polarizability properties. From its temperature dependence, thermodynamic parameters (ΔG°, ΔH°,ΔS°) for this equilibrium were determined in water, n-propanol and n-octanol. The zwitterion is not stable in aprotic solvents, in which the cation or lactone is favored according to the acidity or basicity of the medium. Absorption spectra of rhodamine 6G, an esterified dye which does not exhibit the above equilibria, are only modestly altered by most variations of solvent and temperature.
The Sanjiang Plain is situated in northeast China. It is one of the areas where marshes are distributed concentratedly and
widely in China.Carex lasiocarpa marsh and the marsh withCarex lasiocarpa as dominative species are distributed widely. We choseCarex lasiocarpa marsh as observation object in 1990 and 1991. Marsh evapotranspiration includes transpiration of marsh plants and evaporation
of water surface. We used evaporimeters to observe the water level, and calculated the water level amplitude. The evapotranspiration
of the marsh is one or two times more than the evaporation of water surface in growing season. The larger the vegetation coverage,
the greater the daily evapotranspiration. When the vegetation coverage of marsh is less than 10%, the daily evapotranspiration
of marsh is close to the evaporation of water surface. The difference between the evapotranspiration of marsh and the evaporation
of water surface in sunny days is more than that in cloudy days.
An immunoanalytical method based on electrogenerated luminescence was developed in which the labelling compound is the terbium chelate of 2,6-bis[N,N-bis(carboxymethyl)aminomethyl]-4-benzoylphenol which is linked by a thioureido group to the antibody. The sandwich complex of the labelled antibody, antigen and antibody immobilized on the surface of an aluminium electrode is excited by an electrical pulse and, after a short delay from the end of the pulse, light emission is measured. The electroluminescence immunoassay is exemplified by the heterogeneous and homogeneous assays of human pancreatic phospholipase A2, for which both assay methods give a reasonably good linearity in the range 10–300 ng ml−1.
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.
Carboxyquinolines, specifically 2-carboxyquinoline (quinaldic acid), 4-carboxy-2-hydroxyquinoline (kynurenic acid), 2-carboxy-4-hydroxyquinoline, 4,4′-dicarboxy-2,2′-biquinoline and 2,2′-biquinoline-4,4′-dicarboxylic acid dipotassium salt in dialkylated amides are radiolyzed to novel 1,4-dihydroquinolines which emit light on addition of bases in the presence of oxygen giving rise to quinolinones. The overall quantum yields (radiolysis and chemiluminescence) are as high as 2.4×10−5einsteinmol−1. The radiolysis and chemiluminescence mechanisms are discussed. The radiochemiluminescence reactions constitute prospective radiation dosemeters and can be used for analytical applications.
Acridone and 9-benzylacridine in dialkylated amides are radiolyzed to novel acridan derivatives which emit light upon addition of strong bases regenerating acridone. The chemiluminescence quantum yields are as high as 2.4 × 10−2 for acridone, while for the 9-benzylacridine 3.1 × 10−3 einstein mol−1. The acridone quantum yield is higher than those of most classical chemiluminescent reactions. The radiolysis and chemiluminescence mechanisms are discussed. The radiochemiluminescence reactions constitute prospective radiation dosemeters.
An investigation of rhodamine B (R) equilibria included a benzene extraction study, measurement of solubility of R salts and a spectrophotometric study in aqueous solution. The colorless lactone form (R°) undergoes a separation of charges in polar solvents to form an intensely colored violet zwitterion (R +-). R +- can add a proton to the carboxyl group formed by the opening of the lactone ring, giving RH + which has nearly the same absorption spectrum as R +-. Addition of a second proton, probably to one of the nitrogen atoms, yields an orange species RH 2++. A third proton gives RH 3+++, which is yellow. The color changes are explained in terms of canonical structures. The cations are capable of interaction with anions such as chloride, bromide and perchlorate in solution to form ion pairs having the same absorption spectra as the free cations. In addition, R +- and ion pairs containing RH + or RH 2++ undergo dimerization as the concentration is increased, causing an apparent deviation from Beer's law. Most of the equilibrium constants were determined.
The redox potentials of some free radicals in aqueous solution have been determined using a new method. This method is described in detail and is based on the electron transfer properties and characteristics of the donor radical ·RH to a range of acceptor molecules A. The reaction ·RH + A ⇌ R + ·A - + H + occurs at an appreciable rate whenever the redox potential E° 1 (V, at pH 7.0, ∼25°) of A is higher than that of ·RH. The percentage efficiency of this electron transfer process is determined for a large number of acceptors by monitoring the formation of the ·A - radicals at the appropriate pH and wavelength. The fast reaction technique of pulse radiolysis and kinetic absorption spectrophotometry has been used to follow these reactions. Typical "titration" type curves are obtained when plotting the percentage efficiency vs. E° 1 of the acceptors. From the midpoint of such curves, the redox potentials of a number of organic free radicals have been determined. Various radicals derived from aliphatic alcohols, sugars, lactic acid, glycolic acid, lactamide, glycolamide, oxalacetic acid, glycine, and glycine anhydride have been produced and their potentials derived. These radicals are known to undergo acid-base reactions, and the variations of the E° 1 as a function of the acid or base forms of the radicals were studied. In all cases, the ionized (basic) forms of the radicals have significantly lower redox potentials than the acid forms of the radicals, making these radicals more powerful reducing agents. Some aspects of the mechanisms and kinetics of free radical reactions in the literature are discussed and can be rationalized on the basis of the determined redox potentials of these free radicals.
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 number of fluorescence quanta emitted per reaction [dye(oxidized by •OH) + eaq-], denoted φch, has been estimated for several dyes. Two standards were used involving high energy irradiation: (A) Cerenkov light induced fluorescence of the dye itself, (B) p-terphenyl fluorescence in benzene. Both methods agreed within experimental error and gave mean values of φch for Acriflavin, Rhodamine B, Fluorescein (at pH 10), and anthranilic acid of 0.011, 0.018, 0.031, and 0.020, respectively.
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.
New luminescent quantum counters composed of laser dyes dispersed In either poly (vinyl alcohol) (PVA) or poly(vinylpyrrolldone) (PVP) solid matrices are described. The PVA matrix is more compatible with water-soluble dyes and the films detach easily from the supporting glass substrate. The PVP films are more compatible with dyes soluble in organic solvents and form extremely tenacious films which can only be removed by dissolution. A variety of xanthene and coumarln dye quantum counters are reported. Relative spectral flatness and sensitivity as well as usable wavelength range are discussed. Several new systems with superior properties are presented as well as the considerations Involved in the proper selection of a quantum counter.
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.
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.
Hot electrons can be injected from conductor/insulator/electrolyte (C/I/E) junctions into an aqueous electrolyte solution by cathodic pulse-polarization of the electrode. Injected hot electrons induce electrogenerated chemiluminescence of various luminophores including coumarins in fully aqueous solutions. This is based on the tunnel emission of hot electrons into aqueous electrolyte solution, which can result in the generation of hydrated electrons as reducing mediators. These tunnel-emitted electrons allow also the production of highly oxidizing radicals from added precursors. This work shows that coumarin derivatives are suitable candidates as ECL labels for bioaffinity assays or other analytical applications in which detection is based on the ECL of pulse-polarized C/I/E tunnel-emission electrodes in fully aqueous solutions. The mechanisms of the ECL of coumarins are discussed and the analytical applicability of the ECL of three coumarin derivatives is studied.
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.
WsBrige Lsungen von Rhodamin B, Trypaflavin und alkalisch-wrige L-sungen von Fluorescein (pH 13) zeigen nach Bestrahlung mit 2 sec Elektronenimpulsen von 4 MeV ein Nachleuchten, das bis zu 100 sec nach dem Impuls noch beobachtet werden kann. Die Lichtintensitt steigt nach dem Abklingen der Cerenkov-Strahlung zunchst bis zu einem Maximum an. Dieses ist bei Rhodamin B besonders ausgeprgt und tritt bei den anderen Farbstoffen erst nach Zugabe von geringen Mengen KJ in Erscheinung. Das Spektrum des Nachleuchtens ist identisch mit dem durch UV angeregten Fluoreszenzspektrum. Bei [F] 2 10–6M zeigt die Gesamtemission nach Messungen mit Rhodamin B ein Maximum. Die Quantenausbeute ist bei [F]=10–6M von der Grenordnung 10–5 Quanten/eV. Durch wirksame e
aq
–
-Fnger wird das Nachleuchten wirksam gelscht. Durch Zugabe von geringen Mengen KJ wird die Intensitt um ein Vielfaches gesteigert. Nach dem gegenwrtigen Stand der Untersuchungen ist wahrscheinlich, da die Anregung durch die Reaktionen OH+FFox e
aq
–
+FoxF*+OH– und die Sensibilisierung durch J– durch die Reaktionen e
aq
–
+FFred J–+OHJ+OH– J+FredF*+J– geschieht.
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.
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.
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.
A new Chemiluminescence method for the determination of captopril is described. The method is based on the Chemiluminescence reaction of captopril with cerium(IV) in sulphuric acid medium. Rhodamine B is suggested as a fluorescing compound for the energy-transferred excitation. This sensitized-type of Chemiluminescence emission allows quantitation of captopril concentrations in the 0.1–6.0 μM range with a detection limit of 0.037 μM original concentration. The experimental conditions for the reaction are optimized, the possible reaction mechanism is discussed and a direct application on a commercial drug formulation is given.
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.
El and Pl, breakdown and self-colouring phenomena of anodic oxide films on aluminium, which have been found to be closely related with each other, were reviewed according to newly proposed classifications. Thus the hitherto unsolved problems which had caused confusion in complete understanding of the art are almost solved. Firstly galvanostatic anodic behaviours of Al were classified. Secondly it was shown that el is caused either by (1) selected impurities (activators) (Mn, Eu etc) in Al, (2) excited carboxylate ions in the film formed in aliphatic carboxylic acids, or (3) by, ‘flaws’ in the case of el in inorganic electrolytes, which is not the el in the true sense. Thirdly pl is to be seen in aliphatic carboxylic acid films only and the luminescent centres are the same with those of el in the acid. Aromatic acid films show neither el nor pl since the acid can not enter into the films. Further, reference was made to one type of solid-type led (Al—Al2O3-SnO2) constructed. Fourthly, breakdowns have been classified into: (1) electrochemical breakdown including porous film formation, carboxylate breakdown (pitting) and chloride breakdown, and (2) ‘electric breakdown during anodizing’ or ‘electrolytic breakdown’ which is a complicated mixture of electrochemical and electric breakdown at/or through oxide—electrolyte interface, and which includes ‘burning’ (luminescent ‘flame’ propagation process) and sparking (scintillation and arcing). Finally self-colouring of ‘pure’ Al (except colouring by alloying elements or impurities included) has been classified into: (1) colouring by decomposition or cyclisation of excited carboxylate ions in the film (post-el) higher electric field attained across the film. Thick, coloured films show no more el nor pl. (2) colouring by existence of Al0 particles in the interstitial positions of the film by high electric field assisted oxide growth in the case of the so-called ‘hard anodizing’, locally grown coloured spots in inorganic acid, eg sulphuric acid, and of so-called integral colour anodizing in sulphonated aromatic dibasic acid (or higher aliphatic carboxylic acid) plus a small amount of sulphuric acid, typically Kalcolor, Duranodic 300 or Permalux processes for architectural use. These films show neither el nor pl.