Studies on chemiluminescence in the enzymatic and autoxidative transformation of 3,4-dihydroxyphenylalanine into eumelanins

The catechol oxidase-catalysed and autoxidative transformation of 3,4-dihydroxyphenylalanine (DOPA) to eumelanin have been studied by oxygen consumption, energy transfer, absorption and fluorescence spectroscopy. Formation of transient dopachrome (λ_max = 480 nm) and dopalutin (λ_ex = 423 nm, λ_em = 491 nm) have been found in the enzymatic and autoxidative reaction. In the enzymatic reaction, neither a photon emission with quantum yield Φ > 10^?13 nor energy transfer to triplet and singlet energy acceptors (sensitizers such as anthracene derivatives, xanthene dyes and chlorophyll-a) in water and micellar solutions have been found. The autoxidative reaction is chemiluminescent (Φ = 10^?9), the emission occurring in the 400-600 nm range. The excitation energy is not transferred to sensitizers. The effect of various enzymes and traps of active oxygen species as well as the spectral distribution of chemiluminescence indicate that there is no emission from oxygen dimoles. Carbonates and active species of oxygen are shown to participate in the chemiexcitation reaction.     

Effect of temperature on intense green emitting Na2Ca(PO4)F:Mn2+ phosphor

An intense green luminescent Na₂Ca(PO₄)F:Mn²⁺ phosphor has been prepared at high temperature by reduction treatment in a charcoal environment. The emission band of Mn²⁺ was obtained at around 522 nm (green) under 259 nm excitation. Enhancement in emission intensity arising from the thermal treatment is reported. The intense emission of the spectrum was assigned to electronic transitions ⁴T₁ → ⁶A₁ of Mn²⁺ ions. Intense PL emission suggested that temperature employed plays an important role in the present matrix. X‐ray diffraction pattern, photoluminescence and morphology by SEM of the host lattice of phosphors at different temperatures have been reported in this paper. The results obtained show that the present phosphor has potential for application in green emitting phosphors for the lamp industry. Copyright © 2010 John Wiley & Sons, Ltd.     

A Fluorescence Detected Magnetic Resonance Investigation of the Carotenoid Triplet States Associated with Photosystem II of Isolated Spinach Thylakoid Membranes

The carotenoid triplet populations associated with the fluorescence emission chlorophyll forms of Photosystem II have been investigated in isolated spinach thylakoid membranes by means of fluorescence detected magnetic resonance in zero field (FDMR). The spectra collected in the 680–690 nm emission range, have been fitted by a global analysis procedure. At least five different carotenoid triplet states coupled to the terminal emitting chlorophyll forms of PS II, peaking at 682 nm, 687 nm and 692 nm, have been characterised. The triplets associated with the outer antenna emission forms, at 682 nm, have zero field splitting parameters |D| = 0.0385 cm⁻¹, |E| = 0.00367 cm⁻¹; |D| = 0.0404 cm⁻¹, |E| = 0.00379 cm⁻¹ and |D| = 0.0386 cm⁻¹, |E| = 0.00406 cm⁻¹ which are very similar to those previously reported for the xanthophylls of the isolated LHC II complex. Therefore the FDMR spectra recorded in this work provide insights into the organisation of the LHC II complex in the unperturbed environment represented by thylakoid membranes. The additional carotenoid triplet populations, detected by monitoring the chlorophyll emission at 687 and 692 nm, are assigned to carotenoids bound to inner antenna complexes and hence attributed to β-carotene molecules.     

Near infrared emission of singlet oxygen generated in the dark

Singlet oxygen generation is reported from (1) enzymatic reaction and (2) electron transfer reactions of the superoxide anion measured directly with an ultrasensitive near-IR emission spectrophotometer by monitoring the O₂(¹Δ_g) → O₂ (³Σ_g^?) transition at 1268 nm. Near-IR emission spectra from the myeloperoxidase and lactoperoxidase enzymatic systems show only emission of singlet oxygen at 1268nm. The lipoxygenase/Na–linoleate enzymatic reaction exhibits two emissions, 1268 nm and 1288 nm. The latter emission is identified as originating from a peroxy radical. Spectral and kinetic data giving evidence of singlet oxygen generation is obtained from the reaction of potassium superoxide solubilized by 18-crown-6-ether in acetonitrile with a series of organometallic coordination compounds.     

Influence of Tl+ activator ions on the luminescence characteristics of KCl0.5Br0.5:Eu2+ powder phosphors

Photoluminescence (PL) of thallium co‐doped with KCl_0.5Br_0.5:Eu²⁺ powder phosphors display emission bands at 320 and 370 nm attributable to centres involving Tl⁺ ions in addition to characteristic Eu²⁺ emission around 420 nm. Additional PL excitation and emission bandS observed around 260 and 380 nm, respectively, were observed in the double‐doped KCl_0.5Br_0.5:Eu²⁺, Tl⁺ powder phosphors and are attributed to complex centres involving Tl⁺ and Eu²⁺ ions. The enhancement observed in the intensity of Eu²⁺ emission around 420 nm with the addition of TlBr in KCl_0.5Br_0.5:Eu²⁺ powder phosphors is attributed to the energy transfer from Tl⁺ → Eu²⁺ ions. Photostimulated luminescence (PSL) studies of γ‐irradiated KCl_0.5Br_0.5:Eu²⁺, Tl⁺ mixed phosphors are reported and a tentative PSL mechanism in the phosphors has been suggested. Copyright © 2009 John Wiley & Sons, Ltd.     

Luminescence characterization of KBaPO4:RE (RE = Sm3+,Eu3+,Dy3+) phosphors

KBaPO₄ luminescent powdered phosphors doped with rare earth elements (RE = Sm³⁺,Eu³⁺,Dy³⁺) were successfully synthesized using a wet chemical method to identify the most suitable phosphor for solid‐state lighting based on the measurement of their emission spectra at excitation wavelengths. The X‐ray diffraction pattern of the as‐prepared KBaPO₄ was well matched with its standard JCPDS file no. 330996, indicating the formation of the desired compound. Scanning electron microscopy images revealed irregular morphology, the material crystallized particles aggregated and were non‐uniform with particle sizes ranging from 1 to 100 μm. Photoluminescence excitation and emission spectra clearly indicated that the phosphor containing the Sm³⁺‐activated KBaPO₄ phosphors could be efficiently excited at 403 nm and exhibited an emission mainly including two wavelength peaks at 559 nm and 597 nm. The phosphor containing the Eu³⁺‐activated KBaPO₄ phosphors could be efficiently excited at 396 nm and exhibited a bright red emission mainly including two wavelength peaks at 594 nm and 617 nm. The phosphor containing the Dy³⁺‐activated KBaPO₄ phosphors could be efficiently excited at 349 nm and exhibited wavelength peaks at 474 nm and 570 nm.     

Photoluminescence in K3Ca2(SO4)3Cl‐doped Eu3+ phosphor

In this article we report Eu³⁺ luminescence in novel K₃Ca₂(SO₄)₃Cl phosphors prepared by wet chemical methods. The Eu³⁺ emission was observed at 594 nm and 615 nm, keeping the excitation wavelength constant at 396 nm nearer to light‐emitting diode excitation, Furthermore, phosphors were characterized by X‐ray diffraction for the confirmation of crystallinity. The variation of the photoluminescence intensity with impurity concentration has also been discussed. Thus, prominent emission in the red region makes prepared phosphors more applicable for white light‐emitting diodes. Copyright © 2013 John Wiley & Sons, Ltd.     

Luminescence decay from copper(I) complexes of metallothionein

Luminescence in the 600 nm region from aqueous solutions of Zn₇metallothionein (ZnMT) has been studied as a function of the extent of substitution of the ZnMT with Cu⁺. The steady state emission intensity shows a strong correlation with cluster formation. Analysis of the emission decay curves obtained by single photon-counting techniques using a multiexponential decay function, indicates that three components are responsible for the observed emission intensity. A short lived component exhibits a lifetime of 20 ns, while the two longer lived components exhibit lifetimes in the 1 μ range at room temperature. The longer-lived emission is assigned to the excited triplet states of the CuS chromophore, most probably the metal-centered 3d⁹4s¹.     

Nd3+‐Yb3+/Nd3+‐Yb3+‐Li+ co‐doped Gd2O3 phosphors for up and down conversion luminescence

Frequency up‐conversion (UC) emission from the Nd³⁺‐Yb³⁺/Nd³⁺‐Yb³⁺‐Li⁺ co‐doped gadolinium oxide (Gd₂O₃) phosphors prepared by the solution combustion technique in the visible range have been studied by using 980 nm near infrared (NIR) laser diode excitation. The crystalline structure and formation of the cubic phase has been confirmed with the help of X‐ray diffraction (XRD) studies. XRD peak shifts have been found towards the lower diffraction angle side in the case of the Nd³⁺‐Yb³⁺‐Li⁺ co‐doped phosphors. Surface morphology and particle size information have been observed by using field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) analysis. Down‐conversion emission study under 351 nm excitation in the visible region for the Nd³⁺‐Yb³⁺/Nd³⁺‐Yb³⁺‐Li⁺ co‐doped phosphors has been performed. The UC emission bands lying in the green and red region arising from the Nd³⁺ ions have been enhanced by ~260 times, ~113 times due to incorporation of Li⁺ ions in the Nd³⁺‐Yb³⁺ co‐doped phosphors. Photometric characterization has been done for the Nd³⁺‐Yb³⁺/Nd³⁺‐Yb³⁺‐Li⁺ co‐doped phosphors. The present study suggests the capability of the synthesized phosphors in near‐infrared (NIR) to visible upconverter and luminescent device applications.     

A reinvestigation of chlortetracycline fluorescence: effect of pH,metal ions,and environment

The pH dependence of chlortetracycline fluorescence has been reinvestigated. Below pH 7.5, excitation at 400 nm results in an emission band at 530 nm. Above pH 7.5, in addition to the above band, a strong fluorescence is observed at 430 nm on excitation at 345 nm. pK_a values of 3.5 and 7.7 have been determined for the 530- and 430 nm bands, permitting their assignment to the ring A and ring BCD chromophores, respectively. Both bands are shown to be sensitive to divalent metal binding. Excitation energy transfer from the 430-bband to the 530-nm band is observed in metal complexes, with transfer efficiency being greater for the Mg²⁺ chelate than the Ca²⁺ chelate. The effect of solvent and lipid on band intensities and transfer efficiencies is reported. The interaction of chlortetracycline with BSA in the presence of Mg²⁺ leads to the observation of energy transfer from protein tryptophan residues to the 520-nm band of the complex via the intermediacy of the 430-nm band.     

Enhancing the luminescence of Eu3+/Eu2+ ion‐doped hydroxyapatite by fluoridation and thermal annealing

This paper reports a novel way for the synthesis of a europium (Eu)‐doped fluor‐hydroxyapatite (FHA) nanostructure to control the luminescence of hydroxyapatite nanophosphor, particularly, by applying optimum fluorine concentrations, annealed temperatures and pH value. The Eu‐doped FHA was made using the co‐precipitation method followed by thermal annealing in air and reducing in a H₂ atmosphere to control the visible light emission center of the nanophosphors. The intensities of the OH^? group decreased with the increasing fluorine concentrations. For the specimens annealed in air, the light emission center of the nanophosphor was 615 nm, which was emission from the Eu³⁺ ion. However, when they were annealed in reduced gas (Ar + 5% H₂), a 448 nm light emission center from the Eu²⁺ ion of FHA was observed. The presence of fluorine in Eu‐doped FHA resulted in a significant enhancement of nanophosphor luminescence, which has potential application in light emission and nanomedicine.     

Luminescence of bacteriorhodopsin from Halobacterium halobium and its connection with the photochemical conversions of the chromophore

Red luminescence of purple membranes from Halobacterium halobium cells in suspension, dry film or freeze-dried preparations was studied and its emission, excitation and polarization spectra are reported. The emission spectra have three bands at 665–670, 720–730 and at 780–790 nm. The position (maximum at 580 nm) and shape of the excitation spectra are close to those of the absorption spectra. The spectra depend on experimental conditions, in particular on pH of the medium. Acidification increases the long wavelength part of the emission spectra and shifts the main excitation maximum 50–60 nm to the longer wavelength side. Low-temperature light-induced changes of the absorption, emission and excitation spectra are presented. Several absorbing and emitting species of bacteriorhodopsin are responsible for the observed spectral changes. The bacteriorhodopsin photoconversion rate constant was estimated to be about 1 · 10¹¹ s^?1 at ? 196°C from the quantum yields of the luminescence (1 · 10^?3) and photoreaction (1 · 10^?1). The temperature dependence of the luminescence quantum yield points to the existence of two or three quenching processes with different activation energies. High degree of luminescence polarization (about 45–47%) throughout the absorption and fluorescence spectra and its temperature independence show that there is no energy transfer between bacteriorhodopsin molecules and no chromophore rotation during the excitation lifetime. In carotenoid-containing membranes, energy migration from the bulk of carotenoids to bacteriorhodopsin was not found either. Bacteriorhodopsin phosphorescence was not observed in the 500–1100 nm region and the emission is believed to be fluorescence by nature.     

Synthesis and photoluminescence properties of Eu3+‐activated Ba2Mg(PO4)2 phosphor

In this paper, we have reported the photoluminescence (PL) properties of the Ba₂Mg(PO₄)₂:Eu³⁺ phosphor synthesized using a wet chemical method. The preliminary scanning electron microscopy (SEM) investigation of the sample revealed irregular surface morphology with particle sizes in the 10–50 μm range. The strongest PL excitation peak was observed at 396 nm. The emission spectra indicated that this phosphor can be effectively excited by the 396 nm wavelength. Upon 396 nm excitation, the emission spectrum showed characteristics peaks located at 592 nm and 615 nm. These intense orange‐red emission peaks were obtained due to f→f transitions of Eu³⁺ ions. The emission peak at 592 nm is referred to as the magnetic dipole ⁵D₀→⁷F₁ transition and the emission peak at 615 nm corresponded to the electric dipole ⁵D₀→⁷F₂ transition of Eu³⁺. The Commission Internationale de l’Eclairage (CIE) coordinates of the Ba₂Mg(PO₄)₂:Eu³⁺ phosphor were found to be (0.586, 0.412) for wavelength 592 nm and (0.680, 0.319) for wavelength 615 nm situated at the edge of the CIE diagram, indicating high colour purity of phosphors. Due to the high emission intensity and a good excitation profile, Eu³⁺‐doped Ba₂Mg(PO₄)₂ phosphor may be a promising orange‐red phosphor candidate for solid‐state lighting applications.     

Synthesis,effect of capping agents and optical properties of manganese‐doped zinc sulphide nanoparticles

Mn²⁺‐doped ZnS nanoparticles have been successfully synthesized by a chemical precipitation method, using non‐ionic surfactants such as PMMA and PEG. The particles were prepared in an air atmosphere at 80°C. X‐ray diffraction (XRD), transmission electron microscopy (TEM), UV‐visible and photoluminescence (PL) studies were used to investigate the effect of the capping agent on the size, morphology and optical properties of the ZnS–Mn²⁺ nanoparticles. Enhanced PL was observed from the surfactant‐capped ZnS–Mn²⁺ nanoparticles. The PL spectra showed a broad blue emission band in the range 460–445 nm and a Mn²⁺‐related yellow‐orange emission band in the range 581–583 nm. Copyright © 2012 John Wiley & Sons, Ltd.     

Synthesis and photoluminescence of EuII in barium zinc orthosilicate: a novel green color emitting phosphor for white‐LEDs

A series of Eu²⁺‐activated barium orthosilicates (BaZnSiO₄) were synthesized using a high‐temperature solid‐state reaction. A photoluminescence excitation study of Eu²⁺ shows a broad absorption band in the range of 270–450 nm, with multiple absorption peak maxima (310, 350 and 400 nm) due to 4f–5d electronic transition. The emission spectra of all the compositions show green color emission (in the spectral region 450–550 nm with a peak maximum at 502 nm and a shoulder at ~ 490 nm) with appropriate Comission Internationale de l'Eclairage (CIE) color coordinates. The two emission peaks are due to the presence of Eu²⁺ in two different Ba sites in the BaZnSiO₄ host lattice. The energy transfers between the Eu²⁺ ions in BaZnSiO₄ host are elucidated from the critical concentration quenching data based on the electronic multipolar interaction. All Eu²⁺‐activated BaZnSiO₄ phosphor materials can be efficiently excited in the ultraviolet (UV) to near UV‐region (270–420 nm), making them attractive candidate as a green phosphor for solid state lighting–white light‐emitting diodes.     

Thermoluminescence and photoluminescence studies on γ‐ray‐irradiated Ce3+,Tb3+‐doped potassium chloride single crystals

Single crystals of KCl doped with Ce³⁺,Tb³⁺ were grown using the Bridgeman–Stockbarger technique. Thermoluminescence (TL), optical absorption, photoluminescence (PL), photo‐stimulated luminescence (PSL), and thermal‐stimulated luminescence (TSL) properties were studied after γ‐ray irradiation at room temperature. The glow curve of the γ‐ray‐irradiated crystal exhibits three peaks at 420, 470 and 525 K. F‐Light bleaching (560 nm) leads to a drastic change in the TL glow curve. The optical absorption measurements indicate that F‐ and V‐centres are formed in the crystal during γ‐ray irradiation. It was attempted to incorporate a broad band of cerium activator into the narrow band of terbium in the KCl host without a reduction in the emission intensity. Cerium co‐doped KCl:Tb crystals showed broad band emission due to the d–f transition of cerium and a reduction in the intensity of the emission peak due to ⁵D₃–⁷F_j (j = 3, 4) transition of terbium, when excited at 330 nm. These results support that energy transfer occurs from cerium to terbium in the KCl host. Co‐doping Ce³⁺ ions greatly intensified the excitation peak at 339 nm for the emission at 400 nm of Tb³⁺. The emission due to Tb³⁺ ions was confirmed by PSL and TSL spectra. Copyright © 2015 John Wiley & Sons, Ltd.     

Localization of polyphosphates in Saccharomyces fragilis, as revealed by 4′,6-diamidino-2-phenylindole fluorescence

The fluorescent dye 4′,6-diamidino-2-phenylindole has its emission maximum at 456 nm. Fluorescence intensity at this wavelength is significantly increased by various negatively-charged polyelectrolytes. Among several polyelectrolytes tested, polyphosphates appeared to be unique in the sense that they shifted the emission maximum from 456 to 526 nm. Addition of Saccharomyces fragilis cells to a diamidinophenylindole solution caused an immediate shift of the emission maximum to 526 nm, followed by a gradual increase of fluorescence at 456 nm. The 526 nm, but not the 456 nm fluorescence was instantly quenched by non-penetrating cations, like UO²⁺₂. These results suggest a momentary interaction of diamidinophenylindole with polyphosphate, localized outside the plasma membrane, followed by a slow penetration of the dye into the cells, yielding increased fluorescence at 456 nm by interaction of the dye with e.g., nucleic acids. This was confirmed by fluorescence microscopy. After addition of diamidinophenylindole the yeast cells exhibited an immediate green-yellow fluorescence of the membrane, that was suppressed by UO²⁺₂. After longer incubation times the cytoplasm and nucleus developed a blue fluorescence.     

Fluorescence lifetime spectra of in vivo bacteriochlorophyll at room temperature

Fluorescence lifetime spectra of Rhodopseudomonas sphaeroides chromatophores have been measured at room temperature by phase fluorimetry at 82 MHz in order to investigate the heterogeneity of the emission. The total fluorescence was decomposed into two main components. A constant component, F_c, centered at 865 nm, represents about 50% of the total emission from dark-adapted chromatophores (F_o) and has a lifetime of 0.55 ns. A variable component is centered at 890 nm. Upon closing the reaction centers, 5-fold increases take place in both emission yield and lifetime of this component. In the dark-adapted state, its lifetime is about 50 ps and its contribution to the total fluorescence is 70% at 890 nm. In the presence of sodium dithionite, a long-lifetime component ($\text{τ}{}_{\mathrm{<mtext>D</mtext>}}\text{? 4}\text{ns}$) is observed. This probably arises from radical pair recombination between P⁺ and I^? (P, the primary electron donor, is a dimer of bacteriochlorophyll; I, the primary electron acceptor, is a molecule of bacteriopheophytin). Its spectrum is nearly identical to that of the variable component. This emission seems to be present also under nonreducing conditions, although with a much weaker intensity than when the electron acceptor quinone is prereduced.     

Re-examination of the emission properties of alkoxy- and thioalkyl-substituted phthalocyanines

The role of n → π^∗ transitions in the optical spectra of alkoxy- and thioalkyl-substituted phthalocyanines with peripheral substituents is re-examined, based on a detailed analysis of UV-visible absorption, magnetic circular dichroism (MCD) and fluorescence emission spectral data and the results of density functional theory (DFT) and time dependent-density functional theory (TD-DFT) calculations. The nπ^∗ excited states associated with the lone pairs of the peripheral heteroatoms have been proposed as the origin of a second emission peak observed in the 400-600 nm region, which has been assigned as S₂ emission, since S₁ fluorescence associated with the Q band is observed in the near IR region. Our results demonstrate that emission from a photodecomposition product can fully account for this violet emission and that reports of S₂ emission for peripherally substituted phthalocyanines and porphyrazines should be treated with caution.     

Chemiluminescence of cereal products. II. Chemiluminescence spectra

Spectra of ultraweak chemiluminescence (CL) accompanying auto-oxidation and hydration of cereal products have been measured using single photon counting and cut-off filters. The spectra cover the 380–880 nm spectral range with maxima centred around 600 nm. Analytically pure air-dried carbohydrates like agar, cellulose and nitrocellulose give emission too weak for spectral measurements. The emission from water pure carbohydrates is on average 4–12 times higher and emission spectra are similar to those from cereal products. The effect of free radical scavengers, SOD and O*₂ (¹Δ_g)-quenchers on CL spectra indicates a contribution of radical reactions with the participation of excited carbonyls, O₂⁻ and excited molecular oxygen dimoles. Moreover, possible mechanisms of chemi-excitation due to a cooperative H-bond formation during the hydration of carbohydrates and/or recombination of trapped radicals and electron-holes are discussed. It is also postulated that the excitation energy transfer to natural sensitizers occuring in cereal products may account for non-specific broad spectra and differences in the intensity of CL. © 1998 John Wiley & Sons, Ltd.