A high sensitivity system for luminescence measurement of materials

A unique combined and multi‐disciplinary wavelength multiplexed spectrometer is described. It is furnished with high‐sensitivity imaging plate detectors, the power to which can be gated to provide time‐resolved data. The system is capable of collecting spectrally resolved luminescence data following X‐ray excitation [radioluminescence (RL) or X‐ray excited optical luminescence (XEOL)], electron irradiation [cathodoluminescence (CL)] and visible light from light emitting diodes (LEDs) [photoluminescence (PL)]. Time‐resolved PL and CL data can be collected to provide lifetime estimates with half‐lives from microsecond timeframes. There are temperature stages for the high and low temperature experiments providing temperature control from 20 to 673 K. Combining irradiation, time resolved (TR) and TR‐PL allows spectrally‐resolved thermoluminescence (TL) and optically stimulated luminescence (OSL). The design of two detectors with matched gratings gives optimum sensitivity for the system. Examples which show the advantages and multi‐use of the spectrometer are listed. Potential future experiments involving lifetime analysis as a function of irradiation, dose and temperature plus pump‐probe experiments are discussed.     

Probing the optical properties and luminescence mechanism of a UV‐emitting SrBPO5:Ce3+ phosphor

Ce‐doped (1 × 10^?5 to 3.0 mol%) SrBPO₅ phosphors were synthesized using a conventional solid‐state reaction route at 1273 K in an air atmosphere. Phase and morphology of the samples were studied from powder X‐ray diffraction (XRD) patterns and scanning electron microscope (SEM) micrographs, respectively. The band gap energies of the pure and Ce‐doped SrBPO₅ phosphors were calculated from the recorded diffuse reflectance spectra. Photoluminescence (PL) and Ce³⁺ lifetime were recorded at 300 and 77 K. Photoluminescence lifetime measurements revealed two‐lifetime values for Ce³⁺ at both 300 K (17 and 36 nsec) and 77 K (12 and 30 nsec), suggesting the presence of two different environments around Ce³⁺. Time‐resolved emission spectroscopy (TRES) studies confirmed the presence of Ce³⁺ in two different environments. In addition, SrBPO₅:Ce exhibited intense UV emission, signifying its possible use as an efficient sensitizer for solid‐state lighting applications. The effect of γ‐irradiation on PL was also determined. Thermally stimulated luminescence (TSL) glow curves of the γ‐irradiated phosphor, along with trap parameters, dose–response, and the possible TSL mechanism were also investigated. Positron annihilation lifetime spectroscopy was carried out to probe defects present in undoped and Ce‐doped SrBPO₅.     

Rapid,controllable, one‐pot and room‐temperature aqueous synthesis of ZnO:Cu nanoparticles by pulsed UV laser and its application for photocatalytic degradation of methyl orange

Zinc oxide (ZnO) and ZnO:Cu nanoparticles (NPs) were synthesized using a rapid, controllable, one‐pot and room‐temperature pulsed UV‐laser assisted method. UV‐laser irradiation was used as an effective energy source in order to gain better control over the NPs size and morphology in aqueous media. Parameters effective in laser assisted synthesis of NPs such as irradiation time and laser shot repetition rate were optimized. Photoluminescence (PL) spectra of ZnO NPs showed a broad emission with two trap state peaks located at 442 and 485 nm related to electronic transition from zinc interstitial level (I_Zn) to zinc vacancy level (V_Zn) and electronic transition from conduction band to the oxygen vacancy level (V_O), respectively. For ZnO:Cu NPs, trap state emissions disappeared completely and a copper (Cu)‐related emission appeared. PL intensity of Cu‐related emission increased with the increase in concentration of Cu²⁺, so that for molar ratio of Cu:Zn 2%, optimal value of PL intensity was obtained. The photocatalytic activity of Cu‐doped ZnO revealed 50 and 100% increasement than that of undoped NPs under UV and visible irradiation, respectively. The enhanced photocatalytic activity could be attributed to smaller crystal size, as well as creation of impurity acceptor levels (T₂) inside the ZnO energy band gap.     

Synthesis and luminescence properties of CaSnO3:Bi3+ blue phosphor and the emission improvement by Li+ ion

CaSnO₃:Bi³⁺ blue‐emitting phosphor was synthesized using a high‐temperature solid‐state reaction method in air. The crystal structures and luminescence properties were investigated. A broad emission band peaking at ~448 nm upon excitation at 262 and 308 nm was observed in the range 330–680 nm at room temperature due to ³P₁ → ¹S₀ transition of the Bi³⁺ ion. The chromaticity coordinate was (0.1786, 0.1665). The optimal Bi³⁺ ion concentration was ~0.6 mol% in CaSnO₃:Bi³⁺ phosphor. The emission spectrum of CaSnO₃:Bi³⁺ phosphor showed a blue‐shift with increasing temperature from 50 to 300 K due to the influence of temperature on the electron transition of the Bi³⁺ ion. The emission intensity of CaSnO₃:Bi³⁺ phosphor may be increased ~1.45 times by co‐doping Li⁺ ions as a charge compensator and fluxing agent. The luminescence mechanism is explained by a configurational coordinate diagram of Bi³⁺ ion in CaSnO₃:Bi³⁺ phosphor.     

Studies of Electronic Excitations of Rectangular ZnO Nanorods by Electron Energy-Loss Spectroscopy

Electronic excitations of single ZnO rectangular nanorod have been investigated by electron energy-loss spectroscopy in conjunction with scanning transmission electron microscopy (STEM-EELS). We focus primarily on the surface excitations greatly enhanced at the grazing incidence parallel to the surfaces of ZnO nanorods. An uncommon kind of surface excitation known as surface exciton polaritons occurring near interband transitions is found to dominate in the spectral range between the band gap at 3.4 eV and the surface plasmon peak at 15.8 eV. In addition, the dielectric function of ZnO up to 25 eV has also been derived from the bulk excitation spectra using the Kramers–Kronig analysis on a single nanorod. Theoretical EELS simulations are also compared with the experimental results and good agreements are obtained.     

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.     

Graphical and numerical analysis of thermoluminescence and fluorescence F0 emission in photosynthetic material

A set-up for recording thermoluminescence emission together with the constant F₀ fluorescence yield is described briefly. It is driven by a microcomputer through plugged-in cards.Practical aspects of the simulation of TL bands and of decomposition of complex TL signals are examined. A reproducible and linear temperature gradient and the use of photon counting for luminescence detection are important features for further analyzing the recorded signal. The simulation procedure used is a step-by-step calculation of the number of charge recombinations, which is then substracted from the number of remaining charge pairs able to produce luminescence. This procedure consists first of a graphical fitting, followed by a numerical minimization, with a maximum of five simulated components. The quality of the simulation is evaluated by the sum of squares of differences (signal-simulation), related to the signal area. Equivalent decomposition patterns may be found for the same recording and additional information is needed for interpretation of TL data. Averaging signals is feasible, provided that maximum temperatures Tm of averaged bands are sufficiently similar (±3°C). Simultaneous measurement of the antenna fluorescence yield F₀, using an ultra-weak pulsed blue LED, gives an estimate of the luminescence yield. This has to be taken into account in the analysis of the Q band and of high temperature (>40°C) bands.The simulation parameters appear to be dependent on plant growth conditions. Quantitative analysis of thermoluminescence emission could be useful in the study of the effects of climatic factors on the photosynthetic apparatus in plants.Abbreviations PS-II Photosystem II - TL thermoluminescence - F₀ constant fluorescence emission, under ultra-low light intensity - Q_A and Q_B respectively, primary and secondary electron acceptors of Photosystem II - S₂ and S₃ respectively, the two and three positively charged states of the oxygen evolving system - SSD sum of squares of differences between the signal and a simulation (fitting) or between the signal and a smoothed curve (noise)     

Exploration of electron–vibrational interaction in the 5d states of Eu2+ ions in ABaPO4 (A = Li,Na, K and Rb) phosphors

This work reports the theoretical analysis of the electron–vibrational interaction (EVI) in 4f–5d optical transitions of Eu²⁺ ions in ABaPO₄ (A = Li, Na, K and Rb) systems. The EVI parameters were estimated from the recently reported room temperature photoluminescence results, by employing the spectrum‐fitting method. Parameters such as the Huang–Rhys factor, effective phonon energy, Stokes shift and zero‐phonon line position were estimated and are reported here. The estimated EVI parameters were validated by modeling the emission band and establishing the agreement between the experimental and modeled emission bands. Copyright © 2016 John Wiley & Sons, Ltd.     

Fluorescence emission spectra of chloroplasts and subchloroplast preparations at low temperature.

A study was made of the chlorophyll fluorescence spectra between 100 and 4.2 K of chloroplasts of various species of higher plants (wild strains and chlorophyll b mutants) and of subchloroplast particles enriched in Photosystem I or II. The chloroplast spectra showed the well known emission bands at about 685, 695 and 715--740 nm; the System I and II particles showed bands at about 675, 695 and 720 nm and near 685 nm, respectively. The effect of temperature lowering was similar for chloroplasts and subchloroplast particles; for the long wave bands an increase in intensity occurred mainly between 100 and 50 K, whereas the bands near 685 nm showed a considerable increase in the region of 50--4.2 K. In addition to this we observed an emission band near 680 nm in chloroplasts, the amplitude of which was less dependent on temperature. The band was missing in barley mutant no. 2, which lacks the light-harvesting chlorophyll a/b-protein complex. At 4.7 K the spectra of the variable fluorescence (Fv) consisted mainly of the emission bands near 685 and 695 nm, and showed only little far-red emission and no contribution of the band at 680 nm. From these and other data it is concluded that the emission at 680 nm is due to the light-harvesting complex, and that the bands at 685 and 695 nm are emitted by the System II pigment-protein complex. At 4.2 K, energy transfer from System II to the light-harvesting complex is blocked, but not from the light-harvesting to the System I and System II complexes. The fluorescence yield of the chlorophyll species emitting at 685 nm appears to be directly modulated by the trapping state of the reaction center.     

Fluorescence emission spectra of chloroplasts and subchloroplast preparations at low temperature

A study was made of the chlorophyll fluorescence spectra between 100 and 4.2 K of chloroplasts of various species of higher plants (wild strains and chlorophyll b mutants) and of subchloroplast particles enriched in Photosystem I or II. The chloroplast spectra showed the well known emission bands at about 685, 695 and 715–740 nm; the System I and II particles showed bands at about 675, 695 and 720 nm and near 685 nm, respectively. The effect of temperature lowering was similar for chloroplasts and subchloroplast particles; for the long wave bands an increase in intensity occurred mainly between 100 and 50 K, whereas the bands near 685 nm showed a considerable increase in the region of 50-4.2 K. In addition to this we observed an emission band near 680 nm in chloroplasts, the amplitude of which was less dependent on temperature. The band was missing in barley mutant no. 2, which lacks the lightharvesting chlorophyll a/b-protein complex. At 4.7 K the spectra of the variable fluorescence (F_v) consisted mainly of the emission bands near 685 and 695 nm, and showed only little far-red emission and no contribution of the band at 680 nm.From these and other data it is concluded that the emission at 680 nm is due to the light-harvesting complex, and that the bands at 685 and 695 nm are emitted by the System II pigment-protein complex. At 4.2 K, energy transfer from System II to the light-harvesting complex is blocked, but not from the light-harvesting to the System I and System II complexes. The fluorescence yield of the chlorophyll species emittting at 685 nm appears to be directly modulated by the trapping state of the reaction center.     

Energy transfer and bacteriochlorophyll fluorescence in purple bacteria at low temperature

Emission spectra of bacteriochlorophyll a fluorescence and absorption spectra of various purple bacteria were measured at temperatures between 295 and 4 K. For Rhodospirillum rubrum the relative yield of photochemistry was measured in the same temperature region. In agreement with earlier results, sharpening and shifts of absorption bands were observed upon cooling to 77 K. Below 77 K further sharpening occurred. In all species an absorption band was observed at 751-757 nm. The position of this band and its amplitude relative to the concentration of reaction centers indicate that this band is due to reaction center bacteriopheophytin. The main infrared absorption band of Rhodopseudomonas sphaeroides strain R26 is resolved in two bands at low temperature, which may suggest that there are two pigment-protein complexes in this species. Emission bands, like the absorption bands, shifted and sharpened upon cooling. The fluorescence yield remained constant or even decreased in some species between room temperature and 120 K, but showed an increased below 120 K. This increase was most pronounced in species, such as R. rubrum, which showed single banded emission spectra. In Chromatium vinosum three (835, 893 and 934 nm) and in Rps. sphaeroides two (888 and 909 nm) emission bands were observed at low temperature. The temperature dependence of the amplitudes of the short wavelength bands indicated the absence of a thermal equilibrium for the excitation energy distribution in C. vinosum and Rps. sphaeroides. In all species the increased in the yield was larger when all reaction centers were photochemically active than when the reaction centers were closed. In R. rubrum the increase in the fluorescence yield was accompanied by a decrease of the quantum yield of charge separation upon excitation of the antenna but not of the reaction center chlorophyll. Calculation of the F?rster resonance integral at various temperatures indicated that the increase in fluorescence yield and the decrease in the yield of photochemistry may be due to a decrease in the rate of energy transfer between antenna bacteriochlorophyll molecules. The energy transfer from carotenoids to bacteriochlorophyll was independent of the temperature in all species examined. The results are discussed in terms of existing models for energy transfer in the antenna pigment system.     

Isostructural M(RL)2(hfac)2 complexes with RL = 5-(4-[N-tert-butyl-N-aminoxyl]phenyl)pyrimidine

5-(4-(N-tert-Butyl-N-aminoxylphenyl))pyrimidine (RL, 4PPN) forms crystallographically isostructural and isomorphic pseudo-octahedral M(RL)₂(hfac)₂ complexes with M(hfac)₂, M = Zn, Cu, Ni, Co, and Mn. Multiple close contacts occur between sites of significant spin density of the organic radical units. Magnetic behavior of the Zn, Cu, Ni, Co complexes appears to involve multiple exchange pathways, with multiple close crystallographic contacts between sites that EPR (of 4PPN) indicates to have observable spin density. Powder EPR spectra at room temperature and low temperature are reported for each complex. Near room temperature, the magnetic moments of the complexes are roughly equal to those expected by a sum of non-interacting moments (two radicals plus ion). As temperature decreases, AFM exchange interactions become evident in all of the complexes. The closest fits to the magnetic data were found for a 1-D Heisenberg AFM chain model in the Zn(II) complex (J/k = (?)7 K), and for three-spin RL—M—RL exchange in the other complexes (J/k = (?)26 K, (?)3 K, (?)6 K, for Cu(II), Ni(II), and Co(II) complexes, respectively).     

Mass spectrometry and spectroscopic characterization of a tetrameric photosystem I supercomplex from Leptolyngbya ohadii,a desiccation-tolerant cyanobacterium

Cyanobacteria inhabiting desert biological soil crusts face the harsh conditions of the desert. They evolved a suite of strategies toward desiccation-hydration cycles mixed with high light irradiations, etc. In this study we purified and characterized the structure and function of Photosystem I (PSI) from Leptolyngbya ohadii, a desiccation-tolerant desert cyanobacterium. We discovered that PSI forms tetrameric (PSI-Tet) aggregate. We investigated it by using sucrose density gradient centrifugation, clear native PAGE, high performance liquid chromatography, mass spectrometry (MS), time-resolved fluorescence (TRF) and time-resolved transient absorption (TA) spectroscopy. MS analysis identified the presence of two PsaB and two PsaL proteins in PSI-Tet and uniquely revealed that PsaLs are N-terminally acetylated in contrast to non-modified PsaL in the trimeric PSI from Synechocystis sp. PCC 6803. Chlorophyll (Chl) a fluorescence decay profiles of the PSI-Tet performed at 77 K revealed two emission bands at ～690 nm and 725 nm with the former appearing only at early delay time. The main fluorescence emission peak, associated with emission from the low energy Chls a, decays within a few nanoseconds. TA studies demonstrated that the 725 nm emission band is associated with low energy Chls a with absorption band clearly resolved at ～710 nm at 77 K. In summary, our work suggests that the heterogenous composition of PsaBs and PsaL in PSI-Tet is related with the adaptation mechanisms needed to cope with stressful conditions under which this bacterium naturally grows.     

Enhancement of near‐infrared up‐conversion and blue down‐conversion luminescence in LuNbO4:Yb3+,Tm3+ with Ga3+ and Ta5+ substitutions

Under a 980‐nm excitation, the up‐conversion (UC) spectra of LuNbO₄:Yb³⁺,Tm³⁺ powders exhibited predominantly near‐infrared bands (~805 nm) of Tm³⁺ through an energy transfer process from Yb³⁺ to Tm³⁺. Regarding the down‐conversion (DC) luminescence of the powders, the photoluminescence excitation spectra consisted of a broad charge transfer band (270 nm) due to [NbO₄]^3? and sharp band (360 nm) of Tm³⁺, while the corresponding emission spectra exhibited a blue emission at 458 nm. Upon substitution of Ga³⁺ and Ta⁵⁺ for Lu³⁺ and Nb⁵⁺, respectively, both UC and DC luminescence properties were significantly enhanced. For the Ga³⁺ substitution, the increased emission intensity could be explained by the crystal field asymmetry surrounding the Tm³⁺ ions induced by the large difference in ionic radius between Ga³⁺ and Lu³⁺. For the Ta⁵⁺ substitution, we believe that an M′‐LuTaO₄ substructure was formed in the host, which led to the formation of a TaO₆ octahedral coordination instead of a NbO₄ tetrahedral coordination. Consequently, the crystal symmetry of the local structure was modified, and thus the UC and DC luminescence properties were enhanced. The dual‐mode (UC and DC) luminescence demonstrates that LuNbO₄:Yb³⁺,Tm³⁺ has a great potential in the fields of temperature sensing probes, anti‐counterfeiting, and bioapplications.     

Triplet-minus-singlet absorbance difference spectra of reaction centers of Rhodopseudomonas sphaeroides R-26 in the temperature range 24–290 K measured by Magneto-Optical Difference Spectroscopy (MODS)

The recently developed technique of Magneto-Optical Difference Spectroscopy (MODS) [10] has been applied to reaction centers (RC) of the photosynthetic bacterium Rhodopseudomonas sphaeroides R-26. Absorbance changes induced by a magnetic field are measured as a function of wavelength yielding the triplet-minus-singlet (T-S) absorbance difference spectrum. (T-S) spectra thus obtained have been measured from 24–290 K. Going from low to high temperature the (T-S) spectra show the following features:

1. A rapid decrease of positive absorption bands at 809 and 819 nm.
2. A slow appearance of a band shift at 798 nm.
3. A shift of the peak wavelength of the Q_y absorbance band of the primary donor P-860 from 992 to 861 nm, and of its Q_x band from 603 to 600 nm.

The spectra at 24, 66, 116, and 290 K have been analyzed by Gaussian deconvolution. The 800 nm region of the spectrum at 24 K can be decomposed in a combination of two band shifts and an appearing band. The temperature dependence of the spectra in this region is well explained by spectral broadening of the two shifting bands combined with a decrease in intensity of the appearing band when the temperature increases. The two shifting bands in the 800 nm region are identified as the two bands at 803 and 813 nm which together make up the 800 nm band in the absorption spectrum and are assigned to the two accessory RC bacteriochlorophylls (BChls). The band shift of the 813 nm pigment is appreciably larger than that of the 803 nm pigment. The appearing band at 808 nm is attributed to monomeric absorption of ³P-860, the triplet state being localized on one BChl. We find no evidence for admixture of a charge transfer (CT) state of ³P-860 with one of the accessory BChls at higher temperature.     

Photoluminescence and thermoluminescence dosimetry properties of Ag/Y co-doped ZnO nanophosphor for radiation measurements

This work explores the thermoluminescence (TL) and photoluminescence (PL) properties of Ag/Y co-doped zinc oxide (ZnO) nanophosphor. The proposed dosimeter was prepared by the coprecipitation method and sintered at temperatures from 400°C to 1000°C in an air atmosphere. Raman spectroscopy was studied to investigate the structural features of this composition. The new proposed dosimeter revealed two peaks at 150°C and 175°C with a small shoulder at high temperature (225°C). The PL spectrum showed strong green emissions between 500 to 550 nm. The Raman spectrum showed many bands related to the interaction between ZnO, silver (Ag), and yttrium oxide (Y₂O₃). The rising sintering temperature enhanced the TL glow curve intensity. The Ag/Y co-doped ZnO nanophosphor showed an excellent linearity index within a dose from 1 to 4 Gy. The minimum detectable dose (MDD) of the Ag/Y co-doped ZnO nanopowder (pellets) equaled 0.518 mGy. The main TL properties were achieved in this work as follows: thermal fading (37% after 45 days at 1 and 4 Gy), optical fading (53% after 1 h and 68% after 6 h by exposure to sunlight), effective atomic number (27.6), and energy response (flat behavior from 0.1 to 1.3 MeV). Finally, the proposed material shows promising results nominated to be used for radiation measurements.     

Pitfalls, artefacts and open questions in chlorophyll thermoluminescence of leaves or algal cells

Thermoluminescence of intact photosynthetic organisms, leaves or algal cells, raises specific problems. The constitutive S_2/3Q _B ^? B bands constitute major probes of the state of photosystem II in vivo. The presence of a dark-stable acidic lumen causes a temperature downshift of B bands, specially the S₃ B band, providing a lumen pH indicator. This is accompanied by a broadening of the S₃ B band that becomes an envelope of elementary B bands. The occasional A_T, Q and C bands are briefly examined in an in vivo context. It is emphasized that freezing below the nucleation temperature is not necessary for physiological studies, but a source of artefacts, hence should be avoided. In intact photosynthetic structures, a dark-electron transfer from stroma reductants to the quinonic acceptors of photosystem II via the cyclic/chlororespiratory pathways, strongly stimulated by moderate warming, gives rise to the afterglow (AG) luminescence emission that reflects chloroplast energy status. The decomposition of complex TL signals into elementary bands is necessary to determine the maximum temperature T _m and the area of each of them. A comparison of TL signals after 1 flash and 2 flashes prevents from confusing the three main bands observed in vivo, i.e. the S₂ and S₃ B bands and the AG band. Finally, the thermoluminescence bands arising sometimes above 50 °C are mentioned. The basic principles of (thermo)luminescence established on isolated thylakoids should not be applied directly without a careful examination of in vivo conditions.     

Investigation of luminescent banding in solid coral: the contribution of phosphorescence

This study investigates the nature and components of annual luminescent banding in massive Porites coral skeletons, with a view to refining the technique for using this banding to reconstruct past environmental conditions. Three-dimensional excitation-emission-matrix spectroscopy and optical fibre beam delivery have been used to investigate the luminescence properties of the bright and dull bands of solid coral. Six characteristic excitation/emission peaks have been identified: 280/450–600, 340/450, 370/470, 390/485, 420/505 and 450/530 nm. The first peak corresponds to protein-type fluorescence. The others are characteristic of humic acid luminescence. The difference in luminescence intensity between bright and dull bands has been quantified and characterised spectroscopically. The luminescence of the bright bands is up to 25% more intense than their neighbouring dull bands with the greatest increase in relative intensity in the long wavelength emission region, between 500 and 600 nm. The contribution of long-lived phosphorescence to the total luminescence intensity has been determined by time-resolved measurements on the 100 ms timescale. Both bright and dull bands show long-lived phosphorescence with decay times up to 1.5 s. This phosphorescence accounts for about 10% of the total luminescence intensity of bright bands. The difference in phosphorescence intensity between bright and dull bands is substantially greater than the difference in total luminescence intensity: the phosphorescence of bright bands is up to twice as intense as that of dull bands. This suggests that phosphorescence plays an important role in defining luminescent banding in coral. Furthermore, the large observable difference in phosphorescence between bright and dull bands indicates that measurement of phosphorescence profiles across growth bands in corals may prove to be a more sensitive indicator of past environmental conditions than measurements of total luminescence. Received: 18 March 1999 / Accepted: 20 December 1999     

Implication of storage conditions on luminescence response from Photobacterium phosphoreum in free and immobilized form

Bioluminescent bacteria in the form of a cell suspension for on-site hazard analysis are not suitable as in vivo luminescence in free cells fluctuates and may lead to erroneous results. Furthermore, the culture broth cannot be stored for long durations to continue sensing analytes as the luminescence ceases over time. Factors that affect luminescence response include growth dynamism, and ambient environmental conditions. The present study investigated the effect of storage conditions such as temperature (25 ± 2°C, room temperature; 4°C; and −20°C) and ambient aqueous environment (M1: sucrose, 1.02 M; M2, bioluminescent media [tryptone, 10 g L⁻¹; NaCl, 28.5 g L⁻¹; MgCl₂.7H₂O, 4.5 g L⁻¹; CaCl₂, 0.5 g L⁻¹; KCl 0.5 g L⁻¹; yeast extract, 1 g L⁻¹; H₂O, 1 L]; M3, bioluminescent media and 95% glycerol, 1:1 ratio) on the luminescence emission from the calcium alginate-immobilized Photobacterium phosphoreum (S_b) against the cells in free suspension for an extended period. The results indicated that both the parameters that were undertaken markedly affected the luminescence. In the study, S_b showed an enhanced luminescence emission than the control up to 18.5-fold and for a prolonged period which can be efficiently utilized for rapid biosensing of hazardous materials.     

Temperature Broadening of LH2 Absorption in Glycerol Solution

In order to determine the relationship between the pigment–protein and the pigment–pigment interactions, the measurements of absorption spectra of the peripheral light-harvesting complex LH2 from the purple bacteria Rhodobacter sphaeroides solvated in glycerol/buffer solution were carried out in a wide temperature range, from 4 to 250 K. The SDFs used for simulating the temperature dependence of B800 and B850 bands were determined in a parametric form. To fit experimental spectra the overall exciton–phonon coupling had to be assumed to be weak for B850 (λ/2V ≈ 0.3, where λ is the reorganization energy and V is the nearest-neighbor dipole–dipole coupling for bacteriochlorophylls). At physiological temperatures the intermediate nuclear bath dynamics compares with the magnitude of energy gap fluctuations. Slower dynamics with κ ≈ 0.39, where κ is the ratio of the nuclear relaxation rate and the line width parameter, determines the spectral shape of B850 whilst faster modulations characterize B800 (κ ≈2.39). The static disorder for the B800 band is relatively high with the characteristic value of the inhomogeneous bandwidth Γ^inh ≈120 cm⁻¹, while for the B850 band this value is almost equal to the dipole–dipole coupling strength (Γ^inh ≈360 cm⁻¹). It has been found that the LH2 absorption spectrum is likely to be influenced by the temperature dependence of the dielectric constant of the solution in the high temperature range, when the glycerol/buffer solution is in the liquid state.