Synchronous Luminescence Research Articles

Understanding the structure, bonding and reactions of nanocrystalline semiconductors: a novel high-resolution instrumental method of solid-state synchronous luminescence spectroscopy.

This Perspective Article describes the recent advancements in studies of nanocrystalline metal oxides using a novel ultra-high resolution method, solid-state synchronous luminescence spectroscopy (SS-SLS). Semiconductors notably include titanium dioxide and these studies shed light on the detailed electronic structure, composition, and their reactions. First, we critically discuss the limitations of the major existing non-spectroscopic and spectroscopic methods of characterization of electronic structure of nanocrystalline semiconductors and insulators. Second, we describe the foundations and the setup of SS-SLS as an enhanced-resolution, facile, non-contact, non-destructive, and highly capable method of studies of nanomaterials. Third, the following insights are featured which are obtained by SS-SLS, but are not available by other methods: (a) detection of traps of electric charge (specific mid-gap states); (b) discrimination between "surface" and "bulk" sites; (c) in situ studies of composite nanomaterials and mechanisms of reactions, (d) the derivative SS-SLS for accurate determination of energies of absorption and emission. The specific advantages of SS-SLS versus other methods and in direct comparison with "conventional" photoluminescence spectroscopy are highlighted. Finally, new opportunities and challenges of SS-SLS are presented. SS-SLS is an advanced spectroscopic method with significant potential to aid academia and industry in studies of chemo-sensing, photocatalysis, optoelectronic materials, applied surface science, development of instrumental analysis, and studies of mechanisms of surface and "bulk" chemical reactions.

Detection of Luminescence Centers in Colloidal Cd\(_{0.3}\)Zn\(_{0.7}\)S Nanocrystals by Synchronous Luminescence Spectroscopy

With the advantages of selectivity, spectral resolution and reduction of interference on account of light scattering, synchronous luminescence spectroscopy (SLS) is successfully applied to analyze complex mixtures with overlapped emission and/or excitation spectra. In fact, it is difficult to clearly distinguish the contributions of various luminescence centers to low-energy band of semiconductor nanocrystals (NCs). Herein, we report the application of SLS method to detect luminescence centers in colloidal Cd<sub>0.3</sub>Zn<sub>0.7</sub>S NCs. Their conventional luminescence and synchronous luminescence spectra were comparatively investigated. Differently from conventional luminescence spectrum, the emission peaks at 460 and 515 nm were found using SLS method. They are attributed to the emission transitions related to sulfur and zinc/cadmium vacancies. The obtained results are useful to clarify the nature of luminescence centers as well as relaxation mechanism in Cd<sub>x</sub>Zn<sub>1-x</sub>S NCs.

Synchronous Luminescence Spectroscopy as a Tool in the Discrimination and Characterization of Oral Cancer Tissue.

High incidence of oral cancer is primarily due to ongoing tobacco epidemic. In this work, synchronous luminescence spectroscopy (SLS) has been used to characterize and discriminate oral cancer tissue. Spectral deconvolution method is employed to compute the fluorescence intensity, peak wavelength, and full width half maxima for different endogenous fluorophores. The fluorescence measurements were made on 21 normal and 88 oral squamous cell carcinoma biopsy tissues. Besides, variations in relative concentration of collagen, NADH, and FAD, peak shifts and broadening of peaks are observed for tryptophan, NADH, and FAD, in oral cancer tissues indicating both biochemical and micro environmental changes at cellular level. Linear discriminant analysis showed that oral cancer tissue is discriminated with a sensitivity and specificity of 100% and 95.2% respectively.

Regulation of sensitivity of Yb concentration to power-dependent upconversion luminescence colors

Controlling the power density of exciting light is a widely applied technological approach to dynamically tuning emission spectra to yield desirable luminescence properties, which is essential for various applications in laser devices, cancer cell imaging, biomarker molecule detections, thermometers and optoelectronic devices. However, most of upconversion systems are insensitive to power regulation. In this study, a series of Yb/Ho doped NaYF4 microrods with different Yb concentrations was synthesized by using a sodium citrate-assisted hydrothermal method. The dependence of upconversion characteristics of NaYF4:Yb/Ho microrods on Yb concentration and excitation power density are investigated in detail by a laser confocal microscopy system. The emission spectra exhibit discrete upconversion emission characteristic peaks that can easily be assigned to 5F3→5I8 (at about 488 nm), 5F4, 5S2→5I8 (at about 543 nm), 3K7, 5G4→5I8 (at about 580 nm) and 5F5→5I8 (at about 648 nm) transitions of Ho, respectively. The upconversion spectra and synchronous luminescence imaging patterns show that the luminescence ratio of red to green is not only dependent on the Yb concentration, but also sensitive to the excitation power. With Yb concentration increasing from 5% to 60%, the sensitivity of the power-controlled red to green luminescence ratio largely increases from 0.1% to 13.0%, corresponding to a clear luminescent color modification from green to red. These results indicate that the power-tuned red-to-green-luminescence ratio can be used as a method of measuring and evaluating Yb doping concentration. We attribute the sensitivity tuned by Yb concentration to the differences in population approach and upconversion mechanism for the red and green luminescence. By recording the slope of luminescence intensity versus exciting power density in a double-logarithmic presentation, we detect a small slope for the green emission relative to that for the red emission, especially at a high Yb concentration. These results indicate that the red upconversion process may be a three-photon process. The exciting power induced color adjusting is therefore explained by preferential three-photon population of the red emission due to the high 5S2→5G4 excitation rate, which is verified by down-conversions of emission spectra. Our present study provides a theoretical basis for the spectral tailoring of rare-earth micro/nano materials and supplies a foundation for the applications in rare-earth materials.

Visible Emission in Nanocrystalline Rutile: Free Exciton and Water as Probes for Midgap States in Adsorption/Desorption Using “Conventional” and Synchronous Luminescence Spectroscopy

We studied absorption and emission transitions in the visible range through three distinct electronic midgap states in nanocrystalline rutile TiO2 using absorption spectroscopy, “conventional” photoluminescence (PL) spectroscopy, synchronous luminescence spectroscopy for the first time, and complementary methods. We have used free exciton in rutile as an intrinsic midgap state (obtained under the subbandgap photoexcitation at 430 nm) to assign luminescence bands of “green light” (ca. 520 nm) and “green–yellow light” (ca. 560 nm) at 25 °C. Further, we utilized water in the vapor phase as spectroscopic probe to learn about origin of visible emission in rutile. Visible photoluminescence is increased by factor ×20 after water desorption and reversibly decreased (quenched) upon water adsorption in air under ambient conditions. Preferential quenching of “green–yellow light” by adsorbed water occurs versus “green light” as originating from surface vs subsurface oxygen vacancies. Water “adsorption/desorption” was...

Trap emission by nanocrystalline anatase in visible range studied by conventional and synchronous luminescence spectroscopy: Adsorption and desorption of water vapor

We report the spectroscopic study of adsorption and desorption of water vapor on nanocrystalline anatase in air under ambient conditions. We also report the energy level diagram of absorption and emission through the three electronic midgap states of different origin in anatase as determined by absorption spectroscopy, “conventional” PL emission spectroscopy and, for the first time, by synchronous luminescence spectroscopy at 25°C. Under the extrabandgap photoexcitation, visible luminescence from nanocrystalline anatase contains peaks of “blue light” at ca. 450nm and “green light” at ca. 500nm due to the two different midgap states. Photoluminescence in visible range at 25°C is quenched after water vapor adsorption with formation of “hydrated” anatase and increased in “dried” anatase after water desorption. For both “dried” anatase with strong emission of visible light and “hydrated” anatase with weak (quenched) emission, synchronous luminescence spectra are superior to “conventional” PL emission spectra in resolving characteristic emission peaks due to midgap states. Photoexcitation of self-trapped exciton (STE) as intrinsic electronic midgap state in anatase at 420nm results in a strong emission of “green light” at 500nm, which is preferentially quenched upon water vapor adsorption vs. “blue light” at 450nm. Visible PL in anatase decreases when the average nanocrystal size increases within 5–30nm range, suggesting participation of surface midgap states in water adsorption/desorption.

Porous calcium titanate and sorption and desorption of water under ambient conditions: a study by conventional and synchronous luminescence spectroscopy

We report chemical synthesis and characterization of micro/mesoporous CaTiO3 by X-ray diffraction (XRD), nitrogen sorption/desorption, small angle XRD, diffuse reflectance optical spectroscopy, and luminescence spectroscopy under ambient conditions. We determined the energies of absorption and emission transitions through electronic midgap states in micro/mesoporous calcium titanate by optical spectroscopy, “conventional” photoluminescence (PL) spectroscopy and, for the first time, synchronous luminescence spectroscopy at 25 °C. Micro/mesoporous CaTiO3 reversibly sorbs and desorbs water vapor in ambient air, with water sorption capacity being close to the total pore volume. The luminescence of micro/mesoporous CaTiO3 in visible range at room temperature is repeatedly significantly increased upon desorption of water and decreased (quenched) upon sorption of water vapor in air, due to the interactions of the adsorbate with electronic surface midgap states.

Water as probe molecule for midgap states in nanocrystalline strontium titanate by conventional and synchronous luminescence spectroscopy under ambient conditions

Alkaline earth metal titanates are broad bandgap semiconductors with applications in electronic devices, as catalysts, photocatalysts, sorbents, and sensors. Strontium titanate SrTiO3 is of interest in electronic devices, sensors, in the photocatalytic hydrogen generation, as catalyst and sorbent. Both photocatalysis and operation of electronic devices rely upon the pathways of relaxation of excited charge in the semiconductor, including relaxation through the midgap states. We report characterization of nanocrystalline SrTiO3 at room temperature by “conventional” vs. synchronous luminescence spectroscopy and complementary methods. We determined energies of radiative transitions in the visible range through the two midgap states in the nanocrystalline SrTiO3. Further, adsorption and desorption of vapor of water as “probe molecule” for midgap states in the nanocrystalline SrTiO3 was studied, for the first time, by luminescence spectroscopy under ambient conditions. Emission of visible light from the nanocrystalline SrTiO3 is significantly increased upon desorption of water and decreased (quenched) upon adsorption of water vapor, due to interactions with the surface midgap states.

Conventional and cryo-synchronous luminescence spectra of orthorhombic calcium titanate

We report, for the first time, characterization of electronic midgap states in the nanocrystalline orthorhombic calcium titanate by synchronous luminescence spectroscopy at 77K versus the “conventional” PL emission and excitation spectroscopy, and by the complementary spectroscopic and structural methods. We also report the radiative transitions from the three distinct electronic midgap states in nanocrystalline orthorhombic calcium titanate at the extrabandgap and subbandgap photoexcitation. The emission peaks of the nanocrystalline orthorhombic CaTiO3 obtained in the cryo-synchronous luminescence experiments are significantly narrower and much better resolved than those obtained in the “conventional” PL emission spectroscopy experiments under the same conditions.

Discrimination among melanoma, nevi, and normal skin by using synchronous luminescence spectroscopy.

Novel optical spectroscopy and imaging methods may be valuable in the early detection of cancer. This paper reports differences in the luminescence responses of pigmented skin lesions (melanomas and nevi) and apparently normal non-pigmented human skin, based on analyses of synchronous luminescence spectroscopy measurements. Measurements were performed in the excitation range of 330-545 nm, with synchronous intervals varying from 30-120 nm. Normal skin, nevi, and melanomas differ in the way they fluoresce, and these differences are more distinct in the synchronous fluorescence spectra than in the conventional emission and excitation spectra. The differences in the fluorescence characteristics of pigmented and normal skin samples were ascribed to differences in concentrations of endogenous fluorophores and chromophores. Principal component and linear discriminant analysis of the synchronous spectra measured at different synchronous intervals showed that the greatest variance among the sample groups was at the 70 nm interval spectra. These spectra were then used to create partial least squares discriminant analysis-based classification models. Evaluation of the quality of these models from the receiver operating characteristic curves showed they performed well, with a maximum value of 1 for the area under the curve for melanoma detection. Hence, synchronous luminescence spectroscopy coupled with statistical methods may be advantageous in the early detection of skin cancer.

Synchronous luminescence spectroscopic characterization of urine of normal subjects and cancer patients.

Urine is one of the diagnostically potential bio fluids, as it contains many metabolites and some of them are native fluorophores. These fluorophores distribution and the physiochemical properties may vary during any metabolic change or at different pathologic conditions. Since urine is a multicomponent fluid, synchronous luminescence technique, a powerful tool has been adopted to analyse multicomponents in single spectrum and to resolve emission spectrum without much of photobleaching of fluorophores. In this study, urine samples of both normal subjects and cancer patients were characterised using synchronous luminescence spectroscopy with a Stokes shift of 20 nm. Different ratio parameters were calculated from the intensity values of the synchronous luminescence spectra and they were used as input variables for a multiple linear discriminant analysis across normal and cancer groups. The stepwise linear discriminant analysis classifies 90.3% of the original grouped cases and 88.6% of the cross-validated grouped cases correctly.

Fluorescence Detection and Identification of Tagging Agents and Impurities Found in Explosives

The detection and identification of 2,4,6-trinitrotoluene (TNT), 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX), and pentaerythritol tetranitrate (PETN) vapors have proven to be difficult and challenging due to the low vapor pressures of these high explosives. Detecting higher vapor pressure impurity compounds found in TNT and possible tagging agents mandated to be added to plastic explosives (RDX and PETN) would allow for easier vapor detection. The higher vapor pressure nitro compounds of interest are considered to be non-fluorescent; however, once reduced to their amino analogs, they have relatively high quantum yields. The standard reduction products, the reduction products obtained in solution, and the reduction products obtained in vapor phase were analyzed by conventional fluorescence, synchronous luminescence, and derivative spectroscopy. The nitro analogs of the isomers 1,3-diaminobenzene, 1,2-diaminobenzene, and 1,4-diaminobenzene are found as impurities in TNT. We provide for the first time the synchronous luminescence derivative spectra of these isomers; including their individual spectra and a spectrum of an isomeric mixture of the three. Using the standard reduction products associated with these isomers and other aromatic amines, our data suggest that the vapors of two signature impurities, 1,3-dinitrobenzene and 2,4-dinitrotoluene (2,4-DNT), minor impurity compounds, and two possible tagging agents, 2-nitrotoluene (2-NT) and 4-nitrotoluene (4-NT), can be detected and selectively identified using our fluorescence approach. To prove our methodology, we show that we were able to generate, collect, and reduce 2-NT, 4-NT, and 2,4-DNT vapors to their amino analogs. Using our fluorescence approach, these vapors could be detected and selectively identified both individually and in a mixture. Collectively, our data indicate that our method of detecting and identifying higher vapor pressure explosive-like compounds could potentially be used to detect and identify low vapor pressure explosives such as TNT, RDX, and PETN.

Synchronous fluorescence analysis of phytate in food

A novel synchronous fluorescence method is described for determination of phytic acid in food samples. It is based on the formation of a ternary complex between phytate, 1,10-phenanthroline (phen) and Fe3+. The synchronous fluorescence intensity of the solution was accordingly enhanced proportionally to the increased phytate concentration. Synchronous luminescence spectroscopy was adopted in the study, and the Δλ was set to 40 nm. The calibration graph is linear from 0.33 to 32 mg L−1 with a linear equation of I f = 8.770 + 2.980c (R 2 > 0.9994). The method was applied to determine phytate in food samples and the found concentrations of phytic acid in food were in the range of 4.62–24.08 mg g−1 with recoveries of 92.2%–98.3%. The control experiments were performed using UV-spectrophotometry method, and the results showed the method to be reliable.

Simultaneous Determination of Naproxen and Diflunisal using Synchronous Luminescence Spectrometry

Binary mixtures of naproxen and diflunisal can be resolved by using zero-crossing first derivative emission spectrofluorimetry, first derivative constant wavelength synchronous luminescence spectrometry and first derivative constant energy synchronous luminescence spectrometry. These methods do not require any previous separation steps. The lowest quantitation limits for both drugs were obtained with first derivative constant wavelength synchronous luminescence spectrometry (0.002 and 0.015 microg ml(-1) for naproxen and diflunisal, respectively). The measurements were performed in 40% methanolic aqueous medium at pH 8.0 provided by adding 0.02 M phosphate buffer solution. The proposed methods were successfully applied to the simultaneous determination of naproxen and diflunisal in pharmaceuticals and human serum samples with high precision and accuracy. Linearity, accuracy, precision, limits of detection, limits of quantitation, and other aspects of analytical validation are included in the text.

Increasing Selectivity for TNT-Based Explosive Detection by Synchronous Luminescence and Derivative Spectroscopy with Quantum Yields of Selected Aromatic Amines

The detection of explosive material is at the forefront of current analytical problems. A detection method is desired that is not restricted to detecting only explosive materials, but is also capable of identifying the origin and type of explosive. It is essential that a detection method have the selectivity to distinguish among compounds in a mixture of explosives. The nitro compounds found in explosives have low fluorescent yields or are considered to be non-fluorescent; however, after reduction, the amino compounds exhibit relatively high fluorescence. We discuss how to increase selectivity of explosive detection using fluorescence; this includes synchronous luminescence and derivative spectroscopy with appropriate smoothing. By implementing synchronous luminescence and derivative spectroscopy, we were able to resolve the reduction products of one major TNT-based explosive compound, 2,4-diaminotoluene, and the reduction products of other minor TNT-based explosives in a mixture. We also report for the first time the quantum yields of these important compounds. Relative quantum yields are useful in establishing relative fluorescence intensities and are an important spectroscopic measurement of molecules. Our approach allows for rapid, sensitive, and selective detection with the discrimination necessary to distinguish among various explosives.

Three‐dimensional Total Synchronous Luminescence Spectroscopy Criteria for Discrimination Between Normal and Malignant Breast Tissues

Specimens of malignant and normal female human breast tissues were analyzed after surgery by means of synchronous luminescence spectroscopy. Measurements were performed in the ranges of excitation wavelengths from 330 to 650 nm and synchronous wavelengths from 30 to 120 nm to obtain ordinary and first derivative three-dimensional total synchronous luminescence spectra (3d-TSLS) of each specimen. Arithmetic mean of these spectra has been calculated for normal and malignant specimens and analyzed to establish criteria for tissue differentiation. Spectral domain volumes (volumes below luminescence intensity surface) and mean spectral slopes have been calculated and also analyzed as tissue discrimination criteria. The obtained results are discussed in view of the possible relevance of synchronous luminescence spectroscopy in discrimination between normal and malignant breast tissue.

A Fluorescence Spectroscopic Study of Honey and Cane Sugar Syrup

Fluorescence spectroscopic properties of honey and cane sugar syrup were investigated in order to explore the use of optical techniques for detection of adulteration of honey with cane sugar syrup. Measurements showed that while the major contributor to the fluorescence of cane sugar syrup is the reduced form of nicotinamide adenine dinucleotide, the fluorescence of honey is dominated by flavins. The difference in the synchronous luminescence spectra of honey and cane sugar syrup could be used to monitor adulteration of honey by cane sugar syrup.

Spectroscopic determination of poly-aromatic compounds in petroleum contaminated soils

The determination of poly-aromatic hydrocarbons (PAHs) in the soil is of interest because of their carcinogenic and mutagenic activity in biological systems. The present study deals with the rapid application of infrared, fluorescence, synchronous luminescence spectrometries and gas chromatography to detect organic pollutants and their quantity in the soil. Sohxlet extraction followed by column separation was used to isolate organic pollutants. Although several solvent mixtures were used as eluents for the column, the solvent mixture, hexane:dichloromethane (50:50) efficiently extracts the aromatic compounds. Total petroleum hydrocarbons (TPH) measured by IR were found at high concentrations (30810.0 ppm) in the contaminated soil compared with the reference soil (30.0 ppm). Furthermore, the fluorescence results reveal that almost one-fourth of the 30810.0 ppm are aromatic hydrocarbons. In addition, the presence of PAHs such as naphthalene, acenaphthene, fluorene, fluoranthene, phenanthrene, pyrene, benzo(a)pyrene, chrysene, and dibenzo(a,h)anthracene in the polluted soil was determined by using synchronous study.

Implication of mitochondrial involvement in apoptotic activity of fragile histidine triad gene: Application of synchronous luminescence spectroscopy

The fragile histidine triad (FHIT) tumor suppressor gene incorporates the common human chromosomal fragile site at 3p14.2. The structure and expression of the FHIT gene are frequently altered in many cancers. The tumor suppressor activity of the FHIT gene has been previously demonstrated as potentially involving apoptotic induction. Here, mitochondria are implicated as being involved in the apoptotic activity of the FHIT gene. A number of morphological and biochemical events, including the disruption of the inner mitochondrial transmembrane potential (Delta Psi(m)) and the release of apoptogenic cytochrome c protein into the cytoplasm, are characteristic features of the apoptotic program. The proapoptotic activity of the FHIT gene is studied by investigating the loss of Delta Psi(m) in mitochondria and translocation of cytochrome c. Synchronous luminescence (SL) spectroscopy is applied to measure mitochondrial incorporation of rhodamine 123 for direct analysis of alterations in the mitochondrial Delta Psi(m). The SL methodology is based on synchronous excitation in which the excitation and emission wavelengths are scanned simultaneously while a constant wavelength interval is maintained between the excitation and emission monochromators. An enhanced collapse of Delta Psi(m) in apoptotically induced FHIT expressing cells compared to FHIT negative cells is observed. The loss of Delta Psi(m) is greatly restricted in the presence of the apoptotic inhibitor, cyclosporin A. Cytoplasmic translocation of cytochrome c in the FHIT expressing cells as an early event in apoptosis is also demonstrated. It is concluded that Fhit protein expression maintained apoptotic function by altering the Delta Psi(m) and by enhancing cytochrome c efflux from the mitochondria.

Investigation of Lysozyme-Chitobioside Interactions Using Synchronous Luminescence and Lifetime Measurements

Beside their ability to disrupt the outer membranes of some microorganisms, lysozymes also experience interactions with chitins or their fluorescent analogs. It has been well established that chitins bind to the cleft of lysozymes and the subsites of the location of the different N-acetylglucosamines that are parts of chitins have been identified. Moreover, it has been well documented that a 1,4-β-bond must be located between subsite D and subsite E to be cleaved. Nevertheless, a better understanding of the biophysical and biochemical processes is needed. In this paper, pulsed fluorescence was used to further investigate the mechanism by which the binding of fluorescent analogs of chitin (4-methylumbelliferyl chitobiose and 4-methylumbelliferyl chitotriose) to hen egg-white lysozyme results in an increase of their fluorescence intensity. Although such an increase is not observed when these chitobiosides bind to turkey egg-white lysozyme, synchronous fluorescence techniques show that this binding induces a quenching of the native fluorescence of both these proteins. The findings of this study, associated with previously published cyrstallographic data allow us to suggest that the system lysozyme-chitobioside partitions in two three-dimensional conformational states: an enzymatic active conformation and a storage conformation. These states are separated by an energy barrier, with the storage conformation being more populated than the enzymatic active conformation below 45°C.