Fluorescence Spectroscopy and Confocal Microscopy of the Mycotoxin Citrinin in Condensed Phase and Hydrogel Films

The emission spectra, quantum yields and fluorescence lifetimes of citrinin in organic solvents and hydrogel films have been determined. Citrinin shows complex fluorescence decays due to the presence of two tautomers in solution and interconversion from excited-state double proton transfer (ESDPT) process. The fluorescence decay times associated with the two tautomers have values near 1 and 5 ns depending on the medium. In hydrogel films of agarose and alginate, fluorescence imaging showed that citrinin is not homogeneously dispersed and highly emissive micrometer spots may be formed. Fluorescence spectrum and decay analysis are used to recognize the presence of citrinin in hydrogel films using confocal fluorescence microscopy and spectroscopy.     

Linear and Multi-Photon Fluorescence of Thiophene Based Copolymer with Electron-Accepting Side Chains

A novel copolymer poly(thiophene-2,5-diyl-2,5-di-n-octyloxycarbonyl-1,4-phenylene), denoted as P33, is introduced as potential material for photovoltaics, polymer light-emitting diodes, and/or organic transistors. P33 dissolved in chloroform is investigated by steady-state absorption, linear/non-linear fluorescence spectroscopies and time-resolved fluorescence spectroscopy. Molar extinction coefficient, fluorescence quantum yield, and singlet fluorescence lifetime of P33 are determined to be 18,315 M^?1 cm^?1, 0.4, and 810 ps, respectively. The P33 fluorescence fast components of decay times are 1.2 ps, 2.0 ps, and 0.5 ps for increasing wavelengths of 480 nm, 500 nm, and 520 nm, respectively. The fast component is attributed to a transport of nearly instantaneously formed excitons to localized states known as downhill energy transfer. Additionally multi-photon excited fluorescence is observed for pumping with wavelengths of 800 nm and 1200 nm. Two-photon absorption cross-section is determined to be 6.9 GM. These spectroscopic studies provide basic fluorescence characteristics of the novel thiophene copolymer P33.     

Sugar Binding Effects on the Enzymatic Reaction and Conformation Near the Active Site of Pokeweed Antiviral Protein Revealed by Fluorescence Spectroscopy

In various trials for elucidating the physiological function of pokeweed antiviral protein (PAP), studies on the interaction with sugar are essential. The fluorescence titration curves showed that PAP retained the strong affinity against N-acetylglucosamine (NAG) and two sites in one PAP molecule co-operatively participated in the binding. In the complex of PAP with NAG, Trp208 located at the entrance lid site of substrate came closer to Tyr72 about 0.3 Å. Furthermore, the fluorescence anisotropy decay measurement demonstrated that the segmental rotation of Trp208 was enlarged by the binding of PAP with NAG. Such conformational changes around the active site closely correlate with the enzymatic activity of PAP. The N-glycosidase activity of PAP was enhanced more than two times in the presence of NAG. The obtained results consistently suggested the enzymatic activity of PAP would be regulated through the conformation change near the active site induced by the binding with NAG.     

Photophysical study on daunorubicin by fluorescence spectroscopy

Steady-state fluorescence and time-resolved fluorescence intensity decays of daunorubicin have been studied in polar solvents and in aqueous solution by a time-correlated single-photon counting technique. Daunorubicin, quinizarin, and 2,3-dimethylquinizarin show bi-exponential decay. The decay of daunorubicin becomes tri-exponential in the presence of adenosine 5′ monophosphate. The quenching of the fluorescence of daunorubicin by adenosine 5′ monophosphate exhibits downward deviation from the Stern-Volmer linearity, suggesting the existence of fluorophore in two conformers in the ground state differing only in the extent of hydrogen bonding.     

Steady state and time-resolved studies of pyrene in solution and in single microcrystals

Single microcrystals of pyrene have been studied by steady state and time resolved fluorescence microscopy. The fluorescence spectra of microcrystals exhibit vibrational structure unlike the broad spectrum observed in pyrene excimer. A risetime is observed in the decay curves of the concentrated solutions, indicating the excimer formation. In contrast, the fluorescence decay profiles of the single microcrystals are nonexponential in nature and the decay times vary with their size and the wavelength of emission. This behaviour has been explained mainly by considering the pyrene dimer stabilized in the ground state.     

Synthesis and characterization of core-shell europium(III)-silica nanoparticles

Luminescent core-shell europium(III)-silica nanoparticles were prepared using europium(III) chelate core structure and polyvinylpyrrolidone synthesis strategy for silica shell. Europium(III):naphtoyltrifluoroacetone:trioctylphosphineoxide complex was spontaneously agglomerated from organic solvent to water. Polyvinylpyrrolidone was adsorbed onto the core structure and stable silica shell was synthesized using tetraethylorthosilicate. Nanosized particles with a diameter of 71 ± 5 nm and 11 nm shell thickness were obtained with fluorescence decay rate of 517 μs and excitation and emission wavelengths of 334 and 614 nm, respectively.     

The effect of ionisation of silica nanoparticles on their binding to nonionic surfactants in oil–water system: an atomistic molecular dynamic study

ABSTRACT

In this study, the adsorption of nonionic surfactant, triethylene glycol monododecyl ether (C₁₂E₃), on a surface of silica nanoparticle (NP) has been studied with variation in the degree of ionisation (DI) of silica NP using all-atom molecular dynamic simulations in hexadecane–water system. Hydrogen bonding is found to be responsible for the adsorption of C₁₂E₃ on NP, particularly at low DI. We observe that with increasing DI of NP, the amount of adsorption of C₁₂E₃ on NP reduces, which is negligible beyond DI ～ 0.5. The decrease in the adsorption with increasing DI is due to the decrease in the number of hydrogen bonds formed by the silica NP with surfactant molecules. Potential of mean force (PMF) profiles indicate attractive interactions between NP and C₁₂E₃ for DI < 0.5, and for larger DI depletion effect is observed. This work explains the unusual effect of nonionic surfactant on interfacial tension in the presence of silica particles as observed in recent experiments.     

Aqueous Photophysical Parameters of Congo Red

Using steady-state fluorescence spectroscopy and UV-Vis absorbance spectrometry, the salient photophysical parameters of Congo red, a very important biological staining reagent, was determined. Its absorbance was observed at 497.0 nm; its molar absorptivity ε was determined as 6.26 × 10⁴ M–cm. Its fluorescence, when excited at 330.0 nm, was observed at 417.0 nm. The quantum yield, ф, in aqueous solution was determined by two different methods—the relative, or comparative, method and the absolute method. Both methods gave the same value of 0.011. The room temperature fluorescence lifetime τ_o was determined as 2.8 ns using the Strickler–Berg equation.     

Native Fluorescence and Time Resolved Fluorescence Spectroscopic Characterization of Normal and Malignant Oral Tissues Under UV Excitation—an In Vitro Study

The present work aims to investigates the native fluorescence and time resolved fluorescence spectroscopic characterization of oral tissues under UV excitation. The fluorescence emission spectra of oral tissues at 280 nm excitation were obtained. From the spectra, it was observed that the alteration in the biochemical and morphological changes present in tissues. Subsequently, the Full width at Half Maximum (FWHM) of every individual spectra of 20 normal and 40 malignant subjects were calculated. The student’s t-test analysis reveals that the data were statistically significant (p?=?0.001). The fluorescence excitation spectra at 350 nm emission of malignant tissues confirms the alteration in protein fluorescence with respect to normal counterpart. To quantify the observed spectral differences, the two ratio variables R₁?=?I₂₇₅/I₃₁₀ and R₂?=?I₃₁₀/I₃₂₈ were introduced in the excitation spectra. Among them, the Linear Discriminant Analysis (LDA) of R₁ reveals better classification with 86.4 % specificity and 82.5 % sensitivity. The fluorescence decay kinetics of oral tissues was obtained at 350 nm emission and it was found that the decay kinetics was triple exponential. Then the ROC analysis of fractional amplitudes and component lifetime reveals that the average lifetime shows 77 % sensitivity and 70 % specificity with the cut off value 4.85 ns. Briefly, the average lifetime exhibits better statistical significance when compared to fractional amplitudes and component lifetimes.     

Degradation Studies of Polycaprolactone in Banana Fibers Reinforced Thermoplastics Composites

In this paper we report the fabrication, properties and degradation studies of banana fibers–reinforced thermoplastic polymers. In order to impart hydrophobicity to the fibers and also to concomitantly increase interfacial bond strength, which is a critical factor for obtaining better mechanical properties of composites, banana fibers were treated with sodium hydroxide (5% and 10% for 4 h), sebacoyl chloride (SC) (0.5 g, 4 h), or toluene diisocyanate (TDl) (1.5 mL, 4 h). Mechanical properties of banana fibers treated with TDl were not affected to any significant extent, but there was an increase in tensile strength of fibers treated with sodium hydroxide (NaOH). Deterioration in mechanical properties was observed upon SC treatment. In thermograssimetre analogue (TGA) studies fibers showed initial mass loss (6.5%–9.5%) in the 50–150°C temperature region. Major weight loss occurred above 200°C. Scanning electron microscope (SEM) studies revealed an increase in surface roughness after alkali treatment. High density polyethylene (HDPE) modified by blending with poly (ε‐caprolactone) (80:20 w/w) was used as a thermoplastic matrix. Composites were fabricated by using 1 cm long banana fibers; the weight fraction of fibers was varied from 0.05–0.13. An increase in weight fraction of fibers resulted in an increase in tensile strength and modulus and decrease in elongation at break. Thin sheets and dumbbells were used for enzymatic and chemical hydrolysis degradation tests. The degradation of the material was monitored by weight change and loss of mechanical properties. The enzymatic degradation in (PCL) presence of Pseudomonas cepacia lipase (PCL) gave appreciable weight loss in PCL and blended materials.     

A new dual fluorescent probe preparation and its biodiagnose function in vitro

A new dual fluorescent probe was prepared by encapsulating the complex of cerium (III) and quinizarin into silica nanoparticles. Cerium (III) and quinizarin can form a stable dual fluorescent complex in ethanol, but this complex would dissociate in the aqueous medium, which limited its application in biosystem. So, such complex was encapsulated inside the silica nanoparticles to solve this problem. The results indicated that the stability and water solubility of the complex were greatly improved after been encapsulated into the silica nanoparticles. In addition, its dual fluorescent probe function was studied in vitro.     

Analysis of the Luminescence Decay on the SiO<Subscript>2</Subscript> Surface at Different Temperatures within the Multifractal Formalism

The kinetics of fluorescence decay of the Bengalese pink dye adsorbed on the surface of wide-porous silica with addition of anthracene is investigated in the case in which the fluorescence decay slows down due to annihilation. The experimental kinetic dependences recorded at temperatures in the interval from 153 to 273 K are examined by methods of multifractal analysis and computer modeling. A comparison of the experimental dependences with the results of computer modeling has demonstrated that changes in the kinetic luminescence characteristics of the system at different matrix temperatures are caused by reorganization of structural elements in the system leading to changes in ordering and fractal dimension of the reagent molecule distribution. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 3–8, June, 2005.     

Selectivity Improvement for Spectrofluorimetric Determination of Oseltamivir Phosphate in Human Plasma and in the Presence of Its Degradation Product

A simple and sensitive spectrofluorimetric method has been developed and validated for determination of oseltamivir phosphate (OSP). The proposed method is based on condensation reaction of the primary amino group of OSP with ninhydrin and phenylacetaldehyde in buffered medium (pH 6.5). The formed yellow fluorescent product exhibits excitation and emission maxima at 390 and 460 nm, respectively. The selectivity improvement of our proposed method is based on the water insolubility of the oseltamivir carboxylic acid (OSC) the active metabolite of OSP, which contains the same primary amino group as OSP but cannot, condensed with ninhydrin and phenylacetaldehyde reagents. The different experimental parameters affecting the formation and stability of the reaction product were carefully studied and optimized. The fluorescence intensity concentration plot is rectilinear in the range of 2–15 μg ml^?1 with detection and quantitation limits of 0.32 and 0.98 μg ml^?1, respectively. The proposed method was successfully applied for determination of OSP in commercial capsules, suspension and spiked human plasma with good percentage recovery. In addition, the developed procedure was extended to study the stability of OSP under different stress conditions; including acid and alkali hydrolysis, oxidation, photolysis, and thermal degradation. Furthermore, the kinetic of alkaline and acidic degradation of the cited drug were investigated. The apparent first order degradation rate constants were 0.258 and 0.318 K h^?1 with half times of 2.68 and 2.17 h, for acidic and alkaline degradation, respectively.     

Effect of nano-dispersed silica on the ion-conducting behavior of PMMA-based polymer gel electrolytes containing LiPF<Subscript>6</Subscript>

Nano-sized silica poly(methylmethacrylate)-based gel electrolyte containing lithium hexafluorophosphate (LiPF₆) was synthesized by using different binary solvent mixture (propylene carbonate(PC) and dimethylformamide (DMF) in different volume ratio). Role of DMF in PC: Higher DMF content in PC-based electrolyte shows higher ionic conductivity at all polymer content and at wide temperature regions (10-70 °C). A small increment in ionic conductivity at lower content of polymer in liquid/gel electrolyte was observed and having maximum conductivity of 13.12 mS/cm at 25 °C. Stability (mechanically and electrically), viscosity and ionic conductivity of gel electrolytes were improved with the addition of nano-sized silica at ambient temperature. Ionic conductivity of nano-sized silica-based gel electrolyte does not change much over 5^o–70 °C temperature range and is factor-wise only which make indispensable in different electrochemical devices. Also polymer gel electrolyte membranes as such and with dispersed silica nano-particles were characterized through scanning electron microscope to study the morphology of gel matrix.     

Investigation of the spontaneous emission rate of perylene dye molecules encapsulated into three-dimensional nanofibers via FLIM method

The decay dynamics of perylene dye molecules encapsulated in polymer nanofibers produced by electrospinning of polymethyl methacrylate are investigated using a confocal fluorescence lifetime imaging microscopy technique. Time-resolved experiments show that the fluorescence lifetime of perylene dye molecules is enhanced when the dye molecules are encapsulated in a three-dimensional photonic environment. It is hard to produce a sustainable host with exactly the same dimensions all the time during fabrication to accommodate dye molecules for enhancement of spontaneous emission rate. The electrospinning method allows us to have a control over fiber diameter. It is observed that the wavelength of monomer excitation of perylene dye molecules is too short to cause enhancement within nanofiber photonic environment of 330 nm diameters. However, when these nanofibers are doped with more concentrated perylene, in addition to monomer excitation, an excimer excitation is generated. This causes observation of the Purcell effect in the three-dimensional nanocylindrical photonic fiber geometry.     

Modification of nano-fibriform silica by dimethyldichlorosilane

The modification of nano-fibriform silica by dimethyldichlorosilane was studied by transmission electron microscopy, X-ray powder diffraction, infrared spectroscopy, Raman spectroscopy, physical N₂ adsorption techniques, differential thermal and thermogravimetric analysis, scanning electron microscopy, and elemental analyzer.The results show that dimethyl silane derivatives have been successfully covalently grafted on nano-fibriform silica. The polarity of the modified product decreases with the substitution of -OH groups by siloxyl groups. Therefore, the modified product can be easily dispersed in organic solvent and its compatibility with organic molecules is improved. After modification the pore volume decreases and the ductility greatly increases, indicating that the modified product is of a higher strength than before. The study demonstrates that the modified product can be used as an ideal additive to reinforce the strength of organic materials.     

Pulsed laser deposition of Mg–Al layered double hydroxide with Ag nanoparticles

Powdered layered double hydroxides (LDHs)—also known as hydrotalcite-like (HT)—compounds have been widely studied due to their applications as catalysts, anionic exchangers or host materials for inorganic or organic molecules. Assembling thin films of nano-sized LDHs onto flat solid substrates is an expanding area of research, with promising applications as sensors, corrosion-resistant coatings, components in optical and magnetic devices. The exploitation of LDHs as vehicles to carry dispersed metal nanoparticles onto a substrate is a new approach to obtain composite thin films with prospects for biomedical and optical applications. We report the deposition of thin films of Ag nanoparticles embedded in a Mg–Al layered double hydroxide matrix by pulsed laser deposition (PLD). The Ag-LDH powder was prepared by co-precipitation at supersaturation and pH = 10 using aqueous solutions of Mg and Al nitrates, Na hydroxide and carbonate, and AgNO₃, having atomic ratios of Mg/Al = 3 and Ag/Al = 0.55. The target to be used in laser ablation experiments was a dry pressed pellet obtained from the prepared Ag-LDH powder. Three different wavelengths of a Nd:YAG laser (266, 532 and 1064 nm) working at a repetition rate of 10 Hz were used. X-Ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), and secondary ions mass spectrometry (SIMS) were used to investigate the structure, surface morphology and composition of the deposited films.     

Covalent immobilization of lipases on monodisperse magnetic microspheres modified with PAMAM-dendrimer

This paper reported an immobilization of Candida rugosa lipase (CRL) onto PAMAM-dendrimer-grafted magnetic nanoparticles synthesized by a modified solvothermal reduction method. The dendritic magnetic nanoparticles were amply characterized by several instrumental measurements, and the CRL was covalently anchored on the three generation supports with glutaraldehyde as coupling reagent. The amount of immobilized enzyme was up to 150 mg/g support and the factors related with the enzyme activity were investigated. The immobilization of lipase improved their performance in wider ranges of pH and temperature. The immobilized lipase exhibited excellent thermal stability and reusability in comparison with free enzyme and can be reused 10 cycles with the enzymatic activity remained above 90 %. The properties of lipase improved obviously after being immobilized on the dendritic supports. The inactive immobilized lipase could be regenerated with glutaraldehyde and Cu²⁺, respectively. This synthetic strategy was facile and eco-friendly for applications in lipase immobilization.     

Adsorption of firefly luciferase at interfaces studied by total internal reflection fluorescence spectroscopy

The adsorption of luciferase onto silica surfaces was studied by total internal reflection fluorescence (TIRF) spectroscopy. Two model surfaces were used: hydrophilic and hydrophobic silica. Luciferase adsorbed differently on these two surfaces. Initial kinetics of luciferase adsorption onto the hydrophilic surface showed that luciferase adsorbs over an adsorption energy barrier of 3 kT The quantum yield of luciferase fluorescence decreased at the hydrophilic silica surface, which indicated that the protein conformation was altered during adsorption. Luciferase adsorption onto the hydrophobic silica surface proceeded with a small adsorption energy barrier and the fluorescence efficiency of adsorbed protein remained unchanged after adsorption. The affinity of luciferase for luciferin was measured using quenching of luciferase fluorescence with luciferin. The binding constant of the adsorbed luciferase-luciferin complex at the hydrophilic silica surface was two orders of magnitude smaller than the respective binding constant in the solution. Adsorbed luciferase showed an absence of ATP-dependent visible luminescence, indicating that the adsorbed enzyme was not active at either of the two silica surfaces.