Fluorescence of pyrene in the presence of silver nanoparticles on cellulose modified by surface-active substances

Luminescent properties of pyrene in the ordered structure formed by the molecules of a surface-active substance represented by cetyltrimethylammonium bromide on a cellulose matrix preliminarily modified with silver nanoparticles are studied. Elevated intensity of fluorescence in the violet range of the spectrum of pyrene molecules solubilized in these structures is observed. This phenomenon is rationalized by the resonance of electron transitions of monomeric pyrene molecules in this range with plasmonic vibrations in silver nanoparticles.

     

Concentration of polycyclic aromatic hydrocarbons by chemically modified silver nanoparticles

The ability of silver nanoparticles stabilized by cetyltrimethylammonium bromide (CTAB) to concentrate polycyclic aromatic hydrocarbons (PAHs) from aqueous solutions was shown. It was found that fixed PAH molecules are capable of acting as electronic energy donors and of generating sensibilized fluorescence of silver nanoparticles. It was shown by spectral-luminescent investigations of dilute PAH solutions (5 × 10⁻¹⁰−1 × 10⁻⁶ g/ml) in the presence of silver nanoparticles (∼0.7 vol %) that the concentration of PAH molecules from solutions occurs due to its sorption on hydrocarbon CTAB radicals in close contact to the surface of metallic silver. On the basis of the spectral data, the sorption isotherms were obtained and the values of extraction degree and partition coefficients for naphthalene, phenanthrene, anthracene, chrysene, pyrene, and 3,4-benzopyrene were calculated. It was found that the degree of extraction values of the investigated PAHs fall within the range of 73–98%, the partition coefficients (logD) ∼ 6, and the concentration coefficients ∼10⁵.     

Cofluorescence of dyes in nanoparticles from metal ion complexes with diketones

Nanoparticles (NPs) from diketonates of Al³⁺, Sc³⁺, In³⁺ and Ln³⁺ doped with dye molecules are synthesized. The appearance of sensitized fluorescence (cofluorescence) of dye molecules due to energy transfer from the ensemble of complexes forming NPs is revealed in aqueous solutions of these NPs. It is shown that the dye cofluorescence in NPs from Eu complexes occurs as a result of two distinct processes of energy transfer (ET) to dye molecules: from singlet levels of ligands and from Eu³⁺ ions. It is found that the efficiency of ET from Eu³⁺ ions to dyes in NPs from Eu(DBM)₃phen is one order of magnitude higher than the efficiency of ET from S₁-levels of ligands to dyes in NPs from Al complexes with the same ligands. It is shown that the excitation of dye molecules through ligands of NPs results in the enhancement of the intensity of their fluorescence by a factor of 1.5–2 orders of magnitude compared to the excitation of their own first band of absorption.     

Donor–acceptor interactions between resonance-excited silver nanoparticles and halide ions in water solutions

Donor–acceptor interactions between silver nanoparticles (NPs), resonance-excited by optical quanta of light, and halide ions are studied in aqueous solutions. It is shown that deactivation of the plasmon excitation of Ag NP proceeds according to the exchange mechanism of electron transfer. Plasmon excitation quenching constants are determined and a correlation between quenching and the donor properties of halide ions is found. The efficiency of electrostatic interaction between resonantly-excited Ag NPs and halide ions is studied, and their dipole moment is determined.

     

New nanomaterials for control of the luminescence of polycyclic aromatic hydrocarbons

New nanomaterials (ordered structures) have been proposed for control of the luminescence of polycyclic aromatic hydrocarbons by solubilization of the latter into ordered structures. These structures are formed by hydrophilic silver nanoparticles adsorbed on a cellulose matrix—filter paper—through their self organization and by molecules of a cationic surfactant—cetyltrimethylammonium bromide—in concentrations exceeding the critical micelle concentration. The observed enhancement of luminescence of solubilized pyrene molecules in the violet spectral range is due to the resonance of the electronic transitions of monomeric pyrene molecules in this range with a plasmon vibrations in silver nanoparticles.     

Polyelectrolyte effects on the quenching of pyrene fluorescence in solutions of a pyrene substituted poly(acrylic acid)

The quenching of pyrene fluorescence by nitromethane, Tl⁺, Cu²⁺, I^?, and 4-dimethylaminopyridine (DMAP) in aqueous solutions of a pyrene substituted poly(acrylic acid) ( 1 ) was influenced by the “polyelectrolyte effect” of 1 . The efficiency of quenching in solutions of 1 was measured in terms of the Stern–Volmer constants for dynamic and static quenching which were obtained from comparison of the intensity and lifetime of pyrene fluorescence in solutions of 1 and a monomer model compound. The efficiency of quenching in solutions of 1 was always greater at high pH ( 9 ) in comparison to that at low pH ( 4 ). The ionization of carboxylic groups in 1 caused an expansion of the polymer mainchain and concomitant exposure of the pyrene molecules to the aqueous phase and quencher. The polyanion domain of 1 favored the condensation of cationic quenchers and could account for very efficient quenching in case of Cu²⁺ and Tl⁺. A very efficient quenching of pyrene fluorescence in solutions of 1 by DMAP at high pH was attributed to the hydrophobic interactions of DMAP and pyrene moiety. The iodide ions were less efficient quenchers of pyrene fluorescence due to electrostatic repulsion from the polyanion. The efficiency of quenching by nitromethane was not significantly affected by ionization of the carboxylic groups in 1 .     

Delayed fluorescence in a pyrene-doped naphthalene crystal: homofusion and heterofusion of triplet excitons

Prompt and delayed emissions from a naphthalene crystal doped with pyrene have been investigated in the temperature range 77–300 K. It is shown that delayed fluorescence (DF) in this crystal originates from heterofusion and homofusion of two different types of trap triplets at low temperature (T< 130 K) and from homofusion of pyrene triplets at high temperature (T >200 K). Pyrene concentration effects on DF suggest that free triplet excitons are trapped more efficiently at monomeric pyrene sites than at nascent excimeric pyrene sites.     

Effect of silver nanoparticles on luminescent and generation properties of rhodamine 6G in aqueous alcohol solutions

The plasmon effect that silver nanoparticles have on the luminiscent and generation properties of rhodamine 6G molecules in aqueous alcohol solutions is studied. It is found that the intensities of absorption and emission increase when silver nanoparticles are added to aqueous solutions of rhodamine 6G. It is shown that upon the laser photoexcitation of aqueous solutions of rhodamine 6G dye, spontaneous fluorescence occurs that is converted into stimulated laser emission as the pump power grows. It is found that an increase in intensity and a drop in the generation threshold of stimulated emission are observed when silver nanoparticles are added to a solution of rhodamine 6G. It is shown that the dependence of absorbance, the intensity of fluorescence, and the dye’s generation of stimulated emission on the concentration of silver nanoparticles in solution falls as the proportion of alcohol grows.     

Synthesis and adsorption and luminescence properties of hydrophobic silver nanoparticles in the presence of pyrene

A procedure was developed for the synthesis of hydrophobic silver nanoparticles with an average size of 4 nm in two-phase water-organic emulsions. The physical properties of the obtained silver organosol were studied by molecular spectroscopy and electron microscopy. It was found that the synthesized silver nanoparticles had a specific surface of 60–110 m²/g. It was shown that chemically modified silver nanoparticles can be used as an adsorbent for preconcentrating polycyclic aromatic hydrocarbons (using pyrene as an example) from dilute n-hexane solutions followed by luminescence determination at room temperature.     

The Influence of Surface Composition of Nanoparticles on their Interactions with Serum Albumin

Interactions between differently functionalised silver and gold nanoparticles (NPs) as well as polystyrene nanoparticles with bovine serum albumin (BSA) are studied using circular dichroism (CD) spectroscopy. It is found that the addition of NPs to the protein solution destroys part of the helical secondary structure of the protein as a result of surface adsorption. From the loss of free protein and hence the extent of their structural change adsorption equilibrium constants are derived. The results reveal that citrate‐coated gold and silver NPs exhibit much stronger interactions with BSA than polymeric or polymer‐coated metallic NPs. It is therefore concluded that for the particles considered, the influence of surface composition on the interaction behaviour dominates that of the core.     

Unique hydrophobic interactions of pyrene in aqueous solution as effected by polyelectrolytes and surfactants

Pyrene substituents covalently bounded to polyelectrolytes show not only excited-state interactions but also unique ground-state interactions in aqueous solution. The pyrene moieties in pyrenesubstituted ionic molecules also show these interactions when aqueous solutions of these molecules are treated with polyelectrolytes or surfactants well below their critical micelle concentrations. These hydrophobic interactions are revealed by changes in absorption, fluorescence, and excitation spectra. The well-resolved vibrational bands in the absorption and excitation spectra of pyrene become somewhat diffuse, whereas monomer fluorescence is reduced and replaced by excimer fluorescence. The rationale for these results is that the pyrene moieties in these ionic solutions seek out hydrophobic locations on the polyelectrolytes or surfactants, where pyrene aggregation is responsible for these interactions and the corresponding changes in spectra.     

Fluorescence of pyrene in inhomogeneous media containing silver nanoparticles

Pyrene fluorescence in inhomogeneous media based on ionic detergents containing silver nanoparticles with different morphologies is investigated. An increase in pyrene monomer emissions in the spectral range of 400–500 nm is observed, due to the resonance between electronic transitions in pyrene molecules in that region and the plasmonic oscillations of silver nanoparticles.     

Interactions of ionic surfactants with gelatin in fluid solutions and the gel state studied by fluorescence techniques,potentiometry, and measurements of viscosity and gel strength

The photobehavior of pyrene (fine structure of the monomeric fluorescence band; excited state lifetimes; excimer/monomeric fluorescence intensity ratios and excited state quenching rates by oxygen) in fluid solutions of gelatin and in the gel state, both in the absence and presence of ionic surfactants, has been examined. Surfactants considered were sodium dodecylsulfate and dodecyltrimethylammonium bromide. Potentiometric measurements performed by using surfactant selective electrodes allowed for the determination of the binding capacity of the surfactants onto gelatin in the gel state. When complemented with viscosity and gel strength measurements, the sesults obtained allow for a discussion of the effects of the surfactant-gelatin interactions on microscopic properties of the solutions or gels and their relation with macroscopic properties.     

Influence of the polarity of the microenvironment of pyrene on the intensity of its solid-phase luminescence

The sorption of pyrene by a filter paper modified with lead acetate is studied by luminescence methods. Based on changes in the vibrational structure of fluorescence spectra of pyrene, it was found that the polarity of the microenvironment of pyrene adsorbed from solutions in ethanol increases with the concentration of acetic or hydrochloric acid in these solutions. It was revealed that, when an acid is added, pyrene fluoresces and phosphoresces more intensely at room temperature. It is shown that this is associated with a more efficient sorption of pyrene by the filter paper from acid solutions in ethanol.     

Pyrene fluorescence in the presence of nonquenching and dynamic quenching salting-out agents

A new, simple, and rapid method to determine Setschenow (i.e., salting-out) constants for luminescent organic compounds in aqueous solutions has been developed using steady-state and time-resolved fluorescence measurements. Application of the new method was demonstrated using pyrene in the presence of nonquenching (K⁺) and dynamically quenching (Cs⁺) akali metal chloride salt solutions at room temperature. For these two model systems, Setschenow constants for pyrene salting-out by KCl and CsCl were determined to be 0.211 and 0.355 M⁻¹, respectively. We expect that the methodology reported in this paper can be applied equally well to interactions between a variety of salting-out agents and fluorescent biological and/or environmental molecules in aqueous solutions.     

Synchronous-scan fluorescence as a selective detection method for sodium dodecylbenzene-sulfonate and pyrene in environmental samples

Synchronous-scan fluorescence spectra of dodecylbenzene sulfonic acid sodium salt (SDBS) and pyrene in aqueous solution were studied. The concentration ranges of SDBS and pyrene in aqueous solutions were 0.01-10.00 and 0.001-0.050 mg L⁻¹, respectively. The optimized wavelength differences (Δλ) of 46-55 and 38 nm were maintained between excitation and emission wavelengths for SDBS and pyrene, respectively, and they were found to be suitable for effective determination of SDBS and pyrene without mutual interferences; the peaks were observed at λ_ex 229-232 nm (SDBS) and λ_ex 335 nm (pyrene). Linear relationships between synchronous-scan fluorescence spectroscopy (SFS) intensity and concentration of SDBS or pyrene in aqueous solution (Milli-Q water, river water, and mucus of fish gills) were established. It was demonstrated that SFS method was effective for simultaneous analyses of SDBS and pyrene in mixed solution.     

The effect of negatively charged metallic nanocatalysts on their reactions with alkyl radicals

Abstract

Stored electrons on Au and Ag nanoparticles (NPs) have been found to catalyze various reduction processes initiated by ionizing irradiation or light illumination in which the NPs act as “nano-electrodes”. In the present study, we explored the effect of charging colloidal gold and silver NPs on their reaction with methyl radicals, ·CH₃. The results show that charging the colloidal metal NPs by excess of electrons, (NP)^n?, affected their reaction mechanism with methyl radicals and resulted in different product composition in comparison to the analogous reactions with uncharged particles. These results should be considered in photo(electro)catalytic processes when alkyl radicals are formed near the catalyst surface.     

Reduced blinking and long-lasting fluorescence of single fluorophores coupling to silver nanoparticles

The fluorescence signal of single organic fluorophores is characterized by random blinking and irreversible photobleaching. Photoinduced blinking of Cy5 has posed various limitations of this popular near-infrared (NIR) probe in biological applications. Here we show that fluorophore-metal nanoparticle (NP) complexes greatly suppress Cy5 blinking and noticeably reduce photobleaching events. The blinking behavior of single Cy5 molecules was investigated and compared in the absence and the presence of silver nanostructures. A power-law distribution of off time population was observed for single Cy5 molecules. Average off times were compared to evaluate the plasmonic effect of silver nanoparticles on the triplet decay rates. We furthermore demonstrate enhanced photostability in the presence of silver NPs. The results show that plasmonic-controlled fluorescence can lead to a novel physical mechanism to enhance fluorescence intensity, reduce blinking, and increase photostability.     

Enhancing the analytical performance of immunoassays that employ metal-enhanced fluorescence

In this work, we used a model assay system (polyclonal human IgG–goat antihuman IgG) to elucidate some of the key factors that influence the analytical performance of bioassays that employ metal-enhanced fluorescence (MEF) using silver nanoparticles (NPs). Cy5 dye was used as the fluorescent label, and results were compared with a standard assay performed in the absence of NPs. Two sizes of silver NPs were prepared with respective diameters of 60 ± 10 and 149 ± 16 nm. The absorption spectra of the NPs in solution were fitted accurately using Mie theory, and the dipole resonance of the 149-nm NPs in solution was found to match well with the absorption spectrum of Cy5. Such spectral matching is a key factor in optimizing MEF. NPs were deposited uniformly and reproducibly on polyelectrolyte-coated polystyrene substrates. Compared to the standard assay performed without the aid of NPs, significant improvements in sensitivity and in limit of detection (LOD) were obtained for the assay with the 149-nm NPs. An important observation was that the relative enhancement of fluorescence increased as the concentration of antigen increased. The metal-assisted assay data were analyzed using standard statistical methods and yielded a LOD of 0.086 ng/mL for the spectrally matched NPs compared to a value of 5.67 ng/mL obtained for the same assay in the absence of NPs. This improvement of ∼66× in LOD demonstrates the potential of metal-enhanced fluorescence for improving the analytical performance of bioassays when care is taken to optimize the key determining parameters.      

γ-Ray synthesis of starch-stabilized silver nanoparticles with antibacterial activities

Silver nanoparticles (Ag NPs) are fabricated through γ-irradiation reduction of silver ions in aqueous starch solutions. The UV–vis analyses show smaller sizes of Ag NPs produced, with higher yields, as the irradiation doses and/or Ag⁺ concentrations are increased. Higher concentrations of starch enhance the yields of Ag NPs, with no significant effects on their size. The most economical Ag NPs are produced at 5 kGy γ-irradiation of a 2×10⁻³ M solution of AgNO₃ containing 0.5% starch. They show a relatively narrow size distribution, indicated by TEM and its corresponding size distribution histogram. The XRD pattern confirms the face-centered cubic (fcc) Ag NPs embedded in starch molecules. Interactions between these nanoparticle surfaces and starch oxygen atoms are indicated by FT–IR. Antibacterial activities of Ag NPs against Escherichia coli appear dependent on the γ-ray doses applied.