Spectral Investigations on the Fluorescence Quenching of 1,4-dihydroxy-2,3-dimethylanthracene-9,10-dione by Plasmonic Silver Nanoparticles

Silver nanoparticles (Ag NPs) of different sizes have been prepared by Lee and Meisel’s method using trisodium citrate as reducing agent under ultra sonication. Optical absorption and fluorescence emission techniques were employed to investigate the interaction of 1,4-dihydroxy-2,3-dimethyl anthracene-9,10-dione (DHDMAD) with silver nanoparticles. In fluorescence spectroscopic study, we used the DHDMAD and Ag NPs as component molecules for construction of Förster Resonance Energy Transfer (FRET), whereas DHDMAD serve as donor and Ag NPs as acceptor. The surface plasmon resonance (SPR) peak of the prepared silver colloidal solution was observed from 419 nm to 437 nm. The synthesized silver nanoparticles at different heating time intervals were spherical in shape about the size of 25 nm and 55 nm. The fluorescence interaction between silver nanoparticles and DHDMAD confirms the FRET mechanism. According to Förster theory, the distance between silver nanoparticles and DHDMAD and the critical energy transfer distance were calculated and it is increase with heating time.     

Interaction of glucose‐derived carbon quantum dots with silver and gold nanoparticles and its application for the fluorescence detection of 6‐thioguanine

The interaction of glucose‐derived carbon quantum dots (CQDs) with silver (Ag) and gold (Au) nanoparticles (NPs) was explored by fluorescence spectroscopy. Both metal NPs cause an efficient quenching of CQD fluorescence, which is likely due to the energy transfer process between CQDs as donors and metal NPs as acceptors. The Stern–Volmer plots were evaluated and corresponding quenching constants were found to be 1.9 × 10¹⁰ and 2.2 × 10⁸ M^?1 for AgNPs and AuNPs, respectively. The analytical applicability of these systems was demonstrated for turn‐on fluorescence detection of the anti‐cancer drug, 6‐thioguanine. Because the CQD–AgNP system had much higher sensitivity than the CQD–AuNP system, we used it as a selective fluorescence probe in a turn‐on assay of 6‐thioguanine. Under optimum conditions, the calibration graph was linear from 0.03 to 1.0 μM with a detection limit of 0.01 μM. The developed method was applied to the analysis of human plasma samples with satisfactory results.     

Investigating Fluorescence Quenching of ZnS Quantum Dots by Silver Nanoparticles

Water dispersible zinc sulfide quantum dots (ZnS QDs) with an average diameter of 2.9 nm were synthesized in an environment friendly method using chitosan as stabilizing agent. These nanocrystals displayed characteristic absorption and emission spectra having an absorbance edge at 300 nm and emission maxima (λ _emission) at 427 nm. Citrate-capped silver nanoparticles (Ag NPs) of ca. 37-nm diameter were prepared by modified Turkevich process. The fluorescence of ZnS QDs was significantly quenched in presence of Ag NPs in a concentration-dependent manner with K _sv value of 9 × 10⁹ M⁻¹. The quenching mechanism was analyzed using Stern–Volmer plot which indicated mixed nature of quenching. Static mechanism was evident from the formation of electrostatic complex between positively charged ZnS QDs and negatively charged Ag NPs as confirmed by absorbance study. Due to excellent overlap between ZnS QDs emission and surface plasmon resonance band of Ag NPs, the role of energy transfer process as an additional quenching mechanism was investigated by time-resolved fluorescence measurements. Time-correlated single-photon counting study demonstrated decrease in average lifetime of ZnS QDs fluorescence in presence of Ag NPs. The corresponding F?rster distance for the present QD–NP pair was calculated to be 18.4 nm.     

Electrochemical and antibacterial activity of silver nanoparticles synthesized using Mukia madarasapattana leaf extract

In this study, biosynthesis of stable silver nanoparticles (Ag NPs) were prepared using Mukia madarasapattana leaf extract. X-ray diffraction analysis revealed the synthesized silver nanoparticles had face centered cubic crystalline structure. The TEM image showed the silver nanoparticles are not agglomerated, moderately mono dispersed with the size of 15 nm. The high negative zeta potential values indicated the dispersion stability of Ag NPs. Antibacterial activity was carried out against different test microorganisms in silver nanoparticles. The cyclic Voltammetry study showed that Ag NPs have an oxidation peak at 0.61 mV.

     

Effect of Plasmonic Silver Nanoparticles’ Size on Photophysical Characteristics of 4-Aryloxymethyl Coumarins

The effect of plasmonic silver nanoparticles’ size on photophysical characteristics of four biologically active 4-aryloxymethyl coumarins 4-p-tolyloxymethylbenzo[h] coumarin (4PTMBC), 1-(4-iodo phenoxymethyl)-benzo [f] coumarin (1IPMBC), 4-(4-iodo-phenoxymethyl)-benzo [h] coumarin (4IPMBC), and 4-(4-iodo-phenoxymethyl)- 6-methoxy coumarin (4IPMMC) has been studied using absorption and fluorescence spectroscopy. The size of silver nanoparticles has been estimated by field effect scanning electron microscope technique. The absorption maxima of silver nanoparticles are red shifted with increase in their size. The absorption spectral changes of investigated coumarins with the addition of silver nanoparticles of different sizes suggest their possible interaction with silver nanoparticles. Fluorescence quenching has been observed for all the coumarins with the addition of silver nanoparticles of different sizes. The Stern-Volmer (S-V) plots of fluorescence quenching are found to be linear. The magnitude of quenching rate parameter suggests the involvement of static quenching mechanism. Fluorescence data has been used to estimate binding constants and the number of binding sites. The contribution of diffusion and electron transfer processes in fluorescence quenching mechanism has also been discussed. The values of S-V constant and quenching rate parameter are found to decrease with increase in size of silver nanoparticles.

     

Mechanistic Insights into Interaction of Humic Acid with Silver Nanoparticles

Humic acid (HA) is one of the major components of the natural organic matter present in the environment that alters the fate and behavior of silver nanoparticles (Ag NPs). Transformation of Ag NPs happens upon interaction with HA, thereby, changing both physical and chemical properties. Fluorescence spectroscopy and scanning electron microscopy (SEM) were used to analyze the interaction of Ag NPs with HA. In pH and time-dependent studies, the near field electro dynamical environment of Ag NPs influenced the fluorescence of HA, indicated by fluorescence enhancement. SEM revealed not only morphological changes, but also significant reduction in size of Ag NPs after interaction with HA. Based on these studies, a probable mechanism was proposed for the interaction of HA with Ag NPs, suggesting the possible transformation that these nanoparticles can undergo in the environment.     

Zur Abbaubeschleunigung von Pflanzenschutzmittel-Wirkstoffen im Boden

In the current investigation, we report the biosynthesis of silver nanoparticles (Ag NPs) employing extract of Alternaria alternata, which is an eco-friendly process for the synthesis of metallic nanoparticles. Ag NPs were synthesised through the reduction of aqueous Ag⁺ ion using the cell extract of fungus A. alternata in the dark conditions. The synthetic process was relatively fast and Ag NPs were formed within 24 h. UV–visible spectrum of the aqueous medium containing silver ion showed a peak at 435?nm corresponding to the plasmon absorbance of Ag NPs and another peak at 280?nm refers to tyrosine amino acid. The nanoparticles were characterised by scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). The morphology of nanoparticles is found to be spherical mostly, with ranging size of 27–79?nm; as revealed by SEM. The FTIR spectrum analysis indicated that biomolecules were involved in the synthesis of Ag NPs. The presence of the amino groups is expected to pack differently around the Ag NPs. This in turn will influence the self-assembly of nanoparticles on substrates as well as their stability. The present study demonstrates the possible use of biologically synthesised Ag NPs in the field of agriculture, when A. alternata could be used for simple, nonhazardous and efficient synthesis of Ag NPs.     

Fabrication of Monodispersed Silver Nanoparticles and their Collective Sharp Plasmonic Response

Monodispersed silver (Ag) nanoparticles (NPs) were obtained by applying an electric field on unipolar-charged Ag NPs fabricated using a heterogeneous condensation technique in gas media. Well defined and charged Ag NPs were separated based on their sizes and were collected on quartz substrates. Thin films consisting of monodispersed Ag NPs with size ranging from 35 to 120 nm were prepared by varying an applied electric field during the fabrication process. Scanning electron microscope results showed that the samples have uniform size distribution. Coherent oscillations of conduction band electrons in gas medium induced by electromagnetic field and coupling of all similar plasmon resonances due to uniform Ag NPs size produced unique and interesting optical properties. Narrow extinction widths (～41 to ～69 nm) were observed compared to the width of polydispersed Ag sample. The ability to prepare samples in gas media and tune the plasmon resonance by merely varying an electric field during fabrication makes the method simple, fast, and highly economical.     

Comparative Study of Antimicrobial Activity of AgBr and Ag Nanoparticles (NPs)

The diverse mechanism of antimicrobial activity of Ag and AgBr nanoparticles against gram-positive and gram-negative bacteria and also against several strains of candida was explored in this study. The AgBr nanoparticles (NPs) were prepared by simple precipitation of silver nitrate by potassium bromide in the presence of stabilizing polymers. The used polymers (PEG, PVP, PVA, and HEC) influence significantly the size of the prepared AgBr NPs dependently on the mode of interaction of polymer with Ag⁺ ions. Small NPs (diameter of about 60–70 nm) were formed in the presence of the polymer with low interaction as are PEG and HEC, the polymers which interact with Ag⁺ strongly produce nearly two times bigger NPs (120–130 nm). The prepared AgBr NPs were transformed to Ag NPs by the reduction using NaBH₄. The sizes of the produced Ag NPs followed the same trends – the smallest NPs were produced in the presence of PEG and HEC polymers. Prepared AgBr and Ag NPs dispersions were tested for their biological activity. The obtained results of antimicrobial activity of AgBr and Ag NPs are discussed in terms of possible mechanism of the action of these NPs against tested microbial strains. The AgBr NPs are more effective against gram-negative bacteria and tested yeast strains while Ag NPs show the best antibacterial action against gram-positive bacteria strains.     

Synthesis of Silver and Gold Nanoparticles Using Cashew Nut Shell Liquid and Its Antibacterial Activity Against Fish Pathogens

This study reveals a green process for the production of multi-morphological silver (Ag NPs) and gold (Au NPs) nanoparticles, synthesized using an agro-industrial residue cashew nut shell liquid. Aqueous solutions of Ag⁺ ions for silver and chloroaurate ions for gold were treated with cashew nut shell extract for the formation of Ag and Au NPs. The nano metallic dispersions were characterized by measuring the surface plasmon absorbance at 440 and 546 nm for Ag and Au NPs. Transmission electron microscopy showed the formation of nanoparticles in the range of 5–20 nm for silver and gold with assorted morphologies such as round, triangular, spherical and irregular. Scanning electron microscopy with energy dispersive spectroscopy and X-ray diffraction analyses of the freeze-dried powder confirmed the formation of metallic Ag and Au NPs in crystalline form. Further analysis by Fourier transform infrared spectroscopy provided evidence for the presence of various biomolecules, which might be responsible for the reduction of silver and gold ions. The obtained Ag and Au NPs had significant antibacterial activity, minimum inhibitory concentration and minimum bactericidal concentration on bacteria associated with fish diseases.     

DNA Detection Based on Localized Surface Plasmon Resonance Spectroscopy of Ag@Au Biocomposite Nanoparticles

Noble metal nanoparticles (NPs) have attracted much attention due to their unique physical and chemical properties such as tunable surface plasmonics, high-efficiency electrochemical sensing, and enhanced fluorescence. We produced two biosensor chips consisting of Ag@Au bimetallic nanoparticles (BNPs) on a carbon thin film by simple RF-sputtering and RF-plasma-enhanced chemical vapor co-deposition. We deposited Au NPs with average size of 4 nm (Au₁ NPs) or 11 nm (Au₂ NPs) on a sensor chip consisting of Ag NPs with mean size of 15 nm, and we investigated the effect of shell size (Au NPs) on the chemical activities of the resulting Ag@Au₁ BNPs and Ag@Au₂ BNPs. We estimated the average size and morphology of Ag@Au BNPs by scanning electron microscopy (SEM) and atomic force microscopy (AFM) images. X-ray diffraction (XRD) patterns revealed that Ag NPs and Au NPs had face-centered cubic (FCC) structure. We studied aging of the biosensor chips consisting of Ag@Au BNPs by localized surface plasmon resonance (LSPR) spectroscopy for up to 3 months. UV–visible aging of the prepared samples indicated that Ag@Au₁ BNPs, which corresponded to Ag NPs covered with smaller Au NPs, were more chemically active than Ag@Au₂ BNPs. Furthermore, we evaluated changes in the LSPR absorption peaks of Ag@Au₁ BNPs and bare Ag NPs in the presence of a DNA primer decamer at fM concentrations, to find that Ag@Au₁ BNPs were more sensitive biosensor chips within a short response time as compared to bare Ag NPs.

     

Luminescence Enhancement of Carboxyl-Coated CdTe Quantum Dots by Silver Nanoparticles

Metallic nanoparticles (NPs) have been used to improve the sensibility of biosensors and bioassays either by enhancing radiative emission or inducing quenching process on fluorescent probes. The aim of this research was to study the interaction of silver and silver-pectin NPs with water-dispersed carboxyl-coated cadmium telluride (CdTe) quantum dots (QDs). Metallic NPs were observed to change the emission of these fluorophores through local field effects. In a solution-base platform, an increase of 82 % was observed for the CdTe emission due to the interaction of QDs and silver-pectin NPs. QDs interaction with silver NPs without pectin was also investigated and a smaller emission enhancement of 20 % was detected. We observed that the NPs’ nature and QDs’ surface charge and concentration are important parameters for NPs-QDs interaction. Moreover, the presence of the pectin polymer shows to be a key component to the observed fluorescence enhancement.     

Single-Step Synthesis and Surface Plasmons of Bismuth-Coated Spherical to Hexagonal Silver Nanoparticles in Dichroic Ag:Bismuth Glass Nanocomposites

Here, we report for the first time the synthesis of bismuth-coated silver nanoparticles in dichroic bismuth glass nanocomposites by a novel and simple one-step melt quench technique without using any external reducing agent. The metallic silver nanoparticles (Ag NPs) were generated first, and subsequently, metallic bismuth was deposited on the Ag NPs and formed a thick layer. The reduction of Bi³⁺ to Bi^o and subsequently its deposition on the Ag NPs (which were formed earlier than Bi^o) in the K₂O–Bi₂O₃–B₂O₃ (KBB) glass system have been explained by their standard reduction potentials. The UV–vis absorption spectra show a prominent surface plasmon resonance (SPR) absorption band at 575 nm at lower concentrations (up to 0.01 wt%); three bands at 569, 624 and 780 nm at medium concentration (0.02–0.03 wt%); and two weak bands at 619 and 817 nm at highest concentration (0.06 wt%) of silver. They have been explained by the electrodynamics theories. TEM images reveal the conversion of spheroidal (5–15 nm) to hexagonal (10–35 nm) shaped Ag NPs with the increase in concentration of silver (up to 0.06 wt%). SAED pattern confirms the crystalline planes of rhombohedral bismuth and cubic silver. Thermal treatment at 360 °C, which is the glass transformation temperature (T _g) of the sample containing lower concentration of silver (0.007 wt%), shows red-shifted SPR band due to increase in size of NPs. Whereas the sample containing higher concentration (0.06 wt%) of silver under similar treatment exhibited changes in SPR spectral profile happened due to conversion to spherical NPs from hexagonal shape and reduction in size (10–20 nm) of NPs after heat treatment for 65 h. HRTEM images corroborate the different orientations of the NPs. FESEM images reveal hexagonal disk like structure having different orientations. Dichroic nature of the nanocomposites has been explained with the size and shape of Ag nanoparticles. We believe that this work will create new avenues in the area of nanometal–glass hybrid nanocomposites and the materials have significant applications in the field of optoelectronics and nanophotonics.     

Bioengineered silver nanoparticles using Curvularia pallescens and its fungicidal activity against Cladosporium fulvum

Microorganisms based biosynthesis of nanomaterials has triggered significant attention, due to their great potential as vast source of the production of biocompatible nanoparticles (NPs). Such biosynthesized functional nanomaterials can be used for various biomedical applications. The present study investigates the green synthesis of silver nanoparticles (Ag NPs) using the fungus Curvularia pallescens (C. pallescens) which is isolated from cereals. The C. pallescens cell filtrate was used for the reduction of AgNO₃ to Ag NPs. To the best of our knowledge C. pallescens is utilized first time for the preparation of Ag NPs. Several alkaloids and proteins present in the phytopathogenic fungus C. pallescens were mainly responsible for the formation of highly crystalline Ag NPs. The as-synthesized Ag NPs were characterized by using UV–Visible spectroscopy, X-ray diffraction and transmission electron microscopy (TEM). The TEM micrographs have revealed that spherical shaped Ag NPs with polydisperse in size were obtained. These results have clearly suggested that the biomolecules secreted by C. pallescens are mainly responsible for the formation and stabilization of nanoparticles. Furthermore, the antifungal activity of the as-prepared Ag NPs was tested against Cladosporium fulvum, which is the major cause of a serious plant disease, known as tomato leaf mold. The synthesized Ag NPs displayed excellent fungicidal activity against the tested fungal pathogen. The extreme zone of reduction occurred at 50 μL, whereas, an increase in the reduction activity is observed with increasing the concentration of Ag NPs. These encouraging results can be further exploited by employing the as synthesized Ag NPs against various pathogenic fungi in order to ascertain their spectrum of fungicidal activity.     

Pine cone-mediated green synthesis of silver nanoparticles and their antibacterial activity against agricultural pathogens

The medicinal and physicochemical properties of nanoscale materials are strong functions of the particle size and the materials used in their synthesis. The nanoparticle shape also contributes significantly to their medicinal properties. Several shapes ranging from oval, spherical, rods, to teardrop structures may be obtained by chemical methods. Triangular and hexagonal nanoparticles have been synthesized by using a pine cone extract (PCE). Here, we report the discovery that PCE, when reacted with silver nitrate ions, yields a high percentage of thin, flat, single-crystalline nanohexagonal and nanotriangular silver nanoparticles. The nanohexagonal and nanotriangular nanoparticles appear to grow by a process involving rapid reduction with assembly at room temperature at a high pH. The nanoparticles were characterized by UV–Vis absorption spectroscopy, SEM-EDS, TEM, FTIR, and X-ray diffraction analyses. The anisotropy of the nanoparticle shape results in large near-infrared absorption by the particles. Highly anisotropic particles are applicable in various fields, including agriculture and medicine. The obtained silver nanoparticles (Ag NPs) had significant antibacterial action on both Gram classes of bacteria associated with agriculture. Because the Ag NPs are encapsulated with functional group-rich PCE, they can be easily integrated in various applications.     

Biosynthesized silver nanoparticles for inhibition of antibacterial resistance and biofilm formation of methicillin-resistant coagulase negative Staphylococci

The ability of a natural stabilizing and reducing agent on the synthesis of silver nanoparticles (Ag NPs) was explored using a rapid and single-pot biological reduction method using Nocardiopsis sp. GRG1 (KT235640) biomass. The UV–visible spectral analysis of Ag NPs was found to show a maximum absorption peak located at a wavelength position of ∼422 nm for initial conformation. The major peaks in the XRD pattern were found to be in excellent agreement with the standard values of metallic Ag NPs. No other peaks of impurity phases were observed. The morphology of Ag NPs was confirmed through TEM observation, demonstrating that the particle size distribution of Ag NPs entrenched in spherical particles is in a range between 20 and 50 nm. AFM analysis further supported the nanosized morphology of the synthesized Ag NPs and allowed quantifying the Ag NPs surface roughness. The synthesized Ag NPs showed significant antibacterial and antibiofilm activity against biofilm positive methicillin-resistant coagulase negative Staphylococci (MR-CoNS), which were isolated from urinary tract infection as determined by spectroscopic methods in the concentration range of 5–60 µg/ml. The inhibition of biofilm formation with coloring stain was morphologically imaged by confocal laser scanning microscopy (CLSM). Morphological alteration of treated bacteria was observed by SEM analysis. The results clearly indicate that these biologically synthesized Ag NPs could provide a safer alternative to conventional antibiofilm agents against uropathogen of MR-CoNS.     

Probing the interaction of caffeic acid with ZnO nanoparticles

The binding of ZnO nanoparticles (NPs) and caffeic acid (CFA) was investigated using fluorescence quenching, UV/vis absorption spectrscopy, Fourier transform infrared (FTIR) spectroscopy, transmission electron microscopy (TEM) and dynamic light scattering (DLS) at different temperatures. The study results indicated fluorescence quenching between ZnO NPs and CFA rationalized in terms of a static quenching mechanism or the formation of non‐fluorescent CFA–ZnO. From fluorescence quenching spectral analysis, the binding constant (K_a), number of binding sites (n) and thermodynamic properties were determined. Values of the quenching (K_SV) and binding (K_a) constants decrease with increasing temperature and the number of binding sites n = 2. The thermodynamic parameters determined using Van't Hoff equation indicated that binding occurs spontaneously involving the hydrogen bond, and van der Waal's forces played a major role in the reaction of ZnO NPs with CFA. The FTIR, TEM and DLS measurements also indicated differences in the structure, morphology and size of CFA, ZnO NPs and their corresponding CFA–ZnO. Copyright © 2015 John Wiley & Sons, Ltd.     

Optimization of Nanoparticle Size for Plasmonic Enhancement of Fluorescence

This paper reports on the enhancement of fluorescence that can result from the proximity of fluorophores to metallic nanoparticles (NPs). This plasmonic enhancement, which is a result of the localized surface plasmon resonance at the metal surface, can be exploited to improve the signal obtained from optical biochips and thereby lower the limits of detection. There are two distinct enhancement effects: an increase in the excitation of the fluorophore and an increase in its quantum efficiency. This study focuses on the first of these effects where the maximum enhancement occurs when the NP plasmon resonance wavelength coincides with the fluorophore absorption band. In this case, the excitation enhancement is proportional to the square of the amplitude of the electric field. The scale of the enhancement depends on many parameters, such as NP size and shape, metal type, and NP–fluorophore separation. A model system consisting of spherical gold/silver alloy NPs, surrounded by a silica spacer shell, to which is attached a fluorescent ruthenium dye, was chosen and the dependence of the fluorescence enhancement on NP diameter was investigated. Theoretical calculations, based on Mie theory, were carried out to predict the maximum possible enhancement factor for spherical NPs with a fixed composition and a range of diameters. Spherical NPs of the same composition were fabricated by chemical preparation techniques. The NPs were coated with a thin silica shell to overcome quenching effects and the dye was attached to the shell.     

Enhanced chemiluminescence of the luminol–AgNO3 system by Ag nanoparticles

The oxidation reaction of luminol with AgNO₃ can produce chemiluminescence (CL) in the presence of silver nanoparticles (NPs) in alkaline solution. Based on the studies of UV‐vis absorption spectra, photoluminescence (PL) spectra and CL spectra, a CL enhancement mechanism is proposed. The CL emission spectrum of the luminol–AgNO₃–Ag NPs system indicated that the luminophore was still 3‐aminophthalate. On injection of silver nanoparticles into the mixture of luminol and AgNO₃, they catalysed the reduction of AgNO₃ by luminol. The product luminol radicals reacted with the dissolved oxygen, to produce a strong CL emission. As a result, the CL intensity was substantially increased. Moreover, the influences of 18 amino acids, e.g. cystine, tyrosine and asparagine, and 25 organic compounds, including gallic acid, tannic acid and hydroquinone, on the luminol–AgNO₃–Ag NPs CL system were studied by a flow‐injection procedure, which led to an effective method for detecting these compounds. Copyright © 2011 John Wiley & Sons, Ltd.     

Biosynthesis of silver nanoparticles by using of the marine brown alga Padina pavonia and their characterization

Silver nanoparticles (AgNPs) are gaining considerable importance due to their attractive physicochemical properties for many applications. In the present study, (Ag NPs) were synthesized by the reduction of aqueous solutions of silver nitrate (AgNO₃) with powder and solvent extracts of Padina pavonia (brown algae). The obtained nanoparticles exhibited high stability, rapid formation of the biogenic process (2 min -3 h), small size (49.58–86.37 nm) (the diameter of formed nanoparticles was measured by TEM and DLS) and variable shapes (spherical, triangular, rectangle, polyhedral and hexagonal). Preliminary characterization of nanoparticles was monitored by using UV–visible spectroscopy (UV–vis), Transmission Electron Microscopy (TEM), Dynamic Light Scattering (DLS) and finally by Fourier Transform Infrared spectroscopy (FTIR). The ratios of converted Ag NPs were recorded as 88.5; 86.2 and 90.5% in case of P. pavonia powder. extract and chloroform extract, respectively.