Synthesis of gold nanotriangles and silver nanoparticles using Aloe vera plant extract

Biogenic gold nanotriangles and spherical silver nanoparticles were synthesized by a simple procedure using Aloe vera leaf extract as the reducing agent. This procedure offers control over the size of the gold nanotriangle and thereby a handle to tune their optical properties, particularly the position of the longitudinal surface plasmon resonance. The kinetics of gold nanotriangle formation was followed by UV-vis-NIR absorption spectroscopy and transmission electron microscopy (TEM). The effect of reducing agent concentration in the reaction mixture on the yield and size of the gold nanotriangles was studied using transmission electron microscopy. Monitoring the formation of gold nanotriangles as a function of time using TEM reveals that multiply twinned particles (MTPs) play an important role in the formation of gold nanotriangles. It is observed that the slow rate of the reaction along with the shape directing effect of the constituents of the extract are responsible for the formation of single crystalline gold nanotriangles. Reduction of silver ions by Aloe vera extract however, led to the formation of spherical silver nanoparticles of 15.2 nm +/- 4.2 nm size.     

Review of Some Interesting Surface Plasmon Resonance-enhanced Properties of Noble Metal Nanoparticles and Their Applications to Biosystems

Noble metal, especially gold (Au) and silver (Ag) nanoparticles exhibit unique and tunable optical properties on account of their surface plasmon resonance (SPR). In this review, we discuss the SPR-enhanced optical properties of noble metal nanoparticles, with an emphasis on the recent advances in the utility of these plasmonic properties in molecular-specific imaging and sensing, photo-diagnostics, and selective photothermal therapy. The strongly enhanced SPR scattering from Au nanoparticles makes them useful as bright optical tags for molecular-specific biological imaging and detection using simple dark-field optical microscopy. On the other hand, the SPR absorption of the nanoparticles has allowed their use in the selective laser photothermal therapy of cancer. We also discuss the sensitivity of the nanoparticle SPR frequency to the local medium dielectric constant, which has been successfully exploited for the optical sensing of chemical and biological analytes. Plasmon coupling between metal nanoparticle pairs is also discussed, which forms the basis for nanoparticle assembly-based biodiagnostics and the plasmon ruler for dynamic measurement of nanoscale distances in biological systems.     

Nanogold-plasmon-resonance-based glucose sensing

Noble metal nanoparticles are well known for their strong interactions with light through the resonant excitations of the collective oscillations of the conduction electrons on the particles, the so-called surface plasmon resonances. The close proximity of two nanoparticles is known to result in a red-shifted resonance wavelength peak, due to near-field coupling. We have subsequently employed this phenomenon and developed a new approach to glucose sensing, which is based on the aggregation and disassociation of 20-nm gold particles and the changes in plasmon absorption induced by the presence of glucose. High-molecular-weight dextran-coated nanoparticles are aggregated with concanavalin A (Con A), which results in a significant shift and broadening of the gold plasmon absorption. The addition of glucose competitively binds to Con A, reducing gold nanoparticle aggregation and therefore the plasmon absorption when monitored at a near-red arbitrary wavelength. We have optimized our plasmonic-type glucose nanosensors with regard to particle stability, pH effects, the dynamic range for glucose sensing, and the observation wavelength to be compatible with clinical glucose requirements and measurements. In addition, by modifying the amount of dextran or Con A used in nanoparticle fabrication, we can to some extent tune the glucose response range, which means that a single sensing platform could potentially be used to monitor microM --> mM glucose levels in many physiological fluids, such as tears, blood, and urine, where the glucose concentrations are significantly different.     

Obtain Quadruple Intense Plasmonic Resonances from Multilayered Gold Nanoshells by Silver Coating: Application in Multiplex Sensing

Four intense and separate localized surface plasmon resonance (LSPR) absorption peaks have been obtained in the gold-dielectric–gold–silver multilayer nanoshells. The silver coating on the gold shell results in a new LSPR peak at about 400 nm corresponding to the $ {{\left| {\omega_{+}^{-}} \right\rangle}_{Ag }} $ mode. The intense local electric field concentrated in the silver shell at the wavelength of 400 nm indicates that this new plasmonic band is coming from the symmetric coupling between the antibonding silver shell plasmon mode and the inner sphere plasmon. Increasing the silver shell thickness also leads to the intensity increasing of the $ {{\left| {\omega_{+}^{-}} \right\rangle}_{Au }} $ mode and blue shift of $ \left| {\omega_{-}^{+}} \right\rangle $ and $ \left| {\omega_{-}^{-}} \right\rangle $ modes. Therefore, quadruple intense plasmonic resonances in the visible region could be achieved in gold-dielectric–gold–silver multilayer nanoshells by tuning the geometrical parameters. And the quadruple intense plasmonic resonances in the visible region provide well potential for multiplex biosensing based on LSPR.     

Synthesis, Conforming, Linear, and Non-linear Optical Properties of Gold Nanoparticles-sepiolite Compacts

Gold monodispersed nanoparticles (smaller than 10 nm) have been embedded into sepiolite fibres and sintered by spark plasma sintering (SPS) in order to preserve their nanometre size. The optical properties of these nanoparticles have been measured showing the surface plasmon resonance. Plasmon absorption curves have been successfully fitted by using the quasistatic approximation for gold nanoparticles. It has been observed that the sintering process decreases the plasmon width. SPS sintered nanostructured compacts present large values of the non-linear third-order dielectric susceptibility related to the gold concentration.     

Synthesis and characterization of cysteine functionalized silver nanoparticles for biomolecule immobilization

A facile method for the aqueous phase synthesis of cysteine-functionalized silver nanoparticles by potato extract has been reported in the present work. These functionalized nanoparticles were then used for the covalent immobilization of a biomolecule, alkaline phosphatase, on its surface through carbodiimide coupling. Different reaction parameters such as cysteine concentration, reducing agent concentration, temperature, pH and reaction time were varied during the nanoparticles’ formation, and their effects on plasmon resonance were studied using Ultraviolet–visible spectroscopy. Fourier transform infrared spectroscopy was used to confirm the surface modification of silver nanoparticles by cysteine and the particle size analysis was done using particle size analyzer, which showed the average nanoparticles’ size of 61 nm for bare silver nanoparticles and 201 nm for the enzyme-immobilized nanoparticles. The synthesized nanoparticles were found to be highly efficient for the covalent immobilization of alkaline phosphatase on its surface and retained 67 % of its initial enzyme activity (9.44 U/mg), with 75 % binding efficiency. The shelf life of the enzyme-nanoparticle bioconjugates was found to be 60 days, with a 12 % loss in the initial enzyme activity. With a simple synthesis strategy, high immobilization efficiency and enhanced stability, these enzyme-coated nanoparticles have the potential for further integration into the biosensor technology.     

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.     

Numerical Simulation of Extinction Spectra of Plasmonically Coupled Nanospheres Using Discrete Dipole Approximation: Influence of Compositional Asymmetry

We demonstrate plasmon coupling phenomenon between equivalent (homodimer) and non-equivalent (heterodimer) spherical shape noble metal nanoparticle (Ag, Au and Al). A systematic comparison of surface plasmon resonance (SPR) and extinction properties of various configurations (monomer, homodimer and heterodimer) has been investigated to observe the effect of compositional asymmetry. Numerical simulation has been done by using discrete dipole approximation method to study the optical properties of plasmonically coupled metal nanoparticles (MNPs). Plasmon coupling between similar nanoparticles allows only higher wavelength bonding plasmon mode while both the plasmon modes lower wavelength antibonding mode as well as higher wavelength bonding mode in the case of heterodimer. Au monomer of radius 50 nm shows resonance peak at 518 nm while plasmon coupling between Au-Au homodimer results in a spectral red shift around 609 nm. Au-Ag plasmonic heterodimer (radius 50 nm) reveals two resonant modes corresponding to higher energy antibonding mode (422 nm) as well as lower energy bonding mode (533 nm). Further, we have shown that interparticle edge-to-edge separation is the most significant parameter affecting the surface plasmon resonances of MNPs. As the inter particle separation decreases, resonance wavelength shows red spectral shift which is maximum for the touching condition. It is shown that plasmon coupling is a reliable strategy to tune the SPR.

     

Temperature Dependence of Photoluminescence from Silver Nanoparticles

Strong temperature dependence of surface plasmon enhanced photoluminescence from silver nanoparticles embedded in a silica host matrix has been observed. The quantum yield of photoluminescence increases as the temperature decreases. Such an effect has been rationalized as being the result of an increase in the plasmonic enhancement factor as a consequence of the decrease in the plasmon damping constant. The decrease in the damping constant is due to a reduction in the electron–phonon scattering rate with the decrease in temperature. The temperature dependence of the photoluminescence quantum yield is stronger for small nanoparticles which reflects the strengthening of electron–phonon coupling in silver nanoparticles with a decrease of their size.     

Biosynthesis of silver and gold nanoparticles using thermophilic bacterium Geobacillus stearothermophilus

Nanomaterials have assumed a great deal of importance as they often display unique and considerably modified physical, chemical and biological properties as compared to their counterparts of the macroscale. In this study, biogenic synthesis of silver and gold nanoparticles by Geobacillus stearothermophilus has been attempted. The exposure of G. stearothermophilus cell free extract to the metal salts leads to the formation of stable silver and gold nanoparticles in the solution. These nanoparticles were characterized by UV–Vis spectra, FTIR, TEM, and XRD. The silver and gold nanoparticles have absorption maxima at 423 nm and 522 nm respectively. The TEM micrograph revealed the formation of polydispersed particles in the case of silver nanoparticles and monodispersed particles with respect to the gold nanoparticles. High stability of the nanoparticle solution could be attributed to the secretion of certain capping proteins by the bacterium in the reaction mixture. The involvement of these proteins was confirmed by FTIR and SDS PAGE.     

Plasmonic and Energy Studies of Ag Nanoparticles in Silica-Titania Hosts

The present work reports an investigation of surface plasmon resonance (SPR) of silver nanoparticles in SiO₂–TiO₂ hosts. The surface plasmon resonance of silver nanoparticles was observed in the wavelength range 300–400 nm. Numerical calculation of SPR of silver nanoparticles with spherical morphology was done on the basis of discrete dipole approximation (DDA) method. The observed fluorescence spectrum fits well with the theoretically calculated one. The luminescence enhancement is attributed to the strong local electric field which increases the exciting and emitting photons coupled to SPR. An effort has been made to study the surface plasmon mediated excitation energy transfer (EET) between two spherical metal nanoparticles. The van der Waals (vdW) energy between plasmonic silver nanoparticles in the present hosts has been estimated.     

Enhanced Nonlinear Optical Response of Resonantly Coupled Silver Nanoparticle–Organic Dye Complexes

The influence of metal nanoparticles on linear and nonlinear optical properties of surrounding organic molecules has been widely investigated, whereas much less attention has been paid to the influence of molecules on properties of nanoparticles. Here, we employ transient absorption spectroscopy to address the nonlinear optical responses of the resonantly coupled silver nanoparticle–organic dye systems and demonstrate that silver nanoparticles covered with dye molecules show enhanced and spectrally different nonlinear extinction changes from pristine nanoparticles. We identify changes of the plasmon resonance band of nanoparticles induced by excitation of surrounding dye. We attribute these exciton–plasmon coupling effects to the excitation-induced refractive index modifications of the dye layer surrounding a nanoparticle and to the back-transfer of the oscillator strength borrowed by the dye from the nanoparticle.     

Characterization of silver nanoparticles synthesized by using marine isolate Streptomyces albidoflavus

Silver nanoparticles production by the green chemistry approach was investigated using an isolated marine actinomycetes strain. The isolated strain was identified as Streptomyces albidoflavus based on chemotaxonomic and ribotyping properties. The strain revealed production of silver nanoparticles both extracellular and intracellularly. Surface Plasmon Resonance analysis with the function of time revealed that particle synthesis by this strain is reaction time dependent. The produced particles were spherical shaped and monodispersive in nature and showed a single surface plasmon resonance peak at 410 nm. Size distribution histograms indicated production of 10-40- nm-size nanoparticles with a mean size of 14.5 nm. FT-IR spectra of nanopartilces showed N-H, C-H, and C-N stretching vibrations, denoting the presence of amino acid/ peptide compounds on the surface of silver nanoparticles produced by S. albidoflavus. Synthesized nanoparticles revealed a mean negative zeta potential and electrophoretic mobility of -8.5 mV and -0.000066 cm2/Vs, respectively. The nanoparticles produced were proteinaceous compounds as capping agents with -8.5 mV zeta potential and revealed antimicrobial activity against both Gram-negative and -positive bacterial strains. Owing to their small size, these particles have greater impact on industrial application spectra.     

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.     

Anticancer,antimicrobial, antioxidant,and catalytic activities of green-synthesized silver and gold nanoparticles using Bauhinia purpurea leaf extract

The synthesis of metal nanoparticles by green methods attained enormous attention in recent years due to its easiness, non-toxicity, and eco-friendly nature. In the present study, noble metal nanoparticles such as silver and gold were prepared using an aqueous leaf extract of a medicinal plant, Bauhinia purpurea. The leaf extract performed as both reducing and stabilizing agents for the development of nanoparticles. The formations of silver and gold nanoparticles were confirmed by observing the surface plasmon resonance peaks at 430 nm and 560 nm, respectively, in UV–Vis absorption spectrum. Various properties of nanoparticles were demonstrated using the characterization techniques such as FTIR, XRD, TEM, and EDX. The synthesized silver and gold nanoparticles had a momentous anticancer effect against lung carcinoma cell line A549 in a dose-dependent manner with IC₅₀ values of 27.97 µg/mL and 36.39 µg/mL, respectively. The antimicrobial studies of synthesized nanoparticles were carried out by agar well diffusion method against six microbial strains. Silver and gold nanoparticles were also showed high antioxidant potentials with IC₅₀ values of 42.37 µg/mL and 27.21 µg/mL, respectively; it was measured using DPPH assay. Additionally, the nanoparticles were observed to be good catalysts for the reduction of organic dyes.

     

Core–shell silver nanoparticles for optical labeling of cells

Silver nanoparticles have been modified with self-assembled monolayers of hydroxyl-terminated long chain thiols and encapsulated with a silica shell. The resulting core–shell nanoparticles were used as optical labels for cell analysis using flow cytometry and microscopy. The excitation of plasmon resonances in nanoparticles results in strong depolarized scattering of visible light, permitting detection at the single nanoparticle level. The nanoparticles were modified with neutravidin via epoxide–azide coupling chemistry, to which biotinylated antibodies targeting cell surface receptors were bound. The nanoparticle labels exhibited long-term stability in solutions with high salt concentrations without aggregation or silver etching. Labeled cells exhibited two orders of magnitude enhancement of the scattering intensity compared with unlabeled cells.     

Gold@Silver@Gold Core Double-Shell Nanoparticles: Synthesis and Aggregation-Enhanced Two-Photon Photoluminescence Evaluation

A facile, straightforward, and low-cost method is proposed to synthesize gold@silver@gold core double-shell nanoparticles. The technique is a seed-mediated growth protocol that contains four steps of (1) gold seed synthesis, (2) gold seed growth, (3) silver layer coating through silver salt reduction, and (4) gold layer deposition via gold precursor reduction. The prepared nanoparticles had a narrow size distribution and the average particle size of 28 ± 1 nm. Cysteine was introduced to the nanoparticles solution as a coupling agent to assemble nanoparticles. Aggregation-induced two-photon photoluminescence enhancement of three types assembled nanoparticles, i.e., gold@silver@gold, gold@silver, and gold nanoparticles, was studied. It was observed that the assembled core double-shell nanoparticles presented huge enhancement in two-photon photoluminescence signal in comparison with two other nanoparticles. Moreover, the gold@silver@gold nanoparticle is a stable and biocompatible plasmonic nanosystem. This paper provides a novel candidate for two-photon photoluminescence excitation sensing and imaging for biomedical applications.

     

Grating-based surface plasmon resonance detection of core-shell nanoparticle mediated DNA hybridization

In this report, we have investigated enhanced surface plasmon resonance (SPR) detection of DNA hybridization using gold core - silica shell nanoparticles in localized plasmonic fields. The plasmonic fields were localized by periodic linear gratings. Experimental results measured for hybridization of 24-mer single-stranded DNA oligomers suggest that core-shell nanoparticles (CSNPs) on gratings of 400 nm period provide enhanced optical signatures by 36 times over conventional thin film-based SPR detection. CSNP-mediated DNA hybridization produced 3 times larger angular shift compared to gold nanoparticles of the same core size. We have also analyzed the effect of structural variation. The enhancement using CSNPs was associated with increased surface area and index contrast that is combined by improved plasmon coupling with localized fields on gratings. The combined approach for conjugated measurement of a biomolecular interaction on grating structures is expected to lower the limit of detection to the order of a few tens of fg/mm(2).     

Tansy fruit mediated greener synthesis of silver and gold nanoparticles

In this paper we have reported the green synthesis of silver (AgNPs) and gold (AuNPs) nanoparticles by reduction of silver nitrate and chloroauric acid solutions, respectively, using fruit extract of Tanacetum vulgare; commonly found plant in Finland. The process for the synthesis of AgNPs and AuNPs is rapid, novel and ecofriendly. Formation of the AgNPs and AuNPs were confirmed by surface plasmon spectra using UV–Vis spectrophotometer and absorbance peaks at 452 and 546 nm. Different tansy fruit extract concentration (TFE), silver and gold ion concentration, temperature and contact times were experimented in the synthesis of AgNPs and AuNPs. The properties of prepared nanoparticles were characterized by TEM, XRD, EDX and FTIR. Finally zeta potential values at various pH were analyzed along with corresponding SPR spectra.     

Biosynthesis of fluorescent gold nanoparticles using an edible freshwater red alga,Lemanea fluviatilis (L.) C.Ag. and antioxidant activity of biomatrix loaded nanoparticles

Biosynthesis of gold nanoparticles has been accomplished via reduction of an aqueous chloroauric acid solution with the dried biomass of an edible freshwater epilithic red alga, Lemanea fluviatilis (L.) C.Ag., as both reductant and stabilizer. The synthesized nanoparticles were characterized by UV–visible, powder X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared (FT-IR), and dynamic light scattering (DLS) studies. The UV–visible spectrum of the synthesized gold nanoparticles showed the surface plasmon resonance (SPR) at around 530 nm. The powder XRD pattern furnished evidence for the formation of face-centered cubic structure of gold having average crystallite size 5.9 nm. The TEM images showed the nanoparticles to be polydispersed, nearly spherical in shape and have sizes in the range 5–15 nm. The photoluminescence spectrum of the gold nanoparticles excited at 300 nm showed blue emission at around 440 nm. Gold nanoparticles loaded within the biomatrix studied using a modified 2,2-diphenyl-1-picrylhydrazyl (DPPH) method exhibited pronounced antioxidant activity.