Photochemical and photoelectrochemical behaviour of chlorophyll a-smectite conjugate

Chlorophyll a was adsorbed onto smectite to form chlorophyll-smectite conjugate, which became photostable against light illumination. The chlorophyll-smectite conjugate caused the photoreduction of nitro blue tetrazolium, NBT, through the formation of superoxide anion, and the conjugate retained the photosensitive activity even after 20 hour illumination. Furthermore, the conjugate deposited on SnO₂ electrode caused electron transfer under light illumination. The anodic photocurrent rose at ?200 mV versus Ag/AgCl electrode (KCl saturated) and reached a maximum level at approximately +100 mV. The photocurrent spectrum was in good agreement with the absorption spectrum of chlorophyll-smectite conjugate in an aqueous solution and the quantum efficiency was approximately 1% at 670 nm.     

Intense electrochemical sers signal following hydrogen evolution

Stepping the potential of the electrode to the region of rapid hydrogen evolution prior to performing an oxidation—reduction cycle (ORC) can result in the development of surface-enhanced Raman scattering that is more intense by almost an order of magnitude than an electrode subjected to ORC alone. It is found for the case of chloride adsorbed on silver that the spectral characteristics of the metal-surface—chloride stretching mode at ≈240 cm^?1 are essentially the same as that produced without prior hydrogen evolution. The additional enhancement appears to be due to morphological changes caused by the presence of hydrogen bubbles during the reduction of the silver-chloride phase film.     

银－金红石复合纳米微晶的光谱性能

λ>330nm光照射含有金红石型TiO2微晶颗粒的AgNO3溶液,制备出了Ag-TiO2复合超微粒子,与纯的银溶胶相比,复合粒子上银的等离子体吸收峰宽化红移,位于400～600nm。这种复合超微粒子的溶胶,表现出表面增强拉曼散射(SERS)效应。吸附-共振增强可用来解释Ag-TiO2复合粒子的SERS效应。     

利用多孔阳极氧化铝研究载体孔洞尺寸对负载银粒子团聚的影响

 利用阳极氧化方法制备了具有规整的可控孔洞尺寸的多孔Al2O3 膜,并以此模拟实际的催化剂载体制备了负载银催化剂. 采用扫描电镜、能量分散谱、透射电镜、X射线衍射和X射线光电子能谱等手段,研究了多孔阳极氧化铝的孔洞大小对负载的银粒子团聚的影响. 结果表明,载体孔洞尺寸对银粒子团聚可能起到限制作用,而且这种限制作用随载体孔洞尺寸增大而减小. 当载体的孔洞尺寸约为50 nm时,随温度升高银粒子的团聚和生长都不明显; 当载体的孔洞尺寸约为200 nm时,随温度升高银粒子发生一定程度的团聚和生长,但孔洞尺寸的限制作用仍存在. 这种载体尺寸的限制作用可以有效地阻止催化剂活性组分的团聚.     

Plasmonic field enhancement of individual nanoparticles by correlated scanning and photoemission electron microscopy

We present results of a combined two-photon photoemission and scanning electron microscopy investigation to determine the electromagnetic enhancement factors of silver-coated spherical nanoparticles deposited on an atomically flat mica substrate. Femtosecond laser excitation of the nanoparticles produces intense photoemission, attributed to near-resonant excitation of localized surface plasmons. Enhancement factors are determined by comparing the respective two-photon photoemission yields measured for single nanoparticles and the surrounding flat surface. For p-polarized, 400 nm (～3.1 eV) femtosecond radiation, a distribution of enhancement factors is found with a large percentage (67%) of the nanoparticles falling within a median range. A correlated scanning electron microscopy analysis demonstrated that the nanoparticles typifying the median of the distribution are characterized by spherical shapes and relatively smooth silver film morphologies. In contrast, the largest enhancement factors were produced by a small percentage (7%) of particles that displayed silver coating defects that altered the overall particle structure. Comparisons are made between the experimentally measured enhancement factors and previously reported calculations of the localized near-field enhancement for isolated silver nanoparticles.     

Synthesis of highly stable silver nanoparticles using imidazolium-based ionic liquid

Highly stable, aqueous dispersions, and hydrophilic ionic liquid-capped silver nanoparticles with positive surface charge were synthesized by in situ reduction of AgNO₃ with NaBH₄ in the presence of an imidazolium-based ionic liquid, viz., 1-dodecyl-3-methylimidazolium chloride ([C₁₂mim][Cl]) at room temperature. Prepared silver nanoparticles were characterized by UV–vis spectra, transmission electron microscopy (TEM), and zeta potential. UV–visible spectrum of the aqueous medium peaked at 407 nm corresponding to the plasmon absorbance of silver nanoparticles. TEM analysis revealed the spherical shape of the particles with sizes about 9 nm and low polydispersed. The surface charge of the synthesized silver nanoparticles was determined as +5.0 mV. The ionic liquid ([C₁₂mim][Cl]) capped silver nanoparticles were stable for at least 8 months.     

Tuning the properties of a black TiO2-Ag visible light photocatalyst produced by a rapid one-pot chemical reduction

A highly efficient black TiO₂-Ag photocatalytic nanocomposite, active under both UV and visible light illumination, was synthesized by decorating the surface of 25 nm TiO₂ particles with Ag nanoparticles. The material was obtained via a rapid, one-pot, simple (surfactant and complexing agent free) chemical reduction method using silver nitrate and formaldehyde as a metal salt and reducing agent, respectively. The nanocomposite shows an increase of over 800% in the rate of photocatalytic methylene blue dye degradation, compared to commercial unmodified TiO_2, under UV-VIS illumination. Unlike pure TiO₂, the nanocomposite exhibits visible light activation, with a corresponding drop in optical reflectance from 100% to less than 10%. The photocatalytic properties were shown to be strongly enhanced by post-reduction annealing heat treatments in air, which were observed to decrease, rather than coarsen, silver particle size, and increase particle distribution. This, accompanied by a variation in the silver surface oxidation states, appear to dramatically affect the photocatalytic efficiency under both UV and visible light. This highly active photocatalyst could have wide ranging applications in water and air pollution remediation and solar fuel production.     

Synthesis and surface chemistry of nano silver particles

In this report, we present a simple wet chemical route to synthesize nano-sized silver particles, and their surface properties are discussed in detail. Silver nano particles of the size 40–80 nm are formed in the process of oxidation of glucose to gluconic acid by amine in the presence of silver nitrate, and the gluconic acid caps the nano silver particle. The presence of gluconic acid on the surface of nano silver particles was confirmed by XPS and FTIR studies. As the nano silver particle is encapsulated by gluconic acid, there was no surface oxidation, as confirmed by XPS studies. The nano silver particles have also been studied for their formation, structure, morphology and size using UV–Visible spectroscopy, XRD and SEM. Further, the antibacterial properties of these nano particles show promising results for E. Coli. The influence of the alkaline medium towards the particle size and yield was also studied by measuring the pH of the reaction for DEA, NaOH and Na₂CO₃.     

银纳米粒子自组装结构的光谱性质研究

采用自组装方法在玻璃基底表面上构筑了银纳米粒子二维亚单层结构, 进而以对巯基苯胺为耦联分子进行银粒子的二次组装, 构成具有分子尺寸间隙的银粒子簇. 银粒子表面等离子体共振依赖于粒子间距、表面吸附分子和粒子组装方式. 同层内的银粒子相互间电磁偶合可导致银粒子偶极子等离子体共振蓝移; 对巯基苯胺的吸附则使得表面等离子体共振红移. 表面增强拉曼光谱结果表明, 具有分子尺寸间隙的银粒子簇对耦联于粒子间的对巯基苯胺分子的拉曼散射具有极大的增强效应, 同时也使耦联的对巯基苯胺与银粒子间产生更大程度的电荷转移.     

The effects of Au aggregate morphology on surface-enhanced Raman scattering enhancement

We have identified empirically a relationship between the surface morphology of small individual aggregates (<100 Au nanoparticles) and surface-enhanced Raman scattering (SERS) enhancement. We have found that multilayer aggregates generated greater SERS enhancement than aggregates limited to two-dimensional (2D) or one-dimensional structures, independent of the number of particles. SERS intensity was measured using the 730 cm(-1) vibrational mode of the adsorbed adenine molecule on 75 nm Au particles, at an excitation wavelength of 632.8 nm. To gain insight into these relationships and its mechanism, we developed a qualitative model that considers the collections of interacting Au nanoparticles of an individual aggregate as a continuous single entity that retains its salient features. We found the dimensions of the modeled surface features to be comparable with those found in rough metal surfaces, known to sustain surface plasmon resonance and generate strong SERS enhancement. Among the aggregates that we have characterized, a three 75 nm nanoparticle system was the smallest to generate strong SERS enhancement. However, we also identified single individual Au nanoparticles as SERS active at the same wavelength, but with a diameter twice in size. For example, we observed a symmetric SERS-active particle of 180 nm in diameter. Such individual nanoparticles generated SERS enhancement on the same order of magnitude as the small monolayer Au aggregates, an intensity value significantly stronger than predicted in recent theoretical studies. We also found that an aspect of our model that relates the dimensions of its features to SERS enhancement is also applicable to single individual Au particles. We conclude that the size of the nanoparticle itself, or the size of a protrusion of an irregularly shaped single Au particle, will contribute to SERS enhancement provided that its dimensions satisfy the conditions for plasmon resonance. In addition, by considering the ratio of the generated intensities of typical 2D Au aggregates to the enhancement of individual SERS-active particles, a value of approximately 2 is determined. Its moderate value suggests that it is not the aggregation effect that is responsible for much of the observed SERS enhancement but the surface region associated with the SERS-active site.     

High density silver nanoparticle monolayers produced by colloid self-assembly on polyelectrolyte supporting layers

A stable silver nanoparticle suspension was synthesized via the reduction of silver nitrate using sodium borohydride and sodium citrate. The particle's shape and size distribution were measured by various methods. The electrophoretic mobility measurements revealed that the zeta potential of particles was highly negative, increasing slightly with the ionic strength, from -52 mV for I=10(-5) M to -35 mV for I=3×10(-2) M (for pH=5.5). The zeta potential of mica modified by the adsorption of cationic polyelectrolytes: PEI and PAH was also determined using the streaming potential measurements. The modified mica sheets were used as substrates for particle monolayers formed via colloid self assembly. The kinetics of this process, proceeding under diffusion-controlled transport conditions, was quantitatively evaluated by a direct enumeration of particles using the AFM and SEM techniques. Both the kinetics of particle deposition and the maximum surface concentration were determined. From the slope of the initial deposition rates, the equivalent diameter of particles was determined to be 16 nm, in agreement with previous measurements. Based on this finding, an efficient method of determining particle size in suspension was proposed. It was also demonstrated that for higher ionic strengths, the maximum coverage of particle monolayers on PAH modified mica exceeded 0.39. The kinetic data were quantitatively interpreted in terms of the random sequential adsorption (RSA) model using the effective hard particle concept.     

Synthesis and applications of silver nanoparticles

Over the past few decades, nanoparticles of noble metals such as silver exhibited significantly distinct physical, chemical and biological properties from their bulk counterparts. Nano-size particles of less than 100 nm in diameter are currently attracting increasing attention for the wide range of new applications in various fields of industry. Such powders can exhibit properties that differ substantially from those of bulk materials, as a result of small particle dimension, high surface area, quantum confinement and other effects. Most of the unique properties of nanoparticles require not only the particles to be of nano-sized, but also the particles be dispersed without agglomeration. Discoveries in the past decade have clearly demonstrated that the electromagnetic, optical and catalytic properties of silver nanoparticles are strongly influenced by shape, size and size distribution, which are often varied by varying the synthetic methods, reducing agents and stabilizers. Accordingly, this review presents different methods of preparation silver nanoparticles and application of these nanoparticles in different fields.     

Labeled gold nanoparticles immobilized at smooth metallic substrates: systematic investigation of surface plasmon resonance and surface-enhanced Raman scattering

This paper experimentally and theoretically investigates the influence of an underlying metallic substrate (i.e., gold and silver) on the surface plasmon resonance (SPR) of labeled gold nanoparticles and the concomitant impact on the surface-enhanced Raman scattering (SERS) signal from the labels. These experiments employ nanoparticles of varied sizes (30-100 nm) that are coated with a bifunctional Raman scatterer composed of (1) a disulfide for chemisorption to the nanoparticle surface, (2) a succinimidyl ester for formation of a covalent linkage to an amine-terminated self-assembled monolayer on the underlying substrate, and (3) an aryl nitro group with an intrinsically strong Raman active vibrational mode. This approach allows facile systematic assessments of how variations in nanoparticle size, substrate composition, and the gap between the nanoparticle and substrate affect the SPR of the bound particles. Both UV-vis transmission and reflection absorption (incident angle of 58 degrees ) spectroscopy are used to characterize the effect of each of these parameters on SPR. These results are then correlated with SERS enhancement factors (EFs) that were determined by accounting for particle surface concentrations, which were measured by atomic force microscopy, and the absolute number of labels, which were calculated on the basis of the surface area of each of the different-sized particles. All SERS spectra were collected at an incident angle of 58 degrees with respect to the surface normal. As expected, the SPR for particles in solution red-shifts with increasing particle size. More importantly, the SPR moves to even longer wavelengths as the size of immobilized particles increases and as the gap between the immobilized particle and substrate decreases. The red shift is also greater for a gold nanoparticle tethered to a gold substrate compared to a silver substrate. A theoretical model for the extinction of a particle above a flat substrate, corrected for surface scattering, radiation damping, and dynamic depolarization, is also briefly detailed. SPR results calculated with the model are consistent with the shifts observed in the SPR position for each of the manipulated experimental variables. The largest EFs are found for samples with an SPR maximum (lambda(max)) between the wavelengths for laser excitation (633 nm) and the Raman band for the symmetric nitro stretch of the particle coating (690 nm). As an example, an order of magnitude in the SERS enhancement factor is gained for a 60-nm particle immobilized 1.2 nm above a gold substrate (SPR lambda(max) = 657 nm) compared to that for a 30-nm particle (SPR lambda(max) = 596 nm).     

Synthesis and optical properties of poly(N-isopropylacrylamide) nanogel containing silver nanoparticles

Composite nanoparticles representing silver nanoparticle-containing polymer gels have been synthesized. The synthesis comprises two main stages. Initially, monodisperse hydrogel particles with a controlled diameter of approximately 500 nm are obtained by N-isopropylacrylamide polymerization. Then, silver ions are reduced on the surface of the polymer network. Variations in the concentration ratio between reductants and silver nitrate make it possible to produce silver nanoparticles with sizes in a range of 10–30 nm and different packing densities on the gel particle surface. The resultant nanocomposites have been studied by transmission electron microscopy, spectrophotometry, and dynamic light scattering. Depending on the size and packing density of the silver nanoparticles on the polymer particle surface, the plasmon resonance of the nanocomposites varies in a range of 420–750 nm, which determines variations in the color of the colloid from yellow, orange, and red to blue and blue-green. After the inclusion of silver nanoparticles, nanogels of poly(N-isopropylacrylamide) retain their capability for thermosensitive phase transition with a lower critical mixing temperature of 31°C.     

Influence of interfacial water layer on surface properties of silver halides: effect of pH on the isoelectric point

The hypothesis that pH dependent charge of interfacial water affects electrokinetic charge and electrokinetic potential of hydrophobic colloids, but not the (inner) surface potential was tested. It was found that isoelectric points of silver chloride, bromide and iodide shift to the higher pAg values in the acidic solutions, but that surface potential did not depend on pH. Isoelectric points of water at inert surfaces lie in the range 2相似文献   

Surface Enhanced Raman Spectroscopy Using Silver Nanoparticles: The Effects of Particle Size and Halide Ions on Aggregation

A surface enhanced Raman spectroscopy (SERS) investigation of the aggregation of silver nanoparticles formed via LVCC with diameters in the range 5–50 nm were studied. It was found that with 647.1 nm excitation maximum enhancement is observed using particles with 11 nm diameters. Upon addition of sodium halides, enhancement is proportional to the polarizability of the anion. Maximum enhancement was observed when the concentration of the anion is approximately equal to the concentration of the adsorbate.     

Electromagnetic interactions in plasmonic nanoparticle arrays

Single two-dimensional planar silver arrays and one-dimensional linear gold chains of nanoparticles were investigated by dark-field surface plasmon spectroscopy and studied as a function of interparticle distance, particle size, and number of particles. In agreement with recent theoretical predictions, a red shift of the surface plasmon resonance occurring in two-dimensional arrays was found for lattice spacings below 200 nm. This red shift is associated with a significant broadening of the resonance and is attributed to the onset of near-field interactions. We found that the relative contributions of the long-range and short-range interactions in two-dimensional arrays of particles are fundamentally different to those occurring in individual linear chains.     

Chemical effects in surface-enhanced raman scattering: pyridine chemisorbed on silver adatoms on Rh (100)

Surface-enhanced Raman scattering (SERS) has been observed from pyridine chemisorbed on silver adatoms which had been deposited at submonolayer coverage on a Rh (100) substrate under ultrahigh vacuum. The vibrational frequencies measured are characteristic of pyridine chemically adsorbed on the silver adatoms and an enhancement factor of ≈ 15–65 is calculated from the intensity of the scattering. This enhancement factor is in addition to the factor of four electromagnetic enhancement found for flat metal surfaces and therefore provides unambiguous evidence for the existence and magnitude of chemical enhancement in SERS.     

Electromagnetic fields around silver nanoparticles and dimers

We use the discrete dipole approximation to investigate the electromagnetic fields induced by optical excitation of localized surface plasmon resonances of silver nanoparticles, including monomers and dimers, with emphasis on what size, shape, and arrangement leads to the largest local electric field (E-field) enhancement near the particle surfaces. The results are used to determine what conditions are most favorable for producing enhancements large enough to observe single molecule surface enhanced Raman spectroscopy. Most of the calculations refer to triangular prisms, which exhibit distinct dipole and quadrupole resonances that can easily be controlled by varying particle size. In addition, for the dimer calculations we study the influence of dimer separation and orientation, especially for dimers that are separated by a few nanometers. We find that the largest /E/2 values for dimers are about a factor of 10 larger than those for all the monomers examined. For all particles and particle orientations, the plasmon resonances which lead to the largest E-fields are those with the longest wavelength dipolar excitation. The spacing of the particles in the dimer plays a crucial role, and we find that the spacing needed to achieve a given /E/2 is proportional to nanoparticle size for particles below 100 nm in size. Particle shape and curvature are of lesser importance, with a head to tail configuration of two triangles giving enhanced fields comparable to head to head, or rounded head to tail. The largest /E/2 values we have calculated for spacings of 2 nm or more is approximately 10(5).     

Extracellular synthesis of silver bionanoparticles from Aspergillus clavatus and its antimicrobial activity against MRSA and MRSE

New enzymatic approaches using bacteria and fungi for the synthesis of nanoparticles in both intra- and extracellular are playing an advanced key role in pharmacotherapeutics. In the present study we have reported on the use of fungus Aspergillus clavatus for the extracellular synthesis of bionanoparticles from silver nitrate (AgNO₃) solution. The bionanoscale particles were characterized by UV–visible spectroscopy, thin layer chromatography, atomic force microscopy (AFM) and FTIR. The synthesized bionanoscale particle showed a maximum absorption in the visible region of 420 nm. The AFM study of bionanoscale particle ranged in the size of 550–650 nm. The analysis was carried out by TLC and FTIR to identify the biomolecules responsible for the bioreduction of silver ion and capping of the bioreduced silver nanoparticles. The present study analyzes the antimicrobial activity of the silver nanoparticles synthesized from A. clavatus against MRSA and MRSE, which showed the maximum activity against MRSA, followed by MRSE.