Kinetics of Ag300 nanoclusters formation: The catalytically effective nucleus via a steady-state approach

The kinetics of formation of silver nanoparticles consisting of nearly 300 metal atoms is investigated, which were prepared by reduction of silver nitrate with hydrazine in ethylene glycol at 25°C without any stabilizer other than the glycol solvent. The resulting sigmoidal kinetic curves are analyzed by using the 1997 Finke–Watzky two-step mechanism of slow continuous nucleation with subsequent fast autocatalytic surface growth. The kinetics of homogeneous nucleation of metal nanoparticles was analyzed using the assumption about the stepwise adjunction of precursor and the quasi steady-state approximation. The equations were proposed to calculate the concentration of the formed metal nanoparticles and their mean size from the experimentally determined values of the Finke–Watzky rate constants. It is shown that a stepwise nucleation process can be described in the terms of the catalytically effective nucleus concept and that the number of atoms in the catalytically effective nucleus can be estimated.     

The Kinetic Rate Law for the Autocatalytic Growth of Citrate‐Stabilized Silver Nanoparticles

The formation of sodium citrate stabilized spherical silver nanoparticles synthesized by homogeneous nucleation in aqua solutions was monitored by the method of direct potentiometry. It was observed that the kinetic curve of Ag⁺ ions reduction can be described by the Finke–Watzky (FW) autocatalytic two–step mechanism in the case of a large excess of hydrazine and NaOH. To expand the FW minimalistic model, the impact of the starting concentrations of the reagents on the kinetics of Ag nanoparticles synthesis was studied. It was determined that the nucleation stage is limited by the homogeneous process of ions Ag⁺ reduction, and the reaction orders for the all reagents are calculated. A surface–dependent model of Ag nanoparticle growth is proposed, which takes into account the change in the hydrazine and alkali concentrations; the rate constants of the pseudoelementary reactions are also calculated.     

Solution-phase synthesis of silver nanodiscs in HPMC-matrix and simulation of UV-vis extinction spectra using DDA based method

Present investigation demonstrates a very simple seed-mediated route, using hydroxypropyl methyl cellulose (HPMC) as stabilizing agent, for the synthesis of silver nanodiscs in aqueous solution. Central to the concept of seed-mediated growth of nanoparticles is that small nanoparticle seeds serve as nucleation centres to grow nanoparticles to a desired size and shape. It is found that the additional citrate ions in the growth solution play the pivotal role in controlling the size of silver nanodiscs. Similar to the polymers in the solution, citrate ions could be likewise dynamically adsorbed on the growing silver nanoparticles and promote the two-dimensional (2D) growth of nanoparticles. Morphological, structural, and spectral changes associated with the seed-mediated growth of the nanoparticles in the presence of HPMC are characterized using UV–vis and TEM spectroscopic studies. Metal nanoparticles have received increasing attention for their peculiar capability to control local surface plasmon resonance (LSPR) when interacting with incident light waves. Extensive simulation study of the UV–vis extinction spectra of our synthesized silver nanodiscs has been carried out using discrete dipole approximation (DDA) methodology.     

Size of elementary clusters and process period in silver nanoparticle formation

The time dependence of small-angle X-ray scattering (SAXS) curves for silver nanoparticle formation was followed in situ at a time resolution of 0.18 ms, which is 3 orders of magnitude higher than that used in previous reports (ca. 100 ms). The starting materials were silver nitrate solutions that were reacted with reducing solutions containing trisodium citrate. The SAXS analyses showed that silver nanoparticles were formed in three distinct periods from a peak diameter of ca. 0.7 nm (corresponding to the size of a Ag(13) cluster) during the nucleation and the early growth period. The Ag(13) clusters are most likely elementary clusters that agglomerate to form silver nanoparticles.     

Kinetics of channel formation in bilayer lipid membranes (BLMs) and tethered BLMs: monazomycin and melittin

The kinetics of channel formation by the polyene-like antibiotic monazomycin, both in a bilayer lipid membrane (BLM) and in a tethered BLM (tBLM), and by the peptide melittin in a tBLM, is investigated. Stepping the applied potential from a value at which channels are not formed to one at which they are formed yields current vs time curves that are sigmoidal on a BLM, while they show a maximum on a tBLM; in the latter case, sigmoidal curves are obtained by plotting the charge against time. These curves are interpreted on the basis of a general kinetic model, which accounts for the potential-dependent penetration of adsorbed monomeric molecules into the lipid bilayer, followed by their aggregation with channel formation by a mechanism of nucleation and growth. In the case of monazomycin, which is present in the solution in the form of relatively hydrophilic clusters and is adsorbed as such on top of the lipid bilayer, penetration into the bilayer following a potential jump is assumed to be preceded by a potential-independent disaggregation of the adsorbed clusters into adsorbed monomers.     

Insights into the early stages of metal nanoparticle formation via first-principle calculations: the roles of citrate and water

The early stages of silver nanoparticle formation in the presence of citrate and water have been investigated via first-principle theoretical calculations. Our study revealed that the charge density of the clusters is a key factor determining the selectivity among various growth pathways. An optimal charge density appears to control the selection between neutral and charged species in cluster growth; partially positively charged clusters are thermodynamically preferred and can serve as seeds for further growth. They interact favorably with both the solvent, leading to their solubility, and the citrate. The solvent (water) plays an important role in cluster growth both on the energetics of reactions including highly charged clusters and on the geometry of the resulting silver structures by preventing the formation of asymmetric ones (a structure directing action). Contrary to the common belief we found, from an energetic viewpoint, that growth of small clusters is not blocked by the citrate. Citrate, by acting as a reducing agent, opens up new channels for cluster growth involving highly charged species. By regulating the cluster charge, cluster-cluster associations may be promoted by the citrate, providing a new mechanistic interpretation for the effect of citrate concentration on nanoparticle size substantially different from the classic nucleation theory. From the citrate-silver and water-silver cluster interactions, linear free energy relationships have been retrieved that provide insights into metal nanoparticle growth mechanisms.     

Pore formation by 6-ketocholestanol in phospholipid monolayers and its interpretation by a general nucleation-and-growth model accounting for the sigmoidal shape of voltage-clamp curves of ion channels

6-Ketocholestanol (KC), a steroid that differs from cholesterol mainly by the presence of a carbonyl group, forms pores inside a dioleoylphosphatidylcholine monolayer self-assembled on mercury by a mechanism similar to that of channel-forming peptides and proteins. The potential steps responsible for pore formation by KC molecules give rise to potentiostatic charge vs time curves whose sigmoidal shape and potential dependence can be quantitatively interpreted on the basis of a mechanism of nucleation and growth of KC clusters. Pore formation by KC allows the penetration of thallous ions across the otherwise impermeable phosphatidylcholine monolayer, while pore disruption taking place at more negative potentials causes a drop in thallous ion permeation. Pore disruption is also accounted for by a mechanism of nucleation and growth of holes inside the KC clusters. The kinetic model of nucleation and growth is general, and accounts quantitatively for the sigmoidal shape and potential dependence of the classical Hodgkin-Huxley voltage-clamp curves of potassium channels in squid giant axon,(1) using a minimum number of free parameters.     

Modeling interband transitions in silver nanoparticle-fluoropolymer composites

The interband transition contributions to the optical properties of silver nanoparticles in fluoropolymer matrices are investigated. For the materials in this study, nanoparticle synthesis within the existing polymer matrix is accomplished using an infusion process that consists of diffusing an organometallic precursor gas into the free volume of the fluoropolymer and decomposing the precursor followed by metal nanoparticle nucleation and growth. The resulting polymer matrix nanocomposite has optical properties that are dominated by the response of the nanoparticles owing to the broadbanded transparency of the fluoropolymer matrix. The optical properties of these composites are compared to Maxwell-Garnett and Mie theory with results indicating that interband transitions excited in the silver nanoparticles affect the optical absorption over a range of frequencies including the surface plasmon resonance. It is shown that calculations of the optical absorption spectrum using published data for the silver dielectric function do not accurately describe the measured material response and that a classical model for bound and free electron behavior can best be used to represent the dielectric function of silver.     

Conformational dynamics of poly(acrylic acid). A study using surface plasmon resonance spectroscopy

The conformational dynamics of poly(acrylic acid) induced by pH change is reported here. Poly(acrylic acid) immobilized on gold surface was exposed to pH changes, and the conformational changes thus induced were followed in real time using surface plasmon resonance spectroscopy. The temporal profile of the stretching-coiling phenomenon showed a minimum point, which was proposed to be arising due to the contradictory behavior of two different property changes in the polymeric system. Normally surface plasmon resonance (SPR) response would be a convoluted effect of the thickness and refractive index changes, but the behavior observed here, where the SPR response is predominantly governed by either one of the two, is unique and to the author's knowledge is a feature that is observed for the first time. Analysis of the kinetics of the angle change revealed that it takes longer for the polymer to stretch than it takes for it to collapse, with the kinetic rate constants varying by at least an order of magnitude. The SPR angle change as well as the kinetic constants increased linearly with molecular weight. Effect of Ca2+ was studied, and it was found that the polymer was locked in its conformation due to the binding of the multivalent cations.     

Adsorption of alkylthiol-capped gold nanoparticles onto alkylthiol self-assembled monolayers: an SPR study

The kinetics of alkylthiol-capped gold nanoparticle (RS/Au-NP) adsorption to alkylthiol/Au self-assembled monolayers (RS/Au-SAMs) has been studied using SPR (surface plasmon resonance) spectroscopy. Variation of the alkylthiol chain terminus (CH3, COOH) and solvent (H2O, hexane) provides insight into the relative importance of solvation energies (in the context of surface energies) and RS/Au-NP structure on adsorption kinetics. The kinetics, fitted to the Langmuir kinetic model, yield adsorption and desorption rate constants. DeltaG(ads) derived from kinetic data are compared to calculated values of work of adhesion (W(adh)), derived from the corresponding surface energies. The absence of a deltaG(ads) - W(adh) correlation arises because the measured kinetics are not reporting the approach to equilibrium and/or because there are factors (i.e., local chain packing defects, surface hydration differences, or particle-particle interactions) not accounted for in calculated W(adh) values.     

Nanoparticle Size and Concentration Dependence of the Electroactive Phase Content and Electrical and Optical Properties of Ag/Poly(vinylidene fluoride) Composites

This paper describes the processing of silver‐nanoparticle‐doped poly(vinylidene fluoride). The effects of the concentration and size of the filler on the electroactive phase of the polymer and the optical and electrical properties are discussed. Spherical silver nanoparticles incorporated into the poly(vinylidene fluoride) polymeric matrix induce nucleation of the electroactive γ phase. The electroactive phase content strongly depends on the content and size of the nanoparticles. In particular, there is a critical nanoparticle size, below which the filler losses its nucleation efficiency due to its small size relative to that of the polymer macromolecules. Furthermore, the presence of surface plasmon resonance absorption in the composites is observed, which once again shows a strong dependence on the concentration and size of the particles. The absorption is larger for higher concentrations, and for a given concentration increases with particle size. This behavior is correlated to the electrical response and is related to the extra bands and electrons provided by the nanoparticles in the large energy band gap of the polymer.     

Detection of ferulic acid based on the plasmon resonance light scattering of silver nanoparticles

In this contribution, a plasmon resonance light scattering (PRLS) detection method of ferulic acid (FA) is proposed based on the formation of silver nanoparticles (NPs). It was found that, FA acted as a reducing agent in alkaline medium and could be oxidized by AgNO₃, resulting in the formation of silver NPs. The formed silver NPs, which were identified by measuring the plasmon resonance absorption spectra, PRLS spectra and transmission electron microscopy (TEM) image, display characteristic plasmon resonance optical absorption and PRLS band in the visible region. It was found that the PRLS intensity, which could be easily measured using a common spectrofluorometer, was in proportion to the concentration of FA over the range from 0.2 to 2.0 μmol l⁻¹ with the corresponding limits of determination (3σ) of 15.2 nmol l⁻¹. With that, ferulate sodium injection samples have been detected with R.S.D. lower than 3.0% and recoveries over the range of 101.2–104.5%. On the other hand, the present reaction maybe provides the basis of an environmentally friendly approach for the synthesization of silver NPs.     

Effect of Poly(Vinyl Alcohol) on the Size,Shape, and Rate of Silver Nanoparticles Formation

The kinetics of cetyltrimethylammonium bromide (CTAB) stabilized silver nanoparticles have been studied spectrophotometrically at 425 nm (λ_max of silver sol) in the absence and presence of water soluble polymer (poly(vinyl alcohol); PVA). Transmission electron microscopy (TEM), ultraviolet-visible spectroscopy, and viscosity measurements were used to determine the size, shape, and the size distribution of the silver nanoparticles. The reaction follows the same behavior with respect to [CTAB], [tri-sodium citrate], and [Ag⁺] in both the media indicating the silver nonoparticles were formed through the same reaction path. The sigmoid nature of the kinetic curves suggests an autocatalytic path in the growth of nanoparticles. The reaction rate is increased by increasing [CTAB]. The presence of PVA inhibits nucleation and retards the rate of particle growth, absorbance and size of the particles. Polymer-surfactant interactions were analyzed based on the viscosity of the reaction mixture.     

Comparative study of the morphology, aggregation, adherence to glass, and surface-enhanced Raman scattering activity of silver nanoparticles prepared by chemical reduction of Ag+ using citrate and hydroxylamine

Two different silver colloids were prepared by chemical reduction of silver nitrate with trisodium citrate and hydroxylamine hydrochloride to compare their characteristics in relation to their possible use in surface-enhanced Raman scattering (SERS) spectroscopy. The morphology and plasmon resonance of the single nanoparticles and aggregates integrating these colloids were characterized by means of UV-vis absortion spectroscopy and scanning electron microscopy, revealing important differences between each type of nanoparticle as concerns their physical properties. These metallic systems also manifested differences in the aggregation and the adherence to glass surfaces, revealing significant differences in the chemical surface properties of these nanoparticles. SERS and surface-enhanced IR also indicated the presence of interference bands which can overlap the spectra of the analyte, mainly in the case of the citrate colloid. All these differences have an important influence on the applicability of these nanostructured systems in SERS. In fact, the enhancement factor and spectral pattern of the SERS obtained by using alizarin as a molecule probe are different.     

Nucleation and growth of silver nanoparticles monitored by titration microcalorimetry

The nucleation and the growth of silver nanoparticles were studied by spectrophotometry and transmission electron microscopy, with simultaneous recording of the concomitant enthalpy changes. Silver nanoparticles were stabilized by sodium citrate and reduction was brought about by the addition of hydroquinone in aqueous medium. It was established that nucleation is an exothermic process and heat effects are basically determined by the ratio of silver ions to hydroquinone. The process of nanoparticle formation was divided to three phases: the nucleation phase is exothermic, the growth phase is endothermic and further addition of the reducing agent results in the aggregation of silver nanoparticles, which produces a second exothermic heat effect.     

The kinetics of homogeneous and two-step nucleation during protein crystal growth from solution

Many experimental reports for the kinetics of crystal nucleation and growth, from an isothermal solution, point to a sigmoidal-like behavior for the process. Here we consider three different nucleation models from the literature and show that all lead to sigmoidal or sigmoidal-like behavior for the kinetics of nucleation. A two-step nucleation process is known to occur within certain supersaturated protein solutions, and it is demonstrated in this report how the sigmoidal law yields kinetic information for the two-step and homogeneous nucleation modes. We propose here that two-step solute-rich associates form in the solution around seed nuclei that are already present at or near the point in time when the solution is prepared. Using this hypothesis, we are able to model the time-dependent volume of the two-step phase per unit volume of solution and show that this compares well with reported experimental data. A kinetic model is given for the proposed process, which leads to a sigmoidal rate law. Additionally, a relation between the initial and final nuclei densities and the induction time is derived. As a result of this study, the conclusion is that two-step activity increases with increasing initial supersaturation or increasing salt concentration.     

Ag@SiO2纳米颗粒的制备及其在H2O2检测上的应用

利用柠檬酸三钠还原硝酸银制备了银纳米颗粒(AgNPs),然后通过氨水水解正硅酸乙酯(TEOS)的方法,在AgNPs上沉积SiO2,制备出以Ag为核,SiO2为壳的复合纳米颗粒(Ag@SiO2).调节TEOS用量,可以控制SiO2层的厚度.根据AgNPs的局域表面等离激元共振(LSPR)效应,将制得的Ag@SiO2颗粒用于H2O2的检测,检测下限为1μmol/L,并可以通过控制SiO2层的厚度方便地调节Ag@SiO2颗粒与H2O2反应的速率.与传统方法相比,具有简单、快速、成本低的优点.分别运用TEM、紫外-可见分光光度计对反应前后Ag@SiO2颗粒形貌及反应过程中其LSPR吸收的变化进行了表征.     

Multicomponent dynamical nucleation theory and sensitivity analysis

Vapor to liquid multicomponent nucleation is a dynamical process governed by a delicate interplay between condensation and evaporation. Since the population of the vapor phase is dominated by monomers at reasonable supersaturations, the formation of clusters is governed by monomer association and dissociation reactions. Although there is no intrinsic barrier in the interaction potential along the minimum energy path for the association process, the formation of a cluster is impeded by a free energy barrier. Dynamical nucleation theory provides a framework in which equilibrium evaporation rate constants can be calculated and the corresponding condensation rate constants determined from detailed balance. The nucleation rate can then be obtained by solving the kinetic equations. The rate constants governing the multistep kinetics of multicomponent nucleation including sensitivity analysis and the potential influence of contaminants will be presented and discussed.     

Stimulatory and inhibitory effects of alkyl bromide surfactants on beta-amyloid fibrillogenesis

Beta-amyloid peptide (Abeta), in fibrillar form, is the primary constituent of senile plaques, a defining feature of Alzheimer's disease. In solution assays, fibril formation exhibits a lag time, interpreted as a nucleation/condensation-dependent process. The kinetics of fibrillogenesis is controlled by two key parameters: nucleation and elongation rate constants. We characterized the time course of Abeta fibril formation by measuring the scattering caused by peptide aggregates. We report here the interaction of Abeta with three alkylammonium bromides (dodecyl, tetradecyl, and hexadecyl) at supra- and submicellar concentrations and their influence on the kinetic constants. We observed a dual behavior: surfactants promoted or retarded fibril formation in a concentration-dependent manner. Below a determined surfactant concentration (close to the corresponding critical micellar concentration in medium without peptide), surfactants favor aggregation, presumably by means of electrostatic interactions that destabilize the native conformation. Beyond such concentration, the stabilizing effects of the monomer predominate. As a general rule, surfactants delay but do not completely inhibit aggregation.     

Controlled interlinking of Au and Ag nanoclusters using 4-aminothiophenol as molecular interconnects

This work describes the formation of interlinked gold and silver nanoclusters at controlled pH using 4-aminothiophenol (ATP) as a molecular interconnect. UV-visible spectra give on intercrystal plasmon resonance band in the region 550-580 nm. The crystalline heteroassembly formation is also evident from the transmission electron microscopic (TEM) images, whereas X-ray photoelectron spectroscopic (XPS) analysis of the aggregates shows the presence of charged -N species, indicating electrostatic interaction of -N with Ag nanoclusters. Furthermore, electrochemical studies of these heteroassembled systems suggest that silver nanoclusters are not fully passivated by the monolayers of ATP and are accessible for redox reactions.