Aggregation Kinetics Of Silver Nanoparticles Research Articles

Silver nanoparticles aggregative behavior at low concentrations in aqueous solutions

This study describes aggregation kinetics of silver nanoparticles (AgNPs) and particle size distribution (PSD) of silver aggregates at concentrations ranging from 10 mgAg.L−1 to 10 μgAg.L−1. Aggregation kinetics were conducted at different NaCl concentrations for 2 h and at three environmentally NaCl concentrations (9, 86 and 428 mM) for 12 h to determine the critical coagulation concentration (CCC) and PSD. At mgAg.L−1 level, CCC was near 70 mM, and near 150 mM at μgAg.L−1 level. Furthermore, PSD was strongly affected by AgNPs concentration. At 10 and 1 mgAg.L−1, more than 80 % of aggregates had a hydrodynamic diameter (Dh) higher than 200 nm after 2 h, and more than 95 % after 12 h. At 100 and 10 μgAg.L−1, the main aggregate population had a Dh smaller than 200 nm. At mgAg.L−1 level, particle concentration remained high enough to promote interactions between AgNPs and aggregates, favoring the formation of large aggregates, although it was not the case at μgAg.L−1 level. Moreover, at 106 particles. mL−1, AgNPs aggregation was strongly reduced and could cause the presence of a plateau in aggregation kinetics. Evolution of silver aggregates PSD may increase their residence time in the water column and promotes their exportation to the ocean.

Colloidal stability of silver nanoparticles with layer-by-layer shell of chitosan copolymers

The influence of polymeric shell formed by the layer-by-layer (LbL) assembly technique using chitosan or its grafted copolymers with dextran or poly(ethylene glycol) and dextran sulfate on physico-chemical properties and aggregation kinetics of silver nanoparticles (AgNP) was investigated. It is shown that the hydrodynamic diameter of encapsulated AgNP increases linearly with increasing the number of polyelectrolyte layers without sufficient aggregation. The critical concentrations of coagulation (CCC) and Hamaker constants (H) were determined in dependence on electrolyte charge, structure of polycation, and number of bilayers in the shells. While a shell consists of more than two bilayers, the values of critical electrolyte concentration and energy of interparticle interactions H are influenced mainly by copolymer structure. In all cases, the CCC value is more than doubled after formation of two or more bilayers over silver nanoparticle surface. The shells based on chitosan-poly(ethylene glycol) graft copolymer were proven to be the most effective in increasing aggregation stability in NaCl solutions, while chitosan-dextran graft copolymer effectively decreases van der Waals interaction between nanoparticles. Application of grafted copolymers allows one to obtain encapsulated colloids that are stable over a wide range of NaCl concentrations including the isotonic one and improve biocompatibility and targetability of nanovehicles engineered for therapeutic or imaging purposes.

Impact of pH on the stability, dissolution and aggregation kinetics of silver nanoparticles

Widespread usage of silver nanoparticles (AgNPs) in consumer products has resulted in their presence in the aquatic environment. The evolution of the properties of AgNPs with changes in pH and time in terms of colloidal stability, dissolution and aggregation were investigated in a series of short and long-term experiments using freshly synthesized uncoated AgNPs. The solution pH modifies the surface charge and the oxidative dissolution of AgNPs. As a result, the particle behavior varied in acidic and alkaline conditions. The particle size decreased with the increasing pH at a given time frame resulting in lower aggregation in the higher pH regime and increased particle stability. These results have been further proved with the direct evidence obtained using time resolved in situ imaging acquired through Liquid cell transmission electron microscopy (LCTEM). Furthermore, the magnitude of the impact of the pH on the particle properties is higher than the impact of the dissolved oxygen concentration. The derived empirical formulae reflect that the AgNP oxidation depends on both dissolved oxygen and protons while the AgNP dissolution increasing with the increase of either of these. Overall, our results highlight the impact of the solution pH on the evolution of the properties of AgNPs over the time and provide an insight to confidently extend the results to predict the environmental transformation of AgNPs from ideal systems to the real.

Stability and Aggregation Kinetics of Silver Nanoparticles in Water in Oil Microemulsions of Cetyltrimethylammonium Bromide and Triton X-100

Water in oil (W/O) microemulsions are simple preparative route for nanoparticles where water droplets dispersed in oil stabilized by surfactant or surfactant and cosurfactant monolayer act as nanoreactors to carry out chemical reactions. In this work, silver nanoparticles (AgNPs) were prepared in W/O microemulsions of cetyltrimethylammonium bromide (CTAB) and triton X-100 (TX-100) by using AgNO3 and NaBH4 as a precursor salt and reducing agent, respectively. To prepare microemulsions, CTAB or TX-100, 1-pentanol, cyclohexane and water were mixed with different molar ratio. AgNPs were prepared with different [AgNO3] in microemulsions of CTAB with fixed water to surfactant ratio (Wo). Average particle sizes were determined from dynamic light scattering (DLS) measurements. AgNPs prepared from microemulsions of CTAB were unstable while from TX-100, NPs were stable. Aggregation kinetics was investigated by measuring the absorbance at definite time intervals at the absorption maximum, ?max of AgNPs in different media under pseudo-first-order conditions. The aggregation behavior was studied at different [AgNO3]:[NaBH4] and Wo and the parameters were optimized to ensure formation of stable AgNPs without aggregation in microemulsions. This would help tuning the size, stability, and aggregation kinetics of AgNPs by controlling the nature of the surfactant and composition of the microemulsions.

Aggregation Kinetics of SERS-Active Nanoparticles in Thermally Stirred Sessile Droplets

The aggregation kinetics of silver nanoparticles in sessile droplets were investigated both experimentally and through numerical simulations as a function of temperature gradient and evaporation rate, in order to determine the hydrodynamic and aggregation parameters that lead to optimal surface-enhanced Raman spectroscopic (SERS) detection. Thermal gradients promote effective stirring within the droplet. The aggregation reaction ceases when the solvent evaporates forming a circular stain consisting of a high concentration of silver nanoparticle aggregates, which can be interrogated by SERS leading to analyte detection and identification. We introduce the aggregation parameter, Γa ≡ τ(evap)/τ(a), which is the ratio of the evaporation to the aggregation time scales. For a well-stirred droplet, the optimal condition for SERS detection was found to be Γ(a,opt) = kc(NP)τ(evap) ≈ 0.3, which is a product of the dimerization rate constant (k), the concentration of nanoparticles (cNP), and the droplet evaporation time (τ(evap)). Near maximal signal (over 50% of maximum value) is observed over a wide range of aggregation parameters 0.05 < Γa < 1.25, which also defines the time window during which trace analytes can be easily measured. The results of the simulation were in very good agreement with experimentally acquired SERS spectra using gas-phase 1,4-benzenedithiol as a model analyte.

The impact of stabilization mechanism on the aggregation kinetics of silver nanoparticles

The use of silver nanoparticles (AgNPs) for various applications is growing drastically. The increase in use will eventually lead to their release into the environment. The tendency of AgNPs to aggregate and the kinetics of aggregation are major factors that govern their fate in the environment. Dynamic light scattering (DLS) was utilized to investigate the electrolyte-induced aggregation kinetics (NaNO3, NaCl and Ca(NO3)2) of coated and uncoated AgNPs which are electrostatically (H2-AgNPs and Citrate-AgNPs), sterically (polyvinylpyrrolidone (PVP)-AgNPs) and electrosterically (branched polyethyleneimine (BPEI)-AgNPs) stabilized. The aggregation kinetics of the electrostatically stabilized AgNPs was in agreement with the classical Derjaguin–Landau–Verwey–Overbeek (DLVO) theory and the AgNPs exhibited both reaction-limited and diffusion-limited regimes. The H2-AgNPs had critical coagulation concentrations (CCC) of 25, 30 and 3mM in the presence of NaNO3, NaCl and Ca(NO3)2 salts, respectively. The Citrate-AgNPs had CCC of 70, 70 and 5mM in the presence of NaNO3, NaCl and Ca(NO3)2 salts, respectively. The values of the Hamaker constant for the electrostatically stabilized AgNPs were also determined and the values were in agreement with the reported values for metallic particles. The aggregation kinetics for both the sterically and electrosterically stabilized AgNPs (PVP-AgNPs and BPEI-AgNPs) was not in agreement with the DLVO theory and the particles were resistant to aggregation even at high ionic strength and electrolyte valence. The PVP-AgNPs and the BPEI-AgNPs had no critical aggregation concentration value at the investigated ionic strength values.

Aggregation Kinetics of Citrate and Polyvinylpyrrolidone Coated Silver Nanoparticles in Monovalent and Divalent Electrolyte Solutions

The aggregation kinetics of silver nanoparticles (AgNPs) that were coated with two commonly used capping agents-citrate and polyvinylpyrrolidone (PVP)--were investigated. Time-resolved dynamic light scattering (DLS) was employed to measure the aggregation kinetics of the AgNPs over a range of monovalent and divalent electrolyte concentrations. The aggregation behavior of citrate-coated AgNPs in NaCl was in excellent agreement with the predictions based on Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, and the Hamaker constant of citrate-coated AgNPs in aqueous solutions was derived to be 3.7 × 10(-20) J. Divalent electrolytes were more efficient in destabilizing the citrate-coated AgNPs, as indicated by the considerably lower critical coagulation concentrations (2.1 mM CaCl(2) and 2.7 mM MgCl(2) vs 47.6 mM NaCl). The PVP-coated AgNPs were significantly more stable than citrate-coated AgNPs in both NaCl and CaCl(2), which is likely due to steric repulsion imparted by the large, noncharged polymers. The addition of humic acid resulted in the adsorption of the macromolecules on both citrate- and PVP-coated AgNPs. The adsorption of humic acid induced additional electrosteric repulsion that elevated the stability of both nanoparticles in suspensions containing NaCl or low concentrations of CaCl(2). Conversely, enhanced aggregation occurred for both nanoparticles at high CaCl(2) concentrations due to interparticle bridging by humic acid aggregates.

Dissolution-Accompanied Aggregation Kinetics of Silver Nanoparticles

Bare silver nanoparticles with diameters of 82 ± 1.3 nm were synthesized by the reduction of the Ag(NH(3))(2)(+) complex with D-maltose, and their morphology, crystalline structure, UV-vis spectrum, and electrophoretic mobilities were determined. Dynamic light scattering was employed to assess early stage aggregation kinetics by measuring the change in the average hydrodynamic diameter of the nanoparticles with time over a range of electrolyte types (NaCl, NaNO(3), and CaCl(2)) and concentrations. From this the critical coagulation concentration values were identified as 30, 40, and 2 mM for NaNO(3), NaCl, and CaCl(2), respectively. Although the silver nanoparticles were observed to dissolve in all three electrolyte solutions, the aggregation results were still consistent with classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. The dissolution of the silver nanoparticles, which were coated with a layer of Ag(2)O, was highly dependent on the electrolyte type and concentration. In systems with Cl(-) a secondary precipitate, likely AgCl, also formed and produced a coating layer that incorporated the silver nanoparticles. Aggregation of the silver nanoparticles was also examined in the presence of Nordic aquatic fulvic acid and was little changed compared to that evaluated under identical fulvic acid-free conditions. These results provide a fundamental basis for further studies evaluating the environmental fate of silver nanoparticles in natural aquatic systems.