Limits of the fractal dimension for irreversible kinetic aggregation of gold colloids

Does shining light on gold colloids influence aggregation?

In this article we revisit the much-studied behavior of self-assembled aggregates of gold colloidal particles. In the literature, the electrostatic interactions, van der Waals interactions, and the change in free energy due to ligand-ligand or ligand-solvent interactions are mainly considered to be the dominating factors in determining the characteristics of the gold aggregates. However, our light scattering and imaging experiments clearly indicate a distinct effect of light in the growth structure of the gold colloidal particles. We attribute this to the effect of a non-uniform distribution of the electric field in aggregated gold colloids under the influence of light.

Direct observation of aggregative nanoparticle growth: kinetic modeling of the size distribution and growth rate

Direct observations of solution-phase nanoparticle growth using in situ liquid transmission electron microscopy (TEM) have demonstrated the importance of "non-classical" growth mechanisms, such as aggregation and coalescence, on the growth and final morphology of nanocrystals at the atomic and single nanoparticle scales. To date, groups have quantitatively interpreted the mean growth rate of nanoparticles in terms of the Lifshitz-Slyozov-Wagner (LSW) model for Ostwald ripening, but less attention has been paid to modeling the corresponding particle size distribution. Here we use in situ fluid stage scanning TEM to demonstrate that silver nanoparticles grow by a length-scale dependent mechanism, where individual nanoparticles grow by monomer attachment but ensemble-scale growth is dominated by aggregation. Although our observed mean nanoparticle growth rate is consistent with the LSW model, we show that the corresponding particle size distribution is broader and more symmetric than predicted by LSW. Following direct observations of aggregation, we interpret the ensemble-scale growth using Smoluchowski kinetics and demonstrate that the Smoluchowski model quantitatively captures the mean growth rate and particle size distribution.

Fractal structures of single-walled carbon nanotubes in biologically relevant conditions: role of chirality vs. media conditions

Aggregate structure of covalently functionalized chiral specific semiconducting single-walled carbon nanotubes (SWNTs) was systematically studied employing static light scattering (SLS). Fractal dimensions (Df) of two specific chirality SWNTs-SG65 and SG76 with (6, 5) and (7, 6) chiral enrichments-were measured under four biological exposure media conditions, namely: Dulbecco's Modified Eagle Medium (DMEM), Minimum Essential Medium (MEM), Roswell Park Memorial Institute (RPMI) 1640 medium, and 0.9% saline solution. The SWNTs exhibited chiral dependence on Df with SG65 showing more fractal or loosely bound aggregate structures, i.e., lower Df values (range of 2.24±0.03 to 2.64±0.05), compared to the SG76 sample (range of 2.58±0.13 to 2.90±0.08). All the Df values reported are highly reproducible, measured from multiple SLS runs and estimated with 'random block-effects' statistical analysis that yielded all p values to be <0.001. The key mechanism for such difference in Df between the SWNT samples was identified as the difference in van der Waals (VDW) interaction energies of these samples, where higher VDW of SG76 resulted in tighter packing density. Effect of medium type showed lower sensitivity; however, presence of di-valent cations (Ca(2+)) in DMEM and MEM media resulted in relatively loose or more fractal aggregates. Moreover, presence of fetal bovine serum (FBS) and bovine serum albumin (BSA), used to mimic the in vitro cell culture condition, reduced the Df values, i.e., created more fractal structures. Steric hindrance to aggregation was identified as the key mechanism for creating the fractal structures. Also, increase in FBS concentration from 1% to 10% resulted in increasingly lower Df values.

Kinetics of DNA-coated sticky particles

DNA-functionalized particles are promising for complex self-assembly due to their specific controllable thermoreversible interactions. However, there has been little work on the kinetics and the aggregation rate, which depend on the rate of particle encounters and the probability that an encounter results in particles sticking. In this study, we investigate theoretically and experimentally the aggregation times of micron-scale particles as a function of DNA coverage and salt concentration. Our 2-μm colloids accommodate up to 70,000 DNA strands. For full coverage and high salt concentration, the aggregation time is 5 min while for 0.1 coverage and low salt it is 4 days. A simple model using reaction-limited kinetics and experimental oligomer hybridization rates describes the data well. A controlling factor is the Coulomb barrier at the nanometer scale retarding DNA hybridization. Our model allows easy measurements of microscopic hybridization rates from macroscopic aggregation and enables the design of complex self-assembly schemes with controlled kinetics.

Dependence of fractal dimension of DLCA clusters on size of primary particles

It is well known that clusters generated from colloidal aggregation driven by Brownian motion are typical fractal objects with the fractal dimension in the range of 1.75-1.85 under the diffusion-limited cluster aggregation (DLCA) conditions. In this work, we review and analyze the values of the fractal dimension for DLCA clusters experimentally determined in the literature. It is found that the value of the fractal dimension decreases significantly as the primary particle radius increases. Then, we have properly designed the DLCA experiments, using different radii of the primary particles, and determined the fractal dimensions of the generated clusters. Our results have well confirmed that the fractal dimension indeed decreases as the particle radius increases. To explore the mechanism leading to such dependence, we have performed intense computations through the full T-Matrix theory, and we conclude that this is not related to the effect of the intra-cluster multiple scattering on the slope of the scattering structure factor. The large fractal dimensions of the clusters generated by very small nanoparticles could be explained by thermal restructuring due to their low bonding energies, but no clear explanation can be given for the small fractal dimensions of the clusters made of large particles.

Ion-association complexes unite classical and non-classical theories for the biomimetic nucleation of calcium phosphate

Despite its importance in many industrial, geological and biological processes, the mechanism of crystallization from supersaturated solutions remains a matter of debate. Recent discoveries show that in many solution systems nanometre-sized structural units are already present before nucleation. Still little is known about the structure and role of these so-called pre-nucleation clusters. Here we present a combination of in situ investigations, which show that for the crystallization of calcium phosphate these nanometre-sized units are in fact calcium triphosphate complexes. Under conditions in which apatite forms from an amorphous calcium phosphate precursor, these complexes aggregate and take up an extra calcium ion to form amorphous calcium phosphate, which is a fractal of Ca(2)(HPO(4))(3)(2-) clusters. The calcium triphosphate complex also forms the basis of the crystal structure of octacalcium phosphate and apatite. Finally, we demonstrate how the existence of these complexes lowers the energy barrier to nucleation and unites classical and non-classical nucleation theories.

Ligand-dominated temperature dependence of agglomeration kinetics and morphology in alkyl-thiol-coated gold nanoparticles

The stability of nanoparticle suspensions and the details of their agglomeration depend on the interactions between particles. We study this relationship in gold nanoparticles stabilized with different alkyl thiols in heptane. Temperature-dependent interactions were inferred from small-angle x-ray scattering, agglomeration kinetics from dynamic light scattering, and agglomerate morphologies from transmission electron microscopy. We find that the particles precipitate at temperatures below the melting temperatures of the dry ligands. Agglomerates grow with rates that depend on the temperature: Around precipitation temperature, globular agglomerates form slowly, while at lower temperatures, fibrilar agglomerates form rapidly. All agglomerates contain random dense packings rather than crystalline superlattices. We conclude that ligand-ligand and ligand-solvent interactions of the individual particles dominate suspension stability and agglomeration kinetics. The microscopic packing is dominated by interactions between the ligands of different nanoparticles.

Coagulation kinetics of humic aggregates in mono- and di-valent electrolyte solutions

Coagulation behaviors of humic acids (HAs) aggregates in electrolyte solutions at different pHs, valences and concentrations of electrolyte cations were investigated using dynamic light scattering technique in combination of other analytical tools. For monovalent electrolyte sodium chloride (NaCl) solution, at its low concentrations the average hydrodynamic radius (<Rh>) of aggregates kept nearly constant. However, at high NaCl concentrations, <Rh> could be scaled to the time t as <Rh> ∝ t(a), suggesting a diffusion-limited colloid aggregation (DLCA). The coagulation value of NaCl in a buffer at pH 7.1 was calculated to be in a range of 61.3-84.4 mM. Divalent cation Mg(2+) was far more effective in enhancing the HA coagulation, as evidenced by a lower coagulation value (between 1.0 and 1.7 mM) and a more rapid coagulation rate. Such an enhancement could be explained by the combined effects of electrostatic repulsion, complexation and bridging. The highest coagulation rate (d<Rh>/dt) and coagulation value at different pHs followed the order of: acidic > neutral > alkaline, and alkaline > neutral > acidic, respectively. Such a difference was associated with the extent of hydrogen bond and electrostatic repulsion at different protonation/deprotonation states of carboxyl and phenolic -OH groups. Transmission electron microscopic imaging reveals that HA was predominantly globular aggregates with a rough periphery at pH 5.26, and was changed to smooth spherical particles at pH 10.00. These results are useful for better understanding the coagulation behaviors of HAs in both natural and engineered aqueous systems.

Controlling colloidal morphologies by critical Casimir forces

Active control over the assembly of colloidal and nanoparticles has important applications for the design of new nanostructured materials, but it is a difficult task. Here, a new method is presented to control the morphology of colloidal aggregates using critical Casimir forces. Via direct temperature control of critical Casimir forces, the particles are assembled into aggregates with well-defined architecture.

Colloidal aggregation in microgravity by critical Casimir forces

By using the critical Casimir force, we study the attractive strength dependent aggregation of colloids with and without gravity by means of near field scattering. Significant differences were seen between microgravity and ground experiments, both in the structure of the formed fractal aggregates as well as in the kinetics of growth. In microgravity purely diffusive aggregation is observed. By using the continuously variable particle interaction potential we can for the first time experimentally relate the strength of attraction between the particles and the structure of the aggregates.

Effect of hydrodynamic interactions on rapid Brownian coagulation of colloidal dispersions

The rate of rapid Brownian coagulation is investigated for dispersions of spherical particles with particle volume fractions ranging from Øp = 0.003 to 0.1 by the direct numerical simulation method. This method explicitly considers hydrodynamic interactions (HIs) between particles by simultaneously solving for the motions of the dispersed particles and the host fluid. In the dilute limit, the rate of rapid Brownian coagulation decreases to approximately 0.3-0.5 times the theoretical Smoluchowski rate. We compare this result with results of previously reported experiments and theoretical predictions and find a strong correlation between them. This demonstrates that HIs between particles significantly reduce the coagulation rate. Moreover, the volume fraction dependence of the coagulation rate indicates that the coagulation rate increases with increasing volume fraction. At high particle volume fractions, the initial coagulation stage is affected by heterogeneous coagulation process before the steady state is reached.

Tissue and cellular distribution of gold nanoparticles varies based on aggregation/agglomeration status

AIM

The ability of nanoparticles to form larger superstructures of aggregates and agglomerates has been extensively noted in the literature. The in vivo biological impact of these structures, however, has not been assessed. This knowledge gap is especially critical in the safety assessment of nanoparticles to be used for therapeutic purposes.

METHOD/RESULTS

Here we show that when administered intravenously into a mouse model, gold nanoparticle superstructures of reversible agglomerates and irreversible aggregates demonstrate significant differences in organ and cellular distribution compared with the primary particle building blocks. In addition, different structures produced different blood serum chemistry data.

CONCLUSION

These findings raise the possibility for different mechanisms of toxicity between the structures. Such a possibility necessitates complete characterization and stability assessment of nanomaterials prior to their in vivo administration.

Synthesis of pH-responsive particles with shape anisotropy

Seeded emulsion polymerization is used to produce large quantities of shape anisotropic, amphoteric particles in a size range of about 1 μm. Copolymer dicolloids (CDCs) containing pyridine groups are synthesized by swelling spherical, lightly cross-linked polystyrene seeds with a mixture of styrene and pH-responsive monomer 2-vinyl pyridine followed by secondary polymerization to contrast with their analogue homopolymer dicolloids (HDCs) where the swelling step is carried out with styrene alone. After the particles are coated with a nonionic surfactant to minimize van der Waals attractions, surface potentials and aggregation properties of dilute suspensions are studied as functions of pH and ionic strength. Compared to HDCs, which remain stable at all pH values studied (3 < pH < 9) up to an ionic strength of 5 M, the CDC particles show amphoteric behavior with strong attractions under conditions where dipolar interactions are expected to dominate.

Stability of superparamagnetic iron oxide nanoparticles at different pH values: experimental and theoretical analysis

The detection of superparamagnetic nanoparticles using NMR logging has the potential to provide enhanced contrast in oil reservoir rock formations. The stability of the nanoparticles is critical because the NMR relaxivity (R(2) ≡ 1/T(2)) is dependent on the particle size. Here we use a molecular theory to predict and validate experimentally the stability of citric acid-coated/PEGylated iron oxide nanoparticles under different pH conditions (pH 5, 7, 9, 11). The predicted value for the critical surface coverage required to produce a steric barrier of 5k(B)T for PEGylated nanoparticles (MW 2000) was 0.078 nm(-2), which is less than the experimental value of 0.143 nm(-2), implying that the nanoparticles should be stable at all pH values. Dynamic light scattering (DLS) measurements showed that the effective diameter did not increase at pH 7 or 9 after 30 days but increased at pH 11. The shifts in NMR relaxivity (from R(2) data) at 2 MHz agreed well with the changes in hydrodynamic diameter obtained from DLS data, indicating that the aggregation behavior of the nanoparticles can be easily and quantitatively detected by NMR. The unexpected aggregation at pH 11 is due to the desorption of the surface coating (citric acid or PEG) from the nanoparticle surface not accounted for in the theory. This study shows that the stability of the nanoparticles can be predicted by the theory and detected by NMR quantitatively, which suggests the nanoparticles to be a possible oil-field nanosensor.

Study of colloidal particle Brownian aggregation by low-coherence fiber optic dynamic light scattering

The aggregation kinetics of particles in dense polystyrene latex suspensions is studied by low-coherence fiber optic dynamic light scattering. Low-coherence fiber optic dynamic light scattering is used to measure the hydrodynamic radius of the aggregates. The aggregation kinetics data obtained can be fitted into a single exponential function, which is the characteristic of slow aggregation. It is found that the aggregation rate of particles increased with higher electrolyte levels and with larger particle concentrations. The experimental results can be explained by use of the Derjaruin-Landau-Verwey-Overbeer (DLVO) theory.

Kinetics of aggregation of UV-irradiated glyceraldehyde-3-phosphate dehydrogenase from rabbit skeletal muscle. Effect of agents possessing chaperone-like activity

An aggregation test system based on the aggregation of UV-irradiated glyceraldehyde-3-phosphate dehydrogenase (GAPDH) from rabbit skeletal muscle has been proposed. On the basis of the measurements of the enzyme activity and differential scanning calorimetry data a conclusion has been made that UV radiation results in formation of damaged protein molecules with lower thermostability. It was shown that the order of aggregation rate for UV-irradiated GAPDH with respect to the protein was close to 2. This means that such a test system allows detecting the effect of various agents exclusively on the stage of aggregation of unfolded protein molecules. The influence of α-crystallin and 2-hydroxypropyl-β-cyclodextrin (HP-β-CD) on aggregation of UV-irradiated GAPDH was studied. Despite the fact that HP-β-CD accelerates thermal aggregation of non-irradiated GAPDH, in the case of aggregation of UV-irradiated GAPDH HP-β-CD reveals a purely protective effect.

Polypeptide folding-mediated tuning of the optical and structural properties of gold nanoparticle assemblies

Responsive hybrid nanomaterials with well-defined properties are of significant interest for the development of biosensors with additional applications in tissue engineering and drug delivery. Here, we present a detailed characterization using UV-vis spectroscopy and small angle X-ray scattering of a hybrid material comprised of polypeptide-decorated gold nanoparticles with highly controllable assembly properties. The assembly is triggered by a folding-dependent bridging of the particles mediated by the heteroassociation of immobilized helix-loop-helix polypeptides and a complementary nonlinear polypeptide present in solution. The polypeptides are de novo designed to associate and fold into a heterotrimeric complex comprised of two disulfide-linked four-helix bundles. The particles form structured assemblies with a highly defined interparticle gap (4.8±0.4 nm) that correlates to the size of the folded polypeptides. Transitions in particle aggregation dynamics, mass-fractal dimensions and ordering, as a function of particle size and the concentration of the bridging polypeptide, are observed; these have significant effects on the optical properties of the assemblies. The assembly and ordering of the particles are highly complex processes that are affected by a large number of variables including the number of polypeptides bridging the particles and the particle mobility within the aggregates. A fundamental understanding of these processes is of paramount interest for the development of novel hybrid nanomaterials with tunable structural and optical properties and for the optimization of nanoparticle-based colorimetric biodetection strategies.

The model Lysozyme-PSSNa system for electrostatic complexation: Similarities and differences with complex coacervation

We review, based on structural information, the mechanisms involved when putting in contact two nano-objects of opposite electrical charge, in the case of one negatively charged polyion, and a compact charged one. The central case is mixtures of PSS, a strong flexible polyanion (the salt of a strong acid, and with high linear charge density), and Lysozyme, a globular protein with a global positive charge. A wide accurate and consistent set of information in different situations is available on the structure at local scales (5-1000Å), due to the possibility of matching, the reproducibility of the system, its well-defined electrostatics features, and the well-defined structures obtained. We have related these structures to the observations at macroscopic scale of the phase behavior, and to the expected mechanisms of coacervation. On the one hand, PSS/Lysozyme mixtures show accurately many of what is expected in PEL/protein complexation, and phase separation, as reviewed by de Kruif: under certain conditions some well-defined complexes are formed before any phase separation, they are close to neutral; even in excess of one species, complexes are only modestly charged (surface charges in PEL excess). Neutral cores are attracting each other, to form larger objects responsible for large turbidity. They should lead the system to phase separation; this is observed in the more dilute samples, while in more concentrated ones the lack of separation in turbid samples is explained by locking effects between fractal aggregates. On the other hand, although some of the features just listed are the same required for coacervation, this phase transition is not really obtained. The phase separation has all the macroscopic aspects of a fluid (undifferentiated liquid/gas phase) - solid transition, not of a fluid-fluid (liquid-liquid) one, which would correspond to real coacervation). The origin of this can be found in the interaction potential between primary complexes formed (globules), which agrees qualitatively with a potential shape of the type repulsive long range attractive very short range. Finally we have considered two other systems with accurate structural information, to see whether other situations can be found. For Pectin, the same situation as PSS can be found, as well as other states, without solid precipitation, but possibly with incomplete coacervation, corresponding to differences in the globular structure. It is understandable that these systems show smoother interaction potential between the complexes (globules) likely to produce liquid-liquid transition. Finally, we briefly recall new results on Hyaluronan/Lysozyme, which present clear signs of coacervation in two liquid phases, and at the same time the existence of non-globular complexes, of specific geometry (thin rods) before any phase separation. These mixtures fulfill many of the requirements for complex coacervation, while other theories should also be checked like the one of Shklovskii et al.

Colloidal stability of magnetic iron oxide nanoparticles: influence of natural organic matter and synthetic polyelectrolytes

The colloidal behavior of natural organic matter (NOM) and synthetic poly(acrylic acid) (PAA)-coated ferrimagnetic (γFe(2)O(3)) nanoparticles (NPs) was investigated. Humic acid (HA), an important component of NOM, was extracted from a peat soil. Two different molecular weight PAAs were also used for coating. The colloidal stability of the coated magnetic NPs was evaluated as a resultant of the attractive magnetic dipolar and van der Waals forces and the repulsive electrostatic and steric-electrosteric interactions. The conformational alterations of the polyelectrolytes adsorbed on magnetic γFe(2)O(3) NPs and their role in colloidal stability were determined. Pure γFe(2)O(3) NPs were extremely unstable because of aggregation in aqueous solution, but a significant stability enhancement was observed after coating with polyelectrolytes. The steric stabilization factor induced by the polyelectrolyte coating strongly dictated the colloidal stability. The pH-induced conformational change of the adsorbed, weakly charged polyelectrolytes had a significant effect on the colloidal stability. Atomic force microscopy (AFM) revealed the stretched conformation of the HA molecular chains adsorbed on the γFe(2)O(3) NP surface at pH 9, which enhanced the colloidal stability through long-range electrosteric stabilization. The depletion of the polyelectrolyte during the dilution of the NP suspension decreased the colloidal stability under acidic solution conditions. The conformation of the polyelectrolytes adsorbed on the NP surface was altered as a function of the substrate surface charge as viewed from AFM imaging. The polyelectrolyte coating also led to a reduction in magnetic moments and decreased the coercivity of the coated γFe(2)O(3) NPs. Thus, the enhanced stabilization of the coated maghematite NPs may facilitate their delivery in the groundwater for the effective removal of contaminants.

In situ liquid-cell electron microscopy of colloid aggregation and growth dynamics

We report on real-time observations of the aggregation of gold nanoparticles using a custom-made liquid cell that allows for in situ electron microscopy. Process kinetics and fractal dimension of the aggregates are consistent with three-dimensional cluster-cluster diffusion-limited aggregation, even for large aggregates, for which confinement effects are expected. This apparent paradox was resolved through in situ observations of the interactions between individual particles as well as clusters at various stages of the aggregation process that yielded the large aggregates. The liquid cell described herein facilitates real-time observations of various processes in liquid media with the high resolution of the electron microscope.

The thermal structural transition of α-crystallin inhibits the heat induced self-aggregation

-crystallin, the major constituent of human lens, is a member of the heat-shock proteins family and it is known to have a quaternary structural transition at . The presence of calcium ions and/or temperature changes induce supramolecular self-aggregation, a process of relevance in the cataractogenesis. Here we investigate the potential effect of the bovine -crystallin's structural transition on the self-aggregation process. Along all the temperatures investigated, aggregation proceeds by forming intermediate molecular assemblies that successively aggregate in clusters. The final morphology of the aggregates, above and below , is similar, but the aggregation kinetics are completely different. The size of the intermediate molecular assemblies, and their repulsive energy barrier show a marked increase while crossing . Our results highlight the key role of heat modified form of -crystallin in protecting from aggregation and preserving the transparency of the lens under hyperthermic conditions.

Fractal Aspects of Ceramic Synthesis

The concept of fractal geometry is used to describe the structure of silica polymers, colloidal aggregates, and critical systems. We illustrate the interpretation of scattering curves (X-ray, neutron and light) for fractal systems, and review simple growth models which generate fractal structures. We describe the polymerization of silica under various conditions and demonstrate that, depending on chemical conditions, polymerization maps onto simple fractal growth processes. The key factors which control growth are monomer-cluster vs. cluster-cluster growth, and reaction-limited vs. diffusion-limited growth.

Mechanism of aggregation of UV-irradiated β(L)-crystallin

Thermal denaturation and aggregation of UV-irradiated β(L)-crystallin from eye lenses of steers have been studied. The data on size-exclusion chromatography and SDS-PAGE indicated that UV irradiation of β(L)-crystallin at 10 °С resulted in fragmentation of the protein molecule and formation of cross-linked aggregates. Fluorescence data showed that tryptophan fluorescence in the irradiated protein decreased exponentially with the UV dose. Decrease in tryptophan fluorescence is a result of photochemical destruction, but not of conformational changes of protein, because there is no red shift in the fluorescence maximum. The differential scanning calorimetry (DSC) profiles of the samples of UV-irradiated and wild type β(L)-crystallin were registered. The area under curves, which is proportional to the amount of the native protein, decreased exponentially with increasing the irradiation dose. The shape of the DSC profiles for the samples of UV-irradiated β(L)-crystallin was identical to that for wild type β(L)-crystallin. The DSC data allowed estimating the portion of UV-denatured β(L)-crystallin, which is not registered by DSC, and the portion of the combined fraction consisting of native and UV-damaged molecules retaining the native structure. A conclusion has been made that UV-induced denaturation of β(L)-crystallin follows the one-hit model. The study of the kinetics of thermal aggregation of UV-irradiated β(L)-crystallin at 37 °С using dynamic light scattering showed that the initial stage of aggregation was that of formation of the start aggregates with the hydrodynamic radius of 20 nm. Further sticking of the start aggregates proceeded in the regime of reaction-limited cluster-cluster aggregation. Splitting of the aggregate population into two components occurred above a definite point in time.

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.

Paradoxical acceleration of dithiothreitol-induced aggregation of insulin in the presence of a chaperone

The kinetics of dithiothreitol (DTT)-induced aggregation of human recombinant insulin and the effect of α-crystallin, a representative of the family of small heat shock proteins, on the aggregation process have been studied using dynamic light scattering technique. Analysis of the distribution of the particles by size measured in the course of aggregation showed that the initial stage of the aggregation process was the stage of formation of the start aggregates with a hydrodynamic radius (R(h)) of about 90 nm. When studying the effect of α-crystallin on the rate of DTT-induced aggregation of insulin, it was demonstrated that low concentrations of α-crystallin dramatically accelerated the aggregation process, whereas high concentrations of α-crystallin suppressed insulin aggregation. In the present study, at the molar stoichiometric ratio (insulin:α-crystallin) less than 1:0.5, a pronounced accelerating effect of α-crystallin was observed; whereas a ratio exceeding the value of 1:0.6 caused suppression of insulin aggregation. The mechanisms underlying the dual effect of α-crystallin have been proposed. It is assumed that heterogeneous nucleation occurring on the surface of the α-crystallin particle plays the key role in the paradoxical acceleration of insulin aggregation by α-crystallin that may provide an alternative biologically significant pathway of the aggregation process.

Cholesteric liquid crystal devices with nanoparticle aggregation

A broadband cholesteric liquid crystal (CLC) device with a multi-domain structure is demonstrated by using an aggregation of polyhedral oligomeric silsesquioxane (POSS) nanoparticles in the CLC layer. The aggregation pattern of the self-assembled POSS nanoparticles depends on the concentration of POSS doped in the mixture of POSS/CLC and the cooling rate of the mixture from a temperature higher than the clear point. POSS-induced changes in the bulk and surface properties of the cholesteric cells, such as a promotion of homeotropic alignment, help to form a cholesteric structure with a broadband reflection of light; the latter can be used for improvement of bistable CLC devices. A higher POSS concentration and a higher cooling rate both improve the appearance of the black-white CLC device.

Combinatorial targeting and nanotechnology applications

The development of improved methods for targeted cell detection is of general interest in many fields of research and drug development. There are a number of well-established techniques for the study and detection of biomarkers expressed in living cells and tissues. Many of them rely on multi-step procedures that might not meet ideal assay requirements for speed, cost, sensitivity, and specificity. Here we report and further validate an approach that enables spontaneous molecular assembly to generate biologically active networks of bacteriophage (phage) assembled with gold (Au) nanoparticles (termed Au-phage nanoshuttles). Here, the nanoshuttles preserve the cell binding and internalization attributes mediated by a displayed peptide targeted to a cell surface receptor. The organization of such targeted assemblies can be further manipulated to be used as a multimodal detection assembly, and they can be characterized as fractal nanostructures by angle-dependent light scattering fractal dimension analysis. Targeted Au-phage nanoshuttles offer multiple functionalities for nanotechnology-based sensing and reporting, including enhanced fluorescence and improved contrast for darkfield microscopy.

Stochastic qualifier of gel and glass transitions in laponite suspensions

The existence of the important similarities between gelation and glass transition makes it hard to distinguish between the two types of nonergodic states experimentally. Here, we report on a stochastic analysis of the scattered light intensity through a colloidal particles suspension during the gel and glass formation. In this analysis, we exploit the methods developed for complex hierarchical systems, such as turbulence. Using the multiplicative log-normal cascade models, we provide a criterion to distinguish gels from glasses.