Katarzyna Kubiak

Tuning properties of silver particle monolayers via controlled adsorption-desorption processes.

Using the well-defined silver particle suspension, systematic studies of adsorption kinetics on mica modified by poly(allylamine hydrochloride) (PAH) were carried out. The coverage of adsorbed particles was directly determined by AFM and SEM imaging. The dependence of the coverage on adsorption time, bulk suspension concentration, and ionic strength was systematically studied. It was confirmed that adsorption was diffusion controlled, with the initial rate proportional to the bulk concentration of particles. On the other hand, for long adsorption times, the saturation coverage was attained, which increased systematically with the ionic strength of the particle suspension. The adsorption kinetic runs were adequately reflected for the entire range of times and bulk concentration by the random sequential adsorption (RSA) model. Additionally, particle desorption kinetics from previously formed monolayers were studied. The decrease in the surface coverage of particles as a function of time was measured, which allowed one to determine the equilibrium adsorption constant K(a). The binding energy of silver particles (energy minimum depth) derived form these measurements varied between -16.9 kT and -17.8 kT, which suggests that it is mainly controlled by electrostatic interactions. Knowing the equilibrium adsorption constant, a particle adsorption isotherm was theoretically derived using the RSA model. Experimental data obtained for various bulk concentration of particles were in agreement with these theoretical predictions. These measurements suggest that it is feasible to produce uniform silver particle monolayers of desired coverage in the self-assembly process of particles.

Silver particle monolayers — Formation, stability, applications

The formation of silver particle monolayers at solid substrates in self-assembly processes is thoroughly reviewed. Initially, various silver nanoparticle synthesis routes are discussed with the emphasis focused on the chemical reduction in aqueous media. Subsequently, the main experimental methods aimed at bulk suspension characterization are critically reviewed by pointing out their advantages and limitations. Also, various methods enabling the in situ studies of particle deposition and release kinetics, especially the streaming potential method are discussed. In the next section, experimental data are invoked illustrating the most important features of particle monolayer formation, in particular, the role of bulk suspension concentration, particle size, ionic strength, temperature and pH. Afterward, the stability of monolayers and particle release kinetics are extensively discussed. The results obtained by the ex situ AFM/SEM imaging of particles are compared with the in situ streaming potential measurements. An equivalency of both methods is demonstrated, especially in respect to the binding energy determination. It is shown that these experimental results can be adequately interpreted in terms of the hybrid theoretical approach that combines the bulk transport step with the surface blocking effects derived from the random sequential adsorption model. It is also concluded that the particle release kinetics is governed by the discrete electrostatic interactions among ion pairs on particle and substrate surfaces. The classical theories based on the mean-field (averaged) zeta potential concept proved inadequate. Using the ion pair concept the minor dependence of the binding energy on particle size, ionic strength, pH and temperature is properly explained. The final sections of this review are devoted to the application of silver nanoparticles and their monolayers in medicine, analytical chemistry and catalysis.

Formation of PDADMAC monolayers evaluated in situ by QCM and streaming potential measurements

Kinetics of adsorption and monolayer stability of the cationic polyelectrolyte poly(diallyldimethylammonium chloride) (PDADMAC) were determined. Initially, the bulk characteristics of the polyelectrolyte were acquired using the DLS and microelectrophoretic measurements. These comprised the diffusion coefficient and electrophoretic mobility determined as a function of ionic strength at pH 5.8. From these measurements, the hydrodynamic diameter, zeta potential and the amounts of electrokinetic charge per molecule were calculated. Subsequently, the kinetics of PDADMAC adsorption was evaluated under in situ conditions using the quartz crystal microbalance with dissipation (QCM-D) and streaming potential measurements. The latter allowed one to derive the calibration dependencies of the zeta potential on the polyelectrolyte coverage for various ionic strength successfully interpreted in terms of the 3-dimensional (3D) electrokinetic model. Using these data, the PDADMAC desorption kinetics were quantitatively analyzed. In this way, the desorption constants, the equilibrium adsorption constants, and the binding energies of PDADMAC were determined. The energy varied between -20.5 and -19.7 kT, for ionic strength of 10(-3) and 0.15M, respectively. This agree with the proposed model of discrete electrostatic interactions among ion pairs present at the polyelectrolyte chain and the substrate surface. The mean-field electrostatic interactions approach proved inadequate.

Kinetics of Silver Nanoparticle Deposition at PAH Monolayers: Reference QCM Results

The deposition kinetics of silver nanoparticles on Au/SiO2 /PAH substrate was studied under in situ conditions using the QCM method and the ex situ SEM imaging. Because of low dissipation, the Sauerbrey equation was used for calculating the mass per unit area (coverage). Measurements were done for various bulk suspension concentrations, flow rates, and ionic strengths. It was shown that particle deposition for the low coverage regime is governed by the bulk mass transfer step that results in a linear increase of the coverage with the time. A comparison of QCM and SEM results showed that the hydration of the silver monolayers was negligible. This allowed one to derive a universal kinetic equation that describes the mass transfer rates in the cell as a function of the bulk concentration, flow rate, and diffusion coefficient. Measurements were also performed for longer times and for various ionic strengths where the deposition kinetics and the maximum coverage of particles were determined. The experimental data confirmed a significant increase in the maximum coverage with ionic strength. This was interpreted as due to the decreasing range of the electrostatic interactions among deposited particles. These results were adequately interpreted in terms of the extended random sequential adsorption (eRSA) model. Additionally, it was shown that the QCM data matched the ex situ SEM results, indicating that the monolayer hydration was also negligible for higher coverage range. These results derived for the model silver nanoparticle system can be exploited as reference data for the interpretation of protein adsorption kinetics where the dry mass is needed in order to assess the extent of hydration.

Mechanisms of fibrinogen adsorption at the silica substrate determined by QCM-D measurements.

Adsorption kinetics of fibrinogen at a silica substrate was thoroughly studied in situ using the QCM-D method. Because of low dissipation, the Sauerbreys equation was used for calculating the wet mass per unit area (wet coverage of the protein). Measurements were done for various bulk suspension concentrations, flow rates and pHs. These experimental data were compared with the theoretical dry coverage data derived from the solution of the mass transfer equation. In this way, the hydration functions and water factors of fibrinogen monolayers were quantitatively evaluated for various pHs. In the case of pH 7.4 and ionic strength of 0.15 M, the hydration function changed from 0.75 to 0.6 for the dry coverage Γ(d) equal to 0 and 4 mg m(-2), respectively. Interestingly, for pH 7.4 and 4.5 (ionic strength of 10(-2) M) a minimum of the hydration function appeared at Γ(d) ca. 2 mg m(-2). Analytical polynomial expressions were formulated for the interpolation of the experimental results. By using the hydration functions, the fibrinogen adsorption/desorption runs derived from QCM-D measurements were converted to the Γ(d) vs. the time relationships. This allowed to precisely determine the maximum coverage that varied between 1.2 mg m(-2) at pH 3.5 and 4.2 mg m(-2) at pH 7.4 for ionic strength of 0.15 M. These results agree with theoretical modeling and previous experimental data derived by using ellipsometry, OWLS and TIRF. Various fibrinogen adsorption mechanisms were revealed by exploiting the maximum coverage data whose validity was also confirmed by the dissipation vs. the dry mass relationships. Beside significance to basic science, these results enable to develop a robust technique, based on the QCM-D measurements, suitable for precisely determining the dry mass of protein monolayers adsorbed under various physicochemical conditions.

Influence of supporting polyelectrolyte layers on the coverage and stability of silver nanoparticle coatings.

Deposition mechanisms of citrate-stabilized silver nanoparticles 15 nm in diameter on cationic polyelectrolyte supporting layers were evaluated. Initially, the bulk and the electrokinetic properties of cationic polyelectrolytes and their monolayers on mica were determined using in situ streaming potential measurements. Analogously, the size distribution, stability and electrokinetic properties of silver particles were studied using transmission electron microscopy (TEM) and microelectrophoretic measurements. Afterward, the kinetics of silver particle deposition was quantitatively evaluated by a direct enumeration procedure exploiting the atomic force microscopy (AFM) and scanning electron microscopy (SEM) micrographs. Using this method the kinetics of particle adsorption was determined for various polyelectrolyte supporting layers as a function of ionic strength. These experiments were interpreted in terms of the random sequential adsorption (RSA) model. It was found that the highest coverage of 0.35 was obtained for silver monolayers deposited on poly(allylamine hydrochloride) (PAH)-modified mica in the case of higher ionic strength. The release kinetics of nanoparticles was also studied using the SEM and AFM imaging method. Using these experimental data the equilibrium adsorption constant and the binding energy of nanoparticles were calculated by exploiting the RSA approach. The investigations showed that the most stable silver monolayers are obtained for the poly-L-lysine (PLL) supporting layers where the 50% of particle is released after 441h, whereas in the case of PEI the release time was only 9h. These results are consistent with the model of discrete electrostatic interactions among ion pairs. Additionally, the obtained results have practical implication indicating that it is feasible to regulate the rate of silver nanoparticle release by a proper choice of the polyelectrolyte forming the supporting layer.

Evaluation of BACE1 Silencing in Cellular Models

Beta-secretase (BACE1) is the major enzyme participating in generation of toxic amyloid-beta (Aβ) peptides, identified in amyloid plaques of Alzheimers disease (AD) brains. Its downregulation results in decreasing secretion of Aβ. Thus, BACE1 silencing by RNAi represents possible strategy for antiamyloid therapy in the treatment of AD. In this study, a series of newly designed sequences of synthetic and vector-encoded siRNAs (pSilencer, pcPURhU6, and lentivirus) were tested against overexpressed and endogenous BACE1 in several cell lines and in adult neural progenitor cells, derived from rat hippocampus. SiRNAs active in human, mouse, and rat cell models were shown to diminish the level of BACE1. In HCN A94 cells, two BACE1-specific siRNAs did not alter the expression of genes of BACE2 and several selected genes involved in neurogenesis (Synapsin I, βIII-Tubulin, Calbidin, NeuroD1, GluR2, CREB, MeCP2, PKR), however, remarkable lowering of SCG10 mRNA, coding protein of stathmin family, important in the development of nervous system, was observed.

RNA interference in silencing of genes of Alzheimer's disease in cellular and rat brain models

Accumulation of insoluble aggregates of beta-amyloid peptide, a cleavage product of amyloid precursor protein, is thought to be a central step in the pathogenesis of Alzheimers disease. The major enzymes required for the generation of toxic amyloid-beta peptide are beta-(BACE1) and gamma-secretases. Here, we present the rational design and the application of synthetic and lentivirus vector-encoded siRNAs for specific and efficient knockdown of overexpressed and endogenous BACE1, both in dividing and neural stem cells and in a rat brain. We also tested an approach to anti-amyloid therapy by the use of the allele-specific siRNAs to silence the mutant presenilin 1 (L392V PS-1), the main component of gamma-secretase, responsible for development of Familial Alzheimers disease. Reducing the level of beta-amyloid accumulation in the brain could be beneficial for metabolic studies as well as potential therapeutic approach for prevention and treatment of Alzheimers disease.

Fibrinogen adsorption mechanisms at the gold substrate revealed by QCM-D measurements and RSA modeling

Adsorption kinetics of fibrinogen at a gold substrate at various pHs was thoroughly studied using the QCM-D method. The experimental were interpreted in terms of theoretical calculations performed according to the random sequential adsorption model (RSA). In this way, the hydration functions and water factors of fibrinogen monolayers were quantitatively evaluated at various pHs. It was revealed that for the lower range of fibrinogen coverage the hydration function were considerably lower than previously obtained for the silica sensor [33]. The lower hydration of fibrinogen monolayers on the gold sensor was attributed to its higher roughness. However, for higher fibrinogen coverage the hydration functions for both sensors became identical exhibiting an universal behavior. By using the hydration functions, the fibrinogen adsorption/desorption runs derived from QCM-D measurements were converted to the Γd vs. the time relationships. This allowed to precisely determine the maximum coverage that varied between 1.6mgm(-2) at pH 3.5 and 4.5mgm(-2) at pH 7.4 (for ionic strength of 0.15M). These results agree with theoretical eRSA modeling and previous experimental data derived by using ellipsometry, OWLS and TIRF. Various fibrinogen adsorption mechanisms were revealed by exploiting the maximum coverage data. These results allow one to develop a method for preparing fibrinogen monolayers of well-controlled coverage and molecule orientation.