Structure of adsorbed polyelectrolyte monolayers investigated by combining optical reflectometry and piezoelectric techniques

Kinetics of Macroion Adsorption on Silica: Complementary Theoretical and Experimental Investigations for Poly-l-arginine

A comprehensive approach enabling a quantitative interpretation of poly-l-arginine (PARG) adsorption kinetics at solid/electrolyte interfaces was developed. The first step involved all-atom molecular dynamics (MD) modeling of physicochemical characteristics yielding PARG molecule conformations, its contour length, and the cross-section area. It was also shown that PARG molecules, even in concentrated electrolyte solutions (100 mM NaCl), assume a largely elongated shape with an aspect ratio of 36. Using the parameters derived from MD, the PARG adsorption kinetics at the silica/electrolyte interface was calculated using the random sequential adsorption approach. These predictions were validated by optical reflectometry measurements. It was confirmed that the molecules irreversibly adsorbed in the side-on orientation and their coverage agreed with the elongated shape of the PARG molecule predicted from the MD modeling. These theoretical and experimental results were used for the interpretation of the quartz crystal microbalance measurements carried out under various pH conditions. A comprehensive analysis unveiled that the results stemming from the hydrodynamic theory postulating a lubrication-like (soft) contact of the macroion molecules with the sensor adequately reflect the adsorption kinetics. The range of validity of the intuitively used Sauerbrey model was also estimated. It was argued that acquired results can be exploited to control macroion adsorption at solid/liquid interfaces. This is essential for the optimum preparation of their supporting layers used for bioparticle immobilization and shell formation at nanocapsules in targeted drug delivery.

Molecular Structure and Conformation of Biodegradable Water-Soluble Polymers Control Adsorption and Transport in Model Soil Mineral Systems

Water-soluble polymers (WSPs) are used in diverse applications, including agricultural formulations, that can result in the release of WSPs to soils. WSP biodegradability in soils is desirable to prevent long-term accumulation and potential associated adverse effects. In this work, we assessed adsorption of five candidate biodegradable WSPs with varying chemistry, charge, and polarity characteristics (i.e., dextran, diethylaminoethyl dextran, carboxymethyl dextran, polyethylene glycol monomethyl ether, and poly-l-lysine) and of one nonbiodegradable WSP (poly(acrylic acid)) to sand and iron oxide-coated sand particles that represent important soil minerals. Combined adsorption studies using solution-depletion measurements, direct surface adsorption techniques, and column transport experiments over varying solution pH and ionic strengths revealed electrostatics dominating interactions of charged WSPs with the sorbents as well as WSP conformations and packing densities in the adsorbed states. Hydrogen bonding controls adsorption of noncharged WSPs. Under transport in columns, WSP adsorption exhibited fast and slow kinetic adsorption regimes with time scales of minutes to hours. Slow adsorption kinetics in soil may lead to enhanced transport but also shorter lifetimes of biodegradable WSPs, assuming more rapid biodegradation when dissolved than adsorbed. This work establishes a basis for understanding the coupled adsorption and biodegradation dynamics of biodegradable WSPs in agricultural soils.

Enzymatic and Microbial Electrochemistry: Approaches and Methods

The coupling of enzymes and/or intact bacteria with electrodes has been vastly investigated due to the wide range of existing applications. These span from biomedical and biosensing to energy production purposes and bioelectrosynthesis, whether for theoretical research or pure applied industrial processes. Both enzymes and bacteria offer a potential biotechnological alternative to noble/rare metal-dependent catalytic processes. However, when developing these biohybrid electrochemical systems, it is of the utmost importance to investigate how the approaches utilized to couple biocatalysts and electrodes influence the resulting bioelectrocatalytic response. Accordingly, this tutorial review starts by recalling some basic principles and applications of bioelectrochemistry, presenting the electrode and/or biocatalyst modifications that facilitate the interaction between the biotic and abiotic components of bioelectrochemical systems. Focus is then directed toward the methods used to evaluate the effectiveness of enzyme/bacteria-electrode interaction and the insights that they provide. The basic concepts of electrochemical methods widely employed in enzymatic and microbial electrochemistry, such as amperometry and voltammetry, are initially presented to later focus on various complementary methods such as spectroelectrochemistry, fluorescence spectroscopy and microscopy, and surface analytical/characterization techniques such as quartz crystal microbalance and atomic force microscopy. The tutorial review is thus aimed at students and graduate students approaching the field of enzymatic and microbial electrochemistry, while also providing a critical and up-to-date reference for senior researchers working in the field.

Association of Oral Health and Mini Nutritional Assessment in Older Adults: A Systematic Review with Meta-analyses

PURPOSE

To evaluate whether poor oral health is associated with a higher risk of malnutrition based on the Mini Nutritional Assessment (MNA) or MNA-SF (short form) in older adults.

STUDY SELECTION

For this meta-analysis, cohort and cross-sectional studies with adults 65 years and older, reporting oral health outcomes (i.e. edentulism, number of teeth) and either the MNA or MNA-SF were selected. Four electronic databases were searched (Medline via PubMed, Web of Science, Cochrane Library and EMBASE) through June 2020. Risk of bias was assessed with the checklist by the Agency for Healthcare Research and Quality scale.

RESULTS

A total of 928 abstracts were reviewed with 33 studies, comprising 27,559 participants, aged ≥65 being ultimately included. Meta-analyses showed that the lack of daily oral hygiene (teeth or denture cleaning), chewing problems and being partially/fully edentulous, put older adults at higher risk of malnutrition (p<0.05). After adjustment for socio-demographic variables, the included studies reported lack of autonomy for oral care, poor/moderate oral health, no access to the dentist and being edentulous with either no dentures or only one denture were risk factors significantly associated with a higher risk of malnutrition (p<0.05).

CONCLUSION

These findings may imply that once elders become dependent on others for assistance with oral care, have decreased access to oral healthcare, and lack efficient chewing capacity, there is increased risk of malnourishment. Limitations of the study include heterogeneity of oral health variables and the observational nature of the studies. Further studies are needed to validate our findings.

Nanoparticle and bioparticle deposition kinetics

Mechanisms and kinetic of particle deposition at solid surfaces leading to the formation of self-assembled layers of controlled structure and density were reviewed. In the first part theoretical aspects were briefly discussed, comprising limiting analytical solutions for the linear transport under flow and diffusion. Methods of the deposition kinetics analysis for non-linear regimes affected by surface blocking were also considered. Characteristic monolayer formation times under diffusion and flow for the nanoparticle size range were calculated. In the second part illustrative experimental results obtained for micro- and nanoparticles were discussed. Deposition at planar substrates was analyzed with emphasis focused on the stability of layers and the release kinetics of silver particles. Applicability of the quartz microbalance measurements (QCM) for quantitative studies of nanoparticle deposition kinetic was also discussed. Except for noble metal and polymer particles, representative results for virus deposition at abiotic surfaces were analyzed. Final part of the review was devoted to nanoparticle corona formation at polymer carrier particles investigated by combination of the concentration depletion, AFM, SEM and the in situ electrokinetic method. It is argued that the results obtained for colloid particles can be used as reliable reference systems for interpretation of protein and other bioparticle deposition, confirming the thesis that simple is universal.

Poly-L-Arginine Molecule Properties in Simple Electrolytes: Molecular Dynamic Modeling and Experiments

Physicochemical properties of poly-L-arginine (P-Arg) molecules in NaCl solutions were determined by molecular dynamics (MD) modeling and various experimental techniques. Primarily, the molecule conformations, the monomer length and the chain diameter were theoretically calculated. These results were used to interpret experimental data, which comprised the molecule secondary structure, the diffusion coefficient, the hydrodynamic diameter and the electrophoretic mobility determined at various ionic strengths and pHs. Using these data, the electrokinetic charge and the effective ionization degree of P-Arg molecules were determined. In addition, the dynamic viscosity measurements for dilute P-Arg solutions enabledto determine the molecule intrinsic viscosity, which was equal to 500 and 90 for ionic strength of 10-5 and 0.15 M, respectively. This confirmed that P-Arg molecules assumed extended conformations and approached the slender body limit at the low range of ionic strength. The experimental data were also used to determine the molecule length and the chain diameter, which agreed with theoretical predictions. Exploiting these results, a robust method for determining the molar mass of P-Arg samples, the hydrodynamic diameter, the radius of gyration and the sedimentation coefficient was proposed.

Heterogeneity in Lateral Distribution of Polycations at the Surface of Lipid Membrane: From the Experimental Data to the Theoretical Model

Natural and synthetic polycations of different kinds attract substantial attention due to an increasing number of their applications in the biomedical industry and in pharmacology. The key characteristic determining the effectiveness of the majority of these applications is the number of macromolecules adsorbed on the surface of biological cells or their lipid models. Their study is complicated by a possible heterogeneity of polymer layer adsorbed on the membrane. Experimental methods reflecting the structure of the layer include the electrokinetic measurements in liposome suspension and the boundary potential of planar bilayer lipid membranes (BLM) and lipid monolayers with a mixed composition of lipids and the ionic media. In the review, we systematically analyze the methods of experimental registration and theoretical description of the laterally heterogeneous structures in the polymer layer published in the literature and in our previous studies. In particular, we consider a model based on classical theory of the electrical double layer, used to analyze the available data of the electrokinetic measurements in liposome suspension with polylysines of varying molecular mass. This model suggests a few parameters related to the heterogeneity of the polymer layer and allows determining the conditions for its appearance at the membrane surface. A further development of this theoretical approach is discussed.

Effect of Branching Degree of Dendritic Polyglycerols on Plasma Protein Adsorption: Relationship between Hydration States and Surface Morphology

This study focuses on dendritic glycerols and investigates the construction of biocompatible surfaces by understanding how differences in the branching of these molecules change the interactions with the biological components. The two molecules, polyglycerol dendrimer (PGD), which has a completely branched structure, and hyperbranched polyglycerol (HPG), which has an incompletely branched structure, are compared and the differences in branching are evaluated. It is shown that PGD has a little bit more intermediate water than HPG, which reflects the differences in the branching. The effect of surface state on the adsorption of the plasma proteins, human serum albumin (HSA), fibrinogen (Fib), and fibronectin (FN), is discussed by modifying a glass surface using these molecules with different hydration states. The adsorption of HSA decreases to several percent for HPG and 10% for PGD compared to unmodified substrate. Although the adsorption of Fib decreases to 5% for HPG, an increase to 150% is observed for PGD. Since this specific Fib adsorption observed only onto PGD is suppressed in the cases of a mixed solution of HSA and Fib or sequentially using HSA solution and then Fib solution, it is thought that the Vroman effect is suppressed on the PGD-modified surface. Furthermore, when AFM measurements are performed in PBS to understand the surface roughness, PGD is found to be more highly non-uniform. Because of this, the nanometer scale roughness that is significantly observed only on the PGD-modified surface is thought to have an effect on the characteristic adsorption properties of Fib. Thus, although both PGD and HPG with different branching have intermediate water, the proportion differs between PGD and HPG. Therefore, it is found that differences occur in the plasma protein adsorption mechanisms depending on the coordinates and density of hydroxyl groups within the molecules.

Functional Lignin Nanoparticles with Tunable Size and Surface Properties: Fabrication, Characterization, and Use in Layer-by-Layer Assembly

Lignin is the richest source of renewable aromatics and has immense potential for replacing synthetic chemicals. The limited functionality of lignin is, however, challenging for its potential use, which motivates research for creating advanced functional lignin-derived materials. Here, we present an aqueous-based acid precipitation method for preparing functional lignin nanoparticles (LNPs) from carboxymethylated or carboxypentylated lignin. We observe that the longer grafted side chains of carboxypentylated lignin allow for the formation of larger LNPs. The functional nanoparticles have high tolerance against salt and aging time and well-controlled size distribution with R_h ≤ 60 nm over a pH range of 5-11. We further investigate the layer-by-layer (LbL) assembly of the LNPs and poly(allylamine hydrochloride) (PAH) using a stagnation point adsorption reflectometry (SPAR) and quartz crystal microbalance with dissipation (QCM-D). Results demonstrate that LNPs made of carboxypentylated lignin (i.e., PLNPs with the adsorbed mass of 3.02 mg/m²) form a more packed and thicker adlayer onto the PAH surface compared to those made of carboxymethylated lignin (i.e., CLNPs with the adsorbed mass of 2.51 mg/m²). The theoretical flux, J, and initial rate of adsorption, (dΓ/dt)₀, analyses confirm that 22% of PLNPs and 20% of CLNPs arriving at the PAH surface are adsorbed. The present study provides a feasible platform for engineering LNPs with a tunable size and adsorption behavior, which can be adapted in bionanomaterial production.

Controlling Growth of Poly (Triethylene Glycol Acrylate-Co-Spiropyran Acrylate) Copolymer Liquid Films on a Hydrophilic Surface by Light and Temperature

A quartz crystal microbalance with dissipation monitoring (QCM-D) was employed for in situ investigations of the effect of temperature and light on the conformational changes of a poly (triethylene glycol acrylate-co-spiropyran acrylate) (P (TEGA-co-SPA)) copolymer containing 12-14% of spiropyran at the silica-water interface. By monitoring shifts in resonance frequency and in acoustic dissipation as a function of temperature and illumination conditions, we investigated the evolution of viscoelastic properties of the P (TEGA-co-SPA)-rich wetting layer growing on the sensor, from which we deduced the characteristic coil-to-globule transition temperature, corresponding to the lower critical solution temperature (LCST) of the PTEGA part. We show that the coil-to-globule transition of the adsorbed copolymer being exposed to visible or UV light shifts to lower LCST as compared to the bulk solution: the transition temperature determined acoustically on the surface is 4 to 8 K lower than the cloud point temperature reported by UV/VIS spectroscopy in aqueous solution. We attribute our findings to non-equilibrium effects caused by confinement of the copolymer chains on the surface. Thermal stimuli and light can be used to manipulate the film formation process and the film's conformational state, which affects its subsequent response behavior.

Studying Soft Interfaces with Shear Waves: Principles and Applications of the Quartz Crystal Microbalance (QCM)

The response of the quartz crystal microbalance (QCM, also: QCM-D for "QCM with Dissipation monitoring") to loading with a diverse set of samples is reviewed in a consistent frame. After a brief introduction to the advanced QCMs, the governing equation (the small-load approximation) is derived. Planar films and adsorbates are modeled based on the acoustic multilayer formalism. In liquid environments, viscoelastic spectroscopy and high-frequency rheology are possible, even on layers with a thickness in the monolayer range. For particulate samples, the contact stiffness can be derived. Because the stress at the contact is large, the force is not always proportional to the displacement. Nonlinear effects are observed, leading to a dependence of the resonance frequency and the resonance bandwidth on the amplitude of oscillation. Partial slip, in particular, can be studied in detail. Advanced topics include structured samples and the extension of the small-load approximation to its tensorial version.

Particle Deposition to Silica Surfaces Functionalized with Cationic Polyelectrolytes

Positively charged water-solid interfaces are prepared by adsorption of a cationic polyelectrolyte poly(diallyldimethylammonium chloride) (PDADMAC) from aqueous solutions to planar silica substrates. These substrates are characterized by atomic force microscopy (AFM), optical reflectivity, and streaming current measurements. By tuning the amount of adsorbed polyelectrolyte, the surface charge of the substrate can be systematically varied. These substrates are further used to study deposition of sulfate latex nanoparticles, which is also accomplished by optical reflectivity. This deposition process is found to be consistent with an extension of the random sequential adsorption (RSA) model in a semi-quantitative fashion. Such deposition studies were further used to ascertain that the substrates obtained by in situ and ex situ functionalization behave in an identical fashion.

Oscillatory structural forces between charged interfaces in solutions of oppositely charged polyelectrolytes

Forces between negatively charged micron-sized silica particles were measured in aqueous solutions of cationic polyelectrolytes with an atomic force microscope (AFM). In these oppositely charged systems, damped oscillatory force profiles were systematically observed in systems at higher polyelectrolyte concentrations, typically around few g L-1. The wavelength of these oscillations is decreasing with increasing concentration. When the wavelength and concentration are normalized with the cross-over concentration, universal power-law dependence is found. Thereby, the corresponding scaling exponent changes from 1/3 in the dilute regime to 1/2 in the semi-dilute regime. This dependence is the same as in the like-charged systems, which were described in the literature earlier. This common behavior suggests that these oscillatory forces are related to the structuring of the polyelectrolyte solutions. The reason that the oppositely charged systems behave similarly to like-charged ones is that the former systems undergo a charge reversal due to the adsorption of the polyelectrolytes to the oppositely charged surface, whereby sufficiently homogeneous adsorbed layers are being formed. The main finding of the present study is that at higher polyelectrolyte concentrations such oscillatory forces are the rule, including the oppositely charged ones.

Deposition of Synthetic and Bio-Based Polycations onto Negatively Charged Solid Surfaces: Effect of the Polymer Cationicity, Ionic Strength, and the Addition of an Anionic Surfactant

The deposition of layers of different polycations (synthetic or derived from natural, renewable resources) onto oppositely charged surfaces has been studied using ellipsometry and quartz crystal microbalance with dissipation monitoring (QCM-D). Information about the thickness of the deposited layers and their water content was ascertained. The adsorption of the different polycations onto negatively charged surfaces was found to be a complex process, which is influenced by the chemical nature of the polymer chains, ionic strength, polymer concentration and the addition of additives such as surfactants. The experimental picture shows a good agreement with theoretical calculations performed using the Self-Consistent Mean Field (SCF) approach. The results show that the electrostatically-driven deposition can be tuned by modifying the physico-chemical properties of the solutions and the chemical nature of the adsorbed polymer. This versatile approach is a big step forward in aiding the design of new polymers for many industrial applications and, in particular, the design of sustainable washing formulations for cosmetic applications.

Interaction of synthetic and lignin-based sulfonated polymers with hydrophilic, hydrophobic, and charged self-assembled monolayers

There is a need to understand the role of polymer structure on its interaction with surfaces to produce effective functional surfaces. In this work, we produced two anionic polymers of lignin-3-sulfopropyl methacrylate (L-S) and poly(vinyl alcohol-co-vinyl acetate)-3-sulfopropyl methacrylate (PVA-S) with similar charge densities and molecular weights. On the gold-coated surface, we deposited self-assembled monolayers (SAM) bearing different terminal moieties namely, hydroxyl, carboxyl, methyl, and amine groups of alkanethiols. This study highlighted the difference between the interaction of L-S and PVA-S and functionalized self-assembled surfaces. The information was generated using advanced tools, such as an X-ray photoelectron spectroscopy (XPS), and a quartz crystal microbalance with dissipation (QCM-D), which facilitated the correlation development between polymer properties and deposition performance on the functionalized surfaces. The higher deposition of PVA-S than L-S onto OH and COOH surfaces was observed due to its greater hydrogen bonding development and higher solubility. The solubility and structure of PVA-S were also beneficial for its higher adsorption than L-S onto CH₃ and NH₂ surfaces. However, the variation in pH, temperature, and salt significantly affected the adsorption of the macromolecules.

The interaction mechanism of synthetic and lignin based sulfonated materials with well-designed functional surfaces was investigated systematically.

PEGylated surfaces for the study of DNA-protein interactions by atomic force microscopy

DNA-protein interactions are vital to cellular function, with key roles in the regulation of gene expression and genome maintenance. Atomic force microscopy (AFM) offers the ability to visualize DNA-protein interactions at nanometre resolution in near-physiological buffers, but it requires that the DNA be adhered to the surface of a solid substrate. This presents a problem when working in biologically relevant protein concentrations, where proteins may be present in large excess in solution; much of the biophysically relevant information can therefore be occluded by non-specific protein binding to the underlying substrate. Here we explore the use of PLLx-b-PEGy block copolymers to achieve selective adsorption of DNA on a mica surface for AFM studies. Through varying both the number of lysine and ethylene glycol residues in the block copolymers, we show selective adsorption of DNA on mica that is functionalized with a PLL10-b-PEG113/PLL1000-2000 mixture as viewed by AFM imaging in a solution containing high concentrations of streptavidin. We show - through the use of biotinylated DNA and streptavidin - that this selective adsorption extends to DNA-protein complexes and that DNA-bound streptavidin can be unambiguously distinguished in spite of an excess of unbound streptavidin in solution. Finally, we apply this to the nuclear enzyme PARP1, resolving the binding of individual PARP1 molecules to DNA by in-liquid AFM.

Kinetics of Poly-l-lysine Adsorption on Mica and Stability of Formed Monolayers: Theoretical and Experimental Studies

Various physicochemical parameters of poly-l-lysine (PLL) solutions comprising the diffusion coefficient, the electrophoretic mobility, the density, and the intrinsic viscosity were determined for the pH range 3.0-9.2. This allowed us to calculate derivative parameters characterizing the PLL molecule such as: zeta potential, the number of electrokinetic charges, ionization degree, contour length, and cross section area. These data were exploited in theoretical calculations of PLL adsorption kinetics on solid substrates under diffusion transport. A hybrid approach was used comprising a blocking function derived from the random sequential adsorption (RSA) model. In experiments, the PLL adsorption on mica was studied using the streaming potential measurements and interpreted in terms of a general electrokinetic model. This confirmed a side-on adsorption mechanism of the macroion molecules at the examined pH range. Additionally, using this method, the stability of PLL monolayers was determined performing in situ desorption kinetic experiments. In this way, the equilibrium adsorption constant and the energy minimum depth were determined. It was confirmed that the monolayer stability decreases with pH following the decrease in the number of electrokinetic charges per molecule. This confirmed the electrostatic interaction driven adsorption mechanism of PLL. It is also predicted that at pH 5.7-7.4 the monolayers were stable under diffusion-controlled desorption over the time exceeding 100 h. In addition to their significance for basic science, the results obtained in this work can be exploited for developing procedures for preparing stable PLL monolayers of well controlled coverage and electrokinetic properties.

Mechanisms of Fibrinogen Adsorption on Silica Sensors at Various pHs: Experiments and Theoretical Modeling

The adsorption kinetics of human serum fibrinogen at silica substrates was studied using optical waveguide lightmode spectroscopy (OWLS) and quartz crystal microbalance (QCM) techniques. Measurements were performed at pH 3.5, 4, and 7.4 for various ionic strengths. The experimental data were interpreted in terms of a hybrid random sequential adsorption model. This allowed the mass transfer rate coefficient for the OWLS cell and maximum coverages to be determined at various pHs. The appearance of different, pH-dependent mechanisms of fibrinogen adsorption on silica substrates was confirmed. At pH 3.5 the molecules mostly adsorb in the side-on orientation that produces a low maximum coverage of ca. 1 mg m^-2. At this pH, the kinetics derived from the OWLS measurements agree with those theoretically predicted using the convective-diffusion theory. In consequence, a comparison of the OWLS and QCM results allows the water factor and the dynamic hydration of fibrinogen molecules to be determined. At pH 7.4, the OWLS method gives inaccurate kinetic data for the low coverage range. However, the maximum coverage that was equal to ca. 4 mg m^-2 agrees with the QCM results and with previous literature results. It is postulated that the limited accuracy of the OWLS method for lower coverage stems from a heterogeneous structure of fibrinogen monolayers, which consist of side-on and end-on adsorbed molecules. One can expect that the results acquired in this work allow development of a robust procedure for preparing fibrinogen monolayers of well-controlled coverage and molecule orientation, which can be exploited for efficient immunosensing purposes.

Human Serum Albumin Adsorption Kinetics on Silica: Influence of Protein Solution Stability

Adsorption kinetics of human serum albumin (HSA) on silica substrates was studied using optical waveguide lightmode spectroscopy (OWLS) and quartz microbalance (QCM) techniques. Measurements were performed at pH 3.5, 5.6, and 7.4 for various bulk suspension concentrations and ionic strengths. The diffusion coefficient measurements showed that for pH 3.5 the HSA molecules are stable for NaCl concentrations from 10^-3 to 0.15 M. This allowed us to precisely determine the mass transfer rate coefficients for the OWLS and QCM cells. The experimental data were adequately interpreted in terms of a hybrid random sequential adsorption model. The OWLS maximum coverage of HSA at pH 3.5, which is equal to 1.3 mg m^-2, agrees with the QCM result and with previous results derived from streaming potential measurements. Thus, the results obtained at pH 3.5 served as reference data for the analysis of adsorption kinetics at higher pHs. In this way, it was confirmed that the adsorption kinetics of HSA molecules at pH 5.6 and 7.4 was considerably slower than at pH 3.5. This effect was attributed to aggregation of HSA solutions and interpreted in terms of a theoretical model combining the Smoluchowski aggregation theory with the convective diffusion mass transfer theory. New analytical equations were derived that can be used for the interpretation of other protein adsorption from unstable solutions.

Layer-by-layer adsorption: Factors affecting the choice of substrates and polymers

The electrostatic layer-by-layer technique for fabrication of multi-layered structures of various sizes and shapes using flat and colloidal templates coupled with polyelectrolyte layer-forming materials has attracted significant interest among both academic and industrial researchers due to its versatility and relative simplicity of the procedures involved in its execution. Fabrication of the multi-layered structures using the electrostatic layer-by-layer method involves several distinct stages each of which holds great importance when considering the production of a high-quality product. These stages include selection of materials (both template and a pair of construction polyelectrolytes), adsorption of the first polyelectrolyte layer onto the selected templates, formation of the second layer comprised of the oppositely charged polyelectrolyte and guided by the interactions between the two chosen polyelectrolytes, and multi-layering, where a selected number of layers are produced, and which is conditioned by both intrinsic properties of the involved construction materials and external fabrication conditions such as temperature, pH and ionic strength. The current review summarises the most important aspects of each stage mentioned above and gives examples of the materials suitable for utilization of the technique and describes the underlying physics involved.

Kinetics of human serum albumin adsorption at silica sensor: Unveiling dynamic hydration function

Adsorption kinetics of human serum albumin (HSA) at a silica substrate was studied using the QCM-D and AFM methods. Measurements were performed at pH 3.5 for various bulk suspension concentrations and ionic strengths. The QCM experimental data were compared with the dry coverage of HSA derived from AFM and from the solution of the mass transfer equation. In this way, the dynamic hydration functions and water factors of HSA monolayers were quantitatively evaluated as a function of dry coverage for various ionic strengths. Using the hydration functions, the HSA adsorption runs derived from QCM-D measurements were converted to the dry coverage vs. the time relationships. In this way, the maximum coverage of irreversibly bound HSA molecules was determined. It was equal to 0.35 and 1.4 mg m^-2 for NaCl concentration of 0.001 and 0.15 M, respectively. These results agree with previous experimental data derived by streaming potential measurements for mica and with theoretical modeling. Therefore, the side-on mechanism of HSA adsorption at silica sensor at pH 3.5 was confirmed. Also, a quantitative analysis of the desorption runs allowed one to calculate the binding energy of the reversibly bound HSA fraction. Beside significance to basic science, these results enable to develop a robust technique of preparing HSA monolayers at silica sensor of well-controlled coverage and molecule orientation.

Macroion adsorption-electrokinetic and optical methods

Recent studies on macroion adsorption at solid/liquid interfaces evaluated by electrokinetic and optical methods are reviewed. In the first section a description of electrokinetic phenomena at a solid surface is briefly outlined. Various methods for determining both static and dynamic properties of the electrical double layer, such as the appropriate location of the slip plane, are presented. Theoretical approaches are discussed concerning quantitative interpretation of streaming potential/current measurements of homogeneous macroscopic interfaces. Experimental results are presented, involving electrokinetic characteristics of bare surfaces, such as mica, silicon, glass etc. obtained from various types of electrokinetic cells. The surface conductivity effect on zeta potential is underlined. In the next section, various theoretical approaches, proposed to determine a distribution of electrostatic potential and flow distribution within macroion layers, are presented. Accordingly, the influence of the uniform as well as non-uniform distribution of charges within macroion layer, the dissociation degree, and the surface conductance on electrokinetic parameters are discussed. The principles, the advantages and limits of optical techniques as well as AFM are briefly outlined in Section 4. The last section is devoted to the discussion of experimental data obtained by streaming potential/current measurements and optical methods, such as reflectometry, ellipsometry, surface plasmon resonance (SPR), optical waveguide lightmode spectroscopy (OWLS), colloid enhancement, and fluorescence technique, for mono- and multilayers of macroions. Results of polycations (PEI, PAMAM dendrimers, PAH, PDADMAC) and polyanions (PAA, PSS) adsorption on mica, silicon, gold, and PTFE are quantitatively interpreted in terms of theoretical approaches postulating the three dimensional charge distribution or the random sequential adsorption model (RSA). Macroion bilayer formation, experimentally examined by streaming current measurements, and theoretically interpreted in terms of the comprehensive formalism is also reviewed. The utility of electrokinetic measurements, combined with optical methods, for a precise, in situ characteristics of macroion mono- and multilayer formation at solid/liquid interfaces is pointed out.

Layer-by-layer assembled hydrophobic coatings for cellulose nanofibril films and textiles, made of polylysine and natural wax particles

Herein we present a simple method to render cellulosic materials highly hydrophobic while retaining their breathability and moisture buffering properties, thus allowing for their use as functional textiles. The surfaces are coated via layer-by-layer deposition of two natural components, cationic poly-l-lysine and anionic carnauba wax particles. The combination of multiscale roughness, open film structure, and low surface energy of wax colloids, resulted in long-lasting superhydrophobicity on cotton surface already after two bilayers. Atomic force microscopy, interference microscopy, scanning electron microscopy and X-ray photoelectron spectroscopy were used to decouple structural effects from changes in surface energy. Furthermore, the effect of thermal annealing on the coating was evaluated. The potential of this simple and green approach to enhance the use of natural cellulosic materials is discussed.

Formation and stability of manganese-doped ZnS quantum dot monolayers determined by QCM-D and streaming potential measurements

Manganese-doped ZnS quantum dots (QDs) stabilized by cysteamine hydrochloride were successfully synthesized. Their thorough physicochemical characteristics were acquired using UV-Vis absorption and photoluminescence spectroscopy, X-ray diffraction, dynamic light scattering (DLS), transmission electron microscopy (HR-TEM), energy dispersive spectroscopy (EDS) and Fourier transform infrared (FT-IR) spectroscopy. The average particle size, derived from HR-TEM, was 3.1nm, which agrees with the hydrodynamic diameter acquired by DLS, that was equal to 3-4nm, depending on ionic strength. The quantum dots also exhibited a large positive zeta potential varying between 75 and 36mV for ionic strength of 10^-4 and 10^-2M, respectively (at pH 6.2) and an intense luminescent emission at 590nm. The quantum yield was equal to 31% and the optical band gap energy was equal to 4.26eV. The kinetics of QD monolayer formation on silica substrates (silica sensors and oxidized silicon wafers) under convection-controlled transport was quantitatively evaluated by the quartz crystal microbalance (QCM) and the streaming potential measurements. A high stability of the monolayer for ionic strength 10^-4 and 10^-2M was confirmed in these measurements. The experimental data were adequately reflected by the extended random sequential adsorption model (eRSA). Additionally, thorough electrokinetic characteristics of the QD monolayers and their stability for various ionic strengths and pH were acquired by streaming potential measurements carried out under in situ conditions. These results were quantitatively interpreted in terms of the three-dimensional (3D) electrokinetic model that furnished bulk zeta potential of particles for high ionic strengths that is impractical by other experimental techniques. It is concluded that these results can be used for designing of biosensors of controlled monolayer structure capable to bind various ligands via covalent as well as electrostatic interactions.

Interplay between Depletion and Double-Layer Forces Acting between Charged Particles in Solutions of Like-Charged Polyelectrolytes

Direct force measurements between negatively charged silica particles in the presence of a like-charged strong polyelectrolyte were carried out with an atomic force microscope. The force profiles can be quantitatively interpreted as a superposition of depletion and double-layer forces. The depletion forces are modeled with a damped oscillatory profile, while the double-layer forces with the mean-field Poisson-Boltzmann theory for a strongly asymmetric electrolyte, whereby an effective valence must be assigned to the polyelectrolyte. This effective valence is substantially smaller than the bare valence due to ion condensation effects. The unusual aspect of the electrical double layer in these systems is the exclusion of the like-charged polyelectrolyte from the vicinity of the surface, leading to a strongly nonexponential diffuse ionic layer that is dominated by counterions and has a well-defined thickness. As the oscillatory depletion force sets in right after this layer, this condition can be used to predict the phase of the oscillatory depletion force.

The effect of guanidinium functionalization on the structural properties and anion affinity of polyelectrolyte multilayers

Poly(allylamine hydrochloride) (PAH) is chemically functionalized with guanidinium (Gu) moieties in water at room temperature. The resulting PAH-Gu is used to prepare polyelectrolyte multilayers (PEMs) with poly(sodium 4-styrene sulfonate) (PSS) via layer-by-layer deposition. The polyelectrolyte (PE) adsorption processes are monitored real-time by optical reflectometry and a quartz crystal microbalance with dissipation monitoring (QCM-D). Compared to the reference PSS/PAH PEMs, the PSS/PAH-Gu PEMs show a lower amount of deposited PE materials, lower wet thickness, higher stability under alkaline conditions and higher rigidity. These differences are rationalized by the additional Gu-SO3(-) interactions, also affecting the conformation of the PE chains in the PEM. The interactions between the PEMs and various sodium salts (NaCl, NaNO3, Na2SO4 and NaH2PO4) are also monitored using QCM-D. From the changes in the frequency, dissipation responses and supportive Reflection Absorption Infrared Spectroscopy it is concluded that Gu-functionalized PEMs absorb more H2PO4(-) compared to the Gu-free reference PEMs. This can be understood by strong interactions between Gu and H2PO4(-), the differences in the anion hydration energy and the anion valency. It is anticipated that compounds like the presented Gu-functionalized PE may facilitate the further development of H2PO4(-) sensors and ion separation/recovery systems.

Probing effects of polymer adsorption in colloidal particle suspensions by light scattering as relevant for the aquatic environment: An overview

Modification of particle surfaces by adsorption of polymers is a process that governs particle behavior in aqueous environmental systems. The present article briefly reviews the current understanding of the adsorption mechanisms and the properties of the resulting layers, and it discusses two environmentally relevant cases of particle modification by polymers. In particular, the discussion focuses on the usefulness of methods based on light scattering to probe such adsorbed layers together with the resulting properties of the particle suspensions, and it highlights advantages and disadvantages of these techniques. Measurement of the electrophoretic mobility allows to follow the development of the adsorption layer and to characterize the charge of the modified particles. At saturation, the surface charge is governed by the charge of the adsorbed film. Dynamic light scattering provides information on the film thickness and on the behavior of the modified suspensions. The charge and the structure of the adsorbed layer influence the stability of the particles, as well as the applicability of the classical theory of Derjaguin, Landau, Verwey, and Overbeek (DLVO). This fundamental knowledge is presented in the light of environmental systems and its significance for applied systems is underlined. In particular, the article discusses two examples of environmental processes involving adsorption of polymers, namely, the modification of particles by natural adsorption of humic substances and the tailoring of surface properties of iron-based particles used to remediate contaminated aquifers.

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.

Mapping single macromolecule chains using the colloid deposition method: PDADMAC on mica

Monolayers of the cationic polyelectrolyte poly(diallyldimethylammonium chloride) (PDADMAC) on mica were thoroughly characterized using the streaming potential and the colloid deposition methods. Initially, the stability of the monolayers was determined by performing desorption experiments carried out under diffusion-controlled regime. It was shown that the desorption of the polyelectrolyte at the ionic strength range 0.01-0.15 M is negligible over the time of 20 h. The structure of PDADMAC monolayers and orientation of molecules were evaluated using the colloid deposition measurements involving negatively charged polystyrene latex microspheres, 820 nm in diameter. The functional relationships between the polyelectrolyte coverage and latex coverage deposited within 20 h were acquired by direct optical microscope. In this way the influence of ionic strength varied in the range 0.15-0.01 M on the molecule orientation in monolayers was determined. It was shown that for ionic strength of 0.15 M nearly one to one mapping of polyelectrolyte chains by colloid particles can be achieved for PDADMAC coverage below 0.1%. In this way, because of a considerable surface area ratio between the macromolecule and the colloid particle, an enhancement factor of 10(3) can be attained. This behavior was quantitatively interpreted in terms of the random site adsorption model whereas the classical mean-field theory proved inadequate. On the other hand, for lower ionic strength, it was confirmed that an irreversible immobilization of latex particles can only occur at a few closely spaced PDADMAC chains. It was shown that these experimental results were consistent with the side-on adsorption mechanisms of PDADMAC at mica for the above ionic strength.

Adsorbed Mass of Polymers on Self-Assembled Monolayers: Effect of Surface Chemistry and Polymer Charge

The adsorbed mass of polymers on surfaces with different chemistry is presented, and the related adsorption mechanism is discussed. Strong and weak polyelectrolytes of negative and positive charge are studied, as well as an uncharged polymer. Self-assembled monolayers of alkanethiols on gold are used in reflectometry and quartz crystal microbalance (QCM-D) experiments as adsorbing substrates bearing different terminal moieties, namely, methyl, hydroxyl, carboxyl, and amine groups. The various polymer-surface combinations allow the systematic investigation of the role of surface chemistry and polymer charge on adsorbed amount. Interactions of different nature and range drive polymer adsorption: the measured adsorbed amounts reveal information about their relative contribution. When electrostatic chain-surface attraction is present, the largest adsorbed masses are observed. However, significant mass is measured even when an electrostatic barrier to adsorption is present, suggesting the importance of forces of nonelectrostatic origin, which include both hydrophobic interactions and specific forces acting at short distances. This mechanism results in large adsorbed amounts for the adsorption of weak polyelectrolytes, and it is apparent especially in the adsorption behavior of a neutral polymer.

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.

Studying the role of surface chemistry on polyelectrolyte adsorption using gold-thiol self-assembled monolayer with optical reflectivity

Self-assembled monolayers of thiols on gold are employed to study the role of surface chemistry on adsorption of polyelectrolytes to solid substrates. The suitability of these substrates is demonstrated in optical reflectivity, which combines high sensitivity to the possibility to precisely control the hydrodynamic conditions at the solid/water interface. Therefore, this system allows the determination of both the adsorbed amount and the kinetics of adsorption. The behavior of two representative strong polyelectrolytes of opposite charge is discussed as a function of pH and of concentration of a monovalent electrolyte in aqueous solutions. The application of equivalent substrates with varying surface chemistry sheds light on the role of different energetic contributions driving polyelectrolyte adsorption.

Mechanism of chitosan adsorption on silica from aqueous solutions

We present a study of the adsorption of chitosan on silica. The adsorption behavior and the resulting layer properties are investigated by combining optical reflectometry and the quartz crystal microbalance. Exactly the same surfaces are used to measure the amount of adsorbed chitosan with both techniques, allowing the systematic combination of the respective experimental results. This experimental protocol makes it possible to accurately determine the thickness of the layers and their water content for chitosan adsorbed on silica from aqueous solutions of varying composition. In particular, we study the effect of pH in 10 mM NaCl, and we focus on the influence of electrolyte type and concentration for two representative pH conditions. Adsorbed layers are stable, and their properties are directly dependent on the behavior of chitosan in solution. In mildly acidic solutions, chitosan behaves like a weakly charged polyelectrolyte, whereby electrostatic attraction is the main driving force for adsorption. Under these conditions, chitosan forms rigid and thin adsorption monolayers with an average thickness of approximately 0.5 nm and a water content of roughly 60%. In neutral solutions, on the other hand, chitosan forms large aggregates, and thus adsorption layers are significantly thicker (∼10 nm) as well as dissipative, resulting in a large maximum of adsorbed mass around the pK of chitosan. These films are also characterized by a substantial amount of water, up to 95% of their total mass. Our results imply the possibility to produce adsorption layers with tailored properties simply by adjusting the solution chemistry during adsorption.

Polyelectrolyte adsorption, interparticle forces, and colloidal aggregation

This review summarizes the current understanding of adsorption of polyelectrolytes to oppositely charged solid substrates, the resulting interaction forces between such substrates, and consequences for colloidal particle aggregation. The following conclusions can be reached based on experimental findings. Polyelectrolytes adsorb to oppositely charged solid substrates irreversibly up to saturation, whereby loose and thin monolayers are formed. The adsorbed polyelectrolytes normally carry a substantial amount of charge, which leads to a charge reversal. Frequently, the adsorbed films are laterally heterogeneous. With increasing salt levels, the adsorbed mass increases leading to thicker and more homogeneous films. Interaction forces between surfaces coated with saturated polyelectrolyte layers are governed at low salt levels by repulsive electric double layer interactions, and particle suspensions are stable under these conditions. At appropriately high salt levels, the forces become attractive, principally due to van der Waals interactions, but eventually also through other forces, and suspensions become unstable. This situation can be rationalized with the classical theory of Derjaguin, Landau, Verwey, and Overbeek (DLVO). Due to the irreversible nature of the adsorption process, stable unsaturated layers form in colloidal particle suspensions at lower polyelectrolyte doses. An unsaturated polyelectrolyte layer can neutralize the overall particle surface charge. Away from the charge reversal point, electric double layer forces are dominant and particle suspensions are stable. As the charge reversal point is approached, attractive van der Waals forces become important, and particle suspensions become unstable. This behaviour is again in line with the DLVO theory, which may even apply quantitatively, provided the polyelectrolyte films are sufficiently laterally homogeneous. For heterogeneous films, additional attractive patch-charge interactions may become important. Depletion interactions may also lead to attractive forces and suspension destabilization, but such interactions become important only at high polyelectrolyte concentrations.

Effect of peptide secondary structure on adsorption and adsorbed film properties on end-grafted polyethylene oxide layers

Poly-l-lysine (PLL), in α-helix or β-sheet configuration, was used as a model peptide for investigating the effect of secondary structures on adsorption events to poly(ethylene oxide) (PEO) modified surfaces formed using θ solvents. Circular dichroism results showed that the secondary structure of PLL persisted upon adsorption to Au and PEO modified Au surfaces. Quartz crystal microbalance with dissipation (QCM-D) was used to characterize the chemisorbed PEO layer in different solvents (θ and good solvents), as well as the sequential adsorption of PLL in different secondary structures (α-helix or β-sheet). QCM-D results suggest that chemisorption of PEO 750 and 2000 from θ solutions led to brushes 3.8 ± 0.1 and 4.5 ± 0.1 nm thick with layer viscosities of 9.2 ± 0.8 and 4.8 ± 0.5 cP, respectively. The average number of H2O per ethylene oxides, while in θ solvent, was determined as ~0.9 and ~1.2 for the PEO 750 and 2000 layers, respectively. Upon immersion in good solvent (as used for PLL adsorption experiments), the number of H2O per ethylene oxides increased to ~1.5 and ~2.0 for PEO 750 and 2000 films, respectively. PLL adsorbed masses for α-helix and β-sheet on Au sensors was 231 ± 5 and 1087 ± 14 ng cm(-2), with layer viscosities of 2.3 ± 0.1 and 1.2 ± 0.1 cP, respectively; suggesting that the α-helix layer was more rigid, despite a smaller adsorbed mass, than that of β-sheet layers. The PEO 750 layer reduced PLL adsorbed amounts to ~10 and 12% of that on Au for α-helices and β-sheets respectively. The PLL adsorbed mass to PEO 2000 layers dropped to ~12% and 4% of that on Au, for α-helix and β-sheet respectively. No significant differences existed for the viscosities of adsorbed α-helix and β-sheet PLL on PEO surfaces. These results provide new insights into the fundamental understanding of the effects of secondary structures of peptides and proteins on their surface adsorption.

Evaluation of the effects of amphiphilic oligomers in PEI based ternary complexes on the improvement of pDNA delivery

(HEMA-b-NIPAM) was incorporated into PEI/P(HEMA-b-NIPAM)/pDNA ternary complexes through non-electrostatic assembly to enhance the interaction between complexes and cellular/endocellular membranes to improve gene transfection.