Anisotropic Particle Deposition Kinetics from Quartz Crystal Microbalance Measurements: Beyond the Sphere Paradigm

Deposition kinetics of polymer particles characterized by a prolate spheroid shape on gold sensors modified by the adsorption of poly(allylamine) was investigated using a quartz crystal microbalance and atomic force microscopy. Reference measurements were also performed for polymer particles of a spherical shape and the same diameter as the spheroid shorter axis. Primarily, the frequency and dissipation shifts for various overtones were measured as a function of time. These kinetic data were transformed into the dependence of the complex impedance, scaled up by the inertia impedance, upon the particle size to the hydrodynamic boundary layer ratio. The results obtained for low particle coverage were interpolated, which enabled the derivation of Sauerbrey-like equations, yielding the real particle coverage using the experimental frequency or dissipation (bandwidth) shifts. Experiments carried out for a long deposition time confirmed that, for spheroids, the imaginary and real impedance components were equal to each other for all overtones and for a large range of particle coverage. This result was explained in terms of a hydrodynamic, lubrication-like contact of particles with the sensor, enabling their sliding motion. In contrast, the experimental data obtained for spheres, where the impedance ratio was a complicated function of overtones and particle coverage, showed that the contact was rather stiff, preventing their motion over the sensor. It was concluded that results obtained in this work can be exploited as useful reference systems for a quantitative interpretation of bioparticle, especially bacteria, deposition kinetics on macroion-modified surfaces.

Quantifying Nanoparticle Layer Topography: Theoretical Modeling and Atomic Force Microscopy Investigations

A comprehensive method consisting of theoretical modeling and experimental atomic force microscopy (AFM) measurements was developed for the quantitative analysis of nanoparticle layer topography. Analytical results were derived for particles of various shapes such as cylinders (rods), disks, ellipsoids, hemispheres (caps), etc. It was shown that for all particles, their root-mean-square (rms) parameter exhibited a maximum at the coverage about 0.5, whereas the skewness was a monotonically decreasing function of the coverage. This enabled a facile determination of the particle coverage in the layer, even if the shape and size were not known. The validity of the analytical results was confirmed by computer modeling and experimental data acquired by AFM measurements for polymer nanoparticle deposition on mica and silica. The topographical analysis developed in this work can be exploited for a quantitative characterization of self-assembled layers of nano- and bioparticles, e.g., carbon nanotubes, silica and noble metal particles, DNA fragments, proteins, vesicles, viruses, and bacteria at solid surfaces. The acquired results also enabled a proper calibration, in particular the determination of the measurement precision, of various electron and scanning probe microscopies, such as AFM.

Mimicking Pseudo-Virion Interactions with Abiotic Surfaces: Deposition of Polymer Nanoparticles with Albumin Corona

Adsorption of human serum albumin (HSA) molecules on negatively charged polystyrene microparticles was studied using the dynamic light scattering, the electrophoretic and the solution depletion methods involving atomic force microscopy. Initially, the physicochemical characteristics of the albumin comprising the hydrodynamic diameter, the zeta potential and the isoelectric point were determined as a function of pH. Analogous characteristics of the polymer particles were acquired, including their size and zeta potential. The formation of albumin corona on the particles was investigated in situ by electrophoretic mobility measurements. The size, stability and electrokinetic properties of the particles with the corona were also determined. The particle diameter was equal to 125 nm, which coincides with the size of the SARS-CoV-2 virion. The isoelectric point of the particles appeared at a pH of 5. The deposition kinetics of the particles was determined by atomic force microscopy (AFM) under diffusion and by quartz microbalance (QCM) under flow conditions. It was shown that the deposition rate at a gold sensor abruptly vanished with pH following the decrease in the zeta potential of the particles. It is postulated that the acquired results can be used as useful reference systems mimicking virus adsorption on abiotic surfaces.

QCM-D Investigations of Anisotropic Particle Deposition Kinetics: Evidences of the Hydrodynamic Slip Mechanisms

Deposition kinetics of positively charged polymer microparticles, characterized by prolate spheroid shape, at silica and gold sensors was investigated using the quartz microbalance (QCM) technique. Reference measurements were also performed for positively charged polymer particles of spherical shape and the same mass as the spheroids. Primarily, the frequency and bandwidth shifts for various overtones were measured as a function of time. It is shown that the ratio of these signals is close to unity for all overtones. These results were converted to the dependence of the frequency shift on the particle coverage, directly determined by atomic force microscopy and theoretically interpreted in terms of the hydrodynamic model. A quantitative agreement with experiments was attained considering particle slip relative to the ambient oscillating flow. In contrast, the theoretical results pertinent to the rigid contact model proved inadequate. The particle deposition kinetics derived from the QCM method was compared with theoretical modeling performed according to the random sequential adsorption approach. This allowed to assess the feasibility of the QCM technique to furnish proper deposition kinetics for anisotropic particles. It is argued that the hydrodynamic slip effect should be considered in the interpretation of QCM kinetic results acquired for bioparticles, especially viruses.

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.

Adsorption kinetic of myoglobin on mica and silica - Role of electrostatic interactions

Adsorption kinetics of myoglobin molecules on mica and silica was studied using the atomic force microscopy (AFM), the colloid enhancement and the quartz microbalance (QCM) methods. Measurements were carried out for the NaCl concentration of 0.01 and 0.15 M as a function of pH comprising pH 7.4 stabilized by the PBS buffer. The electrophoretic mobility measurements enabled to derive the molecules zeta potential as a function of pH. The isoelectric point appearing at pH 5, is lower than that predicted from the theoretical calculations of the nominal dissociation charge. The AFM investigations confirmed that myoglobin molecules irreversibly adsorb at pH 3.5 yielding well-defined layers of single molecules. These layers were characterized using the colloid enhancement method involving polymer microparticles for pH range 3-9. The microparticle deposition kinetics was adequately interpreted in terms of a hybrid random sequential adsorption model. It is confirmed that the myoglobin layers exhibit a negligible zeta potential at pH equal to 5 in accordance with the electrophoretic mobility measurements. Analogous adsorption kinetic measurements were performed for the silica substrate using QCM and AFM. It is observed that myoglobin molecules irreversibly adsorb at pH 3.5 forming stable layers of single molecules. On the other hand, its adsorption kinetics at larger pHs was much slower exhibiting a poorly defined maximum coverage. This was attributed to aggregation of the myoglobin solutions due to their vanishing charge. The kinetic QCM runs were adequately interpreted in terms of a theoretical model combining the Smoluchowski aggregation theory with the convective diffusion mass transfer theory.

Myoglobin molecule charging in electrolyte solutions

The compensated charge of myoglobin molecule in electrolyte solution is considerably smaller than the nominal charge.

Gold substrates of controlled roughness and electrokinetic properties formed by nanoparticle deposition

The kinetics of positively charged gold nanoparticle self-assembly on oxidized silicon substrates (wafers) under diffusion-controlled transport was studied using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The latter technique allowed the roughness parameters of the monolayer (root mean square) to be determined as a function of the particle coverage. These results were adequately interpreted in terms of a theoretical model developed for surfaces covered by features of spherical shape considering the tip convolution effect. The stability and the electrokinetic characteristics (zeta potential) of the monolayers were also acquired using streaming potential measurements. It was shown that the inversion of the negative zeta potential of the bare substrate (overcharging) occurs at the particle coverage equal to 0.15, and for larger coverages positive zeta potential values were asymptotically attained. Additionally, the desorption kinetics of the particles was investigated by the streaming potential method, which confirmed the stability of the monolayers for a broad range of pHs. It was argued that these results enable to develop an efficient method for the preparation of gold sensors exhibiting a well-controlled surface roughness and electrostatic charge comprising both negative and positive values.

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.

PDDA-Montmorillonite Composites Loaded with Ru Nanoparticles: Synthesis, Characterization, and Catalytic Properties in Hydrogenation of 2-Butanone

The effect of synthesis parameters on the physicochemical properties of clay/ polydiallyldimethylammonium (PDDA)/Ru composites and their applicability in hydrogenation of 2-butanone under very mild conditions (room temperature, atmospheric pressure, and aqueous solution) was studied. Three synthetic procedures were employed, differing in the order of addition of components and the stage at which metallic Ru species were generated. The materials were characterized with XRD (X-ray diffraction), XRF (X-ray fluorescence), EDS (energy-dispersive spectroscopy), AFM (atomic force microscopy), TEM/HRTEM (transmission electron microscopy/high resolution transmission electron microscopy), and TG/DSC (thermal gravimetry/differential scanning microscopy techniques. The study revealed that the method of composite preparation affects its structural and thermal properties, and controls the distribution and size of Ru particles. All catalysts are active in hydrogenation of 2-butanone. For best catalytic performance (100% conversion within 30 min) both the size of Ru particles and the load of polymer had to be optimized. Superior catalytic properties were obtained over the composite with intermediate crystal size and intermediate PDDA load, prepared by generation of metallic Ru species in the polymer solution prior to intercalation. This method offers an easy way of controlling the crystal size by modification of Ru/PDDA ratio.

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.

Formation of hematite nanoparticle monolayers of controlled coverage and structure at polymeric microparticles

The deposition of hematite nanoparticles (22nm and 29nm in diameter) on negatively charged polystyrene microspheres (820nm in diameter) was studied by micro-electrophoretic measurements and AFM. The influence of ionic strength, varied between 10^-4 and 10^-2M, was determined. Initially, the electrophoretic mobility change of microspheres upon the addition of controlled amount of hematite nanoparticles were measured. These dependencies were quantitatively interpreted in terms of the general electrokinetic model. This allowed to determine the coverage of nanoparticles on microspheres under in situ conditions, which increased with ionic strength attaining 0.35 for the ionic strength of 10^-2M and 29 in diameter hematite particles. This effect, attributed to the decreasing range of lateral electrostatic repulsion among deposited particles, was accounted for by the random sequential adsorption model. However, the coverages attained for lower ionic strength exceeded the theoretical predictions. This effect was interpreted in terms of an additional electrostatic screening due to polymeric chains present at the microparticle surface. The acid base properties of the hematite monolayers were also acquired by applying thorough micro-electrophoretic measurements. The obtained results confirmed a feasibility of preparing hematite nanoparticle monolayers on polymeric carrier microspheres having well-defined coverage and structure.

Revealing deposition mechanism of colloid particles on human serum albumin monolayers

Colloid particle deposition was applied in order to characterize human serum albumin (HSA) monolayers on mica adsorbed under diffusion transport at pH 3.5. The surface concentration of HSA was determined by a direct AFM imaging of single molecules. The electrokinetic characteristics of the monolayers for various ionic strength were done by in situ streaming potential measurements. In this way the mean-field zeta potential of monolayers was determined. It was shown that the initially negative potential changed its sign for HSA surface concentrations above 2800μm(-2) that was interpreted as overcharging effect. The monolayers were also characterized by the colloid deposition method where negatively charged polystyrene particles, 810nm in diameter were used. The kinetics of particle deposition and their maximum coverage were determined as a function of the HSA monolayer surface concentration. An anomalous deposition of particles on substrates exhibiting a negative zeta potential was observed, which contradicts the mean-field theoretical predictions. This effect was quantitatively interpreted in terms of the random site sequential adsorption model. It was shown that efficient immobilization of particles only occurs at adsorption sites formed by three and more closely adsorbed HSA molecules. These results can be exploited as useful reference data for the analysis of deposition phenomena of bioparticles at protein monolayers that has practical significance for the regulation of the bioadhesive properties of surfaces.

Revealing fibrinogen monolayer conformations at different pHs: electrokinetic and colloid deposition studies

Adsorption mechanism of human fibrinogen on mica at different pHs is studied using the streaming potential and colloid deposition measurements. The fibrinogen monolayers are produced by a controlled adsorption under diffusion transport at pH of 3.5 and 7.4. Initially, the electrokinetic properties of these monolayers and their stability for various ionic strength are determined. It is shown that at pH 3.5 fibrinogen adsorbs irreversibly on mica for ionic strength range of 4×10(-4) to 0.15 M. At pH 7.4, a partial desorption is observed for ionic strength below 10(-2) M. This is attributed to the desorption of the end-on oriented molecules whereas the side-on adsorbed molecules remain irreversibly bound at all ionic strengths. The orientation of molecules and monolayer structure is evaluated by the colloid deposition measurements involving negatively charged polystyrene latex microspheres, 820 nm in diameter. An anomalous deposition of negative latex particles on substrates exhibiting a negative zeta potential is observed. At pH 3.5 measurable deposition of latex is observed even at low ionic strength where the approach distance of latex particles exceeded 70 nm. At pH 7.4 this critical distance is 23 nm. This confirms that fibrinogen monolayers formed at both pHs are characterized by the presence of the side-on and end-on oriented molecules that prevail at higher coverage range. It is also shown that positive charge is located at the end parts of the αA chains of the adsorbed fibrinogen molecules. Therefore, it is concluded that the colloid deposition method is an efficient tool for revealing protein adsorption mechanisms at solid/electrolyte interfaces.

Mechanism of nanoparticle deposition on polystyrene latex particles

The deposition of positive amidine latex particles (98 nm in diameter) on negative polystyrene latex particles (820 nm in diameter) was studied by SEM imaging, microelectrophoretic and concentration depletion methods involving AFM. The role of ionic strength varied between 10(-4) and 10(-2) M and was systematically studied. The number of deposited positive latex particles (surface coverage) was evaluated by a direct counting procedure exploiting the SEM images. This allowed one to calibrate the results obtained from measurements of the electrophoretic mobility of larger latex particles covered by a controlled amount of the positive latex. These dependencies were quantitatively interpreted in terms of the 3D electrokinetic model previously used for planar interfaces. This allowed us to determine the coverage of nanoparticles on latex carriers under in situ conditions. Additionally, the maximum coverage of the positive latex was determined via AFM where the kinetics of the residual amidine latex deposition on mica was measured. The maximum coverage monotonically increased with ionic strength, attaining 0.52 for 10(-2) M NaCl. This effect was interpreted in terms of reduced electrostatic repulsion among positive latex particles and theoretically accounted for by the random sequential adsorption model. The obtained results have significance for basic science, indicating that the results obtained for curved interfaces (polymeric carrier particles) by the microelectrophoretic method can be exploited to interpret the deposition of nanoparticles and proteins on planar interfaces and vice versa.

Recombinant albumin monolayers on latex particles

The adsorption of recombinant human serum albumin (rHSA) on negatively charged polystyrene latex micro-particles was studied at pH 3.5 and the NaCl concentration range of 10(-3) to 0.15 M. The electrophoretic mobility of latex monotonically increased with the albumin concentration in the suspension. The coverage of adsorbed albumin was quantitatively determined using the depletion method, where the residual protein concentration was determined by electrokinetic measurements and AFM imaging. It was shown that albumin adsorption was irreversible. Its maximum coverage on latex varied between 0.7 mg m(-2) for 10(-3) M NaCl to 1.3 mg m(-2) for 0.15 M NaCl. The latter value matches the maximum coverage previously determined for human serum albumin on mica using the streaming potential method. The increase in the maximum coverage was interpreted in terms of reduced electrostatic repulsion among adsorbed molecules. These facts confirm that albumin adsorption at pH 3.5 is governed by electrostatic interactions and proceeds analogously to colloid particle deposition. The stability of albumin monolayers was measured in additional experiments where changes in the latex electrophoretic mobility and the concentration of free albumin in solutions were monitored over prolonged time periods. Based on these experimental data, a robust procedure of preparing albumin monolayers on latex particles of well-controlled coverage and molecule distribution was proposed.

Fibrinogen monolayer characterization by colloid deposition

Colloid particle deposition was applied to characterize bovine and human fibrinogen (Fb) monolayers on mica produced by controlled adsorption under diffusion transport at pH 3.5. The surface concentration of Fb was determined by AFM enumeration of single molecules adsorbed over the substrate surface. The electrokinetic properties of Fb monolayers for various ionic strength were studied using the in situ streaming potential measurements. It was shown that Fb adsorbs irreversibly on mica for a broad range of ionic strength of 4 × 10(-4) to 0.15 M, NaCl. The overcharging of initially negative mica surface occurred for fibrinogen surface concentrations higher than 1400 μm(-2). The orientation of fibrinogen molecules in the monolayers was evaluated by the colloid deposition method involving negatively charged polystyrene latex microspheres, 820 nm in diameter. An anomalous deposition of negative latex particles on substrates exhibiting a negative zeta potential was observed, which contradicts the mean-field DLVO predictions. Measurable deposition was observed even at low ionic strength where the minimum approach distance of latex particles to the interface exceeds 70 nm (for 6 × 10(-4) M NaCl). This confirms that, at this pH, fibrinogen molecules adsorb end-on on mica assuming extended conformations with the positive charge located mostly in the end part of the αA chains. This agrees with previous experimental and theoretical results discussed in the literature (Santore, M. M.; Wertz Ch. F. Protein spreading kinetics at liquid-solid interfaces via an adsorption probe method. Langmuir 2005, 21, 10172-10178 (experimental); Adamczyk, Z.; Barbasz, J.; Cieśla, M.; Mechanisms of fibrinogen adsorption at solid substrates. Langmuir, 2011, 25, 6868-6878 (theoretical)). This unusual latex deposition on Fb monolayers was quantitatively interpreted in terms of the model developed in ref 55 (Jin, X.; Wang, N. H. L.; Tarjus, G.; Talbot, J. Irreversible adsorption on nonuniform surfaces: the random site model. J. Phys. Chem. 1993, 97, 4256-4258). It was concluded that the colloid deposition method is an efficient tool for revealing protein adsorption mechanisms at solid/electrolyte interfaces.

Nanoemulsion-templated multilayer nanocapsules for cyanine-type photosensitizer delivery to human breast carcinoma cells

There is great clinical interest in developing novel nanocarriers for hydrophobic cyanine dyes used as photosensitizing agents in photodynamic therapy (PDT). In the present study we have employed nanoemulsion-templated oil-core multilayer nanocapsules as robust nanocarriers for a cyanine-type photosensitizer IR-786. These nanoproducts were fabricated via layer-by-layer (LbL) adsorption of oppositely charged polyelectrolytes (PEs), i.e., anionic PSS and cationic PDADMAC on nanoemulsion liquid cores created by dicephalic or bulky saccharide-derived cationic surfactants. All nanocapsules, with different thicknesses of the PE shell and average size <200 nm (measured by DLS) demonstrated good capacity for IR-786 encapsulation. The nanocarriers were visualized by SEM and AFM and their photo-induced anticancer effect and cellular internalization in human breast carcinoma MCF-7/WT cells were determined. Biological response of the cell culture, expressed as dark and photocytotoxicity as well as fluorescence of drug molecules loaded in the multilayer vehicles, analyzed by the FACS and CLSM techniques, have indicated that the delivered IR-786 did not aggregate inside the cells and could, therefore, act as an effective third-generation photosensitizing agent. In vitro biological experiments demonstrated that the properties of studied nanostructures depended upon the PE type and the envelope thickness as well as on the surfactant architecture in the nanoemulsion-based templates employed for the nanocapsule fabrication. Similarity of results obtained for stored (three weeks in the dark at room temperature) and freshly-prepared nanocapsules, attests to viability of this stable, promising drug delivery system for poorly water-soluble cyanines useful in PDT.

Mechanisms of fibrinogen adsorption on latex particles determined by zeta potential and AFM measurements

The adsorption of fibrinogen on polystyrene latex particles was studied using the concentration depletion method combined with the AFM detection of residual protein after adsorption. Measurements were carried out for a pH range of 3.5-11 and an ionic strength range of 10(-3)-0.15 M NaCl. First, the bulk physicochemical properties of fibrinogen and the latex particle suspension were characterized for this range of pH and ionic strength. The zeta potential and the number of uncompensated (electrokinetic) charges on the protein were determined from microelectrophoretic measurements. It was revealed that fibrinogen molecules exhibited amphoteric characteristics, being on average positively charged for pH <5.8 (isolectric point) and negative otherwise. However, the latex particles did not show any isoelectric point, remaining strongly negative for this pH range. Afterward, systematic measurements of the electrophoretic mobility of fibrinogen-covered latex were carried out as a function of the amount of adsorbed protein, expressed as the surface concentration. A monotonic increase in the electrophoretic mobility (zeta potential) of the latex was observed in all cases, indicating a significant adsorption of fibrinogen on latex for pH below 11. It was also proven that fibrinogen adsorption was irreversible, with the maximum surface concentration varying between 2.5 and 5 × 10(3) μm(-2) (weight concentration of a bare molecule was 1.4 to 2.8 mg m(-2)). These measurements revealed two main adsorption mechanisms of fibrinogen: (i) the unoriented (random) mechanism prevailing for lower ionic strength, where adsorbing molecules significantly penetrate the fuzzy polymeric layer on the latex core and (ii) the side-on adsorption mechanism prevailing for pH > 5.8 and a higher ionic strength of 0.15 M. It was also shown that in the latter case, variations in the zeta potential with the protein coverage could be adequately described in terms of the electrokinetic model, previously formulated for planar substrate adsorption. On the basis of these experimental data, an efficient procedure of preparing fibrinogen-covered latex particles of controlled monolayer structure and coverage was envisaged.