Dissipative Quartz Crystal Microbalance Research Articles

Evaluation of Adhesion Forces for the Manipulation of Micro-Objects in Submerged Environment through Deposition of pH Responsive Polyelectrolyte Layers.

Optimization of surface treatment for reversible adhesion of micro-objects in liquid environment for the need in microassembly processes is presented. A spherical borosilicate probe and planar oxidized silicon wafer substrates were modified by deposition of pH sensitive polyelectrolyte films through layer-by-layer technique. Branched polyethylenimine (b-PEI) and poly(sodium styrenesulfonate) (PSS) were deposited in alternating manner on surfaces, and the influence of polyelectrolyte concentration, pH of deposition, and number of layers on the adhesion were successively examined. The multilayer buildup was followed by optical reflectometry (OR) and dissipative quartz crystal microbalance (QCM-D). The adhesion forces were monitored in aqueous environment at variable pH values by colloidal probe AFM microscopy. The thermodynamic work of adhesion was derived from the pull-off forces by using the Johnson-Kendall-Roberts (JKR) model and compared to the work of adhesion determined from contact angle measurements. It was found out that they correlate well, however, the values accessed from JKR model were underestimated, which was attributed mainly to the effect of surface roughness. Obtained results have demonstrated that it is possible to achieve repeatable reversible adhesion with the change of pH of submerged environment by appropriately tailoring the surface properties and therefore the prevailing surface forces.

Cucurbit[8]uril as nanocontainer in a polyelectrolyte multilayer film: a quantitative and kinetic study of guest uptake.

The host-guest chemistry of cucurbit[8]uril (CB[8]) and the layer-by-layer self-assembly technique are combined to obtain a molecular imprinted polyelectrolyte multilayer film for the recognition and binding of a guest molecule. Cucurbit[8]uril as a ready-made binding site is first associated with a polyelectrolyte and then assembled into a polyelectrolyte multilayer film via layer-by-layer deposition. A cationic guest is subsequently included into the nanocontainer due to specific host-guest interactions. The quantitative analysis of both CB[8] and the included guest molecule in dependence of the surface charge of the multilayer film identifies a high nanocontainer density as well as good to excellent binding efficiencies, therefore yielding a promising imprinted nanomaterial with potential applications in filtration or sensor technology. The investigation of the guest molecule uptake kinetics reveals two processes on different time scales, respectively, which are again related to the charge of the multilayer film surface. The combination of the results obtained from both ultraviolet spectroscopy and dissipative quartz crystal microbalance enables us to describe a full picture of several simultaneous processes initiated by the guest molecule.

Functionalized biocompatible polyelectrolyte multilayers for drug delivery: In situ investigation of mechanical properties by dissipative quartz crystal microbalance

Nanostructured polymeric capsules have been applied in different fields, and specifically are regarded as promising for smart drug delivery applications. The physical–chemical and mechanical properties, and thus the permeability of the polyelectrolyte multilayer shell, play an important role in efficient delivery. Quartz crystal microbalance working in liquid has been used for the characterization of the buildup process and of the viscoelastic properties of biocompatible multilayers and of their functionalization by S-layer proteins. Optical and scanning electron microscopy have been used for the morphological characterization of nanostructured capsules obtained at physiological conditions by the assembly of the characterized multilayers onto spherical cores and by their subsequent removal. The proposed functionalized biocompatible capsules can be regarded as promising candidates for smart drug delivery applications.

Swelling and Stability of Polyelectrolyte Multilayers in Ionic Liquid Solutions

Swelling of polyelectrolyte multilayers with aqueous solution of ionic liquid (IL) is examined with a dissipative quartz crystal microbalance. Multilayers are prepared with different combinations of polyelectrolyte, employing PEI(PSS/PAH)4PSS and PEI(PSS/PDADMAC)4PSS. An enhancement of mass coverage and dissipation is found for films in contact with aqueous solution of IL, i.e., 1-hexyl-3-methylimidazolium chloride and 1-methyl-3-methylimidazolium chloride, suggesting incorporation of IL, accompanied by film swelling. Swelling increases with increasing IL concentration. It is strictly reversible up to a stability limit of IL concentration, above which irreversible layer decomposition starts. Each swelling step can be divided into a fast and a slow process, resulting from IL uptake and chain reorganization or layer decomposition, respectively. While absolute IL uptake and swelling limits differ, the concentration dependence is similar in all systems investigated. The swelling is attributed to hydrophobic interactions, since we find a correlation of the swelling with hydrophobicity of IL.

Cyclotides Insert into Lipid Bilayers to Form Membrane Pores and Destabilize the Membrane through Hydrophobic and Phosphoethanolamine-specific Interactions

Cyclotides are a family of plant-derived circular proteins with potential therapeutic applications arising from their remarkable stability, broad sequence diversity, and range of bioactivities. Their membrane-binding activity is believed to be a critical component of their mechanism of action. Using isothermal titration calorimetry, we studied the binding of the prototypical cyclotides kalata B1 and kalata B2 (and various mutants) to dodecylphosphocholine micelles and phosphoethanolamine-containing lipid bilayers. Although binding is predominantly an entropy-driven process, suggesting that hydrophobic forces contribute significantly to cyclotide-lipid complex formation, specific binding to the phosphoethanolamine-lipid headgroup is also required, which is evident from the enthalpic changes in the free energy of binding. In addition, using a combination of dissipative quartz crystal microbalance measurements and neutron reflectometry, we elucidated the process by which cyclotides interact with bilayer membranes. Initially, a small number of cyclotides bind to the membrane surface and then insert first into the outer membrane leaflet followed by penetration through the membrane and pore formation. At higher concentrations of cyclotides, destabilization of membranes occurs. Our results provide significant mechanistic insight into how cyclotides exert their bioactivities.

Molecular Response and Cooperative Behavior during the Interactions of Melittin with a Membrane: Dissipative Quartz Crystal Microbalance Experiments and Simulations

The molecular-level interactions of an antimicrobial peptide melittin with supported membrane were studied by the combination of dissipative quartz crystal microbalance (QCM-D) experiments and computer simulations. We found the response behavior of lipids upon peptide adsorption greatly influence their interactions. The perturbance and reorientation of the lipid in liquid phase facilitate the insertion of melittin in a trans-membrane way, but in solid phase, asymmetrical membrane disruption happens. Apart from the lipid state, the local peptide-to-lipid ratio also affects the insertion capacity of melittin. When the local peptide number density is high, adjacent peptides can cooperatively penetrate into the membrane. This observation explains the occurrence of the conventional "carpet" mechanism.

Growth of Polyelectrolyte Layers Formed by Poly(4-styrenesulfonate sodium salt) and Two Different Polycations: New Insights from Study of Adsorption Kinetics

We report an experimental study on the growth and adsorption kinetics of polyelectrolyte multilayers (PEMs). PEMs composed of poly(diallyldimethyl ammonium chloride) (PDADMAC) and poly(sodium 4-styrenesulfonate) (PSS), and of poly(allylamine hydrochloride) (PAH) and PSS polyelectrolyte pairs were built and studied via dissipative quartz crystal microbalance (D-QCM) and ellipsometry. The results have pointed out that the growth trend of PEMs may be controlled by the assembly conditions that modify the layer structure and subsequently the film features. The study of adsorption kinetics of the layers suggests that, even though interdiffusion may take place during the growth of PEMs, it does not determine the growth mechanism. Analysis of the mechanical properties allows confirmation of the scenario proposed for the explanation of the PEMs growth. Independent of growth type, the adsorption kinetics of the layers is a bimodal process. The results here presented allow us to rule out any correlation between growth mechanism and adsorption dynamics.

Effect of solid surface charge on the binding behaviour of a metal-binding peptide

Over the last decade, solid-binding peptides have been increasingly used as molecular building blocks coupling bio- and nanotechnology. Despite considerable research being invested in this field, the effects of many surface-related parameters that define the binding of peptide to solids are still unknown. In the quest to control biological molecules at solid interfaces and, thereby, tailoring the binding characteristics of the peptides, the use of surface charge of the solid surface may probably play an important role, which then can be used as a potential tuning parameter of peptide adsorption. Here, we report quantitative investigation on the viscoelastic properties and binding kinetics of an engineered gold-binding peptide, 3RGBP(1), adsorbed onto the gold surface at different surface charge densities. The experiments were performed in aqueous solutions using an electrochemical dissipative quartz crystal microbalance system. Hydrodynamic mass, hydration state and surface coverage of the adsorbed peptide films were determined as a function of surface charge density of the gold metal substrate. Under each charged condition, binding of 3rGBP(1) displayed quantitative differences in terms of adsorbed peptide amount, surface coverage ratio and hydration state. Based on the intrinsically disordered structure of the peptide, we propose a possible mechanism for binding of the peptide that can be used for tuning surface adsorption in further studies. Controlled alteration of peptide binding on solid surfaces, as shown here, may provide novel methods for surface functionalization used for bioenabled processing and fabrication of future micro- and nanodevices.

Adsorption of Conditioning Polymers on Solid Substrates with Different Charge Density

The adsorption processes of polymers that belong to two different families (neutral hydrophilic polymers and cationic polysaccharide polymers) onto solid surfaces with different charge density have been studied using dissipative quartz crystal microbalance (D-QCM) and ellipsometry. The polymers studied are very frequently used in the cosmetic industry as conditioning agents. The adsorption kinetics of the polymers involves at least two steps. The total adsorbed amount depends on the charge density of the surface for both types of polymers. The comparison of the adsorbed mass on each layer obtained from D-QCM and from ellipsometry has allowed calculating the water content of the layers that reaches high values for the polymers studied. The analysis of D-QCM results also provided information about the shear modulus of the layers, whose values have been found to be typical of a rubber-like polymer system. The main driving force of the adsorption was found to be the energy of the interactions between chains and surface.

Adsorption Kinetics of a Novel Organic–Inorganic Hybrid Polymer on Silica and Alumina Studied by Quartz Crystal Microbalance

Adsorption kinetics of an organic–inorganic hybrid polymer, Al(OH)3–polyacrylamide (Al-PAM), on silica and alumina were studied by a dissipative quartz crystal microbalance (QCM-D). The effects of molecular weight and aluminum content of Al-PAM on its adsorption at varying solution pH were investigated. For comparison, the adsorption of a commercial partially hydrolyzed polyacrylamide, Magnafloc1011 (MF1011), on both silica and alumina was also studied. For Al-PAM of a given molecular weight and aluminum content, the adsorption of Al-PAM on both silica and alumina was found to be highly dependent on polymer concentration and solution pH, showing stronger and more rapid adsorption on silica than on alumina within the pH range studied. Adsorption rate of Al-PAM on silica was observed to increase with increasing molecular weight and aluminum content, while Al-PAM of lower aluminum content adsorbed more favorably on alumina. Commercially available MF1011 showed negligible adsorption on silica but a stronger a...

PH-Induced Changes in the Fabrication of Multilayers of Poly(acrylic acid) and Chitosan: Fabrication, Properties, and Tests as a Drug Storage and Delivery System

Multilayers of poly(acrylic acid), PAA, and chitosan, CHI, have been built by the layer-by-layer (LbL) method from aqueous solutions at different pH values and analyzed by the dissipative quartz crystal microbalance (D-QCM) and ellipsometry. The results showed that under all of the assembly conditions considered the growth of the films is nonlinear. The thickness of the PAA layers increases as the pH of the assembling solutions decreases, whereas the adsorption of CHI is almost unaffected by the pH conditions. The comparison of the thickness obtained by D-QCM and by ellipsometry has allowed us to calculate the water content of the films, showing that the multilayers are highly hydrated, with an average water content higher than 20%. The analysis of D-QCM data has provided high-frequency values of the complex shear modulus that are in the megapascal range and shows a transition from mainly viscous to mainly elastic behavior for the added PAA layers, depending on the pH. The monomer surface density in each layer (obtained from the combination of ellipsometry and differential refractive index measurements) indicated that the monomer density depends on the assembly conditions. It was found that the adsorption kinetics is a bimodal process, with characteristic times that depend on the number and nature of the layers. Finally, the possibility of using of these multilayers as a drug storage and delivery system has been evaluated.

Influence of the molecular architecture on the adsorption onto solid surfaces: comb-like polymers

The processes of adsorption of grafted copolymers onto negatively charged surfaces were studied using a dissipative quartz crystal microbalance (D-QCM) and ellipsometry. The control parameters in the study of the adsorption are the existence or absence on the molecular architecture of grafted polyethyleneglycol (PEG) chains with different lengths and the chemical nature of the main chain, poly(allylamine) (PAH) or poly(L-lysine) (PLL). It was found out that the adsorption kinetics of the polymers showed a complex behavior. The total adsorbed amount depends on the architecture of the polymer chains (length of the PEG chains), on the polymer concentration and on the chemical nature of the main chain. The comparison of the thicknesses of the adsorbed layers obtained from D-QCM and from ellipsometry allowed calculation of the water content of the layers that is intimately related to the grafting length. The analysis of D-QCM results also provides information about the shear modulus of the layers, whose values have been found to be typical of a rubber-like polymer system. It is shown that the adsorption of polymers with a charged backbone is not driven exclusively by the electrostatic interactions, but the entropic contributions as a result of the trapping of water in the layer structure are of fundamental importance.

Influence of the percentage of acetylation on the assembly of LbL multilayers of poly(acrylic acid) and chitosan

The Layer-by-Layer (LbL) self-assembly of polyelectrolyte multilayers (PEMs) formed by poly(acrylic acid) (PAA) and chitosan (CHI) of two different percentages of acetylation (AC) has been studied by dissipative quartz crystal microbalance (D-QCM) and ellipsometry. The results point out that the non-linear growth (exponential growth) of the films is not modified by the percentage of acetylation of the CHI (AC). The comparison of the thickness obtained by D-QCM and by ellipsometry has allowed us to calculate the water content of the films showing that the multilayers are highly hydrated. This agrees with the values of the complex shear modulus obtained from the analysis of D-QCM data that are in the MPa range, and show a transition from a viscous to mainly elastic behavior depending on the charge density of the CHI chains. The monomer surface density in each layer (obtained from the combination of ellipsometry and differential refractive index measurements) indicated that the mechanism of charge compensation depends on the percentage of acetylation of the CHI. It was found that the adsorption kinetics is a bimodal process with characteristic times that depend on the number and nature of each layer. The load capacity of the multilayers for a β-blocker, propranolol, was found to be higher for the lowest acetylation degree.

Effect of the molecular structure on the adsorption of conditioning polyelectrolytes on solid substrates

The adsorption of polyelectrolytes onto negatively charged substrates has been studied using dissipative quartz crystal microbalance (D-QCM) and ellipsometry. The polymers studied belong to two families of cationic polymers frequently used as hair conditioners. One of the families is formed by four polymers derived from the poly(diallyl-dimethylammonium chloride), and the other one is formed by two polysaccharide polymers. The control parameters in the study of both families of polymers were the charge density and the rigidity of the chains. The adsorption kinetics of the polymers showed a complex behavior with several processes involved. The total adsorbed amount depends on the nature of the polymer chains, on the ionic strength, and on polymer concentration of the solutions. The comparison of the mass of the adsorbed layers obtained from D-QCM and from ellipsometry allowed us to calculate the water content of the layers. This variable takes high values for all the polymers studied, which is related to the non-homogeneous character of the adsorbed layer, as confirmed by AFM. The analysis of D-QCM results also provides information about the shear modulus of the layers, whose values have been found to be in the MPa range as for rubber-like polymer system. The adsorptions of the polyelectrolytes are not driven exclusively by the electrostatic interactions, entropic contributions being of fundamental importance, and arise mainly from the release of counterions during the adsorption process, and/or the number of loops and tails of the adsorbed chains protruding into the solution.

Linear and Exponential Growth Regimes of Multilayers of Weak Polyelectrolytes in Dependence on pH

Multilayers of poly(allyl amine hydrochloride) (PAH) and poly(acrylic acid) (PAA) are formed by layer-by-layer self-assembly at defined pH values and investigated by dissipative quartz-crystal microbalance (QCM-D) and ellipsometry. Five distinct pH regimes with different growth mechanisms and growth laws are identified. At intermediate pH values, a central regime is characterized by linear growth, accompanied by very thin and very rigid films. Adjacent regimes toward higher and lower pH show exponential layer growth, and very soft films are formed. Toward very low and very high pH, again a sharp change of mechanism occurs and the growth law becomes linear again, while very asymmetric and soft layers are formed. The symmetry of the regimes and the respective film properties are attributed to charge dilution of the polyanion or the polycation chain, respectively. In either of the five regimes the mechanism of layer formation is explained by the charge densities along the chains, as supported by ATR-FTIR exp...

Polyelectrolyte Multilayers Containing Triblock Copolymers of Different Charge Ratio

Multilayers formed by the sodium salt of poly(4-styrenesulfonate), PSS, and triblock copolymers of the form PDMAEMA-PCL-PDMAEMA (PDMAEMA corresponding to poly[2-(N,N-dimethylamino)ethyl methacrylate), and PCL to poly(epsilon-caprolactone) have been built by layer-by-layer self-assembly from the aqueous polyelectrolyte solutions. Two types of block copolymers have been used which differ on the type of the amino groups, either hydrochloride or quaternized. This leads to changes in the charge density of the chains for the same content of amino groups. The growth of the multilayers has been followed using dissipative quartz crystal microbalance and ellipsometry techniques. The results show that, independently of the conditions used in the assembling, the film thickness grows linearly with the number of layers. The comparison of the thickness values obtained from D-QCM and ellipsometry has allowed us to calculate the water content of the polymer film. The analysis of the D-QCM data also provides the shear modulus, whose values are typical of a rubber-like polymer system. The analysis of the mass adsorbed calculated by the ellipsometric measurements indicated that the nature of the charge compensation mechanism is extrinsic for all the studied systems, although the degree of extrinsic compensation is strongly dependent on the copolymer used and the concentration in solution. Finally, it was found that the adsorption kinetic of the layers is bimodal for all the films built. Even though the characteristic adsorption times depend on the specific copolymer used, no dependence on the number of layers has been found for a given multilayer.

Studies of Phospholipid Vesicle Deposition/Transformation on a Polymer Surface by Dissipative Quartz Crystal Microbalance and Atomic Force Microscopy

Polymer-supported phospholipid bilayers (PLBs) are popular model systems for the study of transmembrane proteins under conditions close to cellular membrane environments. In this work, by combining the techniques of dissipative quartz crystal microbalance and atomic force microscopy, we investigate the deposition of vesicles on a hydrated cationic poly(diallyldimethylammonium chloride) (PDDA) layer as a function of phospholipid composition and sodium chloride concentration. The vesicles used consist of phospholipid mixtures with varying amounts of net negative charge. Uniform PLBs are formed by either increasing the negative charge density of the vesicles or decreasing sodium chloride concentration, suggesting that the electrostatic attraction between the vesicle and PDDA layer is the driving force for the formation of the PLBs. Our results indicate that the PLB formation is a fast adsorption-rupture process of the vesicles, without passing through a critical vesicle density. We further contend that the fluctuating PDDA support plays a central role for this process. This work provides a framework for understanding the key factors that influence the formation of PLBs.

Adsorption Kinetics and Mechanical Properties of Ultrathin Polyelectrolyte Multilayers: Liquid-Supported versus Solid-Supported Films

Multilayers of sodium salt of poly(4-styrene sulfonate) (PSS) and poly(diallyl dimethyl ammonium) chloride (PDADMAC) have been built layer by layer (LbL) both at the solid/aqueous interface (solid supported) and the air/aqueous interface (liquid supported). For the solid-supported multilayers, the adsorption kinetics and the complex shear modulus were measured using a dissipative quartz crystal microbalance and a null ellipsometer. A bubble tensiometer was used to measure the adsorption kinetics and the elasticity modulus of the liquid-supported multilayers. At the solid/aqueous interface, adsorption kinetics changes with the number of adsorbed layers. However, at the air/aqueous interface, PSS dynamics were the same for all adsorbed layers except the first. Conversely, the adsorption kinetics of PDADMAC at the air/water surface differed between those layers close to the interface and those far from it. Multilayers grow at the air/water interface by an intrinsic-charge-compensation process, whereas, for the same ionic strengths, solid-supported layers deposit by the extrinsic-charge-compensation process. No significant differences were found between the recoverable dilational storage modulus of the liquid-supported multilayers and the real part of the shear modulus of the solid-supported ones built at the same ionic strength. The values of the modulus are in the MPa range, which corresponds to gel-like films. This result is in agreement with the strong hydration degree of the LbL films calculated from ellipsometry measurements.

Salt-induced changes in the growth of polyelectrolyte layers of poly(diallyl-dimethylammonium chloride) and poly(4-styrene sulfonate of sodium)

Multilayers formed by the sodium salt of poly(4-styrene sulfonate), PSS, and poly(diallyldimethylammonium) chloride, PDADMAC, have been built by electrostatic self-assembly from polyelectrolyte aqueous solutions of different ionic strengths. The growth of the multilayers has been followed using dissipative quartz crystal microbalance and ellipsometry. Neutron reflectometry and XPS data indicate that the PSS and PDADMAC layers interpenetrate leading to an almost homogeneous polymer film. The results show that on increasing [NaCl] the growing process changes from a linear to a non-linear regime. The comparison of the thickness values obtained from QCM and ellipsometry has allowed us to calculate the water content of the polymer film. The results agree with those obtained by neutron reflectometry. The analysis of the QCM data has provided values of the complex shear modulus, which are typical of a rubber-like polymer system. The analysis of the mass adsorbed calculated by the ellipsometry measurements indicated that the charge compensation mechanism changes from intrinsic at low ionic strengths to mainly extrinsic at high ionic strengths. Finally, it was found that the response of a polymer film to a change in [NaCl] is rather different for films grown at low or at high ionic strengths.

Adsorption Behaviors of DPPC/MO Aggregates on SiO2 Surfaces

The adsorption kinetics of extruded 1,2-dipalmitoyl- sn-glycero-3-phosphatidylcholine (DPPC)/1-(cis-9-octadecenoyl)- rac-glycerol (monoolein, MO) aggregates on SiO 2 surface at 25 degrees C is investigated in real time, using the dissipative quartz crystal microbalance (QCM) technique. Four adsorption pathways have been identified depending on the molar fraction of MO in the DPPC/MO system: (I) intact vesicle adsorption, (II) vesicle reorganization on a SiO 2 surface, (III) supported lipid bilayer (SLB) formation, and (IV) cubosome adsorption. The results can be understood by the fact that DPPC is a lamellar phase-forming lipid, whereas MO prefers the cubic phase. Therefore, the incorporation of MO in DPPC increases the packing parameter. Equally important, MO also increases the mobility of lipid molecules and lateral pressure in the bilayers as a result of the presence of a unique cis- double bond. Before extrusion, the vesicles size increases with the MO content when X MO <or= 0.7 and cubosomes are formed for X MO >or= 0.8. The extruded DPPC/MO suspensions consist of reformed vesicles for X MO <or= 0.7 and filtered cubosomes for X MO >or= 0.8, all with a uniform diameter of approximately 100 nm. Differential scanning calorimetry (DSC) further indicates that the addition of MO lowers the main phase transition temperature of DPPC and thus makes the hydrophobic interior more fluid.