Quantitative Rheometry of Thin Soft Materials Using the Quartz Crystal Microbalance with Dissipation

Study on the mechanism of polyphenols regulating the stability of pea isolate protein formed Pickering emulsion based on interfacial effects

To improve the stability of pea isolate protein (PPI) Pickering emulsions, this study compared the stability effects of tannic acid (TA), epigallocatechin gallate, and gallic acid on PPI, and found PPI-TA the strongest binding and the best stability. When TA concentration increased from 0 to 0.5 mmol/L, the average particle size, zeta potential, and surface hydrophobicity of PPI-TA particles decreased by 23.1 %, 17.1 %, and 63.3 % respectively. The highest viscosity and elastic storage modulus G' which was also higher than and parallel to the loss modulus G", and the lowest Turbiscan stability index were observed in the emulsion with 0.5 mmol/L TA, indicating an elastic-based gel-like texture. The concentrations of conjugated diene and thiobarbituric acid reactive substances (TBARS) were also reduced by more than 58 %, showing improved oxidative stability. The study provides new insights into the interfacial behavior of PPI-polyphenols and technical support for their applications in food industry.

Migration of natural organic matter and Pseudomonas fluorescens-associated polystyrene on natural substrates in aquatic environments

This study investigated the migration behavior of microplastics (MPs) covered with natural organic matter (NOM) and biofilm on three substrates (silica, Pseudomonas fluorescent and Pseudomonas aeruginosa biofilms) in various ionic strengths, focusing on the alterations in surface properties based on surface energy theory that affected their deposition and release processes. Peptone and Pseudomonas fluorescens were employed to generate NOM-attached and biofilm-coated polystyrene (PS) (NOM-PS and Bio-PS). NOM-PS and Bio-PS both exhibited different surface properties, as increased roughness and particle sizes, more hydrophilic surfaces and altered zeta potentials which increased with ionic strength. Although the deposition of NOM-PS on biofilms were enhanced by higher ionic strengths and the addition of Ca2+, while Bio-PS deposited less on biofilms and more on the silica surface. Both types exhibited diffusion-driven adsorption on the silica surface, with Bio-PS also engaging in synergistic and competitive interactions on biofilm surfaces. Release tests revealed that NOM-PS and Bio-PS were prone to release from silica than from biofilms. The Extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory furtherly demonstrated that mid-range electrostatic (EL) repulsion had significantly impacts on NOM-PS deposition, and structural properties of extracellular polymeric substances (EPS) and substrate could affect Bio-PS migration.

Using the Quartz Crystal Microbalance to Monitor the Curing of Drying Oils

Drying oils such as linseed oil form a polymer network through a complex free-radical polymerization process. We have studied polymerization in this challenging class of polymers using a quartz crystal microbalance (QCM). The QCM is able to measure the evolution of polymer mass and mechanical properties as the oil transitions from a liquid-like to a solid-like state. Measurements using bulk materials and thin films provide information about the initial polymerization phase as well as the evolution of the mass and mechanical properties over the first two years of cure. The temperature-dependent response of the cured linseed oil films was also measured. These results were combined with previously published results obtained from traditional dynamic mechanical analysis to give a unified picture of the properties of these materials across a very broad temperature range.

Establishing Quartz Crystal Microbalance with Dissipation (QCM-D) Coupled with Spectroscopic Ellipsometry (SE) as an Advantageous Technique for the Characterization of Ultra-Thin Film Hydrogels

Despite the considerable significance of utilizing ultra-thin film (utf) hydrogels as multipurpose platforms for biomedical applications, there is still an important lack of adequate characterization techniques suitable for such materials. In this Perspective, the use of quartz crystal microbalance with dissipation (QCM-D) coupled with spectral ellipsometry (SE) is presented as a potential tool for the complete characterization of utf-hydrogels due to its nanometric sensitivity and high versatility. Herein, the fundaments for utf-hydrogel characterization are settled down as far as the QCM-D/SE response is explored under a wide range of different in operando wet working conditions measurements such as temperature or liquid media, among others. Therefore, the design of measuring protocols capable to perform is proposed and compiled, for the first time, complete and precise characterization of the cross-link density, thickness variations (swelling ratio determination), stability analyses, and mechanical studies (including the simultaneous generation of stress-strain curves and the evaluation of the viscoelastic; leading to the final determination of the Poisson's ratio) under different in operando conditions. Finally, the future challenges and implications for the general characterization of soft-thin films are discussed.

Modeling Nonlinear Dynamics of Functionalization Layers: Enhancing Gas Sensor Sensitivity for Piezoelectrically Driven Microcantilever

This article presents a parametrized response model that enhances the limit of detection (LOD) of piezoelectrically driven microcantilever (PD-MC) based gas sensors by accounting for the adsorption-induced variations in elastic properties of the functionalization layer (binder) and the nonlinear motional dynamics of the PD-MC. The developed model is demonstrated for quantifying cadaverine, a volatile biogenic diamine whose concentration is used to assess the freshness of meat. At low concentrations of cadaverine, an increase in the resonance frequency is observed, contrary to the expected reduction due to mass added by adsorption. The study explores the variations in the elastic modulus vis-à-vis the adsorbed mass of cadaverine and derives the resonance frequency to the adsorbed mass response function. We advance a blended technique involving the analysis of atomic force microscopy (AFM) force-distance (f-d) curves and fitting of the quartz crystal microbalance (QCM) impedance response spectrum to deduce the adsorption-induced changes in the viscoelastic properties of the functionalization layer. The findings obtained are subsequently employed in modeling the response function for a structurally nonhomogenous PD-MC, highlighting the significance of the functionalization layer to the global elastic properties. The structural composition of the PD-MC beam adopted herein features a trapezoidal base hosting the actuating piezoelectric stratum and a rectangular free end with a functionalization layer. The Euler-Bernoulli beam theory coupled with Hamilton's principle is used to develop the equation of motion, which is subsequently discretized into a set of nonlinear ordinary differential equations via Galerkin expansion, and the solutions to the first fundamental mode of vibration are determined using the method of multiple scales. The obtained solutions provide a basis for deducing the nonlinear response function model to the adsorbed mass. The derived model is validated by recorded resonance frequency changes resulting from exposure to known concentrations of cadaverine. We demonstrate that the increase in resonance frequency for low concentrations of cadaverine is due to the dominance of the variation of the elastic modulus of the functionalization layer originating from the initial binder-analyte interactions over damping due to added mass. It is concluded that the developed nonlinear response function model can reliably be used to quantify the cadaverine concentration at low concentrations with an elevated Limit of Detection.

Development of broad modulus profile upon polymer-polymer interface formation between immiscible glassy-rubbery domains

Interfaces of glassy materials such as thin films, blends, and composites create strong unidirectional gradients to the local heterogeneous dynamics that can be used to elucidate the length scales and mechanisms associated with the dynamic heterogeneity of glasses. We focus on bilayer films of two different polymers with very different glass transition temperatures ([Formula: see text]) where previous work has demonstrated a long-range (∼200 nm) profile in local [Formula: see text] is established between immiscible glassy and rubbery polymer domains when the polymer-polymer interface is formed to equilibrium. Here, we demonstrate that an equally long-ranged gradient in local modulus [Formula: see text] is established when the polymer-polymer interface ([Formula: see text]5 nm) is formed between domains of glassy polystyrene (PS) and rubbery poly(butadiene) (PB), consistent with previous reports of a broad [Formula: see text] profile in this system. A continuum physics model for the shear wave propagation caused by a quartz crystal microbalance across a PB/PS bilayer film is used to measure the viscoelastic properties of the bilayer during the evolution of the PB/PS interface showing the development of a broad gradient in local modulus [Formula: see text] spanning [Formula: see text]180 nm between the glassy and rubbery domains of PS and PB. We suggest these broad profiles in [Formula: see text] and [Formula: see text] arise from a coupling of the spectrum of vibrational modes across the polymer-polymer interface as a result of acoustic impedance matching of sound waves with [Formula: see text] nm during interface broadening that can then trigger density fluctuations in the neighboring domain.

Enhanced Removal of Photoresist Films through Swelling and Dewetting Using Pluronic Surfactants

Organic photoresist coatings, primarily composed of resins, are commonly used in the electronics industry to protect inorganic underlayers. Conventional photoresist strippers, such as amine-type agents, have shown high removal performance but led to environmental impact and substrate corrosiveness. Therefore, this trade-off must be addressed. In this study, we characterized the removal mechanism of a photoresist film using a nonionic triblock Pluronic surfactant [poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide)] in a ternary mixture of ethylene carbonate (EC), propylene carbonate (PC), and water. In particular, the removal dynamics determined by using a quartz crystal microbalance with dissipation monitoring was compared with those determined by performing confocal laser scanning microscopy and visual observation to analyze the morphology, adsorption mass, and viscoelasticity of the photoresist film. In the absence of the Pluronic surfactant, the photoresist film in the ternary solvent exhibited a three-step process: (i) film swelling caused by the penetration of a good solvent (EC and PC), (ii) formation of photoresist particles through dewetting, and (iii) particle aggregation on the substrate. This result was correlated to the Hansen solubility parameters. The addition of the Pluronic surfactant not only prevented photoresist aggregation in the third step but also promoted desorption from the substrate. This effect was dependent on the concentration of the Pluronic surfactant, which influenced diffusion to the interface between the photoresist and the bulk solution. Finally, we proposed a novel photoresist stripping mechanism based on the synergy between dewetting driven by an EC/PC-to-water mixture and adsorption by the Pluronic surfactant.

Impact of Hydrophobic and Electrostatic Forces on the Adsorption of Acacia Gum on Oxide Surfaces Revealed by QCM-D

The adsorption of Acacia gum from two plant exudates, A. senegal and A. seyal, at the solid-liquid interface on oxide surfaces was studied using a quartz crystal microbalance with dissipation monitoring (QCM-D). The impact of the hydrophobic and electrostatic forces on the adsorption capacity was investigated by different surface, hydrophobicity, and charge properties, and by varying the ionic strength or the pH. The results highlight that hydrophobic forces have higher impacts than electrostatic forces on the Acacia gum adsorption on the oxide surface. The Acacia gum adsorption capacity is higher on hydrophobic surfaces compared to hydrophilic ones and presents a higher stability with negatively charged surfaces. The structural configuration and charge of Acacia gum in the first part of the adsorption process are important parameters. Acacia gum displays an extraordinary ability to adapt to surface properties through rearrangements, conformational changes, and/or dehydration processes in order to reach the steadiest state on the solid surface. Rheological analysis from QCM-D data shows that the A. senegal layers present a viscous behavior on the hydrophilic surface and a viscoelastic behavior on more hydrophobic ones. On the contrary, A. seyal layers show elastic behavior on all surfaces according to the Voigt model or a viscous behavior on the hydrophobic surface when considering the power-law model.

Effect of Noise on Determining Ultrathin-Film Parameters from QCM-D Data with the Viscoelastic Model

Quartz crystal microbalance with dissipation monitoring (QCM-D) is a well-established technique for studying soft films. It can provide gravimetric as well as nongravimetric information about a film, such as its thickness and mechanical properties. The interpretation of sets of overtone-normalized frequency shifts, ∆f/n, and overtone-normalized shifts in half-bandwidth, ΔΓ/n, provided by QCM-D relies on a model that, in general, contains five independent parameters that are needed to describe film thickness and frequency-dependent viscoelastic properties. Here, we examine how noise inherent in experimental data affects the determination of these parameters. There are certain conditions where noise prevents the reliable determination of film thickness and the loss tangent. On the other hand, we show that there are conditions where it is possible to determine all five parameters. We relate these conditions to the mathematical properties of the model in terms of simple conceptual diagrams that can help users understand the model's behavior. Finally, we present new open source software for QCM-D data analysis written in Python, PyQTM.

Portable Quartz Crystal Resonator Sensor for Characterising the Gelation Kinetics and Viscoelastic Properties of Hydrogels

Hydrogel biomaterials have found use in various biomedical applications partly due to their biocompatibility and tuneable viscoelastic properties. The ideal rheological properties of hydrogels depend highly on the application and should be considered early in the design process. Rheometry is the most common method to study the viscoelastic properties of hydrogels. However, rheometers occupy much space and are costly instruments. On the other hand, quartz crystal resonators (QCRs) are devices that can be used as low-cost, small, and accurate sensors to measure the viscoelastic properties of fluids. For this reason, we explore the capabilities of a low-cost and compact QCR sensor to sense and characterise the gelation process of hydrogels while using a low sample amount and by sensing two different crosslink reactions: covalent bonds and divalent ions. The gelation of covalently crosslinked mucin hydrogels and physically crosslinked alginate hydrogels could be monitored using the sensor, clearly distinguishing the effect of several parameters affecting the viscoelastic properties of hydrogels, including crosslinking chemistry, polymer concentrations, and crosslinker concentrations. QCR sensors offer an economical and portable alternative method to characterise changes in a hydrogel material's viscous properties to contribute to this type of material design, thus providing a novel approach.

Practical Use of Quartz Crystal Microbalance Monitoring in Cartilage Tissue Engineering

Quartz crystal microbalance (QCM) is a real-time, nanogram-accurate technique for analyzing various processes on biomaterial surfaces. QCM has proven to be an excellent tool in tissue engineering as it can monitor key parameters in developing cellular scaffolds. This review focuses on the use of QCM in the tissue engineering of cartilage. It begins with a brief discussion of biomaterials and the current state of the art in scaffold development for cartilage tissue engineering, followed by a summary of the potential uses of QCM in cartilage tissue engineering. This includes monitoring interactions with extracellular matrix components, adsorption of proteins onto biomaterials, and biomaterial–cell interactions. In the last part of the review, the material selection problem in tissue engineering is highlighted, emphasizing the importance of surface nanotopography, the role of nanofilms, and utilization of QCM as a “screening” tool to improve the material selection process. A step-by-step process for scaffold design is proposed, as well as the fabrication of thin nanofilms in a layer-by-layer manner using QCM. Finally, future trends of QCM application as a “screening” method for 3D printing of cellular scaffolds are envisioned.

Operando Characterization of Organic Mixed Ionic/Electronic Conducting Materials

Operando characterization plays an important role in revealing the structure-property relationships of organic mixed ionic/electronic conductors (OMIECs), enabling the direct observation of dynamic changes during device operation and thus guiding the development of new materials. This review focuses on the application of different operando characterization techniques in the study of OMIECs, highlighting the time-dependent and bias-dependent structure, composition, and morphology information extracted from these techniques. We first illustrate the needs, requirements, and challenges of operando characterization then provide an overview of relevant experimental techniques, including spectroscopy, scattering, microbalance, microprobe, and electron microscopy. We also compare different in silico methods and discuss the interplay of these computational methods with experimental techniques. Finally, we provide an outlook on the future development of operando for OMIEC-based devices and look toward multimodal operando techniques for more comprehensive and accurate description of OMIECs.

Effect of Surfactant Concentration and Hydrophobicity on the Ordering of Water at a Silica Surface

The performance of nonionic surfactants is mediated by the interfacial interactions at the solid-liquid interface. Here we applied sum frequency generation (SFG) vibrational spectroscopy to probe the molecular structure of the silica-nonionic surfactant solution interface in situ, supplemented by quartz crystal microbalance with dissipation monitoring (QCM-D) and molecular dynamics (MD) simulations. The combined studies elucidated the effects of nonionic surfactant solution concentration, surfactant composition, and rinsing on the silica-surfactant solution interfacial structure. The nonionic surfactants studied include ethylene-oxide (EO) and butylene oxide (BO) components with different ratios. It was found that the CH groups of the surfactants at the silica-surfactant solution interfaces are disordered, but the interfacial water molecules are ordered, generating strong SFG OH signals. Solutions with higher concentrations of surfactant lead to a slightly higher amount of adsorbed surfactant at the silica interface, resulting in more water molecules being ordered at the interface, or a higher ordering of water molecules at the interface, or both. MD simulation results indicated that the nonionic surface molecules preferentially adsorb onto silanol sites on silica. A surfactant with a higher EO/BO ratio leads to more water molecules being ordered and a higher degree of ordering of water molecules at the silica-surfactant solution interface, exhibiting stronger SFG OH signal, although less material is adsorbed according to the QCM-D data. A thin layer of surfactants remained on the silica surface after multiple water rinses. To the best of our knowledge, this is the first time the combined approaches of SFG, QCM-D and MD simulation techniques have been applied to study nonionic surfactants at the silica-solution interface, which enhances our understanding on the interfacial interactions between nonionic surfactants, water and silica. The knowledge obtained from this study can be helpful to design the optimal surfactant concentration and composition for future applications.

Data evaluation for surface-sensitive label-free methods to obtain real-time kinetic and structural information of thin films: A practical review with related software packages

Interfacial layers are important in a wide range of applications in biomedicine, biosensing, analytical chemistry and the maritime industries. Given the growing number of applications, analysis of such layers and understanding their behavior is becoming crucial. Label-free surface sensitive methods are excellent for monitoring the formation kinetics, structure and its evolution of thin layers, even at the nanoscale. In this paper, we review existing and commercially available label-free techniques and demonstrate how the experimentally obtained data can be utilized to extract kinetic and structural information during and after formation, and any subsequent adsorption/desorption processes. We outline techniques, some traditional and some novel, based on the principles of optical and mechanical transduction. Our special focus is the current possibilities of combining label-free methods, which is a powerful approach to extend the range of detected and deduced parameters. We summarize the most important theoretical considerations for obtaining reliable information from measurements taking place in liquid environments and, hence, with layers in a hydrated state. A thorough treamtmaent of the various kinetic and structural quantities obtained from evaluation of the raw label-free data are provided. Such quantities include layer thickness, refractive index, optical anisotropy (and molecular orientation derived therefrom), degree of hydration, viscoelasticity, as well as association and dissociation rate constants and occupied area of subsequently adsorbed species. To demonstrate the effect of variations in model conditions on the observed data, simulations of kinetic curves at various model settings are also included. Based on our own extensive experience with optical waveguide lightmode spectroscopy (OWLS) and the quartz crystal microbalance (QCM), we have developed dedicated software packages for data analysis, which are made available to the scientific community alongside this paper.

Interaction of lignin dimers with model cell membranes: A quartz crystal microbalance and molecular dynamics simulation study

A study of the interaction between cell membranes and small molecules derived from lignin, a protective phenolic biopolymer found in vascular plants, is crucial for identifying their potential as pharmacological and toxicological agents. In this work, the interactions of model cell membranes [supported 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) lipid bilayers] are compared for three βO4 dimers of coniferyl alcohol (G lignin monomer): guaiacylglycerol guaiacol ester with a hydroxypropenyl (HOC₃H₄-) tail (G-βO4'-G), a truncated GG dimer without HOC₃H₄- (G-βO4'-truncG), and a benzylated GG dimer (benzG-βO4'-G). The uptake of the lignin dimers (per mass of lipid) and the energy dissipation (a measure of bilayer disorder) are higher for benzG-βO4'-G and G-βO4'-truncG than those for G-βO4'-G in the gel-phase DPPC bilayer, as measured using quartz crystal microbalance with dissipation (QCM-D). A similar uptake of G-βO4'-truncG is observed for a fluid-phase bilayer of 1,2-dioleoyl-sn-glycero-3-phosphocholine, suggesting that the effect of the bilayer phase on dimer uptake is minimal. The effects of increasing lignin dimer concentration are examined through an analysis of density profiles, potential of mean force curves, lipid order parameters, and bilayer area compressibilities (disorder) in the lipid bilayers obtained from molecular dynamics simulations. Dimer distributions and potentials of mean force indicate that the penetration into bilayers is higher for benzG-βO4'-G and G-βO4'-truncG than that for G-βO4'-G, consistent with the QCM-D results. Increased lipid tail disorder due to dimer penetration leads to a thinning and softening of the bilayers. Minor differences in the structure of lignin derivatives (such as truncating the hydroxypropenyl tail) have significant impacts on their ability to penetrate lipid bilayers.

Metal-Free Antibacterial Additives Based on Graphene Materials and Salicylic Acid: From the Bench to Fabric Applications

The custom functionalization of a graphene surface allows access to engineered nanomaterials with improved colloidal stability and tailored specific properties, which are available to be employed in a wide range of applications ranging from materials to life science. The high surface area and their intrinsic physical and biological properties make reduced graphene oxide and graphene oxide unique materials for the custom functionalization with bioactive molecules by exploiting different surface chemistries. In this work, preparation (on the gram scale) of reduced graphene oxide and graphene oxide derivatives functionalized with the well-known antibacterial agent salicylic acid is reported. The salicylic acid functionalities offered a stable colloidal dispersion and, in addition, homogeneous absorption on a sample of textile manufacture (i.e., cotton fabrics), as shown by a Raman spectroscopy study, thus providing nanoengineered materials with significant antibacterial activity toward different strains of microorganisms. Surprisingly, graphene surface functionalization also ensured resistance to detergent washing treatments as verified on a model system using the quartz crystal microbalance technique. Therefore, our findings paved the way for the development of antibacterial additives for cotton fabrics in the absence of metal components, thus limiting undesirable side effects.

Recent applications of QCM-D for the design, synthesis, and characterization of bioactive materials

The clinical translation of bioactive technologies is lacking compared to the number of novel technologies reported in the literature. This is in part due to the difficulties in characterizing bioactive materials to understand and predict their biological response. To progress the field and increase clinical success, more robust analytical techniques must be utilized when investigating novel bioactive materials. The quartz crystal microbalance with dissipation (QCM-D), a label-free sensing instrument based on an acoustic resonator, is used to quantify mass change and viscoelastic parameters from soft materials at the nanoscale, in situ, with precise temporal resolution and operation in both liquid and gaseous environments. The versatility of QCM-D has enhanced the characterization of bioactive polymers and sensing arrays for advanced applications of novel biotechnologies. In this review, we highlight exciting, recent applications of QCM-D for the investigation of bioactive materials. Attention is given to the dynamic mechanical properties of bioactive materials, discerning protein structure on surfaces, probing cell adhesion and cytoskeletal changes, and biosensing applications. We conclude that QCM-D has untapped utility in the pre-clinical investigation of bioactive materials and further utilization can improve the clinical success of novel technologies.

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.

Bio-inert Properties of TEG Modified Dendrimer Interface

The bioinert interfaces that prevent adhesion of proteins and cells are important for biomaterial applications. In order to design a bioinert interface, the immobilization of an appropriate functional group and the control of molecular density is required. Dendrimer was modified with triethylene glycol (TEG) to display a dense brush structure. TEG with different density and terminal groups were immobilized with a dendrimer template and thiol terminated molecules. The inhibitory effect on protein and bacteria binding was investigated. The physical property of the interface was measured by QCM-admittance to clarify the factor of the bioinert property.

Hysteretic Swelling/Deswelling of Polyelectrolyte Brushes and Bilayer Films in Response to Changes in pH and Salt Concentration

We use a quartz crystal microbalance with dissipation (QCM-D) to investigate the swelling/de-swelling and hysteresis in brushes of weakly ionizable polyanion poly(acrylic acid) (PAA) brushes and bilayers containing a PAA brush and a poly(ethylene imine) (PEI) overlayer. We show that for a long PAA chain (M_w = 39 kDa), at low grafting density (σ < 0.05 chains/nm²) and at a pH value (=4.2) at which it is partially charged, in the low-salt “osmotic brush” regime, the brush height no longer increases with increased grafting density as is seen for shorter brushes and denser grafting, but shows a slight decrease in height, in qualitative agreement with predictions of scaling theory. In a cycle of stepped pH changes, we also show that at a low grafting density of σ = 0.023 chains/nm² and M_w = 39 kDa, there is hysteresis in swelling over timescales of many minutes. For higher grafting densities σ = 0.87 chains/nm² and shorter chains (2 kDa), we see little or no measurable hysteresis, and, at intermediate chain length 14 kDa and grafting density σ = 0.06 chains/nm², hysteresis is observed at short timescales but is greatly reduced at longer timescales. These results are similarly observed when bilayers are made by adsorbing onto the PAA brush a layer of the polycation PEI. In addition, we also note hysteresis in swelling upon changes of salt concentration when pH is fixed. These results show the rich thermodynamics and kinetics of even monolayers and bilayers of polyelectrolyte films.

Outer Membrane c-Type Cytochromes OmcA and MtrC Play Distinct Roles in Enhancing the Attachment of Shewanella oneidensis MR-1 Cells to Goethite

The outer membrane c-type cytochromes (c-Cyts) OmcA and MtrC in Shewanella are key terminal reductases that bind and transfer electrons directly to iron (hydr)oxides. Although the amounts of OmcA and MtrC at the cell surface and their molecular structures are largely comparable, MtrC is known to play a more important role in dissimilatory iron reduction. To explore the roles of these outer membrane c-Cyts in the interaction of Shewanella oneidensis MR-1 with iron oxides, the processes of attachment of S. oneidensis MR-1 wild type and c-type cytochrome-deficient mutants (the ΔomcA, ΔmtrC, and ΔomcA ΔmtrC mutants) to goethite are compared via quartz crystal microbalance with dissipation monitoring (QCM-D). Strains with OmcA exhibit a rapid initial attachment. The quantitative model for QCM-D responses reveals that MtrC enhances the contact area and contact elasticity of cells with goethite by more than one and two times, respectively. In situ attenuated total reflectance Fourier transform infrared two-dimensional correlation spectroscopic (ATR-FTIR 2D-CoS) analysis shows that MtrC promotes the initial interfacial reaction via an inner-sphere coordination. Atomic force microscopy (AFM) analysis demonstrates that OmcA enhances the attractive force between cells and goethite by about 60%. As a result, OmcA contributes to a higher attractive force with goethite and induces a rapid short-term attachment, while MtrC is more important in the longer-term interaction through an enhanced contact area, which promotes interfacial reactions. These results reveal that c-Cyts OmcA and MtrC adopt different mechanisms for enhancing the attachment of S. oneidensis MR-1 cells to goethite. It improves our understanding of the function of outer membrane c-Cyts and the influence of cell surface macromolecules in cell-mineral interactions.IMPORTANCE Shewanella species are one group of versatile and widespread dissimilatory iron-reducing bacteria, which are capable of respiring insoluble iron minerals via six multiheme c-type cytochromes. Outer membrane c-type cytochromes (c-Cyts) OmcA and MtrC are the terminal reductases in this pathway and have comparable protein structures. In this study, we elucidate the different roles of OmcA and MtrC in the interaction of S. oneidensis MR-1 with goethite at the whole-cell level. OmcA confers enhanced affinity toward goethite and results in rapid attachment. Meanwhile, MtrC significantly increases the contact area of bacterial cells with goethite and promotes the interfacial reaction, which may explain its central role in extracellular electron transfer. This study provides novel insights into the role of bacterial surface macromolecules in the interfacial interaction of bacteria with minerals, which is critical to the development of a comprehensive understanding of cell-mineral interactions.

Investigating Adsorbing Viscoelastic Fluids Using the Quartz Crystal Microbalance

There is little research on using the quartz crystal microbalance (QCM) with adsorbing viscoelastic fluids. These fluids are widely encountered but often difficult to study as many are opaque and highly viscous. Since the QCM does not involve any scattering or reflection of input radiation, it has the potential to study these complex fluids to determine the relative viscoelasticity of the bulk fluid and surface adsorption of active species onto different substrates. In the current study, both Newtonian (sucrose) and viscoelastic (sodium polystyrene sulfonate (NaPSS)) fluids were introduced into the QCM, and the sensor responses were compared. QCM responses of Newtonian sucrose solutions matched the Kanazawa and Gordon model (KG model), as expected. The QCM responses with viscoelastic NaPSS solutions were well below those described by the KG model. A viscoelastic model was used to determine the fluid viscosity and shear modulus at a very high frequency. It was found that the viscosity of NaPSS did not change much compared with low-frequency rheometer measurements, but a significant increase in the shear modulus of several orders of magnitude was found at the QCM frequencies. Modifying the KG model frequency shifts by multiplying by the QCM shear wave decay length ratio, X = δ_V/δ_N, we were able to match the measured QCM values in viscoelastic NaPSS solutions. The QCM dissipation values for NaPSS were matched in a similar way by multiplying the KG model by X ^1/3. By changing the QCM sensor from silica (no NaPSS adsorption) to alumina (NaPSS adsorption), it was shown that the adsorption isotherm of NaPSS on alumina could be recovered and fitted with a Langmuir isotherm despite the frequency response being only a small fraction of the total measured QCM signal.

Aqueous polypropylene glycol induces swelling and severe plasticization of high Tg amphiphilic copolymers containing hexafluoroisopropanol groups

Poly(ethylene glycol) (PEG) tends to be considered low fouling, which has led to its use in a wide variety of applications. Amphiphilic polyols, such as Antifoam 204, are commonly used as surfactants in fermentation processes due to their limited toxicity and low cost, but these polyols in aqueous solutions can unexpectedly swell membranes. Here we examine the interactions of PEG or poly(propylene glycol) (PPG) with amphiphilic substituted norbornene copolymers through swelling in dilute aqueous solution. The effect of molecular mass (M_n) of the polyol (PEG and PPG) in aqueous (1 wt% butanol) solution on the swelling and mechanical properties of a series of poly(alkyl norbornene-co-hexafluoroisopropanol norbornene) is systematically investigated using a quartz crystal microbalance with dissipation. At 10 ppm of PEG, the swelling is less than 20% for all of the copolymers examined and the swelling is independent of PEG M_n. Although PPG at the lowest M_n examined leads to similar swelling to PEG, the swelling induced by PPG increases with M_n to reach a maximum at M_n = 3.1 kg mol^-1. Pluronic L121 is similar compositionally to Antifoam 204, but the equilibrium swelling is decreased by nearly a factor of 2, which is attributed to the higher M_n of Pluronic L121. The limited dependence on the alkyl chain in the copolymer suggest that the interactions between the polyol and hexafluoroisopropanol moiety in the copolymer drive the uptake by the membrane through bound water with the unassociated ether in the PPG that increases swelling with increasing M_n, but a finite size effect limits the swelling for sufficiently large polymer additives.

QCM-D assay for quantifying the swelling, biodegradation, and protein adsorption of intelligent nanogels

Environmentally responsive nanomaterials have been developed for drug delivery applications, in an effort to target and accumulate therapeutic agents at sites of disease. Within a biological system, these nanomaterials will experience diverse conditions which encompass a variety of solute identities and concentrations. In this study, we developed a new quartz crystal microbalance with dissipation (QCM-D) assay, which enabled the quantitative analysis of nanogel swelling, protein adsorption, and biodegradation in a single experiment. As a proof of concept, we employed this assay to characterize non-degradable and biodegradable poly(acrylamide-co-methacrylic acid) nanogels. We compared the QCM-D results to those obtained by dynamic light scattering to highlight the advantages and limitations of each method. We detailed our protocol development and practical recommendations, and hope that this study will serve as a guide for others to design application-specific QCM-D assays within the nanomedicine domain.

Interaction of lignin-derived dimer and eugenol-functionalized silica nanoparticles with supported lipid bilayers

The potential to impart surfaces with specific lignin-like properties (i.e. resistance to microbes) remains relatively unexplored due to the lack of well-defined lignin-derived small molecules and corresponding surface functionalization strategies. Here, allyl-modified guaiacyl β-O-4 eugenol (G-eug) lignin-derived dimer is synthesized and attached to mesoporous silica nanoparticles (MSNPs) via click chemistry. The ability of G-eug lignin-dimer functionalized particles to interact with and disrupt synthetic lipid bilayers is compared to that of eugenol, a known natural antimicrobial. Spherical MSNPs (∼150 nm diameter with 4.5 nm pores) were synthesized using surfactant templating. Post-synthesis thiol (SH) attachment was performed using (3-mercaptopropyl) trimethoxysilane and quantified by Ellman's test. The resultant SH-MSNPs were conjugated with the G-eug dimers or eugenol by a thiol-ene reaction under ultraviolet light in the presence of a photo initiator. From thermogravimetric analysis (TGA), attachment densities of approximately 0.22 mmol eugenol/g particle and 0.13 mmol G-eug dimer/g particle were achieved. The interaction of the functionalized MSNPs with a phospholipid bilayers of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (representing model cell membranes) supported on gold surface was measured using Quartz Crystal Microbalance with Dissipation monitoring (QCM-D). Eugenol-grafted MSNPs in PBS (up to 1 mg/mL) associated with the bilayer and increased the mass adsorbed on the QCM-D sensor. In contrast, MSNPs functionalized with G-eug dimer show qualitatively different behavior, with more uptake and evidence of bilayer disruption at and above a particle concentration of 0.5 mg/mL. These results suggest that bio-inspired materials with conjugated lignin-derived small molecules can serve as a platform for novel antimicrobial coatings and therapeutic carriers.

Investigations of the high-frequency dynamic properties of polymeric systems with quartz crystal resonators

Opportunities arising from the use of the rheometric quartz crystal microbalance (RheoQCM) as a fixed frequency rheometer operating at 15 MHz are discussed. The technique requires the use of films in a specified thickness range that depends on the mechanical properties of the material of interest. A regime map quantifying the appropriate thicknesses is developed, based on the properties of a highly crosslinked epoxy sample that is representative of a broad class of polymeric materials. Relative errors in the measured film properties are typically in the range of several percent or less and are minimized by using a power law model to relate the rheological properties at two different resonant harmonics of the quartz crystal. Application of the RheoQCM technique is illustrated by measuring the temperature- and molecular weight-dependent properties of polystyrene and poly(methyl methacrylate) in the vicinity of the glass transition.

Lignin-graft-PLGA drug-delivery system improves efficacy of MEK1/2 inhibitors in triple-negative breast cancer cell line

Aim: Few targeted therapies are available for triple-negative breast cancer (TNBC) patients. Here, we propose a novel alkaline-lignin-conjugated-poly(lactic- co-glycolic acid) (L-PLGA) nanoparticle drug delivery system to improve the efficacy of targeted therapies. Materials & methods: L-PLGA nanoparticles (NPs) loaded with the MEK1/2 inhibitor GDC-0623 were characterized, tested in vitro on MDA-MB-231 TNBC cell line and compared with loaded PLGA NPs. Results: Loaded L-PLGA NPs were less than half the size of PLGA NPs, had slower drug release and improved the efficacy of GDC-0623 when tested in vitro. We demonstrated that GDC-0623 reversed epithelial-to-mesenchymal transition in TNBC. Conclusion: Our findings indicate that L-PLGA NPs are superior to PLGA NPs in delivering GDC-0623 to cancer cells for improved efficacy in vitro.

The Impact of the Metal Interface on the Stability and Quality of a Therapeutic Fusion Protein

Subvisible particle formation, which occurs after the sterile filtration step of the fill/finish process, is a challenge that may occur during the development of biotherapeutics with complex molecular structures. Here, we show that a stainless steel pump head from a rotary piston pump produces more protein aggregates, past the limit of the acceptable quality range for subvisible particle counts, in comparison to a ceramic pump head. The quartz crystal microbalance was used to quantify the primary layer, proteins irreversibly adsorbed at the solid-liquid interface, and the secondary diffuse gel-like layer interacting on top of the primary layer. The results showed that the mass of protein irreversibly adsorbed onto stainless steel sensors is greater than on an aluminum oxide surface (ceramic pump mimic). This suggests that the amount of adsorbed protein plays a role in surface-induced protein aggregation at the solid-liquid interface.

pH-Responsive Copolymer Films Prepared by Surface-Initiated Polymerization and Simple Modification

We report the preparation of pH-responsive, ester/carboxylic acid random copolymer films via simple modification of poly(norbornene diacyl chloride) (pNBDAC), prepared via surface-initiated ring-opening metathesis polymerization, with mixtures of water and ethanol to form carboxylic acid and ethyl ester side groups. The pNBDAC film serves as a compositionally versatile platform to controllably obtain copolymers with multiple functionalities. In modifying the pNBDAC to form the copolymer film, ethanol exhibits a significantly higher reactivity with acyl chloride groups within the film than does water. The magnitude and range of the pH-responsive performance are highly dependent on the carboxylic acid content in the copolymer films, which demonstrates the effect of film hydrophilicity on the pH-responsive switching of ionic barrier properties. The resistance of the film against ion transfer can be decreased by a factor of 10⁴ through pH change, demonstrating pH-induced switching from hydrophobic and insulating to swollen and ion-permeable films. The interactions of the copolymer films with water at different pH values were also explored. When the copolymer contains 34% carboxylic acids, a 4× greater film thickness is obtained in high pH solution than in low pH solution due to ionically driven water swelling. The reversibility of the pH-responsive performance of these copolymer films is high based on measurements using quartz crystal microbalance with dissipation (QCM-D).

“Vanishing mass” in the Sauerbrey world: quartz crystal microbalance study of self-assembled monolayers based on a tripod-branched structure with tuneable molecular flexibility

Complex loadings that appear on a surface with flexible spatial organisation can reveal anti-Sauerbrey behaviour due to their variable interfacial architecture even for an ultrathin monomolecular sensitive layer.

Influence of Sodium Salts on the Swelling and Rheology of Hydrophobically Cross-linked Hydrogels Determined by QCM-D

Hydrophobically modified copolymers provide a versatile platform of hydrogel materials for diverse applications, but the influence of salts on the swelling and material properties of this class of hydrogels has not been extensively studied. Here, we investigate model hydrogels with three different sodium salts with anions chosen from the classic Hofmeister series to determine how these counterions influence the swelling and mechanical properties of neutral hydrogels. The gel chosen was based on a statistical copolymer of dimethylacrylamide and 2-(N-ethylperfluorooctane sulfonamido) ethyl acrylate (FOSA). Our measurements utilize a quartz crystal microbalance with dissipation (QCM-D) to quantify both swelling and rheological properties of these gels. We find that a 1 mol/L solution of Na₂SO₄, corresponding to a kosmotropic anion, leads to nearly a 2.6-fold gel deswelling and correspondingly, the complex modulus increases by an order of magnitude under these solution conditions. In contrast, an initial increase in swelling and then a swelling maximum is observed for a 0.02 mol/L concentration in the case of a chaotropic anion, NaClO₄, but the changes in the degree of gel swelling in this system are not directly correlated with changes in the gel shear modulus. The addition of NaBr, an anion salt closer to the middle of the chaotropic to kosmotropic range, leads to hydrogel deswelling where the degree of deswelling and the shear modulus are both nearly independent of salt concentration. Overall, the observed trends are broadly consistent with more kosmotropic ions causing diminished solubility ("salting out") and strongly chaotropic ions causing improved solubility ("salting in"), a trend characteristic of the Hoffmeister series governing the solubility of many proteins and synthetic water-soluble polymers, but trends in the shear stiffness with gel swelling are clearly different from those normally observed in chemically cross-linked gels and are correspondingly difficult to interpret. The salt specificity of swelling and mechanical properties of nonionic hydrogels is important for any potential application in which a wide range of salt concentrations and types are encountered.

Salt- and pH-induced swelling of a poly(acrylic acid) brush via quartz crystal microbalance w/dissipation (QCM-D)

We infer the swelling/de-swelling behavior of weakly ionizable poly(acrylic acid) (PAA) brushes of 2-39 kDa molar mass in the presence of KCl concentrations from 0.1-1000 mM, pH = 3, 7, and 9, and grafting densities σ = 0.12-2.15 chains per nm2 using a Quartz Crystal Microbalance with Dissipation (QCM-D), confirming and extending the work of Wu et al. to multiple chain lengths. At pH 7 and 9 (above the pKa ∼ 5), the brush initially swells at low KCl ionic strength (<10 mM) in the "osmotic brush" regime, and de-swells at higher salt concentrations, in the "salted brush" regime, and is relatively unaffected at pH 3, below the pKa, as expected. At pH 7, at low and moderate grafting densities, our results in the high-salt "salted brush" regime (Cs > 10 mM salt) agree with the predicted scaling H ∼ Nσ+1/3Cs-1/3 of brush height H, while in the low-salt "osmotic brush" regime (Cs < 10 mM salt), we find H ∼ Nσ+1/3Cs+0.28-0.38, whose dependence on Cs agrees with scaling theory for this regime, but the dependence on σ strongly disagrees with it. The predicted linearity in the degree of polymerization N is confirmed. The new results partially confirm scaling theory and clarify where improved theories and additional data are needed.

Versatile and High-Throughput Polyelectrolyte Complex Membranes via Phase Inversion

High-flux filtration membranes constructed through scalable and sustainable methods are desirable for energy-efficient separations. Often, these criteria are difficult to be reconciled with one another. Polymeric membranes can provide high flux but frequently involve organic solvents in processing steps. Solubility of many polymeric membranes in organic media also restricts their implementation in solvent filtration. In the present work, we report a simple and high-throughput aqueous processing approach for polyelectrolyte complex (PEC) membranes with controllable porosity and stability in various aqueous and organic environments. PECs are materials composed of oppositely charged polymer chains that can form solids in aqueous environments, yet which can be dissolved in very specific salt solutions capable of breaking the interpolymer ion pairs. By exploiting the salt-induced dissolution and subsequent reformation of the complex, nano- to microporous films are rapidly synthesized which resemble membranes obtained through conventional solvent-phase inversion techniques. PECs remain stable in organic solvents because of the low dielectric constant of the environment, which enhances electrostatic interactions, making them suitable for a wide range of water and solvent filtration applications. Here, we elucidate how the polymer-phase behavior can be manipulated to exercise morphological control, test membrane performance for water and solvent filtration, and quantify the mechanical stability of PECs in relevant conditions.

High-Frequency Mechanical Behavior of Pure Polymer-Grafted Nanoparticle Constructs

Polymer-grafted nanoparticle (GNP) membranes show increased gas permeability relative to pure polymer analogs, with this effect evidently tunable through systematic variations in the grafted polymer chain length and grafting density. Additionally, these materials show less deleterious aging effects relative to the pure polymer. To better understand these issues, we explore the solid-state mechanical properties of GNP layers using quartz crystal microbalance (QCM) spectroscopy, which operates under conditions (≈5 MHz) that we believe are relevant to gas transport. The GNP's high-frequency storage moduli exhibit a characteristic increase with increasing nanoparticle (NP) core loading, consistent with past work on the reinforcement of polymers physically well mixed with bare NPs. However, these GNPs show a substantial, nonmonotonic decrease in loss as a function of chain length (at fixed grafting density), with the loss minimum corresponding to the chain length with the maximum gas permeability. We speculate that this feature corresponds to a dynamical transition, where the GNP membranes go from a jammed solid (colloid-like) to liquid-like (polymer-controlled) behavior with increasing chain length.

Control of capillary behavior through target-responsive hydrogel permeability alteration for sensitive visual quantitative detection

DNA hydrogels have received considerable attention in analytical science, however, some limitations still exist in the applications of intelligent hydrogels. In this paper, we describe a way to prepare gel film in a capillary tube based on the thermal reversible principle of DNA hydrogel and the principle of capillary action. Because of the slight change in the internal structure of gel, its permeability can be increased by the addition of some specific targets. The capillary behavior is thus changed due to the different permeability of the hydrogel film. The duration time of the target solution flowing through the capillary tube with a specified length is used to quantify this change. With this proposed method, ultra-trace DNA hydrogel (0.01 μL) is sufficient to realize the sensitive detection of cocaine without the aid of other instruments, which has a low detection limit (1.17 nM) and good selectivity.

DNA hydrogels have received considerable attention in analytical science but limitations still exist in the applications of intelligent hydrogels. Here, the authors describe a DNA hydrogel sensor for quantitative detection of cocaine based on the permeability change in a DNA hydrogel film.

Guanidinium Can Break and Form Strongly Associating Ion Complexes

Guanidinium is one of nature's strongest denaturants and is also a motif that appears in several interfacial contexts such as the RGD sequence involved in cell adhesion, cell penetrating peptides, and antimicrobial molecules. It is important to quantify the origin of guanidinium's ion-specific interactions so that its unique behavior may be exploited in synthetic applications. The present work demonstrates that guanidinium ions can both break and form strongly associating ion complexes in a context-dependent way. These insights into guanidinium's behavior are elucidated using polyelectrolyte complexes (PECs), where interpolymer ion pairs between oppositely charged polymers play an important role in determining material stability. Different polycation-polyanion combinations can span a large range of association affinities, where more strongly associating complexes can remain insoluble in concentrated salt solutions and in extreme pH conditions. This high stability is desirable in several application contexts for PECs, but also renders them challenging to process and, therefore, to study since they cannot be dissolved into polymer solutions. Here we demonstrate that guanidinium salts are very effective in dissolving the poly(styrenesulfonate)/poly(allylamine) (PSS:PAH) complex, which has one of the highest reported polycation-polyanion association affinities. We also demonstrate the importance of charge identity in complexation phenomena by functionalizing guanidinium directly into poly(allylamine), resulting in a complex that remains stable under highly denaturing conditions. The model system of PSS:PAH is used to glean insights into guanidinium's denaturing activity, as well as to broadly comment on the nature of ion-specific interactions in charged macromolecules.

Determination of the Shear Modulus of Thin Polymer Films with a Quartz Crystal Microbalance: Application to UV-Curing

The photoinduced curing of a light-sensitive varnish was followed, based on a change of the film's shear modulus, G, as determined with a quartz crystal microbalance (QCM). The film thickness was in the range of a few hundred nanometers. Both the storage modulus, G', and the loss modulus, G″, were obtained. The analysis is based on a perturbation calculation. The equations differ from the more commonly used set of equations derived from the small-load approximation and the acoustic multilayer formalism (sometimes termed Voigt-model). The discussion revisits assumptions, accuracy, and limits of the technique. Critical to the analysis is a knowledge of the thickness of the electrode underneath the film.