Combined Quartz Crystal Microbalance Research Articles

Planar Polymer Membranes Accommodate Functional Self-Assembly of Inserted Resorcinarene Nanocapsules.

Solid-supported polymer membranes (SSPMs) offer great potential in material and life sciences due to their increased mechanical stability and robustness compared to solid-supported lipid membranes. However, there is still a need for expanding the functionality of SSPMs by combining them with synthetic molecular assemblies. In this study, SSPMs served as a flexible matrix for the insertion of resorcinarene monomers and their self-assembly into functional hexameric resorcinarene capsules. Resorcinarene capsules provide a large cavity with affinity specifically for cationic and polyhydroxylated molecules. While the capsules are stable in apolar organic solvents, they disassemble when placed in polar solvents, which limits their application. Here, a solvent-assisted approach was used for copolymer membrane deposition on solid support and simultaneous insertion of the resorcinarene monomers. By investigation of the molecular factors and conditions supporting the codeposition of the copolymer and resorcinarene monomers, a stable hybrid membrane was formed. The hydrophobic domain of the membrane played a crucial role by providing a sufficiently thick and apolar layer, allowing for the self-assembly of the capsules. The capsules were functional inside the membranes by encapsulating cationic guests from the aqueous environment. The amount of resorcinarene capsules in the hybrid membranes was quantified by a combination of quartz-crystal microbalance with dissipation and liquid chromatography-mass spectrometry, while the membrane topography and layer composition were analyzed by atomic force microscopy and neutron reflectometry. Functional resorcinarene capsules inside SSPMs can serve as dynamic sensors and potentially as cross-membrane transporters, thus holding great promise for the development of smart surfaces.

Surface nanoroughness impacts the formation and stability of supported lipid bilayers

Solid supported lipid bilayers (SLBs) are excellent platforms for studying the biophysical properties of cell membranes, as well as versatile biomimetic films for biotechnology applications. Among the existing approaches used to form SLBs, vesicle fusion and rupture onto solid supports is the most commonly employed one owing to its straightforward procedure. SLBs are typically formed on atomically flat and very hydrophilic surfaces, overlooking the influence of roughness and topography on membrane formation and organization. As a matter of fact, lipid bilayers in vivo are corrugated at the nanoscale level, as a result of interactions with proteins, fibrils, and other components within the intracellular and extracellular environment. Fundamental studies of the effect of surface roughness on SLBs are scarce and restricted to few contributions, where nanoroughness has shown to affect lipid mobility by a 5-fold decrease and inhibit domain growth in phase-separated membranes. In this work, the impact of nanoroughness on the formation and stability of SLBs onto SiO2 surfaces with different degrees of vertical and lateral surface roughness is studied. Combining quartz crystal microbalance with dissipation monitoring (QCM-D) and atomic force microscopy with force spectroscopy (AFM-FS), it is shown that nanoroughness affects the formation of SLBs by increasing the activation energy of vesicle fusion, rupture and spreading, and weakens the stability and lateral organization of the formed SLBs.

Electrostatic interactions control the adsorption of extracellular vesicles onto supported lipid bilayers

Communication between cells located in different parts of an organism is often mediated by membrane-enveloped nanoparticles, such as extracellular vesicles (EVs). EV binding and cell uptake mechanisms depend on the heterogeneous composition of the EV membrane. From a colloidal perspective, the EV membrane interacts with other biological interfaces via both specific and non-specific interactions, where the latter include long-ranged electrostatic and van der Waals forces, and short-ranged repulsive “steric-hydration” forces. While electrostatic forces are generally exploited in most EV immobilization protocols, the roles played by various colloidal forces in controlling EV adsorption on surfaces have not yet been thoroughly addressed. In the present work, we study the adsorption of EVs onto supported lipid bilayers (SLBs) carrying different surface charge densities using a combination of quartz crystal microbalance with dissipation monitoring (QCM-D) and confocal laser scanning microscopy (CLSM). We demonstrate that EV adsorption onto lipid membranes can be controlled by varying the strength of electrostatic forces and we theoretically describe the observed phenomena within the framework of nonlinear Poisson-Boltzmann theory. Our modelling results confirm the experimental observations and highlight the crucial role played by attractive electrostatics in EV adsorption onto lipid membranes. They furthermore show that simplified theories developed for model lipid systems can be successfully applied to the study of their biological analogues and provide new fundamental insights into EV-membrane interactions with potential use in developing novel EV separation and immobilization strategies.

Role of Driving Force on Engineering Layer-by-Layer Protein/Polyphenol Coating with Flexible Structures and Properties

Protein-based coatings are of immense interest due to their rich biological functions. Layer-by-layer (LbL) assembly, as a powerful means of transferring protein functions to the material surface, has received widespread attention. However, the assembly mechanism of protein-based LbL coatings is still far from being explained, not only because of protein structure and function diversity but also characterization limitations. Herein, we monitored in situ the LbL assembly process of tannic acid (TA) and lysozyme (Lyz), a classic pair of polyphenol and protein, by combining quartz crystal microbalance with dissipation monitoring (QCM-D) and spectroscopic ellipsometry (SE). The water content, morphology, mechanical properties, antioxidant activity, and the driving force of TA-Lyz coating engineered under different pH values were analyzed in detail by various techniques. The water content, a key factor in TA-Lyz coatings, increased with increasing assembled pH values, which resulted in a porous morphology, inhomogeneous mechanical distribution, faster assembly growth, and better antioxidant activity in both acellular and cellular levels. In addition, high water content is unfavorable to both entropy and enthalpy changes, and the thermodynamic driving force of TA and Lyz assembly mainly comes from the enthalpy change brought by the noncovalent interaction between TA and Lyz. These results provide new insights into engineering the structure, function, and assembly mechanisms of protein-based coatings.

Investigating the Sequence Specific Adsorption Behavior of Polypeptides at the Solid/Liquid Interface

Polymer adsorption at the solid/liquid interface depends not only on the chemical composition of the polymer but also on the specific placement of the monomers along the polymer sequence. However, challenges in designing polymers with well-controlled sequences have limited explorations into the role of polymer sequence on adsorption behavior to molecular simulations. Here, we demonstrate how the sequence control offered by polypeptide synthesis can be utilized to study the effects small changes in polymer sequence have on polymer adsorption behavior at the solid/liquid interface. Through a combination of quartz crystal microbalance with dissipation monitoring and total internal reflection ellipsometry, we study the adsorption behavior of three polypeptides, consisting of 90% lysine and 10% cysteine, onto a gold surface. We find different mechanisms are responsible for the adsorption of polypeptides and the resulting conformation on the surface. The initial adsorption of the polypeptides is driven by electrostatic interactions between the polylysine and the gold surface. Once adsorbed, the cysteine undergoes a thiol-Au reaction with the surface, altering the conformation of the polymer layer. Our findings suggest the conformation of the polypeptide layer is dependent on the placement of the cysteines within the sequence; polypeptide chains with evenly spaced cysteine groups adopt a more tightly bound "train" conformation as compared to polypeptides with closely grouped cysteine groups. We envision that the methodologies presented here to study sequence specific adsorption behaviors using polypeptides could be a valuable tool to complement molecular simulations studies.

Surface reaction mechanism of atomic layer deposition of niobium oxide: In situ characterization and first-principle study

We report the surface reaction mechanism of the atomic layer deposition (ALD) of niobium oxide films by combining experimental and theoretical studies. Tert-butylimido tris(dimethylamido)niobium [tBuN=Nb(NMe2)3] and ozone (O3) were studied as the precursor and co-reactant. The in situ quartz crystal microbalance (QCM) analysis predicted that two HNMe2 molecules or one H2NtBu molecule were released at 200 °C per one molecule tBuN=Nb(NMe2)3. Since the QCM alone could not distinguish two possible reaction pathways, we compared two pathways by density functional theory (DFT) calculations. We simulated the chemisorption of the precursor molecule on the –OH terminated T-Nb2O5 substrate. The release of two –NMe2 ligands was favorable due to low activation energies of 0.71 and 0.63 eV for the first and second releases. On the other hand, the release of =NtBu was not favorable due to the high activation energy of 1.50 eV. Therefore, the amido and imido ligands were actor and spectator ligands during the first half-reaction of the ALD of niobium oxide films. The combination of QCM and DFT could successfully reveal the surface reaction mechanism of the ALD process using a heteroleptic precursor.

A new methodology combining QCM-D and proteomic profiling enables characterization of protein adsorption on 2D surfaces.

One of the critical features of biomedical material design is controlling the plasma protein adsorption to modulate the material behavior in biological media. Protein adsorption is highly influenced by the material surfaces and the proteins present in the biological medium. Thus, it is necessary to study protein-surface interactions that eventually take place on nanomaterials introduced into the body by the use of human plasma. However, very little information is available about human plasma interaction with planar surfaces under physiological conditions. Due to the limitation of the current characterization techniques to investigate the complicated interaction between the complex milieu of plasma proteins and planar materials, most efforts have focused on single proteins. To face this challenge, we have developed a new methodology based on the combination of quartz crystal microbalance with dissipation monitoring (QCM-D) and liquid chromatography coupled with mass spectrometry (LC-MS) to obtain information about protein-surface interactions on planar surfaces. First, QCM-D allowed us to determine the adsorbed protein mass and layer thickness. After detaching the proteins by a surfactant treatment, LC-MS analysis revealed the proteomic profile. Here, we have investigated three base materials, polystyrene (PS), gold (Au), and silica (SiO2) with or without precoating and compared the protein profiles.

Scanning gel electrochemical microscopy: Combination with quartz crystal microbalance for studying the electrolyte residue

The residue of electrolyte is a main challenge for local electrochemical measurements based on spatially localized contact between the electrolyte and the sample surface, such as scanning droplet cell or scanning electrochemical cell microscopy. By immobilizing electrolyte in the form of gel, scanning gel electrochemical microscopy (SGECM) is expected to overcome this challenge. This work proposes an analytical method for studying the issue of electrolyte residue from gel probes in SGECM. It is based on measuring approach-retract curves of gel probes on a quartz crystal microbalance (QCM), where the resonance frequency is recorded and synchronized with the current response. Verified by SEM observation, the frequency shift after one approach-retract cycle is taken as a measure for systematically compare the extent of electrolyte residue on the Au-coated quartz crystal surface. The results show that the electrolyte residue from freshly prepared gel probes, especially after soaking redox probes, could be reduced to negligible after a few approach-retract cycles. It can also be inhibited by co-electrodeposition of chitosan with tetraethoxysilane for preparing composite gel probe. Electrolyte residue could increase when over-pressing the gel against the sample. The residue of two types of gel probes and microcapillaries is also compared. The combination of QCM with SGECM offers a quality control method for gel probes before quantitative local electrochemical measurements.

The Impact of Surface Charges of Carboxylated Cellulose Nanofibrils on the Water Motions in Hydrated Films.

Cellulose nanofibrils (CNFs) with carboxylated surface ligands are a class of materials with tunable surface functionality, good mechanical properties, and bio-/environmental friendliness. They have been used in many applications as scaffold, reinforcing, or functional materials, where the interaction between adsorbed moisture and the CNF could lead to different properties and structures and become critical to the performance of the materials. In this work, we exploited multiple experimental methods to study the water movement in hydrated films made of carboxylated CNFs prepared by TEMPO oxidation with two different surface charges of 600 and 1550 μmol·g–1. A combination of quartz crystal microbalance with dissipation (QCM-D) and small-angle X-ray scattering (SAXS) shows that both the surface charge of a single fibril and the films’ network structure contribute to the moisture uptake. The films with 1550 μmol·g–1 surface charges take up twice the amount of moisture per unit mass, leading to the formation of nanostructures with an average radius of gyration of 2.1 nm. Via the nondestructive quasi-elastic neutron scattering (QENS), a faster motion is explained as a localized movement of water molecules inside confined spheres, and a slow diffusive motion is found with the diffusion coefficient close to bulk water at room temperature via a random jump diffusion model and regardless of the surface charge in films made from CNFs.

Tailoring a Solvent-Assisted Method for Solid-Supported Hybrid Lipid-Polymer Membranes.

Combining amphiphilic block copolymers and phospholipids opens new opportunities for the preparation of artificial membranes. The chemical versatility and mechanical robustness of polymers together with the fluidity and biocompatibility of lipids afford hybrid membranes with unique properties that are of great interest in the field of bioengineering. Owing to its straightforwardness, the solvent-assisted method (SA) is particularly attractive for obtaining solid-supported membranes. While the SA method was first developed for lipids and very recently extended to amphiphilic block copolymers, its potential to develop hybrid membranes has not yet been explored. Here, we tailor the SA method to prepare solid-supported polymer–lipid hybrid membranes by combining a small library of amphiphilic diblock copolymers poly(dimethyl siloxane)–poly(2-methyl-2-oxazoline) and poly(butylene oxide)-block–poly(glycidol) with phospholipids commonly found in cell membranes including 1,2-dihexadecanoyl-sn-glycero-3-phosphocholine, 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine, sphingomyelin, and 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine-N-(glutaryl). The optimization of the conditions under which the SA method was applied allowed for the formation of hybrid polymer–lipid solid-supported membranes. The real-time formation and morphology of these hybrid membranes were evaluated using a combination of quartz crystal microbalance and atomic force microscopy. Depending on the type of polymer–lipid combination, significant differences in membrane coverage, formation of domains, and quality of membranes were obtained. The use of the SA method for a rapid and controlled formation of solid-supported hybrid membranes provides the basis for developing customized artificial hybrid membranes.

Biosensing Extracellular Vesicle Subpopulations in Neurodegenerative Disease Conditions.

Extracellular vesicles (EVs) are secreted nanoparticles that are involved in intercellular communication and that modulate a wide range of biological processes in normal and disease conditions. However, EVs are highly heterogeneous in terms of origin in the cell, size, and density. As a result, complex protocols are required to identify and characterize specific EV subpopulations, limiting biomedical applications, notably in diagnostics. Here, we show that combining quartz crystal microbalance with dissipation (QCM-D) and nanoplasmonic sensing (NPS) provides a facile method to track the viscoelastic properties of small EVs. We applied this multisensing strategy to analyze small EVs isolated by differential ultracentrifugation from knock-in mouse striatal cells expressing either a mutated allele or wild-type allele of huntingtin (Htt), the Huntington's disease gene. Our results validate the sensing strategy coupling QCM-D and NPS and suggest that the mass and viscoelastic dissipation of EVs can serve as potent biomarkers for sensing the intercellular changes associated with the neurodegenerative condition.

Graphene Oxide/Silver Nanoparticles Platforms for the Detection and Discrimination of Native and Fibrillar Lysozyme: A Combined QCM and SERS Approach.

We propose a sensing platform based on graphene oxide/silver nanoparticles arrays (GO/AgNPs) for the detection and discrimination of the native and toxic fibrillar forms of an amyloid-prone protein, lysozyme, by means of a combination of Quartz Crystal Microbalance (QCM) and Surface Enhanced Raman Scattering (SERS) measurements. The GO/AgNPs layer system was obtained by Langmuir-Blodgett assembly of the silver nanoparticles followed by controlled adsorption of GO sheets on the AgNPs array. The adsorption of native and fibrillar lysozyme was followed by means of QCM, the measurements provided the kinetics and the mechanism of adsorption as a function of protein concentration as well as the mass and thickness of the adsorbed protein on both nanoplatforms. The morphology of the protein layer was characterized by Confocal Laser Scanning Microscopy experiments on Thioflavine T-stained samples. SERS experiments performed on arrays of bare AgNPs and of GO coated AgNP after native, or fibrillar, lysozyme adsorption allowed for the discrimination of the native form and toxic fibrillar structure of lysozyme. Results from combined QCM/SERS studies indicate a general construction paradigm for an efficient sensing platform with high selectivity and low detection limit for native and amyloid lysozyme.

Total internal reflection fluorescence microscopy to study sheet front growth in phospholipid supported lipid membrane formation

Supported lipid bilayer (SLB) based biosensors possess biomedical applications such as in rapid detection of antigens and cytochromes. It is generally believed that the SLB can be formed by adsorbing and spontaneously rupturing vesicles on substrate. Recent findings highlight the importance of investigating the adsorption and rupture of individual vesicles during the SLB formation. Here, we use total internal reflection fluorescence microscopy (TIRFM) to characterize the spatiotemporal kinetics of the front spreading at patch boundary. Owing to the mixture of labeled and unlabeled vesicles individual vesicle or patch on the surface can be identified. The TIRFM is employed to investigate the adsorption, rupture of vesicles, and spreading of the patch front. Combining quartz crystal microbalance with dissipation monitoring (QCM-D) and TIRFM characterizations, we find that the size of vesicle has a significant effect on the front spreading at the patch boundary. Quantification of the number of patches and patches area displays that smaller vesicles are more prone to the formation of patches. The front spreading at the patch boundary is analyzed quantitatively using the average front growth velocity (<inline-formula><tex-math id="M3">\begin{document}$ {v}_{\rm afv} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20220309_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20220309_M3.png"/></alternatives></inline-formula>), which indicates that the <inline-formula><tex-math id="M4">\begin{document}$ {v}_{\rm afv} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20220309_M4.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20220309_M4.png"/></alternatives></inline-formula>of 40-nm vesicles is one order of magnitude larger than that of the 112 nm vesicles. Both theoretical analysis and experimental observation show that the smaller vesicles can attain the higher concentration on the surface (<i>C</i>) and high diffusivity in the medium. The global growth theoretical model (GGM) presents that for the patches with the same surface area and vesicle exposure time, the growth of the patch depends on <i>C</i> and lipid loss percentage during the vesicle rupture. The calculated lipid loss of the smaller vesicles is slightly higher than that of the larger vesicles, while <i>C</i> plays a dominating role in determining the disparity of the patch growth between the different vesicles. This study promotes the understanding of the growth mechanism of patches on the surface. It demonstates the critcial role of the supply of vesicles in this process and provides an enlightenment for investigating the reassembly of lipids on a nanoscale.

Dextran-based polyelectrolyte multilayers: Effect of charge density on film build-up and morphology

We have studied the growth process of thin polyelectrolyte (PE) films fabricated by the layer-by-layer assembly (LbL) and composed of Dextran sulfate with high (DexS H) and low (DexS L) sulfation rate and poly(allylamine hydrochloride) (PAH). Film growths were monitored by combining Quartz Crystal Microbalance with Dissipation monitoring (QCM-D), Surface Plasmon Resonance (SPR) and Atomic Force Microscopy (AFM). Even though, the two films growth up to 10 bilayers, QCM-D showed that polyelectrolyte pairs do not display similar behaviours. (PAH/DexS H) systems lead to linear growth, i.e. amounts deposited increase both for PAH and DexS H, while the PAH/DexS L pair generated zig-zag shaped asymmetric growth. Film water contents were determined by QCM-D solvent exchange and SPR experiments. DexS L contains less water than DexS H and in agreement with the QCM-D dissipation values that suggest the formation of more rigid films in the case of DexS L than DexS H. Surface morphology investigated by AFM display distinct surface patterns since DexS H form thin films with fibril-like morphology covering all the surface while heterogeneous films with “puddle-like” aggregates were imaged in the case of DexS L. Difference of charge compensation and charge neutralisation between both systems likely lead to dissimilar growth mechanisms that are tentatively proposed in this paper.

Label-free characterization of an extracellular vesicle-based therapeutic.

Interest in mesenchymal stem cell derived extracellular vesicles (MSC‐EVs) as therapeutic agents has dramatically increased over the last decade. Current approaches to the characterization and quality control of EV‐based therapeutics include particle tracking techniques, Western blotting, and advanced cytometry, but standardized methods are lacking. In this study, we established and verified quartz crystal microbalance (QCM) as highly sensitive label‐free immunosensing technique for characterizing clinically approved umbilical cord MSC‐EVs enriched by tangential flow filtration and ultracentrifugation. Using QCM in conjunction with common characterization methods, we were able to specifically detect EVs via EV (CD9, CD63, CD81) and MSC (CD44, CD49e, CD73) markers. Furthermore, analysis of QCM dissipation versus frequency allowed us to quantitatively determine the ratio of marker‐specific EVs versus non‐vesicular particles (NVPs) – a parameter that cannot be obtained by any other technique so far. Additionally, we characterized the topography and elasticity of these EVs by atomic force microscopy (AFM), enabling us to distinguish between EVs and NVPs in our EV preparations. This measurement modality makes it possible to identify EV sub‐fractions, discriminate between EVs and NVPs, and to characterize EV surface proteins, all with minimal sample preparation and using label‐free measurement devices with low barriers of entry for labs looking to widen their spectrum of characterization techniques. Our combination of QCM with impedance measurement (QCM‐I) and AFM measurements provides a robust multi‐marker approach to the characterization of clinically approved EV therapeutics and opens the door to improved quality control.

Investigation of the adsorption of a mixture of two anionic surfactants, AOT and SDBS, on silica at ambient temperature

Chemical flooding, one of the Enhanced Oil Recovery (EOR) techniques used to increase oil production, consists in the injection in the well of an aqueous formulation containing various chemical additives such as surfactants. However, its performance can be significantly altered by the loss of surfactants in reservoir rocks. More precisely, surfactant loss due to adsorption on the reservoir rock may have a non-negligible impact on the efficiency of the injected formulation. In this article, we considered the adsorption of a mixture of two anionic surfactants having an important industrial relevance for EOR applications. Adsorption was studied on silica, representative of reservoir rocks such as sandstone, by combining Quartz Crystal Microbalance with Dissipation monitoring (QCM-D) and neutron reflectivity experiments. A preliminary characterization of the surfactant mixture solution demonstrated the formation of unilamellar vesicles in the bulk. Whereas an adsorbed layer was measured with the single AOT vesicles, but not with the SDBS micelles, we observed that mixing both anionic surfactants change the adsorption phenomenon. Indeed, non-negligible adsorption was measured for the mixture even at concentrations where only slight adsorption had been observed with the individual surfactants. This suggests that the structure of the aggregates formed in the bulk has a non-negligible impact on the adsorption. We note that the addition of salt tends to enhance the adsorption by screening the repulsive interactions between both negatively charged surfactants and silica at neutral pH. The present work provides new insights into the description of the adsorption of a mixture of surfactants of same nature in unfavorable conditions.

Understanding the Dehydration Stress in Lipid Vesicles by a Combined Quartz Crystal Microbalance and Dielectric Spectroscopy Study

Herein, 1‐palmitoyl‐2‐oleoyl‐glycero‐3‐phosphocholine (POPC) vesicles are studied using a newly designed setup that combines dielectric and gravimetric (quartz crystal microbalance, QCM) measurements. This setup allows to monitor the molecular dynamics and phase transitions of lipid structures under controlled humidity and temperature conditions. The main relaxation process (R2), related to the intrinsic headgroups' dynamics, is found in all phases and reveals a systematic acceleration with increasing hydration level. In contrast, a faster relaxation mode (R1) is found exclusively in the “liquid‐crystalline,” high‐temperature state, favoring water clusters located in the disordered aliphatic phase as origin. A third process (R3), being slower than the R2 mode, is attributed to a decoupled dynamics of hydration water in the hydrophilic part of the lipid bilayers occurring at higher hydration levels. In addition to molecular dynamics, both QCM and dielectric relaxation spectroscopy (DRS) measurements are demonstrated to yield consistent information on the phase behavior of the POPC samples which show a marked sensitivity to the hydration level. Moreover, contact angle measurements and atomic force microscopy confirm irreversible changes from the initial vesicles to a stacked bilayer structure upon repeated hydration‐dehydration, which manifests in the hydrophobization of the surface and water reduced absorption kinetics.

Synthesis, characterization and application of S-TiO2/PVDF-g-PSSA composite membrane for improved performance in MFCs

Sulfonated titanium dioxide (S-TiO2) was prepared by the grafting of sulfonic groups onto TiO2, and doped into the PVDF-g-PSSA as a highly efficient inorganic additive, thus achieving simultaneous improvement of the proton conductivity and anti-fouling property of the synthesized S-TiO2/PPSSA proton exchange membrane (PEM). The S-TiO2/PPSSA membrane was implemented in microbial fuel cell (MFC) for the first time, and its physicochemical properties and performance in MFC were studied. A new technique of combining Quartz crystal microbalance with dissipation with membrane spin-coated sensors made by a spin coating method was utilized to provide an insight into the anti-fouling property of the prepared membranes. The implementation of Fourier transform infrared spectrometry and X-ray photoelectron spectroscopy demonstrated TiO2 was successfully introduced into the sulphonic groups. The S-TiO2/PPSSA-5.0 membrane has a significant increase in the water uptake (40.9%) and proton conductivity (0.067 S/cm) compared with the PVDF-g-PSSA membrane. QCM-D results indicated that it has better anti-fouling property than pristine membrane and Nafion 117. Moreover, the MFC with the S-TiO2/PPSSA-5.0 membrane exhibited a maximum power density of 130.54 mW/m2, very close to that of Nafion 117 (132.02 mW/m2), but had the highest COD removal of 91% and least bio-fouling adsorption. These results demonstrate that the blending of sulfonated organic polymers and sulfonated inorganic additives can improve the electrochemical and anti-fouling properties of the composite membranes, which is a new concept for the preparation of PEM suitable for MFC. The S-TiO2/PPSSA-5.0 composite PEM is an appropriate candidate separator for the large-scale application of MFCs.

Multifunctional oligomer immobilized on quartz crystal microbalance: a facile and stabilized molecular imprinting strategy for glycoprotein detection

Glycoprotein detection holds great potential for early diagnosis of diverse diseases. For this purpose, the combination of quartz crystal microbalance (QCM) sensor and molecular imprinting has attracted increasing attention. Nonetheless, the recently common imprinted films fabricated on QCM electrode are thick and rigid, lacking flexibility in aqueous phase. Alternatively, small molecules immobilized on the electrode to construct molecular scale film could address this problem, while stabilization of the imprinted sites remains challenging. Herein, a co-assembly complex was obtained by the mixture of template and multifunctional oligomer, which was then immobilized on the amino-modified transducer surface through epoxy-amino reaction to form a protein-imprinted film. Afterward, the remaining epoxy groups in oligomer chains were cross-linked to conserve and stabilize the orientation of imprinted sites after template elution. Template rebinding tests show that cross-linked film has much higher imprinting factors than that of the non-cross-linked counterpart. Furthermore, control proteins that are distinct in properties and structures were employed to demonstrate the selectivity of this approach, and the imprinted assay reveals high affinity and specificity towards template protein. Graphical Abstract.

Deposition of titanium dioxide nanoparticles onto engineered rough surfaces with controlled heights and properties

Understanding the influence of surface roughness on the deposition of nanoparticles is important to a variety of environmental and industrial processes. In this work, slanted columnar thin films (SCTFs) were engineered to serve as an analogue for rough surfaces with controlled height and surface properties. The deposition of titanium dioxide nanoparticles (TiO2NPs) onto alumina- or silica-coated SCTFs (Al2O3-Si-SCTF, SiO2-Si-SCTF) with varying heights (50 nm, 100 nm, and 200 nm) was measured using a combined quartz crystal microbalance with dissipation monitoring (QCM-D) and generalized ellipsometry (GE) technique. No TiO2NP deposition was observed on flat, silica-coated QCM-D sensors or rough, 100 nm thick SiO2-Si-SCTF. TiO2NP deposition onto Al2O3-Si-SCTFs in ultra-pure water was significantly higher than on the flat alumina-coated QCM-D sensor, and deposition increased as the roughness height increased. The nanoparticle attachment was sensitive to the local flow field and the interaction energy between nanoparticles and the QCM-D sensor. At a higher ionic strength condition (100 mM NaCl), TiO2NP aggregates with varying sizes formed a rigid layer on top of SCTFs. For the first time, deposition of nanoparticles was measured as a function of roughness height, and the impact of roughness on the properties of the attached nanoparticle layers was revealed. This finding indicates that key parameters describing surface roughness should be explicitly included into models to accurately predict the transport of nanoparticles in the subsurface.