Survey of oligoethylene glycol-based self-assembled monolayers on electrochemical aptamer-based sensor in biological fluids

The ability to monitor levels of endogenous markers and clearance profiles of drugs and their metabolites can improve the quality of biomedical research and precision with which therapies are individualized. Towards this end, electrochemical aptamer-based (EAB) sensors have been developed that support the real-time monitoring of specific analytes in vivo with clinically relevant specificity and sensitivity. A challenge associated with the in vivo deployment of EAB sensors, however, is how to manage the signal drift which, although correctable, ultimately leads to unacceptably low signal-to-noise ratios, limiting the measurement duration. Motivated by the correction of signal drift, in this paper, we have explored the use of oligoethylene glycol (OEG), a widely employed antifouling coating, to reduce the signal drift in EAB sensors. Counter to expectations, however, when challenged in 37 °C whole blood in vitro, EAB sensors employing OEG-modified self-assembled monolayers exhibit both greater drift and reduced signal gain, compared with those employ a simple, hydroxyl-terminated monolayer. On the other hand, when EAB sensor was prepared with a mix monolayer using MCH and lipoamido OEG 2 alcohol, reduced signal noise was observed compared to the same sensor prepared with MCH presumably due to improved SAM construction. These results suggest broader exploration of antifouling materials will be required to improve the signal drift of EAB sensors.

Investigation of Three-Dimensional Bacterial Adhesion Manner on Model Organic Surfaces Using Quartz Crystal Microbalance with Energy Dissipation Monitoring

Bacterial biofilms reduce the performance and efficiency of biomedical and industrial devices. The initial step in forming bacterial biofilms is the weak and reversible attachment of the bacterial cells onto the surface. This is followed by bond maturation and secretion of polymeric substances, which initiate irreversible biofilm formation, resulting in stable biofilms. This implies that understanding the initial reversible stage of the adhesion process is crucial to prevent bacterial biofilm formation. In this study, we analyzed the adhesion processes of E. coli on self-assembled monolayers (SAMs) with different terminal groups using optical microscopy and quartz crystal microbalance with energy dissipation (QCM-D) monitoring. We found that a considerable number of bacterial cells adhere to hydrophobic (methyl-terminated) and hydrophilic protein-adsorbing (amine- and carboxy-terminated) SAMs forming dense bacterial adlayers while attaching weakly to hydrophilic protein-resisting SAMs [oligo(ethylene glycol) (OEG) and sulfobetaine (SB)], forming sparse but dissipative bacterial adlayers. Moreover, we observed positive shifts in the resonant frequency for the hydrophilic protein-resisting SAMs at high overtone numbers, suggesting how bacterial cells cling to the surface using their appendages as explained by the coupled-resonator model. By exploiting the differences in the acoustic wave penetration depths at each overtone, we estimated the distance of the bacterial cell body from different surfaces. The estimated distances provide a possible explanation for why bacterial cells tend to attach firmly to some surfaces and weakly to others. This result is correlated to the strength of the bacterium-substratum bonds at the interface. Elucidating how the bacterial cells adhere to different surface chemistries can be a suitable guide in identifying surfaces with a more significant probability of contamination by bacterial biofilms and designing bacteria-resistant surfaces and coatings with excellent bacterial antifouling characteristics.

Prediction of Serum Adsorption onto Polymer Brush Films by Machine Learning

Using machine learning based on a random forest (RF) regression algorithm, we attempted to predict the amount of adsorbed serum protein on polymer brush films from the films' physicochemical information and the monomers' chemical structures constituting the films using a RF model. After the training of the RF model using the data of polymer brush films synthesized from five different types of monomers, the model became capable of predicting the amount of adsorbed protein from the chemical structure, physicochemical properties of monomer molecules, and structural parameters (density and thickness of the films). The analysis of the trained RF quantitatively provided the importance of each structural parameter and physicochemical properties of monomers toward serum protein adsorption (SPA). The ranking for the significance of the parameters agrees with our general understanding and perception. Based on the results, we discuss the correlation between brush film's physical properties (such as thickness and density) and SPA and attempt to provide a guideline for the design of antibiofouling polymer brush films.

Roles of interfacial water states on advanced biomedical material design

When biomedical materials come into contact with body fluids, the first reaction that occurs on the material surface is hydration; proteins are then adsorbed and denatured on the hydrated material surface. The amount and degree of denaturation of adsorbed proteins affect subsequent cell behavior, including cell adhesion, migration, proliferation, and differentiation. Biomolecules are important for understanding the interactions and biological reactions of biomedical materials to elucidate the role of hydration in biomedical materials and their interaction partners. Analysis of the water states of hydrated materials is complicated and remains controversial; however, knowledge about interfacial water is useful for the design and development of advanced biomaterials. Herein, we summarize recent findings on the hydration of synthetic polymers, supramolecular materials, inorganic materials, proteins, and lipid membranes. Furthermore, we present recent advances in our understanding of the classification of interfacial water and advanced polymer biomaterials, based on the intermediate water concept.

Quenching Efficiency of Quantum Dots Conjugated to Lipid Bilayers on Graphene Oxide Evaluated by Fluorescence Single Particle Tracking

A single particle observation of quantum dots (QDs) was performed on lipid bilayers formed on graphene oxide (GO). The long-range fluorescence quenching of GO has been applied to biosensing for various biomolecules. We demonstrated the single particle observation of a QD on supported lipid bilayers in this study, aiming to detect the quenching efficiency of lipid and protein molecules in a lipid bilayer by fluorescence single particle tacking (SPT). A single lipid bilayer or double lipid bilayers were formed on GO flakes deposited on a thermally oxidized silicon substrate by the vesicle fusion method. The QDs were conjugated on the lipid bilayers, and single particle images of the QDs were obtained under the quenching effect of GO. The quenching efficiency of a single QD was evaluated from the fluorescence intensities on the regions with and without GO. The quenching efficiency reflecting the layer numbers of the lipid bilayers was obtained.

Interphase Protein Layers Formed on Self-Assembled Monolayers in Crowded Biological Environments: Analysis by Surface Force and Quartz Crystal Microbalance Measurements

We investigated a viscous protein layer formed on self-assembled monolayers (SAMs) in crowded biological environments. The results were obtained through force spectroscopic measurements using colloidal probes and substantiated by exhaustive analysis using a quartz crystal microbalance with an energy dissipation technique. A hydrophobic SAM of n-octanethiol (C8 SAM) in bovine serum albumin (BSA) solution is buried under an adlayer of denatured BSA molecules and an additional viscous interphase layer that is five times more viscous than the bulk solution. C8 SAMs in fetal bovine serum induced a formation of a thicker adsorbed protein layer but with no observable viscous interphase layer. These findings show that a fouling surface is essentially inaccessible to any approaching molecules and thus has a new biological and physical identity arising from its surrounding protein layers. In contrast, the SAMs composed of sulfobetaine-terminated alkanethiol proved to be sufficiently protein-resistant and bio-inert even under crowded conditions due to a protective barrier of its interfacial water, which has implications in the accurate targeting of artificial particles for drug delivery and similar applications by screening any non-specific interactions. Finally, our strategies provide a platform for the straightforward yet effectual in vitro characterization of diverse types of surfaces in the context of targeted interactions in crowded biological environments.

Switchable β-lactoglobulin (BLG) adsorption on protein resistant oligo (ethylene glycol) (OEG) self-assembled monolayers (SAMs)

HYPOTHESIS

Although protein adsorption at an interface is very common and important in biology and biotechnology, it is still not fully understood - mainly due to the intricate balance of forces that ultimately control it. In food processing (and medicine), controlling and manipulating protein adsorption, as well as avoiding protein adsorption (biofilm formation or membrane fouling) by the production of protein-resistant surfaces is of substantial interest. A major factor conferring resistance towards protein adsorption to a surface is the presence of tightly bound water molecules, as is the case in oligo ethylene glycol (OEG)-terminated self-assembled monolayers (SAMs). Due to strong attractive protein-protein and protein-surface interactions observed in systems containing trivalent salt ions, we hypothesize that these conditions may lead to a breakdown of protein resistance in OEG SAMs.

EXPERIMENTS

We studied the adsorption behavior of BLG in the presence of a lanthanum(III) chloride (LaCl₃) at concentrations of 0, 0.1, 0.8 and 5.0 mM on normally protein resistant triethylene glycol-termianted (EG3) SAMs on a gold surface. We used quartz-crystal microbalance with dissipation (QCM-D) and neutron reflectivity (NR) to characterize the morphology of the interfacial region of the SAM.

FINDINGS

We demonstrate that the protein resistance of the EG3 SAM breaks down beyond a threshold salt concentration c^∗ and mirrors the bulk behaviour of this system, showing reduced adsorption beyond a second critical salt concentration c^∗∗. These results demonstrate for the first time the controlled switching of the protein-resistant properties of this type of SAM by the addition of trivalent salt.

Fluorine-containing bio-inert polymers: Roles of intermediate water

Fluorine-containing polymers are used not only in industrial processes but also in medical applications, because they exhibit excellent heat, weather, and chemical resistance. As these polymers are not easily degraded in our body, it is difficult to use them in applications that require antithrombotic properties, such as artificial blood vessels. The material used for medical applications should not only be stable in vivo, but it should also be inert to biomolecules such as proteins or cells. In this review, this property is defined as "bio-inert," and previous studies in this field are summarized. Bio-inert materials are less recognized as foreign substances by proteins or cells in the living body, and they must be covered at interfaces designed with the concept of intermediate water (IW). On the basis of this concept, we present here the current understanding of bio-inertness and unusual blood compatibility found in fluoropolymers used in biomedical applications. IW is the water that interacts with materials with moderate strength and has been quantified by a variety of analytical methods and simulations. For example, by using differential scanning calorimetry (DSC) measurements, IW was defined as water frozen at around -40°C. To consider the role of the IW, quantification methods of the hydration state of polymers are also summarized. These investigations have been conducted independently because of the conflict between hydrophobic fluorine and bio-inert properties that require hydrophilicity. In recent years, not many materials have been developed that incorporate the good points of both aspects, and their properties have seldom been linked to the hydration state. This has been critically performed now. Furthermore, fluorine-containing polymers in medical use are reviewed. Finally, this review also describes the molecular design of the recently reported fluorine-containing bio-inert polymers for controlling their hydration state. STATEMENT OF SIGNIFICANCE: A material covered with a hydration layer known as intermediate water that interacts moderately with other objects is difficult to be recognized as a foreign substance and exhibits bio-inert properties. Fluoropolymers show high durability, but conflict with bio-inert characteristics requiring hydrophilicity as these research studies have been conducted independently. On the other hand, materials that combine the advantages of both hydrophobic and hydrophilic features have been developed recently. Here, we summarize the molecular architecture and analysis methods that control intermediate water and provide a guideline for designing novel fluorine-containing bio-inert materials.

Confinements of Thermoresponsive Dendronized Polymers to Proteins

Crowding environment created by host polymers plays crucial roles in manipulating interactions with proteins and modulating their bioactivity. Here, we report our investigation on interactions between polymers and proteins in...

Local Cross-Coupling Activity of Azide-Hexa(ethylene glycol)-Terminated Self-Assembled Monolayers Investigated by Atomic Force Microscopy

Azide-oligo(ethylene glycol)-terminated self-assembled monolayers (N₃-OEG-SAMs) are promising interfacial structures for surface functionalization. Its many potential applications include chemical/bio-sensing and construction of surface models owing to its cross-coupling activity that originates from the azide group and oligo(ethylene glycol) (OEG) units for non-specific adsorption resistance. However, there are only a few studies and limited information, particularly on the molecular-scale structures and local cross-coupling activities of N₃-OEG-SAMs, which are vital to understanding its surface properties and interfacial molecular design. In this study, molecular-scale surface structures and cross-coupling activity of azide-hexa(ethylene glycol)-terminated SAMs (N₃-EG₆-SAMs) were investigated using frequency modulation atomic force microscopy (FM-AFM) in liquid. The N₃-EG₆-SAMs were prepared on Au(111) substrates through the self-assembly of 11-azido-hexa(ethylene glycol)-undecane-1-thiol (N₃-EG₆-C₁₁-HS) molecules obtained from a liquid phase. Subnanometer-resolution surface structures were visualized in an aqueous solution using a laboratory-built FM-AFM instrument. The results show a well-ordered molecular arrangement in the N₃-EG₆-SAM and its clean surfaces originating from the adsorption resistance property of the terminal EG₆ units. Surface functionalization by the cross-coupling reaction of copper(I)-catalyzed azide-alkyne cycloaddition was observed, indicating a structural change in the form of fluctuating structures and island-shaped structures depending on the concentration of the alkyne molecules. The FM-AFM imaging enabled to provide information on the relationship between the surface structures and cross-coupling activity. These findings provide molecular-scale information on the functionalization of the N₃-EG₆-SAMs, which is helpful for the interfacial molecular design based on alkanethiol SAMs in many applications.

Protein- and Cell-Resistance of Zwitterionic Peptide-Based Self-Assembled Monolayers: Anti-Biofouling Tests and Surface Force Analysis

Peptide-based self-assembled monolayers (peptide-SAMs) with specific zwitterionic amino acid sequences express an anti-biofouling property. In this work, we performed protein adsorption and cell adhesion tests using peptide-SAMs with repeating units of various zwitterionic pairs of amino acids (EK, DK, ER, and DR). The SAMs with the repeating units of EK and DK (EK and DK SAMs) manifested excellent bioinertness, whereas the SAMs with the repeating units of ER and DR (ER and DR SAMs) adhered proteins and cells. We also performed surface force measurements using atomic force microscopy to elucidate the mechanism underlying the difference in the anti-biofouling property. Our measurements revealed that water-induced repulsion with a range of about 8 nm acts between EK SAMs (immobilized on both probe and substrate) and DK SAMs, whereas such repulsion was not observed for ER and DR SAMs. The strength of the repulsion exhibited a clear correlation with the protein- and cell-resistance of the SAMs, indicating that the interfacial water in the vicinity of EK and DK SAMs is considered as a physical barrier to deter protein and cells from their adsorption or adhesion. The range of the repulsion observed for EK and DK SAMs is longer than 8 nm, indicating that the hydrogen bonding state of the interfacial water with a thickness of 4 nm is modified by EK and DK SAMs, resulting in the expression of the anti-biofouling property.

Effect of bound water content on cell adhesion strength to water-insoluble polymers

Adhesion of cells on biomaterials plays an essential role in modulating cellular functions. Although hydration of biomaterials occurs under biological conditions, it is challenging to systematically evaluate the correlation of hydrated water content in biomaterials with the cell adhesion strength. In this report, we investigated the effect of bound water content on the adhesion strength of cells on poly(2-methoxyethyl acrylate) (PMEA) analogue substrates. Water-insoluble PMEA analogues were synthesized to fabricate substrates with a systemically controlled bound water content. To assess the surface properties of their substrates, contact angle measurement, atomic force microscopy (AFM), and fluorescence measurement were conducted. To reflect the effect of bound water of PMEA analogues, the relationship between the bound water content and cell adhesion behavior was evaluated under serum-free condition. From the single cell force spectrometry (SCFS) and microscopic analysis, it revealed that the increment of bound water content on the substrates decreased cell adhesion strength and cell spreading on the substrates. The bound water content exhibited a good correlation with adhesion strength, spreading area, circularity, and aspect ratio of cells. Our findings indicate that the bound water content could contribute to the development of a novel biomaterial and evaluation of cell behaviors on biomaterials. STATEMENT OF SIGNIFICANCE: For coordinating cell functions, such as growth, mobility, and differentiation, modulating the adhesion strength between cells and their environments is important. Although the hydration to biomaterials has been reported to be closely related to a antifouling property, the effect of hydration water on the cell adhesion behavior is not well understood. We present the first demonstration of essential relationship between cell adhesion strength and hydrated water on a biomaterials surface using the water-insoluble polymers with different hydrated water content. The results reveal that the hydrated water content of polymer substrates strong correlation with adhesion strength of cells. Collectively, the hydrated water content of the biomaterials will be a predominant factor affecting the cell adhesion strength and behavior.

Adsorption of Mussel Protein on Polymer Antifouling Membranes: A Molecular Dynamics Study

Biofouling is one of the most difficult problems in the field of marine engineering. In this work, molecular dynamics simulation was used to study the adsorption process of mussel protein on the surface of two antifouling films—hydrophilic film and hydrophobic film—trying to reveal the mechanism of protein adsorption and the antifouling mechanism of materials at the molecular level. The simulated conclusion is helpful to design and find new antifouling coatings for the experiments in the future.

Physical Properties and Reactivity of Microdomains in Phosphatidylinositol-Containing Supported Lipid Bilayer

We characterized the size, distribution, and fluidity of microdomains in a lipid bilayer containing phosphatidylinositol (PI) and revealed their roles during the two-dimensional assembly of a membrane deformation protein (FBP17). The morphology of the supported lipid bilayer (SLB) consisting of PI and phosphatidylcholine (PC) on a mica substrate was observed with atomic force microscope (AFM). Single particle tracking (SPT) was performed for the PI+PC-SLB on the mica substrate by using the diagonal illumination setup. The AFM topography showed that PI-derived submicron domains existed in the PI+PC-SLB. The spatiotemporal dependence of the lateral lipid diffusion obtained by SPT showed that the microdomain had lower fluidity than the surrounding region and worked as the obstacles for the lipid diffusion. We observed the two-dimensional assembly of FBP17, which is one of F-BAR family proteins included in endocytosis processes and has the function generating lipid bilayer tubules in vitro. At the initial stage of the FBP17 assembly, the PI-derived microdomain worked as a scaffold for the FBP17 adsorption, and the fluid surrounding region supplied FBP17 to grow the FBP17 domain via the lateral molecular diffusion. This study demonstrated an example clearly revealing the roles of two lipid microregions during the protein reaction on a lipid bilayer.

Visualization of molecular binding sites at the nanoscale in the lift-up mode by amplitude-modulation atomic force microscopy

We report a new approach to visualize the local distribution of molecular recognition sites with nanoscale resolution by amplitude-modulation atomic force microscopy. By integrating chemical modification of probes, photothermal excitation to drive a cantilever, and lift-up scanning over surface topography, we successfully visualized binding sites provided by streptavidin on a solid surface for biotin attached on an AFM probe. The optimization of measurement conditions was discussed in detail, and the application of the technique was verified with a different ligand-receptor system.

Assembling Surface Linker Chemistry with Minimization of Non-Specific Adsorption on Biosensor Materials

The operation of biosensors requires surfaces that are both highly specific towards the target analyte and that are minimally subject to fouling by species present in a biological fluid. In this work, we further examined the thiosulfonate-based linker in order to construct robust and durable self-assembling monolayers (SAMs) onto hydroxylated surfaces such as silica. These SAMs are capable of the chemoselective immobilization of thiol-containing probes (for analytes) under aqueous conditions in a single, straightforward, reliable, and coupling-free manner. The efficacy of the method was assessed through implementation as a biosensing interface for an ultra-high frequency acoustic wave device dedicated to the detection of avidin via attached biotin. Fouling was assessed via introduction of interfering bovine serum albumin (BSA), IgG antibody, or goat serum. Improvements were investigated systematically through the incorporation of an oligoethylene glycol backbone employed together with a self-assembling diluent without a functional distal group. This work demonstrates that the incorporation of a diluent of relatively short length is crucial for the reduction of fouling. Included in this work is a comparison of the surface attachment of the linker to Si₃N₄ and AlN, both materials used in sensor technology.

Dendronized oligoethylene glycols with phosphonate tweezers for cell-repellent coating of oxide surfaces: coarse-scale and nanoscopic interfacial forces

Coarse-scale and nanoscopic interfacial force measurements unraveled how dendronized oligoethylene glycols with phosphonate tweezers prevent non-specific cell adhesion to oxide surfaces.

Role of interfacial water in determining the interactions of proteins and cells with hydrated materials

Water molecules play a crucial role in biointerfacial interactions, including protein adsorption and desorption. To understand the role of water in the interaction of proteins and cells at biological interfaces, it is important to compare particular states of hydration water with various physicochemical properties of hydrated biomaterials. In this review, we discuss the fundamental concepts for determining the interactions of proteins and cells with hydrated materials along with selected examples corresponding to our recent studies, including poly(2-methoxyethyl acrylate) (PMEA), PMEA derivatives, and other biomaterials. The states of water were analyzed by differential scanning calorimetry, in situ attenuated total reflection infrared spectroscopy, and surface force measurements. We found that intermediate water which is loosely bound to a biomaterial, is a useful indicator of the bioinertness of material surfaces. This finding on intermediate water provides novel insights and helps develop novel experimental models for understanding protein adsorption in a wide range of materials, such as those used in biomedical applications.

Modulation of Platelet-Surface Activation: Current State and Future Perspectives

Modulation of platelet-surface activation is important for many biomedical applications such as in vivo performance, platelet storage, and acceptance of an implant. Reducing platelet-surface activation is challenging because they become activated immediately after short contact with nonphysiological surfaces. To date, controversies and open questions in the field of platelet-surface activation still remain. Here, we review state-of-the-art approaches in inhibiting platelet-surface activation, mainly focusing on modification, patterning, and methodologies for characterization of the surfaces. As a future perspective, we discuss how the combination of biochemical and physiochemical strategies together with the topographical modulations would assist in the search for an ideal nonthrombogenic surface.

Interplays of Interfacial Forces Modulate Structure and Function of Soft and Biological Matters in Aquatic Environments

Water had been considered as a passive matrix that merely fills up the space, supporting the diffusion of solute molecules. In the past several decades, a number of studies have demonstrated that water play vital roles in regulating structural orders of biological systems over several orders of magnitude. Water molecules take versatile structures, many of which are transient. Water molecules act as hydrogen bond donors as well as acceptors and biochemical reactions utilize water molecules as nucleophiles. Needless to say, the same principle holds for the synthetic materials that function under water: the conformation, dynamics and functions of molecules are significantly influenced by the surrounding water. This review sheds light on how the structure and function of soft and biological matter in aquatic environments are modulated by the orchestration of various interfacial forces.

Quantifying the Extent of Hydration of a Surface-Bound Peptide Using Neutron Reflectometry

Establishing how water, or the absence of water, affects the structure, dynamics, and function of proteins in contact with inorganic surfaces is critical to developing successful protein immobilization strategies. In the present article, the quantity of water hydrating a monolayer of helical peptides covalently attached to self-assembled monolayers (SAMs) of alkyl thiols on Au was measured using neutron reflectometry (NR). The peptide sequence was composed of repeating LLKK units in which the leucines were aligned to face the SAM. When immersed in water, NR measured 2.7 ± 0.9 water molecules per thiol in the SAM layer and between 75 ± 13 and 111 ± 13 waters around each peptide. The quantity of water in the SAM was nearly twice that measured prior to peptide functionalization, suggesting that the peptide disrupted the structure of the SAM. To identify the location of water molecules around the peptide, we compared our NR data to previously published molecular dynamics simulations of the same peptide on a hydrophobic SAM in water, revealing that 49 ± 5 of 95 ± 8 total nearby water molecules were directly hydrogen-bound to the peptide. Finally, we show that immersing the peptide in water compressed its structure into the SAM surface. Together, these results demonstrate that there is sufficient water to fully hydrate a surface-bound peptide even at hydrophobic interfaces. Given the critical role that water plays in biomolecular structure and function, these results are expected to be informative for a broad array of applications involving proteins at the bio/abio interface.

Interaction of water with oligo(ethylene glycol) terminated monolayers: wetting versus hydration

Exposure of biorepulsive, oligo(ethylene glycol) (OEG) substituted self-assembled monolayers to water results in its adsorption both onto the surface and, with a higher binding energy, into the OEG matrix.

Proteomic Analysis of Biomaterial Surfaces after Contacting with Body Fluids by MALDI-ToF Mass Spectroscopy

We developed a method to identify proteins adsorbed on solid surfaces from a solution containing a complex mixture of proteins by using Matrix-Assisted Laser Desorption/Ionization-Time of Flight mass (MALDI-ToF mass) spectroscopy. In the method, we performed all procedures of peptide mass fingerprint method including denaturation, reduction, alkylation, digestion, and spotting of matrix on substrates. The method enabled us to avoid artifacts of pipetting that could induce changes in the composition. We also developed an algorithm to identify the adsorbed proteins. In this work, we demonstrate the identification of proteins adsorbed on self-assembled monolayers (SAMs). Our results show that the composition of proteins on the SAMs critically depends on the terminal groups of the molecules constituting the SAMs, indicating that the competitive adsorption of protein molecules is largely affected by protein-surface interaction. The method introduced here can provide vital information to clarify the mechanism underlying the responses of cells and tissues to biomaterials.

Wettability Recovery Behavior Governed by Desorption of Hydroxyl Species in Steel

The radiation induced surface activation (RISA) effect which occurs at the bilayer of metal and oxide due to irradiation contributes to improving the materials surface wettability and enhancing electrothermal characteristics and corrosion resistance. The purpose of the present study is to reveal the correlation between the wettability recovery behavior and the microstructural features of the adsorbed water and/or hydroxyl species on the surfaces. The wettability change due to the RISA effect was investigated in the oxidized austenitic stainless steel with various holding times under darkness after UV irradiation. The microstructure features of the adsorbed water and/or hydroxyl species on the surfaces were characterized by Raman spectroscopy and scanning probe microscopy (SPM). Results show that the contact angle of the specimen after UV irradiation parabolically increased with increasing holding time, regardless of the chemical composition within these two steels. The absorbed water layer was observed in both specimens by Raman analysis, and its intensity decreased with increasing holding time. From the force-distance curves of SPM analysis, the hydrophilic component was rapidly decreased at a holding time below 200 h, then gradually saturated when the holding time increased above 200 h from the SPM analysis. These results reveal that the wettability recovery behavior in oxidized austenitic stainless steels after UV irradiation can be ascribed to two kinds of mechanisms. One is the desorption of the hydrophilic components which are predominant when the contact angle is less than 30°, and the other is the absorption of the hydrophobic components which works mainly when the contact angle is above 30°.

Molecular diffusion and nano-mechanical properties of multi-phase supported lipid bilayers

Understanding the properties of cell membranes is important in the fields of fundamental and applied biology.

Stochastic Binding Process of Blunt-End Stacking of DNA Molecules Observed by Atomic Force Microscopy

Hydrophobic attraction is often a physical origin of nonspecific and irreversible (uncontrollable) processes observed for colloidal and biological systems, such as aggregation, precipitation, and fouling with biomolecules. On the contrary, blunt-end stacking of complementary DNA duplex chain pairs, which is also mainly driven by hydrophobic interaction, is specific and stable enough to lead to self-assemblies of DNA nanostructures. To understand the reason behind these contradicting phenomena, we measured forces operating between two self-assembled monolayers of duplexed DNA molecules with blunt ends (DNA-SAMs) and analyzed their statistics. We found the high specificity and stability of blunt-end stacking that resulted in the high resemblance between the interaction forces measured on approaching and retracting. The other finding is on the stochastic formation process of blunt-end stacking, which appeared as a significant fluctuation of the interaction forces at separations smaller than 2.5 nm. Based on these results, we discuss the underlying mechanism of the specificity and stability of blunt-end stacking.

Nonclassical Interactions of Phosphatidylcholine with Mucin Protect Intestinal Surfaces: A Microinterferometry Study

Albeit many studies demonstrated that the accumulation of phospholipids in the intestinal mucosal surfaces is essential for the protection of colon epithelia against pathogenic bacteria, the mechanism of interactions between phospholipids and the surface protein mucin is not well understood. In this study, the significance of interfacial interactions between phospholipids and mucin proteins was quantified by the combination of an in vitro intestinal surface model and label-free microinterferometry. The model of intestinal surfaces consists of planar lipid membranes deposited on solid substrates (supported membranes) that display mucin proteins at defined surface densities. Following the quantitative characterization of the systems, we monitored the vertical fluctuation of 10 μm-large particles on model intestinal surfaces by using microinterferometry, and calculated the effective interfacial interaction potentials by analytically solving the Langevin equation. We found that the spring constant of interfacial potentials calculated based on a harmonic approximation increased concomitantly with the increase in surface potentials, indicating the dominant role of electrostatic interactions. Intriguingly, the spring constants of particles coated with phospholipids do not follow electrostatic interactions. The spring constant of particles coated with zwitterionic phosphatidylcholine was larger compared to membranes incorporating positively or negatively charged lipids. Our data suggested the presence of another underlying molecular level interaction, such as phosphocholine-saccharide interactions. The fact that phosphatidylcholine sustains the binding capability to enzymatically degraded mucin suggests that the direct delivery of phosphatidylcholine to the damaged mucus is a promising strategy for the better treatment of patients affected by inflammatory bowel diseases.

Analysis of Interaction Between Interfacial Structure and Fibrinogen at Blood-Compatible Polymer/Water Interface

The correlation between the interfacial structure and protein adsorption at a polymer/water interface was investigated. Poly(2-methoxyethyl acrylate)(PMEA), which is one of the best blood compatible polymers available, was employed. Nanometer-scale structures generated through the phase separation of polymer and water were observed at the PMEA/phosphate buffered saline interface. The interaction between the interfacial structures and fibrinogen (FNG) was measured using atomic force microscopy. Attraction was observed in the polymer-rich domains as well as in the non-blood compatible polymer. In contrast, no attractive interactions were observed, and only a repulsion occurred in the water-rich domains. The non-adsorption of FNG into the water rich domains was also clarified through topographic and phase image analyses. Furthermore, the FNG molecules adsorbed on the surface of PMEA were easily desorbed, even in the polymer-rich domains. Water molecules in the water-rich domains are anticipated to be the dominant factor in preventing FNG adsorption and thrombogenesis on a PMEA interface.

Robust Surface Plasmon Resonance Chips for Repetitive and Accurate Analysis of Lignin-Peptide Interactions

We have developed novel surface plasmon resonance (SPR) sensor chips whose surfaces bear newly synthesized functional self-assembled monolayer (SAM) anchoring lignin through covalent chemical bonds. The SPR sensor chips are remarkably robust and suitable for repetitive and accurate measurement of noncovalent lignin-peptide interactions, which is of significant interest in the chemical or biochemical conversion of renewable woody biomass to valuable chemical feedstocks. The lignin-anchored SAMs were prepared for the first time by click chemistry based on an azide-alkyne Huisgen cycloaddition: mixed SAMs are fabricated on gold thin film using a mixture of alkynyl and methyl thioalkyloligo(ethylene oxide) disulfides and then reacted with azidated milled wood lignins to furnish the functional SAMs anchoring lignins covalently. The resulting SAMs were characterized using infrared reflection-absorption, Raman, and X-ray photoelectron spectroscopies to confirm covalent immobilization of the lignins to the SAMs via triazole linkages and also to reveal that the SAM formation induces a helical conformation of the ethylene oxide chains. Further, SPR measurements of the noncovalent lignin-peptide interactions using lignin-binding peptides have demonstrated high reproducibility and durability of the prepared lignin-anchored sensor chips.

Morphology and Physical Properties of Hydrophilic-Polymer-Modified Lipids in Supported Lipid Bilayers

Lipid molecules such as glycolipids that are modified with hydrophilic biopolymers participate in the biochemical reactions occurring on cell membranes. Their functions and efficiency are determined by the formation of microdomains and their physical properties. We investigated the morphology and properties of domains induced by the hydrophilic-polymer-modified lipid applying a polyethylene glycol (PEG)-modified lipid as a model modified lipid. We formed supported lipid bilayers (SLBs) using a 0-10 mol % range of PEG-modified lipid concentration ( C_PEG). We studied their morphology and fluidity by fluorescence microscopy, the fluorescence recovery after photobleaching method, and atomic force microscopy (AFM). Fluorescence images showed that domains rich in the PEG-modified lipid appeared and SLB fluidity decreased when C_PEG ≥ 5%. AFM topographies showed that clusters of the PEG-modified lipid appeared prior to domain formation and the PEG-lipid-rich domains were observed as depressions. Frequency-modulation AFM revealed a force-dependent appearance of the PEG-lipid-rich domain.

Thermosensitive Polymer Biocompatibility Based on Interfacial Structure at Biointerface

The interfacial structure of a thermosensitive biocompatible polymer, poly[2-(2-methoxyethoxy)ethyl methacrylate] (PMe2MA), at the polymer/phosphate-buffered saline (PBS) interface was investigated by atomic force microscopy. A number of nanometer scale protrusions appeared at 37 °C and disappeared at 22 °C, reversibly. This structural change occurred above the lower critical solution temperature of PMe2MA in PBS (19 °C), indicating that the formation of protrusions was explained by the microphase separation of polymer and water at the interfacial region. The protein adsorption and platelet adhesion onto PMe2MA interface were drastically restrained at 22 °C compared to that at 37 °C. Detachment of NIH3T3 cells accompanied by the dissipation of protrusions on the PMe2MA interface was also demonstrated. These results indicate that the protrusions act as the scaffold for the protein adsorption and cell adhesion.

Molecular-scale structures of the surface and hydration shell of bioinert mixed-charged self-assembled monolayers investigated by frequency modulation atomic force microscopy

Water molecules above a bioinert mixed-charged self-assembled monolayer (MC-SAM) surface are highly structured compared to those of bioactive SAM surfaces.

Preparation, characterization, and protein-resistance of films derived from a series of α-oligo(ethylene glycol)-ω-alkenes on H–Si(111) surfaces

Films on Si(111) were prepared by photo-activated grafting of CH₂&#xe001;CH(CH₂)m(OCH₂CH₂)nOCH₃ (m = 8, 9; n = 3–7) by using different vacuum conditions. High vacuum produced a higher thickness (40 Å) and <0.8% fibrinogen adsorption (C₁₀EG₇). Films were stable even after 28 days.

Glucose-responsive hydrogel electrode for biocompatible glucose transistor

In this paper, we propose a highly sensitive and biocompatible glucose sensor using a semiconductor-based field effect transistor (FET) with a functionalized hydrogel. The principle of the FET device contributes to the easy detection of ionic charges with high sensitivity, and the hydrogel coated on the electrode enables the specific detection of glucose with biocompatibility. The copolymerized hydrogel on the Au gate electrode of the FET device is optimized by controlling the mixture ratio of biocompatible 2-hydroxyethylmethacrylate (HEMA) as the main monomer and vinylphenylboronic acid (VPBA) as a glucose-responsive monomer. The gate surface potential of the hydrogel FETs shifts in the negative direction with increasing glucose concentration from 10 μM to 40 mM, which results from the increase in the negative charges on the basis of the diol-binding of PBA derivatives with glucose molecules in the hydrogel. Moreover, the hydrogel coated on the gate suppresses the signal noise caused by the nonspecific adsorption of proteins such as albumin. The hydrogel FET can serve as a highly sensitive and biocompatible glucose sensor in in vivo or ex vivo applications such as eye contact lenses and sheets adhering to the skin.

Shape memory polymer foams prepared from a heparin-inspired polyurethane/urea

Shape memory foams have been prepared using a heparin-inspired polyurea/urethane that displays excellent resistance to platelet adherence.

Terminal-Specific Interaction between Double-Stranded DNA Layers: Colloidal Dispersion Behavior and Surface Force

Double-stranded DNA-grafted nanoparticles (dsDNA-NPs) exhibit a unique dispersion behavior under high-salt conditions depending on the pairing status of their outermost base pairs (pairing or unpairing). The dsDNA-NPs having complementary (i.e., pairing) outermost base pairs spontaneously aggregate under high-salt conditions, but not when their outermost base pairs are mismatched (unpairing). In this study, we used colloidal probe atomic force microscopy to examine how the outermost base pairs affect the interaction between the dsDNA-grafted layers (dsDNA layers). To precisely assess the subtle structural differences in the dsDNA layers, we developed a method for the formation of a homogenous dsDNA layer on gold surfaces using hairpin-shaped DNAs. Homogenous dsDNA layers having complementary (G-C) or mismatched (C-C) outermost base pairs were grafted onto the surfaces of colloidal probes and gold substrates. Force-distance curves measured in an aqueous medium under high-salt conditions revealed that the surface forces between the dsDNA layers were bilateral in nature and were governed by the outermost base pairs. Between complementary outermost dsDNA layers, the surface force changed from repulsive to attractive with an increase in the NaCl concentration (10-1000 mM). The attraction observed under high-salt conditions was attributed to the site-specific interaction proceeded only between complementary dsDNA terminals, the so-called blunt-end stacking. In fact, between mismatched outermost dsDNA layers, the repulsive force was mostly dominant within the same NaCl concentration range. Our results clearly revealed that the pairing status of the outermost base pairs has significant implications for the surface forces between dsDNA layers, leading to the unique dispersion behavior of dsDNA-NPs.

Surface chemistry from wettability and charge for the control of mesenchymal stem cell fate through self-assembled monolayers

Self-assembled monolayers (SAMs) of alkanethiols on gold are highly controllable model substrates and have been employed to mimic the extracellular matrix for cell-related studies. This study aims to systematically explore how surface chemistry influences the adhesion, morphology, proliferation and osteogenic differentiation of mouse mesenchymal stem cells (mMSCs) using various functional groups (-OEG, -CH₃, -PO₃H₂, -OH, -NH₂ and -COOH). Surface analysis demonstrated that these functional groups produced a wide range of wettability and charge: -OEG (hydrophilic and moderate iso-electric point (IEP)), -CH₃ (strongly hydrophobic and low IEP), -PO₃H₂ (moderate wettability and low IEP), -OH (hydrophilic and moderate IEP), -NH₂ (moderate wettability and high IEP) and -COOH (hydrophilic and low IEP). In terms of cell responses, the effect of wettability may be more influential than charge for these groups. Moreover, compared to -OEG and -CH₃ groups, -PO₃H₂, -OH, -NH₂ and -COOH functionalities tended to promote not only cell adhesion, proliferation and osteogenic differentiation but also the expression of α_v and β₁ integrins. This finding indicates that the surface chemistry may guide mMSC activities through α_v and β₁ integrin signaling pathways. Model surfaces with controllable chemistry may provide insight into biological responses to substrate surfaces that would be useful for the design of biomaterial surfaces.

Tumor-Targeted Multimodal Optical Imaging with Versatile Cadmium-Free Quantum Dots

The rapid development of fluorescence imaging technologies requires concurrent improvements in the performance of fluorescent probes. Quantum dots have been extensively used as an imaging probe in various research areas because of their inherent advantages based on unique optical and electronic properties. However, their clinical translation has been limited by the potential toxicity especially from cadmium. Here, a versatile bioimaging probe is developed by using highly luminescent cadmium-free CuInSe2/ZnS core/shell quantum dots conjugated with CGKRK (Cys-Gly-Lys-Arg-Lys) tumor-targeting peptides. This probe exhibits excellent photostability, reasonably long circulation time, minimal toxicity, and strong tumor-specific homing property. The most important feature of this probe is that it shows distinctive versatility in tumor-targeted multimodal imaging including near-infrared, time-gated, and two-photon imaging in different tumor models. In a glioblastoma mouse model, the targeted probe clearly denotes tumor boundaries and positively labels a population of diffusely infiltrating tumor cells, suggesting its utility in precise tumor detection during surgery. This work lays a foundation for potential clinical translation of the probe.

Evaluation of Factors To Determine Platelet Compatibility by Using Self-Assembled Monolayers with a Chemical Gradient

Intercorrelation among surface chemical composition, packing structure of molecules, water contact angles, amounts and structures of adsorbed proteins, and blood compatibility was systematically investigated with self-assembled monolayers (SAMs) with continuous chemical composition gradients. The SAMs were mixtures of two thiols: n-hexanethiol (hydrophobic and protein-adsorbing) and hydroxyl-tri(ethylene glycol)-terminated alkanethiol (hydrophilic and protein-resistant) with continuously changing mixing ratios. From the systematic analyses, we found that protein adsorption is governed both by sizes of proteins and hydrophobic domains of the substrate. Furthermore, we found a clear correlation between adsorption of fibrinogen and adhesion of platelets. Combined with the results of surface force measurements, we found that the interfacial behavior of water molecules is profoundly correlated with protein resistance and antiplatelet adhesion. On the basis of these results, we conclude that the structuring of water at the SAM-water interface is a critical factor in this context.

Spectroscopic ellipsometry meets AFM nanolithography: about hydration of bio-inert oligo(ethylene glycol)-terminated self assembled monolayers on gold

An accurate thickness determination provides insight into the complex vertical morphology of OEG-terminated SAMs.