Protein absorption and ellipsometry in biomaterial research

Determining the Characteristics of Ultrathin Polymer Films: A Spectroscopic Ellipsometry Approach

Ellipsometry is a powerful and convenient technique that is widely used to determine the thickness and optical characteristics of polymer thin films. The determination is accomplished by modeling the measured change in the polarization of an electromagnetic wave upon interacting with the thin film. However, due to the strong statistical correlations between the fit parameters in the model, simultaneous determination of the thickness and the refractive indices of optically anisotropic ultrathin films using ellipsometry remains a challenge. Here, we propose an approach that can be used to obtain reliable values of both the thickness and the optical anisotropy of ultrathin polymer films. The approach was developed by performing spectroscopic ellipsometry measurements on thin films of hydrophobic polystyrene and hydrophilic chitosan of thickness between a few tens to a few hundred nm and whose absolute value of the birefringence differed by approximately an order of magnitude. Careful consideration of the characteristics of the root mean squared error of the fits obtained by modeling the ellipsometry data and the statistical correlations between the fit parameters formed the basis of the proposed approach.

Near-Field Terahertz Morphological Reconstruction Nanoscopy for Subsurface Imaging of Protein Layers

Protein layers formed on solid surfaces have important applications in various fields. High-resolution characterization of the morphological structures of protein forms in the process of developing protein layers has significant implications for the control of the layer's quality as well as for the evaluation of the layer's performance. However, it remains challenging to precisely characterize all possible morphological structures of protein in various forms, including individuals, networks, and layers involved in the formation of protein layers with currently available methods. Here, we report a terahertz (THz) morphological reconstruction nanoscopy (THz-MRN), which can reveal the nanoscale three-dimensional structural information on a protein sample from its THz near-field image by exploiting an extended finite dipole model for a thin sample. THz-MRN allows for both surface imaging and subsurface imaging with a vertical resolution of ∼0.5 nm, enabling the characterization of various forms of proteins at the single-molecule level. We demonstrate the imaging and morphological reconstruction of single immunoglobulin G (IgG) molecules, their networks, a monolayer, and a heterogeneous double layer comprising an IgG monolayer and a horseradish peroxidase-conjugated anti-IgG layer. The established THz-MRN presents a useful approach for the label-free and nondestructive study of the formation of protein layers.

Surface modification of additive manufactured Ti6Al4V scaffolds with gelatin/alginate- IGF-1 carrier: An effective approach for healing bone defects

The study investigates the potential of porous scaffolds with Gel/Alg-IGF-1 coatings as a viable candidate for orthopaedic implants. The scaffolds are composed of additively manufactured Ti6Al4V lattices, which were treated in an alkali solution to obtain the anatase and rutile phases. The treated surface exhibited hydrophilicity of <11.5°. A biopolymer carrier containing Insulin-like growth factor 1 was coated on the samples using immersion treatment. This study showed that the surface-modified porous Ti6Al4V scaffolds increased cell viability and proliferation, indicating potential for bone regeneration. The results demonstrate that surface modifications can enhance the osteoconduction and osteoinduction of Ti6Al4V implants, leading to improved bone regeneration and faster recovery. The porous Ti6Al4V scaffolds modified with surface coating of Gel/Alg-IGF-1 exhibited a noteworthy increase in cell viability (from 80.7 to 104.1%viability) and proliferation. These results suggest that the surface modified scaffolds have potential for use in treating bone defects.

CA19-9 and CEA biosensors in pancreatic cancer

Cancer is a complex pathophysiological condition causing millions of deaths each year. Early diagnosis is essential especially for pancreatic cancer. Existing diagnostic tools rely on circulating biomarkers such as Carbohydrate Antigen 19-9 (CA19-9) and Carcinoembryonic Antigen (CEA). Unfortunately, these markers are nonspecific and may be increased in a variety of disorders. Accordingly, diagnosis of pancreatic cancer generally involves more invasive approaches such as biopsy as well as imaging studies. Recent advances in biosensor technology have allowed the development of precise diagnostic tools having enhanced analytical sensitivity and specificity. Herein we examine these advances in the detection of cancer in general and in pancreatic cancer specifically. Furthermore, we highlight novel technologies in the measurement of CA19-9 and CEA and explore their future application in the early detection of pancreatic cancer.

In Situ Monitoring of Aptamer-Protein Binding on a ZnO Surface Using Spectroscopic Ellipsometry

The dissolution of zinc oxide is investigated using spectroscopic ellipsometry to investigate its suitability as a platform for biosensing applications. The results indicate that once the ZnO surface has been functionalised, it is suitably protected, and no significant dissolving of the ZnO occurs. The binding kinetics of the SARS-CoV-2 spike protein on aptamer-functionalised zinc oxide surfaces are subsequently investigated. Values are extracted for the refractive index and associated optical constants for both the aptamer layer used and the protein itself. It is shown that upon an initial exposure to the protein, a rapid fluctuation in the surface density is observed. After around 20 min, this effect stabilises, and a fixed increase in the surface density is observed, which itself increases as the concentration of the protein is increased. This technique and setup are demonstrated to have a limit-of-detection down to 1 nanomole (nM) and display a linear response to concentrations up to 100 nM.

FcγRIIIA affinity chromatography complements conventional functional characterization of rituximab

Analytical and functional characterization of batches of biologics/biosimilar products are imperative towards qualifying them for pre-clinical and clinical investigations. Several orthogonal strategies are employed to characterize the functional attributes of these drugs. However, the use of conventional techniques for online monitoring of functional attributes is not feasible. Liquid chromatography is one of the crucial unit operations during the downstream processing of biopharmaceuticals. In this work, we have demonstrated the utility of FcγRIIIA affinity chromatography as an independent quantitative functional characterization tool. FcγRIIIA affinity chromatography aided in sequential elution of Rituximab glycoform mixtures, based on varying levels of galactosylation, and thereby the affinity for the receptor protein. The predominant glycans present in the three Rituximab glycoform mixture peaks were G0F, G1F, and G2F, respectively. Dissociation rate constants were derived from the chromatographic elution profiles by the peak profiling method, for the control and glucose stress conditions. The glucose stress conditions did not result in unfavorable binding kinetics of Rituximab and FcγRIIIA. The dissociation rate constants of the glycoform mixture 2, predominantly consisting of G1F, were similar to the dissociation rate constants obtained by surface plasmon resonance. Moreover, the glycosylation profiles obtained from chromatographic estimation can be corroborated with the ADCC activity. However, the ex vivo ADCC reporter assay indicated that there was an increase in the effector activity with increasing glucose stress. Thus, FcγRIIIA affinity chromatography permitted three independent assessments via a single analysis. Such approaches can be utilized as potential process analytical technology (PAT) tools in the biosimilar development process.

Iodination of PEGylated Polymers Counteracts the Inhibition of Fibrinogen Adsorption by PEG

Poly(ethylene glycol), PEG, known to inhibit protein adsorption, is widely used on the surfaces of biomedical devices when biofilm formation is undesirable. Poly(desaminotyrosyl-tyrosine ethyl ester carbonate), PDTEC, PC for short, has been a promising coating polymer for insertion devices, and it has been anticipated that PEG plays a similar role if it is copolymerized with PC. Earlier studies show that no fibrinogen (Fg) is adsorbed onto PC polymers with PEG beyond the threshold weight percentage. This is attributed to the phase separation of PEG. Further, iodination of the PC units in the PC polymer, (I₂PC), has been found to counteract this Fg-repulsive effect by PEG. In this study, we employ surface-sensitive X-ray techniques to demonstrate the surface affinity of Fg toward the air-water interface, particularly in the presence of self-assembled PC-based film, in which its constituent polymer units are assumed to be much more mobile as a free-standing film. Fg is found to form a Gibbs monolayer with its long axis parallel to the aqueous surface, thus maximizing its interactions with hydrophobic interfaces. It influences the amount of insoluble, surface-bound I₂PC likely due to the desorption of the formed Fg-I₂PC complex and/or the penetration of Fg onto the I₂PC film. The results show that the phase behavior at the liquid-polymer interface shall be taken into account for the surface behavior of bulk polymers surrounded by tissue. The ability of PEG units rearranging into a protein-blocking layer, rather than its mere presence in the polymer, is the key to antifouling characteristics desired for polymeric coating on insertion devices.

A Review of the Role of Amelogenin Protein in Enamel Formation and Novel Experimental Techniques to Study its Function

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Amelognein protein plays a vital role in the formation and mineralization of enamel matrix. Amelogenin structure is complex in nature and researchers have studied it with different experimental techniques. Considering its important role, there is a need to understand this important protein, which has been discussed in detail in this review. In addition, various experimental techniques to study amelogenin protein used previously have been tackled along with their advantages and disadvantages. A selection of 67 relevant articles/book chapters was included in this study. The review concluded that amelogenins act as nanospheres or spacers for the growth of enamel crystals. Various experimental techniques can be used to study amelogenins, however, their advantages and drawbacks should be kept in mind before performing analysis.

Surface Characterization of Polymers for Medical Devices

A survey is given on analytical techniques currently applied to the surface characterization of biomedical polymers. The techniques include spectroscopies, thermodynamic and electrochemical measurements and microscopies, respectively. To illustrate the motivation for surface analysis, the hypotheses on the correlations between surface parameters and hemocompatibility of polymers are briefly examined.

The applications of the examined methods are illustrated by a number of examples. These examples include the characterization of cellulose membranes (low-flux hemodialysis membranes) by streaming potential measurements and by inverse contact angle measurements. The use of surface spectroscopies (ATR-FTIR and XPS) is demonstrated by considering the optimization of surface modification procedures of vascular prostheses made from poly(tetrafluoroethylene). Furthermore, the characterization of water-swollen cellulose membranes by scanning force microscopy is shown. Finally, the extended application of physico-chemical surface analysis to the investigation of protein adsorption is considered. An example deals with in situ spectroscopic ellipsometry used to study the adsorption of fibrinogen onto a plasma-deposited hydrophobic fluoropolymer and onto poly(ethyleneoxide)-grafted fluoropolymer, respectively.

Measurements of complex refractive index change of photoactive yellow protein over a wide wavelength range using hyperspectral quantitative phase imaging

A novel optical holographic technique is presented to simultaneously measure both the real and imaginary components of the complex refractive index (CRI) of a protein solution over a wide visible wavelength range. Quantitative phase imaging was employed to precisely measure the optical field transmitted from a protein solution, from which the CRIs of the protein solution were retrieved using the Fourier light scattering technique. Using this method, we characterized the CRIs of the two dominant structural states of a photoactive yellow protein solution over a broad wavelength range (461-582 nm). The significant CRI deviation between the two structural states was quantified and analysed. The results of both states show the similar overall shape of the expected rRI obtained from the Kramers-Kronig relations.

Fast and Sensitive Ellipsometry-Based Biosensing

In this work, a biosensing method based on in situ, fast, and sensitive measurements of ellipsometric parameters (Ψ, ∆) is proposed. Bare silicon wafer substrate is functionalized and used to bind biomolecules in the solution. Coupled with a 45° dual-drive symmetric photoelastic modulator-based ellipsometry, the parameters Ψ and ∆ of biolayer arising due to biomolecular interactions are determined directly, and the refractive index (RI) of the solution and the effective thickness and surface mass density of the biolayer for various interaction time can be further monitored simultaneously. To illustrate the performance of the biosensing method, immunosensing for immunoglobulin G (IgG) was taken as a case study. The experiment results show that the biosensor response of the limit of detection for IgG is 15 ng/mL, and the data collection time is in milliseconds. Moreover, the method demonstrates a good specificity. Such technique is a promising candidate in developing a novel sensor which can realize fast and sensitive, label-free, easy operation, and cost-effective biosensing.

The Diverse Range of Possible Cell Membrane Interactions with Substrates: Drug Delivery, Interfaces and Mobility

The cell membrane has gained significant attention as a platform for the development of bio-inspired nanodevices due to its immune-evasive functionalities and copious bio-analogs. This review will examine several uses of cell membranes such as (i) therapeutic delivery carriers with or without substrates (i.e., nanoparticles and artificial polymers) that have enhanced efficiency regarding copious cargo loading and controlled release, (ii) exploiting nano-bio interfaces in membrane-coated particles from the macro- to the nanoscales, which would help resolve the biomedical issues involved in biological interfacing in the body, and (iii) its effects on the mobility of bio-moieties such as lipids and/or proteins in cell membranes, as discussed from a biophysical perspective. We anticipate that this review will influence both the development of novel anti-phagocytic delivery cargo and address biophysical problems in soft and complex cell membrane.

Multivalent-Ion-Activated Protein Adsorption Reflecting Bulk Reentrant Behavior

Protein adsorption at the solid-liquid interface is an important phenomenon that often can be observed as a first step in biological processes. Despite its inherent importance, still relatively little is known about the underlying microscopic mechanisms. Here, using multivalent ions, we demonstrate the control of the interactions and the corresponding adsorption of net-negatively charged proteins (bovine serum albumin) at a solid-liquid interface. This is demonstrated by ellipsometry and corroborated by neutron reflectivity and quartz-crystal microbalance experiments. We show that the reentrant condensation observed within the rich bulk phase behavior of the system featuring a nonmonotonic dependence of the second virial coefficient on salt concentration c_{s} is reflected in an intriguing way in the protein adsorption d(c_{s}) at the interface. Our findings are successfully described and understood by a model of ion-activated patchy interactions within the framework of the classical density functional theory. In addition to the general challenge of connecting bulk and interface behavior, our work has implications for, inter alia, nucleation at interfaces.

Direct quantification of dual protein adsorption dynamics in three dimensional systems in presence of cells

Understanding the composition of the adsorbed protein layer on a biomaterial surface is of an extreme importance as it directs the primary biological response. Direct detection using labeled proteins and indirect detection based on enzymatic assays or changes to mass, refractive index or density of a surface have been so far established. Nevertheless, using current methodologies, detection of multiple proteins simultaneously and particularly in a three-dimensional (3D) substrates is challenging, with the exception of radiolabeling. Here using fluorescence molecular tomography (FMT), we present a non-destructive and versatile approach to quantify adsorption of multiple proteins within 3D environments and reveal the dynamics of adsorption of human serum albumin (HSA) and fibrinogen (Fib) on 3D polymeric scaffold. Furthermore, we show that serum starved human articular chondrocytes in 3D environment preferentially uptake HSA over Fib and to our knowledge this represents the first example of direct visualization and quantification of protein adsorption in a 3D cell culture system.

STATEMENT OF SIGNIFICANCE

The biomaterial surface upon exposure to biological fluids is covered by a layer of proteins, which is modified over a period of time and dictates the fate of the biomaterial. In this study, we present and validate a new methodology for quantification of protein adsorption on to a three-dimensional polymer scaffold from unitary and binary systems, using fluorescence molecular tomography, an optical trans-illumination technique with picomolar sensitivity. In additional to being able to follow behavior of two proteins simultaneously, this methodology is also suitable for studying protein uptake in cells situated in a polymer environment. The ability to follow protein adsorption/uptake in a continuous manner opens up new possibilities to study the role of serum proteins in biomaterial compatibility.

Visualization of macro-immune complexes in the antiphospholipid syndrome by multi-modal microscopy imaging

The antiphospholipid syndrome (APS) is an autoimmune thrombotic condition that is marked by autoantibodies against phospholipid-binding proteins. The mechanism(s) of thrombogenesis has (have) resisted elucidation since its description over thirty years ago. Nevertheless, a defining aspect of the disorder is positivity for clinical laboratory tests that confirm antibody binding to anionic phospholipids. It is remarkable that, to our knowledge, the binding of proteins from plasmas of APS patients to phospholipid has not been previously imaged. We therefore investigated this with high resolution microscopy-based imaging techniques that have not been previously used to address this question, namely atomic force microscopy and scanning electron microscopy. Atomic force microscopy imaging of APS plasmas incubated on an anionic planar phospholipid layer revealed the formation of distinct complex three-dimensional structures, which were morphologically dissimilar to structures formed from control plasmas from healthy patients. Likewise, scanning electron microscopy analysis of phospholipid vesicles incubated with APS plasmas in suspension showed formation of layered macro-immune complexes demonstrated by the significant agglomeration of a complex proteinaceous matrix from soluble plasma and aggregation of particles. In contrast, plasmas from healthy control samples bound to phospholipid vesicles in suspension generally displayed a more flattened, mat-like appearance by scanning electron microscopy. Scanning electron microscopy of plasma samples incubated on planar phospholipid layers and previously imaged by atomic force microscopy, corroborated the results obtained by mixing the plasmas with phospholipids in solution. Analysis of the incorporated proteins by silver stained SDS-polyacrylamide gel electrophoresis indicated considerable heterogeneity in the composition of the phospholipid vesicle-adsorbed proteins among APS patients. To our knowledge, these results provide the first images of plasma-derived APS immune complexes at high resolution, and show their consistent presence and heterogeneous compositions in APS patients. These findings demonstrate how high resolution microscopic techniques can contribute to advancing the understanding of an enigmatic disorder and may lay additional groundwork for furthering mechanistic understanding of APS.

Change of the isoelectric point of hemoglobin at the air/water interface probed by the orientational flip-flop of water molecules

Nonlinear vibrational spectroscopy reveals that the isoelectric point of proteins can largely change when the proteins are adsorbed at the air/water interface.

Mechanistic investigations of matrix metalloproteinase-8 inhibition by metal abstraction peptide

The mechanism of matrix metalloproteinase-8 (MMP-8) inhibition was investigated using ellipsometric measurements of the interaction of MMP-8 with a surface bound peptide inhibitor, tether-metal abstraction peptide (MAP), bound to self-assembled monolayer films. MMP-8 is a collagenase whose activity and dysregulation have been implicated in a number of disease states, including cancer metastasis, diabetic neuropathy, and degradation of biomedical reconstructions, including dental restorations. Regulation of activity of MMP-8 and other matrix metalloproteinases is thus a significant, but challenging, therapeutic target. Strong inhibition of MMP-8 activity has recently been achieved via the small metal binding peptide tether-MAP. Here, the authors elucidate the mechanism of this inhibition and demonstrate that it occurs through the direct interaction of the MAP Tag and the Zn(2+) binding site in the MMP-8 active site. This enhanced understanding of the mechanism of inhibition will allow the design of more potent inhibitors as well as assays important for monitoring critical MMP levels in disease states.

Decorating Nanoparticle Surface for Targeted Drug Delivery: Opportunities and Challenges

The size, shape, stiffness (composition) and surface properties of nanoparticles (NPs) have been recognized as key design parameters for NP-mediated drug delivery platforms. Among them, the surface functionalization of NPs is of great significance for targeted drug delivery. For instance, targeting moieties are covalently coated on the surface of NPs to improve their selectively and affinity to cancer cells. However, due to a broad range of possible choices of surface decorating molecules, it is difficult to choose the proper one for targeted functions. In this work, we will review several representative experimental and computational studies in selecting the proper surface functional groups. Experimental studies reveal that: (1) the NPs with surface decorated amphiphilic polymers can enter the cell interior through penetrating pathway; (2) the NPs with tunable stiffness and identical surface chemistry can be selectively accepted by the diseased cells according to their stiffness; and (3) the NPs grafted with pH-responsive polymers can be accepted or rejected by the cells due to the local pH environment. In addition, we show that computer simulations could be useful to understand the detailed physical mechanisms behind these phenomena and guide the design of next-generation NP-based drug carriers with high selectivity, affinity, and low toxicity. For example, the detailed free energy analysis and molecular dynamics simulation reveals that amphiphilic polymer-decorated NPs can penetrate into the cell membrane through the "snorkeling" mechanism, by maximizing the interaction energy between the hydrophobic ligands and lipid tails. We anticipate that this work will inspire future studies in the design of environment-responsive NPs for targeted drug delivery.

A study of molecular adsorption of a cationic surfactant on complex surfaces with atomic force microscopy

The study of molecular adsorption on solid surfaces is of broad interest. However, so far the study has been restricted to idealized flat smooth rigid surfaces which are rarely the case in real world applications. Here we describe a study of molecular adsorption on a complex surface of the submicron fibers of a fibrous membrane of regenerated cellulose in aqueous media. We use a cationic surfactant, cetyltrimethylammonium chloride (CTAC), as the adsorbing molecule. We study the equilibrium adsorption of CTAC molecules on the same area of the fibers by sequentially immersing the membrane in pure water, 1 mM and then a 20 mM solution of CTAC. Atomic force microscopy (AFM) is applied to study the adsorption. The force-volume mode is used to record the force-deformation curves of the adsorbed molecules on the fiber surface. We suggest a model to separate the forces due to the adsorbed molecules from the elastic deformation of the fiber. Interestingly, knowledge of the surface geometry is not required in this model provided the surface is made of elastically homogeneous material. Different models are investigated to estimate the amount of the adsorbed molecules based on the obtained force curves. The exponential steric repulsion model fits the force data the best. The amount of the adsorbed surfactant molecules and its dependence on the concentration are found to be reasonable compared to the data previously measured by means of Raman scattering done on a flat surface of silica.

High-performance liquid chromatography as a technique to determine protein adsorption onto hydrophilic/hydrophobic surfaces

The purpose of this study is to evaluate the potential of simple high performance liquid chromatography (HPLC) setup for quantification of adsorbed proteins on various type of plane substrates with limited area (<3 cm(2)). Protein quantification was investigated with a liquid chromatography chain equipped with a size exclusion column or a reversed-phase column. By evaluating the validation of the method according to guidelines of the International Conference on Harmonization of Technical Requirements for Registration of Pharmaceuticals for Human Use (ICH), all the results obtained by HPLC were reliable. By simple adsorption test at the contact of hydrophilic (glass) and hydrophobic (polydimethylsiloxane: PDMS) surfaces, kinetics of adsorption were determined and amounts of adsorbed bovine serum albumin, myoglobin and lysozyme were obtained: as expected for each protein, the amount adsorbed at the plateau on glass (between 0.15 μg/cm(2) and 0.4 μg/cm(2)) is lower than for hydrophobic PDMS surfaces (between 0.45 μg/cm(2) and 0.8 μg/cm(2)). These results were consistent with bicinchoninic acid protein determination. According to ICH guidelines, both Reversed Phase and Size Exclusion HPLC can be validated for quantification of adsorbed protein. However, we consider the size exclusion approach more interesting in this field because additional informations can be obtained for aggregative proteins. Indeed, monomer, dimer and oligomer of bovine serum albumin (BSA) were observed in the chromatogram. On increasing the temperature, we found a decrease of peak intensity of bovine serum albumin as well as the fraction of dimer and oligomer after contact with PDMS and glass surface. As the surface can act as a denaturation parameter, these informations can have a huge impact on the elucidation of the interfacial behavior of protein and in particular for aggregation processes in pharmaceutical applications.

Site-specific covalent attachment of an engineered Z-domain onto a solid matrix: an efficient platform for 3D IgG immobilization

Immobilized antibodies with oriented and homogeneous patterns are crucial to solid-phase molecular recognition assay. Antibody binding protein-based immobilization can effectively present the desired antibodies. However, steadily installing the stromatoid protein with site-specific attachment manner onto a matrix surface remains to be elucidated. In this study, we present an optimal protocol to tightly attach an immunoglobulin G (IgG)-binding protein (Z-domain) through covalent incorporation of Cys-tag and maleimide group onto polystyrene surface to guarantee site-specific, oriented, and irreversible attachment, resulting in a highly efficient platform for three-dimensional IgG immobilization. The actual IgG-binding characteristic of immobilized Z-Cys was investigated by employing affinity chromatography and size exclusion chromatography. And the efficacy and potential of this platform was demonstrated by applying it to the analysis of interaction between rabbit anti-HRP IgG and its binding partner HRP. The proposed approach may be an attractive strategy to construct high performance antibody arrays and biosensors given that the antibody is compatible with the Z-domain.

Neutralized chimeric avidin binding at a reference biosensor surface

We describe the development of a reference biosensor surface, based upon a binary mixture of oligo-ethylene glycol thiols, one of which has biotin at the terminus, adsorbed onto gold as self-assembled monolayers (SAMs). These surfaces were analyzed in detail by X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS) to establish the relationship between the thiol solution composition and the surface composition and structure. We report the use of argon cluster primary ions for the analysis of PEG-thiols, establishing that the different thiols are intimately mixed and that SIMS may be used to measure surface composition of thiol SAMs on gold with a detection limit better than 1% fractional coverage. The adsorption of neutralized chimeric avidin to these surfaces was measured simultaneously using ellipsometry and QCM-D. Comparison of the two measurements demonstrates the expected nonlinearity of the frequency response of the QCM but also reveals a strong variation in the dissipation signal that correlates with the surface density of biotin. These variations are most likely due to the difference in mechanical response of neutralized chimeric avidin bound by just one biotin moiety at low biotin density and two biotin moieties at high density. The transition between the two modes of binding occurs when the average spacing of biotin ligands approaches the diameter of the avidin molecule.

Measurement of polyphenol oxidase activity using optical waveguide lightmode spectroscopy-based immunosensor

Polyphenol oxidase (PPO) is an important quality index during food processing involving heat-treatment and sensitive determination of PPO activity has been a critical concern in the food industry. In this study, a new measurement of PPO activity exploiting an optical waveguide lightmode spectroscopy-based immunosensor is presented using a polyclonal anti-PPO antibody that was immobilized in situ to the surface of a 3-aminopropyltriethoxysilane-treated optical grating coupler activated with glutaraldehyde. When analysed with a purified PPO fraction from potato tubers, a linear relationship was found between PPO activities of 0.0005607-560.7U/mL and the sensor responses obtained. The sensor was applicable to measurement of PPO activity in real samples that were prepared from potato tubers, grapes and Kimchi cabbage, and the analytical results were compared with those obtained by a conventional colorimetric assay measuring PPO activity. When tested for long-term stability, the sensor was reusable up to 10th day after preparation.

Real-Time Monitoring of Critical Care Analytes in the Bloodstream with Chemical Sensors: Progress and Challenges

We review approaches and challenges in developing chemical sensor-based methods to accurately and continuously monitor levels of key analytes in blood related directly to the status of critically ill hospitalized patients. Electrochemical and optical sensor-based technologies have been pursued to measure important critical care species in blood [i.e., oxygen, carbon dioxide, pH, electrolytes (K(+), Na(+), Cl(-), etc.), glucose, and lactate] in real-time or near real-time. The two main configurations examined to date for achieving this goal have been intravascular catheter sensors and patient attached ex vivo sensors with intermittent blood sampling via an attached indwelling catheter. We discuss the status of these configurations and the main issues affecting the accuracy of the measurements, including cell adhesion and thrombus formation on the surface of the sensors, sensor drift, sensor selectivity, etc. Recent approaches to mitigate these nagging performance issues that have prevented these technologies from clinical use are also discussed.

Label-free kinetic analysis of an antibody-antigen interaction using biolayer interferometry

Biolayer Interferometry (BLI) is a powerful technique that enables direct measurement of biomolecular interactions in real time without the need for labeled reagents. Here we describe the analysis of a high-affinity binding interaction between a monoclonal antibody and purified antigen using BLI. A simple Dip-and-Read™ format in which biosensors are dipped into microplate wells containing purified or complex samples provides a highly parallel, user-friendly technique to study molecular interactions. A rapid rise in publications citing the use of BLI technology in a wide range of applications, from biopharmaceutical discovery to infectious diseases monitoring, suggests broad utility of this technology in the life sciences.

An efficient polymeric micromotor doped with Pt nanoparticle@carbon nanotubes for complex bio-media

A highly efficient polymeric tubular micromotor doped with Pt nanoparticle@carbon nanotubes is fabricated by template-assisted electrochemical growth.

Altered glycosylation of complexed native IgG molecules is associated with disease activity of systemic lupus erythematosus

In addition to the redundancy of the receptors for the Fc portion of immunoglobulins, glycans result in potential ligands for a plethora of lectin receptors found in immune effector cells. Here we analysed the exposure of glycans containing fucosyl residues and the fucosylated tri-mannose N-type core by complexed native IgG in longitudinal serum samples of well-characterized patients with systemic lupus erythematosus. Consecutive serum samples of a cohort of 15 patients with systemic lupus erythematosus during periods of increased disease activity and remission were analysed. All patients fulfilled the 1982 American College of Rheumatology classification criteria. Sera of 15 sex- and age-matched normal healthy blood donors served as controls. The levels and type of glycosylation of complexed random IgG was measured with lectin enzyme-immunosorbent assays. After specifically gathering IgG complexes from sera, biotinylated lectins Aleuria aurantia lectin and Lens culinaris agglutinin were employed to detect IgG-associated fucosyl residues and the fucosylated tri-mannose N-glycan core, respectively. In sandwich-ELISAs, IgG-associated IgM, IgA, C1q, C3c and C-reactive protein (CRP) were detected as candidates for IgG immune complex constituents. We studied associations of the glycan of complexed IgG and disease activity according to the physician's global assessment of disease activity and the systemic lupus erythematosus disease activity index 2000 documented at the moment of blood taking. Our results showed significantly higher levels of Aleuria aurantia lectin and Lens culinaris agglutinin binding sites exposed on IgG complexes of patients with systemic lupus erythematosus than on those of normal healthy blood donors. Disease activity in systemic lupus erythematosus correlated with higher exposure of Aleuria aurantia lectin-reactive fucosyl residues by immobilized IgG complexes. Top levels of Aleuria aurantia lectin-reactivity were found in samples taken during the highest activity of systemic lupus erythematosus. Our results show that native circulating IgG complexes from active systemic lupus erythematosus patients expose fucosyl residues and their glycan core is accessible to soluble lectins. Two putative mechanisms may contribute to the increased exposure of these glycans: (1) the canonical N-glycosylation site of the IgG-CH2 domain; (2) an IgG binding non-IgG molecule, like complement or C-reactive protein. In both cases the complexed IgG may be alternatively targeted to lectin receptors of effector cells, e.g. dendritic cells.

Adsorption of Proteins at Solid Surfaces

Ellipsometry has a very high thin film sensitivity and can resolve sub-nm changes in the thickness of a protein film on a solid substrates. Being a technique based on photons in and photons out it can also be applied at solid-liquid interfaces. Ellipsometry has therefore found many in situ applications on protein layer dynamics but studies of protein layer structure are also frequent. Numerous ex situ applications on detection and quantification of protein layers are found and several biosensing concepts have been proposed. In this chapter, the use of ellipsometry in the above mentioned areas is reviewed and experimental methodology including cell design is briefly discussed. The classical ellipsometric challenge to determine both thickness and refractive index of a thin film is addressed and an overview of strategies to determine surface mass density is given. Included is also a discussion about spectral representations of optical properties of a protein layer in terms of a model dielectric function concept and its use for analysis of protein layer structure.

Structure of biodegradable films at aqueous surfaces: X-ray diffraction and spectroscopy studies of polylactides and tyrosine-derived polycarbonates

Three representative polymers of increasing modulus, poly(d,l-lactic acid), PDLLA, poly(desaminotyrosyl-tyrosine ethyl ester carbonate), PDTEC, and the same polymer with iodinated DTE segments, PI2DTEC, were characterized by surface-pressure versus area (Π-A) isotherms and surface sensitive X-ray diffraction techniques. Films of 10-100 Å thickness were prepared for these studies by spreading dilute polymer solutions at air-water interfaces. The general properties of the isotherms and the Flory exponents, determined from the isotherms, vary in accordance with the increasing modulus of PDLLA, PDTEC, PI2DTEC, respectively. The analysis of in situ X-ray reflectivity and grazing incidence X-ray diffraction (GIXD) measurements from films at aqueous surfaces provides a morphological picture that is consistent with the modulus of the polymers, and to a large extent, with their packing in their dry-bulk state. Large absorption of X-rays by iodine enabled X-ray spectroscopic studies under near-total-reflection conditions to determine the iodine distribution in the PI2DTEC film and complement the structural model derived from reflectivity and GIXD. These structural studies lay the foundation for future studies of polymer-protein interactions at aqueous interfaces.

Protein adsorption onto nanozeolite: effect of micropore openings

A clear and deep understanding of protein adsorption on porous surfaces is desirable for the reasonable design and applications of porous materials. In this study, the effect of surface micropores on protein adsorption was systematically investigated by comparing adsorption behavior of cytochrome c (Cyto-c) and Candida antarctica Lipase B (CALB) on porous and non-porous nanozeolites silicalite-1 and Beta. It was found that micropore openings on the surface of nanozeolites played a key role in determining adsorption affinity, conformations, and activities of proteins. Both Cyto-c and CALB showed higher affinity to porous nanozeolites than to non-porous ones, resulting in greater conformational change of proteins on porous surfaces which in turn affected their bio-catalytic performance. The activity of Cyto-c improved while that of CALB decreased on porous nanozeolites. Recognition of certain amino acid residues or size-matching secondary structures by micropore openings on the surface of nanozeolites was proposed to be the reason. Moreover, the pore opening effect of porous nanozeolites on protein behavior could be altered by changing protein coverage on them. This study gives a novel insight into the interaction between proteins and microporous materials, which will help to guide the rational fabrication and bio-applications of porous materials in the future.

Silicon photonic micro-disk resonators for label-free biosensing

Silicon photonic biosensors are highly attractive for multiplexed Lab-on-Chip systems. Here, we characterize the sensing performance of 3 µm TE-mode and 10 µm dual TE/TM-mode silicon photonic micro-disk resonators and demonstrate their ability to detect the specific capture of biomolecules. Our experimental results show sensitivities of 26 nm/RIU and 142 nm/RIU, and quality factors of 3.3x10(4) and 1.6x10(4) for the TE and TM modes, respectively. Additionally, we show that the large disks contain both TE and TM modes with differing sensing characteristics. Finally, by serializing multiple disks on a single waveguide bus in a CMOS compatible process, we demonstrate a biosensor capable of multiplexed interrogation of biological samples.

Development of novel fluorescent probes for the analysis of protein interactions under physiological conditions with medical devices

In this article, a method to analyze protein adsorption on porous, clinically relevant samples under physiologically relevant conditions is described. The use of fluorescent probes was identified as a methodology that would facilitate analysis under a range of conditions including fully competitive conditions where a protein of interest may be labeled in isolation and then allowed to compete with unlabeled proteins on samples that require no specialized surface pretreatment. As a first step, this article describes the covalent labeling of isolated bovine serum albumin (BSA) with fluorescent fluoresceinthioureidoaminocaproic acid, FTCA, giving FTCA-BSA. The fluorescence intensity of FTCA-BSA was then used to monitor the adsorption and desorption of the protein under noncompetitive conditions with two forms of hydroxyapatite discs (silicate-substituted, SA and stoichiometric, HA) in phosphate-buffered saline (PBS) and minimum essential Eagles' medium (MEM). Noncompetitive conditions were used to facilitate the validation of the technique in which data obtained from these experiments were corroborated against data obtained using an established total protein assay method (Quant-IT kit, Invitrogen). These experiments demonstrated that the FTCA-BSA probe had several advantages including a greater sensitivity at lower concentrations and a considerably longer lifetime. The results also demonstrated that the interaction of BSA with SA and HA was also highly temperature- and media-dependent. Under the most physiologically relevant conditions of MEM at 37 °C, BSA was more readily adsorbed to SA with significant differences between biomaterials, but no differences were observed during the desorption process. The use of this method to analyze adsorption under competitive conditions will be the subject of further investigations.

Kinetics of protein unfolding at interfaces

The conformation of protein molecules is determined by a balance of various forces, including van der Waals attraction, electrostatic interaction, hydrogen bonding, and conformational entropy. When protein molecules encounter an interface, they are often adsorbed on the interface. The conformation of an adsorbed protein molecule strongly depends on the interaction between the protein and the interface. Recent time-resolved investigations have revealed that protein conformation changes during the adsorption process due to the protein-protein interaction increasing with increasing interface coverage. External conditions also affect the protein conformation. This review considers recent dynamic observations of protein adsorption at various interfaces and their implications for the kinetics of protein unfolding at interfaces.