Albumin adsorption on alkanethiols self-assembled monolayers on gold electrodes studied by chronopotentiometry

Adsorption of Acacia Gum on Self-Assembled Monolayer Surfaces: A Comprehensive Study Using QCM-D and MP-SPR

The interfacial structuring of Acacia gum at various pH values on self-assembled monolayer (SAM) surfaces was investigated in order to evaluate the respective importance of surface versus biopolymer hydration in the adsorption process of the gum. To this end, SAMs with four different ending chemical functionalities (-CH3, -OH, -COOH, and -NH2) were used on gold surfaces, and the gum adsorption was monitored using multiparametric surface plasmon resonance (MP-SPR) and quartz crystal microbalance with dissipation. Surface modification with alkanethiol and the subsequent adsorption of Acacia gum were also characterized by contact angle measurements using both sessile drop and captive bubble methods. According to MP-SPR results, this study demonstrated that gum adsorbed on all surfaces and that adsorption is the most favorable at both acid pH and hydrophobic environments, i.e., when both the surface and the biopolymer are weakly hydrated and more prone to interfacial dehydration. These results reinforce our recent proposal of interfacial dehydration-induced structuring of biopolymers. Increasing the pH logically decreased the adsorption capacity, especially on a hydrophilic surface, enhancing the hydration rate of the layer. A hydrophilic surface is unfavorable to Acacia gum adsorption except if the surface presents a negative surface charge. In this case, interfacial charge dehydration was promoted by attractive electrostatic interactions between the surface and biopolymers. In the aggregate, the water percentage and the viscoelastic properties were closely related to the properties of the surface function: the negative charge and hydrophobicity significantly increased the hydration rate and viscoelastic properties with the pH, while the positive charge induced a rigid and more dehydrated layer.

Unexpectedly resisting protein adsorption on self-assembled monolayers terminated with two hydrophilic hydroxyl groups

OH-terminated self-assembled monolayers, as protein-resistant surfaces, have significant potential in biocompatible implant devices, which can avoid or reduce adverse reactions caused by protein adhesion to biomaterial surfaces, such as thrombosis, immune response and inflammation. Here, molecular dynamics simulations were performed to evaluate the degree of protein adsorption on the self-assembled monolayer terminated with two hydrophilic OH groups ((OH)2-SAM) at packing densities (Σ) of 4.5 nm-2 and 6.5 nm-2, respectively. The results show that the structure of the (OH)2-SAM itself, i.e., a nearly perfect hexagonal-ice-like hydrogen bond structure in the OH matrix of the (OH)2-SAM at Σ = 4.5 nm-2 sharply reduces the number of hydrogen bonds (i.e., 0.7 ± 0.27) formed between the hydrophobic (OH)2-SAM surface and protein. While for Σ = 6.5 nm-2, the hydrophilic (OH)2-SAM surface can provide more hydrogen bonding sites to form hydrogen bonds (i.e., 6.2 ± 1.07) with protein. The number of hydrogen bonds formed between the (OH)2-SAM and protein at Σ = 6.5 nm-2 is ∼8 times higher than that at Σ = 4.5 nm-2, reflecting the excellent resistance to protein adsorption exhibited by the structure of the (OH)2-SAM itself at Σ = 4.5 nm-2. Compared with a traditional physical barrier effect formed by a large number of hydrogen bonds between the (OH)2-SAM and water at Σ = 6.5 nm-2, the structure of the (OH)2-SAM itself at Σ = 4.5 nm-2 proposed in this study significantly improves the performance of the (OH)2-SAM resistance to protein adsorption, which provides new insights into the mechanism of resistance to protein adsorption on the (OH)2-SAM.

Electrochemical and X-ray Photoelectron Spectroscopy Surface Characterization of Interchain-Driven Self-Assembled Monolayer (SAM) Reorganization

Herein, we report a combined strategy encompassing electrochemical and x-ray photoelectron spectroscopy (XPS) experiments to investigate self-assembled monolayer (SAM) conformational reorganization onto an electrode surface due to the application of an electrical field. In particular, 3-mercaptopriopionic acid SAM (3MPA SAM) modified gold electrodes are activated with a 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) and N-hydroxysulfosuccinimide (NHSS) (EDC-NHSS) mixture by shortening the activation time, from 2 h to 15/20 min, labelled as Protocol-A, -B and -C, respectively. This step, later followed by a deactivation process with ethanolamine (EA), plays a key role in the reaction yields (formation of N-(2-hydroxyethyl)-3-mercaptopropanamide, NMPA) but also in the conformational rearrangement observed during the application of the electrical field. This study aims at explaining the high performance (i.e., single-molecule detection at a large electrode interface) of bioelectronic devices, where the 3MPA-based SAM structure is pivotal in achieving extremely high sensing performance levels due to its interchain interaction. Cyclic voltammetry (CV) experiments performed in K4Fe(CN)6:K3Fe(CN)6 for 3MPA SAMs that are activated/deactivated show similar trends of anodic peak current (IA) over time, mainly related to the presence of interchain hydrogen bonds, driving the conformational rearrangements (tightening of SAMs structure) while applying an electrical field. In addition, XPS analysis allows correlation of the deactivation yield with electrochemical data (conformational rearrangements), identifying the best protocol in terms of high reaction yield, mainly related to the shorter reaction time, and not triggering any side reactions. Finally, Protocol-C’s SAM surface coverage, determined by CV in H2SO4 and differential pulse voltammetry (DPV) in NaOH, was 1.29 * 1013 molecules cm−2, being similar to the bioreceptor surface coverage in single-molecule detection at a large electrode interface.

Enhancement of the anticoagulant capacity of polyvinyl chloride tubing for cardiopulmonary bypass circuit using aluminum oxide nanoscale coating applied through atomic layer deposition

For cardiopulmonary bypass, the polyvinyl chloride (PVC) circuit which can initiate the activation of platelets and the coagulation cascade after blood cell contacting is the possible detrimental effect. Surface coating of the PVC tubing system can be an effective approach to enhance circuit's hemocompatibility. In this study, aluminum oxide (Al₂ O₃ ) thin films were deposited through thermal atomic layer deposition (T-ALD) or plasma-enhanced ALD (PE-ALD) on PVC samples, and the anticoagulation of the Al₂ O₃ -coated PVC samples was demonstrated. The results revealed that Al₂ O₃ deposition through ALD increased surface roughness, whereas T-ALD had a relative hydrophilicity compared with blank PVC and PE-ALD. Whole blood immersion tests showed that blood clots formed on blank PVC and that a large amount of red blood cells was found on PE-ALD substrates, whereas less blood cells were noted in T-ALD samples. Both T-ALD and PE-ALD Al₂ O₃ films did not cause activation of blood cells, as evidenced in CD3⁺ /CD4⁺ /CD8⁺ , CD61⁺ /CD62P⁺ , and CD45⁺ /CD42b⁺ populations. Analysis of serum coagulation factors showed that a lower amount of prothrombin was absorbed on T-ALD Al₂ O₃ samples than that on blank PVC. For albumin and fibrinogen immersion tests, immunostaining and scanning electron microscopy further revealed that a thin albumin layer was absorbed on T-ALD Al₂ O₃ substrates but not on PVC samples. This study revealed that deposition of Al₂ O₃ films by T-ALD can improve anticoagulation of the PVC tubing system.

Surface dependent contact activation of factor XII and blood plasma coagulation induced by mixed thiol surfaces

Studies of the activation of FXII in both platelet poor plasma and in neat buffer solutions were undertaken for a series of mixed thiol self-assembled monolayers spanning a broad range of water wettability. A wide spectrum of carboxyl/methyl-, hydroxyl/methyl-, and amine/methyl-thiol modified surfaces were prepared, characterized, and then utilized as the procoagulant materials in a series of FXII activation studies. X-ray photoelectron spectroscopy was utilized to verify the sample surface's thiol composition and contact angles measured to determine the sample surface's wettability. These samples were then used in in vitro coagulation assays using a 50% mixture of recalcified plasma in phosphate buffered saline. Alternatively, the samples were placed into purified FXII solutions for 30 min to assess FXII activation in neat buffer solution. Plasma coagulation studies supported a strong role for anionic surfaces in contact activation, in line with the traditional models of coagulation, while the activation results in neat buffer solution demonstrated that FXIIa production is related to surface wettability with minimum levels of enzyme activation observed at midrange wettabilities, and no statistically distinguishable differences in FXII activation seen between highly wettable and highly nonwettable surfaces. Results demonstrated that the composition of the solution and the surface properties of the material all contribute to the observation of contact activation, and the activation of FXII is not specific to anionic surfaces as has been long believed.

Selective albumin-binding surfaces modified with a thrombin-inhibiting peptide

Blood-contacting medical devices have been associated with severe clinical complications, such as thrombus formation, triggered by the activation of the coagulation cascade due to the adsorption of certain plasma proteins on the surface of biomaterials. Hence, the coating of such surfaces with antithrombotic agents has been used to increase biomaterial haemocompatibility. Biomaterial-induced clotting may also be decreased by albumin adsorption from blood plasma in a selective and reversible way, since this protein is not involved in the coagulation cascade. In this context, this paper reports that the immobilization of the thrombin inhibitor D-Phe-Pro-D-Arg-D-Thr-CONH2 (fPrt) onto nanostructured surfaces induces selective and reversible adsorption of albumin, delaying the clotting time when compared to peptide-free surfaces. fPrt, synthesized with two glycine residues attached to the N-terminus (GGfPrt), was covalently immobilized onto self-assembled monolayers (SAMs) having different ratios of carboxylate-hexa(ethylene glycol)- and tri(ethylene glycol)-terminated thiols (EG6-COOH/EG3) that were specifically designed to control GGfPrt orientation, exposure and density at the molecular level. In solution, GGfPrt was able to inactivate the enzymatic activity of thrombin and to delay plasma clotting time in a concentration-dependent way. After surface immobilization, and independently of its concentration, GGfPrt lost its selectivity to thrombin and its capacity to inhibit thrombin enzymatic activity against the chromogenic substrate n-p-tosyl-Gly-Pro-Arg-p-nitroanilide. Nevertheless, surfaces with low concentrations of GGfPrt could delay the capacity of adsorbed thrombin to cleave fibrinogen. In contrast, GGfPrt immobilized in high concentrations was found to induce the procoagulant activity of the adsorbed thrombin. However, all surfaces containing GGfPrt have a plasma clotting time similar to the negative control (empty polystyrene wells), showing resistance to coagulation, which is explained by its capacity to adsorb albumin in a selective and reversible way. This work opens new perspectives to the improvement of the haemocompatibility of blood-contacting medical devices.

The effect of adsorbed fibronectin and osteopontin on macrophage adhesion and morphology on hydrophilic and hydrophobic model surfaces

Macrophages play a crucial role in the host response to biomaterials. Here we investigated the effect of adsorbed fibronectin (FN) and osteopontin (OPN), two important proteins for tissue repair, on macrophage adhesion and morphology. Since cell-biomaterial interactions are modulated via proteins adsorbed onto biomaterial surfaces, FN and OPN were adsorbed on model self-assembled monolayers (SAMs) of alkanethiols on gold with different functional terminal groups (CH(3), OH and tetra(ethylene-glycol)). The initial interaction of inflammatory cells with a biomaterial is crucial for the ensuing phases of an inflammatory reaction. For this reason short-term cultures of primary human macrophages were performed. To account for the competitive adsorption of other proteins serum was added to the culture medium and the effect compared with serum-free medium cultures. In the presence of serum hydrophilic surfaces increased macrophage adhesion. In particular, FN induced a higher cell density, while OPN tended to decrease it. In serum-free medium cell adhesion was greater on hydrophobic surfaces, except for OPN-coated SAMs. Importantly, FN no longer enhanced macrophage adhesion, while OPN maintained its inhibitory effect. Cell polarization studies indicated that macrophage morphology variations induced by surface chemistry are overcome by pre-adsorbed OPN. Taken together our results show that in the presence of serum macrophage adhesion is promoted by FN hydrophilic surfaces, but impaired on OPN-coated surfaces. The effects of inhibited macrophage adhesion on macrophage fusion, and its relevance to the initial stages of the inflammatory response to biomaterials are discussed.

Immunoblot analysis of proteins associated with self-assembled monolayer surfaces of defined chemistries

Intact and fragmented proteins, eluted from self-assembled monolayer (SAM) surfaces of alkanethiols of different chemistries (-CH₃, -OH, -COOH, -NH₂), following exposure to human plasma (HP) or human serum (HS), were examined using sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and immunoblotting techniques. The SAM surfaces were incubated for 1 h with 10% (v/v) sterile-filtered, heat-inactivated (h.i.) HS or 1% (v/v) sterile-filtered h.i. HP preparations [both in phosphate buffered saline (PBS)]. Adsorbed proteins were eluted using 10% SDS/2.3% dithioerythritol for characterization of protein profiles. The type of incubating medium may be an important determinant of adsorbed protein profiles, since some variations were observed in eluates from filtered versus control unfiltered h.i. 10% HS or 1% HP. Albumin and apolipoprotein A1 were consistently detected in both filtered h.i 10% HS and 1% HP eluates from all SAM surfaces and from control tissue culture-treated polystyrene (TCPS). Interestingly, Factor H and Factor I, antithrombin, prothrombin, high molecular weight kininogen (HMWK), and IgG were present in eluates from OH, COOH, and NH₂ SAM surfaces and in eluates from TCPS but not in eluates from CH₃ SAM surfaces, following exposure to filtered h.i. 10% HS. These results suggest that CH₃ SAM surfaces were the least proinflammatory of all SAM surfaces. Overall, similar trends were observed in the profiles of proteins eluted from surfaces exposed to filtered 10% HS or 1% HP. However, the unique profiles of adsorbed proteins on different SAM surface chemistries may be related to their differential interactions with cells, including immune/inflammatory cells.

The stability of self-assembled monolayers with time and under biological conditions

The stability of self-assembled monolayers (SAMs) of different functionalities (CH(3)-, OH-, and EG4-) over time under appropriate storage conditions and when immersed in cell culture media was evaluated. X-ray photoelectron spectroscopy (XPS) was used to detect the oxidized sulfur species (S(2p) binding energy from 167 to 168 eV) resulting from oxidation of the surfaces. CH(3)-terminated SAMs stored for a period of 9 weeks in a nitrogen chamber were not altered. The same did not happen with OH- and EG4-SAMs, for which the XPS spectra evidenced oxidized peaks after 2 weeks. Regarding the stability of these surfaces under biological conditions, 30 min of immersion at 37 degrees C in serum-free or 10% fetal bovine serum (FBS) supplemented medium did not induce detectable oxidation. However, a small percentage of oxidized sulfur could have been washed out by the media, as confirmed in studies using SAMs immersed in water. Despite the possible rinsing out of oxidized thiols, high amounts of oxidation can still be detected by XPS. SAMs degradation during ethanol sterilization was not detectable by XPS, although a small increase on the wettability of OH-SAMs was observed. The data suggest that SAMs must be used freshly prepared, being recommended for short-term biological studies.

Mixed-SAM surfaces monitoring CTX-protein part I: Using atomic force microscope measurements

Fast and efficient detection of Cobra cardiotoxin (CTX) protein molecules on biochip surfaces is an example of application in biotechnology. One potential application of mixed self assembled monolayers (SAMs) as chip surfaces yield different binding affinities of the CTX proteins, a series of studies on the interaction force between CTX proteins and the mixed SAMs surfaces formed from mixtures of two thiols with the same/different chain lengths and/or with the same/different terminal groups will be investigated. In these dual papers, the mixed SAMs of n-alkinethiol SAMs of different chain lengths are chosen as the first examples of this series due to the simple functions of the mixed SAMs surface structure. Thus, the adhesion force measurements of CTX protein molecules on mixed SAMs of n-alkinethiol SAMs of different chain lengths: 1-decanethiol (C9) and 1-hexanethiol (C5) with different mixing ratios are developed and conducted using atomic force microscope (AFM). There are two major tasks in Part I of the dual papers: the development of the AFM measurements providing reliable information, and selection of the surface with highest binding affinity among this mixed SAMs group. Results indicate that the adhesion forces for CTX protein molecules on mixed SAMs with mixing ratio (χ(C9)) of 0.25, 0.5, 0.75 and 1, are 1.26, 1.8, 1.38, and 1.25 folds respectively, compared with the adhesion force of CTX protein molecules on the C5 surface only. Therefore, the SAM surfaces of χ(C9) = 0.5 is the best choice as a biomaterial sensor of this group of mixed SAMs because the strongest binding force and highest efficiency. Effects of the loading force of the AFM operation, the radius of curvature of the AFM tip, and the AFM tip endurance as well as control experiments were examined to ensure the quantitative determination of adhesion force for AFM measurement. The physical mechanism of protein adsorption on SAM surfaces will be studied and analyzed by molecular dynamics (MD) simulations and will be reported in Part II of the dual papers to compensate the limited information on the interaction taking place at atomic level that experiments cannot provide.

Profiles of carbohydrate ligands associated with adsorbed proteins on self-assembled monolayers of defined chemistries

Conserved protein-carbohydrate-lipid pathogen-associated molecular patterns (PAMPs) interact with cells of the innate immune system to mediate antigen recognition and internalization and activation of immune cells. We examined if analogous "biomaterial-associated molecular patterns" composed of proteins, specifically their carbohydrate modifications, existed on biomaterials, which can play a role in mediating the innate immune response to biomaterials. To probe for these carbohydrates in the adsorbed protein layer, as directed by the underlying biomaterial chemistry, self-assembled monolayers (SAMs) presenting -CH(3), -OH, -COOH, or -NH(2) were preincubated with serum/plasma, and the presence of carbohydrate ligands of C-type lectin receptors (CLRs) was investigated using lectin probes in an enzyme-linked lectin assay (ELLA). Presentation of CLR ligands was detected on control tissue culture polystyrene (TCPS). Absorbances of mannose or N-acetylglucosamine increased with decreasing incubating serum concentration, whereas absorbances of sialylated epitopes or fucose remained unchanged. Absorbances of alpha-galactose or N-acetylgalactosamine decreased with decreasing incubating serum concentration; beta-galactose was undetectable. Among SAM endgroups, preincubation with 10% serum resulted in differential presentation of CLR ligands: higher alpha-galactose on COOH SAMs than NH(2) or CH(3) SAMs, highest complex mannose on NH(2) SAMs, and higher complex mannose on OH SAMs than CH(3) SAMs. Least sialylated groups were detected on CH(3) SAMs. In summary, biomaterial chemistry may regulate protein adsorption and hence unique presentation of associated carbohydrates. The ultimate goal is to identify the effects of protein glycosylations associated with biomaterials in stimulating innate immune responses.

Label-free DNA biosensors based on functionalized carbon nanotube field effect transistors

A carbon nanotube transistor array was used to detect DNA hybridization. A new approach to ensure specific adsorption of DNA to the nanotubes was developed. The polymer poly (methylmethacrylate(0.6)-co-poly(ethyleneglycol)methacrylate(0.15)-co-N-succinimidyl methacrylate(0.25)) was synthesized and bonded noncovalently to the nanotube. Aminated single-strand DNA was then attached covalently to the polymer. After hybridization, statistically significant changes were observed in key transistor parameters. Hybridized DNA traps both electrons and holes, possibly caused by the charge-trapping nature of the base pairs.

Instability of self-assembled monolayers as a model material system for macrophage/FBGC cellular behavior

Novel self-assembled monolayers (SAMs) designed to present homogenous surface chemistries were utilized to further investigate the material surface chemistry dependent macrophage and foreign-body giant cell (FBGC) behaviors, including macrophage adhesion, fusion, and apoptosis. Contact angle analysis revealed instabilities in the --CH(3) and --COOH terminated SAM surfaces upon incubation in serum-free media (SFM) at 37 degrees C or under dry, room temperature conditions. Further analysis indicated that the --CH(3) terminated SAM surface degraded rapidly within 2 h and loss of sufficient SAM units to be comparable to the gold (Au) control surface, within 24 h of incubation in SFM at 37 degrees C. After 5 days of incubation in SFM at 37 degrees C, the contact angles for the --COOH terminated SAM surfaces increased markedly. AFM analysis confirmed the desorption of --CH(3) terminated SAM molecules from the surface with increased roughness and marked appearance of peaks and valleys within 2 h. A decrease in the thickness of the --COOH terminated SAM surface also suggests molecular desorption over time. No significant changes in contact angle or AFM analyses were observed on the --OH terminated SAM surfaces. Cellular adhesion decreased more rapidly on the Au control and --CH(3) terminated SAM surfaces in comparison to the other surfaces. However by day 10, cellular adhesion, fusion, and apoptosis were comparable on all SAM surfaces and the Au control. These studies suggest that SAM surfaces may not be suitable for long-term studies where material dependent properties are investigated.

Electrochemical Biosensors - Sensor Principles and Architectures

Quantification of biological or biochemical processes are of utmost importance for medical, biological and biotechnological applications. However, converting the biological information to an easily processed electronic signal is challenging due to the complexity of connecting an electronic device directly to a biological environment. Electrochemical biosensors provide an attractive means to analyze the content of a biological sample due to the direct conversion of a biological event to an electronic signal. Over the past decades several sensing concepts and related devices have been developed. In this review, the most common traditional techniques, such as cyclic voltammetry, chronoamperometry, chronopotentiometry, impedance spectroscopy, and various field-effect transistor based methods are presented along with selected promising novel approaches, such as nanowire or magnetic nanoparticle-based biosensing. Additional measurement techniques, which have been shown useful in combination with electrochemical detection, are also summarized, such as the electrochemical versions of surface plasmon resonance, optical waveguide lightmode spectroscopy, ellipsometry, quartz crystal microbalance, and scanning probe microscopy. The signal transduction and the general performance of electrochemical sensors are often determined by the surface architectures that connect the sensing element to the biological sample at the nanometer scale. The most common surface modification techniques, the various electrochemical transduction mechanisms, and the choice of the recognition receptor molecules all influence the ultimate sensitivity of the sensor. New nanotechnology-based approaches, such as the use of engineered ion-channels in lipid bilayers, the encapsulation of enzymes into vesicles, polymersomes, or polyelectrolyte capsules provide additional possibilities for signal amplification. In particular, this review highlights the importance of the precise control over the delicate interplay between surface nano-architectures, surface functionalization and the chosen sensor transducer principle, as well as the usefulness of complementary characterization tools to interpret and to optimize the sensor response.

Cholesterol biosensor based on amino-undecanethiol self-assembled monolayer using surface plasmon resonance technique

Cholesterol oxidase has been covalently immobilized onto 11-amino-1-undecanethiol hydrochloride (AUT) self-assembled monolayer (SAM) fabricated on gold (Au) substrates using glutaraldehyde as a cross-linker. These ChOx/AUT/Au bioelectrodes characterized using contact angle (CA) measurements; electrochemical technique and atomic force microscopy (AFM) have been utilized for the estimation of cholesterol in solution using the surface plasmon resonance (SPR) technique. These biosensing electrodes exhibiting linearity from 50 to 500 mg/dL of cholesterol in solution and sensitivity of 1.23 m0/(mg dL), can be used more than 20 times and have a shelf life of about 10 weeks when stored at 4 degrees C.

Combinatorial protein display for the cell-based screening of biomaterials that direct neural stem cell differentiation

Neural stem cell (NSC) has emerged as a potential source for cell replacement therapy following traumatic injuries and degenerative diseases of the central nervous system. However, clinical applications of NSC further require technological advances especially for controlling differentiation of NSC. This study aimed at developing biomaterials that serve to expand undifferentiated NSC or to induce cells with specific phenotypes. Our approach is to construct composite biomaterials that consist of extracellular matrix components and growth factors. In order to optimize matrix-growth factor combinations, we conducted the parallel and rapid screening of composite biomaterials through assays using cell-based arrays. The photo-assisted patterning of an alkanethiol self-assembled monolayer was employed to achieve site-addressable combinatorial immobilization of natural and synthetic matrices incorporated with growth factors including epidermal growth factor (EGF), ciliary neurotrophic factor (CNTF), nerve growth factor (NGF), and neurotrophin-3 (NT-3). NSC obtained from the rat embryonic striatum was cultured directly on the array to screen for cell adhesion, proliferation, and promotion of neuronal and glial specification. The results showed that the significant number of cells adhered to laminin-1, fibronectin, ProNectin, and poly(ethyleneimine). It was found that cells proliferated most extensively on a spot with immobilized EGF among the spots with different matrix-growth factor combinations. The results also showed that neuronal differentiation was promoted on the spots with immobilized NGF or NT-3, and astroglial differentiation with CNTF. Importantly, observed effects of growth factors were frequently altered depending on the type of co-immobilized matrices, suggesting synergic effects of adhesion and growth factor signals.

Immunosensor interface based on physical and chemical immunoglobulin G adsorption onto mixed self-assembled monolayers

An immunosensor interface based on mixed hydrophobic self-assembled monolayers (SAMs) of methyl and carboxylic acid terminated thiols with covalently attached human Immunoglobulin G (hIgG), is investigated. The densely packed and organised SAMs were characterised by contact angle measurements and cyclic voltammetry. The effect of the non-ionic surfactant, Tween 20, in preventing nonspecific adsorption is addressed by ellipsometry during physical and covalent hIgG immobilization on pure and mixed SAMs, respectively. It is clearly demonstrated that nonspecific adsorption due to hydrophobic interactions of hIgG on methyl ended groups is totally inhibited, whereas electrostatic/hydrogen bonding interactions with the exposed carboxylic groups prevail in the presence of surfactant. Results of ellipsometry and Atomic Force Microscopy, reveal that the surface concentration of covalently immobilized hIgG is determined by the ratio of COOH / CH(3)-terminated thiols in SAM forming solution. Moreover, the ellipsometric data demonstrates that the ratio of bound anti-hIgG / hIgG depends on the density of hIgG on the surface and that the highest ratio is close to three. We also report the selectivity and high sensitivity achieved by chronoamperometry in the detection of adsorbed hIgG and the reaction with its antibody.

The optimal SAM surface functional group for producing a biomimetic HA coating on Ti

Commercial interest is growing in biomimetic methods that employ self assembled mono-layers (SAMs) to produce biocompatible HA coatings on Ti-based orthopedic implants. Recently, separate studies have considered HA formation for various SAM surface functional groups. However, these have often neglected to verify crystallinity of the HA coating, which is essential for optimal bioactivity. Furthermore, differing experimental and analytical methods make performance comparisons difficult. This article investigates and evaluates HA formation for four of the most promising surface functional groups: --OH, --SO(3)H, --PO(4)H(2) and --COOH. All of them successfully formed a HA coating at Ca/P ratios between 1.49 and 1.62. However, only the --SO(3)H and --COOH end groups produced a predominantly crystalline HA. Furthermore, the --COOH end group yielded the thickest layer and possessed crystalline characteristics very similar to that of the human bone. The --COOH end group appears to provide the optimal SAM surface interface for nucleation and growth of biomimetic crystalline HA. Intriguingly, this finding may lend support to explanations elsewhere of why human bone sialoprotein is such a potent nucleator of HA and is attributed to the protein's glutamic acid-rich sequences.

Adsorption of a therapeutic enzyme to self-assembled monolayers: effect of surface chemistry and solution pH on the amount and activity of adsorbed enzyme

The adsorption of a therapeutic enzyme to self-assembled monolayers (SAMs) of different functionalities (X = CH(3)-, OH- and COOH-) was evaluated as a function of solution pH. Radiolabelling studies showed that the enzyme has higher affinity for hydrophobic surfaces than for hydrophilic surfaces, and that the highest adsorption was obtained at the more acidic pH values (4.5 and 5.5), despite the type of surface. IRAS and XPS measurements confirmed this tendency. Dye-binding studies and fluorescence quenching were used to investigate if a pH variation induces any conformational changes on the enzyme. Both methods suggest that lowering the pH from physiological to acidic values triggers an increased exposure of non-polar sites in the enzyme, which may modulate its adsorption behaviour to the more hydrophobic surfaces. At pH 4.5, the enzyme carries a substantial positive net charge and therefore relatively low native-state stability. As a consequence, surface binding may be favoured, irrespective of the type of surface, by providing increased conformational entropy to the enzyme. The specific activity (SA) of the adsorbed enzyme was strongly dependent on the conditions used. A decrease in SA (ca. 30% of control) was observed after adsorption on CH(3)-SAMs for all the pH tested. Adsorption on gold and on the more hydrophilic SAMs (OH- and COOH-) resulted in different degrees of inactivation at the more acidic pH (4.5), and in enzyme activation (up to ca. 230% of control) at higher pH (7-8), near the isoelectric point of the enzyme.