Self-assembled Monolayers Of Mercaptoundecanoic Acid Research Articles

Highly Sensitive Voltammetric Immunosensing of Cancer Biomarkers HER2 and CA125 Using Gold Nanoparticles Anchored Reduced Graphene Oxide Enzyme-Free Nanolabel

Sandwich type voltammetric immunosensors were developed for the quantitative determination of cancer biomarkers, HER2 (human epidermal growth factor receptor 2), and CA125 (cancer antigen 125). Specific antibodies were immobilized on the gold electrodes by using a self-assembled monolayer of mercaptoundecanoic acid (MUA) activated by EDC-NHS ((1-ethyl-3-(3-dimethylamino propyl) carbodiimide and N-hydroxysuccinimide). Antibody conjugated gold nanoparticles decorated reduced graphene oxide (anti-HER2-Au/rGO and anti-CA125-Au/rGO) were used as enzyme-free nano labels for signal amplification. The sensors were characterized thoroughly by spectroscopic, microscopic, and electrochemical methods. Differential pulse voltammetric (DPV) studies in the presence of thionine revealed that the current signal produced is directly related to the biomarker concentration. A linear range of 0.2 pg ml−1 to 100 ng ml−1 was obtained, which indicated the immunosensors to be highly sensitive in lower concentrations of cancer biomarkers. Finally, the fabricated immunosensors were tested with serum samples spiked with HER2 and CA125. The results obtained from the test confirmed the data collected from clinical labs.

Modulating surface plasmon resonance response by an external electromagnetic wave

We fabricated a highly sensitive layered structure of organic molecules and studied the influence of polarization of the external electromagnetic (EM) wave on the surface plasmon resonance response from the layered structure. The layered structure was formed by depositing a self-assembled monolayer (SAM) of mercaptoundecanoic acid (MUA) followed by a single layer of Langmuir-Schaefer (LS) film of traditional calamitic liquid crystal molecule, 4-octyl-4-biphenylcarbonitrile (8CB). The SAM of MUA was found to be non-responsive towards the change in resonance angle (RA) of surface plasmon resonance (SPR) due to the change in polarization of the external EM wave. However, such layer provides a soft-surface platform for the single layer of 8CB molecules which gets perturbed locally due to the incidence of the external EM wave. We obtained an oscillatory modulation of the change in RA due to the change in polarization angle of the EM wave with respect to the plane of incidence. The magnitude of sensitivity was found to be ∼4 milli°/° angle of polarization of the external EM wave. This study strongly suggests that sensitivity of a SPR-based sensor can be controlled by altering the linear state of polarization of the incident external EM wave.

Maleic anhydride and acetylene plasma copolymer surfaces for SPR immunosensing

We report on the successful application of carboxyl-rich plasma polymerized (PP) films as a matrix layer for bioreceptor immobilization in surface plasmon resonance (SPR) immunosensing. Composition and chemical properties of the carboxyl-rich PP films deposited from a mixture of maleic anhydride and acetylene were investigated. Changes in the films stored in air, water, and buffer were studied and the involved chemical changes were described. Performance in SPR immunosensing was evaluated on interactions of human serum albumin (HSA) with a specific monoclonal antibody. The comparison with the mixed self-assembled monolayer of mercaptoundecanoic acid and mercaptohexanol (MUA/MCH) and one of the most widely used surfaces for SPR, the 2D and 3D carboxymethylated dextran (CMD), was presented to show the efficacy of plasma polymerized matrix layers for biosensing. The PP film-based SPR immunosensor provided a similar detection limit of HSA (100ng/mL) as MUA/MCH- (100ng/mL) and 3D CMD (50ng/mL)-based sensors. However, the response levels were about twice higher in case of the PP film-based immunosensor than in case of MUA/MCH-based alternative. The PP film surfaces had similar binding capacity towards antibody as the 3D CMD layers. The response of PP film-based sensor towards HSA was comparable to 3D CMD-based sensor up to 2.5μg/mL. For the higher concentrations (> 10μg/mL), the response of PP film-based immunosensor was lower due to inaccessibility of active sites of the immobilized antibody inside the flat PP film surface. We have demonstrated that due to its high stability and cost-effective straightforward preparation, the carboxyl-rich PP films represent an efficient alternative to self-assembled monolayers (SAM) and dextran-based layers in label-free immunosensing. Graphical abstract.

Mixed Self-Assembled Monolayers of Mercaptoundecanoic Acid and Thiolactic Acid for the Construction of an Enzymatic Biosensor for Hydroquinone Determination

Self-assembled monolayers (SAMs) have attracted much interest due to their potential applications in biosensors, biomolecular electronics and nanotechnology. Due to its homogeneity, ease of preparation and ability to vary both the length of the chains and terminal functional groups, which provide a good versatility for immobilizing biocatalytically active compounds. Mixed SAM (designated as SAMmix), that combines the properties of alkanethiols with different carbon chain lengths or different functional groups, is one of the most attractive methods for promoting different arrangements on the electrode surface. SAMmix of long and short chains are reported to show better electron transfer rates than unicomponent SAMs due to the flexibility of redox species distribution at the SAM interface. Therefore, in this work, a horseradish peroxidase (HRP) biosensor was constructed using methods of physical adsorption and covalent binding for the enzyme immobilization on binary SAMs of 11-mercaptoundecanoic acid (MUA) and thiolactic acid (TLA) on gold surface for biosensor construction. The characterization of the biosensors was evaluated by voltammetry and electrochemical impedance spectroscopy. The immobilization method by covalent binding provided greater stability when compared to the adsorption method. For the preparation of the mixed self-assembled monolayers (SAMmix), the gold electrode was incubated in a solution containing different proportions of MUA and TLA and the best concentration ratio of these molecules was 0.5 and 1.0 mmol L-1, respectively. The results obtained by cyclic voltammetry and electrochemical impedance spectroscopy showed that the electrode surface dramatically changes when varying the concentration of these molecules. The rate of electron transfer was substantially affected because TLA enables an increase in the conductivity due to the formation of SAMmix "islands", whereas MUA although partially block the surface, it gives stability to the monolayer for the enzyme immobilization. The SAMmix obtained using 1 mmol L-1 of MUA had the lowest sensitivity due to the high concentration of the long-chain molecule. The HRP enzyme was efficiently immobilized on the SAMmix, designated as Au-SAMmix-HRP. The HRP enzyme was immobilized by the method of covalent attachment, in which the reaction occurs between the -COOH end groups of the SAM ligands with EDC (N-(3-9 dimethylaminopropyl)-N'-ethylcarbodiimide hydrochloride) and NHS (N-hydroxysuccinimide ester), which favors a more stable immobilization of the enzyme. This biosensor was applied for the determination of hydroquinone (HQ). It was observed that the presence of the enzyme on the monolayer increases the peak current compared to SAMmix, however best results were obtained for determination of HQ when a fixed concentration of H2O2 was added in solution. Therefore, the effect of the H2O2 concentration on the biosensor response was investigated. It was observed that as the concentration of H2O2 was increased, there was a linear increase in the biosensor current response from of 30.0 to 300.0 µmmol L-1 and 0.1 to 1.0 mmol L-1, with sensitivities of 10.35 μA/mol L-1 and and 2.38 μA/mol L-1, respectively. Lower sensitivity was observed at high concentrations of the substrate, probably due to the saturation of the active site of the enzyme, reducing its sensitivity. Michaelis–Menten kinetics, KM app of 0.4 mmol L-1 was obtained, indicating that the electrode architecture employed presents advantages for the fabrication of enzymatic biosensor. The resulting Au-SAMmix-HRP exhibited an excellent electrocatalytic activity toward the hydroquinone, which presents a wide linear range from 3.0 to 30.0 mmol L with a linear equation, I= 2209.69 [HQ] - 0.00534 with a correlation coefficient of 0.985. The standard deviation for the calibration curve was estimated at SB = 0.00101. This data was used to calculate the detection limit and quantification limit of the biosensor which were DL = 1.37 µmol L-1 and QL = 4.57 µmol L-1. The repeatability of the biosensor Au-SAMmix-HRP was performed on a series of 10 measurements (n = 10) in a solution containing HQ concentration of 0.3 mmol L-1 ([H2O2]= 0.3 mmol L-1) in PBS buffer by means of cyclic voltammetry, with 0.99% RSD. The biosensor stability was evaluated after 100 consecutive cycles by cyclic voltammetry in these same conditions and only a slight decay of the cathodic current peak was observed, at about 14 %. The long-term stability of the biosensor was also investigated and the biosensor was stable for at least 6 days without change in the response. After this period, the signal decreased 10% over a period of 15 days. Therefore, the biosensor Au-SAMmix-HRP displayed good reproducibility, sensitivity and stability for the determination of hydroquinone.

Electrochemical analysis of autodisplayed adrenodoxin (Adx) on the outer membrane of E. coli

In this work, adrenodoxin (Adx) was expressed on the outer membrane of E. coli by autodisplay and then the iron–sulfur cluster was incorporated into apo-Adx by an anaerobic reconstitution process. For the determination of the redox potentials of the iron–sulfur clusters of the autodisplayed Adx, E. coli cells with autodisplayed Adx were immobilized on a gold electrode modified with a self-assembled monolayer of mercaptoundecanoic acid (MUA). From the repeated cyclic voltammetry (CV) analysis, the E. coli (10mM HEPES buffer, pH7.0) with autodisplayed Adx showed significant changes in shape with an oxidation peak at +0.4V (vs. Ag/AgCl) and a reduction peak at −0.3V (vs. Ag/AgCl) after the reconstitution process for the incorporation of the iron–sulfur cluster. From the repeated CV analysis in the reduction and oxidation potential ranges, the iron–sulfur clusters of the autodisplayed Adx were observed to undergo reversible redox reactions via direct electron transfer to the MUA-modified gold electrode.

Mixed Self-Assembled Monolayers of Mercaptoundecanoic Acid and Thiolactic Acid for the Construction of an Enzymatic Biosensor for Hydroquinone Determination

A horseradish peroxidase (HRP) biosensor was constructed using binary self-assembled monolayers (SAM) of 11-mercaptoundecanoic acid (MUA) and thiolactic acid (TLA) on gold surface. The advantages of using mixed SAM for the enzyme immobilization is that the long carbon chain molecules act as a support for the enzyme while the short chain molecules favor the electron transfer process. In order to obtain this modified surface, the gold electrode was incubated in a solution containing different proportions of MUA and TLA and the best concentration ratio of these molecules was 0.5 and 1.0 mmol L−1, respectively. The preparation steps and the biosensor response were monitored by electrochemical techniques. The biosensor proposed was applied to determine hydroquinone in a 0.10 mol L−1 phosphate buffer solution containing H2O2 0.3 mmol L−1. The Au-SAMmix-HRP electrode, in the presence of hydrogen peroxide, catalyzes the oxidation of hydroquinone to the corresponding quinone, which is electrochemically reduced back to hydroquinone at −0.08 V vs Ag/AgCl. The analytical curve was linear for hydroquinone concentrations from 5.0 to 30 μmol L−1 and the detection limit was 1.26 μmol L−1. The lifetime of this biosensor was 15 days. The modified electrode displayed good reproducibility, sensitivity and stability for the determination of hydroquinone.

Effects of solvent on the formation of the MUA monolayer on Si and its diffusion barrier properties for Cu metallization

We investigated the effects of solvents, such as ethanol and isooctane, on self-assembly of the mercaptoundecanoic acid (MUA) monolayer on Si and its diffusion barrier properties for Cu metallization. The use of isooctane as a solvent produced MUA self-assembled monolayers (SAMs) (~1.3 nm thick) on Si. These acted as an effective diffusion barrier against Cu diffusion up to 200°C. In contrast, the MUA SAMs produced by ethanol allowed the diffusion of Cu to a MUA-Si interface at 200°C, stimulating the out-diffusion of Si into Cu and thus resulting in the degraded diffusion barrier properties. This was possibly due to the partial formation of interplane hydrogen bonding between the terminal groups of the bound acid and free thiol groups. This provided less dense thiol surface groups, thus leading to poor adhesion of Cu to MUA SAMs. The fabricated Cu/isooctane-assisted MUA source/drain electrode a-Si:H thin film transistors with a channel length of 10 µm exhibited an excellent electron mobility of 0.74 cm /V-s, threshold voltage of −0.51 V, I /I ratio of 3.25 × 10 , specific contact resistance of 4.24 Ω-cm after annealing at 200°C.

Quantification of Interleukin-6 in cell culture medium using surface plasmon resonance biosensors

Interleukin (IL)-6, a multifunctional cytokine, is widely used as an index for illnesses such as inflammatory and autoimmune disorders, coronary artery disease, neurological disease, and gestational problems. It is thus very important to be able to precisely quantify the level of IL-6 for disease diagnosis and any subsequent therapy. Surface plasmon resonance (SPR) biosensors are sensitive in detecting the interaction between biomolecules by sensing the changes in the refractive index on the sensor chip. This study investigated the SPR technique to determine the IL-6 secretion of human fibroblast MRC5-CVI cells induced by lipopolysaccharide (LPS). To reduce non-specific binding, a mixed self-assembled monolayer of mercaptoundecanoic acid (MUA) and mercaptohexanol (MCH) was attached to the sensor, and then used for IL-6 determination using a sandwich type immunoassay. In addition, two antibody immobilization methods were applied to the sensor surface—direct immobilization and indirect immobilization via protein G affinity. The results demonstrated that the direct immobilization method had a better antibody binding capacity on the sensor surface. The level of cellular IL-6 secretion detected by the SPR biosensor showed a consistent correlation with the commercial kit of IL-6 enzyme-linked immunosorbent assay.

Label free capacitive immunosensor for detecting calpastatin — A meat tenderness biomarker

An immunological capacitive biosensor for calpastatin was developed, optimized and applied for the analysis of meat extract samples. Anti-calpastatin antibody was immobilized on a gold electrode modified with a self-assembled monolayer of mercaptoundecanoic acid and Protein A from Staphylococcus aureus, and the obtained immunosensor was inserted as the working electrode in an electrochemical cell of a flow injection system. The dynamic range of the sensor was 20 to 160 ng/mL calpastatin. The electrode could be regenerated and re-used for more than 7 days with minimal reduction in sensitivity. For the analysis of real samples, the target analyte was extracted from the Longissimus dorsi muscle from beef carcasses directly after slaughtering. The extract was analyzed both with the developed immunosensor and microtiter plate ELISA, and a good correlation was obtained. However the immunosensor offers advantages of speed, simplicity, sensitivity and possibility for miniaturization over conventional assays for calpastatin quantification.

Reversible Oxidation of Spinach Ferredoxin at Surface-Modified Electrodes

The oxidation of Spinacia oleracea ferredoxin [2Fe-2S] at modified and unmodified gold and indium tin oxide electrodes was studied by cyclic voltammetry. The results are compared to previous reports and, in some cases, with the results of quantum chemical simulations. We found that protonated 4-mercaptopyridine attached to a gold surface by self-assembly stimulated the oxidation of ferredoxin at acidic pH values but not in a neutral solution. Furthermore, we found that a gold electrode with a poly-l-lysine film attached to a self-assembled monolayer of mercaptoundecanoic acid elicited a reversible oxidation–reduction pair at neutral pH with a midpoint potential near the reported formal potential of spinach ferredoxin. This work compares and discusses the oxidation of ferredoxin by a range of electrode materials.

Self-assembled monolayers as a base for immunofunctionalisation: unequal performance for protein and bacteria detection

Biosensor development strongly depends on the optimisation of surface functionalisation strategies. When gold surfaces are considered, immunofunctionalisation by modification of self-assembled monolayers (SAMs) is one of the preferred approaches. In this respect, SAM-based antibody (Ab) incorporation has shown better performance than Ab physisorption for the detection of proteins and small targets. Reports on bacteria detection are less frequent. In this work, we assess the performance of various SAM-based gold immunofunctionalisation strategies, currently applied to protein detection, in the field of bacteria determination. We present the results for Ab chemical conjugation on mercaptopropanoic acid and mercaptoundecanoic acid SAMs, as well as on a dextranized cysteamine SAM. All the modified surfaces studied were shown to be appropriate for the direct detection of an enzyme-labelled protein, but none succeeded in detecting a bacterial target in a sandwich assay format. Conversely, gold functionalised by Ab physisorption allowed E. coli detection when a sandwich enzyme-linked assay was carried out. The implications of bacteria size and wall complexity are discussed. These results indicate that immunofunctionalisation strategies appropriate for protein detection are not necessarily transferable to work with more complex targets such as bacteria. In this respect, Ab physisorption appears to be a suitable alternative to SAM-based gold functionalisation for bacteria detection.

Probing Protein Adsorption onto Mercaptoundecanoic Acid Stabilized Gold Nanoparticles and Surfaces by Quartz Crystal Microbalance and ζ-Potential Measurements

The adsorption characteristics of three proteins [bovine serum albumin (BSA), myoglobin (Mb), and cytochrome c (CytC)] onto self-assembled monolayers of mercaptoundecanoic acid (MUA) on both gold nanoparticles (AuNP) and gold surfaces (Au) are described. The combination of quartz crystal microbalance measurements with dissipation (QCM-D) and pH titrations of the zeta-potential provide information on layer structure, surface coverage, and potential. All three proteins formed adsorption layers consisting of an irreversibly adsorbed fraction and a reversibly adsorbed fraction. BSA showed the highest affinity for the MUA/Au, forming an irreversibly adsorbed rigid monolayer with a side-down orientation and packing close to that expected in the jamming limit. In addition, BSA showed a large change in the adsorbed mass due to reversibly bound protein. The data indicate that the irreversibly adsorbed fraction of CytC is a monolayer structure, whereas the irreversibly adsorbed Mb is present in form of a bilayer. The observation of stable BSA complexes on MUA/AuNPs at the isoelectric point by zeta-potential measurements demonstrates that BSA can sterically stabilize MUA/AuNP. On the other hand, MUA/AuNP coated with either Mb or CytC formed a reversible flocculated state at the isoelectric point. The colloidal stability differences may be correlated with weaker binding in the reversibly bound overlayer in the case of Mb and CytC as compared to BSA.

Functionalisation of gold surfaces with thiolate SAMs: Topography/bioactivity relationship – A combined FT-RAIRS, AFM and QCM investigation

Immobilisation of rabbit immunoglobulin G (rIgG) was performed by affinity binding to protein A (PrA) covalently bound to three different thiolate self-assembled monolayers (SAMs), (i) a mixed SAM of mercaptoundecanoic acid (MUA) and mercaptohexanol (C6OH) at a molar ratio of 1–3, (ii) a pure SAM of MUA and (iii) a pure SAM of cystamine (CA). A comparative study of anti-rIgG recognition process on these three surfaces was achieved in order to assess the influence of the attachment layer topography and composition upon the sensor quality. Functionalised gold-coated surfaces were characterised by three complementary analytical techniques, namely atomic force microscopy (AFM), polarization modulation–reflection-adsorption infrared spectroscopy (PM-RAIRS) and quartz crystal microbalance (QCM). PM-RAIRS and AFM revealed that the three SAMs were formed on the gold surfaces. AFM observations made it clear that the thiolate and PrA layers were rather homogeneous in the case of pure MUA and CA SAMs, as compared to the MUA/C6OH mixed SAM on which PrA aggregates were observed. Though the highest amount of antibody was bound to the PrA on CA layer, higher anti-rIgG over IgG ratios were measured on the less dense layers of antibody.

Influence of Solvent Environment and Tip Chemistry on the Contact Mechanics of Tip−Sample Interactions in Friction Force Microscopy of Self-Assembled Monolayers of Mercaptoundecanoic Acid and Dodecanethiol

Friction force microscopy measurements have been made for self-assembled monolayers of mercaptoundecanoic acid (C10COOH) and dodecanethiol (C11CH3) in different liquid media. In perfluorodecalin, the friction-load relationship was nonlinear and consistent with adhesion-controlled sliding. The effective range of the attractive force was controlled by using AFM tips functionalized with alkanethiols (chemical force microscopy). Like pairs of interacting molecules yielded data that were characterized by the Johnson-Kendall-Roberts model of contact mechanics, whereas the interaction between dissimilar pairs of molecules fitted the behavior predicted by the Derjaguin-Muller-Toporov model. In ethanol, the adhesive force was much smaller, and sliding was not adhesion-controlled. Under this condition of low adhesion, the friction force varied linearly with the applied load.

Influence of Nanobubbles on the Adsorption of Nanoparticles

A quartz crystal microbalance was used to study the influence of nanobubbles on the adsorption of polystyrene nanoparticles onto surfaces coated with gold, or coated with dodecanethiol or mercaptoundecanoic acid self-assembled monolayers (SAMs). Adsorption of the nanoparticles onto the surface causes the resonant frequency of the quartz crystal to decrease. We found that particles were adsorbed onto the gold-coated quartz crystal in air-rich water, but not in degassed water. This finding supports the long-standing hypothesis that nanobubbles play a key role in the long-range attractive force between hydrophobic surfaces in aqueous solutions. When the experiments were conducted using quartz crystals coated with a hydrophobic dodecanethiol SAM, the nanoparticles were adsorbed onto the surface even in degassed water due to the short-range hydrophobic interactions between the nanoparticles and the dodecanethiol molecules. In contrast, the nanoparticles were adsorbed to a lesser degree onto the hydrophilic mercaptoundecanoic acid-coated crystals due to electrostatic repulsive forces.

Electron conduction across electrode-immobilized neutravidin bound with biotin-labeled ruthenium pentaamine.

Voltammetry is reported here of a self-assembled redox-protein conjugate consisting of neutravidin conjugated with a biotin derivative redox probe, Ru(NH3)5(N-[(N-[(4-pyridyl)methyl]biotinamide], immobilized on gold electrodes modified by self-assembled monolayers of mercaptoundecanoic acid. This voltammetry indicates that self-assembly of the conjugate/electrode electronic interface, driven by electrostatic binding between the monolayer and a single redox probe, favors orientation of the conjugate, resulting in electronic accessibility of the remaining three redox probes.

Electron and ion transport through multilayers of Au nanoclusters covered by self-assembled monolayers

Two types of gold nanoclusters (GNCs), one covered by self-assembled monolayers of mercaptoundecanoic acid (MUA), hexanethiol (C6SH), and ferrocenylhexanethiol (FcC6SH), MHF-GNC, and the other with MUA and C6SH, MH-GNC, were used for the construction of ten GNC layers of two different sequences on an MUA modified Au(111) surface based on the carboxylate/polycation (poly(allylamine hydrochloride), PAH)/carboxylate electrostatic interaction. MHF-GNC was placed either as the layer closest to the gold electrode, i.e. the first layer, or as the outermost layer with MH-GHC in the other layers. A quasi-reversible redox peak with a constant charge corresponding to the redox reaction of the ferrocene moiety of the MHF-GNC monolayer was observed at both electrodes, showing electrons and perchlorate ions could be transferred through the MH-GNC/PAH multilayers. Cross-sectional transmission electron microscopy (TEM) revealed that the size of the immobilized GNC was almost same as that in solution and well-separated GNCs were dispersed rather uniformly within the polymeric matrix. The incorporation of perchlorate ion upon the oxidation of the ferrocene moiety of the MHF-GNC was demonstrated by electrochemical quartz crystal microbalance measurement. Based on the above results, the charge transfer mechanism in the GNC multilayers was discussed.

Immobilized enzyme-linked DNA-hybridization assay with electrochemical detection for Cryptosporidium parvum hsp70 mRNA.

An electrochemical enzyme-linked immobilized DNA-hybridization assay for the detection of Cryptosporidium parvum in water has been developed. The target molecule was a 121-nucleotide sequence from the C. parvum heat shock protein 70 (hsp70 mRNA from U71181 gene). This analyte offers the possibility of distinguishing dead from live oocysts. The assay involves covalent attachment of a primary DNA probe via its 5'-amine-terminus to self-assembled monolayers of mercaptoundecanoic acid to a gold surface. The primary DNA probe was used to capture the target (sequence 1039-1082 of U71181 gene for the mRNA), by hybridization to a 20-base complementary sequence on the target (at sequence 1063-1082). A secondary DNA probe labeled with alkaline phosphatase (AP) was then hybridized to base sequence 1039-1062 on the target. p-Aminophenol, which is enzymatically generated by the immobilized AP from p-aminophenyl phosphate (PAPP), is detected using electrochemistry. The peak current of cyclic voltammograms from a PAPP solution, in which gold-coated silicon wafer modified with the complete assembly of the assay components was incubated, is linear with concentration of the target (5-50 microg/mL, where P1 and P2-AP concentrations are 50 microg/mL). A detection limit of 2 microg/mL (or 146 nM) of the DNA target was obtained. Cross-reactivity tests showed high selectivity for heat-shocked C. parvum. No signal was obtained for either the synthetic DNA for hsp70 of Campylobacter lari, Escherichia coli, Giardia lamblia, Salmonella typhimurium, and Listeria monocytogenes or for the products of heat-shocked whole organisms of E. coli, G. lamblia, Staphylococcus aureus, and Cryptosporidium muris.

Effect of magnesium ions on oriented growth of calcite on carboxylic acid functionalized self-assembled monolayer.

The combined effect of templating and solution additives on calcite crystallization was studied. Self-assembled monolayers of mercaptoundecanoic acid supported on silver, as templates, induced the uniform, oriented nucleation of calcite from the (012) plane. The presence of Mg2+ in the crystallizing solution affected the crystal growth dramatically, due to the selective Mg binding to the calcite planes roughly parallel to the c-axis. Highly homogeneous arrays of oriented crystals with characteristic sizes, shapes, and morphology, depending on the relative concentration of Mg and Ca ions, were synthesized.

Application of Different Lipid Surfaces to Monitor Protein–Membrane Interactions by Surface Plasmon Resonance Spectroscopy

Planar lipid bilayers on sensor chip surfaces have become useful tools to study membrane bound processes by surface plasmon resonance spectroscopy. We immobilized phospholipids on sensor chips by different approaches. First, a self-assembled monolayer of octadecylmercaptan was formed on a blank gold surface and subsequent addition of phospholipids led to formation of a heterobilayer. Second, a self-assembled monolayer of mercaptoundecanoic acid was formed on a gold surface, the carboxy groups of mercaptoundecanoic acid were activated and covalently linked to phosphatidylethanolamine. Addition of phospholipids then led to a bilayer with phosphatidylethanolamine as the lower leaflet. Third, a hydrophobic sensor chip (L1, BIAcore) was used as a binding matrix for phospholipids. These lipid surfaces were tested, whether they are suitable to study proteinamembrane interactions. As biological test system we used the Ca2+-myristoyl-switch of the neuronal Ca2+-binding protein recoverin. All three surfaces were sufficiently stable to monitor the Ca2+-dependent binding of recoverin to membranes.