Electrochemistry and electrocatalytic activities of superoxide dismutases at gold electrodes modified with a self-assembled monolayer

In this article, the electrochemical properties and electrocatalytic activity of three kinds of superoxide dismutases (SODs), that is, bovine erythrocyte copper-zinc superoxide dismutase (Cu/Zn-SOD), iron superoxide dismutase from Escherichia coli (Fe-SOD), and manganese superoxide dismutase from E. coli (Mn-SOD), in the SOD family were studied. It was revealed that the direct electron transfer of the three kinds of SODs could be efficiently promoted by a self-assembled monolayer (SAM) of 3-mercaptopropionic acid (MPA) confined on a gold electrode. The electrochemical properties of the SODs at the MPA-SAM electrode vary with the sort of SOD with respect to the formal potential, reversibility of electrode reactions, kinetic parameters, and pH dependence, suggesting different mechanisms for the electrode reactions of the individual SODs. A combination of the facilitated direct electron transfer and the bifunctional enzymatic catalytic activities of the SODs via a redox cycle of their active metals substantially offered a flexible electrochemical route to determination of O(2)(*)(-) where O(2)(*)(-) can be sensed with the SOD-based biosensors in both anodic and cathodic polarizations. Such an intrinsic feature of the SOD-based biosensors successfully enabled a sensitive determination scheme for O(2)(*)(-) free from the interference from some coexisting electroactive species, such as ascorbic acid (AA) and uric acid (UA). Further potential applications for in vivo determination of O(2)(*)(-) is also suggested.     

Diffusion of self-assembled monolayers of thiols on the gold surfaces covered with polydimethylsiloxane stamps

Thiols can diffuse and form self-assembled monolayers (SAMs) on the gold surfaces covered with polydimethylsiloxane (PDMS) stamps. For the first time, with cells as the indicator of how far alkanethiols had diffused to form SAM, we studied the growth dynamics of SAMs of HS(CH₂)₁₁(OCH₂CH₂)₃OH (EG₃) and HS(CH₂)₁₁(OCH₂CH₂)₆OH (EG₆) on the gold surfaces covered with PDMS stamps. The growth of SAMs is well described by one-dimensional diffusion from a line source of concentration, with surface diffusion coefficient of 193.4 ± 19.2 μm²/min (EG₃) and 95.8 ± 18.9 μm²/min (EG₆).     

Stripe domain analysis of alkanethiol and alkynethiol monolayers self-assembled on the surfaces of gold grains

The self-assembled monolayers (SAMs) of alkanethiols such as 1-dodecanethiol (DDT) on the surface of polycrystalline gold grain on quartz crystal microbalance (QCM) at room temperature and alkynethiols such as phenylethynyl thiol (PET) on the surface of Au(111)/mica under − 65 °C have been fabricated. A multimode scanning probe microscope (SPM) investigation for the first time reveals the formation of stripe domain of DDT SAMs together with roundish domain. This feature was not caused by an annealing treatment but might be induced by coexistence of a few different facets and reconstruction of the surface on the grain. The stripe domain of PET SAMs could also be found on the surface of the grain, which was within the substrate of the gold/mica. It shows that the feature of the stripe domain for organothiol monolayers self-assembled on gold grain surfaces possesses a certain generality. Furthermore, the arrangement of organothiol stripe domain can be well understood from the interpretation of the relationship between the stripe width and width direction, molecule interval distance, molecule length and tilt angles of the thiols.     

A method to study the phase transition and desorption of self-assembled monolayers on planar gold surfaces

In-plane resistance of thin gold films (thickness approximately 1,000 A) after the formation of self-assembled monolayers has been measured as a function of temperature. We attribute the observed increase in the slope of the resistance vs temperature plot in the case of octadecanethiol and octanethiol monolayers at characteristic temperatures to phase transitions. The structural transition of 1,4-benzenedimethanethiol is also manifested similarly. Propanethiol does not show any phase transition, as expected. The observed changes in the alkanethiol monolayers are related to the decrease in the lifetime of the parallel vibrational motion of the adsorbate on the gold surface. The increase in slope is followed by a decrease which is attributed to the commencement of desorption of monolayers from the gold surface.     

Simulation and modeling of self-assembled monolayers of carboxylic acid thiols on flat and nanoparticle gold surfaces

Quantitative analysis of the 16-mercaptohexadecanoic acid self-assembled monolayer (C16 COOH-SAM) layer thickness on gold nanoparticles (AuNPs) was performed using simulation of electron spectra for surface analysis (SESSA) software and X-ray photoelectron spectroscopy (XPS) experimental measurements. XPS measurements of C16 COOH-SAMs on flat gold surfaces were made at nine different photoelectron emission angles (5-85° in 10° increments), corrected using geometric weighting factors and then summed together to approximate spherical AuNPs. The SAM thickness and relative surface roughness (RSA) in SESSA were optimized to determine the best agreement between simulated and experimental surface composition. On the basis of the glancing-angle results, it was found that inclusion of a hydrocarbon-contamination layer on top the C16 COOH-SAM was necessary to improve the agreement between the SESSA and XPS results. For the 16 COOH-SAMs on flat Au surfaces, using a SAM thickness of 1.1 ?/CH(2) group, an RSA of 1.05, and a 1.5 ? CH(2)-contamination overlayer (total film thickness = 21.5 ?) for the SESSA calculations provided the best agreement with the experimental XPS data. After applying the appropriate geometric corrections and summing the SESSA flat-surface compositions, the best fit results for the 16 COOH-SAM thickness and surface roughness on the AuNPs indicated a slightly thinner overlayer with parameters of 0.9 ?/CH(2) group in the SAM, an RSA of 1.06 RSA, and a 1.5 ? CH(2)-contamination overlayer (total film thickness = 18.5 ?). The 3 ? difference in SAM thickness between the flat Au and AuNP surfaces suggests that the alkyl chains of the SAM are slightly more tilted or disordered on the AuNP surfaces.     

Micromorphological effect of calcium phosphate coating on compatibility of magnesium alloy with osteoblast

Abstract

Octacalcium phosphate (OCP) and hydroxyapatite (HAp) coatings were developed to control the degradation speed and to improve the biocompatibility of biodegradable magnesium alloys. Osteoblast MG-63 was cultured directly on OCP- and HAp-coated Mg-3Al-1Zn (wt%, AZ31) alloy (OCP- and HAp-AZ31) to evaluate cell compatibility. Cell proliferation was remarkably improved with OCP and HAp coatings which reduced the corrosion and prevented the H₂O₂ generation on Mg alloy substrate. OCP-AZ31 showed sparse distribution of living cell colonies and dead cells. HAp-AZ31 showed dense and homogeneous distribution of living cells, with dead cells localized over and around corrosion pits, some of which were formed underneath the coating. These results demonstrated that cells were dead due to changes in the local environment, and it is necessary to evaluate the local biocompatibility of magnesium alloys. Cell density on HAp-AZ31 was higher than that on OCP-AZ31 although there was not a significant difference in the amount of Mg ions released in medium between OCP- and HAp-AZ31. The outer layer of OCP and HAp coatings consisted of plate-like crystal with a thickness of around 0.1 μm and rod-like crystals with a diameter of around 0.1 μm, respectively, which grew from a continuous inner layer. Osteoblasts formed focal contacts on the tips of plate-like OCP and rod-like HAp crystals, with heights of 2–5 μm. The spacing between OCP tips of 0.8–1.1 μm was wider than that between HAp tips of 0.2–0.3 μm. These results demonstrated that cell proliferation depended on the micromorphology of the coatings which governed spacing of focal contacts. Consequently, HAp coating is suitable for improving cell compatibility and bone-forming ability of the Mg alloy.     

Influence of an external magnetic field on the formation of self-assembled monolayers of dodecanethiol on polycrystalline gold electrode

This paper reports the preparation of dodecanethiol self-assembled monolayers (C₁₂SH-SAMs) on polycrystalline gold electrodes in a magnetic field. The qualities of C₁₂SH-SAMs were characterized by both cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The results show that highly dense and well-ordered SAMs could form in a relatively short time (3 h), indicating that the rate of SAMs formation increased under an external magnetic field compared with the natural self-assembly process. Moreover, the results of CV and EIS measurements also suggested that the presence of a magnetic field had influenced the qualities of the SAMs; the stronger magnetic intensity can help to obtain much denser and well-ordered SAMs.     

Combination of the quartz crystal microbalance and the electrochemical technique for the characterization of self-assembled monolayers of octadecylmercaptan on gold surfaces

Quartz crystal microbalance (QCM) technique has been applied to the direct mass measurement of a self-assembled monolayer of octadecylmercaptan (OM) on a gold surface deposited onto a quartz-crystal. Repeated rinsing of the modified electrodes with used solvent was conducted to get self-assembled monolayers of OM. From the resonance frequency change, 2.5 nm molecular occupied area of OM on gold was obtained. The advantage of the combination of QCM and electrochemical methods was emphasized for the characterization of self-assembled monolayers.     

Cation-assisted laser desorption/ionization for matrix-free surface mass spectrometry of alkanethiolate self-assembled monolayers on gold substrates and nanoparticles

We propose a new scheme of matrix-free laser desorption/ionization with cation assistance for surface mass spectrometry of self-assembled monolayers (SAMs) of alkanethiolates on gold substrates and gold nanoparticles (NPs). In a proof-of-concept experiment, a simple treatment using an aqueous salt solution such as NaI(aq) was shown to lead to a significant laser desorption/ionization, producing the characteristic (disulfide) ions of alkanethiolate molecules from the monolayers. Further efforts to understand the mechanism were also given, including laser power and salt concentration dependence studies. In the power dependence study, the characteristic ions were found to be produced at low laser power where no gold substrate species was seen. At high laser power, the generation of gold species, Au(+)-Au5(+), resulted in a saturation behavior in the characteristic mass peak for alkanethiolate molecules. In addition, characteristic ions with gold adducts were not observed at any laser power. With increasing salt concentration, the characteristic mass peak was gradually increased. The results suggest that the adduct formation of a cation with alkanethiolates in the monolayers provide a facile pathway to supply a charge to UV laser-desorbed secondary neutrals for mass spectrometric detection. This cation-assisted laser desorption/ionization (CALDI) mass spectrometry was further examined with the SAMs and mixed SAMs with various terminals such as -OH, -OCH3, -NH2, -ethylene (-CH=CH2), and -acetylene (-C[triple bond]CH). The CALDI method was also successfully applied to surface mass spectrometry of monolayer-protected gold NPs (approximately 16 nm diameter) with OH- and COOH-terminated SAMs. The unique advantages of the matrix-free CALDI method may extend our capability in investigations of interfacial chemistry at SAMs as well as mass spectrometric applications using biochips and nanoparticles.     

The influence of trace magnesium content on the phase composition of silicon-stabilized calcium phosphate powders

A novel silicon-stabilized calcium phosphate phase mixture possesses a characteristic phase composition of ∼ 75 wt.% silicon-stabilized α-tricalcium phosphate (Si-α-TCP) with the balance being calcium hydroxyapatite (HA) and traces of β-tricalcium phosphate (β-TCP). Variability in the phase composition has been shown to be caused by trace magnesium (Mg) contained in the calcium nitrate tetrahydrate used to prepare the sol gel. Mg contents between 250 and 300 ppm are sufficient to form significant quantities of β-TCP at the expense of the Si-α-TCP phase.     

Electrochemical and Raman studies of the biointeraction between Escherichia coli and mannose in polydiacetylene derivative supported on the self-assembled monolayers of octadecanethiol on a gold electrode

Here, we describe a new method to study the biointeraction between Escherichia coli and mannose by using supramolecular assemblies composed of polydiacetylene supported on the self-assembled monolayer of octadecanethiol on a gold electrode. These prepared bilayer materials simply are an excellent protosystem to study a range of important sensor-related issues. The experimental results from UV-vis spectroscopy, resonance Raman spectroscopy, and electrochemistry confirm that the specific interactions between E. coli and mannose can cause conformational changes of the polydiacetylene backbone rather than simple nonspecific adsorption. Moreover, the direct electrochemical detection by polydiacetylene supramolecular assemblies not only opens a new path for the use of these membranes in the area of biosensor development but also offers new possibilities for diagnostic applications and screening for binding ligands.     

3D self-assembled plasmonic superstructures of gold nanospheres: synthesis and characterization at the single-particle level

The synthesis of 3D self-assembled plasmonic superstructures of gold nanospheres as well as the characterization of their structural and optical properties at the single-particle level is presented. This experimental work is complemented by FEM (finite element method) simulations of elastic scattering spectra and the spatial |E|(4) distribution for establishing structure-activity correlations in these complex 3D nanoclusters.     

Large-scale graphene transistors with enhanced performance and reliability based on interface engineering by phenylsilane self-assembled monolayers

In this letter, we report the dielectric/graphene interface physics and engineering of large-scale, chemical vapor deposited (CVD) graphene transistors by self-assembling a molecular-scale organosilane monolayer onto the dielectric surface. We show that phenyl-alkyl-terminated self-assembled monolayers (SAM) at the dielectric/graphene interface consistently improve the graphene device performance and reliability. The extrinsic field-effect mobility of large-scale CVD graphene transistors on the phenyl-SAM engineered dielectric is currently up to 2500 cm(2)/(V s) at room temperature, considerably higher than the counterparts without the SAM. In addition, significant reduction on the bias stress instability and hysteresis is achieved by the SAM-based interface engineering. Further analysis reveals that charge injection from graphene to the dielectric/graphene interface dominates the observed hysteresis behavior. For both graphene transistors with and without SAMs, the bias stress stability, that is, Dirac point shift under bias stress, is well described by the stretched exponential model with its fitting parameters clearly indicating different interface properties.     

Nano-indentation at the surface contact level: applying a harmonic frequency for measuring contact stiffness of self-assembled monolayers adsorbed on Au

In this study, the well-ordered alkanethiolate self-assembled monolayers (SAMs) of varied chain lengths and tail groups were employed as examples for nano-characterization on their mechanical properties. A novel nano-indentation technique with a constant harmonic frequency was applied on SAMs chemically adsorbed on Au to explore their contact mechanics, and furthermore to interpret how SAM molecules respond to an infinitesimal oscillation force without pressing them. Experimental results demonstrated that the harmonic contact stiffness along with the measured displacement of SAMs/Au was distinguishable using a dynamic contact modulus with the distinct feature of phase angles. Phase angles resulted from the relaxing continuation of an applied harmonic frequency and mostly influenced by the outermost tail group of SAM molecules. The harmonic contact stiffness of SAM molecules obviously increased with the densely packed alkyl chains and relatively intense agglomeration of the head group at the anchoring site. As a consequence, the result of this work is relevant to contact mechanics at the surface contact level for the distinction of molecular substances attached on a solid surface. Furthermore it is particularly anticipated to identify biological molecules of variable qualities under a fluid-like micro-environment.     

Heterogeneous immunosensing using antigen and antibody monolayers on gold surfaces with electrochemical and scanning probe detection

We report the use of antibody and antigen monolayer immunosurfaces as detection elements in a competitive heterogeneous immunoassay employing either electrochemical or scanning probe detection. Antibody or antigen monolayers were prepared by covalent attachment of the desired immunoreagent to a two-component self-assembled monolayer via amide linkages. More specifically, mixed monolayers of a carboxylic acid-terminated thiol (thioctic acid) and a methyl-terminated thiol (butanethiol) were used to control the surface epitope density. The microscopic structure of the resulting antibody and antigen arrays was characterized by AFM (atomic force microscopy). Individual, surface-confined rabbit IgG antibodies could be directly imaged in contact mode. The average height of the capture antibodies was found to be 7.1 nm; the average antibody diameter, after correcting for tip convolution effects, was determined to be between 7 and 10 nm. The surface epitope density could be varied over approximately 2 orders of magnitude by changing the composition of the mixed monolayer. AFM was also used to characterize the antibody-antigen binding characteristics of these immunosurfaces, and an average binding efficiency of 22.8% was measured for rabbit IgG antibody arrays. In the second part of this study, the electrochemical detection scheme originally developed by Heineman and co-workers was adapted to our system. A calibration data set was measured, and the linear least-squares correlation coefficient (R2) was found to be 0.993. Finally, the electrochemical and scanning probe detection modes were directly compared. We find an excellent correlation between the surface density of antibody-antigen complexes measured by AFM and the electrochemical response of the same immunosurfaces.     

Structural and compositional features of amorphous calcium phosphate at the early stage of precipitation

Precipitates formed at an early stage (during the first 6 h) of the hydroxyapatite crystallization of a solution were studied. A nitrous synthesis was used (0.583M (NH₄)₂HPO₄ and 0.35 M Ca(NO₃)₂·4H₂O solutions at pH 11–12, 21°C, fast mixing, lyophilization of aliquots). Although XRD patterns indicated an amorphous calcium phosphate (ACP), IR spectra revealed apatite nanocrystals in the precipitates. Some amount of free calcium was found in the mother solution by mass spectrometrical analysis of the aliquots. This amount considerably decreased as the synthesis proceeded, however, the decrease had a slight effect on the crystallinity of the precipitates. A new suggestion on the nature of delayed crystallization (under conditions as those in the present study) was proposed. The free calcium adsorbed by the nanoparticles from the solution formed a shell around a particle because the calcium diffusion into the bulk was poor at the low synthesis temperature. As such, the encapsulation delayed the crystallization of the nanoparticles. Evidence for this suggestion was given. New possibilities were proposed for preparation of bioactive materials of desired composition based on the structural and compositional peculiarities of the X-ray diffractionamorphous calcium phosphates.     

The segregation of calcium ions at the surface of magnesium oxide: theory and calculation

Atomistic simulation methods have been used to calculate the energetics of substitution and segregation of calcium ions near and at the {001} and {110}¦ surfaces of magnesium oxide at thermal equilibrium. Defect clusters and complete planes of segregating ions have been considered, thereby including impurity-impurity interaction terms. Calculated enthalpies of segregation of –0.42 and –0.78 eV, derived from different interatomic potentials, are in good agreement with the experimental heat of segregation of –0.78±0.22 eV reported recently by Wynblatt and McCune. A simple statistical mechanical theory leads to an Arrhenius expression for large crystals and the deviation from this behaviour expected in microcrystallites.     

Self-assembled monolayers of semi-fluorinated thiols and disulfides with a potentially antibacterial terminal fragment on gold surfaces

Attempts to elaborate the best organized cationic self-assembled monolayers (SAMs) with sulfur derivatives containing potentially bactericidal quaternary ammonium salt moieties have been performed on gold with the final aim to obtain contact-active antibacterial surfaces. Four molecules bearing two hydrocarbon spacers with different lengths between the sulfur atom and the quaternized nitrogen atom, and two different terminal semi-fluorinated alkyl chains have been synthesised and used in view to evaluate their capacity for leading to the highest densities and the highest organization of potentially active molecules on the metal surface. The formation and quality of SAMs characterized by X-ray photoelectron spectroscopy, Internal Reflexion Infra Red Imaging, contact angle and blocking factor measurements depend on the lengths of both the hydrocarbon spacer and terminal perfluorinated chain.