Side-on Adsorption Research Articles

Kinetics of Poly-l-lysine Adsorption on Mica and Stability of Formed Monolayers: Theoretical and Experimental Studies.

Various physicochemical parameters of poly-l-lysine (PLL) solutions comprising the diffusion coefficient, the electrophoretic mobility, the density, and the intrinsic viscosity were determined for the pH range 3.0-9.2. This allowed us to calculate derivative parameters characterizing the PLL molecule such as: zeta potential, the number of electrokinetic charges, ionization degree, contour length, and cross section area. These data were exploited in theoretical calculations of PLL adsorption kinetics on solid substrates under diffusion transport. A hybrid approach was used comprising a blocking function derived from the random sequential adsorption (RSA) model. In experiments, the PLL adsorption on mica was studied using the streaming potential measurements and interpreted in terms of a general electrokinetic model. This confirmed a side-on adsorption mechanism of the macroion molecules at the examined pH range. Additionally, using this method, the stability of PLL monolayers was determined performing in situ desorption kinetic experiments. In this way, the equilibrium adsorption constant and the energy minimum depth were determined. It was confirmed that the monolayer stability decreases with pH following the decrease in the number of electrokinetic charges per molecule. This confirmed the electrostatic interaction driven adsorption mechanism of PLL. It is also predicted that at pH 5.7-7.4 the monolayers were stable under diffusion-controlled desorption over the time exceeding 100 h. In addition to their significance for basic science, the results obtained in this work can be exploited for developing procedures for preparing stable PLL monolayers of well controlled coverage and electrokinetic properties.

First principles study of oxygen molecule interaction with the graphitic active sites of a boron-doped pyrolyzed Fe-N-C catalyst.

We study the adsorption and the dissociation of O2 molecules on the active sites of a boron-doped pyrolyzed Fe-N-C catalyst using density functional theory. Initially, we determine the possible structure of the FeN4 active site of the pyrolyzed Fe-N-C catalyst doped with a boron atom by considering the presence of a nitrogen atom as a metal-free site. The most stable configuration of the structure occurs when the boron and nitrogen atoms coalesce with the FeN4 site forming a complex site. This structure has higher stability compared to the undoped FeN4 site. The doped FeN4 possesses the unique ability to adsorb an oxygen molecule in a side-on mode due to the presence of the boron-nitrogen pair acting as a supporting site. One O atom of the O2 molecule sticks strongly to the top of the iron atom, while the other binds with the boron atom. This O2 side-on adsorption stretches the O-O bond length by 15%. Furthermore, the examined catalyst surface can dissociate the oxygen molecule easily with only half the energy barrier of the undoped FeN4 structure.

Carbon-supported Co(III) dimer for oxygen reduction reaction in alkaline medium

Non-precious metal electrocatalysts obtained by pyrolysis of precursors of metal, nitrogen, and carbon (MNC) are viewed as an inexpensive replacement for platinum-based electrocatalysts for the oxygen reduction reaction (ORR) in fuel cells. The hypothesized ORR active site structure of typical MNC catalysts consists of a transition metal coordinated to the pyridinic/pyrollic type of nitrogen covalently attached to the edges of the graphitic crystallites. One of the drawbacks of all the reported procedures to synthesize these MNC electrocatalysts is the inability to control the formation of a specific active site structure suitable for ORR. Lack of clarity on the active site structure limits the researcher’s ability to design a synthesis methodology that maximizes the specific active site density. In this study, we have synthesized a Co(III) dimer ([Co2(OH)2(OOCCH3)3(bpy)2] NO3 ⋅ 1.5 H2O) and demonstrated its ORR activity in alkaline medium. The ORR activity and methanol tolerance property of the Co(III) dimer were compared with those of Ketjenblack carbon (used as support for Co(III) dimer) and commercial 20 wt% Pt/C, respectively. Since Co(III) dimer is a molecular material, its characterization by single-crystal X-ray diffraction, nuclear magnetic resonance, and infrared studies revealed the chemical structure unambiguously. Density functional theory calculation predicted the possibility of both end-on and side-on oxygen adsorption at the metal center of the Co(III) dimer.

Diagnostic Features of EPR Spectra of Superoxide Intermediates on Catalytic Surfaces and Molecular Interpretation of Their g and A Tensors

The use of electron paramagnetic resonance spectroscopy to study the superoxide intermediates, generated by end-on and side-on adsorption of the naturally abundant and 17O-enriched dioxygen on catalytic surfaces is discussed. Basic mechanisms of O2 − radical formation via a cationic redox mechanism, an anionic redox mechanism, and an electroprotic mechanism are illustrated with selected oxide-based systems of catalytic relevance. Representative experimental spectra of various complexities are analyzed and their diagnostic features have been identified and interpreted. The molecular nature of the g and A tensors of the electrostatic and covalent superoxide adducts is discussed in detail within the classic and density functional theory based approaches.

Hydrodenitrogenation of pyridine on MoP(0 1 0): Competition between hydrogenation and denitrification

Periodic density functional theory (DFT) calculations were performed to investigate the hydrodenitrogenation (HDN) mechanism of pyridine on MoP(010). The geometries and energies for the adsorbed pyridine were first described, and then the competition between hydrogenation and denitrification of pyridine were analysed to elaborate the HDN mechanism. Our results show that pyridine has side-on and end-on adsorption modes on MoP(010), in which the most stable η4(N1,C2,C3,C5) configuration of the side-on mode facilitates the hydrogenation of pyridine. The favourable HDN pathway of pyridine proceeds along the pyridine→4-Monohypyridine→3,4-Dihydropyridine→1,3,4-Trihydropyridine→1,3,4,5-Tetrahydropyridine→CH(CH2)3CHNH route. The denitrification process could not exhibit the competitive advantage until after the fourth hydrogenation step of 1,3,4,5-Tetrahydropyridine. This theoretical work provides further information on the HDN mechanism of pyridine on MoP(010), and also provides insight into the understanding of competition between hydrogenation and denitrification.

Mapping single macromolecule chains using the colloid deposition method: PDADMAC on mica

Monolayers of the cationic polyelectrolyte poly(diallyldimethylammonium chloride) (PDADMAC) on mica were thoroughly characterized using the streaming potential and the colloid deposition methods. Initially, the stability of the monolayers was determined by performing desorption experiments carried out under diffusion-controlled regime. It was shown that the desorption of the polyelectrolyte at the ionic strength range 0.01–0.15M is negligible over the time of 20h. The structure of PDADMAC monolayers and orientation of molecules were evaluated using the colloid deposition measurements involving negatively charged polystyrene latex microspheres, 820nm in diameter. The functional relationships between the polyelectrolyte coverage and latex coverage deposited within 20h were acquired by direct optical microscope. In this way the influence of ionic strength varied in the range 0.15–0.01M on the molecule orientation in monolayers was determined. It was shown that for ionic strength of 0.15M nearly one to one mapping of polyelectrolyte chains by colloid particles can be achieved for PDADMAC coverage below 0.1%. In this way, because of a considerable surface area ratio between the macromolecule and the colloid particle, an enhancement factor of 103 can be attained. This behavior was quantitatively interpreted in terms of the random site adsorption model whereas the classical mean-field theory proved inadequate. On the other hand, for lower ionic strength, it was confirmed that an irreversible immobilization of latex particles can only occur at a few closely spaced PDADMAC chains. It was shown that these experimental results were consistent with the side-on adsorption mechanisms of PDADMAC at mica for the above ionic strength.

Human fibrinogen adsorption on positively charged latex particles.

Fibrinogen (Fb) adsorption on positively charged latex particles (average diameter of 800 nm) was studied using the microelectrophoretic and the concentration depletion methods based on AFM imaging. Monolayers on latex were adsorbed from diluted bulk solutions at pH 7.4 and an ionic strength in the range of 10(-3) to 0.15 M where fibrinogen molecules exhibited an average negative charge. The electrophoretic mobility of the latex after controlled fibrinogen adsorption was systematically measured. A monotonic decrease in the electrophoretic mobility of fibrinogen-covered latex was observed for all ionic strengths. The results of these experiments were interpreted according to the three-dimensional electrokinetic model. It was also determined using the concentration depletion method that fibrinogen adsorption was irreversible and the maximum coverage was equal to 0.6 mg m(-2) for ionic strength 10(-3) M and 1.3 mg m(-2) for ionic strength 0.15 M. The increase of the maximum coverage was confirmed by theoretical modeling based on the random sequential adsorption approach. Paradoxically, the maximum coverage of fibrinogen on positively charged latex particles was more than two times lower than the maximum coverage obtained for negative latex particles (3.2 mg m(-2)) at pH 7.4 and ionic strength of 0.15 M. This was interpreted as a result of the side-on adsorption of fibrinogen molecules with their negatively charged core attached to the positively charged latex surface. The stability and acid base properties of fibrinogen monolayers on latex were also determined in pH cycling experiments where it was observed that there were no irreversible conformational changes in the fibrinogen monolayers. Additionally, the zeta potential of monolayers was more positive than the zeta potential of fibrinogen in the bulk, which proves a heterogeneous charge distribution. These experimental data reveal a new, side-on adsorption mechanism of fibrinogen on positively charged surfaces and confirmed the decisive role of electrostatic interactions in this process.

Density Functional Theory Study on the Interaction of O2 and H2O2 Molecules with the Active Sites of Cobalt–Polypyrrole Catalyst

We investigate the interaction of O2 and H2O2 molecules with the active sites of cobalt–polypyrrole (Co–Ppy) catalyst using the density functional theory (DFT) calculations. Several configurations of Co–Nx clusters (x = 2, 3) are used to mimic the active sites structures of pyrolized Co–Ppy catalyst. The highest O–O bond elongation of the adsorbed O2 molecule on Co–Nx clusters is of about 17.14%. The O2 side-on adsorption configuration could be realized in the Co–N2 active sites that have bent N–Co–N angles and strong dxz/yz-orbitals characters in their frontier molecular orbitals (FMOs) and in the Co–N3–pyridinic–trimer active site that has no occupied dz2-like-orbital in its FMOs. Depending on the configuration of O2 adsorption on Co–Nx clusters and the amount of extra charges in anti-bonding orbitals (π*) of the adsorbed O2 molecule, the formed H2O2 molecule on Co–Nx clusters will be spontaneously dissociated into OH* molecules; otherwise it will be adsorbed in its molecular form.

Mechanisms of Fibrinogen Adsorption at Solid Substrates at Lower pH

Adsorption of fibrinogen was theoretically studied using the three-dimensional random sequential adsorption (RSA) model. Fibrinogen molecule shape was approximated by the bead model considering the presence of flexible side arms. Various cases were considered inter alia, the side-on adsorption mechanisms and the simultaneous side-on/end-on adsorption mechanism. The latter mechanisms is pertinent to fibrinogen adsorption at lower pH (below isoelectric point of 5.8) where the entire molecule is positively charged. Extensive calculations enabled one to determine the jamming surface concentration (coverage) of molecules adsorbed under the side-on and end-on orientations as well as the total coverage. For the simultaneous side-on/end-on model the maximum surface concentration was 7.29 × 10(3) μm(-2) corresponding to the protein coverage of 4.12 mg m(-2) (without considering hydration). Additionally, the surface blocking functions for different adsorption regimes were determined and analytically approximated for the entire range of coverage by the interpolating polynomials. Using these blocking functions, fibrinogen adsorption kinetics for diffusion controlled transport conditions was evaluated. Comparison of these theoretical results with experimental data was made. It was demonstrated that the simultaneous side-on/end-on model properly reflects the maximum coverage of fibrinogen adsorbed on latex particles determined via the electrokinetic (electrophoretic mobility) and AFM measurements. Also, streaming potential measurements of fibrinogen adsorption kinetics on mica were successfully interpreted in terms of this model. The theoretical results derived in this work have implications for basic science providing information on mechanisms of anisotropic protein adsorption.

Tuning conformations of fibrinogen monolayers on latex particles by pH of adsorption

Adsorption of fibrinogen on polystyrene latex particles was studied using the micro-electrophoretic and the concentration depletion methods. Measurements were carried out for pH 3.5, 7.4 and ionic strength of 0.15M, NaCl. Electrophoretic mobility of latex was determined as a function of the amount of adsorbed fibrinogen, expressed as a surface concentration. A monotonic increase in the electrophoretic mobility (zeta potential) of the latex was observed, indicating a significant adsorption of fibrinogen on latex for pH equal to 3.5 and 7.4, respectively. The anomalous adsorption in the latter case was explained in terms of the heterogeneous charge distribution on the fibrinogen molecule. The stability of fibrinogen monolayers formed on latex was also determined in pH cycling between 3.5 and 9.7. These measurements revealed that fibrinogen adsorption was irreversible, governed by the two main adsorption mechanisms: (i) the unoriented (random) mechanism prevailing for pH=3.5 where adsorbing molecules significantly penetrate the latex particle core and (ii) the side-on adsorption mechanism prevailing for pH equal to 7.4. In both cases, variations in the zeta potential with the fibrinogen coverage were adequately described in terms of the electrokinetic model, previously formulated for particle adsorption on planar substrates. Based on these experimental data, an efficient procedure of preparing fibrinogen monolayers on latex particles of controlled conformations and coverage was envisaged.

Mechanisms of Fibrinogen Adsorption on Latex Particles Determined by Zeta Potential and AFM Measurements

The adsorption of fibrinogen on polystyrene latex particles was studied using the concentration depletion method combined with the AFM detection of residual protein after adsorption. Measurements were carried out for a pH range of 3.5-11 and an ionic strength range of 10(-3)-0.15 M NaCl. First, the bulk physicochemical properties of fibrinogen and the latex particle suspension were characterized for this range of pH and ionic strength. The zeta potential and the number of uncompensated (electrokinetic) charges on the protein were determined from microelectrophoretic measurements. It was revealed that fibrinogen molecules exhibited amphoteric characteristics, being on average positively charged for pH <5.8 (isolectric point) and negative otherwise. However, the latex particles did not show any isoelectric point, remaining strongly negative for this pH range. Afterward, systematic measurements of the electrophoretic mobility of fibrinogen-covered latex were carried out as a function of the amount of adsorbed protein, expressed as the surface concentration. A monotonic increase in the electrophoretic mobility (zeta potential) of the latex was observed in all cases, indicating a significant adsorption of fibrinogen on latex for pH below 11. It was also proven that fibrinogen adsorption was irreversible, with the maximum surface concentration varying between 2.5 and 5 × 10(3) μm(-2) (weight concentration of a bare molecule was 1.4 to 2.8 mg m(-2)). These measurements revealed two main adsorption mechanisms of fibrinogen: (i) the unoriented (random) mechanism prevailing for lower ionic strength, where adsorbing molecules significantly penetrate the fuzzy polymeric layer on the latex core and (ii) the side-on adsorption mechanism prevailing for pH > 5.8 and a higher ionic strength of 0.15 M. It was also shown that in the latter case, variations in the zeta potential with the protein coverage could be adequately described in terms of the electrokinetic model, previously formulated for planar substrate adsorption. On the basis of these experimental data, an efficient procedure of preparing fibrinogen-covered latex particles of controlled monolayer structure and coverage was envisaged.

Fibrinogen Adsorption on Mica Studied by AFM and in Situ Streaming Potential Measurements

Adsorption of fibrinogen from aqueous solutions on mica was studied using AFM and in situ streaming potential measurements. In the first stage, bulk physicochemical properties of fibrinogen and the mica substrate were characterized for various ionic strength and pH. The zeta potential and number of uncompensated (electrokinetic) charges on the protein surfaces were determined from microelectrophoretic measurements. Analogously, using streaming potential measurements, the electrokinetic charge density of mica was determined for pH range 3-10 and the NaCl background electrolyte concentration of 10(-3) and 10(-2) M. Next, the kinetics of fibrinogen adsorption at pH 3.5 and 7.4 in the diffusion cell was studied using a direct AFM determination of the number of molecules per unit area of the mica substrate. Then, streaming potential measurements were performed to determine the apparent zeta potential of fibrinogen-covered mica for different pH and ionic strength in terms of its surface concentration. A quantitative interpretation of these streaming potential measurements was achieved in terms of the theoretical model postulating a side-on adsorption of fibrinogen molecules as discrete particles. On the basis of these results, the maximum coverage of fibrinogen Θ close to 0.29 was predicted, in accordance with previous theoretical predictions. It was also suggested that anomalous adsorption for pH 7.4, where fibrinogen and the mica substrate were both negatively charged, can be explained in terms of a heterogeneous charge distribution on fibrinogen molecules. It was estimated that the positive charge was 12 e (for NaCl concentration of 10(-2) M and pH 7.4) compared with the net charge of fibrinogen at this pH, equal to -21 e. Results obtained in this work proved that the coverage of fibrinogen can be quantitatively determined using the streaming potential method, especially for Θ < 0.2, where other experimental methods become less accurate.

Kinetics of Fibrinogen Adsorption on Hydrophilic Substrates

Irreversible side-on adsorption of fibrinogen, modeled as a linear chain of touching beads of various size, was studied theoretically using the random sequential adsorption (RSA) model. Numerical simulation of the Monte Carlo type enabled one to determine the dependence of the surface blocking function (available surface function) on the protein coverage. These numerical results were interpolated using analytical functions based on a polynomial expansion. The dependence of the jamming coverage on the size of the simulation area was also determined. By an extrapolation of these results to the infinite area size, the maximum surface concentration of fibrinogen for the side-on adsorption was determined to be 2.26 x 10(3) microm(-2). This corresponds to a jamming coverage theta(infinity) of 0.29. It was shown that the blocking function can well be approximated in the limit of high coverage by the dependence C(theta(infinity) - theta)(4). Using this interpolating expression, the kinetics of fibrinogen adsorption under convection and diffusion transport conditions were evaluated for various bulk concentrations of the protein. These kinetic curves were derived by numerically solving the mass transport equation in the bulk with the blocking function used as a nonlinear boundary condition at the interface. It was shown that our theoretical results are in agreement with experimental kinetic data obtained by AFM, ellipsometry, and other techniques for hydrophilic surfaces in the limit of low bulk fibrinogen concentration.

A glucose biosensor based on direct electrochemistry of glucose oxidase immobilized on nitrogen-doped carbon nanotubes

A novel biosensor for glucose was prepared by immobilizing glucose oxidase (GOx) on nitrogen-doped carbon nanotubes (CNx-MWNTs) modified electrode. The CNx-MWNTs membrane showed an excellent electrocatalytic activity toward the reduction of O2 due to its diatomic side-on adsorption on CNx-MWNTs. The nitrogen doping accelerated the electron transfer from electrode surface to the immobilized GOx, leading to the direct electrochemistry of GOx. The biofunctional surface showed good biocompatibility, excellent electron-conductive network and large surface-to-volume ratio, which were characterized by scanning electron microscopy, contact angle and electrochemical impedance technique. The direct electron transfer of immobilized GOx led to stable amperometric biosensing for glucose with a linear range from 0.02 to 1.02mM and a detection limit of 0.01mM (S/N=3). These results indicated that CNx-MWNTs are good candidate material for construction of the third-generation enzyme biosensors based on the direct electrochemistry of immobilized enzymes.

Desorption properties of BSA and LSZ on carbonate ion-modified apatite

Desorption behavior of two proteins; bovine serum albumin (BSA) and egg white lysozyme (LSZ) was followed on hydroxyapatite (HAp) after 72 h-adsorption in a static system, focusing modification effect of carbonate ion on the HAp surface. Desorbed amounts in a 5 mM - phosphate buffer solution (PBS) showed maxima in 2 h for BSA and 1 h for LSZ. The ratio of the maximal desorbed amount to the initially adsorbed one: the desorption rate was chosen as an index of the binding strength of protein to the surface. For LSZ, the desorption rate decreased as increase of the carbonate content, indicating stronger binding. This was ascribed to the increase of the strongly-adsorption site of LSZ: the P site of the c face because the increase of the carbonate in HAp inhibits crystal growth along the c axis and hence increases the c face. On the other hand, any clear dependency was not found for BSA. However, four samples of the various carbonate contents were divided into two groups; one with the higher desorption rate with nearly full and more than full surface coverages and the other with the lower one of 0.42-0.60, assuming to be side-on (multiple-site) adsorption. This suggested that more numbers of weaker binding sites were involved in BSA adsorption on HAp with the higher desorption rate than the lower one.

Adsorption of dioxygen to copper in CuHY zeolite

The adsorption of dioxygen to copper in CuHY zeolites has been studied by means of FTIR spectroscopy and model calculations at the quantum mechanical/molecular mechanics (QM/MM) level. Different Si/Al ratios, substitution patterns and adsorption sites within the cavities of the zeolite lead to a large number of different isomers to be studied. In addition, these parameters control the end-on vs. side-on adsorption of dioxygen. High-level multireference benchmark calculations for the singlet and triplet states of such adsorption complexes corroborate the use of density functional theory for the investigation of these systems. Comparison of the experimental and computed data allows for the identification of a preferred adsorption site and a small number of isomers which appear to be most relevant for the adsorption process. Redshifts of >250 cm(-1) are obtained for the vibrational frequencies of adsorbed O(2).

Density functional theory study of the adsorption of oxygen molecule on iron phthalocyanine and cobalt phthalocyanine

Iron phthalocyanine (FePc) and cobalt phthalocyanine (CoPc) molecules have been studied in the past as possible catalysts for oxygen reduction reactions. In this work, density functional theory (DFT) computation results of oxygen molecule (O2) adsorption on FePc and CoPc molecules are reported. For O2 on FePc, both end-on and side-on configurations are found to be energetically favourable but the end-on adsorption configurations are more stable than the side-on configurations. Moreover, our results predicted that compared to those in the end-on configuration, the adsorbed O2 molecules in the side-on configuration have weaker O–O bonds, which led us to believe that the side-on configurations would be more active in catalysing oxygen reduction reaction. In contrast, we found that only end-on configurations are energetically stable for O2 on CoPc. Based on our DFT calculation results and the corresponding experimental observations, we postulate that on the surface of transition metal phthalocyanine catalyst molecules the side-on adsorption of O2 molecules could promote four-electron oxygen reduction process while the end-on adsorption of O2 molecules would facilitate two-electron oxygen reduction process.

AFM study of BSA adlayers on Au stripes

Thin narrow Au stripes suitable for propagating long-range surface plasmon–polaritons were deposited by evaporation and lift-off on a thermal oxide layer on a silicon substrate, and modified by direct adsorption of bovine serum albumin (BSA). Atomic force microscopy (AFM) measurements reveal that BSA adsorbs onto the Au stripes from phosphate buffer solutions forming an adlayer having an average thickness of about 2 nm (surface mass density of about 2 ng/mm 2). Comparisons with a simple adsorption model suggest the side-on adsorption of a single monolayer of BSA followed by denaturation and flattening. The BSA-coated stripes have an increased surface roughness compared to a virgin stripe.

Carbon Dioxide Activation by Surface Excess Electrons: An EPR Study of the CO2− Radical Ion Adsorbed on the Surface of MgO

The CO2- radical anion has been generated at the surface of MgO by direct electron transfer from surface trapped excess electrons and characterized by electron paramagnetic resonance spectroscopy. Both 13C and 17O hyperfine structures have been resolved for the first time, leading to a detailed mapping of the unpaired electron spin density distribution over the entire radical anion. The magnetic equivalence of the two O nuclei has been ascertained allowing a side-on adsorption structure at low-coordinate Mg2+ ions to be proposed for the surface stabilized radical.

Immobilization of Lysozyme onto Pore-engineered Mesoporous AlSBA-15

In this research, hydrothermally stable mesoporous AISBA-15 materials have been used as adsorbents for systematic research on the lysozyme adsorption. Stability of the AISBA-15 adsorbents and the lysozyme molecules after the adsorption experiments for several days in aqueous solutions were confirmed by X-ray diffraction (XRD) measurement and FT-IR spectroscopy, respectively. The amount of the lysozyme adsorption can easily be controlled by the pore diameter and pore volume the mesoporous adsorbent, but an unreasonable effect of the surface area on the protein adsorption capacity was observed. The results of the effect of the pore diameter on the lysozyme adsorption suggest that the adsorption might partially be influenced by kinetically favorable edge-on type orientation on the confined mesopore. However, the final adsorption amount of the lysozyme can be well regenerated by models based on the side-on adsorption in dense packing. The present research also confirms the importance of appropriate "pore-engineering" for immobilization of bio-function on mesoporous materials.