Sasha Omanovic

Electrochemical investigations of the interaction of C-reactive protein (CRP) with a CRP antibody chemically immobilized on a gold surface

A possibility of using a range of dc and ac electrochemical techniques to probe associative interactions of C-reactive protein (CRP) with CRP antibody (aCRP) immobilized on a gold electrode surface was investigated. It was demonstrated that the investigated electrochemical techniques can be used efficiently to probe these interactions over a wide CRP concentration range, from 1.15 x 10(-5) to 1.15 mg L(-1). The measured sensitivity of the techniques is in the following decreasing order: differential pulse voltammetry, charge-transfer resistance obtained from electrochemical impedance spectroscopy (EIS), cyclic voltammetry, chronoamperometry, and double-layer capacitance deduced from EIS measurements which gave the poorest sensitivity. Measurements of kinetic parameters demonstrated that the associative interactions of CRP with the immobilized aCRP reached quasi-equilibrium after 20-30 min. The kinetics of these interactions was modeled successfully using a two-step kinetic model. In this model, the first step represents reversible CRP-aCRP associative-dissociative interactions, while the second step represents the irreversible transformation of the bound CRP into a thermodynamically stable configuration. It was demonstrated that the thermodynamically stable configuration of CRP starts prevailing after 7 min of interaction of CRP with the immobilized aCRP.

Thin indium oxide film formation and growth: Impedance spectroscopy and cyclic voltammetry investigations

Abstract The solid-state and electrochemical properties of potentiodynamically grown oxide thin films on indium were studied. The electrochemical characteristics of indium electrodes in 0.1 M Na–borate buffer solution; pH 10 have been correlated with the impedance spectroscopy results. The impedance spectra are interpreted in terms of an electrical equivalent circuit (EEC) based on a possible physical model, with the circuit elements representing electrochemical properties of indium and its oxide films. On the basis of the current, capacitance, resistance and Warburg impedance data, the potential regions where specific surface reactions occur could be delineated. The anodic behavior of indium was found to be under mixed control; the initial stage of an In oxide formation involves a faster charging process and a slower surface diffusion process, with an effective diffusion coefficient of the order of 10 −15 cm 2 s −1 . On the top of this layer, which is of monolayer dimensions, the growth of the barrier oxy-hydrate layer occurs via a base catalyzed hydrolysis of the surface oxide. Transformation from a gel-like oxide structure (InOOH) into the thermodynamically favorable and more stable crystalline form (In 2 O 3 ) takes place at high positive potentials. In 2 O 3 was found to expose high electric conductivity, which was manifested through oxygen evolution on its surface. Impedance spectroscopy was found to be a suitable technique for the complex structure characterisation of the surface layer formed anodically on indium.

Indium as a cathodic material: catalytic reduction of formaldehyde

The performance of indium as cathodic material for the electroreduction of small organic molecules is considered. The cathodic reduction of formaldehyde (FA) is an ideal model reaction for this purpose since indium has a very large overpotential for the hydrogen evolution reaction with and without FA. Kinetic sets of the reaction pathways, with respect to the Tafel slope and reaction order, are considered on the basis of quasi-potentiostatic measurements and cyclic voltammetry. The value of the Tafel slope bc≈60mVdec-1 indicates that the protonation of the adsorbed radical is the rate determining step in the proposed CECE mechanism. The reaction order with respect to FA is close to one in the limiting current regions but smaller in the Tafel region. The existence and kinetics of the radicals adsorbed during FA reduction are evidenced by very fast potentiodynamic experiments, with scan rates between 40 and 80Vs-1. Electrochemical measurements are carried out on freshly in situ prepared In-electrodes. During cathodic polarization, the surface oxide film is reduced to In-metal via a solid-state mechanism. The crystallization kinetics of indium in the oxide matrix is also discussed.

Adsorption of fibrinogen on a biomedical-grade stainless steel 316LVM surface: a PM-IRRAS study of the adsorption thermodynamics, kinetics and secondary structure changes

Polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS) was employed to investigate the interaction of serum protein fibrinogen with a biomedical-grade 316LVM stainless steel surface, in terms of the adsorption thermodynamics, kinetics and secondary structure changes of the protein. Apparent Gibbs energy of adsorption values indicated a highly spontaneous and strong adsorption of fibrinogen onto the surface. The kinetics of fibrinogen adsorption were successfully modeled using a pseudo first-order kinetic model. Deconvolution of the amide I bands indicated that the adsorption of fibrinogen on 316LVM results in significant changes in the proteins secondary structure that occur predominantly within the first minute of adsorption. Among the investigated structures, the alpha-helix structure undergoes the smallest changes, while the beta-sheet and beta-turns structures undergo significant changes. It was shown that lateral interactions between the adsorbed molecules do not play a role in controlling the secondary structure changes. An increase in temperature induced changes in the secondary structure of the protein, characterized by a loss of the alpha-helical content and its transformation into the beta-turns structure.

Thin oxide films on indium: impedance spectroscopy investigation of reductive decomposition

Abstract The kinetics of cathodic reduction of an In 2 O 3 film grown on a bare indium substrate under dynamic conditions was studied by cyclic voltammetry and electrochemical impedance spectroscopy (EIS) in borate buffer, pH 10. Reductive decomposition of In 2 O 3 up to metallic In proceeds in the solid phase, via a solid-state mechanism: In 2 SO 3( s ) + 2 H 2 O ( aq ) + 6 e − ↔ 2 In ( s ) + 6 OH − ( aq . Impedance spectra obtained between 40 mHz and 50 kHz, in the potential range of oxide film stability, its decomposition, new metal phase extraction and hydrogen evolution were interpreted on the basis of equivalent electrical circuits using a suitable fitting procedure. EIS has shown to be a valuable tool in the characterization of the solid/liquid interface: (i) the passive In 2 O 3 film behaved as a simple dielectric, the interface of a passive electrode was blocking and in Bode and Nyquist plots a capacitive feature appeared; whereas (ii) in the vicinity of the In In 2 O 3 redox potential a Warburg low-frequency feature was obtained, which was associated with diffusion processes in the solid phase as the rate-determining step; (iii) the interface of the freshly segregated metallic phase was a close-to-perfect RC feature, which was associated with charge transfer as the rds for the hydrogen evolution reaction on the bare metal surface. Cyclic voltammetry has revealed more details of the reduction processes giving further insight into the mechanism of reductive decomposition of In 2 O 3 up to metallic In.

Interaction of flavin adenine dinucleotide (FAD) with a glassy carbon electrode surface.

The interaction of flavin adenine dinucleotide (FAD) with a glassy carbon electrode (GCE) surface was investigated in terms of the FAD adsorption thermodynamics and kinetics, the subsequent electroreduction mechanism, and the corresponding electron‐transfer rate. The kinetics of FAD electroreduction at the GCE was found to be an adsorption‐controlled process. A set of electroreduction kinetic parameters was calculated: the true number of electrons involved in the FAD reduction, n=1.76, the apparent transfer coefficient, αapp=0.41, and the apparent heterogeneous electron‐transfer rate constant, kapp=1.4 s−1. The deviation of the number of exchanged electrons from the theoretical value for the complete reduction of FAD to FADH2 (n=2) indicates that a small portion of FAD goes to a semiquinone state during the redox process. The FAD adsorption was well described by the Langmuir adsorption isotherm. The large negative apparent Gibbs energy of adsorption (ΔGads=−39.7 ±0.4 kJmol−1) indicated a highly spontaneous and strong adsorption of FAD on the GCE. The energetics of the adsorption process was found to be independent of the electrode surface charge in the electrochemical double‐layer region. The kinetics of FAD adsorption was modeled using a pseudo‐first‐order kinetic model.

Electrochemical kinetics of anodic layer formation and reduction on antimony and antimonial lead

Abstract The cyclic voltammetry of lead, antimony and lead + antimony binary alloys has been studied in 0.5 M H 2 SO 4 solution at 25°C between the hydrogen and oxygen evolution potentials and over narrower potential regions. This investigation, together with systematic variation of the scan rates and the positive or negative reversal potential, has revealed more details of the oxidation and reduction processes which give further insight into the nature of reactions in the lead-acid battery. Voltammetry data were interpreted taking into account nucleation and growth models, thermodynamic data and the electronic properties of the surface compounds in the potential region of species containing Pb(II), Pb(IV) and Sb(III). The conditions under which PbO and PbO 2 formation can be seen in the positive sweep have been determined.

Enhancement of biocompatibility of 316LVM stainless steel by cyclic potentiodynamic passivation

Passivation of stainless steel implants is a common procedure used to increase their biocompatibility. The results presented in this work demonstrate that the electrochemical cyclic potentiodynamic polarization (CPP) of a biomedical grade 316LVM stainless steel surface is a very efficient passivation method that can be used to significantly improve the materials general corrosion resistance and thus its biocompatibility. The influence of a range of experimental parameters on the passivation/corrosion protection efficiency is discussed. The passive film formed on a 316LVM surface by using the CPP method offers a significantly higher general corrosion resistance than the naturally grown passive film. The corresponding relative corrosion protection efficiency measured in saline during a 2-month period was 97% +/- 1%, which demonstrates a very high stability of the CPP-formed passive film. Its high corrosion protection efficiency was confirmed also at temperatures and chloride concentrations well above normal physiological levels. It was also shown that the CPP is a significantly more effective passivation method than some other surface-treatment methods commonly used to passivate biomedical grade stainless steels. In addition, the CPP-passivated 316LVM surface showed an enhanced biocompatibility in terms of preosteoblast (MC3T3) cells attachment. An increased thickness of the CPP-formed passive film and its enrichment with Cr(VI) and oxygen was determined to be the origin of the materials increased general corrosion resistance, whereas the increased surface roughness and surface (Volta) potential were suggested to be the origin of the enhanced preosteoblast cells attachment.

The response of fibrinogen, platelets, endothelial and smooth muscle cells to an electrochemically modified SS316LS surface: Towards the enhanced biocompatibility of coronary stents

Modification of a biomedical-grade stainless steel 316LS surface by electrochemical cyclic potentiodynamic passivation (CPP) and the response of fibrinogen (Fg), platelets, endothelial cells (ECs) and smooth muscles cells (SMCs) to this surface was investigated. Polarization modulation infrared reflection absorption spectroscopy revealed a significant difference between the secondary structure of Fg adsorbed on the unmodified and CPP surface, the latter being closer to that of native Fg. This was postulated as the origin of the significantly lower surface density of attached platelets on the CPP surface. The competitive interaction of ECs and SMCs with the surface showed that the ECs/SMCs surface density ratio is significantly higher on the CPP surface over the first 2h of attachment, suggesting faster initial attachment kinetics of ECs on the CPP surface. The presented results thus clearly demonstrate an increase in biocompatibility of the CPP 316LS surface.

Modification of a Nitinol Surface by Phosphonate Self-Assembled Monolayers

Nitinol, as a shape memory alloy, is attractive material for medical implants and devices. Contrary to titanium, corrosion by releasing Ni2+ ions occurs during a long-term contact of Nitinol with (physiological) solutions containing Cl- ions. In order to develop chemically/electrochemically stable surfaces and interfaces, Nitinol was modified by self-assembled monolayers of dodecylphosphonate (-OH and –CH3 terminated) films, which were characterized by XPS, PM-IRRAS and contact angle measurements. Strongly bounded well-ordered films of high homogeneity and resistance were synthesized. An innovative method that allows in situ study of influence of thermal annealing, following SAM formation, on their protecting properties in simulated (physiological) solutions is presented. Changes of structural sensitive impedance parameters were correlated with the changes in the interfacial layer. Effective thermal annealing greatly enhances the quality of the self-assembled alkyl-phosphonate films, which behave as non-ideal dielectrics, i.e., the solid/liquid interfaces formed represent the blocking contact preventing charge-transfer reactions.