Özlem Yavuz

Challenges of Electrochemical Impedance Spectroscopy in Protein Biosensing

Electrochemical impedance spectroscopy (EIS) measurement, performed in the presence of a redox agent, is a convenient method to measure molecular interactions of electrochemically inactive compounds taking place on the electrode surface. High sensitivity of the method, being highly advantageous, can be also associated with nonspecific impedance changes that could be easily mistaken for specific interactions. Therefore, it is necessary to be aware of all possible causes and perform parallel control experiments to rule them out. We present the results obtained during the early stages of aptamer-based sensor development, utilizing a model system of human alpha thrombin interacting with a thiolated DNA aptamer, immobilized on gold electrodes. EIS measurements took place in the presence of iron ferrocyanides. In addition to known method limitations, that is, inability to discriminate between specific and nonspecific binding (both causing impedance increase), we have found other factors leading to nonspecific impedance changes, such as: (i) initial electrode contamination; (ii) repetitive measurements; (iii) additional cyclic voltammetry (CV) or differential pulse voltammetry (DPV) measurements; and (iv) additional incubations in the buffer between measurements, which have never been discussed before. We suggest ways to overcome the method limitations.

Synthesis and the physical properties of MnZn ferrite and NiMnZn ferrite–polyaniline nanocomposite particles

We present a novel approach to synthesize MnZnferrite (MZF)–NiMnZnferrite(NiMZF)–Polyaniline (PANI) particles, where MnZnferrite is the magnetic core, and PANI is the conducting layer. The commercial ferrite and Ni–ferrite particles are primarily used, and the PANI–ferrite particles with a hybrid structure are synthesized via an oxidative electrochemical polymerization of aniline in an aqueous solution, which contains well-dispersed ferrite particles. The influence of ferrite content with respect to the electrical and ferromagnetic properties of the composite has been investigated by Electrochemical Impedance Spectroscopy (EIS) and DC field-cooled, zero-field cooled susceptibilities and M–H loop measurements. The origin of their electrical and ferromagnetic properties is also discussed based on structural characterizations including Fourier Transform Infrared (FT-IR) spectroscopy and X-Ray Diffraction (XRD).

Electrosynthesis and study of carbazole–acrylamide copolymer electrodes

Abstract Polymerisation of N-carbazoles in the presence of acrylamide has been studied by electrochemical polymerisation onto platinum and indium tin oxide (ITO), electrodes from an acetonitrile solution of the monomers and tetraethylamonnium perchlorate-supporting electrolyte. The influence of polymerisation conditions such as electrode potential, monomer concentration, type of solvent and supporting electrolyte on the mechanical and electrochemical properties of final polymers have been studied. The redox behaviour of the polymer-coated electrodes was checked in solution containing ferrocene and dopamine solution by cyclic voltammetry. Depending on the conditions the electrode response may be reversible or quasi reversible. Cationic polymerisation of monomer is initiated either by the monomer cation radical or by the proton generated by the coupling reaction of the cation radical of carbazole. For the characterisation of copolymers Cyclic Voltammetry, UV–vis and FT-IR spectrophotometric, solid state conductivity and SEM measurements were used.

The optical, thermal and electrochemical properties of co-electropolymerised films of acrylamide and carbazole

Abstract The optical, thermal and electrochemical properties of co-polymer films grown, from acrylamide and carbazole monomer mixtures, by electrochemical oxidation in protic and aprotic media have been studied. The availability of protons in the solution is found to enhance not only the growth rate of the co-polymer but also the electrochemical behaviour of the co-polymer films. This is especially so for the thicker films and is due to the relative ease of cation injection (as opposed to loss of anions) in maintaining electroneutrality of the polymer matrix during electrochemical reduction. The optical properties of the layers appear to be independent of the growth medium and starting Acrylamide:Carbazole (AAm:Cz) ratio in that similar values of the refractive index n are obtained ( n =1.59±0.02) for all the films grown, from AAm:Cz ratios ranging from 80:1 to 1:10. Thermal analysis studies over the temperature range 25–400°C show that the 15–25 wt.% losses recorded over this region are associated with the stabilisation, through cross-linking, of the polymer and not with thermal degradation. This stabilisation appears to increase with the amount of carbazole incorporated within the polymer matrix.

Electrochemically induced redox polymerization of acrylamide

Polymerization of acrylamide (AA) has been studied in aqueous solution in the presence of Ce(IV) salt-oxalic acid initiator system in an electrochemical cell with and without separation of anolyte and catholyte. For reactions that required the cathode and anode sections to be analyzed individually, a cell whose compartments were divided by a sintered glass disk of the medium porosity was employed. Polymerization was initiated by a free radical that is formed by the fast reaction of oxalic acid and Ce(IV). The electrolysis of the reaction solution results in regeneration of Ce(IV), which can oxidize oxalic acid to produce radicals. The effect of sulfuric acid and cerium (IV) salt concentration and temperature on the yield of electroinitiated polymerization in different cell designs and structural identification of products were performed. Reaction was also followed by cyclic voltametric measurements, and a mechanism was proposed. Results indicated that the electrolysis method with a divided cell (85% conversion) shows advantages, compared with nonelectrolytic (5% conversion) and with undivided electrochemical cell (25% conversion) methods where a high concentration of initiator was used.

N‐Vinylcarbazole‐Acrylamide Copolymer Electrodes Electrochemical Response to Dopamine

Copolymers of N-vinylcarbazole and acrylamide were synthesized by the free radical polymerization using AIBN as initiator. Copolymer electrodes were prepared by casting from 3% solution on platinum and stainless steel substrate. Solvent was removed by heating at 1 10°C in vacuum 10 - Pa for 5 h. The response of electrodes to dopamine was tested by cyclic voltammetry and results suggest that depending on conditions (i.e., scan rates, thickness of film, composition of copolymer), the electrode shows reversible and stable behavior during the 14 days and it seems to he a suitable sensor electrode to dopamine. Stability of copolymer coating was investigated for stainless steel electrodes in sulfuric acid and it is found that copolymer coating can inhibit the corrosion of stainless steel up to 94%.

Three-dimensional two-photon imaging in polymeric materials

We report image formation via single and two-photon photoinduced fluorescence changes in a polymeric medium with two-photon fluorescence readout of multiplayer structures. Photoinduced acid generation in the presence of a two-photon fluorescent dye possessing strongly basic functional groups (7-benzothiazolyl-9,9-didecyl-2,2-(N,N- diphenylamino)fluorene underwent protonation upon exposure with UV or near-IR (740 nm fs pulses). Solution studies demonstrate formation of monoprotonated and diprotonated species upon irradiation, each resulting in distinctly different absorption and fluorescence properties. The fluorescence of the original, neutral, fluorophore is quenched upon monoprotonation with a concomitant increase in fluorescence at longer wavelengths due to the monoprotonated form. Hence, two channel two-photon fluorescence imaging provides positive or negative image readout capability. Results of solution and solid polymer thin films experiments are presented.

Corrosion inhibition and photoactivity behavior of N-substituted polycarbazole-coated natural pyrite electrode

Abstract Conductive films of poly (N-vinylcarbazole) (PVCz) and polycarbazole (PCz) were coated electrochemically on a natural iron sulfide (FeS2) surface. The corrosion, photocorrosion, and photoactivity characteristics of coated electrodes were investigated in potassium iodide/iodide (KI/I2) redox electrolyte. However, PVCz was obtained by oxidative polymerization of N-vinylcarbazole (NVCz) by ceric ammonium nitrate (CAN) and showed photoconductivity also was deposited on the natural FeS2 surface chemically from toluene/cyclohegzanone solution, and the electrode was tested for photoactivity. Electrochemical coating inhibited corrosion and photocorrosion of FeS2 causing some decrease in photoactivity, while chemical coating of thin, transparent PVCz on the surface enhanced the photoactivity and inhibited the corrosion and photocorrosion.

Electromagnetic applications of conducting and nanocomposite materials

Publisher Summary This chapter reviews previous studies and summarizes the research on electromagnetic interference (EMI) shielding studies with conductive polymers (CPs): polyaniline (PANI), polypyrrole (PPy), and their composites in various frequency ranges. As the use of electronic products and communication devices increases, the need for EMI shielding materials increases as well. CPs are new alternative candidates for EMI shielding applications due to their lightweight, corrosion resistance, ease of processing, and tunable conductivities as compared with typical metals; however, very high level of shielding efficiency cannot be achieved by using CPs only. Therefore, this chapter highlights, reviews, and shows the usage of magnetic particles (MnZnferrites (MZFs)) together with CPs. The results show that chemical and electrochemical polymerization techniques yield homogenous coatings onto ferrites particles, which is important to generate electromagnetic waves from magnetic sources, especially at low frequencies, together with those generated from electric sources by CPs having a single coating with desired shielding properties. CP coated textile studies demonstrate that CP-coated fabrics offer several advantages including increased flexibility, access to a wide range of structures, and have a high absorption level to shielding, which is lacking in traditional shielding materials.

Novel Two-photon Absorbing Polymers

The ease of synthesis, high two-photon absorptivity, luminescence fluorescence properties, and solubility makes these polymer a good candidates for two-photon based applications, such as optical power limiting and two-photon fluorescence imaging, aspects currently under investigation. Also, luminescence properties of fluorenyl bisbenzothiazole-based polymer suggest possible application as blue light emitting diode material.