Electrochemical and Raman spectroscopic study of polyaniline; influence of the potential on the degradation of polyaniline

The polymerization of aniline has been studied employing in-situ electrochemical and Raman spectroscopical techniques. Aniline was polymerized by cyclic voltammetry on a Pt surface in sulfuric acid solutions of aniline. The Raman bands were assigned for degradation products of the overoxidized form of polyaniline. A discussion of the degradation mechanism is given. Received: 12 November 1997 / Accepted: 20 January 1998     

An amperometric lactate sensor based on a NAD+-analog and lactate dehydrogenase coimmobilized on reticulated vitreous carbon

A NAD⁺-analog was coimmobilized with lactate dehydrogenase (LDH) on reticulated vitreous carbon (RVC) to give an amperometric lactate biosensor. Both LDH and the NAD⁺ -analog were bound covalently with carbodiimide to the surface of the porous RVC-material. The electrode was operated in a FIA-arrangement in the presence or absence of a soluble mediator. Meldola Blue. The stability was poor when the electrode was operated at +400 mV (vs. Ag/AgCl) in the absence of mediator but improved most significantly in the presence of 5 μM mediator, so that 65% of the original activity remained after 16 days. The amperometric currents were smaller with regeneration by mediator at −100 mV than with direct electrochemical oxidation at +400 mV, indicating that the additional steps slow down the reaction rate. Linear calibration plots were obtained from the detection limit, 1 μM, to 500 μM lactate with 5 μM mediator, reoxidized at −100 mV. The sample throughput was about 60 h⁻¹.     

Evaluation of non-specific binding suppression schemes for neutravidin and alkaline phosphatase at the surface of reticulated vitreous carbon electrodes

Non-specific binding (NSB) of high-molecular-weight proteins onto electrode surfaces can complicate the application of electroanalytical techniques to clinical and environmental research, particularly in biosensor applications. We present herein various strategies to modify the surface of reticulated vitreous carbon (RVC) electrodes to suppress non-specific binding of biomolecules onto its surface. Non-specific binding and specific binding (SB) of two enzyme conjugates, neutravidin-alkaline phosphatase (NA-ALP) and biotinylated alkaline phosphatase (B-ALP), and also neutravidin itself, were studied using hydroquinone diphosphate (HQDP) as an enzyme substrate for ALP inside the pores of RVC electrodes that had been subjected to various modification schemes. The extent of NSB and SB of these biomolecules inside RVC pores was assessed by measuring the initial rate of generation of an electroactive product, hydroquinone (HQ), of the enzyme-catalyzed reaction, using linear scan voltammetry (LSV) for HQ detection. Electrodes functionalized with phenylacetic acid and poly(ethylene glycol) (PEG) showed low NSB and high SB (when biotin capture ligands were included in the modification scheme) in comparison with unmodified electrodes and RVC electrodes modified in other ways. A simple sandwich bioassay for neutravidin was performed on the RVC electrode with the lowest NSB. A concentration detection limit of 52 ± 2 ng mL⁻¹ and an absolute detection limit of 5.2 ± 0.2 ng were achieved for neutravidin when this assay was performed using a 100 μL sample size.     

Structure of polyaniline formed in different inorganic porous materials: A spectroscopic study

In this work, the electronic and structural characterization of polyaniline (PANI) formed in cavities of zeolites Y (ZY) and Mordenite (MOR) and montmorillonite (MMT) clay having Cu(II) as oxidant agent are presented. The formation of PANI and its structure is analyzed by Resonance Raman, UV-Vis-NIR, FT-IR and N K XANES techniques. In all cases the structure of PANI formed is different from the “free” polymer. The presence of azo bonds linked to phenazine-like rings are observed only for PANI-MMT composites, independent of the kind of oxidant agent employed in the synthesis. The presence of Cu(II) ions leads to the formation of Phenosafranine-like rings. The presence of these phenazine-like rings in the structure of confined PANI chains can also contribute to the enhancement of the thermal stability observed for all composites.     

Amperometric Glucose Biosensor Based on the Deposition of Copper and Glucose Oxidase onto Glassy Carbon Transducer

ABSTRACT

The performance of a first generation glucose amperometric biosensor based on the entrapment of glucose oxidase (GOx) within a net of copper electrodeposited onto activated glassy carbon electrode, is described. The copper electrodeposited offers an efficient electrocatalytic activity towards the reduction of enzymatically-liberated hydrogen peroxide, allowing for a fast and sensitive glucose quantification. The influence of the electrodeposition conditions (pH, potential, time, copper salt and enzyme concentrations) on the response of the bioelectrode was evaluated from the amperometric signals of hydrogen peroxide and glucose. The combination of copper electrodeposition with a nation membrane allows an excellent selectivity towards easily oxidizable compounds such as uric and ascorbic acids at an operating potential of -0.050 V. The response is linear up to 2.0 × 10^?2 M glucose, the detection limit being 1.2 × 10^?3 M.     

The effect of the polyaniline morphology on the performance of polyaniline supercapacitors

The polyaniline (PANI) prepared by the pulse galvanostatic method (PGM) or the galvanostatic method on a stainless steel substrate from an aqueous solution of 0.5 mol/l H₂SO₄ with 0.2 mol/l aniline has been studied as an electroactive material in supercapacitors. The electrochemical performance of the PANI supercapacitor is characterized by cyclic voltammetry, a galvanostatic charge–discharge test and electrochemical impedance spectroscopy in NaClO₄ and HClO₄ mixed electrolyte. The results show that PANI films with different morphology and hence different capacitance are synthesized by controlling the synthesis methods and conditions. Owing to the double-layer capacitance and pseudocapacitance increase with increasing real surface area of PANI, the capacitive performances of PANI were enhanced with increasing real surface area of PANI. The highest capacitance is obtained for the PANI film with nanofibrous morphology. From charge–discharge studies of a nanofibrous PANI capacitor, a specific capacitance of 609 F/g and a specific energy density of 26.8 Wh/kg have been obtained at a discharge current density of 1.5 mA/cm². The PANI capacitor also shows little degradation of capacitance after 1,000 cycles. The effects of discharge current density and deposited charge of PANI on capacitance are investigated. The results indicate that the nanofibrous PANI prepared by the PGM is promising for supercapacitors.     

Characterization of the gold-catalyzed deposition of silver on graphite screen-printed electrodes and their application to the development of impedimetric immunosensors

This paper describes the characterization of the gold-catalyzed deposition of silver on graphite screen-printed electrodes (SPEs) using electrochemical impedance spectroscopy (EIS) and the application of this approach to the development of impedimetric immunosensors. After applying −0.1 V for 45 s, the amount of electrodeposited silver quantitatively changes the magnitude of two elements of the electrical equivalent circuit: the interface capacitance, Ci, and the charge-transfer resistance, R_CT. Better correlations have been found when considering the R_CT since this parameter is almost exclusively dependent on the amount of deposited silver under these experimental conditions. This approach has been successfully applied to the development of an impedimetric immunosensor for aflatoxin M₁. The R_CT magnitude shows good correlation with the amount of gold immobilized on the electrode surface after a competitive assay and thus, with the toxin concentration. This approach has been found sensitive in a wide range of concentrations, from 15 to 1000 free-AFM₁ ppt with a limit of detection of 12 ppt.     

Oxygen reduction on rotating porous cylinder of modified reticulated vitreous carbon

A rotating cylinder porous electrode (RCPE) of reticulated vitreous carbon (RVC) matrix was used for oxygen reduction reaction (ORR) in H₂SO₄ solutions. Cyclic voltammetry and hydrodynamic voltammetric techniques were used for electrochemical characterization of the ORR. Cyclic voltammograms in stationary solutions showed better performance of the anodically oxidized RVC (for periods of 1 and 5 min) for the ORR than the untreated RVC in which the first scan (ORR) after the surface treatment was of no utility, and the second scan was presented here. The hydrodynamic voltammograms obtained at the treated RCPE gave well-defined limiting current plateau with positively shifted onset potential as compared with the untreated (plain) RVC electrode. The analysis of the limiting current data on RCPE and the determination of a limiting current enhancement factor α enabled us to quantify the enhancement extent exerted by the anodic oxidation treatment. An enhancement factor of up to ∼3 was obtained at the RCPE electrode anodically oxidized for 5 min. It was found that the α slightly decreased with the rotation speed depending on the extent of anodic oxidation of RVC. This was attributed to the different mode of mass transfer (diffusion) to the interior of the micropores with different microstructure resulting from different extent of anodic oxidation. X-ray photoelectron spectroscopic and scanning electron microscopic measurements helped us to characterize the anodically oxidized RVC surface.     

Experimental study and modeling of impedance of the her on porous Ni electrodes

The electrochemical activity towards the hydrogen evolution reaction of pressed powder electrodes (Ni---Zn and Ni---Al) was studied in alkaline solutions after leaching out the more active element. These electrodes displayed porous character, and electrochemical impedance spectroscopy was used to characterize surface porosity. The influence of overpotential, temperature, poisons, electrode composition and electrolyte concentration was studied and distinction criteria between faradaic and geometrical effects were formulated. Digital simulations of impedance values in different pore geometries were also carried out. The kinetic parameters of hydrogen evolution were determined. The main factor influencing the electrode activity seems to be the real surface area.     

Optimisation and characterisation of biosensors based on polyaniline

With lower limits of detection and increased stability constantly being demanded of biosensor devices, characterisation of the constituent layers that make up the sensor has become unavoidable, since this is inextricably linked with its performance. This work describe the optimisation and characterisation of two aspects of sensor performance: a conductive polymer layer (polyaniline) and the immobilised protein layer. The influence of the thickness of polyaniline films deposited electrochemically onto screen-printed electrode surfaces is described in this work in terms of its influence on a variety of amperometric sensor performance characteristics: time to reach steady state, charging current, catalytic current, background current and signal/background ratios. The influence of polymer film thickness on the conductivity and morphology of finished films is also presented.

An electrostatic method of protein immobilisation is used in this work and scanning electron microscopy in conjunction with gold-labelled antibodies and back-scattered electron detection has enabled the direct visualisation of individual groups of proteins on the sensor surface. Such information can provide an insight into the performance of sensors under influence of increasing protein concentrations.     

Diamond like carbon coated films for enzyme electrodes; characterization of biocompatibility and substrate diffusion limiting properties

The biocompatibility and substrate diffusion limiting properties for a range of diamond like carbon (DLC) coated microporous polycarbonate and DLC coated dialysis (haemodialysis) membranes have been studied. This characterisation builds upon previous findings where DLC coated membranes imparted enhanced enzyme electrode performance. In this study electrode linear ranges have been extended from 10 mM glucose for a 0.01 μm pore size membrane to 160 mM. These findings correlated with the duration of DLC deposition and associated reductions in permeability for glucose. Permeability coefficient ratios for both microporous and dialysis membranes were also found to be important with low glucose/O₂ permeability ratios imparting extensions in glucose linear response range. DLC coated membranes employed within enzyme electrodes have also been shown to exhibit enhanced haemocompatibility as determined by both sensitivity change and surface deposition of blood components examined by scanning electron microscopy. Correlations are made between the reduced losses in sensor response to biofouling/ working electrode passivation processes, and extended linear ranges that DLC coated membranes may impart to enzyme electrode performance. Particular reference is made to the determination of glucose levels within whole blood.     

Improvement of direct bioelectrocatalysis by cellobiose dehydrogenase on screen printed graphite electrodes using polyaniline modification

Modification of graphite based screen printed electrodes (SPEs) by electrosynthesised polyaniline (PANI) has been applied to improve the electron exchange between cellobiose dehydrogenase (CDH, EC 1.1.99.18) from the ascomycete Myriococcum thermophilum and the surface of the SPE. The redox intermediate layer of the conducting polymer promotes the bioelectrocatalysis providing a higher current for lactose oxidation at a lower potential compared to CDH immobilised on a plain SPE. The current of the SPE|PANI|CDH electrode was more than 5 times higher as compared to that of a SPE|CDH electrode at a potential of 0 mV vs. Ag|AgCl. When comparing the response obtained through direct electron transfer with that obtained through mediated electron transfer, it was clearly observed that the improved current of the SPE|PANI|CDH electrode is due to the specific role of PANI, rather than caused by a rise of enzyme loading. The operational stability of the enzyme electrode based on PANI modified SPE was 5 times higher compared with that based on plain SPE.     

Raman spectroelectrochemical study on the kinetics of electrochemical degradation of polyaniline

The kinetics of electrochemical degradation of polyaniline and a copolymer of aniline and metanilic acid have been studied by in situ Raman spectroscopy at a gold electrode. It has been concluded that probably no drastic changes in polymer structure occur on prolonged electrochemical treatment of polymer films at a high electrode potential (0.8 V vs. Ag/AgCl). Instead, most prominent changes relate to a gradual decrease of an overall intensity of spectra, viz. to gradual degradation of a polymer layer. The degradation proceeds faster at pH 1.0, compared to pH 7.0. The kinetic results obtained have been analyzed following simple 2- or 3-parameter exponential decay equations, and compared with the known degradation rate constants.     

Electrochemical study of polyaniline deposited on a titanium surface

The electrochemical synthesis of polyaniline on a titanium surface in aqueous sulfuric acid solutions with various concentrations of added aniline has been investigated by cyclic voltammetry. By utilizing a more cathodic potential range (up to −0.6 V) for the cyclization than is usual (up to −0.2 V) on Pt and Au electrodes, the new voltammetric waves have been deconvoluted from the already well-known ones for polyaniline. By simultaneous electrochemical and in situ Raman spectroscopic measurements, the Raman bands of polyaniline electrodeposited on a Ti electrode, were assigned for potentials of −0.15 V and −0.6 V. It was found that the new monitored waves were closely related to the so-called “middle” peaks and appear only when the polyaniline reaches an overoxidized state. Received: 7 August 1997 / Accepted: 4 November 1997     

Electrochemical characterisation of macroporous electrodes: Recent advances and hidden pitfalls

Investigating the intrinsic kinetics of macroporous electrodes by means of cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) is an exceptionally challenging task with a multitude of hidden pitfalls. The classical analysis (e.g. the Randles–?ev?ík or the Nicholson methods) cannot be used for macroporous electrodes as these can only be applied to electrodes which exhibit mass transport controlled by planar semi-infinite diffusion and that have spatially uniform redox activity. Unfortunately, a number of recent articles incorrectly interprete data by applying classical analysis to macroporous electrodes. To address this, the CV and EIS analysis methods, which can be used for macroporous electrodes are reviewed and discussed.     

Multi-walled carbon nanotubes-ferroelectric liquid crystal nanocomposites: effect of cell thickness and dopant concentration on electro-optic and dielectric behaviour

Nanocomposites comprise functionalised multi-walled carbon nanotubes (0.00 wt%, 0.05 wt% and 0.07 wt%) and ferroelectric liquid crystals (FLCs) have been studied in the 5-μm- and 12-μm-thickness cells. Effect of anchoring energy and dopant concentration on the mesomorphic, electro-optic and dielectric behaviour of FLC has been explored. Fast switching time and increase in permittivity of non-doped FLCs and resulting nanocomposites as a function of increased cell thickness (from 5 to 12 μm) can be attributed to the change in the anchoring energy and direct current (DC) conductivity of the non-doped and doped systems. π–π stacking between carbon nanotubes and FLC layers give rise to the spontaneous polarisation of nanocomposites. Effect of cell thickness and anchoring energy on bistability are also discussed.     

Microband electrodes of ideal and nonideal geometries: AC impedance spectroscopy

The AC impedance behavior of microband electrode geometries which deviate from the ideal is derived via numerical modelling of the chronoamperometric response under diffusion-only conditions. Specifically attention is given to four electrode shapes in addition to the ideal microband geometry: elevated microband electrodes (with conducting supporting sides), recessed microband electrodes (with insulating pit walls), platform electrodes (with insulating supporting sides) and, for the purposes of comparison, a hypothetical line electrode without any support which permits diffusional mass transport to both sides of the infinitesimally thin electrode. Simple analytical expressions are established for the frequency dependence of the AC impedance in each case.     

Influence of silver-decorated multi-walled carbon nanotubes on electrochemical performance of polyaniline-based electrodes

In this work, silver (Ag) nanoparticles were deposited on multi-walled carbon nanotubes (MWNTs) by chemical reduction while Ag-decorated MWNTs (Ag-MWNTs)/polyaniline (PANI) composites were prepared by oxidation polymerization. The effect of the Ag incorporated into the interface of the composites on the electrochemical performance of the MWNTs/PANI was investigated. It was found that highly dispersed Ag nanoparticles were deposited onto the MWNTs, and the Ag-MWNTs were successfully coated by PANI. According to cyclic voltammograms, the Ag-MWNTs/PANI exhibited significantly increased electrochemical performances compared to MWNTs/PANI and the highest specific capacitance obtained of MWNTs/PANI and 0.15 M Ag-MWNTs/PANI was 162 F/g and 205 F/g, respectively. This indicated that Ag nanoparticles that were deposited onto the MWNTs caused an enhanced electrochemical performance of the MWNTs/PANI due to their high electric conductivity, which resulted in an increase of the charge transfer between the MWNTs and PANI by a bridge effect.     

Discourse on the utilization of polyaniline coatings for surface plasmon resonance sensing of ammonia vapor

Surface plasmon resonance spectroscopy is sensitive to near-surface (<300 nm) chemical and physical events that result in refractive index changes. The non-specific nature of the stimulus implies that chemical selectivity in SPR sensing configurations entirely relies upon the chemical recognition scheme employed. Biosensing applications commonly use surface layers composed of antibodies or enzymes for biomolecular recognition. Monitoring of volatile compounds with SPR spectroscopy, however, has not been widely discussed due to the difficulty in selectively responding to small molecules (<100 Da) in addition to the limited refractive index changes resulting from the interaction between the plasmon wave and volatile compounds.Different strategies explored thus far for sensing of small molecules have relied on optical and electrical changes of the recognition layer upon exposure to the analyte, yielding an indirect measurement. Examples of coatings used for gas-phase sensing with SPR include conducting metal oxides, polymers and organometallic dyes. Electrically conducting polymers, like polyaniline and polypyrrole, display dramatic conductivity changes in the presence of certain compounds. This property has resulted in their routine incorporation into different sensing schemes. However, application of electrically conducting polymers to SPR gas-phase sensing has been limited to a few examples, despite encouraging results.The emeraldine salt form of polyaniline (PAni) demonstrates a decreased electrical conductivity correlated to NH₃ concentration. In this contribution, PAni doped with camphorsulfonic acid (PAni-CSA) was applied to gas-phase sensing of NH₃ by way of SPR spectroscopy. Spectroscopic ellipsometry was used to determine the optical constants (n and k) for emeraldine salt and emeraldine base forms of PAni, confirming the wavelength-dependent response observed via SPR. The analytical performance of the coatings show that a limit of detection of 32 ppm NH₃ based on precision of the mass-flow controllers used and an estimated method limit of detection of ∼0.2 ppm based on three standard deviations of the blank. This is directly comparable to other, more established sensing architectures.