A Selective Cholesterol Biosensor Based on Composite Film Modified Electrode for Amperometric Detection

An amperometric cholesterol biosensor based on immobilization of cholesterol oxidase in a Prussian blue (PB)/polypyrrole (PPy) composite film on the surface of a glassy carbon electrode was fabricated. Hydrogen peroxide produced by the enzymatic reaction was catalytically reduced on the PB film electrode at 0 V with a sensitivity of 39 μA (mol/L)^?1. Cholesterol in the concentration range of 10^?5 ? 10^?4 mol/L was determined with a detection limit of 6 × 10^?7 mol/L by amperometric method. Normal coexisting compounds in the bio‐samples such as ascorbic acid and uric acid do not interfere with the determination. The excellent properties of the sensor in sensitivity and selectivity are attributed to the PB/PPy layer modified on the sensor.     

Bioelectrochemical Response of Polypyrrole Modified Urease Sensor

The preparation method of biological ferment electrode is one of decisive factors that affect its bio-electrochemical responding sign. It demands both suitable indicator electrode and antiwater soluble film that can keep catalytic activity. In this paper,urease was doped into polypyrrole(PPy) film while pyrrole(Py) was polymerized by electrochemical method to formed urease biosensor based on PPy film,then,ferment electrode was combined with CO₂ gas-sensing electrode to assemble a urease biosensor responding to urea.     

Coupling of a Reagentless Electrochemical DNA Biosensor with Conducting Polymer Film and Nanocomposite as Matrices for the Detection of the HIV DNA Sequences

Abstract

This paper describes a reagentless electrochemical DNA biosensor applied to the detection of human immunodeficiency virus (HIV) sequences based on electrochemical impedance spectroscopy (EIS). The novel DNA biosensor has been elaborated by means of an opposite‐charged adsorption Au‐Ag nanocomposite to a conductive polymer polypyrrole (PPy) modified platinum electrode (Pt) and self‐assembly the mercapto oligonucleotide probes onto the surface of modified electrode via the nanocomposite. The duplex formation was detected by measuring the electrochemical impedance signal of nucleic acids in phosphate buffer solution (PBS). Such response is based on the concomitant conductivity changes of the PPy film and nanocomposite. The reagentless scheme has been characterised using 21‐mer synthetic oligonucleotides as models: parameters affecting the hybridization assay were explored and optimized. The detection limit is 5.0×10^?10 M of target oligonucleotides at 3σ. The potential for development of reagentless DNA hybridization analysis in the clinical diagnosis is being pursued.     

An Electrochemical Biosensor with Cholesterol Oxidase/ Sol‐Gel Film on a Nanoplatinum/Carbon Nanotube Electrode

The carbon nanotubes decorated nanoplatinum (CNT‐Pt) were prepared using a chemical reduction method and a novel base electrode was constructed by intercalating CNT‐Pt on the surface of a waxed graphite electrode. The results showed that the nano‐particles of platinum at a waxed graphite electrode exhibits high catalytic activity for the reduction of hydrogen peroxide. The cholesterol oxidase (ChOx), chosen as a model enzyme, was immobilized with sol‐gel on the CNT‐Pt base electrode to construct a biosensor. The current response of the biosensor for cholesterol was very rapid (<20 s). The linear range for cholesterol measurement was 4.0×10^?6 mol/L ?1.0×10^?4 mol/L with a detection limit of 1.4×10^?6 mol/L. The experiments also showed that the ChOx/sol‐gel/CNT‐Pt biosensor was sensitive and stable in detecting cholesterol in serum samples.     

Nonenzymatic dopamine biosensor based on tannin nanocomposite

A dopamine (DA) biosensor was developed based on polypyrrole/tannin/cetyltrimethylammonium bromide (PPy/TA/CTAB) nanocomposite and central composite rotatable design (CCRD) was employed for the optimization of conditions. Chemical polymerization of the PPy/TA in the presence of a cationic surfactant, CTAB, reduced the particle size of composite and a rod-like structure with a lumpy surface and high porosity was observed for nanocomposite justifying the highest current response for the modified electrode. Amperometry and differential pulse voltammetry analyses were applied for all electrochemical measurements and DA detection in the range of 0.5–100 μM. The good adhesion of nanocomposite on the electrode surface, as well as porosity and high surface area of the modified electrode, enhanced the diffusion of DA molecules inside the matrix. Amperometry analysis of the Screen printed carbon electrode/PPy/TA/CTAB modified electrode displayed a good sensitivity of 0.039 μA (μM)⁻¹ toward DA with the limit of detection of 2.9 × 10^–7 M. The modified biosensor also excludes the interfering species of ascorbic acid and uric acid which makes this sensor appropriate for DA determination. The proposed biosensor showed an acceptable reproducibility and repeatability with low relative standard deviations of 4.8% and 4.4%, respectively.     

Pt-polyaniline nanocomposite on boron-doped diamond electrode for amperometic biosensor with low detection limit

Boron-doped diamond electrodes covered with a nanostructured Pt nanoparticle-polyaniline composite have been fabricated and employed as sensitive amperometric sensors with low detection limit. A highly conductive boron-doped diamond thin film (BDD) was prepared by chemical vapor deposition, and its morphology was characterized by scanning electron microscopy and transmission electron microscopy. The nanostructured composite layer was grown on the BDD electrode by electrochemical deposition of polyaniline and Pt nanoparticles. Glucose oxidase (GOx) was then adsorptively immobilized on the modified BDD electrode. The biosensor displays a large surface area, high catalytic activity of the Pt nanoparticles, efficient electron mediation through the conducting polymer, and low background current of the electrode. The biosensor exhibits an excellent response to glucose, with a broad linear range from 5.9 μM to 0.51 mM, a sensitivity of 5.5 μA·mM^?1, a correlation coefficient (R) of 0.9947, and a detection limit of 0.10 μM. The apparent Michaelis-Menten constant (K _M ^app ) and the maximum current density of the electrode are 4.1 mM and 0.021 mA, respectively. This suggests that the immobilized GOx possesses a higher affinity for glucose at the lower K _M ^app , and that the enzymatic reaction rate constitutes the rate-limiting step of the response.     

Bienzymatic amperometric biosensor for glucose based on polypyrrole/ceramic carbon as electrode material

A novel amperometric biosensor utilizing two enzymes, glucose oxidase (GOD) and horseradish peroxidase (HRP), was developed for the cathodic detection of glucose. The glucose biosensor was constructed by electrochemical formation of a polypyrrole (PPy) membrane in the presence of GOD on the surface of a HRP-modified sol-gel derived-mediated ceramic carbon electrode. Ferrocenecarboxylic acid (FCA) was used as mediator to transfer electron between enzyme and electrode. In the hetero-bilayer configuration of electrode, all enzymes were well immobilized in electrode matrices and showed favorable enzymatic activities. The amperometric detection of glucose was carried out at +0.16 V (versus saturated calomel reference electrode (SCE)) in 0.1 M phosphate buffer solution (pH 6.9) with a linear response range between 8.0×10⁻⁵ and 1.3×10⁻³ M glucose. The biosensor showed a good suppression of interference in the amperometric detection.     

Catalyzed microelectrode mediated by polypyrrole/Nafion® composite film for microfabricated fuel cell applications

A novel Si-based micromachined electrode composed of polypyrrole (PPy)/Nafion^® film and electrochemically deposited Pt nanocatalysts was prepared for the microfabricated fuel cell applications. In addition to its high surface area to host nanocatalyst particles, the PPy/Nafion^® composite film offers good electron and proton conductivity and the fabrication of such films is largely compatible with the micromachining process. The resulted catalyzed microelectrodes exhibit high electrochemical active surface area and high catalyst utilization. The corresponding Si-based micro membrane–electrode-assembly demonstrates good cell polarization characteristics using the H₂/O₂ feed.     

Solid-state ion selective electrode based on polypyrrole conducting polymer nanofilm as a new potentiometric sensor for Zn2+ ion

A pencil graphite electrode (PGE) electrodeposited by a polypyrrole conducting polymer doped with tartrazine (termed as PGE/PPy/Tar) was prepared and used as a zinc (II) solid-state ion-selective electrode. For the preparation of the zinc sensor electrode, electrodeposition of a polypyrrole nanofilm was carried out potentiostatically (E _app?=?0.75 V vs SCE) in a solution containing 0.010 M pyrrole and 0.001 M tartrazine trisodium salt. A pencil graphite and Pt wire were used as working and auxiliary electrodes, respectively. The introduced electrode in the current paper can be fabricated simply and was found to possess high selectivity, exhibited wide working concentration range, sufficiently rapid response, potential stability, and very good sensitivity to Zn (II) ion. The sensor electrode showed a linear Nernstian response over the range of 1.0?×?10^?5 to 1.0?×?10^?1 M with a slope of 28.23 mV per decade change in zinc ion concentration. A detection limit of 8.0?×?10^?6 M was obtained. The optimum pH working of the electrode was found to be 5.0.     

Development and Application of a Voltammetric Biosensor Based on Polypyrrole/uricase/graphene for Uric Acid Determination

In the present paper a voltammetric biosensor based on platinum electrode and polypyrrole / uricase / graphene composite, named Pt‐PPy/UOx/Grap has been successfully prepared and validated for uric acid (UA) determination in real samples. The electrochemical behavior has been evaluated by employing cyclic voltammetric. The charge transfer coefficient, α, and the charge transfer rate constant, κs, for electron transfer between PPy/UOx/Grap and Pt were calculated as 0.71, in average, and 48.3 s⁻¹, respectively. Under optimized conditions for the composite (2.5 U mL⁻¹ of UOx, 0.3 mg mL⁻¹ of Grap and 2.9 mmol L⁻¹ of pyrrole) and for the analysis (pH 7.0 in 0.1 mol L⁻¹ phosphate buffer) the method was validated for UA determination at human urine and showed good linearity to UA from 2 up to 24 nmol L⁻¹ (r=0.993) with limits of detection and quantification of 0.541 and 1.805 nmol L⁻¹, respectively. The results obtained for the UA determination at urine presented a relative error lower than 5 %, showing the good performance of the method developed and potential application in UA clinical analysis.     

Amperometric Study of Au‐Colloid Function on Xanthine Biosensor Based on Xanthine Oxidase Immobilized in Polypyrrole Layer

Four kinds of xanthine oxidase (XOD) based amperometric biosensors were fabricated and their analytical performances were compared. Polypyrrole (PPY)/XOD biosensor was constructed by electrochemical oxidation of pyrrole in the solution containing xanthine oxidase and pyrrole in this paper. Colloidal Au was then immobilized on the biosensor. On the other hand, electron mediator, Prussian Blue (PB), was deposited on the electrode before the immobilization of PPY/XOD to enhance electron‐transfer rate and current response. The results showed that PPY/XOD, PPY/XOD/Au‐colloid, PB/PPY/XOD and PB/PPY/XOD/Au‐colloid biosensors exhibit good response to xanthine in 1×10^?6 M and 2×10^?5 M and Michaelis‐Menten constants (K_m) of these biosensors were 242.2, 113.4, 144.5, 43.2 μmol?L^?1, respectively. The dependence of current responses with applied voltages was discussed, and different mechanisms of these biosensors were discussed. It has been found that colloidal Au can enhance the current response at the same concentration of xanthine solution and decrease the energy‐barrier of electron‐transfer reaction on the electrode.     

Impact of polypyrrole incorporation on nickel oxide@multi walled carbon nanotube composite for application in supercapacitors

In this article we report the synthesis of polypyrrole incorporated nickel oxide multi walled carbon nanotube (NiO@NMWCNT/PPy) composites by thermal reduction protocol for supercapacitor applications. The structural and morphological properties of the composites were confirmed by the aid of X-ray diffraction (XRD), Field-emission scanning electron microscope (FE-SEM) with energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS) and Field-emission transmission electron microscopy (FE-TEM) analysis indicating the hexagonal crystal structure of NiO decorated on NMWCNT/Ppy. The electrochemical characteristics of the NiO@MWCNT/PPy composite were analyzed in the presence of 2 M KOH as an electrolyte. The NiO@NMWCNT/PPy nanostructured composite produced a plenty of active sites for ion migration reactions that facilitate the energy storage mechanism. As a proof of concept demonstration, the NiO@NMWCNT/PPy composite was explored as an electrode materials in supercapacitor and exhibited specific capacitance of 395 F g⁻¹ and cyclic stability up to 5000 cycles at 0.5 A g⁻¹. Enhanced performance of composite is attributed to the incorporation of polypyrrole in NiO@NMWCNT. The improved capacitance and cyclic stability demonstrated by the composite indicates the NiO@NMWCNT/PPy to be a promising candidate for supercapacitor applications.     

Impedance DNA Biosensor Using Electropolymerized Polypyrrole/Multiwalled Carbon Nanotubes Modified Electrode

In this paper, we present an electrochemical impedance‐based DNA biosensor by using a composite material of polypyrrole (PPy) and multiwalled carbon nanotubes (MWNTs) to modify glassy carbon electrode (GCE). The polymer film was electropolymerized onto GCE by cyclic voltammetry (CV) in the presence of carboxylic groups ended MWNTs (MWNTs‐COOH). Such electrode modification method is new for DNA hybridization sensor. Amino group ended single‐stranded DNA (NH₂‐ssDNA) probe was linked onto the PPy/MWNTs‐COOH/GCE by using EDAC, a widely used water‐soluble carbodiimide for crosslinking amine and carboxylic acid group. The hybridization reaction of this ssDNA/PPy/MWNTs‐COOH/GCE resulted in a decreased impedance, which was attributed to the lower electronic transfer resistance of double‐stranded DNA than single‐stranded DNA. As the result of the PPy/MWNTs modification, the electrode obtained a good electronic transfer property and a large specific surface area. Consequently, the sensitivity and selectivity of this sensor for biosensing DNA hybridization were improved. Complementary DNA sequence as low as 5.0×10^?12 mol L^?1 can be detected without using hybridization marker or intercalator. Additionally, it was found that the electropolymerization scan rate was an important factor for DNA biosensor fabrication. It has been optimized at 20 mV s^?1.     

Development of In Situ Electrochemical Scanning Electron Microscopy with Ionic Liquids as Electrolytes

We report that ionic liquids (ILs) can be observed by electron microscopy without any charging of the liquid. Based on this, we present an in situ electrochemical scanning electron microscopy (in situ ECSEM) system. The key technology that enables in situ ECSEM is that charges can be removed from an IL by grounding it with a Pt wire, even if the IL is in an insulating glass cell. As a first demonstration, we describe the redox reaction of a polypyrrole (PPy) film accompanied by changes in its thickness when it is polarized by the film‐deposited Pt electrode in the IL. Furthermore, energy‐dispersive X‐ray fluorescence (EDX) analysis can be employed for the electrode polarized in the IL. The component analysis by EDX of PPy in an IL containing K⁺ as a marker, reveals doping of electrolyte cations into the PPy film upon the latter′s reduction and dedoping of cations from the film upon oxidation.     

Electrochemical capacitance performance of polypyrrole–titania nanotube hybrid

In this study, the polypyrrole–titania nanotube hybrid has been synthesized for an electrochemical supercapacitor application. The highly ordered and independent titania nanotube array is fabricated by an electro-oxidation of titanium sheet through an electrochemical anodization process in an aqueous solution containing ammonium fluoride, phosphoric acid and ethylene glycol. The polypyrrole–titania nanotube hybrid is then prepared by electrodepositing the conducting polypyrrole into well-aligned titania nanotubes through a normal pulse voltammetry deposition process in an organic acetonitrile solution containing pyrrole monomer and lithium perchlorate. The morphology and microstructure of polypyrrole–titania nanotube hybrid are characterized by scanning electron microscopy, infrared spectroscopy and Raman spectroscopy. The electrochemical capacitance performance is determined by cyclic voltammetry and charge/discharge measurement. It indicates that the polypyrrole film can been uniformly deposited on both surfaces of titania nanotube walls, demonstrating a heterogeneous coaxial nanotube structure. The specific capacitance of polypyrrole–titania nanotube hybrid is determined to be 179?F?g^?1 based on the polypyrrole mass. The specific energy and specific power are 7.8?Wh?kg^?1 and 2.8?kW?kg^?1 at a constant charge/discharge current of 1.85?mA?cm^?2, respectively. The retained specific capacitance still keeps 85% of the initial capacity even after 200 cycle numbers. This result demonstrates the satisfying stability and durability of PPy–TiO₂ nanotube hybrid electrode in a cyclic charge/discharge process. Such a composite electrode material with highly ordered and coaxial nanotube hybrid structure can contribute high energy storage for supercapacitor applications.     

Electroconductive Hydrogels: Electrical and Electrochemical Properties of Polypyrrole‐Poly(HEMA) Composites

Composites of inherently conductive polypyrrole (PPy) within highly hydrophilic poly(2‐hydroxyethyl methacrylate)‐based hydrogels (p(HEMA)) have been fabricated and their electrochemical properties investigated. The electrochemical characteristics observed by cyclic voltammetry suggest less facile reduction of PPy within the composite hydrogel compared to electropolymerized PPy, as shown by the shift in the reduction peak potential from ?472 mV for electropolymerized polypyrrole to ?636 mV for the electroconductive composite gel. The network impedance magnitude for the electroconductive hydrogel remains quite low, ca. 100 Ω, even upon approach to DC, over all frequencies and at all offset potentials suggesting retained electronic (bipolaronic) conductivity within the composite. In contrast, sustained application of +0.7 V (vs. Ag/AgCl, 3 M Cl^?) for typically 100 min. (conditioning) to reduce the background amperometric current to <1.0 μA, resulted in complete loss of electroactivity. Nyquist plots suggest that sustained application of such a modest potential to the composite hydrogel results in impedance characteristics that resembles p(HEMA) without evidence of the conducting polymer component. PPy composite gels supported a larger ferrocene monocarboxylate diffusivity (D_appt=7.97×10^?5 cm² s^?1) compared to electropolymerized PPy (D_appt=5.56×10^?5 cm² s^?1), however a marked reduction in diffusivity (D_appt=1.01×10^?5 cm² s^?1) was observed with the conditioned hydrogel composite. Cyclic voltammograms in buffer containing H₂O₂ showed an absence of redox peaks for electrodes coated with PPy‐containing membranes, suggesting possible chemical oxidation of polypyrrole by the oxidant     

Cockle shell-derived nanoparticles for optical urea biosensor development based on reflectance transduction

An optical biosensor for urea based on urease enzyme immobilised on functionalised calcium carbonate nanoparticles (CaCO₃-NPs) was successfully developed in this study. CaCO₃-NPs were synthesised from discarded cockle shells via a simple and eco-friendly approach, followed by surface functionalisation with succinimide ester groups. The fabricated biosensor is comprised of two layers. The first (bottom layer) contained functionalised NPs covalently immobilised to urease, and the second (uppermost layer) was alginate hydrogel physically immobilised to the pH indicator phenolphthalein. The biosensor provided a colorimetric indication of increasing urea concentrations by changing from colourless to pink. Quantitative urea analysis was performed by measuring the reflectance intensity of the colour change at a wavelength of 633.16 nm. The determination of urea concentration using this biosensor yielded a linear response range of 30–1000 mM (R² = 0.9901) with a detection limit of 17.74 mM at pH 7.5. The relative standard deviation of reproducibility was 1.14%, with no signs of interference by major cations, such as K⁺, Na⁺, NH?⁺, and Mg²⁺. The fabricated biosensor showed no significant difference with the standard method for the determination of urea in urine samples.     

Urea Biosensor Based on Electrochromic Properties of Prussian Blue

Prussian blue (PB) is an electrochromic material, which can be used as a signal transducer in the formation of optical urea biosensors. The previous researches in electrochromic properties of PB demonstrated the optical PB response to ammonium ions, which occurs when ammonium ions are interacting with PB layer at a constant 0.2 V vs Ag|AgCl|KCl_sat potential. In this work PB optical dependence on ammonium ions concentration was applied in the formation of electrochromic urea biosensor. Biosensor was formed by modifying the optically transparent indium tin oxide (ITO) coated glass electrode (glass/ITO) with Prussian blue layer and immobilizing urease (glass/ITO/PB‐urease). Calibration curve showed the linear dependency (R²=0.995) between the change of maximal absorbance (ΔA) and urea concentration in concentration range varying from 3 mM to 30 mM. The highest sensitivity (4 ΔA M^?1) of glass/ITO/PB‐urease biosensor is in the concentration range from 7 mM to 30 mM. It was determined that working principle of the glass/ITO/PB‐urease biosensor is not related to pH changes occurring during enzymatic hydrolysis of urea.     

Application of Different Zeolites for Improvement of the Characteristics of a pH‐FET Biosensor Based on Immobilized Urease

Different modifications of the zeolites Na⁺‐Beta and LTA were applied for improving the working characteristics of a urea biosensor. The bioselective membrane of the biosensor was based on urease and different zeolites co‐immobilized with bovine serum albumin on the surface of a pH‐FET. It was shown that the biosensors modified with the zeolites H⁺‐Beta30 and H⁺‐Beta50 are characterized by increased sensitivity to urea. The influence of the zeolite concentration on the sensitivity of the biosensors was studied. The optimal concentration of the zeolites H⁺‐Beta30 and H⁺‐Beta50 in the bioselective membrane was 15 %. Different variants of co‐immobilization of urease and zeolite H⁺‐Beta30 were studied and the optimal method was selected. Thus, a general conclusion is that the urea biosensor sensitivity can be improved using zeolite H⁺‐Beta30 for urease immobilization in the bioselective membrane.     

Pt and Pt–Ru nanoparticles decorated polypyrrole/multiwalled carbon nanotubes and their catalytic activity towards methanol oxidation

Conducting polymer composite films comprised of polypyrrole (PPy) and multiwalled carbon nanotubes (MWCNTs) [PPy–CNT] were synthesized by in situ polymerization of pyrrole on carbon nanotubes in 0.1 M HCl containing (NH₄)S₂O₈ as oxidizing agent over a temperature range of 0–5 °C. Pt nanoparticles are deposited on PPy–CNT composite films by chemical reduction of H₂PtCl₆ using HCHO as reducing agent at pH = 11 [Pt/PPy–CNT]. The presence of MWCNTs leads to higher activity, which might be due to the increase of electrochemically accessible surface areas, electronic conductivity and easier charge-transfer at polymer/electrolyte interfaces allowing higher dispersion and utilization of the deposited Pt nanoparticles. A comparative investigation was carried out using Pt–Ru nanoparticles decorated PPy–CNT composites. Cyclic voltammetry demonstrated that the synthesized Pt–Ru/PPy–CNT catalysts exhibited higher catalytic activity for methanol oxidation than Pt/PPy–CNT catalyst. Such kinds of Pt and Pt–Ru particles deposited on PPy–CNT composite polymer films exhibit excellent catalytic activity and stability towards methanol oxidation, which indicates that the composite films is more promising support material for fuel cell applications.