Nanostructured Iron Oxide Platform for Impedimetric Cholesterol Detection

A nanostructured iron oxide (NanoFe₃O₄, particle size ca. 25 nm and roughness ca. 21 nm) film deposited onto a hydrolyzed indium‐tin‐oxide (ITO) coated glass plate has been used to immobilize cholesterol oxidase (ChOx) to fabricate an impedimetric cholesterol sensor. Electrochemical studies reveal that surface charged Fe₃O₄ nanoparticles provide better conformation for ChOx loading resulting in enhanced electron transfer between ChOx and the electrode. Impedimetric response studies of the ChOx/NanoFe₃O₄/ITO bioelectrode exhibit improved linearity (2.5–400 mg/dL), low detection limit (0.25 mg/dL), fast response time (25 s), high sensitivity (86 Ω/mg dL^?1/cm^?2) and a low value of the Michaelis‐Menten constant (K_m, 0.8 mg/dL) with a regression coefficient of 0.997.     

Electrochemical Cholesterol Sensor Based on Tin Oxide‐Chitosan Nanobiocomposite Film

A chitosan (CS)‐tin oxide (SnO₂) nanobiocomposite film has been deposited onto an indium‐tin‐oxide glass plate to immobilize cholesterol oxidase (ChOx) for cholesterol detection. The value of the Michaelis–Menten constant (K_m) obtained as 3.8 mM for ChOx/CS‐SnO₂/ITO is lower (8 mM) than that of a ChOx/CS/ITO bioelectrode revealing enhancement in affinity and/or activity of ChOx towards cholesterol and also revealing strong binding of ChOx onto CS‐SnO₂/ITO electrode. This ChOx/CS‐SnO₂/ITO cholesterol sensor retains 95% of enzyme activity after 4–6 weeks at 4 °C with response time of 5 s, sensitivity of 34.7 μA/mg dL^?1 cm² and detection limit of 5 mg/dL.     

Zinc oxide nanoparticles-chitosan composite film for cholesterol biosensor

Zinc oxide nanoparticles (NanoZnO) uniformly dispersed in chitosan (CHIT) have been used to fabricate a hybrid nanocomposite film onto indium-tin-oxide (ITO) glass plate. Cholesterol oxidase (ChOx) has been immobilized onto this NanoZnO-CHIT composite film using physiosorption technique. Both NanoZnO-CHIT/ITO electrode and ChOx/NanoZnO-CHIT/ITO bioelectrode have been characterized using Fourier transform-infrared (FTIR), X-ray diffraction (XRD), cyclic voltammetry (CV), scanning electron microscopy (SEM) and electrochemical impedance spectroscopy (EIS) techniques, respectively. The ChOx/NanoZnO-CHIT/ITO bioelectrode exhibits linearity from 5 to 300 mg dl⁻¹ of cholesterol with detection limit as 5 mg dl⁻¹, sensitivity as 1.41 × 10⁻⁴ A mg dl⁻¹ and the value of Michaelis-Menten constant (K_m) as 8.63 mg dl⁻¹. This cholesterol biosensor can be used to estimate cholesterol in serum samples.     

Polypyrrole/multiwalled carbon nanotubes‐based biosensor for cholesterol estimation

The nanocomposite electrode comprising of polypyrrole (PPY) and carboxy functionalized multiwalled carbon nanotubes (MWCNT) has been electrochemically fabricated onto indium–tin–oxide (ITO) electrode using p‐toluene sulfonic acid (PTS). Cholesterol oxidase (ChOx) and cholesterol esterase (ChEt) have been immobilized onto this PPY– MWCNT/ITO nanocomposite electrode using N‐ethyl‐N‐(3‐dimethylaminopropyl) carbodiimide and N‐hydroxy succinimide chemistry for estimation of esterified cholesterol. The ChEt–ChOx/PPY–MWCNT/PTS/ITO bioelectrode has been characterized using Fourier transform infrared spectroscopy, electrochemical techniques, and scanning electron microscope. This ChEt–ChOx/PPY–MWCNT/PTS/ITO nanobioelectrode has a response time of about 9 s, linearity of 4 × 10^?4 to 6.5 × 10^?3 M/l of cholesterol oleate concentration, K_m of 0.02 mM, and thermal stability of upto 45°C. This electrode exhibits improved biosensing characteristics compared with other total cholesterol electrodes reported in literature till date and can be used to estimate cholesterol in blood serum samples. Copyright © 2011 John Wiley & Sons, Ltd.     

Nanostructured zirconium oxide based genosensor for Escherichia coli detection

A deoxyribonucleic acid (DNA) biosensor has been fabricated via immobilization of 17 base terminal single stranded DNA (ssDNA) identified from the 16s rRNA coding region of Escherichia coli onto sol–gel derived nanostructured zirconium oxide (NanoZrO₂) film. An oligonucleotide probe with a terminal 5′-phosphate group has been attached to the surface of the electrode via affinity of NanoZrO₂ for phosphate. The results of hybridization studies carried out with the complementary, non-complementary and genomic DNA reveal that ssDNA/NanoZrO₂/ITO bioelectrode has a high selectivity and sensitivity towards hybridization detection with limits of 10^?6–10⁶ pM of complementary DNA.     

Polyaniline nanotubes for impedimetric triglyceride detection

Polyaniline nanotubes (PANI-NT) based film electrophoretically deposited onto indium–tin–oxide (ITO) coated glass plate has been utilized for covalent immobilization of lipase (LIP), via glutaraldehyde (Glu), for triglyceride detection using impedimetric technique. It is shown that fatty acid molecules produced during triglyceride hydrolysis result in change in charge transfer resistance (R_CT) of PANI-NT film with varying triglyceride concentration. LIP/Glu/PANI-NT/ITO bioelectrode has linearity as 25–300 mg dL^?1, sensitivity as 2.59 × 10^?3 KΩ^?1 mg^?1 dL, response time as 20 s and regression coefficient as 0.99. A low value of apparent Michaelis–Menten constant (～0.62 mM) indicates high enzyme affinity to tributyrin. The LIP/Glu/PANI-NT/ITO bioelectrode has been utilized to estimate triglyceride in serum samples.     

Sol–gel derived nanoporous cerium oxide film for application to cholesterol biosensor

Cholesterol oxidase (ChOx) has been immobilized onto sol–gel derived nano-structured cerium oxide (NS-CeO₂) film deposited on indium-tin-oxide (ITO) coated glass substrate. Phase identification of sol–gel NS-CeO₂ film carried out using X-ray diffraction (XRD) yields reflection peak at 29.4° corresponding to (1 1 1) plane with oriented crystallite (34 nm) along c-axis normal to the substrate. Electrochemical studies reveal that NS-CeO₂ provides electroactive surface for the loading of ChOx and enhances electron transfer rate in the ChOx/NS-CeO₂/ITO bioelectrode. The low value of Michaelis–Menten constant (K_m) obtained as 2.08 mM indicates enhanced ChOx affinity to cholesterol. The observed results show application of sol–gel derived NS-CeO₂ for biosensing without any functionalization.     

Horse radish peroxidase immobilized polyaniline for hydrogen peroxide sensor

Horse radish peroxidase (HRP) has been electrochemically entrapped into perchlorate (ClO) doped polyaniline (PANI) film deposited onto indium‐tin‐oxide (ITO) coated glass plate. This HRP‐PANI‐ClO/ITO bioelectrode characterized using Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), UV‐Visible spectroscopy, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) techniques has been utilized for estimation of hydrogen peroxide (H₂O₂). This H₂O₂ sensor exhibits response time of 5 s, linearity from 3 to 136 mM, sensitivity as 0.5638 µA mM^?1 cm^?2 with linear regression of 0.985. The value of the Michaelis–Menten constant (K_m) has been obtained as 1.984 mM. Copyright © 2009 John Wiley & Sons, Ltd.     

Langmuir-Blodgett film based on MEH-PPV for cholesterol biosensor

Cholesterol oxidase (ChOx) has been immobilized onto conducting poly[2-methoxy,5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV)/stearic acid (SA) Langmuir-Blodgett film transferred onto octadecanethiol (ODT) modified gold plate. The ChOx/MEH-PPV/SA LB film bioelectrode exhibits has been characterized by FT-IR, contact angle, and atomic force microscopy. The response of the ChOx/MEH-PPV/SA LB film bioelectrode carried out using differential pulse voltammetry (DPV) studies reveal linearity from 1.29 to 12.91 mM of cholesterol concentration and response time as 30 s. This ChOx/MEH-PPV/SA bioelectrode exhibits values of correlation coefficient as 0.9939, standard deviation as 0.0029 μA and limit of detection as 1.66 mM. UV-visible spectrophotometer studies reveal that 5.2 × 10⁻³ U of ChOx are actively working per cm² area of ChOx/MEH-PPV/SA LB film bioelectrode and this bioelectrode is thermally stable upto 55 °C with reusability of about 60 times.     

Direct Voltammetry of Copper,Zinc‐Superoxide Dismutase Immobilized onto Electrodeposited Nickel Oxide Nanoparticles: Fabrication of Amperometric Superoxide Biosensor

Direct electron transfer of immobilized copper, zinc‐superoxide dismutase (SOD) onto electrodeposited nickel‐oxide (NiOx) nanoparticle modified glassy carbon (GC) electrode displays a well defined redox process with formal potential of ?0.03 V in pH 7.4. Cyclic voltammetry was used for deposition of (NiOx) nanoparticles and immobilization of SOD onto GC electrode. The surface coverage (Γ) and heterogeneous electron transfer rate constant (k_s) of immobilized SOD are 1.75×10^?11 mol cm^?2 and 7.5±0.5 s^?1, respectively. The biosensor shows a fast amperometric response (3 s) toward superoxide at a wide concentration range from 10 µM to 0.25 mM with sensitivity of 13.40 nA µM^?1 cm^?2 and 12.40 nA µM^?1 cm^?2, detection limit of 2.66 and 3.1 µM based on anodically and cathodically detection. This biosensor exhibits excellent stability, reproducibility and long life time.     

Polyaniline/Single‐Walled Carbon Nanotubes Composite Based Triglyceride Biosensor

Nanocomposite film comprising of polyaniline (PANI) and single walled carbon nanotubes (SWCNT) has been fabricated onto indium‐tin‐oxide (ITO) coated glass plate using electrophoretic technique. Co‐immobilization of glycerol dehydrogenase (GDH) and lipase (LIP) has been done via N‐ethyl‐N′‐(3‐dimethylaminopropyl) carbodiimide and N‐hydroxysuccinimide chemistry to explore its application for triglyceride (tributyrin) sensing. Response studies have been done using linear sweep voltammetry revealing that LIP‐GDH/PANI‐SWCNT‐TB/ITO bioelectrode can detect tributyrin in the range of 50 to 400 mg dL^?1 with low Michaelis–Menten constant of 1.138 mM, improved response time of 12 s, high sensitivity as 4.28×10^?4 mA mg^?1 dL and storage stability of about 13 weeks.     

Electroanalytical Responses of Arsenic Oxide,Methanol, and Oxygen at the Ruthenium Oxide–Hexachloroiridate with Platinum Hybrid Film

Electrochemically active ruthenium oxide (RuO_x?nH₂O), ruthenium oxide/hexachloroiridate (RuO_x?nH₂O/IrCl₆^2?), and ruthenium oxide/hexachloroiridate/platinum (RuO_x?nH₂O/IrCl₆^2?/Pt) hybrid films have been prepared from the mixture of Ru³⁺, IrCl₆^2?, and PtCl₆^2? ions in an acidic aqueous solution. The repetitive cyclic voltammetry (CV) has been used for the film preparation process. The electrochemical properties and the growth mechanism of the above mentioned different kinds of hybrid films have been investigated using CV and electrochemical quartz crystal microbalance. The morphological and quantitative analyses have been carried out using scanning electron microscopy, atomic force microscopy and energy dispersive X‐ray. Among these above mentioned films, RuO_x?nH₂O/IrCl₆^2?/Pt hybrid film exhibits promising electrocatalytic activity towards the oxidation of arsenic oxide, methanol and reduction of oxygen. Further, detailed study of electrocatalysis using rotating ring disk electrodes and amperometric methods have been carried out for arsenic oxide oxidation and oxygen reduction reactions at the hybrid films. From the results, the sensitivity of RuO_x?nH₂O/IrCl₆^2?/Pt hybrid film has been calculated for arsenic oxide as 0.7 mA mM^?1; and for oxygen as 1.8 mA mM^?1.     

Electrophoretically deposited polyaniline nanotubes based film for cholesterol detection

Polyaniline nanotube (PANI-NT) based films have been fabricated onto indium-tin-oxide (ITO) coated glass plates via electrophoretic technique. These PANI-NT/ITO electrodes have been utilized for covalent immobilization of cholesterol oxidase (ChOx) using glutaraldehyde (Glu) as cross-linker. Structural, morphological and electrochemical characterization of PANI-NT/ITO electrode and ChOx/Glu/PANI-NT/ITO bioelectrode have been done using FT-IR spectroscopy, SEM, electrochemical impedance spectroscopy and cyclic voltammetry techniques. Response studies of the ChOx/Glu/PANI-NT/ITO bioelectrode have been carried out using both linear sweep voltammetry and UV-Visible spectrophotometry. The results of the biosensing studies reveal that this bioelectrode can be used to detect cholesterol in wide detection range of 25-500 mg/dL with high sensitivity of 3.36 mA mg(-1) dL and fast response time of 30 s at pH 7.4. This bioelectrode exhibits very low value of Michaelis-Menten constant of 1.18 mM indicating enhanced interactions between cholesterol and ChOx immobilized onto this nanostructured PANI matrix.     

Nanostructured Cobalt(II) Tetracarboxyphthalocyanine Complex Supported Within the MWCNT Frameworks: Electron Transport and Charge Storage Capabilities

The electrochemical redox properties of a surface‐confined thin solid film of nanostructured cobalt(II) tetracarboxyphthalocyanine integrated with multiwalled carbon nanotube (nanoCoTCPc/MWCNT) have been investigated. This novel nanoCoTCPc/MWCNT material was characterized using SEM, TEM, zeta analysis and electrochemical methods. The nanoCoTCPc/MWCNT nanohybrid material exhibited an extra‐ordinarily high conductivity (15 mS cm^?1), which is more than an order of magnitude greater than that of the MWCNT‐SO₃H (527 µS cm^?1) and three orders of a magnitude greater than the nanoCoTCPc (4.33 µS cm^?1). The heterogeneous electron transfer rate constant decreases as follows: nanoCoTCPc/MWCNT (k_app≈19.73×10^?3 cm s^?1)>MWCNT‐SO₃H (k_app≈11.63×10^?3 cm s^?1)>nanoCoTCPc (k_app≈1.09×10^?3 cm s^?1). The energy‐storage capability was typical of pseudocapacitive behaviour; at a current density of 10 µA cm^?2, the pseudocapacitance decreases as nanoCoTCPc/MWCNT (3.71×10^?4 F cm^?2)>nanoCoTCPc (2.57×10^?4 F cm^?2)>MWCNT‐SO₃H (2.28×10^?4 F cm^?2). The new nanoCoTCPc/MWCNT nanohybrid material promises to serve as a potential material for the fabrication of thin film electrocatalysts or energy‐storage devices.     

Fabrication of a Sensitive Cholesterol Biosensor Based on Cobalt‐oxide Nanostructures Electrodeposited onto Glassy Carbon Electrode

Electrodeposited cobalt oxide (CoOx) nanomaterials are not only used for immobilization of cholesterol oxidase (ChOx) but also as electron transfer mediator for oxidation of H₂O₂ generated in the enzymatic reaction. Voltammetry and flow injection analysis (FIA) were used for determination of cholesterol. FIA determination of cholesterol with biosensors yielded a calibration curve with the following characteristics: linear range up to 50 μM, sensitivity of 43.5 nA μM^?1 cm^?2 and detection limit of 4.2 μM. The apparent Michaelis‐Menten constant and the response time of the biosensor are 0.49 mM and 15 s, respectively. This biosensor also exhibits good stability, reproducibility and long life time.     

Nanostructured polyaniline-coated anode for improving microbial fuel cell power output

An approach for improving the power generation of a dual-chamber microbial fuel cell by using a nanostructured polyaniline (PANI)-modified glassy carbon anode was investigated. Modification of the glassy carbon anode was achieved by the electrochemical polymerisation of aniline in 1 M H₂SO₄ solution. The MFC reactor showed power densities of 0.082 mW cm^?2 and 0.031 mW cm^?2 for the nano- and microstructured PANI anode, respectively. The results from electron microscopy scanning confirmed formation of the nanostructured PANI film on the anode surface and the results from electrochemical experiments confirmed that the electrochemical activity of the anode was significantly enhanced after modification by nanostructured PANI. Electrochemical impedance spectroscopic results proved that the charge transfer would be facilitated after anode modification with nanostructured PANI.     

Microstructural and Potential Dependence Studies of Urease-Immobilized Gold Nanoparticles–Polypyrrole Composite Film for Urea Detection

Gold nanoparticle–polypyrrole nanocomposite film was electrochemically deposited in a single-step polymerization of pyrrole in the presence of 3-mercaptopropionic acid (MPA)-capped gold nanoparticles (GNPs) and p-toluenesulfonic acid (pTSA) on the surface of an indium tin oxide (ITO)-coated glass plate. The carboxyl functional groups surrounding the GNPs within the polymer matrix were utilized for the immobilization of urease enzyme through carbodiimide coupling reaction for the construction of a Urs/GNP(MPA)–PPy/ITO-glass bioelectrode for urea detection in Tris–HCl buffer. The resulting bioelectrode film was characterized by atomic force microscopy (AFM), high-resolution transmission electron microscopy (HRTEM), contact angle measurement, Fourier transform infrared spectroscopy (FTIR), and electrochemical techniques. The potentiometric response of the bioelectrode made of polymer nanocomposite films of two different thicknesses prepared at 100 and 250 mC cm^?2 charge densities, respectively, was studied towards the urea concentration in Tris–HCl buffer (pH 7.4). The thin polymer nanocomposite film-based bioelectrode prepared at 100 mC cm^?2 charge density exhibited a comparatively good potentiometric response than a thick 250 mC cm^?2 charge density film with a linear range of urea detection from 0.01 to 10 mM with a sensitivity of 29.7 mV per decade.     

Facile Fabrication of a Graphene‐based Electrochemical Biosensor for Glucose Detection

A simple and effective glucose biosensor based on immobilization of glucose oxidase (GOD) in graphene (GR)/Nafion film was constructed. The results indicated that the immobilized GOD can maintain its native structure and bioactivity, and the GR/Nafion film provides a favorable microenvironment for GOD immobilization and promotes the direct electron transfer between the electrode substrate and the redox center of GOD. The electrode reaction of the immobilized GOD shows a reversible and surface‐controlled process with the large electron transfer rate constant (k_s) of 3.42±0.08 s^?1. Based on the oxygen consumption during the oxidation process of glucose catalyzed by the immobilized GOD, the as‐prepared GOD/GR/Nafion/GCE electrode exhibits a linear range from 0.5 to 14 mmol·L^?1 with a detection limit of 0.03 mmol·L^?1. Moreover, it displays a good reproducibility and long‐term stability.     

Peptide Nucleic Acid Immobilized Biocompatible Silane Nanocomposite Platform for Mycobacterium tuberculosis Detection

We have prepared nanocomposite films comprising of 3‐glycidoxypropyltrimethoxysilane (GOPS) and iron‐oxide (Fe₃O₄) onto indium‐tin‐oxide (ITO) glass plate for covalent immobilization of 21‐mer peptide nucleic acid (PNA). These films have been characterized using contact angle, atomic force microscopy (AFM), electrochemical techniques. The electrochemical response of the GOPS/ITO and Fe₃O₄‐GOPS/ITO electrodes has been investigated by hybridization with complementary, non‐complementary and one‐base mismatch using methylene blue as electrochemical indicator. The PNA/Fe₃O₄‐GOPS/ITO bioelectrode exhibits improved specificity and detection limit (0.1 fM) as compared to that of the PNA‐GOPS/ITO bioelectrode (0.1 pM). This PNA/Fe₃O₄‐GOPS/ITO electrode can be utilized for detection of hybridization with the complementary sequence in sonicated M. tuberculosis genomic DNA within 90 s of hybridization time.     

Ionic Liquid‐Hemoglobin‐Carbon Paste Composite Bioelectrode and Its Electrocatalytic Activity

A new carbon ionic liquid paste bioelectrode was fabricated by mixing hemoglobin (Hb) with graphite powder, ionic liquid 1‐ethyl‐3‐methylimidazolium tetrafluoroborate (EMIMBF₄) and liquid paraffin homogeneously. Nafion film was cast on the electrode surface to improve the stability of bioelectrode. Direct electrochemistry of Hb in the bioelectrode was carefully investigated. Cyclic voltammetric results indicated that a pair of well‐defined and quasi‐reversible electrochemical responses appeared in pH 7.0 phosphate buffer solution (PBS), indicating that direct electron transfer of Hb was realized in the modified electrode. The formal potential (E^0′) was calculated as ?0.316 V (vs. SCE), which was the typical characteristic of the electrochemical reaction of heme Fe(III)/Fe(II) redox couple. Based on the cyclic voltammetric results the electrochemical parameters of the electrode reaction were calculated. This bioelectrode showed high electrocatalytic activity towards the reduction of trichloroacetic acid (TCA) with good stability and reproducibility.