Amperometric Glucose Biosensor Based on Glucose Oxidase Encapsulated in Carbon Nanotube–Titania–Nafion Composite Film on Platinized Glassy Carbon Electrode

A highly sensitive and selective glucose biosensor has been developed based on immobilization of glucose oxidase within mesoporous carbon nanotube–titania–Nafion composite film coated on a platinized glassy carbon electrode. Synergistic electrocatalytic activity of carbon nanotubes and electrodeposited platinum nanoparticles on electrode surface resulted in an efficient reduction of hydrogen peroxide, allowing the sensitive and selective quantitation of glucose by the direct reduction of enzymatically‐liberated hydrogen peroxide at ?0.1 V versus Ag/AgCl (3 M NaCl) without a mediator. The present biosensor responded linearly to glucose in the wide concentration range from 5.0×10^?5 to 5.0×10^?3 M with a good sensitivity of 154 mA M^?1cm^?2. Due to the mesoporous nature of CNT–titania–Nafion composite film, the present biosensor exhibited very fast response time within 2 s. In addition, the present biosensor did not show any interference from large excess of ascorbic acid and uric acid.     

Amperometric Tyrosinase Biosensor Based on Carbon Nanotube–Titania–Nafion Composite Film

A highly sensitive and stable amperometric tyrosinase biosensor has been developed based on multiwalled carbon nanotube (MWCNT) dispersed in mesoporous composite films of sol–gel‐derived titania and perfluorosulfonated ionomer (Nafion). Tyrosinase was immobilized within a thin film of MWCNT–titania–Nafion composite film coated on a glassy carbon electrode. Phenolic compounds were determined by the direct reduction of biocatalytically‐liberated quinone species at ?100 mV versus Ag/AgCl (3 M NaCl) without a mediator. The present tyrosinase biosensor showed good analytical performances in terms of response time, sensitivity, and stability compared to those obtained with other biosensors based on different sol–gel matrices. Due to the large pore size of the MWCNT–titania–Nafion composite, the present biosensor showed remarkably fast response time with less than 3 s. The present biosensor responds linearly to phenol from 1.0×10^?7 M to 5.0×10^?5 M with an excellent sensitivity of 417 mA/M and a detection limit of 9.5×10^?8 M (S/N=3). The enzyme electrode retained 89% of its initial activity after 2 weeks of storage in 50 mM phosphate buffer at pH 7.0.     

Electrogenerated Chemiluminescence Ethanol Biosensor Based on Carbon Nanotube‐Titania‐Nafion Composite Film

Mesoporous titania‐Nafion composite doped with carbon nanotube (CNT) has been used for the immobilization of tris(2,2′‐bipyridyl)ruthenium(II) (Ru(bpy)₃²⁺) and alcohol dehydrogenase on an electrode surface to yield a highly sensitive and stable electrogenerated chemiluminescence (ECL) ethanol biosensor. The presence of CNT in the composite film increases not only the sensitivity of the ECL biosensor but also the long‐term stability of the biosensor. The present biosensor responds linearly to ethanol in the wide concentration ranges from 1.0×10^?5 M to 1.0×10^?1 M with a detection limit of 5.0×10^?6 M (S/N=3). The present ECL ethanol biosensor exhibited higher ECL response compared to that obtained with the ECL biosensor based on the corresponding composite without CNT. The present CNT‐based ECL biosensor showed good long‐term stability with 75% of its initial activity retained after 2 weeks of storage in 50 mM phosphate buffer at pH 7.0.     

Electrochemical Behavior of 8‐Azaguanine at DNA Langmuir–Blodgett Modified Glassy Carbon Electrode and Its Analytical Application

A highly sensitive electrochemical biosensor for the detection of trace amounts of 8‐azaguanine has been designed. Double stranded (ds)DNA molecules are immobilized onto a glassy carbon electrode surface with Langmuir–Blodgett technique. The adsorptive voltammetric behaviors of 8‐azaguanine at DNA‐modified electrode were explored by means of cyclic voltammetry and square wave voltammetry. Compared with bare glassy carbon electrode (GCE), the Langmuir–Blodgett film modified electrode can greatly improve the measuring sensitivity of 8‐azaguanine. Under the optimum experimental conditions, the Langmuir–Blodgett film modified electrode in pH 3.0 Britton–Robinson buffer solutions shows a linear voltammetric response in the range of 5.0×10^?8 to 1.0×10^?5 mol L^?1 with detection limit 9.0×10^?9 mol L^?1. The method proposed was applied successfully for the determination of 8‐azaguanine in diluted human urine with wonderful satisfactory.     

Tris(2,2′‐bipyridyl)ruthenium(II) Electrogenerated Chemiluminescence Sensor Based on Platinized Carbon Nanotube–Zirconia–Nafion Composite Films

Mesoporous films of platinized carbon nanotube–zirconia–Nafion composite have been used for the immobilization of tris(2,2′‐bipyridyl)ruthenium (II) (Ru(bpy)₃²⁺) on an electrode surface to yield a solid‐state electrogenerated chemiluminescence (ECL) sensor. The composite films of Pt–CNT–zirconia–Nafion exhibit much larger pore diameter (3.55 nm) than that of Nafion (2.82 nm) and thus leading to much larger ECL response for tripropylamine (TPA) because of the fast diffusion of the analyte within the films. Due to the conducting and electrocatalytic features of CNTs and Pt nanoparticles, their incorporation into the zirconia–Nafion composite films resulted in the decreased electron transfer resistance within the films. The present ECL sensor based on the Pt–CNT–zirconia–Nafion gave a linear response (R²=0.999) for TPA concentration from 3.0 nM to 1.0 mM with a remarkable detection limit (S/N=3) of 1.0 nM, which is much lower compared to those obtained with the ECL sensors based on other types of sol‐gel ceramic–Nafion composite films such as silica–Nafion and titania–Nafion.     

Simultaneous Determination of Thallium and Lead on a Chemically Modified Electrode with Langmuir– Blodgett Film of a p‐tert‐Butylcalix[4]arene Derivative

A new voltammetric sensor, Langmuir–Blodgett (LB) film of a p‐tert‐butylcalix[4]arene derivative modified glassy carbon electrode, was designed and successfully used in simultaneous determination of Tl⁺ and Pb²⁺ by square‐wave anodic stripping voltammetry. Under the optimum experimental conditions, this newly developed sensor reveal good linear response for Tl⁺ and Pb²⁺ in the concentration range of 3×10^?8–4×10^?6 mol L^?1 and 2×10^?7–2×10^?5 mol L^?1 respectively. The detect limits are 2×10^?8 mol L^?1 for Tl⁺ and 8×10^?8 mol L^?1 for Pb²⁺. Using proposed method, Tl⁺ and Pb²⁺ in environment samples were determined with satisfactory results.     

Detection of NADH and ethanol at a graphite electrode modified with titania sol-gel/Meldola’s Blue/MWCNT/Nafion nanocomposite film

For electrocatalytic determination of NADH, a graphite electrode modified with titania sol-gel/Meldola’s Blue/MWCNT/Nafion nanocomposite was proposed. The composition of the matrix film was optimised in terms of the content of carbon nanotubes and Nafion. Incorporation of a redox mediator, Meldola’s Blue, into the nanocomposite film enabled electrocatalytic determination of NADH at a low potential, −50 mV. For determination of ethanol, alcohol dehydrogenase (ADH) was immobilized into the matrix layer. Experimental conditions affecting the biosensor response were examined, including enzyme loading, temperature of measurement and pH of background electrolyte. Assessments of the analytical characteristics of the biosensor were performed with respect to sensitivity, limit of detection, operational stability, repeatability and reproducibility. The proposed biosensor showed electrocatalytic activity toward oxidation of ethanol with sensitivity of 2.24 μA L mmol⁻¹, linear range from 0.05 to 1.1 mmol L⁻¹, and limit of detection of 25 μmol L⁻¹. The apparent Michaelis-Menten constant was 1.24 mmol L⁻¹, indicating a high biological affinity of ADH/titania sol-gel/Meldola’s Blue/MWCNT/Nafion electrode for ethanol. The developed biosensor was tested in determinations of ethanol content in alcoholic beverages.     

One‐Step Electrodeposited Carbon Nanotube/Zirconia/Myoglobin Film for Direct Electron Transfer and Electrocatalysis

A nanobiocomposite film consisted of zirconia, multiwalled carbon nanotubes (MWNTs) and Myoglobin (Mb) was electrochemically deposited on the electrode. Direct electron transfer for the immobilized Mb was realized and high electrocatalytic efficiency toward H₂O₂ was observed. The proposed biosensor via a simple one‐step electrodeposition method displayed a broader linear range and a lower detection limit for H₂O₂, as compared with those CNT or ZrO₂ based biosensor. The linear range is from 2 × 10^?6 M to 1 × 10^?3 M with the detection limit of 6 × 10^?7 M. The present strategy provides a simple and effective method to assemble CNT, ZrO₂ and enzyme nanohybrid on the electrode and expands the scope of CNT‐based electrochemical devices.     

Sensitive and Selective Sensing of Hydrogen Peroxide with Iron‐Tetrasulfophthalocyanine‐Graphene‐Nafion Modified Screen‐Printed Electrode

We developed a novel iron‐tetrasulfophthalocyanine‐graphene‐Nafion (FeTSPc‐GR‐Nafion) modified screen‐printed electrode to determine hydrogen peroxide (H₂O₂) with high sensitivity and selectivity. The nanocomposite film (FeTSPc‐GR‐Nafion) exhibits an excellent electrocatalytic activity towards oxidation of H₂O₂ at a potential of +0.35 V in the absence of enzyme. A comparative study reveals that the FeTSPc‐GR complexes play a dual amplification role. Amperometric experiment indicates that the sensors possess good sensitivity and selectivity, with a linear range from 2.0×10^?7 M to 5.0×10^?3 M and a detection limit of 8.0×10^?8 M. This sensor has been successfully used to develop the glucose biosensor and has also been applied to determine H₂O₂ in sterile water.     

Fabrication of Tyrosinase Biosensor Based on Multiwalled Carbon Nanotubes‐Chitosan Composite and Its Application to Rapid Determination of Coliforms

A tyrosinase (Tyr) biosensor was fabricated by immobilizing Tyr on the surface of multiwalled carbon nanotubes (MWNTs)‐chitosan (Chit) composite modified glassy carbon electrode (GCE). The MWNTs‐Chit composite film provided a biocompatible platform for the Tyr to retain the bioactivity and the MWNTs possessed excellent inherent conductivity to enhance the electron transfer rate. The Tyr/MWNTs‐Chit/GCE biosensor showed high sensitivity (412 mA/M), broad linear response (1.0×10^?8–2.8×10^?5 M), low detection limit (5.0 nM) and good stability (remained 93% after 10 days) for determination of phenol. The biosensor was further applied to rapid detection of the coliforms, represented by Escherichia coli (E. coli) in this work. The current responses were proportional to the quantity of coliforms in the range of 10⁴–10⁶ cfu/mL. After 5.0 h of incubation, E. coli could be detected as low as 10 cfu/mL.     

A Fast Response Strontium Ion‐Selective Electrode Prepared by Sol‐Gel Membrane Technique

A sol‐gel electrode, based on 6‐(4‐nitrophenyl)‐2‐phenyl‐4,4‐dipropyl‐3,5‐diaza‐bicyclo [3,1,0] hex‐2‐ene (NPDBH) as a neutral ionophore, was successfully developed for the detection of Sr²⁺ in aqueous solutions. Theoretical calculations confirmed NPDBH selectivity toward strontium in comparison with some other metal ions. The electrode responds to Sr²⁺ ion with a sensitivity of 29.1±0.4 mV/decade over the range 8.0×10^?7–1.0×10^?1 M. Selectivity coefficients determined by matched potential method (MPM) indicate high selectivity for strontium ions. The electrode has a fast response time of 11 s and a working pH range of 3.0–10.0. The sol‐gel electrode shows detection limit of 7.5×10^?8 M.     

Electrogenerated Chemiluminescence Determination of Dopamine and Epinephrine in the Presence of Ascorbic Acid at Carbon Nanotube/Nafion‐Ru(bpy)$\rm{ {_{3}^{2+}}}$ Composite Film Modified Glassy Carbon Electrode

Based on the inhibition effect of dopamine and epinephrine on Ru(bpy) ‐tripropylamine electrogenerated chemiluminescence system, the excellent properties of carbon nanotube, and the cation permselectivity of Nafion film, an electrogenerated chemiluminescence inhibition method for determination of dopamine and epinephrine in the presence of ascorbic acid at carbon nanotube/Nafion‐Ru(bpy) composite film modified glassy carbon electrode was described. The results showed that the proposed method was sensitive and selective for the determination of dopamine and epinephine. The linear calibration range was from 1.6×10^?9 M to 3.2×10^?5 M and 5×10^?8 M to 6×10^?5 M for dopamine and epinephrine, respectively. 200‐fold excess of ascorbic acid did not interfere with the determination of 1 μM dopamine and epinephrine.     

Direct Electrochemistry and Electrocatalysis of Hemoglobin Based on Nafion‐Room Temperature Ionic Liquids‐Multiwalled Carbon Nanotubes Composite Film

A robust and effective composite film combined the benefits of Nafion, room temperature ionic liquid (RTIL) and multi‐wall carbon nanotubes (MWNTs) was prepared. Hemoglobin (Hb) was successfully immobilized on glassy carbon electrode surface by entrapping in the composite film. Direct electrochemistry and electrocatalysis of immobilized Hb were investigated in detail. A pair of well‐defined and quasi‐reversible redox peaks of Hb was obtained in 0.10 mol·L^?1 pH 7.0 phosphate buffer solution (PBS), indicating that the Nafion‐RTIL‐MWNTs film showed an obvious promotion for the direct electron transfer between Hb and the underlying electrode. The immobilized Hb exhibited an excellent electrocatalytic activity towards the reduction of H₂O₂. The catalysis current was linear to H₂O₂ concentration in the range of 2.0×10^?6 to 2.5×10^?4 mol·L^?1, with a detection limit of 8.0×10^?7 mol·L^?1 (S/N=3). The apparent Michaelis‐Menten constant (K_m^app) was calculated to be 0.34 mmol·L^?1. Moreover, the modified electrode displayed a good stability and reproducibility. Based on the composite film, a third‐generation reagentless biosensor could be constructed for the determination of H₂O₂.     

Electrochemical Biosensing Platforms Using Phthalocyanine‐Functionalized Carbon Nanotube Electrode

Iron‐phthalocyanines (FePc) are functionalized at multi‐walled carbon nanotubes (MWNTs) to remarkably improve the sensitivity toward hydrogen peroxide. We constructed a highly sensitive and selective glucose sensor on FePc‐MWNTs electrode based on the immobilization of glucose oxidase (GOD) on poly‐o‐aminophenol (POAP)‐electropolymerized electrode surface. SEM images indicate that GOD enzymes trapped in POAP film tend to deposit primarily on the curved tips and evenly disperse along the sidewalls. The resulting GOD^@POAP/FePc‐MWNTs biosensor exhibits excellent performance for glucose with a rapid response (less than 8 s), a wide linear range (up to 4.0×10^?3 M), low detection limits (2.0×10^?7 M with a signal‐to‐noise of 3), a highly reproducible response (RSD of 2.6%), and long‐term stability (120 days). Such characteristics may be attributed to the catalytic activity of FePc and carbon nanotube, permselectivity of POAP film, as well as the large surface area of carbon nanotube materials.     

Strong Lithium Polysulfide Chemisorption on Electroactive Sites of Nitrogen‐Doped Carbon Composites For High‐Performance Lithium–Sulfur Battery Cathodes

Despite the high theoretical capacity of lithium–sulfur batteries, their practical applications are severely hindered by a fast capacity decay, stemming from the dissolution and diffusion of lithium polysulfides in the electrolyte. A novel functional carbon composite (carbon‐nanotube‐interpenetrated mesoporous nitrogen‐doped carbon spheres, MNCS/CNT), which can strongly adsorb lithium polysulfides, is now reported to act as a sulfur host. The nitrogen functional groups of this composite enable the effective trapping of lithium polysulfides on electroactive sites within the cathode, leading to a much improved electrochemical performance (1200 mAh g^?1 after 200 cycles). The enhancement in adsorption can be attributed to the chemical bonding of lithium ions by nitrogen functional groups in the MNCS/CNT framework. Furthermore, the micrometer‐sized spherical structure of the material yields a high areal capacity (ca. 6 mAh cm^?2) with a high sulfur loading of approximately 5 mg cm^?2, which is ideal for practical applications of the lithium–sulfur batteries.     

Multi‐walled Carbon Nanotubes Non‐covalently Functionalized with Polyarginine: A New Alternative for the Construction of Reagentless NAD+/Dehydrogenase‐based Ethanol Biosensor

This work reports for the first time the development of a reagentless enzymatic amperometric biosensor for ethanol based on the use of a glassy carbon electrode (GCE) modified with multi‐walled carbon nanotubes (MWCNTs) non‐covalently functionalized with polyarginine (Polyarg) as platform for the robust immobilization of alcohol dehydrogenase (ADH) and NAD⁺. The new strategy allows to obtain an integrated GCE/MWCNTs‐Polyarg/NAD⁺‐ADH ethanol biosensor with important advantages compared to the existing ethanol biosensors: avoids the external addition of the cofactor for each measurement, ensures a fast and sensitive quantification of ethanol due to the intimate interaction of the components, and allows the detection at considerably lower potentials due to the catalytic activity of the carbon nanostructures. These unique properties have made possible a very efficient ethanol quantification with a sensitivity of (1487±6) μA M^?1, detection limit of 0.65 μM, response time of 8 s, and reproducibility of 5.5 % with a very successful application for the quantification of ethanol in different commercial beverages.     

Analytical Biosensing of Hydrogen Peroxide on Brilliant Cresyl Blue/Multiwalled Carbon Nanotubes Modified Glassy Carbon Electrode

A cationic quinine‐imide dye brilliant cresyl blue (BCB) and horseradish peroxidase (HRP) were co‐immobilized within ormosil on multiwalled carbon nanotubes modified glassy carbon electrode for the fabrication of highly sensitive and selective hydrogen peroxide biosensor. The presence of epoxy group in ormosil as organic moiety improves the mechanical strength and transparency of the film and amino group provides biocompatible microenvironment for the immobilization of enzyme. The presence of MWCNTs improved the conductivity of the nanocomposite film. The surface characterization of MWCNT modified ormosil nanocomposite film was performed with scanning electron microscopy (SEM) and atomic force microscopy (AFM). Cyclic voltammetry and amperometry measurements were used to study and optimize the performance of the resulting peroxide biosensor. The apparent Michaelis–Menten constant was determined to be 1.5 mM. The proposed H₂O₂ biosensor exhibited wide linear range from 3×10^?7 to 1×10^?4 M, and low detection limit 1×10^?7 M (S/N=3) with fast response time <5 s. The probable interferences in bio‐matrix were selected to test the selectivity and no significant response was observed in the biosensor. This biosensor possessed good analytical performance and long term storage stability.     

Determination of Copper at a Glassy Carbon Electrode Modified with Langmuir–Blodgett Film of p‐tert‐Butylthiacalix[4]arene

A new type of voltammetric sensor, Langmuir–Blodgett film of p‐tert‐butylthiacalix[4]arene modified glassy carbon electrode, was advanced and used for determining copper at trace levels by differential pulse stripping voltammetry. Calibration plot was found to be linear in the range of 2×10^?8 M to 5×10^?6 M; the detection limit was 2×10^?9 M. Possible recognition mechanism was also discussed. From determination of Copper in real samples (river, lake and tap water) it can be concluded that the method is rapid, sensitive in determining of copper and can be used in the analysis of natural water samples.     

Disposable Amperometric Biosensor Based on Poly‐L‐lysine and Fe3O4 NPs‐chitosan Composite for the Detection of Tyramine in Cheese

An amperometric tyramine biosensor based on poly‐L‐lysine (PLL) and Fe₃O₄ nanoparticles (Fe₃O₄NP) modified screen printed carbon electrode (SPCE) was developed. PLL was formed on the SPCE by the electropolymerization of L‐lysine. Subsequently, Fe₃O₄NP suspension prepared in chitosan (CH) solution was casted onto the PLL/SPCE. Tyrosinase (Ty) enzyme was immobilized onto the modified Fe₃O₄?CH/PLL/SPCE and the electrode was coated with Nafion to fabricate the Ty/Fe₃O₄?CH/PLL/SPCE. Different techniques including scanning electron microscopy, chronoamperometry (i–t curve), cyclic voltammetry and electrochemical impedance spectroscopy were utilized to study the fabrication processes, electrochemical characteristics and performance parameters of the biosensor. The analytical performance of the tyramine biosensor was evaluated with respect to linear range, sensitivity, limit of detection, repeatability and reproducibility. The response of the biosensor to tyramine was linear between 4.9×10^?7–6.3×10^?5 M with a detection limit of 7.5×10^?8 M and sensitivity of 71.36 μA mM^?1 (595 μA mM^?1 cm^?2). The application of the developed biosensor for the determination of tyramine was successfully tested in cheese sample and mean analytical recovery of added tyramine in cheese extract was calculated as 101.2±2.1 %. The presented tyramine biosensor is a promising approach for tyramine analysis in real samples due to its high sensitivity, rapid response and easy fabrication.     

Amperometric Ethanol Biosensors Based on Chitosan‐NAD+‐Alcohol Dehydrogenase Films

The reagentless and oxygen‐independent biosensors for ethanol were developed based on the covalent immobilization of alcohol dehydrogenase (ADH) and its cofactor nicotinamide adenine dinucleotide (NAD⁺) on chitosan (CHIT) chains. The CHIT‐NAD⁺‐ADH structures were adsorbed onto carbon nanotubes (CNT) in order to provide a signal transduction based on the recycling of redox states of NAD cofactor at CNT (detection limit, 8–30 µM ethanol; dynamic range up to 20 mM). The CHIT‐NAD⁺‐dehydrogenase/CNT hybrid material represents a general approach to the development of dehydrogenases‐based electrochemical biosensors. Interestingly, the CHIT‐NAD⁺ solutions preserved their enzymatic activity even after five years of storage at 4 °C.