Electrocatalytic Reduction of H2O2 and Oxygen on the Surface of Thionin Incorporated onto MWCNTs Modified Glassy Carbon Electrode: Application to Glucose Detection

A new H₂O₂ enzymeless sensor has been fabricated by incorporation of thionin onto multiwall carbon nanotubes (MWCNTs) modified glassy carbon electrode. First 50 μL of acetone solution containing dispersed MWCNTs was pipetted onto the surface of GC electrode, then, after solvent evaporations, the MWCNTs modified GC electrode was immersed into an aqueous solution of thionin (electroless deposition) for a short period of time <5–50 s. The adsorbed thin film of thionin was found to facilitate the reduction of hydrogen peroxide in the absence of peroxidase enzyme. Also the modified electrode shows excellent catalytic activity for oxygen reduction at reduced overpotential. The rotating modified electrode shows excellent analytical performance for amperometric determination of hydrogen peroxide, at reduced overpotentials. Typical calibration at ?0.3 V vs. reference electrode, Ag/AgCl/3 M KCl, shows a detection limit of 0.38 μM, a sensitivity of 11.5 nA/μM and a liner range from 20 μM to 3.0 mM of hydrogen peroxide. The glucose biosensor was fabricated by covering a thin film of sol–gel composite containing glucose oxides on the surface of thionin/MWCNTs modified GC electrode. The biosensor can be used successfully for selective detection of glucose based on the decreasing of cathodic peak current of oxygen. The detection limit, sensitivity and liner calibration rang were 1 μM, 18.3 μA/mM and 10 μM–6.0 mM, respectively. In addition biosensor can reach 90% of steady currents in about 3.0 s and interference effect of the electroactive existing species (ascorbic acid–uric acid and acetaminophen) is eliminated. The usefulness of biosensor for direct glucose quantification in human blood serum matrix is also discussed. This sensor can be used as an amperometric detector for monitoring oxidase based biosensors.     

Sensitive Superoxide Biosensor Based on Silicon Carbide Nanoparticles

Direct electron transfer of immobilized superoxide dismutase (Cu, Zn‐SOD) onto silicon carbide (SiC) nanoparticles displays a pair of well defined and nearly reversible redox peaks with formal potential (E°′) of −0.03 V in pH 7.4. The heterogeneous electron transfer rate constant (k_s) and surface coverage (Γ) of immobilized SOD are 11.0±0.4 s⁻¹ and 1.42×10⁻¹¹ mol cm⁻². Biosensor shows fast amperometric response (3s) with sensitivity and detection limit of 1.416 nA μM⁻¹, 1.66 μM, and 1.375 nA μM⁻¹, 2.1 μM for cathodically or anodically detection of superoxide, respectively. This biosensor also exhibits good stability, reproducibility and long life‐time.     

Sensitive Electrochemical Prostate Specific Antigen Aptasensor: Effect of Carboxylic Acid Functionalized Carbon Nanotube and Glutaraldehyde Linker

The performance of carboxylic acid functionalized carbon nanotubes (CNTs_(COOH)), chitosan (Chit), carbon nanotubes‐chitosan (CNTs‐Chit and CNTs_(COOH)‐Chit) for immobilizing of amino‐functionalized ssDNA and fabrication of electrochemical prostate specific antigen (PSA) aptasensor were studied in detail using X‐ray diffraction spectroscopy (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT‐IR) and electrochemical impedance spectroscopy (EIS). The assemblies of capture probe are formed on the surface via two approaches: EDC/NHS chemistry and glutaraldehyde linker. Cyclic voltammetry (CV), differential pulse voltammetry (DPV) and EIS techniques were used to investigate the analytical performance of the PSA aptasensor. Under optimum conditions the sensitivity of 0.0026 µA/(ng/ml) and a limit of detection of 0.75 ng/ml (22 pM) were obtained for PSA detection. This protocol offers a new means for sensitive detection of PSA with some advantages in terms of simplicity, selectivity, ease of use and regenerability.     

Electrodeposition of Cobalt Oxide Nanostructure on the Glassy Carbon Electrode for Electrocatalytic Determination of para‐Nitrophenol

Cobalt oxide nanostructure (CoO_xNS) deposited on the glassy carbon (GC) electrode surface is proposed as a novel electrocatalytic system for the reduction of para‐Nitrophenol. Cyclic voltammetry, electrochemical impedance spectroscopy, atomic force microscopy and scanning electron microscopy were used for characterization of deposited CoO_xNS. CoO_xNS deposited by cycling at positive potentials (0 to +1.3 V) show less charge‐transfer resistance (Rct) and more catalytic activity for the electroreduction of p‐nitrophenol compared to those CoO_xNS obtained by scanning the applied potential throughout a negative V range. The GC/CoO_xNS electrode showed good electrocatalytic activity toward the reduction of p‐nitrophenol at different pH values.     

Highly Sensitive Electrochemical Hydrogen Peroxide Sensor Based on Iron Oxide‐Reduced Graphene Oxide‐Chitosan Modified with DNA‐Celestine Blue

The present study describes a novel and very sensitive electrochemical assay for determination of hydrogen peroxide (H₂O₂) based on synergistic effects of reduced graphene oxide‐ magnetic iron oxide nanocomposite (rGO‐Fe₃O₄) and celestine blue (CB) for electrochemical reduction of H₂O₂. rGO‐Fe₃O₄ nanocomposite was synthesized and characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), X‐ray diffraction (XRD), electrochemical impedance spectroscopy and cyclic voltammetry. Chitosan (Chit) was used for immobilization of amino‐terminated single‐stranded DNA (ss‐DNA) molecules via a glutaraldehyde (GA) to the surface of rGO‐Fe₃O₄. The MTT (3‐(4,5‐Dim ethylt hiazol‐2‐yl)‐2,5‐diphenylt etrazolium bromide) results confirmed the biocompatibility of nanocomposite. Experimental parameters affecting the ss‐DNA molecules immobilization were optimized. Finally, by accumulation of the CB on the surface of the rGO‐Fe₃O₄‐Chit/ssDNA, very sensitive amperometric H₂O₂ sensor was fabricated. The electrocatalytic activity of the rGO‐Fe₃O₄‐Chit/DNA‐CB electrode toward H₂O₂ reduction was found to be very efficient, yielding very low detection limit (DL) of 42 nM and a sensitivity of 8.51 μA/μM. Result shows that complex matrices of the human serum samples did not interfere with the fabricated sensor. The developed sensor provided significant advantages in terms of low detection limit, high stability and good reproducibility for detection of H₂O₂ in comparison with recently reported electrochemical H₂O₂ sensors.     

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.     

Fabrication of Glucose Biosensor Based on Encapsulation of Glucose‐Oxidase on Sol‐Gel Composite at the Surface of Glassy Carbon Electrode Modified with Carbon Nanotubes and Celestine Blue

A simple procedure was developed to prepare a glassy carbon electrode modified with multi walled carbon nanotubes (MWCNTs) and Celestin blue. Cyclic voltammograms of the modified electrode show stable and a well defined redox couple with surface confined characteristic at wide pH range (2–12). The formal potential of redox couple (E′) shifts linearly toward the negative direction with increasing solution pH. The surface coverage of Celestine blue immobilized on CNTs glassy carbon electrode was approximately 1.95×10^?10 mol cm^?2. The charge transfer coefficient (α) and heterogeneous electron transfer rate constants (k_s) for GC/MWCNTs/Celestine blue were 0.43 and 1.26 s^?1, respectively. The modified electrode show strong catalytic effect for reduction of hydrogen peroxide and oxygen at reduced overpotential. The glucose biosensor was fabricated by covering a thin film of sol‐gel composite containing glucose oxides (GOx) on the surface of Celestine blue /MWCNTs modified GC electrode. The biosensor can be used successfully for selective detection of glucose based on the decreasing of cathodic peak current of oxygen. The detection limit, sensitivity and liner calibration rang were 0.3 μM, 18.3 μA/mM and 10 μM–6.0 mM, respectively. The accuracy of the biosensor for glucose detection was evaluated by detection of glucose in a serum sample, using standard addition protocol. In addition biosensor can reach 90% of steady currents in about 3.0 sec and interference effect of the electroactive existing species (ascorbic acid–uric acid and acetaminophen) was eliminated. Furthermore, the apparent Michaelis–Menten constant 2.4 mM, of GOx on the nano composite exhibits excellent bioelectrocatalytic activity of immobilized enzyme toward glucose oxidation. Excellent electrochemical reversibility of redox couple, high stability, technically simple and possibility of preparation at short period of time are of great advantages of this procedure for modification of glucose biosensor.     

Immobilization of flavine adenine dinucleotide onto nickel oxide nanostructures modified glassy carbon electrode: fabrication of highly sensitive persulfate sensor

A simple method was used to fabricate flavin adenine dinucleotide (FAD)/NiOx nanocomposite on the surface of glassy carbon (GC) electrode. Cyclic voltammetry technique was applied for deposition nickel oxide nanostructures onto GC surface. Owing to its high biocompatibility and large surface area of nickel oxide nanomaterials with immersing the GC/NiOx-modified electrode into FAD solution for a short period of time, 10–140 s, a stable thin layer of the FAD molecules immobilized onto electrode surface. The FAD/NiOx films exhibited a pair of well-defined, stable, and nearly reversible CV peaks at wide pH range (2–10). The formal potential of adsorbed FAD onto nickel oxide nanoparticles film, E ^o′ vs. Ag/AgCl reference electrode is −0.44 V in pH 7 buffer solutions was similar to dissolved FAD and changed linearly with a slope of 58.6 mV/pH in the pH range 2–10. The surface coverage and heterogeneous electron transfer rate constant (k _s ) of FAD immobilized on NiOx film glassy carbon electrode are 4.66 × 10⁻¹¹ mol cm⁻² and 63 ± 0.1 s⁻¹, indicating the high loading ability of the nickel oxide nanoparticles and great facilitation of the electron transfer between FAD and nickel oxide nanoparticles. FAD/NiOx nanocomposite-modified GC electrode shows excellent electrocatalytic activity toward S₂O₈²⁻ reduction at reduced overpotential. Furthermore, rotated modified electrode illustrates good analytical performance for amperometric detection of S₂O₈²⁻. Under optimized condition, the concentration calibration range, detection limit, and sensitivity were 3 μM–1.5 mM, 0.38 μM and 16.6 nA/μM, respectively.     

Cosmic string in the BTZ black hole background with time-dependent tension

In this Letter, we study the equation of circular loops with time-dependent tension in the BTZ black hole background. We obtain various cases where cosmic string loops finally collapse to form black holes. Also, we study effect of the BTZ black hole mass and angular momentum on the evolution of cosmic string loops. We find the critical values of initial radii as a limit for the cosmic string loops collapsing to form black holes.