Direct electrochemistry of glucose oxidase based on its direct immobilization on carbon ionic liquid electrode and glucose sensing

Direct electrochemistry of glucose oxidase (GOx) has been achieved by its direct immobilization on carbon ionic liquid electrode (CILE) with a conductive hydrophobic ionic liquid, 1-butyl pyridinium hexafluophosphate ([BuPy][PF₆]) as binder for the first time. A pair of reversible peaks is exhibited on GOx/CILE by cyclic voltammetry. The peak-to-peak potential separation (ΔE_P) of immobilized GOx is 0.056 V in 0.067 M phosphate buffer solution (pH 6.98) with scan rate of 0.1 V/s. The average surface coverage and the apparent Michaelis–Menten constant are 6.69 × 10⁻¹¹ mol·cm⁻² and 2.47 μM. GOx/CILE shows excellent electrocatalytic activity towards glucose determination in the range of 0.1–800 μM with detection limit of 0.03 μM (S/N = 3). The biosensor has been successfully applied to the determination of glucose in human plasma with the average recoveries between 95.0% and 102.5% for three times determination. The direct electrochemistry of GOx on CILE is achieved without the help of any supporting film or any electron mediator. GOx/CILE is inexpensive, stable, repeatable and easy to be fabricated.     

Predominating stable adsorption and direct electrochemistry of glucose oxidase on carbon nanotubes by oxygen-containing groups

Stable adsorption and direct electrochemistry of glucose oxidase (GOx) occurred on nitric acid (HNO3)-treated multi-walled carbon nanotubes (MWNTs) instead of as-received MWNTs, demonstrating the critical roles of oxygen-containing groups in stableadsorption and direct electrochemistry of GOx on carbon nanotubes (CNTs).     

Direct electrochemistry of hemoglobin and its electrocatalytic effect based on its direct immobilization on carbon ionic liquid electrode

Direct electrochemistry of hemoglobin (Hb) has been achieved by its direct immobilization on carbon ionic liquid electrode (CILE). CILE was immersed in a solution containing Hb and ionic liquid, octylpyridinium chloride ([OcPy][Cl]), to directly immobilize Hb on CILE. Cyclic voltammetry of modified electrode exhibited quasi-reversible peaks corresponding to Hb. The oxidation and reduction peak potentials of immobilized Hb in acetate buffer solution, pH 5.0 and at a scan rate of 0.1 V s⁻¹ were obtained at about –150 mV and –290 mV, respectively. The average surface coverage of the electroactive Hb adsorbed on the electrode surface was calculated as 8.4 × 10⁻¹¹ mol cm⁻². Hb retained its bioactivity on modified electrode and showed excellent electrocatalytic activity towards oxygen, hydrogen peroxide and nitrite. Hydrogen peroxide can be determined in the range of 1.0 × 10⁻⁴–5.0 × 10⁻³ M.     

Direct electrochemistry and reagentless biosensing of glucose oxidase immobilized on chitosan wrapped single-walled carbon nanotubes

Single-walled carbon nanotubes (SWCNTs) selectively wrapped by a water-soluble, environmentally friendly, biocompatible polymer chitosan (CHI) were employed for the construction of a bioelectrochemical platform for the direct electron transfer (DET) of glucose oxidase (GOD) and biosensing purposes. Scanning electron microscopy and Raman spectroscopy were used to investigate the properties of the SWCNT-CHI film. The results show that the preferentially wrapped small-diameter SWCNTs are dispersed within the CHI film and exist on the surface of the electrode as small bundles. The DET between GOD and the electrode surface was observed with a formal potential of about ca. -460 mV vs. SCE in phosphate buffer solution. The heterogeneous electron transfer rate constant and the surface coverage of GOD are estimated to be 3.0 s(-1) and 1.3 x 10(-10)mol/cm(2), respectively. The experimental results demonstrate that the immobilized GOD retains its catalytic activity towards the oxidation of glucose. Such a GOD/SWCNT-CHI film-based biosensor not only exhibits a rapid response time, a wide linear rang and a low detection limits at a detection potential of -400 mV but also shows the effective anti-interference capability. Significantly improved analytical capabilities of the GOD/SWCNT-CHI/GC electrode could be ascribed to the unique properties of the individual SWCNTs and to the biocompatibility of CHI.     

Nonenzymatic glucose voltammetric sensor based on gold nanoparticles/carbon nanotubes/ionic liquid nanocomposite

In this paper, a novel nonenzymatic glucose voltammetric sensor based on a kind of nanocomposite of gold nanoparticles (GNPs) embedded in multi-walled carbon nanotubes (MWCNTs)/ionic liquid (IL) gel was reported. The surface morphology of this nanocomposite was characterized using X-ray photoelectron spectrometer (XPS), scanning electron microscope (SEM) and transmission electron microscope (TEM), respectively. It can be found that most of GNPs lie close to the ektexine of MWCNTs and the others have obviously inserted the inner of MWCNTs through the defects or ends of MWCNTs, due to the attraction between GNPs and MWCNTs as well as the repulsion between GNPs and IL. Voltammetry was used to evaluate the electrocatalytic activities of the nanocomposite biosensor toward nonenzymatic glucose oxidation in alkaline media. The GNPs embedded in MWCNTs/IL gel have strong and sensitive voltammetric responses to glucose, owing to a possible synergistic effect among GNPs, MWCNTs and IL. Under the optimal condition, the linear range for the detection of the glucose is 5.0-120 μM with the correlation coefficient of 0.998, based on the oxidation peak observed during cathodic direction of the potential sweep. The kinetics and mechanism of glucose electro-oxidation were intensively investigated in this system. This kind of nanocomposite biosensor is also highly resistant toward poisoning by chloride ions and capable of sensing glucose oxidation in the presence of 20 μM uric acid and 70 μM ascorbic acid. This work provides a simple and easy approach to the detection of glucose in body fluid with high sensitivity and excellent selectivity.     

Polymerized ionic liquid-wrapped carbon nanotubes: The promising composites for direct electrochemistry and biosensing of redox protein

Polymerized ionic liquid-wrapped carbon nanotubes (PIL-CNTs) were firstly designed for direct electrochemistry and biosensing of redox proteins. The CNTs were coated successfully with polymerized ionic liquid (PIL) layer, as verified by transmission electron microscopy (TEM), thermogravimetric analysis (TGA) and Fourier transform infrared (FT-IR) spectroscopy. The PIL-CNTs were dispersed better in water and showed superior electrocatalysis toward O₂ and H₂O₂ comparing to pristine CNTs and the mixture of IL monomer and CNTs. With glucose oxidase (GOD) as a protein model, the direct electrochemistry of the redox protein was investigated on the PIL-CNTs modified glassy carbon (GC) electrode and excellent direct electrochemical performance of GOD molecules was observed. The proposed biosensor (GOD/PIL-CNTs/GC electrode) displayed good analytical performance for glucose with linear response up to 6 mM, response sensitivity of 0.853 μA mM⁻¹, good stability and selectivity.     

Surface nanocrystallization of glassy carbon electrode: Application in direct electrochemistry of glucose oxidase

The surface nanocrystallization of glassy carbon (GC) electrode was carried out using cyclic voltammetry in anhydrous dimethylformamide containing 0.05 M tetra-n-butylammonium bromine, and carbon nanoparticles with diameter of 10–40 nm were formed on the electrode surface. Comparing with the typical GC electrode, the surface-nanocrystalline GC (SNGC) electrode showed higher electrocatalytic activity for direct electrochemistry of glucose oxidase (GOD) due to higher proportion of edge sites presented on the surface of the SNGC electrode. Because of the surface nanocrystallization of the electrode, a pair of well-defined and quasi-reversible redox peaks of the immobilized GOD was observed for the first time on the GC electrode.     

Direct electrochemistry of myoglobin based on DNA accumulation on carbon ionic liquid electrode

Poly-anionic deoxyribonucleic acid (DNA) was accumulated on the positively charged surface of carbon ionic liquid electrode (CILE) with N-butylpyridinium hexafluorophosphate (BPPF₆) as binder, and then myoglobin (Mb) was immobilized onto the DNA film by electrostatic interaction to form Mb/DNA/CILE electrode. The direct electrochemistry of Mb was then investigated in detail. A pair of well-defined, quasi-reversible cyclic voltammetric peaks of Mb was obtained with the formal potentials (E⁰′) at ?0.304 V (vs. SCE) in phosphate buffer solution (PBS, pH 7.0). The Mb/DNA/CILE electrode showed excellent electrocatalytic activity to H₂O₂ and trichloroacetic acid (TCA) in the range of 1.0–160 μmol/L and 0.5–40.0 mmol/L, respectively. The apparent Michaelis–Menten constants (K_M) toward H₂O₂ and TCA were calculated as 0.42 and 0.82 mmol/L. So, the DNA/CILE had potential to study other proteins.     

Direct electrochemistry of glucose oxidase immobilized on porous carbon nanofiber/room temperature ionic liquid/chitosan composite film and its biosensing application

The direct electron transfer of glucose oxidase (GOD) immobilized on a composite matrix based on porous carbon nanofibers (PCNFs), room-temperature ionic liquid (RTIL), and chitosan (CHIT) underlying on a glassy carbon electrode was achieved. The combination of the PCNFs, RTIL, and CHIT provided a suitable microenvironment for GOD to transfer electron directly. In deaerated buffer solutions (pH 7.0), the cyclic voltammetry of the GOD/PCNFs/RTIL/CHIT composite films showed a pair of well-defined redox peaks with the formal potential of −0.45 V (vs. SCE). The synergistic effort of the PCNFs, RTIL, and CHIT also promoted the stability of GOD in the composite film and retained its bioactivity.     

Enhanced direct electron transfer of glucose oxidase based on a protic ionic liquid modified electrode and its biosensing application

A novel glucose oxidase (GOD) biosensor was fabricated with a protic ionic liquid (PIL) N-ethylimidazolium trifluoromethanesulfonate ([EIm][TfO]) as the modifier of a carbon electrode. Due to the excellent conductivity and the conformational changes of the microenvironment around the GOD, the electrochemical and biocatalytic properties of GOD immobilized on the PIL-based electrode were dramatically enhanced. A couple of well-defined redox peaks could be observed, with a formal potential of −0.476 V. The GOD biosensor presented good catalytic activity to the oxidation of glucose in oxygen-saturated phosphate buffer solutions. The cathodic peak currents of GOD decreased along with glucose concentrations. A linear response in the range 0.005–2.8 mM was obtained with a detection limit of 2.5 μM. The sensitivity and the apparent Michaelis–Menten constant (K _m) were estimated to be 14.96 μA mM⁻¹ and 1.53 μM, respectively. In addition, the biosensor remained stable over 30 days, indicating its good chemical and mechanical stability. The glucose content of several serum samples was determined by using the newly developed biosensor, and the results were in good agreement with those obtained by hospital measurements. All results suggested that PILs were a good media for supporting biocatalytic processes on the bioelectrode.     

Biocatalytic anode for glucose oxidation utilizing carbon nanotubes for direct electron transfer with glucose oxidase

Covalently linked layers of glucose oxidase, single-wall carbon nanotubes and poly-l-lysine on pyrolytic graphite resulted in a stable biofuel cell anode featuring direct electron transfer from the enzyme. Catalytic response observed upon addition of glucose was due to electrochemical oxidation of FADH₂ under aerobic conditions. The electrode potential depended on glucose concentration. This system has essential attributes of an anode in a mediator-free biocatalytic fuel cell.     

Biosensor for atrazin based on aligned carbon nanotubes modified with glucose oxidase

A sensitive method was developed for the determination of the hebicide atrazine. It based on the use of glucose oxidase that is self-assembled on aligned carbon nanotubes on the surface of a copper electrode. The surface morphology and electrochemical properties of the electrode were characterized by field emission scanning electron microscopy and cyclic voltammetry. The effects of buffer solution and incubation time on the response of the electrode were investigated. Response to atrazine is linear in the range from 0.58 µM to 42 µM, and the detection limit is 39 nM. The performance of the biosensor was verified by determination of atrazine in environmental water samples.     

Task specific ionic liquid for direct electrochemistry of metal oxides

We present the first report on task specific ionic liquid (TSIL) for direct electrochemical detection of heavy metal oxides including cadmium oxide, copper oxide and lead oxide at room temperature. This TSIL based electrochemical sensor demonstrated a high sensitivity and selectivity towards the online monitoring of these trace metal oxide particulates, along with short detection time, low cost and high accuracy. This novel sensor platform opens new pathways for in-situ monitoring of metal oxide particulates for environmental sensing and decontamination applications.     

Direct electrochemistry and biocatalysis of glucose oxidase immobilized on magnetic mesoporous carbon

A magnetic mesoporous carbon material (i.e., mesoporous iron oxide/C, mesoFe/C) is synthesized for protein immobilization, using glucose oxidase (GOx) as model. Transmission electron microscopy images show that mesoFe/C has highly ordered porous structure with uniform pore size, and iron oxide nanoparticles are dispersed along the wall of carbon. After adsorption of GOx, the GOx-mesoFe/C composite is separated with magnet. The immobilized GOx remains its natural structure according to the reflection–absorption infrared spectra. When the GOx-mesoFe/C composite is coated on a Pt electrode surface, the GOx gives a couple of quasireversible voltammetric peaks at −0.5 V (vs. saturated calomel electrode) due to the redox of FAD/FADH₂. The electron-transfer rate constant (k _s) is ca. 0.49 s⁻¹. The modified electrode presents remarkably amperometric response to glucose at 0.6 V. The response time (t _95%) is less than 6 s; the response current is linear to glucose concentration in the range of 0.2–10 mM with a sensitivity of 27 μA mM⁻¹ cm⁻². The detection limit is 0.08 mM (S/N = 3). The apparent Michaelis–Menten constant (K _m^app) of the enzyme reaction is ca. 6.6 mM, indicating that the GOx immobilized with mesoFe/C has high affinity to the substrate.     

Myoglobin on multi-walled carbon nanotubes modified electrode: direct electrochemistry and electrocatalysis

Electrochemical behavior of myoglobin (Myb) incorporated on multi-walled carbon nanotubes (MWNTs) modified GC electrode is investigated by electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). The results indicate that Myb can be strongly adsorbed on the surface of the MWNT-modified electrode to form an approximate monolayer film. MWNTs can greatly promote the redox of horse heart Myb, on which a pair of well-defined and nearly reversible CV peaks for Myb Fe(III)/Fe(II) redox couple were obtained in pH 7.0 buffers. The formal potential of Myb on MWNT modified electrode shifts linearly with pH with a slope of −56.4 mV/pH, showing that the electron transfer is accompanied by single proton transportation. The high efficiency of the Myb/MWNT modified electrode towards the catalytic electro-reduction of nitric oxide has been observed. Potential application of Myb/MWNT modified electrode as biosensors to monitor NO is proposed.     

Voltammetric sensors based on gel composites containing carbon nanotubes and an ionic liquid

Possibilities of using electrode coatings based on a gel of carboxylated multiwall carbon nanotubes (MWCNTs) in an ionic liquid (1-butyl-3-methylimidazolium hexafluorophosphate, [BMIm]PF₆) for the creation of a voltammetric sensor with electrocatalytic properties with respect to the pharmacological group of catecholamines—levodopa, methyldopa, and carbidopa—are considered. Using cyclic voltammetry, it was found that a glassy carbon electrode coated with a thin layer of an MWCNT–[BMIm]PF₆ gel or an MWCNT–[BMIm]PF₆–Nafion gel-composite induced a decrease in overvoltage (~60 mV), improved the reversibility of the redox reaction, and increased oxidation currents of the studied substances in comparison with an unmodified glassy carbon electrode. The concentration dependence of the analytical signal was linear in the ranges of 1–250, 2–350, and 5–400 mM for carbidopa, levodopa, and methyldopa, respectively. In the determination of the specified substances in diluted urine samples and tableted drugs, the accuracy index was 98–102% and the relative standard deviation, 0.3–5% (n = 5, P = 0.95).     

Sulfonated polyaniline network grafted multi-wall carbon nanotubes for enzyme immobilization, direct electrochemistry and biosensing of glucose

A new nanomaterial was prepared by grafting a layer of sulfonated polyaniline network (SPAN-NW) on to the surface of multi-walled carbon nanotube (MWNT) and effectively utilized for immobilization of an enzyme and for the fabrication of a biosensor. SPAN-NW was formed on the surface of MWNT by polymerizing a mixture of diphenyl amine 4-sulfonic acid (DPASA), 4-vinyl aniline (VA) and 2-acrylamido-2-methyl-1-propane sulfonic acid (APASA) in the presence of amine functionalized MWNT (MWNT-NH₂). The MWNT-g-SPAN-NW was immobilized with glucose oxidase (GOx) to fabricate the SPAN-NW/GOx biosensor. MWNT-g-SPAN-NW/GOx electrode showed direct electron transfer (DET) for GOx with a fast heterogeneous electron transfer rate constant (k_s) of 4.11 s^− 1. The amperometric current response of MWNT-g-SPAN-NW/GOx biosensor shows linearity up to 9 mM of glucose, with a correlation coefficient of 0.99 and a detection limit of 0.11 μM (S/N = 3). At a low applied potential of − 0.1 V, MWNT-g-SPAN-NW/GOx electrode possesses high sensitivity (4.34 μA mM^− 1) and reproducibility towards glucose.     

Biocomposites of covalently linked glucose oxidase on carbon nanotubes for glucose biosensor

The formation of covalently linked composites of multi–walled carbon nanotubes (MWCNT) and glucose oxidase (GOD) with high-function density for use as a biosensing interface is described. The reaction intermediates and the final product were characterized by using FT–IR spectroscopy, and the MWCNT-coated GOD nanocomposites were examined by atomic force microscopy (AFM) and transmission electron microscopy (TEM). Interestingly, it was found that the GOD–MWCNT composites are highly water soluble. Electrochemical characterization of the GOD–MWCNT composites that were modified on a glassy carbon electrode shows that the covalently linked GOD retains its bioactivity and can specifically catalyze the oxidation of glucose. The oxidation current shows a linear dependence on the glucose concentration in the solution in the range of 0.5–40 mM with a detection limit of 30 μM and a detection sensitivity of 11.3 μA/mMcm². The present method may provide a way to synthesize MWCNT related composites with other biomolecules and for the construction of enzymatic reaction-based biofuel cells and biosensors. Supported by grants from the National Natural Science Foundation of China (NSFC, No. 20125515; 90206037; 20375016) and the Natural Science Foundation of Jiangsu Province (Grant No. BK 2004210)     

Ionic liquid/graphene oxide as a nanocomposite for improving the direct electrochemistry and electrocatalytic activity of glucose oxidase

By combination of 1-ethyl-3-methyl immidazolium ethyl sulfate as a typical room temperature ionic liquid (IL) and graphene oxide (GO) nanosheets, a nanocomposite was introduced for improving the direct electrochemistry and electrocatalytic activity of glucose oxidase (GOx). The enzyme on the IL–GO-modified glassy carbon electrode exhibited a quasireversible cyclic voltammogram corresponding to the flavine adenine dinucleotide/FADH₂ redox prosthetic group of GOx. At the scan rate of 100 mV?s^?1, the enzyme showed a peak-to-peak potential separation of 82 mV and the formal potential of ?463 mV (vs Ag/AgCl in 0.1 M phosphate buffer solution, pH?7.0). The kinetic parameters of the charge transfer rate constant, the electron transfer coefficient, and the apparent Michaelis–Menten constant were calculated as 1.36 s^?1 and 0.35 and 2.47 μM, respectively. When the modified electrode was examined as a biosensor for glucose determination, a linear range of 2.5–45 nM with detection limit of 0.175 nM (signal to noise?=?3) was obtained. The biosensor was stable for 2 months.