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).     

An aptamer-based biosensing platform for highly sensitive detection of platelet-derived growth factor via enzyme-mediated direct electrochemistry

In this work, a new label-free electrochemical aptamer-based sensor (aptasensor) was constructed for detection of platelet-derived growth factor (PDGF) based on the direct electrochemistry of glucose oxidase (GOD). For this proposed aptasensor, poly(diallyldimethylammonium chloride) (PDDA)-protected graphene-gold nanoparticles (P-Gra-GNPs) composite was firstly coated on electrode surface to form the interface with biocompatibility and huge surface area for the adsorption of GOD layer. Subsequently, gold nanoclusters (GNCs) were deposited on the surface of GOD to capture PDGF binding aptamer (PBA). Finally, GOD as a blocking reagent was employed to block the remaining active sites of the GNCs and avoid the nonspecific adsorption. With the direct electron transfer of double layer GOD membranes, the aptasensor showed excellent electrochemical response and the peak current decreased linearly with increasing logarithm of PDGF concentration from 0.005 nM to 60 nM with a relatively low limit of detection of 1.7 pM. The proposed aptasensor exhibited high specificity, good reproducibility and long-term stability, which provided a new promising technique for aptamer-based protein detection.     

Polymerized ionic liquid functionalized multi-walled carbon nanotubes/polyetherimide composites

Thin polyetherimide (PEI) films containing 0.1–3 wt.% multi-walled carbon nanotubes (MWCNTs), have been prepared from three types of MWCNTs, namely pristine, oxidized and polymerized ionic liquid (PIL) functionalized CNTs. Oxidized and PIL functionalized CNTs (CNT–PIL) showed better dispersion in the matrix compared to pristine CNTs. For CNT–PIL, alignment of CNTs has been observed in the matrix. Regardless of the type of CNTs, their incorporation led to an increased thermal stability of the polymer matrix. Dynamic mechanical analysis showed that storage modulus increased by up to 25% (3 wt.% CNT–PIL) and an increase in the height of the damping peaks (tan δ). The addition of CNTs did not have any significant influence on the tensile properties and T_g of the polymer, and the electrical conductivity did not decrease in the case of modified CNTs.     

Direct electrochemistry of glucose oxidase entrapped in nano gold particles-ionic liquid-N,N-dimethylformamide composite film on glassy carbon electrode and glucose sensing

The direct electrochemistry of glucose oxidase (GOD) entrapped in nano gold particles (NAs)-N,N-dimethylformamide (DMF)-1-butyl-3-methylimidazolium hexafluophosphate (BMIMPF₆) composite film on a glassy carbon electrode (NAs-DMF-GOD (BMIMPF₆)/GC) has been investigated for first time. The immobilized GOD exhibits a pair of well-defined reversible peaks in 0.050 M pH 5 phosphate solutions (PS), resulting from the redox of flavin adenine dinucleotide (FAD) in GOD. The peak currents are three times as large as those of GOD-NAs-DMF film coated GC electrode (i.e. NAs-DMF-GOD (water)/GC). In addition, the NAs-DMF-GOD (BMIMPF₆) composite material has higher thermal stability than NAs-DMF-GOD (water). Results show that ionic liquid BMIMPF₆, DMF and NAs are requisite for GOD to exhibit a pair of stable and reversible peaks. Without any of them, the peaks of GOD become small and unstable. Upon the addition of glucose, the peak currents of GOD decrease and a new cathodic peak occurs at −0.8 V (versus SCE), which corresponds to the reduction of hydrogen peroxide (H₂O₂) generated by the catalytic oxidation of glucose. The peak current of the new cathodic peak and the glucose concentration show a linear relationship in the ranges of 1.0 × 10⁻⁷ to 1.0 × 10⁻⁶ M and 2.0 × 10⁻⁶ to 2.0 × 10⁻⁵ M. The kinetic parameter I_max of H₂O₂ is estimated to be 1.19 × 10⁻⁶ A and the apparent K_m (Michaelis-Menten constant) for the enzymatic reaction is 3.49 μM. This method has been successfully applied to the determination of glucose in human plasma and beer samples, and the average recoveries are 97.2% and 99%, respectively.     

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.     

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.     

聚合离子液体-碳纳米管复合物在线固相萃取分离白蛋白

合成了聚（1-乙烯-3-乙基咪唑溴代盐）（poly（ViEtIm＋Br-））聚合离子液体,并通过π-π堆积将其固载到多壁碳纳米管上,得到了聚离子液体-碳纳米管复合物.以15mg复合物的填充柱为固相吸附剂,在顺序注射系统中研究了其对白蛋白的在线固相萃取分离纯化.在pH5.2时,对2mL溶液中（50μgmL-1）白蛋白的吸附效率为91%.0.04molL^-1柠檬酸盐缓冲溶液可定量洗脱吸附的白蛋白.整个分离纯化过程耗时500s,富集倍数为5.用该法对人全血中的白蛋白进行分离纯化,经SDS-PAGE凝胶电泳验证,得到了纯度较好的白蛋白.     

Application of multi-walled carbon nanotubes modified carbon ionic liquid electrode for electrocatalytic oxidation of dopamine

A simple, sensitive, and reliable method based on a multi-walled carbon nanotubes (MWNTs) modified carbon ionic liquid electrode (CILE) has been successfully developed for determination of dopamine (DA) in the presence of ascorbic acid (AA). The acid-treated MWNTs with carboxylic acid functional groups could promote the electron-transfer reaction of DA and inhibit the voltammetric response of AA. Due to the good performance of the ionic liquid, the electrochemical response of DA on the MWNTs/CILE was better than that of other MWNTs modified electrodes. Under the optimum conditions a linear calibration plot was obtained in the range 5.0×10(-8) to 2.0×10(-4) mol L(-1) and the detection limit was 1.0×10(-8) mol L(-1).     

Functionalized carbon nanotubes and nanofibers for biosensing applications

This review summarizes recent advances in electrochemical biosensors based on carbon nanotubes (CNTs) and carbon nanofibers (CNFs) with an emphasis on applications of CNTs. CNTs and CNFs have unique electric, electrocatalytic and mechanical properties, which make them efficient materials for developing electrochemical biosensors.We discuss functionalizing CNTs for biosensors. We review electrochemical biosensors based on CNTs and their various applications (e.g., measurement of small biological molecules and environmental pollutants, detection of DNA, and immunosensing of disease biomarkers). Moreover, we outline the development of electrochemical biosensors based on CNFs and their applications. Finally, we discuss some future applications of CNTs.     

Reagentless Glucose Biosensor Based on the Direct Electrochemistry of Glucose Oxidase on Carbon Nanotube‐Modified Electrodes

The direct electrochemistry of glucose oxidase (GOD) was revealed at a carbon nanotube (CNT)‐modified glassy carbon electrode, where the enzyme was immobilized with a chitosan film containing gold nanoparticles. The immobilized GOD displays a pair of redox peaks in pH 7.4 phosphate buffer solutions (PBS) with the formal potential of about ?455 mV (vs. Ag/AgCl) and shows a surface‐controlled electrode process. Bioactivity remains good, along with effective catalysis of the reduction of oxygen. In the presence of dissolved oxygen, the reduction peak current decreased gradually with the addition of glucose, which could be used for reagentless detection of glucose with a linear range from 0.04 to 1.0 mM. The proposed glucose biosensor exhibited high sensitivity, good stability and reproducibility, and was also insensitive to common interferences such as ascorbic and uric acid. The excellent performance of the reagentless biosensor is attributed to the effective enhancement of electron transfer between enzyme and electrode surface by CNTs, and the biocompatible environment that the chitosan film containing gold nanoparticles provides for immobilized GOD.     

Electroanalytical quantification of total dsDNA extracted from human sample using,an ionic liquid modified,carbon nanotubes paste electrode

Direct electrochemistry of dsDNA has been achieved by using an ionic liquid 1-butyl-4-methylpyridinium hexafluorophosphate modified carbon nanotubes paste electrode (IL-CNTPE). Oxidation peaks appeared at 0.93 and 1.26 V (vs. Ag/AgCl) on the IL- CNTPE after preconcentration of dsDNA in pH 5.0 acetate buffer, which were attributed to the oxidation of guanine and adenine residues on the dsDNA molecule structure. Based on the signal of guanine, under the optimal conditions, very low levels of dsDNA can be detected after 60 s accumulation with detection limits of 0.249 mg L⁻ 16 pM. Additionally, human DNA from a healthy volunteer is determined by use of the IL-CNTPE and it is found to be 40 ± 2, 14 pM.     

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.     

Direct electron transfer of PQQ-glucose dehydrogenase at modified carbon nanotubes electrodes

In order to establish efficient enzyme-electrode-contacts for the pyrroloquinoline quinone dependent glucose dehydrogenase (PQQ-GDH) different immobilisation strategies are investigated. Multi-walled carbon nanotubes (MWCNT) on gold electrodes are modified by chemical treatment and by (poly)-aniline derivatives. The electropolymerisation of methoxy-m-anilinesulfonic acid and m-aminobenzoic acid on the MWCNTs allows the covalent coupling of the PQQ-GDH. Such a poly-[ASA-ABA]/MWCNT/Au electrode can achieve current densities of up to 500 μA/cm² at a potential of 100 mV vs. Ag/AgCl. Furthermore investigations with small amounts of free PQQ indicate direct electron transfer between enzyme and electrode.     

Direct electrochemistry and electrocatalysis of hemoglobin on gold nanoparticle decorated carbon ionic liquid electrode

In this paper a carbon ionic liquid electrode (CILE) was fabricated by using ionic liquid 1-ethyl-3-methylimidazolium ethylsulfate ([EMIM]EtOSO₃) as modifier and further gold nanoparticles were in situ electrodeposited on the surface of CILE. The fabricated Au/CILE was used as a new platform for the immobilization of hemoglobin (Hb) with the help of a Nafion film. Electrochemical experimental results indicated that direct electron transfer of Hb was realized on the surface of Au/CILE with a pair of well-defined quasi-reversible redox peaks appeared. The formal peak potential (E⁰′) was obtained as −0.210 V (vs. SCE) in pH 7.0 phosphate buffer solution (PBS), which was the characteristic of Hb heme Fe(III)/Fe(II) redox couple. The fabricated Nafion/Hb/Au/CILE showed excellent electrocatalytic activity to the reduction of trichloroacetic acid (TCA) and the reduction peak current was in proportional to TCA concentration in the range from 0.2 to 18.0 mmol/L with the detection limit as 0.16 mmol/L (S/N = 3). The proposed electrode showed good stability and reproducibility, and it had the potential application as a new third-generation electrochemical biosensor.     

Direct electrochemistry with enhanced electrocatalytic activity of hemoglobin in hybrid modified electrodes composed of graphene and multi-walled carbon nanotubes

A graphene (GR) and multi-walled carbon nanotubes (MWCNT) hybrid was prepared and modified on a 1-hexylpyridinium hexafluorophosphate based carbon ionic liquid electrode (CILE). Hemoglobin (Hb) was immobilized on GR-MWCNT/CILE surface with Nafion as the film forming material and the modified electrode was denoted as Nafion/Hb-GR-MWCNT/CILE. Spectroscopic results revealed that Hb molecules retained its native structure in the GR-MWCNT hybird. Electrochemical behaviors of Hb were carefully investigated by cyclic voltammetry with a pair of well-defined redox peaks obtained, which indicated that direct electron transfer of Hb was realized in the hybrid modified electrode. The result could be attributed to the synergistic effects of GR-MWCNT hybrid with enlarged surface area and improved conductivity through the formation of a three-dimensional network. Electrochemical parameters of the immobilized Hb on the electrode surface were further calculated with the results of the electron transfer number (n) as 1.03, the charge transfer coefficient (a) as 0.58 and the electron-transfer rate constant (k_s) as 0.97 s⁻¹. The Hb modified electrode showed good electrocatalytic ability toward the reduction of different substrates such as trichloroacetic acid in the concentration range from 0.05 to 38.0 mmol L⁻¹ with a detection limit of 0.0153 mmol L⁻¹ (3σ), H₂O₂ in the concentration range from 0.1 to 516.0 mmol L⁻¹ with a detection limit of 34.9 nmol/L (3σ) and NaNO₂ in the concentration range from 0.5 to 650.0 mmol L⁻¹ with a detection limit of 0.282 μmol L⁻¹ (3σ). So the proposed electrode had the potential application in the third-generation electrochemical biosensors without mediator.     

炭气凝胶纳米颗粒固定葡萄糖氧化酶的直接电化学

利用间苯二酚和甲醛在碱性环境下制备炭气凝胶(CA), 通过扫描电镜(SEM)、透射电镜(TEM)、比表面积测试Brunauer-Emmett-Teller (BET)等方法分析载体的形貌结构; 以CA为载体通过吸附法固定葡萄糖氧化酶(GOD)并修饰玻碳(GC)电极, 得到GOD/CA/GC电极. 在0.1 mol·L^-1磷酸盐缓冲溶液中, 利用循环伏安法研究了GOD/CA/GC 电极的直接电化学行为和对葡萄糖的催化性能. 结果表明, 以CA为载体可以很好地固定GOD并保持其生物活性, 在无任何电子媒介体存在时, GOD在电极上实现了直接电子转移, GOD/CA/GC电极对葡萄糖具有很好的电催化性能.     

Direct electrochemistry and electrocatalysis of horseradish peroxidase immobilized in hybrid organic-inorganic film of chitosan/sol-gel/carbon nanotubes

A hybrid organic-inorganic nanocomposite film of chitosan/sol-gel/multi-walled carbon nanotubes was constructed for the immobilization of horseradish peroxidase (HRP). This film was characterized by scanning electron microscopy. Direct electron transfer (DET) and bioelectrocatalysis of HRP incorporated into the composite film were investigated. The results indicate that the film can provide a favorable microenvironment for HRP to perform DET on the surface of glassy carbon electrodes with a pair of quasi-reversible redox waves and to retain its bioelectrocatalytic activity toward H₂O₂.     

Polyphosphonate induced coacervation of chitosan: Encapsulation of proteins/enzymes and their biosensing

Based on the polyphosphonate-assisted coacervation of chitosan, a simple and versatile procedure for the encapsulation of proteins/enzymes in chitosan–carbon nanotubes (CNTs) composites matrix was developed. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), energy dispersive spectrum (EDS) mapping demonstrated the hemoglobin (Hb) uniformly distributed into chitosan–CNTs composites matrix. Raman measurements indicated the CNTs in composites matrix retained the electronic and structural integrities of the pristine CNTs. Fourier transform infrared (FT-IR), ultraviolet–visible (UV–vis) and circular dichroism (CD) spectroscopy displayed the encapsulated Hb preserved their near-native structure, indicating the polyphosphonate–chitosan–CNTs composites possessed excellent biocompatibility for the encapsulation of proteins/enzymes. Electrochemical measurements indicated the encapsulated Hb could directly exchange electron with the substrate electrode. Moreover, the modified electrode showed excellent bioelectrocatalytic activity for the reduction of hydrogen peroxide. Under optimum experimental conditions, the fabricated electrochemical sensor displayed the fast response (less than 3 s), wide linear range (7.0 × 10⁻⁷ to 2.0 × 10⁻³ M) and low detection limit (4.0 × 10⁻⁷ M) for the determination of hydrogen peroxide. This newly developed protocol was simple and mild and would certainly find extensive applications in biocatalysis, biosensors, bioelectronics and biofuel cells.     

Ionic liquid modified GC electrode for the direct electrochemistry of chloroperoxidase

Chloropcroxidase （CPO） was immobilized by konjac glucomannan （KGM） on the 1-butyl-3-methyl imidazolium tetrafluoroborate [BMIM][BF4]/Nafion modified glassy carbon eloctrode. The electrochemical behaviors of the immobilized CPO were investigated by cyclic voltammetry. The results showed that CPO was successfully immobilized on the GCE and underwent fast direct electron transfer reactions with the formal potential at -0.3 V vs. SCE. The modified electrode showed a good catalytic activity for elcctrocatalytical reduction of O2 and H2O2.