Direct electrochemistry of glucose oxidase and sensing glucose using a screen-printed carbon electrode modified with graphite nanosheets and zinc oxide nanoparticles

Progress of Enzymatic and Non-Enzymatic Electrochemical Glucose Biosensor Based on Nanomaterial-Modified Electrode

This review covers the progress of nanomaterial-modified electrodes for enzymatic and non-enzymatic glucose biosensors. Fundamental insights into glucose biosensor components and the crucial factors controlling the electrochemical performance of glucose biosensors are discussed in detail. The metal, metal oxide, and hybrid/composite nanomaterial fabrication strategies for the modification of electrodes, mechanism of detection, and significance of the nanomaterials toward the electrochemical performance of enzymatic and non-enzymatic glucose biosensors are compared and comprehensively reviewed. This review aims to provide readers with an overview and underlying concept of producing a reliable, stable, cost-effective, and excellent electrochemical performance of a glucose biosensor.

Recent advances in ZnO nanostructure-based electrochemical sensors and biosensors

Nanostructured metal oxides, such as zinc oxide (ZnO), are considered as excellent materials for the fabrication of highly sensitive and selective electrochemical sensors and biosensors due to their good properties, including a high specific surface area, high catalytic efficiency, strong adsorption ability, high isoelectric point (IEP, 9.5), wide band gap (3.2 eV), biocompatibility and high electron communication features. Thus, ZnO nanostructures are widely used to fabricate efficient electrochemical sensors and biosensors for the detection of various analytes. In this review, we have discussed the synthesis of ZnO nanostructures and the advances in various ZnO nanostructure-based electrochemical sensors and biosensors for medical diagnosis, pharmaceutical analysis, food safety, and environmental pollution monitoring.

Screen-printed electrochemical biosensor based on a ternary Co@MoS2/rGO functionalized electrode for high-performance non-enzymatic glucose sensing

In this study, cobalt oxides functionalized MoS₂/reduced graphene oxide was synthesized via a facile one-pot hydrothermal approach. Morphology and crystal structure of this ternary nanoarchitecture were characterized through scanning electron microscopy, transmission electron microscopy, Raman spectra and X-ray photoelectron spectroscopy. An ultrasensitive non-enzymatic glucose sensor was developed by decorating this ternary nanohybrid on the working electrode of a screen-printed electrochemical sensor. Cycle sweep voltammetry and amperometry were used to study the electro-catalytic activity of the modified working electrode, which demonstrated superior catalytic activity towards glucose oxidation with an extremely low detection limit of 30 nM. Meanwhile, this sensor showed an excellent selectivity in the presence of interfering species such as uric acid, ascorbic acid, etc. Based on the screen-printed technique, enzyme mimic nanomaterials could be easily introduced into portable devices, which opens the way to take non-enzymatic glucose electrochemical sensing towards point-of-care.

Nanomolar level sensing of glucose in food samples using glucose oxidase confined MWCNT-Inulin-TiO2 bio-nanocomposite

Development of an effective sensor for sensing glucose in commercially available "sugar free" food products is important as people are becoming diabetic health conscious. Although multi-walled carbon nanotubes (MWCNTs) possess interesting electrical properties, their hydrophobic nature limits their applications. Their hydrophilicity can be improved through modification. In the present study, Inulin, that was isolated from Allium sativum L. using hot water diffusion and incorporated with titanium dioxide (TiO₂), was used for the modification of MWCNTs. The as-synthesized MWCNT-Inulin-TiO₂ bio-nanocomposite immobilized with glucose oxidase (GOx) was incorporated into the carbon paste matrix and was utilized for the sensing of glucose in food products. Differential pulse voltammetric studies revealed that the fabricated electrode demonstrated good linear range (1.6 nM to 1 μM) and was sensitive to nanomolar concentrations of glucose with a very low limit of detection up to 0.82 nM and exhibited a long term stability of 150 days.

ZnO/ZnAl₂O₄ Nanocomposite with 3D Sphere-Like Hierarchical Structure for Photocatalytic Reduction of Aqueous Cr(VI)

Three dimensional (3D) ZnO/ZnAl₂O₄ nanocomposites (Zn_nAl-MMO) were synthesized by a simple urea-assisted hydrothermal process and subsequent high-temperature calcination. The as-prepared samples and their precursors were characterized by X-ray diffraction (XRD), Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), UV-Vis diffuse reflectance spectroscopy (DRS), and Photoluminescence spectra (PL). It was observed that the morphology of Zn_nAl-MMO nanocomposites could be tuned from cubic aggregates, hierarchically flower-like spheres to porous microspheres by simply changing the molar ratio of metal cations of the starting reaction mixtures. The photocatalytic performance of ZnO/ZnAl₂O₄ nanocomposites in the photoreduction of aqueous Cr(VI) indicated that the as-prepared 3D hierarchical sphere-like Zn_nAl-MMO nanocomposite showed excellent photocatalytic activity of Cr(VI) reduction under UV light irradiation. The results indicated that the maximum removal percentage of aqueous Cr(VI) was 98% within four hours at 10 mg/L initial concentration of Cr(VI), owing to the effective charge separation and diversion of photogenerated carriers across the heterojunction interface of the composite. Our study put forward a facile method to fabricate hierarchical ZnO/ZnAl₂O₄ composites with potential applications for wastewater treatment.

Highly efficient biosensors by using well-ordered ZnO/ZnS core/shell nanotube arrays

We have studied the fabrication of highly efficient glucose sensors using well-ordered heterogeneous ZnO/ZnS core/shell nanotube arrays (CSNAs). The modified electrodes exhibit a superior electrochemical response towards ferrocyanide/ferricyanide and in glucose sensing. Further, the fabricated glucose biosensor exhibited good performance over an acceptable linear range from 2.39 × 10^-5 to 2.66 × 10^-4 mM, with a sensitivity of 188.34 mA mM^-1 cm^-2, which is higher than that of the ZnO nanotube array counterpart. A low limit of detection was realized (24 μM), which is good compared with electrodes based on conventional structures. In addition, the enhanced direct electrochemistry of glucose oxidase indicates the fast electron transfer of ZnO/ZnS CSNA electrodes, with a heterogeneous electron transfer rate constant (K _s) of 1.69 s^-1. The fast electron transfer is attributed to the high conductivity of the modified electrodes. The presented ZnS shell can facilitate the construction of future sensors and enhance the ZnO surface in a biological environment.

Glucose biosensor based on functionalized ZnO nanowire/graphite films dispersed on a Pt electrode

We present a glucose biosensor based on ZnO nanowire self-sustained films grown on compacted graphite flakes by the vapor transport method. Nanowire/graphite films were fragmented in water, filtered to form a colloidal suspension, subsequently functionalized with glucose oxidase and finally transferred to a metal electrode (Pt). The obtained devices were evaluated using scanning electron microscopy, energy-dispersive x-ray spectroscopy, cyclic voltammetry and chronoamperometry. The electrochemical responses of the devices were determined in buffer solutions with successive glucose aggregates using a tripolar electrode system. The nanostructured biosensors showed excellent analytical performance, with linear response to glucose concentrations, high sensitivity of up to ≈17 μA cm(-2) mM(-1) in the 0.03-1.52 mM glucose concentration range, relatively low Michaelis-Menten constant, excellent reproducibility and a fast response. The detection limits are more than an order of magnitude lower than those achievable in commercial biosensors for glucose control, which is promising for the development of glucose monitoring methods that do not require blood extraction from potentially diabetic patients. The strong detection enhancements provided by the functionalized nanostructures are much larger than the electrode surface-area increase and are discussed in terms of the physical and chemical mechanisms involved in the detection and transduction processes.

A Stimuli-Responsive Biosensor of Glucose on Layer-by-Layer Films Assembled through Specific Lectin-Glycoenzyme Recognition

The research on intelligent bioelectrocatalysis based on stimuli-responsive materials or interfaces is of great significance for biosensors and other bioelectronic devices. In the present work, lectin protein concanavalin A (Con A) and glycoenzyme glucose oxidase (GOD) were assembled into {Con A/GOD}n layer-by-layer (LbL) films by taking advantage of the biospecific lectin-glycoenzyme affinity between them. These film electrodes possess stimuli-responsive properties toward electroactive probes such as ferrocenedicarboxylic acid (Fc(COOH)₂) by modulating the surrounding pH. The CV peak currents of Fc(COOH)₂ were quite large at pH 4.0 but significantly suppressed at pH 8.0, demonstrating reversible stimuli-responsive on-off behavior. The mechanism of stimuli-responsive property of the films was explored by comparative experiments and attributed to the different electrostatic interaction between the films and the probes at different pH. This stimuli-responsive films could be used to realize active/inactive electrocatalytic oxidation of glucose by GOD in the films and mediated by Fc(COOH)₂ in solution, which may establish a foundation for fabricating novel stimuli-responsive electrochemical biosensors based on bioelectrocatalysis with immobilized enzymes.

Electrochemically Functionalized Seamless Three-Dimensional Graphene-Carbon Nanotube Hybrid for Direct Electron Transfer of Glucose Oxidase and Bioelectrocatalysis

Three-dimensional seamless chemical vapor deposition (CVD) grown graphene-carbon nanotubes (G-CNT) hybrid film has been studied for its potential in achieving direct electron transfer (DET) of glucose oxidase (GOx) and its bioelectrocatalytic activity in glucose detection. A two-step CVD method was employed for the synthesis of seamless G-CNT hybrid film where CNTs are grown on already grown graphene film on copper foil using iron as a catalyst. Physical characterization using SEM and TEM show uniform dense coverage of multiwall carbon nanotubes (MWCNT) grown directly on graphene with seamless contacts. The G-CNT hybrid film was electrochemically modified to introduce oxygenated functional groups for DET favorable immobilization of GOx. Pristine and electrochemically functionalized G-CNT film was characterized by electrochemical impedance spectroscopy (EIS), cyclic voltammetry, X-ray photoelectron-spectroscopy, and Raman spectroscopy. The DET between GOx and electrochemically oxidized G-CNT electrode was studied using cyclic voltammetry which showed a pair of well-defined and quasi-reversible redox peaks with a formal potential of -459 mV at pH 7 corresponding to the redox site of GOx. The constructed electrode detected glucose concentration over the clinically relevant range of 2-8 mM with the highest sensitivity of 19.31 μA/mM/cm(2) compared to reported composite hybrid electrodes of graphene oxide and CNTs. Electrochemically functionalized CVD grown seamless G-CNT structure used in this work has potential to be used for development of artificial mediatorless redox enzyme based biosensors and biofuel cells.