ZnO nanoparticle and multiwalled carbon nanotubes for glucose oxidase direct electron transfer and electrocatalytic activity investigation

Electrocatalytic oxidation of low concentration cefotaxime sodium wastewater using Ti/SnO2-RuO2 electrode: Feasibility analysis and degradation mechanism

In this research, Ti/SnO₂-RuO₂ stable anode was successfully prepared by thermal decomposition method, and low concentration cefotaxime sodium (CFX) was degraded by green and sustainable electrocatalytic oxidation technology. The electrocatalytic activity and stability of the Ti/SnO₂-RuO₂ coating electrode were studied according to the polarization curve of oxygen and chlorine evolution. The effects of current density, initial concentration, pH, electrolyte concentration, and other technological parameters on the degradation efficiency were discussed. Orthogonal experiment results indicated that when the current density was 25 mA cm^-2, concentration of electrolyte was 5 mM and the pH value was 7, the best CFX removal rate of 86.33% could be obtained. The degradation efficiency of electrocatalytic oxidation was discussed through electrochemical analysis. Fourier transform infrared spectroscopy was used to analyze the different inlet and outlet stages before and after the degradation of CFX, and the possible degradation process was discussed. Therefore, the electrocatalytic oxidation of Ti/SnO₂-RuO₂ electrode was a clean and efficient technology, which could be widely used in the treatment of CFX wastewater.

MXene nanoflakes decorating ZnO tetrapods for enhanced performance of skin-attachable stretchable enzymatic electrochemical glucose sensor

Continuous painless glucose monitoring is the greatest desire of more than 422 million diabetics worldwide. Therefore, new non-invasive and convenient approaches to glucose monitoring are more in demand than other tests for microanalytical diagnostic tools. Besides, blood glucose detection can be replaced by continuous glucose monitoring of other human biological fluids (e.g. sweat) collected non-invasively. In this study, a skin-attachable and stretchable electrochemical enzymatic sensor based on ZnO tetrapods (TPs) and a new class of 2D materials - transition metal carbides, known as MXene, was developed and their electroanalytical behavior was tailored for continuous detection glucose in sweat. The high specific area of ZnO TPs and superior electrical conductivity of MXene (Ti₃C₂Tx) nanoflakes enabled to produce enzymatic electrochemical glucose biosensor with enhanced sensitivity in sweat sample (29 μA mM^-1 cm^-2), low limit of detection (LOD ≈ 17 μM), broad linear detection range (LDR = 0.05-0.7 mM) that satisfices glucose detection application in human sweat, and advanced mechanical stability (up to 30% stretching) of the template. The developed skin-attachable stretchable electrochemical electrodes allowed to monitor the level of glucose in sweat while sugar uptake and during physical activity. Continuous in vivo monitoring of glucose in sweat obtained during 60 min correlated well with data collected by a conventional amperometric blood glucometer in vitro mode. Our findings demonstrate the high potential of developed ZnO/MXene skin-attachable stretchable sensors for biomedical applications on a daily basis.

Study of ZnO-CNT Nanocomposites in High-Pressure Conditions

Recently, carbon nanotubes (CNTs) have been used extensively to develop new materials and devices due to their specific morphology and properties. The reinforcement of different metal oxides such as zinc oxide (ZnO) with CNT develops advanced multifunctional materials with improved properties. Our aim is to obtain ZnO-CNT nanocomposites by in situ hydrothermal method in high-pressure conditions. Various compositions were tested. The structure and morphology of ZnO-CNT nanocomposites were analyzed by Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry—thermogravimetry (DSC-TG), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and transmission electron microscopy (TEM). These analyses showed the formation of complex ZnO-CNT structures. FT-IR spectra suggest possible interactions between CNT and ZnO. DSC-TG analysis also reveals the formation of some physical bonds between ZnO and CNT, through the appearance of endothermic peaks which could be assigned to the decomposition of functional groups of the CNT chain and breaking of the ZnO-CNT bonds. XRD characterization demonstrated the existence of ZnO nanocrystallites with size around 60 nm. The best ZnO:CNT composition was further selected for preliminary investigations of the potential of these nanocomposite powders to be processed as pastes for extrusion-based 3D printing.

Potentialities of bioinspired metal and metal oxide nanoparticles in biomedical sciences

To date, various reports have shown that metallic gold bhasma at the nanoscale form was used as medicine as early as 2500 B.C. in India, China, and Egypt. Owing to their unique physicochemical, biological, and electronic properties, they have broad utilities in energy, environment, agriculture and more recently, the biomedical field. The biomedical domain has been used in drug delivery, imaging, diagnostics, therapeutics, and biosensing applications. In this review, we will discuss and highlight the increasing control over metal and metal oxide nanoparticle structures as smart nanomaterials utilized in the biomedical domain to advance the role of biosynthesized nanoparticles for improving human health through wide applications in the targeted drug delivery, controlled release drug delivery, wound dressing, tissue scaffolding, and medical implants. In addition, we have discussed concerns related to the role of these types of nanoparticles as an anti-viral agent by majorly highlighting the ways to combat the Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2) pandemic, along with their prospects.

Single-Atom Cobalt-Based Electrochemical Biomimetic Uric Acid Sensor with Wide Linear Range and Ultralow Detection Limit

Uric acid (UA) detection is essential in diagnosis of arthritis, preeclampsia, renal disorder, and cardiovascular diseases, but it is very challenging to realize the required wide detection range and low detection limit. We present here a single-atom catalyst consisting of Co^(II) atoms coordinated by an average of 3.4 N atoms on an N-doped graphene matrix (A-Co-NG) to build an electrochemical biomimetic sensor for UA detection. The A-Co-NG sensor achieves a wide detection range over 0.4-41,950 μM and an extremely low detection limit of 33.3 ± 0.024 nM, which are much better than previously reported sensors based on various nanostructured materials. Besides, the A-Co-NG sensor also demonstrates its accurate serum diagnosis for UA for its practical application. Combination of experimental and theoretical calculation discovers that the catalytic process of the A-Co-NG toward UA starts from the oxidation of Co species to form a Co³⁺-OH-UA*, followed by the generation of Co³⁺-OH + ^*UA_H, eventually leading to N-H bond dissociation for the formation of oxidized UA molecule and reduction of oxidized Co³⁺ to Co²⁺ for the regenerated A-Co-NG. This work provides a promising material to realize UA detection with wide detection range and low detection limit to meet the practical diagnosis requirements, and the proposed sensing mechanism sheds light on fundamental insights for guiding exploration of other biosensing processes.

Catalase immobilization onto magnetic multi-walled carbon nanotubes: optimization of crucial parameters using response surface methodology

In this study catalase (CAT) immobilization onto magnetic multi-walled carbon nanotubes (mMWCNTs) was undertaken and response surface methodology (RSM) employed to determine the optimum immobilization conditions.

Nanomaterials: Electrochemical Properties and Application in Sensors

The unique properties of nanoparticles make them an extremely valuable modifying material, being used in electrochemical sensors. The features of nanoparticles affect the kinetics and thermodynamics of electrode processes of both nanoparticles and redox reactions occurring on their surface. The paper describes theoretical background and experimental studies of these processes. During the transition from macro- to micro- and nanostructures, the analytical characteristics of sensors modify. These features of metal nanoparticles are related to their size and energy effects, which affects the analytical characteristics of developed sensors. Modification of the macroelectrode with nanoparticles and other nanomaterials reduces the detection limit and improves the degree of sensitivity and selectivity of measurements. The use of nanoparticles as transducers, catalytic constituents, parts of electrochemical sensors for antioxidant detection, adsorbents, analyte transporters, and labels in electrochemical immunosensors and signal-generating elements is described.

Fabrication and Characterization of Glucose Biosensors by Using Hydrothermally Grown ZnO Nanorods

Highly oriented ZnO nanorod (NR) arrays were fabricated on a seeded substrate through a hydrothermal route. The prepared ZnO nanorods were used as an amperometric enzyme electrode, in which glucose oxidase (GOx) was immobilised through physical adsorption. The modified electrode was designated as Nafion/GOx/ZnO NRs/ITO. The morphology and structural properties of the fabricated ZnO nanorods were analysed using field-emission scanning electron microscope and X-ray diffractometer. The electrochemical properties of the fabricated biosensor were studied by cyclic voltammetry and amperometry. Electrolyte pH, electrolyte temperature and enzyme concentration used for immobilisation were the examined parameters influencing enzyme activity and biosensor performance. The immobilised enzyme electrode showed good GOx retention activity. The amount of electroactive GOx was 7.82 × 10⁻⁸ mol/cm², which was relatively higher than previously reported values. The Nafion/GOx/ZnO NRs/ITO electrode also displayed a linear response to glucose ranging from 0.05 mM to 1 mM, with a sensitivity of 48.75 µA/mM and a low Michaelis–Menten constant of 0.34 mM. Thus, the modified electrode can be used as a highly sensitive third-generation glucose biosensor with high resistance against interfering species, such as ascorbic acid, uric acid and L-cysteine. The applicability of the modified electrode was tested using human blood samples. Results were comparable with those obtained using a standard glucometer, indicating the excellent performance of the modified electrode.

Recent advances in designing nanomaterial based biointerfaces for electrochemical biosensing cardiovascular biomarkers

Early diagnosis of cardiovascular disease (CVD) is critically important for successful treatment and recovery of patients. At present, detection of CVD at early stages of its progression becomes a major issue for world health. The nanoscale electrochemical biosensors exhibit diverse outstanding properties, rendering them extremely suitable for the determination of CVD biomarkers at very low concentrations in biological fluids. The unique advantages offered by electrochemical biosensors in terms of sensitivity and stability imparted by nanostructuring the electrode surface together with high affinity and selectivity of bioreceptors have led to the development of new electrochemical biosensing strategies that have introduced as interesting alternatives to conventional methodologies for clinical diagnostics of CVD. This review provides an updated overview of selected examples during the period 2005-2018 involving electrochemical biosensing approaches and signal amplification strategies based on nanomaterials, which have been applied for determination of CVD biomarkers. The studied CVD biomarkers include AXL receptor tyrosine kinase, apolipoproteins, cholesterol, C-reactive protein (CRP), D-dimer, fibrinogen (Fib), glucose, insulin, interleukins, lipoproteins, myoglobin, N-terminal pro-B-type natriuretic peptide (BNP), tumor necrosis factor alpha (TNF-α) and troponins (Tns) on electrochemical transduction format. Identification of new specific CVD biomarkers, multiplex bioassay for the simultaneous determination of biomarkers, emergence of microfluidic biosensors, real-time analysis of biomarkers and point of care validation with high sensitivity and selectivity are the major challenges for future research.

Biofunctionalized "Kiwifruit-Assembly" of Oxidoreductases in Mesoporous ZnO/Carbon Nanoparticles for Efficient Asymmetric Catalysis

A mesoporous ZnO/carbon composite is designed for coimmobilization of two oxidoreductases involving a novel "kiwifruit-assembly" pattern. The coimmobilization of (S)-carbonyl reductase II-glucose dehydrogenase on nanoparticles (SCRII-GDH_nano ) exhibits 40-50% higher specific activity than the free enzyme and significantly improves stabilities of enzymes to heat, pH and solvents. It performs asymmetric catalysis of 75 × 10^-3 m substrate with a perfect yield of 100% and an excellent enantioselectivity of 99.9% within 1 h. SCRII-GDH_nano gives an over 72% yield and 99.9% enantioselectivity after it is reused for ten times. Even with a highly concentrated (400 × 10^-3 m) substrate, it shows about 60% yield and 99.9% enantioselectivity within 4 h. SCRII-GDH_nano presents 4.5-8.0-fold higher productivity in 2.0-8.0-fold shorter reaction time than the free enzyme. This work provides a general, facile, and unique approach for the immobilization of two oxidoreductases and gives high catalytic efficiency, long-term and good recycling stabilities by triggering radical proton-coupled electron transfer.

ZnO-nanorods/graphene heterostructure: a direct electron transfer glucose biosensor

ZnO-nanorods/graphene heterostructure was synthesized by hydrothermal growth of ZnO nanorods on chemically reduced graphene (CRG) film. The hybrid structure was demonstrated as a biosensor, where direct electron transfer between glucose oxidase (GOD) and electrode was observed. The charge transfer was attributed to the ZnO nanorod wiring between the redox center of GOD and electrode, and the ZnO/graphene heterostructure facilitated the transport of electrons on the hybride electrode. The glucose sensor based on the GOD-ZnO/CRG/Pt electrode had a high sensitivity of 17.64 μA mM(-1), which is higher than most of the previously reported values for direct electron transfer based glucose biosensors. Moreover, this biosensor is linearly proportional to the concentration of glucose in the range of 0.2-1.6 mM. The study revealed that the band structure of electrode could affect the detection of direct electron transfer of GOD, which would be helpful for the design of the biosensor electrodes in the future.

Optical and surface band bending mediated fluorescence sensing properties of MoS2 quantum dots

Band bending assisted fluorescence sensing of glucose and bovine serum albumin using MoS₂ quantum dots.

Nanomaterials for biocatalyst immobilization – state of the art and future trends

Advantages, drawbacks and trends in nanomaterials for enzyme immobilization.

H2O/air plasma-functionalized carbon nanotubes decorated with MnO2 for glucose sensing

MnO₂ NPs were decorated on the DBD plasma-functionalized MWCNTs in H₂O vapor-saturated air for glucose detection.

Poly(brilliant green) and poly(thionine) modified carbon nanotube coated carbon film electrodes for glucose and uric acid biosensors

Poly(brilliant green) (PBG) and poly(thionine) (PTH) films have been formed on carbon film electrodes (CFEs) modified with carbon nanotubes (CNT) by electropolymerisation using potential cycling. Voltammetric and electrochemical impedance characterisation were performed. Glucose oxidase and uricase, as model enzymes, were immobilised on top of PBG/CNT/CFE and PTH/CNT/CFE for glucose and uric acid (UA) biosensing. Amperometric determination of glucose and UA was carried out in phosphate buffer pH 7.0 at -0.20 and +0.30 V vs. SCE, respectively, and the results were compared with other similarly modified electrodes existing in the literature. An interference study and recovery measurements in natural samples were successfully performed, indicating these architectures to be good and promising biosensor platforms.

DNA-templated synthesis of PtAu bimetallic nanoparticle/graphene nanocomposites and their application in glucose biosensor

In this paper, single-stranded DNA (ss-DNA) is demonstrated to functionalize graphene (GR) and to further guide the growth of PtAu bimetallic nanoparticles (PtAuNPs) on GR with high densities and dispersion. The obtained nanocomposites (PtAuNPs/ss-DNA/GR) were characterized by transmission electron microscopy (TEM), energy-dispersive X-ray spectrometer (EDS), and electrochemical techniques. Then, an enzyme nanoassembly was prepared by self-assembling glucose oxidase (GOD) on PtAuNP/ss-DNA/GR nanocomposites (GOD/PtAuNPs/ss-DNA/GR). The nanocomposites provided a suitable microenvironment for GOD to retain its biological activity. The direct and reversible electron transfer process between the active site of GOD and the modified electrode was realized without any extra electron mediator. Thus, the prepared GOD/PtAuNP/ss-DNA/GR electrode was proposed as a biosensor for the quantification of glucose. The effects of pH, applied potential, and temperature on the performance of the biosensor were discussed in detail and were optimized. Under optimal conditions, the biosensor showed a linearity with glucose concentration in the range of 1.0 to 1,800 μM with a detection limit of 0.3 μM (S/N = 3). The results demonstrate that the developed approach provides a promising strategy to improve the sensitivity and enzyme activity of electrochemical biosensors.

Direct electron transfer of glucose oxidase and biosensing for glucose based on PDDA-capped gold nanoparticle modified graphene/multi-walled carbon nanotubes electrode

In this work, poly (diallyldimethylammonium chloride) (PDDA)-capped gold nanoparticles (AuNPs) functionalized graphene (G)/multi-walled carbon nanotubes (MWCNTs) nanocomposites were fabricated. Based on the electrostatic attraction, the G/MWCNTs hybrid material can be decorated with AuNPs uniformly and densely. The new hierarchical nanostructure can provide a larger surface area and a more favorable microenvironment for electron transfer. The AuNPs/G/MWCNTs nanocomposite was used as a novel immobilization platform for glucose oxidase (GOD). Direct electron transfer (DET) was achieved between GOD and the electrode. Field emission scanning electron microscopy (FESEM), UV-vis spectroscopy and cyclic voltammetry (CV) were used to characterize the electrochemical biosensor. The glucose biosensor fabricated based on GOD electrode modified with AuNPs/G/MWCNTs demonstrated satisfactory analytical performance with high sensitivity (29.72mAM(-1)cm(-2)) and low limit of detection (4.8 µM). The heterogeneous electron transfer rate constant (ΚS) and the apparent Michaelis-Menten constant (Km) of GOD were calculated to be 11.18s(-1) and 2.09 mM, respectively. With satisfactory selectivity, reproducibility, and stability, the nanostructure we proposed offered an alternative for electrode fabricating and glucose biosensing.

Micro- to nanostructured poly(pyrrole-nitrilotriacetic acid) films via nanosphere templates: applications to 3D enzyme attachment by affinity interactions

We report the combination of latex nanosphere lithography with electropolymerization of N-substituted pyrrole monomer bearing a nitrilotriacetic acid (NTA) moiety for the template-assisted nanostructuration of poly(pyrrole-NTA) films and their application for biomolecule immobilization. The electrodes were modified by casting latex beads (100 or 900 nm in diameter) on their surface followed by electropolymerization of the pyrrole-NTA monomer and the subsequent chelation of Cu(2+) ions. The dissolution of the nanobeads leads then to a nanostructured polymer film with increased surface. Thanks to the versatile affinity interactions between the (NTA)Cu(2+) complex and histidine- or biotin-tagged proteins, both tyrosinase and glucose oxidase were immobilized on the modified electrode. Nanostructuration of the polypyrrole via nanosphere lithography (NSL) using 900- and 100-nm latex beads allows an increase in surface concentration of enzymes anchored on the functionalized polypyrrole electrode. The nanostructured enzyme electrodes were characterized by fluorescence microscopy, 3D laser scanning confocal microscopy, and scanning electron microscopy. Electrochemical studies demonstrate the increase in the amount of immobilized biomolecules and associated biosensor performances when achieving NSL compared to conventional polymer formation without bead template. In addition, the decrease in nanobead diameter from 900 to 100 nm provides an enhancement in biosensor performance. Between biosensors based on films polymerized without nanobeads and with 100-nm nanobeads, maximum current density values increase from 4 to 56 μA cm(-2) and from 7 to 45 μA cm(-2) for biosensors based on tyrosinase and glucose oxidase, respectively.

Biofuel cells for biomedical applications: colonizing the animal kingdom

Interdisciplinary research has combined the efforts of many scientists and engineers to gain an understanding of biotic and abiotic electrochemical processes, materials properties, biomedical, and engineering approaches for the development of alternative power-generating and/or energy-harvesting devices, aiming to solve health-related issues and to improve the quality of human life. This review intends to recapitulate the principles of biofuel cell development and the progress over the years, thanks to the contribution of cross-disciplinary researchers that have combined knowledge and innovative ideas to the field. The emergence of biofuel cells, as a response to the demand of electrical power devices that can operate under physiological conditions, are reviewed. Implantable biofuel cells operating inside living organisms have been envisioned for over fifty years, but few reports of implanted devices have existed up until very recently. The very first report of an implanted biofuel cell (implanted in a grape) was published only in 2003 by Adam Heller and his coworkers. This work was a result of earlier scientific efforts of this group to "wire" enzymes to the electrode surface. The last couple of years have, however, seen a multitude of biofuel cells being implanted and operating in different living organisms, including mammals. Herein, the evolution of the biofuel concept, the understanding and employment of catalyst and biocatalyst processes to mimic biological processes, are explored. These potentially green technology biodevices are designed to be applied for biomedical applications to power nano- and microelectronic devices, drug delivery systems, biosensors, and many more.

Iron oxide filled magnetic carbon nanotube-enzyme conjugates for recycling of amyloglucosidase: toward useful applications in biofuel production process

Biofuels are fast advancing as a new research area to provide alternative sources of sustainable and clean energy. Recent advances in nanotechnology have sought to improve the efficiency of biofuel production, enhancing energy security. In this study, we have incorporated iron oxide nanoparticles into single-walled carbon nanotubes (SWCNTs) to produce magnetic single-walled carbon nanotubes (mSWCNTs). Our objective is to bridge both nanotechnology and biofuel production by immobilizing the enzyme, Amyloglucosidase (AMG), onto mSWCNTs using physical adsorption and covalent immobilization, with the aim of recycling the immobilized enzyme, toward useful applications in biofuel production processes. We have demonstrated that the enzyme retains a certain percentage of its catalytic efficiency (up to 40%) in starch prototype biomass hydrolysis when used repeatedly (up to ten cycles) after immobilization on mSWCNTs, since the nanotubes can be easily separated from the reaction mixture using a simple magnet. The enzyme loading, activity, and structural changes after immobilization onto mSWCNTs were also studied. In addition, we have demonstrated that the immobilized enzyme retains its activity when stored at 4 °C for at least one month. These results, combined with the unique intrinsic properties of the nanotubes, pave the way for greater efficiency in carbon nanotube-enzyme bioreactors and reduced capital costs in industrial enzyme systems.

Graphene quantum dots as a new substrate for immobilization and direct electrochemistry of glucose oxidase: application to sensitive glucose determination

Graphene quantum dots (GQD) were introduced as a novel and suitable substrate for enzyme immobilization. Glucose oxidase (GOx) was immobilized on GQD modified carbon ceramic electrode (CCE) and well-defined quasi-reversible redox peaks were observed. The UV-vis photoluminescence spectroscopy, transition electron microscopy, field emission scanning electron microscopy, electrochemical impedance spectroscopy, and cyclic voltammetry techniques were used for characterizing the electrochemical biosensor. The electron transfer coefficient (α) and the heterogeneous electron transfer rate constant (k(s)) for redox reaction of GOx were found to be 0.48 and 1.12 s(-1), respectively. The developed biosensor responds efficiently to glucose presence over the concentration range 5-1270 μM with the detection limit 1.73 μM (S/N=3) and sensitivity 0.085 μA μM(-1) cm(-2). The high value of surface coverage GOx-GQD|CCE (1.8×10(-9) mol/cm(2)) and the small value of Michaelis-Menten constant (0.76 mM) confirmed an excellent loading of the enzyme and a high affinity of biosensor to glucose. High performance of the biosensor is attributed to the large surface-to-volume ratio, excellent biocompatibility of GQD, porosity of GQD|CCE, and the abundance of hydrophilic edges as well as hydrophobic plane in GQD which enhances the enzyme absorption on the electrode surface.

Recent advances in ZnO nanostructures and thin films for biosensor applications: review

Biosensors have shown great potential for health care and environmental monitoring. The performance of biosensors depends on their components, among which the matrix material, i.e., the layer between the recognition layer of biomolecule and transducer, plays a crucial role in defining the stability, sensitivity and shelf-life of a biosensor. Recently, zinc oxide (ZnO) nanostructures and thin films have attracted much interest as materials for biosensors due to their biocompatibility, chemical stability, high isoelectric point, electrochemical activity, high electron mobility, ease of synthesis by diverse methods and high surface-to-volume ratio. ZnO nanostructures have shown the binding of biomolecules in desired orientations with improved conformation and high biological activity, resulting in enhanced sensing characteristics. Furthermore, compatibility with complementary metal oxide semiconductor technology for constructing integrated circuits makes ZnO nanostructures suitable candidate for future small integrated biosensor devices. This review highlights recent advances in various approaches towards synthesis of ZnO nanostructures and thin films and their applications in biosensor technology.