Layer by Layer Immobilized Horseradish Peroxidase on Zinc Oxide Nanorods for Biosensing

Preparation of electrochemical horseradish peroxidase biosensor with black phosphorene-zinc oxide nanocomposite and their applications

In this work, a novel and sensitive electrochemical biosensor was constructed based on a black phosphorene (BP) and nanosized zinc oxide (ZnO@BP) nanocomposite as a modifier, which was used for the immobilization of horseradish peroxidase (HRP) on a carbon ionic liquid electrode (CILE). The ZnO@BP nanocomposite was synthesized by a simple in situ hydrothermal method with stripped black phosphorus nanoplates and ZnO. The ZnO@BP and HRP-modified electrode was developed by a casting method. ZnO@BP with highly conductivity, large surface area and good biocompatibility could maintain the bioactivity of HRP and accelerate the electron transfer rate. Cyclic voltammetry was used to study the direct electrochemistry of HRP on the Nafion/HRP/ZnO@BP/CILE with the appearance of a pair of distinct redox peaks. The constructed electrochemical HRP biosensor exhibited excellent electrocatalytic effects on the reduction of trichloroacetic acid and sodium nitrite. Real samples were detected with satisfactory results, which demonstrated the potential applications of this electrochemical HRP biosensor.

A Bioinspired Five-Coordinated Single-Atom Iron Nanozyme for Tumor Catalytic Therapy

Single-atom nanozymes (SAzymes) represent a new research frontier in the biomedical fields. The rational design and controllable synthesis of SAzymes with well-defined electronic and geometric structures are essential for maximizing their enzyme-like catalytic activity and therapeutic efficacy but remain challenging. Here, a melamine-mediated pyrolysis activation strategy is reported for the controllable fabrication of iron-based SAzyme containing five-coordinated structure (FeN₅ ), identified by transmission electron microscopy imaging and X-ray absorption fine structure analyses. The FeN₅ SAzyme exhibits superior peroxidase-like activity owing to the optimized coordination structure, and the corresponding catalytic efficiency of Fe-species in the FeN₅ SAzyme is 7.64 and 3.45 × 10⁵ times higher than those in traditional FeN₄ SAzyme and Fe₃ O₄ nanozyme, respectively, demonstrated by steady-state kinetic assay. In addition, the catalytic mechanism is jointly disclosed by experimental results and density functional theory studies. The as-synthesized FeN₅ SAzyme demonstrates significantly enhanced antitumor effect in vitro and in vivo due to the excellent peroxidase-like activity under tumor microenvironment.

Fabrication of Hierarchical Nanoreactor based on COFs for Cascade Enzyme Catalysis

A hierarchical nanoreactor based on covalent organic frameworks (COFs) was fabricated using polystyrene spheres (PSs) as template. When applied in cascade catalysis, the nanoreactor improved significantly the catalytic activity of...

Fabrication of the Ni/ZnO/BiOI foam for the improved electrochemical biosensing performance to glucose

The Ni foam decorated with ZnO/BiOI core-shell p-n junction nanorods was prepared and employed as an enzyme loading matrix to detect glucose. The detection potential was decreased significantly (0.3 V) and the sensitivity was enhanced largely (115.2 μA mM^-1 cm^-2). The metal-semiconductor foam can afford the porous surface for loading enzymes and achieving the multiple catalysis. More important, the built-in electric field and electron well in the p-n junction interface provide the driving force for electron transport. It was an effective strategy to enhance the biosensing performance by the rational design of p-n junction.

3D-printed graphene direct electron transfer enzyme biosensors

Three-dimensional (3D) printing technology offers attractive possibilities for many fields. In electrochemistry, 3D printing technology has been used to fabricate customized 3D-printed electrodes as a platform to develop bio/sensing, energy generation and storage devices. Here, we use a 3D-printed graphene/polylactic (PLA) electrode made by additive manufacturing technology and immobilize horseradish peroxidase (HRP) to create a direct electron transfer enzyme-based biosensors for hydrogen peroxide detection. Gold nanoparticles are included in the system to confirm and facilitate heterogeneous electron transfer. This work opens a new direction for the fabrication of third-generation electrochemical biosensors using 3D printing technology, with implications for applications in the environmental and biomedical fields.

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.

Evaluation of the Corrosion Resistance Properties of Electroplated Chitosan-Zn1-xCuxO Composite Thin Films

Novel chitosan-zinc copper oxide (Zn_1-xCu_xO) composites were electrochemically synthesized through galvanostatic deposition. The prepared chitosan-based composite thin films were elaborately investigated to determine their structural, morphological, compositional, impedance, and corrosion properties. X-ray diffraction analysis was performed to reveal their structural orientation of composite thin films. Energy dispersive analysis by X-ray evidently confirmed the existence of Zn, Cu, and O in the composite thin films. Nyquist plots revealed that the chitosan-Zn_1-xCu_xO thin films had obvious semi-circular boundaries, and higher resistance was observed for chitosan-ZnO due to the grain boundary effect. Corrosion properties were evaluated using both an electrochemical method and the ASTM weight gain method, which revealed good corrosion rates of 34 and 35 × 10^-3 mm/y, respectively, for chitosan-ZnO thin film.

Liver Cancer Detection by a Simple, Inexpensive and Effective Immunosensor with Zinc Oxide Nanoparticles

Regular monitoring of blood α-fetoprotein (AFP) and/or carcino-embryonic antigen (CEA) levels is important for the routine screening of liver cancer. However, AFP and CEA have a much lower specificity than des-γ-carboxyprothrombin (DCP) to detect liver cancer. Therefore, the study reported here was designed, to develop a screen-printed DCP immunosensor incorporating zinc oxide nanoparticles, for accurate determination of DCP. The designed immunosensor shows low detection limits for the detection of DCP: 0.440 ng/mL (based on impedance measurement), 0.081 ng/mL (based on real part of impedance measurement) and 0.078 ng/mL (based on imaginary part of impedance measurement), within the range of 3.125 ng/mL to 2000 ng/mL. In addition, there was little interference to DCP determination by molecules such as Na⁺, K⁺, Ca²⁺, Cl⁻, glucose, urea, and uric acid. It is therefore concluded that the DCP immunosensor developed and reported here is simple, inexpensive and effective, and shows promise in the rapid screening of early-stage liver cancer at home with a point-of-care approach.

Immunosensing procedures for carcinoembryonic antigen using graphene and nanocomposites

Two-dimensional (2D) graphene, sp²-hybridized carbon, and its two major derivatives, graphene oxide (GO) and reduced graphene oxide (rGO) have played an important role in immunoassays (IAs) and immunosensing (IMS) platforms for the detection of carcinoembryonic antigen (CEA), an implicated tumor biomarker found in several types of cancer. The graphene family with high surface area is functionalized to form stable nanocomposites with gold nanoparticles (AuNPs) and electron mediators. The capture anti-CEA antibody (Ab) with high density can be anchored on AuNPs of such composites to provide remarkable detection sensitivity, significantly below the level found in normal subjects and cancer patients. Electrochemical and fluorescence/chemiluminescence-quenching properties of graphene-based nanocomposites are exploited in various detection schemes. Future endeavors are envisioned for the development of an array platform with high-throughput for CEA together with other tumor biomarkers and C-reactive protein, a universal biomarker for infection and inflammation. The ongoing efforts dedicated to the replacement of a lab-based detector by a cellphone with smart applications will further enable cost-effective and frequent monitoring of CEA in order to establish its clinical relevance and provide tools for real-time monitoring of patients during chemotherapy.

An Electrochemical Glucose Sensor Based on Zinc Oxide Nanorods

A glucose electrochemical sensor based on zinc oxide (ZnO) nanorods was investigated. The hydrothermal sol–gel growth method was utilized to grow ZnO nanorods on indium tin oxide-coated glass substrates. The total active area of the working electrode was 0.3 × 0.3 cm² where titanium metal was deposited to enhance the contact. Well aligned hexagonal structured ZnO nanorods with a diameter from 68 to 116 nm were obtained. The excitonic peak obtained from the absorbance spectroscopy was observed at ~370 nm. The dominant peak of Raman spectroscopy measurement was at 440 cm⁻¹, matching with the lattice vibration of ZnO. The uniform distribution of the GO_x and Nafion membrane that has been done using spin coating technique at 4000 rotations per minute helps in enhancing the ion exchange and increasing the sensitivity of the fabricated electrochemical sensor. The amperometric response of the fabricated electrochemical sensor was 3 s. The obtained sensitivity of the fabricated ZnO electrochemical sensor was 10.911 mA/mM·cm² and the lower limit of detection was 0.22 µM.

Protein adsorption onto nanomaterials for the development of biosensors and analytical devices: a review

An important consideration for the development of biosensors is the adsorption of the biorecognition element to the surface of a substrate. As the first step in the immobilization process, adsorption affects most immobilization routes and much attention is given into the research of this process to maximize the overall activity of the biosensor. The use of nanomaterials, specifically nanoparticles and nanostructured films, offers advantageous properties that can be fine-tuned to maximize interactions with specific proteins to maximize activity, minimize structural changes, and enhance the catalytic step. In the biosensor field, protein-nanomaterial interactions are an emerging trend that span across many disciplines. This review addresses recent publications about the proteins most frequently used, their most relevant characteristics, and the conditions required to adsorb them to nanomaterials. When relevant and available, subsequent analytical figures of merits are discussed for selected biosensors. The general trend amongst the research papers allows concluding that the use of nanomaterials has already provided significant improvements in the analytical performance of many biosensors and that this research field will continue to grow.

Layer-by-layer assembled carbon nanotube-acetylcholinesterase/biopolymer renewable interfaces: SPR and electrochemical characterization

Developing simple, reliable, and cost-effective methods of renewing an inhibited biocatalyst (e.g., enzymatic interfaces) on biosensors is needed to advance multiuse, reusable sensor applications. We report a method for the renewal of layer-by-layer (LbL) self-assembled inhibition-based enzymatic interfaces in multiwalled carbon nanotube (MWCNT) armored acetylcholinesterase (AChE) biosensors. The self-assembly process of MWCNT dispersed enzymes/biopolymers was investigated using surface plasmon resonance (SPR). The LbL fabrication consisted of alternating cushion layers of positively charged CNT-polyethylenimine (CNT-PEI) and negatively charged CNT-deoxyribonucleic acid (CNT-DNA) and a functional interface consisting of alternating layers of CNT-PEI and negatively charged CNT-acetylcholine esterase (CNT-AChE, pH 7.4). The observed SPR response signal increased while assembling the different layers, indicating the buildup of multiple layers on the Au surface. A partial desorption of the top enzymatic layer in the LbL structure was observed with a desorption strategy employing alkaline treatment. This indicates that the strong interaction of CNT-biopolymer conjugates with the Au surface was a result of both electrostatic interactions between biopolymers and the surface binding energy from CNTs: the closer the layers are to the Au surface, the stronger the interactions. In contrast, a similar LbL assembly of soluble enzyme/polyelectrolytes resulted in stronger desorption on the surface after the alkaline treatment; this led to the investigation of AChE layer removal, permanently inhibited after pesticide exposure on glassy carbon (GC) electrodes, while keeping the cushion layers intact. The desorption strategy permitted the SPR and electrochemical electrode surfaces to be regenerated multiple times by the subsequent self-assembly of fresh PEI/AChE layers. Flow-mode electrochemical amperometric analysis demonstrated good stability toward the determination of acetylcholine with 97.1 ± 2.7% renewability. Our simple, inexpensive approach shows the potential of renewable LbL self-assembled functional interfaces for multiple uses in a wide field of applications such as biosensing, various biotechnological processes, and the food and health industries.

Macroporous ZnO foams by high internal phase emulsion technique: synthesis and catalytic activity

Zinc(II) oxide nanoparticles were used for the stabilization of dicyclopentadiene (DCPD)-water-based high internal phase emulsions (HIPEs), which were subsequently cured using ring-opening metathesis polymerization (ROMP). The morphology of the resulting ZnO-pDCPD nanocomposite foams was investigated in correlation to the nanoparticle loading and nanoparticle surface chemistry. While hydrophilic ZnO nanoparticles were found to be unsuitable for stabilizing the HIPE, oleic acid coated, yet hydrophobic ZnO nanoparticles were effective HIPE stabilizers, yielding polymer foams with ZnO nanoparticles located predominately at their surface. These inorganic/organic hybrid foam-materials were subsequently calcined at 550 °C for 15 min to obtain inorganic macroporous ZnO foams with a morphology reminiscent to the original hybrid foam, and a specific surface area of 1.5 m(2) g(-1). Longer calcination time (550 °C, 15 h) resulted in a sea urchin like morphology of the ZnO foams, characterized by higher specific surface area of 5.5 m(2) g(-1). The latter foam type showed an appealing catalytic performance in the catalytic wet air oxidation (CWAO) process for the destruction of bisphenol A.

Catalytic activity and stability of glucose oxidase/horseradish peroxidase co-confined in macroporous silica foam

Investigation of the catalytic activity and stability of enzymes in confined nano/microspace provides valuable contributions to the fundamental understanding of biological reactions taking place on a mesoscopic scale within confined spaces. In this paper, macroporous silica foam (MSF) is used as a nanoreactor to co-confine glucose oxidase (GOD) and horseradish peroxidase (HRP). Then, the enzymatic cascade reactions, which act in tandem inside nanoreactors, for oxidation of glucose and 3,3',5,5'-tetramethylbenzidine (TMB) were studied. The catalytic kinetic parameters of apparent Michaelis constant (K(m)(app)) and maximum rate (V(max)) were obtained from Lineweaver-Burk plot by UV-vis spectrometry. Results showed that the catalytic activity of the co-confined enzymes is reduced compared to that of free enzymes in solution at room temperature. The stabilities of co-confined enzymes in denaturing agents, such as guanidinium chloride (GdmCl) and urea, were higher than those of free enzymes in solution. When employing a co-confined bienzyme system as a biosensor for the detection of glucose, a wider linear range of glucose was obtained for the co-confined bienzyme system than for free enzymes in solution.

Process of in situ forming well-aligned zinc oxide nanorod arrays on wood substrate using a two-step bottom-up method

A good nanocrystal covering layer on wood can serve as a protective coating and present some new surface properties. In this study, well-aligned ZnO nanorods (NRs) arrays were successfully grown on wood surface through a two-step bottom-up growth process. The process involved pre-sow seeds and subsequently their growing into NRs under hydrothermal environment. The interface incorporation between wood and ZnO colloid particles in the precursor solution during the seeding process was analyzed and demonstrated through a schematic. The growth process of forming well-aligned ZnO NRs was analyzed by field-emission scanning electron microscopy and X-ray diffraction, which showed that the NRs elongated with increased reaction time. The effects of ZnO crystal form and capping agent on the growth process were studied through different viewpoints.

A novel amperometric biosensor based on ZnO nanoparticles-modified carbon paste electrode for determination of glucose in human serum

Zinc oxide nanoparticles-(ZnONPs)modified carbon paste enzyme electrodes (ZnONPsMCPE) were developed for determination of glucose. The determination of glucose was carried out by oxidation of H2O2 at +0.4 V. ZnONPsMCPE provided biocompatible microenvironment for GOx and necessary pathway of electron transfer between GOx and electrode. The response of GOx/ZnONPsMCPE was proportional to glucose concentration and detection limit was 9.1 × 10(-3) mM. Km and Imax, were calculated as 0.124 mM and 2.033 μA. The developed biosensor exhibits high analytical performance with wide linear range (9.1 × 10(-3)-14.5 mM), selectivity and reproducibility. Serum glucose results allow us to ascertain practical utility of GOx/ZnONPsMCPE biosensor.

Horseradish Peroxidase Immobilization on Amine Functionalized Carbon Nano Tubes: Direct Electrochemistry and Bioelectrocatalysis

Horseradish peroxidase (HRP) was successfully immobilized on amine functionalized TiO2-coated multiwalled carbon nanotubes (NH2 TiO2 CNTs) by a convenient and efficient method. Electrochemical impedance spectroscopy, cyclic voltammetry and amperometry were applied to characterize the HRP/NH2- TiO2 - CNT nano-composite. These techniques showed that the NH2 TiO2CNTs greatly enhance the electron transfer between HRP and the modified electrode. Owing to the redox reaction of the electroactive centre of HRP, the HRP/NH2-TiO2-CNTs modified electrode exhibited a pair of quasi-reversible peaks with a peak-to-peak separation (Δ Ep) of 70.6 m V and a formal potential ( E°’) of - 367.65 m V (versus Ag/AgCl) in phosphate buffer solution. The charge transfer coefficient (a) and the apparent charge transfer rate constant (ks) were found to be 0.34 and 2.08 s-1 respectively. The prepared biosensor responded to H2O2 with a linear range, detection limit, sensitivity and response time of 1.0 × 10−9 to 1.0 × 10 −7 M, 0.786nM, 28.4 μA A nM−1 and 3 s, respectively.

ZnO/Cu nanocomposite: a platform for direct electrochemistry of enzymes and biosensing applications

Unique structured nanomaterials can facilitate the direct electron transfer between redox proteins and the electrodes. Here, in situ directed growth on an electrode of a ZnO/Cu nanocomposite was prepared by a simple corrosion approach, which enables robust mechanical adhesion and electrical contact between the nanostructured ZnO and the electrodes. This is great help to realize the direct electron transfer between the electrode surface and the redox protein. SEM images demonstrate that the morphology of the ZnO/Cu nanocomposite has a large specific surface area, which is favorable to immobilize the biomolecules and construct biosensors. Using glucose oxidase (GOx) as a model, this ZnO/Cu nanocomposite is employed for immobilization of GOx and the construction of the glucose biosensor. Direct electron transfer of GOx is achieved at ZnO/Cu nanocomposite with a high heterogeneous electron transfer rate constant of 0.67 ± 0.06 s(-1). Such ZnO/Cu nanocomposite provides a good matrix for direct electrochemistry of enzymes and mediator-free enzymatic biosensors.