Enzyme mimic nanomaterials as nanozymes with catalytic attributes

Nanozymes are super-efficient nanomaterials with enzyme-like characteristics, as the name suggests. In the last decade, efforts have been made to develop "artificial enzymes," which are alternatives to natural enzymes. As nanoscience and nanotechnology advance, nanozymes, which are catalytic nanomaterials having enzyme-like properties, have fascinated researchers' attention. Nanozymes with unique physicochemical properties and nanomaterials that mimic catalytic activity have gained a special interest in the industrial sectors. However, several constraints have hampered their effective deployment in industrial processes, including denaturation, time-consuming manufacturing, overall high cost-ratio, and reutilization challenges. After a brief overview of nanozyme research, an analysis of the similarities and differences between nanozymes and natural and synthetic enzymes is presented. Because of their distinct properties, nanozymes stand out in this comparison. Nanozymes have exhibited a variety of applications leveraging the physiochemical properties of nanomaterials, ranging from in vitro detection to enzyme substitution in biological systems. In addition, nanozymes have introduced a new field called nanozymology, which blends nanotechnology with enzymology.

A Short Overview of Biological Fuel Cells

This short review summarizes the improvements on biological fuel cells (BioFCs) with or without ionomer separation membrane. After a general introduction about the main challenges of modern energy management, BioFCs are presented including microbial fuel cells (MFCs) and enzymatic fuel cells (EFCs). The benefits of BioFCs include the capability to derive energy from waste-water and organic matter, the possibility to use bacteria or enzymes to replace expensive catalysts such as platinum, the high selectivity of the electrode reactions that allow working with less complicated systems, without the need for high purification, and the lower environmental impact. In comparison with classical FCs and given their lower electrochemical performances, BioFCs have, up to now, only found niche applications with low power needs, but they could become a green solution in the perspective of sustainable development and the circular economy. Ion exchange membranes for utilization in BioFCs are discussed in the final section of the review: they include perfluorinated proton exchange membranes but also aromatic polymers grafted with proton or anion exchange groups.

Energy Harvesting Untethered Soft Electronic Devices

Advances in wearable and stretchable electronic technologies have yielded a wide range of electronic devices that can be conformably worn by, or implanted in humans to measure physiological signals. Moreover, various cutting-edge technologies for battery-free electronic devices have led to advances in healthcare devices that can continuously measure long-term biosignals for advanced human-machine interface and clinical diagnostics. This report presents the recent progress in battery-less, wearable devices using a wide range of energy harvesting sources, such as electromagnetic energy, mechanical energy, and biofuels. Additionally, this report also discusses the principles and working mechanisms of near/far-field communications, triboelectric, thermoelectric, and biofuel technologies.

Charge Transfer and Biocompatibility Aspects in Conducting Polymer-Based Enzymatic Biosensors and Biofuel Cells

Charge transfer (CT) is a very important issue in the design of biosensors and biofuel cells. Some nanomaterials can be applied to facilitate the CT in these bioelectronics-based devices. In this review, we overview some CT mechanisms and/or pathways that are the most frequently established between redox enzymes and electrodes. Facilitation of indirect CT by the application of some nanomaterials is frequently applied in electrochemical enzymatic biosensors and biofuel cells. More sophisticated and still rather rarely observed is direct charge transfer (DCT), which is often addressed as direct electron transfer (DET), therefore, DCT/DET is also targeted and discussed in this review. The application of conducting polymers (CPs) for the immobilization of enzymes and facilitation of charge transfer during the design of biosensors and biofuel cells are overviewed. Significant attention is paid to various ways of synthesis and application of conducting polymers such as polyaniline, polypyrrole, polythiophene poly(3,4-ethylenedioxythiophene). Some DCT/DET mechanisms in CP-based sensors and biosensors are discussed, taking into account that not only charge transfer via electrons, but also charge transfer via holes can play a crucial role in the design of bioelectronics-based devices. Biocompatibility aspects of CPs, which provides important advantages essential for implantable bioelectronics, are discussed.

Effect of Surface and Bulk Properties of Mesoporous Carbons on the Electrochemical Behavior of GOx-Nanocomposites

Biofuel cell (BFC) electrodes are typically manufactured by combining enzymes that act as catalysts with conductive carbon nanomaterials in a form of enzyme-nanocomposite. However, a little attention has been paid to effects of the carbon nanomaterials' structural properties on the electrochemical performances of the enzyme-nanocomposites. This work aims at studying the effects of surface and bulk properties of carbon nanomaterials with different degrees of graphitization on the electrochemical performances of glucose oxidase (GOx)-nanocomposites produced by immobilizing GOx within a network of carbon nanopaticles. Two types of carbon nanomaterials were used: graphitized mesoporous carbon (GMC) and purified mesoporous carbon (PMC). Graphitization index, surface functional groups, hydrophobic properties, and rate of aggregation were measured for as-received and acid-treated GMC and PMC samples by using Raman spectrometry, X-ray photoelectron spectroscopy (XPS), contact angle measurement, and dynamic light scattering (DLS), respectively. In addition to these physical property characterizations, the enzyme loading and electrochemical performances of the GOx-nanocomposites were studied via elemental analysis and cyclic voltammetry tests, respectively. We also fabricated BFCs using our GOx-nanocomposite materials as the enzyme anodes, and tested their performances by obtaining current-voltage (IV) plots. Our findings suggest that the electrochemical performance of GOx-nanocomposite material is determined by the combined effects of graphitization index, electrical conductivity and surface chemistry of carbon nanomaterials.

Nano-Immobilized Biocatalysts for Biodiesel Production from Renewable and Sustainable Resources

The cost of biodiesel production relies on feedstock cost. Edible oil is unfavorable as a biodiesel feedstock because of its expensive price. Thus, non-edible crop oil, waste oil, and microalgae oil have been considered as alternative resources. Non-edible crop oil and waste cooking oil are more suitable for enzymatic transesterification because they include a large amount of free fatty acids. Recently, enzymes have been integrated with nanomaterials as immobilization carriers. Nanomaterials can increase biocatalytic efficiency. The development of a nano-immobilized enzyme is one of the key factors for cost-effective biodiesel production. This paper presents the technology development of nanomaterials, including nanoparticles (magnetic and non-magnetic), carbon nanotubes, and nanofibers, and their application to the nano-immobilization of biocatalysts. The current status of biodiesel production using a variety of nano-immobilized lipase is also discussed.

Fabrication of enzyme-based coatings on intact multi-walled carbon nanotubes as highly effective electrodes in biofuel cells

CNTs need to be dispersed in aqueous solution for their successful use, and most methods to disperse CNTs rely on tedious and time-consuming acid-based oxidation. Here, we report the simple dispersion of intact multi-walled carbon nanotubes (CNTs) by adding them directly into an aqueous solution of glucose oxidase (GOx), resulting in simultaneous CNT dispersion and facile enzyme immobilization through sequential enzyme adsorption, precipitation, and crosslinking (EAPC). The EAPC achieved high enzyme loading and stability because of crosslinked enzyme coatings on intact CNTs, while obviating the chemical pretreatment that can seriously damage the electron conductivity of CNTs. EAPC-driven GOx activity was 4.5- and 11-times higher than those of covalently-attached GOx (CA) on acid-treated CNTs and simply-adsorbed GOx (ADS) on intact CNTs, respectively. EAPC showed no decrease of GOx activity for 270 days. EAPC was employed to prepare the enzyme anodes for biofuel cells, and the EAPC anode produced 7.5-times higher power output than the CA anode. Even with a higher amount of bound non-conductive enzymes, the EAPC anode showed 1.7-fold higher electron transfer rate than the CA anode. The EAPC on intact CNTs can improve enzyme loading and stability with key routes of improved electron transfer in various biosensing and bioelectronics devices.

Enzymatic Biofuel Cells on Porous Nanostructures

Biofuel cells (BFCs) that utilize enzymes as catalysts represent a new sustainable and renewable energy technology. Numerous efforts have been directed to improve the performance of the enzymatic BFCs (EBFCs) with respect to power output and operational stability for further applications in portable power sources, self-powered electrochemical sensing, implantable medical devices, etc. The latest advances in EBFCs based on porous nanoarchitectures over the past 5 years are detailed here. Porous matrices from carbon, noble metals, and polymers promote the development of EBFCs through the electron transfer and mass transport benefits. Some key issues regarding how these nanostructured porous media improve the performance of EBFCs are also discussed.

Fabrication of a biofuel cell improved by the π-conjugated electron pathway effect induced from a new enzyme catalyst employing terephthalaldehyde

A model explaining the π-conjugated electron pathway effect induced by a novel cross-linker adopted enzyme catalyst is suggested and the performance and stability of an enzymatic biofuel cell (EBC) adopting the new catalyst are evaluated. For this purpose, new terephthalaldehyde (TPA) and conventional glutaraldehyde (GA) cross-linkers are adopted on a glucose oxidase (GOx), polyethyleneimine (PEI) and carbon nanotube (CNT)(GOx/PEI/CNT) structure. GOx/PEI/CNT cross-linked by TPA (TPA/[GOx/PEI/CNT]) results in a superior EBC performance and stability to other catalysts. It is attributed to the π bonds conjugated between the aldehyde of TPA and amine of the GOx/PEI molecules. By π conjugation, electrons bonded with carbon and nitrogen are delocalized, promoting the electron transfer and catalytic activity with an excellent EBC performance. The maximum power density (MPD) of an EBC adopting TPA/[GOx/PEI/CNT] (0.66 mW cm(-2)) is far better than that of the other EBCs (the MPD of EBC adopting GOx/PEI/CNT is 0.40 mW cm(-2)). Regarding stability, the covalent bonding formed between TPA and GOx/PEI plays a critical role in preventing the denaturation of GOx molecules, leading to an excellent stability. By repeated measurements of the catalytic activity, TPA/[GOx/PEI/CNT] maintains its activity to 92% of its initial value even after five weeks.

BioCapacitor: A novel principle for biosensors

Studies regarding biofuel cells utilizing biocatalysts such as enzymes and microorganisms as electrocatalysts have been vigorously conducted over the last two decades. Because of their environmental safety and sustainability, biofuel cells are expected to be used as clean power generators. Among several principles of biofuel cells, enzyme fuel cells have attracted significant attention for their use as alternative energy sources for future implantable devices, such as implantable insulin pumps and glucose sensors in artificial pancreas and pacemakers. However, the inherent issue of the biofuel cell principle is the low power of a single biofuel cell. The theoretical voltage of biofuel cells is limited by the redox potential of cofactors and/or mediators employed in the anode and cathode, which are inadequate for operating any devices used for biomedical application. These limitations inspired us to develop a novel biodevice based on an enzyme fuel cell that generates sufficient stable power to operate electric devices, designated "BioCapacitor." To increase voltage, the enzyme fuel cell is connected to a charge pump. To obtain a sufficient power and voltage to operate an electric device, a capacitor is used to store the potential generated by the charge pump. Using the combination of a charge pump and capacitor with an enzyme fuel cell, high voltages with sufficient temporary currents to operate an electric device were generated without changing the design and construction of the enzyme fuel cell. In this review, the BioCapacitor principle is described. The three different representative categories of biodevices employing the BioCapacitor principle are introduced. Further, the recent challenges in the developments of self-powered stand-alone biodevices employing enzyme fuel cells combined with charge pumps and capacitors are introduced. Finally, the future prospects of biodevices employing the BioCapacitor principle are addressed.

Effective antifouling using quorum-quenching acylase stabilized in magnetically-separable mesoporous silica

Highly effective antifouling was achieved by immobilizing and stabilizing an acylase, disrupting bacterial cell-to-cell communication, in the form of cross-linked enzymes in magnetically separable mesoporous silica. This so-called "quorum-quenching" acylase (AC) was adsorbed into spherical mesoporous silica (S-MPS) with magnetic nanoparticles (Mag-S-MPS), and further cross-linked for the preparation of nanoscale enzyme reactors of AC in Mag-S-MPS (NER-AC/Mag-S-MPS). NER-AC effectively stabilized the AC activity under rigorous shaking at 200 rpm for 1 month, while free and adsorbed AC lost more than 90% of their initial activities in the same condition within 1 and 10 days, respectively. When applied to the membrane filtration for advanced water treatment, NER-AC efficiently alleviated the biofilm maturation of Pseudomonas aeruginosa PAO1 on the membrane surface, thereby enhancing the filtration performance by preventing membrane fouling. Highly stable and magnetically separable NER-AC, as an effective and sustainable antifouling material, has a great potential to be used in the membrane filtration for water reclamation.

Cross-linked glucose oxidase clusters for biofuel cell anode catalysts

The efficient localization of increased levels of active enzymes onto conducting scaffolds is important for the development of enzyme-based biofuel cells. Cross-linked enzyme clusters (CEC) of glucose oxidase (GOx) constrained to functionalized carbon nanotubes (CEC-CNTs) were generated in order to evaluate the potential of using CECs for developing GOx-based bioanodes functioning via direct electron transfer from the GOx active site to the CNT scaffold. CEC-CNTs generated from several weight-to-weight ratios of GOx:CNT were examined for comparable catalytic activity to free GOx into the solution, with CEC-CNTs generated from a 100% GOx solution displaying the greatest enzymatic activity. Scanning transmission electron microscopic analysis of CEC-CNTs generated from 100% GOx to CNT (wt/wt) ratios revealed that CEC clusters of ∼78 µm2 localized to the CNT surface. Electrochemical analysis indicates that the enzyme is engaged in direct electron transfer, and biofuel cells generated using GOx CEC-CNT bioanodes were observed to have a peak power density of ∼180 µW cm(-2). These data indicate that the generation of nano-to-micro-sized active enzyme clusters is an attractive option for the design of enzyme-specific biofuel cell powered implantable devices.

BioRadioTransmitter: a self-powered wireless glucose-sensing system

BACKGROUND

Although an enzyme fuel cell can be utilized as a glucose sensor, the output power generated is too low to power a device such as a currently available transmitter and operating system, and an external power source is required for operating an enzyme-fuel-cell-based biosensing system. We proposed a novel biosensor that we named BioCapacitor, in which a capacitor serves as a transducer. In this study, we constructed a new BioCapacitor-based system with an added radio-transmitter circuit and a miniaturized enzyme fuel cell.

METHODS

A miniaturized direct-electron-transfer-type compartmentless enzyme fuel cell was constructed with flavin adenine dinucleotide-dependent glucose dehydrogenase complex-based anode and a bilirubin-oxidase-based cathode. For construction of a BioRadioTransmitter wireless sensing system, a capacitor, an ultra-low-voltage charge-pump-integrated circuit, and Hartley oscillator circuit were connected to the miniaturized enzyme fuel cell. A radio-receiver circuit, comprising two field-effect transistors and a coil as an antenna, was used to amplify the signal generated from the biofuel cells.

RESULTS

Radio wave signals generated by the BioRadioTransmitter were received, amplified, and converted from alternate to direct current by the radio receiver. When the capacitor discharges in the presence of glucose, the BioRadioTransmitter generates a radio wave, which is monitored by a radio receiver connected wirelessly to the sensing device. Magnitude of the radio wave transmission frequency change observed at the radio receiver was correlated to glucose concentration in the fuel cells.

CONCLUSIONS

We constructed a stand-alone, self-powered, wireless glucose-sensing system called a BioRadioTransmitter by using a radio transmitter in which the radio wave transmission frequency changes with the glucose concentration in the fuel cell. The BioRadioTransmitter is a significant advance toward construction of an implantable continuous glucose monitor.

Stable Flexible Electrodes With Enzyme Cluster Decorated Carbon Nanotubes for Glucose-Driven Power Source in Biosensing Applications

Over the years, implantable sensor technology has found many applications in healthcare. Research projects have aimed at improving power supply lifetime for longevity of an implanted sensor system. Miniature power sources, inspired from the biofuel cell principle, can utilize enzymes (proteins) as catalysts to produce energy from fuel(s) that are perennial in the human body. Bio-nanocatalytic hierarchical structures, clusters made of enzyme molecules, can be covalently linked to the electrode’s surface to provide better enzyme loading and sustained activity. Carbon nanotube base electrodes, with high surface area for direct electron transfer, and enzyme clusters can achieve efficient enzymatic redox reaction. A redox pair of such bioelectrodes can make up a power source with improved performance. In this study, we have investigated high throughput processes for coupling enzyme catalysts with power harvesting mechanisms via a screen printing process and solution processing. The process incorporates enzyme (glucosse oxidase and catalase) micro-/nanocluster immobilization on the surface of carboxylated (functionalized) carbon nanotubes with screen printed electrodes. The 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide and N-hydroxysulfosuccinimide amide linkage chemistries were used to bind the enzyme molecules to nanotube surface, and bis[sulfosuccinimidyl] suberate (BS3) was used as the cross-linker between enzymes. Optimized enzyme cross-linking was obtained after 25 min at room temperature with 0.07 mmol BS3/nmol of enzymes, with which 44% of enzymes were immobilized onto the surface of the bioelectrode with only 24% enzyme activity lost. A cell, redox pair of bioelectrodes, was tested under continuous operation. It was able to maintain most of the enzyme activity for 7 days before complete deactivation at 16 days. Thus, the power harvesting mechanism was able to produce power continuously for 7 days. The results were also analyzed to identify impeding factors such as competitive inhibition by reaction byproduct and cathode design, and methods to rectify them have been discussed. Coupling this new and improved nanobiopower cell with a product removal mechanism and enzyme mutagenesis should provide enzyme protection and longevity. This would bring the research one step closer to development of compatible implantable battery technology for medical applications.

Automatic, sequential power generation for prolonging the net lifetime of a miniature biofuel cell stack

A sequential power generation system for prolonging the net lifetime of a miniature biofuel cell stack has been developed. The system consists of layered chambers of enzyme fuel cells designed to be exposed sequentially to fuel solution by automatically switched fuel flow. The cell chambers were initially separated by magnetized plastic covers sealed with a degradable glue, poly(lactic-co-glycolic acid) (PLGA). The time that the cover was opened by attraction with an external magnet, thereby activating the following cell, was adjustable from a few hours to a few weeks by controlling the weight ratio of Fe(3)O(4) in the covers and the molecular weight of PLGA. By using sequential power generation in this way, the power output of the system was stable for longer periods, and therefore the net lifetime of the stack has been extended as compared with that of a single biofuel cell.

Nanobiocatalysis for protein digestion in proteomic analysis

The process of protein digestion is a critical step for successful protein identification in bottom-up proteomic analyses. To substitute the present practice of in-solution protein digestion, which is long, tedious, and difficult to automate, many efforts have been dedicated for the development of a rapid, recyclable and automated digestion system. Recent advances of nanobiocatalytic approaches have improved the performance of protein digestion by using various nanomaterials such as nanoporous materials, magnetic nanoparticles, and polymer nanofibers. Especially, the unprecedented success of trypsin stabilization in the form of trypsin-coated nanofibers, showing no activity decrease under repeated uses for 1 year and retaining good resistance to proteolysis, has demonstrated its great potential to be employed in the development of automated, high-throughput, and on-line digestion systems. This review discusses recent developments of nanobiocatalytic approaches for the improved performance of protein digestion in speed, detection sensitivity, recyclability, and trypsin stability. In addition, we also introduce approaches for protein digestion under unconventional energy input for protein denaturation and the development of microfluidic enzyme reactors that can benefit from recent successes of these nanobiocatalytic approaches.

Glucose Driven Nanobiopower Cells for Biomedical Applications

Power supply is an important aspect of micronanobiomedical devices. Implantable devices are required to stay inside of the body for longer period of time to provide continuous monitoring, detection, and therapeutics. The constricted areas of the human body, accessed by these devices, imply that the power source should not increase the payload significantly. Conventional on-board power sources are big, as compared with the device themselves, or involve wire-outs. Both provisions are liable to develop complications for sensor/actuator implant packaging. A plausible approach can be innovative solutions for sustainable bio-energy harvesting. Research studies have reported feasibility of miniature power sources, running on redox reactions. The device design, reported in this study, is a combination of nano-engineered composites and flexible thin film processing to achieve high density packaging. Of which, the end goal is production of energy for sensor applications. Both the bio-electrodes were successfully functionalized by amide bond cross-linkage between the carbon nanotube surface and the enzyme molecules: catalase and glucose oxidase for cathode and anode, respectively. The nanocomposite based biopower cell was evaluated as a steady power supply across the physiological range of glucose concentration. The power cell was able to deliver a steady power of 3.2 nW at 85 mV for glucose concentrations between 3 mM and 8 mM. Electron microscopy scanning of the functionalized electrode surface and spectroscopic evaluation of nanotube surface were used for evaluation of the biofunctionalization technique. Cyclic voltametric (CV) scans were performed on the cathodic and anodic half cells to corroborate bioactivity and qualitatively evaluate the power cell output against the redox peaks on the CV scans. The importance of these results has been discussed and conclusions have been drawn pertaining to further miniaturization (scale down) of the cell.