NAD-dependent dehydrogenase bioelectrocatalysis: the ability of a naphthoquinone redox polymer to regenerate NAD

Surfactant-Free Formate/O2 Biofuel Cell with Electropolymerized Phenothiazine Derivative-Modified Enzymatic Bioanode

A formate (HCOO-) bioanode was developed by utilizing a phenothiazine-based electropolymerized layer deposited on sucrose-derived carbon. The electrode modified with NAD-dependent formate dehydrogenase and the electropolymerized layer synergistically catalyzed the oxidation of the coenzyme (NADH) and fuel (HCOO-) to achieve efficient electron transfer. Further, the replacement of carbon nanotubes with water-dispersible sucrose-derived carbon used as the electrode base allowed the fabrication of a surfactant-free bioanode delivering a maximum current density of 1.96 mA cm-2 in the fuel solution. Finally, a separator- and surfactant-free HCOO-/O2 biofuel cell featuring the above bioanode and a gas-diffusion biocathode modified with bilirubin oxidase and 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate) was fabricated, delivering a maximum power density of 70 μW cm-2 (at 0.24 V) and an open-circuit voltage of 0.59 V. Thus, this study demonstrates the potential of formic acid as a fuel and possibilities for the application of carbon materials in bioanodes.

Semi-Implantable Bioelectronics

Developing techniques to effectively and real-time monitor and regulate the interior environment of biological objects is significantly important for many biomedical engineering and scientific applications, including drug delivery, electrophysiological recording and regulation of intracellular activities. Semi-implantable bioelectronics is currently a hot spot in biomedical engineering research area, because it not only meets the increasing technical demands for precise detection or regulation of biological activities, but also provides a desirable platform for externally incorporating complex functionalities and electronic integration. Although there is less definition and summary to distinguish it from the well-reviewed non-invasive bioelectronics and fully implantable bioelectronics, semi-implantable bioelectronics have emerged as highly unique technology to boost the development of biochips and smart wearable device. Here, we reviewed the recent progress in this field and raised the concept of "Semi-implantable bioelectronics", summarizing the principle and strategies of semi-implantable device for cell applications and in vivo applications, discussing the typical methodologies to access to intracellular environment or in vivo environment, biosafety aspects and typical applications. This review is meaningful for understanding in-depth the design principles, materials fabrication techniques, device integration processes, cell/tissue penetration methodologies, biosafety aspects, and applications strategies that are essential to the development of future minimally invasive bioelectronics.

Advances in electrochemical cofactor regeneration: enzymatic and non-enzymatic approaches

Nicotinamide adenine dinucleotide(NAD(P)H) is a metabolically interconnected redox cofactor serving as a hydride source for the majority of oxidoreductases, and consequently constituting a significant cost factor for bioprocessing. Much research has been devoted to the development of efficient, affordable, and sustainable methods for the regeneration of these cofactors through chemical, electrochemical, and photochemical approaches. However, the enzymatic approach using formate dehydrogenase is still the most abundantly employed in industrial applications, even though it suffers from system complexity and product purity issues. In this review, we summarize non-enzymatic and enzymatic electrochemical approaches for cofactor regeneration, then discuss recent developments to solve major issues. Issues discussed include Rh-catalyst mediated enzyme mutual inactivation, electron-transfer rates, catalyst sustainability, product selectivity and simplifying product purification. Recently reported remedies are discussed, such as heterogeneous metal catalysts generating H⁺ as the sole byproduct or high activity and stability redox-polymer immobilized enzymatic systems for sustainable organic synthesis.

Rapid Entrapment of Phenazine Ethosulfate within a Polyelectrolyte Complex on Electrodes for Efficient NAD+ Regeneration in Mediated NAD+-Dependent Bioelectrocatalysis

Over the past two decades, the designs of redox polymers have become critical to the field of mediated bioelectrocatalysis and are used in commercial glucose biosensors, as well as other bioelectrochemical applications (e.g., energy harvesting). These polymers are specifically used to immobilize redox mediators on electrode surfaces, allowing for self-exchange-based conduction of electrons from enzymes far from the electrode to the electrode surface. However, the synthesis of redox polymers is challenging and results in large batch-to-batch variability. Herein, we report a rapid entrapment of mediators for NAD⁺-dependent bioelectrocatalysis within reverse ionically condensed polyelectrolytes. A high ionic strength aqueous solution of oppositely charged polyelectrolytes, composed of cationic polyguanidinium (PG) chloride and anionic sodium hexametaphosphate (P6), undergoes phase inversion into a solid microporous polyelectrolyte complex (PEC) when introduced into a low ionic strength aqueous solution. The ionic strength-triggered phase inversion of PGP6 solutions was investigated as a means to entrap mediators on the surface of electrodes for mediated bioelectrocatalysis. Compared to the traditional cross-linked immobilizations using redox polymers, this phase inversion takes place within seconds and requires up to 60 min for complete stabilization. In this work, redox mediator phenazine ethosulfate (PES) was entrapped within PGP6 on electrode surfaces for nicotinamide adenine dinucleotide (NAD⁺)-dependent bioelectrocatalysis. In the bulk solution, NAD⁺-dependent dehydrogenase enzymes catalyze the oxidation of the substrate while reducing NAD to reduced nicotinamide adenine dinucleotide (NADH). The resulting NADH is reoxidized to NAD⁺ by the entrapped PES that gets reduced on the electrode, completing the NAD⁺-regeneration-based bioelectrocatalysis. To show the use of these new materials in an application, biofuel cells were evaluated using four different anodic enzyme systems (alcohol dehydrogenase, lactate hydrogenase, glycerol dehydrogenase, and glucose dehydrogenase).

Development of a Sensitive Self-Powered Glucose Biosensor Based on an Enzymatic Biofuel Cell

Biofuel cells allow for constructing sensors that leverage the specificity of enzymes without the need for an external power source. In this work, we design a self-powered glucose sensor based on a biofuel cell. The redox enzymes glucose dehydrogenase (NAD-GDH), glucose oxidase (GOx), and horseradish peroxidase (HRP) were immobilized as biocatalysts on the electrodes, which were previously engineered using carbon nanostructures, including multi-wall carbon nanotubes (MWCNTs) and reduced graphene oxide (rGO). Additional polymers were also introduced to improve biocatalyst immobilization. The reported design offers three main advantages: (i) by using glucose as the substrate for the both anode and cathode, a more compact and robust design is enabled, (ii) the system operates under air-saturating conditions, with no need for gas purge, and (iii) the combination of carbon nanostructures and a multi-enzyme cascade maximizes the sensitivity of the biosensor. Our design allows the reliable detection of glucose in the range of 0.1-7.0 mM, which is perfectly suited for common biofluids and industrial food samples.

Enzymatic Bioreactors: An Electrochemical Perspective

Biocatalysts provide a number of advantages such as high selectivity, the ability to operate under mild reaction conditions and availability from renewable resources that are of interest in the development of bioreactors for applications in the pharmaceutical and other sectors. The use of oxidoreductases in biocatalytic reactors is primarily focused on the use of NAD(P)-dependent enzymes, with the recycling of the cofactor occurring via an additional enzymatic system. The use of electrochemically based systems has been limited. This review focuses on the development of electrochemically based biocatalytic reactors. The mechanisms of mediated and direct electron transfer together with methods of immobilising enzymes are briefly reviewed. The use of electrochemically based batch and flow reactors is reviewed in detail with a focus on recent developments in the use of high surface area electrodes, enzyme engineering and enzyme cascades. A future perspective on electrochemically based bioreactors is presented.

Advancing the fundamental understanding and practical applications of photo-bioelectrocatalysis

Photo-bioelectrocatalysis combines the natural and highly sophisticated process of photosynthesis in biological entities with an abiotic electrode surface, to perform semi-artificial photosynthesis. However, challenges must be overcome, from the establishment and understanding of the photoexcited electron harvesting process at the electrode to the electrochemical characterization of these biotic/abiotic systems, and their subsequent tuning for enhancing energy generation (chemical and/or electrical). This Feature Article discusses the various approaches utilized to tackle these challenges, particularly focusing on powerful multi-disciplinary approaches for understanding and improving photo-bioelectrocatalysis. Among them is the combination of experimental evidence and quantum mechanical calculations, the use of bioinformatics to understand photo-bioelectrocatalysis at a metabolic level, or bioengineering to improve and facilitate photo-bioelectrocatalysis. Key aspects for the future development of photo-bioelectrocatalysis are presented alongside future research needs and promising applications of semi-artificial photosynthesis.

NAD(P)-dependent glucose dehydrogenase: Applications for biosensors, bioelectrodes, and biofuel cells

This review discusses the physical and chemical properties of nicotinamide redox cofactor dependent glucose dehydrogenase (NAD(P) dependent GDH) and its extensive application in biosensors and bio-fuel cells. GDHs from different organisms show diverse biochemical properties (e.g., activity and stability) and preferences towards cofactors, such as nicotinamide adenine dinucleotide (NAD⁺) and nicotinamide adenine dinucleotide phosphate (NADP⁺). The (NAD(P)⁺) play important roles in biological electron transfer, however, there are some difficulties related to their application in devices that originate from their chemical properties and labile binding to the GDH enzyme. This review discusses the electrode modifications aimed at immobilising NAD⁺ or NADP⁺ cofactors and GDH at electrodes. Binding of the enzyme was achieved by appropriate protein engineering techniques, including polymerisation, hydrophobisation or hydrophilisation processes. Various enzyme-modified electrodes applied in biosensors, enzymatic fuel cells, and biobatteries are compared. Importantly, GDH can operate alone or as part of an enzymatic cascade, which often improves the functional parameters of the biofuel cell or simply allows use of cheaper fuels. Overall, this review explores how NAD(P)-dependent GDH has recently demonstrated high potential for use in various systems to generate electricity from biological sources for applications in implantable biomedical devices, wireless sensors, and portable electronic devices.

Data describing the cofactor additives effect on bioelectrocatalytic activity of «crude» extracts

«Crude» extracts obtained via simple ultrasonic disintegration of microbial cell membrane are perspective bioelectrocatalysts. This extract contains all the necessary enzymes and cofactors required for oxidative or reductive conversion. The technology of synthesis of «crude extract» is simpler and less costly in comparison with technology of obtaining pure enzymes. Dialysis of the obtained extracts was performed with different molecular weight cut-off (3.5 kDa, 12-14 kDa, 25 kDa, 50 kDa). The obtained data show that after dialysis extracts lose their dehydrogenase and bioelectrocatalytic activity due to the loss of cofactors. However, the addition of NAD and NADP cofactors leads to a recovery of activity. The obtained data demonstrate that the concentration of the cofactor directly affects the rate of the bioelectrocatalytic reaction. Also, the obtained data indicate that the composition of the enzyme systems of the extract includes succinate dehydrogenase. Analyzing this data set can provide insight on increase of the electrocatalytic activity of a new type of bioelectrocatalyst.

Direct bioelectrocatalysis by redox enzymes immobilized in electrostatically condensed oppositely charged polyelectrolyte electrode coatings

The ionic induced phase inversion of two oppositely charged electrolytes for enzyme immobilization and its application in bioelectrocatalysis.

Catechol-Based Capacitor for Redox-Linked Bioelectronics

A common bioelectronics goal is to enable communication between biology and electronics, and success is critically dependent on the communication modality. When a biorelevant modality aligns with instrumentation capabilities, remarkable successes have been observed (e.g., electrodes provide a powerful tool to observe and actuate biology through its ion-based electrical modality). Emerging biological research demonstrates that redox is another biologically relevant modality, and recent research has shown that advanced electrochemical methods enable biodevice communication through this redox modality. Here, we briefly summarize the biological relevance of this redox modality and the use of redox mediators to enable access to this modality through electrochemical measurements. Next, we describe the fabrication of a catechol-chitosan redox capacitor that is redox-active but nonconducting and thus offers a unique set of molecular electronic properties that enhance access to redox-based information. Finally, we cite several recent studies that demonstrate the broad potential for this capacitor to access redox-based biological information. In summary, we envision the redox capacitor will become a vital component in the integrated circuitry of redox-linked bioelectronics.

Tackling the Challenges of Enzymatic (Bio)Fuel Cells

The ever-increasing demands for clean and sustainable energy sources combined with rapid advances in biointegrated portable or implantable electronic devices have stimulated intensive research activities in enzymatic (bio)fuel cells (EFCs). The use of renewable biocatalysts, the utilization of abundant green, safe, and high energy density fuels, together with the capability of working at modest and biocompatible conditions make EFCs promising as next generation alternative power sources. However, the main challenges (low energy density, relatively low power density, poor operational stability, and limited voltage output) hinder future applications of EFCs. This review aims at exploring the underlying mechanism of EFCs and providing possible practical strategies, methodologies and insights to tackle these issues. First, this review summarizes approaches in achieving high energy densities in EFCs, particularly, employing enzyme cascades for the deep/complete oxidation of fuels. Second, strategies for increasing power densities in EFCs, including increasing enzyme activities, facilitating electron transfers, employing nanomaterials, and designing more efficient enzyme-electrode interfaces, are described. The potential of EFCs/(super)capacitor combination is discussed. Third, the review evaluates a range of strategies for improving the stability of EFCs, including the use of different enzyme immobilization approaches, tuning enzyme properties, designing protective matrixes, and using microbial surface displaying enzymes. Fourth, approaches for the improvement of the cell voltage of EFCs are highlighted. Finally, future developments and a prospective on EFCs are envisioned.

Enzyme-Based Glucose Sensor: From Invasive to Wearable Device

Blood glucose concentration is a key indicator of patients' health, particularly for symptoms associated with diabetes mellitus. Because of the large number of diabetic patients, many approaches for glucose measurement have been studied to enable continuous and accurate glucose level monitoring. Among them, electrochemical analysis is prominent because it is simple and quantitative. This technology has been incorporated into commercialized and research-level devices from simple test strips to wearable devices and implantable systems. Although directly monitoring blood glucose assures accurate information, the invasive needle-pinching step to collect blood often results in patients (particularly young patients) being reluctant to adopt the process. An implantable glucose sensor may avoid the burden of repeated blood collections, but it is quite invasive and requires periodic replacement of the sensor owing to biofouling and its short lifetime. Therefore, noninvasive methods to estimate blood glucose levels from tears, saliva, interstitial fluid (ISF), and sweat are currently being studied. This review discusses the evolution of enzyme-based electrochemical glucose sensors, including materials, device structures, fabrication processes, and system engineering. Furthermore, invasive and noninvasive blood glucose monitoring methods using various biofluids or blood are described, highlighting the recent progress in the development of enzyme-based glucose sensors and their integrated systems.

FAD-Dependent Glucose Dehydrogenase Immobilization and Mediation Within a Naphthoquinone Redox Polymer

Electrochemically-active polymers (redox polymers) are useful tools for simultaneous immobilization and electron transfer of enzymes at electrode surfaces, which also serve to increase the localized concentration of the biocatalyst. The properties of the employed redox couple must be compatible with the target biocatalyst from both an electrochemical (potential) and biochemical standpoint. This chapter details the synthesis of a naphthoquinone-functionalized redox polymer (NQ-LPEI) that is used to immobilize and electronically communicate with flavin adenine dinucleotide-dependent glucose dehydrogenase (FAD-GDH), yielding an enzymatic bioanode that is able to deliver large catalytic current densities for glucose oxidation at a relatively low associated potential.

Tailoring Biointerfaces for Electrocatalysis

Bioelectrocatalysis is an expanding research area due to the use of this type of electrocatalysis in electrochemical biosensors, biofuel cells, bioelectrochemical cells, and biosolar cells. This feature article discusses recent advancements in tailoring the biointerface between electrodes and biocatalysts for facile electrocatalysis. This includes the design of pyrene moieties for directing the orientation of biocatalysts on electrode surfaces and mediation as well as the rational design of redox polymers for self-exchange-based electron transport to/from biocatalysts and the electrode and the use of bioscaffolding techniques for designing the bioelectrode structure. However, recent advances in the past decade have shown the importance of hybrid bioelectrocatalytic systems, and future work will be needed to use these same pyrene, redox polymer, and bioscaffolding techniques for hybrid bioelectrocatalysis.