Effects of pore size and surface properties of MgO-templated carbon on the performance of bilirubin oxidase–modified oxygen reduction reaction cathode

Emerging Trends of Bilirubin Oxidases at the Bioelectrochemical Interface: Paving the Way for Self-Powered Electrochemical Devices and Biosensors

Bilirubin oxidases (BODs) [EC 1.3.3.5 - bilirubin: oxygen oxido-reductase] are enzymes that belong to the multicopper oxidase family and can oxidize bilirubin, diphenols, and aryl amines and reduce the oxygen by direct four-electron transfer from the electrode with almost no electrochemical overpotential. Therefore, BOD is a promising bioelectrocatalyst for (self-powered) biosensors and/or enzymatic fuel cells. The advantages of electrochemically active BOD enzymes include selective biosensing, biocatalysis for efficient energy conversion, and electrosynthesis. Owing to the rise in publications and patents, as well as the expanding interest in BODs for a range of physiological conditions, this Review analyzes scientific literature reports on BOD enzymes and current hypotheses on their bioelectrocatalysis. This Review evaluates the specific research outcomes of the BOD in enzyme (protein) engineering, immobilization strategies, and challenges along with their bioelectrochemical properties, limitations, and applications in the fields of (i) biosensors, (ii) self-powered biosensors, and (iii) biofuel cells for powering bioelectronics.

Hierarchical Structure of Gold and Carbon Electrode for Bilirubin Oxidase-Biocathode

Biofuel cells (BFCs) with enzymatic electrocatalysts have attracted significant attention, especially as power sources for wearable and implantable devices; however, the applications of BFCs are limited owing to the limited O₂ supply. This can be addressed by using air-diffusion-type bilirubin oxidase (BOD) cathodes, and thus the further development of the hierarchical structure of porous electrodes with highly effective specific surface areas is critical. In this study, a porous layer of gold is deposited over magnesium-oxide-templated carbon (MgOC) to form BOD-based biocathodes for the oxygen reduction reaction (ORR). Porous gold structures are constructed via electrochemical deposition of gold via dynamic hydrogen bubble templating (DHBT). Hydrogen bubbles used as a template and controlled by the Coulomb number yield a porous gold structure during the electrochemical deposition process. The current density of the ORR catalyzed by BOD without a redox mediator on the gold-modified MgOC electrode was 1.3 times higher than that of the ORR on the MgOC electrode. Furthermore, the gold-deposited electrodes were modified with aromatic thiols containing negatively charged functional groups to improve the orientation of BOD on the electrode surface to facilitate efficient electron transfer at the heterogeneous surface, thereby achieving an ORR current of 12 mA cm^-2 at pH 5 and 25 °C. These results suggest that DHBT is an efficient method for the fabrication of nanostructured electrodes that promote direct electron transfer with oxidoreductase enzymes.

High-performance paper-based biocathode fabricated by screen-printing an improved mesoporous carbon ink and by oriented immobilization of bilirubin oxidase

In this study, the performance of a paper-based, screen-printed biofuel cell with mesoporous MgO-templated carbon (MgOC) electrodes was improved in two steps. First, a small amount of carboxymethyl cellulose (CMC) was added to the MgOC ink. Next, the cathode was modified with bilirubin prior to immobilizing the bilirubin oxidase (BOD). The CMC increased the accessibility of the mesopores of the MgOC, and subsequently, the performance of both the bioanode and biocathode. CMC also likely increased the stability of the electrodes. The pre-modification with bilirubin improved the orientation of the BOD, which facilitated direct electron transfer. With these two steps, an open circuit potential of 0.65 V, a maximal current density of 1.94 mA cm⁻², and a maximal power density of 465 μW cm⁻² was achieved with lactate oxidase as bioanode enzyme and lactate as fuel. This is one of the highest reported performances for a biofuel cell.

Cu2O nanoparticles anchored on carbon for the efficient removal of propofol from operating room wastewater via peroxymonosulfate activation: efficiency, mechanism, and pathway

Anesthetic drug wastage has increasingly become the main resource of operating room sewage, which poses a great risk to the safety of humans and other organisms. Propofol is the most widely used anesthetic drug in the world, and also occupies the largest proportion of the total anesthetic wastage in the operating room. In this work, a 2D Cu2O anchored carbon catalyst (Cu2O@NC) was prepared by the assembly-pyrolysis process and successfully applied to peroxymonosulfate (PMS) activation. We took propofol as a typical example and investigated the removal activity through heterostructure-enhanced advanced oxidation processes (AOPs). Through the degradation process, propofol can be removed from 20 ppm to ultralow levels within 5 min using the PMS/Cu2O@NC system. The degradation pathway of propofol was deduced through quantum chemical calculation and LC/GC-MS results. The final products were verified as CO2 and H2O. Moreover, sulfate radicals (SO4˙-) proved to be the dominant reactive oxidation species by radical scavenger experiments and ESR results. In addition, it has great universality for various pharmaceuticals such as tetracycline (TC), amoxicillin (AMX), cephalexin (CPX), and norfloxacin (NFX). Our work provided the possibility to treat operation room sewage in a rapid, high-efficiency, and feasible way.

From Enzyme Stability to Enzymatic Bioelectrode Stabilization Processes

Bioelectrocatalysis using redox enzymes appears as a sustainable way for biosensing, electricity production, or biosynthesis of fine products. Despite advances in the knowledge of parameters that drive the efficiency of enzymatic electrocatalysis, the weak stability of bioelectrodes prevents large scale development of bioelectrocatalysis. In this review, starting from the understanding of the parameters that drive protein instability, we will discuss the main strategies available to improve all enzyme stability, including use of chemicals, protein engineering and immobilization. Considering in a second step the additional requirements for use of redox enzymes, we will evaluate how far these general strategies can be applied to bioelectrocatalysis.

Rational Surface Modification of Carbon Nanomaterials for Improved Direct Electron Transfer-Type Bioelectrocatalysis of Redox Enzymes

Interfacial electron transfer between redox enzymes and electrodes is a key step for enzymatic bioelectrocatalysis in various bioelectrochemical devices. Although the use of carbon nanomaterials enables an increasing number of redox enzymes to carry out bioelectrocatalysis involving direct electron transfer (DET), the role of carbon nanomaterials in interfacial electron transfer remains unclear. Based on the recent progress reported in the literature, in this mini review, the significance of carbon nanomaterials on DET-type bioelectrocatalysis is discussed. Strategies for the oriented immobilization of redox enzymes in rationally modified carbon nanomaterials are also summarized and discussed. Furthermore, techniques to probe redox enzymes in carbon nanomaterials are introduced.

Direct Electron Transfer-Type Bioelectrocatalysis of Redox Enzymes at Nanostructured Electrodes

Direct electron transfer (DET)-type bioelectrocatalysis, which couples the electrode reactions and catalytic functions of redox enzymes without any redox mediator, is one of the most intriguing subjects that has been studied over the past few decades in the field of bioelectrochemistry. In order to realize the DET-type bioelectrocatalysis and improve the performance, nanostructures of the electrode surface have to be carefully tuned for each enzyme. In addition, enzymes can also be tuned by the protein engineering approach for the DET-type reaction. This review summarizes the recent progresses in this field of the research while considering the importance of nanostructure of electrodes as well as redox enzymes. This review also describes the basic concepts and theoretical aspects of DET-type bioelectrocatalysis, the significance of nanostructures as scaffolds for DET-type reactions, protein engineering approaches for DET-type reactions, and concepts and facts of bidirectional DET-type reactions from a cross-disciplinary viewpoint.

Challenges and Opportunities of Carbon Nanomaterials for Biofuel Cells and Supercapacitors: Personalized Energy for Futuristic Self-Sustainable Devices

Various carbon allotropes are fundamental components in electrochemical energy-conversion and energy-storage devices, e.g., biofuel cells (BFCs) and supercapacitors. Recently, biodevices, particularly wearable and implantable devices, are of distinct interest in biomedical, fitness, academic, and industrial fields due to their new fascinating capabilities for personalized applications. However, all biodevices require a sustainable source of energy, bringing widespread attention to energy research. In this review, we detail the progress in BFCs and supercapacitors attributed to carbon materials. Self-powered biosensors for futuristic biomedical applications are also featured. To develop these energy devices, many challenges needed to be addressed. For this reason, there is a need to: optimize the electron transfer between the enzymatic site and electrode; enhance the power efficiency of the device in fluctuating oxygen conditions; strengthen the efficacy of enzymatic reactions at the carbon-based electrodes; increase the electrochemically accessible surface area of the porous electrode materials; and refine the flexibility of traditional devices by introducing a mechanical resiliency of electrochemical devices to withstand daily multiplexed movements. This article will also feature carbon nanomaterial research alongside opportunities to enhance energy technology and address the challenges facing the field of personalized applications. Carbon-based energy devices have proved to be sustainable and compatible energy alternatives for biodevices within the human body, serving as attractive options for further developing diverse domains, including individual biomedical applications.