Supramolecular Immobilization of Laccase on Carbon Nanotube Electrodes Functionalized with (Methylpyrenylaminomethyl)anthraquinone for Direct Electron Reduction of Oxygen

Applications and immobilization strategies of the copper-centred laccase enzyme; a review

Laccase is a multi-copper enzyme widely expressed in fungi, higher plants, and bacteria which facilitates the direct reduction of molecular oxygen to water (without hydrogen peroxide production) accompanied by the oxidation of an electron donor. Laccase has attracted attention in biotechnological applications due to its non-specificity and use of molecular oxygen as secondary substrate. This review discusses different applications of laccase in various sectors of food, paper and pulp, waste water treatment, pharmaceuticals, sensors, and fuel cells. Despite the many advantages of laccase, challenges such as high cost due to its non-reusability, instability in harsh environmental conditions, and proteolysis are often encountered in its application. One of the approaches used to minimize these challenges is immobilization. The various methods used to immobilize laccase and the different supports used are further extensively discussed in this review.

Modulating the adsorption orientation of methionine-rich laccase by tailoring the surface chemistry of single-walled carbon nanotubes

Achieving fast electron transfer process between oxidoreductase and electrodes is pivotal for the biocathode of enzymatic biofuel cells (EBFCs). However, in-depth understanding of the interplay mechanism between enzymes and electrode materials remains challenging when designing and constructing EBFCs. Herein, atomic-scale insight into the direct electron transfer (DET) behavior of Thermus thermophilus laccase (TtLac) with a special methionine-rich β-hairpin motif adsorbed on the carboxyl-functionalized carbon nanotube (COOH-CNT) and amino-functionalized carbon nanotube (NH₂-CNT) surfaces were disclosed by multi-scale molecular simulations. Simulation results reveal that electrostatic modification is an effective way to tune the DET behavior for TtLac on the modified-CNTs electrode surface. Surprisingly, the positively charged TtLac can be attracted by both negatively charged COOH-CNT and positively charged NH₂-CNT surfaces, yet only the latter is capable to trigger the DET process due to the 'lying-on' adsorption orientation. Specifically, the T1 copper site is near the methionine-rich β-hairpin motif, which is the key binding site for TtLac binding onto the NH₂-CNT surface via electrostatic interaction, π-π stacking and cation-π interaction. Moreover, TtLac on the NH₂-CNT surface undergoes less conformational changes than those on the COOH-CNT surface, which allows the laccase stability and catalytic efficiency to be well preserved. These findings provide a fundamental guidance for future design and fabrication of methionine-rich laccase-based EBFCs with high power output and long lifespan.

The Structure of Bilirubin Oxidase from Bacillus pumilus Reveals a Unique Disulfide Bond for Site-Specific Direct Electron Transfer

Efficient oxygen-reducing biocatalysts are essential for the development of biofuel cells or photo-bioelectrochemical applications. Bilirubin oxidase (BOD) is a promising biocatalyst for oxygen reduction processes at neutral pH and low overpotentials. BOD has been extensively investigated over the last few decades. While the enzyme’s internal electron transfer process and methods to establish electrical communication with electrodes have been elucidated, a crystal structure of BOD from bacterial origin has never been determined. Here we present the first crystal structure of BOD from Bacillus pumilus (BpBOD) at 3.5 Å resolution. Overall, BpBOD shows high homology with the fungal enzymes; however, it holds a unique surface-exposed disulfide bond between Cys229 and Cys322 residues. We present methodologies to orient the T1 site towards the electrode by coupling the reduced disulfide bond with maleimide moiety on the electrodes. The developed configurations were further investigated and revealed improved direct electron transfer rates with the electrodes. The work presented here may contribute to the construction of rationally designed bioanodes or biocathode configurations that are based on redox-active enzymes.

A review: Evolution of enzymatic biofuel cells

Ever-growing demands for energy, the unsustainability of fossil fuel due to its scarcity and massive impact on global economies and the environment, have encouraged the research on alternative power sources to work upon for the governments, companies, and scientists across the world. Enzymatic biofuel cells (eBFCs) is one category of fuel cell that can harvest energy from biological moieties and has the future to be used as an alternative source of energy. The aim of this review is to summarize the background and state-of-the-art in the field of eBFCs. This review article will be very beneficial for a wide audience including students and new researchers in the field. A part of the paper summarized the challenges in the preparation of anode and cathode and the involvement of nanomaterials and conducting polymers to construct the effective bioelectrodes. It will provide an insight for the researchers working in this challenging field. Furthermore, various applications of eBFCs in implantable power devices, tiny electronic gadgets, and self powered biosensors are reported. This review article explains the development in the area of eBFCs for several years from its origin to growth systematically. It reveals the strategies that have been taken for the improvements required for the better electrochemical performance and operational stability of eBFCs. It also mentions the challenges in this field that will require proper attention so that the eBFCs can be utilized commercially in the future. The review article is written and structurized in a way so that it can provide a decent background of eBFCs to its reader. It will definitely help in enhancing the interest of reader in eBFCs.

Graphene and carbon nanotubes interfaced electrochemical nanobiosensors for the detection of SARS-CoV-2 (COVID-19) and other respiratory viral infections: A review

Recent COVID-19 pandemic has claimed millions of lives due to lack of a rapid diagnostic tool. Global scientific community is now making joint efforts on developing rapid and accurate diagnostic tools for early detection of viral infections to preventing future outbreaks. Conventional diagnostic methods for virus detection are expensive and time consuming. There is an immediate requirement for a sensitive, reliable, rapid and easy-to-use Point-of-Care (PoC) diagnostic technology. Electrochemical biosensors have the potential to fulfill these requirements, but they are less sensitive for sensing viruses/viral infections. However, sensitivity and performance of these electrochemical platforms can be improved by integrating carbon nanostructure, such as graphene and carbon nanotubes (CNTs). These nanostructures offer excellent electrical property, biocompatibility, chemical stability, mechanical strength and, large surface area that are most desired in developing PoC diagnostic tools for detecting viral infections with speed, sensitivity, and cost-effectiveness. This review summarizes recent advancements made toward integrating graphene/CNTs nanostructures and their surface modifications useful for developing new generation of electrochemical nanobiosensors for detecting viral infections. The review also provides prospects and considerations for extending the graphene/CNTs based electrochemical transducers into portable and wearable PoC tools that can be useful in preventing future outbreaks and pandemics.

Flexible electrochemical uric acid and glucose biosensor

Fully integrated uric acid (UA) and glucose biosensors were fabricated on polydimethylsiloxane/polyimide platform by facile one step laser scribed technique. The laser scribed graphene (LSG) on the thin polyimide film was functionalized using pyrenebutanoic acid, succinimide ester (PBSE) to improve the electrochemical activity of the biosensors. The LSG was further decorated with platinum nanoparticles (PtNPs) to promote the electrocatalytic activity towards the oxidation of UA. Glucose oxidase was immobilized on the PtNPs modified surface for selective detection of glucose. The fabricated biosensors were characterized via scanning electron microscopy (SEM), Energy dispersive X-ray (EDX), X-ray photoelectron spectroscopy (XPS), and electrochemical methods (cyclic voltammetry and amperometry measurements). Outstanding electrocatalytic activities toward oxidation of UA and glucose were demonstrated. A wide detection range of 5 µM to 480 µM UA with a high sensitivity of 156.56 µA/mMcm² and a calculated detection limit (LOD) of 0.018 μM (S/N = 3) were achieved for the UA biosensor. The glucose biosensor exhibited a detection range of 5 µM to 3200 µM with a sensitivity of 12.64 µA/mMcm² and an LOD of 2.57 µM (S/N = 3). These integrated biosensors offer great promise for potential applications in wearable UA and glucose sensing due to their good sensitivity, selectivity, and stability properties.

Functionalizable Glyconanoparticles for a Versatile Redox Platform

A series of new glyconanoparticles (GNPs) was obtained by self-assembly by direct nanoprecipitation of a mixture of two carbohydrate amphiphilic copolymers consisting of polystyrene-block-β-cyclodextrin and polystyrene-block-maltoheptaose with different mass ratios, respectively 0–100, 10–90, 50–50 and 0–100%. Characterizations for all these GNPs were achieved using dynamic light scattering, scanning and transmission electron microscopy techniques, highlighting their spherical morphology and their nanometric size (diameter range 20–40 nm). In addition, by using the inclusion properties of cyclodextrin, these glyconanoparticles were successfully post-functionalized using a water-soluble redox compound, such as anthraquinone sulfonate (AQS) and characterized by cyclic voltammetry. The resulting glyconanoparticles exhibit the classical electroactivity of free AQS in solution. The amount of AQS immobilized by host–guest interactions is proportional to the percentage of polystyrene-block-β-cyclodextrin entering into the composition of GNPs. The modulation of the surface density of the β-cyclodextrin at the shell of the GNPs may constitute an attractive way for the elaboration of different electroactive GNPs and even GNPs modified by biotinylated proteins.

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.

Synthetic strategies in construction of organic macromolecular carrier-drug conjugates

Many metabolic inhibitors, considered potential antimicrobial or anticancer drug candidates, exhibit very limited ability to cross the biological membranes of target cells. The restricted cellular penetration of those molecules is often due to their highhydrophilicity. One of the possible solutions to this problem is a conjugation of an inhibitor with a molecular organic nanocarrier. The conjugate thus formed should be able to penetrate the membrane(s) by direct translocation, endocytosis or active transport mechanisms and once internalized, the active component could reach its intracellular target, either after release from the conjugate or in an intact form. Several such nanocarriers have been proposed so far, including macromolecular systems, carbon nanotubes and dendrimers. Herein, we present a comprehensive review of the current status of rational design and synthesis of macromolecular organic nanocarrier-drug conjugates, with special attention focused on the mode of coupling of a nanocarrier moiety with a "cargo" molecule through linking fragments of non-cleavable or cleavable type.

Mechanically Interlocked Carbon Nanotubes as a Stable Electrocatalytic Platform for Oxygen Reduction

Mechanically interlocking redox-active anthraquinone onto single-walled carbon nanotubes (AQ-MINT) gives a new and advanced example of a noncovalent architecture for an electrochemical platform. Electrochemical studies of AQ-MINT as an electrode reveal enhanced electrochemical stability in both aqueous and organic solvents compared to physisorbed AQ-based electrodes. While maintaining the electrochemical properties of the parent anthraquinone molecules, we observe a stable oxygen reduction reaction to hydrogen peroxide (H₂O₂). Using such AQ-MINT electrodes, 7 and 2 μmol of H₂O₂ are produced over 8 h under basic and neutral conditions, while the control system of SWCNTs produces 2.2 and 0.5 μmol, respectively. These results reveal the potential of this rotaxane-type immobilization approach for heterogenized electrocatalysis.

Bismuth Vanadate/Bilirubin Oxidase Photo(bio)electrochemical Cells for Unbiased, Light-Triggered Electrical Power Generation

The construction of bias- and donor-free photobioelectrochemical cells for the generation of light-triggered electrical power is presented. The developed oxygen reduction biocathodes are based on bilirubin oxidase (BOD) that originates from Myrothecium verrucaria (MvBOD) and a thermophilic Bacillus pumilus (BpBOD). Methods to entrap the BOD with 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) redox molecules in a polydopamine layer are presented. A pH-independent, positively charged pyrenebetaine linker was synthesized, utilized, and led to a threefold improvement to the bioelectrocatalytic current. Both the developed polydopamine/ABTS/MvBOD and the pyrenebetaine/BpBOD biocathodes were further coupled with BiVO₄ /cobalt phosphate water-oxidation photoanodes to construct biotic/abiotic photobioelectrochemical cells, which generated power outputs of 0.74 and 0.85 mW cm^-2 , respectively. The presented methods are versatile, show the strength of biotic/abiotic hybrids, and can be further used to couple different redox enzymes with electrodes.

Efficiency of Site-Specific Clicked Laccase-Carbon Nanotubes Biocathodes towards O2 Reduction

A maximization of a direct electron transfer (DET) between redox enzymes and electrodes can be obtained through the oriented immobilization of enzymes onto an electroactive surface. Here, a strategy for obtaining carbon nanotube (CNTs) based electrodes covalently modified with perfectly control-oriented fungal laccases is presented. Modelizations of the laccase-CNT interaction and of electron conduction pathways serve as a guide in choosing grafting positions. Homogeneous populations of alkyne-modified laccases are obtained through the reductive amination of a unique surface-accessible lysine residue selectively engineered near either one or the other of the two copper centers in enzyme variants. Immobilization of the site-specific alkynated enzymes is achieved by copper-catalyzed click reaction on azido-modified CNTs. A highly efficient reduction of O₂ at low overpotential and catalytic current densities over -3 mA cm^-2 are obtained by minimizing the distance from the electrode surface to the trinuclear cluster.

A hydrogen/oxygen hybrid biofuel cell comprising an electrocatalytically active nanoflower/laccase-based biocathode

Enzymatic fuel cells (EFCs) are one of the promising next-generation energy conversion systems.

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.

Mimicking the bioelectrocatalytic function of recombinant CotA laccase through electrostatically self-assembled bioconjugates

Unprecedented 3D nanobiosystems composed of recombinant CotA laccases and citrate-stabilised gold nanoparticles have been successfully achieved by an electrostatic self-assembly strategy. The bioelectrochemical reduction of O2 driven by CotA laccase at the spore coat was mimicked. Consequently key insights into its bioelectrocatalytic function were unravelled.

Non-Covalent Functionalization of Carbon Nanotubes for Electrochemical Biosensor Development

Carbon nanotubes (CNTs) have been widely studied and used for the construction of electrochemical biosensors owing to their small size, cylindrical shape, large surface-to-volume ratio, high conductivity and good biocompatibility. In electrochemical biosensors, CNTs serve a dual purpose: they act as immobilization support for biomolecules as well as provide the necessary electrical conductivity for electrochemical transduction. The ability of a recognition molecule to detect the analyte is highly dependent on the type of immobilization used for the attachment of the biomolecule to the CNT surface, a process also known as biofunctionalization. A variety of biofunctionalization methods have been studied and reported including physical adsorption, covalent cross-linking, polymer encapsulation etc. Each method carries its own advantages and limitations. In this review we provide a comprehensive review of non-covalent functionalization of carbon nanotubes with a variety of biomolecules for the development of electrochemical biosensors. This method of immobilization is increasingly being used in bioelectrode development using enzymes for biosensor and biofuel cell applications.

5,5-Dithiobis(2-nitrobenzoic acid) pyrene derivative-carbon nanotube electrodes for NADH electrooxidation and oriented immobilization of multicopper oxidases for the development of glucose/O2 biofuel cells

We report the functionalization of multi-walled carbon nanotubes (MWCNTs) electrodes by a bifunctional nitroaromatic molecule accomplished via π-π interactions of a pyrene derivative. DTNB (5,5'-dithiobis(2-nitrobenzoic acid)) has the particularity to possess both electroactivable nitro groups and negatively charged carboxylic groups. The integration of the DTNB-modified MWCNTs was evaluated for different bioelectrocatalytic systems. The immobilized DTNB-based electrodes showed electrocatalytic activity toward the oxidation of the reduced form of nicotinamide adenine dinucleotide (NADH) with low overpotential of -0.09V vs Ag/AgCl at neutral pH. Glucose dehydrogenase was successfully immobilized at the surface of DTNB-based electrodes and, in the presence of NAD⁺, the resulting bioelectrode achieved efficient glucose oxidation with high current densities of 2.03mAcm^-2. On the other hand, the aromatic structure and the negatively charged nature of the DTNB provoked orientation of both laccase and bilirubin oxidase onto the electrode, which enhanced their ability to undergo a direct electron transfer for oxygen reduction. Due to the proper orientation, low overpotentials were obtained (ca. 0.6V vs Ag/AgCl) and high electrocatalytic currents of about 3.5mAcm^-2 were recorded at neutral pH in O₂ saturated conditions for bilirubin oxidase electrodes. The combination of these bioanodes and bilirubin oxidase biocathodes provided glucose/O₂ enzymatic biofuel cells (EBFC) exhibiting an open-circuit potential of 0.640V, with an associated maximum current density of 2.10mAcm^-2. Moreover, the fuel cell delivered a maximum power density of 0.50mWcm^-2 at 0.36 V.

Non-covalent double functionalization of carbon nanotubes with a NADH oxidation Ru(II)-based molecular catalyst and a NAD-dependent glucose dehydrogenase

We report the double functionalization of multiwalled carbon nanotube electrodes by two functional pyrene molecules. In combination, an immobilized Ru(II)-based NADH oxidation catalyst and glucose dehydrogenase achieve highly efficient glucose oxidation with low overpotential of -0.10 V and high current densities of 6 mA cm(-2).

Recent progress in oxygen-reducing laccase biocathodes for enzymatic biofuel cells

This review summarizes different approaches and breakthroughs in the development of laccase-based biocathodes for bioelectrocatalytic oxygen reduction. The use of advanced electrode materials, such as nanoparticles and nanowires is underlined. The applications of recently developed laccase electrodes for enzymatic biofuel cells are reviewed with an emphasis on in vivo application of biofuel cells.

Laccase wiring on free-standing electrospun carbon nanofibres using a mediator plug

The enzyme laccase was wired on a free-standing electrospun carbon fiber mat using a cross-linker plug based on the pyrene modified electron shuttle ABTS.

Recent advances in carbon nanotube-based enzymatic fuel cells

This review summarizes recent trends in the field of enzymatic fuel cells. Thanks to the high specificity of enzymes, biofuel cells can generate electrical energy by oxidation of a targeted fuel (sugars, alcohols, or hydrogen) at the anode and reduction of oxidants (O2, H2O2) at the cathode in complex media. The combination of carbon nanotubes (CNT), enzymes and redox mediators was widely exploited to develop biofuel cells since the electrons involved in the bio-electrocatalytic processes can be efficiently transferred from or to an external circuit. Original approaches to construct electron transfer based CNT-bioelectrodes and impressive biofuel cell performances are reported as well as biomedical applications.