Nanostructured carbon electrodes for laccase-catalyzed oxygen reduction without added mediators

Immobilization of Cellulase Enzymes on Single-Walled Carbon Nanotubes for Recycling of Enzymes and Better Yield of Bioethanol Using Computer Simulations

The utilization of microbial cellulase enzymes for transforming plant biomass into biofuel or bioethanol, which can serve as a substitute for fossil fuel, is a subject of growing interest. Nonetheless, large-scale production of biofuel using cellulases is not economically feasible as the extraction of these enzymes from diverse microorganisms is an expensive process. To address this issue, immobilizing the enzyme to a substrate material, e.g., carbon nanotubes (CNTs), to recycle without a significant decline in its catalytic activity is a promising solution. Due to the hydrophobic nature of CNTs, we employed molecular docking and network analysis methodologies to identify potential CNT-binding sites on the outer surface of a wild-type cellulase enzyme, CelS. Classical molecular dynamics simulations of CNT-bound CelS through one of the selected binding sites resulted in negligible changes in the secondary structure of the enzyme and its catalytic domain, implying the least possible effect on the catalytic activity post-immobilization. Furthermore, our study reveals that while the unfolding near the CNT-binding region in CelS is more pronounced when the enzyme is interacting with a wider CNT, resulting in enhanced contact area and improved binding affinity, its impact on the overall CelS structure is relatively less significant when compared to thinner CNTs. Particularly, CNTs of diameter ∼12 Å can serve as a favorable option for substrate materials in cellulase immobilization. Our study also provides critical insights into the binding mechanisms between cellulase and CNTs, which could lead to the development of more efficient biocatalysts for biofuel production.

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.

Recent advances in carbon nanotubes-based biocatalysts and their applications

Enzymes have been incorporated into a wide variety of fields and industries as they catalyze many biochemical and chemical reactions. The immobilization of enzymes on carbon nanotubes (CNTs) for generating nano biocatalysts with high stability and reusability is gaining great attention among researchers. Functionalized CNTs act as excellent support for effective enzyme immobilization. Depending on the application, the enzymes can be tailored using the various surface functionalization techniques on the CNTs to extricate the desirable characteristics. Aiming at the preparation of efficient, stable, and recyclable nanobiocatalysts, this review provides an overview of the methods developed to immobilize the various enzymes. Various applications of carbon nanotube-based biocatalysts in water purification, bioremediation, biosensors, and biofuel cells have been comprehensively reviewed.

Stacked-Cup Carbon Nanotube Flexible Paper Based on Soy Lecithin and Natural Rubber

Stacked-cup carbon nanotubes (SCCNTs) are generally referred to as carbon nanofibers (CNFs). SCCNTs are much less expensive to fabricate and are regarded as good polymer modifiers suitable for large-scale production. Flexible, SCCNT-based soy lecithin biocomposites were fabricated using liquid natural rubber latex as binder. Natural polymers and the SCCNTs were dispersed in a green solvent using a benchtop high-pressure homogenizer. The inks were simply brush-on painted onto cellulose fiber networks and compacted by a hydraulic press so as to transform into conductive paper-like form. The resulting flexible SCCNT papers demonstrated excellent resistance against severe folding and bending tests, with volume resistivity of about 85 Ω·cm at 20 wt % SCCNT loading. The solvent enabled formation of hydrogen bonding between natural rubber and soy lecithin. Thermomechanical measurements indicated that the biocomposites have good stability below and above glass transition points. Moreover, the SCCNT biocomposites had high through-plane thermal conductivity of 5 W/mK and 2000 kJ/m³K volumetric heat capacity, ideal for thermal interface heat transfer applications.

Bacterial exopolysaccharides as a modern biotechnological tool for modification of fungal laccase properties and metal ion binding

Four bacterial EPSs extracted from Rhizobium leguminosarum bv. trifolii Rt24.2, Sinorhizobium meliloti Rm1021, Bradyrhizobium japonicum USDA110, and Bradyrhizobium elkanii USDA76 were determined towards their metal ion adsorption properties and possible modification of Cerrena unicolor laccase properties. The highest magnesium and iron ion-sorption capacity (~ 42 and ~ 14.5%, respectively) was observed for EPS isolated from B. japonicum USDA110. An evident influence of EPSs on the stability of laccase compared to the control values (without EPSs) was shown after 30-day incubation at 25 °C. The residual activity of laccases was obtained in the presence of Rh76EPS and Rh1021EPS, i.e., 49.5 and 41.5% of the initial catalytic activity, respectively. This result was confirmed by native PAGE electrophoresis. The EPS effect on laccase stability at different pH (from 3.8 to 7.0) was also estimated. The most significant changes at the optimum pH value (pH 5.8) was observed in samples of laccase stabilized by Rh76EPS and Rh1021EPS. Cyclic voltamperometry was used for analysis of electrochemical parameters of laccase stabilized by bacterial EPS and immobilized on single-walled carbon nanotubes (SWCNTs) with aryl residues. Laccases with Rh76EPS and Rh1021EPS had an evident shift of the value of the redox potential compared to the control without EPS addition. In conclusion, the results obtained in this work present a new potential use of bacterial EPSs as a metal-binding component and a modulator of laccase properties especially stability of enzyme activity, which can be a very effective tool in biotechnology and industrial applications.

Carbon nanotubes as nanovectors for intracellular delivery of laronidase in Mucopolysaccharidosis type I

The immobilization of proteins on carbon nanotubes (CNTs) has been widely reported mainly for the preparation of sensors while the conjugation of enzymes for therapeutic purposes has scarcely been considered. Herein we report, to the best of our knowledge, the first example of intracellular delivery of a therapeutic enzyme by means of CNTs, retaining its activity. Mucopolysaccharidosis I is a rare genetic disease characterized by the deficiency or absence of the activity of the α-l-iduronidase (IDUA) enzyme. We evaluated the capacity of the recombinant form of the human IDUA enzyme, laronidase (Aldurazyme®), conjugated with CNTs to be internalized by fibroblasts from subjects affected with Mucopolysaccharidosis type I and the capacity of the enzyme to retain its activity after internalization. The enzyme was successfully delivered into the lysosomal space and the enzymatic activity of the conjugate was preserved after internalization up to 48 hours. This paves the way towards the use of such a kind of construct for therapeutic applications.

Improving O2 reduction at an enzymatic biocathode: mimicking the lungs

Here, we demonstrate the use of phospholipid micelles to enhance O₂ concentrations by two-fold at the surface of a bilirubin oxidase biocathode. Specifically, 1,2-diarachidoyl-sn-glycero-3-phosphocholine was used in a glucose enzymatic fuel cell to limit power losses due to O₂ transport, even in a quiescent solution.

Chitosan improves stability of carbon nanotube biocathodes for glucose biofuel cells

We demonstrate a novel combined chitosan–carbon-nanotube–enzyme biocathode with a fibrous microstructure that improves the performance by creating a protective microenvironment, preventing the loss of the electrocatalytic activity of the enzyme, and providing good oxygen diffusion.

Enhanced performance of electrospun carbon fibers modified with carbon nanotubes: promising electrodes for enzymatic biofuel cells

New nanostructured electrodes, promising for the production of clean and renewable energy in biofuel cells, were developed with success. For this purpose, carbon nanofibers were produced by the electrospinning of polyacrylonitrile solution followed by convenient thermal treatments (stabilization followed by carbonization at 1000, 1200 and 1400° C), and carbon nanotubes were adsorbed on the surfaces of the fibers by a dipping method. The morphology of the developed electrodes was characterized by several techniques (SEM, Raman spectroscopy, electrical conductivity measurement). The electrochemical properties were evaluated through cyclic voltammetry, where the influence of the carbonization temperature of the fibers and the beneficial contribution of the carbon nanotubes were observed through the reversibility and size of the redox peaks of K3Fe(CN)6 versus Ag/AgCl. Subsequently, redox enzymes were immobilized on the electrodes and the electroreduction of oxygen to water was realized as a test of their efficiency as biocathodes. Due to the fibrous and porous structure of these new electrodes, and to the fact that carbon nanotubes may have the ability to promote electron transfer reactions of redox biomolecules, the new electrodes developed were capable of producing higher current densities than an electrode composed only of electrospun carbon fibers.

Carbon Nanotubes: A Review on Structure and Their Interaction with Proteins

Carbon nanotubes (CNTs) are allotropes of carbon with a nanostructure that can have a length-to-diameter ratio greater than 1,000,000. Techniques have been developed to produce nanotubes in sizeable quantities, including arc discharge, laser ablation, and chemical vapor deposition. Developments in the past few years have illustrated the potentially revolutionizing impact of nanomaterials, especially in biomedical imaging, drug delivery, biosensing, and the design of functional nanocomposites. Methods to effectively interface proteins with nanomaterials for realizing these applications continue to evolve. The high surface-to-volume ratio offered by nanoparticles resulted in the concentration of the immobilized entity being considerably higher than that afforded by other materials. There has also been an increasing interest in understanding the influence of nanomaterials on the structure and function of proteins. Various immobilization methods have been developed, and in particular, specific attachment of enzymes on carbon nanotubes has been an important focus of attention. With the growing attention paid to cascade enzymatic reaction, it is possible that multienzyme coimmobilization would be one of the next goals in the future. In this paper, we focus on advances in methodology for enzyme immobilization on carbon nanotubes.

Recent Developments of Nanostructured Electrodes for Bioelectrocatalysis of Dioxygen Reduction

The recent development of nanostructured electrodes for bioelectrocatalytic dioxygen reduction catalysed by two copper oxidoreductases, laccase and bilirubin oxidase, is reviewed. Carbon-based nanomaterials as carbon nanotubes or carbon nanoparticles are frequently used for electrode modification, whereas there are only few examples of biocathodes modified with metal or metal oxide nanoparticles. These nanomaterials are adsorbed on the electrode surface or embedded in multicomponent film. The nano-objects deposited act as electron shuttles between the enzyme and the electrode substrate providing favourable conditions for mediatorless bioelectrocatalysis.

Thermoresponsive poly(N-isopropylacrylamide) gel for immobilization of laccase on indium tin oxide electrodes

We report on the properties of hydrogel matrix for the immobilization of laccase on conductive supports. The poly(N-isopropylacrylamide) gel is attached firmly to the indium-tin oxide (ITO) electrode, following its silanization with dimethylethoxyvinylsilane. The enzyme entrapped in the gel structure remained active longer than in the solution, and its redox and catalytic properties could be investigated by voltammetric methods. The reduction signals of the active sites, T1 and T2, of the Cerrena unicolor laccase were determined to be 0.79 and 0.38 V, respectively. The laccase catalytic activity toward oxygen in poly(N-isopropylacrylamide) was found to depend strongly on temperature. Reversible swelling/shrinking of the matrix was studied at 30 and 35 degrees C. Shrinking of the gel at higher temperature considerably decreased the efficiency of the catalytic reaction, however, interestingly, did not lead to irreversible changes in the enzyme structure. At temperatures below that corresponding to volume phase transition, the catalytic properties of the film were fully restored. High catalytic efficiency of the gel immobilized enzyme made it possible to employ the gel covered electrode for monitoring oxygen in solutions.