Prolongation of electrode lifetime in biofuel cells by periodic enzyme renewal

A Strep-Tag Imprinted Polymer Platform for Heterogenous Bio(electro)catalysis

Molecularly imprinted polymers (MIPs) are artificial receptors equipped with selective recognition sites for target molecules. One of the most promising strategies for protein MIPs relies on the exploitation of short surface-exposed protein fragments, termed epitopes, as templates to imprint binding sites in a polymer scaffold for a desired protein. However, the lack of high-resolution structural data of flexible surface-exposed regions challenges the selection of suitable epitopes. Here, we addressed this drawback by developing a polyscopoletin-based MIP that recognizes recombinant proteins via imprinting of the widely used Strep-tag II affinity peptide (Strep-MIP). Electrochemistry, surface-sensitive IR spectroscopy, and molecular dynamics simulations were employed to ensure an utmost control of the Strep-MIP electrosynthesis. The functionality of this novel platform was verified with two Strep-tagged enzymes: an O2-tolerant [NiFe]-hydrogenase, and an alkaline phosphatase. The enzymes preserved their biocatalytic activities after multiple utilization confirming the efficiency of Strep-MIP as a general biocompatible platform to confine recombinant proteins for exploitation in biotechnology.

Biocatalytic Membranes for Carbon Capture and Utilization

Innovative carbon capture technologies that capture CO₂ from large point sources and directly from air are urgently needed to combat the climate crisis. Likewise, corresponding technologies are needed to convert this captured CO₂ into valuable chemical feedstocks and products that replace current fossil-based materials to close the loop in creating viable pathways for a renewable economy. Biocatalytic membranes that combine high reaction rates and enzyme selectivity with modularity, scalability, and membrane compactness show promise for both CO₂ capture and utilization. This review presents a systematic examination of technologies under development for CO₂ capture and utilization that employ both enzymes and membranes. CO₂ capture membranes are categorized by their mode of action as CO₂ separation membranes, including mixed matrix membranes (MMM) and liquid membranes (LM), or as CO₂ gas-liquid membrane contactors (GLMC). Because they selectively catalyze molecular reactions involving CO₂, the two main classes of enzymes used for enhancing membrane function are carbonic anhydrase (CA) and formate dehydrogenase (FDH). Small organic molecules designed to mimic CA enzyme active sites are also being developed. CO₂ conversion membranes are described according to membrane functionality, the location of enzymes relative to the membrane, which includes different immobilization strategies, and regeneration methods for cofactors. Parameters crucial for the performance of these hybrid systems are discussed with tabulated examples. Progress and challenges are discussed, and perspectives on future research directions are provided.

In vitro and in vivo detection of lactate with nanohybrid-functionalized Pt microelectrode facilitating assessment of tumor development

Accelerated glucose uptake and "aerobic glycolysis" of tumor cells generates a high-level lactate in extracellular space and within tumor tissue, which is thought to be a hallmark of tumor and closely correlated with tumor development. Here, we report the development of an enzyme-free electrochemical sensing platform based on a Pt-microneedle electrode functionalized with Au nanoparticles (Au-NPs) decorated polydopamine nanospheres (PDA-NSs), and explore its practical application in in vitro and in vivo detection of lactate in different biological samples. Our results demonstrate that in virtue of the nanostructured merits and high electrocatalytic activity, the resultant nanohybrid-microelectrode exhibits good sensitivity and selectivity to the nonenzymatic electrochemical detection of lactate, with a detection limit of 50 μM, a liner range of 0.375-12 mM, and a sensitivity of 11.25 mA mM^-1 cm^-2, as well as a good anti-interference ability to other active small molecules. The platform quantifies lactate in complex bio-fluids, including cancerous and non-cancerous cell culture media, as well as serum samples, with detecting time 7.5-fold faster than does a clinically-used approach. Moreover, owing to miniaturized size and satisfactory electrochemical performance, the sensor achieves in vivo recording of lactate-related characteristic voltammetric signals within a living tumor, which are positively correlated with tumor burden and growth. Therefore, the platform cannot only be employed for cancer metabolic investigation, but also potentially for clinical assessment of tumor progression, and even clinical diagnosis of other lactate metabolism disorders.

Degradation of 2,4-dichlorophenol contaminated soil by ultrasound-enhanced laccase

In this paper, ultrasound was used to enhance the degradation effect of laccase for 2,4-dichlorophenol (2,4-DCP) in soil. The degradation effect and mechanism of the ultrasound-enhanced laccase were investigated. From the results, the degradation rate of 2,4-DCP can reach as high as 51.7% under the following conditions: reaction period was 21 h, pH = 5.5, ultrasound power was 240 W, duty cycle was 50%, and moisture content was 50%. Using the ultrasound-enhanced laccase, the degradation rate of 2,4-DCP was significantly higher than that using only laccase or only ultrasound. In addition, when ultrasound was used, the optimum pH for the degradation of 2,4-DCP using laccase was increased, making the degradation technology more practical. The analysis results from high-performance liquid chromatography (HPLC) and gas chromatography-mass spectrometry (GC-MS) revealed the degradation pathway of 2,4-DCP in soil: first, 2,4-DCP gradually became phenol through dechlorination, then the small molecular organic matter was generated from the hydroxyl radical or laccase reaction.

A Hybrid Microbial–Enzymatic Fuel Cell Cathode Overcomes Enzyme Inactivation Limits in Biological Fuel Cells

The construction of optimized biological fuel cells requires a cathode which combines the longevity of a microbial catalyst with the current density of an enzymatic catalyst. Laccase-secreting fungi were grown directly on the cathode of a biological fuel cell to facilitate the exchange of inactive enzymes with active enzymes, with the goal of extending the lifetime of laccase cathodes. Directly incorporating the laccase-producing fungus at the cathode extends the operational lifetime of laccase cathodes while eliminating the need for frequent replenishment of the electrolyte. The hybrid microbial–enzymatic cathode addresses the issue of enzyme inactivation by using the natural ability of fungi to exchange inactive laccases at the cathode with active laccases. Finally, enzyme adsorption was increased through the use of a functionally graded coating containing an optimized ratio of titanium dioxide nanoparticles and single-walled carbon nanotubes. The hybrid microbial–enzymatic fuel cell combines the higher current density of enzymatic fuel cells with the longevity of microbial fuel cells, and demonstrates the feasibility of a self-regenerating fuel cell in which inactive laccases are continuously exchanged with active laccases.

Carbon-coated magnetic nanoparticles as a removable protection layer extending the operation lifetime of bilirubin oxidase-based bioelectrode

One of the factors hindering the development of enzymatic biosensors and biofuel cells in real-life applications is the time-dependant degradation of the biocatalysts on electrode surfaces. In this work, we present a new practical approach for extending the operation lifetimes of bioelectrocatalytic assemblies based on bilirubin oxidase (BOD). As evident by both spectroscopic and electrochemical measurements, an adsorption of carbon-coated magnetic nanoparticles (ccMNPs) onto a BOD/carbon nanotubes-deposited surface yields a stable bioelectrocathode system for mediatorless oxygen reduction. As compared to electrodes, which were stored without a preliminary interaction with the ccMNPs, an 80% increase in the active enzymatic content and the electrocatalytic performance was evident for the modified assemblies over a course of one month. As the full removal of the protective particles before the measurement requires only a single step applying an external magnetic force, the method is shown to be simple, reproducible, and easy to implement. Combined with the high efficiency in preserving the enzymatic stability and bioelectrocatalytic currents, the findings suggest a promising methodology for enhancing the lifetimes of bioelectronic applications.

An air-breathing enzymatic cathode with extended lifetime by continuous laccase supply

We present a novel concept of an air-breathing enzymatic biofuel cell cathode combined with continuous supply of unpurified laccase-containing supernatant of the white-rot fungus Trametes versicolor for extended lifetime. The air-breathing cathode design obviates the need for energy-intensive active aeration. In a corresponding long-term experiment at a constant current density of 50 µA cm^-2, we demonstrated an increased lifetime of 33 days (cathode potential above 0.430 V vs. SCE), independent of enzyme degradation. The obtained data suggest that theoretically a longer lifetime is feasible. However, further engineering efforts are required to prevent clogging and fouling of the supply tubes. These results represent an important step towards the realization of enzymatic biofuel cell cathodes with extended lifetime and enhanced performance.

Decolourisation of Acid orange 7 in a microbial fuel cell with a laccase-based biocathode: Influence of mitigating pH changes in the cathode chamber

Biocathodes may be a suitable replacement of platinum in microbial fuel cells (MFCs) if the cost of MFCs is to be reduced. However, the use of enzymes as bio-cathodes is fraught with loss of activity as time progresses. A possible cause of this loss in activity might be pH increase in the cathode as pH gradients in MFCs are well known. This pH increase is however, accompanied by simultaneous increase in salinity; therefore salinity may be a confounding variable. This study investigated various ways of mitigating pH changes in the cathode of MFCs and their effect on laccase activity and decolourisation of a model azo dye Acid orange 7 in the anode chamber. Experiments were run with catholyte pH automatically controlled via feedback control or by using acetate buffers (pH 4.5) of various strength (100mM and 200mM), with CMI7000 as the cation exchange membrane. A comparison was also made between use of CMI7000 and Nafion 117 as the transport properties of cations for both membranes (hence their potential effects on pH changes in the cathode) are different. Results show that using Nafion 117 membrane limits salinity and pH changes in the cathode (100mM acetate buffer as catholyte) leading to prolonged laccase activity and faster AO7 decolourisation compared to using CMI7000 as a membrane; similarly automatic pH control in the cathode chamber was found to be better than using 200mM acetate buffer. It is suggested that while pH control in the cathode chamber is important, it does not guarantee sustained laccase activity; as salinity increases affect the activity and it could be mitigated using a cation selective membrane.

Improving the performance of a biofuel cell cathode with laccase-containing culture supernatant from Pycnoporus sanguineus

Laccases are multicopper oxidoreductases that can be used in biofuel cells to improve cathode performance by cathodic oxygen reduction. Here we present a laccase from the ligninolytic white-rot fungus Pycnoporus sanguineus that, in contrast to the Trametes versicolor laccase, can be produced in the absence of inducers in a standard culture medium. After 7days of cultivation the activity of this laccase in culture supernatant reached 2.5U/ml, which is high enough for direct application of the supernatant in biofuel cells. The highest current density of 115.0±3.5μA/cm(2) at 400mV vs. SCE was obtained at pH 5 with a buckypaper cathode with a laccase-containing culture supernatant. The enzyme also showed electrocatalytic activity at pH 6 and 7. These results not only present a new cost-efficient laccase for improving cathode performance, but also show that new laccases with different catalytic properties can be suitable for biofuel cells.

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.

Overcoming bottlenecks of enzymatic biofuel cell cathodes: crude fungal culture supernatant can help to extend lifetime and reduce cost

Enzymatic biofuel cells (BFCs) show great potential for the direct conversion of biochemically stored energy from renewable biomass resources into electricity. However, enzyme purification is time-consuming and expensive. Furthermore, the long-term use of enzymatic BFCs is hindered by enzyme degradation, which limits their lifetime to only a few weeks. We show, for the first time, that crude culture supernatant from enzyme-secreting microorganisms (Trametes versicolor) can be used without further treatment to supply the enzyme laccase to the cathode of a mediatorless BFC. Polarization curves show that there is no significant difference in the cathode performance when using crude supernatant that contains laccase compared to purified laccase in culture medium or buffer solution. Furthermore, we demonstrate that the oxygen reduction activity of this enzymatic cathode can be sustained over a period of at least 120 days by periodic resupply of crude culture supernatant. This is more than five times longer than control cathodes without the resupply of culture supernatant. During the operation period of 120 days, no progressive loss of potential is observed, which suggests that significantly longer lifetimes than shown in this work may be possible. Our results demonstrate the possibility to establish simple, cost efficient, and mediatorless enzymatic BFC cathodes that do not require expensive enzyme purification procedures. Furthermore, they show the feasibility of an enzymatic BFC with an extended lifetime, in which self-replicating microorganisms provide the electrode with catalytically active enzymes in a continuous or periodic manner.