Electron transfer from Proteus vulgaris to a covalently assembled, single walled carbon nanotube electrode functionalised with osmium bipyridine complex: application to a whole cell biosensor

Electric wiring of bacteria using redox polymers and selective measurement of metabolic activity in the presence of surrounding planktonic bacteria

Non-electroactive bacteria (n-EAB), constituting the majority of known bacteria to date, have been underutilized in electrochemical conversion technologies due to their lack of direct electron transfer to electrodes. In this study, we established an electric wiring between n-EAB (gram-positive Bacillus subtilis and gram-negative Escherichia coli) and an extracellular electrode via a ferrocene-polyethyleneimine-based redox polymer (Fc-PEI). Chronoamperometry recordings indicated that Fc-PEI can transfer intracellular electrons to the extracellular electrode regardless of the molecular organization of PEI (linear or branched) and the membrane structure of bacteria (gram-positive or -negative). As fluorescence staining suggested, Fc-PEI improves the permeability of the bacterial cell membrane, enabling electron carriers in the cell to react with Fc. In addition, experiments with Fc-immobilized electrodes without PEI suggested the existence of an alternative electron transfer pathway from B. subtilis to the extracellular Fc adsorbed onto the cell membrane. Furthermore, we proposed for the first time that the bacteria/Fc-linear PEI modified structure enables selective measurement of immobilized bacterial activity by physically blocking contact between the electrode surface and planktonic cells co-existing in the surrounding media. Such electrodes can be a powerful analytical tool for elucidating the metabolic activities of specific bacteria wired to the electrode even within complex bacterial communities.

Enhancing Microbial Electron Transfer Through Synthetic Biology and Biohybrid Approaches: Part II : Combining approaches for clean energy

It is imperative to develop novel processes that rely on cheap, sustainable and abundant resources whilst providing carbon circularity. Microbial electrochemical technologies (MET) offer unique opportunities to facilitate the conversion of chemicals to electrical energy or vice versa by harnessing the metabolic processes of bacteria to valorise a range of waste products including greenhouse gases (GHGs). Part I (1) introduced the EET pathways, their limitations and applications. Here in Part II, we outline the strategies researchers have used to modulate microbial electron transfer, through synthetic biology and biohybrid approaches and present the conclusions and future directions.

Electronic control of redox reactions inside Escherichia coli using a genetic module

Microorganisms regulate the redox state of different biomolecules to precisely control biological processes. These processes can be modulated by electrochemically coupling intracellular biomolecules to an external electrode, but current approaches afford only limited control and specificity. Here we describe specific electrochemical control of the reduction of intracellular biomolecules in Escherichia coli through introduction of a heterologous electron transfer pathway. E. coli expressing cymAmtrCAB from Shewanella oneidensis MR-1 consumed electrons directly from a cathode when fumarate or nitrate, both intracellular electron acceptors, were present. The fumarate-triggered current consumption occurred only when fumarate reductase was present, indicating all the electrons passed through this enzyme. Moreover, CymAMtrCAB-expressing E. coli used current to stoichiometrically reduce nitrate. Thus, our work introduces a modular genetic tool to reduce a specific intracellular redox molecule with an electrode, opening the possibility of electronically controlling biological processes such as biosynthesis and growth in any microorganism.

Membrane fusion and drug delivery with carbon nanotube porins

The plasma membrane protects cell content from the outer environment. Overcoming this obstacle and delivering drugs efficiently into cells still remain a major hurdle for modern pharmacology and medicine. We developed and demonstrated a simple nanomaterial platform—a dimer of small-diameter carbon nanotube porins (CNTPs)—which functions as a potent membrane fusogen. Molecular simulations revealed a distinct fusion mechanism. CNTP-studded vesicles loaded with a chemotherapeutic agent, doxorubicin, delivered the drug to cancer cells, killing a majority of them. Our work provides new opportunities for understanding membrane fusion mechanisms, designing synthetic fusogens, and developing simple and efficient drug carriers for delivery of therapeutics and vaccines.

Drug delivery mitigates toxic side effects and poor pharmacokinetics of life-saving therapeutics and enhances treatment efficacy. However, direct cytoplasmic delivery of drugs and vaccines into cells has remained out of reach. We find that liposomes studded with 0.8-nm-wide carbon nanotube porins (CNTPs) function as efficient vehicles for direct cytoplasmic drug delivery by facilitating fusion of lipid membranes and complete mixing of the membrane material and vesicle interior content. Fusion kinetics data and coarse-grained molecular dynamics simulations reveal an unusual mechanism where CNTP dimers tether the vesicles, pull the membranes into proximity, and then fuse their outer and inner leaflets. Liposomes containing CNTPs in their membranes and loaded with an anticancer drug, doxorubicin, were effective in delivering the drug to cancer cells, killing up to 90% of them. Our results open an avenue for designing efficient drug delivery carriers compatible with a wide range of therapeutics.

Toward nanobioelectronic medicine: Unlocking new applications using nanotechnology

Bioelectronic medicine aims to interface electronic technology with biological components and design more effective therapeutic and diagnostic tools. Advances in nanotechnology have moved the field forward improving the seamless interaction between biological and electronic components. In the lab many of these nanobioelectronic devices have the potential to improve current treatment approaches, including those for cancer, cardiovascular disorders, and disease underpinned by malfunctions in neuronal electrical communication. While promising, many of these devices and technologies require further development before they can be successfully applied in a clinical setting. Here, we highlight recent work which is close to achieving this goal, including discussion of nanoparticles, carbon nanotubes, and nanowires for medical applications. We also look forward toward the next decade to determine how current developments in nanotechnology could shape the growing field of bioelectronic medicine. This article is categorized under: Therapeutic Approaches and Drug Discovery > Emerging Technologies Nanotechnology Approaches to Biology > Nanoscale Systems in Biology Diagnostic Tools > Biosensing.

Use of a Thermophile Desiccation-Tolerant Cyanobacterial Culture and Os Redox Polymer for the Preparation of Photocurrent Producing Anodes

Oxygenic photosynthesis conducted by cyanobacteria has dramatically transformed the geochemistry of our planet. These organisms have colonized most habitats, including extreme environments such as the driest warm desert on Earth: the Atacama Desert. In particular, cyanobacteria highly tolerant to desiccation are of particular interest for clean energy production. These microorganisms are promising candidates for designing bioelectrodes for photocurrent generation owing to their ability to perform oxygenic photosynthesis and to withstand long periods of desiccation. Here, we present bioelectrochemical assays in which graphite electrodes were modified with the extremophile cyanobacterium Gloeocapsopsis sp. UTEXB3054 for photocurrent generation. Optimum working conditions for photocurrent generation were determined by modifying directly graphite electrode with the cyanobacterial culture (direct electron transfer), as well as using an Os polymer redox mediator (mediated electron transfer). Besides showing outstanding photocurrent production for Gloeocapsopsis sp. UTEXB3054, both in direct and mediated electron transfer, our results provide new insights into the metabolic basis of photocurrent generation and the potential applications of such an assisted bioelectrochemical system in a worldwide scenario in which clean energies are imperative for sustainable development.

Extracellular electron transfer features of Gram-positive bacteria

Electroactive microorganisms possess the unique ability to transfer electrons to or from solid phase electron conductors, e.g., electrodes or minerals, through various physiological mechanisms. The processes are commonly known as extracellular electron transfer and broadly harnessed in microbial electrochemical systems, such as microbial biosensors, microbial electrosynthesis, or microbial fuel cells. Apart from a few model microorganisms, the nature of the microbe-electrode conductive interaction is poorly understood for most of the electroactive species. The interaction determines the efficiency and a potential scaling up of bioelectrochemical systems. Gram-positive bacteria generally have a thick electron non-conductive cell wall and are believed to exhibit weak extracellular electron shuttling activity. This review highlights reported research accomplishments on electroactive Gram-positive bacteria. The use of electron-conducting polymers as mediators is considered as one promising strategy to enhance the electron transfer efficiency up to application scale. In view of the recent progress in understanding the molecular aspects of the extracellular electron transfer mechanisms of Enterococcus faecalis, the electron transfer properties of this bacterium are especially focused on. Fundamental knowledge on the nature of microbial extracellular electron transfer and its possibilities can provide insight in interspecies electron transfer and biogeochemical cycling of elements in nature. Additionally, a comprehensive understanding of cell-electrode interactions may help in overcoming insufficient electron transfer and restricted operational performance of various bioelectrochemical systems and facilitate their practical applications.

Tailoring the Electrochemical Properties of Carbon Nanotube Modified Indium Tin Oxide via in Situ Grafting of Aryl Diazonium

Our ability to tailor the electronic properties of surfaces by nanomodification is paramount for various applications, including development of sensing, fuel cell, and solar technologies. Moreover, in order to improve the rational design of conducting surfaces, an improved understanding of structure/function relationships of nanomodifications and effect they have on the underlying electronic properties is required. Herein, we report on the tuning and optimization of the electrochemical properties of indium tin oxide (ITO) functionalized with single-walled carbon nanotubes (SWCNTs). This was achieved by controlling in situ grafting of aryl amine diazonium films on the nanoscale which were used to covalently tether SWCNTs. The structure/function relationship of these nanomodifications on the electronic properties of ITO was elucidated via time-of-flight secondary ion mass spectrometry and electrochemical and physical characterization techniques which has led to new mechanistic insights into the in situ grafting of diazonium. We discovered that the connecting bond is a nitro group which is covalently linked to a carbon on the aryl amine. The increased understanding of the surface chemistry gained through these studies enabled us to fabricate surfaces with optimized electron transfer kinetics. The knowledge gained from these studies allows for the rational design and tuning of the electronic properties of ITO-based conducting surfaces important for development of various electronic applications.

Electronic communication of cells with a surface mediated by boronic acid saccharide interactions

The fabrication of a molecularly tailored surface functionalised with a saccharide binding motif, a phenyl boronic acid derivative is reported. The functionalised surface facilitated the transfer of electrons, via unique electronic interactions mediated by the presence of the boronic acid, from a macrophage cell line. This is the first example of eukaryotic cellular-electrical communication mediated by the binding of cells via their cell-surface saccharide units.

Fast, Ultrasensitive Detection of Reactive Oxygen Species Using a Carbon Nanotube Based-Electrocatalytic Intracellular Sensor

Herein, we report a highly sensitive electrocatalytic sensor-cell construct that can electrochemically communicate with the internal environment of immune cells (e.g., macrophages) via the selective monitoring of a particular reactive oxygen species (ROS), hydrogen peroxide. The sensor, which is based on vertically aligned single-walled carbon nanotubes functionalized with an osmium electrocatalyst, enabled the unprecedented detection of a local intracellular "pulse" of ROS on a short second time scale in response to bacterial endotoxin (lipopolysaccharide-LPS) stimulation. Our studies have shown that this initial pulse of ROS is dependent on NADPH oxidase (NOX) and toll like receptor 4 (TLR4). The results suggest that bacteria can induce a rapid intracellular pulse of ROS in macrophages that initiates the classical innate immune response of these cells to infection.

Crossing Over: Nanostructures that Move Electrons and Ions across Cellular Membranes

Critical biological processes such as energy generation and signal transduction are driven by the flow of electrons and ions across the membranes of living cells. As a result, there is substantial interest in creating nanostructured materials that control transport of these charged species across biomembranes. Recent advances in the synthesis of de novo and protein nanostructures for transmembrane ion and electron transport and the mechanistic understanding underlying this transport are described. This body of work highlights the promise such nanostructures hold for directing transmembrane transport of charged species as well as challenges that must be overcome to realize that potential.

Application of nanomaterials in microbial-cell biosensor constructions

Microbial cell biosensors, where cells are in direct connection with a transducer enabling quantitative and qualitative detection of an analyte, are very promising analytical tools applied mainly for assays in the environmental field, food industry or biomedicine. Microbial cell biosensors are an excellent alternative to conventional analytical methods due to their specificity, rapid detection and low cost of analysis. Nowadays, nanomaterials are often used in the construction of biosensors to improve their sensitivity and stability. In this review, the combination of microbial and other individual cells with different nanomaterials (carbon nanotubes, graphene, gold nanoparticles, etc.) for the construction of biosensors is described and their applications are provided as well.

Cellobiose dehydrogenase modified electrodes: advances by materials science and biochemical engineering

The flavocytochrome cellobiose dehydrogenase (CDH) is a versatile biorecognition element capable of detecting carbohydrates as well as quinones and catecholamines. In addition, it can be used as an anode biocatalyst for enzymatic biofuel cells to power miniaturised sensor-transmitter systems. Various electrode materials and designs have been tested in the past decade to utilize and enhance the direct electron transfer (DET) from the enzyme to the electrode. Additionally, mediated electron transfer (MET) approaches via soluble redox mediators and redox polymers have been pursued. Biosensors for cellobiose, lactose and glucose determination are based on CDH from different fungal producers, which show differences with respect to substrate specificity, pH optima, DET efficiency and surface binding affinity. Biosensors for the detection of quinones and catecholamines can use carbohydrates for analyte regeneration and signal amplification. This review discusses different approaches to enhance the sensitivity and selectivity of CDH-based biosensors, which focus on (1) more efficient DET on chemically modified or nanostructured electrodes, (2) the synthesis of custom-made redox polymers for higher MET currents and (3) the engineering of enzymes and reaction pathways. Combination of these strategies will enable the design of sensitive and selective CDH-based biosensors with reduced electrode size for the detection of analytes in continuous on-site and point-of-care applications.

Tailoring 3D single-walled carbon nanotubes anchored to indium tin oxide for natural cellular uptake and intracellular sensing

The ability to monitor intracellular events in real time is paramount to advancing fundamental biological and clinical science. We present the first demonstration of a direct interface of vertically aligned single-walled carbon nanotubes (VASWCNTs) with eukaryotic cells, RAW 264.7 mouse macrophage cell line. The cells were cultured on indium tin oxide with VASWCNTs. VASWCNTs entered the cells naturally without application of any external force and were shown to sense the intracellular presence of a redox active moiety, methylene blue. The technology developed provides an alluring platform to enable electrochemical study of an intracellular environment.

Electrochemical communication between microbial cells and electrodes via osmium redox systems

Electrochemical communication between micro-organisms and electrodes is the integral and fundamental part of BESs (bioelectrochemical systems). The immobilization of bacterial cells on the electrode and ensuring efficient electron transfer to the electrode via a mediator are decisive features of mediated electrochemical biosensors. Notably, mediator-based systems are essential to extract electrons from the non-exoelectrogens, a major group of microbes in Nature. The advantage of using polymeric mediators over diffusible mediators led to the design of osmium redox polymers. Their successful use in enzyme-based biosensors and BFCs (biofuel cells) paved the way for exploring their use in microbial BESs. The present mini-review focuses on osmium-bound redox systems used to date in microbial BESs and their role in shuttling electrons from viable microbial cells to electrodes.

Application of nanotechnology to control bacterial adhesion and patterning on material surfaces

Bacterial adhesion and biofilm formation on surfaces raises health hazard issues in the medical environment. Previous studies of bacteria adhesion have focused on observations in their natural/native environments. Recently, surface science has contributed in advancing the understanding of bacterial adhesion by providing ideal platforms that attempt to mimic the bacteria's natural environments, whilst also enabling concurrent control, selectivity and spatial control of bacterial adhesion. In this review, we will look at techniques of how nanotechnology is used to control cell adhesion on a planar scale, in addition to describing the use of nanotools for cell micropatterning. Additionally, it will provide a general background of common methods for nanoscale modification enabling biologist unfamiliar with nanotechnology to enter the field.

Regeneration of pyrolyzed photoresist film by heat treatment

A simple, time-, and cost-effective procedure is described for regenerating film-modified or deactivated pyrolyzed photoresist film (PPF) surfaces. Heating for 30 min at 545 ± 25 °C in argon at a flow rate of 1 L min(-1) removes covalently bound thin organic films, attached via electrografting from aryldiazonium salt solutions. The heat-treated surfaces exhibit improved electrochemical characteristics compared to those prior to modification and can be reused for solution-based electrochemical measurements and for electrografting. The same treatment reactivates PPF electrodes that have been deactivated by exposure to adsorbates from air or solution. X-ray photoelectron spectroscopy (XPS), atomic force microscopy, and water contact angle measurements establish that the regeneration procedure does not lead to significant changes in oxygen content, roughness, or hydrophobicity of PPF surfaces. XPS measurements also confirm the complete removal of covalently attached organic films after heat treatment but reveal a specific interaction between grafted nitrophenyl films and PPF which results in a small amount of N incorporation in the surface.