Microbial biosensors: A review

Biofilm's morphology design for high sensitivity of bioelectrochemical sensor: An experimental and modeling study

High sensitivity is essential for the application of bioelectrochemical system-based sensor (BES sensor) in water quality early-warning, where the electroactive biofilm is of vital importance as it delivers a responsive electric signal to toxic substances. This study artificially designed the morphology of a naturally formed biofilm by employing a serrated knife to scrape the biofilm and thus obtained a reduced thickness and roughness. Then it was further cut by half to halve the biomass. BES sensors equipped with control and processed biofilms were operated under constant anode potential (CAP) and tested at different Cu(II) concentrations to study their sensitivities. Results revealed that the scraped biofilms delivered much increased sensitivity towards Cu(II) shock, which was attributed to a reduced thickness as illustrated by macroscopic and microscopic morphology analysis. Another finding was that biomass per unit interfacial area, rather than the biomass, also affected the sensitivity. To further describe how the inner biofilm responded the toxicity after morphology design, a one-dimension mass transfer model was developed to simulate the mass transfer of Cu(II) in the biofilms with different thicknesses. The relative threshold value of inlet Cu(II) concentration was employed to fit the modeling and experimental results, indicating that decreased biofilm thickness was beneficial for improving the sensitivity.

Immobilisation of electrochemically active bacteria on screen-printed electrodes for rapid in situ toxicity biosensing

Microbial biosensors can be an excellent alternative to classical methods for toxicity monitoring, which are time-consuming and not sensitive enough. However, bacteria typically connect to electrodes through biofilm formation, leading to problems due to lack of uniformity or long device production times. A suitable immobilisation technique can overcome these challenges. Still, they may respond more slowly than biofilm-based electrodes because bacteria gradually adapt to electron transfer during biofilm formation. In this study, we propose a controlled and reproducible way to fabricate bacteria-modified electrodes. The method consists of an immobilisation step using a cellulose matrix, followed by an electrode polarization in the presence of ferricyanide and glucose. Our process is short, reproducible and led us to obtain ready-to-use electrodes featuring a high-current response. An excellent shelf-life of the immobilised electrochemically active bacteria was demonstrated for up to one year. After an initial 50% activity loss in the first month, no further declines have been observed over the following 11 months. We implemented our bacteria-modified electrodes to fabricate a lateral flow platform for toxicity monitoring using formaldehyde (3%). Its addition led to a 59% current decrease approximately 20 min after the toxic input. The methods presented here offer the ability to develop a high sensitivity, easy to produce, and long shelf life bacteria-based toxicity detectors.

A Coculture Based Tyrosine-Tyrosinase Electrochemical Gene Circuit for Connecting Cellular Communication with Electronic Networks

There is a growing interest in mediating information transfer between biology and electronics. By the addition of redox mediators to various samples and cells, one can both electronically obtain a redox "portrait" of a biological system and, conversely, program gene expression. Here, we have created a cell-based synthetic biology-electrochemical axis in which engineered cells process molecular cues, producing an output that can be directly recorded via electronics-but without the need for added redox mediators. The process is robust; two key components must act together to provide a valid signal. The system builds on the tyrosinase-mediated conversion of tyrosine to L-DOPA and L-DOPAquinone, which are both redox active. "Catalytic" transducer cells provide for signal-mediated surface expression of tyrosinase. Additionally, "reagent" transducer cells synthesize and export tyrosine, a substrate for tyrosinase. In cocultures, this system enables real-time electrochemical transduction of cell activating molecular cues. To demonstrate, we eavesdrop on quorum sensing signaling molecules that are secreted by Pseudomonas aeruginosa, N-(3-oxododecanoyl)-l-homoserine lactone and pyocyanin.

Development of an epigenetic tetracycline sensor system based on DNA methylation

Bacterial live cell sensors are potentially powerful tools for the detection of environmental toxins. In this work, we have established and validated a flow cytometry readout for an existing bacterial arabinose sensor system with DNA methylation based memory function (Maier et al., 2017, Nat. Comm., 8:15336). Flow cytometry readout is convenient and enables a multiparameter analysis providing information about single-cell variability, which is beneficial for further development of sensor systems of this type in the future. We then designed a tetracycline sensor system, because of the importance of antibiotics pollution in the light of multi-resistant pathogens. To this end, a tetracycline trigger plasmid was constructed by replacing the araC repressor gene and the ara operator of the arabinose trigger plasmid with the tetR gene coding for the tetracycline repressor and the tet operon. After combination with the memory plasmid, the tetracycline sensor system was shown to be functional in E. coli allowing to detect and memorize the presence of tetracycline. Due to a positive feedback between the trigger and memory systems, the combined whole-cell biosensor showed a very high sensitivity for tetracycline with a detection threshold at 0.1 ng/ml tetracycline, which may be a general property of sensors of this type. Moreover, acute presence of tetracycline and past exposure can be detected by this sensor using the dual readout of two reporter fluorophores.

Developments of Riboswitches and Toehold Switches for Molecular Detection-Biosensing and Molecular Diagnostics

Riboswitches and toehold switches are considered to have potential for implementation in various fields, i.e., biosensing, metabolic engineering, and molecular diagnostics. The specific binding, programmability, and manipulability of these RNA-based molecules enable their intensive deployments in molecular detection as biosensors for regulating gene expressions, tracking metabolites, or detecting RNA sequences of pathogenic microorganisms. In this review, we will focus on the development of riboswitches and toehold switches in biosensing and molecular diagnostics. This review introduces the operating principles and the notable design features of riboswitches as well as toehold switches. Moreover, we will describe the advances and future directions of riboswitches and toehold switches in biosensing and molecular diagnostics.

A Strategy Employing a TF-Splinting Duplex Nanoswitch to Achieve Single-Step, Enzyme-Free, Signal-On Detection of l-Tryptophan

Transcription factor (TF)-based metabolite detection mainly depends on TF-regulated gene expression in cells. From TF activation to gene transcription and translation, the signal travels a relatively long way before it is received. Here, we propose a TF-splinting duplex DNA nanoswitch to detect metabolites. We show its feasibility using tryptophan repressor (TrpR) to detect l-tryptophan as a model. The assay has been optimized and characterized after obtaining a proof of concept, and the detection of l-tryptophan in complex biological samples is feasible. Unlike an equivalent gene expression approach, the whole process is a single-step, enzyme-free, and signal-on method. It can be completed within 20 min. This proposed TF-splinting duplex has the potential to be applied to the quick and convenient detection of other metabolites or even TFs.

Label-Free Biosensors for Laboratory-Based Diagnostics of Infections: Current Achievements and New Trends

Infections pose a serious global public health problem and are a major cause of premature mortality worldwide. One of the most challenging objectives faced by modern medicine is timely and accurate laboratory-based diagnostics of infectious diseases. Being a key factor of timely initiation and success of treatment, it may potentially provide reduction in incidence of a disease, as well as prevent outbreak and spread of dangerous epidemics. The traditional methods of laboratory-based diagnostics of infectious diseases are quite time- and labor-consuming, require expensive equipment and qualified personnel, which restricts their use in case of limited resources. Over the past six decades, diagnostic technologies based on lateral flow immunoassay (LFIA) have been and remain true alternatives to modern laboratory analyzers and have been successfully used to quickly detect molecular ligands in biosubstrates to diagnose many infectious diseases and septic conditions. These devices are considered as simplified formats of modern biosensors. Recent advances in the development of label-free biosensor technologies have made them promising diagnostic tools that combine rapid pathogen indication, simplicity, user-friendliness, operational efficiency, accuracy, and cost effectiveness, with a trend towards creation of portable platforms. These qualities exceed the generally accepted standards of microbiological and immunological diagnostics and open up a broad range of applications of these analytical systems in clinical practice immediately at the site of medical care (point-of-care concept, POC). A great variety of modern nanoarchitectonics of biosensors are based on the use of a broad range of analytical and constructive strategies and identification of various regulatory and functional molecular markers associated with infectious bacterial pathogens. Resolution of the existing biosensing issues will provide rapid development of diagnostic biotechnologies.

Structure and Functions of Hydrocarbon-Degrading Microbial Communities in Bioelectrochemical Systems

Bioelectrochemical systems (BESs) exploit the interaction between microbes and electrodes. A field of application thereof is bioelectrochemical remediation, an effective strategy in environments where the absence of suitable electron acceptors limits classic bioremediation approaches. Understanding the microbial community structure and genetic potential of anode biofilms is of great interest to interpret the mechanisms occurring in BESs. In this study, by using a whole metagenome sequencing approach, taxonomic and functional diversity patterns in the inoculum and on the anodes of three continuous-flow BES for the removal of phenol, toluene, and BTEX were obtained. The genus Geobacter was highly enriched on the anodes and two reconstructed genomes were taxonomically related to the Geobacteraceae family. To functionally characterize the microbial community, the genes coding for the anaerobic degradation of toluene, ethylbenzene, and phenol were selected as genetic markers for the anaerobic degradation of the pollutants. The genes related with direct extracellular electron transfer (EET) were also analyzed. The inoculum carried the genetic baggage for the degradation of aromatics but lacked the capacity of EET while anodic bacterial communities were able to pursue both processes. The metagenomic approach provided useful insights into the ecology and complex functions within hydrocarbon-degrading electrogenic biofilms.

A Fast and Easily Parallelizable Biosensor Method for Measuring Extractable Tetracyclines in Soils

Quantification of extractable antibiotics in soils is important to assessing their bioavailability and mobility, and ultimately their ecotoxicological and health risks. This study aimed to establish a biosensor method for detecting extractable tetracyclines in soils (Alfisol, Mollisol, and Ultisol) using whole-cell biosensors containing a reporter plasmid (pMTGFP or pMTmCherry) carrying fluorescent protein genes tightly controlled by tetracyclines-responsive control region (tetRO). This whole-cell biosensor method can simultaneously measure 96 or more samples within 6 h and is easily parallelizable, whereas a typical high-performance liquid chromatography (HPLC) method may require 7 times more of analysis time and much greater cost to achieve similar analytical throughput. The biosensor method had a detection limit for each of six tetracyclines between 5.32-10.2 μg/kg soil, which is considered adequate for detecting tetracyclines in ethylenediaminetetraacetic acid (EDTA) extracts of soils. Relative standard deviation was between 19.8-51.2% for the biosensor Escherichia coli DH5α/pMTGFP and 2.98-25.8% for E. coli DH5α/pMTmCherry, respectively, suggesting that E. coli DH5α/pMTmCherry was superior to E. coli DH5α/pMTGFP for detecting extractable tetracyclines in soils. This new, fast, easily parallelizable, and cost-effective biosensor method has the potential for measuring extractable concentrations of tetracyclines for a large number of soil samples in large-scale monitoring studies.

Microbial electrochemical technologies: Electronic circuitry and characterization tools

Microbial electrochemistry merges microbiology, electrochemistry and electronics to provide a set of technologies for environmental engineering applications. Understanding the electronic concepts is crucial for effectively adopting these systems, but the importance of electronic circuitry is often overlooked by microbial electrochemistry researchers. This review provides the background on the electronics and electrochemical concepts involved in the study of microorganisms interacting with electricity, and their applications in microbial electrochemical technology (MET). The potentiostat circuitry is described along with its working principles. Electrochemical analyses are presented together with the rational and parameters employed to study MET devices and electroactive microorganisms. Finally, future directions are delineated towards the adoption of MET, and the related electronics, in environmental engineering applications.

P-N Junction-Based Si Biochips with Ring Electrodes for Novel Biosensing Applications

In this work, we report on the impedance of p-n junction-based Si biochips with gold ring top electrodes and unstructured platinum bottom electrodes which allows for counting target biomaterial in a liquid-filled ring top electrode region. The systematic experiments on p-n junction-based Si biochips fabricated by two different sets of implantation parameters (i.e. biochips PS5 and BS5) are studied, and the comparable significant change of impedance characteristics in the biochips in dependence on the number of bacteria suspension, i.e., Lysinibacillus sphaericus JG-A12, in Deionized water with an optical density at 600 nm from OD₆₀₀ = 4–16 in the electrode ring region is demonstrated. Furthermore, with the help of the newly developed two-phase electrode structure, the modeled capacitance and resistance parameters of the electrical equivalent circuit describing the p-n junction-based biochips depend linearly on the number of bacteria in the ring top electrode region, which successfully proves the potential performance of p-n junction-based Si biochips in observing the bacterial suspension. The proposed p-n junction-based biochips reveal perspective applications in medicine and biology for diagnosis, monitoring, management, and treatment of diseases.

A novel biosensor for zinc detection based on microbial fuel cell system

Microbial fuel cell (MFC) biosensors are self-sustainable device for monitoring of various substrates; however, for heavy metals detection are still scarce. In this study, E. coli BL21 was engineered to express the zntR, ribB, and oprF genes with P_zntA promoter, which could sense zinc (Zn²⁺) for riboflavin and porin production. The engineered strain produced high levels of riboflavin (2.4-3.6 μM) and improved cell membrane permeability, with a positive correlation of Zn²⁺ (0-400 μM). The strain was then employed in MFC biosensor under the following operational parameters: external resistance 1000 Ω, pH 9, and temperature 37 °C for Zn²⁺ sensing. The maximum voltages (160, 183, 260, 292, and 342 mV) of the constructed MFC biosensor have a linear relationship with Zn²⁺ concentrations (0, 100, 200, 300, and 400 μM, respectively) (R² = 0.9777). An Android App was developed for the biosensor system that could sense Zn²⁺ in real-time and in situ. The biosensor was applied to wastewater with different Zn²⁺ concentrations and the results showed that the detection range for Zn²⁺ was 20-100 μM, which covers common Zn²⁺ safety standards. The results obtained with developed MFC biosensor were comparable to conventional methods such as colorimetric, flame atomic absorption spectroscopy (FAAS), and inductively coupled plasma optical emission spectroscopy (ICP-OES). In summary, MFC biosensor with biosynthetic strain is an efficient and affordable system for real-time monitoring and sensing of heavy metals.

Modelling the energy harvesting from ceramic-based microbial fuel cells by using a fuzzy logic approach

Microbial fuel cells (MFCs) is a promising technology that is able to simultaneously produce bioenergy and treat wastewater. Their potential large-scale application is still limited by the need of optimising their power density. The aim of this study is to simulate the absolute power output by ceramic-based MFCs fed with human urine by using a fuzzy inference system in order to maximise the energy harvesting. For this purpose, membrane thickness, anode area and external resistance, were varied by running a 27-parameter combination in triplicate with a total number of 81 assays performed. Performance indices such as R² and variance account for (VAF) were employed in order to compare the accuracy of the fuzzy inference system designed with that obtained by using nonlinear multivariable regression. R² and VAF were calculated as 94.85% and 94.41% for the fuzzy inference system and 79.72% and 65.19% for the nonlinear multivariable regression model, respectively. As a result, these indices revealed that the prediction of the absolute power output by ceramic-based MFCs of the fuzzy-based systems is more reliable than the nonlinear multivariable regression approach. The analysis of the response surface obtained by the fuzzy inference system determines that the maximum absolute power output by the air-breathing set-up studied is 450  μ W when the anode area ranged from 160 to 200 cm², the external loading is approximately 900 Ω and a membrane thickness of 1.6 mm, taking into account that the results also confirm that the latter parameter does not show a significant effect on the power output in the range of values studied.

Impedimetric transducers based on interdigitated electrode arrays for bacterial detection - A review

Application of the impedance spectroscopy technique to detection of bacteria has advanced considerably over the last decade. This is reflected by the large amount of publications focused on basic research and applications of impedance biosensors. Employment of modern technologies to significantly reduce dimension of impedimetric devices enable on-chip integration of interdigitated electrode arrays for low-cost and easy-to-use sensors. This review is focused on publications dealing with interdigitated electrodes as a transducer unit and different bacteria detection systems using these devices. The first part of the review deals with the impedance technique principles, paying special attention to the use of interdigitated electrodes, while the main part of this work is focused on applications ranging from bacterial growth monitoring to label-free specific bacteria detection.

Microbial Fuel Cell-Based Biosensors

The microbial fuel cell (MFC) is a promising environmental biotechnology that has been proposed mainly for power production and wastewater treatment. Though small power output constrains its application for directly operating most electrical devices, great progress in its chemical, electrochemical, and microbiological aspects has expanded the applications of MFCs into other areas such as the generation of chemicals (e.g., formate or methane), bioremediation of contaminated soils, water desalination, and biosensors. In recent decades, MFC-based biosensors have drawn increasing attention because of their simplicity and sustainability, with applications ranging from the monitoring of water quality (e.g., biochemical oxygen demand (BOD), toxicants) to the detection of air quality (e.g., carbon monoxide, formaldehyde). In this review, we summarize the status quo of MFC-based biosensors, putting emphasis on BOD and toxicity detection. Furthermore, this review covers other applications of MFC-based biosensors, such as DO and microbial activity. Further, challenges and prospects of MFC-based biosensors are briefly discussed.

Microbial Fuel Cell-Based Biosensors

The microbial fuel cell (MFC) is a promising environmental biotechnology that has been proposed mainly for power production and wastewater treatment. Though small power output constrains its application for directly operating most electrical devices, great progress in its chemical, electrochemical, and microbiological aspects has expanded the applications of MFCs into other areas such as the generation of chemicals (e.g., formate or methane), bioremediation of contaminated soils, water desalination, and biosensors. In recent decades, MFC-based biosensors have drawn increasing attention because of their simplicity and sustainability, with applications ranging from the monitoring of water quality (e.g., biochemical oxygen demand (BOD), toxicants) to the detection of air quality (e.g., carbon monoxide, formaldehyde). In this review, we summarize the status quo of MFC-based biosensors, putting emphasis on BOD and toxicity detection. Furthermore, this review covers other applications of MFC-based biosensors, such as DO and microbial activity. Further, challenges and prospects of MFC-based biosensors are briefly discussed.

Strategies of Immobilizing Cells in Whole-cell Microbial Biosensor Devices Targeted for Analytical Field Applications

This review summarizes the development of whole-cell biosensors with a special focus on device development and cell immobilization. Integration of biosensor functions in a device will pave the way for field applications in remote areas and resource-limited settings. Firstly, an introduction to the field of whole-cell biosensors is provided, followed by examples of genetic engineering of cells in order to fulfill sensor functions. A framework of requirements to enable future field applications of biosensors is elaborated. A special focus is on different cell immobilization techniques ranging from polymers, to microfluidic devices, immobilization on paper and combinations of these methods. Looking at globally successfully implemented point of care devices such as a home pregnancy test or a blood glucose meter, we conclude the review with thoughts on long-term stability, portability, ease of use and user safety design guidelines for whole-cell biosensor devices.

Cell-based biosensors: Recent trends, challenges and future perspectives

Existing at the interface of biology and electronics, living cells have been in use as biorecognition elements (bioreceptors) in biosensors since the early 1970s. They are an interesting choice of bioreceptors as they allow flexibility in determining the sensing strategy, are cheaper than purified enzymes and antibodies and make the fabrication relatively simple and cost-effective. And with advances in the field of synthetic biology, microfluidics and lithography, many exciting developments have been made in the design of cell-based biosensors in the last about five years. 3D cell culture systems integrated with electrodes are now providing new insights into disease pathogenesis and physiology, while cardiomyocyte-integrated microelectrode array (MEA) technology is set to be standardized for the assessment of drug-induced cardiac toxicity. From cell microarrays for high-throughput applications to plasmonic devices for anti-microbial susceptibility testing and advent of microbial fuel cell biosensors, cell-based biosensors have evolved from being mere tools for detection of specific analytes to multi-parametric devices for real time monitoring and assessment. However, despite these advancements, challenges such as regeneration and storage life, heterogeneity in cell populations, high interference and high costs due to accessory instrumentation need to be addressed before the full potential of cell-based biosensors can be realized at a larger scale. This review summarizes results of the studies that have been conducted in the last five years toward the fabrication of cell-based biosensors for different applications with a comprehensive discussion on the challenges, future trends, and potential inputs needed for improving them.

Electrochemical inhibition bacterial sensor array for detection of water pollutants: artificial neural network (ANN) approach

This work reports on further development of an inhibition electrochemical sensor array based on immobilized bacteria for the preliminary detection of a wide range of organic and inorganic pollutants, such as heavy metal salts (HgCl₂, PbCl₂, CdCl₂), pesticides (atrazine, simazine, DDVP), and petrochemicals (hexane, octane, pentane, toluene, pyrene, and ethanol) in water. A series of DC and AC electrochemical measurements, e.g., cyclic voltammograms and impedance spectroscopy, were carried out on screen-printed gold electrodes with three types of bacteria, namely Escherichia coli, Shewanella oneidensis, and Methylococcus capsulatus, immobilized via poly l-lysine. The results obtained showed a possibility of pattern recognition of the above pollutants by their inhibition effect on the three bacteria used. The analysis of a large amount of experimental data was carried out using an artificial neural network (ANN) programme for more accurate identification of pollutants as well as the estimation of their concentration. The results are encouraging for the development of a simple and cost-effective biosensing technology for preliminary in-field analysis (screening) of water samples for the presence of environmental pollutants.

Sensitive and Specific Whole-Cell Biosensor for Arsenic Detection

Whole-cell biosensors (WCBs) have been designed to detect As(III), but most suffer from poor sensitivity and specificity. In this paper, we developed an arsenic WCB with a positive feedback amplifier in Escherichia coli DH5α. The output signal from the reporter mCherry was significantly enhanced by the positive feedback amplifier. The sensitivity of the WCB with positive feedback is about 1 order of magnitude higher than that without positive feedback when evaluated using a half-saturation As(III) concentration. The minimum detection limit for As(III) was reduced by 1 order of magnitude to 0.1 µM, lower than the World Health Organization standard for the arsenic level in drinking water, 0.01 mg/liter or 0.13 µM. Due to the amplification of the output signal, the WCB was able to give detectable signals within a shorter period, and a fast response is essential for in situ operations. Moreover, the WCB with the positive feedback amplifier showed exceptionally high specificity toward As(III) when compared with other metal ions. Collectively, the designed positive feedback amplifier WCB meets the requirements for As(III) detection with high sensitivity and specificity. This work also demonstrates the importance of genetic circuit engineering in designing WCBs, and the use of genetic positive feedback amplifiers is a good strategy to improve the performance of WCBs.IMPORTANCE Arsenic poisoning is a severe public health issue. Rapid and simple methods for the sensitive and specific monitoring of arsenic concentration in drinking water are needed. In this study, we designed an arsenic WCB with a positive feedback amplifier. It is highly sensitive and able to detect arsenic below the WHO limit level. In addition, it also significantly improves the specificity of the biosensor toward arsenic, giving a signal that is about 10 to 20 times stronger in response to As(III) than to other metals. This work not only provides simple but effective arsenic biosensors but also demonstrates the importance of genetic engineering, particularly the use of positive feedback amplifiers, in designing WCBs.

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.

Electrochemical assessment of the interaction of microbial living cells and carbon nanomaterials

This work considers the effects of various carbon nanomaterials and fibres on bioelectrocatalytic and respiratory activity of bacterial cells during the oxidation of ethanol in the presence of an electron transport mediator. Gluconobacter oxydans sbsp. industrius VKM B-1280 cells were immobilised on the surfaces of graphite electrodes and had an adsorption contact with a nanomaterial (multi-walled carbon nanotubes, thermally expanded graphite, highly oriented pyrolytic graphite, graphene oxide, reduced graphene oxide). The electrochemical parameters of the electrodes (the polarisation curves, the value of generated current at the introduction of substrate, the impedance characteristics) were measured in two-electrode configuration. Modification by multi-walled carbon nanotubes led to the increase of microbial fuel cell (MFC) electric power by 26%. The charge transfer resistance of modified electrodes was 47% lower than unmodified ones. Thermally expanded and pyrolytic graphites had a slight negative effect on the electrochemical properties of modified electrodes. The respiratory activity of bacterial cells did not change in the presence of nanomaterials. The data can be used in the development of microbial biosensors and MFC electrodes based on Gluconobacter cells.

Towards the optimisation of ceramic-based microbial fuel cells: A three-factor three-level response surface analysis design

Microbial fuel cells (MFCs) are an environment-friendly technology, which addresses two of the most important environmental issues worldwide: fossil fuel depletion and water scarcity. Modelling is a useful tool that allows us to understand the behaviour of MFCs and predict their performance, yet the number of MFC models that could accurately inform a scale-up process, is low. In this work, a three-factor three-level Box-Behnken design is used to evaluate the influence of different operating parameters on the performance of air-breathing ceramic-based MFCs fed with human urine. The statistical analysis of the 45 tests run shows that both anode area and external resistance have more influence on the power output than membrane thickness, in the range studied. The theoretical optimal conditions were found at a membrane thickness of 1.55 mm, an external resistance of 895.59 Ω and an anode area of 165.72 cm2, corresponding to a maximum absolute power generation of 467.63 μW. The accuracy of the second order model obtained is 88.6%. Thus, the three-factor three-level Box-Behnken-based model designed is an effective tool which provides key information for the optimisation of the energy harvesting from MFC technology and saves time in terms of experimental work.

Dynamic Culture and Selective Extraction of Target Microbial Cells in Self-Assembled Particle Membrane-Integrated Microfluidic Bioreactor Array

Various microfluidic devices have overcome many disadvantages common to conventional bioreactor systems by enabling active manipulation of cell-culture conditions, monitoring of cellular responses in high-throughput mode, and extraction of target cells in a relatively rapid and low-cost manner. However, existing microfluidic devices still have limitations, including the complexity of their operation and a lack of availability of dynamic control of the chemical environment. Here, we present a novel microfluidic bioreactor array device capable of not only the stable and dynamic programing of cell-culture environments but also the selective extraction of target cells. This device comprises 64 microchambers in a 16 × 4 array format, and each microchamber is integrated with a robust and nanoporous membrane on one side and an H-shaped entrance on the other. The membrane made of self-assembled particles allowed continuous and sequential delivery of various nutrients containing gene inducers to compartmentalized microbial cells, thereby enabling dynamic cell culturing. Additionally, the H-shaped entrance was used for local and selective blocking of the microchamber by employing UV-curable material, thereby enabling the retrieval of target cells from the device while sequestering nontarget cells in the microchambers. Our results demonstrated that the targeted rare cells could be isolated and separated from a mixture of cells by repeating the extraction procedure. Therefore, we anticipate that this microfluidic bioreactor array device will be widely used for not only screening/extraction but also off-chip postanalyses of various microorganisms.

Enhancement of gasworks groundwater remediation by coupling a bio-electrochemical and activated carbon system

Here, we show the electrical response, bacterial community, and remediation of hydrocarbon-contaminated groundwater from a gasworks site using a graphite-chambered bio-electrochemical system (BES) that utilizes granular activated carbon (GAC) as both sorption agent and high surface area anode. Our innovative concept is the design of a graphite electrode chamber system rather than a classic non-conductive BES chamber coupled with GAC as part of the BES. The GAC BES is a good candidate as a sustainable remediation technology that provides improved degradation over GAC, and near real-time observation of associated electrical output. The BES chambers were effectively colonized by the bacterial communities from the contaminated groundwater. Principal coordinate analysis (PCoA) of UniFrac Observed Taxonomic Units shows distinct grouping of microbial types that are associated with the presence of GAC, and grouping of microbial types associated with electroactivity. Bacterial community analysis showed that β-proteobacteria (particularly the PAH-degrading Pseudomonadaceae) dominate all the samples. Rhodocyclaceae- and Comamonadaceae-related OTU were observed to increase in BES cells. The GAC BES (99% removal) outperformed the control graphite GAC chamber, as well as a graphite BES and a control chamber both filled with glass beads.

Rapid determination of 1,3-propanediol in fermentation process based on a novel surface-enhanced Raman scattering biosensor

The production of 1,3-propanediol (1,3-PDO) is an important fermentation process. However, 1,3-PDO could not be distinguished separately and efficiently in fermentations previously because it has a highly similar molecular structure to the feedstock glycerol (GLY) and by-product lactic acid (Lac), which leads to the difficulty of quantification. In this paper, a low-cost and environmentally friendly biosensor based on surface-enhanced Raman scattering (SERS) technique was developed. Using it, the concentration of 1,3-PDO and Lac in a fermentation solution can be determined directly from their respective characteristic peaks in Raman spectroscopy. Moreover, by analyzing the respective contributions of 1,3-PDO, Lac, and GLY to the integrated intensities of the 2920 cm^-1 Raman peak common to these three substances, the concentration of GLY could also be quantified. SERS study on various 1,3-PDO:GLY and Lac:GLY molar ratios were conducted to establish the proportional relationships of these compounds by analyzing the relationship between the concentration and the Raman peak intensities. The 1,3-PDO:Lac:GLY with serial concentration gradient was carried out to verify the relationship between the concentration and the Raman peak intensities by the high-performance liquid chromatography (HPLC) with relative deviations <25%. Concentrations of 1,3-PDO and Lac as low as 1 g/L and concentration of GLY as low as 4 g/L were analyzed to determine the limit of detection. Therefore, this new method allows the rapid quantification of 1,3-PDO, Lac and GLY concentrations on a SERS-based biosensor.

Fully Differential Piezoelectric Button-Like Mode Disk Resonator for Liquid Phase Sensing

We present a unique lateral shear resonance mode excited in a microelectromechanical (MEM) disk resonator. We refer to this proposed mode as the button-like (BL) mode. The BL mode has a characteristic lateral strain profile (based on the sum of orthogonal strain components in the plane of fabrication) which resembles a shirt button, hence our choice of name for this mode. The strain profile of the BL mode is highly suited for piezoelectric transduction. Like the more widely reported wine-glass (WG) or elliptical mode, the BL mode offers feedthrough cancellation through fully differential transduction. However, compared to the WG mode, the BL mode possesses a higher coupling coefficient ( [Formula: see text]) and a higher quality ( Q ) factor for the same disk radius. These advantages make the BL mode highly attractive for realizing electrically addressed MEM resonators for liquid-phase sensing. This paper examines various design aspects pertaining to the BL mode: tether geometry, characterization setup, size of disk, and even the effect of the gap around the disk on the Q factor. The highest Q factor measured in water is 410 based on a disk with a radius of [Formula: see text]. The lowest motional resistance in water is 1.36 [Formula: see text] based on a disk with a radius of [Formula: see text].

Methods to Assess the Antioxidative Properties of Probiotics

Probiotics prove useful in correcting and preventing numerous health conditions, including those having severe impact on society, e.g., obesity and cancer. Notably, these capabilities of probiotics appear to be associated with their antioxidant properties. The mechanisms of antioxidant action of probiotics range from immediate biochemical scavenging of reactive substances to induction of signaling events leading to increased capacity of the host's cytoprotective systems. Since the antioxidant effects of probiotics significantly vary in types and details, a broad selection of methods of assessment of these properties is required in order to identify, characterize, and develop novel probiotics for medical purposes, as well as to explain the mechanisms of action of probiotics already in use in healthcare. This review revises the versatile toolbox, which can be used to assess the antioxidant properties of probiotics.

Aptasensors as a new sensing technology developed for the detection of MUC1 mucin: A review

Mucin 1 protein (MUC1) is a membrane-associated glycoprotein overexpressed in the majority of human malignancies and considered as a predominant protein biomarker in cancers. Owing to the crucial role of MUC1 in cancer dissemination and metastasis, detection and quantification of this biomarker is of great importance in clinical diagnostics. Today, there exist a wide variety of strategies for the determination of various types of disease biomarkers, especially MUC1. In this regard, aptamers, as artificial single-stranded DNA or RNA oligonucleotides with catalytic and receptor properties, have drawn lots of attention for the development of biosensing platforms. So far, various sensitivity-enhancement techniques in combination with a broad range of smart nanomaterials have integrated into the design of novel aptamer-based biosensors (aptasensors) to improve detection limit and sensitivity of analyte determination. This review article provides a brief classification and description of the research progresses of aptamer-based biosensors and nanobiosensors for the detection and quantitative determination of MUC1 based on optical and electrochemical platforms.

Actuation of chitosan-aptamer nanobrush borders for pathogen sensing

We demonstrate a sensing mechanism for rapid detection of Listeria monocytogenes in food samples using the actuation of chitosan-aptamer nanobrush borders. The bio-inspired soft material and sensing strategy mimic natural symbiotic systems, where low levels of bacteria are selectively captured from complex matrices. To engineer this biomimetic system, we first develop reduced graphene oxide/nanoplatinum (rGO-nPt) electrodes, and characterize the fundamental electrochemical behavior in the presence and absence of chitosan nanobrushes during actuation (pH-stimulated osmotic swelling). We then characterize the electrochemical behavior of the nanobrush when receptors (antibodies or DNA aptamers) are conjugated to the surface. Finally, we test various techniques to determine the most efficient capture strategy based on nanobrush actuation, and then apply the biosensors in a food product. Maximum cell capture occurs when aptamers conjugated to the nanobrush bind cells in the extended conformation (pH < 6), followed by impedance measurement in the collapsed nanobrush conformation (pH > 6). The aptamer-nanobrush hybrid material was more efficient than the antibody-nanobrush material, which was likely due to the relatively high adsorption capacity for aptamers. The biomimetic material was used to develop a rapid test (17 min) for selectively detecting L. monocytogenes at concentrations ranging from 9 to 107 CFU mL-1 with no pre-concentration, and in the presence of other Gram-positive cells (Listeria innocua and Staphylococcus aureus). Use of this bio-inspired material is among the most efficient for L. monocytogenes sensing to date, and does not require sample pretreatment, making nanobrush borders a promising new material for rapid pathogen detection in food.

Optimal set of electrode potential enhances the toxicity response of biocathode to formaldehyde

The autotrophic biocathode was promising as a broad spectrum, rapid-responding and sensitive sensing element for the early warning of toxicants in water. However, we found that the baseline current and the responsivity strongly relied on the cathode potential. Here we poised cathode potentials at 0, -0.2 and -0.4 V to investigate the effect of electrode potential on the sensor responsivity. With formaldehyde as the tested toxicant, the biocathode poised at -0.2 V had the highest baseline current (118.2 ± 10.7 A m^-2) and the lowest toxicity response concentration (0.00148%), which exhibited a 6-64 times higher response ratio (1.4 × 10⁴ A%^-1 m^-3) than those controlled at 0 V (2.3 × 10³ A%^-1 m^-3) and -0.4 V (2.2 × 10² A%^-1 m^-3). First derivative of cyclic voltammetries revealed that the biocathode acclimated at -0.2 V had a highest main peak centered at 0.301 ± 0.006 V and several minor peaks between -0.2 to 0.2 V. Bacterial community analysis showed that Proteobacteria and Bacteroidetes families closely related to the sensing performance. Interestingly, Nitrospirae was obviously acclimated at -0.2 V, indicating that bacteria belonging to this phylum possibly contributed to the highest responsivity as well. Our findings revealed that the optimal set of electrode potential was critical to promote the toxicity responses of biocathode to the formaldehyde, and the differences were mainly from the microbial communities selected by different cathode potentials.

Electrochemical aptamer-based sensors for food and water analysis: A review

Global food and water safety issues have prompted the development of highly sensitive, specific, and fast analytical techniques for food and water analysis. The electrochemical aptamer-based detection platform (E-aptasensor) is one of the more promising detection techniques because of its unique combination of advantages that renders these sensors ideal for detection of a wide range of target analytes. Recent research results have further demonstrated that this technique has potential for real world analysis of food and water contaminants. This review summaries the recently developed E-aptasensors for detection of analytes related to food and water safety, including bacteria, mycotoxins, algal toxins, viruses, drugs, pesticides, and metal ions. Ten different electroanalytical techniques and one opto-electroanalytical technique commonly employed with these sensors are also described. In addition to highlighting several novel sensor designs, this review also describes the strengths, limitations, and current challenges this technology faces, and future development trend.

Recent progress on cell-based biosensors for analysis of food safety and quality control

Food quality and safety has become a subject of major concern for authorities and professionals in the food supply chain. Rapid methods, particularly biosensors, have exceptional specificity and sensitivity, rapid response times, low cost, relatively compact size, and are user friendly to operate. Cell-based biosensors are portable, and provide the biological activity of the analyte suitable for an initial screening of food. In this overview, the utilization of cell-based biosensors for food safety and quality analyses, such as detecting toxins, foodborne pathogens, allergens, and evaluating toxicity and function are summarized. Our results will promote the future development of cell-based biosensors in the food field.

Application of eukaryotic and prokaryotic laccases in biosensor and biofuel cells: recent advances and electrochemical aspects

Laccases exhibit a wide range of applications, especially in the electrochemical field, where they are regarded as a potential biotic component. Laccase-based biosensors have immense practical applications in the food, environmental, and medical fields. The application of laccases as biocathodes in enzymatic biofuel cells has promising potential in the preparation of implantable equipment. Extensive studies have been directed towards the potential role of fungal laccases as biotic components of electrochemical equipment. In contrast, the potential of prokaryotic laccases in electrochemistry has been not fully understood. However, there has been recent and rapid progress in the discovery and characterization of new types of prokaryotic laccases. In this review, we have comprehensively discussed the application of different sources of laccases as a biocatalytic component in various fields of application. Further, we described the potential of different types of laccases in bioelectrochemical applications.

Bioelectrochemical Systems for Removal of Selected Metals and Perchlorate from Groundwater: A Review

Groundwater contamination is a major issue for human health, due to its largely diffused exploitation for water supply. Several pollutants have been detected in groundwater; amongst them arsenic, cadmium, chromium, vanadium, and perchlorate. Various technologies have been applied for groundwater remediation, involving physical, chemical, and biological processes. Bioelectrochemical systems (BES) have emerged over the last 15 years as an alternative to conventional treatments for a wide variety of wastewater, and have been proposed as a feasible option for groundwater remediation due to the nature of the technology: the presence of two different redox environments, the use of electrodes as virtually inexhaustible electron acceptor/donor (anode and cathode, respectively), and the possibility of microbial catalysis enhance their possibility to achieve complete remediation of contaminants, even in combination. Arsenic and organic matter can be oxidized at the bioanode, while vanadium, perchlorate, chromium, and cadmium can be reduced at the cathode, which can be biotic or abiotic. Additionally, BES has been shown to produce bioenergy while performing organic contaminants removal, lowering the overall energy balance. This review examines the application of BES for groundwater remediation of arsenic, cadmium, chromium, vanadium, and perchlorate, focusing also on the perspectives of the technology in the groundwater treatment field.