A Green Microbial Fuel Cell-Based Biosensor for In Situ Chromium (VI) Measurement in Electroplating Wastewater

Single-Chamber Microbial Fuel Cell with an Innovative Sensing Component for Real-Time Continual Monitoring of a Wide Range of Cr(VI) Concentrations in Wastewater

Hexavalent chromium (Cr(VI)) is toxic, carcinogenic, and harmful to biological systems. Common detection methods, such as colorimetry, atomic absorption spectrometry, ion chromatography, and biological systems, can only be used in the laboratory and do not provide real-time feedback. To address these limitations, the current study cloned the ChrB gene, which exhibits high specificity in detecting Cr(VI), and the ChrA gene, which exhibits high Cr(VI) tolerance, into Escherichia coli. This recombinant strain, ChrA-ChrB-E. coli, was integrated into a single-chamber microbial fuel cell for accurate continual monitoring over a wide range of Cr(VI) concentrations. ChrA-ChrB-E. coli thrived in temperatures from 25 °C to 45 °C and pH levels between 5 and 8. Its ability to reduce Cr(VI) remained consistent across Cr(VI) forms, carbon sources, and oxyanions. Cyclic voltammetry was employed to verify the electrical activity of the biosensor. The biosensor exhibited a detection limit of 0.0075 mg/L. Under conditions simulating the regulatory emission limit for Cr(VI) of 0.5 mg/L in industrial wastewater, the biosensor achieved a response time of 20 s during continual operation. When tested with synthetic wastewater containing Cr(VI) concentrations from 0.02 to 150 mg/L, the system exhibited high adaptability and facilitated stable monitoring (relative standard deviation ≤ 2.7%). Additionally, the biosensor's accuracy (-1.73% to 2.5%) matched that of traditional batch methods, highlighting its suitability for real-time Cr(VI) monitoring in aquatic environments.

Improving the practicality of recombinant Escherichia coli biosensor in detecting trace Cr(VI) by modifying the cryogenic storage conditions of biosensors and applying simple pretreatment

Hexavalent chromium (Cr(VI)) is a global environmental pollutant. To reduce the risk caused by Cr(VI), a simple, accurate, reproducible, and inexpensive method for quantifying Cr(VI) in water and soil should be developed. In this study, three types of recombinant Escherichia coli biosensors (namely T7-lux-E. coli, T3-lux-E. coli, and SP6-lux-E. coli biosensor) containing promoters (T7, T3, and SP6), chromate-sensing regulator chrB, and the reporter gene luxAB were constructed. This study investigated the effects of cryogenic freezing temperature and time on trace Cr(VI) measurement by using recombinant E. coli biosensors. The results indicated that the activity of thawed frozen SP6-lux-E. coli cells stored at -20 °C for 270 days did not differ from that of freshly prepared cells. Turbidity and conductivity in water samples and organic matter in soil interfered with Cr(VI) measurement using the biosensor. The SP6-lux-E. coli biosensor exhibited a wide measurement range and a low deviation of <5% for measuring Cr(VI) in various Cr(VI)-contaminated water and soil samples and required only a simple pretreatment or extraction process even after 270-day storage at -20 °C. To the best of our knowledge, this is the first study to report the use of recombinant biosensors for accurately measuring Cr(VI) in both water and soil.

A review of biosensor for environmental monitoring: principle, application, and corresponding achievement of sustainable development goals

Human health/socioeconomic development is closely correlated to environmental pollution, highlighting the need to monitor contaminants in the real environment with reliable devices such as biosensors. Recently, variety of biosensors gained high attention and employed as in-situ application, in real-time, and cost-effective analytical tools for healthy environment. For continuous environmental monitoring, it is necessary for portable, cost-effective, quick, and flexible biosensing devices. These benefits of the biosensor strategy are related to the Sustainable Development Goals (SDGs) established by the United Nations (UN), especially with reference to clean water and sources of energy. However, the relationship between SDGs and biosensor application for environmental monitoring is not well understood. In addition, some limitations and challenges might hinder the biosensor application on environmental monitoring. Herein, we reviewed the different types of biosensors, principle and applications, and their correlation with SDG 6, 12, 13, 14, and 15 as a reference for related authorities and administrators to consider. In this review, biosensors for different pollutants such as heavy metals and organics were documented. The present study highlights the application of biosensor for achieving SDGs. Current advantages and future research aspects are summarized in this paper.Abbreviations: ATP: Adenosine triphosphate; BOD: Biological oxygen demand; COD: Chemical oxygen demand; Cu-TCPP: Cu-porphyrin; DNA: Deoxyribonucleic acid; EDCs: Endocrine disrupting chemicals; EPA: U.S. Environmental Protection Agency; Fc-HPNs: Ferrocene (Fc)-based hollow polymeric nanospheres; Fe₃O₄@3D-GO: Fe₃O₄@three-dimensional graphene oxide; GC: Gas chromatography; GCE: Glassy carbon electrode; GFP: Green fluorescent protein; GHGs: Greenhouse gases; HPLC: High performance liquid chromatography; ICP-MS: Inductively coupled plasma mass spectrometry; ITO: Indium tin oxide; LAS: Linear alkylbenzene sulfonate; LIG: Laser-induced graphene; LOD: Limit of detection; ME: Magnetoelastic; MFC: Microbial fuel cell; MIP: Molecular imprinting polymers; MWCNT: Multi-walled carbon nanotube; MXC: Microbial electrochemical cell-based; NA: Nucleic acid; OBP: Odorant binding protein; OPs: Organophosphorus; PAHs: Polycyclic aromatic hydrocarbons; PBBs: Polybrominated biphenyls; PBDEs: Polybrominated diphenyl ethers; PCBs: Polychlorinated biphenyls; PGE: Polycrystalline gold electrode; photoMFC: photosynthetic MFC; POPs: Persistent organic pollutants; rGO: Reduced graphene oxide; RNA: Ribonucleic acid; SDGs: Sustainable Development Goals; SERS: Surface enhancement Raman spectrum; SPGE: Screen-printed gold electrode; SPR: Surface plasmon resonance; SWCNTs: single-walled carbon nanotubes; TCPP: Tetrakis (4-carboxyphenyl) porphyrin; TIRF: Total internal reflection fluorescence; TIRF: Total internal reflection fluorescence; TOL: Toluene-catabolic; TPHs: Total petroleum hydrocarbons; UN: United Nations; VOCs: Volatile organic compounds.

Removal mechanisms of aqueous Cr(VI) by anaerobic fermentation sludge

After fermentation, activated sludge contains many acid-producing bacteria and their metabolites, which have a good reducing effect. Various active groups (e.g., hydroxyl, amino, carboxyl, and phosphate) on microbial cell surfaces can adsorb heavy metals through complexation or chelation, forming heavy metal precipitates and thereby reducing the toxicity of heavy metals. However, the effects and mechanisms of using sludge after anaerobic fermentation to remove Cr(VI) are unclear, such as the dominance of direct versus indirect biological reduction, the contribution of abiotic effects, and the influence of fermentation conditions. This paper compares Cr(VI) removal in fermented and unfermented sludges. After fermentation for 24 h, 99.9% of the Cr(VI) (50 mg/L) in anaerobic sludge was removed within 7 h, which was twice the rate in unfermented activated sludge. A series of comparative experiments demonstrated that Cr(VI) removal primarily occurred through biological effects (about 92%), which included biological reduction and biosorption. 16SrRNA gene sequencing revealed that Cr(VI) transformation primarily occurred through direct biological reduction, with the related genera being Trichococcus, Acetobacter, Aeromonas, and Tolumonas. Fourier-transform infrared (FTIR) spectroscopy results showed that the C = O and C-O functionalities on sludge were likely involved in the Cr(VI) conversion. Majority of the Cr(VI) in the system was reduced to Cr(III) and existed in the suspension, with a small amount deposited on the sludge surface. The X-ray photoelectron spectroscopy (XPS) results indicated that the majority of Cr was present as reduced Cr(III) on the sludge. These results demonstrate that after fermentation in an aqueous environment, activated sludge is an effective medium for the remediation of Cr(VI). These results are useful for designing a green and sustainable bioreduction system for the remediation of Cr(VI)-polluted water.

Microbial Fuel Cell-Based Biosensors and Applications

The sustainable development of human society in today's high-tech world depends on some form of eco-friendly energy source because existing technologies cannot keep up with the rapid population expansion and the vast amounts of wastewater that result from human activity. A green technology called a microbial fuel cell (MFC) focuses on using biodegradable trash as a substrate to harness the power of bacteria to produce bioenergy. Production of bioenergy and wastewater treatment are the two main uses of MFC. MFCs have also been used in biosensors, water desalination, polluted soil remediation, and the manufacture of chemicals like methane and formate. MFC-based biosensors have gained a lot of attention in the last few decades due to their straightforward operating principle and long-term viability, with a wide range of applications including bioenergy production, treatment of industrial and domestic wastewater, biological oxygen demand, toxicity detection, microbial activity detection, and air quality monitoring, etc. This review focuses on several MFC types and their functions, including the detection of microbial activity.

A Review of Recent Advances in Microbial Fuel Cells: Preparation, Operation, and Application

The microbial fuel cell has been considered a promising alternative to traditional fossil energy. It has great potential in energy production, waste management, and biomass valorization. However, it has several technical issues, such as low power generation efficiency and operational stability. These issues limit the scale-up and commercialization of MFC systems. This review presents the latest progress in microbial community selection and genetic engineering techniques for enhancing microbial electricity production. The summary of substrate selection covers defined substrates and some inexpensive complex substrates, such as wastewater and lignocellulosic biomass materials. In addition, it also includes electrode modification, electron transfer mediator selection, and optimization of operating conditions. The applications of MFC systems introduced in this review involve wastewater treatment, production of value-added products, and biosensors. This review focuses on the crucial process of microbial fuel cells from preparation to application and provides an outlook for their future development.

Enhanced Pb(II) removal from water using conductive carbonaceous nanomaterials as bacterial scaffolds: An experimental and modelling approach

This study investigated the abilities of electrochemically active bacteria (EAB) as bio-catalysts for Pb(II) bioreduction in an attached growth reactor. Electrically conductive carbon nanofibers (CNF) were used as bacterial scaffolds to facilitate electron transfer between EAB and Pb(II). Our results demonstrated that the CNF attached growth reactor can remove > 80% and > 90% of Pb(II) from solution at 0.5 mg/L and 5.0 mg/L concentrations, respectively, and maintained Pb(II) levels < 0.10 mg/L over repeated Pb(II) dosages dosed at all concentrations. Experimental measurements by SEM-EDX and ICP-OES provided evidence that Pb(II) was reduced to Pb⁰, suggesting that Pb(II) was adsorbed onto CNFs, and subsequently bioreduced. Numerical modelling was used to estimate Pb(II) concentrations along the CNFs via capturing Pb(II)'s diffusive transport, and the removal mechanisms governed by EAB and CNF adsorption. The model simulation results demonstrated that Pb(II) bioreduction was dominant in the reactor and the significance of Monod kinetics, biofilm thickness and Pb(II) dosage concentration on its removal. This study is the first to prove EAB's ability to treat Pb(II) from contaminated waters in an attached growth configuration. The findings here demonstrate that CNFs can boost biological treatment efficacies in wastewater and/or water treatment to meet stringent water guidelines.

A critical review on early-warning electrochemical system on microbial fuel cell-based biosensor for on-site water quality monitoring

The microbial fuel cell (MFC) sensor is a very promising self-powered self-sustainable system for early warning water quality detection. These sensors are cost-effective, biodegradable, compact in design, and portable in nature are favorable for real-time in situ water quality monitoring. This review represents the mechanism action behind the toxicity detection, optimization strategies, process parameters, role of biofilm, the role of external resistance, hydrodynamic study, and mathematical modeling for improving the performance of the sensor. Additionally, the techno-economic prospect of this MFC-based sensor and its challenges, limitations are addressed to make it economically more favorable for commercial use. The future direction is also explored based on the sensor's disadvantages and limitations. Comprehensively, this review covered all the possible directions of MFC sensor fabrication, their application, recent advancement, prospects challenges, and their possible solutions.

A critical review on microbe-electrode interactions towards heavy metal ion detection using microbial fuel cell technology

Implicit interaction of electroactive microbes with solid electrodes is an interesting phenomenon in nature, which supported development of bioelectrochemical systems (BESs), especially the microbial fuel cell (MFCs) for valorization of low-value waste streams into bioelectricity. Intriguingly, the metabolism of interacted microbes with electrode is affected by the microenvironment at electrodes, which influences the current response. For instance, when heavy metal ions (HMIs) are imposed in the medium, the current production decreases due to their intrinsic toxic effect. This event provides an immense opportunity to utilize MFC as a sensor to selectively detect HMIs in the environment, which has been explored vastly in recent decade. In this review, we have concisely discussed the microbial interaction with electrodes and mechanism of detection of HMIs using an MFC. Recent advancement in sensing elements and their application is elaborated with a future perspective section for follow-up research and development in this field.

Highly Sensitive Luminescent Bioassay Using Recombinant Escherichia coli Biosensor for Rapid Detection of Low Cr(VI) Concentration in Environmental Water

In this study, we constructed a recombinant Escherichia coli strain with different promoters inserted between the chromate-sensing regulator chrB and the reporter gene luxAB to sense low hexavalent chromium (Cr(VI)) concentrations (<0.05 mg/L); subsequently, its biosensor characteristics (sensitivity, selectivity, and specificity) for measuring Cr(VI) in various water bodies were evaluated. The luminescence intensity of each biosensor depended on pH, temperature, detection time, coexisting carbon source, coexisting ion, Cr(VI) oxyanion form, Cr(VI) concentration, cell type, and type of medium. Recombinant lux-expressing E. coli with the T7 promoter (T7-lux-E. coli, limit of detection (LOD) = 0.0005 mg/L) had the highest luminescence intensity or was the most sensitive for Cr(VI) detection, followed by E. coli with the T3 promoter (T3-lux-E. coli, LOD = 0.001 mg/L) and that with the SP6 promoter (SP6-lux-E. coli, LOD = 0.005 mg/L). All biosensors could be used to determine whether the Cr(VI) standard was met in terms of water quality, even when using thawing frozen cells as biosensors after 90-day cryogenic storage. The SP6-lux-E. coli biosensor had the shortest detection time (0.5 h) and the highest adaptability to environmental interference. The T7-lux-E. coli biosensor—with the optimal LOD, a wide measurement range (0.0005–0.5 mg/L), and low deviation (−5.0–7.9%) in detecting Cr(VI) from industrial effluents, domestic effluents, and surface water—is an efficient Cr(VI) biosensor. This unprecedented study is to evaluate recombinant lux E. coli with dissimilar promoters for their possible practice in Cr(VI) measurement in water bodies, and the biosensor performance is clearly superior to that of past systems in terms of detection time, LOD, and detection deviation for real water samples.

Microbial Fuel Cell Technology—A Critical Review on Scale-Up Issues

Microbial fuel cell (MFC) technology has attracted a great amount of attention due to its potential for organic and inorganic waste treatment concomitant with power generation. It is thus seen as a clean energy alternative. Modifications and innovations have been conducted on standalone and hybrid/coupled MFC systems to improve the power output to meet the end goal, namely, commercialization and implementation into existing wastewater treatment plants. As the energy generated is inversely proportional to the size of the reactor, the stacking method has been proven to boost the power output from MFC. In recent years, stacked or scale-up MFCs have also been used as a power source to provide off-grid energy, as well as for in situ assessments. These scale-up studies, however, encountered various challenges, such as cell voltage reversal. This review paper explores recent scale-up studies, identifies trends and challenges, and provides a framework for current and future research.

Biosensors for wastewater-based epidemiology for monitoring public health

Public health is attracting increasing attention due to the current global pandemic, and wastewater-based epidemiology (WBE) has emerged as a powerful tool for monitoring of public health by analysis of a variety of biomarkers (e.g., chemicals and pathogens) in wastewater. Rapid development of WBE requires rapid and on-site analytical tools for monitoring of sewage biomarkers to provide immediate decision and intervention. Biosensors have been demonstrated to be highly sensitive and selective tools for the analysis of sewage biomarkers due to their fast response, ease-to-use, low cost and the potential for field-testing. This paper presents biosensors as effective tools for wastewater analysis of potential biomarkers and monitoring of public health via WBE. In particular, we discuss the use of sewage sensors for rapid detection of a range of targets, including rapid monitoring of community-wide illicit drug consumption and pathogens for early warning of infectious diseases outbreaks. Finally, we provide a perspective on the future use of the biosensor technology for WBE to enable rapid on-site monitoring of sewage, which will provide nearly real-time data for public health assessment and effective intervention.

Bio- and Biomimetic Receptors for Electrochemical Sensing of Heavy Metal Ions

Heavy metals ions (HMI), if not properly handled, used and disposed, are a hazard for the ecosystem and pose serious risks for human health. They are counted among the most common environmental pollutants, mainly originating from anthropogenic sources, such as agricultural, industrial and/or domestic effluents, atmospheric emissions, etc. To face this issue, it is necessary not only to determine the origin, distribution and the concentration of HMI but also to rapidly (possibly in real-time) monitor their concentration levels in situ. Therefore, portable, low-cost and high performing analytical tools are urgently needed. Even though in the last decades many analytical tools and methodologies have been designed to this aim, there are still several open challenges. Compared with the traditional analytical techniques, such as atomic absorption/emission spectroscopy, inductively coupled plasma mass spectrometry and/or high-performance liquid chromatography coupled with electrochemical or UV-VIS detectors, bio- and biomimetic electrochemical sensors provide high sensitivity, selectivity and rapid responses within portable and user-friendly devices. In this review, the advances in HMI sensing in the last five years (2016-2020) are addressed. Key examples of bio and biomimetic electrochemical, impedimetric and electrochemiluminescence-based sensors for Hg2+, Cu2+, Pb2+, Cd2+, Cr6+, Zn2+ and Tl+ are described and discussed.

Highly efficient removal of victoria blue R and bioelectricity generation from textile wastewater using a novel combined dual microbial fuel cell system

A novel combined dual microbial fuel cell (MFC) system was developed for the continuous removal of Victoria Blue R (VBR) and electricity generation. Anaerobic and aerobic VBR-degrading bacteria, Shewanella putrefaciens and Acinetobacter calcoaceticus, respectively, were applied simultaneously. The effects of various factors on the performance of the novel system in the continuous mode were investigated, and optimal operating parameters for the system were determined. The optimal liquid retention time for continuous treatment was 36 h. The optimal external resistances of connected MFCs were 390 Ω and 1300 Ω. When artificial wastewater containing 1000 mg/l of VBR was fed continuously into the system, the VBR removal efficiency achieved was 98.7%. In addition, the acute toxicity of the effluent was decreased by a factor of 21.1-22.3, indicating that the system could detoxify VBR intermediates. VBR degradation involved a stepwise demethylation process, which occurred mainly in the first MFC, whereas aromatic ring opening, sequential deamination reaction, and carbon oxidation occurred mainly in the second MFC. When actual VBR-containing wastewater (75-262 mg/l) was introduced, the removal efficiencies of VBR, chemical oxygen demand, colority, NH₃, and bioelectricity generation were >99.8%, >96.6%, >88.0%, 100%, and >194.8 mW/m², respectively and the original inoculated strains remained dominant. Therefore, the combined dual MFC system could be applied to the treatment of actual VBR-containing wastewater.

Recent Advances in Electrochemical Monitoring of Chromium

The extensive use of chromium by several industries conducts to the discharge of an immense quantity of its various forms in the environment which affects drastically the ecological and biological lives especially in the case of hexavalent chromium. Electrochemical sensors and biosensors are useful devices for chromium determination. In the last five years, several sensors based on the modification of electrode surface by different nanomaterials (fluorine tin oxide, titanium dioxide, carbon nanomaterials, metallic nanoparticles and nanocomposite) and biosensors with different biorecognition elements (microbial fuel cell, bacteria, enzyme, DNA) were employed for chromium monitoring. Herein, recent advances related to the use of electrochemical approaches for measurement of trivalent and hexavalent chromium from 2015 to 2020 are reported. A discussion of both chromium species detections and speciation studies is provided.

Application of enhanced bioreduction for hexavalent chromium-polluted groundwater cleanup: Microcosm and microbial diversity studies

Hexavalent chromium (Cr⁶⁺) is a commonly found heavy metal at polluted groundwater sites. In this study, the effectiveness of Cr⁶⁺ bioreduction by the chromium-reducing bacteria was evaluated to remediate Cr⁶⁺-contaminated groundwater. Microcosms were constructed using indigenous microbial consortia from a Cr⁶⁺-contaminated aquifer as the inocula, and slow-releasing emulsified polycolloid-substrate (ES), cane molasses (CM), and nutrient broth (NB) as the primary substrates. The genes responsible for the bioreduction of Cr⁶⁺ and variations in bacterial diversity were evaluated using metagenomics assay. Complete Cr⁶⁺ reduction via the biological mechanism was observed within 80 days using CM as the carbon source under anaerobic processes with the increased trivalent chromium (Cr³⁺) concentrations. Cr⁶⁺ removal efficiencies were 83% and 59% in microcosms using ES and NB as the substrates, respectively. Increased bacterial communities associated with Cr⁶⁺ bioreduction was observed in microcosms treated with CM and ES. Decreased bacterial communities were observed in NB microcosms. Compared to ES, CM was more applicable by indigenous Cr⁶⁺ reduction bacteria and resulted in effective Cr⁶⁺ bioreduction, which was possibly due to the growth of Cr⁶⁺-reduction related bacteria including Sporolactobacillus, Clostridium, and Ensifer. While NB was applied for specific bacterial selection, it might not be appropriate for electron donor application. These results revealed that substrate addition had significant impact on microbial diversities, which affected Cr⁶⁺ bioreduction processes. Results are useful for designing a green and sustainable bioreduction system for Cr⁶⁺-polluted groundwater remediation.

Removal of toluene vapor in the absence and presence of a quorum-sensing molecule in a biotrickling filter and microbial composition shift

Toluene is highly toxic and mutagenic, and it is generally used as an industrial solvent. Thus, toluene removal from air is necessary. To solve the problem of reducing high toluene concentrations with a short gas retention time (GRT), a quorum-sensing molecule [N-(3-oxododecanoyl)-L-homoserine lactone] (OHL) was added to a biotrickling filter (BTF). In this study, a BTF was used to treat synthetic and natural waste gases containing toluene. An extensive analysis was performed to understand the removal efficiency, removal characteristics, and bacterial community of the BTF. The addition of 20 μM OHL to the BTF significantly improved toluene removal, and more than 99.2% toluene removal was achieved at a GRT of 0.5 min when natural waste gas containing toluene (590-1020 ppm or 2.21-3.83 g m^-3) was introduced. The maximum inlet load for toluene was 337.9 g m^-3 h^-1. Moreover, the BTF exhibited satisfactory adaptability to shock loading and shutdown operations. Pseudomonadaceae (33.0%) and Comamonadaceae (26.3%) were predominant bacteria in the system after a 98-day operation. These bacteria were responsible for toluene degradation. The optimal moisture content and low pressure drop for system operations demonstrated that the BTF was energy and cost efficient. Therefore, processing through a BTF with OHL is a favorable technique for toluene treatment.

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.

Three-Stage Single-Chambered Microbial Fuel Cell Biosensor Inoculated with Exiguobacterium aestuarii YC211 for Continuous Chromium (VI) Measurement

Chromium (VI) [Cr(VI)] compounds display high toxic, mutagenic, and carcinogenic potential. Biological analysis techniques (e.g., such as enzyme-based or cell-based sensors) have been developed to measure Cr(VI); however, these biological elements are sensitive to the environment, limited to measuring trace Cr(VI), and require deployment offsite. In this study, a three-stage single-chambered microbial fuel cell (SCMFC) biosensor inoculated with Exiguobacterium aestuarii YC211 was developed for in situ, real-time, and continuous Cr(VI) measurement. A negative linear relationship was observed between the Cr(VI) concentration (5–30 mg/L) and the voltage output using an SCMFC at 2-min liquid retention time. The theoretical Cr(VI) measurement range of the system could be extended to 5–90 mg/L by connecting three separate SCMFCs in series. The three-stage SCMFC biosensor could accurately measure Cr(VI) concentrations in actual tannery wastewater with low deviations (<7%). After treating the wastewater with the SCMFC, the original inoculated E. aestuarii remained dominant (>92.5%), according to the next-generation sequencing analysis. The stable bacterial community present in the SCMFC favored the reliable performance of the SCMFC biosensor. Thus, the three-stage SCMFC biosensor has potential as an early warning device with wide dynamic range for in situ, real-time, and continuous Cr(VI) measurement of tannery wastewater.

Optimization of sensing performance in an integrated dual sensors system combining microbial fuel cells and upflow anaerobic sludge bed reactor

Two bio-cathode microbial fuel cells (MFCs) with Upflow anaerobic sludge blanket (UASB) are integrated to construct a UASB-MFC dual sensors system, developed to solve the problems of low accuracy and less information about the single sensor system. The bio-cathode MFC is developed as the biosensor for UASB operation performance and online monitoring. Owing to the biosorption of anaerobic microbes, the MFC in the suspended layer is more suitable for effluent chemical oxygen demand (COD) real-time monitoring, and the MFC in the sludge layer is more suitable for total volatile fatty acid (TVFA) real-time monitoring. Electrochemical analysis discovers that the lower electron transfer resistance determines the higher sensitivity of MFC in the suspended layer. The difference in species abundance indicates that TVFA has a stronger inhibitory effect on MFC in the sludge layer. The novel UASB-MFC dual sensors system shows promising potential for COD and TVFA simultaneous online monitoring, which can enhance the reliability of anaerobic reaction.

A Tube-Integrated Painted Biosensor for Glucose and Lactate

Developing a simple and direct approach for sensitive, specific, and rapid detection of metabolic compounds is of great importance for a variety of biological, medical, and food applications. Tubes are a highly portable and accessible container shape which are widely used for scientific research in cell biology and chemical synthesis, and which are also of great use in domestic health care applications. Here, we show for the first time the development of a tube-based painted amperometric biosensor for the detection of glucose and lactate. The sensor was prepared by printing carbon graphite and silver/silver chloride inks on the interior wall of the tube and then immobilizing glucose oxidase or lactate oxidase on the sensor. The sensor showed a sensitive, rapid, and reliable detection of glucose and lactate. We anticipate that these results could open new avenues for the development of painted biosensors, and toward advanced biosensor applications.

A Novel Early Warning System Based on a Sediment Microbial Fuel Cell for In Situ and Real Time Hexavalent Chromium Detection in Industrial Wastewater

Hexavalent chromium (Cr(VI)) is a well-known toxic heavy metal in industrial wastewater, but in situ and real time monitoring cannot be achieved by current methods used during industrial wastewater treatment processes. In this study, a Sediment Microbial Fuel Cell (SMFC) was used as a biosensor for in situ real-time monitoring of Cr(VI), which was the organic substrate is oxidized in the anode and Cr(VI) is reduced at the cathode simultaneously. The pH 6.4 and temperature 25 °C were optimal conditions for the operation. Under the optimal conditions, linearity (R² = 0.9935) of the generated voltage was observed in the Cr(VI) concentration range from 0.2 to 0.7 mg/L. The system showed high specificity for Cr(VI), as other co-existing ions such as Cu²⁺, Zn²⁺, and Pb²⁺ did not interfere with Cr(VI) detection. In addition, when the sediment MFC-based biosensor was applied for measuring Cr(VI) in actual wastewater samples, a low deviation (<8%) was obtained, which indicated its potential as a reliable biosensor device. MiSeq sequencing results showed that electrochemically active bacteria (Geobacter and Pseudomonas) were enriched at least two-fold on the biofilm of the anode in the biosensor as compared to the SMFC without Cr(VI). Cyclic voltammetry curves indicated that a pair of oxidation/reduction peaks appeared at −111 mV and 581 mV, respectively. These results demonstrated that the proposed sediment microbial fuel cell-based biosensor can be applied as an early warning device for real time in situ detection of Cr(VI) in industrial wastewaters.