Swift Acid Rain Sensing by Synergistic Rhizospheric Bioelectrochemical Responses

Glucose-based biofuel cells and their applications in medical implants: A review

In glucose biofuel cells (G-BFCs), glucose oxidation at the anode and oxygen reduction at the cathode yield electrons, which generate electric energy that can power a wide range of electronic devices. Research associated with the development of G-BFCs has increased in popularity among researchers because of the eco-friendly nature of G-BFCs (as related to their construction) and their evolution from inexpensive bio-based materials. In addition, their excellent specificity towards glucose as an energy source, and other properties, such as small size and weight, make them attractive within various demanding applied environments. For example, G-BFCs have received much attention as implanted devices, especially for uses related to cardiac activities. Envisioned pacemakers and defibrillators powered by G-BFCs would not be required to have conventional lithium batteries exchanged every 5-10 years. However, future research is needed to develop G-BFCs demonstrating more stable power consistency and improved lifespan, as well as solving the challenges in converting laboratory-made implantable G-BFCs into implanted devices in the human body. The categorization of G-BFCs as a subcategory of different biofuel cells and their performance is reviewed in this article.

Living electrochemical biosensing: Engineered electroactive bacteria for biosensor development and the emerging trends

Bioelectrical interfaces made of living electroactive bacteria (EAB) provide a unique opportunity to bridge biotic and abiotic systems, enabling the reprogramming of electrochemical biosensing. To develop these biosensors, principles from synthetic biology and electrode materials are being combined to engineer EAB as dynamic and responsive transducers with emerging, programmable functionalities. This review discusses the bioengineering of EAB to design active sensing parts and electrically connective interfaces on electrodes, which can be applied to construct smart electrochemical biosensors. In detail, by revisiting the electron transfer mechanism of electroactive microorganisms, engineering strategies of EAB cells for biotargets recognition, sensing circuit construction, and electrical signal routing, engineered EAB have demonstrated impressive capabilities in designing active sensing elements and developing electrically conductive interfaces on electrodes. Thus, integration of engineered EAB into electrochemical biosensors presents a promising avenue for advancing bioelectronics research. These hybridized systems equipped with engineered EAB can promote the field of electrochemical biosensing, with applications in environmental monitoring, health monitoring, green manufacturing, and other analytical fields. Finally, this review considers the prospects and challenges of the development of EAB-based electrochemical biosensors, identifying potential future applications.

Exogenous Electroactive Microbes Regulate Soil Geochemical Properties and Microbial Communities by Enhancing the Reduction and Transformation of Fe(III) Minerals

Electroactive microbes can conduct extracellular electron transfer and have the potential to be applied as a bioresource to regulate soil geochemical properties and microbial communities. In this study, we incubated Fe-limited and Fe-enriched farmland soil together with electroactive microbes for 30 days; both soils were incubated with electroactive microbes and a common iron mineral, ferrihydrite. Our results indicated that the exogenous electroactive microbes decreased soil pH, total organic carbon (TOC), and total nitrogen (TN) but increased soil conductivity and promoted Fe(III) reduction. The addition of electroactive microbes also changed the soil microbial community from Firmicutes-dominated to Proteobacteria-dominated. Moreover, the total number of detected microbial species in the soil decreased from over 700 to less than 500. Importantly, the coexistence of N-transforming bacteria, Fe(III)-reducing bacteria and methanogens was also observed with the addition of electroactive microbes in Fe-rich soil, indicating the accelerated interspecies electron transfer of functional microflora.

Structure evolution of air cathodes and their application in electrochemical sensor development and wastewater treatment

Cathode structure and material are the most important factors to determine the performance and cost of single chamber air-cathode microbial fuel cell (MFC), which is the most promising type of MFC technology. Since the first air cathode was invented in 2004, five major structures (1-layer, 2-layer, 3-layer, 4-layer and separator-support) have been invented and modified to fit new material, improve power performance and lower MFC cost. This paper reviewed the structure evolution of air cathodes in past 18 years. The benefits and drawbacks of these structures, in terms of power generation, material cost, fabrication procedure and modification process are analyzed. The practical application cases (e.g., sensor development and wastewater treatment) employed with different cathode structures were also summarized and analyzed. Based on practical performance and long-term cost analysis, the 2-layer cathode demonstrated much greater potential over other structures. Compared with traditional activated-sludge technology, the cost of an MFC-based system is becoming competitive when employing with 2-layer structure. This review not only provides a detailed development history of air cathode but also reveals the advantages/disadvantages of air cathode with different structures, which will promote the research and application of air-cathode MFC technology.

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.

Regulation of rhizospheric microbial network to enhance plant growth and resist pollutants: Unignorable weak electric field

The union of Plant Growth-Promoting Bacteria (PGPB) and rhizosphere confers a series of functions beneficial to plant. However, the lack of an opearable in situ method limits the further understanding on the mechanism. In this study, a weak electric field was designed to regulate rhizospheric microflora in a constructed root-splitting reactor. Compared with the control, the aboveground and underground biomass of rice seedling increased by 17 % and 18 % (p < 0.05) respectively under the exist of weak electric field of 0.14 V/cm. The joint action of rhizosphere and PGPB displayed the detoxification ability in the condition of soluble petroleum hydrocarbons, where the height, stem diameter, biomass and root vigor of the plant was increased by 58 %, 32 %, 43 % and 48 % respectively than the control. The selective reproduction of endophytes and ectophytes (denitrifying, auxin-producing, hydrocarbon-degrading and electroactive bacteria) was observed under applied weak electric field, which enhanced the nitrogen utilization, cellular metabolic activity and resistance to toxic organics of plant. This was further confirmed by the up-regulated OTUs related to the hydrocarbon degradation function, tryptophan metabolism and metabolism of nicotinate and nicotinamide. Moreover, the weak electric field also enhanced the transfer ability of partial endophytes grown in the root to improve plant stress resistance. The results in this work inspired an exercisable method for in situ enrichment of PGPB in the rhizosphere to cope with food crisis and provided a new way to deal with sudden environmental events.

Innovative Cost-Effective Nano-NiCo2O4 Cathode Catalysts for Oxygen Reduction in Air-Cathode Microbial Electrochemical Systems

Microbial electrochemical systems (MESs) can harvest bioelectricity from varieties of organic matter in wastewater through electroactive microorganisms. Oxygen reduction reaction (ORR) in a cathode plays an important role in guaranteeing high power generation, which can be enhanced by cathode catalysts. Herein, the tiny crystalline grain nanocrystal NiCo₂O₄ is prepared via the economic method and utilized as an effective catalyst in air-cathode MESs. The linear sweep voltammetry results indicate that the current density of 2% nano-NiCo₂O₄/AC cathode (5.05 A/m²) at 0 V increases by 20% compared to the control (4.21 A/m²). The cyclic voltammetries (CVs) and the electrochemical impedance spectroscopy (EIS) showed that the addition of nano-NiCo₂O₄ (2%) is efficient in boosting the redox activity. The polarization curves showed that the MESs with 2% nano-NiCo₂O₄/AC achieved the highest maximum power density (1661 ± 28 mW/m²), which was 1.11 and 1.22 times as much as that of AC and 5% nano-NiCo₂O₄. Moreover, the adulteration of nano-NiCo₂O₄ with a content of 2% can not only enable the electrical activity of the electrode to be more stable, but also reduce the cost for the same power generation in MESs. The synthetic nano-NiCo₂O₄ undoubtedly has great benefits for large-scale MESs in wastewater treatment.

Detection and Characterization of Bacterial Biofilms and Biofilm-Based Sensors

Microbial biofilms have caused serious concerns in healthcare, medical, and food industries because of their intrinsic resistance against conventional antibiotics and cleaning procedures and their capability to firmly adhere on surfaces for persistent contamination. These global issues strongly motivate researchers to develop novel methodologies to investigate the kinetics underlying biofilm formation, to understand the response of the biofilm with different chemical and physical treatments, and to identify biofilm-specific drugs with high-throughput screenings. Meanwhile microbial biofilms can also be utilized positively as sensing elements in cell-based sensors due to their strong adhesion on surfaces. In this perspective, we provide an overview on the connections between sensing and microbial biofilms, focusing on tools used to investigate biofilm properties, kinetics, and their response to chemicals or physical agents, and biofilm-based sensors, a type of biosensor using the bacterial biofilm as a biorecognition element to capture the presence of the target of interest by measuring the metabolic activity of the immobilized microbial cells. Finally we discuss possible new research directions for the development of robust and rapid biofilm related sensors with high temporal and spatial resolutions, pertinent to a wide range of applications.

Arsenopyrite weathering in acid rain: Arsenic transfer and environmental implications

Arsenopyrite is widely distributed and weathers readily in the nature, releases As and pollutes the surrounding environment. Acid rain is acidic in nature as contains sulfur oxides (SO_x) and nitrogen oxides (NO_x), and is a typical hazardous material to human. When arsenopyrite encounters acid rain, their interaction effect may aggregate environmental degradation. In this work, the weathering behavior of arsenopyrite in simulated acid rain was studied using the electrochemical techniques and surface analysis. Cyclic voltammetry and Raman and XPS confirmed that FeAsS was oxidized to Fe²⁺, AsO₃^3- and S⁰ at the initial phase, then, Fe²⁺ was converted to Fe³⁺, S⁰ transformed to SO₃^2- and ultimately to SO₄^2-, and AsO₃^3- to AsO₄^3- with the accumulation of H⁺. Polarization curve revealed higher temperature or higher acidity of acid rain increased the weathering trend and rate of arsenopyrite, and electrochemical impedance spectroscopic measurements showed the causes behind this to be smaller resistance and greater capacitance at the double layer and passivation film. Arsenopyrite weathering rate and temperature has a relationship: lnk = -3824.8/T + 10.305, via a transition state with activation enthalpy 29.37 kJ mol^-1 and activation entropy - 167.40 J mol^-1 K^-1. This study provides a rapid and quantitative in-situ electrochemical method for arsenopyrite weathering and an improved understanding of arsenopyrite weathering in acid rain condition. The results have powerful implications for the remediation and management of As-bearing sites affected by mining activities in acid rain area.

Microbial fuel cells for in-field water quality monitoring

The need for water security pushes for the development of sensing technologies that allow online and real-time assessments and are capable of autonomous and stable long-term operation in the field. In this context, Microbial Fuel Cell (MFC) based biosensors have shown great potential due to cost-effectiveness, simplicity of operation, robustness and the possibility of self-powered applications. This review focuses on the progress of the technology in real scenarios and in-field applications and discusses the technological bottlenecks that must be overcome for its success. An overview of the most relevant findings and challenges of MFC sensors for practical implementation is provided. First, performance indicators for in-field applications, which may diverge from lab-based only studies, are defined. Progress on MFC designs for off-grid monitoring of water quality is then presented with a focus on solutions that enhance robustness and long-term stability. Finally, calibration methods and detection algorithms for applications in real scenarios are discussed.

A highly sensitive bioelectrochemical toxicity sensor and its evaluation using immediate current attenuation

Electroactive biofilm (EAB) sensor had shown great potential in the field of early warning of toxicants in water because of the low-cost and broad-spectrum. However, the traditional calculation of sensitivity strongly relied on the time and concentration gradient which weakened time-efficiency of the sensor. Moreover, the sensitivity could be further improved to respond trace concentrations. Here EAB sensors with different substrate concentrations were formed to respond different concentrations formaldehyde ranging from 1 ppm to 50 ppm and immediate current attenuation (ICA) was induced to evaluate the sensitivity. The ICA (~70 s) exhibited a shorter time than that calculated by calculable sensitivity (CS) and current attenuation (CA), which not only achieved the response of trace concentration but also improved the time-efficiency of the sensor. The EAB formed with 0.1 g/L acetate (EAB-0.1) had a 380% higher sensitivity than that formed with 1.0 g/L acetate (EAB-1.0), leading to a significant electrochemical toxicity response to 1 ppm of formaldehyde. The results of electrochemical response coefficient confirmed that EAB-0.1 was 1.5-6.3 times of that formed with acetate from 0.2 to 1.0 g/L, which was related with microbial community and component of EAB as described in our previous study. Our findings demonstrated that calculation of sensitivity could be optimized to reflect time-efficiency and EAB with limit acetate could be applied in trace toxicant detection.

An electroactive biofilm-based biosensor for water safety: Pollutants detection and early-warning

Besides serving in wastewater treatment and energy generation fields, electroactive biofilm (EAB) has been employed as a sensitive bio-elements in a biosensor to monitor water quality by delivering electrical signals without additional mediators. Increasing studies have applied EAB-based biosensor in specific pollutant detection, typically biochemical oxygen demand (BOD) detection, as well as in early-warning of composite pollutants. Based on a comprehensive review of literatures, this study reveals how EAB outputs electrical signal, how we can evaluate and improve this performance, and what information we can expect from EAB-based biosensor. Since BOD detection and early-warning are normally confusing, this study manages to differentiate these two applications through distinguished purposes and metrics. Based on the introductions of progresses and applications of EAB-based biosensors so far, several novel strategies toward the future development of EAB-based biosensors are proposed.

Acetate limitation selects Geobacter from mixed inoculum and reduces polysaccharide in electroactive biofilm

Bioelectrochemical systems (BESs) are widely investigated as a promising technology to recover bioenergy or synthesize value-added products from wastewaters. The performance of BES depends on the activity of electroactive biofilm (EAB). As the core of BES, it is still unclear how the EAB is formed from mixed inoculum, and how exoelectrogens compete with non-exoelectrogens. Here we confirmed that microbial community composition and the morphology of EAB on the electrode including the thickness and porosity of the biofilm are critical for the performance of BES, and these properties can be simply controlled by the substrate concentration during EAB formation. The EAB formed with 0.1 g/L of acetate (EAB-0.1) exhibited a 90% higher current density than that formed with 1.0 g/L acetate (EAB-1.0). EAB-0.1 had a 50% higher electroactivity per biomass and a 20% thinner thickness than EAB-1.0, which was partly due to the 54% decrease of insulative polysaccharide in biofilm. Limited acetate also imposed a selective pressure to enrich Geobacter up to 88% compared to 72% when acetate was abundant. Our findings demonstrate that a highly active EAB can be formed by limiting substrate concentration, providing a broader understanding of the EAB formation process, the ecology of interspecies competitions and potential applications for bioenergy recovery and trace toxicant detection in the future.

Unignorable toxicity of formaldehyde on electroactive bacteria in bioelectrochemical systems

Formaldehyde poses significant threats to the ecosystem and is widely used as a toxicity indicator to obtain electrical signal feedback in electroactive biofilm (EAB)-based sensors. Although many optimizations have been adopted to improve the performance of EAB to formaldehyde, nearly no studies have discussed the toxicity of formaldehyde to EAB. Here, EABs were acclimated with a stable current density (8.9 ± 0.2 A/m²) and then injected with formaldehyde. The current density decreased by 27% and 98% after the injection of 1 and 10 ppm formaldehyde, respectively, compared with that in the control. The ecotoxicity of formaldehyde caused the irreversible loss of current with 3% (1 ppm) and 81% (10 ppm). Confocal laser scanning microscopy and scanning electron microscopy results showed that the redox activity was inhibited by formaldehyde, and the number of dead/broken cells increased from 2% to 40% (1 ppm) and 91% (10 ppm). The contents of the total protein and extracellular polymer substances decreased by more than 28% (1 ppm) and 75% (10 ppm) because of the cleavage reaction caused by formaldehyde. Bacterial community analysis showed that the proportion of Geobacter decreased from 81% to 53% (1 ppm) and 24% (10 ppm). As a result, the current production was significantly impaired, and the irreversible loss increased. Toxicological analysis demonstrated that formaldehyde disturbed the physiological indices of cells, thereby inducing apoptosis. These findings fill the gap of ecotoxicology of toxicants to EAB in a bioelectrochemical system.

Molecular Transformation of Crude Oil Contaminated Soil after Bioelectrochemical Degradation Revealed by FT-ICR Mass Spectrometry

Bioremediation is a low-cost approach for crude oil spill remediation, but it is often limited by electron acceptor availability. In addition, the biodegradation products of crude oil contaminants are complex, and transformation pathways are difficult to decipher. This study demonstrates that bioelectrochemical systems (BESs) can be effective in crude oil degradation by integrating biological and electrochemical pathways, and more importantly, it provides the first understanding on the daughter products of bioelectrochemical hydrocarbon degradation. Using electrospray ionization (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and two-dimensional gas chromatography (GC × GC), the results showed that the active BES reactor improved the total petroleum hydrocarbon (TPH) degradation by ∼70% than open circuit control reactors. After separating the daughter products into nine fractions (MA1-MA9) according to the molecular weight (m/z 200-1000) by modified aminopropyl silica (MAPS) fractionation, we found that active BES remediation resulted in 50% more polar, oxygen-containing naphthenic (NAP) acids. The MA4 fraction (centered at ∼550 Da) increased by 47%, and MA5 and MA7 fractions with higher molucular weight increased by a maximum of ∼7- and 9-fold, respectively. These results are in accordance with the variation of bulk elemental compositions in O₂ species, where daughter transformation products doubled relative to parent oil extract. The contribution of newly generated NAP acids was mainly from higher-order oxygen species (O₅-O₆) with increased hydrophobicity in conjunction with a decreased abundance in lower-order oxygen species (O₁). Overall, the study suggests that n-alkane degradation occurred via β-oxidation to oxygenated transformation products with lower molecular weight, such as n-alcohols in O₁ class and subsequently to n-fatty acids in O₂ class.

Foliar spray of TiO2 nanoparticles prevails over root application in reducing Cd accumulation and mitigating Cd-induced phytotoxicity in maize (Zea mays L.)

Cadmium (Cd) pollution is considered one of the global environmental issues due to its adverse effects on plant and human health. With the rapid development of nanotechnology and the practical application of engineered nanoparticles (ENPs) in agriculture, the mechanisms underlying the interactions between NPs and heavy metal on their uptake, accumulation, and phytotoxicity in crops are still not fully understood. Therefore, the impact of TiO₂ NPs (0, 100, 250 mg/L) and Cd (0, 50 μM) co-exposure on hydroponic maize (Zea mays L.) was determined under two exposure modes. Results showed that root co-exposure to TiO₂ NPs and 100 mg/L Cd significantly enhanced Cd uptake and produced greater phytotoxicity in maize than foliar exposure to TiO₂ NPs. Meanwhile, plant dry weight and chlorophyll content showed a reduction of 45.3% and 50.5%, respectively, when compared with single Cd treatment. In addition, the accumulation of Ti in shoots and roots increased by 1.61 and 4.29 times, respectively when root exposure to 250 mg/L TiO₂ NPs. By contrast, foliar exposure of TiO₂ NPs could markedly decrease shoot Cd contents from 15.2% to 17.8% and had a stronger influence on alleviating Cd-induced toxicity via increasing superoxide dismutase (SOD) and glutathione S-transferase (GST) activities and upregulating several metabolic pathways, including galactose metabolism and citrate cycle, alanine, aspartate and glutamate metabolism, as well as glycine, serine and threonine metabolism. This study provides a new strategy for the application of TiO₂ NPs in crop safety production in Cd contaminated soils.

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.

Innovative operation of microbial fuel cell-based biosensor for selective monitoring of acetate during anaerobic digestion

Volatile fatty acids (VFAs) especially acetate concentration have been proved to be a sensitive and reliable indicator for many anaerobic processes such as anaerobic digestion (AD). Microbial fuel cells (MFC) have been demonstrated as a promising VFAs sensor due to simple reactor design and operating conditions among microbial electrochemical biosensors. However, the conventional MFC biosensors may fail to distinguish between VFAs and other organics as real digestates containing complex organics and microbes are fed into anode directly. In the present study, an MFC based biosensor was developed and operated in a smart way for selective acetate detection. In the biosensor, acetate ions contained in the AD sample was first fed into the cathode, and then acetic ion transferred through the membrane from the cathode to anode chamber where it was further used as the sole substrate by pre-enriched electroactive biofilm for the current generation. A linear correlation between the current density and acetate concentrations (0.5-20 mM) at varied reaction time (1-5 h) was established. Then, the interference from propionate, butyrate, isobutyrate, and glucose on the performance of the biosensor was evaluated. Furthermore, the influence of sample temperatures (37 and 55 °C) was also studied. Finally, the VFAs content in real AD effluent with this biosensor was measured. The results corresponded well with gas chromatographic measurements. This simple, and reliable biosensor could serve as a promising alternative method for acetate detection in the AD process or any other acetate-rich fluids.