A laminar-flow microfluidic device for quantitative analysis of microbial electrochemical activity

Microfluidic membraneless microbial fuel cells: new protocols for record power densities

The main hurdle in leveraging microfluidic advantages in membraneless MFCs is their low electrode area-normalized power. For nearly a decade, maximum power densities have remained stagnant, while at the same time macrosystems continue to gather pace. To bridge this growing gap, we showcase a strategy that focuses on (i) technology improvements, (ii) establishment of record areal power densities, and (iii) presentation of different normalization methods that complement areal power densities and enable direct comparisons across all MFC scales. Using a pure-culture Geobacter sulfurreducens electroactive biofilm (EAB) in a new membraneless MFC that adheres to the strategy above, we observed optimal anode colonization, resulting in the highest recorded electrode areal power density for a microfluidic MFC of 3.88 W m-2 (24.37 kW m-3). We also consider new power normalization methods that may be more appropriate for comparison to other works. Normalized by the wetted cross-section area between electrodes accounts for constraints in electrode/electrolyte contact, resulting in power densities as high as 8.08 W m-2. Alternatively, we present a method to normalize by the flow rate to account for acetate supply, obtaining normalized energy recovery values of 0.025 kW h m-3. With these results, the performance gap between micro- and macroscale MFCs is closed, and a road map to move forward is presented.

Employment of osmotic pump as a novel feeding system to operate the laminar-flow microfluidic microbial fuel cell

Laminar-flow microfluidic microbial fuel cell (LMMFC) has attracted attention due to the advantage of the liquid-liquid interface between anolyte and catholyte without the use of membrane as a separator resulting in less fabrication cost. Unlike previous studies of LMMFC using syringe pumps, this study proposes the use of osmotic pumps to feed anolyte and catholyte in the microchannel without any additional power supply. The osmotic pump was constructed with two cylindrical chambers separated by a forward osmosis membrane, with the initial draw solution concentration of 90 g l^-1 NaCl. We have, for the first time, demonstrated using the osmotic pumps to deliver both anolyte and catholyte and create co-laminar flow in LMMFC. Under the catholyte and anolyte flow rates of 18 ml/h and 40 ml/h respectively, LMMFC cultivated with Shewanella oneidensis produced the maximum power density of 87 mW m^-2 and current density of 747 mA m^-2 with the internal resistance of 1660 Ω. Further studies are warranted to develop osmotic pumps-fed LMMFC into a potential platform for portable biosensors.

Microfluidic Microbial Bioelectrochemical Systems: An Integrated Investigation Platform for a More Fundamental Understanding of Electroactive Bacterial Biofilms

It is the ambition of many researchers to finally be able to close in on the fundamental, coupled phenomena that occur during the formation and expression of electrocatalytic activity in electroactive biofilms. It is because of this desire to understand that bioelectrochemical systems (BESs) have been miniaturized into microBES by taking advantage of the worldwide development of microfluidics. Microfluidics tools applied to bioelectrochemistry permit even more fundamental studies of interactions and coupled phenomena occurring at the microscale, thanks, in particular, to the concomitant combination of electroanalysis, spectroscopic analytical techniques and real-time microscopy that is now possible. The analytical microsystem is therefore much better suited to the monitoring, not only of electroactive biofilm formation but also of the expression and disentangling of extracellular electron transfer (EET) catalytic mechanisms. This article reviews the details of the configurations of microfluidic BESs designed for selected objectives and their microfabrication techniques. Because the aim is to manipulate microvolumes and due to the high modularity of the experimental systems, the interfacial conditions between electrodes and electrolytes are perfectly controlled in terms of physicochemistry (pH, nutrients, chemical effectors, etc.) and hydrodynamics (shear, material transport, etc.). Most of the theoretical advances have been obtained thanks to work carried out using models of electroactive bacteria monocultures, mainly to simplify biological investigation systems. However, a huge virgin field of investigation still remains to be explored by taking advantage of the capacities of microfluidic BESs regarding the complexity and interactions of mixed electroactive biofilms.

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.

Microfluidic bioanalytical flow cells for biofilm studies: a review

Bacterial biofilms are among the oldest and most prevalent multicellular life forms on Earth and are increasingly relevant in research areas related to industrial fouling, medicine and biotechnology. The main hurdles to obtaining definitive experimental results include time-varying biofilm properties, structural and chemical heterogeneity, and especially their strong sensitivity to environmental cues. Therefore, in addition to judicious choice of measurement tools, a well-designed biofilm study requires strict control over experimental conditions, more so than most chemical studies. Due to excellent control over a host of physiochemical parameters, microfluidic flow cells have become indispensable in microbiological studies. Not surprisingly, the number of lab-on-chip studies focusing on biofilms and other microbiological systems with expanded analytical capabilities has expanded rapidly in the past decade. In this paper, we comprehensively review the current state of microfluidic bioanalytical research applied to bacterial biofilms and offer a perspective on new approaches that are expected to drive continued advances in this field.

Fast Start-Up Microfluidic Microbial Fuel Cells With Serpentine Microchannel

Microfluidic microbial fuel cells (MMFCs) are promising green power sources for future ultra-small electronic devices. The MMFCs with co-laminar microfluidic structure are superior to other MMFCs according to their low internal resistance and relative high power density. However, the area for interfacial electron transfer between the bacteria and the anode is quite limited in the typical Y-shaped device, which apparently restricts the current generation performance. In this study, we developed a membraneless MMFC with serpentine microchannel to enhance the interfacial electron transfer and promote the power generation of the device. Owing to the merit of laminar flow, the proposed MMFC was working well without any proton exchange membrane (PEM). At the same time, the serpentine microchannel greatly increased the power density. The S-MMFC catalyzed by Shewanella putrefaciens CN32 achieves a peak power density of 360 mW/m² with the optimal channel configuration and the flow rate of 5 ml/h. Meanwhile, this device possesses much shorter start-up time and much longer duration time at high current plateau than the previous reported MMFCs. The presented MMFC appears promising for biochip technology and extends the scope of microfluidic energy.

Electricity generation of a laminar-flow microbial fuel cell without any additional power supply

Laminar-flow microbial fuel cells (LFMFCs) utilize the co-laminar flow feature in the microchannel as a virtual barrier to separate the anolyte and catholyte. However, for LFMFCs reported before, syringe pumps were always used to drive the fluid and form the co-laminar flow of anolyte and catholyte in the microchannel, reducing the net power output and the efficiency of the whole system. In this study, a laminar-flow microbial fuel cell (LFMFC) without any additional power supply is proposed. The LFMFC is successfully started-up after inoculation for 90 h. The anode biofilm distribution becomes sparser along the flow direction due to the thicker boundary layer and unfavorable crossover from the catholyte downstream. Moreover, the LFMFC delivers a maximum volumetric power density of 3200 W m⁻³, which is higher than that of previous LFMFCs without membranes. Considering the practical application of LFMFC as a power source, the cell voltage responses to different conditions are further investigated. When the external resistance is switched between 1000 Ω and 4000 Ω, it takes the LFMFC 10 minutes to reach a stable voltage output. However, the voltage response to the intermittent supply takes 1 h to reach a stable value. Additionally, short-term cold storage has little effect on bacterial metabolic activity and cell voltage.

A novel laminar-flow microbial fuel cell without any additional power supply is proposed.

Dynamic Flow Characteristics and Design Principles of Laminar Flow Microbial Fuel Cells

Laminar flow microbial fuel cells (MFCs) are used to understand the role of microorganisms, and their interactions with electrodes in microbial bioelectrochemical systems. In this study, we reported the flow characteristics of laminar flow in a typical MFC configuration in a non-dimensional form, which can serve as a guideline in the design of such microfluidic systems. Computational fluid dynamics simulations were performed to examine the effects of channel geometries, surface characteristics, and fluid velocity on the mixing dynamics in microchannels with a rectangular cross-section. The results showed that decreasing the fluid velocity enhances mixing but changing the angle between the inlet channels, only had strong effects when the angle was larger than 135°. Furthermore, different mixing behaviors were observed depending on the angle of the channels, when the microchannel aspect ratio was reduced. Asymmetric growth of microbial biofilm on the anode side skewed the mixing zone and wall roughness due to the bacterial attachment, which accelerated the mixing process and reduced the efficiency of the laminar flow MFC. Finally, the magnitude of mass diffusivity had a substantial effect on mixing behavior. The results shown here provided both design guidelines, as well as better understandings of the MFCs due to microbial growth.

Single-Genotype Syntrophy by Rhodopseudomonas palustris Is Not a Strategy to Aid Redox Balance during Anaerobic Degradation of Lignin Monomers

Rhodopseudomonas palustris has emerged as a model microbe for the anaerobic metabolism of p-coumarate, which is an aromatic compound and a primary component of lignin. However, under anaerobic conditions, R. palustris must actively eliminate excess reducing equivalents through a number of known strategies (e.g., CO₂ fixation, H₂ evolution) to avoid lethal redox imbalance. Others had hypothesized that to ease the burden of this redox imbalance, a clonal population of R. palustris could functionally differentiate into a pseudo-consortium. Within this pseudo-consortium, one sub-population would perform the aromatic moiety degradation into acetate, while the other sub-population would oxidize acetate, resulting in a single-genotype syntrophy through acetate sharing. Here, the objective was to test this hypothesis by utilizing microbial electrochemistry as a research tool with the extracellular-electron-transferring bacterium Geobacter sulfurreducens as a reporter strain replacing the hypothesized acetate-oxidizing sub-population. We used a 2 × 4 experimental design with pure cultures of R. palustris in serum bottles and co-cultures of R. palustris and G. sulfurreducens in bioelectrochemical systems. This experimental design included growth medium with and without bicarbonate to induce non-lethal and lethal redox imbalance conditions, respectively, in R. palustris. Finally, the design also included a mutant strain (NifA^*) of R. palustris, which constitutively produces H₂, to serve both as a positive control for metabolite secretion (H₂) to G. sulfurreducens, and as a non-lethal redox control for without bicarbonate conditions. Our results demonstrate that acetate sharing between different sub-populations of R. palustris does not occur while degrading p-coumarate under either non-lethal or lethal redox imbalance conditions. This work highlights the strength of microbial electrochemistry as a tool for studying microbial syntrophy.

A laminar flow microfluidic fuel cell for detection of hexavalent chromium concentration

An electrochemical hexavalent chromium concentration sensor based on a microfluidic fuel cell is presented. The correlation between current density and chromium concentration is established in this report. Three related operation parameters are investigated, including pH values, temperature, and external resistance on the sensor performance. The results show that the current density increases with increasing temperature and the sensor produces a maximum regression coefficient at the catholyte pH value of 1.0. Moreover, it is found that the external resistance has a great influence on the linearity and current densities of the microfluidic sensor. Owing to the membraneless structure and the steady co-laminar flow inside the microchannel, the microfluidic sensor exhibits short response time to hexavalent chromium concentration. The laminar flow fuel cell sensor provides a new and simple method for detecting hexavalent chromium concentration in the industrial wastewater.

Review of micro/nanotechnologies for microbial biosensors

A microbial biosensor is an analytical device with a biologically integrated transducer that generates a measurable signal indicating the analyte concentration. This method is ideally suited for the analysis of extracellular chemicals and the environment, and for metabolic sensory regulation. Although microbial biosensors show promise for application in various detection fields, some limitations still remain such as poor selectivity, low sensitivity, and impractical portability. To overcome such limitations, microbial biosensors have been integrated with many recently developed micro/nanotechnologies and applied to a wide range of detection purposes. This review article discusses micro/nanotechnologies that have been integrated with microbial biosensors and summarizes recent advances and the applications achieved through such novel integration. Future perspectives on the combination of micro/nanotechnologies and microbial biosensors will be discussed, and the necessary developments and improvements will be strategically deliberated.

Microscale microbial fuel cells: Advances and challenges

The next generation of sustainable energy could come from microorganisms; evidence that it can be seen with the given rise of Electromicrobiology, the study of microorganisms' electrical properties. Many recent advances in electromicrobiology stem from studying microbial fuel cells (MFCs), which are gaining acceptance as a future alternative "green" energy technology and energy-efficient wastewater treatment method. MFCs are powered by living microorganisms with clean and sustainable features; they efficiently catalyse the degradation of a broad range of organic substrates under natural conditions. There is also increasing interest in photosynthetic MFCs designed to harness Earth's most abundant and promising energy source (solar irradiation). Despite their vast potential and promise, however, MFCs and photosynthetic MFCs have not yet successfully translated into commercial applications because they demonstrate persistent performance limitations and bottlenecks associated with scaling up. Instead, microscale MFCs have received increasing attention as a unique platform for various applications such as powering small portable electronic elements in remote locations, performing fundamental studies of microorganisms, screening bacterial strains, and toxicity detection in water. Furthermore, the stacking of miniaturized MFCs has been demonstrated to offer larger power densities than a single macroscale MFC in terms of scaling up. In this overview, we discuss recent achievements in microscale MFCs as well as their potential applications. Further scientific and technological challenges are also reviewed.

Towards the understanding of the effect of oxygen on the electrocatalytic activity of microbial biofilms: a spectroelectrochemical study

Metal-respiring bacteria oxidize an organic substrate and transfer the liberated electrons to the electrode. Molecular oxygen interrupts the current flow by cutting the electrical circuit wiring the cell metabolism to the electrode.

Microbial catalysis in bioelectrochemical technologies: status quo, challenges and perspectives

Over the past decade, microbial electrochemical technologies, originally developed from an interesting physiological phenomenon, have evolved from a rush of initiatives for sustainable bioelectricity generation to a multitude of specialized applications in very different areas. Genetic engineering of microbial biocatalysts for target bioelectrochemical applications like biosensing or bioremediation, as well as the discovery of entirely new bioelectrochemical processes such as microbial electrosynthesis of commodity chemicals, open up completely new possibilities. Where stands this technology today? And what are the general and specific challenges it faces not only scientifically but also for transition into commercial applications? This review intends to summarize the recent advances and provides a perspective on future developments.