Electroactive biofilms from activated sludge: Mechanistic insights into electron transport on nanostructured electrodes for the development of biosensors and microbial fuel cell devices.

Exopolysaccharides matrix affects the process of extracellular electron transfer in electroactive biofilm

Increasing industrialization and urbanization have contributed to a significant rise in wastewater discharge and exerted extensive pressure on the existing natural energy resources. Microbial fuel cell (MFC) is a sustainable technology that utilizes wastewater for electricity generation. MFC comprises a bioelectrochemical system employing electroactive biofilms of several aerobic and anaerobic bacteria, such as Geobacter sulfurreducens, Shewanella oneidensis, Pseudomonas aeruginosa, and Ochrobacterum pseudiintermedium. Since the electroactive biofilms constitute a vital part of the MFC, it is crucial to understand the biofilm-mediated pollutant metabolism and electron transfer mechanisms. Engineering electroactive biofilm communities for improved biofilm formation and extracellular polymeric substances (EPS) secretion can positively impact the bioelectrochemical system and improve fuel cell performance. This review article summarizes the role of electroactive bacterial communities in MFC for wastewater treatment and bioelectricity generation. A significant focus has been laid on understanding the composition, structure, and function of electroactive biofilms in MFC. Various electron transport mechanisms, including direct electron transfer (DET), indirect electron transfer (IET), and long-distance electron transfer (LDET), have been discussed. A detailed summary of the optimization of process parameters and genetic engineering strategies for improving the performance of MFC has been provided. Lastly, the applications of MFC for wastewater treatment, bioelectricity generation, and biosensor development have been reviewed.

Single-walled carbon nanotubes (SWCNTs) outperform other materials due to their high conductivity, large specific surface area, and chemical resistance. They have numerous biomedical applications, including the magnetization of the SWCNT (mSWCNT). The drug loading and release properties of see-through pectin hydrogels doped with SWCNTs and mSWCNTs were evaluated in this study. The active molecule in the hydrogel structure is allantoin, and calcium chloride serves as a cross-linker. In addition to mixing, absorption, and swelling techniques, drug loading into carbon nanotubes was also been studied. To characterize the films, differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA), Fourier transform infrared (FTIR) spectroscopy, surface contact angle measurements, and opacity analysis were carried out. Apart from these, a rheological analysis was also carried out to examine the flow properties of the hydrogels. The study was also expanded to include N-(9-fluorenyl methoxycarbonyl)glycine-coated SWCNTs and mSWCNTs as additives to evaluate the efficiency of the drug-loading approach. Although the CNT additive was used at a 1:1000 weight ratio, it had a significant impact on the hydrogel properties. This effect, which was first observed in the thermal properties, was confirmed in rheological analyses by increasing solution viscosity. Additionally, rheological analysis and drug release profiles show that the type of additive causes a change in the matrix structure. According to TGA findings, even though SWCNTs and mSWCNTs were not coated more than 5%, the coating had a significant effect on drug release control. In addition to all findings, cell viability tests revealed that hydrogels with various additives could be used for visual wound monitoring, hyperthermia treatment, and allantoin release in wound treatment applications.

Redox potential-induced regulation of extracellular polymeric substances in an electroactive mixed community biofilm

The ongoing research in the sustainable energy sector has shown tremendous potential of microbes or electro-active biofilms (EABfs). These biofilms act as the important component of bioprocessing technologies that are based on bio-electrochemical systems (BESs). EABfs exhibit unique characteristics including redox reactions and resilience against otherwise harmful products that make BESs promising for important applications in energy recovery in the form of electricity or hydrogen or even production of fuels or chemicals from CO2. A deeper understanding of the mechanisms of EABfs characteristics is considered essential for the optimization of BESs for practical applications. To this end, a wide range of characterization techniques based on electrochemical, visual and chemical methods have been employed for the analyses of EABfs. These techniques can provide very valuable and wide-ranging information about EABfs that include performance, morphology and biofilm composition. Especially, significant attention has been paid to developing non-destructive visual techniques for EABfs characterization. The goal is to obtain in-situ information of EABfs functioning for industrial-scale development of BESs. Visual techniques are considered extremely useful for EABfs monitoring studies that can complement the information obtained with other characterization techniques. In this perspective, we have provided a short overview of various visual characterization techniques that have been proposed to study EABfs for the optimization of BESs.

Chapter 39 - Use of biofilm bacteria to enhance overall microbial fuel cell performance

Chapter 13 - Nanomaterials supporting indirect electron transport

Response of electroactive biofilms from real wastewater to metal ion shock in bioelectrochemical systems

Bioelectrochemical anaerobic ammonium oxidation (anammox) systems allow eco-friendly removal of nitrogen from reject wastewater coming from biogas processing as the anammox bacteria have previously shown to have c-type cytochromes acting in the extracellular electron transport (EET) mechanism between the bacteria and electrode. The anammoxosome compartment present in anammox bacteria features a highly curved membrane and contains tubular structures along with electron-dense particles that contain iron, which could enhance the process of EET and enhance nitrogen removal by properly applied potentials. In this study, nitrogen removal was investigated in the electrostimulated anammox nitrogen removal (EANR) cells operated comparatively at open circuit and at applied potentials of - 300mV, - 500mV, and - 700mV vs. Ag/AgCl. At peak performance (at - 700mV vs. Ag/AgCl), the EANR showed up to 140% higher specific nitrogen removal rate (11.2 ± 0.3g N/m2/day) compared to the control reactors without applied potential (8.3 ± 0.2g N/m2/day). The microbial community on the cathode with the applied potential had a higher relative proportion of unclassified Candidatus Brocadia (7.5%) compared to inoculum (> 0.01%), in contrast to cathode without potential (0.74%) and control (0.2%). The EANR system demonstrated to achieve ammonium and nitrite removal efficiencies of 91% and 53%, respectively, during a 24-h test cycle from an initial TN concentration of ~ 100mg N/L. After 150h, it achieved complete removal of all nitrogen compounds, reaching a 100% removal efficiency. The EANR would be very useful in the establishment of field-scale bilateral anammox-bioelectrochemical technology combining microbial fuel cell bioanodes and EANR biocathodes for wastewater treatment.

This work addresses the need for kinetic studies to identify potentially exploitable mechanisms involved in generating current from flow-based bioelectrochemical systems (BES). Unlike most kinetic studies, which focus on electron transport, we focus on chemical mass transport by controlling the relevant experimental parameters. This is accomplished using a microfluidic 3-electrode setup that recorded output current (I) from a mature Geobacter sulfurreducens electroactive biofilm (EAB) while accurate control is applied over acetate concentration ([Ac]) and flow rate (Q). Additionally, the flow mode (tangential and perpendicular) is controlled to apply expansive or compressive sheer forces against the EAB. A detailed analysis of the effects of the control variables on the current is based on data collected for nearly 1 year, the longest timeframe for a microfluidic BES experiment to date. All experimental parameters affect output, but we find that age is the dominant factor. After nearly 1 year, current densities were as high as 29.5 A m−1, which is higher than in any reported 3-electrode experiment on G. sulfurreducens EAB. We conclude that flow-based deacidification of the EAB led to increases to outputs during early growth stages, whereas at later times, the increases were related to improved EAB permeability to acetate. Additionally, after 5 months each flow mode provokes complementary kinetic properties based on measurements of apparent enzyme/substrate affinity (KM(app)) and maximum current (Imax) values. Therefore, in addition to providing fundamental insights into BES functionality, these findings also open the door to practical applications and offer a road map to further experimental optimization.

The accumulation of protons in electro-active biofilms (EABfs) has been reported as a critical parameter determining produced currents at the anode since the very beginning of the studies on Bio-electrochemical systems (BESs). Even though the knowledge gained on the influence of this parameter on the produced currents, its influence on EABfs growth is frequently overlooked. In this study, we quantified EABfs thicknesses in real-time and related them to the produced current at three buffer concentrations, two anode potentials and two acetate concentrations. The thickest EABfs (80 μm) and higher produced currents (2.5 A.m−2) were measured when a 50 mM buffer concentration was used. By combining the measured EABfs thicknesses with the pH in the anolyte, a simple model was developed to identify buffer limitations. Buffer limited EABfs with thicknesses of 15 and 42 μm were identified at −0.3 V vs Ag/AgCl when 10 and 50 mM buffer concentrations were used, respectively. At −0.2 V vs Ag/AgCl, the thicknesses of buffer limited EABfs decreased to 13 and 20 μm, respectively. The model also estimated buffer and acetate diffusion rates in EABfs and allowed to determine the boundary between a buffer and acetate limited EABfs. The diffusion rates reported in this study and the definition of the boundary between buffer and acetate limited EABfs provide a powerful tool to avoid limitations, leading to higher produced currents at the anode.

Though there are many experiments on conduction and electronic transport mechanism in carbon nanotubes (CNTs), a systematic study of quantum conductance of bundled single-wall carbon nanotube (SWCNTs) with the disorder is essential for nanoelectronic and optoelectronic applications. Although a single SWCNT has excellent conductance and a longer mean free path, a disorder in SWCNT crystal impairs conduction. The disorder affects conductance severely and, in this work, we studied quantum conductance of metallic and semiconducting bundled SWCNTs with diameters of 0.8 nm and 1 nm. If bundled SWCNTs have non-ballistic conduction, its quantum conductance reduces significantly. Along with disorder strength, the conductance relies on the diameter and length of SWCNTs.

Optimization of bio-electrochemical systems (BESs) relies on a better understanding of electro-active biofilms (EABfs). These microbial communities are studied with a range of techniques, including electrochemical, visual and chemical techniques. Even though each of these techniques provides very valuable and wide-ranging information about EABfs, such as performance, morphology and biofilm composition, they are often destructive. Therefore, the information obtained from EABfs development and characterization studies are limited to a single characterization of EABfs and often limited to one time point that determines the end of the experiment. Despite being scarcer and not as commonly reported as destructive techniques, non-destructive visual techniques can be used to supplement EABfs characterization by adding in-situ information of EABfs functioning and its development throughout time. This opens the door to EABfs monitoring studies that can complement the information obtained with destructive techniques. In this review, we provide an overview of visual techniques and discuss the opportunities for combination with the established electrochemical techniques to study EABfs. By providing an overview of suitable visual techniques and discussing practical examples of combination of visual with electrochemical methods, this review aims at serving as a source of inspiration for future studies in the field of BESs.

Research progress and trend of antibiotics degradation by electroactive biofilm: A review

Although conventional antibiotics have evolved as a staple of modern medicine, increasing antibiotic resistance and the lack of antibiotic efficacy against new bacterial threats is becoming a major medical threat. In this work, we employ single-walled carbon nanotubes (SWCNTs) known to deliver and track therapeutics in mammalian cells via intrinsic near-infrared fluorescence as carriers enhancing antibacterial delivery of doxycycline and methicillin. SWCNTs dispersed in water by antibiotics without the use of toxic bile salt surfactants facilitate efficacy enhancement for both antibiotics against Staphylococcus epidermidis strain showing minimal sensitivity to methicillin. Doxycycline to which the strain did not show resistance in complex with SWCNTs provides only minor increase in efficacy, whereas the SWCNTs/methicillin complex yields up to 40-fold efficacy enhancement over antibiotics alone, suggesting that SWCNT-assisted delivery may circumvent antibiotic resistance in that bacterial strain. At the same time SWCNT/antibiotic formulations appear to be less toxic to mammalian cells than antibiotics alone suggesting that nanomaterial platforms may not restrict potential biomedical applications. The improvement in antibacterial performance with SWCNT delivery is tested via 3 independent assays—colony count, MIC (Minimal Inhibitory Concentration) turbidity and disk diffusion, with the statistical significance of the latter verified by ANOVA and Dunnett’s method. The potential mechanism of action is attributed to SWCNT interactions with bacterial cell wall and adherence to the membrane, as substantial association of SWCNT with bacteria is observed—the near-infrared fluorescence microscopy of treated bacteria shows localization of SWCNT fluorescence in bacterial clusters, scanning electron microscopy verifies SWCNT association with bacterial surface, whereas transmission electron microscopy shows individual SWCNT penetration into bacterial cell wall. This work characterizes SWCNTs as novel advantageous antibiotic delivery/imaging agents having the potential to address antibiotic resistance.