Microbial Fuel Cells as Effective Tools for Energy Recovery and Antibiotic Detection in Water and Food.

This work demonstrates that microbial fuel cells (MFCs), optimized for energy recovery, can be used as an effective tool to detect antibiotics in water-based environments. In MFCs, electroactive biofilms function as biocatalysts by converting the chemical energy of organic matter, which serves as the fuel, into electrical energy. The efficiency of the conversion process can be significantly affected by the presence of contaminants that act as toxicants to the biofilm. The present work demonstrates that MFCs can successfully detect antibiotic residues in water and water-based electrolytes containing complex carbon sources that may be associated with the food industry. Specifically, honey was selected as a model fuel to test the effectiveness of MFCs in detecting antibiotic contamination, and tetracycline was used as a reference antibiotic within this study. The results show that MFCs not only efficiently detect the presence of tetracycline in both acetate and honey-based electrolytes but also recover the same performance after each exposure cycle, proving to be a very robust and reliable technology for both biosensing and energy recovery.

Ultrasonic Spray Coating to Optimize Performance of Bio-Electrochemical Systems.

This work investigates the optimization of carbon-based electrodes employed in bio-electrochemical systems (BES) through the deposition of nanostructured layers of poly(3,4-ethylene-dioxy-thiophene) poly(styrene-sulfonate) (PEDOT:PSS) on commercial carbon paper electrodes via ultrasonic spray coating (USC). This innovative application of USC demonstrated that uniform and controlled depositions of PEDOT:PSS can be successfully performed on carbon-based electrodes. To this end, the morphology and spatial uniformity of depositions were verified via scanning electron microscopy and Raman spectroscopy. Electrochemical characterizations of fabricated electrodes demonstrated a more than two-fold increase in the electrochemical active surface area with respect to bare carbon paper. A lab-scale experiment on BES was performed, selecting microbial fuel cells (MFCs) as the reference devices. Devices featuring USC-deposited PEDOT:PSS electrodes showed a three-fold-higher energy recovery with respect to control cells, reaching a maximum value of (13 ± 2) J·m-3. Furthermore, the amount of PEDOT:PSS required to optimize MFCs' performance is in line with values reported in the literature for other deposition methods. In conclusion, this work demonstrates that USC is a promising technique for application in BES.

A Nanofiber-Based Gas Diffusion Layer for Improved Performance in Air Cathode Microbial Fuel Cells.

This work investigates a new nanostructured gas diffusion layer (nano-GDL) to improve the performance of air cathode single-chamber microbial fuel cells (a-SCMFCs). The new nano-GDLs improve the direct oxygen reduction reaction by exploiting the best qualities of nanofibers from electrospinning in terms of high surface-area-to-volume ratio, high porosity, and laser-based processing to promote adhesion. By electrospinning, nano-GDLs were fabricated directly by collecting two nanofiber mats on the same carbon-based electrode, acting as the substrate. Each layer was designed with a specific function: water-resistant, oxygen-permeable polyvinylidene-difluoride (PVDF) nanofibers served as a barrier to prevent water-based electrolyte leakage, while an inner layer of cellulose nanofibers was added to promote oxygen diffusion towards the catalytic sites. The maximum current density obtained for a-SCMFCs with the new nano-GDLs is 132.2 ± 10.8 mA m-2, and it doubles the current density obtained with standard PTFE-based GDL (58.5 ± 2.4 mA m-2) used as reference material. The energy recovery (EF) factor, i.e., the ratio of the power output to the inner volume of the device, was then used to evaluate the overall performance of a-SCMFCs. a-SCMFCs with nano-GDL provided an EF value of 60.83 mJ m-3, one order of magnitude higher than the value of 3.92 mJ m-3 obtained with standard GDL.

3D Composite PDMS/MWCNTs Aerogel as High-Performing Anodes in Microbial Fuel Cells.

Porous 3D composite materials are interesting anode electrodes for single chamber microbial fuel cells (SCMFCs) since they exploit a surface layer that is able to achieve the correct biocompatibility for the proliferation of electroactive bacteria and have an inner charge transfer element that favors electron transfer and improves the electrochemical activity of microorganisms. The crucial step is to fine-tune the continuous porosity inside the anode electrode, thus enhancing the bacterial growth, adhesion, and proliferation, and the substrate's transport and waste products removal, avoiding pore clogging. To this purpose, a novel approach to synthetize a 3D composite aerogel is proposed in the present work. A 3D composite aerogel, based on polydimethylsiloxane (PDMS) and multi-wall carbon nanotubes (MWCNTs) as a conductive filler, was obtained by pouring this mixture over the commercial sugar, used as removable template to induce and tune the hierarchical continuous porosity into final nanostructures. In this scenario, the granularity of the sugar directly affects the porosities distribution inside the 3D composite aerogel, as confirmed by the morphological characterizations implemented. We demonstrated the capability to realize a high-performance bioelectrode, which showed a 3D porous structure characterized by a high surface area typical of aerogel materials, the required biocompatibility for bacterial proliferations, and an improved electron pathway inside it. Indeed, SCMFCs with 3D composite aerogel achieved current densities of (691.7 ± 9.5) mA m-2, three orders of magnitude higher than commercial carbon paper, (287.8 ± 16.1) mA m-2.

Living Bacteria Directly Embedded into Electrospun Nanofibers: Design of New Anode for Bio-Electrochemical Systems.

The aim of this work is the optimization of electrospun polymeric nanofibers as an ideal reservoir of mixed electroactive consortia suitable to be used as anodes in Single Chamber Microbial Fuel Cells (SCMFCs). To reach this goal the microorganisms are directly embedded into properly designed nanofibers during the electrospinning process, obtaining so called nanofiber-based bio-composite (bio-NFs). This research approach allowed for the designing of an advanced nanostructured scaffold, able to block and store the living microorganisms inside the nanofibers and release them only after exposure to water-based solutions and electrolytes. To reach this goal, a water-based polymeric solution, containing 5 wt% of polyethylene oxide (PEO) and 10 wt% of environmental microorganisms, is used as the initial polymeric solution for the electrospinning process. PEO is selected as the water-soluble polymer to ensure the formation of nanofiber mats offering features of biocompatibility for bacteria proliferation, environment-friendliness and, high ionic conductivity. In the present work, bio-NFs, based on living microorganisms directly encapsulated into the PEO nanofiber mats, were analyzed and compared to PEO-NFs made of PEO only. Scanning electron microscopy allowed researchers to confirm the rise of a typical morphology for bio-NFs, evidencing the microorganisms' distribution inside them, as confirmed by fluorescence optical microscopy. Moreover, the latter technique, combined with optical density measurements, allowed for demonstrating that after electrospinning, the processed microorganisms preserved their proliferation capability, and their metabolic activity after exposure to the water-based electrolyte. To demonstrate that the energy-production functionality of exo-electrogenic microorganisms was preserved after the electrospinning process, the novel designed nanomaterials, were directly deposited onto carbon paper (CP), and were applied as anode electrodes in Single Chamber Microbial Fuel Cells (SCMFCs). It was possible to appreciate that the maximum power density reached by bio-NFs, which resulted in being double of the ones achieved with PEO-NFs and bare CP. SCMFCs with bio-NFs applied as anodic electrodes reached a current density value, close to (250 ± 5.2) mA m-2, which resulted in being stable over time and was comparable with the one obtained with carbon-based electrode, thus confirming the good performance of the whole device.

Electrospun PEO/PEDOT:PSS Nanofibers for Wearable Physiological Flex Sensors.

Flexible sensors are fundamental devices for human body monitoring. The mechanical strain and physiological parameters coupled sensing have attracted increasing interest in this field. However, integration of different sensors in one platform usually involves complex fabrication process-flows. Simplification, even if essential, remains a challenge. Here, we investigate a piezoresistive and electrochemical active electrospun nanofibers (NFs) mat as the sensitive element of the wearable physiological flex sensing platform. The use of one material sensitive to the two kinds of stimuli reduces the process-flow to two steps. We demonstrate that the final NFs pH-Flex Sensor can be used to monitor the deformation of a human body joint as well as the pH of the skin. A unique approach has been selected for pH sensing, based on Electrochemical Impedance Spectroscopy (EIS). A linear dependence of the both the double layer capacitance and charge transfer re-sistance with the pH value was obtained by EIS, as well as a linear trend of the electrical resistance with the bending deformation. Gauge factors values calculated after the bending test were 45.84 in traction and 208.55 in compression mode, reflecting the extraordinary piezoresistive behavior of our nanostructured NFs.

Design and Optimization of Piezoresistive PEO/PEDOT:PSS Electrospun Nanofibers for Wearable Flex Sensors.

Flexible strain sensors are fundamental devices for application in human body monitoring in areas ranging from health care to soft robotics. Stretchable piezoelectric strain sensors received an ever-increasing interest to design novel, robust and low-cost sensing units for these sensors, with intrinsically conductive polymers (ICPs) as leading materials. We investigated a sensitive element based on crosslinked electrospun nanofibers (NFs) directly collected and thermal treated on a flexible and biocompatible substrate of polydimethylsiloxane (PDMS). The nanostructured active layer based on a blend of poly(ethylene oxide) (PEO) and poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonate) (PEDOT:PSS) as the ICP was optimized, especially in terms of the thermal treatment that promotes electrical conductivity through crosslinking of PEO and PSS, preserving the nanostructuration and optimizing the coupling between the sensitive layer and the substrate. We demonstrate that excellent properties can be obtained thanks to the nanostructured active materials. We analyzed the piezoresistive response of the sensor in both compression and traction modes, obtaining an increase in the electrical resistance up to 90%. The Gauge Factors (GFs) reflected the extraordinary piezoresistive behavior observed: 45.84 in traction and 208.55 in compression mode, which is much higher than the results presented in the literature for non-nanostructurated PEDOT.

Electrospun Nanofibers: from Food to Energy by Engineered Electrodes in Microbial Fuel Cells.

Microbial fuel cells (MFCs) are bio-electrochemical devices able to directly transduce chemical energy, entrapped in an organic mass named fuel, into electrical energy through the metabolic activity of specific bacteria. During the last years, the employment of bio-electrochemical devices to study the wastewater derived from the food industry has attracted great interest from the scientific community. In the present work, we demonstrate the capability of exoelectrogenic bacteria used in MFCs to catalyze the oxidation reaction of honey, employed as a fuel. With the main aim to increase the proliferation of microorganisms onto the anode, engineered electrodes are proposed. Polymeric nanofibers, based on polyethylene oxide (PEO-NFs), were directly electrospun onto carbon-based material (carbon paper, CP) to obtain an optimized composite anode. The crucial role played by the CP/PEO-NFs anodes was confirmed by the increased proliferation of microorganisms compared to that reached on bare CP anodes, used as a reference material. A parameter named recovered energy (Erec) was introduced to determine the capability of bacteria to oxidize honey and was compared with the Erec obtained when sodium acetate was used as a fuel. CP/PEO-NFs anodes allowed achieving an Erec three times higher than the one reached with a bare carbon-based anode.

Tuning Porosity and Functionality of Electrospun Rubber Nanofiber Mats by Photo-Crosslinking.

The present work proposes a versatile and efficient method to fabricate rubber nanofiber membranes with a controlled morphology and tailored functionality, based on the application of photoinduced thiol-ene cross-linking reactions to electrospun mats. Besides preventing the polymer cold flow and freezing the structure obtained by electrospinning, the photocuring step finely controls the morphology of the nanofiber mats, in terms of the fiber diameter up to the nanometer range and of the membrane porosity. Nanofiber membranes are also made chemically resistant, while retaining their flexibility. Finally, the proposed approach allows imparting specific functionalities to the rubber nanofibers: the type and concentration of the functional groups can be precisely tuned by changing process parameters (i.e., thiol/ene stoichiometric ratio and irradiation dose). Active chemical groups that remain available on the surface of the nanofibers can be used for further material modifications, as here proven by two target reactions. This key result is also demonstrated with electrospun membranes embedded into a microfluidic chip, opening the way to advanced functional flexible devices.

Biohybrid Cathode in Single Chamber Microbial Fuel Cell.

The aim of this work is to investigate the properties of biofilms, spontaneously grown on cathode electrodes of single-chamber microbial fuel cells, when used as catalysts for oxygen reduction reaction (ORR). To this purpose, a comparison between two sets of different carbon-based cathode electrodes is carried out. The first one (Pt-based biocathode) is based on the proliferation of the biofilm onto a Pt/C layer, leading thus to the creation of a biohybrid catalyst. The second set of electrodes (Pt-free biocathode) is based on a bare carbon-based material, on which biofilm grows and acts as the sole catalyst for ORR. Linear sweep voltammetry (LSV) characterization confirmed better performance when the biofilm is formed on both Pt-based and Pt-free cathodes, with respect to that obtained by biofilm-free cathodes. To analyze the properties of spontaneously grown cathodic biofilms on carbon-based electrodes, electrochemical impedance spectroscopy is employed. This study demonstrates that the highest power production is reached when aerobic biofilm acts as a catalyst for ORR in synergy with Pt in the biohybrid cathode.

Microwave-Assisted Synthesis of Reduced Graphene Oxide/SnO2 Nanocomposite for Oxygen Reduction Reaction in Microbial Fuel Cells.

We report on an easy, fast, eco-friendly, and reliable method for the synthesis of reduced graphene oxide/SnO2 nanocomposite as cathode material for application in microbial fuel cells (MFCs). The material was prepared starting from graphene oxide that has been reduced to graphene during the hydrothermal synthesis of the nanocomposite, carried out in a microwave system. Structural and morphological characterizations evidenced the formation of nanocomposite sheets, with SnO2 crystals of few nanometers integrated in the graphene matrix. Physico-chemical analysis revealed the formation of SnO2 nanoparticles, as well as the functionalization of the graphene by the presence of nitrogen atoms. Electrochemical characterizations put in evidence the ability of such composite to exploit a cocatalysis mechanism for the oxygen reduction reaction, provided by the presence of both SnO2 and nitrogen. In addition, the novel composite catalyst was successfully employed as cathode in seawater-based MFCs, giving electrical performances comparable to those of reference devices employing Pt as catalyst.

Dispelling clichés at the nanoscale: the true effect of polymer electrolytes on the performance of dye-sensitized solar cells.

In the field of dye-sensitized solar cells, polymer electrolytes are among the most studied materials due to their ability to ensure both high efficiency and stability, the latter being a critical point of these devices. Hundreds of polymeric matrices have been proposed over the years, and their functionalization with several groups, the variation of their molecular weight and the tuning of the crosslinking degree have been investigated. However, the true effect that polymeric matrices have on the cell parameters has often been addressed superficially, and hundreds of papers justify the obtained results with a simple bibliographic reference to other systems (sometimes completely different). This work proposes a system of nanoscale growth and crosslinking of a polymer electrolyte inside a nanostructured photoanode. Electrochemical and photovoltaic parameters are carefully monitored as a function of thickness and degree of penetration of the electrolyte. The results derived from this study refute many clichés generally accepted and taken for granted in many literature articles, and ­ for the first time ­ a compromise between the amount of polymer, cell efficiency and stability is achieved.