Natalia J. Sacco

Analytical applications of microbial fuel cells. Part I: Biochemical oxygen demand

Microbial fuel cells (MFCs) are bio-electrochemical devices, where usually the anode (but sometimes the cathode, or both) contains microorganisms able to generate and sustain an electrochemical gradient which is used typically to generate electrical power. In the more studied set-up, the anode contains heterotrophic bacteria in anaerobic conditions, capable to oxidize organic molecules releasing protons and electrons, as well as other by-products. Released protons could reach the cathode (through a membrane or not) whereas electrons travel across an external circuit originating an easily measurable direct current flow. MFCs have been proposed fundamentally as electric power producing devices or more recently as hydrogen producing devices. Here we will review the still incipient development of analytical uses of MFCs or related devices or set-ups, in the light of a non-restrictive MFC definition, as promising tools to asset water quality or other measurable parameters. An introduction to biological based analytical methods, including bioassays and biosensors, as well as MFCs design and operating principles, will also be included. Besides, the use of MFCs as biochemical oxygen demand sensors (perhaps the main analytical application of MFCs) is discussed. In a companion review (Part 2), other new analytical applications are reviewed used for toxicity sensors, metabolic sensors, life detectors, and other proposed applications.

Analytical applications of microbial fuel cells. Part II: Toxicity, microbial activity and quantification, single analyte detection and other uses.

Microbial fuel cells were rediscovered twenty years ago and now are a very active research area. The reasons behind this new activity are the relatively recent discovery of electrogenic or electroactive bacteria and the vision of two important practical applications, as wastewater treatment coupled with clean energy production and power supply systems for isolated low-power sensor devices. Although some analytical applications of MFCs were proposed earlier (as biochemical oxygen demand sensing) only lately a myriad of new uses of this technology are being presented by research groups around the world, which combine both biological-microbiological and electroanalytical expertises. This is the second part of a review of MFC applications in the area of analytical sciences. In Part I a general introduction to biological-based analytical methods including bioassays, biosensors, MFCs design, operating principles, as well as, perhaps the main and earlier presented application, the use as a BOD sensor was reviewed. In Part II, other proposed uses are presented and discussed. As other microbially based analytical systems, MFCs are satisfactory systems to measure and integrate complex parameters that are difficult or impossible to measure otherwise, such as water toxicity (where the toxic effect to aquatic organisms needed to be integrated). We explore here the methods proposed to measure toxicity, microbial metabolism, and, being of special interest to space exploration, life sensors. Also, some methods with higher specificity, proposed to detect a single analyte, are presented. Different possibilities to increase selectivity and sensitivity, by using molecular biology or other modern techniques are also discussed here.

Voltamperometric Discrimination of Urea and Melamine Adulterated Skimmed Milk Powder

Nitrogen compounds like urea and melamine are known to be commonly used for milk adulteration resulting in undesired intoxication; a well-known example is the Chinese episode occurred in 2008. The development of a rapid, reliable and economic test is of relevance in order to improve adulterated milk identification. Cyclic voltammetry studies using an Au working electrode were performed on adulterated and non-adulterated milk samples from different independent manufacturers. Voltammetric data and their first derivative were subjected to functional principal component analysis (f-PCA) and correctly classified by the KNN classifier. The adulterated and non-adulterated milk samples showed significant differences. Best results of prediction were obtained with first derivative data. Detection limits in milk samples adulterated with 1% of its total nitrogen derived from melamine or urea were as low as 85.0 mg·L−1 and 121.4 mg·L−1, respectively. We present this method as a fast and robust screening method for milk adulteration analysis and prevention of food intoxication.

Performance of planar and cylindrical carbon electrodes at sedimentary microbial fuel cells

This paper presents data obtained using an indigenous microbial community contained in anaerobic sediments (mud) collected from the shore of the Río de La Plata River (South America). After the sedimentary microbial fuel cells were assembled the evolution of current and power vs. time was studied. Two types of commercially available graphite materials were used as electrodes, which differ mainly in shape and size. In some experiments, an external carbon source (acetate) increased the power generation rate. The maximum power density observed in the aforementioned condition was 19.57 ± 0.35 and 8.72 ± 1.39 mW/m(2) using rod and graphite disk electrodes, respectively. The better performance of the rod electrodes can be explained, at least in part, by an enhanced rate of mass transport by radial diffusion. DGGE fingerprints were used to study the electrogenic community growing over the electrodes.

Assessing the effect of oxygen and microbial inhibitors to optimize ferricyanide-mediated BOD assay.

Methods for short-term BOD analysis (BOD(st)) based on ferricyanide mediator reduction have succeeded in overcoming some problems associated with the standard BOD test analysis (BOD(5)) such as long-term incubations (5 days), the need to dilute samples and low reproducibility. Here we present a bioassay where a Klebsiella pneumoniae environmental strain successfully reduces ferricyanide without de-aeration of the samples with linear BOD(5) ranges between 30 and 500 mg L(-1) or 30 and 200 mg L(-1), using glucose-glutamic acid solution (GGA) or OECD standards respectively. We further propose a new assay termination solution that allows higher reproducibility and standardization of the cell-based assay, employing formaldehyde (22.7 g L(-1)) or other compounds in order to stop ferricyanide reduction without affecting the amperometric detection and therefore replace the centrifugation step normally used to stop microbial-driven reactions in ferricyanide-mediated bioassays. These improvements led to an accurate determination of real municipal wastewater samples.

Isolation and Characterization of a Novel Electrogenic Bacterium, Dietzia sp. RNV-4.

Electrogenic bacteria are organisms that can transfer electrons to extracellular electron acceptors and have the potential to be used in devices such as bioelectrochemical systems (BES). In this study, Dietzia sp. RNV-4 bacterium has been isolated and identified based on its biochemical, physiological and morphological characteristics, as well as by its 16S rRNA sequence analysis. Furthermore, the current density production and electron transfer mechanisms were investigated using bioelectrochemical methods. The chronoamperometric data showed that the biofilm of Dietzia sp. RNV-4 grew as the current increased with time, reaching a maximum of 176.6 ± 66.1 mA/m2 at the end of the experiment (7 d); this highly suggests that the current was generated by the biofilm. The main electron transfer mechanism, indicated by the cyclic voltammograms, was due to secreted redox mediators. By high performance liquid chromatography, canthaxanthin was identified as the main compound involved in charge transfer between the bacteria and the solid electrodes. Dietzia sp. RNV-4 was used as biological material in a microbial fuel cell (MFC) and the current density production was 299.4 ± 40.2 mA/m2. This is the first time that Dietzia sp. RNV-4 has been electrochemically characterized and identified as a new electrogenic strain.

Stability of Lipogels with Low Molecular Mass Gelators and Emollient Oils

The influence of preparation methods on stability of lipogels was studied. The objectives of this study were to evaluate the ability of different low molecular solid ingredients used as excipients in pharmaceutical and cosmetical products to form lipogels with emollient liquids of different polarities as well as to evaluate the stability of the lipogels obtained and the spreading ability of stable lipogels. The lipogels were prepared by heating the mix of oil and gelator a 100°C with two different forms of cooling: slow cooling of the without stirring and quick cooling with stirring. The stability tests were one year of storage at room temperature, centrifugation and three months at 40°C. None of the lipogels prepared with slow cooling and without stirring were stable in all stability tests. Eight of the formulations with quick cooling and stirring were stables in all stability tests: six with 12-hydroxystearic acid, one with hydrogenated castor oil, and one with beeswax as gelators. The lipogels with 12-hydroxystearic acid as gelator do not spread on skin or form clusters that spread after pressing with the fingers. The two lipogels with castor oil have good spreading ability on the skin.

The Calculus of HLB Values of Polyoxyethylene Fatty Acid Esters from Quality Control Data

In this article, we obtained equations that permit us to calculate the hydrophilic-lipophilic balance (HLB) value of polyoxyethylene esters from quality control data of the raw materials (fatty acids and polyethylene glycol) and the finished product (surfactant). These data include the acid value of the fatty acid, the hydroxyl value of the polyethylene glycol, and the hydroxyl value of the surfactant. These calculations permit us, moreover, to know the mean molecular masses of fatty acids, of polyethylene glycol, of monoester and diester, and the proportion of polyoxyethylene monoester and polyoxyethylene diester.

Rapid and label-free differentiation of bacterial strains using low frequency electrochemical impedance spectroscopy

Microinterdigitated electrode structures (μIDES) have widely been used for electrochemical cell impedance spectroscopy (ECIS). For instance ECIS has been used to study biofilm formation and growth of a wide range of microorganisms to detect bacterial infections, food contaminations, biocorrosion, and biofouling. In this study we describe the development of a label-free bioassay to determine the presence of different bacteria strains using ECIS. Following initial characterization of embedded gold μIDES using solutions of increasing conductivities, impedance spectra ranging from 10Hz to 300 kHz in the presence of Bacillus subtilis DMS10 and Shewanella oneidensis MR-1 were recorded over a period of approx. 5 hours. Frequency analysis showed that at 30 Hz the impedance response differences were higher and the relative standard deviations were smaller between each strain and the medium. In contrast to standard cell impedance sensing, which is performed at frequencies higher than 10 kHz to detect cell membrane polarizations, our low frequency results indicate that ionic changes at the electrode-solution double layer (Cdt) resulting by individual bacterial metabolism, can also be used to non-invasively and label-free differentiate microorganisms.

Metabolism of Klebsiella pneumoniae freeze-dried cultures for the design of BOD bioassays.

Aims:  The survival rate of freeze‐dried cultures is not enough information for technological applications of micro‐organisms. There could be serious metabolic/structural damage in the survivors, leading to a delay time that can jeopardize the design of a rapid biochemical oxygen demand (BOD) metabolic‐based bioassay. Therefore, we will study the metabolic activity (as ferricyanide reduction activity) and the survival rate (as colony‐forming units, CFU) of different Klebsiella pneumoniae freeze‐dried cultures looking for stable metabolic conditions after 35 days of storage.