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This work reports on Fe2O3 and ZnO materials for lactate quantification. In the synthesis, the bi-phase γ-/α-Fe2O3 and γ-/α-Fe2O3/ZnO nanoparticles (NPs) were obtained for their application in a lactate colorimetric sensor. The crystalline phases of the NPs were analyzed by XRD and XPS techniques. S/TEM images showed spheres with an 18 nm average and a needle length from 125 to 330 nm and 18 nm in diameter. The γ-/α-Fe2O3 and γ-/α-Fe2O3/ZnO were used to evaluate the catalytic activity of peroxidase with the substrate 3,3,5,5-tetramethylbenzidine (TMB), obtaining a linear range of 50 to 1000 μM for both NPs, and a 4.3 μM and 9.4 μM limit of detection (LOD), respectively. Moreover, γ-/α-Fe2O3 and γ-/α-Fe2O3/ZnO/lactate oxidase with TMB assays in the presence of lactate showed a linear range of 50 to 1000 µM, and both NPs proved to be highly selective in the presence of interferents. Finally, a sample of human serum was also tested, and the results were compared with a commercial lactometer. The use of ZnO with Fe2O3 achieved a greater response toward lactate oxidation reaction, and has implementation in a lactate colorimetric sensor using materials that are economically accessible and easy to synthesize.

In this work, unsupported Pd aerogel catalysts were synthesized for the very first time by using microwaves as a heating source followed by a lyophilization drying process and used towards formic acid electro-oxidation in a microfluidic fuel cell. Aerogels were also made by heating in a conventional oven to evaluate the microwave effect during the synthesis process of the unsupported Pd aerogels. The performance of the catalysts obtained by means of microwave heating favored the formic acid electro-oxidation with H2SO4 as the electrolyte. The aerogels' performance as anodic catalysts was carried out in a microfluidic fuel cell, giving power densities of up to 14 mW cm-2 when using mass loads of only 0.1 mg on a 0.019 cm2 electrode surface. The power densities of the aerogels obtained by microwave heating gave a performance superior to the resultant aerogel prepared using conventional heating and even better than a commercial Pd/C catalyst., Mexican Council for Science and Technology (CONACYT) through project Ciencia de Frontera Grant no. 845132 and estancia Sabatica Conacyt 2021-2. Grant PID2020-113001RB-I00 funded by MCIN/AEI/ 10.13039/501100011033 and by the European Union NextGenerationEU/PRTR; Grant PCI2020-112039 funded by MCIN/AEI/ 10.13039/501100011033 and by the European Union NextGenerationEU/PRTR.

A microfluidic biofuel cell (µBFC) can be defined as a BFC where the delivery of fuel and removal of waste takes place via a microfluidic channel incorporating electrode structures. These devices typically do not require a physical barrier to separate the anodic and cathodic compartments. One type of µBFC is a flow-through device that consists of porous electrodes where the fuel and oxidant flow-through the electrodes. The implementation of flow-through µBFC technologies allow for higher enzyme loadings and high surface area to volume ratios to promote the surface-based electrochemical reactions catalyzed by the immobilized enzymes. Although µBFC have been evaluated for glucose and ethanol mainly, they have never evaluated the use of lactate as fuel. Lactate/O2 BFC can have high theoretical energy densities of up to 507 Wh/L using a sinlge enzyme and the complete oxidation of lactate could theoretically result in a maximum energy density of 3041 Wh/L. For this reason, lactate is an interesting fuel source for alternative energy generation. In this work, lactate oxidase (LOx) was coupled with a ferrocene-based redox polymer (dimethylferrocene-modified linear polyethylenimine, FcMe2-LPEI) as the bioanode and laccase (Lc) was compared connected to pyrene-anthracene and anthracene-modified carbon nanotubes (PyrAn-MWCNT and An-MWCNT) to facilitate the direct electron transfer at the biocathode and TBAB-modified Nafion was added and it was deposited onto a TCP electrode using a brush and allowed to dry for 2 h. Both electrodes were evaluated in two µBFC configurations using different concentrations of lactate, in the range found in sweat (0, 2, 10, 20 and 40 mM). The µBFC with an air-breathing cathode was fabricated using two plates made of PMMA [poly-(methyl methacrylate)] with a silicon elastomer film (Silastic®) compressed between the plates used to seal the device and define the channel pattern. In the top plate, an air-intake window was constructed with a dimension of 0.22 cm2 enabling oxygen delivery from the air. The FcMe2-LPEI/LOx bioanode was characterized by oxidation and reduction peak potentials of 0.28 and 0.16 V (vs. SCE). In the presence of 70 mM lactate, the limiting current density was 213 mA cm-2, corresponding to the mediated oxidation of lactate by LOx. The catalytic kinetics of the LOx bioanode were studied by amperometry similar current density of 204.1 mAcm-2 using 70 mM lactate and an apparent Michaelis constant of 8.9 ± 1.1 mM calculated by double-reciprocal evaluation of the Lineaweaver-Burk equation. On other hand, two different modifications of An-MWCNT and PyrAn-MWCNT were performed by cyclic voltammograms in 0.1 M PBS at pH 5.6 and. An-MWCNT/Lc electrodes resulted in a current density of 68 mA cm-2 without stirring, while PyrAn-MWCNT/Lc electrodes yielded a current density of 111.3 mA cm-2. Therefore, we utilized the PyrAn-MWCNT/Lc electrodes for µBFC experiments. The performance of the µBFC using 0.05 M PBS and a pH of 5.6 as anolyte and catholyte achievement a maximum OCP of 0.67 V using 20 and 40 mM of lactate, 1700 ± 77 mA cm-2 and 305 ± 20 mW cm-2 using 40 mM lactate, respectively. In order to take advantage of the µBFC design that allows the use of individual electrolyte inlets for the anode and cathode, the pH of the anolyte was changed to pH 7.4 (0.05 M), as appropriate for use in physiological conditions, and the pH of the catholytewas kept at 5.6 (0.05 M). With these change, the OCP increased to 0.73 V and remained constant at concentrations ranging from 2 to 40 mM of lactate, achieving maximum current and power densities of 2087 ± 84 mA cm-2 and 402 ± 23 mW cm-2 using 10 mM of lactate. This is a 19% and 24% enhancement in performance compared to employing the same pH. Also, the effect of flow rate was evaluated using a concentration of 10 mM lactate in 0.05 M at pH 7.4 for the anolyte and pH 5.6 for the catholyte. The OCP of the µBFC was 0.73 V. The results show that a flow rate of 3mL h-1 allowed the OCP to reach a maximum in less time, 6.9 min, compared to a flow rate of 9 mL h-1 which resulted in an improved current density of 1875 mA cm-2 although the average stabilization time was 11.2 min. Interestingly, the current density begins to decrease at a flow rate of 12 mL h-1, perhaps due to flow-induced mechanical instabilities in the CNT immobilization. This is the highest performing lactate/oxygen biofuel cell utilizing a single enzyme system. Future work will focus on incorporating multi-enzyme cascades to further improve performance.

Oxygen reduction reaction (ORR) remains as the limiting reaction in fuel cells for its successful used as energy source due to its slow kinetic and, noble metals such as Pt and Pd are still some of the most active materials. This is the reason why attention has focused on finding another kind of catalyst that allows decreasing the usage of noble metals, either through using non-metallic materials or using less noble metals with specific morphologies such as core-shell structures. In addition, hydrogen evolution reaction (HER) is a key reaction for electrochemical hydrogen compression. The kinetic limiting steps for ORR and HER can be decreased using mixtures of metal transition oxides (MMTOs) as alternative electrocatalysts; several MMTOs have been reported in literature presenting good catalytic activity and great stability, these qualities make them a sustainable solution to traditional ORR catalysts. In this work we describe the use of Mo, Co and Ni oxides as electrocatalysts for ORR. The research was carried out using three different combinations MoNi, MoCo-W (W as particle stabilizer) and NiCo. The electrocatalytic activity was studied by rotating disc electrode (RDE), the kinetic parameters were obtained from the Koutecky-Levich and Tafel plots. X-ray diffraction (XRD) indicated that the MoCo-W and MoNi materials have an amorphous structure, while NiCo presented the diffraction peaks corresponding to Co3O4 and Ni. The particle size and morphology were studied by Transmission electron microscopy (TEM), where all materials presented sizes below 20 nm. These materials showed overpotentials lower than 170 mV for both, ORR and HER compared with commercial Pt/C (30 wt.%). Finally, a tolerance study was performed where, MoCoW and NiCo exhibited excellent tolerance to several alcohols (methanol, ethanol, glycerol, ethylene glycol).

Microfluidic fuel cells are a low-power demand fuel cells which work without the need of a physical membrane and have a wide range of potential applications. Biological catalysts such as immobilized enzymes can be used in electrodes to carry out the oxidation processes of different organic fuels using the immobilization technology of enzymes; they are carried out under moderate conditions of pH and temperature, and the specificity of the catalytic reactions. The alcohol dehydrogenase enzyme has been used for the development of bioanodes for the oxidation of different alcohols such as ethanol and methanol. In this work, the immobilization of this enzyme was carried out using nanotubes of titanium dioxide for the development of electrodes for an air-breathing microfluidic fuel cell. TiO2 nanotubes has excellent properties such as pH resistance, superior mechanical strength, good biocompatibility making it a great candidate for the process of immobilization of the enzyme alcohol dehydrogenase. TiO2 nanotubes were fabricated by electrochemical anodic oxidation on Ti foils. The foils were pre-treated with sand-paper, thereafter immersed in ethanol, placed in ultrasonic bath, dried under N2 flow. An ethylene glycol-based solution with a concentration of 0.1M NH4F (96% purity, Alfa-Aesar) and 2% w/w deionized water was used as electrolyte. The anodization times used was 1 h with a voltage step of 60 V via a power source. Using this nanostructure, the enzymatic electrode was constructed using a catalityc ink with enzyme, tetrabutylammonium bromide and Nafion to carry out the immobilization process. We carried out the characterization of the electrode developed by means of different electrochemical and kinetic techniques, where it was demonstrated that the enzyme is present and active in the electrode, as well as its capacity to oxidize ethanol. Then the bioanode was evaluated in the microfluidic device also using an inorganic cathode of Pt / C, obtaining a good performance over an open circuit potential greater than 0.93V.

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