Flexible electronics can be developed with a low-cost and simple fabrication process while being environmentally friendly. Conductive silver inks have been the most applied material in flexible substrates. This study evaluated the performance of different conductive ink formulations using silver nanoparticles by studying the material properties, the inkjet printing process, and application based on electrical impedance spectroscopy using a buffer solution. Silver nanoparticles synthesis was carried out through chemical reduction of silver nitrate; then, seven conductive ink formulations were produced. Properties such as resistivity, viscosity, surface tension, adhesion, inkjet printability of the inks, and electrical impedance of the printed electrodes were investigated. Curing temperature directly influenced the electrical properties of the inks. The resistivity obtained varied from 3.3 × 10 0 to 5.6 × 10 −06 Ω.cm. Viscosity ranged from 3.7 to 7.4 mPa.s, which is suitable for inkjet printing fabrication. By using a buffer solution as an analyte, the printed electrode pairs presented electrical impedance lower than 200 Ω for all the proposed designs, demonstrating the potential of the formulated inks for utilization in flexible electronic devices for biological sensing applications. Inkjet printing has been investigated as an alternative production tool for the fabrication of conductive elements and devices in the field of flexible electronics. This fabrication technique deposits particles of the material with desirable electrical properties onto a substrate, after which, the printed pattern is converted into conductive elements 1. There are benefits related to the inkjet printing, namely, a simple fabrication process, low cost, reduction of material waste, and excellent adequacy to several substrates 2-4. This printing process involves the storage of ink in a cartridge and the ejection of an exact amount of material through the nozzles 5. Therefore, the fabrication of flexible circuits, sensors, and other printed materials represents a great technological advancement compared with other standard methods, such as drop casting or stamping 6. Silver remains one of the best options for application as a conductive ink and adhesive, compared to other electrically conductive fillers. This is mainly due to its high electrical and thermal conductivity, chemical stability, relatively low cost (compared to gold or graphene, for example), and the ability of its oxide form to conduct electricity 2. Additionally, silver nanoparticles have a low melting point, which promotes the generation of conductive thin films in relatively low temperatures, this is vital to applications in flexible substrates, such polymers and papers 4,7,8. Different methods can be used for the synthesis and stabilization of silver nanoparticles. One of the most popular approaches is chemical reduction, using a variety of organic and inorganic reducing agents 7,9. Depending on the method used silver nanoparticles can be fabricated with different morpholo...

We report an electrochemical biosensor combined with recombinase polymerase amplification (RPA) for rapid and sensitive detection of severe acute respiratory syndrome coronavirus 2. The electrochemical biosensor based on a multi-microelectrode array allows the detection of multiple target genes by differential pulse voltammetry. The RPA reaction involves hybridization of the RPA amplicon with thiol-modified primers immobilized on the working electrodes, which leads to a reduction of current density as amplicons accumulate. The assay results in shorter “sample-to-answer” times than conventional PCR without expensive thermo-cycling equipment. The limits of detection are about 0.972 fg/μL (RdRP gene) and 3.925 fg/μL (N gene), which are slightly lower than or comparable to that of RPA assay results obtained by gel electrophoresis without post-amplification purification. The combination of electrochemical biosensors and the RPA assay is a rapid, sensitive, and convenient platform that can be potentially used as a point-of-care test for the diagnosis of COVID-19.

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Lactate is an important organic molecule that is produced in excess during anaerobic metabolism when oxygen is absent in the human organism. The concentration of this substance in the body can be related to several medical conditions, such as hemorrhage, respiratory failure, and ischemia. Herein, we describe a graphene-based lactate biosensor to detect the concentrations of L-lactic acid in different fluids (buffer solution and plasma). The active surface (graphene) of the device was functionalized with lactate dehydrogenase enzyme using different substances (Nafion, chitosan, and glutaraldehyde) to guarantee stability and increase selectivity. The devices presented linear responses for the concentration ranges tested in the different fluids. An interference study was performed using ascorbic acid, uric acid, and glucose, and there was a minimum variation in the Dirac point voltage during detection of lactate in any of the samples. The stability of the devices was verified at up to 50 days while kept in a dry box at room temperature, and device operation was stable until 12 days. This study demonstrated graphene performance to monitor L-lactic acid production in human samples, indicating that this material can be implemented in more simple and low-cost devices, such as flexible sensors, for point-of-care applications.