A comparative study of several screen-printed electrodes modified with electrochemically reduced graphene oxides (ERGO) by means of a constant current has been carried out. The ERGOs were produced from graphene oxides (GO) with different controlled functional groups contents and locations. The experimental reduction conditions were optimized for each particular GO to generate ERGO-modified electrodes with the aim of improving the performance of the dopamine redox process. The ERGO-modified electrodes exhibited a larger electroactive area, a higher reversibility of the redox process, faster kinetics, and greater double-layer capacitance. Analysis of the reduction degree and residual functional groups by means of X-ray photoelectron spectroscopy (XPS) after and before reduction enabled us to determine the relation between the electrochemical behavior and morphology of the electrodes. A hydrazine-reduced graphene oxide showed the best analytical performance with a higher sensitivity (0.259 μA/μM vs. 0.090 μA/μM in the case of a bare electrode), and a linear range from 1-100 μM. It was found that both the experimental reduction conditions and starting graphene material are critical for obtaining a modified electrode with a suitable electrochemical behavior and properties and that a constant current is a suitable technique for the reduction of graphene oxides, especially for screen-printed electrodes.Graphene, a two-dimensional carbon material with highly useful properties including a large surface area and high mechanical, thermal and chemical stabilities, has emerged as a very interesting material for electrochemical applications, especially in (bio)sensors due to its excellent properties such as high electron conductivity, fast heterogeneous electron-transfer rate at the graphene sheet edges and basalplane defect sites, large surface area and good biocompability. 1-4 The most suitable graphene material for modifying electrochemical sensors is graphene oxide (GO) due to its high capacity to be dispersed in aqueous solvents that allows a more efficient electrode modification. GO is usually produced by the oxidation of graphite, a methodology based on the Hummers 5 or Brodie method, 6 which is considered to be the best alternative for the preparation of graphene on a large scale. 7 Additionally, the preparation of GO can be modified by using graphites with different crystalline structures or by different preparation methods 8 giving rise to graphene materials of different sheet size, structure and properties, variables that can also be tuned to improve the electrochemical performance of electrodes.Although several studies on GO improving electrode properties have been reported, 9,10 in most cases a previous GO reduction is carried out in order to decrease the number of oxygen-based functional groups to obtain better electrochemical properties. 11,12 And again the properties of the reduced graphene oxides are influenced not only by the parent GO but also by the reduction process followed. 13 Several authors have report...

The aim of this study was to develop a sensitive lateral flow immunoassay (LFIA) for the rapid detection of Escherichia coli (E. coli) O157:H7, a pathogen contributor to diseases and fatalities worldwide. Au nanoparticles with high stability, uniform size, and shape were synthesized and coated with heterobifunctional PEG polymer with carboxyl groups, and they were bioconjugated to be used as label in sandwich-LFIA. Then, a silver enhancement strategy was developed as an accessible, rapid, and cost-effective approach for signal amplification to reduce the limit of detection (LOD). The optimal results were achieved when a solution of silver nitrate and hydroquinone/citrate buffer was added to the strips for 4 min. This led to a decrease in the visual LOD from 2 × 106 (CFU mL−1) to 2 × 103 (CFU mL−1), resulting in a threefold improvement in sensitivity compared to the conventional LFIA system. The specificity of the system was evaluated by using non-target bacteria (E. coli BL21 and E. coli T515) and its reliability was determined by testing commercial food samples (milk, tap water, and orange juice), demonstrating its effectiveness for quickly detecting pathogenic bacteria in food products. Graphical Abstract

Heavy metals have a huge impact on the environment due to their toxicity and bioaccumulation capacity. A great variety of methods can be used to the determination. The electrochemical methods are one of the best options due to their sensitivity, selectivity, and rapidness. They are based on the use of electrochemical cells made of different materials depending on the analyte to determine. An eco-friendly cheap option that has earned big importance is paper-based electrodes, which are formed by a cellulose matrix modified with conductive inks. Carbon ink is the most used, and it can be modified with nanoparticles to increase sensitivity. Alternatively, metallic surfaces or “films” such as mercury or bismuth can improve the determination of heavy metals because of their interactions with the film. This chapter focuses on the methods to determination of heavy metals based on their affinity with different nanomaterials or films on low-cost electrode substrates.