Diego A. Peixoto’s research while affiliated with Federal University of Uberlândia and other places

Monitoring nitrate in aquatic systems is of fundamental importance since its presence at high levels can result in adverse effects on human health. Thus, in this work, manufactured carbon black (CB)/polylactic acid (PLA)-based 3D-printed electrochemical sensors modified with electrodeposited silver particles (AgPs) for nitrate analysis in real water samples. Raman and FT-IR spectra, scanning electron microscopy images, and analysis by energy-dispersive spectroscopy confirmed the presence of AgPs on the porous carbonaceous surface. The preliminary electrochemical studies showed that using the modified electrode (CB-PLA/AgPs) an incredible increase in the electrochemical response (around 16.5-fold in terms of current density) was obtained for the electroreduction of nitrate at around − 1.08 V vs. Ag|AgCl|KCl(sat.) compared to the unmodified electrode (CB-PLA). The linear sweep voltammetry technique was employed whose instrumental parameters have been carefully optimized. Under optimized conditions, a linear range between 5 and 80 mg L⁻¹ (R² > 0.99) was achieved, with a detection limit of 2.7 mg L⁻¹, which is below the maximum level permitted (50 mg L⁻¹) by the World Health Organization (WHO). Repeatability (intra-electrode, n = 8) and reproducibility (inter-electrode, n = 3) studies were performed, and RSDs < 2.1% were found, demonstrating good precision of the analysis and reproducible manufacturing process of the sensors. Moreover, the proposed sensor proved to be selective in the presence of other inorganic compounds frequently found in environmental waters. Importantly, in the recovery tests, percentage values between 91 and 117% confirmed the accuracy and reliability of the analyses. Thus, the developed strategy can be useful for nitrate sensing in real water samples in remote locations.

Metal hexacyanoferrates (HCF), which are produced by hydrothermal synthesis, have received increasing attention for different applications due to their unique physicochemical properties, such as three-dimensional framework with large open sites and metal sites with different oxidation states. The hydrothermal method is a simple and established synthesis technique, with the ability to synthesize unstable materials near the melting point, the possibility to synthesize large and good quality crystals and to obtain tunable size particles, controlled agglomeration, and low contamination by impurities, and in which the properties of products depend on the formation mechanism and hydrothermal conditions. Focusing on the structure of HCF including crystallography and morphology from the literature, this review discusses the effects of synthesis conditions (metal precursor, hydrothermal temperature, and duration) and post-treatment (acid addition and calcination) on the formation of metal hexacyanoferrates. Furthermore, structural modifications and current progress toward the application of HCF as catalysts, sensors, adsorbents, electrochromic displays, and energy storage are also summarized in this review.

Zn1-xNixO and Zn1-xFexO structures were synthesized by the microwave-assisted hydrothermal method. The best photocatalytic degradation of rhodamine B (RhB) and 4-nitrophenol (4-NP) were achieved by the Zn0.96Ni0.04O and Zn0.99Fe0.01O. The specificity of each dopant showed significance in the positions of the impurity energy levels, which ended up influencing the electron-hole separation and transport, as demonstrated by the photoluminescence emissions. The morphological analysis revealed that besides inhibiting the growth of particles, the incorporation of dopant ions into the ZnO lattice triggered a nucleation process, consequently changing their morphology. Density functional theory (DFT) calculations showed that the Fe³⁺ 3d orbitals generate energy levels below the conduction band (CB) while for Ni²⁺, the levels were found to be spread in a broad energy range above the valence band (VB). The synergistic effect of band gaps alteration, inhibition of electron-hole pair recombination and appearance of new trapping energy sites justifies the superior photocatalytic activity.

This study reports on the effect of the alkali source in pure ZnO and Ag-doped ZnO samples formation quickly synthesized using the microwave-assisted hydrothermal (MAH) method. The flower-like morphology of ZnO and Ag-doped ZnO samples was controlled by the presence of ammonia under reaction conditions, which provided slower ZnO nucleation. In the presence of sodium hydroxide, the precursor species were instantaneously formed, promoting a rapid ZnO formation and the obtainment of Ag0 in the Ag-doped ZnO sample. The presence of Ag+ ions into ZnO wurtzite structure created defects, contributing to a local structural disorder, as observed by Raman spectra. All the samples studied herein presented antifungal activity for Saccharomyces cerevisiae, which was influenced because of the presence of Ag+ ions into ZnO lattice and the morphologies of the samples.