Evaluation of the detection and quantification limits in electroanalysis using two popular methods: application in the case study of paraquat determination

In this work, the reduction reaction of paraquat herbicide was used to obtain analytical signals using electrochemical techniques of differential pulse voltammetry, square wave voltammetry and multiple square wave voltammetry. Analytes were prepared with laboratory purified water and natural water samples (from Mogi-Guacu River, SP). The electrochemical techniques were applied to 1.0 mol L-1 Na2SO4 solutions, at pH 5.5, and containing different concentrations of paraquat, in the range of 1 to 10 mu mol L-1, using a gold ultramicroelectrode. 5 replicate experiments were conducted and in each the mean value for peak currents obtained -0.70 V vs. Ag/AgCl yielded excellent linear relationships with pesticide concentrations. The slope values for the calibration plots (method sensitivity) were 4.06 x 10(-3), 1.07 x 10(-2) and 2.95 x 10(-2) A mol(-1) L for purified water by differential pulse voltammetry, square wave voltammetry and multiple square wave voltammetry, respectively. For river water samples, the slope values were 2.60 x 10(-3), 1.06 x 10(-2) and 3.35 x 10(-2) A mol(-1) L, respectively, showing a small interference from the natural matrix components in paraquat determinations. The detection limits for paraquat determinations were calculated by two distinct methodologies, i.e., as proposed by IUPAC and a statistical method. The values obtained with multiple square waves voltammetry were 0.002 and 0.12 mu mol L-1, respectively, for pure water electrolytes. The detection limit from IUPAC recommendations, when inserted in the calibration curve equation, an analytical signal (oxidation current) is smaller than the one experimentally observed for the blank solution under the same experimental conditions. This is inconsistent with the definition of detection limit, thus the IUPAC methodology requires further discussion. The same conclusion can be drawn by the analyses of detection limits obtained with the other techniques studied.Secretaria de Educacao do Estado de Sao PauloSecretaria de Educacao do Estado de Sao PauloConselho Nacional de Desenvolvimento Cientifico e Tecnologico (CNPq)Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (CNPq

Use of Hg-Electroplated-Pt Ultramicroelectrode for Determining Elemental Sulphur in Naphtha Samples

This paper describes the applicability of a Hg-electroplated-Pt ultramicroelectrode in the quantification of elemental sulphur in naphtha samples by square-wave voltammetry. A reproducible deposition methodology was studied and is reported in this paper. This methodology is innovative and relies on the quality of the mercury stock solution to obtain reproducible surfaces required for the analytical methodology. All analyses were performed using a Hg-electroplated-Pt ultramicroelectrode (Hg-Pt UME) due to the low sensibility of such devices to ohmic drops in resistive solutions. The responses of the peak areas in voltammetric experiments were linear in all of the range studied. The method developed here is accurate and reproducible, with a detection limit of 0.010 mg L-1 and a good recovery range for both standard solutions of elemental sulphur (85 to 99%) and real naphtha sample (79%). These results attest to the potential for the application of this electroanalytical methodology in determining elemental sulphur in naphtha samples containing mercaptans and disulphides.CNPqCNPqCenpes/PetrobrasCenpes/Petrobra

Nanogravimetric study of lead underpotential deposition on selenium thin films as a semiconductor alloy formation procedure

An electrochemical quartz crystal microbalance Au electrode modified with a Se thin film was used to investigate the electrochemical behavior of lead ad-atoms using underpotential deposition (UPD) conditions. A specific quasi-reversible process was observed during the reduction of Pb2+ on Se thin films in perchloric acid media. The charge density of Pb ad-atoms on Se thin film (46.86 mu C cm(-2)) suggests a recovery of 0.1 monolayers, which is in good agreement with EQCM data. The Se thin film can be successfully alloyed with Pb atoms that are deposited by chronoamperometry using time intervals large enough to allow for diffusion toward the inner Se phase. Linear sweep voltammetry combined with EQCM in perchloric acid was used to characterize the amount of Pb absorbed in the Se thin film. These findings offer a new strategy for alloy formation in semiconductor films using UPD as an effective tool to quantify the exact amount of the incorporated metal.Fundacao de Amparo a Pesquisa do Estado de Sao Paulo [FAPESP-Proc. 04/09906-3]Fundacao de Amparo a Pesquisa do Estado de Sao PauloConselho Nacional de Desenvolvimento Cientifico e TecnologicoConselho Nacional de Desenvolvimento Cientifico e Tecnologic

Aplicação de Diagramas V de Gowin como Ferramenta de Avaliação de Aprendizagem em Experimentos Realizados com Simulador Virtual

Em tempos de isolamento social, medida adotada para combater a disseminação do vírus SARS-CoV-2, causador da COVID-19, as aulas presenciais se tornaram remotas e, dessa forma, para garantir a segurança de todos, necessita-se usar ferramentas passíveis de serem aplicadas a distância. Dessa forma, o presente trabalho traz a aplicação de uma atividade avaliativa sobre o ensino de gases para alunos do primeiro semestre do curso de Bacharelado em Química da Universidade de São Paulo. A atividade avaliativa foi composta por inicial experimentação com simuladores virtuais e a aprendizagem foi avaliada usando questionários e diagramas de V de Gowin construídos pelos estudantes, utilizando-se da análise qualitativa dos dados. Após aplicar a atividade, foi possível constatar que o uso do simulador permitiu a compreensão submicroscópica do comportamento dos gases e 97% dos alunos relataram que o uso dessa ferramenta facilita a compreensão dos conteúdos abordados. O uso do diagrama V de Gowin, por sua vez, se apresentou como uma ferramenta importante avaliativa, permitindo verificar se a aprendizagem do aluno se deu de forma significativa, o que não foi possível observar através do uso de um questionário, uma forma de avaliação mais tradicional. Por fim, segundo o relato dos alunos, 90% deles afirmam que a atividade facilita a associação de fenômenos do cotidiano com a teoria estudada e 80% deles desejam mais atividades utilizando metodologias de ensino alternativas tais como as que foram empregadas neste trabalho

Graphene-encapsulated materials: Synthesis, applications and trends

Graphene-based materials (GBM) are an exceptional type of materials that offer unprecedented application capabilities to the scientific and technologic community. The encapsulation of different materials such as drugs, nanoparticles, polymers, oxides and cells by GBM is leading to outstanding hybrid materials with unprecedented behaviours promising a myriad of advantageous applications, including micro/nanomotors, biosensing platforms, bio/imaging agents, drug delivery systems, potential tumour treatment alternatives, environmental remediation platforms, advanced batteries and novel supercapacitors. We present an overview on graphene-encapsulated materials and their most important synthesis pathways. In addition, we explore the synergistic functionalities provided by these composites and highlight the state-of-the-art related to energy, environmental and bio-applications, among others. Finally, we discuss their challenges and future outlooks.This work was supported by: The European Commission Program, FP7-OCEAN, SMS Project (613844) and MINECO (Spain, MAT2014-52485-P, BIO2013-49464-EXP, RTC-2014-2619-7). ICN2 acknowledges support from the Severo Ochoa Program (MINECO, Grant SEV-2013-0295). Nanobiosensors and Bioelectronics Group acknowledges the support from Secretaria d’Universitats i Recerca del Departament d’Economia i Coneixement de la Generalitat de Catalunya (2014 SGR 260). The author SASM thanks FAPESP (Proc. No. 2013/17053-0) and CNPq (Proc. No. 302762/2013-2) for the financial support. The author LFS thanks FAPESP (Proc. No. 2012/08750-6 and 2013/20701-3).Peer Reviewe

The use of mixed self-assembled monolayers as a strategy to improve the efficiency of carbamate detection in environmental monitoring

We report a simple strategy to obtain an efficient enzymatic bioelectrochemical device, in which the enzyme acetylcholinesterase (AChE) was immobilized on gold electrodes functionalized with mixed self-assembled monolayer (SAMmix) of 11-mercaptoundecanoic acid (11-MUA) and 2-mercaptoethanol (C2OH). Development of the modified electrodes included the chemical adsorption of SAMmix on gold surface followed by immersion in AChE solution, resulting in the final Au/SAMmix/AChE configuration. For comparison, the electrochemical performance of Au/11-MUA/AChE and Au/C2OH/AChE electrodes were also investigated. The performance of the modified electrodes toward acetylthiocholine hydrolysis was investigated via cyclic voltammetry and chronoamperometric measurements, revealing a fast increase in anodic current with a well-defined peak upon addition of acetylthiocholine iodide to the electrolytic solution. The anodic currents for Au/SAMmix/AChE electrodes were significantly higher than those for Au/11-MUA/AChE and Au/C2OH/AChE electrodes. The AU/SAMmix/AChE architecture provided by the SAMmix surface promoted a high oxidation current of thiocholine at 0.31 V without the need of electron mediators. The chronoamperometric biosensor developed here provided a linear response to carbaryl in the concentration range of 0 to 1.75 µM. The detection limit and quantification values for carbaryl were found to be 3.45 × 10-10M and 1.15 × 10-9 M, respectively. Michaelis-Menten kinetics, KMapp, of 0.46 mM was obtained, indicating that the electrode architecture employed is advantageous for fabrication of enzymatic devices via physical adsorption process with improvement of the biocatalytic properties.FAPESP (08/00546-5)CNPqCAPE

Enzymatic Solid-Phase Reactor Based on Silica Organofunctionalized with p-Phenylenediamine for Electrochemical Detection of Phenolic Compounds

A new biomaterial, based on silica organofunctionalized with p-phenylenediamine (p-PDA) and the enzyme peroxidase, was used in the development of an enzymatic solid-phase reactor. The analytical techniques used in the characterization showed that the organic ligand was incorporated into the silica matrix. Thus, the silica modified with p-PDA allowed the incorporation of peroxidase by the electrostatic interaction between the carboxylic groups present in the enzyme molecules and the amino groups attached to the silica. The enzymatic solid-phase reactor was used for chemical oxidation of phenols in 1, 4-benzoquinone that was then detected by chronoamperometry. The system allowed the analysis of hydroquinone with a detection limit of 83.6 nmol L-1. Thus, the new material has potential in the determination of phenolic compounds river water samples.FAPESPFAPESP [2010/11049-2, 2010/11567-3

Electrochemical Determination of Norepinephrine by Means of Modified Glassy Carbon Electrodes with Carbon Nanotubes and Magnetic Nanoparticles of Cobalt Ferrite

This study describes the electrochemical preparation of the electrocatalytic oxidation/reduction of noradrenaline in modified glassy carbon of cobalt ferrite nanoparticles and carbon nanotubes (GC/MWCNT/FCo98). The cobalt ferrite powder was characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The optimum conditions found in an electrode composition were 4 μL of cobalt ferrite and 10 μL of carbon nanotubes in 0.1 mol L−1 PBS at pH 7.0. The electrode displays electrochemical behavior in a wide potential range (−0.4 to 1.0 V vs. Ag/AgCl), high conductivity, and electrode stability/durability in 0.1 mol L−1 PBS. Catalytic oxidation of noradrenaline was performed at the unmodified GC electrode at +0.60 V vs. Ag/AgCl and current of 0.17 μA and modified GC with cobalt ferrite nanoparticles and carbon nanotubes at +0.54 V vs. Ag/AgCl and current of 0.23 mA. With regard to the anodic peak current (Ipa) versus noradrenaline concentration by means of the amperometric method at the modified electrode, (which is linear in the 0.16 and 1.91 mmol L−1 concentration range), the concentration limit was 0.76 μmol L−1. In this way, the modified electrode GC/MWCNT/FCo98 was found to be a promising application for the determination of this neurotransmitter in the area of neuroscience

Carbon nanotubes modified with antimony nanoparticles: A novel material for electrochemical sensing

In this study, a novel material for the electrochemical determination of bisphenol A using a nanocomposite based on multi-walled carbon nanotubes modified with antimony nanoparticles has been investigated. The morphology, structure, and electrochemical performance of the nanocomposite electrodes were characterised by field emission gun scanning electron microscopy, energy-dispersive X-ray spectroscopy, and cyclic voltammetry. A scan rate study and electrochemical impedance spectroscopy showed that the bisphenol A oxidation product is adsorbed on nanocomposite electrode surface. Differential pulse voltammetry in phosphate buffer solution at pH 6, allowed the development of a method to determine bisphenol A levels in the range of 0.5-5.0 mu mol L-1, with a detection limit of 5.24 nmol L-1 (1.19 mu g L-1). (C) 2012 Elsevier Ltd. All rights reserved.FAPESPFAPESP [2010/11567-3, 2010/11049-2]CNPqCNP