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The Pechini synthesis method, presents chemical homogeneity as its main advantage, enabling the desired phase to be obtained. This synthesis usually occurs at pH 7. In this study, Bi2Fe4O9 was synthesized using the Pechini method at different pHs (pH= 3, 5, 7, 8, 10 and 12) to verify if the pH conditions can influence in its photocatalytic activity. The materials obtained at different conditions were characterized by using X rays diffraction (XRD), UV/Vis diffuse reflectance and scanning electron microscopy by field emission (MEV/FEG).It was possible to synthesize the material under conditions already mentioned, but it was not observed significant differences in the chemical structure of the obtained materials from the characterization techniques. However, they presented a small variation in the band gap values, morphological aspects and photocatalytic activities. All the materials proved to be a good semiconductor, maintaining high degradation rates. The photocatalyst synthesized at pH 3 (BFO - 1) presented a degradation rate of approximately 95% for the group chromophore and 93% for the aromatic group of the structure of the tartrazine yellow dye, proving to be a promising photocatalyst for industrial applications. This result is particularly important because BFO – 1 showed higher catalyst activity since the chromophore of the aromatic group was degraded. The highlights of this study is the synthesis of Bi2Fe4O9 performed at pH=3, since it reduces the quantity of reagents comparing to the conventional Pechini method and presented high photocatalytic activity. DOI: http://dx.doi.org/10.17807/orbital.v13i2.1486

This paper describes the preparation and characterization of CG/Co(OH)2 electrode obtained by electrodeposition of cobalt hydroxide on expanded carbon graphite electrode (CG) for potential application as energy storage devices. The CG electrodes used in this work were obtained from exhausted batteries. Firstly, the CG electrode was submitted to an anodic polarization at 2 V in H2SO4 solution, in order to increase the active surface area at different times. Thereafter, the electrodeposition of the Co(OH)2 was carried out by applying different reduction potentials over different times in order to optimize the electrodeposition process. Scanning electron microscopy, X-ray diffraction and Raman spectroscopy results confirmed the expansion of the bare CG and the Co(OH)2 electrodeposition. Cyclic voltammetry, galvanostatic charge/discharge curves and electrochemical impedance spectroscopy were used to evaluate the electrochemical performance of the modified electrode. It was observed that the parameters of the modification process affect the morphology of the electrodeposited cobalt hydroxide. The best performance was found for the CG/Co(OH)2 modified electrode obtained by CG expanded during 750 s, followed by the electrodeposition conditions of −1.25 V during 250 s, presenting a capacitiy of 3.4 C cm−2 in an applied current density of 1.0 mA cm−2. In addition, this modified electrode also presented a retention capacity of 91% after 1400 cycles. Keywords: Carbon graphite, Cobalt hydroxide, Energy storage device

Efforts have been made on the development of new modified electrodes to be used in the fast determination of neurotransmitters, either in commercial drugs or in biological samples. Determination of dopamine (DA), for example, is of great importance since the lack of this neurotransmitter is related to many neurological disorders, including Parkinson’s and Alzheimer’s diseases. In this paper, we present a detailed electrochemical characterization, as well as DA detection studies of paste electrodes incorporating carbon materials in different allotropic forms, including carbon black modified with intrinsically conducting polymers (Eeonomers®), pristine carbon black, graphite, and carbon nanotubes. Emphasis is put on the smaller particle size and larger specific surface area of CB Eeonomers® materials, which led to an improved electroanalytical performance for the developed devices. The electrodes fabricated with Eeonomers® modified with polyaniline exhibited the higher current response towards DA detection, in addition to the ability of distinguishing DA from its natural interferent, ascorbic acid. Furthermore, a central composite design was used to investigate the influence of pH and electrode composition (proportion of Eeonomers®) on the electrochemical sensing of DA. A greater sensitivity was achieved for 50:50 (w/w) KP20/KPy20 electrode at pH 7.0. The optimized devices showed to be promising tools to perform quick, cheap and sensitive detection of this neurotransmitter in bioanalytical systems.

In the present work, gold nanoparticles were synthesized through a green route by using, for the first time, polysaccharides extracted from pineapple gum (PG) as the reducing and capping agent. The obtained nanoparticles (AuNPs-PG) were characterized by UV-VIS, FTIR, TEM, FESEM, EDX, XRD, and zeta potential measurements, which confirmed that PG was effective to produce AuNPs with an average diameter of 10.3 ± 1.6 nm. The AuNPs-PG were employed as the modifier of glassy carbon paste electrodes (CPE/AuNPs-PG), which were applied as sensitive electrochemical sensors to the determination of the antihistamine drug promethazine hydrochloride (PMZ). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements showed that the AuNPs-PG could enhance the electronic transfer properties of the glassy carbon paste, which was due to their large surface area and high electrical conductivity. After optimization of the instrumental parameters of square wave voltammetry (SWV) through a Box-Behnken factorial design, a linear relationship between the anodic peak current and PMZ concentration was obtained in the range from 2.0 to 15.7 μmol L−1 in McIlvaine buffer solution pH 5.0. The detection and quantification limits were found to be equal to 1.33 and 4.44 μmol L−1, respectively. The developed sensors could successfully quantify PMZ in different commercial pharmaceutical formulations, with satisfactory levels of accuracy and precision. In addition to improving the analytical features of the electrodes, hemocompatibility assays carried out on erythrocytes and leukocytes showed that the AuNPs-PG do not exhibit toxic effects on the referred cells. This interesting behavior enables their use in biocompatible electrochemical sensing platforms as well as for future biomedical investigations.

A insercao de oxidos mistos em eletrodos de carbono cerâmico a partir do processo sol-gel pode proporcionar materiais com alta pureza e homogeneidade, sem a segregacao de fases e alta estabilidade, combinando as propriedades mecânicas e quimicas da silica gel com as propriedades quimicas dos oxidos metalicos (MxOy). Esse recurso pode fornecer ao dispositivo propriedades interessantes, uma vez que a distribuicao homogenea dos oxidos, pode minimizar a resistividade proporcionada pela rede de silica. Alem disso, a insercao de sitios acidos possibilita a imobilizacao de diferentes mediadores de eletrons, os quais permanecem estaveis sobre a superficie dos ECC, aumentando a seletividade e sensibilidade de tais eletrodos. Desta forma, neste trabalho teve-se como objetivo incorporar o pentoxido de niobio (Nb2O5) no ECC/SiO2-SnO2, de modo reduzir ainda mais a resistencia eletrica interna da matriz de silica, obtendo-se o eletrodo de carbono cerâmico, designado como ECC/SiO2-SnO2-Nb2O5. O material obtido foi caracterizado por espectroscopia de FTIR, a qual indicou que a rede de SiO2 e pouco perturbada pelo incorporacao dos oxidos, mostrando evidencias da formacao da ligacao Si-O-M (onde M=Sn ou Nb), a qual foi corroborada pela tecnica de XPS. Espectros Raman evidenciaram um aumento da relacao de intensidade entre as bandas D e G (ID/IG), indicando que a medida que os oxidos sao inseridos na estrutura da SiO2, ocorre um aumento do grau de desordem do sistema. A partir das tecnicas morfologicas, MEV e EDS evidenciou-se que o Sn e o Nb mostraram-se bem distribuidos pela superficie. O ECC/SiO2-SnO2 e o ECC/SiO2-SnO2-Nb2O5 apresentaram melhores respostas voltametricas que os sistemas sem a adicao destes (ECC/SiO2), apresentando um processo difusional de transferencia de massa e com respostas de corrente estaveis apos 100 ciclos voltametricos, com variacao de Ipa e Ipc abaixo de 5%. Tais resultados demonstram a potencialidade de utilizacao destes materiais no desenvolvimento de sensores.

Este artigo descreve a construcao de um eletrodo de baixo custo e reutilizavel, utilizando eletrodo de carbono grafite (CG) obtido de baterias de zinco-carbono exauridas, modificados com microparticulas de ouro. O eletrodo de CG foi modificado pela eletrodeposicao potenciostatica de microparticulas de ouro e aplicado como sensor eletroquimico para deteccao simultânea de dopamina (DA) e acido urico (AU). Os efeitos do tempo de deposicao, potencial e concentracao do precursor tetracloroaurato(III) de hidrogenio (HAuCl4.3H2O) na formacao do eletrodeposito foram sistematicamente estudados, utilizando microscopia eletronica de varredura (FEG-SEM), difracao de raios X (XRD) e metodos eletroquimicos incluindo voltametria ciclica e cronoamperometria. Os resultados confirmaram que as particulas de ouro possuem boa atividade eletrocatalitica para a deteccao eletroquimica simultânea de dopamina e acido urico na faixa de 9,9 a 90,0 mol L-1 para DA e na faixa de 0,13 a 0,51 mmol L-1 para AU com limites de deteccao de 2,6 μmol L-1 e 58,9 μmol L-1, respectivamente. Esses resultados indicaram que o eletrodo CG reutilizado de baterias exauridas alem de ser ambiental correto, apresentar baixo custo tem potencial aplicacao como sensor eletroquimico.

A silsesquioxane based on a silica matrix and 4-(aminomethyl)pyridine group was successfully synthesized using the sol–gel process with the proposal of two different carbonic chain arrangments in the structure of the silica polymer, observed employing NMR techniques. The silsesquioxane proposed succeeds an improvement of the adsorption of monomer electroactive species on the surface of the electrode obtained through films assembly, this fact was attributed to the presence of two coordinating nitrogen in new silsesquioxane. In this study, the Layer-by-Layer (LbL) technique was shown suitable to be used to produce electroactive electrodes with indium tin oxide electrode (ITO) coated by tetrasulfonated nickel phthalocyanine (NiTsPc) alternated with the silsesquioxane. Voltammetric techniques were used to investigate the electrochemical behavior of the nitrite oxidation, which exhibited an enhanced current response at the modified electrode. The proposed electroddisplayed a current response which varied linearly with the concentration of nitrite in the range 0.113 to 0.860 mmol L−1 with a detection limit of 0.026 mmol L−1, a sensitivity of 32.06 µA (mmol L−1) and relative standard deviation of approximately 3%.

In this work, we report the construction, characterization, and the potential application of carbon-ceramic electrodes prepared with functionalized multi-walled carbon nanotubes (CCE/f-MWCNT) to detect two sulfonamide drugs: sodium sulfacetamide (SFC) and sulfadiazine (SFD). Prior to the construction of CCE/f-MWCNT, MWCNT samples were functionalized by using an acid treatment (HNO3/HClO4), and then characterized by FTIR spectroscopy, XRD, FESEM, and Raman spectroscopy. These techniques were also employed to characterize the prepared carbon-ceramic electrodes, and the obtained results demonstrated that the MWCNT were properly dispersed in the silica matrix. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy experiments showed that the f-MWCNT could enhance the electrochemical performance of the electrodes, as a consequence of its high electrical conductivity and large surface area. The antibacterial drugs were determined with CCE/f-MWCNT by employing CV and differential pulse voltammetry (DPV). A linear relationship between DPV peak currents and SFC and SFD concentrations was verified separately in the range from 9.9 to 177.0 µmol L− 1 in 0.04 mol L− 1 BR buffer solution pH 6.0. Detection and quantification limits were found to be 1.06 and 3.54 µmol L− 1 for SFC and 2.31 and 7.71 µmol L− 1 for SFD, respectively. Besides the good sensitivity, the CCE/f-MWCNT showed reproducibility and specificity, since its response was not significantly affected by the presence of electroactive interfering compounds. The promising analytical performance of the CCE/f-MWCNT was confirmed by determining SFC and SFD in commercial veterinary formulations, with percent recoveries ranging from 99.9 to 101.1% for SFC and from 99.8 to 102.2% for SFD.

The application of carbon paste electrodes modified with porphyran-capped gold nanoparticles (CPE/AuNps-PFR) to detect an important anticancer drug, 5-fluorouracil (5-FU), is described. Gold nanoparticles (AuNps) were synthesized through a green one-pot route, by using porphyran (PFR) (a sulfated polysaccharide extracted from red seaweed) as reducing and stabilizing agent. The reaction temperature and the concentrations of AuCl4− and PFR for AuNps-PFR synthesis were optimized by using a 23 full factorial design with central point assayed in triplicate. The smallest particle size (128.7 nm, obtained by DLS) was achieved by employing a temperature of 70 °C and AuCl4− and PFR concentrations equal to 2.5 mmol L−1 and 0.25 mg mL−1, respectively. The AuNps-PFR nanocomposite was characterized by UV–vis spectroscopy, FTIR, DLS, TEM, XRD and zeta potential, which proved that PFR was effective at reducing and capping the AuNps. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) experiments showed that the nanocomposite could enhance the electrochemical performance of the electrodes, as a consequence of the high conductivity and large surface area presented by the AuNps. The CPE/AuNps-PFR was able to electrocatalyze the oxidation of 5-FU by CV and differential pulse voltammetry (DPV). A linear relationship between the DPV peak currents and 5-FU concentration was verified in the range from 29.9 to 234 μmol L−1 in 0.04 mol L−1 BR buffer solution pH 8.0. Detection and quantification limits were found to be 0.66 and 2.22 μmol L−1, respectively. Besides the good sensitivity, CPE/AuNps-PFR showed reproducibility and did not suffer significant interference from potentially electroative biological compounds. The good analytical performance of the modified electrode was confirmed for determining 5-FU in pharmaceutical formulations, with good percent recoveries (ranging from 96.6 to 101.4%) and an acceptable relative standard deviation (RSD = 2.80%).

In this manuscript, we describe the synthesis and electrochemical characterization of polyaniline—gum arabic nanocomposites and graphene oxide (PANI-GA/GO) modified electrodes with a detailed study concerning their supercapacitive properties. The electrode modification was carried out by using the Layer-by-Layer technique (LbL), where the PANI-GA nanocomposite dispersion was used as polycation and the GO colloidal dispersion as polyanion. The bilayer growth was followed by both UV–vis spectroscopy and cyclic voltammetry, and an increase in the characteristic PANI absorption and in the electrochemical signal was verified, confirming the electrode build up. Galvanostatic charge-discharge curves (GCDC) were performed to evaluate the supercapacitive properties of the modified electrodes, these results showed the dependence of the specific capacitance with the number of bilayers, where values of CS around 15 mF cm−2 (i = 0.1 mA cm−2) were found. Electrochemical impedance spectroscopy confirmed the pseudocapacitive properties of the modified electrodes, showing an increase in the low-frequency capacitance with the number of bilayers. Hereby the (PANI-GA/GO)-LbL electrodes were shown to be good candidates for active materials in supercapacitors.

In this paper, the 3- n -propylpyridinium silsesquioxane chloride (SiPy + Cl − ) polymer was used as a stabilizing agent for the synthesis of gold nanoparticles (AuNps). The formation of AuNPs-SiPy + Cl − was confirmed by UV–vis spectroscopy from the plasmon band at 521 nm, which showed good distribution with size of about 5.0 nm, observed by transmission electron microscopy (TEM) and good stability (ζ = + 38.5 mV) obtained by zeta potential experiments. The surface of the glassy carbon electrode (GCE) was modified with gold nanoparticles (AuNPs-SiPy + Cl − ) and subsequent formation of thiolactic acid (TLA) self-assembled monolayer (SAM). For the biosensor construction, horseradish peroxidase (HRP) was covalently immobilized on the surface modified with SAM. The steps of the formation of this biosensor was confirmed by electrochemical impedance spectroscopy (EIS) and field-effect scanning electron microscopy (SEM-FEG). The GCE/AuNps/TLA/HRP biosensor was applied for the detection of cathecol (CT). Under optimized experimental conditions, the biosensor showed an excellent electrocatalytic activity for CT in presence of 0.03 mmol L −1 H 2 O 2 in the range of 6.0 at 46.0 μmol L −1 , with a low detection limit (LOD = 0.852 μmol L − 1) and good sensitivity (0.026 μA.mol L −1 ). The modified electrode displayed good reproducibility for the determination of catechol and long-term stability of approximately 25 days. The formation of a TLA monolayer on the gold nanoparticles modifying GCE provided a suitable, simple, and low cost platform that could effectively be used to immobilize different enzymes.

The preparation of a carbon ceramic electrode modified with SnO2 (CCE/SnO2) using tin dibutyl diacetate as precursor was optimized by a 23 factorial design. The factors analyzed were catalyst (HCl), graphite/organic precursor ratio, and inorganic precursor (dibutyltin diacetate). The statistical treatment of the data showed that only the second-order interaction effect, catalyst × inorganic precursor, was significant at 95% confidence level, for the electrochemical response of the system. The obtained material was characterized by scanning electron microscopy (MEV), X-ray diffraction (XRD), RAMAN spectroscopy, XPS spectra, and voltammetric techniques. From the XPS spectra, it was confirmed the formation of the Si–O–Sn bond by the shift in the binding energy values referred to Sn 3d3/2 due to the interaction of Sn with SiOH species. The incorporation of SnO2 provided an increment of the electrode response for levofloxacin, with Ipa = 147.0 μA for the ECC and Ipa = 228.8 μA for ECC/SnO2, indicating that SnO2 when incorporated into the silica network enhances the electron transfer process. Under the optimized working conditions, the peak current increased linearly with the levofloxacin concentration in the range from 6.21×10−5 to 6.97×10−4 mol L−1 with quantification and detection limits of 3.80×10−5 mol L−1 (14.07 mg L−1) and 1.13×10−5 mol L−1 (4.18 mg L−1), respectively.

This paper reports the electrocatalytic properties of layer by layer (LbL) films formed by gold nanoparticles stabilized in propylpyridinium silsesquioxane polymer (AuNPs-SiPy + ) as polycation, and prussian blue nanoparticles (PB) as polyanion with synergistic effect in the H 2 O 2 reduction. Under optimized synthesis conditions by factorial design, the TEM images of gold nanoparticles showed spherical shape with diameter ranging from 2 to 35 nm. All modified electrodes with inorganic materials were explored by spectroscopy (UV-Vis and FTIR) and electrochemical techniques (Cyclic Voltammetry, Chronoamperometry and Electrochemical Impedance Spectroscopy). We demonstrated that the presence of gold nanoparticles greatly enhanced the electroactive properties of the multilayer films providing interesting electrochemical features and excellent sensitivity in the electrocatalytic reduction of hydrogen peroxide, compared to electrode containing only the silsesquioxane polymer as cationic polyelectrolyte.

The interaction of surfactants and hydrophobic polymers in solution has attracted substantial interest due to the broad application in industry. In this study, the interactions of the nonionic surfactant polyethylene glycol-p-(1,1,3,3 tetramethylbutyl)-phenyl ether (Triton X) and anionic surfactant sodium dodecyl sulfate (SDS) with a cationic polymer 3-chloride-n-propylpyridinium silsesquioxane (SiPy+Cl−) and their effects on the layer-by-layer (LbL) film assembly with nickel tetrasulfonated phthalocyanine (NiTsPc) have been analyzed by surface tension, viscosity, and Brewster angle microscopy. As Triton X and SDS surfactants are inserted in the solution containing the polycation SiPy+Cl−, they are immediately incorporated in the hydrophobic nanodomains present in the silsesquioxane skeleton. The association between polyelectrolytes and oppositely charged surfactants starts at very low surfactant concentrations far below the critical micelle concentration, named critical aggregation concentration (cac), which was determined for both surfactants in the presence of SiPy+Cl− (cac Triton X = 2.4 × 10−6 and SDS = 8.2 × 10−7 mol L−1). The coexistence of mixed and pure micelles containing only Triton X and SDS also occurs in higher surfactant concentrations. The implication of these aggregates adsorption on the surface was analyzed in thin films by using the LbL technique. Multilayered films were obtained in presence of surfactants, (T-SiPy/NiTsPc) n or (S-SiPy/NiTsPc) n to compare with LbL films in absence of surfactant (SiPy/NiTsPc) n , and these films were characterized by UV–Vis, FTIR, and Raman spectroscopies to analyze the molecular interactions and to investigate the presence of specific interactions between the polyelectrolytes. The formation of LbL film in the architectures (T-SiPy/NiTsPc) n or (S-SiPy/NiTsPc) n is more efficient in relation to (SiPy/NiTsPc) n LbL films since the presence of the surfactants causes a greater amount of material to be deposited on each bilayer formed. The results of UV–Vis indicate that the film in the presence of SDS has a higher amount of material added to each bilayer. The electrochemical studies of (S-SiPy/NiTsPc)3 film show that the charge transfer resistance is lower than the one observed for the other architectures. Thus, the characteristic hopping electron effect of LbL film may be responsible for lower capacitance and lower resistance of the films. When the surfactant SDS is present, a higher oxidation peak current of dopamine is seen.

This work describes the electrochemical characterization toward supercapacitive properties of a modified electrode based on MnO2 electrodeposited onto carbon felt electrode (CFE/MnO2). Raman analysis confirmed the electrode modification and FEG-SEM images showed a 3D network structure with a homogeneous deposit of the MnO2 film. The morphology of the electrodeposited film showed to be dependent on the electrodeposition applied potential and time. Cyclic voltammetry and galvanostatic charge/discharge curve results showed that the charge storage process is reversible and a combination of EDLC and pseudocapacitive behavior. Under optimized conditions (1.2 V and 600 s), the modified electrode presented a specific capacitance of 541 F g−1 in an applied current density of 0.2 A g−1, which was attributed to a favored accessibility of the electrolyte on the film porous due to morphological issues. Besides, the modified electrode revealed a good capacitance retention of 80% in an applied current density of 1.0 A g−1 after 1000 cycles.

This paper describes the synthesis of starch-stabilized silver nanoparticles (AgNPs-St) for the development of electrochemical sensors. Layer-by-Layer (LbL) films were evaluated with the AgNPs-St, as polyanion, alternating with 3- n -propylpyridinium silsesquioxane chloride (SiPy + Cl − ), as polycation, obtaining the (SiPy + Cl-/AgNPs-St) n where n is the bilayers number. UV–vis characterization of (SiPy + Cl − /AgNPs-St) n LbL films showed a linear increase of the absorbance of surface plasmon resonance band at 400 nm with bilayers number. Atomic force microscopy revealed an increase in thickness of the (SiPy + Cl − /AgNPs-St) n LbL films from n = 2 to n = 10. These LbL films were employed for the electroanalytical determination of triiodide using the differential pulse voltammetry (DPV). The DPV parameters were optimized by 2 3 factorial design. Two linear ranges were found with detection limits of 5.56 × 10 −6 and 1.51 × 10 −5 mol L −1 and quantification limits 1.85 × 10 −5 and 5.04 × 10 −5 mol L −1 .

This manuscript presents the electrochemical impedance spectroscopy study regarding the influence of the pH and the range of the applied potential on the IR drop of polyaniline – Gum arabic/graphene oxide (PANI-GA/GOx) electrodes modified by the Layer-by-Layer technique. Using the equivalent circuit modeling methodology, it was discussed the IR drop dependence on parameters such as series and charge-transfer resistances and the double-layer capacitance. Series resistance related to the (PANI-GA/GOx) film conductivity showed to be very influenced by the pH, where a lower value of pH favored the protonation of the polyaniline-based film, increasing the overall conductivity leading to lower values of IR drop. Moreover, the IR drop also showed to be dependent on the range of the applied potential, reflecting in a change at the electrode/electrolyte interface, where an increase at the charge-transfer resistance was found at higher potentials, probably due to the pernigraniline state of PANI.

Nitrogen-doped carbon nanotubes (N-CNTs) were synthesized at various temperatures ranging from 650 to 950 °C, at 100 °C intervals, by chemical vapor deposition technique (CVD). The synthesized N-CNTs were employed as modifiers of glassy carbon paste electrodes (GCPE), which were further used as electrochemical sensors for the determination of dopamine (DA) and epinephrine (EP), two important catecholamines that display biological roles as neurotransmitters and hormones. The results revealed that electrodes modified with the N-CNTs synthesized at 650 °C (GCPE/N-CNT650) and 950 °C (GCPE/N-CNT950) presented better electrocatalytic activities and sensing capabilities than the others (GCPE/N-CNT750 and GCPE/N-CNT850). N-CNT950 had the highest graphitization and the highest powder conductivity, whereas N-CNT650 had the lowest graphitization and the lowest powder conductivity. However, both N-CNT950 and N-CNT650 had the highest nitrogen contents, 3.5 and 3.8 at.%, respectively, which probably enhanced their number of electroactive sites to interact with DA and EP molecules. This result is of high significance, because the use of the lowest temperature (650 °C) in the CVD process, yielded N-CNTs with improved electrocatalytic activity and less energy consumption during CVD.

Apolipoprotein E4 (ApoE4) has a key role on the onset and progression of Alzheimer's disease (AD), since it favours the deposition of toxic amyloid-beta (Aβ) aggregates in the brain. These effects might result from the interaction between ApoE4 and specific DNA promoters related to cellular autophagy pathways and to the expression of neuroprotective proteins, like sirtuin-1. Herein, we modified gold electrodes with mixed self-assembled monolayers of 6-mercapto-1-hexanol and thiolated DNA oligonucleotides related to CLEAR (associated with autophagic processes that enable the clearance of toxic species, such as Aβ) and SirT1 (related to the expression of sirtuin-1) promoter sequences. The interactions of the immobilized DNA sequences with isoforms of ApoE (ApoE4/ApoE3/ApoE2) were investigated by differential pulse voltammetry (DPV) and electrochemical impedance spectroscopy (EIS) measurements. By monitoring current and charge transfer resistance (Rct) variations, CLEAR showed to interact specifically with ApoE4, whereas SirT1 showed a higher affinity to ApoE4 compared to ApoE3 and ApoE2. To the best of our knowledge, this is the first report about the application of electrochemical techniques to investigate the sequence-specific interaction between ApoE isoforms and CLEAR and SirT1 oligonucleotides.

Pediatric adrenocortical carcinoma (pACC) is a rare and aggressive malignancy of high occurrence in Southern Brazil. pACC is characterized by the usual overproduction of dehydroepiandrosterone sulfate (DHEAS), whose detection in serum or plasma can be effective to the early diagnosis of the disease. Therefore, the present paper reports, for the first time, the construction and application of a label-free impedimetric immunosensor to detect DHEAS, which was based on the modification of an oxidized glassy carbon electrode with arginine-functionalized gold nanoparticles (AuNPs-ARG) and anti-DHEA IgM antibodies (ox-GCE/AuNPs-ARG/IgM). AuNPs-ARG was synthesized by a green route, and characterized by UV–VIS spectroscopy, FTIR, TEM, DLS, and XRD. The construction of ox-GCE/AuNPs-ARG/IgM was optimized through factorial design and response surface methodology. Cyclic voltammetry and electrochemical impedance spectroscopy measurements were employed to characterize the optimized immunosensor. The DHEAS detection principle was based on the variation of charge transfer resistance (∆Rct) relative to the Fe(CN)64−/3− electrochemical probe after immunoassays in the presence of the biomarker. A linear relationship between ∆Rct and DHEAS concentration was verified in the range from 10.0 to 110.0 µg dL−1, with a LOD of 7.4 µg dL−1. Besides the good sensitivity, the immunosensor displayed accuracy, stability, and specificity to detect DHEAS. The promising analytical performance of ox-GCE/AuNPs-ARG/IgM was confirmed by quantifying DHEAS in real patient plasma samples, with results that were comparable to the reference chemiluminescence assay. Our results suggest that the presented immunosensor can find clinical applications in the early diagnosis of pACC and to monitor DHEAS levels in other adrenal pathologies.

This paper describes the preparation of carbon paste electrodes (CPE) modified with porphyran (CPE/PFR), a sulfated polysaccharide extracted from red algae, and its application for the electroanalytical determination of sodium sulfacetamide (SFC) and sulfadiazine (SFD) by square wave voltammetry (SWV). The CPE/PFR was characterized by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in presence of the [Fe(CN) 6 ] 3-/4- redox probe, and the electrochemical behavior of the drugs on the modified electrode surface was studied by CV. Experimental conditions such as pH and voltammetric parameters were optimized in order to achieve the best analytical current signal. The CPE/PFR showed higher current density values ( j ) and lower charge transfer resistance compared to the unmodified CPE, suggesting that the PFR had a positive effect on the electrode response. A good linear relationship between the oxidation peak current and the concentration of SFC and SFD was obtained by SWV for the CPE/PFR over the range of 4.98 to 47.6 μmol L -1 for both drugs, in BR buffer solution 0.04 mol L -1 by using SWV. Detection limit (DL) and quantification limit (QL) were found to be 0.75 and 2.51 μmol L -1 for SFC and 1.47 and 4.91 μmol L -1 for SFD, respectively. Besides the good sensitivity, the CPE/PFR showed several advantages such as stability, ease of preparation and good reproducibility. The promising analytical performance of the developed sensor was finally confirmed at determining the studied drugs in pharmaceutical formulations.

Graphene oxide (GO) obtained by electrochemical exfoliation method was used to prepare a composite with the inorganic polymer, 3-n-propylpyridinium silsesquioxane chloride polymer, SiPy + . This composite (SiPy + (GO)) was applied as polyelectrolyte to form LbL films with nickel tetrasulfophthalocyanine (NiTsPc 4 − ) as polyanion. The absorbance of the NiTsPc 4 − Q bands linearly increased with the number of deposited bilayers (SiPy + (GO)/NiTsPc 4 − ) n LbL films. From the graphics of absorbance as a function of the number of bilayers, it was possible to observe that the amount of NiTsPc 4 − deposited on the FTO substrate is higher to the SiPy + (GO)/NiTsPc 4 − than SiPy + / NiTsPc 4 − films, as a consequence of the effective π–π interaction between the graphene and phthalocyanine molecules. The Raman spectra of the film with graphene oxide, SiPy + (GO)/NiTsPc 4 − showed the GO “D” bands at 1339 cm − 1 , and the “G” bands at 1556 cm − 1 . The presence of GO in the SiPy + (GO)/NiTsPc 4 − film was confirmed by the presence of “2D” band at 2686 cm − 1 , which is not observed in the SiPy + /NiTsPc 4 − film. Cyclic voltammetric and EIS studies showed that the presence of graphene oxide nanoparticles in the film configuration considerably increased its conductivity when compared to the film without GO (SiPy + /NiTsPc 4 − ) which favors its application for the development of electrochemical sensors.

This paper describes the construction, evaluation, and application of an electrochemical biosensor for the determination of carbaryl. The biosensor was constructed using the self assembled monolayer technique with gold substrate modified with nanostructured monolayer of fourth-generation poly(amidoamine) with a cystamine core (PAMAM-G4) and covalent immobilization of acetylcholinesterase enzyme (AChE). The surface coverage was investigated by cyclic voltammetry and linear voltammetry. The results of partial surface coating and electrochemical desorption showed that the studied time interval of 12 h is necessary to form and stabilize the monolayer time. The biosensor electrochemical properties were investigated using cyclic voltammetry and electrochemical impedance spectroscopy. Parameters affecting the biosensor response such as enzyme amount, concentration of glutaraldehyde, and inhibition time were optimized. Based on the inhibition effect of carbaryl on the AChE activity, this biosensor showed a linear range from 1.0 at 9.0 μmol L−1 and detection limit of 0.032 μmol L−1.

Porphyran is a sulfated polysaccharide obtained from Porphyra genus. Carbon paste electrodes (CPEs) containing natural ionic polysaccharides have been studied. This study describes the voltammetric behavior of porphyran modified CPE obtained before and after chemical desulfation of the polysaccharides by alkaline treatment (PA) and solvolysis (PD) from the native fraction (PN). The cyclic voltammograms for all porphyran modified electrodes indicated a redox current peaks increase in the same potential range when compared to the unmodified CPEs. The CPEs were modified with different amounts of porphyran and those which showed the most intense anodic peaks were: 10 (EN), 20 (EA) and 40 µg (ED). In addition, the anodic peak current increased linearly with the square root of the scan rate, indicating that the redox process was diffusion-controlled. In conclusion, the electrochemical experiments demonstrated that electron transfer on the porphyran-modified CPEs surface is more efficient, conducting higher current and offering lower resistivity.

This paper describes the construction, evaluation, and application of an electrochemical sensor for the determination of dopamine (DA) in the presence of ascorbic acid (AA) and uric acid (UA). Satisfactory results were achieved for the simultaneous determination of DA and UA, it was verified that it is possible to detect AA in the presence of DA, but high concentrations of AA interfere in detection of DA. The sensor was constructed using the layer-by-layer (LbL) technique with the modification of the surface of indium tin oxide coated glass (ITO) substrate with nanostructured films comprising a 3-n-propylpyridinium silsesquioxane polymer (SiPy+Cl−) and nickel(II) tetrasulfophthalocyanine (NiTsPc). Using the square wave voltammetry technique (SWV), the LbL modified electrodes produced at different pHs (pH 2 and 8) were used to determine DA in the presence UA, and the electrochemical responses of the electrodes were compared. It was observed that the electrodes with the highest concentration of monomeric species showed the highest current intensity and the lowest peak potential for the DA and UA analytes in analysis of DA and UA, individually and simultaneously, with peak potential separation of 460 mV versus Ag/AgCl. Applying SWV, a linear dynamic range of 10–99 μmol L−1 and 100–900 μmol L−1 with detection limit of 16.8 μmol L−1 and 58.3 μmol L−1 was obtained for DA and UA, respectively. The method has good selectivity and sensitivity, and it was successfully applied to the simultaneous determination of DA and UA in spiked human urine sample.

Zika virus (ZIKV) has recently become a global health challenge due to its rapid geographical expansion, since it is associated with serious neurological anomalies such as Guillain-Barre syndrome and microcephaly. Currently, the techniques for ZIKV diagnosis require labor-intensive, expensive and lengthy tests using sophisticated equipment. Moreover, false-positive or false-negative results can occur. In the present work, a DNA biosensor to detect ZIKV in real human serum samples was developed using an oxidized glassy carbon electrode (ox-GCE) modified with silsesquioxane-functionalized gold nanoparticles (AuNPs-SiPy). This nanohybrid was characterized by UV-Vis, FTIR and Raman spectroscopies, DLS, and XRD. The conditions for the immobilization of a ZIKV ssDNA probe on the electrode surface (ox-GCE-[AuNPs-SiPy]) were optimized by univariate and multivariate analysis. The optimized biosensor was characterized by CV, EIS and AFM experiments. The ZIKV target recognition was based on the variation of the charge transfer resistance (ΔRct) of the redox marker ([Fe(CN)6]3-/4-) used and the roughness (Rq) of the electrode surface. The proposed biosensor presented a LOD of 0.82 pmol L-1, with a linear range of 1.0 x10-12 – 1.0 x10-6 mol L-1. Moreover, the reported device showed a suitable stability and satisfactory sensitivity and selectivity to quantify ZIKV in human serum samples, which suggests its promising clinical applications for the early diagnosis of ZIKV-associated pathologies.

The preparation and characterization of carbon ceramic electrodes (CCEs) produced by the sol–gel method was reported. Different parameters for the manufacture of CCE were studied. Some factors such as the preparation method, type of precursor and catalyst, directly influence the conductive properties, mechanical and morphological characteristics of CCEs, and these are reflected in their electrochemical response. Such factors were evaluated using a 24 (two factors and four levels) factorial design analyzing the anodic peak currents (Ipa) and the variation of peak potential (ΔEp) responses of the respective cyclic voltammograms in presence of potassium ferrocyanide. Analyzing the set of results obtained from cyclic voltammetry, a higher reversibility was observed for the electrodes prepared by basic catalysis using methyltrimethoxysilicate, MTMS, as Si precursor. It is suggested that the obtained material presents a porous morphological structure, providing higher reversibility and better definition of voltammetric peaks. These electrodes were characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XDR).

In this paper, LbL films were prepared between 3-n-propylpyridinium silsesquioxane polymer (SiPy+Cl−) and copper (II) tetrasulfophthalocyanine (CuTsPc), denoted as (SiPy+Cl−/CuTsPc)n. The multilayer film modified electrode (SiPy+Cl−/CuTsPc)2 showed excellent electrocatalytic activity for H2O2 reduction with a wide linear range of response, high sensitivity and low detection limit. This film was modified with glucose oxidase (GOx) and a film of Nafion (Nf) which was used to prevent the GOx from leaking off. FTIR, fluorescence spectrometry and CD measurements showed that the GOx enzyme retains its native secondary structure when immobilized on the LbL film. The resulting (SiPy+Cl−/CuTsPc)2(SiPy+Cl−/GOx/Nf) biosensor displayed a current response which varies linearly with the concentration of glucose in the range from 1 to 10 mmol L−1 with a detection limit of 0.16 mmol L−1, a sensitivity of 1.397 × 10−7 μA (mmol L−1)−1 and a fast response time. The apparent Michaelis–Menten constant ( k m app ) was 10 mmol L−1. In addition, the biosensor also exhibited good reproducibility, excellent stability and long life-time with no decrease in the activity of enzyme over a one week period and remained active in the films for periods longer than three weeks.

This study presents the preparation of a nanostructured films with an amphiphilic meso-porphyrin whose side chains are derived from cardanol, a byproduct of the cashew industry. The Langmuir films were prepared on acidified water to reducing the self-aggregation of the complex and the mean molecular area for the meso-porphyrin measured by surface pressure isotherms was 177 A2 and confirmed by compressibility modulus. The 700 nm band observed in the Langmuir films supports the coexistence of monomers and dimers similar to the spectra of the solution (CHCl3/CH3COOH). The formation of bright block domains with different sizes and shapes for the monolayer disappears after surface pressure starts to increase (π ≥ 2 mN m−1), as observed by Brewster angle microscopy measurements. Results of UV–vis spectra of the Langmuir films and FTIR reflection of the Langmuir–Blodgett (LB) films of meso-porphyrin indicated that the molecules exhibit preferential orientation with the planes perpendicular to the water (Langmuir) and parallel to the substrate (LB) surface. The LB films were tested as a promethazine sensor, which detected a concentration of 2 μM, a value greater than those obtained by other nanostructured systems.

In this paper the preparation and electrochemical characterization of an immunosensor for Chagas disease is described. The results obtained indicating the possibility of the modification of ITO surface by formation of a LbL film constructed by the intercalation of PVS as polyanion and AgNP-SiPy+ as polycation. The LbL films were then modified by incorporation of Chagas ATG on their surface. These modified LbL films showed a pronounced impedance change when in the presence of the Chagas ATB indicating that this kind of film could be used in a device for diagnosis of Chagas disease.

A (SiPy+Cl−/CuTsPc)5 layer-by-layer film was employed for the electroanalytical determination of promethazine hydrochloride in BR buffer pH 5.0 with peaks at 0.48 and 0.79 V. After optimisation of the square-wave parameters (f=100 s−1, a=40 mV and ΔEs=2 mV), the peak at 0.79 V was used for quantification and a detection limit of 8.71×10−9 mol L−1 and a quantification limit of 9.31×10−8 were calculated. The applicability of this procedure was tested on commercial formulations of promethazine hydrochloride by observing the stability, specificity, recovery and precision of the procedure in complex samples, without any preliminary treatment.

This paper shows the simultaneous electrochemical determination of dypirone (DIP) and paracetamol (PAR) by differential pulse voltammetry technique (DPV) using an unmodified carbon paste electrode. Because of the overlapping of the voltammetric peaks of DIP and PAR, the multivariate calibration methodology based on Partial Least Square Regression (PLSR) was proposed. The data pre-treatment used in this process was mean centering and to choose the principal component number a cross validation procedure was used (leave-one-out). Four principal components were necessary to obtain the lowest PRESS (Prediction Residual Error Sum of Squares). The statistics showed that this model explains approximately 95.5% of the variance from the data set. Using this model, high correlation between real and predicted concentrations was observed. However, for low concentrations of PAR the relative error increased to 25%. Comparing RMSEP (Root Mean Square of Error Prediction) between PAR and DIP, it was observed that it was lower for DIP probably due to higher analytical information in the voltammograms for this analyte when compared to the electrochemical process of PAR, which presented only one potential peak due to its irreversible oxidation. Esse artigo descreve a determinação simultânea de dipirona (DIP) e de paracetamol (PAR) utilizando a técnica de Voltametria de Pulso Diferencial (VPD) e eletrodo de pasta de carbono não modificado. Devido à sobreposição dos picos voltamétricos de DIP e PAR, a metodologia de calibração multivariada baseada na Regressão de Mínimos Quadrados Parciais (PLSR) foi proposta. O conjunto de voltamogramas obtidos na presença de ambos os analitos em diferentes concentrações foi pré-processado pelos dados centrados na média. Para escolha do número de componentes principais, um procedimento de validação cruzada foi empregado, sendo que quatro componentes principais foram necessárias para obtenção dos menores valores de PRESS (Prediction Residual Error Sum of Squares). Esse modelo explicou aproximadamente 95,5% da variância do conjunto de dados. Os dados obtidos utilizando-se esse modelo mostraram uma alta correlação entre as concentrações reais e previstas. Entretanto, para baixas concentrações do PAR, os erros relativos aumentaram para 25%. Comparando-se os valores de RMSEP (Root Mean Square of Error Prediction), entre PAR e DIP, foi observado que este foi menor para DIP, provavelmente devido a maior quantidade de informação analítica apresentada pelos voltamogramas desse analito quando comparado ao processo redox de PAR, o qual devido a sua oxidação irreversível apresenta apenas um único pico.

This paper shows an application of a first generation porphyrin, FeTPPCl 5,10,25,20-tetraphenylporphyrin iron(III) chloride (FeTPPCl or FeTPP+) immobilized on a inorganic matrix, SiO2/Nb2O5 (niobium oxide grafted on silica gel surface, designated as SiNb) to the preparation of a carbon paste electrode (CPE). The carbon paste modified with a FeTPP+ immobilized on SiO2/Nb2O5 (SiNb), designated as CPE/SiNb/FeTPP, was applied for simultaneous electrochemical determination of dopamine (DA) and ascorbic acid (AA). Because of the overlapping of the voltammetric peaks of DA and AA, the multivariate calibration methodology based on partial least square regression (PLSR) was proposed. The data pre-treatment used in this process was mean centering and to choose the principal components number a cross validation procedure was used (leave-one-out). Five principal components were necessary to obtain the lowest PRESS (prediction residual error sum of squares). The statistics showed that this model explains approximately 95.2% of the variance from the data set. Using this model, high correlation between actual and predicted concentrations was observed, mainly for higher dopamine (maximum relative error of 8%) and ascorbic acid (maximum relative error of 10%) concentrations. For low analytes concentrations the relative error increases to 35% for AA. Considering the similarity of the voltammetric response of the analytes, the results obtained were satisfactory and showed the promissory capability of the CPE/SiNb/FeTPP coupled multivariate calibration methods for simultaneous voltammetric analysis of DA and AA.

The preparation and electrochemical characterization of a carbon paste electrode (CPE) modified with mer-[Ru0Cl3(dppb)(4-pic)] (dppb=Ph2P(CH2)4PPh2, 4-pic=CH3C5H4N), referred to as Rupic, were investigated. The CPE/Rupic system displayed only one pair of redox peaks, with a midpoint potential at 0.28 V vs. Ag/AgCl, which were ascribed to RuIII/RuII charge transfer. This modified electrode presented the property of electrocatalysing the oxidation of dopamine (DA) and ascorbic acid (AA) at 0.35 V and 0.30 V vs. Ag/AgCl, respectively. Because the oxidation for both AA and DA practically occurred at the same potential, distinguishing between them was difficult with cyclic voltammetry. This limitation was overcome using Partial Least Square Regression (PLSR), which allowed us, with the optimised models, to determine four synthetic samples with prediction errors (RMSEP) of 5.55´10-5 mol L-1 and 7.48´10-6 mol L-1 for DA and AA, respectively. A preparação e caracterização eletroquímica do eletrodo de pasta de carbono (EPC) modificado com o mer-[RuCl3(dppb)(4-pic)] (dppb= Ph2P(CH2)4PPh2, 4-pic= CH3C5H4N), denominado Rupic, foi investigada. O sistema EPC/Rupic apresentou apenas um par de picos com potencial médio de 0,28 V vs. Ag/AgCl, o qual foi atribuído ao processo de transferência de elétrons do RuIII/RuII. Este eletrodo modificado apresentou a propriedade de eletrocatalisar a oxidação da dopamina (DA) e do ácido ascórbico (AA) em 0,35 V e 0,30 V, respectivamente. Como a oxidação dos analitos, AA e DA, ocorreu praticamente no mesmo potencial, não foi possível a distinção dos sinais voltamétricos utilizando-se a voltametria cíclica. Esta limitação foi resolvida empregando-se a Regressão de Mínimos Quadrados Parciais (PLSR), a qual permitiu a determinação de quatro amostras sintéticas, com erros de previsão (RMSEP) de 5,55´10-5 mol L-1 e 7,48´10-6 mol L-1 para DA e AA, respectivamente.

In this work, a new porphyrin, the 5,10,15,20-tetrakis-(2,6-difluoro-3-sulfonatophenyl) porphyrinato iron(III) chloride (denoted as FeTsP) was immobilized on SiO2/Al2O3 (SiAl) coated with n-propylpyridiniumsilsesquioxane polymer (SiPy+Cl−). The FeTsP was adsorbed on SiAl/SiPyCl by an ion exchange reaction, obtaining a modified solid, SiAl/SiPy/FeTsP, where the porphyrin complex was strongly adhered. Cyclic voltammograms of the SiAl/SiPy/FeTsP carbon paste electrode showed an irreversible response, with an oxidation peak at Epa=0.40 V and nondefined reduction peak at Epc=0.15 V (vs. SCE). These peaks were not observed for the nonmetallated porphyrin, indicating that they probably correspond to the Fe(III)/Fe(II) process. Studies made in solutions having different pH, (between pH 2 and 9) using the modified electrode showed that the peak potentials and the current density were not affect by pH changes, indicating that the iron porphyrin is very stable and strongly entrapped in the matrix. The modified electrode presented the property to electrocatalyze the eletrooxidation of hydrazine at 0.41 V (vs. SCE), at pH 7. The potentiality of the SiAl/SiPy/FeTsP electrode as a sensor for hydrazine was evaluated by the using the chronoamperometric technique. A linear response was obtained in the concentration range between 5×10−5 and 6×10−4 mol L−1 of hydrazine.

Hexacyanoferrate ion, [Fe(CN)6]4−, was immobilized by an ion-exchange reaction on the propylpyridiniumsilsesquioxane chloride polymer thin-film-coated SiO2/Al2O3 surface. The amount of [Fe(CN)6]4− immobilized was 0.22 mmol g−1 with a surface coverage of 9.6×10−6 mmol cm−2. A carbon paste electrode made with this material was prepared and its electrochemical properties studied. The electrode presented two well-defined redox peaks with midpoint potentials, Em, of 0.152 V vs SCE. This potential was not significantly affected by pH changes between 2 and 9.5. The electrode showed much reproducible responses and was successfully used to study the electrochemical oxidation of cysteine.