Search

Made available in DSpace on 2021-06-25T11:17:47Z (GMT). No. of bitstreams: 0 Previous issue date: 2021-09-15 Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) In the present study, we report the use of an electrochemical sensor for uric acid using a molecularly imprinted polymer (MIP) as a molecular recognition platform. The MIP was based on an azopolymer electropolymerized in the presence of uric acid used as a template molecule. The azo group of the polymer contributes to the quantifiable redox capacitance of the electrode surface, which can be probed by impedance/capacitance spectroscopy. Thus, the advantage of the developed sensor is its ability to perform direct measurements without a soluble redox probe. The molecularly imprinted polymer can sensitively transduce the recognition and binding of specific analytes by redox capacitance. Through electrochemical impedance spectroscopy (EIS) and electrochemical capacitive spectroscopy (ECS), unusual approaches were able to obtain different analytical curves with distinct characteristics, reaching a limit of detection of 0.160 μmol L−1. Department of Chemistry and Biochemistry School of Science and Technology – Sao Paulo State University (UNESP), Rua Roberto Simonsen 305 Department of Chemistry and Biochemistry School of Science and Technology – Sao Paulo State University (UNESP), Rua Roberto Simonsen 305 FAPESP: 2016/09017-1 CNPq: 301298/2017-3 CAPES: 88882.434480/2019-01

The preparation and electrochemical characterization of hausmannite-type manganese oxide to use as a sensing material for sodium ion is described. This paper reports a new via synthetic to obtain of the hausmannite-type manganese oxide and its application in the construction of modified electrode as a voltammetric sensor. The electrochemical activity of hausmannite-type manganese oxide is controlled by intercalation/deintercalation of the sodium ions within the oxide lattice. The detection is based on the measurement of anodic current generated by oxidation of Mn III –Mn IV at electrode surface. The best electrochemical response was obtained for a sensor composition of 20% (w/w) hausmannite oxide in the paste, a TRIS buffer solution of pH 6.0–7.0 and a scan rate of 50 mV s −1 . A sensitive linear voltammetric response for sodium ions was obtained in the concentration range of 2.01 × 10 −5 –2.09 × 10 −4 mol L −1 with a slope of 355 μA L mmol −1 and a detection limit of 7.50 × 10 −6 mol L −1 using cyclic voltammetry. The use of hausmannite has significantly improved the selectivity of the sensor compared to the birnessite-type manganese oxide modified electrode. Under the working conditions, the proposed method was successfully applied to determination of sodium ions in urine samples.

An amperometric oxygen sensor based on a polymeric nickel– salen ( salen = N , N ′-ethylene bis (salicylideneiminato)) film coated platinum electrode was developed. The sensor was constructed by electropolymerization of nickel– salen complex at platinum electrode in acetonitrile/tetrabutylammonium perchlorate by cyclic voltammetry. The voltammetric behavior of the sensor was investigated in 0.5 mol L −1 KCl solution in the absence and presence of molecular oxygen. Thus, with the addition of oxygen to the solution, the increase of cathodic peak current (at −0.25 V vs. saturated calomel electrode (SCE)) of the modified electrode was observed. This result shows that the nickel– salen film on electrode surface promotes the reduction of oxygen. The reaction can be brought about electrochemically, where the nickel(II) complex is first reduced to a nickel(I) complex at the electrode surface. The nickel(I) complex then undergoes a catalytic oxidation by the molecular oxygen in solution back to the nickel(II) complex, which can then be electrochemically re-reduced to produce an enhancement of the cathodic current. The Tafel plot analyses have been used to elucidate the kinetics and mechanism of the oxygen reduction. A plot of the cathodic current vs. the dissolved oxygen concentration for chronoamperometry (fixed potential = −0.25 V vs. SCE) at the sensor was linear in the 3.95–9.20 mg L −1 concentration range and the concentration limit was 0.17 mg L −1 O 2 . The proposed electrode is useful for the quality control and routine analysis of dissolved oxygen in commercial samples and environmental water. The results obtained for the levels of dissolved oxygen are in agreement with the results obtained with a commercial O 2 sensor.

The electrochemical behavior of a carbon paste electrode modified (CPEM) with nickel-doped potassium strontium niobate (KSr 2 Ni 0.75 Nb 4.25 O 15− δ ) of tetragonal tungsten bronze (TTB)-type structure was investigated as a new sensor for dipyrone. The optimum conditions were found in an electrode composition (in mass) of 20% KSr 2 Ni 0.75 Nb 4.25 O 15− δ , 65% graphite and 15% mineral oil in 0.5 mol L −1 KCl solution at pH 6.0. The electrode exhibits reversible electrochemical behavior in a wide potential range (0.1–0.7 V vs. SCE), high conductivity, and stability/durability electrode in 0.50 mol L −1 KCl solution. The estimated surface concentration was found to equal 1.08 × 10 −9 mol cm −2 . The oxidation of dipyrone was performed at carbon paste electrode unmodified at +0.54 V vs. SCE and carbon paste electrode modified with KSr 2 Ni 0.75 Nb 4.25 O 15− δ at +0.45 V vs. SCE. This result shows that the nickel-doped potassium strontium niobate on electrode surface promotes the oxidation of dipyrone. The reaction can be brought about electrochemically, where the niobium(IV) is first oxidation to a niobium(IV) oxide at the electrode surface. The niobium(IV) then undergoes a catalytic reduction by the dipyrone in solution back to the niobium(III), which can then be electrochemically re-oxidized to produce an enhancement of the anodic current. The identification of the oxidation product of dipyrone was made by mass spectrometry for elucidation of response mechanism of the sensor. Anodic peak current ( I pa ) vs. dipyrone concentration for amperometric method at the modified electrode was linear in the 3.5 × 10 −5 and 3.1 × 10 −4 mol L −1 concentration range and the detection limit was 5.1 × 10 −6 mol L −1 dipyrone. At higher concentrations (>3.1 × 10 −4 mol L −1 ), deviation from linearity occurs. Under this condition, an electrocatalytic reaction takes place and proceeds through a mechanism similar to the Michaelis–Menten model.

An oxovanadium-salen complex (N,N � -ethylene-bis(salicylideneiminato) oxovanadium) thin film deposited on a graphite–polyurethane electrode was investigated with regard to its potential use for detection of l-dopa in flow injection system. The oxovanadium(IV)/oxovanadium(V) redox couple of the modified electrode was found to mediate the l-dopa oxidation before its use in the FIA system. Experimental parameters, such as pH of the carrier solution, flow rate, sample volume injection and probable interferents were investigated. Under the optimized FIA conditions, the amperometric signal was linearly dependent on the l-dopa concentration over the range 1.0 × 10 −6 to 1.0 × 10 −4 mol L −1 (Ianodic A) = 0.01 + 0.25 [l-dopamol L −1 ]) with a detection limit (S/N = 3) of 8.0 × 10 −7 mol L −1 and a sampling frequency of 90 h −1 was achieved. For a concentration of 1.0 × 10 −5 mol L −1 l-dopa, the R.S.D. of nine consecutive measurements was 3.7%.

The electrochemical behavior of a carbon paste electrode modified (CPEM) with N,N′-ethylenebis(salicylideneiminato)oxovanadium(IV) complex ([VIVO(Salen)]) was investigated as a new sensor for cysteine. Cyclic voltammetry at the modified electrode in 0.1 mol L−1 KCl solution (pH 5.0) showed a single-electron reduction/oxidation of the couple VO3+/VO2+. The CPEM with [VO(Salen)] presented good electrochemical stability in a wide pH range (4.0–10.0) and an ability to electrooxidate cysteine at 0.65 V versus SCE. These results demonstrate the viability of the use of this modified electrode as an amperometric sensor for cysteine determination.

The potentiometric response of a λ-MnO2-based graphite–epoxy electrode for the determination of lithium ions was examined. The electrode response to lithium ions was linear in the concentration range 10−6–3.3×10−2 mol/l with a slope of 58.2 mV/decade (at 25°C) over a wide pH range (4–10); no such linear response was found for the ions of other alkali metals or alkaline-earth metals. The logarithm of the selectivity coefficients for Li+ against Na+, K+, Cs+, Mg2+, Ca2+, Sr2+and Ba2+ was found to be −1.5, −2.0, −2.2, −2.0, −2.0, −2.1 and −2.1, respectively. The response time of the proposed electrode was lower than 30 s and its lifetime greater than 6 months.

The potentiometric response of a novel λ-MnO 2 -based graphite–epoxy electrode for the determination of acids and bases was examined. The electrode was prepared by the incorporation of manganese (IV) oxide in a graphite–epoxy matrix. The data obtained in various acid–base titrations were compared with those using a glass electrode in the same conditions. The proposed electrode presented a linear response in the pH range 2 to 13 with a slope of −53.6±0.5 mV/pH (at 25°C) and a response time of ca. 10 s. The lifetime of this electrode was higher than 3 months (ca. 1500 determinations) and it showed an excellent performance for pH determination and end-point indication potentiometric titrations in the determination of acids and bases. The miniaturization of the proposed electrode is also possible.