Christiana Andrade Pessoa

Direct electron transfer: an approach for electrochemical biosensors with higher selectivity and sensitivity

The most promising approach for the development of electrochemical biosensors is to establish a direct electrical communication between the biomolecules and the electrode surface. This review focuses on advances, directions and strategies in the development of third generation electrochemical biosensors. Subjects covered include a brief description of the fundamentals of the electron transfer phenomenon and amperometric biosensor development (different types and new oriented enzyme immobilization techniques). Special attention is given to different redox enzymes and proteins capable of electrocatalyzing reactions via direct electron transfer. The analytical applications and future trends for third generation biosensors are also presented and discussed.

Preliminary electrochemical study of phenothiazines and phenoxazines immobilized on zirconium phosphate

Abstract The electrochemical properties of one phenothazine (Methylene Blue) and one phenoxazine (Nile Blue) dye immobilized on zirconium phosphate were investigated by incorporation into a carbon paste electrode. The dyes presented a very stable redox property. The midpoint potentials ( E m ) were 150 and 50 mV vs. SCE for Methylene Blue and Nile Blue, respectively. The composition and pH of the supporting electrolyte did not affect the E m , in contrast to those observed for the dyes in solution phase or when adsorbed on carbon. These electrodes showed a good activity in a test made for electrocatalytic oxidation of NADH.

Ferrocenecarboxylic acid adsorbed on Nb2O5 film grafted on a SiO2 surface: NADH oxidation study

Abstract Ferrocenecarboxylic acid was adsorbed on a Nb 2 O 5 film grafted on a porous SiO 2 surface, resulting in a modified silica, presenting the following characteristics: amount of attached Nb 2 O 5 =0.58 mmol g −1 , amount of ferrocenecarboxylic acid (HOOCFeCp 2 )=0.12 mmol g −1 and specific surface area S BET =294 m 2 g −1 with an average surface density δ =0.24 molecules nm −2 or an average intermolecular distance of 2 nm. The organometallic complex is adhered to the surface by forming the NbOOC bond and, thus, the electroactive species is not easily leached out from the surface under various oxidation–reduction cycles of the central metal in a cyclic voltammetry experiment. The midpoint potential, E 1/2 , is observed at 0.29 V versus SCE and it remains practically unchanged in various supporting electrolyte solutions and also is not significantly affected by pH changes between 1 and 7.5. An electrode made with this material, SiNb–O 2 CFeCp 2 , mediates NADH oxidation at 0.35 V in solution at pH 6. Kinetics studies were evaluated by using the rotating disk electrode technique in order to understand the mechanism of NADH oxidation on the SiNb–O 2 CFeCp 2 modified electrode.

Emprego de monocamadas auto-organizadas no desenvolvimento de sensores eletroquímicos

Self-assembled monolayers (SAMs) modified electrodes exhibit unique behavior that can greatly benefit electrochemical sensing. This brief review highlights the applications of SAM modified electrodes in electroanalytical chemistry. After a general introduction, which includes the approaches for SAM development, different electrochemical systems for detecting inorganic and organic species are described and discussed. Special attention to the coupling of biological sensing element to the SAM is given, which can selectively recognize the analyte. Future prospects are also evaluated.

Cobalt Porphyrin Immobilized on a Niobium(V) Oxide Grafted – Silica Gel Surface: Study of the Catalytic Oxidation of Hydrazine

meso-Tetracarboxyphenylporphyrin (4,4′,4″,4′′′(21H-23H-Porphine-5,10,15,20-tetrayl) tetrakis-(benzoic acid)), designated as H2TCPP, was efficiently immobilized onto niobium oxide grafted on the silica gel surface, SiO2/Nb2O5, by the -COO-Nb bond formed between the porphyrin carboxyl groups and Nb2O5. This immobilized porphyrin, SiO2/Nb2O5/H2TCPP, was then in situ metallated with Co(II) in dimethylformamide solution. The metallation process was followed by inspection of the UV-vis spectra which showed a decrease in the number of Q bands (a2u → eg transition) after the metallation, indicating that the symmetry of the porphyrin ring changed from D2h to D4h upon incorporation of Co(II). This material, when incorporated into a carbon paste electrode, electrocatalyzed hydrazine oxidation. Amperometric studies showed the potential of the modified electrode as a sensor for determination of hydrazine. The response time of the modified electrode was less than 1 s. A linear response was obtained between 1×10−5 and 5×10−4 mol L−1 at pH 7.

Cobalt(II) metallated hematoporphyrin IX and protoporphyrin IX immobilized on niobium(V) oxide grafted on a silica gel surface: electrochemical studies

Abstract Niobium oxide thin film grafted on porous silica gel was prepared and used to immobilize protoporphyrin IX and hematoporphyrin IX. The porphyrins adhered strongly to the surface probably by the COONb chemical bond formed between the porphyrin carboxyl groups and Nb2O5. The immobilized hematoporphyrin IX and protoporphyrin IX, SiO2/Nb2O5/H2HMP and SiO2/Nb2O5/H2PP, respectively, were further metallated by means of in situ reaction with CoCl2 giving rise to SiO2/Nb2O5/CoHMP and SiO2/Nb2O5/CoPP. The effectiveness of the reactions in producing the metallated adsorbed complexes was investigated by electronic absorption spectroscopy. Graphite paste electrodes of the metallated adsorbed porphyrins were prepared in order to study the electrocatalytic reduction of dissolved dioxygen by the cyclic voltammetry technique. The dioxygen eletroreduction was observed at −0.18 and −0.16 V versus SCE in pH 6 for hematoporphyrin and protoporphyrin complexes, respectively. The cathodic peak potentials at different solution pH remained practically constant between pH 4 and 7 for both materials. The current intensities for a fixed potential plotted against dissolved dioxygen concentrations showed linear responses between 2 and 12 ppm.

Electrochemical behavior of pyrroloquinoline quinone immobilized on silica gel modified with zirconium oxide.

Pyrroloquinoline quinone (PQQ) was immobilized on the silica gel surface modified with zirconium oxide, designated as Si:Zr, by the carboxylic groups of the PQQ molecule and the zirconium oxide on the silica surface. The electrochemistry of PQQ immobilized on the Si:Zr matrix, incorporated in a carbon paste electrode, was evaluated using the cyclic voltammetry technique. The Si:Zr:PQQ-modified electrode showed a redox couple at E(m)=(E(pa1)+E(pc))/2=-0.150 V vs SCE at pH 7, close to that observed in aqueous solution, and another oxidation peak, E(pa2)=-0.100 V vs SCE. Studies in different pH solutions in the range of 3-7 showed that the first oxidation peak, E(pa1), is highly dependent on the solution pH shifting from to -0.175 to 0.100 V vs SCE, while E(pa2) remains practically constant at 0.100 V as the pH decreases from 7 to 3. The immobilized PQQ electrode presented the property to electrocatalyze the NADH at 150 mV vs SCE. The effect of addition of Ca(2+) ions on the electrode electroactivity for the NADH oxidation was also verified. Different from that observed for the PQQ immobilized on other electrode materials, the Ca(2+) ions did not influence the electrocatalytical response; however, the electrode stability was considerably improved in the presence of Ca(2+) ions, indicating that the matrix surface has a great influence on the electrochemical behavior of PQQ.

COPPER (II) TETRASULPHO-PHTHALOCYANINE ENTRAPPED IN A PROPYLPYRIDINIUMSILSESQUIOXANE POLYMER IMMOBILIZED ON A SiO2/Al2O3 SURFACE: USE FOR ELECTROCHEMICAL OXIDATION OF ASCORBIC ACID

ABSTRACT Copper (II) tetrasulphophthalocyanine, CuPc4−, was encapsulated in a n-propylpyridinium silsesquioxane polymer, SiPy+Cl−, immobilized on the SiO2/Al2O3 surface. The material obtained incorporated in a carbon paste electrode, presented the property of electrocatalysing the oxidation of ascorbic acid at 0.18 vs SCE. It was observed that the anodic peak potential at different solution pH remained practically constant between pH 5 and 8 and shifted to more positive potentials below pH 5. The electrode showed a very reproducible response and was chemically stable under various oxidation-reduction cycles. By amperometric measurements, a linear response was obtained in the range of 1 × 10−4–1.4 × 10−3 mol l−1 of ascorbic acid. The modified electrode was used to determine ascorbic acid (vitamin C) in tablets. The results were validated by comparison with the results obtained by using the standard 2,6-dichlorophenolindophenol titration method.

Simultaneous voltammetric determination of dopamine and ascorbic acid using multivariate calibration methodology performed on a carbon paste electrode modified by a mer-[RuCl3(dppb)(4-pic)] complex

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.

Otimizaçâo das condições de preparação de eletrodos à base de carbono cerâmico utilizando-se planejamento fatorial

Different parameters of carbon ceramic electrodes (CCE) preparation, such as type of precursor, carbon material, catalyst amount, among others, significantly influence the morphological properties and consequently their electrochemical responses. This paper describes a 23 factorial design (2 factors and 3 levels with central point replicates), which the factors analyzed were catalyst amount (HCl 12 mol L-1), graphite/precursor ratio, and precursor type (TEOS - tetraethoxysilane and MTMOS - methyltrimetoxysilane). The design resulted in a significant third order interaction for peak current values (Ipa) and a second order interaction for potential difference (ΔE), between thefactors studied, which could not be observed when using an univariated study.