Graciliano de Oliveira Neto

Construction and evaluation of an optical pH sensor based on polyaniline-porous Vycor glass nanocomposite

This work describes the preparation of a polyaniline (Pani)‐porous Vycor glass (PVG) nanocomposite and its use as sensing phase in an optical fibre pH sensor. Nanocomposites of Pani‐PVG were prepared by in situ polymerisation of aniline absorbed inside the pores of a PVG (Corning 7930) with an average porous size of 8 nm. The optical sensor was constructed by fixing a PVG slide onto a distal end of bifurcated optical fibre bundle, with a cyanoacrylic resin. The sensor response was found to be reversible in the pH range from 5 to 12 and linear from pH 7.4 to 9.5. Response times of 4, 8 and >16 min were obtained for slide thickness’ of 0.5, 1 and 1.5 mm, respectively. Changes of temperature, ranging from 20 to 408C, showed minor effect on the dynamic range. Similarly, the ionic strength (0.15, 0.30 and 0.50 mol l ˇ1 ) and the nature of the ions (NaCl, KCl and NaClO4) showed minor influence on the sensor response. Leaching of Pani was not observed and the sensor lifetime was determined as being at least 5 months. These results indicate that a Pani‐PVG nanocomposite is suitable for the construction of optical pH sensors, with good analytical performance, since the glass slides can be prepared with good reproducibility and durability. # 2001 Elsevier Science B.V. All rights reserved.

Electrochemical sensor for NADH based on Meldola's blue immobilized on silica gel modified with titanium phosphate

By means of cyclic voltammetry and amperometry, the evaluation of a modified electrode of Meldolas Blue immobilized on titanium phosphate coated onto a silica gel surface is described. The adsorbed Meldolas Blue was incorporated into a carbon paste electrode, and electrochemical behavior of the electron mediator was investigated under different conditions. The midpoint potential was found to be −30 mV vs sce and the ratio of cathodic and anodic peak current was about 1. Changing the pH of the solution between 3 and 8 did not shift the midpoint potential but for pH values lower than 3 the midpoint potential shifted to more positive potentials. The nature of the anion of the supporting electrolyte did not affect the response of the electrode, but the nature of the cation significantly influenced the voltammetric response when a sodium salt was used as electrolyte. The electrocatalytic oxidation of reduced nicotinamide adenine dinucleotide (NADH) was verified and the response time of the sensor was found to be 10s by amperometry, applying 0 mV of potential vs sce. The linear response range was 1.0 10−5 −5.0 10−5 M and the stability was very good with 500 measurements being carried out with the same electrode, obtaining a rsd lower than 4%.

Electrochemical Sensor for Hydrazine Based on Silica Modified with Nickel Tetrasulfonated Phthalocyanine

Nickel tetrasulfonated phthalocyanine (NiTsPc) immobilized onto titanized silica gel (ST) showed good stability and redox properties. The cyclic voltammograms obtained with a carbon paste electrode modified with ST-NiTsPc presented an enhanced peak current at 450 mV (vs. SCE) which did not shift with solution pH. The redox process was attributed to NiII/NiIII, which presented a good electrocatalytical activity for oxidizing hydrazine in neutral solution. The process is controlled by the electron transfer step and no diffusion limitation was observed. The modified electrode was very sensitive for hydrazine, giving a detection limit of 1.0 × 10−5 mol L−1, for an applied potential of 450 mV (vs. SCE). A linear response range was observed between 1.0 × 10−4 and 6.0 × 10−4 mol L−1, fit by the equation I = −0.1 (± 0.1) + 7.9 (± 0.3) [Hydrazine], with a correlation coefficient of 0.998 for n = 6. The repeatability for 11 measurements was excellent, with a relative standard deviation of 3.7 %. The active material, nickel tetrasulfonated phthalocyanine modified silica, was stable for at least 6 months, losing only 10 % of its signal over this period.

A new amperometric biosensor for fructose using a carbon paste electrode modified with silica gel coated with Meldola's Blue and fructose 5-dehydrogenase

Abstract A carbon paste modified with silica gel coated with Meldolas Blue and fructose 5-dehydrogenase was used to construct a new biosensor for fructose. An efficient electron transfer mediation from the reduced enzyme to the carbon paste electrode was verified, at a low applied potential of 20mV vs. SCE, where interfering reactions are minimized. A linear response range for 0.1 to 0.8 mmoll −1 fructose was adjusted by the equation I = 0.0150( ± 0.006) + 0.618(± 0.011) [fructose], with a correlation coefficient of 0.999 and an rsd of 0.68% for n = 7. The biosensor was used to determine fructose in sweets and fruit jellies and the results present good recovery, higher than 96% in both cases. A stable response is observed over two months, when stored in a refrigerator, or for 300 measurements in continuous use.

Solid-phase spectrofluorimetric determination of acetylsalicylic acid and caffeine in pharmaceutical preparations using partial least-squares multivariate calibration.

PLS-1, a variant of the partial least-squares algorithm was used for the solid-phase spectrofluorimetric determination of acetylsalicylic acid (ASA) and caffeine (CF) in pharmaceutical formulations. The method allows the simultaneous quantification of the analytes, as the closely overlapping spectral bands are efficiently solved. Sample preparation prior to analysis is not required. The calibration set consisted of 83 samples with 50-170mgg(-1) ASA plus 5-20mgg(-1) CF; another set of 25 samples was used for external validation. Agreement between predicted and experimental concentrations was fair (r=0.987 and 0.974 for ASA and CF models). For both models, the prediction performance was evaluated in terms of the coefficient of variability (CV), relative predictive determination (RPD), and ratio error range (RER). The final PLS-1 models were used for the determination of ASA and CF in pharmaceutical formulations.

Determination of salicylate in blood serum using an amperometric biosensor based on salicylate hydroxylase immobilized in a polypyrrole-glutaraldehyde matrix

Abstract The use of an amperometric biosensor for the salicylate determination in blood serum is described. The biosensor is based on salicylate hydroxylase (EC 1.14.13.1) electropolymerized onto a glassy carbon-working electrode with polypyrrole and glutaraldehyde, to improve the biosensor lifetime. The hexacyanoferrate (II) was also incorporated to work as a redox mediator to minimize possible interferences. The salicylate is enzymatically converted to catechol, which is monitored amperometrically by its electrooxidation at+0.170 V versus SCE (saturated calomel electrode). Salicylate determination was carried out maintaining the ratio between β-NADH and salicylate at 4:1 (30°C). The amperometric response of the biosensor was linearly proportional to the salicylate concentration between 2.3×10 −6 and 1.4×10 −5 mol l −1 , in 0.1 mol l −1 phosphate buffer (pH 7.8), containing 0.1 mol l −1 KCl and 5.0×10 −4 mol l −1 Na 2 H 2 EDTA, as supporting electrolyte. The recovery studies, in the presence of several interfering compounds, showed recoveries between 96.4 and 104.8%. The useful lifetime of the biosensor in the concentration range evaluated was at least 40 days, in continuous use. Blood serum samples analyzed by this biosensor showed a good correlation compared to the spectrophotometric method (Trinder) used as reference, presenting relative deviations lower than 7.0%.

Development of a tubular periodate electrode for flow-injection determination of glycerol

Periodate electrodes without inner reference solution based on tetraoctylammonium periodate plus solvent mediator (dibutyl phthalate or 2-nitrophenyl octylether) were constructed. Linear dynamic range, practical detection limit, slope, stability, selectivity coefficients, pH dependence, response time and lifetime were evaluated. A tubular version was further developed and coupled to a flow-injection system for glycerol determination in samples relevant to the industrial production of soaps, detergents and similar. The method involves glycerol oxidation by periodate with potentiometric evaluation of its consumption. The influence of oxidizing agent concentration (10(-5)-10(-2)M NalO(4)), ionic strength (0.0-1.0M Na(2)SO(4)) and mean resident time were investigated and the feasibility of using a single-fine manifold was discussed. The proposed system handles about 40 samples/hr, is very stable and suitable to industrial control. Results within the 1000 and 5000 mg/l range glycerol are precise (r.s.d. <0.005) and in fair agreement with conventional procedures. Baseline drift or noise is not observed and a thermostat water bath is not required. A noteworthy feature is the almost linear relationship between glycerol concentration and recorded peak height which is a consequence of combined effects of reaction kinetics and electrode Nernstian response.

Potentiometric study using chemically modified silica gel with pyridinium as membrane for ClO4- ions

A potentiometric sensor for the perchlorate anion was developed by mixing chemically modified silicagel with pyridinium perchlorate, with an epoxy polymer and graphite. The electrode showed Nernstian response between 1.0 × 10−2 and 1.0 × 10−3M perchlorate concentrations. The electrode showed high selectivity to this ion at solutions pH between 5.5 and 8.0. The presence of IO4−, NO3−,Br−, IO3−, Cl− and SO42− ions in the solutions, had only small interference in the electrode response in the range mentioned.

A Modified Carbon Paste Electrode with Silica Gel Coated with Meldola's Blue and Salicylate Hydroxylase as a Biosensor for Salicylate

Abstract A biosensor for salicylate was developed using a modified carbon paste electrode with silica gel coated with Meldolas Blue and salicylate hydroxylase. This biosensor detects the catechol produced in the enzymatic reaction through its electroxidation on the mediator. The influence of NADH/substrate ratio was verified and the biosensor presented response when this ratio was higher than 1.8. The pH of the medium significantly influences the results, requiring control within a narrow range of pH. The effect of the applied potential in the biosensor response showed that 200 mV vs SCE is the best potential to use in the measurements. The biosensor has a good linear response range of 10 to 60 μmol L−1, adjusted by the equation I = 11.7 (×1.3) + 6.78 (×0.03)[salicylate], with high sensitivity. The relative standard deviation for 6 measurements was 3.7%. The biosensor showed good stability for 20 days when used daily or for one month when stored in a refrigerator.

A new amperometric biosensor for salicylate based on salicylate hydroxylase immobilized on polipyrrole film doped with hexacyanoferrate

Abstract An amperometric biosensor for salicylate detection was developed by immobilizing salicylate hydroxylase via glutaraldehyde onto a polypyrrole film doped with hexacyanoferrate, supported on a glassy carbon electrode surface. The sensor monitors the catechol produced in the enzymatic reaction on the film surface, at an applied potential of 150 mV vs. SCE. A [NADH]/[salicylate] ratio between 2 and 4 gave the best response. The biosensor presented the best performance in a solution with pH = 7.4. The response time was about 40 s. A linear range of response was observed for salicylate concentrations between 1.0 × 10 −5 and 1.0 × 10 −4 mol l −1 and the equation adjusted for this curve was I = (−0.04 ± 0.01) + (11.4 ± 0.2)[salicylate] with a correlation coefficient of 0.999 for n = 6. The biosensor retains its activity for at least 10 days despite daily use. The results obtained using the biosensor for salicylate determination, in three different samples of antithermic drugs, presented a good correlation with the standard colorimetric method.