Detecção piezelétrica não-gravimétrica de boro empregando cristal de quartzo com eletrodo separado e superfície quimicamente modificada / Non-gravimetric piezoelectric detection of boron by using quartz crystal with electrode separated and chemically modified surface

AUTOR(ES)
DATA DE PUBLICAÇÃO

2003

RESUMO

Piezoelectric quartz crystals (PQCs) with electrode-separated configuration and chemically modified surface were used to develop a boron sensor. A detection cell was built in such a way that a stainless-steel electrode was held at a short distance (0.3 mm) from the PQC surface, whose original gold electrode was removed. By using impedance spectroscopy, oscillation frequency measurements, and simulations of impedance spectra of the cell equivalent circuit, it was observed that the physico-chemical properties (conductivity, permissivity, density and viscosity) of the solution affect in different ways the characteristic frequencies of the cell (resonance and oscillation). In order to make the electrodeless PQC surface selective to boron, several chemical modification strategies were tested with the aim of anchoring poliols (manitol and N-metilglucamine) via silanization reactions or deposition of modified polymers. Best results were obtained with PQC coated with a thin film (0.1 to 0.5 µm) of a polymer, which was the product of the polyepichlorohydrin modification with N-methylglucamine (NG). The sensor was evaluated in a flow injection analysis system (FIA), which used a carrier solution containing disodic ethylenediaminetetraacetate (EDTA) 50 mmol/L (pH 8.5). It was observed that the mechanism of detection of this sensor is predominantly nongravimetric, owing to an increase in the stiffness of the covering film, because of the complex formation involving boron and the NG groups, which were incorporated in the polymer. Moreover, the sensor shows an interesting characteristic that is the boron retention, which allows decreasing the limit of detection (LOD) of the FIA method by increasing the injected sample volume. Boron is eluted from the sensor film with 1 mol/L manitol solution. For 160 and 1000 µL volumes of borate solution, the LODs were 2 and 0.3 µmol/L, respectively. It is possible to detect down to 3 ng of boron in the injected sample volume. The effect of several ions (Mg 2+, Ca2+, Al3+, Cr3+, Mn2+, Fé3+, Co2+, Cu2+, Ni2+, Zn2+, Sn2+, Pb2+, HPO42-, SiO32-, F-, Cl-, SO42-, NO3-, CH3COO-, CO32-, VO3-, Sb(OH)6-AsO4-, AsO3-, SeO3-, SeO4- e Ge(OH) 62-) was evaluated. Only the germanate ion showed a significant interference, thus, the sensor can also be suitable for this species. However, this interference can be eliminated by the addition of sulfide to the sample. The non-interference of the metal ions is due to the complex formation with EDTA present in the carrier solution. Polyols at high concentration in the samples affect the sensor response, because these compounds compete with the sensor film for the analyte. An automated FIA equipment was built and successfully applied for determination of boron in samples such as mineral water, grape juice, vinegar, wine, and a standard reference material (river water).

ASSUNTO(S)

flow injection analysis chemical surface modification boron cristais piezelétricos de quartzo modificação química de superfície análise por injeção em fluxo piezoelectric quartz crystal boro sensores sensors

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