M. Teresa S.R. Gomes

Thin-film electrochemical sensor for diphenylamine detection using molecularly imprinted polymers

This work reports on the development of a new voltammetric sensor for diphenylamine based on the use of a miniaturized gold electrode modified with a molecularly imprinted polymer recognition element. Molecularly imprinted particles were synthesized ex situ and further entrapped into a poly(3,4-ethylenedioxythiophene) polymer membrane, which was electropolymerized on the surface of the gold electrode. The thickness of the polymer layer was optimized in order to get an adequate diffusion of the target analyte and in turn to achieve an adequate charge transfer at the electrode surface. The resulting modified electrodes showed a selective response to diphenylamine and a high sensitivity compared with the bare gold electrode and the electrode modified with poly(3,4-ethylenedioxythiophene) and non-imprinted polymer particles. The sensor showed a linear range from 4.95 to 115 μM diphenylamine, a limit of detection of 3.9 μM and a good selectivity in the presence of other structurally related molecules. This sensor was successfully applied to the quantification of diphenylamine in spiked apple juice samples.

Detection of volatile amines using a quartz crystal with gold electrodes

Abstract A quartz crystal with gold electrodes is used to detect and quantify volatile amines. The magnitude of the observed frequency decrease is highly dependent on the roughness of the surface of the gold electrodes and sensitivity can be increased by rubbing them with alumina. Water increases the observed signal, leading to important errors in the amine quantification. However, the presence of minor amounts of water is readily detected, as the plotted frequency signal shows a marked distortion. The effect of water on the interaction between amine and gold was studied by infrared spectroscopy.

Quantification of CO2, SO2, NH3, and H2S with a single coated piezoelectric quartz crystal

Abstract Previous experiments showed that a quartz crystal coated with tetramethylammonium fluoride tetrahydrate (TMAF) could be used to quantify CO2. It was also known that TMAF absorbed SO2 and H2S, although there were no studies involving quartz crystal microbalances (QCMs) with the latter gas. The remarkable stability of the frequency of the TMAF-coated crystals was reason enough to test them for a series of gases of environmental interest: CO2, SO2, NH3, H2S, and CO. Studies of sensitivity of TMAF to those gases are reported, along with a detailed discussion of the signal shape. Traps of specially prepared molecular sieves were used in an attempt to separate some of the analytes.

Comparison of two methods for coating piezoelectric crystals

Abstract Quartz piezoelectric crystals were coated with triethanolamine using two procedures: spray and syringe methods. The reproducibility of both coating methods was evaluated comparing the relative standard deviation of batches of four crystals coated by each of the procedures. The relative standard deviation obtained with the spray method is a hyperbolic function of the coating frequency and is, at least, three times lower than the one calculated from results of the syringe method, which is not frequency dependent. An explanation for the poorer reproducibility of the syringe method is given, based on the radial sensitivity theory and the non-uniformity of the coating distribution over the crystal, as evidenced by microscopic photographs.

Development of an electronic nose to identify and quantify volatile hazardous compounds.

A new electronic nose was developed to identify the chemical compound released when a 2.5-L flask was broken inside a 3 m x 3 m x 2.5 m store-room. Flasks of 10 different hazardous compounds were initially present in the room: ammonia, propanone, hexane, acetic acid, toluene, methanol, tetrachloromethane, chloroform, ethanol and dichloromethane. Besides identification, quantification of the compound present in the air was also performed by the electronic nose, in order to evaluate the risk level for room cleaning. An array of six sensors based on coated piezoelectric quartz crystals was used. Although none of the individual sensors was specific for a single compound, an artificial neural network made it possible to identify and quantify the released vapour, among a series of 10 compounds, with six sensors. The neural network could be simplified, and the number of neurons reduced, provided it was used just for the identification task. Quantification could be performed later using the individual calibration of the sensor most sensitive to the identified compound.

Detection of CO2 using a qaurtz crystal microbalance

Abstract Several amines have been evaluated with respect to stability, sensitivity and reversibility, regarding the analysis of carbon dioxide. The sensitivity of the amines has been related to their molecular structure and reactivity, and THEED was shown to be the best choice. However, 1,2-diaminoethane, 1,3-diaminopropane and 1,2-diaminopropane can be used, at least at 1 °C, with a smaller sensitivity and for shorter periods.

Aluminium migration into beverages : Are dented cans safe?

Aluminium (Al) migration from cans to beer and tea was studied along time. Analyses of Al in the canned drinks were performed till the sell-by date, and, in seven months, aluminium migration was found to increase 0.14 mg L(-1) in beer, and 0.6 mg L(-1) in tea. This study included dented cans from which aluminium migration into tea was found to be particularly severe. Al concentration in dented canned tea increased 9.6 mg L(-1) in seven months.

The quality of our drinking water: Aluminium determination with an acoustic wave sensor

A new methodology based on an inexpensive aluminium acoustic wave sensor is presented. Although the aluminium sensor has already been reported, and the composition of the selective membrane is known, the low detection limits required for the analysis of drinking water, demanded the inclusion of a preconcentration stage, as well as an optimization of the sensor. The necessary coating amount was established, as well as the best preconcentration protocol, in terms of oxidation of organic matter and aluminium elution from the Chelex-100. The methodology developed with the acoustic wave sensor allowed aluminium quantitation above 0.07 mg L(-1). Several water samples from Portugal were analysed using the acoustic wave sensor, as well as by UV-vis spectrophotometry. Results obtained with both methodologies were not statistically different (alpha=0.05), both in terms of accuracy and precision. This new methodology proved to be adequate for aluminium quantitation in drinking water and showed to be faster and less reagent consuming than the UV spectrophotometric methodology.

Quartz crystal microbalance with gold electrodes as a sensor for monitoring gas-phase adsorption/desorption of short chain alkylthiol and alkyl sulfides

A quartz crystal microbalance (QCM) with gold electrodes was used as a sensor to monitor the gas-phase adsorption/desorption of ethanethiol, propane-1-thiol, 2-methylpropane-2-thiol, 2-methylpropane-1-thiol, butane-1-thiol, butane-2-thiol, pentanethiol, methyl sulfide and ethyl methyl sulfide. The results show that the QCM is able to distinguish adsorbed layers with different binding energies from the gold.

The quantification of sodium in mineral waters using a quartz crystal microbalance

A sensor for sodium based on a piezoelectric quartz crystal is proposed. The quartz crystal was coated with 5% of bis[(12-crown-4)methyl] dodecylmethylmalonate, 33% of PVC and 62% of NPOE to which KTpClPB in a 22% molar proportion to the ionophore was added. Coating amounts producing a frequency decrease around 18 kHz show optimum linear calibration ranges for the analysis of sodium in commercial mineral waters. Besides sensitivity, coating stability and selectivity of the sensor over other cations were adequate for those analyses. The results obtained analysing commercial mineral waters by the proposed method are not significantly different (alpha=0.05) from the ones obtained by atomic spectrometry.