Guilherme Braga

Guilherme S. Braga, Leonardo G.Paterno, John Paul H.Lima, Fernando J.Fonseca, Adnei M.de Andrade
The influence of deposition parameters, namely polymer concentration and pH of the deposition solution, cleaning, and drying steps on the morphology and electrical characteristics of polyaniline and sulfonated polystyrene (PANI/PSS) nanostructured films deposited by the self-assembly technique is evaluated by UV–Vis spectroscopy, optical and atomic force microscopy, and electrical resistance measurements. It is found that stirring the cleaning solution during the cleaning step is crucial for obtaining homogenous films. The stirring of the cleaning solution also influences the amount of PANI adsorbed in the films. In this regard, the drying process seems to be less critical since PANI amount and film thickness are similar in films dried with N2 flow or with an absorbent tissue. It is observed, however, that drying with N2 flow results in rougher films. As an additional point, an assessment of the influence of the deposition method (manual versus mechanical) on the film characteristics was carried out. A significant difference in the amount of PANI and film thickness between films prepared by different human operators and by a homemade mechanical device was observed. The variability in film thickness and PANI adsorbed amount is smaller in films mechanically assembled.

Guilherme S.Braga, Leonardo G.Paterno, Fernando J.Fonseca
2-Methylisoborneol (MIB) and geosmin (GSM) are sub-products from algae decomposition and, depending on their concentration, can be toxic; otherwise, they give unpleasant taste and odor to the water. For water treatment companies it is important to constantly monitor their presence in the distributed water and avoid further customer complaints. Lower-cost and easy-to-read instrumentation would be very promising in this regard. In this study, we evaluate the potentiality of an electronic tongue (ET) system based on non-specific polymeric sensors and impedance measurements in monitoring MIB and GSM in water samples. Principal component analysis (PCA) applied to the generated data matrix indicated that this ET was capable to perform with remarkable reproducibility the discrimination of these two contaminants in either distilled or tap water, in concentrations as low as 25 ng L−1. Nonetheless, this analysis methodology was rather qualitative and laborious, and the outputs it provided were greatly subjective. Also, data analysis based on PCA severely restricts automation of the measuring system or its use by nonspecialized operators. To circumvent these drawbacks, a fuzzy controller was designed to quantitatively perform sample classification while providing outputs in simpler data charts. For instance, the ET along with the referred fuzzy controller performed with a 100% hit rate the quantification of MIB and GSM samples in distilled and tap water. The hit rate could be read directly from the plot. The lower cost of these polymeric sensors allied to the special features of the fuzzy controller (easiness on programming
and numerical outputs) provided initial requirements for developing an automated ET system to monitor odorant species in water production and distribution.

Eduardo M. Rodríguez; Marco R. Cavallari; Guilherme S. Braga; Estrella F. González Rodríguez; Fernando J. Fonseca
Three types of Organic Thin Film Transistors (OTFTs) based on poly-3-hexylthiophene (P3HT) were evaluated for discrimination of solutions from the four basic tastes: sweet, bitter, salty and sour. High-k titanium oxynitride dielectric was employed altogether with thermal silicon oxide and different channel geometries to improve taste identification. Tastants as sucrose, sodium chloride and hydrochloric acid were detected even below the human threshold. The present results demonstrate a good potential of these devices to operate under low voltage in aqueous media and to compose electronic tongues based in OTFTs for beverages and food classification.

Sandra M. Zoldan, Guilherme S. Braga, Fernando J. Fonseca, Mercedes C. Carrão-Panizzi
An electronic tongue system was tested as a fast and efficient analytical tool for flavor evaluation of soybean genotypes. Grain samples of 25 soybean lines were analyzed using 0.25 g of milled samples added to 100 mL of distilled water and mixing for one minute on a magnetic stirrer. An aliquot (50 mL) from the filtered liquid was used for the analysis on a pre-fixed frequency of 1 kHz and alternate tension of 50 mV. Two analyses were conducted in a complete randomized design with three replicates. Electrical response (capacitance) of eight polymeric chemical sensors used to analyze the soybean lines were submitted to Principal Component Analysis (PCA). In the spatial distribution of the PCA graphic, the lines close to each other were similar, while the distant ones showed different characteristics. The electronic tongue system was efficient in discriminating the flavor of soybean lines.

Marco R. Cavallari, José E. E. Izquierdo, Guilherme S. Braga, Ely A. T. Dirani, Marcelo A. Pereira-da-Silva, Estrella F. G. Rodríguez and Fernando J. Fonseca
Electronic devices based on organic thin-film transistors (OTFT) have the potential to supply the demand for portable and low-cost gadgets, mainly as sensors for in situ disease diagnosis and environment monitoring. For that reason, poly(3-hexylthiophene) (P3HT) as the active layer in the widely-used bottom-gate/bottom-contact OTFT structure was deposited over highly-doped silicon substrates covered with thermally-grown oxide to detect vapor-phase compounds. A ten-fold organochloride and ammonia sensitivity compared to bare sensors corroborated the application of this semiconducting polymer in sensors. Furthermore, P3HT TFTs presented approximately three-order higher normalized sensitivity than any chemical sensor addressed herein. The results demonstrate that while TFTs respond linearly at the lowest concentration values herein, chemical sensors present such an operating regime mostly above 2000 ppm. Simultaneous alteration of charge carrier mobility and the threshold voltage is responsible for pushing the detection limit down to units of ppm of ammonia, as well as tens of ppm of alcohol or ketones. Nevertheless, P3HT transistors and chemical sensors could compose an electronic nose operated at room temperature for a wide range of concentration evaluation (1–10,000 ppm) of gaseous analytes. Targeted analytes include not only biomarkers for diseases, such as uremia, cirrhosis, lung cancer, and diabetes, but also gases for environment monitoring in food, cosmetic, and microelectronics industries.

Guilherme S. Braga, Peter A. Lieberzeit, Fernando J. Fonsecaa
Herein we report the development of a selective chemical sensor based on molecularly imprinted polymers (MIP) and quartz crystal microbalance (QCM) to detect isoborneol (ISO) in aqueous samples. Their response scales linearly with isoborneol concentration and exhibits a sensitivity of -16.23 Hz mM-1 cm-2 at the active area, with a gradient of -25 Hz mM-1. Moreover, they respond rapidly, reversibly, and reproducibly. Sensor layers are stable within a period of four weeks during use as compared to a few days for natural receptors. Moreover, the ISO-MIP sensor calculated theoretical detection limit is 0.58 mM.

Marco R. Cavallari; Guilherme S. Braga; Mauro F. P. da Silva; José E. E. Izquierdo; Leonardo G. Paterno; Ely A. T. Dirani; Ioannis Kymissis; Fernando J. Fonseca
In this contribution, the selectivity toward a diabetes biomarker was demonstrated by a non-specific impedance-metric chemical sensor array from blends of graphene oxide (GO)-based materials as a multivariate system for simultaneous aqueous and gaseous analyte investigation. The electrical impedance of bare graphene either oxidized or after reduction (RGO) displayed high specificity toward ammonia. The sensitivity of GO thin-film capacitance was 10.4 %/ppm of ammonia dissolved in ultrapure water, whereas RGO resistance featured 1.8 %/ppm to gaseous ammonia. However, composites with metal oxides, despite even providing a superior sensitivity to ammonia, completely alter the sign of sensor response to enable the distinction of alcohols. Ceria and cyclodextrin allowed GO to operate in the air at room temperature with improved stability and faster response of approximately 60 s. These materials made for an increase in sensitivity to acetone of 11 and 3.2 times, respectively, compared to RGO. Therefore, GO-based composites, as well as the junction of electronic nose and tongue arrays were fundamental to enable the separation of acetone from alcohols and ammonia after principal component analysis.