Electrochemical and spectroscopic studies of the oxidation mechanism of the herbicide propanil

Electrochemical oxidation of propanil in deuterated solutions was studied by cyclic, differential pulse, and square wave voltammetry using a glassy carbon microelectrode. The oxidation of propanil in deuterated acid solutions occurs at the nitrogen atom of the amide at a potential of +1.15 V vs Ag/ AgCl. It was also found that, under the experimental conditions used, protonation at the oxygen atom of propanil occurs, leading to the appearance of another species in solution which oxidizes at +0.60 V. The anodic peak found at +0.79 V vs Ag/AgCl in deuterated basic solutions is related to the presence of an anionic species in which a negative charge is on the nitrogen atom. The electrochemical data were confirmed by the identification of all the species formed in acidic and basic deuterated solutions by means of NMR spectroscopy. The results are supported by electrochemical and spectroscopic studies of acetanilide in deuterated solutions.

Molecularly imprinted electrochemical sensor for ochratoxin A detection in food samples

A novel electrochemical sensor for ochratoxin A (OTA) detection was fabricated through the modification of a glassy carbon electrode (GCE) with multiwalled carbon nanotubes (MWCNTs) and a molecularly imprinted polymer (MIP). The MWCNTs dramatically promoted the sensitivity of the developed sensor, while polypyrrole (PPy) imprinted with OTA served as the selective recognition element. The imprinted PPy film was prepared by electropolymerization of pyrrole in the presence of OTA as a template molecule via cyclic voltammetry (CV). The electrochemical oxidation of OTA at the developed sensor was investigated by CV and differential pulse voltammetry (DPV). The developed MIP/MWCNT/GCE sensor showed a linear relationship, when using DPV, between peak current intensity and OTA concentration in the range between 0.050 and 1.0 μM, with limits of detection (LOD) and quantification of 0.0041 μM (1.7 μg/L) and 0.014 μM (5.7 μg/L) respectively. With the developed sensor precise results were obtained; relative standard deviations of 4.2% and 7.5% in the evaluation of the repeatability and reproducibility, respectively. The MIP/MWCNT/GCE sensor is simple to fabricate and easy to use and was successfully applied to the determination of OTA in spiked beer and wine samples, with recoveries between 84 and 104%, without the need of a sample pre-treatment step.

Electrochemical immunosensor for multiplexed detection of food-borne pathogens using nanocrystal bioconjugates and MWCNT screen-printed electrode

Bacterial food poisoning is an ever-present threat that can be prevented with proper care and handling of food products. A disposable electrochemical immunosensor for the simultaneous measurements of common food pathogenic bacteria namely Escherichia coli O157:H7 (E. coli), campylobacter and salmonella were developed. The immunosensor was fabricated by immobilizing the mixture of anti-E. coli, anticampylobacter and anti-salmonella antibodies with a ratio of 1:1:1 on the surface of the multiwall carbon nanotube-polyallylamine modified screen printed electrode (MWCNT-PAH/SPE). Bacteria suspension became attached to the immobilized antibodies when the immunosensor was incubated in liquid samples. The sandwich immunoassay was performed with three antibodies conjugated with specific nanocrystal ( -E. coli-CdS, -campylobacter-PbS and -salmonella-CuS) which has releasable metal ions for electrochemical measurements. The square wave anodic stripping voltammetry (SWASV) was employed to measure released metal ions from bound antibody nanocrystal conjugates. The calibration curves for three selected bacteria were found in the range of 1 × 103 – 5 × 105 cells mL−1 with the limit of detection (LOD) 400 cells mL−1 for salmonella, 400 cells mL−1 for campylobacter and 800 cells mL−1 for E. coli. The precision and sensitivity of this method show the feasibility of multiplexed determination of bacteria in milk samples.

Produção de hidrogénio através da gasificação da biomassa

A procura de uma forma limpa de combustível, aliada à crescente instabilidade de preços dos combustíveis fósseis verificada nos mercados faz com que o hidrogénio se torne num combustível a considerar devido a não resultar qualquer produto poluente da sua queima e de se poder utilizar, por exemplo, desperdícios florestais cujo valor de mercado não está inflacionado por não pertencer à cadeia alimentar humana. Este trabalho tem como objetivo simular o processo de gasificação de biomassa para produção de hidrogénio utilizando um gasificador de leito fluidizado circulante. O oxigénio e vapor de água funcionam como agentes gasificantes. Para o efeito usou-se o simulador de processos químicos ASPEN Plus. A simulação desenvolvida compreende três etapas que ocorrem no interior do gasificador: pirólise, que foi simulada por um bloco RYIELD, combustão de parte dos compostos voláteis, simulada por um bloco RSTOIC e, por fim, as reações de oxidação e gasificação do carbonizado “char”, simuladas por um bloco RPLUG. Os valores de rendimento dos compostos após a pirólise, obtidos por uma correlação proposta por Gomez-Barea, et al. (2010), foram os seguintes: 20,33% “char”, 22,59% alcatrão, 36,90% monóxido de carbono, 16,05%m/m dióxido de carbono, 3,33% metano e 0,79% hidrogénio (% em massa). Como não foi possível encontrar valores da variação da composição do gás à saída do gasificador com a variação da temperatura, para o caso de vapor de água e oxigénio, optou-se por utilizar apenas vapor na simulação de forma a comparar os seus valores com os da literatura. Às temperaturas de 700, 770 e 820ºC, para um “steam-to-biomass ratio”, (SBR) igual a 0,5, os valores da percentagem molar de monóxido de carbono foram, respetivamente, 56,60%, 55,84% e 53,85%, os valores de hidrogénio foram, respetivamente, 17,83%, 18,25% e 19,31%, os valores de dióxido de carbono foram, respetivamente, 16,40%, 16,85% e 17,93% e os valores de metano foram, respetivamente, 9,00%, 8,95% e 8,83%. Os valores da composição à saída do gasificador, à temperatura de 820ºC, para um SBR de 0,5 foram: 53,85% de monóxido de carbono, 19,31% de hidrogénio, 17,93% de dióxido de carbono e 8,83% de metano (% em moles). Para um SBR de 0,7 a composição à saída foi de 54,45% de monóxido de carbono, 19,01% de hidrogénio, 17,59% de dióxido de carbono e 8,87% de metano. Por fim, quando SBR foi igual a 1 a composição do gás à saída foi de 55,08% de monóxido de carbono, 18,69% de hidrogénio, 17,24% de dióxido de carbono e 8,90% de metano. Os valores da composição obtidos através da simulação, para uma mistura de ar e vapor de água, ER igual a 0,26 e SBR igual a 1, foram: 34,00% de monóxido de carbono, 14,65% de hidrogénio, 45,81% de dióxido de carbono e 5,41% de metano. A simulação permitiu-nos ainda dimensionar o gasificador e determinar alguns parâmetros hidrodinâmicos do gasificador, considerando que a reação “water-gas shift” era a limitante, e que se pretendia obter uma conversão de 95%. A velocidade de operação do gasificador foi de 4,7m/s e a sua altura igual a 0,73m, para um diâmetro de 0,20m.

Síntese de derivados de ácidos fenólicos e estudo da sua actividade antioxidante

A peroxidação lipídica, além de causar sérios danos no corpo humano, é a principal causa da deterioração dos alimentos afectando a sua cor, aroma e valor nutricional, o que conduz a uma diminuição do seu ciclo de vida. O fenómeno de oxidação pode ser retardado com recurso a antioxidantes, de origem natural ou sintética, que inibam a formação de espécies reactivas, ou que reajam com estas, formando posteriormente radicais com menor grau de reactividade. Nesta dissertação procedeu-se à síntese e elucidação estrutural de novos antioxidantes, derivados dos ácidos 3,4-diidroxibenzóico (PCA) e 3,4-diidroxifenilacético (DOPAC), e à avaliação da sua actividade anti-radicalar e antioxidante. Os novos antioxidantes sintetizados foram caracterizados usando RMN de 1H e de 13C, FTIR e EM-IE. A avaliação da actividade antioxidante foi realizada com base no método do radical 2,2- difenil-1-picrilhidrazilo (DPPH·) e por técnicas electroquímicas (voltametria de impulso diferencial e voltametria cíclica). Os resultados obtidos permitiram concluir que a eficácia anti-radicalar (AE) é determinada por aspectos da estrutura molecular dos compostos, nomeadamente pela presença de grupos hidroxilo no anel aromático e também de grupos extensores relativamente à posição do grupo carboxílico. Os resultados permitiram verificar que o DOPAC apresenta a mais elevada eficiência anti-radicalar dos compostos em estudo, incluindo o trolox e o ácido gálhico (compostos de referência). Com base nos resultados obtidos concluiu-se que, um menor potencial de oxidação conduz a uma melhor actividade anti-radicalar dos compostos. De facto, verificou-se para o PCA e respectivos ésteres o maior potencial de oxidação e também a menor eficiência anti-radicalar. Em contrapartida, os antioxidantes com maior eficiência anti-radicalar, DOPAC, trolox e ácido gálhico, apresentaram menor potencial de oxidação.

New insights into the oxidation pathways of apomorphine

A detailed study of the oxidative behaviour of apomorphine in aqueous media is reported. Resorting to the synthesis of apomorphine derivatives it was possible to identify all the anodic oxidation peaks of apomorphine, which are related to the oxidation of the catechol and tertiary amine groups. These findings were revealed to be important since they could lead to a better understanding of the biological interactions of apomorphine and gain insight into its metabolic pathways. During the voltammetric studies, it was also found that apomorphine forms a complex with borate through the catechol group leading to an increase of its oxidation potential. This property could be very useful with regard to the stabilization of apomorphine solutions since it could drastically reduce its autoxidation.

Voltammetric oxidation of drugs of abuse I. Morphine and metabolites

A detailed study of the electrochemical oxidative behavior of morphine in aqueous solution is reported. Through the synthesis of several metabolites and derivatives, pseudomorphine, morphine N-oxide, normorphine, dihydromorphine and 2-(N,N-dimethylaminomethyl)morphine, and their voltammetric study it was possible to identify the oxidation peaks for morphine. The anodic waves are related with the oxidation of phenolic and tertiary amine groups. It is also possible to verify that a poorly defined peak observable during morphine oxidation is not a consequence of further oxidation of pseudomorphine but due to formation of a dimer during phenolic group oxidation. The results obtained and especially those regarding the formation of a new polymer based on a C O coupling could be useful for clarifying the discoloration phenomenon occurring during storage of morphine solutions as well as leading to a better understanding of its oxidative metabolic pathways.

Voltammetric oxidation of drugs of abuse II. Codeine and metabolites

The oxidation of codeine on glassy carbon electrodes has been studied in detail using differential pulse voltammetry. The results obtained using a glassy carbon electrode clearly show a much more complex oxidation mechanism than that previously reported when platinum and gold electrodes were used. To clarify the codeine oxidative profile, several metabolites and analogues of this alkaloid, codeine N-oxide, norcodeine, dihydrocodeine, acetylcodeine and 6- chlorodesoxycodeine, were synthesized and studied. It was deduced that the anodic waves observed in codeine oxidation are related to the presence of methoxy, hydroxy and tertiary amine groups. Due to the similarity of potentials at which these oxidative processes take place, at some pHs an overlap of peaks occurs and only one anodic wave is observed.

Voltammetric oxidation of drugs of abuse III. Heroin and metabolites

The oxidative behavior of heroin in aqueous solution is reported. In order to identify its oxidation peaks, several metabolites, 6-monoacetylmorphine, 3-monoacetylmorphine and norheroin, were synthesized and their electrochemical behavior studied using differential pulse voltammetry. The anodic waves observed for heroin correspond to the oxidation of the tertiary amine group and its follow-up product (secondary amine), and to the oxidation of the phenolic group obtained from hydrolysis, at alkaline pHs, of the 3-acetyl group. The results enabled a new oxidative mechanism for heroin to be proposed in which a secondary amine, norheroin, and an aldehyde are obtained. The voltammetric behavior of 6-monoacetylmorphine and morphine was found to be similar demonstrating that the presence of an acetyl substituent on the 6-hydroxy group does not have a relevant influence on the peak potential of the wave resulting from oxidation of the 3-phenolic group.

Static and hydrodynamic monitoring of citalopram based on its electro-oxidation behavior at a glassy-carbon surface

The electrooxidative behavior of citalopram (CTL) in aqueous media was studied by cyclic voltammetry (CV) and square-wave voltammetry (SWV) at a glassy-carbon electrode. The electrochemical behaviour of CTL involves two electrons and two protons in the irreversible and diffusion controlled oxidation of the tertiary amine group. The maximum analytical signal was obtained in a phosphate buffer (pH ¼ 8.2). For analytical purposes, an SWV method and a flow-injection analysis (FIA) system with amperometric detection were developed. The optimised SWV method showed a linear range between 1.10 10 5–1.20 10 4 molL 1, with a limit of detection (LOD) of 9.5 10 6 molL 1. Using the FIA method, a linear range between 2.00 10 6–9.00 10 5 molL 1 and an LODof 1.9 10 6 molL 1 were obtained. The validation of both methods revealed good performance characteristics confirming applicability for the quantification of CTL in several pharmaceutical products.

Electrochemical oxidation of propanil and related N-substituted amides

The electrochemical behaviour of propanil and related N-substituted amides (acetanilide and N,N-diphenylacetamide) was studied by cyclic and square wave voltammetry using a glassy carbon electrode. Propanil has been found to have chemical stability under the established analytical conditions and showed an oxidation peak at +1.27V versus Ag/AgCl at pH 7.5. N,N-diphenylacetamide has a higher oxidation potential than the other compounds of +1.49V versus Ag/AgCl. Acetanilide oxidation occurred at a potential similar to that of propanil, +1.24V versus Ag/AgCl. These results are in agreement with the substitution pattern of the nitrogen atom of the amide. A degradation product of propanil, 3,4-dichloroaniline (DCA), was also studied, and showed an oxidation peak at +0.66V versus Ag/AgCl. A simple and specific quantitative electroanalytical method is described for the analysis of propanil in commercial products that contain propanil as the active ingredient, used in the treatment of rice crops in Portugal.

Maltol determination in food by microwave assisted extraction and electrochemical detection

An electrochemical method is proposed for the determination of maltol in food. Microwave-assisted extraction procedures were developed to assist sample pre-treating steps. Experiments carried out in cyclic voltammetry showed an irreversible and adsorption controlled reduction of maltol. A cathodic peak was observed at -1.0 V for a Hanging Mercury Drop Electrode versus an AgCl/Ag (in saturated KCl), and the peak potential was pH independent. Square wave voltammetric procedures were selected to plot calibration curves. These procedures were carried out with the optimum conditions: pH 6.5; frequency 50 Hz; deposition potential 0.6 V; and deposition time 10 s. A linear behaviour was observed within 5.0 × 10-8 and 3.5 × 10-7 M. The proposed method was applied to the analysis of cakes, and results were compared with those obtained by an independent method. The voltammetric procedure was proven suitable for the analysis of cakes and provided environmental and economical advantages, including reduced toxicity and volume of effluents and decreased consumption of reagents.

Celiac disease diagnosis and gluten-free food analytical control

Celiac disease (CD) is an autoimmune enteropathy, characterized by an inappropriate T-cell-mediated immune response to the ingestion of certain dietary cereal proteins in genetically susceptible individuals. This disorder presents environmental, genetic, and immunological components. CD presents a prevalence of up to 1% in populations of European ancestry, yet a high percentage of cases remain underdiagnosed. The diagnosis and treatment should be made early since untreated disease causes growth retardation and atypical symptoms, like infertility or neurological disorders. The diagnostic criteria for CD, which requires endoscopy with small bowel biopsy, have been changing over the last few decades, especially due to the advent of serological tests with higher sensitivity and specificity. The use of serological markers can be very useful to rule out clinical suspicious cases and also to help monitor the patients, after adherence to a gluten-free diet. Since the current treatment consists of a life-long glutenfree diet, which leads to significant clinical and histological improvement, the standardization of an assay to assess in an unequivocal way gluten in gluten-free foodstuff is of major importance.

FIA potentiometric system based on periodate polymeric membrane sensors for the assessment of ascorbic acid in commercial drinks

Ascorbic acid is found in many food samples. Its clinical and technological importance demands an easyto- use, rapid, robust and inexpensive method of analysis. For this purpose, this work proposes a new flow procedure based on the oxidation of ascorbic acid by periodate. A new potentiometric periodate sensor was constructed to monitor this reaction. The selective membranes were of PVC with porphyrin-based sensing systems and a lipophilic cation as additive. The sensor displayed a near-Nernstian response for periodate over 1.0x10-2–6.0x10-6 M, with an anionic slope of 73.9 ± 0.9 mV decade-1. It was pH independent in acidic media and presented good selectivity features towards several inorganic anions. The flow set-up operated in double-channel, carrying a 5.0x10-4 M IO- 4 solution and a suitable buffer; these were mixed in a 50-cm reaction coil. The overall flow rate was 7 ml min-1 and the injection volume 70 µl. Under these conditions, a linear behaviour against concentration was observed for 17.7–194.0 µg ml-1, presenting slopes of 0.169 mV (mg/l)-1, a reproducibility of ±1.1 mV (n = 5), and a sampling rate of ~96 samples h-1. The proposed method was applied to the analysis of beverages and pharmaceuticals.

Food consumption, physical activity and socio-economic status related to BMI, waist circumference and waist-to-height ratio in adolescents

Objective: To examine the association between obesity and food group intakes, physical activity and socio-economic status in adolescents. Design: A cross-sectional study was carried out in 2008. Cole’s cut-off points were used to categorize BMI. Abdominal obesity was defined by a waist circumference at or above the 90th percentile, as well as a waist-to-height ratio at or above 0?500. Diet was evaluated using an FFQ, and the food group consumption was categorized using sex-specific tertiles of each food group amount. Physical activity was assessed via a self-report questionnaire. Socio-economic status was assessed referring to parental education and employment status. Data were analysed separately for girls and boys and the associations among food consumption, physical activity, socio-economic status and BMI, waist circumference and waist-to-height ratio were evaluated using logistic regression analysis, adjusting the results for potential confounders. Setting: Public schools in the Azorean Archipelago, Portugal. Subjects: Adolescents (n 1209) aged 15–18 years. Results: After adjustment, in boys, higher intake of ready-to-eat cereals was a negative predictor while vegetables were a positive predictor of overweight/ obesity and abdominal obesity. Active boys had lower odds of abdominal obesity compared with inactive boys. Boys whose mother showed a low education level had higher odds of abdominal obesity compared with boys whose mother presented a high education level. Concerning girls, higher intake of sweets and pastries was a negative predictor of overweight/obesity and abdominal obesity. Girls in tertile 2 of milk intake had lower odds of abdominal obesity than those in tertile 1. Girls whose father had no relationship with employment displayed higher odds of abdominal obesity compared with girls whose father had high employment status. Conclusions: We have found that different measures of obesity have distinct associations with food group intakes, physical activity and socio-economic status.