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Selenium is included in selenoprotein sequences, which participate in enzymatic processes necessary to preserve optimal health. Some lactic acid bacteria carry out the biotransformation of inorganic selenium in their metabolism. The complete biochemical mechanism of selenium biotransformation is still unknown; however, it is known that both the selenocysteine synthesis process and its subsequent incorporation into selenoproteins include serine as part of the action of seryl-RNAt synthetase. Therefore, the aim of this work was to determine the effect of serine during the biotransformation of selenium and the subsequence growth of Streptococcus thermophilus in a minimal medium. Two culture media were prepared, one enriched with the minimum inhibitory concentration of selenite (as Na 2 SeO 3 ) and the other as a mixture of the minimum inhibitory concentration of selenite and serine. The absorbed selenium concentration was measured by inductively coupled plasma, and the selenocysteine identification was performed by reverse-phase HPLC. In the second culture medium, decreases in both times, the adaptation and the logarithmic phase, were observed. According to the results, it was possible to establish that the presence of serine allowed the biotransformation of selenite into selenocysteine by Strep. thermophilus.

In this work, a procedure using solid phase microextraction in combination with capillary electrophoresis was developed for the determination of oxytetracycline in milk samples. The method involves the synthesis of poly(1-allyl-3-methyl imidazolium) chloride film on a stainless-steel bar via electropolymerization and its use as an adsorbent for oxytetracycline (OT) by an ionic exchange mechanism. The coated fiber is then immersed in milk samples for retention of oxytetracycline residues, followed by elution, drying, and reconstitution before analysis with capillary electrophoresis. The proposed method achieves a limit of detection of 70 µg L−1 with adequate precision and uncertainty, making this methodology appropriate for the determination of OT in milk samples. The method was applied to the pre-concentration and quantification of oxytetracycline in ten commercial milk samples. Two tested samples were positive for the presence of oxytetracycline but the concentration was below the maximum residue limit according to the international normative standard. The proposed methodology was evaluated according to the Eco-Scale approach, and the total score of 51 indicated that the methodology proposed is both green and acceptable despite the multi-stage character. SPME-CE methodology allows us to perform the sample pre-treatment and determination of OT in an effective and greener way, decreasing the number of steps during the analysis and the generation of waste.

It is well known that Pb(II) is considered a highly toxic metal. The slight difference between toxic and permissible levels in drinking water is a matter of concern; therefore, highly sensitive and selective techniques have been proposed for quantification, such as the electrochemical ones. Here, an easy, simple, low-cost, and highly selective sensor based on carbon paste electrodes (CPE) and ion-imprinted polymers (IIP) is proposed for Pb(II) analysis in real water samples. Recognition cavities, selective to Pb(II), were synthesized based on a cross-linked polymer using vinyl pyridine. A modified CPE was constructed by a mixture of graphite powder, IIP, and paraffin oil. By voltammetry studies, a notable difference was observed in the electrochemical response of the electrodes modified with IIP and those with non-imprinted polymer (NIP), confirming the existence of the recognition cavities in the IIP. The construction and analysis parameters related to the analytical response Pb(II) (anodic current intensity of stripping voltammetry), were optimized; the selectivity was also studied considering potential interference ions. A linear concentration range from 3.3 mg l-1 to 33 mg l-1 and a limit of detection of 0.99 mg l-1 were achieved. Pb(II) was successfully quantified in real complex samples without previous treatment.