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The interaction of estrone and estradiol with b-cyclodextrins (bCD) was investigated by differential pulse voltammetry (DPV) and high-performance liquid chromatography (HPLC) in mixed media. The co-solvent influence on the tendency of these estrogens to form inclusion complexes with bCD was examined. Thus, acetonitrile (MeCN) and ethanol (EtOH) were used in a mixed aqueous medium containing phosphate buffer. The association constant of the inclusion complexes (K a ) of estrone and estradiol with bCD were determined in two different media by using both voltammetric and chromatographic techniques. Estradiol was found to bind to bCD with higher affinities than estrone, irrespective of the medium. We have also found a clear influence of the co-solvent on the K a value, which means a competition of co-solvent molecules with estrogens for binding to the cavity of bCD. Consequently, interaction between bCD and the steroids is weakened when acetonitrile is used.

A detailed investigation of the electrochemical reduction of 2-nitroimidazole ͑2-NIm͒ in a mixed aqueous medium was carried out by means of cyclic voltammetry ͑CV͒ at a mercury electrode. The voltammetric behavior of 2-NIm in 60% dimethylformamide ͑DMF͒ ͑0.1 M tetrabutyl ammonium perchlorate, TBAP͒/40% aqueous buffer ͑0.3 M KCl + 0.015 M citric acid + 0.03 M boric acid͒ is pH-dependent, changing from one irreversible reduction peak at acid pHs to two reduction peaks at alkaline pHs. At pH 2.5 it is possible to obtain a voltammetric pK which is due to the protonation-deprotonation equilibrium produced by the hydroxylamine derivative formed from the reduction of 2-NIm. This indicates that 2-NIm is reduced to the unprotonated hydroxylamine derivative above pH 2.5 and to its protonated form below it. At pH Ͼ 7 it is possible to observe a cyclic voltammetric couple due to the one-electron reduction of 2-NIm to produce the corresponding nitro radical anion. Furthermore, the nitro radical anion disproportionates with a rate constant, k 2 , of 6.78 ϫ 10 5 M s −1 and a half-life, t 1/2 , of 1.5 ms for the first half-life. The voltammetric behavior of 2-NIm in aqueous mixed medium is substantially different from that described in nonaqueous medium; in fact, only in the aqueous medium is it possible to study in isolation the nitro radical anion.

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In this work, a new method to quantify nitrofurantoin in aqueous media using a flow injection system connected to commercially available screen-printed carbon nanofibers was developed. A pretreatment of the screen-printed carbon nanofibers electrode with BrittonÀRobinson buffer/N,N-dimethylformamide was applied to enhance the nitrofurantoin peak current signal in one step. The developed method was demonstrated to be sensible, reproducible, easy, and inexpensive. With a low detection limit, it is applicable to real samples. The results indicate that it is highly applicable for the detection of nitrofurantoin in several matrices. When urine samples without any pretreatment were analyzed, the method proved reproducible and sensible and had a low detection limit. The use of screenprinted electrodes has advantages over other modified electrodes as a glassy carbon due to its versatility.