The interaction between mercury (Hg) and selenium (Se) is one of the best known examples of biological antagonism, yet the underlying mechanism remains unclear. This review focuses on the possible pathways leading to the Hg-Se antagonism, with an emphasis on the potential Hg-Se compounds that are responsible for the antagonism at the molecular level (i.e., bis[methylmercuric]selenide, methylmercury selenocysteinate, selenoprotein P-bound HgSe clusters, and the biominerals HgSe(x)S(1-x)). The presence of these compounds in biological systems has been suggested by direct or indirect evidence, and their chemical properties support their potentially key roles in alleviating the toxicity of Hg and Se (at high Hg and Se exposures, respectively) and deficiency of Se (at low Se exposures). Direct analytical evidences are needed, however, to confirm their in vivo presence and metabolic pathways, as well as to identify the roles of other potential Hg-Se compounds. Further studies are also warranted for the determination of thermodynamic properties of these compounds under physiological conditions toward a better understanding of the Hg-Se antagonism in biota, particularly under real world exposure scenarios.

A new chemical demethylation pathway for methylmercury under physiologically and environmentally relevant conditions is reported. The pathway involves the reaction between methylmercury and a selenoamino acid (L-selenocysteine, L-selenoglutathione, D,L-selenopenicillamine, or L-selenomethionine) via the formation of bis(methylmercuric)selenide and dimethylmercury as intermediates. The final degradation product is HgSe(s).

The electrochemical detection of chemical warfare agent (CWA) mimics was explored using multiwalled carbon nanotubes (MWCNTs) on indium tin oxide (ITO) surfaces in connection with ferrocene-amino acid conjugates. Various ferrocene-amino acid conjugates were synthesized and utilized as the recognition layer for the detection of CWA mimics. The ferrocene-amino acid conjugates were noncovalently attached to the pretreated MWCNTs on the ITO surface and reacted with CWA mimics, upon which the electrical properties of the MWCNTs and the Fc group were affected significantly. Alternating current voltammetry and capacitance-based detection offered large dynamic ranges for the detection of methylphosphonic acid, diethyl cyanophosphonate, ethylmethylphosphonate, and pinacolyl methylphosphonate in water. Electrochemical measurements showed dramatic changes upon the electrostatic interaction between the CWA mimics and the ferrocene-amino acid conjugates immobilized on MWCNTs on ITO surfaces. Electrochemical sensing in connection with MWCNTs is shown to be a promising analytical tool for the trace-level detection of CWA mimics in aqueous solutions.

The cystamine conjugate [(BocNH)Fc(CO)CSA]2 was prepared by coupling cystamine with the N-protected ferrocene amino acid derivative BocHN-Fc-COOH and was fully characterized by spectroscopic methods and by single-crystal X-ray diffraction. The cystamine conjugate forms films on gold substrates, which upon deprotection of the amino group, react with chemical warfare agent (CWA) mimics, upon which the redox properties of the Fc group are affected significantly. Cyclic voltammetry shows 50(5) mV anodic shifts of the Fc redox potentials after exposure to EtSCH2CH2Cl, a simulant for sulfur mustard HD (MA), and (NC)(EtO)2P(O), a simulant for nerve agent Tabun (NA). Exposure to MA and NA causes an increase in 2.3 and 4.5 ng mass, respectively, in QCM which indicates ca. 70% efficiency in Boc-deprotection. Ellipsometry measured a film thickness increase from 6(+/-1) A for the deprotected film to 10(+/-4) A for the film modified with MA and to 7(+/-2) A for the film modified with NA. AFM measurements show changes in the thickness and morphology of the film after reaction with MA and NA. The surfaces were analyzed by X-ray photoelectron spectroscopy (XPS) and clearly show the attachment of the cystamine conjugate on the surface and its reaction with CWA mimics.