Molecular recognition is the key step in a wide range of controlled separation and chemical transformation processes, with enzymes performing this task with an unsurpassed degree of selectivity. Enzymes contain only 20 simple amino acids, yet it remains difficult to rationalize or even predict these stereospecific recognition events. Nonetheless, the rational design of receptors able to recognize amino acids stereospecifically is attracting considerable interest because therapeutic drugs, that may be developed from chiral amino acid intermediates, are increasingly required in enantiomerically pure form. Early work has stimulated the development of efficient receptors based on small molecules, but binding of amino acids with high and predictable stereospecificity remains difficult to achieve. Directed molecular evolution, on the other hand, does select for RNA sequences or antibodies that bind amino acids with high specificity, but typically without providing insights into the molecular recognition mechanisms involved. Here we show that a rationally designed metal complex formed from a trivalent cobalt ion and a tetradentate ligand binds natural amino acids, including the simple yet challenging amino acid alanine, with high and predictable regio- and stereospecificity. We expect that our approach will allow the binding as well as separation and stereospecific catalytic formation of its target amino acids.

Here we present a quartz crystal microbalance (QCM) sensor for the highly selective and sensitive detection of Hg 2+ ion, a toxic chemical species and a hazardous environmental contaminant. Hg 2+ ion can be quantitatively measured based on changes in the resonance frequency of QCM following mass changes on the QCM sensor surface. The high selectivity for Hg 2+ ion in this study can be obtained using a thymine-Hg 2+-thymine pair, which is more stable than the adeninethymine base pair in DNA. On the other hand, gold nanoparticles (AuNPs) and their size-enhancement techniques were used to amplify the QCM signals to increase the sensitivity for Hg 2+ ion. With this strategic approach, the proposed QCM sensor can be used to quantitatively analyze Hg 2+ ion with high selectivity and sensitivity. The detection limit was as low as 98.7 pM. The sensor failed to work with other metal ions at concentrations 1000-times higher than that of the Hg 2+ ion. Finally, the recovery does not exceed 10% of the original value for the detection of Hg 2+ ion in tap and bottled water. The results indicate acceptable accuracy and precision for practical applications.