Corrin-based chemosensors allow the rapid and selective colorimetric detection of endogenous biological cyanide. The color change from orange to violet can be easily observed with the "naked eye" (Deltalambda(max) = 51 nm). The methodology works directly in the biological matrix without time-consuming sample pretreatment and the use of special reaction conditions. It was possible to study the enzymatic release of cyanide from its biological precursor linamarin with diffuse reflectance UV-visible (DRUV-vis) spectroscopy on a freshly prepared biological surface. These experiments were accompanied by stopped-flow kinetic measurements under homogeneous conditions. Detection in the biological sample is based on the selective coordination of cyanide to the metal-based chemosensor as proven by UV-vis and (1)H NMR spectroscopy. Examples of applications during food manufacturing are given.

Substitution of Co(III)-bound water by cyanide allows the rapid colorimetric detection of micromolar amounts of cyanide with cobalt corrinoids. Negatively charged side chains at the periphery of these metal-based sensors increase the selectivity of cyanide versus thiocyanate, the most disturbing anion, by a factor of 30 through additional supramolecular interactions in water.

This short-review discusses the recent developments in the colorimetric detection of cyanide with different types of receptors. Significant progress in terms of selectivity, sensitivity and straightforwardness has been observed for either organic-, main group- or transition metal-based sensors. Our group has developed a simple and highly specific system for the optical sensing of cyanide based on the conformational switch of commercially available vitamin B12.

Colorimetric solid phase with spatially separated extraction and detection zones as a rapid, effective and economic method for the optical detection of cyanide in complex samples is described. The system is seven times more sensitive for the optical detection of cyanide than the same class of chemical sensors used under homogeneous conditions. The application of the method in the detection of (i) endogenous cyanide in colored plant samples and of (ii) hydrogen cyanide in tobacco smoke is shown. The optical detection of multiple anions within a single sample has been demonstrated in principle for the detection of both CN(-) and SCN(-). Immobilized aquacyano-corrinoids and immobilized vitamin B12 are applied as chemical sensors, and cyanide is qualitatively identified by the violet color (λ(max) = 583 nm) of the corresponding dicyano-complex. Quantitative determinations with diffuse reflectance spectroscopy (DRUV-vis) are possible in the linear range up to 0.2 mg/L with a LOD of 1 μg/L. Alkyl-modified silica particles are employed for immobilization of the indicator on the surface of the solid phase (detection zone), and for removal of colored hydrophobic interferents (extraction zone).

A two-step "naked-eye" screening of selective metal-based chemosensors for biologically important anions such as cyanide, phosphate, and oxalate from commercially available metal indicators and metal ions has been developed. Applications in food safety control have been demonstrated.