Cations common in biological systems

In biological studies, apart from alkali and alkaline earth cations, zinc sensing is very important, especially in neuroscience. Binding of zinc and other (often toxic) transition metal ions requires receptors of different structure and coordination properties. Polypyridines, dendritic pyridines, and thiacrown ethers are the receptors of choice. 

Linear and cyclic polyamines [13], as well as linear oligoesters [14], are also efficient chelators for zinc ions, used especially for zinc imaging in cultured cells. Depending on the target cell and/or intercellular structure, various sensor architectures are applied. 

Another metal present in biological media in relatively high concentrations is iron. Most iron ions are strongly coordinated within proteins, although a small fraction of iron ions is loosely bound with carboxylates and phosphates, phospholipids, glycans, and sulphonates. This fraction is usually called labile iron and constitutes a pool of iron ions accessible for incorporation into functional metalloproteins. In contrast to closed shell cations, rather than increasing the fluorescence quantum 

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