Synthesis and properties of a potential extracellular fluorescent probe for potassium

Development of a new, selective, high affinity, fluorescent probe CD222, 2, for potassium is described. Coincidentally two related probes, CDF18, 3, and CTF18, 4, were prepared and showed selectivity for the cations sodium and calcium, respectively, and possible reasons for these differences are discussed. The bicyclic probe CD222, 2, with observed KD values for K+ ranging from 1 to 10 mmol dm–3, has potential applications as an extracellular probe for potassium.