Chemical Sensors for Ion Recognition

The design and synthesis of molecular recognition agents for chemical and biological sensing applications has become an active area of research in recent years. Our research in this arena has focused on development of molecular recognition agents for chemical sensing of ions such as cesium and strontium in DOE tank wastes. Calix[4]crown ethers have been shown to be highly selective towards complexation of alkali and alkaline-earth metal ions. Furthermore, some derivatives of 1,3-alternate calix[4]crown-6 ethers exhibit high selectivity towards cesium in both acidic and alkaline media. We have utilized these attractive features of calix[4]crown ethers to synthesize fluorophore bearing cesium selective molecular recognition agents¹. The sensing phenomenon observed in such systems results from the change in emission intensity of the reporter molecule (anthracene) upon accommodation of a guest molecule (metal ion) within the host cavity (crown ether). The probe is non-or weakly emissive in the absence of ions because the fluorescence of the reporter molecule, anthracene, is quenched by Photo-induced Electron Transfer (PET) process. These optical probes can signal the presence of cesium ions in solution by showing an increase in the emission intensity of the fluorophore upon cesium complexation. Thus, the sensitivity of the probe is proportional to the strength of the emission signal produced after complexation of the guest ion by the receptor. We have observed 54-fold enhancement in the emission intensity of the fluorophore upon cesium ion complexation. Molecular recognition agents capable of sensing two different metal ions (e.g. K+ and Cs+) in solution upon changing the pH of the media have also been synthesized².

1. Ji, H-F.; Dabestani, R.; Brown, G. M. J. Chem. Soc. Chem. Comm. 1999, 609.
2. Ji, H-F.; Dabestani, R.; Brown, G. M. J. Am. Chem. Soc. 2000,122, 9306.