Fluorescent cryptand recognises radioactive pollutant

The first fluorescent sensor for radioactive pertechnetate anions has been developed by researchers in Italy and Switzerland.

Technetium-99 (⁹⁹Tc) is a radioactive product of nuclear fission, whose presence in nuclear waste is a significant concern due to its long half-life. It is mainly handled on an industrial scale as the pertechnetate anion (⁹⁹TcO₄^-). If released accidentally, environmental contamination with ⁹⁹TcO₄^- can last thousands of years and poses a serious hazard. Valeria Amendola and co-workers at the University of Pavia and the University of Zurich have designed the first supramolecular host molecule that can fluorescently sense the presence of ⁹⁹TcO₄^-.

Using a fluorescent probe to sense ⁹⁹TcO₄^- could have significant advantages over current radiometric methods using b-counting by liquid scintillation (LSC), explains Amendola. ‘Direct determination of ⁹⁹Tc by LSC is hard to achieve because of the presence of complex matrices, and sometimes insufficient sensitivity. The high sensitivity of optical chemosensors would allow the detection of low levels of the target anion in contaminated water samples, without special instrumentation.’

Encapsulating ⁹⁹TcO₄^- using non-covalent interactions is more challenging than encapsulating smaller, more commonly occurring anions as its large size and low charge density make electrostatic and hydrogen bonding interactions less effective. Amendola’s fluorescent aza-cryptand is carefully designed to have a cavity which is almost exactly the right size and shape to contain a ⁹⁹TcO₄^- anion.

The fluorescence of the receptor in acidic, aqueous solution is quenched by adding ⁹⁹TcO₄^- anions, but not by other, more commonly occurring anions. This is the most crucial aspect of this work, according to Phil Gale, an anion coordination chemistry expert at the University of Southampton in the UK. ‘The most exciting aspect of Amendola's work is that the hexaprotonated receptor selectively binds and senses pertechnetate in the presence of other common anions such as chloride, sulfate and nitrate. This selectivity should allow this receptor to be developed for sensing of pertechnetate in the environment.’

Amendola’s team are now looking to improve their design to sense and potentially trap ⁹⁹TcO₄^- more effectively. ‘Our future challenges are the development an OFF–ON type chemosensor for pertechnetate, based on the switching on of the signal upon binding and the development of new materials and devices for both sensing and extraction of the radioactive pollutant.’