Development of a reusable colorimetric calcium sensor based on a calix[4]arene-functionalised glass surface

A dual functional chromogenic calix[4]arene that contains both hydroxy azophenols and triazoles for colorimetric sensing and cationic recognition, respectively, has been synthesised. This system was further functionalised with dimethallylsilane to permit grafting to a glass surface. This grafting may be used to produce a surface modified glass slides that act as reusable ion-selective colorimetric sensors. UV-vis spectral studies revealed that this compound is capable of binding the calcium cation in aqueous media and producing an easily detectable colour change. The same is true for the modified surface set-up. For the free receptor in acetonitrile solution, the absorption maximum undergoes a red shift from 390 to 496 nm upon exposure to calcium cations. In the case of the modified surface, exposure to acetonitrile solution of calcium perchlorate leads to a change in colour from orange to red. Washing with acetone serves to remove the metal cations from the calix[4]arene-functionalised glass and restore the original orange colour. Then the system is ready for reuse. Specifically, following treatment with acetone, the glass may be submerged in a metal solution once again. In the case of exposure to calcium salts, this re-exposure results in a colour change analogous to what was seen for a pristine system. This process may be repeated several times.