Imino nitrile sensors for selective fluoride detection

Abstract

Fluoride ions exist in abundance throughout nature, and human exposure to fluoride is inevitable. High levels of fluoride intake (over 2 ppm) induces acute and chronic illnesses such as gastric disorders, dental and skeletal fluorosis, and urolithiasis. To minimize the potentially harmful exposure, fluoride selective detectors must be developed that gives colorimetric response for easy detection. In this thesis, we developed a one-pot synthesis of 4 different sensors that are based on diaminomaleonitrile units tethered on aldehydes. The structures provide bright color change visible even to the naked eye at $F^-$ ion concentrations as low as 2 ppm. Furthermore, when tested along with similar anions such as $CN^-$, $I^-$, $Br^-$, $Cl^-$, $NO_3^-$, $PO_4^{3-}$, $OAc^-$, and $HSO_4^-$, the sensors only displayed visible color change in the presence of fluoride, indicating high selectivity. $HF_2^-$ ion generation is clearly observed in $^{19}F$ NMR, which is a direct evidence for the participation of nitrogens in the donor moiety providing its unique difference in the selectivity. This thesis sets forth an unprecedented selectivity of $F^-$ ions over $CN^-$ ions, previously not achieved by others. Additionally, the detection limit of 2 ppm complies with World Health Organization (WHO) standards.