Fabrication of calix[4]arene-attached mesoporous ammonium molybdophosphate–silica hybrid and its application as an adsorbent for cesium ions

The cesium cation is a common contaminant in medical and industrial products as well as in nuclear wastes, and it can present a serious risk to human health. Thus, the development of novel and facile methods for detection and elimination of cesium ions is important for environmental and biological systems. Mesoporous ammonium molybdophosphate–silica hybrid particles (AMP/Al–SiO2) were prepared, and the particles were immobilized on the calix[4]arene-based receptor 1 by a covalent bond between the carboxylic acid and amine, yielding the hybrid product AMP/Al–SiO2-1. In an evaluation of Cs+ adsorption on both particles, 57.8 mg g−1 of Cs+ was adsorbed by AMP/Al–SiO2 while 72.2 mg g−1 was adsorbed by AMP/Al–SiO2-1. The increased adsorption capacity of AMP/Al–SiO2-1 was due to a synergistic effect of the complex formation between 1 and Cs+ by a coordination bond. Additionally, the fluorescence intensity of AMP/Al–SiO2-1 was enhanced upon addition of Cs+. No significant fluorescence changes were observed in the parallel experiments with other metal ions such as Na+, K+, Mg2+, Rb+, Ca2+, Ba2+, Ag+, Ni2+, Zn2+ and Sr2+ ions. These findings confirmed that the AMP/Al–SiO2-1 can be useful as a chemoprobe for selective detection of Cs+ ions over a range of other metal ions in aqueous solution. We also determined that the fluorescence changes of AMP/Al–SiO2-1 were almost constant between pH 3 and 12. These results imply that the AMP/Al–SiO2-1 is applicable as a portable fluorogenic sensor for detection of Cs+ ions in environmental field studies.