Optical sensor based on nanomaterial for the selective detection of toxic metal ions

A heterogeneous "naked-eye" colorimetric and spectrophotometric cation sensor, SNT-1, was prepared by immobilization of the azo-coupled macrocyclic receptor I on a silica nanotube (SNT) via sol-gel reaction. The optical sensing ability of SNT-1 was studied by addition of metal ions such as Ag+, Co2+, Cd2+, Pb2+, Zn2+, Fe3+, CU2+, and Hg2+ (all as nitrates) in water. Upon the addition of Hg2+ in suspension SNT-1 resulted in a color change from yellow to violet. This is novel rare example for chromogenic sensing of a specific metal ion by inorganic nanotubes. On the other hand, no significant changes in color were observed in the parallel experiments with Co2+, Cd2+, Pb2+, Zn2+, Fe3+, Cu2+, and Ag+. These findings confirm that SNT-1, can be useful as chemosensors for selective detection of Hg2+ over a range of metal ions. More interestingly, after addition of NO3- and ClO4- SNT-1 was observed to change color from yellow to violet and pink, respectively. However, no color changes were observed upon addition of Cl-, Br-, I-, SCN-, or SO42-. Furthermore, the extraction ability of SNT-1 was also estimated by measuring the amount of Hg2+ adsorbed by ion chromatography, showing that 95 % of the Hg2+ ion is extracted by SNT-1. This suggests that SNT-1 is potentially useful as a stationary phase for the separation of Hg2+, in liquid chromatography. In order to extend the above performance to a portable chemosensor kit, SNT-1 was coated as a thin film of 50 mu m thickness onto a glass substrate. The supported SNT-1 also changed from yellow to violet when dipped into Hg2+ solution. On the other hand, no significant change in color was observed in other metal-ion solutions. The results imply that the supported SNT-1 is applicable as a portable colorimetric sensor for detection of Hg2+, in the field.