Optical Sensor Based on Nanomaterial for the Selective Detection of Toxic Metal Ions

Abstract

AbstractA heterogeneous “naked‐eye” colorimetric and spectrophotometric cation sensor, SNT‐1, was prepared by immobilization of the azo‐coupled macrocyclic receptor 1 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 μm 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.