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 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 + , Co 2+ , Cd 2+ , Pb 2+ , Zn 2+ , Fe 3+ , Cu 2+ , and Hg 2+ (all as nitrates) in water. Upon the addition of Hg 2+ 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 Co 2+ , Cd 2+ , Pb 2+ , Zn 2+ , Fe 3+ , Cu 2+ , and Ag + . These findings confirm that SNT‐ 1 can be useful as chemosensors for selective detection of Hg 2+ over a range of metal ions. More interestingly, after addition of NO 3 – and ClO 4 – 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 SO 4 2– . Furthermore, the extraction ability of SNT‐ 1 was also estimated by measuring the amount of Hg 2+ adsorbed by ion chromatography, showing that 95 % of the Hg 2+ ion is extracted by SNT‐ 1 . This suggests that SNT‐ 1 is potentially useful as a stationary phase for the separation of Hg 2+ 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 Hg 2+ 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 Hg 2+ in the field.