Abstract
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.