Heterogeneous "naked-eye" colorimetric and spectrophotometric cation sensors were prepared by immobilization of an azobenzene-coupled receptor onto mesoporous silica (AR-SiO(2)) or titania nanoparticles (AR-TiO(2)) via sol-gel or hydrolysis reactions. The optical sensing ability of AR-SiO(2) was studied by addition of metal ions such as K(+), Ca(2+), Sr(2+), Co(2+), Cd(2+), Pb(2+), Zn(2+), Fe(3+), Cu(2+) and Hg(2+) ions (all as chlorides) in aqueous solution. Upon the addition of Hg(2+) ion in suspension, the AR-SiO(2) resulted in a color change from yellow to deep red. No significant color changes were observed in the parallel experiments with K(+), Ca(2+), Sr(2+), Co(2+), Cd(2+), Pb(2+), Zn(2+), Fe(3+) or Cu(2+) ion. These findings confirm that the AR-SiO(2) can be useful as chemosensors for selective detection of Hg(2+) ion over a range of metal ions in aqueous solution. Also, the color change of AR-SiO(2) was independent of the presence of anions NO(3)(-), ClO(4)(-), Br(-) and I(-). We also prepared a portable chemosensor kit by coating a 4 microm thick film of AR-TiO(2) onto a glass substrate. We found that this AR-TiO(2) film detects Hg(2+) ion at pH 7.4 with a sensitivity of 28 nM. Finally, we tested the effect of pH on AR-TiO(2) with Hg(2+) ion between pH 1.0 to 11.0. The absorbance and color changes of AR-TiO(2) were almost constant between pH 4 and 11. The results imply that the AR-TiO(2) film is applicable as a portable colorimetric sensor for the detection of Hg(2+) ion in the environmental field.