Functionalized monolayers on mesoporous silica and on titania nanoparticles for mercuric sensing

Heterogeneous naked-eye'' colorimetric and spectrophotometric cation sensors were prepared by immobilization of an azobenzene-coupled receptor onto mesoporous silica (AR-SiO2) or titania nanoparticles (AR-TiO2) via sol-gel or hydrolysis reactions. The optical sensing ability of AR-SiO2 was studied by addition of metal ions such as K+, Ca2+, Sr2+, Co2+, Cd2+, Pb2+, Zn2+, Fe3+, Cu2+ and Hg2+ ions (all as chlorides) in aqueous solution. Upon the addition of Hg2+ ion in suspension, the AR-SiO2 resulted in a color change from yellow to deep red. No significant color changes were observed in the parallel experiments with K+, Ca2+, Sr2+, Co2+, Cd2+, Pb2+, Zn2+, Fe3+ or Cu2+ ion. These findings confirm that the AR-SiO2 can be useful as chemosensors for selective detection of Hg2+ ion over a range of metal ions in aqueous solution. Also, the color change of AR-SiO2 was independent of the presence of anions NO3-, ClO4-, Br- and I-. We also prepared a portable chemosensor kit by coating a 4 mm thick film of AR-TiO2 onto a glass substrate. We found that this AR-TiO2 film detects Hg2+ ion at pH 7.4 with a sensitivity of 28 nM. Finally, we tested the effect of pH on AR-TiO2 with Hg2+ ion between pH 1.0 to 11.0. The absorbance and color changes of AR-TiO2 were almost constant between pH 4 and 11. The results imply that the AR-TiO2 film is applicable as a portable colorimetric sensor for the detection of Hg2+ ion in the environmental field.