Fabrication of TiO₂/γ-CD films for nitro aromatic compounds and its sensing application via cyclic surface-polarization impedance (cSPI) spectroscopy

In order to achieve high sensitive and selective detection for nitro aromatic compounds related with landmines, γ-cyclodextrin (γ-CD) hosts were introduced onto the TiO₂ ultrathin layer. The surface of the Pt electrode was modified with ultrathin TiO₂ gel film immobilized with γ-CD molecular hosts through the surface sol-gel process. The surface modification of the electrode was confirmed by QCM (9 MHz) and X-ray photoelectron spectra (XPS) experiments. As a result, an alternate assembly film of Ti(O-ⁿBu)₄ and γ-CD prepared on the gold coated QCM as a substrate showed linearly growth, and judging from the large carbon and oxygen ratios of XPS spectra in the TiO₂/γCD film of the Pt electrode surface, it was confirmed that γ-CD hosts must be adsorbed onto the TiO₂ ultrathin layers. Furthermore, the resulting γ-CD-immobilized TiO₂/γ-CD ultrathin films for the high sensitive detection of nitro aromatic compounds were examined by cyclic surface-polarization impedance (cSPI) spectroscopy which is a newly developed chemical sensor measuring electrochemical impedance change due to constant phase element (CPE) behavior on the electrode surface under controlled electrode potential. Consequently the resulting TiO₂/γ-CD films showed specific and sensitive detection for 2,4-dinitrotoluene (2,4-DNT) among the examined guest molecules. Moreover, a high impedance response value for 2,4-DNT compared to structurally related other molecules was achieved by the specific host-guest interaction, estimated to be 1.6-3.4 times larger at the concentration of 2 × 10^-8 M.