Chemical Sensing of Metal Ions Using a Silica-Micelle Mesophase Doubly Functionalized by a Fluorogenic Ionophore and a Masking Agent
673 Since the discovery of surfactant-templated silica mesophases, 1-3 several types of mesoporous thin films (MTFs) 4-8 have been synthesized during this decade. The MTFs obtained as-deposited films were calcined or washed in solvents; subsequently, the surfactant templates were removed from the mesopore to use the silica framework and free pore spaces. However, the silica-micelle mesophase is potentially capable of material collection, identical to the manner in which the micelles and the
... icelles and the cyclodextrin derivatives in aqueous solutions collect the hydrophobic species. 9 Nanochannels containing the silica-micelle mesophase will play a key role as an unconventional chemical field for chemical separation and sensing. Although a separation membrane using the silica-micelle mesophase has been demonstrated by Teramae and his coworkers, 10 this concept offers a novel approach for constructing sensing devices based on solvent extraction and molecular recognition within the nanospaces. In this paper, we report on a novel chemical-sensing method based on a silica-micelle mesophase wherein both a fluoroionophore and a masking agent are embedded. The fluoroionophore 8-hydroxyquinoline-5-sulfonic acid (QS), which is a typical fluorogenic chelating agent for a variety of metal ions, 11 and 1,10-phenanthroline (Phen) as a masking agent were introduced into the silica-micelle mesophase of the MTFs by adsorption from its aqueous solution. A precursor solution for MTFs was prepared by referring to a report by Fan et al., 7 and deposited on microwells (diameter, 4 mm; depth, 25 μm). Optically transparent films were formed on the bottom of the wells. The X-ray diffraction patterns strongly support the fact that the as-deposited film has a two-dimensional hexagonal structure, and not a lamella mesophase, as discussed in a previous paper (see Supporting Information). 8 Microwells having MTFs functionalized by QS and Phen were used as a sensor array. A fluorescence image of the microliter plate was captured by a 1/4-inch color charge-coupled device (CCD) without an additional cooling system for noise reduction under UV irradiation at 365 nm (see Supporting Information). Aqueous solutions of QS, Phen, and metal ions used for these experiments were buffered at pH 5.0 by 0.1 M acetic acid-sodium acetate. QS was introduced into the MTF by dropping 10 μl of a QS aqueous solution onto MTF deposited on the bottom of a microwell. After aging for 10 min under saturated steam, the droplet in the well was sucked by a micropipette, and the well was then dried in air for 10 min. As a control experiment, nonmesoporous silica films deposited by a precursor solution without surfactants were also examined. The fluorescence intensities of QS observed on both films were plotted as a function of the concentration of the QS solution used for the introduction process (see Supporting Information). The fluorescence intensity observed on the MTF increases with the QS concentration and approaches saturation, while the fluorescence on the nonmesoporous film cannot be observed. This result strongly supports the fact that the QS molecules are embedded into the silica-micelle mesophase, and not attached onto the film surface. The fluorescence response of the MTFs can be well fitted by a Langmuir adsorption isotherm, and the adsorption coefficient, Kads, can be evaluated as 6.4 × 10 4 M -1 . Anionic species, such as the QS, may adsorb at the ionic interface between the silica framework and the hydrophobic core of the micelles, which was categorized by Zink and his coworkers. 12 The fluorogenic responses of the QS-embedded MTFs to Al 3+ , Mg 2+ , Zn 2+ , and Cd 2+ were examined. Those microwells treated with a 3 mM QS solution were filled with 10 μl of each metal solution and aged for 20 min. Subsequently, the solutions were removed, and the microwells were dried in air for 10 min. The fluorescence intensities were measured and plotted as a function of the metal concentration, as shown in Fig. 1 . The intensities of Al 3+ and Zn 2+ increased significantly with the metal concentration, while that of Cd 2+ increased gradually, and that of Mg 2+ remained constant. These results indicate that complexation between QS and metal ions occurred successfully in the vicinity of the nanochannels, reflecting the order of the binding constants of QS for each metal ion in water and micelle. 11,13 Furthermore, fluorescence images of the microwells were easily obtained by the conventional color CCD, as shown in the inset of Fig. 1 . It is feasible to conduct simultaneous 2011 We report on a chemical-sensing method based on the silica-micelle mesophase wherein both a fluoroionophore and a masking agent are embedded. Using this method, a highly selective detection of metal ions in an aqueous solution has been successfully demonstrated. Furthermore, simultaneous analyses of multisamples using a sensor array composed of functionalized mesoporous thin films were demonstrated for the first time.