Abstract:

Objective Luminescent metal-organic frameworks (MOFs) are a new type of inorganic-organic hybrid materials made from an assembly of metal ions with organic linkers. They have excellent applications in the fields of fluorescence detection, radiometric nanothermometers, luminescent thin films, and so on. However, the inherent characteristics of particle materials restrict their further application in practice. Therefore, it is of great importance to deposit MOFs onto suitable substrates to reduce aggregation and improve utilization.
Method Flexible fluorescent pH sensing materials were prepared using cotton fabric as the substrate. Terbium nitrate hexahydrate, fumaric acid, and oxalic acid were used to prepare micro-nano Terbium-metal-organic frameworks (Tb-MOFs) particles by adding the sodium acetate as a blocking agent with the same group of the ligands. Then, the Tb-MOFs particles were coated on the surface of the cotton fabric by hot pressing method to obtain Tb-MOFs-cotton fabric (Tb-MOFs-CF). The morphology, crystallization, fluorescence, and pH sensing properties of the Tb-MOFs-CF were characterized and analyzed.
Results The Tb-MOFs particles with uniform particle size were synthesized by hydrothermal method, and the particle size in the range of 4-6 μm (Fig. 2). The dense and uniform micro-nano Tb-MOFs were formed on the surface of the cotton fabric, and the Tb-MOFs-CF has the same white appearance and crisscross textures with the pristine cotton fabric (Fig. 3). The Tb-MOFs-CF exhibited the characteristic peaks of the pristine cotton fabric and Tb-MOFs, indicating the hot press process has no significant influence on the crystal structure of Tb-MOFs (Fig. 4). The water contact angel test of the pristine cotton fabric and Tb-MOFs-CF are 0°, indicating that both have the good hydrophilicity (Fig. 5). The optical images of the Tb-MOFs-CF is blue-green under UV light, as the cotton fabric is blue and Tb-MOFs is green (Fig. 6), which revealed the good fluorescent properties of Tb-MOFs-CF. The fluorescence spectra of the Tb-MOFs-CF in the solution of different pH (from 1-13) using 510 nm as the excitation wavelength showed that there are mainly two peaks from 375-650 nm (Fig. 7). The broad peak about 415 nm is the emission of the cotton fabric, and the peaks at about 510 nm is the typical peak of the Tb ions. The position, shape, and range of the peaks remain unchanged, but the intensity changes obviously with different pH. With the increase of the pH from 1-5, the fluorescence intensity decreased obviously. Then, with the increase of the pH from 8-13, the fluorescence intensity showed no significant change. However, there were obviously changes in fluorescence intensity in the pH 1, 7, and 13. The curve of fluorescence intensity vs. pH showed that at acidic conditions, the fluorescence intensity decreased obviously, while at alkaline conditions, the fluorescence intensity leveled out (Fig. 8). Especially in the range of pH from 1-5, the fluorescence intensity was in linear relation with pH. Therefore, the pH dependence of the Tb ions could be used as the fluorescent sensor. The brightness of fluorescence color for Tb-MOFs-CF was different under UV light, in the pH 1, 7, and 13, demonstrating the Tb-MOFs-CF could be applied for detecting acid-base (Fig. 9). Besides, the color of the Tb-MOFs-CF under UV light had no difference with different wash times, indicating the Tb-MOFs is firmly adhered on the Tb-MOFs-CF (Fig. 10).
Conclusion The micro-nano Tb-MOFs are uniformly coated on cotton fabric and retain their crystal morphology with good adhesion. Tb-MOFs-CF demonstrates excellent fluorescence properties and exhibits rapid response in the detection of pH. Especially at acidic conditions, the characteristic emission peak of Tb ions shows a linear relationship with pH.

Key words: terbium metal-organic framework, pH value detecting, fluorescence property, rare earth luminescence material, cotton fabric