The rational design of covalent organic frameworks (COFs) with hydrochromic properties is of significant value because of the facile and rapid detection of water in diverse fields. In this report, we present a thiazole-linked COF (TZ-COF-6) sensor with a large surface area, ultrahigh stability, and excellent crystallinity. The sensor was synthesized through a simple three-component reaction involving amine, aldehyde, and sulfur. The thiazole and methoxy groups confer strong basicity to TZ-COF-6 at the nitrogen sites, making them easily protonated reversibly by water. Therefore, TZ-COF-6 displayed color change visible to the naked eye from yellow to red when protonated, along with a red shift in absorption in the ultraviolet-visible diffuse reflectance spectra (UV-vis DRS) when exposed to water. Importantly, the water-sensing process was not affected by polar organic solvents, demonstrating greater selectivity and sensitivity compared to other COF sensors. Therefore, TZ-COF-6 was used to detect trace amounts of water in organic solvents. In strong polar solvents, such as N,N-dimethyl formamide (DMF) and ethanol (EtOH), the limit of detection (LOD) for water was as low as 0.06% and 0.53%, respectively. Even after 8 months of storage and 15 cycles, TZ-COF-6 retained its original crystallinity and detection efficiency, displaying high stability and excellent cycle performance.

The strategic construction of heterojunctions through a simple and efficient strategy is one of the most effective means to boost the photocatalytic activity of semiconductor materials. Herein, a thiazole-linked covalent organic framework (TZ-COF) with large surface area, well-ordered pore structure, and high stability was developed. To further boost photocatalytic activity, the TZ-COF was synthesized in situ on the surface of Cu₂O through a simple multicomponent reaction, yielding an encapsulated composite material (Cu₂O@TZ-COF-18). In this composite, the outermost COF endows the material with abundant redox active sites and mass transfer channels, while the innermost Cu₂O exhibits unique photoelectric properties. Notably, the synthesized Cu₂O@TZ-COF-18 was proven to have the heterojunction structure, which can efficiently restrain the recombination of photogenerated electron–hole pairs, thereby enhancing the photocatalytic performance. The photocatalytic degradation of tetracycline demonstrated that 3-Cu₂O@TZ-COF-18 had the highest photocatalytic efficiency, with the removal rate of 96.3% within 70 min under visible light, which is better than that of pristine TZ-COF-18, Cu₂O, the physical mixture of Cu₂O and TZ-COF-18, and numerous reported COF-based composite materials. 3-Cu₂O@TZ-COF-18 retained its original crystallinity and removal efficiency after five cycles in photodegradation reaction, displaying high stability and excellent cycle performance.