Manipulating Active Sites of 2D Metal-Organic Framework Nanosheets with Fluorescent Materials for Enhanced Colorimetric and Fluorescent Ammonia Sensing

2D metal-organic frameworks (MOFs) offer high surface area and unique accessibility to active adsorption sites making them appealing for gas sensing applications. 2D MOFs-based sensors are gaining traction for detecting hazardous flu-gases such as ammonia selectively at low concentrations. Fluorescent and colorimetric sensing are promising techniques offering high sensitivity, selectivity, and rapid response in simple applications. In this work, Zn-BTC is synthesized as 2D-MOFs nanosheet with approximate thickness of 2.52 nm via a fast, facile, direct synthesis technique. The introduction of 8-hydroxyquinoline during synthesis forms fluorescent compounds with zinc (ZnQ) which is encapsulated and decorated onto Zn-BTC. Inherent charges on ZnQ lead to the agglomeration of multiple 2D-flakes forming ZnQ@Zn-BTC multi-flaked nano-discs. The synthesized material shows visible color change upon exposure to ammonia from white to ivory. In addition, selective fluorescence quenching is observed under ultraviolet illumination (lambda(ex) = 365 nm) when ZnQ@Zn-BTC is exposed to ammonia. The limit of detection reaches 0.27 ppm as a dried film for gaseous sensing and 60.8 nm in liquid phase fluorescence quenching, respectively. The observed high sensitivity and selectivity are attributed to the manipulation of active sites of 2D-MOFs nanosheet with ZnQ. Functionalization also limits the degradation and breakdown of ZnQ@Zn-BTC.

Automated summary

2D metal-organic frameworks (MOFs) have high surface area and active adsorption sites, making them appealing for gas sensing applications. In this study, a 2D-MOFs nanosheet, Zn-BTC, with approximately 2.52 nm thickness was synthesized using a fast and facile technique. The introduction of 8-hydroxyquinoline resulted in the formation of fluorescent compound ZnQ, which was encapsulated and decorated onto Zn-BTC. The synthesized material exhibited visible color change and fluorescence quenching upon exposure to ammonia, with detection limits of 0.27 ppm and 60.8 nm in gaseous and liquid phase sensing, respectively.