Zinc-encapsulating covalent organic frameworks for enhanced chemiresistive NH3 sensing at room temperature

Abstract

Ammonia (NH₃) is a hazardous gas used in industry, agriculture, and biomedical applications, and the development of efficient room-temperature and low-concentration ammonia detection sensors is essential. However, conventional sensors, including metal oxides, nanocomposites, and MOFs, require highly elevated temperatures (200–500 °C), leading to high energy consumption and less durability. To overcome these challenges, we developed functionalized zinc-encapsulated covalent organic frameworks (Zn@COFs) using a facile metal-doping approach. COFs doped with zinc have a modulated electronic environment, increased active sites, efficient charge transfer, and enhanced gas interactions. The incorporation of Zn²⁺ into the COF frameworks was confirmed by IR, TEM-EDAX, ¹³C CP MAS NMR spectra (CO peak at ∼183 ppm, and imine CN peaks at ∼148 and ∼146 ppm) and XPS (CO peak at 527.84 eV, CN at 399.2 eV; Zn 2p_3/2 peak at 1042 eV, and Zn 2p_1/2 at 1019 eV). Among the synthesized frameworks, Zn@COF-3 exhibited exceptional NH₃ sensing at a concentration of 1 ppm at room temperature, with a rapid response time (26 s) and recovery time (18 s), outperforming pristine COFs and Zn@COFs. This superior performance is attributed to its rich active sites (CO), high surface area (335 m² g⁻¹), porosity, strong NH₃ adsorption energy (−282 kJ mol⁻¹), and low energy gap (2.65 eV), as confirmed by DFT calculations. Additionally, Zn@COF-3 shows excellent selectivity and long-term stability over 30 days. This Zn@COF-based approach yields next-generation ammonia sensors, featuring energy-efficient, highly selective, and room-temperature chemiresistive sensors for industrial, environmental, and biomedical applications.