Single-site Zn (II) strategy enhanced humidity sensing performance of 2, 6-DPA grafted UiO-66-NH2 for breath and finger detections

Metal-organic frameworks (MOFs) have been proved to be outstanding candidates for humidity sensors, however, pure MOFs are still limited in hysteresis, integration, and intelligence. In this investigation, inspired by a single-site-ion strategy for catalysis, UiO-66-NH2 is first grafted by 2, 6-DPA (2, 6-dipicolinic acid) molecules (named as U-DPA) and then combined by Zn (II) ions (noted as U-DPA-Zn). Taking advantage of electrolyte and MOF, U-DPA-Zn exhibits significantly enhanced humidity sensing performance than UiO-66-NH2 and U-DPA. U-DPA-Zn possesses the highest sensitivity (Sr = 8.07 × 103), the best linearity (R2 = 0.994), the shortest response-recovery time (τres = 2 s, τrec = 105 s), the smallest hysteresis (ΔH = 1.81%), as well as good repeatability in a wide relative humidity (RH) range (11%-97%). Thanks to its excellent properties, U-DPA-Zn shows great potential in the field of monitoring human respiration (breath detection) and water evaporation on human skin (finger detection). Additionally, the humidity sensing mechanism is discussed based on complex impedance analysis, revealing the single Zn (II) make giant contribution to MOFs. This work provides an optional material for humidity sensors and further a new strategy to improve the humidity sensitivity of pure MOFs.