Solution-Processable MOF-on-MOF System Constructed via Template-Assisted Growth for Ultratrace H₂S Detection

Conductive metal-organic frameworks (c-MOFs) hold promise for highly sensitive sensing systems due to their conductivity and porosity. However, the fabrication of c-MOF thin films with controllable morphology, thickness, and preferential orientation remains a formidable yet ubiquitous challenge. Herein, we propose an innovative template-assisted strategy for constructing MOF-on-MOF (Ni₃(HITP)₂/NUS-8 (HITP: 2,3,6,7,10,11-hexamino-tri (p-phenylene))) systems with good electrical conductivity, porosity, and solution processability. Leveraging the 2D nature and solution processability of NUS-8, we achieve the controllable self-assembly of Ni₃(HITP)₂ on NUS-8 nanosheets, producing solution-processable Ni₃(HITP)₂/NUS-8 nanosheets with a film conductivity of 1.55×10^-3 S ⋅ cm^-1 at room temperature. Notably, the excellent solution processability facilitates the fabrication of large-area thin films and printing of intricate patterns with good uniformity, and the Ni₃(HITP)₂/NUS-8-based system can monitor finger bending. Gas sensors based on Ni₃(HITP)₂/NUS-8 exhibit high sensitivity (LOD~6 ppb) and selectivity towards ultratrace H₂S at room temperature, attributed to the coupling between Ni₃(HITP)₂ and NUS-8 and the redox reaction with H₂S. This approach not only unlocks the potential of stacking different MOF layers in a sequence to generate functionalities that cannot be achieved by a single MOF, but also provides novel avenues for the scalable integration of MOFs in miniaturized devices with salient sensing performance.