Hierarchically porous ZIF-67-based Au with enhanced electromagnetic, chemical, and mass-transfer properties for flexible gas–liquid SERS sensing

Metal–organic framework materials exhibit considerable potential as molecularly selective surface-enhanced Raman spectroscopy (SERS) substrates because of their microporous structures, which enrich small molecules while excluding larger ones. In this study, we develop a template-assisted chemical-etching strategy to prepare layered tuneable SERS substrates based on hierarchical porous zeolitic imidazolate framework-67 (HP-ZIF-67) with a rhombic dodecahedral structure. The synergistic SERS enhancement mechanisms of HP-ZIF-67, which combine electromagnetic (EM) and chemical (CM) effects, were systematically studied through numerical simulations and experiments. Calculations revealed that under 633-nm laser excitation, the contributions of the EM and CM effects to the total SERS enhancement factor of HP-ZIF-67 were 60% and 40%, respectively. The hierarchical porous structure enhanced the fluid-flow flux over the microporous ZIF-67 because the increased pore radius reduced the viscous resistance and facilitated rapid molecular transport through the interconnected macro-meso-channels. Precise modulation of the CM and EM effects, combined with enhanced mass transfer, facilitated the development of HP-ZIF-67 and HP-ZIF-67@Au as efficient SERS sensors. An investigation of the relationship between pore-size distribution and EM effects revealed the pivotal role of light confinement by whispering-gallery-mode microcavities in enhancing the SERS performance. The optimised HP-ZIF-67@Au composites functioned as flexible and highly sensitive in situ SERS sensors for gases and liquids, including volatile organic-compound gas and liquid-pesticide residues. This study introduces a novel design concept and provides a robust theoretical foundation for the future development of exhaled-breath point-of-care diagnostic devices and sweat-based wearable biomedical sensors. Graphical abstract