MOF-on-MOF heterostructured composites for food safety analysis: construction strategies, sensing applications and sample pretreatment.

Food safety requires rapid, sensitive and reliable methods for detecting contaminants in complex food matrices. Metal-organic frameworks (MOFs), owing to their high surface areas, tunable pores and abundant functional sites, have shown considerable potential in food sensing and sample pretreatment. Recently, MOF-on-MOF heterostructures have attracted increasing attention because their hierarchical architectures and tunable interfaces can integrate complementary adsorption, catalytic and signal-transduction functions. This review summarizes the major construction strategies and structural types of MOF-on-MOF materials, with emphasis on epitaxial growth, seed-mediated assembly, interfacial engineering and post-synthetic conversion. Particular attention is given to the distinction between intact MOF-on-MOF heterostructures, MOF-on-MOF-derived materials and related MOF-based hybrids. Recent applications in electrochemical and optical sensing, biosensing-related platforms, and sample pretreatment by magnetic solid-phase extraction (MSPE), solid-phase microextraction (SPME) and dispersive micro-solid-phase extraction (DµSPE) are critically reviewed, together with emerging applications in food preservation and freshness monitoring. Structure-function relationships and analytical performance are discussed beyond sensitivity alone, considering matrix effects, selectivity, reproducibility and stability. Finally, current challenges in mechanistic understanding, material safety, batch-to-batch consistency and scalable fabrication are highlighted. Future priorities include robust real-matrix validation, reproducible synthesis, portable multimodal analysis and more sustainable fabrication, providing guidance for the practical development of MOF-on-MOF materials in food-safety analysis.