*2.5. Metal-Organic Frameworks*

In recent years, more and more metal-organic frameworks (MOFs) have been found to effectively coordinate polymer nanomaterials and have received extensive attention as effective quenching materials. MOFs are a kind of crystalline nanomaterial composed of metal ions and organic ligands. Due to their water dispersibility, adjustability, biocompatibility, low cost, controllable shape, ultra-high porosity, and high specific surface area [61], MOFs have been increasingly used in biosensors, electrocatalysis, energy storage, and conversion. The large specific surface area and ultra-high porosity of MOFs provide more reaction sites for aptamers and targets. Organic ligands with rich functional groups make MOFs easy to be functionalized with various molecules and materials [62]. In addition, MOF compositions take a variety of forms (e.g., nanosheets, cages, tubes, rods, cubes, etc.) and can be easily adjusted according to the selection of various organic connectives and metal ions [63].

Aptamers are securely fixed in the MOFs by encapsulation. The main framework of MOFs can facilitate various interactions with analytes through functional groups in organic ligands, thus achieving high sensitivity and high selectivity recognition. Therefore, MOFs can be used as signal probes for different detection methods. Zhang et al. [64] used a Zr-based MOF embedded with three kinds of aptamer. Ling et al. [65] prepared streptavidin functionalized zirconium porphyrin MOF (PCN-222@SA) using a covalent method as a signal nanoprobe. Introducing this signal nanoprobe into the sensor surface significantly amplified the electrocatalytic current. Zhang et al. [66] synthesized a coreshell nanostructured composite material composed of Fe (III)-based MOF (Fe-MOF) and mesoporous Fe3O4@C nanocapsules (Fe-MOF@mFe3O4 @MC) that exhibited excellent electrochemical activity, water stability, and high specific surface area, resulting in strong biological binding to heavy metal ion targeting aptamer chains.
