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  • Metal organic frameworks MOFs have emerged over the

    2022-05-17

    Metal–organic frameworks (MOFs) have emerged over the past two decades with the potentials to act as promising materials in gas storage, chemical separation, catalyst, magnetism, sensing, and drug delivery etc (Du et al., 2013, Zhou and Kitagawa, 2014). Additionally, the secondary interactions, such as π–π stacking, hydrogen bonding, and electrostatic force, can be formed between special functional groups on MOFs linkers and negatively charged nucleic VU 0364439 sequences (Liu et al., 2017b, Zhang et al., 2017a). Thus, a series of MOFs-based biosensors have been designed to detect various targeted analytes, such as MOF/Au/G-quadruplex (Shao et al., 2018), zinc-methylimidazolate framework-8 (Pan et al., 2018), and NH2-Ni-MOF (Wang et al., 2018b). Although several kinds of MOFs have been explored to determine H2O2 released from living cells by electrochemical methods (Li et al., 2018a), the direct detection of living cancer cells has not been achieved. Consequently, it would be highly desirable to explore MOFs-based scaffolds for simultaneously detection of HER2 and living cancer cells. In the previous work, bimetallic MnFe Prussian blue analogue coupled to gold nanoparticles has been developed for the determination of HER2 and living MCF-7 cells, only giving a limitation of detection of 0.247 pg mL−1 and 36 cell mL−1 toward HER2 and MCF-7 cells, respectively (Zhou et al., 2019). Lately, many synthetic efforts toward MOFs have focused on those with group IV transition metal ions, especially zirconium and hafnium (Takata et al., 2015). Zr and Hf MOFs with linear dicarboxylate ligands generally adopt the well documented UiO-66 topology, where M6O4(OH)4 clusters (M = Zr or Hf) are linked in three dimensions by bridging organic ligands (Morris et al., 2017). In virtue of the excellent biocompatibility, chemical stability, strong bioaffinity, and high binding interactions of Zr–O–P between MOF frameworks and DNA strands (Chen et al., 2016), some Zr-MOF-based aptasensors have been fabricated to sensitively determine different analytes (Guo et al., 2017, Liu et al., 2018), and UiO-66-N3 to create the first MOF nanoparticle-nucleic acid conjugates (Morris et al., 2014). Nevertheless, most Zr-MOFs suffer from their poor electrochemical activity (Vermoortele et al., 2013), thus limiting the applications in electrochemical biosensors. Owing to their high surface areas, tunable pore sizes, and excellent adsorbability, MOFs could be served as effective solid multifunctional matrixes for other components, such as metal nanoparticles (Duan et al., 2018), metal oxide NPs (Falcaro et al., 2016), and quantum dots (Zhao et al., 2014). Among them, carbon dots (CDs), as carbon-based photoluminescent nanomaterials (< 10 nm in size), may show great potentials in developing sensitive, stable, and cost-effective aptasensor (Zhang et al., 2017b). Further, CDs can be applied to enhance the immobilization of aptamer probes, with the exception of their hydrophobicity, chemical stability, electro-conductivity, biocompatibility, cost efficiency, and easy synthesis (Tuteja et al., 2016). Based on the above analysis, aiming at developing a novel bifunctional electrochemical aptasensor for simultaneously detecting HER2 and living cancer cells (MCF-7), we have prepared a bimetallic ZrHf-MOF coupling with CDs (represented by CDs@ZrHf-MOF) and explored it as the scaffold for anchoring HER2 aptamer (Scheme 1). Combining the advantages of large specific area, high stability, strong bioaffinity toward biomolecules, and excellent biocompatibility of ZrHf-MOF and good fluorescence, high electrochemical activity, and outstanding biosensing performance of CDs (Wang et al., 2015), the fabricated CDs@ZrHf-MOF-based electrochemical aptasensor not only can be applied to sensitively detect the trace HER2 in human serum sample, but also can determine the living cancer cells. The electrochemical CDs@ZrHf-MOF-based aptasensor displays extremely low limitation of detections (LODs) of 19 fg mL−1 toward HER2 and 23 cell mL−1 for MCF-7 cells, also with good selectivity, stability, reproducibility, and acceptable applicability. This work could supply a promising approach for early diagnosis of both cancer marker and living cancer cells.