**3.4 Metal-organic frameworks**

With their nice monodisperse structure and large surface area, metal-organic frameworks (MOFs) represent advanced hierarchical nanostructures that have been regarded as suitable QD loading hosts for amplification and stabilization of signals [37].

GQDs/UiO-66 NC were prepared by Abdolmohammad-Zadeh et al. resulting from terephthalic acid (TA) oxidation with H2O2 resulting from cholesterol oxidation with ChOx. As a result of the H2O2 oxidation of TA, a highly fluorescent product was produced, enabling a direct correlation between the intensity of fluorescence and the amount of cholesterol in solution [38].

The bi-coreactants, TEOA@AuNPs and CdS QDs@MOF, were prepared by Wei et al. to synergistically enhance the ECL signal. A TEOA@AuNPs nanocarrier was used to carry CEA aptamers via Au-S bonds in addition to acting as coreactants. After CEA's aptamer was bound to Ru(bpy)3 2+, the ECL signal was also weakened because of increased impedance between Ru(bpy)3 2+ and the bi-coreactants. An ECL biosensor for CEA analysis has been successfully constructed using the ECL system [39].

According to Yan et al., a fluorescent aptasensor was constructed by encapsulating CHAs in MOF-5-NH2 and self-cycling CHAs (scCHAs). Patulin (PAT) in apple juice can be detected sensitively using a dual amplification strategy. As a result of the 17 s CS and 17 s AP hairpins, the 17 s AP includes an aptamer domain, which allows cyclic amplification without the addition of an extra aptamer. Further, hybridization strength between hairpins and aptamer domains was optimized to obtain the strongest signal change following PAT binding to aptamer domains [40].

Using a direct encapsulation method, Cui et al. prepared CdS QDs@MOF-5 composites for ultrasensitive cTnI detection [41].

CdSe nanocomposites were synthesized in situ using MIL-101. The as-prepared composites showed high ECL activity and sensing selectivity because of MIL-101's highly selective adsorption and efficient accumulation abilities [42].

For diverse sensing applications, MIL-53(Al)@CdS, MIL-53(Fe)@CdS, and MIL-53(Al)@CdTe are among the QD-loaded composites described in this study [43, 44].

Some MOFs can also serve as catalysts in the ECL process, in addition to their loading capacity. CdTe QDs were loaded on both the internal framework and external surface of NH2 MIL-88(Fe) by Zhou et al. for signal amplification [45]. CdTe@ IRMOF-3@CdTe composites were used as ECL labels in a sandwich immunosensor developed by Zhuo et al. As a coreactant accelerator, 2-amino terephthalic acid (2-NH2-BDC) acted as an organic ligand of IRMOF-3 in the ECL process to increase the intensity and sensitivity of the ECL process [46].

According to Du et al., CdS@TiO2 emits a highly transcendent ECL, whereas curcumin-Au NPs incorporated in ZIF-8 led to an intense reduction in ECL from CdS@ TiO2. ECL-RET acceptors have also been developed from Ag nanoclusters (Ag NCs) [47].
