1. Monometallic MOFs
In general, transition metal ions, such as Cu, Co, Ni, etc., can be used as the metal centers of MOFs because these metals possess various stable valence states that allow for easy electron gain and loss, making them potential catalysts for electrode reactions. The unique electronic properties of these metals facilitate the binding of reactants and make it easier for the products to leave 
. Additionally, the open metal active sites on the MOFs’ surface can act as electron-transfer channels, which can significantly enhance the electrocatalytic activity of the electrode 
. By adjusting the electronic properties and chemical structures of the MOFs, the catalytic activity of the electrode can be optimized for glucose detection.
As an example of monometallic MOFs, a Cu-MOF-based enzymeless electrochemical sensing system was constructed for chronoamperometric determination of glucose in alkaline media 
. The synthesized Cu-MOFs have a high specific surface area and porous structure. The carbon paste (CP)-based sensing electrode has good electrical conductivity. This synergistic effect leads to a good electrocatalytic activity for glucose oxidation. The sensor exhibited long-term stability, good selectivity, and anti-interference ability. It was applied to the determination of glucose in serum with satisfactory results 
. The Cu-MOF-based sensor demonstrates the potential of using MOFs in developing enzymeless electrochemical glucose sensors.
Chen et al. developed a simple and efficient diffusion control strategy to prepare ZIF-67 hollow nanoprisms (ZIF-67 HNPs) for the preparation of enzymeless glucose sensors 
. The hollow structure provided a pathway for the rapid diffusion of the guest molecule in the MOF material. The sensor exhibited good electrocatalytic activity for glucose oxidation. The enzymeless electrochemical sensor achieved satisfactory results in the analysis of glucose in human serum samples with good reproducibility, selectivity, and long-term storage stability 
. The use of ZIF-67 HNPs in the construction of enzymeless electrochemical glucose sensors also represents a significant advancement in MOF-based sensing materials.
2. Bimetallic MOFs
Kim et al. successfully synthesized Cu@Ni-based bimetallic MOFs with a spherical structure by a two-step hydrothermal reaction 
. The MOF material was modified on the GCE for enzymeless glucose sensors in alkaline solutions. The bimetallic MOFs material exhibited better electrocatalytic activity compared to the monometallic MOF material. Electrochemical analysis showed that the sensor has good electrocatalytic activity for glucose oxidation with good selectivity. Compared to monometallic MOF material, the Cu@Ni MOF exhibited better electrocatalytic activity, mainly due to the synergistic effect of Cu and Ni in the MOF material 
. The use of bimetallic MOF materials in enzymeless electrochemical glucose sensors represents a significant advancement in the development of MOF-based sensing materials.
Kong et al. obtained a Cox
-LDHs (LDH = layered double hydroxide) structure by adjusting the molar ratio of cobalt and nickel, which gradually transformed from a yolk-shell structure to a hollow structure 
. The resulting electrochemical glucose sensor showed that LDH with a hollow structure has better performance than the yolk-shell structure because the hollow structure has a shorter electron-transfer pathway than the yolk-shell structure. Meanwhile, for the hollow layered double hydroxide (HLDH), Co0.33
-HLDH has a higher response current than Co0.21
-HLDH, which may be attributed to its fine structure and more reasonable molar ratio of cobalt to nickel 
. The use of LDH materials with a hollow structure represents a significant advancement in the development of MOF-based sensing materials for glucose detection.
3. Composites of MOFs with Carbon Nanomaterials
Due to the poor electrical conductivity of MOFs 
, carbon materials are often used to enhance the conductivity of MOFs. When carbon nanotubes, graphenes, and other carbon nanomaterials with large surface area are incorporated, the overall surface area of the composite materials of MOFs with carbon nanomaterials could be increased compared to MOFs alone. Therefore, carbon-based nanomaterials have a favorable effect on the catalytic oxidation of glucose by MOFs.
In the study by Wang et al., layered three-dimensional Ni(TPA)-MOFs were synthesized by a solvothermal method 
. Then, single-walled carbon nanotubes (SWCNTs) were mixed with Ni(TPA)-MOFs by ultrasonication. The Ni(TPA)-SWCNT-modified GCE was prepared as an electrochemical glucose sensor. The results showed that the oxidation reaction and electrocatalytic activity of the nanocomposite with SWCNT for glucose were significantly improved compared to the single component Ni(TPA)-MOFs, while SWCNTs could enhance the conductivity and adsorption capacity of MOFs for the catalytic process 
. The electrochemical glucose sensor has excellent selectivity and is capable of fast response (<5 s) for glucose detection. For the glucose assay with actual serum samples, the sensor had results in good agreement with the automated biochemical analyzer. These results demonstrate the high accuracy and promising applications of the sensor in rapid glucose analysis 
Zheng et al. prepared composites of Cu-MOFs and carbon nanohorns (CNHs) with electrocatalytic activity of Cu-MOFs on glucose oxidation 
. The effective glucose biosensing interface was constructed via the synergistic effect of Cu-MOFs and carbon nanohorns nanocomposites. The resulting sensor displayed a wide linear detection range and a low detection limit for glucose analysis. The results were comparable to those based on the clinical method (hexokinase method). The determination of glucose in commercial pear juice also demonstrated the reliability and accuracy of the sensor 
. Zhang et al. developed a composite material of Ni-MOFs and carbon nanotubes (Ni-MOFs/CNTs) by in situ self-assembly in carbon nanotube suspensions 
. The resulting Ni-MOFs has a porous structure, facilitating close contact between glucose and the active center of Ni-MOFs for efficient electrocatalytic oxidation of glucose. The chronoamperometric study showed that the resulting glucose sensor exhibited good reproducibility, repeatability, long-time stability, and high selectivity 
Wu et al. successfully prepared composites of Cu-MOFs/multi-walled carbon nanotubes (MWCNTs) (Cu-MOFs/MWCNTs) 
. A high-performance enzymeless glucose sensor was constructed based on the Cu-MOFs/MWCNT-modified GCE using a layer-by-layer electrodeposition method 
. The multilayer composite membrane on the electrode could effectively increase the exposure of active sites, as well as the reaction contact surface area for catalytic oxidation of glucose 
. Chen et al. prepared graphene nanosheet (GS)@ZIF-67 composites with ordered layered nanostructures by loading polyhedral ZIF-67, in situ, on both sides of the physically exfoliated graphene nanosheets at room temperature 
. The synthesized GS@ZIF-67 complexes exhibited higher catalytic activity for glucose oxidation compared to individual components. The resulting sensor exhibited a good performance with good stability and selectivity for glucose analysis. The glucose sensor was also successfully applied to the glucose detection in human serum with satisfactory results 
. The ordered layered nanostructures of the GS@ZIF-67 composites provided a large specific surface area and abundant active sites, which contributed to the excellent electrocatalytic performance of the sensor for glucose oxidation.
4. Composites of MOFs with Metal Nanoparticles
Meng et al. synthesized Ag@ZIF-67 nanocomposites by continuous deposition reduction 
. The glucose catalysis was investigated with different Ag loading densities. The results demonstrate that increasing Ag loading density from 0% to 0.5% shortened the response time of the modified electrode by more than 2-fold and increased the sensitivity by 2.5-fold. Ag-0.5%@ZIF-67/GCE showed good electrocatalytic performance with good selectivity and stability 
. Chen et al. prepared Au@Ni-BTC by microwave-assisted synthesis 
. The results showed that Au nanoparticles were uniformly deposited on Ni-BTC microspheres. The resulting electrochemical glucose sensor displayed good performance with high selectivity. It was successfully applied for serum sample analysis. The improved glucose sensing performance may be due to the synergistic effect of Au nanoparticles and Ni-BTC, which promoted faster charge transfer at the electrode 
Zhang et al. developed a self-supported Cu/Ni-MOF-modified electrode as an enzymeless glucose sensor by electrodepositing Cu nanoparticles on Ni-MOF derivatives 
. The porous structure and carbon support of Ni-MOFs ensured fast electron-transfer kinetics for electrocatalytic oxidation of glucose at the electrode. The Cu nanoparticles influenced the array structure of the Ni-MOF-derived membrane, thereby improving its electrochemical performance in glucose detection. The electrochemical glucose sensor exhibited good reusability, reproducibility, and stability. The sensor performance was compared to that using the glucose-6-phosphate dehydrogenase for the determination of glucose concentrations in human serum samples. The experimental results further demonstrated the practical feasibility of the modified electrode with Cu/Ni-MOFs 
It is important to note that the compounding of MOFs with metal nanoparticles, such as Au, Ag, Pt, etc., can effectively improve the conductivity of MOFs; however, sometimes, the specific surface area, the porosity, and the active sites of the composite materials decrease because the metal nanoparticles encroach on the pore channels of MOFs, leading to reduced amounts of accommodated glucose. This would have a negative impact on the catalytic performance of MOFs. Therefore, the optimization of the synthesis process and the precise control of the doping ratio and loading density of metal nanoparticles are crucial for achieving high-performance MOF nanoparticle-composite electrochemical glucose sensors.
5. Composites of MOFs with Metal Oxides
For the enhancement of the catalytic performance of MOFs, metal oxides such as CuO, NiO, Fe3O4, TiO2, Co3O4, Mn3O4, etc., can also be utilized. The introduction of such metal oxides has a remarkable effect on catalytic glucose oxidation by MOFs. However, the disadvantages of metal oxides lie in that they may also encroach on the pore channels of MOFs, making the number of accommodated reactants inevitably reduced, thus affecting the catalytic performance of MOFs adversely. Therefore, the doping ratio of metal oxides with MOFs needs to be well controlled for the enhanced catalytic performance.
Zhang et al. developed CuO nanoparticle-modified multilayer Ce-MOFs composites (CuO/Ce-MOFs) by in situ precipitation 
. Ce-MOFs and CuO/Ce-MOFs were characterized by electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and thermogravimetric analysis. CuO nanoparticles were found to be uniformly distributed on the surface of Ce-MOFs. The SEM and TEM images of CuO/Ce-MOF exhibit a composite structure where CuO nanoparticles were well distributed on the Ce-MOF surface 
. The XRD peaks indicated that CuO nanoparticles were successfully synthesized on Ce-MOF surface. The XPS results confirmed the presence of a mixed valence state (Ce3+
) in the Ce-MOF, while the Cu 2p peaks were found in the survey spectrum of CuONPs/Ce-MOF. The thermogravimetric analysis displayed two stages of weight loss of Ce-MOF. After modification with CuO nanoparticles onto Ce-MOF, the thermal weight loss of Ce-MOF was decreased by 20%, indicating the successful synthesis of CuO/Ce-MOF 
. The CuO/Ce-MOF-modified electrodes showed good electrocatalytic activity for glucose in 0.1 M NaOH. Furthermore, the enzymeless glucose sensor was constructed using the modified electrode. The sensor exhibited excellent performance with good repeatability, reproducibility, stability, and selectivity. The results of glucose detection in human serum were satisfactory 
. Shu et al. synthesized Ni-MOF/Ni/NiO/C nanocomposites by an efficient one-step calcination method and constructed an enzymeless electrochemical glucose sensor by immobilizing the composites on the GCE with Nafion membranes 
. The glucose sensor exhibited decent performance with good reproducibility, long-term stability, and good selectivity. In addition, the constructed high-performance sensor was used to monitor glucose levels in human serum and satisfactory results were obtained 
. The study suggests that Ni-MOF/Ni/NiO/C nanocomposites can be a promising material for the development of high-performance enzymeless glucose sensors.
6. MOFs on Metal Foams
It is important to note that the performance of MOF-based glucose sensors can be influenced by factors such as the morphology, pore size, specific surface area, and composition of MOFs, as well as the preparation methods used to modify the electrodes. Zhou et al. synthesized [Mn2
]·2DMF on copper foam (Cu foam, CF) by a one-step hydrothermal method 
. Because the [NiS4
] nucleus has multiple redox states, the [NiS4
] center in the highly oxidized state could oxidize glucose to gluconolactone. Thus, the enzymeless electrochemical glucose sensor was constructed based on the resulting modified electrode to catalyze the enzymeless oxidation of glucose. The enzymeless glucose sensor displayed excellent performance with satisfactory stability and reproducibility 
Zhang et al. have successfully prepared MIL-53 (NiFe) MOFs on nickel foam (Ni foam, NF) for use as a self-supporting electrode for enzymeless glucose detection 
. The sensor has high sensitivity and a low detection limit due to the abundant active sites in MIL-53 (NiFe) MOFs and its good stability in alkaline solutions. In addition, the molecular sieve effect of MIL-53 (NiFe) MOFs showed significant anti-interference ability, even at interfering concentrations as high as 20% glucose concentration. In addition, thermal treatment was performed to remove residual TPA from the MOF channel, which increased the linear range of the assay to 2–1600 μM 
. Overall, this study demonstrates the potential of MIL-53 (NiFe) MOFs as a promising candidate for enzymeless glucose detection. Chen et al. successfully prepared Prussian blue analogues (PBA)/ZIF nanocomposites on NF 
. The electrochemically active PBA particles and NF substrate can effectively reduce the transfer resistance and greatly improve electron transfer and mass transfer efficiency. The resulting electrode based on CoFe-PBA/Co-ZIF/NF were used for the highly selective and sensitive detection of glucose. The sensor was successfully used for the detection of glucose in human serum 
The direct in situ growth of MOFs on Cu and Ni foams can improve the electrical conductivity of MOFs and thus enhance its catalytic activity. This method can effectively retain the intact pore channels of MOFs without encroachment. However, the reproducibility and stability of the electrodes prepared by this method need to be improved because it is difficult to control in situ growth uniformity. Further improvement in stability and reproducibility is needed to meet the commercialization needs for sensor construction.
7. Pyrolyzed MOFs
Pyrolysis of MOFs has been shown to transform crystalline MOFs into amorphous derivatives with a similar chemical composition. Amorphous materials obtained from the pyrolysis of MOFs have attracted a lot of attention in the field of catalysis due to their rich catalytic active sites and enhanced catalytic activities. The amorphous structure of these materials provides a higher degree of disorder and a greater number of defects, which leads to the exposure of more active sites and improved catalytic properties. Additionally, the high surface area and unique morphology of the resulting amorphous materials allow for better mass transfer, facilitating access to the active sites 
. These amorphous derivatives can be used as promising catalysts for various chemical reactions, including glucose oxidation for enzymeless glucose sensors.
Han et al. developed a novel method to prepare a high-performance Co3
-based glucose sensor for the first time by rational design of a novel Co-MOF precursor 
. The Co-MOF precursor exhibits a one-dimensional chain in the molecular structure and is further extended by hydrogen bonding to form a three-dimensional framework. The Co-MOF crystals were further stacked into microfloral clusters and then transformed into curved sheet-like Co3
after heat treatment at 450 °C in an air atmosphere. The curved Co3
-modified GCE (Co3
/GCE) was used to prepare the enzymeless glucose sensor. The resulting sensor worked well at low and high concentrations with sensitivities of 254.21 μA·mM−1
and 102.80 μA·mM−1
, respectively. The electrochemical glucose sensor has good anti-interference ability, reproducibility, and stability 
Zhou et al. investigated the heat treatment of ZIF-67 hollow spheres at various temperatures to obtain amorphous derivatives that have high catalytic activity as electrocatalysts for oxygen evolution reaction and enzymeless glucose sensing 
. The ZIF-67 hollow spheres were heat-treated at 260 °C for 3 h, and intermediate products with amorphous structures were formed during the transformation of ZIF-67 to Co3
nanocrystals. The chemical composition of the amorphous derivatives was similar to that of ZIF-67, but their carbon and nitrogen contents were significantly higher than those of the crystalline samples at 270 °C and above. The catalytic activity of the amorphous Co3
derivative was significantly higher than that of the crystalline Co3
as an electrocatalyst for the oxygen evolution reaction and enzymeless glucose sensing 
. Overall, the amorphous Co3
derivatives obtained by heat treatment of ZIF-67 hollow spheres could be a promising candidate for the development of efficient electrocatalysts and glucose sensors.
8. Other MOFs
The catalytic oxidation of glucose can be enhanced by compounding MOFs with MOFs. The development of new core-shell MOFs for glucose sensing has a broad research prospect. Lu et al. prepared a core-shell electrochemical sensor based on MOF@MOF composites for enzymeless sensing of glucose in alkaline media 
. The UiO-67@Ni-MOFs core-shell composite was synthesized by growing Ni-MOFs on UiO-67 cores under the modulation of polyvinylpyrrolidone (PVP). In the sensor system, UiO-67 with large specific surface area and good conductivity was used to accelerate the electron-transfer rate of UiO-67@Ni-MOFs. The UiO-67@Ni-MOF composite exhibited higher electrocatalytic activity for glucose oxidation compared to UiO-67 and Ni-MOF alone. The resulting sensor was 5–550 μM and 550 μM–3.9 mM with a response time within 5 s and a detection limit of 0.98 μM. The sensor exhibited satisfactory reliability and accuracy for glucose level detection in human serum samples 
Xiao et al. developed a novel electrochemical glucose sensor based on Ni3
@ZIF-67 composites by an ion-exchange method using ZIF-67 as a carrier 
. The electrocatalytic activity of Ni3
@ZIF-67/GCE for the oxidation of glucose was significantly improved compared to the GCE, Ni3
/GCE, and ZIF-67/GCE. The significant improvement in glucose oxidation was mainly attributed to the unique structure of Ni3
@ZIF-67, the mutual promotion of the redox reaction at the Ni(III)/Ni(II) and Co(IV)/Co(III) interfaces, and the increase in surface area. The sensitivity of the resulting sensor was 2783 μA·mM−1
, the linear range was 1.0 μM–4.0 mM, and the detection limit was 0.7 μM (S/N = 3). In addition, Ni3
@ZIF-67/GCE was successfully applied to human serum glucose detection with a good recovery rate (92–109%) 
Sun et al. synthesized heterohybrid materials of ZIF-67@ graphene nanosheets (GSs) (ZIF-67@GS) with a layered morphology by an in situ synthesis method 
. The high electrical conductivity of graphene nanosheets could effectively enhance the electrochemical activity of ZIF-67. Subsequently, by chemical etching phytic acid (PA), PA-functionalized ZIF-67@GS (PA-ZIF-67@GS) was formed with a unique core-shell structure. The metal active sites, the electrochemically active surface area, and the electron-transfer kinetics of chemically etched ZIF-67@GS were further significantly improved. The prepared PA-ZIF-67@GS hybrids exhibited excellent electrocatalytic activity for the oxidation of glucose, resulting in an ultra-sensitive enzymeless electrochemical sensing platform