# Metal-Organic Frameworks Toward Electrocatalytic Applications
## 4 Recent Progress in Utilizing MOFs for Electrocatalysis and Relevant Applications
### Electroanalysis  
If the electrode surface can selectively react to a certain species electrochemically, the electrochemical system can be utilized to determine the concentration of the targeted species with current response as the output signal. Most of such amperometric sensors rely on the two-step EC\({}_{\text{cat}}\) mechanism [125], which implies that these electrochemical sensing systems in fact belong to electrocatalytic applications.  
Figure 7: The use of a porphyrinic Zr-MOF (metal–organic framework) PCN-223(Fe), for electrocatalytic oxygen reduction [110]. Lime green: Zr; red: O; grey: C; light slate blue: N; orange: Fe. Hydrogen atoms are not shown for clarity.  
The most commonly seen example is the amperometric glucose sensor [1], which has been widely available on the market. During the past five years, numerous zinc-based and copper-based MOFs have been utilized in electrochemical sensing systems operated in phosphate buffer solutions or alkaline electrolytes to detect a range of species including hydrogen peroxide [126; 127; 128; 129], glucose [130], L-cysteine [131], L-tryptophan [132], ascorbic acid [132; 133], dopamine [134], and nitrite [135; 136]. In 2015, [PERSON] et al. demonstrated the use of a Zr-MOF, MOF-525, to electrochemically detect nitrite in aqueous electrolytes with a natural pH (see Figure 8a) [43]. The free-base porphyrinic linkers can be served as the active sites to catalyze nitrite oxidation, and the framework was found to keep intact after the electrochemical operations. Since the sensing performance was strongly limited by the sluggish redox hopping in MOF-525, the authors also designed the nanocomposites composed of MOF-525 nanocrystals interconnected by graphene nanoribbons to improve the electrochemical sensing performance (see Figure 8b) [137]. A similar strategy was also utilized by [PERSON] et al.; they synthesized the composite material consisting of MOF-525 nanocrystals and poly(3,4-ethylenedioxythiophene) (PEDOT) nanotubes, and the resulting material was found to outperform both the MOF-525 and PEDOT nanotubes for the electrochemical detection of dopamine [138]. Recently, [PERSON] and coworkers also reported the use of MOF-525/macroporous carbon composite for the electrochemical sensing application toward luteolin [139]. On the other hand, [PERSON] et al. demonstrated the polymerization of aniline monomers in the presence of another Zr-MOF, UiO-66-NH\({}_{2}\), to synthesize the composite material with Zr-MOF and polyaniline; the obtained material was utilized for the electrochemical detection of cadmium ions [140]. Besides, the same group also designed another porphyrinic Zr-MOF (MMPF-6(Fe)) incorporated with gold nanoparticles for the electrochemical detection of hydroxylamine [141]. Another similar example was demonstrated by [PERSON] and coworkers, who utilized UiO-66-NH\({}_{2}\) to immobilize gold-palladium nanoparticles and applied the resulting material for electrochemical nitrite sensing [142]. Beyond these Zr-MOFs, [PERSON] et al. reported the encapsulation of microperoxidase-11 into an aluminum-based MOF, PCN-333 (Al), and utilized the material for electrochemical detection of hydrogen peroxide [143].