# A Review of Electrochemical Sensors for the Detection of Glyated Hemoglobin
## 3 Indirect Type Electrochemical HbA1c Sensors
### MIC Type  
The molecular imprinting catalyst is the formation of molecular recognition sites in a polymer by performing synthesis in the presence of a target template. Therefore, MIPs can selectively identify target analytes. The use of MIP technology is the most common, stable, scalable, and economical method at present [108; 109]. In this technology, a functional monomer is combined with imprinted molecules and fixed by a cross-linking agent, and then, the imprinted molecules are washed away. The goal is to develop a polymer material with high affinity and selectivity to bind the target substance (Figure 6) [82; 108]. The content of the template monomer used in the preparation process is usually less than 5%, while that of the cross-linking agent is as high as 95% [110]. MIC is an artificial enzyme catalyst. Molecular imprinting technology can be used to construct molecular recognition elements, and the premise of detecting FV by sensor elements based on the enzyme method is to develop a catalytic center, which is used to mimic fructosamine dehydrogenase [81]. [PERSON] et al. found that polyvinyl imidazole (PVI) could be used as an oxidant in the oxidation reaction of FV in the presence of electron receptors under alkaline conditions. Similarly, the same team used a carbon paste electrode to fix PVI and built an amperometric sensor to detect FV, with a linear response range of 0.02-0.7 mM [81]. These results indicated that PVI can be used as a catalyst for oxidizing fructosamine compounds and HbA1c. This team also prepared an artificial fructosyl amine dehydrogenase, a polymer catalyst prepared by copolymerizing allylamine, 1-vinylimidazole, and 4-vinylphenylboronate to improve the selectivity of MIC  
Figure 6: (**A**) The working principle of the molecularly imprinted sensor; (**B**) A molecularly imprinted sensor that specifically recognizes FV (this figure was adopted from [108]). to FV, showing a typical response curve of amperometric enzyme sensors for FV in the range of 0.2-0.8 mM [111]. Subsequently, the team used a higher concentration of buffer solution to improve the selectivity and sensitivity of MIC to FV by changing the operating conditions [112]. When 100 mM of potassium phosphate buffer were used instead of 10 mM, the sensitivity ratio of FV and Fru-\(\epsilon\)-Lys with the sensor increased significantly from 1.9 to 5.7. However, the synthesized MIC was characterized by low flexibility, and most of the active catalyst sites were located inside the polymer. Therefore, the FV oxidation activity of the MIC needs to be improved [113]. [PERSON] proved that changing the flexibility of the polymer chain can improve the FV oxidation activity of the MIC [113]. A more water-soluble polyacrylamide gel with a relatively low cross-linking degree and macroporous structure was used to improve the conformational flexibility of the polymer. This material was fixed on a gold electrode, and an amperometric sensor was prepared that could detect 0.05 to 0.6 mM FV. Therefore, this soluble MIC sensor achieves the detection range required for HbA1c measurement. In brief, existing indirect type HbA1c detection sensors that work by detecting FV/FVH have several performance advantages. Specifically, the detection time is generally only 2-120 s; the pH value of the measurement system is near that of human physiology (7.0-7.5). The linear detection range of FV by indirect sensors based on nanotechnology can reach 3000 uM at most, and the detection limit can reach at least 0.05 uM. The linear detection range of FV by indirect sensors based on imprinting technology can reach 800 uM, and the minimum detection limit is 50 uM. Due to the unique properties of nanomaterials, including adjustable chemical properties, large surface-to-volume area, strong biocompatibility, good stability, excellent conductivity, high sensitivity, and non-participation in redox reaction, electrochemical HbA1c sensors based on nanotechnology have attracted extensive attention (e.g., see [87]).