# A Review of Electrochemical Sensors for the Detection of Glyated Hemoglobin
## 1 Introduction  
Diabetes mellitus (DM) is one of the three most harmful non-communicable diseases to humans [1], with an estimated global prevalence of 9.3% (463 million people) in 2019, which is projected to increase to 10.2% (578 million) by 2030 [2]. DM is a group of metabolic diseases associated with high blood glucose [3]. Conventionally, diabetes detection is based on glucose sensing, which is a continuous process, such as impaired fasting glucose (IFG) and impaired glucose tolerance (IGT), and which can easily cause diagnosis errors [4, 5]. However, more recently, glycated hemoglobin (HbA1c) has been shown as an index of blood glucose levels in patients in the past 60 to 90 days, and therefore, it could be an excellent biomarker for continuous glucose monitoring. This protein is a stable product of a non-enzymatic reaction of glucose and human hemoglobin (Hb) \(\beta\)-chain N-terminal valine in serum, and its concentration is insensitive to short-term fluctuations in glucose [6, 7, 8]. Therefore, HbA1c levels reflect the long-term glucose levels of a patient, which can improvediabetes diagnostic accuracy [9] and is crucial for the diagnosis of diabetes [10]. HbA1c levels are defined as the ratio of HbA1c concentration to total hemoglobin concentration and are \(\sim\)4-6.5% for a normal person, while the clinical reference range for its concentration is 5-20% [4], and the physiological levels range from 3 to 13 mg/mL in human blood samples [5]. In addition, current diagnostic criteria for diabetes include the requirement of monitoring fasting blood glucose or plasma glucose measured 2 h after an oral glucose tolerance test (OGTT). By contrast, HbA1c is more convenient, requiring no preparation, and has the lowest intra-individual variation [11]. Several clinical methods are currently available for determining the level of HbA1c in bodies, including liquid chromatography [12], electrophoresis [13], affinity chromatography [14], ion exchange chromatography [15], and immunoassays [16]. Although the effectiveness of these methods has been demonstrated in clinical practice, they require expensive and professional equipment, operation by experienced professionals, and complicated testing processes [17]. In contrast, electrochemical methods require no professional equipment or well-trained operators, and the testing processes are simple and quick [18]. Furthermore, using captured biomolecules, proteins, or antibodies to activate the surface of the electrodes and enable repeatable electrical output for HbA1c detection has significant applications in point-of-care testing (POCT) [19; 20]. Here, we focus exclusively on electrochemical sensing and biosensing reports of HbA1c detection by using different materials. Depending on the target analyte, electrochemical-based HbA1c sensors can be divided into two categories: \"direct sensors,\" based on detecting HbA1c directly, and \"indirect sensors,\" based on detecting FV/FVH indirectly [7; 21; 22]. With the direct sensors, bio-affinity molecules are pre-modified on the surface of electrodes to capture HbA1c, and then, the concentration of HbA1c can be directly determined according to the change in electrical signals generated from electrochemical reactions [23]. Moreover, based on the detection signals, direct type sensors can be further classified into three main types: (a) amperometric sensors, (b) potentiometric sensors, and (c) impedetric sensors [23]. On the other hand, indirect sensors are built to detect the FV/FVH released from HbA1c by protease digestion [24], which has a specific proportional relationship with HbA1c in terms of moles [25]. Indirect sensors are also based on the measurement of electrical signals generated by the redox reaction during the oxidation of FV/FVH by FAO/FPOX/MIC, which is directly proportional to the concentration of HbA1c [26]. To the best of our knowledge, existing electrochemical indirect sensors of HbA1c provide only electrical currents as output; therefore, we will classify these sensors based on the enzyme types.