# A Review of Electrochemical Sensors for the Detection of Glyated Hemoglobin
## 2 Direct Type Electrochemical HbA1c Sensors
### Amperometric Sensors  
The amperometric HbA1c sensor detects biomolecules by the change in current as the output signal. This type of sensor was first developed in 2002, in which HbA1c molecules were attached to the electrode surface by a cellulose membrane pre-modified with globin [29]. This ground-breaking work verified the capability and promising potential of amperometric sensors for detecting HbA1c. Moreover, antibodies, boric acid and its derivatives, ferrocene and its derivatives, and nucleic acid aptamers can be used to construct amperometric sensors [22; 25; 30]. Since HbA1c is a protein with reduction property, in general, the detection mechanism of the amperometric HbA1c sensor is that the electrode modification substance oxidizes HbA1c to produce a redox reaction, or HbA1c hinders the oxidation current value of redox media [31; 32]. **Boric acid-based. ** Under weakly alkaline conditions, boric acid can covalently bind to the diastatic cis-diol bonds on the surface of HbA1c [33]. [PERSON] et al. [34] proposed a method of using boric acid-polyamine G4 dendrimer-modified electrodes, verified successful binding of HbA1c at a content ranging from 2.5% to 15%, and simultaneously measured the electrochemical current generated as ferrocene methanol was catalyzed by glucose oxidase (GO\({}_{x}\)) on the electrode surface. The current value can be used as an indicator of the combination of HbA1c and the boric acid layer. Furthermore, to detect HbA1c in whole human blood, [PERSON] et al. developed a competitive electrochemical HbA1c biosensor based on the cis-diol interaction between HbA1c and a boronate recognition group [35]. Predetermined concentrations of HbA1c and activated GO\({}_{x}\) were simultaneously dropped onto the surface of boric acid-modified electrodes, and the two species competed for the limited binding sites (Figure 2A). The experimental results provided a linear response within the content  
Figure 1: Classification of electrochemical sensors for HbA1c detection. range of 4.5-15%. This biosensor holds great potential for the determination of HbA1c in whole blood samples without labeling with antibodies, dyes, or fluorescent materials. However, since boric acid can also combine with other sugar substances [36], the detection specificity is poor, and the entire blood sample needs to be pretreated before detection. Recently, a certain modification of boric acid was performed to achieve specific binding of HbA1c. [PERSON] et al. developed a dual-electrode sensor (SPCE/CNT-Nf@Hb-Nf and SPCE*/AQBA-HbA1c) to detect Hb and HbA1c in whole blood samples simultaneously. Anthraquinone boric acid was prepared through electrooxidation with anthraceric boric acid as the raw material. The SPCE*/AQBA electrode could be identified using a specific borate-diol and recombined with the applied HbA1c [18]. Phenylboronic acid (PBA) is obtained by replacing one of the hydroxyl groups in boric acid with a phenyl group. This compound can bind HbA1c by a borate bond, and the catalyst or redox-active species should be attached to the electrode to obtain an electrical signal [37]. A poly(terthiophene benzoic acid) (pTTBA)-modified electrode was immersed in the HbA1c solution, and the current generated from the reduction reaction between HbA1c and hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)) was used to determine the HbA1c level. The linear dynamic detection range varied only from 0.1 to 1.5% [38], and it was not suitable for clinical use. Another research group, [PERSON] et al., used a conducting self-assembled monolayer (SAM) of mercaptophenyl boronic acid (MPBA) to bind HbA1c [39]. The authors labelled a gold screen-printed electrode with a ferrocene-tagged anti-HbA1c antibody (FcAb) as a tracer molecule. The produced current was proportional to the amount of HbA1c between 5% and 16%, significantly expanding the detection range. In another study, researchers coated a layer of phenylboronic acid-modified pyrrolouinoline quinine (PBA-PQQ) onto the surface of a glassy carbon disc electrode [40] (Figure 2C).