# Current Status of HbA1c Biosensors
## 2 HbA1c Biosensors
### Affinity Biosensors Based on Boronic Acid Derivatives
#### 2.1.3 Biosensors Based on 3-Aminophenyl Boronic Acid (APBA)  
[PERSON] and coworkers reported the synthesis of a APBA-graphene oxide (GO) conjugate via the amide linkage confirmed by IR spectroscopy [30], and its application in the determination of glycosylated hemoglobin (GHb) by EIS. It appears that the high conductivity and wide surface area of GO enhance the performance of these APBA-GO modified biosensors. Eggshell membranes (ESMs), a natural porous fiber, are used as a low-cost protein-immobilization platform. By using 3-APBA-modified ESMs biosensor, the HbA1c can be measured in a clinically relevant range (2.3\(\sim\)14%) with a detection limit of 0.19% [31]. The EIS biosensors exhibit good reproducibility, precision and selectivity. Although further optimization shortens the time of HbA1c measurement [32], the need for sophisticated instrumentation severely limits the EIS-based biosensor from being used for clinical application. In contrast, the features of simplicity and rapid measurement increase the possibility of the amperometric method for the design of portable devices. Since the boronate-based biosensors lack selectivity between glycated Hb and sugar molecules, one can solve this by use of the heme iron as catalyst as well as by carrying out an adequate pretreatment of the sample to remove plasma interferences. [PERSON] and coworkers have fabricated a disposable biosensor using APBA chemically bonded to a poly(terthiophenebenzoic acid) (pTTBA)/AuNPs-modified screen-printed electrode [33] (Figure 1C). The detection of HbA1c is by measurement of the response current of catalytic reduction H\({}_{2}\)O\({}_{2}\) by the heme of the protein on the interface. The sensor has been used successfully for the analysis of simple filtration-prepared finger-prick blood samples, exhibiting a linear response to HbA1c levels of 0.1\(\sim\)1.5% in amperometric mode and 0.5\(\sim\)6% in impedometric mode, respectively. An indirect competitive assay for HbA1c determination was developed in which HbA1c competes with alizarin red S (ARS) for binding to phenylboronic acid (PBA) to form diol-boronic complexes in solution [34]. The potential shift of ARS is correlated with the competitive binding of HbA1c to PBA, and is proportional to the concentration of HbA1c. In addition, the total Hb concentration is measured using HCF as the redox indicator to give the result of % HbA1c in line with that measured by a reference method. Although the above methods can provide direct electrochemical detection of HbA1c, they require addition of an exogenous redox indicator, such as a redox mediator or H\({}_{2}\)O\({}_{2}\), due to the relative electrochemical inactivity of HbA1c itself. A label-free and enzyme-free HbA1c sensor was prepared by modifying the electrode surface with poly(3-aminophenylboronic acid) (PAPBA) NPs [35]. The peak current measured by differential pulse voltammogram (DPV) decreases with increasing HbA1c concentrations from 1.0 to 156 \(\upmu\)M, owing to the fact that the HbA1c bonded with the PAPBA interface blocks the ion-flux-channel of the PAPBA NPs. The sensor also showed high selectivity toward HbA1c over glucose, serum albumin, ascorbic acid, uric acid, dopamine and Hb. A label-free voltammetric HbA1c sensor was fabricated by a composite multilayer of APBA-pyrroloquinolinequinone (PQQ) coupled with electrodeposition of reduced graphene oxide (ERGO) on the glassy carbon electrode (GCE) [36]. The APBA/PQQ/ERGO/GCE not only displays a high sensitivity for HbA1c determination with the linear range is 9.4-65.8 \(\upmu\)g/mL, but also can be applied for whole blood samples. Besides the conventional electrochemical detection methods, [PERSON] and coworkers also employed a home-built surface plasmon resonance (SPR) biosensor to study interactions between the APBA monolayer and HbA1c as functions of pHs and salt concentrations [37]. The surface of an Au-coated SPR probe was modified with 4,4-dithiodibutyric acid (DTBA)-APBA monolayers in order to bind HbA1c.