# Trends in Quantification of HbA1c Using Electrochemical and Point-of-Care Analyzers
## 2 Sensors for Quantification of HbA1c
### Electrochemical Sensors for Direct HbA1c Detection Using Boronate Chemistry
The hemolyzed sample was immobilized on the electrode surface and changes in the impedance were used for the label-free quantification of HbA1c between 2.3 and 14%. A ferro-ferricyanide couple was employed as the redox mediator in this study [41]. The study employed centrifugation to remove other glycated moieties that could potentially interfere with the analysis. While this is beneficial for the accuracy of measurement, it also makes the process time-consuming and tedious. In a similar strategy, interdigitated gold electrodes modified with a self-assembled monolayer of cysteamine covalently linked to APBA through glutaraldehyde were employed for quantification of HbA1c using impedance measurements. Hemolyzed whole blood was immobilized on the electrode surface for the measurements. The sensor exhibited a linear range of 0.1-8.36% of HbA1c with a LoD of 0.024% [42]. However, the clinical utility of this system could be improved with an increase in the upper threshold of detection. Use of gold electrodes also reduces the cost-effectiveness of the measurement. In a similar study employing EIS for quantification of HbA1c, gold electrodes modified with a self-assembled monolayer of T3 BA were employed to achieve a detection range between 0.08 and 8.4% of HbA1c [43]. The sample preparation in this case involved removal of glycated albumin by centrifugation to avoid interferences from other glycated species in the sample. However, this additional step will not only increase the total analysis time, but also restrict the portability of the system. Graphene oxide (GO) modified with APBA has also been employed for the impedimetric-based quantification of HbA1c. In this work, 1-Ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC), a coupling agent, was employed to facilitate the formation of an amide bond between the carboxylic group of GO and the amine group of APBA. The synthesized GO-APBA was drop-casted onto a clean glassy carbon electrode and employed for further electrochemical studies. The sensor could detect HbA1c in the range of 8% [44]. However, the study did not specify the limit of detection, linear range of detection of HbA1c, or the effect of interferents on the sensor performance. Therefore, the true clinical effectiveness of this system could not be predicted. [PERSON] et al. developed a ring-shaped interdigitated electrode (RSIDE) that works on impedance spectroscopy for the quantification of HbA1c. Here, the gold electrode surface was modified with a SAM of T3 BA. The sensor design was evaluated on a computational platform using COSMOL simulations, which was then translated to a biosensor chip for quantifying HbA1c. The sensor measured HbA1c in the range of 1-100 mg/L. The cheap, label-free, and easy micro-electro-mechanical system (MEMS) fabrication of the sensor can be effectively translated to a point-of-care diagnostic device, provided validation with real patient samples is demonstrated [45]. Recently, a SPCE with a nanointerface comprising bovine serum albumin (BSA), multi-walled carbon nanotubes (MWCNTs), and glutaraldehyde (GA) was reported for measurement of HbA1c. The BSA-MWCNT composite in PBS was crosslinked to the electrode by using glutaraldehyde, followed by immobilization of APBA using EDC-NHS coupling. The BSA imparted an anti-fouling nature to the electrode surface. The fabricated sensor detected HbA1c between 2 and 15% in the samples. To improve the sensitivity of the sensor, the working electrode was further modified by incorporation of the anti-HbAlc antibody. The sensor also evaluated unprocessed human serum [46]. However, the use of antibodies on the electrode surface could lead to increased costs and a reduced shelf-life.