# 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 resulting 4-MPBA/AuNF/SPCE was characterized and then employed for the quantification of HbA1c and exhibited a linear range between 5 ug/mL and 1 mg/mL of HbA1c [36]. In a different approach, a boronic acid-modified gold electrode was fabricated by dipping the electrode in a DMSO (dimethyl sulfoxide) solution of 3,3'-Dithio-bis-propionic acid N-hydroxysuccinimide ester (DTSP) to form an amine-reactive self-assembled monolayer (SAM). The electrode was then incubated in a 0.5% (\(w/w\)) ethanolic solution of poly(amidoamine) generation 4 (PAMAM) dendrimer and then in 1 mM of 4-formylphenylboronic acid (FPBA). The sensing strategy employs a unique \"backfilling\" assay scheme (Figure 3), where after addition of the sample containing HbA1c, periodate-treated activated glucose oxidase (GOx) was made to react with the unreacted amine groups on the PAMAM-FPBA electrode, which produced the electrochemical response. The signal intensity decreased with increased concentrations of HbA1c due to steric interference of HbA1c on the catalytic efficiency of GOx, thereby providing a stable, quantitative measurement of HbA1c in the sample. The sensor exhibited a detection range of 2.5-15%, which was well within the clinical levels for diagnosis. However, the sensor requires further validation with clinical samples to establish its diagnostic potential [37]. The same group have also reported an interface comprising a conjugate of cysteamine and FPBA that were allowed to react for 4 h at 60 \({}^{\circ}\)C under stirring conditions to obtain the Cys-FPBA\({}_{2}\) conjugate. The gold electrodes were dipped in Cys-FPBA\({}_{2}\) for the formation of a boronate-modified SAM on the working electrode. The electrochemical response was obtained using the backfilling method with GOx. The Hb was separated from whole blood by zinc-induced precipitation. The sensor exhibited a detection range of 4.5-15%. The sensor performance was evaluated with a small sample size of human samples. Though promising, the sensor preparation involves a larger number of steps, which increased the detection time [38]. Figure 3: Scheme representing the backfilling assay–mediated amperometric detection of HbA1c employing boronic acid derivatives. Reprinted with permission from [37]. Poly(3-aminophenyl boronic acid) (PAPBA) nanoparticles obtained by polymerization of APBA have been employed as a thin-film nanointerface material over SPCE for detection of HbA1c [39]. The sensor exhibited a linear range between 0.975 and 156 uM of HbA1c when measured using differential pulse voltammetry and was stable when used in the presence of interferents such as dopamine, ascorbic acid, glucose, uric acid, and bovine serum albumin. However, the sensor did not account for potential errors arising due to variations in the total hemoglobin levels. Recently, a novel interface of anthraquinone boronic acid (AQBA) that was formed by electrochemical oxidation of 2-anthraceneboronic acid (ANBA) on pre-anodized SPCE, designated as AQBA/SPCE*, was employed for HbA1c quantification [40]. The sensor performance remained unaffected in the presence of interferents such as ascorbic acid, uric acid, dopamine, glucose, fructose, mannose, and hemoglobin. The sensor detected HbA1c in the linear range of 31.2-500 nM. An AQBA-free electrode was used for detecting Hb. The sensor exhibited good correlation with the results obtained from the conventional HPLC method for HbA1c in patient samples. Several studies have employed electrochemical impedance spectroscopic (EIS) measurements to measure the rate of charge transfer between the electrode and the redox probe, which is influenced the blocking effect caused by binding of HbA1c with the recognition element (boronic acid derivatives). An earlier study had employed screen-printed platinum electrodes, surface-coated with a fibrous mesh of eggshell membrane, incorporating APBA through glutaraldehyde crosslinking.