# A Review of Electrochemical Sensors for the Detection of Glyated Hemoglobin
## 2 Direct Type Electrochemical HbA1c Sensors
### Impedimetric Sensors  
Accurate and rapid detection has always been a research hotspot in the field of medical diagnosis. Impedance sensors are effective in detecting the reaction mechanism of the modified electrode interface and provide a fast detection method by studying the conductivity and chemical conversion process with electrochemistry [68]. In general, the detection mechanism of impedetric HbA1c sensors is that the accumulation of HbA1c on the biosensor film changes the resistance characteristics of the electrode interface [68]. These sensors can detect affinity interactions (e.g., antibody-antigen interactions) without labeling in real time [69; 70; 71; 72]. [PERSON] et al. reported a novel sensor with HbA1c immobilized on a gold electrode covered by an SAM of thiophene-3-boronic acid (T3 BA), using K\({}_{3}\)Fe(CN)\({}_{6}\) and K\({}_{4}\)Fe(CN)\({}_{6}\) as a redox probe. The rate of charge transfer between the electrode and  
Figure 4: (**A**) Schematic diagram of HbA1c microsensor; (**B**) Mixed SAMs method; (**C**) Seed mediated growth nano-gold method; (**D**) HbA1c test strip prepared by SAM and nanotechnology; (**E**) Voltage responses of three kinds of immunosensor in the simulated blood sample to HbA1c (reprint permission of A–E has been requested from [63; 64]).  
the redox probe is related to the concentration of HbA1c [73]. Since HbA1c is difficult to distribute evenly on the sensor surface, the stability of the detection results is not excellent. Although the redox agent can significantly improve electron transfer, it may also diminish the activity of the electrode/SAM interface over time.  
Fortunately, [PERSON] et al. found that the electrode could detect surface binding behavior within a specific frequency range, even without a special redox reagent [74]. [PERSON] et al. [75] achieved the detection of HbA1c without redox reagent in the frequency range of 20-1000 Hz. The sensor consisted of a pair of parallel electrodes integrated into a microfluidic device that was modified by an SAM of T3 BA. This sensor can be easily integrated into a microfluidic device, consuming a low amount of the sample. Furthermore, [PERSON] et al. [76] proposed a circular gold finger-like electrode on this basis, which still required no redox reagents. The electrode could measure HbA1c concentrations from 1 to 100 ng/uL at frequencies ranging from 0.5 to 20 kHz. This strategy makes it more suitable for POCT applications. Moreover, [PERSON] et al. proposed a novel 3D paper-based electrochemical impedance device combined with haptoglobin (Hp)-modified and APBA-modified eggshell membranes (ESMs) that was highly responsive within the clinically relevant total concentration range (0.5-20 g/dL) and that of HbA1c (2.3-14%) and reduced the data acquisition time 15-fold [77]. This micro-fast sensor not only shows great potential for POCT but is also a unique platform for off-site clinical diagnosis. The interface materials, detection mechanisms, and detection results of direct type sensors were compiled and are shown in Table 1.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Type** & **Electrode/Interface** & **Detection** & **Mechanisms of Detection** & **Sample** & **References** \\ \hline \multicolumn{5}{c}{Dend-FPBA electrode/poly(amidoamine)} \\  & & 2.5–15\%/NA & \begin{tabular}{c} GO\({}_{\text{x}}\) catalyzes the \\ oxidation of \\ reference \\ \end{tabular} & \begin{tabular}{c} HbA1c \\ reagent \\ \end{tabular} & [34] \\  & & Au/Si electrode/Cys-FPBA2, & 4.5–15\%/NA & \
\begin{tabular}{c} GO\({}_{\text{x}}\) catalyzes the \\ oxidation of \\ \end{tabular} &
\begin{tabular}{c} HbA1c \\ reagent \\ \end{tabular} & [35] \\  & & GO\({}_{\text{x}}\) & & \\  & & & & \\  & & & & \\  & & & & \\  & & & & \\  & & & & \\  & & & & \\  & & & & \\ \end{tabular}
\end{table}
Table 1: Direct type electrochemical HbA1c sensors.