Portable Electrochemical Impedance Spectroscopy Device Implementation for Alzheimer's Disease-Related Protein
Authors: Wang Yu-Rong , Chien Shang Chi , Lin Yu Chien , Chen Jung-Chih 
Published: September 2023

Electrochemical impedance spectroscopy (EIS) is an electrochemical technique to measure the impedance varieties of a system based on the alternating current (AC) potential frequency. This technique is high accuracy and frequently used to distinguish the influences between different components. The charge transfer resistance and electric double layer phenomenon commonly occur in these components in an electrochemical system. A slight sinusoidal excitation with multiple frequency has been used to process the phenomenon to impedance values. These values can be simulated as an equivalent circuit to analyze the resistance, amplitude, and phase angle in the actual electrochemical cell [1].
Alzheimer's disease (AD) is the most common form of amnesia. However, AD symptoms are difficult to be conscious of in the early stage, and current screening methods are hard to popularize. Current research generally agrees that AD has a considerable association with beta-amyloid (Aβ) precipitation [2]. Aβ is the proteolytic fragment derived from amyloid precursor proteins (APP). These proteins can be used to screen the pathogenesis of AD. Biological detection chips capture these proteins and then use commercial instruments for detection. The biological detection chips are fabricated using a screen-print-electrode (SPE) chip which contains three gold electrodes, and the fabrication steps show in Figure 1(A). 11-Mercaptoundecaoic acid (11-MUA) was used as a self-assembled monolayer (SAM) to functionalize the biological chip. 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC)/N-hydroxysuccinimide (NHS) have been used to activate the functional group on SAM to bond the antibody for specific detection of Aβ. Finally, block the residual activated groups with ethanolamine. After modifying the sensors, the protein was applied for measuring immediately.
This research aims to popularize a portable device that can achieve rapid and accurate monitoring of early AD. In this study, an AD5933 chip was utilized as the main chip for a portable EIS device to compare with the commercial instrument CHI920D. This integrated circuit chip can be applied as an impedance meter. The AD5933 is embedded on a board that contains an analog front-end (AFE) circuit. This AFE circuit can bias the direct current (DC) output from 1.48 V to 0 V and AC output from 1.98V p-p to 0.1V p-p. Also, we applied a 25 kHz clock signal for AD5933 to sweep frequency in 10 Hz to 20 Hz with 1 Hz resolution. This portable device is controlled by NI Myrio1900, and NI LabVIEW system with a laptop, as shown in Figure 1(B).
To analyze the signal gathered from the Aβ, a simulated actual electrochemical cell with an equivalent current is needed. The test frequency has been fixed between 10 to 20 Hz due to the limitation of AD5933. The original Randles circuit is the frequently used circuit applied to impedance calculation [1]. To optimize the current there are some conditions that can be ignored. For instance, the resistance of redox solution (RΩ), which the resistance is too small compared with the whole electrochemical cell. The Warburg element (Zw) gives a low impact on the frequency interval. Thus, the circuit can be optimized and simplified as a simple parallel resistor-capacitor (RC) circuit. To evaluate system performance, the same biological detection chips were used for both portable devices (Figure 1(B)) and CHI920D (Figure 1(C)). Both calculation method is sample charge transfer resistance (Rct) divided by blocking charge transfer resistance (Rct). The correlation coefficient R2 of these sensors is 0.97 (Figure 1(D)), which means the portable system result is identical to the CHI920D.
In conclusion, our research accomplished the system with low frequency and low AC and DC measuring, indicating a higher sensitivity for early AD detection. The limitation of this research is the biological detection chips. These modified chips are time-sensitivities that lead to conservation becoming difficult. The experimental results suggest that this portable is a potential candidate for wide screening of AD, and arranging the portable device in pairs might accomplish the point of care of AD. Furthermore, this device can also develop with different biomarkers, depending on additional antibody modification.

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Hardy, J.A., et al., Alzheimer's disease: the amyloid cascade hypothesis. Science, 1992. 256(5054): p. 184-186.

Figure 1