Amplification Strategy of FET Biosensor Signal for Sensitive Detection of Prion Proteins
Authors: Wustoni Shofarul , Hideshima Sho , Kuroiwa Shigeki , Nakanishi Takuya , Osaka Tetsuya 
Published: February 2020

Field effect transistor (FET) biosensor was considered as a valuable tool for detection of chemical and biological molecules over the past decades [1]. The basic principle of FET biosensor is by converting the variation in the surface potential of gate insulator into the conductivity change in the semiconductor channel. Such kind of direct detection can interestingly offers a simple, label-free and reliable sensing mechanism. However, to configure highly sensitive sensor, the FET signal is strongly depend on the electric field resulting from the binding of charged molecule to the gate surface. Hence, the charge of target molecule is a key factor for electrically-based sensing in which the FET biosensor in some cases hampers to be applied for the detection of low charge density of molecules and neutral molecules.
In this research, we designed two approaches of charge amplification including (i) addition of metal ions as second ligand, and (ii) chemical modification via coupling reaction as illustrated at Figure 1. These strategies were performed into the target protein that can increase the charge of proteins, leading to the enhancement of FET signal. Here, we used and detected prion proteins as a model of the target protein. Highly sensitive method for detecting prion proteins is a top priority for early diagnosis and minimizing the spread of the diseases [2], and the amplified FET signal is of utmost importance to meet the goal for improving the sensitivity of prion detection to the lowest concentration as possible.
Firstly, by utilizing thiamine as probe molecule, the surface of FET biosensor can specifically interact with the prion protein as confirmed by FET response, atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) analysis [3]. Then, the sensitivity of thiamine-immobilized FET to the prion protein in human serum sample before the use of amplification step was quantitatively determined and achieved at the concentration of 400 pM, which is also lower than the cut-off value (2 nM). Furthermore, the amplification process was applied. As for approach (i), by taking the advantage of specific interaction between prion protein and copper (II) ions (Cu2+), a model of dual-ligand binding consist of thiamine immobilized surface, prion protein and Cu2+was performed and successfully generated an enhancement of FET response. As for approach (ii), we demonstrated the feasibility of protein modification with conventional coupling reaction, converting the carboxylate group (anionic) to the amino group (cationic) of prion protein. These strategies can result higher electrical signals in prion detection based on FET biosensor. The detail of experimental procedure and mechanism will be discussed further in the conference. In summary, the FET biosensor device could provide a promising technique and a pave way for highly sensitive detection of prion protein.
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Figure 1