Signal Amplification Techniques By Effective Use of the Interfacial Region for Field Effect Transistor (FET) Based Biosensing
Authors: Hideshima Sho , Wustoni Shofarul , Kuroiwa Shigeki , Nakanishi Takuya , Osaka Tetsuya 
Published: February 2020

Highly sensitive method is greatly desirable for detecting protein biomarkers, which exist in most cases at very low concentration. Such performance would also be of utmost importance to the development of early detection test. In the past few years, we have demonstrated sensitive diagnosis of emerging protein diseases by using field effect transistor (FET) biosensor. Among the recent proposed biosensor devices, FET can offer a simple operation and applicable integration as well as miniaturization that are useful for large-scale production and low-cost application test. This promising FET biosensor principally detects the charged target molecules that specifically interact with probe molecule onto the FET gate surface within the Debye length. Therefore, we can construct strategy for signal amplification with the approaches from the probe molecules as well as from the charged target molecule. To date, we have successfully achieved several techniques to improve sensing signal for FET biosensor by applying small probe molecules[1-3] and by enhancing net charges of target molecules[4-6].

Application of small probe molecules

Utilization of small probe molecule is beneficial to improve sensitivity of FET biosensor due to the molecular interaction taking place closer to the sensing surface, which leads to the increase of detectable region of charged target molecules within the Debye length. We have shown a good sensitivity of FET biosensor modified with antigen-binding fragment (Fab) instead of native antibody for detecting cancer biomarkers[1]. In addition, by taking the advantages of small probes, we succeeded sensitive detection for influenza virus-related protein and amyloid β protein (Aβ) using glycan[2] and Congo red[3], respectively.

Enhancement of net charges of target molecules

In this approach, the main idea is to intensify the charge of target molecules that cause an increase of charge density on the sensing surface. For the first case, we have designed a unique dual-ligand binding by adding the second ligand (metal ions) into the target of prion proteins, which clearly showed an enhancement of the sensitivity of FET biosensor[4]. Additionally, we performed a potent model of surfactant conjugated with target molecule that is useful for signal amplification using FET biosensor[5]. In both of these two examples, signal amplification was mainly due to additional charge by introducing other ions or molecules. Furthermore, we also established a straightforward strategy to enhance the charge of protein by direct modification of the concerned target molecule with chemical modification[6]. This technique proved a significant enhancement of signal amplification for the FET-based biosensing detection with high sensitivity.
As described above, we proposed two strategies in the signal amplification for FET biosensors and demonstrated their usefulness. These strategies would be applicable to the detection of other proteins, with adjustment to a certain degree depending on binding affinity and amino acid residues of proteins.
Acknowledgements
This work is partly supported by the Center of Innovation Program, Building of Consortia for the Development of Human Resources in Science and Technology and Grant-in-Aid for Young Scientists (B) (16K17498), all from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.
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