Recently, mental stress-related diseases, such as integration disorder syndrome and depression, affect people's health. Previously, the relation between concentration of stress-markers and mental stress condition has been reported, suggesting that a simple and daily detection of these markers from biological samples is important to prevent the disease. Secretory immunoglobulin A (s-IgA) which is present in the non-invasive biological samples, such as sweat or saliva, is one of the candidates to measure the mental stress. For the detection of s-IgA, field effect transistor (FET) biosensor is a promising device to easily and quickly detect biomolecules, as the FET characteristics can be responded to the change of surface potential caused by the adsorption of charged molecules. To achieve a sensitive detection by using FET biosensors, small receptors have been applied for an effective use of charge-recognition region, Debye length ^[1]. In this study, jacalin, a small tetrameric lectin from plant (66 kDa), was selected as a receptor to detect s-IgA. Each monomer of jacalin has a glycan-binding site to the hinge region of IgA, thus the sensitivity expected to be improved due to increasing the amounts of specifically adsorbed s-IgA.

The SiO₂ gate surface was exposed to O₂ plasma (200 W for 1 min) in order to introduce hydroxyl groups reacting with triethoxysilane groups of self-assembled monolayer (SAM). Then, FET was immersed in 1%(v/w) 3-aminopropyltriethoxysilane toluene solution at 60ºC for 7 min in an argon atmosphere, followed by the cross-linking by glutaraldehyde. After the modification with the cross-linker, jacalin was immobilized on FET gate surfaces, and ethanolamine capping was performed to prevent the non-specific adsorption of contaminating molecules. The electrical responses were calculated by the FET characteristics before and after the addition of target solution. The FET characteristics were measured by sweeping the gate-voltage from -2.0 V to 0 V with 0.1 V drain voltage in 0.01 × phosphate buffered saline (pH 7.4).

The immobilization of jacalin through the aminopropylsilane SAM was confirmed from atomic force microscopic images on the surface morphology change of the FET gate surfaces. Additionally, to optimize the immobilization condition, the relationship between concentration of jacalin solution and magnitudes of electrical responses were examined. Then, the specificity of jacalin-immobilized FET biosensor to s-IgA were confirmed by the comparison of responses caused by the addition of s-IgA and human serum albumin (HSA). The ΔV_g was shifted in a positive direction due to the negative charges of s-IgA (isoelectric point = 4.5-5.0). On the other hand, when the HSA was added to the FET biosensor, the responses was scarcely observed, suggesting that the jacalin-immobilized surface specifically captured the s-IgA (Figure 1). In addition, a correlation between the FET responses and s-IgA from 100 ng/mL to 100 μg/mL, including the fluctuations of concentration in human sweat sample, was clearly obtained. These results suggested that jacalin could be useful as a receptor for FET biosensors to achieve a simple and sensitive detection of s-IgA.

[1] S. Cheng, K. Hotani, S. Hideshima, S. Kuroiwa, T. Nakanishi, M. Hashimoto, Y. Mori and T. Osaka, Materials, 7, (4), 2490-2500, (2014).