Effect of the Receptor Size on the Sensitivity of Field Effect Transistor Biosensor for Label-Free Detection of Cancer Biomarker
In this work, we developed an FET biosensor for label-free detection of alpha-fetoprotein (AFP), a liver cancer marker. In order to improve the sensitivity of FET biosensor, this biosensor was designed by immobilizing antigen binding fragment (Fab) with small size as a receptor (Figure 1). The immobilization of Fab molecules was carried out on the SiO2 gate surface of FET devices modified with monolayer of 3-aminopropylsilane by using glutaraldehyde as a cross-linker. The gate voltage (Vg)-drain current (Ids) characteristics were measured before and after the reaction between cancer biomarker (antigen) in the test solution and receptor (antibody or Fab) on the gate. A shift of threshold voltage (ΔVg), evaluated from a linear region of the FET characteristics was discussed as an FET response. We have experimentally compared the sensitivity of the Fab-immobilized FET biosensors with that of antibody-immobilized one (Figure 2). Electrical measurement results showed that the use of Fab results in more than twice higher response to AFP of the same concentration than the use of antibody. It should be noted that the response of Fab-immobilized FET to 100 pg/mL AFP is still greater than the response of antibody-immobilized FET to 10 ng/mL AFP. Considering the fact that both FETs shows the response as small as 5 mV to 1 μg/mL human serum albumin (HSA) added as a negative control, it was demonstrated that, compared to the antibody-immobilized FET, the Fab-immobilized FET has similar specificity to AFP and detection ability to two-digit lower concentrations.
Thus, the sensitivity of FET biosensor for label-free detection of cancer biomarker was improved by the use of Fab with small size instead of antibody. The calibration scheme of Fab-immobilized FET biosensor will be discussed at the conference.
This work was financially supported in part by Grants for Excellent Graduate Schools (Practical Chemical Wisdom), and by Global COE program “Center for Practical Chemical Wisdom”, both from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.
 D. Niwa, K. Omichi, N. Motohashi, T. Homma, T. Osaka, Sens. Actuators B, 108, 721 (2005).
 S. Hideshima, R. Sato, S. Inoue, S. Kuroiwa, T. Osaka, Sens. Actuators B, 161, 146 (2012).