Effect of Electrochemically Grafted Aryl-Based Monolayer on Nonspecific Electrical Signal of Field-Effect-Transistor-Based Biosensor
Authors: Himori Shogo , Nishitani Shoichi , Sakata Toshiya 
Published: February 2020

An artificially well-designed biointerface is required for a new strategy of biosensor such as an enzyme-free glucose sensor. A solution-gate field-effect transistor (FET) is based on the potentiometric measurement of intrinsic ionic or biomolecular charges at the gate surface of the FET without enzymatic reactions. A platform based on this measurement method is suitable for the detection of biomarkers in a noninvasive, real-time, and label-free manner. Moreover, an extended-Au-gate FET enables a highly-sensitive detection of small biomarkers, because the Au surface exhibits a strong catalytic action, resulting in the oxidation of organic compounds such as uric acid and glucose. However, the specific detection of small biomarkers, which involves a signal to noise ratio, with the Au electrode is not sufficient for a real sample containing impurities. To prevent the nonspecific signals based on impurities, the surface of Au electrode should be chemically covered with a functional membrane. Therefore, we propose to chemically graft the aryl-derivative film on the Au electrode in this study, because the surface coverage of aryl-derivative film can be electrochemically controlled in a simple way. In particular, we design a Au film/solution interface for the specific detection of small biomarkers and investigate the effect of aryl-derivative monolayer on the prevention of nonspecific signals based on non-targeted small biomarkers.
A Au(/Cr) film was fabricated by sputtering on a glass substrate. The Au electrode was modified by nitro group-terminated aryldiazonium monolayer in acetonitrile including 1 mM 4-nitrobenzenediazonium tetrafluoroborate (NBD), 25 mM tetrabutylammonium hexafluorophosphate (nBuPF6), and 2 mM DPPH via cyclic voltammetry (CV). DPPH was used as radical scavenger to make aryl-derivative monolayer. The immobilization density of aryl-derivative monolayer was controlled via the CV cycles. On the other hand, the same method was applied for the modification of aryl-derivative multilayer in the solvent without DPPH. The change in the surface potential was monitored using the aryl-derivative film-grafted FET. In particular, uric acid was used as a small interference molecule and introduced onto the Au electrode with the aryl-derivative monolayer or aryl-derivative multilayer, the density of which was varied. The concentration of added uric acid was changed in the range of 5 µM to 1 mM. The thickness of aryl-derivative film was analyzed using laser microscope. Besides, the surface coverage was calculated by the reduction from nitro groups of NBD to amino groups. In addition, the surface properties of aryl-derivative films were evaluated by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM).
The uric acid response for the aryl-derivative monolayer-grafted FET was smaller than that for the aryl derivative multilayer-grafted FET. That is, the aryl-derivative monolayer-grafted FET suppressed the nonspecific electrical signal based on the interaction of uric acid with the Au surface. This is because the surface coverage by the aryl-derivative multilayer was insufficient, that is, there were the exposed part of the Au surface among the grafted aryl molecules, through which uric acid approached to the Au surface, while the density of the aryl-derivative monolayer grafted on the Au surface was higher than that of the aryl-derivative multilayer. Moreover, the effect of the aryl-derivative monolayer on the prevention of nonspecific signal was enhanced with increasing the CV cycles for electrografting, resulting in the increase of immobilization density. Thus, a platform based on such a monolayer film electrografted via diazonium chemistry is suitable for controlling the potential response based on the interference of small molecules in biosamples.
Our work suggests a new strategy for the specific detection of small biomarkers using the extended-Au-gate FET biosensor. Now, we design a polymeric nanofilter interface on the Au electrode, which is polymerized from the aryl-derivative films, to utilize the Au electrode with the high sensitivity. In this scheme, the polymeric nanofilter has a functional group to capture small interference molecules outside the diffusion layer near the Au electrode. Biomacromolecules such as proteins can’t access to the Au surface, which means the prevention of nonspecific adsorptions, owing to the high density of grafted films. In addition, small interference molecules can’t also access to the Au surface because the functional groups in the polymeric nanofilter catches them. Then, a target small biomarker can only access to the Au surface and be detected owing to a catalytic action. For this scheme, it is very important to control the density of aryl-derivative films.

Figure 1