1.Cyclodextrin-modified silicon nanowire field-effect transistor as chemicals sensors .Investigation of the avidin binding behavior by silicon nanowire field-effect transistor

In the past few years, silicon nanowire field-effect transistors (SiNW-FETs) have been widely constructed as ultrasensitive biological and chemical sensors, because of their tunable electrical properties and biocompatibility. Successful applications of SiNW-FET sensors have been developed for detecting ions, DNA, proteins, and virus. In this work, we studied two sensing systems. The first one is that a SiNW with its surface modified by cyclodextrin molecule can be viewed as organic molecular recognition sensor. Cyclodextrins compose of 5 or more α-D-glucopyranoside units linked 1→4, as in amylase. Typical cyclodextrins contain a number of glucose monomers ranging from 6 to 8 units in a ring, creating a cone shape. It is well known that the molecule is captured in the cyclodextrin cavity by non-covalent forces. Some similar studies are performed by carbon nanotube-FETs. However, the sensing mechanism is still unclear. It is interesting to understand the sensing mechanism with cyclodextrin-modified SiNW-FET. The other one is the investigation of the binding of avidin to the biotin modified SiNW-FET (termed biotin/SiNW-FET). The dissociation constant of avidin-biotin is measured to be Kd ≈ 10-15 M. We delivered avidin to the biotin/SiNW-FET surface by injecting an avidin sample into a microfluidic channel system. We measured the response times of biotin/SiNW-FET to avidin at various concentrations. Theixperimental results were explained by a theoretical model, considering that the avidin molecules move to the surface of biotin/SiNW-FET by mass-transfer, diffusion, and convection. That mass-transfer controlled behavior for avidin is dominant in lower concentration condition. For higher concentration, the response time may be limited by the binding rate of avidin onto the biotin/SiNW-FET, instead of by mass-transfering to the biotin/SiNW-FET. The mass-transfer-controlled model is therefore concluded valid in diluted solution.