Polysilicon nanowire with liquid gate control for pH sensing

Polysilicon nanowire based sensors have garnered great potential in serving as highly sensitive, label-free and real-time sensing for broad range of applications, that include but not limited to pH values, DNA molecules, proteins and single viruses. In this research, two distinct types of polysilicon nanowires are fabricated, one has an array of nanowires with a 100 nm width and the other is a single nanowire with 100 nm width. Top-down fabrication method is utilized to fabricate the polysilicon nanowire from silicon wafer using the conventional photolithography and reactive ion etching processes. The fabricated polysilicon nanowire have an approximately 100 nm in width, is then undergo surface modification, which is the nanowire is immersed into a 2% 3-aminopropyltriethoxysilane (APTES) to create a molecular binding chemistry, which results in amino (NH2) and silanol (SiOH) groups at the nanowire surface. Since the surface of the polysilicon is hole-dominated (p-type material), it responds well to changes in pH values. In this research, pH sensing is performed based on several types of standard aqueous pH buffer solutions (pH 2, pH 4, pH 7, pH 10 and pH 12) to demonstrate the electrical response of the sensor. At low pH, NH2 group is protonated, resulting in high proton ion acts as a positive gate. At high pH, SiOH group is deprotonated, resulting in bringing negative charges at the polysilicon nanowire surface and acts as a negative gate voltage. The sensitivity of the polysilicon nanowire attained was 207.1 fS/pH for array nanowire and 8.91 fS/pH for single nanowire, which shows excellent properties for pH sensing.