Preparation of Silicon Nanowire Field-effect Transistor for Chemical and Biosensing Applications

Summary

We describe key steps for biosensing by using polysilicon nanowire field-effect transistors, including the preparation of the device and the immobilization and confirmation of a DNA molecular probe on the nanowire surface, as well as conditions for DNA sensing.

Abstract

Surveillance using biomarkers is critical for the early detection, rapid intervention, and reduction in the incidence of diseases. In this study, we describe the preparation of polycrystalline silicon nanowire field-effect transistors (pSNWFETs) that serve as biosensing devices for biomarker detection. A protocol for chemical and biomolecular sensing by using pSNWFETs is presented. The pSNWFET device was demonstrated to be a promising transducer for real-time, label-free, and ultra-high-sensitivity biosensing applications. The source/drain channel conductivity of a pSNWFET is sensitive to changes in the environment around its silicon nanowire (SNW) surface. Thus, by immobilizing probes on the SNW surface, the pSNWFET can be used to detect various biotargets ranging from small molecules (dopamine) to macromolecules (DNA and proteins). Immobilizing a bioprobe on the SNW surface, which is a multistep procedure, is vital for determining the specificity of the biosensor. It is essential that every step of the immobilization procedure is correctly performed. We verified surface modifications by directly observing the shift in the electric properties of the pSNWFET following each modification step. Additionally, X-ray photoelectron spectroscopy was used to examine the surface composition following each modification. Finally, we demonstrated DNA sensing on the pSNWFET. This protocol provides step-by-step procedures for verifying bioprobe immobilization and subsequent DNA biosensing application.

Introduction

Silicon nanowire field-effect transistors (SNWFETs) have the advantages of ultra-high sensitivity and direct electrical responses to environmental charge variation. In n-type SNWFETs for example, when a negatively (or positively) charged molecule approaches the silicon nanowire (SNW), the carriers in the SNW are depleted (or accumulate). Consequently, the conductivity of the SNWFET decreases (or increases)¹. Therefore, any charged molecule near the SNW surface of the SNWFET device can be detected. Vital biomolecules including enzymes, proteins, nucleotides, and many molecules on the cell surface are charge carriers and can be monitored using SNWFETs. With s....

Protocol

1. Fabrication and Preservation of pSNWFET Devices

1. Device Fabrication
   Note: The pSNWFET was fabricated using a sidewall spacer technique as previously reported^23,24.
   1. Prepare the gate dielectric layer.
      1. Cap a 100-nm-thick thermal oxide (SiO₂) layer on a Si substrate by using the wet oxidation process²⁵ (O₂ and H₂ process gas at 980 °C for 4 hr).
      2. Deposit a 50-nm-thick nitride (Si₃N₄) layer by using low-pressure chemical vapor deposition (LPCVD)²⁵ at 980 °C for 4 hr.
   2. Deposit a....

Results

Various SNWFETs have been reported to serve as transducers of biosensors (Table 1). Single-crystalline SNWFETs (sSNWFETs) and pSNWFETs show comparable electric properties as transducers in aqueous solutions, and both have been reported to have many biosensing applications. An advantageous feature of the pSNWFET device used in this study is its simple and low-cost fabrication procedure. Figure 1a shows the key steps involved in fabricating the pSNWFET. An .......

Discussion

Commercializing the top-down and bottom-up fabrication approaches for sSNWFETs is considered difficult because of its cost^32,33, SNW position control^34,35, and its low production scale³⁶. By contrast, fabricating pSNWFETs is simple and low cost³⁷. Through the top-down approach and combination with the sidewall spacer formation technique (Figure 1), the size of the SNW can be controlled by adjusting the duration of reactive plasma etching. The procedures for pre.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Acetone	ECHO	AH-3102
(3-Amonopropyl)triethoxysilane (APTES), ≥98%	Sigma-Aldrich	A3648	Danger
Ethanol, anhydrous, 99.5%	ECHO	484000203108A-72EC
Glutaraldehyde solution (GA), 50%	Sigma-Aldrich	G7651	Avoid light
Sodium cyanoborohydride, ≥95.0%	Fluka	71435	Danger and deliquescent
Sodium phosphate tribasic dodecahydrate, ≥98%	Sigma	04277
Phosphoric acid, ≥99.0%	Fluka	79622	Deliquescent
Photoresist (iP3650)	Tokyo Ohka Kogyo Co., LTD	THMR-iP3650 HP
Synthetic oligonucleotides, HPLC purified	Protech Technology
Tris(hydroxymethyl)aminomethane (Tris), ≥99.8%	USB	75825
Keithley 2636 System SourceMeter	Keithley
SR830 DSP Lock-In Amplifier	Stanford Research Systems
SR570 Low-noise Current Preamplifier	Stanford Research Systems
Ni PXI Express	National Instruments