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.
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.
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 suitable modifications, particularly immobilizing a biomolecular probe on the SNW, a SNWFET can be developed into a label-free biosensor.
Surveillance using biomarkers is critical for diagnosing diseases. As shown in Table 1, several studies have used NWFET-based biosensors for label-free, ultra-high-sensitivity, and real-time detection of various biological targets, including a single virus2, adenosine triphosphate and kinase binding3, neuronal signals4, metal ions5,6, bacterial toxins7, dopamine8, DNA9-11, RNA12,13, enzyme and cancer biomarkers14-19, human hormones20, and cytokines21,22. These studies have demonstrated that NWFET-based biosensors represent a powerful detection platform for a broad range of biological and chemical species in a solution.
In SNWFET-based biosensors, the probe immobilized on the SNW surface of the device recognizes a specific biotarget. Immobilizing a bioprobe usually involves a series of steps, and it is critical that every step is properly performed to ensure the proper functioning of the biosensor. Various techniques have been developed for analyzing the surface composition, including X-ray photoelectron spectroscopy (XPS), ellipsometry, contact angle measurement, atomic force microscopy (AFM), and scanning electron microscopy (SEM). Methods such as AFM and SEM provide direct evidence of bioprobe immobilization on the nanowire device, whereas methods such as XPS, ellipsometry, and contact angle measurement are dependent on parallel experiments performed on other similar materials. In this report, we describe the confirmation of each modification step by using two independent methods. XPS is used to examine the concentrations of specific atoms on polysilicon wafers, and variations in the electric properties of the device are measured directly to confirm the charge variation on the SNW surface. We employ DNA biosensing by using polycrystalline SNWFETs (pSNWFETs) as an example to illustrate this protocol. Immobilizing a DNA probe on the SNW surface involves three steps: amine group modification on the native hydroxyl surface of the SNW, aldehyde group modification, and 5'-aminomodified DNA probe immobilization. At each modification step, the device can directly detect the variation in the charge of the functional group immobilized on the SNW surface, because the surface charges cause local interfacial potential changes over the gate dielectric that alter the channel current and conductance1. Charges surrounding the SNW surface can electrically modulate the electric properties of the pSNWFET device; therefore, the surface properties of the SNW play a crucial role in determining the electrical characteristics of pSNWFET devices. In the reported procedures, the immobilization of a bioprobe on the SNW surface can be directly determined and confirmed through electric measurement, and the device is prepared for biosensing applications.
Commercializing the top-down and bottom-up fabrication approaches for sSNWFETs is considered difficult because of its cost32,33, SNW position control34,35, 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…
The authors have nothing to disclose.
This research was financially supported by Ministry of Science and Technology, Taiwan (104-2514-S-009 -001, 104-2627-M-009-001 and 102-2311-B-009-004-MY3). We thank the National Nano Device Laboratories (NDL) for its valuable assistance during device fabrication and analysis.
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 |