Published: July 22nd, 2013
We describe the device fabrication and measurement protocol for carbon nanotube based high frequency biosensors. The high frequency sensing technique mitigates the fundamental ionic (Debye) screening effect and allows nanotube biosensor to be operated in high ionic strength solutions where conventional electronic biosensors fail. Our technology provides a unique platform for point-of-care (POC) electronic biosensors operating in physiologically relevant conditions.
The unique electronic properties and high surface-to-volume ratios of single-walled carbon nanotubes (SWNT) and semiconductor nanowires (NW) 1-4 make them good candidates for high sensitivity biosensors. When a charged molecule binds to such a sensor surface, it alters the carrier density5 in the sensor, resulting in changes in its DC conductance. However, in an ionic solution a charged surface also attracts counter-ions from the solution, forming an electrical double layer (EDL). This EDL effectively screens off the charge, and in physiologically relevant conditions ~100 millimolar (mM), the characteristic charge screening length (Debye length) is less than a nanometer (nm). Thus, in high ionic strength solutions, charge based (DC) detection is fundamentally impeded6-8.
We overcome charge screening effects by detecting molecular dipoles rather than charges at high frequency, by operating carbon nanotube field effect transistors as high frequency mixers9-11. At high frequencies, the AC drive force can no longer overcome the solution drag and the ions in solution do not have sufficient time to form the EDL. Further, frequency mixing technique allows us to operate at frequencies high enough to overcome ionic screening, and yet detect the sensing signals at lower frequencies11-12. Also, the high transconductance of SWNT transistors provides an internal gain for the sensing signal, which obviates the need for external signal amplifier.
Here, we describe the protocol to (a) fabricate SWNT transistors, (b) functionalize biomolecules to the nanotube13, (c) design and stamp a poly-dimethylsiloxane (PDMS) micro-fluidic chamber14 onto the device, and (d) carry out high frequency sensing in different ionic strength solutions11.
When a charged molecule binds to a SWNT or NW electronic sensor, it can either donate/accept electrons or act as a local electrostatic gate. In either case, the bound molecule can alter the charge density in the SWNT or NW channel, leading to a change in the measured DC conductance of the sensor. A large variety of molecules15-20 have been successfully detected by studying DC characteristics of the nanosensors during such binding events. Even though charge-detection based sensing mechanism has many advantages including label-free detection21, femto-molar sensitivity22, and electronic read out capability15; it is effective on....
1. Catalyst Patterning for SWNT Growth
A scanning electron microscope image of SWNT transistor with a suspended top gate is shown in Figure 7a. The gate dimensions are critical for suspension25. The current design dimensions are (length x width x thickness = 25 μm x 1 μm x 100 nm). The gate electrode consists of 50 nm Cr/50 nm Au; a thick chrome layer adds more strength to suspended structure. The suspended structure is confirmed by absence of leakage current between top gate and drain (Figure 7b).
The growth of carbon nanotubes depends not only on furnace conditions but also substrate cleanliness. The optimal gas flow rate, temperature and pressure for growth have to carefully calibrated and once fixed they are more or less stable. Even with these conditions being met, we found that growth depends on the patterned catalyst area, amount of catalyst and substrate cleanliness. Hence, we incorporated multiple catalyst pit sizes to account for the variability in growth. A one hour high temperature anneal step helped re.......
We thank Prof. Paul McEuen at Cornell University for early discussion. The work is supported by the start-up fund provide by the University of Michigan and the National Science Foundation Scalable Nanomanufacturing Program (DMR-1120187). This work used the Lurie Nanofabrication Facility at University of Michigan, a member of the National Nanotechnology Infrastructure Network funded by the National Science Foundation.....
|Reagents which were provided within Lurie Nanofabrication Facility (University of Michigan) are marked as LNF in the catalogue column. Chemicals which require protective equipment (gloves, safety goggles, face mask, apron) and/or fume hood are denoted with PPE in comments section.
|Silicon wafers (P-type, <100>, 500-550 μm thick)
|Silicon Valley Microelectronics
|SPR 220 3.0
|Dow (Rohm and Haas)
|AZ Electronic Material Corporation
|J T Baker
|J T Baker
|Buffered Hydrofluoric Acid
|1-Pyrene Butanoic Acid, succinimidyl ester
|Biotin PEO Amine
|EZ- Link PEG2 Biotin, # 21346
|Polydimethylsiloxane Elastomer Base and Curing Agent
|Sylgard 184 Elastomer Kit
|Phosphate Buffer Saline System
|Equipment provided by Lurie Nanofabrication Facility (University of Michigan) is denoted as LNF in Catalogue column.
|GCA 200 Autostepper
|Low Pressure Chemical Vapor Deposition Tool
|CNT growth Furnace
|Easy Tube 3000 (LNF)
|Petri dish (150mm)
|Polyethylene Tubing PE-50
|New Era Pump Systems
|BD Safety-Lok Syringes
|Stanford Research Systems
|8648B, 9kHz -2GHz
|DL Instruments, LLC
|Sentro Tech Corp
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