Fabrication of Carbon Nanotube High-Frequency Nanoelectronic Biosensor for Sensing in High Ionic Strength Solutions

Summary

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.

Abstract

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 impeded^6-8.

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 mixers^9-11. 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 frequencies^11-12. 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.

Introduction

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 molecules^15-20 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....

Protocol

1. Catalyst Patterning for SWNT Growth

1. Begin with a silicon wafer with a low pressure chemical vapor deposition (CVD) grown 500 nm Si₃N₄/ 500 nm SiO₂ film on top.
2. Spin coat a layer of photoresist (PR) at 500 rpm for 5 sec and then 4,000 rpm for 40 sec.
3. Bake the wafer at 115 °C for 90 sec.
4. Use a photomask with rectangular pits for catalysts (Figure 1) and expose the wafer in UV (365 nm) irradiance of 300 mJ/cm² for 0.3 sec. After exposure bake the wafer at 115 °C for 90 sec.

Tip: Design pits of different size....

Results

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).

Discussion

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.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
REAGENTS
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)	Megaposit SPR	PPE
AZ 300MIF	AZ Electronic Material Corporation		PPE
Acetone	J T Baker	9005-05	PPE
Isopropanol (IPA)	J T Baker	9079-05
Buffered Hydrofluoric Acid	Transene		PPE
1-Pyrene Butanoic Acid, succinimidyl ester	Molecular Probes	P130	PPE
Biotin PEO Amine	Thermo Scientific	EZ- Link PEG2 Biotin, # 21346	PPE
Streptavidin	Invitrogen	S 888	PPE
Dimethylformamide	MP Biomedicals	0219514791	PPE
Polydimethylsiloxane Elastomer Base and Curing Agent	Dow Corning	Sylgard 184 Elastomer Kit	PPE
SU-8 2015	Microchem	Y111064	PPE
SU-8 Developer	Microchem	Y020100	PPE
Silanizing agent	Sigma Aldrich	452807	PPE
Hydrogen	Purity Plus	LNF
Ethylene	Purity Plus	LNF
Argon	Purity Plus	LNF
Phosphate Buffer Saline System	Sigma Aldrich	PBS1
EQUIPMENT
Equipment provided by Lurie Nanofabrication Facility (University of Michigan) is denoted as LNF in Catalogue column.
GCA 200 Autostepper	GCA	LNF
Low Pressure Chemical Vapor Deposition Tool	Tempress	LNF
e-beam Evaporator	Enerjet	LNF
CNT growth Furnace	First Nano	Easy Tube 3000 (LNF)
Photomasks	Nanofilm	LNF
Petri dish (150mm)		LNF
Desiccator	Bel-Art	F420100000
Biopsy Punch	Ted Pella	15071/78
Scalpel	Ted Pella	548
Polyethylene Tubing PE-50	VWR	20903-414
Syringe Pump	New Era Pump Systems	NE-1000
Syringe	Fisher Scientific	BD Safety-Lok Syringes
Syringe Needles	Fisher Scientific	14-821-13A
DAQ card	National Instruments	779111-01
GPIB connector	National Instruments	778032-51
Lock-in Amplifier	Stanford Research Systems	SR 830
Frequency Generator	HP Agilent	8648B, 9kHz -2GHz
Bias Tee	Picosecond	5575A-104
Current Preamplifier	DL Instruments, LLC	DL 1211
BNC cables	Allied Electronics	665-xxxx
SMA cables	Sentro Tech Corp	SCF65141