Charge-Based Biosensor Using Carbon
Nanotube Transistors Array
Presenter: Jui-Ping Chiang
Outline
• Introduction of field effect detection of DNA
hybridization
• Experiments
– Fabrication
– Hybridization
– Measurements
• Results
Detection of hybridization
• Hybridization: combining
complementary, single-stranded
nucleic acids into a single molecule
• A single strand of DNA with 15 bases
long has a charge of -15q (2.4 aC)
• Charges adsorbed on the
electrolyte-insulator-silicon (EIS)
structure produce a field effect to
shift the flat band potential and/or
change the capacitive/resistive
impedance across the EIS structure
20qe
10qe
DNA
hyb.
Sensor
Sensor
• In a very simplified model, the threshold voltage shift is
V

Q
/C
given by 
eff
• Express the effective capacitance as:
Ceff

 buffer 
Area ldebye
2 buffer z 2 e 2 c
k BT
e.g. For 10mM TE buffer used during measurement,
Ceff
 2.29 10 5 F 2
cm
Area
• For DNA hybridization, we get positive threshold voltage
shift
Compared to fluorescent method...
Fluorescent
Field effect
•
•
•
•
Pros:
– Well-established
– Very sensitive (sensitivity
depends of sophistication of
detection instrument)
– 3 zeptomolar (10-21 M) has
been reported
Cons:
– Instrument-heavy:
photomultiplier, electronics
– Labeled
Other labeled methods:
Electrochemical redox…
•
•
Pros:
– Simple device structure,
scalable
– Probe attached to oxide:
protocols are compatible
– label-free
– Sensitivity around nM range
Cons:
– Real-monitoring not possible,
unless charge-less PNA
probes are used
Other labeled methods: surface
plasmon resonance; ellipsometry;
THz-transmission analysis;
capacitance measurement; SAW
devices…
Device fabricaiton:
carbon nanotube growth
•
Prepare iron catalyst:
- Brief iron evaporation in UHV
•
CNT growth in CVD furnace
850 ºC
ethylene
hydrogen
methane
argon
•
Cool down
•
Most single walled, and contain both
semiconducting and metallic type in
roughly 2:1 proportion (metallic CNTs
would be burned out later)
exhaust
Device fabrication: microlithography
Carbon nanotube mats on
silicon dioxide substrate
Spin on photoresist, prebake at 90 ºC for 1 min
carbon nanotubes
silicon dioxide
silicon
UV exposure
Metallization
gold
Photoresist
development
mask
photoresist
Final structure:
Lift-off
S
Similar steps to mask
active area during
oxygen plasma etch
D
Al2O3
SiO2
• The active area between source
and drain electrodes where the
nanotube channel sits is 10um
long and 50um wide.
D
S
DNA immobilization and hybridization
1.
2.
3.
4.
5.
6.
Aluminium oxide surface hydroxylation: O2 plasma
Exposed to 3-mercapto-propyl-trimethoxy-silane (MPTMS) vapor
10 mM of Acrydite™-modified probe oligonucleotide dissolved in TE buffer (pH
7), was pipetted to the chip. Dry overnight
Measure the transfer characteristics (IDS-VGS curves)
Incubate the target DNA of on the chip up to 2 hrs in 100% humidity
Wash the whole chip throughly in TE buffer
Measurement
45
before wash
40
after wash
•
Applied fixed 100 mV bias between the
drain and source electrode.
IDS was recorded as VGS was swept to
generate transistor transfer
characteristic curve.
IDS (A)
•
35
30
25
20
15
10
-1
-0.5
0
VGS (V)
0.5
1
1.5
Results
26
probe only
1uM hybridization
dehybridization wash
24
22
IDS (A)
20
18
16
14
fluorescent image of 1uM
complementary target
hybridization
12
10
8
-1
-0.5
0
0.5
1
1.5
VGS (V)
• The shift in threshold voltage, which
arises from the additional surface
charges, serves as the hybridization
signal
fluorescent image after
dehybridization wash to remove
target DNA
1.4
•
Threshold voltage shift of all
transistors are positive upon
DNA hybridization, and the
recovery upon target DNA
denaturation are universal
The amount of voltage shift
should not depend on the
initial position of the
transistor’s threshold voltage
1.3
threshold voltage (V)
•
1.2
1.1
1
0.9
0.8
probe only
1uM hybridization
dehybridization wash
0.7
0
5
10
15
device no.
20
25
30
threshold voltage shift (mV)
• For different target DNA concentrations
80
complementary
non complementary
60
The non-complementary
signal serves as the noise floor
40
20
0
10 nM
100 nM
1 uM
target concentration
– Sensitivity: 10~100 nM
– To be useful for diagnostic applications, the sensitivity of 1 pM or better is
needed
– Compelling estimates that suggest the feasilibity of achieving this level of
detection through improved probe immobilization chemistry, better
hybridization protocols and improved performance by using single CNT
transistors
Thanks for your attention.