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TRANSCRIPT
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Fabrication of Nanoscale BLM Biosensors
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Presentation Contents
Objectives
Background
Fabricated devices
Signal Processing
Current Goals
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Objectives
Fabrication of a stable platform for
transducing signals through artificial BLMs Allow for the most stable BLM possible
Analysis of BLM impedance characteristics
Including signals produced with proteins
Packaging of a sensor with analytic
capabilities on-chip
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BLMs
Composed of a hydrophilic polar head andhydrophobic non polar tail
5nm thickness with .5nm2 area / lipid molecule
BLMs have high resistances and high capacitances
An Artist's conception of ion channels in a lipid bilayer membrane
(taken from Hille, B., 1992.Ionic Channels of Excitable Membranes.
Sinauer, Sunderland, Massachusetts.)
Bilayer Lipid Membranes
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Why use a BLM/protein system?
Biosensors based on natural receptors
(proteins) with BLMs provide a sensitive andselective method of sensing chemical species(ions or molecules)
Upon binding with analytes, transport proteins
change their transport behavior across BLMs These types of sensors are unique in that
they have molecular recognition as well assignal tranduction properties.
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Electrochemical ImpedanceSpectroscopy (EIS)
A small amplitude sinusoidal voltage is
applied across the device The frequency dependant impedance is
measured as a magnitude and phase angle
device
electrodes
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Electrochemical ImpedanceSpectroscopy (EIS)
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Electrochemical ImpedanceSpectroscopy (EIS)
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Electrochemical ImpedanceSpectroscopy (EIS)
Every circuit element has a transfer function
Transfer functions are used to derive the resistance andcapacitance of the system
Component Current Vs.Voltage Impedance
resistor E= IR Z = R
inductor E = L di/dt Z = jwL
capacitor I = C dE/dt Z = 1/jwC
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Electrochemical ImpedanceSpectroscopy (EIS)
The most basic circuit
model utilized is
Zel
This circuit has a function of
ZelZmZt
RjwC
RZm
jwCRZm
1
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Electrochemical ImpedanceSpectroscopy (EIS)
Assuming some knowledge of the circuit
structure, a transfer function can be derivedand the circuit parameters can be extracted.
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Electrochemical ImpedanceSpectroscopy (EIS)
Unfortunately, these systems can be far more
complicated due to a variety of other parasiticinteractions
A primary source of these complications is the Si
substrate itself which is highly conductive. This
presents a low conductance, high capacitancepathway when combined with the membrane.
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Electrochemical ImpedanceSpectroscopy (EIS)
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Fabrication Requirements
Hold a stable membrane
Smooth and clean surface Preferably oxide surface
Porous surface
Allow for signals to be passed through
membrane/proteins Pore size should be small to increase the stability of
suspended region and prevent lipids from forming
conformally to the surface
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Fabrication Requirements
Measure signals with a high S/N ratio
Need a high resistance, low capacitance substrate Prevents capacitive coupling, capacitive signal
leakage
High resistance allows for signals to be measured
only through the membrane area Good electrode placement
i.e. Ag/AgCl electrodes for Cl- measurement
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Porous alumina substrates
Designed by Xinquin Jiang (Spencer group)
Utilizes porous alumina formed
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Porous Alumina Substrate Fabrication
Use LPCVD (Low Pressure Chemical Vapor
Deposition) to coat a 4 DSP (Double sided polish)wafer with Silicon Nitride
Si3N4
Si
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Porous Alumina Substrate Fabrication
Etch a 180 micron x 180 micron square
window on the backside of the substrate
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Porous Alumina Substrate Fabrication
Use KOH as a wet etchant to etch through the Si substrate
KOH preferentially etches crystal plane, resulting in a V-
groove
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Porous Alumina Substrate Fabrication
Evaporate a thin layer of Al onto the front side
of the substrateAl
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Porous Alumina Substrate Fabrication
Anodize the aluminum
Al(metal) Al2O3
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Porous Alumina Substrate Fabrication
Etch the backside to remove the Si2N3
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Porous Alumina Substrate Fabrication
Alumina film characteristics can be adjusted by use of phosphoricacid and anodization conditions
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Porous Alumina Substrate Fabrication
BLM can then be deposited
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Signals obtained from this system
Our results are comparable to state of the art
systems The results do require some amount of
interpretation
This is because the systems on which the BLMs
reside are not identical.
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Si substrates have a much lower resistance
and higher capacitance than quartz substrates
Sample AREA Impedance
0.1 Hz 1 Hz 10 Hz
Quartz plus oxide 88 mm2 46.25 G 14.02 G 1.67 G
Silicon, N-type
0.005-0.02 -cm
88 mm2 1.51 M 173 k 21.32 k
Silicon plus oxide 88 mm2 559.6 M 53.58 M 5.66 M
Silicon/Nitride/Alumina (no H2PO4
etching)
88 mm2 25.21 M 4.197 M 494 k
Silicon/Nitride/Alumina (no H2PO4
etching)
12.6 mm2 18.91 M 3.85 M 503 k
Silicon/Nitride/Alumina (H2PO4 etch
20 min)
88 mm2 1.63 M 133 k 25.02 k
Silicon/Nitride/Alumina (H2PO4 etch
20 min)
12.6 mm2 3.26 M 488.5 k 72.32 k
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Proposed Structure
Change of Silicon substrate for SiO2
Difficulty in etching through the wafer HF wet etch is isotropic
Dry etching of SiO2 has a maximum rate of
100nm/minute which is 5000 minutes for a 500um
wafer.
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Proposed Structure
Cut 100um diameter holes in a quartz
substrate with a micromachining laser
Quartz
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Proposed Structure
Cut 100um diameter holes in a quartz
substrate with a micromachining laser
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Proposed Structure
Anodize the aluminum
Al(metal) Al2O3
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Proposed Structure
Coat the surface with a polymer (polyimide or
adhesive wax)
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Proposed Structure
Adhere the Si and quartz surfaces (hot press)
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Proposed Structure
Dry etch the Si wafer (Bosch etch process) at
a rate of 1um/minute. Dry etch polymer (RIE)
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Proposed Structure
BLM can then be deposited
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The Next Step
Addition of proteins
The proteins are the mechanism by which theenvironment is actually measured
Measurements will be made at a single frequency
that is chosen to maximize sampling while
remaining in the resistive regime Optimally this frequency will be in the kHz range
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Hirano from Nihon University used a patchclamp to measure current openings from a
single gramicidin protein in response to
different concentrations of ferritin avidin
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Opening percentage vs. FA concentration
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Conclusion
We have developed a system to hold
membranes at a high resistance over apatterned substrate
Current readings are feasible and should
generate readable results due to the larger
number of measurement proteins
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Wadsworth Center
(State of NY)
http://www.cnf.cornell.edu/index.html