voltage-gating in synthetic nanopores induced by cobalt ions michael sullivan, undergraduate: george...
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Voltage-Gating in Synthetic Nanopores
Induced by Cobalt Ions
Michael Sullivan, Undergraduate:George Mason University
IM SURE Fellow, 2006
Dr. Zuzanna S. Siwy, Professor and Mentor:Department of Physics and Astronomy, University
of California, Irvine
Matthew R. Powell & Eric Kalman, Graduate Students:
University of California, Irvine
Outline
• Motivation for Studies of Single Nanopores
• Preparation, Behavior of Single Nanopores
• The Impact of Cobalt
• Conclusions and Future Applications
Motivation
• Small scale offers fundamental insight into physics on the nanoscale.
Motivation• Structure models biological channels.
Preparing Single Nanopores
Linear accelerator
UNILAC, GSI Darmstadt, Germany
Polyethylene terephthlalate(PET), Hostaphan, RN12
Preparing Single Nanopores
Linear accelerator
UNILAC, GSI Darmstadt, Germany
• Heavy ions (e.g. Pb, Au, U) are fired with total kinetic energy equivalent to ~10% velocity of light
•1 Heavy Ion → 1 Latent Track → 1 Pore
track
Preparing Conical Pores•Pores made conical through chemical etching.
Latent Track
L =
12
µm
DConical Pore
→d
Why Conical?Cylindrical Pore
d
L =
12
µm
DConical Pore
→d
21
4
d
LR
dD
LR
4
2 >>
Why Conical?
• Asymmetry induces interesting reactions and interactions.– Rectifies Ion Current
• Under equal but opposite electrochemical potentials, allow more ions to flow in the direction of d→D than D→d.
• Biological channels are asymmetrical.
One-Sided Etching Technique that We Used
Etch solution
9 M NaOH
HCOOH + OH- HCOO- + H2O
Neutralization
Stopping medium
1 M KCl + 1 M HCOOH
PET
PET
Recording Ion Current
Voltage Applied Here
I
U
0.1 M KCl 0.1 M KCl
Voltage Applied Here
K+ Ion Transfer with + Applied Voltage
K+ Ion Transfer with - Applied Voltage
Recording Ion Current
PET Film
Ground
Working Electrode
The Conductivity Cell
Recording Ion Current
The Data Acquisition Setup
Molecular Devices, Inc.
I - V Curve f or KCl
-2500
-2000
-1500
-1000
-500
0
500
1000
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
Voltage (V)
Cu
rren
t (p
A)
I1
I2
• I1 > I2 because of Ion Current Rectification
Recording Ion Current Time Series (I-t Curves)
• Identical setup to I-V Curves, but current is read over time.
• Useful when current is voltage dependent, yet not constant over time.
+500 mV -100 mV
-600 mV -1000 mV
I-t Readings w/o Cobalt
+500 mV -100 mV
-600 mV -1000 mV
I-t Readings w/ Cobalt
0.1 M KCl + 0.1 mM Co2+ @ -1V
-400
-300
-200
-100
0
100
200
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
Voltage (V)
Cu
rren
t (p
A) 0.1 M KCl
1 uM Co
3 uM Co
10 uM Co
30 uM Co
0.1 mM Co
0.3 mM Co
I-V Curves for Cobalt Concentrations
Differences Between PoresBoth w/ 0.1 M KCl + 0.1 mM Co2+
@ -1000 mV
•Different Timescales
•Different Magnitudes
•Same “Shape”
Quantitative
Qualitative
• Different divalent cations produce different characteristic responses:
• Possible application for sensing molecules.
Future Applications
0.1 M KCl + 0.1 mM Co2+0.1 M KCl + 0.1 mM Ca2+
Future Work• Needed: A mathematical model
Koper, Sluyters. J. Electroanal. Chem. 303 (1990) 73.
Acknowledgements
• Thank you to the following people and organizations for making this experience possible:– My mentor, Zuzanna S. Siwy– My graduate students, Matt Powell and
Eric Kalman– UROP and the IM-SURE Program– National Science Foundation