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