1. Asymmetric ion track nanopores with highly-tapered profile: geometrical and current-voltage characteristics P.Yu. Apel 1, I.V. Blonskaya 1, S.N. Dmitriev 1, O.L. Orelovitch 1, B.A. Sartowska 2 1 Flerov Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research, Joliot-Curie str. 6, 141980 Dubna, Russia 2 Institute of Nuclear Chemistry and Technology, Dorodna str. 16, 03-195, Warsaw, Poland 24 th ICNTS Bologna, September 2

2. Preamble. Fabrication of ion track conical nanopores Irradiation with single ions at UNILAC (GSI) R. Spohr; German Patent DE 2951376 C2 (filed 20.12.1979, issued 15.09.1983); United States Patent No. 4369370 (1983) Sample in which single ion track is produced

3. I U NaOH Acidic solution PET foil Apel P.Yu, Korchev E.Y., R.Spohr, Z.Siwy, M.Yoshida. Nucl. Instrum. Meth. B184 (2001) 337 + Electrical current registered after breakthrough Electrical field assisted one-sided chemical etching Preamble. Fabrication of ion track conical nanopores

4. Preamble. Diode-like behavior of the conical nanopore in electrolyte solutions KCl U I I (nA) U (V) Ion-track asymmetric nanopores resemble properties of biological ion channels Transport properties of the asymmetric nanopores are determined by the size and shape of the narrow tip The pore tip is cation-selective The pore walls are negatively charged due to COO - groups pH3 pH8

5. Preamble. Single nanopores as resistive- pulse sensors for biological molecules Translocation of single- stranded DNA through the alpha-hemolysin channel Principle of the method "This translocation of DNA movie was made by Dr. Alek Aksimentiev using VMD and is owned by the Theoretical and Computational Biophysics Group, NIH Resource for Macromolecular Modeling and Bioinformatics, at the Beckman Institute, University of Illinois at Urbana-Champaign."

6. Motivation: Asymmetric nanopores as models of non-cylindrical channels, including biological ion channels Asymmetric nanopores for molecular sensors (resistive-pulse technique) Asymmetric nanopores for micro- and nanofluidics Goals of this work: Development of methods allowing control over the shape of ion track nanopores Study of geometrical and transport properties of nanopores having different profiles

7. Surfactant-controlled etching of profiled pores in ion-irradiated polymer foils The ratio between the alkali diffusion flux and the surfactant diffusion flux determines the profile Surfactant molecules have a size of a few nanometers and block entrances of the “new- born” track pores

8. Experimental Polymer foils: Polyethylene terephthalate (PET) Hostaphan 5, 12 and 23 um thick Irradiation with Kr ions (250 MeV), U-400 cyclotron Track densities 10 4 -10 5 cm -2 Etching and subsequent measurement of ionic conductance in KCl solutions Conductometric cell with Ag/AgCl electrodes Track densities 10 7 - 3  10 9 cm -2 Etching and subsequent SEM and FESEM studies of pore structure JSM-840 (SEM) LEO-1530 (FESEM)

9. Treatment with UV PET Latent track Photo-oxidized layer Experimental. Fabrication of nanopores with asymmetric profile NaOH + surfactant 280 nm < < 400 nm, 7 W/m 2 on the sample surface; exposure time: 24 h

10. Experimental Surfactant : Dowfax 2A1 (sodium dodecyl diphenyloxide disulfonate) - Why? Easily soluble in alkaline solutions Stable in alkaline solutions

11. Experimental. Control over the pore profile by etching conditions 6M NaOH + 0.05% Dowfax, 60 o C Highly-tapered pore profile Slightly-tapered pore profile 3M NaOH + 0.05% Dowfax, 60 o C

12. Asymmetric pores with highly-tapered profile Kr ions, 5x10 7 cm -2, etched in 6M NaOH + 0.05% Df, 60 o C, 5 min Surface pre- treated with UV PET foil 5 um thickPET foil 12 um thick Apel P.Yu., Blonskaya I.V., Dmitriev S.N., Orelovitch O.L., Sartowska B. Nanotechnology, 2007, 18, 305302

13. FESEM image of the pore tip (cross-section) d = 30-50 nm   18 o PET 12 um thick, Kr ions, 5x10 7 cm -2, 6M NaOH + 0.05% Df 5 min etching Asymmetric pores with highly-tapered profile

14. Highly-tapered pore profile Current-voltage characteristics of a many-pore membrane PET 23um 84 Kr n=5e4 cm -2 6M NaOH + 0.05%DF, 60 0 C, 5 min one-sided UV 24 hours Well-pronounced rectification, especially high in 0.1M KCl The rectification is observed even for tip radii considerably larger than Debye length D = (  о  RT / 2 F 2 C o ) 1/2 which is equal to ~ 1 nm in 0.1 М KCl

15. Rectification ratio for highly-tapered pores Dependence on electrolyte concentration 5 min etching6.5 min etching8 min etching ~50 nm~100 nm~70 nm

16. Slightly-tapered pore profile Current-voltage characteristics for a many-pore membrane, normalized to one pore PET 23um 84 Kr n=5e4 cm-2 3M NaOH + 0.05%DF, 60 0 C, 8 min one-sided UV 24 hours 500 nm d = 25 nm D  120 nm Small rectification!

17. Rectification ratio I (-1V)/ I (+1V) depending on pore size and pore profile 100 nm Effective pore diameter = diameter of a cylindrical pore having the same electrical conductance in 1M KCl

18. Comparison with theoretical predictions ( based on the Poisson and Nernst-Planck eqs) (P.Ramirez, P.Yu.Apel, J.Cervera, S.Mafe. Nanotechnology 19 (2008) 315707) Trumpet-like pores: low rectification ratio Bullet-like pores: high rectification ratio Qualitatively, experimental data on rectification are in agreement with theoretical prediction for nanopores with different shapes of the tip Quantitatively, the theory does not predict such a high rectification effect for the bullet-like pores with d = 30-70 nm d = 4 nm

19. 3 M NaOH (6-10) M NaOH PET or polycarbonate foil Fabrication of asymmetric ion track nanopores using asymmetric surfactant- assisted etching + surfactant Temperature 60 o C

20. Shape ion track nanopores produced by asymmetric surfactant-assisted etching Pore length = 5 um Small pore diameter  50 nm Large pore diameter  900 nm Etching conditions: Upper surface: 3 M NaOH + surf. Bottom surface: 8 M NaOH Remark: surprisingly, such pores show low rectification of electrical current

21. Conclusions New procedures for the production of ion-track asymmetrical nanopores in polymer foils are suggested: - asymmetric photooxidation and symmetric surfactant-assisted etching; - asymmetric surfactant-assisted etching The methods allow control of pore profile and enable us to fabricate asymmetric nanopores other than conical Ionic transport through the asymmetric pores strongly depends on the shape of the narrow tip Rectification produced by highly-tapered nanopores is higher than theoretically predicted Rectification is maximum at an electrolyte concentration of about 0.1 mol/L, i.e. close to the salt concentration in human body

22. Acknowledgements R. Neumann B. Schiedt R. Spohr C. Trautmann (MR group GSI) A. Presz (INCT, Warsaw) P.Ramirez (UP, Valencia)

23. Thank you!