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Unformatted text preview: DOI: 10.1007/s00339-002-1982-7 Appl. Phys. A 76, 781785 (2003) Materials Science & Processing Applied Physics A z. siwy 1,2, d. dobrev 1 r. neumann 1 c. trautmann 1 k. voss 1 Electro-responsive asymmetric nanopores in polyimide with stable ion-current signal 1 Gesellschaft fr Schwerionenforschung (GSI), Planckstr. 1, 64291 Darmstadt, Germany 2 Silesian University of Technology, Strzody 9, 44-100 Gliwice, Poland Received: 23 May 2002 / Accepted: 2 September 2002 Published online: 15 January 2003 Springer-Verlag 2003 ABSTRACT For the preparation of a single asymmetrically shaped nanopore in a polyimide membrane, Kapton foils were irradiated with single heavy ions and subsequently etched from one side in sodium hypochlorite (NaOCl). The other side of the membrane was protected from etching by a stopping medium containing a reducing agent for hypochlorite ions (OCl ). The resulting conical nanopore rectified ion current and exhibited a stable ion-current flow. PACS 81.16.-c; 81.07.De; 61.82.-d; 05.60.-k 1 Introduction Track etching is a widely used method for pro- ducing membranes with pores having diameters from tens of nanometers to the micrometer range, offering high aspect ratio and homogeneity of the pores . More recently, an increased interest in the production of narrower pores with openings down to a few nanometers has developed . In particular, novel biotechnological filtration and sensing pro- cesses require extremely small pores with diameters similar to those of biochannels [2, 3]. Achieving this goal is much easier with conical pore geometry than with the commonly used cylindrical shape. Moreover, such asymmetric pores al- low higher fluxes of transported media because their flow and electrical resistance are defined by the narrow tip aper- ture. To meet these needs, the technique of producing conical nanopores in polyethylene terephthalate (PET) has recently been developed . However, the application of membranes with pores having diameters in the nanometer range is highly limited due to their unfavorable transport properties; namely the fact that ion currents through the PET nanopores are not stable, but fluctuate strongly in time. The amplitude of such fluctuations may even reach 100% of the signal , and pores of diameter approximately 2 nm tend to close com- pletely. These properties result from the chemical structure of the polymer as well as from the radiation damage and the etch- ing process. Polymeric chains are broken and subsequently modified chemically during etching, forming so-called dan- gling ends. According to the existing concepts of polymers Fax: +49-6159 / 712-179, E-mail: Z.Siwy@gsi.de in solution, they move continuously in a more or less random fashion [9, 10]. The broken chains also seem to contribute to the formation of a gel layer, which makes the transport through the pores even less defined . Another drawback of pores in PET is the very small opening angle of only several...
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