Chemical Modification of track-etched dingle conical nanopores inducing inversed inner wall polarity

Nanopores are currently considered as promising candidates for a variety of applications, including separation techniques and biological sensing [1,2]. The pore surface properties are crucial for all these applications, since they influence the transport properties (e.g. hydrophobicity, selectivity) and chemical groups on the inner pore walls may serve as binding sites for analytes [3]. Therefore, it is of highest interest to be able to control the wall properties. Track-etched pores in polymer films open the possibility to directly chemically modify their surfaces. In polyimide (PI), mainly carboxyl groups are created during the track-etching process [4], which can be modified by different methods. One has to keep in mind, however, that most of these methods involve organic solvents, which might affect the polymer. This is of minor importance for somewhat large pores [5], but becomes crucial when going down to nanometer sizes. The pores we were interested in here, are single conical pores in 12 μm thick PI, with only a few nm opening on one side, similar to those which have recently been used successfully for the detection of DNA molecules [6]. These pores are characterized by asymmetric I-V curves, which originate from their charged surfaces. These surface charges are influenced by the pH of the surrounding electrolyte (negative at neutral and basic pH (COO) and neutral at acidic pH in the I-V curves can be seen as an indication for a (COOH), and the surface charge is therefore reflected in the shape of the I-V curves at different pH values [7]. The rectification vanishes for neutral surfaces and is reversed for positive ones [8]. Therefore, changes in the I-V curves can provide an indication for a change in surface charge. The method chosen here for modification uses only water soluble chemicals to avoid any undesirable effects on the pore shape. The reaction is the following: The free carboxylic groups are first activated by Nhydroxysuccinimide (NHS) esters via N-(3-dimethylaminopropyl)-N-ethylcarbodiimide hydrochloride (EDC) [9] and subsequently react with an amine; this reaction yields amide bonds. The COOgroups are therefore converted into NH2 terminal groups.