Biosensing with Functionalized Single Asymmetric Polymer Nanochannels Mubarak Ali,* Birgitta Schiedt, Reinhard Neumann, Wolfgang Ensinger Introduction Solid-state nanochannels fabricated in ion-tracked polymer membranes have a great range of applications in bio- technology, [1] where they are suitable for sensing bio- molecules, [2] and act as stimuli-responsive devices, [3] and molecular filters of high selectivity [4] as well as nanofluidic diodes. [5] Therefore, for all these applications, it is highly desirable to control the channel-surface properties, i.e. to functionalize the surface in order to match specific requirements concerning hydrophobicity, selectivity, and to achieve desired interactions with molecules of interest. It has been proven, that the asymmetric nanochannels in polymer membranes rectify the ionic current [6] (i.e. preferential transports of cations (anions) from the narrow entrance towards the wide opening of the channel) similar to voltage-gated biological ion channels. The rectification of ionic current occurs due to asymmetry in the electro- chemical potential inside the asymmetric nanochannel having fixed surface charges. Previously, Wei et al. have also reported the rectification of ionic current at the nanopipet electrode systems and investigated that the current-voltage (I-V) behavior depends sensitively on the size of electrodes as well as on the concentration and pH value of the electrolyte solution. [6e] Single asymmetric nanochannels in polyimide (PI) membranes have been used for the detection of DNA [7a] and porphyrin molecules, [7b] which cause the blockage of ionic current during their translocation through the channel. Recently, based on the rectification behavior, these nanochannels were successfully used for the sensing of organic analytes (crown ether) [7c] and drug molecules [7d] in the electrolyte solution used for measuring the I-V curves. Previously, Siwy et al. have reported the protein sensing [8] with gold coated asymmetric nanotubes, where the incorporation of molecular recognition element was achieved via the chemisorption of thiol molecules. Recently, we have demonstrated the protein sensing [9] by incorpor- ating an electrostatic self-assembly of bifunctional macro- molecule (biotinylated poly(allylamine hydrochloride) having biorecognition moieties in its backbone. I-V curves Communication M. Ali, W. Ensinger Technische Universita ¨t Darmstadt, Fachbereich Material-u. Geowissenschaften, Fachgebiet Chemische Analytik, Petersenstraße 23, D-64287 Darmstadt, Germany E-mail: m.ali@gsi.de B. Schiedt MPI, Universite ´ Val d’Essonne, 91025 Evry Cedex, France R. Neumann GSI Helmholtzzentrum fu ¨r Schwerionenforschung GmbH, Planckstr. 1, D-64291 Darmstadt, Germany In this work, we describe the direct covalent attachment of protein recognition elements (biotin) with the carboxyl groups present on the walls of polyimide nanochannels. Sub- sequently, these biotinylated channels were used for the bio-specific sensing of protein analytes. Moreover, surface charge of these asymmetric nanochannels was reversed from negative to positive via the conversion of carboxyl groups into terminated amino groups. The nega- tively charge (carboxylated) and positively charged (aminated) channels were further used for the electrochemical sensing of bovine serum albumin (BSA, pI ¼ 4.7). These biorecognition events were assessed from the changes in the ionic current flowing through the nanochannel. 28 Macromol. Biosci. 2010, 10, 28–32 ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/mabi.200900198