Fluoride-induced modulation of ionic transport in asymmetric nanopores functionalized with "caged" fluorescein moieties.

Mubarak Ali,Javier Cervera,Patricio Ramirez,Ishtiaq Ahmed,Christof M Niemeyer,Saima Nasir,Wolfgang Ensinger

doi:10.1039/c6nr00292g

Abstract

We demonstrate experimentally and theoretically a nanofluidic fluoride sensing device based on a single conical pore functionalized with "caged" fluorescein moieties. The nanopore functionalization is based on an amine-terminated fluorescein whose phenolic hydroxyl groups are protected with tert-butyldiphenylsilyl (TBDPS) moieties. The protected fluorescein (Fcn-TBDPS-NH2) molecules are then immobilized on the nanopore surface via carbodiimide coupling chemistry. Exposure to fluoride ions removes the uncharged TBDPS moieties due to the fluoride-promoted cleavage of the silicon-oxygen bond, leading to the generation of negatively charged groups on the fluorescein moieties immobilized onto the pore surface. The asymmetrical distribution of these groups along the conical nanopore leads to the electrical rectification observed in the current-voltage (I-V) curve. On the contrary, other halides and anions are not able to induce any significant ionic rectification in the asymmetric pore. In each case, the success of the chemical functionalization and deprotection reactions is monitored through the changes observed in the I-V curves before and after the specified reaction step. The theoretical results based on the Nernst-Planck and Poisson equations further demonstrate the validity of an experimental approach to fluoride-induced modulation of nanopore current rectification behaviour.

Highlights

Over the recent years, the community working on host–guest and supramolecular chemistry has paid much attention to miniaturize anion sensing devices.[1]
Single asymmetric nanopores were fabricated in 12 μm thick polyethylene terephthalate (PET) membranes irradiated with swift heavy ions by the well-established asymmetric tracketching technique.[12]
The nanopore functionalization is based on an amine-terminated fluorescein whose phenolic hydroxyl groups are protected with tert-butyldiphenylsilyl moieties

Summary

Introduction

The community working on host–guest and supramolecular chemistry has paid much attention to miniaturize anion sensing devices.[1]. We demonstrate here a nanofluidic fluoride sensing device based on a single asymmetric pore functionalized with “caged” fluorescein moieties. On exposure to fluoride ions, the uncharged TBDPS moieties are removed due to the fluoridepromoted cleavage of the silicon–oxygen bond, leading to the generation of negatively charged functionalities on the pore surface.

Results

Conclusion