Confining Spirocyclic Fluorescein in an Asymmetric Solid-State Nanochannel: A Simple and Versatile Design Concept for Fabricating Integrated Nanofluidic Diodes with Adjustable Surface Chemistry.

Using small molecules to integrate multifunctional surfaces within a nanopore is an effective way to endow smart responsibilities of nanofluidic diodes. However, the complex synthesis of the small molecules hinders their further application in achieving multifunctional surfaces. Here, a simple and versatile design concept is reported for fabricating bioinspired integrated nanofluidic diodes with adjustable surface chemistry by confining a spirocyclic fluorescein derivative, 6-aminofluorescein (6-AF), within an asymmetric track-etched nanopore. The pH-dependent open-close of lactone ring in 6-AF allows facile fabrication of a pH-gated nanofluidic diode, confirmed with finite element simulations. This pH-gated nanofluidic diode also shows high specificity for sensing 3-nitropropionic acid (3-NPA), indicating its potential applications in food safety. Moreover, three functional nanofluidic diodes are successfully constructed via a regioselective Vilsmeier reaction between 6-AF and N-methylformanilide, the electrophilic addition reaction between 6-AF and propargyl bromide, and a highly controllable reduction process between 6-AF and NaBH/I. The combination of asymmetric nanopores with small molecules not only expands traditional fluorescent spirocyclic molecules to the realm of electrochemistry but also offers valuable insights for the achievement of novel fluorescence-electrochemical coupling detection methods. Besides, the introduction of spirocyclic small molecules to asymmetric nanopores serves as an inspiration source to explore new design concepts for nanofluidic devices.