Modulating Electrostatic Interactions to Control the Analyte Transport in Nanochannels

Ion-receptor binding is a key mechanism for various biological responses that greatly inspire biomimetic approaches in technologies ranging from nanomedicine to energy storage and active membrane separation. Interaction between analytes and nanopores has been reported to either favour the transport (electrochemical studies performed in the millimolar concentration regime) or to slow down the diffusion in nanochannels (single molecule investigations in the nanomolar range). Here, we propose a simple and inexpensive fluorescence setup for monitoring sub-micromolar diffusion, which effectively bridges these two concentration regimes, and show that at micromolar concentration, electrostatic interactions between the analyte (Ru(bpy)32+) and nanochannel walls slow down the transport by ca. 20% due to hopping diffusive behavior. The occurrence of this mechanism was previously investigated with single molecule FCS techniques, and it is here confirmed even in bulk measurements conducted at micromolar concentration. Furthermore, we demonstrate that electrostatic interactions can be (i) switched off by changing the pH to acidic, or can be (ii) finely tuned by adding a competitor divalent cation (Ca2+), which effectively competes with the cationic analyte (Ru(bpy)32+) for the negatively charged walls, allowing smoother diffusion through the nanochannels.