Ion-receptor binding is a key mechanism underlying various biological responses, which greatly inspires biomimetic approaches in technologies ranging from nanomedicine to energy storage and active membrane separation. Interaction between analytes and nanopores has been reported to either favor 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 submicromolar 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 the diffusion mediated by transient surface adsorption. The occurrence of this mechanism has been previously investigated using single-molecule FCS techniques, and it is confirmed here, even in bulk measurements conducted at micromolar concentrations. 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.
Varol et al. (2025) studied this question.