Calcium Chloride as an Ionic Response Modulator in Metal Organic Framework-filled Nanopores (MOF@SSNs): Enhancing Ionic Current Saturation and Selectivity

We studied ionic transport properties of UiO-66 metal-organic framework-modified solid-state nanochannels (MOF@SSNs) embedded in polyethylene terephthalate (PET) membranes, focusing on the effect of calcium ions from chloride salt (CaCl2) acting as ionic response modulator. We observed a behavior known as ionic current saturation (ICS) regime in a broad pH range, which can be attributed to specific binding of divalent calcium ions to free-carboxylate moieties present in the MOF-filled nanochannels. Such binding provokes a surface charge increase and causes the ICS regime to dominate the response even in alkaline aqueous environments, which were previously shown to feature simple ohmic regimes. The primary ionic transport mechanism operating involves the presence of (mesoscopic) constructional porosity arising from defects and gaps generated during MOF formation within PET nanochannels, rather than intrinsic MOF microporosity also present. The hereby discussed example illustrates how, through straightforward chemical modification, ionic transport properties of the nanochannels can be modulated to feature specific responses necessary for high-impact applications such as ion selective transport, biosensing, or energy generation.