Smart polymer-based calcium-ion self-regulated nanochannels by mimicking the biological Ca2+-induced Ca2+ release process

Abstract In nature, ion channels play key roles in controlling ion transport between cells and their surroundings. Calcium ion (Ca²⁺)-induced Ca²⁺ release (CICR), a critical control mechanism for Ca²⁺ channels, occurs due to a Ca²⁺ concentration gradient working in synergy with ryanodine receptors, which are famously known as “calcium sparks”. Inspired by this self-regulated biological process, a smart Ca²⁺ concentration-modulated nanochannel system was developed by integrating a poly{N-isopropylacrylamide-co-acrylamide-[4-(trifluoromethyl) phenyl]-2-thiourea_0.2-co-acrylamide-DDDEEKC_0.2} (denoted as PNI-co-CF₃-PT_0.2-co-DDDEEKC_0.2) three-component copolymer onto the nanochannels of a porous anodic alumina (PAA) membrane. In this smart polymer design, the DDDEEKC hepta-peptide unit has an extraordinary binding affinity with Ca²⁺ through coordination bonds, while CF₃-PT functions as a hydrogen bond mediation unit, facilitating the remarkable conformational transition of the PNI main chain in response to Ca²⁺-specific adsorption. Due to these futures, the dynamic gating behaviors of the modified nanochannels could be precisely manipulated by the Ca²⁺ concentration. In addition, the sensitive Ca²⁺ response, as low as 10 pM with a high specificity toward Ca²⁺ capable of discriminating Ca²⁺ from other potential interference metal ions (e.g., K⁺, Cu²⁺, Mg²⁺, Zn²⁺, Fe³⁺, and Al³⁺), remarkable morphological change in the nanochannel and satisfactory reversibility indicate the great potential of Ca²⁺-responsive polymers for the fabrication of biodevices and artificial nanochannels.