pH-temperature cooperative dual-responsive nanogating device

Inspired by natural biological ion channels, artificial nanochannels that can respond to external stimuli have been attracting much interests in recent years. However, design of the artificial nanochannel mimicking the organism functions and sensitive to complex signals remains one large challenge for current technique. In this work, we experimentally demonstrate a novel biomimetic nanogating device which displays advanced features of dual-stimuli cooperative respond. This feature originates from the combination of fixed charge and the cooperative configuration change of the immobilized functionalization molecule. The mentioned nanogating device is designed based on artificial polyethylene terephthalate (PET) membrane. We utilize the well-developed ion track-etching technique to produce conical nanochannel, which is an important component of the gating device. After soaking in water overnight, the asymmetrical nanochannel distributed between membrane surfaces is modified with poly-L-lysine through two-steps chemical modification method. The immobilized amphoteric chains can exert influence on the inner surface charges due to the pH-sensitive moieties. At low pH values, the amino groups are positively charged as a result of protonation. Changing the experimental pH from acidic to alkaline values, a significant decrease in positive charges and the deprotonated carboxyl group with negative charges can be observed. The ion transportation properties of the nanochannels can be investigated by measuring the current-voltage (I-V) characteristics. This phenomenon allows for switching the polarity of ion transport from anion-selective to cation-selective by controlling solution pH. The diode- like behavior is quantified by measuring the current rectification ratios. A definite plus is the existence of different configurations of the polymer chains. Poly-L-lysine adopts different conformation when the external pH and temperature change. At pH lower than pKa of poly-L-lysine, the polymer chain mainly adopt random coil structure which is insensitive to the experimental temperature. Higher pH than pKa induces α-helix structure, which is subject to the temperature. When raising the temperature, α-helix structure is observed to transform to β-sheet structure. Circular dichroism (CD) spectra and contact- angle measurements are taken to support the conformational change of PLL chains. This configuration change exerts influence on the effective nanochannel diameter, which leads to significant increase in the gating ratio. Thus, we have constructed the nanogating which is pH-temperature cooperative responsive. This system could potentially promote further research on advanced complicated functionalization smart nanochannel systems.