A pH‐Gating Ionic Transport Nanodevice: Asymmetric Chemical Modification of Single Nanochannels

2010 WILEY-VCH Verlag Gmb Ion channels that regulate ion permeation through cell membranes are very important for the implementation of various significant physiological functions in life processes. The components of these channels are asymmetrically distributed between membrane surfaces. Inspired by these asymmetrical nanochannels – including both the various components of ion channels that are not uniform in distribution and the structural asymmetry – the generation of artificial nanochannels has strong implications for the simulation of the different ionic transport processes as well as the enhancement of the functionality of biological ion channels. Recently, we and others have successfully developed simple and functional pH-controllable nanochannels. Here, we further develop the concept of a smart nanochannel system, which is not subject to the solution environment restriction of the chemical modification, by using a plasma asymmetric chemical modification approach. Compared to other systems, this responsive nanochannel system has the advantage that it provides simultaneous control over the pH-tunable asymmetric and pH gating ionic transport properties. This highly effective method can be used in the near future to build smarter, biologically inspired nanochannel machines with more precisely controlled functions by designing more complicated functional molecules. There has been rapid progress in developing chemical properties and chemical modification of the interior surface of the nanochannels with functional molecules, such as specific ions, light, pH, and temperature. In order to achieve different functionalities of artificial nanochannels, various methods have been invented, such as electroless deposition, solution chemical modification, electrostatic self-assembly and to only cover the whole inner surface of the nanochannels. Plasma technology, which mainly includes plasma etching and plasma modification, offers an effective method for nanoscale surface engineering of materials. Plasma etching has often been used in template synthesis of nanomaterials. Plasma modification can functionalize a specific local area precisely, whether via symmetric or asymmetric chemical modification. This advantage can provide the potential variety of nanoscale features and new properties for developing advanced nanomaterials. Even though multiple nanochannel membranes using plasma modification have been studied for the control of the water permeability of polymeric membranes, we still need an optimal system, such as a single nanochannel system, for studying transport properties of different ionic or molecules in a confined space, without having to average the effects of multiple channels. Therefore, in this work, we developed a perfect pH gating ionic transport nanodevice using plasma asymmetric chemical modification, and this approach can be considered as a platform for developing more ways of the precise asymmetric chemical modification of the interior surface of nanochannels, which have strong implications for the simulation of different ionic transport processes as well as the enhancement of ion channel functionality. As shown in Figure 1, we prepared a single nanochannel membrane with the well-developed ion track etching technology. In this work the etched single nanochannel is symmetric and hour-glass shaped (see Supporting Information, Fig. S1). Diameter measurements of hour-glass shaped nanochannels were conducted with a commonly used electrochemical method. The opening at the base was usually 250–300 nm wide and the narrow center (tip) was 10–30 nm wide. Only one side of the nanochannel was treated by plasma-induced grafting in the vapor phase of the distilled acrylic acid (AAc), which became a pH-responsive polymer, poly acrylic acid (PAA), after plasma-induced graft polymerization. To explore the surface properties of the PET films before and after plasma treatment, the films have been studied by contact angle (CA) measurements. The CA of the PET film (Hostaphan RN12 Hoechst, 12mm thick) surfaces after plasma treatment (Fig. 2b) was smaller than the one before treatment (Fig. 2a). The results of the CA measurements showed that the plasma treatment could lead to a visible change of the surface wettability (from 66.68 1.38 to 36.78 6.98), which indicated a change of the chemical composition. This result may spark further experimental approaches to study the relationship between the wettability and ionic transport properties within a confined space at the nanoscale. PAA is a weak polyelectrolyte that adopts a coiled conformation dependant upon the degree of dissociation or protonation, which is controlled by the pH and ionic strength of the surrounding solution, due to ionic repulsion between anionic groups. PAA contains carboxylic groups that become ionized at pH values above its pKa of 4.7 (Fig. 1). [14a] The conformation of this weak polyelectrolyte changes from coiled at low pH to stretched at high pH. The effect of pH on chain conformation on the surface is schematically outlined in Figure 1 (right insert). At low pH values the formation of intramolecular hydrogen bonds in PAA prevails,