Olfactory Sensory Neuron‐Mimetic CO2 Activated Nanofluidic Diode with Fast Response Rate

DOI: 10.1002/adma.201405564 The CO 2 -activated nanofl uidic devices were constructed on an ion-track-etched [ 12 ] conically shaped nanopores (Figure S1, Supporting Information) in poly(ethylene terephthalate) (PET) membranes through a “grafting-from” method (More details in the Supporting Information). Atom transfer radical polymerization (ATRP) initiator was immobilized covalently with carboxylic groups on the nanopores surface. Then, imidazolecontaining monomer, 1-(4-vinylbenzyl)-1H-imidazole (VBI), was graft-polymerized through ATRP method ( Figure 2 a, some details in the Supporting Information). X-ray photoelectron spectroscopy (XPS) technique was applied to monitor the modifi cation. As shown in Figure S2 (Supporting Information), the XPS spectrum of primary PET exhibits only O1s and C1s signals. When grafted with poly[1-(4-vinylbenzyl)-1H-imidazole] (PVBI), the N1s signal appears, indicating successful PVBIgrafting (Figure S3, Supporting Information). The membrane surface properties before and after modifi cation were studied by contact angle (CA) measurements. The results show that obvious changes of the surface wettability occur. The water CA of the primary membrane is ≈35°, and increases to ≈90° after PVBI modifi cation (Figure 2 b). The ion current through the nanopores was measured in KCl solution (0.1 M ) under a scanning transmembrane electrical potential from −2 to +2 V. [ 13 ] The experiments were carried out at room temperature in fresh air (ca. 400 ppm). Ion current is ca. 61.14 nA at 2 V, ca. −108.29 nA at −2 V. When immobilized with PVBI, ion current decreases obviously to 3.10 nA at 2 V, ca. −3.15 nA at −2 V (Figure 2 c). The nanochannels turn to be a closed state. The I – V curves testify that PVBI is successfully incorporated on the nanochannels surface, as we will discuss later. The imidazole group in the PVBI can be protonated in CO 2 solution. [ 14 ] The major variation occurs in the narrow part of the pore, where the effects of the surface charge are higher, increasing the ionic transport property and the ion current. Therefore, the closed state of PVBI-grafted nanochannels is to be opened by CO 2 . After bubbling CO 2 in the solution, the transmembrane ion current increases to 300 nA ( Figure 3 a). Meanwhile, CO 2 induces surface charge change from neutral to positive and the anions pass preferentially from the tip to the base of the nanochannels due to the asymmetric conical shape, while cations are rejected, which show their ionic transport preference in one direction and determine the direction of the ion rectifi cation. Thereby, the current prefers to fl ow from the base to the tip, which is opposite to the direction of anions (Figure S4, Supporting Information). The nanochannels rectifi es the current, and the ion rectifi cation ratio (the ratio of absolute values of currents at a given voltage 2 vs −2 V) reaches as high as ≈23 (Figure 3 b). Meanwhile, the ions are not free to transport through the nanochannels with anion selectivity. The CO 2 is an important environmental stimulus that regulates many organisms’ behaviors, e.g., fi nding mates, seeking food or hosts, and avoiding predators. [ 1 ] A subset of olfactory sensory neurons (OSNs) is considered to be specialized for sensing CO 2 [ 2 ] The CO 2 response threshold of OSNs in mice is at nearatmospheric concentration of ≈0.066%. [ 3 ] This ability enables the mice to run away from the high concentration CO 2 places, avoiding unconsciousness or even death. [ 4 ] Ion channels, embedded in the cell membranes of OSNs, play crucial roles in the CO 2 -activated signaling transduction. CO 2 is fi rstly enzymecatalyzed into bicarbonate ions by carbonic anhydrase II (CAII). Then the product bicarbonate ions directly activates guanylyl cyclase-D and cyclic nucleotide-gated ion channels are opened to allow cation infl ux into the cells, resulting in neuronal depolarization to fi re bursts of action potentials. [ 3 ]