Illuminating Biomimetic Nanochannels: Unveiling Macroscopic Anticounterfeiting and Photoswitchable Ion Conductivity via Polymer Tailoring

Abstract

Artificial photomodulated channels represent a significant advancement toward practical photogated systems because of their remote noncontact stimulation. Ion transport behaviors in artificial photomodulated channels, however, still require further investigation, especially in multiple nanochannels that closely resemble biological structures. Herein, we present the design and development of photoswitchable ion nanochannels inspired by natural channelrhodopsins (ChRs), utilizing photoresponsive polymers grafted anodic aluminum oxide (AAO) membranes. Our approach integrates spiropyran (SP) as photoresponsive molecules into nanochannels through surface-initiated atom transfer radical polymerization (SI-ATRP), creating a responsive system that modulates ionic conductivity and hydrophilicity in response to light stimuli. A key design feature is the reversible ring-opening photoisomerization of spiropyran groups under UV irradiation. This transformation, observable at the molecular level and macroscopically, allows the surface inside the nanochannels to switch between hydrophobic and hydrophilic states, thus efficiently modulating ion transport via changing water wetting behaviors. The patternable and erasable polySP-grafted AAO, based on a controllable and reversible photochromic effect, also shows potential applications in anticounterfeiting. This study pioneers achieving macroscopic anticounterfeiting and photoinduced photoswitching through reversible surface chemistry and expands the application of polymer-grafted structures in multiple nanochannels.

Keywords: anodic aluminum oxide; anticounterfeiting; biological nanochannel; ion conductivity; photoswitchable; spiropyran.

Figure 1

Conceptual design and mechanism illustration of polySP-modified photoswitchable ion nanochannels. (a) Graphical illustrations of biological ChR gates. Opening and closing operations are triggered via irradiation. (b) Schematic illustration of the spiropyran molecule, spiropyran monomer, and polySP-grafted nanochannel. (c) Graphical illustrations of the photoswitchable polySP system, in which the hydrophilicity and on-demand ionic conductivity can be controlled via light irradiation. (d) Graphical illustrations of rewritable and patternable anticounterfeiting polySP-AAO membranes.

Figure 2

Characterizations of the polySP-modified photoswitchable ion nanochannels. (a) Photograph and graphical illustration of AAO nanochannels. (b) Schematic description of the modification procedures. (c,e) SEM images of the pristine AAO membranes: (c) top-view and (e) side-view. (d,f) SEM images of the polySP-modified AAO membranes: (d) top-view and (f) side-view. (g-i) XPS spectra: (g) survey, (h) C 1s scans, and (i) N 1s scans of the pristine AAO, initiator-immobilized AAO, and polySP-grafted AAO. (j) TGA curves of pristine AAO, initiator-immobilized AAO, and polySP-grafted AAO.

Figure 3

Photoswitching of the ring-opening reaction of the spiropyran molecules and polymers. (a) Reversible ring-opening reaction mechanism of the spiropyran derivative (SPOH). (b,c) UV–vis absorption spectra of the SPOH solutions (b) during UV and (c) visible light irradiations for different times. (d,e) Reflectance spectra and photographs of polySP-modified nanochannels (d) during UV irradiation and (e) visible light irradiations for different times. (f) Multiple ring-opening isomerization of the polySP-modified nanochannels as monitored by recording 20 cycles.

Figure 4

Rewritable and patternable properties of photoswitchable polySP-AAO membranes as anticounterfeiting materials. (a) Schematic illustration of the selective isomerization regions upon short-visible light laser. (b,c) Photographs of the patternable and erasable polySP-grafted AAO (b) written by short-visible light laser and (c) patterned by photomask with NYCU logo. (d) Reversible transformations of SP, MC, and protonated merocyanine (MCH⁺) containing polymers. (e) UV–vis absorption spectra of the SPOH solutions in methanol before and after UV irradiations in different pH values. (f) Reflectance spectra of polySP-modified nanochannels before and after UV irradiations in different pH values. (g) Schematic illustration of the selective acidic treatment upon UV irradiation. Patterns at the regions without UV irradiation are invisible. (h) Photographs of the patternable polySP-grafted AAO for the use as anticounterfeiting materials.

Figure 5

Photoswitching of the wettabilities in photoswitchable ion nanochannels. (a) Schematic illustration of the reversible wettabilities by polySP-modified nanochannels. (b) Plots and images of water contact angles of the pristine AAO, initiator-immobilized AAO, polySP-grafted AAO, and polyMC-grafted AAO. (c) Asymmetric water contact angle of a sample that is shone with UV light on half of the region. (d) Plots of water contact angles upon UV and visible light irradiations in different cycles. (e–h) Photoswitching of the electrochemical properties in the polySP photoswitchable ion nanochannels via controllable wettabilities. (e) Illustration of the electrode configuration for the electrochemical measurements. (f) Working mechanism and internal structure of the polySP-AAO. (g,h) Electrochemical impedance spectra of the polySP-AAO: (g) upon UV and (h) visible light irradiations.

Figure 6

Photoswitching of the electrochemical properties in the polySP-AAO using different parameters. (a) Illustration of the electrode configuration, parameters, and mechanism. (b,c) Summarized plots of the impedance and reflectance changes: (b) under UV and (c) visible light irradiations for different times. (d,e) Controllable impedances with different UV irradiation areas: (d) graphical illustration of the polySP-AAO under different photomasks and (e) electrochemical impedance spectra and summarized plot of the polySP-AAO with different UV coverages. (f,g) Electrochemical impedance spectra of the polySP-AAO with different pore sizes for (f) single cycle and (g) different cycles.