Tailoring Polymer Coatings and Grafting Structures for Photoswitchable Ionic Transport in Solid-State Nanochannels

Abstract

Photoresponsive ion nanochannels have gained significant attention for their ability to regulate ionic transport in response to external stimuli. The potential of molecular and polymeric architectures in the nanochannels to further enhance and modulate these behaviors, however, remains underexplored. In this work, we explore the integration of spiropyran-based polymers into anodic aluminum oxide (AAO) nanochannels, resulting in tailored photoresponsive behaviors. Spiropyran undergoes reversible ring-opening isomerization upon UV irradiation, which leads to changes in the packing and polarity of polymer chains within the nanochannels. The polySp-coated and polySp-grafted AAO systems, fabricated via solution wetting and surface-initiated atom transfer radical polymerization (SI-ATRP), exhibit unique macroscopic and microscopic responses, including reversible color changes, wettability adjustments, and modulation of ion transport under UV and visible light. These findings demonstrate the potential of spiropyran-functionalized nanochannels for applications in optical information storage, photogated materials, and sensors. By manipulating molecular architecture and nanoconfinement, this work paves the way for the design of next-generation photoswitchable systems with enhanced multifunctionality.

Keywords: Anodic aluminum oxide; Ion conductivity; Photochromism; Photoresponsive; Spiropyran.

Figure 1

Conceptual illustration of the polySp‐based nanochannels. (a) Graphical illustration, photograph, and chemical structures of pure AAO nanochannels and polySp‐coated and polySp‐grafted AAO nanochannels. (b) Schematic illustration of the photoswitable SP polymer electrolyte system. (c) Photoisomerization of SP and related hydrophobicity changes via UV and visible lights.

Figure 2

Structural characterizations of the polySp‐based AAO nanochannels. (a–c) Side‐view SEM images of (a) pure AAO and (b) polySp‐coated and (c) polySp‐grafted nanochannels. (d) Chemical structures of the SP molecules and polymer. (e) FTIR spectra of SP molecules and polymer. (f) Overall XPS survey spectra and (g) high‐resolution XPS spectra centered on C 1s and N 1s scans of pure AAO and polySp‐coated and polySp‐grafted nanochannels.

Figure 3

Photoswitching and the ring‐opening reaction of the spiropyran and merocyanine solutions and polySp‐based AAO membranes. (a) Reaction mechanism and photographs of the solutions before and after UV irradiations. (b) UV–vis absorption spectra of the SpOH and Sp copolymer solutions before and after UV irradiations. The SpOH and Sp copolymer are dissolved in methanol and chloroform, respectively, due to their differing solubility. (c) UV–vis reflectance spectra of the polySp‐coated and polySp‐grafted AAO nanochannels before and after UV irradiations.

Figure 4

Photoswitchable color changes of the polySp‐based nanochannels. (a) Schematic illustration of different polySp‐modified AAO nanochannels via molecules and polymers solution wetting and grafting. (b,c) Photographs of Sp‐based nanochannels (b) before and (c) after UV irradiations. (d) UV–vis absorption spectra of the SpOH solutions in different pH values. (e, f) Photographs of Sp‐based nanochannels after acid modifications: (e) Sp‐rinsed and (f) polySp‐grafted AAO nanochannels before and after UV irradiations.

Figure 5

Photoswitching of the electrochemical properties in the polySP‐based AAO nanochannels. (a) Schematic illustration of the electrode configuration and mechanism. (b) Water contact angle plots and images of the pure AAO and polySp‐coated and polySp‐grafted nanochannels before and after UV irradiations. The measurement is performed five times on the same sample, with the error bars representing the standard deviation. (c, d) Electrochemical impedance spectra of the (c) polySp‐coated and (d) polySp‐grafted nanochannels upon UV irradiations. (e) Plots of impedance and (f) impedance changes of the polySp‐coated and polySp‐grafted nanochannels with different UV irradiation times.