Ultraviolet–visible/fluorescence behaviors of a spiropyran/polydimethylsiloxane composite film under acid vapors

Abstract

A spiropyran (SP)/polydimethylsiloxane (PDMS) composite film was prepared in this study, and its ultraviolet-visible/ fluorescence behaviors under acid vapors were investigated. With UV irradiation, the ring-closed SP in the PDMS matrix transformed to a ring-opened merocyanine, which contained phenolate oxygen and could be further protonated by the hydrogen offered by the acid vapor. Such a protonation process endowed the composite film with not only an obvious color change from purple to yellow but also a strong fluorescent emission. As a result, some common volatile acids, hydrochloric acid, hydrofluoric acid, nitric acid, trifluoroacetic acid, formic acid, acetic acid, and propionic acid, could be distinguished with this method either via color or fluorescent emission. Meanwhile, the film showed good recyclability during the processes of irradiation and protonation. The method, therefore, displayed good processing in real time and was fast and recyclable; this makes it suitable as an alternative for applications related to environmental and safety monitoring of acid vapors.

Intelligent Environmental Sensing: Fabrication of Switchable, Reusable, and Highly Sensitive Gas Sensors with Spiropyran-Grafted Anodic Aluminum Oxide Templates

“…Strong acids with p K a values less than one, such as hydrochloric acid and nitric acid, can provide enough protons to protonate the MC-AAO membranes, forming H-AAO membranes completely in 5 s. By contrast, weak acids with low vapor pressures, such as acetic acid, could not provide enough protons to protonate the MC-AAO membranes in 5 s. For formic acid with intermediate acidity, protons can still be provided to protonate the MC-AAO membranes, which is probably caused by the fast rate of protonation. Interestingly, the H-AAO membranes transformed back to the MC-AAO membranes after they were exposed to base vapors following the deprotonation and neutralization steps. , The reversible transformation between the MC-AAO and H-AAO membranes was demonstrated using a hydrochloric acid and ammonia solution (Supporting Information, Video S2). First, the purple colored MC-AAO membrane changed rapidly to yellow colored H-AAO membrane after exposed to hydrochloric acid vapors; similarly, the yellow colored H-AAO membrane changed to purple colored MC-AAO membrane again after exposed to ammonia solution vapors.…”

“…22,23,34 The phenolate oxygen of merocyanine on the MC-AAO membranes can be further protonated by acid vapors to become H-AAO membranes with the color changing from purple to yellow. 7,16 However, for the MC-AAO membranes exposed to acetic acid, the color changed from purple to pale yellow. A graphical illustration of acidochromism between MC-AAO and H-AAO membranes is shown in Figure 4a.…”

“…It is one of the mostly used light-responsive materials; − typically, upon irradiation with UV light, spiropyran undergoes ring-opening isomerization and transforms into merocyanine. Similarly, merocyanine turns back to spiropyran upon irradiation with visible light by ring-closing isomerization. , Moreover, the phenolate oxygen in merocyanine can be protonated in the presence of an acid; likewise, the protonated merocyanine can easily transform back to merocyanine with the addition of a base. , Furthermore, because of their effective responsive properties, simple synthesis, and favorable fatigue resistance, a variety of intelligent sensors based on spiropyran have been widely developed, such as metal ion sensing, pollutant sensing, solvent sensing, and acid vapor sensing. − Specifically, many sensors to detect acid and base vapors based on spiropyran devices have been investigated and reported mainly because of their various advantages such as desirable portability and a simple fabrication process. For example, Zhao and Tian and Fragouli et al have embedded spiropyran in PDMS to develop smart flexible acid vapor sensors; , Lee et al have developed a microfluidic gas sensor based on a photochromic spiropyran-embedded PDMS composite; Fragouli et al have created acid vapor sensors using silk fibroin/poly­(ethylene oxide)/spiropyran nanofibrous mats; Yin et al have developed various spiropyran derivatives with different substituents on chemical structures, making the materials suitable for sensing the environment for extreme acid or base .…”

“…Interestingly, the H-AAO membranes transformed back to the MC-AAO membranes after they were exposed to base vapors following the deprotonation and neutralization steps. 7,16 The reversible transformation between the MC-AAO and H-AAO membranes was demonstrated using a hydrochloric acid and ammonia solution (Supporting Information, Video S2). First, the purple colored MC-AAO membrane changed rapidly to yellow colored H-AAO membrane after exposed to hydrochloric acid vapors; similarly, the yellow colored H-AAO membrane changed to purple colored MC-AAO membrane again after exposed to ammonia solution vapors.…”

Progressive Readout Platform Based on Photoswitchable Polyacrylic Nanofibers Containing Spiropyran in Photopatterning with Instant Responsivity to Acid–Base Vapors

“…Photoresponsive sensors for detection and determination of metal ions, pH, and acid and ammonia vapors are of great interest because of their fast and visual response without any need for sophisticated instruments. − The phenolate ion of the zwitterionic MC isomer could interact with protonic acids (such as HCl, HF, and acetic acid) and convert to the MCH + form in a reversible manner, accompanied by variation in the absorption spectra and color change. The performance and sensitivity of photoresponsive SP-based sensors return to SP ⇌ MC switching rate, photofatigue resistance, and their photostability.…”