Thioxanthylium Cations: Highly Reversible Hydrochromic Mate‐rials with Tunable Color and Moisture Sensitivity

Abstract

Hydrochromic materials, which change color in response to moisture, are highly sought-after due to some unique applications such as moisture detection, humidity monitoring, anti-counterfeiting, and sweat-pore mapping. We herein report a new thioxanthene-9-ols/thioxanthylium cations hydrochromic system. The hydrochromic thioxanthylium cations can be generated/regenerated by exposing a chemically stable thioxanthene-9-ol to trifluoroacetic acid (TFA). The intensely colored thioxanthylium cations can be chemically reverted to the colorless thioxanthene-9-ol. Interestingly, the conversion between thioxanthene-9-ol and its cation species was found to be highly reversible in most cases (up to 50 activation-deactivation cycles). The color and moisture-sensitivity of thioxanthylium cations were also validated to be tunable via functionalization of the thioxanthene core with different aliphatic and aromatic groups at its 9 or 2,7positions. Finally, paper probes were prepared using solutionimmersion and inkjet-printing techniques to demonstrate the system's potential applications in humidity sensing and anticounterfeiting [a] Dr.

Responsive hydrogel dressings for intelligent wound management

“…However, infected wounds are usually filled with bacteria that release lactic and acetic acid, causing a wound site to be at a slightly acidic environment of pH 4.5-6.5. [102][103][104] Therefore, pH-sensitive hydrogel patches can make use of these changes in pH levels induced by bacteria growth and design the polymer to release the loaded drugs only when applied to these acidic wound sites in a sustained manner. [105] However, these hydrogels face problems of poor mechanical strength especially after swelling, low solubility, low biodegradability, and biocompatibility especially for the synthetic hydrogels.…”

“…When a wound is inflicted, internal body fluids are exposed, which raise the skin's pH to physiological pH of 7.4, which will return to its initial pH as the wound heals. However, infected wounds are usually filled with bacteria that release lactic and acetic acid, causing a wound site to be at a slightly acidic environment of pH 4.5–6.5 [102–104] . Therefore, pH‐sensitive hydrogel patches can make use of these changes in pH levels induced by bacteria growth and design the polymer to release the loaded drugs only when applied to these acidic wound sites in a sustained manner [105] .…”

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“…16b. 189 UV-responsive self-indicating polymers can be incorporated into coatings to provide visual indications of UV exposure.…”

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“…Due to their versatile stimuli-responsive functions and cost-effective preparation, smart polymer materials, such as nanoparticles, films and hydrogels, are attracting great interest for extensive applications such as drug carriers, coatings, molecular sensing, artificial muscles, and thermoelectric materials. [9][10][11][12][13][14] Among the stimuli-responsive polymers, reversibly switchable smart polymer materials and surfaces can achieve reversible "on-off" switching to tune their structures and properties several times. 9,15,16 In nature, several creatures, e.g., cuttlefish, chameleon and gray treefrog, are masters at reversibly changing their appearance such as colour and pattern in response to their surrounding environment.…”

“…Due to their versatile stimuli-responsive functions and cost-effective preparation, smart polymer materials, such as nanoparticles, films and hydrogels, are attracting great interest for extensive applications such as drug carriers, coatings, molecular sensing, artificial muscles, and thermoelectric materials. 9–14…”