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Fluorescent Hydrogels with Tunable Nanostructure and Viscoelasticity for Formaldehyde Removal

Song, Shasha, Song, Aixin, Feng, Lei, Wei, Guangcheng, Dong, Shuli, Hao, Jingcheng
ACS applied materials 2014 v.6 no.20 pp. 18319-18328
ammonium compounds, aqueous solutions, cations, cesium, electrostatic interactions, fluorescence, formaldehyde, gelation, home furnishings, hydrogels, lithium, nanomaterials, pollutants, potassium, potassium hydroxide, salts, sodium, strength (mechanics), temperature, viscoelasticity, water content
Hydrogels with ultrahigh water content, ∼99 wt %, and highly excellent mechanical strength were prepared by 4′-para-phenylcarboxyl-2,2′:6′,2″-terpyridine (PPCT) in KOH aqueous solution. The self-assembled structure, rheological properties, and the gel–sol transformation temperature (Tgₑₗ–ₛₒₗ) of PPCT/KOH hydrogels that depend on PPCT and KOH concentrations were studied, indicating easily controllable conditions for producing hydrogels in PPCT and KOH mixtures. An important finding was that the hydration radius (Rₕ) of cations (M⁺ = Li⁺, Na⁺, K⁺, Cs⁺, NH₄⁺, (CH₃)₄N⁺, (CH₃CH₂)₄N⁺, (CH₃CH₂CH₂)₄N⁺, (CH₃CH₂CH₂CH₂)₄N⁺) plays a vital role in gelation of PPCT/MOH systems. To produce hydrogels in PPCT/MOH systems, the Rₕ of M⁺ must be in a suitable region of 3.29 to 3.58 Å, e.g., K⁺, Na⁺, Cs⁺, and the capability of M⁺ for inducing PPCT to form hydrogels is K⁺ > Na⁺ > Li⁺, which is followed by the Hofmeister series. The hydrogels of PPCT and KOH mixtures are responsive to external stimuli including temperature and shearing force, and present gelation-induced enhanced fluorescence emission property. The states of being sensitive to the stimuli can readily recover to the original hydrogels, which are envisaged to be an attracting candidate to produce self-healing materials. A typical function of the hydrogels of PPCT and KOH mixtures is that formaldehyde (HCHO) can speedily be adsorbed via electrostatic interaction and converted into nontoxic salts (HCOOK and CH₃OK), making it a promising candidate material for HCHO removal in home furnishings to reduce indoor environmental pollutants.