Fluorescent
Hydrogels with Tunable Nanostructure and
Viscoelasticity for Formaldehyde Removal
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Abstract
Hydrogels with ultrahigh water content,
∼99 wt %, and highly
excellent mechanical strength were prepared by 4′-<i>para</i>-phenylcarboxyl-2,2′:6′,2″-terpyridine (PPCT)
in KOH aqueous solution. The self-assembled structure, rheological
properties, and the gel–sol transformation temperature (<i>T</i><sub>gel–sol</sub>) 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 (<i>R</i><sub>h</sub>) of cations (M<sup>+</sup> = Li<sup>+</sup>, Na<sup>+</sup>, K<sup>+</sup>, Cs<sup>+</sup>, NH<sub>4</sub><sup>+</sup>, (CH<sub>3</sub>)<sub>4</sub>N<sup>+</sup>, (CH<sub>3</sub>CH<sub>2</sub>)<sub>4</sub>N<sup>+</sup>, (CH<sub>3</sub>CH<sub>2</sub>CH<sub>2</sub>)<sub>4</sub>N<sup>+</sup>, (CH<sub>3</sub>CH<sub>2</sub>CH<sub>2</sub>CH<sub>2</sub>)<sub>4</sub>N<sup>+</sup>) plays a vital role in gelation
of PPCT/MOH systems. To produce hydrogels in PPCT/MOH systems, the <i>R</i><sub>h</sub> of M<sup>+</sup> must be in a suitable region
of 3.29 to 3.58 Å, e.g., K<sup>+</sup>, Na<sup>+</sup>, Cs<sup>+</sup>, and the capability of M<sup>+</sup> for inducing PPCT to
form hydrogels is K<sup>+</sup> > Na<sup>+</sup> > Li<sup>+</sup>,
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 CH<sub>3</sub>OK), making
it a promising candidate material for HCHO removal in home furnishings
to reduce indoor environmental pollutants