Thermo-sensitive hydrogels for forward osmosis with NIR light-induced freshwater recovery

Abstract

We report a near-infrared light-responsive hydrogel material consisting of two parts of a monomer, an ionic liquid monomer (TVBP) and N-isopropylacrylamide (NIPAM), whose crosslinker is diacrylate containing poly(propylene glycol), all three of which are temperature sensitive. Interestingly, the ionic liquid monomer TVBP can also generate a relatively high osmotic pressure, allowing the hydrogels to draw fresh water from brackish water through a semi-permeable membrane. These properties make it possible to use it for desalination by forward osmosis (FO). It is possible to skillfully recover fresh water (recovery rate was about 95%) from hydrogels through photothermal conversion generated by irradiation with near-infrared light-emitting diode (NIR LED) light onto croconaine dye-doped silica nanoparticles (NIR-800@SiO₂ NPs). The nanoparticles, which were designed and synthesized for the first time by us, can flexibly respond to NIR light (in this paper, λ_max = 760 nm) while exhibiting inertness to sunlight, making the water drawing and desalination process weather-independent (e.g., when the sun shines or when it is dark). The proposal of this scheme is expected to provide a considerable reference for continuous water desalination.