Graphene-Conjugated Upconversion Nanoparticles as Fluorescence-Tuned Photothermal Nanoheaters for Desalination

Multiphoton (980 nm) absorbing lanthanide doped upconversion nanoparticles (UCNPs) are emerging as fluorophores and photothermal agents but are limited by the low quantum yield of their visible fluorescence. In a way similar to metal-enhanced fluorescence, we demonstrate that a monolayer of graphene could quench the 540 nm (green) fluorescence from the core-UCNPs by 3 times, and a bilayer graphene could enhance the green fluorescence from the silica (SiO₂) coated core-shell (cs) UCNPs by 30 times. This graphene-aided fluorescence tuning in the engineered UCNPs can be translated to photothermal conversion using a 980 nm excitation. Infrared thermal imaging and thermocouple measurements both indicate the local temperature to scale with fluorescence. From the dynamic response of local temperature, we could estimate the photothermal conversion efficiency of the core- and cs-UCNP/graphene surfaces to be 65 and 46%, respectively. The photothermally generated heat on these nanoheater surfaces can be used for desalination of salt water. We demonstrate >96% salt recovery from saline water dispersed on the UCNP coated substrate under 980 nm irradiation. Around 9 mg/cm² of water could be evaporated in 10 min from a 1 cm² area of a 1 mL column of saturated saline water on the photothermally active UCNP/graphene material independent of the substrate (copper or SiO₂/Si) under 7.96 W/cm² of 980 nm irradiation. Normal sunlight (∼100 mW/cm²) could evaporate saturated saline water from the cs-UCNP/graphene coated substrate in ∼200 s, compared to ∼600 s on the surface without the photothermal agent.