Optimized core-shell lanthanide nanoparticles with ultrabright Ce 3+ -modulated second near-infrared emission for "lighting" plants.

Second near-infrared (NIR-II, 1000-1700 nm) photon-mediated fluorescence imaging has attracted extensive interest in the field of bioimaging. However, NIR-II fluorescent nanoprobes competent for plant imaging have been rarely developed. Herein, lanthanide-doped nanoparticle (LDNP) optimal core-shell structure and ultrabright NIR-II emission were developed for "lighting" plants. The Ce 3+ -doped active shell coated on the NaErF 4 :Tm core enables dual-mode red upconversion (UC) and NIR-II downconversion (DC) emission of LDNPs upon 980 nm laser excitation. Under the optimized doping content, the intensities of red UC and NIR-II DC emission were respectively boosted by 5- and 19-fold those of the core nanoparticles, which endowed LDNPs with ideal NIR-II emissive capabilities for optical imaging of plants. Significantly, the NIR-II fluorescent signal affords much higher signal-to-noise rate than the red UC. LDNPs were modified with polyethyleneimine to enable outstanding hydrophilicty and facilitate their uptake by plants. Arabidopsis thaliana and Nicotiana benthamiana were chosen as plant models for NIR-II imaging studies. The toxic effect of LDNPs after being transported into Brassica rapa chinensis was systematically studied on mice. The NIR-II imaging strategy offers a promising method for studying the uptake and transport of nanoparticles in plants.