Fig. 1. NIR-II/SWIR luminescent nanoparticles for the SARS-CoV-2 pEr-P³-RBD nanovaccine.

(A) Schematic of the pEr-P³-RBD (RBD conjugated to ─NH₂ groups on the pEr-P³ surface via EDC chemistry). P³ refers to cross-linked three layers of hydrophilic polymers coated on the pEr nanoparticles. (B) TEM image of pEr nanoparticles comprised the NaErF₄ core and NaYF₄ shell; scale bar, 20 nm. (C) Energy level diagram of the erbium showing the NIR-II down conversion pathway. (D) Absorption (red curve) and emission (blue curve) spectra of pEr nanoparticles in cyclohexane (prior to P³ coating). The dashed vertical line shows the excitation wavelength (975 nm) for pEr nanoparticles. a.u., arbitrary units. (E) DLS spectra of pEr in cyclohexane (gray), hydrophilic pEr-P³ in 1×PBS buffer (blue). The average size for pEr nanoparticles is ~25.5 nm, and that for pEr-P³ nanoparticles is ~55.5 nm.