We prepared poly-L-aspartic acid (PASP) functionalized NaYF4:Yb(3+),Er(3+) upconversion nanoparticles (UCNP-PASP). These nanoparticles can give red upconversion emission under excitation at 915 nm, whose wavelength of emission and excitation is located in the optical window of biological tissue. Dynamic laser scatting and zeta potentials of UCNP-PASP were used to study their stabilities in different aqueous solution. To understand the mechanism of the red emission of UCNP-PASP, photoluminescence spectra of samples were recorded before and after modification with PASP, poly acrylic acid (PAA), and poly(ether imide) (PEI) ligands under excitation at 915 and 980 nm, respectively. The cytotoxicity of the UCNP-PASP was also examined on a A549 cell and KB cell by 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl-tetrazolium bromide (MTT) assay. Moreover, the PASP-functionalized UCNP was employed as a potential biomarker for in vitro and in vivo experiments of upconversion luminescence imaging.

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