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a The curve of the zeta potentials of OA-free NaYF4:Yb,Er NPs at different pH values. b The FTIR spectra of the complexes of amine acid coupling with NaYF4:Yb,Er NPs. a new-NaYF4:Yb,Er, b–e new-NaYF4:Yb,Er coupled with glycine, threonine, glutamic acid, and lysine, respectively
