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Fig. 2. (a) The fluorescence spectra of core and core@shell UCNPs. Dependence of fluorescence intensity (Im) on the excitation power density (P) for (b) NaGdF4:Yb,Er core UCNPs, (c) NaGdF4:Yb,Er@NaYF4 core@shell UCNPs, and (d) NaGdF4:Yb,Er@NaYF4:Yb core@shell UCNPs.