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Fluorescence signals obtained from Y2O3:Yb,Ho,Tm@Y2O3@SiO2(COO−) UPNBs (Yb/Tm/Ho = 25/0.2/0.2 (a), 25/0.2/0.5 (b), 25/0.2/1.0 (c), 25/0.2/2.0 (d), 20/0.2/2.0 (e) and 20/0.2/2.0 mol % (f)). Both absolute intensities and relative intensity ratios at different wavelengths are used for coding purposes; for example (147), (139) and (334) are distinguishable codes.
