Figure 3

Photoluminescence spectrum of emission of Eu: Gd ₂ O ₃ suspended in (a) lung serum, (b) PBS at pH 5 for periods up to 170 hours. The emission peaks at 590, 613, and 623 correspond to ⁵ D ₀  → F ₂ ,⁵ D ₀  → ⁷ F ₁ and ⁵ D ₀  → ⁷ F ₃ transitions respectively. The intensity of the signal is proportional to the number of Eu ions present in the Gd₂O₃ nanoparticles. A very small measurable reduction in emission, and hence in mass of Eu present in a particle, is discernible after 170 hours.