Ca3(PO4)2:Er3+,Yb3+: An upconversion phosphor for in vivo imaging

Using a sol-gel process, we have synthesized compounds isostructural with the biogenic mineral whitlockite and containing calcium phosphate, β-Ca3(PO4)2, codoped with Er3+ and Yb3+ in various concentrations and ratios. Their particle size was determined to be ∼42–57 nm by atomic force microscopy and ∼93 nm by dynamic light scattering. The particles were shown to be weak anion exchangers (their zeta potential is −13.3 mV). The observed luminescence of the β-Ca3(PO4)2:Er3+,Yb3+ phosphates in the visible spectral region (λ = 0.525, 0.550, and 0.650 μm) under IR excitation (λ = 0.98 μm) is due to the upconversion mechanism and is acceptable for in vivo imaging in terms of safety (green emission) and intensity. Biocompatibility testing results demonstrate that the β-Ca3(PO4)2:Er3+,Yb3+ phosphates meet the relevant biosafety and nontoxicity criteria.