Optimizing Blue Persistent Luminescence in (Sr_1−δBa_δ)₂MgSi₂O_7:Eu²⁺,Dy³⁺ via Solid Solution for Use in Point-of-Care Diagnostics

Inorganic persistent luminescent phosphors are an excellent class of optical reporters for enabling sensitive point-of-care diagnostics, particularly with smartphone-based biosensing devices in testing formats such as the lateral flow assay (LFA). Here, the development of persistent phosphors for this application is focused on the solid solution (Sr 1−δ Ba δ ) 2 MgSi 2 O 7:Eu 2+,Dy 3+ (δ = 0, 0.125, 0.25, 0.375), which is prepared using a high-temperature solid-state reaction as confirmed by synchrotron X-ray powder diffraction. The substitution of barium for strontium enables control over the Eu 2+ 5d-orbital crystal field splitting (CFS) as a tool for tuning the emission wavelength while maintaining luminescence lifetimes >9 min across the composition range. Thermoluminescence measurements of the solid solution provide evidence that trap states contribute to the persistent lifetimes with the trap depths also remaining constant as a function of composition. Time-gated luminescence images of these compounds are captured on a smartphone arranged in a layout to mimic a point-of-care test and demonstrate the viability of using these materials as optical reporters. Moreover, comparing the blue-emitting (Sr 0.625 Ba 0.375 ) 2 MgSi 2 O 7: Eu 2+,Dy 3+ and the green-emitting SrAl 2 O 4:Eu 2+,Dy 3+ in a single LFA-type format shows these two compounds can be detected and resolved simultaneously, thereby permitting the development of a multiplexed LFA.