The invention described here is a series of material compositions with the general formula (Sr1-δBaδ)2-j-kMgSi2O7:EujDyk wherein δ is between 0 and 0.5 and j and k are greater than about 0.0005 and less than about 0.3 that can be used as diagnostic reporters. The presence of the rare-earth ions in the compound allow the material to absorb light in the ultraviolet (UV) to near UV region (≈365 nm to 410 nm) and re-emit the absorbed light in the blue region (≈440 nm to ≈475nm) of the electromagnetic spectrum. The long luminescence lifetimes (>10 minutes) and variable emission wavelength depending on the value of δ allows fine-tuning of the optical properties. The particle size can be reduced using a combination of mechanical separation, wet-milling, and fraction allowing these phases to be used as reporter materials in lateral flow assay (LFA) devices. Measuring the luminescence of these compounds in a LFA-type format coupled to a smartphone-based time-gated imaging application subsequently demonstrates the viability of these materials as optical reporters in point-of-care diagnostics. Moreover, comparing the blue-emitting (Sr0.675Ba0.375)2MgSi2O7:Eu2+,Dy3+ and the green-emitting SrAl2O4:Eu2+,Dy3+ in a single LFA-type format shows these two compounds can be detected and resolved simultaneously thereby permitting the development of a multiplexed LFA.