Smartphone-readable RPA-LFA for the highsensitivity detection of Leishmania kDNA using nanophosphor reporters

Early diagnosis of infectious diseases improves outcomes by enabling earlier delivery of effective treatment, and helps prevent further transmission by undiagnosed persons. We demonstrated a proof-of-concept assay combining isothermal amplification and lateral flow assay (LFA) for early diagnosis of cutaneous leishmaniasis, a vector-borne infectious disease that affects ca. 700,000 to 1.2 million people annually. Conventional molecular diagnostic techniques based on polymerase chain reaction (PCR) require complex apparatus for temperature cycling. Recombinase polymerase amplification (RPA) is an isothermal DNA amplification method that has shown promise for use in low-resource settings. Combined with lateral flow assay as the readout, RPA-LFA can be used as a point-of-care diagnostic tool with high sensitivity and specificity, but reagent costs can be problematic. In this work, we developed a highly-sensitive smartphone-based RPA-LFA for the detection of Leishmania panamensis DNA using blue-emitting [(Sr_0.625Ba_0.375)_1.96Eu_0.01Dy_0.03]MgSi₂O₇ (SBMSO) persistent luminescent nanophosphors as LFA reporters. The greater detectability of nanophosphors allows the use of a reduced volume of RPA reagents, potentially reducing the cost of RPA-LFA. The limit of detection (LOD) of RPA with gold nanoparticle-based LFA readout is estimated at 1 parasite per reaction, but LOD can be 100-fold better, 0.01 parasites per reaction, for LFA based on SBMSO. This approach may be useful for sensitive and cost-effective point-of-care diagnosis and contribute to improved clinical and economic outcomes, especially in resource-limited settings.