Persistent Luminescent Nanophosphors as Reporters for Sensitive Diagnostics

Light-based detection methods are fundamental in modern medical testing, as light offers a highly detectable and quantifiable signal for sensitive analyte detection. A variety of luminescent labels and reporter technologies have been developed including fluorescent molecules, quantum dots, and chemiluminescent enzymes. Each of these light-based reporters, however, has fundamental disadvantages that can preclude their use in certain diagnostics applications, including stability, the need for substrates to produce the signal for enzymatic reporters, and the necessity for often expensive and cumbersome optical hardware to achieve sensitive readout. These disadvantages of conventional light-based technologies can be overcome by using persistent luminescence, a unique phenomenon in which an inorganic phosphor emits light for several minutes to hours after photoexcitation. By leveraging the long emission of persistent phosphors, sensitive assay readout requires only a suitable excitation source and detector, unlike conventional fluorescence methods. Persistent luminescent phosphors are typically highly optically stable, and are easily surface-modified to be rendered chemically stable for a variety of environments. Here we demonstrate the use of strontium aluminate nanoparticles as reporters in point-of-care diagnostics. Strontium aluminate is one of the brightest known persistent luminescent phosphors, making it an ideal candidate for leveraging the potential of persistent luminescence for developing highly sensitive assays. We present a facile strategy for isolating nanophosphors from bulk powders, and rendering the particles water-stable for use in aqueous environments. Bioconjugate chemistry strategies for effectively immobilizing affinity reagents on the nanophosphors for sensitive analyte detection are established. The nanophosphors are shown to give better sensitivity in lateral flow assays, a widely used point-of-care testing format, than conventional colorimetric labels. We also demonstrate a simple, low-cost smartphone-based platform for time-resolved luminescence imaging for readout of assays run with nanophosphors. The smartphone-based platform gives excellent sensitivity and can enable quantitative readout at low analyte concentrations. Additionally, we present an alternate route for preparing nanophosphors using a microwave-assisted reverse micelle synthesis strategy. This process can produce persistent luminescent phosphors with comparable luminescence properties to phosphors prepared via conventional solid state synthesis, with a smaller average particle size, and using significantly shorter annealing times.