An Ultrabright Lanthanide-Doped Luminescent Nanotag for a Smartphone-Based Visual and Portable Lateral Flow Immunoassay of Procalcitonin and Rapid Diagnosis of Bacterial Infection

The lateral flow immunoassay (LFIA), as a popular point-of-care-testing (POCT) technology, has attracted increasing attention, but its analytical performance, especially detection sensitivity, relies on the photophysical properties of the nanotags used. The exploitation of high-performance, robust tags featuring facile fabrication and excellent water solubility remains a great challenge. Herein, we reported a one-pot hydrothermal preparation of diverse polymers (e.g., PAA, PEI, and PVA)-modified YVO₄:Eu nanoparticles (NPs). The polymer-coated YVO₄:Eu NPs revealed abundant surface functional groups (e.g., carboxyl, amino, and hydroxyl), good water solubility, excellent colloidal stability, and strong red emission (absolute quantum yield over 50%). To demonstrate their potential in biolabeling, a routine sandwich LFIA was designed for quick detection of procalcitonin (PCT) using PAA-YVO₄:Eu as the nanotags. Through the luminescence image collection and the corresponding RGB color value transformation, the LFIA test strips were applied for a smartphone-based portable and quantitative detection of PCT (0.2–50 ng/mL) with a limit of detection of 0.06 ng/mL in buffer. Especially, the same quantitative fitting was obtained in human serum with a slightly higher detection limit (0.16 ng/mL), demonstrating high robustness of the test strips in a complex biological matrix. For a clinical validation, in comparison with the chemiluminescent immunoassay, the proposed test strips revealed a good analytical performance (sensitivity: 88.9%, specificity: 95.2%, κ value: 0.83). This work also provides an opportunity to build versatile LFIA platforms for other targets using the developed YVO₄:Eu probes.