Sensitive and rapid determination of heat shock protein 70 using lateral flow immunostrips and upconversion nanoparticle fluorescence probes

Heat shock protein 70 (Hsp70), belonging to the heat shock protein (HSP) family, is reported to be a potential diagnostic biomarker. In this work, a lateral flow immunostrip was fabricated for the sensitive and rapid determination of Hsp70 by the incorporation of fluorescence and upconversion nanoparticle probes. The upconversion nanoparticles (UCNPs, size ∼39 nm, λex = 980 nm; λem = 540 nm) consisting of a NaYF4:Yb/Er core and polyacrylic acid-modified shell were covalently coupled with Hsp70 antibodies to form the signal probe, which was characterized by dynamic light scattering and zeta potential analyses. The lateral flow assay (LFA) was constructed based on the sandwich-type immunoassay using a sample pad, a test pad, and an adsorption pad on a PVP backing. Hsp70 antibody, IgG antibody and the signal probe were separately dropped on the test zone, the control zone of the test pad, and the sample pad, respectively. In the sandwich LFA, since two antibodies bind to Hsp70 antigenic epitopes, i.e. specific binding, it provided superior specificity and high sensitivity, making it an ideal sensing platform for complex samples like serum Hsp70 samples. The important parameters for the preparation of the lateral flow immunostrips were optimized. Under the optimized conditions, Hsp70 can be detected using the increased fluorescence intensity of UCNPs with a wide linear range from 0.11 to 12 ng mL-1, low detection limit of 0.06 ng mL-1, small sample volume (120 μL), short assay time (15 min) and good reproducibility. The fluorescence method was successfully applied in the determination of Hsp70 in serum samples with good recovery. By combining the accessibility of the lateral flow immunostrips and upconversion nanoparticles, the fluorescence method can serve as a point-of-care testing method for protein assays with high sensitivity and fast detection.