Homogeneous Immunosorbent Assay Based on Single-Particle Enumeration Using Upconversion Nanoparticles for the Sensitive Detection of Cancer Biomarkers

Prostate-specific antigen (PSA) is an intercellular glycoprotein produced primarily by the prostate gland, which is commonly chosen as the initial target for the early diagnosis of prostate cancer. In this work, we demonstrate a simple yet sensitive sandwich-type single-particle enumeration (SPE) immunoassay for the quantitative detection of PSA in a flow chamber. The design is based on the luminescence resonance energy transfer (LRET) between upconversion nanoparticles (UCNPs) and gold nanoparticles (GNPs). The carboxyl group-functionalized UCNPs are conjugated with anti-PSA detection antibodies (Ab₁) and serve as the luminescence energy donor, while GNPs are modified with anti-PSA capture antibodies (Ab₂) and act as the energy acceptor. In the presence of target antigen (i.e., PSA), the specific immnuoreaction brings the donor and acceptor into close proximity, resulting in quenched luminescence. Through statistical counting of the target-dependent fluorescent particles on the glass slide surface, the quantity of antigens in the solution is accurately determined. The dynamic range for PSA detection in Tris-buffered saline (TBS) is 0-500 pM and the limit-of-detection (LOD) is 1.0 pM, which is much lower than the cutoff level in patients' serum samples. In the serum sample assay, comparable LOD was also achieved (i.e., 2.3 pM). As a consequence, this method will find promising applications for the selective detection of cancer biomarkers in clinical diagnosis.