Digital Immunoassay for Biomarker Detection Based on Single-Particle Laser Ablation ICP MS

Single-particle (digital) immunoassays offer significantly lower limits of detection (LODs) than traditional immunoassays, making them suitable for the detection of low-abundance biomarkers. The most common approach for digital detection is based on counting individual labels. Here, we introduce a novel dot-blot particle-linked immunosorbent assay (PLISA) with digital readout utilizing laser ablation (LA) of photon upconversion nanoparticle (UCNP) labels from the nitrocellulose substrate. Compared to conventional LA, our approach allows desorption of intact nanoparticles and their precise counting by single-particle inductively coupled plasma mass spectrometry (SP ICP MS), thus counting individual UCNP-labeled immunocomplexes. Digital signal processing filters instrument noise and nanoparticle aggregates, minimizing potential errors. The immunoassay and LA SP ICP MS readout were optimized using human serum albumin, a kidney damage biomarker, as a model analyte, obtaining LODs of 0.18 and 0.12 ng/mL for the reference upconversion luminescence (UCL) and LA SP ICP MS readout, respectively. Building upon these optimized conditions, we developed PLISA for prostate-specific antigen, the key prostate cancer biomarker, with LODs of 2.4, 1.4, and 0.3 pg/mL for the UCL, analog, and digital LA SP ICP MS readout, respectively. The LOD in the sub-pg/mL range highlighted the advantage of particle counting and its ability to detect low-abundance biomarkers, as superior performance was achieved compared to the UCL and analog LA ICP MS readout. Finally, clinical serum samples of patients tested for prostate cancer were analyzed, and a strong correlation with the reference electrochemiluminescence method confirmed the potential of LA SP ICP MS for clinical diagnostics.