Quantitation of Femtomolar‐Level Protein Biomarkers Using a Simple Microbubbling Digital Assay and Bright‐Field Smartphone Imaging

Quantitating ultra-low concentrations of protein biomarkers is critical for early disease diagnosis and treatment. However, most current point-of-care (POC) assays are limited in sensitivity. Herein, we introduce an ultra-sensitive and facile microbubbling assay for the quantification of protein biomarkers with a digital-readout method that requires only a smartphone camera. We used machine learning to develop a smartphone application for automated image analysis to facilitate accurate and robust counting. Using this method, post-prostatectomy surveillance of prostate specific antigen (PSA) can be achieved with a detection limit (LOD) of 2.1 fm (0.060 pg mL