Ultrasensitive Single-Molecule Immunoassays Built on Digital Techniques for Low Amounts of Biomarker Detection

The molecular-level identification of diseases is recognized as a crucial aspect of precision medicine, playing an important role in the early diagnosis and surveillance of various diseases. Digital single-molecule immunoassay (SMI) is an emerging technique that enables the precise quantitative detection of biomolecules at concentrations as low as 10^–18 mol/L, thereby facilitating early disease diagnosis and clinical monitoring. This review provides a comprehensive overview of the principles and instrumentation associated with digital SMI technologies, which are classified into four types based on their sensing mechanisms, key characteristics, and evolving development trends aimed at being stronger, smarter, and simpler (3S). First, a detailed description of microarray-based and microdroplet-based SMI techniques is provided. These approaches achieve a ″stronger″ single-molecule detection performance and reliable stability. Subsequently, recent advancements in digital SMIs that employ nonchip and homogeneous formats are discussed. These innovations extend classical digital SMIs while significantly reducing assay complexity, thereby making the detection process both ″smarter″ and ″simpler.″ Furthermore, this review delves into the distinctive clinical applications of ultrasensitive digital SMIs in the detection of neurological disorders, cancers, and infectious diseases, highlighting the specific methodologies utilized. Finally, the current status of digital SMI technology is summarized, along with future prospects for its development.