Confinement-enabled nanobiosensing: from chemical signal amplification to programmable single-molecule diagnostics.

For rare targets in complex samples, ultrasensitive biosensing requires not only strong signal amplification but also reliable discrimination of target-derived signals from the background. Confinement-enabled nanobiosensing addresses this challenge by organizing molecular recognition, amplification and readout across nanoscale interfaces and microscale compartments. In this Feature Article, we review how chemical amplification strategies, from plasmonic nanogold probes and enzyme-triggered cascades to CRISPR-Cas signal networks, are being coupled with droplets and microwell arrays to convert ensemble signals into digital molecular counts. We further highlight programmable microfluidics, particularly digital microfluidics, as an automation layer for sample handling, washing, reagent exchange and compartment sealing. Recent dual-digital immunoassay platforms illustrate how fluidic digitization and molecular digitization can be integrated for single-molecule diagnostics. We conclude by outlining challenges in interface stability, scalable fabrication, multiplexed readout and clinical translation.