Digital droplet-enhanced Raman spectroscopy for ultrasensitive quantification via coarsening effect

Ultrasensitive and quantitative detection is vital for early-stage disease diagnosis, biochemical threat identification, and environmental monitoring. However, existing techniques often suffer from limited molecular specificity or reliance on external labels. For instance, digital polymerase chain reaction (dPCR) enables absolute quantification but involves time-consuming sample preparation and is confined to nucleic acid targets. Surface-enhanced Raman spectroscopy (SERS), though label-free and molecularly specific, is hindered by hotspot variability and poor reproducibility, limiting its effectiveness in trace detection. Here, we report a novel coarsening-enabled digital droplet-enhanced Raman spectroscopy (dDERS) platform for absolute quantification of diverse molecules down to the attomolar level. Our approach eliminates the coffee-ring effect by utilizing a droplet spray to compartmentalize nanoparticles and target molecules into atomized droplets, which are deposited onto a pre-prepared substrate composed of hydrophilic micropillars embedded in oil-infused nanotextures. The microdroplet array is precisely patterned through the spontaneous climbing on oil meniscus. This enables digital sensing at the single-molecule level and statistical quantification using Poisson analysis. The dDERS platform rapidly generates microdroplet array with precise nanoparticle aggregation for Raman signal localization. By addressing long-standing physical limit in hotspot control and statistical limit in quantification reproducibility, our platform advances the state-of-the-art in SERS-based trace detection by several orders of magnitude and is applicable to a broad range of label-free molecules with characteristic Raman signals.