Digital SERS bioanalysis of single-enzyme biomarkers

Digital bioanalysis enables highly sensitive detection of biomolecules at the single-molecule level, making it a widely used technique in biomedical research. However, conventional approaches typically rely on fluorescence detection of single-enzyme reactions, which limits molecular selectivity and the ability to analyze multiple targets simultaneously. To address these limitations, we developed a digital bioanalysis platform based on surface-enhanced Raman scattering spectroscopy and microchamber arrays decorated with silver nanoparticles. This platform achieves a million-fold amplification of Raman signals from products generated by single-enzyme reactions, enabling precise digital counting of enzyme biomarkers with high molecular selectivity and multiplexing capability. We applied this platform to detect and distinguish two closely related enzyme biomarkers, acetylcholinesterase (AChE) and butyrylcholinesterase. By leveraging the sharp and distinct Raman spectral signatures of the reaction products, the platform achieved multiplexed biomarker quantification with femtomolar-level sensitivity. As a proof-of-concept, the platform successfully quantified AChE in human cerebrospinal fluid within 8.5 min, highlighting its potential utility in clinical diagnostics, particularly for differentiating types of dementia based on subtle differences in enzyme levels. Hence, this study presents a valuable alternative to fluorescence-based digital bioanalysis by offering enhanced molecular selectivity and multiplexing capability. Its application extends the scope of digital bioanalysis and broadens its capacity to quantify multiple biomarkers in complex biological samples with high precision and efficiency.