Ultrasensitive SERS nanoprobe-based multiplexed digital sensing platform for the simultaneous quantification of Alzheimer's disease biomarkers.

Alzheimer's disease (AD) is a severe neurodegenerative disease that requires early diagnosis to manage its progression. Although the simultaneous detection of multiple AD biomarkers is expected to facilitate early assessment of AD risk, the lack of multiplexed sensing platforms for accurately quantifying multiple AD biomarkers remains a challenge. Here, we present a multiplexed digital sensing platform based on bumpy core-shell (BCS) surface-enhanced Raman spectroscopy (SERS) nanoprobes for ultrasensitive, quantitative, and simultaneous detection of Aβ42 and Aβ40 as AD biomarkers, enabling the accurate determination of the Aβ42/Aβ40 ratio. We synthesized BCS SERS nanoprobes with distinct Raman reporters to generate unique, intense, and reproducible SERS signals, offering single-nanoparticle sensitivity and quantification capabilities. These nanoprobes were subsequently employed in SERS-based immunoassays combined with digital SERS analysis for multiplexed quantification. The proposed platform accurately and quantitatively detected Aβ42 and Aβ40 across a range of five orders of magnitude, with a limit of detection of 8.7× 10-17 g/mL (1.9 × 10-17 M) for Aβ42 and 1.0 × 10-15 g/mL (2.3 × 10-16 M) for Aβ40, surpassing the performance of conventional enzyme-linked immunosorbent assays. Based on the exclusive detection of Aβ42 and Aβ40 using distinct SERS nanoprobes, the proposed sensing platform can also accurately quantify Aβ42 and Aβ40 at clinically relevant levels in both cerebrospinal fluid and blood plasma. Therefore, this sensing platform can be used to accurately and reliably determine the Aβ42/Aβ40 ratio, thus serving as an effective tool for the early diagnosis of AD.