Dual-Mode Method for the Sensitive Detection of β-Secretase (BACE1) Based on Surface-Enhanced Raman Scattering and Dark-Field Microscopy

β-Secretase (BACE1), a key enzyme to producing neurotoxic β-amyloid, is a potential biomarker of Alzheimer's disease (AD). Developing a sensitive and efficient detection method for BACE1 activity is significant for AD progression evaluation. Due to the poor cleavage efficiency and acidic working conditions of BACE1, developing probes with high stability and strong signals is challenging for its detection. This work proposed a dual-mode BACE1 detection method based on surface-enhanced Raman scattering and dark-field microscopy. 4-Mercaptobenzoic acid (4-MBA), as the internal Raman reporter of Au@Ag nanoparticles (NPs), shows stable and enhanced Raman signals in an acidic environment. The plasmonic Au^MBA@Ag-Fe₃O₄ NPs are prepared by assembling Au^MBA@Ag NPs on the surface of Fe₃O₄ NPs with the assistance of the BACE1 substrate peptides, which aggregate the Raman signals on the surface of Fe₃O₄ NPs. The presence of BACE1 cleaves its substrate peptides, releasing Au^MBA@Ag NPs from Fe₃O₄ NPs. As a result, the Raman intensity of 4-MBA significantly decreased, and the characteristic localized surface plasmon resonance (LSPR) scattering of Au^MBA@Ag-Fe₃O₄ NPs changed obviously. Thus, a dual-mode sensor based on Raman and LSPR was constructed to detect BACE1. The linear range and detection limit were superior to those of previously reported strategies. The innovative Au^MBA@Ag-Fe₃O₄ NPs present remarkable application potential for detecting BACE1.