Chiral Gold Nanostructure Monolayers as SERS Substrates for Ultrasensitive Detection of Enantiomer Biomarkers of Alzheimer's Disease.

Sensitive and specific biomarkers are needed for early diagnosis of neurodegenerative diseases, such as Alzheimer's disease (AD). Herein, a new type of chiral gold nanostructure induced by D-/L-cysteine-leucine dipeptides with a g-factor of 0.1 was successfully synthesized. To enhance the discrimination performance, the chiral gold nanostructures were assembled into D-/L-Au monolayers. As surface-enhanced Raman scattering (SERS) substrates, the D-/L-Au monolayers simultaneously deliver molecular structural specificity and enantioselectivity within a single spectrum, which can be a versatile, label-free chiral discrimination strategy for the detection of D-/L-kynurenine (Kyn). The mechanism was unveiled to involve high enantioselective adsorption energies between L- and D-Kyn on the lattice plane (221), resulting in enantioselective sensing. The results showed that L-Au monolayer reached a limit of detection (LOD) of 3.7 nM for L-Kyn, while D-Au monolayer reached a LOD of 3.6 nM for D-Kyn, respectively. Notably, there was a significant difference in D-Kyn levels between AD patients and healthy individuals in serum samples, a distinction not observed for L-Kyn, which positioned D-Kyn as a potential novel biomarker for clinical prediagnosis of AD patients, marking the first report of its kind worldwide. This study provides a robust tool for biomedical science and clinical diagnostics.