Surface-Enhanced Raman Scattering Enantioselective Detection of Gastric Cancer-Related d -Amino Acids in Saliva Based on Enzyme-Mediated Cascade Reaction

The unusual d -amino acids ( d -AAs), as the counter enantiomer of usual l -amino acids ( l -AAs), have evoked increasing attention because of their potential relevance with diseases. Accordingly, it is essential to establish sensitive and selective detection methods for d -AAs without the interferences from l -AAs. The surface-enhanced Raman scattering (SERS) technique is efficacious for the detection of molecules but routinely ineffective in enantiomeric differentiation. d -Proline ( d -Pro) and d -alanine ( d -Ala) are regarded as biomarkers of gastric cancer. Herein, Raman-active boronate modified SERS chips are constructed to develop a d -amino acid oxidase (DAAO)-mediated cascade reaction-based SERS enantioselective assay for d -Pro and d -Ala. The principle is that DAAO selectively catalyzes the deamination of d -Pro and d -Ala, and the produced H 2 O 2 oxidizes boronate to present a new SERS peak at 883 cm –1 for quantitative analysis in a ratiometric way. A linear range from 20 to 400 μmol/L and a limit of detection down to 14.8 μmol/L are reached. In addition, interferences from l -AAs and many other possible species coexisting in biofluids with the detection of d -Pro and d -Ala are ignorable. Enzyme-mediated cascade reaction-based SERS chips are further utilized for saliva sample analysis, and the total levels of d -Pro and d -Ala in salivary samples from gastric cancer patients are much higher than those of healthy persons. This work provides a solution for SERS enantioselective analysis and noninvasive screening chiral biomolecules for disease diagnosis.