A versatile technique based on surface-enhanced Raman spectroscopy for label-free detection of amino acids and peptide formation in body fluids.

As an effective analytical method, surface-enhanced Raman spectroscopy (SERS) is widely used in the detection of nucleic acids, amino acids, and other biomolecules. However, obtaining the SERS signal of nonaromatic amino acids is still a challenge. In this work, excess sodium borohydride was used as a reducing agent to prevent the surface of silver nanoparticles from being coated with AgO to enable amino acid molecules to reach the surface of silver nano-substrates. Calcium ions were used as aggregators for silver nano-substrates to successfully achieve the label-free and accurate fingerprint determination of various nonaromatic amino acids. Different types of amino acids were distinguished based on the changes in their peak intensity that were obtained using colorless and transparent organic dichloromethane (DCM) as the internal standard. A Raman signal for low-concentration amino acids in body fluids was detected, and the detection limit for tyrosine was 5 ng/mL. Moreover, the physical and chemical properties of peptides and the formation of peptide chains were further analyzed. The proposed method can potentially be applied to protein sequencing.