  Surface-enhanced Raman spectroscopy (SERS) is a sensitive label-free optical
method that can provide fingerprint Raman spectra of biomolecules such as DNA,
amino acids and proteins. While SERS of single DNA molecule has been recently
demonstrated, Raman analysis of single protein sequence was not possible
because the SERS spectra of proteins are usually dominated by signals of
aromatic amino acid residues. Here, we used electroplasmonic approach to trap
single gold nanoparticle in a nanohole for generating a plasmonic nanocavity
between the trapped nanoparticle and the nanopore wall. The giant field
generated in the nanocavity was so sensitive and localized that it enables SERS
discrimination of 10 distinct amino acids at single-molecule level. The
obtained spectra are used to analyze the spectra of 2 biomarkers (Vasopressin
and Oxytocin) made of a short sequence of 9 amino-acids. Significantly, we
demonstrated identification of single non-aromatic amino acid residues in a
single short peptide chain as well as discrimination between two peptides with
sequences distinguishable in 2 specific amino-acids. Our result demonstrate the
high sensitivity of our method to identify single amino acid residue in a
protein chain and a potential for further applications in proteomics and
single-protein sequencing.
