Intrinsic Detection of Proteins

In intrinsic peptide/protein SERS spectra, the most intensive Raman bands belong to amino acids containing aromatic or S-S groups. Halas and co-workers prepared Au nanoshells to compare SERS spectra of three dipeptides containing Cys and one aromatic amino acid Phe, tyrosine (Tyr) or Trp (Wei et al. 2008). They 

8 Intrinsic SERS spectra of peptides, a SERS spectra of Try-Cys from eight different measurements, b a comparison of SERS with MRS spectra (adapted with permission from Wei et al. 2008. Copyright 2008 American Chemical Society) 

The main problem of protein SERS study is that the protein native structure can be disturbed by its interaction with a metal surface. SERS spectra of oxy- and deoxy- haemoglobin indicated a partial denaturation of haemoglobin when adsorbed on Ag colloidal NPs (Smulevich and Spiro 1985). Biocompatible coating of the metal surface by SAMs of alkanethiols terminated with NH2- or COOH- groups mediating protein immobilization was used to preserve its native structure (Murgida and Hildebrandt 2006). Another way to keep the natural protein structure is the preparation of a specific SERS-active substrate where the protein molecule is more 

A silver staining can significantly increase the sensitivity of label-fiee protein SERS detection. It is available for detection of proteins separated by gel electrophoresis. The basic mechanism underlying silver staining of proteins is that silver ions bound to amino acid side-chains (primary COO-, -SH) are reduced to free silver by a reductant (citric acid, formaldehyde, or photoreduction). The 

9 SERS spectra of amino acids and proteins obtained using iodide-modified Ag NPs. A SERS spectra of (a) lysozyme, (b) avidin, (c) BSA, (d) Phe and (e) Tip. B Plot of the relative SERS intensity of 752/1000 cm to the ratio of Trp/(Trp and Phe) for the three proteins (reproduced with permission from Xu et al. 2014. Copyright 2014 American Chemical Society) 

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