Label-free detection of binary mixtures of proteins using surface-enhanced Raman scattering

The surface-enhanced Raman scattering (SERS) spectra of five binary mixtures of proteins were studied, including mixtures of apomyoglobin (apMb) and lysozyme, insulin and lysozyme, Mb and cytochrome c (cyt c), Mb and apoMb, and cyt c and human serum albumin. For mixtures in which both proteins did not contain chromophores, the variations in the intensities of SERS signals with changes in the relative concentration of the two proteins were largely proportional to the relative concentration of the protein mixtures. Similar results were obtained for mixtures in which both the proteins contained chromophores. However, for mixtures in which one protein contained chromophores and the other did not, the SERS intensities of the protein mixtures showed a bell-shaped variation with changes in the compositions of the mixtures. The present study provides new insights into the application of SERS spectroscopy to label-free detection in a binary of mixture of proteins. The complexities in both SERS phenomena and protein structures should be considered in the analysis of the SERS spectra of protein mixtures. By monitoring the heme band of a protein containing a heme and the NO3 band of a protein without a heme, the simultaneous detection of both proteins in a binary mixture may be possible. It has also been found that the larger the size of the protein, the smaller is the SERS intensity of that protein. Copyright (C) 2011 John Wiley & Sons, Ltd.