Tip-Enhanced Raman Spectroscopy Can See More:  The Case of Cytochrome c

The heme protein, cytochrome c (Cc) has been studied using tip-enhanced Raman spectroscopy (TERS). By virtue of its sensitivity and superior spatial resolution, TERS detected both the heme and amino acid vibrational bands of Cc using resonance excitation at 532 nm. This is in contrast to conventional surface-enhanced Raman spectroscopy (SERS) where ensemble information is obtained, leading to the strongest Raman bands obscuring the weaker ones; i.e., only the resonantly enhanced heme bands are observed. Matrix-assisted laser desorption/ionization mass spectrometry supported the interpretation of the Raman data by showing that Cc remains intact on Ag surfaces. This work demonstrates the sensitivity of TERS for analyzing proteins (complete with band assignments) and that a collection of TER spectra gives a more complete description of large biomolecules. The latter is an important advantage usually not found in SERS measurements.