SERS detection of cell surface and intracellular components of microorganisms using nano-aggregated Ag substrate

The intracellular and cell surface composition and structural features of gram-positive and gram-negative bacteria were identified using near-infrared surface-enhanced Raman scattering (SERS). The structural differences of components that reside in the cell envelope are manifested by their SERS spectra, e.g. gram-negative vs. gram-positive. Silver particles were used as a SERS substrate by exploiting the existence of strong local electromagnetic fields (hot spots) within nanoscale aggregates of the particles. The aggregation of silver nanoparticles was induced by magnesium ions. These hot spots reduce the screening length of the double layer. The obtained SERS spectra showed excellent quality having unprecedented high signal to noise ratio, which again enabled identification of numerous cellular components, such as proteins, polysaccharides, lipids and nucleic acids with excellent structure-spectrum correlation. The SERS spectra associated with these features give rise to intense bands that provide information about the chemical composition of the bacterial cell envelope and can also detect low concentration compounds. We demonstrate the high sensitivity of our SERS based method for detection of bacteria and its capability to differentiate between bacterial species based on minor chemical differences on their outer cell envelope.