The Use of Surface Enhanced Raman Scattering for the Detection of Dipicolinic Acid on Silver Nanoparticles

ABSTRACTWe report on the use of surface enhanced Raman scattering (SERS) for the detection of dipicolinic acid (DPA) adsorbed on a silver (Ag) nanoparticle substrate. We have examined the interaction of DPA with Ag nanoparticles in a slightly basic, aqueous solution and determined that the molecule adsorbs as a dipicolinate anion on the metal surface. For micro molar or lower DPA concentrations in the colloid solution, no SERS activity from the adsorbed molecule was observed until nanoparticle aggregation was induced by reducing the pH with the addition of nitric acid. Following aggregation, the SERS response exhibited vibrational bands associated with both the pyridine ring and the carboxylate moieties in the adsorbed dipicolinate species. With proper control of the colloidal solution chemistry, the dipicolinate vibrational features could be observed in the SERS spectra at concentrations as low as 20 nano molar, a limit determined by the presence of solution-based contaminants on the Ag surface. In addition to the controlled DPA analyte studies, SERS was also able to detect the release of this molecule from Bacillus globigii spores, a non-toxic simulant for Bacillus anthracis, which demonstrated the potential of this optical spectroscopy for the detection of biological and chemical agents.