Reproducible surface-enhanced Raman spectroscopy of small molecular anions

A gold-coated silicon substrate having an array of pyramidal shaped holes is shown to provide a reproducible surface-enhanced Raman spectra (SERS) in a number of inorganic ions such as NO(2)(-), CO(3)(2-), N(3)(-), and NO(3)(-) deposited on the substrate as 10(-3) to 10(-4) molar aqueous solutions of their salts. Of particular interest is the observation of a SERS effect in NO(3)(-), the anion of ammonium nitrate, a commonly used terrorist explosive, suggesting the potential for sensitive detection of this material. An unusual increase in the frequency of the NO(2)(-) bending mode frequency is observed in the SERS spectra of NNO(2). Density Functional Theory calculations of the frequencies of the normal modes of vibration of NO(2)(-) bonded to gold predict an upward shift of the frequencies compared with the calculated results for a free NO(2)(-), suggesting a possible explanation for the shifts.