para-nitroaniline isomers (ONA, MNA, and PNA, respectively) were investigated with surface-enhanced infrared absorption (SEIRA) and surface-enhanced Raman spectroscopy (SERS) via adsorption of the individual isomers on vacuum-deposited silver films and on silver powders of varying physical dimensions using temperature-programmed desorption (TPD). PNA exhibited a strong SERS signal, whereas the ONA and MNA isomers showed more moderate enhancement. Only the MNA isomer exhibited SEIRA. Varying the polarity of the deposition solvent changed the MNA adsorbate orientation and the degree of vibrational enhancement. Solvent experiments with MNA were the first to show SEIRA enhancement of the -NH 2 symmetric and antisymmetric stretch modes. TPD and vibrational experiments show evidence for nitroaniline intermolecular hydrogen bonding on 100-µm silver powder while suggesting stronger nitroaniline adsorption to 100-nm silver powder. Resonance effects, adsorbate geometries, and hydrogen-bonding variations combined with changes in the strength of adsorption explain the differences in SEIRA versus SERS enhancement of the individual isomers, solvent effects, and desorption characteristics.

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