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Modified SERS (surface-enhanced Raman scattering) substrates are based on self-assembled monolayers (SAMs) formed from compounds such as alkanethiols. Chain lengths ranging from ethanethiol to octadecanethiol were used to investigate properties of modified SERS substrates. These properties include determining the magnitude of the SERS electromagnetic enhancement, developing a sensitivity factor for detecting aromatic compounds, and evaluating SERS substrates for performance characteristics such as stability and solvent effects. A SERS electromagnetic enhancement was determined to be 2.1 × 103 for detecting aromatic compounds. The effects of experimental conditions on the SERS detection process were addressed. SERS sensitivity was shown to be highly dependent upon a correlation between the SERS interfacial distance dependence and a hydrophobic effect exhibited by the alkyl chain of the thiol. The distance dependence was more significant than the hydrophobic effect for detecting aromatic compounds. For comparison to the electromagnetic enhancement, a SERS sensitivity factor was determined for detecting benzene in water and in organic solvents. Experimental stability issues, such as solvent and laser exposure, were shown to affect the sensitivity of modified SERS substrates. 1-Propanethiol exhibited the strongest sensitivity and stability under all solvent and experimental conditions.
Kennedy et al. (1999) studied this question.