Faezeh Mohaghegh

• The contribution of the chemical enhancement mechanism to the SERS process has been studied using a Kretschmann configuration (KC) with a thin silver layer attached to the totally reflecting surface for reproducibility of the results. After studying fundamental properties, additionally, the KC was applied for specific applications. The basic KC setup has been optimized and the observed enhancement was investigated in detail. SERS studies have been performed on a monolayer of Nile blue, Crystal Violet, and 4-Nitrobenzenethiol. Under resonance conditions for the coupling of the light to the surface plasmons, a decay of the Raman line intensities has been observed over time, converging to constant signal levels. By analyzing this time-dependent intensity variation for the single vibrational modes, we found a mode-dependent Raman deactivation rate. This process has also been investigated for different angles of incidence of the beam (varying the resonance condition), and a clear dependence on the strength of the coupling to the plasmons for the behavior of the different Raman lines has been found. Although, a uniform enhancement of the electromagnetic fields of exciting and scattered light can be assumed for a given angle of incidence when considering the electromagnetic enhancement (EME) mechanism, which usually dominates the SERS process, the relative enhancements were found to be strongly mode-dependent, which is a clear indication of an electronic effect. Obviously, the KC allows for the observation of a dominating chemical enhancement (CE) in these single-layer SERS experiments. Knowing that only chemisorbed molecules should contribute to the CE, we switched this mechanism off by introducing an isolating interim layer such as Octadecanethiol, 4- Nitrobenzenethiol, or MoO2 between the metal substrate and the molecules (Nile Blue). The intensity ratio between the signals taken under resonance and off-resonance conditions was significantly reduced even when these interim layers were only few nanometers thick where EME should not be considerably affected. This was supporting the assumption that CE is the major contribution in the KC SERS experiment. Due to the domination of the CE, which only involves chemisorbed molecules, the KC SERS arrangement appears to be attractive for applications where thin layers of molecules have to be studied. As an example, we have demonstrated the detection and quantization of a low concentration (100 nM) of Moxifloxacin (Moxi).