Raman signal enhancement

This example illustrates several important lessons. First, this application takes advantage of Raman signal enhancements to measure concentrations normally considered too low for Raman spectroscopy. Extremely strong Raman signal is a hallmark of carotenoid measurements. Traditionally, this enhancement has been considered a resonance effect but others believe it is due to coupling between 7r-electrons and phonons.37 It can be beneficial to use such enhancements whenever possible. 

The extremely small cross sections for conventional Raman scattering, typically 10 111 to 10-25 cm2/molecule has in the past precluded the use of this technique for single-molecule detection and identification. Until recently, optical trace detection with single molecule sensitivity has been achieved mainly using laser-induced fluorescence [14], The fluorescence method provides ultrahigh sensitivity, but the amount of molecular information, particularly at room temperature, is very limited. Therefore, about 50 years after the discovery of the Raman effect, the novel phenomenon of dramatic Raman signal enhancement from molecules assembled on metallic nanostructures, known as surface-enhanced Raman spectroscopy or SERS, has led to ultrasensitive single-molecule detection. 

Raman Signal Enhancement Using a Bandpass Filter... 

For a Stokes-Raman signal enhanced by SERS, the above equation should be modified to... 

Figure 5. (a) Normal Raman spectrum of adenine on Si substrate (b) SERS spectrum obtained at same location on metal substrate after spraying Ag sol. Raman signal enhancement larger than 1000 were observed. Si bands from substrate as well as other analyte bands are also observed in enhanced mode. 

The molecular orbital molecule-surface interaction generates new electronic states that can be resonant with light laser at lower frequencies than the states of either the molecule or the surface. This is the so called chemical enhancement, the other SERS important mechanism (12). The results presented here demonstrate that the signals of the Raman spectrum of the target nitroaromatic HE TNT and DNT are enhanced strongly by contact with the metal oxide particles. Since Raman signal enhancement is clear in this experiment, other mechanisms could be responsible for the enhancement factors found. 

Figure 7. (a) CoO (b) CoO + TNT (20 1 mix). CoO does not participate in Raman signal enhancement under current experimental conditions. 

Vibrational hyper-Raman scattering [40-43]. Another nonlinear phenomenon observed for metal/molecule system is the surface-enhanced hyper-Raman scattering (SEHRS), consisting in the emission from a sample irradiated with iight at a frequency radiation with frequency 2 > com, where com is a frequency t3q)ical of the molecule (such as a vibrational frequency). For h3q)er-Raman signals enhancement factors of 10 have been measured [41]. 

Although the core-shell method can significantly improve the SERS activity of transition metals, it did not break the limitation of the surface generality. Recently, Tian s group developed method by which SERS can be obtained from virtually any surface. They coated Au nanoparticles with ultrathin silica shells. The Au core provides Raman signal enhancement and the silica shell prevents the core from coming into direct contact with probe/ analyte molecules or the surface over which these particles are spread. They called this method shell-isolated nanoparticle-enhanced Raman spectroscopy shown in Fig. 12 (SHINERS). ... 

An excellent example of this was the discovery of Raman signal enhancement when an analyte is placed on a roughened silver substrate." This discovery led to a new analytical technique termed surface-enhanced Raman spectroscopy (SERS). However, a lot is unknown about the mechanism of this anomalous signal enhancement. In the literature, the enhancement has been attributed to a combination of electromagnetic and chemical mechanisms " however, until a great control over the substrates atomistic composition and surface morphology can be achieved, the full extent of each mechanism wiU not be understood. Of course, the production of controllable nanostructured substrates through SERS is currently a heavily studied research area. " ... 

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