Surface Raman signals, distinction

SERS biosensors have also been used in conjunction with reporter molecules which have strong, distinct Raman spectra as labels. Some such molecules are thiophenol (TP), 2-naphthalenethiol (NT) [39], 5,5-dithiobis(succinimidyl-2-nitro-benzoate) (DSNB, a derivative of dithiobis (benzoic add)) [40], (4-mercaptobenzoic acid, 4MBA) [41], etc. Because the Raman signal is usually quite weak, high-power lasers are required to elicit a measurable signal. However, it is not easy to miniaturize such lasers. Hence, enhancement of the signal by surface modification (ie, SERS) and/or using reporter molecules makes this technique more amenable to use in POC scenarios. 

An example of the results for such an approach is shown in Fig. 2.5 (from Ref [1]), which we now discuss. Methylene Blue (MB) is used here as a probe molecule and is transferred onto gold NP arrays by dipping them into a 10 // M MB solution for 5 minutes. The arrays are plasma-cleaned before any dipping, to ensure that no contaminants are previously adsorbed on the NPs. The MB molecules are therefore adsorbed directly onto the gold surfaces, with possibly some molecules further away from the surface if several monolayers are present. Three NP arrays are used with distinct LSP resonances at 612 nm (array Al), 667 nm (A2) and 713 nm (A3), as evidenced in the extinction spectra of Fig. 2.5(b). The resonance of A2 is close to the peak absorption and fluorescence of MB and would be considered as the standard situation for most MEF experiments (except for the direct adsorption onto the metal). Al and A3 have resonances much further away on either side of the MB fluorescence spectrum. The MEF spectra (corrected for the ITO background), shown in Fig. 2.5(c), exhibit a broad spectrum underneath the SERS (Raman) peaks. The SERS peaks clearly confirm the presence of MB on the NPs surface. The accompanying broad signal is attributed to the modified MB fluorescence (MEF), initially for two reasons ... 

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