In a typical SERS measurement protocol, 2.5 μL of an check details R6G solution in ethanol 80 μM in concentration was Selleck MAPK inhibitor applied onto the surface of the substrate under study. The average surface area occupied by the dye droplet spread on the substrate was around 7 mm2. Measurements were mainly taken using radiation from a He-Ne laser (wavelength 632.8 nm, power in the beam spot approximately 5 mW). The laser beam spot diameter was around
20 μm, and the signal accumulation time came to 10 s (the signal was averaged over 10 measurements). With the test conditions remaining the same, SERS signals were measured from the R6G dye applied onto GNR-Si and GNR-OPC substrates differing in thickness of the opal-like film. Figure 5 shows the SERS spectra of the 80 μM rhodamine 6G solution applied onto a GNR-Si (spectrum 1) and a GNR-OPC (spectrum 2) substrate excited at 632.8 nm. Evidently, the integral analytical enhancement [42] of the GNR-OPC substrate is from two to five times as high as that of the simple fractal-like GNR assembly
on silicon. A common property of SERS measurements is that the integral enhancement depends on the particular Raman line selected for the purpose. The fundamental Selleckchem Fludarabine SERS enhancement [41, 42] is determined by several important factors that are difficult to take into account for mesoporous substrates. For a detailed discussion of this point, the readers are referred to the comprehensive analysis by Le Ru et al. [36]. Figure 5 SERS spectra of 80
μM rhodamine 6G solution applied onto GNR-Si (1) and thin GNR-OPC (2) substrates. Excited at 632.8 nm. In Figure 6, we compare between the SERS spectra of the 80 μM rhodamine 6G solution applied onto ‘thin’ and ‘thick’ GNR-OPC substrates. This classification roughly corresponds to the number of the deposited silica layers, which is less than 10 in the former case and more than 10 in the latter. However, in both cases, the pores between silica spheres are densely covered by GNRs, but GNRs fail these to cover the silica spheres completely. Surprisingly enough, the maximum SERS enhancement is observed with thin rather than thick substrates (cf. spectra 1 and 2 in Figure 6). It should be noted that the elevated tail in SERS spectrum 2 is due exactly to a thick silica film contribution. For thin substrates, the baseline is flat (similar to that for spectrum 1 in Figure 6). Moreover, for extremely thick substrates (about 1 to 2 mm thick), the SERS enhancement falls down, and we observe a monotonous contribution from the underlying silica opal (data not shown). Figure 6 SERS spectra of 80 μM rhodamine 6G solution applied onto thin (1) and thick (2) GNR-OPC substrates. Excited at 632.8 nm. Taking into account the analytical SERS enhancement coefficient of GNR-Si substrates [33] (2.5 × 103), we estimate the analytical enhancement coefficient of GNR-OPC substrates to be on the order of 104. We suppose that the additional SERS enhancement in the GNR-OPC substrates is due to several factors.