For routine sensing applications, surface-enhanced Raman scattering (SERS) spectroscopy needs to be simple, stable for a long time, reproducible and inexpensive. To meet these requirements we develop SERS substrates based on self-assembled silver-coated gold nanorods (Au/AgNRs) (Fig.1) [1] and 2D colloid crystal-mediated assemblies of gold nanorods [2].
Figure 1. Schematic fabrication of a SERS substrate of PEG-coated Au/Ag NRs on a silicon wafer (top). TEM images of self-assembled monolayers on formvar-coated grids for NR-0 (a), NR-1 (b), and NR-2 (c) rods. Panel (d) shows the geometrical model for as-prepared Au NRs and the anisotropic growth of silver shells.
Figure 1 demonstrates the scheme of nanorods-based SERS substrate fabrication on a silicon wafer (top). TEM images show self-assembled monolayers synthesized on formvar-coated grids of NR-0 (Fig.1, a), NR-1 (Fig.1, b), and NR-2 (Fig.1, c). The anisotropic growth of silver shells and the geometrical model for as-prepared Au NRs based on TEM statistics are presented in Fig.1,d.
Figure 2. SERS spectra of 8 µM R6G (a-c) from substrates of self-assembled NR-2 (a), NR-1 (b), and NR-0 (c) and the Raman spectrum of 10 mM R6G (d). Panel (e) demonstrates electric field distribution in the middle section of random NR-2 monolayer calculated by FDTD simulation.
Figure 2 shows that even 2-nm silver coating of AuNRs (NR-2) is sufficient to increase the analytical enhancement factor to 25000 in comparison with 1000 for Au NRs (NR-0). In agreement with recent data by Contreras-Caceres et al. [3], we observed remarkable analytical SERS enhancement even for thin (1 to 2 nm) silver coatings of AuNRs.
3D finite-difference time-domain (FDTD) simulations confirm the electromagnetic mechanism of enhancement owing to generation of hot spots near the end-to-end and end-to-side nanoparticle contacts (Fig. 2, e). The proposed technology can be easily reproduced in routine laboratory practice as it is very simple, robust, and cheep. Indeed, in a typical synthesis, we fabricate 1 L of nanorods and then nanorods powder [4] for 50000 measurements. As result, the cost of all reagents for one measurement can be reduced to one cent.
Assembling of NRs on a 2D colloidal silica crystal (CC) resulted in the formation of ring-like chains with preferential tail-to-tail orientations along the hexagonal boundaries (Fig.3, c,d). To the best of our knowledge, this is the first time that such unique nanostructures have been prepared [2]. In agreement with previously published data [1], the tail-to-tail orientation of assembled GNRs produces a strong local electromagnetic enhancement in the gaps between the GNR ends. Owing to great number of tail-to-tail packing of NRs, this substrate demonstrates an analytical SERS enhancement 10 to 15 times higher than that for randomly assembled NRs.
For comparative purposes, we performed additional experiments using 282-nm silica CCs decorated with 25-nm gold nanospheres (GNSs). In general, the overview (Fig. 3,a) and enlarged (Fig. 3,b) SEM images demonstrate chain-like NS structures similar to those shown in Fig. 3c and Fig. 3d. The bright rings in Fig. 3a correspond to the adsorbed GNSs. The enlarged panel (b) illustrates some preferential chain-like assembling of GNSs along the hexagonal boundaries of the CC. However, near the crystal vertices, the chain-like order of gold NSs is violated and several contacting particles are clearly seen.
Figure 3. Overview of 282-nm SiNP CCs decorated with 25-nm colloidal GNSs (a) and GNRs (c). The bright rings correspond to adsorbed gold nanoparticles. The enlarged panels (b,d) illustrate preferential chain-like assembling of gold nanoparticles along the hexagonal boundaries of CCs. The arrows indicate preferential tail-to-tail assemblies of GNRs.
Figure 4 (a) shows a comparison of SERS efficiencies for two types of substrates: 2D silica CC+GNRs and 2D silica CC+GNSs. The laser power was equal to 7 mW and the acquisition time was equal to 5 s for assembled GNSs and 1 s for assembled GNRs. Note that for calculations of the AEF, a particular SERS intensity should be normalized to the DTTC concentration and to the acquisition time. It follows from Fig. 4 that the maximal SERS enhancement is observed for GNRs (the spectrum 3), whereas the spectrum 1 has no distinguished peaks at the same 5 μM concentration of DTTC even after five-fold increase in the acquisition time. When the DTTC concentration was increased up to 50 μM, the measured SERS intensities were still weak and their magnitudes become comparable to those in the spectrum 3 after the acquisition time was increased from 1 s to 5 s. To calculate the ratio of the AEFs for both substrates (GNR- and GNS-based), we selected two characteristic peaks near 820 cm-1. The dashed lines in Fig. 4 (a) illustrate determination of the base-corrected intensities, which are 4690 a.u. and 4030 a.u for GNR- and GNS-based substrates, respectively. Taking into account the corresponding DTTC concentrations (5 and 50 μM) and acquisition times (1 and 5 s), we get the following ratio: AEF(GNRs)/AEF(GNSs) =4690/[4030/10/5] = 58. This means that the average AEF for assembled GNRs is almost 60 times greater as compared to that for GNS-based substrates.
Figure 4. (a) SERS spectra of 5 mM (1) and 50 mM (2) DTTC solutions on silica CCs+GNSs substrate in comparison with SERS spectrum of 5 mM DTTC on silica CCs+GNRs substrate (3). The acquisition time was 5 and 1 s for spectra (1, 2) and (3), respectively. The dashed lines illustrate determination of base-corrected SERS intensities for the 822 cm-1 peak. (b) Raman spectra of a 500 μM DTTC solution on silica CC (1) and on a silicon wafer (2), respectively. Curves (3) and (4) show SERS spectra for 500 nM and 50 nM DTTC measured with GNRs and GNRs on CCs substrates, respectively.
Comparative Raman spectra of DTTC solutions for different substrates are shown in Fig. 4 (b). It follows from Fig. 4 (b) that CCs themselves (curve 1) do not provide any SERS enhancement as compared to the reference Raman spectrum (2). Moreover, the smallest Raman intensities were observed with the CC substrate. The GNRs assembled on the silicon wafer demonstrate an AEF of about 500 to 700, depending on the particular SERS peak. Finally, we note good qualitative and quantitative agreement between SERS spectra 3 (GNRs) and 4 (CCs +GNRs). This means that the average AEF is about 10 (500 nM/50 nM) as compared to the GNR substrate and about 5000 as compared to the silicon wafer. Specifically, for the 840 cm-1 peak, the calculated AEF is about 16; therefore the total AEF is about 8000, with the wafer substrate as the benchmark reference.
To summarize, we have demonstrated [2] that the assembling of GNRs on 2D CCs gives rise to ringlike chains with some preferential tail-to-tail orientation along the hexagonal boundaries between closely packed silica spheres. Owing to this structure, the 2D SiNP CC + GNR substrates are more effective SERS enhancers as compared to self-assembled GNRs. Specifically, the ratio of corresponding AEFs is about of 15. Even a very thin (1−2 nm) silver coating of Au NRs has brought about a 25-fold increase in the AEF: from 103 for Au NRs to 2.5×104 for Au−Ag NRs. The comparative study with 2D SiNP + GNS substrate gave strong evidence for greater SERS efficiency of GNR-based 2D CC substrates, which is almost 60 times greater as compared with that for GNS-based one. In view of their relative simplicity and large-scale reproducibility, such substrates can be useful in the routine chemical or biomedical SERS sensing of small amounts of analytes.
ACKNOWLEDGMENTS
The work was supported by the Government of the Russian Federation (grants 14.Z50.31.0004 and 14.B25.31.0019 to support scientific research projects implemented under the supervision of leading scientists at Russian institutions and Russian institutions of higher education).
REFERENCES
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