**3. Nanoplates**

Triangular nanoplates are the circular, triangular or hexagonal nanomaterials in which one of the dimensions is much smaller than the other two. The most commonly used SERS active substrate nanoplates are the Au and Ag nanotriangles and nanohexagons. Because of the sharp corners and edges, the nanotriangles exhibit the strong E-field enhancements. **Figure 3** shows the morphology and optical property of nanotriangles [17].

to the straight edge nanotriangles. The most widely used Ag nanotriangles synthetizing methods include light-induced synthesis and wet chemistry methods. For the Ag nanotriangles synthetizing methods, polyvinylpyrrolidone (PVP), cetyltrimethylammonium chloride (CTAC) or CTAB acting as the surfactants can induce the Ag growth to the platelike morphologies with sharp corners and edges [19]. For Au nanotriangles, Scarabelli et al. reported the controllable synthesis of Au nanotriangles with tunable size and high yield (95%) upon simple purification [17]. Tan et al. synthesized Ag nanotriangles with different edge lengths from 30 up to 210 nm with corresponding LSPR band between 485 nm and 1130 nm [20]. Ag nanotriangles SERS substrate was also used for biosensing and measured the E-field of adenosine triphosphate (ATP) with two different incident

**Figure 3.** (A) TEM images of Au nanotriangles with different edge lengths (scale bar 500 nm). (B) UV-vis-NIR spectra of the Au nanotriangles depicted in TEM images (a–f). (C) SERS performance of Au NTs in solution for thiophenol (TP) excited at 785 nm and concentrations varying between 10−5 and 10−8 M. Adapted from Ref. [17] with permission,

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light wavelengths [20].

Copyright American Chemical Society.

Au and Ag nanotriangles exhibit the excellent SERS signal because of the high generation of E-field, good assembling behavior and the tunability of the LSPR bands. Although the nanotriangles own the better optical property than that of NRs, the synthetic difficulties of nanotriangles restrict its application as the SERS active substrate materials. Study has shown that the Ag nanotriangles with sharp corners and edges exhibit the larger Raman enhancement than the rounded corner nanotriangles as shown in **Figure 4** [18]. In addition, the zigzag edges of nanotriangles can generate a further enhancement compared

Precisely Controllable Synthesized Nanoparticles for Surface Enhanced Raman Spectroscopy http://dx.doi.org/10.5772/intechopen.73086 59

**Figure 3.** (A) TEM images of Au nanotriangles with different edge lengths (scale bar 500 nm). (B) UV-vis-NIR spectra of the Au nanotriangles depicted in TEM images (a–f). (C) SERS performance of Au NTs in solution for thiophenol (TP) excited at 785 nm and concentrations varying between 10−5 and 10−8 M. Adapted from Ref. [17] with permission, Copyright American Chemical Society.

The results indicated that the Ag NRs show the high Raman enhancement than the Au NRs because of the stronger E-field of Ag NRs than that of Au NRs. This phenomenon can be attributed to higher plasmonic intensity at the tips of Ag NRs and higher Rayleigh scatter-

**Figure 2.** (A) TEM image of the Au NRs seeds. (B) TEM image and (C) SEM image of the Au @AuPd NRs. (D) STEM images of Au@AuPd NRs taken along the [100], [110], and [001] axes (the first column, from the top down), and the corresponding elemental mappings for Au (the second column) and Pd (the third column); the last column shows combined mappings, in which the simultaneous presence of Au and Pd appears yellowish. (E) Successive SERS spectra

collected on a planar platform made of Au@AuPd NRs. Copyright American

Triangular nanoplates are the circular, triangular or hexagonal nanomaterials in which one of the dimensions is much smaller than the other two. The most commonly used SERS active substrate nanoplates are the Au and Ag nanotriangles and nanohexagons. Because of the sharp corners and edges, the nanotriangles exhibit the strong E-field enhancements. **Figure 3**

Au and Ag nanotriangles exhibit the excellent SERS signal because of the high generation of E-field, good assembling behavior and the tunability of the LSPR bands. Although the nanotriangles own the better optical property than that of NRs, the synthetic difficulties of nanotriangles restrict its application as the SERS active substrate materials. Study has shown that the Ag nanotriangles with sharp corners and edges exhibit the larger Raman enhancement than the rounded corner nanotriangles as shown in **Figure 4** [18]. In addition, the zigzag edges of nanotriangles can generate a further enhancement compared

shows the morphology and optical property of nanotriangles [17].

ing for Au NRs.

Chemical Society.

58 Raman Spectroscopy

of the reduction of 4-nitrothiophenol by H2

**3. Nanoplates**

to the straight edge nanotriangles. The most widely used Ag nanotriangles synthetizing methods include light-induced synthesis and wet chemistry methods. For the Ag nanotriangles synthetizing methods, polyvinylpyrrolidone (PVP), cetyltrimethylammonium chloride (CTAC) or CTAB acting as the surfactants can induce the Ag growth to the platelike morphologies with sharp corners and edges [19]. For Au nanotriangles, Scarabelli et al. reported the controllable synthesis of Au nanotriangles with tunable size and high yield (95%) upon simple purification [17]. Tan et al. synthesized Ag nanotriangles with different edge lengths from 30 up to 210 nm with corresponding LSPR band between 485 nm and 1130 nm [20]. Ag nanotriangles SERS substrate was also used for biosensing and measured the E-field of adenosine triphosphate (ATP) with two different incident light wavelengths [20].

**Figure 4.** (A) The simulation results of Ag nanotriangles with curved, straight and zigzag edges, using finite-difference time-domain (FDTD) calculations. (B and C) E-field amplitude patterns of nanotriangles with E-field along the x-axis (Ex) and y-axis (Ey), for B and C, respectively. Adapted from Ref. [18] with permission, Copyright Wiley-VCH.

dynamic process, reductant concentration and the selective absorption facet of surfactants, the Au NCs can be obtained with requested shape and single crystal structure [23]. The simulation results indicated that the dipolar LSPR charges of Ag and Au NCs tend to accumulate at corner sites [24, 25]. Due to the strong LSPR and hot spots highly localized at corners, NCs are excellent candidates as SERS substrates. When the other symmetric Au NPs such as rhombic dodecahedra and octahedra were used as the SERS active substrate materials, the SERS signal

**Figure 5.** (A) Low- and (B) high-magnification SEM images of Ag NCs. (C) The TEM image of the same batch of Ag NCs. Inset: Diffraction pattern of an individual cube. (D) The XRD pattern of the Ag NCs. Adapted from Ref. [22] with

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Because certain facets show higher chemical activities, concave NCs can also be prepared via modifications of the seed-mediated growth method, where CTAC provides control over the concave morphology of the final product. Thereby, the concave NCs can effectively enhance the performance as the SERS active substrates. The LSPR band of concave NCs red-shift obviously compared to the NCs with flat faces. Because of the sharp corner of concave NCs, the

of these Au NPs was still observed [26].

permission, copyright science.

higher E-field enhancements can be expected [27].
