**4. Nanocubes**

From the synthetic angle, Ag nanocubes (NCs) can be easily obtained with high crystallinity, monodispersion and uniformity than that of Au NCs. In the most commonly used Ag NCs synthetizing method, the Ag ions of the AgNO<sup>3</sup> are reduced by ethylene glycol in the presence of HCl and PVP [21, 22]. These Ag NCs were single crystals and were characterized by a slightly truncated shape bounded by {100}, {110} and {111} facets (**Figures 5** and **6**).

The synthesis of monodisperse Au NCs is still a huge challenge. The key parameters such as reproducibility and fine size control still require further optimization. The most commonly used synthetizing method is also based on seed-mediated growth, in combination with CTAC, CTAB or CPC (cetylpyridinium chloride) acting as the surfactants. By precisely controlling the Precisely Controllable Synthesized Nanoparticles for Surface Enhanced Raman Spectroscopy http://dx.doi.org/10.5772/intechopen.73086 61

**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 permission, copyright science.

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 of these Au NPs was still observed [26].

**4. Nanocubes**

60 Raman Spectroscopy

synthetizing method, the Ag ions of the AgNO<sup>3</sup>

From the synthetic angle, Ag nanocubes (NCs) can be easily obtained with high crystallinity, monodispersion and uniformity than that of Au NCs. In the most commonly used Ag NCs

**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.

ence of HCl and PVP [21, 22]. These Ag NCs were single crystals and were characterized by a

The synthesis of monodisperse Au NCs is still a huge challenge. The key parameters such as reproducibility and fine size control still require further optimization. The most commonly used synthetizing method is also based on seed-mediated growth, in combination with CTAC, CTAB or CPC (cetylpyridinium chloride) acting as the surfactants. By precisely controlling the

slightly truncated shape bounded by {100}, {110} and {111} facets (**Figures 5** and **6**).

are reduced by ethylene glycol in the pres-

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

Au ion can be reduced by ascorbic acid and CTAB or CTAC acting as the surfactants or using N,N-dimethyl-formamide (DMF) as solvent and reductant in the presence of PVP as surfactant. For template-based methods, using mesoporous silica as the template, Au nanotips can be grown on the surface of mesoporous silica. The seed-mediated growth method can also be used to synthetize Ag nanostars. In the presence of the sodium polyacrylate as the seeds, Ag

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http://dx.doi.org/10.5772/intechopen.73086

63

The UV-vis-NIR spectra of Au nanostars indicate that it shows a plasmon band in the range from 600 nm to 1200 nm, corresponding to tailor the sharpness and/or AR of the tips as shown in **Figure 7** [29, 30]. EELS mapping showed an extremely high E-field intensity at the tips of

Nanostars show a higher E-field at the resonance wavelength than that of NRs or nanospheres with the similar dimers. The SERS detection limit can achieve to enhance the Raman signal via the plasmon coupling between the adjacent Au tips or between the Au tips and Au core. Au nanostars acting as the SERS active substrate material can achieve an ultra-sensitive 4-mercaptobenzoic acid (4-MBA) detection, with the detection limits as low as 10 fM [32]. Based on the SIE-MoM, by increasing the surface coverage, the relatively constant enhancement can be observed [33].

**Figure 7.** (a) Optical properties of Au nanostars with different branching degrees. An increase in the branching produces a red-shift in the corresponding spectra. Adapted from Ref. [30] with permission, Copyright IOP Publishing. (b and c) TEM images of Au nanostars. (d) SERS spectra of Texas red (TR) dye bound to Au nanostar dimers with average gaps of 7 nm (curve (i)) and 13 nm (curve (ii)), and TR dye bound to Au nanostar monomer on DNA origami (curve (iii)) and bulk TR dye (curve (iv)) recorded using 532 nm laser. Adapted from Ref. [34] with permission, Copyright American Chemical Society.

ion can be reduced to Ag nanostars by ascorbic acid as the reductant.

the nanostars, which can enhance the Raman signal efficiently [31].

**Figure 6.** (A–D) TEM images of nanocages at degrees of galvanic replacement. (E) UV-vis-NIR spectra of nanocages with varying Au amounts. (F and G) SERS spectra of 1,4-BDT with nanocages used as the SERS substrate. Adapted from Ref. [28] with permission, copyright Royal Society of Chemistry.

When the inner atoms are etched, the nanocubes can be transformed to nanocages. Galvanic replacement is the most common method for synthetizing hollow nanocages. Au nanocages can be created by galvanic replacement by using Ag NCs as the templates with chloroauric acid in water phase [21]. A valuable property of Au nanocages is the red-shifting of LSPR bands into the NIR range, which is particularly useful for biological sensing and detecting applications. In addition, the galvanic replacement can also produce the bimetallic Au-Ag alloy nanocages, in which the SERS intensity shows a strong relationship between excitation wavelength and Au [28].
