**5. Nanostars**

Nanostars and nanoflowers, so-called branched NPs, are formed by a central body and several arms or tips. These nanoarms and tips can effectively enhance the Raman scattering signal. A lot of synthetic methods based on wet chemistry, for example, the seed-mediated growth and other methods, have been reported for the preparation of nanostars for Au, Ag and other noble metals. For the seed-mediated growth method, in the presence of Ag ions, 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 ion can be reduced to Ag nanostars by ascorbic acid as the reductant.

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

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].

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].

**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.

Nanostars and nanoflowers, so-called branched NPs, are formed by a central body and several arms or tips. These nanoarms and tips can effectively enhance the Raman scattering signal. A lot of synthetic methods based on wet chemistry, for example, the seed-mediated growth and other methods, have been reported for the preparation of nanostars for Au, Ag and other noble metals. For the seed-mediated growth method, in the presence of Ag ions,

**5. Nanostars**

62 Raman Spectroscopy

[28] with permission, copyright Royal Society of Chemistry.

**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.
