**2. Nanorods**

In the early twenty-first century, the seed-mediated Au and Ag NRs growth method was developed, and this method has become the most popular approach for Au and Ag NRs synthetizing [8]. In general, this method comprises two steps: (1) the synthetizing of the small metal NPs, that is, so-called "seeds" and (2) the overgrowing of the seed to the larger, mature metal NPs. In addition, the shape of the mature metal NPs can be controlled by using the different surfactants, salt precursors and solvents. In this method, the separation of the nucleation and growth of the nanocrystals synthesis process lead to precise control of shape and size and the high uniformity of the metal NRs. In 2001, the precisely size controlled Ag NRs were successfully synthetized by using silver nitrate as the Ag precursor, ascorbic acid (A.A.) as the reductant and cetyltrimethylammonium bromide (CTAB) as the surfactant with the presence of Ag seeds and NaOH [9]. By reducing the amounts of NaOH, the ARs of the Ag NRs increased. As a pioneering Au NRs synthetizing research, the high-crystallinity colloidal Au NRs have been prepared with a high 90% shape yield by Nikoobakht and El-Sayed [10]. But the Ag ions are always needed in the high shape yield Au NRs synthetic method [11].

the most important factor restricting its applications [6]. However, the bottleneck can be overcome when the molecules are adsorbed onto a rough surface of noble metal NPs by enhancing the vibrational excitation of the adsorbed molecules or materials in a process commonly known as SERS. To develop size, shape and chemical composition, controllable anisotropic NPs to generate the "hot spot" can maximize the Raman signal to enhance the molecule detecting capacity. A wide variety of anisotropic noble metal NPs have been developed as a suitable candidate for SERS detecting and sensing. By controlling the size, shape, chemical component and structure of the NPs, the absorption range of noble metal NPs can be tuned form the visible to the near-infrared (NIR) range, and the resonances of noble metal NPs occur in the visible and NIR range of the electromagnetic spectrum [7]. Synthetizing the precisely controllable anisotropic noble metal NPs has proven to be an extremely powerful tool to tune the plasmon resonance for rendering them suitable for

By varying the diameters of spherical noble metal NPs, the plasmon resonances of NPs can only be tuned in a relatively narrow wavelength range (always only tens of nanometers). However, the anisotropic shape NPs provide one or several additional degrees of freedom, which allows controlling the plasmon resonance wavelengths, ranging from the visible to the NIR range, by varying the aspect ratio (AR) of the nanorods (NRs) or other topography

This chapter focuses on recent researches on the most commonly used anisotropic Ag, Au, and so on noble metal NPs for SERS application. Herein, we focus on the intrinsic shapedependent SERS property of the precisely controlled anisotropic NPs, synthesized mainly in

In the early twenty-first century, the seed-mediated Au and Ag NRs growth method was developed, and this method has become the most popular approach for Au and Ag NRs synthetizing [8]. In general, this method comprises two steps: (1) the synthetizing of the small metal NPs, that is, so-called "seeds" and (2) the overgrowing of the seed to the larger, mature metal NPs. In addition, the shape of the mature metal NPs can be controlled by using the different surfactants, salt precursors and solvents. In this method, the separation of the nucleation and growth of the nanocrystals synthesis process lead to precise control of shape and size and the high uniformity of the metal NRs. In 2001, the precisely size controlled Ag NRs were successfully synthetized by using silver nitrate as the Ag precursor, ascorbic acid (A.A.) as the reductant and cetyltrimethylammonium bromide (CTAB) as the surfactant with the presence of Ag seeds and NaOH [9]. By reducing the amounts of NaOH, the ARs of the Ag NRs increased. As a pioneering Au NRs synthetizing research, the high-crystallinity colloidal Au NRs have been prepared with a high 90% shape yield by

ultrasensitive detecting.

56 Raman Spectroscopy

parameters of NPs.

solution.

**2. Nanorods**

The typical UV-vis-NIR spectra of Au and Ag NRs show two obvious plasmon bands, corresponding to the transverse (short wavelength) and longitudinal modes (long wavelength). The transverse mode at ca. 520 nm can be hardly tuned, but the longitudinal mode can be precisely controlled by tuning the AR of noble metal NRs, and the intensity is much stronger than the transverse mode as shown in **Figure 1** [12]. The simulation and experimental studies of the external electromagnetic fields (E-field) distribution indicated that the high localized E-field enhancement at the noble metal NRs tips, so-called hot spots, along the dipolar longitudinal mode [13, 14].

Noble metal NRs are the most commonly used anisotropic SERS active substrate materials. Tailoring the LSPR relative to the laser excitation wavelength is one of the most significant approaches to obtain a high SERS signals. The search has indicated that the Ag NRs can generate the higher E-field than that of Au NRs under the same wavelength of incident light. In addition, the E-field intensity increases with the increasing of AR for Ag NRs [15]. Han's group developed an efficient platform for investigating the kinetics of catalytic reactions with SERS by using Au@AuPd NRs as the SERS active materials as shown in **Figure 2** [16].

**Figure 1.** The optical properties and morphology of Au nanorods with different AR. (A) The UV-vis-NIR spectra of Au NRs with different ARs. (B) Relationship between longitudinal LSPR and AR. (C–F) Representative TEM images of Au NRs with different ARs. Scale bars: 100 nm. Adapted from Ref. [12] with permission, Copyright Royal Society of Chemistry.

**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 of the reduction of 4-nitrothiophenol by H2 collected on a planar platform made of Au@AuPd NRs. 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 scattering for Au NRs.
