**1. Introduction**

The utilization of the enhancement effect of Raman scattering by using the plasmonic effect for nanoparticles (NPs) is mainly determined by the physical and chemical properties of the surface-enhanced Raman spectroscopy (SERS) active substrates. SERS is a powerful technique for ultrasensitive detection through the enhancing of electromagnetic fields (E-field) generated by localized surface plasmon resonance (LSPR) effect [1–4]. It can provide specific fingerprint information about a wide range of target molecules [5]. Noble metal NPs (e.g., Au, Ag, Cu, etc.) display a variety of plasmonic behaviors. The extremely inefficient inelastic light scattering of Raman scattering of the molecules or materials is

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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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 ultrasensitive detecting.

Nikoobakht and El-Sayed [10]. But the Ag ions are always needed in the high shape yield

Precisely Controllable Synthesized Nanoparticles for Surface Enhanced Raman Spectroscopy

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

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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

**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.

SERS by using Au@AuPd NRs as the SERS active materials as shown in **Figure 2** [16].

Au NRs synthetic method [11].

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 parameters of NPs.

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 solution.
