2. Electrostatic Self-Assembly and Laser Ablation in Liquid (LAL) Method

Laser ablation of metal target in colloidal medium will one step produce the nano-sized metal NPs and wrapping of the oxides layer. The physical and chemical properties of oxides shell depend on diverse parameters, including the metal, colloidal type, temperature and pH of the colloidal medium.

### 2.1. Charged properties of colloids

interaction with incident light. However, in many specific applications, the surfaces of these plasmonic materials need to be modified or coated by organic or inorganic species to achieve core-shell structured nanomaterials with functional properties [5, 6]. Among them, stable semiconducting oxide-coated plasmonic NPs, which couple strong bandgap absorption and plasmon absorption of the light, possess a relatively sensitive surface and have been extremely applied in photo/electro catalysis [7–9], chemical analysis [10, 11], and solar cells [12, 13]. In those applications, the thickness of the shell has been accepted as a fundamental and an important parameter. In particular, the shell is commonly required to be thin enough for maximizing the short-range local surface plasmon resonance (LSPR) effect of the plasmonic metal core. Typically, the wrapping shell thickness is often expected to not exceed 10 nm to ensure strong LSPR effect in the sensitive surface-enhanced Raman scattering (SERS) detection of target analytes [14, 15]. As a result, facile approaches to obtain ultrathin oxides wrapped plasmonic metal NPs are in urgent demands for high performance SERS-based detection

In the literature, conventional methods including Stöber method [16], hydrothermal method [17], and sol-gel method [18] have been extremely applied to prepare oxide layers coated plasmonic NPs. For example, Yoshio et al. prepared core-shell structured Ag@SiO2 NPs through borohydride reduction method [16]. Kim et al. deposited 60 nm of TiO2 shell layer on gold NPs via microwave-assisted hydrothermal method [17]. In general, these wet chemical fabrications are usually relied on two steps: the formation of the core and subsequent wrapping of the shell, in which an organic substance with multifunctional groups is commonly utilized as a bridge to connect the metal core and the oxide shell. The as-obtained products are often encapsulated with surfactants or the media molecules that cause severe interfering signals in SERS detection and deteriorate the performances in practical applications. What's more, most of these mentioned methods are difficult to achieve ultrathin wrapping layer due to the difficulty in controlling the nucleation and growth stages of the shell. The present reports in literature illustrate that the thickness of the shell is generally thicker than 10 nm, which is beyond or close to the working distance limit of LSPR. And a very few reports regarding one-pot synthesis method could be found for the ultrathin oxides wrapped plasmonic metal NPs [9, 19, 20]. In addition to the wet chemical methods, atomic layer deposition (ALD) has been widely reported to realize homogenous oxide shell thin to monoatomic layers [21–23]. For example, Qian et al. fabricated Au@TiO2 core-shell structured NPs by using an electrochemistry controlled atomic layer deposition [22]. However, despite of its ultrathin and uniform oxide shell characters, it is time-consuming and tedious in operation and commonly restricted to deposit Al2O3 or SiO2 layers. In total, controllable and flexible methods to facilely and one-pot prepare the ultrathin and uniform oxide-coated

Recently, some important progresses have been made in fabrication of ultrathin and uniform oxide-wrapped plasmonic NPs [24, 25]. In this chapter, we introduce a universal strategy for wrapping NPs based on colloidal electrostatic attraction and self-assembly on the plasmonic NPs. Using this strategy and via one-step laser ablation of noble metal targets in the hydrolysis-induced hydroxide sol solutions at room temperature, the oxide shell-wrapped plasmonic NPs with several tens of nanometers in size could be obtained, such as Au@oxides (Fe2O3, Al2O3, In2O3, CuO, and ZnO) as well as Pt@TiO2 and Pd@TiO2.

plasmonic metal NPs free of contaminations are still expected.

application.

202 Plasmonics

We propose that the surface charge status of both colloids and metal NPs are crucial to the selfassembly formation of oxides shells. Since the surface of the colloids attracts the anions and cations, these ions are distributed in a diffusive state at the two-phase interface to form a diffusion double layer [26–28]. The electric double layer can be divided into two parts by the Stern plane as Stern layer and the diffusion layer. The double layer theory suggests some possible behaviors of the colloids in solution, such as repelling each other and staying stable or attracting each other and coagulating. A parameter quantitatively describes charged properties of the colloids is Zeta potential [27, 28], which refers to the potential of the Shear plane relative to the solution at infinity. The Zeta potential can be positive or negative, which suggests different charged properties of the colloids. The larger the absolute value of Zeta potential usually indicates the more charges on colloids and better stability. If two kinds of colloids charged differently close to each other, a strong electrostatic attraction between them will occur.

### 2.2. Colloidal electrostatic self-assembly

A key issue in the preparation of core-shell-structured NPs is how to efficiently attach the shell materials or its precursors to the preformed core particles. Considering the charging characteristics of colloidal NPs, if two colloidal NPs with different charge properties are brought close to each other, a strong Coulomb attraction will attract them together. When the sizes of the two colloidal NPs differ greatly, the small colloids should be adsorbed onto the surface of the big one to complete the electrostatic assembly process. And such method has been reported for the preparation of Au-wrapped magnetic Fe3O4 NPs [29], and Au-wrapped silica NPs [30]. However, the oxides wrapped plasmonic metal NPs are rarely reported.

In order to achieve the ultrathin oxide wrapping layer, the oxides or its precursor colloids used in the colloidal electrostatic self-assembly should be small enough. Generally, the artificially assisted methods usually results in bigger oxides or its precursor colloids (most of them are no less than 5 nm). Generally, metal cations (Fe3+, Al3+, Cu2+, Ti4+, Sn4+, etc.) are hydrolyzed in solution to form corresponding hydroxide colloids. The size of these hydroxide colloids is generally at several nanometers [31, 32], which is suitable for such a colloidal electrostatic self-assembly. And most of them are positively charged. On the other hand, many plasmonic metal NPs tend to adsorb anions on the surface and carry negative charges [33, 34]. Strong electrostatic attraction between the two kinds of colloids will occur when they are close enough to each other. The small hydroxide colloids will be attached on the surface of the plasmonic metal NPs and a monolayer hydroxide wrapping layer would be formed on the metal NPs due to the colloidal self-assembly, as schematically shown in Figure 1a and b. After dehydration treatment by annealing or heating, the hydroxides shell will be transformed to corresponding oxides (Figure 1c).

Obviously, such a self-assembly process should be a flexible and universal, which is suitable to fabricate a series of core-shell NPs. And the thickness of the shell is highly relied on the size of the colloids produced by the hydrolysis, which could be simply controlled by the pH value and temperature of the colloidal precursor. This has been confirmed by a one-step laser ablation of plasmonic metal target in hydrolyses induced hydroxides sol solutions.

### 2.3. Laser ablation in hydroxides sol solutions

In order to avoid and remove interferences from other substances (such as surfactants), the laser ablation in liquid method, typically been accepted as a chemical green approach, has been adopted to verify the colloidal electrostatic self-assembly strategy.

In the laser-based synthesis procedure, metal foils are usually utilized as the ablation target while metal salt solutions are used as the liquid medium, as shown in Figure 2. When a pulsed laser is focused onto the surface of metal target, a localized high-temperature and high-pressure plasma involving atoms, ions, electrons, and clusters is generated. From the moment on formation, the

plasma adiabatically expands at supersonic speeds and transfer heat to the surrounding liquid medium. The quenched plasma nucleates and gradually grows up to form plasmonic NPs [35, 36]. Commonly, the newly formed metal NPs are negatively charged and, when they are dispersed into the colloidal solution, they rapidly absorb the cations in the solution. The small hydroxide colloids, which are formed through hydrolysis of the metal cations, in the solution are positively charged. As a result, the metal NPs will electrostatically attract with the positively charged hydroxide colloids. The hydroxide colloids will form a nano-sized layer around the metal core, forming a core-shell structured NP. With the ongoing laser ablation, the hydroxidebased core-shell NPs will absorb the latter arrived laser and slowly dehydrated to oxide shell due

Figure 2. Schematic illustration of the laser ablation for ultrathin oxide layer-wrapped plasmonic metal NPs via the

Ultrathin Oxide Wrapping of Plasmonic Nanoparticles via Colloidal Electrostatic Self-Assembly and their…

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Based on the aforementioned formation process, pulsed laser ablation in colloidal solution will

to the laser-induced heating.

colloidal electrostatic self-assembly strategy.

3. LAL-induced oxides wrapped metal NPS

one-pot fabricate the oxides wrapped metal NPs.

Figure 1. Schematic illustration for the fabrication strategy of ultrathin oxide layer-wrapped metal NPs based on the electrostatic colloidal attraction and self-assembly. (a) Adsorption or attachment of hydroxide colloids on a metal NP due to the electrostatic attraction. (b) Formation of monolayer hydroxide colloidal shell by colloidal self-assembly on the metal NP. (c) Formation of ultrathin oxide shell layer on the metal NP by dehydration [24].

Ultrathin Oxide Wrapping of Plasmonic Nanoparticles via Colloidal Electrostatic Self-Assembly and their… http://dx.doi.org/10.5772/intechopen.79573 205

Figure 2. Schematic illustration of the laser ablation for ultrathin oxide layer-wrapped plasmonic metal NPs via the colloidal electrostatic self-assembly strategy.

plasma adiabatically expands at supersonic speeds and transfer heat to the surrounding liquid medium. The quenched plasma nucleates and gradually grows up to form plasmonic NPs [35, 36]. Commonly, the newly formed metal NPs are negatively charged and, when they are dispersed into the colloidal solution, they rapidly absorb the cations in the solution. The small hydroxide colloids, which are formed through hydrolysis of the metal cations, in the solution are positively charged. As a result, the metal NPs will electrostatically attract with the positively charged hydroxide colloids. The hydroxide colloids will form a nano-sized layer around the metal core, forming a core-shell structured NP. With the ongoing laser ablation, the hydroxidebased core-shell NPs will absorb the latter arrived laser and slowly dehydrated to oxide shell due to the laser-induced heating.
