NANOSHELLS *
CHAPTER 8 CHAPTER 8
8.2 TYPES OF NANOSHELLS
Nanoshells can be obtained by different methods, based on the size and the intended use of the nanoshell. When synthesizing nanoshells, it is important to achieve controlled and uniform coating of core particles with the shell material. Several methods of synthesizing nanoshells have been estab- lished, but controlling the thickness of shell material coating on the core particles is difficult. These methods for synthesizing nanoshells suffer from the disadvantage of nonuniform coating. A favorable method to obtain desired coating involves surface precipitation of inorganic molecular precursors on
177 8.2 TypES Of NANOSHELLS
particles, and removal of the core by thermal procedure, depending on the type of the shell (Pradeep, 2009). There are two types of nanoshells: oxide nanoshells and metal nanoshells. Fig. 8.1 illustrates different types of core–shell particles.
8.2.1 OXIDE NANOSHELLS
Oxide nanoshells are formed from the oxide core–shell particles with a hollow core. This group includes silica, titania, and zirconia nanoshells. One of the unique features of oxide nanoshells is that they have a hollow core and a covering made of oxide. The most important application of this type of nanoshell is in the field of encapsulation of molecules and spectroscopy.
8.2.1.1 Hollow silica nanoshells
Silica provides a few advantages for use as a protecting agent. Silica is chemically unreactive, so it does not interact with the reactions that take place within the core of nanoshells. It is optically transparent, which allows us to study the spectroscopy of the nanoshell systems easily. These sil- ica nanoshell systems are useful in the study of the photochemistry of molecules and fluorophores (Pradeep, 2009).
FIGURE 8.1
Variety of core–shell particles. (A) Surface-modified core particles anchored with shell particles. (B) More shell particles reduced on to core to form a complete shell. (C) Smooth coating of dielectric core with shell.
(D) Encapsulation of very small particles with dielectric material. (E) Embedding number of small particles inside a single dielectric particle. (f) Quantum bubble. (G) Multishell particle.
From Kalele, S., Gosavi, S., Urban, J., & Kulkarni, K. (2006). Nanoshell particles: synthesis, properties and applications. Current Science, 15.
178 CHAPTER 8 NANOSHELLS
There are several steps required for the synthesis of silica-covered gold core–shell particles. The steps are the following:
1. Gold colloids are prepared.
2. 3-Aminopropyltrimethoxysilane (APS) is used as the primer and stabilizer to make gold surface vitreophilic, allowing for a thin layer of silica to form on gold colloid core.
3. Another precursor of silica, sodium silicate solution, is added at the appropriate pH to get a thicker layer of shell using the Stöber process.
The Stöber process is used to make large particles of silica with desired properties. This process allows formation of controlled silica particles within the range of 500 nm to 2 µm. The chemicals used for this reaction are tetraethoxysilane (TEOS) as silica precursor, water, ethanol, and ammonia. The silica particles obtained from the reaction mixture have a small size distribution and can be controlled by adjusting the pH of the solution, the composition of reactants, and the temperature. This process is currently used in various manufacturing processes. The hydrolysis of TEOS with water is very slow and ammonia is used as a catalyst. Hydrolysis promotes the formation of gel structures and ammonia is a morphological catalyst, which produces spherical particles. The reactions involved in the Stöber process are (Kalele, Gosavi, Urban, & Kulkarni, 2006):
Si(OC H )2 5 4 +4H O2 →Si(OH)4 +4C H OH2 5 (8.1) Si(OH)4 →(in presenceof NH SiO3) 2 +2H O2 (8.2) The first reaction shows the hydrolysis of TEOS and the second reaction shows the condensation of silica. Both reactions are base catalyzed. These reactions give the particles a negative charge, which stabilizes the surface. This method can be used to synthesize silica-covered gold and silver core–shell particles.
Procedures to make gold particles with definite size are available, and it is thus possible to obtain core–shell particles with a fixed core size. Using the core–shell particles, nanoshells can be attained by removing the core material. This requires using a suitable procedure to remove the core that does not affect the shell structure. One of the most used procedures for this application is using a cyanide ion for gold particles and ammonia for silver particles. To get nanoshells from silica-covered gold core shells, sodium cyanide is added to the solution, which dissolves the core of the particle (Pradeep, 2009). The dissolution of the core is observed using absorption spectroscopy from the disappearance of the surface plasmon peak of the gold nanoparticle. The reaction is shown below:
4Au+8NaCN+2H O2 +O2 → 4NaOH+4NaAu(CN)2 (8.3)
8.2.2 METAL NANOSHELLS
Metal nanoshells have a dielectric core composed of silica, which is different from oxide nanoshells in terms of structure. The structure of metal nanoshells provides a significant of optical properties that be adjusted based on the thickness of the shell. Gold nanoshells are part of metal nanoshells that provides opportunities for applications in the field of cancer treatment (Pradeep, 2009).
179 8.2 TypES Of NANOSHELLS
8.2.2.1 Gold nanoshells
One type of metal nanoshells is gold nanoshells, which are very useful in the field of cancer detection and treatment. The properties of gold nanoshells can be adjusted to scatter or adsorb light in a broad spectral range, nearly including infrared (IR). Near IR (NIR) is a wavelength region that provides maximum pen- etration of light through the tissue. This allows designing nanoshells that can be used for therapeutic and diagnostic applications. Gold nanoshells provide several advantages over silica nanoshells with respect to optical properties and absorption. The optical properties and absorption of gold nanoshells can be altered based on the thickness of the gold layer on silica core particles. The gold surface of the nanoshells provides the advantage of attaching different biomolecules. The gold surface is biocompatible, so it does not present any challenges to the functionality of the body. By using polyethylene glycol (PEG), the gold surface can be tailored to attach the desired type of molecules (Pradeep, 2009) (Fig. 8.2).
The following procedure is used to synthesize nanoshells:
1. Using the Stöber method, 100 nm diameter silica nanoparticles are prepared.
2. The surface silica particles are functionalized using APTES.
3. Small gold colloids are grown using the Duff and Baiker method and are adsorbed on silica nanoparticles.
4. More gold is grown into the nucleation sites using potassium carbonate and formaldehyde (Fig. 8.3).
8.2.2.2 Silver nanoshells
Silver nanoshells on silica nanoparticles can be prepared by a seed growth approach. This method allows us to get the desired optical properties of the silver nanoshells by adjusting the thickness of the silver shell. The Mie resonance of silver nanoshells takes place at energies different from any bulk interband transition. This allows silver colloid to have a stronger and sharper plasmon resonance than
FIGURE 8.2
Gold nanoshell spectral range. UV–visible spectrum of dispersed nanoshells fabricated with 96 nm diameter core and 22 nm thick gold shells.
From Pradeep, T. (2009). Nano the essentials: Understanding nanoscience and nanotechnology. New Delhi: Tata McGraw-Hill Publishing Limited Company.
180 CHAPTER 8 NANOSHELLS
gold. A significant advantage of silver colloid is that the plasmon resonance of a solid silver nanoparti- cle appears at a shorter wavelength than of gold nanoshells (Pradeep, 2009).
The following procedure is used to synthesize nanoshells:
1. Silica sphere core is treated with amine-terminated surface silanizing agent.
2. The terminated amine groups are used as attachment points from small colloidal silver particles.
They are used for growth of a silver nanoshell overlayer.
3. Silver particles are grown using the standard sodium citrate route.
Step 3 is used to adjust the thickness of the silver shell as required (Fig. 8.4).
FIGURE 8.3
Synthesis of gold nanoshell having silica as core.
With permission from Naomi Halas - Optics and Photonics News (2002). Vol. 13, Issue 8, pp. 26–30.
FIGURE 8.4
fabrication procedure of silver nanoshells. As described in the text, using a silica sphere and a salinizing agent, silver particles are grown and adjusted as desired.
From Pradeep, T. (2009). Nano the essentials: Understanding nanoscience and nanotechnology. New Delhi: Tata McGraw-Hill Publishing Limited Company.
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