Metallic Nanoparticles for Biomedical Applications
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3 Bottom-Up Methods
In this method, the building block of NPs is atoms or molecules. NPs are formed from
these building blocks through various phenomena and chemical reactions. The topdown method is compared with the bottom-up approach, as given in Fig. 3. Different
approaches in bottom-up methods are solid-state methods, liquid state synthesis, gas
phase, biological, microfluidic-based, and other methods. In all methods, building
blocks are generated at first, followed by the assembling of building blocks to form
NPs of desired size and shape [34].
3.1 Solid-State Methods
Here, the material is deposited on a surface in the form of thin-film or as a nanomaterial. Two types of deposition approaches exist; physical vapour deposition (PVD) and
chemical vapour deposition (CVD). In PVD, techniques like laser ablation is used,
which further causes the formation of plasma of ablated target, and they are deposited
on a substrate to produce a thin film. MNPs are deposited on carbon nanotubes in this
manner. Even though the method is simple, it is unable to provide a higher volume of
material, and the procedure is expensive. The chemical reaction of gaseous molecules
of atoms to form the thin film is utilized in depositing thin films on the substrate in
CVD. Three types of CVD are used; thermally active chemical vapour deposition
(TACVD), plasma-enhanced chemical vapour deposition (PECVD), and photoinitiated chemical vapour deposition (PICVD). In the form of volatile molecules, the
target material is deposited on the substrate. In the case of temperature-sensitive
material such as polymer substrate, TACVD is not applicable. For PECVD, plasma
is generated inside the void chamber with the help of microwave and inductively
induced electric current. Due to the requirement of specific operating conditions,
PECVD suffers from scale-up issues. For PICVD, low energy treatment, as well as a
Fig. 3 Overview and comparison of synthesis of metallic nanoparticles; top-down method and
bottom-up method. Redrawn from [34]
37
3 Bottom-Up Methods
In this method, the building block of NPs is atoms or molecules. NPs are formed from
these building blocks through various phenomena and chemical reactions. The topdown method is compared with the bottom-up approach, as given in Fig. 3. Different
approaches in bottom-up methods are solid-state methods, liquid state synthesis, gas
phase, biological, microfluidic-based, and other methods. In all methods, building
blocks are generated at first, followed by the assembling of building blocks to form
NPs of desired size and shape [34].
3.1 Solid-State Methods
Here, the material is deposited on a surface in the form of thin-film or as a nanomaterial. Two types of deposition approaches exist; physical vapour deposition (PVD) and
chemical vapour deposition (CVD). In PVD, techniques like laser ablation is used,
which further causes the formation of plasma of ablated target, and they are deposited
on a substrate to produce a thin film. MNPs are deposited on carbon nanotubes in this
manner. Even though the method is simple, it is unable to provide a higher volume of
material, and the procedure is expensive. The chemical reaction of gaseous molecules
of atoms to form the thin film is utilized in depositing thin films on the substrate in
CVD. Three types of CVD are used; thermally active chemical vapour deposition
(TACVD), plasma-enhanced chemical vapour deposition (PECVD), and photoinitiated chemical vapour deposition (PICVD). In the form of volatile molecules, the
target material is deposited on the substrate. In the case of temperature-sensitive
material such as polymer substrate, TACVD is not applicable. For PECVD, plasma
is generated inside the void chamber with the help of microwave and inductively
induced electric current. Due to the requirement of specific operating conditions,
PECVD suffers from scale-up issues. For PICVD, low energy treatment, as well as a
Fig. 3 Overview and comparison of synthesis of metallic nanoparticles; top-down method and
bottom-up method. Redrawn from [34]