32
K. Illath et al.
Synthesis of NPs can be classified into two; top-down and bottom-up approaches.
In the top-down method, bulk materials are etched to produce NPs, whereas, in a
bottom-up approach, NPs are formed from the atoms or molecules through nucleation, aggregation, growth, and crystallization processes. Nucleation is the process
during which nuclei (seed) from the parent is formed to produce crystals. It may
happen either homogeneously or heterogeneously in the solution. Several theories
explain nanoparticle formation; classical nucleation theory (CNT), LaMer’s nucleation and growth mechanism, two-step nucleation, and pre-nucleation cluster mechanism [12–16]. As per CNT, nucleation starts from the nuclei, which are formed
from the monomers within the bulk liquid as a new phase, and they are considered as the building blocks of NPs. These nuclei aggregate together to form critical
nucleation of a new phase. According to LaMer’s concept, nucleation and growth
processes can be separated in time, as shown in Fig. 1a. A prenucleation process
happens within a short time to produce nuclei needed for monodispersity. Once the
nuclei are formed, nucleation starts and continues until the concentration reaches
critical limiting supersaturation. Soon after that, the growth process begins. During
this process, nuclei monomers are consumed, and gradually, their concentration
decreases. The growth process can be either a diffusion-limited or reaction-controlled
growth process. Another theory explains NP formation through two-step nucleation
and growth mechanism. The two-step nucleation mechanism starts before nucleation, generally in the existence of amorphous metastable clusters or disordered
liquid in homogenous solution [17]. During the first step, nuclei are formed directly
from the precursor as a result of a pseudo-first-order reaction. In the second step,
autocatalytic growth of nuclei happens and results in the formation of the particle
[17, 18]. Another theory is the prenucleation cluster mechanism, which says that
prenuclei (small solutes in their lowest kinetic energy) are formed from the chemical
species of atoms or molecules and are considered to be initiated during the phase
transition [17]. Polte et al. studied the mechanism of Au NPs formation using in situ
small-angle X-ray scattering at millisecond resolution [11]. Figure 1b illustrates the
result of the study. Au NPs were synthesized using sodium borohydride (NaBH 4 )
as a reducing agent through the chemical reduction method (explain later). Initially,
the gold precursor has reduced into nuclei Au (0), which is very fast, usually within
200 ms nuclei are formed. It is followed by the coalescence of nuclei into bigger
particles, as illustrated.
1.3 Metallic Nanoparticles
NPs synthesized from the metallic precursors are known as MNPs [5]. Among MNPs,
Au NPs have been studied in detail across the research sectors and have provided
new insights, especially in biomedical applications. Gold is a material that is considered as a quintessential noble metal, and its NPs exhibit unique properties such as
non-toxicity, ease of functionalization, physicochemical, catalytic, and biological
K. Illath et al.
Synthesis of NPs can be classified into two; top-down and bottom-up approaches.
In the top-down method, bulk materials are etched to produce NPs, whereas, in a
bottom-up approach, NPs are formed from the atoms or molecules through nucleation, aggregation, growth, and crystallization processes. Nucleation is the process
during which nuclei (seed) from the parent is formed to produce crystals. It may
happen either homogeneously or heterogeneously in the solution. Several theories
explain nanoparticle formation; classical nucleation theory (CNT), LaMer’s nucleation and growth mechanism, two-step nucleation, and pre-nucleation cluster mechanism [12–16]. As per CNT, nucleation starts from the nuclei, which are formed
from the monomers within the bulk liquid as a new phase, and they are considered as the building blocks of NPs. These nuclei aggregate together to form critical
nucleation of a new phase. According to LaMer’s concept, nucleation and growth
processes can be separated in time, as shown in Fig. 1a. A prenucleation process
happens within a short time to produce nuclei needed for monodispersity. Once the
nuclei are formed, nucleation starts and continues until the concentration reaches
critical limiting supersaturation. Soon after that, the growth process begins. During
this process, nuclei monomers are consumed, and gradually, their concentration
decreases. The growth process can be either a diffusion-limited or reaction-controlled
growth process. Another theory explains NP formation through two-step nucleation
and growth mechanism. The two-step nucleation mechanism starts before nucleation, generally in the existence of amorphous metastable clusters or disordered
liquid in homogenous solution [17]. During the first step, nuclei are formed directly
from the precursor as a result of a pseudo-first-order reaction. In the second step,
autocatalytic growth of nuclei happens and results in the formation of the particle
[17, 18]. Another theory is the prenucleation cluster mechanism, which says that
prenuclei (small solutes in their lowest kinetic energy) are formed from the chemical
species of atoms or molecules and are considered to be initiated during the phase
transition [17]. Polte et al. studied the mechanism of Au NPs formation using in situ
small-angle X-ray scattering at millisecond resolution [11]. Figure 1b illustrates the
result of the study. Au NPs were synthesized using sodium borohydride (NaBH 4 )
as a reducing agent through the chemical reduction method (explain later). Initially,
the gold precursor has reduced into nuclei Au (0), which is very fast, usually within
200 ms nuclei are formed. It is followed by the coalescence of nuclei into bigger
particles, as illustrated.
1.3 Metallic Nanoparticles
NPs synthesized from the metallic precursors are known as MNPs [5]. Among MNPs,
Au NPs have been studied in detail across the research sectors and have provided
new insights, especially in biomedical applications. Gold is a material that is considered as a quintessential noble metal, and its NPs exhibit unique properties such as
non-toxicity, ease of functionalization, physicochemical, catalytic, and biological