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Li et al. Microstructures 2023;3:2023024 https://dx.doi.org/10.20517/microstructures.2023.09 Page 3 of 20

Figure 1. Structures of MNCs of Ag Pt and Ag Pt [22] . Copyright 2018, American Chemical Society.
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method for preparing MNCs. Next, we review how MNCs can enhance the PCR process as catalysts. We
then classify the MNCs into two major categories: noble and nonnoble MNCs and summarize their
advantages for PCR applications. Finally, we discuss the current challenges and prospects of MNCs for PCR.
We hope this review will stimulate further research and innovation in this emerging field.

ADVANTAGES OF MNCS AND COMMON SYNTHESIS METHODS
Advantages of MNCs
MNCs show a very different energy level structure in comparison with large-size nanoparticles (as shown in
Figure 2). MNCs change the electron energy level near the Fermi level from quasi-continuous to discrete,
resulting in energy level splitting or energy gap widening [32,33] . The discrete energy levels allow the electrons
in MNCs to jump between energy levels and interact with light, which enhances the separation of electrons
and holes . Additionally, MNCs also have a small energy gap and absorb light in the visible and near-
[34]
infrared regions [35,36] . Furthermore, MNCs are stable against oxidation or reduction by photogenerated
electrons or holes [37,38] . More importantly, MNCs have a high specific surface area, a high fraction of low-
coordinated atoms, a quantum size effect, a tunable composition, and a unique surface structure (e.g.,
pocket-like sites) due to their ultra-small size. These unique features of MNCs make them an emerging class
of photocatalytic materials with increasing interest . Moreover, their precise atomic-level structures enable
[39]
fundamental research on the photocatalytic mechanisms, which provides theoretical guidance for the design
of new photocatalytic materials and improves catalytic performance .
[40]

The synthesis methods of MNCs
There has been much research into the synthesis of MNCs, so we have summarized the commonly reported
synthesis methods in the literature as follows :
[41]
[42]
(1) Reduction growth method (as shown in Figure 3 a→c): The metal core is created by reducing the
respective metal ions. Therefore, the strength of the chosen reducing agent and the kinetic regulation of the
reduction process are crucial for synthesizing the desired product.

(2) Seed growth method (as shown in Figure 3 c→d): Smaller-sized MNCs are used as seeds to induce
[43]
nucleation, resulting in the growth of larger-sized MNCs over time. The regulation of the crystal growth
process is noteworthy.