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Bimetallic nanomaterials (BMNs) are one kind of innovative nanomaterials, referring to nano-bimetallic alloy, intermetallic compounds, or the combination of two kinds of metallic nanoparticles. Compared with monometallic nanomaterials, BMNs perform similar or even better physical and chemical properties in the medical field. BMNs possess excellent physical and chemical properties, such as easy surface modification, superior photothermal properties, multiple catalytic properties, delicate sensitivity, and good stability. Synthesis methods of bimetallic nanomaterials. The preparation methods of BMNs commonly used for cancer therapy, such as co-reduction method, hydrothermal method, seed-mediated growth method, and electrodeposition method.
Generally, doping with the second metal is considered to possess superior performance than monometallic nanomaterials (MMNs), due to the addition of more active sites by the formation of metal polar bonds and irregular arrangement.[1][2] At the same time, the combination of the two metals is more likely to form complex structures with enhanced surface plasmon resonance (SPR) effects such as hollow structures, porous structures, and core-shell structures.[3] Therefore, regulating the optical, electrical, chemical, and biological properties by controlling their size, shape, and composition during synthesis is particularly important in the study of monometallic nanomaterials (BMNs).[4] In order to achieve the design and effects, many methods have been extensively tested in the past decades. Among them, there is an important problem that the ratio of the two metals and the shape of BMNs are difficult to control.[5] BMNs with controlled composition can be synthesized by co-reduction method and hydrothermal method, and their shapes are mostly spherical and massive. BMNs synthesized by seed-mediated method usually perform complex shapes, while the BMNs obtained by electrodeposition are mostly nanowires and nanofilms. In order to obtain suitable BMNs for cancer therapy, a systematic understanding of the synthesis methods is required.
Co-reduction is one of the most straightforward and convenient methods to prepare BMNs due to its simple operation, low cost, and short reaction time. Co-reduction, also known as the one-pot method, is usually used when two precursors containing metal elements are mixed, and the metal ions are reduced to form alloys or intermetallic compounds.[6][7] In addition, the BMNs’ morphology and structure can be tailored by the reaction temperature, the addition of surfactants, the reducing agent, and the nature of coordination ligand. Many common BMNs such as gold/silver core-shell structures, gold/silver nanowires, and gold/platinum nanoparticles (NPs) are synthesized by this method. More sophisticated nanomaterials with special shapes and structures can be synthesized by co-reduction in combination with other methods.[8]
Hydrothermal method is another widely used method, which is similar to the co-reduction method. The metal precursors are promoted to decompose and reduce after heating. The hydrothermal method is generally applicable to reactions with lower reduction potentials that are not easily reduced directly. Up to date, some BMNs synthesized from metal precursors with lower reduction potentials have been prepared with this method, such as CuNi,[9][10] Ni-Fe,[11] CoNi,[12] and NiRe.[13] Besides, the hydrothermal method is not only suitable for metal precursors with low reducibility but also widely used in the synthesis of many BMNs (PtCu, PtPd, et al., for instance).[14][15] In a word, the hydrothermal method is simple to operate and easy to synthesize large quantities of BMNs.
Seed-mediated growth has been proven an effective method for synthesizing plasmonic noble metal nanocrystals.[16] This method has been applied in the synthesis of BMNs because the prepared nanocrystals have well-defined morphology, size, and surface composition. Typically, seed-mediated methods play an important role in the preparation of anisotropic metal structures and hierarchical epitaxial core/shell structures.[17][18][19][20] Seed-mediated growth method has emerged in the field of synthesizing BMNs due to simple operation, strong repeatability, and high yield. In particular, the shapes and components of BMNs can be accurately regulated by the seed-mediated method, which is difficult to obtain by other methods. It is believed that this method will be more extensively used in the future.
BMNs exhibit various and complex morphologies based on spatial arrangements, which are related to abundant physicochemical properties. Common nanomaterials used in disease treatment are mainly manifested as nanospheres, nanorods, nanoclusters, nanostars, and nanoflowers, which have large surface areas and unique optical properties. BMNs with specific morphology and structure can be obtained by different synthesis methods for the treatment of tumor.
In summary, many complex and diverse morphologies can be exhibited in BMNs that indicate the suitable characteristics for tumor therapy, such as good biocompatibility, drug-loading capacity, photothermal and catalytic properties. BMNs have a broad development prospect in tumor therapy by virtue of these abundant morphologies. BMNs have been widely used in biomedical fields, especially in biosensing and imaging, due to their unique catalytic and optical properties. In addition, many studies have shown that BMNs are also promising nanotechnology in cancer therapy.
However, it is still challenging to find suitable synthetic methods from many methods that can be used to produce BMNs for cancer therapy. It has been found that BMNs with zero dimension, such as spheres, rods, stars, and flowers, are usually used in cancer treatment. These structures are usually synthesized by seed-mediated and hydrothermal methods, where the regulation of BMNs' morphology has been well studied. In addition, it is also suggested that the growth pattern and mechanisms of all BMNs can be thoroughly explored by molecular simulation to establish a material library so that nanostructures with specific properties and shapes can be designed more clearly to reply to complex environments and various therapeutic applications.