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1. Introduction

Pathogenic bacteria are abundant in the environment. They spread quickly and can easily cause adverse reactions, long-lasting health effects, or even death. Infection originating from the invasion of pathogens into the body is an acute threat to humans, causing diseases, such as pneumonia, gastritis, and sepsis, which can lead to tissue damage, organ failure, and death [1]. Antibiotics have immense benefits in the fight against a diverse range of pathogens. However, mutations are one strategy that bacteria employ to enhance their resistance to antibiotics, leading to the advent of a large number of multidrug-resistant (MDR) strains, which markedly lowers the therapeutic ability of antibiotics. There is increasing concern regarding the generation of antibiotic resistance, as bacteria vigorously persist to emerge with flexible countersteps against conventional antibiotics [2]. This is one of the most notable health-related matters of the 21st century [3]. Infectious diseases caused by MDR bacteria and the abundance of these bacteria have increased at an alarming rate, especially penicillin-resistant Streptococcus pneumoniae, methicillin-resistant Staphylococcus aureus, vancomycin-resistant Enterococcus faecium, ceftazidime-resistant Klebsiella pneumonia and Escherichia coli, fluoroquinolone-resistant Pseudomonas aeruginosa, and multi-antibiotic resistant Acinetobacter baumannii [4,5]. In addition, various foodborne pathogens associated with Gram-positive bacteria, such as Bacillus cereus, Campylobacter jejuni, Clostridium perfringens, Clostridium botulinum, Cronobacter sakazakii, E. coli, Listeria monocytogenes, Salmonella enteritidis, Shigella dysenteriae, S. aureus, Vibrio furnissii, and Yersinia enterocolitica, are causing a large number of diseases, with major effects on public health and safety [6].

The production of new antibiotics requires enormous economic and labor demands and is a time-consuming process. Hence, the development of alternative, unconventional strategies to treat infectious diseases has become highly advisable [7]. The combination of one antimicrobial agent with other antimicrobial agents has many advantages, such as increased biological activity, reduced adverse effects, and increased antimicrobial toxicity of the combined elements. In synergism, one antimicrobial agent influences the activity of the other, and they finally act more efficiently and effectively together due to their different mechanisms of individual action. This may be considered a new approach to solve the problem of bacterial resistance and reduced antimicrobial susceptibility. Thus far, many alternative strategies have been developed to combat MDR bacteria. Among them, several in vitro studies have confirmed the significant antimicrobial activities of combinations of essential oils/plant extracts, conventional antibiotics/plant extracts, and phytochemicals/antibiotics [8].

Nanotechnology has important commercial applications in the fields of biology and medicine, particularly in the areas of drug delivery, diagnosis, tissue engineering, imaging, and bacterial infections [9]. Nanomaterials have special properties owing to their small dimensions; electrical, mechanical, optical, and magnetic properties; thermal stability; and high surface-to-volume ratio [10]. Nanomaterials are considered favorable alternatives to antibiotics for controlling bacterial infections due to a diverse range of factors, such as their size, morphology, surface charge, stability, and concentration in the growth medium. The surface coatings of nanoparticles (NPs) play an important role in influencing the antimicrobial properties of nanomaterials [11,12]. Depending on the number of metals, metal and metal oxide NPs are divided into mono-, bi-, tri-, and quadrometallic types. Among these, bi-, tri-, and multimetallic NPs have attracted the greatest interest due to their enhanced catalytic properties and favorable characteristics compared with monometallic NPs [13]. Multimetallic NPs are novel materials that incorporate two or more metals to make alloys with different functionalities and tunable properties, such as catalytic and optical properties. Multimetallic NPs can be modified by controlling their structure, chemical composition, and morphology to achieve maximum synergistic performance [14]. The addition of one or more metals into the NPs is expected to bring combinatorial effects, such as alteration of the electron structure, deduction of the lattice distance, and improvements in the total electronic charge shift [15]. Therefore, the study of the multidimensional space is warranted.

Monometallic NPs possess only one type of metal with specific chemical and physical properties, such as Ag, Au, Zn, Pd, Cu, Ti, Si, Al, Se, and Mg, which have been used for their antimicrobial activity for centuries. Among these, Ag NPs are the most effective as they are able to kill both Gram-positive and Gram-negative bacteria, and they are even effective against drug-resistant species [16]. Moreover, metal oxide NPs, such as Ag₂O, ZnO, CuO, TiO₂, NiO, Fe₃O₄, α-Fe₂O₃, CaO, MgO, Al₂O₃, CeO₂, Mn₃O₄, and ZrO₂ NPs, have highly potent antibacterial effects against a wide spectrum of microorganisms [17]. Similarly, metal sulfide and metal–organic framework (MOF) nanomaterials, such as AgS-, FeS-, CdS-, and ZnS-MOFs and Mn-, Cu-, and Zn-based MOFs, have demonstrated antimicrobial activities [18]. Bimetallic NPs are formed via the integration of two different types of metal atoms to form a single nanometric material with varying structures, morphologies, and properties [19]. Bimetallic NPs can be tuned by selecting the appropriate metal precursors to achieve the desired shape, size, structure, and morphology according to the configuration of atoms, and this finally leads to the formation of alloy, core–shell, and aggregated nanoparticle types [13]. Bimetallic NPs, such as Ag/Au, Ag/Cu, Au/Pt, Au/Pd, Ag/Fe, Fe/Pt, Cu/Zn, Cu/Ni, Au/CuS, and Fe₃S₄/Ag NPs, have distinctive surface activities. In addition, bimetallic oxide NPs, such as MgO/ZnO, CuO/ZnO, and Fe₃O₄/ZnO NPs, due to tensile strain and synergism between the constituent metals, often exhibit unique antibacterial performance.

Similarly, trimetallic NPs are made from three different metals for lowering metal consumption, atomic ordering, and fine-tuning the size and morphology of these NPs. Trimetallic NPs exhibit higher catalytic selectivity/activity and efficiency in various applications, such as biomedical, antimicrobial, catalytic, active food packaging, and sensing applications. Moreover, owing to the presence of three metals, there are some possibilities for different structures and morphologies, such as core–shell, mixed structure, subcluster segregated, and multishell [20]. To change their catalytic performance, trimetallic NPs were further designated as alloys and intermetallic NPs by altering the atomic distribution and surface compositions of different metals [21]. Trimetallic NPs exhibit innovative physicochemical properties owing to their synergistic or multifunctional effects for diverse potential applications when compared with monometallic and bimetallic nanomaterials. However, to date, only monometallic, bimetallic, and very few trimetallic NPs have been reported for their antimicrobial effects.

2. Multimetallic NPs

Multimetallic NPs, comprising two or more different metals to form alloy or core–shell nanocomposites, have attracted considerable attention as novel materials due to their unique functionalities. The combined action of different metals and metal oxides in a chemical transformation enhances the catalytic performance of multimetallic NPs [24]. Multimetallic NPs with binary, ternary, and quaternary combinations usually have special characteristics, with enhanced chemical, optical, and catalytic properties when compared with mono- and bimetallic NPs, because of the synergistic effects between different metals [14]. Metallic NPs are classified as monometallic, bimetallic, trimetallic, quadrometallic, and so on based on the number of metallic ingredients.

2.1 Monometallic NPs

As the name suggests, monometallic NPs compose a single metal species, which compels for the catalytic characteristics of the nanoparticle. Based on the type of metal atom and properties, monometallic NPs are in different forms, like metallic, magnetic, transition metal, and oxide. They can be synthesized by chemical reduction and green synthetic methods, and their structure can be stabilized by various functional groups. Many studies have reported on a wide range of monometallic and metal oxide NPs, and these are used in various catalytic, medical, agricultural, active food packaging, nano-biosensor construction, industrial, and environmental applications. The order of atoms at the nanoscale, which differs from the bulk materials, is due to not only the large surface-area-to-volume ratio but also the specific electronic structure, plasmon excitation, and quantum confinement. In addition, the increased number of kinks, short-range ordering, chemical properties, and ability to store excess electrons also enhance activity. Recent studies on the antimicrobial activity of monometallic and metal oxide NPs are summarized in Table 1, highlighting the size, bacterial strains tested, mode of action, and fabrication techniques used.

Table 1. Antimicrobial activity of monometallic and metal oxide nanoparticles (NPs).

2.2 Bimetallic NPs

Bimetallic NPs have attracted huge attention due to their modified properties, and they can be prepared in different sizes, shapes, and structure with a combination of different metals.

Extensive studies in the past decade investigated the use of bimetallic NPs as a new advancement in the field of research and as technological domains to increase efficiency. Owing to the distinct catalytic and synergistic properties between two different metals, bimetallic NPs have potential applications in more fields than their corresponding monometallic one. Depending on their physical and chemical interactions, the spatial overlapping and distribution of two atoms can lead to the formation of a core–shell or simply an alloy due to the impact of individual metals [60]. The addition of a second metal is a major technique for tuning the geometric and electronic structures of NPs to increase their catalytic activity and selectivity. The size, shape, and morphology of alloy or core–shell NPs are comparatively different from those of the individual metals, thereby creating novel opportunities for a range of biomedical applications [61]. The antimicrobial activity of bimetallic NPs has been assessed against numerous types of pathogenic bacteria, especially E. coli, P. aeruginosa, and S. mutans, which are mainly responsible for human epidemics. Bimetallic NPs exhibit remarkable performance compared with commonly used antibiotics and other antimicrobial treatments, as pathogens cannot develop resistance to them because they suppress the generation of biofilms and accelerate other correlated processes [62].

2.3 Trimetallic NPs

Trimetallic NPs have favorable properties, such as physical, chemical, and tunable properties, when compared with mono- and bimetallic NPs, which result in multiple applications for these NPs. These favorable properties are due to multifunctional or synergistic effects produced by the three metals present in the same system [20]. The addition of a third metal or metal oxide into the composite supposedly generates a combinatorial effect and introduces several possibilities for different morphologies, structures, and chemical compositions to improve catalytic activity, selectivity, and specific performance [63]. Trimetallic NPs exhibit improved reactivity because of the electronic and synergistic effects of the different elements and the geometric arrangements of the metal surrounding the absorbing atoms. At this coherence, the properties of the materials are altered due to electron transfer effects, lattice mismatching, and surface rearrangement [64]. Consequently, trimetallic NPs, such as Fe/Co/Ni supported on multiwalled carbon nanotubes (MWCNTs), show efficient and enhanced bifunctional performance for the oxygen reduction and oxygen evolution reactions [65]. Similarly, Cu/Au/Pt, with high catalytic activity and excellent killing performance for biosensing and cancer theranostics [66], and Pd/Cu/Au, as excellent temperature sensors and fluorescence detectors of H₂O₂ and glucose [67], have attracted attention as promising catalysts. Hence, trimetallic NPs exhibit potent antibacterial activity and have been found to be more effective agents than bimetallic and monometallic NPs, even at lower metal concentrations.

2.4. Quadrometallic NPs

Higher-order quadrometallic NPs are made from four different metals for various applications with different fabrication methods. Recently, solid-state dewetting of Ag/Pt/Au/Pd quadrometallic NPs on sapphire has been prepared successfully with tunable localized surface plasmon resonance [68]. Similarly, Ag/Cu/Pt/Pd quadrometallic NPs prepared by seed-mediated growth, where small Pt and Pd NPs were attached on the surface of AgCu Janus bimetallic NPs as seeds in an aqueous solution [69], and heterostructured Pt/Pd/Rh/Au tetrahexahedral multimetallic NPs were synthesized through alloying/dealloying with Bi in a tube furnace [70]. Moreover, to date, there has been no investigation into the antimicrobial properties of quadrometallic and multimetallic NPs. Table 2 summarizes bi- and trimetallic NPs used for antibacterial activity with an array of sizes, strains tested, mechanisms, and synthetic methods.

Table 2. Antimicrobial activity of bimetallic and trimetallic NPs.

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This entry is adapted from the peer-reviewed paper 10.3390/molecules26040912