Greener synthesis and medical applications of metal oxide nanoparticles

Abstract

Over the past decade, the subject of “greener chemistry" and chemical processes has been emphasized. The “greener chemistry” improves environmental efficiency in reducing the consumption of resources and energy and achieving a stable economic development of the environment. Nanotechnology is investigating nanoscale materials that have applications in the area of biotechnology and nanomedicine alongside several other significant applications such as cosmetics, drug delivery, and biosensors. The different shapes and sizes of nanoparticles can be synthesized with physical, chemical, or biological methods. The tendency to produce nanomaterials, especially metal oxides, and use them, is increasing because of their exciting properties in the nanoscale. However, metal oxide nanoparticles produced by chemical methods have significant concerns due to hazardous and toxic chemicals and their environmental damage. The production of metal oxide nanoparticles using the principles of greener chemistry has found a special place in research. Increased awareness of greener chemistry and biological processes has necessitated using environmentally friendly methods for the production of non-toxic nanomaterials. Plants and polymeric materials as renewable and inexpensive sources have received particular attention to prepare nano biomaterials. The use of plants to synthesize metal oxide nanoparticles because of the non-use toxic pollutants is one of the environmentally friendly methods, and that's why this type of synthesis is called greener synthesis. In this review, we exhibit a total sight of greener synthesis methods for producing metal oxide nanoparticles and their medical applications.

Introduction

Science and technology are moving at a high rate for emancipating greener nanotechnology. Nanotechnology is one of the most exciting topics used to create and use materials with interatomic structural properties. In terms of nanotechnology indicators, the nanoparticles range in size from 1 to 100 nm and show dimensions on a scale of one billionth (10⁻⁹) of a meter [[1], [2], [3]]. Nanoparticles are the most common materials in science and technology, and their remarkable properties have led to a variety of applications in the chemical [4], medical [5], pharmaceutical [6], electronic [7], agricultural [8,9], and chemical conversion catalysts [10]. Among the types of nanoparticles, metallic nanoparticles are in the range of 10–1000 nm with the maximum surface area attribute relative to volume. This feature provides ease of contact with other particles on a large scale [11]. The integration of science and engineering based on an intermediate pattern has created a frontier common nanotechnology and greener nanotechnology. Therefore, nanoparticles offer completely new or developed properties based on specific features such as distribution, size, and morphology [12]. Conventional methods in the synthesis of nanoparticles require the use of reducing agents [13], stabilizers, high-temperature cycles [14], and expensive manufacturing costs [15], which are problems of these methods. Also, the use of chemical agents can be considered a potential hazard to human health and the environment. In the last two decades, studies on the greener synthesis of nanoparticles called “Greener Chemistry” in early 1991 have been named by Anastas, a member of the U.S. Environmental Protection Agency (EPA), to minimize environmental damage in the preparation and production of human-required chemicals [16]. The most important expressions of greener chemistry are (i) the innovation, design, and use of chemical products and (ii) procedures to decrease or remove the use and production of hazardous materials [17]. Furthermore, they can have used as optimal nanoparticles with advanced catalytic activity for specific applications. One of these nanoparticles is the biosynthesis of metal oxide nanoparticles (MONPs) are used for greener synthesis without the use of strict, noxious, and expensive chemicals [18,19]. The MONPs and metal are rapidly being developed for use in research centers and human health applications. They have been noted not only for their wide variety of physical and chemical properties but also for their antibacterial activity [20]. The nanoparticle production technologies are diverse and follow the two conventional approaches, including “Top-Down” [21] and “Bottom-Up” [22] in three subcategories: physical, chemical, and biological. The bottom-up method unlike the top-down method leads to the formation of nanoparticles through the “nucleation” mechanism [23]. Despite the widespread use of chemical and physical techniques for the synthesis of nanoparticles, they are not only costly but also harmful to the environment and organisms. Widespread exploitation of the resulting products, requires high energy consumption, the use of harmful organic solvents, the manufacture of new disposal intermediates, and ultimately biological hazards and environmental pollution [24]. Other synthesis methods for MONPs are summarized in (Fig. 1). Factors affecting these processes are metal composition (preventing uncontrolled particle growth), solvent (particle mass avoidance), reducing (particle shape control) and stabilization (solubility in different solvents). These materials act by modifying the rate and mechanism of the electrochemical reaction effective in flame retardant and sono-electrochemical physicochemical processes [25,26].

To counter the limitations of the methods outlined, much attention has been paid to the “greener synthesis” from the subgroup bottom-up approach in materials science and technology to develop environmentally friendly products [27]. Biosynthesis or greener synthesis of nanoparticles is an eco-friendly route in the field of chemistry and biotechnology that is increasingly popular and can be considered as a solution to the diagnosis and treatment, global pollution, and toxicity problems [28,29]. Synthesis of many nanoparticles has so far been reported using the bio-synthesis approach using plant extract [30], fruit extract [31], algae [32], enzymes [33], cyanobacteria [34], fungi [34,35] and bacteria [36,37]. The key merits of “greener synthesis” such as minimizing waste, reducing pollution, and using standard solvent systems and natural resources (such as organic systems) as well as renewable raw materials can be explained. Therefore, nanomaterials' greener synthesis leads to increasing efficiency and environmental adaptability through direct regulation, control, and repair process [23]. Biological precursors alter different reaction parameters such as temperature, pressure, solvent, and pH conditions in greener synthesis methods. Therefore, plant biodiversity due to availability, large herbal extracts accompanies a simple and easy process for producing large-scale MONPs known as biogenic nanoparticles. Also, metal salts can be reduced to metallic nanoparticles through various plant extracts containing phytochemicals (e.g., leaves such as ketones, aldehydes, phenols, amides, terpenoids, and ascorbic acids) [38].

The MONPs are used as nanoparticles with many benefits such as efficient, easily synthesizable, modifiable, and catalytic due to their porous structure with high thermal/chemical stability and large surface areas (>100 m²g^-1). The MONPs as nanomaterials with useful capabilities and unique properties have been used in different applications such as optics [39], electronics [40,41], hyperthermia treatment [42,43], catalyst [44], Magnetic Resonance Imaging (MRI) [45,46], magnetic targeted drug delivery [47], and cell & nucleic acid separations [[48], [49], [50]]. Briefly, it illustrates the applications of MONPs in Fig. 2.

Magnetic MONPs that are biologically functional have potential and exciting applications in the biomedical fields [51],e.g. in hyperthermia process and cancer treatments due to their specific absorption rate and intrinsic loss power [52]. In addition to functional studies and toxicity studies of MONPs, the physicochemical analysis should also be performed. In recent years, researchers have divided the groups into inert and reactive materials because of the different capacities of MONPs (50), which explains nanoparticles' properties due to toxicity [53]. Therefore, MONPs should be designed and manufactured only with the desired material properties that do not contain any toxins. The MONPs include silica (51, 52), alumina (52), zirconia (53, 54), titania (55), ceria (56, 57), nickel oxide [54], zinc oxide [55], silver oxide [56], manganese oxide [57], magnesium oxide [58], calcium oxide [59], lead oxide [60,61], cobalt oxide [62], iron oxide [63,64], selenium oxide [65], and etc. Therefore, in this review, we attempt to describe recent explorations, while explaining each of the MONPs in terms of greener synthesis methods, their physicochemical properties, and their functional potentials have been discussed. It is hoped that the present collection will provide researchers with the necessary insights and clarity.