Recent development in the green synthesis of titanium dioxide nanoparticles using plant-based biomolecules for environmental and antimicrobial applications

Abstract

Nanoparticles (NPs) have properties for potential applications in different areas but their conventional production processes are often linked to secondary health and environmental impacts. Aggressive research endeavors are being undertaken to identify greener NPs synthesis strategies. Green synthesis is an evolving area with the objective of an efficient and environmental friendly development of NPs. Biological substances including bacteria, fungi, yeast, algae, and plant have been demonstrated to be applicable for the synthesis of TiO₂ NPs. This work is dedicated to phyto-based biosynthesis of TiO₂ NPs. Biomolecules in plants such as terpenoids, flavones, ketones, aldehydes, proteins, amino acids, vitamins, alkaloids, tannins, phenolics, saponins, and polysaccharides can play vital roles as a reducing, capping and stabilizing agent in the formation of TiO₂ NPs. A systemic comparison is made with the focus on the bioreduction ability of different plant extracts to produce specific TiO₂. Important results of applicable instrumentation techniques for characterizing the TiO₂ NPs are also elucidated to identify the superior qualities of the NPs. Moreover, photocatalytic and antimicrobial applications of the phytosynthesized TiO₂ NPs are also critically discussed. The main scopes are the current status of plant- assisted biosynthesis of TiO₂ NPs and future prospects of this promising area of research.

Introduction

NPs are usually defined as particles with sizes in the range of 1–100 nm. These particles are remarkable because they usually exhibit unique physical and chemical properties that are substantially different compared to their macro range particles [1]. Different shapes and extremely small size of nanomaterials can influence their physical, chemical and optical properties such as surface area, solubility, melting point, dielectric constant etc. Among the unique features on nano scale particles is the decrease in the melting point. The particles’ optical band gap usually increases when their sizes are reduced. NPs can also emit multiple colors that rely on the absorption of certain wavelengths by NPs of different sizes. Based on the amount of NPs generated annually in the industrial sector, metal oxide NPs are the most widely used [2]. Out of three most frequently produced NPs i.e., TiO₂, ZnO, and SiO₂, two of them are indeed metal oxides.

Among all metal oxides, TiO₂ NPs are of the most important scientific concern in photocatalytic, antimicrobial and antibacterial successful applications based on their favorable properties [3], [4], [5], [6], [7], [8], [9], [10]. Photocatalytic wastewater treatment based on nano-sized TiO₂ is a highly effective process for degrading and detoxifying recalcitrant organic and inorganic pollutants from the wastewater [11], [12]. TiO₂ NP's are primarily used as a semiconductor material based on its important characteristics such as low cost, strong oxidizing strength, high chemical stability, high refractive index and the presence of oxygen vacancies in its lattice. The world's production of TiO₂ had exceeded 10,000 tons per year in 2011 [13]. TiO₂ NPs have a wide range of applications like as photocatalysts [14], sensors [15] and antimicrobial agents [16]. They can also be used for the decomposition of different microorganisms such as bacteria and viruses as well as cancer cells. They are also critical components in ultraviolet light resistant oxides, toothpastes, papers, food colorants, paints, plastics and inks. TiO₂ NP's are the best harvesters of sunlight and can typically absorb 3–4 percent of solar energy. As such, they are known as effective photocatalysts for the synthesis of hydrogen while also beneficial for decomposition of the toxic organic compounds in water [17].

Generally, three different methods are used to synthesize NPs i.e., physical, chemical, and biological methods. Physical methods include thermal decomposition, laser irradiation and electrolysis. For instance, the synthesis process is usually performed at very high temperatures in the thermal decomposition method. Physical methods are usually more energy-intensive and costly to prepare nanoparticles as they often involve the use of expensive vacuum systems and their supporting equipment [18]. The chemical reduction approach using chemicals such as sodium borohydride or sodium citrate as reducing agents is, by far, the most commonly used method for the chemical synthesis of NPs.

The chemical synthesis of NPs usually involves the use of costly, toxic and harmful chemicals under certain conditions and their exposure to the environment could give rise to major eco-toxicological issues and limits their applications in clinical field [19]. Moreover, many researchers have shown that metal oxide NPs with desired properties, which are often toxic to the test organism, had been successfully synthesized using non-green chemical methods [20], [21], [22], [23]. To mitigate the toxicity of these NPs, greener synthesis approaches have been explored recently [24]. The use of different biomolecules in this area is developing very rapidly due to the ease of NPs formation and some potential results reported. This intensive research area contributes to the emergence of various effective biological synthesis pathways for the NPs production [25]. This is supported by the growing number of publications from 2005 to 2020 as presented in Fig. 1.

The green synthesis of metal/metal-oxide nanoparticles (NPs) has attracted considerable attention to be explored for the potential application in the field of bio-based nanotechnology. A quick, environmental-friendly and less toxic way to synthesize NPs from biodegradable products such as plant extracts, microbes and enzymes is a motivation of many material scientists [18], [26]. Biodegradables such as enzymes, vitamins and proteins, as well as other pure and naturally occurring materials have been reported as effective reagent for NPs’ synthesis as shown in Fig. 2. In comparison to conventional industrial manufacturing process, synthesis methods using plant extracts seem to have advantages, in terms of their handling of reagents and process safety [27]. The fact that many of the recent works concerning the NPs synthesis have described plant extracts as the key reagents to be the prospect of the future. Phytochemicals are generally thought to effectively produce NPs, although the precise mechanism is yet to be concretely explained. Some phytochemicals can help initialize the formation of NP cores when the NP colloidal suspension develops. The phytochemicals could act as capping agents in order to stabilize the forming NPs. In addition to the formation of phyto-nano blends by mixing the bioactive agents with prefabricated NPs, the biologically active phytochemicals can also be loaded onto the NP surfaces to generate functionalized NPs in a simple single pot synthesis [28], [29].

In comparison to more conventional nanoparticles, there are still barriers to overcome. The control over sizes and shapes of greenly produced NPs has been the main limitation. The shapes and sizes of different phytochemical compositions with predefined molecular dimensions found in the plant are usually predetermined. Moreover, dissimilarities in the active components can even be caused by the composition of similar plants grown in various geographical areas or harvested in different seasons. This would, in turn, influence the size and shape of the precipitated NPs. These would also lead to a decline in their market value as commercial nanoparticles are usually perfectly suited. Thus, it is sometimes even more difficult to find the right applications and markets for phyto-based NPs. In addition, plant extracts contain an abundance of active ingredients compared to the preparation solutions used in chemical methods. Nevertheless, the positive aspects are deemed to greater than the negative aspects, which further encourage researchers to optimize their process for phyto-based NPs production.