An overview of the plant-mediated green synthesis of noble metal nanoparticles for antibacterial applications

Abstract

Metallic nanoparticles (MNPs) with attractive physicochemical properties are utilized in several biomedical domains, such as antimicrobial agents. Many researchers have tried to explore novel aspects of the respective worth of MNPs, arising from their small size. The increasing demand for MNPs has attracted research interest on developing simple, rapid, inexpensive, and environmentally friendly processes for the synthesis of MNPs. Among the developed types of MNPs, noble metal nanoparticles (NMNPs), especially silver (Ag), gold (Au), and platinum (Pt), have received much attention, due to their excellent properties and diverse applications. For example, microbial contaminations are one of the most severe issues faced in food industries, medical device application, and water treatment. To address these issues, nanoscale materials and nanostructures, especially NMNPs, have been developed, offering novel antimicrobial features. Herein, we review the available green approaches for producing NMNPs (AgNPs, AuNPs, and PtNPs) using different plant extracts, and discuss the antibacterial influences and biocompatibility of these NPs. We highlight the developments of NMNPs for resolving existing toxicity concerns, their antimicrobial effects, as well as the future challenges in this field.

Introduction

The unique properties of nanoparticles (NPs) can be helpful in many practical applications, including industrial and biomedical applications [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35]. NPs have established a great attention in medical and medicine biotechnology [36], [37], [38], [39]. In 1857, Faraday investigated the existence of metal nanoparticles (MNPs) in solution, and Mie quantitatively explained their color, as in the medieval era MNPs were used to decorate cathedral windows [40]. Among known MNPs, noble metal nanoparticles (NMNPs) [such as- Pt, Ag, Au] and their combinational analogs have received much attention owing to their novel chemical, photo-chemical, electrical, and optical characteristics. For example, nanosilver (Ag) is used in clinical care and consumer products, as Ag ions are toxic for a wide array of pathogenic microbes. In addition, many important applications of gold nanoparticles (AuNPs) have been reported towards biological and environmental fields, owing to AuNPs’ antifungal, anti-diabetic, and antibacterial effects [41], [42], [43]. Recently, platinum nanoparticles (PtNPs) have attracted great interest for various biological applications, including cancer therapy, antimicrobial agents, hyperthermia, controlled release of drugs, bio-imaging, bio-sensing, and photo-ablation therapy [44].

Various novel approaches have been explored for NPs synthesis, including photochemical and chemical procedures, radiolysis, electrochemical, and radiation techniques [45], [46]. Nevertheless, high energy requirements, toxic chemicals, and low yield are the main subjects into consideration in this field. In 1962, the publication of “Silent Spring” by Rachael Carson triggered a severe regulation of pesticide use and initiated a global reflection of the environmental dangers arising from chemical processes [47]. Over the past two decades, due to increasing demands for sustainable processes in the chemical industry, green chemistry has been supported by the Environmental Protection Agency (EPA), aiming to prevent or reduce the use of dangerous and toxic chemicals [48]. In this context, John C. Warner and Paul Anastas suggested in 1998, 12 vital principles (see Fig. 1), elaborating on the requirements of green chemistry and helping their implementation in chemical processes. These principles could help prepare novel procedures for designing processes and devices that are accessible, biocompatible, and inexpensive [49].

Plant-mediated fabrication of NPs is assuredly not a troublesome method to follow. A metallic salt is produced by plant-based extracts and the response is usually completed within a timespan of minutes up to 2 h. Hence, in contrast to green synthesis methods based on microorganisms (MO), plant-mediated fabrication does not require the complex endeavors for developing microbial cultures or bearing the danger of leaks, avoiding thus dangerous effects on the human health and the environment. The extracts of different plant sections, including flowers, latexes, fruits, peels, roots, leaves, and seeds have been utilized for the biosynthesis of MNPs. This strategy has attracted great attention during the last decade particularly for AuNPs and AgNPs, which are safer compared to other MNPs [41], [50], [51]. AuNPs obtained from the seed, leaf, pod extract, and seed shell of Cola nitida showed potent anticoagulant, thrombolytic, and catalytic effects [52], [53]. Plant-based fabrication of AgNPs by the extract of Alfalfa (Medicago sativa) roots was the first report on the plant-mediated nanotechnology [54]. Bioactive compositions present in plants (reducing agents) facilitate the reduction of metallic ions to MNPs with fine shapes, distinct size, and significant antibacterial efficiency [55]. Research has shown that the controlled release of metals from MNPs produces superoxide radicals (•O⁻²) and hydroxide radicals (•OH) in pathogenic bacteria. When the level of these reactive oxygen species (ROS) exceeds the scavenging capacity of the bacterial cell, they can damage the cell. ROS produced by NPs could establish the cytotoxicity of resistant bacterial cells. For example, the catalytic reaction between dissolved oxygen molecules and Ag can generate a great amount of ROS [56], [57], [58], [59].

Researchers have shown that metal-green NPs can exhibit antibacterial properties [60]. For instance, AgNPs with antibacterial effects were produced using apple aqueous extracts [61]. Ag acts as antibacterial material against a broad range of over 650 MOs from various classes, including gram-negative and positive bacteria, viruses, and fungi. Therefore, AgNPs could be a promising candidate to overcome emerging bacterial resistance [55]. It has been reported that the AuNPs synthesized via a green route can be used in the development of antibacterial and catalytic products [62]. Despite numerous publications on the green fabrication of Au and AgNPs, there is limited research available on the green synthesis and antibacterial effect of PtNPs [63]. Biosynthesis of NPs with anti-cancer, antioxidant, and antimicrobial properties can become possible through the collaboration of various natural science disciplines [64].

This is the first review article discussing the mechanisms of plant-mediated synthesis of three types of MNPs (Ag, Au, and Pt), their antibacterial activities and cytotoxicity, and the developments of MNPs for resolving the cytotoxicity matters. As ROS production in MNPs is vital for establishing injuries or modulating cell function, we reviewed it and its related effects. Given the absence of relevant information, the effect of ROS on the biological systems is still unknown and needs more research. However, modulation and characterization of the MNP-induced ROS generation are required for the use of MNPs in biomedical and industrial applications. Therefore, a summary of previous findings on the mechanism of the ROS function in biological and physiological systems will motivate future research on the investigation of the cytotoxicity of the ROS released from MNPs and their potential diagnostic and therapeutic applications.