Green Synthesis of Nanoparticles and Their Energy Storage, Environmental, and Biomedical Applications

Abstract

Green synthesis offers a superior alternative to traditional methods for producing metal and metal oxide nanoparticles. This approach is not only benign and safe but also cost-effective, scalable, and straightforward, operating under ambient conditions. Notable metals and metal oxide nanoparticles, such as manganese oxides, iron oxides, silver, and gold, have been produced using various bio-reductants derived from plant extracts. These biological agents not only expedite the reduction process but also stabilize the nanoparticles, serving dual roles as reducing and capping agents. This review presents the green synthesis of nanoparticles (NPs) obtained from biogenic wastes and plant extracts. The green-synthesized nanostructured MnO₂ nanoparticles are evaluated as a potential photocatalyst for water treatment and as an electrode material in lithium-ion batteries and supercapacitors. The green-derived iron oxide nanoparticles are examined as promising antioxidant, anti-inflammatory, and anti-diabetic agents. Additionally, this review discusses the green synthesis of precious metal nanoparticles, specifically silver (Ag NPs) and gold (Au NPs), highlighting their potential medical applications in areas like antiviral treatments and cancer therapy.

1. Introduction

Modern nanotechnology research is gaining prominence due to its groundbreaking and encouraging outcomes in numerous sectors [1]. This rapidly evolving field has a profound influence on life science, especially in the biotechnology and biomedical science areas [2,3,4]. Nanotechnology is pioneering the development of multifunctional nanomaterials for various applications such as energy storage, optical devices, medical devices, solar cells, biomedical and drug delivery, foods, cosmetic and paints, textile industry, etc. [5]. These span areas such as biomedical science, catalytic activity, beauty products, electric chemistry, electronic devices, energy technology, food processing, medical treatment, mechanical work, membrane modification, transparent products, drugs, sensors, aerospace sector, textiles, and water purification, highlighting nanomaterials’ unparalleled versatility, efficacy, and adaptability [6,7,8,9,10]. Various nanoparticle morphologies are currently developed such as nanoshells, nanorods, nanoneedles, nanospheres, nanocubes, nanoplates, etc. Nanotechnology, as defined by the National Science Foundation of the United States, encompasses the study of systems and materials with dimensions from 1 to 100 nm. This field delves into the control of physical and chemical properties and the integration of these nanoscale components into larger structures [11,12]. Nanoparticles (NPs) have garnered significant interest owing to their distinct chemical and physical attributes when compared to their bulk counterparts and their expansive surface area [13,14]. In the past decade, the National Nanotechnology Initiative in the US has allocated over USD 27 billion to this domain. Concurrently, the European Commission has channeled approximately EUR 1.1 billion into nanotechnology ventures under the Horizon 2020 initiative [15,16].

A search on Web of Science revealed numerous articles on nanoparticles produced through green methods, reflecting global initiatives in the nanoparticle domain [17]. As population growth drives urbanization and industrial expansion, there is an increase in chemical waste, contributing to environmental degradation. Exploring nature and its inherent molecules for nanoparticle biosynthesis is favorable as it is cost-effective, environmentally benign, and energy-efficient [18]. NPs are typically produced using two primary methods: bottom-up and top-down. The top-down approach often involves the breakdown of bulk substances into NPs, while the bottom-up approach includes the assembly of atoms into NPs [19]. As illustrated in Figure 1, the top-down strategy employs various preparation techniques such as lithography, ball milling, etching, and sputtering to produce nanomaterials and nanoparticles. In contrast, the bottom-up approach utilizes several methods, including chemical vapor deposition, sol–gel processes, spray pyrolysis, laser pyrolysis, and atomic/molecular condensation [20].

The green synthesis of nanoparticles, rooted in nanobiotechnology [21], has become a central area of focus in nanotechnology research [22,23,24,25,26]. The biosynthesis of nanoparticles stands out due to its environmentally friendly, pure, cost-effective, and versatile nature, often carried out at room temperature [27,28,29,30,31,32]. The adoption of biosynthesis is crucial in avoiding the generation of toxic or hazardous byproducts, emphasizing the need for straightforward and benign production techniques [20]. Practically, the green synthesis of nanoparticles is not only cost-effective but can also be efficiently conducted under ambient conditions. During a one-step synthesis process, a natural bio-reductant extract is mixed with a metallic salt solution. The ensuing redox reaction rapidly produces nanomaterials. This reaction has been observed to demand comparatively low initiation energy [33,34].

Despite extensive physicochemical research in the nanotechnology field, the production of silver and gold NPs is highly performed using biosynthesis. However, there have been limited studies delving into the green synthesis and potential applications of other metallic NPs [35,36]. Given the vast potential of plants as sources of bio-reductants, the evolution of green methodologies for nanoparticle preparation is regarded as a pivotal advancement in nanotechnology [37].

Green synthesis aims to safeguard the environment by substituting harmful chemicals. As such, yeast, fungi, bacteria, algae, and plant extracts are favored as reducing agents over toxic chemicals for the preparation of metal and metal oxide nanoparticles [38]. Biosynthesis not only refines the size and morphology of nanoparticles but is also more eco-friendly compared to other physicochemical methods [39]. Biological sources, particularly plant extracts, impart stabilizing and capping effects to the synthesized nanoparticles. This makes them both more stable and less hazardous than conventionally produced counterparts [40].

While numerous articles delve into the green synthesis of metal oxides, only a handful explore the use of plant extracts in this context [41]. Given its advantages over conventional synthesis methods, there is a recent surge in research focused on the plant-mediated green synthesis of nanoparticles [42]. Plants are preferred in green synthesis due to their inherent reducing agents like citric acid, ascorbic acid, flavonoids, reductases, dehydrogenases, and extracellular electron shuttlers, which facilitate the biosynthesis of metal nanoparticles [43]. Factors such as plant extract concentration, metal salt concentration, reaction duration, solution pH, and temperature play crucial roles in determining the quality, morphology, and crystallite size of the resultant nanoparticles [44,45,46,47].

This review focuses on the recent advancements in the biosynthesis of select metal and metal oxide NPs. We also explore the advantages of biosynthesis over traditional synthesis methods, highlighting its simplicity, cost-effectiveness, and eco-friendliness. Moreover, we delve into the natural compounds present in plant extracts that drive the reduction reactions. Our aim is to outline the green synthesis procedure, detail various characterization tools employed to study green-synthesized metal and metal oxide-based nanoparticles, and shed light on their current applications. Ultimately, we hope to enrich the existing literature in this domain, offering insights that might guide researchers in their future work. Numerous nanoparticles, including gold [48], platinum [49], iron oxide [50,51], copper [52,53], palladium [54,55], zinc oxide [56], and silver [57,58,59] have been prepared from the extracts of natural sources. While the utilization of plant extracts in nanoparticle production is well-documented in scientific literature, there are limited studies on the green synthesis of MnO₂ NPs [60,61]. Even fewer investigations delve into the harnessing of extracts from natural resources for the fabrication of α-MnO₂ nanomaterials. This review will provide an in-depth exploration of this niche topic. Applications of green-synthesized nanoparticles including energy storage, environmental, and biomedical are also described.

2. Green Synthesis

While numerous procedures exist for preparing nanostructures, it is imperative to avoid harmful chemicals and foster green synthesis techniques that produce nanoparticles with equivalent characteristics to those prepared using conventional methods [45,46]. Singh et al. [20] outline the primary advantages of green synthesis, i.e., use at large scale production, eco-friendly approach, biological component acting as reducing and capping agent, and saving energy due to no requirements of high energy and high pressure. Leveraging green nanotechnology helps circumvent detrimental consequences. Moreover, bio-nanotechnology profoundly influences the development of nanostructures by diminishing or eradicating pollutants, thereby addressing current environmental dilemmas, as depicted in Figure 2.

2.1. Green Synthesis of NPs from Biogenic Wastes

Effective solid waste management is essential to prevent potential health hazards and mitigate environmental impacts. Both industrial and domestic waste streams, if mismanaged, can pose significant risks to the environment and public health by facilitating the spread of infections. However, when properly processed and repurposed, biowaste can offer sustainable solutions and reduce its adverse effects [62]. Waste management has profound implications for humans, animals, and the environment, both locally and globally. In developing nations, the surge in waste production parallels population growth and economic development. Both biodegradable and non-biodegradable wastes pose challenges and potential risks, which, if mismanaged or improperly disposed of, can negatively impact communities. Notably, wastes from plants and animals are biodegradable [63]. The four “R’s” of waste management strategies consist of reduce, reuse, recycle, and recovery that can mitigate the risks associated with waste mismanagement [63].

Approximately 80% of total biomass stems from post-harvest agricultural waste. Often, this waste is incinerated, leading to significant emissions of greenhouse gases, smog, and other pollutants. Such emissions contribute to climate change, air contamination, and adverse human health effects. Common kitchen waste, such as peels from fruits and vegetables, is biodegradable and can be broken down by bacteria and other decomposers. However, when biodegradable waste accumulates in vast quantities, it poses environmental risks. Such waste can foster microbial growth, with certain bacteria potentially spreading infectious diseases to humans, flora, and fauna. Additionally, incinerating these wastes releases greenhouse gases like methane and CO₂. Waste dumps, meanwhile, provide breeding grounds for disease-carrying rodents and mosquitoes. As the evidence of climate change’s impact mounts, researchers are continuously seeking methods to mitigate the harm caused by unsustainable disposal practices [64].

Moreover, waste from the food industry, particularly fruit waste, and agricultural crop residues are abundant and typically require no pre-treatment. This accessibility positions biodegradable wastes as potential raw materials for the green synthesis of metal and metal oxide nanoparticles. Leveraging these wastes for nanoparticle synthesis not only mitigates environmental pollution but also supports waste reduction, reuse, and economic enhancement by decreasing high-energy waste accumulation. In green synthesis methodologies, nanomaterials are derived utilizing natural biological processes [64].

Nearly all agro-industrial byproducts and food residues contain phenolic compounds with functional groups that facilitate reduction and ensure structural stability. Flavonoids, present in fruit-derived biomass waste, can chelate and transform metal ions into NPs. This property underlines their application in NP synthesis [63]. For the bio-fabrication of metal oxide nanostructures, fruit peels from sources like banana, Citrus sinensis, jackfruit, lemon, mango, Musa paradisiaca, pomegranate, tangerine, Punica granatum, Garcinia mangostana, Citrus aurantifolia, and Nephelium lappaceum have been documented [65]. Recent studies have leveraged fruit waste, specifically lemon and orange peels, in the synthesis of α-MnO₂ NPs [66,67].

2.2. Green Synthesis of NPs from Plant Extracts

The term “green synthesis” refers to the utilization of natural resources, such as plant extracts [68]. Plants function as primary producers for crafting metals and metal oxide nanostructures [69]. Relative to microorganisms, plant extracts emerge as superior biological agents for nanoparticle synthesis. They are abundant, stable, environmentally benign, cost-effective, and safe to use, boasting a broad spectrum of metabolic functionalities [70,71,72]. Furthermore, plant extracts offer rapid metal ion reduction and are more scalable. Consequently, research on nanoparticle production predominantly centers on plants, which have consistently demonstrated higher success rates in comparison to other “green” methods [73].

Over the past three decades, the harnessing of plant extracts for the eco-friendly synthesis of NPs has garnered significant attention. Numerous plant species have been explored for nanoparticle production, each offering a unique composition of organic reducing agents [74]. The advantages of this green synthesis approach, leveraging plant extracts, encompass biocompatibility, medical relevance, and scalability [75]. These eco-friendly synthesized nanoparticles find diverse applications in pharmaceuticals, encompassing novel drug formulations, targeted drug delivery, diagnostic procedures, and the creation of functional nanodevices [76]. Consequently, this approach is vital in pioneering new treatments to tackle various global health threats [77]. The burgeoning commercial demand for nanomaterials has accelerated the quest for sustainable nanoparticle synthesis methods [78]. Research indicates that various plant components, including leaves [79,80], seeds [81], flowers [82], fruits [83], latex [84], tubers [85], bark [86], and even cultured tissues [87], are potential sources for nanoparticles production.

Commonly available fruits, vegetables, and herbs with rich polyphenol content can serve as reducing or antioxidant agents in nanoparticle synthesis. The primary advantages of using plant extracts in green nanoparticle synthesis include energy efficiency, ready availability, and the capability for production under ambient conditions at room temperature [88,89,90]. The formation of metallic nanoparticles is driven by the antioxidant properties inherent in the plant extracts [91,92]. These plants are rich in bioactive compounds, including sugars, vitamins, and polyphenols, which facilitate the reduction of ions to atoms [93,94]. Notably, the primary phytochemicals contributing to nanoparticle formation encompass terpenoids, flavones, ketones, amides, aldehydes, and carboxylic acids [95].

Polyphenolic compounds feature an aromatic ring bonded to hydroxyl groups and carbon atoms. Notably, the antioxidant activity tends to increase with a higher count of hydroxyl groups [96,97,98,99]. Plant leaf extracts are rich in biomolecules, including carbohydrates and proteins, which serve as reducing agents in the synthesis of metal and metal oxide nanoparticles. Additionally, proteins in these extracts, endowed with functional amino groups (-NH₂), play an active role in reducing metal ions. Functional groups found in phytochemicals, such as C-O-C, -C-O-, and -C=C, can further facilitate the formation of metal and metal oxide nanoparticles [20].

Utilizing aqueous plant extracts for nanoparticle synthesis is a straightforward method, necessitating only the plant extract as a reducing agent and a metal-ion solution, as depicted in Figure 3. In general, the green synthesis process of nanoscale metals includes the following: obtention of plant extract using a specific solvent, mixture with metal salt solution, and a subsequent step to obtain the target nanoscale metal. The mechanism through which the plant leaf extract facilitates the green synthesis of nanoparticles is illustrated in Figure 4 [99]. The metal ions (I) bind to the reducing metabolites (phytochemicals) and stabilizing agents, and are reduced to metal atoms (II). The resulting complex of the metal ion and metabolite interacts with similar complexes forming a small metal nanoparticle (III).

3. Metal and Metal Oxide-Based Nanoparticles

Researchers have increasingly turned their attention to transition-metal nanoparticles due to their vast array of applications. Oxide nanoparticles derived from transition metals find uses across a multitude of human-centric industries [100]. Transition metals, characterized as d-block elements, have electron configurations with incomplete d-orbitals. Their properties are notably different from those of main group elements [101]. The ability of these metals to exhibit multiple oxidation states renders them particularly intriguing to researchers, leading to diverse oxide configurations and further expanding their potential applications [98].

Traditional methods have been employed to produce transition metal nanoparticles and their oxides, with these techniques meticulously honed through rigorous research and experimentation to optimize the physicochemical attributes of the resulting nanostructures. However, these methods present challenges when it comes to large-scale production. Thus, there is a pressing need to identify scalable approaches for manufacturing metal and metal oxide NPs [102].

Green-synthesized nanosized metals and metal oxides exhibit enhanced reactivity owing to their organic capping molecules and crystalline imperfections, setting them apart from those produced chemically. Furthermore, green synthesis facilitates production at an industrial scale [103,104].

Previous studies demonstrate that a diverse range of natural biological materials can be employed in nanoparticle fabrication. This includes plants [105,106,107,108,109], bacteria [110,111], fungi [112,113,114,115], algae [116,117,118,119], viruses [120], and yeast [121,122,123], as illustrated in Figure 5. These biological entities possess metabolites capable of reducing metallic salts to produce nanoparticles. Moreover, these substances not only serve as reducing agents but also play a role in stabilizing the nanoparticles [124]. Recent studies have effectively highlighted the applications of metals and metal oxides produced through green synthesis methods. These applications include cancer treatment [125], drug delivery systems [126], antioxidant therapy [127], virus prevention [128], photocatalytic actions [129], water purification [130], and energy storage [66,67].

Green synthesis offers an environmentally friendly and cost-effective approach to produce metal and metal oxide nanoparticles on an industrial scale. Significant bioactive substances found in vegetable and fruit wastes, such as alkaloids, amino acids, enzymes, phenolics, proteins, polysaccharides, tannins, saponins, vitamins, and terpenoids, serve as reducing agents in this process [131,132]. Agro-wastes can be harnessed for nanoparticle production. In agriculture, even weeds can be repurposed as bioreactors for producing metal and metal oxide nanoparticles [133].

Plants serve as nature’s cost-effective and environmentally friendly chemical factories [67]. A crucial role is played by phytochemicals present in various parts of plants, including roots, leaves, stems, and fruits [134,135,136]. Figure 6 showcases the biologically active phytochemicals found in plants [105]. Notable phytochemicals in plant extracts include amides, carboxylic acids, aldehydes, ketones, sugars, terpenoids, and flavones. Due to their functional groups, these phytochemicals can reduce metal ions more rapidly than bacteria or fungi, making them efficient for nanoparticle synthesis [137,138].

Subsequent sections of this study will delve into the recent applications of nanoparticles, including those of metals and their oxides, produced through green synthesis using plant extracts. Specifically, we will spotlight manganese oxide, iron oxide, silver, and gold nanoparticles. A significant portion of this exploration emphasizes the green synthesis and applications of α-MnO₂ NPs, which are crafted from redox reactions involving KMnO₄ and natural reducers, including plant and waste extracts.

4. Mn Oxide NPs

Manganese has distinguished itself among the diverse nanomaterials available today [139]. Representing the twelfth most abundant element on Earth and the third most prevalent transition metal following Fe and Ti [140], Mn nanoparticles are mainly found as oxides or as composites with other metals (bimetallic or polymetallic) [139]. Owing to their expansive surface area, structural adaptability, and distinctive physicochemical properties, Mn and its oxides are gaining prominence in nanomaterial development [141]. Consequently, nanostructured Mn and Mn oxide materials hold a significant advantage in a variety of applications, including catalysis, energy storage, gas sensors, magnetic materials, electrochromic devices, and high-temperature solar selective absorbers [142].

Manganese oxides, including MnO, MnO₂, Mn₂O₃, Mn₃O₄, and Mn₅O₈, stand out among the 3d transition metal oxides due to their diverse structural transformations. These Mn oxide nanoparticles present significant promise for advanced nanotechnology applications [143]. With their advantageous chemical properties, Mn oxides find potential applications in batteries, catalysts, drug delivery, magnetic materials, molecular sieves, optoelectronics, and solar cells [144]. Additionally, manganese oxides are cost-effective, possess high capacitance, and are environmentally benign, making them a safer alternative to certain other compounds, such as various chalcogenides [145,146].

The structural diversity of Mn oxide nanoparticles encompasses a broad spectrum of chemical and physical attributes. Various Mn oxide nanostructures, including nanobelts, nanorods, nanosheets, nanotubes, nanowires, and nanofibers, have been fabricated using diverse techniques. Among these, MnO₂ is particularly notable. As the most stable Mn oxide under ambient conditions, it boasts appealing physicochemical properties. Consequently, many researchers have focused on evaluating the efficacy of MnO₂ and its significance in a range of applications [147].

4.1. MnO₂ NPs

Nanostructured manganese dioxide (MnO₂) holds promise as a sustainable material suitable for a host of crucial and globally beneficial applications [148]. This potential stems not only from its unique structures and favorable properties [66,149], such as cost-effectiveness, accessibility [150], non-toxicity, and versatile oxidation states [151,152], but also from its outstanding physicochemical characteristics and structural adaptability. This versatility makes it ideal for use across multiple sectors, including as catalysts [153], absorbents for toxic metals [154], and ion and molecular sieves [155]. Additionally, MnO₂ serves roles as an artificial oxidase [156], a depolarizer in the Leclanché dry cell [157], an inorganic pigment in ceramics, an electrode material for solar cells, and in applications like photocatalysis [158], supercapacitors [159,160,161], and Li-ion and Li-air batteries [162,163,164,165].

4.2. Crystal Structure of MnO₂ NPs

MnO₂ exists in various crystalline forms, including α-, β-, γ-, δ-, and λ-MnO₂. The diverse crystallographic forms arise from the unique configurations in which MnO₆ octahedra are arranged within the MnO₂ nanoparticles. Each form has a distinctive tunnel or interlayer structure. The multifaceted connections—either vertex or edge-sharing—between MnO₆ octahedral structures give rise to these tunnel structures of varying dimensions [166].

Specifically, α-, β-, and γ-MnO₂ exhibit a 1D tunnel structure. In contrast, δ-MnO₂ showcases a 2D layered structure, and λ-MnO₂ presents a 3D spinel structure [167]. These structures can be differentiated by the tunnel size and the count of octahedral subunits, represented as (n × m).

Table 1. Crystallographic data of α-, β-, γ-, δ-, λ-MnO₂.

Crystal Structure	α-MnO2	β-MnO2	γ-MnO2	δ-MnO2	λ-MnO2
Chemical name Crystal structure Lattice parameter (Å) Tunnel shape Tunnel size (Å)	hollandite tetragonal a = 9.96 c = 2.85 (2 × 2) 4.6	pyrolusite tetragonal a = 4.39 c = 2.87 (1 × 1) 1.89	nsutite intergrowth a = 9.65 c = 4.43 (1 × 1), (1 × 2) 1.82, 2.3	birnessite rhombohedral ahex = 2.94 chex = 21.86 interlayer distance 7.0	spinel cubic a = 8.04

lists their fundamental crystallographic data, while their structural schematic diagrams are provided in Figure 7 [168]. For instance, the α-MnO₂ structure, depicted in Figure 7a, consists of double chains of edge-sharing MnO₆ octahedra. Within α-MnO₂, octahedral MnO₆ form 1D (2 × 2) tunnels by connecting at their vertices [168,169]. The β-MnO₂ structure, represented in Figure 7b, demonstrates a 1D (1 × 1) tunnel created by a singular chain of edge-sharing MnO₆ octahedra. Figure 7c reveals γ-MnO₂ to be a composite of a disorganized mix of ramsdellite (1 × 2) and pyrolusite (1 × 1) domains. The 2D layered structure of δ-MnO₂, seen in Figure 7d, possesses an interlayer spacing of approximately 7 Å. Lastly, λ-MnO₂, illustrated in Figure 7e, displays a 3D spinel configuration.

The physicochemical properties of MnO₂ nanostructures are intricately influenced by their crystallographic attributes, encompassing geometry, lattice parameters, and tunnel dimensions [170]. Figure 8 depicts MnO₂ nanostructures of diverse morphologies such as nanosheets, nanotubes, nanorods, nanowires, nanoflakes, nanoneedles, nanoflowers, nanourchins, and nanospheres [171]. The varied morphological forms of the MnO₂ structure emerge based on the synthesis techniques employed [172].

5. Synthesis of Nanostructured MnO₂

Numerous conventional methods have been successfully employed to produce MnO₂ nanomaterials (MDOs) through MnO⁻⁴ and Mn⁺² redox reactions. These methods include wet chemical, solvothermal, precipitation, co-precipitation, controlled synthesis, sol–gel, hydrothermal, reflux, pyrolysis, sonochemical, low-temperature solution combustion, self-reacting, microemulsion, photochemical, and forced hydrolysis techniques [173,174]. Common synthesis approaches involve the oxidation of Mn(II) in a basic solution, its oxidation by agents such as oxygen, potassium persulfate, and hydrogen peroxide, or the reduction of permanganate through various means [175,176]. Frequently used reducing agents include nitric acid (HNO₃) [177], sodium hydroxide (NaOH) [178], hydrochloric acid (HCl) [179], and ammonium fluoride (NH₄F) [180].

Furthermore, given the connection between the morphology and size of NPs, it is imperative to devise fabrication techniques that allow optimization of attributes like monodispersity, crystallite size, surface area, crystallinity level, and morphology [175].

5.1. Traditional Synthesis of MnO₂ NPs

KMnO₄ is a primary starting material for synthesizing α-MnO₂. Its viability as a Mn source and its inherent capacity to stabilize MnO₂’s crystallographic structure stem from the presence of K⁺ within the 2 × 2 tunnel of α-MnO₂. Factors like pH, synthesis temperature, and reactant concentration have been noted to influence the morphology of α-MnO₂ [181].

In a hydrothermal method, Subramanian [182] employed MnSO₄.H₂O and KMnO₄ as initial materials to produce MnO₂ nanorods across varying reaction durations (1–18 h) using KMnO₄, sulfuric acid, and Cu scrap. Xu et al. [183] developed α-MnO₂ hollow spheres characterized by a loosely connected, mesoporous cluster structure. In another approach, KMnO₄ and HNO₃ were subjected to a hydrothermal procedure, yielding MnO₂ nanowhiskers and nanorods at 120 °C [184]. Using KMnO₄, ethanol, and sulfuric acid, and maintaining a pH of 2 at 60 °C, Liu et al. [185] crafted nearly spherical MnO₂ particles, each around 100 nm in diameter. Additionally, with the same components, α-MnO₂ nanowires with tailored sizes were produced, exhibiting intriguing electrochemical traits [186]. Feng et al. [187] leveraged a one-pot hydrothermal synthesis to produce α-MnO₂ nanorods with lengths reaching 1.2 mm and a crystallite size of 300 nm. Ji et al. [188] utilized a similar technique to yield longer α-MnO₂ nanorods, while Hashem et al. [162] harnessed a redox reaction of ammonium persulfate with manganese sulfate to derive a pure α-MnO₂ phase.

However, these conventional methods for the synthesis of MnO₂ NPs present scalability challenges and are typically intricate, time-intensive, energy-consuming, and expensive. Many require the use of high-cost and hazardous chemicals for reduction and capping, along with intense mixing, prolonged durations, high temperatures, and substantial energy, leading to significant energy losses [189,190]. A critical concern is the residual harmful substances left on the NPs even after multiple washes, potentially harming the environment [191].

Therefore, the search for an efficient, eco-friendly production method for MnO₂ materials is initiated. Recently, efforts have been geared towards mitigating environmental impact. Innovations aim for the creation of harmless, biocompatible, benign, scalable, cost-efficient, and safe MnO₂ materials [192]. Ideally, a straightforward, affordable, and safe method would control MnO₂ nanoparticle size, utilizing eco-friendly reducing agents [69,193]. Plant extracts have shown more promise for the green synthesis of MnO₂ NPs compared to microorganisms, thanks to their diverse biomolecules that serve as both capping and reducing agents [69].

5.2. Green Synthesis of MnO₂ NPs

Traditional methods often rely on costly, outdated, and complex techniques [193]. Given the growing emphasis on ensuring that produced NPs are non-toxic and environmentally friendly, there has been a shift towards using green chemistry principles for their synthesis. This sustainable approach is termed “biosynthesis” or “green synthesis” [194]. Distinct from conventional methods, the biosynthesis approach for preparing MnO₂ NPs emerges as a superior alternative. It not only proves less expensive and less harmful but also boasts several advantages, such as adaptability, simplicity, rapidity, affordability, eco-friendliness, safety, and scalability for nanomaterials production [195].

In green synthesis, traditional pricey chemical reducers like hydrazine hydrate, sodium borohydrate, formaldehyde, and ethylene glycol are supplanted by natural compounds. These compounds, rich in antioxidative elements like flavonoids, tannins, and vitamin C, can address the concerns of cost and contamination [196]. Hence, biosynthesis for MnO₂ NPs is examined through an environmental lens, leveraging waste by-products and allowing production at room temperature [197].

For the synthesis, agents like bacteria, fungi, biopolymers, raw biomaterials, and plant extracts are utilized to create MnO₂ NPs [194,197]. However, refining the size, morphology, and application range of these green-synthesized MnO₂ NPs stands as a central challenge in contemporary green nanotechnology [198]. Recent findings have highlighted the successful production of MnO₂ NPs through biosynthesis in a single step at room temperature, cutting energy costs, overall expenses, and the need for harmful solvents and organic materials [199,200]. In this method, plant extract, serving as a natural reducing agent, is combined with a KMnO₄ solution. The KMnO₄’s full reduction by the plant extract shifts the mixture’s color from purple to black within an hour at room temperature. Afterward, to remove potassium ions, the resulting precipitate is repeatedly rinsed with distilled water and separated via filtration. This precipitate is dried for a day at 90 °C and subsequently baked for five hours at 300 °C in a standard atmosphere [67]. The resulting α-MnO₂ NPs have exhibited promising outcomes in contemporary applications, including lithium batteries, supercapacitors, and photocatalysis [66,67,172].

6. Plant Extracts for Green-Synthesized MnO₂ NPs and Recent Applications

The properties of green-synthesized MnO₂ NPs are influenced by the type and concentration of natural reducing agents found in plant extracts. These extracts predominantly contain antioxidants, notably, polyphenols that play a crucial role in the green fabrication of MnO₂ NPs [201,202]. When employing plant extracts for nanoparticle synthesis, the chosen extract is simply mixed with an aqueous Mn solution and stirred at room temperature for a short period. Compounds within the plant extract, such as flavonoids, polysaccharides, polyphenols, terpenoids, and tannins, facilitate the reduction of manganese ions [203,204,205,206].

Numerous plant extracts have been found capable of reducing metal salts to their corresponding metal oxides. Ullah et al. [207] harnessed Bryophyllum pinnatum leaf extract to produce MnO₂ NPs ranging from 4 to 18 nm in diameter. Meanwhile, Dewi and Yulizar [208] utilized Euphorbia heterophylla leaf extract to generate MnO₂ NPs with a crystallite dimension of 56 nm. Their reaction mixture was stirred for an hour at 80 °C and subsequently heated for two hours at 500 °C. Other studies have reported the synthesis of MnO₂ nanomaterials using extracts from Gardenia resinifera, Phyllanthus amarus, Kalopanax pictus, Origanum vulgare, Artemisia dracunculus, Sapindus mukorossi, Rosmarinus officinalis, and Vernonia amygdalina [67,175,209,210,211,212,213]. Additionally, Souri et al. [175] highlighted the biosynthesis of MnO₂ nanoparticles utilizing Yucca gloriosa leaf extract. This synthesis was confirmed via XRD, revealing an average particle size of 80 nm as calculated by the Debye–Scherer equation. Another notable precursor is the fruit extract of Acacia concinna, employed in the sol–gel method for the green synthesis of manganese oxide nanomaterials. This natural reducing agent was observed to adjust the nanoparticle size and eradicate detrimental byproducts [67].

The subsequent section delves into the green synthesis of various MnO₂-based NPs using diverse plant and fruit extracts as natural reducers (Figure 9). The discussion extends to cover their recent applications, notably, as photocatalysts in water purification and as electrode materials in energy storage devices like lithium-ion batteries and supercapacitors.

6.1. Lemon Juice and Lemon Peel Extracts

Lemon juice is primarily composed of water. The acidity of lemon juice predominantly stems from citric acid, which accounts for about 5% by volume (or 48 g L⁻¹). Other contributors to its acidity include malic acid and smaller quantities of ascorbic acid (which provides 40 mg of vitamin C per 100 g of lemon) and tartaric acid [198]. Citrus fruit peels, like those of lemons, serve as potent antioxidants [214,215]. They are abundant in several reducing agents, notably p-coumaric acid, flavonoid glycoside, and β-sitosterol [216,217]. Figure 10 showcases the molecular structures of the antioxidative compounds present in both lemon juice and lemon peel [66].

ZnO nanoparticles were synthesized using Citrus aurantifolia extract, which is abundant in citric acid and ascorbic acid (vitamin C) [218]. Polyphenols and ascorbic acid present in citrus fruit fibers have been found to enhance the crystallization of metal oxides, as reported by Ahmad et al. [219].

The tetragonal α-phase of the MnO₂ polymorph has garnered significant attention compared to other polymorphs due to its expansive 2 × 2 tunnels. These tunnels are conducive for the movement and accommodation of foreign ions [220], such as K⁺ found in cryptomelane-type K_xMn₈O₁₆ compounds [221]. Zhang et al. highlighted that K_0.25Mn₂O₄ nanofibers possess remarkable lithium insertion abilities, exhibiting superior charge capacities and a high-rate capability [222]. Hashem et al. [66] reported the successful production of MnO₂ NPs using lemon juice and lemon peel (designated as J-MnO₂ and P-MnO₂, respectively) at room temperature without producing harmful waste. This was achieved through a green synthesis approach, using a redox reaction between KMnO₄ and natural reducing agents: specifically, lemon juice for J-MnO₂ and lemon peel extracts for P-MnO₂.

All XRD peaks corresponded accurately to α-MnO₂ (JCPDS card No. 44-0141). The reflection (121) associated with the cryptomelane structure had the most pronounced intensity. The P-MnO₂ NPs exhibited superior crystallinity in comparison to the J-MnO₂ NPs. The observed differences in crystallinity between the two compounds can be attributed to the specific type of reducing agent employed. This distinction is evident in the varying patterns depicted in the XRD of J-MnO₂ and P-MnO₂, as shown in Figure 11 [66]. It is well recognized that employing diverse carboxylic acids combined with the chelate-assisted sol–gel technique can yield products with markedly different morphologies and structural defects [223].

The surface morphology of the two green-synthesized compounds was examined using SEM and TEM, as displayed in Figure 11c [66]. Distinct morphologies are evident in the figure. For J-MnO₂, a cluster of extremely small particles with sizes less than 10 nm can be seen in image (iii). The diminished size of J-MnO₂’s primary particles can be attributed to its limited crystallinity. Conversely, P-MnO₂ exhibits a different morphology. It consists of uniform nanorods with a crystallite size of 17 nm, as illustrated in image (iv). It is evident that the morphology of α-MnO₂ nanoparticles is profoundly affected by the choice of reducing agent. The lemon juice extract, rich in citric and ascorbic acids, yields the tiniest nanoparticles, whereas the more complex reducing agents in lemon peel foster the formation of nanorod structures with a more pronounced crystallite size.

To evaluate the electrochemical properties and discharge performance of J-MnO₂ and P-MnO₂ NPs for potential use as cathodes in lithium-ion batteries, galvanostatic charge–discharge studies were conducted. Cycle tests in the voltage range of 1.5–3.5 V vs. Li⁺/Li⁰ were carried out at various current densities ranging from 10 to 400 mA g⁻¹ (with 1C = 260 mA g⁻¹). As depicted in Figure 12a,b, the superior performance of P-MnO₂ cells is evident across discharge current densities from 10 to 400 mA g⁻¹ [66]. The P-MnO₂ cell exhibits a reversible specific capacity of 50 mA h g⁻¹ at 400 mA g⁻¹, whereas the specific capacity of the J-MnO₂ cell diminishes, indicating lattice disarray. Both compounds exhibited reduced Coulombic efficiency during the first cycle. However, by the second cycle, the cells showcased high rechargeability, achieving efficiency close to 98%. The cycling stability of J-MnO₂ and P-MnO₂ electrodes is highlighted in Figure 12c. After 50 cycles, the capacity retention of P-MnO₂ was 73%, while J-MnO₂ managed 55%. Reports suggest that the enhanced electrochemical performance of P-MnO₂ stems from its superior crystallinity.

6.2. Black and Green Tea Extracts

Both black and green tea extracts, though structurally distinct, contain flavonoids. These are powerful antioxidants that have been employed to convert KMnO₄ to MnO₂ through redox reactions. The high phenolic content of these teas imparts them with potent antioxidant capabilities [224].

Green tea is especially rich in Epigallocatechin gallate (EGCG), a renowned antioxidant. Unlike black tea, green tea avoids fermentation and the subsequent oxidation process. This oxidation in black tea transforms catechins into the more complex aflavins and arubigins. Consequently, green tea exhibits enhanced antioxidant activity in comparison to black tea. However, it is important to note that while oxidation alters the type of flavonoids present, it does not affect their quantity or antioxidant potency [197,225,226]. Figure 13 highlights the key flavonoid concentrations in both black and green teas [69].

In 2018, Abuzeid et al. [69] developed nanosized MnO₂ NPs using both green and black tea extracts, termed as GT-MnO₂ and BT-MnO₂, respectively. Figure 14a illustrates their crystal structures, as determined by XRD analysis. Reflections in α-MnO₂ (JCPDS card No. 44-0141) are indexed across both. GT-MnO₂ displays a crystalline nature with a Scherrer crystallite size of roughly 8.0 nm, in contrast to the amorphous or highly disordered BT-MnO₂ which has a crystallite size of about 4.4 nm. This XRD pattern discrepancy between GT-MnO₂ and BT-MnO₂ is attributable to their differing crystallinity. Experimental data suggest that the redox reaction facilitated by black tea is slower compared to that by green tea. This difference, stemming from variances in flavonoid structures and antioxidant power, impacts the MnO₂ structure’s crystallinity. The potency of the reducing agent, governing ion extraction from the 2 × 2 tunnels and K⁺ ion movement, also affects the synthesis methods. Raman spectra, depicted in Figure 14b, were used to further explore the MnO₂ NPs’ crystal structures, focusing on the influence of the reducing agents [69]. The Raman spectrum for GT-MnO₂ displays distinct bands at 181, 380, 510, 577, 630, and 754 cm⁻¹. In contrast, BT-MnO₂’s Raman bands are broad and less defined, suggesting a highly disordered structure in the material. Key insights can be drawn from the band positions: the tetragonal 2 × 2 tunnel structure is revealed by the high-frequency signals at 577 and 630 cm⁻¹ (A_g modes). The low-frequency band at 181 cm⁻¹ arises from the translational motion of [MnO₆], and the band at 380 cm⁻¹ is attributed to O-Mn-O bending vibrations. Furthermore, the band situated at 754 cm⁻¹ is linked to antisymmetric Mn-O stretching vibrations. These distinctive Raman characteristics align with findings for α-MnO₂ documented in prior research [227,228]. Notably, the crystallization process of MnO₂ NPs affects the intensity of the ν₅₇₇ and ν₆₃₀ bands. This consistency between Raman and XRD results underscores GT-MnO₂’s superior crystallinity over BT-MnO₂ [69].

Figure 14c,d present the galvanostatic charge–discharge profiles of MnO₂//Li cells using GT-MnO₂ and BT-MnO₂ as the positive electrode materials. The profiles of these half-cells were obtained at a consistent current density of 10 mA g⁻¹ (C/26) and within the potential window of 1.5–3.5 V vs. Li⁺/Li⁰ [69]. The discharge cell potential steadily decreases throughout the entire discharge span, revealing two pseudo-plateaus. Each plateau is characterized by an S-shaped curve, indicative of a topotactic reaction during lithium insertion into the electrodes. However, in the case of BT-MnO₂, the voltage drop is markedly steeper, a characteristic typically observed in disordered electrode materials [229]. This electrochemical behavior aligns with the structural observations made earlier. The highly disordered BT-MnO₂ exhibits an initial specific capacity of approximately 236 mA h g⁻¹, while the well-crystallized GT-MnO₂ sits at around 198 mA h g⁻¹. The expansive tunnel (4.6 Å) accommodates a significant quantity of electrochemically inactive K⁺ ions, which remain lodged at the 4e sites, leading to the reduced starting capacity. For GT-MnO₂, potassium occupies more than half of these 4e sites, whereas for BT-MnO₂, it is just above a quarter. This inert cation might hinder the ingress of Li ions into the tunnel and also obstruct the ion movement during the discharge phase [230]. GT-MnO₂’s Coulombic efficiency showcases impressive rechargeability, approaching 99%, even at a steady current density of 200 mA g⁻¹ (=0.75 C). This remains consistent except for the initial two cycles. The cycling behavior over 54 cycles for MnO₂//Li cells in lithium-ion batteries is depicted in Figure 14e [69].

GT-MnO₂ demonstrates superior capacity retention compared to BT-MnO₂. After the third cycle, GT-K_yMnO₂ exhibits a discharging capacity of 161 mA h g⁻¹, which gently reduces to 141 mA h g⁻¹ by the 54th cycle at a C/10 rate. The capacity degradation for GT-K_yMnO₂ averages 0.25% per cycle, while it is 0.58% for BT-K_yMnO₂. Both materials experience some capacity loss post the initial cycle. GT-MnO₂’s irreversible capacity caps at 30 mA h g⁻¹, retaining 70% of its inaugural capacity after 20 cycles. In contrast, BT-MnO₂ holds onto roughly 62% of its capacity. This diminished initial capacity suggests that certain lithium ions became ensnared within the cell’s internal voids during the cell fabrication [231].

6.3. Broccoli Vegetable Extract

Broccoli, akin to cabbage and cauliflower, boasts antioxidant, antibacterial, and anticancer attributes [232,233,234,235]. It is rich in polyphenols, particularly flavonoids [234]. Various parts of broccoli, including its leaves, flowers, and other tissues, house these flavonoids and phenolic acids [232]. Figure 15 depicts the antioxidant constituents in broccoli extract, namely, α-lipoic acid, sulforaphane, and coenzyme Q10 [235].

Figure 16a presents the XRD pattern of K_yMnO₂ nanoparticles (NPs) synthesized using broccoli extract [235]. The most prominent peak at 2θ = 37° corresponds to the (211) plane of the tetragonal α-MnO₂ phase, with no detectable defects. The subdued intensity indicates low crystallinity, while the peak broadening implies that the α-K_yMnO₂ NPs are of nanoscale dimensions. Using the half-width at half-maximum of the (211) diffraction line, observed at 2θ ≈ 37.6, the calculated particle size is approximately 4.4 nm. This highlights the polycrystalline character of the α-K_yMnO₂ NPs.

Nitrogen adsorption/desorption at 77 K was conducted over a relative pressure range of P/P₀ = 0.0–1.0, where P and P₀ denote equilibrium and saturation pressures, respectively, to determine the Brunauer–Emmett–Teller (BET) specific surface area. As the P/P₀ value increases, the volume of N₂ adsorbed on the isotherm curve also grows. The emergence of a hysteresis loop signifies the hierarchical mesoporous structure of α-K_yMnO₂. At P/P₀ = 0.97, the volume of N₂ adsorbed is estimated to be 450 cm³ g⁻¹. The inset in Figure 16b [236] showcases the mesoporous nature of the green-synthesized α-K_yMnO₂, determined via the Barrett–Joyner–Halenda (BJH) method [236].

Suib et al. [237] suggested that mesoporosity arises from the aggregation of MnO₂ nanomaterials, either in the form of nanorods or nanoneedles, along their lateral facets. The mesopore size distribution, in the range of 1–20 nm, primarily displays a single peak at 10.4 nm, accompanied by a cumulative pore volume of 0.950 cm³ g⁻¹. Notably, the BET surface area of MnO₂ NPs biosynthesized using broccoli extract registers at 161 m² g⁻¹. This is superior to MnO₂ structures derived from alternative methods such as the microemulsion technique (123 m² g⁻¹), hydrothermal synthesis (150 m² g⁻¹), silica templating coupled with the ion-exchange approach (142 m² g⁻¹), mild reactions [238], and exfoliation [239]. Thus, green synthesis emerges as a potent strategy for fabricating mesoporous, pure α-phase K_yMnO₂ characterized by minute particle dimensions and an expansive surface area.

The potassium concentration inside the 2 × 2 tunnels, which is considered to be positive to stabilize the tetragonal structure, was determined by thermogravimetry (TG). Figure 16c presents the differential weight dw/dT corresponding to the rapid weight loss in the region of 400–600 °C, which depends on the concentration of tunneled foreign ions (potassium or ammonium) in the cryptomelane K_yMn₈O₁₆ structure. According to the decomposition temperature of 504 °C shown in Figure 16c, a concentration of potassium is estimated to be y = 0.035, which is close to the value obtained from ICP measurements.

Hashem et al. [235] investigate the galvanostatic charge–discharge curves of α-K_0.03MnO₂//Li cells over 50 cycles at a constant current density of 30 mA g⁻¹. The MnO₂ structure, which possesses two distinct coordination sites for Li⁺, exhibits a topotactic behavior for Li⁺ insertion, characterized by a gradual voltage decline marked by two pseudo-plateaus and an S-shaped profile. As subsequent cycles commence, these plateaus shift to higher potentials. Over four consecutive cycles, the material’s capacity diminishes from 211 to 198 mAhg⁻¹. The pronounced alterations in the electrochemical profile during the second cycle have been highlighted in various studies [240,241,242]. Figure 15d–f underscores the commendable rate capability and cycle stability of α-K_0.03MnO₂ when employed as a cathode in lithium-ion batteries, operating within voltages of 1.5 to 4.0 V and current densities ranging from 0.1 C to 10 C [235]. As the current density increases, there is a decline in specific capacity; however, the charge and discharge profiles remain largely unaltered, as depicted in Figure 16d. Throughout the assessed C-rate spectrum, the characteristic S-shaped profile persists. The modified Peukert plot, which plots discharge capacity against C-rate, exhibits a near semi-logarithmic trend, as illustrated in Figure 16e. At 10C, the α-K_0.03MnO₂ electrode delivers a specific capacity of 32 mAh g⁻¹. The α-K_0.03MnO₂ electrode showcases commendable reversibility, as seen in Figure 15f. This is highlighted by its efficiency, which remains an impressive 98.8% at 0.1C rate over 100 cycles. This robust electrochemical stability suggests that the cationic exchange (Li⁺ vs. K⁺) during Li⁺ integration into the α-K_0.03MnO₂ structure is minimal.

6.4. Orange Juice and Orange Peel Extracts

Millions of tons of oranges are produced globally, with a significant portion dedicated to industrial orange juice extraction. This industry generates large amounts of byproducts, including orange peels and segments. The orange peel, accounting for 50%–65% of the fruit’s weight, is rich in 7.1% protein, 12.79% crude fiber, and bioflavonoids. These bioflavonoids possess antioxidant properties, making them suitable for NP production [67]. Addressing the vast amounts of orange peel waste is essential to avert potential environmental harm and other adverse effects [243,244].

Orange juices and peels are rich in ascorbic acid, flavonoids, phenolic compounds, and pectin. The primary components of orange juice are organic acids, sugars, and phenolic compounds, including sucrose, glucose, fructose, and citric, malic, and ascorbic acids. Additionally, orange juice contains phenolic substances such as flavanones, hydroxybenzoic acids, hydroxycinnamic acids, ferulic acid, hesperidin, and narirutin [245].

Orange peels are primarily composed of polyphenolic and flavonoid compounds. Prominent among these flavonoids are hesperidin, narirutin, naringin, and eriocitrin [246]. The glycosides hesperidin and naringin endow orange peel extracts with their antioxidant activity. Furthermore, orange peel molasses contains coniferin and phlorin, which aid in radical scavenging and support the sustainable recycling of orange peels [247]. Skiba et al. [248] reported the use of orange peel extract in the fabrication of silver NPs using a plasma chemical extraction process, along with the degradation of methylene blue in sunlight. Abuzeid et al. [67] employed orange peel extract for the green synthesis of MnO₂ nanomaterials. These were then utilized as electrodes for supercapacitors, representing an innovative approach to repurpose the vast residue from orange production. Notably, MnO₂ is widely used as an electrode in both electrochemical supercapacitors and batteries [249].

Supercapacitors offer high power in short time spans. These devices are vital for high-power applications due to their cost-effectiveness, low maintenance, safety, rapid charging, and extended cycle life [250,251,252]. While supercapacitors may have a lower energy density than lithium-ion batteries, they bridge the gap, providing a balance between the high energy density of batteries and the power density of electrochemical capacitors [253,254,255].

X-ray powder diffraction patterns of synthesized OP-MnO₂ ad OJ-MnO₂ using orange peel and orange juice, respectively, are presented in Figure 17a,b. The prominent peaks characteristic of α-MnO₂ were identified in alignment with the reference (JCPDS No. 44-0141) as documented in a previous study [256]. It has been highlighted that the potency of the reducing agent can influence the amount of K⁺ integrated within the 2 × 2 tunnels of α-MnO₂. The presence of potassium plays a pivotal role in fortifying the α-MnO₂ structure. Materials with a lesser degree of crystallization tend to have reduced K⁺ concentrations within the α-MnO₂ framework, which can be attributed to the utilization of a less potent reducing agent [66,69,256].

The mesoporous nature of the prepared OJ-MnO₂ and OP-MnO₂ samples was confirmed from BET experiments. The pore size was estimated to be 7.25 and 6.75 nm for OJ-MnO₂ and OP-MnO₂, respectively. The calculated BET surface area according to the Barrett–Joyner–Halenda method was 5.63 and 8.40 m² g⁻¹ for OJ-MnO₂ and OP-MnO₂, respectively

Figure 17c–e present the charge and discharge data (CD) utilized to compute the specific capacitance (SC) of both OJ-MnO₂ and OP-MnO₂ NPs based on the following equation [67]:

C_sp = I Δt/m ΔV

(1)

where m is the total mass of materials coated on the glassy carbon electrode, I is the discharging current (A), Δt is the discharging time (s), and ΔV is the voltage range.

At current densities of 15, 5, 2, and 0.5 A g⁻¹, the specific capacitance values for OJ-MnO₂ NPs are 18.5, 25, 33, and 50 F g⁻¹, respectively (Figure 17f). In contrast, at the same current densities, the specific capacitance values for OP-MnO₂ are 61, 85, 107, and 139 F g⁻¹. Notably, the specific capacitance values for OP-MnO₂ are approximately two and a half times those of OJ-MnO₂. This significant difference can be explained by two primary factors. Firstly, OP-MnO₂ possesses a larger surface area and smaller particle size, as evident from the BET surface area data [67]. Secondly, OJ-MnO₂ has a substantial concentration of K⁺ ions lodged within its 2 × 2 tunnel. This increased presence of K⁺ ions in the 2 × 2 tunnel restricts the easy insertion and extraction of H⁺ ions. Moreover, it is important to note that these K⁺ ions act as inert components, thereby reducing the overall capacitance value, as detailed in the study [67].

Cycle stability is a pivotal aspect for electrochemical supercapacitors [257,258,259]. An investigation was conducted on the cycle stability of OP-MnO₂ electrodes over 500 cycles, using a current density of 3 A g⁻¹ and employing the charge/discharge method within voltage parameters ranging from −0.2 to 1.2 V. As depicted in Figure 17g [67], there is a noticeable uptrend in capacitance retention for the first 450 cycles relative to the inaugural cycle. Interestingly, by the 500th cycle, the electrode’s capacitance had reverted to its initial value. Specifically, the electrode began with 119 F g⁻¹ during the 1st cycle, peaked to 137 F g⁻¹ by the 350th cycle (indicating a capacitance retention of 115%), and then circled back to 119 F g⁻¹ on the 500th cycle, showcasing a complete 100% capacitance recovery.

6.5. Moringa and Cinnamon Herb Extracts

Moringa oleifera is renowned for its myriad benefits, encompassing health, nutrition, commercial, and clinical attributes, primarily due to its potent antioxidant properties. This plant is a rich source of various vitamins, minerals, amino acids, fatty acids, glucosinolates, and phenolic compounds. Specifically, Moringa oleifera leaves are enriched with vitamin C, amino acids, and beta-carotene. Delving deeper, compounds present in this plant include flavonoids, L-ascorbic acid, retinol, niacin, thiamine, chlorogenic acid, tocopherol, caffeic acid, O-coumaric acid, gallic acid, and riboflavin, all of which possess remarkable reducing properties.

Cinnamon, a potent spice, has held medicinal significance for millennia. In modern medicine, cinnamon is recognized for its ability to reduce blood glucose, cholesterol, and blood pressure levels. This spice boasts antiparasitic, antibacterial, antioxidant, and free-radical scavenging properties. Key natural antioxidants found in cinnamon, such as cinnamaldehyde, eugenol, borneol, cinnamyl acetate, cinnamic acid, and coumarin, are responsible for its therapeutic benefits [260,261,262].

Moringa and cinnamon extracts are recognized for their potent antioxidant properties and their excellent capacity to reduce KMnO₄ to α-MnO₂, as recently explored by Abuzeid et al. [172]. Figure 18a presents the XRD characteristics of the biosynthesized MnO₂ nanoparticles M-MnO₂ and C-MnO₂ using moringa and cinnamon extracts, respectively. Based on the JCPDS data (card no. 44 0141), the characteristic peaks for both compounds can be attributed to the tetragonal α-MnO₂, with no additional defects observed. Both compounds exhibit low crystallinity, suggesting a nanosized structure. This is evident from the reduced intensity and broadening of these peaks. The electronic properties of C-MnO₂ and M-MnO₂ have been investigated using UV-Vis diffuse reflectance spectroscopy.

C-MnO₂ exhibits a higher reflectance intensity compared to M-MnO₂. This difference can be attributed to the formation of defect-induced energy levels in the nanoparticles during the synthesis process. The intermediate optical response to visible light results in the absorption band at 282 nm [263]. The band gap (E_g) value of the green-synthesized C-MnO₂ and M-MnO₂ compounds was determined using the Kubelka–Munk equation, as shown in Figure 18b [172]. C-MnO₂ and M-MnO₂ possess band gap values of 1.42 and 1.39 eV, respectively. The variation in the band gap energy between these compounds can be attributed to disparities in their internal electronic structures and particle sizes [263,264]. Owing to their narrow band gap (1–2 eV), manganese oxides can act as photocatalysts in the visible light spectrum [265,266]. MnO₂ is considered a potent catalyst because of its porous nature, available lattice oxygen, and the presence of multiple valence states of manganese ions, such as Mn⁴⁺/Mn³⁺ and Mn³⁺/Mn²⁺. In addition to its notable catalytic properties, the widespread availability and affordability of MnO₂ make it a preferred choice for organic dye removal. Moreover, MnO₂ displays a diverse range of crystal structures and morphologies, enhancing its suitability for photocatalytic applications [267].

The photocatalytic efficiency of green-synthesized MnO₂ nanoparticles using Y. gloriosa leaf and curcumin extracts for degrading acid orange as an organic contaminant was previously studied, with decomposition results reported [140]. Green-synthesized M-MnO₂ and C-MnO₂ nanoparticles have been examined for their potential in photocatalytically degrading methylene blue and Congo red under visible sunlight. For the tests, 100 mL of each of the Congo red and methylene blue dye solutions (10 ppm) was combined with 0.05 g of the respective nanoparticles. The mixtures of M-MnO₂ or C-MnO₂ with the dyes were stirred in the dark for 30 min before being exposed to visible sunlight [172]. The absorbance of Congo red and methylene blue at 464 nm was determined using a JASCO V630 UV-Vis spectrophotometer to gauge the degradation efficiency. The following equation was employed to calculate the photodegradation efficiency (Ph in %):

Ph (%) = [(C_o − C_i)/C_o ] × 100,

(2)

Ph (%) = [(A_o − A_i)/A_o ] × 100,

(3)

in which C_o and A_o represent the initial concentration and absorbance of MB and Congo red prior to radiation, and C_i and A_i represent the concentration and absorbance of MB and Congo red, respectively, after a specific period of time of exposure. Figure 18c illustrates the alteration of the methylene blue (MB) highest absorbance peak (664 nm) of C-MnO₂ and M-MnO₂ after 140 min of exposure to sunlight [172]. After 140 min of exposure to visible light, there was a notable reduction in the absorbance intensity of MB. The photodegradation efficiency for MB dye using M-MnO₂ reached 96%. Meanwhile, C-MnO₂ displayed a slightly reduced photodegradation efficiency of 89% over the same exposure duration. Figure 18d,e display the photodegradation percentages of MB and CR dyes over time under visible light exposure. Both C-MnO₂ and M-MnO₂ exhibit commendable photocatalytic activity in degrading MB and CR. Between the two, M-MnO₂ consistently outperforms C-MnO₂ across all time intervals. Specifically, M-MnO₂ achieves a photocatalytic activity of 96% for MB and 93% for CR, while C-MnO₂ registers 89% for MB and 91% for CR. The superior photocatalytic performance of M-MnO₂ can be attributed to its narrower band gap. Studies have noted that both C-MnO₂ and M-MnO₂ nanoparticles exhibit urchin-like morphologies, composed of interconnected nanowires with particle sizes ranging from 4 to 10 nm. This structure significantly amplifies their photocatalytic activity [172]. Figure 17f elucidates the mechanism of the photodegradation process, showing how sunlight activates M-MnO₂ and C-MnO₂ to produce highly reactive radicals such as OH⁻, O₂⁻, and H₂O molecules.

The bandgap values for M-MnO₂ and C-MnO₂ nanoparticles suggest that both compounds can be effectively activated by sunlight’s visible spectrum. Upon activation, electrons (e⁻) are excited from the valence band to the conduction band in both M-MnO₂ and C-MnO₂, creating electron–hole pairs. These photo-excited electrons in the conduction band can subsequently interact with the dissolved oxygen (O₂) adsorbed on the surfaces of M-MnO₂ or C-MnO₂, yielding superoxide anion radicals (O₂⁻). Simultaneously, the holes in the valence band can react with hydroxide ions (OH⁻) and water molecules (H₂O) to produce hydroxyl radicals (OH⁻). These radicals, O₂⁻ and OH⁻, being highly reactive, are instrumental in breaking down organic molecular pollutants. The described mechanism provides insights into the photodegradation process of dyes, specifically methylene blue (MB) and Congo red (CR) [268].

MnO₂ NPs → photon energy (hʋ) → α-MnO₂ (h + VB + e⁻ CB),

(4)

e⁻ + O₂ → O₂⁻,

(5)

3O₂⁻ + 2H₂O + 2H⁺→3H₂O₂ + O₂,

(6)

h + VB + OH⁻→OH⁻,

(7)

h + VB + H₂O → OH⁻ + H⁺,

(8)

OH⁻/O₂⁻ + MB dye → non-toxic compounds (CO₂ + H₂O).

(9)

The superior photocatalytic activity of M-MnO₂ over C-MnO₂ can be attributed to its narrower band gap and reduced light scattering, enhancing its ability to absorb more light, as evident from the diffuse reflectance measurements. These findings suggest the potential for creating high-performing, cost-effective photocatalysts tailored for environmental and water treatment applications. While prior studies on MnO₂ have demonstrated rapid degradation, they often relied on UV radiation as the light source, which comes with higher operational costs [172]. In this study, the photocatalytic experiment was conducted using direct visible sunlight on an immediately prepared dye solution, presenting a practical and cost-effective approach. Given the escalating production of industrial wastewater, particularly from the textile industry, there is an urgent need to commercialize this environmentally friendly method. Further market research and exploration are crucial for scaling up this innovative approach to address the increasing environmental challenges.

7. Iron Oxide Nanoparticles

Iron and iron oxide nanoparticles are distinguished by their unique chemical, optical, and magnetic properties. Among the sixteen identified iron oxides, hematite (α-Fe₂O₃), maghemite (γ-Fe₂O₃), and magnetite (Fe₃O₄) are the most prevalent. Due to health, environmental, and economic considerations, biogenic production is often preferred [269]. The green synthesis of iron oxide NPs represents a burgeoning area in nanotechnology [270]. The efficiency in producing iron and iron oxide NPs is largely influenced by the quantity and concentration of antioxidants in plant extracts. Notably, plants with a high phenolic content exhibit potential reductive capability [271,272].

The green synthesis process of iron and iron oxide NPs typically involves combining plant extracts with iron precursors. A change in the solution’s color signifies the reduction of iron salts to iron ions [273]. V. Ramalingam et al. [274] successfully synthesized hematite nanoparticles using guava and drumstick leaf extracts as natural solvents. The extract solution was stirred at 80 °C for 1 h. Subsequently, a ferrous salt solution was mixed with the extract, stirred for another hour, and then filtered.

In the medical field, the technique of utilizing grain extracts to synthesize iron oxide nanoparticles is gaining traction. The UV-visible spectrum of iron oxide NPs produced using extract of Echinochloa frumentacea grains is presented in Figure 19a, showcasing a prominent absorbance band at 289 nm [269]. Figure 19b depicts the XRD of the iron oxide NPs, confirming their significant crystallinity. Using Scherrer’s formula, the average particle size was determined to be 26 nm. The EDX analysis of iron oxide nanoparticles reveals the following composition: 30.80 at.% of iron, 64.11 at.% of oxygen, and 5.09 at.% of carbon as the predominant elements within the nanoparticles.

Magnetite (Fe₃O₄) NPs were biosynthesized using an aqueous extract of Carica papaya leaves extract at room temperature. Figure 20 displays the XRD pattern and SEM image of the produced magnetite (Fe₃O₄) NPs, revealing a plate-like morphology characterized by coarse grains and encapsulated structures. Using Scherrer’s formula, the crystallite size of the magnetite was determined to be approximately 33 nm [275].

7.1. Iron Oxide NP Applications

Diverse medical applications of iron oxide nanomaterials synthesized using Echinochloa frumentacea grain extract have been reported by Velsankar et al. [269]. These applications include antioxidant, anti-inflammatory, and anti-diabetic activities.

7.1.1. Antioxidant Activity

The antioxidant activity of iron oxide NPs was evaluated to assess their potential bioactivity. Using the DPPH method, various concentrations of nanoparticles were tested, with ascorbic acid serving as the reference standard. The results, presented in Figure 21a [269], detail the determined percentage activities. Specifically, iron oxide NPs exhibited antioxidant activities of 36.08% at 20 μg mL⁻¹, 55.24% at 40 μg mL⁻¹, 72.05% at 60 μg mL⁻¹, and 95.10% at 80 μg mL⁻¹. In comparison, ascorbic acid demonstrated antioxidant activities of 29.50%, 44.26%, 62.10%, and 84.20% at the same concentrations. The efficacy of the antioxidant activity was gauged by determining the IC50 value, which indicates the concentration of a substance needed to achieve 50% activity. A lower IC50 value signifies greater effectiveness. Remarkably, the IC50 value for iron oxide NPs (34.5 μg mL⁻¹) was found to be lower than that of the standard ascorbic acid (46.45 μg mL⁻¹).

7.1.2. Anti-Inflammatory Activity

To assess the protective potential of the nanoparticles, an anti-inflammatory study was conducted using egg albumin. Different concentrations of iron oxide nanoparticles were tested, with diclofenac sodium serving as the benchmark. The observed results are presented in Figure 21b [269]. The iron oxide nanoparticles exhibited anti-inflammatory activities of 34.81% at 20 μg mL⁻¹, 53.95% at 40 μg mL⁻¹, 75.18% at 60 μg mL⁻¹, and 92.10% at 80 μg/mL. In comparison, diclofenac sodium showed anti-inflammatory activities of 28.50%, 45.80%, 67.25%, and 83.76% at the same concentrations. Notably, the iron oxide nanoparticles demonstrated superior effectiveness, as indicated by their lower IC50 value (35.88 μg mL⁻¹) compared to the standard diclofenac sodium (43.92 μg mL⁻¹).

7.1.3. Anti-Diabetic Activity

To assess their potential in biomedicine, the anti-diabetic properties of iron oxide NPs were investigated. The study utilized varying concentrations of iron oxide nanoparticles, with acarbose serving as the reference standard. The outcomes of this analysis, expressed in percentages, are illustrated in Figure 21c [269]. The iron oxide nanoparticles exhibited anti-diabetic activities of 38.10% at 20 μg mL⁻¹, 54.89% at 40 μg mL⁻¹, 78.25% at 60 μg mL⁻¹, and 91.68% at 80 μg/mL. In comparison, acarbose demonstrated anti-diabetic effects of 30.43%, 45.98%, 63.90%, and 80.96% at the same concentrations. Significantly, the iron oxide nanoparticles showcased a superior efficacy, reflected by their lower IC50 value (34.18 μg mL⁻¹) compared to the standard acarbose (44.49 μg mL⁻¹).

8. Silver Nanoparticles

Noble metals like gold (Au NPs) and silver nanoparticles (Ag NPs) have garnered attention due to their unique ability to interact with electromagnetic radiation, resulting in a myriad of features, including surface plasmon resonance (SPR). This interaction imparts these nanoparticles with remarkable optical properties, electrical conductivity, and chemical stability [276,277]. Such attributes have paved the way for their utilization in diverse sectors such as medical science, gene therapy, thermotherapy, and environmental enhancements [278,279]. Among the various methods to produce these nanoparticles, green synthesis, especially for silver and gold NPs, stands out. It has been gaining traction due to its eco-friendliness, distinctive properties, and adaptability [280,281]. This section delves into the various plant extracts utilized for the eco-friendly synthesis of these precious metal nanoparticles. The biosynthesis process involving plant extracts combined with AgNO₃ or HAuCl₄ solutions is straightforward, eliminating the need for elevated temperature or pressure conditions. Additionally, this section further elaborates on the applications of green-synthesized Ag and Au NPs. An overview of the uses for both silver and gold nanoparticles include antimicrobial agents, antioxidants, medical devices, cancer treatments, electronic devices, etc. [282].

Alghoraibia et al. [283] and [284] determined that the total phenolic content in the plant extract was directly correlated with its antioxidant capacity. Thus, a richer content of phenolic compounds led to a more significant reduction of metallic ions. Gopinath et al. [285] employed the extract of Tribulus terrestris to synthesize silver NPs. Analytical techniques, including TEM and AFM, revealed that these nanoparticles possess a spherical morphology with an average diameter of 22 nm. Additionally, an aqueous leaf extract from Ocimum gratissimum was utilized to produce Ag NPs. Mehata [286] reported the green route synthesis of silver nanoparticles using plant/ginger extracts with enhanced surface plasmon resonance and degradation of textile dye. The impact of ginger extract on the fabrication of Ag NPs was observed by varying the amount of ginger extract from 0.5 mL to 2.0 mL in silver salt, and measuring the absorption spectra at each concentration (Figure 22a). The effect of pH on ginger-capped Ag NPs was analyzed by maintaining the pH of the solution in the range of 7 to 12 by using NaOH. Figure 22b showcases the XRD analysis, highlighting distinct peaks characteristic of Ag NPs. This pattern includes the (111), (200), (220), (311), (331), and (222) crystallographic planes. The sharp diffraction peaks observed at (111), (200), and (220) indicate the formation of nanosized particles with a face-centered cubic structure of silver [287]. The SEM analysis of Ag NPs synthesized using Areca catechu showed spherical nanoparticles. Conversely, Ag NPs derived from the bulb of Allium cepa L. exhibited a cubical shape [288].

8.1. Ag NP Applications

Green synthesis, using Tamarindus indica fruit shell extract, has been employed to produce silver nanoparticles, which are applied in the treatment of human breast cancer [289]. Silver nanoparticles derived from plant extracts have been employed in drug delivery and, in some instances, direct disease treatment. When loaded with anti-infective agents, these synthesized nanoparticles could serve as even more potent nanocarriers [290]. Moreover, silver nanoparticles have been effectively synthesized from medicinal plant extracts, such as bamboo leaves. These plants are rich in phytochemicals, including flavonoids, phenolic acids, and lactones, endowing them with antiviral and antibacterial properties [291].

Silver possesses both antibacterial and anti-inflammatory properties, making Ag-based nanoparticles ideal for applications such as wound dressings, various detergents, shampoos, clothing, and toothpaste. Compared to other chemical agents and drugs, Ag nanoparticles have shown exceptional efficacy against antibiotic-resistant bacteria, multidrug-resistant tumors, and parasitic diseases, and have also acted as antioxidants and antifungals. Therefore, the synthesis of eco-friendly and biocompatible Ag nanoparticles is of paramount importance [292]. Numerous studies have indicated the efficacy of Ag nanoparticles against a variety of cancer cell lines and viruses. Smaller Ag nanoparticles can readily enter cells either by simple diffusion or through membrane transporters, ion channels, or receptors.

8.2. Antiviral Activity

Silver nanoparticles possess potent virucidal properties and have demonstrated efficacy against an array of viruses, including the human immunodeficiency virus (HIV), various hepatitis strains (A, B, C, and E), and the herpes simplex virus (HSV-1 and 2). Upon contact with viral membranes, silver nanoparticles can permeate the cell, subsequently damaging the viral genome (be it DNA or RNA) and interacting with viral membrane glycoproteins. These interactions with the cell’s genetic material curtail its ability to replicate, thereby limiting the spread of infections [293].

The significance of silver nanoparticles has been underscored during the COVID pandemic due to their proven antiviral properties. Specifically, they have demonstrated potential in inhibiting the proliferation of the SARS-CoV-2 virus by obstructing viral nucleotide replication. They engage with electron-donor groups found in bacterial enzymes, such as sulfur, oxygen, and nitrogen. This interaction deactivates the enzymes, depriving the cell of energy and quickly leading to its demise [294].

The exact mechanism by which silver nanoparticles neutralize viruses remains elusive. However, it is established that they bind to the outer structural proteins of viruses, hindering early infection stages by either blocking the virus’s attachment and penetration, or destabilizing surface proteins that compromise the virions’ structural integrity [295]. Typically, silver nanoparticles latch onto viral surface proteins with sulfhydryl groups, disintegrating disulfide bonds, which influence viral infections [296]. Furthermore, these nanoparticles also engage with viral nucleic acids, obstructing intracellular viruses [297].

The antiviral efficacy of silver nanoparticles is size-dependent. Research indicates that particles around 10 nm in size are especially potent, as their interactions with viral proteins are notably more stable compared to larger sized particles [298]. Concentration also plays a role in the antiviral activity of silver nanoparticles. Studies suggest that they are most effective against viruses when used in concentrations ranging between 10 and 100 ppm [299]. Figure 23 provides a schematic depiction of the viral pre-treatment processes (top panel), subsequent cell treatments (middle panel), and preliminary cell treatments (bottom panel), illustrating the efficacy of silver nanoparticles against extracellular SARS-CoV-2.3A [300].

9. Gold Nanoparticles

Gold nanoparticles (Au NPs) are garnering significant attention from scientists due to their unique physicochemical properties. These include impressive thermal stability, robust chemical resilience, biocompatibility, non-toxicity, ease of synthesis, excellent dispersibility, high electrical and optical conductivity, a substantial surface area to volume ratio, and multifunctionality. Their stability under physiological conditions further bolsters their appeal across diverse disciplines such as biology, medicine, and materials science. Historically, gold has held a revered place in traditional medical practices, especially in ancient China and India, where it was believed to promote fertility and longevity [301,302,303].

9.1. Au NP synthesis

In contemporary applications, Au NPs serve as antibacterial agents, tools for tumor detection and diagnosis, treatments for inflammatory conditions, and diagnostic instruments in fluorescence tomography [304,305]. Their distinct optical properties have spurred investigations for a wide array of applications, ranging from medicine, catalysis, drug delivery, crystal growth, biolabeling, phase transfer, DNA melting assays, to therapeutic uses [306]. Owing to their versatility, Au NPs can be fashioned into various morphologies and sizes, encompassing spheres, rods, cages, belts, wires, octahedrons, prisms, and stars [307,308,309,310,311].

Given the significant role of Au NPs in medical treatments, this study focuses on the green synthesis of AuNPs and their emerging applications in cancer therapy and medical diagnostics. The push towards green synthesis arises from the need for a non-toxic, environmentally friendly, and biocompatible approach, as opposed to traditional methods that often involve harmful chemicals.

Various biological entities, such as plant tissues, bacteria, microorganisms, and other organisms, have been leveraged for the biosynthesis of Au NPs. Of these, plant-based biosynthesis stands out as an appealing method for the mass production of Au NPs, primarily due to its cost-effectiveness and the ability to produce nanoparticles with precise size and morphology [312]. Thus, utilizing plant extracts in the production of Au NPs may pave the way for innovative applications in the near future [313]. The green synthesis approach using plants typically involves various plant parts, including leaves, bark, stems, and roots. These are finely chopped and boiled in distilled water to produce an extract. Subsequent purification processes, such as centrifugation and filtration, ensure the quality of the extract [314,315].

Proteins containing functional amino groups (-NH₂) in plant-based extracts can actively participate in the reduction process of Au NPs [316]. Phytochemicals, such as flavones, alkaloids, and phenolic compounds, contain functional groups (e.g., -C-O-C-, -C-O-, -C=C-, and -C=O-) that facilitate the formation of Au NPs. In such a green synthesis approach, harmful chemicals like sodium borohydride (NaBH₄) are substituted with non-toxic phytochemicals. These phytochemicals not only act as reducing agents but also serve as stabilizers or capping agents during the biosynthesis of Au NPs [317,318]. Jannathul and Lalitha proposed a mechanism for the biosynthesis of Au NPs using different plant sources [304]. Organometallic Au NPs have been prepared from diazonium tetrachloro aurate(III) complexes using a mild reducing agent, borohydride, via a simple method. This synthesis approach requires a revaluation process of isolated compounds from medicinal herbs with computational studies which will be helpful in producing Au NPs at a large scale for pharmaceutical applications. The green synthesis process begins by reducing metal ions from their monovalent or divalent oxidation states to the zero-valent state. The growth of the reduced metal atoms follows. Ultimately, the gold metallic salt solution undergoes reduction from Au³⁺ to Au⁰ due to the extract, culminating in the one-step synthesis of Au NPs in just a matter of minutes [319]. The diversity in sizes and shapes of the resulting nanoparticles can be attributed to variations in the composition and concentration of reducing agents present in the plant extracts [320].

For the biosynthesis of Au NPs, various fruit peel extract solutions have been utilized [321]. Pomegranate peel extracts, for instance, are used in the green synthesis of Au NPs, serving both as reducing agents and stabilizers or capping agents. The production of Au nanoparticles is evidenced by the color transition from gold to pink in the reaction mixture at room temperature [322].

In one study, Au NPs with diameters between 80 to 150 nm were produced using Plumeria alba flower extracts, which functioned as both reducing and encapsulating agents for the Au NPs. Mishra et al. demonstrated that the zero-calorie sweetener Stevia rebaudiana leaves can produce octahedral Au nanoparticles with sizes ranging from 8 to 20 nm [323]. Under ambient conditions (28 °C) and a pH range of 3.4–10.2, an aqueous leaf extract of Cassiem auriculata facilitated the conversion of gold ions into 15–25 nm nanoparticles [324]. There are also reports on the use of phytochemicals from cinnamon as reducing and stabilizing agents in the biosynthesis of gold nanoparticles [325]. Rapid reduction of Au ions using geranium leaf extract yielded gold NPs with rod and prismatic morphologies [326]. Moreover, the green synthesis of gold nanoparticles using extracts from the green and red residual parts of watermelon has been documented [327]. These synthesized gold NPs were analyzed using UV-Vis spectra, XRD, energy-dispersive spectra (EDS), and SEM. The UV-Vis spectrum of Au NPs synthesized from red and green watermelon extracts (3:1 ratio watermelon: AuCl₄⁻) displays a strong absorption peak at 560 nm. XRD pattern shows the presence of crystalline Au, with reflections at 2θ of 38.4°, 44.6°, 64.5°, and 77.8° corresponding to the (111), (200), (220), and (311) planes. SEM studies show the growth of various Au nanoparticle shapes produced using the red watermelon extract, and Au triangular nanoparticles generated using the green watermelon extract [327].

9.2. Green Au NP Applications in Cancer Therapy and Diagnosis

Cancers are characterized by the uncontrolled division of cells [320]. Despite advances in medical science, cancer continues to claim millions of lives annually [328]. Traditional treatments for cancer encompass a range of methods, including surgery, immunotherapy, radiotherapy, chemotherapy, targeted therapies, hormone therapies, and bone marrow transplantation [329]. Typically, cancer patients initiate their treatment regimen with surgery or chemotherapy [330]. However, these conventional treatments often present challenges. They can lead to the emergence of drug-resistant cells, have detrimental effects on healthy cells, promote rapid drug metabolism, and consequently diminish the effective treatment duration [331]. Targeted therapy, which focuses on cancer cells directly, mitigates these adverse effects and enhances therapeutic efficiency. As a result, innovative targeted therapeutic drugs have been increasingly sought after in recent times [332].

Nanotechnology, an evolving domain, employs nanoscale particles for both therapeutic and diagnostic applications [333,334,335,336,337,338,339,340,341,342,343,344,345,346]. Notably, cancer treatment stands out as one medical arena where nanoparticles hold immense potential [333]. Thanks to their diverse surface chemistries, nanomaterials can be paired with molecularly targeted drug delivery components, such as antibodies, facilitating the selective elimination of tumor cells. Au NPs synthesized through green methods have recently garnered significant attention in the field. Au NPs exhibit cytotoxic effects on cancer cells while sparing healthy ones, positioning them as promising anticancer agents [335]. These nanoparticles have been employed for targeted drug delivery in cancer treatments [336]. It is notable that the anticancer efficacy of biosynthesized Au NPs varies, with their potency and cellular interactions hinging on the specific natural extracts used during their formation [337]. The various biological effects and potential applications of green-synthesized metal nanoparticles are depicted in Figure 24.

Gold nanoparticles derived from Corchorus olitorius extract have shown significant inhibitory effects on three types of cancer cells: hepatocellular cancer HepG-2, breast adenocarcinoma MCF-7, and colon cancer HCT-116 [339]. Fazel et al. further explored the anticancer potential of gold nanoparticles green-synthesized using cocoa extract [339]. Au NPs are versatile carriers capable of delivering hydrophobic and hydrophilic drugs, herbal medicines, peptides, antibodies, short interfering ribonucleic acids, antibiotics, chemotherapy agents, and small molecules directly to tumor sites without affecting healthy or surrounding tissues. Additionally, the protective layer of Au NPs shields the drug from enzymatic degradation in the bloodstream [340,342].

Utilizing biosynthesized Au NPs as nanocarriers for herbal medicinal compounds has the potential to revolutionize cancer treatment by minimizing adverse effects. Owing to their ability to house multiple drug molecules, these nanoparticles can also target and penetrate specific cells more efficiently [335]. Figure 25 depicts the anticancer mechanisms of Au NPs [335]. These mechanisms include alterations in cell permeability, mitochondrial dysfunction, the release of reactive oxygen species (ROS) leading to oxidative stress and DNA damage, modifications in the chemical composition of proteins and DNA, and cell cycle arrest [343,344,345].

This section highlights recent studies examining the antitumor effects of various green-synthesized Au NPs on different cancer cell types. Hoshyar et al. [346] employed crocin, a compound abundant in phytochemicals, to produce Au NPs in a single step and in an eco-friendly manner. Crocin acts to neutralize harmful free radicals, thereby shielding cells from oxidative harm. The anti-proliferative effects of Crocin-Au NPs on human breast cancer cells were evaluated using the neutral red assay. An LDH (lactate dehydrogenase) analysis was conducted to ascertain the release rate of LDH enzymes from the cytoplasm. After 24 and 48 h of incubation, the IC50 (half-maximal inhibitory concentration) values of crocin-Au NPs stood at 1.8 mg mL⁻¹ ± 0.08 and 1.2 mg mL⁻¹ ± 0.04, respectively. These findings indicate a dose- and time-dependent decline in cancer cell viability. The observed results are attributed to the rapid internalization of the synthesized crocin-Au NPs by the cancer cells and the subsequent release of conjugated crocin from the Au NPs. Vinayagam et al. [347] green-synthesized Au NPs using Cynodon dactylon (Bermuda grass). The cytotoxic effects of these Au NPs were examined on the MCF-7 breast cancer cell line and the NIH 3T3 noncancerous cell line. Remarkably, even at elevated concentrations, the NIH 3T3 cell line exhibited negligible cytotoxicity. Furthermore, Au NPs derived from dragon fruit extract demonstrated notable anticancer activity against MCF-7 cells. Due to their diminutive size and expansive surface area, the Au NPs effectively targeted tumor sites. A remarkable inhibition of approximately 80% was achieved against MCF-7 cells at a concentration of 500 g mL⁻¹ after 48 h of treatment [348].

10. Future Prespectives

The main aim of this review is to provide the details of biosynthesis and its actual life applications that will help academics working in this developing field and act as a helpful manual for readers with a large interest in this subject. An eco-friendly strategy is needed to minimize waste in response to environmental catastrophes. The increasing growth in the population and the development of modern technology has created a demand for cheap, safe, and effective materials. This has encouraged researchers to develop a scalable green manufacturing process for the preparation of nanomaterials using different local plant extracts that have significant antioxidant capabilities without using harmful chemical reducing agents or chemical solvents.

Biogenic metallic and metal oxides can be synthesized by various organisms, such as plants, fungi, bacteria, yeasts, and algae, which induce considerable modifications of the material properties. Thus, the use of plants’ extracts provides wonderful advancements in nanobiotechnology. Meanwhile, Figure 26 depicts future perspectives and applications of biosynthesized manganese-based NPs (i.e., Mn, MnO, MnO₂, Mn₃O₄, etc.). They include electrode materials for energy storage (lithium-ion batteries and supercapacitors), energy conversion (optical and magnetic sensors), environmental (dye degradation, water purification) and biomedical applications (DNA purification).

These applications require to scale up from lab-scale to industrial-scale production of these green-synthesized nanomaterials for them to be used in different important sectors like industry, energy, water purification, medical treatment, etc. It is also necessary to extensively investigate the use of biological waste components in the biosynthesis of nanoparticles. So, using extracts from plants to create nanomaterials could have an enormous effect in the future. Several efforts may be listed for the production of nanomaterials from fruit extracts. Significant research is being conducted on various capping agents, including biomolecules and polysaccharides, which can function as chelating, reducing, and capping agents in the production of nanoparticles. As a result, the resultant nanoparticles have protection from additional reactions and aggregation, enhancing their stability and lifetime.

In the drug delivery system, these biogenic nanoparticles might be the future thrust for the biomedical field because they are opening up new promise for the treatment of many diseases and offering new perspectives on traditional approaches to therapy. The achievement of nanoparticle-based medication in clinical studies will help future studies and provide new therapies for some untreatable diseases. As discussed in this review, many plant and biowaste extracts are used as precursors in the biosynthesis of some metal and metal oxide nanoparticles like MnO₂, iron oxide, Ag, and Au nanoparticles. But some issues require further investigation in future studies. (1) The mechanism by which plant extracts help synthesize nanoparticles is yet unknown. (2) The impact of temperature and reaction time on the morphology of nanomaterials also needs to be investigated. The focus of future efforts will be on further modifications to the structure of biosynthesized MnO₂ nanoparticles by loading them in polymers to further enhance their electrochemical characteristics as storage potential electrode materials in the energy sector for lithium-ion batteries and supercapacitors.

The hetero-nanostructure of MnO₂ NPs and AuNPs with the medium of DNA is an interesting topic. In a recent report [349], MnO₂ nanosheets are regarded as a new class of two-dimensional nanomaterials with several attractive properties with enormous progress in biomedical fields. The MnO₂@Au nanostructures are thus applied to the analysis of miRNA. Duplex-specific nuclease (DSN)-catalyzed digestion, hybridization chain reaction (HCR), and catalytic hairpin assembly (CHA) are utilized for signal amplification. Analyzing the optical responses of the nanocomponents, a highly sensitive analysis of target miRNA can be achieved.

In terms of contamination-free utilization, biowastes are determined to be more favorable than other raw materials. This new way of synthesizing nanoparticles is simple, non-toxic, highly stable, and economic, with nanomaterial synthesis offering up novel avenues and a better future. Green nanoparticle production utilizing waste materials will enable researchers to create safer nanoparticles and gain knowledge about NP health and safety issues. So, forthcoming work will focus extensively on using different vegetable and fruit waste in the green synthesis of different NPs as a trial to create a green environment with zero contamination.

More study is needed to use biowaste-mediated nanoparticles, especially in the medical sector, for therapeutic medications, antibacterial activity, water purification, and food packaging. Additional research studies are being carried out on how to use NPs made from biowaste in smart and fashionable technology. Because of this, there is still great potential to make new green pathway methods that are based on green synthesis. Overall, algae and waste products for the biosynthesis of nanomaterials are promising nanotechnology sectors that may enhance nanoscience.

Green synthesis offers outstanding opportunities to enhance nanomaterial manufacturing on an industrial scale. Much future research is needed to create potential “green” materials and for the synthesis of nanoparticles to support industrial and commercial activities for the development of nanomaterials’ megafactories. The majority of research studies have been carried out on a small scale in research labs, but it is necessary to investigate and apply nanoparticles on a large scale in the fields of agriculture, environment, medicine, catalytic activity, and other sciences to meet the future demands of the growing world population. The main advantage of green synthesis is the availability of huge quantities of waste, as only process refinement is required to minimize manufacturing costs. Also, these green nanoparticles could help in solving the current energy crisis by being used in electronic devices that are powered by energy.

11. Concluding Remarks

The demand for safe and cost-efficient materials suitable for a diverse range of applications—from energy storage devices, biomedicine, and drug delivery, to cancer therapies, the medical industry, biosensors, textiles, water treatment, cosmetics, and dye degradation—has witnessed a surge in recent years. This escalating interest has prompted researchers to delve deeper into these issues. The quest is on for the evolution of eco-friendly and economical methods, with the anticipation that the future would necessitate novel, stable, non-toxic, and cost-effective materials. In this context, expanded research into the biosynthesis of NPs could prove immensely advantageous.

This paper provides an overview of the green synthesis of metal and metal oxide NPs, and contrasts these methods with conventional approaches. This review highlights the potential of various natural extracts as replacements for traditional components or solvents, eliminating the need for additional capping agents or typical industrial surfactants that are challenging to remove post NP synthesis. Green synthesis emerges as an uncomplicated, eco-friendly, cost-effective, energy-efficient, and scalable method for crafting nanomaterials.

Table 2. Summary of green-synthesized metal and metal-oxide nanoparticles using biological agents.

Material	Biological Agent	Particle Size (nm)	Application
MnO2 MnO2 KyMnO2 α-MnO2 α-MnO2 Fe3O4 Ag Ag Au Au	Peel and juice of lemon Black and green tea Broccoli extract Peel and juice of orange Moringa oleifera Echinochloa frumentacea Ginger rhizome Fungi Plumeria alba flower Cassiem auriculata leaf	17 8 4.4 7.2 (pore size) 4–10 33 22 1–100 80–150 15–25	Cathode in Li-ion batteries Cathode in Li-ion batteries Cathode in Li-ion batteries Electrode of supercapacitor Photocatalysts (dye degradation) Antioxidant, anti-inflammatory, and anti-diabetic activities Degradation of textile dye Antiviral (HIV, SARS) Cancer therapy Cancer therapy

lists the biosynthesized NPs with their potential applications investigated in this article.

Among the nanoparticles with a myriad of applications are those made of manganese oxides, iron oxides, silver, and gold. This review delves into the green synthesis of MnO₂-based NPs using an array of plant extracts known for their reductive capacities, attributed to biomolecules like flavonoids, terpenoids, polyphenols, alkaloids, saponins, proteins, and amino acids. Interestingly, NPs produced with plant extracts often exhibit superior properties than those crafted through conventional techniques. The composition and concentration of reducing agents in plant extracts play pivotal roles in determining the size, shape, and morphology of the resultant nanoparticles. Furthermore, this review offers insights into the large-scale fabrication of α-MnO₂ nanomaterials and their subsequent deployment as photocatalysts in water purification, as well as electrode materials in energy storage mediums such as lithium-ion batteries and supercapacitors.

This paper also covers the biosynthesis and versatile applications of iron oxide, silver, and gold NPs. For instance, iron oxide NPs synthesized from Echinochloa frumentacea grain extracts emerge as potential candidates for applications in the medical realm, given their antioxidant, anti-inflammatory, and anti-diabetic properties. Additionally, the utilization of diverse plant extracts in the green synthesis of precious metal NPs, like silver and gold, and their myriad applications, form a crucial part of this comprehensive review.