Abstract

General background: Nanotechnology has emerged as a vital field for the development of functional materials at the nanoscale, with zinc oxide nanoparticles (ZnO NPs) gaining attention due to their high thermal stability, cost-efficiency, and antimicrobial properties. Specific background: Traditional physical and chemical synthesis methods, while effective, often pose environmental risks and scalability challenges. In contrast, green synthesis utilizing plant extracts offers a sustainable alternative. Knowledge gap: Despite growing interest, a comprehensive understanding of how different plant species and zinc salts affect ZnO NP morphology and functionality remains limited. Aims: This review aims to consolidate recent studies on the green synthesis of ZnO NPs using diverse plant extracts and zinc precursors, focusing on synthesis parameters and resulting nanoparticle characteristics. Results: Analysis reveals that variables such as molarity, temperature, and reaction time significantly influence particle size (8–66 nm), shape (spherical, hexagonal, rod-like), and bioactivity (antibacterial, anticancer, antioxidant). Novelty: This work systematically compares a wide array of plant-mediated synthesis strategies, offering a unique reference for optimizing ZnO NP production. Implications: These findings highlight the potential of eco-friendly synthesis routes for biomedical, environmental, and industrial applications, supporting further advancement in sustainable nanotechnology.
Highlight :

• The review emphasizes the eco-friendly and cost-effective green synthesis of ZnO nanoparticles using various plant extracts.

• It discusses how synthesis conditions (e.g., temperature, molarity, reaction time) significantly affect size and morphology of ZnO NPs.

• Biosynthesized ZnO NPs exhibit diverse biomedical and environmental applications, including antimicrobial, anticancer, and photocatalytic uses.

Keywords : Nanotechnology, Green Synthesis, ZnO Nanoparticles, Plant Extracts, Biomedical Applications

Introduction

Nanotechnology refers to the precise manipulation, control, and fabrication of novel materials or devices at the nanometer scale (10^-9m), exhibiting distinct properties from those of bulk materials. Nanoparticles are regarded as the fundamental building blocks of nanotechnology, serving as the essential foundation for various nanostructured technologies and materials. Nanomaterials can be acquired through natural means, such as viruses and proteins, or unintentionally, such as nanoparticles created from diesel combustion. They can also be intentionally manufactured using specific fabrication processes. Nanoparticles are particles that have one or more diameters between 1 and 100 nm. They can exist as individual particles, or they can be grouped together as aggregates or agglomerates [1–3]. The nanomaterials have exceptional properties against the bulk matter. First, the properties of material such as energy change at the nanometer level. Nanomaterials possess two notable characteristics. Firstly, they can be constructed atom by atom using two distinct production techniques (bottom-up and top-down). Secondly, nanomaterials exhibit the remarkable attribute of having a high surface-to-volume ratio in comparison to bulk materials [4,5]. These superior properties make the field of nanotechnology attractive to many researchers to synthesis and optimize new materials in nano scale range [5]. In addition, the preparation of nanoparticles is cost effective, facile, and simple and there is no need to large amount for an application. Thus, a clear enhancement has been obtained in fabricating nanoparticles with desired size and morphology for applications [6,7]. However for non-toxic, ecofriendly production of nanoparticles the green synthesis methods were employed by using different substrates including plants, bacteria, fungi, and algae for generating metal oxide particles at a large scale while ensuring their purity [8]. Thus , among verity of many metals and metal oxides and despite of their novel properties ,ZnO nanoparticles has been chosen because of high thermal stability , more cost effective production and the good antibacterial activity , also the wide range of applications such as electronics, communications, sensors, cosmetics and environmental protection [10-12] The present review primarily centers on the significance of ZnO nanoparticles and various synthesis methods, with a particular emphasis on biological approaches, characterization techniques employed, and factors influencing the properties and dimensions of biosynthesized ZnO nanoparticles.

Approaches for producing ZnO NPs

Nanostructures can be created through the process of synthesis, and nanoparticles can be produced using various methods. These techniques involve synthesizing dry particles as well as dispersing nanoparticles in liquid. Nanostructures can be formed either by assembling atoms together or by reducing the size of micro particles to nanoparticles [13]. The synthesis of nano particles can be achieved through two distinct approaches, known as the top-down approach and the bottom-up approach [14]. The top-down technique entails the fragmentation of the bulk material into structures or particles at the Nano scale. Top-down synthesis techniques are an expansion of the methods used to create particles that are on the scale of microns. Top-down techniques are fundamentally more straightforward and rely on either removing or dividing bulk material or on shrinking bulk production procedures to create the desired structure with suitable attributes. The primary issue with the top-down strategy lies in the inherent flaws of the surface structure. For instance, nanowires produced using lithography exhibit surface roughness and can harbor a significant amount of contaminants and structural flaws. Aerosol spray, gas-phase condensation, atomic force manipulation, electron beam lithography, and high-energy wet ball milling are a few instances of these techniques [15]. The term "bottom-up approach" refers to the process of constructing material at the molecular, atomic, and cluster level by adding one molecule, atom, or cluster at a time. During the assembly process, physical forces are applied to the nanostructure to merge the particles into a bigger entity. Nanotechnologists mostly like the bottom-up method for making complex nanostructures because it lets them precisely control particle size, which improves optical, electrical, and other properties [16]. Additionally, the methods for synthesizing nanoparticles can be categorized into three classes: physical, chemical, and biological processes [17] as shown in Fig.(1). In comparison between the mentioned methods, the physical and chemical types have some drawbacks such as the difficulty to achieve the expected narrow size, having control over the surface chemistry and the structure of the nanoparticles, costly, non-eco-friendly as lead to the release of harmful by products. Moreover they caused energy consumption, environmental pollution and biological risk [18-22] While the biological method referred to the green chemistry and considered the best because its eco-friendly, biocompatible, facile technique, cheap and can deal with the limitations of nanoparticle synthesis. The biological system employed algae and plants [23], bacteria [24], yeast [25], fungi [26], and human cells [27, 28], and used the reductive and stabilizing agents of their proteins and metabolites to convert metal ions into their particular nanoparticles. Nanotechnology researchers are interested in green synthesis. Many studies have demonstrated that microorganisms and plant components can produce zinc oxide nanoparticles [29–31].

Figure 1.The synthesis and approaches of nanoparticles.

Plants-mediated synthesis of ZnO nanoparticles.

It has been proven that plants can be used to synthesis nanoparticles by eco-friendly, facile, simple and safe methods to use. Utilizing plant extracts is a straightforward and efficient method for synthesizing metal or metal oxide nanoparticles in large quantities. This method is simpler and easier compared to using bacteria or fungi, as plants contain abundant medicinal compounds that can reduce complex metallic ions into simple ions. The concept of utilizing metal reduction on nano-sized materials was derived from the observation of metallic ions accumulating in plant cells and tissues [32]. Plants are frequently chosen as the primary biological source for producing ZnO NPs due to their cost-effectiveness in production and handling, minimal environmental impact, simplicity in manufacturing, and their resilience against potential harm from microorganisms. Additionally, plant extracts are commonly prepared using distilled water as solvents, which present fewer health risks compared to microbial-assisted ZnO NPs synthesis [33]. A great number of leaves have been utilized in the production of ZnO nanoparticles. ZnO nanoparticles can be produced through biological processes.Successful Biosynthesis of zinc oxide nanoparticles using plant extract is presented in table (1).

Table 1

Conclusion

In conclusion, the review underscores the significance of ZnO nanoparticles in nanotechnology and the promising role of green synthesis methods in addressing environmental concerns. The comprehensive discussion of various green synthesis methods using plant extracts, alongside a summary of successful ZnO NP synthesis experiments, demonstrates the feasibility and effectiveness of this approach. Factors influencing the synthesis process, such as molarity, temperature, pH value, and reaction time, have been explored, emphasizing the need for careful consideration during the synthesis and characterization phases. The ability to control these parameters contributes to tailoring the size and morphology of the ZnO nanoparticles for specific applications. As researchers continue to explore and optimize these methods, the potential for ZnO nanoparticles to contribute to advancements in various fields remains a subject of ongoing interest and investigation.

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