The synthesis and application of nanometals have become a focal point of nanotechnology over the past few decades. Research in this area spans multiple disciplines, including material science, biological science, electronics, medicine, the pharmaceutical industry, and environmental science. Nanometals can be synthesized using various protocols, each with its own set of advantages and challenges. Traditional methods, while effective, often pose significant drawbacks such as environmental toxicity, high costs, high energy consumption, and potential health hazards. In contrast, the green synthesis of nanometals has garnered considerable attention in recent years as a more sustainable and eco-friendlier alternative to produce various nanomaterials, such as carbon nanoparticles, nanofibers, nanocomposites, carbon dots, quantum dots, and metal-based nanoparticles [1]. This approach utilizes biogenic pathways, leveraging natural resources such as plant extracts or microorganisms to produce nanometals. The significance of green synthesis lies in its ability to mitigate the adverse effects associated with conventional methods. By employing biological entities in the synthesis process, green methods reduce the environmental footprint, lower production costs, and minimize health risks, making them a compelling choice for sustainable nanotechnology [2, 3]. The importance of green synthesis extends beyond environmental benefits. Biogenic approaches often result in nanometals with unique properties that can be tailored for specific applications, enhancing their effectiveness in various fields [4, 5].
Recently, various waste materials have been utilized as initial feedstocks for producing NMs [1, 6, 7]. Waste materials can be divided into two categories: biomass and industrial waste. Biomass waste includes unprocessed or raw plant parts such as leaves, roots, bark, stems, fruits, shoots, flowers, and seeds. Many researchers have successfully used extracts derived from these plant parts for the efficient biosynthesis of nanomaterials [1, 6, 7]. Extractives, also called secondary metabolites, include phenolic acids, terpenoids, alkaloids, and flavonoids which can act as reducing as well as capping agents, which results in the synthesis of nanoparticles by reducing metal ions, followed by nucleation and, finally, growth [6–9]. The continuous availability, sustainability and potential of waste material for the production of high-value products are factors that have attracted researchers to work upon plant biomass-based nanoparticle production [10].
The present study aimed to utilize discarded plant leaves from four different plants, teak, neem, lantana and senna, to produce copper nanoparticles (CuNPs). Teak and neem are important agroforestry tree species largely planted by farmers in different planting systems, such as blocks, bunds, and paired row systems, in conjunction with different agriculture and horticulture crops [11]. As part of management practices, large quantities of leaf biomass are generated on farms after pruning and lopping, which are unutilized or otherwise discarded as waste. Lantana and senna are the major invasive alien species in India and pose threats to many ecosystems, including forests. The National Biodiversity Authority, Ministry of Environment Forests and Climate Change, Government of India, reported that these two species have rapidly multiplied and spread in different ecosystems, severely affected ecosystem functions and services and caused biodiversity loss [12]. Management practices aimed at eradicating these species also generate large quantities of leaf biomass in the country. In this context, we aimed to use these biomass waste effectively for the synthesis of CuNPs.
The green synthesis of Cu-NPs from leaf extracts has been reported as a remedy for most of the limitations associated with the physical and chemical methods used for Cu-NP production. Cu-NPs produced via this method showed good morphological, chemical, and biological activities [13]. Several extracts from plants such as Asparagus adscendens Roxb. Root [14], Nerium oleander leaf [15], Tinospora cardifolia leaf [16], and from trees such as Prunus nepalensis fruit [17], Celastrus paniculatus leaf [18], Alstonia scholaris leaf [19], Eucalyptus globulus leaf [20] and Ziziphus zizyphus leaf [21] have been reported to be good reducing and stabilizing agents for the synthesis of Cu-NPs.
CuNPs have a wide range of applications in nanoparticle synthesis, biological system catalysis, electrical wiring, and optical, biomedical and antimicrobial activities [22, 23]. Copper nanoparticles are antimicrobial because they are toxic to microorganisms and nontoxic to animal cells; therefore, they are considered to be safe for humans [24]. The antifungal and antibacterial mechanisms of copper nanoparticles (CuNPs) have been extensively studied, with many reports highlighting their applications in the food industry and plant disease management. The primary mechanism involves the binding of CuNPs to the cell membranes of microorganisms, resulting in cytoplasmic leakage and subsequent cell death [25, 26]. Factors such as particle size, shape, and charge significantly influence the antifungal activity of CuNPs [27, 28]. In forestry applications, the antifungal properties of CuNPs are important for controlling fungal pathological disease and decay in plants and wood [29, 30]. The present study is the initial phase of the institute level project, which aims to prevent wood decay and damage caused by biological agents, including microorganisms, through the application of eco-friendly copper nanoparticles. In this context, we aimed to characterize green-synthesized CuNPs, including their antifungal characteristics, through systematic research.