## 1 Introduction  
Zinc oxide (ZnO) is a well-engineered compound that has received remarkable interest globally due to its distinctive properties and usages in various applications, such as pharmaceuticals, cosmetics, photonics, and photocatalysis [1; 2; 3; 4; 5; 6]. ZnO nanoparticles (NPs) can be synthesized through many physiochemical routes, such as sol-gel processes, co-precipitation, laser vaporization, microemulsion, and ball milling [7; 8; 9; 10]  
Commonly, these preparation methods face several limitations, such as the high cost of equipment, the large area required for equipment set up, and additional use of capping agents, stabilizers and toxic chemicals [11]. Most of these chemical methods are not environmentally friendly due to the use of harsh chemicals for stabilizing processes, which will bind to the ZnO NPs, and limit their biological applications [1; 11]. To overcome these limitations, green chemistry procedures have attractedsignificant scientific attention and have provided a new path for material researchers, because they are safe and environmentally friendly methods, which do not produce toxic by-products. Developing simple and green methods for synthesizing ZnO NPs is thus important, and remains a challenge for researchers [11; 12]. According to the literature, plants extracts have been proposed as novel alternatives to chemical methods for synthesis of NPs [12; 13; 14; 15; 16; 17]; for example, Iran and most parts of Asia have a rich source of valuable plants for this purpose [12; 13]. It should be noted that most nanoparticles and plant extracts pose a threat to the ecosystem and human beings. Unregulated use in terms of their toxicity may affect the biological system in unanticipated ways. Better control of the synthetic parameters will undoubtedly help to improve not only the physiochemical properties of ZnO NPs but also their compatibility [14; 18]. Extracts of citrus fruits or peels have been successfully used in producing metal and various metal oxide NPs, such as gold (Au) NPs, silver (Ag) NPs, and titanium dioxide (TiO\({}_{2}\)) NPs [19; 20; 21]. In addition, several types of fruit and plant extracts have been used for the synthesis of ZnO NPs, such as _Limonia acidissima_, _Tabernaemontana divaricata_, _Cochlosperum religiosum_, _Conyza canadensis_, _Citrus maxima_, _Boswellia coalifoliaula_, _Echinacea_ spp., _Salvadora oleoides_, and _Aristolchia indica_[22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33]. [PERSON] et al. [33] reported the influence of _Cuminum cyminium_ leaf extract on ZnO NP synthesis, which showed a good crystallite structure and high sensitivity to Gram-negative bacteria. In a similar study, _Hibiscus sabdarifa_ and _Acalypha indica_ extracts presented suitable optical and antibacterial properties [34]. Furthermore, ZnO NPs have shown promising properties in photocatalytic applications; it has been reported that _Parkia roxburghii_ extract/ZnO NPs showed excellent degradation of methylene blue (MB) and Rhodamine B dyes, reaching about 98% [34]. _Sambucus ebulus_ (elderberry or danewort), which is a natural plant with novel nutritional properties, grows in most parts of South America, Asia, North Africa, and the United States [35]. This plant is rich in biomolecules for reducing metal ions and stabilizing nanoparticles; _S. ebulus_ leaf extract contains enormous numbers of carbonyl, carboxyl, and hydroxyl groups in polyphenols and flavonoids [35]. These compounds can have a positive effect on human health because they can act as antioxidants and anti-inflammatory and anticancer agents. Due to its easy collection, natural origin, and remarkable biological properties, it could be an ideal replacement for expensive modern chemical surfactants. As mentioned, _S. ebulus_ contains high amounts of natural antioxidants, such as flavonoids, lectins, anthocyanins, vitamin C, and phenolic compounds which may be considered as suitable stabilizing agents [35].