Agriculture is the main source of our food, feed, fiber, and fuel. Fibers are one of the important agro-productivity sources. Fibers can originate from two main sources, natural and synthetic fibers. Natural fibers refer to the fibers obtained from plants, animals, and minerals. Concerning plants, natural fibers may originate from leaves, seeds, bark, fruit, and stalks as sources of fiber, whereas animal ones are derived from silk, wool, and hair as well as mineral fibers like asbestos
[1]. The main properties of natural fibers may include renewable, safe, non-polluting, and legitimate sources of fiber, which could be employed for the manufacturing of several composites in the future
[1]. Several recent publications issued on different applications of natural fibers, such as important sources for producing polymers (e.g.,
[1][2][3]). These applications may include the following fields, such as biomedicine
[2], 4D printing technology
[4], aerospace
[5], filtration of water/wastewater
[6], textiles
[7], additive manufacturing applications
[8], the automotive industry
[9][10], the construction industry
[11], thermal structure engineering
[12], food packaging
[13], harvesting and the storage of energy
[14]. The water–energy–food (WEF) nexus has a strong relationship with nanofibers, which can support their components through mainly the following activities: food packaging
[15], energy processing
[16], and water handling
[17].
Nanofibers are a kind of fiber that can be in natural and synthetic nanoforms (less than 100 nm). These fibers have several distinctive properties compared with natural fibers such as high porosity, large specific surface area, and high size uniformity
[6]. Apart from natural fibers, nanofibers have been applied in many major sectors of our life, including agricultural
[18], pharmaceutical
[19], biomedical
[20], and industrial fields
[21] of the global water
[22][23], energy
[24], and food sectors
[25]. Biotechnological applications of nanofibers have gained sound interest from scientists and industrial workers due to their promising roles in many fields such as delivering bioactive compounds
[26], and the biomedical field
[27]. The suggested roles of nanofibers in the main sectors of water, energy, and food are linked to the security of these sectors, and their nexus. Nanofibers in the water sector were discussed in many publications focusing on water treatment
[6], water purification
[28], and wastewater treatment
[23]. Additional studies on energy and food industry applications of nanofibers could be noticed, which mainly focused on harvesting/storage of energy
[24] or food packaging
[25].
2. Nanofibers in Water, Energy and Food
What are the main properties of nanofibers? Nanofibers are polymeric fibers on a nano-scale, which have certain properties and can be produced using both natural and synthetic polymers. Plant natural fibers are the main sources for producing such fibers along with animal and mineral sources, which depend on the used plant fraction such as leaves, stems, stalks, and seeds (
Figure 1). The most common plant fiber sources may include seed fibers from cotton (
Gossypium arboretum L.), fruit fibers from coconut (
Cocos nucifera L.), bark fibers from jute (
Corchorus olitorius L.), leaf fibers from sisal (
Agave sisalana L.), and banana (
Musa sp.). Synthetic or artificial polymers of nanofibers are widely used due to their easy production, low cost, and higher mechanical properties, but they can cause long-term environmental and human health problems due to their nonbiodegradable disposal, toxic nature, and their persistence in the ecosystem for a long time
[29]. Nanofibers possess several desired properties such as extremely high porosity, low density, high specific surface area, and a highly porous matrix. Nanofibers also have specific functionalities due to their large specific area, which allow immobilizing nanoparticles (NPs), metal–organic frameworks, and zeolites
[29].
Figure 1. The main sources of natural fiber resources that can be used in the agro-productivity, whereas (A) refers to the plant sources of natural fibers, (B) animal sources, and (C) is mineral sources.
Due to the previous advantages, nanofibers have many applications such as drug delivery systems
[30], industrial building design
[31], biomedicine
[32][33], 4D printing industry
[34], textile industry
[35][36], and wastewater remediation
[37]. Nanofibers could be produced by electrospinning and non-electrospinning techniques (
Figure 2). Several unique properties of electrospun nanofibers are well-known compared with other bulk materials such as adjustability of pore sizes, large surface area, and porosity, as well as the extracellular matrix
[26]. These production methods with advantages and disadvantages can be presented in
Figure 3. Microbial sources for producing nanofibers are considered an important approach, which is applied to many fields such as bacterial cellulose nanofiber
[38], microbial fuel cells
[39], microbial polysaccharides for controlled drug release
[40], and monitoring of food packaging
[41]. These fields will be discussed in the following sections.
Figure 2. The main basic information on nanofibers, their definition, classification, and applications.
Figure 3. Suggested advantages (in the yellow boxes) and disadvantages (in the pink boxes) of producing methods of nanofibers.
The electrospinning method is the most widely used compared with other techniques for producing electrospun fibers due to its efficiency, operational simplicity, practicality, cost-effectiveness, and versatility
[26]. The production of nanofibers is not only the main limiting factor but also the properties of these fibers, which should be evaluated by scanning electron microscopy (SEM) and other methods. Certain properties of nanofibers, which are important for their functions, will be discussed in the next section including physico-chemical, mechanical, and biological characteristics. On the microscopic level of nanofibers, SEM images of the fabrics revealed that the enhancement in fiber strength can be attributed to the formation of structures resembling bamboo nodes or wheat stem nodes within the fibers. The production of nano-sized fibers and the use of these fibers to create non-woven fabric was achieved by our lab (Nano Food Lab, Debrecen University, Debrecen, Hungary). These nanofibers were applied as specialized filtering materials for gases (air) and liquids, as well as for manufacturing functional filters. Several scientific attempts for producing nanofibers in our lab were developed on different levels including a laboratory scale, a pilot-scale device, and a full-scale manufacturing facility. Design, development, and optimization of nanofiber formation using different parameters (e.g., flow rate, voltage, electrode distance, surface, time, and solution concentration) were evaluated. Construction of a full-scale manufacturing facility included design, implementation, test production, and machine adjustment, as well as mass production of products, quality control, and implementation of a quality assurance system.