Biotechnology of Nanofiber in Water, Energy, and Food Sectors

Abstract

Natural resources including water, energy, and food have an increase in demand due to the global population increases. The sustainable management of these resources is an urgent global issue. These resources combined in a very vital nexus are called the water–energy–food (WEF) nexus. The field of nanotechnology offers promising solutions to overcome several problems in the WEF nexus. This review is the first report that focuses on the suggested applications of nanofibers in the WEF sectors. An economic value of nanofibers in WEF sectors was confirmed, which was mainly successfully applied for producing clean water, sustainable energy, and safe food. Biotechnological solutions of nanofibers include various activities in water, energy, and food industries. These activities may include the production of fresh water and wastewater treatment, producing, converting, and storing energy, and different activities in the food sector. Furthermore, microbial applications of nanofibers in the biomedicine sector, and the most important biotechnological approaches, mainly plant tissue culture, are the specific focus of the current study. Applying nanofibers in the field of plant tissue culture is a promising approach because these nanofibers can prevent any microbial contamination under in vitro conditions, but the loss of media by evaporation is the main challenge in this application. The main challenges of nanofiber production and application depend on the type of nanofibers and their application. Different sectors are related to almost all activities in our life; however, enormous open questions still need to be answered, especially the green approach that can be used to solve the accumulative problems in those sectors. The need for research on integrated systems is also urgent in the nexus of WEF under the umbrella of environmental sustainability, global climate change, and the concept of one’s health.

1. Introduction

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].

This study, as far as we know, is the first report discussing the relationship between nanofibers and the WEF nexus. The biotechnological applications of nanofibers in WEF and biomedical sectors also were included. This work was also designated to highlight the promising applications of nanofibers in the field of plant tissue culture and other further applications (e.g., mainly on a large scale in the industrial fields).

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].

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.

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 (Figure 4). 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.

2.1. Nanofibers in the Water Sector

Nanofibers have certain properties that support their function including physico-chemical, mechanical, and biological characteristics (Figure 5). These properties have a great role during water filtration and wastewater treatment. They allow nanofibers to be preferable for many water processes such as wastewater treatment [37], desalination [42,43], removal of heavy metals [23], and eco-remediation of highly concentrated wastewater streams [44]. Water-based nanofibers are considered attractive alternatives to organic solvents, which are used during the electrospinning methods [45]. This kind of fiber can be applied to nanofibers such as water-resistant polyvinyl alcohol nanofibers [46], modified cellulose nanofibers [47], and cellulose succinate nanofiber [16]. Such hydrophilic cellulose nanofiber as a separator has a high porosity (60%), and a hydrophilic surface, which can enhance a strong electrostatic repulsion between facilitating ion transportation and fibrils and increase negative surface charges [16]. Microbial nanofibers are considered a favorable alternative to petrochemical-based membranes due to their higher flux rate, higher tensile strength, flux recovery, and favorable stability ratios, as well as sustainable and eco-friendly membranes [48].

For a long time, nanofibers synthesized using polymers synthetized from petroleum caused several environmental problems because they are non-degradable and persistent in the environment. So, green nanofibers are considered a promising approach on the industrial and academic levels to produce plant-derived nanofiber polymers instead of synthetic polymers derived from petroleum due to their sustainable, renewable, completely biodegradable, and eco-friendly nature [29,49,50]. More published studies on green nanofibers are presented in

Table 1. Studies on green nanofibers and their applications.

Nanofiber Type	Plant Source	The Main Findings	Refs.
Cellulose nanofiber	Cassava pulp	Nanofibers improved the mechanical properties of injectable hydrogels for meniscus tissue engineering by promoting cell viability, and physico-chemical properties	[56]
Green flexible nanofibers (GNF)	Rice straw	Isolated GNF was used in combination with ZnO-NPs to prepare flexible nano-paper films for electronic components	[57]
Lignin-containing cellulose nanofibers (LCNFs)	Sugar cane bagasse	Sustainable and eco-friendly nanofibers for producing LCNFs using microwave-assisted natural deep eutectic solvent with high-performance	[58]
Lignin-based electrospun nanofiber	Bamboo pulp	This nanofiber membrane decorated with photo-Fenton Ag@MIF-100(Fe) was with good wettability and high porosity as a green, sustainable, and efficient membrane for methylene blue dye removal and degradation (99%)	[37]
Starch-based green electrospun nanofiber	Natural polymer of pullulan	Green nanofibers as fast-dissolving nanofibers for delivery bioactives (cumin) in food and pharmaceutical fields	[59]
MXene–cellulose nanofiber (CNF) composites	Cellulose from biomass (wood, cotton, hemp, etc.)	Green nanofibers are low cost and have biodegradability, process-ability, and a hydrophilic behavior, which leads to acceptable dispersion in water treatment by using the vacuum-assisted filtration	[60]
Cellulose nanofibers	Bamboo chips	Cellulose nanofiber-blended polylactic acid composite can be produced from bamboo under high-pressure steam and high-temperature for low-strength biodegradable polymer	[61]
Lignin-containing cellulose nanofiber/glycerol composite	Poplar pulp	This nanofiber composite can be applied for engineering light management, solar cells, food packaging, anti-glare film, and flexible optoelectronic devices	[62]
Nano complexes-based nanofibers	Cellulose extracted from rice straw	Polyvinyl alcohol/NC/cellulose nanocrystals could be applied for multi-functional water treatment and removing pathogenic microorganisms and other pollutants	[63]
Nanofiber-hydrogel composite	Dried chamomile flower	A polyamide/Pistacia atlantica (P.a) gum nanofiber and PEBAX/Polyvinyl alcohol/Ag hydrogel showed a high anti-microbial activity towards the E. coli	[64]
Cellulose nanofibers (CNFs)	Sugar beet pulp	CNFs had higher hydrophilicity, and tensile strength properties, whereas a decrease in air porosity when coated paper sheets after hydrothermal-depectinated pulp coating	[65]
Cellulose nanofibers (CNFs)	Salicornia ramosissima waste	Produced nanofibers can be used as reinforcing agents of polymeric composites, food packaging, pharmaceutical medical, and cosmetic industries	[66]
Cellulose nanofiber-polyvinyl alcohol	Oil palm bunches	A good nanocomposite film has water vapor transmission rate and transparent film layer (1294.82 and 1395.91 g/(m2 0.24 h), resp. for gas barrier application on paper packaging	[67]
Cellulose nanofiber	Artificial produced by a company	Using basil essential oil and corn starch coated nanofiber to prolong the shelf life of the mandarin orange by maintaining the fruit quality, and reducing weight loss of the coated fruits	[68]
Stabilized cellulose nanofibers	From wood fiber	Coating nanofibers with chitosan NPs as edible coating for maintaining fruit quality against antifungal activities (Rhizopus stolonifera and Penicillium digitatum)	[69]

The 2D layered materials are called “MXenes”.

. Along with green nanofibers, animal sources of nanofibers were reported such as chitin from crab shells for coloring dyes [51], and wool keratin nanofibers for the biomedical field [52]. It has also recently reported on developing green tires using chitin nanofibers (from prawn shells), natural rubber, and carbon black as a sustainable and circular economy approach [53]. Interestingly, a new green approach for the wearable biosensors that can be used in monitoring the physiological metabolism in biofluids (e.g., sweat, blood, saliva, urine, and tears) was reported by detecting glucose and uric acid [54] or a non-green approach [55].

The microbial role in water treatment is noteworthy during water/wastewater processing. Many studies indicated this potential approach of applying microbes as immobilized carriers during wastewater treatment (e.g., [46,70,71]). Apart from microbial nanofibers, the microbial immobilization of wastewater treatment has proved its efficiency in removing and biodegradation of many types of wastewater pollutants for a low cost, highly efficient, high surface area, chemically and physically stable carriers, and excellent biological compatibility [72]. More concerns were focused on the microbial removal of emerging pollutants, and the green catalysts using enzymes such as laccase during wastewater treatment [73]. Concerning the suggested mechanism of nanofibers during the water treatment, this is presented in Figure 6, as well as the list of some suggested kinds of nanofibers as well.

Nanofibers have been used in the sector of water treatment as adsorbents, electrochemical electrodes, photocatalytic materials, and membranes for removing specific pollutants such as persistent organic pollutants, heavy metals, emerging pollutants, and oily molecules. Electrospun nanofiber membranes have many advantages during water treatment, such as a homogeneous pore distribution, high surface area to volume ratio, a wide variety of polymers, ease of fabrication, high porosity, and high hydrophobicity [29]. Whereas, the disadvantages may include high voltage, differential pore size and fiber thickness, and toxic solvents. The mechanisms of removing pollutants (e.g., heavy metals and others) from wastewater using nanofibers involve the photocatalytic process, adsorption, ion exchange, diffusion dialysis, electrostatic processes, and metal–organic framework crystal or polymers [73]. The application of nanofibers as adsorbents mainly depends on their functional groups in the polymeric matrix, which have high selectivity for removing the target pollutants [29].

2.2. Nanofibers in Energy Sector

The energy sector represents a crucial resource for our life. At all times, a global urgent concern is needed for searching for new untraditional sources for producing and maximizing energy production. This approach could be achieved by applying different reliable agro-wastes as renewable, excellent, and affordable resources [74]. At the same time, the accumulation of agro- and industrial waste causes harmful environmental issues, which should be valorized into energy. The most important suggested approaches for the energy crisis may involve microbial and green energy, as sustainable strategies for producing energy apart from fossil sources. Microbial energy conversion or microbial energy technologies are sources where microbes can make fuels out of raw organic materials and convert the chemical energy in the agro-biomass into chemical energy in the form of hydrogen or ethanol through the bioenergy process (Figure 7) [75]. The main applications of nanofibers in the energy sector are lithium-ion batteries, solar cells of crystalline silicon using nanofiber, dye-sensitized solar cells, supercapacitors for electrospun carbon nanofibers, hydrogen storage using nanofiber membranes, and pressure-retarded osmosis (Figure 8). Using biomass-based nanofibers in energy storage has many advantages such as high specific surface area, unique porous structure, low cost, and easy availability [76].

Concerning green energy, cellulose nanofibers (CNFs) in addition to wood and other lignocellulose materials (e.g., agro-residues) are the most common renewable materials that can be used for producing this kind of energy [77]. Green nanofibers have a specific surface area, and high strength, and are lightweight, stiff, renewable, and biodegradable [59]. CNFs can be applied in different fields such as membranes and polymer films [47,48], reinforcing agents in plastic composites [56], wastewater treatment [21], food additives [15], paper coatings [65], energy storage [16], medicines [78] and cosmetic products [66]. It is worth mentioning that carbon nanofibers derived from nanocarbon materials have several integrated advantages, such as reducing chemical functionalization [62], chemical and structural flexibility [79], bulk production potential [80], better electrical conductivity [81], and use during different electrochemical processes [74]. Furthermore, generated carbonaceous NPs from biomass have shown a significant potential for use in bioimaging, fuel cells, medicinal delivery, catalysis, carbon fixation, and gas sensors [74]. Many kinds of nanofibers could be listed and their application for energy harvesting is as follows:

• Carbon nanofiber network-based Zn-ion capacitor delivers [82];
• Three-dimensional electrospun porous nanofiber materials for energy storage [83];
• SiO₂ nanofiber-reinforced P-based microporous polymer electrolytes [84];
• Electrospinning polyvinylidene fluoride nanofibers [85];
• Carbon nanofibers decorated with copper–metal–organic frameworks [86];
• Nanofiber-reinforced clay (montmorillonite)-based 2D nanofluidic for osmotic harvesting energy [87];
• Electrospun nanofiber for electrochemical energy storage/conversion [88];
• Electrospun metal–organic framework-derived nanofibers, as a next-generation technology for electrochemical energy storage [89];
• BaTiO₃:La embedded nanofiber membrane for wireless power transmission and harvesting energy [90];
• Electrospun organic piezoelectric nanofibers due to their biocompatibility, eco-friendliness, flexibility, and biodegradability for energy harvesting and other bioapplications [91].

On the other hand, the biotechnological approach to energy can be found in microbial fuel cells (MFCs), which are considered sustainable bioelectrochemical systems. In an MFC system, electroactive microbes (e.g., bacteria or yeast) can be utilized in the biological treatment of wastewater via biochemical reactions to convert directly chemical energy into electrical energy [92]. The electricity can be generated in MFC systems through biodegrading organic pollutants in water bodies to obtain a new eco-friendly energy technology that has no biofouling, low operating cost, and a wide range of substrate sewage treatments [93,94]. Several recent studies focused on microbial energy under different conditions as presented in

Table 2. Studies on the role of nanofibers in improving microbial fuel under certain cases.

Nanofiber Type	Method of Producing	The Adapted and Used Nanofibers	Refs.
Polyacrylonitrile nanofibers	Electrospun nanofibers	Enhanced oxygen reduction upon Ag-Fe-doped polyacrylonitrile@UiO-66-NH2 nanofibers	[92]
Nitrogen-doped carbon nanofiber	Electrospinning	N-doped carbon nanofiber (Co/CoP/Co2P@N-CNF, Co/CoS2@N-CNF)	[98]
Polyvinylidene fluoride nanofibers	Electrospinning	Free molding polyvinylidene fluoride @Ag nanofiber	[99]
Carbon nanofiber-decorated graphite rods	Electrospinning	An effective and low-cost anode using a polyvinylidene fluoride electrospun nanofiber for industrial wastewater-driven microbial fuel cells	[100]
Aligned C-nanofiber-bacteria hybrid	Electrospinning and co-filtration method	This nanofiber has high-power output, low cost, facial fabrication, high-performance, and cost-effective microbial fuel cells in a large-scale	[101]
Carbon nanofiber on (CoCu@N-CNFs)	Metal–organic frameworks and electrospinning	Bimetal Cu/Co-N-doped porous C-nanofibers have high anti-bacterial capacity, inhibited the biofouling on the cathode surface, and best electrochemical activities were exhibited when Co to Cu was 1:1	[97]
Fe-Co bimetallic with carbon nanofibers	Electrospinning and metal–organic framework	Enhances the bioelectrocatalysis activity and promotes the direct electron transfer process of electroactive bacteria to improve the sluggish extracellular electron transport process of microbial fuel cell anode	[96]
Carbon nanofibers	Electrospinning	Decorated electrospinning C-nanofibers can save more exoelectrogens colonization, higher electrocatalytic activity, and more efficient interspecies interactions in the microbial fuel cell	[92]
N-doped carbon nanofibers	Electrospinning	N-doped C-nanofibers embedded in vertical-grown nanosheets improved the surface area of the material, preventing the agglomeration of Co and Co-Fe alloy NPs as a new approach for regenerating and conversion of the clean energy	[94]
Bacteria/electrospun oriented C-nanofibers	Electrospinning	These nanofibers have a self-supporting anode using filtration strategy; electrospinning technology for bacteria colonization and fully utilized interior surface area	[102]
Polymerized nanofiber polyaniline	In situ oxidative chemical polymerization	Cystobasidium slooffiae JSUX1 yeast enhancing both hydrogen production and bioelectricity from xylose in microbial fuel cells by polymerized nanofiber polyaniline	[95]

. These MFC systems used nanofibers, as mentioned in many studies, with a focus on converting and storing energy [92,95,96,97].

2.3. Nanofibers in the Food Sector

The production, handling, preservation, and storage of global food are essential items for the optimum management of food security. They need a suitable fiber or polymer for improving and controlling the properties of food. Maintaining freshness and food quality via food packaging has gained a great concern. As a multidisciplinary area, food packaging involves several sciences in addition to food science, which includes mainly food chemistry, food engineering, and food microbiology [103]. Recently, food packaging has had great progress thanks to the use of nanofibers (

Table 3. Use of nanofibers in food packaging and multifunctional food packaging.

Nanofiber Type	Functional Item	The Outcome from Used Nanofibers	Refs.
Gelatin/chitosan nanofibers	Curcumin	Electrospun nanofibers have the potential as multifunctional packaging in protecting and monitoring the freshness of protein-rich animal foods (e.g., seafood and meat)	[106]
Gelatin-based nanofibers	Xanthan gum and propolis	Nanofibers containing propolis showed a homogenous morphology with high antibacterial and antioxidant activities	[107]
Gelatin/zein-based nanofibers	Cinnamaldehyde/thymol	Used nanofiber films showed blocking for UV light, excellent antioxidant, antibacterial, and reduced water vapor permeability	[108]
Gelatin/pullulan nanofibers	Carvacrol/cyclodextrin	Studied nanofibers recorded the potential of carvacrol as a promising antioxidant and antibacterial that accelerated shelf-life test at 40 °C during active food packaging and as a bioactive compound	[109]
Carboxymethyl chitosan/polyethylene oxide nanofibers	Nisin from Lactococcus lactis	Nanofiber had mechanical strength and good antibacterial activity that extended the shelf life of packed bass fish from 9 days to 15 days as preserved aquatic product	[110]
Polyurethane nanofiber	Polyurethane-enrolled Fe3O4-NPs	Green synthesis of Fe3O4-NPs using food waste (molasses) promoted packaging cheese as an antimicrobial and maintained quality, freshness, and extended its shelf-life to 40 days	[111]
Ag-coated nanofiber	Nitrite detection	Gluten/zein film-based Ag-coated nanofiber was effective for rapid nitrite detection in food and the nitrite content ranged from 10–1 to 10−4 mol L−1	[112]
Gluten/zein nanofibers	Anise essential oil/β-cyclodextrin	Suggested sustainable, antioxidant, antimicrobial-loaded nanofibers against Staphylococcus aureus and Escherichia coli as a promising tool for active food packaging	[113]
Sulfonated cobalt phthalocyanine–titanium dioxide NPs	Ethylene scavenging capability	Studied nanofiber inhibited antibacterial activity of both Escherichia coli and Staphylococcus aureus (over 90%) and ethylene removal during vegetable and fruit preservation	[114]
Lycium barbarum polysaccharide nanofibers	Eugenol–silk fibroin NPs	Studied nanofibers were antibacterial, thermal stable, and encapsulated eugenol into silk fibroin NPs suppressed its volatility and extended the needed time against bacteria activity	[115]
La2NiO4 functionalized carbon nanofiber	Vanillin	Studied nanofiber was applied for detecting the vanillin in studied food samples (i.e., ice cream and chocolate), through a promising electrochemical sensing strategy	[116]
Composite films of cellulose nanofibers	Packaging film of halochromic C-dots; anthocyanin	The film had antimicrobial, antioxidant, and UV barrier properties, as an intelligent and active food packaging by changing color from red to colorless yellow during storage at 25 °C for 48 h during storage of fish, pork, and shrimp	[117]
Poly (lactic acid)/polyethylene glycol nanofibers	Peppermint essential oil	This hydrophobic nanofiber enhanced strawberries’ shelf-life by decreasing water vapor permeability, reducing weight loss after 5 days, and increasing firmness of fruits	[118]
Chitosan/cellulose nanofiber	γ-Cyclodextrin/curcumin	Coating film is used to extend the shelf-life of tomato, banana, and cut apple slices by reducing water loss, and microbial attack due to its excellent antimicrobial/antioxidative properties	[119]
Polycaprolactone/chito-oligosaccharide nanofiber-films	(EGCG)/2-(HP-β-CD)	This film had thermal stability as a good antibacterial agent for post-harvest fruit packaging due to its antifungal nanofiber membrane activities	[120]

Abbreviation: Epigallocatechin gallate (EGCG)/2-hydroxypropyl-β-cyclodextrin (HP-β-CD).

). Several applications of nanofibers in the food industry include three sectors: food analysis (antibiotics, food compositions, pathogens, and pesticide residues), food production (beverage filtration and delivery system for functional food), and food packaging (Figure 9). Using nanofibers in food packaging is a major contributor in the food sector, which may include the following applications, such as active packaging, edible packaging, gas sensors, food freshness sensors, intelligent packaging, time–temperature indicators, moisture removal packaging, and ethylene removal packaging [104]. In addition, pH indicators, functional bioactive packaging, antibacterial (thymol, gallic acid, curcumin), antifungal (curcumin, essential oils, etc.), and antioxidants (ascorbate, tocopherol, phenols) are common applications [105].

It is well-documented that the common methods for preparing nanofibers may include centrifugal spinning, electrospinning, and solution blow spinning toward active food packaging [121]. This active packaging of food is a crucial strategy for food maintenance, extension of their shelf life, and ensuring integrity, freshness, and safety, to meet the consumer demand for higher quality, healthier, and safer food [121]. What are the main advantages of nanofibers for packaging food? Nanofibers have been applied in food packaging because of their high porosity, specific surface area, and loading capacity of active compounds. There are several kinds of nanofibers that can be applied in the food industry, in general, and for food packaging, in particular, depending on the purpose, and which bioactives are needed (Table 3). For intelligent food packaging, electrospun nanofibers are considered to be promising tools due to their higher sensitivity and accurate pH labels [122]. In seeking sustainable and green approaches in the food sector, great progress in plant-based natural fibers has been achieved in the sector of food packaging [13]. These natural green fibers have several benefits compared with synthetic fiber, which includes lighter weight, low density and cost, superior life cycle, biodegradability, and good mechanical properties [123]. The main sources of natural nanofibers for food packaging involve cellulose [15], chitosan from chitin [124], gelatin from collagen [107], and silk fibroin from silkworms [125], and starch-based polymers can also be used to produce biodegradable food packaging [126].

2.4. Nanofibers in Water–Energy–Food Nexus

It is well-documented that by 2050, the global population will be around 9.7 billion, which means greater demand for all the major resources of our lives. This demand rate differs from sector to sector, for example, it increased by 70, 50, and 20–55% in the food, energy, and water sectors, respectively [127]. Therefore, these main sectors of water, energy, and food combine a crucial nexus (WEF) that requires urgent global management, as confirmed by several studies discussing this nexus from different points of view (e.g., [128,129,130]). This nexus has a very close relation to global climate change [131], the sustainable use of these resources [132], the sustainability of desalination [133], the ecosystem [134], and rainwater harvesting in arid zones [135]. This WEF nexus has a very closed relationship among all its components (water, energy, and food), which the factors of each component depend on each other, as well as it is impossible to secure any resource (e.g., food) without securing the others (i.e., water and energy) and vice versa [129,136].

Several applications of nanotechnology have been globally added to new strategies for improving and sustaining the components of this nexus, but still, environmental risks associated with this science need to be fully understood [128,137]. In the current study, nanofibers are the nano-form that can be applied to the components of this WEF nexus (Figure 10). Several research areas can be identified under the WEF nexus and apply nanofibers, such as using nanofiber-based biosorbents for ecological remediation [22,37], applying the microbial nanofibers for wastes and organic pollutants for remediation [138,139], the approach of producing nanofibers from food waste by converting energy using microbial fuel cells [101], and the general approach of how to use nanofibers in producing products of water–energy–food in integrated system research. The microbial role in this nexus and its components are very clear and involve several fields in all components such as biotechnological applications in agriculture, healthcare, and energy [140]. Under the WEF nexus and toward integrated system research, many investigations are needed including different relationships among these resources, in addition to integrated nexus management for enhancing the resilience and sustainability of the whole nexus system [129]. More suggested research areas and gaps are shown in Figure 11 in the entire nexus as well as in each one.

Green technologies under this nexus (WEF) have great outputs, which can achieve many goals of global sustainable development [141]. These green technologies are a hot topic, which has spread in the water, energy, and food sectors. These technologies mainly depend on plant species and their used part and/or microbe species (bacteria, fungi, or yeast). This technology produced different kinds of nanofibers (green and microbial), which have several excellent properties such as degradability, green, and biocompatibility, in addition to their high porosity, and high specific surface area [142]. Under the WEF nexus, green research is expected to include the following issues: green nanofibers for water activities, green techno-food for all food industries, and green energy production and handling. The main global issues including one health concept, sustainability, climate change, and the environment can be managed by using microbial nanofibers. These former questions are closely related to nanotechnology, whose nano-applications are a key approach to solving many environmental problems [143]. This approach was confirmed in many published studies such as the importance of environmental sustainability and health systems/healthcare as one concept [144], nano-based smart farming [145], eco-sustainability under nano additives for diesel engines [146], and waste management for sustainable future [147].

3. Most Common Applications of Nanofibers

It is well known that natural fibers derived from plants have gained great concern as a sustainable alternative to synthetic materials depending on plant species and the part used. Several cultivated plants have been used in fiber production such as the stem of the following plants Himalayacalamus falconeri [148], Lankaran acacia L. [149], Ficus benjamina L. [150], Waltheria indica L. [151], Myriostachya wightiana [152], and Cyperus platystylis R. Br. [153]. Chemical treatment is the main process for producing natural fibers, which may involve alkaline, acetylation, benzoylation, peroxide, potassium permanganate, silane, and stearic acid in addition to the surface treatments [6]. The biomedical sector has major applications of nanofibers among other different sectors including water, energy, and food. Enormous applications of nanofibers in biomedicine and therapy sectors are well-known and shown in Figure 12. Nanofiber composites can be used to deliver drugs, genes, or other biomolecules. Furthermore, they can be applied for engineering different tissues, from the skin through the blood to neural tissues as summarized in Figure 12. The therapeutic application of nanofiber composites is extensive, they can be used orally, transdermally, or intravenously in different types of therapies (Figure 12).

These applications may involve the delivery of drugs [30], bionanocomposites [154], and genes [155], as well as different applications in the field of tissue engineering for many human organs including bone [156], tendon [157], skin [158], cartilage [159], skeletal muscle [78], cardiac [160], vascular [161], and neural tissues [162]. Nanofibers exist in every branch of the medical sector, which contributes to several fields such as tissue engineering [163,164], regenerative medicine [165], and sensing and biomedical approaches [166]. These applications may involve drug delivery carriers [30], wound dressing [167], and facemasks [168]. These nanofibers are useful for tissue regeneration by mimicking a porous topography, enhancing the adhesion cells, and improving physiological acceptability and mechanical strength [169].

4. Further Applications of Nanofibers

Nanotechnology is one of the fastest growing fields of advanced technology and has been proven to have an essential role in achieving significant development in agricultural systems and many different scopes of science and technology [170]. It has an extremely wide range of applications that could be employed for modern industry, agriculture as nanofertilizers [171], nanopesticides, the food sector, electronics, bioengineering, renewable energy, medicine, pharmacy, cosmetics, and sensor technology [172,173]. Nanotechnology has changed the delivery properties of metal elements into plant cells and has affected the biological systems including plant survival, biosynthesis, antioxidant status, enzyme activity, growth, and development processes, and it has an active interference with the environment [174]. Nanoparticles (NPs) have proven to exhibit catalytic, optical, antibacterial, and antifungal properties [175,176]. With the advancement of nanotechnology, numerous potential applications of nanoparticles have attracted considerable attention due to their unequaled physiochemical, non-toxic properties, and practical effects of their very tiny molecules as well as the low cost and stability at high temperatures [170].

Plant tissue cultures are the center of plant biology and play a significant role in the conservation of plant resources, mass propagation, genetic manipulation, bioactive metabolites production, and plant improvement [177]. In recent years, plant tissue culture applications using nanotechnology, called plant nanobiotechnology, have become feasible and more popular [173]. The promising applications in plant tissue culture field include callus induction and proliferation [170,173,174,175], biodiversity preservation of plants by cryopreservation [178], embryogenic callus formation and plant development via somatic embryogenesis [172,179,180], and in vitro shoot multiplication and plant regeneration or organogenesis [170,177,181,182,183,184].

Moreover, NPs have been used in plant tissue cultures to improve in vitro seed germination for commercial production of seedlings by nano-priming [185], antifungal nano-composite [186], copper oxide nanoparticles [187], and iron nanoparticles [188]. NPs also have been applied for enhancing root induction [170,182,186] and stimulating the production of bioactive compounds as they can be used as elicitors in plant tissue cultures [177,180,189,190,191,192,193,194]. On the other hand, NPs may induce or reduce the somaclonal variation of in vitro cultured plants [177]. Genetic and phenotype variations that could be stimulated in vitro by Ag-NPs are very desirable in breeding programs and plant improvement [195]. Furthermore, nanoparticles could be utilized for the genetic transformation of plants via gene delivery “Nanoparticles-based delivery technique” into the plant genome, with the purpose of improving the quality and quantity of agricultural crops [196].

In addition to the direct actions of NPs on in vitro cultures, silver and cobalt nanoparticles (Ag-NPs, Co-NPs) have been used to overcome leaf abscission and enhance the growth, development, and survival rate of the in vitro propagated plantlets. They act as ethylene inhibitors [197]. Ethylene could accumulate in culture vessels and negatively affect the growth of the tissues by increasing the activity of hydrolytic enzymes leading to abnormal phenomena of leaf abscission, yellowing, and hyperhydricity of some species accordingly, which decreases the shoot quality [182]. Ag-NPs were successfully applied to reverse hyperhydricity and regain the normal growth of in vitro shoots [198]. NPs were proven to have the ability to enhance the tolerance of in vitro plants to abiotic stress. The application of iron nanoparticles could significantly mitigate the harmful effects of salinity or drought on the plant tissues cultured in vitro. This is an efficient method to produce plant materials that are tolerant to various abiotic stresses [188,199,200].

Several in vitro studies reported the antimicrobial behavior of NPs. They have the potential use to reduce the fungal and bacterial contamination of tissue-cultured plants [176,186], consequently ensuring aseptic environmental culture conditions. However, they should be used at optimal concentrations, to avoid possible toxicity to plant tissues, due to the cytotoxic behavior of the element itself (i.e., copper). CuO-NPs and Ag-NPs are considered the most effective antimicrobials commonly used in plant tissue cultures mainly for in vitro propagation of plants. They are applied by supplementing the culture media or as an explant disinfectant by surface sterilization [186,197,201,202,203]. Titanium dioxide nanoparticles (TiO₂-NPs) showed potential for inhibition of bacterial growth in plant tissue culture media [204].

Preliminary trials on the possible use of the nanofiber as an antimicrobial in plant tissue cultures were carried out as reported in both

Table 4. Some trials on the possible application of nanofibers in the plant tissue technique.

The Trials and Their Details	Observations
Trial 1: The plastic cover of the jar, which contained a sterilized medium without plants, was replaced by nanofiber in one layer and wrapped with plastic stretch and rubber.	After 10 days, the medium was very clean, where there was no contamination noticed but the problem was that a part of the medium was evaporated (Figure 13a). After five weeks, the medium was nearly evaporated but still clean, and the fiber started to decompose due to its exposure to the dry air (Figure 13b).
Trial 2: To overcome the evaporation of the medium, the nanofiber is used in two or double layers to cover the jar which contains sterilized medium.	After 10 days, the medium evaporated but less than one layer, however still without contamination (Figure 13c).
Trial 3: To decrease the evaporation of the medium, we reduced the surface area of the nanofiber exposure to the dry air (we try to modify the nanofiber to decrease the evaporation to a minimum, after that we can try using plants. We made a double-face nanofiber and made a hole in the plastic cover of the jar.	After 10 days, the medium evaporated but still clean (Figure 13d).
Trial 4: Double-layer nanofiber without modified plastic cover.	The medium became dry after 10 days and the explants (Royal Gala apple shoots) died (Figure 13e).
Trial 5: Double-layer nanofiber with modified plastic cover.	The medium evaporated partially after 10 days, and the explants started to grow but the medium dried after a while and the plants died (Figure 13f).

and Figure 13. These experiments were performed in order to answer the following questions: what kind of nanofibers did we apply? What is the main source of these nanofibers? What is the main target of this application? What are the main results of these experiments? To what extent can we use nanofibers in the plant tissue culture field? What are the main limitations and future perspectives of this application? More justifications and validations can be extracted from these experiments by answering the previous questions.

This study was carried out using the electrospinning method for preparing the nanofibers produced by Dispomedicor Ltd. (Hajdúböszörmény, Hungary). The nanofiber was between two melt-blown nonwoven PE layers. The nanofiber was used as a closure of culture vessels. We aimed to determine whether nanofiber could be used to eliminate microbial contamination in plant tissue culture media or not, and to what extent these nanofibers allow good ventilation for enhancing the growth and quality of tissue-cultured plants as well. The recorded observations for these initial experiments indicated that nanofibers are promising tools and could be used efficiently to eliminate microbial contamination in the culture medium but they need some more modifications in further studies to control the high rate of evaporation from the medium, which may cause stress to the cultured tissues, and to keep the explants alive so they can grow and develop. Accordingly, obtain clean and high-quality tissue-cultured plants. Therefore, future investigations are needed on these kinds of nanofibers for more applicability. The future aim is to use these nanofibers for closing the culture vessels, not only at a small scale in the scientific labs, but also in the tissue culture companies that use big bioreactors, and/or in the nurseries of plant propagation for commercial production in a large scale.

We successfully developed a laboratory-scale electrospinning device capable of producing nanofibers from different composition solutions. By combining atomization and electrospinning, we managed to develop and apply nanofiber-producing heads suitable for industrial-scale production. Various nano-powder additives for the fibers were tested and developed using a 3% graphite, 3% nano-selenium, and 10–12% PVB alcohol solution for nanofiber production (unpublished data). Further remaining tasks may include controlling and depositing the fibers produced by the atomization-based electrospinning technology onto a carrier, further investigation of the filtration efficiency of nano-fabrics, and exploring the potential applications of nanofibers with nanoparticle additives. Industrial optimization of these nanofibers may significantly contribute to the advancement and potential commercial applications of the developed technology.

5. General Discussion

In this section, some questions will present more justifications to highlight the potential of applied nanofibers in different sectors. What are the economic benefits of nanofibers in WEF sectors? To what extent can nanofibers be applied in the plant tissue culture sector? Are nanofibers promising tools for producing sustainable, clean, and safe water/energy/food? What are the main challenges of nanofiber production and application? What are the open questions that still need to be answered? It is well documented that nanofibers were produced using the electrospinning method several years ago. The electrospinning nanofibers are preferred over other methods due to their high quality.

Based on green and sustainable photocatalysis technology, photocatalytic fibers are considered innovative strategies for the remediation of water and air environments in addition to energy conversion [205,206]. They are promising for the degradation of volatile and gaseous pollutants in the air because of their high light utilization efficiency, high specific surface area, easy regeneration, and sustainability [207]. Many recent studies reported on the effective photocatalytic degradation using nanofibers in oilfield-produced water treatment [208], degradation of organic dyes [209], and for degradation of antibiotics using heterojunction photocatalytic nanomaterials [210] or ceramic nanofibers [211].

What is the scientific research progress concerning nanofibers in the energy sector? Many recently published articles answered this question by presenting different published materials. These publications focused mainly on sectors of energy production and the harvesting of energy, tools of green energy, energy conservation and storage, as well as nanofibers for producing microbial “bioelectricity” energy. Nanofibers are considered promising tools in both energy and water sectors when both can be applied for producing energy and water remediation at the same time [212]. Using nanofibers for the separation of oil spills from water is an emerging technology for treating oil/water emulsions [213]. More improvement in nanofibers and their functional structures or physicochemical characteristics are needed to increase their efficiency for energy generation and storage [214]. Applying piezoelectric catalysis is an engineering strategy for water treatment (sterilization, degradation of organic pollutants) and energy regeneration (CO₂ reduction, H₂ production). Water and energy are rich sectors, and scientific research every day can find innovative [215] solutions like applying polymeric polyvinylidene fluoride nanofibers in such fields [216].

Concerning the economic value of nanofibers in WEF sectors, these benefits are common in the applications of these sectors, which were mainly successfully applied for producing clean water, sustainable energy, and safe food. Applying nanofibers in the field of plant tissue culture is a promising approach because these nanofibers can prevent any microbial contamination under in vitro conditions, but the loss of media by evaporation is the main challenge in this application. The main challenges of nanofiber production and application depend on the type of nanofibers and their application. In the case of biomedical applications, the major sector for applying nanofibers, there have been many recent advancements in the fabrication of nano-fibers and their technology [217,218]. For WEF sectors, each one has many challenges such as water and air pollution, food pollution and spoilage, agro-waste burning and loss, whereas the possible solutions using nanofibers may involve green nanofibers, green techno-food, green energy production, integrated farming water/energy/food, and plant factory for water/energy/food production. Regarding the open questions that still need to be answered, there are many questions for each sector such as to what extent can nanofiber monitor moisture changes in food packaging? Which nanofiber criteria can save the sustainable alternatives of WEF? Which microbial biotechnological approaches are needed using renewable bioprocessing for future energy systems? To what extent can nanofibers deliver different bioactives in the field of food and pharmaceuticals?

6. Conclusions and Future Perspectives

During our life, there is a need for water, energy, and food, as essentialities for livelihood. There is an increased global demand for these issues, which may pose an increased decline in their abundance in the future. The WEF nexus has a very strong link with these resources, which should be investigated on all levels including local, national, and global ones. WEF nexus has sustainable and green approaches when we can use many applications of nanotechnology (mainly in this study, nanofibers). After the amazing progress in nanotechnology, nanofibers have penetrated several fields, nearly all aspects of human life, due to their distinguished characteristics. The main fabrication methods of nanofibers may include drawing, self-assembly, template assist, interfacial polymerization, phase separation, and electrospinning. The electrospinning technique is a sophisticated method enabling both researchers and companies to fabricate nanofibers with a variety of arrangements, architecture from different sources of materials, and morphology.

Therefore, the production and storage of energy, obtaining fresh and clear water, and maintaining the food and packaging are the main suggested benefits of nanofibers in this review. More biotechnological applications were discussed in the three previous sectors, primarily microbial applications in such areas. A novel application of nanofibers in the plant tissue culture field was presented in this review. Due to the very wide biotechnological applications of nanofibers, more highlights were shed on the biomedical sector as well. Furthermore, enormous questions still need to be answered, which mainly focus on integrated research in such areas.

In general, there is no doubt that water, energy, and food are essential resources, which attracted several workers to focus on them and this is the main motivation for this review. These resources used nanofibers in their activities to produce the needed water, energy, and food for human life. This potential is great in biomedicine applications, which do not need to be proven as they are very common in our lives. The integration production of energy, water, and food at the same time can be achieved when these nanofibers can be applied for using agro-wastes for producing nanofibers for energy/water treatments. What about the limitations of this study? This study still needs more experiments on field plant tissue culture to know how and when can we use the nanofibers in such fields. What about the research gaps/future perspectives that should be identified based on this review? The main aim of this study was to identify to what extent can nanofibers be applied in the field of plant biotechnology. However, this was not the only aim of this study. The green application of nanofibers and microbial nanofibers is a great area for the society of researchers to find a sustainable solution on the farm and industry level as well.