Various methods are used for the manufacturing of CNTs and three of them have been mentioned in this current review. The first one is known as arc discharge (AD) method. It is the most conventional method in which an arc is produced by direct current using two electrodes. Carbon rods are set in an inert chamber with a gas (26). The gas itself is provided at a steady rate that permits fast accumulation of carbon at the cathode. The deposition process is further enhanced by the presence of plasma in the chamber. Two significant parameters govern the method of synthesis, the first one is the controlled arc current, and the second one is the inert gas pressure. The mode of action of this process includes keeping the pressure of the non-reactive gas consistent in the chamber and steady supply of electric current. It results in the positive electrode to be drawn closer to the negative terminal and it results in the production of an arc. This process results in a rise in temperature to such as extent that plasma is formed, and electrodes become intensely hot. CNTs are then deposited at the negative electrodes and thus this procedure can be used for the synthesis of SWCNTs and MWCNTs. CNTs produced by this method have a diameter of ~2–20 nm respectively (27). The conditions required for synthesis include a voltage of 25–60 V, current being 100A and graphite as the carbon source (28).

The second method employed for the manufacturing of CNTs is known as laser ablation (LA). In this method, a laser beam is utilized to vaporize carbon (29). The graphene surface is heated via laser and the graphene rods themselves are composed of cobalt and nickel which are subjected in an oven having a temperature of about 1,200°C in an inert atmosphere. Bigger particles are separated with the help of two pulses to produce CNTs. CO₂ laser can be utilized for the delivery of SWCNTs while another important parameter found to be the deciding factor for increased CNT diameter, is the power of the laser employed for synthetic process. This method is different from the previous ones as it doesn't use heat above 1,200°C, pressure up to 500 torr and usage of graphite.

The third technique to manufacture CNTs is known as Chemical Vapor Deposition (CVD) method. This technique uses a carbon source along with a catalyst and produces CNTs having irregular and dispersed graphene sheaths (30). CVD uses hydrogen gas and other processed gases such as N₂ and H₂. The gas decomposition process causes CNTs to act as catalysts and be grown on the substrate. These are then implanted with nanoparticles including Ni, Co, and Fe. The hydrocarbon deposits on the substrate and decomposes, it is then connected to the metal particles generating CNTs later on (31–33). Apart from these three methods, flame synthesis, gas phase catalytic (HiPCO), aerosol precursor method, core shell polymer, arc water process, microsphere method plasma method, low temperature route, nebulized spray and fluidized bed method can also be utilized for manufacturing CNTs for a number of applications (17). The techniques and different methods of fabricating CNTs are exhibited in Figure 2.

Functionalization of CNTs can be performed by utilizing two processes; (a) Covalent bonding and (b) Noncovalent (van der Waals bonds). The former includes functional groups (COOH, OH, and NH₂) attached to sp² carbon structure. These functional groups are adhered to carbon nanotubes and result in better solubility parameters and disruption. The latter includes Van der Waals and π–π interactions. Surface modification can be performed with noncovalent bonds with surfactants, sidewall functionalization and noncovalent hexahedral polymers respectively. These modifications provide useful properties to CNTs including thermodynamic and interactive traits. The sidewall functionalization includes sp² to sp³ interaction with the help of nucleophilic attack on aromatic rings. The noncovalent functionalization is categorized by the presence of amphiphilic entities including surfactants (34). These molecules possess hydrophilic and hydrophobic portions and are capable of reducing surface tension among two immiscible liquids. The use of surfactants with CNTs results in better dispersion properties and functionality of CNTs. Endohedral functionalization involves the inner space modification of CNTs. The inner space of CNTs can be modified with the help of various mechanisms and thus the space is considered as a nano reactor environment that can be modified for useful applications and properties. External surface can also be modified in an approach known as exohedral functionalization in which functional groups can be attached to the surface of CNTs to enhance their properties and use in various applications (34). The mechanism of working of CNTs and its relation to conductivity is illustrated in Figure 3.

Food supplements are important in the regulation of health and are considered as driving nutritional factors toward a sound life. They are incorporated into the diet to prevent deficiencies by bridging the gap between diet and essential nutrients (35). Severe diseases and health issues such as cardiovascular disorders, anemia, and carcinogenesis can arise due to the disruption of metabolic pathways in vivo, due to the lack of nutrition in diet, comprising of vitamins and essential supplementary ingredients such as antioxidants. The supplementary food ingredients are degraded during processing steps and need to be made a part of food again. A solution to this problem is tackled by nanotechnology which improves food quality with the help of several mechanisms. One such mechanism is known as Nanoencapsulation (NE) that has been procured specially because of its distinctive features for efficient nano-scale encapsulation, enhanced stability, and better-controlled release of encapsulated materials. NE is a highly replicable methodology as it conserves the flavor of food items. Various food supplements have been processed using this novel strategy (36). Details of some encapsulated materials can be found in the following text.

Anthocyanins are nano encapsulated due to being extremely reactive. Nanoencapsulation of cyanidin-3-0-glucoside within the center cavity of a modified kernel of Glycine max enhances the stability of the kernel via the H2 subunit of ferritin (rH2) whereas rutin (a dietary flavonoid), is used less in the food sector due to its instability. However, NE of ferritin nanocages enhanced the physical properties of rutin in contrast to non-NE. Usually, increasing the bioavailability of the biologically active compounds is much more common by nanoemulsions. In addition, biologically active compounds like carbohydrates, lipids, vitamins, and proteins are susceptible to degradation by the enzymes that are active in the acidic environment of the stomach and duodenum. Their lower stability is owed to their activity and less solubility traits. Countering this problem, nanoencapsulation allows these biologically active compounds to withstand these adverse factors thus making them more soluble, tougher, and stable. Daily foods may consist of nanomaterials immobilized on small edible capsules for the provision of health benefits and the efficient delivery of important micronutrients including some vitamins (19). Nano-fortification can also be performed with the help of CNTs and carbon NPs (37). The properties of nanomaterials like MWCNTs and SWCNTs alter at the nanoscale as compared to their bulk counterparts due to which they are occupied in crop production. CNTs usually don't dissolve in aqueous media due to their high hydrophobicity but they have the capacity to form van der Waals forces. Specifically, SWCNTs have a hydrophilic nature and are large in size due to which they can't cross the cell walls of plants. Thus, MWCNTs can cross the cell wall barrier and provide useful properties regarding quality of food products. Seed germination is enhanced by the incorporation of MWCNTs in Solanum lycopersicum where MWCNTs show no toxicity toward crops including Lactuca sativa, Cucumis sativus, Triticum, and Zea mays. Research shows that long-term exposures didn't exhibit toxicity problems, however the positive impact was seen as observed by Lahiani et al. on the growth of crops. Lahiani concluded that different concentrations of CNTs act as different stimulus for plant growth. In case of S. lycopersicum, a lesser concentration (25 μg/ml) gave better germination results as compared to 100 μg/ml treatment. In addition, magnetic NPs can be used to aid the process as the chemicals are protected and nanomaterials are directed inside the structure of the plants (38). Nile reported that SWCNTs are utilized in the food industry for the manufacturing of honey and wine. Patel found out that a low concentration of MWCNTs enhances the growth of mustard seeds and increases the rate of germination for S. lycopersicum seeds respectively. In addition, it was reported that the rate of root growth was enhanced by non-functionalized CNTs in C. sativus, Z. mays, and Allium cepa respectively (28). Thus, nanomaterials aid the nutrition at the nanoscale and can help us to evade different complications related to food processing and growth. Additionally, the accumulation of cholesterol can be prevented by nutraceuticals possessing phytosterols, β-carotenes, and lycopene and lead to better quality parameters (39). Figure 4 shows the role of CNTs and their applications in the food industry.

The detection of numerous deteriorated supplements is extremely crucial for ensuring better health and that is why the utilization of various nanosensors and nanostructures are coming into play. Supplements such as folic acid (FA) and ascorbic acid (AA) can be detected in several fruit juices, Triticum flour and milk samples by nanostructure based CNTs. The number of degraded supplements can be calculated using CNTs and it can be made sure if the basic requirements and the minimum quantities of the supplementary ingredients are being fulfilled in the diet or not. FA plays a vital role in the prevention of complications in pregnancy such that its deficiency can lead to fetal abnormalities during development in the neural tube. In addition, FA is not synthesized in the human body due to which a minimum dose having 400 μg of synthetic FA needs be daily incorporated in the diet by pregnant women especially as it is essential for fetal growth and development (40). Unstable key food ingredients like folate and vitamin B9, H₂O₂ and tryptophan can also be detected by PtCb SWCNT, DWCNT and MWCNT by amperometry and cyclic voltammetry techniques (19). In a research, efficient identification and detection of bacterial cells without labeling, SWNT-mediated potentiometric aptamer biosensor was implemented in milk having 6 cfu/ml and malus juice having 26 cfu/ml respectively (3). Quality control is a crucial step in food safety and processing as it ensures that supplements do not exceed their normal values and it can lead to dangerous health implications and complications, thus deteriorating the quality of life (41).

Phycotoxins intoxicate humans causing digestive and neurological disorders, as well as respiratory stress, skin problems and prove to be fatal during the lifetime (72). CNTs has been used previously in mice models in the healthcare sector for studying various processes (73). Shafiq et al. (19) studied better ways to detect palytoxin and microcysin-LR utilizing immunoassays and electro-chemiluminiscence properties of MWCNT and SWCNT. Currently electro chemical biosensors synthesized via novel nanomaterials, such as Boron CNTs (single and multi-walled) are utilized in the identification of toxins in food. Zhang et al. synthesized a novel biosensor for the identification of cyanide in Prunus armeniaca. In another research, CNTs and other nanomaterials were utilized by transforming the CNTs into functionalized c-CNTs for field applications (22). For the detection of biochemical contaminants within food as explained by Girigoswami et al. (74), various technologies have been devised that help in overcoming issues related to food biosafety, using nanomaterial-based biosensors. These techniques are efficient in solving various hurdles in the field of biosensing and result in better, feasible and faster results as compared to their counterpart techniques. All the techniques used in the research were based on the principles of electrochemical sensing and fluorescent signaling respectively (74). In another research, H₂O₂ was detected by ferrocene-grafted carbon nanotube in a non-enzymatic manner by using an electrochemical process (75). The foregoing accounts make it clear that CNTs can be used as efficient tools in biosensing toxins and prove to be useful in food safety applications in the coming future (76). Figure 5 Illustrates the manufacturing steps regarding biosensor fabrication.

Serious health risks are linked with chemical exposure from the environment (77). The risk of chronic diseases such as neurodegenerative diseases, diabetes and cancer along with decreased fertility and birth defects are all linked to exposure to different pesticides (77, 78). The detection of these types of contaminants and their estimation is very crucial for the safety of people (23, 79). Owing to their excellent adsorption properties, CNTs have been utilized for isolation and extraction of pesticides from different food samples, with help of various techniques including the application of Solid-Phase Extraction (SPE) and Solid-Phase Micro-Extraction (SPME) (19). The detection of compounds is linked to the extraction techniques used for their quantification. These techniques include High performance liquid chromatography (HPLC), Liquid chromatography (LC), and Gas chromatography (GC). The use of CNTs is permissible owing to their cheap and effective methods of fabrication, thus allowing the detection of pesticides with 15-folds of sensitivity as compared to counterpart techniques (80). In research, c-MWCNTs gave better sensitivity values regarding biosensing mechanisms. In a research Chen et al. (29) employed MWCNTs to synthesize a acetylcholinesterase (AChE)-based electrochemical sensor for feasible and effective pesticide detection in food samples. These processes are very important since they allow the detection of toxins and pesticides with high sensitivity (81, 82). This useful trait regarding CNTs can prove to be useful in future bioremediation applications and treatment of soil and water.

CNTs have been employed in medical applications for the detection of pathogens including Escherichia coli and Salmonella, which can be detected through the application of SWCNT based fluorescence microscopy (19). E. coli is a gram-negative bacterium which poses great threat to humans and causes a range of ailments. E. coli encompasses galactose-binding surface proteins that have strong interactions that can be detected by Galactose single-walled nanotubes (Gal-SWNTs) (3). Real-time detection of bacteria can be also be done with CNT-based sensors in which MWCNT can be used for the screening of Enterobacter cloacae by assay methods (83). SWCNT-based immunosensor working on the principle of electrochemical impedance is utilized in onsite identification of Listeria monocytogenes, which is a pathogenic bacterial strain and causes many diseases in living things (84). SWCNTs use covalent bonding to detect Salmonella via biosensing mechanism. In the process of detection, firstly, the ssDNA probe solution at room temperature was incubated on electrodes for about 2 h. The sensor exhibited a concentration in the typical range of 1 × 10⁻⁹ mol/L of DNA sensitivity. In another experiment Salmonella was detected using PCR but using deposition mechanism of ITO (indium tin oxide) to MWCNTs which resulted in amino-modified apt sensor process. Researchers were able to detect 148 bp invA gene expressed in both strains of Salmonella within a limit of 103 cfu-m. In another experiment, SWCNTs-based nanosensor was utilized for the detection of strains coupled with anti-E. coli antibodies and found the limit to be 102 cfu-m. Later researchers verified the process by with Staphylococcus aureus that gave negligible variation in the signal thus confirming the sensitivity and specificity results regarding E. coli (85). This phenomenon shows the importance of CNTs in the detection of pathogenic strains which can be useful in food protection processes and health safety, in the future.

Food colorings and preservatives are considered toxic, when utilized beyond permissible dose limits. In a research, Ionic-liquid nanomaterials conjugated with MWCNTs were used to detect a red colored carcinogenic dye known as Sudan-I in chili powder and sauce. They can also be used to detect sunset yellow and tartrazine which can pose health threats when consumed in higher quantities. Similarly, nanoparticles of zinc oxide and nanocomposites conjugated with CNTs have been used to detect sudan and bisphenol A (hazardous molecule expelled from containers of plastic) (19). Functionalized CNTs contain oxide nanoflowers which helps in ultra-sensitive identification of preservatives in food including tert-butylhydroquinone, by utilizing 3D copper (86). Some innovative techniques based upon field effect transistor (FET) and voltammetry (cyclic) utilized MWCNTs and SWCNTs coupled with various nanomaterials for the detection of metals including Cd ions, and various dyes like tartrazine, sunset yellow, and Bisphenol A. In case of Sudan 1, HPLC was also used along with CNTs treatment (19). MWCNTs coupled with tyrosine have been used as nanoadsorbents to expel the toxic additive coloring known as methylene blue (87). This research shows the ability of CNTs to be utilized as detective materials and their potential to be used in food applications and safety, respectively (88, 89). Figure 6 illustrates the applications of CNTs as described in this review.