Graphene being an allotrope of carbon is considered one of the best products to be produced for industries throughout the world. Its characteristics and properties make it one of a kind and enable the applications in rather a very vast scenario. One such is in the food and beverage packing industry.
Food and beverages are items related to the health and well-being of humans so they need to be cared for cautiously. Graphene plays a major and vast role in maintaining the hygiene of food and beverages and due to this role, food industries are benefiting a lot from graphene.
Graphene is carbon’s allotrope which contains a single layer of atoms that is arranged in a 2-dimensional honeycomb lattice. ‘ene’ in graphene means that carbon’s graphite allotrope consists of stacked layers of graphene. In a graphene sheet, each atom is linked by σ-bond to the 3 nearest neighbors, contributing one of the electrons to a conduction band that therefore extends over the complete sheet. The bonding is of the same type that was seen in polycyclic aromatic hydrocarbons and carbon nanotubes, and in glassy carbon and fullerenes partially. Graphene is a semimetal with rare but remarkable electronic characteristics because of these conduction bands, and they are best explained by the theories for massless relativistic particles.
Linear, instead of quadratic, energy’s dependence on momentum is displayed by the charge carriers in graphene, and there can be a production of field-effect transistors with graphene, displaying bipolar conduction. Over long distances, the charge transport is ballistic however nonlinear and large diamagnetism and large quantum oscillations are displayed by the material. Electricity and heat are both very efficiently conducted by graphene along its plane. Graphite has a black color as it is a kind of material that absorbs light of all visible wavelengths completely. Although, due to its extreme thinness, a single graphene sheet is almost transparent. As compared to the strongest steel of the same thickness, this material is about 100 times stronger.
The global graphene market
In 2012, graphene’s global market was $9 million, with the most demand coming from development and research in composites, electric batteries, electronics, and semiconductors.
Properties of graphene
2630 m2/g is graphene’s theoretical specific surface area (SSA) which is way more than what the value is for carbon nanotubes (CNTs) or carbon black until now (less than 900 m2/g typically) and is the same as activated carbon. Graphene is carbon’s only form or solid material in which every single one of the atoms is available to react chemically from 2 sides because of the 2-dimensional structure. Special chemical reactivity is possessed by the atoms at the graphene sheet’s edges. As compared to any of the allotropes, the highest ratio of edge atoms is possessed by graphene. The chemical reactivity of the graphene sheet is increased by the defects within a graphene sheet.
Between oxygen gas and single-layer graphene’s basal plane, the reaction’s onset temperature is less than 530 K (260 °C). At extremely low temperatures, graphene burns (350 °C (620 K)). X-ray photoelectron spectroscopy and infrared spectroscopy analyze graphene whereas commonly, nitrogen- and oxygen-containing functional groups modify graphene. Although, the structures should be well controlled for the determination of graphene’s structures with nitrogen- and oxygen-functional groups.
0.763 mg per square meter is graphene’s (2-dimensional) density. One of the strongest materials that have ever been tested is graphene. 130 GPa (19,000,000 psi) is graphene’s intrinsic tensile strength (with 50-60 GPa of representative engineering tensile strength to stretch freestanding graphene of large-area). Graphene’s stiffness (Young’s modulus) is almost 1 TPa (150,000,000 psi). This was illustrated by the Nobel announcement by the statement that a cat of 4 kilograms can be supported by a 1 square meter graphene hammock but it would weigh almost the same as one of the whiskers of the cat, at 0.77 mg (about 0.001% of the weight of paper of 1 m2).
Angle based graphene
With negligible strain, large-angle bent graphene can display a 2-dimensional carbon nanostructure’s mechanical robustness. In monolayer graphene, remarkable carrier mobility can be preserved, even with extreme deformation.
An atomic force microscope (AFM) has been used to measure the suspended graphene sheet’s spring constant. There was a suspension of graphene sheets over SiO2 cavities in which stress was applied to the sheet by using an AFM tip for testing the sheet’s mechanical characteristics. As compared to bulk graphite, 0.5 TPa is the stiffness whereas its spring constant is in the 1–5 N/m range. Applications like NEMS as resonators and pressure sensors can result because of these intrinsic characteristics. As far as scrolling is concerned, flat graphene sheets are not stable because of its out of plane ductility and large surface energy, for instance, bending into a shape of the cylinder, which is its lower-energy state.
In graphene, due to its potential for applications in thermal management, thermal transport is an active area of research as it has gained a lot of attention. At room temperature, pyrolytic graphite has a thermal conductivity of 2000 W⋅m−1⋅K−1 whereas suspended graphene has 5300 W⋅m−1⋅K−1 of remarkably large thermal conductivity according to the early measurements. Although, there are further studies on more defected more scalable graphene derived by CVD is incapable of reproducing such measurements of high thermal conductivity, forming a broad range of thermal conductivities for suspended single-layer graphene between 1500-2500 W⋅m−1⋅K−1.
Variations in the processing conditions and quality of graphene and uncertainties in measurement can cause a large range of the reported thermal conductivity. Moreover, at room temperature, 500-600 W⋅m−1⋅K−1 is the thermal conductivity when single-layer graphene is supported on an amorphous material due to graphene lattice wave’s scattering by the substrate, and for few-layer graphene encased in amorphous oxide, it can be even lesser. Similarly, for bilayer graphene, suspended graphene’s thermal conductivity can be decreased to almost 500 – 600 W⋅m−1⋅K−1 because of polymeric residue.
One of the largest industries in the world is the food industry and it is worth numerous trillions of dollars. The main purpose of this global and complex enterprise is to ensure consumed food’s safety and high quality. Nanomaterial-based applications are very important now in the food industry while the food industry lags industries of automobiles and electronics in adopting new technologies. Nanotechnology has helped in several ways for enhancing food’s safety, overall quality, nutrient delivery, shelf life, texture, and taste. For instance, silver (nanoparticles) and titanium dioxide (TiO2) nanoparticles (NPs) have been utilized in storage containers as antimicrobial agents for beverages and foods.
US Food and Drug Administration
In bottled water, Ag salts direct addition up to 17 μg/kg is allowed as disinfectant according to the US Food and Drug Administration (FDA). Although, concerns have also been raised by the increase in the usage of the engineered nanomaterials in the food industry regarding their effect, influence, and potential toxicity on human health. For instance, there were investigations on the toxicity of TiO2-NPs that were extracted from chewing gum. Despite being considered safe, according to some studies, titanium oxide (TiO2) nanoparticles can pass through the GI (gastrointestinal) tract and then its slow distribution and accumulation in other organs. There are ongoing investigations on our understanding of the engineered nanoparticle’s cytotoxicity utilized in foodstuffs.
Use of biosensors
Analytical accuracy, sensitivity, and speed can be improved by the biosensors incorporating nanomaterials that are required for detecting the presence of adulterants or molecular contaminants in complex food matrices. In biosensors, gold nanoparticles (AuNPs) are utilized popularly since the aggregation of the gold nanoparticles, results in the change in visibly perceptible color, signaling the tested analyte’s presence. On using carbon nanotubes (CNTs), Au nanorods, and AuNPs, they specifically helped for the detection of the presence of pathogens, chemical contaminants, aromas, gases, or respond to the changes in the environmental conditions. Novoselov et al. discovered graphite’s 2-dimensional, one atom thick nanosheet which is known as graphene.
High electron mobility
High electron mobility of 250,000 cm2/V s is possessed by it along with remarkable mechanical characteristics and excellent thermal conductivity (5000 W m−1 K−1),(for instance, 1 TPa Young’s modulus), good biocompatibility, good electron transferability, and large specific surface area (more than 100 m2 g−1 ). Chemical vapor deposition (CVD) can synthesize high-quality graphene with no structural defects.
Moreover, electrochemical exfoliation or chemical oxidation can be done to derive reduced GO (rGO) or graphene oxide (GO) from graphite. Over the past years, graphene has had various exciting usages in numerous fields of technology, engineering, and science. Also, due to the presence of some interesting developments ensuring the safety and quality of food, there is a rise in graphene-based applications.
Fungicides, insecticides, herbicides, and pesticides are commonly used for improving agricultural productivity. However, if they stay in high enough concentrations in the food chain, they will turn potentially toxic, so one should evaluate food’s safety and quality before delivering it to the consumer market. Colorants, preservatives, and other additives are used for improving shelf life or/and the appeal of the consumer.
The presence of some of these agents should be evaluated as they are deleterious. In order to produce precise, reliable, and rapid information, we need new analytical technologies and methods. There is a new sorbent in extraction from food, biological, and environmental samples, and it is graphene. Chlorophenols, squalene, methyl parathion, phenols, pyrethroid pesticides, carbamate pesticides, sulfonamide antibiotics, adenosine, and cocaine are some of the extractions.
As compared to other sorbents like multi-walled carbon nanotubes, single-walled carbon nanotubes, graphitic carbon, silica, and C18, the Graphene-based sorbents are more superior regarding material’s cost, ease of elution, sorption capacity, and extracted analyte’s recovery. Foodstuffs CVD-synthesized graphene is utilized for synthesizing thin films of numerous nanomaterials by utilizing thermochemical vapor-phase reactions in a vacuum furnace for achieving desired material’s deposition at high temperature (~1000 C) on a substrate.
CVD can be used to synthesize graphene by flowing methane (carbon source) and hydrogen gases with a metal catalyst on a metal-film substrate. Decomposition of methane occurs, leaving the atoms of carbon deposited on the substrate for forming graphene layers.
Food as the carbon source
Monolayer graphene was synthesized by Ruan et al. by using food materials (chocolates, cookies) as the carbon source. Under the flow of H2/Ar, a high-quality graphene film was obtained at 1050 C on a Cu foil. No major disorder (D) bands were exhibited by these films in their Raman spectra which indicated the presence of law defects. According to the large 2 D/G ratios, the synthesized graphene was a monolayer film.
Freestanding monolayers were formed by glucose’s calcinations with dicyandiamide to oligo-layered graphene, and layered graphic carbon nitride (g-C3N4) functioned as a sacrificial template and undergoing total thermolysis at 750 celsius. At high temperatures, graphene-like sheets were liberated in the subsequent steps. Although, this method produces graphene which has nitrogen atoms in the graphene lattice, mainly in pyridinic nitrogen form and a little amount of graphitic nitrogen.
In beverages and food
Polymer nanotechnology has been used to develop food packaging materials with enhanced performance. There has been the development of polymeric nanocomposite packaging and it helps in delaying the oxidation of beverages and foods and control the growth of microbes. For food packaging, nanocomposites of PHBV-g-MWCNTs were designed by Yu et al.
Young’s Modulus and tensile strength were improved by 172 and 88% with 7 wt% nanofiller addition. Nanocomposite film’s WVP and water uptake were showed. As compared to neat PHBV, nanocomposite’s decreased WVP and water uptake were caused by the nanofiller. If the loading of PHBV-g-MWCNT increased to 7 wt% from 0, it lessened the uptake of water and the WVP values by 33% and 67%, respectively. Just like PHBV, the decrease was contributed to the nanocomposite’s crystalline structure.
Therefore, acceptable gas barrier characteristics and desired permeability degree were provided by the polymer/ MWCNT nanocomposite. Although, materials like this may possess material costs, high production, and additional additives are required too.
Regulations about food’s safety
There are some complications regarding the governmental regulations about these nanocomposite’s recycling, safety, and usage. Polymer/Cloisite nano clay packaging’s antimicrobial activity was studied by Hong and Rhim. Film’s antimicrobial activity was tested against pathogenic bacteria like Escherichia Coli, Salmonella typhimurium, Listeria monocytogenes, and Staphylococcus aureus. The content and type of organoclay and polymer matrix determine the nanocomposite’s antimicrobial characteristics. The coatings are made to protect products of poultry and meat. Some of the essential factors regarding commercial packing materials are design modifications, recyclability, environmental stability, and the size of the packaging.
Extraction and detection of toxins
Several microalgae species produce Lipophilic marine toxins (LMTs) and they are bio-accumulated frequently in filter-feeding molluscan shellfish (clams, oysters, and mussels). Humans can be majorly intoxicated by consuming phycotoxin-contaminated marine products (for instance, diarrhetic shellfish poisoning, which is a worldwide syndrome and it leads to various side effects like vomiting, nausea, abdominal cramps, diarrhea, and gastrointestinal disorder. Graphene’s potential to purify LMTs as an SPE sorbent was investigated by Shen et al.
The performance of Several commercial SPE cartridges and other sorbents was compared with graphene’s performance. Those other sorbents are Strata-X, hydrophilic-lipophilic balance, multi-walled carbon nanotubes, and C18. Tissues from commercially available shellfish were also analyzed by applying this method. Graphene as PT-SPE sorbent was used to obtain the best extraction efficiencies. The reason for attaining 90% extraction efficiencies was its double-sided polyaromatic scaffold structure, and it was also the reason for its high loading capacity and ultrahigh-specific surface area. Low LOD, high extraction efficiency, remarkable reproducibility, linearity, and specificity, was exhibited by this graphene-based sorbent.
Extraction and quantification of aflatoxins
To extract and quantify alfa-toxins in peanuts, they used GO and attained an extraction efficiency that was more than what was attained when they used HPLC i.e. 85%. Fungi make secondary metabolites known as Mycotoxins and they can cause death and disease in animals and humans. Algae Karenia Brevis produced a neurotoxin, Brevetoxin B (BTX-2). If one consumes brevetoxin-contaminated shellfish, it can result in intoxication and death too, thus causing respiratory irritation in the coastal areas through aerosol exposure.
Detection of pesticides
Commonly, Pesticides (for instance, ditalimfos, vinclozolin, folpet, procymidone, diethofencarb, pirimicarb, carbaryl, carbofuran, diuron, thidiazuron, and pymetrozine) are utilized in agriculture for thwarting diseases. Despite the risks of various concomitants remaining, the usage of these chemicals is extremely effective. Due to their inherent toxicity and high biological activity, human health gets adversely affected by the pesticide residues in foods. 12 of the 26 broadly utilized pesticides were listed by the US Environmental Protection Agency (EPA) as carcinogens in the USA. Excessive and improper usage of these pesticides results in them being accumulated in the food chain slowly. Natural water sources can be contaminated by these poisonous chemicals.
Some common methods
In order to pre-concentrate trace analytes from food or environmental samples, common methods like SPE and liquid-liquid extraction (LLE) are needed. Although, organic solvents are required in large quantity and SPE is known to be extremely tedious. In environmental samples, graphene was utilized as an adsorbent in SPE and LLE for pre-concentrating and for extracting pesticide compounds because of its very large surface area and remarkable adsorption capacity.
Amine-modified graphene was used by Guan et al. for cleaning up the fatty acids and other interfering substances from oil crops’ acetonitrile extracts. One of the best-tested sorbents as a reversed-dispersive SPE (r-DSPE) clean-up sorbent was this amine-modified graphene. It was efficiently used to check the 31 pesticides in the oil crops.
Antibacterial characteristics of graphene
Antibacterial characteristics are shown to be possessed by GO against E. Coli (Escherichia coli). There have been observations regarding improved antibacterial characteristics on Ag-functionalized graphene materials. According to Hu and co-workers, E. Coli’s growth can be effectively inhibited by rGO nanosheets, GO, and water-dispersible graphene derivatives, while exhibiting minimal cytotoxicity. Cell metabolic activity for Escherichia Coli lessened to 13% from 70% after incubation for 2 hours at 37 C with 20 μg/mL of GO nanosheets.
Metabolic activity of E. coli
Similarly, E. coli DH5α cells’ metabolic activity was lessened to 24% on treating them for 2 hours at 37 C with rGO nanosheets of 85 μg/mL, and according to colony counting, E. coli survived but only less than 10%. E. Coli’s cell membrane was destroyed by the rGO nanosheets according to TEM studies. With 20 μg/mL and 85 μg/mL of rGO nanosheets, A549’s cell viability(a mammalian cell line) was lessened to 47% and 15%
Antibacterial effects of sheets
As compared to GO nanosheet’s antibacterial effect, rGO nanosheet’s antibacterial effect was less, whereas as compared to GO nanosheets, the cytotoxicity of rGO nanosheets was considerably greater. Oxidative stress or their cell membrane being damaged because of having direct contact with graphene sheet’s extremely sharp edges might be the reason for the mechanism of E. Coli’s such irreversible GO-induced or rGO-induced cellular damage. A microwave-irradiation method was used by Chook et al. for synthesizing AgNPs on GO with a narrow size distribution. As compared to the activity against gram-positive bacteria, the antibacterial activity displayed by both Ag-GO and AgNPs samples was stronger against the gram-negative bacteria.
According to reports, the antibacterial performance by using AgNPs was not as efficient as the antibacterial performance by using Ag-GO but with Ag in a lower concentration. Therefore, AgNPs can be utilized in a smaller amount by the synergistic effect between AgNPs and GO with no compromising on the antibacterial characteristics. This causes concerns about excessive usage of Ag and it makes Ag-GO composite a potential material for applications of wound dressing and food packaging. Due to antibiotics being excessively used, various human pathogenic bacteria show resistance to most of the clinically approved antibiotics.
Graphene is highly responsible for the authenticity of food and beverages in the food industry. Packing food items is a long and hard process as it requires a safe and genuine method so that the food can be stored in a way that can keep it fresh and long-lasting and for this entire purpose graphene has been benefitting the food industries for a long time.