Chapter 1: Carbon-reinforced Polymer Nanocomposites Against Infectious Diseases

In the present era, everyone is aware of the importance of the terms ‘nano’ and ‘the nano effect’ owing to the abundant adoption of this technology in the development of many uses with different real-time applications, such as nano-SIM cards, nanocars, nanodevices, nanomedicine, nanoscaffolds, etc., with their outstanding mechanical, thermal, electrical, magnetic, and optical properties.¹  The use of nanotechnology is extensively seen in the electronics, food, biomedical, and packaging industries owing to their interesting morphology, high tensile strength, biocompatibility, low toxicity, nanosize, and large surface area. The development of nanomaterials and their derivatives has advanced their use predominantly to solve biomedical and health-related issues. Research on the development of smart and low-cost materials has been prioritized. For the growth of society through nanoscience and nanotechnology, advances in the preparation of new sustainable nanocomposite materials are essential.

Nanocomposites are materials that have different dimensions in which at least one phase must be in the nano range. Nanocomposites are composed of two parts, namely a filler part and the matrix. The various types of filler materials used for the preparation of nanomaterials are nanotubes (NTs), metal oxide and ceramic nanoparticles (NPs), and polymeric nanomaterials. Depending on the type of matrix, nanocomposites are broadly classified into three types, namely ceramic matrix nanocomposites, metal matrix nanocomposites, and polymer matrix nanocomposites. All of these nanocomposite materials possess unique thermal, mechanical, and pharmacological properties with different morphologies that differ from those of their bulk substances. The enhanced properties at the nano level encouraged applications in different areas such as packaging, electronics, and biomedical uses. Different types of nanocomposites and their synthesis routes and applications are shown in Figure 1.1.

Polymer matrix nanocomposites have been shown to be more effective in biomedical applications owing to their high biocompatibility and high pharmacological activity. Researchers all over the world are continuously involved in developing new composite materials from biopolymers, which have been proved to be more sustainable for use in the biomedical field. Widely used materials include natural and synthetic polymer matrix nanocomposites and polymers reinforced with carbon nanotubes (CNTs), graphite, graphene and inorganic NPs, focusing on light weight, easy processing, recyclability, oxygen and moisture permeability, solvent resistance, thermal stability, barrier properties, pharmaceutical features, and high strength, which inspired many scientists to work on treatments against infections caused by different agents.^2,3  In addition to these properties, the high conductivity, effective adsorption, and high thermal stability of such composites attracted attention towards their application as materials useful in defence, electronics, sensor, and biomedical areas.⁴  The phenomenon of the dispersion of nanofillers on different matrices shows the reinforcement that is extensively observed in the case of polymer nanocomposites bearing superior properties. The multipurpose uses of the polymer matrix are illustrated in Figure 1.2.

One of the greatest challenges that the whole world is facing, along with biomedical science, is the problem of tackling the concurrent occurrence of a pandemic due to parasitic pathogen infection. An infection is defined as the attack of a pathogenic parasite on healthy host tissue through toxin production with replication of the pathogen cells. This infection causes serious illness in the host body and in many cases efficiently kills the host if the immune system is unable to fight back properly. The host body for infection can be a plant, animal, or human depending on the nature of the pathogen involved. Plants are least susceptible to pathogenic infection whereas the trend of infection is high in the case of animals and humans. Many pathogens are found in nature that can cause infection in both animals and humans. The transmissibility factor of pathogens helps them to cause severe, deadly pandemics such as COVID-19. The majority of infections are caused by bacteria and viruses, and some infections are caused by other pathogens such as fungi, protozoa, worms, and prion proteins. Traditional research has been ongoing aimed at the development of vaccines and drugs to control pathogen infections. A few of the drugs and vaccines have been registered for use in society for specific infection control. However, in many cases, there are no suitable drug candidates available to combat infection. For two decades, with the rapid increase in sustainable nano-biomaterials development, there has been bright hope in biomedical science regarding the availability of infection-resistant drugs and vaccines.

Direct and indirect transmission of the viruses, bacteria, worms, prion proteins, protozoa, and fungi from the outside to the inside of living cells poses serious threats to humans, sometimes even taking lives. The spread and transmission of infected microbes can occur by coughing, sneezing, and direct and indirect touching, and also by air- and water-borne microbes. The combating effect of nanocomposites has increased substantially in the biomedical field by fighting many serious issues, including working against many agents such as bacteria, viruses, worms, etc. To counter these infectious agents and to achieve their complete eradication, the development of drug delivery modes, scaffolds, catheters, and stents, tissue engineering, and anti-infectious drugs has been advancing, and polymer matrix nanocomposites have come into the picture.^5,6 

Carbon-reinforced polymer nanocomposites (CRPNCs) are one category of sustainable nanomaterials that have attracted attention in the field of material science and technology owing to safety aspects, quality, renewability, low cost, ready availability, high surface area, conductivity, thermal stability, and porosity, planar structure, regular framework, nanosize and thickness, flat surface, and good mechanical strength.⁷  In the current scenario, CRPNCs have shown high efficacy against infectious diseases. These sustainable nanomaterials are made up of a polymer matrix reinforced with carbon-based materials such as CNTs, fullerenes, graphene, nanodiamond, carbon dots, etc.⁸  In some infections caused by bacteria, fungi, and protozoa, the reinforced composite material efficiently destroys the cell wall of the pathogen with the production of reactive oxygen species (ROS), which leads to the death of the pathogen and the curing of the disease. In the case of viral infections, CRPNCs suitably enhance the production of interferons for the protection of healthy cells.^9,10  The mixed properties derived from these composites evidenced many broad and superior properties.

This chapter discusses the various types of CRPNCs used against infectious agents. Their functionalization, factors, influences, and behaviour with respect to the infectious agents causing disease are explained in depth. Polymers act as matrices and the reinforcement is carried out with many carbon nanofillers such as carbon fibre, CNTs, nanodiamond, fullerenes, graphene, graphene oxide, and reduced graphene oxide. In the treatment of infectious diseases, several challenges faced by the nanocomposites have been resolved by modification of the embedded properties of the filler–matrix nanocomposite. CRPNCs have been widely employed for the treatment of infectious diseases in many areas depending on the mechanical strength, antimicrobe resistance, antioxidant parameters, etc. The low cost, large molecules, multifunctional and easily synthesized polymers, high surface area, and porous and tuneable thermal, electrical, and mechanical attributes of the carbon-based matrix have contributed to and improved the performance of such nanocomposites in harsh conditions. In summary, this chapter discusses the types, composition, reinforced activity, and biological response and development of CRPNCs.

Living beings, especially humans, are quite hostile to microbes causing infections and their related health threats. The agents for infectious disease are microbes, worms, and a few pathogenic proteins. Most infections are the result of the pathogenic activities of microorganisms and easy transmission via air, water, and soil. The variable shapes and sizes affect the infection rates and fatalities among living beings. A detailed discussion of several types of infection-causing microbes is presented below. Figure 1.3 illustrates different types of infectious agents and the infections they cause.

Bacteria are unicellular organisms that have a cell wall with other cellular materials such as cytoplasm, ribosomes, and nucleoids. Numerous types of bacteria are present on this green planet, and they are considered one of the first living organisms on Earth. Bacteria can be classified into different types from different aspects, but with regard to their infection-causing ability they can be classified into two types: symbiotic bacteria and parasitic bacteria. Symbiotic bacteria are conventionally non-pathogenic whereas parasitic bacteria are pathogenic. Many of the infections in animals and humans are caused by pathogenic bacteria and represent a global challenge. The rapid multiplication of bacteria cells in the host body leads to infection, causing serious illness. The major infectious diseases seen in humans include Acinetobacter infection, actinomycosis, Arcanobacterium haemolyticum infection, bacterial meningitis, bacterial vaginosis, cavities (dental caries), Carrion’s disease, cat-scratch disease, chancroid, and cholera, whereas the major bacterial infectious diseases observed in both humans and animals include anaplasmosis, anthrax, Bacillus cereus-caused illness, bacterial pneumonia, bartonellosis, brucellosis, bubonic plague, Burkholderia infection, Buruli ulcer, campylobacteriosis, cellulitis, chlamydia, Chlamydia pneumoniae infection, Clostridioides difficile colitis, and diphtheria.^11,12  Being ubiquitous, bacteria play a significant role in the maintenance of a healthy life. As bacterial infections and their transmissions are common and occur by infected guests reaching susceptible hosts from the environment through air, water, food, and physical contact, they are also easy to treat compared with viral and fungal infections. To inhibit bacterial growth, antibiotics and antibacterial agents have been increasingly used depending on the chemical structure, target site, and types of bactericidal agents.¹³ 

Fungal infection is one of the most common infections observed in living cells in the form of yeasts or moulds. They grow like plant species. The primary agents responsible for these infections are the genera Candida, Cryptococcus, and Aspergillus, all of which cause serious infections. The reason lies with the cell itself or is caused by host factors. They can affect the cells and tissues of the living being with varying infection rates. The aggravated cells multiply to large numbers in a very short time and diagnosis can be difficult, leading to high fatality rates.¹⁴  At the present time, fungal infections caused by SARS-CoV-2 are suspected to be due to superinfected agents affecting the lungs, kidneys, heart, and eye tissues.^15,16  To face such challenges, appropriate measures should be taken, such as the recording of fungal distribution and drug resistance capacity, and consideration of risk factors. Fungal disease is very chronic once a suitable host has been found, and can be life threatening if it is not treated early.

Viruses can replicate inside the host itself if there is a chance of a growth-permissible environment. Common viral diseases include AIDS, hepatitis, COVID-19, chickenpox, smallpox, cold sores, colds, flu, etc., caused by direct contact or intake of the virus into the body. Viruses exist in many shapes and sizes, including helical, complex, simple, cylindrical, icosahedral, and envelope types. They are made up of nucleotides derived from genes, a protein for coating, and phospholipids to act as an envelope outside the cell for their survival purposes. They can affect other infected and non-infected agents including humans, plants, animals, bacteria, and fungi. The study of and research on these agents have been quite difficult owing to their small size and fast growth rate, affecting major organs of living beings. Recent viral infections include the dangerous COVID-19, which created havoc in the world by causing a life-threatening epidemic. This RNA virus causes severe pulmonary damage over time by mutation transmitting from one host to another.¹⁷  The infection was difficult to control because of its unknown structure and the rate of invasion was too fast. Effective interventions and research to reduce viral infections are predominantly based on the use of vaccines,¹⁸  drugs, and systematic treatments.

Infections due to protozoa are caused by Plasmodium agents targeting tissue damage, leading to diseases that mainly affect the respiratory tract. Their size lies in the micrometre range. Protozoa such as Entamoeba, Acanthamoeba, Balamuthia, Leishmania, Trypanosoma, Trichomonas, Lophomonas, Cryptosporidium, Cyclospora, Toxoplasma, Plasmodium, Enterocytozoon, and Balantidium, cause dangerous clinical conditions such as AIDS, malign haemopathies, giardia, toxoplasmosis, etc.¹⁹  Diseases such as malaria and chagas disease are mainly dominated by protozoa infections and cause the highest mortality due to protozoa infection. The immunological response of the parasitic host antigen and cytokine profiles gives the infection profile of damaged tissue. The detection, finding, and treatment of these parasitic protozoa these days depend on methodology, availability of commercial kits, and modern molecular biological techniques.²⁰ 

Worms resemble helminths with varying micron sizes. Parasitic worms are spread and transmitted to the living body by drinking polluted water, contact with infected soil, eating undercooked and date-expired food, and mosquito bites. The common diseases mainly caused by worms are ascariasis, schistosomiasis, amoebiasis, cryptosporidiosis, and filariasis.²¹  The worms affect the intestines and stomach of humans and reduce the nutritional value of the intake of food and water.²²  To face this challenge, treatment to eradicate the worms should be carried out by restoring the required energy, proteins, and micronutrients.

Prion diseases caused by prion proteins are extremely dangerous as they affect the delicate parts of humans such as neurological tissues. The prion proteins first malfunction and then form aggregates called prions that cause infectious brain disease, and as this affects the neurological path the condition is very difficult to treat. The diseases caused by prion proteins are Creutzfeldt–Jakob disease (CJD), Gerstmann–Sträussler–Scheinker syndrome, and fatal insomnia.²³  The chemical modification of prion proteins might be one of the reasons for this infective nature.²⁴  Painful and incurable diseases are not easy to treat but the symptoms may be eased with supportive care. Early measures should be taken to control prion-causing diseases to avoid severe chronic health issues.

To combat these infectious agents and to reduce the fatality rate, it is wise to use suitable drugs with formulated anti-infective resistance and supportive care. To simplify these issues further, reinforced polymers came into the picture with an additive manufacturing process that enhanced the biological and pharmaceutical performance.²⁵  The wide range of beneficial properties and the existence of various forms of carbon have challenged many issues such as improper alignment, formation of cracks and voids, and unsuccessful adhesion. The properties of CRPNCs depend on the dimensions of the carbon fillers. For a better understanding, different types of carbon-based fillers with various dimensions are illustrated in Figure 1.4.

Carbon fibres are broadly obtained naturally from plant sources such as rice husk, palm, banana, jute, cotton, coir, etc., and artificially from graphene–basalt, glass, etc. Polymers such as epoxies, polyethylene (PE), polyesters, and polyacrylonitrile (PAN) act as templates or matrices for the development of CFRPNCs. These fibres have been adopted for several uses as a consequence of their light weight, availability, low cost, natural origin, biodegradability, low toxicity, high strength, and durability.²⁶  The reinforcement of high-strength carbon fibres as a filler of the chemical environment of the polymer matrix contributes strengthened and useful CFRPNCs by polymerization, in situ, one-pot, and sol–gel synthetic routes. The fibre content, morphology, size, shape, composition, orientation, and preparation method impact the quality of CFRPNCs. The extraordinary multifunctional nature, biocompatibility, zwitterionic effect, anticorrosive properties, high flexural strength, and stiffness damping of these nanocomposites enhanced their importance in biomedical fields to establish the optimum characteristics of the material for a desired application.

Matharu et al. designed graphene oxide (GO)–poly(methyl 2-methylpropenoate) (PMMA) CFRPNCs via a three-step synthesis. These ∼1.4 mm sized 8 wt% CFRPNCs show effective antibacterial properties against Escherichia coli bacteria with strong bioactivities of 46–85% compared with than GO–poly(vinyl alcohol) (PVA) fibres due to the presence of the non-water-soluble polymer PVA.²⁷  The incorporation of the naturally derived polymeric fibre chitosan (CS) in a poly(butylene adipate-co-terephthalate) (PBAT) matrix prepared using the solution casting technique showed outstanding activity against fungal and bacterial growth. The biocompatibility, biodegradability, low cost, and hydrophilic nature of CS led to the easy and effective incorporation of NPs in the polymer matrix. The interaction of the amine (–NH₂) groups of the CS polymer creates leakage in the fungal and bacterial cell walls by removing the proteinaceous and other intracellular constituents of the bacteria. These CFRPNCs show effective antifungal and antibacterial activity against the fungi Aspergillus niger and Mucor rouxii, and Gram-positive Staphylococcus aureus and Bacillus subtilis and Gram-negative Salmonella enteritidis and E. coli bacteria. The ductility reduction, high water permeability, and better strength improved the anti-infective resistance focusing on strong H-bonding interactions between the hydroxyl (–OH) and –NH₂ groups of CS and the carbonyl moieties of the polymer. The study suggested that prepared CFRPNCs can act as potential agents for the inhibition of fungal and bacterial growth and can be used in antimicrobial packaging systems for food and medical applications.²⁸  de Faria et al. functionalized GO–Ag–CS-embedded poly(lactide-co-glycolide) (PLGA) CFRPNC, which prevented bacterial colonization on its surfaces. The 356 nm diameter fibre nanocomposites were formed by the interlinking of the carboxyl (–COOH) group of GO and the –NH₂ group of the polymer. The direct interaction of CFRPNC with the bacterial cell wall showed an efficacy of 99% and 76% inactivation growth rate for Gram-negative (E. coli and Pseudomonas aeruginosa) and Gram-positive (S. aureus) bacteria by an ROS mechanism.²⁹ 

The risk of infection post-surgery always scares the patient. To avoid this problem to some extent, the development of a graphene fibre-based polycaprolactone (PCL) matrix CFRPNC was achieved by melt spinning. The release of the antibacterial agent chlorhexidine (CHX) works successfully with controlled and sustainable kinetics. The increased mechanical strength of CFRPNCs due to the amount of graphene present in it showed elastic modulus and tensile strength increments of +175% and +115%, respectively. Even the trapping of the required drug and its release occurred with the porous graphene filler in the nanocomposite, causing an improved anti-infective environment for the patient.³⁰  The biodegradability, size, cylindrical and round morphology, drug release capacity of the fibres, and suitable chemical interactions between carbon filler and polymer matrix led to an effective inhibition effect on the growth of infection-causing agents.

A rolled graphene sheet closed by hemifullerenes is called a carbon nanotube (CNT). CNT-reinforced polymer nanocomposites (CNTRPNCs) possess good physical and pharmacological properties. With excellent physical properties such as electrical, mechanical, magnetic, thermal, and optical properties, these polymer nanocomposites have been used for antimicrobial applications, drug delivery, nerve cell generation, and catalysts.³¹  The size, thickness, functionalization, and easy production make CNTs very useful for several applications. A CNTRPNC facilitates the interaction between the infected cell and itself and protects itself from the adsorption of non-specific proteins. CNT-based polymer nanocomposites can prevent bacterial growth and biofilm adhesion due to the functionalization, CNT length, electronic structure facilitating ionic conductivity, limited hydrophobic surface, and the presence of a large surface area.³² 

Joo et al. prepared a useful CNTRPNC by using CNT as filler and poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA) as a matrix via grafting and atom transfer radical polymerization (ATRP). This CNT-based CNTRPNC showed an outstanding antimicrobial effect against yeast, and an antibacterial effect against Gram-positive S. aureus and Gram-negative E. coli. The highest viability loss was observed for Gram-negative E. coli bacteria with the highest sheet resistance of <9.68 × 10³ Ω per square. The solubility, stimuli responsiveness, and quaternary ion formation of this CNT-based polymer nanocomposite contribute to the effective inhibition of microbial growth.³³  The long, thin, and round CNTs induce antibacterial and antifungal activities by breaking the infected cells. CNT-based CS reduces the enzyme activities of infected fungal and bacterial cells and facilitates DNA breaking.³⁴  It has been demonstrated that CNTRPNCs show fascinating anti-infective mechanisms towards fungi, bacteria, and viruses such as Candida albicans, E. coli, B. subtilis, P. aeruginosa, and S. aureus by following the steps (i) interaction of CNTRPNCs with the infective microbial surface, (ii) destruction of protein and DNA, and (iii) ROS formation.³⁵  The peptide-based CNTRPNCs, otherwise called host defence peptides (HDPs), also kill the infection-causing microorganisms through multiple steps such as alteration of the cell membrane, membrane permeabilization, activation of autolysin, inhibition of biomolecule synthesis, inhibition of certain enzymes, induction of cell and membrane damage, DNA destruction, protein dysfunction, and release of secondary products, such as ROS.³⁶ 

The functionalization of hydrazide and hydrazone derivatives, epoxides, and hydroxyl and amine groups resulted in improved antimicrobial activities against the growth of Enterococcus faecalis, Staphylococcus epidermidis, E. coli, Aspergillus niger, Cryptococcus neoformans, and Candida tropicalis as judged by their higher inhibition zone diameters using the agar well diffusion method.³⁷  Liu et al. fabricated a CNT-based CRPNC as a poly(lactic acid) (PLA)–CNTs–CS composite and studied its antimicrobial behaviour. It was reported that an increase in the CS content in the composite led to the enhancement of the mechanical properties, solubility, shelf life, swelling ratio of the fibre films, and antimicrobial activity of the composite fibre films against Botrytis cinerea, Rhizopus, S. aureus, and E. coli, making it useful for fruit and vegetable preservation.³⁸  Xiang et al. discussed the activity of CNTs for biomedical and therapeutic purposes. The CNT-based CRPNCs show reduced neuron damage in the brain and spinal cord owing to the presence of accumulated prion proteins in the human body. The prion protein formation is caused by the accumulation of misfolded proteins, oxidative stress, apoptosis, inflammation, perturbation of vesicle trafficking, and synapse dysfunction.³⁹  The structural modification, multifunctionalized moieties, divergent synthesis procedure, composition, and reduced self-aggregation originated from powerful van der Waals interactions, and greater interaction between the MWCNTs and microorganisms led to an enhanced anti-infective mechanism against the growth of harmful microorganisms. The easy preparation and fabrication of CNT-based RPNCs can improve the microbial growth inhibition rate and provide a sustained eradication procedure to fight recurrent and deadly infection-causing microbes such as bacteria, worms, protozoa, etc.

Nanodiamonds (NDs) are a type of carbon-based nanomaterial used in different applications such as bioimaging, biomedical devices, and implants. The surface composition, biocompatibility, enough acid anhydride groups, small size, and presence of a partially oxidized and negative surface of NDs made them potent antimicrobial agents with a new nanomaterial. Generally, they are composed of a non-toxic sp³-carbon diamond core and a graphitized outer layer containing sp²-hybridized partially oxidized carbon, which allow the unique morphologies and functionalization through electrostatic interactions among the groups present.⁴⁰  The PLGA–polypropylene–PLA-embedded NDRPNCs have shown great efficacy in biomedical use. The ease of surface functionalization and fascinating mechanical and interesting surface morphology has been widely used to reinforce biopolymers and the participation of hydrophilic groups led to the creation of good antibacterial activities with the presence of water-soluble antimicrobial drugs.⁴¹  ND-reinforced PMMA–PLA nanocomposites possess very good antibacterial activity depending on the weight percentage of doping of NDs in the composite, leading to good doping and depositional capacity.⁴²  Xie et al. synthesized silicone–ND–polydimethylsiloxane–polyurea nanocomposites and studied their antifouling activity. These NDRPNCs are non-toxic and environmentally friendly. The presence of perfluoro and polysiloxane over the top surface of the CRPNC improved the self-healing properties and performance, showing an effective reduction in and control over antimicrobial growth on its surface.⁴³  The incorporation of NDs in the polymer nanocomposites greatly impacted the reduction of infective agent growth due to the interesting silicone modification in the nanocomposite, good adhesion and toughness, presence of hydrogen bonds, size effects, surface area, and carbon-based hybridizations. The biocide-free, eco-friendly nature, high strength, and oxygen abundance nature of the ND-incorporated polymer nanocomposites provide strong evidence for utilization against antimicrobial growth.

Being a carbon allotrope, closed or semi-closed mesh-like structured fullerenes are interconnected by double and single bonds and are useful for biomedical purposes. They possess sp²- and sp³-hybridized carbon atoms having a high transport charge ability and electron affinity. The production of peroxides (hydrogen peroxide, H₂O₂), superoxide (˙O₂⁻), singlet oxygen (¹O₂), and hydroxyl radicals (˙OH) also enhances the inhibition of microbial growth depending on the electrostatic interaction. The positive surface charge of fullerenes and the functional groups in the polymer in the CRPNCs enhance the efficacy against Gram-negative E. coli bacteria. Fullerene-reinforced polymer nanocomposites (FRPNCs) are mainly useful for bone tissue engineering and tissue generation and antimicrobial growth. The FRPNCs have been shown to be effective against bovine leukaemia virus (BLV) and Lactobacillus rhamnosus and Lactobacillus acidophilus bacteria.⁴⁴  The incorporation of fullerenes in a natural polymer such as CS and cyclodextrin polysaccharides to give cyclodextrin–CS–fullerene nanocomposite CRPNCs showed significant antibacterial action against C. albicans, S. aureus, and E. coli.⁴⁵  The activity, membrane permeability, elasticity, mesh-type structure, interesting size, and photodynamic surface possessed by fullerene CRPNCs are very useful properties for antimicrobial performance.

Carbon dots (CDs) have been extensively used in numerous energy storage and biomedical applications depending on their quantum size, size effect, irregular surface, surface defects, and fluorescent properties. ROS production depends on the temperature effect and photothermal agents are responsible for the effective generation of reactive oxygen species. The fluorescent-type quasi-spherical CDs show high water solubility and high stability, are environmentally benign, have good biocompatibility, are cost-effective, and have a spherical structure with an average size of 10 nm. As CDs are among the best photothermal agents, they show significant antibacterial resistance following the photodynamic inactivation mechanism in Gram-positive (S. aureus) and Gram-negative (E. coli), Klebsiella pneumoniae, and P. aeruginosa bacterial species and against multidrug-resistant pathogens.⁴⁶  The strategic development of CD incorporation in polymer nanocomposites (CDRPNCs) creates more opportunities for various applications depending on the high abundance of CDs as the precursor, low cost, easy synthesis methods, large surface area, and interesting morphology. The CDRPNCs are produced by sol–gel and green chemistry methods, hydrothermal processing, ultrasonic wave use, and microwave radiation and oxidation methods.⁴⁷  Wu et al. discussed some CDPRNCs for application in photodynamic antimicrobial chemotherapy (PACT) using multidrug resistance (MDR) concepts. To combat and prevent the MDR effect without any side effects, these CRPNCs seem to be most useful in exhibiting powerful photodynamic effects. These can kill both bacterial and cancer cells under visible light irradiation, hence presenting themselves as strong candidates for application against serious infective agents.⁴⁸ 

The viruses from the plague, chickenpox, Zika, swine flu, Ebola, and SARS-Cov-19 have caused havoc in the daily routine of humans, and have taken billions of lives over the last few centuries. Although they are inactive and incapable of growing on their own, they cause life-threatening diseases once they get a chance in a suitable environment. They are otherwise called parasites. Currently, the world is still facing the SARS-CoV-2 attacks that began in 2019, and to combat this deadly pandemic CDs and their derived carbon quantum dots (CQDs) incorporated in polymer nanocomposites have contributed extensively. They act as effective antiviral agents owing to their biocompatibility, small size, good sustainability, environmental friendliness, and potential candidature. The encapsulation of antiviral drugs into CQD–DNA CDRPNCs through electrostatic interactions and adsorption helps with easy, sustainable, and effective controlled drug delivery. These CDRPNCs also affect the viral growth in living organisms occurring in the following steps: viral entry–uncoating, their replication, and release in cell lines. This growth is prevented by the presence of CQDs in the CDRPNCs depending on their composition, chemical moieties, topography, surface morphology, and hydrogen and covalent bondings.⁴⁹  Maruthapandi et al. designed an interesting CD-initiated polypyrrole (PPY)–CuO composite via a one-step sonochemical method and studied its antibacterial behaviour against E. coli and S. aureus. The best antibacterial action of these CDRPNCs was observed at a concentration of 1 mg mL⁻¹ of the composite involving 0.234 mg mL⁻¹ of CuO, which destroyed the bacterial cell wall after ROS generation. The ROS production in this case also depends on the synergistic effect between the polymer and CuONPs. Even the aromatic ring and amine chemical moiety generated from PPY also participated in the bacterial DNA damage with the production of an electrostatic interaction leading to electrolyte loss from the bacterial cell wall, causing its death.⁵⁰ 

Two-dimensional, single-sheet, sp²-hybridized graphene has been extensively used as an important precursor for derived species such as GO and reduced GO (rGO). There are many resources available in nature that can be extracted by mechanical separation, chemical vapour deposition, exfoliation, etc. The honeycomb-shaped graphene sheet itself possesses antibacterial properties, which are enhanced when incorporated with a bioactive polymer matrix. The use of graphene increased after its mixing with polymers, which enhanced the conductive nature of graphene-reinforced polymer nanocomposites (GRPNCs). These have been widely used in pharmaceutical, sensor, and energy storage device applications.⁵¹  The antimicrobial properties of graphene are followed by three important mechanisms: membrane stress, oxidative stress, and electron transfer pathways. Graphene–poly(N-vinylcarbazole) (PVK) underwent van der Waals interactions via physical mixing and blending with the chemical reaction between these two precursors and showed efficacy against numerous bacteria such as B. subtilis, E. coli, Rhodococcus opacus, and Cupriavidus metallidurans. Chemical interaction develops between the π–π stacking of graphene and the aromatic rings of PVK.⁵²  The zwitterionic effect, production of quaternary ammonium salts, grafting, and multifunctional, polyionic complex formation of polymer nanocomposites facilitate the inhibition of microbial growth on any surface. There are several GRPNCs, such as graphene-reinforced and Ni–P-incorporated polyethylenimine, graphene–PMMA, graphene–PLA, graphene–polyindole, and graphene–polyurethanes, that can eliminate 99.99% of bacterial growth due to their efficient dispersion rate, controlled flow direction, and limited agglomeration effect, depending on their synthesis routes.⁵³  Mousavi et al. discussed graphene-incorporated poly(ethylene glycol) (PEG) and its probable pH-controlled drug release properties with antiviral and antibacterial agents residing in it, prepared by condensation polymerization, chemical vapour deposition, and exfoliation methods.⁵⁴  Joshi et al. designed electrochemical immunosensors to detect the presence of the influenza virus H1N1, which is a worldwide threat, by using thermally decomposed rGO and shellac. The high stability and reproducibility due to the presence of the phenolic –OH moiety made this method effective for the inhibition of microbial growth. The limits of detection of H1N1 in phosphate-buffered saline and saliva samples were found to be 26 and 33 PFU mL⁻¹, making these GRPNCs successful immune sensors.⁵⁵ 

GO with a hexagonal structure is the most common form. The morphology of GO looks like a transparent paper-like structure. The synthesis routes such as epitaxial growth, layer-by-layer assembly, chemical and mechanical exfoliation, and laser-assisted approaches generated an improved version of graphene as GO. GO materials are versatile to use for different purposes as they are oxygen-rich, cost-effective, and easy to prepare and handle. However, the underlying disadvantages such as limited-scale preparation, high production cost, low production yield, and toxicity led scientists to hybridize GO with other nanocomposites such as metal oxides and polymers to improve such issues. It has been reported that when different forms (graphite, graphite oxide, GO, and rGO) were tested, GO showed the lowest toxicity, and exhibited significant microbial inhibition of >80% for planktonic microbial cells, C. metallidurans, B. subtilis, and E. coli.⁵⁴  The toxicity and low-scale production of GO-based material can be improved by the development of new GO-reinforced polymer nanocomposites (GORPNCs) with superior composite properties. The other reasons for enhanced antimicrobial activity lie in the size dependency attribute of GO, the smaller sheet size prepared via sonication, the oxidative stress caused by the high defect density of GO, and the contact-based interaction between the infected cell and itself.⁵⁶  The excellent catalytic, thermal, mechanical, and electrical properties, renewability, biocompatibility, large surface area, small size, and easy preparation of such GORPNCs are favourable attributes.

Kumar et al. discussed in detail several GO-functionalized polymers such as PVA–CS–GO against E. coli and B. subtilis via an Agar Diffusion Assay (ADA) evaluation method with an inhibition effect up to 1.25 and 1.40 mm, PDMS–GO–DMA against E. coli and S. aureus through a plate count method with a degree of inhibition of ∼40%, 2 wt% GO–CS against E. coli and S. aureus via ADA, and 10 nm sized GO@PEG@AgNPs against E. coli and S. aureus, which demonstrated that GO@PEG@AgNPs have enhanced antibacterial properties.⁵⁷  The outbreak during the recent COVID-19 pandemic caused by the coronavirus was reduced with the practical application of GORPNCs. The use of surface coating strategies with biopolymers such as amino acids, peptides, and naturally rich GO became an important pathway for a sustainable solution to control such dangerous diseases. Technically graphene and its derivatives have the strength to fight against Ebola, dengue, Zika, human coronavirus 229E, and African swine flu by inhibiting their growth through cell wall penetration, ROS and agglomerate formation, and negatively charged interactions with positively charged capsids, but the coordination of GO along with the polymer makes it more effective for antiviral activity. The trapped pores and hydrophobic groups in the polymer make this composite more useful for coronavirus treatment by controlling the growth of viruses.⁵⁸ 

Cheong et al. synthesized the GO-reinforced polymer nanocomposites (GORPNCs) GO–PEG and expanded GO (EGO)–PEG via a coupling reaction with new amide bond formation. An antifungal study was performed for C. albicans using the in vitro cut-well method, showing extensive shrinkage and porosity deformations. The participation of Zn and Cu metals at different weight percentages in these GORPNCs exhibited effective antifungal behaviour in broth dilution assays. A 30 : 70 combination Cu in GO–PEG showed a minimum inhibitory concentration (MIC₅₀) of 185–225 μm mL⁻¹. The viability results indicated that GO–PEG appeared to be as good an antifungal agent as CuNPs, which can be explained by the high colloidal stability with CuNP concentration increment.⁵⁹  The complex preparation of GO–CS GORPNCs has limited demand but still has been extensively used in wound healing treatment depending on the permeability to bacteria, high surface area, prosody-driven drug delivery path, and high exudate absorption capacity.⁶⁰  Similar work by Khan et al. was performed using numerous bacteria, fungi, and wound-healing treatments. They designed GO and its derivative-based nanocomposites to study the pharmaceutical activities of bacteria (P. aeruginosa and S. aureus) and fungi (Saccharomyces cerevisiae and Candida utilis) and the interesting results are dependent on the laser-activated modified surface of GO.^61,62  There are many nerve diseases involving the presence of prion proteins, which cause spongiform encephalopathy with an abnormal mass that often leads to fatal brain infection. This disease is not curable but can be controlled to some extent by the use of GO–PMMA GORPNCs.⁶³  Their outstanding thermal and biomedical properties, unique surface, and planar structure have led to a great demand for their substantial development as antibacterial and antimicrobial agents influenced by their interesting functionalities and rich oxygen groups.

Improved rGO is prepared from GO by chemical reduction in the presence of numerous reducing agents at different temperatures. It is composed of mixed sp² and sp³ carbons and also planar geometries. The catalytic and mechanical activities of rGO have resulted in important biomedical uses.⁶⁴  The high surface area, the possibility of unsaturated bond formation, environmentally benign properties, hydrogen–electrostatic interactions, and π–π stacking provide a good platform for conjugation with polymer nanocomposites to produce better pharmaceutical agents such as rGO-reinforced polymer nanocomposites (rGORPNCs). Ouyang et al. designed water-soluble poly-l-lysine–reduced graphene oxide–copper nanoparticle (PLL–rGO–CuNP) nanocomposites and studied their antibacterial activity against Gram-negative E. coli and Gram-positive S. aureus. The composite showed additive antibacterial activity, and the CuNPs on PLL–rGO were more stable than those on polyvinylpyrrolidone, resulting in a long-term additive antibacterial effect depending on the presence of multifunctional rGO and the band gap of Cu metal in the composites.⁶⁵ 

Studies have suggested that the biocidal activity of rGO is higher than those of GO and graphene, especially for Gram-negative E. coli bacteria with an outer membrane, whereas it was less effective towards Gram-positive S. aureus without an outer membrane. The toxicity in the case of rGO is also greater owing to the presence of sharp edges during the contact interaction.⁶⁶  Vatandost et al. designed natural-based rGORPNCs derived from rGO and polyphenol-containing green tea extract and studied the pharmaceutical properties. It exhibits a strong bacterial growth resistance for E. coli and S. aureus with more antioxidant activity.⁶⁷  The rapid and easy preparation of rGORPNCPs through several simple synthesis techniques with specific monitoring and recognition capabilities provides a fruitful strategy in the fight against infections from bacteria, fungi, viruses, and other agents.⁶⁸  Table 1.1 lists different types of CRPNCs, their biomedical uses, and factors responsible for eradicating infectious diseases.

In view of the present health scenario of the globe, it can be readily hypothesized that the major worldwide health alert in the future will be infections arising as a result of antimicrobial resistivity. The major challenge will be to design sustainable drugs or vaccines that will suppress the antimicrobial resistivity to tackle infectious diseases caused by parasitic microorganisms as discussed above. The proper selection of carbon-based filler materials with a bioactive polymer matrix with additional biocomponents will be highly effective in developing natural or modified carbon-reinforced hybrid polymer composites for dealing with infectious diseases by suppressing antimicrobial resistivity.^69–72  However, some challenges that are yet to be solved can be overcome by a better understanding of the science behind carbon fillers, polymer matrix, their combination, incorporation of other natural molecules, their interaction, and morphology. The mechanisms, pathways, and the participation of CRPNCs in the inhibition of microbe growth by damaging the microbe cell wall and DNA destruction are shown in Figure 1.5.

In this chapter, a summary of different types of carbon-reinforced polymer nanocomposites and their experimental activity against various infectious diseases is presented. Infection-causing agents such as bacteria, viruses, protozoa, fungi, helminths, and prion proteins are discussed with their host attack and cell damage mechanisms. Different studies have been reported for the preparation of types of CRPNCs and their activity against model pathogens. The chemical structure, nanosize, quantum effect, multifunctional attributes, stability, thermal, mechanical, and electrical behaviour, and interesting morphology have led to the adoption of CRPNCs for different pharmaceutical purposes in recent years. The excellent stability, sensitivity, and accuracy of these nanocomposites have influenced their activity against these life-threatening diseases. Exceptionally, the low-cost preparation methods, superior activities, and environmentally benign and in a few cases green synthetic methods of CRPNCs have motivated the use of these materials against the growth of infective agents and led to a reduction in fatalities.

ATRP

Atom transfer radical polymerization

BLV

Bovine leukaemia virus

CDRPNCs

Carbon dot-reinforced polymer nanocomposites

CDs

Carbon dots

CFRPNCs

Carbon fibre-reinforced polymer nanocomposites

CJD

Creutzfeldt–Jakob disease

CNTRPNCs

Carbon nanotube-reinforced polymer nanocomposites

CNTs

Carbon nanotubes

–COOH

Carboxyl

CQDs

Carbon quantum dots

CRPNCs

Carbon-reinforced polymer nanocomposites

CS

Chitosan

FRPNCs

Fullerene-reinforced polymer nanocomposites

GO

Graphene oxide

GORPNCs

Graphene oxide-reinforced polymer nanocomposites

GRPNCs

Graphene-reinforced polymer nanocomposites

H₂O₂

Hydrogen peroxide

HDPs

Host defence peptides

MDR

Multidrug resistance

NDRPNCs

Nanodiamond-reinforced polymer nanocomposites

NDs

Nanodiamonds

–NH₂

Amine

NPs

Nanoparticles

NTs

Nanotubes

–OH

Hydroxyl

˙OH

Hydroxyl radical

˙O₂−

Superoxide

¹O₂

Singlet oxygen

PACT

Photodynamic antimicrobial chemotherapy

PAN

Polyacrylonitrile

PBAT

Poly(butylene adipate-co-terephthalate)

PCL

Polycaprolactone

PDMAEMA

Poly[2-(dimethylamino)ethyl methacrylate]

PE

Polyethylene

PEG

Poly(ethylene glycol)

PLA

Poly(lactic acid)

PLGA

Poly(lactide-co-glycolide)

PLL

Poly-l-lysine

PMMA

Poly(methyl 2-methylpropenoate)

PPY

Polypyrrole

PVA

Poly(vinyl alcohol)

PVK

Poly(N-vinylcarbazole)

rGO

Reduced graphene oxide

rGORPNCs

Reduced graphene oxide-reinforced polymer nanocomposites

ROS

Reactive oxygen species

The authors declare that there is no conflict of interest in publishing this chapter.

The authors acknowledge research support grants awarded by University Grants Commission (UGC) India and a DS Kothari Post-Doctoral fellowship to Dr Biswajit Parhi. Dr Debasrita Bharatiya acknowledges Veer Surendra Sai University of Technology (VSSUT), Burla, India, for its support with research facilities as a Research Associate (RA).