The main biomedical applications of CNTs are presented in Figure 2 and are discussed in the following subsections.
Figure 2. The main biomedical applications of carbon nanotubes.
2.1. CNTs as Biosensors
Given their remarkable attributes, CNTs have been recognized as innovative nanoprobes
[37]. Their high aspect ratio, conductivity, chemical robustness, as well as sensitivity
[38], along with their rapid electron-transfer rate
[39], render them highly suitable for biosensing purposes. The fundamental aspect of CNTs-based biosensors involves the surficial immobilization of biomolecules, thereby enhancing recognition, as well as facilitating the signal transduction process. The biosensors are commonly classified as electrochemical and electronic CNTs-based biosensors, as well as optical ones, resting on their target sensing and communication mechanisms. CNTs have gained significant acclaim as advantageous materials for enhancing electron transfer, making them well-suited for integrating electrochemical and electronic biosensors
[40].
2.1.1. Sensing of Glucose
Several glucose biosensors utilizing CNTs in conjunction with glucose oxidase have been developed
[26][41].
Compared to the first-generation sensor where the electrode and glucose oxidase were used
[42], the second generation of glucose sensor incorporates an electron carrier within the enzyme electrode
[43]. In this phase, the electron medium actively participates within the reaction, replacing the role of O
2. The enhanced exclusivity and acuteness of the second-generation sensor have resulted in its commercial exploitation in glucose sensing. Enzyme-only sensors tend to be unsteady, as enzymes are susceptible to deactivation and detachment. In order to address this, enzyme immobilization has been identified as an effective approach to assure enzyme robustness and recyclability
[44]. Additionally, leveraging the unique properties of the material can further enhance enzyme efficiency. For example, glucose oxidase can be anchored using polymers, as well as a vertical array of CNTs (VACNTs)
[45]. Chemical vapor deposition (CVD) is utilized to modify the CNT wall with polyaniline (PANI) for fixing glucose oxidase (GOx), resulting in improved electrode stability and accelerated electron transfer. The sensor’s range of linearity spans between 2 and 426 mM, having a detectable limit equal to 1.1 mM. Importantly, it is not affected by interference from uric acid and ascorbic acid. To overcome enzyme activity reduction or denaturation and to facilitate enzyme adsorption, metal or metal oxide nanoparticles are employed for enzyme immobilization. A ZnFe
2O
4-CNTs-glucose oxidase composite has been fabricated for use in glucose sensing
[46]. The as-mentioned sensor demonstrated stable catalytic efficiency for 20 days, as well as recyclability >5 times, benefiting from the ZnFe
2O
4′s-increased catalytic effectiveness, the enhanced surface area ratio of CNTs, and the elevated loading capacity of GOx.
An electrode consisting of Au nanoparticles/MnO
x-VO
x/CNT/GCE was synthesized using electrochemical pulsed deposition by combining metal oxides
[47]. MnO
x-VO
x was applied as a surficial modification of a glassy carbon electrode (CE) with CNT (CNT/GCE), while Au nanoparticles were deposited onto the oxide film. Glucose detection exhibited a linear range of 0.1–1.0 mM, and the detection limit was measured at 0.02 mM. Another approach involved the utilization of CNTs altered with platinum nanoparticles as electrodes for glucose detection
[48]. The researchers initially formulated composite solutions comprising carboxyl functionalized MWCNTs and chemically derived graphene (CDG). These solutions were then deposited as thin films onto the gold electrode, followed by the deposition of Pt nanoparticles through an electrochemical process. The linear range for glucose detection was 0.5–13.5 mM, with a low detection limit (1.3 mM).
Ensuring long-lasting durability and optimal current density for continuous monitoring are crucial considerations for glucose sensors. The rate of alteration also impacts the efficiency of CNT materials. The presence of lignosulfonate can influence the composites’ electrical effectiveness containing multi-walled CNTs (MWCNTs), subsequently affecting their electrocatalytic functionality
[49]. The chemical alteration of glucose oxidase can enhance its efficiency and selectivity simultaneously. By combining glucose oxidase and MWCNTs modified with β-cyclodextrin, the sensor exhibited 95.8% stability in current retention over a span of 14 days
[50].
In contrast to the conventional one-factor-at-a-time (OFAT) approach, several researchers have suggested the use of a design of experiments (DOE) approach to simultaneously maximize the current density and robustness
[51].
The enzymes’ chemical features are composed of proteinic molecules that are prone to denaturation when exposed to pH value modifications, temperature, and various environmental factors. This leads to decreased sensor stability and affects the effectiveness of detection. While physical immobilization helps preserve the enzymes, it does not guarantee a reduction in activity. Enzyme leakage occurs when enzymes detach and access the solution, rendering them ineligible for involvement in the reaction. To address the issue of poor stability, a nGOx/N-CNTs-Chi/GCE (Chitosan/Glassy CE) sensor was developed
[52]. A polymerization approach was suggested to enclose glucose oxidase in single-molecule enzyme nanocapsules (SMENs) in situ. The synthesized SMENs of GOx-substituted traditional glucose oxidase, improving enzyme activity preservation. In comparison to traditional CNTs, n-doped CNTs (NCNTs) were employed to enhance the catalytic conductivity, specific surface area, and biocompatibility. Another approach utilized glucose oxidase micro-particles instead of independent enzymes
[53]. The relevant sensitivity persisted >86% for 9 days. Additionally, the issue of enzyme leakage was effectively addressed through the implementation of a hollow fiber membrane with a gradient structure. The range of linear detection spanned from 0 to 24 mM. The enzymes’ catalytic efficiency was intimately linked to temperature. Microencapsulated phase change materials (MEPCMs) were employed to inhibit enzyme leakage and attain temperature control, thereby enhancing the enzyme-sensing performance at increased temperatures
[54].
Several crucial aspects are considered during the development of enzymatic sensors to enhance robustness and sensitivity. Firstly, effective screening is carried out to identify enzymes with high activity. Secondly, efforts are made to maintain the activity and stability of the enzymes. Lastly, reliable fixation of the enzymes to the semiconductor is ensured. To address the aforementioned challenges, a third generation of glucose sensors resting on direct electron transfer (DET) has emerged. These sensors operate without the need for media or chemical reactions, enabling a direct transfer of electrons from glucose to the electrode. In one study, a GOx-Chit-CNT/ITO electrode was created by modifying the composite chitosan-CNTs onto an indium tin oxide (ITO) electrode
[55]. Various solutions with diverse mass ratios of CNTs were prepared, and it was found that chitosan-CNT85 exhibited the optimal performance. In the long run, glucose detection biosensors are expected to be transformed into wearable devices for convenient in vitro sensing
[56]. Another approach involved utilizing a dual enzyme system consisting of glucose oxidase and horseradish peroxidase in a CNTs-EVA composite membrane
[57]. The glucose oxidase/horseradish peroxidase/CNT-EVA electrode was instantly employed for glucose detection in sweat. A non-intrusive glucose sensor utilizing two electrodes demonstrated superior performance compared to existing invasive glucose meters, presenting sensitivity equal to 14.45 ± 2.97 mA mM
−1 cm
−2 [58].
Zhu and his team
[59] employed non-woven fabrics made of CNTs (CNTFs) to detect glucose by incorporating glucose oxidase into a polyvinyl alcohol solution. In addition, Gaitán and co-researchers focused on the impact of surficial chemistry, as well as glucose oxidase-coated MWCNTs’ structure in electrochemical glucose sensing
[60].
Non-enzymatic sensors possess several advantageous features, such as long-lasting durability, low-cost, enhanced sensitivity, effective electron transfer, and strong electro-catalytic efficiency
[61][62]. These benefits overcome the constraints of enzyme-based sensors, which are prone to instability and storage difficulties. Consequently, numerous research studies have been conducted
[63][64][65], encouraging advancements in glucose sensor technology. Ideal glucose sensors should exhibit an extensive linear span, decreased limit of detection, and increased sensitivity. While enzyme-based sensors offer a broad linear range and sensitivity, their fixation and preparation present challenges. Moreover, enzyme stability issues can lead to discrepancies. Consequently, extensive studies are dedicated to the fabrication of non-enzymatic sensors to enhance their sensing capabilities. Non-enzymatic sensors typically involve the modification of electrodes with metal nanoparticles. For instance, an electrode composed of stacked cup CNTs (SCCNT) modified with nickel oxide (NiO) was utilized
[66]. Another approach involved a bimetallic nanoparticle-altered electrode (Fe, Ni/CNTs/GCE) that harnessed the advantageous electrochemical features of Fe and Ni
[67]. The impact of an NiO layer’s thickness on sensing has been also investigated. Pt nanoparticles have been employed to modify MWCNTs, with the functionalization of 3-mercaptophenylboronic acid (3MPBA) attached to the platinum particle surface
[68]. The linear range of sensing was 0–10 mM, presenting a detection limit equal to 4.5 mM. Moreover, PdNi@f-MWCNT/GCE was produced by modifying functionalized MWCNTs with Pd and Ni nanoparticles
[69]. This particular sensor presented a linear range of 0.01–1.4 mM, and an exceptionally decreased LOD of 0.026 mM.
Transition metal oxide/carbon hybrid electrodes offer enhanced sensitivity and improved electron transfer efficiency. A fluorescent nanosensor utilizing a hybrid nanocomplex of cobalt and CNTs (Co-CNT) was documented
[70]. This sensor employs unlabeled and enzyme-free fluorescence ratio approaches for the simultaneous sensing of H
2O
2 and glucose. The detection sensitivity for H
2O
2 reached 150 nM, while for glucose, it was also 150 nM. A straightforward approach was proposed for the preparation of a cobalt-containing electrode using melamine foam
[71]. The glucose sensor employing the CoeCo
3O
4/CNT/CF/GCE electrode exhibited a detection range of 1.2 mM to 2.29 mM. The catalytic properties of binary metal composites outperformed those of metal oxides, making Ni and Co-based alloys with functionalized MWCNTs (f-MWCNTs) highly effective in glucose detection
[72]. The combination of cobalt phthalocyanines (CoPc) with single-walled CNTs (SWCNTs) and reduced graphene oxide (rGO) allowed for a rapid response, responding within 1.2 s to changes in glucose concentrations in saliva
[73].
With a focus on nanomaterials, researchers have become increasingly dedicated to nano-structured design and novel approaches
[74]. An electrode composed of Co
3O
4/NCNTs, based on a metal–organic skeleton, was created for detecting glucose and H
2O
2 [75]. A unique dodecahedral CNT array was achieved through the carbonization of a specific zeolitic imidazolate framework material (ZIF-67), with Co
3O
4 nanoparticles encapsulated within the CNTs ends. Furthermore, to enhance the robustness and catalytic effectiveness of binary metal composites, porous carbon materials were coupled with a CoCu organic framework catalyst, resulting in the development of nitrogen-doped CNTs@MOF@CoCu biosensors
[76].
Cu
2O presents itself as a favorable option for a metal oxide electrode. Along with material selection, the utilization of various nanostructures can enhance the performance of detection. For instance, a non-enzymatic sensor employed a copper nanowire-bilayer (CuNW-CNT-BL) configuration
[77]. The sensor presented a linear range of 10–2000 mM, characterized by a limit of quantification equal to 1.1 nM and a low detection limit equal to 0.33 nM. Moreover, this sensor demonstrated selectivity towards uric acid, dopamine, and various molecules. In the Cu
2O-MSs/S-MWCNTs/GCE composite electrode, the Cu
2O material exhibited a spherical design
[78]. By incorporating sulfur atoms, the electrocatalytic performance of CNTs was enhanced. The linear range spanned from 4.95 mM to 7 mM, with a comparatively decreased detection limit (1.46 mM). In order to enhance manufacturing accessibility, researchers have explored novel approaches, like microfluidic technology, which enables the regulation and sensing of complex fluids at the micro-scale. A decreased-fluid-consuming sensor built upon Pt-Ni NPs-MWCNTs/screen-printed CE was also introduced
[79]. Additionally, microfluidic cloth-based analytical devices (mCADs) with multiple empty channels have been synthesized for enzyme-free glucose sensing
[80]. The utilization of a method named photoelectrochemical (PEC) monitoring involved CdS quantum dots and poly (dimethyl diadly ammonium chloride)-functionalized MWCNTs, enabling the efficient and straightforward detection of glucose in saliva possessing a decreased detection limit (15.99 nM).
2.1.2. Sensing of DNA
Deoxyribonucleic acid (DNA) plays a crucial role as a genetic information carrier in organisms, and its analysis can aid in the timely identification and management of diseases
[81]. Cancer, a major cause of mortality, benefits greatly from early detection, increasing the chances of successful treatment. For instance, urokinase plasminogen activator (uPA) serves as a biomarker for distinct categories of prostate cancer
[82], while alpha-fetoprotein (AFP) is associated with liver cancer
[83]. Colorectal cancer diagnosis could be facilitated through detecting the glycoprotein CEACAM5
[84]. However, conventional methods, like polymerase chain reaction (PCR), for the clinical detection of diseases have limitations, including the risk of contamination
[85]. Consequently, there is a requirement for economical and exceptionally sensitive methods for biomolecule sensing in medical applications. CNT-based biosensors have emerged as promising candidates
[86]. Over time, notable strides have been taken in the evolution of DNA sensors based on CNTs, employing both optical
[87] and electrochemical techniques
[88] in the medical field.
For instance, HIV is a contagious illness that devastates the immune system and carries a high fatality rate. Detecting the infection at an early stage can impede its progression. A wearable device utilizing a paper electrode MOF-based sensor exhibited remarkable tensile attributes and demonstrated promising prospects. Furthermore, it showcased excellent detection capabilities
[89]. The detection range spanned from 10 nM to 1 mM, with an impressively low limit of detection (0.13 nM). The unexpected emergence of COVID-19
[90] and the escalating infection incidents have presented significant difficulties in swiftly identifying nucleic acids. By utilizing ss-DNA-modified SWCNTs, it binding with the RNA of the virus target becomes possible. Moreover, the ss-DNA can be substituted to capture other viruses, making it a versatile and adaptable solution
[91].
CNTs were modified with extremely spinning Fe atoms to perform label-free cancer gene testing repeatedly
[85][92]. A Pt-interdigitated electrode surface was coated with Au/Fe
2O
3 magnetic nanoparticles and CNTs prepared from two different types of nanoparticles for influenza virus detection using an altered DNA probe approach
[93]. A label-free DNA sensor with a 3D spatial framework, consisting of four DNA types, was developed
[94]. Furthermore, CNTs selectively integrate with DNA that was immobilized on an ITO electrode, and the detection of DNA was achieved using the electrochemical luminescence approach
[95]. In this procedure, CNTs had a vital role in suppressing electrochemical reactions and augmenting electro-chemiluminescence signals. In order to surpass the complexity of fabricating CNTS electrodes on supporting matrixes, like polymer-based and glassy CEs, self-standing three-dimensional CNTs structures have been introduced. Prakash and his team recently developed a standalone CNTs aerogel electrode for exclusive DNA sensing, achieving a detection limit equal to 1 pM
[89]. The as-mentioned electrode, imposed with a multi-directional CNT network, exhibited favorable electrical features, enabling increased sensitivity in DNA sensing. A sensor using AuNPs/Chit-MWCNTs/GCE was prepared with methylene blue as a procedure indicator
[96]. CNTs/chitosan, Au nanoparticle thin films, and ss-DNA probes were successively altered on the surface of a glassy CE. Additionally, polymer- and MWCNT-modified electrodes were used in DNA, drugs, and interaction sensing by modifying double-stranded DNA (ds-DNA)
[97].
Nano-biosensors utilizing a field-effect transistor (FET) offer remarkable advantages, including high label-free detection specificity. In this approach, the probe-ss-DNA is immobilized within the interval between the nanotube and the Au-gate, forming a double-stranded DNA structure when coupled with the target ss-DNA
[98]. However, the substrate used in traditional FETs can hinder the sensing performance. To overcome this limitation, a suspended CNT FET was proposed, which separates the CNTs from the substrate and exhibits a decreased detection limit
[99]. Liang and his team further enhanced the sensitivity by optimizing the FET structure, introducing a Y
2O
3 thin film between the Au nanoparticles and the CNT layers
[100]. Additionally, nitrogen-containing CNTs with a chiral structure can detect DNA damage, which is associated with aging, gene mutation, and carcinogenesis, providing insights into pathogenesis
[101]. Nanobridges developed by wrapping MWCNTs with elongated DNA strands can detect DNA damage resulting from various free radicals
[102]. Another innovative approach is the CNT thin-film-transistor (CNT-TFT) sensor, which incorporates peptide electrodes at the ends of CNTs’ channels and functionalizes CNTs. This enables the quantitative detection of cancer cell genes, expanding the range of sensing and reducing the detection limit
[103].
2.1.3. Sensing of Proteins and Amino Acids
Proteins, which are involved in various physiological activities, play a crucial role in body metabolism
[104]. Despite the use of multiple techniques for protein detection, there is still a need for rapid and simple sensing methods
[105][106]. Recently, CNTs have been utilized in point-of-care protein sensing, with the aim of increasing sensitivity and reducing the response time. Janssen and co-researchers designed a label-free biosensor based on CNTs to detect bovine serum albumin, which demonstrated a detection limit of 2.89 ng/mL
[107]. This biosensor incorporates cellulose paper, SWCNTs, and BSA-specific antibodies, utilizing the principle of electrical percolation for the simplified and rapid quantification of BSA. Additionally, Huang et al. introduced an Fe
3O
4 nanoparticle–CNT-based biosensor for the visual detection of Rabbit IgG without complicated sample processing
[108]. The biosensor employs Fe
3O
4 nanoparticles decorated on the surface of magnetized CNTs, which are altered with specific antibodies. This approach favored the immediate sensing of target proteins in whole blood possessing a low detection limit (10 ng/mL). Such low-cost and visual protein detection methods hold promise for clinical diagnosis. Furthermore, a redox probe-free electrochemical biosensor was developed by incorporating copper nanoparticle-doped CNTs as the electroactive film and a plastic antibody used as the bio-recognition constituent
[109]. The biosensor eliminated the need for a redox probe-reading stage in conventional electrochemical detection. It exhibited a detection limit equal to 1.0 ng/mL for protein detection, and its application in serum samples suggests its potential for direct sample readings during in situ biomedical analysis.
CNT-FET biosensors employing the chemo-dosimetric detection principle have been fabricated for cysteine’s detection by
[110]. The molecular interaction among the selective chemo-dosimeter and cysteine induces a chemical alteration, affecting the surficial distribution of charge of CNTs and triggering electrical responses in the CNT-FET. The acquired data demonstrate that the as-mentioned biosensor enabled label-free, selective, and highly sensitive cysteine sensing (LOD = 0.45 fM). Additionally, the interaction between serine amino acid and CNTs was investigated
[111]. The chirality of serine amino acid significantly influences CNTs’ dielectric features, proposing the CNT-based sensors’ potential to discriminate chiral molecules. Furthermore, a dual-signal electrochemical biosensor was constructed using chiral CNTs coupled with platinum nanoparticles and cyclodextrin
[112]. This biosensor exhibited the ability to differentiate among amino acids, specifically between tyrosine and tyrosine, presenting the limits of detection equal to 0.107 nM and 0.133 nM, respectively.
2.1.4. Sensing of Neurotransmitters
Neurotransmitters like dopamine, acetylcholine, and norepinephrine are crucial chemical components involved in cell-to-cell communication
[113]. Acetylcholine, for instance, is related to the nervous system and is linked to the increased threat of Alzheimer’s disease
[114]. Epinephrine contributes to regulating blood pressure and heart function
[115]. Dopamine serves as a hormone and neurotransmitter, and insufficient levels of it can cause conditions like Parkinson’s disease and mental dysfunctions. Typically, neurotransmitters are absent in isolation within the body but exist alongside other biomolecules like ascorbic acid, uric acid, and tyrosine
[116], which are crucial for preserving normal physiological function. To enable the simultaneous sensing of dopamine, ascorbic acid, and uric acid without interference, a green reduction process was employed using Pelargonium graveolens extract to reduce hydroxylated MWCNTs
[117], followed by hybridization with Pb nanotubes. The detection platform involved modifying a graphite electrode through the hybridization of reduced carboxylated MWCNTs and Pd nanotubes.
Guan and his team conducted a hydroxylation process on both SWCNTs and MWCNTs in order to acquire the MWCNT-COOH/SWCNT-OH-modified glassy CE sensor
[118]. The arrangement of the two types of nanotubes in a staggered distribution created a simple grid structure, which facilitated electron mobility. Metal-based electrodes incorporating CNTs are commonly used in electrochemical sensors. For instance, GC/Ni-CNT electrodes have been employed to sense dopamine and adrenaline
[119]. PPy has been extensively utilized in biosensor fabrication, and in this case, sodium dodecyl sulfate was used to modify the sensor, resulting in the overoxidized PPy/sodium dodecyl sulfate-modified MWCNT electrode
[120]. This sensor demonstrated high sensitivity in detecting DA while minimizing interference from glucose and other substances. Another innovative approach involved the development of a molecularly imprinted polymer (MIP)/MWCNT/graphene aerogel composite electrode (MIP/MWCNT/GAs/GCE), where the MIP was created through electro-polymerization of pyrrole
[121]. Initially, MWCNT/graphene aerogels were synthesized, while the resulting mixture was applied onto the glassy CE surface using drop casting. Furthermore, a novel type of graphene quantum dots was fabricated from glucose
[122], and these dots were integrated with modified MWCNTs to enable the sensing of dopamine. The linear detection range was 250 nM–250 mM, with a detection limit equal to 95 nM. Gupta et al. suggested a CNT array-based method for dopamine detection, utilizing increasingly densified CNTs fibers
[123]. The range of detection of this approach was 1 nM–100 mM, with an LOD of 32 pM.
Uric acid and dopamine are present alongside ascorbic acid in living organisms, making their detection crucial. Electrochemical techniques are extensively employed in neurotransmitter detection because of their benefits, including heightened sensitivity, rapid response, and cost effectiveness
[124]. To detect dopamine and uric acid, a sensitive and selective biosensor was developed using Au nanoparticle-loaded MWCNTs altered with PCA
[125]. Serotonin, comprising an essential gastrointestinal neurotransmitter, has also garnered significant attention in medical and life sciences. A remarkably sensitive electrochemical sensor for serotonin sensing was developed by using Cu
2O encapsulated in CNTs with platinum nanoparticles
[126]. The as-described sensor exhibited exceptional electro-catalytic performance towards serotonin oxidation, achieving a low detection limit (3 nM). Furthermore, MWCNTs modified with capsaicin were employed to alternate glassy CEs towards the quantitative sensing of xanthurenic acid, dopamine, epinephrine, ascorbic acid, and uric acid at a micromolar level
[127]. A ternary nanocomposite of N-doped CQDs@Fe
2O
3/MWCNTs enabled the sensing of uric acid and xanthine, characterized by detection limits equal to 0.106 mM and 0.092 mM, respectively
[128]. The practical applicability of this nanocomposite was demonstrated in a human urine sample.
2.2. CNTs for Drug Delivery
CNTs have gained tremendous attention as an efficient means of transferring several drug molecules into vital cells due to their unique structure, which facilitates non-intrusive permeation across biological membranes
[129]. Typically, drug molecules are anchored to CNT sidewalls either through covalent or non-covalent bonds with functionalized CNTs
[130]. Nevertheless, each of these methods comes with its own set of benefits and limitations. Covalent interactions provide stability to the drug-loaded CNTs both outside and inside cells, but they lack controlled drug release within the cancer cells’ cellular micro-environment that comprises a limitation of the drug delivery system. On the other hand, non-covalent interactions allow for gradual drug release in the acidic tumor sites’ conditions but suffer from robustness issues at extracellular pH values
[131]. Therefore, utilizing the inner hollow cavity of CNTs for drug loading offers an optimal solution by providing drug isolation from the physiological environment. To overcome these challenges related to drug release, researchers have investigated various external stimuli, either individually or in combination, including temperature, electric field, and light. For instance, a chitosan-modified CNT with a thermosensitive polymer (NIPAAm) and NIPAAm-co-Vim was developed and encapsulated in bovine serum albumin at body temperature (37 °C) to evaluate the temperature-responsive discharge of biomolecules. Bovine serum albumin was released just above the lower critical solution temperature of polyVIm (38–40 °C)
[18][132].
Shi and co-researchers
[133] employed an electric field to release ibuprofen from a hybrid hydrogel that consisted of sodium alginate, bacterial cellulose, and MWCNTs. Estrada and his team
[134] investigated the temperature and NIR light-responsive discharge of methylene blue from a hydrogel composed of MWCNTs and k-carrageenan. CNTs have been utilized as carriers for a range of drugs, such as doxorubicin, paclitaxel, docetaxel, and oxaliplatin, in both in vitro and in vivo cancer therapy. Modified CNTs for drug delivery purposes have been thoroughly researched, developing strategies to enhance water solubility and entrap drug molecules
[135]. Jain and Mehra, in 2015, compared the cancer-targeting abilities of doxorubicin-loaded folic acid and estrone-anchored PEG-modified MWCNTs in tumor-bearing mice, showing improved survival with the use of DOX/ES-PEG-MWCNTs
[136]. Calcium phosphate-crowned MWCNTs as nanocapsules for intracellular drug transfer have been also explored, demonstrating pH-stimulated dissolution and controlled discharge within lysosomes
[137]. Xu and co-researchers
[138] developed an amine-PEG-modified polydopamine-CNT nanosystem for drug transfer to enhance the biocompatibility of CNTs. Finally, Picaud et al.
[139] carried out theoretical investigations on the loading and discharge of cisplatin from CNTs.
2.3. CNTs for Cancer Treatment
Since cancer remains a major health issue, several studies have focused on increasing the understanding of the molecular mechanisms that mediate tumorigenesis
[140], oxidative-stress
[141], and cellular senescence and aging
[142], and others have tried to provide alternative therapeutic approaches
[15][16][17][18].
CNTs possess versatile properties that make them highly valuable in biomedical applications. They are extensively utilized as carriers for anticancer drugs, genes, and proteins in chemotherapy, making them a promising option
[143]. Additionally, their enhanced absorption of NIR light enables effective photothermal therapy. Su and his team
[144] synthesized iRGD-PEI modified MWCNTs, which were then conjugated with candesartan. This functionalized nanocomposite targeted αvβ3-integrin and AT1R receptors on tumor endothelium and lung cancer cells, respectively. The combination of candesartan with [pAT (2)] resulted in the downregulation of VEGF and the effective inhibition of angiogenesis
[144].
Another study by Zhang and his team
[19] involved the design of a nanocomposite consisting of DOX-loaded MWCNTs and magneto-fluorescent CQDs for combined chemo- and photothermal therapy
[19]. The negatively charged GdN@CQDs-MWCNTs enabled the bonding with positively charged DOX molecules. This nanocomposite exhibited strong NIR irradiation absorption. In in vivo photothermal therapy experiments, laser irradiation of tumor sites led to a temperature increase of 51.8 °C in the mice provided with GdN@CQDs-MWCNTs/DOX-EGFR, while the control group showed no significant change in temperature. The localized heating effect facilitated the release of DOX and led to successful photothermal therapy, as demonstrated by a reduction in the tumor volume.
In a recent study, a nanosystem consisting of TAT-chitosan functionalized MWCNTs loaded with DOX was utilized by Dong et al.
[145] to combine chemo and photothermal therapy. To promote apoptosis in cancer cells, a PEG-coated CNT-ABT737 nanodrug was employed, which targeted the mitochondria
[146]. The discharge of the nanodrug in the cytosol induced apoptosis in lung cancer cells by disrupting the mitochondrial membrane. In vivo, this approach showed impressive therapeutic effectiveness, further enhanced by the localized heating effect triggered by NIR light irradiation. Another development involved an Au nanoparticle-coated CNT ring that exhibited exceptional Raman and optical signal characteristics, leading to enhanced photoacoustic signal and photothermal conversion behavior
[147]. The as-mentioned composite indicated promising results in image-guided cancer therapy. Additionally, the photothermal cancer-killing efficacy was improved by the surface plasmon resonance absorption of Au in SWNT-Au-PEG-FA nanomaterials
[148].
2.4. CNTs for Reinforcing Tissue Engineering Scaffolds
The utilization of CNTs in composite reinforcements of tissue engineering scaffolds has predominantly emphasized increasing their mechanical attributes
[149]. Traditional scaffold materials, like hydrogels and fibrous scaffolds, are commonly pliable to emulate the inherent rigidity of tissues, resulting in decreased structural robustness and support. However, by introducing CNTs into the as-mentioned materials, their mechanical properties can be significantly improved. For instance, Shin et al. conducted a study where they incorporated CNTs into gelatin hydrogels, resulting in notable enhancement of tensile strength
[150]. Similarly, Sen and his team illustrated the enhanced tensile strength of CNTs-reinforced polystyrene and polyurethane fibrous membranes
[151]. Lately, researchers have explored the versatile capabilities of CNTs in tissue scaffold engineering, particularly their ability to confer electrical conductivity. Most biomaterials utilized in tissue engineering are electrically insulating due to their composition of non-conductive polymers
[152][153][154]. Nevertheless, specific applications, like neural and cardiac tissues, require conductive scaffolds to efficiently transfer electrical signals and support optimal electro-physiological operations. For instance, Kam and co-researchers conducted experiments where they employed electrical stimulation to neural stem cells cultured on CNT–laminin composite thin films, resulting in increased activity potential and the functional neural networks’ differentiation
[153]. Similarly, Shin and his team cultured cardiomyocytes on CNT-reinforced gelatin hydrogels, which exhibited increased electro-physiological efficiency and eventually produced functional cardiac tissue
[155]. The aforementioned studies demonstrate the capability of CNTs to successfully supply electrical conductivity to previously non-conductive biomaterials.
2.5. Cytotoxicity of CNTs
Although CNTs have been successfully applied in biomedical engineering, there is a growing apprehension regarding their harmlessness. Several contemporary in vitro studies have documented the elevated cytotoxicity of CNTs, which can be attributed to factors such as cellular uptake, aggregation, and induced oxidative stress
[156][157][158]. The contradictory results regarding the CNTs’ biocompatibility primarily arise from variations in their properties (such as size, surficial characteristics, and functionalization), as well as the different testing protocols employed (including in vitro and in vivo studies, cell type, tissues, and animals tested). Furthermore, the insufficient elimination of metal catalysts utilized during CNT preparation has been associated with increased cytotoxicity
[157]. The majority of in vivo studies have indicated the minimal toxicity of CNTs and their clearance through renal pathways, although some accumulation in organs like lungs, liver, and spleen has been observed, potentially leading to inflammation
[156][157][158]. Nevertheless, cytotoxicity appears to demonstrate increased fluctuation and significance at the cellular level, as indicated by numerous in vitro cell culture studies
[159][160]. Given the expanding use of CNTs in diverse biomedical fields, it is crucial to conduct comprehensive biological evaluations considering the various chemical and physical properties of CNTs to ascertain their pharmaco-kinetics, cytotoxicity, and optimized dosages. Nonetheless, a plethora of research studies have demonstrated that functionalizing CNTs utilizing biocompatible polymers or surfactants can effectively minimize toxicity and prevent aggregation
[161][162].