Multi Walled Carbon Nanotubes in Biology and Health Research

Multi-walled carbon nanotubes (MWCNTs) have gained attention in biology and health research due to their high surface area, mechanical strength, and electrical conductivity. These properties make them promising for drug delivery, biosensing, and tissue engineering. However, their interactions with biological systems raise concerns about safety, biocompatibility, and functional effectiveness.

Understanding their behavior in biological environments is essential for harnessing their potential while addressing associated risks.

Structural Features

MWCNTs consist of multiple concentric graphene layers rolled into cylindrical structures, held together by van der Waals forces. Their outer diameter typically ranges from 10 to 100 nanometers, with inner diameters as small as a few nanometers. The number of walls influences their mechanical properties, electrical conductivity, and surface reactivity, distinguishing them from single-walled nanotubes. The interlayer spacing, about 0.34 nm, is comparable to that in graphite and affects molecular interactions with the nanotube surface.

Structural integrity is influenced by chirality and defect density. Chirality determines electronic properties, affecting whether a nanotube behaves as a metallic or semiconducting material. Defects, such as vacancies and pentagon-heptagon pairs, alter mechanical strength and chemical reactivity. While pristine MWCNTs exhibit exceptional tensile strength—often exceeding 100 GPa—defects introduce weaknesses that affect stability in biological environments. These imperfections also create reactive sites for functionalization, improving compatibility with biological molecules.

The high aspect ratio of MWCNTs, often exceeding 1000:1, affects their interactions with cells and tissues. Their elongated shape enables penetration into biological membranes, influencing cellular uptake and intracellular trafficking. Studies indicate that MWCNTs enter cells via endocytosis or direct membrane penetration, depending on size and surface chemistry. Their flexibility also plays a role; some exhibit rigid, needle-like properties, while others bend, impacting biodistribution and aggregation tendencies.

Techniques For Production

MWCNT synthesis requires precise control over temperature, catalyst composition, and carbon feedstock to achieve desired structural and functional properties. Chemical vapor deposition (CVD) is the most widely used method due to its scalability and ability to produce high-purity nanotubes with controlled morphology. In this process, a hydrocarbon gas—such as methane, ethylene, or acetylene—is introduced into a reaction chamber containing a metal catalyst, typically iron, cobalt, or nickel nanoparticles. At temperatures between 600°C and 1200°C, the hydrocarbon decomposes, allowing carbon atoms to nucleate around the catalyst, forming concentric graphene layers.

CVD efficiency depends on catalyst particle size and substrate selection. Smaller catalyst particles promote thinner-walled nanotubes, while larger ones result in thicker structures. Substrates like silicon wafers or quartz influence nanotube alignment and density, particularly when growth is guided by patterned catalyst deposition. Plasma-enhanced CVD (PECVD) introduces an electric field to generate plasma, enhancing gas dissociation and enabling lower-temperature synthesis—beneficial for applications like biosensors and microelectrode arrays.

Alternative methods, including arc discharge and laser ablation, have distinct advantages. Arc discharge generates an electric arc between graphite electrodes in an inert gas atmosphere, vaporizing carbon atoms that condense into nanotube structures. This method yields high-quality MWCNTs with fewer defects but requires extensive purification to remove residual catalysts and amorphous carbon. Laser ablation, which uses a high-powered laser to vaporize a graphite target containing metal catalysts, produces nanotubes with well-defined diameters and fewer structural irregularities but is less practical for large-scale manufacturing due to high energy consumption.

Purification Approaches

MWCNT synthesis produces unwanted byproducts, including residual metal catalysts, amorphous carbon, and graphitic impurities. These contaminants affect dispersion, surface reactivity, and biological interactions, necessitating effective purification strategies. Acid treatment is a common method, using strong acids like nitric acid (HNO₃) or sulfuric acid (H₂SO₄) with hydrochloric acid (HCl) to dissolve metal impurities while oxidizing amorphous carbon. However, prolonged exposure introduces defects that alter electrical and mechanical properties.

To minimize structural damage, alternative techniques such as thermal annealing and ultracentrifugation have been explored. Thermal annealing selectively removes amorphous carbon by heating MWCNTs in an oxidative atmosphere at 400°C to 700°C, requiring careful temperature control to prevent excessive oxidation. Ultracentrifugation separates impurities based on density differences, isolating unwanted materials without chemical modifications—an advantage for preserving pristine nanotube structures.

Filtration and electrophoretic techniques offer additional refinement. Membrane filtration, often combined with surfactant-assisted dispersion, removes larger agglomerates and metal residues. Electrophoretic purification applies an electric field to separate charged impurities from nanotubes suspended in a liquid medium, reducing metal content while maintaining nanotube length. Combining multiple purification steps, such as acid washing followed by ultracentrifugation, yields higher purity MWCNTs with fewer structural defects, improving their performance in biological environments.

Surface Modification Strategies

Enhancing MWCNT functionality for biological applications requires surface modifications that improve solubility, biocompatibility, and targeted interactions. Due to their hydrophobicity and tendency to aggregate in aqueous environments, functionalization is necessary for effective dispersion and biological integration. Covalent modification involves attaching functional groups such as carboxyl (-COOH), hydroxyl (-OH), or amine (-NH₂) moieties through oxidation or direct chemical reactions. This enhances solubility and provides reactive sites for conjugation with biomolecules like proteins, peptides, and nucleic acids.

Non-covalent strategies preserve nanotube structure while improving biological compatibility. Surfactants like sodium dodecyl sulfate (SDS) and Pluronic block copolymers stabilize dispersions by adsorbing onto the surface, preventing aggregation without altering electronic properties. Similarly, π–π stacking interactions with aromatic molecules, such as pyrene-based compounds, enable stable functionalization while maintaining conductivity—useful for biosensing and neural interfacing.

Analytical Tools For Characterization

Characterizing MWCNTs is essential for assessing structural integrity, purity, and functional properties in biological applications. Spectroscopic, microscopic, and thermal techniques provide a comprehensive understanding of their attributes. Raman spectroscopy reveals crystallinity and defect density, with the D-band (associated with defects) and G-band (graphitic order) serving as key structural indicators.

Electron microscopy techniques, including transmission electron microscopy (TEM) and scanning electron microscopy (SEM), visualize nanotube morphology, wall thickness, and aggregation tendencies. TEM reveals concentric graphene layers and structural irregularities, while SEM provides insights into surface topology and dispersion. Thermogravimetric analysis (TGA) quantifies purity by measuring weight loss at specific temperatures, distinguishing between carbonaceous impurities and metal catalysts. X-ray photoelectron spectroscopy (XPS) detects surface functional groups, assessing oxidation levels and confirming chemical modifications. These tools ensure precise evaluation of MWCNTs for safe and effective biological applications.

Dispersion Behavior In Liquids

MWCNT dispersion in liquid environments affects their usability in biological applications, as aggregation alters cellular interactions and functional efficiency. Their hydrophobicity and strong van der Waals forces promote bundling, making dispersion a critical aspect of preparation. Dispersing agents help counteract this tendency. Surfactants like SDS and Triton X-100 adsorb onto nanotube surfaces, reducing inter-tube attraction and improving solubility. Biocompatible polymers such as polyethylene glycol (PEG) and polyvinylpyrrolidone (PVP) provide additional stabilization while enhancing biomedical suitability.

Ultrasonication, often used with surfactants or polymers, disrupts nanotube aggregates with high-frequency sound waves, temporarily improving dispersion. However, prolonged ultrasonication can introduce defects, altering electrical and mechanical properties. Ionic strength and pH also influence stability, as electrostatic interactions between charged functional groups impact aggregation. In physiological conditions, where ionic strength is high, tailored surface modifications are necessary to maintain dispersion.

Interactions With Biomolecules

MWCNT interactions with biological molecules influence applications such as drug delivery and biosensing. Proteins, lipids, and nucleic acids adsorb onto nanotube surfaces, forming a corona that alters bioavailability and cellular uptake. The composition of this corona depends on nanotube surface chemistry, with hydrophilic modifications favoring interactions with polar biomolecules, while unmodified nanotubes bind more strongly to hydrophobic protein and lipid regions. This adsorption can affect protein structure and function, potentially altering enzymatic activity or immune recognition.

DNA and RNA interactions with MWCNTs have been explored for gene delivery and biosensing, leveraging π–π stacking between nucleobases and the nanotube surface. These interactions facilitate genetic material transport into cells, but stability and controlled release remain key considerations. Lipid membranes also play a role in cellular uptake, as MWCNTs can insert into bilayers, disrupting membrane integrity or facilitating endocytic pathways. The extent of these interactions depends on nanotube length, diameter, and surface charge, requiring careful design to optimize biocompatibility and functionality for biomedical applications.