Functionalized Carbon Nanotubes

Quick Summary: Functionalized Carbon Nanotubes

  • What they are — pristine SWCNT or MWCNT with reactive surface groups (COOH, OH, NH₂) added by chemical treatment
  • Why they matter — dramatically improved dispersion in water and polar solvents, plus reactive handles for conjugation and matrix bonding
  • Compared to pristine CNTs — better dispersibility, easier to formulate, slightly lower aspect ratio after acid oxidation
  • Key applications — biomedical scaffolds, drug delivery, polymer composites, conductive inks, sensors, dispersion-critical formulations
  • Available chemistries — COOH (carboxyl), OH (hydroxyl), NH₂ (amine)
  • Choosing the right one — see the SWCNT Buying Guide for functionalization selection by application

Functionalized Carbon Nanotubes

Pristine CNTs won’t bond to your matrix. They aggregate in water. They have no reactive handles for your conjugation chemistry. Functionalization solves all of this — but only when the surface chemistry is matched to your specific application and matrix.

Cheap Tubes has supplied COOH, OH, NH₂, F, and nitrogen-functionalized SWCNTs and MWCNTs to research labs and development teams since 2005 — acid-functionalized and plasma-functionalized grades, with representative characterization data for each. Used in drug delivery, composite reinforcement, biosensor development, and specialty polymer applications. If our material doesn’t meet published specifications, we’ll replace it or refund your order. Ships from Vermont.

In stock. Contact us for custom functionalization density, surface group ratios, or volume pricing.

Used in published research: Pharmaceutics 2021  ·  Biology 2021

What Is CNT Functionalization?

Carbon nanotube functionalization is the controlled attachment of chemical groups to nanotube surfaces, either covalently (forming direct C–X bonds to the sp² carbon lattice) or non-covalently (through π–π stacking, wrapping, or physisorption). The purpose is to convert pristine CNTs — chemically inert, hydrophobic, and strongly bundled — into CNTs that can be dispersed in specific solvents or matrices, bond to target surfaces, or participate in chemical reactions. The type and density of surface groups determines which applications become accessible.

Schematic diagram of carbon nanotube functionalization showing covalent sidewall groups (-COOH, -OH, -NH2, -F), endcap functionalization, non-covalent polymer wrapping, and defect-site functionalization
Four modes of CNT functionalization. Covalent methods (acid, plasma) introduce reactive surface groups; non-covalent wrapping preserves the sp² lattice and electrical properties.

Types of Functionalized CNTs

Functional Group Symbol Key Properties Primary Applications
Carboxylic acid -COOH Disperses in water and polar solvents; reactive toward amines and alcohols Polymer composites (epoxy, PA, PC), biomedical, covalent coupling
Hydroxyl -OH Moderate polarity; reacts with isocyanates, silanes, epoxides Polyurethane composites, surface coatings, silane coupling
Amine -NH₂ Basic, highly water-dispersible; reacts with COOH, epoxides, aldehydes Drug delivery, biosensors, crosslinked epoxy composites
Fluorine -F Highly reactive toward nucleophiles; enables further functionalization Covalent attachment chemistry, fluoropolymer composites
Nitrogen-doped -N, N₂ Electron donor; improves catalytic activity and electrochemical behavior Electrocatalysis (ORR), supercapacitors, fuel cells

Functionalization Methods

Acid Functionalization

The most widely used method treats CNTs with concentrated nitric acid, sulfuric acid, or mixtures of both at elevated temperature. This oxidizes the tube surfaces and ends, introducing carboxylic acid (-COOH) and hydroxyl (-OH) groups primarily at defect sites and tube ends (which are more reactive than intact sidewalls). Acid functionalization also removes residual catalyst metals from CVD synthesis, reducing metal content to <1 wt%. The drawback is that it introduces additional lattice defects and shortens the tubes, which reduces aspect ratio and can lower mechanical properties in composites.

Plasma Functionalization

Plasma treatment exposes CNTs to reactive gas plasmas (oxygen, nitrogen, ammonia, or fluorine plasmas) that attach functional groups to the surface with minimal internal lattice damage. Plasma functionalization is gentler than acid treatment — it preserves tube length and introduces fewer structural defects — while still providing effective surface group attachment and catalyst removal. It is the preferred method when preserving mechanical and electrical properties is important alongside the functionalization.

Where Functional Groups Locate on CNTs

The sp² graphene lattice of CNT sidewalls is relatively chemically inert. Functional groups attach preferentially at higher-energy sites: end caps (which have 5-membered rings), pre-existing lattice defects, and sites of incomplete hexagon closure. This means the density of functional groups is higher at tube ends and defect sites than on pristine sidewalls. The degree of functionalization is typically quantified by the COOH or other group content as a weight percentage, measured by thermogravimetric analysis (TGA).

Benefits of Functionalization for Composite Applications

In polymer matrix composites, the limiting factor for CNT performance is typically the quality of the CNT–matrix interface. Pristine CNTs bond to most polymer matrices only through weak van der Waals forces, producing poor stress transfer and pull-out at low strains. COOH-functionalized CNTs can form covalent ester or amide bonds with epoxy, polyamide, and polyurethane matrices — dramatically improving interfacial shear strength. This translates to 30–60% higher composite tensile modulus and 20–40% higher strength compared to pristine CNT composites at equivalent loading.

For dispersion in aqueous biological or analytical applications, -COOH and -NH₂ groups provide colloidal stability through electrostatic repulsion (high negative or positive zeta potential) without requiring surfactants that would need to be removed in downstream processing. This makes functionalized CNTs the preferred choice for drug delivery systems, biosensors, and diagnostic conjugation chemistry.

Selecting the Right Functionalized CNT Grade

Cheap Tubes offers COOH, OH, NH₂, F, and nitrogen-functionalized grades for both single-walled (SW/DWNT) and multi-walled CNTs, across diameter ranges from 8 nm to >50 nm. Graphitized functionalized grades (COOH on graphitized MWCNTs) provide higher conductivity alongside functional surface groups. Key selection parameters are the degree of functionalization required (light functionalization preserves electrical conductivity; heavy functionalization improves dispersibility at the cost of some conductivity), tube diameter (affects aspect ratio, percolation threshold, and surface-to-volume ratio), and the target matrix chemistry.

For custom functionalization density, specific surface group ratios, or volume pricing, contact our technical team. SDS documentation is available from our SDS page.

Frequently Asked Questions

Why functionalize carbon nanotubes?

Pristine carbon nanotubes are highly hydrophobic and aggregate strongly via van der Waals forces, which makes them difficult to disperse in polar solvents and polymer matrices. Functional groups (-COOH, -OH, -NH2, -F, etc.) attached to the sidewall and tube ends provide chemical handles for: improved dispersion in polar media, covalent bonding to polymer chains for stronger composites, conjugation to biomolecules for biomedical applications, and surface charge that enables stable aqueous suspensions without surfactants.

What is the difference between -COOH, -OH, and -NH2 functionalization?

Carboxyl (-COOH) groups are introduced by acid oxidation (HNO3 or HNO3/H2SO4 reflux) and provide pH-responsive water dispersibility plus reactive sites for amide and ester chemistry. Hydroxyl (-OH) groups give similar dispersion benefits with milder reactivity, useful for polyurethane and epoxy systems. Amine (-NH2) groups are introduced via diamine coupling to -COOH and provide direct bonding to epoxy resins and conjugation chemistry to biomolecules via standard NHS/EDC coupling.

Does functionalization affect electrical conductivity?

Yes, but the effect is application-dependent. Functional groups disrupt the sp2 carbon network, which reduces single-tube electrical conductivity. However, in real composites, the percolation threshold typically drops because functionalized CNTs disperse far more uniformly. The net effect is usually higher composite conductivity at a given loading despite the per-tube loss. For applications where peak conductivity matters (transparent conductors, OFETs), pristine SWCNTs are preferred.

How is functionalization quantified?

Functional group density is measured by TGA (mass loss in the 200 to 600 C range corresponds to functional groups), XPS (atomic percent of N, O, F at the surface), Boehm titration (for acidic groups specifically), and Raman spectroscopy (D/G ratio increases with covalent functionalization). Each batch ships with TDS reporting the relevant characterization for the specific functional group; full XPS or TGA reports are available on request.

Can I conjugate biomolecules to functionalized CNTs?

Yes. -COOH and -NH2 functionalized MWCNTs are the standard substrates for biomolecule conjugation. Use EDC/NHS chemistry to couple amines from a peptide, antibody, or aminated DNA strand to surface -COOH groups, or use glutaraldehyde or NHS-PEG-NHS linkers between -NH2 surfaces and amine-bearing biomolecules. For drug delivery and imaging applications, follow standard sterile-handling and endotoxin-free protocols; we offer additional acid-purified grades for biomedical research.