nanotube suspensions
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COOH Functionalized, Surfactant Free Carbon Nanotube Suspensions
Our carboxylation process yields nanotubes with a strong negative charge that keeps them suspended in water at up to 2 gram/liter concentrations without the presence of a surfactant. Lower concentrations can be achieved in a number of other solvents that can be used as a nanotube paint, ink, etc. To create carboxyl (COOH) functional groups on the sidewall of the carbon nanotubes, we treat them in a mixture of hot nitric & sulfuric acids. After washing the nanotubes to remove excess acid, they may be dispersed. We offer suspended, carboxylated nanotubes in water, ethanol, acetone or dimethylformamide (DMF). These nanotube paint - like suspensions are prepared from our standard single and multiwall carbon nanotubes, and are stable for months in our lab. Below the product offerings is a review, covering details of preparing and working with suspensions or inks aka (Nink). These can be inkjet printed thru Fuji Dimatix printers, or other printers. Suspensions and paints having different concentrations, special solvents, or large volumes are available; send a request by email. Separately we offer suspensions in various solvents that are made from pristine nanotubes. These typically involve the use of surfactants to assist in stabilization.
For our line of inkjet printing inks (Nink) based on nanotubes, click here.
Please note Suspensions are made to order and can sometimes take a week. If you need your suspension sooner, please email us directly at
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nanotube suspensions
Nanotubes Solvent Volume
100 ml 1 L
Hollow, MWCNT
DiH2O $100 $600
DMF, Ethanol, THF,Acetone Backordered Backordered
Bamboo MWCNT BPD15L5-20-COOH
DiH2O $100 $600
DMF, Ethanol, THF,Acetone Backordered Backordered
DiH2O $125 $800
DMF, Ethanol, THF,Acetone Backordered Backordered
Related products:
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  • Ultrasonic dispersion equipment
  • Nink- nanotube ink for inkjet printing
  • PECVD-MWCNT suspensions harvested from aligned arrays

  • Keywords:
    Suspension, solution, carbon nanotube, gram per liter, g/l, surfactant, solvent, stabilization, ink, paint, stable, dispersion, disperse
    Carbon nanotube suspensions can be made in a number of ways, and with a number of fluids. Nanotubes are not solubilized, rather they are suspended as individual particles in the fluid, like an ink or paint. This brief is an introduction to nanotube paints and suspensions, and explains how they are stabilized and what concentrations are feasible. It will be useful to start by modeling the nanotubes as rigid rods in a liquid suspsension, to determine how far apart they may be for a given concentration.

    When particles with a large aspect ratio (like carbon nanotubes) is dispersed in a solvent fluid, they will rotate, due to Brownian motion or in the presence of a velocity gradient (flow). This rotation creates a large effective hydrodynamic radius, and each nanotube sweeps out a spherical volume, as shown below. For a nanotube with an aspect ratio of a = L/d, the ratio (R) of the nanotube's actual volume Vactual = (0.785 a d3) to the volume of the swept out sphere Vsphere = (0.523a3d3) reduces to: R = 3/(2a2) Aspect ratios for carbon nanotubes are ~2000, For example, MWNT are often 20 microns long and 10nm diameter, and SWNT are often 2 micron long and 1nm diameter. hydrodynamic radius of nanotube in suspension

    The volumetric ratio R for nanotubes of this aspect ratio is 4x10-5 vol%, which is less than 1 milligram per liter! (see Ref 1) At any greater concentration, the nanotubes will intrude upon each otherís hydrodynamic spheres and hinder rotation. This is the primary reason that nanotube paint or suspensions are difficult to make and build viscosity quickly.
    We can conclude that the motion of a nanotube in a fluid at any appreciable concentration will be severely restricted by its neighbors, who essentially fix the nanotube in place. In this arrangement, there should be an average separation distance (s) between nanotubesurfaces that is relatively constant. If we model the separation distance as a square lattice, the volume fraction of nanotubes can be written in terms of the nanotube diameter d, aspect ratio a, surface-to-surface spacing s, as follows: When we consider that the aspect ratio a is much larger than s/dn, this equation simplifies to a quadratic: In this figure, we plotted the ratio s/dn against the resulting volume percent CNT in the suspension. In practice, we can make stable 1-2 gram per liter (g/l)r suspensions of high aspect ratio nanotubes in a number of solvents. This corresponds to 0.1 vol% loading, and the separation distance is ~30dn.
    So in a 1g/liter dispersion of 5nm diameter nanotubes, the nearest neighbor resides only 150nm away. The major take-away from this section is that nanotubes can be expected to entangle and agglomerate, unless we provide some means to make nanotubes repel each other.

    From the model section, we saw that nanotubes will interact with each other's hydrodynamic radius at any appreciable concentration. This would result in entanglement and the creation of large agglomerates, if there was no repulsive force to keep them from aggregating. This is exactly what we observe. When pristine nanotubes are suspended in water, they will aggregate and form a loose sediment on the container bottom, with a clear supernatant. To provide a repulsive force, we need to electrically charge the nanotubes, so that they repel each other and keep agglomeration from occurring. This may not completely stop gravity induced settling, but any sediment will then be made up of individual particles, not agglomerates.
    We can create electrical repulsive charges on the carbon nanotubes several ways. Chemical functionalization of the nanotubes will create ionizable groups on the nanotube sidewall. The carboxyl group, COOH, will loose its hydrogen and become COO- at any pH <8-9. A carboxylated nanotube will ionize in suspension and the repulsive forces between COO- groups will hold the nanotubes apart, preventing agglomeration. To evaluate the concentration of COOH groups, we use a titration method. Typical carboxyl concentrations run ~5-7wt%. The zeta potential is another good measure of the charge on our carboxylated nanotubes. The zeta potential as a function of pH for our carboxylated hollow PD15L520-COOH nanotubes is shown below. NanoLab's carboxylated carbon nanotubes can have zeta potential values near -70mV. COOH functionalized nanotube zeta potential

    Wrapping pristine nanotubes with molecules that can ionize will accomplish the same result. Sodium polyacrylate is one poly-anionic molecule that can be wrapped around a nanotube. In suspension, the sodium ionizes and disperses in the paint or ink medium, while the remaining backbone (which is wrapped around the nanotube) acquires a negative charge. Surfactants, which have a hydrophobic tail and a hydrophilic head, can also be used to impart charge to the nanotubes. Since pristine nanotubes are inherently hydrophobic, the tails of molecules like SDS (sodium dodecyl sulfate) will stick to the nanotube surface. The heads, which contain both the sulfate and the sodium ions, stick out into the suspension. Again, the sodium departs, and leaves a net negative charge on the nanotubes.
    NanoLab offers two types of surfactants to help customers disperse un-functionalized nanotubes. The first is NanoSperse AQ, and it is suitable for use in highly polar, protic, high dielectric constant solvents like water and ethanol. The second is NanoSperse AC, which we recommend for the preparation of suspensions in lower dielectric constant, aprotic solvents, like acetone, DMF,THF, etc. Note that adding extra salts containing cations such as Na+ or Ca2+ to a negatively charged nanotube suspension can destabilize it. Salt induced precipitation of nanotubes is well known.

    Preparing Suspensions- Ultrasonicators
    When making a nanotube ink or a paint, NanoLab uses an ultrasonic probe to disperse nanotubes in fluids. The collapse of the cavitation bubbles made by the ultrasonicator creates areas of high shear that teases apart clusters of nanotubes effectively, so the nanotubes are separated and suspend-able. Some other high shear mixers are also effective. Probe sonicators do a much better job at making stable suspensions than ultrasonic bath units. In a bath sonicator, the sound must pass through the tank wall, through the bath, through the beaker wall, and still have enough energy to create cavitation bubbles that will disperse the nanotubes. In contrast, the probe is immersed directly in the nanotube suspension, so the transfer is more efficient. We use a 500W Misonix probe sonicator, and it has served us wonderfully. We are a distributor for Misonix probes, and offer the same pricing. Sonicators are noisy, though, and aerosols can be created, so NanoLab recommends a ventilated sound enclosure to make life safer and more enjoyable in the lab.
    We established in the previous sections that nanotubes will agglomerate unless they are charged or prevented from touching via steric hinderance. A good ultrasonicator will disperse nanotubes, but it will not stop agglomeration, so the nanotubes must either be functionalized, or stabilized with a surfactant or other polyionic species. When nanotubes are properly functionalized, they should require no surfactant to keep them separated. For pristine or purified nanotubes, we recommend that nanotubes be added to the solvent first, and the suspension sonicated to break up the nanotube clusters. Surfactants should be predissolved in a small amount of the dispersing liquid, and added gradually to the nanotube suspension, between sonication steps. The Misonix probe sonicators allow for pulse operation. The nanotubes are dispersed during the pulse, and surfactants have time to attach to the surfaces between pulses. NanoLab sells stable suspensions, inks and paints in a number of solvents, including dimethly formamide - DMF (CH3)2NC(O)H , ethanol ( C2H5OH ), acetone ((CH3)2CO) and tetrahydrofuran aka THF, (CH2)4O See Ultrasonic dispersion equipment.

    With any suspension, if there is a density difference between the particles and the fluid, there is a tendency for settling due to gravity. The settling rate depends upon a lot of factors, including the viscosity of the fluid, the surface charge on the particles, and the dielectric constant of the fluid. The best examples of well stabilized suspensions of small particles are paints and inks. Our carbon nanotubes have a density near 1.3g/cm3. Carboxylated suspensions in water are stable for months, and can be centrifuged to 3000rpm for 10 minutes without evidence of settling. Dispersions of nanotubes in low density solvents (e.g. THF p=0.889g/cm3) have a higher rate of settling, but can be stable for many days.

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    1. R.L. Pober, M.T. Strauss, "Nanotubes in Liquids; Effective Thermal Conductivity," J. Appl. Phys. 2006, 100, 084328, pp1-9
    2. X. Peng, J. Jia, X. Gong, Z. Luan, B. Fan, "Aqueous stability of oxidized carbon nanotubes and the precipitation by salts," Journal of Hazardous Materials, Volume 165, Issues 1-3, 15 June 2009, Pages 1239-1242
    This page was updated on 03/29/2017