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NanoComposix’s gold nanorods are surface plasmon resonant (SPR) rod-shaped nanoparticles with narrow size distributions and uniform shape. The gold nanorods have absorption peaks that are tunable throughout the visible and short wave near-infrared (SWNIR) regions of the electromagnetic spectrum. Our purification methods remove ammonium surfactants used during synthesis and replace it with citrate, producing a stable, easily displaceable surface chemistry to use as a base for functionalization with a wide variety of molecules (silica, DNA, antibodies, etc.). We also offer PEG-functionalized nanorods, as well as carboxyl-terminated surfaces for bioconjugation.

Gold nanorods with a high aspect ratio (length/width) have a longer peak resonant wavelength than gold nanorods with a lower aspect ratio. The three resonances that are available as standard products are listed to the right.

Resonance Typical Aspect Ratio
660 nm 2.7
800 nm 3.6
980 nm 6.1

By tuning the synthesis conditions to control the nanorod length and diameter, nanorods with peak resonance from 600–1100 nm can be produced. To receive nanorods with a custom resonance wavelength peak please contact us.

Surface Functionalization

The surface of gold nanorods is very important for use in a variety of applications. Most gold nanorods available commercially are coated with cetyl trimethylammonium bromide (CTAB), a common surfactant used during nanorod synthesis. CTAB coated nanorods are toxic in biological systems and are readily destabilized when transferred to different solvents. NanoComposix’s gold nanorods are CTAB free and, like all our materials, highly purified to remove all residual reactants following synthesis.

The gold nanorod surface options available include a covalently-bound 5 kDa methoxy-terminated PEG, which has been shown to have a high degree of salt stability, and a discrete carboxylic acid-terminated PEG, (PEG)12-Carboxyl, in which the free acid functional group is available for subsequent coupling chemistry and can provide anchor points for covalent protein attachment. Also available is the citrate surface option which can be readily functionalized with other ligands, biomolecules and DNA. Other custom surfaces are available, and include PVP, lipoic acid, and BPEI.


Gold nanorods are utilized in biomedical imaging, drug delivery, optical filters, and photothermal therapy. Example publications of nanorod use in biomedical applications are provided below:

Frequently Asked Questions

  1. What is CTAB and why is its removal important?
    Many nanorod synthetic recipes utilize a positively-charged surfactant, cetyl trimethylammonium bromide (CTAB), which forms a bilayer coating that strongly adsorbs to the gold nanorod surface. This bilayer is critical in forming the desired rod-shaped morphology due to it restricting growth normal to the {110} and {100} crystal lattice faces. The CTAB also provides stabilization through electrostatic repulsion in aqueous solutions due to its positive surface charge. Unfortunately, the positively charged CTAB molecules exhibit high levels of cytotoxicity – the degree to which a material is destructive towards cells – and is challenging to displace without destabilizing the nanorods. Thus, the same surfactant that is largely responsible for the anisotropic shape of the gold during growth also hinders its use in bio-applications and interferes with subsequent binding of molecules to the nanorod surface. In some cases, additional polymer layers can be added around the CTAB and used as an anchor for subsequent binding but for most applications it is preferable to bind directly to the gold surface.
  2. Why do other companies primarily sell CTAB coated rods?
    Attempts by others to remove the CTAB bilayer have had limited success yielding aggregated nanorods, only partially exchanged surfaces, or capping over the CTAB bilayer with another agent. NanoComposix has been able to solve this problem through a specialized surface exchange protocol. Utilizing this protocol we are able to displace the CTAB bilayer and ultimately replace it with a citrate capping agent that keeps the gold nanorods stable in aqueous solution. With a citrate stabilized surface, standard surface binding techniques can be utilized to generate a variety of surfaces.
  3. How did you confirm that there is no CTAB remaining on the nanorods?
    We are able to prove that we have displaced the CTAB by using Matrix-Assisted Laser Desorption/Ionization Time of Flight Mass Spectrometry (MALDI-TOF MS) analysis and monitoring the intensity of the bromine peaks found in the CTAB. Bromine has two naturally occurring isotopes. One of the isotopes has a molecular weight of 79 and an abundance of 50.52%. The other isotope has a molecular weight of 81 and an abundance of 49.48%. Therefore, these two peaks appear next to each other at similar intensity levels on the MADLI-TOF spectrum. A half-log serial dilution was made and analyzed with decreasing concentrations of CTAB correlating to decreasing bromine peak intensities. Our citrate capped gold nanorods were then analyzed and compared against the serial dilution curve. Using this method we found that the CTAB concentration of our citrate capped gold nanorods has been reduced down to a concentration below 10 uM. Samples were also sent off-site for independent analysis using Direct Sample Analysis Time of Flight Mass Spectrometry (DSA-TOF MS). This independent analysis also confirmed that the CTAB levels were undetectable in the sample.
  4. What is the shape purity of your nanorods?
    The shape purity of our nanorods depends on the peak plasmon resonance of the nanorod. Typical shape purities for our 660 nm, 800 nm, and 980 nm resonant nanorods are 95%, 90%, and 80%, respectively.
  5. Why is my nanorod peak wavelength resonance different than the value on your specification sheet?
    At the nanoscale, the size and shape of particles can change over time. Typically this is due to reorganization of the atoms within the particle or due to a process call Ostwald ripening where atoms are released from particles with high curvature (very small particles or shaped particles) and those atoms redeposit on other particles. Since the plasmon resonance is so sensitive to aspect ratio, even low rates of Ostwald ripening can cause a noticeable shift in the peak resonant wavelength. At nanoComposix, we guarantee that the supplied nanorods will have at least 90% of their peak absorbance at the specified wavelength.
  6. Why is the Hydrodynamic Diameter listed on my CoA smaller than the TEM sizing values?
    The reported Hydrodynamic Diameter of the gold nanorods does not provide a direct measure of either the nanorod diameter or length, unlike DLS measurements of spherical nanoparticles. Literature reports suggest that the measured value corresponds to a rotational diffusion of the nanorods, rather than a translation diffusion of the nanorods through the bulk solution. The DLS measurement of the nanorods is still useful for confirming that the particles are well dispersed in solution and unaggregated; the presence of strongly-scattering aggregates would significantly increase the reported hydrodynamic diameter and indicate that the nanorods have become destabilized. A hydrodynamic diameter in the 5–25 nm range is normal and can also depend on the resonance and surface.
  7. I’d like a nanorod with a different resonance or a different surface. Are these available?
    We can make nanorods with resonant wavelengths from 600 nm–1100 nm and with all of our standard surfaces (citrate, PVP, lipoic acid, BPEI, PEG, silica shelled). In addition, we can covalently bind antibodies or other targeting molecules to the surface. Please contact us for a quote for a custom nanorod wavelength or surface coating.