Frequently Asked Questions

Table of contents:

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General FAQ

  1. I can’t find the material or size that I’m looking for. Can you make it for me?
    We routinely fabricate custom materials for our customers. Please see our custom synthesis page for more information.
  2. Can you coat nanomaterials with other capping agents or biomolecules?
    We routinely functionalize our materials with alternative capping agents or biomolecules for our customers. Please see our custom synthesis page for more information.
  3. How are your materials characterized?
    All of our nanomaterials are provided with specification sheets that include TEM images, particle size statistics and histogram based on 100 individual nanoparticle measurements, UV-Visible extinction spectrum and pH. Hydrodynamic diameter (dynamic light scattering) and zeta potential are also measured for spherical nanoparticles > 10 nm.
  4. What is your production volume?
    Our proprietary technologies allow us to fabricate 10's, 100's or 1000's of grams per batch, and to provide significant discounts for large quantity orders. Please contact us for more information.
  5. My nanoparticles settle out of solution. Is this normal?
    Yes, it is normal for larger gold and silver nanoparticles to settle to the bottom of the storage container. This is completely reversible, simply shake the container for 10-30 seconds until the nanoparticles have redispersed into the solution prior to using the material.

Gold Nanoparticles

  1. Do you offer "bare" or "uncapped" gold nanoparticles?
    We frequently receive inquiries from customers asked for nanoparticles with bare surfaces. It is important to recognize that all nanoparticles require some type of capping agent or stabilizer on the surface, as truly "bare" nanoparticles would remain stable for only a few seconds before irreversibly aggregating. This is because nanoparticles can be stabilized by two forces: electrostatic repulsion (capping agents are charged, and therefore nearby particles repel each other before van der Waals forces can pull them together), or steric repulsion (the capping agent is large enough that it physically gets in the way of van der Waals forces pulling them together.

    However, our citrate and tannic acid capping agents are designed such that they can be displaced by other capping agents. Tannic acid nanoparticles are more stable than citrate coated nanoparticles at high concentrations, and tannic acid can be displaced by reactive groups with high affinity for the gold particle surface. Citrate capped nanoparticles are more stable than tannic capped nanoparticles in higher ionic strength solutions, and are the best choice when using physisorption for subsequent surface modification. For more information regarding our standard capping agents, please visit our Surfaces course listings.

  2. The peak resonance wavelength of my nanoshells or nanorods is not exactly at the wavelength advertised. Is my material out of specification and will this affect my experiment?
    For gold nanoshells and nanorods, our specification is that the OD at the advertised wavelength is at least 90% of the value of the peak OD. We set it this way to ensure that the particles are functionally equivalent between lots even if the peak wavelength varies slightly from the advertised wavelength. These materials have relatively broad extinction peaks, so even if the peak resonance is shifted from the wavelength advertised, the material retains most of its absorption and scattering properties across a wide range of wavelengths.
Don't see your question about gold nanoparticles? View additional Gold FAQs & learn more at our Knowledge Base.

Silver Nanoparticles

  1. Do silver nanoparticles oxidize?
    Yes, silver will oxidize in the presence of sulfur and oxygen. Please see our Guidelines for Nanotoxicology Researchers Using nanoComposix Materials for additional details.
  2. Are your silver nanospheres amorphous or crystalline?
    We consider our silver nanospheres to be polycrystalline, as can be seen by the different lines and contrasts of the particles in TEM images. It is the nature of silver nanoparticles to have the silver atoms reduce into a somewhat regular crystal structure since it is a more stable form, even if it subdivides into many crystal domains within the same particle. By having numerous crystal domains, the particles are able to maintain a near spherical shape. These crystal domains can sometimes be seen as lines in the particles or patches that are darker than the rest of the particle.
  3. Do silver nanoparticles have antimicrobial properties?
    Yes, recent research has demonstrated that silver ions are an effective antimicrobial yet have low toxicity for humans. Silver nanoparticles are garnering increasing attention for their antiviral properties and are being investigated for various applications in the medical field. For more information, visit our page about Antimicrobial Silver Nanoparticles.
  4. How do I tell if the silver nanoparticles that I've purchased have gone bad?
    Well-dispersed silver nanoparticles typically have a yellow color in solution and a distinct plasmon resonance. Monitoring the UV-Visible signature of silver nanoparticles over time is a good method of ensuring that the particles are still "good". If there is a destabilization event, the color will usually change dramatically and it is clear that the particles have aggregated.
Don't see your question about silver nanoparticles? View additional Silver FAQs & learn more at out Knowledge Base.

Silica Nanoparticles

  1. Are your silica nanosphere amorphous or crystalline?
    Amorphous. Colloidal silica particles are non-crystalline, meaning that the atoms do not have long-range order, resembling the structure of bulk glass.
  2. What are silica nanoparticles used for?
    Due to the versatility of silica in terms of porosity, surface chemistry, and nanoparticle size, silica has a wide range of applications, ranging from drug delivery and catalysis to its use as an ingredient in paint and cement. Visit our page about Silica Nanoparticle Applications to learn more.
  3. How many amines are on the surface of your aminated silica?
    Based on the amount of reagent used during the surface functionalization step and the surface area available for the ligand to bind, we calculate a maximum of ~2.5 amine groups/nm2 at the particle surface. This is consistent with literature reports, which estimate approximately two amine groups/nm2. Depending on orientation, packing density, and other factors, only a portion of the amines may be accessible for conjugation. Further, in some cases there are also amine groups that are incorporated into the silica network below the particle surface, and which contribute to the zeta potential of the particle and can be detected using different characterization methods. Because they are embedded within the silica shell, however, these amines are not accessible for conjugation.
Don't see your question about silica nanoparticles? View additional Silica FAQs & learn more at our Knowledge Base.

Lateral Flow

  1. Do you sell nitrocellulose membranes and backing cards for lateral flow assays?
    We can likely provide nitrocellulose membranes, conjugate pads, sample pads, wick pads, and/or backing cards by individual request. Please contact us to inquire about the availability of these materials.
  2. Which of your nanoparticles offers the greatest sensitivity?
    Our 150 nm ultra-bright Gold Nanoshells offer up to a 20x sensitivity enhancement compared to 40 nm gold nanoparticles. Click here to learn more.
  3. What is the advantage of NHS-activated nanoparticles over the carboxyl-functionalized surfaces?
    BioReady NHS Gold Nanospheres and Nanoshells can be covalently conjugated to primary amines (-NH2) of proteins in a simplified procedure compared to the carboxyl surface. These nanoparticles are surface functionalized with an active NHS ester to generate gold nanoparticle-antibody amide bonds, eliminating the need for the user to perform the intermediary EDC/Sulfo-NHS chemistry steps. The particles are supplied as a lyophilized powder that can be resuspended with a buffer to covalently bind to an added antibody. This coupling reaction is rapid, simple, robust, and requires little optimization.
  4. What items are included in your covalent conjugation kits?
    For a complete list of items included in our covalent (carboxyl-functionalized) conjugation kits, follow the links below:
  5. Do you offer any sample protocols to help me get started with my conjugation?
    We sure do! Follow the link to view our library of Protocols & White Papers.
Can't find your question here? See additional Lateral Flow FAQs & learn more at our Knowledge Base.

Nanoparticle Characterization & Quality Control

  1. How are your materials characterized?
    All of our nanomaterials are provided with specification sheets that include TEM images, particle size statistics and histogram based on 100 individual nanoparticle measurements, UV-Visible extinction spectrum and pH. Hydrodynamic diameter (dynamic light scattering) and zeta potential are also measured for spherical nanoparticles > 10 nm.
  2. How do you measure nanoparticle size and CV?
    Mean nanoparticle size is calculated by measuring 100 individual nanoparticles as imaged by TEM. The coefficient of variation is calculated by dividing the standard deviation of the nanoparticle size by the mean nanoparticle size, and multiplying by 100 to get a percentage. For instance, a 50 nm diameter nanosphere with a standard deviation of 3 nm would have a CV of 6%: (3/50) × 100 = 6%.
  3. Can you send me the raw data for the characterization measurements reported on the Certificate of Analysis (CoA) for my material?
    Absolutely. We can send individual characterization measurement data or the complete data package by request. Our prices for this are listed below. Contact us to request your data, and please include the lot numbers for the data you're interested in with your request.

    Characterization Data Purchase Options

    $250 a la carte, or $600 for Full Data Package (all six items listed below)
    • TEM images: original full-size image files
    • TEM sizing data: 100 particle count measurements
    • UV-Vis: spectrophotometer measurement data and dilution info
    • DLS: summary data table; additional $15 for PDF report generated by Malvern software
    • Zeta: summary data table; additional $15 for PDF report generated by Malvern software
    • ICP-MS report: contact us for details
  4. When I characterize your particles using TEM, I see a bimodal size distribution. Aren’t nanoComposix particles supposed to be monodisperse?
    NanoComposix Nanoxact and BioPure nanoparticles are unagglomerated and monodisperse, and each batch of high quality materials is extensively characterized before being shipped to our customers. Some customers have provided us with TEM images where there is a second population of smaller silver nanoparticles visible with TEM analysis. In many of these cases TEM grids with amine, thiol, and carboxy functional were utilized for imaging. The functional groups on these grids can serve as nucleation points for dissolved silver ions resulting in the appearance of small silver nanoparticles that are not present in the colloidal solution. Therefore, we recommend that customers use carbon-coated formvar TEM grids to image silver nanoparticles. More information on this phenomenon can be found in "Generation of Metal Nanoparticles from Silver and Copper Objects", by R. Glover, et al, ACS 2011
  5. Why does my specification sheet list the hydrodynamic diameter and zeta potential as "N/A"?
    Both DLS and zeta potential characterization measure small changes in light scattering as nanomaterials move in solution. Gold and silver nanoparticles with diameters of < 10 nm have very small scattering cross sections, and at dilute concentrations do not scatter enough photons to achieve a signal to noise ratio acceptable for an accurate reading. In some cases, more concentrated solutions can be measured to obtain an acceptable DLS or zeta potential and, in these cases, we do report the values for sizes < 10 nm. Alternatively, centrifugal particle sizing can be used to measure the size distribution of nanoparticles in this size range and we can perform this as a custom service upon request.
  6. How do I measure DLS for gold or silver nanospheres?

    Some of our gold and silver nanosphere materials require dilution for DLS in order to achieve a reliable measurement. For materials suspended in water or a citrate buffer we will normally dilute with Milli-Q (or DI) water as needed. If the diluted sample is going to be stored or measured after more than a few minutes, we recommend diluting citrate-stabilized materials with a citrate buffer for better stability.

    Our dilution factor depends on the material:

    • 5–20 nm BioPure Gold or Silver (1 mg/mL): measured neat (no dilution)
    • 30–40 nm BioPure Gold or Silver (1 mg/mL): dilute 200 uL sample with 800 µL water
    • 50–60 nm BioPure Gold or Silver (1 mg/mL): dilute 100 uL sample with 900 µL water
    • 70–80 nm BioPure Gold or Silver (1 mg/mL): dilute 50 uL sample with 950 µL water (or similar dilution factor like ~100 µL sample into 2 mL water)
    • 90–100 nm BioPure Gold or Silver (1 mg/mL): dilute 20 uL sample with 980 µL water (or similar dilution factor like ~60 µL sample into 3 mL water)

    Note: for NanoXact materials (0.05 mg/mL gold or 0.02 mg/mL silver), all sizes should be measured neat with no dilution.

    Very small particles like the 20 nm silver or smaller have very low scattering cross sections, so they must be measured at very high concentrations to be able to measure them by DLS. We normally measure these solutions neat (no dilution), but it can still be difficult to obtain reliable results. In all cases, we recommend checking the quality report for their measurement to determine if the results are reliable, and making sure the count rates are within 200–700 kcps for best results.

Plasmonics & Optical Properties

  1. What is the effect of different environments (e.g. water or air) on the spectrum of silver nanoparticles? How does shelling the nanoparticles with silica affect the spectra?
    The shape and peak wavelength of the plasmon resonance of silver nanoparticles is influenced by the refractive index of the media it is suspended in. When the particles are in water (n=1.33) the resonance of 80 nm silver nanoparticles is predicted by Mie Scattering theory to be ~462 nm . In air (n=1.0) the peak plasmon resonance of the nanoparticles is predicted to be 398 nm, a shift of 64 nm. In air, if the silver nanoparticle is shelled with a very thick shell of a material that has the same refractive index of water, the peak plasmon wavelength will be close to 462 nm. If the silver is shelled with a very thin shell of material, the peak plasmon resonance will be close to 398 nm. If the shell is of intermediate thickness, the peak will be somewhere between these two extremes (398 nm – 462 nm). Thus, by adjusting the thickness of the shell, the peak resonance of a silica coated silver nanoparticles can be tuned to a particular value.
    Since the refractive index of silica is n=1.43, a value greater than water, the silica shell will shift the peak plasmon resonance of a silver nanoparticle suspended in water to a longer wavelength than a silver nanoparticle with no shell. For example, a 50 nm silica shell on an 80 nm silver nanoparticle in water has a predicted peak resonance wavelength of 487 nm, a 25 nm shift compared to an unshelled silver nanoparticle.
  2. I'm looking for a nanoparticle with specific optical properties. How can I determine what particle size is most appropriate for my application?
    This is a common questions due to the unique size and shape dependent optical properties of gold and silver nanoparticles. Please see our Plasmonics webpage for information regarding the optical properties of our standard products, or our Online Mie Theory Simulator for information regarding absorption and scattering splits and silica shelling. Still have questions? Please contact us, and we'd be glad to help you!
Learn more about The Science of Plasmonics.

TEM Characterization Services

  1. What types of nanoparticles can be imaged?
    Successful imaging of nanoparticles with a TEM depends on the contrast of the material that you are analyzing compared to the background. TEM grids are prepared by drying nanoparticles on a copper grid that is coated with a thin layer of carbon. Materials that have different electron densities than the amorphous carbon film are easily imaged (for example, silver and gold) whereas polymers or biomolecules that have similar electron densities to amorphous carbon can be difficult to image.
    • High Contrast / Easily Imaged:
      • Most metals: gold, silver, copper, aluminum
      • Most oxides: silica, aluminum oxide, titanium oxide
      • Particles made from polymers
      • Carbon nanotubes, quantum dots, magnetic nanoparticles
    • Low Contrast / Difficult to Image
      • Biomolecules / Dendrimers
      • Some polymers
    If you're unsure whether your sample is appropriate for imaging please contact us.
  2. What information can I provide to maximize image quality?
    To obtain high quality TEM images often requires optimization of sample preparation techniques that are specific to each material that is being analyzed. There are many factors that can influence the quality of the image.The more information that you can provide on your sample the higher the chance that our first pass at imaging your sample will be successful. We highly suggest that you perform the following steps when preparing your sample for TEM imaging:
    • Provide the nanomaterial in a pure state.The grid preparation involves the drying of your sample onto a grid and all residual salts, polymers, biomolecules, or other particulates will be dried and imaged along with your nanoparticles. Highest quality images will be obtained when you can isolate the nanomaterial from all residual reactant components using centrifugation/wash steps, filtration, or dialysis.
    • Provide as much information as possible with your sample. Details on the size, shape, material, and concentration as well as the solvent and the concentration of residual reactants will help us determine if the sample needs additional processing before images can be obtained.
    For questions about sample preparation please contact us.
  3. I received my images but they are not what I was expecting. What options do I have?
    In some cases, the images will be different from what you are expecting. This can be due to a wide range of reasons including sample preparation, residual reactants, or low concentration or low contrast of the particles. Options for next steps:
    • Re-imaging: In some cases, simply re-imaging the sample at a lower/higher dilution or capturing additional images will provide the data that you need. Send an email to describing what you would like to achieve. A new sample will be prepared and an additional TEM analysis charge will be billed to your order.
    • New Preparation: Preparation optimization can be extremely challenging for TEM samples. Typically, nanoparticles are dispersed in a compatible solvent and drop cast on a carbon coated TEM grid. Residual chemicals in solution can coat the nanoparticles during the drying process. A number of different techniques can be tried to improve the sample quality image. Contact to set up a time to discuss your sample and the potential for achieving better images. We’ll provide a quote for the preparation optimization.
    • Sample Processing: Depending on your sample, it may not be possible to obtain high quality images in the as received form. Washing the nanoparticles may be necessary to isolate the nanoparticles from other residual reactants. Contact to set up a time to discuss sample processing that may generate improved images.

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