Gold Nanoparticle Frequently Asked Questions

Important reference for frequently asked questions regarding gold nanoparticles


What is an OD-mL and why don't you sell by gold weight or molarity?

OD-mL is short for Optical Density per milliliter. We believe this is the best way to sell our products because this is the only way to truly measure the concentration (number) of the gold nanoparticles you are purchasing. Before we ship the product we measure the optical density with our NIST traceable UV-VIS spectrophotometer. This analysis gives the exact number of gold nanoparticles per milliliter. When we see competitors selling their products by gold weight, we know that they are determining this property by the amount of gold used for production. Depending on the process, the amount of gold that is used versus the amount of gold that ultimately winds up as gold nanoparticles can vary 10-fold. (Usually always at the customer's expense!!). If you'd like to know more about Optical Density - mL and how its used, please visit our info note at:

Info Note 804: UV-VIS Nomenclature and Units

Quantum Dots are purchased by molarity. We often are asked why we don't sell ours by molarity too. The bottom line is that gold nanoparticles are a completely different product than quantum dots. Whereas quantum dots may come in molarity that are 100 times that of our products, our gold nanoparticles are 10-50 times the size with 5 orders of magnitude greater absorption and scattering. Please see our info notes comparing these products.

Your conjugated products seem really expensive - $500 for a mL. Why?

Actually, the contrary is true, they are comparatively inexpensive. First, though they come in 1 mL sizes, they come highly concentrated (> 50x). Further, our yield in processing is about 50%, so it takes 100 mL of gold nanoparticles to make 1 mL of extremely high concentrated product. Being highly concentrated, the customer has the option to dilute as needed - which is very popular. The polymer cages have the additional advantages of 1) long shelf life 2) storage at room temperatures 3) reduced aggregation 4) reduced non-specific binding 5) the ability to be further concentrated upwards of 5000 times 5) covalent bonds 6) operation in pHs from 3 to 11 and 7) cloaking effect in in vivo applications. Many of these advantages are included in our pending patent. So whereas other companies sell Protein A adsorbed (no covalent bond) onto their gold nanoparticles for $10/OD-mL, our covalently bonded, polymer caged, custom conjugations to your antibodies are about the same price. 

Can I get a free sample? I only want 1 mL of your nanoparticles to evaluate.

Our products demand a very high level of quality in the preparation and packaging prior to shipment. This is very time consuming and expensive. To help our customers in the evaluation of our product, we provide extensive data on our website. Further, we provide introductory kits for both our spherical gold and our gold nanorods. These come with four different sizes of 25 mL each, perfect for evaluation. Additionally, we offer 1mL volumes on all of our spheres and rods.

But I don't know if my application will work? I want to try your gold nanoparticles to see.

We offer our Gold, Silver and Platinum Partnerships that may help circumvent this problem and offer a potential solution - through Partnership. Please go to our Partner Development Programs for more information.

What is the concentration of your particles?

For all of our products, the concentration is given on the web page and on the data sheets. To convert weight concentration (mg/mL) to particle concentration (nps/mL), or to convert our spherical gold particle concentration to weight concentration, and for other handy conversions, download: FAQz conversions.xls.

Why do my spherical nanoparticles all appear red? The 30 nm particles have a peak absorbance (absorption and scattering) at 523 nm, right in the middle of the green. Shouldn't they be green? Shouldn't the 80 nm spherical gold nanoparticles, with an absorbance peak at 550 nm appear yellow?

Great question, one that few truly understand. To actually see the resonance scattering effect, place the nanoparticles in a dark room and illuminate the bottle with a collimated white light source (flashlight). Looking at about 170 degrees from illumination (almost directly behind the flashlight), you'll see the green and yellow scattering of the gold nanoparticles.

Spherical Gold

Are your colloids pure gold?

Yes, with a capping agent. In order to correctly grow in solution, all nanoparticles require a capping agent. Like citrate, our patent pending method utilizing a proprietary capping agent that is also a carboxylic acid. However, our capping agent results in highly monodisperse nanoparticles with a reduced surface charge, both of which are highly advantageous properties in nanotechnology applications. Like citrate, our capping agent is easily replaced with chemistry or electrochemistry. This includes simple adsorption of antibodies to the surface, as well as more complex chemistries that involve thiolated attachments.

I notice that your nanoparticles settle out of solution when stored for weeks at a time. Is this normal?

Yes, especially for larger nanoparticles. There are two types of settling, reversible and irreversible aggregation. If stored under the proper conditions, settling of our particles is reversible and the nanoparticles are easily resuspended with a gentle shaking motion.

How do you calculate the size distribution of a batch?

In the past we sampled 20-30 nanoparticles from multiple TEM pictures. Using different imaging techniques, we calculate distribution curves manually. More recently we have turned to our Malvern Zetasizer Nano ZS Dynamic Light Scattering (DLS) instrument. This instrument samples approximately 1,000,000 particles. Under the right conditions, this instrument not only sizes the particles correctly, but gives a polydispersity measurement. This measurement is called the Polydispersity Index (PDI). Proper results on the Zetasizer are directly proportional to the monodispersity of the sample. It is our belief that Zetasizers have not achieved gold nanoparticle market acceptance because no other vendor has been able to manufacture gold nanoparticles at the monodispersity necessary for accurate results. However, we do this for every batch manufactured. For every bare particle order, the customer receives a Certificate of Analysis showing the results of our DLS. Our product specifications have been confirmed in a study by the NIH/NICHD, NIST, and by hundreds of customers.

How long does it take to manufacture a batch of spherical nanoparticles?

Although our manufacturing methods are highly characterized and completely automated, in order to make nanoparticles with the reproducibility we require is no slow task. Some sizes of our nanoparticles take up to nine days to manufacture. Conjugations can take another two to three days.

What is the shelf life of your bare spherical nanoparticles?

If stored and handled correctly, the shelf life is > 1 year. We have been manufacturing our nanoparticles for over two years and have yet to see a definitive lifetime. For more information about shelf life, please go here ALL ABOUT SHELF LIFE .

What is your return policy?

There are no returns for our items since they are not resalable. However, we are happy to replace products if they are found not to be within our stated specifications. Please go to return policy.

Gold Nanorods

I notice your nanorods can be just about any color, not just red like the spherical gnps. Wouldn't this have great applications in multichannel lateral flow?


We have bought some gold nanorods from you and we would like to know the thickness of the CTAB coating because we are interested in the interaction between these CTAB coated nanorods with the surrounding. Do you know the thickness of the CTAB coating?

Theory is that it forms a bilayer. If it is completely linear, then the bilayer thickness would be ~6nm.

What is the thickness of the polymer cages you use for your in vivo nanorod products?

Unfortunately, this is proprietary and part of our growing list of intellectual property.