Introduction
Let’s answer the question first. To do so, we need to know the size of the Au particle and to assume it is monodisperse. In
fact, one purpose of this application note is to explore the meaning of monodispersity, despite its literal definition, one
size. We chose 5 nm diameter Au nanoparticles. Now the earth has an average circumference of 40,000 km or 4 x 1016
nm. So we need 8 x 1015 particles. That’s a lot of particles. Yet, it corresponds to a total volume of just 5 x 10-4 cm3, approximately.
Gold’s density is 19.3 g/cm3. So we need about 10 mg of Au. Not much. Of course, given that high density, it
will be hard to keep the particles from sinking into the oceans as we circumscribe the earth, not to mention the tiny tweezers
required to manipulate the nanoparticles. But our main goal is to figure out if the size claimed is correct and if the
distribution is one size.
Experimental Method
The Au was purchased from nanoComposix Inc.1. The NanoXact 5 nm diameter sample was selected with tannic acid
used as the surface agent to provide stability. The concentration is 0.05 mg/mL and it was used without dilution. The accompanying
literature claims monodispersity with a CV of 8-15% and a mean size within 2 nm of 5.0 nm. The TEM picture
from the manufacturer is shown here as Fig. 1.
Notice there are less than 150 total primary particles. The majority are single
primary particles. But some apparent doublets and even more apparent
triplets are visible. But are they true aggregates or forced together during
sample preparation for the picture? With so few particles, it is hard to
tell. Likewise, it is hard to be precise about the mean diameter and CV with
so few particles.
DLS was performed using the Brookhaven NanoBrook Omni with a nominal
35 mW diode laser at 658 nm wavelength. Ten measurements were
made using the BI-SM50, a 50 μL disposable cell.