Measurement of the number concentration of gold nanoparticle suspension by scanning electron microscopy

Measuring the number concentration (NC) of nanoparticles (NPs) in suspension has become important because of its use as an index of the quality and efficacy of medical and industrial products. Although scanning electron microscopy (SEM) can reveal individual particles, it is not commonly used for measurement owing to difficulties in sample preparation. We used a micro-liquid dispenser to prepare a specimen of a gold NP suspension, and by combining SEM and gravimetry we measured the NC. The dispenser places a small droplet of suspension onto a Si substrate without splashing, confining the particles in a small area. This method enabled us to observe and count all particles in the area by SEM without loss. The average weight of droplets was determined from weight increments of multiple ejections from the dispenser into a reservoir on an analytical balance. Dividing the NP number by single droplet weight gives NC in g(-1). Our simple method gave a precise measurement of the NC of Au NPs with a relative expanded uncertainty of <6% (k = 2) by identifying all particles in a droplet.