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Ask an Expert: I would like to measure the Optical Density (OD) of gold nanoparticles. Also, how do I convert OD into other units used to measure concentration?

Optical density (OD) is a common method/unit used to quantify the concentration of solutes or nanoparticles. As per Beer-Lambert’s law, the absorbance (or more properly termed “extinction”) of a particular sample is proportional to the concentration of solutes that absorb a particular wavelength of light.


Using a spectrophotometer the optical density of a liquid sample with dispersed solute or nanoparticles can be quantified by comparing the intensity of incident light that enters the sample with the intensity of light that passes through the sample (transmitted light). Optical density can be defined at any wavelength, but in the case of nanoparticles it is given at the wavelength of peak absorbance (the longitudinal surface plasmon resonance (LSPR).

Formula for optical density:

Calculating optical density: Formula





In the case of a colloidal nanoparticle sample, the optical density can be expressed as:

OD = (molar extinction coefficient) X (molar concentration) X (pathlength of light (1 cm in a standard cuvette))
Where molar absorbance coefficient = the absorbance of the solution per unit length per mole of solute.

Other units to measure concentration of gold nanoparticles

Please use this conversion chart that helps compare different units of gold nanoparticle concentration (including particles/mL, molarity, OD, and mg/mL)


Related Links:

Plasmonic properties of gold nanorods

Ask an Expert: What is the difference between Optical Density, Absorbance and Extinction of gold nanoparticles? 


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