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).
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