Sizing Single Gold Nanoparticles with Bright-Field Microscopy

We present an experimental procedure for determining the diameter of spherical gold nanoparticles (AuNPs) using a basic bright-field microscopy apparatus. We achieved high-contrast imaging by constructing a bright-field microscopy system with ultrabright LEDs (incorporating low-cost components), employing Köhler illumination near the coherence limit, and using digital processing to perform image averaging and background subtraction. Our system allows for the detection of 80 nm, 150 nm, and 300 nm diameter AuNPs immobilized on functionalized glass substrates. Through-focus images of the particles show characteristic contrast inversion, from where we find a nearly linear relationship between the minimum intensity contrast and particle diameter (as determined from scanning electron microscopy) for the three sizes studied and for three different illumination wavelengths covering the corresponding AuNP plasmon band (λ = 460 nm, 520 nm, and 620 nm). This behavior was found to be consistent with the corresponding scattering and absorption cross-sections of the AuNPs under the illumination wavelengths considered.