A.E. Saunders, D.A. Garcia, S.J. Oldenburg
Keywords: gold nanoparticle, calibration, reference material, electron microscopy
Summary:The ability to understand and predict the properties of nanomaterials relies on an accurate measurement of their size, shape, number density, and other physical properties. One challenge associated with many measurement techniques is the lack of available reference materials with narrow specification ranges that can be used for instrument calibration. This talk describes the fabrication of colloidal gold nanoparticles with exceptionally narrow size and shape distributions and their use as calibration standards for electron microscope and particle counting instruments. Currently used calibration standards for electron microscopy include biologically-derived materials (catalase crystals) and inorganic materials (gold and polymer nanoparticles and evaporated metal films), which typically have large variance in properties or which cannot be used to verify calibration across large magnification ranges. As an alternative material, we use colloidal synthesis techniques to produce gold nanocrystals with nearly perfect spherical shape in the size range between 20-200 nm and low coefficients of variation (CV). To facilitate measurement of statistically-relevant numbers of particles, the gold nanocrystals can be coated with uniform polymer coatings with low electron density, which allows large-area monolayer coatings of particles to be deposited onto grids for measurement. The gold particles are primarily single crystalline, with few lattice defects, allowing the lattice spacings to be easily measured at high magnification for calibrating high-resolution TEMs. At lower magnification, the large interparticle separation in the polymer-coated particles allows automated image processing methods to be used for generating magnification-dependent scaling factors. Techniques for standardizing image analysis to obtain consistency among different instruments will be discussed. The narrow size distribution of the core particles also makes them useful as the basis for other particle properties measurements, including their use as a particle number standard for calibrating single-particle tracking and particle counting instruments. Recent efforts and comparison between different measurement techniques will be described.