Diffusion-Ordered NMR Spectroscopy as a Reliable Alternative to TEM for Determining the Size of Gold Nanoparticles in Organic Solutions

Diffusion-ordered spectroscopy is used to determine gold nanoparticle sizes. Traditional characterization of nanoparticles has centered on imaging by electron microscopy and plasmon resonance absorption in UV-visible electronic spectra. We present a convenient method to characterize gold nanoparticles using diffusion-ordered NMR spectroscopy (DOSY). 2D DOSY NMR is used to calculate diffusion constants and the diameter of solubilized gold nanoparticles capped with 1-dodecanethiol (C-12) or 1-octanethiol (C-8) in three deuterated solvents. The distributions of nanoparticle sizes strongly correlate with transmission electron microscopy (TEM) image analysis. C12 and C8 capped nanoparticle sizes were found to be 4.6 and 2.7 nm by TEM as compared to estimates of 4.6 +/- 0.3 and 2.5 +/- 0.1 nm based on 2D DOSY NMR data. This demonstrates that reliable size characterization of nanoparticles with NMR active nuclei CH in this study) in their protective groups (alkane thiols in this study) can be achieved by a widely available NMR method (DOSY) in place of electron microscopy.