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. C 12 and C 8 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 ( 1 H 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.