Characterizing gold nanoparticles by NMR spectroscopy

Gold nanoparticles have attracted considerable attention in recent research because of their wide applications in various fields such as material science, electrical engineering, physical science, and biomedical engineering. Researchers have developed many methods for synthesizing different kinds of gold nanoparticles, where the sizes and surface chemistry of the nanoparticles are considered to be the two key factors. Traditionally, the sizes of nanoparticles are determined by electron microscopy whereas the surface chemistry is characterized by optical spectroscopies such as infrared spectroscopy and Raman spectroscopy. Compared with that, nuclear magnetic resonance (NMR) spectroscopy provides a more advanced and convenient way for size determination and surface chemistry investigations by combining one- and multiple-dimensional NMR spectroscopy and diffusion-order NMR spectroscopy. Here, we show a thorough study that NMR spectroscopy can be applied to characterize small thiol-protected gold nanoparticles, including size determination, surface chemistry investigation, and structural study. The results show that the nanoparticles' sizes determined by NMR agree well with transmission electron microscopy results. Furthermore, the ligand densities of nanoparticles were determined by quantitative NMR spectroscopy, and the structures of ligands capped on the surfaces were studied thoroughly by one- and multiple-dimensional NMR spectroscopy. In this work, we establish a general method for researchers to characterize nanostructures by using NMR spectroscopy.