In Jin's case, x-ray crystallography revealed that the gold nanoparticle is chiral. Chemists typically probe the internal chiral structure of gold nanoparticles using a technique called circular dichoism spectroscopy. When pure chiral molecules are exposed to circularly polarized light, each isomer absorbs the light differently, resulting in a unique and of opposite sign spectrum for each isomer. The process of creating the gold nanoparticles, however, often results in a 50/50 mix of each isomer, known as racemates.
"Because the spectrum is of opposite sign for each isomer, they cancel each other out and the net optical response is zero. This makes circular dichoism (CD) spectroscopy useless when it comes to determining the chirality of gold nanoparticles in 50/50 mixtures," said Gil, associate research professor of chemistry and director of the Department of Chemistry's NMR Facility.
Since Jin couldn't use circular dichoism spectroscopy, Gil was able to use NMR to help Jin distinguish between his gold nanoparticles' left- and right-handed isomers.
NMR spectroscopy takes advantage of the physical phenomenon wherein some nuclei wobble and spin like tops, emitting and absorbing a radio frequency signal in a magnetic field. By observing the behavior of these spinning nuclei, scientists can piece together the chemical structure of the compound.
In 1957, scientists observed that the hydrogen atoms of a freely rotating methylene (CH2) group produced two different frequencies if they were close to a chiral center. Jin's gold nanoparticles, which have a chiral core, are cushioned by several chemical groups, including freely rotating methylene groups. Gil reasoned that the nanoparticles' chiral core should induce the methylene group's two hydrogen atoms to give off different frequencies, a phenomenon known as diastereotopicity.
Gil and Jin comp
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Carnegie Mellon University