Confocal Raman Microscope Exposes Diffusion Kinetics
Using confocal Raman microscopy, researchers at the University of Utah in Salt Lake City are measuring the diffusion of molecules into polymer particles. The work could help people get their medicine continuously and could help clean up the environment. The diffusion of molecules out of polymers plays an important role in the action of time-release drugs, and the uptake of molecules into polymers is a basis of environmental remediation.
“These diffusion kinetics govern the time profile of release,” said chemistry professor Joel M. Harris. He and his students have found that the diffusion rate depends on the local solvent concentration, as they expected. They also discovered an unanticipated variation in diffusion.
In confocal Raman microscopy, as with other confocal methods, a pinhole restricts the collected radiation so that only those signals from a small volume appear at the detector. By its nature, the technique probes molecular vibrational and rotational modes.
In the custom-built Raman confocal microscope, 647.1-nm light from a krypton-ion laser is expanded and sent into a cell to create Raman scattering, which is used for the analysis of the chemical composition of the small confocal volume. Researchers used the setup to investigate the diffusion of molecules into polymers, important in the design of time-release drugs. Courtesy of Joel M. Harris, University of Utah.
In their work, the investigators use a custom setup that incorporates a Nikon inverted fluorescence microscope and a Coherent krypton-ion laser operating at 647.1 nm for a light source. They use a Chromex monochromator and an Andor CCD cooled to –60 °C to obtain Raman spectra from regions less than 1 µm in diameter in the particles.
In a demonstration of the technique, they monitored carbon-nitrogen stretching modes of the solvent acetonitrile in 75-µm-diameter polystyrene particles in contact with acetonitrile/water mixtures to determine the concentration and changes in relative concentration of the solvent within the particles. Acquiring a spectrum took 30 s, which was not a problem because of the relatively slow diffusion process involved, but Harris noted that this would not be suitable for all polymers.
When they analyzed the results, the researchers found an increase in diffusion that was especially pronounced for high concentrations of the solvent and at long contact times. The deviation, they discovered, was caused by the particles swelling in volume by as much as 9 percent over 90 minutes because the solvent accumulated within them.
Harris attributed the discovery to confocal Raman microscopy, noting that the observation of the phenomenon required the ability to monitor the local concentration of molecules at a specific location within the sample particle.
Future investigations will measure the transport of mixed solvents and examine the effects of particle size and cross-linking density. Other planned research will study changes in polymer structure upon incorporation of strongly interacting solvents.
Analytical Chemistry, online March 1, 2006, doi:10.1021/ac052056n.
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