Quantum Dots Provide Enhanced Medical Imaging

Semiconductor nanocrystals known as quantum dots (QDs) promise to provide greatly enhanced capabilities for medical imaging and diagnostics. These new nanoscale materials have increased intensity of fluorescent light emission when illuminated with excitation radiation, have longer lifetimes for fluorescing, and provide a much broader spectrum of excited colors than that obtainable with conventional materials. One application of these properties – the increased brightness – is illustrated in the figures below. “(QDs) are bright, photostable fluorophores that have a broad excitation spectrum but a narrow Gaussian emission at wavelengths controllable by the size of the material. QDs allow for efficient multicolor imaging of biological samples and should be especially useful for fluorescence imaging in living tissues, where signals can be obscured by scattering and competing intrinsic emissions” (quote from the reference given in the figure).

Experiments that are more recent have also shown that with appropriate surface coatings the QDs may be made stable for in vivo imaging for periods up to four months. “That QD surfaces can control serum lifetime and patter of deposition suggests many medical uses. … Finally, thanks to the high stability of amp-coated QDs, experiments using molecules and cells tagged with QDs and injected into animals may permit antigen trafficking and cellular migration over very long time scales, with unprecedented sensitivity and resolution” (B. Ballou, et al. Bioconjugate Chemistry, 2004, 15, 79-86).

Other researchers have developed new coatings that make QDs appear as protein-like entities and thus are not seen by live cells as toxic. “In addition to the capacity to paint and observe many different proteins with separate colors, quantum dots can be used for the ultimate detection sensitivity: observing a single molecule. Until now, tracking and following a single protein in the cell has been extremely challenging and was the equivalent of searching for the proverbial needle in a haystack. By using the new methods developed at UCLA, and observing with a fluorescence microscope and high-sensitivity imaging cameras, researchers can track a single protein tagged with a fluorescent quantum dot inside a living cell in three dimensions and within a few nanometers of accuracy” (press release by UCLA April 18, 2004).

		Comparison of quantum dot fluorescence with conventional methods for accomplishing in vivo imaging through skin of mice for dynamic angiography of mice
			From D.R. Larson, et al. Science, 2003, 300:1434-1436
				In vivo imaging of same area with conventional methods at same depth with five times the illuminating power
In vivo imaging of vasculature of mouse with quantum dots