After a rainstorm, when the sun finally breaks through the clouds, colors glimmer and swirl on the oily surfaces of puddles that have accumulated on the ground. For physicist Anna K. Swan, this simple phenomenon provides the basis for the new imaging technology she is developing to look at very small objects such as strands of DNA.

Swan’s ultimate goal is to build a better microscope. Today’s best microscopes fall a hundred times short of revealing DNA. A single base pair of DNA measures only .3 nm, and a strand of 20 pairs measures only 6 nm in length.

The new technology will help advance detection of genetic defects. Today,scientists infer genetic defects by testing to see if two strands of DNA fit together properly. A microscope able to determine the shape of a strand would allow scientists to detect defects through direct observation.

The basic physics of rainbows in puddles provides the foundation for Swan’s imaging technology. A thin layer of oil floats on the water. As sunlight penetrates the layer of oil, some of it reflects off the oil, and some continues through and reflects off the water. The light, coming from one direction, reflects back from different directions off the oil and water. This skews the wavelengths, causing interference that eliminates some colors from the visible spectrum.

The eye sees reflected blues, reds, even iridescent pinks and greens, depending on the thickness of the oil. Because the oil isn’t spread evenly, it produces a swirled rainbow in the puddle.

Swan applies this basic principle of reflected light to observe strands of DNA. She stands the strands upright, like blades of grass, onto glass layered on top of a silicon base.Then she attaches fluorescent material to the top or bottom of the strand. By observing how light reflects from the fluorescent material and the glass, she can determine the strand’s length. By coupling two strands of DNA, the same light will reflect differently, revealing more about the shape of the strand.

The Center for Nanotechnology and Nanobiotechnology has helped Swan make progress on DNA imaging by connecting her lab in the BU Photonics Center to biomedical engineer Charles Cantor’s laboratory. Biologist Lev Moiseev, who received his doctorate for his work on this project in 2003, provided expertise on DNA and fluorescence microscopy, and assisted in preparing materials for imaging.

Researchers in Swan’s photonics lab are focused on engineering the microscope. The scope consists of two lenses on either side of the plate holding DNA strands. Laser light beams through the lenses, hits the subject, and reflects back through the lenses into a spectrometer that collects and analyzes the reflected light. As they encounter problems, the researchers refine the design. In a recent iteration, they eliminated the fiber optic cables carrying the laser light because the cables caused interference. Now mirrors direct the laser beams. Ultimately, they want to collect enough information about an object from the microscope to create rotating 3-D computer images of the DNA.

This new kind of microscope will also help advance the basic understanding of cell biology. Today,biologists use electron microscopes to look at the structure of a cell, a process that kills the cell by encasing it in conducting material. According to Swan, since her new technology uses light, which does not require this destructive step, “we can look at life as it unfolds.”

To learn more about the Center for Nanoscience and Nanobiotechnology, see
http://nanoscience.bu.edu/. To learn more about CNN projects, see http://ultra.bu.edu/.

— Elizabeth Dougherty

Swan is also investigating the potential of carbon nanotubes as light sources, similar to LEDs. The image shows how a nanotube begins to vibrate when it is "plucked" with laser light that resonates with the tube's electronic structure. The frequency of the nanotube's vibration is shown on the horizontal axis, as the energy of the laser is varied on the verticle axis.