SERS detects DNA without labels
Technique might be useful for drug discovery.
David L. Shenkenberg
DNA detection has revolutionized everything from forensics to diagnostic testing in medicine.
Because most current detection methods are based on fluorescent labels that require costly and complex chemistry to implement, researchers at Rice University in Houston developed surface-enhanced Raman spectroscopy (SERS) for detecting DNA without labels.
Using a mathematical spectral correlation function that they developed, the investigators quantitatively determined the reproducibility of their technique to detect single- and double-stranded DNA molecules of various lengths and sequences under a range of experimental conditions. This function also can quantify the conformational distribution of the DNA molecules as long as they are in close proximity to the Raman-enhancing nanoparticles.
The surface-enhanced Raman signal from adenine was extremely strong. (a = adenine, b = thermally pretreated thiolated single-stranded DNA, c = thermally pretreated thiolated double-stranded DNA). Reprinted with permission of the Journal of the American Chemical Society.
The particles that the researchers used had a silica core and a gold shell and were deposited onto a microscope slide. They pipetted the DNA in solution onto the slide, where it bound to or remained unbound from the nanoparticles.
The scientists performed Raman spectroscopy on the DNA samples using a Renishaw Raman microscope equipped with an excitation laser that emits at 785 nm. Thus, they synthesized their tunable silica-gold nanoparticles to have maximum absorbance at that particular wavelength. They used a 63× magnification Leitz water-immersion lens corresponding to a sampling area of 3 × 30 μm and set the microscope to capture Raman spectra in 20 s.
Among thiolated single- and double-stranded DNA molecules, the researchers observed a strong Raman signal from adenine regardless of the DNA sequence, suggesting that the surface-enhanced Raman signal from adenine could be used to detect DNA. Pretreating the DNA with heat improved the signal — probably because heating ordered the DNA by uncoiling all the strands uniformly — whereas before, the DNA molecules were coiled randomly.
The investigators attempted to detect the binding of two platinum-containing chemotherapy agents to DNA, transplatin and cisplatin. They detected the binding of cisplatin, but not transplatin. They believe that cisplatin binding reduces the reproducibility of the Raman signal because it more greatly disrupts the DNA conformation, whereas transplatin does not bind as much between the strands. Their measurements were nearly 100 percent reproducible, according to their mathematical assessment. These experiments were detailed in the April 23, 2008, issue of the Journal of the American Chemical Society.
The researchers noted that the information gained in this study may be used to develop an all-optical chemical sensor of molecules that bind to DNA, which would be useful for drug discovery and development.
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