Sometimes big surprises come in small packages. An example can be found in recent research involving gold nanoparticles coated with a protective layer of thiol-modified polyethylene glycol, or thiol-PEG. Researchers recently investigated the surface-enhanced Raman scattering (SERS) of these nanoparticles and found them to be bright and stable even in harsh conditions. When conjugated to tumor-targeting antibodies, the nanoparticles were bright enough to highlight single cells and could be detected within tissue up to two centimeters deep.These results are better than those from other nanoparticles, said research group member Shuming Nie. “This class of nanoparticle tags is biocompatible, nontoxic and is considerably brighter than semiconductor quantum dots.”Nie is an Emory University biomedical engineering professor and the director of the Emory-Georgia Tech Cancer Nanotechnology Center in Atlanta. Others in the research group were from Emory University School of Medicine and from Georgia Institute of Technology, also in Atlanta.What makes these results even more surprising is that they run counter to expectations. Thiol-modified PEG is a nontoxic, hydrophilic polymer commonly used to improve drug biocompatibility and system circulation. It was thought that adding the coating to a gold nanoparticle would lead to the crowding out of any reporter dye or fluorophore. However, the researchers found that PEG stabilized the reporter, a counterintuitive discovery made initially by postdoctoral fellow Ximei Qian.By using gold nanoparticles, the location of a tumor in this mouse model can be determined clearly when illuminated by a 785-nm laser. The beam excites gold nanoparticles that have accumulated at the tumor site, allowing surface-enhanced Raman spectroscopy (SERS) to highlight the cancer. The nanotags (inset) have a gold core, a thiol-PEG protective coating (outer ring) and reporters (stars) for specific biomarkers (ovals). Courtesy of Ximei Qian and Shuming Nie, Emory University.The researchers started with 60-nm-diameter gold nanoparticles because studies have shown that these are the most efficient for SERS with red (630 to 650 nm) excitation and also are excellent with near-infrared (785-nm) excitation. Absorption from water and hemoglobin is at a minimum in the near-in-frared spectral range, an important consideration for in vivo imaging.They encoded the gold nanoparticles with a Raman reporter, such as malachite green, crystal violet or Nile blue, and encapsulated the nanoparticles with a layer of thiol-modified PEG. Once the coating layer was complete, adding more thiol-PEG had little effect. The layer increased the hydrodynamic diameter of the nanoparticles by 20 nm. However, there was virtually no effect on the gold plasmonic resonance spectra. Moreover, the spectra were stable for months at room temperature and were not altered by strong acids, bases or organic solvents. The pegylated gold nanoparticles were approximately 200 times as bright as semiconductor quantum dots, and they had approximately one-fourth the diameter.The researchers conjugated the nanoparticles with an antibody fragment specific for human epidermal growth factor receptors. They used the nanoparticles on human cancer cells and xenograft tumors in animal models, obtaining the in vivo SERS spectra with a handheld Raman system from DeltaNu of Laramie, Wyo., operating it with a 785-nm excitation source. The investigators found that the targeted nanoparticles accumulated in the tumor at 10 times the rate of the nontargeted ones and that they could detect the nanotags at up to two centimeters in depth.Nie noted that these results were not unexpected, given the earlier findings. “We were not surprised by these sensitivity results because the pegylated gold particles are so bright and stable under in vivo conditions.”With further development, he added, these nanoparticles could enable the detailed molecular profiling of single cancer cells and could allow the sensitive wavelength-multiplexed detection of tumors in vivo. They also might serve as a platform integrating molecular imaging and therapy. Nature Biotechnology, January 2008, pp. 83-90.