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Gold stars offer new rewards

BioPhotonics
Jun 2006
Gary Boas

The size- and shape-dependent emission of noble metal nanoparticles makes them useful for applications such as sensing and labeling and even for nanoscale optical waveguides. For sensing, these particles can detect molecular binding because their plasmon resonance wavelength shifts in response to changes in the local environment.

Researchers at Rice University in Houston recently described star-shaped gold nanoparticles that have a high dielectric sensitivity and multiple spectral peaks — each corresponding to one of the starlike tips. These characteristics could make them highly efficient localized surface plasmon resonance sensors, said Jason H. Hafner, principal investigator in the study. The particles also could provide a simple and cheap means of transducing molecular affinity.


Researchers have synthesized star-shaped gold nanoparticles for localized surface plasmon resonance sensing. The nanostars’ high dielectric sensitivity and multiple spectral peaks recommend them for a variety of applications.


The scientists happened upon the nanostars while they were exploring other nanoparticle shapes. “We were trying a shortcut for gold nanorod synthesis in which we used commercial gold colloids as the seed particle,” Hafner said. “We found that the exact growth conditions that yielded nanorods with the standard seed yielded nanostars with the commercial seed.”

Other groups also recently described star-shaped gold nanoparticles, but those synthesized by Hafner and colleagues were unique in that they were relatively small and featured a complex three-dimensional structure: aspects that lend themselves to biological sensing.

The researchers characterized the optical properties of the nanostars using single-particle spectroscopy with a setup based on an inverted optical microscope made by Carl Zeiss of Oberkochen, Germany, and a halogen light source. They performed the measurements with dark-field microscopy, using both epi-illumination with a 0.9-NA objective and transmitted light with a 0.75-NA oil-immersion objective. An imaging spectrometer and a thermoelectrically cooled electron-multiplying CCD camera, both from PI/Acton of Trenton, N.J., imaged the nanostars and detected their spectra.


Scanning electron microscopy images show the structure and heterogeneity of the nanostars. The scientists will need to address the latter before the nanostars can be developed as commercial sensors.


Hafner foresees a number of benefits of the nanostars. Nonspherical particles such as gold nanorods can detect two-dimensional nanoscale orientation of molecules, which has significant implications for biological research. Because of their shape and multiple spectral peaks, the nanostars can detect three-dimensional nanoscale orientation. This, combined with the high dielectric sensitivity and sharp near-infrared resonances, makes them ideal nanoparticles for a variety of biomedical applications, he said.

Currently, though, the researchers cannot produce nanostars of uniform shape. As a result, each has a different number and arrangement of starlike tips, leading to a large, unwieldy ensemble spectrum. “This problem would need to be solved through synthesis or purification before commercial nanostar-based [localized surface plasmon resonance] sensors could be made,” Hafner said. “This is why we are studying single nanostars.”

They plan to address a variety of questions using the single nanostars, he added. For example: Can one observe the binding of single molecules by this technique? What is the sensing limit for a small peptide? What is the “dielectric constant” of a single molecule?

Nano Letters, April 2006, pp. 683-688.


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