GOLD NANOMATERIALS APPLICATIONS: THE ROLE OF THE SHAPE (NANORODS AND NANOSTARS)
Gold nanomaterials have attracted attention scientific community over the last two decades. Gold nanospheres are a more well-known nanomaterial, in fact, they are widely employed in different bioapplications, and even gold nanospheres are used in commercial devices such as lateral flow assay (rapid test devices).
The plasmonic properties of gold nanomaterials, as well as their biocompatibility and versatile surface functionalization, turn them into an attractive alternative for cancer imaging and treatment.
People usually refer to new properties or advantage of nanomaterials when moving from bulk material to nanoscale. However, regarding gold nanomaterials apart from nanomaterial size, the shape of gold nanocrystal also plays a key role in the final physical and chemical properties.
Anisotropic gold nanomaterials which shapes differ from conventional spherical gold nanoparticles, show modified optoelectronic properties contributing to improve and expand the traditional applications of gold nanosphere. In spite of being composed by the same element at the nanometer scale, shape variation leads to a modification of the final optoelectronic properties of the nanomaterial, from the UV-Vis range to the near infrared.
Within this category, one example of anisotropic gold nanomaterial is nanorods. Gold Nanorods are rod shape gold nanomaterials whose LSPR (Localized Surface Plasmon Resonance) moves from UV-Vis range to the near infrared. This region of electromagnetic spectra is quite valuable to imaging or analytical purposes. Nanorods possess two plasmon resonance modes, one mode associated with oscillations in the longitudinal axis and another associated to the short dimension of the nanorod. In this case, LSPR bands are tunable varying the aspect ratio (length vs diameter). Longer aspect ratios give longer absorption peak wavelengths in the electromagnetic spectra.
The possibility of varying their plasmon wavelength varying their aspect ratio makes nanorods in an alternative for imaging purposes with a different excitation wavelength in the infrared region. Furthermore, nanorods with different aspect ratio can be employed for simultaneous imaging of distinct biomarkers.
Gold nanorods applications include biological applications such as drug delivery, imaging, photothermal therapy or sensing applications employed as a label such as biosensors or immunoassays among other optoelectronics or photonic applications.
Another innovative anisotropic nanomaterial is gold nanostars (also known as gold nanourchins). Gold Nanostarts are branched gold nanoparticles whose optical properties also differ from the conventional gold nanosphere. In this case, the sea urchin or star shape of the nanomaterials leads to LSPR band to even longer wavelength than gold nanorods. Apart from this valuable redshift of the gold nanostar plasmon, a larger enhancement of the electromagnetic field at the tip of the star takes places, a valuable phenomenon for SERS or SEF sensing.
Anisotropic gold nanomaterials are innovative nanomaterials, but with the advantage that their gold surface chemistry is quite popular, therefore their surface can be easily modified with a variety of ligands or biomolecules employing known protocols.
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Liu Y., Yuan H., Fales A. W., Register J. K., Vo-Dinh T., Multifunctional gold nanostars for molecular imaging and cancer therapy. Frontiers in Chemistry 3 51 2015.