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Forschungsarbeit

Gold nanorods purification methods

By Sarah Jessl (05.11.2014)

My research module is based on gold nanorods and a targeted purification after the synthesis.
Over the last decade anisotropic gold nanoparticles have become a focus in research. Nanoparticles in general are a very promising field because of their shape- and size-dependent properties. Therefore they show a wide range of applications in electronics, diagnostics, plasmonics, biological sensing and catalysis. Gold nanorods in particular exhibit fluorescence in the visible spectrum with a 6 to 7 orders higher quantum efficiency than that of the metal itself and they show a very high surface enhanced raman scattering (SERS) spectroscopy enhancement. Gold nanorods (GNRs) show two plasmon modes: the longitudinal and the transverse one. The longitudinal mode occurs along the long axis of the gold nanorod, the transverse mode perpendicular to this axis. The absorption maxima of these modes are dependent on the aspect ratio (AR) of the nanorods. The longitudinal mode exhibits a very pronounced red-shift with increasing AR. Especially in biological imaging, cancer and tumor therapy, biomolecular sensing and optical cellular imaging those unique optical properties of gold nanorods are applied. Due to the properties' dependence on the size and shape of the GNRs, a lot of research is currently done to provide a better understanding of the synthesis' mechanism and parameters. Different methods for nanorods and nanowires have been reported. Seed-mediated methods are very promising for the synthesis of GNRs with different ARs and thus further developed including fine-tuning of the different parameters. Two different main seed-mediated mechanisms can be distinguished: one with citrate capped seeds leading to penta-twinned rods and one with N-Cetyl-N,N,N-trimethylammonium bromide (CTAB) capped seeds producing single crystalline rods.


Due to the size-and shape-dependent optical properties of nanoparticles monodisperse solutions are needed to optimally use the properties of each shape and size. Since it is not easily possible to synthesize nanorods without any by-products, purification afterwards is essential for further applications, especially in the gold nanorod synthesis with high ARs. Consequently this is the topic of my current research. Four different purification methods are examined: separation by addition of warm CTAB[1], by addition of sodium chloride[2], by addition of a gold(III)chloridetrihydrate[3] and by centrifugation with a gradient[4], as it can be seen in Figure 1. Characterization of the purified substrates is mostly done by scanning electron microscopy (SEM) and UV-Vis-spectra.

[Bildunterschrift / Subline]: Figure 1: Overview of the cleaning process with the standard cleaning step by centrifugation and sedimentation inwater (1) and the more difficult purification step (2) to separate platelets, rods and triangles. On the right side is and overview of the used purification methods for the second purification step and their.

Purification of nanorods from nanoplates via the addition of a certain amount of sodium-chloride (NaCl) was demonstrated before by Guo et al. [2] By adding NaCl the colloidal stabilities of the particles are influenced and they show different sedimentation behaviors., e.g. rods stay in the solution and plates sediment. Nevertheless, up to now only aggregates were found, indicating that the salt concentration was too high and destabilized the nanoparticles. A further analysis of the aggregates as well as a step-by-step NaCl addition in relation to the concentration of the nanoparticles will be necessary to improve this purification method.

Another method used is the addition of warm CTAB [1], creating a liquid crystalline phase that is then used for separation. Currently it is not possible to only accumulate long rods in the sediment and by-products in the supernatants. The supernatants, however, showed a significant amount of by-products and the sediment has a lower concentration of by-products. By improving the procedure further by using longer sedimentation times between the steps or combining it with the previously mentioned one this purification method could work out quite well.

Long rods were also purified as described by the addition of a CTAB/gold(III)chloridetrihydrate solution. [3] By adding this solution, gold nanoparticles are oxidized again, their oxidation speeds depending on their shape. Gold nanorods get oxidized very slowly, thus they sediment still while other shapes don’t anymore. Looking at the changes in UV-Vis-Spectra and SEM pictures it can be said that the purification clearly worked. It was definitely proven that the shape of the nanoparticles can be altered by the addition of the CTAB/gold(III)chloridetrihydrate solution which holds a lot of possibilities especially in adjusting the length of nanorods. By addition of the right amount of gold(III)chloridetrihydrate it could be possible to tailor any rod-length which would be great since currently it is not possible to achieve specific medium nanorod length by synthesis.

Additionally it is possible to use a gradient with glycol and water that enables a separation. The different layers in the gradient mixture after centrifugation can be seen clearly, but still need to be analyzed. This method, however, is also very promising for the separation of different rod sized as well as the separation of long rods from by-products.
In conclusion, in my research module a lot of techniques are very promising and can be further improved to achieve tailored gold nanorods for various applications.

References:
[1] Jana, N. R. Chem. Commun. 2003, 1950 - 1951.
[2] Guo, Z. et al. Chem. Commun. 2011, 47, 4180 - 4182.
[3] Khanal et al. J. Am. Chem. Soc. 2008, 130, 12634 – 12635.
[4] Akbulut, O. et al. Nano Letters 2012, 12, 4060 – 4064, Tyler, T. P. et al. J. Phys. Chem. Letters 2012, 3, 1484 – 1487.


mailto: Sarah Jessl
Sarah Jessl
* 1988

Scientific Career
  • 2007-2008
  • Wofford-University, Spartanburg, SC, USA; Auditing in General und Organic Chemistry
  • 2008-2011
  • B.Sc. in Polymer- und Kolloidchemie, Universität Bayreuth
  • since 10/2011
  • M.Sc. Polymer Science, Universität Bayreuth
  • since 12/2011
  • Elite Graduate Program Macromolecular Science
  • since 10/2014
  • PhD in Cambridge, UK

Preise und Auszeichnungen
  • * Jugend forscht: Sonderpreis „Umwelt” (2002)
  • * E-fellows.net Online Stipendium (since 2007)
  • * Deutschlandstipendium (04/2013-03/2014)

Publikationen
  • * Hanske, C., Müller, M. B., Bieber, V., Tebbe, M., Jessl, S. et al., The Role of Substrate Wettability in Nanoparticle Transfer from Wrinkled Elastomers: Fundamentals and Application toward Hierarchical Patterning, Langmuir, 2012, 8, 2563-2570.