Gold nanorods: Non-toxic coating aids anticancer agents

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  • Published: Dec 15, 2011
  • Author: David Bradley
  • Channels: Atomic
thumbnail image: Gold nanorods: Non-toxic coating aids anticancer agents

Nanorod coatings

Inductively coupled plasma-optical emission spectrometry (ICP-OES) has been used in a study of coated gold nanorods that have potential as anticancer agents.

Nanoscopic gold can be used as a contrast agent in medical imaging as well as a treatment for cancer. Now, Leonid Vigderman, Pramit Manna and Eugene Zubarev of the Rice University in Houston, Texas, USA, have developed a novel process for pre-treating gold nanorods that might open up new applications in medicine and research.

The researchers explain that because cancer cells are generally more sensitive to changes in temperature than healthy tissues, the local heating of such malignant cells can be exploited in targeted heat treatments. The use of gold nanorods introduced into cancer cells would absorb incident near-infrared light and so produce photoinduced hyperthermia.

Inroads to nanorods

Unfortunately, making viable gold nanorods through standard techniques does not produce the optimal structure. Such nanorods are produced in a concentrated solution of cetyl trimethylammonium bromide (CTAB), which gives them a double coating of CTAB. The CTAB is present only as a deposited layer and is not bound chemically to the gold, which means in the aqueous environment of physiological conditions, the CTAB molecules can slowly dissolve. This obvious problem with such loose-fitting CTAB layers on the gold nanorods is that CTAB is highly toxic. The CTAB is essential to success with the gold nanorods, however, as without it the nanorods form aggregates and so become ineffective in photoinduced hyperthermia.

Researchers have tried various techniques in order to make nanorods both biocompatible and active. However, the loss of toxic CTAB from the nanorods remains an ongoing problem and activity is often compromised by the earlier approaches to pre-treatment disrupting the uptake of the nanorods into cancer cells and so lowering efficacy significantly.

The US team has now found a workaround that precludes toxicity and enhances activity simply by swapping in a sulfur-hydrogen group, to make (16-mercaptohexadecyl)trimethylammonium bromide (MTAB). It is possible to completely replace CTAB with MTAB on gold nanorod, as demonstrated by proton nuclear magnetic resonance (NMR) spectroscopy. Importantly, the presence of sulfur in MTAB gives the molecule a chemical hook with which to covalently bond to the gold nanorods. This means the coatings are stable in aqueous solution and even if the coated gold nanorods are freeze-dried. The team says the nanorods can thus be stored indefinitely as a dry, brown powder but readily dissolve in water ready for use.

Cell death by heating

They have carried out tests on cell cultures to demonstrate that MTAB gold nanorods are non-toxic, even at high concentrations. They have also shown by scanning electron microscopy (SEM) that the nanorods are absorbed in large quantities by breast cancer cells, type MCF-7. The researchers estimate that under experimental conditions, a single cell can take up more than two million nanorods as quantified by inductively coupled plasma-optical emission spectrometry (ICP-OES). Such a rate of uptake would be useful in a putative photoinduced hyperthermia treatment for cancer.

Credit: Wiley/Angewandte Inductively coupled plasma-optical emission spectrometry (ICP-OES) has been used in a study of coated gold nanorods that have potential as anticancer agents.
Breast cancer cells can take up millions of coated gold nanorods for potential therapy

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