AsianScientist (Dec. 20, 2017) – Scientists in China have added gold nanodots to nanorods made of bismuth sulfide to kill tumor cells with heat upon irradiation with near-infrared light (NIR). They published their findings in Angewandte Chemie.
In photothermal therapy, an agent is introduced into a tumor and then the region is irradiated with NIR light, a wavelength that penetrates far into tissue without causing damage. The agent absorbs the NIR light and converts it to heat.
The local overheating kills off the tumor cells while healthy tissue is protected. Ideally, the photothermal agent can simultaneously act as a contrast agent for diagnostic imaging, such as computed tomography (CT), which can be used to locate the tumor.
In the present study, a research group led by Professor Zhang Haiyuan at the Chinese Academy of Sciences added gold nanodots to the surface of the semiconductor, bismuth sulfide (Bi2S3), to improve the photothermal performance of the nanorods.
When exposed to NIR light, negatively charged electrons in Bi2S3 are excited to such a degree that they jump to a higher energy level called the conduction band. This leaves behind positively charged holes. Recombination of the electrons and holes releases energy, which is transferred into the Bi2S3 crystal lattice, causing it to vibrate. This vibrational energy is released into the environment as heat. Certain defects, known as deep level defects, in the crystal lattice promote this type of electron-hole recombination.
Increasing the number of deep defects or introducing more electrons into these deep defects could increase the photothermal efficacy of Bi2S3 nanomaterials. This is where the gold atoms play a role. Gold atoms bind sulfur atoms and keep them out of their lattice positions, resulting in more defects.
In addition, the contact points between the Bi2S3 and gold provide the excited electrons with an energy level that allows them to return more easily to the energy level where there are substitution errors. This enables the electrons to fall more easily into the deep defect ‘trap.’
As the gold-coated nanorods preferentially aggregate in tumor cells, they are very visible as contrast agents in CT scans of tumors in mice. The researchers also demonstrated that the gold version of the nanorods had a significantly higher inhibitory effect on tumor growth than gold-free nanorods under irradiation with NIR.
When used to treat mice bearing tumors, the nanorods that contained gold nanodots shrank the tumors to a point where some of them were no longer palpable after a fourteen-day course of treatment. Importantly, the researchers did not observe toxic side effects or damage in the tissues surrounding the tumors. Further studies may one day allow these nanorods to be used to treat cancer in humans.
The article can be found at: Cheng et al. (2017) Deep-Level Defect Enhanced Photothermal Performance of Bismuth Sulfide–Gold Heterojunction Nanorods for Photothermal Therapy of Cancer Guided by Computed Tomography Imaging.
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Source: Wiley.
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