Nanoparticles probe deeper

NUS Chemistry Professor Liu Xiaogang has led a multidisciplinary team of researchers from Singapore, Korea, China and the United States to develop a hybrid nanoparticle structure that brings promise of application in deep tissue bio-imaging and drug delivery. The results of this study were published in Advanced Materials in March.

This novel structure was achieved by interacting two types of nanoparticles — upconversion nanoparticles and gold nanoparticles. Upconversion nanoparticles are so-named because of its unique ability to convert light from low-energy to high-energy, and lower the wavelength. They are luminescent, inorganic, and comprise of metal ions.  When near-infrared (NIR) light is shone on upconversion nanoparticles, the resultant light emitted is reduced in wavelength and goes from being invisible to the human eye to visible, giving out light in the colours of the rainbow. Upconversion nanoparticles have various potential applications, including in currency and merchandise to prevent counterfeiting.

This hybrid nanoparticle structure enables DNA strands to be attached to the upconversion nanoparticles and Prof Liu, who is also the 2017 University Awards Outstanding Researcher awardee, sees broader possibilities in screening for cancer markers and bio-imaging. “We can design the particles to selectively bind to cancer cells so then we can differentiate healthy cells and cancer cells by just looking at the luminescence,” he elaborated. 

We can design the particles to selectively bind to cancer cells so then we can differentiate healthy cells and cancer cells by just looking at the luminescence.

Another use of the particles is as a site-specific drug delivery system. The DNA strands are able to encapsulate drug molecules to carry them specifically to the cancer cells. When the NIR light laser is shone onto the particles, the light emitted also produces heat; which causes the heat-sensitive DNA strands to open, releasing the drugs into the cancer cells. Thus, the drug can be more precisely injected into specific areas of the body where it is most needed. Such site-specific drug delivery would reduce the amount of dosage of medicine required for treatment, and potentially its cost too.

Other research for bio-imaging and site-specific drug delivery typically use fluorescent organic molecules, which are compounds that contain carbon. These compounds usually need an ultraviolet (UV) light source to cause them to emit light, said Prof Liu. “A beam of UV light is likely to produce hydroxyl radicals that can decompose the materials very quickly, which is why organic molecules are not very photostable. These inorganic particles are very stable and the emission is very consistent. We don’t see photo degradation or photo bleaching, which means the emission is quenched,” he added.

Another reason for the choice of inorganic nanoparticles is the penetration of the light source. UV light sources can only go up to two millimetres below the surface of the skin, while studies by Prof Liu’s team have showed that NIR light can go up to two centimetres below the skin, giving a greater breadth of possibilities for imaging and drug delivery.