Researchers routinely image biological tissue using optical techniques that make use of the intrinsic light emission of nanoparticles in the near-infrared range of the electromagnetic spectrum (which lies between 700 and 2500 nm). This is because near-infrared photons are much less scattered by tissue than those at visible light wavelengths, which means that they can penetrate tissue much more efficiently. Near-infrared light also minimizes biological autofluorescence.

Afterglow light emission is an intrinsic luminescence process that generally occurs thanks to the slow release of photons from energy traps in materials once they have been heated up – with laser light, for example. Although afterglow imaging shows much promise for in vivo imaging, most of the optical afterglow agents made so far are not very bright. What is more, they are based on inorganic nanoparticles containing rare-earth heavy-metal ions, such as europium, praseodymium and chromium, which are toxic for biological cells.

Afterglow intensity of SPNs is more than 100-fold brighter

A team of researchers led by Kanyi Pu of Nanyang Technological University in Singapore has now discovered that semiconducting polymer nanoparticles (SPNs) based on poly(phenylenevinylene)s can emit long-lasting afterglow light. The particles, which are a class of optically active photonic materials and are less than 40 nm in size, are completely organic, are biodegradable and contain biologically benign ingredients so they are not toxic, says Pu.

“The afterglow intensity of the SPNs is more than 100-fold brighter than that of traditional NIR inorganic afterglow agents and the signal is detectable through the body of a live mouse,” he tells nanotechweb.org. “Our polymer nanoparticles also emit at the longest wavelength yet (of 780 nm) and have the longest half-life (of 396 s) of all such agents. To compare, existing rare-earth doped inorganic nanoparticles, for example, emit light in the 510 to 716 nm range and have a half-life of less than 200 s.”

Light-induced formation of unstable chemical defects

According to the researchers, the afterglow luminescence in the SPNs involves the light-induced formation of unstable chemical defects in the material that generate photons.

Pu and colleagues say they are able to obtain high-contrast images of lymph nodes and tumours in living mice with their afterglow agents and that the signal-to-background ratio of the particles is up to 127-times higher than that obtained by NIR fluorescence imaging. The particles can also be developed into smart molecular probes that emit afterglow light only in the presence of biomarkers.

“We indeed show that the SPN-based probe turns on its afterglow in the presence of antioxidant molecules called biothiols, which allows us to image drug-induced liver injury in mice. The probe can detect the hepatotoxicity within 30 minutes of the drug being delivered. This is much shorter compared to the time required for observing histological changes in the liver, which can take around three hours.”

SPN brightness is being further increased

Drug-induced hepatotoxicity has been a long-standing concern for modern medicine and is one of the most common reasons that the US Food and Drug Administration withholds drug approval, he adds. Biothiols including cysteine (Cys), homocysteine (Hcy) and glutathione (GSH) make up a major portion of the body’s antioxidants that defend against oxidative stress. Real-time in situ imaging of biothiol levels, as done in this work, could thus be a good way of evaluating drug-induced hepatotoxicity.

The team, reporting its work in Nature Biotechnology doi:10.1038/nbt.3987, says that it is now busy further increasing the brightness of its SPNs. “After this, we will be working on making them renal-clearable from the body,” adds Pu.