This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: A team of researchers from University of Toronto Engineering has created a new type of dye-sensitized nanoparticle that can detect target chemicals at very low concentrations while also distinguishing between molecules with very similar shapes. When bound to their target molecules, the nanoparticles absorb light in the form of low-energy photons and use it to emit high-energy photons.
This chemical-sensing ability could help pharmaceutical manufacturers detect impurities or enable researchers to find tiny traces of chemical pollutants in groundwater. "Organic molecules called fluorophores have been used for decades to absorb light and convert it into colorful emissions, but the process only works in one direction," says Professor Kai Huang, senior author on a paper published in Journal of the American Chemical Society that describes the new particles. "With fluorophores, the excitation frequency has to be higher than the emission frequency, which means that they convert high-energy photons into low-energy photons.
What makes our dye-sensitized nanoparticles special is that they are capable of upconversion, meaning that they can absorb light in the form of low-energy photons and emit higher-energy ones. "For example, you could excite them with near-infrared light, which can easily be produced with low-cost lasers, and they would glow bright green in response." Huang says that the difference between the excitation and emission frequencies makes it easier to sort the signal from the noise. "It's like the difference between stargazing at night versus the daytime," he says.
"The stars shine at the same brightness all the time, but during the day the sun is so powerful that it overwhelms them. Shifting the excitation frequency lower produces a zero-autofluorescence background in the samples you are analyzing, while the luminescent nanoprobes keep shining; it is like turning off the sun, so you can see the stars better." In the nanoparticles, the upconversion is made possible by ions of ytterbium and erbium, part of the chemical family of elements known as lanthanides. Previously, the typical approach to making these chemical-sensing agents resulted in nanoparticles shaped like flat hexagons.
In these particles, ytterbium and erbium ions were embedded in a host matrix made of sodium, yttrium and fluorine, like chocolate chips in a cookie. The dyes are organic molecules coated on the outside, analogous to the icing. When infrared light is shone on the particles, the dyes absorb the light energy and pass it on to the ytterbium ions, which act as an energy relay to pass it on to the erbium ions.
The erbium ions do the upconversion, with the energy then re-emitted as green light. "But there's a problem: If you pack the ytterbium atoms in too densely, they start to absorb not only the energy coming in, but also the energy coming out," says Jiaze Wu, a doctoral student in Huang's lab and lead author of the new paper. "This is called back-energy transfer: It means that the energy that would have been emitted by the erbium ions as green light instead gets bounced back to the ytterbium relay and never reaches the surface." Wu, Huang and the team overcame this trade-off by changing the recipe.
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