Researchers working at the University of Toronto have constructed nanoparticles capable of detecting infinitesimal amounts of chemicals and distinguishing between nearly identical molecules. When the particles attach to the chemical they are designed to detect, they emit a bright green light. What sets them apart is their ability to convert infrared light into a green signal.
The particles are dye-sensitized nanoparticles that take in low-energy photons and turn that energy into higher-energy photons. That process is called upconversion.
How the Particles Work
The upconversion process is carried out by nanoparticles that contain ions of ytterbium and erbium, two chemical elements within the lanthanide family. Researchers have drawn a comparison between the design of these nanoparticles and chocolate-chip cookies, with the ytterbium and erbium ions embedded inside like chips set within a host material made from sodium, yttrium and fluorine. Organic dye molecules cover the outside, functioning much the same way as icing.
When particles are struck by infrared light, dye molecules catch the incoming energy first. It passes through ytterbium ions, which serve as a relay, then moves on to erbium ions. The erbium ions carry out the upconversion step, allowing stored energy to come out as green light.
The issue at hand is how densely ytterbium atoms are packed. When pressed together too tightly, the atoms begin to absorb not just incoming energy but also energy that comes out — a phenomenon known as back-energy transfer. Instead of erbium ions giving off green light, that energy gets redirected back to the ytterbium relay and never escapes to the surface.
The Layered Design
To get around this problem, researchers rebuilt the nanoparticles from the ground up, changing both what they were made of and how they were organized. The new host material combines lithium, lutetium and fluorine. The particles themselves got a shape overhaul, shifting from flat hexagons into more three-dimensional, diamond-shaped forms.
A particle is made up of separate regions: a dense core, followed by an inner shell, then an outer shell. The concentration of ytterbium ions increases with each layer, from the core outward.
The new paper’s lead author, Jiaze Wu, a PhD student in Huang’s lab, says “We were able to create nice gradient: the concentration of embedded ytterbium ions gets denser as you go through each layer, with the core being the most dense.”
“This arrangement enabled us to pack in much more ytterbium. In our particles, the light energy coming in flows almost entirely in one direction, inward toward the erbium ions.”
To arrive at this design, the team did not simply try things until they worked. Instead, they leaned heavily on computer modeling, simulating dozens of possible chemical formulations and particle shapes before manufacturing the most promising versions in the laboratory.
“We used Monte Carlo simulations and density functional theory to simulate how the energy would interact between different parts of the nanoparticle, right down to the atomic or even subatomic level,” says undergraduate student Weixiang Ben, who led the computational work.
“That’s how we showed that this core-shell-shell structure could actually function as a one-directional energy tunnel for incoming light.”
Brighter Nanoparticles
The layered structure resolves the back-energy transfer issue. Inside the core, ytterbium ions take in infrared light, then transfer that energy through erbium ions. The green signal then passes out without being re-absorbed by ytterbium around it.
According to the researchers, these particles can reach up to 150 times the brightness of earlier versions of the same type of nanoparticle.
In the Journal of the American Chemical Society, senior author Professor Kai Huang offers an explanation that makes the benefit easy to see.
“It’s like the difference between stargazing at night versus the daytime,” he says.
“The stars shine 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 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.”
| Particle Design | Older Versions | New Design |
|---|---|---|
| Shape | Flat hexagonal | Diamond-shaped |
| Host matrix | Sodium, yttrium, fluorine | Lithium, lutetium, fluorine |
| Ytterbium arrangement | Embedded like chips | Dense core, then shells |
| Light flow | In and out | Almost entirely inward |
| Brightness | Not stated | Up to 150 times brighter |
One day, particles like these might offer a cheaper means of trapping dangerous drug impurities or trace pollutants, using nothing more than simple low-cost lasers. The design has been laid bare, and the research has been made public.
What matters here is the arrangement of layers. Packing more ytterbium ions into the core created a gradient that keeps energy flowing in one direction. The ytterbium ions in the core absorb the incoming infrared light and pass it through to the erbium ions, which upconvert it into green light.
The green light appears without being taken back in by ytterbium around it, a difficulty that earlier versions could not avoid. This produces a brighter signal.
Source material: “New nanoparticles make hidden chemical differences light up,” ScienceDaily.
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