Toronto engineers graded ytterbium through a core and two shells to get more green light out

Upconversion buys one specific thing: distance between the light you shine in and the light you count. Excite a sample above the readout frequency and everything in it that fluoresces answers in your detection window. Push the excitation below the readout, and in an ordinary sample almost nothing answers. "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," Huang said.

The older particles lost ground on the way back out. Infrared light hits the organic dye on the surface, the dye hands the energy to ytterbium ions, the ytterbium relays it to erbium, and the erbium performs the upconversion and re-emits green light. Both ions are lanthanides. More ytterbium collects more light, and it also gives the outgoing photon more chances to be intercepted. "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," Jiaze Wu, the lead author, said.

The recipe change is where the ytterbium went. The host crystal was sodium, yttrium and fluorine; it is now lithium, lutetium and fluorine, so two of the three elements were replaced and only the fluorine stayed. Across the core and the two shells, the ytterbium is graded. "We were able to create a nice gradient: The concentration of embedded ytterbium ions gets denser as you go through each layer, with the core being the most dense," Wu said. "In our particles, the light energy coming in flows almost entirely in one direction, inward toward the erbium ions."

Sensitivity and selectivity are separate measurements, and the architecture described here works on the first one. Brightness against a dark background sets how little of something you can see. Telling apart two molecules of nearly the same shape is the job of the binding chemistry at the particle's surface, which here is the organic dye coating the outside. The phys.org account of the work describes both capabilities. It gives no detection limit, does not name the look-alike molecules resolved, and does not say what sample the particles were tested in. A pharmaceutical quality-control group reading that cannot yet put this next to the assay it runs today, and groundwater is a harder matrix than a clean buffer.

Monte Carlo simulations and density functional theory were used to test dozens of formulations and geometries before the particles were made in the lab.

The excitation source is ordinary hardware. "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 said.