Cardiff University goes for gold in woodlice

18 Apr 2023

Four-wave mixing microscopy spots and identifies metal nanoparticles in living systems.

The progress of man-made nanoparticles through the natural environment is of great concern to researchers, but tracking them in living systems has always been challenging.

A project at Cardiff University has now shown how ingested gold nanoparticles can be identified inside the humble woodlouse, potentially indicating how the particles and similar contaminants might be monitored.

"Gold nanoparticles are used extensively for biological research applications owing to their biocompatibility and photostability and are available in a large range of shapes and sizes," said Cardiff University's Wolfgang Langbein.

"By using gold nanoparticles, which would not normally be present in the woodlice diet, we can study the journey of nanoparticles inside complex biological systems."

Published in Applied Physics Letters, the research aimed to tackle the long-standing lack of microscopy techniques having the required 3D spatial resolution, sensitivity, specificity, and applicability in situ, ideally on the feely moving specimen, to image such nanoparticles.

X-ray fluorescence can be applied to the task, but it is time consuming, expensive, unsuited to biological systems and does not yield a 3D picture of particle distribution.

The Cardiff project investigated whether four-wave mixing (FWM) microscopy, in which interactions between two or three wavelengths in a nonlinear medium produce new wavelengths of light that can then be detected, might be an answer. The FWM effect has been put to use by designers in the creation of new laser emitters or other novel sources of photons.

In the Cardiff platform, pump laser pulses absorbed by gold nanoparticles lead to a heating effect which changes the dielectric function of the gold, creates a surface plasmon effect, and alters the light scattering behavior. This change in scattering, monitored via a separate probe laser pulse, allows the individual gold nanoparticles to be located in the 3D cellular environment.

Tracking metals from the environment in animals

To test the theory on living specimens, gold nanoparticles of 10-nanometers radius were fed to the common woodlouse, animals that can inhabit metal-contaminated areas and ingest particles from their environment, enabling them to serve as model organisms for the study of metallic species in natural systems.

The digestive glands of the woodlice were collected and the gold nanoparticles inside were imaged by FWM. They were detected "with high contrast, locating them with sub-cellular resolution in the volume, despite the significant light scattering from these multi-cellular organs," according to the project's paper.

"By precisely pinpointing the fate of individual gold nanoparticles in the hepatopancreas of woodlice, we can gain a better understanding of how these organisms sequester and respond to metals ingested from the environment," said Langbein. "Tracking this metal within these organisms is the first step enabling further study to determine, for example, if gold is collected within specific cells, or if it can interfere with the metabolisms in high doses."

FWM also allowed the project to distinguish the gold nanoparticles from any other metal particles ingested by the woodlice and accumulated in the same digestive gland, thanks to the characteristic dynamics of the non-linear optical effect. The same principle might now be used to spot other contaminant nanoparticles in organisms like fish larvae and human cell cultures.

"Considering its simplicity and 3D imaging capabilities, FWM microscopy holds great potential to complement synchrotron-based x-ray methods when investigating metal nanoparticles inside biological systems," wrote the project.