PNAS: The concentration of toxins is seen with the naked eye

John A.
from Harvard University.
Researchers at the Paulson School of Engineering and Applied Sciences (SEAS) have developed a rapid and cost-effective method for detecting a family of compounds known as amphiphilic compounds prevalent in liquids, which are used to detect diseases such as early tuberculosis and cancer, as well as to detect toxins
in drugs, food, medical devices and water supplies.

The study was recently published in the Proceedings of the National Academy of Sciences (PNAS).

Endotoxins are a common amphiphilic substance that can contaminate water, causing serious illness and death
.
Today's gold standard for endotoxin testing requires the use of compounds found only in horseshoe crab blood, making the testing process expensive and unsustainable
.
Cheaper alternatives are not sensitive
enough for the detection of amphiphilic organisms.

The new test, developed by Joanna Aizenberg, professor of materials science Amy Smith Berylson, professor of chemistry and chemical biology, and Xiaoguang Wang, assistant professor of chemical and biomolecular engineering at Ohio State University and former postdoc in Aizenberg's lab, uses microstructured surface rolling droplets to detect ultra-low concentrations of amphiphilic molecules
.

The team demonstrated techniques for detecting levels of pathogenic amphiphilic endotoxins in water, compounds that can be toxic
even at extremely low concentrations.

The microstructured surface consists of thousands of circular micropillars uniformly covered with self-assembled long-tailed molecules, forming a smooth, frictionless interface
except for the edges of the microcolumns.
Here, there are voids in the coating - similar to molecular pits
.
As the droplet rolls along the surface, if it has no amphiphilic molecules, it hits a crater and stops, unable to overcome friction
at the disordered edge.

But a droplet with a higher content of amphiphilic molecules will continue to roll because the amphiphilic molecules will fuse with the long-tail molecules, filling the voids on the surface – like a paving machine on a smooth surface
.
In the case of endotoxins, when amphiphilic molecules are deposited on the surface of droplets, they connect with other amphiphilic molecules in the droplet, forming larger and larger compounds that eventually slow down and stop
the droplet.
The position where the droplet stays on the surface tells you how many potholes it has patched, and you can know the concentration
of the parent.

"Our surface provides a rapid, portable way to detect amphiphilic molecules in droplets that you can see
with the naked eye," Aizenberg said.
"There's no other way to detect the low levels
we've seen in our tests without using expensive or complicated equipment.
"

The researchers also developed a model to predict how different concentrations of amphiphilic compounds would interact
with the surface of the structure.
By varying the size, shape, and distance between columns, as well as the molecular coating, the surface can be adjusted to detect specific types of amphiphilic molecules
at specific concentrations.

"Our method is broadly applicable to any type of amphiphilic substance," Wang said
.
"With this universal method, we have already detected endotoxins at levels associated with water quality testing, but the test can be further optimized to detect lower concentrations of endotoxins
.
"