UCLA researchers show how world's smallest 'coffee ring' may help biosensors detect disease

The field of biosensing has recently found an unlikely partner in the quest for increased sensitivity: coffee rings. The next time you spill your coffee on a table, look at the spot left after the liquid has evaporated, and you'll notice it has a darker ring around its perimeter that contains a much higher concentration of particles than the centre.

Because this "coffee ring" phenomenon occurs with many liquids after they have evaporated, scientists have suggested that such rings can be used for examining blood or other fluids for disease markers by using biosensing devices. But a better understanding of how these rings behave at the micro- and nano-scale would probably be needed for practical bionsensors.

"Understanding micro- and nano-particle transportation within evaporating liquid droplets has great potential for several technological applications, including nanostructure self-assembly, lithography patterning, particle coating, and biomolecule concentration and separation," said Chih-Ming Ho, the Ben Rich–Lockheed Martin Professor at the UCLA Henry Samueli School of Engineering and Applied Science and director of the UCLA Center for Cell Control.

"However, before we can engineer biosensing devices to do these applications, we need to know the definitive limits of this phenomenon. So our research turned to physical chemistry to find the lowest limits of coffee-ring formation."

A research group led by Ho, a member of the National Academy of Engineering, has now found the definitive microscopic minimal threshold of coffee-ring formation, which can be used to set standards for biosensor devices for multiple disease detection, as well as other uses. The research appears in the current issue of the Journal of Physical Chemistry B and is available online.

"If we consider human blood, or saliva, it has a lot of micro- and nano-scale molecules or particles that carry important health information," said Tak-Sing Wong, one of the researchers and a postdoctoral scholar in UCLA Engineering's department of mechanical and aerospace engineering. "If you put this blood or saliva on a surface, and then it dries, these particles will be collected in a very small region in the ring. By doing so, we can quantify these biomarkers by various sensing techniques, even if they are very small and in a small amount in the droplets."