Using tiny tweezers to characterize bioinspired nanocarriers

Oil and water are two very different substances that typically don’t mix, except in mayonnaise, milk, salad dressings, and other so-called “emulsions” — stable mixtures of water droplets and fatty molecules. Oil and water emulsions are used in everything from drug delivery to food processing and cosmetics. However, studying how single particles within these emulsions change in response to, for example, pH shifts in the digestive tract has proved challenging.

Now, for the first time, NCCR Bio-Inspired Materials researchers at the University of Fribourg have used “optical tweezers” to trap and characterize individual particles within an oil and water emulsion while changing the pH. Their findings may help not only to better understand how fat is digested in the body, but also inform the production of improved food formulations and drug nanocarriers.

“No one before managed to look at the ultrastructural features of a digesting oil-in-water emulsion droplet at the single-particle level in real time using optical tweezers,” says NCCR principal investigator Stefan Salentinig, professor of experimental physical chemistry at the University of Fribourg.

Salentinig and his team have previously found that, as milk is digested, it forms highly organized nanostructures that could be key for transporting nutrients that do not dissolve in water through the digestive tract.

The researchers also showed that these structures may be used as tiny shuttles to transport antimicrobial drugs and release the drugs in response to changes in pH. However, the effect of pH on the nanostructures remained mysterious. So, Salentinig teamed up with NCCR PI Frank Scheffold, professor of physics at the University of Fribourg, whose group has been working on optical tweezers — intense beams of laser light that can be used to capture and manipulate tiny objects. This Nobel Prize-winning technology, developed in the 1970s, is now used in many applications, from trapping fat droplets inside living cells to studying the workings of individual enzymes.

“Optical tweezers are one of the most brilliant innovations in science and allow us to manipulate objects on the microscale,” Scheffold says. “We adapted and contributed with new developments to this technology, in terms of making it faster and higher-precision, and we have implemented some of these learnings in this study.”

Scheffold, Salentinig, and their teams combined optical tweezers with microscopy, microfluidics and other analytic methods in a way that allowed them to capture and characterize droplets of the fatty molecule oleic acid in water under changing pH conditions.

The researchers found that at low pH, the oleic acid droplets do not have a specific structure. “They look like a simple oil droplet surrounded by water,” Salentinig says. But as the pH increased, the molecules began to arrange in very specific nanostructures.

At neutral pH, water from the outside started to enter the droplets, and aggregates of oleic acid filled with water formed honeycomb-like structures. As the pH increased, these so-called liquid crystalline structures transformed into tiny fat sponges whose pores were filled with water. At high pH, onion-shaped vesicles are formed; these structures are made of several concentric layers of oleic acid and water. The researchers reported their findings in the Journal of Colloid and Interface Science.

“By changing the pH, one can tune the structures and induce the release of a drug, a molecule, a nutrient — whatever is loaded into these carriers,” Salentinig says. Understanding how the structures of these nano-shuttles change with pH, he adds, would allow researchers to target drugs to specific organs or influence the feeling of satiety in people.

Salentinig and Scheffold continue collaborating on an ambitious new project that uses optical tweezers to capture individual droplets, manipulate their structure, and map their composition in real time using state-of-the-art techniques. Looking at the composition of the droplets in real time would allow the researchers to monitor the release of drugs and other molecules.

Reference: Manca, M.; Zhang, C.; Scheffold, F.; Salentinig, S., Optical tweezer platform for the characterization of pH-triggered colloidal transformations in the oleic acid / water system, J. Colloid Interface Sci., 2022, 627, 610–620.