New tool could end filtration guesswork

Mark A. DeSorbo

BLACKSBURG, Va.—Take a basketball and attach it to the end of a diving board, and you've got a life-size version of a sub-micron tool that could eliminate the guess work in determining how to remove contamination from solutions and usher in a new age of filtration technology.

While it sounds more like an object of art, the description best describes a 2-micron sphere of carboxylated polystyrene at the end of a 5-micron-thick cantilever, which has been fashioned inside an atomic force microscope that Tracy L. Cail, a doctoral candidate at Virginia Polytechnic Institute, is using to develop a new method for measuring the sticking and repulsion efficiency between microorganisms and surfaces.

The geological sciences student says the new measurement starkly contrasts prior methodology. In the past, scientists have used theoretical equations or soil-packed columns for water composition tests to determine how to filter contamination from solutions.

“Neither of those approaches explains what happens on the smallest scale,” Cail says. “This will allow us to better understand how nanoscale bacteria adheres to surfaces, and how to remove it.”

While Cail plans to introduce actual bacteria in future experiments, she is presently evaluating the attraction between silica glass collector surfaces and spheres of carboxylated polystyrene, which mimic bacteria in size, shape and surface charge.

“Using atomic force microscopy allows you to measure the forces between microbes and surfaces; values that had only been theoretically calculated,” she adds. “Just like there is a gravitational force between the earth and the moon, there is electrostatic or capillary attraction between particles.”

Referring back to the basketball-diving board contraption, Cail's advisor, Michael Hochella, a professor of geochemistry, explains that precise measurements with the microscope are extrapolated when the ball (the sphere, in this case) is moved toward and apart from a given surface.

“Just before it touches, you measure the attraction or repulsion it has to the surface with the microscope,” he adds. “There's also a third force, adhesion, that can be measured, and that is how great a force is exerted when the sphere is pulled away from the surface.”

These measurable forces will unlock the secrets of nanotechnology, which Hochella says are crucial as hosts of industries embrace a new sub-micron focus of gigantic potential.

“We have to understand the physical, chemical and mechanical properties, and characteristics of nanotechnology,” he adds.

Cail's research, Hochella says, will pave the way for that, saying she has been able to take measurements that describe the movements of colloids, pinpoint pathogenic or toxic bacteria within those particle clusters, and determine how contamination can be removed.

“Traci is applying this to the transfer of colloids and bacteria through porous media. That could be a ground water aquifer or the movement of particles and microorganism through a HEPA filter,” he adds.

Cail, however, insists that she is merely elaborating on prior studies: “The research group I come from studies how microbes attach to surfaces, but sticking efficiency has not been measured experimentally before using the atomic force microscope. This is another improvement, an advance in technology, and the results look good.”

Cail, whose research was funded through a grant from the nanoscience technology program of the National Science Foundation, is not the first to use an atomic force microscope to measure nano-Newtons of force.

Former Virginia Tech student Stephen Lower, an assistant professor at the University of Maryland, was the first to use the atomic microscope to measure the forces between bacterial cells. Treavor Kendall, another doctoral candidate, is using the device to calculate the forces bacteria generates to grab and consume molecules of iron.

“Whether you are doing a measurement with an entire intact bacterial cell or within a molecule that is produced by the bacteria, you can do measurements on the affinity,” says Hochella.

“When you bring surfaces together,” he adds, “there are reasons why they are interested in getting together, and that's why these very fundamental measurements will have an impact on how future contamination control technology is developed.”


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