Sniffing For Sea Mines

In the opening minutes of the 1981 James Bond movie “For Your Eyes Only,” a British spy ship inadvertently hauls in an underwater mine and sinks. In today’s world of war and terrorism, the threat of underwater weapons is all too real. The Navy wants to detect underwater explosives remotely, and thanks to a new study, it may have a way to do it (Anal. Chem., DOI: 10.1021/ac2009788).

According to lead author André Adams, a research chemist at the U.S. Naval Research Laboratory, remotely detecting underwater explosives would be useful in two scenarios: First, when the Navy decommissions a weapons testing area, it must ensure that no unexploded ordnance remains. The Navy also must actively monitor ports and harbors for underwater threats from terrorists. Some trained divers specialize in spotting ordnance, but going in the water is dangerous. Other less risky possibilities, like sonar and magnetometry, can’t distinguish harmless metal junk from explosive hazards.

Adams and his team developed what they call a “high-throughput microfluidic immunosensor” for TNT. The sensor contains 39 parallel 2.5-cm long microfluidic channels, each coated with TNT-targeting antibodies preloaded with a fluorescent TNT analog. The assay works by displacement: As water is pumped through the chip, if TNT is in solution it knocks the fluorescent analog from the antibody, producing a signal in a downstream fluorescence detector.

Designed to maximize interactions between any TNT in solution and surface-bound antibodies, the team’s short, parallel channels improved sensitivity and flow rates, and reduced back-pressure (or flow resistance) problems that had plagued earlier microfluidic designs for undersea sensors, Adams says. A system with high back-pressure requires more power to drive liquid through the device, which is a problem for autonomous, battery-powered sensors.

When the researchers tested their device, they found it could detect as little as 0.01 ng TNT per milliliter of solution at flow rates as high as 6 ml/min. That’s one to two orders of magnitude better sensitivity, Adams says, collected up to 60 times faster than previous incarnations of microfluidic sensors. The device even worked when submerged in seawater.

Kenneth Johnson, a senior scientist at theMonterey Bay Aquarium Research Institute, calls the system’s design “beautiful,” especially in its approach to back-pressure. He says he may try to incorporate its design into environmental sensors he builds for non-explosive contaminants.

But it’s just a first step, Johnson cautions. Deployable sensor systems must be autonomous, with their own detectors, pumps, electronics, and power. “They are a long ways away from running chemical analyses under the ocean,” he says.

Diane Blake of Tulane University School of Medicine, who also develops environmental sensors, calls the device “a step in the right direction.” She adds, “If anyone can do it, this group can.”

In fact, the researchers may already have. Adams says the team has now built a paint-can-sized prototype containing every component but the power systems. They tested the system in the ocean continuously for about a day and are now analyzing the data, he says.