The nano-sub, as its developers call it, is so small (no more than billionths of a metre across, or a tiny fraction of the width of a human hair) it has to be built and monitored by special high-resolution microscopes.

Constructed in the Rice University laboratory of chemist and nano-engineer James Tour, the motor-equipped subs, powered by ultraviolet light, have tail-like propellers that drive them forward.

The motors, comprising little more than a handful of atoms, run at speeds of more than a million revolutions per minute. This enables the tiny submarines to travel at up to 15 metres per second – extraordinarily fast on the molecular scale.

"These are the fastest-moving molecules ever observed in solution and promise to transform the world of nanotechnology," said Professor Tour, whose research is published in the American Chemical Society journal Nano Letters.

While the submersibles cannot yet be steered, their minute molecular motors are powerful enough to drive the subs through solutions of moving molecules of about the same size. "This is akin to a person walking across a basketball court with 1000 people throwing basketballs at him," Professor Tour said.

His group has extensive experience with molecular machines, including single-molecule cars with four wheels, axles and independent suspensions that can be "driven" across a surface.

Professor Tour said many microscopic machines with motors had been created over the years. "But most have either used or generated toxic chemicals," he added.

A motor first developed by a Dutch group proved suitable for the subs, which were produced in a 20-step chemical synthesis.

The motors operate more like a bacterium's lash-like appendage, or "tail", than a propeller and complete each revolution in four steps. When excited by light, the double bond holding the rotor to the body becomes a single bond, allowing it to rotate a quarter step.

As the motor seeks to return to a lower energy state, it jumps adjacent atoms for another quarter turn. The process repeats as long as the light is on.

The scientists also made submersibles with no motors, slow motors and ones that paddled back and forth. All versions have pontoons that fluoresce red when excited by a laser, Professor Tour said. This helped experts at North Carolina State University to measure how well the nano-subs moved.

He said future nano-subs would carry tiny cargoes for a variety of purposes. "We've proven the concept beyond doubt," he explained. "The next step is to find more potential applications."

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