US scientists say they have invented a tiny water-purifying device, which is about the size of a person’s fingertip and harnesses light to kill harmful bacteria. In laboratory experiments, the ultra thin 1cm-by-2cm miniature tablet-like device, which was placed in a water container, killed 99.9 percent of bacteria in just 20 minutes, according to a study published last month in the Nature Nanotechnology journal by a team from Stanford University in California. “Researchers have always…