A physics preprint from Jiangang Zhu and colleagues at Washington University in St. Louis, Missouri, explains how to count nanoparticles, determine their mass, and measure their radius. Everyone from solar cell researchers to cosmetics manufacturers need metrics on their nanoparticles. But nanodetectors are not yet commonplace. Now, Zhu's team has developed a technique that precludes fluorescence tagging and uses mechanical resonance to detect and determine nanoparticles instead. A tiny resonating toroid on a stalk - an ultra-high-Q microresonator - is all they need for an accurate and reliable detector, the whole system could be built on to a single lab-on-a-chip type device.