Scientists have known for decades that small clusters of atoms, known as nanoparticles, often behave differently from the same stuff in bulk. Gold nanoparticles, for example, catalyze chemical reactions that larger gold chunks cannot. And semiconductor-based nanoparticles emit different colors of light based solely on tiny changes in their size. So it’s no surprise that chemists come up with myriad ways to synthesize nanoparticles, such as those shown in the image above. However, until now, they haven’t had a way to systematically combine different elements on the nanoscale and test the results. Today, researchers report online in Science that they have done just that, synthesizing a library of nanoparticles made of different combinations of five different metals. The researchers used the ultrasharp tip of an atomic force microscope as a quill, which they dipped into five different liquid, polymer-based inks. Each ink was spiked with metal ions: gold, silver, copper, nickel, or cobalt. After dabbing different combinations of the inks in tiny blobs on a flat surface, the researchers solidified the particles using two separate heat treatments. The first, at low temperature, pulled the metal ions together out of the ink. The second, at high temperature, burned away the ink and bound the metals into solid particles. In the end, the researchers created all 31 different particle combinations of five metals. Now, they are testing those particles for a wide variety of optical, electronic, and catalytic behaviors, in hopes of finding the next big nanotechnology breakthrough.