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Dispersing small, bimetallic nanoparticles For applications of nanoparticles in sensing and catalysis, smaller nanoparticles are often more effective because they expose more active surface sites. The properties of metallic nanoparticles can also be improved by creating bimetallic alloys, but typical synthetic methods yield larger nanoparticles where the metals are poorly mixed. Wong et al. show that well-mixed bimetallic ∼1-nm-diameter nanoparticles can be made on silica supports. To do this, they exploited strong electrostatic adsorption, in which the metal precursors are strongly adsorbed onto the surface by controlling pH relative to the surface point of zero charge. Their method was successful for a wide range of metal alloys. Science, this issue p. 1427 A scalable and generalizable synthetic method yields ultrasmall bimetallic nanoparticles that are fully alloyed. Supported nanoparticles containing more than one metal have a variety of applications in sensing, catalysis, and biomedicine. Common synthesis techniques for this type of material often result in large, unalloyed nanoparticles that lack the interactions between the two metals that give the particles their desired characteristics. We demonstrate a relatively simple, effective, generalizable method to produce highly dispersed, well-alloyed bimetallic nanoparticles. Ten permutations of noble and base metals (platinum, palladium, copper, nickel, and cobalt) were synthesized with average particle sizes from 0.9 to 1.4 nanometers, with tight size distributions. High-resolution imaging and x-ray analysis confirmed the homogeneity of alloying in these ultrasmall nanoparticles.