A nanoparticle used in 3D printing is disclosed herein. In an example, the nanoparticle can comprise: at least one metal oxide, which absorbs infrared light in a range of from about 780 nm to about 2300 nm and is shown in formula (1): MmM′On  (1) wherein M is an alkali metal, m is greater than 0 and less than 1, M′ is any metal, and n is greater than 0 and less than or equal to 4; and a bilayer-forming surfactant encapsulating at least a portion of the metal oxide, wherein the nanoparticle has a diameter of from about 0.1 nm to about 500 nm.