This letter presents a method for the release and sorting of gold nanoparticles (AuNP) from a soft substrate with an optothermal mechanical (OTM) method. The OTM method relies on the heating of AuNPs on a soft substrate via a continuous-wave laser. An AuNP absorbs the energy from the focused beam and transfers the heat to the substrate beneath it. The induced heat causes a rapid thermal expansion of the substrate, imparting an impulse to the AuNP and releasing it from the substrate. The release of an AuNP from a PMMA substrate of different thicknesses is studied. The release angle is determined by the polymer surface thickness, which affects the printing accuracy. Therefore, the release of an AuNP from a soft substrate of different thicknesses is investigated. The heat that can be generated within an AuNP depends on its size upon absorption. Generally, a smaller AuNP needs higher laser intensity to overcome the Van der Waal force to be released from the substrate. Therefore, the mixed sizes of AuNPs can be selected and sorted by their different release threshold. In this paper, the successful sorting of two types of AuNPs from a substrate to a receiver substrate in air condition is demonstrated. The OTM method paves a new way to separate and purify nanoparticles directly in a gaseous environment.
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