We have studied plasmonic optical tweezers (POT) for nanomaterials such as DNA and polymers. These nanomaterials would be efficiently trapped by a plasmon-enhanced optical force. However, plasmon excitation also leads to a photothermal effect. Such heat generation has frequently hindered POT. Recently, we have developed a novel optical trapping technique; Nano-Structured Semi-Conductor-Assisted (NASSCA) optical tweezers. In NASSCA optical tweezers, we used a metal-free black silicon with a nanoneedles structure on the surface. NASSCA optical tweezers presents a useful and powerful manipulation technique without heat generation.