Highly Efficient Metal-Free Two-Dimensional Luminescent Melem Nanosheets for Bioimaging

Two-dimensional (2D) luminescent materials have received tremendous attention for their intrinsic properties and promising practical applications. Achieving 2D luminescent materials with high photoluminescence (PL) efficiency is still a great challenge. Here, ultrathin metal-free 2D luminescent nanosheets of 2,5,8-triamino-tri-s-triazine (melem) are synthesized through a facile liquid exfoliation process assisted by ultrasound. The as-obtained melem nanosheets distribute in the size range from a few nanometers to around 150 nm with a thickness of about 5 to 6 atomic layers. Melem nanosheets exhibit efficient blue emission with a PL efficiency as high as 77.09%, much higher than the heavily explored 2D luminescent g-C₃N₄ nanosheets. The high efficiency of melem nanosheets comes from the absence of atom vacancies and the low carrier mobility. Benefiting from the easy synthesis, good stability, low cell toxicity, and high efficiency, melem nanosheets are successfully applied as bioimaging materials on human breast cancer cells, requiring no extra treatments such as surface coating or functionalization. These metal-free 2D luminescent melem nanosheets hold great potential for various applications including bioimaging and other biorelated applications.