Boron nitride nanosheets, quantum dots, and dots: Synthesis, properties, and biomedical applications

This review examines three aspects of hexagonal boron nitride (h-BN) nanomaterials: properties, synthesis methods, and biomedical applications. We focus the scope of review on three types of h-BN nanostructures: boron nitride nanosheets (BNNSs, few-layered h-BN, larger than ∼100 nm in lateral dimensions), boron nitride quantum dots (BN QDs, smaller than ∼10 nm in all dimensions, with inherent excitation-dependent fluorescence), and boron nitride dots (BN dots, smaller than ∼10 nm in all dimensions, wide bandgap without noise fluorescence). The synthesis methods of BNNSs, BN QDs, and BN dots are summarized in top-down and bottom-up approaches. Future synthesis research should focus on the scalability and the quality of the products, which are essential for reproducible applications. Regarding biomedical applications, BNNSs were used as nanocarriers for drug delivery, mechanical reinforcements (bone tissue engineering), and antibacterial applications. BN QDs are still limited for non-specific bioimaging applications. BN dots are used for the small dimension to construct high-brightness probes (HBPs) for gene sequence detections inside cells. To differentiate from other two-dimensional materials, future applications should focus on using the unique properties of BN nanostructures, such as piezoelectricity, boron neutron capture therapy (BNCT), and their electrically insulating and optically transparent nature. Examples would be combining BNCT and chemo drug delivery using BNNSs, and using BN dots to form HBPs with enhanced fluorescence by preventing fluorescence quenching using electrically insulating BN dots.