Synthesis of amino acid-functionalized BNNSs. Amino acids, arginine (Arg), tryptophan (Trp), cysteine (Cys), aspartic acid (Asp), and Serine (Ser), were selected based on their different Rside chain properties. Different amino acids (1.5 g) were dissolved in deionized (DI) water (15 mL) at 50 °C for 10 min. Then, 0.5 g bulk hBN powder was added to each amino acid solution, followed by 4-h ultrasonication (Thermo Fisher, 40 kHz). All post-ultrasonicated samples were centrifuged at 2,000 rpm for 5 min. The supernatant was then filtered by 0.2 μm nylon membrane, washed with 100 mL DI water, and oven-dried at 60 °C. Different mass ratios of hBN powder:amino acids and different ultrasonication times were also examined for optimization. Finally, BNNSs functionalized by Arg (BNNS-Arg) were selected.