C3N4 nanosheet: sonopiezoelectric effect to boost NO therapy

Abstract

With various physiological effects, nitric oxide (NO) holds significant promise in anticancer. Because their functions are concentration-dependent, the controllable NO supplement and high release capacity are vital to realize the multifaceted capabilities. Here, C₃N₄ nanosheet was prepared as new NO donor, revealing the specific release profile to tumor microenvironment (TME, weak acidity and high expressed H₂O₂). The NO release is initiated with the protonation of N (high electronegativity) and the subsequent oxidation of active -NH_x species. Unlike the organic NO donors, inorganic C₃N₄ nanosheets don't need extra carriers, which is associated with high N atom ratio to grant the great release capacity. Furthermore, the as-synthesized C₃N₄ nanosheet exhibits piezoelectric feature, enabling reactive oxygen species (ROS) generation under ultrasound (US) treatment for sonopiezoelectric therapy (SPT). The sufficient ROS allows for the amplification of NO release triggered by US. The intracellular NO bubbles also enhance the contrast for ultrasonic imaging, guarantying real-time monitoring of the therapeutic effect. Additionally, NO can capture ROS in situ to generate peroxynitrite anion (·ONOO⁻) with stronger oxidability to damage mitochondria and DNA seriously. Without US, C₃N₄ nanosheet still can produce the moderate amount of NO triggered by TME to downregulate immune checkpoints (PD-L1), normalize vascular, and relieve hypoxia, all of which contributes to the overall anticancer efficacy. The synergic therapy (NO + SPT) also could enhance immunogenic cell death (ICD), thereby stimulating anticancer immune response for the inhibition of metastasis and recurrence, efficiently.