The use of light in medicine has significantly advanced diagnostics and treatments. Light-matter interactions enable optical imaging and diagnostics, while photoirradiation with photosensitizers facilitates photothermal therapy, photodynamic effects, and other biological processes. Two-dimensional (2D) nanomaterials, like graphene and its analogs (MoS₂, MXenes, black phosphorus, etc.), have shown promise in biomedical applications. Borophene, the lightest 2D material, exhibits unique optical, electronic, and chemical properties due to its polymorphism and anisotropic structure, making it a promising candidate for various applications, including optically transparent electrodes and photoluminescence. However, borophene fabrication remains challenging due to the covalent bonding framework in bulk boron. Existing methods include molecular beam epitaxy (MBE), chemical vapor deposition (CVD), and top-down strategies like thermal oxidation and liquid-phase exfoliation. This research focuses on a novel light-induced tumor theranostic application of borophene fabricated through a top-down acid selective etching method.

The introduction section extensively reviews the literature on light-based medical applications, highlighting advancements in biomedical optics and the use of 2D nanomaterials in this field. It emphasizes the unique properties of borophene compared to other 2D materials and discusses the challenges associated with its fabrication. Existing fabrication methods, including MBE, CVD, and top-down approaches, are reviewed, underscoring the novelty of the acid selective etching method presented in the current study.

Borophene was produced via acid selective etching of AlB₂. HCl etching dissolved Al, leaving borophene, while HF etching dissolved B, yielding Al sheets. This method yielded ultrathin, large borophene nanosheets. The characterization techniques included X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy (EDS) mapping, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM). The photothermal performance was evaluated, and polydopamine (PDA) was used to enhance absorption and biocompatibility, forming B@PDA. In vitro cytotoxicity was assessed using CCK-8 assay in various cancer cell lines. Photothermal killing effect was evaluated using laser irradiation (808 nm). Intracellular tracking of B@PDA (using Cy5 labeling) was performed to investigate the endocytosis process and its pH sensitivity. Reactive oxygen species (ROS) levels were also measured.

The study successfully demonstrated a novel acid selective etching method for producing borophene from AlB₂. HCl etching selectively removes Al, leaving behind borophene, while HF does the opposite. The resulting borophene nanosheets were ultrathin (~4 nm thick) and large (up to ~600 nm lateral size). The borophene exhibited a significant photothermal effect with an extinction coefficient comparable to Ge nanosheets and higher than gold nanorods and antimonene. PDA modification further enhanced the photothermal effect and biocompatibility. B@PDA showed low cytotoxicity and a significant dose-dependent photothermal killing effect in various cancer cell lines. The cellular uptake of B@PDA was enhanced in acidic tumor environments due to the pH-sensitive nature of PDA. The B@PDA was found to target mitochondria.

The findings demonstrate the potential of chemically exfoliated borophene as a safe and effective agent for photothermal cancer therapy. The biodegradability of borophene addresses the long-term toxicity concerns associated with non-degradable nanoparticles like gold. The enhanced cellular uptake in acidic tumor environments improves therapeutic efficacy. The targeting of mitochondria suggests a potential mechanism for ROS-mediated cell death. This work opens new avenues for exploring the diverse applications of borophene in various technological fields.

This study successfully synthesized borophene via a novel acid selective etching method, demonstrating its potential for light-induced tumor theranostics. The biocompatible B@PDA nanoplatform exhibited high photothermal efficiency, low cytotoxicity, and enhanced cellular uptake in acidic tumor microenvironments. Future research should focus on in vivo studies to further evaluate the therapeutic efficacy and safety of this promising material.

The study primarily focused on in vitro experiments. Further in vivo studies are necessary to confirm the therapeutic efficacy and biodistribution of B@PDA. The long-term effects and potential toxicity of borophene degradation products need to be investigated. The study used a limited set of cancer cell lines, and further investigation with a broader range of cell types is needed.