Biodegradable black phosphorus nanosheets have emerged as a promising nanomaterial for near-infrared (NIR)-triggered reactive oxygen species (ROS) generation and hyperthermia, offering advantages in cancer theranostics and other biomedical applications. Their unique properties stem from a tunable bandgap, high photothermal conversion efficiency, and inherent biodegradability, setting them apart from other two-dimensional (2D) materials such as molybdenum disulfide (MoS2). This article examines the thickness-dependent bandgap tuning of black phosphorus (BP), strategies to prevent surface oxidation, and their application in dual-modal imaging and therapy.

The electronic structure of BP nanosheets is highly dependent on their thickness. Monolayer BP exhibits a direct bandgap of approximately 2.0 eV, which decreases as the number of layers increases. Bulk BP possesses a bandgap of around 0.3 eV, making it a semiconductor with strong light-matter interaction in the NIR region. This thickness-dependent bandgap allows precise tuning of optical and electronic properties for specific applications. For instance, few-layer BP nanosheets (3-5 layers) demonstrate optimal NIR absorption, enabling efficient photothermal conversion and ROS generation under laser irradiation. In contrast, MoS2, another widely studied 2D material, has a bandgap ranging from 1.2 eV (bulk) to 1.8 eV (monolayer), limiting its NIR absorption efficiency compared to BP.

A critical challenge in BP nanosheet applications is their susceptibility to surface oxidation under ambient conditions, leading to degradation and loss of functionality. Several strategies have been developed to enhance stability while preserving biodegradability. Surface passivation with polymers such as polyethylene glycol (PEG) or poly(lactic-co-glycolic acid) (PLGA) forms a protective layer that minimizes exposure to oxygen and water. Another approach involves covalent functionalization with aryl diazonium salts, which not only improves stability but also enhances biocompatibility. Additionally, encapsulation within metal-organic frameworks (MOFs) or mesoporous silica coatings has been shown to significantly prolong shelf life without compromising the nanosheets' therapeutic performance. These strategies contrast with MoS2, which exhibits greater environmental stability but lacks the same degree of biodegradability.

BP nanosheets exhibit exceptional photothermal conversion efficiency, often exceeding 30%, making them ideal for hyperthermia-based therapies. Under NIR irradiation, the absorbed light energy is converted into heat, inducing localized temperature increases that can selectively ablate cancer cells. Simultaneously, BP generates ROS, including singlet oxygen and hydroxyl radicals, through photoexcitation and energy transfer to surrounding oxygen molecules. This dual mechanism enhances therapeutic efficacy compared to single-modality approaches. MoS2, while also capable of photothermal therapy, typically requires higher laser intensities to achieve comparable heating effects and lacks the same ROS-generating capability.

In addition to therapeutic applications, BP nanosheets serve as contrast agents for dual-modal imaging. Their strong NIR absorption enables high-resolution photoacoustic imaging, where laser-induced ultrasound waves provide detailed anatomical information. Furthermore, BP exhibits intrinsic fluorescence in the visible to NIR range, allowing fluorescence imaging for real-time tracking. The combination of these imaging modalities improves diagnostic accuracy and treatment monitoring. MoS2, though used in photoacoustic imaging, does not exhibit significant fluorescence, limiting its utility in multimodal imaging platforms.

Biodegradability is a key advantage of BP over many other 2D materials. Upon completion of their function, BP nanosheets degrade into nontoxic phosphates and phosphonates, which are naturally metabolized by the body. This property reduces long-term toxicity concerns and eliminates the need for surgical removal, a significant drawback of non-degradable nanomaterials like gold nanoparticles or carbon nanotubes. MoS2, while less toxic than some alternatives, degrades more slowly and can accumulate in tissues over time.

Comparative studies between BP and MoS2 highlight the former's superior performance in NIR-triggered applications. BP's broader absorption spectrum, higher photothermal efficiency, and ROS generation capability make it more versatile for combined therapy and imaging. However, MoS2 remains relevant in scenarios requiring prolonged stability or where ROS generation is undesirable. The choice between these materials depends on specific application requirements, with BP being particularly advantageous in biodegradable, multifunctional theranostic platforms.

Future research directions include optimizing BP nanosheet synthesis for large-scale production, further improving stability without compromising biodegradability, and exploring combination therapies with chemotherapy or immunotherapy. Advances in surface engineering and functionalization will expand the scope of BP-based nanomaterials in precision medicine.

In summary, biodegradable black phosphorus nanosheets represent a significant advancement in NIR-responsive nanomaterials, offering tunable bandgaps, high photothermal efficiency, and dual-modal imaging capabilities. Their controlled degradation and superior performance compared to MoS2 position them as a leading candidate for next-generation theranostic applications. Continued development of oxidation-resistant coatings and scalable fabrication methods will further enhance their clinical potential.