Recent developments in two-dimensional molybdenum disulfide-based multimodal cancer theranostics

Abstract

Recent advancements in cancer research have led to the generation of innovative nanomaterials for improved diagnostic and therapeutic strategies. Despite the proven potential of two-dimensional (2D) molybdenum disulfide (MoS₂) as a versatile platform in biomedical applications, few review articles have focused on MoS₂-based platforms for cancer theranostics. This review aims to fill this gap by providing a comprehensive overview of the latest developments in 2D MoS₂ cancer theranostics and emerging strategies in this field. This review highlights the potential applications of 2D MoS₂ in single-model imaging and therapy, including fluorescence imaging, photoacoustic imaging, photothermal therapy, and catalytic therapy. This review further classifies the potential of 2D MoS₂ in multimodal imaging for diagnostic and synergistic theranostic platforms. In particular, this review underscores the progress of 2D MoS₂ as an integrated drug delivery system, covering a broad spectrum of therapeutic strategies from chemotherapy and gene therapy to immunotherapy and photodynamic therapy. Finally, this review discusses the current challenges and future perspectives in meeting the diverse demands of advanced cancer diagnostic and theranostic applications.

Graphical Abstract

Introduction

In the post-COVID era, the urgency for further advancements in cancer theranostics has become increasingly intense [1, 2]. While conventional therapeutic and diagnostic modalities have benefits, they also have inherent limitations. These limitations include unavoidable adverse reactions due to the nonspecificity of chemotherapy agents, the inability to eliminate micrometastases resulting from the localized nature of surgery and radiation therapy, and the inadequate imaging resolution provided by commonly used contrast agents [3,4,5]. Consequently, these limitations underscore the urgent need for the development of more efficacious theranostic strategies. Notably, emerging nanotechnologies, particularly two-dimensional (2D) materials, have been positioned at the forefront of research in accelerating cancer theranostics [6, 7]. Among the various 2D materials recently explored, 2D transition metal dichalcogenides (TMDs) stand out for their promising advantages. These advantages include (1) their small size, which allows for the enhanced permeation and retention (EPR) effect [8, 9]; (2) their potential for ensuring biocompatibility, physiological stability and safety [10, 11]; and (3) their high specific surface area, which facilitates the easy loading or delivery of functional molecules, imaging or therapeutic agents, thus combining both imaging and therapeutic functionalities [12,13,14].

Among all known TMDs, 2D molybdenum disulfide (MoS₂) has garnered considerable attention in the biomedical field because of its exceptional inherent properties [15, 16]. These properties include photoluminescence (PL) resulting from an indirect bandgap and distinct fluorescence properties; remarkable photothermal properties, such as strong absorption in the near-infrared (NIR) region; an outstanding photothermal conversion rate; enzyme-like catalytic activity; and a potent X-ray attenuation ability attributed to the high atomic number of Mo [17,18,19,20]. Consequently, 2D MoS₂ has been utilized in applications such as optical imaging, computerized tomography (CT), photothermal therapy (PTT), and catalytic therapy. Like other 2D nanomaterials, 2D MoS₂ has an ultrahigh surface-to-volume ratio, thereby acting as a versatile nanoplatform to load various agents for tumor treatment [21]. However, challenges such as limited biocompatibility and a propensity to aggregate in physiological environments have restricted the widespread application of conventional 2D MoS₂ [22, 23]. The growing demand for multifunctionality in biomedical fields has necessitated the optimization of 2D MoS₂. Surface modification strategies, involving the introduction of functional molecules such as biocompatible polyethylene glycol (PEG) and targeted folic acid (FA), have been employed to confer increased physiological stability, enhanced biocompatibility, and improved targeting capabilities to 2D MoS₂ [24, 25]. Furthermore, the incorporation of other nanoparticles can provide additional functionality and expand the range of applications for 2D MoS₂. For instance, the attachment of superparamagnetic nanoparticles can endow 2D MoS₂ with magnetic properties, broadening its application in magnetic resonance imaging (MRI) [26].

While still in its nascent stage, the study of 2D MoS₂ has made notable strides in biomedicine in recent years. Given that a comprehensive review of 2D MoS₂ for synergistic applications in imaging, therapy, and drug delivery remains elusive, this review aims to fill that gap by summarizing relevant advancements. In this context, we summarize the synthetic strategies utilized and elucidate the properties retained following surface modification. Furthermore, we emphasize the potential biomedical applications of these properties, with a particular focus on bioimaging and tumor therapy. Future prospects and challenges are also discussed, aiming to chart a course toward the development of more efficacious 2D MoS₂-based cancer theranostic strategies.

Evaluation of 2D MoS₂ performance in nanomedicine

Unlike early 2D materials such as graphene, which is hampered by high toxicity, black phosphorus that is limited by instability, and hexagonal boron nitride that is limited by its insulating nature and lack of strong NIR absorption, 2D MoS₂ displays minimal toxicity and robust chemical stability [27,28,29]. Its outstanding optical properties, catalytic functionality, and high specific surface area, coupled with the advantages of a small size for drug delivery, show significant potential in cancer theranostics [30]. Additionally, 2D MoS₂ is amenable to scalable, high-quality synthesis, making it a promising candidate for transformation [31]. However, the practical application of 2D MoS₂ in tumor diagnosis and treatment faces considerable challenges. The optimization of synthesis methods is crucial for producing biomedical-grade 2D MoS₂ to increase its clinical scalability. Improvements in therapeutic efficacy and safety are also necessary. These issues can be partially addressed through surface modifications, which can impart greater biocompatibility, physiological stability, and targeting ability to 2D MoS₂ [32]. We evaluate 2D MoS₂ from the perspectives of synthesis, inherent characteristics, and tolerance properties, aiming to align its biomedical specifications with the requirements of cancer theranostics.

Scalability

Different synthetic strategies produce 2D MoS₂ nanomaterials with distinct properties [33]. Currently, various synthesis strategies have been investigated with two main approaches: top-down strategies, including mechanical exfoliation, chemical exfoliation and liquid-phase exfoliation (LPE), and bottom-up strategies, including chemical vapor deposition (CVD) and hydrothermal or solvothermal methods [34,35,36,37]. Top-down strategies involve peeling or etching large-sized MoS₂ into mono- or few-layered nanosheets. In contrast, bottom-up strategies refer to the process of assembling small building blocks (atomic or molecular building blocks) into relatively large nanoscale MoS₂ [38]. In the following sections, we briefly describe these classic synthetic methods while considering their feasibility and limitations from a biomedical perspective.

Physical exfoliation, or mechanical exfoliation, is a typical top-down method used to synthesize 2D MoS₂. In this process, a tape and a substrate are usually required to peel and hold thin films, respectively. Due to the weak van der Waals force between layers, 2D MoS₂ is easily exfoliated from a bulk crystal, adhered to the tape, transferred to and retained on the appropriate substrate [39]. The simplicity of the operation and the cleanness of the obtained 2D MoS₂ inspired physical exfoliation as a promising synthetic strategy, but it has not been popularized because the uncontrollability of the peel size or thickness and the low production yield limit its use in biomedical applications [33, 40].

Chemical exfoliation, achieved through the intercalation of ions or molecules, is another method used to produce 2D MoS₂ [35]. Among various candidates, alkali metals, particularly lithium (Li), are prominent because of their high reduction potential, reactivity, and mobility within the MoS₂ layers [41]. Morrison et al. submerged MoS₂ powder in n-butyllithium to prepare Li ion (Li⁺)-intercalated MoS₂. Following the transfer of the intercalated material into water, the Li in the gaps reacts vigorously with the water to evolve hydrogen, resulting in the adjacent layers separating and dispersing in the water [42]. Although high throughput can be achieved by successive intercalation, the conventional Li intercalation method is time-consuming and requires water-free conditions, which limits its universality [38, 43]. Fortunately, some methods, such as microwave/ultrasonication-assisted intercalation, have accelerated the reaction process and improved efficiency [44]. In addition, the toxicity of residual Li⁺ is a serious concern, especially in biomedical applications [38]. Therefore, several rational non-Li intercalants, such as potassium ions and ammonia/ammonium ions, have been explored for more secure applications [45, 46]. Importantly, the use of some natural polymers and biomacromolecules, such as chitosan (CS) and bovine serum albumin (BSA), as intercalants could improve the dispersity and biocompatibility of 2D MoS₂ in biological systems [47, 48].

LPE proceeds in three stages: bulk MoS₂ slab dispersion in a specific solvent, ultrasonication, and centrifugation [49]. Due to the acoustic cavitation effects, MoS₂ slabs are successfully exfoliated into nanosheets upon ultrasonication. In addition, appropriate solvents can provide shear force and weaken the interactions between layers, which commits to the preparation of 2D MoS₂ dispersions in collaboration with ultrasonication [50]. However, commonly used dispersants such as N-methylpyrrolidone present challenges in removal because of the strong adsorption mediated by the specific surface energy of 2D MoS₂ and the high boiling point of organic solvents [51, 52]. Not only could this strong adsorption lead to flake deposition, but the toxicity of these solvents limits the broad application of 2D MoS₂, especially in the context of biomedicine [50, 52, 53]. Strikingly, the use of easy-to-remove, low-toxicity or even nontoxic surfactants such as deoxyribonucleic acid (DNA)/ribonucleic acid (RNA) nucleotides has partly solved this problem, achieving the safe and efficient production of nanosheets, especially for biomedical purposes [54].

CVD is a typical bottom-up method for the low-cost and large-scale production of MoS₂ monolayers [55]. In this process, the source materials, generally sulfur powder and Mo-based oxide/chloride, are first sublimated such that the precursor reagents are in the gas state. After being transported through the inert carrier gas, the reactant vapor then diffuses to the substrate and adsorbs onto the substrate surface. The adsorbed atoms (Mo and S) subsequently diffuse along the substrate surface and react with each other to form films [56]. Unfortunately, the difficulty in transferring 2D MoS₂ into physiological solutions and its instability under physiological conditions limit its biomedical applications [38, 57].

As one of the simplest synthesis methods, the hydrothermal/solvothermal method has received much attention. This process involves 2D MoS₂ crystallization from an aqueous/organic solution after the hydrothermal reaction between source materials [58]. Generally, the reaction is conducted in a Teflon-lined stainless-steel autoclave at moderate temperature and high pressure with Mo and S precursor solutions as reactants, which are usually S-containing salt/organic and Mo-containing salt/oxide. As the reaction ends and the reaction system cools down, 2D MoS₂ crystallizes [33, 41]. It is important to note that this preparation method may not be universally applicable, as specific adjustments might be required depending on the reactants and desired properties of the final product. Due to its ability to grow high-quality, uniform, pure and even biocompatible 2D MoS₂ materials, the hydrothermal/solvothermal method has been widely adopted and is the choice in the biomedical field [38, 41]. For instance, with ammonium tetrathiomolybdate as the precursor, Wang et al. prepared PEGylated 2D MoS₂ via a solvothermal reaction in a PEG-400 aqueous solution [24]. Notably, PEGylated nanosheets have high colloidal stability and biocompatibility, indicating their excellent application value in biomedicine.

Scalability is a crucial factor when considering practical clinical applications. The synthesis of 2D MoS₂ must be feasible on a large scale while maintaining consistency and quality. Chemical exfoliation offers higher throughput, especially with advancements such as microwave/ultrasonication-assisted intercalation, which improve the efficiency and scalability [35]. LPE is also scalable, particularly when nontoxic surfactants are used to simplify solvent removal and reduce toxicity, making it viable for industrial applications [54]. CVD can produce large-area 2D MoS₂ at low cost and is suitable for industrial applications, although it requires modifications for stability in physiological solutions [55]. The hydrothermal/solvothermal method is noted for its ability to produce high-quality, uniform 2D MoS₂ under moderate conditions, making it cost-effective and highly efficient for large-scale production [41]. Thus, CVD and hydrothermal/solvothermal methods are promising for scalable production for consistent and reproducible theranostic applications, while improved chemical exfoliation and LPE techniques also show potential.

Toxicity and biosafety

To date, toxicity remains a critical obstacle hindering the widespread application of 2D nanomaterials. Even though 2D MoS₂ is less hazardous than some other materials, such as graphene and its analogs, its toxicity cannot be overlooked [59, 60]. To this end, toxicological studies have demonstrated that 2D MoS₂ has mild toxicity, which is primarily linked to increased oxidative stress and physical disruption. Oxidative stress is a major contributor to nanotoxicity, such as organelle damage and cell death [61]. A study of Kupffer cells revealed that the dissolution of nanosheets and the release of hexavalent Mo contribute to mitochondrial reactive oxygen species (ROS) generation, inducing caspase 3/7-mediated apoptosis. This effect is dose dependent, with statistically significant reductions in cell viability observed at concentrations greater than 25 μg/mL [62]. Importantly, even chronic low-dose (1 μg/mL) exposure (7 days) can induce severe toxic effects. HaCaT keratinocytes exhibited a loss of cell membrane integrity, mitochondrial dysfunction, endoplasmic reticulum (ER) disorder, and nuclear damage after chronic exposure. These changes are attributed to the disruption of cell membrane integrity due to high cellular internalization and increased oxidative stress caused by faster electron transfer from MoS₂, which seizes electrons from the mitochondrial membrane, disturbing normal electron transport. The function of cell organelles is subsequently compromised, resulting in a failure to maintain normal metabolic activity [63]. ROS generation induced by chemically exfoliated MoS₂ also mediates DNA cleavage, another key mechanism of nanotoxicity [64]. In addition to oxidative stress induction, 2D MoS₂ can directly cause physical damage to cells and organelles. For example, the internalization of 2D MoS₂ disrupts cell membrane integrity, as mentioned above [63]. The 2D MoS₂ can penetrate the mitochondrial lipid membrane through hydrophobic interactions, causing heterogeneous lipid packing and resulting in damage [65]. In addition to intrinsic toxicity, the distribution state of 2D MoS₂ is also a significant factor influencing its toxicity. Wang et al. showed that aggregated MoS₂ induced strong proinflammatory and profibrogenic responses in vitro and acute lung inflammation in mice, whereas exfoliated MoS₂ had little or no effect [22]. This finding underscores the importance of further modification and functionalization not only to increase the biocompatibility of 2D MoS₂ itself but also to reduce aggregation and increase physiological stability to improve safety.

Given that the inherent toxicity of 2D MoS₂ and damage, such as inflammation caused by nanosheet aggregation, can highlight the concern of wide bioapplication and clinical translation, optimizing the biocompatibility and physiological stability of 2D MoS₂ remains essential [22, 62, 66]. In this respect, many polymers or biomimetic molecules, such as PEG, CS and lipids, have been extensively used to functionalize 2D MoS₂ for increased biosafety. PEG is considered the preferred polymer in drug delivery systems because of its structural flexibility, amphiphilicity, and especially well-established safety profile [67]. Hao et al. prepared MoS₂-PEG by anchoring lipoic acid-conjugated PEG (LA-PEG) to the surface defect sites of 2D MoS₂ via Mo-S bonding [68]. As expected, PEG provided 2D MoS₂ with enhanced biocompatibility. Severe toxicity in vitro and in vivo was not detected even at high concentrations, and further long-term toxicity within a reasonable dose range could be neglected in light of the almost complete clearance of MoS₂-PEG within 30 d. Remarkably, PEGylation also greatly increased the physiological stability, with MoS₂-PEG being fairly stable in diverse physiological solutions, including RPMI-1640 medium, phosphate-buffered saline (PBS), and fetal bovine serum. On the other hand, CS, a natural and abundant biopolymer, is usually used as a biocompatible agent [69]. Yin et al. introduced CS to the surface of 2D MoS₂ via physical interactions to synthesize MoS₂-CS [70]. MoS₂-CS nanosheets are well dispersed in water and other physiological buffers and exhibit greater physiological stability and biocompatibility than MoS₂. Lipids have emerged as excellent candidates for surface coatings because of their ability to improve the solubility of drugs, their biodegradability, and their biocompatibility [71]. Building on this foundation, Xie et al. developed MoS₂-lipid nanocomposites [72]. Lipid modification significantly improved the stability and biocompatibility of 2D MoS₂. Unlike untreated MoS₂, which exhibited severe coagulation in water, PBS, and cell culture media over 48 h, MoS₂-lipid maintained excellent dispersibility and stability in all three environments. Additionally, lipid modification increased the hydrophilicity of MoS₂, resulting in a much lower BSA adsorption rate than that of unmodified MoS₂, thus indicating the enhanced biocompatibility of MoS₂-lipid.

Targeting

As targeting is the functional basis of efficient and safe tumor diagnosis and treatment, achieving precise targeting should always be our goal [73]. Due to the small size of ordinary 2D MoS₂, targeting is achieved primarily through the EPR effect, also referred to as passive targeting [74]. Furthermore, modification with adaptive biological molecules such as PEG can enhance the passive targeting capability of 2D MoS₂. This expectation is reasonable since PEGylation helps reduce reticuloendothelial system uptake, thus improving accumulation in the tumor [75]. However, relying solely on the EPR effect is inadequate in practice, whereas smart and precise targeting is sought. This demand has encouraged the development of active targeting and response targeting methods based on high-specificity targeting ligands and sensitive stimulus-responsive elements, respectively [76, 77]. Currently, targeting ligands, including peptides such as arginine–glycine–aspartate (RGD) and small molecules such as FA and hyaluronic acid (HA), have been widely utilized to decorate 2D MoS₂ [78,79,80]. The corresponding receptors are typically overexpressed on tumor cells instead of healthy cells, thereby presenting tumor specificity and conferring tumor targeting to 2D MoS₂. For example, RGD selectively binds to ανβ3 integrin, which is commonly overexpressed in tumor cells such as HeLa cells. Additionally, the FA receptor is upregulated in various types of epithelial cancers and 90% of ovarian carcinomas and is expressed at low levels in healthy tissues. HA can specifically bind to cluster determinant 44 (CD44), which is overexpressed on various tumor cell surfaces. Guided by these ligands, 2D MoS₂ attaches to tumor cells expressing the corresponding receptors for internalization, enabling improved targeting and bioavailability [81, 82]. In another dynamic targeting strategy, 2D MoS₂ is modified with responsive molecules or chemical bonds such as HA and disulfide bonds (S–S). These modifications respond to internal (pH, enzymes, redox, etc.) or external (light, heat, ultrasound, etc.) stimuli for effective targeting [80,81,82,83]. For example, hyaluronidase and glutathione (GSH), which are abundant in the tumor microenvironment (TME) but absent in normal tissues, can efficiently cleave HA and reduce S–S. This property endows 2D MoS₂ with responsive targeting and the precise release of loaded substances from MoS₂-HA or MoS₂-SS as carriers.

Approaches for single-mode imaging and therapy

The early diagnosis of cancer and subsequent effective therapies are undeniably critical for a good prognosis [84]. Various noninvasive bioimaging techniques, such as MRI, photoacoustic imaging (PAI), and fluorescence imaging (FI), can be employed for early diagnosis [85,86,87]. Through the development of tailored imaging agents, these powerful techniques have demonstrated efficacy in the identification and characterization of diseases in their nascent stages [88]. Due to their unique optical and photothermal properties and additional or enhanced characteristics conferred by loading, 2D MoS₂ and its compounds have displayed immense potential in these imaging modalities as imaging agents. In addition to imaging applications, 2D MoS₂ is also a promising therapeutic agent for cancer treatment, especially as a photothermal agent (PTA), due to its excellent photothermal properties [57]. Furthermore, 2D MoS₂ has been investigated as an outstanding catalyst for therapeutic interventions because of its high surface area, abundant defects, and sulfur vacancies [89, 90]. In this work, we present the advantages of 2D MoS₂ in approaches for single-mode imaging and therapy, including FI, PAI, PTT and catalytic therapy.

2D MoS₂ in fluorescence imaging

FI has attracted considerable interest and has emerged as a rapidly evolving and promising imaging method in the biomedical context [91]. Its attractiveness lies in its high sensitivity, impressive resolution, quick feedback, and substantial safety due to its noninvasive nature and absence of ionizing radiation [92, 93]. These advantageous characteristics have positioned FI as an indispensable tool in the exploration of various biological phenomena. Notably, the unique inherent PL or fluorescence characteristics of 2D MoS₂ make them an optimal choice for FI. Building on these findings, Qi et al. utilized 2D MoS₂ as a fluorescent label for imaging HepG2 cells [94]. Their experiments revealed a crucial finding: numerous 2D MoS₂ molecules with strong fluorescence were internalized into cells. When the cells were irradiated with broad-band excitation light, specifically UV (300–400 nm), blue (400–500 nm), and green (500–600 nm) light, the fluorescently labeled cells appeared blue, green, and red, respectively, in the fluorescence images. The cells exhibited vibrant colors corresponding to the specific wavelength of light used, which allowed easy differentiation. Additionally, they showed clear morphologies in the bright field image, highlighting the effectiveness of 2D MoS₂ as a tool for detailed cellular studies.

Theoretically, coupling these strongly fluorescent 2D MoS₂ with fluorescent dyes may result in a dual fluorescence effect. More importantly, the use of 2D MoS₂ as a carrier enhances the accumulation of fluorescent dyes through the EPR effect, thereby potentially generating greater effectiveness. Unfortunately, the fluorescence emission quenching or decreasing property of 2D MoS₂ renders that phenomenon invalid [95]. Wang et al. solved this problem to some extent through an “intermediary” strategy [96]. In this frontier work, MoS₂-PEG was embedded into mesoporous silica nanoparticles (MSNs), playing the role of an “intermediary”, and then the aggregation-induced emission fluorogen PhENH₂ was chemically grafted onto the MSNs, triggering robust fluorescence emission (Fig. 1A, B). FA was also linked to the surface of the MSNs, leading to the synthesis of PhENH₂-MoS₂-FA MSNs. Interestingly, after an incubation with PhENH₂-MoS₂-FA MSNs, MDA-MB-231 cells, which highly express FA receptors, exhibited potent red fluorescence. This fluorescence intensity notably surpassed that of HepG2 cells, which lack FA receptors, emphasizing the targeting efficacy of these nanoparticles.

Fig. 1

Copyright 2019, Elsevier. Reproduced with permission [102]. Copyright 2020, Elsevier

A Schematic diagram of the preparation of PhENH₂-MoS₂-FA MSNs and their fluorescence effect. B Fluorescence spectra of PhENH₂-MoS₂-FA MSNs dispersed in water. C Schematic diagram of the sensing principle of AgNC@MoS₂ probe. D UV–Vis absorption spectra and synchronous scanning fluorescence spectra of AgNC@MoS₂ probe. Reproduced with permission [96].

In addition to directly targeting tumor cells for imaging, 2D MoS₂ can also be used for molecular detection imaging, allowing researchers the ability to distinguish tumor cells from normal cells. Due to oxidative metabolism disorders, more hydrogen peroxide (H₂O₂) accumulates in tumor cells than in normal cells [97]. These findings indicate that H₂O₂ and H₂O₂-induced oxidative stress could serve as markers for tumor cells. Based on this information, Liu et al. layered 2D MoS₂ with ortho-phenylenediamine (OPD) to quantify the intracellular concentrations of H₂O₂ and thus distinguished cells under pathological conditions [98]. During this procedure, 2D MoS₂, which possesses high peroxidase (POD)-mimicking catalytic activity, decomposed H₂O₂ into hydroxyl radicals (OH·) to oxidize OPD. The resulting OPD oxide then emitted fluorescence with a maximum at ~ 557 nm upon excitation at 450 nm. This light emission characteristic enabled a clear distinction between human foreskin fibroblasts and HeLa cells via intensity feedback (the fluorescent signal was low for the former but higher for the latter) [98, 99].

Although the fluorescence quenching property of 2D MoS₂ is regrettable on direct imaging, as mentioned above, encouragingly, this property is used in the “switching” strategy to provide a higher target-to-background ratio for FI. Briefly, the use of 2D MoS₂ as a quencher maintains the fluorescence in an “off” state in circulation, while the signal turns to an “on” state with the degradation of nanoparticles mediated by some specific molecules and the consequent release of the dye at the target site [100]. Building on this strategy, Jia et al. grafted an adenosine triphosphate (ATP) aptamer (Apt)-labeled photosensitizer, chlorine e6 (Ce6), onto 2D MoS₂ for intracellular ATP imaging. Upon internalization into cancer cells, these compounds accumulate in ATP-rich lysosomes. Here, the intracellular ATP binds to the Apt, resulting in the release of Ce6-Apt and consequently the restoration of Ce6 fluorescence [101]. Furthermore, Xu et al. synthesized a AgNC@MoS₂ probe by assembling fluorescent DNA-silver nanoclusters (DNA-AgNCs) with the ATP Apt as a template and LA-PEG-FA on 2D MoS₂ (Fig. 1C) [102]. This method allows positive targeting imaging and a quantitative analysis of intracellular ATP concentrations (Fig. 1D). In practical terms, when AgNC@MoS₂-PEG-FA is taken up by tumor cells overexpressing the FA receptor, the recognition of ATP by the probe triggers the release of DNA-AgNCs and the recovery of fluorescence. The released DNA-AgNCs can be measured via inductively coupled plasma‒mass spectrometry. This step is crucial, as it provides accurate quantitative information and enables a quantitative analysis of intracellular ATP concentrations. In summary, 2D MoS₂ offers significant potential for direct cellular imaging and molecular monitoring. However, the application of 2D MoS₂ in clinical FI remains elusive primarily due to the need for enhancements in brightness, photostability, and safety, necessitating further effectiveness and biocompatibility modifications.

2D MoS₂ in photoacoustic imaging

As an emerging imaging modality, PAI has attracted much attention because of its excellent superiority with respect to higher resolution than traditional MRI, deeper penetration than optical imaging, and lack of radiation exposure [103, 104]. When exposed to pulsed light, instant thermoelastic expansion occurs in the photoabsorption tissue and creates an ultrasound wave, which is subsequently detected by an ultrasonic transducer and converted into an image [105]. Benefiting from the pronounced light absorbance, high photothermal conversion rate, ability to produce ultrasonic waves and exceptional resolution improved by the generation of air microbubbles, 2D MoS₂ is believed to be a promising contrast agent for PAI [12, 106]. In this context, Chen et al. prepared single-layer 2D MoS₂ nanosheets and explored their potential for augmenting photoacoustic (PA) signals [107]. The nanosheets were efficiently internalized into glioma cells. This internalization resulted in strong PA signals with an impressively sensitive detection limit of approximately 100 cells. Encouraging outcomes were also observed in subcutaneous and orthotopic tumor-bearing mice. The tumor tissue nearly 1.5 mm below the skull was clearly visualized. Nevertheless, relying solely on simple 2D MoS₂ when encountering gliomas at even deeper sites is insufficient, necessitating the utilization of more advanced contrast agents. For this reason, Liu et al. introduced indocyanine green (ICG), a dye with a high NIR extinction coefficient, onto the surface of 2D MoS₂, obtaining MoS₂-ICG with higher sensitivity [108, 109]. This adjustment provided clear benefits. The absorption peak (800 nm) of MoS₂-ICG was significantly greater than that of 2D MoS₂ (675 nm). This shift allowed PAI to be conducted at longer wavelengths, which could lower the background noise and increase the penetration depth. Remarkably, the PA signal observed upon 800 nm laser excitation of MoS₂-ICG was approximately 16-fold greater than that of MoS₂. This enhancement exhibited a concentration-dependent profile and was coupled with outstanding photostability. In practice, this enhancement translated to an impressive imaging depth of up to 3.5 mm when MoS₂-ICG was used for in vivo PAI of orthotopic gliomas in the brain. This depth was almost twice that achieved using 2D MoS₂, suggesting the significant potential of MoS₂-ICG in PAI applications. In summary, the advancements obtained in PAI applications using 2D MoS₂ and MoS₂-ICG highlight the critical role of contrast agents in enhancing the capabilities of PAI. While 2D MoS₂ shows remarkable efficacy in visualizing tumor tissues near the surface, its limitations at greater depths necessitate the exploration of modified agents such as MoS₂-ICG. This finding underscores the importance of continuous optimization to meet demands for higher-resolution imaging, particularly in deep tissue imaging.

2D MoS₂ in photothermal therapy

PTT is a technique that utilizes the principle of converting absorbed light into heat, thereby inducing thermal damage in the tumor [110]. In this process, PTAs possess excellent photothermal properties and safety and are critical factors for achieving high therapeutic efficacy [111]. Among various PTAs, 2D MoS₂ stands out because of its admirable optical absorption, photothermal conversion efficiency, and biocompatibility.

In a recent study, Chen et al. synthesized PEGylated MoS₂ nanosheets (MoS₂-PPEG) with poly(acrylic acid) (PAA), resulting in the ability of the obtained nanosheets to degrade (Fig. 2A) [112]. Notably, the MoS₂-PPEG demonstrated exceptional stability and photothermal properties. For example, upon 300 s of NIR laser irradiation, the temperature of the MoS₂-PPEG dispersion at a concentration of 200 µg/mL increased from 25.8 to 51.4 °C, whereas the corresponding temperature change for water was only 1.3 °C. In subsequent in vivo investigations, the temperature of MoS₂-PPEG reached 52.1 °C, whereas the temperature of PBS increased approximately 1 °C. Intriguingly, while the tumor volume of the mice treated with MoS₂-PPEG or NIR laser irradiation alone continuously increased throughout the detection period, PTT with MoS₂-PPEG dramatically suppressed tumor growth (Fig. 2B). Moreover, the body weights of the mice remained stable throughout the treatment period, suggesting a high tolerance to PTT (Fig. 2C).

Fig. 2

Copyright 2017, American Chemical Society. Reproduced with permission [114]. Copyright 2021, Elsevier

A Schematic diagram of MoS₂-PPEG preparation and in vivo PTT. Relative tumor volume (B) and body weight (C) of mice under different treatments. D Schematic diagram of precise PTT with APT modified 2D MoS₂. E Relative tumor volume of mice under different treatments. Reproduced with permission [112].

Enhanced targeting modifications have been engineered to ensure high specificity in recognizing and efficiently targeting tumor cells. Nucleic acid Apts, which are single-stranded oligonucleotides, exhibit excellent specificity and affinity for their target molecules [113]. In related work, Pang et al. modified 2D MoS₂ with amino-nucleic Apt and BSA, creating MoS₂-BSA-Apt noted for its precise targeting ability and favorable biocompatibility (Fig. 2D) [114]. When exposed to 808 nm NIR laser irradiation, these nanosheets demonstrated a potent ability to ablate tumors. Remarkably, following the injection of MoS₂-BSA-Apt and subsequent 808 nm laser irradiation for 10 min, the breast tumors in the mice were entirely eliminated after 14 days of PTT. This finding contrasted sharply with the outcomes observed in mice treated with 2D MoS₂ and laser irradiation alone, which failed to effectively inhibit the growth of tumors (Fig. 2E). Furthermore, Liu et al. prepared CD44-targeted 2D MoS₂, referred to as the MoS₂@protein A@CD44 antibody (Ab) [115]. This complex was achieved by coating anti-CD44 Ab on the 2D MoS₂ surface through the connection of a layer of Protein A mixed with bovine BSA. Upon cellular uptake, CD44-targeted MoS₂ led to the depletion of membrane CD44 in lysosomal degradative compartments, resulting in a decrease in RhoA activation (GTP-RhoA) and its downstream epithelial‒mesenchymal transition (EMT)-related transcription factors, thereby weakening tumor progression. This result was also expected, as CD44 is known to induce the EMT, a process during which tumor cells lose their original epithelial phenotype and acquire a mesenchymal phenotype characterized by increased stemness, chemoresistance, and invasiveness [116]. Moreover, PTT utilizing MoS₂@CD44Ab induced an increase in the intracellular temperature, which disrupted the activation of these signals and further hampered the biological behavior of the tumor.

Enhanced photothermal therapeutic efficacy is anticipated with the combination of additional PTAs and 2D MoS₂, with the potential of heterojunction architectures to enhance the efficient separation and migration of photoexcited charges [117]. Carbon dots (CDs) have been extensively investigated due to their advantages, such as their excellent safety profile and tunable photochemical properties [118]. Despite the lower photothermal efficiency of CDs, their heterojunctions combined with 2D MoS₂ have exhibited a markedly enhanced photothermal effect compared with that of single 2D MoS₂. Expanding on this concept, Geng et al. reported 0D/2D/0D sandwich heterojunctions, such as NIR-CD/MoS₂, with 0D-N-doped CDs and 2D MoS₂ nanosheets [119]. The results revealed that the absorbance and photothermal conversion efficiency were remarkably greater than those of the pristine NIR-CDs and MoS₂. This enhanced performance was also evident in in vivo studies, where the temperature of the tumors in the NIR-CD/MoS₂ group rapidly increased upon exposure to 808 nm laser irradiation, resulting in complete inhibition of 4T1 tumor growth and no occurrence of pulmonary metastatic nodules at an ultralow power density. Conversely, the tumors in the saline, single NIR-CD/MoS₂, and saline with laser irradiation groups rapidly increased in size within 18 days, and a considerable number of metastatic nodules was observed. Collectively, these examples exemplify the satisfactory efficacy of 2D MoS₂ and its nanocomposites in PTT. Each study contributes uniquely to the field, either through advancements in biocompatibility modifications, targeting strategies, or the development of new heterojunction architectures. Future research should continue to explore additional optimizations, further improving the therapeutic outcomes of PTT.

2D MoS₂ in catalytic therapy

Nanocatalytic therapy has recently emerged as a promising alternative for tumor treatment, leveraging nanozymes to induce catalytic reactions in vivo that generate abundant ROS. These ROS have the potential to significantly impair and even annihilate malignant cells [120]. Among these nanozymes, 2D MoS₂, which functions similarly to POD, offers a stable and cost-effective platform for catalytic activity with outstanding and adjustable catalytic activity in vivo [121, 122].

Recently, Wang et al. highlighted an innovative nanocatalyst, BTO/MoS₂@CA, which responds to the acidic conditions of the TME and is activated by ultrasound (Fig. 3A) [123]. This nanocatalyst is constructed from few-layer MoS₂ nanosheets grown on the surface of piezoelectric tetragonal barium titanate (T-BTO) and is further modified with pH-responsive cinnamaldehyde (CA). In the acidic TME, CA initiates the breakdown of endogenous molecules to release substantial quantities of H₂O₂. The generated H₂O₂ is subsequently transformed into OH· through a series of reactions catalyzed by POD-like BTO/MoS₂ (Fig. 3B). The effectiveness of this reaction is influenced by the ultrasound-induced microscopic pressure, which segregates the positive and negative charges generated by BTO, thereby increasing the interaction between MoS₂ and H₂O₂ and leading to continuous OH· production (Fig. 3C). Furthermore, MoS₂ contributes to the depletion of GSH in cancer cells, intensifying oxidative stress. This stress reduces the levels of GSH-POD-4, disrupting the redox balance and inducing tumor cell ferroptosis (Fig. 3D). The in vivo studies underscore the superior antitumor efficacy of BTO/MoS₂@CA under ultrasound. Compared with the control groups treated with PBS and CA + ultrasound, the BTO/MoS₂@CA + ultrasound group exhibited significant tumor growth inhibition, characterized by pronounced tumor necrosis, hemorrhage, and infiltration of inflammatory cells.

Fig. 3

Copyright 2023, Wiley

A Schematic diagram of BTO/MoS₂@CA catalytic activity for tumor treatment. B Reaction scheme for H₂O₂ decomposition and TMB oxidation. C Schematic illustration of the surface positive and negative charges distribution of BTO driven by ultrasound. D Schematic diagram of ferroptosis mediated by BTO/MoS₂@CA. Reproduced with permission [123].

The high specific surface area and abundant surface active sites of 2D MoS₂ facilitate its role as a cocatalyst support for single-atom catalysis, increasing its potential in cancer catalytic therapy [124]. In this context, Yang et al. developed a 2D composite nanocatalyst, termed MoS₂@SA-Fe-PEG (MSFP), by integrating atomically dispersed iron (Fe) onto a MoS₂ support and modifying the surface with amine-polyethylene glycol (NH₂-PEG) [125]. This configuration harnesses the Fenton reaction, a well-known Fe-mediated redox process, to decompose H₂O₂ into OH· at the atomic level [126, 127]. The doped Fe atoms catalyze Fenton reactions, while the abundant sulfur vacancy defects generated on both the flat surface and edge of the nanosheet after Fe doping increase the surface electron density and thereby favor electron capture by H₂O₂ to promote OH· production. Importantly, the MoS₂ support serves as a cocatalyst, enhancing the reduction of Fe³⁺ to Fe²⁺ through its Mo⁴⁺ sites, thereby facilitating the Fenton process efficiently. Comparative studies underscore the superior catalytic anticancer capabilities of MSFP to traditional nanocatalysts. In contrast to the control group (Fe²⁺ or MoS₂-PEG treatment), which exhibited significant tumor proliferation, the MSFP-treated group displayed distinct tumor suppression and prolonged lifetimes [125]. These studies illustrate significant advancements in the field of nanocatalytic therapy using 2D MoS₂-based nanozymes. As an emerging field, this topic warrants further in-depth research, including enhancing the catalytic efficiency, selectivity, specificity, and biocompatibility of these nanozymes. Future research should focus on the interplay between their intrinsic structures and external environments to optimize these systems and improve their safety and efficacy in clinical settings [128].

Synergistic strategies in imaging and therapy

Despite recent achievements in cancer diagnosis and treatment, single imaging modalities and therapies often lack the required diagnostic precision and therapeutic efficacy, indicating a strong desire to realize multimodal imaging (MMI), combination therapy, and even highly integrated platforms [129,130,131]. In particular, due to its large surface area and fascinating photochemical properties, 2D MoS₂ was developed as a promising platform for MMI and cancer treatment.

2D MoS₂ in multimodal imaging

Importantly, limitations remain for each single imaging modality that must be improved, such as the restricted tissue penetration of optical techniques, the radioactivity of CT, and the low sensitivity of MRI [132]. Fortunately, MMI, which is founded on the aforementioned modalities, takes the best out of the worst, whereby the integration of unique strengths and the remedy for each other's shortcomings provide novel insights toward improving the imaging capability [130]. In this regard, as a promising contrast agent, 2D MoS₂ is adaptable to multiple imaging methods simultaneously, leading to more possibilities for the advancement of MMI.

Indeed, numerous MMI methods that combine two or more imaging techniques, such as FI, CT, MRI and PAI, have been extensively investigated in recent years [130, 133]. Among these methods, the dual imaging technique based on 2D MoS₂ has tremendous potential, considering the diverse combinations of intrinsic properties, such as excellent fluorescence properties and photothermal properties, and has emerged as a representative approach in MMI [134, 135]. For example, the dual-modality platform of MoS₂-HA can serve as a fluorescer and PA contrast agent with both fluorescence and PAI capabilities (Fig. 4A) [136]. Subsequent in vivo bioimaging experiments revealed that the fluorescence signal from MoS₂-HA was markedly observable in the primary tumor (Fig. 4B). In terms of the PA signals, MoS₂-HA demonstrated an astounding 1860-fold increase on average compared with that of PBS, far exceeding the several 100-fold enhancements achieved by traditional PA agents (Fig. 4C) [136, 137].

Fig. 4

Copyright 2018, Wiley. Reproduced with permission [143]. Copyright 2022, Wiley

A Schematic diagram of theranostic HA-MoS₂ conjugates. B In vivo fluorescence imaging of HA-MoS₂ conjugates. C The PA amplitude enhancement of HA-MoS₂ conjugates compared to the control (PBS). D Schematic diagram of hydrogel loading MoS₂ nanosheets preparation and synergistic PTT/PDT for cancer. Tumor photographs (E), relative tumor volume (F) and body weight (G) of mice under different treatments. Reproduced with permission [136].

In addition, by leveraging its strong fluorescence quenching ability and broad absorption in the NIR region, 2D MoS₂ has emerged as a versatile candidate for FI quenchers and PAI contrast agents, enabling multimodal molecular detection and imaging. Li et al. introduced an activatable nanoprobe (MoS₂@polydopamine (PDA)-PEG/peptide, MPPF) engineered for dual-mode near-infrared fluorescence (NIRF)/ratiometric PAI to detect endogenous furin activity [138]. The 2D MoS₂ undergoes a coating process with PDA and PEG, after which it is covalently functionalized with Cy7-labeled furin substrate peptides. The enzymatic cleavage of these peptides by furin triggers the release of Cy7 molecules from the MPPF nanoprobes, thereby restoring their fluorescence. Concurrently, this process disrupts the fluorescence resonance energy transfer from Cy7 to MoS₂ and facilitates the efficient removal of small Cy7 molecules from tumor tissues, resulting in a prompt decrease in the PA signal at 768 nm (PA768). In contrast, the PA signal at 900 nm (PA900), attributed to 2D MoS₂, decreases gradually due to its substantial size and slow tumor clearance. This coordinated alteration in NIRF and ratiometric PA signals thus represents a novel method for the real-time visualization of endogenous furin activity. These examples underscore the critical role of 2D MoS₂ as a MMI platform in advancing the precision of biomedical imaging and overcoming the limitations of traditional imaging modalities. These advancements are pivotal for the progression of personalized medicine, ensuring targeted and effective treatments.

2D MoS₂ in combination therapy

The need for increased antitumor efficacy has led to a gradual shift from monotherapy to multimodal therapy. This trend further underscores the need to develop a complementary strategy aimed at achieving synergistic therapy with the desired efficacy [139]. In this context, we discuss a synergistic PTT/photodynamic therapy (PDT) strategy as an example. Importantly, drug delivery for combination therapy is not discussed here but will be addressed in the next section.

PDT is capable of generating ROS to kill cancer cells, especially in deep tissues [140]. This capacity significantly offsets the limited effectiveness of PTT in these regions. Conversely, PTT has been proven to improve tumor oxygenation, a crucial advantage in countering the tumor hypoxia commonly observed after prolonged PDT [141, 142]. Evidently, the integration of these therapies presents synergistic potential that exceeds the capabilities of each treatment. In a recent study, Qi et al. investigated the application of 2D MoS₂ in a hydrogel context [143]. This study developed an injectable polysaccharide hydrogel loaded with BSA-MoS₂ nanosheets for synergistic PTT/PDT in breast cancer (Fig. 4D). The hydrogel enables the sustained presence of BSA-MoS₂ around tumor cells for prolonged therapeutic efficacy. Upon NIR irradiation, BSA-MoS₂ in the hydrogel demonstrates a remarkable ability to annihilate tumor cells via photothermal conversion. Additionally, under 808 nm laser irradiation, a large amount of ROS is generated within cells, augmenting the efficacy of the PDT modality in tumor management. Highlighting the synergistic impact of PTT/PDT, the weights of the tumors treated with the BSA-MoS₂ hydrogel and laser were significantly lower than those of the PBS group and even the BSA-MoS₂ with laser group (Fig. 4E-G). These findings underscore the significant potential and importance of 2D MoS₂ in multimodal treatment strategies. Such approaches could not only reduce the intensity of each treatment modality, thereby mitigating adverse reactions, but also significantly enhance the therapeutic efficacy compared with single-modality therapies. This finding supports a shift toward more personalized medical approaches.

Synergistic strategies in cancer-targeting drug delivery

The high specific surface area of 2D MoS₂ makes it an excellent drug carrier. Through surface modification, the hydrophilicity, stability, and biocompatibility of 2D MoS₂ can be improved, making it an ideal delivery platform that provides an effective solution for drug insolubility and the degradation of genes and immune reagents [38, 144, 145]. Effective cancer treatment relies not only on the design of therapeutic agents but also on their ability to reach and penetrate tumor tissues efficiently. In this regard, 2D MoS₂ with enhanced stability and biocompatibility allows relatively long circulation times in the bloodstream, increasing the likelihood of reaching tumor sites. The small size of 2D MoS₂ enables it to exploit the EPR effect, allowing preferential accumulation in tumor tissues due to the characteristic leaky vasculature and poor lymphatic drainage of tumors [146]. Within the TME, 2D MoS₂ can be efficiently taken up by tumor cells through endocytosis, followed by the subsequent release of drugs [147]. Moreover, 2D MoS₂ conjugated with targeting ligands can achieve increased tumor specificity and reduce off-target effects [148]. The transport oncophysics of 2D MoS₂ is still insufficiently described. Issues such as the fluid dynamics of 2D MoS₂ and methods to overcome the extracellular matrix barrier require further investigation to improve drug delivery efficiency [149, 150]. Undeniably, 2D MoS₂ has shown promising potential in cancer-targeting drug delivery. In the following paragraphs, we discuss the progress in drug delivery-based combination therapies (Table 1).

Table 1 Functionalized 2D MoS₂ for drug delivery and theranostic

Chemotherapy-based combination therapy

The atomically thin planar structure and extraordinary surface-area-to-volume ratio of 2D MoS₂ makes it an ideal drug carrier for drug delivery applications [38]. In addition, its combination with antitumor agents, such as doxorubicin (DOX), further endows 2D MoS₂ with the ability to treat different malignancies [151]. In view of the photothermal effects and chemotherapeutic potential conferred by drug loading, the combination of these two modalities is anticipated to generate superior therapeutic outcomes.

In a pioneering work, Liu et al. documented the tremendous potential of 2D MoS₂ in drug delivery and cancer combination therapy [79]. Specifically, drug loading ratios of up to ~ 239% for DOX were achieved by electrostatic attraction. This performance is a substantial advancement considering the typical range of 10–30% observed in conventional nanoparticle-based drug delivery systems. Following PEG modification, the resulting MoS₂-PEG-DOX nanosheets exhibited enhanced biocompatibility, leading to a safer antitumor response. However, single injections of MoS₂-PEG-DOX did not yield satisfactory therapeutic outcomes, as evidenced by slightly delayed tumor growth in the absence of NIR laser irradiation. In sharp contrast, the combination of chemotherapy and PTT with MoS₂-PEG/DOX and NIR irradiation led to the dramatic inhibition of tumor growth [19].

Although MoS₂-PEG has shown high biocompatibility and efficacy in chemo-photothermal therapy, the linear polymers represented by PEG could be subject to more rapid clearance from the blood circulation than their branched counterparts, potentially leading to inadequate treatment durability [152]. Wang et al. developed branched hyperbranched polyglycidyl (HPG)-modified MoS₂ (MoS₂-HPG) by absorbing HPG onto the surface of MoS₂ to address this issue, and this molecule displayed excellent dispersion, stability, biocompatibility, and, most importantly, a prolonged in vivo circulation time. Additionally, MoS₂-HPG was utilized as a drug carrier to deliver DOX, producing superior antitumor effects through chemo-photothermal therapy [153]. Based on these findings, MoS₂-HPG was further employed for the codelivery of DOX and chloroquine (CQ). Importantly, MoS₂-HPG exhibited a high degree of loading efficiency for DOX and CQ, with rates of 88.9% and 92.4%, respectively. Furthermore, the release of drugs from the nanosheets was significantly enhanced by laser irradiation, resulting in the efficient eradication of incubated multidrug-resistant HeLa cells [154]. Nonetheless, exogenous substances can ultimately be eliminated by the immune system, underscoring the need for endogenous materials such as erythrocyte (RBC) membranes to prolong the retention time in the blood and enhance therapeutic efficacy [155]. Li et al. designed an RBC membrane-camouflaged MoS₂-based nanosystem for DOX delivery to address this issue [156]. In this system, the RBC membrane contributes to the synergistic effect of chemotherapy and PTT by MoS₂-DOX, enabling the nanoparticles to effectively enter tumor tissue with enhanced hydrophilicity, immune evasion capability and long circulation properties.

Tumor-specific ligands such as FA, peptides and biotin were incorporated onto the surface of 2D MoS₂ to further improve the therapeutic efficacy and minimize unwanted effects. Murugan et al. constructed an FA-based targeting core–shell nanoparticle, MoS₂@ barium titanate (BT)-PDA-FA (MBPF), with BT and PDA providing excellent biocompatibility [157]. This platform was then adapted to carry gemcitabine (Gem), amalgamating chemotherapy with PTT. Through FA receptor-mediated endocytosis, MBPF was internalized, and Gem was released into the cytoplasm. Upon exposure to the NIR laser, heat was generated, simultaneously promoting Gem release and increasing the therapeutic effect. In addition, compared with Gem-loaded MBPF (34.6%), MBPF + NIR (39.8%), and MoS₂@BT (31.8%), breast cancer cells exposed to MBPF + Gem + NIR exhibited increased cytotoxicity (81.3%). In addition, Mo et al. chemically attached the ανβ3 integrin binding peptide RGD to MoS₂-PEG [78]. They further combined it with thiolated DOX (SH-DOX) via a disulfide linkage, leading to the creation of the RGD/MoS₂/DOX nanodrug system, termed MoS₂-PEG-RGD-succinimidyl 3-[2-pyridyldithio] propionate (SPDP)-DOX (MPRS-DOX) (Fig. 5A). This innovative methodology endows MoS₂ with the ability to target ανβ3 integrin and respond to GSH, resulting in an increase in antitumor efficiency (Fig. 5B, C). In vitro experiments revealed that HeLa cells overexpressing ανβ3 integrin exhibited a viability rate of approximately 67% when treated with the MPRS-DOX nanodrug containing 1.25 μg/mL DOX for 48 h. This percentage was notably less than the 98% viability observed in cells treated with MPRS alone. Moreover, when the dose of the MPRS-DOX nanodrug was increased to 10 μg/mL DOX, the viability of the HeLa cells decreased to 22.2%. Impressively, when ανβ3 integrin was combined with 808-nm NIR laser irradiation, the cytotoxic impact on ανβ3 integrin-overexpressing tumor cells intensified, reducing HeLa cell viability to 45% (Fig. 5D).

Fig. 5

Copyright 2022, Elsevier

A Schematic diagram of MoS₂/RGD/DOX nanodrug preparation for targeted delivery, intracellular GSH-triggered DOX release, and synergistic chemo/photothermal antitumor therapy. B Fluorescence microscope images showing the uptake of MPRS-DOX by Hela cells. C Fluorescence microscope images showing Hela cells apoptosis in different treatment groups. D Cell viability of Hela cells under different treatments. Reproduced with permission [78].

Similarly, a MoS₂-based biotin-functionalized nanoplatform was developed for targeted delivery. Due to the heightened demand for biotin for the accelerated growth of cancer cells, biotin receptors (BiRs) are overexpressed in cancer cells, making them important targets for cancer diagnosis and treatment [158, 159]. In light of this information, Liu et al. developed an innovative MoS₂-based nanoplatform, DOX-Biotin-BSA-polyethyleneimine (PEI)-lipoic acid (LA)-MoS₂-LA-PEG (DOX-BBPL-MoS₂-LP), which was enhanced with LA-PEG and BSA to increase its dispersibility and colloidal stability and incorporated biotin to enable the specific targeting of human cervical cancer cells [147]. The engineered nanoplatform generates considerable heat upon NIR light (808 nm) stimulation, not only ablating tumors but also facilitating the targeted release of DOX. This process enables potent synergy between PTT and chemotherapy. In addition, Chen et al. utilized a biotin-enhanced nanodrug delivery system to achieve the codelivery of curcumin (Cur) and erlotinib (Er) [160]. Cur is known to inhibit Er resistance by maintaining Ikappa-B expression and reducing phosphatidylinositol kinase levels in the epidermal growth factor receptor downstream signaling pathway [161, 162]. This action facilitates the release of the apoptotic proteins caspase-3 and caspase-9, thereby promoting the apoptosis of tumor cells and enhancing the effectiveness of chemotherapy [163]. Upon NIR irradiation, MoS₂-PEG-Biotin efficiently converts the absorbed light into heat, enabling the photothermal ablation of cancer cells and consequently enhancing the antitumor efficacy. Research indicates that MoS₂-PEG-Biotin-Cur/Er has a remarkable tumor growth inhibition rate of approximately 95.6% and significantly reduces the tumor volume under NIR irradiation, which is attributed mainly to the dynamic combination of enhanced synergistic chemotherapy and PTT [160]. Notably, 2D MoS₂ has been extensively investigated as a carrier for chemotherapeutic drug delivery. The innovative approaches discussed, including high drug loading capacities, enhancements in biocompatibility, and novel delivery strategies, underscore the potential of 2D MoS₂ to serve as an exemplary carrier. The integration of 2D MoS₂ with various functional modifications demonstrates its versatility as a platform for enhancing the efficacy and safety of chemotherapy and PTT.

Gene therapy-based combination therapy

Small interfering RNA (siRNA)-induced gene therapy has been identified as a promising and innovative approach for cancer treatment, as it can effectively inhibit tumor development by suppressing the expression of specific genes [164, 165]. Nonetheless, the naked form of siRNA is susceptible to degradation by both intracellular and extracellular nucleases, while its negatively charged nature hinders its cellular uptake [166]. The key to successful gene therapy relies heavily on the development of suitable siRNA carriers for efficient siRNA delivery into cells. In this context, the potential use of 2D MoS₂ in siRNA delivery has been explored, given its exceptional ability to deliver chemotherapeutic agents.

Polo-like kinase 1 (PLK1), a well-known oncogene, is a critical regulator of DNA replication [167]. Kou et al. synthesized MoS₂-PEG-PEI, wherein PEI provided a positive charge to bind to and deliver the negatively charged PLK1 siRNA [168]. Following MoS₂-induced siRNA transfection, the efficacy of PLK1 silencing achieved with MoS₂-PEG-PEI/siPLK1 was comparable to that achieved with Lipofectamine 2000 at an N/P ratio of 20. Notably, as the N/P ratio increased, the proportion of apoptotic cells treated with MoS₂-PEG-PEI/siPLK1 increased, indicating the efficacy of MoS₂ as a transfection carrier and the promise of gene therapy. In addition, the synergy of PTT and gene therapy is expected to yield more favorable outcomes. Kong et al. constructed a generation 5 (G5) poly(amidoamine) dendrimer-MoS₂ (G5-MoS₂) platform for the combination B-cell lymphoma-2 (Bcl-2) gene silencing and PTT of tumors (Fig. 6A) [169]. The G5 dendrimers possess both a compact size and positive surface potential. These characteristics are pivotal, as they aid in the delivery of the Bcl-2 siRNA, enabling the downregulation of the Bcl-2 protein in cancer cells and inhibiting their growth. On the other hand, under photothermal conditions, the destruction of cancer cell skeletons through hyperthermia-induced mechanisms can be achieved, resulting in synergistic cancer cell therapy [169, 170]. The evidence of its effectiveness is clear: a marked decrease in cell viability was observed when cells were treated with G5-MoS₂/siRNA polyplexes and laser irradiation, in contrast to cells treated with standalone siRNA with or without laser treatment. In support of these findings, in vivo experiments revealed that while G5-MoS₂/siRNA treatment inhibited tumor growth to some extent, coupling G5-MoS₂/siRNA treatment with laser irradiation not only considerably suppressed growth but also achieved complete tumor elimination. This finding underscores the strong synergy between PTT and gene silencing [169].

Fig. 6

Copyright 2017, American Chemical Society. Reproduced with permission [173]. Copyright 2024, Elsevier

A Schematic diagram of the synthesis of G5-MoS₂ nanosheets and combinational gene silencing and PTT. B Schematic illustration of preparation of HAPM@siPDL1 and HAPM@siPDL1-mediated PD-L1 down-regulation and mPTT. Reproduced with permission [169].

Instead of directly inhibiting or eliminating tumors, as with traditional oncogenes, innovative platforms are being developed to silence immune checkpoint-related genes to enhance antitumor immune responses. Programmed cell death protein 1/programmed death-ligand 1 (PD-1/PD-L1) is a classic immune checkpoint that enables tumor cells to foster an immunosuppressive TME through the overexpression of PD-L1 [171, 172]. In light of these findings, Ye et al. developed a nanoplatform that carries PD-L1 siRNA (siPDL1), resulting in potent antitumor immune reactions [173]. A 6-azidohexyl guanidine-rich α-helical polypeptide and HA were combined with 2D MoS₂ to form HAPM@siPDL1 through complexation with siPDL1 to construct the nanoplatform (Fig. 6B). After targeted accumulation in tumors, HAPM@siPDL1 provoked efficient cytosolic delivery and PD-L1 silencing due to the potent membrane-penetrating capability of the polypeptides, resulting in attenuated immunosuppression. Importantly, the combination with mild PTT, characterized by tumor cell ablation in the low-temperature range (42–46 °C), complements this effect by enhancing immunogenicity through tumor cell apoptosis and antigen release [174]. This synergistic action with siPD-L1 maximized the antitumor immune response, effectively increasing immunosuppression and reinstating immune surveillance. It has also displayed remarkable antitumor efficacy, with an approximately 95% tumor inhibition rate observed in mice treated with HAPM@siPDL1 under NIR irradiation within the 18-day observation period [173]. From delivering siRNAs that target oncogenes such as PLK1 to engaging immune checkpoints such as PD-L1, 2D MoS₂ has versatile potential for both direct cancer treatment and immunomodulation. Building on this foundation, the integration of gene silencing with PTT represents a more effective approach to cancer therapy. Future research should not be confined to siRNAs alone; broader gene therapy strategies, including the use of messenger RNA and antisense oligonucleotides, should be explored to broaden the therapeutic potential [175].

Immunotherapy-based combination therapy

In recent years, immunotherapy has emerged as a powerful clinical strategy for cancer treatment. By employing the immune system to eradicate cancer cells, immunotherapy has demonstrated robust antitumor activity while mitigating metastasis and recurrence [176]. Various agents, such as cytosine–phosphate–guanine (CpG) and anti-PDL1 antibodies (aPDL1), are often used to augment the activation of the immune system against cancer cells [177, 178]. Importantly, combination with delivery nanoplatforms could further increase cancer immunotherapy efficiency and reduce off-target adverse effects by increasing the accumulation within tumor tissues and the internalization of immunotherapeutic components [179].

Currently, 2D MoS₂ is being investigated as a vehicle for immunoreagents. For example, Han et al. fabricated MoS₂-PEG-CpG to deliver the immune adjuvant CpG for photothermal-enhanced immunotherapy [180]. The mammalian immune system recognizes CpG through Toll-like receptor 9. This interaction leads to the release of anticancer cytokines, activates helper T-cell 1-based cellular and humoral effector functions, and subsequently induces potent cytotoxic T lymphocyte (CTL) activity [177, 181, 182]. However, these nucleic acids cannot easily cross the cell membrane due to their negative charges, and they are vulnerable to degradation by nucleases [183, 184]. Therefore, the delivery of CpG via MoS₂ represents a highly desirable solution to this challenge. Importantly, 2D MoS₂ loaded with CpG has been applied to target head and neck squamous cell carcinoma, a cancer type characterized by a highly immunosuppressive TME [185, 186]. Li et al. coated medium-sized and CpG-loaded 2D MoS₂ with low PEI_0.8 k coverage to synthesize CpG@MM-PLs (Fig. 7A) [145]. CpG@MM-PLs effectively promoted dendritic cell (DC) maturation and CTL function, thereby potentially reversing immunosuppression in this challenging context. In vivo, CpG@MM-PL treatment effectively reduced tumor growth (Fig. 7B-D). Notably, when combined with immune checkpoint blockade, CpG@MM-PLs exhibited an increased capacity to retard tumor growth, which was significantly greater than that of single CpG@MM-PL treatment (Fig. 7E-G).

Fig. 7

Copyright 2023, Wiley

A Schematic diagram of CpG@MM-PL preparation and its antitumor therapy. B Schematic diagram and timeline of antitumor treatment with CpG@MM-PL. Tumor growth (C), survival curves (D) of mice with different treatments. E Schematic diagram and timeline of antitumor treatment with CpG@MM-PL combined with PD-1 antibody. Tumor growth (F), survival curves (G) of mice with different treatments. Reproduced with permission [145].

In addition to mere combination therapies, the codelivery of immune enhancers and checkpoint inhibitors has been successfully implemented. Tang et al. pioneered a codelivery platform, MoS₂-aPDL1-V9302, designed for delivering both aPDL1 and V9302, aiming to amplify the anticancer immune response in triple-negative breast cancer cells [187]. The glutamine transporter inhibitor V9302 selectively targets alanine–serine–cysteine transporter 2, effectively obstructing glutamine absorption and thus reversing the nutritional deprivation exerted by cancer cells on immune cells [188]. Importantly, V9302 prompts the strategic repositioning of tumor-infiltrating lymphocytes from the periphery of the tumor to its core [189]. This shift creates a conducive environment for immune checkpoint inhibitors to function, specifically by blocking additional PD-L1. The resulting synergy between V9302 and aPDL1 is evident in their ability to significantly increase lymphocyte proliferation, enhance lymphocyte functionality, and increase the effectiveness of anti-PDL1 therapy. In vivo experiments have shown that the administration of MoS₂-aPDL1-V9302 leads to elevated glutamine levels in the tumor interstitial fluid, which correlates with a substantial increase in activated CTLs, effectively inhibiting the growth of tumors [187]. In conclusion, by facilitating the delivery of immune adjuvants such as CpG and aPDL1, 2D MoS₂ nanoplatforms address significant challenges such as immune suppression within the TME. The use of 2D MoS₂ not only improves the stability and efficacy of the immunoreagents but also synergistically enhances their therapeutic outcomes, particularly when combined with other treatments such as PTT and checkpoint inhibitors, demonstrating promising capabilities in advancing cancer immunotherapy.

Photodynamic therapy-based combination therapy

2D MoS₂ has been leveraged not only for its role in catalyzing the generation of ROS for PDT but also as a carrier for agents to increase PDT efficiency. Xu et al. employed glucose-HPG-functionalized MoS₂ (GPM) for the targeted delivery of the PDT dyes triphenylphosphonium (T) and glibenclamide (G) conjugated with Cy7.5 (Cy7.5-TG@GPM) (Fig. 8A) [190]. This formulation achieves dual tumor- and subcellular-targeted PDT. Specifically, Cy7.5-TG@GPM can target tumor cells expressing high levels of glucose transporter 1, ensuring significant tumor uptake. Upon exposure to NIR, Cy7.5-T and Cy7.5-G are released, which target the mitochondria and ER, respectively. This release triggers the production of ROS that impair mitochondria and provoke ER stress, leading to the death of tumor cells. In detail, ROS-induced ER stress not only initiates the proapoptotic signaling cascade but also cooperatively promotes the release of cytochrome C from the mitochondria, causing tumor cell apoptosis. Moreover, such mitochondrial dysfunction disrupts ATP production, which in turn downregulates the expression of multidrug resistance (MDR)-related P-glycoprotein, thus contributing to overcoming MDR. The in vivo studies revealed remarkable success in inhibiting tumor growth and reversing MDR in multidrug-resistant HeLa cell tumors in nude mice, highlighting its potential for advanced therapeutic strategies (Fig. 8B).

Fig. 8

Copyright 2022, Elsevier. Reproduced with permission [191]. Copyright 2024, Elsevier

A Schematic illustration of mitochondria and ER-targeting synergistic PDT by Cy7.5-TG@GPM under NIR irradiation. B Relative tumor volume of mice under different treatments. C Schematic illustration for the fabrication of MoS₂-TFPy and its synergistic anti-tumor PTT/PDT. Antitumor efficacy in vitro (D, relative cell viability) and in vivo (E, relative tumor volume) under different treatments. Reproduced with permission [190].

In addition to the potent outcomes of 2D MoS₂ in PDT, further investigations have focused on the synergistic application of PDT and PTT to achieve better efficacy. In a pioneering work, Li et al. synthesized a novel nanocomposite, MoS₂-TFPy, by intercalating aggregation-induced emission luminogens (AIEgens) mercapto-PEG-5-(4-(diphenylamino) phenyl) furan-2-pyridine (TFPy-SH) into the layers of MoS₂ nanosheets (Fig. 8C) [191]. This process not only expanded the interlayer spacing of the MoS₂ nanosheets but also induced a crystal phase transformation from 2H to 1 T. As a consequence of this structural transformation, compared with the pristine material, MoS₂-TFPy demonstrated an enhanced capability for photothermal conversion. The incorporation of positively charged AIEgens granted MoS₂-TFPy the ability to accurately target mitochondria and increase the efficiency of ROS generation, establishing it as a highly effective photosensitizer in PDT applications. The therapeutic platform built on MoS₂-TFPy integrated the enhanced PTT effect of MoS₂-TFPy with the PDT efficacy of TFPy-SH, culminating in a synergistic '1 + 1 > 2' effect on tumor therapy. Comparative in vitro and in vivo studies revealed that the antitumor efficacy of individual PTT (MoS₂ + NIR) or PDT (MoS₂ + white light) treatments and simple combinations of PTT and PDT (MoS₂ + TFPy-SH + white light + NIR) were significantly inferior to that of the combined MoS₂-TFPy + white light + NIR treatment, thereby underscoring the superior synergistic antitumor effect (Fig. 8D, E). These studies highlight the significant potential of 2D MoS₂ in PDT and its combination with PTT for enhanced cancer treatment. By serving as a carrier for PDT agents and facilitating their targeted delivery to tumor cells, 2D MoS₂ not only improves the efficiency of ROS generation but also enhances subcellular targeting, contributing to the comprehensive eradication of cancer cells. Furthermore, the synergistic use of MoS₂ in combined PDT and PTT treatments represents a powerful strategy to overcome drug resistance and achieve superior therapeutic outcomes.

Synergistic strategies for precision theranostic applications

The advancement of precision medicine has spurred significant interest in the development of an efficient theranostic platform that integrates imaging and therapeutic functionalities [192, 193]. In this context, 2D MoS₂ offers a promising protocol for the simultaneous application of diagnostic bioimaging and combination therapy based on PTT or drug delivery (Table 1).

The exploration of dual-mode imaging-guided combination therapy has garnered significant attention in the field of cancer treatment. This emerging approach integrates diagnostics and therapeutics, aiming to increase the precision and efficacy of cancer therapy. In a recent study, Lu et al. engineered a versatile nanoplatform, designated ionic liquid (IL)-MoS₂-PEG-block-poly(IL) (PEG-b-PIL)@DOX, for PA/thermal imaging-guided synergistic PTT/chemotherapy (Fig. 9A) [194]. This nanosheet was exfoliated with an antitumor IL and further tailored with PEG-b-PIL to improve its biodegradability, biocompatibility, and physiological stability. Its excellent photothermal properties endow it with exceptional capabilities in PAI, thermal imaging and PTT. In addition, its advantageous surface-area-to-volume ratio facilitates the efficient delivery of DOX for chemotherapy. The results revealed a pronounced PA signal at the tumor location at 12 and 24 h after intravenous administration, with the signal gradually increasing over time (Fig. 9B). Thermal imaging revealed a significant increase in temperature to 55.8 °C under 808 nm laser irradiation for 10 min, which was significantly greater than that observed with the saline injection (37.1 °C). Moreover, the group treated with IL-MoS₂-PEG-b-PIL@DOX exhibited markedly superior tumor growth inhibition compared with the groups treated with IL-MoS₂-PEG-b-PIL and free DOX, underscoring the enhanced efficacy of the combination of chemo-photothermal therapy (Fig. 9C). In a parallel study, Xia et al. exploited the unique properties of MoS₂ to facilitate a PA/thermal imaging-guided combination of PTT and piezocatalytic therapy (PCT) [195]. Benefiting from the photothermal effect, MoS₂-PEG is employed not only as a potent contrast agent for PAI and thermal imaging but also as an efficacious PTA for PTT. Additionally, MoS₂-PEG exhibits a remarkable piezotronic effect, converting mechanical vibration energy into electrical energy under the stimulation of ultrasound-mediated micropressure. This conversion triggers ROS generation for cancer PCT to further kill cancer cells. After an intravenous injection of MoS₂-PEG nanosheets into 4T1 tumor-bearing mice, significant PA signals and an increase in temperature were observed under 1064 nm laser irradiation within just 10 min. Notably, the tumors were almost completely inhibited in the experiments where the tumors were treated with MoS₂-PEG nanosheets combined with NIR and ultrasound.

Fig. 9

Copyright 2023, Wiley. Reproduced with permission [196]. Copyright 2022, Elsevier

A Schematic diagram of IL-MoS₂-PEG-b-PIL preparation and its application in synergistic photothermal/chemotherapy and PAI for cancer. B Quantitative analysis of PA value. C Relative tumor volume of mice after different treatments. D Schematic diagram of 2D MoS₂-based platform facilitating CT/PA/thermal imaging-guided photothermal-selective chemotherapy. CT (E) and PA (F) values of tumors in nude mice after MPT (I) and MPTF (II) intravenous injection. Thermal profiles (G) of tumors in nude mice under NIR irradiation after I and II intravenous injection. H Relative tumor volume of mice under different treatments. Reproduced with permission [194].

Based on the promising results achieved through dual imaging, MMI-guided combination therapy has been further explored to advance the acquisition of precision medicine. In a recent study, Li et al. covalently blended PEGylated α-tocopheryl succinate (α-TOS) and FA on 2D MoS₂ to construct a comprehensive treatment platform, MoS₂-PEG-TOS-FA (MPTF) (Fig. 9D) [196]. Based on excellent CT/PA/thermal imaging capacities conferred by high atomic numbers and photothermal effects, MPTF could be employed to locate ovarian tumors preoperatively using MMI (Fig. 9E-G). The inclusion of FA ensures targeted delivery, meaning that MPTF will predominantly accumulate within tumors, paving the way for more effective treatments [197]. α-TOS can induce tumor cell apoptosis with no toxicity to healthy tissues [198]. Building on these advantages, highly efficient PTT could be activated to completely ablate the entire solid tumor under safe NIR irradiation (Fig. 9H). Finally, locally infiltrating and metastatic cancer cells are killed by α-TOS to prevent recurrence. Notably, the excellent efficacy and safety of synergistic therapy resulted in a 100% survival rate of tumor-bearing mice over 91 days [196]. Furthermore, Hu et al. developed 1-methyl-tryptophan (1-MT)-cisplatin (Pt)-PPDA@MoS₂ complexes, realizing CT/PA/thermal imaging-guided chemo-photothermal immunotherapy [199]. NIR laser irradiation (PTT) combined with Pt (chemotherapy) can destroy tumor cells and effectively induce immunogenic cell death and DC maturation, significantly bolstering T-cell-mediated antitumor immune responses. This effect is further amplified by 1-MT, which blocks the immune checkpoint associated with indoleamine 2,3-dioxygenase-mediated tryptophan metabolism. This blockade interferes with tryptophan metabolism, hindering the development of tumor-regulatory T cells and fostering the activation of T-cell-driven immunotherapy [199,200,201]. Impressively, the use of 1-MT-Pt-PPDA@MoS₂ with laser therapy led to complete tumor eradication in just 8 days, underscoring the unparalleled potency of this trimodal therapeutic approach. In addition, the excellent photothermal properties and X-ray absorption ability of the nanoplatform enabled comprehensive CT/PA/thermal imaging of the tumors. As expected, the CT/PAI signals of the tumors tended to increase with increasing Mo concentrations. The thermal images were also obviously obtained under 808 nm laser irradiation for 300 s, with the temperature in the tumor region increasing to 61 °C, which was much higher than that of the injection of PBS (less than 40 °C) [199].

Based on their inherent properties, loading and delivering various imaging components and therapeutic agents will lead to more possibilities in cancer therapeutics and diagnostics. For example, Liu et al. designed functionalized nanosheets termed MoS₂-HA-diethylenetriaminepentaacetic acid (DTPA)-gadolinium (Gd)/gefitinib (Gef) (Fig. 10A) [202]. These nanosheets were crafted by surface decoration with HA, conjugation with Gd ions via DTPA and the physical incorporation of Gef. The functionalized nanoplatform is capable of achieving the targeted codelivery of Gd-based contrast agents and the anticancer drug Gef, enabling both MRI and synergistic chemo-photothermal therapy. By incorporating HA as a targeting ligand, the nanosheets can efficiently direct the loaded Gd toward cancer cells that overexpress the HA receptor [203]. This targeted approach results in enhanced relaxivity, which is 3.3 times greater than that of commercial contrast agents such as DTPA-Gd, and high-resolution images of the tumor. In terms of therapy, the nanoplatform effectively converts the absorbed NIR light into heat, which not only induces the photothermal ablation of cancer cells but also spurs the release of Gef, laying the foundation for effective synergistic therapy. When measured against either chemotherapy or PTT alone, this combined approach is significantly more effective in inhibiting tumor growth in mice injected with lung cancer cells [202]. Similarly, Cai et al. revealed an integrated nanocomposite, gambogic acid (GA)/MoS₂/BSA-cadmium trioxide (Gd₂O₃)-HA (GMH), where 2D MoS₂ is functionalized with BSA-Gd₂O₃ and further augmented with GA, serving as both a chemotherapeutic and a heat shock protein 90 (HSP90) inhibitor [204, 205]. This composite is designed for MRI-guided combined low-temperature PTT (43–45 °C) and chemotherapy (Fig. 10B). BSA-Gd₂O₃, an MRI contrast agent, provides benefits such as excellent biocompatibility, a straightforward synthesis process, and a high relaxation rate [206]. Impressively, the T1 relaxation rate (r1) of MoS₂/BSA-Gd₂O₃ was 3.5 times higher than that of conventional Gd-DTPA. GA is particularly critical, as it specifically inhibits the overexpression of HSP90 when subjected to elevated temperatures, thereby reducing tumor thermoresistance and facilitating distinct types of apoptosis at relatively low temperatures [205]. Consequently, the integration of GA with PTT enables the killing of tumors at low temperatures without harming adjacent normal organs and effectively provides deep-seated tumors with sufficient heat to minimize recurrence. Consistent with our theoretical predictions, PTT with MoS₂/BSA-Gd₂O₃-HA resulted in only partial inhibition of tumor growth at a relatively low temperature (43 °C), whereas mice treated with GMH exhibited significant tumor inhibition [204, 205]. The aforementioned studies illustrate the advanced utilization of MoS₂ nanoplatforms in dual-mode and MMI-guided combination therapies. These approaches represent significant progress in overcoming MDR, enhancing therapeutic efficacy, and facilitating comprehensive tumor imaging. Innovations in MoS₂-based theranostics are paving the way for more precise and effective treatments, advancing personalized medicine in oncology.

Fig. 10

Copyright 2019, Elsevier. Reproduced with permission [204]. Copyright 2021, Elsevier

A Schematic diagram of HA-grafted MoS₂ as a carrier for co-delivering Gef and Gd-based contrast agents for MRI guided chemo-photothermal therapy. B Schematic illustration of GA/MoS₂/BSA-Gd₂O₃-HA for MRI-guided combined low-temperature PTT and chemotherapy. Reproduced with permission [202].

Challenges and future perspectives

The exploration of 2D MoS₂ as a theranostic platform for cancer treatment exemplifies how the primary objectives of modern oncology align with those of nanomedicine. This alignment emphasizes the development of innovative methods by integrating various treatment modalities and diagnostic techniques. This review highlights the potential of 2D MoS₂ to revolutionize cancer treatment through the development of single-mode theranostic and synergistic platforms that combine diagnostic and targeted therapeutic functionalities, aligning with the precision medicine paradigm. However, the path from the research bench to the patient bedside is laden with challenges that must be meticulously navigated.

One of the most pressing challenges is the ability to control synthesis process of 2D MoS₂ nanocomposites to ensure the uniformity, reproducibility, and scalability necessary for clinical applications [57, 207]. The complexity of the materials requires precise control over the synthesis process, which is crucial for the clinical translation of MoS₂-based theranostics. Moreover, given the unknown potential risks, evaluating the toxicity of nanomaterials is crucial. While the low toxicity of 2D MoS₂ has been preliminarily reported in cellular and murine models, the path to clinical translation requires extensive, systematic investigations [62, 65, 68]. Further assessments at both the cellular and subcellular levels are essential to fully explore the implications of these therapies, including potential side effects on organelle structure and cellular signaling pathways [208]. In murine models, toxicity is often assessed by evaluating the accumulation in organs such as the liver and lungs, as well as weight loss. However, a comprehensive assessment of biotoxicity should encompass additional systems, including the cardiovascular, reproductive, and nervous systems, to provide a holistic understanding of the potential risks [209]. Additionally, more extensive use of larger animal models and human-mimicking systems, such as organoids and microfluidic systems, is necessary to accurately represent patient biology and tumor heterogeneity [210, 211]. In conclusion, a thorough understanding of the long-term toxicity and systemic effects of these nanomaterials is essential for establishing their clinical viability.

The pursuit of increased targeting capabilities is paramount to enhance therapeutic efficacy and minimize side effects. Research on drug delivery strategies utilizing 2D MoS₂ platforms is still in its infancy. After 2D MoS₂ is successfully delivered to tumors by leveraging overexpressed receptors on tumor cell membranes and the EPR effect, utilizing both endogenous and exogenous stimuli to achieve smart regulation within tumors is crucial. Future research should focus on innovative targeting mechanisms that are responsive to biological cues such as ROS, hypoxia, and external stimuli such as ultrasound or electric fields [212]. Furthermore, the range of drugs delivered using 2D MoS₂ remains limited. Considering the rapid advancements in the application of 2D materials in drug delivery, future research on 2D MoS₂ should delve deeper into exploring its potential uses in new domains. This information is pertinent in the delivery of biologics, such as oncolytic viruses, where novel delivery mechanisms may significantly increase therapeutic efficacy [213]. Additionally, the complexity and dynamics of cancer require a deeper understanding of the antitumor effects of 2D MoS₂ nanocomposites and the exploration of potential therapeutic mechanisms.

With increasing focus on the idea of precision medicine, theranostic platforms based on 2D MoS₂ still need to be optimized. In addition to the current research into the therapeutic applications of 2D MoS₂, other treatment strategies that leverage its outstanding physicochemical properties, such as radiotherapy and sonodynamic therapy, are attracting significant interest [214]. Employing a more diversified therapeutic strategy will unlock the prospects and potential for precision medicine. For instance, Cai et al. constructed a MoS₂/BSA-Gd₂O₃ complex, where the inclusion of Gd₂O₃ enables the platform to be well suited for MRI applications [204]. Similarly, other metal elements, such as copper, can also act as functional reagents. This diverse combination of nanoparticles provides substantial scope for the development of innovative MoS₂ composites. Moreover, the heterojunction strategy should also be widely promoted within the 2D MoS₂ platform to meet the demands of precision medicine. For example, Yu et al. used a myeloid-derived suppressor cell membrane to encapsulate Fe₃O₄, thereby enhancing targeting [215]. A neutrophil membrane was used to coat black phosphorus to improve biocompatibility [216]. These strategies exemplify how similar approaches could be beneficially extended to 2D MoS₂ composites. Exploring therapeutic platforms more broadly and in greater depth should be the direction of our future research to fully harness the potential of 2D MoS₂ in medical applications.

Despite the continuously growing list of nanodrugs in patent and clinical trials, the clinical translation of 2D nanodrugs remains largely unexplored [217]. To date, few patents involving 2D MoS₂ have been reported (Table 2), and there have been no related clinical trials. This underscores the need for substantial efforts to advance this field. Achieving scalability, reducing toxicity, and optimizing therapeutic efficacy are crucial steps in the pathway to clinical translation, as mentioned above. Historically, the failure of many nanodrugs during phase II trials has been attributed predominantly to poor efficacy, suggesting that after ensuring biosafety, enhancing therapeutic efficacy is highly important [217]. Beyond the inherent challenges, the clinical translation process for 2D MoS₂ is marked by uncertainty, a common issue for 2D nanodrugs, especially for borderline products combining multiple technologies. Establishing clear regulatory guidelines for the use of 2D nanodrugs in cancer treatment is essential to ensure their successful translation through clinical trials [218, 219].

Table 2 Summary of recent patents on 2D MoS₂-based cancer theranostics

The advancement of 2D MoS₂-based theranostic applications from the laboratory to clinical settings requires a concerted effort involving multidisciplinary research teams and medical practitioners. The concept of 2D MoS₂ as an "all-in-one" theranostic platform is a beacon of innovation in the field of nanomedicine, representing a significant stride toward the goal of precision medicine in cancer treatment, and it is poised to play a pivotal role in the future of oncology.