Ding-Kun Ji

Cécilia Ménard-Moyon

Alberto Bianco
email: a.bianco@ibmc-cnrs.unistra.fr

Physically-triggered nanosystems based on two-dimensional materials for cancer theranostics

Keywords: 2D Materials, Nanomaterials, Graphene, Therapy, Diagnosis, Theragnosis

There is an increasing demand to develop effective methods for treating malignant diseases to improve healthcare in our society. Stimuli-responsive nanosystems, which can respond to internal or external stimuli are promising in cancer therapy and diagnosis due to their functionality and versatility. As a newly emerging class of nanomaterials, two-dimensional (2D) nanomaterials have attracted huge interest in many different fields including biomedicine due to their unique physical and chemical properties. In the past decade, stimuli-responsive nanosystems based on 2D nanomaterials have been widely studied, showing promising applications in cancer therapy and diagnosis, including phototherapies, magnetic therapy, drug and gene delivery, and non-invasive imaging.

Here, we will focus our attention on the state-of-the-art of physically-triggered nanosystems based on graphene and two-dimensional nanomaterials for cancer therapy and diagnosis. The physical triggers include light, temperature, magnetic and electric fields.

Introduction

Developing simple, effective, and reliable methods to treat diseases is an important way to improve healthcare in our society. The development of new drug delivery methods will have a significant effect on the drug theranostic (combination of therapy and diagnosis) efficacy [START_REF] Santini | A controlled-release microchip[END_REF]. However, conventional drug administration procedures generally require a high dose to achieve therapeutic levels at the pathological sites, leading to various undesired effects, such as toxicity to normal cells and tissues, and often multidrug resistance [START_REF] Mura | Stimuli-responsive nanocarriers for drug delivery[END_REF][START_REF] Van Elk | Nanomedicines for advanced cancer treatments: transitioning towards responsive systems[END_REF]. To improve the theranostic efficiency, various novel drug delivery systems have been successfully explored to enhance drug accumulation at the disease sites as well as to minimize side effects. This is possible with the help of nanotechnology and has confirmed to be effective by some preclinical studies [START_REF] Liu | The smart drug delivery system and its clinical potential[END_REF][START_REF] Ding | Recent advances in stimuli-responsive release function drug delivery systems for tumor treatment[END_REF].

Stimuli-responsive nanosystems are specialized nanosized systems that can adjust to surrounding environments, manage transport of ions and molecules, change wettability and adhesion of different species under stimuli, or convert chemical and biochemical signals into optical, electrical, thermal and mechanical signals, and vice versa [START_REF] Stuart | Emerging applications of stimuli-responsive polymer materials[END_REF]. They provide a new horizon in the development of nanosystems for many biomedical uses. In general, these stimuli can be divided into two modes: i) internal stimuli including temperature, pH, redox potential and enzymes; and ii) external stimuli comprising light, surrounding temperature, electric field, magnetic field, radiofrequency and ultrasounds [START_REF] Van Elk | Nanomedicines for advanced cancer treatments: transitioning towards responsive systems[END_REF][START_REF] Liu | Internal stimuli-responsive nanocarriers for drug delivery: design strategies and applications[END_REF]. Owing to the high local concentration, reduced overall injection dose, and reduced systemic toxicity, these stimuli-responsive nanosystems pave a new way for personalized medicine [START_REF] Yu | Stimuli-responsive cancer therapy based on nanoparticles[END_REF]. Until now, many types of nanomaterials have been used to construct stimuli-responsive nanosystems for therapy and diagnosis, such as polymers [START_REF] Taghizadeh | Classification of stimuli-responsive polymers as anticancer drug delivery systems[END_REF][START_REF] Kamaly | Degradable controlled-release polymers and polymeric nanoparticles: mechanisms of controlling drug release[END_REF], carbon materials [START_REF] Yang | Stimuli responsive drug delivery systems based on nano-graphene for cancer therapy[END_REF][START_REF] Georgakilas | Noncovalent functionalization of graphene and graphene oxide for energy materials, biosensing, catalytic, and biomedical applications[END_REF][START_REF] Augustine | Recent advances in carbon based nanosystems for cancer theranostics[END_REF], and inorganic metal nanoparticles (NPs) [START_REF] Yao | External-stimuli responsive systems for cancer theranostic[END_REF][START_REF] Ahmad | Recent advances in magnetic nanoparticle design for cancer therapy[END_REF].

As a newly emerging class of nanomaterials, 2D nanomaterials (Figure 1) have aroused increasing interest since the discovery of exfoliated graphene in 2004, which led to the Nobel Prize in Physics in 2011 [START_REF] Novoselov | Electric field effect in atomically thin carbon films[END_REF][START_REF] Chimene | Two-dimensional nanomaterials for biomedical applications: emerging trends and future prospects[END_REF][START_REF] Bhimanapati | Recent advances in two-dimensional materials beyond graphene[END_REF]. In addition to graphene nanosheets, other graphene-like 2D materials, such as transition metal dichalcogenides (TMDs), transition metal oxides, graphitic carbon nitride (g-C 3 N 4 ), hexagonal boron nitride (hBN), 2D clays like layered double hydroxides and laponite™, black phosphorus (BP) monolayers, and MXene (i.e. Ti 3 C 2 , Ti 2 C, Ta 4 C 3 , etc.) monolayers, have attracted extensive attention in various fields including nanoelectronic devices, transparent conductors, energy, and catalysis [START_REF] Koski | The new skinny in two-dimensional nanomaterials[END_REF]. 2D nanomaterials are highly diverse in terms of mechanical, chemical, and optical properties as well as size, shape, biocompatibility, and biodegradability. These various characteristics make them interesting nanomaterials for biomedical applications, including drug and gene delivery, bioimaging, biosensing, and cancer therapies [START_REF] Wang | Biological and environmental interactions of emerging two-dimensional nanomaterials[END_REF][START_REF] Kurapati | Biomedical uses for 2D materials beyond graphene: current advances and challenges ahead[END_REF]. 2D nanomaterials possess the highest specific surface area of all known materials. Therefore, they can adsorb various types of molecules including drugs, proteins, nucleic acids, fluorescent probes with high efficiency through covalent or non-covalent interactions and enable superior controlled release by external stimuli (Figure 2) [START_REF] Georgakilas | Noncovalent functionalization of graphene and graphene oxide for energy materials, biosensing, catalytic, and biomedical applications[END_REF][START_REF] Mo | Nucleic acid-functionalized transition metal nanosheets for biosensing applications[END_REF]. Simultaneously, a range of functional NPs, such as iron oxide NPs (IONPs), Au NPs, and inorganic quantum dots, can be adsorbed onto the surface of 2D nanomaterials to endow them with additional characteristics like magnetism, radioactivity, electrochemical properties, etc. In addition, 2D nanomaterials typically have ultrathin structure and can respond to light, which is useful in optical therapies, including photothermal therapy (PTT) and photodynamic therapy (PDT) [START_REF] Li | Recent advances in synthesis and biomedical applications of two-dimensional transition metal dichalcogenide nanosheets[END_REF]. Considering these characteristics, 2D materials were demonstrated to be excellent platforms for theranostic uses [START_REF] Zhou | Material chemistry of graphene oxide-based nanocomposites for theranostic nanomedicine[END_REF]. Recently, a certain number of review articles focusing on the applications of 2D nanomaterials in biosensing, phototherapies, and imaging have appeared [START_REF] He | Photoluminescence architectures for disease diagnosis: from graphene to thin-layer transition metal dichalcogenides and oxides[END_REF][START_REF] Gong | Two-dimensional transition metal dichalcogenide nanomaterials for combination cancer therapy[END_REF][START_REF] Chen | Two-dimensional nanomaterials for cancer nanotheranostics[END_REF][START_REF] Chen | Two-dimensional graphene analogues for biomedical applications[END_REF].

2D nanomaterials

However, 2D nanomaterial research in biomedical field is still at its early stage of maturation. Here, we focus our attention on physically-triggered nanosystems for therapy and diagnosis based on graphene and other 2D nanomaterials. The physical stimuli include light, temperature, magnetic and electric fields (Figure 3). [START_REF] Liu | Iron oxide decorated MoS 2 nanosheets with double PEGylation for chelator-free radiolabeling and multimodal imaging guided photothermal therapy[END_REF]. Both of them copyright 2015 American Chemical Society. This review will assess the state-of-the-art developments of physically-triggered nanosystems for therapy and diagnosis. The design and working mechanisms of each physically-triggered nanosystem will be discussed in dedicated sections. Finally, the challenges and future directions, development and exploitation of physically-triggered nanosystems based on 2D materials will be highlighted.

Graphene-based nanosystems for therapy and diagnosis

Graphene, with its atomically thin, honeycomb lattice that consists of sp 2 -hybridized carbons, exhibits remarkable electronic, thermal, optical, and mechanical properties [START_REF] Novoselov | Electric field effect in atomically thin carbon films[END_REF].

As a novel nanocarbon material, graphene and its derivative GO have received much attention in materials science and biotechnology. Significant progress has been made in the use of graphene in biosensing [START_REF] Tian | Graphene and graphene-like two-denominational materials based fluorescence resonance energy transfer (FRET) assays for biological applications[END_REF][START_REF] Li | Nanomaterial-based activatable imaging probes: from design to biological applications[END_REF], drug delivery [START_REF] Liu | Graphene and graphene oxide as new nanocarriers for drug delivery applications[END_REF][START_REF] Goenka | Graphene-based nanomaterials for drug delivery and tissue engineering[END_REF], PTT/PDT [START_REF] Erol | Recent advances in bioactive 1D and 2D carbon nanomaterials for biomedical applications[END_REF], and imaging [START_REF] Yoo | Graphene-based nanomaterials for versatile imaging studies[END_REF]. Graphene-based materials have been developed as physically-triggered nanosystems (Table 1) for therapy and diagnosis due to the following reasons: 1) their high specific surface area results in π-stacking interactions with the aromatic rings of drugs and photosensitizers (PSs) as well as hydrophobic forces, suggesting that graphene can be used as carrier to release payloads to the target sites; 2) due to the oxygenated functional groups, GO and reduced GO (rGO) can adsorb not only many drugs by electrostatic interactions and hydrogen bonding, but also other molecules to offer additional optical and magnetic properties, which could be useful to provide contrast for in vivo multimodal imaging; 3) owing to high photothermal conversion efficiency, graphene possesses high ability to transfer the absorbed radiation into thermal energy. The resulting temperature change at the target site usually causes structural damages to cells and intracellular proteins leading to cell death.

Light-triggered nanosystems for therapy and diagnosis

Among external stimuli, light is the most flexible and can be used as a safe and useful energy source. Light irradiation is particularly interesting because intensity and wavelength can be easily tuned [START_REF] Erol | Recent advances in bioactive 1D and 2D carbon nanomaterials for biomedical applications[END_REF][START_REF] Karimi | Smart micro/nanoparticles in stimulus-responsive drug/gene delivery systems[END_REF]. UV, visible, or near-infrared (NIR) regions of the light spectrum can be all used to trigger drug or gene release from appropriately designed nanosystems [START_REF] Zhang | Recent progress in light-triggered nanotheranostics for cancer treatment[END_REF][START_REF] Shim | Light-switchable systems for remotely controlled drug delivery[END_REF]. Among them, NIR is the best choice for applications in the biomedical field due to its better transmission through the tissues (penetration in the tissue of about 10 cm). In addition, NIR causes less damage to cells than UV and visible light, because hemoglobin, water, and lipids barely absorb light in this region [START_REF] Yi | UV light stimulated encapsulation and release by polyelectrolyte microcapsules[END_REF]. In the following section, we will selectively discuss light-triggered drug delivery and light-triggered therapies based on PTT and PDT using graphene.

Photothermal therapy using graphene

PTT is a powerful technology, which employs photoabsorbing agents to generate heat from appropriate light irradiation, leading to thermal ablation of cells. Ideal photothermal agents should exhibit strong absorbance in the NIR, which is a transparent window for biological tissues, and efficiently convert the absorbed energy into heat [START_REF] Bai | Graphene: A versatile platform for nanotheranostics and tissue engineering[END_REF]. In addition, the agents used in PTT should be non-toxic and possess high tumor accumulation ability to improve the therapeutic efficacy. To date a range of inorganic and organic photothermal agents including graphene have been developed for PTT in cancer research.

In 2010 Yang et al. reported the first success of using polyethylene glycol (PEG)-modified GO (PEG-GO) for efficient in vivo PTT on mice bearing 4T1 tumor cells [START_REF] Yang | Graphene in mice: ultrahigh in vivo tumor uptake and efficient photothermal therapy[END_REF]. The authors observed that this nanoconjugate (10-50 nm lateral size) possessed a strong NIR absorbance. After intravenous (i.v.) injection, high tumor accumulation of PEG-GO was obtained due to the enhanced permeability and retention (EPR) effect in tumor tissues.

Under NIR laser irradiation (at 808 nm), the surface temperature of the tumors reached ∼50°C, leading to efficient tumor ablation. Following the reduction of GO leading to partial restoration of the aromatic and conjugated character of graphene, it was found that PEGylated rGO exhibited 6-fold higher NIR absorption than PEG-GO. The mass extinction coefficient for PEG-rGO at 808 nm was 24.6 L•g -1 •cm -1 , nearly twice that of gold nanorods [START_REF] Robinson | High performance in vivo near-IR (﹥1 µm) imaging and photothermal cancer therapy with carbon nanotubes[END_REF]. After attaching the targeting Arg-Gly-Asp (RGD) peptide to PEG-rGO, the conjugate was taken up by human glioblastoma U87MG cancer cells with high selectivity.

When exposed to 808 nm irradiation, U87MG cells were completely destroyed within 8 min [START_REF] Robinson | Ultrasmall reduced graphene oxide with high near-infrared absorbance for photothermal therapy[END_REF]. To further improve the photothermal conversion efficiency of graphene, some graphene-based nanomaterials with new structures were designed and fabricated. For example, Akhavan et al. fabricated a 2D rGO nanomesh (rGONM) via photocatalytic degradation of rGO [START_REF] Akhavan | Graphene nanomesh promises extremely efficient in vivo photothermal therapy[END_REF]. The rGONM has abundant uniform pores (~8 nm) on its surface.

After functionalization with PEG, this rGONM exhibited about 4.2-and 22.4-fold higher NIR absorption at 808 nm than PEG-rGO and PEG-GO. Following the modification with the RGD peptide and cyanine 7 (Cy7), this multi-functional rGONM was utilized for in vivo tumor targeting and fluorescence imaging (FI) of U87MG tumors. After intravenous injection of a low concentration (10 µg•mL -1 ) of conjugate and exposure to low laser power (0.1 W•cm -2 ) irradiation, this nanosystem was able to induce a complete tumor ablation.

Photodynamic therapy using graphene

PDT is an optical non-invasive treatment modality that uses appropriate laser light to activate exogenous PSs inside the target cells, resulting in the formation of reactive oxygen species (ROS) such as singlet oxygen ( 1 O 2 ) or free radicals that ultimately lead to cell apoptosis and/or necrosis [START_REF] Shim | Light-switchable systems for remotely controlled drug delivery[END_REF][START_REF] Cheng | Functional nanomaterials for phototherapies of cancer[END_REF]. PDT is especially suitable for epithelial tumors, such as skin or breast tumors because they are easily reached by visible and NIR light.

PDT has obtained regulatory approval for clinical applications in various diseases, like age-related macular degeneration [START_REF] Pegaz | Encapsulation of porphyrins and chlorins in biodegradable nanoparticles: The effect of dye lipophilicity on the extravasation and the photothrombic activity. A comparative study[END_REF], psoriasis [START_REF] Bonnett | Photobleaching of sensitisers used in photodynamic therapy[END_REF], and some oncological diseases [START_REF] Bugaj | Targeted photodynamic therapy -a promising strategy of tumor treatment[END_REF][START_REF] Talreja | Photodynamic therapy for unresectable cholangiocarcinoma: contribution of single operator cholangioscopy for targeted treatment[END_REF]. A wide range of PS molecules, most of which containing porphyrin structures, have been combined with graphene and applied in PDT. In GO-based PDT nanosystems, GO usually plays the role of platform for high PS loading due to its water dispersibility and extremely high specific surface area, allowing to improve accumulation of PSs in tumor site. Moreover, due to its broad absorption spectrum from 200 to 1000 nm, GO can quench the fluorescence and phototoxicity of PSs via energy transfer during the blood circulation (in normal vasculature) [START_REF] Chen | Protease-triggered photosensitizing beacon based on singlet oxygen quenching and activation[END_REF]. The complex remains stable and keeps the phototoxicity of PSs "off" until it is activated by stimuli resulting in desorption of the PS molecules from graphene. GO-based smart nanosystems can remotely control the photoactivity of PSs and have the potential to be ideal carriers of PSs that do not work until they reach the site of action (i.e., tumor sites), leading to minimum damage to healthy tissues and blood cells.

In 2010 Dong et al. designed the first PDT nanosystem containing graphene by loading the photosensitizer zinc phthalocyanine (ZnPc) on the surface of a PEGylated GO via π-stacking and hydrophobic interactions [START_REF] Dong | Poly(ethylene glycol) conjugated nano-graphene oxide for photodynamic therapy[END_REF]. Compared to other photosensitizer carriers such as polymeric micelles and silica NPs, PEGylated GO showed a higher photosensitizer loading efficiency. Under irradiation by a Xe light, this nanosystem led to a lethal effect on human MCF-7 breast cancer cells within 5 min. To increase the accumulation of PSs at the tumor site, Huang et al. designed and prepared folic acid (FA)-conjugated GO as drug delivery system for targeted PDT [START_REF] Huang | Folic acid-conjugated graphene oxide loaded with photosensitizers for targeting photodynamic therapy[END_REF]. In this work, chlorin e6 (Ce6) was loaded on FA-modified GO via hydrophobic interactions and π-stacking. Ce6 was released from GO thanks to acidic pH in lysosomes (pH 4~5). The FA-GO-Ce6 could specifically target human stomach cancer MGC-803 cells causing approximately 90% loss of viability under irradiation at 633 nm, demonstrating a high photodynamic efficacy. Li et al. constructed hyaluronic acid (HA)-GO conjugates with a high loading of Ce6 for PDT [START_REF] Li | Hyaluronic acid-conjugated graphene oxide/photosensitizer nanohybrids for cancer targeted photodynamic therapy[END_REF]. In this work, the activity of Ce6 adsorbed onto HA-GO was mostly switched off in aqueous solution to ensure biocompatibility, but it was quickly recovered after its release from the carrier surface upon cellular uptake. Under irradiation at 670 nm, the PDT efficiency of the HA-GO/Ce6 nanohybrid was 10 times higher than that of free Ce6 in HeLa cells.

Beside these promising results, some challenges still exist in photosensitizer delivery.

Among them, phototoxicity to normal cells is one of the most urgent issue that needs to be solved. Undesired delivery usually makes the photosensitizer to stay in the skin and the eyes several days and even weeks. The ROS induced by the PSs can damage the vascular system causing burns, swelling, pain and other serious side effects [START_REF] Dolmans | Photodynamic therapy for cancer[END_REF]. To reduce these effects, Wei et al. developed a phototoxicity on/off nanosystem by attaching pyropheophorbide-a (PPa) and a monoclonal antibody (mAb) targeting the integrin α v β 3 , on GO surface [START_REF] Wei | A graphene oxide based smart drug delivery system for tumor mitochondria-targeting photodynamic therapy[END_REF]. Due to π-π interactions between PPa and GO, the phototoxicity of PPa was switched off in aqueous solutions. When this nanosystem was in non-aqueous environment, the π-π interactions between PPa and GO were broken leading to recovery of the phototoxicity of PPa. In vitro results demonstrated that this nanosystem displayed low toxicity (off-state) into cytoplasm, but when in contact with mitochondria, it penetrated into them. In this lipid environment, the phototoxicity was turned "on" (on-state) to generate 1 O 2 to kill cancer cells. This targeting system improved the PS pharmacokinetics and ultimately enhanced the treatment outcome with low side effects (Figure 4). [START_REF] Wei | A graphene oxide based smart drug delivery system for tumor mitochondria-targeting photodynamic therapy[END_REF]. Copyright 2016 Royal Society of Chemistry.

Light-triggered nanosystems for drug and gene delivery

Since Dai's group first reported that graphene can be used for drug delivery [START_REF] Liu | PEGylated nanographene oxide for delivery of water-insoluble cancer drugs[END_REF], a range of 'smart' systems based on this material have been designed to increase the release of anticancer drugs and reduce side effects in cancer treatment. Among them, light-triggered drug delivery nanosystems have attracted tremendous interest due to several advantages. These advantages include: 1) the mild photothermal effect (43-45°C) caused by light that has the capacity to increase the permeability of the cell membrane accumulating more nanomaterials in tumor region [START_REF] Zhang | Synergistic effect of chemo-photothermal therapy using PEGylated graphene oxide[END_REF][START_REF] Sherlock | Photothermally enhanced drug delivery by ultrasmall multifunctional FeCo/graphitic shell nanocrystals[END_REF]; 2) the photothermal effect (photoinduced local heat generation) that can change the binding energy between graphene and the drugs, leading to rapid and substantial drug release upon NIR light; and 3) the photothermal effect that has the ability to induce the structural degradation of the carriers or the vesicles leading to enhanced drug release.

In 2012, Kurapati et al. developed NIR-light responsive GO-poly(allylamine hydrochloride) (PAH) capsules and demonstrated the release of the encapsulated anticancer drug doxorubicin (DOX) [START_REF] Kurapati | Near-infrared light-responsive graphene oxide composite multilayer capsules: a novel route for remote controlled drug delivery[END_REF]. These capsules possessed a high and broad absorption (400-1200 nm). Upon irradiation with a NIR laser (at 1064 nm) for 45 s, the capsules were completely ruptured by the heat locally generated by the irradiation. Simultaneously almost 85% of DOX was released from the capsules proving the attractive properties of this system for on-demand drug delivery. In another approach, Kim et al. explored nanosized PEG-branched polyethyleneimine (BPEI)-rGO conjugate by covalently grafting rGO with BPEI and PEG to photothermally trigger cytosolic drug delivery [START_REF] Kim | Photothermally triggered cytosolic drug delivery via endosome disruption using a functionalized reduced graphene oxide[END_REF]. The authors found that PEG-BPEI-rGO could release DOX, previously complexed to this system, to cancer cells under NIR laser irradiation. Moreover, with the help of GO-induced photothermally endosomal disruption and the proton sponge effect of BPEI, PEG-BPEI-rGO could easily escape the endosomes to kill more efficiently cancer cells.

Similarly, Chen et al. developed a functionalized nanosized rGO-PEG with high loading of resveratrol (RV) for remotely NIR controlled drug delivery towards 4T1 breast cancer in vitro and in vivo [START_REF] Chen | One-step reduction and PEGylation of graphene oxide for photothermally controlled drug delivery[END_REF]. The results showed that after cellular uptake, rGO-PEG-RV was encapsulated into the endosomes and then merged with lysosomes, which offered an endogenous acidic medium causing a slow RV release. Under NIR irradiation at 808 nm the photothermal effect of rGO-PEG-RV further promoted a rapid RV release (Figure 5). In the last decade, gene therapy has drawn great attention in both the academic world and industry. This therapeutic modality is expected to be an effective strategy for the treatment of cancers, cardiovascular diseases, viral infections, and other genetic dysfunctions [START_REF] Yin | Functionalized 2D nanomaterials for gene delivery applications[END_REF]. As one of the non-viral carriers, GO has shown the highest gene loading due to the interactions between oligonucleotide fragments and its surface modified with appropriate polymers to complex nucleic acids [START_REF] Feng | Graphene based gene transfection[END_REF]. Similar to the release mechanism of drugs, light irradiation can be also used as stimulus to remotely control the gene release. In 2013, Feng et al. found that the dual-polymer-functionalized GO conjugate (GO-PEG-PEI) showed good gene transfection efficiency [START_REF] Feng | Polyethylene glycol and polyethylenimine dual-functionalized nano-graphene oxide for photothermally enhanced gene delivery[END_REF]. Under a low power NIR laser irradiation, the mild photothermal heating could increase the cell membrane permeability to enhance the cellular uptake of GO-PEG-PEI. Moreover, the GO conjugate could serve as a NIR light controllable siRNA delivery platform to silence polo-like kinase 1 gene in human breast cancer cells (MDA-MB-435s cells). In another work, Kim et al. observed that PEG-BPEI-rGO had a high gene transfection efficiency without observable cytotoxicity in PC-3 and NIH/3T3 cells [START_REF] Kim | Photothermally controlled gene delivery by reduced graphene oxide-polyethylenimine nanocomposite[END_REF]. Moreover, under NIR irradiation, locally induced heat could accelerate endosomal escape of PEG-BPEI-rGO, thus displaying enhanced gene transfection efficiency.

Light-triggered nanosystems for combined therapies

We already discussed that light-triggered nanosystems have remarkable potential in PTT to destruct tumors. However, PTT alone is unlikely to eradicate cancer cells at once because of the restricted phototreated area and depth-dependent heat in the tumor. To date, chemotherapy is still a major therapeutic approach for the treatment of a wide variety of cancers. Unfortunately, chemotherapy suffers from the low tumor uptake of the drugs, unavoidable side effects and multidrug resistance. In this context, combined therapies are able to complement limitations of individual therapies to achieve a better efficacy. Combined with PTT, light-triggered drug delivery has been employed in a wide range of cancer treatments, such as gene-photothermal synergistic therapy [START_REF] Feng | Polyethylene glycol and polyethylenimine dual-functionalized nano-graphene oxide for photothermally enhanced gene delivery[END_REF][START_REF] Kim | Photothermally controlled gene delivery by reduced graphene oxide-polyethylenimine nanocomposite[END_REF] or photodynamic-photothermal therapy [START_REF] Tian | Photothermally enhanced photodynamic therapy delivered by nano-graphene oxide[END_REF], showing a better therapeutic response compared to mono-therapies.

Combined PTT-chemotherapy is an effective method to improve cancer treatment, because PTT can increase the sensitivity of chemotherapy and synergistically improve the therapeutic effects. Zhang et al. found that PEG-GO has a very high DOX loading capacity (142.5 wt%), which is far beyond other common drug carriers (generally below 100 wt%) [START_REF] Zhang | Synergistic effect of chemo-photothermal therapy using PEGylated graphene oxide[END_REF]. Drug release from GO was triggered by both pH changes and NIR laser irradiation. developed a pH-responsive nanocarrier by conjugating nano-GO (NGO) with PEG and positively charged PAH, the latter subsequently modified with 2,3-dimethylmaleic anhydride (DA) [START_REF] Feng | Smart pH-responsive nanocarriers based on nano-graphene oxide for combined chemo-and photothermal therapy overcoming drug resistance[END_REF]. The obtained conjugate was stable and negatively charged under physiological pH (7.4). Once in a slightly acidic pH (e.g. 6.8), it was rapidly converted into positively charged nanocarriers with markedly enhanced cellular uptake. After being loaded with DOX, the obtained NGO-PEG-DA/DOX responded to pH change of tumor microenvironment inducing an accelerated drug release under the cell lysosomal pH (5.0-5.5), especially in drug-resistant MCF-7/ADR cells. Taking advantage of the NIR absorbance of NGO and combination of PTT and chemotherapy, the system offered enhanced cell-killing ability for both wild-type MCF-7/WT cells and drug-resistant cancer MCF-7/ADR cells (Figure 6). Alternatively, Hu et al. developed a protein-graphene-protein (PGP) capsule using rGO and lactoferrin which exhibited precise on-demand drug release under highly controlled NIR irradiation [START_REF] Hu | Photoresponsive protein-graphene-protein hybrid capsules with dual targeted heat-triggered drug delivery approach for enhanced tumor therapy[END_REF]. This system displayed the capacity to effectively avoid the premature release of the cargo before it was induced by NIR light at the target site, therefore avoiding unintended side effects. Combined thermal-chemotherapy in vivo demonstrated that this nanosystem led to glioma tumor growth suppression not only in the phototreated area but also in the widely surrounding tumor cells, with little side effects (Figure 7). Recently, graphene-family nanomaterials have been used to form hybrids with mesoporous silica [START_REF] Wang | Multifunctional mesoporous silica-coated graphene nanosheet used for chemo-photothermal synergistic targeted therapy of glioma[END_REF][START_REF] Tang | An aptamer-targeting photoresponsive drug delivery system using "off-on" graphene oxide wrapped mesoporous silica nanoparticles[END_REF], CuS NPs [START_REF] Bai | Multifunctional PEG-GO/CuS nanocomposites for near-infrared chemo-photothermal therapy[END_REF], Ag NPs [START_REF] Shi | A tumor-targeting near-infrared laser-triggered drug delivery system based on GO@Ag nanoparticles for chemo-photothermal therapy and X-ray imaging[END_REF], and gold nanorods [START_REF] Song | Sequential drug release and enhanced photothermal and photoacoustic effect of hybrid reduced graphene oxide-loaded ultrasmall gold nanorod vesicles for cancer therapy[END_REF], in light-triggered nanosystems for combined therapy in many malignant tumors. The resulting rGO/GO hybrids usually displayed superior properties than rGO/GO alone, resulting in higher drug loading, stronger NIR absorption, and higher photothermal conversion efficiency.

Temperature-triggered nanosystems for therapy

Many diseases including tumors and inflammatory diseases typically lead to increased temperature in lesions (~40-42°C) compared to normal tissue (~37°C) [START_REF] Liu | Internal stimuli-responsive nanocarriers for drug delivery: design strategies and applications[END_REF]75].

Temperature differences between normal and tumor tissue have also been used to diagnose early-stage tumors [START_REF] Yoshida | Relationship between microvessel density and thermographichot areas in breast cancer[END_REF][START_REF] Tepper | Thermographic investigation of tumor size, and its correlation to tumor relative temperature, in mice with transplantable solid breast carcinoma[END_REF]. With the aim to exploit the spontaneous temperature of the tissues, a range of temperature-triggered nanosystems have been developed for drug delivery to tumor by modifying graphene with thermoresponsive polymers [START_REF] Yang | Stimuli responsive drug delivery systems based on nano-graphene for cancer therapy[END_REF]. In this type of systems, graphene serves as carrier to load drugs, while the polymers with low critical solution temperature (LCST) serve as triggers responding to the temperature and inducing the release of drugs in the diseased tissues [START_REF] Karimi | Temperature-responsive smart nanocarriers for delivery of therapeutic agents: applications and recent advances[END_REF]. Ideally, a temperature-triggered nanosystem should retain the drugs at body temperature, and rapidly deliver them at the disease site by spontaneous hyperthermia [START_REF] Mura | Stimuli-responsive nanocarriers for drug delivery[END_REF][START_REF] Liu | Internal stimuli-responsive nanocarriers for drug delivery: design strategies and applications[END_REF].

As the best-known thermosensitive polymer, poly(N-isopropylacrylamide) (PNIPAM) and its derivatives have attracted constant attention in graphene-based temperature-triggered systems, since their corresponding LCST is about 32°C, which is close to the physiological temperature of the human body [START_REF] Schild | Poly(N-isopropylacrylamide): experiment, theory and application[END_REF]. PNIPAM-based graphene complexes have been investigated as carriers for drug delivery. These examples show that temperature-triggered nanosystems based on graphene have big potential application in cancer therapies. However, until now no temperature-triggered nanosystem has been used for in vivo applications as the LCST was always lower than that of the body temperature (37°C). Therefore, further work needs to be done, such as modifying PNP to reach higher LCST.

Magnetic field-triggered nanosystems for therapy and diagnosis

Magnetic field is considered one of the best external stimuli compared to other stimuli such as light irradiation, ultrasound or electric field. Indeed, it permits no depth-penetration limit in the human body and remote-management without physical or chemical contact with the body [START_REF] Yu | Stimuli-responsive cancer therapy based on nanoparticles[END_REF][START_REF] Yu | Magnetic nanoparticle-based cancer therapy[END_REF]. Magnetic field mainly has three applications in the biomedical field including targeting transport, magnetic resonance imaging (MRI) and magnetothermal stimulation. Although graphene does not have paramagnetic characteristics, thanks to the large surface area and oxygen-containing active groups (in GO/rGO), a range of magnetic NPs (MNPs) have been adsorbed onto its surface resulting in various magnetic field-responsive nanosystems. So far, magnetic field-triggered nanosystems based on graphene have been used for many biomedical applications including magnetic targeting for drug delivery, magnetic hyperthermia (MHT), and MRI-guided therapy [START_REF] Alegret | Recent advances of graphene-based hybrids with magnetic nanoparticles for biomedical applications[END_REF].

Magnetic field-triggered nanosystems for targeted drug delivery and photothermal therapy

Magnetic field is like a pair of "invisible hands", which can move magnetic NPs to a target location. Magnetic field-responsive nanosystems possess an intrinsic ability to respond to an external field. Thus, magnetic targeting represents an alternative to commonly used passive targeting due to the EPR effect and positive targeting (i.e. by conjugation of FA or HA). When exposed to an external magnetic field, the responsive nanosystems can be magnetized, they can move thanks to magnetic driving forces and concentrate at specific sites [START_REF] Yu | Stimuli-responsive cancer therapy based on nanoparticles[END_REF]. Hence, this powerful tool has the ability to deliver drugs directly to target sites and maintain drug concentration during the treatment time, thus reducing side effects due to non-specific accumulation in normal tissues.

As the most widely used magnetic materials, Fe 3 O 4 can be combined to graphene for targeted drug delivery due to its superparamagnetic properties, low toxicity, and favorable biocompatibility in physiological environments [START_REF] Alegret | Recent advances of graphene-based hybrids with magnetic nanoparticles for biomedical applications[END_REF]. Yang et al. first prepared a superparamagnetic GO-Fe 3 O 4 hybrid through chemical precipitation [START_REF] Yang | Superparamagnetic graphene oxide-Fe 3 O 4 nanoparticles hybrid for controlled targeted drug carriers[END_REF]. This nanohybrid had a high DOX loading capacity (108 wt%) and was further modified with folate to endow the hybrid with a double targeting ability (magnetic targeting and positive targeting). This multifunctional GO was quickly delivered into the targeted tumor cells SK3 cells, which overexpress folate receptors and show toxicity to HeLa cells after loading DOX [START_REF] Yang | Multi-functionalized graphene oxide based anticancer drug-carrier with dual-targeting function and pH-sensitivity[END_REF]. This CPT and methotrexate (MTX) to tumor site by the induction of an external magnetic field [START_REF] Shen | Graphene oxide-Fe 3 O 4 nanocomposite for combination of dual-drug chemotherapy with photothermal therapy[END_REF]. Combined with PTT under NIR irradiation, the multidrug synergistic delivery system displayed an enhanced capacity to effectively inhibit sarcoma tumor growth leading to necrosis.

MRI-guided cancer diagnosis and therapy

MRI is a powerful and widely used clinical diagnostic technology with high resolution and deep tissue penetration [START_REF] Ahmad | Recent advances in magnetic nanoparticle design for cancer therapy[END_REF]. MNPs with unique magnetic properties are able to improve the signal-to-noise ratio in MRI and have been widely applied as MRI contrast agents (CAs). Under magnetic field, the MNPs not only can drive drugs to specific sites, but they can also act as CAs to assess the distribution of the molecules in vivo and to visualize the

Magneto-thermal therapy

Magnetic hyperthermia is a new generation of tumor thermal therapy method triggered by alternating magnetic field (AMF), which is associated to the unique ability of MNPs (with superparamagnetic behavior) to generate local heat to ablate cancer cells [START_REF] Ahmad | Recent advances in magnetic nanoparticle design for cancer therapy[END_REF]. Compared to PTT, magnetic hyperthermia can target and penetrate deeply inside the biological systems. Moreover, the magnetic field does not cause adverse effect on biological tissues, which is distinctly beneficial for non-invasive in vivo applications [START_REF] Alegret | Recent advances of graphene-based hybrids with magnetic nanoparticles for biomedical applications[END_REF]. Magneto-thermal therapy has been widely explored for cancer treatment. Copyright 2016 American Chemical Society.

Electrically-triggered nanosystems for drug delivery

In the last years, electrically-triggered nanosystems (ETNs) have been developed as alternative remote-controlled delivery systems. The use of an electric field has several advantages for on-demand drug release, including reliability and precise control over the magnitude, duration, and intervals of pulses. Indeed, it offers the possibility to accurately regulate drug release levels [START_REF] Karimi | Smart micro/nanoparticles in stimulus-responsive drug/gene delivery systems[END_REF][START_REF] Zhang | Advanced review of graphene-based nanomaterials in drug delivery systems: Synthesis, modification, toxicity and application[END_REF][START_REF] Zhao | Nano-engineered electro-responsive drug delivery systems[END_REF]. Until now, a range of electrically-triggered nanosystems based on graphene (GETNs) including hydrogels have been designed for remote-controlled delivery. Most of GETNs consist of conductive polymers or conductive particles, which respond to an electric stimulus, and graphene, which not only improves the stability of the hydrogels, but also increases the drug loading. For example, Servant et al. prepared a graphene-based hydrogel as electroactive scaffold to control drug release (Figure 10) [START_REF] Servant | Graphene-based electroresponsive scaffolds as polymeric implants for on-demand drug delivery[END_REF]. The electroactive scaffold with enhanced mechanical, electrical, and thermal properties demonstrated controlled drug release upon the on/off application of a low direct current (DC) voltage (10 V) both in test tubes and in vivo. electrically-triggered drug release [START_REF] Kenna | Electro-stimulated release from a reduced graphene oxide composite hydrogel[END_REF]. In this work, HA was used to improve the stability of the rGO dispersions. The rGO had the capacity to increase the mechanical strength and electrochemical properties of the electroconductive hydrogel. This work showed that an electrical stimulation (-0.6 V or + 0.6 V) could increase electrostatic repulsion between methyl orange (a model drug) and rGO to accelerate the drug release.

Multi-modal imaging nanosystems for therapy and diagnosis

Non-invasive tumor imaging is an important aspect throughout the entire process of cancer theranostics. Owing to large surface area, graphene can be easily modified with a range of molecules to prepare many multifunctional systems with outstanding performances, such as magnetism, fluorescence, radioactivity, plasmonic properties, etc.

[ [START_REF] Zhou | Material chemistry of graphene oxide-based nanocomposites for theranostic nanomedicine[END_REF][START_REF] Bai | Graphene: A versatile platform for nanotheranostics and tissue engineering[END_REF]. Multifunctional GO conjugates possess great potential in non-invasive imaging technology including MRI, positron emission tomography, X-ray computed tomography, fluorescence imaging, photoacoustic imaging (Figure 11). In the last ten years, a series of multimodal imaging systems based on graphene have been explored to localize tumors, track cancer cells, follow drug delivery and monitor the cancer treatment in real time. After labeling with a radioactive or a fluorescent compound, this rGO derivative showed good ability for triple-modal fluorescence/MRI/PAI in vivo for 4T1 tumor imaging and imaging-guided PTT to destroy tumor cells (Figure 11B). In another system, after grafting magnetic NPs and gold NPs on GO, the obtained conjugate (GO-IONP-Au) with both magnetic and plasmonic properties was successfully used for dual-modal magnetic resonance and X-ray imaging to monitor the efficiency of PTT (Figure 11A) [START_REF] Shi | Graphene-based magnetic plasmonic nanocomposite for dual bioimaging and photothermal therapy[END_REF]. In the meantime, it was found that Ag NPs can also endow GO with plasmonic properties to achieve X-ray imaging-guided drug delivery and PTT to ablate breast tumor [START_REF] Hu | Photoresponsive protein-graphene-protein hybrid capsules with dual targeted heat-triggered drug delivery approach for enhanced tumor therapy[END_REF]. developed a plasmonic gold nanorod hybridized rGO vesicle with amplified photoacoustic performance and photothermal effects (Figure 11C) [START_REF] Song | Sequential drug release and enhanced photothermal and photoacoustic effect of hybrid reduced graphene oxide-loaded ultrasmall gold nanorod vesicles for cancer therapy[END_REF]. After labeling with the radioactive element 64 Cu, the multifunctional GO showed an excellent ability to localize tumor and allowed visualization of the shape and size of the tumor via PET and PAI.

Physically-triggered nanosystems based on 2D materials for therapy and diagnosis

Encouraged by the success and versatility of graphene within physically-triggered nanosystems, extensive efforts have equally contributed to the development of graphene-related materials (GRMs) for biomedical applications (Table 2). Due to the diversity of the constituent elements, 2D materials possess tunable optical, electrical, electrochemical, and thermal properties [START_REF] Koski | The new skinny in two-dimensional nanomaterials[END_REF][START_REF] Tan | Ultrathin two-dimensional multinary layered metal chalcogenide nanomaterials[END_REF]. It is therefore possible to use external stimulations (laser, heat, electric field, magnetic field, ultrasound) to more precisely trigger therapeutic actions and to combine multiple diagnostic and therapeutic functions in a single 2D system. Until now, physically-triggered nanosystems based on GRMs have been widely explored for phototherapies, drug delivery, combination therapies and non-invasive imaging. Reprinted from Chen et al. [START_REF] Chen | One-pot synthesis of MoS 2 nanoflakes with desirable degradability for photothermal cancer therapy[END_REF]. Copyright 2017 American Chemical Society.

Similar to MoS 2 , WS 2 nanosheets also exhibit promising photothermal conversion capability for PTT. WS 2 nanosheets with a size of 50-100 nm were synthesized [START_REF] Cheng | PEGylated WS 2 nanosheets as a multifunctional theranostic agent for in vivo dual-modal CT/photoacoustic imaging guided photothermal therapy[END_REF]. After PEGylation, the mass extinction coefficient of PEG-WS 2 at 808 nm was calculated to be 23.8 L•g -1 •cm -1 , which is similar to rGO. Following mice injection (either intratumorally or intravenously) of PEG-WS 2 , the surface temperature of the tumor quickly raised to about 65°C within 5 min by 808 nm irradiation at the laser power density of 0.8 W•cm -2 . Liu et al.

prepared WS 2 nanosheets intercalated with NH 4 + via a bottom-up hydrothermal method [START_REF] Liu | Stable metallic 1T-WS 2 ultrathin nanosheets as a promising agent for near-infrared photothermal ablation cancer therapy[END_REF]. These materials with a size of 150 nm possessed highly hydrophilic character and strong NIR absorption. Following the intratumor injection into tumor-bearing mice, the temperature of the tumor increased to 50°C under NIR laser irradiation within 4 min leading to tumor disappearance after 4 days.

As another representative TMD material, molybdenum diselenide (MoSe 2 ) possesses an characteristic absorption peak located at ∼ 800 nm, giving them a great potential as NIR photothermal transducers [START_REF] Lei | Simultaneous Exfoliation and functionalization of MoSe 2 nanosheets to prepare "smart" nanocomposite hydrogels with tunable dual stimuli-responsive behavior[END_REF][START_REF] Chen | 2D transition metal dichalcogenide nanosheets for photo/thermo-based tumor imaging and therapy[END_REF]. A number of reports have reported the utilization of As a new member of the 2D materials family, black phosphorus has received much interest for its unique 2D layered structure and layer-dependent bandgap of 0.3-2.0 eV.

Because of its excellent optical properties such as strong NIR absorption and high photothermal conversion efficiency, BP has recently been adopted for PTT. Zhao et al.

reported the functionalization of BP with Nile Blue (NB) dye via diazonium chemistry [START_REF] Zhao | Stable and multifunctional dye-modified black phosphorus nanosheets for near-infrared imaging-guided photothermal therapy[END_REF].

The resulting NB@BP displayed good biocompatibility and strong NIR fluorescence (680 nm). Under 808 nm NIR laser irradiation (1.0 W•cm -2 ) NB@BP exhibited excellent photothermal conversion ability, and the temperature increased by ∼27.5°C after 10 min.

As a result of the stable NIR fluorescence and photothermal characteristics, the NB@BP were successfully used in both in vivo NIR fluorescence imaging and photothermal tumor ablation.

MXenes, another class of interesting 2D materials, owning the general formula of M n+1 X n , where M is an early transition metal and X represents C and/or N (being n = 1, 2, or 3) [START_REF] Naguib | Two-dimensional nanocrystals produced by exfoliation of Ti 3 AlC 2[END_REF]. Due to their strong NIR absorption and high photothermal conversion efficiency,

MXenes have attracted considerable attentions as PTT agents in biomedicine. Ti 3 C 2 nanosheets showed a higher photothermal conversion efficient (30.6%) than Au nanorods (21%) [START_REF] Zeng | A Plasmon-assisted optofluidic (PAOF) system for measuring the photothermal conversion efficiencies of gold nanostructures and controlling an electrical switch[END_REF]. After modifying Ti 3 C 2 with soybean phospholipid (SP), the Ti 3 C 2 -SP complex could achieve a NIR light-induced tumor ablation without recurrence by either localized intratumoral injection or intravenous injection [START_REF] Lin | Two-dimensional ultrathin MXene ceramic nanosheets for photothermal conversion[END_REF]. To endow MXenes with more functionalities to improve their theranostic applications, MnOx was grown on the surface of Ti 3 C 2 to construct MnOx/Ti 3 C 2 nanosheets for PAI/MRI-guided PTT [START_REF] Dai | Biocompatible 2D titanium carbide (MXenes) composite nanosheets for pH-responsive MRI-guided tumor hyperthermia[END_REF]. In tumor microenvironment (mild acidic conditions), MnOx was degraded to Mn 2+ to enhance T 1 -weighted MRI for PTT guidance and monitoring. These nanosheets showed a high photothermal stability and a photothermal conversion efficiency of 22.9%. The MnOx/Ti 3 C 2 nanosheets achieved a highly efficient tumor ablation and tumor-growth suppression (Figure 13). Reprinted from Dai et al. [START_REF] Dai | Biocompatible 2D titanium carbide (MXenes) composite nanosheets for pH-responsive MRI-guided tumor hyperthermia[END_REF]. Copyright 2017 American Chemical Society.

Photodynamic therapy using 2D materials

Similar to graphene, 2D GRMs can be loaded with PSs and used for remotely-triggered PDT for cancer treatment owing to their high surface area [START_REF] Ma | Fe III -doped two-dimensional C 3 N 4 nanofusiform: A new O 2 -evolving and mitochondria-targeting photodynamic agent for MRI and enhanced antitumor therapy[END_REF]. Moreover, due to their excellent photoelectric conversion performance, a range of GRMs have the inherent ability to produce ROS and can thus be used as PSs for PDT [START_REF] Lin | Graphitic-phase C 3 N 4 nanosheets as efficient photosensitizers and pH-responsive drug nanocarriers for cancer imaging and therapy[END_REF][START_REF] Wang | Ultrathin black phosphorus nanosheets for efficient singlet oxygen generation[END_REF][START_REF] Wan | Robust two-photon visualized nanocarrier with dual targeting ability for controlled chemo-photodynamic synergistic treatment of cancer[END_REF][START_REF] Chen | Graphitic carbon nitride nanosheet@metal-organic framework core-shell nanoparticles for photo-chemo combination therapy[END_REF][START_REF] Ji | Targeted intracellular production of reactive oxygen species by a 2D molybdenum disulfide glycosheet[END_REF].

Graphitic-phase carbon nitride nanosheets, one of the newly emerging 2D layered nanomaterials, with high stability, good biocompatibility, low toxicity, and extremely high photoluminescence quantum yields can be obtained from direct sonication of bulk g-C 3 N 4 in water. Lin et al. exploited g-C 3 N 4 nanosheets for cancer imaging and therapy [START_REF] Lin | Graphitic-phase C 3 N 4 nanosheets as efficient photosensitizers and pH-responsive drug nanocarriers for cancer imaging and therapy[END_REF]. In [START_REF] Ji | Targeted intracellular production of reactive oxygen species by a 2D molybdenum disulfide glycosheet[END_REF]. Due to the specific glycoligand-receptor recognitions, the galactose-modified MoS 2 sheets could be selectively internalized into the HepG2 cells that highly express the galactose receptors. They showed a good ability to produce ROS under visible light irradiation leading to apoptosis of the cancer cells (Figure 14).

Combined photothermal therapy and drug/gene delivery

Due to the high surface-to-volume ratio and ultrahigh surface area, 2D GRMs are regarded as ideal carriers with high drug loading capacity. Indeed, GRMs have been demonstrated as efficient carriers for drugs and genes [START_REF] Chen | Two-dimensional nanomaterials for cancer nanotheranostics[END_REF][START_REF] Yin | Functionalized 2D nanomaterials for gene delivery applications[END_REF]. In addition, the unique Similarly, 2D WS 2 also showed the ability for combined PTT and drug delivery. Yang et al.

loaded IONPs and mesoporous silica shells onto PEG-WS 2 (WS 2 -IO@MS-PEG) [START_REF] Yang | Two-dimensional magnetic WS 2 @Fe 3 O 4 nanocomposite with mesoporous silica coating for drug delivery and imaging-guided therapy of cancer[END_REF].

The multifunctional WS 2 exhibited high NIR and X-ray absorbance, as well as strong superparamagnetic properties. Combined PTT/chemotherapy was then carried out in vivo using WS 2 -IO@MS-PEG/DOX, achieving a remarkably synergistic therapeutic effect superior to the respective mono-therapies.

Recently, light-triggered gene delivery based on GRMs has attracted increasing attention to remove some obstacles in cellular gene delivery such as low uptake by cells and undesired release efficiency of payloads at targeting site [START_REF] Kong | Dendrimer-modified MoS 2 nanoflakes as a platform for combinational gene silencing and photothermal therapy of tumors[END_REF]. 16).

Combined photodynamic and photothermal therapy

Photosensitizers, light and oxygen are the three key components for generation of 1 O 2 in PDT [START_REF] Cheng | Functional nanomaterials for phototherapies of cancer[END_REF]. However, the hypoxic microenvironments of tumors usually lead to low generation of 1 O 2 , which limits its therapeutic efficacy. To solve this problem, PTT has been used to improve PDT efficiency. Mild hyperthermia not only increases the cellular uptake of PSs, but it also increases intratumoral blood flow which further transports more oxygen into the cancer cells [START_REF] Gong | Two-dimensional transition metal dichalcogenide nanomaterials for combination cancer therapy[END_REF]. Combination of PTT and PDT has been demonstrated being a highly efficient approach for cancer therapy.

Magnetic-triggered nanosystems for therapy and diagnosis

Similar to graphene, a range of 2D could be moved and hijacked to tumor site under external magnetic field. This smart nanosystem not only showed an enhanced photothermal ablation of cancer cells in vivo upon exposure to 808 nm NIR laser, but it also showed the ability to be used as T 2 -weighted MRI CA.

Multi-modal nanosystems for therapy and diagnosis

Up to now, a range of multimodal GRMs have been used to offer better accuracy for cancer diagnosis and therapy. Due to their large surface area, 2D graphene-like materials can adsorb different types of imaging molecules and nanoparticles such as fluorophores, Gd, radioactive elements, IONPs and other NPs, to allow them to be used for many non-invasive imaging modalities including MRI, FI, and CT. In addition, due to the diversity of elemental composition, the intrinsic properties of GRMs can be exploited for multimodal imaging. For example, both MoS 2 and WS 2 nanosheets possess strong X-ray attenuation ability. They have been used as X-ray Cas for CT imaging, leading to enhanced contrast images compared to some commercial Cas [START_REF] Cheng | PEGylated WS 2 nanosheets as a multifunctional theranostic agent for in vivo dual-modal CT/photoacoustic imaging guided photothermal therapy[END_REF][START_REF] Yin | High-throughput synthesis of single-layer MoS 2 nanosheets as a near-infrared photothermal-triggered drug delivery for effective cancer therapy[END_REF]. Due to their strong NIR absorption, GRMs (such as MoS 2 , TiS 2 [START_REF] Qian | Two-dimensional TiS 2 nanosheets for in vivo photoacoustic imaging and photothermal cancer therapy[END_REF]) can also generate ultrasound signal for PAI.

In 2014, Cheng et al. found that PEG-modified WS 2 nanosheets possessed strong X-ray attenuation ability and high NIR absorbance [START_REF] Cheng | PEGylated WS 2 nanosheets as a multifunctional theranostic agent for in vivo dual-modal CT/photoacoustic imaging guided photothermal therapy[END_REF]. Its Hounsfield units (HU) value (about 22.01 HU L•g -1 ) was higher than commercial CT CA iopromide (15.9 HU L•g -1 ). After either i.t. or i.v. injection, the PEG-WS 2 nanosheets served as Cas for CT and PAI to monitor PTT of 4T1 tumor. Furthermore, Gd 3+ has been grown onto PEG-WS 2 to endow WS 2 with strong contrast in T 1 -weighted MRI for allowing triple-modal CT/MRI/PAI and imaging-guided PTT and radiotherapy [START_REF] Cheng | Bottom-up synthesis of metal-ion-doped WS 2 nanoflakes for cancer theranostics[END_REF]. In this multimodal nanosystem, the two-dimensional transition metal dichalcogenides intrinsic properties enabled PAI and PTT, while both W and Gd elements attenuated X-ray irradiation to enable CT imaging and enhanced radiotherapy (Figure 17).

Conclusion and perspectives

In summary, we have reviewed the recent developments of physically-triggered nanosystems based on 2D materials from design to biomedical applications. The physical triggers mainly include light, heat, magnetic and electric field. These systems with unique properties provide various innovative methods for cancer theranostics, including phototherapies, magnetic therapy, remotely controlled chemotherapy, and non-invasive imaging. Although tremendous efforts have been made to improve the performance of physically-triggered nanosystems, to date no such systems have been utilized in clinic.

The physically-triggered theranostics based on 2D materials is still in its early stage of development and requires considerable efforts on several aspects listed below:

1. Biocompatibility of 2D materials. The biocompatibility is one of the primary challenges to exploit 2D nanomaterials for biomedical applications. To date, there is a huge amount of studies on graphene-family nanomaterials (but it is difficult to generalize) and much less on newly emerged other 2D materials. There has been a wide consensus that 2D materials can pose potential risks to living organisms due to their toxicity to various living organisms and cells [START_REF] Chen | Biodegradation of carbon nanotubes, graphene, and their derivatives[END_REF][START_REF] Bianco | Graphene: safe or toxic? the two faces of the medal[END_REF][START_REF] Yang | Behavior and toxicity of graphene and its functionalized derivatives in biological systems[END_REF]. Many efforts have been made

to study their toxicity mechanism and try to alleviate their toxicity. Some studies have shown that functionalized graphene and 2D materials do not produce any cytotoxic effect and do not cause immune response [START_REF] Singh | Amine-modified graphene: thrombo-protective safer alternative to graphene oxide for biomedical applications[END_REF][START_REF] Duch | Minimizing oxidation and stable nanoscale dispersion improves the biocompatibility of graphene in the lung[END_REF][START_REF] Zhang | Distribution and biocompatibility studies of graphene oxide in mice after intravenous administration[END_REF]. Simultaneously, some studies have demonstrated that graphene can be degraded by human myeloperoxidase in the presence of hydrogen peroxide [START_REF] Kurapati | Dispersibility-dependent biodegradation of graphene oxide by myeloperoxidase[END_REF] and signs of in vivo degradation have been evidenced [START_REF] Girish | Confocal Raman imaging study showing macrophage mediated biodegradation of graphene in vivo[END_REF]. Furthermore, some functionalized 2D materials, such as MoS 2 [START_REF] Chen | One-pot synthesis of MoS 2 nanoflakes with desirable degradability for photothermal cancer therapy[END_REF][START_REF] Kurapati | Enzymatic biodegradability of pristine and functionalized transition metal dichalcogenide MoS 2 nanosheets[END_REF], MnO 2 [START_REF] Zhang | Manganese dioxide nanosheets-based redox/pH-responsive drug delivery system for cancer theranostic application[END_REF][START_REF] Chen | Break-up of two-dimensional MnO 2 nanosheets promotes ultrasensitive pH-triggered theranostics of cancer[END_REF], BP [START_REF] Wang | Ultrathin black phosphorus nanosheets for efficient singlet oxygen generation[END_REF][START_REF] Tao | Black phosphorus nanosheets as a robust delivery platform for cancer theranostics[END_REF] can be degraded in physiological environments and transformed into ions, which can be easily eliminated from the body. However, the long-term toxicology evaluation in vivo is still needed to demonstrate the safety of 2D material-based nanomedicines for further use in clinic.

Standardization of 2D materials.

It is widely accepted that the size, shape, and surface modification of 2D materials have an important influence on their toxicology and performances, as well as they critically impact their fate in physiological environments [START_REF] Bianco | Graphene: safe or toxic? the two faces of the medal[END_REF][START_REF] Yang | The influence of surface chemistry and size of nanoscale graphene oxide on photothermal therapy of cancer using ultra-low laser power[END_REF][START_REF] Ji | An insight into graphene oxide associated fluorogenic sensing of glycodye-lectin interactions[END_REF]. However, it is difficult to precisely control size and shape of 2D materials with current preparation technology. 2D materials obtained by similar methods can even possess large size span and structural differences. This lack of control makes the systematic evaluation of their biological interactions challenging.

Therefore, there is immense interest in preparing 2D nanomaterials with uniform size and shape by innovative preparation technology.

3. The diversity of 2D materials. Since isolation of graphene in 2004, about 30 different 2D materials with unique properties have been explored and utilized as physically-triggered nanosystems. It is only a drop in the ocean compared with the huge number of 2D materials. Therefore, 2D materials, which may have novel properties for biomedical applications definitely need investigation.

4. Working modes of nanosystems. Physically-triggered nanosystems should be controlled by efficient stimuli to adapt to our demand. However, due to the complexity of physiological environment, it is unavoidable that they can be disturbed by extensive interference factors. In addition, each physical trigger has its own limitations. For example, shallow tissue penetration is an issue for light-triggered phototherapy, especially PTT, whereas magnetic therapies usually need a high dose of magnetic NPs to be efficient. By integrating the advantages of each physical trigger, developing multimodal physically-triggered nanosystems will allow overcoming these limitations.

Potential applications of nanosystems in immunotherapy.

As mentioned above, with a deep insight of intrinsic characteristic of 2D materials, different anti-tumor applications of physically-triggered nanosystems have been constantly discovered. However, there is still much room to further find new biomedical applications of physically-triggered nanosystems to improve the cure rates for the treatment of tumors. Recently, cancer immunotherapy based on nanomaterials is attracting growing attention. Indeed, it was found that phototherapy using nanomaterials can trigger anti-tumor immune response to efficiently inhibit tumor metastasis bringing a significant improvement of cure rates [START_REF] Liang | Emerging nanomedicine approaches fighting tumor metastasis: Animal models, metastasis-targeted drug delivery, phototherapy, and immunotherapy[END_REF], giving new horizons in immunotherapy and vaccine development [START_REF] Fan | Toward biomaterials for enhancing immune checkpoint blockade therapy[END_REF][START_REF] Irvine | Synthetic nanoparticles for vaccines and immunotherapy[END_REF].

Encouraged by this discovery, other nanosystems have been developed for treating tumor metastasis by combined PTT and immunotherapy [START_REF] Nam | Chemo-photothermal therapy combination elicits anti-tumor immunity against advanced metastatic cancer[END_REF][START_REF] Liu | Synergistic immuno photothermal nanotherapy (SYMPHONY) for the treatment of unresectable and metastatic cancers[END_REF][START_REF] Tao | Engineered, self-assembled near-infrared photothermal agents for combined tumor immunotherapy and chemo-photothermal therapy[END_REF]. Considering the unique photothermal and photodynamic features of 2D materials, in our opinion, cancer immunotherapy of physically-triggered nanosystems based on 2D materials could be a new direction for cancer treatment.

An ideal "smart" nanosystem should be like a nanorobot that can automatically identify a disease and not release its payloads until triggered by stimuli at the target site, thus destroying targeting cells and not normal cells and tissues. In addition, its residence time, working position in vivo should be precisely controlled in real time. There is no doubt that it is not easy to achieve these objectives, but we do believe that it is just a matter of time before getting the perfect nanosystem with unremitting joint efforts of material scientists, chemists, physicists, biologists, and medical doctors.

Contents 1 .

 1 Introduction 2. Graphene-based nanosystems for therapy and diagnosis 2.1. Light-triggered nanosystems for therapy and diagnosis 2.1.1 Photothermal therapy using graphene 2.1.2 Photodynamic therapy using graphene 2.1.3 Light-triggered nanosystems for drug and gene delivery 2.1.4 Light-triggered nanosystems for combined therapies 2.2 Temperature-triggered nanosystems for therapy 2.3 Magnetic field-triggered nanosystems for therapy and diagnosis 2.3.1 Magnetic field-triggered nanosystems for targeted drug delivery and photothermal therapy 2.3.2 MRI-guided cancer diagnosis and therapy 2.3.3 Magneto-thermal therapy 2.4 Electrically-triggered nanosystems for drug delivery 2.5 Multi-modal imaging nanosystems for therapy and diagnosis 3. Physically-triggered nanosystems based on 2D materials for therapy and diagnosis 3.1 Light-triggered nanosystems based on 2D materials for therapy and diagnosis 3.1.1 Photothermal therapy using 2D materials 3.1.2 Photodynamic therapy using 2D materials 3.1.3 Combined photothermal therapy and drug/gene delivery 3.1.4 Combined photodynamic and photothermal therapy 3.2 Magnetic-triggered nanosystems for therapy and diagnosis 3.3 Multi-modal nanosystems for therapy and diagnosis 4 Conclusion and perspectives

Figure 1 .

 1 Figure 1. Illustration of 2D nanomaterials including graphene, graphene oxide (GO), MoS 2 , WS 2 , g-C 3 N 4 , Bi 2 S 3 , black phosphorus, and MXenes.

Figure 2 .

 2 Figure 2. Schematic diagram for the design of physically-triggered nanosystems.

Figure 3 .

 3 Phototherapy

Figure 4 .

 4 Figure 4. The synthesis and action procedure of PPa-GO-mAb. (a) Use of PPa-GO-mAb showing phototoxicity switching when mitochondria are targeted. (b) Scheme of the synthesis of PPa-GO-mAb and its two states in different environments. Adapted from Wei et al. [56]. Copyright 2016 Royal Society of Chemistry.

Figure 5 .

 5 Figure 5. Schematic illustration of photothermally controlled action mechanism of rGO-PEG-RV inducing cell death (NrGO stands for nanosized rGO). Adapted from Chen et al. [62]. Copyright 2014 Elsevier Ltd.

  photothermal treatment alone.Combined PTT-chemotherapy can be also used to overcome drug resistance.Feng et al.

Figure 6 .

 6 Figure 6. Schematic illustration of the acidic extracellular environment-induced charge reverse of NGO-PEG-DA/DOX complex, its cellular uptake and intracellular acidic environment-triggered DOX release. Compared to wild-type MCF-7 cells (MCF-7/WT), drug-resistant MCF-7/ARD cells, with a higher expression of P-glycoprotein, showed a rapid drug efflux. Adapted from Feng et al. [68]. Copyright 2014 WILEY-VCH Verlag GmbH & Co.

Figure 7 .

 7 Figure 7. (a) Schematic illustration of core-shell PGP capsules encapsulating DOX; (b) Under NIR irradiation, the PGP capsules with and without loading DOX exhibited different cell killing efficacy. (c) In vivo study of PGP capsules. Fluorescence images of RG2 tumor-bearing mice 1 and 5 days after intravenous injection of the PGP capsules. (d) Infrared thermal images of RG2 tumor-bearing mice under PGP+NIR treatment. (e) RG2 tumor treated by PGP+NIR light (middle) and PGP-DOX+NIR light (right) after 2 months. (f) Mouse weight as a function of post-treatment time displaying no weight loss for 30 days in all groups. Adapted from Hu et al. [69]. Copyright 2014 WILEY-VCH Verlag GmbH & Co.

  work provided a new strategy to improve targeted drug delivery and it also allowed the use of GO-Fe 3 O 4 hybrid as an ideal drug carrier for tumor combination therapy. In parallel, Fan et al. developed a graphene nanosheet-carbon nanotube-iron oxide NP hybrid (GN-CNT-Fe 3 O 4 ), which exhibited superparamagnetic properties that were exploited for magnetic targeting[START_REF] Fan | The preparation and drug delivery of a graphene-carbon nanotube-Fe 3 O 4 nanoparticle hybrid[END_REF]. The hybrid was able to bind the anticancer drug 5-fluorouracil (5-FU) displaying a pH-activated 5-FU release profile to kill HepG2 cells. Because both graphene and Fe 3 O 4 -based nanoparticles were shown to absorb NIR light, it has been demonstrated that magnetic GO can be used as magneto-photothermal agents for targeted PTT[START_REF] Akhavan | Zinc ferrite spinel-graphene in magneto-photothermal therapy of cancer[END_REF] or combination of PTT with chemotherapy to treat cancers[START_REF] Shen | Graphene oxide-Fe 3 O 4 nanocomposite for combination of dual-drug chemotherapy with photothermal therapy[END_REF]. Shen et al. explored superparamagnetic GO-Fe 3 O 4 conjugates, which can specifically deliver both

  tumors and the circulatory system. Graphene-based MNPs with enhanced physiological stability have been widely used for MRI-guided cancer therapy and diagnosis including MRI-guided PTT and MRI-guided drug delivery. Generally, MRI CAs are divided into two types: T 1 and T 2 CAs, depending on the signal enhancement or decay. Due to the presence of paramagnetic Fe 3+ and its property of high saturation magnetization, Fe 3 O 4 NPs are considered good candidates for T 2 -weighted MRI. In 2010, Cong et al. firstly prepared a magnetic rGO by in situ high-temperature decomposition of Fe 3+ magnetite NPs on the rGO sheets [91]. This work opened the prologue of graphene-based magnetic theranostic. To increase the physiological stability and reduce the cytotoxicity of the GO-based magnetic NPs, Chen et al. modified Fe 3 O 4 with aminodextran on GO and successful applied them for cellular MRI tests [92]. Subsequently, Liu's group and others [93-96] developed a series of Fe 3 O 4 modified graphene derivatives, which served as T 2 -weighted MRI CAs. Combined with PTT and drug delivery (i.e. DOX or 5-Fu), these magnetic GO nanosystems can get cancer treatment and diagnosis together in a single platform showing the ability to monitor the progress of cancer treatment in real time with a big potential in clinical applications. As the most frequently employed T 1 CAs in clinics, paramagnetic gadolinium complexes were also used in combination with GO for MRI. Zhang et al. chemically conjugateddiethylenetriaminepentaacetic acid (DTPA) to GO, followed by Gd(III) complexation, to form a T 1 MRI CA[START_REF] Zhang | Graphene oxide based theranostic platform for T 1 -weighted magnetic resonance imaging and drug delivery[END_REF]. This complex exhibited significantly a higher r 1 relaxivity enhancement than the commercially used Magnevist™. This new MRI CA was internalized into cells, allowing cellular MRI, and simultaneously delivered DOX to lesions to kill HepG2 cells. Diffusion-weighted magnetic resonance imaging (DW-MRI) is a clinical imaging method which provides image contrast based on the differences in diffusion of water molecules. It is able to assess the extent of tissue damage after treatment by monitoring the apparent diffusion coefficient (ADC). Fu et al. used (DW-MRI to monitor GO-mediated PTT under different conditions[START_REF] Fu | Diffusion-weighted magnetic resonance imaging for therapy response monitoring and early treatment prediction of photothermal therapy[END_REF]. In this work, a GO-IONP-PEG conjugate was driven to the tumor site by a magnetic field. After NIR irradiation for 5 min, the temperature of the tumor tissue increased from 34.5°C to 65.2°C leading to complete 4T1 breast cell ablation. After PTT, the animals were scanned by DW-MRI at different time points and the mean ADC value changes in tumors were monitored. The result showed an increase of ADC value (>80%) within 48 h which associated with effective tumor growth suppression and extended animal survival. This work suggested ADC as an effective early prognosis marker for PTT, showing a good prospect for future development and clinical translation of the technology (Figure8).

Figure 8 .

 8 Figure 8. Schematic illustration of experimental design. GO-IONP-PEG was prepared by

  Zhu et al. created a magneto-induced drug delivery system made of a DOX-GO-Fe 3 O 4 -PEI gel for breast cancer treatment [99]. After exposure to an AMF of 488 kHz for 30 min, the increased temperature (7°C) in tumor enhanced the release of DOX improving tumor treatment. At the same time, Swain et al. observed that GO modified with poly(vinyl alcohol) (PVA), polyvinylpyrrolidone (PVP), and Fe 3 O 4 has the ability to load and release both hydrophobic and hydrophilic drugs [100]. Under AMF at 250 kHz, it took less than 16 min for the polymerA-GO-Fe 3 O 4 to reach the stable hyperthermia temperature of 42°C. This local generated heat was able to enhance the drug release from GO destroying MCF-7 cells in vitro and in vivo. Recently, Deng et al. developed a dual triggered hybrid microcapsule comprising GO, IONP and polysaccharides via a layer-by-layer techniquefor on-demand drug release[START_REF] Deng | Hybrid iron oxide-graphene oxide-polysaccharides microcapsule: A micro-matryoshka for on-demand drug release and antitumor therapy in vivo[END_REF]. This nanosystem can release its payload by a dual-responsive trigger induced by both NIR and magnetic hyperthermia, making it highly controllable. The synergistic effect of hyperthermia (PTT and MHT) and chemotherapy successfully destroyed cervical cancer cells in vitro and in vivo (Figure9).

Figure 9 .

 9 Figure 9. (A) Schematic illustration of hybrid microcapsules (hMC) created via layer-by-layer assembly. (B) Confocal laser scanning microscopy images of dead cells incubated with different samples: control (without samples), microcapsules, and hybrid microcapsules. (C) In vivo study of hybrid microcapsules. Adapted from Deng et al. [101].

Figure 10 .

 10 Figure 10. Electroresponsive graphene/polymethylmethacrylate hydrogel hybrids for on-demand drug delivery. (A) The preparation process of graphene/hydrogel hybrid. (B) Drug release upon on/off application of a DC electric field and photos of the hydrogel before and after electrical stimulation. Reprinted from Servant et al. [104], Copyright WILEY-VCH Verlag GmbH & Co.

Figure 11 .

 11 Figure 11. (A) In vivo dual-modal MR/X-ray imaging of 4T1 tumor-bearing mice: (i) MRI, (ii) X-ray imaging. Adapted from Shi et al. [95]. Copyright 2013 Elsevier Ltd. (B) Multimodal

3. 1 3 . 1 . 1

 1311 photothermal stability[START_REF] Wang | Biocompatible PEGylated MoS 2 nanosheets: Controllable bottom-up synthesis and highly efficient photothermal regression of tumor[END_REF]. After intravenous administration, PEG-MoS 2 accumulated in the tumor via the EPR effect. Upon irradiation at 808 nm laser, the local generated heat caused the temperature increase of tumor tissues leading to ablation of 4T1 cancer cells. Recently, in order to enhance biocompatibility of MoS 2 , Chen et al. developed a PEGylated MoS 2 (PEG-MoS 2 ) by one-pot hydrothermal reaction in the presence of poly(acrylic acid) [112]. It was found that PEG-MoS 2 accumulated in tumor site and caused temperature increase up to 52.1°C, significantly suppressing the growth of subcutaneously xenografted 4T1 tumor in vivo upon NIR irradiation. More importantly, this nanosheet degraded into water-soluble ions MoO 4 2-under physiological condition (pH =

Figure 12 .

 12 Figure 12. Schematic diagram of preparation of PEG-MoS 2 via one-pot hydrothermal reaction. The obtained PEG-MoS 2 nanosheets showed good degradability in physiological environment (pH = 7.4) which can be used for in vivo photothermal cancer therapy.

MoSe 2 -

 2 based nanomaterials as photothermal agents for PTT[START_REF] Lei | Hydrophilic MoSe 2 nanosheets as effective photothermal therapy agents and their application in smart devices[END_REF][START_REF] Yuwen | Aqueous phase preparation of ultrasmall MoSe 2 nanodots for efficient photothermal therapy of cancer cells[END_REF][START_REF] Wang | Polydopamine coated selenide molybdenum: A new photothermal nanocarrier for highly effective chemo-photothermal synergistic therapy[END_REF][START_REF] Zhong | Facile synthesis of biocompatible MoSe 2 nanoparticles for efficient targeted photothermal therapy of human lung cancer[END_REF]. For example, Lei et al. modified MoSe 2 nanosheets with a biocompatible polymer PVP[START_REF] Lei | Hydrophilic MoSe 2 nanosheets as effective photothermal therapy agents and their application in smart devices[END_REF]. The resulting hydrophilic MoSe 2 nanosheets displayed good biocompatibility and a high photothermal conversion efficiency. Their mass extinction coefficient at 808 nm was calculated to be 11.1 L•g -1 •cm -1 , ∼ 3-fold higher than that of GO nanosheets [3.6 L•g -1 •cm -1 ] [109]. Under irradiation at 808 nm for 15 min, PVP-coated MoSe 2 nanosheets effectively killed HeLa cells at the concentration of 100 µg•mL -1 . Furthermore, these MoSe 2 layers were successfully encapsulated into a thermoresponsive PNIPAM hydrogel matrix. This nanocomposite hydrogel exhibited a remote-controlled photoresponsive capability for drug delivery and PTT towards cancer cells in vitro. Bi 2 Se 3 nanosheets have recently been demonstrated to be promising light-triggered nanosystem. In 2013, Li et al. found that Bi 2 Se 3 nanoplates can absorb NIR laser light and effectively convert the laser energy into heat [121]. After modification with PVP, the Bi 2 Se 3 nanosheets had good water dispersibility and biocompatibility. Under the irradiation of 808 nm laser, they successfully induced complete eradication of solid malignant tumors in mice. Recently, in order to improve tumor targeting and clearance of the nanosheets, Li et al. reported a strategy based on the transportation of Bi 2 Se 3 nanosheets within macrophage vehicles for cell-mediated therapy [122]. Compared with bare Bi 2 Se 3 , the Bi 2 Se 3 -laden-macrophages, after intravenous injection, showed prolonged blood circulation and could overcome the hypoxia-associated drug delivery barrier to target the tumor efficiently and dramatically enhance the efficiency of photothermal cancer therapy.

Figure 13 .

 13 Figure 13. Schematic illustration of the synthetic process and MRI/PAI-guided photothermal tumor therapy of MnOx/Ti 3 C 2 -SP nanosheets. (a) Scheme of the exfoliation process for 2D Ti 3 C 2 nanosheets. (b) Scheme of the whole synthetic procedure for MnOx/Ti 3 C 2 -SP nanosheets. (c) Schematic illustration of theranostic functions of MnOx/Ti 3 C 2 -SP nanosheets, i.e., MRI/PAI-guided efficient PTT ablation of cancer.

  photosensitizers for the generation of singlet oxygen under visible light with a high 1 O 2 quantum yield (about 0.91)[START_REF] Wang | Ultrathin black phosphorus nanosheets for efficient singlet oxygen generation[END_REF]. Under a short light irradiation time, a small amount of BP nanosheets exhibited a notable MDA-MB-231 breast tumor growth inhibition.Interestingly, the nanosheets were completely degraded into biocompatible phosphorus oxides under light irradiation, thus offering a great therapeutic potential for cancer treatment.Glycoligand-protein interactions are pivotal biological events that are also involved in human diseases such as cancer, influenza, and inflammation [134]. Ji et al. have developed MoS 2 glycosheets by the supramolecular self-assembly of fluorophore-labeled glycoligands on MoS 2[START_REF] Ji | Targeted intracellular production of reactive oxygen species by a 2D molybdenum disulfide glycosheet[END_REF]. Due to the specific glycoligand-receptor recognitions, the

Figure 14 .

 14 Figure 14. (a) Structures of the dicyanomethylene-4H-pyran-tagged glycoligands DK1, DK2, and DK3 (Gal = galactose; GalNAc = N-acetyl galactosamine; Lac = lactose). (b) Schematic illustration of the supramolecular self-assembly between 2D MoS 2 and DK glycoligand to form the glycosheet, and their targeted intracellular production of singlet oxygen through two stages: (i) the receptor-mediated endocytosis, and (ii) the light irradiation. Adapted from Ji et al. [133]. Copyright 2016 WILEY-VCH Verlag GmbH & Co.

Figure 15 .

 15 photothermal conversion efficiency (24.4%)[START_REF] Yin | High-throughput synthesis of single-layer MoS 2 nanosheets as a near-infrared photothermal-triggered drug delivery for effective cancer therapy[END_REF]. DOX was loaded onto MoS 2 -CS and it was found that NIR light-induced local hyperthermia induced the release of DOX from MoS 2 -CS, resulting in enhanced antitumor efficacy for the treatment of pancreatic cancer (Figure15).

Figure 16 .

 16 Figure 16. Schematic illustration of sequential pDNA delivery using MoS 2 -PEI-PEG by photothermally triggered endosomal escape followed by redox-mediated polymer detachment and DNA release. Adapted from Kim et al. [141]. Copyright from 2016 Wiley-VCH Verlag GmbH & Co.

Figure 17 .

 17 Figure 17. In vivo triple-modal imaging in 4T1 tumor-bearing mice before and 24 h after i.v. injection of PEG-WS 2 :Gd 3+ and combined PTT and radiotherapy. (A) In vivo CT images of mice. (B) T 1 -weighted MR images of mice. (C) Quantified MR and CT signals of tumors from mice based on images in (A) and (B). (D) In vivo PA images of tumors in mice. (E) IR thermal images of mice without or with i.v. injection of PEG-WS 2 /Gd 3+ under 808 nm laser irradiation taken at different time intervals. (F) Tumor volume growth curves of mice after various treatments. (G) Micrographs of H&E-stained tumor slices. Adapted from Cheng et al. [29] Copyright 2015 American Chemical Society.

  For example, Wang et al. prepared a targeting peptide-modified mesoporous silica-coated graphene nanosystem[START_REF] Wang | Multifunctional mesoporous silica-coated graphene nanosheet used for chemo-photothermal synergistic targeted therapy of glioma[END_REF]. This complex adsorbed DOX with a high loading efficiency (about 127 wt%) and displayed a high absorption in the NIR window and an efficient heat transformation. Cytotoxicity experiments demonstrated that combined therapy triggered by NIR light provoked the highest rate of glioma cell death compared to that of single chemotherapy or PTT. Bai et al. developed PEG-GO/CuS by growing CuS NPs on PEG-GO surface for cervical cancer treatment[START_REF] Bai | Multifunctional PEG-GO/CuS nanocomposites for near-infrared chemo-photothermal therapy[END_REF]. The PEG-GO/CuS complex exhibited a high DOX loading efficiency (90 wt%) and an enhanced DOX release by NIR irradiation (at 980 nm). In a tumor-bearing mouse model, the chemo-photothermal effect of PEG-GO/CuS/DOX showed the ability to significantly inhibit mouse cervical tumor growth and avoid tumor recurrence.

  Unfortunately, in vitro and in vivo experiments to demonstrate the potential of this hydrogel have not been performed. In another work, Pan et al. functionalized graphene sheet (GS) with PNIPAM via click chemistry [81]. The resulting PNIPAM-GS

	exhibited a phase transition temperature at 33°C and possessed a superior capability of
	binding camptothecin (CPT) with a loading ratio of 18.5 wt%, which is two times greater
	than that of PEG-NGO (PEG modified NGO) [57] and four times that of FA-NGO (folic
	acid-modified GO) [82]. The in vitro drug release experiments showed that 16.9% and
	19.4% of CPT were released after 72 h at 37°C in water and PBS, respectively, affording a
	strong potency to kill metastatic skin cancer cells. Wang et al. synthesized GO-PNP

Sun et al.

developed the first GO/PNIPAM hydrogel by covalently binding GO sheets and PNIPAM-co-acrylic acid microgels directly in water

[START_REF] Sun | A one-step strategy for thermal-and pH-responsive graphene oxide interpenetrating polymer hydrogel networks[END_REF]

. This hydrogel exhibited dual thermal and pH responses with good reversibility showing potential biological applications as carrier for controlled drug delivery. hybrids [PNP: PNIPAM-poly(ethylene oxide)] with different PNP coverage by assembling PNP on GO nanosheets

[START_REF] Wang | Thermo-sensitive graphene oxide-polymer nanoparticle hybrids: synthesis, characterization, biocompatibility and drug delivery[END_REF]

. The resulting GO-PNP exhibited a phase transition temperature at 34.8°C, which is about 2.8°C higher than that of the homopolymeric PNIPAM (32°C). This synthesized hybrid displayed high DOX loading efficiency (~91 wt%). About 22% of DOX was released upon temperature change to 37°C, achieving ~80% apoptosis of HepG2 cells.

  Because of the depth-dependent decrease of laser intensity, a high-power laser is generally required to eradicate tumor by PTT. The high temperature may cause unavoidable collateral damage to normal tissue and exceed the tolerance of patients. To address this problem, Zhang et al. used PEI-WS 2 -siRNA as platform for gene-photothermal synergistic therapy of tumors in mild conditions[START_REF] Zhang | Multifunctional WS 2 @poly(ethylene imine) nanoplatforms for imaging guided gene-photothermal synergistic therapy of cancer[END_REF]. As a member of the family of apoptosis inhibitors, survivin siRNA could inhibit heat shock response and thus makes cancer cells more sensitive to heat at mild temperature. A high inhibition of liver tumor (91.7%) without recurrence was observed after combined PTT/gene silencing. In order to increase gene delivery efficiency,Kim et al.

	increased by ~2.5 times in HCT116 cells and by ~4 times in B16F1 cells upon 808 nm
	laser irradiation (Figure
	immobilized PEI and PEG on MoS 2 (MoS 2 -PEI-PEG), which is sensitive to both external
	(light) and internal stimuli (glutathione, GSH), achieving efficient gene delivery by a
	step-by-step approach [141]. The MoS 2 -PEI-PEG carrying plasmid DNA (pDNA) entered
	into human colon cancer cells (HCT116) by endocytosis. When the nanosystem was
	trapped inside the endosomes, NIR laser was used to generate local heat that resulted in
	endosomal membrane rupture, inducing endosomal escape of MoS 2 -PEI-PEG-pDNA.

  Yong et al. employed WS 2 nanosheets functionalized with bovine serum albumin (BSA) as drug carriers to adsorb the PS MB for combined PTT and PDT treatment of cancer cells[START_REF] Yong | WS 2 nanosheet as a new photosensitizer carrier for combined photodynamic and photothermal therapy of cancer cells[END_REF]. Due to the effective energy transfer between MB and WS 2 , singlet-oxygen generation by MB was efficiently inhibited by the WS 2 nanosheets. Under NIR irradiation (808 nm), the local heat generated by BSA-WS 2 not only killed cancer cells, but also triggered MB release resulting in the efficient restoration of singlet-oxygen generation for PDT application. The in vitro results showed that the efficacy of the combined PDT/PTT treatment of cancer cells was higher than that of PDT and PTT alone. Furthermore, Liu et al. first applied PDT/PTT combined therapy in animal experiments[START_REF] Liu | Combined photothermal and photodynamic therapy delivered by PEGylated MoS 2 nanosheets[END_REF]. In this work,

	PEGylated MoS 2 nanosheets were exploited as drug carrier of Ce6. Under 808 nm laser
	irradiation, mild heating increased cell membrane permeability and promoted cellular
	uptake, inducing enhanced intracellular delivery of Ce6. The nanosheets significantly
	killed cancer cells upon exposure to 808 nm laser irradiation (PTT) and 660 nm (PDT),
	achieving synergistic effect in delaying 4T1 breast tumor growth. Another report showed
	that PEG-MoS 2 decorated with Au NPs (MoS 2 -PEG-Au) displayed enhanced
	photothermal conversion efficiency compared to PEG-MoS 2 and a Ce6 loading capacity of
	23.1 wt% [144]. Upon 808 nm laser irradiation, over 45% of Ce6 was released from MoS 2
	surface. In vivo experiments using two lasers (660 nm and 808 nm) showed that PTT (808
	nm) remotely induced Ce6 release and ROS generation (660 nm), thus achieving
	synergistic effect of PTT and PDT for antitumor therapy.

  GRMs have been explored for MRI and magnetic-triggered drug delivery after coating with IONPs or gadolinium complexes. For example, Anbazhagan et al. developed a core-shell magnetic hybrid made of MoS 2 and In another work [138], WS 2 nanosheets were pre-adsorbed with IONPs, and after coating with PEGylated mesoporous silica shell (MS), the obtained WS 2 -IONP@MS-PEG displayed strong superparamagnetic properties and high NIR absorbance, and it was used for T 2 -weighted MRI. Yu et al. explored magnetic targeted PTT by the combination of MoS 2 flakes and Fe 3 O 4 NPs [146]. In this work, MoS 2 -Fe 3 O 4

gadolinium

[START_REF] Anbazhagan | MoS 2 -Gd chelate magnetic nanomaterials with core-shell structure used as contrast agents in vivo magnetic resonance imaging[END_REF]

. This system exhibited enhanced paramagnetic properties with 4.5 times longer water proton spin-lattice relaxation time when compared to commercial gadolinium Cas. MRI revealed a hyperintense signal in vivo and showed that it was easily cleared from the body.

  Liu et al. prepared multifunctional theranostic nanosheets based on MoS 2 for multimodal imaging-guided cancer therapy[START_REF] Liu | Iron oxide decorated MoS 2 nanosheets with double PEGylation for chelator-free radiolabeling and multimodal imaging guided photothermal therapy[END_REF]. In this work, MoS 2 nanosheets were functionalized with IONPs and positron-emitting radioisotope64 Cu without using a chelating agent for PET, PAI and MRI. Under the guidance of the triple imaging modalities, in vivo PTT using MoS 2 -IONP-PEG achieved effective tumor ablation in 4T1 animal tumor model. Very recently, Meng et al. decorated MoS 2 with Cu 1.8 S NPs via in situ growth [148]. After conjugating MoS 2 /Cu 1.8 S with a targeting aptamer and PEG, multifunctional MoS 2 hybrid

	nanosheets with a high photothermal conversion efficiency (32.5%) were obtained. The
	fluorescent MoS 2 and Cu 1.8 S NPs allowed photoluminescence imaging and PAI,
	respectively, as well as photothermal imaging for multimodality imaging diagnosis of

cancer. In addition, it enabled to target A549 cells specifically and deliver molecular beacon probe with the miRNA-155 to detect intracellular microRNA and DOX for chemotherapy under NIR irradiation. This targeted chemo-photothermal therapy

Acknowledgments

This work was supported by the Centre National de la Recherche Scientique (CNRS), by the Agence Nationale de la Recherche (ANR) through the LabEx project Chemistry of Complex Systems (ANR-10-LABX-0026_CSC) (to A.B.), and by the International Center for Frontier Research in Chemistry (icFRC). The authors gratefully acknowledge financial support from ANR (ANR-15-GRFL-0001-05) (G-IMMUNOMICS project).