Porphyrin‐Based Metal–Organic Frameworks for Biomedical Applications

Abstract Porphyrins and porphyrin derivatives have been widely explored for various applications owing to their excellent photophysical and electrochemical properties. However, inherent shortcomings, such as instability and self‐quenching under physiological conditions, limit their biomedical applications. In recent years, metal–organic frameworks (MOFs) have received increasing attention. The construction of porphyrin‐based MOFs by introducing porphyrin molecules into MOFs or using porphyrins as organic linkers to form MOFs can combine the unique features of porphyrins and MOFs as well as overcome the limitations of porphyrins. This Review summarizes important synthesis strategies for porphyrin‐based MOFs including porphyrin@MOFs, porphyrinic MOFs, and composite porphyrinic MOFs, and highlights recent achievements and progress in the development of porphyrin‐based MOFs for biomedical applications in tumor therapy and biosensing. Finally, the challenges and prospects presented by this class of emerging materials for biomedical applications are discussed.


Introduction
Porphyrins are macromolecular heterocyclic compounds composed of porphin (C 20 H 14 N 4 )s ubstituted by various functional groups at the meso-position or b-position. [1] In addition, free-base porphyrins can be coordinated with numerous metal ions at the porphyrin center to form metal complexes,also known as metalloporphyrins. [2] In fact, many porphyrins and metalized porphyrin derivatives,s uch as cytochrome,h eme,a nd chlorophyll, exist in nature and play significant roles in organisms.T he family of porphyrins discovered up to now have been comprehensively studied. [3] Due to their large p-aromatic system, porphyrins display excellent chemical and thermal stability as well as distinct photophysical and electrochemical properties,w hich can be regulated by the substitution patterns on porphin and the coordinated metal ions.B esides the coordination of metal ions at the porphyrin center, the periphery of porphyrins can also be bound to metal ions. [4] Intricate binding modes enable porphyrins to form desirable molecular cages or framework solids. [5] Porphyrins and porphyrin derivatives are an important class of organic chromophores with considerable absorption in the visible section of the electromagnetic spectrum. Ty pically,t he absorption spectrum shows the strongest absorption around 400-450 nm (Soret band) and as eries of absorption bands between 500 and 700 nm (Q-bands) with gradually reduced intensity. [1] Porphyrins and porphyrin derivatives have been used in various applications due to their characteristics and versatile functions ( Figure 1). Forinstance,owing to their pronounced visible-light absorption and energy-transfer properties,p orphyrins can be used in light-harvesting, solar cells,a nd molecular electronics. [6] Thec hemical catalytic activity of porphyrins,e specially metalloporphyrins,r ender them efficient photocatalysts,e lectrocatalysts,a nd biomimetic cata-lysts. [7] Ther egulation of the optical and electronic properties of porphyrins by the coordination of metal ions and the axial coordination of molecules enable their application in molecular recognition and metal-ion sensing. [8] More importantly,p orphyrins are crucial in many biological processes.They have potent biological properties,such as biocompatibility,effective clearance,long residence time in tumors,f ew side effects,a nd the mimicking of various biological functions,w hich are extremely useful for biomedical applications. [9] Forexample,porphyrins have been widely developed as photosensitizers for photodynamic therapy (PDT). At the same time,t heir fluorescence characteristics make porphyrin-based photosensitizers valuable systems for Porphyrins and porphyrin derivatives have been widely explored for various applications owing to their excellent photophysical and electrochemical properties.H owever,i nherent shortcomings,such as instability and self-quenching under physiological conditions,limit their biomedical applications.I nrecent years,m etal-organic frameworks (MOFs) have received increasing attention. The construction of porphyrin-based MOFs by introducing porphyrin molecules into MOFs or using porphyrins as organic linkers to form MOFs can combine the unique features of porphyrins and MOFs as well as overcome the limitations of porphyrins.T his Review summarizes important synthesis strategies for porphyrin-based MOFs including porphyrin@MOFs,p orphyrinic MOFs,a nd composite porphyrinic MOFs,and highlights recent achievements and progress in the development of porphyrin-based MOFs for biomedical applications in tumor therapyand biosensing.Finally,t he challenges and prospects presented by this class of emerging materials for biomedical applications are discussed.
From the Contents

Introduction 5011
fluorescence imaging-guided therapy. [9b] However,t he drawbacks of most porphyrins and porphyrin derivatives,i n particular instability,e nzymatic degradation, nonspecific targeting, and propensity to self-quench under physiological conditions,s eriously limit their biomedical applications.T o overcome these problems,v arious carriers have been developed to encapsulate,p hysically adsorb,o rc ovalently bind porphyrins and porphyrin derivatives,s uch as micelles, [10] liposomes, [11] carbon nanotubes, [12] inorganic nanoparticles, [13] and polymer nanoparticles. [14] Metal-organic frameworks (MOFs) are ac ategory of hybrid porous coordination polymers with two-or threedimensional (2D/3D) topologies assembled by the coordination of metal ions/secondary building units (SBUs) with organic linkers.Asanemerging class of material, MOFs have recently attracted increasing attention because of their multiple merits including ultrahigh porosity, [15] tunable pores, adjustable structure and composition, unsaturated metal sites,a nd functional diversity. [16] Furthermore,t he particle size of MOFs can be easily decreased down to the nanoscale, and nanoscale MOFs (NMOFs) are suitable as delivery systems.C ompared with other types of carriers,N MOFs are more promising as versatile nanoplatforms for biomedical applications,s uch as drug delivery,t umor therapy,b ioimaging, and biosensing. [17] Accordingly,i tw as proposed that the integration of porphyrins into MOFs could lead to multifunctional carriers having promising characteristics and versatile functional components.F rom this perspective,p orphyrin-based MOFs are ideal biomaterials and their full potential has not yet been tapped. Moreover,the conceptual construction of porphyrinbased MOFs matches well with advanced design concepts for highly active biomaterials that maximize integrated functional building units while minimizing inactive constituents. [18] Therefore,much effort has been devoted to the development of various porphyrin-based MOF platforms for biomedical applications.T he loading of porphyrins into MOF channels and the decoration of porphyrins on the MOF surface are efficient strategies for the synthesis of porphyrin@MOFs that enhance the stability of the porphyrin and facilitate potential applications. [19] Interestingly,free-base porphyrins and metalloporphyrins can also be used as organic linkers to assemble porphyrinic and metalloporphyrinic MOFs,r espectively.I n Jiajie Chen is currently studying Materials Science and Engineering at University of Shanghai for Science and Technology and has been in the Master's program since 2017. His research interests focus on the synthesis of multifunctional porphyrinic MOFsf or biomedicala pplications in cancer therapy and biosensing.  this way,t he self-aggregation and self-quenching of the porphyrins can be prevented and the physicochemical properties improved. Due to the highly active frameworks and improved performance of porphyrins in MOFs,p orphyrinbased MOFs present apromising opportunity for biomedical applications.
Numerous excellent reviews in the past two decades have separately addressed porphyrins and MOFs,and their respective potential biomedical applications.H owever,s of ar ac omprehensive review linking porphyrins to MOFs for biomedical applications has been missing. In this Review,we summarize the latest achievements and progress in the field of porphyrin-based MOFs,from synthesis strategies all the way to functions and applications.W ed iscuss numerous biomedical applications to provide aconcise overview and motivation for further work in this promising direction. Furthermore,we also discuss the challenges and prospects for biomedical applications of porphyrin-based MOFs.

Synthesis of Porphyrin-Based MOFs
At remendous number of MOFs,f or example ZIFs (zeolite imidazolate frameworks), [20] MILs (MatØriaux de l'Institut Lavoisier), [21] and UiOs (Universitetet iOslo), [22] have been successfully synthesized, and provide efficient platforms to encapsulate or deliver pharmaceuticals,imaging agents,a nd enzymes.F or developing biomedical nanoplatforms,the downsizing of MOFs to the nanoscale (10-100 nm) is essential because of the significant influence on the sizedependent function and biodistribution of administered particles.T odate,avariety of strategies have been proposed for the synthesis of NMOFs and have been summarized in several reviews. [23] Particularly for the synthesis of NMOFs, several efficient methods have also been developed, including hydrothermal/solvothermal, sonochemical, and mechanochemical methods,m icrowave-assisted synthesis,a nd reverse microemulsion. However,there have been no review articles on the synthesis of porphyrin-based MOFs.
Ty pically,p orphyrin-based MOFs can be categorized as porphyrin@MOFs and porphyrinic MOFs,a ccording to the distinct structures assembled from porphyrins and MOFs.For porphyrin@MOFs,f ree-base porphyrins or metalloporphyrins are integrated as guest molecules into MOFs via encapsulation in the pores or through adsorption or grafting on the surface.I np orphyrinic/metalloporphyrinic MOFs, free-base porphyrins/metalloporphyrins act as organic linkers and coordinate with metal ions or SBUs. In addition, porphyrinic/metalloporphyrinic MOFs can be combined with other functional components,l ike magnetic nanoparticles,p hotothermal agents,a nd fluorescent quantum dots,t o form multifunctional platforms.T he morphology,s tructure, and composition of porphyrin-based MOFs are vital for biomedical applications,and much effort has been devoted to synthesizing porphyrin-based MOFs to meet the requirements of biomedical applications.

Synthesis of Porphyrin@MOFs
To synthesize porphyrin@MOFs,f ree-base porphyrins or metalloporphyrins are typically encapsulated into the pores or decorated on the surface of MOFs,a nd in situ formation and post-synthesis methods are mainly used for these porphyrin-integrating processes ( Figure 2). [19] Thei ns itu formation method refers to ao ne-pot reaction of MOF precursors (i.e., metal nodes and organic linkers) with freebase porphyrins/metalloporphyrins.D uring the formation of the frameworks,f ree-base porphyrins/metalloporphyrins are entrapped inside MOFs like a" ship in ab ottle". In contrast, post-synthesis methods realize the encapsulation into pores or grafting on the surface of MOFs through host-guest reactions between synthetic MOFs and free-base porphyrins/metalloporphyrins;t hese interactions include hydrogen bonding, van der Waals forces,e lectrostatic interactions, p-p stacking interactions,c ovalent bonds,and even coordinative bonds. [24] To date,avariety of porphyrin@MOFs have been successfully synthesized, and some typical examples are summarized in Table 1.
When in situ formation methods are used to encapsulate porphyrins in the interior of MOFs,s mall pore windows can prevent the leakage of porphyrins as well as facilitate the diffusion of smaller molecules.E ddaoudi et al. were the first to successfully decorate MOFs with porphyrins by in situ formation in 2008. [25] Theencapsulation of cationic 5,10,15,20tetrakis(1-methyl-4-pyridinio)porphyrin (TMPyP) in rho-ZMOF (indium-imidazole-dicarboxylate-based zeolite-like metal-organic framework) was achieved via the one-pot reaction of In 3+ ,4 ,5-imidazoledicarboxylica cid (H 3 ImDC), and TMPyP in an N,N'-dimethylformamide (DMF)/acetonitrile (CH 3 CN) solution. Furthermore,M n, Co,C u, and Zn ions could be used for the metalation of the encapsulated free-base porphyrins.
Due to the simplified synthesis and high efficiency of porphyrin encapsulation by the in situ formation method, many groups have attempted to synthesize porphyrin@MOFs as functional platforms for biomedical applications.A mong them, porphyrin@UiO-66 and porphyrin@MIL-88(Fe) have been successfully synthesized, in which the free-base porphyrins/metalloporphyrins are protected by encapsulation and which also contain biocompatible Zr and Fe ions. [26] For example,D ong et al. reported the synthesis of ap orphyri-n@UiO-66 material in which UiO-66 is used to encapsulate 5,15-di(4-carboxyphenyl)-10,20-bis(4-iodophenyl)porphyrin zinc(II) (DTPP(Zn)-I 2 )b yt he in situ formation method. [26a] Thel oading of DTPP(Zn)-I 2 in UiO-66 was estimated to be ca. 1.4 wt %, which was much higher than that obtained by the normal impregnation method (0.06 wt %).
Forp ost-synthesis functionalization, the construction of porphyrin@MOFs generally requires appropriate interactions between the porphyrins and MOFs,a nd the following issues require consideration: 1) MOFs should be activated to eliminate solvent molecules in the pores or channels before decoration.
2) Thesize and shape of the porphyrins should match the dimensions of the MOFs pores or channels;t his allows the entrance of porphyrins throughout the integration process.
3) Bond formation should be triggered between the porphyrin and the framework.

Synthesis of Porphyrinic MOFs
Porphyrins are macromolecular heterocyclic compounds that can be used as organic linkers to coordinate with metal ions or SBUst of orm porphyrinic MOFs.S uch porphyrinic MOFs feature porphyrin functionality but also provide high porosity to host further secondary functional components.In 1991 Robson et al. described the first porphyrinic MOFs, [28] which were assembled from 5,10,15,20-tetrakis(4-pyridyl)por-phyrin palladium(II) (TPyP(Pd)) as linkers and Cd 2+ ions as nodes.I nterestingly,S BUs, also termed metal clusters,c ould be formed through stable metal-oxygen bonds or metalnitrogen bonds.S uch units play ak ey role in improving structural stability,g enerating more coordination sites,p romoting framework extension, preventing structural interpenetration, and expanding pores in MOFs.Therefore,SBUs as nodes can be used to construct robust porphyrinic MOFs with high stability,and recently increasing attention has arisen due to their potential biomedical applications. [29] In 2002, Suslick et al. reported the first stable porphyrinic MOF (PIZA-1, porphyrinic Illinois zeolite analogue No.1 )w ith SBUsasnodes, [30] which was synthesized via self-assembly of trinuclear Co II -carboxylate clusters and Co III -metalloporphyrin linkers through as olvothermal treatment. These porphyrinic MOFs have large and refillable tridirectional channels and exhibit significant hydrophilic character.
Ther ational choice of porphyrins plays av ital role in regulating the pores,s hapes,a nd sizes of porphyrinic MOFs during the synthesis. [31] Carboxy-based porphyrins,s uch as TCPP and its metalized molecules (TCPP(M)), have been widely used as linkers to synthesize porphyrinic MOFs. [31a] Generally,f ree-base porphyrins can form porphyrinic MOFs without metal chelation, while the cores of free-base porphyrins can be pre-metalized or metalized by in situ chelation or post-synthesis chelation with various metal ions to generate metalloporphyrinic MOFs.O nt he other hand, it is interesting that weakly coordinated metal ions can be replaced by other more strongly binding metal ions, [32] and thereby there are alternative approaches for the synthesis of stable porphyrinic MOFs.
To date,various metal nodes,such as Cu, Zn, Co,and Cd ions,h ave been used to assemble with TCPP or TCPP(M) linkers to form 2D MOFs (Figure 3). [6e, 33] Forexample,Zhang et al. reported the first surfactant-assisted method to synthesize homogeneous ultrathin 2D MOF nanosheets (Zn-TCPP) with at hickness of less than 10 nm. [33c] In 2D Zn-TCPP nanosheets,o ne TCPP ligand is linked to four Zn paddlewheel metal nodes (Zn 2 (COO) 4 )a nd the TCPP ligand is Thecoordination of TCPP or TCPP(M) with Mn-, Fe-, Zr-,and Hf-based clusters results in the formation of porphyrinic MOFs with typical 3D topologies ( Figure 3). [34] Zhou et al. synthesized several porphyrinic MOFs (porous coordination networks,PCNs) with different topologies by asolvothermal method; [35]  Interestingly,m any studies have proposed the coordination between metal nodes and two or more kinds of linkers to construct multivariate MOFs for introducing multiple functionalities into MOFs. [36] Here,t he introduction of other organic linkers into porphyrinic MOFs can regulate the physicochemical properties of porphyrins or endow frameworks with additional functions.F or instance,Z hou et al. developed ap hotochromic MOF (SO-PCN,s inglet oxygengenerating porous coordination network), which was constructed by self-assembly of dual linkers,1 ,2-bis(2-methyl-5-(pyridin-4-yl)thiophen-3-yl)cyclopent-1-ene (BPDTE) and TCPP,w ith Zn nodes. [36a] Due to the integration of the photochromic switch BPDTE into the porphyrinic MOF,SO-PCN displayed reversible control over the photosensitization of porphyrins by an energy transfer process upon irradiation at specific wavelengths,and thereby met the requirements for light-controlled applications.
Va rious porphyrinic MOFs with different structures have been synthesized due to the diversity of porphyrins and metal nodes.Similar to other MOFs,the downsizing of porphyrinic MOF crystals to the nanoscale can also be achieved to improve physiochemical and biological properties for biomedical applications.S olvothermal synthesis is by far the most common method for synthesizing porphyrinic NMOFs. [35d, 37] Here,the types of metal nodes and porphyrins, the reaction conditions,solvent species,stoichiometry,molecular modulators,t emperature,a nd reaction time are the key factors to control the size and morphology of porphyrinic NMOFs,w hich should be carefully considered in synthesis process.T able 2s ummarizes typical conditions for the synthesis of porphyrinic MOFs for biomedical applications.

Synthesis of Composite Porphyrinic MOFs
To endow porphyrinic MOFs with multifunctionality,t he incorporation of functional components into porphyrinic MOFs to construct composite porphyrinic MOFs is au seful strategy.T od ate,v arious functional components (e.g., magnetic nanoparticles,u pconversion nanoparticles (UCNPs), biomolecules) have been introduced to form composite porphyrinic MOFs for biomedical applications. [38] Forexample,Zhang et al. reported the growth of porphyrinic MOFs on the surfaces of polydopamine nanoparticles, graphene oxide sheets,a nd gold nanorods to form coreshell composite porphyrinic MOFs. [38g] This strategy was driven by controlling the coordination interactions between the functional groups of the nanostructures and the Zr metal nodes,which not only prevented the self-nucleation of MOFs in solution but also controlled the thickness of porphyrinic MOFs.More interestingly,Lietal. reported in situ growth of porphyrinic MOFs on polyvinylpyrrolidone (PVP)-coated lanthanide-doped UCNPs to form UCNPs/PMOF heterodimers with asymmetric compositions. [38c] In this context, the surface interactions including electrostatic forces,h ydrophobic interactions,a nd covalent or coordinate bonds,a re critical factors to control composite porphyrinic MOFs with well-defined morphology.T herefore, surface modifications are necessary for the fabrication of composite porphyrinic MOFs.T he intricate interactions in composite porphyrinic MOFs also affect the degradation characteristics.F or example,the high affinity of Zr cations to phosphate anions can result in the disassembly of Zr-based porphyrinic MOFs.R en et al. evaluated the degradation degree of platinum-decorated PCN-224 (PCN-224-Pt) and PCN-224 in phosphate-buffered saline (PBS). [39] Ther esults indicated that PCN-224-Pt degraded and released TCPP ligands more slowly than PCN-224 due to the strong covalent interaction between Pt and PCN-224. Moreover,d ecorating the surface of MOFs with biocompatible materials (e.g., polyethylene glycol (PEG), PVP) can improve their stability and prolong circulation time in biological environments. Although av ariety of composite porphyrinic MOFs have been synthesized, the complicated synthesis process,l ow yield, and highly wasteful production still restrict their development in biomedical applications.T able 3s ummarizes some typical examples of composite porphyrinic MOFs for biomedical applications.

Biomedical Applications of Porphyrin-Based MOFs
Porphyrin-based MOFs offer av ariety of attractive features making use of the integration of porphyrins into MOFs,i np articular excellent photophysical and electrochemical properties,porous structure,modular functionalization, and biocompatibility.These characteristics are beneficial for applications of porphyrin-based MOFs in biomedicine, which have attracted more and more attention. Nowadays, numerous interesting biomedical applications,s uch as drug delivery,t umor therapy,b ioimaging and biosensing, have been developed for porphyrin-based MOFs.Inthe following, we summarize recent progress in the area of biomedical applications of porphyrin-based MOFs,i ncluding photodynamic therapy (PDT), synergistic therapy,i maging-guided therapy,and biosensing ( Figure 4).

Photodynamic Therapy with Porphyrin-Based MOFs
PDT is an efficient tumor therapy strategy that requires three essential factors:p hotosensitizer,l ight, and molecular oxygen in cells. [40] As aresult, PDT generates highly cytotoxic reactive oxygen species (ROS) under light irradiation. ROS including singlet oxygen ( 1 O 2 ), superoxide anion radical (O 2 À C), and hydroxyl radical (COH) can induce tumor cell death through apoptosis and/or necrosis,a nd even tumor immunity.B yi ntravenous injection of photosensitizers for accumulation in tumor tissue and local exposure of tumor sites to light, PDT can selectively destroy tumor cells while damage to the surrounding normal cells and tissues is minimized. [41] This approach profits from being non-invasive and has fewer side effects (e.g.,r adiation damage,d rug toxicity) than conventional surgery,radiotherapy,and chemotherapy. Porphyrin and its derivatives are excellent photosensitizers for PDT and several have been approved for clinical trials.F or porphyrin-based MOFs,the high porphyrin loading capacity and free diffusion of oxygen and ROSinthe porous material allow for highly efficient PDT.F urthermore,t he biocompatibility and biodegradability of MOFs result in enhanced biosafety during PDT.S ince the first report on porphyrinic NMOFs for potential tumor PDT by Lin et al. in 2014, [42] numerous porphyrin-based MOFs have been designed for PDT.T he most important advances are summarized in Tables 1-3. 3.1.1. Enhanced Photodynamic Therapy In general, the key factor for PDT is the quantity of 1 O 2 generated by photosensitizers upon light irradiation, which can be regulated by the efficiency of the intersystem crossing (ISC) of the photosensitizers from the singlet state (S 1 )tothe triplet state (T 1 ). [43] Interestingly,t he introduction of heavy atoms into photosensitizers is known to enhance ISC and is the "heavy atom effect". In porphyrin-based MOFs,t he interactions between porphyrins and metal ions can improve the photodynamic properties of porphyrins on account of the "heavy atom effect", and thereby increase the production of 1 Thefirst report on porphyrinic MOFs for PDT was the use of DBP-UiO nanoplatelets,which were assembled from 5,15di(p-benzoato)porphyrin (DBP) as linkers and Hf 4+ ions as metal nodes ( Figure 5). [42] DBP-UiO nanoplates had ah igh porphyrin loading capacity of 77 wt %, and showed at least twice as efficient 1 O 2 production as free DBP.C orrespondingly,D BP-UiO nanoplates presented highly enhanced PDT efficacya gainst human head and neck tumor cells (SQ20B) compared to free DBP.T herefore,t he coordination of porphyrin linkers with heavy metal nodes in porphyrinic MOFs lead to an enhancement on PDT,caused by the "heavy atom effect", and the same phenomenon was observed in other porphyrinic MOFs. [33d, 39, 44] Fore xample,G d-TCPP MOF nanosheets,w hich were linked by TCPP with Gd ions, showed improved photosensitive activity compared to free TCPP. [33d] On the other hand, the encapsulation of porphyrins in MOFs can also enhance ISC together with the 1 O 2 generation. Lei et al. encapsulated TMPyP into HKUST-1a nd found that the narrowed S 1 -T 1 energy gap of TMPyP caused by the interaction between the encapsulated TMPyP and Cu 2+ nodes enhanced the ISC.A saresult, TMPyP@H-KUST-1 exhibited higher 1 O 2 production capability compared to free TMPyP,a nd thereby induced higher phototoxicity to tumor cells. [45] To optimize the photophysical properties and PDT efficacyofporphyrin-based MOFs,the reduction of porphyrins to chlorins by the hydrogenation of the cross-conjugated double bond of porphyrin ring is also an effective strategy, which could result in ared-shift of absorption and an increase of the extinction coefficient. [46] Lin et al. described the first chlorin-based NMOFs (DBC-UiO), which were synthesized from Hf 4+ nodes and 5,15-di(p-benzoato)chlorin (DBC), and found an enhanced PDT effect for colon tumors. [47] There was a13nmred shift in absorption and an 11-fold augmentation in the extinction coefficient of the longest-wavelength Qb and. Consequently,D BC-UiO NMOFs were three times as efficient as DBP-UiO NMOFs in the generation of 1 O 2 and exhibited amuch stronger PDT effect in dual colon tumor cell lines (CT26 and HT29). Therefore,t he use of chlorins to construct NMOFs provides nanoplatforms for realizing highly efficient PDT owing to the improved photophysical properties of the reduced porphyrins.
Generally,t he accumulation of photosensitizers in tumor tissues and further uptake by tumor cells facilitates efficient PDT.I ti sw ell known that the size of NMOFs is of crucial importance during cellular uptake. [48] In many studies the MOF particle size has been optimized to further enhance PDT efficacy. Fore xample,Z hou et al. synthesized PCN-224 with sizes ranging from 30 to 190 nm to investigate the effect of particle size on cellular uptake and PDT efficacy. [35d] These PCN-224 nanoparticles showed different uptake levels in Hela cells,a nd the 90 nm sized PCN-224 had am aximal cellular uptake amount, as determined by inductively coupled plasma mass spectrometry (ICP-MS). Furthermore,the PDT for Hela cells indicated that the 90 nm-sized PCN-224 induced 81 %c ell apoptosis after PDT treatment, which was much higher than that induced by other sized PCN-224, suggesting that the size-dependent cellular uptake of NMOFs determines PDT efficacy.

Light-Controlled Photodynamic Therapy
Controllable generation of 1 O 2 is useful for enhancing PDT efficacy, which requires the release of cytotoxic 1 O 2 only in tumor sites and less damage to normal cells and tissues. [49] Light, as an essential exogenous condition for PDT,has been utilized to control the generation of 1   However,i fS O-PCN is irradiated at l = 365 nm, BPDTE is transformed into the closed state,w hich is lower in energy than the TCPP,a nd thereby energy transfer happens to the closed BPDTE, resulting in the quenching of 1 O 2 generation. Therefore,the introduction of BPDTE in porphyrinic MOFs can modulate 1 O 2 regeneration and this strategy shows great potential for controllable PDT.
Thea bsorption spectra of porphyrins are mostly in the visible light range,r esulting in poor tissue penetration, and thereby there are limits for in vivo PDT with porphyrin-based MOFs.Near-infrared (NIR) light has certain advantages over visible light including deeper tissue penetration, and higher sensitivity and resolution. This makes it attractive to harvest NIR light with porphyrin-based MOFs for NIR-triggered PDT.S ome studies demonstrated that lanthanide-doped UCNPs can transform NIR light to visible light and thereby excite photosensitizers under NIR irradiation for enhanced PDT. [50] Recently,s everal studies reported the combination of porphyrin-based MOFs with UCNPs for NIR-triggered PDT with enhanced therapeutic efficacy. [

Positive Targeted Photodynamic Therapy
In general, nanoparticles have ap assive targeting ability for improving accumulation of their cargos at the tumor site due to the enhanced permeability and retention (EPR) effect. However,surface modification can endow nanoparticles with an active targeting ability,f acilitating precise positioning at the targeted site and promoting cellular uptake.D ue to abundant functional groups and metal nodes on the surface, porphyrin-based MOFs can be easily modified by host-guest reactions,and now various porphyrin-based MOFs have been modified with targeting moieties for enhanced PDT. [35d, 36e,51] Zhou et al. modified PCN-224 NMOFs with folic acid (FA), of which the receptor (FAR) is overexpressed in tumor cells, [35d] through the coordination between the carboxylate groups of FA and Zr 6 clusters.T he results showed that FAmodified PCN-224 NMOFs had better cellular uptake by FAR-positive Hela cells than PCN-224 NMOFs due to the FA receptor-mediated endocytosis,a nd further improved PDT efficacya gainst Hela cells.Z hang et al. reported hyaluronic acid (HA)-coated Zr IV -based porphyrinic NMOFs (PZM) for enhanced PDT due to the CD44-targeting of HA to CD44overexpressed tumor cells. [51] Thei nv ivo results confirmed that more HA-coated PZM NMOFs were distributed in CD44-positive tumor sites compared to PZM NMOFs,a nd they exhibited remarkable tumor growth inhibition.
In addition, modification of porphyrin-based MOFs with specific biomolecules,s uch as cell membranes,D NA,a nd antibodies,a lso results in active targeting ability. [

Tumor Microenvironment-Associated Photodynamic Therapy
Porphyrins show great potential for PDT,but the oxygen dependence limits the therapeutic efficacyo fP DT owing to the hypoxia conditions in most tumor cells/tissues. [53] Moreover, the oxygen consumption in PDT exceeds the oxygen supply by tumor blood vessels,w hich further aggravates hypoxia in tumor cells/tissues,a nd thereby decreases PDT efficacy. [54] To overcome hypoxia in the tumor microenvironment, combining MOFs with other adjuvants (e.g., oxygen carriers,p eroxidase,a nd interference agents of oxygen consumption [51,55] )o ru sing its own components (e.g., Cu, Fe,M nm etal ions in MOF skeletons for Fenton-like reactions [56] )i sa ne fficient strategy to increase the intratumoral O 2 level and thereby achieve better therapeutic efficacyfor PDT.
Ren et al. reported platinum-decorated PCN-224 (PCN-224-Pt) NMOFs for enhanced PDT ( Figure 6). [39] Due to the higher level of H 2 O 2 in the tumor microenvironment, Pt nanoparticles with catalase-like activity could catalyze the decomposition of intratumoral H 2 O 2 to generate O 2 ,w hich could facilitate the further generation of cytotoxic 1 O 2 to kill tumor cells.I nv itro results showed that PCN-224-Pt presented much higher lethality upon 638 nm laser irradiation than that of PCN-224 under the hypoxia conditions.T he in vivo antitumor potential of PCN-224-Pt in an H22 tumorbearing mice model confirmed that tumor growth was completely inhibited after the injection of PCN-224-Pt and subsequent irradiation, but only partial tumor inhibition was observed for PCN-224 after PDT.L in et al. proposed to use 5,10,15,20-tetra(p-benzoato)porphyrins (TBP) as linkers and Fe 3 Oclusters as metal nodes for the synthesis of porphyrinic NMOFs (Fe-TBP), in which the Fe 3 Oc lusters could decompose the intratumoral H 2 O 2 through the Fenton reaction to produce more O 2 for PDT and thereby effectively circumvent cellular hypoxia. [56a] Compared to free TBP and Hf-TBP NMOFs constructed from Hf-based clusters and TBP linkers, Fe-TBP NMOFs showed highest PDT efficacyu nder both normoxic and hypoxic conditions.O bviously,F e-TBP NMOFs are promising for enhanced PDT in hypoxia due to the use of inherent ingredients in porphyrinic MOFs without the addition of other adjuvant agents.
In addition, the increase of the ROSlevel can be achieved by decreasing the glutathione (GSH) level in tumor cells, because GSH can weaken the ROSp roduction from the photosensitizers and further decrease PDT efficacy. [57] To decrease the GSH level in tumor cells,L ie tal. constructed metalloporphyrinic NMOFs (MOF-2) that assembled from Al 3+ nodes and TCPP(Cu) linkers. [58] As the active center of MOF-2, Cu 2+ can specifically bind and absorb GSH and thus decrease the GSH level, thereby increasing the ROSl evel. Compared to NMOFs without Cu 2+ centers (MOF-1), MOF-2 had aG SH-binding role and generated higher concentration of ROS. In vitro results showed that the therapeutic effect of MOF-2 was comparable to that of the antineoplastic drug camptothecin (CPT). These results offer an interesting idea for controlling the tumor microenvironment by directly decreasing the intracellular GSH level to enhance PDT efficacy.
Specific tumor microenvironment-associated controllable 1 O 2 release is also an attractive strategy for enhanced PDT. [59] Fori nstance,T ang et al. developed ab imetallic porphyrinic MOF (NP-1), which could be activated by hydrogen sulfide (H 2 S) signaling molecules to control 1 O 2 release for PDT in the microenvironment of ac olon adenocarcinoma tumor. [59a] NP-1 was synthesized by the self-assembly of TCPP(Zn) linkers and Cu 2+ nodes.I nv itro results showed that NP-1NMOFs were activated by H 2 Si nc ells to induce apoptosis of tumor cells upon irradiation due to the release of Cu 2+ from NP-1 NMOFs.I nv ivo antitumor results also demonstrated that the high H 2 Sl evels in colon adenocarcinoma tumors significantly enhanced the antitumor efficacyo wing to the H 2 S-responsive PDT.
More recently,Jiang et al. found that 2D Cu-TCPP MOF nanosheets exhibited the selective generation of 1 O 2 in the tumor microenvironment and the depletion of GSH, presenting augmented tumor therapeutic efficacy. [60] Here,the TCPP linkers in the nanosheets could be peroxided in the presence of H 2 O 2 and the acidic pH in the tumor microenvironment, and be further reduced to peroxyl radicals (ROOC)w ith the help of peroxidase-like Cu-TCPP MOF nanosheets and Cu 2+ ions. 1 O 2 could be generated in the spontaneous recombination reaction of ROOC according to the Russell mechanism. Furthermore,G SH could be depleted and converted into oxidized glutathione (GSSG) by the cycle conversion of Cu 2+ and Cu 1+ in Cu-TCPP MOF nanosheets.C onsequently,C u-TCPP MOF nanosheets selectively killed tumor cells without side effects both in vitro and in vivo.T hese therapeutic strategies based on the tumor microenvironment can avoid the oxygen dependence and light penetration limitations inherent in PDT,o ffering inspiration for other tumor treatments.

Synergistic Therapy of Porphyrin-Based MOFs
To date,numerous therapy modalities based on nanoplatforms,i ncluding chemotherapy,P DT,p hotothermal therapy (PTT), radiotherapy (RT), immunotherapy,a nd gas therapy, have been developed as effective techniques for treating malignant tumors. [59c, 61] However,s ingle therapy modality often fails to achieve ideal therapeutic efficacyd ue to limitations,such as multidrug resistance,oxygen dependence, nonspecific heating, and serious side effects.H ence,m uch effort has been devoted to developing multifunctional nanoplatforms,w hich integrate two or more therapy modalities into asingle system. Porphyrin-based MOFs exhibit excellent PDT efficacy due to the photophysical properties of porphyrins,a nd porous structures endow them with controlled drug delivery.F urthermore,p orphyrin-based MOFs can integrate other functional components to obtain multifunctionality. Hence porphyrin-based MOFs are able to combine two or more efficient therapy modalities for tumor therapy that can achieve synergistic effects for maximizing therapeutic efficacy.

Synergistic Chemo-and Photodynamic Therapy
Thecombination of PDT and chemotherapy is an efficient dual-modality therapy approach that has shown enhanced therapeutic efficacy and negligible toxicity. [62] Here,PDT can generate toxic ROSi nt umor cells/tissues that induce oxidative damage to intracellular protein and DNA, making the tumor cells highly sensitive to toxic chemotherapeutic drugs. [56b,63] Thus,the use of lower doses of chemotherapeutic drugs could bring the desired antitumor effect with reduced toxicity for normal cells/tissues.T he open porosity,s table structure,a nd low cytotoxicity of porphyrin-based MOFs make them suitable drug delivery vehicles for encapsulating chemotherapeutics,a nd the intrinsic nature of porphyrinbased MOFs supports satisfactory drug-release behavior to enhance therapeutic efficacy. [64] Hence,d rug-loaded porphyrin-based MOFs show great potential for synergistic chemoand photodynamic therapy.
Yine tal. reported biocompatible zirconium porphyrinic NMOFs (NPMOFs) for synergistic chemotherapy and PDT. [37a] NPMOFs showed ad oxorubicin (DOX)l oading efficiency as high as 109 %(w/w), which was attributed to the noncovalent interactions between DOXa nd NPMOFs,s uch as p-p stacking effects,hydrophobic interactions,and electrostatic interactions.D rug release profiles showed av ery slow DOX release rate in an ormal biological environment (only 10.3 %ofDOX released at pH 7.0 after 72 h), but afast DOX release in the tumor microenvironment (58.5 %o fD OX released at pH 5.0 after 72 h). This drug release behavior is beneficial for high therapeutic efficacy against tumor cells with few side effects for normal cells.A tt he same time,t he high porphyrin content of NPMOFs (59.8 %) resulted in excellent PDT efficacy.Atumor cell apoptosis rate of 90 % was observed upon synergistic therapy with DOX@NPMOFs upon 655 nm laser irradiation. In vivo therapy for HepG2 tumor-bearing mice achieved an enhanced therapeutic efficacy for DOX@NPMOFs compared to chemotherapy or PDT alone,i ndicating that biocompatible porphyrin-based MOFs are highly promising for synergistic chemotherapy and PDT.
Forl ight-controlled release of toxic chemotherapeutic drugs and enhanced PDT,Z hang et al. developed core-shell composite nanoparticles (AuNR@MOFs) by the growth of porphyrinic MOFs on gold nanorods (AuNR) (Figure 7). [38a] These nanoparticles were used for synergistic chemotherapy and phototherapy due to the well-defined pore structure as the drug carrier,T CPP as the photosensitizer,a nd AuNR as the photothermal agent. With camptothecin (CPT) as amodel drug, AuNR@MOFs achieved controllable CPT release upon 808 nm laser irradiation. On the other hand, AuNR@MOFs exhibited an enhanced ability to produce 1 O 2 compared to single porphyrinic NMOFs upon 660 nm laser irradiation due to the enhanced light absorption and strong electromagnetic field effect of the AuNR surface.C onsequently,A uNR@-MOFs with NIR-triggered drug release and phototherapy showed significant synergistic efficacy for killing tumor cells in vitro and inhibiting tumor growth and metastasis in vivo.

Synergistic Photothermal and Photodynamic Therapy
Similar to PDT,P TT is also at argeted and local therapeutic method with minimal invasiveness and high efficacy, which uses photothermal agents to generate heat upon NIR irradiation to ablate tumor cells and tissues. [41] In contrast to PDT,P TT is an oxygen-independent and ROSfree process mediated by photothermal agents upon NIR irradiation. Hence,t he combination of PDT and PTT for tumor treatment could synergistically enhance the therapeutic effect and reduce side effects. [65] On the one hand, the heat generated by PTT can improve blood flow as well as oxygen supply,a nd thereby intensify the sensitivity of tumor cells to oxygen-dependent PDT;o nt he other hand, the ROS produced by PDT can interfere with tumor physiology and change the microenvironment, and thereby increase the heat sensitivity of the tumor cells. [66] Recently,s everal studies on porphyrin-based MOFs for synergistic PDT and PTT have been reported. Zhang et al. proposed to assemble porphyrinic MOFs on polydopamine (PDA) nanoparticles to form core-shell nanoparticles (PDA@MOF), [38g] and the photodynamic activity of porphyrinic MOFs and the photothermal effect of PDAnanoparticles could contribute to PDT and PTT for tumors.T he results indicated that PDA@MOF nanoparticles efficiently generated intracellular ROSu pon irradiation with 630 nm laser light, and had excellent photothermal conversion upon irradiation with 808 nm laser light. Only 18 %o ft he cells were viable after incubation with PDA@MOF nanoparticles and irradiation with 630 nm and 808 nm laser light. This was much lower than the viability following single laser irradiation, suggesting synergistic tumor inhibition.
Besides composite porphyrinic NMOFs,p orphyrinic NMOFs without the integration of other PTT agents also have been developed for synergistic PTT and PDT,w hich shorten the synthesis procedures and improve biosafety.F or example,Y ang et al. reported novel multifunctional metalloporphyrinic NMOF (Zr-FeP) shuttles for PDT and lowtemperature PTT synergistic treatment. [66] Zr-FeP NMOFs were assembled from Zr 6 clusters and TBP(Fe) metalloporphyrins and loaded with an inhibitor of heat shock protein 70 (Hsp70). Ther esults demonstrated that Zr-FeP NMOFs catalyzed the generation of abundant COH from endogenous H 2 O 2 due to Fenton effect of the ion centers,a nd produced toxic 1 O 2 upon laser irradiation. Furthermore,Z r-FeP NMOFs showed an obvious photothermal effect and the photothermal conversion efficiency was as high as 33.7 %. Consequently,upon 635 nm laser light irradiation, the Hsp70loaded Zr-FeP NMOFs efficiently suppressed the tumor growth by synergistic PDT and low-temperature PTT.
Recently,L ie tal. proposed using Cu-TCPP MOF nanosheets as phototherapy nanosystems for synergistic PTT and PDT. [67] Thed -d energy band transition of Cu 2+ and the thickness of the ultrathin Cu-TCPP MOF nanosheets led to strong NIR absorption and excellent photothermal performance.T he photothermal conversion efficiencyo fC u-TCPP MOF nanosheets at 808 nm irradiation was estimated to be 36.8 %a nd 1 O 2 could be generated upon 660 nm irradiation, which induced efficient synergistic PTT and PDT in vitro and in vivo.

Synergistic Immuno-and Photodynamic Therapy
Immunotherapy has an important status in tumor therapy since it is possible to stimulate an individualso wn immune system to recognize and kill primary tumor cells as well as inhibit metastatic tumor cells. [68] Checkpoint blockade immunotherapy,w hich adopts antibodies or small molecules to block negative immune regulatory pathways for priming antitumor immune,h as been successfully used in clinic. Studies have demonstrated that tumor cells themselves can induce Tc ell apoptosis via the programmed cell death receptor 1( PD-1)/programmed death 1l igand (PD-L1) pathway for immune evasion. Consequently,t he PD-1/PD-L1 checkpoint blockade strategy is proposed to enhance antitumor immunity by inhibiting cytotoxic Tl ymphocyte exhaustion. However,t his type of immunotherapy elicits limited rates of systemic antitumor response for many types of tumors owing to insufficient activation of the host immune systems. [69] Interestingly,P DT can excite adaptive immune responses through the release of inflammatory mediators and cytokines into the intratumoral environment. Therefore,t he combination of checkpoint blockade immunotherapy and PDT can not only promote systematic immune response to inhibit the metastatic tumors,b ut also induce synergistic therapy for treating primary tumors.R ecently,t here have been some reports describing porphyrinic MOFs for synergistic immunotherapy and PDT due to easy construction of porphyrinic MOFs-based nanoplatforms.

Angewandte Chemie
Reviews 5022 www.angewandte.org coordination of tetrabenzoporphyrin linkers with 10-connected Zr 6 clusters,e xhibited efficient 1 O 2 generation in the hypoxic microenvironment for PDT.T he results demonstrated that TBP-NMOF-induced PDT could not only kill the 4T1 murine breast tumor cells,b ut also augment the presentation of tumor-activated antigens,f urther stimulating adaptive immune response for initiating the secretion of inflammatory cytokines (IFN-g,T NF-a)a nd recruiting tumor-infiltrating Tcells (CD4 + ,CD8 + ). Further,they investigated the antitumor effect of low O 2 -dependent PDT and a-PD-1 in vivo.W hen PD-1 antibodies were injected, TBP-NMOF-induced PDT dramatically inhibited the growth of 4T1 tumors without body weight loss.F urthermore,t he combination therapy was more efficient than PDT alone in preventing tumor recurrence and remarkably prevented lung metastasis of 4T1 tumors for 22 days.Similarly,Lin et al. also utilized porphyrinic MOFs (Fe-TBP) and a-PD-L1 for synergistic PDT and immunotherapy. [56a] It is interesting that the Fe 3 Oclusters in Fe-TBP NMOFs could overcome hypoxia by the Fenton reaction and enhance immunogenic PDT for priming tumor immunotherapy.
Obviously,t he use of porphyrinic MOFs for synergistic immuno-and photodynamic therapy has shown great potential for the therapy of many difficult-to-treat tumors and inhibition of metastatic tumor cells,b ecause immunogenic PDT can prime tumor immunotherapy to promote the response rates,a nd thereby significantly promote immunotherapeutic efficacy.I na ddition to checkpoint blockade immunotherapy based on antibodies or small-molecule agents,i tc an be imagined that the use of porphyrinic MOF platforms to combine PDT with other immunotherapeutic strategies would be attractive. [71]

Other Synergistic Therapies
Radiotherapy (RT), which uses ionizing irradiation such as X-rays to destroy localized solid tumors,i sacommon therapy modality in clinic. Numerous nanoplatforms containing high-Z elements,w hich are able to interact with ionizing radiation to produce photo/auger electrons and then produce reactive free radicals to destroy tumor cells,h ave been applied for RT of tumors. [72] Biocompatible and biodegradable porphyrinic MOFs are formed by the coordination of porphyrin linkers and metal nodes,which is ideal to introduce high-Z metal elements into MOFs for use in synergistic PDT and RT.Liu et al. reported Hf-TCPP NMOFs assembled from Hf 4+ and TCPP through as olvothermal process, [44] in which TCPP linkers exhibited good PDT ability.T he Hf 4+ metal centers,a sahigh-Z element, served as ar adio-sensitizer to enhance RT.D ue to the ISC enhanced by heavy Hf 4+ nodes, Hf-TCPP NMOFs were extremely effective in producing 1 O 2 upon 661 nm irradiation for PDT.I nv ivo results on 4T1 breast tumor-bearing mice showed that synergistic PDT and RT with Hf-TCPP NMOFs greatly inhibited tumor growth and had enhanced therapeutic efficacyc ompared to PDT or RT alone.
Gas therapy also has been developed for tumor therapy due to its negligible side effects and enhanced therapeutic efficacy. Va rious gaseous transmitters,s uch as hydrogen sulfide (H 2 S), nitric oxide (NO), and carbon monoxide (CO), can be utilized for gas therapy,w hich can accelerate tumor cell apoptosis,p revent tumor cell proliferation, and selectively protect the activity and physiological function of normal cells. [73] Recently,C heng et al. reported synergistic NO gas therapy and PDT to treat tumors based on l-arginine (l-Arg)-incorporated PCN-224 NMOFs (Figure 9). [52c] Here, l-Arg, anatural NO donor, could release NO for gas therapy via oxidization through ROS, which were provided by PDT with PCN-224 upon 660 nm irradiation. An in vitro MTT assay showed that the viability of 4T1 cells treated with l-Arg-incorporated PCN-224 was lower than that of cells treated with PCN-224 upon 660 nm irradiation. Of special note,ac ell apoptosis assay executed by flow cytometry showed that l-Arg-incorporated PCN-224 was much more effective in PDT under hypoxic conditions than pure PCN-224, proving that NO can sensitize PDT in hypoxic tumors. Furthermore,a ni nv ivo antitumor study showed that the therapeutic efficacyofthe PCN-224 was weaker than that of the l-Arg-incorporated PCN-224 upon irradiation with 660 nm light, indicating that synergistic PDT and gas therapy enhanced antitumor effects to ac ertain extent. This type of nanoplatform with gas therapy and PDT provides an ovel therapy paradigm to overcome the hypoxia barrier for efficient tumor treatment.
Hydrogen gas as aclean energy source has recently been discovered to have curative effects in many diseases,a nd it possesses ahuge potential in tumor treatment due to its high biosafety and satisfactory therapeutic effect. He et al. developed an anoscale Pd-MOF,w hich was assembled from Pd 2+ nodes and TPyP porphyrin linkers,f or hydrogenothermal therapy of tumors. [74] Due to the strong hydrogen-binding and catalytic hydrogenation capacity of Pd, Pd-MOF could be used to deliver highly reductive hydrogen gas.T he results showed that Pd-MOF displayed ah igh hydrogen loading capacity and continuous hydrogen release profile.I np hotothermal experiments Pd-MOF loaded with hydrogen gas showed higher photothermal conversion efficiency (44.2 %) than Pd-MOF alone (27.7 %) at 808 nm irradiation. Further in vitro and in vivo results proved the synergistic antitumor effects of hydrogen gas and PTT.
Selectively cutting off the nutrient supply and metabolic pathways of tumor cells is another efficient strategy for tumor therapy.Z hang et al. designed at umor-targeted cascade bioreactor mem@catalase@GOx@PCN-224 (mCGP) by the encapsulation of glucose oxidase (GOx) and catalase in tumor cell membrane-camouflaged PCN-224 NMOFs for synergistic starvation and PDT. [52b] It was found that mCGP could facilitate microenvironmental oxygenation by catalyzing endogenous H 2 O 2 to generate O 2 by catalase,w hich would subsequently accelerate the consumption of intracellular glucose by GOx and increase the generation of 1 O 2 under 660 nm irradiation. This could cut off tumor glucose supply and disturb glucose metabolism related cellular elements for starvation therapy as well as overcome the hypoxia issue for enhanced PDT.I nv ivo experiments revealed that mCGP possessed acertain antitumor activity without 660 nm irradiation for long-term starvation effects.O nt he other hand, mCGD showed acomplete suppression of 4T1 tumors in the presence of 660 nm irradiation without obvious side effects, which might be attributed to the significant synergistic effects of PDT and long-term tumor starvation therapy.

Imaging-Guided Therapy of Porphyrin-Based MOFs
Nanomaterials have been comprehensively studied as novel imaging agents or functional platforms for bio-imaging, which produce signals or enhance signal contrast at specific tissues.Bio-imaging enables the visualization, evaluation, and quantification of biological processes at the molecular and cellular levels in an oninvasive means,a nd shows great potential in the earlier diagnosis of diseases,on-going assessment of treatments,a nd optimization of disease therapeutic protocols.
[23b] Therefore,t he construction of nanosystems for imaging-guided therapy by the integration of therapy modalities and bio-imaging techniques is promising to maximize the therapeutic efficacyi nat ime-and position-resolved manner. [75] Over the years,p orphyrin-based MOFs have been utilized as promising fluorescence imaging-guided therapy nanosystems owing to their excellent fluorescence and therapeutic properties.I na ddition, imaging modes of porphyrin-based MOFs can be branched to magnetic resonance imaging (MRI), X-ray computed tomography (CT), photoacoustic imaging (PAI), and photothermal imaging (PTI) by employing metal nodes in MOFs or metal centers in porphyrin rings,o rb yi ncorporating other contrast agents in MOFs. Fluorescence imaging has been apowerful imaging mode for both in vitro and in vivo imaging because it is noninvasive and offers high signal sensitivity.Fluorescence imaging relies on the properties of fluorophores that absorb the photon energy in ac ertain band of wavelengths and then emit new photon energy in aband of longer wavelengths.Fluorescence imaging is rapid and suitable for high-throughput screening. [76] Porphyrins as popular fluorophores have been incorporated into MOFs for both in vitro and in vivo fluorescence imaging. [52a, 77] Theu se of fluorescence imaging to guide therapy allows the visualization of the distribution and evolution of the tumors/particles during treatment, and thereby ensures selectively controllable therapy with high efficacya nd safety.F luorescence imaging-guided therapy nanosystems based on porphyrin-based MOFs can be easily constructed for efficient therapy due to the combination of fluorescence and photosensitivity from porphyrins.
In 2017, Yine tal. developed the first biocompatible porphyrinic NMOFs for fluorescence imaging-guided tumor therapy ( Figure 10). [37a] Porphyrinic NMOFs (NPMOFs) with ahigh TCPP content (59.8 %) achieved efficient fluorescence imaging.T he results showed that the fluorescence of TCPP was observed at 651 nm with aw eaker shoulder at 710 nm when excited at 514 nm, but only one emission peak at 689 nm for NPMOFs. [78] Thestrong red emission and long Stokes shift of NPMOFs resulted in ah igh signal-to-noise ratio and resolution in fluorescence imaging due to the low excitation interference,a utofluorescence,a nd scattering light from the biological tissue.Inamouse model, the absorption, distribution, metabolism, and excretion (ADME) processes of NPMOFs were observed by tracking the fluorescent trajectory,indicating the biocompatibility of NPMOFs in mammals. Furthermore,i nH epG2 tumor-bearing mice,t he in vivo fluorescence of NPMOFs clearly confirmed the distributions of NPMOFs in tumors with high resolution and signal-tonoise ratio,suggesting that porphyrinic NMOFs are promising as fluorescence imaging-guided therapy platforms for earlier tumor diagnosis and enhanced therapy.O nt he other hand, composite porphyrinic NMOFs have also been constructed for fluorescence imaging-guided therapy. [38a, 79] Fore xample, Zhang et al. assembled porphyrinic NMOFs on AuNRs to form core-shell nanocomposites for fluorescence imagingguided PDT,PTT,and chemotherapy. [38a] Theintroduction of porphyrinic NMOFs made the nanocomposites suitable for PDT,chemotherapy,and fluorescence imaging,while AuNRs exhibited excellent PTT properties.
In addition, fluorescence imaging-guided therapy can also be used for real-time monitoring and assessment of the therapeutic efficacyaccording to the changes of fluorescence signals,a nd can be used to further improve the therapeutic efficacya nd avoid under-o ro vertreatment. Lei et al. designed ap hotosensitive and caspase-responsive porphyr-in@MOF nanoprobe,w hich was assembled from porphyrin (TMPyP), folate targeting-motif,a nd ac aspase-sensitive fluorescent dye,i.e., Cy3-labeled caspase-3 substrate peptide, in MOFs. [45] Here,the fluorescence of Cy3 was quenched until the activated caspase-3 specifically cleaved the peptide and Cy3 was released from the MOF for fluorescence imaging, resulting in ac aspase-responsive sensing strategy for monitoring cell apoptosis.T hese results demonstrated that biocompatible TMPyP@MOFs provided enhanced 1 O 2 yield and folate targeting tumor therapy as well as the ability for in situ imaging of therapeutic efficacyv ia caspase-3 activation. Thes witch-on signal provides an effective way to image intracellular caspase activity for real-time monitoring and evaluation of treatment effects.

MRI-Guided Therapy
MRI is another noninvasive medical method to obtain images of the human anatomy and physiological processes with high spatial resolution. It utilizes an external magnetic field to detect radiofrequencys ignals produced by protons, typically the hydrogen atoms in water from fat and tissues. Thea cquired signals can be utilized to compare different tissues,distinguish lesions from healthy tissues,and construct anatomical maps of the human body. [80] Compared to fluorescence imaging,M RI possesses the advantages of better spatial resolution, higher soft tissue contrast, and unlimited penetration depth, but exhibits the disadvantage of low sensitivity.M RI contrast agents are typically used to reduce proton relaxation times and further improve the image quality.F or porphyrin-based MOFs,t he metal nodes of the MOF skeleton, the chelation of paramagnetic metal ions in porphyrin rings,a nd the incorporation of other contrast agents in MOFs make MRI-guided therapy possible.
Recently,W ang et al. reported that gadolinium porphyrinic NMOF sheets (Gd-TCPP) displayed ah igh relaxation rate (40.8 mm À1 s À1 )a nd 1 O 2 production upon 660 nm irradiation due to the high Gd 3+ content in the MOFs and the TCPP photosensitizer.
[33d] Therefore,G d-TCPP nanosheets are potent for T 1 -weighted MRI-guided PDT.Yin et al. proposed to bind paramagnetic metal ions into porphyrin rings by chelation to develop metalloporphyrinic MOFs for T 1weighted MRI-guided therapy. [81] Through the chelation of paramagnetic Mn ions in porphyrin rings,metalloporphyrinic MOFs possessed excellent T 1 -weighted MR contrast capacity and high photothermal conversion and heat-responsive NO release.I nv ivo MRI experiments showed that metalloporphyrinic MOFs were efficiently accumulated at the tumor site after intravenous injection into mice,a nd tumor growth was completely inhibited when exposed to 808 nm laser for PTT and NO gas therapy,indicating the potential for MRI-guided therapy.

Multimode Imaging-Guided Therapy
Single-mode bio-imaging is usually insufficient for comprehensive imaging and cannot provide the complete characterization needed to diagnose diseases due to its inherent limitations,s uch as limited signal sensitivity,t issue penetration depth, and spatial resolution. To overcome these problems,v arious studies have concentrated on incorporating multimode imaging properties in as ingle MOF-based system. [82] Forp orphyrinic MOFs,t he use of functional components of the MOFs themselves or the assembly of other imaging agents can achieve multimode imaging-guided therapy,w hich could avoid inherent limitations of singlemode imaging-guided therapy and provide more accurate localization of lesions/particles as well as guidelines of therapy.
Fore xample,Y in et al. developed ab iocompatible nanocomposite (Fe 3 O 4 @C@PMOF) with fluorescence imaging and MRI functions for dual-mode imaging-guided phototherapy ( Figure 11). [79] Specifically,F e 3 O 4 @C cores were selected as T 2 -weighted MRI contrast agents and photothermal agents, and porphyrinic MOF shells had fluorescence imaging and PDT abilities.S uch dual-mode imaging nanoplatforms achieved high sensitivity of fluorescence imaging as well as deep penetration and great spatial resolution of MRI, and more precise in vivo information could be provided for enhancing the safety and therapeutic efficacy. Following intravenous injection of Fe 3 O 4 @C@PMOF nanocomposites in MCF-7 tumor-bearing mice,2 2h later the fluorescence imaging showed that nanocomposites were primarily localized in the liver region rather than in other organs.T he tumor area slowly brightened and became the brightest site in the mice after 26 h, indicating that the accumulation of nanocomposites at the tumor site had gradually increased. Ther esults were also confirmed by dramatic dimming at the tumor site in T 2 -weighted MR imaging.O wing to their high tumor accumulation, Fe 3 O 4 @C@PMOF nanocomposites were used for imaging-guided phototherapy;a no bvious PTT/PDT synergetic effect was observed for MCF-7 breast tumor but the toxicity for normal tissues was low.
Clearly,i ti sa lso valuable to develop porphyrinic MOFs without additional components for multimode imagingguided therapy due to its simplicity and efficiency. Wu et al. reported ultrathin 2D Cu-TCPP MOF nanosheets for dualmode imaging-guided phototherapy. [67] TheCu 2+ nodes in Cu-TCPP nanosheets offered excellent photothermal properties for PTT,P TI, and T 1 -weighted MRI. Meanwhile,p orphyrin TCPP provided the ability to produce 1 O 2 for PDT.T he results indicated that Cu-TCPP nanosheets have tremendous potential for synergistic PTT and PDT,g uided by PTI and MRI.
CT is an important medical imaging technique,w hich is based on the attenuation of X-rays by aspecimen resulting in 3D images with high spatial resolution. [83] Thehigh-Z number elements in MOFs can be chosen as CT contrast agents.I n addition, PA Iisarecently developed imaging technique that integrates optical excitation and ultrasound detection. [84] This technique displays high selectivity and deep penetration, and can be used to acquire high-resolution and strong-contrast tissue images.T oc ombine the merits of PTI, CT,a nd PA I, Yang et al. developed multifunctional metalloporphyrinic NMOF (Zr-FeP) shuttles as an "all-in-one" nanoplatform to realize trimode imaging-guided therapy. [66] In vivo PA I experiments demonstrated that am uch stronger PA signal was generated in the tumor tissue 2hafter tail vein injection of nanoshuttles into mice,i ndicating the considerable accumulation of nanoshuttles in the tumor.Z r-FePM OF nanoshuttles exhibited good CT capability due to the high-Z component for CT imaging. Additionally,t he outstanding photothermal performance of Zr-FeP MOF nanoshuttles Figure 11. A) Fluorescence images of the tumor-bearing mouse before and after injection of Fe 3 O 4 @C@PMOF(red arrow refers to the liver region, yellow arrow refers to the tumor region). B) T 2 -weighted MRI of the tumor-bearing mouse (upper red dot lines refer to the liver region, lower red dot lines refer to the tumor region). C) Infrared thermal photographs of tumor-bearingmice in different groups (black arrows refer to the tumor region). D) Changes of relative tumor volume in vivo after various treatments (black and red arrows refer to the injection and irradiation time points, respectively). (A-D) Reproduced with permission. [79] Copyright 2017, Nature PublishingG roup. endowed remarkable PTI ability in vivo,and after treatment with Zr-FeP MOF nanoshuttles the tumor site temperature rapidly increased by 17 8 8Cupon exposure to 635 nm laser for 5min. Therefore,t he fascinating trimode imaging capability of Zr-FeP MOF nanoshuttles would be ap owerful tool for precise tumor diagnosis and imaging-guided therapy.

Biosensing
Alarge number of physiological species play akey role in regulating cellular functions and physiological activities,o f which the changes in the concentrations and types indicate disruptions or pathological changes in physiological environments.T herefore,s pecific and sensitive biosensing systems have been increasingly used to detect and monitor the variations for disease diagnosis and biological processes in living organisms.P orphyrins and their derivatives have been widely studied as small-molecule biosensors,d ue to their excellent physicochemical characteristics that can be used for signal interaction with host molecules.Moreover,porphyrins can mimic many biological functions.For example,porphyrins can reversibly bind with gaseous compounds and can undergo photophysical and/or redox processes mediated by the target analytes. [8a] Interestingly,h emin, which is also called iron protoporphyrin IX, has been utilized for biosensing, because it can act not only as ar edox mediator based on the electrochemical activity of the Fe III /Fe II reversible redox pair, but also as ac atalyst based on its peroxidase-like activity.I n recent years,M OFs have also been extensively employed to construct biosensors due to their unique merits. [17b,76a, 85] In particular,the porous structure and high specific surface area of MOFs provide high catalytic activity.I na ddition, the designed shape and size as well as specific chemical interactions between guest molecules and MOFs endow them with high selectivity for their target molecules.

Biosensing of Biomacromolecules
DNAi st he most essential genetic material of living beings,a nd various strategies have been proposed to detect DNAf or diagnosis of diseases and biological processes. Among these strategies,E Cb iosensing has been extensively adopted for the detection of specific DNAo wing to its high sensitivity and selectivity. [85] ForD NA electrochemical biosensors,p orphyrin-based MOFs with excellent electrocata-lytic activity can be utilized as external probes for the production and amplification of the electrochemical signal. [26d, 86] Fore xample,d ue to the peroxidase-like catalytic activity of TCPP(Fe) metalloporphyrins,L ei et al. encapsulated TCPP(Fe) into HKUST-1 and subsequently conjugated it with streptavidin (SA) to construct an smart signal-transduction platform for hairpin DNAd etection (Figure 12 A-C). [86a] Here,alabel-free electrochemical DNAbiosensor was designed by using TCPP(Fe)@MOF-SA as asignal probe and hairpin DNAa sa na llosteric switch. Thea llosteric switch of hairpin DNAo nt he glassy carbon electrode (GCE) surface could form SA aptamer that could selectively bind the synthetic TCPP(Fe)@MOF-SA via biorecognition affinity. Thus,t he conjugated TCPP(Fe)@MOF-SA on the GCE surface acted as am imetic catalyst;i nt he presence of H 2 O 2 , it could greatly enhance the oxidation of o-phenylenediamine (o-PD) to 2,2'-diaminoazobenzene,w hich could serve as ag ood electrochemical signal readout indicator. Such a" signal-on" electrochemical biosensor was proved to detect hairpin DNAw ith ad etection limit down to 0.48 fm (S/N = 3), with al inear range over six orders of magnitude and good feasibility in complex serum matrixes.
On the other hand, porphyrinic MOFs for fluorescence biosensing have also been developed for DNAd etection by utilizing the fluorescence quenching mechanism. [33c, 87] For example,W ang et al. used ultrathin Cu-TCPP MOF nanosheets to design afluorescence biosensor for rapid, sensitive, and multiplex detection of DNAs. [87] Due to the large surface area and large number of accessible active sites on the nanosheets,t hree dye-labeled ssDNAp robes could be easily absorbed on the surface,leading to fluorescence quenching of the dye-labeled ssDNAbyFçrster resonance energy transfer (FRET). When as pecific target was hybridized with the ssDNAprobes to form dsDNA, the formed dsDNAdetached from Cu-TCPP MOF nanosheets owing to the weak physical adsorption between Cu-TCPP MOF nanosheets and dsDNA, and thereby the fluorescence "turned on". Consequently,Cu-TCPP MOF nanosheets exhibited excellent performance for quantitatively detecting pathogenic DNAw ith ad etection limit in the pm range.
Immunoassays are of great interest for the clinical early diagnosis of various tumors,such as prostate specific antigen (PSA) analysis for diagnosis of prostate cancer. PEC bioanalysis is ap otential bioanalytical method based on the photoinduced electron transfer (PET) processes among analyte,p hotoactive species,a nd electrode upon photoirradiation, which has been widely used in the detection of various biological molecules.Recently,Zhang et al. reported an ew non-enzyme PEC immunoassay using DNA-mediated nanoscale PCN-222 to detect PSA. [88] Thes s-DNA-tagged antibody (Ab-DNA) was conjugated on PCN-222 to form Ab-DNA-functionalized NMOF complexes (Ab-DNA-NMOFs). Ab-DNA-NMOFs,asan active photocathode PEC substrate, displayed ar emarkably enhanced photocurrent response in the presence of dopamine (DA) in O 2 -containing aqueous media. Further functionalized with polyamidoamine dendrimer, Ab-DNA-NMOFs could serve as an efficient PEC signal transduction system for non-enzyme PSA immunoassays with hypersensitivity,abroad calibration range of 1pgmL À1 to 10 ng mL À1 ,a nd adetection limit of 0.2 pg mL À1 .
In addition, ECL, electrochemical redox-induced light emission, possesses several advantages including low background interference,s imple operation, and high sensitivity and space-time controllability.H an et al. proposed to use porphyrin@MOFs for PSA sensing via the ECL strategy. [89] In this study,F e-MIL-88 MOFs were used as aplatform to load hemin and gold nanoparticles (AuNPs) and further graft Gquadruplex DNAzyme for synergetic catalytic amplification of luminol. Thep DNA-Au-Hemin-MIL-DNAzyme probes acted as quenchers and enhancers in the fabrication of at arget-induced ratiometric ECL biosensor for PSA detection. Ther esults demonstrated that the pDNA-Au-Hemin-MIL-DNAzyme-based biosensor was highly sensitive and could be used for the accurate analysis of PSA with al inear detection range from 0.5 to 500 ng mL À1 and adetection limit of 0.058 ng mL À1 (S/N = 3).
Enzymes and proteins play important roles in metabolic regulation, cell growth, and other biological processes.Some of these biomolecules have been accepted as important biomarkers for early-stage disease diagnosis,prognosis judgement, and pathogenesis studies.S everal different types of porphyrin-based MOFs have been used in the construction of electrochemical biosensors to enhance the electrochemical signal for enzyme or protein biosensing. Yuan et al. reported multifunctional hemin-encapsulated Fe-MIL-88 MOFs,which were the first example for the construction of an electrochemical aptasensor for thrombin (TB) detection (Figure 12 D-F). [26c] Such porphyrin@MOFs served as the support for the immobilization of recognition molecules (thrombin binding aptamer (TBA)) and were also conjugated with GOx for signal amplification. GOx oxidizes glucose into gluconic acid accompanied by the production of H 2 O 2 ,w hich can be further electrocatalyzed by hemin to amplify the electrochemical signal. Such an electrochemical biosensor showed arelatively low detection limit of 0.068 pm and aw ide linear range from 0.0001 nm to 30 nm with ah igh correlation coefficient of 0.9961, indicating that the constructed biosensor could be used to detect TB quantitatively.Lei et al. designed an electrochemical biosensor that was composed of iron metalloporphyrinic NMOFs (PorMOFs) for telomerase activity detection. [90] PorMOFs exhibited excellent electrocatalytic activity towards O 2 reduction, which was used to detect telomerase with ad etection limit of 30 cells mL À1 and assess the telomerase activity even in single Hela cell. Moreover, porphyrin-based MOFs were also applied to construct PEC and ECL biosensors for protein detection. Fore xample, Zhang et al. developed as imple PCN-222 based PEC biosensor for label-free detection of ap hosphoprotein (a-

Angewandte Chemie
Reviews 5028 www.angewandte.org Ther apid and sensitive detection of organophosphorus pesticides has gained more and more attention due to the environmental pollution and human health risks.S ue tal. synthesized Cu-H MOFs/NECF composites consisting of ballflower-like Cu-hemin MOFs grown on the fibers of 3D nitrogen-containing melamine carbon foam (NECF) and immobilized AChE on Cu-H MOFs/NEC for trichlorfon detection. [96] TheCu-H MOFs/NECF-based biosensor exhibited great performance for trichlorfon detection with abroad linear range (0.25-20 ng mL À1 ), low detection limit (0.082 ng mL À1 ), and good stability,a nd thereby facilitated trace detection of organophosphorus pesticides.

Ion Sensing
Several metal ions are essential elements in the human body and play vital roles in cellular activities,w hile their excess would be harmful for health. In particular, heavy metal ions are easily accumulated in the human body via the food chain, which thereby results in adverse effects.Obviously,itis of great importance to selectively detect metal ions in cells or in the environment. Forp orphyrin-based MOFs,t he coordination capability of metal ions in the porphyrin rings endows them with the ability to construct fluorescence "turn-off" or "turn-on" biosensors.I n2 013, Li et al. proposed to use porphyrinic MOF-525 as af luorescence "turn-off" biosensor for Cu 2+ sensing. [97] Thep orphyrin moieties in MOF-525 provided accessible recognition sites for fluorescence quenching through energy/charge transfer to determine the amount of Cu 2+ ions.M OF-525 fluorescent probes showed great potential for rapid and specific detection of Cu 2+ in practical water samples and for the study of copper-related diseases like Wilsonsa nd Alzheimersd iseases.R ecently,anovel fluorescent porphyrinic MOF probe,where the UiO-66(OH) 2 MOF was encapsulated into PCN-224, was developed by Lu et al. for detecting Cu 2+ in complex water samples. [98] Similarly,d ue to the fluorescence quenching ability of Cu 2+ toward PCN-224, this fluorescent probe exhibited excellent Cu 2+ detection with al imit-of-detection (LOD) value as low as 0.068 nm,which is lower than the Cu 2+ concentration limit in drinking water regulated by the WHO.
Compared to fluorescence "turn-off" biosensors,f luorescence "turn-on" biosensors can avert the wrong response to sensing signals and more conveniently identify in ad ark background. [36c] Forexample,Jiang et al. developed afluorescence "turn-on" biosensor based on Pd II metalloporphyrinic MOFs (PCN-222(Pd)) for Cu 2+ sensing. [99] Here,P CN-222-(Pd) was used as acatalytic Heck reaction system to produce highly fluorescent "turn-on" signal in the presence of Cu 2+ , due to the stronger binding affinity of Cu 2+ over Pd 2+ to the nitrogen atoms in the porphyrin and the conversion of nonfluorescent aniline to fluorescent ring-closing product via Pd 0catalyzed Heck reaction. Consequently,t he PCN-222(Pd) biosensor exhibited highly selective and sensitive sensing of Cu 2+ in aqueous solution with av ery low detection limit. In addition, porphyrinic MOFs were also utilized in the construction of electrochemical biosensors for detecting metal ions. [100] Additionally,although anion sensing is more challenging, selective sensing of anions by porphyrin-based MOFs has also been reported. Fori nstance,C ao et al. developed Zr-based MOFs with the mixed linkers 1,3,6,8-tetra(4-carboxylphenyl)pyrene (TBAPy) and TCPP (Zr(TBAPy) 5 (TCPP)) as afluorescence "turn-on" biosensor for S 2À sensing in aqueous environments. [36c] Ther esults showed that the fluorescence signal of Zr(TBAPy) 5 (TCPP) was intensified remarkably after addition of S 2À ,but other anions including SO 4 2À ,CNS À , COOH À ,B r À ,I À ,I O 3 À ,F À ,H SO 3 À ,C l À ,a nd NO 3 À did not alter its fluorescent intensity.E ddaoudi et al. encapsulated TMPyP(Pt) porphyrins in rho-ZMOF for the selective sensing of various anions in aqueous and methanolic solutions. [8c] 3. 4

.4. pH Sensing
Generally,pHplays an important role in maintaining the stability,n ormal morphology,a nd function of cells and organisms.D ue to the spectral characteristics that change upon protonation/deprotonation of the pyrrole ring in porphyrins with ac hange in pH, [101] porphyrin-based MOFs could be used to construct pH sensors,showing great potential in pH monitoring,d isease diagnosis,a nd even evaluating therapeutic efficacy.In2013, Zhou et al. reported for the first time Zr-based porphyrinic MOF PCN-225 and metalloporphyrinic MOF PCN-225(Zn) for pH sensing. [35b] Ther esults demonstrated that the fluorescence intensity of PCN-225 has astrong correlation with the pH of the solution in the range of pH 0-10.2. Them ost acidic solution showed the weakest fluorescence,w hile the pH 10.2 solution gave the highest intensity.F urthermore,p H7-10 was the most sensitive pH range for intensity response.Luetal. developed an effective dual-emission fluorescent MOF composite probe (RB-PCN) that was constructed by the encapsulation of Fe 3 O 4 nanoparticles in PCN-224 followed by modification with RBITC. [102] Ther esults showed that RB-PCN could sensitively detect ab road range of pH changes.I tc ould also be used to monitor the pH changes in living cells by fluorescence confocal imaging and determine the pH values in actual water samples by fluorescence spectroscopy.

Other Applications of Porphyrin-Based MOFs
In addition to the biomedical applications mentioned above,p orphyrin-based MOFs also have valuable applications in other biologically related fields.F or example,G uo et al. synthesized spherical Cu-TCPP(Cu) MOFs,a nd subsequently encapsulated Ag nanoparticles in MOFs to form Ag/Cu-TCPP(Cu) composites for antibacterial application. [103] Ther esults showed that antibacterial effect of Ag/ Cu-TCPP(Cu) composites was superior to that of penicillin, and the cytotoxicity of Ag/Cu-TCPP(Cu) composites was lower than that of naked Ag nanoparticles and Ag ions in vitro.F urthermore,A g/Cu-TCPP(Cu) composites could also significantly promote wound healing in vivo.
More recently,Q ue tal. utilized ultrathin Cu-TCPP(Fe) MOF nanosheets to physically adsorb GOx to construct acomposite nanocatalyst for in vivo wound healing. [104] Here, Angewandte Chemie Reviews 5030 www.angewandte.org Cu-TCPP(Fe)/GOx could continuously convert nontoxic glucose into gluconic acid and H 2 O 2 through the action of GOx, and the subsequent pH decrease from 7t o3 -4 could dramatically activate the peroxidase-like activity of Cu-TCPP(Fe) MOF nanosheets.S ubsequently,t he produced H 2 O 2 could be further transformed to toxic COH radicals by the Fenton reactions of Cu and Fe ions.I nv itro and in vivo experiments showed that this cascade catalytic conversion of nontoxic glucose to toxic COH radicals endowed the Cu-TCPP(Fe)/GOx nanocatalyst with robust antibacterial activity and negligible biotoxicity.

Toxicity and Safety Concerns
Regarding porphyrin-based MOFs for biomedical applications,their potential toxicity is of high concern. Considering the construction of MOFs from metal nodes and organic linkers,i ti sapriority to choose nontoxic or hypotoxic building components to improve the biocompatibility and reduce the toxicity.F ortunately,porphyrins as biocompatible motifs are beneficial for the MOFs biosafety.H owever,t he toxicity of MOFs was found to be closely related to their metal nodes. [76a] Theb iocompatible metal ions (e.g.,Z r, Fe, Zn, Ca, Mg) are possible nodes for the construction of biofriendly porphyrin-based MOFs due to their acceptable toxicity for biomedical applications. [106] Especially for the synthesis of composite porphyrinic MOFs,t he additional functional components also need to be considered for potential damage to the tissues and organisms.I na ddition to the composition of porphyrin-based MOFs,t he physicochemical natures of MOFs including size distribution, shape, surface chemistry,a nd dose-dependent properties are also important factors in terms of toxicology. [106,107] All these parameters strongly affect the absorption, distribution, metabolism and excretion, biodegradation, and clearance of porphyrin-based MOFs,and further determine their potential toxicity in vivo.T od ate,t hese aspects of porphyrin-based MOFs have not been fully investigated, while the multifunctionality has been deeply explored in most biomedical studies.
Understanding the intricate interactions between the physicochemical properties,biological performance,and biosafety could contribute to the successful translation to practical biomedical applications.T herefore,t he relevant studies of porphyrin-based MOFs in biomedical applications need to be developed in depth. Firstly,i ti si mportant to use various progressive characterization techniques to accurately determine the physicochemical parameters of MOFs (e.g., size,s hape,s urface chemistry,p orosity) and reasonably evaluate their merits and limitations in order to understand and ultimately modulate the structure-function relationship for MOFs. [108] Next, investigating the correlation between MOFs and biological systems is the basis for correlating the MOFs features with the toxicity results.Recently,F aria et al. proposed the MIRIBEL (Minimum Information Reporting in Bio-Nano Experimental Literature) standard for different studies on bio-nano interactions,w hich normalizes material characterization, biological characterization, and experimen-tal details for enhancing reproducibility and quantitative comparisons of bio-nano materials. [109] In the development of porphyrin-based MOFs for biomedical applications,s uch standard should be referenced to facilitate safety evaluation and material optimizations.F urthermore,g reat progress has been made with respect to mechanism-based hazard assessment of nanomaterials and advanced tools have emerged for their safety evaluation. [110] Therefore,not only for porphyrinbased MOFs,r esearchers need to accelerate the use of emerging methods and tools,combining research results with bioinformatics,t os cientifically and quantitatively correlate the data outcome of biomaterials and their toxicity results for their practical application in biomedicine.

Conclusions and Perspectives
Porphyrins and their derivatives have been often used as photosensitizers for PDT,f luorophores for fluorescence imaging,a nd sensors for biosensing due to their excellent photophysical properties.Asanew type of porous materials, MOFs possess high porosity,t unable structures,m ultifunctionality,biocompatibility,and biodegradability.Studies have demonstrated that the construction of porphyrin-based MOFs can not only overcome the limitations of free porphyrins like instability and self-quenching issues in physiological environments,b ut also integrate the functionalities of MOFs and porphyrins as well as improve the physicochemical properties. In this Review,r ecent achievements and progress in the construction of porphyrin-based MOFs and their biomedical applications have been highlighted and discussed.
To date,v arious methods have been used to construct porphyrin@MOFs,p orphyrinic MOFs,a nd composite porphyrinic MOFs in order to meet the requirements of biomedical applications.A mong them, many strategies including heavy-atom substitution of porphyrinic MOFs, controlling the particle size,c ontrollable design, and modification with targeting moieties or functional components have been proposed to enhance the therapeutic efficacy of porphyrin-based MOFs in PDT.A dditionally,t he rational design of functional components in MOFs or adjuvant agents makes it possible to interfere with survival mechanisms and alter the tumor microenvironment to achieve better results in terms of PDT efficacy.Obviously,the optimized construction of porphyrin-based MOFs shows great potential for enhanced PDT of tumors.
Another efficient strategy for enhancing therapeutic efficacyi st he implementation of multimodality therapy in asingle system to overcome the limitations of single-modality therapy.P orphyrin-based MOFs have exhibited ag reat capacity for combining PDT with other therapy modalities, including chemotherapy,P TT,i mmunotherapy,a nd gas therapy,t oa chieve synergistic effects for maximizing therapeutic efficacy and improving safety.However,the majority of current studies focus on dual-modality therapies,but there are few reports on tri-or more modality therapy based on porphyrin-based MOFs,w hich might offer more significant therapeutic effects.
Porphyrin-based MOFs have also been developed for imaging-guided therapy to further optimize the therapeutic process and thereby enhance therapeutic efficacy. Due to the excellent fluorescent properties and photosensitization of porphyrins,p orphyrin-based MOFs could realize fluorescence imaging-guided therapy.O nt he other hand, metal nodes in MOFs or the incorporation of contract agents in MOFs endow porphyrin-based MOFs with the potential for MRI-, CT-, PA I-or PTI-guided therapy.S uch "all-in-one" nanoplatforms provide more accurate diagnosis and guidance for treatment, and are of great attractive for tumor therapy.
Besides the exploration of tumor therapy functions, porphyrin-based MOFs have been utilized to construct biosensors for detecting various biological species,s uch as biomacromolecules,s mall molecules,a nd ions as well as pH values and gases.O wing to the combined excellent physicochemical properties of MOFs and porphyrins,p orphyrinbased MOFs types of biosensors show significant performance in biosensing with high sensitivity and selectivity, acceptable reproducibility,g ood stability,a nd biocompatibility.S ome porphyrins including hemin, TCPP,a nd its metal derivatives are often used to construct porphyrin-based MOF type biosensors.H owever,i tc an be anticipated that the biosensing performance could be further improved with more interesting porphyrins.O nt he other hand, porphyrin-based MOF type biosensors for cell sensing and in vivo sensing need further investigation for the development of biosensors in living biological systems.
Despite the growing number of impressive progress reports on porphyrin-based MOFs,t heir development in biomedical applications is still at the early stage and faces many challenges.F or porphyrin@MOFs,t he porphyrin loading capacity and premature leakage and self-quenching of porphyrins should be considered during the synthesis process. Actually,p orphyrinic MOFs are more promising candidates for biomedical applications.H owever,t he influence of size, shape,composition, and surface chemistry of porphyrin-based MOFs on therapeutic efficacy and biosensing ability should be further optimized to meet the requirements of personalized therapy,diagnosis,and detection. On the other hand, the design of "all-in-one" nanoplatforms based on composite porphyrinic MOFs would be an efficient strategy to enhance therapeutic efficacy in tumor therapy and biosensing ability. Actually,r esearchers should establish straightforward synthesis protocols for these designed MOF materials,a st he simpler the preparation, the more likely it is that the product will be relevant for biomedical applications in the real world. Finally,i ts hould be re-emphasized that thoroughly and deeply investigating the correlation between the fundamental parameters of porphyrin-based MOFs and their biosafety including the pharmacokinetics,b iodistribution, biocompatibility,l ong-term toxicity,a nd clearance from the body,i s crucial for possible clinical translations.I nc onclusion, porphyrin-based MOFs have great potential for applications in tumor therapy and biosensing, and more advances and progress in the biomedical field will be achieved in the future.