ARTICLESPUBLISHED ONLINE: 13 DECEMBER 2009 | DOI: 10.1038/NMAT2608

Porous metal–organic-framework nanoscalecarriers as a potential platform for drugdelivery and imagingPatricia Horcajada1*, Tamim Chalati2, Christian Serre1, Brigitte Gillet3, Catherine Sebrie3,Tarek Baati1, Jarrod F. Eubank1, Daniela Heurtaux1, Pascal Clayette4, Christine Kreuz4,Jong-San Chang5, Young Kyu Hwang5, Veronique Marsaud2, Phuong-Nhi Bories6, Luc Cynober6,Sophie Gil7, Gérard Férey1, Patrick Couvreur2 and Ruxandra Gref2*

In the domain of health, one important challenge is the efficient delivery of drugs in the body using non-toxic nanocarriers. Mostof the existing carrier materials show poor drug loading (usually less than 5 wt% of the transported drug versus the carriermaterial) and/or rapid release of the proportion of the drug that is simply adsorbed (or anchored) at the external surface ofthe nanocarrier. In this context, porous hybrid solids, with the ability to tune their structures and porosities for better druginteractions and high loadings, are well suited to serve as nanocarriers for delivery and imaging applications. Here we showthat specific non-toxic porous iron(III)-based metal–organic frameworks with engineered cores and surfaces, as well as imagingproperties, function as superior nanocarriers for efficient controlled delivery of challenging antitumoural and retroviral drugs(that is, busulfan, azidothymidine triphosphate, doxorubicin or cidofovir) against cancer and AIDS. In addition to their highloadings, they also potentially associate therapeutics and diagnostics, thus opening the way for theranostics, or personalizedpatient treatments.

For nanocarriers, the requirements for ensuring an efficienttherapy are to (1) efficiently entrap drugs with high payloads,(2) control the release and avoid the ‘burst effect’ (important

release within the first minutes), (3) control matrix degradation,(4) offer the possibility to easily engineer its surface to controlin vivo fate and (5) be detectable by imaging techniques. Moreover,entering a new stage of molecular medicine requires the associationof therapeutics and diagnostics to make personalized patienttreatment a reality. A step forward aims at conceiving a nanocarrierthat could serve both as drug carrier and as diagnostic agent (satisfycriteria (4) and (5)), to evaluate drug distribution and treatmentefficiency (theranostics).

Currently, for delivery, some materials are being used (forexample, liposomes, nanoemulsions, nanoparticles or micelles;refs 1–5) but are, for the most part, unsatisfactory; better routesare therefore necessary to address the limitations. Very recently,our group6,7 (ibuprofen storage/long time release) and thoseof R. Morris8,9 (gas delivery of NO for antithrombosis andvasodilatation) and Lin10–13 (imaging) introduced a new pathwayby using hybrid porous solids14 (or metal–organic frameworks(MOFs)) for this purpose. However,most of thematerials describedin these publications (that is, Co-, Ni- and Cr-based MOFs) werenot compatible with biomedical and pharmaceutical applications,and, with few exceptions10–13,15–17, they were not engineered asnanoparticles to enable controlled drug release by intravenous

1Institut Lavoisier (CNRS 8180) & Institut universitaire de France, Université de Versailles, 78035 Versailles Cedex, France, 2Faculté de Pharmacie (CNRS8612), Université Paris-Sud, 92296 Châtenay-Malabry, France, 3CNRS 2301, 91190 Gif-sur-Yvette France and CNRS8081, Université PARIS-Sud 91405Orsay, France, 4Laboratoire de Neurovirologie, SPI-BIO, CEA, 92260 Fontenay aux Roses Cedex, France, 5Catalysis Center for Molecular Engineering, KoreaResearch Institute of Chemical Technology (KRICT), PO Box 107, Yusung, Daejeon 305-600, Korea, 6Laboratoire de Biochimie—HôpitalHôtel-Dieu—AP-HP 75004 Paris, France, 7EA 2706, Faculté de Pharmacie, Université Paris-Sud, 92296 Châtenay-Malabry, France.*e-mail: horcajada@chimie.uvsq.fr; ruxandra.gref@u-psud.fr.

administration. To circumvent these problems, the strategy of thepresent paper (Fig. 1) was to take advantage of the character andperformance of suitable iron(iii) carboxylate MOFs. Their non-toxic nature and potential for nanoparticle synthesis (nanoMOFs),coupled with unusually large loadings of different drugs andimaging properties, make them ideal candidates for a new valuablesolution in the field of drug-delivery nanocarriers.

MOFs result from the assembly, exclusively by strong bonds, ofinorganic clusters and easily tunable organic linkers (carboxylates,imidazolates or phosphonates14). This huge family presents highand regular porosities (φ up to 4.7 nm; pore volume up to2.3 cm3 g−1) enabling, for instance, the entrapment of largeamounts of greenhouse gases18. They can show simultaneouslyhydrophilic and hydrophobic entities, as well as tunable pore sizeand connectivities, which can be adapted to the physico-chemicalproperties of each drug and its medical application19,20. Moreover,the high structural flexibility of someMOFs (refs 21, 22) enables theadaptation of their porosity to the shape of the hostedmolecule.

We have synthesized, in biologically and environmentallyfavourable aqueous or ethanolic medium, some non-toxic iron(iii)carboxylate MOFs (MIL-53, MIL-88A, MIL-88Bt, MIL-89, MIL-100 and MIL-101_NH2; MIL = Materials of Institut Lavoisier;refs 23–27) and have adapted the synthesis conditions to obtainthese materials as nanoparticles (see Methods and SupplementarySections S1 and S7; Figs S1–S5 and S11–S12), which were

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NATURE MATERIALS DOI: 10.1038/NMAT2608 ARTICLES

CORONA

Biodistribution

Targeting

~ 200 nm

CORE

8 Å

Biodegradable porous iron carboxylates

Controlled release of challenging drugs

Imaging

MIL-536–11 ÅMIL-88

24–29 ÅMIL-100

29–34 ÅMIL-101

Cidofovir

Busulfan

Azidothimidinetriphosphate

Doxorubicin

Figure 1 | Scheme of engineered core–corona porous iron carboxylates for drug delivery and imaging.

100 nm200 nm 200 nm

MIL-100 MIL-88A MIL-88A-PEG

Figure 2 | Scanning electron micrographs of MIL-100 (left), MIL-88A (centre) and PEGylated MIL-88A nanoparticles (right).

characterized in terms of biocompatibility, degradability andimaging properties (Figs 1 and 2). Their efficiency as drug carrierswas tested with four challenging anticancer or antiviral drugs(busulfan (Bu), azidothymidine triphosphate (AZT-TP), cidofovir(CDV) and doxorubicin (doxo)), which, except the latter, couldnot be successfully entrapped using existing nanocarriers (Table 1).Some cosmetic molecules, such as caffeine (liporeductor), urea(hydrating agent), benzophenone 3 and benzophenone 4 (UVAand UVB filters) were also tested. For biological applications, thenanoMOF surfaces were engineered by coating with several relevantpolymers28 (see Methods); this treatment prevented aggregationof the nanoparticles but did not improve the results. Finally, thepotential of these nanoMOFs as contrast agents is reported.

The first step of the study was to evaluate the performancesof the pure nanosized iron carboxylates in terms of degradabilityand cytotoxicity. Their in vitro degradation under physiologicalconditions (see Supplementary Fig. S10) shows that, in the

case of MIL-88A (fumarate) and MIL-100 (trimesate), a majordegradation occurred after seven days of incubation at 37 ◦C. Thenanoparticles lose their crystallinity and release large quantitiesof their ligands (72 and 58wt% of the fumaric and trimesicacids, respectively), indicating a reasonable in vitro degradabilityof the MOF nanoparticles. Interestingly, in the case of MIL-88A, the degradation products, iron and fumaric acid, areendogenous (see Supplementrary Section S7), and show low toxicityvalues (LD50(Fe) = 30 g kg−1, LD50(fumaric acid) = 10.7 g kg−1;LD50(trimesic acid) = 8.4 g kg−1) and LD50(terephthalic acid)> 6.4 g kg−1 (refs 29–32).

The nanoMOF cytotoxicity, studied in vitro (MTT assay; ref 33)on mouse macrophages (see Supplementary Section S8), was low(57± 11 µgml−1 for MIL-88A) and comparable with that of thecurrently available nanoparticulate systems34. Acute in vivo toxicityexperiments were then carried out after intravenous administrationof nanoMOFs inWistar female rats (see Supplementary Section S7).

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ARTICLES NATURE MATERIALS DOI: 10.1038/NMAT2608

Table 1 | Structure description, particle size, drug loading (wt%) and entrapment efficiency (below the drug loading values inparentheses, wt%) in several porous iron(III) carboxylate nanoparticles.

MIL-89 MIL-88A MIL-100 MIL-101 _NH2 MIL-53

Organic linkerMuconicacid

HOO

OOH

FumaricacidHO OH

OO

Trimesic

OHO

HO OH

O O

acid terephthalicacid NH

2

HO

OO

OH

Amino Terephthalicacid

OO

HO OH

Crystallinestructure

Flexibility Yes Yes No No YesPore size (Å) 11 6 25 (5.6)

29 (8.6)29 (12)34 (16)

8.6

Particle size (nm) 50–100 150* 200 120 350*

Bu loading (efficiency) (%)

O

O O

sO

OOs

13.4×3.5amphiphilic

9.8(4.2)

8.0(3.3)

25.5(31.9)

-14.3(17.9)

AZT-TP loading (efficiency) (%)

N NN

OH

OH

OHOH

OH

O O

O OO

O

O

PP

PHO

N

NH2

11.9×9.1hydrophilic

-0.60(6.4)

21.2(85.5)

42.0(90.4)

0.24(2.8)

CDV loading (efficiency) (%)

NH2

NN O

O

O

P

OH

HO

HO

10.8×7.7hydrophilic

14(81)

2.6(12)

16.1(46.2)

41.9(68.1)

-

Doxorubicin loading(efficiency)(%)

OH

OH

OH

OH

OO

O O

ONH2

MeO

HO

15.3× 11.9hydrophobic

- -9.1(11.2)

- -

OOH

Ibuprofen loading(efficiency) (%) 10×5

hydrophobic- -

33(11.0)

-22(7.3)

Caffeine loading(efficiency) (%)

NNN

N O

O

CH3

CH3

CH3

6.1×7.6amphiphilic

- -24.2(16.5)

-23.1(15.7)

Urea loading(efficiency)(%)

CNH2H2N

O

4.1×3.1hydrophilic

- -69.2(2.1)

-63.5(1.9)

Benzophenone 4 loading(efficiency) (%)

O

COH

O CH3SO3H

12.0×7.2hydrophilic

- -15.2(22.8)

-5(7.5)

Benzophenone 3 loading(efficiency)(%)

O

O

OH12.1×5.6hydrophobic

- -1.5(74.0)

- -

*Bimodal distribution of sizes, with micrometric particles.

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Time (days)

Rele

ased

dru

g (%

)Doxo

CDV AZT-TP

11 12 13 140 1 2 3 4 5

0

50

100

Figure 3 | CDV (black), doxo (red) and AZT-TP (green) delivery undersimulated physiological conditions (PBS, 37 ◦C) from MIL-100nanoparticles. All experiments were carried out in quadruplicate.

Three different loaded porous iron(iii) carboxylate nanoparticleswere used. They were built up either from a hydrophilic aliphaticlinker, fumarate (MIL-88A), from a hydrophilic aromatic linker,trimesate (MIL-100), or from a hydrophobic aromatic linker,tetramethylterephthalate (MIL-88Bt) (see Methods). Doses upto the highest possible injectable amounts were administrated(220mg kg−1 for MIL-88A and MIL-100, and 110mg kg−1 forMIL-88Bt). Different indicators (the animal behaviour, bodyand organ weights and serum parameters) were evaluated upto three months after injection (see Supplementray Section S7;Figs S13 and S14). Their comparison with control groups didnot show significant differences between them, except a slightincrease in the spleen and liver weights, attributed to the fastsequestration by the reticuloendothelial organs of the nanoMOFsnot protected by a PEG (polyethylene glycol) coating. As allthe body organ weights were back to normality one to threemonths after injection (see Supplementary Figs S13 and S14),the phenomenon was fully reversible. The absence of immune orinflammatory reactions after nanoparticle administration supportstheir lack of toxicity. Moreover, the absence of activation ofcytochrome P-450 suggests a direct excretion of the polyacids,in agreement with their high polarity. Finally, in vivo subacutetoxicity assays were carried out by injecting up to 150mg ofMIL-88A kg−1 d−1 during four consecutive days. No significanttoxic effects were observed up to ten days after administration (seeSupplementary Figs S15–S17).

The non-toxicity of the iron nanoMOFs, proved above, ledus to investigate their ability to entrap anticancer and antiviraldrugs. Chemotherapy indeed plays a key role in the treatmentof cancer in children. Thanks to its efficiency, three out of fourchildren can now be cured. Nevertheless, 25% of paediatric cancerpatients go uncured, and chemotherapy-induced long-term sideeffects justify the continued development of new strategies tofight childhood cancer. Research in paediatric oncology is nowencouraged and supported by European legislation (PaediatricUse Marketing Authorization, PUMA) and new internationalorganizations, such as the consortium Innovative Therapies forChildren with Cancer (ITCC).

In this context, the amphiphilic antitumoural drug busulfan(Bu) is widely used in combination high-dose chemotherapyregimes for leukaemias, especially in paediatrics, because it rep-resents a good alternative to total-body irradiation35,36. However,Bu possesses a poor stability in aqueous solution and an impor-tant hepatic toxicity due to its microcrystallization in the hepaticmicrovenous system (hepatic veno-occlusive diseases37).Moreover,the current encapsulation of Bu in known drug nanocarriers, such

as liposomes or polymeric nanoparticles, is not satisfactory becauseloading never exceeds 5–6wt% (ref. 38), rendering our search ofefficient nanocarriers an attractive challenge.

Bu was loaded in the preformed nanoMOFs by soaking insaturated drug solutions (Supplementary Table S2, Fig. S18).Table 1 shows the maximum amounts of drug adsorbed in severalporous iron carboxylates. The Bu loading in the rigid mesoporousMIL-100 may be considered as exceptionally high (25wt%). Thisresult is five times higher than the best system of polymernanoparticles (5–6wt%; ref. 38) and 60 times higher than withliposomes (0.4 wt%; refs 37, 39). Owing to their lower porevolumes, Bu entrapment in microporous flexible structures (MIL-88A, MIL-53, MIL-89) is lower than for MIL-100, but significantlylarger than for the existing materials. Consequently, the use ofporous iron carboxylates as nanocarriers could represent importantprogress for Bu therapy, especially because smaller amounts ofsolids would be required to deliver the needed dose of this drug.Indeed, considering the actual intravenous dosage of Bu (Busilvex,Bu solution inN ,N ′-dimethylacetamide; ref. 40), the total amountof MIL-88A or MIL-100 to be administered would be around100 and 20mg kg−1 d−1, respectively, for four days. Moreover,Bu-loaded nanoMOFs could avoid the use of toxic organic solvents(N ,N ′-dimethylacetamide) during administration and reduce theliver toxicity mentioned above (hepatic veno-occlusive disease37,39)owing to the entrapment of Bu in its molecular form withinthe pores. We have verified on cell culture experiments that thenanoMOFs were able to release Bu in its active form. Studies onhuman leukaemia and human multiple myeloma cells in culturehave shown that Bu has the same activity whether it is in itsfree form or entrapped in the nanoMOFs (see SupplementarySection S9; Fig. S19). In the same way, we have confirmed thetotal absence of cytotoxicity of the empty MIL-100 nanoparticlesin the same cell lines.

In addition to alkylating agents such as Bu, nucleoside analoguesare also of major importance in the treatment of cancer andviral infections. They include the monophosphorylated form ofthe antiviral phosphonate cidofovir, and the triphosphorylatedform of azidothymidine, which are the active forms of theseanti-cytomegalovirus and anti-HIV compounds, and doxorubicin,one of the most effective agents in the treatment of breast cancer.However, the clinical use of nucleoside analogues is limited by theirpoor stability in biological media, often resulting in short half-livesand low bioavailabilities37, as well as sometimes partial resistanceto the drug41. The important hydrophilic character of nucleosideanalogues also strongly limits their intracellular penetration owingto their low membrane permeability42,43. Some nanocarriers werepreviously developed to circumvent these inconveniences, but showpoor efficiencies together with ‘burst effects’44.

The performance of iron carboxylates therefore indicated majorpromise for the entrapment of all the above important drugs(Table 1). In the case of AZT-TP and CDV, this was achievedby simply soaking the preformed dried nanoMOFs in aqueoussolutions of the drugs. Even if the concentration of the drug inthe solution was low, the active molecules could be loaded withhigh efficiency (in most cases, higher than 80%); the nanoMOFsact as remarkable molecular ‘sponges’. For instance, MIL-100nanoparticles load up to 25, 21, 16 and 29wt%of Bu, AZT-TP, CDVand doxo, respectively. An unprecedented capacity of 42wt% can beachieved for AZT-TP and CDV with MIL-101_NH2 nanoparticles(Table 1; Supplementary Section S10 and S11; Table S3–S5),compared with 1wt% values reported in the literature for thesedrugs in usual nanocarriers41.

A progressive release of the three active molecules (AZT-TP,CDV and doxo) is observed using MIL-100 nanoparticles (Fig. 3),with no ‘burst effect’. The comparison between kinetics of drugdelivery and the degradation profiles suggests that the delivery

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ARTICLES NATURE MATERIALS DOI: 10.1038/NMAT2608

dm

s

st

li

li

k

dm

st

dm

CONTROL 220 mg kg¬1 MILL-88A_nanoa d

b e

c f

Figure 4 |Magnetic resonance images. The images were acquired with gradient echo (a, c, d, f) or spin echo (b, e) sequence of control rats (left; a–c) andrats injected with 220 mg kg−1 MIL-88A (right; d–f), in liver (a, b, d, e) and spleen (c, f) regions. 30 min after injection, product effect is observable on theliver and spleen. (dm, dorsal muscle; k. kidney; li, liver; s, spleen; st, stomach.)

process is governed mainly by diffusion from the pores and/ordrug–matrix interactions and not by theMOF degradation. Indeed,the total delivery of AZT-TP occurred within 3 days, when onlyapproximately 10% of MIL-100 was degraded. Moreover, testscarried out in nanoparticles with smaller pore size than the drugdimensions have shown very low drug capacities and ‘burst’ releasekinetics. This suggests that, in this last case, the drug was adsorbedonly onto the external surface and not within the pores (seeSupplementary Information, Fig. S20).

The promising data obtainedwithAZT-TP inMIL-100 nanopar-ticles incited us to evaluate, in vitro in human peripheral bloodmononuclear cells infected by HIV-1-LAI (see Supplementary Sec-tion S10), the anti-HIV activity of AZT-TP. A significant anti-HIVactivity was observed only for (AZT-TP)-charged nanoparticles(about 90% inhibition of HIV replication) for a concentration of200 nM in AZT or AZT-TP. In parallel, the empty nanoparticlesdemonstrated no cytotoxic effects, even at the highest tested dose(10 µgml−1 of nanoparticles).

From the above results, it is clear that porous iron(iii)carboxylates currently represent the best nanocarriers for the drugrelease of important drugs. Their unprecedented encapsulationcapacities apply to a large number of challenging drugs, notonly hydrophilic (AZT-TP, CDV, urea and benzophenone 4) butalso hydrophobic (doxorubicin, ibuprofen and benzophenone 3)and amphiphilic (busulfan and caffeine) molecules (Table 1; seeSupplementary Section S13; Table S6). The adaptive internalmicroenvironment (for example, amphiphilic polar metal andnon-polar linker) of the pores of this family of solids could probablyexplain the exceptional qualities of these porousmaterials.

Finally, we have investigated the potential of the nanoMOFsas contrast agents. We first proved by Mössbauer spectroscopythat the MOFs themselves (and not eventual iron oxide and/orhydroxide degradation products) act as contrast agents. Magneticresonance imagingmeasurements have beenmade onWistar femalerats 30min after injection of 220, 44 and 22mg kg−1 suspensionsof MIL-88A nanoparticles (Fig. 4 and Supplementary Section S14).Both gradient echo and spin echo sequences show that the treatedorgans are darker than the normal ones (Fig. 4d–f versus Fig. 4a–c.).The resulting aspects of the liver and the spleen are indeed differentbetween control and treated rats (Supplementary Figs S21 and S22).Also, three months after injection, the liver and spleen returned toa similar appearance to that of the untreated animals (results notshown). This is in accordance with the temporary accumulation ofthe nanoparticles in these organs, as discussed previously.

The favourable in vivo detection of the iron carboxylate MOFnanoparticles makes them interesting candidates for contrastagents, and, to the best of our knowledge, this represents thefirst example for iron-based MOFs. However, some examples ofMOFs based on Gd (ref. 12) or Mn (ref. 14) as potential contrastagents have been recently reported. The efficiency of our iron-based nanoMOFs is directly related to their relaxivity, in otherwords their capacity to modify the relaxation times of the waterprotons in the surrounding medium when a magnetic field isapplied. The higher the quantity and the mobility of the metalcoordinated water in the first and second coordination spheres,the higher the relaxivity. In this sense, our MOF nanoparticlespossess not only paramagnetic iron atoms in their matrix, but alsoan interconnected porous network filled with metal coordinated

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Table 2 |Transversal (r2) relaxivities of MIL-88A andMIL-100 nanoparticles, PEGylated or not, measured at 9.4 T.

Fe (mmol l−1) PEG (wt%) r2 (s−1 mM−1)

MIL-88 A 0.428 0 56MIL-88 A+PEG 0.364 13.6 95MIL-100 0.187 0 73MIL-100+PEG 0.15 13.3 92

and/or free water molecules. Table 2 shows the relaxivity of the ironfumarate MOF (MIL-88A) nanoparticles under a 9.4 T magneticfield. Relaxivity values r1 could not be measured, but r2 of MIL-88A nanoparticles are of the order of 50 s−1 mM−1, which canbe considered as sufficient for in vivo use45. The relaxivity valuesare related not only to the iron content, but also to the size ofthe nanoparticles. The PEGylated nanoparticles showed slightlyhigher r2 values than the non-PEGylated ones. The PEG coatingmay modify the nanoparticle relaxivities in two opposite ways46:increasing the size of individual nanoparticles and decreasingtheir aggregation. These results show that the iron-based core isresponsible for the favourable relaxivities and imaging propertiesof the MOF nanoparticles. Their framework contains (1) watermolecules strongly coordinated to the Lewis acid metal sites, as wellas (2) free water molecules, probably in exchange with these boundwater molecules, diffusing through the interconnected pores. Thepresence of this last type of water molecule should induce an effecton the relaxation times of the water protons, resulting in goodimaging properties.

In conclusion, our porous iron carboxylate nanoMOFs havemany advantages when used as non-toxic and biocompatibledrug nanocarriers. In terms of synthesis, they are obtained inaqueous or ethanolic solutions, instead of using organic solvents,and provide an example of what ‘green’ technology can affordfor biomedical applications. In the biomedical sense, they act asmolecular sponges, encapsulating drugs with different polarities,sizes and functional groups by immersion in correspondingsolutions. This simple entrapment method has been applied topreviously challenging antitumoural and antiviral drugs, as well ascosmetic agents. Progressive release was obtained under simulatedphysiological conditions. Moreover, anti-HIV activity of AZT-TPloaded nanoMOFs has been proven.

These results open new perspectives for improved treatmentwith anticancer and antiviral drugs and for the developmentof adapted formulations in paediatrics (using Bu nanoMOFs).Finally, the iron-based cores are endowed with good relaxivities,whichmakes these nanoparticles candidates formagnetic resonanceimaging (contrast) agents. These complementary properties mightopen new opportunities to use nanoMOFs for the eventual goalas theranostic agents.

MethodsThe syntheses of nanoscale (Fig. 2) porous iron(iii) carboxylates (labelled MIL-n)with different topologies and compositions (iron trans,trans-muconate (MIL-89;refs 21–24)), fumarate (MIL-88A; refs 21–24), tetramethylterephthalate (MIL-88Bt;refs 21–24), terephthalate (MIL-53; ref. 25), trimesate (MIL-100; ref. 26) andaminoterephthalate (MIL-101 _NH2; ref. 27) were optimized by an appropriatechoice of the reaction conditions (conventional solvothermal or microwavesynthesis, solvent, additives, iron source, concentrations, energy, temperature andtime) (see supplementary Section S1). These porous iron(iii) carboxylates are builtup from the assembly of either oxo-centred trimers of iron octahedra (MIL-88,MIL-89, MIL-100, MIL-101 _NH2) or chains of corner sharing octahedra (MIL-53)and di- or tri-carboxylate linkers, leading either to microporous flexible solids(MIL-88, MIL-89, MIL-53) or mesoporous rigid frameworks (MIL-100, MIL-101_NH2) (Fig. 1). The structure and composition of the resulting nanoparticles wereanalysed using X-ray powder diffraction, thermogravimetric analysis and infraredspectroscopy. In the case of MIL-53, MIL-88A, MIL-89, MIL-100 and MIL-101_NH2, the synthesis could be carried out in water or ethanol.

In most cases, the nanoparticles’ mean diameter, determined by both scanningelectronmicroscopy and quasi-elastic light scattering investigations, was lower than200 nm, compatible with the intravenous route of administration (see Table 1).The nanoparticle size distribution of MIL-53 and MIL-88A was bimodal, probablyowing to the competition between nucleation and growth during the crystallizationprocess and to an aggregation of the particles.

To control crystal growth, PEG chains with only one terminal reactive group(amino or carboxyl) were added during the course of the synthesis process (seeSupplementary Section S2). Thus, PEG led to the formation of a superficial PEG‘brush’ sterically protecting the nanoparticles from aggregation. Zeta-potentialmeasurements clearly indicated that neutral PEG chains were located at the surfaceof the nanoparticles. Zeta-potential values of uncoated MIL-100 (−14mV) wereshifted to almost neutral values (−2mV) in the case of PEGylated MIL-100, andfrom 17 to 2mV in the case of PEGylated MIL-88A. This is in accordance withpreviously reported data on PEG-coated nanoparticles2.

Bound PEG could be removed only after particle degradation under acidicconditions, supporting the fact that it was firmly bound to the nanoparticlesthrough coordination of its amino or carboxyl end-group with the metal centres.Indeed, when PEG with two non-reactive monomethoxy end-groups was addedto the reaction mixture, a negligible surface modification occurred. Thus, PEG wassuccessfully bound to the nanoparticles’ surface, and PEG contents up to 17 wt%were obtained, of the same order of magnitude as those described as being sufficientto ensure ‘stealth’ properties (see Supplementary Section S2).

Received 16 December 2008; accepted 11 November 2009;published online 13 December 2009

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AcknowledgementsWe acknowledge E. Legenre, M. Belle, F. Kani, C. Bellanger and E. Jubeli for theirhelp with the experiments. We are grateful to J-M. Greneche, H. Chacun, M. Apple,C. Bories, H.Hillarieu, and O. David for their collaboration. We thank K. Storck,V. Huyot and R. Yousfi for their technical assistance with the AZT-TP experiments.This work was partially supported by the CNRS, Université Paris Sud, Université deVersailles Saint-Quentin, EU funding through the ERC-2007-209241-BioMOFs, ERCand KOCI through the Institutional Research Program of KRICT. KRICT’s authorsthank You-Kyong Seo for his experimental assistance.

Author contributionsP.H., nanoMOF synthesis, surface modification of nanoparticles, drug and cosmeticencapsulation tests, toxicity assays, degradation tests,in vivomagnetic resonance imaging;C. Serre, nanoMOF synthesis, surface modification of nanoparticles, drug and cosmeticencapsulation tests, degradation tests, imaging applications; T.C., nanoMOF synthesis,PEG modification, drug encapsulation and delivery, in vitro toxicity assays, degradationtests, in vitro magnetic resonance imaging; B.G. and C. Sebrie, imaging applications;T.B., in vivo toxicity assays, nanoMOF degradation tests, doxorubicin encapsulation anddelivery; J.F.E., nanoMOF degradation tests; D.H., synthesis of nanoparticles of MIL-101_NH2; P. Clayette and C.K., anti-HIV activity of MIL-100 nanoparticles; J.-S.C. andY.K.H., synthesis of nanoparticles of MIL-100 and MIL-53 in water; V.M., busulfanactivity tests; P.-N.B. and L.C., liver function evaluation in the in vivo toxicity assays;S.G., activity of Cyp-450 in the in vivo toxicity assays; G.F., nanoMOF synthesis, surfacemodification of nanoparticles; P. Couvreur, drug encapsulation and delivery, toxicityassays, surface modification of nanoparticles; R.G., drug encapsulation and delivery,toxicity assays, surfacemodification of nanoparticles, imaging applications.

Additional informationThe authors declare no competing financial interests. Supplementary informationaccompanies this paper on www.nature.com/naturematerials. Reprints and permissionsinformation is available online at http://npg.nature.com/reprintsandpermissions.Correspondence and requests formaterials should be addressed to P.H. or R.G.

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