Improving stabilization and responsiveness of iron oxide nanoparticles with a triblock copolymer

J. Diricq,a,b D. Stanicki,b D. Mertz,c Ph. Dubois,a S. Laurent b and L. Mespouille a

a Laboratory of Polymeric and Composite Materials (LPCM), Center of Innovation and Research in Materials and Polymers (CIRMAP) b Department of General, Organic and Biomedical Chemistry, NMR and Molecular Imaging Laboratory

University of Mons, 23 Place du Parc, B-7000 Mons – Belgium c Institut de Physique et Chimie des Matériaux de Strasbourg IPCMS, UMR 7504, CNRS-ECPM-Université de Strasbourg, 23 rue du Loess BP 43, 67034 Strasbourg cedex 2, France

In the last years, superparamagnetic iron oxide nanoparticles (SPIONs) have attracted particular interests in the biomedical field owing to their magnetic properties, leading to widespread applications as

hyperthermia for drug delivery and contrast agent in MRI [1], Despite promising throughputs in hot biomedical fields, their high tendency to aggregate in physiological conditions limits their use currently. However, this

undesired effect can be by-passed by the use of various coatings as silica, citric acid, dextran or synthetic polymers. Among them, synthetic polymers offers lots of advantages as the ease to tune their functionality,

topology and macromolecular parameters.

In the present work, we report the preparation and characterization of functional block copolymers composed of polymethacrylic acid (PMAAc) to ensure anchoring on SPIONs surface, a PEO block for stealth

properties and stabilization in water and poly(N-isopropylacrylamide) (PNiPAAm) as a thermo-responsive block. Reversible Addition-Fragmentation Chain Transfert (RAFT) polymerization has been chosen as the

most reliable polymerization process. Indeed, RAFT polymerization is controlled polymerization process well-suited for both methacrylates and acrylamides and allow the preparation of very well-defined polymer

architecture. More interestingly, the RAFT end-chain can be easily hydrolyzed to release a thiol end-group on the shell of the coated SPIONs, allowing easy grafting of specific ligands for tumor vectorization.

Introduction

One pot synthesis of triblock copolymer : MAAc and NiPAAm copolymerization

The polymerization of MAAc and of NiPAAm have been conducted according to a protocol adapted from literature [3]. Typically, the polymerization

of MAAc was carried out in a mix of water and iso-propanol (précise le rapport volumique) for 4 hours at 70°C using 4,4'-azobis(4-cyanovaleric

acid) as radical precursor and MeO-PEO-RAFT macro-CTA. After 4 hours of reaction, a solution of NiPAAm was added and the whole solution was

allowed to stir at 70 °C for 18 extra hours. Various DP ranging from à compléter

Grafting of SPIONs

[1] Thomas, International Journal of Molecular Sciences, 14, 15910 – 15930 (2013). [2] Lu, Macromolecules, 44, 7233 – 7241 (2011).

[3]Chaduc, Macromolecules, 45, 1241 – 1247 (2012).

[4]Ruiz, Nanoscale, 5, 11400 – 11408 (2013).

References Conclusion

Scheme 2 : Synthetic strategy for (i) the RAFT polymerization of MAAc from PEO-RAFT CTA; (ii) and chain extention by copolymerization of NiPAAm in a One-Pot, two steps approach

The grafting of aminated SPIONs (a-SPIONS) has been

conducted at CNRS (Strasbourg). Aminated iron oxide

nanoparticles, EDCI and MeO-PEO-PMAAc (previously

prepared) were used for the optimization of the grafting [4].

MeO-PEO-PMAAc and EDCI were first dissolved in a PBS

buffer, then a small amount of a-SPIONs was add and the

whole solution is allowed to stir at room temperature for the night.

In this research, macro-RAFT have have been successfully prepared by Steglich esterification. Triblock

copolymers composed by PEO, MAAc and NiPAAm have been synthetised by RAFT with good control

of the macromolecular parameters. Coating of SPIONs with triblock copolymer needs to be done in an

other way to avoid this aggregation.

Synthesis of a PEO-macro-RAFT agent : MeO-PEO-RAFT

The macro-RAFT agent was obtained according to a strategy derived from the literature [2], involving the esterification reaction of MeO-PEO-OH (Mw = 2000 g.mol-1) with a slight excess (1.X eq) 4-cyano-4-

[(ethylsulfanylthiocarbonyl)sulfanyl]pentanoic acid (commercial RAFT agent) using N-(3-Dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (EDCI) as coupling agent in dichloromethane.

Scheme 1 : Synthesis of the macro-RAFT agent : MeO-PEO-RAFT

Figure 1 : ESI-MS spectrum of MeO-PEO-RAFT. Na+ as cationisation agent, mono-cherged Figure 2 : 1H-NMR of MeO-PEO-RAFT in CDCl3

Both 1H NMR and ElectroSpray Ionisation-Mass Spectrometry

(ESI-MS) attests for the quantitative esterification of the PEO.

Indeed, 1H-NMR spectrum (Figure 2) confirms the formation of

the ester link with a typical signal appearing at 4.25 ppm (Hc).

The good intensity ratio between protons a and c tends to

indicate a total conversion of hydroxyl protons. Mass spectrum

confirms also the formation of the macro-RAFT agent with a

signal at X uma, corresponding to X specy.

Synthesis of Polymethacrylic acid based-block copolymers by RAFT polymerization

Figure 3 : 1H-NMR of MeO-PEO-PMAAc-PNiPAAm in DMF

Samples were taken during the polymerization to be sure no unreactant

monomer was found in solution. 1H-NMR and size exclusion

chromatography (SEC) show the triblock is obtained with good

macromolecular parameters. Unfortunately, DP could not be verified by 1H-NMR.

Figure 4 : Transmission Electron Microscopy of a-SPIONs

coated with MeO-PEO-PMAAc

By TEM, the coating seems to be effective between particles, but aggregation could not be avoided.

Synthetic strategy