Poly(glycidyl methacrylate)-functionalized magnetic nanoparticles as platforms for linking functionalities, bioentities and organocatalysts

Abstract

New magnetic core shell nanoparticles were synthesized consisting of a magnetite core and a poly(glycidyl methacrylate) shell. Magnetite nanoparticles were first coated by bis(methacryloyloxyethoxy)phosphate as a reactive alkene-containing anchor followed by radical polymerization of glycidyl methacrylate. The poly(glycidyl methacrylate) coated MNPs represent versatile magnetic nano-platforms capable of linking various N-, S- and O-nucleophiles by opening the oxirane ring. In this way bioentities such as α-amino acids could be introduced directly or via first introducing a binucleophile followed by acylation (biotin) or thiol-ene click chemistry (cinchonine). Comprehensive high resolution XPS spectroscopy investigations were used as a major tool for analyzing the various types of magnetic nanoparticles. The results are attractive for application in biomedicine, organocatalysis and nanotechnology.

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1. Introduction

Magnetic nanoparticles (MNPs) have found wide application in medicine as drug delivery systems, contrast reagents or hyperthermia mediators, in separation of biomaterials or noxious compounds and as supports for catalysts.^1–6 In these applications the special features of MNPs such as control by external magnetic fields, ability of electromagnetic heating, adsorption properties and colloidal behaviour are exploited. In most of these cases the MNPs have to be covered by anchoring compounds or polymers leading to core–shell nanoparticles. These shells protect the magnetic core on the one hand but also can provide special properties such as tuned colloidal stability, selective adsorption and the possibility to link applicatory functions on the other hand.

Glycidyl methacrylate (GMA) is a commercial monomer that is often used to prepare homo or copolymers by various polymerization methods. These polymers contain oxirane rings as reactive groups, which can easily react with nucleophiles by ring opening reactions affording hydroxyalkyl esters with the respective nucleophile linked to the terminus of the resulting hydroxyalkyl groups.

Glycidyl methacrylate was also used to establish polymer shells for magnetic nanoparticles where micro-sized objects were obtained in most cases.^7–18 A number of them were applied as supports for antibodies, enzymes, or for separation purposes. Patents claim their usage as magnetic painting, for separation of proteins or other biomolecules and microorganism.^19–22 Most of these cases concern copolymers with other acrylates or styrene and the composition of the materials were insufficiently analyzed. The synthesis of such magnetic poly(glycidyl methacrylate) coated objects was achieved in several ways, such as mixing the ready made polymer with magnetic nanoparticles, generation of the magnetic nanoparticles in the presence of the polymer or from magnetic nanoparticles and glycidyl methacrylate monomer by polymerization in presence of MNPs stabilized by surfactants. The latter methodology has the drawback that the polymer can unintentionally be formed in the surrounding solution without enclosing the MNPs. To address this problem it is advantageous to use starting MNPs that are covered with active stabilizers, which are able to join the ongoing polymerization in a type of copolymerization thus establishing a covalent bond between the polymer and the surface anchors. So far, this methodology has been applied to the preparation of glycidyl-functionalized MNPs by using penta(propylene glycol) methacrylate phosphate, acrylic acid or 4-aminobenzoic acid as primary stabilizer as well as.^23–25

We report here the synthesis of new MNPs covered with poly(glycidyl methacrylate) (pGMA) shells starting with MNPs coated with bis methacryloyloxyethoxy phosphate as stabilizer capable of joining the polymerization of GMA by means of their C=C double bonds. The resulting core–shell NPs are platforms for linking various applicatory functions by nucleophilic ring opening of the glycidyl–oxirane moieties as exemplified by the introduction of selected bioentities or organocatalysts. Furthermore, turning the reactivity of the pGMA coated MNPs towards nucleophilic amino or mercapto functionality are demonstrated. Extensive high resolution XPS studies will shed light onto the phenomenon in how far the epoxy-moieties in the shells are accessible by the respective nucleophiles.

2. Materials and methods

2.1 Reagents

All chemicals used were purchased from Sigma Aldrich and used as such.

2.2 Synthesis of MNPs 3

Magnetic nanoparticles (MNPs) were synthesized by the well-known co-precipitation method from FeCl₃·6H₂O and FeCl₂·4H₂O in the presence of aqueous ammonia with a ratio 2 : 1. The resulting magnetite particles were surface-functionalized in situ under argon by the addition of bis[2-(methacryloyloxy)ethyl]phosphate 2 and strong stirring at 80 °C for 2 h (Scheme 1). The particles were then repeatedly washed with water and dried providing MNPs 3 and further on submitted to free radical polymerization with glycidyl methacrylate.

Scheme 1 Synthesis of pGMA-coated MNPs 5 by primary coverage of MNP 1 with bis(methacryloyloxyethoxy)phosphate 2 and subsequent polymerization with GMA.

2.3 Synthesis of MNPs 5 by polymerization of glycidyl methacrylate in the presence of stabilized MNPs 3

MNPs 3 (2.32 g) were used for surface mediated free radical polymerization of GMA 4 (7.1 g, 50 mmol) in the presence of 2,2′-azobis(2-methylpropionitrile) (AIBN) (0.19 g, 1.2 mmol) as radical initiator at 70 °C in dry dimethylformamide (DMF) for 24 h. The MNPs 5 were separated from the aqueous phase with the help of a Nd magnet by decantation. They were washed successively with water, ethanol and acetone.

2.4 Nucleophilic ring opening of the oxirane ring – synthesis of MNPs 7, 10 and 13

MNPs 7a and 7c. MNPs 5 (0.464 g) were dispersed in ethylenediamine 6a (19.52 g, 320 mmol) and stirred at room temperature for 48 h. The resulting MNPs 7a were then washed several times with ethanol, water and acetone and dried at 60 °C. For the transformation into MNPs 7c, biotin 8 (3.17 g, 13 mmol) was dissolved at 70 °C in 60 mL of DMF, the mixture was stirred and cooled down to 40 °C. At this temperature 1,1′-carbonyldiimidazole (3.24 g, 19.5 mmol, 1.5 eq.) dissolved in DMF (9 mL) was added to the solution and a white precipitate was formed while the mixture was stirred for 2 h. MNPs 7a (0.3 g) dispersed in DMF (30 mL) were added. After stirring for further 3 h the resulting MNPs 7c were separated by magnetic decantation, washed successively several times with DMF, water and ethanol to remove all unreacted starting materials and secondary products.

MNPs 7b. MNPs 5 (0.464 g) were suspended in chloroform (20 mL) and a high excess of N_α-Boc-L-lysine (6.4 g, 26 mmol) 6b was dissolved in this suspension. After standing at rt for 48 h the resulting MNPs 7b were separated by magnetic decantation, washed and dried.

MNPs 10a. 1,2-Ethanedithiol 9a (1.88 g) was added to a cold suspension (5 °C) of MNPs 5 (0.464 g) in distilled water (40 mL) was cooled to 5 °C. The reaction mixture was allowed to stand at room temperature for 24 h. After the completion of reaction the MNPs 10a were magnetically separated and washed several times with ethyl acetate, ethanol and acetone to remove the high excess of 1,2-ethanedithiol. Further, the MNPs 10a (0.3 g) were dispersed in toluene (100 mL) and used in a thiol-ene reaction with cinchonine 11 (0.842 g, 2.6 mmol) in the presence of AIBN (0.256 g, 1.56 mmol). The mixture was stirred at rt over night. The resulting MNPs 10c were magnetically separated and washed two times with toluene and ethanol, three times with water and ones with acetone, later on dried at 60 °C.

MNPs 10b. A solution of cysteine 9b (0.484 g, 4 mmol) in water (20 mL) was cooled to 5 °C and pH 7 was adjusted by addition of 1 M aqueous NaOH. The solution was allowed to warm up to room temperature and MNPs 5 (0.464 g) were suspended in it. After stirring at rt for 24 h the resulting MNPs 10b were magnetically separated, washed successively several times with water and dried at 60 °C.

MNPs 13. A solution of N-Boc-4-hydroxy-L-proline 12 (4.62 g, 20 mmol) and boron trifluoride diethyl etherate (BF₃·Et₂O) (1.42 g, 10 mmol) in toluene was added dropwise to a suspension of MNPs 5 (0.464 g) in toluene (30 mL). After 18 hours the resulting MNPs 13 were magnetically separated, washed several times with toluene, ethyl acetate, ethanol and water and dried at 60 °C.

2.5 Instrumentation

The chemical surface analysis for the functionalized magnetic nanoparticles was performed by X-ray Photoelectron Spectroscopy (XPS). XPS spectra were recorded using a XPS spectrometer SPECS equipped with a dual-anode X-ray source Al/Mg, a PHOIBOS 150 2D CCD hemispherical energy analyzer and a multi-channeltron detector with vacuum maintained at 1 × 10⁻⁹ Torr. The AlK_Kα X-ray source (1486.6 eV) operated at 200 W was used for XPS investigations. The XPS survey spectra were recorded at 30 eV pass energy, 0.5 eV per step. The high resolution spectra for individual elements were recorded by accumulating 10–15 scans at 30 eV pass energy and 0.1 eV per step. The powdered sample was pressed on an indium foil to allow the XPS measurements. A cleaning of the samples surface was performed by argon ion bombardment (500 V). Data analysis and curve fitting was performed using Casa XPS software with a Gaussian–Lorentzian product function and a non-linear Shirley background subtraction.

Infrared absorption spectra in the 400–4000 cm⁻¹ spectral range were recorded with a spectrophotometer JASCO FTIR-6100, on a pressed pellet prepared from the MNPs powder embedded in KBr. The morphology of the MNPs was determined by TEM using 1010 JEOL and Hitachi H9000NAR transmission electron microscopes. Magnetic measurements were performed at room temperature by using a Vibrating Sample Magnetometer Cryogenics. TGA was performed by a Pyris 1 TGA (Perkin Elmer) in a temperature range from 30 to 800 °C with heating rate of 30 °C min⁻¹ under nitrogen.

3. Results and discussion

3.1 Synthesis of pGMA coated MNPs as magnetic platforms and their functionalization

The starting magnetite nanoparticles 1 were obtained in a straight forward way by the well-known co-precipitation method from Fe(II) and Fe(III) and further stabilization with bis(methacryloyloxyethoxy)phosphate 2. The stabilizer shells of the resulting MNPs 3 proved to be useful in joining an ongoing polymerisation of functionalized acrylates before.²⁶ In the present study, they were covered with pGMA by mixing with a solution of GMA 4 in THF in the presence of AIBN as initiator (Scheme 1). The resulting pGMA–MNPs 5 were easily separated by an external magnet and washing successively with water, ethanol and acetone.

The presence of reactive oxirane groups renders the MNPs 5 as an attractive magnetic nano-platform whereon applicatory functions can be linked by nucleophilic opening of the oxirane rings. This feature was exemplified by the introduction of practically interesting N-, S- and O-nucleophiles.

Thus reaction of MNPs 5 with 1,2-diaminoethane and N-Boc-lysine as N-nucleophiles were performed neat or in chloroform, respectively, leading to MNPs 7a and 7b. The former reaction is common in functionalization of polyglycidyl methacryle (for a recent reference see ref. 27). MNPs 7b can be expected to act as an organocatalyst after deprotection while the former represents a magnetic nano-platform itself allowing reaction of the unchanged amino groups with electrophilic partners. In this way, biotin was introduced by amide formation resulting in MNPs 7c (Scheme 2). Biotin is widely used in biochemistry and biomedicine as linking unit for streptavidin and avidin-containing entities.

Scheme 2 Linking of amino functions to pGMA coated MNPs 5 by ring opening of the oxirane rings and introduction of biotin.

1,2-Ethandithiol and cysteine were chosen as S-nucleophiles for linking with glycidyl-functionalized MNPs 5 (Scheme 3). The reactions were performed either neat (9a) or in methanol (9b). Since the latter was used as hydrochloride, sodium hydroxide was applied to deliberate the free base. In principle, cysteine could also add via the amino group. But reported cases of the reaction of cysteine to oxiranes confirm an S–C bond rather than an N–C bond formation.²⁸ Cysteine moieties such as found in MNPs 10b are known to exhibit organocatalytic properties.²⁹ The mercaptoethyl functionalized MNPs 10a represent magnetic nano-platforms prone to thiol-ene click chemistry.³⁰ This property was exploited to link cinchonine as a naturally occurring alkaloid widely used in organocatalysis.³¹ The reaction was performed with AIBN as radical initiator (Scheme 3).

Scheme 3 Linking of thiol functions to pGMA coated MNPs 5 by ring opening of the oxirane rings and subsequent introduction of cinchonine by thiol-ene click reaction.

In order to verify the possibility to link O-nucleophiles to glycidyl-functionalized MNPs 5 N-Boc-protected 4-hydroxyproline 12 was used as a natural α-amino acid occurring in collagen, which was also applied as organocatalyst using the hydroxyl group as linking point to various supports.^26,32 The reaction was carried out in ether in the presence of BF₃·Et₂O as catalyst and led to proline-functionalized MNPs 13 (Scheme 4).

Scheme 4 Linking of N-protected 4-hydroxyproline to pGMA coated MNPs 5 by ring opening of the oxirane rings.

3.2 Characterization

In considering the potential applications of the synthesized magnetic core–shell nanoparticles their surface had to be very well characterized. Since NMR-spectroscopy as the standard method for characterization of organic compounds is not applicable to magnetic core–shell nanoparticles, we followed up the chemical transformations starting from MNPs 3 by detailed XPS investigations and FTIR spectroscopy.

The presence of magnetite in the cores of the MNPs 3 was confirmed by XPS spectroscopy. Due to spin–orbit j–j coupling, the Fe 2p core levels split into 2p_1/2 and 2p_3/2 components, situated at binding energies (B.E.) values ∼711 eV and ∼724 eV, respectively.³³ The deconvolution of Fe 2p spectrum obtained for the starting phosphate stabilized MNPs 3 (see Fig. 1) shows peaks due to magnetite with contributions from both Fe²⁺ and Fe³⁺ ions, the former octahedrally coordinated and the later distributed over either octahedrally or tetrahedrally sites. Considering the above mentioned issues, the obtained Fe 2p spectrum can be well fit to three main peaks and two satellites in the Fe 2p_3/2 region with a repeated scheme predicted to be at half the intensity for the Fe 2p_1/2 component since Fe 2p_3/2 has degeneracy of four states whilst Fe 2p_1/2 has only two. The peak located at 710 eV was attributed to Fe²⁺ ions, with a corresponding satellite at 717.3 eV. The Fe³⁺ octahedral species was found at a binding energy of 712 eV and Fe³⁺ tetrahedral species were observed at 714 eV in accordance with reported literature.^34–37 The Fe³⁺/Fe²⁺ ratio on the surface of the sample was found to be 1.8, this value being close to the value 2 characteristic for magnetite.

Fig. 1 The XPS Fe 2p deconvoluted spectra of MNPs 3.

The C 1s XPS spectrum of MNPs 3 can be deconvoluted into four components located at binding energies of 284.2 eV (C=C), 285.7 eV (C–C), 286.7 eV (C–O) and 288.1 eV (C=O) (Fig. 2). The P 2p spectrum of the same sample (inset Fig. 2) shows an energy maximum at 132.9 eV that is assigned to phosphate groups.

Fig. 2 High resolution XPS C 1s spectrum of MNPs 3.

After covering MNPs 3 with pGMA the resulting MNPs 5 show characteristic peaks at 285.0 eV (C–C), 286.4 eV (C–O) and 288.5 (C–O–C=O) in the C 1s XPS spectrum (Fig. 3). In addition, a peak at 287.6 eV (C–O–C) appeared typical for the oxirane ring.³⁸ The C=C group found in the precursor 3 (peak at 284.2 eV, Fig. 2) is not any longer present in the C 1s spectrum at this stage. This observation confirms that the polymerisation reaction involved the C=C bonds of the anchors at the MNPs 3. The O 1s XPS spectrum of MNPs 5 (Fig. 4) displays four contributions at 530.3 eV (Fe–O), 531.7 eV (C=O), 533.4 eV (oxirane, C–O–C) and 534.9 eV (water adsorbed during washing). The Fe 2p XPS spectrum of MNPs 5 (Fig. S1†) has lower intensity than in the precursor 3 (Fig. 1). This is due to the fact that in addition to the elemental concentration XPS intensities are dependent upon the mean free path of the electrons as well as the absorption efficiency of the material. Similar shape was observed in the Fe 2p spectra for all further investigated sample (not shown) due to the well covered magnetite core by the polymer shell.

Fig. 3 High resolution XPS C 1s spectrum of MNPs 5.

Fig. 4 High resolution XPS O 1s spectrum of pGMA coated MNPs 5.

Fig. 5 shows the C 1s XPS peak of the ethylenediamine modified MNPs 7a consisting of three features at binding energy 285.0 eV (C–C, C–N), 286.3 eV (C–O) and 288.4 eV (C=O). Since a signal at around 287.6 eV as found in the precursor 5 is missing, it can be assumed that almost all oxirane moieties were cleaved affording alcohol entities at least in the outer shell up to a depth of about 7 nm. The deconvolution of the N 1s spectrum (inset Fig. 5) reveals two peaks at binding energies 398.5 eV and 400.6 eV attributed to sp³ bonding.³⁹ They confirm the existence of primary and secondary amino groups. Like in case of the precursors 5 the deconvolution of the O 1s XPS spectrum of 7a (Fig. 6) shows four contributions at 530.3 eV (Fe–O), 531.4 eV (C=O), 532.8 eV (OH), 535 eV (H₂O), but the peak at 533.4 eV (in 5) is shifted to 532.8 eV (in 7a) due to the transformation of the oxirane ring into alcohol moieties.

Fig. 5 High resolution XPS C 1s spectrum of 1,2-diaminoethane modified MNPs 7a. Inset: XPS N 1s spectrum.

Fig. 6 High resolution XPS O 1s spectrum of MNPs 7a.

When biotin was linked to amino-functionalized MNPs 7a by amide formation to form MNPs 7c, the deconvoluted C 1s XPS spectrum (Fig. 7) shows three peaks at 288.9 eV, 286.3 eV, 285.0 eV. They are attributed to C=O, C–O and C–C, respectively. The incorporation of the biotin moiety is confirmed by the S 2p XPS spectrum (inset Fig. 7). The high resolution S 2p XPS spectrum shows a well resolved spin orbit doublet peak at 164.2 eV and 165.4 eV attributed to S 2p_1/2 and S 2p_3/2, respectively. Their positions are consistent with reported values for C–S–C.⁴⁰ In addition, a weak contribution is found from a second doublet at 168.8 eV and 167.6 eV indicating that a few biotin moieties are oxidized to the respective sulfone. This phenomenon was also observed during the introduction of biotin into other shells of MNPs.⁴⁰

Fig. 7 High resolution XPS C 1s spectrum of 7c. Inset: XPS S 2p spectrum.

XPS spectroscopy also allowed proving the ring opening of the oxirane moieties of MNPs 5 by sulphur nucleophiles. Fig. 8 presents the XPS results of the 1,2-ethandithiol derivatized MNPs 10a. The high resolution C 1s XPS spectrum displays peaks at thee binding energies: 284.6 (C–C), 285.9 (C–S, C–O, C–N) and 288.4 eV (C=O). The presence of sulphur is confirmed in the S 2p XPS region (inset Fig. 8), wherein a doublet is found at 163.5 eV and 164.6 eV in a 1 : 2 ratio, which is assigned to the S 2p_3/2 and S 2p_1/2 according to literature results.⁴¹

Fig. 8 High resolution XPS C 1s spectrum of 10a. Inset: XPS S 2p spectrum.

The C 1s XPS spectrum of the cysteine adduct 10b (see Fig. S2†) was deconvoluted giving four peak maxima corresponding to C–C (285 eV), C–N and C–O (286.6 eV), C=O (288.7 eV) and COOH (290.6 eV). However, C atoms of the oxirane ring may also be present in a small amount hidden by the superposition of the low binding energy tail of carboxyl and high binding energy tail of carbonyl components. This is suggested by the full weight half maximum (FWHM) of the high energy component, which is larger than the other, an indicator of an increased number of chemical bonds contributing to this XPS peak. As a conclusion, the more bulky cysteine can not reach all C=C-double bonds of the shell as the smaller ethane-1,2-dithiol does. The S 2p XPS spectrum of 10b (inset of Fig. S2†) shows a weak unresolved S 2p peak at 162.4 eV, which is assigned to thioether of the cysteine molecules present at the sample surface.⁴² The data are noisy due to both the reduced amount of sulphur present in the surface and the time constraints limiting the number of scans that could be acquired. However the presence of only a single binding sulphur species in the 10b spectrum suggests homogeneous bonding.

The C 1s XPS spectrum of the cinchonine-modified MNPs 10c (Fig. 9), consist of four components at binding energies 284.5 eV (C=C), 285.2 eV (C–C, C–S), 286.0 eV (C–N, C–O) and 288.9 eV (C=O). The presence of peaks attributed to aromatic C=C bond in the spectrum suggest that the thiol-ene reaction of the thiol groups at the MNPs 10a and cinchonine was successful. For further confirmation, the N 1s XPS spectrum of 10c was analyzed in detail (inset Fig. 9). It was deconvoluted into two peaks at 399.3 eV and 401.3 eV corresponding to C–N and C=N bonds, respectively,⁴³ as found in the cinchonine moiety.

Fig. 9 High resolution XPS C 1s spectrum of 10c. Inset: XPS N 1s spectrum.

The C 1s XPS spectrum of the proline-functionalized MNPs 13 (Fig. S3†) was deconvoluted into four components with peaks at 284.5 (C–C), 285.9 (C–O) and 288.0 eV (C=O). Since the peak at 287.6 eV of the precursors 5 disappeared it can be assumed that all oxirane moieties at the surface are ring opened by hydroxyproline. The presence of the proline moiety was proved by a peak at 399.6 eV in the N 1s core level XPS spectrum (inset Fig. S3†) typical for amide functionality.

Some of the transformations performed with MNP 5 were investigated by TGA (Fig. 10). As expected, the addition of nucleophiles to MNPs 5 (48.4% mass loss) leading to MNPs 7c (59.2% mass loss), 10b (53.6% mass loss), 10c (71% mass loss) and 13 (50.3% mass loss) resulted in an increase of the mass of the organic shell.

Fig. 10 TGA of MNPs 5, 7c, 10b, 10c and 13.

Although not as informative as XPS the FTIR spectroscopy also contributed to the structure elucidation of the MNPs. Thus the strong absorption band located at around 580 cm⁻¹ in the FTIR spectra of MNPs 3 and 5 is attributed to Fe–O bond of magnetite found in the cores (Fig. 11). Another important band found in both types of MNPs 3 and 5 is situated at around 1049 cm⁻¹ and is specific for P–O bonds stemming from bis(methacryloyloxyethoxy)phosphate 2. As expected, the band owing to stretching C=O vibrations at 1722 cm⁻¹ can be seen in both FTIR spectrum of MNPs 3 and 5, but in increased intensity in the latter case due to additional carbonyl groups introduced by glycidyl methacrylate. The spectrum of magnetic nanoparticles 5 in the range of 2800–3000 cm⁻¹ shows the presence of hydrocarbon groups, i.e. CH, CH₂ and CH₃ characteristic of poly(glycidyl methacrylate).

Fig. 11 FTIR spectra of MNPs 3 stabilized by bis(methacryloyloxyethoxy)phosphate, magnetite pGMA core shell nanoparticles 5 and biotin functionalized MNPs 7c.

The transformation of MNPs 5 into 7c results in the appearance of an additional carbonyl band at 1696 cm⁻¹ attributed to the amide and urea functionality (Fig. 11). For further FTIR spectra see ESI.†

The pGMA-coated MNPs and their derivative form suspensions with partial aggregation. TEM image of proline functionalized MNPs 13 (Fig. 12) reveals aggregated magnetite NP coated with a relatively high amount of polymer material as compared with the precursor MNPs 3.

Fig. 12 TEM image of MNPs 3 (left, bar size 100 nm) and proline functionalized MNPs 13 (right, bar size 50 nm).

The magnetic properties of all MNPs were investigated by magnetometry confirming supraparamagnetic behaviour in each case. The values of saturation magnetization step down with the introduction of additional functionality increasing the weight of the organic shell (Fig. 13), e.g., 43 emu g⁻¹ (3) via 28 emu g⁻¹ (5) to 23 emu g⁻¹ (7c). The saturation magnetization of MNPs 10c (11 emu g⁻¹) is much lower than that of the poly(glycidyl methacrylate) MNPs 5 (Fig. 13) because additional mass is introduced by two subsequent steps (transformation from 5 into 10a and further into 10c). On the other hand, a small reduction of the saturation magnetization (Fig. 13) was observed in the one-step transformation of MNPs 5 into the proline functionalized MNPs 13 (28 emu g⁻¹ versus 24 emu g⁻¹, respectively). Regardless of all these differences, all MNPs exhibit saturation magnetizations high enough for the envisaged practical application in biomedicine, biotechnology and catalysis.

Fig. 13 Magnetization versus applied magnetic field at room temperature of MNPs 3, 5, 7c, 10c and 13.

4. Conclusion

In summary, new magnetic core shell nanoparticles 5 were synthesized consisting of a magnetite core and a poly(glycidyl methacrylate) shell. Magnetite nanoparticles 1 stabilized by bis(methacryloyloxyethoxy) phosphate as reactive alkene-containing anchor capable of joining the ongoing polymerization of glycidyl methacrylate 4 in a type of copolymerization were used in this methodology. The poly(glycidyl methacrylate) coated MNPs 5 represent versatile magnetic nano-platforms capable of linking various functionalized N-, S- and O-nucleophiles by opening the oxirane ring. In this way bio entities such as α-amino acids could be linked directly or via first introducing a binucleophile (1,2-diaminoethane, 1,2-ethandithiol) followed by acylation (biotin) or thiol-ene click chemistry (cinchonine), respectively. Comprehensive high resolution XPS spectroscopy investigations were used as a major tool for analyzing the various types of magnetic nanoparticles confirming the expected transformation and sometimes providing information about the effectiveness of those transformations. The results are attractive for application in biomedicine, organocatalysis and nanotechnology.

Acknowledgements

The authors wish to acknowledge, Dr Cristian Leostean (INCDTIM) for conducting magnetic measurements, Dr Lucian Barbu (INCDTIM) for TEM investigations and Dr Björn Kobin (Humboldt-University Berlin) for TGA investigations. This work was supported by the Romanian Ministry of Education and Research under the research project PN-II-RU-TE-2011-3-0130.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra05126j

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