A. Glaria‡
a,
S. Soulé‡b,
N. Hallali‡a,
W.-S. Ojoa,
M. Mirjoleta,
G. Fuksa,
A. Cornejoa,
J. Alloucheb,
J. C. Dupinb,
H. Martinezb,
J. Carreya,
B. Chaudreta,
F. Delpech*a,
S. Lachaize*a and
C. Nayral*a
aLPCNO, Université de Toulouse, CNRS, INSA, UPS, 135 Avenue de Rangueil, 31077 Toulouse, France. E-mail: slachaiz@insa-toulouse.fr; fdelpech@insa-toulouse.fr; cnayral@insa-toulouse.fr
bInstitut des Sciences Analytiques et de Physico-Chimie pour l’Environnement et les Matériaux, Université de Pau et des Pays de l’Adour, Hélioparc, 2 av. Président Angot, F-64053 Pau, France
First published on 17th September 2018
This work provides a detailed study on the synthesis and characterization of silica coated iron nanoparticles (NPs) by coupling Transmission Electronic Microscopy (TEM), X-ray Photoelectron Spectroscopy (XPS) and magnetic measurements. Remarkably, iron NPs (of 9 nm of mean diameter) have been embedded in silica without any alteration of the magnetization of the iron cores, thanks to an original protocol of silica coating in non alcoholic medium. Tuning the synthesis parameters (concentration of reactants and choice of solvent), different sizes of Fe@SiO2 composites can be obtained with different thicknesses of silica. The magnetization of these objects is fully preserved after 24 h of water exposure thanks to a thick (14 nm) silica layer, opening thus new perspectives for biomedical applications. Hyperthermia measurements have been compared between Fe and Fe@SiO2 NPs, evidencing the self-organization of the free Fe NPs when a large amplitude magnetic field is applied. This phenomenon induces an increase of heating power which is precluded when the Fe cores are immobilised in silica. High-frequency hysteresis loop measurements allowed us to observe for the first time the increase of the ferrofluid susceptibility and remanence which are the signature of the formation of Fe NPs chains.
We report here our results on this novel silica coating method. We explored the influence of key parameters such as the solvent and the reactants concentrations. We particularly paid attention to characterize the nature of the interface between Fe(0) and SiO2 to evaluate the oxidation due to the coating step. The magnetic properties have been studied and compared to the ones of the pristine Fe NPs and then followed by magnetic hyperthermia measurements. Finally, the impact of the exposition to air and to water has been evaluated.
In the case of Fe@SiO2 samples, NPs settled down fastly thus ϕ could only be estimated, but not precisely known. The volume concentration was estimated by measuring the NP powder height inside the Schlenk – which permits to evaluate the volume of material – and then to divide it by the iron mass. When using this estimation, discrepancy between the magnetic method and the calorimetric one could reach a factor as large as 2.3. Therefore, to offset the difference between calculated magnetization values and the real ones we calculated the area (Aloop) of the estimated hysteresis loop as a function of AMF amplitude value (μ0Hmax). We compared this curve to the one corresponding to the specific losses obtained from temperature measurements (ATemp). The Aloop (μ0Hmax) curve was multiplied by a corrective factor xcorr (ranging from 0.7 to 2.3) so as to obtain a good correspondence with the ATemp (μ0Hmax) curve (see ESI†-Fig. 4). This correcting factor was then applied to the magnetic hysteresis loops. After this treatment, the magnetization curves are expected to represent quantitatively the high-frequency magnetic properties of the samples.
The metallic NPs were then used as seeds for the growth of the silica material which proceeded over a few days once introduced in the reaction medium. The growth of the silica shell is based on a protocol derived from our previous study on silica formation in a non-alcoholic medium.17 The hydrolysis and the condensation of 1 equivalent of tetraethyl orthosilicate (TEOS) occur in an organic solvent (tetrahydrofuran (THF) or dimethoxyethane (DME)), using 1-butylamine (BA) as a catalyst and 2 equivalents of water (minimum amount to allow the silica formation while avoiding an excess of water in the medium). In these conditions, the water exposure of iron NPs is minimized and the kinetic of silica formation is significantly slowed down (compared to classical Stöber protocols). The amine plays also the role of stabilizing agent during and after the synthesis by interacting with the silica surface. The formation of the silica NPs occurs by a slow release in solution of spherical nano-objects from a large silicated network. The coating process has been optimized by playing with the concentration of reactants and with the choice of the solvent, which appear as two levers to control the rate of silica formation. The influence of concentration has been studied in THF and the effect of the solvent has been evaluated by using DME instead of THF. Thus, a set of four samples, as reported in Table 1, is presented here and has been fully characterized, using Transmission Electronic Microscopy (TEM), elemental analysis, magnetic measurements and XPS.
Sample | Experimental conditions | Remark | |||||
---|---|---|---|---|---|---|---|
Molar ratios | Time (days) | Solvent | [TEOS] (mol L−1) | ||||
TEOS | BA | H2O | |||||
1 | 2 | 1 | 4 | 7 | THF | 0.09 | Standard |
2 | 2 | 1 | 4 | 7 | DME | 0.09 | Solvent effect |
3 | 1 | 0.5 | 2 | 7 | THF | 0.045 | Concentration effect |
4 | 2 | 1 | 4 | 2 | DME | 0.09 | Time effect |
The TEM pictures of the obtained hybrid NPs (Fe@SiO2) (samples 1, 2, 3, 4) are reported in Fig. 2 showing the formation of a silica shell surrounding the iron cores. It is worth noting that, whatever the conditions, no uncoated iron core is observed. The standard procedure, used for sample 1, leads to spheroidal hybrid NPs with a size of 79 (12) nm. For a same reaction time of 7 days, depending on the experimental conditions the final size of the Fe@SiO2 NPs can be tuned. It is increased to 89 (14) nm when using DME instead of THF (sample 2) and decreased to 67 (11) nm when reducing the concentration of the reactants compared to the standard protocol (sample 3). These results are in good agreement with the study reported for silica particles alone which showed the same tendency when playing on these two parameters. Here, the introduction of a seed material in the medium did not drastically modify the hydrolysis/condensation process.
Fig. 2 TEM pictures of (a) sample 1, (b) sample 2 and (c) sample 3 after 7 days of reaction and (d) sample 4 after 2 days of reaction. |
To analyze the different steps of the coating, aliquots have been harvested during the reaction (see Table 2). For sample 1, we observed the formation of individualized particles right after day 2 with a large amount of aggregates (micron-size) still present. While the size of the hybrid NPs increased over the days, the number of aggregates was reduced for a complete disappearance at day 7. This tendency was also noted for sample 2 but more pronounced with respect to the increase rate of the NPs size and the disappearance of the aggregates. Indeed, the first hybrid NPs were observable after 48 hours with a size at this stage already approaching the final one obtained using the standard procedure. Then over the days, the NPs population tends to be more homogeneous, with a slight increase in size (12%). When the concentration of the reactants was reduced, i.e. sample 3, the reaction was slowed down so that the uncoated Fe NPs were still observable at day 2. Then, aggregates were formed embedding the iron cores and giving rise at day 7 to hybrid NPs with a size of ca. 67 nm, nearly 12 nm smaller than the ones obtained in the standard conditions. Nevertheless, these smaller hybrid NPs were still accompanied by a few aggregates. Assuming that the hydrolysis/condensation formation process followed the one described for silica nanomaterials alone, it seems to indicate that the release of hybrid NPs was not yet finished. In the case of sample 4, the use of DME favors a higher rate of formation of the silica and by limiting the time of reaction at 2 days, the silica thickness around the iron cores can be restrained and is then only about 4–5 nm. Individualized hybrid particles are not yet a majority and mainly undefined silica aggregates are observed. Interestingly depending on the experimental conditions the number of Fe NPs encapsulated can also be varied (see Table 2). For sample 1, the number of Fe NPs remained relatively low with an average value of 5 Fe NPs per final hybrid NP. When the solvent was changed for DME, which is prone to increase the reaction rate, the number of Fe NPs was more than doubled up to an average value of 13. However, the iron contents of samples 1 and 2 continue to be quite similar (28.9% for sample 1 and 27.4% for sample 2). For sample 3, the slow reaction process seemed to promote also a higher number of encapsulated NPs closed to the one observed for sample 2, but here the iron content is doubled (57.2%).
Sample | dTEM,1d | dTEM,2d | dTEM,3d | dTEM,4d | dTEM,7d | Nbr. of Fe NPs | % Fe |
---|---|---|---|---|---|---|---|
1 | agg. | 53 (12) agg. | 65 (14) agg. | 68 (12) agg. | 79 (12) | 5 (3) | 28.9 |
2 | agg. | agg. | 74 (12) agg. | 70 (10), 95 (14) | 89 (14) | 13 (7) | 27.4 |
3 | Fe NPs | Fe NPs | agg. | 62 (12) agg. | 67 (11) agg. | 11 (6) | 57.2 |
4 | agg. | agg. | — | — | — | — | 35.0 |
Sample | Ms (A m2 kgFe−1) | MR (A m2 kgFe−1) | |Δμ0H| (mT) |
---|---|---|---|
Fe NPs | 221 (3) | 68 (1) | 1 |
1 | 180 (31) | 76 (13) | 4 |
2 | 205 (37) | 85 (15) | 6 |
2-Air | 88 (12) | 50 (7) | 68 |
2-Water | 190–217 | 75 (10) | 0 |
3 | 184 (16) | 85 (7) | 4 |
4 | 197 (28) | 90 (12) | 1 |
4-Air | 86 (12) | 69 (9) | 34 |
4-Water | 163–191 | 52 (7) | 6 |
Fig. 3 Hysteresis loops of the different samples reported in Table 3 and performed at 4 K. |
The Fe2p3/2 peak on the Fe2p spectrum of Fe NPs (Fig. 4a) clearly evidences the presence of iron in the oxidation states 0, +II and +III located respectively around 707 eV, 709 eV and 711 eV. Note that the peak also displays the satellite structure of Fe(II) around 715 eV, increasing the peak asymmetry towards the high binding energies. On the contrary, the shake-up satellite of Fe(III), expected at 8.5 eV from the Fe2p3/2 component, is not clearly identified because it overlaps with the Fe2p1/2 components (Fe(0) and Fe(II)). The deconvolution of iron metal enables to estimate the relative proportion of Fe(0) and of Fe(II/III), thus at the surface the estimated Fe(II/III)/Fe(0) ratio is 1.5 (Table 4). The O1s core peak of pure iron NPs displays three components (Fig. 4b). The main peak at 529.6 eV (10.0 at%) corresponds to Fe–O–Fe bonds. The two other components located at 531.5 eV (10.1 at%) and 533.4 eV (3.1 at%) can be attributed respectively to Fe–O–C (carboxylate ligand) ‘bidentate’ environments and ‘monodentate’ in accordance with the C–O component at 286.3 eV (10.4 at%) and OC–O (288.7 eV, 4.2 at%) on the C1s spectrum.27 These results clearly point out that at the surface of the NPs (considering the XPS depth analysis), a part of oxidized iron atoms (Fe–O) is detected in different forms, Fe–O–C due to surface carboxylates ligands as well as Fe–O–Fe. However, as shown earlier, these atoms do not induce any detectable effect on the magnetic properties. XPS quantitative analyses (Table 5) also reveal an important atomic percentage of carbon (66.7% at) in agreement with the presence of ligands (palmitic acid and hexadecylamine) at the NPs surface.
Sample | Fe (0) (rel. %) | Fe(II/III) (rel. %) | Fe(II/III)/Fe(0) |
---|---|---|---|
Fe NPs | 40.8 | 59.2 | 1.5 |
4 | 7.9 | 92.1 | 11.7 |
4-Air | 5.5 | 94.5 | 17.2 |
4-Water | — | 100 | — |
Fe NPs | Sample 4 | |||
---|---|---|---|---|
BE (eV) (FWHM (eV)) | At% | BE (eV) (FWHM (eV)) | At% | |
C1s | 285.0 (1.3) | 52.1 | 285.0 (1.6) | 11.1 |
286.3 (1.7) | 10.4 | 286.3 (1.8) | 4.9 | |
288.7 (1.6) | 4.2 | 288.6 (1.6) | 1.6 | |
Total at% | 66.7 | 17.6 | ||
O1s | 529.6 (1.4) | 10.0 | 530.8 (1.9) | 5.3 |
531.5 (2.0) | 10.1 | 533.4 (2.3) | 53.4 | |
533.4 (1.9) | 3.1 | |||
Total at% | 23.2 | 58.7 | ||
Fe2p3/2(Fe(0) | 706.3 | 4.1 | 706.8 | 0.2 |
Fe2p3/2 (Fe(II/III)) | 6.0 | 2.8 | ||
Total at% | 10.1 | 3.0 | ||
Si2p3/2–1/2 | — | — | 103.9–104.5 (2.1–2.1) | 20.0 |
N1s | — | — | 0.7 |
On sample 4, XPS quantitative analysis (Table 5) reveals the prevalence of silicon (20.0 at%) and oxygen (58.7 at%) but also the presence of carbon (17.6 at%) and nitrogen (0.7 at%) due to amine surface ligands. Due to the XPS depth analysis (which is around 5 nm, while 4–5 nm is the silica thickness estimated from TEM pictures on sample 4), the total atomic percentage of iron deduced from XPS analysis is low, i.e. 3.0 at%. However, we can notice the shift of the envelope maximum of the Fe2p3/2 peak towards high binding energies after the silica coating (Fig. 4a) which reveals the further oxidation at the interface with the silica shell. Thus, the Fe(II/III)/Fe(0) ratio is 11.7 for Fe@SiO2 compound versus 1.5 for iron NPs. The O1s spectrum (Fig. 4b) displays a component at 530.8 eV (5.3 at%) attributed to Fe–O–Si bonds characteristic of the interface. Another component is evidenced at 533.4 eV assigned to SiO2 environment, in agreement with the Si2p3/2 core peak located at 103.9 eV (Fig. 5c). These results are in agreement with the location of a thin iron oxide layer between the iron core and the silica shell, as detected by the magnetic characterization.
Fig. 5 (a) Fe2p, (b) O1s and (c) Si2p XPS core peaks of Fe@SiO2 NPs and Fe@SiO2 NPs exposed to air and water. |
To evaluate the air and water stability of the hybrid NPs, we have chosen to compare the behaviors of two different types of silica shells: sample 2, which exhibits large silica shells around the iron cores (thickness of the silica layer about 14 nm) and sample 4, which presents thinner silica layers (about 4–5 nm of thickness). We thus exposed during 24 h two different aliquots of each sample (2 and 4) to the different media (air and water) and measure the resulting magnetic properties.
Whatever the silica shell thickness (samples 2 and 4), unlike before water exposure, hysteresis loops (Fig. 6) do not saturate at high field. This phenomenon is consistent with the formation of paramagnetic species, such as electrically insulated Fe(II) and/or Fe(III). Their proportion (%para), and their contribution (Mpara) on the magnetic signal measured by VSM (MVSM) at 4 K, was determined using the following expression:
MVSM = %paraMpara + %ferroMferro with %para + %ferro = 1 |
Mferro is the ferromagnetic contribution to the magnetic signal . The saturation value of the Mferro signal is called Ms,ferro. We assumed that the paramagnetic magnetization Mpara comes from magnetically independent Fe(II) or Fe(III) species (see ESI† for details of the calculation). The analysis process is illustrated in Fig. 7a. The paramagnetic contribution can be equally well fitted assuming segregated Fe(II) or Fe(III) species, so the shape of the hysteresis loop does not allow us to discriminate between the two. Depending on the type of species assumed, the value of %para is slightly different, but the trends are similar. Here, the effect of water exposure causes a significant increase in the paramagnetic species proportion, going from ∼3% to ∼14% (see Fig. 7b), identically for samples 2 and 4. On the contrary, the fall of Ms,ferro is less important for sample 2 than for sample 4 (Fig. 7c).
Fig. 7 (a) Illustration of the method used and based on Mferro formula to determine the paramagnetic contribution %para and Ms,ferro value. Mpara is obtained theoretically using the Brillouin function applied for magnetically independent Fe(II) or Fe(III) species (see ESI† for details of the calculation). This method is illustrated on sample 2 after water exposure and %para is determined to obtain a perfect saturated hysteresis loop (Mferro) considering Fe(II) as paramagnetic species. (b) and (c) Paramagnetic contribution %para and Ms,ferro value obtained for each sample considering either Fe(II) or Fe(III) (the non-zero value of %para for NPs Fe sample is not significant but could be linked to a slight spin canting of Fe(0) surface spins). |
XPS analysis of sample 4 after water exposure shows a drastic modification of the silica shell (Fig. 5b and c, ESI†-Fig. 3). The chemical shift of −0.8 eV of both Si2p(SiO2) and O(SiO2) is consistent with a chemical modification of the silica shell leading to less condensed species.28 Besides, the relative intensity of the O1s component at 530.9 eV increases significantly (Fig. 5b) and can be attributed to the formation of FeOSi which is coherent with the apparition of Fe(II/III) paramagnetic species evidenced by magnetic measurements. A reasonable hypothesis could be the formation of iron silicate species by diffusion of iron into the less condensed silica shell. In this case, the chemical modification of the silica precludes the direct interpretation of the Fe2p3/2 peak shifts, that we will not discuss, since the probed zones (before and after water exposure) are not comparable anymore.
To sum-up, the consistency between XPS and VSM measurements allows us to draw up the following evolution scheme concerning the Fe NPs and Fe@SiO2 NPs: (i) before the synthesis of the silica shell, the Fe NPs are essentially composed of Fe(0). A small percentage of iron atoms have a higher oxidation degree, such as Fe(II) and/or Fe(III). (ii) After the silica shell growth, a slight layer of iron oxide appears and the magnetization of the iron cores decreases slightly. This result is not surprising with an interface necessarily composed of Fe–O–Si. No significant difference in magnetic properties between the different Fe@SiO2 NPs samples was observed. (iii) After air exposure, whatever the thickness of the silica shell, the magnetization as well as the Fe(0) content of iron cores have greatly decreased, proving that the silica shell is not adapted to protect iron cores from their oxidation under air. (iv) After water exposure, the silica shell is drastically modified. Iron II or III species were produced due to the probable diffusion of water and then migrated into silica to form segregated paramagnetic species (in identical proportion regardless of the silica shell thickness). However, the comparative study between sample 2 and sample 4 shows that a greater thickness of shell allows limiting more effectively the fall of the magnetization of the iron cores during water exposure.
Fig. 8 SAR values for each sample obtained from calorimetric measurements under AMF at 93 kHz and an amplitude range from 0 to 70 mT. |
Fig. 9 Magnetic hysteresis loops measured at different magnetic field amplitude: (a) Fe NPs, (b) sample 1, (c) sample 2, (d) sample 3, and (e) sample 4. |
Since SAR amplitude depends strongly on the amplitude of the applied magnetic field and frequency, a convenient way to compare the heating properties of MNPs measured under various experimental conditions is to calculate their intrinsic loss parameter (ILP), defined by the equation with Hmax the AMF maximum value (in A m−1), f, the AMF frequency (in Hz) and SAR the specific absorption rate (in W kg−1).29 The ILP is commonly expressed in nHm2 kg−1 (see Table 6). The evolution of ILP as a function of the magnetic field is shown in ESI†-Fig. 5. For the three Fe@SiO2 samples, ILP is rather independent of magnetic field amplitude so its average value is provided in Table 6. For the Fe NP samples, it increases significantly with the magnetic field amplitude, so the ILP range is not displayed in Table 6, but its evolution in function of magnetic field is provided in ESI†-Fig. 5. The origin of this phenomenon is discussed at the end of this section.
ILP (nHm2 kgFe−1) | |
---|---|
Sample 1 | 0.40 (5) |
Sample 2 | 0.43 (12) |
Sample 3 | 0.24 (5) |
Sample 4 | 0.31 (6) |
As shown in Fig. 8, 9 and Table 6, samples 1, 2 and 4 present similar specific loss values, larger than the ones of sample 3. In this case, the iron content of the hybrid NPs is higher than in samples 1, 2, 4, and can influence the amplitude of magnetic interactions. SAR values are indeed very sensitive to the latter.30
The modest ILP values (in view of the results from the literature ref. 31) of the Fe@SiO2 NPs could be explained by the diameters of the cores. Using iron NPs with diameter in the range 16–20 nm, which displays much larger SAR values than the present one, could permit to increase these ILP values.7 However, the strong magnetization of such large MNPs tends to force their agglomeration in solution, impeding the coating process.
Finally, the Fe sample presents, at low magnetic field, heating and magnetic properties almost similar to the Fe@SiO2 samples. However, when the magnetic field is above 25 mT, Fe sample heating power becomes much larger than the other samples. Hysteresis loops in Fig. 9 show that this increase is associated with both a tilt of the hysteresis loops and an increase in remanence. Moreover, the minor loops at low magnetic field are not included any more in the loop at larger magnetic field, evidencing a change of regime. These features have been measured several times and are reproducible. They can be interpreted as a signature of the mobility of the free Fe NPs in solution during AMF application: above 25 mT, they self-organize in chains and/or have their anisotropy axis oriented along the magnetic field, which increases the ferrofluid susceptibility and remanence. Since ILP can be linked to magnetic susceptibility of ferrofluid (as detailed in ESI†), the increase of this last parameter as a function of magnetic field amplitude due to magnetic interaction could explain the ILP dependence on magnetic field amplitude for Fe NPs sample. The influence of the formation of chains of MNPs on heating properties has been reported by many groups.8,32–34 It is however the first time to our knowledge, that the signature of this formation (i.e. the increase of the ferrofluid susceptibility and remanence), is observed clearly with high-frequency hysteresis loop measurements. Moreover, in our case, a change of regime is observed, the formation of chains occurring only at large magnetic fields. Since the heating properties of Fe@SiO2 NPs and Fe NPs at low magnetic field are similar, the weaker heating power of the Fe@SiO2 NPs at large AMF compared to Fe NPs must not be interpreted as the signature of degraded properties due to the coating. The higher heating power of the Fe NPs is the consequence of their self-organization whereas the iron cores are immobilized inside the silica for Fe@SiO2 NPs during the magnetic field application. As example, the influence of NPs immobilization on their SAR decrease had been described for iron oxide NPs embedded in agarose or polyvinyl alcohol.35 In our case, silica can be seen as a more drastic freezing matrix, precluding any shift of the shelled iron NPs.
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8ra06075d |
‡ Equal contribution. |
This journal is © The Royal Society of Chemistry 2018 |