Silane Functionalisation of Iron Oxide Nanoparticles

Silane Functionalisation of Iron Oxide Nanoparticles Sam G. Ogdena*, David Lewisb, Joe G. Shaptera a

School of Chemistry, Physics and Earth Sciences, Flinders University, Bedford Park, SA, 5001 b

Carl Zeiss Vision, Cooroora Crescent, Lonsdale, SA, 5160

ABSTRACT Silane encapsulated magnetic iron oxide nanoparticles were synthesized through a sequential approach. The nanoparticles were synthesised via a coprecipitation method to form Fe3O4 nanoparticles with an average particle size of 8.3 ± 2.3nm. Iron oxide nanoparticles were then coated with 3-glycidoxypropyltrimethoxysilane (GPTMS) to form coreshell type nanoparticles. Coating was performed using a base catalysed sol-gel process involving the direct condensation of GPTMS onto the nanoparticle surface. Elemental composition and crystal structure of the uncoated nanoparticles were determined by XRD. The coated nanoparticles were characterised with infra-red spectroscopy and energy dispersive xray spectroscopy (EDX) to confirm the presence of silane on the nanoparticles. TEM analysis and Scherrer broadening analysis of XRD were used to determine particle size and morphology of both coated and uncoated nanoparticles.

1.

INTRODUCTION

Iron oxide is the common name for any of three different minerals: Wustite (FeO), Magnetite (Fe3O4) and Hematite/Maghemite (α-Fe2O3/γ-Fe2O3). Nanoparticle Fe3O4 and Fe2O3 are the subject of a large amount of research in a variety of areas from biological system control1 to magnetic data storage2 and medical imaging,3-6 due to the change in their magnetic properties at small size ranges (sub-25nm for Fe3O4,7 sub-10nm for Fe2O38). Of particular interest are the superparamagnetic properties shown by these minerals as a result of the increase of relaxation time due to the decreasing particle size.8 One of the favoured methods for producing hydrophilic iron nanoparticles has been the “co-precipitation” technique. In this approach iron (II) and iron (III) salts are reacted in basic conditions under a N2 atmosphere to produce Fe3O4 nanoparticles with moderate size dispersion.6, 9-13 The basic conditions can be obtained by adding ammonia or sodium hydroxide to the solution.6, 7 The nanoparticles formed varied in size according to reaction conditions but have been reported to have sizes of 8-10nm 10, 11 with a variation of about a nanometre.10, 11 The reduction of the salts was performed in aqueous media and results in the formation of magnetite nanoparticles (Fe3O4) (Eq1.1): Fe2+(aq) + 2Fe3+(aq) + 8OH-(aq) Æ Fe3O4(s) + 4H2O

(1)

Upon the addition of heat and with aeration the nanoparticles are oxidised to form maghemite (Fe2O3) (Eq1.2): (2) This resulted in an increase in the formal oxidation state of the iron from a mixture of both Fe2+/Fe3+ in the magnetite, which has a formal equation of Fe2O3 FeO, to a Fe3+ only oxidation state. The crystal structure of the nanoparticle remained the same in either mineral.8 For this reason the nanoparticles are synthesised at low temperature and under N2. Coating agents are often used on iron oxide nanoparticles to make them biocompatible. Some of the most commonly used reagents for this are silicon alkoxides. The formation of silane coatings onto inorganic nanoparticles has often Smart Materials V, edited by Nicolas H. Voelcker, Helmut W. Thissen, Proc. of SPIE Vol. 7267, 72670A · © 2008 SPIE CCC code: 0277-786X/08/$18 · doi: 10.1117/12.810679 Proc. of SPIE Vol. 7267 72670A-1 2008 SPIE Digital Library -- Subscriber Archive Copy

followed a sol-gel technique5, 6, 11, 14-18 Iron oxide nanoparticles can be coated with silanes and the resulting nanoparticles used for medical imaging. Here iron oxide nanoparticles will be formed via a coprecipitation reaction and coated with (3glycidoxypropyl)trimethoxysilane (GPTMS).

Fig. 1: Structure of GPTMS

Due to the high pH required for the synthesis of the nanoparticles6, 9-13 the addition of GPTMS was hindered by the possibility of polymerisation via an epoxide ring opening mechanism.19 As such the nanoparticles were separated via centrifugation and then redispersed in various aqueous media in order to obtain the most monodisperse and stable solution possible. The dispersed nanoparticles were with GPTMS following a modified Stöber process.20 The nanoparticles produced were imaged using transmission electron microscopy (TEM), while Energy Dispersive X-ray Spectroscopy (EDX) and X-ray Diffraction Spectroscopy (XRD) were used to determine nanoparticle composition and the size of the nanoparticles was determined from TEM images and using Scherrer Broadening from XRD data. IR spectroscopy was also used to confirm silane attachment.

2.

EXPERIMENTAL

FeCl2 4H2O, FeCl3 6H2O, dimethyl sulfoxide (DMSO) and 1.0M NaOH solution was purchased from Sigma-Aldrich and used as received without further purification. (3-glycidoxypropyl)trimethoxysilane and Oleic Acid was purchased from Aldrich and used as received. Iron oxide nanoparticles were initially created following a modified coprecipitation technique.6, 9-13

2.1 Formation of iron oxide nanoparticles FeCl2 4H2O (0.01mol) and FeCl3 6H2O (0.02mol) salts were dissolved in 100mL water and added with continuous stirring to 100mL NaOH (1.0M) solution under a nitrogen atmosphere. The supernatant solution was decanted off and the concentrated nanoparticles centrifuged at 3000rpm and washed three times with water. Subsequent centrifugation followed each wash. The washed nanoparticles were examined using TEM, XRD and IR. 2.2 Iron oxide nanoparticle dispersion Nanoparticle dispersion in various media was tested over a range of pH and ionic strengths.2 Standard solutions with pH ranging from pH=0 – pH=14 and NaCl concentrations of 0M and 2M were prepared. These were then combined to produce a range of pH and ionic strengths in order to test the redispersability of the prepared nanoparticles. The ionic strength of each solution was calculated and the pH measured. 2%w/w aqueous dispersions of the nanoparticles formed via the coprecipitation syntheses were created in each of the salt free solutions. This 2%w/w solution (0.1mL) was then added to 9.9mL of dispersant solution and the resulting dispersions were sonicated for 10 minutes before being left to stand for 72 hours and the stability of the mixtures observed. TEM grids were prepared of the nanoparticles in the resulting solutions.

Proc. of SPIE Vol. 7267 72670A-2

2.3 Silanisation via sol-gel Silanisation followed a modified Stöber method.5, 11, 14, 15, 20 Initially a 2%w/w aqueous suspension of peptised nanoparticles was prepared. Two grams of this solution was added to 200mL of a 4:1 solution of ethanol and water. One millilitre of 25% NH4OH solution was added as a catalyst prior to 10 minutes sonication to homogenise the solution. GPTMS (1mL) was added slowly with mechanical stirring and the solution stirred for 3 hours. The resulting solution was centrifuged to collect the nanoparticles. Two further solutions were made with no catalyst and 1mL of 35% HCl catalyst respectively. A final silanisation was performed using the same conditions as the initial reaction, however using propan-2-ol as the alcohol in a 4:1 ratio with H2O.

2.4 Characterisation Nanoparticles were separated with an external magnetic field and washed twice with ultrapure water. Redispersed nanoparticles were centrifuged out and IR spectra taken. Infra-red spectra (IR) were obtained as KBr discs (1-5mg sample, 150mg KBr) formed using a Pye Univar press. Spectra were obtained using a Nicolet Avater 370MCT (Thermo Electron Corporation). The spectrometer was fitted with a transmission accessory and all spectra were recorded using Omnic version 7 software. Spectra were recorded over a range of 500-4000cm-1, at a resolution of 2cm-1 and taken as an average of 128 scans. XRD patterns were taken at the Adelaide Museum and were performed using room temperature powder X-ray diffraction (XRD) using a Huber G760 100mm image plate Guinier Camera with Co Kα_1 radiation (λ= 1.78892 Å). The samples were ground in acetone, spread uniformly on a MYLAR film and mounted on the sample oscillation unit for data collection. TEM images of all samples were prepared after 3 hours by dipping TEM grids into the solution. Excess solution was drained off by placing the grids on filter paper. All samples for TEM analysis were prepared on 300 mesh carbon coated copper grids. TEM experiments were performed on a EM 200 High Resolution Transmission Electron Microscope (Phillips Corporation) fitted with an EDX Detector (EDAX, Mahwah, NJ, USA) at an accelerating voltage of 200keV. Images were captured using Gatan version 5 software and analysed using Gatan version 5 and ImageJ version 1.34.

3. RESULTS AND DISCUSSION There were three desirable properties sought in the initial formation of nanoparticles; magnetically active, the ability to be directly coated with silane (hydroxy surface groups) and the ability to readily be redispersed in order to try to achieve homogenous coating of the nanoparticles. The coprecipitation preparation technique yielded black precipitates that responded to an external magnetic field. This black precipitate contained nanoparticles as in Fig. 2(a) and the particle sizes were measured to be 8.4nm ± 2.3nm. XRD was used to characterise the as synthesised nanoparticles. The XRD spectrum shown in Fig. 2b for nanoparticles synthesised showed a strong correlation to fitted crystal structure data for Fe3O4.The peak broadening observed in the spectra was due to the small size of the crystalline domains in the sample. The nanoparticles formed by this technique were formed with single crystalline domains, 6, 9-13, 21-26 and so the nanoparticle size could be calculated with the Scherrer formula,25, 26 The mean diameter found using peak broadening of the nanoparticles was 8nm.27 This was in close agreement with the average particle size obtained from the TEM data. The nanoparticles were also examined using HRTEM (Fig. 2c). From these images it was possible to see lattice fringes as a result of interference between electrons passing through the crystal lattices. These fringes were measured to be 0.454nm, close to the literature value (0.480nm) of the [113] plane in magnetite (Fe3O4).14, 28


(b)

(a)

10

40

30

50 60 2 theta (deg)

70

80

LI

100

(c) O.454nn

4 Fig. 2: (a) TEM image of nanoparticles as formed. (b) XRD pattern for Fe3O4 nanoparticles27 The black line in both shows the measured data, the red trace is the calculated pattern using the crystal structural data for Fe3O4 and idealised experimental parameters. The corresponding planes have bee added from the literature.11, 25 (c) HRTEM image of Fe3O4 nanoparticles. Lattice spacing shown for the visible lattice. Spacing corresponds to [113] lattice14, 28

3.2 Redispersion of uncoated nanoparticles in saline media

The stability of each sample was qualitatively analysed by observing the amount of sedimentation over a 72 hour period. Fe3O4 nanoparticles showed rapid sedimentation in all but the most acidic media, with complete sedimentation in all samples, other than those with pH < 4, after 48 hours. Only samples with pH = 0 maintained stable solutions. The sedimentation was caused by two factors; change in the surface potential with pH and shortening of the electric double layer (EDL) with increased ionic strength. The sedimentation at high pH was likely due to loss of surface charges on the nanoparticles as shown in Equation 3. The change in pH changed the surface potential on the nanoparticles, through deprotonation of the nanoparticles with increasing pH. This was analogous to the hydrolysis of Fe2+/Fe3+ ions in aqueous solutions,29 where the complexing water molecules are deprotonated converting from H2O molecules at low pH; to OH- at increased pH and one proton is disassociated; and finally converting into O- at very high pH as all protons are removed from the surface.29 If the assumption was made that this process will occur on the oxygen groups on the surface of the nanoparticle then the rapid sedimentation of the nanoparticles in pH > 10 may have been due to a change in the bonding of the Fe-O groups on the surface from hydroxy groups to double bonded oxides as shown in equation 3. This change will reduce the surface charge and thus change the length of the EDL by altering the number of counter ions. Hence the volume of solution required to neutralise the nanoparticles’ surface charge becomes smaller allowing the nanoparticles to agglomerate and thus precipitate.


(3)

Table 1: Sedimentation of Fe3O4 in saline solutions

Sample #

pH

Ionic Strength

1

-0.2

1.45

2a

3.1

0.015

2b

3.1

2c

3.1

2d

Sedimentation Over Time 12

24

36

48

72

Solution

Solution

Solution

Solution

Colloid

Colloid

Colloid

Partial

Complete

0.0725

Colloid

Colloid

Colloid

Partial

Complete

0.145

Colloid

Partial

Complete

3.1

0.29

Colloid

Partial

Complete

2e

3.0

1.45

Colloid

Partial

Complete

3a

7.8

0.015

Colloid

Partial

Complete

3b

7.1

0.0725

Colloid

Partial

Complete

Solution

3c

7.0

0.145

Colloid

Partial

Complete

3d

7.1

0.29

Colloid

Partial

Complete

3e

6.7

1.45

Colloid

Partial

Complete

4a

5.9

0.015

Colloid

Partial

Complete

4b

6.3

0.0725

Colloid

Partial

Complete

4c

5.9

0.145

Colloid

Partial

Complete

4d

6.0

0.29

Colloid

Partial

Complete

4e

6.5

1.45

Colloid

Partial

Complete

5a

10.9

0.015

Partial

Complete

5b

10.8

0.0725

Partial

Complete

5c

10.7

0.145

Complete

5d

10.7

0.29

Complete

5e

10.4

1.45

Complete

6

12.7

1.45

Complete


When Fe3O4 nanoparticles were dispersed into acidic media, as demonstrated in Fig. 3 there was no observable change in morphology, however there was a noticeable increase in dispersion. Dispersed nanoparticles precipitated back out of solution after the addition of 1M NaOH.

*

20 nm

Fig. 3: TEM image of Fe3O4 nanoparticles dissolved in acid showing increased dispersion to the as prepared nanoparticles.

As a result of the increased dispersion of nanoparticles in acidic solutions Fe3O4 nanoparticles were subjected to an acid wash to peptise the nanoparticles before silanisation occurred. 3.3 Silanisation Sol-gel silanisation occurred under three different catalysts; NH4OH, no catalyst and HCl, and in two different solvents; an ethanol/water mix and a propan-2-ol/water mix. When an ethanol/water mix was used the resulting nanoparticles agglomerated readily. The nanoparticles still display a high degree of crystallinity after the coating process as can be seen in Fig. 4b, indicating that they were Fe3O4 nanoparticles, rather than Si based nanoparticles, which would be amorphous in character. The EDX spectrum shown in Fig. 4c shows a large iron peak with a minor silicon peak meaning that some silane has bonded to the iron nanoparticles. The agglomeration of the nanoparticles demonstrates that the nanoparticles were not homogeneously coated or monodisperse, however the change in morphology of the agglomerates between the coated and uncoated nanoparticles indicated a change in the surface properties of the coated nanoparticles. This can be seen in Fig. 4a and Fig. 4b which showed a clear change in morphology from dense agglomerates in the uncoated nanoparticles to chain-like agglomerates in the coated nanoparticles.


(b)

(a)

2Onm

(c)

1.40

2.40

3.40

4.40

5.40

6.40

1.40

0.40

0.40

Fig. 4: Base catalysed sol-gel coating of Fe3O4 nanoparticles. (a) Nanoparticles before coating, (b) Nanoparticles after sol-gel coating and (c) EDX spectra of nanoparticles after coating

According to Osterholtz and Pohl17 this agglomeration was to be expected when using a base catalyst as the condensation rate is increased under basic conditions, leading to condensation before all species are hydrolysed. However Schubert and Hüsing18 suggested that base catalysis results in rapid monomer consumption by pre-existing clusters, creating larger clusters but limiting the amount of gelation. This concept was also the basis of the method employed by Stöber et al.20 and formed the basis of the majority of iron-oxide coating techniques employed in the literature currently.5, 6, 11, 14-16, 30, 31 This may be explained by the epoxide ring on GPTMS reacting with a Lewis base19, 32 in a ring opening reaction as shown in equation 4:

(4) The most prevalent Lewis bases in this sol-gel system are H2O and ethanol. Water especially would react with the epoxide ring, forming a diol. Further reaction of the hydroxyl groups in this diol with hydrolysed silane groups would lead to the formation of a polyether.19 Even with a slow reaction rate, the volume of ethanol/water involved in the reaction would lead to ring opening reactions occurring. When an acid catalyst was used the resulting morphology, of the silanised nanoparticles was very similar to the nanoparticles coated in a base catalyst. This result was in agreement with the Stöber et al.20 and Schubert and Hüsing18, and was a result of both epoxide ring openings and the preference of acid catalysed silane systems to react with terminal silanes.18, 20 When no catalyst was added to the solution, no observable reaction occurred. The nanoparticles before and after silanisation maintained the same morphology and the EDX spectra indicated no/minimal silane in the agglomeration.


The difficulty when using GPTMS in these silanisations was that it has a more non-polar character than the reagents used previously in the literature. (TEOS 5, 14, 16, APS 11, 31) For this reason the base-catalysed attachment was repeated using propan-2-ol/water as the solvent. This was primarily for two reasons. Firstly, the decreased polarity of propan-2-ol compared with ethanol was thought to help redispersion of coated nanoparticles and increase the chain extension of the coating silane. Secondly, the increased steric bulk surrounding the hydroxyl group in propan-2-ol compared with ethanol would limit its ability to effectively act as a Lewis base in ring opening catalysation. It was thought that this would in turn lead to decreased polymerisation. However as can be seen in Fig. 5 attachment using propan-2-ol/water as the sol-gel solvent resulted in agglomerates of coated nanoparticles of similar size and morphology to those formed when ethanol/water was used as the solvent.

(b)

(a)

I

y,i'

;i.i$

a

-

c.

.lyc ..!t

Fig. 5: Agglomerates formed in (a) ethanol/water and (b) propan-2-ol/water solvents.

The major difficulty involved was in redispersing the nanoparticles effectively, while limiting the amount of water (Lewis-base) in the system. Deng, et al.5 claimed that the nanoparticles do not disperse evenly in solvents with an alcohol: water ratio greater then 4:1. When ratios of 5:1 and 10:1 were investigated, rapid agglomeration was observed in the former and eventual phase separation in the latter when the nanoparticles were introduced into the system. 3.4 Infra-red analysis IR spectroscopy was also used to determine the composition of the nanoparticles. The Fe3O4 spectra shown Fig. 6 contains a dominant OH stretch at ~3400cm-1, an OH wag at ~1600cm-1. The OH peaks in the sample are likely to be from a combination of OH groups on the surface of the nanoparticle and any water trapped inside the crystal lattice of the nanoparticles. The lack of peaks around 800cm-1 in this spectrum indicated that the samples have undergone complete conversion of the iron salts to Fe3O4 and do not contain any iron oxyhydroxide impurities. Typically Fe-OH peaks from these species appear at around 800cm-1.33 IR was used to further confirm the presence of silane on the surface of the nanoparticles. Nanoparticles were magnetically separated from solution and an IR spectrum taken this spectrum was compared to the spectrum of the uncoated nanoparticles as seen in Fig. 6.


E

3,600

3,100

2,600

2,100

1600

1,100

600

Wavenumber (cm1)

Fig. 6: IR spectra of coated and uncoated iron oxide nanoparticles. Upper trace is uncoated magnetite (Fe3O4) and the lower trace is magnetically separated, coated magnetite.

The presence of a triplet of peaks at ~900cm-1 from Si-O bonds in the coated nanoparticle spectrum was indicative that the silanisation was successful. The increase in peak height of the CH, CH2 (~2900cm-1) doublet when compared with the OH stretch (3300cm-1) and the shift in OH stretch from 3400cm-1 to 3300cm-1 may also be the result of the silanisation.

4.

CONCLUSION

Fe3O4 nanoparticles were formed in solution with an average size of 8.4nm ± 2.3nm. Fe3O4 nanoparticles were insoluble in pH>1 however formation of stable sols of Fe3O4 was possible using acid peptisation. From TEM images and EDX data silanisation of the iron oxide nanoparticles had occurred during the sol-gel process. This was confirmed by IR spectra of separated nanoparticles. Due to agglomeration of silanised nanoparticles it was difficult to tell whether homogenous silanisation was obtained, however the change in morphology and size of the resulting agglomerates compared to the agglomerates formed by the uncoated nanoparticles, indicated that the surface properties of the nanoparticles had been altered.

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