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Colloidal dispersion of magnetite nanoparticles via in situ preparation with sodium polyoxyalkylene di-phosphonates
Colloids and Surfaces A: Physicochemical and Engineering Aspects 173 (2000) 61 – 71 www.elsevier.nl/locate/colsurfa
Colloidal dispersion of magnetite nanoparticles via in situ preparation with sodium polyoxyalkylene di-phosphonates Isabelle Dumazet-Bonnamour, Pierre Le Perchec * Laboratoire des Mate´riaux Organiques a` Proprie´te´s Spe´cifiques, LMOPS, BP 24, 69390 Vernaison, France Received 3 August 1999; accepted 3 November 1999
Abstract Hydrosoluble polymers attached to inorganic surfaces allow production of dispersed state particles. Functionalized end-chain polymers were used to prepare stable aqueous magnetite colloidal dispersions. The magnetite nanoparticles were synthesized from coprecipitation of aqueous Fe3 + and Fe2 + ions in the presence of sodium polyoxyalkylene di-phosphonates. Transmission electron microscopy (TEM), dynamic light scattering (DLS), pyroanalysis techniques and isotherm determinations are reported to characterize the nanosized particles. Because of the structure of the interface with the polymer tightly anchored to the surface and the biocompatibility of the polyoxyalkylene chain, new developments might be found in the forthcoming time. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Dispersion; Particles; Nanoscale; Magnetite; Polymer
1. Introduction Well dispersed magnetic particles have been found of great interest for magnetic tape, disks and toner in laser printing [1] and as magnetic guidance and probe materials in biology [2–7]. Since new synthetic approaches of nanoscaled magnetic particles are based on size distribution control and improved magnetic properties, functionalized polymers have been increasingly recommended as dispersing agents for aggregates [8]. Coating and encapsulation of magnetic particles * Corresponding author. Tel.: +33-0478-0222-68; fax: + 33-0478-0271-87. E-mail address: [email protected] (P. Le Perchec).
with natural polymers such as proteins and polyglucosides [9], synthetic ones such as polyelectrolytes [10–12] and non-ionic polymers bearing lateral functionality (PVA) [13,14] have been used in order to obtain colloidal suspensions. Polymers randomly adsorb on surface involve affinities to the surface. Homo- and co-polymers produce adsorbed layers which consist of segments laying flat on the surface, and non-adsorbed loops and tails which ensure electrosteric stabilization of the particles. However colloidal stabilization depends on polymer layer thickness and it is known that particle destabilisation may occur by the lack of layer thickness or by polymer bridging mechanism. Because of this limitation, the molecular weight has to remain moderate while a brush
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monolayer adsorption mode will better satisfy steric stabilization of dispersed particles. Recent work has shown that strongly adsorbed monolayer on particles can be obtained from v-end-chain functionalized nonionic polymer [15]. This block co-polymer is segmented into one strongly adsorbed group and a hydrosoluble nonadsorbed chain. We report on the use of such synthetic hydrosoluble polymer end-chain groups as a dispersing agent for magnetite particles. The building block of particle – polymer associations has been obtained either from polymer treatment of commercially available preformed magnetite particles or by in situ synthesis of magnetite particles in the presence of the polymer. The objectives are to get well defined stable nanosized magnetic particles with narrow size distributions and with a biocompatible character.
primary aminopolyoxyalkylene issued by anionic polymerization of ethylene oxide initiated by potassium aminoalkylalcoholate [16] or from commercially available monofunctionalized Jeffamine®. The di-phosphonate derivatives were prepared by means of the Mannich–Modritzer reaction [17]. The following nonionic water soluble polymers containing di-phosphonate sodium groups at chain-end have been used for the production of magnetite nanoparticle dispersion:
2. Materials and methods
n= R1 R2
2.1. Preparation of poloxyyalkylene di-phosphonates. Polyoxyalkylene di-phosphonates used in this paper have been prepared starting either from
20 H; CH3 CH3
50 H H
70 H; CH3 CH3
50PEO-P2, Mn = 2200 g mol − 1, Dp = 1.2, 50PEOP2 derived from 50POE-NH2 as described in [17],
Fig. 1. Adsorption isotherm of the 20PAO-P2, 50PEO-P2 and 70PAO-P2 polymer on Fe3O4 particle surface.
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2.2. Studies on preformed magnetite particles Preformed magnetite particles were purchased from Aldrich, powder with particle size less than 5 mm and specific area of 18 m2 g − 1.
2.2.1. Sample preparation Preformed magnetite particles were suspended in aqueous polymer solutions at different concentrations (0BCi B 2 mmol l − 1) and the pH was set up at 11 by addition of a small volume of concentrated sodium hydroxide (5N). The sample was sonicated for 2 min with a Sonimasse high ultrasound power device and stirred for 18 h at ambient temperature in order to reach an equilibrium. The magnetite content was 2.5 wt.% in a suspension volume of 20 ml for all samples. 2.2.2. Polymer adsorption isotherm determination Equilibrated magnetite suspensions were ultracentrifugated with a Beckman JA 21 centrifuge at 20°C (20 000 rpm), the supernatant was then isolated and the polymer content was determined. The adsorbed amount G (mmol m − 2) (Eq. (1)) was calculated by ICP-AES analyses from initial (Ci) to final (Cf) polymer concentrations. (C − Cf)V G= i (1) MAsp where V is the volume of the aqueous solution, M is the magnetite weight and Asp is the particles specific area (18 m2 g − 1 Aldrich particles). 2.3. ‘In situ’ preparation of polymer-magnetite particles
Fig. 2. Transmission electron micrographs (magnification G= 17 000) of preformed magnetite particles treated with 70PAOP2 polymer at different initial concentrations of polymer (Ci); (a) Ci = 0 mmol l − 1; (b) Ci = 0.63 mmol l − 1.
20PAO-P2, Mn =1400 g mol − 1, (18 ethylene oxide units and 2 propylene oxide units) and 70PAO-P2, Mn =3600 g mol − 1, 70PAO-P2 (62 ethylene oxide units and 8 propylene oxide units) derive from Jeffamine® samples (PAO-NH2) gifted by Hunstmann Company.
The most convenient method for the synthesis of magnetite particles is obtained by coprecipitation from a solution of Fe2 + and Fe3 + salts14. To 5 ml aqueous solutions of polymer concentrations (0–10 mmol l − 1), 2.4 ml of aqueous solution of 720 mg of FeCl36H2O and 290 mg of FeCl24H2O were added dropwise under N2 gas flow and vigorous stirring. The mixture was then made alkaline (pH 10) by the addition of 25% ammonia water solution. After 20 min of reaction, the resulting black material was collected by magnetic force and washed with water to remove free polymer.
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2.4. Polymers modified particles characterization 2.4.1. Polymer adsorption The nPAO-P2 adsorption was determined in both cases (preformed and ‘in situ’) by ICP-AES plasma ionization analysis (Fison ARL 3520) of the phosphorous element or by pyroanalysis [18] of the carbon element from the magnetite powder which was washed with water, ethanol and chloroform in order to remove salts and free polymer. This technique was monitored by a programmed temperature furnace under an inert atmosphere. A helium stream was used to sweep the receptor sample in the pyrex tube, while downstream helium containing oxygen was used to burn the volatile effluents in a combustion zone containing active oxidation catalysts. The quantitative CO2 determination from the combustion zone was detected by a highly sensitive IR detector.
the size distribution of the coated magnetite particles issued from ‘in situ’ preparation. DLS techniques have been investigated at different concentration levels of the polymer. The light scattering spectrometer is equipped with a Spectra-Physics argon ion laser (l= 514 nm) as vertical polarized light source. The initial sample solutions were diluted using distilled water, filtered through a 0.22 mm pore size millipore filter and placed in a thermostated scattering cell at 22°C. The intensity–intensity time correlation function measurements were carried out in the self-beating mode by using a Brookhaven BI 8000-channel correlator at angular u= 90°. Correlation functions were accepted when the measured and the calculated baselines were in good agreement (with a difference of less than 1%). The NNLS method (non negative linear square) was used for the data analysis of the measured intensity time correlation function.
2.4.2. Light scattering analysis Dynamic light scattering (DLS) was used to determine the average hydrodynamic radius and
2.4.3. Transmission electron microscopy The magnetite particles were observed under a transmission electron microscope (TEM) Philipps
Fig. 3. X-ray diffraction pattern of magnetite particles made from in situ preparation.
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Fig. 4. Programmed-temperature profiles and CO2 detection of magnetite-adsorbed polymer samples.
CM 120. The suspension was deposited on a copper grid recovered with a formvar membrane.
2.4.4. Specific surface area Specific surface area was measured by nitrogen BET method with a Sorptomatic 1900 apparatus. 2.4.5. X-ray diffraction measurements X-ray diffraction measurements were carried out by using a Siemens D5000 diffractometer.
Table 1 Determination of adsorbed polymer amount on the Fe3O4 surface by pyroanalysis Graph n° 0 1 2 3
Ci (mmol l−1) 0 0.7 1.3 1.6
Integrale
Weight lost (%)
4.5 31.3 40.1 62.1
1.5 6.6 9.5 11.2
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Fig. 5. Carbon analysis from IR-pyroanalysis detection on magnetite powder versus the initial polymer concentration used for the dispersion of magnetite particles.
3. Results and discussion
3.1. Adsorption le6el on preformed particles We have developed our study on preformed particles. A significant adsorption level was found on various concentration scales of the polymer. The adsorption isotherms were built up from three different polymers nPAO-P2 lengths (n = 20, 50, 70) by ICP-AES analyses (Fig. 1). As shown on Fig. 1, two different adsorption steps were obtained. The adsorption levels were similar in cases of 50POE-P2 and 20PAO-P2 showing a Langmuir type adsorption regime (0.3 mmol m − 2) followed by a sterically controlled adsorption step (0.5 mmol m − 2). In the case of 70PAO-P2 both steps were found to be sterically
controlled while the level of adsorption is increased (0.5 mmol m − 2). The minimum amount of adsorbed polymer that is required corresponds to the crossover concentration between the ‘mushroom’ and ‘brush’ regimes [19]. In the case of n=20, 50, the influence of the polymer on the adsorption profile appears not to be sensitive to the molecular weight increase with neither the hydrophilic–hydrophobic. However, the 70PAOP2 expressed a slight difference, the adsorption of which appear to be controlled by steric effect of the hydrosoluble segment.
3.2. Size determination by TEM on preformed particles The nanoscaled particle dispersion was attempted under sonicated solutions of preformed
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particles in the presence of polymer. The TEM micrographs (Fig. 2) were set up which show very weak change in particles size. Preformed particles treated with 70PAO-P2 polymer led to produce size aggregates which had been decreased from 1 to 0.2 mm diameter. Obviously this treatment afforded only partial particle disaggregation despite the efficient level of adsorption.
3.3. ‘In situ’ prepared magnetite particles X-ray diffraction pattern was carried out on in situ magnetite powder. The peaks listed in standard reference tables (as shown on Fig. 3) clearly correspond to Fe3O4 powder as describe previously.
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Because of the solid phase evolution during in situ production of magnetite particles, the ICPAES analyses led only to evidence of the adsorption level of the polymer. In order to obtain quantitative results of the adsorbed level, pyroanalysis was found to be a more convenient and accurate method.
3.3.1. Pyroanalysis The oxidative pyroanalysis technique was applied to magnetite samples. The response factors were established between the integrated signals (Fig. 4), and the weight of C detected element. The accuracy of the detection was determinated as 1%, from consecutive runs. Therefore, the signal treatment gives the weight of C element present in
Fig. 6. Transmission electron micrographs (magnification G= 17 000) of dispersed magnetic nanoparticles; micrographs of in situ synthetic nanoparticles with 70PAO-P2 polymer at different initial concentration polymer (Ci); (a) Ci =0 mmol l − 1; (b) Ci =0.65 mmol l − 1; (c) Ci = 1.28 mmol/l; (d) Ci = 2.42 mmol l − 1. Bis: transmission electron micrographs of in situ synthetic nanoparticles with 70PAO-P2 polymer at initial concentration polymer Ci =2.42 mmol l − 1 (Fig. 6 (d)) at higher resolution (G =200 000).
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Fig. 7. DLS analysis of magnetite particles.
any volatile magnetite samples (Table 1). The pyroanalysis of washed – dried particles has shown large and efficient adsorption of the polymer at the surface sensitive to the concentration of the polymer in the solution. Fig. 4 shows CO2 detection diagrams obtained by pyroanalysis on magnetite powder, in case of addition of the polymer on in situ prepared particles. The diagram n°0 represents the reference: the pyroanalysis of the synthesized magnetite without polymer. The following diagrams n° 1,2,3 demonstrate the increasing amount of adsorbed polymer on the magnetite particles. These diagrams allowed the determination of the adsorbed polymer amount and the polymer weight loss (Table 1), since the polymer decomposition temperature was found around 260°C. The in situ synthesis was allowed to produce highly fine nanoparticles. The colloidal suspensions don’t flocculate or precipitate after standing several days. From dry washed particles, specific surface area (Asp) was measured by BET method and was found to be Asp =108 m2 g − 1 with a narrow micropores radius distribution from 10 to 90 A, . In contrast, in the presence of the polymer 70PAO-P2, the Asp was widely lower than previously with Asp =4 m2 g − 1. This result firmly sustains that the polymer was adsorbed on the magnetite surface nanoparticles.
The thermal analyses showed that the polymer was adsorbed on the magnetite surface since no significant elimination nor fragmentation was found before the polymer decomposition temperature. Fig. 5 compares results obtained from pyroanalysis detection on preformed to in situ particles treatment by 70PAO-P2. As reported on this Fig. 5, a large increase in adsorption was found when the in situ method was used.
3.3.2. Size Determination by TEM and DLS analyses TEM analyses allowed us to follow the effi-
Fig. 8. Size distribution of magnetite particles completely dispersed.
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Fig. 9. Magnetization curve of dispersed magnetite suspension in water.
ciency of the polymer contribution on the dispersion state of the magnetite particles in water. At the same magnification level (G = 17 000) as reported for preformed particles (Fig. 2) the in situ germs synthesis produced fine nanoparticles (Fig. 6). The picture Fig. 6 (a) represents the magnetite particles synthesized without polymer; Fig. 6 (b) shows large aggregate production which is relatively sensitive to the 70PAO-P2 concentration; whereas pictures Fig. 6 (c) and (d) show dispersed particles where the concentration was raised to 2.5 mmol l − 1. According to the polymer concentration change in the aqueous phase (from 0 to 10 mmol l − 1), an efficient and regular dispersion of magnetite particles can be visualized. The more the polymer concentration in water solution increases the more the aggregate size decreases allowing the production of stable dispersed magnetite particle suspensions. The analysis was completed from high resolution TEM microscopy pattern which led to precise and accurate determination of the morphological parameter of the particles. At the magnification
G=200 000 (Fig. 6 (Bis)), it is shown that particles have narrow size distribution between 5–10 nm. The assembly looks like chaplet wires of light density material; although single particles appear well defined in size. DLS analyses have confirmed the production of self-assembly dispersed particles since the apparent particle size lay down from 22 to 40 nm. As shown in Fig. 7 at low polymer concentrations Ci B 3 mmol l − 1, two domains of size particles were found while at upper polymer concentrations Ci \ 8 mmol l − 1, a simple distribution was observed. In case of Ci = 10 mmol l − 1, a narrow single distribution was observed with an average particle size of 30 nm (Fig. 8). Such behavior is in agreement with the production of loose assembly as shown by high resolution microscopy (Fig. 6 Bis) which will be sensitive to the ligand concentration. This result is strong evidence on the controlled size effect of the polymeric 70PAO-P2 agent and
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might allow the production of particle size content of any average size.
3.4. Magnetic properties of Fe3O4 in suspension form The magnetization curve for 70PAO-P2-treated magnetite particles is shown in Fig. 9. Diamagnetic high field susceptibility is from the diamagnetic background signal of water and glass sample holder (primary signal on Fig. 9). The measure of the water in the glass sample holder allowed us to obtain the corrected magnetization curve (Fig. 9) at ambient temperature. The remaining magnetization Mr (magnetization remaining after removal of the applied field) was less than 5% and consequently negligible. Superparamagnetic behavior was observed [20]. The saturation magnetization, Ms, was obtained by extrapolation plots of magnetization M versus 1/H =0. This extrapolation gave Ms =75.7 (emu cgs g − 1). Eqs. (2) and (3) (concerning only Fe3O4) can be used to calculate the surface average diameter D1 and D2 corresponding respectively to the low field (H tends to 0) and to the high field (1/H tends to 0).
n n
D1 = 534.51 D2 = 17.2
T 1 d Ms ds H
T ds Ms ds −dH
1/3
=12 nm
(2)
H[0
1/3
=5 nm
(3)
with ds: signal value d at saturation (unit arbitrary); dH : value signal at field H = 10 000 (Oe); K =1.38066 10 − 16 erg K − 1 (Boltzmann constant); Ms = 89.9 uem cgs g − 1 for Fe3O4; T= 300 K.
3.5. Discussion Fine nanosized magnetite particles appear to be easily produced in the presence of sodium endchain polyoxyalkylene di-phosphonates under conventional conditions. The particles size was found homogeneous around 10 to 30 nm depending on the analytical method.
The DLS method for measuring particle size include the magnetite radius and the hydrated polymer at the magnetite surface. The magnetic radius obtained from magnetization curve was about 5 to 12 nm in scale as found by others [11,14] in accordance with TEM analyses, while a noticeable discrepancy appears when the DLS volume is considered. Since the hydrated polyoxyalkylene radii do not exceed 2 to 2.5 nm giving rise to a maximum 5 nm increase of the particle diameter, the resulting DLS size measurements should lead to particles of approximately 20 nm. This suggests that a large portion of the particle assembly consists of associated water, solvated ions and counterions with the monolayer adsorbed polymer. The differences bear resemblance to the results of Hwang et al. [11], who also found similar diameters from TEM and SM (9 and 12 nm, respectively), and larger ones from DLS (19 nm). The facility to reach the equilibrium state of adsorption is obviously dependent on the average molecular weight of the polymer trough steric and conformational effects of the water soluble chain, the lower molecular weight being able to more efficiently adsorb than the higher one as shown from the isotherms. A second argument relates to the tightness and the selectivity of the polymer irreversibly connected to the surface. This result was demonstrated since after several washings, the stability on the colloidal was found unchanged and no elimination of polymer was detected. Because of the nonionic character of the adsorbed polymer one can speculate about the electroneutrality of the particles obtained in these circumstances. In contrast to multifunctional agents such as polyelectrolyte or polyvinyl alcohol, end-chain functionalized hydrosoluble polyoxyalkylene is more likely to produce fine nanosized particles under monolayer brush type model association at the surface. This association made relevant a flexible thin and regular polymer distribution at the surface with a calculated thickness of approximately 2 nm for a 50POE-P2 chain length. Another improvement associated to unsymetrical polymer agent lay in the fact that the dispersing agent would highly facilitate further chemical modifications on the opposite end-chain
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site offering some interesting opportunities for grafting biologically active agents.
4. Conclusion In conclusion we have found sodium polyoxyalkylene di-phosphonate polymers efficient agents in producing nanoscale magnetic particles under conventional synthesis scheme. Interestingly there is a tight dependence of the nanoscale distribution on the concentration of the polymer which acts through a brush model adsorption. Moreover the unsymmetrical nature of the polymer will offer the ability to graft reactive probes or monomers to produce magnetic active materials of nanosize shapes.
Acknowledgements This work was accomplished at the LMOPSCNRS laboratory in Solaize (France). The authors thank J-P. Dalmon, H. Mozzanega for magnetic measurements; A. Rivoire, I. Bornard and M. Esnouf for TEM measurements; P. Marotte for BET analysis, F. Melis for DLS analyses and S. Brunel for assistance in the laboratory.
References [1] T.E. Karis, J. Appl. Polym. Sci. 59 (1996) 1405.
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[2] X. Li, Z. Sun, J. Appl. Polym. Sci. 58 (1995) 1991. [3] M. Shimomura, N. Sugiyama, T. Yamauchi, S. Miyauchi, Polym. J. 30 (1998) 350. [4] M. Shinkai, H. Honda, T. Kobayashi, Biocatalysis 5 (1991) 61. [5] M. Alves da Silva, H.D. Burrows, S.J. Formosinho, M.H. Gil, A.R. Laurenc¸o, F.J.A. Paula, A.P. Piedade, Mater. Lett. 11 (1991) 96. [6] S.S. Davis, L. Illum, J.G. McVie, E. Tomlinson (Eds.), Microspheres and Drug Therapy, Elsevier, Amsterdam, 1984, p. 353. [7] M. Suzuki, M. Shinkai, M. Kamihira, T. Kobayashi, Biotechnol. Appl. Biochem. 21 (1995) 335. [8] D.H. Napper, Polymeric Stabilization of Colloidal Dispersion, Academic Press, New York, 1983. [9] A. Wooding, M. Kilner, D.B. Lambrick, IEEE Trans. Magn. 24 (1988) 1650. [10] A. Wooding, M. Kilner, D.B. Lambrick, J. Colloid Interface Sci. 149 (1992) 98. [11] D. Mendenhall, Y. Geng, J. Hwang, J. Colloid Interface Sci. 184 (1996) 519. [12] P.A. Dresco, V.S. Zaitsev, R.J. Gambino, B. Chu, Langmuir 15 (1999) 1945. [13] H. Yokoi, T. Kantoh, Bull. Chem. Soc. Jpn. 66 (1993) 1536. [14] J. Lee, T. Isobe, M. Senna, Colloids Surf. A: Physicochem. Eng. Aspects 109 (1996) 121. [15] M. Mosquet, Y. Chevalier, S. Brunel, J.P. Guicquero, P. Le Perchec, J. Appl. Polym. Sci. 65 (1997) 2545. [16] M. Mosquet, Y. Chevalier, P. Le Perchec, J-P. Guicquero, Macromol. Chem. Phys. 198 (1997) 2457. [17] M. Mosquet, PhD Thesis, University of Lyon I, 1994. [18] B. Fixari, P. Le Perchec, M. Bigois, Fuel Sci. Technol. Int. 9 (1991) 321. [19] M. Mosquet, Y. Chevalier, J.P. Guicquero, P. Le Perchec, New J. Chem. 21 (1997) 143. [20] J.-A. Dalmon, in: B. Imelik, J.C. Vedrine (Eds.), Catalyst Characterisation: Physical Techniques for Solid Materials, Plenum Press, New York, 1994 Chapter 21.
Colloidal dispersion of magnetite nanoparticles via in situ preparation with sodium polyoxyalkylene di-phosphonates Isabelle Dumazet-Bonnamour, Pierre Le Perchec * Laboratoire des Mate´riaux Organiques a` Proprie´te´s Spe´cifiques, LMOPS, BP 24, 69390 Vernaison, France Received 3 August 1999; accepted 3 November 1999
Abstract Hydrosoluble polymers attached to inorganic surfaces allow production of dispersed state particles. Functionalized end-chain polymers were used to prepare stable aqueous magnetite colloidal dispersions. The magnetite nanoparticles were synthesized from coprecipitation of aqueous Fe3 + and Fe2 + ions in the presence of sodium polyoxyalkylene di-phosphonates. Transmission electron microscopy (TEM), dynamic light scattering (DLS), pyroanalysis techniques and isotherm determinations are reported to characterize the nanosized particles. Because of the structure of the interface with the polymer tightly anchored to the surface and the biocompatibility of the polyoxyalkylene chain, new developments might be found in the forthcoming time. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Dispersion; Particles; Nanoscale; Magnetite; Polymer
1. Introduction Well dispersed magnetic particles have been found of great interest for magnetic tape, disks and toner in laser printing [1] and as magnetic guidance and probe materials in biology [2–7]. Since new synthetic approaches of nanoscaled magnetic particles are based on size distribution control and improved magnetic properties, functionalized polymers have been increasingly recommended as dispersing agents for aggregates [8]. Coating and encapsulation of magnetic particles * Corresponding author. Tel.: +33-0478-0222-68; fax: + 33-0478-0271-87. E-mail address: [email protected] (P. Le Perchec).
with natural polymers such as proteins and polyglucosides [9], synthetic ones such as polyelectrolytes [10–12] and non-ionic polymers bearing lateral functionality (PVA) [13,14] have been used in order to obtain colloidal suspensions. Polymers randomly adsorb on surface involve affinities to the surface. Homo- and co-polymers produce adsorbed layers which consist of segments laying flat on the surface, and non-adsorbed loops and tails which ensure electrosteric stabilization of the particles. However colloidal stabilization depends on polymer layer thickness and it is known that particle destabilisation may occur by the lack of layer thickness or by polymer bridging mechanism. Because of this limitation, the molecular weight has to remain moderate while a brush
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monolayer adsorption mode will better satisfy steric stabilization of dispersed particles. Recent work has shown that strongly adsorbed monolayer on particles can be obtained from v-end-chain functionalized nonionic polymer [15]. This block co-polymer is segmented into one strongly adsorbed group and a hydrosoluble nonadsorbed chain. We report on the use of such synthetic hydrosoluble polymer end-chain groups as a dispersing agent for magnetite particles. The building block of particle – polymer associations has been obtained either from polymer treatment of commercially available preformed magnetite particles or by in situ synthesis of magnetite particles in the presence of the polymer. The objectives are to get well defined stable nanosized magnetic particles with narrow size distributions and with a biocompatible character.
primary aminopolyoxyalkylene issued by anionic polymerization of ethylene oxide initiated by potassium aminoalkylalcoholate [16] or from commercially available monofunctionalized Jeffamine®. The di-phosphonate derivatives were prepared by means of the Mannich–Modritzer reaction [17]. The following nonionic water soluble polymers containing di-phosphonate sodium groups at chain-end have been used for the production of magnetite nanoparticle dispersion:
2. Materials and methods
n= R1 R2
2.1. Preparation of poloxyyalkylene di-phosphonates. Polyoxyalkylene di-phosphonates used in this paper have been prepared starting either from
20 H; CH3 CH3
50 H H
70 H; CH3 CH3
50PEO-P2, Mn = 2200 g mol − 1, Dp = 1.2, 50PEOP2 derived from 50POE-NH2 as described in [17],
Fig. 1. Adsorption isotherm of the 20PAO-P2, 50PEO-P2 and 70PAO-P2 polymer on Fe3O4 particle surface.
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2.2. Studies on preformed magnetite particles Preformed magnetite particles were purchased from Aldrich, powder with particle size less than 5 mm and specific area of 18 m2 g − 1.
2.2.1. Sample preparation Preformed magnetite particles were suspended in aqueous polymer solutions at different concentrations (0BCi B 2 mmol l − 1) and the pH was set up at 11 by addition of a small volume of concentrated sodium hydroxide (5N). The sample was sonicated for 2 min with a Sonimasse high ultrasound power device and stirred for 18 h at ambient temperature in order to reach an equilibrium. The magnetite content was 2.5 wt.% in a suspension volume of 20 ml for all samples. 2.2.2. Polymer adsorption isotherm determination Equilibrated magnetite suspensions were ultracentrifugated with a Beckman JA 21 centrifuge at 20°C (20 000 rpm), the supernatant was then isolated and the polymer content was determined. The adsorbed amount G (mmol m − 2) (Eq. (1)) was calculated by ICP-AES analyses from initial (Ci) to final (Cf) polymer concentrations. (C − Cf)V G= i (1) MAsp where V is the volume of the aqueous solution, M is the magnetite weight and Asp is the particles specific area (18 m2 g − 1 Aldrich particles). 2.3. ‘In situ’ preparation of polymer-magnetite particles
Fig. 2. Transmission electron micrographs (magnification G= 17 000) of preformed magnetite particles treated with 70PAOP2 polymer at different initial concentrations of polymer (Ci); (a) Ci = 0 mmol l − 1; (b) Ci = 0.63 mmol l − 1.
20PAO-P2, Mn =1400 g mol − 1, (18 ethylene oxide units and 2 propylene oxide units) and 70PAO-P2, Mn =3600 g mol − 1, 70PAO-P2 (62 ethylene oxide units and 8 propylene oxide units) derive from Jeffamine® samples (PAO-NH2) gifted by Hunstmann Company.
The most convenient method for the synthesis of magnetite particles is obtained by coprecipitation from a solution of Fe2 + and Fe3 + salts14. To 5 ml aqueous solutions of polymer concentrations (0–10 mmol l − 1), 2.4 ml of aqueous solution of 720 mg of FeCl36H2O and 290 mg of FeCl24H2O were added dropwise under N2 gas flow and vigorous stirring. The mixture was then made alkaline (pH 10) by the addition of 25% ammonia water solution. After 20 min of reaction, the resulting black material was collected by magnetic force and washed with water to remove free polymer.
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2.4. Polymers modified particles characterization 2.4.1. Polymer adsorption The nPAO-P2 adsorption was determined in both cases (preformed and ‘in situ’) by ICP-AES plasma ionization analysis (Fison ARL 3520) of the phosphorous element or by pyroanalysis [18] of the carbon element from the magnetite powder which was washed with water, ethanol and chloroform in order to remove salts and free polymer. This technique was monitored by a programmed temperature furnace under an inert atmosphere. A helium stream was used to sweep the receptor sample in the pyrex tube, while downstream helium containing oxygen was used to burn the volatile effluents in a combustion zone containing active oxidation catalysts. The quantitative CO2 determination from the combustion zone was detected by a highly sensitive IR detector.
the size distribution of the coated magnetite particles issued from ‘in situ’ preparation. DLS techniques have been investigated at different concentration levels of the polymer. The light scattering spectrometer is equipped with a Spectra-Physics argon ion laser (l= 514 nm) as vertical polarized light source. The initial sample solutions were diluted using distilled water, filtered through a 0.22 mm pore size millipore filter and placed in a thermostated scattering cell at 22°C. The intensity–intensity time correlation function measurements were carried out in the self-beating mode by using a Brookhaven BI 8000-channel correlator at angular u= 90°. Correlation functions were accepted when the measured and the calculated baselines were in good agreement (with a difference of less than 1%). The NNLS method (non negative linear square) was used for the data analysis of the measured intensity time correlation function.
2.4.2. Light scattering analysis Dynamic light scattering (DLS) was used to determine the average hydrodynamic radius and
2.4.3. Transmission electron microscopy The magnetite particles were observed under a transmission electron microscope (TEM) Philipps
Fig. 3. X-ray diffraction pattern of magnetite particles made from in situ preparation.
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Fig. 4. Programmed-temperature profiles and CO2 detection of magnetite-adsorbed polymer samples.
CM 120. The suspension was deposited on a copper grid recovered with a formvar membrane.
2.4.4. Specific surface area Specific surface area was measured by nitrogen BET method with a Sorptomatic 1900 apparatus. 2.4.5. X-ray diffraction measurements X-ray diffraction measurements were carried out by using a Siemens D5000 diffractometer.
Table 1 Determination of adsorbed polymer amount on the Fe3O4 surface by pyroanalysis Graph n° 0 1 2 3
Ci (mmol l−1) 0 0.7 1.3 1.6
Integrale
Weight lost (%)
4.5 31.3 40.1 62.1
1.5 6.6 9.5 11.2
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Fig. 5. Carbon analysis from IR-pyroanalysis detection on magnetite powder versus the initial polymer concentration used for the dispersion of magnetite particles.
3. Results and discussion
3.1. Adsorption le6el on preformed particles We have developed our study on preformed particles. A significant adsorption level was found on various concentration scales of the polymer. The adsorption isotherms were built up from three different polymers nPAO-P2 lengths (n = 20, 50, 70) by ICP-AES analyses (Fig. 1). As shown on Fig. 1, two different adsorption steps were obtained. The adsorption levels were similar in cases of 50POE-P2 and 20PAO-P2 showing a Langmuir type adsorption regime (0.3 mmol m − 2) followed by a sterically controlled adsorption step (0.5 mmol m − 2). In the case of 70PAO-P2 both steps were found to be sterically
controlled while the level of adsorption is increased (0.5 mmol m − 2). The minimum amount of adsorbed polymer that is required corresponds to the crossover concentration between the ‘mushroom’ and ‘brush’ regimes [19]. In the case of n=20, 50, the influence of the polymer on the adsorption profile appears not to be sensitive to the molecular weight increase with neither the hydrophilic–hydrophobic. However, the 70PAOP2 expressed a slight difference, the adsorption of which appear to be controlled by steric effect of the hydrosoluble segment.
3.2. Size determination by TEM on preformed particles The nanoscaled particle dispersion was attempted under sonicated solutions of preformed
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particles in the presence of polymer. The TEM micrographs (Fig. 2) were set up which show very weak change in particles size. Preformed particles treated with 70PAO-P2 polymer led to produce size aggregates which had been decreased from 1 to 0.2 mm diameter. Obviously this treatment afforded only partial particle disaggregation despite the efficient level of adsorption.
3.3. ‘In situ’ prepared magnetite particles X-ray diffraction pattern was carried out on in situ magnetite powder. The peaks listed in standard reference tables (as shown on Fig. 3) clearly correspond to Fe3O4 powder as describe previously.
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Because of the solid phase evolution during in situ production of magnetite particles, the ICPAES analyses led only to evidence of the adsorption level of the polymer. In order to obtain quantitative results of the adsorbed level, pyroanalysis was found to be a more convenient and accurate method.
3.3.1. Pyroanalysis The oxidative pyroanalysis technique was applied to magnetite samples. The response factors were established between the integrated signals (Fig. 4), and the weight of C detected element. The accuracy of the detection was determinated as 1%, from consecutive runs. Therefore, the signal treatment gives the weight of C element present in
Fig. 6. Transmission electron micrographs (magnification G= 17 000) of dispersed magnetic nanoparticles; micrographs of in situ synthetic nanoparticles with 70PAO-P2 polymer at different initial concentration polymer (Ci); (a) Ci =0 mmol l − 1; (b) Ci =0.65 mmol l − 1; (c) Ci = 1.28 mmol/l; (d) Ci = 2.42 mmol l − 1. Bis: transmission electron micrographs of in situ synthetic nanoparticles with 70PAO-P2 polymer at initial concentration polymer Ci =2.42 mmol l − 1 (Fig. 6 (d)) at higher resolution (G =200 000).
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Fig. 7. DLS analysis of magnetite particles.
any volatile magnetite samples (Table 1). The pyroanalysis of washed – dried particles has shown large and efficient adsorption of the polymer at the surface sensitive to the concentration of the polymer in the solution. Fig. 4 shows CO2 detection diagrams obtained by pyroanalysis on magnetite powder, in case of addition of the polymer on in situ prepared particles. The diagram n°0 represents the reference: the pyroanalysis of the synthesized magnetite without polymer. The following diagrams n° 1,2,3 demonstrate the increasing amount of adsorbed polymer on the magnetite particles. These diagrams allowed the determination of the adsorbed polymer amount and the polymer weight loss (Table 1), since the polymer decomposition temperature was found around 260°C. The in situ synthesis was allowed to produce highly fine nanoparticles. The colloidal suspensions don’t flocculate or precipitate after standing several days. From dry washed particles, specific surface area (Asp) was measured by BET method and was found to be Asp =108 m2 g − 1 with a narrow micropores radius distribution from 10 to 90 A, . In contrast, in the presence of the polymer 70PAO-P2, the Asp was widely lower than previously with Asp =4 m2 g − 1. This result firmly sustains that the polymer was adsorbed on the magnetite surface nanoparticles.
The thermal analyses showed that the polymer was adsorbed on the magnetite surface since no significant elimination nor fragmentation was found before the polymer decomposition temperature. Fig. 5 compares results obtained from pyroanalysis detection on preformed to in situ particles treatment by 70PAO-P2. As reported on this Fig. 5, a large increase in adsorption was found when the in situ method was used.
3.3.2. Size Determination by TEM and DLS analyses TEM analyses allowed us to follow the effi-
Fig. 8. Size distribution of magnetite particles completely dispersed.
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Fig. 9. Magnetization curve of dispersed magnetite suspension in water.
ciency of the polymer contribution on the dispersion state of the magnetite particles in water. At the same magnification level (G = 17 000) as reported for preformed particles (Fig. 2) the in situ germs synthesis produced fine nanoparticles (Fig. 6). The picture Fig. 6 (a) represents the magnetite particles synthesized without polymer; Fig. 6 (b) shows large aggregate production which is relatively sensitive to the 70PAO-P2 concentration; whereas pictures Fig. 6 (c) and (d) show dispersed particles where the concentration was raised to 2.5 mmol l − 1. According to the polymer concentration change in the aqueous phase (from 0 to 10 mmol l − 1), an efficient and regular dispersion of magnetite particles can be visualized. The more the polymer concentration in water solution increases the more the aggregate size decreases allowing the production of stable dispersed magnetite particle suspensions. The analysis was completed from high resolution TEM microscopy pattern which led to precise and accurate determination of the morphological parameter of the particles. At the magnification
G=200 000 (Fig. 6 (Bis)), it is shown that particles have narrow size distribution between 5–10 nm. The assembly looks like chaplet wires of light density material; although single particles appear well defined in size. DLS analyses have confirmed the production of self-assembly dispersed particles since the apparent particle size lay down from 22 to 40 nm. As shown in Fig. 7 at low polymer concentrations Ci B 3 mmol l − 1, two domains of size particles were found while at upper polymer concentrations Ci \ 8 mmol l − 1, a simple distribution was observed. In case of Ci = 10 mmol l − 1, a narrow single distribution was observed with an average particle size of 30 nm (Fig. 8). Such behavior is in agreement with the production of loose assembly as shown by high resolution microscopy (Fig. 6 Bis) which will be sensitive to the ligand concentration. This result is strong evidence on the controlled size effect of the polymeric 70PAO-P2 agent and
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might allow the production of particle size content of any average size.
3.4. Magnetic properties of Fe3O4 in suspension form The magnetization curve for 70PAO-P2-treated magnetite particles is shown in Fig. 9. Diamagnetic high field susceptibility is from the diamagnetic background signal of water and glass sample holder (primary signal on Fig. 9). The measure of the water in the glass sample holder allowed us to obtain the corrected magnetization curve (Fig. 9) at ambient temperature. The remaining magnetization Mr (magnetization remaining after removal of the applied field) was less than 5% and consequently negligible. Superparamagnetic behavior was observed [20]. The saturation magnetization, Ms, was obtained by extrapolation plots of magnetization M versus 1/H =0. This extrapolation gave Ms =75.7 (emu cgs g − 1). Eqs. (2) and (3) (concerning only Fe3O4) can be used to calculate the surface average diameter D1 and D2 corresponding respectively to the low field (H tends to 0) and to the high field (1/H tends to 0).
n n
D1 = 534.51 D2 = 17.2
T 1 d Ms ds H
T ds Ms ds −dH
1/3
=12 nm
(2)
H[0
1/3
=5 nm
(3)
with ds: signal value d at saturation (unit arbitrary); dH : value signal at field H = 10 000 (Oe); K =1.38066 10 − 16 erg K − 1 (Boltzmann constant); Ms = 89.9 uem cgs g − 1 for Fe3O4; T= 300 K.
3.5. Discussion Fine nanosized magnetite particles appear to be easily produced in the presence of sodium endchain polyoxyalkylene di-phosphonates under conventional conditions. The particles size was found homogeneous around 10 to 30 nm depending on the analytical method.
The DLS method for measuring particle size include the magnetite radius and the hydrated polymer at the magnetite surface. The magnetic radius obtained from magnetization curve was about 5 to 12 nm in scale as found by others [11,14] in accordance with TEM analyses, while a noticeable discrepancy appears when the DLS volume is considered. Since the hydrated polyoxyalkylene radii do not exceed 2 to 2.5 nm giving rise to a maximum 5 nm increase of the particle diameter, the resulting DLS size measurements should lead to particles of approximately 20 nm. This suggests that a large portion of the particle assembly consists of associated water, solvated ions and counterions with the monolayer adsorbed polymer. The differences bear resemblance to the results of Hwang et al. [11], who also found similar diameters from TEM and SM (9 and 12 nm, respectively), and larger ones from DLS (19 nm). The facility to reach the equilibrium state of adsorption is obviously dependent on the average molecular weight of the polymer trough steric and conformational effects of the water soluble chain, the lower molecular weight being able to more efficiently adsorb than the higher one as shown from the isotherms. A second argument relates to the tightness and the selectivity of the polymer irreversibly connected to the surface. This result was demonstrated since after several washings, the stability on the colloidal was found unchanged and no elimination of polymer was detected. Because of the nonionic character of the adsorbed polymer one can speculate about the electroneutrality of the particles obtained in these circumstances. In contrast to multifunctional agents such as polyelectrolyte or polyvinyl alcohol, end-chain functionalized hydrosoluble polyoxyalkylene is more likely to produce fine nanosized particles under monolayer brush type model association at the surface. This association made relevant a flexible thin and regular polymer distribution at the surface with a calculated thickness of approximately 2 nm for a 50POE-P2 chain length. Another improvement associated to unsymetrical polymer agent lay in the fact that the dispersing agent would highly facilitate further chemical modifications on the opposite end-chain
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site offering some interesting opportunities for grafting biologically active agents.
4. Conclusion In conclusion we have found sodium polyoxyalkylene di-phosphonate polymers efficient agents in producing nanoscale magnetic particles under conventional synthesis scheme. Interestingly there is a tight dependence of the nanoscale distribution on the concentration of the polymer which acts through a brush model adsorption. Moreover the unsymmetrical nature of the polymer will offer the ability to graft reactive probes or monomers to produce magnetic active materials of nanosize shapes.
Acknowledgements This work was accomplished at the LMOPSCNRS laboratory in Solaize (France). The authors thank J-P. Dalmon, H. Mozzanega for magnetic measurements; A. Rivoire, I. Bornard and M. Esnouf for TEM measurements; P. Marotte for BET analysis, F. Melis for DLS analyses and S. Brunel for assistance in the laboratory.
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