Langmuir 1995,11, 4617-4622
4617
Self-Assembled Monolayer Coatings on Nanosized Magnetic Particles Using 16-MercaptohexadecanoicAcid Qingxia Liu and Zhenghe Xu* Department of Mining and Metallurgical Engineering, McGill University, Montreal, Quebec, Canada H3A 2A7 Received July 6, 1995. In Final Form: September 26, 1995@ A bolaamphiphile,16-mercaptohexadecanoicacid (MHA),was deposited on nanosized magnetic particles (y-FezO3)using a self-assemblymethod. The coated particles were characterized using X-ray photoelectron spectroscopy(XPS)and diffusereflectance infrared Fourier transform spectroscopy (DRIFTS).The molecular orientation of surfactant on the surface was inferred from critical surface tension values determined by film flotation and confirmed by infrared spectrum. Under the experimental conditions,MHA anchors onto the y-FezO3 surface by chemical bonding between the carboxylate head group and iron on the surface, leaving the thio or disulfide groups reactive. The self-assembledMHA film on y-FezO3 is immobilized and resistant to acid and base attack. The potential applications of the fabricated magneticparticles in biological cell separation and gold recovery are discussed.-
Introduction The surface coating of surfactants on various types of substrates has potential applications in a variety of areas, including non-linear optical devices, waveguides, microstructural electronics, optical and biological sensors, material protection (corrosion resistance), raw material recovery, and tribology,' to name a few. The preparation and characterization of self-assembled (SA) monolayer coatings of various organic surfactants on flat metal or metal oxide surfaces have been reported in a number of publications. These include alkykilane surfactant on hydroxylated surfaces, such as silica and aluminum oxide, alkanethiolates on gold, silver, and copper, alcohol and amines on platinum, and carboxylic and hydroxamic acids on aluminum oxide and silver oxide.2-10 In more recent publications, bolaamphiphiles with two reactive head groups at both ends of the molecule are used to manipulate the architecture of organic films on flat metal or metal oxide surfaces such as gold, silica, and aluminum ~ x i d e . ~ However, J - ~ ~ few publications have described the preparation and characterization of SA coatings using bolaamphiphiles on metal or metal oxide powders, particularly of nanosized magnetic iron oxides which have @
Abstract published in Advance A C S Abstracts, November 1,
1995. (UUlman, A. A n Introduction to Ultrathin Organic Film From Langmuir-Blodgett to Self-Assembly;Academic Press: Boston, MA, 1991. (2)(a) Pomerantz, M.; Segmuller, A.; Netzer, L.; Sagiv, J. Thin Solid Films 1986,132,153.(b) Netzer, L.; Iscovici, R.; Sagiv, J. Thin Solid Films 1983,99, 235. (3)Allara, D. L.; Nuzzo, R. G. Langmuir 1985,1 , 52. (4)Schlotter, N.E.; Porter, M. D.; Bright, T. B.; Allara, D. L. Chem. Phys. Lett. 1986,132,93. (5)Laibinis, P. E.; Hickman, J. J.;Wrighton, M. S.; Whitesides, G. M. Science 1989.245.845. ( 6 ) Bain, C. D.'; Troughton, E. B.; Tao, Yu-Tai; Evall, J.;Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. SOC.1989,111, 321. (7)Walczak, M. M.; Chung, C.; Stole, S. M.; Widrig, C. A.: Porter, M. D. J. Am. Chem. SOC.1991,113,2370. (8)Parikh, A. N.; Allara, D. L.; Azouz, I. B.; Rondelez, F. J. Phys. Chem. 1994,98,7577. (9) Yoon, R.-H.; Flinn, D. H.; Guzonas; D. A.Colloids Sure 1994,87, 163. (10)Folkers, J. P.; Gorman, L. B.; Laibinis, P. E.; Buchholz, S.; Whitesides, G. M.; Nuzzo, R. G.Langmuir 1996,11, 813. (11) Allara, D. L.; Hebard, A. F.;Padden, F. J.;Nuzzo, R. G.;Falcone, D. R. J. Vac. Sci. Technol. 1983,AI (2),376-382. (12)(a) Ihs, A.; Liedberg, B. J . Colloid Interface Sci. 1991,144,283. (b) Uvdal, K.;Bodo, P.; Liedberg, B. J. Colloid Interface Sci. 1992,149, 163. (13)Goss, C. A.; Charych; D. H.; Majda, M. Anal. Chem. 1991,63, 85. (14)Smith,E. L.;Alves,L.A.;Andergg,J.W.;Porter,M. D.;Siperko, L. M. Langmuir 1992,8, 2707.
potential applications in drug delivery, raw material recovery, biological cell separation, magnetic fluids, magnetic ink, and magnetic memory media.15-19 Cell removal using magnetic separation represents a powerful alternative technique that is growing rapidly in diagnostic, biotechnological, and biomedical applications.20 One of the methods to prepare magnetic carriers is to coat microsize spheres containing small paramagnetic particles with appropriate antibodies, specific to the cell type to be removed. The spheres attach to the specific type of cells in a mixing chamber and are then exposed to a magnetic field gradient. The small spheres, with their load of selected cells, are captured and thus separated from the rest of stream. Another application of biocompatible magnetic carriers is in the detection of bacteria, viruses, and parasites as reviewed recently.21 The key to these applications is to engineer the magnetic particles with desired antibodies attached to their surfaces. An attractive approach is to use the self-assembly method with different surfactants through which the desired functional groups on the surface would remain reactive to the probing molecules. Selecting the reactive functional groups of surfactants to be coated on the surface of magnetic particles is based on their affinity for the probing molecules. Recently, the cross-linking properties of tetrameric antibody complexes have been used in labeling targeted cells with submicrometer dextran iron oxides. The labeled cells can then be readily removed using a magnetic separation method.22 It is noted that tetrameric antibody complexes are formed by cross linking of thiol in each of the antibody molecules through the formation of disulfide bonds. This finding led us to explore a new method for putting functional groups on iron oxide particles. The basic idea is to self-assemble bolaamphiphiles on iron oxides in such a way that one functional group anchors to the surface through chemical bond formation and the (15)Rozenfeld,0.; Koltypin,Y.; Bamnoker, H.; Margel, S.;Gedanken,
A.Langmuir, 1994,10,3919.
(16)Liu, Qingxia; Friedlaender, F. J. Min. Eng. 1994,7 (4),449. (17)Molday, R.S.; Mackenzie, D. J. Immunological Methods 1982, 52,353. (18)Saito, S.,Ed. Fine Ceramics, Elsevier: Amsterdam, 1988. (19)Goldman, P. In Electronic Ceramics: Properties, Deuices, and Applications; Levinson, L. M., Ed.; Dekker: New York, 1988. (20) Sonti, S. V.; Bose, A. J. Colloid Interface Sci. 1996,170, 575. (2.1) Olsvik, 0.; Popovic, T.; Skjerve, E.; Cudjoe, K. S.; Hornes, E.; Ugestad, J.; Uhlen, M. Clin. Microbiol. Rev. 1994,7,43. (22)Thomas, T. E.; Abraham, S. J. R.; Blackmore, E. W.; Lansdorp, P.M. J.Immunol. Methods 1992,154,245.
0743-7463/95/2411-4617$09.00/0 0 1995 American Chemical Society
4618 Langmuir, Vol. 11, No. 12, 1995 other remains reactive to the molecules or elements (metals)to be removed or recovered. The magnetic carriers prepared as such do not contain biological products and have no toxicity for such applications. In this communication, preliminary studies describing the SA coating on nanosized magnetic particles (y-Fez03) with 16-mercaptohexadecanoic acid (MHA) are reported. Self-assembled monolayer coatings on magnetic particles using stearic acid, citric acid, and 3,3’-dithiodipropionic acid (DTDPA) are also characterized to elucidate the reactivity of polar groups with y-FezO3. On the basis of previous studies, it is expected that the carboxylic head group (-COOHI of MHA anchors to the surface of the magnetic particles so that the thiol (-SH) on the other end remains available for different reactions. The magnetic particles thus fabricated are considered to have the potential applications in biological cell separation, in magnetic separation of minerals, and as fillers in polymer matrices.
Materials and Methods Materials. y-FezO3 powders (99+%) from Alfa Chemicals were dried in a vacuum oven at 12 psi and 120 “C for 24 h prior to the self-assembly experiments. The purpose of drying the particles was to remove physisorbed water molecules. The average particle size was found by transmission electron microscopy (TEM) to be about 30 nm. Stearic acid (99%),citric acid (99%),3,3’-dithiodipropionicacid (99%),chloroform (HPLC grade), hexane (HPLC grade), and hexadecanethiol(96%) from Aldrich were used as received. The procedures as reported in the literaturez3were used in synthesizing 16-mercaptohexadecanoic acid (99%). All other chemicals were reagent grade. Water, purified with a Millipore water treatment system, was used in all experiments. All glassware was cleaned with “piranha solutions” (1:3 HzOflzS04) at 70 “C for about 20 min and then rinsed with distilled water until the pH ofthe effluent was neutral (pH = 6-7). Note: “piranhaso1utions”react violently with many organic materials and should be handled with extreme care. Procedures. Surfactant solutions (3 mM) were prepared in chloroform. y-FezO3 powder (50 mg) was gently mixed with 25 mL of surfactant solution in a 40-mL vial while bubbling the nitrogen through the solution. The vial was then sealed and shaken for 24 h using a laboratory shaker (New Brunswick Scientific,Inc., USA). The treated particles were separated from solution by a hand magnet and rinsed repeatedly with chloroform, followed by dry hexane to remove unbound surfactant. The particles were then dried in a vacuum oven (12 psi) at 40 “C for 12 h and stored under nitrogen prior to characterization. X P S . XPS spectra were obtained on an ESCALAB Mark-I1 instrument with a Mg & anode (hv = 1253.6 eV) at a take-off angle normal to the sample. The source X-ray was not filtered and the instrument was calibrated against the C1, band (284.8 eV). The spectra were recorded using a band-pass energy of 20 eV corresponding to an energy resolution of 1.2 eV. The powder samples, placed in a copper cup, were maintained under a Torr for -1 h in the sample background pressure of 1 x chamber before spectral acquisition. The spectra presented here were corrected for background charging by determining the Cl, (284.8 eV) signal both at the outset and at the end of a series of narrow scans for each sample. No significant charging was detected. Band-fitting and spectral deconvolution were performed using the program S u r f - S ~ f t . ~ ~ DRIFTS. The coated samples were characterized by diffise reflectance infrared Fourier transform spectroscopy (DRIFTS, Nicolet advanced diffuse reflectance accessory) using a Bruker IFS 66 FTIR spectrometer equipped with a narrow-band MCT detector. A sample of finely crushed KESr was used as the background. DRIFTS spectra were obtained using 100 scans at nominal resolution of 4.0 cm-l and presented without base-line correction. (23) Pale-Grosdemange, C.; Simon, E. S.; Prime, K. L.; Whitesides, G. M. J. Am Chem. SOC.1991, 113, 12. (24)Programme Surf-SoR, Ecole Polytechnique de Montreal, Quebec, Canada.
Letters Table 1. Band Maxima (eV) Obtained from XPS Dataa ~ ~ _ _ _ _ _ _ _ _ ~ binding energy (eV) system SzP 01, Fezp C1, 724.3 ndb 529.8 710.7 nd ~~
~
y-FezOa + stearic acid
nd
513.4 529.6
724.3 710.7
288.3 284.6
y-FezOa + MHA
163’3
531.4 529.6
724.3 710.7
288.3 284.6
a
Band positions are accurate to f0.7 eV. * nd, not detected.
Film Flotation. Surface wettability is a sensitive way to characterize surfaces.2s Film flotation, proposed by Williams and Fuerstenau,26provides a simple yet accurate method to study the wetting characteristics of powder samples. In this communication, the surface wettability of coated particles was characterized by film flotation, from which the critical surface tension was derived. Methanol was used to adjust the surface tension of the liquid. Twenty milligrams of coated particles was placed gently on the solution-air interface and left for about 10 min. The particles remaining on the air-liquid interface were skimmed off. The sinks and floated particles were weighed separately after filtration and drying. The floated (lyophobic) fraction was plotted against surface tension of the liquid, from which the critical surfacetension ofwetting(defhed as the surface tension of liquid with 50% of particles flotating) was obtained. Two additional surface tension values for a complete wetting (0%floats) and nonwetting (100%floats) conditions can be found from this plot, and the difference between these two values serves as a measure of surface heterogeneity, in this case, of selfassembled particles.
Results and Discussion X P S . XPS spectra of y-FezO3 with and without selfassembled layers are shown in Figure 1. The band positions in XPS spectra are given in Table 1. The band positions of iron-Fez, and oxygen-01, for untreated y-FezO3 are consistent with those reported previo~sly.~’No significant spectral changes in the Fezpband were observed when y-Fe203was treated by either stearic acid or MHA. The bands corresponding to 01, (spectra a and b) became broader slightly and skewed toward high binding energy, probably due to the contributions of conjugated oxygens from carboxylic acid interacting with surface iron. The most significant spectral change is, however, the appearance of two CI, bands at 288.3 and 284.6 eV, when the y-FezO3 powders were treated by stearic acid and MHA. In the latter case, a sulfur band SZ, at 163.3 eV was observed as well. These spectral changes indicate the presence of stearic acid and MHA on y-Fe203. It is well documented2I that the band at 288.3 eV is characteristic of carbon in a carboxylic environment (COO), while the band at 284.6 eV is characteristic of carbon in a hydrocarbon chain (C-C). The ratio of area under the C1, band of high binding energy to that of low binding energy was calculated to be ca. 1:17 and 1:15 for y-FezO3 self-assembled with stearic acid and MHA, respectively. These values are in excellent agreement with those derived from the molecular structure. A similar calculation showed that the ratio of area under the C1, band (at 284.6 eV) to that of Fezp (at 707 eV, normalized by the sensitivity factors of the elements and the experimental number of cycles) was 3.7 and 3.6 for y-Fe203 particles coated with stearic acid and MHA, respectively. (25)Schrader, M. E., Loeb, G. I., Ed. Modern Approaches to Wettability: Theory and Application; Plenum Press: New York, 1992. (26) Williams, M. C.; Fuerstenau, D. W. Int. J. Miner. Proess. 1987, 20,
153-157.
(27)Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer Corp.: Eden Prairie, MN, 1992.
Letters
Langmuir, Vol. 11, No. 12, 1995 4619
1
,
~
"
-732
I
'
"
'
1
-716
-724
~
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I
~,
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,
Binding Energy (eV)
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-287
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-283
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-535
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Binding Energy (eV)
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,
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Binding Energy (eV)
Binding Energy (eV)
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Figure 1. XPS spectra ofnarrowscans for the elements ofinterests on y-FezOssurfaceswith self-assembled(a)MHA and (b)stearic acid and (c) untreated. Considering a 5-8 nm sampling depth ofXPS and a length of 2.4 nm for a fully extended stearic acid molecule, the carbon to iron ratio of 3.7 suggests that a densely packed surfactant layer is assembled on the y-FezO3 particles. The ratio for MHA (3.6) is almost the same as for stearic acid, indicating that the monolayer packing of MHA is similar to that of stearic acid on y-FezOsparticles. (Note: MHA contains 15 carbons while stearic acid contains 17 carbons in the hydrocarbon chain.) The SZPband at 163.7 eV for y-Fe& coated by MHA is characteristic for -SH or -S-S- groups. For comparison, the band positions of different sulfur groups as
reportedzs are given in Table 2. It is evident from Table 2 that the binding energy of SzPis expected to increase as the thio or disulfide group is further oxidized, appearing at around 166-168 eV. These bands were not observed in our experiments, suggesting that our procedure does not induce oxidation of the thio group to sulfate, which is consistent with literature results. XPS is, however, unable to distinguish whether the sulfur on y-Fez03 is in the (28) Volmer-Uebing, M.; Stratmann, M. Appl. Surf Sci. 1992, 55,
19 *".
(29) Nishikana, Y.; Kimura, K.; Matsuda, A.; Kenpo, T. Appl. Spectrosc. 1992, 46, 11, 1695.
4620 Langmuir, Vol. 11, No. 12, 1995
Letters
Table 2. Binding Energies of the Snp ( e n in Organic and Inorganic System* binding energy fwhmb a
Fe-S
R-SH
161.5 f 0.5 2.0
163.8 & 0.2 2.0
R-S-S-R
R-SO2-
163.9 f 0.2 2.0
Fe2(S04)3
R-SO3-
166.6 f 0.5 2.2
168.6 f 0.6 2.2
168.8 2.2
* 0.2
From ref 28. Full-width at half maximum. 1
I
I
A
N r-
'1
I
: : f
C
3100
2900
3000
2800
2700
1800
1700
1600
1500
1400
1300
1200
Wavenumber (cm-1) Wavenumber (cm-1) Figure 2. Infrared spectra in the high- (A) and low-frequency (B)regions for (a)MHA in KBr, (b) MHA on y-FezOa,and (c) y-FezO3. Table 3. Vibrational Mode Assignments and Band Positions (cm-') for MHA Dispersed in KBr and Self-Assembled on y-Fe20s sample
a
mode assignment
KJ3r
monolayer
va(CH2) vs(CH2) v(C=O) va(C0O-) vs(C0O-)
2924 2851 1702 nd
2924 2851
nd
nda 1527 1433
nd, not detected.
form of thio or disulfide as the band positions of Sppfor both are within the resolution limit of the instrument used here. DRIFTS. Infrared spectra of y-FezO3 particles with and without MHA self-assembled layers were obtained using DRIFTS, a surface-sensitive technique suitable for powder samples. The high (3100-2700 cm-') and low (1800-1200 cm-l) wavenumber regions of the spectra, which contain diagnostic spectral features, are given in Figure 2. For comparison, the spectrum ofMHAdispersed in KBr is also included in this figure. The assignment of the bands is given in Table 3. The four distinct bands at 2924,2851,1527, and 1433 cm-l for y-Fe203treated with MHA (spectrum b) are due to the molecular vibrations of asymmetric and symmetric stretching of CH2 and COOgroups, respectively, confirming the self-assembly of MHA on y-FezO3. There is no band observed in the spectral region above 3000 cm-l where characteristic bands ofCH3 stretching vibrations appear. This is in agreement with the molecular structure of MHA which does not contain any CH3 group. Two bands assigned to CH3 stretching vibrations were observed in this spectral region when stearic acid was assembled on y-FezOa. It is worth mentioning that a direct spectral feature of thiols (i.e. C-S-H) was not observed. The weak infrared absorption by C-S and S-H stretching vibrations appears to be
responsible for these two bands being undetected. (Note: The C-S band at 600-700 cm-l overlaps with the iron oxide vibrational bands.) Two distinct spectral differences between the spectrum of bulk MHA (spectrum a) and that of MHA on y-FezO3 (spectrum b) were observed. In the low wavenumber region, the carbonyl band at 1703 cm-l for bulk MHA was displaced to a lower wavenumber, 1527 cm-l, and associated with this shift is the appearance of a new band at 1433cm-' when the MHAwas present on y-FezO3. This spectral change indicates binding between the carboxylate group and metal on y-FezO3 occurred, transforming a carboxylic acid functionality to a carboxylate functionality with both oxygen atoms interacting with the metal on the surface.14 However, this spectral feature is different from that of MHA self-assembled on gold substrates where the carboxyl group did not react with surface and remained exposed to the en~ir0nment.l~ In this case, the reactivity of the thiol with gold is stronger than that of the carboxyl group. In contrast, our observations show stronger reactivity of the carboxylate group with y-FezO3 than the thio group, leaving the thio group exposed to the environment. The above experimental evidence demonstrated that by controlling the relative reactivity of polar groups of a bolaamphiphile with a substrate, a surface of desired functional groups can be fabricated using the self-assembly method. It is important to note the absence of band at 1703 cm-l (spectrum b) assigned to the C=O stretching mode of the carboxyl group, indicating that all carboxyl groups on MHA are intimately bound to the y-FeZO3 surface. This finding suggests that MHA only forms a monolayer on y-FezO3. In the high wavenumber region, the band positions for CH2 stretching remained the same for both bulk and SA MHA. However, the CH2 stretching bands for MHA on y-FenO3 are sharper with a narrow band width a t half band height, indicating more ordered (crystalline) polymethylene chains and hence confirming the formation of
Langmuir, Vol. 11,No. 12, 1995 4621
Letters
20
30
40
50
60
70
80
Surface Tension (mN/m)
Figure 3. Partition curve of the film flotation using y-FezOs self-assembled with (a)stearic acid, (b) MHA, (c) DTDPA, and (d) citric acid and untreated.
a densely packed surfactant monolayer. It should be noted that the quantitative geometrical orientation of MHA on y-Fe2O3 powder cannot be determined directly using the DRIFTS technique. Film Flotation. The results of film flotation using y-Fe2O3 with and without surface coatings are shown in Figure 3. The untreated y-Fe203 particles were readily wetted by water as expected for high-energy surfaces of oxides. A similar result was obtained for the y-Fe2O3 particles treated with citric acid, on which highly hydrophilic carboxylic acid groups were exposed to water. The y-Fe2O3 particles treated with 3,3'-dithiodipropionic acid (DTDPA) showed moderate hydrophobicity with 20% of particles floating on the air-water interface (Figure 3c). In contrast, the particles treated with stearic acid and MHA were strongly hydrophobic,all the particles floating on the air-water interface. The critical surface tensions of 27 and 33 mN/m at the low limit (0% floats), and 32 and 43 mN/m at the high limit -(100% floats) of the film flotation curves were obtained for y-Fe203 particles treated with stearic acid and MHA, respectively. For stearic acid coated y-Fe2O3 particles,the critical surface tension for complete wetting (27 mN/m) approached the surface tension of normal hydrocarbon liquids (25 mN/m), or CH3 terminated crystalline monolayers (22 mN/m),lBO confirming a compact monolayer on y-Fe203 terminated with methyl groups. The small difference (5 mN/m) between the two limits (Figure 3a) implies a homogeneous surface, i.e. suggests the formation of a relatively uniform monolayer. The critical surface tension of MHA-coated y-Fe2O3 particles remained low (33 mN/m), confirming that hydrophilic carboxylic groups anchored onto the surfacewith relatively (30) Israelachvili, J. Intermolecular & Surface Forces, 2nd ed.; Academic Press: San Diego, CA, 1992. (31) Fuhrhop, J. H.; Kon, J.'Membranesand Molecular Assemblies: the Synkinetic Approach; Cambridge University Press: Cambridge, 1994.
hydrophobic thio or/and disulfide groups exposed to the environment. However, this value is slightly higher than that for stearic acid coated particles, which can be attributed to the relatively polar nature of thiol or disulfide groups compared to methyl groups. The difference between the two limiting critical surface tension values (10 mN/m) increased slightly, showing an increased surface heterogeneity of MHA coated particles compared to stearic acid coated particles. It is possible that some of thio groups were oxidized to disulfide.35 The presence of these two sulfur species with different polarity may have contributed to the increased surface heterogeneity. It is interesting to note the significant difference in surface wettability of y-Fe203 particles coated with MHA and DTDPA. As shown in insert c of Figure 3, two carboxylic groups of DTDPA are expected to anchor to y-Fe2O3 particles with a disulfide group being exposed to water. Therefore; thiol or/and disulfide are the terminating group for both MHA and DTDPA coated y-Fe203 particles. The significantlyhigher wettability of DTDPAcoated particles compared to MHA-coated particles seems to be related to the lower DTDPA surface coverage, suggesting that long chain hydrocarbon association of amphiphiles is required to self-assemble a densely packed monolayer, such as MHA on y-Fe2O3. The architecture of self-assembled surfactant layers using various types of amphiphiles is shown schematically in Figures 3 and 4. Stability. To examine the nature of the packing of various surfactants on the y-Fe2O3 particle surface and to investigate the stability of self-assembled layers to harsh environments, leaching experiments were conducted by placing the treated particles in acidic (pH = 3) and alkaline (pH = 10) media. These experiments showed that the surface coatings with stearic acid and MHA are stable in aqueous solutions over a pH range from 3 to 10 for a few days. No ferric ions were leached into the solution, indicating that the surface coatings are tightly packed. After the leachate was removed and the powder dried, the same values of critical surface tension were obtained, suggestingthat the surfactant layer prepared with stearic acid and MHA remained stable under the test conditions. In contrast, ferric ions were detected (by sodium thiocyanide titration) in leachate of DTDPA-coated y-Fe2O3 exposed to distilled water. These observations confirm the packing information inferred from DRIFTS and film flotation. General Observations. These preliminary studies demonstrate the success of fabricatingnanosized magnetic particles with special surface functionalitiessuch as thio and/or disulfide groups. The molecular orientation of MHA self-assembled on y-FesO3, which is shown schematically in Figure 4a, was inferred from surface characterization and film flotation. Stable monolayer films were obtained by self-assembly of MHA on y-Fe2O3 from chloroform solutions. However, no significant difference was observed in the present study when the film was selfassembled from ethanol solutions, although there may be some subtle difference in molecular packing density and nanostructure of assembled domains if they are present. The former can be confirmed by quantitative analysis using titration methods, while the latter may be confirmed by image analysis using atomic force micro~copy.~~ It is important to mention that the characterization techniques used in this study are not suitable to derive such information on fabricated surfactant layers on fine powder surfaces as layer thickness, quantitative packing density, (32) Sarid, D. Scanning Force Microscopy: with Applications to Electronic,Magnetic and Atomic Force;Oxford University Press: Oxford, 1991.
4622 Langmuir, Vol. 11, No. 12, 1995
Letters
(b) Cell
0 -SH e* -s-s0 -COOH Figure4. Schematicrepresentation of y-FezOs coatedwith (a) MHA monolayer, (b) coated with a slightly oxidized MHA monolayer, (c) activated with an antibody, and (d) attached to a targeted cell.
and geometric orientation or to distinguish thiols from disulfides. Complementary techniques% should be used to obtain this information for powder samples and the detailed research is in progress. The magnetic particles fabricated with thio or disulfide groups have potential applications in various areas. The special affinity of thio with antibodies makes fabrication of thio-containing magnetic particles of special interest. It has been shown that monoclonal antibodies (mAb), usually containing disulfides in the basic unit, are readily cross-linked to another incubated antibody, forming tetrameric antibody complexes. The thio-containing magnetic particles (carriers) as shown in Figure 4a can, therefore, be sensitized with a mAb of specific paratopes, such as anti-glycophorin,generating sensitized magnetic antibodies. These magnetic antibodies (Figure 4c) recognize glycophorin, one kind of protein (Mand N blood group antigens) in red blood cell membranes. When the
sensitized magnetic particles are introduced into a biological system, the cells containing glycophorin will be captured by magnetically sensitized media (Figure 4d). These fractions may then be separated using magnetic separationmethods. In the case where the thiol fabricated on magnetic particles gets oxidized (Figure 4b), the disulfide can be readily reduced nonenzymatically by glutathione to a thio functionality, thus activating magnetic particles for cell ~ e p a r a t i o n . ~ ~ It is also well-known that thio and disulfidegroups have strong affinity with precious metals, such as gold, silver, and ~ o p p e r . The ~ ~ ?fabricated ~~ magnetic particles (Figure 4a,b) of large surface area could, therefore, be used to capture gold and silver from their leachates. The metalloaded magnetic particles can then be readily separated from leaching solutions using magnetic separation. The results of a laboratory silver and copper loading test using fabricated magnetic particles will be presented in a separate communication. In addition, the approach can be readily extended to the fabricationof magneticparticles with other customized functional groups by controlling the reactivity of hnctional groups of a bolaamphiphile with magnetic particles. The functionalized magnetic particles can be used to recover secondary resources or to remove toxic species from industrial effluentby controlling the reactivity of functional groups with targeted species.
Acknowledgment. The help from Dr. M. Shi and Dr. S. Brienne in obtaining the XPS and DRIFTS spectra, respectively, and the valuable discussions with them are gratefblly acknowledged. We wish to thank Professor J. A. Finch for his comments and suggestions and Professor B. Lennox and Ms. Antonella Badia for providing us the 16-mercaptohexadecanoic acid. The financial support from Natural Sciencesand Engineering Research Council of Canada is also acknowledged. LA950975A (33) Mathews, C. K.; van Holds, K. E. Biochemistry; The Benjamin/ Cummings Publishing Company, Inc.: Reading, MA, 1990; Chapter 14, p 500. (34) Wood, R.,Kim, D. S.; Basilid, C. I.; Yoon, R.-H. Colloids Surf 1995,94, 67. (35)Zhong, C.H.; Poter, M. D. J.Am. Chem. SOC.1994,116,1161611617.
