Langmuir 2002, 18, 5511-5515
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Raman Spectroscopic Investigation of Maghemite Ferrofluids Modified by the Adsorption of Zinc Tetrasulfonated Phthalocyanine Jean C. O. Silva, Marcelo H. Sousa, Francisco A. Tourinho, and Joel C. Rubim* Instituto de Quı´mica da Universidade de Brası´lia, C.P. 04478, 70919-970, Brası´lia, DF, Brazil Received January 24, 2002. In Final Form: April 29, 2002 Maghemite (γ-Fe2O3) ferrofluids modified by the adsorption of tetrasulfonated zinc phthalocyanine (ZnPcTs) have been characterized by resonance Raman, Fourier transform infrared (FTIR), and fluorescence spectroscopies. While no signal from the adsorbate was observed from the FTIR measurements, the Raman results have shown good signal-to-noise ratios for the adsorbate at surface coverages down to 0.2. A fluorescence signal from the ZnPcTs in solution was observed only for ZnPcTs concentrations corresponding to a monolayer coverage or for higher ZnPcTs concentrations. Langmuir adsorption plots using Raman and fluorescence data have shown that the ZnPcTs adsorbs on the maghemite surface with an adsorption equilibrium constant of ca. 102, suggesting that electrostatic forces govern the adsorption process. It has been observed that for coverages above a monolayer the colloidal solution becomes unstable and aggregation occurs. Experiments performed with a modified Gouy method have shown that the ZnPcTs-modified maghemite ferrofluids have magnetic properties comparable to those of the maghemite ferrofluid and are stable for at least 24 h after sample preparation.
Introduction These days, a keyword in science is nanomaterials. Chemistry has, with no doubt, the major role in the development and design of these kinds of materials. One of these materials is known as magnetic fluid or ferrofluid. Originally, magnetic fluids were obtained by the mechanical mulling of magnetite (Fe3O4) and dispersion of the nanocrystallites into an appropriate solvent in order to get a stable colloidal solution of magnetite nanoparticles.1,2 These nanoparticles present a magnetic dipole, but they are randomly oriented in solution and therefore behave as liquids. Once a magnetic field is applied to the solution, the nanoparticles get oriented, lose some liquid characteristic properties, and gain other properties that are characteristic of solids. It is also possible to obtain magnetic fluids (or ferrofluids) by chemical ways. The preparation procedure usually involves the precipitation of magnetite from an Fe(II)/Fe(III) solution with an alkaline solution.3,4 By chemically oxidizing the magnetite magnetic fluid, one can obtain the maghemite (γ-Fe2O3) ferrofluid,5 and changing Fe(II) by an appropriate metallic ion, one can obtain the electric double layered ferrofluids with the formula [MIIOFe2O3] where M ) Zn(II), Cu(II), Ni(II), Mn(II), and Co(II).6,7 The surface chemistry and structure of this kind of material are very important since they determine the stability of the colloidal solution and therefore its magnetic property. One of the interesting tasks concerning magnetic fluids is to obtain such a material that can be stable in * To whom correspondence should be addressed. (1) Pappell, S. S. U.S. Patent 3,215,572, 1965. (2) Rosensweig, R. E.; Kaiser, R.; Miskolczy, G. J. Colloid Interface Sci. 1969, 29, 680. (3) Khalafala, S.; Reiners, G. W. U.S. Patent 3,764,540, 1973. (4) Massart, R. IEEE Trans. Magn. MAG. 1981, 17, 1247. (5) Caubil, V.; Massart, R. J. Chem. Phys. 1987, 84, 967. (6) Tourinho, F. A.; Franck, R.; Massart, R. J. Mater. Sci. 1990, 25, 3249. (7) Sousa, M. H.; Tourinho, F. A.; Depeyrot, J.; da Silva, G. J.; Lara, M. C. F. L. J. Phys. Chem. B 2001, 105, 1168.
physiological pH, that is, a biocompatible magnetic fluid. This can be done by just modifying the nanoparticle surface chemistry as, for instance, adsorbing glutamate and aspartate on its surface.8 Once one gets a magnetic fluid that can be stable in vivo, one can think of its use in drug delivery to handle localized diseases. There are some kinds of localized diseases, as for instance skin cancer, that can be treated by laser photodynamic therapy (PDT).9 Usually a drug (e.g., porphyrin, a photosensitive chemical) is administered to the patient and a period of time is observed before the laser therapy is performed in the local area. This kind of process has several side effects, since the drug is spread out in the patient’s entire body.9 Water soluble metal phthalocyanines (MPcs), as for instance the tetrasulfonated Zn-phthalocyanine (ZnPcTs), have proved to be powerful candidates in the treatment of cancer by laser PDT.10,11 In our goal to obtain magnetic fluids as a drug delivery vehicle for laser PDT, we present in this work maghemite ferrofluids that were modified by the adsorption of ZnPcTs. The obtained material is stable in solution and was characterized by resonance Raman and fluorescence spectroscopies. It will be shown that as in other surface phenomena related to nanomaterials, resonance Raman spectroscopy is a powerful tool in the characterization of surface-modified magnetic fluids. Experimental Section The magnetic fluid used in this work comprises a water dispersion of maghemite nanoparticles and was prepared according to a previously described procedure.12 In agreement with X-ray measurements, the maghemite particles present a mean (8) Sousa, M. H.; Rubim, J. C.; Sobrinho, P. G.; Tourinho, F. A. J. Magn. Magn. Mater. 2001, 225, 67. (9) Sibata, C. H.; Colussi, V. C.; Oleinck, N. L.; Kinsella, T. J. Braz. J. Med. Biol. Res. 2000, 33, 869. (10) Phillips, D. Prog. React. Kinet. 1995, 22, 175. (11) Bonnett, R. Chem. Soc. Rev. 1995, 24, 19. (12) Tourinho, F. A.; Depeyrot, J.; da Silva, G. J.; Lara, M. C. L. Braz. J. Phys. 1998, 28, 413.
10.1021/la020078o CCC: $22.00 © 2002 American Chemical Society Published on Web 06/07/2002
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diameter of 10 nm.13 The maghemite concentration in the magnetic fluid is ca. 2.42 × 1016 particles/cm3 8 corresponding to 1 mol/L. The pH of the stock solution of maghemite ferrofluid is 2, which means that the maghemite particle surface is positively charged.14 The tetrasulfonated zinc phthalocyanine (ZnPcTs) was purchased from Porphyrin Products Inc. and used as received. Its high purity was verified by capillary electrophoresis and mass spectrometry.15 Maghemite magnetic fluids with different ZnPcTs surface coverages were prepared by the addition of different amounts from a stock solution of ZnPcTs (3 × 10-4 mol/L) to 5.0 × 10-3 L aliquots of the magnetic fluid at a 1.0 × 10-2 mol/L concentration. The Raman measurements were done with these solutions, while the fluorescence measurements were performed after dilution of the same solutions with distilled water in order to achieve a magnetic fluid concentration of 5.0 × 10-4 mol/L. The amount of ZnPcTs added to each magnetic fluid aliquot was calculated considering the number of molecules necessary to cover the surface of the particles at the desired surface coverage. These calculations were done considering the particles’ average surface area, the concentration of the particles in solution, and the ZnPcTs surface area, assuming it adsorbs flat on the maghemite surface. The ZnPcTs molecular area was calculated using a molecular mechanics software based on the MM2 force field, and the obtained value was 1.59 × 10-14 cm2. Solid samples corresponding to ZnPcTs-modified maghemite were prepared by adding acetone to the corresponding ferrofluid solution. The addition of acetone causes particle aggregation. The resulting solid is filtered, washed with acetone, and dried in the dark. A redispersed solution of the corresponding solid ZnPcTs-modified maghemite ferrofluid, or its peptization, is readily obtained by just adding distilled water to the desired amount of solid. The Raman spectra were acquired on a Renishaw Raman System 3000 using a 80× microscope objective. The excitation source was the 632.8 nm line from a Spectra-Physics He/Ne laser. The Raman spectra from the ferrofluid solutions were obtained from samples in glass capillaries. The laser power at the sample was ca. 70 µw and 1 mW when measuring solids and solutions, respectively. The fluorescence measurements were performed on a Spex spectrofluorimeter, and the excitation source was a Xe lamp. The fluorescence spectra (emission and excitation) were obtained from samples in a 1 cm optical path quartz cuvette. The FTIR spectra were obtained on a Bomen instrument (BS series) equipped with a diffuse reflectance accessory from Spectratech. The experimental procedure used for the verification of the magnetic properties of the investigated samples is an adaptation of the Gouy method.7 In this case, we have used a magnet above a balance ((0.001 g) and determined the apparent mass change caused by each ZnPcTs-modified maghemite magnetic fluid sample. The magnetic particle concentration and the sample volume were maintained as constants.
Results and Discussion We have observed for FePc,16 which has D4h symmetry, that excitation at resonance causes the a2g silent modes to become active. In the case of ZnPcTs, the presence of four SO3- groups led to at least four structures considering the SO3- moieties as punctual charges bound to the benzene rings at the “meta” positions. The four structures so generated have C4h, D2h, C2v, and Cs symmetries. Taking into account steric hindrances and minimum energy configuration, the C4h will be considered as the symmetry of the ZnPcTs. (13) Sousa, M. H.; Tourinho, F. A.; Rubim, J. C. J. Raman Spectrosc. 2000, 31, 185. (14) Campos, A. F.; Tourinho, F. A.; da Silva, G. J.; Lara, M. C. F. L.; Depeyrot, J. Eur. Phys. J. E 2001, 6, 29. (15) Santos, M. R. Ph.D. Thesis, Instituto de Quı´mica da Universidade de Sa˜o Paulo, Brazil, 2000. (16) Corio, P.; Aroca, R.; Rubim, J. C. Langmuir 1998, 14, 4162.
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Figure 1. (a) FTIR (diffuse reflectance) and (b) Raman spectra of the solid (powder) ZnPcTs. Table 1. Raman Wavenumbers Observed in the Resonance Raman Spectra of FePC and ZnPcTs and the Respective Tentative Assignment FePc1
ZnPc Ts
symmetry C4h
1608 w 1528 vs 1451 s 1432 w 1398 w 1340 s 1306 s 1192 m 1140 m 1108 m 955 m 831 m 750 s 680 s 593 m 482 m
1611 wa 1517 vs 1448 w 1424 s 1399 w 1332 vs 1275 m 1214 m 1179 w 1128 w 963 m 845 w 750 s 693 s 600 m 509 m
ag ag ag ag eg ag ag ag ag ag ag ag ag ag ag ag
a Relative intensities: vw ) very weak, w ) weak, m ) medium, s ) strong, and vs ) very strong.
Figure 1 presents the resonance Raman and the Fourier transform infrared (FTIR) spectra of ZnPcTs. Note that some of the most intense Raman bands are not observed in the FTIR spectrum. This observation strongly supports the above assignment of the ZnPcTs as belonging to the C4h point group. Taking these symmetry considerations, Table 1 displays the tentative assignment for the Raman wavenumbers observed in the ZnPcTs as compared to the FePc. The area of the ZnPcTs molecule calculated as described in the Experimental Section was done considering a C4h symmetry. The resonance Raman spectra of ZnPcTs-modified maghemite magnetic fluids are displayed in Figure 2. For comparison purposes, the resonance Raman spectrum of solid ZnPcTs is also displayed. The spectra displayed in Figure 2 were baseline corrected in order to eliminate the photoluminescence background. The inset of Figure 2 displays two original spectra before baseline correction in order to show that both ZnPcTs-modified maghemite samples, solid and solution, are fluorescent. Note that almost all Raman wavenumbers for the solid ZnPcTs are observed in the modified ferrofluids and their relative intensities are quite similar. The major differences are observed when comparing the solid-state spectra of ZnPcTs (Figure 2c) and ZnPcTs-modified maghemite (Figure 2a). The ZnPcTs-modified maghemite spectrum (Figure 2a) shows a broad feature at 720 cm-1. This feature is due to the strongest maghemite phonon13 that is not observed in the solution spectrum (Figure 2b). In a solution of pH 2, the ZnPcTs has its absorption maximum at 634 nm. This means that the spectra
Raman Investigation of Maghemite Ferrofluids
Figure 2. Raman spectra of (a) ZnPcTs/maghemite solid, (b) ZnPcTs/maghemite redispersed (see text for details), and (c) ZnPcTs solid. The inset shows the original spectrum for the maghemite/ZnPcTs ferrofluid solution and solid before baseline correction.
presented in Figure 2 are resonance enhanced. The fact that almost no significant changes are observed in the resonance Raman spectra of ZnPcTs and maghemite modified by ZnPcTs can be explained by the strong electronic delocalization in this molecule. We have observed16 that the FePc resonance Raman spectrum and the surface-enhanced resonance Raman spectrum (SERRS) of FePc adsorbed on a silver electrode are quite similar as in the case here. In the particular case of the ZnPcTs, we may have covalent interactions between the molecule and the maghemite surface, but due to its anion characteristics the interaction with the substrate must be essentially electrostatic and involves the SO3- moieties. Raman signals characteristic of these moieties were not observed, suggesting that they do not take part in the chromophore bonding and are therefore not resonance enhanced. The spectrum displayed in Figure 2b shows a good signal-to-noise ratio considering that a monolayer of ZnPcTs is adsorbed. Resonance Raman spectra from ZnPcTs in solution are quite difficult to obtain due to the strong fluorescence. It seems that part of the ZnPcTs fluorescence is quenched due to adsorption. The FTIR spectrum of the ZnPcTs-modified maghemite shows only the strong absorptions due to maghemite, and no signal from ZnPcTs is observed. In the study of the adsorption of aspartate, glutamate, and methylene blue on the maghemite surface, we were also not able to observe characteristic vibrational bands of the adsorbates while their adsorption on maghemite was well characterized by Raman spectroscopy.8,17,18 The results of Figure 2 encouraged us to investigate ZnPcTs-modified maghemites at lower coverages. Figure 3 displays resonance Raman spectra of ZnPcTs-modified maghemites at different ZnPcTs concentrations and the corresponding calculated surface coverages. We have observed that the Raman intensity tends to a constant value as the ZnPcTs concentration increases. These results are better seen in Figure 4. Note that for ZnPcTs concentrations above 1.75 × 10-4 mol/L, corresponding to the value calculated for a monolayer coverage, the Raman intensity tends to remain constant suggesting a surface saturation. The interaction of ZnPcTs with the maghemite surface is very strong suggesting that the adsorption equilibrium (17) Rubim, J. C.; Sousa, M. H.; Silva, J. C. O.; Tourinho, F. A. In Progress in Surface Raman Spectroscopy: Theory, Techniques & Applications; Tian, Z. Q., Ren, B., Eds.; Xiamen University Press: Xiamen, China, 2000; p 129. (18) Rubim, J. C.; Silva, J. C.; Sousa, M. H.; Tourinho, F. A. Braz. J. Phys. 2001, 31, 402.
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Figure 3. Raman spectra of the maghemite ferrofluid at the following ZnPcTs concentrations: (a) 3.50 × 10-5 mol/L, (b) 7.00 × 10-5 mol/L, and (c) 1.75 × 10-4 mol/L.
Figure 4. Raman intensity at 1517 cm-1 (2) and emission intensity for the maghemite ferrofluid band at 435 nm (0) and for the free ZnPcTs in solution at 680 nm (O) vs ZnPcTs concentration normalized by the ferrofluid concentration.
Figure 5. Emission spectra of maghemite ferrofluid solutions at the following ZnPcTs concentrations: (a) 0.00, (b) 1.74 × 10-6 mol/L, (c) and (d) 4.36 × 10-6 mol/L, (e) 6.10 × 10-6 mol/L, and (f) 8.75 × 10-6 mol/L.
is completely displaced in the direction of the adsorption process, that is, the ZnPcTs equilibrium concentration is much lower than that of the adsorbed ZnPcTs. Since ZnPcTs is a strong fluorophore, we decided to investigate the fluorescence spectra of ZnPcTs-modified maghemite solutions. Figure 5 displays emission spectra from a ZnPcTs-maghemite ferrofluid for different ZnPcTs concentrations. The emission in the region of magnetic fluid (λem ) 435 nm) was obtained by exciting at 268 nm, while the emission spectra in the region of the ZnPcTs (λem ) 680 nm) were obtained at 304 nm excitation. The emission band at ca. 435 nm is intrinsic of the magnetic fluid, and it increases in intensity as the ZnPcTs concentration increases. Note that for ZnPcTs concentrations equal to
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the adsorption equilibrium constant, and θ is the surface coverage. For the Raman measurements, we considered that the surface coverage θ is given by the expression
θ)
I Imax
where Imax is the Raman intensity at saturation and I is the Raman intensity corresponding to the ZnPcTs concentration. This means that saturation occurs at θ ) 1. For the florescence measurements, we considered the following expression:
θ) Figure 6. Langmuir type adsorption isotherm for the adsorption of ZnPcTs on the maghemite surface as obtained from Raman (0) and fluorescence (9) measurements.
or above 4.36 × 10-6 mol/L, the emission signal of the ZnPcTs in solution increases with the ZnPcTs concentration. This amount of ZnPcTs is the one estimated to obtain a monolayer coverage for the amount of ferrofluid used. For ZnPcTs concentrations lower than this value, no signal from the ZnPcTs in solution is observed. These results suggest, as expected, that the ZnPcTs, a tetra-anion, is strongly adsorbed on the positively charged maghemite surface and that after a monolayer coverage is achieved the excess of ZnPcTs stays in solution. The formation of aggregates and multilayers is quite impossible due to the electrostatic repulsion between the SO3- groups of different molecules. Mass spectrometry analysis of ZnPcTs solutions has shown that only the monomer is present in water solutions. The same could not be said for the disulfonated species, since even at 10-6 mol/L concentration the dimeric species was detected by mass spectrometry.15 To correlate the fluorescence signal and the surface coverage, a graphic displaying the area under the emission band at 435 nm versus ZnPcTs concentration in solution is displayed in Figure 4. For comparison purposes, the band intensity of the ZnPcTs emission at 680 nm as a function of ZnPcTs concentration is also displayed. Considering the ZnPcTs surface area and the maghemite average surface, as mentioned in the Experimental Section, the ZnPcTs concentration expected for a monolayer coverage is 4.36 × 10-6 mol/L. Note that a significant increase in the emission at 435 nm is observed for ZnPcTs concentrations near this value and that the ZnPcTs emission at 680 nm starts to be observed above this concentration level. Note also that the Raman and fluorescence results agree very well. The ZnPcTs concentration was divided by the ferrofluid concentration. This was done since the amount of maghemite used in each type of measurement (Raman and fluorescence) was different and, therefore, so was the total area. The maghemite concentration in the Raman measurements was 40 times larger than that used in the fluorescence experiment. Figure 6 exhibits a Langmuir type adsorption isotherm considering the results displayed in Figure 4 as obtained from Raman and fluorescence measurements. The Langmuir isotherm is given by the equation
c 1 ) +c θ K where c is the equilibrium concentration of ZnPcTs, K is
I - I0 Imax - I0
where I0 is the band intensity at 435 nm in the absence of ZnPcTs, Imax is the band intensity when the system saturates, and I is the band intensity corresponding to the ZnPcTs concentration. In both cases, we have normalized the ZnPcTs concentration by the ferrofluid concentration. The slopes found by the linear regression analysis were 0.693 and 0.805 for the Raman and fluorescence results, respectively. The difference between these values is ca. 15% which is reasonable considering the experimental error. The adsorption constants were 9.33 × 103 and 9.59 × 105 for the Raman and fluorescence data, respectively. To compare the adsorption constants obtained in the Raman and fluorescence experiments, we need to normalize them by the ferrofluid concentration. Doing so, the obtained values are 1.87 × 102 and 4.78 × 102, for Raman and fluorescence, respectively. These values have the same order of magnitude, indicating that as mentioned above the interaction of ZnPcTs with the maghemite surface is mainly governed by electrostatic forces. The expression for the Langmuir isotherm considers the adsorbate concentration at equilibrium. Note that we have used the ZnPcTs total concentration since we were not able to obtain the equilibrium concentration by any method at our disposal. Therefore, we believe that the adsorption equilibrium constant must be higher than the value obtained above, since the ZnPcTs equilibrium concentration is very low. For additions of ZnPcTs above the concentration calculated for a monolayer coverage, the colloidal solutions are no longer stable and flocculation occurs. The aggregation of the modified maghemite particles can explain the nonlinear response of the emission intensity at 680 nm on the ZnPcTs concentration. Indeed, we have observed that modified ferrofluid solutions corresponding to 1.22.0 monolayers of ZnPcTs present solid particles deposited on the flask bottom after 1 h of preparation. This colloidal instability, or aggregation, is due to the fact that the ZnPcTs negative charges start to neutralize the positively charged maghemite particle surface, that is, the electrostatic repulsive forces responsible for the colloid stability become weaker and particles tend to aggregate. To verify the magnetic property of the ZnPcTs-modified maghemite ferrofluids, we have performed some measurements in a modified Gouy balance.7 In this experiment, we have measured the apparent mass change induced by a magnetic field on the magnetic fluid. Figure 7 displays the observed mass changes for the maghemite and ZnPcTsmodified maghemites at different surface coverages. These results show that the deflection remains constant up to a ZnPcTs surface coverage of ca. 0.5, indicating that the magnetic properties of the ZnPcTs-modified maghemite
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(i) Resonance Raman spectroscopy has proved to be a powerful tool in the characterization of modified magnetic fluids, making possible the identification of the surface species even in solution which could not be done by infrared spectroscopy. (ii) The ZnPcTs has a C4h symmetry that is not modified upon adsorption on the maghemite surface. Its interaction with the maghemite surface is governed by an adsorption equilibrium constant of the order of 102 which is consistent with an interaction based on electrostatic forces. (iii) ZnPcTs maghemite ferrofluids with a ZnPcTs coverage close to 0.5 are stable and are fluorescent, which is an interesting result for photodynamic therapy. Figure 7. Apparent mass change measurements of the ZnPcTsmodified maghemite magnetic fluids with different surface coverage after (0) 1 h and (2) 24 h of sample preparation.
ferrofluids are unchanged. The mass changes remain almost constant after 24 h of sample preparation. For ZnPcTs surface coverages above 0.5, the deflection decreases, reducing the apparent mass change under the magnetic field. This result can be interpreted as the modified maghemite particles forming aggregates causing a decrease on the colloidal stability of the ferrofluid. For surface coverages larger than 1.0, the aggregation is so high that precipitation starts to occur, decreasing even more the measured deflection. Conclusion The results presented in this work lead to the following conclusions:
(iv) Maghemite ferrofluids modified by the adsorption of ZnPcTs form stable magnetic fluids for ZnPcTs coverages between 0.1 and 0.5. For higher coverages, the colloidal stability degenerates, and flocculation occurs for surface coverages larger than 1. The magnetic properties of the ZnPcTs-modified maghemites for θ e 0.5 remain stable for at least 24 h after preparation. Acknowledgment. The authors greatly thank the Laborato´rio de Espectroscopia Molecular do Instituto de Quı´mica da USP where the Raman measurements were performed. The authors also thank the Laborato´rio de Fotoquı´mica do Instituto de Quı´mica da USP where the fluorescence measurements were done. Fellowship grants from CNPq are also acknowledged. LA020078O
