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Magnetic and spectroscopic properties of Polyacrylamide-CoFe2O4 magnetic hydrogel
Journal of Molecular Structure 1036 (2013) 386–391
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc
Magnetic and spectroscopic properties of Polyacrylamide-CoFe2O4 magnetic hydrogel E. Alverog˘lu a, H. Sözeri b,⇑, U. Kurtan c, M. Sß enel c, A. Baykal c a
Department of Physics, Istanbul Technical University, 34469 Maslak, Istanbul, Turkey TUBITAK-UME, National Metrology Institute, PO Box 54, 41470 Gebze-Kocaeli, Turkey c Department of Chemistry, Fatih University, 34500 B. Cekmece, Istanbul, Turkey b
h i g h l i g h t s " Polyacrylamide (PAAm) hydrogels containing magnetic CoFe2O4 nanoparticles (NPs) have been synthesized. " Pyranine (POH) molecules were used as fluoroprobe to investigate interaction of NPs with polymer strands. " CoFe2O4 NPs have trapped in the gel so that it cannot move through the gel even the gel is swollen. " CoFe2O4 NPs have superparamagnetic behavior. " These features make the gel very suitable for applications like magnetic adsorbents.
a r t i c l e
i n f o
Article history: Received 8 August 2012 Received in revised form 31 October 2012 Accepted 5 December 2012 Available online 19 December 2012 Keywords: Magnetic hydrogels Magnetic properties Electrical properties Fluorescence spectroscopy
a b s t r a c t This study investigates synthesis and characterization of polyacrylamide (PAAm) hydrogels containing ferromagnetic CoFe2O4 nanoparticles. Structural, electrical, and magnetic characterization of the gels have been performed with X-ray powder diffractometry, Scanning electron microscopy, DC conductivity, magnetization and fluorescence spectroscopy techniques. Fluorescence and electrical measurements show that nanoparticles have trapped in the gel so they cannot move through the gel even if the gel is swollen and the voltage is applied. Pyranine molecules diffuse easily through the gel due to the presence of ferromagnetic nanoparticles. As number of diffused pyranine molecules increases current densities of the magnetic hydrogel increase. Magnetization measurements reveal that CoFe2O4 nanoparticles do not diffuse out of the gel during swelling. As a result, total magnetization of the gel do not change as volume of the gel increases. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction The interest in functionalized hydrogels has been increased due to the possible applications of these materials in controlled drug release, water treatment, biosensors, contrast agents, etc. Depending on the application, hydrogels can be functionalized by using some additives like cell penetrating peptides [1], aniline oligomers [2] and magnetic nanoparticles [3–6]. An approach often utilized in the synthesis of magnetic hydrogels is cross-linking the gel in the presence of magnetic nanoparticles. The impact of the embedding of iron oxide nanoparticles (Fe2O3) to the formation and structure of poly(acrylamide) (PAAm) hydrogels has been investigated in several works [7,8]. Cobalt ferrite (CoFe2O4) is another interesting ferromagnetic material because of its very high magneto crystalline anisotropy accompanied by a reasonable saturation ⇑ Corresponding author. Tel.: +90 262 679 5000; fax: +90 262 679 5001. E-mail address: [email protected] (H. Sözeri). 0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molstruc.2012.12.009
magnetization [9–11]. Therefore, it can be used to prepare a hydrogel sensitive to the external stimuli of magnetic field. In a recently published work, carbonyl-coated magnetic cobalt nanoparticles were surface-functionalized with a vinyl group and integrated covalently in a poly(2-hydroxyethyl methacrylate) (PHEMA) hydrogel network cross-linked by ethylene glycol dimethacrylate (EGDMA) by a free-radical polymerization process. High particle loading of up to 60 wt.% without effecting the stability was succeeded [12]. In the literature, fluoroprobes are widely used for monitoring various processes and functions on a microscopic level [13,14]. This technique is based on the interpretation of the change in anisotropy, emission spectra or intensity and viewing the lifetimes of injected fluoroprobes to monitor the change in their microenvironment [15–18]. Because of their special features, fluorescence spectroscopy is sensitive and powerful method for analysis of many compounds. In our previous work, we have investigated the interaction of Co2+ ions with pyranine molecules which are
E. Alverog˘lu et al. / Journal of Molecular Structure 1036 (2013) 386–391
used as fluoroprobe. We have shown that Co2+ ions and pyranine molecules form stable clusters which can be obtained only during polymerization [5]. In another work, photoluminescence properties of CoFe2O4– Cr2O3–SiO2 magnetic nanocomposite has been investigated by without addition of any external fluorescent marker. TEM images have evidenced the formation of spherical cobalt ferrite nanoparticles homogenously dispersed inside the silica matrix, with diameters below 5 nm for annealing temperature of 600 °C. The fluorescent nanocomposites were further utilized for staining the cultured HeLa cells for fluorescence imaging detection. New type of CoFe2O4–Cr2O3–SiO2 fluorescence magnetic nanocomposite has been prepared and utilized as photocatalyst for degradation of methylene blue in aqueous solution [19–21]. Moreover there is few studies in literature about spectroscopic properties of CoFe2O4 nanoparticle system [22,23]. In these studies it is seen that there are five resonance scattering peaks at 400, 470, 510, 800 and 940 nm for aqueous CoFe2O4 nanoparticles. It is a nonlinear scattering medium. Additionally, motility of CoFe2O4 nanoparticle-labeled microtubules in magnetic fields has been studied [23]. This work involves in situ synthesis of CoFe2O4 nanoparticles in PAAm gel and investigation of interactions between fluoroprobe molecules and CoFe2O4 nanoparticles by fluorescence and conductivity measurements. Besides, structural properties of hydrogels have been investigated by X-ray diffractometry, scanning/transmission electron microscopy and FT-IR spectroscopy. Stability of hydrogels were tested by magnetization measurements with samples having different swelling ratios. 2. Experimental
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Transmission electron microscopy (TEM) analysis was performed using a JEOL JEM 2100 microscope. A drop of diluted sample in alcohol was dripped on a TEM grid. Varian Cary Eclipse fluorescence spectrophotometer was used for fluorescence measurements. Model 6487 Picoammeter/Voltage Source Keithley was used for DC electrical measurements. 2.3. In situ synthesis of CoFe2O4 nanoparticles in PAAm gel Poly(acrylamide) (PAAm) hydrogel was synthesized by employing free radical polymerization using N,N-methylenebisacrylamide (MBA) as a cross-linker and APS/TEMED as a redox-initiating pair following the usual procedure [24]. 300 mg of acrylamide (Am) was dissolved in pure water. After the Am was dissolved, 4 mg of MBA was added to the solution as a crosslinker. The polymerization was initiated by adding 15 mg of APS and 4 ll of TEMED to the monomer mixture. The solution turned into a highly viscous liquid immediately which finally formed into a solid gel within a few minutes at 25 °C. To be sure the reaction was complete; it was left for 24 h. The gels obtained were removed from petri dishes and washed with distilled water to get rid of any unreacted compounds. Then, PAAm hydrogel were immersed in iron (III) nitrate tetrahydrate (Fe(NO3)34H2O) and Cobalt (II) nitrate monohydrate (Co(NO3)2H2O) (molar ratio of Co to Fe is 1:2) containing solution. After one day, iron and cobalt ion absorbed PAAm hydrogel was immersed in 2 M ammonia solution. The prepared PAAm–CoFe2O4 magnetic hydrogel then immersed in 10 4 M pyranine solution in pure water (5.3 10 2 mg/ml) and hold 1 day for the adsorption of the fluorescence probe. The synthesis of PAAmCoFe2O4 magnetic hydrogel was simply shown in Scheme 1.
2.1. Chemicals 3. Results and discussion Acrylamide (Am), Fe(NO3)34H2O, Co(NO3)2H2O, N,N-methylenebis(acrylamide) (MBA), NaOH, ammonium persulfate (APS), N,N,N,N-tetramethylethylenediamine (TEMED), 8-hydroxypyrene1,3,6-trisulfonic acid (pyranine). 2.2. Instrumentations Structural and morphology of the samples were investigated by Fourier transform infrared spectroscopy (FT-IR), X-ray diffractometry (XRD), (TGA) and high resolution transmission electron microscopy (HR-TEM). The crystallite size and phase fractions of the samples were determined from the XRD patterns (Rigaku Smart Lab, Cu Ka radiation). Fourier transform infrared (FT-IR) spectra were recorded in transmission mode with a Perkin Elmer BX FT-IR infrared spectrometer. The powder samples were ground with KBr and compressed into a pellet. FTIR spectra in the range 4000–400 cm 1 were recorded in order to investigate the nature of the chemical bonds formed. The magnetic characterization of the samples was performed at room temperature using a vibrating sample magnetometer (LDJ Electronics Inc., Model 9600) in an applied field of 15 kOe.
3.1. XRD analysis Phase investigation of the crystallized product was performed by XRD and the powder diffraction pattern of PolyacrylamideCoFe2O4 magnetic hydrogel is presented in Fig. 1. The broad area between 2h = 20–30° is due to the PAAm gel. The XRD powder pattern indicates that the product is CoFe2O4, and the diffraction peaks are broadened owing to very small crystallite size. All of the observed diffraction peaks are indexed by the cubic structure of CoFe2O4 (JCPDS No. 22-1086) revealing a high phase purity of the product. The mean size of the crystallites was estimated from the diffraction pattern by using the Scherrer equation [25]. The observed seven peaks were fitted for with the following Miller indices: (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1) and (4 4 0). The average crystallite size, D, was obtained as 11 nm. 3.2. TEM–SEM analysis Morphology of the samples can be determined either by scanning electron microscopic (SEM) and transmission electron microscopic (TEM) studies. The surface morphology of Polyacrylamide-CoFe2O4 magnetic hydrogel is shown in Fig. 2. A randomly
Scheme 1. Schematical representation of in situ synthesis of Polyacrylamide-CoFe2O4 magnetic hydrogel.
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Fig. 1. XRD pattern of Polyacrylamide-CoFe2O4 magnetic hydrogel.
aggregated structure throughout the gel network was observed from SEM micrographs (Fig. 2a). TEM micrographs (Fig. 2b) clearly illustrate the formation of well defined magnetic nanostructures throughout the hydrogel networks. 3.3. FT-IR analysis The FT-IR spectra of Polyacrylamide-CoFe2O4 magnetic hydrogel is shown in Fig. 3. After heating the product at 100 °C, pellet was prepared for the FT-IR measurements. The inorganic lattice vibration for Fe3O4 appears in the range 400–700 cm 1. As prepared powder presents characteristic peaks that are exhibited by the commercial magnetite powder: metal–oxygen band, m1, observed at 590 cm 1 corresponds to intrinsic stretching vibrations of the metal at tetrahedral site (Mtetra ? O), whereas metal–oxygen band observed at 445 cm 1; m2, is assigned to octahedral-metal stretching (Mocta ? O) [26–29]. FT-IR spectrum of nanocomposites hydrogels are shown in Fig. 2. Magnetic hydrogel showed a broad band in the range of 3400–3585 cm 1, attributed to the ANH asymmetric and AOH. The absorption bands at 2924 cm 1 that appeared in all hydrogel spectrums is resulting from stretching frequency of ACH3 groups [30]. Poly(acrylamide) (PAAm) hydrogel exhibited broad bands at 3430 and 1664 cm 1 due to amide groups stretching of poly(acrylamide) chains [31]. 3.4. Swelling ratio of the gels The neat and magnetic hydrogel were dried completely at 40 °C. Then, the dried gels were swollen in excess of distilled water and the masses of the swelling gels were measured with balance at certain time periods. As seen from Fig. 4 that swelling ratio (mGel/mDried Gel) of CoFe2O4-PAAm magnetic hydrogel excessively higher than neat PAAm gel. Nanomagnetic particles changed the gel morphology and gel was turn into more homogeny so they were swollen much more than their neat form. The hydrogels loaded with iron and cobalt ions were treated with ammonia and nanoparticles are formed immediately. They are stabilized inside the hydrogel networks. During formation of the particles the free space within the hydrogel networks was slightly increased which allows diffusion of more water molecules. As a result, higher swelling capacity was observed in CoFe2O4 containing hydrogels compared to pure one. 3.5. Magnetization measurements Magnetization curves of pure CoFe2O4 particles and hydrogels containing CoFe2O4 nanoparticles are shown in Fig. 5a and b,
respectively. They exhibit no remanence and no coercivity at room temperature. Besides, magnetization of the particles does not reach to a saturation even at high external fields and is low compared to bulk magnetization of this material (74.08 emu/g) [32]. These are all typical features of superparamagnetic particles whose grain sizes are within the single domain limit. For such particles, magnetization reversal occurs due to the coherent rotation of spins. Reduced magnetization can be explained by surface the difference in spin ordering at the surface of particles over that in the bulk [33,34]. The surface has negligible magnetization and behaves like a death layer. Surface effects dominates the properties of the nanoparticles since decreasing the particle size increases the surface to core spin ratio. Magnetic behavior of superparamagnetic particles can be described by Langevin function with the assumption that they are weakly or non-interacting particles. Average particle size can be determined by fitting Langevin function to the measured M–H hysteresis curve. Thus, mean magnetic moment (l) of particles can be determined. When this is inserted in l = MspqD3/6, where q is the density of the magnetite particles 5.3 g/cm3, and average particle size is found to be 16 ± 1 nm. Magnetic hydrogel was swelled in different mass ratios, ms/m0, from 1 to 10 to investigate whether there is a change in the magnetic properties. Swollen gels appeared to be the same magnetization with the collapsed one, as presented in Fig. 5b. This indicates that CoFe2O4 nanoparticles do not diffuse out of the gel during water intake. This feature may be very suitable for industrial applications of such gels, for example, in waste water treatment and metal extraction from the polluted water. 3.6. Spectroscopic measurements Fig. 6 shows emission spectra of gel and solution which consists nanoparticles and dye molecules for 390 nm and 400 nm excitation. All curves exhibit free pyranine emission (510 nm wavelength peak) [35–37] so pyranine molecules can diffuse into gel which is containing nanoparticle. Additionally, there is a peak at 420 nm wavelength, this peak seem from only the spectra taken from the gel. When pyranine molecules diffused into the gel, they are affected by pH values of nanoparticles embedded gel media and acted as in lower pH [36]. This 420 nm wavelength peak does not seem when pyranine molecules diffused into neat PAAm gel [36]. In Ref. [36] this peak is placed at longer wavelength (430 nm), so we can say that nanoparticles affected pH value of gels media and thus pyranine molecules act as in lower pH media. But there is no other peak except from the free pyranine peaks in spectrum taken from solution. This shows that pyranine molecules and nanoparticles does not affected each other in the solution. Pyranine molecules emit the light as if in lower pH when nanoparticles embedded in the neat PAAm gel. Excitation spectra of solution are seen in Fig. 7. These spectra are extremely different from well known absorption spectrum of pyranine [38,39]. We do not give the absorption spectrum of pyranine to avoid of repetition. All absorption peaks shift to shorter wavelength when solution contained nanoparticles. This shifting shows that nanoparticles affected electrostatically to energy levels of pyranine [40]. 3.7. Electrical measurements DC electric measurement carried on gel prepared in the slice shapes of thickness 2 mm. Gel was swollen to a certain swelling ratio (mswollen gel/mdried gel) in pure water, and then placed between platinum electrodes, and sealed from the air against drying. The current densities per unit mass, J/mgel, of the gel were measured against the applied voltage (Fig. 8) and the swelling ratio (Fig. 9)
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Fig. 2. (a) SEM and (b) TEM images of Polyacrylamide-CoFe2O4 magnetic hydrogel.
Fig. 3. FT-IR spectra of CoFe2O4-PAAm magnetic hydrogel.
Fig. 4. The ratio, mGel/mDried Gel vs. swelling time, t, for neat PAAm gel and CoFe2O4 nanoparticles containing PAAm gel at room temperature.
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Fig. 5. M–H hysteresis curves of (a) CoFe2O4 nanoparticles, (b) CoFe2O4 containing PAAm hydrogels with different swelling ratios.
of the gels. Here, we should note that conduction occurs due the presence of charged ions in the gel. Possible free charges in our system are H+ and OH ions coming from water entrapped in the gel, Na+ ions and SO3 subgroups due to the pyranines [41,42]. All of these charges have contribution to the conduction when gel is swollen, i.e., m/m0 > 1. Fig. 8 shows that normalized current densities increase with increasing applied voltage. If gel is dried, there is no current pass through the gel since ions cannot move if the gel is in collapse state [41,42]. When the gel is swollen, current starts to flow. Peaks, seen in current density–swelling ratio curves, correspond to different blobs in PAAm gels [41–43]. The structure of the PAAm gel can be regarded as a set of blobs (or clusters) with long polymer chains, which connects these blobs to each other. When PAAm gels swell, these blobs are rearranged in such a way that small ones are expelled from the big ones, creating low concentration regions. It is seen that one small and two big peaks (i.e., blobs) in Fig. 9, but neat Fig. 6. Emission spectra of CoFe2O4–PAAm magnetic hydrogel and solution for 390 nm and 400 nm excitation wavelength.
Fig. 7. Excitation spectra of solution for 420 nm and 510 nm wavelength emission.
Fig. 8. Normalized current density vs. applied voltage curves for varying swelling ratio of CoFe2O4–PAAm magnetic hydrogel.
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References
Fig. 9. Normalized current density vs. swelling ratio of CoFe2O4–PAAm magnetic hydrogel curves for varying applied voltage.
PAAm gel shows three more distinct peaks [41,42]. Nanoparticles synthesized in situ in PAAm gel causes the some peaks less distinguishable. So, gels got more homogeneous than their neat form. Yilmaz et al. showed that current density per unit mass decreases P Bij t exponentially in time and can be expressed as mJ [42], ij Aij e gel the number of terms in the series corresponds to the number of generations of these blobs. Moreover, magnitude of current density of magnetic hydrogel for an applied voltage of 10 V and having swelling ratio of 2.16 is 140 mA g 1 cm2, see Figs. 8 and 9. This value is much bigger than that of the pyranine doped PAAm gel without magnetic filler (7 mA g 1 cm 2) as presented in Ref. [41]. This means that magnetic nanoparticles made diffusion of pyranine molecules into the gel easier. 4. Conclusion The ferromagnetic CoFe2O4 nanoparticles were successfully synthesized by in situ in polyacrylamide hydrogels. Electrical and spectroscopic measurements show that synthesizing nanoparticles by in situ in PAAm gel causes the gel to become more homogeneous so that fluoroprobe molecules can diffuse easily. Magnitude of the DC current (140 mA g 1 cm 2) increases considerably due to the presence of nanoparticles, which enable the gel suck up more ionic molecules, compared to that of the pure PAAm gel (7 mA g 1 cm 2) see Ref. [41]. Moreover nanoparticles change the absorption of pyranine molecules diffused into the gel, because energy level of pyranine molecules were changed by nanoparticles and this causes the shifting in excitation spectrum. The synthesized gel appeared to be magnetically stable as swelling ratio of the gel increases. In other words, magnetization of the gel does not change during the water intake. This property may be suitable for applications of waste water treatment and metal extraction from the polluted water. Acknowledgements This work is supported by Fatih University under BAP Grant no. P50021104-B. H. Sözeri thanks to Mr. C. Berk and Dr. Ö. Duygulu for taking SEM and TEM micrographs.
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Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc
Magnetic and spectroscopic properties of Polyacrylamide-CoFe2O4 magnetic hydrogel E. Alverog˘lu a, H. Sözeri b,⇑, U. Kurtan c, M. Sß enel c, A. Baykal c a
Department of Physics, Istanbul Technical University, 34469 Maslak, Istanbul, Turkey TUBITAK-UME, National Metrology Institute, PO Box 54, 41470 Gebze-Kocaeli, Turkey c Department of Chemistry, Fatih University, 34500 B. Cekmece, Istanbul, Turkey b
h i g h l i g h t s " Polyacrylamide (PAAm) hydrogels containing magnetic CoFe2O4 nanoparticles (NPs) have been synthesized. " Pyranine (POH) molecules were used as fluoroprobe to investigate interaction of NPs with polymer strands. " CoFe2O4 NPs have trapped in the gel so that it cannot move through the gel even the gel is swollen. " CoFe2O4 NPs have superparamagnetic behavior. " These features make the gel very suitable for applications like magnetic adsorbents.
a r t i c l e
i n f o
Article history: Received 8 August 2012 Received in revised form 31 October 2012 Accepted 5 December 2012 Available online 19 December 2012 Keywords: Magnetic hydrogels Magnetic properties Electrical properties Fluorescence spectroscopy
a b s t r a c t This study investigates synthesis and characterization of polyacrylamide (PAAm) hydrogels containing ferromagnetic CoFe2O4 nanoparticles. Structural, electrical, and magnetic characterization of the gels have been performed with X-ray powder diffractometry, Scanning electron microscopy, DC conductivity, magnetization and fluorescence spectroscopy techniques. Fluorescence and electrical measurements show that nanoparticles have trapped in the gel so they cannot move through the gel even if the gel is swollen and the voltage is applied. Pyranine molecules diffuse easily through the gel due to the presence of ferromagnetic nanoparticles. As number of diffused pyranine molecules increases current densities of the magnetic hydrogel increase. Magnetization measurements reveal that CoFe2O4 nanoparticles do not diffuse out of the gel during swelling. As a result, total magnetization of the gel do not change as volume of the gel increases. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction The interest in functionalized hydrogels has been increased due to the possible applications of these materials in controlled drug release, water treatment, biosensors, contrast agents, etc. Depending on the application, hydrogels can be functionalized by using some additives like cell penetrating peptides [1], aniline oligomers [2] and magnetic nanoparticles [3–6]. An approach often utilized in the synthesis of magnetic hydrogels is cross-linking the gel in the presence of magnetic nanoparticles. The impact of the embedding of iron oxide nanoparticles (Fe2O3) to the formation and structure of poly(acrylamide) (PAAm) hydrogels has been investigated in several works [7,8]. Cobalt ferrite (CoFe2O4) is another interesting ferromagnetic material because of its very high magneto crystalline anisotropy accompanied by a reasonable saturation ⇑ Corresponding author. Tel.: +90 262 679 5000; fax: +90 262 679 5001. E-mail address: [email protected] (H. Sözeri). 0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molstruc.2012.12.009
magnetization [9–11]. Therefore, it can be used to prepare a hydrogel sensitive to the external stimuli of magnetic field. In a recently published work, carbonyl-coated magnetic cobalt nanoparticles were surface-functionalized with a vinyl group and integrated covalently in a poly(2-hydroxyethyl methacrylate) (PHEMA) hydrogel network cross-linked by ethylene glycol dimethacrylate (EGDMA) by a free-radical polymerization process. High particle loading of up to 60 wt.% without effecting the stability was succeeded [12]. In the literature, fluoroprobes are widely used for monitoring various processes and functions on a microscopic level [13,14]. This technique is based on the interpretation of the change in anisotropy, emission spectra or intensity and viewing the lifetimes of injected fluoroprobes to monitor the change in their microenvironment [15–18]. Because of their special features, fluorescence spectroscopy is sensitive and powerful method for analysis of many compounds. In our previous work, we have investigated the interaction of Co2+ ions with pyranine molecules which are
E. Alverog˘lu et al. / Journal of Molecular Structure 1036 (2013) 386–391
used as fluoroprobe. We have shown that Co2+ ions and pyranine molecules form stable clusters which can be obtained only during polymerization [5]. In another work, photoluminescence properties of CoFe2O4– Cr2O3–SiO2 magnetic nanocomposite has been investigated by without addition of any external fluorescent marker. TEM images have evidenced the formation of spherical cobalt ferrite nanoparticles homogenously dispersed inside the silica matrix, with diameters below 5 nm for annealing temperature of 600 °C. The fluorescent nanocomposites were further utilized for staining the cultured HeLa cells for fluorescence imaging detection. New type of CoFe2O4–Cr2O3–SiO2 fluorescence magnetic nanocomposite has been prepared and utilized as photocatalyst for degradation of methylene blue in aqueous solution [19–21]. Moreover there is few studies in literature about spectroscopic properties of CoFe2O4 nanoparticle system [22,23]. In these studies it is seen that there are five resonance scattering peaks at 400, 470, 510, 800 and 940 nm for aqueous CoFe2O4 nanoparticles. It is a nonlinear scattering medium. Additionally, motility of CoFe2O4 nanoparticle-labeled microtubules in magnetic fields has been studied [23]. This work involves in situ synthesis of CoFe2O4 nanoparticles in PAAm gel and investigation of interactions between fluoroprobe molecules and CoFe2O4 nanoparticles by fluorescence and conductivity measurements. Besides, structural properties of hydrogels have been investigated by X-ray diffractometry, scanning/transmission electron microscopy and FT-IR spectroscopy. Stability of hydrogels were tested by magnetization measurements with samples having different swelling ratios. 2. Experimental
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Transmission electron microscopy (TEM) analysis was performed using a JEOL JEM 2100 microscope. A drop of diluted sample in alcohol was dripped on a TEM grid. Varian Cary Eclipse fluorescence spectrophotometer was used for fluorescence measurements. Model 6487 Picoammeter/Voltage Source Keithley was used for DC electrical measurements. 2.3. In situ synthesis of CoFe2O4 nanoparticles in PAAm gel Poly(acrylamide) (PAAm) hydrogel was synthesized by employing free radical polymerization using N,N-methylenebisacrylamide (MBA) as a cross-linker and APS/TEMED as a redox-initiating pair following the usual procedure [24]. 300 mg of acrylamide (Am) was dissolved in pure water. After the Am was dissolved, 4 mg of MBA was added to the solution as a crosslinker. The polymerization was initiated by adding 15 mg of APS and 4 ll of TEMED to the monomer mixture. The solution turned into a highly viscous liquid immediately which finally formed into a solid gel within a few minutes at 25 °C. To be sure the reaction was complete; it was left for 24 h. The gels obtained were removed from petri dishes and washed with distilled water to get rid of any unreacted compounds. Then, PAAm hydrogel were immersed in iron (III) nitrate tetrahydrate (Fe(NO3)34H2O) and Cobalt (II) nitrate monohydrate (Co(NO3)2H2O) (molar ratio of Co to Fe is 1:2) containing solution. After one day, iron and cobalt ion absorbed PAAm hydrogel was immersed in 2 M ammonia solution. The prepared PAAm–CoFe2O4 magnetic hydrogel then immersed in 10 4 M pyranine solution in pure water (5.3 10 2 mg/ml) and hold 1 day for the adsorption of the fluorescence probe. The synthesis of PAAmCoFe2O4 magnetic hydrogel was simply shown in Scheme 1.
2.1. Chemicals 3. Results and discussion Acrylamide (Am), Fe(NO3)34H2O, Co(NO3)2H2O, N,N-methylenebis(acrylamide) (MBA), NaOH, ammonium persulfate (APS), N,N,N,N-tetramethylethylenediamine (TEMED), 8-hydroxypyrene1,3,6-trisulfonic acid (pyranine). 2.2. Instrumentations Structural and morphology of the samples were investigated by Fourier transform infrared spectroscopy (FT-IR), X-ray diffractometry (XRD), (TGA) and high resolution transmission electron microscopy (HR-TEM). The crystallite size and phase fractions of the samples were determined from the XRD patterns (Rigaku Smart Lab, Cu Ka radiation). Fourier transform infrared (FT-IR) spectra were recorded in transmission mode with a Perkin Elmer BX FT-IR infrared spectrometer. The powder samples were ground with KBr and compressed into a pellet. FTIR spectra in the range 4000–400 cm 1 were recorded in order to investigate the nature of the chemical bonds formed. The magnetic characterization of the samples was performed at room temperature using a vibrating sample magnetometer (LDJ Electronics Inc., Model 9600) in an applied field of 15 kOe.
3.1. XRD analysis Phase investigation of the crystallized product was performed by XRD and the powder diffraction pattern of PolyacrylamideCoFe2O4 magnetic hydrogel is presented in Fig. 1. The broad area between 2h = 20–30° is due to the PAAm gel. The XRD powder pattern indicates that the product is CoFe2O4, and the diffraction peaks are broadened owing to very small crystallite size. All of the observed diffraction peaks are indexed by the cubic structure of CoFe2O4 (JCPDS No. 22-1086) revealing a high phase purity of the product. The mean size of the crystallites was estimated from the diffraction pattern by using the Scherrer equation [25]. The observed seven peaks were fitted for with the following Miller indices: (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1) and (4 4 0). The average crystallite size, D, was obtained as 11 nm. 3.2. TEM–SEM analysis Morphology of the samples can be determined either by scanning electron microscopic (SEM) and transmission electron microscopic (TEM) studies. The surface morphology of Polyacrylamide-CoFe2O4 magnetic hydrogel is shown in Fig. 2. A randomly
Scheme 1. Schematical representation of in situ synthesis of Polyacrylamide-CoFe2O4 magnetic hydrogel.
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Fig. 1. XRD pattern of Polyacrylamide-CoFe2O4 magnetic hydrogel.
aggregated structure throughout the gel network was observed from SEM micrographs (Fig. 2a). TEM micrographs (Fig. 2b) clearly illustrate the formation of well defined magnetic nanostructures throughout the hydrogel networks. 3.3. FT-IR analysis The FT-IR spectra of Polyacrylamide-CoFe2O4 magnetic hydrogel is shown in Fig. 3. After heating the product at 100 °C, pellet was prepared for the FT-IR measurements. The inorganic lattice vibration for Fe3O4 appears in the range 400–700 cm 1. As prepared powder presents characteristic peaks that are exhibited by the commercial magnetite powder: metal–oxygen band, m1, observed at 590 cm 1 corresponds to intrinsic stretching vibrations of the metal at tetrahedral site (Mtetra ? O), whereas metal–oxygen band observed at 445 cm 1; m2, is assigned to octahedral-metal stretching (Mocta ? O) [26–29]. FT-IR spectrum of nanocomposites hydrogels are shown in Fig. 2. Magnetic hydrogel showed a broad band in the range of 3400–3585 cm 1, attributed to the ANH asymmetric and AOH. The absorption bands at 2924 cm 1 that appeared in all hydrogel spectrums is resulting from stretching frequency of ACH3 groups [30]. Poly(acrylamide) (PAAm) hydrogel exhibited broad bands at 3430 and 1664 cm 1 due to amide groups stretching of poly(acrylamide) chains [31]. 3.4. Swelling ratio of the gels The neat and magnetic hydrogel were dried completely at 40 °C. Then, the dried gels were swollen in excess of distilled water and the masses of the swelling gels were measured with balance at certain time periods. As seen from Fig. 4 that swelling ratio (mGel/mDried Gel) of CoFe2O4-PAAm magnetic hydrogel excessively higher than neat PAAm gel. Nanomagnetic particles changed the gel morphology and gel was turn into more homogeny so they were swollen much more than their neat form. The hydrogels loaded with iron and cobalt ions were treated with ammonia and nanoparticles are formed immediately. They are stabilized inside the hydrogel networks. During formation of the particles the free space within the hydrogel networks was slightly increased which allows diffusion of more water molecules. As a result, higher swelling capacity was observed in CoFe2O4 containing hydrogels compared to pure one. 3.5. Magnetization measurements Magnetization curves of pure CoFe2O4 particles and hydrogels containing CoFe2O4 nanoparticles are shown in Fig. 5a and b,
respectively. They exhibit no remanence and no coercivity at room temperature. Besides, magnetization of the particles does not reach to a saturation even at high external fields and is low compared to bulk magnetization of this material (74.08 emu/g) [32]. These are all typical features of superparamagnetic particles whose grain sizes are within the single domain limit. For such particles, magnetization reversal occurs due to the coherent rotation of spins. Reduced magnetization can be explained by surface the difference in spin ordering at the surface of particles over that in the bulk [33,34]. The surface has negligible magnetization and behaves like a death layer. Surface effects dominates the properties of the nanoparticles since decreasing the particle size increases the surface to core spin ratio. Magnetic behavior of superparamagnetic particles can be described by Langevin function with the assumption that they are weakly or non-interacting particles. Average particle size can be determined by fitting Langevin function to the measured M–H hysteresis curve. Thus, mean magnetic moment (l) of particles can be determined. When this is inserted in l = MspqD3/6, where q is the density of the magnetite particles 5.3 g/cm3, and average particle size is found to be 16 ± 1 nm. Magnetic hydrogel was swelled in different mass ratios, ms/m0, from 1 to 10 to investigate whether there is a change in the magnetic properties. Swollen gels appeared to be the same magnetization with the collapsed one, as presented in Fig. 5b. This indicates that CoFe2O4 nanoparticles do not diffuse out of the gel during water intake. This feature may be very suitable for industrial applications of such gels, for example, in waste water treatment and metal extraction from the polluted water. 3.6. Spectroscopic measurements Fig. 6 shows emission spectra of gel and solution which consists nanoparticles and dye molecules for 390 nm and 400 nm excitation. All curves exhibit free pyranine emission (510 nm wavelength peak) [35–37] so pyranine molecules can diffuse into gel which is containing nanoparticle. Additionally, there is a peak at 420 nm wavelength, this peak seem from only the spectra taken from the gel. When pyranine molecules diffused into the gel, they are affected by pH values of nanoparticles embedded gel media and acted as in lower pH [36]. This 420 nm wavelength peak does not seem when pyranine molecules diffused into neat PAAm gel [36]. In Ref. [36] this peak is placed at longer wavelength (430 nm), so we can say that nanoparticles affected pH value of gels media and thus pyranine molecules act as in lower pH media. But there is no other peak except from the free pyranine peaks in spectrum taken from solution. This shows that pyranine molecules and nanoparticles does not affected each other in the solution. Pyranine molecules emit the light as if in lower pH when nanoparticles embedded in the neat PAAm gel. Excitation spectra of solution are seen in Fig. 7. These spectra are extremely different from well known absorption spectrum of pyranine [38,39]. We do not give the absorption spectrum of pyranine to avoid of repetition. All absorption peaks shift to shorter wavelength when solution contained nanoparticles. This shifting shows that nanoparticles affected electrostatically to energy levels of pyranine [40]. 3.7. Electrical measurements DC electric measurement carried on gel prepared in the slice shapes of thickness 2 mm. Gel was swollen to a certain swelling ratio (mswollen gel/mdried gel) in pure water, and then placed between platinum electrodes, and sealed from the air against drying. The current densities per unit mass, J/mgel, of the gel were measured against the applied voltage (Fig. 8) and the swelling ratio (Fig. 9)
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Fig. 2. (a) SEM and (b) TEM images of Polyacrylamide-CoFe2O4 magnetic hydrogel.
Fig. 3. FT-IR spectra of CoFe2O4-PAAm magnetic hydrogel.
Fig. 4. The ratio, mGel/mDried Gel vs. swelling time, t, for neat PAAm gel and CoFe2O4 nanoparticles containing PAAm gel at room temperature.
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Fig. 5. M–H hysteresis curves of (a) CoFe2O4 nanoparticles, (b) CoFe2O4 containing PAAm hydrogels with different swelling ratios.
of the gels. Here, we should note that conduction occurs due the presence of charged ions in the gel. Possible free charges in our system are H+ and OH ions coming from water entrapped in the gel, Na+ ions and SO3 subgroups due to the pyranines [41,42]. All of these charges have contribution to the conduction when gel is swollen, i.e., m/m0 > 1. Fig. 8 shows that normalized current densities increase with increasing applied voltage. If gel is dried, there is no current pass through the gel since ions cannot move if the gel is in collapse state [41,42]. When the gel is swollen, current starts to flow. Peaks, seen in current density–swelling ratio curves, correspond to different blobs in PAAm gels [41–43]. The structure of the PAAm gel can be regarded as a set of blobs (or clusters) with long polymer chains, which connects these blobs to each other. When PAAm gels swell, these blobs are rearranged in such a way that small ones are expelled from the big ones, creating low concentration regions. It is seen that one small and two big peaks (i.e., blobs) in Fig. 9, but neat Fig. 6. Emission spectra of CoFe2O4–PAAm magnetic hydrogel and solution for 390 nm and 400 nm excitation wavelength.
Fig. 7. Excitation spectra of solution for 420 nm and 510 nm wavelength emission.
Fig. 8. Normalized current density vs. applied voltage curves for varying swelling ratio of CoFe2O4–PAAm magnetic hydrogel.
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References
Fig. 9. Normalized current density vs. swelling ratio of CoFe2O4–PAAm magnetic hydrogel curves for varying applied voltage.
PAAm gel shows three more distinct peaks [41,42]. Nanoparticles synthesized in situ in PAAm gel causes the some peaks less distinguishable. So, gels got more homogeneous than their neat form. Yilmaz et al. showed that current density per unit mass decreases P Bij t exponentially in time and can be expressed as mJ [42], ij Aij e gel the number of terms in the series corresponds to the number of generations of these blobs. Moreover, magnitude of current density of magnetic hydrogel for an applied voltage of 10 V and having swelling ratio of 2.16 is 140 mA g 1 cm2, see Figs. 8 and 9. This value is much bigger than that of the pyranine doped PAAm gel without magnetic filler (7 mA g 1 cm 2) as presented in Ref. [41]. This means that magnetic nanoparticles made diffusion of pyranine molecules into the gel easier. 4. Conclusion The ferromagnetic CoFe2O4 nanoparticles were successfully synthesized by in situ in polyacrylamide hydrogels. Electrical and spectroscopic measurements show that synthesizing nanoparticles by in situ in PAAm gel causes the gel to become more homogeneous so that fluoroprobe molecules can diffuse easily. Magnitude of the DC current (140 mA g 1 cm 2) increases considerably due to the presence of nanoparticles, which enable the gel suck up more ionic molecules, compared to that of the pure PAAm gel (7 mA g 1 cm 2) see Ref. [41]. Moreover nanoparticles change the absorption of pyranine molecules diffused into the gel, because energy level of pyranine molecules were changed by nanoparticles and this causes the shifting in excitation spectrum. The synthesized gel appeared to be magnetically stable as swelling ratio of the gel increases. In other words, magnetization of the gel does not change during the water intake. This property may be suitable for applications of waste water treatment and metal extraction from the polluted water. Acknowledgements This work is supported by Fatih University under BAP Grant no. P50021104-B. H. Sözeri thanks to Mr. C. Berk and Dr. Ö. Duygulu for taking SEM and TEM micrographs.
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