pubs.acs.org/Langmuir © 2010 American Chemical Society
Controlled Fabrication of Polyethylenimine-Functionalized Magnetic Nanoparticles for the Sequestration and Quantification of Free Cu2þ Ian Y. Goon,† Chengcheng Zhang,† May Lim,† J. Justin Gooding,‡ and Rose Amal*,† †
ARC Centre of Excellence for Functional Nanomaterials, School of Chemical Engineering and ‡ School of Chemistry, University of New South Wales, Sydney NSW 2052, Australia Received March 25, 2010. Revised Manuscript Received May 25, 2010
Presented herein is a detailed study into the controlled adsorption of polyethylenimine (PEI) onto 50 nm crystalline magnetite nanoparticles (Fe3O4 NPs) and how these PEI-coated Fe3O4 NPs can be used for the magnetic capture and quantification of ultratrace levels of free cupric ions. We show the ability to systematically control the amount of PEI adsorbed onto the Fe3O4 magnetic nanoparticle surfaces by varying the concentration of polymer during the adsorption process. This in turn allows for the tailoring of important colloidal properties such as the electrophoretic mobility and aggregation stability. Copper adsorption tests were carried out to investigate the effectiveness of PEI-coated Fe3O4 NPs in copper remediation and detection. The study demonstrated that the NPs ability to bind with copper is highly dependent on the amount of PEI adsorbed on the NP surface. It was found that PEI-coated Fe3O4 NPs were able to capture trace levels (∼2 ppb) of free cupric ions and concentrate the ions to allow for detection via ICP-OES. More importantly, it was found that due to the amine-rich structure of PEI, the PEI-coated Fe3O4 NPs selectively adsorb toxic free cupric ions but not the less toxic EDTA complexed copper. This unique property makes PEI-coated Fe3O4 NPs a novel solution for the challenge of separating and quantifying toxic cupric ions as opposed to the total copper concentration of a sample.
Introduction Toxic metal ions are commonly found in waste streams of many industrial processes and have been known to cause severe health problems.1 Copper, for instance, becomes toxic to some forms of aquatic life even at concentrations as low as ∼5 ppb.2 As a result, stringent regulations have been imposed on the discharge of copper-containing wastewater from the semiconductor, smelting, and other copper intensive industries.3 Conventional treatment of copper-containing wastewater involves the use of ion exchange resins which are costly and often suffer from low efficiency often due to their poor selectivity.4 Another important aspect of copper remediation is the ability to detect and remove low concentrations of free cupric ions (Cu2þ). This is because the true toxic effects of copper arise from cupric ions (Cu2þ), and easily dissociable inorganic complexes, rather than metallic copper or its stable organic complexes.5,6 Free cupric ions, however, are difficult to quantify using standard analytical techniques such as ICP-OES or AAS because these techniques cannot be used to distinguish between the toxic Cu2þ and other more biologically stable forms of copper.7 Research into the use of micro- and nanoscale particles as tools for heavy metal remediation has increased in recent times because *Corresponding author: Tel þ6129385 4361, Fax þ 6129385 5966, e-mail
[email protected].
(1) Jarup, L. Br. Med. Bull. 2003, 68(1), 167–182. (2) Cruz, L. A.; Roberts, C.; Reiley, M.; Santore, R.; Paquin, P.; Chapman, G.; Mitchell, J.; Delos, C.; Meyer, J.; Mathew, R.; Linton, T. K. Aquatic Life Ambient Freshwater Quality Criteria - Copper. In EPA, U.S., Ed., Washington, DC, 2007. (3) Mazon, A. F.; Fernandes, M. N. Bull. Environ. Contam. Toxicol. 1999, 63(6), 797–804. (4) Maketon, W.; Zenner, C. Z.; Ogden, K. L. Environ. Sci. Technol. 2008, 42(6), 2124–2129. (5) Gooding, J. J.; Chow, E.; Finlayson, R. Aust. J. Chem. 2003, 56(2-3), 159–162. (6) Rensing, C.; Maier, R. M. Ecotoxicol. Environ. Saf. 2003, 56(1), 140–147. (7) Liu, G. Z.; Nguyen, Q. T.; Chow, E.; Bocking, T.; Hibbert, D. B.; Gooding, J. J. Electroanalysis 2006, 18(12), 1141–1151.
Langmuir 2010, 26(14), 12247–12252
the high surface areas of the nanoparticles allow for enhanced heavy metal uptake.8-10 The use of nanoparticles in water treatment, however, is limited by the fact that costly separation steps are subsequently required to capture and recover the particles.11 Magnetic nanoparticles present a viable solution to this problem because they can easily be separated in a magnetic field. Recent studies involving the use of chitosan-coated magnetic nanoparticles in heavy metal remediation have shown promising results which highlight the viability of magnetic nanoparticles as a means to effectively capture environmental contaminants and subsequently separate the nanoparticles from the treated water.11,12 An important aspect in the fabrication of magnetite particles for their use in heavy metal remediation is the attachment of a functional polymer on the surfaces of the Fe3O4 NPs. The use of polymers for surface modification is particularly attractive as it provides the Fe3O4 NPs with specific properties, including stability against aggregation13 and optimal particle surface charge14 in addition to the availability of functional groups for the adsorption of heavy metals.15 One polymer that has attracted attention for possible use a functional polymer is polyethyleneimine (PEI). PEI is a water-soluble polycation consisting of primary, secondary, and tertiary amine functional groups.16 PEI has shown exciting (8) Zhong, L.-S.; Hu, J.-S.; Cao, A.-M.; Liu, Q.; Song, W.-G.; Wan, L.-J. Chem. Mater. 2007, 19(7), 1648–1655. (9) Sljukic, B.; Wildgoose, G. G.; Crossley, A.; Jones, J. H.; Jiang, L.; Jones, T. G. J.; Compton, R. G. J. Mater. Chem. 2006, 16(10), 970–976. (10) Xiao, L.; Wildgoose, G. G.; Crossley, A.; Knight, R.; Jones, J. H.; Compton, R. G. Chem.;Asian J. 2006, 1(4), 614–622. (11) Liu, X.; Hu, Q.; Fang, Z.; Zhang, X.; Zhang, B. Langmuir 2009, 25(1), 3–8. (12) Chang, Y.-C.; Chen, D.-H. J. Colloid Interface Sci. 2005, 283(2), 446–451. (13) Boyer, C.; Bulmus, V.; Priyanto, P.; Teoh, W. Y.; Amal, R.; Davis, T. P. J. Mater. Chem. 2009, 19(1), 111–123. (14) Hajdu, A.; Illes, E.; Tombacz, E.; Borbath, I. Colloids Surf., A 2009, 347 (1-3), 104–108. (15) Caruso, F. Adv. Mater. 2001, 13(1), 11–22. (16) Perrine, T. D.; Landis, W. R. J. Polym. Sci., Part A: Polym. Chem. 1967, 5(8), 1993–2003.
Published on Web 06/08/2010
DOI: 10.1021/la101196r
12247
Article
potential as an effective gene delivery agent17-19 and as a functional coating to enhance the performance of adsorbents in packed bed columns in the treatment of industrial wastewater.20-23 The effectiveness of PEI in these applications is attributed to its amine-rich structure, with a high number amino nitrogens that can be protonated. This makes PEI a macromolecule with a high cationic charge density potential. This unique structure has been exploited in biomedical as well as environmental remediation applications. The structure enables the chelation of PEI with heavy metal ions for effective heavy metal remediation; PEI also forms stable complexes with DNA, which results in good transfection efficiencies for gene therapy applications.17 The properties of PEI make it an ideal candidate for coating onto magnetic nanoparticles because it allows for the heavy metal binding ability of PEI to be combined with the ability to magnetically manipulate the particles. We have shown in our previous work that PEI can be used for the controlled attachment of gold nanoparticles onto Fe3O4 NPs.24 Others have used PEI-coated Fe3O4 particles for the purpose of magnetically controllable enzymes25 and magnetically enhanced gene delivery.26-29 Of the heavy metals tested for adsorption by PEI, cupric ions in particular have been shown to coordinate favorably with PEI.4,30 Moreover, because of the affinity of PEI to cupric ions rather than copper metal or its organic complexes, PEI-coated Fe3O4 NPs have the potential to be utilized as tools to concentrate low quantities of cupric ions and subsequently enable their quantification using standard analytical techniques such ICP-OES or AAS. In addition, an important, but often overlooked, aspect of functionalizing magnetic nanoparticles with macromolecules is the ability to systematically control the amount of polymer adsorbed onto the particle surface. The main reason for this is the fact that each nanoparticle-polymer combination has a unique adsorption profile, and the need for elucidation of this profile is dependent on the intended application of the polymer functionalized nanoparticles. In the case of PEI-coated Fe3O4 NPs for use in biomedical or environmental applications, determining the adsorption profile of PEI onto the Fe3O4 NPs is vital in ensuring the optimal performance of these nanoparticles because the amount of PEI adsorbed will determine the nanoparticle surface charge, their resistance to aggregation, and the number of available functional groups on the particle surface.29 Previous studies into polymer adsorption on nanoparticles have demonstrated that the key factors affecting adsorption are nanoparticle shape and dimensions, polymer chain length and concentration, and total salt concentration.31 (17) Boussif, O.; Lezoualc’h, F.; Zanta, M. A.; Mergny, M. D.; Scherman, D.; Demeneix, B.; Behr, J. Proc. Natl. Acad. Sci. U.S.A. 1995, 92(16), 7297–7301. (18) Akinc, A.; Thomas, M.; Klibanov, A. M.; Langer, R. J. Gene Med. 2005, 7(5), 657–663. (19) Godbey, W. T.; Wu, K. K.; Mikos, A. G. J. Biomed. Mater. Res. 1999, 45(3), 268–275. (20) Ghoul, M.; Bacquet, M.; Morcellet, M. Water Res. 2003, 37(4), 729–734. (21) Chanda, M.; Rempel, G. L. React. Funct. Polym. 1997, 35(3), 197–207. (22) Bayramoglu, G.; Arica, M. Y. Chem. Eng. J. 2008, 139(1), 20–28. (23) Amara, M.; Kerdjoudj, H. Talanta 2003, 60(5), 991–1001. (24) Goon, I. Y.; Lai, L. M. H.; Lim, M.; Munroe, P.; Gooding, J. J.; Amal, R. Chem. Mater. 2009, 21(4), 673–681. (25) Dekker, R. F. H. Appl. Biochem. Biotechnol. 1989, 22(3), 289–310. (26) Scherer, F.; Anton, M.; Schillinger, U.; Henke, J.; Bergemann, C.; Kr€uger, A.; G€ansbacher, B.; Plank, C. Gene Ther. 2002, 9(2), 102–109. (27) Yiu, H. H. P.; McBain, S. C.; Haj, A. J. E.; Dobson, J. Nanotechnology 2007, 43, 435601. (28) Wang, X.; Zhou, L.; Ma, Y.; Li, X.; Gu, H. Nano Res. 2009, 2(5), 365–372. (29) Arsianti, M.; Lim, M.; Marquis, C. P.; Amal, R. Langmuir 2010, 26(10), 7314–7326. (30) Steinmann, L.; Porath, J.; Hashemi, P.; Olin, A˚. Talanta 1994, 41(10), 1707–1713. (31) Gittins, D. I.; Caruso, F. J. Phys. Chem. B 2001, 105(29), 6846–6852.
12248 DOI: 10.1021/la101196r
Goon et al.
In this paper, we report a facile method of coating Fe3O4 NPs with PEI and demonstrate how these nanoparticles can be used for the dual purposes of sequestration and quantification of cupric ions. We show how varying the PEI concentration during the coating process allows for the systematic control of PEI attachment onto the Fe3O4 NPs. A detailed analysis is then provided of how the extent of PEI coating is vital in tailoring the colloidal electrostatic charge and aggregation stability. Finally, we quantify the copper adsorption properties of the synthesized PEI-coated Fe3O4 NPs and show how these particles are able to selectively capture and concentrate free cupric ions as opposed to copper complexed with EDTA for detection via ICP-OES.
Experimental Section Materials. Polyethyleneimine (PEI, branched, Mw ∼ 25 kDa) and copper(II) sulfate (CuSO4 3 5H2O) were obtained from SigmaAldrich (Sydney, Australia). Iron(II) sulfate (FeSO4 3 7H2O), potassium nitrate (KNO3), hydrochloric acid (HCl), nitric acid (HNO3), sodium hydroxide (NaOH), ethylenediaminetetraacetic acid (EDTA), and sodium chloride (NaCl) were obtained from Ajax Chemicals (Sydney, Australia). All chemicals were used as received with no further purification. Synthesis of 50 nm Fe3O4 NPs. Cubic Fe3O4 cores were synthesized following a method reported by Sugimoto and Matijevic32 with slight modifications. Briefly, Fe3O4 NPs were precipitated by mixing 0.7 g of FeSO4 with 80 mL of Milli-Q water followed by the addition of 10 mL of 2.0 M KNO3 and 10 mL of 1.0 M NaOH and in an oxygen-free environment. The initially formed Fe(OH)2 was heated at 90 °C in a water bath for 2 h, during which the Fe(OH)2 was oxidized to Fe3O4 NPs. Particles were magnetically separated from reaction mixture by placing a magnet below the reaction vessel (250 mL Schott bottle) for 5 min to capture all magnetic particles before discarding the reaction solution. The collected Fe3O4 NPs were rinsed five times with Milli-Q water and suspended in 80 mL of Milli-Q water, yielding a suspension of Fe3O4 NPs (2.4 g/L, pH 7). PEI Coating of Fe3O4 NPs. A stock solution of 20 g/L PEI solution was first made up by adding 2 g of PEI (in the form of a viscous liquid) into 100 mL of Milli-Q water. From this stock solution, several solutions with concentrations ranging from 10 to 100 mg/L PEI were made up. A 20 mL aliquot of each concentration of PEI solution was added to 48 mg of Fe3O4 NPs (obtained by magnetically separating the Fe3O4 NPs from 20 mL of above stock solution). The pH of the resulting mixtures was adjusted to pH 8 using small amounts of 0.1 M NaOH or HNO3 solutions. The 20 mL mixture of Fe3O4 NPs and PEI were then sonicated for 2 min with an ultrasonic probe (Misonix S4000 ultrasonic liquid processor) in order to fully disperse the particles in the PEI solution. Immediately after sonication, the particles were placed in an 80 °C oven for 4 h to allow for the adsorption of PEI onto the Fe3O4 NPs. Particles were magnetically separated from reaction mixture before discarding reaction solution. The collected Fe3O4 NPs were rinsed five times with Milli-Q water and suspended in 10 mL of Milli-Q water followed by 2 min of sonication with an ultrasonic probe to break up any loosely agglomerated particles, yielding a suspension of PEI-coated Fe3O4 NPs (2.4 g/L, pH 7). Total Organic Carbon (TOC) Analysis for PEI Quantification. To determine the concentration of PEI adsorbed onto the Fe3O4 NPs, 3 mL of the PEI-coated Fe3O4 NPs was dissolved in 3 mL of aqua regia (1:3 HNO3:HCl) and diluted to 40 mL (total volume) with Milli-Q water. Samples were then analyzed for organic carbon content, which was then used to quantify PEI content. The amount of PEI attached onto the Fe3O4 NPs is correlated to the TOC value obtained because PEI is the only source of organic carbon after dissolution of the particles. (32) Sugimoto, T.; Matijevic, E. J. Colloid Interface Sci. 1980, 74(1), 227–243.
Langmuir 2010, 26(14), 12247–12252
Goon et al.
Article
Figure 1. (a) TEM image of synthesized Fe3O4 NPs. (b) HRTEM image showing the crystal lattice of the synthesized Fe3O4 NPs. (c) HRTEM image showing a layer of PEI coating the Fe3O4 NPs. (d) Size distribution histogram of the Fe3O4 NPs.
Copper Adsorption Test. To determine the ability of PEIcoated Fe3O4 NPs to remove Cu2þ from solution, a 1 L stock solution of 5 ppm of Cu2þ solution was made up by dissolving 19.6 mg of CuSO4 3 5H2O in 1 L of Milli-Q water and adjusting the pH to 5.5. Then 3 mL of 2.4 g/L PEI-coated Fe3O4 NPs was mixed with 7 mL of 5 ppm of Cu2þ solution yielding a total Cu2þ concentration of 3.5 ppm. This mixture was sonicated for 1 min with an ultrasonic probe to disperse to particles and left to mix in an ultrasonic bath for 1 h to adsorb Cu2þ ions. After 1 h, the particles were magnetically separated from solution, and the copper content of the reaction solution was analyzed by inductively coupled plasma-optical emission spectroscopy (ICP-OES). The difference in copper content before and after mixing with the PEI-coated Fe3O4 NPs yields the amount of Cu2þ successfully adsorbed by the particles. Selective Cupric Ion Quantification. To determine if PEIcoated Fe3O4 NPs binds specifically to cupric ions compared to organic copper complexes, 7.2 mg of 2.4 g/L PEI-saturated Fe3O4 NPs (62.5 mg/L PEI mixing concentration) was mixed with 3 ppm (with respect to copper) of CuSO4 solution, Cu-EDTA organic complex solution, and various mixtures of CuSO4 and CuEDTA complex. The mixtures were sonicated for 1 min with an ultrasonic probe to disperse to particles and left to mix in an ultrasonic bath for 1 h. This was followed by ICP-OES analysis to determine the copper adsorption as described above. EDTA, a widely used chelating agent for heavy metals, forms a stable chelating complex with all cations at a 1:1 ratio regardless of the charge of the cation.4 The Cu-EDTA complex was hence prepared with a Cu:EDTA molar ratio of 1:1 by mixing 100 mL of 0.01 M CuSO4 with 100 mL of 0.01 M EDTA (yielding at 318 ppm of Cu solution) and adjusting the pH to 5.5; the complex was then diluted to 1 ppm. At a concentration of 0.01 M, the equilibrium constant for Cu-EDTA (log K) complexation has been reported to be 20.33,34 In addition to this, speciation studies carried out by Ogden and co-workers also show that in an equimolar ratio of Cu2þ and EDTA at pH >2 the [Cu(EDTA)]2- complex represents close to 100% of all possible chemical species.4 Characterization. Transmission electron microscopy (TEM) images were obtained using a Philips CM 200 TEM operating at 200 kV. Electrophoretic mobility measurements to characterize the particle surface charge and its effect on aggregation were carried out by using the Brookhaven ZetaPALS analyzer. Particle size distributions were obtained based on dynamic light scattering (DLS) principles with a Brookhaven 90 Plus particle sizer. Total organic carbon (TOC) measurements to determine PEI concentrations were performed by a Shimadzu TOC-VCSH analyzer. The concentrations of the Fe3O4 NPs and the amounts of copper adsorbed were determined using an Optima 3000D inductively coupled plasma optical emission spectrometer (ICP-OES). Fe3O4 NPs particle elemental compositions were characterized via X-ray (33) Juang, R. S.; Chen, Y. J.; Huang, I. P. Sep. Sci. Technol. 1999, 34(15), 3099– 3112. (34) Morel, F. M. M.; Hering, J. G. Principles and Applications of Aquatic Chemistry; Wiley: New York, 1993; pp 332-333.
Langmuir 2010, 26(14), 12247–12252
photoelectron spectroscopy (XPS) on a Thermo Scientific VGESCALAB 220-iXL spectrometer with a monochromated Al KR source (1486.6 eV).
Results and Discussion Characterization of the Synthesized PEI-Coated Fe3O4 NPs. Because of the multitude of different methods reported for synthesizing Fe3O4 NPs which each produce Fe3O4 colloids with differences in size, morphology, chemical composition, and magnetic properties, it was important to first characterize the Fe3O4 NPs synthesized in this work. It can be seen from the TEM image presented in Figure 1a that the Fe3O4 NPs are cuboidal, with an average primary particle size of ∼50 nm (note: average face-centered diagonal) (Figure 1d). The HRTEM image presented in Figure 1b reveals the well-ordered lattice-fringe patterns, which show the highly crystalline morphology of the Fe3O4 NPs. XPS analysis of the PEI-coated Fe3O4 confirmed that the dominant phase of synthesized particles was magnetite (see Supporting Information for detailed discussion). The saturation magnetization of the Fe3O4 NPs was measured by SQUID magnetometry to be 88.4 emu/g; this high saturation magnetization is particularly important for speedy magnetic capture of the particles after the adsorption process. TEM images were collected to characterize the Fe3O4 NPs before and after coating with PEI. It can be seen by comparing the HRTEM images in Figure 1b,c that a distinct layer of PEI was formed around the NPs after PEI coating. The pH of PEI and Fe3O4 NPs mixtures during PEI adsorption has an important effect on the extent of PEI adsorption due to the fact that the pH determines the electrostatic charge on both the PEI and Fe3O4 NPs. A pH of 8 was selected to maximize the charge differential between PEI (þve) and Fe3O4 NPs (-ve). The cationic nature of PEI over a wide range of pH is due to the presence of large numbers of proton accepting amine groups. Typically, the maximum extent of PEI adsorption onto the Fe3O4 NPs was obtained between pH 6 and 8; at other pH values, less PEI was adsorbed due to less favorable charge conditions for electrostatic adsorption.35,36 Controlled PEI Attachment and Its Effect on Particle Properties. In this study, particular attention was paid to the concentration of 25 kDa MW PEI during mixing to determine its effect on the amount of PEI adsorbed onto the Fe3O4 NPs. Determining the amount of PEI attached to the particle surface involved the complete dissolution of the Fe3O4 NPs using aqua regia, followed by total organic carbon (TOC) analysis of the dissolved particle solution. Samples of Fe3O4 NPs which were mixed with different concentrations of PEI were dissolved and (35) Illes, E.; Tombacz, E. J. Colloid Interface Sci. 2006, 295(1), 115–123. (36) Chibowski, S.; Patkowski, J.; Grzadka, E. J. Colloid Interface Sci. 2009, 329(1), 1–10.
DOI: 10.1021/la101196r
12249
Article
Figure 2. Adsorption isotherm for 25 kDa PEI onto Fe3O4 NPs.
Goon et al.
Figure 4. DLS measurements showing the extent of Fe3O4 NPs aggregation over time (pH 7, Milli-Q H2O).
Figure 3. Curves showing the dependence of the electrophoretic mobility of the Fe3O4 NPs coated with different amounts of PEI on pH (10 mM NaCl background electrolyte).
analyzed for organic carbon. Figure 2 shows the adsorption behavior of PEI onto Fe3O4 NPs at pH 8. It was observed that increasing the concentration of the PEI in the mixing solution resulted in an increased amount of PEI adsorbed of the Fe3O4 NPs. The increase in PEI adsorption can be attributed to the greater number of PEI chains present which favors adsorption onto the Fe3O4 NP sufaces. At a PEI equilibrium concentration of around 50 mg/L, a plateau is reached; this is indicative of a saturation of the NP surfaces with PEI, and it can be seen that no further adsorption of PEI occurs at concentrations above 50 mg/L. Analysis of the dependence of the electrophoretic mobility on pH in Figure 3 shows that increasing the amount of PEI adsorbed onto the Fe3O4 NPs generally increases the electrophoretic mobility of the particles. It can also be seen that the isoelectric point of the Fe3O4 NPs shifts toward a higher pH as more PEI is attached. This was attributed to the fact that increasing the amount PEI molecules adsorbed onto the particle surface led to an increasing number of amine groups on the particle; this in turn resulted in more hydroxyl ions (higher pH) being required to neutralize the positive charge on the amine groups. Dynamic light scattering (DLS) measurements to track the intensity average hydrodynamic diameter of the Fe3O4 NPs with respect to time after being subjected to ultrasonication with a probe are presented in Figure 4. The increasing size of the particles as time passes is indicative of the aggregation of the colloid. It can be seen in Figure 4 that the 50.0 mg/L PEI samples remain stable for the duration of the experiment, while for the 10.0 mg/L PEI samples, the size gradually increases soon after sonication is ceased. This result indicates that higher amounts of adsorbed PEI leads to significantly enhanced aggregation stability. Fe3O4 12250 DOI: 10.1021/la101196r
NPs without any coating (not shown) began to aggregate immediately after sonication, reaching 1 μm in size within a few minutes. The enhanced aggregation stability with increasing PEI amount is due largely to the steric hindrance and electrostatic repulsion between the Fe3O4 NPs as a result of the PEI coating.37 This enhanced stability is important particularly in copper adsorption applications, where a greater dispersion of particles allows for greater exposure of PEI on the Fe3O4 NPs surfaces to copper ions, which in turn increases the opportunity for capture. The improved resistance to aggregation, however, has implications for the magnetic collection of the Fe3O4 NPs after copper capture. Basak et al. have recently shown that polymer-coated magnetic nanoparticles are attracted to an applied magnetic field at a slower rate compared to uncoated magnetic nanoparticles due to their increased stability against aggregation.38 This is due to the fact that aggregated nanoparticles experience a greater magnetic attraction force as a result of the increased dimensions of the agglomerate. Our experiments revealed that this was indeed the case for our PEI-coated Fe3O4 NPs, where a 10 mL of the 50.0 mg/L sample took 2 min to be attracted to a single neodymium magnet (6 mm thickness 6 mm radius) placed at the bottom of a 10 mL sample tube; this is in comparison to 30 s for the 10.0 mg/L Fe3O4 NPs sample. Performance of PEI-Coated Fe3O4 as Magnetic Adsorbents for Cu2þ Remediation. Figure 5 shows the dependence of Cu2þ removal on the amount of 25 kDa PEI coating. It can be seen by comparing Figures 2 and 5 that copper adsorbed for the 25 kDa reaches a maximum at the same point as PEI adsorbed. The increasing removal of copper as a result of the increase in amount of PEI on Fe3O4 NPs is thus related to the number of amine groups available for coordination with Cu2þ (see Supporting Information, S4). The PEI-saturated Fe3O4 NPs were found to have a fixed Cu2þ adsorption capacity which was found to be ∼160.0 mg of Cu2þ/g of adsorbent. This result highlights the importance of ensuring optimal PEI coating on the Fe3O4 NPs to allow for maximum copper adsorption. The Cu2þ adsorption capacity of the PEI-coated Fe3O4 compares favorably to commercially available ion exchange resins (such as the commonly used Amberlite and Amberjet resins) which have Cu2þ adsorption capacities ranging from 10 to 130 mg of Cu2þ/g of resin.39,40 The reason for the improved performance of the PEI-coated Fe3O4 (37) Lim, J. K.; Majetich, S. A.; Tilton, R. D. Langmuir 2009, 25(23), 13384– 13393. (38) Basak, S.; Brogan, D.; Dietrich, H.; Ritter, R.; Dacey, R. G.; Biswas, P. Int. J. Nanomed. 2009, 4, 9–26. (39) Marshall, W. E.; Wartelle, L. H. Water Res. 2006, 40(13), 2541–2548. (40) Rengaraj, S.; Yeon, J.-W.; Kim, Y.; Jung, Y.; Ha, Y.-K.; Kim, W.-H. J. Hazard. Mater. 2007, 143(1-2), 469–477.
Langmuir 2010, 26(14), 12247–12252
Goon et al.
Figure 5. Copper removal performance of Fe3O4 NPs coated with different amounts of PEI, pH 5.5.
arises from the Cu2þ chelating properties of PEI combined with the nanoscale size of the particles which allow for a greater surface area of contact between the Cu2þ and PEI-coated Fe3O4; this allows for greater adsorption of Cu2þ.11 In addition to improved Cu2þ capture, PEI-coated Fe3O4 has the added advantages of enhanced recovery via magnetic fields and the ability to quantify captured analyte. It is important to note that the copper adsorbed by PEI-coated Fe3O4 NPs was significantly influenced by pH. At pH6, hydrolysis of the Cu2þ occurs, which leads to the formation of copper hydroxide precipitates. A possible reason for the influence of pH on Cu2þ adsorption is the possibility that PEI desorbs from the Fe3O4 surface when the pH changes. To determine the extent of PEI desorption from the Fe3O4 at different pH values, PEI-coated Fe3O4 NPs were suspended in solutions of different pH values ranging from pH 2 to 11 and mixed by stirring continuously for 12 h. TOC analysis carried out on the solutions revealed that at pH < 10 no TOC was detected (and hence no PEI desorption). In our experiments involving the attachment of PEI onto Fe3O4, we have found that the PEI-coated particles are stable against aggregation for long periods (>24 h) at a range of pH values of
