Binary Short-Range Colloidal Assembly of Magnetic Iron Oxides

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Binary Short-Range Colloidal Assembly of Magnetic Iron Oxides Nanoparticles and Fullerene (nC60) in Environmental Media Saikat Ghosh,† Nihar R. Pradhan,‡ Hamid Mashayekhi,† Stefan Dickert,‡ Rukshan Thantirige,‡ Mark T. Tuominen,‡ Shu Tao,§ and Baoshan Xing*,† †

Stockbridge School of Agriculture, University of Massachusetts, Amherst, Massachusetts 01003, United States Department of Physics, University of Massachusetts, Amherst, Massachusetts 01003, United States § Laboratory for Earth Surface Processes, College of Urban and Environmental Sciences, Peking University, Beijing 100871, China ‡

S Supporting Information *

ABSTRACT: Colloidal assembly of nC60 fullerene with naturally abundant magnetic iron oxide NPs will affect their fate and transformation in environmental media. In solution, fullerene association to aggregating iron oxide NPs/clusters greatly enhanced the overall colloidal stability. Development of depletion-mediated structured fullerene layers between pure and surface modified γFe2O3 NPs possibly resulted in such stabilization. Here, we also report that on air−water interface, association of fullerene to pure and humic acid (HA7) coated γFe2O3 NPs led to the formation of ordered assemblies, e.g., binary wires and closed-packed “crystalline” and “glassy” structures in the presence and absence of electrolytes suggesting immobilization of the former. The interaction of fullerene to Fe3O4 NPs and clusters also produced ordered assemblies along with amorphous aggregates. Fullerene interaction with Fe3O4 NPs in low concentration of HA1 and Na+ at pH 6 formed dendritic growth and polycrystalline circular assemblies on air−water interface. HRTEM study further revealed that the monolayer circular assemblies were highly ordered but structural degeneracy was evident in multilayers. Therefore, interfacial assemblies of fullerene with iron oxide NPs resulted in short-range periodic structures with concomitant immobilization and reduction in availability of the former, especially in soils or sediments rich in the latter.

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INTRODUCTION The unique material properties of C60 fullerene nanoparticles (NPs) have prompted their usage in several commercial products.1,2 Furthermore, combustion byproducts and aerosols containing fullerenes and its derivatives enriched their atmospheric concentration.3 The enhanced production and application of fullerene also raised a serious concern regarding their environmental release and subsequent toxicity to organisms.4−6 In spite of strong hydrophobicity, solvent exchange and extended mixing produce stable fullerene suspension in water with negative surface charge.7 Furthermore, natural organic matter (NOM) mediated electrostatic and steric stabilization can increase fullerene bioavailability and subsequent toxicity.8,9 Presently, majority of the studies illustrate NOM mediated fate and transport of engineered NPs. However, Delay and Frimmel10 reported that colloidal interaction of engineered NPs with naturally abundant suspended particulates can also control the fate and transport of the former. Among the particulates, iron oxides minerals are ubiquitous in most soils. Boedk et al. (1998) reported that iron concentration in regular soil ranged between 0.2% to 55%.11 However, significant variation in iron concentration was observed in different soil types with high abundance in clayey-soil.12 Previously, Buffle et al.13 identified association © XXXX American Chemical Society

of small-sized iron oxide colloids to large silica particles in a natural lake water sample. A compartmental multimedia simulation study has illustrated that engineered NPs accumulation to sediments are incumbent upon their attachment to naturally abundant suspended solids.14 Therefore, it is foreseeable that attachment of fullerene to pure and NOM-modified mineral colloids may affect the environmental fate and transport of the former. Soil, being the sink of numerous anthropogenic activities, can significantly control the transport or attenuation of several contaminants including heavy metal ions and others.15 Porewater chemistry and humic acid (HA), an integral component of NOM can mobilize radionuclides in saturated sand columns.16 Zhang et al.17 have corroborated the importance of air−water interface regarding colloid retention in unsaturated porous media due to electrostatic forces and pore blocking. Similarly, Wan and Wilson18 specified significant retention of colloids on air−water interface in unsaturated columns compared to water saturated columns. Therefore, transport of Received: April 13, 2014 Revised: July 24, 2014 Accepted: September 15, 2014

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dx.doi.org/10.1021/es501846s | Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Environmental Science & Technology

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aqueous phase and compared with standard solutions of fullerene in toluene. Fe3O4 NPs ranged between 2 and 6 nm diameters were synthesized from coprecipitation of Fe3+ and Fe2+ salts.35 Ferrimagnetic γFe2O3 NPs (25−50 nm) used for these experiments were obtained from Nanoamor (Houston, TX) and were not surface modified. The magnetic force emanating from γFe2O3 and Fe3O4 NPs were evaluated with magnetic force microscopy (MFM) imaging (Supporting Information (SI) Figure S1). The detailed procedure is provided in the supplementary section. HA1 and HA7 fractions were isolated from the Amherst peat soil after sequential extraction. Our previous studies have displayed lower polarity of the latter HA fractions obtained from sequential extraction.36,37 The detailed description regarding preparation of HA7-coated γFe2O3 NPs was described in our previous work.38 Fe3O4 NPs suspension in 50 mM NaCl with 20 mg/L HA1 was prepared by adding specific volume of 500 mg/L HA1 fraction to Fe3O4 NPs suspension in 50 mM NaCl solution at pH 6. Minimum volume of 0.1 M NaOH and 0.1 M HCl were added to adjust the suspension pH. A sonicator probe was employed for 5 min to disperse the mixture followed by overnight shaking with a mechanical shaker. The colloidal behavior of pure and surface modified iron oxide NPs and fullerene was investigated by measuring hydrodynamic diameter (Dn) and zeta potential (ζ) with dynamic light scattering techniques (DLS, Brookhaven 90 Plus, U.S.A.). Sample preparation techniques are provided in the SI. Binary assembly formation of pure and HA-coated iron oxide NPs and fullerene was investigated with high resolutiontransmission electron microscopy (HR-TEM, JEOL 200FX, U.S.A.), TEM (JEOL, JEM 100-CX, U.S.A.), atomic force microscopy (AFM) (Dimension 3100 model, Digital Instruments, U.S.A. and MFP-3D, Asylum Research, U.S.A., AFM), and scanning electron microscopy (SEM) (JEOL JSM-7001F). ImageJ software was used for image processing. HRTEM samples were prepared by adding a minute volume (1−2 μL) of the binary mixture to the carbon/Formvar coated TEM grids (Electron Microscopy Sciences, U.S.A.) followed by overnight drying in a humid chamber. TEM grids were put on a paraffin film placed over a water-saturated cellulose filter paper in a covered Petri-dish to ensure very high relative humidity. The samples were removed from the humid chamber prior to the HRTEM imaging. To study the binary crystal formation, 15 μL of the binary mixtures were deposited on silica substrates kept over a paraffin film, which was placed over a water-saturated cellulose filter paper inside a Petri-dish. The whole Petri-dish was further wrapped with paraffin film and kept inside a ziplock bag which resulted in extremely slow-drying environment (approximately 48−60 h). We have also ascertained the effect of fast drying on binary assembly in a 2:1 volume ratio of 0.4 mg/L Fe3O4 NPs and 8 mg/L fullerene. The mixture was allowed to sorb on a freshly cleaved mica surface for 15−20 min followed by N2 mediated drying.

anthropogenic NPs through unsaturated porous media or “vadose zone” may profoundly attenuate or mobilize depending upon their interaction on air−water interface.14,17,19,20 Among the mineral constituents, natural abundance of iron in the Earth’s crust enriches our environment with iron oxides of biotic and abiotic origins.21 Among the iron oxides, magnetite and maghematite are naturally abundant in soil, lacustrine, and marine environments and sediments.22,23 Several studies have shown that these iron oxide rich sediments play a significant influence in the mobility of trace metals.24−26 Soil mineral particles on air−water interface are subjected to alternate wetting and drying cycles, which will affect several environmental processes including nutrient transport in plants.27 Ladd et al.28 indicated that drying facilitates formation of less hydrated crystalline compounds of lower solubility; hence, nutrient immobilization. Therefore, colloidal association and subsequent immobilization of fullerene with pure and HAmodified iron oxide NPs on air−water interface would be important to fully comprehend their environmental fate, transport, and subsequent availability. In this article, we investigated interaction of fullerene with pure and structurally different HA-modified magnetic iron oxides (γFe2O3 and Fe3O4 NPs) and clusters under varying solution conditions. We would also like to identify whether mutual association of fullerene to pure and HA-modified iron oxide NPs produces any ordered assemblies on air−water interface under high relative humidity conditions. It is foreseeable that formation of thermodynamically favorable ordered structures substantially prohibits fullerene transport through unsaturated air−water interface. Most of the binary colloidal assembly studies are conducted by the material scientists to engineer long-range binary superlattices and 3D colloid crystals. In long-range binary assemblies or superlattices, small-sized monodisperse NPs are arranged in a regular array to produce membranes with a few micrometer lateral dimensions having packing symmetry and stoichiometry.29 However, in case of short-range assemblies this is limited, especially in hydrophobically modified circular self-assemblies.30 Low dielectric (ε) property of the nonaqueous solvents and minimal interparticle interaction energy (kBT) are prerequisite to generate such long-range structures with diverse applications.31−34 However, investigations on binary assembly of these engineered NPs with naturally abundant NPs/clusters in aquatic conditions (high dielectric) or at air−water interface will enrich our knowledge on the fate and transport of the former upon environmental release. Considering the complexity of the natural aquatic medium, formation of short-range binary assemblies are anticipated between fullerene and pure and HA-modified magnetic iron oxide NPs/clusters on air− water interface. The combined influence of electrostatic, van der Waals, magnetic dipolar and entropic depletion forces on colloidal assembly formation are discussed.

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MATERIAL AND METHODS An aqueous suspension of C60 fullerene was prepared using a solvent exchange method. In brief, a small volume of saturated fullerene solution in toluene was added to deionized water followed by sonication. The purple-colored solution turned into a yellow colored suspension with evaporation of toluene. The entire procedure regarding preparation of aqueous suspension of fullerene was described in our previous work.7 Fullerene concentration in the suspension was determined with a UV− visible spectrophotometer after extraction with toluene from

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RESULTS AND DISCUSSIONS Colloidal Interaction of γFe2O3 NPs and Fullerene in Solution. DLS measurements revealed that γFe2O3 NPs were strongly aggregated and produced large clusters (Dn= 646 nm) at pH 6 (SI Figure S2). Saturation magnetization (Ms) data obtained from our previous study and MFM imaging showed that γFe2O3 NPs were highly magnetic (dipolar attraction); hence, undergo aggregation in the absence of surface B



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Figure 1. (a) AFM phase (15°) image of the 2:1 binary mixture of 30 mg/L γFe2O3 NPs and 8 mg/L fullerene on silicon substrate produced parallel and adjoining wires. (b) HRTEM image of the binary wires revealed γFe2O3 NPs (dark) were separated by self-assembled fullerene particles (light). (c) AFM image of the 2:1 binary mixture of 30 mg/L γFe2O3 NPs and 8 mg/L fullerene on silicon substrate formed flux-closed triangular and complex ring. (d) γFe2O3 NPs and fullerene binary mixture produced magnetically driven stripe-chain phase (inset image) along with closed-loop assembly as indicated in the HRTEM image. (e) HRTEM image of the close-packed binary colloidal assembly of 1:1 mixture of 30 mg/L γFe2O3 NPs and 8 mg/L fullerene. The lattice fringes and directional pattern of the binary assembly structures are indicated by the arrows. (f) γFe2O3 NPs and fullerene formed close-packed structure, where γFe2O3 NPs (dark) were separated by the self-assembled fullerene. (g) Close-packed binary assembly of 1:1 mixture of 30 mg/L HA7-coated γFe2O3 NPs in 25 mM NaCl and 8 mg/L fullerene at pH 6. (h) Lower magnification image of the binary assembly on the same slide.

approach of fullerene to HA7-coated γFe2O3 NPs. However, specific interactions, such as H-bonding and π−π interactions, and nonspecific hydrophobic interaction between fullerene and amphiphilic HA7 moieties adsorbed on γFe2O3 NPs/clusters or desorbed from the γFe2O3 NPs surface cannot be overruled. DLS data illustrated that HA7-induced surface modification of γFe2O3 NPs decreased the average Dn along with reversal of surface charge (ζ) (SI Figure S2c,d). HA7-mediated electrosteric stabilization along with loss of surface magnetic moments, enhanced the overall colloidal stability of γFe2O3 NPs. Previous studies have shown that capping of magnetic iron oxide NPs with HA or polymeric materials caused loss in surface magnetic moments.38,46 The obtained Dn and ζ data further illustrated that HA7-coated γFe2O3 NPs started aggregating in the presence of 50 mM NaCl. But, addition of equal volume of fullerene (8 mg/L) to the suspension reduced the average Dn. Minimization in electrostatic repulsions due to electrolytemediated charge screening and reduced interparticle separation of coated γFe2O3 NPs along with concurrent entropic depletion of fullerene in the intermediate zone possibly augmented colloidal stability. In addition, abated electrostatic repulsion between the electrical double layers of fullerene clusters, in the presence of electrolyte may also favor fullerene crowding around HA7-coated γFe2O3 NP.47 Interfacial Association of Pure and HA-Modified γFe2O3 NP and Fullerene. In contrast to the general entropic concept associated with disorder, a number of studies have highlighted that entropic depletion interaction can mediate the binary assembly of particles on air−water interface.32,42−45 In order to simulate naturally occurring air−water interface and avoid fast drying, binary assemblies of magnetic iron oxide NPs with fullerene were investigated by drying the binary suspensions on TEM grids under high relative humidity

modifiers.38 The large aggregate size of γFe2O3 NPs/clusters is in assertion with the low |ζ| data. DLS data further indicate lower size distribution (Dn = 27 nm) for fullerene clusters in contrast to γFe2O3 NPs clusters at pH 6 (SI Figure S2), which is in compliance with the obtained charge data (ζ). However, binary mixture comprised of equal volume of 8 mg/L fullerene and 50 mg/L γFe2O3 NPs suspensions caused fullerene depletion into the magnetic clusters with concomitant reduction in Dn and ζ (SI Figure S2a,b). Our visual observation clearly showed enhanced stability with reduced precipitation in the binary mixture compared to sedimentation of large-sized magnetic clusters in pure γFe2O3 NPs suspension. The depletion repulsion interaction due to layering of small-sized fullerene around γFe2O3 NPs might have caused reduction in aggregate size.39,40 Moreover, minimization of electrostatic attraction between fullerene and large γFe2O3 NPs clusters further suffices the depletion conditions; hence evoking that iron oxide NPs can impact fullerene transport and bioavailability in aqueous environments. Crocker et al.40 observed the enhanced colloidal stability of a binary suspension comprised of small and large spheres at relatively higher volume fraction of small spheres through depletion repulsion interaction. However, entropic depletion attraction with subsequent aggregation prevails in the binary mixtures especially at lower volume fraction of small spheres.40−45 This is in corroboration to the model of Asakura and Oosawa, where overlapping of excluded volume of large spheres can increase the total volume accessible by the small spheres with concomitant increase in entropy and reduction in free energy.40−45 The natural abundance of HA molecules with spatial and temporal variation can cause encapsulation of the iron oxide NPs. The ionization of the HA moieties adsorbed on iron oxide NPs and subsequent electrostatic barrier possibly inhibit close C



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HA7 coated-γFe2O3 NPs.49 Dinsmore et al.50 deduced from geometric arguments that the accessibility of small spheres in the overlapping excluded volume zone of large spheres rises when large particles approach a substrate/wall with subsequent enhancement in entropy. We also found weakly ordered “glassy” binary assembly on the same slide (SI Figure S4c,d) through depletion interactions. Therefore, colloidal association of fullerene with naturally abundant pure and NOM-modified γFe2O3 NPs can produce close-packed crystalline and amorphous assemblies in the presence of electrolytes especially on the unsaturated air−water interface; hence, may reduce fullerene bioavailability and transport through soil columns. Furthermore, it is likely that fullerene may be more readily released from amorphous structures than crystalline 2D binary assemblies as free volume entropy of fullerene is maximum in the latter.51 Colloidal Interaction and Assembly Formation between Fe3O4 NPs and Fullerene. The association of Fe3O4 NPs/clusters and fullerene was investigated at varying pHs and ionic strengths. In addition, the implication of lower HA concentration, and Na+ on assembly formation was assessed in solution and at air−water interface. From visual observation, DLS data and MFM investigations, Fe3O4 NPs (Dn = 82 nm) have much higher colloidal stability compared to γFe2O3 NPs at pH 6. Weak magnetic dipolar attraction (SI Figure S1) and relatively stronger electrostatic repulsions between Fe3O4 NPs as detected from the ζ at pH 6 have contributed to stabilization (SI Figure S5a,b). However, electrostatic attraction upon fullerene mixing to Fe3O4 NPs/clusters have led to surface charge neutralization along with strong aggregation as seen from the Dn and ζ data (SI Figure S5a,b). We presume fullerene depletion to Fe3O4 NPs clusters along with electrostatic attraction at this pH. Comparison between TEM and HRTEM images of pure and fullerene dispersed Fe3O4 NPs illustrate that pure Fe3O4 NPs were aggregated even in acidic pH conditions in the absence of surface coating (SI Figure S6). Therefore, it is likely that barring electrostatic attraction mediated amorphous aggregates, fullerene depletion into the Fe3O4 NPs clusters (formed from magnetic attraction) contributed to ordered assembly formation with subsequent accumulation on air−water interface. A 2:1 mixture of 40 mg/L Fe3O4 NPs and 8 mg/L fullerene displayed several magnetically driven structures (Figure 2a and SI Figure S7) on air−water interface at pH 6. Magnetic dipolar effect that stemmed from enhanced local concentration of Fe3O4 NPs and van der Waals attractions have resulted in such binary wire formation.52 HRTEM investigation displayed staggered and doublet binary wires as indicated by the dashed ellipses, where Fe3O4 NPs (dark) were separated by fullerene particles (light) (Figure 2a). The formation of doublet and staggered chains was due to magnetic dipolar orientation of the latter particles relative to the alignment of magnetic dipole of the first two particles (Figure 2a).53 Along with parallel wires, several other magnetically driven binary assemblies with flux closed structures were observed (SI Figure S7). Different types of binary structures along with wires originated from the same point but angularly orientated were also seen (SI Figure S7b). Relatively higher moment associated with the large sized Fe3O4 NPs, at the origin of the chains possibly align the magnetic moments of the other Fe3O4 NPs to constitute this structure. An incomplete triangular assembly was formed, where binary wires produced the arms of the triangle (SI Figure S7c). Philipse and Mass54 reported several magnetically driven flux-

conditions. Furthermore, fast drying methods were avoided as it led to quick crystallization of the binary mixture which hindered viewing of fine structures. A 2:1 binary mixture of 30 mg/L γFe2O3 NPs and 8 mg/L fullerene revealed magnetically driven binary assembly formation. In-plane magnetic dipolar interactions originating from higher local concentration of γFe2O3 NPs and fullerene depletion have led to formation of wires, triangles, and ring structures (Figure 1). AFM investigation of the mixture exhibited formation of binary wires in parallel to each other, where γFe2O3 NPs were separated by the fullerene (Figure 1a and SI Figure S3). HRTEM investigation of the same mixture confirmed that the binary chains were formed by alternate dark (γFe2O3 NPs) and bright (self-assembled fullerene) particles (Figure 1b). AFM height image also showed closed-loop triangular and complex ring structures on the same sample, prepared on silicon wafer substrate (Figure 1c). Therefore, it is evident from HRTEM and AFM imaging that the binary mixtures produced chain, and flux-closed transition states due to enhanced magnetic dipolar force that evolved from increased local concentration of γFe2O3 NPs. HRTEM imaging also showed that binary assembly of γFe2O3 NPs and fullerene produced linear striped-chain phase along with circular structure (Figure 1d). A close look at the image portrayed that alternate linear chains of γFe2O3 NPs (dark) were separated by the self-assembled fullerene (bright) (inset image of Figure 1d). The elevated horizontal magnetic field and alignment of γFe2O3 NPs dipoles in one direction and fullerene-induced separation between linear γFe2O3 NPs assemblies possibly resulted in this formation. This is in line with the stripe-tile phase formation in the binary assembly of paramagnetic and diamagnetic micro spheres assembled in a relatively concentrated ferrofluid medium.48 AFM investigation of the 2:1 mixture of 30 mg/L γFe2O3 NPs and 8 mg/L fullerene showed formation of chain along with closed-loop binary assembled structures (SI Figure S3c). In addition to the magnetic dipole-induced self-assembled binary wires, a relatively lower concentration of γFe2O3 NPs in the mixture produced square lattices on air−water interface as detected by HRTEM imaging (Figure 1e−f and SI Figure S4). Unlike high dipole−dipole strain mediated binary wire formation, weak dipolar force formed intermediate energy square lattices. Khalil et al.48 observed binary square-lattices of paramagnetic and diamagnetic microparticles in a ferrofluid medium. A 1:1 binary mixture of 30 mg/L γFe2O3 and 8 mg/L fullerene produced 2D square assembly, and the arrows designate the growth directions and lattice fringes (Figure 1e). Another close-packed binary assembly formation was detected with the same suspension composition as shown in the SI (Figure S4a). A multilayer close-packed binary assembly of γFe2O3 NPs and fullerene was also identified (Figure 1f). Therefore, on air−water interface enhanced fullerene depletion and structuring around γFe2O3 NPs can produce 2D colloidalassembly with subsequent immobilizations. However, majority of the γFe2O3 NPs in the natural environment are surface modified due to sorption of NOM molecules, especially in the top of the soil/sediment column. HRTEM investigation of the 1:1 binary mixture of 30 mg/L HA7-coated γFe2O3 NPs in 25 mM NaCl and fullerene produced close-packed binary assemblies (Figure 1g,h). The shrinking of the electrical double layer and minimization of long ranged electrostatic repulsions between the coated γFe2O3 NPs favored entropic depletion of fullerene with structured layers. These structured fullerene layers are creating an energy barrier to prevent attachment of D



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led to assembly formation on air−water interface upon slow drying along with immobilization (Figure 2b). To understand whether complex environmental conditions allow binary assembly formation on air−water interface, we investigated colloidal association of fullerene to a mixture containing 40 mg/L Fe3O4 NPs suspension in the presence of 20 mg/L HA1 and 50 mM NaCl at pH 6. First, Fe3O4 NPs started aggregating in the presence of 50 mM NaCl at pH 6 as complemented by Dn and ζ data (SI Figure S8a,b). However, addition of 20 mg/L HA1 and subsequent shaking of the mixture caused overcharging with reduction in aggregate size (SI Figure S8a and b). A slight increment in aggregate size was observed in the 1:1 binary mixture of fullerene and HA1modified Fe3O4 NPs in NaCl electrolyte. Absence of electrostatic attraction between fullerene and HA1-modified Fe3O4 NPs, and electrolyte-mediated charge screening possibly facilitated fullerene depletion as discussed earlier. HRTEM image of this binary mixture revealed dendritic growth along with circular assembly formation (Figure 2c). The inset image clearly shows the ordered monolayer binary assembly, where Fe3O4 NPs (black) are separated by the self-assembled fullerene. These specific spots where circular binary assembly structures were formed are likely to be the “potential well”, where enhanced local concentrations of fullerene and HA1modified Fe3O4 NPs have crossed a threshold limit (SI Figure S8c). A magnified HRTEM image of the dendritic binary assembly further demonstrated that HA1-modified Fe3O4 NPs were separated by the self-assembled fullerene (Figure 2d). A close-look at the binary assembly structure and corresponding selected area electron diffraction (SAED) data showed polycrystalline ring pattern (Figure 2e). HRTEM image further clarifies dendritic growth of the binary assembly adjoining circular assemblies (Figure 2f). Comparison between the multilayer and monolayer circular binary assembly elicits existence of structural degeneracy in the former. Therefore, fullerene transport through unsaturated porous media could be strongly inhibited due to their immobilization on air−water interface through formation of thermodynamically favorable short-range binary assemblies. These spontaneously formed binary assemblies can hinder fullerene availability and transport through soil columns, especially rich in iron oxide NPs. Crystallization of the Binary Mixtures of Iron Oxide NPs and Fullerene. As noted earlier, alternate wetting-drying cycles especially in calcareous soils can transform nutrient mobility. The binary mixtures of iron oxide NPs and fullerene produced 3D crystals under extremely slow drying environment. AFM phase image of the 2:1 binary mixture of 40 mg/L Fe3O4 NPs and 8 mg/L fullerene elicited epitaxial growth of plate like triangular binary crystals on silicon substrates; hence immobilized on air−water interface (Figure 3a). AFM and SEM imaging of the 2:1 binary mixture of 30 mg/L γFe2O3 NPs and 8 mg/L fullerene further exhibited regular aggregate formation through crystallization of the binary mixture under slow drying environment (Figure 3b,c). Even at relatively fast drying condition, 2:1 binary mixture of 0.4 mg/L Fe3O4 and fullerene NPs (8 mg/L) formed a flower-like 2D binary assembly on mica surface (Figure 3d). Therefore, it is likely that soils rich in iron oxide NPs facilitate fullerene attenuation by forming 2D short-range assembly or 3D crystals. To the best of our knowledge, this study reported for the first time that colloidal attachment of fullerene to naturally abundant magnetic iron oxide NPs/clusters could influence environmental fate and transport and subsequent availability of

Figure 2. HRTEM images of (a) magnetically driven doublet and staggered binary chains produced from 2:1 mixture of Fe3O4 NPs and fullerene. (b) 1:2 binary mixture of Fe3O4 NPs and fullerene at pH 6.5 produced assembly. (c and d) 1:1 mixture of 40 mg/L Fe3O4 NPs in 20 mg/L HA1 and 50 mM NaCl and fullerene produced circular binary assembly and dendritic growth of the assembled structure. The inset image shows ordered monolayer structure, where Fe3O4 NPs (dark) were separated by fullerene (light). (e) The diffraction data obtained from the assembled structures as indicated by the dotted circle display polycrystalline ring pattern (in the inset). (f) Magnified data revealed adjacent binary rings are joined by the dendritic growth.

closed ring structures by single domain Fe3O4 NPs isolated from magnetotactic bacteria. The 1:2 mixture of 4 mg/L Fe3O4 NPs and 2.67 mg/L fullerene also formed assembled structures (SI Figure S7d). The electron diffraction pattern (inset image of SI Figure S7d) exposed single and polycrystalline nature of the binary assembly as displayed from the rings and spots. pH-induced surface charge neutralization of Fe3O4 NPs caused significant aggregation as recorded from Dn and ζ measurements at pH 6.5 (SI Figure S5c,d). However, addition of higher volume fraction of fullerene to aggregated Fe3O4 NPs suspension at pH 6.5 enhanced the overall colloidal stability. The proximity of the suspension pH to the point of zero charge (PZC) and overlapping of the excluded volume regions of Fe3O4 NPs/clusters favored fullerene depletion with subsequent enhancement in colloidal stability. HRTEM investigation of the 1:2 mixture of 40 mg/L Fe3O4 NPs and 8 mg/L fullerene E



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with pure and NOM-modified iron oxide colloids in the presence and absence of naturally abundant electrolyte.

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ASSOCIATED CONTENT

* Supporting Information S

Additional information including DLS, MFM/AFM, TEM, and HRTEM data of the pure and assembled structures. This material is available free of charge via the Internet at http:// pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Massachusetts Agricultural Experimental Station (MAS00475) and NSF (CMM10531171). The authors thank Prof. Richard Vachet (Dept. of Chemistry, UMass Amherst) and Prof. Dulasiri Amarasiriwardane (Hampshire College, Amherst) for their help and valuable insight regarding this work. The authors also acknowledge Dr. Anasua Bose for her assistance in editing the manuscript.

Figure 3. Influence of drying on crystallization of binary mixture. (a) AFM image of the epitaxial growth of binary crystals of Fe3O4 NPs and fullerene under very slow drying environment. (b) AFM image of binary crystals of γFe2O3 and fullerene with truncated edges and triangular structure. (c) SEM image of binary crystal of γFe2O3 and fullerene. (d) AFM phase image of flower like binary assembly of Fe3O4 NPs and fullerene obtained after sorption on mica surface followed by drying under N2.

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REFERENCES

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the former. Incorporation of fullerene to the aggregated iron oxide NPs clusters enhanced the overall colloidal stability through entropic depletion stabilization. Electrolyte-mediated charge screening of HA7-coated γFe2O3 NPs and subsequent overlapping of excluded volume facilitates fullerene depletion; hence affecting colloidal stability and fullerene mobility. On air−water interface, binary mixture of magnetic iron oxide NPs and fullerene produced an array of entropy driven assemblies including wires and close-packed structures with resultant immobilization. However, colloidal attachment of HA7-coated γFe2O3 NPs with fullerene produced close-packed binary assembly with “weakly crystalline” and “glassy” domains in the presence of Na+ ion. Electrolyte-induced charge screening and quenching of electrical double layer and subsequent fullerene depletion facilitated the binary assembly formation. The colloidal association of Fe3O4 NPs with fullerene in acidic and neutral pHs produced depletion mediated assemblies along with amorphous aggregates, especially in the case of former. The weak crystallinity or loosely bound amorphous aggregates may lead to release of fullerene particles with relative ease with sudden change in environmental conditions. HA1 and electrolyte mediated binary association of fullerene with Fe3O4 NPs had dendritic growth and polycrystalline circular binary assembly on air−water interface. The formation of thermodynamically favorable short-range periodic binary assembly with subsequent immobilization of fullerene on air− water interface may alleviate fullerene mobility and subsequent exposure. Our study has also indicated that under slow drying conditions, mixtures of magnetic iron oxide NPs and fullerene produced binary crystals; hence likely reduced environmental availability or flow through the “vadose zone”. In addition, the precursor of naturally abundant goethite minerals displayed similar binary assembly formation on air−water interface (SI Figure S9). Thus, environmental fate, transport, and availability of fullerene will be strongly affected due to their interaction F



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Binary Short-Range Colloidal Assembly of Magnetic Iron Oxides

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