Source: https://pubs.rsc.org/EN/content/articlehtml/2016/en/c5en00089k?page=search
Timestamp: 2019-04-20 20:17:04+00:00

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Advances in nanoscale engineering and material science not only allow for the precise size, shape and composition control of engineered nanoparticles, but also for tunable surface modification techniques needed aqueous stability for advanced environmental applications. In this report, we have systematically designed, synthesized, and evaluated water stable, 8 nm superparamagnetic iron oxide nanoparticles with a rational series of 13 bilayer surface structures. Detailed synthesis strategies, process efficiencies, and fundamental properties of each resulting particle suspension are summarized and compared under environmentally relevant conditions. Findings directly advances current knowledge for needed control/tailoring of particle surface chemistries with regard application of engineered superparamagnetic nanoparticles, which have broad, yet unique potential in next generation remediation and sensing environmental technologies, among others.
Room temperature, aqueous-based, co-precipitation methods, using ferrous and ferric salts, are widely used to obtain relatively large amounts of ‘hydrophilic’ iron oxide NPs.3 Although the procedure is simple and convenient, this method is limited due to poor control over particle size (i.e. broad size distribution) along with the formation of large, non-stable aggregates.9,22 In contrast, iron oxide NPs with tunable sizes, narrow size distribution, and high (even single) crystallinity can be synthesized via decomposition of iron precursor(s) at high temperature (over 260 °C) in organic, apolar solvents.23,24 Various iron precursors, including iron cupferron, iron pentacarbonyl, iron acetylacetonate, and iron oleate have been used to prepare monodisperse iron oxide NPs for a range of particle sizes (4–30 nm).3,25 For such methods, oleic acid is widely used as a surface stabilizer during synthesis.5 The carboxylate group from oleic acid is chemisorbed onto the oxide surface as the NP is formed, with the hydrophobic long alkyl chain (C18) interfacing with the nonpolar solution. As a result, the NPs synthesized from this approach are monodispersed and stable in apolar organic solvents (e.g. hexane, chloroform, toluene, etc.).5,26 For effective phase transfer into water, additional surface modification is required.
Fig. 1 Schematic representation for the phase transfer of NPs from nonpolar solvent to aqueous solution facilitated by a bilayer structure formation process.
In this work, 8 nm, monodisperse iron oxide, superparamagnetic NPs were prepared by pyrolysis of iron carboxylate salts in the presence of oleic acid and 1-octadecene. These particles were correspondingly transferred into water via a tailored ligand addition (bilayer formation) approach, which were characterized over a range of transfer conditions, including 13 different ligand types. Bilayer phase transfer procedures were optimized and reported based on transfer yield as a function of sonication amplitude, sonication time and surfactant concentration used for each ligand. Resulting aqueous NPs suspensions were characterized through transmission electron microscope (TEM) and surface charge via zeta (ζ) potential measurements. For each of the 13 surface stabilization strategies (bilayers), NP aggregation kinetics and long-term colloidal stabilities are quantitatively described as a function of ionic strength/type and storage time using time-resolved dynamic light scattering (TR-DLS).
Iron(III) oxide (hydrated, catalyst grade, 30–50 mesh), oleic acid (technical grade, 90%), 1-octadecene (technical grade, 90%), oleic acid (OA, 99%), elaidic acid (EA, 99.0%), sodium stearate (SA, 99.0%), sodium palmitate (PA, 98.5%), sodium myristate (MA, 99%), sodium laurate (LA, 99%), sodium decanoate (DA, 98%), sodium monododecyl phosphate (SDP), sodium dodecyl sulfate (SDS, 99.0%), dodecyltrimethylammonium bromide (C12TAB, 98%), N,N-​dimethyl-​N-​dodecylglycine betaine (EMPIGEN), sodium chloride (ACS reagent, 99.0%), calcium chloride dihydrate (ACS reagent, 99%), and nitric acid (trace metal grade) were all purchased from Sigma-Aldrich. Sodium ricinolate (RA, 90%) and sodium dodecylbenzenesulfonate (SDBS, 95%) were purchased from TCI America. Reagent grade of hexane, acetone, and ethanol were purchased and used without purification.
Iron oxide NPs were prepared using a method previously reported.24 To synthesize 8 nm iron oxide NPs, 0.178 g FeO(OH) fine powder, 2.26 g oleic acid and 5.0 g 1-octadecene were stirred in a three-neck flask equipped with a heating mantle and temperature controller. The system was kept at 120 °C for 1 h to remove residual water and then heated to 320 °C for 1 h under argon condition. After the reaction at high temperature, the resulting brown-black colloid was purified by acetone and methanol; for details, the synthesized colloid (10 ml) was collected in a centrifuge tube mixing with 40 ml of ethanol/acetone solution and centrifuged at 6000 rpm for 15 min. This procedure was repeated 4–5 times to remove unreacted iron salts, excess organic moieties. Purified iron oxide NPs were finally collected in hexanes and stored at 4 °C.
The purified NPs were transferred from hexane to water by ligand addition (bilayer) method using a probe sonicator.10,31 Specifically, 1.0 mL of NPs in hexane solution (1–5 g L−1) and variable amounts of ligand were added to 10 mL of ultrapure water (Millipore, 18.2 Ω) in a glass vial. The mixture of organic and aqueous phase was then subjected to a probe sonicator (UP 50H, Dr. Hielscher, GMHB) for 3–6 min at various amplitude (60–75%) and full cycle. The cloudy and colored suspension after sonication was kept stirred for 1 day for the evaporation of residual hexane. The aqueous phase was collected and the NPs were purified via ultracentrifugation (Sorvall WX Ultra 80, Thermo scientific), membrane filtration (Ultrafiltration cellulose membranes, 100 KDa MWCO), followed by redispersion and filtration through a syringe filter (pore size of 0.2 um, Millipore).
NPs core size was characterized by transmission electron microscopy (TEM, FEI Tecnai G2 Spirit) operated at 120 kV. TEM samples were prepared by placing a small drop (10 uL) of the diluted NPs suspension on a carbon coated copper grids (Electron Microscopy Sciences) and left to dry at room temperature (22 ± 0.5 °C). The average diameter (with size distribution) was obtained by counting more than 1000 randomly chosen NPs from the TEM micrographs using ImageJ software (National Institutes of Health).
To determine the iron concentration of NPs in both hexane and water, iron oxide NPs were digested by strong nitric acid (10%) and analyzed with inductively coupled plasma optical emission spectroscopy (ICP-OES, Perkin Elmer ELAN DRC).
Bi-layers coating surface concentrations were quantified using a total organic carbon analyzer (TOC-L, Shimadzu Scientific Instrument, Inc., MD; 680 °C), similar to others studying inorganic nanoparticles stabilized by organic coatings.33,34 All NPs samples were diluted to the same concentration (as Fe). Before measurement, samples were acidified (HCl) to remove any inorganic carbonates.
Magnetization measurements were carried out with a superconducting quantum interference device (SQUID) magnetometer (Quantum Design MPMS 5XL SQUID). Magnetization and hysteresis data were collected at a temperature of 300 K up to 5 T with powders of NPs.
Serial concentrations of salt stock solutions (ACS grade NaCl and CaCl2) were prepared and filtrated (pore size of 0.2 μm, Millipore) before use. All DLS and zeta potential measurements were conducted at room temperature (22 ± 0.5 °C).
The hydrodynamic diameters and zeta (ζ) potentials of NPs in water were measured by dynamic light scattering (Zetasizer, Malvern Nano ZS, UK). Triplicate samples were prepared and measured for the size and zeta potential analysis. The average value and the standard deviation of size and zeta potential were calculated from at least 5 measurements.
where k is the initial aggregation rate constant at examined salt concentrations and kfast is the aggregation rate under diffusion-limited (fast) aggregation conditions.
Fig. 1 schematically illustrates the general procedure of the bilayer phase transfer approach used in this study. 8.1 ± 0.6 nm iron oxide NPs were prepared from decomposition of iron carboxylate in organic media (1-octadecene) and surface stabilized (in organic phase) with oleic acid. These materials were chosen as they are not only relevant to environmental sensing and remediation applications,4,8,10,11,14 but also because they can be synthesized as highly monodispersed NPs with narrow size distribution as seen from Fig. 1. SQUID analysis indicates these NPs are superparamagnetic at 300 K with negligible coercivity, thus net magnetization of zero in the absence of an external magnetic field (Fig. S1 in the ESI†).
Bilayer formations were achieved by mixing varied amounts of select surfactant with iron oxide NPs suspension in hexane and ultrapure water (two phases) via probe sonication, as detailed in Tables S1–S12 for each ligand in the ESI.† This process of particle transfer and subsequent stabilization from hexane into water is visualized in Fig. S2 in the ESI.† For all systems described, when NPs are transferred from hexane into water, they remain monodispersed as observed from TEM micrographs (Fig. 1 and Fig. S3 in the ESI†) and DLS data (Table 1).
For each ligand, Tables S1–S12 in the ESI† show the detailed hydrodynamic diameter and transfer yield as a function of sonication amplitude, sonication time and surfactant concentration tested. The sonication amplitude used in our study ranges between 60% and 75% (UP 50H, 50 watts, 30 kHz). We observed that it was not efficient to transfer these NPs from hexane into water when the sonication amplitude was below 50%, and while under high sonication amplitude (>90%), aggregates of NPs were seen during the transfer process. Typically, phase transfer process was completed in a short time (3–6 min), and longer sonication time did not further improve the phase transfer efficiencies. After phase transfer and purification (ultracentrifugation, ultrafiltration and syringe filtration described above), aqueous NP suspensions were characterized by DLS at room temperature and stored in the dark. For most, the hydrodynamic diameter of NPs coated with bilayered surface stabilizers are around 20 nm, indicating these NPs have a very thin and compact (bilayer) coating structures. Oleic acid (OA) bilayer coated NPs have the smallest hydrodynamic size of 15.6 ± 1.3 nm while the decanoic acid (DA) coated NPs have the largest hydrodynamic size of 25.1 ± 0.7 nm. Phase transfer yield (%) was calculated by measuring the total iron concentration transferred to water compared to initial iron concentration added to the hexane solution. The highest observed yield was 95% for SDP coated NPs, which as a phosphate has the highest formal charge (−2) head group of all ligands evaluated, while C12TAB and elaidic acid surfactant reached a maximum of 47% and 26%, respectively. Generally, most of the surfactants allow for relatively high transfer efficiencies but never reaching 100% due to the difficulty to mixing limitations and interfacial partitioning of/at the two different phases.31 For optimized phase transfer conditions, particle coating densities, as measured by total organic carbon (TOC), for the 13 surfactants (as outer layers) range from 80–105 ppm (ca. 4900–6650 mol of outer layer surfactant per mol NP, or as 3.16–5.0 × 10−5 mol outer layer surfactant per m2 of NP as tabulated in Table S13, in the ESI†). Similar coating densities allow for surface charge, aggregation kinetic(s), and colloidal stability comparisons, which are discussed below.
Fig. 2 Zeta potentials of bilayer coated NPs as a function of pH.
Fig. 3 Aggregation profiles of DA-NPs in the presence of (a) NaCl and (b) CaCl2 at pH 7.0.
Fig. 4 Attachment efficiencies of OA-NPs and EA-NPs as functions of (a) NaCl and (b) CaCl2 concentrations at pH 7.0.
The CCC values for OA-NPs are 710 mM for NaCl and 10.6 mM for CaCl2, while the CCC values for EA-NPs are 260 mM for NaCl and 7.4 mM CaCl2. The α value for OA-NPs is comparatively smaller than that of EA-NPs under the same electrolyte concentration in the reaction-limited regime. We hypothesize that cis (oleyl) forms of unsaturated–unsaturated oleyl carbon chains may lead to enhancement in bilayer stability due, in part to, stronger van der Waals (primarily as London type forces) interactions, as the tails are sterically aligned, with lower (molecular) degree(s) of freedom, compared to the relatively straighter EA-(trans-)unsaturated–OA-(cis-)unsaturated carbon chain.
Fig. 5 Attachment efficiencies of DA-NPs (C10), LA-NPs (C12), MA-NPs (C14), PA-NPs (C16), and SA-NPs (C18) as functions of (a) NaCl and (b) CaCl2 concentrations at pH 7.0.
The effect of head group functionality on NPs colloidal stability is compared in Fig. 6. LA, SDS, SDP, C12TAB, and EMPIGEN have similar size (C12) and type (unsaturated, aliphatic) of hydrophobic tails but vary with regard to functional head groups. Carboxylate group functionalized LA-NPs have CCC values of 16 mM NaCl and 0.5 mM CaCl2. SDS coated NPs with a sulfate functional head group has CCC values of 45 mM NaCl and 1.4 mM CaCl2. With phosphate based hydrophilic head group, SDP-NPs have a higher CCC values in NaCl (250 mM) and CaCl2 (3.6 mM). The highest CCC values were obtained when positive and zwitterionic functionalized second layer was applied. C12TAB and EMPIGEN coated NPs have the CCC values of 555 mM and 766 mM for NaCl, 11.1 mM and 11.3 mM for CaCl2, respectively.
Fig. 6 Attachment efficiencies of LA-NPs, SDS-NPs, SDP-NPs, C12TAB-NPs, and EMPIGEN-NPs as functions of (a) NaCl and (b) CaCl2 concentrations at pH 7.0.
Fig. 7 Attachment efficiencies of OA-NPs and RA-NPs as functions of (a) NaCl and (b) CaCl2 concentrations at pH 7. Attachment efficiencies of SDS-NPs and SDBS-NPs as functions of (c) NaCl and (d) CaCl2 concentrations at pH 7.0.
In addition to short-term aggregation studies, long-term stability of all materials in water was evaluated. Libraries of samples were sealed and stored in the dark at room temperature and DLS size measurements were taken at 1 day (d), 1–2 weeks (wk), 1–6 months (mo) and 1 year (yr). Fig. 8 shows the change in hydrodynamic diameter of NPs in ultrapure water as a function of time. The initial hydrodynamic diameters for these NPs were around 20 nm and the sizes were still below 50 nm after one year of storage. For example, RA coated NPs demonstrate negligible size change (<2 nm) during the entire time (one year).
Fig. 8 Long-term stability of bilayer coated NPs in ultrapure water.
This work is supported by American Chemical Society's Petroleum Research Fund (#52640-DNI10), the National Science Foundation (NSF) (CBET, #1236653 and #1437820), and U.S. Army Corps of Engineers (W912HZ-13-2-0009-P00001). TEM, DLS, Ultracentrifugation, and ICP-OES measurements were provided by the Nano Research Facility (NRF) at Washington University in St. Louis, a member of the National Nanotechnology Infrastructure Network (NNIN), which is supported by the NSF (#ECS-0335765). All SQUID measurements were performed at the UCSB MRL Shared Experimental Facility, which is supported by the NSF MRSEC Program Award No. DMR 1121053 (and is a member of the NSF-funded Materials Research Facilities Network).
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