Article pubs.acs.org/JPCC
Colloidal Stability and Magnetophoresis of Gold-Coated Iron Oxide Nanorods in Biological Media Swee Pin Yeap,† Pey Yi Toh,† Abdul Latif Ahmad,† Siew Chun Low,† Sara A. Majetich,‡ and JitKang Lim*,†,‡ †
School of Chemical Engineering, Universiti Sains Malaysia, Nibong Tebal, Penang 14300, Malaysia Department of Physics, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States
‡
S Supporting Information *
ABSTRACT: Magnetic iron oxide nanorods are coated with a gold colloid (Fe/Au NRs) to form core−shell particles that combine magnetic and plasmonic properties in a single nanostructure. Three different macromolecules are employed to surface functionalize the nanorod in order to promote colloidal stability of these particles in elevated ionic strength media (∼154 mM NaCl equivalent) that are appropriated for biomedical applications. With a 10 000 molecular weight poly(ethylene glycol) (PEG), the NRs flocculated and sedimented within a few minutes. However, Pluronic F127 or poly(diallyldimethylamonium chloride) (PDDA) coatings yielded stable dispersions for up to 20 h. These NRs exhibit two absorbance peaks at 530 nm and ∼740 nm corresponding to the transverse and longitudinal surface plasmon resonances (SPR). In addition to dynamic light scattering (DLS), spectrophotometry can also be used to monitor dispersion stability because the 530 nm SPR peak shape changes when agglomerates form. The magnetophoretic migration time of these particles, monitored by suspension opacity measurement by light dependent resistor (LDR) under low gradient magnetic separation (LGMS), was prolonged from 1.5 min to ∼8 min after surface functionalization.
1. INTRODUCTION Gold coated iron oxide nanoparticles that exhibit both plasmonic and magnetic functionality are of interest for many biomedical applications. Magnetic nanoparticles (MNPs) have found use in guided drug delivery,1,2 gene therapy,3 cell sorting,4 magnetic imaging,5 intracellular tracking,6 and guidance.7 Most of these applications require the rapid delivery of MNP to targeted cells via magnetophoresis under the influence of an externally applied magnetic field. Gold NPs have also been a subject of intensive biomedical research,8,9 since Au is nontoxic and chemically inert,10 and has a surface plasmon resonance (SPR)11 that can be utilized in sensing or in optical hyperthemia. In addition, gold nanostructures are beneficial for their relative ease of surface biofunctionalization, through either physisorption or well-established thiol chemistry.12 Both magnetic and plasmonic properties depend on particle geometry. In a rod-shaped MNP, shape anisotropy orients the magnetic dipole moment along the long axis, and stabilizes it with respect to thermal fluctuations.13,14 The magnetic moment aligns in the direction of the magnetic field gradient, so a nanorod (NR) will have less viscous drag during magnetophoresis than a spherical particle of the same volume.15,16 Gold nanorods with a high aspect ratio possess two plasmon resonances, corresponding to the oscillation of electrons along the longitudinal and transverse axes.17 These features have stimulated efforts to synthesize hybrid iron/iron oxide and gold NRs.15,18−20 Here having gold as the outer shell improves © 2012 American Chemical Society
the ease of biofunctionalization and facilitates plasmonic imaging.15,21 To fully realize the biomedical applicability of gold coated MNPs, it is crucial to maintain their colloidal stability in high ionic strength environments.22,23 This often requires grafting a layer of macromolecules onto the MNP, since the electrostatic repulsion between nanoparticles is suppressed by Debye screening due to the presence of free ionic species.22,24 The most common macromolecules employed to disperse spherical gold-coated MNPs are amphiphilic polyether triblock copolymers such as Pluronic22 and cationic polyelectrolyte polyethyleneimine.23,24 Since the magnetic nanorods used in this study are not superparamagnetic at room temperature (see the Supporting Information for the M−H curve), the magnetic forces between particles are also significant, and without a surface coating, magnetic NRs form large agglomerates.25 In addition, the electrostatic double layer repulsion between the uncoated particles is screened at high ionic strength within phosphate buffered saline (PBS) solution. Therefore, it is necessary to coat the particles with a steric layer to reduce the van der Waals and magnetostatic interactions. A schematic of the particles studied here is shown in Figure 1. The gold shell provides a good anchoring platform, resulting in the strong adsorption of macromolecules. However, the increased surface Received: June 22, 2012 Revised: September 30, 2012 Published: October 3, 2012 22561
dx.doi.org/10.1021/jp306159a | J. Phys. Chem. C 2012, 116, 22561−22569
The Journal of Physical Chemistry C
Article
Figure 1. (a) Schematic of gold-coated iron oxide NRs (dark core = iron oxide, lighter dots = gold nanoclusters) coated with adsorbed macromolecules. There is a thin layer of PDDA (not shown) used to bind the iron oxide core to the Au clusters for all NRs. In this study, the “bare” gold-coated iron oxide NRs are surface functionalized with one of three different types of macromolecules: (b) PDDA, (c) Pluronic F127, and (d) PEG, as shown as a black line in (a).
that of iron oxide nanoparticles in water (33−39 zJ),28 and this would also enhance the van der Waals interaction between gold-coated iron oxide MNPs.22 For most biomedical applications involving MNPs, magnetic collection or manipulation should be as rapid as possible.4,7 The manipulation, recovery, and collection rate of MNPs using external magnetic fields are dependent on the particle size, shape, concentration, and magnetic field gradient.15,16,29,30 For both high-gradient (∇B⃗ > 1000 T m−1) and low-gradient magnetic separation (∇B⃗ < 100 T m−1) (HGMS and LGMS), rapid magnetophoretic separation can be achieved due to the cooperative aggregation of MNPs leading to the formation of large clusters, followed by migration of the entire aggregate.31,32 Under these conditions, it is critical for the interacting MNPs to lose their colloidal stability through field-induced reversible aggregation. If this hypothesis is correct, the kinetics of the magnetophoresis process should be highly dependent on the surface coating of the MNPs. Here we studied the influence of colloidal stability on the magnetophoretic collection rate under LGMS. The analysis provides valuable information concerning the kinetics of magnetophoresis, a process that is still not wellunderstood.33 In this work, three different macromolecules, Pluronic F127, cationic polyelectrolyte poly(diallyldimethylamonium chloride) (PDDA), and polyethylene glycol (PEG), were employed to stabilize gold-coated iron oxide NRs in phosphate buffered saline solution (PBS). PBS has the typical ionic strength (154 mM NaCl) and pH (7.4) of physiological media and is a common biological buffer. Dynamic light scattering (DLS) was used to measure the hydrodynamic diameter(s) of the dispersed species. When recorded as a function of time, DLS enabled quantitative comparison of the colloidal stability. The optical transmission, or opacity, of the suspension was used to characterize the same phenomena. We measured the opacity as
roughness, seen in the transmission electron micrograph of Figure 2, tends to increase the strength of van der Waals attractions at small separation, and therefore lowers the energy barrier to flocculation.26 The Hamaker constant of gold in water is ∼300 zJ,27 almost an order of magnitude higher than
Figure 2. Transmission electron microscopy (TEM) image of the Aucoated iron oxide NRs after adsorption of Pluronic F127, transfer to PBS, and deposition onto a TEM grid. The tiny black dots covered the entire surfaces of the rod-like particles corresponding to the gold clusters. 22562
dx.doi.org/10.1021/jp306159a | J. Phys. Chem. C 2012, 116, 22561−22569
The Journal of Physical Chemistry C
Article
a function of time in a suspension of NRs without a polymer coating, in zero magnetic field, to study the combined effects of aggregation and sedimentation. When measured in a magnetic field gradient, it monitored the dynamics of aggregate formation and the kinetics of magnetophoresis processes.33,34 This study improves the understanding of the competitive nature of colloidal stability and rapid magnetophoresis of magnetic NRs in physiological media.
Table 1. Electrophoretic Mobility of Iron Oxide Nanorods and Gold-Coated Iron Oxide Nanorods with Different Adsorbed Macromolecules in 0.1 mM NaCl Solutiona electrophoretic mobility (μm cm V−1 s−1) iron oxide NRs Au-coated iron oxide NR bare PEG Pluronic F127 PDDA
2. EXPERIMENTAL SECTION Synthesis and Steric Stabilization of Gold-Coated Nanorods. Iron oxide NRs employed in this work were generously provided to us by TODA American, Inc. Following our previously described method,15 the magnetite nanorods were coated with PDDA (Mw < 100 000) to promote the attachment of 1.5−3.0 nm gold clusters made by Duff’s method.35 The electrostatic interaction between the positively charged PDDA-coated iron oxide NRs (zeta potential, ζ = +25.6 mV) and the negatively charged gold clusters (ζ = −3 mV) promotes nonspecific binding of these small gold nanocolloids onto the PDDA decorated nanorods (Figure 2). To sterically stabilize the gold-coated NR dispersion, ∼10 mg L−1 of the NRs were subjected to the macromolecules of interest (Pluronic F127, low molecular weight PDDA, and PEG) in DI water. A typical macromolecule concentration of 1.0 mg mL−1 was employed to ensure that there are at least 5 orders of magnitude more polymer molecules than would be needed to saturate the nanoparticle surface in the final suspension of 5 × 1010 particles mL−1. This concentration was estimated by assuming the nanorods have the density of bulk magnetite (5.2 g cm−3) and average dimensions of ∼380 nm in length and ∼40 nm in diameter (from Figure 2). Even though the physical dimension of the nanrod employed in this study is larger than the conventional spherical MNP used for biomedical applications, we envisage that this species of particles would have enormous benefits for high yield cell separation,36 and the assembly of a multicellular array37 as driven by a micrometer sized magnetic nanowire (>10 μm). The sterically stabilized NR suspension was subjected to ultrasonication (using a Fisher Scientific ultrasonic probe FS14) for 10 min and then incubated for at least 48 h with continuous gentle agitation in an end-to-end rotating mixer at 30 rpm, with intermittent ultrasonication, to allow full adsorption of macromolecules. The electrophoretic mobility of the Fe/Au NRs was measured (Malvern Instruments Nanosizer) before and after surface functionalization to verify the successful attachment of the macromolecules (Table 1). The uncoated NRs are positively charged due to the PDDA used to promote gold attachment. The adsorption of neutral macromolecules, such as PEG and Pluronic, causes a relative decrease in the electrophoretic mobility, while adsorbing more PDDA increases it (as seen in Table 1). The surface-modified NRs in 0.1 mM NaCl solution were collected by a NdFeB permanent magnet,. while the supernatant was decanted, and the retentate was resuspended into 154 mM ionic strength (10 mL), pH 7.4 PBS solution under ultrasonication for 10 min. Monitoring Colloidal Stability and Magnetophoresis. The colloidal stability of the Fe/Au NRs before and after macromolecule adsorption was probed by DLS (Malvern Instruments Nanosizer). The light scattering intensity autocorrelation function was fit by the CONTIN algorithm to produce an intensity-weighted distribution of equivalent spherical hydrodynamic diameters.38 DLS was also employed to
−2.735 +1.061 +0.653 +0.481 +2.927
a
All of these measurements are conducted under low ionic strength environments to ease the electrophoretic mobility measurement and verify the successful adsorption of macromolecules onto the Au-coated iron oxide NRs.
probe the dynamic behavior of NR aggregation by recording shifts in the particle size distribution to higher values together with the appearance of heterogeneous peaks, and a broadening of the size distribution (as quantified by the standard deviation of the measurements). To quantify the transient behavior of magnetophoresis, we placed a light dependent resistor (LDR) on one side of the cuvette containing the NR dispersion opposite a light source (Figure 3). The initial dark purple dispersion becomes
Figure 3. Pictorial representation of the method employed to measure the opacity of the dispersion during magnetophoresis. The light transmitted through the NR dispersion reaches the photoresistor on the other side of the cuvette, roughly 1 cm from the bottom. (a) Prior to magnetophoretic collection, the NRs are distributed uniformly and block most of the light. (b) After magnetophoretic collection, most of the NRs are localized at the cuvette wall next to the permanent magnet and almost all light passes through.
progressively more transparent, allowing more light to be detected by the LDR. As the light intensity detected by the LDR increases, its voltage output decreases. The LDR voltage at time t, V(t), is converted into a normalized opacity θ:33 θ (t ) =
V (t ) − Vmin V (0)
(1)
where V(0) is the initial readout and Vmin is the lowest voltage achieved, corresponding to full removal of the NRs from the illuminated zone. Magnetic collection was achieved by placing a N50-grade NdFeB permanent magnet (Ningbo YuXiang E&M Int’l Co.,Ltd.) with a surface magnetic field of ∼6.0 kG (measured by an AlphaLab Model GM2 DC Magnetometer) perpendicular to the light beam (Figure 3b). This arrangement applied a low field gradient (
