- Email: [email protected]
Design and synthesis of plasmonic magnetic nanoparticles
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 311 (2007) 78–83 www.elsevier.com/locate/jmmm
Design and synthesis of plasmonic magnetic nanoparticles JitKang Lima, Robert D. Tiltona,b, Alexander Eggemanc, Sara A. Majetichc, a
Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, PA 15213-3890, USA b Biomedical Engineering, Carnegie Mellon University, Pittsburgh, PA 15213-3890, USA c Physics Carnegie Mellon University, Pittsburgh, PA 15213-3890, USA Available online 20 December 2006
Abstract Core–shell nanoparticles containing both iron oxide and gold are proposed for bioseparation applications. The surface plasmon resonance of gold makes it possible to track the positions of individual particles, even when they are smaller than the optical diffraction limit. The synthesis of water-dispersible iron oxide-gold nanoparticles is described. Absorption spectra show the plasmon peaks for Au shells on silica particles, suggesting that thin shells may be sufficient to impart a strong surface plasmon resonance to iron oxide-gold nanoparticles. Dark field optical microscopy illustrates the feasibility of single-particle detection. Calculations of magnetophoretic and drag forces for particles of different sizes reveal design requirements for effective separation of these small particles. r 2006 Elsevier B.V. All rights reserved. Keywords: Magnetic nanoparticle; Plasmon; Dark-field microscopy; Magnetophoresis; Biosensing
1. Introduction Applications involving the controlled motion of magnetic micro- or nano-particles in microfluidic systems are diverse and growing rapidly. Some of the most interesting developments are in the fields of microbiology and biomedicine where magnetic particles are used to target, label, manipulate and separate biomaterials such as cells, enzymes, antigens and DNA [1–3]. Current clinical cell/ biomolecule sorting devices are based on immunoaffinity columns or on high-gradient magnetic separation (HGMS) columns utilizing either micrometer-sized polymeric beads doped with magnetite, or nanometer-size iron dextran colloids, conjugated to targeting antibodies [4,5]. For HGMS, the two most important design parameters are (a) the magnetic moment of a magnetic label, and (b) the generation of a high magnetic field gradient [6]. Different strategies have been proposed to generate high gradient magnetic fields using magnetic dipole [7] or quadrupole sorting [8]. However, magnetophoretic microfluidic systems are also promising [9,10], and may be better suited for Corresponding author. Tel.: +1 412 268 3105; fax: +1 412 681 0648.
E-mail address: [email protected] (S.A. Majetich). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.1169
bioseparation involving extremely small amounts of material that could later be amplified, such as DNA purification or the separation of cell subpopulations with different degrees of surface antigen expression. Here we describe a new type of magnetic nanoparticle (Fig. 1) that combines the advantages of magnetite for magnetic separation with the surface plasmon resonance of gold, which enables single particles to be tracked optically, even though they are smaller than the diffraction limit. The gold shells exhibit strong plasmonic bands in their UV–visible absorption spectra, providing a way to detect and quantitatively characterize the separation effectiveness in situ. Conventional cytometric analysis requires fluorescent probe labelling, and hence extra effort is needed to conjugate fluorophore molecules onto the cell/magnetic particle complex. An additional advantage of the proposed gold shells is their extremely large molar extinction coefficient, greater than 105 M1 cm1 at wavelengths in the 500–600 nm range [11]. Resonant Rayleigh scattering efficiencies equivalent to 106 fluorescent dye molecules [12] have been reported for gold nanoparticles. In addition, this plasmonic response is not vulnerable to the photobleaching effects that arise during prolonged and/or high intensity illumination of fluorescent tags.
ARTICLE IN PRESS J. Lim et al. / Journal of Magnetism and Magnetic Materials 311 (2007) 78–83
79
Fig. 1. Left: a nanoshell particle with magnetite core and gold shell. Right: a composite nanoparticle with silica core, decorated by magnetite nanoparticles and coated with a gold shell.
Fig. 2. (a) TEM micrograph and (b) characteristic electron diffraction rings of magnetite particles in toluene.
2. Particle synthesis Synthesis of nanoshell and/or composite nanoparticles proceeds first by the synthesis of individual magnetite, Au, and silica nanoparticles. These are then assembled and subjected to further growth to create the gold shell, either with a magnetite core and a gold shell or a with a silica core, and then shells of magnetite and gold nanoparticles. The size of the constituent particles can have a dramatic effect on the magnetic properties (iron oxide particles) and plasmonic behavior (gold particles) and so synthetic control is very important to obtain a high degree of uniformity. A hightemperature polyol process developed by Sun et al. [13] was used to make magnetite nanoparticles in organic solvent. Oleic acid (OA) and oleylamine (OY) were used as capping agents that sterically stabilize the particles in toluene/hexane. After the first step, 8 nm iron oxide nanoparticles with high crystallinity are obtained. Based on the lattice spacing corresponding to five strongest electron diffraction intensity rings (Fig. 2), the nanoparticles are identified as either magnetite or maghemite but the observed black color suggests that they are predominantly magnetite. These nanoparticles then serve as seeds for further growth. After two reaction stages, the average diameter of the magnetite nanoparticles was 11.0571.7 nm (shown in Fig. 2), with the size distribution shown in Fig. 3. A modified Sto¨ber process [14] was used to make silica spheres with a diameter of 50 nm. This involves the slow
Fig. 3. Iron oxide particles size distribution. Scion Image Beta 4.03 for Windows 95 to XP was used to identify individual particles and ImageJ 1.36 was used to determine the average particle diameters.
base hydrolysis of tetraethyl orthosilicate in ethanol. High monodispersity and spherical shape is ensured by slow diffusional growth over 24 h. Controlling the volume ratio of ammonium hydroxide to tetraethyl orthosilicate allows the particle size to be altered between 50 and 500 nm. Ultrafine gold nanoparticles with an average size of 2 nm
ARTICLE IN PRESS 80
J. Lim et al. / Journal of Magnetism and Magnetic Materials 311 (2007) 78–83
were produced by the technique developed by Duff et al. [15]. The particle size mismatch between the gold and iron oxide or the silica particles was crucial to ensure the formation of a core–shell structure. The magnetite nanoparticles were transferred from toluene into water by a modified version of the method proposed by Maceira et al. [16]. Fig. 4 illustrates the major steps. Magnetite nanoparticles are first washed in ethanol to get rid of excess surfactant, OA/OY. The nanoparticles are then collected with a permanent magnet and redispersed into 10 wt% tetramethylammonium hydroxide (TMAOH) solution in water following 10 min of ultrasonication. Afterwards the nanoparticles are collected with a permanent magnet and redispersed into fresh 10 wt% TMAOH, and incubated overnight. The final washing step, using ethanol, removes remaining OA and OY from the sol. Without this step, the particles will flocculate over time. Several washing steps in TMAOH were needed to ensure complete replacement of OA/OY with negatively charged surface groups. These particles have a zeta potential of 41.9 mV, measured in deionized water, which indicates the replacement of OA/OY with a negatively charged surface. Evidence of the removal of the organic layer from the surface of the nanoparticles was also provided by FTIR spectroscopy measurements before and after the transferring steps. The collected FTIR spectra (Fig. 5) show peaks at 2871 and 2922 cm1, corresponding to the symmetric and asymmetric CH2 stretching modes of the oleyl group. These peaks are strong for the nanoparticle suspension in toluene, but the intensity dramatically decreased after washing and phase transfer into water. In deionized water, gold, magnetite and silica particles are all negatively charged. Further surface modification is needed to promote surface binding of magnetite or silica particles with gold particles. Fig. 6 shows a generic representation of major steps involved in producing the gold shell structure. The seeding stage is necessary to overcome the difference in surface energies of the different component materials. Gold has a much higher surface energy than either silica or
magnetite, and so it will not deposit evenly over these surfaces. Attaching gold seeds creates nucleation sites for further metal deposition, and also alters the effective surface energy of the oxide particle allowing an even metal layer to be deposited. After this the seeded particles can grow to full core–shell structure by using formaldehyde as a mild reducing agent. 3-Aminopropyltriethoxy silane (APS) [17] and 11-mercaptoundecanoic acid were selected as two possible means to modify the surface of magnetite particles that serve to promote further binding of gold nanoparticles (see Fig. 7). It was found that APS-coated magnetite particles quickly became unstable in water, whereas 11-mercaptoundecanoic acid modified nanoparticles were stable for up to a month in aqueous suspension. This observation makes 11mercaptoundecanoic acid more attractive as a surface functionalization agent compared to APS. This functiona-
Fig. 5. FTIR spectra for 11 nm nanoparticles in toluene and in deionized water. Absorption peaks for the two-phonon feature of the diamond crystal attenuated total reflection (ATR) optical element appear at 2000 and 2200 cm1; the peaks for the oleic acid/oleylamine CH2 stretching modes appear at 2871 and 2922 cm1 and are significantly diminished in intensity after rinsing in TMAOH solution.
Fig. 4. Major steps involved to transfer magnetite nanoparticles from organic solvent into water (a) replacement of oleic acid (OA) and olelyamine (OY) surface with charged species (b) additional washing of particles to encourage further OA/OY replacement (c) removal of the remaining OA/OY.
ARTICLE IN PRESS J. Lim et al. / Journal of Magnetism and Magnetic Materials 311 (2007) 78–83
81
Fig. 6. Major steps for core–shell structure formation (a) surface modification of iron-oxide particles (b) gold particles attachment onto surface modified iron oxide particles (c) complete coarsening of gold seeded iron oxide particles forming core–shell structure.
Fig. 7. TEM micrograph of 11-meracaptoundecanoic acid and APS modified magnetite particles after attachment of a small number of gold particles and transfer to water. Because the particles are charge stabilized, rather than sterically stabilized, they flocculate during drying. Individual gold nanoparticles are evident on the iron oxide particle surfaces.
lization deposits thiol groups onto the iron oxide surface that are then accessible to bind to gold nanoparticles. 3. Nanoparticle optical characterization and dark-field imaging
Fig. 8. UV–Vis spectra of 50 nm silica cores with gold seeds or shells attached. The spectrum of an aqueous suspension of the silica particles seeded with individual 2 nm gold nanoparticles is shown to illustrate the red-shift of the plasmon resonance after growing the seeds into complete gold shells.
Nanoparticles of gold and silver are of interest for optical applications because they exhibit a surface plasmon resonance at optical frequencies. This results in strong absorption and scattering of light by the particles at specific frequencies. One interesting property of the shell structures proposed is that the resonant frequency can be varied over wider frequency ranges compared to simple homogeneous nanoparticles [18–20]. A good explanation of surface plasmon resonances and the optical properties of nanoshells can be found in Refs. [18,19]. An important conclusion drawn from earlier studies is that the resonance condition for the core–shell particles is determined by the ratio of the core-radius to the total radius of the particle. In this way the resonance can be
‘tuned’ by varying the shell thickness. This can be seen in Fig. 8 where UV–vis absorption spectra are shown for gold-seeded 50 nm silica cores, along with 10 and 20 nm thick, complete gold shells on the same silica cores. The gold nanoparticles have a characteristic resonance at 520 nm, and as the particles merge to form a shell the resonance shifts to longer wavelengths. As the shell thickens a further red-shift as far as 560 nm is seen. An extinction coefficient of 104 M1 cm1 can be estimated using Beer’s Law, which is comparable to previous measurements in Ref. [11]. The presence of magnetite within the cores compared to silica will lead to a small redshift of the plasmon peak.
ARTICLE IN PRESS J. Lim et al. / Journal of Magnetism and Magnetic Materials 311 (2007) 78–83
82
Another important effect seen from scattering theory is that the scattered intensity from the core-shell particles varies as the sixth power of the total particle radius, but there are only small absorptive losses, as seen by the increased intensity of the plasmon peak in Fig. 8. This makes the particles very suitable for imaging by optical dark-field microscopy. Dark-field microscopy collects only light that has been scattered obliquely by the sample. If no sample is present, and the microscope slide is otherwise smooth, no image is detected. In the presence of a sample of strongly scattering particles, a bright spot appears against the dark background. Although any small particle will scatter light, the strong scattering associated with the surface plasmon resonance of Au or Ag nanostructures makes individual nanoparticles readily detectable. Observation of Ag or Au particles as small as 30 nm have been reported [21], which is significantly smaller than the optical diffraction limit. Fig. 9 is an optical dark-field micrograph of 50 nm silica particles with 10 nm gold shells. The strong scattering that was evident in the UV–vis spectrum causes the particles to appear very distinct. The micrograph was obtained with a 10 oil immersion objective lens. This indicates that these thin gold shells are sufficient for darkfield microscopy detection of individual nanoparticles. 4. Requirements for magnetic separation For known values of the magnetic field B and magnetic field gradient XB, the magnetic force Fmag, given by F mag ¼
DwV p ðrBÞB, m0
(1)
and the magnetophoretic velocity, umag, given by umag ¼
F mag , 6p Zr
(2)
can be calculated [10,22]. Here Z is the viscosity of the medium and r is the particle radius, Vp is the particle volume and Dw is the dimensionless difference in magnetic susceptibility between the particle and the fluid. The magnetophoretic velocity umag is proportional to the square of the particle radius and to the magnetic susceptibility of the particle, umag / r2 wp .
(3)
Since the magnetophoretic behavior of the nanoparticles is highly size dependent, a series of calculations were carried out to probe the size-dependent effects of magnetophoresis for both single magnetite particles and composite nanoparticles. Very large magnetic field gradients are needed in order to overcome viscous drag forces on small nanoparticles, so that magnetophoretic forces dominate over Brownian motion. Using micropatterned wires, field gradients as high as 5000 T/m are possible [3]. Fig. 10 shows how the magnetite nanoparticle velocity depends on the flux gradient. For 15 nm particles to have a velocity of 10 mm/ min, they would require a field gradient of 35 T/m. This field gradient is readily achievable by high gradient magnetic separators [23,24] but due to the spatial variation of the field gradient, nanopatterned elements [25] or nanowires are more promising for the manipulation of individual particles. A further design consideration for magnetic separation of small nanoparticles is that the magnetophoretic force will decay rapidly as a function of distance from the surface of the magnetic element. Fig. 11 shows the magnetophoretic force on particles of different diameters as a function of the distance from the magnetic pole. For 30 nm particles, the viscous drag force is 8 1018 N, roughly an order of magnitude smaller than the magnetophoretic force at the surface of the ferromagnetic element. However, at distances greater than 1.5 mm from the surface, the drag force overwhelms magnetophoresis and the particle is not moved significantly. 5. Conclusions
Fig. 9. Dark-field optical micrograph showing bright spots associated with the strong scattering from individual 50 nm silica cores with 10 nm thick gold shells. The particles were in aqueous suspension over a glass cover slip. Particles were observed to undergo Brownian motion, confirming that spots correspond to discrete nanoparticles.
The design of bi-functional nanoparticles with magnetic cores for magnetic separation and Au shells for plasmonic optical imaging has been described, along with the status of efforts to realize these particles. Magnetite nanoparticles decorated with gold nanoparticles have been prepared and are stable in aqueous suspension. Using silica as a stand-in core particle material, it was shown that an incomplete Au shell is insufficient for a strong surface plasmon peak, but individual particles with a 10 nm shell can clearly be observed as they undergo Brownian motion using darkfield optical microscopy. Future work will explore the ability to separate these particles using magnetophoresis while imaging their trajectories optically. Preliminary
ARTICLE IN PRESS J. Lim et al. / Journal of Magnetism and Magnetic Materials 311 (2007) 78–83
83
Fig 10. (a) Particle velocity in water as a function of the diameter and the local magnetic field gradient. Here the magnetic field at the surface of the micropatterned pole tip is 0.064 T. (b) Magnetophoretic force as a function of distance, for different particle diameters.
calculations indicate that large magnetic field gradients will be required, such as those achievable with micropatterned wires or ferromagnetic elements. Acknowledgments This work was supported in part by the NSF through grant number ECS-0304453 and by the Pennsylvania Infrastructure Technology Alliance. The authors also thank Fred Lanni of the Carnegie Mellon University Biological Sciences Department for assistance in the optical dark-field microscopy experiments. References [1] M.A.M. Gijs, Microfluid Nanofluid 1 (2000) 22. [2] M. Tondra, M. Granger, R. Fuerst, et al., IEEE Trans. Magn. 37 (2001) 2621. [3] M. Berger, J. Castelino, R. Huang, et al., Electrophoresis 22 (2001) 3883. [4] K. Auditore-Hargreaves, S. Heimfeld, R.J. Berenson, Bioconj. Chem. 5 (1994) 287. [5] J. Ugelstad, P. Stenstad, L. Kilaas, E. Hornes, et al., Blood Purification 11 (1993) 349. [6] J. Chalmers, M. Zborowski, L. Moore, et al., Biotechnol. Bioeng. 59 (1998) 10.
[7] L. Moore, M. Zborowski, L.P. Sun, et al., J. Biochem. Biophys. Meth 37 (1998) 11. [8] M. Zborowski, L.P. Sun, L. Moorem, et al., J. Magn. Magn. Mater. 194 (1999) 224. [9] Y. Sahoo, E.P. Furlani, J. Phys. D 39 (2006) 1724. [10] N. Pamme, A. Manz, Anal. Chem. 76 (2004) 7250. [11] T.R. Jensen, M. Duval Malinsky, C.L. Haynes, et al., J. Phys. Chem. B 104 (2000) 10549. [12] J. Yguerabide, E.E. Yguerabide, Anal. Biochem. 262 (1998) 157. [13] S.H. Sun, H. Zeng, D.B. Robinson, et al., J. Am. Chem. Soc. 126 (2004) 273. [14] W. Sto¨ber, A. Fink, E. Bohn, J. Colloid Interface Sci. 26 (1968) 62. [15] D. Duff, A. Baiker, P.P. Edwards, Langmuir 9 (1993) 2301. [16] V.S. Maceira, L.M. Liz-Marzan, M. Farle, Langmuir 20 (2004) 6946. [17] M. Takafuji, S. Ide, H. Ihara, et al., Chem. Mater. 16 (2004) 1977. [18] R.D. Averitt, D. Sarkar, N.J. Halas, Phys. Rev. Lett. 78 (22) (1997) 4217. [19] R.D. Averitt, S.L. Westcott, N.J. Halas, J. Optic. Soc. Am. B 16 (10) (1999) 1824. [20] A.L. Aden, M. Kerker, J. Appl. Phys. 22 (1951) 601. [21] A.D. McFarland, R.P. VanDuyne, Nano Lett. 3 (2003) 1057. [22] G.P. Hatch, R.E. Stelter, J. Magn. Magn. Mater. 225 (2001) 262. [23] L. Sun, M. Zborowski, L.R. Moore, et al., Cytometry 33 (1998) 469. [24] T. Schnieder, L.R. Moore, Y. Jing, et al., J. Biochem. Biophys Meth 68 (2006) 1. [25] B.B. Yellen, G. Friedman, Adv. Mater. 16 (2004) 111.
Journal of Magnetism and Magnetic Materials 311 (2007) 78–83 www.elsevier.com/locate/jmmm
Design and synthesis of plasmonic magnetic nanoparticles JitKang Lima, Robert D. Tiltona,b, Alexander Eggemanc, Sara A. Majetichc, a
Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, PA 15213-3890, USA b Biomedical Engineering, Carnegie Mellon University, Pittsburgh, PA 15213-3890, USA c Physics Carnegie Mellon University, Pittsburgh, PA 15213-3890, USA Available online 20 December 2006
Abstract Core–shell nanoparticles containing both iron oxide and gold are proposed for bioseparation applications. The surface plasmon resonance of gold makes it possible to track the positions of individual particles, even when they are smaller than the optical diffraction limit. The synthesis of water-dispersible iron oxide-gold nanoparticles is described. Absorption spectra show the plasmon peaks for Au shells on silica particles, suggesting that thin shells may be sufficient to impart a strong surface plasmon resonance to iron oxide-gold nanoparticles. Dark field optical microscopy illustrates the feasibility of single-particle detection. Calculations of magnetophoretic and drag forces for particles of different sizes reveal design requirements for effective separation of these small particles. r 2006 Elsevier B.V. All rights reserved. Keywords: Magnetic nanoparticle; Plasmon; Dark-field microscopy; Magnetophoresis; Biosensing
1. Introduction Applications involving the controlled motion of magnetic micro- or nano-particles in microfluidic systems are diverse and growing rapidly. Some of the most interesting developments are in the fields of microbiology and biomedicine where magnetic particles are used to target, label, manipulate and separate biomaterials such as cells, enzymes, antigens and DNA [1–3]. Current clinical cell/ biomolecule sorting devices are based on immunoaffinity columns or on high-gradient magnetic separation (HGMS) columns utilizing either micrometer-sized polymeric beads doped with magnetite, or nanometer-size iron dextran colloids, conjugated to targeting antibodies [4,5]. For HGMS, the two most important design parameters are (a) the magnetic moment of a magnetic label, and (b) the generation of a high magnetic field gradient [6]. Different strategies have been proposed to generate high gradient magnetic fields using magnetic dipole [7] or quadrupole sorting [8]. However, magnetophoretic microfluidic systems are also promising [9,10], and may be better suited for Corresponding author. Tel.: +1 412 268 3105; fax: +1 412 681 0648.
E-mail address: [email protected] (S.A. Majetich). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.1169
bioseparation involving extremely small amounts of material that could later be amplified, such as DNA purification or the separation of cell subpopulations with different degrees of surface antigen expression. Here we describe a new type of magnetic nanoparticle (Fig. 1) that combines the advantages of magnetite for magnetic separation with the surface plasmon resonance of gold, which enables single particles to be tracked optically, even though they are smaller than the diffraction limit. The gold shells exhibit strong plasmonic bands in their UV–visible absorption spectra, providing a way to detect and quantitatively characterize the separation effectiveness in situ. Conventional cytometric analysis requires fluorescent probe labelling, and hence extra effort is needed to conjugate fluorophore molecules onto the cell/magnetic particle complex. An additional advantage of the proposed gold shells is their extremely large molar extinction coefficient, greater than 105 M1 cm1 at wavelengths in the 500–600 nm range [11]. Resonant Rayleigh scattering efficiencies equivalent to 106 fluorescent dye molecules [12] have been reported for gold nanoparticles. In addition, this plasmonic response is not vulnerable to the photobleaching effects that arise during prolonged and/or high intensity illumination of fluorescent tags.
ARTICLE IN PRESS J. Lim et al. / Journal of Magnetism and Magnetic Materials 311 (2007) 78–83
79
Fig. 1. Left: a nanoshell particle with magnetite core and gold shell. Right: a composite nanoparticle with silica core, decorated by magnetite nanoparticles and coated with a gold shell.
Fig. 2. (a) TEM micrograph and (b) characteristic electron diffraction rings of magnetite particles in toluene.
2. Particle synthesis Synthesis of nanoshell and/or composite nanoparticles proceeds first by the synthesis of individual magnetite, Au, and silica nanoparticles. These are then assembled and subjected to further growth to create the gold shell, either with a magnetite core and a gold shell or a with a silica core, and then shells of magnetite and gold nanoparticles. The size of the constituent particles can have a dramatic effect on the magnetic properties (iron oxide particles) and plasmonic behavior (gold particles) and so synthetic control is very important to obtain a high degree of uniformity. A hightemperature polyol process developed by Sun et al. [13] was used to make magnetite nanoparticles in organic solvent. Oleic acid (OA) and oleylamine (OY) were used as capping agents that sterically stabilize the particles in toluene/hexane. After the first step, 8 nm iron oxide nanoparticles with high crystallinity are obtained. Based on the lattice spacing corresponding to five strongest electron diffraction intensity rings (Fig. 2), the nanoparticles are identified as either magnetite or maghemite but the observed black color suggests that they are predominantly magnetite. These nanoparticles then serve as seeds for further growth. After two reaction stages, the average diameter of the magnetite nanoparticles was 11.0571.7 nm (shown in Fig. 2), with the size distribution shown in Fig. 3. A modified Sto¨ber process [14] was used to make silica spheres with a diameter of 50 nm. This involves the slow
Fig. 3. Iron oxide particles size distribution. Scion Image Beta 4.03 for Windows 95 to XP was used to identify individual particles and ImageJ 1.36 was used to determine the average particle diameters.
base hydrolysis of tetraethyl orthosilicate in ethanol. High monodispersity and spherical shape is ensured by slow diffusional growth over 24 h. Controlling the volume ratio of ammonium hydroxide to tetraethyl orthosilicate allows the particle size to be altered between 50 and 500 nm. Ultrafine gold nanoparticles with an average size of 2 nm
ARTICLE IN PRESS 80
J. Lim et al. / Journal of Magnetism and Magnetic Materials 311 (2007) 78–83
were produced by the technique developed by Duff et al. [15]. The particle size mismatch between the gold and iron oxide or the silica particles was crucial to ensure the formation of a core–shell structure. The magnetite nanoparticles were transferred from toluene into water by a modified version of the method proposed by Maceira et al. [16]. Fig. 4 illustrates the major steps. Magnetite nanoparticles are first washed in ethanol to get rid of excess surfactant, OA/OY. The nanoparticles are then collected with a permanent magnet and redispersed into 10 wt% tetramethylammonium hydroxide (TMAOH) solution in water following 10 min of ultrasonication. Afterwards the nanoparticles are collected with a permanent magnet and redispersed into fresh 10 wt% TMAOH, and incubated overnight. The final washing step, using ethanol, removes remaining OA and OY from the sol. Without this step, the particles will flocculate over time. Several washing steps in TMAOH were needed to ensure complete replacement of OA/OY with negatively charged surface groups. These particles have a zeta potential of 41.9 mV, measured in deionized water, which indicates the replacement of OA/OY with a negatively charged surface. Evidence of the removal of the organic layer from the surface of the nanoparticles was also provided by FTIR spectroscopy measurements before and after the transferring steps. The collected FTIR spectra (Fig. 5) show peaks at 2871 and 2922 cm1, corresponding to the symmetric and asymmetric CH2 stretching modes of the oleyl group. These peaks are strong for the nanoparticle suspension in toluene, but the intensity dramatically decreased after washing and phase transfer into water. In deionized water, gold, magnetite and silica particles are all negatively charged. Further surface modification is needed to promote surface binding of magnetite or silica particles with gold particles. Fig. 6 shows a generic representation of major steps involved in producing the gold shell structure. The seeding stage is necessary to overcome the difference in surface energies of the different component materials. Gold has a much higher surface energy than either silica or
magnetite, and so it will not deposit evenly over these surfaces. Attaching gold seeds creates nucleation sites for further metal deposition, and also alters the effective surface energy of the oxide particle allowing an even metal layer to be deposited. After this the seeded particles can grow to full core–shell structure by using formaldehyde as a mild reducing agent. 3-Aminopropyltriethoxy silane (APS) [17] and 11-mercaptoundecanoic acid were selected as two possible means to modify the surface of magnetite particles that serve to promote further binding of gold nanoparticles (see Fig. 7). It was found that APS-coated magnetite particles quickly became unstable in water, whereas 11-mercaptoundecanoic acid modified nanoparticles were stable for up to a month in aqueous suspension. This observation makes 11mercaptoundecanoic acid more attractive as a surface functionalization agent compared to APS. This functiona-
Fig. 5. FTIR spectra for 11 nm nanoparticles in toluene and in deionized water. Absorption peaks for the two-phonon feature of the diamond crystal attenuated total reflection (ATR) optical element appear at 2000 and 2200 cm1; the peaks for the oleic acid/oleylamine CH2 stretching modes appear at 2871 and 2922 cm1 and are significantly diminished in intensity after rinsing in TMAOH solution.
Fig. 4. Major steps involved to transfer magnetite nanoparticles from organic solvent into water (a) replacement of oleic acid (OA) and olelyamine (OY) surface with charged species (b) additional washing of particles to encourage further OA/OY replacement (c) removal of the remaining OA/OY.
ARTICLE IN PRESS J. Lim et al. / Journal of Magnetism and Magnetic Materials 311 (2007) 78–83
81
Fig. 6. Major steps for core–shell structure formation (a) surface modification of iron-oxide particles (b) gold particles attachment onto surface modified iron oxide particles (c) complete coarsening of gold seeded iron oxide particles forming core–shell structure.
Fig. 7. TEM micrograph of 11-meracaptoundecanoic acid and APS modified magnetite particles after attachment of a small number of gold particles and transfer to water. Because the particles are charge stabilized, rather than sterically stabilized, they flocculate during drying. Individual gold nanoparticles are evident on the iron oxide particle surfaces.
lization deposits thiol groups onto the iron oxide surface that are then accessible to bind to gold nanoparticles. 3. Nanoparticle optical characterization and dark-field imaging
Fig. 8. UV–Vis spectra of 50 nm silica cores with gold seeds or shells attached. The spectrum of an aqueous suspension of the silica particles seeded with individual 2 nm gold nanoparticles is shown to illustrate the red-shift of the plasmon resonance after growing the seeds into complete gold shells.
Nanoparticles of gold and silver are of interest for optical applications because they exhibit a surface plasmon resonance at optical frequencies. This results in strong absorption and scattering of light by the particles at specific frequencies. One interesting property of the shell structures proposed is that the resonant frequency can be varied over wider frequency ranges compared to simple homogeneous nanoparticles [18–20]. A good explanation of surface plasmon resonances and the optical properties of nanoshells can be found in Refs. [18,19]. An important conclusion drawn from earlier studies is that the resonance condition for the core–shell particles is determined by the ratio of the core-radius to the total radius of the particle. In this way the resonance can be
‘tuned’ by varying the shell thickness. This can be seen in Fig. 8 where UV–vis absorption spectra are shown for gold-seeded 50 nm silica cores, along with 10 and 20 nm thick, complete gold shells on the same silica cores. The gold nanoparticles have a characteristic resonance at 520 nm, and as the particles merge to form a shell the resonance shifts to longer wavelengths. As the shell thickens a further red-shift as far as 560 nm is seen. An extinction coefficient of 104 M1 cm1 can be estimated using Beer’s Law, which is comparable to previous measurements in Ref. [11]. The presence of magnetite within the cores compared to silica will lead to a small redshift of the plasmon peak.
ARTICLE IN PRESS J. Lim et al. / Journal of Magnetism and Magnetic Materials 311 (2007) 78–83
82
Another important effect seen from scattering theory is that the scattered intensity from the core-shell particles varies as the sixth power of the total particle radius, but there are only small absorptive losses, as seen by the increased intensity of the plasmon peak in Fig. 8. This makes the particles very suitable for imaging by optical dark-field microscopy. Dark-field microscopy collects only light that has been scattered obliquely by the sample. If no sample is present, and the microscope slide is otherwise smooth, no image is detected. In the presence of a sample of strongly scattering particles, a bright spot appears against the dark background. Although any small particle will scatter light, the strong scattering associated with the surface plasmon resonance of Au or Ag nanostructures makes individual nanoparticles readily detectable. Observation of Ag or Au particles as small as 30 nm have been reported [21], which is significantly smaller than the optical diffraction limit. Fig. 9 is an optical dark-field micrograph of 50 nm silica particles with 10 nm gold shells. The strong scattering that was evident in the UV–vis spectrum causes the particles to appear very distinct. The micrograph was obtained with a 10 oil immersion objective lens. This indicates that these thin gold shells are sufficient for darkfield microscopy detection of individual nanoparticles. 4. Requirements for magnetic separation For known values of the magnetic field B and magnetic field gradient XB, the magnetic force Fmag, given by F mag ¼
DwV p ðrBÞB, m0
(1)
and the magnetophoretic velocity, umag, given by umag ¼
F mag , 6p Zr
(2)
can be calculated [10,22]. Here Z is the viscosity of the medium and r is the particle radius, Vp is the particle volume and Dw is the dimensionless difference in magnetic susceptibility between the particle and the fluid. The magnetophoretic velocity umag is proportional to the square of the particle radius and to the magnetic susceptibility of the particle, umag / r2 wp .
(3)
Since the magnetophoretic behavior of the nanoparticles is highly size dependent, a series of calculations were carried out to probe the size-dependent effects of magnetophoresis for both single magnetite particles and composite nanoparticles. Very large magnetic field gradients are needed in order to overcome viscous drag forces on small nanoparticles, so that magnetophoretic forces dominate over Brownian motion. Using micropatterned wires, field gradients as high as 5000 T/m are possible [3]. Fig. 10 shows how the magnetite nanoparticle velocity depends on the flux gradient. For 15 nm particles to have a velocity of 10 mm/ min, they would require a field gradient of 35 T/m. This field gradient is readily achievable by high gradient magnetic separators [23,24] but due to the spatial variation of the field gradient, nanopatterned elements [25] or nanowires are more promising for the manipulation of individual particles. A further design consideration for magnetic separation of small nanoparticles is that the magnetophoretic force will decay rapidly as a function of distance from the surface of the magnetic element. Fig. 11 shows the magnetophoretic force on particles of different diameters as a function of the distance from the magnetic pole. For 30 nm particles, the viscous drag force is 8 1018 N, roughly an order of magnitude smaller than the magnetophoretic force at the surface of the ferromagnetic element. However, at distances greater than 1.5 mm from the surface, the drag force overwhelms magnetophoresis and the particle is not moved significantly. 5. Conclusions
Fig. 9. Dark-field optical micrograph showing bright spots associated with the strong scattering from individual 50 nm silica cores with 10 nm thick gold shells. The particles were in aqueous suspension over a glass cover slip. Particles were observed to undergo Brownian motion, confirming that spots correspond to discrete nanoparticles.
The design of bi-functional nanoparticles with magnetic cores for magnetic separation and Au shells for plasmonic optical imaging has been described, along with the status of efforts to realize these particles. Magnetite nanoparticles decorated with gold nanoparticles have been prepared and are stable in aqueous suspension. Using silica as a stand-in core particle material, it was shown that an incomplete Au shell is insufficient for a strong surface plasmon peak, but individual particles with a 10 nm shell can clearly be observed as they undergo Brownian motion using darkfield optical microscopy. Future work will explore the ability to separate these particles using magnetophoresis while imaging their trajectories optically. Preliminary
ARTICLE IN PRESS J. Lim et al. / Journal of Magnetism and Magnetic Materials 311 (2007) 78–83
83
Fig 10. (a) Particle velocity in water as a function of the diameter and the local magnetic field gradient. Here the magnetic field at the surface of the micropatterned pole tip is 0.064 T. (b) Magnetophoretic force as a function of distance, for different particle diameters.
calculations indicate that large magnetic field gradients will be required, such as those achievable with micropatterned wires or ferromagnetic elements. Acknowledgments This work was supported in part by the NSF through grant number ECS-0304453 and by the Pennsylvania Infrastructure Technology Alliance. The authors also thank Fred Lanni of the Carnegie Mellon University Biological Sciences Department for assistance in the optical dark-field microscopy experiments. References [1] M.A.M. Gijs, Microfluid Nanofluid 1 (2000) 22. [2] M. Tondra, M. Granger, R. Fuerst, et al., IEEE Trans. Magn. 37 (2001) 2621. [3] M. Berger, J. Castelino, R. Huang, et al., Electrophoresis 22 (2001) 3883. [4] K. Auditore-Hargreaves, S. Heimfeld, R.J. Berenson, Bioconj. Chem. 5 (1994) 287. [5] J. Ugelstad, P. Stenstad, L. Kilaas, E. Hornes, et al., Blood Purification 11 (1993) 349. [6] J. Chalmers, M. Zborowski, L. Moore, et al., Biotechnol. Bioeng. 59 (1998) 10.
[7] L. Moore, M. Zborowski, L.P. Sun, et al., J. Biochem. Biophys. Meth 37 (1998) 11. [8] M. Zborowski, L.P. Sun, L. Moorem, et al., J. Magn. Magn. Mater. 194 (1999) 224. [9] Y. Sahoo, E.P. Furlani, J. Phys. D 39 (2006) 1724. [10] N. Pamme, A. Manz, Anal. Chem. 76 (2004) 7250. [11] T.R. Jensen, M. Duval Malinsky, C.L. Haynes, et al., J. Phys. Chem. B 104 (2000) 10549. [12] J. Yguerabide, E.E. Yguerabide, Anal. Biochem. 262 (1998) 157. [13] S.H. Sun, H. Zeng, D.B. Robinson, et al., J. Am. Chem. Soc. 126 (2004) 273. [14] W. Sto¨ber, A. Fink, E. Bohn, J. Colloid Interface Sci. 26 (1968) 62. [15] D. Duff, A. Baiker, P.P. Edwards, Langmuir 9 (1993) 2301. [16] V.S. Maceira, L.M. Liz-Marzan, M. Farle, Langmuir 20 (2004) 6946. [17] M. Takafuji, S. Ide, H. Ihara, et al., Chem. Mater. 16 (2004) 1977. [18] R.D. Averitt, D. Sarkar, N.J. Halas, Phys. Rev. Lett. 78 (22) (1997) 4217. [19] R.D. Averitt, S.L. Westcott, N.J. Halas, J. Optic. Soc. Am. B 16 (10) (1999) 1824. [20] A.L. Aden, M. Kerker, J. Appl. Phys. 22 (1951) 601. [21] A.D. McFarland, R.P. VanDuyne, Nano Lett. 3 (2003) 1057. [22] G.P. Hatch, R.E. Stelter, J. Magn. Magn. Mater. 225 (2001) 262. [23] L. Sun, M. Zborowski, L.R. Moore, et al., Cytometry 33 (1998) 469. [24] T. Schnieder, L.R. Moore, Y. Jing, et al., J. Biochem. Biophys Meth 68 (2006) 1. [25] B.B. Yellen, G. Friedman, Adv. Mater. 16 (2004) 111.