Preparation of plasmonic magnetic nanoparticles and their light scattering properties

Abstract

Fe₃O₄@SiO₂@Au nanoparticles (NPs) that have plasmonic and magnetic properties were prepared by simple immobilization method of Au NPs to silica coated magnetic NPs. The Fe₃O₄@SiO₂@Au exhibit 5 times higher light scattering compared to the same sized gold NPs. The experimental results were supported by the simulations.

---

Gold nanoparticle (NP) composite systems have been widely studied and continue to draw considerable attention in various applications, including surface-enhanced spectroscopy,¹ super-resolution optical imaging,^2,3 and ultrasensitive biosensing, as such complexes can generate surface plasmons that absorb light and enhance an electromagnetic field by the collective and resonant excitation of free electrons.⁴ Additionally, inter-particle plasmonic coupling can create a dramatically enhanced electromagnetic field; the strength of which is determined by the shape, size, and composition of the NPs.^5–7

Multifunctional materials can be made more effectively by utilizing the advantages of materials.^8,9 Especially, magnetic materials combined with plasmonic property have the potential to be manipulated for targeting, labeling, and separation of various types of biological samples.^10–12 A facile method for preparing monodispersed plasmonic magnetic nanoparticles (MNPs) has potential for use in a broad range of applications.

In this work, we present a plasmonic and magnetic material using gold NPs immobilized on a silica shell, which encapsulates a magnetic core (Fe₃O₄@SiO₂@Au) and demonstrate their properties using scattering-based analysis and magnetic manipulation. So far, NPs-assembled structures have shown excellent surface-enhanced Raman scattering (SERS) properties, both theoretically and experimentally. To the best of our knowledge, the light-scattering properties of gold-immobilized silica NPs have not been studied until now. Here, we demonstrate that NPs-assembled structures exhibit better light-scattering properties than individual gold NPs of the same size.

The Fe₃O₄@SiO₂@Au NPs have a core–shell structure, with an 18 nm magnetic core and silica shell with a thickness of 40 nm. On the surface of the silica shell, ca. 5.5 nm gold NPs are immobilized and act as the plasmonic substrate (Fig. 1a). The schematic illustration of the Fe₃O₄@SiO₂@Au NPs is shown in Fig. 1a and the transmission electron microscopy (TEM) images and their corresponding illustrations are shown in Fig. 1b and c.

Fig. 1 Fabricated Fe₃O₄@SiO₂@Au nanoparticles (NPs) (a) illustration of Fe₃O₄@SiO₂@Au NPs. TEM images and corresponding illustration of (b) silica coated MNPs, and (c) gold NPs immobilized MNPs.

First, MNPs (18 nm) were utilized to make silica coated MNPs (100 nm), which act as the template for the immobilization of gold NPs. This silica shell coated MNP template was prepared using a literature method.¹³ Briefly, the NPs were prepared using a ligand exchange and a modification of the Stöber method. Because the MNPs were stabilized by oleic acid, their surface was hydrophobic. To apply the Stöber method, we changed the ligand to amphiphilic PVP to induce a hydrophilic surface. Oleate-stabilized NPs in chloroform were mixed with PVP dissolved in a mixture of DCM and DMF to form a clear dispersion. This mixture was kept at 100 °C overnight to exchange the oleate ligands with the excess PVP. The ligand exchange process did not affect the properties of the MNPs, such as size, magnetic properties, and shape. In addition, the particles can be dispersed in hydrophilic solvents such as EtOH. To make 100 nm silica-coated MNPs, ammonium hydroxide and TEOS was added to the PVP coated MNPs. The TEM images in Fig. 1b show that the silica-coated MNPs have a magnetic core and silica shell. The average particle size is approximately 100 nm and more than 95% of the NPs have a single magnetic core.¹³

The silica-coated MNPs were then functionalized with thiol groups using 3-mercaptopropyltriethoxysilane (MPTS) in ethanol to allow the facile introduction of the gold NPs. When we compared the thiol and the amino group's affinity toward Au NPs, the thiol group exhibited a five-fold stronger bond than the amino group, which has the stronger affinity with gold.^14,15

To make the plasmonic material, Au NPs of different size (5.5, 10, and 25 nm) were added, and Au NPs immobilized silica coated MNPS were selectively collected from the unbounded excess Au NPs by applying a magnetic force.

All the Au NPs prepared have a strong affinity toward the thiol modified silica-coated MNPs. However, the probability of cross-linking increases with the size of the NPs, as shown in ESI Fig. 1,† and therefore, the 5.5 or 10 nm Au NPs seemed to be more suitable. In this study, we used the 5.5 nm Au NPs. The UV absorbance of the Au NPs from Fe₃O₄@SiO₂@Au NPs was observed at the 530 nm after immobilization of the Au NPs as shown in Fig. 2a. By increasing the ratio of the Au NPs to the thiol modified silica-coated MNPs, the UV absorbance also increased (see ESI Fig. 2†).

Fig. 2 Properties of the Fe₃O₄@SiO₂@Au NPs (a) UV-Vis spectra of MNP@SiO₂ (blue line) and MNP@SiO₂@Au (red line). (b) Field-dependent magnetization of silica-coated MNP–Au NPs at 300 K and (inset) corresponding photograph of silica-coated MNPs separated by a magnetic field at room temperature. (c) Dispersed MNP@SiO₂@Au in various solvents.

We could not find any remaining Au NPs after collection of the Au NPs-immobilized MNPs by magnetic force even when up to approximately 100 times of Au NPs were added to the MNPs (analyzed by color and UV absorbance). When the ratio reached over 125 times, the absorbance of Au NPs-immobilized NMPs was not increased any further, which indicates that the Au NPs were saturated on the surface of the NMPs (see ESI Fig. 3†). The resulting Fe₃O₄@SiO₂@Au NPs are visualized in Fig. 1c and each MNP has approximately 120 Au NPs (see ESI Fig. 4†). The field-dependent magnetization of the silica-coated MNPs at 300 K exhibited superparamagnetism, and the saturated magnetization was approximately 0.04 emu g⁻¹, as shown in Fig. 2b. The magnetic separation is a critical step to the facile preparation of the plasmonic NPs, which have potential for bio-application. In addition, these particles are well dispersed in several solvents such as water, EtOH, DMSO, PBS buffer, and Tris buffer (Fig. 2c and ESI Fig. 5†).

Fig. 3a and b show the comparison of the dark-field scattering images of the Au NPs (ca. 100 nm diameter) and Fe₃O₄@SiO₂@Au NPs, respectively taken with a dark-field microscope equipped with a 20× objective lens. Single Fe₃O₄@SiO₂@Au NP that is of 100 nm size and contains 120 Au NPs of 5.5 nm size exhibits 5 times brightness in light scattering when compared to single Au NPs of ca. 100 nm diameter (Fig. 3 and ESI Fig. 6†). Because the signal intensity can be normalized by the structure which is collected Au NPs, the signal intensity can be similar.

Fig. 3 Dark-field scattering anaysis. Dark-field scattering images of (a) Au NPs of ca. 100 nm diameter and (b) Fe₃O₄@SiO₂@Au NPs, (c) comparison of light scattering intensities of Au NPs and Fe₃O₄@SiO₂@Au NPs.

In order to gain insight into the enhanced scattering of Fe₃O₄@SiO₂@Au NPs, the scattering efficiency of Au NPs of various assembly were simulated using DDA calculations (Fig. 4). Fig. 4a shows the scattering efficiency spectra of an Au NP monomer of 5.5 nm size and an Au NP dimer with different distances (1, 2, 3, 5, 10 and 20 nm) between the Au NPs and Fig. 4b shows the maximum scattering efficiency ratio of each of the Au NPs divided by 2 in order to get insights of scattering contribution of each Au NP except for Au NP monomer. The scattering efficiency of the Au NP dimer with 1 nm distance was enhanced approximately 2.5 times when compared with the scattering efficiency of a single Au NP. When the distance between the Au NPs was increased, the scattering efficiency gradually decreased, but that of each dimer was at least 1.3 times higher than that of an individual Au NP since two particles contributed in light scattering.¹⁶

Fig. 4 Theoretical optical properties of Au NPs scattering spectrum. (a) Theoretical scattering-efficiency spectra of Au NP dimers for various dimer distances. (b) Maximum scattering efficiency at l_max in dependence of the dimer distance. (c) The structures of the assembled Au NPs along with incident light and polarization directions. (d) Theoretical scattering-efficiency spectra of the assembled Au NPs structures. (e) Maximum scattering efficiency at l_max for the different assembled Au NPs structures, divided by the number of Au NPs in the assembly.

For further investigation of the scattering efficiency enhancement when the number of Au NPs interacting collectively is increased within single Fe₃O₄@SiO₂@Au NPs, the scattering efficiency of the assembled Au NPs (Fig. 4c) were calculated in terms of different number of Au NPs with a given inter-particle distance of 3 nm as a representative example of a moderately enhanced system (Fig. 4d). Fig. 4e is a plot of maximum scattering efficiency of each assembly divided by number of Au NPs in the assembly in order to get insights of scattering contribution of each Au NPs. The scattering efficiency of each of the assembled structures were enhanced when compared with the Au NP monomer and the scattering efficiency of the Au NP decamer was increased about ca. 5 times when compared with the Au NP monomer.

Several factors could affect to the light scattering (ex, silica core as shown in ESI Fig. 7†). In this study, we presented some possibility of light-scattering enhancement.

Conclusions

We have described a facile preparation method of Fe₃O₄@SiO₂@Au NPs that exhibit 5 times higher light scattering intensity when compared to the sized individual Au NPs in dark-field microscope analysis. In addition, we have also shown its magnetic properties and that these particles were well dispersed in several solvents. The NPs show the potential as a label in on-chip immunoassays for both magnetic manipulation and plasmonic sensing in a single entity. The surface plasmon resonance of gold makes it possible to track the positions of individual NPs using dark-field microscopy. In addition, these NPs have potential application in microfluidic and cell sorting applications.

Acknowledgements

This study was supported by the Bio & Medical Technology Development Program of the National Research Foundation (NRF) funded by the Korean government (Ministry of Education; MOE) (no. 2013-035616), by the Ministry of Science, ICT & Future Planning (2014-A002-0065), and the Korean Health Technology R&D Project funded by the Ministry of Health & Welfare (no. HI13C-1299-010013).

Notes and references

1. K. L. Wustholz, A. I. Henry, J. M. McMahon, R. G. Freeman, N. Valley, M. E. Piotti, M. J. Natan, G. C. Schatz and R. P. Van Duyne, J. Am. Chem. Soc., 2010, 132, 10903–10910.
2. I. E. Sendroiu, L. K. Gifford, A. Luptak and R. M. Corn, J. Am. Chem. Soc., 2011, 133, 4271–4273.
3. B. Huang, Curr. Opin. Chem. Biol., 2010, 14, 10–14.
4. Y. Zhang, B. de Boer and P. W. M. Blom, Phys. Rev. B: Condens. Matter Mater. Phys., 2010, 81, 085201.
5. Y. Li, J. Liu, Y. Zhong, J. Zhang, Z. Wang, L. Wang, Y. An, M. Lin, Z. Gao and D. Zhang, Int. J. Nanomed., 2011, 6, 2805–2819.
6. J. Ding, Q. Gao, D. Luo, Z.-G. Shi and Y.-Q. Feng, J. Chromatogr. A, 2010, 1217, 7351–7358.
7. H. Kang, S. Jeong, Y. Park, J. Yim, B. H. Jun, S. Kyeong, J. K. Yang, G. Kim, S. Hong, L. P. Lee, J. H. Kim, H. Y. Lee, D. H. Jeong and Y. S. Lee, Adv. Funct. Mater., 2013, 23, 3719–3727.
8. B. H. Jun, M. S. Noh, J. Kim, G. Kim, H. Kang, M. S. Kim, Y. T. Seo, J. Baek, J. H. Kim, J. Park, S. Kim, Y. K. Kim, T. Hyeon, M. H. Cho, D. H. Jeong and Y. S. Lee, Small, 2010, 6, 119–125.
9. B. H. Jun, G. Kim, J. Baek, H. Kang, T. Kim, T. Hyeon, D. H. Jeong and Y. S. Lee, Phys. Chem. Chem. Phys., 2011, 13, 7298–7303.
10. B.-H. Jun, G. Kim, M. S. Noh, H. Kang, Y.-K. Kim, M.-H. Cho, D. H. Jeong and Y.-S. Lee, Nanomedicine, 2011, 6, 1463–1480.
11. M. S. Noh, B.-H. Jun, S. Kim, H. Kang, M.-A. Woo, A. Minai-Tehrani, J.-E. Kim, J. Kim, J. Park, H.-T. Lim, S.-C. Park, T. Hyeon, Y.-K. Kim, D. H. Jeong, Y.-S. Lee and M.-H. Cho, Biomaterials, 2009, 30, 3915–3925.
12. F. H. Lin and R. A. Doong, J. Phys. Chem. C, 2011, 115, 6591–6598.
13. W.-Y. Rho, H.-M. Kim, S. Kyeong, Y.-L. Kang, D.-H. Kim, H. Kang, C. Jeong, D.-E. Kim, Y.-S. Lee and B.-H. Jun, J. Ind. Eng. Chem., 2014, 20, 2646–2649.
14. X. Wang, C. Wang, L. Cheng, S.-T. Lee and Z. Liu, J. Am. Chem. Soc., 2012, 134, 7414–7422.
15. H. Cai, K. Li, M. Shen, S. Wen, Y. Luo, C. Peng, G. Zhang and X. Shi, J. Mater. Chem., 2012, 22, 15110–15120.
16. B. Khlebtsov, A. Melnikov, V. Zharov and N. Khlebtsov, Nanotechnology, 2006, 17, 1437–1445.

---

Footnotes

† Electronic supplementary information (ESI) available: Experimental details and TEM images, light-scattering spectra and absorption spectra of the Fe₃O₄@SiO₂@Au NPs. See DOI: 10.1039/c5ra00513b

‡ These authors contributed equally to the work.

---