Plasmonics DOI 10.1007/s11468-014-9708-1
Plasmonic Property and Stability of Core-Shell Au@SiO2 Nanostructures Jinsheng Liu & Caixia Kan & Bo Cong & Haiying Xu & Yuan Ni & Yuling Li & Daning Shi
Received: 22 January 2014 / Accepted: 20 March 2014 # Springer Science+Business Media New York 2014
Abstract Au nanorod (Au NR) is one of the most studied colloidal nanostructures for its tunable longitudinal surface plasmon resonance (SPRL) property in the near infrared region. And surface coating Au NRs into core-shell nanostructures is particularly important for further investigation and possible applications. In this paper, Au NRs colloids were synthesized using an improved seed method. Then asprepared Au NRs were coated with SiO2 to form a coreshell nanostructure (Au@SiO2) with different shell thickness. And the influence of SiO2 shell on the SPRL of Au NRs was investigated based on the experimental results and FDTD simulations. Under the 808 nm laser irradiating, the stability of Au@SiO2 was studied. Compared with Au NRs, the Au@SiO2 is stable with increasing laser power (up to 8 W), whereas Au NRs undergo a shape deformation from rod to spherical nanoparticle when the laser power is 5 W. The high stability and tunable optical properties of core-shell structured Au@SiO 2, along with advantages of SiO 2 , show that Au@SiO2 composites are promising in designing plasmonic photothermal properties or further applications in nanomedicine.
J. Liu : C. Kan (*) : B. Cong : H. Xu : Y. Ni : Y. Li : D. Shi (*) College of Science, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, People’s Republic of China e-mail:
[email protected] e-mail:
[email protected] C. Kan : D. Shi Key Laboratory for Intelligent Nano Materials and Devices of the Ministry of Education, Nanjing, China J. Liu Jincheng College, Nanjing University of Aeronautics and Astronautics, Nanjing, China
Keywords Noble metal nanoparticles . Surface plasmon resonance . Core-shell nanostructure . FDTD simulations . Au nanorod
Introduction Combining high yield fabrication with theoretical modeling, size, and shape control of noble metal nanostructures has been greatly advanced in the fields of optoelectronic devices [1, 2], plasmon-enhanced spectroscopy [3, 4], bioimaging, and therapeutics [5, 6], and so forth [7–11]. Particularly, Au nanorod (Au NR) is one of the most studied colloidal nanostructures due to its distinct surface plasma resonances (SPR) corresponding to the width and length, known as the transverse mode (SPRT) and longitudinal mode (SPRL) [12, 13]. The SPRT is weak and located at ∼520 nm, while the strong SPRL mode can be tuned in the visible-near infrared (Vis-NIR) region [14–17]. This optical tunability has brought several advantages to Au NRs. For example, the accessible NIR SPRL extinction is consistent with the transparent window of human tissue facilitating widespread biophotonics applications in bioimaging and NIR photothermal cancer therapy [5, 6, 17]. The SPRL is highly sensitive to medium, surface charge density, aspect ratios, surface coating, and aggregation degree or assembly mode [18–22]; therefore, they have promising applications in chemical and biological sensing [22–25]. For bioapplications, Au NRs need to be highly stable, biocompatible, sensitive, and targetable [5, 6, 17, 26]. Monodispersed colloidal Au NRs are often prepared by seed-mediated method in the presence of cationic surfactant CTAB as a “soft template” with continuous optimization [27–31]. However, CTAB is a kind of biological toxicity molecular, which is often used as a “soft template” to direct the formation of Au NRs. In general, colloidal Au NRs covered by CTAB bilayer can be kept stable in an aqueous
Plasmonics
solution. Unfortunately, the concentration of CTAB decreases after centrifugation, and the CTAB coverage on Au NRs would be dissolved by an alcohol solution in further investigation, resulting in Au NRs aggregating and damping and even disappearance of SPRL property. Under this case, surface coating with biological compatibility molecular or medium was usually proposed for further research. Silica (SiO2), as a gain medium for plasmonic property and its advantages of biological compatibility and chemical stability, was a promising coating medium, especially for surface coating of Au NRs [32–34]. The SiO2 coating can be used not only in keeping the stability of AuNRs for further treatment, but also in increasing plasmonic property, biocompatibility, and SERS signal for single molecule resolution [5, 6, 35, 36]. Smooth, uniform SiO2 shells have been successfully deposited on a number of colloidal particles of metals, metal oxides, and semiconductors based on the stober method [36–38]. For surface coating of CTAB-stabilized Au NRs, as the PEG-terminated molecules selectively bind to the ending surfaces of nanorods, one proposed method is attempting to replace the CTAB bilayer with a polyethylene glycol (PEG)terminated molecules, followed by growth of SiO2 shells in ethanol or isopropanol solution [36–40]. Recently, this technique was modified by Gorelikov et al. to directly encapsulate CTAB-stabilized nanoparticles without an intermediary coating step [21, 41–44]. In this paper, we present a rapid method for Au NRs surface coating with homogeneous SiO2 shells into core-shell structured Au@SiO2 nanocomposite. The key point in the coating procedure is adjusting the concentration of CTAB just above the critical micelle concentration (∼1 mM) [42, 43]. Then, Au@SiO2 composites with different shell thickness were obtained through changing the SiO2 precursor (TEOS) and ethanol. Compared with Au NRs colloid, the stability of coreshell Au@SiO2 nanostructure was investigated under 808 nm laser irradiation. In the optical property study, FDTD simulations were applied for further understanding of the shell influence on the SPR property of Au NRs. Fig. 1 TEM images of asprepared Au NRs (a) and SiO2coated Au NRs (b). Insertion of (b) is the magnified image of core-shell structured Au@SiO2
a
Experimental Section Materials Sodium tetrahydridoborate (NaBH 4, 99 %, Sinopharm Chemical Reagent Co. Ltd.), cetyltrimethyl ammonium bromide (CTAB, 99 %, Nanjing Robiot Co. Ltd.), tetrachloroaurate (HAuCl4 ·H2O, 99.9 %, Shanghai Chemical Reagent Co. Ltd.), ascorbic acid (AA, ≥99.7 %, Sinopharm Chemical Reagent Co. Ltd.), silvernitrate (AgNO3, >99.8 %, Sinopharm Chemical Reagent Co. Ltd.), tetraethoxysilane (TEOS, ChengDu Kelong Chemical Co. Ltd), and deionized water (18.25 M) were used in the experiment. All reagents were used without further purification. Synthesis of Au Nanorods (NRs) Au NRs colloids were synthesized through a “seed” method, as reported previously [10]. Briefly, the seed solution was prepared by dissolving HAuCl4 (0.05 mL, 0.05 M) in an aqueous solution mixed with CTAB (10 mL, 0.1 M). Then, ice-cold NaBH4 (0.6 mL, 0.01 M) was injected into the solution under vigorous stirring. The growth solution was obtained by successively adding AgNO3 (0.24 mL, 0.02 M), HAuCl4 (0.5 mL, 0.05 M), and AA (0.13 mL, 0.1 M) into 49 mL of 0.1 M CTAB aqueous solution. Then, 0.06 mL “seed” solution was added into the as-prepared growth solution for the growth of Au NRs at 25 °C. After 12 h, the obtained sample was purified by centrifugation to remove the excess CTAB and dispersed into water. Surface coating Au NRs with SiO2 A modified stober method for SiO2 coating Au NRs reported by Gorelikov et al. was applied in the surface coating step, 0.04 mL NaOH (0.1 M) solution was added into the asobtained Au NRs colloid (4 mL) with a certain CTAB concentration under stirring. Then, suitable TEOS (such as
b
Plasmonics
Absorption
GNRs Au @ SiO2
400
600
800
1000
Wavelength (nm)
Fig. 2 UV–vis-NIR spectra of as-prepared Au NRs (solid line) and coreshell Au@SiO2 nanostructure (dash line)
0.04 mL TEOS mixed in a certain amount of ethanol) was injected into Au NRs colloid under gentle rotary shaking for the hydrolysis of TEOS and SiO2 coating on the surface of Au NRs. Au@SiO2 core-shell structure was obtained after 20 h. The product was washed off three times by centrifugation with ethanol at 5,000 RPM (for 15 min), and dispersed in water for characterization. Characterization The absorption spectra of the samples were collected on violet-visible-near infrared (UV–vis-NIR) spectrometer (UV6300) in the range of 200–1,100 nm. The cleaning samples were deposited on copper grids covered by an amorphous carbon film and HRTEM grids for transmission electron microscopy (TEM: JEOL-100CX) and high-resolution TEM (HRTEM: JEOL-2011) measurements. Finite-Difference Time-Domain (FDTD) Simulation FDTD Solutions (Lumerical, Inc.) was utilized to study both the near- and far-field electromagnetic responses of metal, by solving Maxwell’s curl equations on a discretized grid. The absorption cross section and near-field enhancement were calculated to evaluate the properties of plasmon-resonant local Fig. 3 TEM images of the sample with (a) and without (b) additional CTAB after ethanol washing. Insertion of (b) is the sample of the supernatant solution
a
fields on the Au NRs. In our simulations, a nanorod was surrounded by a virtual boundary with an appropriate size. The individual nanorod was modeled as a cylinder with two hemispherical end caps. The length and diameter of the Au NRs were 52 and 12 nm, respectively. The structure of SiO2 coating was designed as a hollow cylinder shell. A total field scattered field (TFSF) source with the wavelength range from 550 to 950 nm was launched into the boundary to simulate a propagating plane wave interacting with the targets. The targets and the surrounding medium inside the boundary were divided into 0.5 nm meshes. The dielectric function of Au NRs was used from Johnson and Christ and the Au NRs were assumed to be embedded in water with a refractive index of 1.33, and the silica refractive index used was 1.459. The electric field polarization was along the long axial direction of the Au NRs. Stability of Au NRs and Au@SiO2 Under Laser Irradiation The setup for this procedure was composed of a 10-mm path length quartz cuvette and continuous 808 nm laser coupled out through an optical fiber (GKFCM-808, laser spot size 2 mm). The laser power was obtained from the P-I curves. The laser light illuminated on the Au NRs colloid in the cuvette. The temperature of the colloid is reasonably assumed to be uniform in the solution for the same testing data at different positions.
Results and Discussion The microstructure and growth mechanism of the Au NRs synthesized by the “seed” method were studied in detail, as reported previously [10]. Figure 1a shows the TEM image of the as-prepared Au NRs, which have an average diameter of 12 nm and a length of 52 nm. Figure 1b presents the TEM image of Au NRs after SiO2 coating. It can be seen that most of the Au NRs, together with some Au nanoparticles, have been successfully coated with homogeneous SiO2, and
b
Plasmonics
Fig. 4 Schematic for the exchange of TEOS with CTAB and the SiO2 coating process
formed core-shell Au@SiO2 nanostructure. Also, there are a few SiO2 nanospheres without Au NRs cores. The aspect ratio of the obtained Au NRs is ∼4.3, giving rise to a SPRL peak centered at about 800 nm, as indicated by the solid line in Fig. 2. After SiO2 coating, the SPRT position remains constant whereas the longitudinal one has an obvious wavelength red-shifting of ∼12 nm (see the dashed line). As it is known that the SPRL position is sensitive to the dielectric constant of surrounding. After the formation of SiO2 shell, the SPRL of Au NRs red-shifts with increasing local refractive index [45], which agreed well with the gans theory [46]. In the experiment, it is found that certain CTAB coverage on the surface of Au NRs is very important for the SiO2 capping. And 0.3 mL CTAB (0.1 M) solution was usually added into the purified colloidal Au NRs to keep the CTAB concentration just above the critical micelle concentration (1 mM) [42, 43]. Moreover, if the Au NRs were encapsuled
Fig. 5 TEM images of Au NRs coated with SiO2 shells of different thickness
unsuccessfully by SiO2 shell, Au NRs would aggregate with the addition of ethanol in the washing process, and the optical property changes obviously. Therefore, when ethanol was used to wash the sample after capping, the color of colloid with additional CTAB has no changes, indicating the stable of Au@SiO2 dispersion. Also, unsuccessful SiO2 capping occurs when the Au NRs colloid has no additional CTAB. Figure 3a shows the TEM image of the SiO2 capping sample with additional CTAB after ethanol washing and Fig. 3b shows the TEM image of the samples without additional CTAB after ethanol washing (insertion is TEM image of the sample of the supernatant solution). Therefore, CTAB molecules play an important role not only as surfactant in stabilizing Au NRs but also in SiO2 coating. The formation of SiO2 coatings involve base-catalyzed hydrolysis of SiO2 precursor TEOS, followed by nucleation and condensation of SiO2 onto the surface of the Au NRs. The exchange of TEOS with CTAB and the SiO2 coating process were illustrated by the schematic diagrams shown in Fig. 4. As it is known that the hydrolysis of TEOS can be proceeded by acid–base catalysis, and the basehydrolysis is a nucleophilic reaction, favoring for the primary SiO2 granule polycondensation on the surface of Au NRs core. By controlling the coating time, volume of TEOS/ethanol, and the concentration of Au NRs, we were able to vary the shell thickness. Figure 5a–d shows the TEM images of Au
a
b
c
d
Absorption
Plasmonics
400
Au NRs before SiO2 coating 10nm 20nm 30nm 40nm
600
800
1000
Wavelength(nm)
Fig. 6 UV–vis-NIR spectra of Au@SiO2 with different SiO2 thickness corresponding to the samples of Fig. 5a–d
NRs coated with SiO2 shells of different thickness. With increasing the volume of TEOS/ethanol mixture to the Au NRs, the average thickness of the SiO2 shell is estimated about 10 nm, 20 nm, 30 nm, and 40 nm, respectively. The SiO2 coatings in Fig. 5b are nearly ellipsoid with a uniform SiO2 coverage on the Au NRs surface. With increasing TEOS, the SiO2 shells are nearly spheres, i.e., the SiO2 shells are nonuniform, and some SiO2 nanospheres without Au NRs core can be seen. Figure 6 shows the optical spectra of Au@SiO2
b a
GNRs 10nm 20nm 30nm 40nm
Absorption
Fig. 7 a Geometry of Au@SiO2 with geometrical parameters used in the analysis (l=52 nm, w= 12 nm, h=10, 20, 30, 40 nm). b FDTD-calculated absorption spectra of Au@SiO2 with uniform SiO2 shell thickness increasing from 10 nm to 40 nm
with different SiO2 shell thickness. It is indicated that SPRL red-shifts (from 800 nm to 820 nm) gradually with increasing the thickness of the SiO2 shells (corresponding to Fig. 5a–c). And compared with the Au@SiO2 with 30 nm SiO2 shell (see Fig. 5c), there is no obvious SPRL shift for Au NRs coated with spherical SiO2 shell of ∼40 nm in thickness (Fig. 5d). For further understanding of the influence of surface SiO2 coating on the SPRL property of Au NRs, the finite-difference time-domain (FDTD) solutions were applied to simulate the SPR property of one Au NR coated with SiO2 shell. In the first FDTD solutions, the size of Au NR was 12 nm in dimension and 52 nm in length, and the Au NR was coated by SiO2 shell with uniform thickness, as shown in Fig. 7a. Figure 7b shows the calculated absorption spectra of Au@SiO2 with shell thickness being 10 nm, 20 nm, 30 nm, and 40 nm, respectively. It is found that the SPRL red-shifts obviously from 835 to 865 nm when increasing the SiO2 shell thickness. And the FDTD calculations show a very similar evolution to that of the experimental measurements (Fig. 6). Probably due to geometrical and environmental differences between the experimental sample and the calculating mode, the SPRL position and shifting are not the same. Then, FDTD simulations were also used to analyze the influences caused by the non-uniform SiO2 coating. In the simulation, the same sized Au NR was
700
750
800
850
900
Wavelength(nm)
b a Absorption
Fig. 8 a Geometry of Au@SiO2 with geometrical parameters used in the analysis (t=40 nm, h decreases from 40 to 10 nm). b FDTD-calculated absorption spectra of Au@SiO2 with different SiO2 shell thickness on two head surfaces of Au nanorod
40nm 35nm 30nm 20nm 10nm 840
750
850
860
870
800
880
850
Wavelength(nm)
900
950
Plasmonics
a
400
b GNRs 2W 3W 4W 5W
600
Au@SiO2 5W 6W 7W 8W
Absorption
Absorption
Fig. 9 (a–b) UV–vis-NIR spectra of Au NRs and Au@SiO2 colloids after irradiating 5 min by 808 laser under different power. (c–d) TEM images of Au NRs and Au@SiO2 colloids after irradiated 5 min under 5 and 8 W laser powers, respectively
800
1000
400
c
used, keeping the thickness of the transverse SiO2 shell 40 nm constant and decreasing the longitudinal SiO2 thickness (see Fig. 8a). As the thickness of the SiO2 coating on the two heads decrease from 40 nm to 10 nm, the SPRL blue-shifts very slightly from 865 nm to 860 nm, as shown in Fig. 8b and magnified spectra in inset. It is indicated that non-uniform coating has an obvious impact on the SPRL shift of the colloid spectra, which can even partly explain the differences between the experimental measurements and the FDTD simulations, corresponding to Fig. 6 and Fig. 7b, respectively. Additionally, the stability of Au NRs before and after SiO2 coating was investigated with the assistance of 808 nm laser irradiation. Figure 9 showed the absorption spectra of Au NRs and core-shell Au@SiO2 after irradiating, respectively, for 5 min under different laser power. For CTAB-stabilized Au NRs, the SPRL of Au NRs had no obvious changes when the laser power was less than 4 W, as shown in Fig. 9a. If the laser power exceeded 4 W, the intensity of SPRL decreased sharply. As the colloid was irradiated by the 5 W laser, the temperature increased quickly, and the Au NRs aggregated. However, for SiO2 coated Au NRs, when the laser power was less than 7 W, the optical spectra had no obvious changes. As the laser power increased to 8 W, the absorption intensity over the whole wavelength decreased and the SPRL shown in Fig. 9b. Images in Fig. 9c–d are TEM results of Au NRs and Au@SiO2 colloids after irradiating for 5 min under 5 W and 8 W, respectively. From the TEM image of d Au NRs, it can be seen that melting and reshaping occurred during the irradiating process, and rod shape turned from rounded cylinder to ellipsoid or sphere [47]. For the Au@SiO2 colloid after
600
800
1000
Wavelength(nm)
Wavelength(nm)
d
irradiation, most of Au NRs are stable with discreted SiO2 coating. Combing Fig. 9b with Fig. 9d, we speculated that the Au-Si bonding can be partially destroyed at high temperature during the strong laser irradiation [48]. The stability and optical properties of Au@SiO2, together with advantages of SiO2, such as gain medium for plasmonic property, biological compatibility, and chemical stability indicated promising potentials for core-shell structured Au@SiO2 in further applied research.
Conclusions In summary, we have successfully deposited the mesoporous SiO2 shell onto the surface of the Au NRs into core-shell Au@SiO2 nanostructures. The thickness of SiO2 shell can be achieved by increasing the addition of as-prepared mixture of TEOS and anhydrous ethanol to the reaction solution. It is also found that the concentration of CTAB is very important for the SiO2 coating. The influence of uniform and nonuniform SiO2 shell on the SPR of Au NRs was investigated based on the experimental results and FDTD simulations. Compared with Au NRs, the Au@SiO2 nanostructure is stable with increasing 808 nm laser power (up to 8 W), whereas Au NRs undergo a shape deformation from rod to spherical particle when the laser power is less than 5 W. The stability and optical properties of core-shell Au@SiO2, along with the advantages of SiO2, show that Au@SiO2 composites are promising in designing plasmonic photothermal properties and further applications in nanomedicine.
Plasmonics Acknowledgments This study was financially supported by National Natural Science Foundation of China (Nos. 11274173, 11374159, 61222403) and Fundamental Research Funds for the Central Universities (NZ2013304, NJ20140005). TEM measurements in Nanjing Medical University are also gratefully acknowledged.
References 1. Gutiérrez-Sánchez C, Pita M, Vaz-Domínguez C, Shleev S, De Lacey AL (2012) Gold nanoparticles as electronic bridges for laccase-based biocathodes. J Am Chem Soc 134:17212–17220 2. Li H, Yuan K, Zhang Y, Wang J (2013) Synthesis of Au-SiO2 asymmetric clusters and their application in ZnO nanosheet-based dye-sensitized solar cells. Appl Mater Interfaces 5:5601–5608 3. Zijlstra P, Paulo PMR, Orrit M (2012) Optical detection of single non-absorbing molecules using the surface plasmon resonance of a gold nanorod. Nat Nanotechnol 7:397–382 4. Liu G, Li Y, Duan G, Wang J, Liang C, Cai W (2012) Tunable surface plasmon resonance and strong SERS performances of Au openingnanoshell ordered arrays. ACS Appl Mater Interfaces 4:1–5 5. Mallick S, Sun IC, Kim K, Yil DK (2013) Silica coated gold nanorods for imaging and photo-thermal therapy of cancer cells. J Nanosci Nanotechnol 13:3223–3229 6. Zhang Z, Wang L, Wang J, Jiang X, Li X, Hu Z, Ji Y, Wu X, Chen C (2012) Mesoporous silica-coated gold nanorods as a light-mediated multifunctional theranostic platform for cancer treatment. Adv Mater 24:1418–1423 7. Hu Y, Lu L, Liu J, Chen W (2012) Direct growth of size-controlled gold nanoparticles on reduced graphene oxide film from bulk gold by tuning electric field: effective methodology and substrate for surface enhanced Raman scattering study. J Mater Chem 22:11994–12000 8. Cohen-Karni T, Jeong KJ, Tsui JH, Reznor G, Mustata M, Wanunu M, Graham A, Marks C, Bell DC, Langer R, Kohane DS (2012) Nanocoposite gold-silk nanofibers. Nano Lett 12:5403–5406 9. Yang SC, Kobori H, He CL, Lin MH, Chen HY, Li CC, Kanehara M, Teranishi T, Gwo S (2010) Plasmon hybridization in individual gold nanocrystal dimers: direct observation of bright and dark modes. Nano Lett 10:632–637 10. Li H, Kan C, Yi Z, Ding X, Cao Y, Zhu JJ (2010) Synthesis of one dimensional gold nanostructures. J Nanomater ID: 962718(8pp) 11. Kan C, Zhu J, Wang C (2009) Ag nanoparticle-filled polymer shell formed around Au nanoparticle core via ultrasound-assisted spherulite growth. J Cryst Growth 311:1565–1570 12. Kan C, Wang C, Li H, Qi J, Zhu J, Li Z, Daning Shi D (2010) Gold microplates with well-defined shapes. Small 6:1768–1775 13. Pérez-Juste J, Pastoriza-Santos I, Liz-Marzán LM, Mulvaney P (2005) Gold nanorods: synthesis, characterization and applications. Coord Chem Rev 249:1870–1901 14. Zhong L, Zhou X, Bao S, Shi Y, Wang Y, Hong S, Huang Y, Wang X, Xie Z, Zhang Q (2011) Rational design and SERS properties of sideby-side, end-to-end and end-to-side assemblies of Au nanorods. Mater Chem 21:14448–14455 15. Ni W, Mosquera RA, Pérez-Juste J, Liz-Marzán LM (2010) Evidence for hydrogen-bonding-directed assembly of gold nanorods in aqueous solution. J Phys Chem Lett 1:1181–1185 16. Wang Y, DePrince AE, Gray SK, Lin XM, Pelton MP (2010) Solvent-mediated end-to-end assembly of gold nanorods. J Phys Chem Lett 1:2692–2698 17. Luo T, Huang P, Gao G, Shen G, Fu S, Cui D, Zhou C, Re Q (2011) Mesoporous silica-coated gold nanorods with embedded indocyanine green for dual mode X-ray CT and NIR fluorescence imaging. Opt Express 19:17030–17039
18. Chen H, Kou X, Yang Z, Ni W, Wang J (2008) Shape- and sizedependent refractive index sensitivity of gold nanoparticles. Langmuir 24:5233–5237 19. Fava D, Nie Z, Winnik MA, Kumacheva E (2008) Evolution of selfassembled structures of polymer-terminated gold nanorods in selective solvent. Adv Mater 20:4318–4322 20. Sun Z, Ni W, Yang Z, Kou X, Li L, Wang J (2008) pH-controlled reversible assembly and disassembly of gold nanorods. Small 4: 1287–1292 21. Gorelikov I, Matsuura N (2008) Single-step coating of mesoporous silica on cetyltrimethyl ammonium bromide-capped nanoparticles. Nano Lett 8:369–373 22. Huang H, Liu X, Hu T, Chu PK (2010) Ultra-sensitive detection of cysteine by gold nanorod assembly. Biosens Bioelectron 25:2078– 2083 23. Wang L, Zhu Y, Xu L, Chen W, Kuang H, Liu L, Agarwal A, Xu C, Kotov NA (2010) Side-by-side and end-to-end gold nanorod assemblies for environmental toxin sensing. Angew Chem Int Ed 49:5472– 5475 24. Huang H, Qu C, Liu X, Huang S, Xu Z, Zhu Y, Chu PK (2011) Amplification of localized surface plasmon resonance signals by a gold nanorod assembly and ultra-sensitive detection of mercury. Chem Commun 47:6897–6899 25. Wang J, Zhang P, Li CM, Li YF, Huang CZ (2012) A highly selective and colorimetric assay of lysine by molecular-driven gold nanorods assembly. Biosens Bioelectron 34:197–201 26. Tallury P, Payton K, Santra S (2008) Silica-based multimodal/ multifunctional nanoparticles for bioimaging and biosensing applications. Nanomedicine 3:579–592 27. Wu H-L, Kuo C-H, Huang MH (2010) Seed-mediated synthesis of gold nanocrystals with systematic shape evolution from cubic to trisoctahedral and rhombic dodecahedral. Langmuir 26:12307– 12313 28. Liu X, Xu H, Xia H, Wang D (2012) Rapid seeded growth of monodisperse, quasi-spherical, citrate-stabilized gold nanoparticles via H2O2 reduction. Langmuir 28:13720–13726 29. Jana NR, Gearheart L, Murphy CJ (2001) Seed-mediated growth approach for shape-controlled synthesis of spheroidal and rod-like gold nanoparticles using a surfactant template. Adv Mater 13:1389– 1393 30. Nikoobakht B, El-Sayed MA (2003) Preparation and growth mechanism of gold nanorods (NRs) using seed-mediated growth method. Chem Mater 15:1957–1962 31. Ye X, Jin L, Caglayan H, Chen J, Xing G, Zheng C, Doan-Nguyen V, Kang Y, Engheta N, Kagan CR, Murray CB (2012) Improved sizetunable synthesis of monodisperse gold nanorods through the use of aromatic additives. ACS Nano 6:2804–2817 32. Liz-Marzán LM, Mulvaney P (2003) The assembly of coated nanocrystals. J Phys Chem B 107:7312–7326 33. Rodríguez-Fernández J, Pastoriza-Santos I, Pérez-Juste J, de Abajo FJG, Liz-Marzán LM (2007) The effect of silica coating on the optical response of sub-micrometer gold spheres. J Phys Chem C 111:13361–13366 34. Liu W, Zhu Z, Deng K, Li Z, Zhou Y, Qiu H, Gao Y, Che S, Tang Z (2013) Gold nanorod@chiral mesoporous silica core– shell nanoparticles with unique optical properties. J Am Chem Soc 135:9659–9664 35. Li N, Yu Z, Pan W, Han Y, Zhang T, Tang B (2013) A nar-infrared light-triggered nanocarrier with reversible DNA valves for intracellular controlled release. Adv Funct Mater 23:2255–2262 36. Chen YS, Frey W, Kim S, Kruizinga P, Homan K, Emelianov S (2011) Silica-coated gold nanorods as photoacoustic signal nanoamplifiers. Nano Lett 11:348–354 37. Pastoriza-Santos I, Pérez-Juste J, Liz-Marzán LM (2006) Silicacoating and hydrophobation of CTAB-stabilized gold nanorods. Chem Mater 18:2465–2467
Plasmonics 38. Fernández-López C, Mateo-Mateo C, Álvarez-Puebla RA, PérezJuste J, Pastoriza-Santos I, Liz-Marzán LM (2009) Highly controlled silica coating of PEG-capped metal nanoparticles and preparation of SERS-encoded particles. Langmuir 25:13894–13899 39. Juvé V, Cardinal MR, Lombardi A, Crut A, Maioli P, Pérez-Juste J, Liz-Marzán LM, Fatti ND, Vallée F (2013) Size-dependent surface plasmon resonance broadening in nonspherical nanoparticles: single gold nanorods. Nano Lett 13:2234–2240 40. Wang G, Chen Z, Chen L (2011) Mesoporous silica-coated gold nanorods: towards sensitive colorimetric sensing of ascorbic acid via target-induced silver overcoating. Nanoscale 3:1756–1759 41. Ming T, Zhao L, Chen HJ, Woo KC, Wang J, Lin HQ (2011) Experimental evidence of plasmophores: plasmon-directed polarized emission from gold nanorod–fluorophore hybrid nanostructures. Nano Lett 11:2296–2303 42. Lee S, Anderson LJ, Payne CM, Hafner JH (2011) Structural transition in the surfactant layer that surrounds gold nanorods as observed by analytical surface-enhanced Raman spectroscopy. Langmuir 27: 14748–14756
43. DuChene JS, Almeida RP, Wei WD (2012) Facile synthesis of anisotropic Au@SiO2 core-shell nanostructures. Dalton Trans 41: 7879–7882 44. Wu C, Xu QH (2009) Stable and functionable mesoporous silicacoated gold nanorods as sensitive localized surface plasmon resonance (LSPR) nanosensors. Langmuir 25:9441–9446 45. Zhan Q, Qian J, Li X, He S (2010) A study of mesoporous silica-encapsulated gold nanorods as enhanced light scattering probes for cancer cell imaging. Nanotechnology 21:12, 055704 46. Quinten M (2011) Optical properties of nanoparticle system: mie and beyond. Wiley-VCH 47. Ni W, Ba H, Lutich AA, Jäckel F, Feldmann J (2012) Enhancing single-nanoparticle surface-chemistry by plasmonic overheating in an optical trap. Nano Lett 12:4647–4650 48. Wang W, Cheng W, Xing X, Mo G, Cai Q, Chen Z, Wu Z (2010) Temperature-induced interfacial change in Au@SiO2 core-shell nanoparticles detected by extended x-ray absorption fine structure. J Phys Chem C 114:41–49
