Surface confined successive growth of silver nanoplates on a solid substrate with tunable surface plasmon resonance

Abstract

A strategy for direct growth of Ag nanoplates on a solid substrate was developed by using successive seed-mediated growth approach. The size and surface density of grown Ag nanoplates increased by repeated successive growth with tunable LSPR properties and the prepared Ag nanoplate grown substrates showed excellent performance as a SERS platform.

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Introduction

The controlled synthesis of anisotropic metal nanostructures has been an important issue because their optical property—specifically, localized surface plasmon resonance (LSPR) known as collective oscillation of free electrons on metal surfaces^1–3—is mostly dependent on their shape and size.^4,5 Especially, Ag nanoplates have attracted much attention due to their highly anisotropic structure and high potential for practical application as surface enhanced Raman scattering (SERS)-based sensing platforms.^6–8 To date, several size- and shape-controlled synthetic strategies have been developed for Ag nanoplates such as photochemical processes in the presence of citrate ligands^9–12 and thermal transformation processes in the presence of various surfactants and polymers such as polyvinylpyrrolidone (PVP),^13–15 poly(diallyl dimethylammonium) chloride (PDDA),¹⁶ polystyrene sulfonate,¹⁷ cetyltrimethylammonium bromide (CTAB),^18,19 di(2-ethylhexyl) sulfosuccinate (AOT)^20,21 and sodium dodecyl sulfate (SDS).²² In addition to the synthetic strategies, the development of a high density array of Ag nanoplates on a solid substrate is also important to fabricate more robust SERS platforms because the SERS signal of Raman active molecules is expected to be more reproducible and highly enhanced on solid substrates coated with Ag nanostructures.^23,24

Recently, Yin²⁵ and Xia²⁶ groups reported the successive growth of Ag nanoplates by using triangular Ag nanoplates as seeds and trisodium citrate and PVP as shape-directing agents having different affinity to {111} and {100} crystal facets. Those reports showed that Ag seeds could be grown toward a selective direction under appropriate synthetic conditions. In contrast to those successful demonstrations in solution phase synthesis, the assembly of the synthesized Ag nanoplates with high density on a solid substrate still remains a challenging subject because of cumbersome purification, surface functionalization and surface immobilization processes.

Very recently, Duley and co-workers reported that Ag nanoplates of various sizes were successfully assembled on hydrophobic solid substrates by immersing in Ag nanoplate suspensions for approximately 20 h and the resulting films showed high SERS activity in the tunable wavelength region.²⁷ However, this post-assembly approach provided only a laterally assembled structure due to strong face-to-face interactions. Therefore, the hot spot in narrow gaps of Ag nanoplates was randomly formed at the restricted sites and it is difficult to further tune LSPR properties for improvement of SERS activity once the Ag nanoplates were laterally immobilized on substrates.

To be more flexible and compatible with mass production of active SERS platforms, direct and successive growth of Ag nanoplates would be a more attractive approach for the preparation of Ag nanoplate films on a solid substrate with tunable LSPR properties and many hot spots. To date, the seed-mediated chemical growth,²⁸ electrochemical growth,²⁹ and galvanic displacement^30–34 have been investigated to directly synthesize Ag nanostructures on solid substrates. However, this seed-mediated chemical growth method could not provide Ag nanoplate films with high density and tunable LSPR properties and requires CTAB as a shape-directing agent. The seed-mediated electrochemical growth requires electrochemical equipment and PVP as a shape-directing agent. Those shape-directing agents make further functionalization of Ag nanoplates with other ligands difficult, which is often necessary to develop sensing platforms, due to a cumbersome ligand exchange process. On the other hand, the galvanic displacement method is limited in applicable substrates because only specific semiconductor substrates are suitable for this method. Therefore, it is urgent to develop Ag nanoplate growth strategy on a solid substrate which is simple and applicable to various substrates while providing high density and tunable LSPR properties of Ag nanoplates for large scale fabrication of a robust SERS-platform.

Here, we demonstrate a seed-mediated successive growth of Ag nanoplates on solid substrates loaded with Ag nanoparticle (AgNP) in the presence of citrate ligands (Scheme 1). The citrate ligands preferentially bind to {111} crystal facets on both top and bottom faces of growing Ag nanoplates to slow down vertical growth versus lateral growth and could be readily replaced with thiol-terminated ligands for flexible surface modifications and SERS-based sensing after the nanoplate synthesis.²⁶ The size and density of Ag nanoplates grown on substrates were successfully tuned by simply repeating the growth cycles. This simple synthetic strategy led to an increased surface area and tunable LSPR properties such as red-shifted in-plane dipole plasmon resonance and strong plasmon coupling effect which resulted in a significantly augmented SERS signal by ∼200 and ∼300 fold at 514 and 633 nm excitation sources, respectively, after 5 growth cycles as compared to the AgNP loaded-substrates.

Scheme 1 Successive growth of Ag nanoplates on polyallylamine hydrochloride (PAA)-functionalized solid substrates.

Experimental

Materials

Silver nitrate, L-ascorbic acid, sodium borohydride, PAA (M_w 15 000 Da) and 4-aminothiophenol (4-ATP) were purchased from Aldrich Chemical Co. (Milwaukee, WI, USA). Hydrogen peroxide (30%), trisodium citrate and rhodamine 6G (R6G) were purchased from Junsei (Japan). Sulfuric acid was purchased from Samchun (Seoul, Korea). Ethanol was purchased from Merck (Darmstadt, Germany). 500 nm SiO₂/P⁺⁺ Si substrates (500 μm thickness) and 4 inch quartz wafers (500 μm thickness) were respectively purchased from STC (Japan) and i-Nexus (Stamford, USA). All chemicals were used as received.

Synthesis of AgNP

0.5 mL of AgNO₃ (59 mM) and 1 mL of trisodium citrate (34 mM) were added to 98 mL of water and stirred for 10 min. Then, 0.5 mL of NaBH₄ (20 mM) aged for 2 h was added to the mixed solution simultaneously with stirring. The reaction mixture was further stirred for 1 h and suspended for 24 h at room temperature.

Immobilization of AgNP on PAA-functionalized substrates

The substrates (quartz and Si wafers) were cleaned by immersing into piranha solution (H₂SO₄ : H₂O₂ (30%) = 3 : 1, Warning: piranha solution is explosive and extremely corrosive) at 150 °C for 30 min, rinsed with water and ethanol and dried under a stream of nitrogen. The cleaned substrates were immersed in the PAA aqueous solution (1 mg mL⁻¹) for 30 min, rinsed with water and ethanol and dried under a stream of nitrogen. The PAA-treated substrates were incubated with the AgNP suspension for 1 h, rinsed with water and ethanol and dried under a stream of nitrogen. The AgNP loaded substrates were stored in the dark until use.

Successive growth of Ag nanoplates on AgNP loaded substrates

AgNP loaded substrates were incubated in 5 mL of reduction solution consisting of ascorbic acid (1.2 mM) and trisodium citrate (0.4 mM) for 3 min. After incubation, 5 mL of AgNO₃ (0.6 mM) was added drop-wise to the reduction solution and incubated for 5 min for seed-mediated growth. After growing reaction, the substrates were rinsed with water and ethanol and dried under a stream of nitrogen. The Ag nanoplate grown substrates were repeatedly used for the next growing reaction by applying the same procedure up to 5 times.

Coating of 4-ATP and R6G onto the grown Ag nanoplates

AgNP loaded and Ag nanoplate grown substrates were respectively immersed in 1 mL of 1 μM and 1 mM 4-ATP ethanolic solutions for 24 h, washed with water and ethanol and dried under a stream of nitrogen. All the substrates were also respectively immersed in 1 mM aqueous solution of R6G for 12 h, washed with water and ethanol and dried under a stream of nitrogen. To find the detectable concentration range of R6G on AgNP loaded and Ag nanoplate grown substrates, all the substrates were respectively immersed in aqueous R6G solution with varying concentrations from 1 × 10⁻⁴ to 1 × 10⁻⁹ M for 2 h, washed with water and ethanol and dried under a stream of nitrogen.

Characterization

The surface morphology changes of AgNP into Ag nanoplate on substrates were observed by S-4800 field emission scanning electron microscopy (Hitachi, Japan). SERS spectra of 4-ATP on silver nanostructures were collected by LabRAM HR UV/vis/NIR (Horiba Jobin Yvon, France) using an Ar ion CW laser (514.5 nm) and HeNe laser (633 nm) as excitation sources (15 mW) focused through a BXFM confocal microscope equipped with an objective lens (50×, numerical aperture = 0.50). The observation of LSPR changes by shape evolution of silver nanostructures was carried out by a UV-2550 (Shimadzu, Japan).

Results and discussion

We first synthesized spherical AgNP (average 10 ± 5 nm in diameter) with LSPR band at 395 nm (Fig. S1a in the ESI†) by reduction of Ag⁺ ions using NaBH₄ in the presence of citrate ligands. It is well known that the AgNP synthesized under the above conditions have stacking faults and twin defects on their side facets with high surface energy, which led to preferential attachment of adatoms to lower the local nucleation energy for the formation of a new atomic layer toward lateral direction (Fig. S1b†).²⁶ For the immobilization of the AgNP as seeds, piranha-treated Si substrates were first immersed in a PAA solution (1 mg mL⁻¹) to introduce positive charges on the substrate surface. Then, the PAA-treated Si substrates were immersed into the AgNP suspension for 1 h to immobilize AgNP on the surface by an electrostatic interaction between PAA-treated substrates and citrate-stabilized AgNP (Fig. 1a).

Fig. 1 SEM images of surface-immobilized AgNP (a) and subsequently grown Ag nanoplates on Si substrates through 1–5 growth cycles (b–f).

Next, the seed-mediated growth proceeded by applying a slightly modified solution phase successive growth method to our surface-confined seed-mediated growth system. Specifically, the AgNP loaded substrates were immersed into 5 mL of reduction solution consisting of 1.2 mM ascorbic acid and 0.4 mM trisodium citrate for 3 min to fully induce the binding of citrate ligands on the immobilized AgNP surface. Next, 5 mL of 0.6 mM AgNO₃ was slowly added to the reduction solution containing AgNP loaded substrates and incubated for 5 min to induce seed-mediated lateral direction growth. This one round of growth gave Ag nanoplates with triangular, trapezoidal and hexagonal shapes on the surface with broad size distribution and random orientation, although un-grown AgNP still existed on the surface. The edge length of the Ag nanoplates grown on the surface was mostly 85 ± 10 nm or 120 ± 10 nm and the number of Ag nanoplates with each edge length was 21 and 9 per μm² of surface area, respectively. The compositions of triangular, trapezoidal and hexagonal Ag nanoplates were 55, 32 and 10%, respectively. The Ag nanoplates had an uniform thickness of ∼25 nm as measured from vertically oriented Ag plates on the surface (Fig. 1b).

Ag nanoplates were not formed on solid substrates without pre-loaded AgNP (Fig. S2†). When the AgNP loaded substrates were incubated in the growing solution over 5 min, the color of the growing solution changed from colorless to emerald green after 5 min of incubation, indicating that the self-nucleation occurred in the solution phase during seed-mediated growth on the surface, mainly resulting in irregular, relatively large Ag nanostructures (Fig. S3†). The self-nucleation was also observed in a solution without AgNP loaded substrate. To minimize this self-nucleation, the successive seed-mediated growth of Ag nanoplates was performed for less than 5 min for each growth cycle with thorough rinsing and drying steps. As another approach to decrease the self-nucleation, Ag precursor could be changed from Ag⁺ ion to Ag⁺–citrate complex to slow down the reaction rate of Ag reduction.³⁵

The successive growth was carried out by simply repeating the growth process until the desired size and density of Ag nanoplates were obtained. As the growth cycle was repeated up to 5 times, the number of Ag nanoplates with an edge length 120 ± 10 nm continuously increased from 9 to 16, 20, 23 and 31 per μm² but the number of Ag nanoplates with an edge length 85 ± 10 nm decreased from 21 to 24, 18, 17 and 7 per μm² (Fig. 1c–f). The changed composition of large Ag nanoplates was attributed to the sequential growth of un-grown AgNP and further growth of Ag nanoplates that were small in size on the surface. It is noteworthy that the total number of Ag nanoplates remained nearly constant after 2 growth cycles, indicating that the transition of AgNP to Ag nanoplate almost completed within 2 growth cycles, further 3 growth cycles resulted in enlargement of grown Ag nanoplates with shape transition and there was almost no desorption of Ag nanostructures from the substrates and adsorption of Ag nanostructures formed in solution phase by self-nucleation on the substrates. In addition to the size change of Ag nanoplates, the composition of triangular, trapezoidal and hexagonal Ag nanoplates also changed respectively during repeated growth cycles from 55, 32 and 10% to 14, 32 and 54% after 5 growth cycles with no noticeable thickness change (Fig. 1c–f). This shape transition of Ag nanoplates induced by the truncation of triangular nanoplates is generally observed in successive growth of Ag nanoplates in the solution phase.^25,26

The optical properties of the Ag nanoplate grown quartz substrates by repeated growth cycles were monitored by using UV-vis-NIR spectroscopy (Fig. 2a). The typical LSPR band of AgNP from out-of-plane dipole resonance was broadened by immobilization on the PAA-treated surface owing to slight aggregation on the surface.²⁸ After the first round of the seed-mediated growth, the out-of-plane dipole resonance band of AgNP was red-shifted from 390 to 405 nm by shape transition of AgNP (10 ± 5 nm in diameter) to Ag nanoplates (∼25 nm in thickness) and a new absorption band appeared around 770 nm, which corresponds to in-plane dipole plasmon resonance band originated from the growth of Ag nanoplates with narrowing the gaps between them (Fig. 2a). While the absorbance of out-of-plane and in-plane dipole resonance bands almost linearly increased with 2–5 growth cycles, the shape of the out-of-plane dipole resonance band showed no change but that of the in-plane dipole plasmon resonance band was broadened with 2–5 growth cycles (Fig. 2). The linear increase of both band intensities and broadening of in-plane dipole resonance band could be attributed to the continuing shape transition of un-grown AgNP during the first growth cycle into Ag nanoplates and further increased the edge length of the already-formed Ag nanoplates by the first growth cycle from 85 ± 10 nm to 120 ± 10 nm with 2–5 growth cycles, which were shown by SEM data (Fig. 1a–f). The data also well concurred with previously reported spectra of Ag nanoplates.²⁷ Therefore, the LSPR properties of the substrates were successfully tuned by the seed-mediated successive growth toward a lateral direction. After 5 growth cycles, the additional growth cycles led to gradual saturation in absorption derived from out-of-plane and in-plane dipole resonance bands (Fig. 2 and S4†).

Fig. 2 UV-vis-NIR spectra (200–1000 nm) of surface-immobilized AgNP and subsequently grown Ag nanoplates on quartz substrates through 1–10 growth cycles (a). Changes of absorbance at 405 and 770 nm respectively corresponding to out-of-plane and in-plane dipole plasmon resonance bands through 1–10 growth cycles (b). Reproducible UV-vis-NIR spectra were obtained on 4 different positions of each substrate (Fig. S4†).

As a control, the AgNP loaded substrate was continuously incubated in a single growth solution for 30 min, equivalent to the total growth time applied for five successive growth cycles. In this control, no significant changes of Ag nanostructure (Fig. S5†) and LSPR properties (Fig. S6†) were observed after initial 5 min incubation, even though fresh Ag⁺ ion, citrate and ascorbic acid were added to the reaction mixture. The data suggested that the seed-mediated growth reaction on the surface terminated within 5 min, probably due to competitive self-nucleation. Therefore, the successive lateral growth, not continuous growth, is an effective approach to obtain Ag nanostructures with different densities on a substrate with varying LSPR properties.

The applicability of the Ag nanoplates grown on the solid substrate as a SERS platform was examined by using 4-aminothiophenol (4-ATP) as a model compound. The AgNP loaded and the Ag nanoplate grown substrates were immersed in an ethanolic solution of 4-ATP (1 × 10⁻⁶ M) for 24 h to induce adsorption of 4-ATP and analyzed by Raman spectroscopy with 514 and 633 nm lasers as excitation sources. Although the 633 nm laser is not matched with out-of-plane dipole resonance of AgNP, 633 nm laser was used as an excitation source because the Ag nanoplates grown substrates showed in-plane dipole resonance at around 633 nm. Thus, the SERS signal could be further enhanced by 633 nm excitation source with repeated successive growth cycles.²⁹ The SERS spectra of 4-ATP on the AgNP loaded substrate showed five characteristic bands at 1077, 1143, 1391, 1435 and 1577 cm⁻¹ (Fig. 3a, AgNP), unlike a normal Raman spectrum of bulk solid 4-ATP (Fig. S7†), due to the charge transfer from AgNP to 4-ATP.^36,37 The band intensity at 1435 cm⁻¹ in SERS spectrum of 4-ATP on the AgNP loaded substrate continuously increased 205- and 296-fold with 514 and 633 nm excitation sources, respectively, by five successive growth cycles (Fig. 3a and b).

Fig. 3 SERS spectra of 4-ATP on the surface-immobilized AgNP and on the subsequently grown Ag nanoplates on Si substrates through 1–5 growth cycles by using 514 (a) and 633 nm (b) excitation sources. Average SERS signal intensity at 1577 cm⁻¹ (c) and EF value (d) of 4-ATP with standard deviation were plotted versus the number of growth cycles. The average SERS signal intensity and EF value with standard deviation were calculated from 5 measurements at different positions of each substrate. The data showed homogeneous Raman signal intensities from the entire surface.

To investigate the origin of SERS signal enhancement, the average enhancement factor (EF) of 4-ATP molecules adsorbed on the AgNP loaded and the Ag nanoplate grown substrates was calculated from five spectra obtained from five different positions on each substrate. The EF was defined as³⁷

EF = I_SERSN_bulk/I_RamanN_surface

where I_SERS and I_Raman are respectively the intensities of the vibrational mode at 1577 cm⁻¹ in SERS (Fig. 3c) and normal Raman spectra (Fig. S6†), and N_bulk and N_surface are respectively the number of 4-ATP molecules exposed to laser spot (1 μm in diameter) under normal Raman and SERS conditions. The N_bulk (8.9 × 10⁹) was obtained by taking the laser spot size, the penetration depth (2 μm), the molecular weight (125.19 g mol⁻¹) and the density of solid 4-ATP (1.18 g cm⁻³) into account.³⁸ The determination of N_surface on the AgNP loaded substrates and the Ag nanoplates grown substrates was not simple because of random orientation and broad distribution of size and shape of Ag nanostructures. Therefore, we employed the previously reported method which was developed for determination of N_surface on vertically grown Ag nanoplate coated substrates (Fig. S8†).³⁸ The approximate EF values of 4-ATP on the AgNP loaded substrates were respectively determined as 6.2 × 10² and 8.0 × 10² with 514 and 633 nm excitation sources and both values increased to 5.9 × 10⁴ and 9.4 × 10⁴ with the corresponding excitation sources after five repeated growth cycles (Fig. 3d and S8†). The higher EF of 4-ATP with 633 nm than that with 514 nm laser as an excitation source on the Ag nanoplate grown substrates could be ascribed to the augmented absorbance at 633 nm than at 514 nm, which was accompanied by repeated growth cycles. In addition to the SERS signal enhancement, the successive growth of Ag nanoplates on substrates provided decent homogeneous SERS EF on the overall surface of the substrates (Fig. 3). To confirm that the successive growth of Ag nanoplates is needed for SERS signal enhancement, the Ag nanoplates grown substrates by continuous growth were immersed in an ethanolic solution of 4-ATP (1 × 10⁻⁶ M) for 24 h and analyzed by Raman spectroscopy. The SERS signal of 4-ATP was not further enhanced on the control substrates prepared by continuous growth for more than 10 min (Fig. S9†).

Since there is a debate on the origin of 4-ATP SERS effect on AgNP,³⁹ rhodamine 6G (R6G) was next used as another SERS active molecule for Raman analysis to further investigate the applicability of a substrate with grown Ag nanoplates as a SERS platform. The AgNP loaded substrates and the Ag nanoplates grown substrates were immersed in 1 × 10⁻³ M aqueous solution of R6G for 12 h for the adsorption of R6G on the substrates and analyzed by Raman spectroscopy with 514 and 633 nm laser as an excitation source. The SERS spectra of R6G obtained on the AgNP loaded substrates with 514 nm laser showed typical six characteristic bands at 1309, 1360, 1418, 1506, 1572 and 1648 cm⁻¹ (Fig. 4a). The band intensity at 1648 cm⁻¹ in SERS spectrum of R6G on the AgNP loaded substrates continuously increased up to 65.1-fold on the Ag nanoplates grown substrates prepared by 5 growth cycles (Fig. 4a). By contrast, when 633 nm laser was used as an excitation source, there was no SERS signal of R6G on the AgNP loaded substrates because LSPR band of AgNP was not matched with the excitation source. However, after the first seed-mediated growth, the characteristic bands of R6G appeared in the SERS spectra and the band intensity at 1648 cm⁻¹ was enhanced up to 24.8-fold after 5 growth cycles (Fig. 4b). This result indicated that the SERS signal enhancement on the successively grown Ag nanoplates is originated from the increased surface area and the augmented absorbance at excitation wavelength by tuning of LSPR properties resulting in red-shift of in-plane dipole resonance and dipole resonance coupling.

Fig. 4 SERS spectra and band intensity at 1648 cm⁻¹ of R6G on the surface-immobilized AgNP and on the subsequently grown Ag nanoplates on Si substrates through 1–5 growth cycles obtained with 514 (a) and 633 nm (b) excitation sources. The average band intensity at 1648 cm⁻¹ of R6G with standard deviation was calculated from 5 measurements at different positions of each substrate. The data show homogeneous Raman signal intensities from the entire surface.

For further confirmation of the enhanced SERS activity, the AgNP loaded and the Ag nanoplate grown substrates were immersed in aqueous solutions of R6G with varying concentrations from 1 × 10⁻⁴ to 1 × 10⁻⁹ M for 2 h. The SERS signal of R6G on the AgNP loaded substrate was detected only at 1 × 10⁻⁴ M, the highest concentration we tested. On the other hand, the SERS signal of R6G on the Ag nanoplate substrate after three successive growths was detectable down to 1 × 10⁻⁶ M and the detectable concentration of R6G was further reduced to 1 × 10⁻⁷ M after five successive growth cycles (Fig. 5 and S10–S15†). Therefore, the detectable concentration range increased by three orders of magnitude by using the Ag nanoplates prepared through the seed mediated successive growth developed in the present study. The detection limit could be improved due to the increment of surface area and electric-field enhancement induced by the augmented absorbance at the excitation wavelength in Raman spectroscopy.

Fig. 5 SERS spectra of R6G on the surface-immobilized AgNP (a) and on the subsequently grown Ag nanoplates on Si substrates through 1–5 growth cycles (b–f) obtained with 514 nm excitation source. The substrates were respectively incubated in aqueous R6G solutions with varying concentrations from 1 × 10⁻⁴ to 1 × 10⁻⁹ M to compare their detectable concentration range of R6G.

Conclusions

We developed a strategy for the direct growth of Ag nanoplates on solid substrates based on successive seed-mediated growth using a citrate ligand as a shape-directing agent. The size and density of Ag nanoplates grown on the substrates increased successively without a noticeable change of their thickness. This simple synthetic approach without requiring expensive, complicated equipment or a lithographic process allowed us to control the LSPR properties of the fabricated Ag nanoplate substrates. We believe that the present seed-mediated direct growth approach on a solid substrate will provide a useful route to fabricate the Ag nanostructure loaded substrates with high density for various applications including SERS-based sensors.

Acknowledgements

This work was supported by the Basic Science Research Program (2011-0017356) through the National Research Foundation of Korea (NRF) and by the Research Center Program (EM1302) of IBS (Institute for Basic Science) funded by the Korean government.

References

1. D. L. Feldheim and C. A. Foss, Metal Nanoparticles: Synthesis, Characterization, and Applications, CRC Press, 1st edn, 2002, p. 338.
2. N. A. F. Al-Rawashdeh, M. L. Sandrock, C. J. Seugling and C. A. Foss, Jr, J. Phys. Chem. B, 1998, 102, 361.
3. N. A. F. Al-Rawashdeh and C. A. Foss, Jr, Nanostruct. Mater., 1997, 9, 383.
4. T. K. Sau, A. L. Rogach, F. Jäckel, T. A. Klar and J. Feldmann, Adv. Mater., 2010, 22, 1805.
5. A. I. Henry, J. M. Bingham, E. Ringe, L. D. Marks, G. C. Schatz and R. P. Van Duyne, J. Phys. Chem. C, 2011, 115, 9291.
6. M. Rycenga, C. M. Cobley, J. Zeng, W. Li, C. H. Moran, Q. Zhang, D. Qin and Y. Xia, Chem. Rev., 2011, 111, 3669.
7. M. E. Stewart, C. R. Anderton, L. B. Thompson, J. Maria, S. K. Gray, J. A. Rogers and R. G. Nuzzo, Chem. Rev., 2008, 108, 494.
8. N. L. Rosi and C. A. Mirkin, Chem. Rev., 2005, 105, 1547.
9. R. C. Jin, Y. C. Cao, E. C. Hao, G. S. Metraux, G. C. Schatz and C. A. Mirkin, Nature, 2003, 425, 487.
10. Y. G. Sun and Y. Xia, Adv. Mater., 2003, 15, 695.
11. M. Maillard, P. R. Huang and L. Brus, Nano Lett., 2003, 3, 1611.
12. C. Xue, G. S. Metraux, J. E. Millstone and C. A. Mirkin, J. Am. Chem. Soc., 2008, 130, 8337.
13. Y. Sun, B. Mayers and Y. Xia, Nano Lett., 2003, 3, 675.
14. G. S. Métraux and C. A. Mirkin, Adv. Mater., 2005, 17, 412.
15. Y. Yang, S. Matsubara, L. Xiong, T. Hayakawa and M. Nogami, J. Phys. Chem. C, 2007, 111, 9095.
16. H. Chen, Y. Wang and H. Dong, Inorg. Chem., 2007, 46, 10587.
17. D. Aherne, D. M. Ledwith, M. Gara and J. M. Kelly, Adv. Funct. Mater., 2008, 18, 2005.
18. S. Chen and D. L. Carroll, Nano Lett., 2002, 2, 1003.
19. S. Chen and D. L. Carroll, J. Phys. Chem. B, 2004, 108, 5500.
20. M. Maillard, S. Giorgio and M. P. Pileni, Adv. Mater., 2002, 14, 1084.
21. M. Maillard, S. Giorgio and M. P. Pileni, J. Phys. Chem. B, 2003, 107, 2466.
22. J. Yang, Q. Zhang, J. Y. Lee and H. P. Too, J. Colloid Interface Sci., 2007, 308, 157.
23. S. Wang, D. F. P. Pile, C. Sun and X. Zhang, Nano Lett., 2007, 7, 1076.
24. A. Gopinath, S. V. Boriskina, W. R. Premasiri, L. Ziegler, B. M. Reinhard and L. Dal Negro, Nano Lett., 2009, 9, 3922.
25. Q. Zhang, Y. Hu, S. Guo, J. Goebl and Y. Yin, Nano Lett., 2010, 10, 5037.
26. J. Zeng, X. Xia, M. Rycenga, P. Henneghan, Q. Li and Y. Xia, Angew. Chem., Int. Ed., 2011, 50, 244.
27. X. Y. Zhang, A. Hu, T. Zhang, W. Lei, X. J. Xue, Y. Zhou and W. W. Duley, ACS Nano, 2011, 5, 9082.
28. K. Aslan, J. R. Lakowicz and C. D. Geddes, J. Phys. Chem. B, 2005, 109, 6247.
29. G. Liu, W. Cai, L. Kong, G. Duan and F. Lü, J. Mater. Chem., 2010, 20, 767.
30. S. Y. Sayed, B. Daly and J. M. Buriak, J. Phys. Chem. C, 2008, 112, 12291.
31. Y. Sun, Chem. Mater., 2007, 19, 5845.
32. Y. Sun and G. P. Wiederrecht, Small, 2007, 3, 1964.
33. Y. Sun, J. Phys. Chem. C, 2010, 114, 857.
34. Y. Sun, Nanoscale, 2011, 3, 2247.
35. S. Djokic, Bioinorg. Chem. Appl., 2008, 436458.
36. M. Osawa, N. Matsuda, K. Yoshii and I. Uchida, J. Phys. Chem., 1994, 98, 12702.
37. Z. Zhang, F. Xu, W. Yang, M. Guo, X. Wang, B. Zhang and J. Tang, Chem. Commun., 2011, 47, 6440.
38. Y. Sun, L. Wang, L. Sun, C. Guo, T. Yang, Z. Liu, F. Xu and Z. Li, J. Chem. Phys., 2008, 128, 074704.
39. Y. F. Huang, H. P. Zhu, G. K. Liu, D. Y. Wu, B. Ren and Z. Q. Tian, J. Am. Chem. Soc., 2010, 132, 9244.

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Footnote

† Electronic supplementary information (ESI) available: Detailed experimental procedure and additional data. See DOI: 10.1039/c3ra44280b

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