Stable and Functionable Mesoporous Silica-Coated Gold Nanorods

Apr 21, 2009 - Chuanliu Wu and Qing-Hua Xu*. Department of .... Langmuir 2013 29 (41), 12633-12637. Abstract | Full Text ... Tingting Zhao , Hao Wu , ...
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Stable and Functionable Mesoporous Silica-Coated Gold Nanorods as Sensitive Localized Surface Plasmon Resonance (LSPR) Nanosensors Chuanliu Wu and Qing-Hua Xu* Department of Chemistry, National University of Singapore 3 Science Drive 3, Singapore 117543, Singapore Received February 23, 2009. Revised Manuscript Received March 29, 2009 Core-shell structured Au NRs with a surface-exposed gold core and a mesoporous silica shell (MS Au NRs) were demonstrated as a promising platform for localized surface plasmon resonance (LSPR) based molecular sensing. Mesoporous silica shell not only allows the Au NRs core to be directly exposed to their surrounding environment but also stabilizes Au NRs dispersion in various water-organic mixtures and pure organic solvents. The LSPR band of MS Au NRS displays a stable and linear response in spectral shift to the changes in their surrounding refractive index with a sensitivity of 325 nm/RIU. To demonstrate the application of MS Au NRs as LSPR nanosensors in molecular sensing, the plasmon response to molecular adsorbates (GSH) was demonstrated. MS Au NRs provide a more stable and sensitive response than CTAB-capped Au NRs in GSH sensing. In addition, we have also demonstrated that the LSPR response of Au NRs is highly sensitive to changes of local refractive index in mesoporous silica shell, which renders the feasibility of using MS Au NRs as effective molecule-sensing platforms when mesoporous silica shells were functionalized with various chemical and biological ligands.

Introduction Localized surface plasmon resonance (LSPR) of metal nanoparticles is highly sensitive to the refractive index of their surfacebound molecules and surrounding environment, which is the basis of using LSPR for molecular sensing.1,2 Over the past a few years, LSPR nanosensors based on metal nanoparticle arrays have been demonstrated to act as effective platforms for the detection of biological targets, binding kinetics, and protein conformational changes.2-5 The sensitivity of LSPR-based sensing has been greatly improved by selecting optimized nanoparticle structures, improving instrumental resolution, and shifting from ensemble to single-nanoparticle detection.2 For example, Hicks et al. developed a plasmonic structure termed “film over nanowell”, which exhibits a good refractive index sensitivity of 538 nm/RIU.6 Dahlin and co-workers improved the instrumental resolution of LSPR nanosensors, yielding a spectral shift precision of less than 5  10-4 nm and a noise level of less than 5  10-6 extinction units.7 Recently, progress in single-nanoparticle sensors has demonstrated improved absolute detection limit and higher spatial resolution in LSPR-based sensing.8-10 McFarland and co-workers reported a real-time molecular sensing with zeptomolar sensitivity by using single silver nanoparticle-based LSPR nanosensors.10 These studies suggest that LSPR nanosensors *Corresponding author. E-mail: [email protected]. (1) Willets, K. A.; Van Duyne, R. P. Annu. Rev. Phys. Chem. 2007, 58, 267. (2) Anker, J. N.; Hall, W. P.; Lyandres, O.; Shah, N. C.; Zhao, J.; Van Duyne, R. P. Nat. Mater. 2008, 7, 442. (3) Baciu, C. L.; Becker, J.; Janshoff, A.; Sonnichsen, C. Nano Lett. 2008 8, 1724. (4) Zhao, J.; Das, A.; Zhang, X. Y.; Schatz, G. C.; Sligar, S. G.; Van Duyne, R. P. J. Am. Chem. Soc. 2006, 128, 11004. (5) Hall, W. P.; Anker, J. N.; Lin, Y.; Modica, J.; Mrksich, M.; Van Duyne, R. P. J. Am. Chem. Soc. 2008, 130, 5836. (6) Hicks, E. M.; Zhang, X. Y.; Zou, S. L.; Lyandres, O.; Spears, K. G.; Schatz, G. C.; Van Duyne, R. P. J. Phys. Chem. B 2005, 109, 22351. (7) Dahlin, A. B.; Tegenfeldt, J. O.; Hook, F. Anal. Chem. 2006, 78, 4416. (8) Novo, C.; Funston, A. M.; Mulvaney, P. Nat. Nanotechnol. 2008, 3, 598. (9) Muskens, O. L.; Billaud, P.; Broyer, M.; Fatti, N.; Vallee, F. Phys. Rev. B 2008, 78, 205410. (10) McFarland, A. D.; Van Duyne, R. P. Nano Lett. 2003, 3, 1057.

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based on metal nanoparticle arrays provides a powerful platform for chemical and biological sensing. In addition to LSPR sensors based on nanoparticle arrays, suspension-based LSPR sensors were also used for various chemical and biological sensing.11-13 Liu and co-workers reported a nanoplasmonic molecular ruler based on DNA-conjugated gold nanoparticles for label-free, quantitative, and real-time measurement of nuclease activity.11 To improve the sensitivity of suspension-based LSPR sensing, Yu et al. utilized gold nanorods to fabricate sensitive LSPR sensors for multiplex detection of biological targets.12 Recently, Huang and co-workers developed a photostable single silver nanoparticle-based LSPR biosensor for real-time detection of single cytokine molecules and their binding reactions.13 Compared with array-based sensing, LSPR sensing in solution-phase possesses several intrinsic advantages. For example, the binding kinetics of nanosenors in solution phase can be accelerated by efficient mixing, while the binding kinetics in nanoparticle array-based sensors is limited by diffusion to the surface.14 Solution-phase sensing eliminates the separation and washing steps and thus is more time-saving.14 Furthermore, suspension-based LSPR sensors allow applications for singlenanoparticle detections inside cells and tissues where fixed arrays are unable to penetrate. Despite these successful applications and attractive advantages, suspension-based LSPR sensing is still not nearly as prevalent as nanoparticle arrays. The application of LSPR sensing in solution phase is greatly hampered by the aggregation of metal nanoparticles and the difficulty of surface labeling.12,15 To alleviate the spurious spectral shift caused by aggregations, the (11) Liu, G. L.; Yin, Y. D.; Kunchakarra, S.; Mukherjee, B.; Gerion, D.; Jett, S. D.; Bear, D. G.; Gray, J. W.; Alivisatos, A. P.; Lee, L. P.; Chen, F. Q. F. Nat. Nanotechnol. 2006, 1, 47. (12) Yu, C. X.; Irudayaraj, J. Anal. Chem. 2007, 79, 572. (13) Huang, T.; Nallathamby, P. D.; Xu, X.-H. N. J. Am. Chem. Soc. 2009 130, 17095. (14) Wilson, R.; Cossins, A. R.; Spiller, D. G. Angew. Chem., Int. Ed. 2006 45, 6104. (15) Sun, Z. H.; Ni, W. H.; Yang, Z.; Kou, X. S.; Li, L.; Wang, J. F. Small 2008, 4, 1287.

Published on Web 04/21/2009

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conjugation of biological ligands with nanoparticles needs to be carried out in the presence of carefully selected stabilizers and under mild conditions.11-13 In addition, unpredictable aggregation of LSPR nanosensors also limits their further applications in complicated biological environments. Designing stable and easily functionalized LSPR nanosensors is highly necessary for suspension-based sensing. Here we propose a core-shell nanoparticle with a metal core and a mesoporous silica shell to act as stable and functionalizable LSPR nanosensors with high sensitivity. By coating onto the surface of various nanoparticles, silica has been known as an ideal platform to endow nanoparticles with excellent stability, biocompatibility, and flexible surface modification.16-20 Mesoporous silica shell allows the encapsulated nanoparticle surface to be directly exposed to its surrounding environment,20 which renders the sensitivity of LSPR sensing. In addition, such core-shell nanoparticles provide a mesoporous shell with high surface area and close proximity to metal surface, which can be used for conjugation of chemical or biological ligands and the subsequent molecular sensing (for specific sensing). Recently, nanoparticles with a mesoporous silica shell have attracted a lot of attention in the field of multimodal bioimaging,19 drug delivery,21 and catalyst.22 In this study, we demonstrate that mesoporous silicacoated gold nanorods (MS Au NRs) can be used as a promising platform for LSPR-based molecular sensing. The longitudinal plasmon of MS Au NRs is highly sensitive to the changes in the refractive index of the surrounding medium with a sensitivity of 325 nm/RIU, which is comparable to that of CTAB-capped Au NRs. To explore the application of MS Au NRs as moleculesensing platforms, the LSPR response to glutathione (GSH) was demonstrated. We have also demonstrated that LSPR is also sensitive to the organic molecules functionalized inside mesoporous silica shell.

The solution was then centrifuged at 10 000 rpm for 15 min to remove the excess CTAB surfactant. The obtained Au NRs were collected and redispersed in 70 mL of deionized water by sonication for further use. Au NRs with different size can be synthesized by changing the concentration of AgNO3 in the growth solution.23 Preparation of MS Au NRs. Mesoporous silica coating was performed according to a previously published procedure.20 0.10 mL of 0.1 M NaOH solution was added to 10 mL of the prepared Au NRs upon stirring. After that, 30.0 μL of 20% TEOS in methanol was injected three times at a 30 min interval. The reaction was allowed to react for 24 h. The obtained MS Au NRs were then centrifuged and washed with methanol and ethanol at least five times to remove the CTAB molecules.20 Then the nanoparticles were dispersed in 1.5 mL of deionized water. Preparation of N-MS Au NRs. Modification of mesoporous silica shell with amino groups was achieved by a typical co-condensation method.19,24 0.10 mL of 0.10 M NaOH solution was added to 10 mL of the prepared Au NRs upon stirring. After that, 30.0 μL of 20% TEOS in methanol together with 10 μL of 2% APTS in methanol was injected three times at a 30 min interval. The reaction was allowed to further proceed for 24 h. The obtained N-MS Au NRs were then washed with methanol and ethanol five times to remove the CTAB molecules. Then the nanoparticles were dispersed in 1.5 mL of dimethylformamide (DMF). Conjugation of RITC with N-MS Au NRs. 0.5 mL of the prepared N-MS Au NRs in DMF was added to 5 mL of DMF containing 2 mg of RITC under stirring. The mixture was allowed to react for 10 h. The nanoparticles were then centrifuged and washed with ethanol to remove the unreacted RITC molecules. Instrumentation. A Shimadzu UV 2450 spectrophotometer was used for recording extinction spectra. Particle characterizations were performed using a JEOL JEM-3010 electron microscope (300 kV). Samples for TEM measurements were prepared by placing a drop of the colloidal solution on a carbon-coated copper grid.

Experimental Section Synthesis of Au NRs. Au NRs were synthesized in aqueous solution by a typical seed-mediated, CTAB surfactant directed procedure.23 Seed and growth solutions were made as described below. Preparation of Seed Solution. 2.5 mL of CTAB solution (0.20 M) was mixed with 2.5 mL of HAuCl4 (0.6 mM) by stirring. 0.30 mL of ice-cold NaBH4 (0.01 M) was added into the solution, which resulted in formation of a brownish-yellow solution. The obtained solution was stirred for another 2 min kept at room temperature as the seed solution. Preparation of Growth Solution. 1.5 mL of HAuCl4 (50 mM) and 0.224 mL of AgNO3 (50 mM) were added to 100 mL of 0.10 M CTAB solution at room temperature. After gentle mixing of the solution, 1.25 mL of 0.08 M ascorbic acid was slowly added into the mixture, upon which the growth solution changes the color from dark yellow to colorless. 0.20 mL of the seed solution was then subsequently added to the growth solution at 27-30 °C. The color of the solution gradually changed within 10-20 min. The reaction was allowed to further proceed for 10 h. (16) Roca, M.; Haes, A. J. J. Am. Chem. Soc. 2008, 130, 14273. (17) Pastoriza-Santos, I.; Perez-Juste, J.; Liz-Marzan, L. M. Chem. Mater. 2006, 18, 2465. (18) Irit, N.; Bendikov, T. A.; Doron-Mor, I.; Barkay, Z.; Vaskevich, A.; Rubinstein, I. J. Am. Chem. Soc. 2007, 129, 84. (19) Kim, J.; Kim, H. S.; Lee, N.; Kim, T.; Kim, H.; Yu, T.; Song, I. C.; Moon, W. K.; Hyeon, T. Angew. Chem., Int. Ed. 2008, 47, 8438. (20) Gorelikov, I.; Matsuura, N. Nano Lett. 2008, 8, 369. (21) Kim, J.; Lee, J. E.; Lee, J.; Yu, J. H.; Kim, B. C.; An, K.; Hwang, Y.; Shin, C. H.; Park, J. G.; Hyeon, T. J. Am. Chem. Soc. 2006, 128, 688. (22) Joo, S. H.; Park, J. Y.; Tsung, C. K.; Yamada, Y.; Yang, P. D.; Somorjai, G. A. Nat. Mater. 2009, 8, 126. (23) Nikoobakht, B.; El-Sayed, M. A. Chem. Mater. 2003, 15, 1957.

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Results and Discussion Au NRs were synthesized in aqueous solutions by a typical seed-mediated, CTAB surfactant directed procedure.23 Mesoporous silica coatings were achieved by a sol-gel process reported recently.20,22 After removing the excess CTAB surfactant by centrifugation, the as-prepared Au NRs were dispersed in an aqueous solution. The core-shell structured nanoparticles were formed by injecting a silica alkoxide precursor (tetraethyl orthosilicate, TEOS) into CTAB-capped Au nanorod solution and the solution were kept stirring for 24 h. The ultimate mesoporous Au NRs were obtained after removal of CTAB surfactant molecules by washing with methanol and ethanol.20 To completely remove the absorbed CTAB, the silica-coated nanoparticles were washed a minimum of 5 times in our experiments (the LSPR peak of MS Au NRs will not shift after 4-5 washings). Figure 1 shows the typical TEM images of the prepared MS Au NRs. The MS Au NRs consisted of a single-Au nanorod core and a uniform mesoporous silica shell. The TEM images clearly show that silica shells (thickness of ∼8 nm) were composed of disordered mesopores, similar to the previously reported CTABbased mesoporous silica systems. The mesoporous structure of the coatings will allow the exposure of the encaged Au NRs to the surrounding environments20 and thus enable a sensitive response of the LSPR peak wavelength of Au NRs to refractive index changes caused by molecule adsorption or ligand recognition in molecular sensing. (24) Hoffmann, F.; Cornelius, M.; Morell, J.; Froba, M. Angew. Chem., Int. Ed. 2006, 45, 3216.

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Figure 2. Normalized extinction spectra of CTAB-capped Au NRs (red line) and mesoporous (MS) Au NRs (black line). Figure 1. TEM images of the prepared MS Au NRs.

The effects of the silica coating on the LSPR of Au NRs are shown in Figure 2. Comparing the extinction spectra of MS Au NRs and CTAB-capped Au NRs, the LSPR band blue shifts upon the silica deposition, whereas the transverse plasmon band remains nearly unchanged. This effect can be ascribed due to a smaller overall refractive index of the mesoporous silica coating around Au nanorod surface compared to that of CTAB coating. Although the refractive index of silica itself may not be less than that of CTAB molecules, mesoporous silica shell allows the Au NRs to be directly exposed to its surrounding solvent environment, which will cause a large decrease of their overall surface refractive index. The refractive index of the solvents (rather than that of silica) is the dominant factor that determines the position of the plasmon band. The refractive index of the water solvent is smaller than that of CTAB, which is responsible for the observed blue shift of the LSPR plasmon band. This is further supported by the change of the extinction spectrum of MS NRs during the process of the removal of CTAB surfactant from the freshly prepared raw Au NRs. When the CTAB surfactant molecules were removed by washing with methanol and ethanol, a blue shift of longitudinal surface plasmon was observed (data shown in Supporting Information). There are some other possible mechanisms that might also cause the blue shift of the LSPR band during the silica coating processes. An increase in the electron density of Au NRs and the subsequent changes in particle morphology caused by electron transfer from strong reducing agents will result in a blue shift of LSPR band.25 In our experiments, no reducing agent was used during the mesoporous silica coating and washing processes; thus, the possibility of chargetransfer-induced blue shift of LSPR can be excluded. Self-assembly of Au NRs into a side-by-side orientation may also result in a blue shift of LSPR band and a considerable shape change in plasmon spectrum.15 This possibility could be excluded considering the fact that the LSPR peak shape of Au NRs remained nearly unchanged after the coating of mesoporous silica. Changes in particle morphology during the coating processes, such as small change in aspect ratio, could also cause a blue shift of the LSPR peak. Because it is very difficult to determine the subtle change in the aspect ratio of the Au NRs, such a possibility cannot be excluded. However, the significant blue shift of LSPR band upon the removal of CTAB surfactant molecules suggests that the major mechanism of the blue shift is due to the refractive index change. (25) Novo, C.; Mulvaney, P. Nano Lett. 2007, 7, 520.

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It is worth mentioning that our observation of spectral shift of the plasmon band of Au nanoparticles upon coating of the mesoporous silica is different from the previously reported results on nonporous silica-coated gold spheres and rods. Coating of a nonporous silica shell generally results in a separation between the metal surface and its solvent environments by a hermetic silica shell and an intermediate organic coating and therefore leads to a red shift of the plasmon band.17,26 In molecule-sensing applications, most organic molecules have a higher refractive index than buffer solution.2 Molecular binding can thus be monitored by a red shift of surface plasmon band of LSPR nanosensors. Although coating the nonporous silica can help to stabilize the Au nanoparticles, high refractive index of surface coating will obscure the contributions from refractive index changes caused by molecular binding and thus make them less attractive in the application of molecular sensing. Coating the Au NRs with a mesoporous structure will allow the exposure of Au NRs to the surrounding environments with lower refractive index, which will cause less interference to the contribution from surface coating. The LSPR of the proposed MS Au NRs will therefore be more sensitive to changes of their surface states due to molecular binding, making it a more sensitive nanosensor. To explore the LSPR sensitivity of MS Au NRs to local changes in refractive index, the extinction spectra of MS Au NRs in water-diethylene glycol (DEG) liquid mixtures of varying volume ratios were recorded. As illustrated in Figure 3, the longitudinal surface plasmon band systematically shifts to longer wavelength as the solvent refractive index increases. The refractive index of liquid mixtures was calculated via a Lorentz-Lorenz equation.27,28 Linear regression analysis of the data (Figure 3b, black line) yielded a refractive index sensitivity of 325 nm per refractive index unit (RIU), which is comparable to that of Au NRs in other reports (typically in the range of 150-400 nm/RIU, depending on the particle size).28,29 This result also suggests that mesoporous silica shell indeed allows the exposure of the encaged Au NRs to their surrounding environments. For direct comparison, the LSPR response of CTAB-capped Au NRs to refractive index changes was also recorded (data shown in Supporting Information). The plot of the LSPR peak shift is shown in Figure 3b (red line). A nonlinear response of the LSPR peak wavelength to the change of local refractive index was observed. (26) Pastoriza-Santos, I.; Sanchez-Iglesias, A.; de Abajo, F. J. G.; Liz-Marzan, L. M. Adv. Funct. Mater. 2007, 17, 1443. (27) Mehra, R. Proc.;Indian Acad. Sci., Chem. Sci. 2003, 115, 147. (28) Chen, H. J.; Kou, X. S.; Yang, Z.; Ni, W. H.; Wang, J. F. Langmuir 2008, 24, 5233. (29) Mayer, K. M.; Lee, S.; Liao, H.; Rostro, B. C.; Fuentes, A.; Scully, P. T.; Nehl, C. L.; Hafner, J. H. ACS Nano 2008, 2, 687.

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Figure 3. (a) Normalized extinction spectra of MS Au NRs in water-DEG liquid mixtures of varying volume ratios. (b) Dependence of the longitudinal plasmon shifts of MS Au NRs (black squares) and CTAB-capped Au NRs (red circles) on the refractive index of the liquid mixtures.

The nonlinear response could be understood as a consequence of formation of a CTAB bilayer on the Au nanorod surface for CTAB-capped Au NRs dispersed in aqueous solution. The CTAB bilayer could be easily destroyed by organic solvents, such as methanol and ethanol. The formation and stability of CTAB micelle or bilayer structure will also be affected by the presence of DEG in aqueous solution.30 The nonlinear response could be resulted from disruption of CTAB bilayer capping on the surface of Au NRs in the water-DEG mixtures. We also performed the experiment by using CTAB-capped Au NRs with a different plasmon peak (different size) and even more complicated nonlinear responses were observed, suggesting a lower stability of CTAB-capped Au NRs. The low stability of CTAB-capped Au NRs makes them difficult for their applications in LSPR-based sensing. However, mesoporous silica coating helps to stabilize the LSPR of Au NRs dispersion in various water-organic mixtures and pure organic solvents. MS Au NRs can provide a stable and linear response of LSPR shift to changes in their surface refractive index (Figure 3), thus make them attractive in the application of molecular sensing. In addition to peak wavelength shift, differential spectra could also be used to monitor the changes of extinction spectra.31 Figure 4a shows the differential spectra derived from normalized extinction spectra in Figure 3a. The differential spectra were obtained by subtraction of normalized extinction spectra of MS AU NRs in the mixture solvent by that of MS Au NRs in pure aqueous solution. The effect of the changes in refractive index on the LSPR resonance is more clearly observed in the (30) Kolay, S.; Ghosh, K. K.; MacDonald, A.; Moulins, J.; Palepu, R. M. J. Solution Chem. 2008, 37, 59. (31) Raschke, G.; Kowarik, S.; Franzl, T.; Sonnichsen, C.; Klar, T. A.; Feldmann, J.; Nichtl, A.; Kurzinger, K. Nano Lett. 2003, 3, 935.

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Figure 4. (a) Differential spectra derived from normalized extinction spectra in Figure 3a. (b) Dependence of differential extinction on the refractive index of the liquid mixtures.

differential spectra. This method could also be used to quantitatively determine the sensitivity of the LSPR band to the refractive index change. A plot of the maximum intensity of differential spectrum with the refractive index give a straight line, as shown in Figure 4b, yielding a sensitivity of ∼4.00 normalized extinction unit (NEU) per RIU. Compared with peak wavelength shiftbased detection, the differential spectrum reflects the shift of the whole spectrum instead of peak wavelength only and thus provides a more precise and sensitive evaluation of the changes in LSPR-related spectra. To demonstrate the application of MS Au NRs as LSPR nanosensors in molecular sensing, the plasmon response to molecular adsorbates (GSH) was demonstrated. GSH is the most abundant thiol species in cells and is involved in many important biochemical processes. It has been found that GSH possesses high affinity to the gold surface, which is the basis of using gold nanoparticles for GSH-related sensing, drug release, and nanoparticle preparation.32-35 Figure 5a shows the normalized extinction spectra of MS Au NRs before (black line) and after (red line) binding of GSH. A red shift (6 nm) of longitudinal surface plasmon band of MS Au NRs is clearly seen after GSH binding. The differential spectrum shown in Figure 5a (blue line) indicates that the red shift in the extinction spectrum leads to a differential extinction of ∼0.12 NEU. To illustrate the advantages of the MS Au NRs in sensing applications, GSH sensing by using CTABcapped Au NRs was also performed as a contrast experiment. The spectra before and after binding with GSH are shown in (32) Sudeep, P. K.; Joseph, S. T. S.; Thomas, K. G. J. Am. Chem. Soc. 2005 127, 6516. (33) Hong, R.; Han, G.; Fernandez, J. M.; Kim, B. J.; Forbes, N. S.; Rotello, V. M. J. Am. Chem. Soc. 2006, 128, 1078. (34) Shichibu, Y.; Negishi, Y.; Tsukuda, T.; Teranishi, T. J. Am. Chem. Soc. 2005, 127, 13464. (35) Verma, A.; Simard, J. M.; Worrall, J. W. E.; Rotello, V. M. J. Am. Chem. Soc. 2004, 126, 13987.

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Figure 5. (a) Normalized extinction spectra of MS Au NRs before (black line) and after (red line) binding of GSH (1.6 mM) and their differential spectrum (blue line). (b) Normalized extinction spectra of CTAB-capped Au NRs before (black line) and after (red line) binding of GSH (1.6 mM) and their differential spectrum (blue line).

Figure 5b. The binding of GSH to the CTAB-capped Au NRs results in a slight red shift (1.5 nm) of its LSPR band and a differential extinction of 0.075 NEU. Comparison of the spectra in Figure 5a,b suggests that MS Au NRs provide a more sensitive response than CTAB-capped Au NRs in GSH sensing. These results further demonstrate the advantage of using mesoporous silica coating and the exposed gold surface in molecule-sensing applications. MS Au NRs can also be used for quantitative analysis of GSH. Figure 6a shows the normalized extinction spectra and the corresponding differential spectra of MS Au NRs in the presence of different concentrations of GSH. We can see that the peak of their surface plasmon band gradually shifted to the longer wavelength. The subtle spectral shift of the plasmon band was clearly manifested in the differential spectra. A plot of changes in extinction maximum of their differential spectra (differential extinction, DE) versus the GSH concentration is shown in Figure 6b. Upon gradual increase of GSH concentration, the differential extinction progressively increased with the addition of GSH until saturation (with a clear transition point around 100 μM). Furthermore, the concentration-dependent response can be fitted with the Langmuir binding isotherm36,37 (details are shown in Supporting Information), as shown in Figure 6b (right). The fact that the dependence of the plasmon shift on the GSH concentration obeys the Langmuir binding isotherm forms the basis of quantitative molecular sensing. (36) Ni, W. H.; Yang, Z.; Chen, H. J.; Li, L.; Wang, J. F. J. Am. Chem. Soc. 2008, 130, 6692. (37) Chen, Y. F.; Rosenzweig, Z. Anal. Chem. 2002, 74, 5132.

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Figure 6. (a) Normalized extinction spectra of MS Au NRs in the presence of different concentrations of GSH (0, 1.6, 3.2, 6.4, 12.8, 25.6, 51.2, 102.4, 204.8, 409.6, 819.2, 1638 μM) and their differential spectra (inset). (b) Dependence of the differential extinction on the GSH concentration. The inset of (b) displays their Langmuir binding isotherm.

Figure 7. Normalized extinction spectra of N-MS Au NRs before (black line) and after (red line) conjugation with RITC molecules.

Mesoporous silica coating provides Au NRs with high surface areas, which are feasible for the conjugation of chemical or biological ligands and the subsequent molecular sensing. To demonstrate the LSPR response of Au NRs to the conjugates inside the mesoporous silica shell, we prepared amino groupmodified mesoporous Au NRs (N-MS Au NRs) via a typical co-condensation method.24 The extinction spectrum of the LSPR band of N-MS Au NRs is similar to that of MS Au NRS (see Supporting Information), suggesting that morphology of N-MS Au NRs remained nearly unchanged during the modification processes. The LSPR sensitivity of N-MS Au NRs to local changes in refractive index is 324 nm/RIU (or 3.73 NEU/RIU), similar to that of MS Au NRs (data shown in Supporting Information). Amino groups inside mesoporous silica shell can be used for further conjugation with various chemical or biological ligands. Figure 7 shows the normalized extinction DOI: 10.1021/la900646n

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spectra of N-MS Au NRs before and after conjugation with RITC molecules. A notable red shift of LSPR band was observed upon the conjugation with RITC. The result indicates that the LSPR response of Au NRs is highly sensitive to changes of local refractive index in mesoporous silica shell, which renders the feasibility of using MS Au NRs as effective molecule-sensing platforms when mesoporous silica shells were functionalized with various chemical and biological ligands.

Summary In summary, we have demonstrated that core-shell structured Au NRs with a surface-exposed gold core and a mesoporous silica shell can be a promising platform for LSPR-based molecular sensing. Mesoporous silica shell not only allows the Au NRs core to be directly exposed to its surrounding environment but also stabilizes Au NRs dispersion in various water-organic mixtures and pure organic solvents. The LSPR band of MS Au NRs displays a stable and linear response in spectral shift to the

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changes in their surrounding refractive index. The GSH-sensing experiment has also been demonstrated by using these MS Au NRs, confirming the feasibility of their applications as LSPR nanosensors in molecular sensing. In addition, mesoporous silica coating provides Au NRs with high surface area and functionizable silica matrix, which facilitates the application of MS Au NRs in molecular sensing where various chemical and biological ligands are required to be functionalized onto the gold surface. These results can be also extended to other mesoporous silicacoated metal nanoparticles. Acknowledgment. The authors thank the Faculty of Science, National University of Singapore (R-143-000-302-112 and R-143-000-341-112), for financial support. Supporting Information Available: Figures S1-S5, Table S1, and details of Langmuir binding isotherm. This material is available free of charge via the Internet at http://pubs.acs.org.

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