- Email: [email protected]
shell nanorods
Thin Solid Films 520 (2012) 7002–7005
Contents lists available at SciVerse ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Langmuir–Blodgett films of gold/silica core/shell nanorods E. Fülöp a, N. Nagy a, A. Deák a,⁎, I. Bársony a, b a b
Research Institute for Technical Physics and Materials Science, HAS, P.0. Box 49, H-1525 Budapest, Hungary Department of Nanotechnology, Pannon University, P.0. Box 125, H-8200 Veszprem, Hungary
a r t i c l e
i n f o
Article history: Received 16 December 2011 Received in revised form 12 June 2012 Accepted 2 July 2012 Available online 27 July 2012 Keywords: Gold nanoparticle Core/shell nanoparticle Langmuir–Blodgett film Optical property Plasmonics
a b s t r a c t We report the preparation of Langmuir- and Langmuir–Blodgett films of mesoporous silica coated gold nanorods. The silica coating on the gold nanorods was found to prevent the aggregation of the plasmonic particles trapped at the air/water interface. Due to the small aspect ratio of the gold core and the presence of the silica shell, the orientational alignment of the nanorods in the Langmuir–Blodgett film is hindered. After particle deposition, no plasmon coupling was observed, which enables the design of the resulting film's optical property at the particle level. By using mesoporous silica as the shell material, the accessibility of the metal core's surface is preserved. Organic dye (Rhodamine 6G) was found to be able to penetrate into the mesoporous shell of the gold nanorods, resulting in a red shift of the longitudinal plasmon mode. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Rod shaped gold nanoparticles support transversal and longitudinal plasmon modes, where the longitudinal peak's position can be tuned in a wide wavelength range by engineering the particle's aspect ratio. The longitudinal mode was shown to be very sensitive to the dielectric constant of the surrounding medium [1,2]. Due to the large electric field enhancement at the nanorods' ends, the particles were investigated as platforms for surface enhanced Raman scattering [3,4], in photovoltaic applications [5] and as anti-tumor agents [6,7]. For perspective optical and optoelectronic applications, solid supported monolayers of particles are of specific interest. On the other hand, the combination of the special optical properties of plasmonic nanorods with high surface-area mesoporous systems is gaining increasing attention. This is of interest, e.g., for light-triggered release of drugs from mesoporous containers [8] or in-situ seeded growth of gold nanoparticles with special morphology [9]. Besides the high surface area, the mesoporous coating on the solid supported gold nanorods can be of further advantage, since the physical dimensions of the pores impose size restriction on the molecules entering or leaving the shell. High density macroscopic arrays of such nanocomposites can enable their use as sensing platforms based on optical methods, without the need for sophisticated instrumentation. Among the numerous approaches to producing nanoparticulate monolayer, the Langmuir–Blodgett (LB) technique is a real layer-bylayer method offering great control over the particle deposition and enables the preparation of real monolayers with high particle surface density. The main problem associated with the LB-film preparation of ⁎ Corresponding author. Tel.: +36 1 392 2602; fax: +36 1392 2226. E-mail address: [email protected] (A. Deák). 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2012.07.097
metallic nanoparticles is related to the strong van der Waals interaction between them due to the large Hamaker constant, which can easily induce particle aggregation [10] especially for larger (>30 nm) particles. To prevent this, a protective silica shell can be used to shield these attractive forces by increasing the minimum particle–particle distance. This protective coating, however, can prevent the interaction of the particle's near field with its environment due to the rapid decay of the electric field from the nanoparticle's surface. Here we show that this approach can be used for the bottom-up fabrication of large arrays of gold/silica core/shell nanoparticles. Due to the absence of particle aggregation, the optical properties of the individual particles are not affected, enabling control over the resulting film's optical properties. By using mesoporous silica as the shell material, the accessibility of the metal core's surface is preserved, which is demonstrated by the infiltration of the LB-film of the core/ shell nanoparticles by fluorescent dye. 2. Experimental details 2.1. Materials Sodium borohydride, ReagentPlus® 99% (NaBH4); tetrachloroauric (III) acid trihydrate, ACS reagent (HAuCl4·3H2O); cetyltrimethyl ammonium bromide, SigmaUltra 99% (CTAB); tetraethyl orthosilicate, puriss., 99% (TEOS); L-ascorbic acid, ACS reagent 99%; silver nitrate, 99.9999% metal basis (AgNO3); Rhodamine 6G dye, ~95% (R6G) and methanol, puriss. p.a. (max. 0.005% H2O) were purchased from Sigma-Aldrich. Ammonium hydroxide, 32% from Scharlau, Sodium hydroxide, a.r. (NaOH) from Reanal and chloroform, ultra-resi analyzed, 99.8%, from J. T. Baker were used. All chemicals were used as received.
E. Fülöp et al. / Thin Solid Films 520 (2012) 7002–7005
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For all experiments ultrapure water with a resistivity of 18.2 MΩ/cm was used. 2.2. Nanoparticle synthesis CTAB-capped nanorods were synthesized and in a single step coated with a mesoporous silica shell according to literature, resulting in disordered pores in the silica coating with ca. 4 nm diameter and ca. 2 nm wall thickness [11]. To coat the nanorods, 15 ml of the as-synthesized nanorod sol was centrifuged at 7500 rpm for 30 min and redispersed in 10 ml of water. The pH was adjusted to ~10–11 using 100 μl 0.1 M NaOH. Under magnetic stirring, 3×30 μl 20% TEOS in MeOH was injected into the reaction vessel at 30 minute intervals. The zeta-potential of the particles was characterized using electrophoretic light scattering technique (Malvern Zetasizer Nano ZS). The samples were subjected to repeated centrifugation and redispersion in methanol (6 times) to remove the positively charged surfactant molecules until a zeta-potential value about −30 mV was reached, and small residual core-free silica particles were also removed. The particle size and the shell thickness were measured using a Philips CM20 transmission electron microscope. For the film preparation the particles were used without any further surface-modification. 2.3. Langmuir–Blodgett film preparation The Langmuir- and LB-film preparation was carried out as reported by us earlier for Stöber silica particles [12]. Silicon substrates were washed with acetone, water, 2% hydrofluoric acid solution and finally with water. Glass slides (Menzel Gläser microscope slides, AA00000102E) were washed with ethanol and water. To spread the nanoparticles at the air/water interface in a Langmuir-trough (KSV2000), the particles dispersed in methanol were diluted with chloroform (1:2 volume ratio) and sonicated for 5 min right before spreading. The nanoparticles were spread at the water/air interface using a Hamilton syringe. After 10 min, when chloroform evaporated, the particles were compressed with a barrier speed of 0.4 cm 2/s. As the surface pressure reached the value of ca. 1 mN/m, the compression speed was lowered to 0.2 cm 2/s. The Langmuir–Blodgett films were prepared by vertical deposition (4 mm/min), at ca. 80% of the collapse pressure. Film morphology was investigated using a LEO 1540 XB field emission scanning electron microscope (SEM) with an acceleration voltage of 5.00 kV. The optical properties of the film were studied using UV–vis spectroscopy (Agilent 8453). To investigate the accessibility of the mesopores of the silica shells, the LB-films were immersed in 1 mM Rhodamine 6G aqueous solution for 1 min. Immediately after immersion, they were extensively rinsed with water and dried in air. 3. Results and discussion The gold cores of the prepared nanoparticles are ~ 23 nm thick and ~ 40 nm long (aspect ratio ~ 1.75), the silica shell thickness is about 15 nm. In Fig. 1a the normalized extinction spectra of the CTAB-capped and the silica-coated nanorods in water are shown. The longitudinal peak of the nanorods is at 615 nm. For the core/ shell particles this longitudinal peak red-shifts ca. 10 nm, due to the higher effective refractive index in the particle's vicinity as the result of silica shell formation. Previous experiments and calculations on the shift of the longitudinal mode predict higher (~ 17 nm) red-shift for similar nanoparticles [13]. This can be attributed to the smaller aspect ratio of the nanorods used in the present work, since the refractive index sensitivity of the nanorods depends on their aspect ratio [14]. Besides the slight red-shift, broadening of the longitudinal mode due to aggregation induced near-field coupling [15] could not be observed. After washing the particles several times, the spectrum is blue-shifted, resulting in a spectrum almost identical with the starting
Fig. 1. (a) Normalized extinction spectra of the CTAB-capped and the silica-coated gold nanorod sol, before and after washing. (b) Surface pressure vs. area isotherm of the core/shell nanoparticles.
rod's extinction. The coated particles could be transferred into a methanol or methanol–chloroform mixture without aggregation. The core/shell nanorods were spread onto the water's surface in a Langmuir-trough without any surface-modification. This was possible due to the surface energy of the silica coating, which enabled earlier the preparation of monolayers of pure silica nanoparticles in a wide size range [16–20]. The monolayer of the particles was clearly visible at the water/air interface. On the surface pressure (Π) vs. area (A) isotherm (Fig. 1b) of the monolayer, the surface pressure remains zero over a large area range. The steep increase of the surface pressure below a certain area value indicates that the core/shell particles form a weakly cohesive film at the water/air interface [21]. The typical collapse pressure values of the Langmuir monolayers were around 25 mN/m, which are comparable to that of Stöber silica nanoparticles [16]. Plasmon coupling related change of the color of the film upon compression, found earlier for silver nanoparticles [22], could not be observed. The LB-films of the nanoparticles were defect free on the macroscopic scale, showing uniform coverage of the substrate (Fig. 2a). The SEM image of the film (Fig. 2b) reveals, that the particles are not arranged in a hexagonally close packed (hcp) structure as observed earlier for plain silica nanoparticles [17,18,22]. The lower ordering of the particles can be attributed to the anisometric shape of the particles. Side-by-side ordering of the nanoparticles, as found earlier for rods with higher aspect ratio at water/air interface [23–25], could not be observed. This can be attributed to the presence of the silica shell, which increases the possible minimum distance between the lower aspect ratio gold cores. Additionally, the particles possess -30 mV ζ-potential, which induces
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E. Fülöp et al. / Thin Solid Films 520 (2012) 7002–7005
Fig. 2. (a) Langmuir–Blodgett monolayer of the gold/silica nanoparticles on glass support. (b) SEM image of the LB-film.
longer ranged repulsive interaction between the particles. The above factors seem to cancel the ordering effect of the van der Waals interaction between the anisometric cores [26]. The extinction spectra of the LB-film (Fig. 3a) reveal that the particles preserved their ensemble optical property after the monolayer deposition. The longitudinal and transversal plasmon peaks in the spectrum of the LB-monolayer (dotted line) agree well with the peaks obtained for core/shell particles in solution (solid line). There is no sign for near-field coupling between the particles, which is consistent with the large inter-particle separation observed with SEM.
To investigate the accessibility of the pores of the shell after the LB-film preparation, the same LB-films were soaked in the aqueous solution of 1 mM Rhodamine 6G. This resulted in a significant increase in the extinction around 530 nm (Fig. 3a), indicating the presence of the dye in the film. At the wavelength corresponding to the longitudinal plasmon resonance of the nanorods (ca. 615 nm), the absorption of the dye can be neglected (see Fig. 3b dashed line). The intensity of the longitudinal peak, however, increases and the peak red-shifts, indicating that the average refractive index in the gold rods' vicinity increased [27]. This can be attributed to the dye molecules, diffusing into the mesopores of the silica shell, as reported for plain mesoporous silica structures earlier [28,29]. To verify the adsorption of the dye molecules into the mesopores, first the extinction of the dye molecules adsorbed on the supporting glass substrate was measured (Fig. 3b). The extinction caused by the molecules adsorbed on the surface of the core/shell nanoparticles was estimated using a 146 nm Stöber silica nanoparticle monolayer (Fig. 3b) as model sample. Using these two values, the amount of dye adsorbed at the core/shell nanoparticles could be estimated from the extinction spectra of the LB-films (see the Appendix A for details on the calculation). The amount of dye present at the core/ shell nanoparticles is 2.6 times higher due to the pores, compared to the plain silica particles with the same outer surface area. When the LB-films of the core/shell particles, which were previously infiltrated with the dye, were placed in pure water, the original longitudinal plasmon mode of the gold/silica nanoparticles was recovered (see Fig. 4), indicating the removal of the dye from the pores. 4. Conclusion In this study we report about the preparation of Langmuir- and Langmuir–Blodgett films of gold/silica core/shell nanorods. This bottom-up approach allows the particles to preserve their optical property during the film preparation, which enables control over the resulting films extinction spectra at the particle level. We found, that the mesoporous shell of the particles is capable of hosting organic dye molecules, resulting in a significant change in the film's optical properties. By controlling the pore size of the mesoporous coating and decorating the silica surface with functional groups, the model system presented here could be used as sensing platform for molecules of different size and functionality. Acknowledgment András Deák gratefully acknowledges the support of the LMU Excellent Research Fellowship. This work was supported by the
Fig. 3. (a) Extinction spectra of the LB-films prepared from the core/shell nanorods before and after Rhodamine 6G treatment. For comparison the spectrum of the core/ shell nanorods in water and the substrate spectrum are also shown. (b) Extinction spectra of the substrate and a 146 nm Stöber silica particle LB-film before and after contact with Rhodamine 6G.
Fig. 4. Extinction spectra of the LB-films prepared from gold/silica core/shell nanorods. The spectra show the effect of soaking the films in pure water after the dye adsorption.
E. Fülöp et al. / Thin Solid Films 520 (2012) 7002–7005
János Bolyai Research Scholarship and grants from NKTH in projects “TFSOLAR2” and “PVMET08”. The help of János Volk and Levente Illés with SEM and György Sáfrán with TEM analysis and Dániel Zámbó with the R6G experiments is appreciated. We thank Andrey Lutich and Weihai Ni for the fruitful discussions. Appendix A. Calculations on the R6G adsorption The amount of molecules adsorbed on the glass substrate was determined by immersing the substrate into the dye solution and measuring the absorption spectra. From this, the absorption contribution of the molecules adsorbed on the two sides of the substrate can be determined. To be able to estimate the amount of dye adsorbed on the surface of the core/shell nanoparticles, monolayers of 146 nm Stöber silica nanoparticles were used as model surface, since – just like the shell of the core/shell nanorods – they are also prepared by the hydrolysis and condensation of TEOS in alkaline environment, and the surface charge of the nanoparticles is almost identical to that of gold/silica core/shell nanoparticles [30]. Concerning the theoretical surface coverage of the substrate by the Stöber silica nanoparticles in a monolayered LB-film, its value is ca. 91%, which corresponds to a hexagonally close packed (hcp) structure. For a real sample, however, the packing of the particles is never perfect. The surface coverage for our sample is only 80%, as calculated from the SEM picture of the LB-films. Similarly, for the core/shell particles the surface coverage based on SEM images accounts for ca. 66%. If we take now an arbitrary substrate area, the average surface area of the particle ensemble over that specific substrate area can be calculated based on the surface coverage determined from the SEM images, and the surface area of a single particle. From the above considerations the ratio between the total surface of the core/shell and core-free particles (φ) for an arbitrary substrate area is φ¼
Acore=shell 51919 nm2 ¼ ¼ 0:78: Asilica 66652 nm2
ðA:1Þ
This means that the total surface area of the core/shell particles is 78% of that of the Stöber silica particles. The absorption caused by the dye molecules located at the nanoparticles (ΔEcore/shell and ΔEsilica) can be calculated from the extinction spectra in Fig. 3a–b after subtracting the contribution of the dye molecules adsorbed on the supporting glass substrate at the Rhodamine main absorption wavelength (~ 530 nm). The ratio of these extinction surpluses (γ) is γ¼
ΔΕcore=shell 0:014 ¼ 2; ¼ 0:007 ΔΕsilica
ðA:2Þ
7005
indicating that in the LB-film of the core/shell nanoparticles there is twice the amount of dye as compared with the Stöber silica LB-film. In the LB-film of the core/shell nanoparticles, however, the total surface area of the particles is 78% of that of the Stöber silica nanoparticles. Taking this into account the final ratio between the amount of dyes adsorbed by the core/shell and Stöber silica nanoparticles gets γ 2 ¼ ¼ 2:6: φ 0:78
ðA:3Þ
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Contents lists available at SciVerse ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Langmuir–Blodgett films of gold/silica core/shell nanorods E. Fülöp a, N. Nagy a, A. Deák a,⁎, I. Bársony a, b a b
Research Institute for Technical Physics and Materials Science, HAS, P.0. Box 49, H-1525 Budapest, Hungary Department of Nanotechnology, Pannon University, P.0. Box 125, H-8200 Veszprem, Hungary
a r t i c l e
i n f o
Article history: Received 16 December 2011 Received in revised form 12 June 2012 Accepted 2 July 2012 Available online 27 July 2012 Keywords: Gold nanoparticle Core/shell nanoparticle Langmuir–Blodgett film Optical property Plasmonics
a b s t r a c t We report the preparation of Langmuir- and Langmuir–Blodgett films of mesoporous silica coated gold nanorods. The silica coating on the gold nanorods was found to prevent the aggregation of the plasmonic particles trapped at the air/water interface. Due to the small aspect ratio of the gold core and the presence of the silica shell, the orientational alignment of the nanorods in the Langmuir–Blodgett film is hindered. After particle deposition, no plasmon coupling was observed, which enables the design of the resulting film's optical property at the particle level. By using mesoporous silica as the shell material, the accessibility of the metal core's surface is preserved. Organic dye (Rhodamine 6G) was found to be able to penetrate into the mesoporous shell of the gold nanorods, resulting in a red shift of the longitudinal plasmon mode. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Rod shaped gold nanoparticles support transversal and longitudinal plasmon modes, where the longitudinal peak's position can be tuned in a wide wavelength range by engineering the particle's aspect ratio. The longitudinal mode was shown to be very sensitive to the dielectric constant of the surrounding medium [1,2]. Due to the large electric field enhancement at the nanorods' ends, the particles were investigated as platforms for surface enhanced Raman scattering [3,4], in photovoltaic applications [5] and as anti-tumor agents [6,7]. For perspective optical and optoelectronic applications, solid supported monolayers of particles are of specific interest. On the other hand, the combination of the special optical properties of plasmonic nanorods with high surface-area mesoporous systems is gaining increasing attention. This is of interest, e.g., for light-triggered release of drugs from mesoporous containers [8] or in-situ seeded growth of gold nanoparticles with special morphology [9]. Besides the high surface area, the mesoporous coating on the solid supported gold nanorods can be of further advantage, since the physical dimensions of the pores impose size restriction on the molecules entering or leaving the shell. High density macroscopic arrays of such nanocomposites can enable their use as sensing platforms based on optical methods, without the need for sophisticated instrumentation. Among the numerous approaches to producing nanoparticulate monolayer, the Langmuir–Blodgett (LB) technique is a real layer-bylayer method offering great control over the particle deposition and enables the preparation of real monolayers with high particle surface density. The main problem associated with the LB-film preparation of ⁎ Corresponding author. Tel.: +36 1 392 2602; fax: +36 1392 2226. E-mail address: [email protected] (A. Deák). 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2012.07.097
metallic nanoparticles is related to the strong van der Waals interaction between them due to the large Hamaker constant, which can easily induce particle aggregation [10] especially for larger (>30 nm) particles. To prevent this, a protective silica shell can be used to shield these attractive forces by increasing the minimum particle–particle distance. This protective coating, however, can prevent the interaction of the particle's near field with its environment due to the rapid decay of the electric field from the nanoparticle's surface. Here we show that this approach can be used for the bottom-up fabrication of large arrays of gold/silica core/shell nanoparticles. Due to the absence of particle aggregation, the optical properties of the individual particles are not affected, enabling control over the resulting film's optical properties. By using mesoporous silica as the shell material, the accessibility of the metal core's surface is preserved, which is demonstrated by the infiltration of the LB-film of the core/ shell nanoparticles by fluorescent dye. 2. Experimental details 2.1. Materials Sodium borohydride, ReagentPlus® 99% (NaBH4); tetrachloroauric (III) acid trihydrate, ACS reagent (HAuCl4·3H2O); cetyltrimethyl ammonium bromide, SigmaUltra 99% (CTAB); tetraethyl orthosilicate, puriss., 99% (TEOS); L-ascorbic acid, ACS reagent 99%; silver nitrate, 99.9999% metal basis (AgNO3); Rhodamine 6G dye, ~95% (R6G) and methanol, puriss. p.a. (max. 0.005% H2O) were purchased from Sigma-Aldrich. Ammonium hydroxide, 32% from Scharlau, Sodium hydroxide, a.r. (NaOH) from Reanal and chloroform, ultra-resi analyzed, 99.8%, from J. T. Baker were used. All chemicals were used as received.
E. Fülöp et al. / Thin Solid Films 520 (2012) 7002–7005
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For all experiments ultrapure water with a resistivity of 18.2 MΩ/cm was used. 2.2. Nanoparticle synthesis CTAB-capped nanorods were synthesized and in a single step coated with a mesoporous silica shell according to literature, resulting in disordered pores in the silica coating with ca. 4 nm diameter and ca. 2 nm wall thickness [11]. To coat the nanorods, 15 ml of the as-synthesized nanorod sol was centrifuged at 7500 rpm for 30 min and redispersed in 10 ml of water. The pH was adjusted to ~10–11 using 100 μl 0.1 M NaOH. Under magnetic stirring, 3×30 μl 20% TEOS in MeOH was injected into the reaction vessel at 30 minute intervals. The zeta-potential of the particles was characterized using electrophoretic light scattering technique (Malvern Zetasizer Nano ZS). The samples were subjected to repeated centrifugation and redispersion in methanol (6 times) to remove the positively charged surfactant molecules until a zeta-potential value about −30 mV was reached, and small residual core-free silica particles were also removed. The particle size and the shell thickness were measured using a Philips CM20 transmission electron microscope. For the film preparation the particles were used without any further surface-modification. 2.3. Langmuir–Blodgett film preparation The Langmuir- and LB-film preparation was carried out as reported by us earlier for Stöber silica particles [12]. Silicon substrates were washed with acetone, water, 2% hydrofluoric acid solution and finally with water. Glass slides (Menzel Gläser microscope slides, AA00000102E) were washed with ethanol and water. To spread the nanoparticles at the air/water interface in a Langmuir-trough (KSV2000), the particles dispersed in methanol were diluted with chloroform (1:2 volume ratio) and sonicated for 5 min right before spreading. The nanoparticles were spread at the water/air interface using a Hamilton syringe. After 10 min, when chloroform evaporated, the particles were compressed with a barrier speed of 0.4 cm 2/s. As the surface pressure reached the value of ca. 1 mN/m, the compression speed was lowered to 0.2 cm 2/s. The Langmuir–Blodgett films were prepared by vertical deposition (4 mm/min), at ca. 80% of the collapse pressure. Film morphology was investigated using a LEO 1540 XB field emission scanning electron microscope (SEM) with an acceleration voltage of 5.00 kV. The optical properties of the film were studied using UV–vis spectroscopy (Agilent 8453). To investigate the accessibility of the mesopores of the silica shells, the LB-films were immersed in 1 mM Rhodamine 6G aqueous solution for 1 min. Immediately after immersion, they were extensively rinsed with water and dried in air. 3. Results and discussion The gold cores of the prepared nanoparticles are ~ 23 nm thick and ~ 40 nm long (aspect ratio ~ 1.75), the silica shell thickness is about 15 nm. In Fig. 1a the normalized extinction spectra of the CTAB-capped and the silica-coated nanorods in water are shown. The longitudinal peak of the nanorods is at 615 nm. For the core/ shell particles this longitudinal peak red-shifts ca. 10 nm, due to the higher effective refractive index in the particle's vicinity as the result of silica shell formation. Previous experiments and calculations on the shift of the longitudinal mode predict higher (~ 17 nm) red-shift for similar nanoparticles [13]. This can be attributed to the smaller aspect ratio of the nanorods used in the present work, since the refractive index sensitivity of the nanorods depends on their aspect ratio [14]. Besides the slight red-shift, broadening of the longitudinal mode due to aggregation induced near-field coupling [15] could not be observed. After washing the particles several times, the spectrum is blue-shifted, resulting in a spectrum almost identical with the starting
Fig. 1. (a) Normalized extinction spectra of the CTAB-capped and the silica-coated gold nanorod sol, before and after washing. (b) Surface pressure vs. area isotherm of the core/shell nanoparticles.
rod's extinction. The coated particles could be transferred into a methanol or methanol–chloroform mixture without aggregation. The core/shell nanorods were spread onto the water's surface in a Langmuir-trough without any surface-modification. This was possible due to the surface energy of the silica coating, which enabled earlier the preparation of monolayers of pure silica nanoparticles in a wide size range [16–20]. The monolayer of the particles was clearly visible at the water/air interface. On the surface pressure (Π) vs. area (A) isotherm (Fig. 1b) of the monolayer, the surface pressure remains zero over a large area range. The steep increase of the surface pressure below a certain area value indicates that the core/shell particles form a weakly cohesive film at the water/air interface [21]. The typical collapse pressure values of the Langmuir monolayers were around 25 mN/m, which are comparable to that of Stöber silica nanoparticles [16]. Plasmon coupling related change of the color of the film upon compression, found earlier for silver nanoparticles [22], could not be observed. The LB-films of the nanoparticles were defect free on the macroscopic scale, showing uniform coverage of the substrate (Fig. 2a). The SEM image of the film (Fig. 2b) reveals, that the particles are not arranged in a hexagonally close packed (hcp) structure as observed earlier for plain silica nanoparticles [17,18,22]. The lower ordering of the particles can be attributed to the anisometric shape of the particles. Side-by-side ordering of the nanoparticles, as found earlier for rods with higher aspect ratio at water/air interface [23–25], could not be observed. This can be attributed to the presence of the silica shell, which increases the possible minimum distance between the lower aspect ratio gold cores. Additionally, the particles possess -30 mV ζ-potential, which induces
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Fig. 2. (a) Langmuir–Blodgett monolayer of the gold/silica nanoparticles on glass support. (b) SEM image of the LB-film.
longer ranged repulsive interaction between the particles. The above factors seem to cancel the ordering effect of the van der Waals interaction between the anisometric cores [26]. The extinction spectra of the LB-film (Fig. 3a) reveal that the particles preserved their ensemble optical property after the monolayer deposition. The longitudinal and transversal plasmon peaks in the spectrum of the LB-monolayer (dotted line) agree well with the peaks obtained for core/shell particles in solution (solid line). There is no sign for near-field coupling between the particles, which is consistent with the large inter-particle separation observed with SEM.
To investigate the accessibility of the pores of the shell after the LB-film preparation, the same LB-films were soaked in the aqueous solution of 1 mM Rhodamine 6G. This resulted in a significant increase in the extinction around 530 nm (Fig. 3a), indicating the presence of the dye in the film. At the wavelength corresponding to the longitudinal plasmon resonance of the nanorods (ca. 615 nm), the absorption of the dye can be neglected (see Fig. 3b dashed line). The intensity of the longitudinal peak, however, increases and the peak red-shifts, indicating that the average refractive index in the gold rods' vicinity increased [27]. This can be attributed to the dye molecules, diffusing into the mesopores of the silica shell, as reported for plain mesoporous silica structures earlier [28,29]. To verify the adsorption of the dye molecules into the mesopores, first the extinction of the dye molecules adsorbed on the supporting glass substrate was measured (Fig. 3b). The extinction caused by the molecules adsorbed on the surface of the core/shell nanoparticles was estimated using a 146 nm Stöber silica nanoparticle monolayer (Fig. 3b) as model sample. Using these two values, the amount of dye adsorbed at the core/shell nanoparticles could be estimated from the extinction spectra of the LB-films (see the Appendix A for details on the calculation). The amount of dye present at the core/ shell nanoparticles is 2.6 times higher due to the pores, compared to the plain silica particles with the same outer surface area. When the LB-films of the core/shell particles, which were previously infiltrated with the dye, were placed in pure water, the original longitudinal plasmon mode of the gold/silica nanoparticles was recovered (see Fig. 4), indicating the removal of the dye from the pores. 4. Conclusion In this study we report about the preparation of Langmuir- and Langmuir–Blodgett films of gold/silica core/shell nanorods. This bottom-up approach allows the particles to preserve their optical property during the film preparation, which enables control over the resulting films extinction spectra at the particle level. We found, that the mesoporous shell of the particles is capable of hosting organic dye molecules, resulting in a significant change in the film's optical properties. By controlling the pore size of the mesoporous coating and decorating the silica surface with functional groups, the model system presented here could be used as sensing platform for molecules of different size and functionality. Acknowledgment András Deák gratefully acknowledges the support of the LMU Excellent Research Fellowship. This work was supported by the
Fig. 3. (a) Extinction spectra of the LB-films prepared from the core/shell nanorods before and after Rhodamine 6G treatment. For comparison the spectrum of the core/ shell nanorods in water and the substrate spectrum are also shown. (b) Extinction spectra of the substrate and a 146 nm Stöber silica particle LB-film before and after contact with Rhodamine 6G.
Fig. 4. Extinction spectra of the LB-films prepared from gold/silica core/shell nanorods. The spectra show the effect of soaking the films in pure water after the dye adsorption.
E. Fülöp et al. / Thin Solid Films 520 (2012) 7002–7005
János Bolyai Research Scholarship and grants from NKTH in projects “TFSOLAR2” and “PVMET08”. The help of János Volk and Levente Illés with SEM and György Sáfrán with TEM analysis and Dániel Zámbó with the R6G experiments is appreciated. We thank Andrey Lutich and Weihai Ni for the fruitful discussions. Appendix A. Calculations on the R6G adsorption The amount of molecules adsorbed on the glass substrate was determined by immersing the substrate into the dye solution and measuring the absorption spectra. From this, the absorption contribution of the molecules adsorbed on the two sides of the substrate can be determined. To be able to estimate the amount of dye adsorbed on the surface of the core/shell nanoparticles, monolayers of 146 nm Stöber silica nanoparticles were used as model surface, since – just like the shell of the core/shell nanorods – they are also prepared by the hydrolysis and condensation of TEOS in alkaline environment, and the surface charge of the nanoparticles is almost identical to that of gold/silica core/shell nanoparticles [30]. Concerning the theoretical surface coverage of the substrate by the Stöber silica nanoparticles in a monolayered LB-film, its value is ca. 91%, which corresponds to a hexagonally close packed (hcp) structure. For a real sample, however, the packing of the particles is never perfect. The surface coverage for our sample is only 80%, as calculated from the SEM picture of the LB-films. Similarly, for the core/shell particles the surface coverage based on SEM images accounts for ca. 66%. If we take now an arbitrary substrate area, the average surface area of the particle ensemble over that specific substrate area can be calculated based on the surface coverage determined from the SEM images, and the surface area of a single particle. From the above considerations the ratio between the total surface of the core/shell and core-free particles (φ) for an arbitrary substrate area is φ¼
Acore=shell 51919 nm2 ¼ ¼ 0:78: Asilica 66652 nm2
ðA:1Þ
This means that the total surface area of the core/shell particles is 78% of that of the Stöber silica particles. The absorption caused by the dye molecules located at the nanoparticles (ΔEcore/shell and ΔEsilica) can be calculated from the extinction spectra in Fig. 3a–b after subtracting the contribution of the dye molecules adsorbed on the supporting glass substrate at the Rhodamine main absorption wavelength (~ 530 nm). The ratio of these extinction surpluses (γ) is γ¼
ΔΕcore=shell 0:014 ¼ 2; ¼ 0:007 ΔΕsilica
ðA:2Þ
7005
indicating that in the LB-film of the core/shell nanoparticles there is twice the amount of dye as compared with the Stöber silica LB-film. In the LB-film of the core/shell nanoparticles, however, the total surface area of the particles is 78% of that of the Stöber silica nanoparticles. Taking this into account the final ratio between the amount of dyes adsorbed by the core/shell and Stöber silica nanoparticles gets γ 2 ¼ ¼ 2:6: φ 0:78
ðA:3Þ
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