Article pubs.acs.org/JPCC
Highly Controlled Plasmonic Emission Enhancement from MetalSemiconductor Quantum Dot Complex Nanostructures Hiroyuki Naiki,*,† Akito Masuhara,‡ Sadahiro Masuo,§ Tsunenobu Onodera,† Hitoshi Kasai,† and Hidetoshi Oikawa† †
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai, Miyagi 980-8577, Japan ‡ Department of Organic Device Engineering, Graduate School of Science and Technology, Yamagata University, Jonan 4-3-16, Yonezawa, Yamagata 992-8510, Japan § Department of Chemistry, School of Science and Technology, Kwansei Gakuin University, Gakuen 2-1, Sanda, Hyogo 669-1537, Japan ABSTRACT: We have fabricated well-defined nanostructures such as SiO2-coated Ag nanoparticles (NPs) connected with quantum dots (QDs) (Ag/SiO2-QDs) so as to control the fluorescence enhancement induced by localized surface plasmon resonance. Namely, the distance between Ag NP and QD should be noted as a controllable model to investigate the fluorescence enhancement effect. Actually, highly monodispersed Ag NP as a core was first coated with five thicknesses of SiO2 as a shell, and then QDs were specifically adsorbed onto the surface of the amino-functionalized SiO2-coated Ag NPs. As a result, the fluorescence intensity increased with the shell thickness as a result of excitation enhancement. On the other hand, the fluorescence intensity decreased when the shell thickness became thinner because of the induced quenching. Therefore, the distance between Ag NPs and QDs should be optimized to control and enhance the fluorescence intensity. the fluorescence lifetime and an increase in the fluorescence quantum yield. These fluorescence enhancement effects are strongly dependent on structural factors in metal nanostructures and fluorophores. For instance, the optimal distance between metal nanostructures and fluorophores is reported to be 5−30 nm to bring about efficiently excitation enhancement.27,28 In addition, the spectral overlap between LSPR spectra from metal nanostructures and the emission spectra from fluorophores is also important for controlling the emission enhancement.6,7,25,26 Namely, it is necessary to optimize the distance between the metal nanostructures and the fluorophore at a nanometer scale and to control spectral overlap in the visible light range for a highly efficient application of fluorescence enhancement. In the previous study, we have investigated the single-photon emission behavior from a single CdSe/ZnS quantum dot located near inhomogeneous Ag nanostructures using a photon correlation technique in combination with pulsed laser excitation.29,30 As a result, the probability of single-photon emission decreased with an increase in the fluorescence intensity, with a decrease in the fluorescence lifetime, and with suppression of the blinking. We have interpreted these facts as follows: Single QD indicates the multiphoton emission
1. INTRODUCTION Because of unique optoelectronic properties of metal nanostructures, physicochemical interactions between molecules and metal nanostructures are essentially attractive, for example, surface enhanced Raman scattering1 and fluorescence enhancement2,3 induced by the localized surface plasmon resonance (LSPR) toward potential applications such as the single molecule imaging technique,4 ultrafast photodetectors,5,6 and highly efficient solar cells.7 Thereby, the LSPR enhancement effects have been extensively studied for the purpose of understanding the fundamental enhancement dynamics so as to apply these enhancement effects toward device applications.2,3,8−11 Especially, fluorescence enhancement has been revealed in metal spherical and polygonal nanoparticles,12−14 nanowires,15,16 core/shell nanoparticles,17 and continuous films.18−23 Thus, so-called plasmonics may open a new fundamental science and extensive applications. The origin of fluorescence enhancement is due to the enhanced electromagnetic field of LSPR, which may lead to two enhancement ways so as to affect the photophysical process in fluorophores. One is the increase in the numbers of photoexcitation and photoexcited fluorophores per unit time induced by the enhanced electromagnetic field from LSPR, which is the so-called “excitation enhancement”.24 The other is an increase in the radiative decay rate from photoexcited fluorophores; that is to say, the transfers of excitation energy from fluorophores to metal nanostructures and the subsequent radiation of photons from metal nanostructures, called “emission enhancement”,6,7,25,26 resulting in a shortening of © XXXX American Chemical Society
Special Issue: Nanostructured-Enhanced Photoenergy Conversion Received: June 2, 2012 Revised: August 17, 2012
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for 15 h at 313 K. The obtained Ag/SiO2 NPs were collected by centrifugation at 5000g for 10 min. The resulting aminofunctionalized Ag/SiO2 NP dispersion liquid was diluted with 2 mL of the total volume so that the absorbance was about 0.25. Next, 100 μL of QDs dispersion liquid (0.55 μM) was added to adsorb onto the surface of the amino-functionalized Ag/SiO2 NPs for 15 h. Optical properties were measured with a UV−vis absorption spectrophotometer (V-570, Jasco) and a fluorescence spectrophotometer (F-7000, Hitachi).
to interact in conjunction with the excitation enhancement and emission enhancement. Recently, a single metal nanoparticle connected with single QD nanostructures was reported;17,31−33 however, these nanostructures would still not be satisfied and enough from the viewpoints of the optimal distance and the spectral overlap to efficiently control the fluorescence enhancement. As a result, complex nanostructures composed of metal NPs and QDs have never been accomplished to control singlephoton emission. In the present article, we have fabricated a well-defined Ag/ SiO2-QD that is a coated Ag nanoparticle connected with QD nanostructures as a model to investigate the fluorescence enhancement effect.
3. RESULTS AND DISSUCUSION Figure 1 displays the TEM image and the corresponding size histogram of the Ag NPs, which are almost spherical, and the
2. EXPERIMENTAL SECTION 2-1. Materials. Silver nitrate, L-ascorbic acid, poly(vinyl pyrrolidone) (PVP, Mw: 40 000), sodium chloride, sodium hydroxide, and 50% dimethylamine (DA) aqueous solution were purchased from Wako Pure Chemical Industries Ltd. N′(2-Aminoethyl)-N-(3-trimethoxysilylpropyl)ethane-1,2-diamine, and CdSe/ZnS QDs (Lumidot, λem: 610 nm) were acquired from Sigma-Aldrich. Tetraethyl orthosilicate (TEOS) was purchased from Alfa Aesar Co. All chemicals were used as received without further purification. 2-2. Fabrication of SiO2-Coated Ag Nanoparticle Connected with Quantum Dots. Monodispersed and welldefined Ag nanoparticles (Ag NPs) were prepared by the reduction of AgCl colloids with ascorbic acid.34 A 170 mg portion of PVP and 170 mg of silver nitrate were first dissolved in 40 mL of pure water under stirring. Subsequently, 400 μL of 5.0 M sodium chloride aqueous solution was further added and stirred for 15 min at 298 K under dark conditions. The color of the resulting dispersion liquid was white and turbid because of the formation of AgCl colloids. To reduce AgCl colloids, the asprepared AgCl colloid dispersion liquid was added into 360 mL of a mixed aqueous solution that contained 2800 mg of Lascorbic acid and 40 mL of 0.5 M sodium hydroxide. The obtained dispersion liquid was further stirred for 2 h under dark conditions. The formed Ag NPs were collected by centrifugation at 8000g for 60 min and washed twice with pure water and ethanol. The Ag NPs were coated with a SiO2 shell (Ag/SiO2 NPs), five different thicknesses of the SiO2 shell, by using a modified Stöber method (sol−gel reaction).35 Namely, 8 mL of Ag NPs dispersion liquid was diluted with 20 mL of ethanol, and 750 μL of 50% DA solution was further added so as to adjust the pH and accelerate the sol−gel reaction. After ultrasonicating for 30 min to completely disperse the Ag NPs, the different amounts of TEOS ethanol solution (1 M: 50, 150, 300, 500, and 800 μL) were added to form the SiO2 shell for 12 h at 313 K. The Ag/SiO2 NPs were collected again by centrifugation at 5000g for 30 min, washed with ethanol, and then redispersed in ethanol. The size and shape of the obtained Ag/SiO2 NPs were characterized using a transmission electron microscope (TEM, H-7650, Hitachi). To adsorb CdSe/ZnS QDs on Ag/SiO2 NPs, we modified the shell surface of the Ag/SiO2 NPs by employing an amino silane coupling agent. A 4 mL portion of Ag/SiO2 NPs dispersion liquid and 330 μL of a 50% DA solution were added to 6 mL of ethanol and then ultrasonicated for 30 min. As an amino silane coupling agent, 190 μL of N′-(2-aminoethyl)-N(3-trimethoxysilylpropyl)ethane-1,2-diamine ethanol solution (16 μM) was used, and the reaction was allowed to proceed
Figure 1. TEM image and the diameter histogram of Ag NPs. Scale bar represents 100 nm.
size distribution is very narrow: 54 ± 2 nm. This value is the reported optimal size to provide the highest enhancement factor.4 Lakowicz et al. have verified that the diameter of Ag NPs is optimal from 50 to 70 nm to provide the strength distribution of the enhanced electromagnetic field of LSPR and the depression of competitive quenching.4 Thus, the resulting Ag NPs would be expected to indicate high fluorescence enhancement. The TEM images of the five kinds of Ag/SiO2 NPs and the histograms of the shell thicknesses are shown in Figure 2. To control the excitation enhancement effect,27,28 it is necessary to optimize the distance between the Ag NP and QD. Hence, we have employed the SiO2 shell so as to finely control this distance at the nanometer scale. In a previous paper,35 ammonia was commonly used to adjust the pH for the purpose of accelerating the sol−gel reaction. However, a Ag[NH3]2+ complex was formed in this case, which would decompose the once-formed Ag NPs under base conditions.35 Instead of ammonia, we have inhibited the decomposition of Ag NPs by employing the DA solution in the present experiment. These SiO2 shells also increased with increasing amounts of TEOS. As shown in Figure 2, the average thicknesses are 7 ± 1, 15 ± 1, 24 ± 2, 31 ± 1, and 38 ± 2 nm, respectively. The optimal distance is reported to be 5−30 nm, at which the excitation enhancement effect strongly appears.27,28 Thus, we have successfully fabricated the appropriate Ag/SiO2 NPs so that the distance between the resulting Ag NP and QD could be B
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Figure 2. TEM images and shell thickness histograms of SiO2-coated Ag NPs (Ag/SiO2 NPs) with different shell thicknesses. Average shell thickness: (a) 7, (b) 15, (c) 24, (d) 31, and (e) 38 nm. Scale bar represents 100 nm.
enhancement effect affects the enhanced fluorescence induced by the LSPR. Klimov et al. have previously reported the fabrication of SiO2-coated Au NPs connected with QD (Au/SiO2-QDs) nanostructures and fluorescence enhancement in the range of 10−25 nm in a SiO2 shell.36,37 This enhancement is due to interactions in conjunction with the electromagnetic field of LSPR induced by excitation light and with an energy transfer from the QDs to Au NPs caused by the spectral overlap between the Au NPs’ LSPR spectra and the emission spectra from the QDs. Especially, the energy transfer may be inversely proportional to the third power of the distance from the Au NPs to the QDs.36 In other words, the fluorescence enhancement from Au/SiO2-QDs occurs despite a thinner SiO2 shell than that of the presented case. On the contrary, it should be noted that the enhanced fluorescence from the present Ag/SiO2-QDs resulted from only the electromagnetic field effect of the LSPR. Namely, in the case of a thick SiO2 shell, which possibly caused suppression of the quenching, this fluorescence enhancement primarily occurred through an excitation enhancement of the electromagnetic field. Thus, we have successfully observed fluorescence enhancement from Ag/ SiO2-QDs with a shell thickness >24 nm, as shown in Figure 3c, and this fluorescence enhancement, which was controlled only by excitation enhancement, was clearly achieved to fabricate SiO2 shells having five different thicknesses in single nanometer order.
well-controlled with high uniformity for discussion of the fluorescence enhancement phenomenon. Figure 3 shows (a) the absorption spectra of Ag/SiO2 NPs with different thicknesses, (b) absorption spectra of Ag/SiO2QDs with different thicknesses and QDs, and (c) fluorescence spectra of Ag/SiO2-QDs, and QDs in ethanol dispersion liquid. In Figure 3a and b, all absorption spectra were scarcely changed before and after adsorbing QDs with Ag/SiO2-QDs, that is, Ag/ SiO2-QDs are well dispersed in ethanol liquid. In the fluorescence spectra (Figure 3c), the fluorescence intensity of Ag/SiO2-QDs with thin shell thicknesses (shell thicknesses 7 and 15 nm) decreased less than that of only QDs. On the other hand, the other nanostructures (shell thicknesses 24, 31, and 38 nm) gave a stronger fluorescence intensity as compared with only QDs. In this study, ∼1000-fold QDs per Ag/SiO2 NP were added to evaluate the optical properties of Ag/SiO2-QDs by measurement with absorption and fluorescence spectroscopy. However, it was estimated that ∼100 QDs per Ag/SiO2 NP were actually adsorbed by considering the concentration of the amino silane coupling agent. The residue and unadsorbed QDs could not optically interact with Ag/SiO2 and could not influence the enhancement and quenching in fluorescence intensity. Indeed, the concentration of “QDs” in Figure 3b and c is essentially equal to that of the added amount of QDs in the Ag/SiO2 NP dispersion. In other words, the prepared Ag/SiO2QDs were not collected and washed to remove excess QDs in the present study, and the concentrations of QDs in the present Ag/SiO2-QD samples were the same as that of only QD samples as a reference. Namely, the changes in the fluorescence intensity were dependent on only the shell thicknesses of the Ag/SiO2-QDs. The quenching of the fluorescence intensities is responsible for a too close distance between the Ag NPs and the QDs, whereas the optimal distance for the excitation
4. CONCLUSIONS We have successfully fabricated highly monodispersed and almost spherical Ag/SiO2-QDs and confirmed the fluorescence enhancement by controlling the shell thickness. Ag/SiO2-QDs with thick shells show enhanced fluorescence due to the excitation enhancement. On the other hand, in the case of Ag/ SiO2-QDs with a thin shell, the fluorescence intensity decreased C
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ACKNOWLEDGMENTS This work was supported by KAKENHI (Grant-in-Aid for Scientific Research) on Priority Area “Strong PhotonsMolecules Coupling Fields (No. 470)” from the Ministry of Education, Culture, Sports, Science and Technology, Japan.
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Figure 3. (a) Absorption spectra of Ag/SiO2 with different thicknesses, (b) absorption spectra of Ag/SiO2-QDs with different thicknesses and QDs, and (c) fluorescence spectra of Ag/SiO2-QDs and QDs (λem: 610 nm) dispersed in ethanol. The excitation wavelength is 405 nm.
because of quenching. These kinds of metal/SiO2-QD nanostructures would be useful for investigation of singlephoton emission behavior to clearly understand the exciton dynamics of QDs interacting with LSPR. To further discuss the fluorescence enhancement, we are now preparing a similar nanostructure having a gold NP, Au/ SiO2-QDs, and will investigate the enhanced fluorescence, depending on the excitation wavelength, the spectral overlap, etc. in the near future.
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REFERENCES
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[email protected]. Notes
The authors declare no competing financial interest. D
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