ARTICLE pubs.acs.org/JPCC
Coupling of Nanoparticle Plasmons with Colloidal Photonic Crystals as a New Strategy to Efficiently Enhance Fluorescence Cheng-an Tao,† Wei Zhu,† Qi An,† Haowei Yang,† Weina Li,† Changxu Lin,† Fuzi Yang,‡ and Guangtao Li*,† †
Key Laboratory of Organic Optoelectronic and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, People’s Republic of China ‡ Electromagnetic Materials Group, School of Physics, University of Exeter, Exeter EX4 4QL, United Kingdom
bS Supporting Information ABSTRACT: In this paper, we present a new strategy for efficiently enhancing fluorescence, based on the coupling of surface plasmons of metal particles with optical properties of colloidal photonic crystals. It is found that such coupling can effectively improve the near-field effect and induced plasmon effect of metal particles but also be favorable for the extraction of the emission, collectively leading to a further significant enhancement of metal-enhanced fluorescence. Finite-difference time-domain (FDTD) simulation proved the related enhancement mechanism for the designed hybrid system. As a proof of concept, an enhancement factor up to 260-fold has been experimentally achieved by using rhodamine B as test molecule, which greatly transcends those of two individual plasmonic and photonic components and is also much larger than the simple plus effects of the two components.
’ INTRODUCTION Fluorescence has become the dominant sensing technology in life science and has been making fast advancement in optical imaging, because of its well-known advantages, such as simplicity, sensitivity, and availability of organic dyes with diverse spectral properties. In this context, further enhancement of fluorescence could significantly improve the performance (e.g., sensitivity, quality of imaging, or photostability) of the related systems, in particular for cases of low-quantum-yield fluorophores, and thus has drawn considerable attention in recent years.111 Various methods have been developed to further amplify the fluorescence signals. The essence of these methods, however, is related to three basic mechanisms: excitation enhancement, quantum efficiency (QE) enhancement, and extraction enhancement. As one efficient method, metal-enhanced fluorescence (MEF) has attracted much attention and been widely reported.610,1220 The unique plasmonic structure of metal surface/nanoparticle is responsible for the observed enhancement through dramatically modifying the optical properties of a locally situated fluorophore. Concretely, the presence of the localized surface plasmon resonance (LSPR) resulting from the interactions of incident light with free electrons in metal surface gives rise to a local concentration of the incident field and, hence, strong enhancement of the excitation field of nearby fluorescent molecules (near-field effect).21 Furthermore, interaction of the excited-state fluorophores with the surface plasmon electrons results in an increase in the radiative decay rate, and as a result, the QE of fluorophores in proximity to the metal surface is greatly enhanced r 2011 American Chemical Society
(induced plasmon effect).2224 Both near-field effect and induced plasmon effect lead to an enhanced fluorescence through excitation enhancement and QE enhancement. However, it is conceivable that if an appropriate optical element is coupled with the plasmonic structure of metal nanoparticle that can further improve the nearfield effect and induced plasmon effect but also be favorable for the extraction of the emission, then a further enhancement of metal-enhanced fluorescence should be possible. Under the consideration noted above, in this paper, a new strategy for significantly enhancing fluorescence has been developed based on the coupling of localized surface plasmons with unique optical properties of colloidal photonic crystals (PCs). Three-dimensional PCs were produced by using monodisperse polystyrene nanospheres. The photonic band gap (PBG) of the prepared PCs can be tuned by changing the nanosphere size. The top layer of PCs was deposited with Ag nanoparticles, and the resulting hybrid structure was used as substrate for fluorescence enhancement. As a proof of concept, dye molecules (e.g., fluorescein and rhodamine B) were immobilized on metal particles in this work. It is found that the effective matching (coupling) of PBG of photonic crystals with the located surface plasmon resonance (SPR) of metal particles at the excitation wavelength will lead to a significant enhancement of fluorescence intensity of dyes, much more than the simple addition of both Received: February 18, 2011 Revised: August 2, 2011 Published: August 09, 2011 20053
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metal and PC fluorescence enhancement. Finite-difference timedomain (FDTD)25 calculations show that the introduction of PC structure significantly improves the near-field effect and the extraction of the emission on the metal surface, indicating that the fabricated hybrid structures are efficient platforms (substrates) for fluorescence enhancement. In the present case, the experimental results confirmed an extreme increase in fluorescence intensity up to 260-fold as compared to MEF (∼42-fold) and PC (∼71-fold).
’ EXPERIMENTAL SECTION Materials. 1,2-Bis(trimethoxysilyl)ethane (BTME) and (3-mercaptopropyl)triethoxysilane (MPTES) were purchased from Alfa Aesar Co. Silver nitrate, sodium citrate, hydrochloric acid, styrene, methacrylic acid, sodium persulfate, and other chemicals were purchased from Beijing Chemical Co. Deionized water was further purified with RF ultrapure water system. Preparation of Thiol-Doped Organosilica Precursor Sols. BTME (3.54 g), water (0.38 g), and HCl (12.3 μL, 0.07 M) were mixed [mole ratios 1:2.12:(8.63 105)], and the resulting reaction mixture was refluxed at 60 °C for 90 min. Water (1.25 mL) and HCl (65.4 μL, 1 M) were then added. After the mixture was stirred at room temperature for 15 min, the formed sol was also further diluted with ethanol. MPTES (0.39 g) was added to introduce thiol groups. Before spinning-coating process, the formed sol was further diluted 2 times to get a proper viscosity. Preparation of PC-Ag, PC, and Ag Samples. The PC-Ag sample was fabricated as follows: the PC films were fabricated on glass substrates by homemade monodispersed polystyrene microspheres through a vertical deposition method.26 They were infiltrated with silica sol by a spin-coating process that formed a thin cover layer. After being aged for 2 days, the films were immersed into silver colloids, which were formed by the reduction of a warmed solution of silver nitrate and sodium citrate. After the films were rinsed with water, the PC-Ag samples were obtained. As control experiments, the PC samples were prepared by immersion in water without silver colloids. The silver colloidal films were fabricated by the same procedure with glass instead of PC films. Following a similar approach reported in literature,27 we deposited dye molecules onto the hybrid structure. Concretely, dye molecules were solvated in ethanol (1 mM), and the solution was bubbled by N2 flow to deposit on the surface of the hybrid structure. For all samples the same deposition conditions, including concentration of dye, flow of N2, and bubbled time, were applied. The incident angle of excitation light is about 60° from the sample surface. Steady-state emission spectra were recorded by a LS 55 spectrofluorometer (Perkin-Elmer) with excitation at 540 nm. Characterization. The spin coater was a Laurell WS-4006NPP/LITE. Reflectance spectra in the range 400750 nm were collected by use of an Ocean Optics USB2000 fiber-optic diodearray dual-channel spectrometer interfaced with an ocular tube of an Olympus BX51 binocular microscope by fiber optics using the microscope’s light source (Olympus U-LH100-3) and a 20 workingdistance objective (Olympus 20/0.45 MPLanFL N). Fluorescence spectra were measured by a LS 55 spectrofluorometer (Perkin-Elmer). Scanning electron microscopy (SEM) images were obtained with a field-emission scanning electron microscope (LEO-1503, Germany), after the samples were sputtered with a thin layer of gold. Transmission electron microscopy (TEM) images were obtained with JEOL JEM-1200 (Japan).
Figure 1. Schematic illustration of coupling metal nanoparticle plasmons with colloidal photonic crystals as a strategy to efficiently enhance fluorescence.
Finite-Difference Time-Domain Calculations. The FDTD method was employed to determine the electric field intensities and distributions at the surface of 50 nm silver nanoparticles by FDTD Solution software.28 In the excitation enhancement calculation, the simulation region is set to 2R 3.464R 4500 nm with a mesh accuracy of 2. Photonic crystals contain 15 monolayers that consist of PS spheres with radius R. Ag particle is set at the center point of XY plane and 65 nm away from PC surface. The incident field is defined as a plane wave with a wavevector that forms a 60° angle with the PC surface. The boundary conditions are set to Block periods for X and Y and set to PML for Z. To minimize simulation time and maximize the resolution of the field enhancement regions around the silver particle, a mesh override region was set to 0.5 nm around the 50 nm Ag particle. The overall simulation time was set to 1000 fs and calculated at wavelengths of 450 and 540 nm. In the radiative power enhancement simulation, it is assumed that the excitation stage of fluorescence has occurred, and the fluorophore is now emitting dipole radiation. A dipole source radiating from 300 to 800 nm is used to mimic the emission of the dye. It is set at the center point of XY plane and 100 nm away from PC surface; that is, it is spaced 10 nm from Ag particle. The simulation region is set to 2R 3.464R 4500 nm, while the boundary conditions are set to Block periods for X and Y and set to PML for Z. We set a frequency domain surface monitor 200 nm away from the PC surface to measure the directional power radiated by the system by integrating the real part of the Poynting vector. All of the calculations are done with the assumption of a background relative dielectric constant of 1.0.
’ RESULTS AND DISCUSSION Mechanism of our strategy is illustrated in Figure 1. Photonic crystals are structural materials, in which the periodic modulation of the dielectric constant over the structure generates a forbidden gap of photonic states (photonic band gap, PBG),29 in a similar way as that in semiconductor crystals. As a consequence of this unique feature, the propagation of incident light with the wavelength in the bandgap region is forbidden, and the corresponding light wave incidents are evanescent and therefore penetrate the photonic crystal slab to a limit depth. A resonant evanescent electric field is then formed, and high-intensity light is localized near the PC surface. When the silver particles are placed on the surface of PC and at the same time the resonance mode of PC matches the LSPR of Ag particles, the near field around the 20054
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particles will be extremely enhanced. That is the major special consequence of the designed coupling. Additionally, due to the strong coherent scattering effects of photonic crystals, the fluorescence emission from the mirror plasmons of metal particles will be effectively extracted by PC. Besides, the QE enhancement effect of metal particles takes place as well. Clearly, the coupling of suitable photonic crystals with metal nanoparticle forms an effective hybrid system, in which the total fluorescence signals can be amplified simultaneously through increasing the quantum efficiency of fluorescent dye, the electric-field intensity experienced, and the collection efficiency of fluorescence emission. The fluorescence intensity (IF) of fluorophore can be described as follows: IF ¼ Iexe QEηext
ð1Þ
where Iexe is the intensity of the excitation field, QE is the quantum efficiency of the fluorophore, and ηext is the extraction efficiency. Enhancement factor (EF) is defined as the ratio of the fluorescence intensity observed from moleculesplasmon system to that from molecules adsorbed on a glass substrate, and it can be described by the product of |L(ωexc)|2 and Z(ωflu),23,30 as shown in eq 2. The term |L(ωexc)|2 is the enhancement factor of local electromagnetic field intensity near the metal particles at the excitation frequency, which represents the ability of the metal to concentrate the electromagnetic energy. The second factor, Z(ωflu), describes the radiative yield enhancement of the excited dye molecules on the two substrates. EF ¼
IF Iexe QEηext ¼ ¼ jLðωexc Þj2 Zðωflu Þ IF;0 Iexe;0 QE0 ηext;0
ð2Þ
On the basis of eq 2, the two enhancement factors were calculated by the FDTD method. The FDTD is a rigorous computational electrodynamics method that can accurately describe plasmonic effects and PC resonances. Ag nanoparticles with diameter of about 50 nm (Figure S1, Supporting Information) and four PCs with different lattice constants (Figure 2) were prepared and used in our case. Figure S2 (Supporting Information) shows the reflection spectra of the PCs along ΓL and L + 30° directions. Figure S3 (Supporting Information) maps the calculated photonic band gap location and reflection efficiency of the PCs as a function of the launch angle along the LW direction and for S-polarized incidence. We see an excellent agreement between simulation and experiment of the reflection spectra. At first, the excitation enhancement effect [|L(ωexc)|2] was evaluated. Two excitation wavelengths of 450 and 540 nm were chosen, corresponding respectively to the fluorescein and rhodamine B dyes used. Figure 3panels ae display the calculated distribution of electric field intensity around 50 nm Ag nanoparticle coupled with four different PCs at a wavelength of 450 nm. The cyan photonic crystal (cyan PC) is the best one that can match well with SPs of Ag colloids (see Figure 3c). It is clearly seen that the electric field is localized around the particle. The maximum is about 2 nm space from the particle surface, and the intensity is fast diminishing further away from the surface. Remarkably, it is found that through the coupling of PC structure the near-field intensity around the particles was enhanced extremely; that is, the photons were dramatically localized around the silver particle relative to that without PC. For the sake of clarity, the maximum of the electric field intensity of Ag particles without and with coupled PC is plotted in Figure 4 (top). It can be seen that the
Figure 2. SEM top-view and cross-section images of the prepared violet, cyan, yellow, and pink colloidal photonic crystal films made of PS colloid spheres with average diameters of (a, a0 ) 190, (b, b0 ) 210, (c, c0 ) 240, and (d, d0 ) 260 nm.
enhancement of the electrical field intensity [|L(ωexc)|2] has been up to 30-fold coupled with Cyan PC at 450 nm, while only about 7-fold for individual Ag colloid without PC. When the excitation wavelength shifts to 540 nm, the best coupled PC accordingly changes to yellow PC (Figure 3fj). In this case, the enhancement factor increases to 18-fold, while it is less than 5-fold for individual Ag colloid. Clearly, the excitation enhancement effect of metal nanoparticles can be significantly improved through coupling with matched PC. The thickness of the infiltrated silica overlayer is about 40 nm, and the radius of the silver particles used is about 25 nm. On the basis of these data, the separation between the Ag nanoparticle and the photonic crystal (PC) plane was set at 65 nm in our calculations. As mentioned above, a largely enhanced local electric field can be produced around the Ag nanoparticles within the photonic band gap. The effect comes from the strongly reflected light from the PC within the band gap, which can effectively couple with the LSPR of the Ag nanoparticles. In our case, the cyan photonic crystal (cyan PC) is the best one that can match well with SPs of Ag colloids for excitation of 450 nm, while the yellow photonic crystal (yellow PC) is the best one that can match well with SPs of Ag colloids for excitation of 540 nm. The calculations were also performed for separations of 55 and 75 nm separately, and the results are shown in Figures S4 (excitation wavelength 450 nm) and S5 (excitation wavelength 540 nm) in Supporting Information. It is clearly seen that whether the separation is 55 or 75 nm, similar results were obtained as the case of the separation of 65 nm. 20055
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Figure 3. FDTD simulated near-field intensity distribution around a 50 nm Ag nanoparticle at a wavelength of 450 nm (ae) or 540 nm (fj) without (a, f) or with coupled photonic crystals (be and gj) (65 nm away from Ag nanoparticle), and the incident angle propagates along L + 30°.
Generally, the electric field distribution around metal particle is calculated in a total field scattered field (TFSF). Unfortunately, due to the immobilization of Ag particles on the surface of PC, which is a periodic structure, the TFSF source cannot be used in our case under Bloch boundary condition. In our simulation, the incident light source was set to plane wave source, and the incident angle was 30°, not normal incidence. Under this simulation condition, the value (only 7- and 5-fold) of the electric field enhancement is different from that by TFSF, but the comparison of the electric field around the silver sphere without and with
underlying PC layer is not affected when all the simulations are performed under the same conditions. The electric field distribution around a single 50 nm Ag nanoparticle in air under TFSF source condition was calculated (Figure S6, Supporting Information), and also the calculated scattering spectrum is shown in Figure S7a (Supporting Information). Indeed, the electric field intensity enhancement of the Ag nanoparticles at the wavelengths of 450 and 540 nm are found to be 17- and 12-fold, larger than 7- and 5-fold. The main scattering peak for 50 nm Ag particle is about 370 nm. The electric field intensity enhancement 20056
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Figure 5. Radiated power enhancement factor for dipoles located at 10 nm spaced distance from particle and 100 nm away from the surface of photonic crystals. The dipole orientation was averaged over all solid angles.
Figure 4. Calculated electric field maximum intensity around 50 nm Ag nanoparticle at wavelength of (top) 450 nm and (bottom) 540 nm coupled with different photonic crystals compared to that without PC.
of the Ag nanoparticles at the wavelength of 370 nm will be much larger (over 120-folds), as shown in Figure S7b (Supporting Information). The second enhancement factor [Z(ωflu)] can be expressed in the form of radiative power enhancement under the assumption that the excitation stage of fluorescence has occurred and the fluorophore is emitting dipole radiation. This enhancement factor in the total radiated power can be inferred by integrating the normal flux passing through a closed surface containing the system [an isolated dipole (excited fluorophore) with or without silver nanoparticle] and is given as follows:31 Zðωflu Þ ¼
γrad Prad ¼ γ0 P0
ð3Þ
)
where P0 is the radiated power of a classical dipole in air, Prad is the radiated power of the dipole close to the silver nanoparticle, γ0 and γrad are the radiative decay rates of the dipole in air and close to the silver nanoparticle, respectively. Equation 3 implies that an enhancement in the total radiated power is indicative of a corresponding increase in the relative radiative decay rate of the system.18 In the case of SPs coupled with PC, the detection device lies opposite the PC. The extraction enhancement should be taken into account in calculation. Therefore, the combination factor [Z (ωflu)] of QE enhancement and extraction enhancement is expressed as follows: )
Prad; QEηext γ η ¼ rad ext ¼ QE0 ηext;0 γ0 ηext;0 P0;
)
)
Z ðωflu Þ ¼
ð4Þ
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)
)
where P0, and Prad, are the radiated power flux through the detection plane of a classical dipole in air and in proximity to the silver nanoparticle, respectively. The separation between the dye molecule and the Ag nanoparticle is very crucial for determining the emission enhancement. Generally, the metal-assisted fluorescence enhancement occurs in the range of 515 nm away from metal surface. In our calculation/simulation, in order to check the effect of different photonic crystals on the emission enhancement, a separation of 10 nm was set for all samples. Figure S8 (Supporting Information) displays the calculated enhancements of dye molecules located at 5, 10, and 15 nm away from silver particles. Clearly, the maximum enhancements of dye molecules at 5, 10, and 15 nm occur at nearly the same wavelength. This result implies that the choice of separation (515 nm) between the dye molecule and the Ag nanoparticle will have no influence on the comparison results of the effect of different photonic crystals on the emission enhancement. The radiated power enhancement factor for dipoles located at 10 nm spaced distance from particle and 100 nm away from the surface of photonic crystals was calculated. As shown in Figure 5, all of the PCs can enhance the radiation of dipole, but the highest enhancement is located around the SPR peak of Ag nanoparticles. The scattering component of the Ag nanoparticles also plays an important role in the reradiation of the coupled system. The maximum enhancement changes from short to long wavelength along with the stop-band position of photonic crystal varying from violet PC to pink PC. This result reflects the contribution of the underlying polystyrene photonic crystal in the hybrid system. To confirm the mentioned extraction effect does exist in our hybrid system, the ratio of forward radiative power flux (to the detector) to backward one was calculated (Figure 6). Clearly, all the PCs exhibit the extraction effect. The enhancement emission occurred at the wavelength corresponding to the photonic stopband of the PC. For example, for emission wavelength around 580 nm, the yellow PC was the best one to display the extraction effect. FDTD calculations described above show that the coupling of photonic crystal structure with the plasmonic structure of metal nanoparticle can further improve the near-field effect and induced plasmon effect of metal nanoparticle and the extraction effect, indicating that the proposed photonicplasmonic hybrid system should be a more efficient substrate for fluorescence enhancement. To experimentally confirm the validity of this dx.doi.org/10.1021/jp2016165 |J. Phys. Chem. C 2011, 115, 20053–20060
The Journal of Physical Chemistry C strategy, rhodamine B (RhB) is selected as the fluorescent model molecule and immobilized on the surface of the hybrid substrate. The experimental geometry is illustrated in Figure 7a, and Figure S9 (Supporting Information) shows the SEM images of the constructed substrate, in which silica gel filled the cavities of PC to avoid the infiltration of Ag nanoparticles inside the PS colloidal crystals. There are some small aggregates on the surface of the PC, but we found that the extinction wavelength of Ag colloidal film in water has no evident difference from that of the discrete Ag particle colloidal solution (Figure S10, Supporting Information). There is no appearance of a new SPR peak that plays a decisive role in the fluorescence enhancement. Thus, the aggregation was not taken into account in our analysis. Excitation light
Figure 6. Ratio of forward radiative power flux (to the detector) to backward one for dipoles located at 10 nm spaced distance from particle and 100 nm away from the surface of photonic crystals. The dipole orientation was averaged over all solid angles.
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with wavelength 540 nm is used, and an angle of 60° with respect to the surface is fixed, while the detector is kept at an angle of 30° with respect to the surface. Excitation and emission spectra of RhB in water are shown in Figure S11 (Supporting Information). The enhancement factor (EF) is defined as the ratio of FL intensity at 581 nm of RhB dye on different substrates to FL intensity of RhB on reference glass substrate under identical experimental conditions. The EF of Ag particle is about 42-fold. When Ag particle is combined with PC structure to form a photonicplasmonic hybrid substrate, indeed, the EF has been improved significantly (Figure 7b). Up to 260-fold enhancement was observed for the yellowAg substrate, in which SP of Ag particles was efficiently coupled with yellow PC. On the basis of FDTD calculations described above, yellow PC matches best with SPs of Ag particles at the excitation wavelength (540 nm) of RhB. The maximum enhancement factor could be achieved on the yellow PCAg substrate. Our experimental results are in excellent agreement with the FDTD simulation. As control experiments (Figure S12, Supporting Information), the enhancement factors of RhB on sole PC substrates were also determined and are shown in Figure 7c. From comparison of enhancement factors of RhB on different substrates (Figure 7d), we could see that the enhancement factor of silver particle coupled with yellow PC (260-fold) is much larger than the sum of the factors of Ag film (∼42-fold) and yellow PC (∼71-fold). This result clearly indicates that the observed enhancement effect results indeed from the coupling of metal particles rather than from the separate enhancement of Ag particle and PC elements. Recently, a few reports have also demonstrated the combination of photonic and plasmonic systems as a means to regulate
Figure 7. (a) Experimental geometry used; (b) enhancement factors of fluorescence intensity of RhB on different substrates (glass, Ag particles, and PCAg hybrid systems); (c) enhancement factors of fluorescence intensity of RhB on different PC substrates as control experiments; (d) comparison of enhancement factors of fluorescence intensity of RhB on Ag particle, yellow PC substrate, and Ag particles coupled with yellow PC with respect to that on glass substrate. (Inset) Fluorescence emission spectra of RhB on different substrates. 20058
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The Journal of Physical Chemistry C light.3236 For example, Song and co-workers33 have deposited metal film on the surface of different opals to tune surface plasmon resonances (SPR) and to enhance the energy transfer efficiency of donors between the opal and the metal film. Reboud et al.34 reported a photoluminescence enhancement of a dyedoped polymer film deposited on a silver layer through the introduction of a nanoimprinted two-dimensional photonic structure into the samples. Although the enhancement effect of metal surface plasmons and the light extraction capability of PC were used to enhance photoluminescence in the designed system, there is no direct coupling between plasmonic and photonic elements at either excitation or emission wavelength. Very recently, Lopez and co-workers35 studied the emission characteristics of monolayers of dye-doped polystyrene spheres deposited on thin gold substrates. It was found that electric field extends onto both the polymer spheres and the metal surface, leading to emission enhancement of organic dyes contained in the spheres through the formed hybrid plasmonicphotonic modes waveguide-surface plasmon polariton (WG-SPP) as compared to reference PCs deposited on flat dielectric substrates. However, it is also realized that in such a hybrid system the electric field confinement effect of hybrid WG-SPP mode is much smaller than the original waveguide-like WG-like and SPP-like modes of plasmonic and photonic elements. In addition, Cunningham and co-workers36 coupled the discrete metal nanoparticles to photonic crystal surface resonant modes and applied them for Raman studies. Despite these developments, it should be noted that the work described here represents the first example of coupling PBGinduced surface mode of PC with SPs of metal particles to enhance fluorescence. Compared to the previous reports, our new strategy and the designed hybrid system allow simultaneous improvement of three enhancement factors of fluorescence (excitation, quantum efficiency, and extraction enhancement), and as a consequence, the fluorescence emission can be effectively enhanced.
’ CONCLUSION In summary, a new strategy to efficiently enhance fluorescence was developed on the basis of coupling surface plasmons of metal particles with optical properties of colloidal photonic crystals. It is found that such coupling can effectively improve the near-field effect and induced plasmon effect of metal particles but also can be favorable for the extraction of the emission, collectively leading to a further significant enhancement of metal-enhanced fluorescence. FDTD simulation proved the related enhancement mechanism. As a demonstration, an enhancement factor up to 260fold has been experimentally achieved by using rhodamine B as a test molecule, which greatly transcends those of individual plasmonic and photonic components and is also much larger than the simple additive effects of the two components. Additionally, at variance with expensive and time-consuming fabrication methods used in the microelectronic industry, the hybrid plasmonic/ photonic system described here is constructed by the selfassembly of colloidal particles. Therefore, we believe that the designed hybrid system based on our strategy could serve as an easily accessible platform for fluorescence enhancement of diverse fluorophores and have great potential in detecting fluorescenttagged analytes at low concentrations, DNA expression analysis and protein diagnostic assays, and any other fluorescence-based chemical biosensors.
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’ ASSOCIATED CONTENT
bS
Supporting Information. Twelve figures, showing detailed calculated reflection map of PC, fluorescence spectra, and others as described in the text. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail
[email protected]; tel (+86) 10-6279-2905; fax (+86) 10-6279-2905.
’ ACKNOWLEDGMENT We acknowledge financial support from the NSFC (20533050, 20772071, and 50673048), 973 Program (2006CB806200), and the transregional project (TRR61) ’ REFERENCES (1) Ganesh, N.; Zhang, W.; Mathias, P. C.; Chow, E.; Soares, J. A. N. T.; Malyarchuk, V.; Smith, A. D.; Cunningham, B. T. Enhanced fluorescence emission from quantum dots on a photonic crystal surface. Nat. Nanotechnol. 2007, 2, 515–520. (2) Zhang, Y. Q.; Wang, J. X.; Ji, Z. Y.; Hu, W. P.; Jiang, L.; Song, Y. L.; Zhu, D. B. Solid-state fluorescence enhancement of organic dyes by photonic crystals. J. Mater. Chem. 2007, 17, 90–94. (3) Bhasikuttan, A. C.; Mohanty, J.; Nau, W. M.; Pal, H. Efficient fluorescence enhancement and cooperative binding of an organic dye in a supra-biomolecular host-protein assembly. Angew. Chem., Int. Ed. 2007, 46, 4120–4122. (4) Liu, H. L.; Hou, X. L.; Pu, L. Enantioselective precipitation and solid-state fluorescence enhancement in the recognition of alphahydroxycarboxylic acids. Angew. Chem., Int. Ed. 2009, 48, 382–385. (5) Ray, K.; Chowdhury, M. H.; Szmacinski, H.; Lakowicz, J. R. Metal-enhanced intrinsic fluorescence of proteins on silver nanostructured surfaces toward label-free detection. J. Phys. Chem. C 2008, 112, 17957–17963. (6) Chu, L. Q.; Forch, R.; Knoll, W. Surface-plasmon-enhanced fluorescence spectroscopy for DNA detection using fluorescently labeled PNA as “DNA indicator”. Angew. Chem., Int. Ed. 2007, 46, 4944–4947. (7) Chowdhury, M. H.; Ray, K.; Gray, S. K.; Pond, J.; Lakowicz, J. R. Aluminum nanoparticles as substrates for metal-enhanced fluorescence in the ultraviolet for the label-free detection of biomolecules. Anal. Chem. 2009, 81, 1397–1403. (8) Ellard, J. M.; Zollitsch, T.; Cummins, W. J.; Hamilton, A. L.; Bradley, M. Fluorescence enhancement through enzymatic cleavage of internally quenched dendritic peptides: A sensitive assay for the AspN endoproteinase. Angew. Chem., Int. Ed. 2002, 41, 3233–3236. (9) Ye, B. C.; Yin, B. C. Highly sensitive detection of mercury(II) ions by fluorescence polarization enhanced by gold nanoparticles. Angew. Chem., Int. Ed. 2008, 47, 8386–8389. (10) Kim, K.; Lee, Y. M.; Lee, H. B.; Shin, K. S. Silver-coated silica beads applicable as core materials of dual-tagging sensors operating via SERS and MEF. ACS Appl. Mater. Inter. 2009, 1, 2174–2180. (11) Wolfbeis, O. S. Nanoparticle-enhanced fluorescence imaging of latent fingerprints reveals drug abuse. Angew. Chem., Int. Ed. 2009, 48, 2268–2269. (12) Geddes, C. D.; Lakowicz, J. R. Metal-enhanced fluorescence. J. Fluoresc. 2002, 12, 121–129. (13) Fu, Y.; Lakowicz, J. R. Enhanced fluorescence of Cy5-labeled DNA tethered to silver island films: Fluorescence images and timeresolved studies using single-molecule spectroscopy. Anal. Chem. 2006, 78, 6238–6245. 20059
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