Fluorescence Enhancement by Surface Plasmon Polaritons on

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Fluorescence Enhancement by Surface Plasmon Polaritons on Metallic Nanohole Arrays Peng-Feng Guo,† Shan Wu,‡ Qin-Jun Ren,§ Jian Lu,§ Zhanghai Chen,§ Shou-Jun Xiao,*,† and Yong-Yuan Zhu*,‡ †

State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing National Laboratory of Microstructures, Nanjing University, Nanjing 210093, China., ‡National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China., and §Department of Physics, Surface Physics Laboratory, Advanced Materials Laboratory, Fudan University, Shanghai 200433, China

ABSTRACT A maximum fluorescence enhancement of 11 times was achieved by surface plasmon polaritons (SPPs) on a silver nanohole array region in reflection mode, compared to that on the nonarray area. A 30 nm dielectric SiO2 film was sputtered on the silver film as a spacer to separate the fluorophore from silver for attenuation of the fluorescence quenching. An array period of 550 nm and a nanohole radius of 100 nm were optimized to match the most efficient fluorescence excitation and emission of a boron-dipyrromethene fluorophore (BODIPY 630/650) on the array region. SECTION Nanoparticles and Nanostructures

urface plasmon polaritons (SPPs) represent an electromagnetic surface wave that is confined to the near vicinity of the dielectric-metal interface and propagates along the interface. This confinement leads to enhancement of the electromagnetic field at the interface, which results in an extraordinary sensitivity of SPP-based analytical methods and spectroscopies to analyze the light-matter and biomolecule-surface interactions at the interface or in a thin film.1,2 A key issue for SPP-based spectroscopies is the coupling of light to the SPP resonance. The indispensable wave vector conservation is needed for coupling light into SPP resonance, where the SPP wave vector kSPPs = k0[εdεm/ (εd þ εm)]1/2 (εd and εm are the frequency-dependent permittivities of the metal and the dielectric material, respectively, and k0 is the wave vector of light in free space). A usual way for optical excitation of SPPs is prism coupling, which has been greatly developed in the past several decades.3,4 The periodic nanoholes in metal films have been proved as a new and valid approach to achieve coupling between the light and SPPs.5-7 Compared to the prism coupling, the nanohole array coupling exhibits a planar geometry, which is more suitable for miniaturization as part of an apparatus in industrialized production, and shows broad application prospects. By means of SPP coupling with nanohole arrays, extraordinary optical transmission can be achieved and used for developing SPP-based chemo- and biosensors.8-11 An important application of SPPs is to develop the SPP-enhanced fluorescence spectroscopy, which has been reported previously by prism coupling.4,12,13 The mechanism for fluorescence enhancement has been demonstrated as the unusual excitation and decay processes when SPP resonance happens on metal surfaces.14-16 Only a few works reported the SPP-enhanced fluorescence spectroscopy in transmission mode by means of nanohole arrays.17,18

S

r 2009 American Chemical Society

In this Letter, we describe a SPP-enhanced fluorescence spectroscopy on the metallic nanohole arrays in reflection mode, which theoretically applies much stronger SPPs to excite fluorescence than those in transmission mode. The AFM images of nanohole array structures on a silver film and a SiO2-coated silver film are shown in Figure S1 of Supporting Information. The array's period and the nanohole's radius were changed for optimization. Typically, a 140 nm silver film was first deposited by sputtering on a germanium-coated (5 nm as a seed layer) quartz substrate. The nanohole array was fabricated using a focused ion beam. Then, a SiO2 layer (30 nm) was sputtered onto the chip surface to ensure a spatial separation between the fluorescent molecule and the metal surface. Introduction of a dielectric SiO2 layer aimed to attenuate the fluorescence quenching by metal.4,19,20 Surface modification of the chip in Figure 1 was initiated by 30% v/v APTEs (aminopropyltriethoxysilane) in anhydrous toluene to produce a 3 nm siloxane film. Then, an amine-reactive BODIPY 630/650-X SE (6-(((4,4-difluoro-5-(2-thienyl)-4-bora3a,4a-diaza-s-indacene-3-yl)styryloxy)acetyl) aminohexanoic acid succinimidyl ester) in Figure 1b was grafted onto the chip surface to introduce a uniform fluorophore monolayer. The wave vector conservation provided by the periodic nanostructure in metal films can be indicated by the equation kSPPs = (p(2π/D)μ1 ( q(2π/D)μ2,1 where μ1 and μ2 are the reciprocal lattice vectors of a periodic structure respectively, D is the period of the nanohole array, and p and q are integer numbers corresponding to different propagation directions of the excited SPPs. A numerical simulation by the finite Received Date: October 12, 2009 Accepted Date: November 30, 2009 Published on Web Date: December 03, 2009

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DOI: 10.1021/jz900119p |J. Phys. Chem. Lett. 2010, 1, 315–318

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Figure 1. (a) Schematic illustration of surface modification and fluorescence labeling on SiO2-coated silver nanohole arrays; (b) molecular structures of APTEs and BODIPY 630/650-X SE.

Figure 2. Numerical simulation of the SPP distribution by the finite difference time domain method (FDTD)21 in the near vicinity of metallic nanohole arrays; (a) and (b) are the SPP electric field distribution in x-z plane at periods of D = 350 and at 550 nm, respectively. Other parameters used for simulation are the frequency of the electric field (4.74 1014 Hz, corresponding to the wavelength 633 nm), the thickness of the Ag film (140 nm), and the thickness of the SiO2 film (30 nm). (c) The square of the electric field (E2) along the Z direction on a nanohole's edge (D = 550 nm, the location of the nanohole's edge is denoted in (b) with a red line).

difference time domain method (FDTD)21 was used to obtain the distribution of the electric field at the dielectric-metal interface. For the SiO2 (30 nm)/metal interface, an effective dielectric constant εd(eff) = 1.3 was optimized. The Drude form dielectric function was used for silver with a plasma frequency of 1.374 1016 rad/s and a collision frequency of 3.21 1013 Hz.22 The dielectric constant of SiO2 was 2.27. Figure 2 shows the calculated field intensities distributed in the x-z plane for the sample with periods of D = 350 and 550 nm, respectively, corresponding to the frequency 4.74 1014 Hz (or the wavelength 633 nm, which is near the excitation wavelength of 630 nm for the fluorescent molecule of BODIPY 630/650). For a nanohole array with D = 550 nm, we achieved a great enhancement of the electric field at 633 nm that matches the SPP resonance condition. For an array with D = 350 nm, its comparatively lower coupling efficiency can


explain the minute enhancement of the electric field. For the D = 550 nm array, with a definition that z = 0 is at the metal/ substrate interface, we show in Figure 2c a distinct magnification of the square of the electric field (E2) along the Z direction at a nanohole's edge noted by a red line in Figure 2b. Two local maxima of E2 are achieved at the quartz/Ag and SiO2/air interfaces. However, the largest enhancement is at the interface of SiO2/air, where the fluorescent molecules are located. As we know, fluorescence is mainly modulated by two parameters, excitation rate and quantum yield. The excitation rate is directly proportional to the square of the electric field (E2). The higher E2 region is restricted on the metal nanohole array region, which forecasts stronger excitation of fluorescence. The other quantum yield (Q) is defined as Q = Γ/ (Γ þ knr), where Γ is the radiative decay rate and knr the nonradiative decay rate. The fluorescence excitation and

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decay can be shown as a simplified Jablonski diagram in Figure 3. The fluorescence excitation rate on the nanohole array can be divided into three parts, direct photon excitation rate (Edir), reflected photon excitation rate on a metal surface (Eref), and SPPs' excitation rate (ESPPs). Different from the ordinary photon excitation in free space, the excitation rate can be increased by Eref and especially by ESPPs. Simultaneously, new decay channels will emerge on the metallic nanohole array area. In a free space, after a Stokes shift, there are only two decay modes, Γdir and knr, while on the metallic nanohole array, there are two additional radiative decay rates, Γref (fluorescence emission rate by reflected excitation) and ΓSPPs (fluorescence emission rate by SPPs' excitation). Γref and especially ΓSPPs accelerate the decay rate. Simultaneously, the nonradiative part of knr decreases. These new radiative decay channels lead to a higher quantum yield, which can be explained as the result of rapid energy transfer to SPPs.16 Therefore, the fluorescence emission in a nanohole array is enhanced by the SPPs' excitation and radiative decay rates. Fluorescence measurement was performed in reflection mode on a confocal microphotoluminescence system (JY LabRAM high-resolution single spectrometer with a laser for excitation at 632.8 nm; for the experimental setup and details, please see Supporting Information). The fluorescence of BODIPY 630/650 was excited and recorded at the normal direction to the chip surface. To get a fluorescence spectrum,

five spots were chosen discretionarily on the array area for measurements, and then, these spectra were counterpoised to get an average fluorescence spectrum. In order to study the influence of periodicity on fluorescence, arrays were constructed with different periods from 300 to 700 nm and with an average nanohole radius of 100 nm. The typical reflection spectra were recorded in Figure 4a. Their maximum fluorescence intensities after subtraction of the reference fluorescence from the SiO2-coated flat silver surface without nanohole arrays are plotted against the array periods in Figure 4b. Obviously, the strongest fluorescence was obtained at D = 550 nm, in accordance with the most efficient excitation of SPP resonance. In this case, the fluorescence intensity was enhanced 11 times over that on the reference surface. Additionally, the detected fluorescence is strongly dependent on the nanohole radius. Figure 5 shows the fluorescence spectra obtained from four different nanohole radii with the same period of 550 nm. When the array was constructed with an average nanohole radius of 50 nm, the SPP-mediated fluorescence enhancement was inconspicuous. The maximum fluorescence enhancement was obtained at an average nanohole radius of 100 nm, while other nanohole radii led to much less fluorescence enhancement. This can be explained with the change of Fourier coefficients.23 Here, the Fourier coefficient of the (1, 0) reciprocal lattice vector relies on both the periodicity and the hole shape. The variation of Fourier coefficients implies a redistribution of intensities of the surface diffracted waves.22 The maximum fluorescence at the hole radius of 100 nm from Figure 5 indicates its corresponding largest Fourier coefficient and vice versa. In conclusion, the SPP resonance from the metallic periodic nanohole array was confirmed to enhance the fluorescence of a fluorophore monolayer. A 30 nm dielectric SiO2 film was used to separate the fluorophore monolayer from the silver film for attenuation of the fluorescence quenching by silver. We also optimized the chip with a period of 550 nm and a nanohole radius of 100 nm to achieve the most efficient fluorescent emission for the dye, BODIPY 630/650. This kind

Figure 3. Simplified Jablonski diagram for molecular fluorescence excitation and decay on a metallic nanohole array.

Figure 4. (a) Typical reflection fluorescence spectra for arrays with periods of 300, 350, 400, 450, 500, 550, 600, 650, and 700 nm. All arrays adopt the average nanohole radius of 100 nm. A spacer layer of 30 nm of SiO2 separates the fluorophore monolayer from the silver film to attenuate the fluorescence quenching. The fluorescence from the SiO2-coated flat silver surface without nanohole arrays is treated as the reference. (b) Plot of the fluorescence intensity at 650 nm after subtraction of the reference against the array period.


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(12) Figure 5. Reflection fluorescence spectra with different nanohole radii. Their nanohole radii are 50, 100, 150, and 180 nm. The same period (550 nm) and the same thickness of the SiO2 spacer layer (30 nm) were adopted for all chips.

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of SPP-enhanced fluorescence could lead to an analytical approach with high sensitivity and with a portable miniaturized setup.

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SUPPORTING INFORMATION AVAILABLE Experimental details, AFM images of the structured nanohole arrays, and the schematic drawing of the JY LabRAM high-resolution single spectrometer. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. Fax: þ86 25 83314502. Tel: þ86 25 83621001. E-mail: [email protected] (S.-J.X.); [email protected] (Y.-Y.Z.).

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ACKNOWLEDGMENT This work was supported by the National

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Natural Science Foundation of China (Grants 10874079, 20721002, and 20827001), the National Basic Research Program of China (Grants 2007CB925101 and 2010CB630703), and the Scientific Research Foundation of the Graduate School of Nanjing University.

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