Enhanced Fluorescence Emission from Single Molecules on a Two

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Enhanced Fluorescence Emission from Single Molecules on a Two-Dimensional Photonic Crystal Slab with Low Background Emission Takahiro Kaji,* Toshiki Yamada,* Rieko Ueda, and Akira Otomo Advanced ICT Research Institute, National Institute of Information and Communications Technology, 588-2 Iwaoka, Nishi-Ku, Kobe 651-2492, Japan

bS Supporting Information ABSTRACT: Enhancement of fluorescence from single molecules of perylene diimide (PDI) on tantalum pentoxide (Ta2O5) two-dimensional photonic crystal (PC) slabs with low background emission was observed using a single-molecule fluorescence spectroscopy technique. The fluorescence intensity of the single molecules on the PC was more than 3 times higher than that of the molecules without the PC. The performance of the PC slabs with a 100 objective lens was evaluated by steady-state and time-resolved ensemble fluorescence measurements. The mechanism of the enhancement was attributed to coupling of an excitation laser and fluorescence to the PCs’ modes. The present results yield new insights into a highly sensitive investigation of single molecules and regulation of fluorescence emission from them via the effect of PCs. SECTION: Nanoparticles and Nanostructures etection of fluorescence from single molecules using an optical microscope1,2 has been widely employed for sensing biological molecules,3
6 studying photochemical and photophysical processes of organic molecules,7,8 and characterizing a single-photon source,9 because it enables us to understand the molecules’ individual characteristics, which are difficult to reveal using ensemble-averaged measurements, as well as to utilize their nature as a single quantum system. Techniques that can efficiently couple light to molecules are significant for improving the efficiency of these detections and expanding the coverage of the variety of the molecules for these studies. One technique for this purpose utilizes enhanced local field around the metal nanostructures.10
12 However, controlling the distance of the molecules to the metal structures usually has trouble obtaining the desired enhancement effect because of the competing effects of increased excitation rate and quenching due to energy transfer to the structures.13 On the other hand, photonic crystals (PCs),14,15 which have a periodic structure with a similar scale to the light wavelength and the ability to control light propagation, are an excellent candidate for this purpose. One of the properties of PCs is control of spontaneous radiation16,17 of the light emitter due to the Purcell effect18 or the effect of a photonic band gap,19,20 which could also lead to regulation of the molecules’ excited state21
23 resulting from the accelerated or inhibited rate of their spontaneous emission. Meanwhile, enhancement of fluorescence emission has been achieved as a result of efficient coupling of the excitation and fluorescence light to PCs’ modes.24,25 Of particular note, two-dimensional PC slabs26,27 that are compatible with fabrication by lithographic processes have attracted much attention for observing the enhanced fluorescence emission from quantum dots and organic dyes with one-photon24,28 and

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two-photon25,29 excitations. However, although the emission from single quantum dots or semiconductors has been enhanced by using PC cavities,30
32 there is a problem in applying PCs to the enhancement for single molecules; that is, the background emission of PCs, which is also greatly enhanced by the PCs’ effect,33 disturbs the signal from the molecules, which is much weaker than that from the quantum dots under the same excitation power. Particularly, since materials with a high refractive index (>2) such as titanium dioxide (TiO2)24 and silicon nitride (SiN)25 show some background emissions, efforts to reduce background emission have been devoted34,35 to fabricating PCs applicable for enhancement of very weak signals. In the present study, we investigated the fluorescence emission of single molecules on two-dimensional PC slabs made of tantalum pentoxide (Ta2O5) that has a high refractive index (∼2) and a high transparency in a broad wavelength range (0.3
10 μm), based on our previous report of fabrication of Ta2O5 PCs with extremely low background emission.35 We measured the fluorescence from single molecules on PCs with different lattice constants, and observed the enhanced fluorescence emission from the molecules on the PCs. This appears to be the first demonstration of enchantment of fluorescence from single molecules via the effect of two-dimensional PCs. These results provide insight into improving the efficiency of fluorescence detection from single molecules and regulation of fluorescence emission from them via the effect of PCs. Received: May 24, 2011 Accepted: June 21, 2011 Published: June 21, 2011 1651

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Figure 1. (a) Schematic diagram of a Ta2O5/quartz PC slab. (b) SEM image of a PC with a lattice constant of 360 nm fabricated on the Ta2O5/ quartz substrate. (c) Structure of perylene diimide (PDI).

A schematic diagram of the two-dimensional PC slab is shown in Figure 1a. A Ta2O5 layer was deposited on a quartz substrate (Fujiwara Scientific Company) (refractive index: 1.46) by electron beam (EB) evaporation (Eiko Engineering). The pattern of the hexagonal lattice was fabricated on the Ta2O5 layer by EB lithography (ELS-7700, Elionix) and reactive ion etching (RIE) (RIE-10NR, Samco) using CHF3 plasma as detailed in our previous paper.35 A refractive index and a thickness (d) of the Ta2O5 film were respectively estimated to be 2.04 and 164 nm by using an ellipsometer (M-2000UI
Ni, J. A. Woolam). The etched depth of the Ta2O5 layer was about 115 nm, as measured by a stylus surface profiler (Dektak, ULVAC). Figure 1b shows a scanning electron microscope (SEM) image of a PC with a lattice constant (a) of 360 nm. The ratio of the radius (r) of the air holes to the lattice constant (a) was 0.36 in the image. The PC substrate was spin-coated at 3000 rpm with a toluene solution containing 7  10
6 or 1  10
11 M of N,N0 -bis(2,6-dimethylphenyl)perylene-3,4,9,10-tetracarboxylic diimide (PDI) (Santa Cruz Biotechnology) (Figure 1c) and 0.05 wt % of poly(methyl methacrylate) (PMMA) (molecular weight: ∼15000, SigmaAldrich). The thickness of the PMMA layer after drying was estimated at less than 5 nm by the stylus surface profiler and the ellipsometer, ensuring the minimized alteration of the PC structures after the spin coating. A circularly polarized second harmonic beam (488 nm) of a picosecond Ti:sapphire laser (Tsunami, Spectra Physics) with 8 MHz repetition was led into an inverted microscope (Eclipse TE2000-U, Nikon) with a XYZ piezo stage (P517.3CD, Physik Instrumente) and focused into the PC substrates using a 100 (LU Plan Fluor, Nikon) (N.A. 0.90) objective lens. The fluorescence was collected by the same objective lens and transmitted through a dichroic mirror, a

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100-μm pinhole, and edge filters (BLP01-488R-25, Semrock, and 10SWF-600-B, 10SWF-550-B or 10LWF-550-B, Newport), and detected by an intensified charge coupled device (ICCD) (C5909, Hamamatsu Photonics) with a spectrometer (TRIAX 320, Horiba) for steady-state fluorescence measurements, or a photomultiplier tube (PMT) (H7422P-40-MOD, Hamamatsu Photonics),36 which is connected to a time-correlated singlephoton counting (TCSPC) module (SPC-630, Becker & Hickl), for time-resolved fluorescence measurements and single-molecule fluorescence measurements. A finite-difference time-domain (FDTD) simulation was carried out using a commercial software package (FullWAVE, RSoft) to obtain photonic band structures. Ensemble fluorescence measurements were carried out to evaluate the performance of the PCs in the case of using a 100 objective lens with a high numerical aperture (N.A. 0.90). A substrate spin-coated with a 7  10
6 M PDI solution was used. Steady-state fluorescence spectra of PDIs on the PCs with various lattice constants and that of PDIs without the PC are shown in Figure 2a. The black solid line shows a spectrum of PDIs without the PC. Among the lattice constants from 300 to 800 nm and without the PC, the strongest intensity was obtained at the lattice constant of 360 nm. Figure 2b shows the lattice constant dependence of enhancement factors, which are defined as ratios of the fluorescence intensities on the PCs to that without the PC.25,28,35 Enhancement factors of 6 and 5 were obtained at the lattice constants of 360 and 390 nm, respectively. By contrast, a large enhancement factor of 24 was obtained at a lattice constant of 360 nm by using a 20 objective lens with a low numerical aperture (N.A. 0.45) (see Supporting Information). Although a high numerical aperture objective lens, which covers a broad range of angles for irradiation and collection of light, is useful for single-molecule fluorescence spectroscopy, it is rather difficult for the lens to obtain a remarkable enhancement effect by the PCs. This is because a contribution of coupling of incident light at a specific angle to specific modes of the PCs is relatively small in the case of using a high numerical aperture lens in addition to a high efficiency of collecting photons in a broad range of angles by the lens itself. Figure 2c shows lattice constant dependences of fluorescence intensities in the region of 500
550 nm and 550
650 nm, respectively corresponding to the first and second peaks in the spectra (hereafter referred to as “peak 1” and “peak 2”), the total fluorescence intensities, and ratios of the intensities of peak 1 to those of peak 2. The strongest fluorescence intensity of peak 1 was observed at a lattice constant of 360 nm. On the other hand, the strongest intensity of peak 2 was observed at a lattice constant of 460 nm. At a lattice constant of 390 nm, both peak 1 and peak 2 were sufficiently enhanced with a peak 1/peak 2 ratio similar to that without the PC. The shift of the peak 1/peak 2 ratio can be explained by a shift of wavelengths of modes that can couple with the fluorescence light, as will be discussed later. To understand the mechanism of the fluorescence enhancement, time-resolved ensemble fluorescence measurements were performed using a TCSPC technique (see Supporting Information for detail). Figure 2d summarizes mean lifetimes obtained for peak 1, peak 2, and total fluorescence. All of the fluorescence lifetimes with the PCs increased slightly at the larger lattice constants, and the lifetimes reached a value of that without the PC (∼2.3 ns). Since there was no correlation between the lifetimes and the fluorescence intensities for peak 1, peak 2, and the total fluorescence, the mechanism of the fluorescence enhancement is attributed to an increased electromagnetic field 1652

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Figure 2. (a) Steady-state fluorescence spectra of PDIs without a PC and on PCs with lattice constants of 300, 330, 360, 390, 460, 600, and 800 nm. (b) Lattice constant dependence of enhancement factors of the fluorescence intensities. (c) Lattice constant dependence of fluorescence intensities of peak 1 (the region of 500
550 nm in the spectra) (circles) and peak 2 (the region of 550
650 nm in the spectra) (squares), the total fluorescence intensities (triangles), and the ratios (diamonds) of the fluorescence intensities of peak 1 to those of peak 2. A dashed line shows the ratio of the spectrum without the PC. (d) Lattice constant dependences of mean lifetimes of peak 1 (circles) and peak 2 (squares), and the total fluorescence (triangles) from PDIs on the PCs and those without (near) the PCs (diamonds). The fluorescence lifetimes were obtained by TCSPC (see Supporting Information for details).

of the excitation laser and/or an increased collection efficiency of the fluorescence light, resulting from coupling of the excitation laser and/or the fluorescence to the PCs’ modes, instead of the Purcell effect,18 which increases the rate of a spontaneous emission. The lifetime shortening of PDIs on the Ta2O5/quartz substrate compared with PDIs in the previous reports (4
5 ns)37,38 can be due to the interaction of the PDIs in the very thin PMMA layer with the Ta2O5 surface in addition to the slight modification of the spontaneous emission rate of the dipoles near a mirror interface39 with the material of the high refractive index. For single-molecule measurements, we used a substrate that was spin-coated with a 1  10
11 M PDI solution. Figure 3a shows a fluorescence image of PDIs on the substrate without the PC acquired by the confocal microscope with stage scanning. Fluorescence spots that can correspond to single molecules were observed. After acquiring the fluorescence image, the focal spot of the objective lens was moved to each fluorescence spot, and then a time trace of fluorescence intensity of a molecule was obtained as shown in Figure 3c. For obtaining time traces, a relatively weak laser power (3.5 μW) was chosen to investigate the effect of the PCs without a saturation effect. Nearly all the fluorescence time traces exhibited one-step blinking and/or onestep photobleaching due to photodegradation of the molecules after a certain period of irradiation. This ensures that these spots correspond to single PDI molecules.7,11,12 The fluorescence intensity of the time trace (14.8 counts/bin (296 counts/s)) was obtained by subtracting the mean background intensity from the mean signal intensity in the time trace (Figure 3c). Figure 3b

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Figure 3. Confocal fluorescence images of PDIs without a PC (a) and on a PC (b) with a lattice constant of 390 nm. Representative fluorescence time traces of single PDIs without the PC (c) and on the PC (d) with a lattice constant of 390 nm. The bin time is 50 ms. A picosecond 488 nm laser of 3.5 (for a, c, and d) and 0.6 (for b) μW was employed for the fluorescence images and time traces. A laser power was measured before entering the inverted microscope.

shows a fluorescence image of PDIs on a PC with a lattice constant of 390 nm. One of the time traces from the spots in Figure 3b is shown in Figure 3d. The fluorescence intensity of the time trace was 92.9 counts/bin (1858 counts/s). The fluorescence intensities of the time traces that exhibited clear one-step photobleaching and were corrected by subtracting their background intensities without the PC and on the PCs with lattice constants of 320, 390, 460, and 600 nm are summarized as histograms in Figure 4. Scanning for exploration of the molecules was carried out for the same area size of the three different PCs for each lattice constant, and 56 ( 14 of the time traces of single molecules were obtained for each lattice constant. The maximum fluorescence intensities from the PDIs without the PC and on the PC with a lattice constant of 390 nm were 24.8 and 97.7 counts/ bin, respectively. The average and the standard deviation (σ) of the distributions for no PC and PCs with lattice constants of 320, 390, 460, and 600 nm were 10.0, 12.9, 34.0, 24.5, and 21.0 counts/bin, and 5.8, 8.4, 23.0, 16.2, and 13.0 counts/bin, respectively. The average and the standard deviation for PCs with lattice constants of 390 nm were larger than those for no PC and PCs with other lattice constants. This indicates that the fluorescence enhancement of the single molecules is attributed to the effect of the two-dimensional PCs, as evaluated by the ensemble measurements. The distribution of the fluorescence intensities without the PC is explained by the distribution of dipole orientation. On the other hand, the extended distributions with the PCs can be understood by considering both of the distribution of dipole orientation and the distribution of light intensity near the PC surface including some part of the top of the slab and the bottom of the holes, where near-field intensity is locally enhanced.24 We obtained enhancement factors of 1.3, 3.4, 2.5, and 2.1 from the averages of the fluorescence intensities of the single molecules at lattice constants of 320, 390, 460, and 600 nm, respectively. The slightly larger enhancement factors obtained from the ensemble measurements compared to those 1653

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Figure 4. Distributions of the fluorescence intensity of the time traces from single PDIs without the PC and on the PCs with lattice constants of 320, 390, 460, and 600 nm. Normal distribution curves of the distributions are also shown.

from the single-molecule measurements suggest that the fluorescence enhancement in the ensemble measurements can be partially due to the increased number of the excited molecules as a result of not only a large surface area of the structures but also a slightly extended confocal volume by the effect of the PCs using confocal microscopy. Since some part of the excitation and fluorescence lights that are resonant with the PCs’ modes can propagate within the PC slab, the molecules outside the focal spot can be slightly excited, and the fluorescence from them can be slightly detected even with the confocal pinhole in the ensemble measurements. Photonic band structures of Ta2O5/quartz PC slabs were computed by using a three-dimensional FDTD method.14,29 A three-dimensional periodic unit cell containing a slab, upper air, and a lower quartz plate was used in the calculation. Photonic band structures at lattice constants of 360, 390, and 460 nm are shown in Figure 5a,b,c, respectively. The black solid line corresponds to a light line (ω = ck) in air, where ω is a frequency, c is the speed of light, and k is an in-plane wave vector. Since the high numerical aperture objective lens (N.A. 0.90) covers a broad range of angles (0
64°), the emitted light can couple to many modes that are located above a dotted line (ω = ck/sin θ) corresponding to an incident angle (θ) of 64° in the photonic band structure. By contrast, the Gaussian beam of the incident laser can more efficiently couple to the modes near the Γ point (or in the small incident angles). Figure 5d shows distributions of wavelengths of the modes that couple to the light with the incident angles of 0
64° (N.A. 0.90), 0
27° (N.A. 0.45), and 0
12° (N.A. 0.20) at lattice constants of 360, 390, and 460 nm. As can be seen, the distribution of the wavelengths of those modes shifts to longer wavelength with increasing lattice constant. At lattice constants of 360 and 390 nm, the blue line corresponding to the wavelength of the laser (488 nm) intersects with many modes near the Γ point (or in the small incident angles), indicating that the large fluorescence enhancement at these lattice constants is partly attributed to the

Figure 5. Photonic band structures of the Ta2O5/quartz PC slabs with lattice constants of 360 (a), 390 (b), and 460 nm (c) along the ΓM direction computed by a three-dimensional FDTD method. Filled and open squares are transverse-electric-like (TE-like (even)) modes and transverse-magnetic-like (TM-like (odd)) modes, respectively. The black solid line, dotted line, dashed line, and dotted-dashed line represent a light line and lines corresponding to incident angles (θ) of 64° (N.A. 0.90), 27° (N.A. 0.45), and 12° (N.A. 0.20), respectively. (d) Distributions of wavelengths of the modes (TE-like and TM-like modes along the ΓM direction) that are located above the lines of 64° (N.A. 0.90), 27° (N.A. 0.45), and 12° (N.A. 0.20)) at lattice constants of 360, 390, and 460 nm. The blue and green lines correspond to the wavelengths of the laser (488 nm) and the fluorescence peaks (534 and 572 nm) of PDI, respectively. See Supporting Information for the full photonic band structure and the distributions of wavelengths of the modes along the ΓK direction.

locally enhanced light of the excitation laser. On the other hand, the shift of the ratio of the peak intensities in Figure 2c can be understood from the improved fluorescence collection efficiency according to the shift of the distribution of the wavelengths of the modes in Figure 5d; that is, the fluorescence of peak 1 (500
550 nm) and the fluorescence of peak 2 (550
650 nm) are respectively enhanced as a result of the coupling of the fluorescence light to many modes at lattice constants of 360 and 390 nm, and at lattice constants of 390 and 460 nm. In summary, we demonstrated the enhancement of the fluorescence from single PDI molecules via the effect of a twodimensional PC slab, using a slab made of Ta2O5 exhibiting extremely low background emission. The fluorescence intensity of the single molecules on the PCs was shown to be more than 3 1654

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The Journal of Physical Chemistry Letters times higher than that of the molecules without the PC. The results of the ensemble fluorescence measurements and the analysis using the photonic band structures suggested the mechanism of the fluorescence enhancement was attributed to the coupling of the excitation laser and fluorescence to the PCs’ modes. The results lead the way toward highly sensitive fluorescence detection from biological single molecules and investigation of photochemical and photophysical processes of single molecules, along with control of their emission and regulation of their excited state via the use of PCs.

’ ASSOCIATED CONTENT

bS Supporting Information. Steady-state fluorescence spectra and enhancement factors of PDIs on PCs using a 20 objective lens, results of time-resolved fluorescence measurements of PDIs on PCs using a 100 objective lens, and a full photonic band structure and distributions of wavelengths of the modes along the ΓK direction. This information is available free of charge via the Internet at http://pubs.acs.org. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]; phone: +81-78-969-2149 (T.K.). E-mail: [email protected]; phone: +81-78-969-2257 (T.Y.). Fax: +81-78-969-2259.

’ ACKNOWLEDGMENT This work was partly supported by Grants-in-Aid for Young Scientists (B) (22750013) and Scientific Research (C) (20510112) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan. ’ REFERENCES (1) Rigler, R.; Orrit, M.; Basch
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