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A method to tune fluorescence lifetime of Eu-PMMA films is proposed, which consists of self-assembled gold nanorods on glass substrate covered by Eu-PMMA shell. The fluorescence lifetime is tunable in a wide range, and depends on aspect ratio and mutual distance of gold nanorods. In a single red color emission channel, more than six distinct fluorescence lifetime populations ranging from 356 to 513 μs are obtained. Through theoretical calculation, we attribute tunable fluorescence lifetime to the change of radiative and nonradiative decay rate and density of photon states. In addition, we use these as-prepared Eu-PMMA films for security data storage to demonstrate optical multiplexing applications. The optical multiplexing experiments show an interesting pseudo-information “8” and conceal the real messages “2” and “6”.
© 2016 Optical Society of America
1. Introduction
Lanthanide doped organic fluorescence films have attracted considerable attention due to their widespread applications in planar optical waveguide [1], solar cell [2] and organic light-emitting diode (OLED) [3]. However, previous researches mainly focused on enhancing fluorescence intensity, the importance of manipulating fluorescence lifetime was usually neglected. In fact, lanthanide doped polymer films with tunable fluorescence lifetime are good candidates for optical multiplexing. On one hand, conventional optical multiplexing is usually achieved by utilizing dyes and quantum dots with different colors [4,5 ], while their emission band widths are relatively broad, leading to inevitable spectral overlap and thus restricting the degree of multiplexing. Lanthanide doped organic fluorescence films exhibit superior performance in thermal endurance, photostability and mechanical properties without losing features of narrow bandwidth emission (<10 nm) [6,7 ], which is appropriate for optical multiplexing applications, such as data storage [8], optical encoding [9] and security printing [10]. On the other hand, with the development of fluorescence lifetime imaging microscopy and relevant techniques [11,12 ], optical multiplexing can use different fluorescence lifetime as an additional dimension [13,14 ], which enables one-color emissive fluorophores to generate more populations and hence greatly extends its capability. Therefore, it is highly desirable to prepare lanthanide doped polymer films with tunable fluorescence lifetime.
To tune the fluorescence lifetime of lanthanide doped polymer films, plasmonic nanostructures are seductive candidates since there is no need to modify the configuration of organic compounds which is not a trivial task. Furthermore, they can generate strong electromagnetic field at the metal surface [15,16 ], which can take fluorescence enhancement as the starting point rather than some other fluorescence quenching methods. So far, many strategies have been conducted to fabricate plasmonic nanostructures, such as top-down and bottom-up approaches [17,18 ]. Among these strategies, colloidal gold nanorods together with covalent self-assembly monolayer technology are good candidates, not only because they facilitate large-scale production without high cost, but also for their tunable plasmonic band [19]. In addition, to the best of our knowledge, there has been no report on tuning fluorescence lifetime of lanthanide doped polymer films for multiplexing, despite some reports on enhancing their fluorescence intensity using plasmonic nanostructures [7,20 ].
In this study, we propose a facile approach for tunable fluorescence lifetime in lanthanide doped polymer films composed of self-assembled gold nanorods on glass substrate covered by Eu-incorporated poly(methylmethacrylate) (PMMA) shell. The fluorescence lifetime of these Eu-PMMA films is tunable, and depends on aspect ratio (AR) and distance between gold nanorods. In a single red color emission channel, more than six distinct fluorescence lifetime populations ranging from 356 to 513 μs are obtained. In addition, we use these as-prepared Eu-PMMA films for security data storage to demonstrate optical multiplexing applications.
2. Sample preparation
A library of gold nanorods with various AR was prepared by seed-mediated growth procedure [18]. Before self-assembly of gold nanorods onto glass substrate, the glass slides were washed in a piranha bath (30% H2O2 mixed in a 1:4 ratio with concentrated H2SO4 at 60 °C), following with deionized water, and dried by N2 stream. The washed glass slides were then immersed in methanol solutions containing 10% (3-mercaptopropyl)trimethoxysilane (MPTMS) overnight. Afterward, these MPTMS-derivatized glass slides were rinsed with methanol, and immersed in a 1 nM colloidal gold nanorods solution for a given duration, varying from 0.5 to 36 hours. To form fluorescent nanolayers, 2 mg of europium thenoyltrifluoroacetonate (Eu(ttfa)3) and 250 mg of PMMA were dissolved in 50 ml of chloroform followed by spin-coating onto the pre-treated glass substrate. For the ease of comparison, all procedures and parameters were optimized and kept constant throughout the experiment.
3. Results and discussion
3.1 Optical performance and morphology of prepared gold nanorods
Figure 1(a) shows colorful images of gold nanorods solution varying from red, purple to blue, cyan, green and brown. UV-vis extinction spectra of these gold nanorods were recorded on a Shimadzu 1700 spectrometer and shown in Fig. 1(b), which demonstrates their plasmon band maxima located at 533, 620, 653, 675, 690 and 720 nm, respectively. Transmission electron microscopy (TEM) images of these gold nanorods were taken on a JEOL 2010 electron microscope and shown in Figs. 1(c)–1(h). The TEM results show a tight particle size distribution, which are consistent with their narrow extinction spectra. Moreover, the plasmon wavelengths of these gold nanorods correspond to AR 1.1, AR 1.5, AR 2.0, AR 2.3, AR 2.6 and AR 2.8, respectively. It should be noted that gold nanorods with AR 1.1 are more inhomogeneous than AR 2.8 ones, which is probably due to that both AR 1.1 ones and AR 2.8 ones have some small particles in solution before purifying and the AR 2.8 ones are easier to separate from smaller particles in the process of centrifugation. It also should be noted that gold nanorods usually have two extinction peaks. The one located at about 525 nm comes from collective oscillations of electrons along transverse direction, and the other one comes from longitudinal direction which is sensitive to AR [19]. For AR 1.1 ones, these two peaks are close to each other and have overlaps, thus the measured one extinction peak is an average result including these two peaks, which is verified by their plasmon wavelengths located at about 550 nm. These experimental results indicate that changing AR will lead to a change of plasmon wavelength, and a larger AR corresponds to a longer plasmon wavelength.
Fig. 1 (a) Colloidal gold nanorods solutions with six different colors. (b) UV-vis extinction spectra of these gold nanorods. TEM images of these gold nanorods with (c) AR 1.1, (d) AR 1.5, (e) AR 2.0, (f) AR 2.3, (g) AR 2.6, and (h) AR 2.8. The scale bar is 50 nm.
3.2 Eu-PMMA films with gold nanorods of different AR
Figure 2(a) describes the prototype of Eu-PMMA films with tunable fluorescence lifetime, in which gold nanorods self-assemble onto thiol-functionalized glass substrates and then covered by Eu-incorporated PMMA shell. Taking AR 2.0 of gold nanorods as an example, atomic force microscope (AFM) images were obtained by NanoScope III with tapping mode and shown in Fig. 2(b), which indicates a successful self-assembly of gold nanorods onto the surface of glass substrate. Except for AR, all the parameters were kept constant including the immersion time. A thickness about 30 nm of PMMA film was then coated onto the substrate by adjusting the concentration of Eu-doped PMMA solution and rate of spin-coating at 0.5% w/v and 3000 rpm, respectively [21]. The photoluminescence (PL) spectra of these Eu-PMMA films with gold nanorods of different AR were measured by a spectrophotometer (Edinburgh Instruments, FLS920) and shown in Fig. 2(c). Upon UV excitation at 340 nm, these Eu-PMMA films exhibit a typical Eu3+ emission pattern with a main emission peak at 612 nm (5 D 0→7 F 2) and several side peaks centered at 579, 591 and 654 nm, yielding a single red color emission. The full-width at half-maximum (FWHM) of this main emission peak is about 9 nm, which is a little narrower than that of Eu(ttfa)3 (~10 nm). Compared with PL spectra of Eu(ttfa)3 [15], this narrower FWHM in Eu-PMMA films is a consequence of the fine structure occurs at 623 nm. In comparison with PL spectra of Eu-PMMA films without gold nanorods [Fig. 2(c)], it can be observed that there is no distinguishable profile difference in main emission peak. These comparisons indicate that this fine structure and narrower FWHM are presumably due to the complexation between Eu(ttfa)3 and PMMA in the inner coordination sphere leading to a change of crystal field strength and symmetry felt by the europium ions. Moreover, the narrower FWHM of main emission peak makes the probability of spectral overlap smaller, which is appropriate for optical multiplexing applications. The fluorescence lifetime of Eu-PMMA films from 5D0 to 7F2 was measured using the time-correlated single-photon counting technique with a FSP920 spectrophotometer. As shown in Fig. 2(d), the fluorescence lifetime is sensitive to AR, which becomes longer with increasing AR from 1.1 to 2.0, and then gradually decreases with further increasing AR to 2.8. Interestingly, these Eu-PMMA films exhibit separated fluorescence decay curves ranging from 356 to 513 μs, and their fluorescence lifetime values are shown in the caption of Fig. 2. These experimental results indicate that tunable fluorescence lifetime of Eu-PMMA films is AR dependence, which can be attributed to the change of plasmon wavelength. Together with Fig. 2(c), we can further find that these tunable fluorescence lifetimes take fluorescence enhancement as starting point, although the minimum enhanced factor is about 1.3 at AR 2.8. A stronger fluorescence enhancement might be obtained by introducing a layer of spacer, because Eu3+ ions random distributed in the polymer films will lead to no enhancement for emitters far away (> 60 nm), an enhanced emission at intermediate distances and quenching at short distances (< 5 nm) [20].
Fig. 2 (a) Schematic illustration of Eu-PMMA films with gold nanorods. (b) AFM image of representative gold nanorods (AR 2.0) self-assemble onto the surface of glass substrate. (c) PL spectra of these Eu-PMMA films with various gold nanorods, and without gold nanorods (ref). The inset plots radiative decay rate vs. AR. (d) The fluorescence lifetimes of these Eu-PMMA films are 356 ± 2.8%, 375 ± 2.6%, 420 ± 3.2%, 442 ± 2.0%, 471 ± 2.9% and 513 ± 3.1% μs.
3.3 Eu-PMMA films with gold nanorods under different self-assemble time
Considering a longer immersion time can bring about higher particle density of colloidal gold nanoparticles deposited onto functionalized substrate surfaces [22], and thus a closer distance between gold nanorods. Figures 3(a)–3(c) show the distance between gold nanorods is gradually smaller than 50 nm after 2 hours immersion time, and reaches to about 5 nm after 18 hours immersion time. Figure 4(a) shows fluorescence decay curves of these Eu-PMMA films with gold nanorods of AR 2.0 under distinct immersion time. The fluorescence lifetime of Eu3+ ions (5 D 0→7 F 2) increase from 430 to 511 μs with increasing immersion time from 0.5 to 6 hours, and then decrease to 461 μs with further prolonging the immersion time to 36 hours. These fluorescence decay curves are a little crowded than those ones shown in Fig. 2(d), and their fluorescence lifetime values are shown in the caption of Fig. 4. Although the interval is not as well-separated as those ones in Fig. 2(d), fortunately these fluorescence lifetimes are still lying in the microsecond region, which is superior for practical applications, because shorter lifetime on the order of nanoseconds cannot be easily separated from background noise by time-gated technique, and longer lifetime on the order of milliseconds is less favorable for high-throughput analysis [11]. Moreover, from another perspective, if just selecting the ones at 0.5, 6 and 36 hours, the mutual interval of fluorescence lifetime is about 40 μs, and thus they are still well-separated and appropriate for fluorescence-lifetime optical multiplexing. The fluorescence intensity, shown in Fig. 4(b), increases with increasing immersion time from 0.5 to 6 hours and then decreases with further prolonging the immersion time. Compare Fig. 4(a) with Fig. 2(d), these fluorescence lifetimes show a similar trend, which inspired us to find out whether the plasmonic properties are also changed in Eu-PMMA films when changing distance between gold nanorods.
Fig. 3 AFM images of gold nanorods (AR 2.0) self-assemble onto the surface of substrate under different immersion time of (a) 2 hours, (b) 6 hours and (c) 18 hours.
Fig. 4 (a) Fluorescence decay curves and (b) PL spectra of Eu-PMMA films with AR 2.0 under different immersion time. The inset plots relative radiative decay rate vs. self-assembly time. The fluorescence lifetimes of these Eu-PMMA films are 430 ± 2.3%, 445 ± 2.1%, 461 ± 2.7%, 486 ± 2.9% and 511 ± 2.9% μs.
For simplicity, finite difference time domain (FDTD) calculations of gold nanorods with different distance are based on side-by-side prototype, because gold nanorods deposited onto a substrate usually trend to assemble side by side rather than end to end for lower surface energy [19]. The configuration of simulations followed the fabricated sample structure, which consisted of two gold nanorods on a glass substrate covered by Eu-PMMA shell. The individual nanorod was modeled as a cylinder with two hemispherical end caps. The side-by-side distance between two nanorods varied from 50 to 5 nm. The dielectric function of gold nanorods was used from Johnson and Christ [23]. The silica refractive index used was 1.459. The refractive index of Eu-PMMA was measured using a Metricon 2010 prism coupler system and the refractive index at 612 nm is 1.52 using the Cauchy’s equation [24]. A total field scattered field (TFSF) source with the wavelength ranging from 500 to 700 nm was launched into the boundary to simulate a propagating plane wave interacting with the targets. The source was perpendicularly oriented to the substrate surface. The mesh size for the calculation of the electric field distribution was 0.5 nm.
Figures 5(a)–5(c) show the electric field distribution around gold nanorods (AR 2.0) coated by Eu-PMMA shell with different side-by-side distance ranging from 50 to 20 and 5 nm, respectively. The color bar shows the enhancement factor of local electric field at 612 nm. The local electric field intensity maximum first increases and then decreases with decreasing distance between gold nanorods, which is consistent with the trend of fluorescence intensity [Fig. 4(b)]. In addition, Fig. 5(d) illustrates the extinction spectra of these gold nanorods with different distances, in which the longitudinal plasmon maximum gradually blue shifts from 630 to 575 nm with decreasing distance from 50 to 5 nm. Interestingly, Fig. 2(c) shows the fluorescence intensity maximum occurs at AR 2.0 indicating that these ones actually have a good spectral overlap with Eu3+ ion emission wavelength at 612 nm. This experimental result implies a blue shift of plasmon wavelength after gold nanorods deposited onto the substrate, because the original plasmon band maxima located at 653 nm in solution. In addition, experimental results in the reference [25] reporting that blue shifts occur in the longitudinal plasmon wavelength after gold nanorods deposited onto the substrate with a dense monolayer. These results indicate that depositing gold nanorods onto the substrate with a higher particle density will lead to a change of plasmon wavelength even at the same AR. Thus tunable fluorescence lifetime of Eu-PMMA films with different self-assemble time [Fig. 4(a)] can also be attributed to the change of plasmon wavelength.
Fig. 5 The electric field distribution around gold nanorods (AR 2.0) coated by Eu-PMMA shell with different distance of (a) 50 nm, (b) 20 nm, and (c) 5 nm. (d) The calculated extinction spectra of these gold nanorods in Eu-PMMA films with decreasing distances. The incident light propagates along the z direction, and the electric field polarizes along x direction.
3.4 Theory on tunable fluorescence lifetime
The above experimental and theoretical results indicate that, whether changing AR or distance, tunable fluorescence lifetime of Eu-PMMA films can be attributed to the change of plasmon wavelength. To have further insight into why changing plasmon wavelength can lead to tunable fluorescence lifetime, radiative decay rate (Ar) is calculated. Considering magnetic dipole emission is less affected by the local crystal field of different host materials [7] and the decreasing degree of electric dipole emission (5D0→7F2) is larger than that of magnetic dipole emission (5D0→7F1) when the Eu3+ ions are very close to the plasmonic nanoparticles [20], the relative change of Ar can approximately be seen from the intensity ratio of (5 D 0→7 F 2)/(5 D 0→7 F 1). The calculated Ar is plotted in Fig. 2(c) inset and Fig. 4(b) inset, respectively. As shown in Fig. 2(c) inset, Ar is sensitive to the change of plasmon wavelength, increasing to a maximum and then gradually decrease, which is similar to those ones shown in Fig. 4(b) inset. Changing plasmon wavelength leading to a variation of Ar is probably due to that the energy is transferred from Eu3+ excitation state to gold nanorods and then excite their local surface plasmon resonance (LSPR); subsequently, the LSPR transforms into radiation light (612 nm) with different rate, namely Ar; a larger Ar means a good matching between Eu3+ excitation state and LSPR, which can be finely tailored by changing plasmon wavelength of gold nanorods.s
Considering fluorescence lifetime (τ) can be expressed as:
where Knr is nonradiative decay rates. Figures 2(c)–2(d) and Figs. 4(a) and 4(b) show that τ firstly increases together with an increase of Ar. According to the Eq. (1), we can deduce that the Knr is decreasing in this process, and the decreasing extent of Knr is greater than the increasing extent of Ar [26]. This result means that a longer fluorescence lifetime is due to reducing energy loss caused by nonradiative decay Knr. And then τ decreases together with a decrease of Ar, which indicates an increase of Knr. In addition, this increasing degree of Knr is stronger than the decreasing degree of Ar, which can be deduced from Eq. (1). These results demonstrate that tunable fluorescence lifetime is due to the competition between Ar and Knr, and a good matching between Eu3+ excitation state and LSPR can greatly reducing Knr leading to a longer fluorescence lifetime.From another point of view, transition rates (R) is the sum of Ar and Knr, and thus fluorescence lifetime can also be express as:
where n, c, ω, μ, ρ(ω) and Eex are refractive index, the speed of light in vacuum, frequency, transition dipole of the emitter, density of photon states and the intensity of excited electric field, respectively. From FDTD calculation results, it can be found that these gold nanorods mainly change fluorescence emission process at 612 nm rather than excitation process at 340 nm, and thus the Eex is constant. As shown in Fig. 2(d) and Fig. 4(a), τ firstly increases which indicates a decrease of ρ(ω) when changing plasmon wavelength towards a good overlap with Eu3+ emission wavelength, and then τ decreases indicating an increase of ρ(ω). Therefore, from another perspective, tunable fluorescence in these Eu-PMMA is due to the change of density of photon states ρ(ω).3.5 Optical multiplexing applications
Thanks to well-separated and tunable fluorescence lifetime of these Eu-PMMA films, we decide to use them for security data storage in the time-domain via a novel method of acrostic poem. Analogous to acrostic poems that real message is hidden in a noticeable scene, a blended pattern [Fig. 6(a) ] comprised of two overlapping numbers of “2” and “6” were made of Eu-PMMA films with three kinds of different fluorescence lifetime. As shown in Fig. 6(c), three kinds of fluorescence lifetime denoted as “2”, “2+6” and “6” stem from Eu-PMMA films with AR 2.8, 1.5 and 2.0, respectively. Upon the UV radiation, the concealed image [Fig. 6(a)] is easily translated into number “8” [Fig. 6(b)], which is a wrong message. For decoding the real message, the fluorescence lifetime of each edge was measured and saved in a datasheet, which was a recovery form of Fig. 6(d). Different combination of labeled pattern can receive the real message. For instance, covering all the edges including “2” and “2+6”, a red “2” could be captured; covering all the edges including “6” and “2+6”, a red “6” could be captured. After two times of covering, the real message “2” and “6” can be recovered and shown in Fig. 6(e).
Fig. 6 Fluorescence-lifetime optical multiplexing. (a) A photograph of a pattern consists of seven-segment Eu-PMMA films, (b) which is easily translated into “8”. (c)-(e) Recovered real messages are “2” and “6” due to their distinct fluorescence lifetime.
4. Conclusion
In summary, we report a facile approach to tune fluorescence lifetime in lanthanide doped polymer films consisting of self-assembled gold nanorods on glass substrate covered by Eu-PMMA shell. The fluorescence lifetime is tunable, and depends on aspect ratio and mutual distance of gold nanorods. Through theoretical calculation, we attribute tunable fluorescence lifetime to the change of plasmon wavelength, and from two perspectives, namely radiative and nonradiative decay rate and density of photon states, to explain these phenomena. Finally, these Eu-PMMA films are used for security data storage and show an interesting pseudo-information number “8”. We believe that this method should be easily extended to other material type of substrates and morphology of nanoparticles, besides facile glass substrates and gold nanorods used in this work. On the other hand, using Eu-PMMA films for multiplexing may also stimulate new concepts on conventional lanthanide-based fluorescent materials for novel applications.
Acknowledgment
This work is financially supported by the International Cooperation Program and 973 Program (2012CB921900) sponsored by MOST. Research Fund for the Doctoral Program of Higher Education (20130031110010). National Natural Science Foundation of China (Grant Nos. 61138004 and 11274180). Applied Basic Research Programs of Science and Technology Commission Foundation of Tianjin (No. 15JCYBJC17200).
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