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Abstract
The Au@SiO2 nanorods with two plasmonic resonance bands are used to enhance and tune the emission of binary lanthanide (Eu/Tb) complexes. The emissions of Tb and Eu ions are both enhanced, the maximum enhancement is over 100-fold. Meanwhile the ratio and relative intensity of the red/green bands is altered by the strong coupling between complexes and nanorods, tuning the color of emission from green to yellow under excitation of 292 nm and improving the color purity from orange to red under excitation of 360 nm. The underlying physics of the lanthanide complex-plasmonic nanorod composite system is analyzed, which deepen the understanding of the interaction between complexes and plasmon nanoparticles.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Luminescent lanthanide compounds can exhibit abundant emission colors based on their 4f-4f or 5d-4f transitions. Their unique spectroscopic properties such as narrow bands and long lifetimes originate from 4f-4f transitions within ions [1–6]. All of these properties attract much attention for their potential applications in various fields, such as fluorescence probes and labels in biological systems [7,8] and display devices [9–13]. However, the luminescence intensities need be further enhanced due to the low quantum yield. One traditional way to obtain the excellent luminescent properties in lanthanides composites is the addition of two or more types of ligands [14, 15]. Organo-Eu and organo-Tb based materials as the color resources are good choices because of their good luminescent properties. Detailed studies on polymer doped with this pair are very important as the incorporation of lanthanides in a polymeric matrix can be very useful to produce wide color tunability. Now, the tunable properties of Organo-Eu and organo-Tb based materials usually be realized by efficient Tb to Eu energy transfer [16,17].
Another strategy based on metal enhanced luminescence has been explored for the enhancement of luminescence properties of lanthanide complexes [18–22]. The plasmonic nanoparticles (NPs), such as Ag, Au, Pt and so on, which support the so-called Local Surface Plasmon Resonance (LSPR), can influence the spectroscopic properties of materials due to the resonant interactions between plasmonic and luminescent phase by matching the LSPR with the excitation or emission bands. The effects may lead to enhancement or deterioration of luminescence intensity, excitation and emission tuning, alterations of luminescence lifetimes, quantum yield, etc.[23–27]. Gold nanorod with two plasmonic resonance bands, responding to its transverse LSPR (T-LSPR) and longitudinal LSPR (L-LSPR) respectively, is an ideal candidate to affect the luminescence of material, which is often used in emission [28–32]. However, the study about the interaction between plasmonic nanoparticles and two types of complexes is limited. The actual mechanism of metal enhanced luminescence is still debated at present. Moreover, the use of a solution process for the polymer emitters and the plasmonic nanoparticles can result in a much simple and less expensive fabrication process.
In this work, The red light emitting Eu(dbm)3phen and the green light emitting Tb(TMHD)3 (Tris(acetylacetonato)(1,10-phenanthroline)terbium) co-doped PMMA films combined with Au@SiO2 nanorods are prepared. The enhancements of luminescence for the two types of complexes are both obtained by matching the T-LSPR with the Tb complex and the L-LSPR with Eu complex. Meanwhile the emissions of Eu/Tb co-doped PMMA films are tuned by using plasmonic nanorods. Under the excitation of 292 nm, the emission enhancement factor is over 40-fold at 612 nm and it’s nearly 20-fold at 545 nm, this led to the significant change of the ratio and relative intensity of the bands in ${}^\textrm{5}{\textrm{D}_\textrm{0}} \to {}^\textrm{7}{\textrm{F}_\textrm{2}}\textrm{/}{}^\textrm{5}{\textrm{D}_\textrm{4}} \to {}^\textrm{7}{\textrm{F}_\textrm{5}}$ and the emission color can be tuned from green to yellow by distributing nanorods. Under the excitation of 360 nm, the emission enhancement is more than 100-fold at 612 nm and the emission is tuned from orange to red. The tuned and enhanced emission and mechanism is detailed in terms of photoluminescence studies (PL), decay profiles, energy level diagram and underlying physics.
2. Material and methods
The complexes of Tb(TMHD)3 and Eu(dbm)3phen with molar ratio 1:1 were dissolved in chloroform solution of Polymethyl methacrylate (PMMA). The Eu-Tb co-doped PMMA solutions was stirred for about 2 hours at ambient temperature to be used. 0.25 ml Au@SiO2 solutions with different concentrations, which were synthesized following our previous method [19,20], were drop-casted onto cleaned silicon wafers and then dried in vacuum oven at 60 °C. The nanorods were dispersed on the silicon wafers with the evaporating of water. The distribution of nanorods depends on the concentration of nanorods [21]. Then the prepared Eu-Tb co-doped PMMA solution was spin-coated at 3000 rpm for 60 seconds on the nanorods covered wafer, the thickness of films is about 100 nm.
The size and morphology of nanorods were determined by Transmission Electron Microscope (TEM) under an acceleration voltage of 200 kV (JEM-2010). The distribution of nanorods were observed by Scanning Electron Microscopy (SEM) with an acceleration voltage of 10 kV. The absorption spectrum of the nanorods was recorded using the UV-vis-NIR spectrophotometer system (HITACHI U3310). Optical properties and decay profiles of samples were measured by spectrophotometer system (Edinburgh FLS920). The luminescence decay curves were obtained using the time-correlated single-photon counting technique with the FSP920 spectrophotometer and the luminescence decay time was estimated from the luminescence decay curves by using the FSP920’s software. All the samples were excited at wavelengths of 292 nm and 360 nm, which corresponding to the absorption of ligands of complexes Tb(TMHD)3 and Eu(dbm)3phen, respectively.
3. Results and discussion
In the rare earth complexes, an appropriate organic ligand is usually employed to function as an antenna to absorb the light and subsequently transfer the excitation energy to the central emissive rare earth ion [33]. The luminescent intensity is strongly dependent on ligand absorption and energy transfer from ligand to metal [34]. The steady-state excitation and emission spectra of Tb(TMHD)3 and Eu(dbm)3phen alone are displayed in Fig. 1. As shown in Fig. 1(a), the ligand of Tb(TMHD)3 shows an intense excitation band ranging from 280 nm to 320 nm and a peak located at 292 nm. The main emission locates at 545 nm which comes from the transition (${}^5{D_4} \to {}^7{F_5}$), and other weak emissions locate at 490 nm, 582 nm, and 622 nm are Tb characteristic f-f transitions. As shown in Fig. 1(b), the ligand Eu(dbm)3phen shows an intense wide excitation band ranging from 320 nm to 400 nm and the peak locates at 360 nm. Under the excitation of 360 nm, the hypersensitive transition ${}^5{D_0} \to {}^7{F_2}$ consists of a strong band at 612 nm. The ${}^5{D_0} \to {}^7{F_0}$, ${}^5{D_0} \to {}^7{F_1}$ and ${}^5{D_0} \to {}^7{F_3}$ transitions situated at 580 nm, 592 nm and 651 nm are very weak.
As known, many references have confirmed the energy transfer efficiency (ET) between Tb and Eu by varying the concentration of Eu [16,17], we also get the similar results. We prepared the Eu/Tb co-doped PMMA films with different Tb/Eu molar ratios by varying the concentration of Eu. The emission spectra of samples under excitation of 292 nm are shown in Fig. 2(a). As shown, the mixture of complexes displays two main emissions at 545 nm and 620 nm. The emission of Tb decreases and the emission of Eu increases with increasing of Eu complexes, which confirm the energy transfer from Tb to Eu. The ET efficiencies ($\eta$) can be calculated using following formula [35,36]:
Where ${\textrm{I}_{\textrm{s0}}}$ is the emission intensity of Tb in the absence of Eu, ${\textrm{I}_\textrm{s}}$ is the emission intensity of Tb in the presence of Eu. We calculated the energy transfer efficiency. The results are shown in Fig. 2(b). The ET efficiency is increased and then stabilized with the increasing of Eu complex. The ET efficiency is about 90% when the molar ratio of Eu and Tb is 1:1. The luminescence decay is another useful probe to study the energy transfer process. The Eu and Tb ions doped in PMMA host have two kinds of decay time values due to the interaction between complex and host, indicating two different emitting sites exist around RE in the mixed systems- one being unbounded complex, other-complex bounded with polymer PMMA. So the luminescence decay of Eu/Tb complexes obey double exponential function for our samples [19,37,38]. When monitoring transition of 545 nm under excitation of 292 nm, the PL decay profiles are shown in Fig. 2(c). The obtained average decay time is smaller than that in absence of Eu complex, which further confirm the energy transfer from Tb to Eu [16]. The fast initial component observed in the decay curves in Fig. 2(c) is attributed to the increased non-radiative energy transfer from Tb to Eu with the increasing of Eu concentration.
Fig. 2. (a) the emission spectra, (b) ET efficiency and (c) lifetime of 545 nm under excitation of 292 nm.
In order to further tune the emission of Eu/Tb two types of complexes, the effect of Au@SiO2 nanorods is employed. The TEM image and plasmonic absorption spectrum of nanorods are shown in Fig. 3. Figure 3(a) shows the morphology of nanorods. As shown, the thickness of SiO2 shell is about 20 nm. The shell had a very small influence on the LSPR of nanorods, only small shifts (less than 10 nm) in plasmon maxima [29]. Figure 3(b) shows the plasmon bands of nanorods, the nanorods have two plasmon bands of ∼480-550 nm and 600-750 nm and peaks located at 520 nm and 670 nm, responding to its T-LSPR and L-LSPR respectively [37,39]. Furthermore, the L-LSPR mode has a larger absorption than the T-LSPR mode. By comparing plasmon bands with the emissions of Eu and Tb complexes, the L-LSPR strongly overlaps with the main emission band of Eu, the T-LSPR strongly overlaps with the main emission band of Tb. The Eu/Tb co-doped polymers combined with nanorods are chosen to tune the luminescence behavior in binary complexes. The surface images of Eu/Tb co-doped PMMA films with nanorods are shown in Fig. 4. Figure 4(a) shows the surface of sample without nanorods, which is almost smooth. Figures. 4(b)– 4(d) shows the surface of samples with nanorods of 0.05 nM, 0.50 nM and 1.00 nM. In Fig. 4(b), there are some small bumps. It can be seen that some bumps become bigger in Fig. 4(c). In Fig. 4(d), there are many fantastic prominences, which means the nanoparticles change from nanorods to clusters because of aggregation of nanoparticles at the high concentration [21].
Fig. 4. SEM images of samples with and without nanorods. (a)Eu/Tb-PMMA film without nanorods, Eu-PMMA films with nanorods, the concentration is (b) 0.05 nM, (c) 0.5 nM, (d) 1.00 nM.
Under excitation of 292 nm, the emission spectra with and without nanorods are shown in Fig. 5(a). Ref. is the emission in absence of nanorods, others show the emission of samples with different concentrations of nanorods. The addition of nanorods led to the significant change of the spectroscopic properties of samples. The corresponding enhancement factors at 490 nm, 546 nm and 612 nm are shown in Fig. 5(b). The three enhancement factors all increase and reach a maximum, then decrease along with increasing of nanorods. The enhancement factor is greatest for the emission at 612 nm, which is about 42 times, followed by the emission at 490 nm, and it is smallest at 545 nm. These results demonstrate that the enhancement is very sensitive to the distribution of nanorods. The band ratio of ${}^5{D_0} \to {}^7{F_2}/{}^5{D_4} \to {}^7{F_5}$ (red/green) changes due to the different emission enhancements of the two bands, which results in the change of emission color. To demonstrate the tunable color, we further show the band ratio in Fig. 5(c). Due to the greater enhancement at 612 nm, the band ratio in presence of nanorods is greater than that in absence of nanorods. It increases gradually and reaches to a maximum, and then slightly decreases with the increasing of nanorods.
Fig. 5. PL spectroscopy results for samples with nanorods. Under excitation of 292 nm: (a) the emission spectra, (b) the corresponding enhancement factors at 490 nm, 546 nm and 612 nm, (c) band ratio of red to green emission . Under excitation of 360 nm: (d) the emission spectra, (e) the corresponding enhancement factors at 490 nm, 546 nm and 612 nm, (f) band ratio of red to green emission.
Then we excited the samples at wavelength of 360 nm, which corresponding to the excitation wavelength of Eu complex, the emission spectra are shown in Fig. 5(d). The emission spectra mainly exhibit the emission at 612 nm. The emission of Tb is very weak because the ligand of Tb complex nearly has no absorption at 360 nm. Also we found that the band around 518 nm appeared in Fig. 5(d). The band comes from the scattering of nanorods, because there are strong scattering at the resonant band of nanorods. The light scattering effect of nanorods also be a mechanism responsible for the PL enhancement of Eu/Tb co-doped PMMA films [40].
In presence of nanorods, the luminescence is obviously enhanced. The enhancement factors are shown in Fig. 5(e). Similar with the enhancement at excitation of 292 nm, the enhancement at 612 nm is greatest, followed by 490 nm and 545 nm. The greatest enhancement at 612 nm is over 100-fold. The overall enhancement factor under excitation of 360 nm is greater than that at 292 nm. The band ratio of red to green emission is shown in Fig. 5(f). The band ratio is significantly increased in presence of nanorods. It increases and reaches a maximum, then it decreases again with the increasing of the nanorods.
To reflect the true color of luminescence, the CIE 1931(x,y) coordinates on CIE chromaticity diagram under the two excitation are shown in Fig. 6. The exactly coordinates are shown in Table 1. Under excitation of 292 nm, the CIE chromatic coordinate can tune from x = 0.375, y = 0.575 to x = 0.465, y = 0.466 by tuning the concentration of nanorods. The emitting color of complexes tune from green to yellow, which indicate them as promising light emitting materials potentially application in multi-colour display [41,42]. Under the excitation of 360 nm, the obtained emission color is orange and its CIE chromatic coordinate locates at x = 0.589, y = 0.389 in absence of nanorods. In presence of nanorods, the emission color is tuned toward the red and the coordinate is changed to x = 0.633, y = 0.352 when the concentration of the nanorods is 0.5 nM. Then the color returns to orange-red, the coordinate locates at x = 0.62, y = 0.36 when the concentration of nanorods is 1.0 nM. The emitting color of complexes shift from orange emission to red emission by tuning the concentration of nanorods, which indicate that we can improve the red saturation by engineering it emission spectrum through nanorods.
Fig. 6. CIE coordinates of mixture complex in presence of nanorods under excitations of 292 nm and 360 nm.
Table 1. The CIE Chromatic Coordinate of Samples.
The enhanced and tuned emissions are all related to the strong coupling between the complexes and the nanorods. When an emitter is near to a nanorod, its PL is modified in multiple ways, which may include reshaping of its spectrum, intensity enhancement or quenching, modulation of radiative and non-radiative decay rates and more [29,43,44]. The Au@SiO2 nanorods we used exhibit strong LSPR bands in the ranges of ∼480-550 nm and 600-750 nm, which overlap with the main emission bands. Therefore, the nanorods mainly change the emission cross section (radiative decay rate) relative to the nonradiative decay, rather than the increased excited-state population. The mechanism is shown in Fig. 7. For Eu and Tb complexes, the ligand absorb the energy in the UV region and transfer the energy to Eu and Tb ions, which results the luminescence of complex. And when excited at 292 nm, the energy transfer from Tb to Eu also occurs, which results the emission of Eu complex [16,17]. In presence of nanorods, the nanorods couple with Eu/Tb complexes when excited the samples, the nanorods increase the radiative decay rates, which result in the increased luminescence. The enhancement factor is consistent with strength of the plasmon absorption at the corresponding wavelength, which demonstrate the enhancement of emission depends on the overlap between LSPR and emission bands of materials. The non-monotonic relation between the emission enhancement factor and density of nanorods indicates that the Förster resonance energy transfer (FRET) also plays an important role in complex-nanorod coupled system. With further increasing of nanorods, there are more efficient nonradiative FRET, which induced the decreased luminescence. Therefore, there are two competing mechanisms that ultimately determine the emission enhancement in presence of nanorods. The plasmon resonance enhances radiative decay rates in presence of nanorods, at the same time, the nanorods at high concentrations introduce additional nonradiative mechanism resulting in luminescence quenching.
Fig. 7. Schematic of the energy absorption, excited electron migration and emission processes in Lanthanide complexes in presence of nanorods.
To further explore the reason for the increased emission, we also perform time-resolved PL measurements at 612 nm and 545 nm under the two excitations, the results are shown in Figs. 8(a)–8(c). And the decay time was estimated from the luminescence decay curves. The results are shown in Fig. 8(d) and Table 2. As shown in Figs. 8(a), 8(c) and Table 2, for emission at 612 nm, the PL decay of Eu are increased in presence of nanorods and also obeys the bi-exponential function. The short decay time and the long decay time are both increased and then decreased again with the increasing of nanorods. The average decay time increases from 441 µs to 572 µs under excitation of 360 nm, and the greatest decay time is obtained when the concentration of nanorods is 0.5 nM. The change is similar under the excitation of 292 nm as shown in Table 2. For the emission at 545 nm, as shown in Fig. 8(b), the fast initial component become more obvious at high concentrations of nanorods, which indicates that there are FRET from complexes to nanorods besides the energy transfer from Tb to Eu [45]. The short decay time is decreased, the long decay time is increased and then decreased again with the increasing of the nanorods. The average decay time increases from 698 µs to 780 µs under excitation of 292 nm, the greatest decay time is obtained when the concentration of nanorods is 0.17 nM. They are consistent to the enhancement of luminescence. In all cases, the average decay time of the Tb/Eu system is gradually increased and then decreased again with the increasing of nanorods concentrations.
Fig. 8. Luminescence decay profiles of samples with nanorods. Under excitation of 292 nm: (a) emission at 612 nm, (b) emission at 545 nm. Under excitation of 360 nm: (c) emission at 612 nm. And (d) the average decay time values of the samples in presence of nanorods.
Table 2. The Decay Time Values of The Samples with Nanorods
The lifetime can be expressed as
where ${{\gamma }_\textrm{r}}$ is the radiative decay rate and ${{\gamma }_{\textrm{nr}}}$ is the nonradiative decay rate. The luminescence intensity $\textrm{I}$ and the quantum yield ${\textrm{q}_{0}}$ of organic materials in a free space at a certain emission wavelength satisfy the equations [46]:Fig. 9. The relative change of ${{\gamma }_\textrm{r}}$ for the main emission under excitation of 292 nm and 360 nm.
To further explain the underlying physics of a composite system comprising an emitter near a plasmonic structure, we explain the results from Purcell effect. The Purcell factor F represents the radiative decay rate enhancement of the emitter close to the plasmonic nanostructure [47]:
4. Conclusions
The luminescent of Eu and Tb complexes co-doped polymer are successfully editing by plasmonic Au@SiO2 nanorods. Due to effective nanorods and complexes coupling, the luminescent is obviously enhanced. Under the excitation of 292 nm, the maximum enhancement is over 40-fold for Eu emission and over 20-fold for Tb emission. Under the excitation of 360 nm, the enhancement is over 100-fold at 612 nm. Meanwhile, the interactions between complexes and nanorods led to significantly changes of the band ratio of red and green emissions, the color tune from green to yellow under excitation of 292 nm and improve the red saturation under excitation of 360 nm. The luminescent materials we used can be potentially applied for biomedical, analytical and optical sensors, etc.
Funding
National Natural Science Foundation Program of China (11504390, 61405085, 61775089); Natural Science Foundation of Shandong Province (ZR2017BA013, ZR2017BF009, ZR2018MA039, ZR2019MF068); Shandong Province Higher Educational Science and Technology Program (J17KA175); Alliance fund of shandong provinical key laboratory (SDKL20016038); The special construction project fund of shandong province Taishan scholars..
References
1. Z. R. Hong, C. J. Liang, R. G. Li, W. L. Li, D. Zhao, D. Fan, and S. T. Lee, “Rare earth complex as a high-efficiency emitter in an electroluminescent device,” Adv. Mater. 13(16), 1241–1245 (2001). [CrossRef]
2. Y. Hasegawa and T. Nakanishi, “Luminescent lanthanide coordination polymers for photonic applications,” RSC Adv. 5(1), 338–353 (2015). [CrossRef]
3. Y. Cui, H. Xu, Y. Yue, Z. Guo, J. Yu, Z. Chen, J. Gao, Y. Yang, G. Qian, and B. Chen, “A luminescent mixed-lanthanide metal-organic framework thermometer,” J. Am. Chem. Soc. 134(9), 3979–3982 (2012). [CrossRef]
4. S. Hussain, X. Chen, W. T. A. Harrison, S. Ahmad, M. R. J. Elsegood, I. U. Khan, and S. Muhammad, “Synthesis, Thermal, Structural Analyses, and photoluminescent properties of a new family of malonat-containing lanthanide coordination polymers,” Front. Chem. (Lausanne, Switz.) 7, 260 (2019). [CrossRef]
5. H. Xu, Q. Sun, Z. F. An, Y. Wei, and X.G. Li, “Electroluminescence from europium complexes,” Coord. Chem. Rev. 293-294(15), 228–249 (2015). [CrossRef]
6. L. N. Bochkarev, A. V. Rozhkov, V. A. Ilichev, G. A. Abakumov, and M. N. Bochkarev, “Green light emitting norbornene based terbium cotaining coploymer, synthesis, photo- and electroluminescent properties,” Synth. Met. 190, 86–91 (2014). [CrossRef]
7. W. W. Zhong, “Nanomaterials in fluorescence-based biosensing,” Anal. Bioanal. Chem. 394(1), 47–59 (2009). [CrossRef]
8. S. H. Kim, J. R. Gunther, and J. A. Katzenellenbogen, “Monitoring a coordinated exchange process in a four component biological interaction system: development of a time resolved terbium based one donor/ three acceptor multicolor FRET system,” J. Am. Chem. Soc. 132(13), 4685–4692 (2010). [CrossRef]
9. S. SeethaLekshmi, A. R. Ramya, M. L. P. Reddy, and S. Varughnese, “Lanthanide complex-derived white-light emitting solids: A survey on design strategies,” J. Photochem. Photobiol., C 33, 109–131 (2017). [CrossRef]
10. J. G. Bunzli and C. Piguet, “Taking advantage of luminescent lanthanide ions,” Chem. Soc. Rev. 34(12), 1048–1077 (2005). [CrossRef]
11. B. N. Li, L. Liu, G. R. Fu, Z. Zhang, H. Y. Li, X. Q. Lü, and W. K. Wong, “Color-tunable to direct white-light and application for white polymer light emitting diode (WPLED) of organo-Eu and organo-Tb doping polymer,” J. Lumin. 192, 1089–1095 (2017). [CrossRef]
12. Y. Lu and B. Yan, “Lanthanide organic-inorganic hybrids based on functionalized metal-organic frameworks (MOFs) for a near-UV white LED,” Chem. Commun. 50(97), 15443–15446 (2014). [CrossRef]
13. D. Liu, Y. N. Zhou, J. L. Zhao, and Y. L. Xu, “An intensive green emitting terbium complex using a newly designed daromatic hyperbranched polyester as an efficient antenna ligand,” J. Mater. Chem. C 5(44), 11620–11630 (2017). [CrossRef]
14. S. Petoud, G. Muller, E. G. Moore, J. Xu, J. Sokolnicki, J. P. Piehi, U. N. Le, S. M. Cohen, and K. N. Raymond, “Brilliant Sm, Eu, Tb, and Dy Chiral Lanthanide Complexes with Strong Circularly Polarized Luminescence,” J. Am. Chem. Soc. 129(1), 77–83 (2007). [CrossRef]
15. D. J. Zhang, X. M. Wang, Z. A. Qiao, D. H. Tang, Y. L. Liu, and Q. S. Huo, “Synthesis and characterization of novel lanthanide(III) complexes-functionalized mesoporous silica nanoparticles as fluorescent nanomaterials,” J. Phys. Chem. C 114(29), 12505–12510 (2010). [CrossRef]
16. S. Gopi, S. K. Jose, E. Sreeja, P. Manasa, N.V. Unnikrishnan, and C. Joseph, “Tunable green to red emission via Tb sensitized energy transfer in Tb/Eu codoped alkali fluoroborate glass,” J. Lumin. 192, 1288–1294 (2017). [CrossRef]
17. A. F. Y. Matsushita, A. A. C. C. Pais, and A. J. M. Valente, “Energy transfer and multicolour tunable emission of Eu,Tb(PSA)Phen composites,” Colloids Surf., A 569, 93–101 (2019). [CrossRef]
18. X. J. Wang, Y. R. Qu, Y. L. Zhao, and H. B. Chu, “Effect of the composition of lanthanide complexes on their luminescence enhancement by Ag@SiO2 core-shell nanoparticles,” Nanomaterials 8(2), 98 (2018). [CrossRef]
19. J. Zhang, F. Song, Z. He, Y. Liu, Z. Chen, S. Lin, L. Huang, and W. Huang, “Wide-Range tunable fluorescence lifetime and ultrabright luminescences of Eu-Grafted plasmonic core shell nanoparticles for multiplexing,” Small 12(3), 397–404 (2016). [CrossRef]
20. Q. R. Wang, Q. Shi, S. H. Li, D. Zhang, and W. J. Wang, “Influence of fluorescence of Eu(dbm)3phen doped films by gold nanorods,” J. Lumin. 177, 295–298 (2016). [CrossRef]
21. Q. R. Wang, J. Zhang, X. Sang, D. Zhang, Q. Shi, S. H. Li, and W. J. Wang, “Enhanced Luminescence and prolonged lifetime of Eu-PMMA films based on Au@SiO2 plasmonic hetero-nanorods,” J. Lumin. 204, 284–288 (2018). [CrossRef]
22. H. Nabika and S. Deki, “Enhancing and quenching functions of silver nanoparticles on the luminescent properties of europium complex in the solution phase,” J. Phys. Chem. B 107(35), 9161–9164 (2003). [CrossRef]
23. C. D. Geddes, “Metal enhanced fluorescence,” Phys. Chem. Chem. Phys. 15(45), 19537 (2013). [CrossRef]
24. U. J. Krull, “Localized surface plasmon resonance: nanostructures, bioassays and biosensing- a review,” Anal. Chim. Acta 706(1), 8–24 (2011). [CrossRef]
25. Y. Chen and H. Ming, “Review of surface plasmon resonance and localized surface plasmon resonance sensor,” Photonic Sens. 2(1), 37–49 (2012). [CrossRef]
26. M. K. Hossain, M. Kitajima, K. imura, and H. Okamoto, “Interstitial dependent enhanced photoluminescence: A Near field microscopy on single spheroid to dimmer, tetramer, and few particles gold nanoassembly,” J. Phys. Chem. 121(4), 2344–2354 (2017). [CrossRef]
27. J. Zhang, X. Li, S. Li, J. C. Zhang, X. Yan, G. F. Yu, D. P. Yang, and Y. Z. Long, “Ultrasensitive fluorescence lifetime tuning in patterned polymer composite nanofibers with plasmonic nanostructures for multiplexing,” Macromol. Rapid Commun. 40(5), 1800022 (2019). [CrossRef]
28. N. Gandra, C. Protz, L. Tian, R. Tang, B. Xu, S. Achilefu, and S. Singamaneni, “Probing distance-dependent plasmon enhanced near infrared fluorescence using polyelectrolyte multilayers as dielectric spacers,” Angew. Chem., Int. Ed. 53(3), 866–870 (2014). [CrossRef]
29. N. S. Abadeer, M. R. Brennan, W. L. Wilson, and C. J. Murphy, “Distance and plasmon wavelength dependent fluorescence of molecules bound to silica-coated gold nanorods,” ACS Nano 8(8), 8392–8406 (2014). [CrossRef]
30. F. W. Kang, J. J. He, T. Y. Sun, Z. Y. Bao, F. Wang, and D. Y. Lei, “Plasmonic dual-enhancement and precise color tuning of gold nanorod@SiO2 coupled core-shell-shell Upconversion nanocrystals,” Adv. Funct. Mater. 27(36), 1701842 (2017). [CrossRef]
31. Y. X. Xue, C. J. Ding, Y. Y. Rong, Q. Ma, C. D. Pan, E. Wu, B. T. Wu, and H. P. Zeng, “Tuning plasmon enhancement of single nanocrystal upconversion luminescence by varying gold nanorod diameter,” Small 13(36), 1701155 (2017). [CrossRef]
32. R. March, S. Natalla, G. Szymon, and L. Stefan, “Luminescent-plasmonic, lanthanide-doped core/shell nanomaterials modified with Au nanorods upconversion luminescence tuning and morphology transformation after NIR laser irradiation,” J. Alloys Compd. 762, 621–630 (2018). [CrossRef]
33. J. C. G. Buenzli, “On the design of highly luminescent lanthanide complexes,” Coord. Chem. Rev. 293-294, 19–47 (2015). [CrossRef]
34. Q. Li, T. Li, and J. G. Wu, “Luminescence of Europium and Terbium complexes incorporated in Poly(Vinyl Pyrrolidone) Matrix,” J. Phys. Chem. B 105(49), 12293–12296 (2001). [CrossRef]
35. X. L. Wu, W. N. Bai, O. Hai, Q. Ren, J. L. Zheng, and Y. H. Ren, “Tunable color of Tb3+/Eu3+/Tm3+ co-activated K3La(PO4)2 via energy transfer : A single phase white emitting phosphor,” Opt. Laser Technol. 115, 176–185 (2019). [CrossRef]
36. C. Y. Cao, H. K. Yang, J. W. Chung, B. K. Moon, B. C. Choi, J. H. Jeong, and K. H. Kim, “Hydrothermal synthesis and enhanced photoluminescence of Tb3+ in Ce3+/Tb3+ doped KGdF4 nanocystals,” J. Mater. Chem. 21(28), 10342–10347 (2011). [CrossRef]
37. J. Zhang, F. Song, S. X. Lin, S. J. Liu, and Y. L. Liu, “Tunable fluorescence lifetime of Eu-PMMA films with plasmonic nanostructures for multiplexing,” Opt. Express 24(8), 8228–8236 (2016). [CrossRef]
38. I. Malina, V. Kampars, and S. Belyakov, “Luminescence properties of 2-benzoyl-1,3-indandione based Eu3+ ternary and tetrakis complexes and their polymer films,” Dyes Pigm. 159, 655–665 (2018). [CrossRef]
39. S. E. Lohse and C. J. Murphy, “The quest for shape control: a history of gold nanorods synthesis,” Chem. Mater. 25(8), 1250–1261 (2013). [CrossRef]
40. P. Chen, Z. Y. Xiong, X. Y. Wu, M. Shao, Y. Meng, Z. H. Xiong, and C. H. Gao, “Nearly 100% efficiency enhancement of CH3NH3PbBr3 perovskite light-emitting diodes by utilizing plasmonic Au nanoparticles,” J. Phys. Chem. Lett. 8(17), 3961–3969 (2017). [CrossRef]
41. L. Gu, H. Shi, L. Bian, M. Gu, K. Ling, X. Wang, H. Ma, S. Cai, W. Ning, L. Fu, H. Wang, S. Wang, Y. Gao, W. Yao, F. Huo, Y. Tao, Z. An, X. Liu, and W. Huang, “Colour-tunable ultra-long organic phosphorescence of a single-component molecular crystal,” Nat. Photonics 13(6), 406–411 (2019). [CrossRef]
42. Z. An, C. Zheng, Y. Tao, R. Chen, H. Shi, T. Chen, Z. Wang, H. Li, R. Deng, X. Liu, and W. Huang, “Stabilizing triplet excited states for ultralong organic phosphorescence”,” Nat. Mater. 14(7), 685–690 (2015). [CrossRef]
43. A. Das, C. C. Mao, S. Cho, K. Kim, and W. Park, “Over 2018 fold enhancement of upconversion luminescence using water dispersible metal insulator metal nanostructures,” Nat. Commun. 9(1), 4828 (2018). [CrossRef]
44. M. Runowski, “Color tunable up conversion emission luminescent plasmonic, core/shell nanomaterials-KY3F10:Yb3+,Tm3+/SiO2-NH2/Au,” J. Lumin. 186, 199–204 (2017). [CrossRef]
45. C. Chen, C. Midelet, S. Bhuckory, N. Hildebrandt, and M. H. V. Werts, “Nanosurface energy transfer form long lifetime erbium donors to gold nanoparticles,” J. Phys. Chem. C 122(30), 17566–17574 (2018). [CrossRef]
46. J. R. Albani, “Principles and applications of fluorescence spectroscopy,” Blackwell (2007).
47. O. Bitton, S. N. Gupta, and G. Haran, “Quantum dot plasmonics: from weak to strong coupling,” Nanophotonics 8(4), 559–575 (2019). [CrossRef]
