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Tunable PL increase_3DFe3O4 NPs_Hu_Nanotech_2016

Nanotechnology 27 (2016) 245709 FHuet al Figure 4. (a) Fluorescence curves of DMTPS-DCV in THF/water mixtures with different f w . Concentration: 10 mM; excitation wavelength: 449 nm; (b) enhanced fluorescence curves of PCs/ DMTPS-DCV (1.0 × 10 −7 M) regulated with the increasing external magnetic field from 6.8–38.5 mT; and (c) the maximum fluorescence curves of PCs/DMTPS-DCV and pure DMTPS-DCV (1.0 × 10 −7 M). added a kind of dye molecule with aggregation-induced emission to further observe the fluorescence enhancement of the liquid PCs system. DMTPS-DCV exhibits the feature of AIE, probably due to the extended p-conjugation [36], which emits strongly at 582 nm in dilute THF solution (figure 4(a)). The addition of water into the THF solution gradually increased its emission with a red-shift of the peak from 582–612 nm (orange-red light emission) probably owing not only to the DCV groups, which could form J-aggregates with their planar structures, but also to the polarity effect of water. The highest emission intensity was recorded in its THF/water mixtures with f w of 90% due to aggregate formation with its hydrophobic nature, which was employed in the following fluorescence enhancement test. Similar to the previous results, the fluorescence intensity increased with an increase of magnetic field from 6.8 mT–22.2 mT and then decreased with an increase of magnetic field from 22.2 mT–38.5 mT (figure 4(b)). The maximum fluorescence enhancement was obtained when the emission wavelength of DMTPS-DCV (λ em = 612 nm and λ ex = 514 nm) overlapped the photonic bandgap of the liquid PC system, giving a 12.3-fold fluorescence enhancement for PCs/DMTPS-DCV (1.0 × 10 −7 M, f w = 90%) (figure 4(c)). 3.3. Simulation of fluorescence enhancement with the liquid Fe 3 O 4 PC system The liquid Fe 3 O 4 PC structure and reflectance were calculated using FDTD software, which can investigate the optical properties (such as the reflectance, transmittance and absorbance) of finite-sized PCs. A frequency-domain field profile monitor was used to record the reflectance over the simulation Figure 5. (a) The schematic of the simulative liquid PC structure; (b) reflectance calculated using the FDTD method with a plane wave source; the setting inter-particle distances of the PC model from left to right are 245, 265, 280, 295, 310, 335 and 360 nm, respectively, and (c) calculated reflectance using four point sources in the various bandgap positions of PCs (λ), which was corrected with the modulation coefficients. region. In order to accurately simulate the liquid Fe 3 O 4 PC with a tunable photonic bandgap, Fe 3 O 4 sphere (diameter of 108 nm) arrays were constructed with a hexagonal arrangement in the x-y plane in a reduplicative arrangement of six layers in the x-z plane (figure 5(a)). Six layers have already met the requirement of the maximum reflectivity in calculation. The refractive index of Fe 3 O 4 and water were set as 2.42 and 1.33. Periodic boundary conditions were applied to the x- and y- directions to describe an infinite array and perfectly matched layers were set to the z-boundaries as a boundary condition. Considering these 3D-PCs are surrounded by water and THF, reflectance and transmittance should be recalculated using the complex refractive index with the following formula (1) [37]: eTHF - eeff eHO- e 2 eff dTHF + ( 1 - dTHF) = 0 ( 1) eTHF + eeff eHO+ e 2 eff where δ THF is the volume fraction of THF in the solution; ε eff , ε THF and ε H2O are the dielectric constants for the solution, THF and water. Figure 5(b) shows that the calculated reflectance of the Fe 3 O 4 PC system was related to the interparticle distance, and the maximum reflectance intensity (corresponding photonic bandgap) was well matched with the setting conditions. The results indicated an excellent agreement between the simulation and experiment of the reflection spectra when the inter-particle distance was given according to the experimental section. In order to simulate the interaction of dye molecules and PCs, four point light sources [38] with the wavelengths of 556, 575, 612 or 667 nm (replace the λ em of R6G, RB, DMTPS-DCV or Cy5, respectively) and an x-z plane monitor were placed to collect the reflectance spectra. As is shown in 5

Nanotechnology 27 (2016) 245709 figure 5(c), the calculated reflectance intensity reached a maximum value for DMTPS-DCV (point light source is 612 nm) when the inter-particle distance was set as 230 nm and the corresponding bandgap of PC was 611 nm. The spectra were corrected with the modulation coefficients according to the previous reference [39]. It is consistent that the maximum reflectance intensity was achieved while the wavelength of the point light sources overlapped with the bandgap position of a precisely given 3D photonic structure. 4. Conclusion This work introduces the Fe 3 O 4 NPs suspension as a tunable liquid fluorescence enhancement substrate. The suspension can assemble to a periodic PC by application of an external magnetic field, which reduces the inter-particle distance in a reversible manner, and further modifies the optical properties. In addition, fluorescence enhancement was achieved by coupling the position of the photonic bandgap of the liquid PC with various emission wavelengths, giving a maximum of 12.3-fold fluorescence enhancement for PCs/DMTPS-DCV (1.0 × 10 −7 M, f w = 90%). The work may control the enhancement factor reliably and reversely, which is a significant step forward in employing such non-metallic base and liquid photonic crystal substrates for fluorescence detection. The dynamic modulation and precise control of the photonic bandgap of the liquid Fe 3 O 4 PC system may significantly contribute to more accurate and flexible applications of fluorescence enhancement. This method, combined with the surface-enhanced fluorescence based on noble metal particles, would find wide applications. Acknowledgments We thank Professor B Z Tang and his group for providing us with the experiment reagent. The project was supported by the National Basic Research Program of China (973 Program) (Grant No. 2012CB932903), Major Research Plan of National Natural Science Foundation of China (No. 91433111), Qing Lan Project, Collaborative Innovation Center of Suzhou Nano Science & Technology, the Priority Academic Program Development of Jiangsu Higher Education Institutions. References [1] Aslan K, Lakowicz J R and Geddes C 2005 Rapid deposition of triangular silver nanoplates on planar surfaces: application to metal-enhanced fluorescence J. Phys. Chem. B 109 6247–51 [2] Lakowicz J et al 2004 Advances in surface-enhanced fluorescence J. Fluoresc. 14 425–41 [3] Maenosono S 2003 Modeling photoinduced fluorescence enhancement in semiconductor nanocrystal arrays Chem. Phys. Lett. 376 666–70 [4] Uematsu T, Maenosono S and Yamaguchi Y 2005 Photoinduced fluorescence enhancement in mono- and FHuet al multilayer films of CdSe/ZnS quantum dots: dependence on intensity and wavelength of excitation light J. Phys. Chem. B 109 8613–8 [5] Andrew P and Barnes W L 2004 Energy transfer across a metal film mediated by surface plasmon polaritons Science 306 1002–5 [6] Willard D, Carillo L, Jung J and Orden A V 2001 Quantum dots as resonance energy transfer donors in a model proteinprotein binding assay Nano Lett. 1 469–74 [7] Peng H, Strohsahl C, Leach K, Krauss T and Miller B 2009 Label-free DNA detection on nanostructured Ag surfaces ACS Nano 3 2265–73 [8] Stewart M, Anderton C, Thompson L, Maria J, Gray S, Rogers J and Nuzzo R G 2008 Nanostructured plasmonic sensors Chem. Rev. 108 494–521 [9] Fu C, Ossato G, Long M, Digman M, Gopinathan A, Lee L, Gratton E and Khine M 2010 Bimetallic nanopetals for thousand-fold fluorescence enhancements Appl. Phys. Lett. 97 203101 [10] Kinkhabwala A, Yu Z, Fan S, Avlasevich Y, Mullen K and Moerner W E 2009 Large single-molecule fluorescence enhancements produced by a bowtie nanoantenna Nat. Photon. 3 654–7 [11] Anger P, Bharadwaj P and Novotny L 2006 Enhancement and quenching of single-molecule fluorescence Phys. Rev. Lett. 96 113002 [12] Yablonovitch E 1987 Inhibited spontaneous emission in solidstate physics and electronics Phys. Rev. Lett. 58 2059–62 [13] John S 1987 Strong localization of photons in certain disordered dielectric superlattices Phys. Rev. Lett. 58 2486–9 [14] Do Y, Kim Y, Song Y, Cho C, Jeon H, Lee Y, Kim S and Lee Y H 2003 Enhanced light extraction from organic lightemitting diodes with 2D SiO 2 /SiNx photonic crystals Adv. Mater. 15 1214–8 [15] Chen J, von Freymann G, Choi S, Kitaev V and Ozin G A 2006 Amplified photochemistry with slow photons Adv. Mater. 18 1915–9 [16] Lodahl P, van Driel A, Nikolaev I, Irman A, Overgaag K, Vanmaekelbergh D L and Vos W L 2004 Controlling the dynamics of spontaneous emission from quantum dots by photonic crystals Nature 430 654–7 [17] Qiao F, Zhang C, Wan J and Zi J 2000 Photonic quantum-well structures: multiple channeled filtering phenomena Appl. Phys. Lett. 77 3698–700 [18] Li H, Wang J, Lin H, Xu L, Xu W, Wang R M, Song Y L and Zhu D B 2010 Amplification of fluorescent contrast by photonic crystals in optical storage Adv. Mater. 22 1237–41 [19] Shkunov M N, Vardeny Z, DeLong M, Polson R, Zakhidov A and Baughman R H 2002 Tunable, gap-state lasing in switchable directions for opal photonic crystals Adv. Funct. Mater. 12 21–6 [20] Ganesh N and Cunningham B T 2006 Photonic-crystal nearultraviolet reflectance filters fabricated by nanoreplica molding Appl. Phys. Lett. 88 071110 [21] Hu F, Lin H, Zhang Z, Liao F, Shao M W, Lifshitz Y and Lee S T 2014 Smart liquid sers substrates based on Fe 3 O 4 / Au nanoparticles with reversibly tunable enhancement factor for practical quantitative detection Sci. Rep. 4 7204 [22] Ondic L, Dohnalova K, Ledinsky M, Kromka A, Babchenko O and Rezek B 2011 Effective extraction of photoluminescence from a diamond layer with a photonic crystal ACS Nano 5 346–50 [23] Fornasari L, Floris F, Patrini M, Canazza G, Guizzetti G, Comoretto D and Marabelli F 2014 Fluorescence excitation enhancement by Bloch surface wave in all-polymer onedimensional photonic structure Appl. Phys. Lett. 105 053303 [24] Ge J P, Hu Y, Biasini M, Beyermann W and Yin Y D 2007 Superparamagnetic magnetite colloidal nanocrystal clusters Angew. Chem. Int. Ed. 46 4342–5 6

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