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Tunable PL increase_3DFe3O4 NPs_Hu_Nanotech_2016

Nanotechnology 27 (2016) 245709 The ultraviolet–visible absorption spectra were recorded by a Thermo Scientific Evolution 220 Diode Array Spectrophotometer. FHuet al 2.4. Theoretical calculations In order to theoretically model the fluorescence enhancement coupling in the liquid Fe 3 O 4 system with a photonic bandgap, finite difference time domain (FDTD) simulations were performed with FDTD Solutions from Lumerical Solutions, Inc. (Vancouver, Canada). The mesh size used in the calculations was 1 nm × 1nm × 1 nm. The simulation time was set at 500 fs, ensuring the fields decayed completely before the termination of the simulation. 3. Results and discussion 3.1. Magnetic field induced PC system Highly water dispersible magnetite Fe 3 O 4 NPs were synthesized via a hydrothermal approach. The SEM showed uniform dispersed spherical Fe 3 O 4 NPs with a diameter of 108 ± 6.0 nm (figures 1(a) and (b)). PAA was selected as a surfactant to confer the particles with a uniform size and excellent dispersibility in water. Moreover, the size of the Fe 3 O 4 NPs can be precisely controlled from 80–226 nm by simply adjusting the amount of PAA (figure S1 in the supporting information). Figure 1(c) shows that a typical Fe 3 O 4 NP is composed of small primary crystals with a size of 6–8 nm. A high-resolution TEM (HRTEM) image (figure 1(d)) measuring the distance between two adjacent planes gives a value of 0.30 nm and 0.48 nm, which correspond to the lattice spacing of (220) and (111) planes of cubic magnetite, respectively. XRD measurements also confirm the excellent crystallinity of magnetite particles (figure S2). All the diffraction peaks from 20°–70° can be indexed to the corresponding (2 2 0), (3 11), (4 00), (5 11) and (4 40) diffraction planes of face-center-cubic phase Fe 3 O 4 . The lattice parameter calculated from the XRD pattern is a = 0.8434 ± 0.0021 nm, which is in good agreement with the standard (a = 0.8396 nm, JCPDS Card No. 19-0629). Since each Fe 3 O 4 NP is composed of small primary nanocrystals, the Fe 3 O 4 NPs retain the superparamagnetic behaviour at room temperature (figure S3). Without an external magnetic field, the Fe 3 O 4 NPs are randomly dispersed in water and display an intrinsic brown colour. When a magnetic field is applied, the Fe 3 O 4 NPs gradually form chain-like structures along the magnetic field lines and then assemble into a 3D ordered array in the water [28, 29]. The PC system is successfully established when the attractive magnetic force and repulsive electrostatic force achieve a balance. The inter-particle distance between the Fe 3 O 4 NPs can be tuned with a large scale (nearly 100 nm) through controlling the applied magnetic field intensity, capable of forming tunable PC periodicity. Diffraction occurs when the periodicity of the assembled structure and the wavelength of the incident light satisfy the Bragg condition [30]. Figure 2. (a) Photographs of PCs in solution formed in response to an external magnetic field, which were taken from above the beaker with a vertical magnetic field; the electric current increases and then the magnetic field strength increases gradually from right to left; (b) reflection spectra of PCs in response to an external magnetic field. Diffraction peaks blue-shift (from right to left) as the magnetic field increases from 6.8–38.5 mT. Figures 2(a) and (b) show the photographs and reflection spectra of an aqueous solution of Fe 3 O 4 NPs with a diameter of 108 nm (ca. 6.8 mg ml −1 ) in response to varying magnetic fields achieved by controlling the electric current of an electromagnet, whose intensity can be modulated from 3.3–39.6 mT by increasing the electric current from 0.01–0.14 A (figure S4). The diffraction peak was blue-shifted from 694–492 nm as the magnetic field increases from 6.8–38.5 mT. It also means that the bandgap positions of the PCs decrease from 694–492 nm, while the interplanar spacing decreases from 261–185 nm along with the increasing electromagnetic field strength calculated with Bragg’s Law (l = 2ndsin q; λ is the diffraction wavelength, n is the refractive index of water, d is the lattice plane spacing, and θ = 90° is the Bragg angle) [31]. Note that the inter-particle distance can be determined with the intensity of the magnetic field (figure S5), which is crucial for the accuracy of the 3D model in the 3D-finite FDTD simulation. The optical response of PCs based on superparamagnetic particles can be conveniently tuned by changing the external magnetic field intensity fast and reversibly. Such a PC system possesses a large tunability of bandgap covering the whole visible spectrum. 3.2. Enhanced fluorescence intensity Organic dyes, including R6G, RB, Cy5 and DMTPS-DCV, were dispersed into a magnetic field induced PC system, in which the emission wavelengths (λ em ) of R6G (556 nm), RB (575 nm), Cy5 (667 nm), and DMTPS-DCV (612 nm) overlap the bandgaps of the Fe 3 O 4 PC system (figure S6). In order to systematically investigate the influence factors when applying the liquid PC system on fluorescence enhancement, the first 3

Nanotechnology 27 (2016) 245709 Figure 3. (a) Enhanced fluorescence curves of PCs/R6G (1.0 × 10 −10 M) regulated with the increasing external magnetic field from 6.8–38.5 mT; (b) the maximum fluorescence curves of PCs/R6G and pure R6G (1.0 × 10 −10 M); (c) enhanced fluorescence curves of PCs/R6G (1.0 × 10 −9 M) regulated with increasing magnetic field; (d) the maximum fluorescence curves of PCs/R6G and pure R6G (1.0 × 10 −9 M); (e) enhanced fluorescence curves of PCs/R6G (1.0 × 10 −8 M) regulated with increasing magnetic field; and (f) the maximum fluorescence curves of PCs/R6G and pure R6G (1.0 × 10 −8 M). three dyes were prepared with water as the solvent, while DMTPS-DCV was dissolved in the THF/water mixture. Figure S7 shows the chemical structures of four organic dye molecules and the schematic diagram of the liquid PC system based on Fe 3 O 4 NPs (red ball) with a regular inter-particle distance in response to an external magnetic field. For fluorescence experiments, 20 μl of 1.0 × 10 −9 M R6G solution was dropped into 180 μl as-prepared Fe 3 O 4 NPs dispersion. The mixture was transferred in a quartz cuvette (∼380 μl in volume) with a quartz window, which was then placed onto an electromagnet. Both the cuvette and electromagnet were positioned under the Raman microscope, whose laser was focused on the surface of the Fe 3 O 4 dispersion through the quartz window (figure S8). As shown in figure 3(a), the fluorescence intensity of PCs/R6G (1.0 × 10 −10 M) increased from 152.1–742.3 with an increase of the magnetic field from 6.8 mT–29.8 mT. The intensity decreased from 742.3–259.4 with an increase of the magnetic field from 29.8 mT–38.5 mT. Compared with pure R6G (1.0 × 10 −10 M) with the same depth of laser, the PCs/ R6G (1.0 × 10 −10 M) shows a 6.5-fold fluorescence enhancement (figure 3(b)). Moreover, another two concentrations of R6G (1.0 × 10 −9 M and 1.0 × 10 −8 M) were prepared in the liquid PC system in order to demonstrate the effect of the concentration on fluorescence enhancement (figures 3(c)–(f)). The fluorescence intensity consistently FHuet al increased with an increase of magnetic field from 6.8 mT– 29.8 mT and then decreased with an increase of magnetic field from 29.8 mT–38.5 mT (figures 3(c) and (e)), giving a 4.8-fold of PCs/R6G (figure 3(d), 1.0 × 10 −9 M) and 4.5- fold of PCs/R6G (figure 3(f), 1.0 × 10 −8 M) fluorescence enhancement, respectively. For a given mixture of liquid PC/ fluorescent dye molecules, high fluorescence enhancement can be obtained from a low concentration of dye molecules. The same phenomenon was also observed in the other two water-soluble dye molecules (figure S9). For liquid PCs/RB (1.0 × 10 −9 M) and PCs/Cy5 (1.0 × 10 −10 M), the maximum fluorescence enhancements were as high as 4.9-fold and 5.5- fold, respectively. Note that the excitation wavelength (λ ex ) is 514 nm for R6G/RB and 633 nm for Cy5, respectively. Interestingly, the maximum fluorescence intensity was obtained when the emission peaks of dye molecules overlapped with the photonic bandgap of the liquid Fe 3 O 4 system (table S1). For instance, when the photonic bandgap (the maximum reflectance position was 549 nm) matched well with the emission peak of R6G (λ em = 556 nm), the strongest fluorescence intensity and a maximum 6.5-fold enhancement factor were observed. Manipulation of maximum enhancement can be easily realized for RB (λ em = 575 nm) and Cy5 (λ em = 667 nm) through regulating the bandgap position to approximate the respective emission wavelength. This is mainly attributed to the efficient reflection of the PC surface [32]. When the emission wavelength just overlaps with the photonic bandgap of the PC, this light is unable to propagate into the interior of the PC due to the internal Bragg diffraction. Furthermore, the excitation light (λ ex = 514 and 633 nm) can also be reflected as the excitation wavelength is also in the range of the photonic bandgap, which is in favour of fluorescence enhancement due to the excitation of more dye molecules. Note that this liquid Fe 3 O 4 PC system possesses prodigious advantages compared with solid ones due to its dynamic modulation and precise control of the photonic bandgap (nearly covering the whole visible spectrum). In addition, PC enhanced fluorescence has been demonstrated as an effective means for amplifying the excitation provided to surface-bound fluorescent molecules while simultaneously enhancing fluorescence emission collection efficiency. It is reported that PC surfaces provide a consistent and highly efficient platform for the enhancement of fluorescence by heightening the excitation field using their optical resonance properties [33]. Therefore, it is vital to investigate the interactions of the PC system with dye molecules in the interface. A novel phenomenon of aggregation-induced emission (AIE) was first found by Tang’s group [34]. The emission of organic dyes (e.g. propeller-like siloles or silacyclopentadiene derivatives, hexaphenylsilole, and tetraphenylethene) was weakly or non-emissive in solution but became highly luminescent when aggregated in solutions or cast into solid films. The weak or non-emission of silole molecules in solution is attributed to the active intramolecular rotations of multiple phenyl rings on their periphery, whereas the intense luminescence in the condensed phase is caused by the restriction of intramolecular rotations [35]. Inspired by their work, we 4

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