2 months ago

Tunable PL increase_3DFe3O4 NPs_Hu_Nanotech_2016

Nanotechnology Nanotechnology 27 (2016) 245709 (7pp) doi:10.1088/0957-4484/27/24/245709 Tunable fluorescence enhancement based on bandgap-adjustable 3D Fe 3 O 4 nanoparticles Fei Hu, Suning Gao, Lili Zhu, Fan Liao, Lulu Yang and Mingwang Shao Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou, 215123, Jiangsu, People’s Republic of China E-mail: and Received 6 January 2016, revised 19 March 2016 Accepted for publication 19 April 2016 Published 11 May 2016 Abstract Great progress has been made in fluorescence-based detection utilizing solid state enhanced substrates in recent years. However, it is still difficult to achieve reliable substrates with tunable enhancement factors. The present work shows liquid fluorescence enhanced substrates consisting of suspensions of Fe 3 O 4 nanoparticles (NPs), which can assemble 3D photonic crystal under the external magnetic field. The photonic bandgap induced by the equilibrium of attractive magnetic force and repulsive electrostatic force between adjacent Fe 3 O 4 NPs is utilized to enhance fluorescence intensity of dye molecules (including R6G, RB, Cy5, DMTPS-DCV) in a reversible and controllable manner. The results show that a maximum of 12.3-fold fluorescence enhancement is realized in the 3D Fe 3 O 4 NP substrates without the utilization of metal particles for PCs/DMTPS-DCV (1.0 × 10 −7 M, water fraction ( f w ) = 90%). S Online supplementary data available from Keywords: fluorescence, iron oxides, photonic crystal, tunability (Some figures may appear in colour only in the online journal) 1. Introduction Fluorescence-based detection is vitally significant in a wide range of applications from environmental monitoring to disease diagnostics, owing to its high sensitivity, selectivity, availability and cost-effectiveness. In order to expand its advantages, increasing the luminescence of molecular materials, help is being sought from various enhancement methods, such as metal-enhanced fluorescence [1, 2], photoinduced fluorescence enhancement [3, 4], fluorescence resonance energy transfer [5, 6] and surface plasmon resonance [7, 8]. All the above methods are effective for fluorescence enhancement. Yet, the involved experimental processes are usually complicated or lack uniformity over large areas. In addition, when metal nanostructures (metallic thin films [9, 10] and nano-particles [11]) are utilized to enhance local field intensities with surface plasmon modes, photo quenching is an inevitable defect on metallic surfaces, which greatly reduces the photo-stability and increases absorption of energy through non-radiative processes. While a spacing layer (∼10 nm) is usually needed to reduce the quenching effect, the signal enhancement factor will also decrease. Finally, the effect of such solid-based enhancement is nonadjustable, restricting their practical applications. Photonic crystals (PCs) were first defined by Yablonovitch and John in 1987 [12, 13]. These materials can modulate spontaneous emission of embedded optically-active materials by using the photonic bandgap due to their unusual light manipulation properties [14–17]. Light propagates at reduced group velocity owing to resonant Bragg scattering near the bandgap of a PC, capable of enhancing optical gain and leading to stimulated emission [18, 19]. Enhanced fluorescence based on PCs, typically comprising a 1D or 2D periodic surface structure, also occupied a special position because PC resonators are composed of 0957-4484/16/245709+07$33.00 1 © 2016 IOP Publishing Ltd Printed in the UK

Nanotechnology 27 (2016) 245709 FHuet al The results showed that the coupling of fluorescence emission and the unique modulated bandgap of the colloidal PCs yielded substrates with controllable fluorescence enhancement and good reproducibility. Changing the externally applied magnetic field modifies the inter-particle distance (d) of the PC and results in various bandgap positions, which overlaps with the emission peak of the fluorophores. Consequently, fluorescence enhancements with four organic dye molecules (diverse emission wavelength) were obtained due to the wide range tunability (nearly covering the whole visible spectrum) and a maximum of 12.3-fold fluorescence enhancement was realized for PCs/DMTPS-DCV (1.0 × 10 −7 M, water fraction ( f w ) = 90%). 2. Experimental section 2.1. Materials Figure 1. Representative SEM images of magnetite Fe 3 O 4 NPs at low (a) and high (b) magnifications. The average diameter of the Fe 3 O 4 NPs is 108 ± 6.0 nm by measuring 80 NPs. A typical TEM image of a Fe 3 O 4 NP with a diameter of 108 nm (c) and (d) HRTEM image taken from the bordering of the NP. dielectric materials and do not quench fluorophores close to their surface by resonant energy transfer [20]. In addition, the possibility of tuning the photonic bandgap and precisely controlling the PCs is of particular interest [21], which can be further utilized to enhance the fluorescence emission in a controllable manner. Recently, Rezek et al realized enhanced fluorescence extraction by periodical nanopatterning of the film’s surface into a form of a 2D photonic crystal, which shows approximately 6-fold enhancement of the vertical PL intensity [22]. Fornasari et al demonstrated a ten times fluorescence enhancement by using an all-polymer 1D photonic crystal structure capped with a fluorescent organic film [23]. For enhanced fluorescence substrates combined with non-metallic and photonic crystals, there is still much room for improvement of the enhancement factor. Iron oxide colloids, one class of photonic bandgap materials, have been recently found to form magnetically induced self-assembled periodic structures [24, 25]. Significantly, Yin et al have developed a series of work based on monodispersed colloidal building blocks, which have been applied in biochemical sensors, colour paints and inks and reflective display units [26, 27]. The optical response of magnetic colloidal PCs can be conveniently tuned by changing the strength of the external magnetic field. These excellent works gave us the inspiration to tunably enhance the fluorescence emission of dye molecular materials in the liquid by 3D PC based on Fe 3 O 4 nanoparticles (NPs). It was a simple, effective and practical strategy to tune fluorescence intensities fast and reversibly. Several organic dyes, including Rhodamine 6G (R6G), Rhodamine B (RB), cyanine dye (Cy5) and three dimethyltetraphenylsiloles dicyanovinyl (DMTPS-DCV), were employed as probe molecules. Ammonium ferric citrate (FeC 6 H 5 O 7 · NH 4 OH), polyacrylic acid (PAA, Mw = 3000), hydrazine hydrate (N 2 H 4 · H 2 O, 85%) and tetrahydrofuran (THF) were all purchased from Alfa Aesar Co. R6G, RB and Cy5 were purchased from Sinopharm Group Chemical regent Co., Ltd (Shanghai, China). DMTPS-DCV was presented to this work by B Z Tang’s group in the Department of Chemistry, Institute of Molecular Functional Materials, Hong Kong University of Science & Technology. Other reagents were of analytical reagent grade and used without further purification. 2.2. Synthesis of Fe 3 O 4 NPs In a typical procedure, 0.45 g FeC 6 H 5 O 7 · NH 4 OH was dissolved in 18 ml water to form a clear solution. Under stirring of an electric mixer, 6 ml PAA was added dropwise. After 1 h, 6ml N 2 H 4 · H 2 O (85%) was added. The final mixture was stirred for another 40 min and subsequently transferred into a 50 ml Teflon-lined stainless-steel autoclave. The autoclave was maintained at 180 °C for 10 h. After it was cooled to room temperature naturally, the black product was collected and washed with distilled water several times, and then dispersed in distilled water. 2.3. Characterization The phase and crystallography of the products were characterized by x-ray powder diffraction (XRD, a Philips X’ pert PRO MPD diffractometer) with Cu Kα radiation (λ = 0.154 06 nm). A scanning rate of 0.05° s −1 was applied to record the pattern in the 2θ range of 20°–80°. The size and morphology of the samples were performed on a Zeiss Supra 55 field scanning electron microscopy (SEM) operating in high vacuum mode at 15 kV accelerating voltages. Fluorescence spectra were measured with a fluoromax-4 spectrofluorimeter and an HR 800 Raman spectroscopy (J Y, France) equipped with a synapse CCD detector and a confocal Olympus microscope. Fluorescence spectra were collected by an objective with a magnification of ×50 (Olympus) with a numerical aperture of 0.90 and an accumulation time of 1 s. 2

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