Electrothermally Driven Fluorescence Switching by ... - ACS Publications

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Electrothermally Driven Fluorescence Switching by Liquid Crystal Elastomers Based On Dimensional Photonic Crystals Changxu Lin,†,‡,⊥ Yin Jiang,‡,∥,⊥ Cheng-an Tao,§ Xianpeng Yin,‡ Yue Lan,‡ Chen Wang,‡ Shiqiang Wang,‡ Xiangyang Liu,*,† and Guangtao Li*,‡ †

Research Institute for Biomimetics and Soft Matter, Fujian Provincial Key Laboratory for Soft Functional Materials Research, College of Physical Science and Technology, Xiamen University, 361005 Xiamen, P.R. China ‡ Key Lab of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China § College of Science, National University of Defence Technology, Changsha 410073, P. R. China ∥ Beilun Science and Technology Bureau, Ningbo, 315800, P. R. China S Supporting Information *

ABSTRACT: In this article, the fabrication of an active organic−inorganic one-dimensional photonic crystal structure to offer electrothermal fluorescence switching is described. The film is obtained by spin-coating of liquid crystal elastomers (LCEs) and TiO2 nanoparticles alternatively. By utilizing the property of LCEs that can change their size and shape reversibly under external thermal stimulations, the λmax of the photonic band gap of these films is tuned by voltage through electrothermal conversion. The shifted photonic band gap further changes the matching degree between the photonic band gap of the film and the emission spectrum of organic dye mounting on the film. With rhodamine B as an example, the enhancement factor of its fluorescence emission is controlled by varying the matching degree. Thus, the fluorescence intensity is actively switched by voltage applied on the system, in a fast, adjustable, and reversible manner. The control chain of using the electrothermal stimulus to adjust fluorescence intensity via controlling the photonic band gap is proved by a scanning electron microscope (SEM) and UV−vis reflectance. This mechanism also corresponded to the results from the finite-difference time-domain (FDTD) simulation. The comprehensive usage of photonic crystals and liquid crystal elastomers opened a new possibility for active optical devices. KEYWORDS: photonic crystals, liquid crystal elastomers, electrothermal driven, fluorescence switching, nanostructured materials

1. INTRODUCTION Fluorescence has been widely utilized in detection and imaging due to its advantages such as sensitivity and efficiency, and the availability of organic dyes with diverse spectral properties. Fluorescence switches, which turn external stimuli (e.g., photo,1,2 electrical,1,3 mechanical,4 thermal,5 pH,6,7 and specific chemical8,9) into the change of fluorescence intensity, have attracted much attention. A most direct pathway for switching is the introduction of responsive units by the covalent or noncovalent connection with fluorophores. Via responsive units, external stimuli change the spatial organization or the energy distribution of fluorophores.10 Most of these strategies required an inevitable molecular design and time-consuming © XXXX American Chemical Society

synthesis process for a functional structure, and an even more subtle control over the balance between thermodynamics and dynamics in the self-assembly.11,12 Looking beyond the modification of fluorophores themselves, researchers make the manipulation of generated fluorescence emission photons a more important breakthrough direction. The enhancement factor is adjustable by various means to contribute in this direction. Two main enhancement mechanisms have been developed to further amplify the fluorescence signals as the Received: December 6, 2016 Accepted: March 15, 2017 Published: March 15, 2017 A

DOI: 10.1021/acsami.6b15619 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Scheme 1. Schematic Image of Organic/Inorganic Hybrid 1D-PC by Alternating Spin-Coating of TiO2 Suspensions and Liquid Crystal Polymer Precursors, and Its Mechanism of Electrothermally Driven Regulation of the EF of Organic Dye’s Fluorescence Intensity to Realize Fluorescence Switching

fields.36 Among these, electric field is one of the most easily controllable stimulus sources to make responsive PCs applicable in practical optical instruments. On the other hand, constituents and the arrangement in the fabrication of particular PC systems should be chosen to offer appropriate driving patterns and to maximize the response. The organic/inorganic hybrid 1D-PCs have good balance on stability and responsiveness, unlike too fixed all-inorganic 1D-PCs or all-polymer/ organic 1D-PCs with too small a refractive-index difference and too complicated fabrication processes. They also had excellent benchmark optical properties resulting from the large difference of refractive indices. The liquid crystal elastomers combine the orderliness of liquid crystals and the elasticity of rubber37 and form an anisotropic polymer network responding to external stimuli, such as heat,38 light,39 and electric fields.40 Their shape changing ability and corresponding reversibility make them capable of fabricating soft actuators, sensors and artificial muscles,33,41 and responsive PCs in this case. Titania was widely used in the inorganic layers of hybrid 1D-PCs based on its higher refractive index difference and easy film forming ability with spin-coating. Examples of organic/inorganic hybrid 1D-PC were made with different fabrication methods and were presented with full color range, fast, reversible, and sensitive responses to water vapor.42 Zhang and Yang et. al used 1D-PC to build a pH responsive fluorescence switch for CdSe quantum dots.7 We have shown in our previous work that the PBG and corresponding structural color of LCE-based inverse opaline films could be electrothermally tuned.43 The fast responsive rate and perfect reversibility of the LCE based 3D-PC film in this work supported the LCE to be a suitable candidate material for manipulating the PBG for PC-based optical applications including fluorescence switching. In this work by using the LCE based 1D-PC (LCE-PC), a method for active fluorescence switching was realized with a series of cascade steps starting from applying voltage which was literally forming a control chain. The method combined the optical characteristics of 1DPC and the responsive manner of LCE. It was also more favorable for fluorescent molecules uniformly distributing on the surface of 1D-PC than 3D-PC as in the previous work. As shown in Scheme 1, the temperature changed with the applied voltage on/off through an electrothermal conversion layer from spin-coated graphite nanoparticles. The changing temperature induced reversible deformation of LCEs and caused the PBG of the fabricated LCE-PC to shift. The PBG was shifted to

surface plasmon resonance (SPR)13 and photonic crystal resonance (PCR).14 Other than tuning the electromagnetic field near fluorophores with metallic surface plasmon in the SPR route,15,16 the PCR regulates the propagation of fluorescent photon in a larger scale with photonic crystals (PCs) as guides and mirrors.17,18 The photonic band gap (PBG) of PCs can be designed with multiple resonances and coupled with either fluorescent excitation or emission. The strength of PCR enhancement came from the increased local electric field at the PC surface (enhanced excitation) and the increased collection efficiency of emitted photons (enhanced extraction) and the resulting low energy loss. PCs were first introduced into the fluorescence enhancement by Ye et al. with specific one-dimensional photonic crystals (1D-PCs) with the help of the high local field of light generated by a photonic state localized at the defect layer.19 Fluorescent molecules were later moved onto the surface of 1D-PC for application in fluorescence imaging and biosensing.14 Other types of PC like 3D-PC20,21 and 2D-PC22 had also been reported to serve as the Bragg reflector mirror for enhancement, by matching (or overlapping) their PBG with a fluorescent emission spectrum or joining with SPR mechanisms.23 The accessible problem in distributing the fluorescent molecules of these examples was overcome by overlaying 2DPC on the Ag substrate in our latest work.24 So far, most methods mentioned above could be categorized as the static fluorescence enhancement. They all targeted one predetermined organic dye by using one specific PC structure with a nonadjustable enhancement factor. Given all those intrinsic wavelengths of fluorescent excitation and emission of different organic dyes, it is too tedious to customize a specific PC structure for each combination of excitation and emission. Meanwhile, the development of modern optics has been in great need of fluorescence switches with variable enhancement factors (EFs) in different applications.25 The separation of fluorophores and manipulation units in the PCR mechanism offers the flexibility to incorporate various fluorophores. It is very appealing to provide a versatile switch for different fluorophores by using precast and integrated PC elements under the same strategy. The key step is to make PCs responsive to physical or chemical external stimuli in the regulation of the PBG.26 So far, quite a few external field stimulations have been applied in responsive PCs, such as temperature change,27 pH,7,28,29 mechanical force,30,31 light,32−34 electric field,35 or magnetic B


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ACS Applied Materials & Interfaces Scheme 2. Synthesis Route of the Photo-Crosslinkable Side-Chain Liquid Crystal Polymers

8.6 Hz, 2H; ArH), 6.05 (m, 1H, CH), 5.40 (m, 2H, CH2), 4.64 (d, J = 5.2 Hz, 2H; CH2O). 2.2.4. 4-Methoxyphenyl 4-(Allyloxy)benzoate (M2). A similar synthetic step as that for M1 was followed except for using 4methoxyphenol to replace 4-cyanophenol. 1H NMR (300 MHz, CDCl3, δ): 8.14 (d, J = 8.9 Hz, 2H; ArH), 7.11 (d, J = 8.9 Hz, 2H; ArH), 6.96 (m, 4H, ArH), 6.05(m, 1H, CH), 5.41 (m, 2H, CH2), 4.62 (d, J = 5.2 Hz, 2H; CH2O), 3.82 (s, 3H, OCH3). 2.2.5. (4-(Allyloxy)phenyl)(4-hydroxyphenyl)methanone. A similar synthetic step as that for M1 was performed except for using hydroquinone to replace 4-cyanophenol (the molar ratio of 4-allyloxybenzoyl chloride and hydroquinone = 1:5). 2.2.6. 4-((Trimethylsilyl)oxy)phenyl 4-(Allyloxy)benzoate (M3). Bis(trimethylsilyl)amine (0.392 g, 2.43 mmol) was added dropwise to a 90 °C solution of (4-(allyloxy)phenyl)(4-hydroxyphenyl)methanone (0.5 g, 1.85 mmol) in toluene (20 mL). After addition, the mixture was stirred for 2 h at 90 °C. The desired product was recrystallized from n-hexane to give white crystals (0.54 g, 1.57 mmol, 85% yield). 1H NMR (300 MHz, CDCl3, δ): 8.13 (d, J = 8.9 Hz, 2H; ArH), 7.02 (m, 4H, ArH), 6.86 (d, J = 8.9 Hz, 2H; ArH), 6.05(m, 1H, CH), 5.41 (m, 2H, CH2), 4.62 (d, J = 5.2 Hz, 2H; CH2O), 0.28 (s, 9H, Si(CH3)3). 2.3. Preparation of Photo-Cross-Linkable Side-Chain Liquid Crystals Polymers. To synthesize polymer 1 (Scheme 2), liquid crystal monomers M1 (24 mmol), M2 (24 mmol), 4-M3 (12 mmol), and 2,4,6,8-tetramethyl-cyclotetraslioxane (15 mmol) were placed in a 100 mL round-bottomed flask. 40 mL of purified toluene and 50 μL of 1% (w/w)-Pt cyclooctadiene platinum(II) chloride [Pt(COD)Cl2] in DCM were then added. The mixture was refluxed for 2 h to make sure the hydrosilylation reaction was complete. When the solution cooled down to room temperature, 0.14 mmol of NaOH in 3 mL of ethanol was added, and after stirring uniformly, the mixture was refluxed for another 8 h for ring-opening polymerization and deprotection. Then, the solution was evaporated, and the viscous product was dissolved in DCM. The solution was poured in cold methanol to precipitate the polymer and centrifuged at 6000 rpm for 5 min. This process was repeated three times, and the solvent was then evaporated. The polymer was freeze-dried for 1 day. In the next step for polymer 2, a solution of acryloyl chloride (1.35 mL, 16.60 mmol) was added dropwise to an ice-cooled solution of the obtained polymer above and trimethylamine (2.35 mL, 16.75 mmol) in DCM (10 mL). After addition, the mixture was stirred for 12 h at room temperature, and the solvent was evaporated. The residue was dissolved in DCM, and then the solution was poured in cold methanol to precipitate the photo-cross-linkable polymer and centrifuged at 8000 rpm for 5 min. The final product was dissolved in a small amount of toluene for fabrication of the 1D-PC. 2.4. Preparation of TiO2 Suspensions. The TiO2 suspensions were synthesized using a procedure based on the hydrolysis of the TPT, followed by peptization of the precipitates and particle growth under hydrothermal conditions.44 Briefly, 20 g of TPT was added to 36 g of deionized water and stirred for 1 h. The fast TPT hydrolysis

mismatch or match with the emission spectrum of a particular organic dye mounted on the surface of PC film. The mismatch lowered the EF and turned off the fluorescence virtually, while restoration of the match status turned on fluorescence.

2. EXPERIMENTAL SECTION 2.1. Chemicals. Titanium(IV) isopropoxide (TPT), tetramethylammonium hydroxide, 2,4,6,8-tetramethylcyclotetraslioxane, rhodamine B (RhB), azobisisobutyronictrile (AIBN), graphite powders, Pt(COD)Cl2, acryloyl chloride, and triethylamine (TEA) were purchased from Acros. Liquid crystal monomers (4-cyanophenyl 4(allyloxy)benzoate (M1), 4-methoxyphenyl 4-(allyloxy)benzoate (M2), and 4-((trimethylsilyl)oxy)phenyl 4-(allyloxy)benzoate (M3)) were provided by our collaborator. Poly(vinyl alcohol) (PVA), dichloromethaneother (DCM), toluene, dimethylformamide (DMF), tetrahydrofuran (THF), and other chemicals were purchased from Beijing Chemical Co. and used without further purification. Deionized water was further purified with an RF ultrapure water system. The glass substrates were cut into 25 mm × 25 mm pieces, soaked in a mixture of 98% H2SO4/40% H2O2 (volumetric ratio 7:3) overnight, then rinsed with ethanol several times, and finally dried with an N2 stream. 2.2. Synthesis of the LC Monomer. 2.2.1. 4-Allyloxy Benzoic Acid. 4-Hydroxybenzoic acid (4.05 g, 29.4 mmol), allyl bromide (5.35 mL, 61.5 mmol), and potassium carbonate (10.0 g, 72.5 mmol) were suspended in DMF (30 mL) and stirred for 48 h. Then, water (10 mL) and dichloromethane (25 mL) were added, and the layers were separated. The aqueous layer was extracted three times by DCM (30 mL). The combined organic layers were dried over Na2SO4, and the solvent was evaporated in vacuo. To the residue, 2 N NaOH (50 mL) and ethanol (10 mL) were added, and the solution was heated to reflux for 5 h. A pH of 1 was obtained by conc HCl, and the precipitated solid was filtered, washed with ice-cooled water−methanol 5:1, and dried in vacuo over CaCl2 to give the allyl ether (4.7 g, 26.4 mmol, 90% yield). 2.2.2. 4-Allyloxy-benzoyl Chloride. 4-Allyloxy benzoic acid (3.2 g, 18.0 mmol) was suspended in thionyl chloride (25 mL) and heated to reflux for 8 h. Thionyl chloride was removed in vacuo. Toluene (20 mL) was added, and the solvent was removed in vacuo again. This procedure was repeated twice. The obtained 4-allyloxy-benzoyl chloride (2.8 g, 14.3 mmol, 80% yield) was directly used in the next reaction step. 2.2.3. 4-Cyanophenyl 4-(allyloxy)benzoate (M1). 4-Cyanophenol (2.0 g, 16.8 mmol), triethylamine (15 mL), and 4-allyloxy-benzoyl chloride (3.3 g, 16.8 mmol) were suspended in THF and stirred for 2 h at 0 °C and another 2 h at room temperature. After the reaction, a large amount of deionized water was added to the mixture to precipitate the solid. The desired product was filtered and purified using column chromatography to yield 2.8 g (60% yield) of product. 1 H NMR (300 MHz, CDCl3, δ): 8.13 (d, J = 8.9 Hz, 2H; ArH), 7.73 (d, J = 8.6 Hz, 2H; ArH), 7.35 (d, J = 8.6 Hz, 2H; ArH), 7.01 (d, J = C


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Figure 1. (A) FT-IR spectra of liquid crystal polymer 1 (red) and polymer 2 (black) reacting with acryloyl chloride (see Scheme 2); (B) DSC of the as prepared photo-cross-linked liquid crystal elastomer film. g, glass phase; n, nematic phase; and i, isotropic phase. produces a white solid. The precipitate was filtered and washed several times with deionized water. Next, the solid was placed in a Teflon reactor and dispersed in 4.0 mL of 0.6 M tetramethylammonium hydroxide. Peptization takes place by heating in an oven at 120 °C for 3 h. After this, a colloidal suspension of anatase crystallites was obtained. Further centrifugation at 14000 rpm for 10 min was carried out to eliminate large aggregates. 2.5. Preparation of the Graphite Layer. The electrothermal conversion layer based on conductive graphite was obtained by spincoating a 10% (w/w) dispersion of graphite nanoparticles in ethyl acetate on a glass substrate (20 s, 1000 rpm). The thickness of the graphite layer in need could be controlled by the concentration of the dispersion or the rotational speed of a spin coater. The electrical leads were attached onto both ends of the graphite layer (Scheme 1). When electrical voltage is applied between the two leads connected at the graphic layer, the ohmic heating produced in the graphite layer can actuate the LCE. 2.6. Fabrication of Organic/Inorganic Hybrid 1D-PC. In order to get films of different thickness, the TiO2 suspensions were diluted with different volumes of deionized water, and the polymer solution was diluted with different volumes of toluene (containing a small amount of the initiator AIBN) before use. The 1D-PCs were fabricated by spin-coating the TiO2 suspensions and the polymer precursor alternately, and both the TiO2 suspensions and the polymer precursor were spin-coated at 3000 rpm for 30 s onto a glass substrate. Each polymer layer was polymerized under UV-irradiation (365 nm, 1.38 mW/cm2 at 20 cm2) for 1 h. Following a similar approach reported in the literature,45 the dye molecules were mixed with the PVA solution (the concentration of RhB is about 10−5 M) and placed onto the surface of 1D-PC by spin-coating at 2000 rpm for 30 s. 2.7. Characterization. 1H NMR spectra were recorded on a JEOL JNMECS 300 MHz instrument using tetramethylsilane as an internal reference. SEM images were obtained with a field-emission scanning electron microscope (LEO-1503, Germany), after the samples were sputtered with a thin layer of gold. The spin-coater was a Laurell WS400-6NPP/LITE. Reflectance spectra in the range of 400−750 nm were collected by use of an Ocean Optics USB2000 fiber-optic diodearray dual-channel spectrometer interfaced with an ocular tube of an Olympus BX51 binocular microscope by fiber optics using the microscope’s light source (Olympus U-LH100-3) and a 20× working distance objective (Olympus 20×/0.45 MPLanFL N). Fluorescence spectra were measured by a LS 55 spectrofluorometer (PerkinElmer). Fourier transform infrared (FT-IR) spectra were collected with an IFS66v/S FT-IR spectrometer (Bruker). The phase transition behaviors were examined by differential scanning calorimetry (DSC 2910) at heating rates of 10 K·min−1. The voltage was applied using a function generator Katherine 4200. 2.8. FDTD Simulation. In the simulation, the refractive indices of meso-TiO2 and LCEs were set as 2.45 and 1.5, respectively. For the 1D-PC reflectance simulation, the simulation region was 50 × 50 ×

3000 nm3, and an override mesh region with 1 nm resolution involving the 1D-PC structure was created to give a more accurate calculation result. A plane-wave source was set at 1 μm from the top of the structure incident along the -Z axis. The boundary conditions of X, Y, and Z were periodic, periodic, and PML, respectively. The simulation time was set to 1000 fs. Meanwhile, a time monitor indicated that 1000 fs is enough. For the fluorescence enhancement simulation, a dipole source was used as an excited dye molecule. The boundary conditions of X, Y, and Z were block, block, PML, respectively. The dipole source was set 110 nm above the 1D-PC for the dye molecule dispersed in ca. 220 nm PVA film upon the structure.

3. RESULTS AND DISCUSSION 3.1. Structural Characterization of LCE-PC. Prior to the structural characterization, the synthesis of polymer 1 and the polymer 2 was characterized. The acryloyl moiety was grafted on the side chain hydroxyl of polymer 1 for photo-cross-linking. The process was verified on the FT-IR spectra by the complete disappearrance of the hydroxyl stretching at 3440 cm−1 after the graft reaction between polymer 1 and acryloyl chloride (Figure 1A). In our work, we found that the polymer synthesis worked well, and the used conditions cannot harm the aromatic esters of the mesogens. The degree of polymerization was determined by GPC and is ca. 15.7. The preservation of ester units during polymerization was confirmed by the FT-IR data (Figure 1A). The characteristic stretching vibrations of CO and C−O of esters in the resulting polymer (polymer 1) were observed at 1750 cm−1 band and 1250 cm−1, respectively. In our case, the typical broad hydrogen-bonding band at ca. 3000 cm−1 as well as the broad absorption shoulder at 2700−2500 cm−1 for the carboxyl group was not detected. The transition temperature of LCE was identified in the DSC curve. They were 31 °C from the glass phase to the nematic phase and 130 °C from the nematic phase to isotropic phase (TNI) (Figure 1B). The fluctuation after the isotropic state came from the baseline correction when the DSC instrument switched to the temperature reducing section. Active 1D-PC was constructed by spin-coating the TiO2 suspension in deionized water and liquid crystal polymer precursor in toluene solution alternatively. The basic spincoating parameter was 3000 rpm for 30 s. Films of different thicknesses were obtained by varying the initial concentrations of solutions/suspensions. Each polymer layer was polymerized under excess UV-irradiation (365 nm) for 1 h. The first layer of each bilayer was the LCE polymer, and the last layer of it was titania in all experiments, and the total number of bilayers was D


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Figure 2. (A) Cross-sectional SEM images and (B) magnified image of a 1D-PC with eight layers.

N. The structure of obtained LCE-PC was exhibited by the cross-sectional images of the layered structures in the SEM. As shown in the cross-sectional SEM image in Figure 2A, four bilayers of the 1D-PC were identified clearly. The thickness of the LCE layer or TiO2 layer was almost the same in each bilayer, and the multilayered structure was uniform over a large area. The average thicknesses of the LCEs layer and titania layer were calculated to be 208 and 85 nm from the magnified image, respectively (Figure 2B). The position of the maximum Bragg-diffraction (λmax), the reflectivity (R), and the PBG width (Δω) of 1D-PC complies with the following equations:46 mλmax = 2(nLhL + nHhH) ⎡ n − n (n /n )2N ⎤2 s L H ⎥ R=⎢ 0 ⎣ n0 + ns(nL /nH)2N ⎦

Δω =

4ωmax −1⎛ |nL − nH| ⎞ sin ⎜ ⎟ π ⎝ nL + nH ⎠

(1) Figure 3. Optical properties of the 1D-PC with total numbers of bilayers (N).

(2)

wt %) gave λmax at 438, 460, 532, 564, 620, and 660 nm. Similarly by single variation, the concentration of TiO2 suspensions in preparation structure colors (along corresponding λmax) of violet (478 nm), green (539 nm), yellow (571 nm), and red (644 nm) were obtained across the full visible light spectrum (Figure 4B). The feasible thickness controllability of both titania and LCE layer offered a two-pronged solution to obtain virtually any color in need. Additionally, the bandwidth was broadening with the thickness of either the LCEs or the titania layer increasing, which was a direct result of the proportional correlation between the PBG width and λmax from eq 1 and between the λmax and the layer thicknesses from eq 3. The uniformity of the 1D-PC structure color covered a large area of at least 25 mm × 25 mm, and this was maintained for months. 3.2. Characterization of Optical Property of the Film. In our work, the graphite layer deposited on the surface of the glass substrate was employed to electrothermally drive our LCE-PC device. Following the suggestion of the reviewer, we measured the temperature evolution on the graphite layer using an infrared thermometer when different electrical voltages were applied. Basically, the increasing temperature from direct heating and that from the electrothermal transition of the graphite layer had no difference in essence. As shown in Figure S1, the temperature on the graphite layer increases nearly lineally with the increase of the voltage applied. When 35 V is applied, the temperature reaches 130 °C, at which the phase transition of the prepared LCE from the nematic phase to isotropic phase (TNI) occurred, as shown in the DSC

(3)

where ωmax = 2πc/λmax with c being the speed of the light in vacuum, m the diffraction order, hL and hH the thicknesses of the low-refractive-index and high-refractive-index materials, and nL and nH the respective refractive indices. n0 and ns are the refractive indices of the surrounding environment and of the substrate, respectively. N is the number of bilayers. From the equations above, it is known that the optical properties of 1D-PC were determined by the number of bilayers, the refractive indices of the two components and the physical thicknesses. UV−vis reflectance of samples containing a stepwise increase of bilayer number N are shown in Figure 3. The reflectance grew higher, and the photonic bandwidth became narrower with increasing N at the same time, which is in good agreement with the previous theory describing the thickness dependence of the optical response of photonic crystal slabs.47 Equation 1 reveals the correlation between λmax and the thickness of any layer (or the period). So by selecting the proper period of alternative refractive indices, the maximum wavelength of the Bragg-diffraction peak can be adjusted across the full visible spectrum range. In our work, the thicknesses of the layers were readily controlled by manipulating either the concentration of two precursor solution/suspension of the deposition. As shown in the reflectance spectra (Figure 4A), the single variation of thickness of the LCEs layer by controlling the concentration of the LCP precursor solution (from 5 to 10 E


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Figure 4. (A) Photographs and the corresponding normalized reflectance spectra of 1D-PC with a fixing thickness of the titania layer and different thicknesses of LCEs layers (from 5−10 wt % of LCP toluene solution). (B) Photographs and the corresponding normalized reflectance spectra of 1D-PC with a fixing thickness of the LCE layer and different thicknesses of titania layers (from 3−12 wt % of TiO2 suspension).

the LCE was located between TiO2 layers, it is found that probably due to the confinement effect applying lower voltage cannot induce obvious change in optical properties (stopband) (Figure 5). Only when 35 V was used to reach the clearing temperature of the elastomer, a large shift of the stopband was observed. Additionally, we also found that the phase transition of LCE could not immediately induce actuation and that the observed shift of the stopband took more than 10 s to reach a stable position (blue line in Figure 5). Thus, it is possible to monitor the shift around the clearing temperature, as shown in Figure 5 (green and red lines). It was noteworthy that the thin LCE layers were cramped between two hard inorganic layers, so they showed a much smaller shape deformation than their performance in bulk state. 3.3. Relationship between Fluorescence Enhancement Factor and Photonic Band-Gap. The RhB was chosen as a representative dye to investigate the relationship of fluorescence enhancement and the PBG of the LCE-PC. Taking advantage of the 1D-PC structure, the RhB was also placed onto the surface of as made LCE-PC films by simply spin-coating. Our discussion was based on the comparison of the FDTD simulation data and real results of LCE-PC and final RhB/LCE-PC. LCE-PC samples with different λmax in need were fabricated by controlling the continuously adjustable precursor concentration mentioned above. Examples were LCE-PC-480 (denoted for PC of λmax = 480 nm, so is later ones), LCE-PC-540, LCE-PC-580, LCE-PC-620, all with similar 50% reflectivity. Their reflection spectra are shown in Figure 6A. The reflection spectra calculated from the FDTD method based on the experimental parameters of the corresponding samples in Figure 6A are mentioned above (Figure 6B). The correspondence of simulated and experimental λmax was good at the first order Bragg diffraction peaks. The higher secondary or tertiary diffraction of real samples was weaker than the related simulated results due to the unavoidable quality loss from defects in fabrications. On the basis of these results, the RhB was placed on the surface for fluorescence switching (denoted as RhB/LCE-PCλmax in Figure 6C), and RhB showed the different fluorescence emission intensities on the surface LCE-PC of different λmax wavelengths. For the RhB/LCE-PC-580 whose λmax matched

measurement (Figure 1B). Indeed, when 35 V was applied on our LCE-PC device, a large jump of the stopband was observed (Figure 5). Liquid crystal elastomer films have huge

Figure 5. Normalized reflectance spectra of LCE-PC tuned reversibly by voltage. Probably due to the confinement effect, we found that applying lower voltage (>35 V) cannot induce an obvious change in optical properties (stopband). Only when 35 V was used to reach the clearing temperature of the elastomer, a large shift of the stopband was observed.

deformability after being heated above the nematic−isotropic transition temperature (TNI), and this shape change process is reversible. In our system, liquid crystal moieties in LCEs layers are most likely to align perpendicular to the film surface (approximately homotropic) after spin-coating and polymerization on the substrate. When a voltage of 35 V was applied to the fabricated 1D-PC to heat LCE layers and approach their TNI, the liquid crystal moieties turned isotropic, and LCE layers started to contract along the nematic director axis of liquid crystals. Then, the reduction of thicknesses of the LCEs layers caused a rapid blue shift of the λmax (as shown in Figure 5, the shift is almost 40 nm from 560 to 520 nm). Once the voltage was turned off, the thicknesses of the LCEs layers and the λmax recovered to their initial states. The discussion above had been simplified by excluding the effect of the refractive index change of the LCE layers during electrothermal manipulation. Because F


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Figure 6. (A) Experimental reflection spectra of as-prepared 1D-PC with specified λmax. (B) The corresponding calculated spectra were produced by the FDTD method. (C) The fluorescence emission spectra of RhB loaded PVA films coated on surfaces of 1D-PC (with specified λmax) and glass. (D) FDTD simulated enhancement factors of the fluorescence intensity of RhB on 1D-PC with specified λmax; the intersections of the straight line and curves are the relative RhB EFs by using 1D-PC with different λmax values.

Figure 7. (A) Fluorescence intensity of RhB/LCE-PC switched reversibly by tuning the voltage on/off. (B) Normalized reflectance spectra of LCEPC changed reversibly by tuning the voltage on/off. Inset photographs in A, a and b are in situ optical images from the microscope with different fluorescence intensities. The voltage applied was 35 V.

PC should also provide a better effect of fluorescence enhancement. The fluorescence intensity (IF) of the fluorophore can be described as follows:48

the maximum emission wavelength of RhB, a 7-fold enhancement appeared at the maximum emission wavelength of RhB/ PVA on the LCE-PC compared with the control sample (RhB/ PVA film on glass). Similarly, about 5-fold, 4-fold, and 3-fold fluorescence enhancements had been obtained on the LCE-PC620, LCE-PC-540, and LCE-PC-480, respectively. The fluorescence enhancement was positively correlated to the matching degree between the PBG of LCE-PC and fluorescence emission of rhodamine B. It served as the fundamental of the fluorescence switching. Furthermore, with increasing the bilayer number N, the higher reflectivity of 1D-

IF = IexeQEηext

(4)

where Iexe is the intensity of the excitation field, QE is the quantum efficiency of the fluorophore, and ηext is the extraction efficiency. EF is defined as the ratio of the fluorescence intensity observed from a molecules-PC system (IF) to that from molecules absorbed on a glass substrate (IF,0). In our G


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ACS Applied Materials & Interfaces system, metal nanoparticles did not exist, and the fluorophore (RhB) was uniformly distributed in the PVA film instead of just absorbing on the 1D-PC surface, so the QE and Iexe were almost the same value and could be removed. The fluorescence enhancement was mainly attributed to enhanced emission generated on the surface of 1D-PC, which acted as the dielectric mirror for the emission propagation. Equation 5 is derived from eq 4. EF =

IexeQEηext η IF = ≈ ext IF ,0 Iexe ,0QEOηext ,0 ηext ,0

provide an additional enlargement of tuning range of the fluorescence intensity. The reversibility of electrothermally driven fluorescence switching was further verified. Modulated fluorescence emission intensity of the RhB/LCE-PC-580 surface during four voltage on−off cycles is given in Figure 8. No loss of switching effect

(5)

On the basis of eq 5, the enhancement factor was calculated by the FDTD method. As shown in Figure 6D, the intersections of the straight line and curves represented the relative EFs of RhB by using 1D-PC with different Bragg diffraction peaks, and the calculation results (EFc,λ) generally had the same trend with the experimental data (EFe,λ). In detail, a reducing sequence of EFc,580 > EFc,620 > EFc,540 agreed with the experimental data, only with an exception of EFc,480, which was approximately equal to EFc,540. The EFc,480 was smaller than EFe,540, which meant the experimental EFe,480 was smaller than the calculated EFc,480. Either the calculated or experimental EF480 was mainly caused by the ηext in the secondary Bragg diffraction peak unlike the other EFs (ηext in λmax). The differences between the experimental and calculated results were calculated to the higher secondary Bragg diffraction peaks in simulation results. Experimental and simulation results both evidenced that the EF of fluorescence intensity can be regulated in our system through selecting the active LCE-PC of specific λmax. 3.4. Electrothermally Driven Fluorescence Switching. On the same theoretical basis, fluorescence switching had been achieved by electrothermally tuning EF through the linkage of the λmax of PBG to EF. As shown in Figure 7A, the RhB/LCEPC-580 sample which had the highest EF performance was chosen to verify the feature. Once the voltage was applied on the LCE-PC, the λmax of its PBG exhibited a blue shift of the λmax from 580 to 540 nm within 15 s of response time (Figure 7B). For the RhB/LCE-PC, the shift led the fluorescence emission intensity to decrease to about the 20% level. The fluorescent optical photo exhibited the corresponding dimming of RhB original excited emission (Figure 7A, inset). After turning off the voltage, the EF was restored. In this process, voltage was first converted into heat by the graphite conversion layer. The temperature of the sample was brought up close to the TNI temperature of liquid crystals at 130 °C when they became isotropic. The liquid crystal started to contract along the nematic director axis. Along with the reflection spectra shifting to the limit, the fluorescence enhancement of RhB was tuned down to the lowest EF. The key factor of the fluorescence EF change was the matching degree of the emission spectra of RhB to the continuous adjustable λmax of LCE-PC. After turning off the voltage, the thicknesses of the LCEs layers and the λmax could restore to the initial state, delivering the restored EF. This voltage on−off cycle virtually induced a fluorescence switching operation which was reversible over a large range. It was noteworthy that the fluorescence intensity of photograph b (λmax blue-shifted to 540 nm when the voltage was turned on) was much weaker than that in the original λmax = 540 nm system (shown in Figure 6C). This deviation came from the simultaneous fluorescence intensity loss of RhB with increasing temperature.49,50 The temperature factor became a complementary measure to

Figure 8. Modulated fluorescence emission intensity of the RhB on the 580 nm 1D-PC surface during four voltage on/off cycles.

appeared in these cycles offered. In addition, the electrothermal conversion interface layer based on graphite in our system had rapid temperature response to the applied on/off voltage, providing the capability to change fluorescence intensity in tens of seconds.

4. CONCLUSION In conclusion, an active and reversible approach to electrothermal fluorescence switching was realized. A fluorescent 1DPC film was constructed from alternative spin-coating of TiO2 and liquid crystal elastomers and finally a layer of rhodamine B. On the basis of this structure and the liquid crystal responsiveness of LCEs, the photonic band gap of 1D-PC was tuned by applying voltage to it via the electrothermal conversion on the graphite layer. For switching the fluorescence from rhodamine B, the λmax of PBG of 1D-PC was accurately set to 580 nm by selecting layer thickness by changing the concentration of precursors. The electrothermally induced change of λmax was about 40 nm for this sample. On the basis of the change, the enhancement factor was lowered to the switch off status with the mismatching between the photonic band gap and fluorescence emission of rhodamine B. The FDTD simulation of PBG and fluorescence emission revealed a good coincidence with the experimental results, offering us a more theoretical design ability with the 1D-PC switching system. The reversible characteristics of the switching system were also verified. It is believed that this electrothermally driven fluorescence switching system may lead to applications in optical imaging and storage devices. Further work will focus on two or three organic dyes with different emission wavelengths alternating enhancement and switching of their fluorescence intensity by using just one piece of this active LCE-PC film.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b15619. H


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Measurement of the temperature evolution on a graphite layer using an infrared thermometer when different electrical voltages were applied (PDF)

AUTHOR INFORMATION

Corresponding Authors

*(X.L.) E-mail: [email protected]. *(G.L.) E-mail: [email protected]. ORCID

Changxu Lin: 0000-0001-6017-9872 Guangtao Li: 0000-0003-4127-1715 Author Contributions ⊥

C.L. and Y.J. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by National Science Foundation of China (No. 21473098, 21121004, 21421064, 5750873051, 205333050, 21403077, and U1405226), MOST (2013CB834502), and Transregional Project (TRR61), the “111” Project (B16029), the Fujian Provincial Department of Science & Technology (2014H6022), and the 1000 Talents Program from Xiamen University. We thank Professor Hong Tang for offering the 4-((trimethylsilyl)oxy)phenyl 4-(allyloxy) benzoate.

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