Luminescent photonic crystals with multi-functionality and tunability

We develop a general method to incorporate aggregation-induced emission luminogens into photonic crystals (PCs) and the resulting luminescent PCs display diverse structural colors in response to water stimulation.


Introduction
In biological systems, the effects of photonic crystals (PCs) have created unique and amazing structural colors, building a colorful world. 1 Many living organisms can exhibit iridescent color or switch different colors by tuning their PC structures for social communication, sexual attraction, or environmental camouage. 2 For example, Pentapodus paradiseus can reversibly alter color from blue to red under different osmotic pressures; 2b Tmesisternus isabellase can change its structural color from gold to red in response to humidity variation; 3 Paracheirodon innesi can convert from cyan, in the normal state, to yellow, in the stressful state; 2a and Furcifer pardalis can switch from green to orange upon excitation. 4 Such structural color changes are critically dependent on external stimulation. One popular approach, energy-based stimuli, for example use photons, heating, pressure and chemicals. On the other hand, coloration can be achieved by exploiting water as a medium-active stimulus, because it is much easier to be obtained by living organisms. However, studies of water-active stimuli are very limited although there have been studies regarding animal evolution. 5 Inspired by Nature, it is believed that such water-active colortuning is of great excitement and would bring a lot of potential applications.
So far, research on articial PCs has been focused on incorporating self-assembled PC structures into a stimuliresponsive matrix (e.g., polymers or hydrogels). 2c,6 In comparison to natural PCs, they are much less smart. A major problem is the single functionality depending on the periodic crystal packing. That is, only structural color changes can be realized by adjusting the packing pattern, which limits their applications in the eld of multi-function materials. Thus it is of great interest to endow PCs with internal functional properties besides such structure-induced properties.
There are many approaches for tuning the structural colors of PCs. One is designed to change the lattice spacing through the application of external stimuli on the matrix. 7 Typical examples are swelling/deswelling hydrogel/PC composites using water, 8 or stretching/releasing PDMS/PC materials via mechanical stress. 9 An alternative method is self-tuning the PCs (i.e., internal tuning), 10 such as swelling/shrinking the hydrophilic layer of the PS-co-PDMAA opal hydrogel. 11 However, both these approaches are rarely applied in one system. Herein, we realized multi-dimensional tuning by exploiting the internalexternal tuning synergy, where both the color and intensity are able to be modulated.
Photonic crystals are known for their ability to control the propagation of light via the photonic band gap (PBG), and displaying the emitted light. 12 The question is whether miraculous phenomena would arise when both reected and emitted light were combined together? However, traditional uorophores are subjected to aggregation-caused quenching (ACQ), where their emissions are weakened in the solid state. Conversely, aggregation-induced emission luminogens (AIEgens) display excellent emission efficiencies in the solid state and have full color emissions. 13 Thus, such AIEgens are the most promising materials for high emitting PCs.
We are interested in fabricating AIEgen-functionalized PC structures, which are promising materials for sensors. Recently, we reported the silole-inltrated SiO 2 inverse opal PC for detecting organic vapors (VOCs). 14 The crystallization of AIEgens was transformed with a spontaneous color change of the PC upon encountering the VOCs. Another example is the use of a silole-based PC lm as a uorescent probe to selectively detect Hg 2+ and Fe 3+ . 15 However, multi-functional and multi-responsive luminescent PCs have so far not been demonstrated.
Here, we report a unique PC system composed of AIEgenbased poly(methyl methacrylate) (PMMA) nanoparticles (NPs), where the color of both the reected and emitted light can be tuned simultaneously in response to water stimulation. This simple and controllable system allows the modulation of singleor multi-emission from only one material. Induced by the wateractive stimulus, the PBG of the PC is bathochromically shied, leading to a red-shi in emission with a narrow full width at half maximum (FWHM). We nd that such a luminescent PC system could respond to both external (e.g., swelling by water) and internal tuning (e.g., swelling by ethanol), where the intensity and wavelength of emission are changed. Thus, it is believed that this system is a good dual-sensor for detecting humidity and alcohol.
Characterization SEM images were collected on a JEOL-7100F scanning electron microscope operating at an accelerating voltage of 20 kV. Photoluminescence (PL) spectra were recorded on a Perkin-Elmer LS 55 spectrouorometer. The reectance spectra were detected on a HR-4000CG-UV-NIR spectrometer.

Synthesis of luminescent photonic crystals
The luminescent PCs were prepared via a modied emulsion polymerization following a literature method. 17 In brief, MMA (3 mL, 47.7 mg) was dispersed into 25 mL water, which contained SDBS (5 mg). The mixture was heated to 80 C with stirring. Then, the AIEgen in THF (2 mL, 2.5 mg mL À1 ) was dropwise added into the above solution. Aer adding K 2 S 2 O 8 (0.06 g in 1.5 mL H 2 O), the polymerization was carried out at 80 C for 1 h. The luminescent nanoparticles were centrifuged and washed with water three times. Finally, the luminescent PC hydrogels were obtained using centrifugation at 12 000 rpm for 15 min.

The external tuning of PCs upon water-active stimulus
Both the band gap and emission peak of the AIEgen-based PCs were bathochromically shied upon adding water. Aer centrifugation, such a PC hydrogel (50 mL) was added into a quartz cell. Then deionized water (0, 25, 50, 75, 100, 125, and 150 mL) was added into the cuvette. Aer sonication for 2 min, the reectance and PL spectra were collected. The same procedure was used for the response to the stimulation of ethanol.

Results and discussion
The AIEgen-loaded PMMA nanoparticles were synthesized via a modied emulsion polymerization. 17 Specically, AIE1 NFTPE  (b) and enlarged image (c) of NFTPE functionalized poly(methyl methacrylate) photonic crystals.
( Fig. 1a), exhibiting yellow-green emission, was dissolved in THF solution, and mixed with hot MMA/SDBS aqueous solution, followed by addition of K 2 S 2 O 8 as initiator. Aer 1 h incubation at 80 C, the solution turned to a light creamy yellow, indicating MMA polymerization. Driven by hydrophobic interactions, NFTPE are encapsulated by PMMA NPs, with the hydrophilic ends of SDBS dissolved in water. The resulting luminescent NPs were washed with water through centrifugation. To induce the PC hydrogel assembly, the puried NPs were then centrifuged at high speed (12 000 rpm), illustrated in Fig. 1a, aer which an opal structure was obtained. Fig. 1b and c show the scanning electron microscopy (SEM) images of the PC aer drying, where the NPs with a uniform diameter of 120 nm are in a face-centered cubic arrangement. It is believed that the uniform size and centrifugation speed of the luminescent NPs play critical roles in making the high performance AIE/PMMA PC. The former is a prerequisite, and the latter is the determining factor. If the speed is lower than 12 000 rpm, the PC cannot form, while large aggregates would occur at higher centrifugation speeds (>12 000 rpm).
The resulting PC hydrogel has a violet color under light, with a narrow PBG, peaking at 460 nm (Fig. 2b, black curve). Because each NP was encapsulated with a layer of aqueous solution, the PC was able to respond to the water stimulation. As shown in the inset of Fig. 2b, the PC exhibited a colour change from violet to blue, green, yellow, orange and nally to red, upon increasing the volume of water from 0 to 150 mL. Fig. 2b demonstrates the reection spectra that reveal the PBGs, positioned at 460, 484, 502, 543, 580, 613, and 618 nm, respectively. The color appearance of the PC can be described using Bragg's law (eqn (1)), where l is the wavelength of the reected light, d is the PC lattice space, and q is the angle between the incident light and diffracting plane. 18 When q is xed, d is a key factor that determines the PBG (i.e., l) of the PC. Usually, the NP size plays an important role in tuning d. For example, in a traditional copolymer PC system, the addition of water enlarges the NP size by swelling the hydrophilic block, resulting in an increase in the PC lattice space. 8a In our system, however, the hydrophobic NFTPE/PMMA NPs were surrounded by SDBS aqueous solution; thus, it is believed that the water only tuned the aqueous medium which expands the PC lattice space (Fig. 2a), and consequently the PBG is bathochromically shied. Moreover, the structure color of the swelled NFTPE/PMMA PC can be returned to violet aer centrifugation (Fig. S2 †). This is because the PC lattice space was reduced by extracting the water.
To our surprise, the luminescent PC has unique emission properties under the stimulation of water. It works as a lter (Fig. 3a), promoting the emission peak of NFTPE to red-shi from 583 to 692 nm, and narrowing the FWHM from 137 to 92 nm ( Fig. 3b-e). As discussed above, such a water-active stimulus resulted in the red-shi of the PBG (Fig. 2b). Thus, we speculate that the PBG of the PC should play a critical role in tuning the AIEgen emission. To understand the effects of the PBG-induced shi, we must carefully check the relationship between the PBGs and the emission spectra under different conditions.
We chose the emission of the NFTPE aggregates formed in a THF/water ¼ 1 : 9 solution as a reference, which emitted at 630 nm (Fig. 3b-e and S1, † blue curve). Before adding water, the emission of the NFTPE/PMMA PC is located at 583 nm, which is blue-shied compared to that of the pure NFTPE aggregates (Fig. S1a †). This may be ascribed to the twisted intramolecular charge transfer (TICT) characteristic of NFTPE. 19 It is well known that such TICT is sensitive to variation in the solvent polarity. Thus, the NFTPE emission is blue-shied from a polar solution (i.e., THF/water ¼ 1 : 9 mixture) to a weak polar one (i.e., PMMA). As shown in Fig. S1a, † the PBG of the PC hydrogel (460 nm, black curve) is at the le of the emission of NFTPE without adding water, far from the emission of AIE1 with minimum overlap. In this case, the PBG shows little effect on the emission shi. On increasing the water content, it is found that the PBG plays an increasingly prominent role in tuning the emission of NFTPE. At a low water content (V adding water ¼ 25 mL), the PBG slightly overlapped with the blue band-edge of the Fig. 2 (a) Schematic illustrating the swelling process of the AIEgenloaded photonic crystals. (b) Photographs (inset) and corresponding reflection spectra of the NFTPE/PMMA photonic crystal hydrogels upon adding different amounts of water: 0, 25, 50, 75, 100, 125, 150 mL (by volume), respectively. The color changed from violet to blue, green, green, yellow, orange and red, respectively. emission, inducing the NFTPE emission at 611 nm (Fig. S1b †). When the water volume was increased from 25 to 75 mL, the PBG was further red-shied which enhanced its intimate contact with the emission spectra ( Fig. 3b and c). The PBG has the ability to inhibit the propagation of photons if their energy is inside the PBG. That is, the emission is quenched in the wavelength range of the PBG. 12,20 For example, in Fig. 3b and c, the emissions around 502 and 543 nm were suppressed. This may be due to (a) the forbidden effect of the PBG which impedes the emitted light from being detected, and (b) the energy transfer where the suppressed energy in the blue-wavelength region (i.e., inside the PBG) is transferred to the low energy region. Therefore, the emission was bathochromically shied from 611 to 633 nm, accompanied by the shrinking of the FWHM from 127 to 108 nm ( Fig. 3b and c). On further increasing the water content to 125 and 150 mL, the PBGs completely overlapped with the emission spectra, resulting in the emissions below 613 and 618 nm being fully suppressed ( Fig. 3d and e). This was due to the intramolecular energy transfer, and the NFTPE/PMMA PC emitted red light at 676 and 692 nm, with the FWHM reduced to 99 and 92 nm, respectively. The PBG-controlled emission allows the selective choice of an arbitrary light from just one material. Importantly, such a PBGinduced lter effect might be capable of modulating the broad emissions of AIEgens, beneting their optoelectronic and biological applications. It is well known some AIEgen materials can change their colors and emission bands due to the transition of the crystalline structure or molecular packing induced by mechanical force or heating. However, in our system, the PBGtuned emission change breaks such a traditional mode, and might open a new door to realize the emission modulation of all AIEgens, not only limited to materials with mechanochromic properties.
Aer normalization, it is clear that the emission peak of AIE1/PMMA PC exhibits a red shi in response to the increase in water level (Fig. 4a). Furthermore, a linear relationship between the emission peak versus the volume of added water was identied, as shown in Fig. 4b, which allows the quantitative detection of humidity. Such PBG-controlled emission shi is attributed to water permeation leading to the swelling of the aqueous medium, which could be regarded as an external stimuli-tuning process.
Since organic solvents can inuence the mobility of PMMA NPs, we studied the inuence of ethanol on the emission of the NFTPE/PMMA PC. In Fig. 4c, the emission of the luminescent PC was red-shied when an ethanol/water mixture (25 mL) containing variable ethanol contents (V% ¼ 0, 25%, 50%, 75%, and 100%, respectively) was added. At a low ethanol level (V% ¼ 0), only the water-active stimuli dominates. With increasing ethanol content, the emission red-shi results from two factors: one is the swelling of the aqueous medium  induced by the water-ethanol synergy (dened as external tuning); the other is the expansion of the luminescent NPs conducted by the ethanol (dened as internal tuning). Although such external-internal synergy-active tuning is gentle (red-shi of 34 nm), a linear relationship between the emission peak and the volume of added ethanol can also be achieved (Fig. 4d). Thus, the moderate and double tuning could be applied to detect the alcohol in wine. In contrast to the wateractive stimuli, in cases where ethanol participated in the tuning, the emission intensities were sharply reduced, especially at a high ethanol level (Fig. 4c). We postulated that this was due to two possibilities: (a) the PC structure might be partially destroyed by ethanol; (b) the aggregated and rigid AIEgen NFTPE might become loose due to the swelling of ethanol, and thus the molecular motion of NFTPE is active, giving rise to a decrease in emission intensity.
With this new understanding, another AIEgen TPE-containing diacrylate (AIE2, Fig. S3a †) was employed to study the PBGcontrolled emission shiing. Because of two active C]C double bonds, AIE2 could chemically crosslink with MMA during the emulsion polymerization, forming luminescent polymer NPs. 16a This behavior is different from AIE1 NFTPE which is only physically incorporated into the PMMA NPs. However, aer centrifugation at high speed, the resulting AIE2/PMMA PC demonstrated a similar phenomenon to NFTEP/PMMA upon stimulation with water. As shown in Fig. S3b †, the reection peak displayed a red shi from 411 to 450, 482, 529, 563, 583 and 600 nm, when the water volume was progressively increased from 0 to 25, 50, 100, 125, 150, and 175 mL, respectively. Consequently, the AIE2/PMMA PC exhibited bright structure colors, varying from violet, to blue, sky blue, green, yellow-green, yellow, and to red (Fig. S3b, † inset).
The aggregates of AIE2 assembled in a THF/water ¼ 1 : 9 solution emitted blue light, peaking at 470 nm (Fig. 5a, blue curve). However, in the absence of added water, the obtained AIE2/PMMA PC emitted at 490 nm (Fig. 5a, red curve), showing a red-shi compared to the pure AIE2 aggregates. In this case, the PBG-induced emission shi has come into play. In Fig. 6a, the reection spectrum of AIE2/PMMA PC overlapped with the blue band-edge of its emission, thus resulting in the light inside the PBG (i.e., below 430 nm) being suppressed. On the principle of intramolecular energy transfer, the emission is bathochromically shied with a decrease in the FWHM from 99 to 88 nm. If destroying the PC structure using a large amount of water, the emission of AIE2/PMMA NPs was recovered to 460 nm (Fig. 5b). This indicates that AIE2 was polymerized with MMA through covalent bonds and uniformly distributed in the PC, and that the synthesized polymer aggregates were more sensitive to the variation in the surrounding environment due to their higher hydrophobicity. 16a To further investigate the effect of the PBG in modulating the emission of AIE2, we added different volumes of water into the AIE2/PMMA PC. With the increase in water content from 25 to 75 and 100 mL, the PBG band (black curve in Fig. 6) shied from high energy to the middle and then to the low energy region of the emission spectra. As a result, the AIE2 emission peak moved to long wavelengths from 529, to 562 and 587 nm, respectively, accompanied by a lessening of the FWHM from 80 to 68 and 66 Fig. 5 (a) The PL spectra of the AIE2 aggregates (blue) formed in THF : H 2 O ¼ 1 : 9 solution, complemented with the fluorescence (red) and reflection spectra (black) of the AIE2/PMMA photonic crystal before adding water. (b) The PL spectra of AIE2/PMMA nanoparticles after destroying the photonic crystal structure. nm (Fig. 6a-c). Such phenomena are consistent with those occurring in the NFTEP/PMMA PC system (Fig. 3). Thus, it is further proved that the PBG plays a dominant role in tuning the AIE emission.
On increasing the volume of added water to 125 mL, an unconventional phenomenon was observed, where the emission of the AIE2/PMMA PC was red-shied to 622 nm, and simultaneously a new peak appeared at 470 nm with a relatively low emission intensity (Fig. 6d). On adding more water into the PC system, the AIE2 emission still split into two peaks. In Fig. 6e and f, for example, the emission was further shied to 637 and 656 nm, and new emissions were observed at 466 and 460 nm, respectively. However, in these two cases, the intensities of the new emission peaks became stronger than those of the shied ones. This observation was in contrast to the NFTPE/PMMA PC system, which only produced an emission red-shi in response to the water stimulation. We realized that such an unconventional phenomenon is dependent on the optical properties of the AIEgens themselves. For AIE1 NFTPE emission, the energy population is concentrated at 500-750 nm, and the PBG is always on the le side of its emission spectrum (Fig. 3), so that the light with little energy population is blocked because of the PBG effect, promoting the red-shi of emission based on the intramolecular energy transfer. However, AIE2 exhibits blue emission, for which the energy population lies at a short wavelength (i.e., 400-580 nm). Thus, when the PBG shied to the right side of the AIE2 emission spectrum, such as 563 nm or even further, it displayed a weak role in suppressing the blue emission, giving rise to the re-emergence of the characteristic peak of AIE2. On the other hand, the emission inside the PBG wavelength was inhibited, resulting in the splitting of the emission spectrum, and the red-shi of emission due to the intramolecular energy transfer. Compared to the blue emission, the relative emission intensity of the shied peak was gradually reduced. This is ascribed to the synergy of the following factors: (a) the red light has no contribution to the emission of AIE2; (b) the PBG suppresses partial red emission; and (c) little energy is transferred. Such a unique phenomenon provides an opportunity to gain two color emissions from a single material by combining the PC properties and AIEgen photophysical properties. Therefore, it is believed that such a luminescent PC as a good intramolecular lter would be able to create a designed multiplex light.
In the presence of the water-active stimuli, the emission change of AIE2/PMMA PC is evidently reected in Fig. 7a. The emission peak and the water volume perfectly conform to a linear relationship (Fig. 7b). We also found that the AIE2/ PMMA PC was capable of tuning its emission intensity and wavelength in response to the variation in ethanol, and a linear relationship was obtained ( Fig. 7c and d). These results indicate that the AIE2/PMMA PC possesses dual-functionality to quantitatively test humidity and alcohol. Although different AIEgens were used, the similarity in the tunability and multi-function response is a strong indication for the generality of our AIE/ PMMA PC system.

Conclusions
In this study, we exploited a simple method, emulsion polymerization combined with centrifugation, for the generic fabrication of luminescent photonic crystals from AIEgens. The resulting AIE/PMMA photonic crystal exhibits a prominent water-responsive optical property change, where a linear relationship was achieved. By carefully tuning the photonic crystal structure, its reection and emission color were red-shied at the same time. It was found that the ability of the PBG to selectively modulate and narrow emission allowed the luminescent PC to act as an intramolecular lter. This unique property thus gives insights into tuning an arbitrary emission color from only one material. Meanwhile, the choice of AIEgen is important, particularly for the modulation of two emission colors. Our understanding of the emission-change mechanism allowed the use of different solvents for the modulation. Further success has been achieved in the ethanol-conducted emission change.
The luminescent photonic crystal system, with multi-functionality and controllability, could be potentially useful as a humidity and alcohol sensor, as well as a photoelectric device component. Thus, the perfect encounter between AIEgens and photonic crystals will surely bring new opportunities to greatly exploit AIEgens in biological and optoelectronic applications. Fig. 7 (a) Normalized PL spectra of the AIE2/PMMA photonic crystal on adding different amounts of water: 0, 25, 50, 100, 125, 150, and 175 mL (by volume), respectively; (b) linear relationship between the emission peak and the volume of added water. (c) PL spectra of the AIE2/PMMA photonic crystal with an added ethanol/water mixture with different ethanol amounts: 0, 25%, 50%, 75%, and 100% (volume fraction, V%), respectively; (d) linear relationship between the emission peak and the amount of added ethanol (f E ).