Dual-Modal Invisible Photonic Crystal Prints from Photo/ Water Responsive Photonic Crystals

Invisible photonic crystal prints (IPCPs) are appealing for anti-counterfeiting and information protection due to their capability in hiding and showing the encrypted information under speci ﬁ c conditions. Herein, the dual-modal IPCPs based on responsive photonic crystals (PCs) with tunable photoluminescent (PL) and structural colors are reported. The responsive PCs are prepared by the self-assembly of dyes doped silica particles into non/swellable polymers. The independent design and control over the optical response of the PL of the particles and structural colors of PCs enable the successful encryption of dual-modal patterns. Under normal conditions, the as-fabricated IPCP shows a fake pattern with uniform structural colors and thus hides the encrypted pattern due to the similar particles size, lattice constant, and refractive index of each region. In striking contrast, PL and structural colors-based new patterns can be instantly and reversibly revealed when the IPCP is exposed to UV illumination or soaked in water. This work provides a new concept in designing and fabricating dual-modal IPCPs, and facilitates their applications in the ﬁ elds of color display, antifake package, and multilevel anti-counterfeiting. R – DA, PC – MA, and PC – DA, can be fabricated with similar protocols when the R-SiO 2 and SiO 2 particles were used to replace F-SiO 2 particles. The pattern of the PCs can be controlled through the well-developed region selective polymerization strategy with the help of mask. Therefore, the IPCPs were prepared by integration of different PCs onto the substrate. Characterization : The morphologies of F-SiO 2 , R-SiO 2 , SiO 2 , and the assembly structures of PCs were investigated by the HITACHI SEM- SU8010. The optical microscope images and microscopic re ﬂ ectance spectra were obtained on an Olympus BXFM re ﬂ ection-type microscope operated in dark ﬁ eld mode. The re ﬂ ectance and backscattering spectra at different angles were measured by a NOVA spectrometer (Hamamatsu, S7031). The UV – vis absorption spectra were conducted on SHIMADZU UV-3600Plus spectrophotometer. The PL spectrum was recorded on HORIBA Instruments Incorporated Fluorolog-3 instrument. The zeta-potential and particle sizes of dye-doped silica particles were determined by Zetasizer Nano ZS90 (Malvern Instruments Ltd.) at a ﬁ xed angle of 90º. Dynamic scattering of light (DLS) of the samples were prepared freshly before the measurement by diluting the products with water.

DOI: 10.1002/adpr.202000197 Invisible photonic crystal prints (IPCPs) are appealing for anti-counterfeiting and information protection due to their capability in hiding and showing the encrypted information under specific conditions. Herein, the dual-modal IPCPs based on responsive photonic crystals (PCs) with tunable photoluminescent (PL) and structural colors are reported. The responsive PCs are prepared by the selfassembly of dyes doped silica particles into non/swellable polymers. The independent design and control over the optical response of the PL of the particles and structural colors of PCs enable the successful encryption of dual-modal patterns. Under normal conditions, the as-fabricated IPCP shows a fake pattern with uniform structural colors and thus hides the encrypted pattern due to the similar particles size, lattice constant, and refractive index of each region. In striking contrast, PL and structural colors-based new patterns can be instantly and reversibly revealed when the IPCP is exposed to UV illumination or soaked in water. This work provides a new concept in designing and fabricating dual-modal IPCPs, and facilitates their applications in the fields of color display, antifake package, and multilevel anti-counterfeiting.
new pattern with different structural colors can be revealed once the IPCP is immersed in water due to the swelling effect of PCs. The hiding and revealing of the pattern into the IPCP between the pristine and UV irradiation/swelling state is instant and reversible. Although the combination of fluorescence and structural color has been reported previously, [77][78][79] these works focused on the enhancement of fluorescence by PC or detection, which are quite different from this work. The IPCP prepared by this work provides a new concept in the design and fabrication of multiresponsive IPCPs and will promote their application in multistage anti-counterfeiting, information storage, and delivery. Figure 1 shows the schematic illustration of the fabrication processes of the IPCP, which includes 1) the synthesis of dye-doped silica particles; 2) the self-assembly of the PL silica particles into PCs that can respond to UV irradiation and water, respectively; and 3) arrangement of these PCs with designed patterns in a desired way.

Results and Discussion
The PL silica particles with diverse fluorescent colors are prepared simply by the introduction of silane dyes into the reaction system of Stöber silica particles. Here, we use the (3-aminopropyl)triethoxysilane (APS) to form a covalent bond with the fluorescein isothiocyanate isomer I (FTIC) or rhodamine B isothiocyanate (RITC) through the click reaction between the amidogen from APS and isothiocyanate groups from dyes. The covalent bond can prevent leakage of dye from the particle, thereby contributing to the steady fluorescent intensity of the particles. For easy comparison, the FITC-and RITC-doped silica particle are named as F-SiO 2 and R-SiO 2 , respectively. The transmission electron microscope (TEM) and scanning TEM (STEM) images (Figure 2a) of the F-SiO 2 demonstrate the high uniformity of the particles. Furthermore, the element mapping images of the particles prove that Si, O, N, and S are uniformly dispersed across the particles, implying the dyes are homogeneously doped into the particles. When the FITC was replaced by RITC or the fabrication is in the absence of dye, R-SiO 2 and SiO 2 particles with uniform size was also obtained (Figure 2b), respectively. These particles with or without dyes show similar particle size, polydispersity index (<0.03), and ξ-potential (Figure 2c), suggesting that the encapsulation of dyes into silica particles has negligible effect on the size and the surface property of the silica particles. Similar to the Stöber silica particles, the size of the dye doped silica particles can be easily adjusted through controlling the reaction parameter, such as the amount of ammonium hydroxide ( Figure S1, Supporting Information).
Dissimilar to the whitish color of Stöber silica, the F-SiO 2 and R-SiO 2 particles show yellow and pink appearance, respectively, due to their diverse absorption characteristics at visible range ( Figure S2, Supporting Information). The weak absorption spectra of R-SiO 2 and F-SiO 2 can be attributed to the low concentration of the organic dyes into the particles and scattering of light by the colloidal particles. Since the low concentrations of dyes are embedded into the particles, the light was significantly scattered by the colloidal particles during the absorption test, thereby contributing to the weak absorption. In the view of following encryption, taking F-SiO 2 as an example, it is better to select UV light than the visible light (Ex max : 495 nm) as the excitation wavelength because the visible light may disturb the showing of information. Therefore, we select UV light (365 nm) as excitation wavelength as the light source for the fluorescent. Under the excitation of UV light (365 nm, Figure 2d,e), the F-SiO 2 and R-SiO 2 colloidal solutions exhibit brilliant green and orange colors with www.advancedsciencenews.com www.adpr-journal.com corresponding PL peak located at 522 and 584 nm, respectively. The RITC molecules are chemically doped into the network of silica particles with strong covalent bonding, which means that the RITC molecules will not escape from the particles and thus possesses a stable fluorescent intensity. The transparent supernatant solution ( Figure S3, Supporting Information) separated from the R-SiO 2 colloidal solution by centrifugation after being stored for 6 months firmly demonstrates the stability of the R-SiO 2 particles. The mass fractions of RITC and FITC in the particles are 0.054% and 0.195%, respectively, implying the dye molecules are probably isolated from each other and thus avoid the aggregation efficiently. For example, the fluorescence of the R-SiO 2 particles is much brighter than that of RITC powder ( Figure S4, Supporting Information), strongly supporting the aforementioned assumption. In contrast, the pure Stöber silica particles show neglectable PL property. The aforementioned results demonstrate that dyes doped silica particles with uniform size, different appearances, and PL properties can be fabricated in an efficient and simple way. PCs can be prepared when the dye-doped silica particles are self-assembled into the ordered structures. Here, the F-SiO 2 , R-SiO 2 , and SiO 2 particles are used to be assembled into the poly(ethylene glycol) methacrylate (PEGMA) to fabricate PCs, respectively, which are named as PC-F-MA, PC-R-MA, and PC-MA, accordingly, where F/R and MA represent the F/R-SiO 2 and PEGMA, respectively. The volume fraction of the particles and PEGMA is designed to be 40% and 60%, respectively. The as-prepared PCs show similar reflection wavelengths around at 525 nm and thus vivid green colors (Figure 3a,b). The microscope image of the PC-R-MA ( Figure 3c) shows that the reflection signal is originated from the numerous microcrystals with bright green color. These microcrystals with similar size can be regarded as colloids assemblies through the stacking of particles at microscale. When the volume fraction of particle exceeds to a critical value (20% for most case), the particles will spontaneously be precipitated out from the solution and selfassembled into ordered structures through balancing the electrostatic repulsion and attraction interactions between the particles. The assembly of the particles will be stopped once the volume fraction of the particles is below to 20%. This means not all the particles are participated into the ordered structures, and small part of them are randomly dispersed into the system.
The ordered structures of PC-R-MA can be confirmed by its SEM images and angle-dependent reflection spectra. Under SEM (Figure 3d), the particles of the most top layer are packed into long-range ordered structure. Different from the close packed structures of traditional PCs, the particles are nonclosely packed with PEGMA filled between the interparticle gaps. The crosssectional SEM images (Figure 3e,f ) indicate both the (100) and (111) planes with large areas can be observed, indicating the particles are assembled into face center cubic structure. The angleresolved reflection spectra of PC-R-MA show that its reflection wavelength blueshifts from 524 to 428 nm (Figure 3g,h) as the incident detector angle simultaneously varied from 0 to 60 , further suggesting its highly ordered structure. Except for PEGMA, the dyes doped silica particles also can be assembled into other acrylates with distinct structures, such as trimethylolpropane ethoxylate triacrylate (ETPTA) and poly(ethylene glycol) diacrylate (PEGDA). For example, PC-R-DA and PC-F-DA showing similar structural colors and reflection wavelengths to that of www.advancedsciencenews.com www.adpr-journal.com PC-R-MA and PC-F-MA can be prepared when the PEGMA was replaced by PEGDA. Based on the aforementioned results, we can achieve two basic conclusions: 1) the monodispersed functional silica particles with diverse PL colors can be prepared in a straightforward and efficient way, which provides a PL channel for construction of IPCPs and 2) PCs with similar structural colors can be prepared using particles with different PL colors and acrylates with different structures. The independent control of the PL color and structures of the acrylate allows us a new chance in fabricating IPCPs with dual-modal patterns in response to the UV light and another stimulus, respectively.
The as-prepared PCs with similar structural colors show diverse and abundant optical response when exposed to UV illumination and water (Figure 4a  In striking contrast, the PC-R-MA shows bright orange color when it is illuminated by UV light. The PL wavelength of the PC is located at around 584 nm (Figure 4c), similar to that of R-SiO 2 particles, indicating the ordered structures of the PC has a little effect on its PL property. The structural and PL color can be reversibly and instantly switched under the alternative illumination of white and UV light, respectively (Figure 4d).
The PC-R-MA can further respond to the water due the swelling effect of PEGMA. The reflection wavelength of this PC redshifts from 525 to 624 nm ( Figure 4e) and corresponding color turns form green to red in seconds when it was immersed in water. In contrast, no obvious change in reflection signal can be observed when the PC-R-MA is replaced by PC-R-DA (Figure 4f ). The different shift of the reflection peak position between the PC-R-MA and PC-R-DA can be ascribed to the dissimilar swelling behavior of PEGMA and PEGDA. The PEGMA has abundant hydrophilic groups (-OH), which contributing to the good swelling, leading to the expansion of lattice constant and thus the redshift of reflection wavelength of the PC into water. This can be well explained by the modified Bragg-Snell law: mλ ¼ 2d (n 2 À sin 2 (90 À θ)) 1/2 , where m and λ are the diffraction order and reflection wavelength, respectively. The d and n are the lattice constant and refractive index of PCs, respectively. θ is angle between the incident/reflective light and sample. Under SEM, the lattice constants of PC-R-MA at pristine and swelling state are determined to be 146.0 and 168.3 nm (Figure 4g,h), respectively. The increase in lattice constant of PC will lead to the redshift of the reflection wavelength and thus www.advancedsciencenews.com www.adpr-journal.com a red structural color. Conversely, the PEGDA cannot be swelled due to the lack of hydrophilic groups, thereby almost no change can be observed in the aspect of the appearance and reflection signal of PC-R-DA. It is worth noting that the large color contrast of PC-R-MA between the pristine and swelling state is benefiting from the large volume fraction of PEGMA (60%) in the PC. Both the dynamic microscope images (Figure 4i) and reflection spectra (Figure 4j) of PC-R-MA swelled in water suggest the swelling process can be accomplished instantly within 30 s. After drying, the appearance and reflection of the PC-R-MA would return back to the pristine state due to the shrinkage of the lattice constant of the PC. Similar to that of UV-visible light cycles, the switch of the reflection signal of the PC-R-MA between the dried and swelling state is fully reversible (Figure 4k). In addition, we test the R-PC-R-Ma film after repeated use for 20 times in encryption environment of water. The similar fluorescence intensity before and after repeated usage ( Figure S5, Supporting Information) suggests the good stability of the fluorescent PCs.
Through careful observation, one may find that the swelling structural color and fluorescent color are not consistent under the same conditions. For instance, PC-R-MA and PC-R-DA should exhibit same fluorescent color under UV (but yellow and pink in the article); SiO 2 , R-SiO 2 , and F-SiO 2 in PC-PEGMA system should exhibit same swelling structural color (but yellow, brown, and red in the article). Moreover, the original color of the pattern is green, whereas the fluorescent color of F-SiO 2 particle is also green. Therefore, fluorescent emission overlaps with the photonic bandgap, probably preventing the outward emission of fluorescence. The reasons may be very complicated probably due to the complex interactions between the PC and organic dyes. The colors of the PC composites encapsulation of organic dyes are not pure reflective, transmissive, or fluorescent color, but probably the mixture of them. Especially, the effect of photonic bandgap on the absorption and emission of organic dyes is still unclear and need numerous efforts and investigations in the future. Fortunately, for all the PCs, they have similar green colors, which is extremely important for the encryption process.
IPCP can be fabricated by the combinations of PCs with similar structural color but dissimilar PL and water-responsive structural colors. Here, IPCP with two encrypted patterns was achieved through the region selective assembly process (Figure 5a), which allows us to modify the optical response of the PCs of each region in a straightforward and efficient way. The W, KY, E, US, and L of the IPCP is composed by PC-R-DA, PC-MA, PC-R-MA, PC-F-DA, and PC-DA, respectively. At normal conditions, the www.advancedsciencenews.com www.adpr-journal.com as-prepared IPCP (Figure 5b) shows the pattern of "WKEYULS" with similar structural colors, and the encrypted information cannot be recognized at this state. In the presence of UV irradiation, the "WE" and "US" with brilliant yellow and green PL colors, respectively, can be instantly revealed (Figure 5c). In contrast, new patterns of "KEY" with bright red color (Figure 5d) are shown in seconds when the IPCP is immersed into the water. The encrypted information can be hided again into the background after removing the UV light or drying, and the switch between the pristine and UV illumination/drying state is fully reversible. In addition, other IPCP (Figure 5e) also can be fabricated with similar procedures but different designed patterns (Figure 5f,g), which exhibit reversibly dual-modal color contrasts under UV light and water.

Conclusion
In summary, IPCPs with dual-modal invisible patterns were generated by the integration of the UV light and water-responsive PCs, which was prepared by the assembly of the dyes doped silica particles into the PEGMA/PEGDA with tunable swelling ability. The independent control over the optical response of the PCs by UV illumination from the PL particles and swelling in water caused by the polymer is the key to the successful fabrication. At normal conditions, the IPCP shows a fake pattern with similar structural color of each region. A new PL pattern was obtained when the IPCP is exposed to the UV light. In addition, another new pattern was achieved once the IPCP is soaked into water due the swelling effect of PCs. The hiding and showing of the encrypted patterns between the pristine and PL/swelling state is instant and reversible. This work demonstrates a new insight and way in the construction of multifunctional IPCPs and will facilitate their applications in display, information protection, and high-level anti-counterfeiting.
Synthesis of SiO 2 , F-SiO 2 , and R-SiO 2 Particles: FTIC (0.1 mmol) and RITC (0.02 mmol) were dissolved into ethanol solution (20 mL) containing APS (0.2 mL), respectively. The mixed solutions were stirred for 12 h to form APS-FITC and APS-RITC precursors before usage. Afterward, TEOS (64 mL) and the APS-FITC (20 mL) [or APS-RITC solution (20 mL)] precursor solutions were added into the mixture containing ethanol (800 mL), H 2 O (56 mL), and NH 4 OH (32 mL). After stirring for 5 h, the products were purified by centrifugation-washing process with excessive ethanol. The F-SiO 2 and R-SiO 2 particles showed yellow and pink colors, respectively. The Stöber silica particles were synthesized by similar procedures with TEOS as the silane resource.
Fabrication of IPCPs: For the fabrication of PC-F-MA, F-SiO 2 particles (0.04 mL) were dispersed into the mixture of ethanol (1 mL) and PEGMA (0.06 mL) containing 5% photoinitiator. Then, the mixed solution was heated at 373 K for 1 h and a nearly transparent precursor solution was obtained. This precursor solution was sandwiched between two glass slides with an interval of 0.09 mm, followed by UV light illumination (365 nm, 4.8 mW cm À2 ) for 3 min. One side of the PC-F-MA film was fixed on the glass by the commercially available NOA61 with the help of UV light. The PC-F-DA film can be obtained with similar procedures when the PEGMA was replaced by PEGDA. Other PCs, such as PC-R-MA, PC-R-DA, PC-MA, and PC-DA, can be fabricated with similar protocols when the R-SiO 2 and SiO 2 particles were used to replace F-SiO 2 particles. The pattern of the PCs can be controlled through the well-developed region selective polymerization strategy with the help of mask. Therefore, the IPCPs were prepared by integration of different PCs onto the substrate.
Characterization: The morphologies of F-SiO 2 , R-SiO 2 , SiO 2 , and the assembly structures of PCs were investigated by the HITACHI SEM-SU8010. The optical microscope images and microscopic reflectance spectra were obtained on an Olympus BXFM reflection-type microscope operated in darkfield mode. The reflectance and backscattering spectra at different angles were measured by a NOVA spectrometer (Hamamatsu, S7031). The UV-vis absorption spectra were conducted on SHIMADZU UV-3600Plus spectrophotometer. The PL spectrum was recorded on HORIBA Instruments Incorporated Fluorolog-3 instrument. The zetapotential and particle sizes of dye-doped silica particles were determined by Zetasizer Nano ZS90 (Malvern Instruments Ltd.) at a fixed angle of 90º. Dynamic scattering of light (DLS) of the samples were prepared freshly before the measurement by diluting the products with water.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.