Dynamic Control of Multiple Optical Patterns of Cholesteric Liquid Crystal Microdroplets by Light-Driven Molecular Motors

Introduction

Data encryption and anti-counterfeiting technologies are crucial for society as governments, companies, and customers routinely use these techniques which can help protect authentic paper currency, documents, microelectronics, goods, and most importantly pharmaceuticals where counterfeit medicines are now great threats to public health.^1,2 The key to high-level security labels is the storage of multiple distinct forms of information; for example, digital codes, patterns, or images, within one physical unit where each information level can be individually addressed in a highly precise and independent manner by different stimuli. Photo-fluorochromic materials are considered to be among the most promising candidates for their reversible emission output in response to light.^3–5 The fluorescent compound is usually concealed as hidden information and only shows when the light of a specific wavelength is applied. Instead of fluorescent systems that involve complicated construction of orthogonal photoactive molecular structures or require distinct energy levels of composite materials to enhance fluorescent intensity or to avoid quenching,^6–13 nature has provided an alternative approach by taking advantage of periodic structures, which is usually referred to as structural color. For instance, morpho butterflies selectively reflect the light of certain wavelengths,^14–17 while the beetle Chrysina gloriosa reflects left circularly polarized (LCP) light due to helicoidally arranged nested arcs.^18,19 In addition, the Panther chameleon is able to actively change color by modulation of pigment-containing organelles and nanostructures of guanine nanocrystals in dermal iridophores.²⁰ Taking inspiration from these fascinating biosystems, we envisioned that by dynamic tuning of the self-organized structures of artificial photonic materials, distinct optical states can be obtained and switching achieved due to their responsive nature, which might result in multi-level high-performance anti-counterfeit labels. However, it represents a formidable challenge as the second layer or hidden information that carries completely different images or codes has to be read with fast response and high resolution without cross-talk to the first layer.

Cholesteric liquid crystals (CLCs) are photonic materials that adopt periodical structures.²¹ The liquid crystal (LC) molecules are aligned parallel in each layer and slightly twisted along the normal direction of layers in a helical fashion, enabling selective reflection, that is, a photonic band gap (PGB) of circularly polarized light (CPL) with the same helicity as the LC matrix. The central wavelength of the selectively reflected light is correlated with the CLC’s helical pitch length (P), according to Bragg’s law. When P of CLCs lies in the range of the wavelengths of visible light, the material appears vividly colored, which is referred to as structural color, notably observed in morpho butterflies and Chrysina gloriosa. Introducing a photo-responsive chiral dopant to control the pitch by light has proven to be a practical approach to dynamically modulate the optical properties of CLCs as it offers remote, spatial, and temporal controllability, thereby enabling various applications in functional stimuli-responsive soft materials.^21–23 Illustrative examples of photo-chromic chiral dopants that are widely used include overcrowded alkenes,²⁴ azobenzene,^25–29 dithienylcyclopentene,^5,30–33 fulgide,^34–36 and spirooxazine derivatives.^37,38 Among these chiral dopants, light-driven molecular motors, based on overcrowded alkenes, offer unique features due to the intrinsic helicity; they undergo not only geometrical isomerization but axially helical changes as well during the rotary motion triggered by light and heat.^39,40 The photo-triggered rotation of motors induces reorganization of the superstructure of CLCs resulting in changes both in the pitch and handedness, which distinguishes the motors from most chiral switches. Motor-doped CLCs have enabled the discovery of rotating surfaces,^41–43 reconfigurable LC droplets,⁴⁴ supramolecular vortices,⁴⁵ adaptive optical materials,^46–48 and soft actuators.^49,50 These pioneering studies established a solid basis for exploiting dynamic control over multiple levels of optical information by modulation of superstructures of CLSs using molecular motors.

Here, we report responsive optical labels based on motor-doped CLC microdroplets that were prepared by a microfluidic technique (Figure 1). Structurally defined spherical CLCs are known to provide rich, strong, and angle-independent structural colors that help enhance the resolution of the created images.^31,51 Two distinct patterns were embedded by programmable construction of CLC microspheres in pre-defined arrays. The label displayed first-layer information, that is, colored letters “BLF” under natural light. When observed under LCP light or irradiated with UV light, “BLF” disappeared, and instead the letters “RUG” were shown (Figure 1). The completely different second or hidden information was accessed without cross-talk to the first-layer information solely by taking advantage of manipulation of the superstructure of CLC microdroplets. In addition, we prepared responsive devices, including a UV light indicator, wearable door pass, and digital codes of wines for daily practical use, demonstrating the potential of our novel strategy for advanced data encryption and an anti-counterfeiting technique.

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Experimental Methods

Materials

Nematic liquid crystal E7 (99 wt %) and chiral agent R5011(≥95 wt %) were purchased from Jiangsu Hecheng Display Technology Co., Ltd. (Nanjing, China). Polyvinyl alcohol (PVA; Mn = 13,000 g/mol) and octadecyl triethoxy silane (OTS; ≥97 wt %) were purchased from Sigma-Aldrich Reagent Co., Ltd. (Shanghai, China). Dimethylsiloxane prepolymer (supporting curing agent) was purchased from Dow Corning Inc. (Midland, Michigan, United States). Wedge cells (KCRK-07, tanθ = 0.0785) were provided by Japan EHC Co., Ltd. (Tokyo, Japan). The photo-responsive chiral dopants motor 1 was synthesized following procedures reported in previous works,⁴¹ and chemicals and solvents (analytical grade) were used as obtained without any other purification. Deionized water (18.25 MΩ·cm⁻¹ at 25 °C) was prepared using a Milli-Q Plus water purification system (Wortel Water Treatment Equipment Co. Ltd., Sichuan, China).

Characterization of the molecule motors 1

Solution circular dichroism (CD) spectra were recorded on a JASCO J-715 spectropolarimeter (JASCO, Inc., Easton, Maryland, United States) at −20 °C. Enantiopure motor was obtained by chiral high-performance liquid chromatography (HPLC) (ODH; Hexane:DCM = 1:99, flow rate 1 mL/min). Helical twisting powers (HTP; wt %) of the molecular motor 1 were measured in the nematic liquid crystal E7, by using the Grandjean–Cano wedge method.⁴¹ To induce planar anchoring, a glass substrate was thoroughly cleaned with water, acetone, and ethanol, and spin-coated with a PVA layer. The coated substrate was rubbed with velvet in a certain direction. Two of the substrates were put together by the UV-curing spacer to construct an LC cell with a spacing distance of 35 μm. This yielded the following values for each motor enantiomer: (S,P)- 1: HTP = +90 μm⁻¹; (S,M)- 1: HTP  = −59 μm⁻¹; (R,M)- 1: HTP  = −90 μm⁻¹, (R,P)- 1: HTP = +59 μm⁻¹. The HTP of the R5011 dopants was (R)-5011 HTP  = +120 μm⁻¹.⁵² These values were used to prepare the CLCs.

Preparation of the CLC microdroplets and characterization

CLC was obtained by doping with chiral agents to nematic liquid crystal with definite weight concentration. The blue, green, and red-responsive CLC mixtures were prepared by adding 4.0, 3.29, and 2.74 wt % (S)- 1 to E7, respectively. Similarly, 3.28 and 2.79 wt % R5011 chiral dopants were added to E7 to prepare blue and green non-photoresponsive CLC mixtures. CLC droplets were produced using a flow-focusing capillary microfluidic device. The capillary microfluidic device was homemade by coaxially assembling two cylindrical [outer diameter = 1.0 mm, inner diameter (ID) = 500 μm] capillaries in a square capillary (ID) = 1.0 mm) to have a tip-to-tip separation of 120 μm, which were connected using a tee connector. Assembled capillaries and tees were bonded and sealed using acrylate adhesive finally. The end of the injection capillary tube was tapered to 70–80 μm in tip diameter using a capillary puller (P-1000, Sutter Instrument, Novato, California, United States). A larger tip diameter (130–150 μm) of the collection capillary was obtained by using sandpaper. The injection/collection capillaries were respectively treated with OTS and Piranha solution (H₂SO₄:H₂O₂ = 7/3, v/v) to make it hydrophobic/hydrophilic. In the experiment, CLC mixtures of different colors were respectively installed as the inner phase, then 3.0 wt % PVA aqueous solution was used as the outer phase, which was injected into the corresponding flow channel of the microfluidic device with a pressure syringe pump (TYD01-01, Leadfluid, Baoding; Heibei, China). Under combined effects of shear force, surface tension, and viscosity, CLC droplets with uniform sizes of 30–150 μm can be obtained by changing the volumetric flow rates of the two fluidic phases. The droplet formation process was observed with an optical microscope (OM; CKX41, Olympus, Tokyo, Japan) equipped with a high-speed camera (Phantom MIRO M110, Vision Research, Wayne, New Jersey, United States). The prepared droplets were collected in a 5 mL vial for observation and storage. Irradiation experiments were performed using a light-emitting diode (LED) lamp (Thorlabs, Shanghai, China) at 365 nm. All the optical phenomena and structural changes of CLC droplets were observed and recorded via a polarizing optical microscope (POM; Axio Vert.A1, Zeiss, Oberkochen, Germany). Optical fiber spectrometers (Ocean Optics 2000+; Ocean Spectroscopy, Dunedin, Florida, United States) were used to characterize the reflectance spectra at normal incidence.

Fabrication of the geminate information labels

There were two kinds of information labels constructed in the experiment, rigid label and flexible label. The rigid label was a 21 × 21 “pixelated” array fabricated by lithography on a glass substrate. The size of each pixel was 2.0 mm × 2.0 mm × 400 μm. The flexible label was the same pixelated black array of polydimethylsiloxane (PDMS), which was fabricated by reverse modeling a positive model of a resin substrate prepared by three-dimensional (3D) printing. The black PDMS substrate was processed by O₂ plasma to become hydrophilic. After the collection of the photo-responsive or non-photo-responsive CLC microdroplets fabricated by the glass capillary microfluidic device, the excess PVA aqueous solution was removed to concentrate the density of microdroplets in the vial. Then the CLC microdroplets were selectively drawn to fill the array for constructing information labels with special patterns by using glass droppers. Particularly, the glass dropper was treated with Piranha solution (H₂SO₄:H₂O₂ = 7/3, v/v) to become hydrophilic, and the orifice diameter was smaller than the size of the pixels (1.5 mm).

Results and Discussion

Photo-responsive CLC microdroplets

Light-driven rotary motor molecule 1 was chosen as the photo-responsive chiral dopant (Figure 2a). The preparation and separation of both enantiomers of 1 were carried out according to our reported procedure⁴¹ and the enantiomer purity of the compounds was determined by chiral HPLC (see Supporting Information Figure S1). The chiral inversion of molecular motor 1 during illumination was verified by circular dichroism spectra (see Supporting Information Figure S2). (S,P)-stable- 1 adopted right-handed axial helicity and its HTP was 90 μm⁻¹ when used as a chiral dopant in the LC material E7. Upon irradiation with UV light, it underwent photo-isomerization and resulted in the formation of the (S,M)-unstable- 1 isomer with opposite axial chirality, which induced a change of the HTP from 90 to −59 μm⁻¹ (Figure 2a). After finishing the irradiation, stable isomer (S,P)- 1 was regenerated by a thermal helix inversion step completing the rotary cycle. The large variation in HTP value and the dynamic tuning of the chirality at will during the rotary motion of 1 played key roles in the modulation of the optical properties of CLC.⁴⁸ Distinct from traditional LC thin film approaches, in the present study, motor-doped CLCs were confined into spherical geometries, which allowed the exploration of the LC microdroplets for their rich structural color^53–55 and more importantly, the microspheres could be manipulated to assemble in arrays and formed specific patterns by dedicated programming.^31,51,56 A flow-focusing glass capillary microfluidic technique was employed to prepare high-flux structurally defined and monodisperse CLC microdroplets with controllable sizes. The dispersed phase comprised photo-responsive CLCs composed of (S,P)- 1 in liquid crystal E7 while the continuous phase was a 3.0 wt % PVA aqueous solution (Figure 2b and see Supporting Information Figure S3). The addition of PVA not only stabilized the cholesteric droplets against coalescing but also facilitated the planar anchoring of the liquid crystal molecules at the droplet surfaces.^31,57–61 By adjusting the flow rates of both phases, monodisperse CLC droplets with specific diameters were generated and collected (see Supporting Information Figure S4). Figure 2c shows OM images of microdroplets containing 4.0 wt % (S,P)- 1. The single blue reflection spot located at the core of the sphere confirmed the proper orientation of the motor and LC molecules and showed that the helical axes were radially aligned and perpendicular to the surface within the droplets.³¹ Upon UV irradiation, the initial blue color showed a redshift, according to Bragg’s law, due to the decrease of HTP originating from the formation of (S,M)-unstable- 1. The central reflective wavelength was followed quantitatively during the irradiation, and it shifted from 490 to 628 nm within 34 s and finally to 648 nm when the motor reached the photostationary state (PSS) (see Supporting Information Figure S5a), showing a red reflective color (Figure 2c; see Supporting Information Movie S1). When the UV irradiation was stopped, the superstructure of CLC gradually regenerated to the original state, concomitant with the central color changing from red to the initial blue (Figure 2c and see Supporting Information Figure S5b and Movie S1). Microdroplets with different initial central colors could be obtained by doping (S)- 1 with different concentrations and the corresponding change of color upon irradiation was monitored and detailed as shown in the Supporting Information (see Supporting Information Figures S6 and S7). We next investigated the effect of the droplet size. A diameter of 60 μm was established to show the most optimal condition as microspheres with this diameter were found to respond with good stability and reversibility while being fatigue resistant (see Supporting Information Figures S8 and S9). The reversible color tuning process was expected to involve a change of the axial chirality of CLC, induced by the unidirectional rotary motion of (S)- 1, and concomitant helical reorganization of the superstructure of CLC microdroplets was observed during the irradiation (see Supporting Information Figure S10). To further confirm this hypothesis, microdroplets with identical sizes prepared by (S)- 1 and (R)- 1, respectively, were mixed together. Figure 2d shows that the blue color of microspheres with opposite chirality can be observed under natural light. However, when the sample was investigated under right circularly polarized (RCP) white light, the central reflection color of the (R)- 1-doped microspheres switched off while the microspheres containing (S)- 1 showed the intact color (Figure 2d). The disappearance of the central reflection spot was attributed to the selective reflection of CLC; thus, in agreement with the chirality of the motor dopant (see Supporting Information Figure S11). Accordingly, the opposite phenomenon was observed when the droplets were detected by LCP light, which switched off the blue color of the (S)- 1 microsphere in this scenario (Figure 2d). We then continued to perform the irradiation keeping the samples monitored by RCP light (Figure 2e). The blue color of microdroplets with (S)- 1 redshifted and disappeared when PSS was reached, indicating the formation of opposite chirality of the microspheres. In contrast, microdroplets with (R)- 1 showed a red color at the PSS state while the central color was completely blocked at the initial state. Removal of the UV irradiation resulted in the reversible process and recovery of the original state (Figure 2e). These experimental data confirmed that the central reflective color of motor-doped CLC microdroplets, that is, structural color, was dynamically manipulated not only by UV but by LCP or RCP light as well, thereby providing unique opportunities for the preparation of advanced responsive photonic systems.

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Photo-responsive labels carrying single or dual information

A mold with a rose pattern was prepared by photolithography (see Supporting Information Figure S12). Blue and green light-responsive CLC droplets doped with (S)- 1 were respectively filled into the petal, stem, and stamen parts (Figure 3a and see Supporting Information Figure S13). Under UV light irradiation, the petals and stems turned yellow, starting from blue and green, respectively; after stopping the irradiation, the original color could be gradually regenerated (Figure 3a and see Supporting Information Movie S2). It should be noted that the infrared irradiated microdroplets at the PSS state, however, exhibited a pale-yellow color originating from the pigment color of motors rather than the structural color of CLCs. The reason might be that the infrared reflection at 648 nm was not sharp enough when compared to the pigment color of the motor; hence, the pale yellow became visible in this system. Subsequently, we constructed a label carrying letters information by the same method. Blue and green CLC droplets doped with (S)- 1 were filled into pixels with pre-defined arrays. The details of this pattern programming are shown in Figure 3b. Under natural light, the blue letters of “SCNU” could be clearly read-out with green as the background color since green and blue provided enough color contrast and therefore could be distinguished by the naked eye. Upon UV irradiation, the label became yellow and the information about SCNU disappeared. After the UV irradiation was stopped, the letters “SCNU” reappeared changing from green to the original blue color while the background color recovered to the initial green (Figure 3b; see Supporting Information Movie S3). In addition, as shown by previous experiments (Figure 2d), the structural color was directly related to the axial chirality of the motor. Therefore, when read by LCP, the central reflective color of the label was completely blocked, showing only a pale-yellow color with the disappearance of all the information embedded (see Supporting Information Figure S14). When the label was brought to natural light, SCNU was detected again (see Supporting Information Figure S14 and Movie S4). Moreover, when a motor with opposite chirality, that is, (R)- 1 was employed, the letters “SCNU” were observed under natural light, while RCP light was used instead of LCP light to switch off the reflective color in this case (see Supporting Information Figure S15). The above experiments showed that single encrypted information could be hidden either by UV light or CP light. Based on these findings we envisioned that a second information layer could be concealed under the first layer and might become visible when the first layer was temporally erased.

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Non-photoresponsive chiral dopant R5011 was employed for the preparation of advanced multilevel optical labels involving motor- 1. R5011 had a large HTP value (HTP = +120 μm⁻¹), which is close to that of motor- 1.⁵² Therefore, it could provide a strong blue and green structural color compared to motor- 1. More importantly, when the central reflective color of the CLC microdroplet containing R5011 was blocked under LCP, it presented a blue color as the pigment color. The blue pigment color offered enough color contrast against the pigment color of motor droplets, that is, pale-yellow color, ensuring the read-out of information when pre-stored in R5011 by the naked eye. Therefore, we expected that the combination of motor-doped and non-photoresponsive CLC microdroplets could provide two approaches for the extraction of the hidden information. First, when UV light was applied, motor microspheres became pale-yellow while blue micro-droplets of R5011 remained intact, thereby exhibiting embedded information. Alternatively, when LCP light was employed, the central reflective color of both the microdroplets with motor and R5011 were switched off and the blue pigment color of R5011 provided good visibility of the hidden information beyond the pale-yellow pigment color of motor droplets. An intricate geminate label composed of two different messages was created as depicted in Figure 4a. Four different microdroplets were programmably filled with pre-defined patterns into the pixels. The label showed the blue letters “BLF” under natural light. To our delight, upon UV irradiation, “BLF” disappeared completely within 5 s and the letters “RUG” were observed instead. After removal of UV light, the label displayed “BLF” again. Interestingly, when the label was placed under the LCP light, “BLF” disappeared immediately and only “RUG” was seen (see Supporting Information Movie S5). Switching between these states, the information could be modulated several times without significant fatigue. Encouraged by the above result, two distinct images that involved more complicated and overlapping patterns were designed (Figure 4b). Respective microdroplets were filled in a “pixelated” black array (21 × 21) of PDMS (each pixel is 2.0 mm × 2.0 mm × 400.0 μm) according to the pre-defined blueprint. A green panda was shown under natural light. When the device was placed under UV light, the panda image disappeared and the letters “I love you 2022” were observed. After ceasing illumination with UV light, the original panda image was regained (Figure 4b). The hidden pattern of “I love you 2022” could be extracted by LCP light as well (Figure 4b and see Supporting Information Movie S6), showing a successful, fast switching between two completely distinct information features using either UV or LCP light.

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Responsive devices for practical use

With the above experimental results in hand, we then decided to move one step forward to prepare devices and labels for practical daily use. First, a label with “MOTOR” letters was built and each letter contained a different color (see Supporting Information Figure S16). The structural color of the labels under white light is angular-independent, owing to the omnidirectional Bragg reflections produced by the three-dimensional microdroplets. The rich structural color was well-preserved when the label was viewed from 0° to 80°, which provided a solid basis for advanced responsive optical devices (see Supporting Information Figure S16). Second, a warning indicator of UV sterilizer was created as depicted (Figure 5, See Supporting Information Figure S17), and placed inside the machine. When the machine functioned, Chinese characters meaning “BE CAREFUL” were shown, warning that the UV light source was switched on (Figure 5a). After turning off the machine, no UV light was detected; therefore, the warning letters disappeared on the label (Figure 5a and see Supporting Information Movie S7). In addition, we employed 3D printing to prepare a soft wearable tag that could be placed on the sleeve of a student’s cloth (for preparation details, see Supporting Information Figure S18). The letters “SAFE” were shown on the tag if the student stood indoors (Figure 5b). As the student stood outdoors, the letter “SAFE” disappeared (see Supporting Information Movie S8) when the UV intensity of sunlight reached 100.0 mW/cm², reported to be harmful to human skin.^62,63 Moreover, we prepared a wearable identity card: This card displayed the name “BLF” of one of the main authors, and when detected by LCP, the affiliation information “RUG” was shown, allowing the researcher to enter areas that were only opened to university faculty (Figure 5c and see Supporting Information Movie S5). Besides the indicator and ID card, optical devices for anti-counterfeiting and data security were prepared. An anti-counterfeiting label was created (see Supporting Information Figure S19). Figure 5d displays a label that is adhering to the top of a wine bottle. The letter “M” was shown initially, but when the label was checked under the LCP light, the letter “T” was detected, confirming the authenticity of the product (see Supporting Information Movie S9). Finally, we constructed a QR code with unique features (Figure 5e): By design, the QR code was composed of green CLC droplets doped with (R)- 1, and the background color of the label was created by blue CLC droplets doped with (S)- 1 (see Supporting Information Figure S20). Under the condition of natural light, one could observe a green QR code with blue as the background color. Since the color contrast between these two colors was not sharp enough, the QR code could not be recognized by a mobile phone, and the information carried was not extracted (Figure 5e). When viewed under RCP light, the background blue color of the label remained while the green reflective color of the code was shut off, resulting in dark droplets. It further reduced the color contrast; therefore, no signal was observed after the code was scanned. In contrast, under LCP light, the QR code retained a bright green color, and the blue reflective background color was turned off leaving dark yellow as the background. It enhanced the color contrast between the code and background; thus, enabling successful scanning by phone. The carried information “MOUTAI” was clearly shown (Figure 5e and see Supporting Information Movie S10). It is worth noting that UV irradiation of the code would not result in a successful scan but instead the disappearance of all the information (see Supporting Information Figure S21 and Movie S11). Unlike the conventional QR codes that were easy to duplicate, optical patterns prepared by our strategy showed ample opportunities for high-level data security and advanced encryption.

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Conclusions

We present here a strategy to dynamically control the optical patterns of CLC microdroplets by light-driven molecular motors. Motor 1 was doped in nematic LC and the formed CLC mixture was further processed into structurally defined and monodisperse microspheres by capillary microfluidic technique, providing striking reflective color, that is, structural color. The photo-triggered unidirectional rotation of the motor induced not only changes in the HTP value but an opposite helical orientation of the superstructure in CLCs as well, which resulted in changes in both the structural color and the selective reflection of CPL. The motor-doped microdroplets were programmed to fill in pixels with pre-defined arrays to form certain patterns, exhibiting the corresponding information. Under UV irradiation or LCP light, the information was erased and could be recovered by removal of UV light or LCP light. In addition, geminate labels that carried two completely different information patterns were prepared. The hidden image was concealed behind the first layer under natural light and decrypted with high resolution and fast response upon UV or LCP light without cross-talk to the first layer. Moreover, responsive devices for daily practical use, including a UV indicator, as well as labels for a wearable ID card and a wine bottle were prepared. In particular, QR codes created by motor microdroplets with different helicities offered unique features, in particular, the ability that the encrypted digital message could only be extracted by specific CPL. Our strategy of dynamical control over optical patterns of responsive devices solely by taking advantage of tuning the superstructures of CLC microdroplets provided a non-fluorescence-based approach for constructing advanced labels where dual distinct information could be stored and addressed individually by different stimuli without cross-talk to each other. It offered major potential to greatly enhance the level of data security against counterfeiting, providing a versatile platform for the protection and encryption of authentic information.

Supporting Information

Supporting Information is available and includes experimental methods and characterization data for molecular motor and CLC droplets, HPLC, and CD studies, OM images, and reflective spectrum (PDF), and internal structural changes of photo-responsive liquid crystal droplets and dual information switching of photo-responsive labels (MP4)

Conflict of Interest

There is no conflict of interest to report.

Funding Information

This work was supported financially by the National Key R&D Program of China (grant no. 2020YFE0100200), Science and Technology Projects in Guangzhou (grant no. 202201000008), and Guangdong Provincial Key Laboratory of Optical Information Materials and Technology (grant no. 2017B030301007), the Netherlands Ministry of Education, Culture and Science (Gravitation Program 024.001.035 to B. L. F.). S. X. acknowledges the fellowship of China Postdoctoral Science Foundation (grant no. 2022M711224).