Circularly polarised luminescent inks with low crosstalk and high efficiency for 3D information encoding from liquid crystal microcapsules

Abstract

Cholesteric liquid crystals (CLCs) possess inherent circular dichroism and selective reflection, making them promising candidates for optical applications. However, the fluidic nature of small molecule CLCs limits their direct integration into ink-based printing systems for fabricating solid-state devices. To address this challenge, a scalable fabrication strategy was developed to produce rhodamine 6G-doped CLC microcapsules (R6G-CLCMs) using interfacial polymerisation. This encapsulation process enhances the structural stability of CLCs whilst preserving their key optical properties. The resulting CLCMs allow fine-tuned modulation of multiple optical characteristics, including structural colouration, colour absorption, fluorescence emission, and circularly polarised luminescence (CPL). By systematically adjusting the dye concentration, capsule shell thickness, and film thickness, flexible composite films achieved a maximum g_lum value of −1, with significantly reduced optical crosstalk among the structural colouration, colour absorption, and fluorescence emission. The fabricated R6G-CLCMs were successfully incorporated into water-based inks, demonstrating compatibility with mould casting, blade coating, and direct dispensing, which facilitates scalable fabrication of flexible optical devices. These composite films displayed pronounced structural colouration dependent on the viewing angle, fluorescence intensity, and well-defined CPL emission. Furthermore, encrypted code boards derived from R6G-CLCMs demonstrated potential for secure visual information storage. This capability underscores the broader utility of the system beyond optical functions. This study demonstrates a versatile CPL-active material platform with broad applicability in anti-counterfeiting technologies, flexible photonic coatings, and high-density three-dimensional optical data encoding.

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Introduction

Circularly polarised luminescence (CPL) occurs when emitted light exhibits unequal intensities of left- and right-handed circularly polarised components, typically favouring one helical-twist direction. This property has attracted significant attention because of its potential applications in quantum computing, information encryption, three-dimensional (3D) display systems, and next-generation optoelectronics.^1,2 CPL-active materials have been developed by using two strategies: covalently incorporating chiral centres within luminescent molecules and incorporating achiral fluorescent dyes into chiral host environments. The first approach produces intrinsically chiral emitters with stable CPL, but complex synthetic procedures are often required, which restricts structural diversity and scalability. The second strategy provides greater material versatility; however, it presents challenges in optimising the loading amount of fluorescent dyes, maintaining compatibility among components, and preserving processability for device fabrication.^3–5

Cholesteric liquid crystals (CLCs) are quasi-one-dimensional photonic crystals consisting of rod-like molecules that self-organise into a helical structure, resulting in selective wavelength reflection via Bragg reflection.^6–9 Fluorescent dye-doped CLCs can readily produce CPL due to their exceptional light-modulation properties and broad material compatibility, often achieving a luminescence dissymmetry factor (g_lum) higher than 0.1.^10–12 In contemporary studies, polymer-dispersed liquid crystals (PDLCs) and polymer-stabilised liquid crystals (PSLCs) are commonly used to fabricate CPL-active films because they suppress the intrinsic fluidity of pure CLCs.^13–19 However, incorporating polymers can introduce optical imperfections, including light scattering and distortion of the photonic bandgap. Furthermore, fabrication processes that involve polymer curing often result in film shrinkage, phase separation, and non-uniform alignment, ultimately reducing the CPL efficiency.

CLC microcapsule (CLCM) technology has emerged as a promising strategy to overcome the limitations associated with PDLCs or PSLCs. The radial alignment of the helical structure, dispersibility, and film-forming capability of CLCMs are preserved by encapsulating CLCs within polymer shells, enabling the use of CLCMs as functional optical inks compatible with scalable printing techniques, including mould casting, blade coating, and direct dispensing.^20–25 Recent studies further demonstrate that CLCM-based systems can exhibit robust CPL performance. For example, Lin et al. reported a photo-triggered full-colour CPL system in which a chiral fluorescent donor and photochromic acceptors were incorporated into CLC matrices, illustrating the potential for tunable, multimodal optical functionality. Under ultraviolet (UV) irradiation, a Förster resonance energy transfer process occurs from the chiral donor to the photochromic acceptors, enabling reversible modulation of emission wavelength and CPL intensity, with the g_lum reaching up to −0.11.²⁶ Jiang et al. demonstrated upconversion CPL (UC-CPL) by incorporating upconversion nanoparticles (UCNPs) into CLCMs. Through tuning the photonic bandgap and excitation wavelength, and optimising the UCNP concentration, they achieved a g_lum value of −0.47.²⁷ In addition, Zhang et al. utilised coacervation to fabricate dye-doped CLCMs (DDCLCMs) with 1 wt% coumarin 7 (C7), achieving an exceptionally high g_lum value of 1.6.²⁸ Similarly, Li et al. fabricated carbon dot-doped CLCMs via coacervation, achieving a g_lum value of 1.4 and a photoluminescence quantum yield of 1.12 at a doping amount of 1 wt%.²⁹ Although luminophor-doped CLCMs have demonstrated promising CPL efficiency, systematic studies on dye-doped CLCM systems, particularly concerning optical crosstalk among multiple optical modes and their integration with scalable printing techniques, still remain challenging. Addressing these knowledge gaps is essential for advancing high-performance CPL materials toward practical applications.

In this study, a dye-doped CLCM composite film was developed to achieve low optical crosstalk, strong CPL performance, and compatibility with scalable printing processes. Commercially available rhodamine 6G (R6G) was selected as the emissive dopant because of its high quantum yield, excellent photostability, and environmental responsiveness.^30–34 The influence of R6G concentration, polymer shell thickness, and film thickness on encapsulation was systematically investigated. The optimised R6G-CLCM aqueous inks demonstrated excellent compatibility with scalable fabrication techniques, enabling their integration into flexible films and patterned architectures. At a doping concentration of 0.25 wt%, the system achieved a high g_lum value of −1, indicating robust CPL activity under practical processing conditions. This performance underscores the effectiveness of the encapsulation strategy in preserving chiral optical properties and its adaptability for real-world device fabrication. This approach establishes a versatile material platform for next-generation CPL inks by combining tunable optical activity with structural flexibility. Potential applications include information-rich optical labeling, flexible photonic coatings and 3D information encoding, where stability, ease of processing, and strong chiroptical responses are crucial.

Results and discussion

Synthesis and characterisation of R6G-CLCMs

As shown in Scheme 1a, R6G-CLCMs were synthesised through interfacial polymerisation. Dichloromethane was used as a co-solvent to enhance the dissolution of R6G, facilitating efficient dissolution of R6G within the liquid crystal (LC) oil phase. The resulting solution was then emulsified in an aqueous polyvinyl alcohol (PVA) medium, after which radical polymerisation of methyl methacrylate (MMA) was initiated using azobisisovaleronitrile (ABVN). Upon heating, ABVN-generated radicals initiated the polymerisation of MMA, which produced linear poly(methyl methacrylate) (PMMA) chains that encapsulated the CLC droplets (Fig. S1). Uniform R6G-CLCMs were successfully isolated after centrifugation and washing. Meanwhile, the incorporation of the chiral dopant S5011 imparted left-handed circular polarisation to the CLCMs.

Scheme 1 Schematic illustration of the synthesis and versatile processing routes of R6G-CLCMs. (a) Preparation of R6G-CLCMs via interfacial polymerisation. (b) Fabrication of films by mould casting. (c) Large-area film formation by blade coating. (d) Production of encrypted code boards for information anti-counterfeiting applications.

The successful encapsulation of R6G and CLCs within PMMA microcapsules was confirmed by differential scanning calorimetry (DSC, Fig. S2), thermogravimetric analysis (TGA, Fig. S3), Fourier-transform infrared spectroscopy (FTIR, Fig. S4), and energy-dispersive spectroscopy (EDS, Fig. S5). The resulting R6G-CLCMs exhibited an average particle size of ∼12 µm and displayed characteristic “Maltese cross” patterns under polarised optical microscopy (POM), confirming both precise optical alignment and enhanced mechanical stability (Fig. S6). As the dye concentration increased, the absorption colour of the microcapsules became markedly more distinct (Fig. S7), while the size distribution became progressively narrower. This trend was reflected in a reduction in the standard deviation from σ = 4.03 µm (C1, μ = 12.14 µm) to σ = 2.74 µm (C5, μ = 11.39 µm) (Fig. S8). For film preparation, the purified microcapsules were dispersed in a 15 wt% PVA solution at a 1 : 1.5 mass ratio. To minimise the coffee-ring effects during the mould-casting process, 0.01 wt% Silok 120 was added to the mixture (Scheme 1b–d and Fig. S9). Environmental tolerance evaluations revealed that the microcapsules retained both structural integrity and optical performance across the pH range of 5–10. Exposure to ethanol concentrations (>40 vol%) disrupted the LC alignment and weakened the PMMA shell, resulting in pronounced CLC leakage (Fig. S10).³⁵ These findings indicate that R6G-CLCMs exhibit strong acid–base resistance but display limited tolerance to ethanol.

Tunable optical properties via R6G concentration modulation

A critical step in evaluating the CPL performance of the R6G-based system is to compare their photophysical behaviour in two distinct environments: the dissolved state within LCs (R6G-CLC) and the encapsulated state within microcapsules (R6G-CLCMs). Encapsulation can significantly influence light–matter interactions, alter emission efficiency, and modify the chiral organisation, all of which directly affect the CPL output. Spectral analysis revealed that R6G-CLCMs exhibited multiple excitation bands, reflecting the complex microenvironment of the capsule matrix. The strongest emission intensity was observed at 531 nm, which was selected as the representative excitation wavelength for subsequent concentration-dependent studies (Fig. S11). This wavelength is critical because it corresponds to the most efficient radiative transition of the encapsulated system, providing a reliable basis for evaluating CPL amplification under different doping levels.

R6G was dissolved in CLCs and subsequently introduced into LC cells with a uniform thickness of 25 µm. Systematic variations in the concentration of the chiral dopant S5011 enabled precise tuning of the Bragg reflection across the visible spectrum. The reflectance and fluorescence spectra of R6G-CLC in the red, green, and blue regions (Fig. 1a and b) exhibited enhanced fluorescence around 554 nm when the R6G emission overlapped with the photonic band gap (PBG). In this case, the PBG acts to enhance the fluorescence emission by regulating the spectral interaction between the R6G emission and the cholesteric structure.³⁶ A progressive blue shift of the PBG was produced by a gradual increase in the R6G concentration, starting from 0.05 wt%, with higher concentrations leading to more pronounced shifts of the PBG (Fig. 1c). The fluorescence intensity peaked at 0.1 wt% R6G but decreased significantly at a higher concentration due to R6G molecular aggregation, which induces inner filter effects and nonradiative energy dissipation (Fig. 1d).^37–39

Fig. 1 (a) Bragg reflection spectra of R6G-CLCs in red, green, and blue regions. (b) Fluorescence spectra of R6G-CLCs with red, green, and blue structural colours (0.1 wt% R6G). (c) Reflection spectra and (d) fluorescence spectra of R6G-CLCs with different R6G concentrations. (e) Reflection spectra and (f) fluorescence spectra of R6G-CLCMs with different R6G concentrations. (g) Reflection spectra and (h) fluorescence spectra of R6G-CLCM films with different thicknesses (20 µm, 55 µm, and 85 µm).

As shown in Fig. 1e, spectral analysis of the resulting R6G-CLCM films revealed a slight red shift in the PBG as the dye concentration increased. This shift became particularly pronounced at 0.25 wt% R6G, where a distinct shoulder peak emerged near 625 nm, attributable to interference with the intrinsic absorption band of R6G.⁴⁰ Concurrently, as the R6G concentration was increased from 0.05 wt% to 0.25 wt%, the size distribution of the microcapsules became progressively narrower (Fig. S8). This narrowing promoted denser and more ordered packing in films of comparable thickness, thereby enhancing constructive optical interference and leading to higher peak reflectance values. Fluorescence analysis further revealed that R6G-CLCM films reached maximum fluorescence intensity at 0.15 wt% R6G (Fig. 1f) relative to pure CLCs, which showed maximum emission at 0.1 wt% R6G (Fig. 1c). This different concentration was attributed to the partial dissolution of the dye within the PVA matrix. Notably, the fluorescence intensity did not decrease significantly at higher dye concentrations in the CLCMs, most likely because the R6G molecules were physically immobilized within the PMMA shell. These isolated R6G molecules fluoresced independently within the shell phase and were not subjected to aggregation-induced quenching in the CLC matrix.⁴¹ To further evaluate the influence of the fabrication method and resulting thickness on optical performance, films were prepared using mould-casting (Scheme 1b) and blade-coating (Scheme 1c), respectively. The two techniques yielded films with markedly different thicknesses, corresponding to distinct variations in reflectance and fluorescence intensity (Fig. 1g and h). Mould-cast films were thicker (55 µm and 85 µm, measured with a digital micrometer) compared to thinner blade-coated films (∼20 µm, determined by the blade gap). The increased thickness of the mould-cast films resulted in higher reflectance and stronger fluorescence. This enhancement of optical performance was attributed to the greater packing density of R6G-CLCMs in thicker films, which intensified constructive optical interference under normal incidence and consequently increased both reflectance and fluorescence. Notably, the reflectance of the 85 µm film was lower than that of the 55 µm film. This decrease is attributed to stronger dye absorption in the thicker film, where the structural colour reflection was suppressed by the extended optical path length and increased dye aggregation. As a result, the reflectance peak was reduced despite the higher capsule density.

Multiscale CPL modulation in R6G-CLCM films

R6G typically emits unpolarised fluorescence under standard conditions. However, when incorporated into a left-handed CLC matrix, the dye molecules align helically, following the periodic helical CLC structure. This chiral arrangement enables R6G-CLCMs to selectively reflect left-handed circularly polarised (LCP) light, whereas the transmitted emission emerges predominantly as right-handed circularly polarised (RCP) light. As shown in Fig. 2a and b, doping the CLC matrix (Bragg reflection centred at 554 nm) with 0.1 wt% R6G resulted in a maximum dissymmetry factor g_lum value of −0.6 at the emission peak. This relatively high g_lum value demonstrates the efficiency of the helical CLC framework in transferring chiral optical information to the fluorophore. To further investigate environmental effects, the CPL performance of R6G-CLCMs was compared between their aqueous suspension and solid film states. In the aqueous dispersion, the microcapsules exhibited a significantly lower g_lum value of −0.018 (Fig. 2d). This decrease arises from the preservation of the spherical morphology and the random orientation of their helical axes in solution, which leads to partial cancellation of CPL signals. In contrast, the solid-state films restrict random orientation and enhance cooperative alignment, thereby maintaining stronger CPL activity. The observed emission behaviour confirms that R6G-CLCMs satisfy the Bragg reflection condition (λ = n̄p cos θ). Under normal incidence (θ ≈ 0°), the helical axes of CLCs are oriented predominantly perpendicular to the substrate. This orientation increases the effective optical path length and enhances the Bragg-selective reflection of circularly polarized luminescence by the cholesteric structure,⁴² thereby amplifying the CPL emission. In the solid-film state, this effect produced a g_lum value as high as −0.11 (Fig. 2f). A distinct blue shift of the CPL peak was observed, with the emission maximum shifting from 554 nm in the aqueous dispersion (Fig. 2c) to 523 nm in the film state (Fig. 2e). This spectral shift is primarily attributed to microcapsule deformation during the film formation process. The effective helical pitch decreases as the originally spherical capsules are compressed into more elliptical geometries, thereby shifting the Bragg reflection band and corresponding CPL emission toward a shorter wavelength.

Fig. 2 (a) CPL spectrum and (b) g_lum of R6G-CLCs in a liquid crystal cell. (c) CPL spectrum and (d) g_lum of R6G-CLCMs in aqueous dispersion. (e) CPL spectrum and (f) g_lum of R6G-CLCMs in film state.

Although a fraction of R6G can diffuse into the PMMA shell, the fluorescence originating from PMMA is unpolarised and therefore contributes minimally to the overall CPL performance. To systematically investigate the role of shell thickness on optical behaviours, two types of R6G-CLCMs were synthesised by varying the MMA monomer content (0.8 g and 1.2 g) whilst keeping all the other preparation conditions constant. An increase in MMA content produced thicker microcapsule shells, which consequently entrapped a greater fraction of R6G molecules within the shell region.⁴³ The shell thicknesses of R6G-CLCMs prepared with 0.8 g and 1.2 g MMA are shown in Fig. S12. For a direct comparison of their optical responses, films with identical thickness (55 µm) were fabricated using the mould-casting method. Scanning electron microscopy (SEM) images of individual microcapsules and film cross-sections are shown in Fig. S13. Interestingly, films composed of thin-shell capsules (0.8 g MMA) exhibited a higher reflectance, whereas those with thick shells (1.2 g MMA) displayed stronger fluorescence intensity (Fig. S14). As illustrated in Fig. 3a1, increasing the R6G concentration progressively diminished the intrinsic structural colouration of R6G-CLCM films. This effect arises from the dominance of the dye absorption and the enhanced aggregation of R6G molecules within the PMMA shell, which together produce a pronounced pink-orange appearance under daylight. Chromaticity analysis conducted under CIE 1931 standard conditions (D65 source, 2° observer angle) further corroborates this observation. The red circles in the chromaticity diagram represent the structural colour measured on black substrates, which minimises interference from absorbed light and daylight. In contrast, the blue circles correspond to the dye absorption measured on white substrates, which suppresses structural reflection and enhances the intrinsic colour of the dye. As shown in Fig. 3a2, the red-circled data points display wide scattering, indicating substantial displacement in structural colouration with increasing R6G concentration. Meanwhile, the blue-circled points exhibit a concentrated shift toward the pink-orange region, indicating progressively stronger dye absorption. Excitation at 360 nm was adopted to mitigate optical interference during fluorescence imaging. As shown in Fig. 3a1, increasing the R6G concentration in the thick-shell samples produced a pronounced redshift in the fluorescence emission, which is visible in UV-excited images and further supported by the emission peak evolution, as shown in Fig. S15. This redshift can be attributed to the aggregation of R6G molecules within the thicker PMMA shells, where the increased shell volume facilitates closer dye–dye interactions. In contrast, the thin-shell microcapsules (0.8 g MMA) retained their intrinsic structural colour across all R6G concentrations and exhibited only minor redshifts in fluorescence emission with increasing dye loading (Fig. 3a3). This relative stability arises because the reduced shell volume restricts the extent of dye aggregation, thereby preserving photonic bandgap reflection and maintaining consistent emission properties. The corresponding chromaticity diagrams (Fig. 3a4) and the peak evolution trends in Fig. S15 corroborate these findings and highlight the superior optical stability of the thin-shell R6G-CLCMs.⁴⁴

Fig. 3 (a1) Photographs and (a2) corresponding CIE chromaticity coordinates of thick-shell R6G-CLCM films (1.2 g MMA) under daylight and UV irradiation. (a3) Photographs and (a4) corresponding CIE chromaticity coordinates of thin-shell R6G-CLCM films (0.8 g MMA) under daylight and UV irradiation. (b) CPL spectra, (c) glum values, and (d) variation of the maximum glum at the strongest fluorescence emission peak with R6G concentration for mould-cast thick-shell R6G-CLCM films (1.2 g MMA) at various R6G concentrations. (e) CPL spectra, (f) g_lum values, and (g) variation of the maximum g_lum at the strongest fluorescence emission peak with R6G concentration for mould-cast thin-shell R6G-CLCM films (0.8 g MMA). (h) CPL spectra, (i) g_lum values, and (j) variation of the maximum g_lum value at the strongest fluorescence emission peak with R6G concentration for blade-coated thin-shell R6G-CLCM films (0.8 g MMA).

Previous studies have shown that the device thickness influences the performance of CPL-active materials.⁴⁵ At identical dye concentrations and comparable film thicknesses (∼55 µm), R6G-CLCM films with thin PMMA shells consistently exhibited higher g_lum values compared with their thick-shell counterparts (Fig. 3c and f). The thin-shell configuration reached a maximum g_lum value of approximately −0.36 at 0.25 wt% R6G (Fig. 3g), whereas the thick-shell samples remained below −0.16 across all tested concentrations (Fig. 3d). To systematically evaluate these effects, thin-shell R6G-CLCM samples were further studied by varying the dye concentration and the film thickness. When a blade gap of ∼20 µm was used to fabricate thin-shell films with increased R6G loading, the fluorescence intensity was enhanced, whereas the reflectance was slightly reduced. However, the g_lum values showed a pronounced decline under these conditions (Fig. 3j and Fig. S16). This behaviour indicates that a greater fraction of R6G molecules becomes incorporated into the PMMA shell at higher dye concentrations rather than aligning along the CLC helix. Because these shell-localised dyes emit unpolarised fluorescence that is uncoupled from the CLC's helical structure, they act as a non-chiral background signal, thereby reducing the overall CPL efficiency of the system. Furthermore, at a fixed dye concentration of 0.15 wt%, the R6G-CLCM films exhibited thickness-dependent CPL performance. The 55 µm film exhibited a higher g_lum value of −0.29, whilst the 20 µm film showed a lower value of −0.25 (Fig. 3g and j). This decline in CPL efficiency for thinner films can be attributed to inadequate vertical stacking of microcapsules, which limits cooperative helical alignment and weakens chiral signal amplification. These findings demonstrate that enhanced fluorescence intensity does not necessarily translate into higher g_lum values; thus, CPL performance in this work is governed by the shell thickness. To the best of our knowledge, this study represents the first systematic investigation into the role of the microcapsule shell thickness on CPL efficiency whilst simultaneously addressing the optical crosstalk between dye absorption and structural colouration in dye-doped CLCM systems.

To enhance g_lum, a 55 µm-thick film was fabricated using the mould-casting method, incorporating 0.25 wt% R6G-doped thin-shell microcapsules. Due to the diffuse scattering nature of microcapsule-based films, the Bragg reflection was first characterised with a fiber-optic spectrometer in diffuse reflectance mode, yielding a reflection peak at 559 nm (Fig. 1e). Subsequent CPL measurements conducted using a JASCO-300 spectrometer exhibited a pronounced CPL peak at approximately 525 nm (Fig. 4d), indicating a hypsochromic shift of 29 nm relative to the initial Bragg reflection. To obtain a more accurate determination of the Bragg reflection, UV-visible diffuse reflectance was performed using an integrating sphere. The measurements revealed that the actual Bragg reflection peak for the optimised R6G-CLCM film occurred at approximately 526 nm (Fig. 4a), which was notably misaligned with the fluorescence emission peak. To align these peaks, the helical pitch was carefully adjusted by modifying the chiral dopant concentration. This optimisation produces a 0.25 wt% R6G-doped film with a Bragg reflection at 558 nm (Fig. 4a), precisely coinciding with the fluorescence emission peak (557 nm), thereby achieving a maximum g_lum value of −1 (Fig. 4b and d). The residual 15 nm blue shift of the CPL peak relative to the adjusted Bragg reflection (Fig. 4c) can be attributed to photonic bandgap coupling effects.⁴⁶ The achieved g_lum value significantly exceeded previously reported values for pure CLC systems (−0.6, Fig. 2d). The optimised film also exhibited excellent chiroptical selectivity under daylight and UV irradiation (Fig. 4e). Under daylight, the film exhibited pronounced polarisation-dependent structural colouration. Vivid structural colours were observed when viewed through a left-handed circular polariser (S-CPR), whereas structural colouration was significantly reduced or even absent when examined through a right-handed polariser (R-CPR). In the latter case, only the intrinsic absorption of the randomly oriented dye molecules was visible. Upon UV excitation, the films generated strongly polarised emission, with the fluorescence intensity measured through L-CPR substantially exceeding that observed through R-CPR. This asymmetry provides direct evidence of the preferential alignment of R6G chromophores within the chiral CLC matrix. The highest fluorescence intensity was detected in the absence of polarisation filters, indicating the combined contributions of both polarised and non-polarised emission components.

Fig. 4 (a) Bragg reflection peaks before and after optimisation. (b) Fluorescence emission spectrum. (c) CPL spectrum. (d) g_lum spectrum. (e) Photographs of the optimised film under daylight, the left-handed circular polariser (S-CPR), and the right-handed circular polariser (R-CPR), as well as under UV irradiation (UV), the left-handed circular polariser (UV-S-CPR), and the right-handed circular polariser (UV-R-CPR).

Patterned multi-mode colouration films and anti-counterfeiting information

The mould-casting method was used to fabricate an apple-pattern film, which displayed pronounced angle-dependent optical properties when positioned on black substrates (Fig. 5a). The majority of the CLC helical axes were oriented perpendicular to the viewing direction, thereby maximising the structural colouration intensity through constructive interference under Bragg's law at normal incidence (θ ≈ 0°). In contrast, dye absorption became pronounced at an oblique viewing angle (θ ≈ 90°), and the CLC structural colour was completely suppressed. This angular optical response was attributed to the deformation of microcapsules into ellipsoidal geometries during film formation, as confirmed by SEM (Fig. S12), which promoted preferential alignment of CLC domains. Furthermore, at oblique orientations, the elongated optical path length through PMMA shells encapsulating R6G significantly intensified the perceptible dye absorption at oblique orientations. The effective optical contrast generated by the underlying black substrate was progressively diminished because its visual contribution decreased at larger viewing angles. As shown in Fig. 5a2, the apple pattern exhibited a notable increase in fluorescence intensity as the viewing angle shifted from 0° to −90° under UV irradiation. This trend corresponds to the enhanced dye absorption observed at oblique orientations, reinforcing the angle-dependent optical response. Additional patterned films were fabricated using the mould-casting method (Fig. S17). In addition, a free-standing film was produced on a polyethylene terephthalate (PET) substrate via blade coating with a 20 µm doctor blade, which exhibited pronounced circular polarisation characteristics (Fig. 5b1) whilst maintaining excellent mechanical flexibility. These films demonstrated reversible optical switching behaviour: the dye absorption band became dominant in determining optical visibility in the curled configuration, whereas the CLC phase was predominant in the flat configuration (Fig. 5b2).

Fig. 5 (a) Mould cast patterned films observed under daylight (a1) and UV irradiation (a2). (b1) Blade coated films on a PET substrate photographed under daylight (top), left-handed CPL filter (middle), and right-handed CPL filter (bottom). (b2) Free-standing films demonstrating flexibility and peelability. (c) Dot-array “code board” fabricated by direct dispensing, imaged under daylight and UV irradiation (scale bar: 1 cm).

As illustrated in Scheme 1d, a laser-engraved “code board” mould was fabricated on a transparent acrylic sheet, consisting of a 5 × 18 array of raised micro-blocks (0.3 × 0.3 × 0.2 mm). To facilitate film detachment and repeated mould reuse, the patterned surface was coated with a thin film of 15 wt% PVA solution using an MP1000 dispensing system and subsequently dried. Two types of functional inks were prepared to demonstrate selective optical encoding: one composed of R6G-CLCMs and the other of dye-free CLCMs. Polarized optical microscopy (POM) images of the dye-free CLCMs are shown in Fig. S18. Both inks were blended with a 15 wt% PVA solution at a microcapsule-to-PVA ratio of 1 : 1 (w/w) to ensure uniform dispersion and film-forming capability. The inks were precisely deposited into predetermined grid positions on the mould using a MP1000 dispensing system, producing a regular patterned array (Fig. 5c). Following natural drying at room light, the patterned film was gently peeled from the acrylic mould, with the PVA interlayer acting as a flexible supporting substrate. Under daylight, the array appeared as a series of uniform green dots resulting from structural colouration, which effectively concealed the embedded information. However, upon UV irradiation, only the R6G-CLCM region emitted intense fluorescence, revealing the hidden message “BIGC”. This contrast between daylight camouflage and UV-selective emission demonstrates the dual-ink system's optical encryption potential.

Using the same strategy as the code board, a 21 × 21 QR code template was fabricated by laser engraving, followed by coating with a thin PVA layer (Fig. 6a). Two encoding schemes were implemented. In the first scheme, a single-ink QR code was fabricated using R6G-CLCM inks (Fig. 6b). Under ambient light, the QR code exhibited structural colour against the black background. Upon UV irradiation, it showed light pink fluorescence; however, due to interference from the UV source, the overall appearance adopted a pink-blue tint. Despite this colour shift, the QR code remained recognizable using a smartphone, successfully decoding to the text “R6G-CLCMs.” Polarised imaging (Fig. S19) reveals that, under daylight, the QR code exhibited a bright green structural colour. The colour appeared more vivid when viewed through a left-handed circular polarizer, whilst it largely disappeared under a R-CPR. The incomplete extinction was attributed to thickness non-uniformity resulting from surface gradients formed during the dispensing and drying of the microcapsules on the flat PVA layer. Under UV irradiation, the fluorescence intensity was strongest without a polariser, relatively strong with an L-CPR, and decreased with an R-CPR, confirming that both polarised and non-polarised emission components contributed, as observed in optimised thin-film samples. Similarly, angle-dependent observations (Fig. S20) further showed a pink shift in the QR code appearance, indicating a transition from cholesteric structural colouration to dye absorption colouration.

Fig. 6 Dispensing fabrication and optical encryption performance of QR code patterns. (a) Schematic illustration of the QR code preparation process using the dispensing method. (b) QR code generated with R6G-CLCM inks, which can be successfully decoded under both daylight (left) and UV irradiation (right) using a smartphone. (c) Dual-ink QR code systems prepared with R6G-CLCMs and dye-free CLCMs (red box region), where the incorporation of dye-free CLCMs serves as an encryption element, rendering the code unreadable under daylight but successfully decoded under UV irradiation. (d) The same dual-ink QR code was placed on a white substrate, which remains unreadable under daylight but can be successfully decoded under UV irradiation (scale bar: 1 cm).

In the second scheme, a dual-ink system was implemented. Building upon the first scheme, a red-box region was encoded using dye-free CLCMs (Fig. 6c). Under daylight, the dye-free CLCM regions displayed a structural colour identical to that of the R6G-CLCM regions, thereby preventing smartphone recognition and achieving an encryption effect. Under UV irradiation, however, only the R6G-CLCMs emitted fluorescence, enabling the smartphone to successfully decode the text “R6G-CLCMs”. This strategy thus provides dual-level anti-counterfeiting functionality. When the QR code was transferred onto a white substrate (Fig. 6d), the structural colour of R6G-CLCMs was significantly weakened, and the QR code appeared as a faint pink pattern dominated by dye absorption. Due to the low contrast, the QR code could not be effectively recognized under ambient light. Upon UV excitation, however, fluorescence from the R6G-CLCM regions enabled successful decoding. Fig. 6d is captured on a non-fluorescent white board under irradiation of one D65 standard light source. In contrast, when put it on ordinary white paper, background fluorescence from the substrate interfered with recognition, further highlighting the robustness of the R6G-CLCM ink system for encryption applications. A demonstration of QR code recognition is given in Video S1.

Conclusions

Water-based R6G-CLCM inks with tunable multimodal optical characteristics were developed, integrating vivid structural colouration, strong dye absorption, and angle-dependent fluorescence emission. By optimising the dye concentration (0.25 wt% R6G), shell thickness (0.8 g MMA), and film thickness (55 µm), the optical response was precisely controlled, yielding a maximum luminescence dissymmetry factor (g_lum) of −1. The optimised R6G-CLCM films exhibited minimal optical crosstalk among structural colouration, fluorescence, and dye absorption, enabling high-contrast CPL. Furthermore, these optical inks demonstrated broad compatibility with scalable fabrication approaches, including mould-casting, blade coating, and direct dispensing. This versatility highlights their potential in advanced photonic applications, particularly in anti-counterfeiting technologies, flexible photonic coatings, and high-density 3D optical data encoding.

Author contributions

Data curation: Shenghao Hua. Funding acquisition: Xiaojuan Wu and Yinjie Chen. Methodology: Shenghao Hua. Supervision: Xiaojuan Wu and Yinjie Chen. Resources: Yu Liu. Validation: Shenghao Hua and Haixiao Yang. Writing – original draft: Shenghao Hua. Writing – review and editing: Shenghao Hua, Xiaojuan Wu, Yinjie Chen, Yajun Zhang, Haifeng Yu and Quan Li.

Conflicts of interest

There are no conflicts to declare.

Data availability

All data supporting the findings of this study are available within the article and the accompanying supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d5tc04178c.

Acknowledgements

This work was supported by the National Key R&D Program of China (Grant No. 2024YFB3612600), the National Natural Science Foundation of China (Grant No. 52472086), the Beijing Natural Science Foundation (Grant No. JR25028), the Project of Cultivation for Young Top-Notch Talents of Beijing Municipal Institutions (Grant No. BPHR202203071), the Project of Construction and Support for High-level Innovative Teams of Beijing Municipal Institutions (Grant No. BPHR20220107), the Beijing Institute of Graphic Communication College-level Projects (Grant No. Ea202402 and Ec202501), the Platform Construction Project of Beijing Institute of Graphic Communication (Grant No. KYCPT202510), and the Beijing Nova Program (Project No. 20240484683).

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Footnote

† These authors contributed equally to this work.

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