1. Introduction

Nowadays, there is a growing need for optical security techniques such as encoding, encryption, recognition, secure identification, watermarking verification, brand protection, authentication, personal data management, protection of diagnostic devices from counterfeiting, etc [1]. Information security systems based on optical methods have been widely developed and studied in the recent years compared to electronic encryption methods, because of an optical encryption benefits from parallel processing and multiple degrees of freedom or multidimensional characteristics, such as amplitude, phase, wavelength, and polarization [2].

Quick Response (QR) code that was intended for the data storage and fast reading applications has been very extensively investigated. The QR code is one of the 2D matrix codes, which belongs to a larger set of machine-readable codes [3]. QR coding is employed to achieve noise-free decrypted results applications including: information storage (advertising, museum art description), redirection to websites, track and trace (for transportation tickets or brands), identification (flight passenger information, supermarket products), protection against counterfeited medications and fake medicines, etc [4–6].

On the other hand, stimuli-responsive photo-functional materials which can be reused for multiple times, are progressively emerging and have shown potential in the application of rewritable media. Several external-stimuli responsive materials exhibiting color or luminescence switching have been developed for the fabrication of rewritable paper [7,8]. Among the different stimuli, light is the most attractive one that can be delivered instantly in a precise location, and in the form of different wavelengths, to which different photoresponsive molecules selectively respond [9,10].

Due to the light-driven molecular switching properties, photochromic compounds have been incorporated into many intriguing systems, including molecular actuators, the surface of the nanoparticles, optical memory devices, holographic gratings, and drug delivery vesicles [11–17]. One of the most unique examples of molecular switches is spiropyran (SP), whose closed-ring form, a hydrophobic isomer transforms into a highly polar, open-ring merocyanine (MC) form, upon exposure to UV light, whereas the reverse reaction can be induced by visible light, or by heat [18,19]. The use of self-assembling materials, as liquid crystals, combined with polymeric matrices has been proposed as an emerging concept that enables to tailor the optical and photonic properties of the mixtures that became suitable for overt and covert anti-counterfeiting systems. Cholesteric liquid crystals (CLC) are able to self-assemble in a simple one-dimensional photonic crystal with unique optical properties: 100% selective reflection of circularly polarized light and ability to change their selective reflection wavelength changing external or internal factors (electric, magnetic and acoustic fields, temperature, local order, etc.). These properties of CLC structures make them good candidates for tracing, authentication and identification technologies [20–23]. SP doped CLC systems have many advanced optical attributes quite different from those of isotropic liquids, semiconductors and polymer doped ones: extra high non-destructive solubility of SP in liquid crystal host, which can vary from 1% to 4% (by weight); high orientational order parameters of SP molecules given by the spatial orientation of the molecules of the CLC host [24].

Here, we report on a rewritable media based on an SP doped polymer dispersed cholesteric liquid crystal (SPCLC) film, to produce high quality, rewritable visual QR codes. Herein, the reversible writing and erasing occur through the coloration or discoloration of the photochromic material. The SPCLC film was fabricated from an SP doped CLC mixture, encapsulating in a polymer matrix. The prepared film exhibits a light controllable spatiotemporal photosensitivity, which can be used as an optical recording medium, with a micrometric spatial resolution and a good contrast ratio due to the confinement of the CLC and SP moieties inside the polymeric microcapsules. The encapsulation grants robustness and stability of the film that can undergo several tens of writing/erasing cycles. Further, the high solubility of SP molecules inside the liquid crystal host leads to an increased sensitivity of the film to light irradiation.

2. Sample preparation and experimental setup

To fabricate an SPCLC film, commercially available and certified compounds were used: nematic matrix-BL-036, optically active dopant (OAD) MLC-6247 (both from Merck), photochromic material-1',3′,3′-Trimethyl-6-nitro-1',3′-dihydrospiro[chromene-2,2'-indole] (SP), and macromonomer Poly(vinyl alcohol) – PVA with an average molecular weight -Mw 85,000-124,000, 99 + % hydrolyzed (both from Sigma-Aldrich). An SP doped CLC mixture was prepared mixing the nematic host, OAD, and SP with the following concentration ratio in weight: 96% [74.6% BL-036 + 25.4%MLC-6247 ] + 4% SP. Thin film was prepared as follows: 1 g of the SP doped CLC mixture was added to 10 mL of a 17 weight % PVA solution in distilled water. As the suitable emulsifying agent, 0.5 mL of icy acetic acid was doped to the mixture. The prepared composite was poured into a glass vial and stirred at 1600 rpm for 30 min at 75 °C. After this procedure, a creamy white emulsion was obtained. By controlling all stages of microencapsulation, such as stirring speed, temperature and concentration ratio of the constituents we have obtained the microcapsules with the predictable sizes.

In order to prepare a uniform film with the desired thickness, was utilized a drop casting-spreading method. At first, the prepared emulsion was let to degas for 1 h, then was dropped on the glass substrate treated by deionized water and was spread homogeneously using a metal stripe. The prepared sample was stored for 48 hours at room temperature. After the water evaporation, a film was detached carefully from the glass substrate. As a result, we obtained process-perfect, uniform elastic film, which meets the requirements for the practical applications. Prepared SPCLC film contains a phase–separated composites aggregated in the form of microcapsules with desired packages and spatial distributions. The dispersive medium of the polymer matrix preserves at most the initial properties of the SP doped composite and conditions the production of polymer films with high elasticity and photosensitivity. The microcapsules produced in such a manner tend to be uniform in size, in a range from 5 to 15 μm. The film thickness measured was about 30 μm. Prepared film is not sensitive to humidity and it can operate in ambient conditions and large temperature range. The surface morphology and a structure of the SPCLC film were investigated by using an optical microscope, Fig. 1.

 

Fig. 1 Optical microscope image of an SPCLC film. The size of the microcapsules ranges from 5 to 15 μm.

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To write/erase the images on an SPCLC film, a U-shaped flash lamp (UFL) with a flash duration of 500 microseconds and with a power of 0, 04 J/pulse, was used. The rest period between the generated flashes was 10 seconds. For a light-induced generation of MC, a 340–400 nm band-pass filter was utilized. The reverse switching to the non-colored form was obtained by using the same light source with a 540–630 nm bandpass filter. The distance from the UFL to the samples was adjusted to 3 cm. The photo-switching behavior and the absorption spectra of the SPCLC films were investigated by a fiber optic spectrometer (Avaspec-2048, Avantes), at room temperature. The photo recording procedure has been implemented using inverse grayscale photomasks. QRs on the photomasks were created employing free online barcode generator. Appropriate photomasks were placed firmly on the SPCLC films to obtain a good optical contrast, Fig. 2.

 

Fig. 2 Schematic of the experimental setup used to record QRs on the SPCLC films; (1) UFL; (2) band-pass filter; (3) inverse grayscale photomask; (4) recorded image on SPCLC film.

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To show the UV light-induced absorbance, we irradiated an SPCLC film by UFL. For the recording of absorbance before and after UV irradiations, a spectrometer was used. To maximally reduce the light scattering of SPCLC film caused by the encapsulated drops, the optical fibers were firmly attached to the SPCLC film. During the very short interval of time (500 microseconds), the maximum of absorbance of SPCLC film centered at 585 nm was increased from 0.06 to 1.26 arbitrary units, Fig. 3.

 

Fig. 3 Absorbance plots and image of SPCLC film before (a), and after (b), exposures by UFL source.

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The resolution of the images recorded on the SPCLC film can be defined by the average size of microcapsules. Considering that the recorded images are assembled with 10 μm size dots (an average size of microcapsules) one can conclude that the maximum achievable resolution is 100 lines per millimeter. In Fig. 4, shown the rhombus-shaped pixels recorded on the SPCLC film, using an inverse grayscale photomask.

 

Fig. 4 Recorded pixels on the SPCLC film as they look under the optical microscope.

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3. Results and discussions

In order to calculate the static contrast ratio of the recorded images, the general equations which relate the absorbance a (λ) to the transmittance T (λ) were used. In particular, $A(\lambda )=\lg I_0(\lambda )/I(\lambda )$ and$T(\lambda )=I(\lambda )/I_0(\lambda )={10}^{-A}$, where $I_0(\lambda )$is the incident light intensity and $I(\lambda )$is the transmitted light intensity, respectively. As the light source, a fiber-coupled halogen lamp (12V, 20 W, Osram) equipped with band-pass filters (cut-on wavelength 535 nm and cut-off wavelength 590 nm) was used. A power/energy meter was utilized to measure the light absorbance in the bright and dark parts of the recorded images. The light absorbance$A_w$, measured in the “white” spots of the image can be found using the equation:

An average attenuation of the transmitted light intensity measured for (i, j) spots at λ = 585 nm, was found as:

 

Fig. 5 Recorded image of QR code; static contrast ratio equal to 22.

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The photo-stability over the time has also to be considered when dealing with photochromic SP dyes. The number of recordings/erasing cycles, that a system can undergo, is a critical experimental parameter. To study the feasibility of repeated cycles, experiments on the fatigue resistance of SPCLC films upon UV and visible light irradiations were carried out. In a cycle, a system is transformed from the SP configuration to the MC one, which can be returned back thermally or optically. If the degree of degradation in a cycle is x, the not degraded fraction y after n cycles will be y = (1− x)^n.. For very small x and very large n, this expression can be approximated as y ≈1− nx. In the experiments, an average measured reduction of the absorption after ten cycles on an SPCLC film was found to be 0.08 (yield = 92%).

We have approximated this dependence for all cycles and built an appropriated plot. Furthermore, we performed optical measurements to compare the calculated dependence on the experimental results. Considering the differences in the peak absorption bands of SP and MC and this approximated value were used to calculate the absorption and to compare experimental results during 75 cycles. Optical measurements were performed to compare the calculated absorbance (Fig. 6. line A) as a function of the number of cycles with the experimental results (Fig. 6. line B). The exposure time for UV/Vis irradiations was set to 500 microseconds. Using the UFL and two 340–400 nm and 540–630 nm band-pass filters, multiple times recording/erasing irradiation experiments were performed at room temperature. Absorptions were recorded immediately after each irradiation. The time interval between two cycles was set to 10 seconds. The linear estimation for the absorption approximates a more complex behavior as, after the first ten cycles, the efficiency was found to be practically constant with no evident hysteresis. The absorption of MC form started to lower after around 45 cycles, and smoothly decreased with increasing the number of cycles. As demonstrated in Fig. 6, even after 70 erasing-recording cycles, the absorption efficiency of MC form is high enough to obtain images with yet acceptable optical contrast.

 

Fig. 6 Absorbance under recording/erasing cycles. Spots A (red) show the calculated dependence of absorbance on the number of cycles and spots B (blue) the experimental results.

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As shown in Fig. 6, in this system, the reduction of absorption could be due to the fact that the MC molecules polarity plays an important role in the interaction with the liquid crystal. In the MC form, molecules have a reduced solubility in the CLC host. This could cause an aggregation of MC molecules that hinders an easy transition to the SP form. In Fig. 7. are demonstrated the different images recorded on the same SPCLC film, using UFL light source. Despite the fact that the SPCLC film was erased and rewritten multiple times, the images on the film retain a high spatial resolution and good readout quality.

 

Fig. 7 Photograph of the same SPCLC film, which was erased and rewritten multiple times.

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Another promising development associated with the information encryption technique is the luminescence printing because it provides easy visualization of barcodes, localization of signals, and fast readout. It is known that MC form emits a strong red luminescence light under UV/Visible excitation. While the SP isomer does not exhibit strong emission, a ring-opening one results in the appearance of an intense emission band and subsequently, the resulting red emission can be “turned on/off” [19]. Accordingly, it would be of particular interest to use the luminescence property of MC molecules to imprint rewritable luminescent QRs for anti-counterfeiting and security applications [25,26]. For the recording of luminescent QRs, the above-described actions were performed. For the reading of QR luminescent images was utilized a low-intensity halogen lamp, to avoid a fast erase procedures. Herein, a halogen lamp was equipped with 540–630 nm bandpass filter. Luminescent spectra were recorded using a spectrometer. In Fig. 8, in the left part of the picture shown a luminescence image of QR, imprinted on an SPCLC film, and in the right part of the picture shown the emission spectra of an SPCLC film upon excitation with the halogen lamp. Seemingly, a faint emission occurring after the “erase” procedure, indicates that some quantity of photochromic molecules remains in an MC isomeric condition.

 

Fig. 8 Luminescent image of QR code, recorded on SPCLC film (a) and the luminescent spectra of the SPCLC film upon excitation with a halogen lamp (b); in the right picture the curves “on/off” correspond the light emissions under “write/erase” cycles.

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As with the non-luminescence circumstance, the luminescent QRs can undergo several tens of writing/erasing cycles.

4. Conclusions

In summary, for the first time is demonstrated the fabrication of a rewritable QRs, imprinted on SP doped polymer dispersed CLC films, suitable for anti-counterfeiting and security applications. By alternating UV and visible lights, images on the photochromic films can be written/erased up to 70 times, without compromising their readability. Low-cost, simple-to-fabricate, simple-to-operate and quick update feature of the films promise to produce high-quality visual QR codes that can be scanned and read out using conventional devices, such as smartphones. Furthermore, the stimuli-responsive luminescence switching properties of the proposed films may be widely acceptable to the development of novel functional materials based information storage holographic systems and devices suitable for the high-resolution spatiotemporal imaging in biological samples.