Construction of anti-counterfeiting pattern on the cellulose film by in-situ regulation strategies

Cellulose and its derivatives films are increasingly used in paper, packaging, and other fields, but there is limited research on its films with anti-counterfeiting patterns. Here, we demonstrate an approach to prepare cellulose film with a micro-millimeter scale pattern by lithium chloride (LiCl) crystal template, which grows out by the in-situ generation in the film formation process. The surface morphologies of the as-prepared film were systematically controlled by regulating the solution viscosity (η) and the surrounding temperature. The imprinting of LiCl crystal was retained on the film surface after rinsing. Moreover, the crystallinity of patterned film was significantly improved, and about 62 MPa increased tensile strength than the film without a pattern. Incredibly, these patterns were similar to fingerprint size, which exhibited an excellent function to unlock fingerprint locked phones and showed potential in the anti-counterfeiting yield.


Introduction
Due to the great coding capacity and extensive application, visually (Braeckmans et al. 2003;Han et al. 2012;Huang et al. 2010) and spectrally (Lee et al. 2010(Lee et al. , 2014, encoded taggants have proven tempting as 'identifiers' to convey information about tagged items. On the other hand, most designs are insufficiently appropriate as 'security tags' for verification since these taggants are prone to cloning threats. Hypothetically, any deterministic encoding system eventually leads to counterfeiting by definition and code replication. As a result, a nondeterministic encoding system is required for robust authentication to ensure distinct and fundamentally irreproducible code outcomes. A physical object with an intrinsic, unique, fingerprint-like property is known as a physical unclonable function (PUF). A random pattern with various features achieves its individuality. (i) It is constructed on Abstract Cellulose and its derivatives films are increasingly used in paper, packaging, and other fields, but there is limited research on its films with anti-counterfeiting patterns. Here, we demonstrate an approach to prepare cellulose film with a micromillimeter scale pattern by lithium chloride (LiCl) crystal template, which grows out by the in-situ generation in the film formation process. The surface morphologies of the as-prepared film were systematically controlled by regulating the solution viscosity (η) and the surrounding temperature. The imprinting of LiCl crystal was retained on the film surface after rinsing. Moreover, the crystallinity of patterned the object's microstructure being randomly disordered throughout the production procedure; (ii) It is simple to construct yet impossible to duplicate, even by the producer; (iii) It is simple to understand yet difficult to anticipate.; and (iv) It is entirely unclonable by characterization. Due to the desirable encoding features, basic information about the tag does not have to be hidden for unclonable functions to work. They merely depend on the uniqueness of their physical properties. The uniqueness should initiate from the usual variance in the PUF production protocol. Utilizing biomimetic fingerprint-like structures was one of the most crucial ways of synthesizing PUF anti-counterfeiting materials. Liquid crystals and the random wrinkling of polymer films both contained artificial fingerprintlike features that may be read out via imaging. Nakayama et al. (Ohtsubob 2012) showed that the photopolymerized ultraviolet (UV)-curable liquid crystals produce polarized patterns fixed in a random texture. Furthermore, Bae et al. (Bae et al. 2015) designed unique artificial fingerprints coated with disc-shaped polymer particles by randomly wrinkling silica films with remarkable encoding capability. Unfortunately, it is limited by a sensitive identification technique with 80% reliability, subjected to manual verification in practical application.
Recently, sensitive recognition technologies were improved and fused into PUF design. For example, Tian et al. (Tian et al. 2016) developed a material having electromagnetically sensitive areas by covering a randomly folded polymer sheet with plasmonic nanostructures. The plasmonic film was encoded with Raman tags that give bright surface-enhanced Raman scattering (SERS) signals. The inclusion of Raman tags offers stochastic chemical patterns on top of the nondeterministic structural patterns and expands the PUF key's encoding capability and the number of potential responses. However, they still suffer from some limitations, such as (i) orientation-sensitive optical codes (Baughman et al. 2002), (ii) short lifespan of fluorescent material (Feng et al. 2019), and (iii) narrow-ranged product applicability (Buchanan et al. 2005). Moreover, its patterned size mainly focused on the micro-nano level, and it was challenging to produce a clear visual appearance and judge anti-counterfeiting. On the other hand, overt macrofeatures let users authenticate the authenticity of a pack. Such characteristics are pronounced and complicated, but they are difficult to duplicate (Herder et al. 2014;Pappu et al. 2002). As a result, PUF taggants are not yet versatile enough for an industry scalable authentication technique (Bazrafshan 2020;Gassend et al. 2002;Geng et al. 2016).
According to the above literature, the preparation method of the current PUF was often that particles with fluorescent or structural colors were embedded into the matrix to form an anti-counterfeiting function under external stimulation. The process is very complex. Meanwhile, the materials of anti-counterfeiting taggants were mainly from the petroleum-based polymer matrix, which has caused severe white pollution. But, bio-based polymers could overcome this shortcoming, which had been widely applied in the packaging and other yields (Cazón and Vázquez 2021;Liu et al. 2021;Mahmud et al. 2020). Cellulose is one such renewable and degradable natural polymer Mahmud et al. 2021), which film has high mechanical strength, excellent hydrophilic properties, good thermal resistance, and a good potential to replace petroleum-based films (Azeredo et al. 2017;Zhang et al. 2020;Zhao et al. 2019). Surprisingly, limited reports were found regarding the macro-pattern on the surface of cellulose films as an anti-counterfeiting code.
Herein, the in-situ growth of LiCl crystal on the surface of natural cellulose film at different temperatures was investigated. The surface morphologies of the film were systematically controlled by regulating the solution viscosity and temperature. The morphology of LiCl crystal was retained on the surface of cellulose film after rinsing and achieved the different macroscopic patterned film. The size of the pattern was in the micron range, which was the same level as the human fingerprint. This method was easy to operate and did not need complicated instruments and equipment. The pattern on the surface of natural cellulose film was crisp, complex, and challenging to reproduce, which exhibited a potential application in anti-counterfeiting.
Ltd., Fushan, China. Anhydrous LiCl was obtained from Shanghai Mclin Biochemical Technology Co. Ltd., Shanghai, China; and N, N-dimethyl acetamide (DMAc) was purchased from Guoyao Pharmaceutical Group Co. Ltd., Jiangxi, China. Carboxymethyl cellulose (CMC, DP = 700) and Tri-cellulose acetate (DP = 700) were purchased from Haidong Qinghua Co. Ltd., Zhangjiagang, China; In preparing patterned cellulose films, a dissolution method was initially adopted to prepare cellulose solution, according to our previous reports (Zhang et al. 2021a, b). In brief, cellulose was initially agitated in DMAc solution for 30 min at 160 °C, and DMAc was squeezed out at around 180 °C. Then, the solution was cooled to ambient temperature and left there until a translucent cellulose solution developed. The solution was made on the basis of weight. Then, the solution was poured onto a clean glass plate. A smooth glass rod with copper wire at both ends was used to scrape the plate's casting solution. The glass plate was then inserted into the baking oven, where the temperature was set to 70, 80, 90, 100, and 120 °C, respectively. After about two hours, most of the DMAc had been evaporated, and a polarizing optical microscope (POM) observed the crystal morphology of LiCl on the surface of cellulose film. The glass plate was then immersed in a water bath for 6 h to remove the residual LiCl crystals and DMAc at room temperature. After the LiCl/DMAc was removed entirely, the wet film was taken out and dried naturally at room temperature. The print of LiCl crystals was retained on the cellulose substrates even after rinsing. Finally, the pattern on the cellulose film was engraved by eliminating LiCl crystal and monitored by a stereo-microscope. Similarly, the solution of CMC and Tri-cellulose acetate and patterns were achieved.
During the characterization, the viscosity of cellulose solution was measured by a digital viscometer (NDJ-5 S, Pingxuan Scientific Instruments Co. Ltd., Shanghai, China). A stereo microscope (XTL-770E, Chang Fang Instruments Co. Ltd., Shanghai, China) was used to track the film formation process and take pictures. The morphology of crystals in the film was observed by a polarizing microscope (Nicon LV100POL, Japan). Alpha-Step IQ (2D benchtop surface profiler), HRP 350 series provides comprehensive surface metrology analysis and surface topography control solutions.

Results and discussion
As shown in Fig. 1, the cellulose solution was coated on glass (a-b), after film formation (c), the solvent was slowly evaporated at different ambient temperatures, and lithium chloride crystals (d-f) with different morphologies were gradually formed on the films. The method for preparing the patterned film is simple and convenient compared with the procedure for preparing the patterned polymer film reported in the literature (Bazrafshan 2020;  Gassend et al. 2002). The morphology of lithium chloride crystals formed in the process can be observed by Polarized light microscopy (Fig. 2).
The films with divergent like a LiCl crystal were obtained in Fig. 2 (a 1 , a 2 , and a 3 ). The concentration of the film-forming solution was 1.0, 1.5, and 2 wt %, respectively. The DP of cellulose was about 700. It was noticed that with the setting of the growth temperature at 90 °C, the crystal size was decreased with the increase of cellulose concentration. Cellulose film retained crystal morphology print after rinsing (Fig. 2b 1 , b 2, and b 3 ). The daisy and chrysanthemum-like crystals appeared on the film's surface when the DP of cellulose was 1400 (Fig. 2a 5 and a 6 ), while the size was also different due to the solution concentration variation.
Considering the formation process of the crisp daisy and chrysanthemum-like print, we further investigated the effect of temperature (80-120 °C) on the morphology of LiCl crystal ( Figure S1 and S2). It was observed that the size of the pattern increased first and then decreased in the process of increasing temperature from 80 to 120 °C ( Figure S3). Meanwhile, an increasing number of branching was obtained with the increasing temperature ( Figure S1 and S2). However, few regular patterns were obtained once the temperature was lowered than 90 °C ( Figure  S1 and S3). It showed that the formation of a perfect crystal needed enough activation energy.

Fig. 2
Polarization microscopic (yellow) before rinsing and stereo-microscope picture (green) after rinsing on the surface of cellulose films formed at temperature for 90 °C (DP = 700, a 1 , b 1 for 1.0 wt%; a 2 , b 2 for 1.5 wt%; a 3 , b 3 for 2.0 wt%), (DP = 1400, a 4 , b 4 for 1 wt%; a 5 , b 5 for 1.5 wt%; a 6 , b 6 for 2.0 wt%) Moreover, the print clarity was improved with the increase of concentration (viscosity) or DP according to the stereo-microscope picture ( Figure S1 and S2) and characterization of the surface profilometer ( Fig. 3E 4 and E 5 , Table S2 and S3). The main reason was that the growing DP and concentration could increase the viscosity of the solution, thus improving the matrix stability, which was conducive to stabilizing the arrangement of LiCl crystal. The higher temperature was beneficial to LiCl crystals' growth and formed a relatively perfect patterned structure under relatively stable substrates.
To further verify the effect of different viscosity (0.031, 2.02, and 19.94 Pa. S) on the morphology of LiCl crystal, the variable cellulose concentrations (1.0, 2.0, and 4.0 wt%) with DP of 300 were used to produce patterned films using the same technique at 100 °C. While the viscosity was increased, the morphology of LiCl crystals on the cellulose film surface was changed from a divergent pattern ( Fig. 3A and A 1 ) to a daisy-like pattern ( Fig. 3B and B 1 ). The pattern became curlier when concentration was increased to 4.0 wt % ( Fig. 3C and C 1 ). It means that the pattern can be regulated by changing the cellulose solution concentration at a specific temperature. Moreover, it was observed that the petal width for daisy or line for divergent and the ridge height were different, although these films had a similar pattern. The characterization of the surface profilometer ( Fig. 3E 1 and E 2 , Table S2 and S3) exhibited the width between the neighboring ridge and ridge height at a distance of 1 and 2 mm from the center. Furthermore, it was identified that the width between the neighboring ridge and ridge height would continue to increase with increasing distance from the center. This trend was similarly observed with the increase of the cellulose solution viscosity (Fig. 3E 4 and Table S2). However, only a minor difference was observed in the height ridge and the width between neighboring ridges when the temperature was raised to 100 °C (Table S3). We understand that by changing the combination of concentration, temperature, and viscosity, many patterns of different heights and widths could be generated. The crisp pattern was like the unique characteristics of the fingerprint of humans (Fig. 3E 3 ), which may be used in the anti-counterfeiting field.
During the experiment, it was observed that various LiCl morphologies could be formed. Besides, their print can be retained on the surface of cellulose film even after rinsing. To further explore the effect of the substrate on LiCl crystals' formation, we first investigated the growth process of its crystal from the evaporation of the pure solvent (DMAc, without cellulose) at different temperatures. As shown in Figure  S4, the morphologies of LiCl crystals were almost the same octahedral structure at different temperatures and concentrations of LiCl after evaporation. However, it was evident that the size of the LiCl crystal increased with the increase in temperature and concentration ( Figure S4.b). A similar phenomenon was also observed in a previous report while NaCl crystal morphology was produced with the assistance of other compounds such as formamides and urea (Zhang et al. 2011). Since both LiCl and NaCl belong to the cubic crystal system, it explains a similar growth mechanism. The carbonyl oxygen in acetamide had a strong interaction with lithium ions, thus stabilizing the surface of the LiCl crystal (1 1 1), leading to the formation of the octahedral crystal ( Figure S4.c). This control experiment also exhibited that the cellulose solution induced some influence on the growth of LiCl crystal.
In order to further make sure the mechanism of forming LiCl crystal with different morphology, which was due to the kinds of matrix or solvent, CMC (containing hydroxyl group) and tri-cellulose acetate (little hydroxyl group) were selected as growth matrix for LiCl crystal, where solvents were DMAc and a different solvent, respectively. The morphology evolution of LiCl crystals in LiCl/DMAc and LiCl/water solvent systems was compared and discussed for the CMC matrix. Cubic crystals could also be formed in the CMC matrix regenerated from LiCl/water at 100 °C, almost the same as pure solvent systems. At the same time, divergent patterns were only produced in the CMC matrix regenerated from LiCl/DMAc ( Figure S5: a and b), which was similar to that from cellulose substrate. The formation processing of LiCl crystal could be observed on the surface of CMC films regenerated from solution due to its slower growth rate. It was recorded at 20, 60, and 120 s for 0.5 wt% (Fig. 4a 1 , b 1 , c 1 ) and 2.0 wt% (Fig. 4a 2 , b 2 , c 2 ) CMC solution, respectively. The pattern started from cubic LiCl crystals, evolved into the octahedral structure, and finally formed divergent or similar structures. For Tri-cellulose acetate substrates, a similar result was also obtained, although it had few hydroxyl groups. This pattern can also be achieved in other polymer matrices. Therefore, the experimental phenomena further revealed that the interaction between LiCl and amide solvent was the prerequisite for forming diversity morphology on a polymer film.
Accordingly, the LiCl crystal growth in the cellulose matrix and its influencing factors were schematically presented in Fig. 4I, II, III. The solvent evaporated slowly when the temperature was relatively lower (T = 70, 80 °C). The cellulose solution was easy to flow and could not provide a stable matrix for the further growth of LiCl crystals at a low viscosity. Meanwhile, there was insufficient activation energy to form a perfect crystal at a low temperature. Consequently, the LiCl crystals were disseminated as particles in the matrix (Fig. 4I). While the solution viscosity was increased, the cellulose solution provided a stable matrix for the growth of the LiCl crystal. The free LiCl molecule released by the evaporation of DMAc began to rearrange and gradually grew along a crystal plane of LiCl to form an octahedral structure. After that, these octahedral LiCl crystals could aggregate and form dendritic LiCl crystals on the film surface (Fig. 4II). With the further increase in temperature (T = 90, 100, 120 °C), enough growth activation energy could be provided. Meanwhile, the evaporation rate of the solvent accelerated and increased the solution viscosity and stability. Finally, the perfect crystal could be formed, such as divergent, daisy, chrysanthemum, and other morphologies. It was noticed that the summary of the growth and regulation law for LiCl crystal on cellulose would be suitable for guiding the preparation of the patterned film.
The XRD curves of cellulose film with different patterns mainly showed the characterization peaks at 2θ = 12°, 20° (cellulose II) (Fig. 5a) (Zhang et al. 2021a, b). The crystallinity order of film formed at different conditions was as follows: Dendritic film (80 °C) > Divergent film (90 °C) > Daisy film (100 °C) > Film without a pattern (25 °C). It could be seen that cellulose crystallinity was increasing firstly and then decreasing when the temperature ranged from 25 to 100 °C. The daisy-like film (100 °C) showed higher toughness among the patterned films, which had a large area of the stress-strain curve. The tensile strength of films formed at high temperatures (80, 90 and 100 °C) was improved, more obviously comparing the film at 25 °C, which was more than 140 MPa. The main reason was that LiCl was more easily crystallized due to the solvent evaporation quickly at higher temperatures ( Figure S2 and S3). Crystal LiCl could be a nucleating agent during the film formation, which improved the crystallinity of cellulose film and favored the increase of tensile strength.
Interestingly, the pattern on the cellulose film exhibited an excellent anti-counterfeiting function similar to the fingerprint. As shown in Figure S6, the forefinger fingerprint was collected and stored on the phone. The phone locked was readily unlocked by the fingerprint printed on the pattern ( Figure S6:c). But it was not unlocked by finger with patterned cellulose film. However, it can work after the pattern is collected and stored on the phone (Fig. 5e ). Therefore, the patterned film has played a role in identifying information in this unlock process, in which writing and reading information parts have been recorded by the video (Figure S6). In addition, the anti-counterfeiting function was sustained after one year. In this process, thirty trials were given every month and found all responsive in one second ( Figure S7). It is also worth noting that the artificially fingerprinted taggant held up well in harsh conditions such as low (− 20 °C) or high temperature (60 °C) ( Figure S7), RH > 65 (exposure time < 1 h). The results showed that cellulose film with these patterns could be used as an anti-counterfeiting key like a fingerprint. Compared with the traditional anti-counterfeiting pattern (Bazrafshan 2020;Gassend et al. 2002;Geng et al. 2016), the patterned film is easy to prepare and does not need external stimulation and mainly relies on the complexity of the pattern to achieve its anti-counterfeiting function.

Conclusions
In conclusion, a dendritic, daisy, diverging, chrysanthemum-like pattern on the cellulose surface was firstly successfully prepared by in-situ regulation of Fig. 3 A (DP = 300, 1.0 wt% for cellulose solution), B (DP = 300, 2.0 wt% for cellulose solution), C (DP = 300, 4.0 wt% for cellulose solution), Polarization microscopic picture (yellow: A, B, C) before washing and Stereo-microscope picture (green: A 1 , B 1 , C 1 ) after washing at temperature for 100 °C. E 1 and E2 for surface roughness measurements of lines aa 0 and a 1 a 2 . E3 for the model of fingerprints. E4 and E5 for change of the height and width of the ridge with the increase of viscosity and temperature ◂ the morphology of LiCl crystal. The morphology of the LiCl crystal could be regulated by the changing temperature and viscosity of the solution. The dendritic pattern can be obtained when the viscosity was less than 0.8 Pa.s at 70-90 °C; a divergent pattern appeared when the viscosity was between 0.8 Pa.s and 3.8 Pa.s at 90-100 °C, and the daisy pattern would appear with the further increase of viscosity. These patterns could be retained on the cellulose surface after removing the LiCl crystal. The patterned film exhibited higher crystallinity, which tensile strength was more than 140 MPa and provided a basis for the possible application. The patterns on the film could be collected by phone and used as unlock triggers. Furthermore, countless points of temperature and viscosity, including their possible combination, could form hundreds of billions of patterned films with different heights and widths of the ridge, which was like a fingerprint from the human, exhibiting potential in anti-counterfeiting fields. Fig. 4 Polarized microscope image of the growth process for LiCl crystals on the surface of CMC films from different concentration at the temperature for 100 °C, (a 1 ~ c 1 , C = 4.0 wt%); (a 2 ~ c 2 , C = 12.0 wt%). Schematic illustration for the growth process of LiCl crystal on the surface of the cellulose film (I, II, III) Fig. 5 a XRD patterns of cellulose films with different patterns, b Mechanical properties of cellulose film with different patterns, c, d, e pattern information stored on the phone used as the key of unlocking