Water-Soluble and Low-Toxic Ionic Polymer Dots as Invisible Security

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Water-Soluble and Low-Toxic Ionic Polymer Dots as Invisible Security Ink for Multi-Stage Information Encryption Dejian Chen, Caiyan Cui, Na Tong, Haifeng Zhou, Xinchen Wang, and Ruihu Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18638 • Publication Date (Web): 10 Dec 2018 Downloaded from http://pubs.acs.org on December 11, 2018

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ACS Applied Materials & Interfaces

Water-Soluble and Low-Toxic Ionic Polymer Dots as Invisible Security Ink for Multi-Stage Information Encryption Dejian Chen,†,|| Caiyan Cui,† ,¶,|| Na Tong,†,‡ Haifeng Zhou,§ Xinchen Wang,‡ Ruihu Wang†* †

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of

Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China. ‡

State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry,

Fuzhou University, Fuzhou, Fujian 350108, China. §

School of Chemical and Material Engineering, Jiangnan University, Wuxi, Jiangsu 214122,

China. ¶

University of Chinese Academy of Sciences, Beijing, 100049, China.

KEYWORDS ionic polymers, anion exchange, polymer dots, water-soluble, ink.

1 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT Nanodots are attractive stimuli-responsive luminescence materials for anti-counterfeiting and information encryption. However, their applications are limited by low water solubility and single-mode information identification by naked eyes under UV light illumination. Herein, we report one type of new nanodots, main-chain imidazolium-based ionic polymer dots (IPDs). There is no edge effect in IPDs, the ionic groups are homogenously distributed in the entire dot. IPDs exhibit high water solubility, good stability, narrow size distribution, low toxicity and exceptional optical performance without the additional modification. Written information using aqueous IPDs solution is invisible in natural light, but can be recognized by a portable UV lamp. Moreover, they can be further encrypted and decrypted using easily available and nontoxic sodium carbonate and acetic acid, respectively. The encrypted information is invisible in natural light and/or UV light. This study provides new prospective for high-level data recording and security protection by using water-soluble IPDs as invisible security ink.

INTRODUCTION Data recording, anti-counterfeiting and information encryption technologies have been important issues in our daily lives and military fields.1,2 A variety of stimuli-responsive luminescence materials have been developed and utilized as security ink in these fields as they offer advantages in easy handling, high-throughput and facile design.3-8 However, written information is usually visible by naked eyes under UV light illumination. Such single-stage anticounterfeiting and encryption techniques based on fluorescence switching were relatively less effective in the protection of confidential information.9-12 It is of great significance to develop high-level techniques for information coding and security protection, in which information is


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readable only under special environments, and could not be identified without correct information about original ink. The design and synthesis of new luminescence materials with task-specific groups as security ink are pivotal for multiple encryption of information. Metal-free nanodots have captured considerable interests as attractive luminescence materials because of their outstanding characteristics, such as versatile fluorescent properties, high stability, excellent biocompatibility and nontoxic features.13-17 Among them, polymer dots (PDs) possess prominent properties as security ink in anti-counterfeiting and information encryption. PDs could overcome disadvantages of common luminescence materials, such as poor photostability of organic dyes, environmental hazards of heavy metal-containing quantum dots, broad emission band and ill-defined structure of carbon dots.18,19 Although considerable efforts have been made, their applications mainly focus on anti-counterfeiting and single-mode information encryption, there are still great challenges associated with high-level security patterns. Meantime, most of PDs are only soluble in organic solvents. To form a stable and transparent aqueous solution, the additional steps are usually required to tune their surface chemistry using hydrophilic groups or to combine with water-soluble polymers.20-23 For example, the ionic liquids-modified carbon dots have been reported to improve their performance in anticounterfeiting, bioimaging and ﬂuorescence detection.23-25 It is well known that fluorescence mechanism of nanodots could be classified as the eigenstate emission based on the quantum confinement effect and the surface defect emission based on the edge effect.26-28 The surface post-modification of nanodots will inevitably disturb their original luminescence properties of task-specific functional groups and complicate their structures. To avoid the disturbance of surface defect on the performance of nanodots, it is highly desirable to design one type of new



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polymer nanodots with well-defined structures and composites that are soluble and stable in water without the assistance of surfactants or additives. The main-chain imidazolium-based ionic polymers (ImIPs) are one of potential candidates. The modular nature of ImIPs synthesis allows for tailor-making their structures and properties through judicious selection of building units at the molecular level, which provides tremendous opportunities to tune their emission color by the incorporation of different luminescent cationic groups into the polymer backbones.29-33 Particularly, ImIPs show dominant light absorption in UV region, but their emission occurs in the range of visible light with the single-color peaks, which are ideal characteristics as invisible security ink. Since the interactions between host cationic backbones and counter anions could produce detectable luminescent responses, their fluorescent intensity could be tuned rapidly and reversibly through anionic exchange.28-33 The attractive fluorescence sensing properties could provide a system of chemical encryption, in which ImIPs are used as fluorescent ink, and the anions for fluorescent quenching serve as encryption algorithm. By further encrypting information using these anions, recorded information is impossible to mimic and counterfeit unless original information is revealed using an appropriate decryption reagent. Considering that the performance of ImIPs is dominantly dependent on cations and anions, it could be speculated that the judicial combination of host cationic backbones and charge-balanced anions could generate water-soluble ionic polymer dots (IPDs) as invisible security ink. As a proof-of-concept study, herein, we present a facile bottomup method for the synthesis of water-soluble main-chain imidazolium-based IPDs. IPDs show rapid and reversible stimuli-responsive luminescence, and are preferable as fluorescent ink for high-level and multi-stage information encryption, the requirements for imperceptibility, robustness and security of data storage are fully satisfied.


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EXPERIMENTAL SECTION Synthesis of tris(4-imidazolylbenzylidene)triaminoguanidinium chloride (TIG-Cl) Triaminoguanidinium chloride (281 mg, 2 mmol) was dissolved in H2O (3 mL) and ethanol (6 mL). A mixed solution of 4-(1H-imidazol-1-yl)benzaldehyde (1.085 g, 6.3 mmol) and ethanol (3 mL) was slowly added to the above hot mixture. After stirring and heating under reflux overnight, the resulting mixture was cooled to ambient temperature before a large body of amorphous yellow powder was obtained, then the desired product was collected, washed with cold EtOH and dried in vacuo. Yield: 1.077 g (82%). 1H NMR (400 MHz, [D6]DMSO): δ 8.75 (s, 3H), 8.48 (s, 3H), 8.11 (d, J = 8.2 Hz, 6H), 7.92 (s, 3H), 7.83 (d, J = 8.2 Hz, 6H), 7.19 (s, 3H). 13

C NMR (100 MHz, [D6]DMSO): δ 150.30, 149.11, 138.67, 136.10, 132.55, 130.38, 129.87,

120.72, 118.42 ppm. FTIR spectrum (KBr, cm-1): 3418 (m), 3117 (m), 3056 (w), 1645 (w), 1613 (w), 1514 (s), 1400 (w), 1365 (w), 1335 (m), 1309 (m), 1284 (m), 1258 (m), 1182 (w), 1115 (m), 1057 (m), 1019 (m), 964 (m), 909 (m), 829 (m), 765 (m), 736 (m), 657 (m), 618 (w), 586 (w), 543 (m). Synthesis of IPDs A mixture of TIG-Cl (0.1 mmol, 60.2 mg) and 2,4,6-tris(4-(bromomethyl)phenyl)-1,3,5triazine (0.1 mmol, 58.8 mg) in 55 mL DMSO/DMF mixed solvent (v/v: 7/4) was heated at 100 o

C for 24 h to form an orange red solution. The resulted solution was then subjected to dialysis to

completely remove the DMF, DMSO and non-reactive monomer. Resultant IPDs aqueous solution was stored in the dark for further characterization. FT-IR spectra (KBr, cm-1): 3424 (s), 3126 (m), 1695 (w), 1649 (m), 1606 (m), 1551 (m), 1515 (s), 1454 (m), 1417 (m), 1362 (s), 1308 (m), 1266 (w), 1213 (m), 1190 (m), 1107 (w), 1071 (m), 1048 (m), 1016 (m), 954 (w), 846 (m), 816 (m), 795 (m), 757 (m), 697 (w), 610 (m), 604 (w), 531 (m).



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In vitro cytotoxicity study The in vitro viability tests were performed based on methyl thiazolyl tetrazolium (MTT) method. Hela and HepG2 cells from exponential cultures were seeded into 96-well culture plates in 100 mL of medium per well at a density of 5000 cells per wel under 37 oC and 5% CO2 for 24 h. The medium was then replaced with 100 mL of medium containing different concentrations of IPDs solution (0, 5, 10, 20, 50 and 100 µg mL-1) and incubated in quadruplicate for 24 h. 80 µL of medium and 20 µL (5 mg mL-1) of MTT solution were then added to each well, and the wells were further incubated for 4 h. DMSO (150 mL) was used to completely liberate the formazan crystals. The absorbance at 570 nm was measured for the calculation of the cell survival rate.

Scheme 1. Schematic illustration for the synthesis of IPDs as well as the process of information coding, reading, encryption and decryption, RESULTS AND DISCUSSION As depicted in Scheme 1, IPDs were readily synthesized by the quaternization reaction of tris(4-imidazolylbenzylidene)triaminoguanidinium

chloride

(TIG-Cl)

and

2,4,6-tris(4-


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(bromomethyl)phenyl)-1,3,5-triazine (TPT-Br) in DMSO/DMF. After removing most of organic solvents under reduced pressure, the dialysis of the resultant solution in water provided yellow aqueous IPDs solution. It should be mentioned that the use of the mixed solvent plays an important role in the synthesis of IPDs. When the reaction of TIG-Cl and TPT-Br was conducted in DMSO under the same conditions, no colloidal particles were detected in yellow solution, while the use of DMF gave rise to yellow precipitates (Figure S1). The counter anions also have pivotal effects on the formation of water-soluble IPDs, the quaternization reactions of TIG-Cl and

2,4,6-tris(4-(chloromethyl)phenyl)-1,3,5-triazine

(TPT-Cl)

or

tris(4-

imidazolylbenzylidene)triaminoguanidinium bromide (TIG-Br) and TPT-Br under the same conditions generated the precipitates (Figure S2). IPDs display high water solubility. The concentration of IPDs in water could reach 59 mg mL-1, which is about 30 times higher than reported one-dimensional conjugated PDs modified by hydrophilic groups, such as PDHF, PF-10BT, and PF-5DTBT.9 Attractively, high concentration of aqueous IPDs solution is very stable in air, no appreciable floats or precipitates are detected after stored in air for at least one month (Figure S3). High solubility and stability of IPDs in water is mainly attributed to high polarity of the ionic backbone. The imidazolium- and guanidinium- groups are homogeneously located in the host backbone to form a highly crosslinked cationic network, counter bromide and chloride reside on their proximity as chargebalanced anions, resulting in the lack of surface defect. Thus, the performance of IPDs could be modulated through exchanging anions in the entrie material, which is different from those in surface-modified nanodots by ionic liquids.23-25 Their electrostatic interactions prevent IPDs from aggregation or coalesence in water. The pH value of the aqueous IPDs solution is 6.5, and the zeta potential is measured to be 16.39 mV. The high positive zeta potential further shows



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high stability of aqueous IPDs solution. To the best of our knowledge, this is the first report for the nanodots containing homogenously distributed hydrophilic groups in the entire dot. The host network and counter halide anions in IPDs show relatively weak interaction, the electron-transfer transitions from counter anions to cationic host network could modulate its energy of n−π* transition, which provides tremendous possibility to adjust their fluorescent properties through facile anion exchange.

Figure 1. (a) TEM image of IPDs, the inset is particle size distribution. (b) Particle size distribution based on dynamic light scattering, the inset photograph shows the Tyndall effect generated by a red laser. (c) FTIR and (d) solid-state 13C NMR spectra of TIG-Cl, TPT-Br and IPDs.


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Transmission electron microscopy (TEM) image in Figure 1a shows that IPDs are well dispersed with narrow size distribution, and the average particle size is 9.0 ± 0.2 nm, which is similar to the results from dynamic light scattering in water (Figure 1b). The chemical structure of IPDs was defined by Fourier transformed infrared (FTIR) and solid-state 13C NMR spectra. In the FTIR spectra (Figure 1c), IPDs contain almost all of the characteristic peaks of TIG-Cl and TPT-Br. The moderate peaks at 603 and 1416 cm-1, corresponding to the stretching vibration of C-Br and methylene groups in TPT-Br, respectively, are greatly attenuated in IPDs. These results suggest the occurrence of quaternization reaction between TIG-Cl and TPT-Br. In the solid-state 13

C NMRspectrum of IPDs (Figure 1d), the characteristic peak of methylene carbon shifts from

30.8 ppm in TPT-Br to 51.4 ppm in IPDs, which further suggests successful quaternization of TIG-Cl and TPT-Br. The resonance peaks at 145 and 168.7 ppm correspond to the triaminoguanidinium and triazine carbon atoms, respectively. Other broad peaks from 105 to 140 ppm are assigned to the phenyl and imidazolium carbon atoms. The UV-vis spectrum of aqueous IPDs solution exhibits a broad absorption peak at 230-400 nm with the maximum absorption centered at 275 nm (Figure S4), which could be assigned to the n-π* transition of ionic host backbone. As the rise of the IPDs concentration, the peak intensity increases gradually. Notably, there is no detectable absorption in the range of visible light, which is favorable for invisible security ink in anti-counterfeiting and information encryption. The absorbance per unit length of aqueous IPDs solution versus the concentration at different wavelengths is shown in Figure 2a. The slopes of straight line at each wavelength fit the Lambert-Beer law well, further indicating good water solubility of IPDs. The maximum value of



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the absorptivity is recorded as 12.0 mL mg-1 cm-1 at λ a 2aa nm, which is much smaller than those in reported nanomaterials,40,41 indicating that IPDs are preferable as fluorescent ink.

Figure 2. (a) Lambert-Beer plots for different concentration of aqueous IPDs solution at different wavelengths. (b) Fluorescence spectra of aqueous IPDs solution. (c) Fluorescence intensity of IPDs upon consecutive heating and cooling from 1a to 4a oC for five times. (d) HeLa and HepG2 cell viability after incubating with IPDs for 24 h, the quantitative assays are based on standard MTT method. (e) The effects of various anions on fluorescence intensity of aqueous IPDs solution.


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In the fluorescent spectra of aqueous IPDs solution, a blue emission peak centered at 465 nm is observed upon 348 nm excitation (Figure 2b). In contrast to fluorescent spectra of TIG-Cl and TPT-Br (Figure S5), the obvious redshift shows the crosslink-induction emission effect of these building units in IPDs. Further investigations reveal that the positions of the emission peaks in IPDs have no obvious shift at various excitation wavelengths (Figure S6). This excitation-independent emission behavior is attributed to uniform distribution of the building units in IPDs. IPDs possess excellent photostability, there is no significant reduction in fluorescent intensity under continuous UV exposure for 6 h (Figure S7). The effects of temperature on the luminescence were further investigated. Figure S8 displays slightly thermal dependence of fluorescent intensity of IPDs when the temperatures of aqueous solution change from 15 to 45 oC, the intensity gradually attenuates upon increasing temperature. Different from carbon dots that display remarkable temperature-dependent spectral shifts,42 the fluorescent spectra of aqueous IPDs solution have no shift within the investigated temperature window. Interestingly, there is no detectable fluorescence bleaching after consecutive heating and cooling for five cycles (Figure 2c), which is important for use as invisible security ink in various environments. Besides high stability and excellent water solubility, IPDs show good biocompatibility and low cellular toxicity. After HeLa and HepG2 cells were fed with different concentrations of aqueous IPDs solution (0-100 µg mL-1) for 24 h, the relative cell viability slightly decreases with the increment of IPDs concentration (Figure 2d). More than 80% live HeLa and HepG2 cells are retained even in 100 µg mL-1 of aqueous IPDs solution, which is much higher than that of reported polyethyleneimine-based PDs.43,44 Additionally, the morphologies of HeLa and HepG2 have no obvious change after 24 h (Figure S9).



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One of major merits of IPDs is that fluorescent intensity could be flexibly adjusted through facile anion exchange. The fluorescent response behavior of aqueous IPDs solution toward different anions was monitored by liquid fluorescence spectroscopy based on typical emission of IPDs at 465 nm. As shown in Figure 2e, there was no appreciable fluorescence variation when Br- (20 µL, 0.1 mmol L-1) was added into aqueous IPDs solution (3 mL, 0.5 mg mL-1). However, a slight attenuation was observed when the same amount of Cl- and CH3COO- were added. Using the same amount of HCO3-, SO42-, NO3-, SO32-, NO2-, CO32-, I- and PO43- resulted in gradual attenuation of the fluorescence of IPDs. Obvious fluorescence quenching was observed after the addition of ClO4-, CrO42-, MnO4- and Cr2O72-, but these anions are not suitable for information encryption owing to their toxicity and the intrinsic color of their aqueous solution even at a low concentration. The anion-dependent fluorescence responsive characteristics were further investigated by taking CO32- as an exemplification (Figure S10). When aqueous sodium carbonate solution (20 µL, 0.1 mM) was added into aqueous IPDs solution, the luminescence of IPDs was significantly weakened after 1 min, and the elongation of time has no detectable effect on the fluorescence, suggesting fast kinetics for anion exchange between IPDs and CO32-. Impressively, when excess acetic acid was added to the exchanged IPDs solution, the fluorescence of the aqueous solution almost recovered to that of IPDs solution before exchange (Figure S12). Thus, the fluorescence of aqueous IPDs solution can be switched off and on through the addition of sodium carbonate and acetic acid, respectively. Interestingly, the carbonate-exchanged IPDs solution is not only stable under UV lamp illumination for at least 5 h, but also invisible by naked eyes in natural light and UV light, which holds great promise in the applications for multiple-stage encryption and decryption.


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Figure 3. Photographs for Chinese or English characters written by (a) fountain pen (b) brush pen and (c) inkjet printer under natural light and UV light. (d) Photographs for English characters “dots” in second encryption and decryption process by reversible anion exchange reaction, d1, d3 and d4 in UV light, (d2) in natural light. The outstanding optical properties and low toxicity of aqueous IPDs solution motivate us to explore their application in invisible security ink. A new strategy for information coding, encryption and decryption was designed by using the transparent aqueous IPDs solution with the concentrations of 1 mg mL-1. In the first encryption stage, the Chinese or English characters were written or printed on commercially available A4 paper using fountain pen, brush pen and inkjet printer, respectively. After A4 paper was dried at room temperature, these characters on A4 paper were almost invisible with naked eyes or cameras in natural light (Figure 3a, 3b and 3c). Upon illumination using a portable UV lamp, the Chinese or English characters were clearly



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identified without any other interference pattern. In addition, the IPDs ink has good compatibility with various writing or templating tools, the filter paper and silica gel plate could also be used as carriers for information recording (Figure S13). The multiple-stage encryption and decryption of the IPDs ink was further investigated. After the English characters “dots” was recorded on A4 paper, the information was encrypted using nontoxic and easily available sodium carbonate. After the recorded information was immersed into aqueous sodium carbonate solution for 5 min, the fluorescence of IPDs was quenched immediately, there is only a weak background fluorescence of A4 paper. The English characters “dots” are concealed, no pattern of information could be observed either under natural light or UV light illumination after the encryption. Notably, when the encrypted paper was immersed into household white vinegar or diluted acetic acid for 10 min, the encrypted “dots” could be clearly identified using a portable UV lamp at room temperature. In the process, acetic acid reacts with sodium carbonate to consume CO32- and form CH3COO- as charge-balanced anions, thus original fluorescence is recovered. Moreover, the Chinese or English characters could maintain their stability for at least three months in atmospheric environment, which indicate that the IPDs-based security ink is very suitable for long-term storage of confidential information.

CONCLUSIONS Metal-free and water-soluble IPDs have been presented through facile quanternization reaction, they are promising invisible security ink for high-level information coding, encryption and decryption. To the best of our knowledge, this is the first report for the nanodots containing homogenously distributed hydrophilic groups in the entire dot. Written information using aqueous IPDs solution possesses outstanding stability, and can be further encrypted using


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carbonate and be quickly decrypted using acetic acid. In contrast to conventional fluorescent ink, recorded information using IPDs ink is more reliable in long-term storage and secrecy maintenance, the encryption and decryption reagents are easily available, inexpensive and environmentally benign, which greatly alleviate public concerns towards health, safety and environment in confidential data protection. This work not only provides one type of new watersoluble and low-toxic polymer nanodots as invisible security ink, but also opens a new avenue for applications in advanced anti-counterfeiting, confidential information storage and secret communication.

ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website. Materials synthesis and characterization data AUTHOR INFORMATION Corresponding Author * [email protected] Author Contributions || Dejian

Chen and Caiyan Cui contributed equally to this work.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21471151 and 21673241) and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB20000000).



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Water-Soluble and Low-Toxic Ionic Polymer Dots as Invisible Security

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