Rewritable Anticounterfeiting Polymer Inks Based on Functionalized

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Rewritable Anticounterfeiting Polymer Inks Based on Functionalized Stimuli-Responsive Latex Particles Containing Spiropyran Photoswitches: Reversible Photopatterning and Security Marking Amin Abdollahi, Keyvan Sahandi-Zangabad, and Hossein Roghani-Mamaqani* Department of Polymer Engineering, Sahand University of Technology, P.O. Box 51335-1996, Tabriz 51368, Iran

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S Supporting Information *

ABSTRACT: Increase of safety in security documents by using anticounterfeiting inks based on fluorochromic and photochromic compounds has attracted a great deal of attention in the recent years. Herein, we developed novel functionalized stimuli-responsive latex particles containing spiropyran (1 wt %) by semicontinuous emulsifier-free emulsion polymerization, which are usable as anticounterfeiting inks for marking on security documents and also photopatterning on cellulosic papers. The size and morphology of the latex particles were characterized by scanning electron microscopy and dynamic light scattering and their functionality was characterized by Fourier-transform infrared spectroscopy. All the stimuli-responsive latexes are composed of spherical particles with different hydroxyl, epoxy, and carboxylic acid functional groups, and the size of the particles varies in the range of 400−900 nm. Additionally, the latex particles undergo a remarkable light-induced size variation (aggregation− disaggregation) upon UV illumination (365 nm), depending on the functional group type, as a result of π−π stacking interactions and also electrostatic attractions between the different particles. The photochromic behavior, kinetics of the SP ⇌ MC isomerization, photoswitchability, and photofatigue-resistant characteristics of the prepared latexes were extensively investigated. The results display that the photochromic behavior and SP ⇌ MC isomerization can significantly be influenced by the polar interactions between the functional groups and MC molecules. As a novel application, the prepared stimuli-responsive latexes were used as anticounterfeiting inks for writing on cellulosic paper and also security marking on several monies, where the written phrase displayed red fluorescence emission and coloration under and after UV illumination (365 nm), respectively. Additionally, the latexes were sprayed on cellulosic papers to prepare stimuliresponsive papers for investigation of their photopatterning ability under UV irradiation and different masking. The presence of functional groups and large particle sizes are the main effective factors for stabilization of the latex particles on cellulosic papers. This is the first report on application of functionalized stimuli-responsive latex particles containing spiropyran as anticounterfeiting inks for security marking and photopatterning on cellulosic papers, directly and without using further additives. KEYWORDS: photochromic polymer, functional latex, spiropyran, anticounterfeiting ink, photopatterning, security marking pers.23−27 Depending on the induced stimulus, spiropyran can display different stimuli-chromic characteristics such as photochromism,28−30 mechanochromism,31−34 solvatochromism,26,35,36 ionochromism,37−39 acidochromism,32,40−42 piezochromism,43 basochromism,44 electrochromism,45−47 gelochromism,48,49 and thermochromism.50−52 Spiropyran undergoes reversible isomerization between the ring-closed, colorless, nonpolar, and nonplanar spiropyran (SP) and ringopened, high-polar zwitterionic, colored, and planar merocyanine (MC) forms upon UV (or other stimulants) and visible light irradiation.53,54 Hence, it is believed that spiropyran is a multistimuli-chromic compound with excellent photostability,

1. INTRODUCTION Stimuli-responsive polymers and their applications have significantly increased in the past few years. These smart polymers change their physicochemical properties in response to variation in environmental conditions such as temperature, pH, and light as the most significant stimuli.1−5 Nowadays, light-responsive photochromic polymers have attracted a great deal of attention because of the advantages of light-triggered smart polymers such as fast and facile responsivity, response reversibility, controllability out of the system, accessibility, and nondestructivity of light as the stimulus.6−8 In photochromic compounds, spiropyran because of its unique and extensive stimuli-chromic properties is considered largely in development of anticounterfeiting inks,9−11 photopatterning,12,13 smart drug-delivery systems,14−16 optical-data storage devices,17−19 chemosensors,20−22 and stimuli-responsive pa© XXXX American Chemical Society

Received: August 30, 2018 Accepted: October 11, 2018

A

DOI: 10.1021/acsami.8b14865 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

remarkable photofatigue-resistant characteristics can be developed for anticounterfeiting inks and photopatterning on cellulosic papers and security documents. Photochromic inks are an important, interesting, and novel research area in smart materials, which have not largely been focused on.9 According to the literature, the need for development of novel anticounterfeiting materials and polymers based on photochromic compounds has greatly been increasing in the recent years, which is mainly on account of the unique characteristics of photochromism for increase of security in different documents. Hence, photochromic inks can display a very interesting role in development of anticounterfeiting and security documents. For this purpose, photochromic compounds should be incorporated into polymer matrices for increasing of photochromic efficiency, life time, and also minimizing the cost. Among different photochromic polymeric systems, latex particles are the most usable polymer carriers for spiropyran, which can be developed as photochromic latexes for anticounterfeiting and rewritable ink applications. Functionalization of such photochromic latex particles can lead to their chemical attachment on the cellulosic substrates and therefore higher stability of the inks in a paper matrix. To date, a wide range of studies have focused on photochromic latex particles; however, they have not covered applications such as anticounterfeiting rewritable polymer inks and smart printings. One of the most important studies on anticounterfeiting inks based on spiropyran was reported by Tsai and co-workers.11 They prepared anticounterfeiting inks based on polymer dots bearing spiropyran. These anticounterfeiting inks cannot directly be used and should be doped into flexible substrates and physically stabilized. In this study, the functionalized photochromic latexes can directly be used as an ink without addition of any chemicals. Most importantly, these latex particles can be attached on cellulosic papers physically and chemically. We developed solvent-free and attachable polymeric anticounterfeiting inks based on spiropyran-containing photochromic latex particles with different functional groups at their surface for the first time. In comparison with the previous works,26,27,57 we prepared spiropyran-based photochromic polymer particles by a facile and fast strategy, which can directly be used as a smart ink for marking of security documents. These functionalized photochromic latex particles were prepared by the semicontinuous emulsifier-free emulsion copolymerization of MMA and 1′-(2-acryloxyethyl)-3′,3′-dimethyl-6-nitrospiro(2H-1-benzopyran-2,2′-indoline) (SPEA) (1 wt %). Functionalization of the latex particles was performed by the addition of GMA, HEMA, and MAA monomers (5 wt %) at the end of the polymerization reaction. Functionality, morphology, particle size, and particle size distribution of the latex particles were analyzed in detail. Reversible light-induced particle size variation was also investigated by measurement of the particle size and transmittance of the latexes before and after UV illumination (365 nm). Photochromic properties, kinetic of the SP to MC and MC to SP isomerization, solvatochromism, photofatigue-resistant characteristics, reversibility, and photoswitchability of the diluted latexes were also examined. Finally, the prepared latexes were used as an anticounterfeiting ink for writing, marking, and photopatterning on the cellulosic papers and security documents for increasing of safety and security.

reversibility, responsivity, and also sensitivity. Such characteristics of spiropyran are significantly influenced by its interaction with the surrounding media, especially hydrogen boding and polar−polar interactions.35,36,55 Therefore, protection of spiropyran from environmental degradations of such strong interactions is an important requirement for development of stimuli-chromic polymers. Chemical incorporation of spiropyran into the hydrophobic or less-polar polymer matrices such as polystyrene (PS) or poly(methylmethacrylate) (PMMA) improves the efficiency of its stimuli-chromic properties, for instance, photostability, reversibility, and also sensitivity.12,26,56−59 On the other side, chemical attachment of the photochromic polymers to different matrices can result in decrease of environmental degradation effects. This is carried out by functionalization of the photochromic polymers by using functional monomers such as glycidyl methacrylate (GMA), hydroxyethyl methacrylate (HEMA), and methacrylic acid (MAA). In addition to chemical bonding, such functional groups can stabilize the polymer chains in different substrates by physical forces such as hydrogen bonding and polar−polar interactions. In recent years, a wide range of studies have focused on the preparation and investigation of photochromic polymer particles containing spiropyran moieties; however, a few of them have investigated attachment of the particles to different matrices by physical and chemical interactions.12,26,44,60 For instance, Sun and co-workers introduced spiroxazine-doped PS latex particles and nanocrystalline cellulose (CNC) into the paper matrix, where the PS particles and CNC were used as carriers for spiroxazine.23,60 The spiroxazine molecules were stabilized in PS and CNC carriers by physical interactions. Also, spiroxazine-containing PS and CNC were incorporated on cellulose fibers by the weak physical interactions. Mahdavian’s group developed functionalized and nonfunctionalized polymer nanoparticles with different characteristics.12,26,27,44,57 For instance, epoxy-functionalized photochromic polymer nanoparticles were prepared by using of acrylated spiropyran as a comonomer in the semicontinuous emulsion copolymerization of methyl methacrylate (MMA) and GMA.26 Because of the presence of epoxy functional groups at the surface of particles, the latex particles are capable of chemical attachment to different substrates by the ringopening reaction of epoxy functionalities with appropriate nucleophiles. They also prepared photochromic cellulosic papers by chemical incorporation of epoxy-functionalized latex particles on the cellulose fibers by a chemical reaction between the hydroxyl functional groups of cellulose and epoxy functionalities of photochromic latex particles.27,61 The prepared stimuli-responsive papers can be used as an optical polarity- and pH-sensor and smart papers. Recently, Tian and co-workers prepared photochromic papers by introduction of a polyelectrolyte composite layer based on cationic chitosan and anionic carboxyl-containing spiropyran on pulp fibers by using the layer-by-layer assembly technique.25 Incorporation of spiropyran without polymeric carriers into the polar matrix such as cellulosic paper can lead to decrease of photochromic properties and also increase of negative photochromism and irreversibility. Also, Trosien et al. developed a new class of photochromic papers by incorporation of UV-attachable photochromic copolymers based on spiropyran on the cellulosic fibers, which finally resulted in the development of paper-based colorimetric UV sensors.62 Such photochromic polymer particles with unique photochromic properties and B


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Figure 1. Preparation of the functional photochromic latex particles containing spiropyran by emulsifier-free emulsion polymerization.

Table 1. Procedures for Preparation of the Functional Photochromic Latex Particles with Their Characteristics sample

LB

LB-SP

CLB-SP

HLB-SP

ELB-SP

Water (mL) NaHCO3 (g) KPS (g) NaCl (g) SPAN 80 (g) SPEA (g) CSPEA × 10−3 (mol/L) MMA (g) GMA (g) HEMA (g) MAA (g) pH Conversion (%)

36 0.15 0.06 0.04 0.045 0 0 4.3 0 0 0 8 98

36 0.15 0.06 0.04 0.045 0.045 2.21 4.3 0 0 0 8 99

36 0.35 0.06 0.04 0.045 0.045 2.21 4.3 0 0 0.225 6 97

36 0.2 0.06 0.04 0.045 0.045 2.21 4.3 0 0.225 0 6.5 98

36 0.15 0.06 0.04 0.045 0.045 2.21 4.3 0.225 0 0 7 99

the functionalized stimuli-responsive latex particles, functional monomers such as MAA, HEMA, and GMA (5 wt % with respect to MMA content) were added into the reaction mixture 1 h after the addition of SPEA, and the polymerization reaction was continued for 2 h to form the carboxylated photochromic latex particles (CLB-SP), hydroxylated photochromic latex particles (HLB-SP), and also epoxidized photochromic latex particles (ELB-SP), respectively. According to Table 1, an appropriate amount of SPAN 80 was added to the prepared latex samples and stirring was continued for 24 h for full incorporation of the nonionic surfactant on the latex particles. The total coagulation and conversion were, respectively, less than 1 wt % and more than 95%, which are measured by the gravimetric method and presented in Section 3 of the Supporting Information. The total amount of the incorporated SPEA in the latex particles was determined by coagulation of the latexes and absorption measurement of the remaining serum by UV−vis analysis with respect to a standard solution. It was found that more than 95% of the added SPEA was incorporated in these samples. Final photochromic latex samples were obtained after three cycles of centrifugation in 7500 rpm for 15 min. The chemical structure of the latex samples was characterized by 1H NMR analysis, as displayed in Figures S2−S6 (Supporting Information). The sample preparation method is also mentioned in Section 2 of the Supporting Information. In addition, number-average degree of polymerization (DP) and molecular weight (MW) of the samples were measured by 1H NMR spectra and are reported in Table 1. 2.4. Preparation of the Rewritable Anticounterfeiting Polymer Ink and Photopatterns. The prepared stimuli-responsive latexes (10 wt %) were used as anticounterfeiting and rewritable inks for writing and imprinting on the security papers and also for preparation of stimuli-responsive papers for application in photopatterning. Stimuli-responsive papers were prepared by spraying the

2. EXPERIMENTAL SECTION 2.1. Materials. 2,3,3-Trimethylindolenin, 2-bromoethanol, and 2hydroxy-5-nitrobenzaldehyde, used for synthesis of (R/S)-2-(3′,3′dimethyl-6-nitro-3′H spiro[chromene-2,2′-indol]-1′-yl)ethanol (SPOH), and GMA were purchased from Sigma-Aldrich. Acryloyl chloride, MMA, HEMA, MAA, potassium persulfate (KPS), SPAN 80, sodium hydrogen carbonate (NaHCO3), sodium chloride (NaCl), and all of the solvents were supplied by Merck Chemical Company. Distilled-deionized (DI) water was used in all recipes and all of the materials were used without further purification. 2.2. Synthesis of SPOH and SPEA. SPOH was prepared according to the reported procedure by Raymo.28 Also, SPOH was modified to the spiropyran ethyl acrylate (SPEA) monomer by the previously published procedure by Abdollahi and co-workers.26 The chemical structure of SPEA was characterized by 1H NMR spectroscopy [dimethyl sulfoxide (DMSO, 400 MHz)] and the resulted spectrum is presented in Figure S1 (Supporting Information). 2.3. Preparation of the Functionalized Stimuli-Responsive Latex Particles. The functionalized stimuli-responsive latex particles with a solid content of about 10 wt % were prepared by the semicontinuous emulsifier-free emulsion polymerization method, as shown in Figure 1. According to Table 1, the required amounts of KPS, NaCl, and NaHCO3 were dissolved in DI water (45 mL) and transferred into the reactor under continuous flow of nitrogen gas, and then MMA (4.3 g) was slowly added to the mixture. After that, the reactor temperature was increased to 75 °C for start of the polymerization under reflux. After 3 h, an aqueous solution of SPEA (1 wt % with respect to the total amount of MMA) was added dropwise into the reactor, and the reaction was continued for 3 h to reach a monomer conversion of above 95%. The prepared sample at this point is photochromic latex particles (LB-SP). For preparation of C


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ACS Applied Materials & Interfaces prepared latexes on the cellulosic papers and drying in 50 °C for 2 h. The latexes were directly used without adding any chemical or solvent. 2.5. Characterization. Identification of different functional groups in polymeric particles was carried out by using an FT-IR BRUKER-IFS48 spectrophotometer (Germany). Sample preparation was performed after coagulation of the latex particles with concentrated sulfuric acid, drying at 50 °C, and dispersion in KBr pellets. Characterization of the chemical structure of SPEA and the latex samples was carried out by proton nuclear magnetic resonance (1H NMR) spectroscopy using a Bruker DPX 400 MHz apparatus. Measurement of the particle size, its distribution, and also zeta potential was carried out by using ZETASIZER NANO ZSP dynamic light scattering (DLS, Malvern, United Kingdom) at 25 °C. For this purpose, the initial latexes were diluted up to 100-fold in DI water. Scanning electron microscopy (SEM) micrographs were taken by a Tescan Mira III (Czech Republic). A drop of diluted latex samples was placed on the sample holder and dried in vacuum at 25 °C. Then, it was put in vacuum, evacuated, and a layer of gold was deposited under flushing with argon by using EMITECH K450x sputter-coating (England). Photochromic properties and transmittance of the samples were investigated by UV−vis analysis by using an i3 UV−vis spectrophotometer, Hanon Instruments (China). For this reason, the initial latex was diluted to about 0.1 wt %. To evaluate the photochromic properties, the excitation was done by a UV lamp (365 nm, 6 W/m2), CAMAG 12VDC/VAC (50/60 Hz, 14VA, Switzerland). Also, the source for visible light was a common LED lamp (8 W/m2). The UV and visible irradiation time was set at 5 min for all the samples. Also, DLS analysis for investigation of lightinduced particle size variation upon UV irradiation was carried out by using of UV (365 nm) and visible light irradiation in a dark room. Finally, the DLS spectra were simultaneously recorded there.

particles can induce chemical and physical attachments of the particles on different substrates, especially cellulosic papers, which are significant matrices for anticounterfeiting inks. 3.1. Characterization of the Functional Groups. Characterization of the functional groups in the stimuliresponsive latex particles was carried out by Fourier-transform infrared spectroscopy (FTIR) analysis, and the results are shown in Figure 2. For this purpose, the latexes were

Figure 2. FTIR spectra of the functionalized stimuli-responsive latex particles.

coagulated, washed with DI water, dried at 40 °C, powdered, and used in preparation of KBr pellets. Because of the low amount of the functional monomers (about 5 wt % with respect to the total amount of MMA), the resulted FTIR spectra do not display significant differences for all the samples. All the spectra display characteristic peaks of PMMA as the matrix. Therefore, the observed broad peak in the range of 3300−3500 cm−1 in CLB-SP and HLB-SP spectra was attributed to the stretching vibrations of O−H functional groups in MAA and HEMA. This peak is observable with a weak intensity for ELB-SP, which resulted from the hydrogen bonding between epoxide functional groups and moisture or can be attributed to the opening of epoxide rings. 3.2. Characterization of the Size and Morphology of the Functionalized Stimuli-Responsive Latex Particles. Emulsifier-free emulsion polymerization is a very significant method for preparation of polymer particles with a size in the range of 400−900 nm and also a low polydispersity index. Because of the absence of surfactant in the polymerization feed, this method is appropriate for preparation of polymer particles with potential applications in biomedical and biological fields. The absence of ionic surfactants decreases surface tension and correspondingly leads to growth of the particles. It should be pointed out that using of an ionic surfactant in normal emulsion polymerization or miniemulsion polymerization results in particles with a size in the range of 40−200 nm, which is not appropriate for anticounterfeiting inks. It is required that the latex particles be stabilized on the paper surface and do not diffuse within the cellulosic fibers.12,26,63,64 For simplicity of discussion on the prepared latex characteristics, some of the polymerization conditions for all of the samples were considered as the same. For example, temperature and stirring speed were set at 75 °C and 600 rpm, respectively. However, the other conditions such as monomer

3. RESULTS AND DISCUSSION The zwitterionic MC form of spiropyran is colored after UV irradiation and displays red fluorescence under UV illumination. Such smart properties are applicable for development of anticounterfeiting and rewritable smart inks and also photopatterning.9 Here, we developed a novel class of functionalized photochromic latex particles based on spiropyran via semicontinuous emulsifier-free emulsion polymerization, as shown in Figure 1. The procedure used for preparation of the functionalized stimuli-responsive latexes and also DP and MW of the samples calculated from 1H NMR spectra (Figures S2− S6, Supporting Information) are presented in Table 1. The photochromic latexes can chemically and physically be attached to different substrates because of their surface functional groups. Such smart inks should be protected from environmental degradation and the other distractive effects by polymer carriers. In the current study, PMMA was used for introduction of spiropyran into the high-polar cellulosic matrix. DLS analysis and measuring of transmittance were used for investigation of the light-induced particle size variation upon UV irradiation. Photofatigue-resistant characteristics of the latexes were also examined by UV−vis analysis. Finally, applications of the prepared functionalized stimuli-responsive latex particles as anticounterfeiting and rewritable inks in security documents and in photopatterning on cellulosic papers were carefully examined. It was experimentally observed that addition of the second monomer resulted in instability and coagulation of the latex particles, especially in the case of MAA. This problem can be dissolved by increasing the amount of NaHCO3 in the polymerization feed, as displayed in Table 1. It should be pointed out that the prepared latexes display desirable stability without any coagulation at the end of polymerization. Functional groups at the surface of the latex D


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Figure 3. SEM images of LB (A−A‴), LB-SP (B−B‴), CLB-SP (C−C‴), ELB-SP (D−D‴), and HLB-SP (E−E‴).

feeding were varied according to the recipes presented in the Experimental Section. Figure 3 displays SEM images for the prepared stimuli-responsive latex particles containing spiropyran and different functional groups. The LB sample, which has no functional groups and spiropyran in its composition, displayed spherical polymer particles with sizes in the range of 500−700 nm and also a narrow size distribution, as shown in the SEM images (Figure 3A−A‴). Figure 3B−B‴ shows a similar morphology and particle size distribution with a different particle size for LB-SP in comparison with the LB sample. The remarkable decrease in particle size of LB-SP can result from the incorporation of spiropyran into the latex particles and increasing the surface charge because of the zwitterionic nature of the MC form on the particle’s surface, which finally resulted in increase of surface tension. As mentioned in the previous sections, conversion of spiropyran into the zwitterionic MC form after dissolving in water resulted in variation of the ionic strength of the medium. Here, the decrease of particle size is due to the presence of MC molecules on the surface of the particles and correspondingly increase of surface tension. Because of the absence of surfactant in our strategy, surface charges that resulted from the MC form increased the electrostatic interactions and also surface tension. This is similar to the results reported by Jin and co-workers for self-assembly of spiropyran block copolymers under UV irradiation, where the size of the

micelles was reduced remarkably after UV irradiation as a result of surface tension increment.65 Other samples display an increase in particle size by functionalization with different functional groups (Figure 3C−C‴,D−D‴,E−E‴). It is notable that decrease of polarity of the functional groups resulted in increase of the particle size, where the CLB-SP samples with carboxylic acid functional groups, HLB-SP-containing hydroxyl functional groups, and also ELB-SP with epoxy functional groups display particle sizes in the range of 500−600, 700− 800, and 800−900 nm, respectively. Such an increase in the particle size can be attributed to the decrease of surface tension by decrease of surface polarity, which finally resulted in growth of particles. 3.3. Photoswitchable Variation of the Particle Size. Spiropyran molecules on the surface of the particles can induce aggregation−disaggregation, assembly−disassembly, or change of particle size in response to UV illumination because of interparticle physical interactions (electrostatic attractions and π−π stacking) between the MC forms.66−68 Hence, average particle size, zeta potential, and also particle size distribution were measured for the diluted latex samples (100-fold) before and after UV illumination by DLS analysis and the obtained results are presented in Figure 4. All the latex particles displayed negative zeta potential in the range of −30 to −40 mV, because of the presence of SO4− at the surface of the particles because of using KPS as the initiator. As discussed in E


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Figure 4. DLS and zeta-potential analysis for the stimuli-responsive latexes bearing spiropyran and different functional groups before and after UV irradiation (365 nm): LB (A), LB-SP (B), CLB-SP (C), ELB-SP (D), and HLB-SP (E).

Figure 5. Color change of the functionalized stimuli-responsive latexes under UV and visible light irradiations and also schematic illustration of the corresponding light-induced particle size variation.

the previous section on SEM images, incorporation of spiropyran into the latex particles (LB-SP sample) resulted in the decrease of particle size by increasing the surface tension. The results obtained from DLS analysis for functionalized latexes before UV illumination confirmed the observations in SEM images, where the particle size increased by decrease of

polarity of the functional groups. Therefore, the highest particle size was observed for the ELB-SP sample (850 nm), the lowest particle size was shown in the CLB-SP sample (550 nm), and HLB-SP displayed a moderate particle size (750 nm). It should be noted that the SEM images were taken from the dried latexes and the resulted particle size is smaller in F


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Figure 6. Variation of transmittance with time for the stimuli-responsive latexes under UV and visible light irradiation at λmax of the samples: LB-SP (A) at 557 nm, CLB-SP (B) at 557 nm, ELB-SP (C) at 552 nm, and HLB-SP (D) at 556 nm.

respectively. Such a notable particle size variation for the ELBSP sample is due to the physical interactions between epoxy functional groups and MC molecules on the particle surface leading to bimodal particle size distribution upon UV irradiation. On the other hand, the zeta potentials of the CLB-SP and ELB-SP samples display a significant reduction (from −39.1 to −35.6 mV and −37.3 to −33.4 mV, respectively) after UV illumination as a result of latex particle assembly. This can confirm the interactions between the MC molecules on the surface of the particles with the carboxylic acid and epoxy functional groups, which resulted in a remarkable increase of particle size after UV illumination for these samples compared to LB-SP and HLB-SP. As displayed in Figure 5, not only the MC molecules interacted with themselves by packing the positive and negative charges in their zwitterionic structure, but also the zwitterionic MC molecules can be physically bonded to the carboxylic acid and epoxide functionalities. For this reason, the zeta potential of LB-SP and HLB-SP samples undergoes a slight increase after UV irradiation. Therefore, it can be claimed that the surface charge was not influenced by UV irradiation and formation of the MC molecules, because of the interactions of opposite charges with themselves in the MC structure. As expected, the observed particle size variation is photoswitchable and reversible under alternated UV and visible light irradiation because this light-induced particle size variation resulted from spiropyran and its reversible isomerization. The variation of UV transmittance as a function of particle size is a key point for investigation of the particle size variations in colloidal systems. Because of the increase of the reflective index, this parameter decreases by the increase of particle size. Therefore, variation of transmittance as a function of time under UV and visible light irradiation was investigated by UV− vis spectroscopy at the λmax of the MC isomer, which is different for each of the samples. For this purpose, diluted latex

comparison with the size obtained from the DLS results. This is mainly because of the swelling of particles in the latex state by water. On the other hand, the amount of difference in particle size extracted from the DLS results and SEM images depends on the polarity of the surface functional groups. The less-polar particles undergo maximum size reduction (ELB-SP, LB-SP, and LB) and the polar ones (HLB-SP and CLB-SP) display a minimum decrease in particle size. Indeed, the polar groups trapped higher amounts of water molecules by hydrogen bonding and consequently display a minimum decrease in particle volume. However, the less-polar particles cannot trap water molecules and display higher decreases in particle volume after drying. According to the DLS results for latex samples before and after UV illumination (Figure 4), the particle size can be modulated by UV irradiation when spiropyran molecules are incorporated at the surface of the particles. Upon UV irradiation and formation of the MC molecules on the particle surface, particles were assembled or aggregated by physical attractive interactions because of the formation of electrostatic attractions and π−π stacking interactions between the MC molecules, as shown in Figure 5. The size variation for different latex samples is influenced by different factors. The maximum size variation was observed for the CLB-SP sample, where it undergoes a remarkable size increase from 550 nm before UV irradiation to above 10 μm upon UV irradiation. This can be attributed to the interaction of the carboxylic acid functional groups with the MC form or the light-induced coagulation because of the variation of the ionic strength by MC molecules. It should be noted that the measurement range of the DLS instrument is 1 nm to 10 μm. The size of the CLB-SP particles increased to above 10 μm after UV illumination, which it is out of the standard range and cannot be observed in reported DLS curves. Light-induced increasing of particle sizes for the LB-SP, ELB-SP, and HLB-SP samples are 390, 3150, and 300 nm, G


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Figure 7. UV−vis spectra of the functionalized stimuli-responsive latex particles before and after UV illumination (365 nm): LB-SP (A), CLB-SP (B), ELB-SP (C), and HLB-SP (D).

3.4. Photochromic Properties of the Stimuli-Responsive Latex Particles. Spiropyran displays a complex photochromic behavior in different media, especially polar environments such as water and cellulose. Investigations of photoc h r om i sm , p h ot o f a t ig ue - r e s i st a n t ch a ra c t e r i st i c s , photoswitchability, and also reversible isomerization kinetics are the most important studies for photochromic materials based on spiropyran. Accordingly, these properties are investigated for the prepared functionalized stimuli-responsive latexes in the current section in order to develop an applicable anticounterfeiting and rewritable polymeric ink, which can be used in photopatterning. The photochromic properties and also color change of the prepared latexes are macroscopically observable in Figure 5. The prepared latexes were diluted to 0.1 wt % (100-fold) and directly used for investigation of photochromic properties by UV−vis analysis. The photochromic behavior of the samples was examined before and after UV light (365 nm) illumination for 5 min. The colorless SP form was converted to the colored MC form upon UV irradiation, which can be characterized by a broad absorption peak in the range of 400−700 nm, as displayed in UV−vis spectra of the latex samples in Figure 7. All the samples display maximum absorption in the range of 550−560 nm, which is the same for different samples and reported as λmax in the spectra of Figure 7. As expected, all of the functionalized stimuli-responsive latex samples display suitable photochromism because of the sharp absorption peak of the MC form (Figure 7) and also notable color change after UV irradiation (Figure 5). It should be pointed out that the observed weak peak in the range of 400−500 nm in the spectra of CLB-SP was attributed to the establishment of the MC form by protonation with H+ and formation of the protonated merocyanine (MCH) form. Protonation of the phenolate anion in the MC structure is due to the presence of carboxylic acid functional groups at the surface of CLB-SP particles. The

samples (100-fold, 0.1 wt %) were exposed to UV irradiation (365 nm), and the transmittance was measured every 20 s. Then, this measurement was repeated for the samples under visible light irradiation at similar periods and the results are shown in Figure 6. Accordingly, the UV transmittance decreased by switching the UV light on and it returns to the initial value by visible light illumination. As previously mentioned, the particle size and also the light reflectance were increased upon UV irradiation and formation of the MC molecules, leading to a decrease of transparency. As displayed in Figure 6, all of the samples display an illumination timedependent decrease in the transmittance during UV illumination, where the slope of the transparency decrease was dramatically increased with irradiation time. These observations are due to the increase of particle size upon UV irradiation in all the samples, but the intensity and value of the particle size variation are different for latexes containing various functional groups. The transparency variations are more intense for the CLB-SP and ELB-SP samples because of their maximum particle size variation. On the other hand, the transmittance was increased upon visible light irradiation and displayed a similar time dependency, which is mostly increased by irradiation time. The MC form was converted to the SP form under visible light irradiation and resulted in the decrease of particle size by disassembly of particles because of dissociation of the interparticle interactions. The results obtained from transparency changes under UV and visible illumination by UV−vis spectroscopy confirmed the results of the DLS analysis for light-induced variation of particle size. Also, this responsivity is fully reversible under alternated UV and visible light irradiation. It should be pointed out that measuring the transmittance variation as a function of UV irradiation time was performed for confirmation of the results obtained from DLS analysis, and it cannot display increase of the particle size upon UV irradiation alone. H


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Figure 8. Normalized and exponential fitting curves of the SP to MC and MC to SP isomerization kinetics for the latex samples at the corresponding λmax under UV and visible light irradiation: LB-SP (A), CLB-SP (B), ELB-SP (C), and HLB-SP (D).

Table 2. Coloration and Discoloration Rate Equations and the Corresponding Kinetics Parameters for the Prepared StimuliChromic Latex Particles under UV and Visible Light Irradiation for Samples Containing Different Spiropyran Contents SP to MC

MC to SP 2

−1

samples

equation

R

kc1 (s )

T1/2 (s)

LB-SP CLB-SP ELB-SP HLB-SP

0.058 − 0.058 exp(−0.003t) 0.06 − 0.06 exp(−0.0036t) 0.06461 − 0.06461 exp(−0.0060t) 0.05326 − 0.05326 exp(−0.0058t)

0.99 0.99 0.99 0.99

0.0030 0.0036 0.0060 0.0058

215.50 188.43 115.55 117.12

a

equation

R2

kc2 (s−1)

T1/2 (s)a

0.0064 + 0.0635 exp(−0.0102t) −0.0007 + 0.0683 exp(−0.0030t) 0.0012 + 0.0686 exp(−0.0106t) 0.0102 + 0.0781 exp(−0.0082t)

0.99 0.99 0.99 0.99

0.010 0.003 0.010 0.0082

78.08 251.93 67.14 102.02

The time required to reach half of the final absorbance for coloration

a

results obtained from UV−vis spectra display remarkable photochromic behavior for the functionalized stimuli-responsive latexes. The polar functional groups cannot significantly influence their photochromism, which was observed in the previous studies as the negative photochromism.26,27,36 On the other hand, the effect of the different functional groups on the SP to MC and MC to SP isomerization rate is not clear and should be carefully investigated by UV−vis analysis. The photochromic properties of spiropyran, especially isomerization kinetics, in a polymer matrix can significantly be influenced by different factors such as UV and visible illumination time, polarity, pH, and chain flexibility (glass transition temperature, Tg).12,26,27,36,44 Hence, to clarify the effect of different functional groups on the photochromism, kinetics of the SP to MC and MC to SP isomerization, respectively, under UV and visible light irradiation were studied for all the stimuli-responsive latexes by UV−vis spectroscopy. For this purpose, the absorbance intensities were measured at a specific λmax for each of the samples in different time intervals of UV (365 nm) and visible light irradiation. Therefore, the primarily measured absorbance values were converted to the normalized ones (with respect to the absorbance of each sample before UV and visible light irradiation) and plotted versus irradiation time, as shown in

Figure 8. On the other hand, we used Origin software (Professional 2017) for fitting the resulted data with suitable curves to extract the kinetic equations and rate constants as the most important kinetics parameters. For this purpose, the SP to MC isomerization kinetics was fitted by using the BoxLucas 1 model and also the MC to SP isomerization (back reaction) was fitted by using the ExpDec1 model, leading to suitable exponential equations, as presented in eqs 1 and 2. Normalized absorbance (t ) = A(1 − exp(−kc1t )) Normalized absorbance (∞)

(1)

Normalized absorbance (t ) = A 0 + A exp( −kc2t ) Normalized absorbance (∞)

(2)

where A and A0 are the constant values and kc1 and kc2 represent the rate constants for the SP to MC (eq 1) and MC to SP (eq 2) photoisomerization upon UV and visible light irradiation, respectively. The normalized absorbance values at time t and ∞ were extracted from Figure 8. It is noteworthy that kc could be a good indication of the susceptibility or responsivity of the functionalized stimuli-responsive latex particles toward UV and visible light irradiation during photoisomerization. The concentration of spiropyran in the I


Research Article


Figure 9. Photofatigue-resistant characteristics of the prepared photochromic latexes upon alternating UV (365 nm for 5 min) and visible light (5 min) irradiation: LB-SP (A), CLB-SP (B), ELB-SP (C), and HLB-SP (D).

of the MC to SP form, where further visible light illumination time is required to complete the MC to SP isomerization. It should be pointed out that the UV-induced interparticle interactions leading to a variety of particles sizes can display a significant role in control of the MC to SP isomerization kinetics. Finally, it can be concluded that the kinetics of the SP to MC and MC to SP isomerization in the functionalized stimuli-responsive latex particles were influenced by interactions of the SP or MC molecules with different functional groups. Where such interactions lead to the stability of the MC form (polar interactions such as hydrogen bonding), maximum kc1 and minimum kc2 are obtained. Also, stabilization of the SP molecules by nonpolar interactions and reduction of their ground-state energy levels resulted in minimum kc1 and maximum kc2 values. However, the kinetics and photochromic properties can significantly be influenced by interactions of the MC molecules together with the surrounding media.36,69 As a key point, fast SP to MC isomerization and slow MC to SP isomerization can be observed in media where the MC structures are highly stable. However, stability of the SP form results in fast MC to SP and slow SP to MC isomerizations. The MC and SP structures can be stabilized in the polar and nonpolar environments, respectively. Investigation of the photoswitchability and photofatigueresistant characteristics is the most important analysis for evaluation of the life-time of photochromic behavior in stimuliresponsive polymers, which can remarkably influence the final applications of the prepared photochromic materials. Spiropyran is a very sensitive photochromophore in environmental interactions such as chemical reactions, polarity, pH, and temperature. Hence, its protection from environmental degradation in high-polar media is a very important requirement for application of the final polymer in advanced systems such as chemosensors, anticounterfeiting inks, and photopatterning.9,38,70 Recent studies show that incorporation of

polymer particles is constant (1 wt %) and does not influence the isomerization kinetics; therefore, kinetics parameters are independent from the spiropyran concentration. The only influential factor on isomerization kinetics is polarity of the functional groups because the MC form is remarkably affected by polar interactions.26,27,36 As displayed in Figure 8, both the SP to MC and MC to SP isomerization kinetic curves were fitted as well by eqs 1 and 2, respectively, and the corresponding extracted parameters such as R2 (regression), kc, and T1/2 (the time required to reach half of the final absorbance for coloration) are presented in Table 2. The obtained R2 values reveal that the kinetics of the SP to MC and MC to SP isomerization are adapted by eqs 1 and 2 as well. According to the results of Table 2, the maximum kc1 and minimum T1/2 for the SP to MC isomerization are observed for the HLB-SP and ELB-SP samples as a result of their polar hydroxyl and epoxy functional groups. The CLB-SP sample displays the lowest kc1 and the highest T1/2 in comparison with the HLB-SP and ELB-SP samples, even though the polarity of the carboxylic acid functional group is higher than that of hydroxyl and epoxy. This contrary observation can be attributed to the establishment of the MC form as the MCH structure and reduction of the measured absorbance intensity in the kinetics study, because kinetic results were obtained at λmax of the MC form, which is different from the λmax of the MCH isomer. The LB-SP, HLB-SP, and ELB-SP samples display similar kinetic parameters for the MC to SP isomerization, where the values of kc and T1/2 for these samples are in the same range. On the other hand, minimum kc and maximum T1/2 of the MC to SP isomerization reaction were observed for the CLB-SP sample, which can be attributed to the establishment of the MC form by hydrogen bonding with carboxylic acid functionalities known as negative photochromism. This phenomenon stabilizes the MC form and reduces the rate of back isomerization reaction for conversion J


Research Article

ACS Applied Materials & Interfaces photochromic compounds such as spiropyran into the hydrophobic polymer particles can lead to significant improvements in photostability, photoswitchability, and also reversibility.12,26,27,36,44,57,59,61,63 Here, the photoswitchability and photofatigue-resistant characteristics of the prepared photochromic latexes are investigated by UV−vis analysis under alternating UV and visible light irradiation to develop an applicable photochromic latex for using in anticounterfeiting documents. For this purpose, the diluted latex samples (0.1 wt %) were used for measurement of absorbance intensity in specified λmaxs after 5 min of UV irradiation (365 nm) and 5 min of visible light irradiation for 15 cycles and the resulted curves from absorbance versus cycle number are displayed in Figure 9. The corresponding absorbance intensity was immediately measured after each irradiation within a 5 min interval between each cycle. As expected, photochromic latex particles displayed good photoswitchability after each cycle and also significant photofatigue resistance after 15 UV−vis irradiation cycles, where the negative photochromism and decrease of photoresponsivity as disadvantages for anticounterfeiting inks and photopatterning applications were not observed for any sample. Additionally, the observations in the current analysis confirm the reversibility of the light-induced particle size variation for a long time. All the advantages resulted from the role of the polymer carrier, chemical incorporation of spiropyran, and also hydrophobic polymer matrix for protection of spiropyran from environmental degradation and negative photochromism especially in highpolar media. The obtained results confirmed the potential applications of the functionalized stimuli-responsive latexes for using as ink in anticounterfeiting documents and also coatings on a surface for photopatterning. 3.5. Applications of the Stimuli-Responsive Latexes as Rewritable Anticounterfeiting Polymer Inks, Security Marking, and Reversible Photopatterning. The functionalized stimuli-responsive latex particles containing spiropyran (1 wt %) displayed photochromic properties and photofatigueresistant characteristics and can be used as anticounterfeiting inks for writing on general cellulosic papers and security documents and also in photopatterning. As shown in Figure 10, the prepared latexes (10 wt %) were used as ink for loading into a pen and then writing on a cellulosic paper. It should be

pointed out that the potentials of all the prepared latexes for these applications were examined and similar results were obtained. Therefore, only the photographs obtained from the CLB-SP sample have been presented. As displayed in Figure 10, before UV irradiation and under visible light, the written phrases and chemical structures are discolored and invisible; however, the written phrases and chemical structures are colored and visible after UV irradiation (365 nm) for 1 min. This coloration can be attributed to the light-induced isomerization of the SP to MC form, which is fully reversible under alternating UV and visible light irradiation. These inks are solvent-free (water-based) and can be stable on the paper surface because of their functional groups that can be attached to the cellulose by physical or chemical interactions. Also, their large particle size (400−900 nm) prevents their their diffusion into the cellulose fibers and makes them stable on the paper’s surface. To the best of our knowledge, this is the first report on such anticounterfeiting and rewritable inks based on photochromic latex particles. These latex particles (LB-SP, CLB-SP, HLB-SP, and ELBSP) were used for increasing the security of several monies and similar results were observed for all the samples, and the results for the CLB-SP sample are shown in Figure 11 and also Figure S7 (Supporting Information). For this purpose, some points of a sample money were marked by a pen (containing the latex particles as the ink). The marked points are discolored before UV irradiation, have red fluorescence emission under UV irradiation, and are colored after UV illumination, because of the reversible fluorochromic and photochromic properties of the spiropyran molecules in the latex particles. As a result, the functionalized stimuli-responsive latex particles can be used as a fluorochromic and photochromic material for security marking in high-security documents such as money, but it needs further investigations in the near future by using different fluorochromic and photochromic compounds, for which our group (Research Group for Innovation in Smart Polymers, RGISP) is conducting ongoing studies in the RGISP lab. Photopatterning is the most famous application of the stimuli-responsive materials based on photochromic compounds. Hence, photopatterning of the prepared functionalized stimuli-chromic latexes was investigated by spraying the latexes (LB-SP, CLB-SP, HLB-SP, and ELB-SP) with 10 wt % of solid content on the cellulosic papers for preparation of photochromic papers. The resulted stimuli-responsive papers display notable color change and photopatterning upon UV irradiation, as shown in Figure 12 for the CLB-SP sample. The photopatterns were induced by using different masks under UV irradiation. It was notable that the prepared stimuliresponsive papers display excellent reversible photopatterning characteristics under different masks and UV illumination. As a summary, we reported the most significant and interesting applications for a novel class of functionalized stimuliresponsive latex particles in different advanced fields such as anticounterfeiting and rewritable inks and also photopatterning on stimuli-responsive papers for the first time. This is a facile and fast approach for designing and preparation of such advanced smart systems.

4. CONCLUSIONS In this study, a facile approach was developed for preparation of novel functionalized stimuli-responsive latex particles containing spiropyran (1 wt %) by semicontinuous emulsi-

Figure 10. Application of the functionalized stimuli-responsive latex particles (CLB-SP) as an anticounterfeiting and rewritable smart ink for writing on cellulosic papers. K


Research Article


Figure 11. Application of the functionalized stimuli-responsive latex particles (CLB-SP) as an anticounterfeiting and rewritable smart ink for secured marking in security documents.

0.98) for the SP ⇌ MC isomerization as a function of time. The prepared photochromic latexes bearing spiropyran moieties display remarkable photoswitchability and photofatigue-resistant characteristics under alternating UV and visible light irradiation cycles. Applications of these latexes as anticounterfeiting inks for writing on cellulosic papers and also security marking on a sample money display that the written phrases show red fluorescence emission and coloration, respectively, under and after UV illumination (365 nm). Investigation of photopatterning for latex-sprayed cellulosic papers under UV irradiation and different masks confirmed that these latexes have potential applications in photopatterning. The presence of functional groups and large particle size are the main effective factors for stabilization of the latex particles on cellulosic papers by physical and chemical interactions.

■

Figure 12. Coating of the functionalized stimuli-responsive latex (CLB-SP) on cellulosic papers and investigation of photopatterning abilities.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b14865.

fier-free emulsion polymerization, investigation of their application as anticounterfeiting inks in security documents, and also their use in photopatterning on cellulosic papers. The results obtained from SEM, FTIR, and DLS (zeta potential) analysis display that all the functionalized stimuli-responsive latexes have spherical particles with sizes in the range of 400− 900 nm and different functional groups. Incorporation of spiropyran and polar functional groups at the surface of the particles can reduce the particle size by increasing the surface tension. The latex particles undergo a remarkable light-induced particle size variation upon UV illumination (365 nm) as a result of interparticle interactions of electrostatic attractions and π−π stacking interactions between MC molecules with themselves and also functional groups on the surface of the particles. The values of the light-induced particle size variation have been affected by the type of the functional groups on the particles’ surface, where the maximum and minimum variations were observed for latex particles bearing carboxylic acid and hydroxyl functionalities, respectively. The results obtained from kinetic studies display that the photochromic behavior and SP ⇌ MC isomerization can be influenced by polar interactions between functional groups and MC molecules, which is known as negative photochromism. Exponentially fitted kinetic equations display interesting relationships (R2 >

1

■

H NMR spectra of SPEA, 1H NMR spectra for all the stimuli-responsive latex samples, further experiments for determination of solid content of latexes, and also additional images for security marking experiments on different monies (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Amin Abdollahi: 0000-0002-6415-7547 Keyvan Sahandi-Zangabad: 0000-0003-1980-8731 Hossein Roghani-Mamaqani: 0000-0001-6681-7679 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The Iran National Science Foundation (INSF) is greatly appreciated for its financial support (grant number: 97003824). L


Research Article


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