Dual Colorimetric and Fluorescent Authentication Based on

Aug 17, 2017 - Dual Colorimetric and Fluorescent Authentication Based on Semiconducting Polymer Dots for Anticounterfeiting Applications. Wei-Kai Tsai...
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Dual Colorimetric and Fluorescent Authentication Based on Semiconducting Polymer Dots for Anticounterfeiting Applications Wei-Kai Tsai, Yung-Sheng Lai, Po-Jung Tseng, Chia-Hsien Liao, and Yang-Hsiang Chan* Department of Chemistry, National Sun Yat-sen University, 70 Lien Hai Road, Kaohsiung, 80424, Taiwan S Supporting Information *

ABSTRACT: Semiconducting polymer dots (Pdots) have recently emerged as a novel type of ultrabright fluorescent probes that can be widely used in analytical sensing and material science. Here, we developed a dual visual reagent based on Pdots for anticounterfeiting applications. We first designed and synthesized two types of photoswitchable Pdots by incorporating photochromic dyes with multicolor semiconducting polymers to modulate their emission intensities and wavelengths. The resulting full-color Pdot assays showed that the colorimetric and fluorescent dualreadout abilities enabled the Pdots to serve as an anticounterfeiting reagent with low background interference. We also doped these Pdots into flexible substrates and prepared these Pdots as inks for pen handwriting as well as inkjet printing. We further applied this reagent in printing paper and checks for high-security anticounterfeiting purposes. We believe that this dual-readout method based on Pdots will create a new avenue for developing new generations of anticounterfeiting technologies. KEYWORDS: semiconducting polymer dots, anticounterfeiting, fluorescent polymer inks, photoswitchable, full-color patterning

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INTRODUCTION Fast-growing counterfeiting markets in currency, fuels, medicines, and products in the food industry have led to serious economic losses and health risks to consumers, manufacturers, and governments. Traditional anticounterfeiting technologies range from watermarks, holograms, security inks, taggants, and metal threads to radio frequency identification. These authentication features can be either overt or covert to offer different levels of protection. However, due to the rapid development of the high-tech computerized equipment in recent years, high-quality counterfeiting products could be easily produced in a short period of time. Therefore, there is an urgent demand to develop innovative and highly scalable anticounterfeiting and security technologies to deter counterfeiting. In the past few years, new generations of anticounterfeiting systems based on nanotechnologies have been developed.1−6 Among these technologies, colorimetric and fluorometric functional nanomaterials are of particular interest because these nanomaterials can be made invisible to naked eyes under room light and can be easily implemented onto different surfaces.1 Moreover, they can be integrated with stimuli-responsive materials, which alter their physical or optical properties in response to external stimuli, to add additional security features, making them more difficult to forge. Herein, we report a novel strategy to develop lightresponsive fluorescent nanomaterials in which photochromic spiropyran molecules were incorporated into semiconducting polymer dots (Pdots) with full-color emission for anticounterfeiting purposes. Recently, Pdots have attracted enormous interest owing to their extraordinary fluorescence brightness, © XXXX American Chemical Society

good photostability, fast radiative rate, facile surface functionalization, and high biocompatibility.7−24 Besides, their advanced applications in photoacoustic imaging and photothermal activation of neurons have also been studied.25,26 Our group has also shown that Pdots can be extensively applied in biological imaging and sensing, as well as material science.27−31 Even with the unique optical properties of Pdots, the adoption of Pdots in anticounterfeiting applications is still very rare and preliminary.32,33 Moreover, the reported works focus only on the fluorescent properties of Pdots by preparing Pdots as fluorescent inks, but they often suffer from serious background interference on fluorescent substrates. To make Pdots even more practical in anticounterfeiting applications, we were inspired to design Pdots with colorimetric and fluorescent dualreadout abilities. By integrating photochromic dyes with Pdots, here, we were able to reversibly modulate the absorption, emission intensity, and fluorescence wavelength of the Pdots upon alternative illumination with light of two different wavelengths. The colorimetric and fluorescent dual-readout approach could offer complementary signals at the same time to avoid possible background interference from colored substrates and/or fluorescent surfaces. Additionally, we further fabricated these Pdots as security inks for pen writing and inkjet printing. We then applied this reagent on printing paper, checks, and currency to demonstrate its practical use. Received: June 22, 2017 Accepted: August 17, 2017 Published: August 17, 2017 A

DOI: 10.1021/acsami.7b08993 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces Scheme 1. Schematic Showing the Preparation of Photoswitchable Pdots for Anticounterfeiting Applicationsa

a

Schematic showing two routes for the preparation of photoswitchable Pdots for anticounterfeiting purposes. (A) Hydroxyl functionalized spiropyran and 0−50% mol 2-butyl-1-octanol was first conjugated to poly(styrene-co-maleic anhydride) polymers, and the resulting polymers were mixed with PFBTDBT/PPE/PFBT polymers in THF to coprecipitate in water to form Pdots with different emission colors. (B) Hydrophilic spiropyran derivative, Py-Bips, was directly blended with PFBTDBT/PPE/PFBT polymers in THF and then injected into water via nanoprecipitation to form Pdots.

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RESULTS AND DISCUSSION The aim of this work is to design full-colored fluorescent (blue, yellow, and red) polymer dots with light-tunable absorption and emission. We first synthesized and then incorporated two types of spiropyran derivatives into Pdot matrices to investigate their photoswitching efficiency. After that, the optimized Pdots could be used for anticounterfeiting applications. Here, we selected three semiconducting polymers, PFBTDBT, PFBT, and PPE, because they have distinct emission wavelengths (red, green, and blue, respectively) under the same excitation source of 365 nm, allowing us to develop full-color (RGB) fluorescent patterns. Design and Synthesis of Photoswitchable Pdots. Scheme 1 illustrates our strategies for the preparation of photoswitchable Pdots. Here, we introduced two methods and then compared their switching performance. For route A, we first synthesized spiropyran with hydroxyl groups and then covalently bound to poly(styrene-co-maleic anhydride) (PSMA) polymers. The spiropyran-grafted polymers were further blended with semiconducting polymers (e.g., PFBTDBT, PPE, and PFBT) in tetrahydrofuran and quickly injected into H2O under sonication to generate polymer-SP Pdots. It is worth mentioning that we also introduced 2-butyl-1octanol onto the PSMA backbone to form polymer-SPB Pdots and investigated how the steric hindrance affected the

photoswitching efficiency of Pdots (vide infra). It has been recently reported that the photoswitching capability of spiropyran could be greatly improved in aqueous solution if the nitro substituent in the 6 position of the chromene moiety of spiropyran was removed.34 It was suggested that the nitrosubstituted spiropyran would involve triplet states in the photochemical opening reaction and could easily undergo hydrolysis in its ring-opening merocyanine form.35 Therefore, we devised route B where we first synthesized an unsubstituted and water-soluble spiropyran derivative, Py-Bips (Scheme 1B), after which we blended Py-Bips with semiconducting polymers during nanoprecipitation to form Py-Bips embedded Pdots. Because Py-Bips is relatively hydrophilic while semiconducting polymers are hydrophobic, we added sodium dodecyl sulfate, an amphiphilic surfactant, to help the encapsulation of Py-Bips inside the polymer matrices (see Experimental Section). It is worth mentioning that Py-Bips could not be directly conjugated to PSMA like spiropyran because we found that Py-Bips was not chemically stable enough to undergo dimethylaminopyridine-catalyzed reactions. We called the Pdots formed by route A and B to be polymer-SP (without branched alkanes)/ polymer-SPB (with branched alkanes) and polymer-Py, respectively. Selection of Suitable Spiropyran Molecules for Development of Photoswitchable Pdots. We first assessed B

DOI: 10.1021/acsami.7b08993 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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leaching of the embedded materials from the Pdot core, and therefore, we here selected route A for our subsequent studies. The absorption and emission spectra of three different types of polymer-SP Pdot solutions before and after 365 nm UV irradiation are shown in Figure 1B, and their corresponding photographs are displayed in Figure 1C. Before UV irradiation, the strong fluorescence of PPE (400−500 nm), PFBT (500− 600 nm), and PFBTDBT (600−700 nm) could be clearly observed. After UV irradiation, the fluorescence of all three Pdots was quenched by over 98% accompanied by a new emission peak at ∼680 nm (merocyanine). Besides, a strong absorption of ca. 570 nm from merocyanine could be seen. These results indicate that the full-color (RGB) fluorescence photomodulation could be achieved via light-controllable energy transfer. The particle sizes of three types of Pdots prepared by use of route A were also characterized by dynamic light scattering and transmission electron microscopy, ranging from 19 to 22 nm (Figure S1). We further doped these Pdots into transparent poly(vinyl alcohol) (PVA) films to assess their feasibility for use in flexible substrates. As shown in Figure 1D, the resulting polymer-SP PVA films (PFBT, PPE, and PFBTDBT from left to right) exhibit strong RGB emissions before UV irradiation. After UV irradiation with a flowershaped photomask on top of the PVA films, we clearly observed a dark-purple flower pattern in the center. The pattern can be subsequently erased by the illumination of visible light. The fluorescence and absorption spectra of doped Pdots in PVA films were also measured (Figure S2), and no significant change could be observed as compared to the Pdots in water. Optimization of Photoswitching Efficiency of Pdots. We further investigated how the steric hindrance of the polymer structures affected the photoconversion efficiency and the number of switching cycles of Pdots. By taking PFBT as an example, we fabricated 50 mol % bulky yet flexible 2-butyl-1octanol and 50 mol % spiropyran onto PSMA and then blended it with semiconducting polymers to form polymer-SPB Pdots. We found that the incorporation of branched alkanes could provide improved fatigue-resistance of Pdots (Figure 2B) as compared to nonbranched ones (Figure 2A). The results indicate that these flexible long-chain alkanes could provide extra free volume among polymer chains, allowing more efficient photoisomeriation of spiropyran/merocyanine. To further improve the fatigue-resistant property of Pdots, we added an additional amount (50%, w/w) of bare PSMA during polymer-SP Pdot preparation. As shown in Figure 2C, we found that additional PSMA inside Pdots indeed further enhanced the fatigue-resistant property of Pdots. We speculate that it is likely due to the bulky and rigid configuration of PSMA, which could provide more space and thereby prevailed over flexible/compressible 2-butyl-1-octanol. Therefore, we added PSMA into polymer-SPB Pdots to achieve optimal fatigue-resistance performance. As shown in Figure 2D, we can clearly observe that the blending of PSMA and the incorporation of branched alkanes into Pdot matrices significantly increased the fatigue-resistance of polymer-SP Pdots. For practical use in anticounterfeiting applications, the photoswitching rate of Pdots is an important factor to be evaluated. We compared the light-induced conversion rates of PFBT-SP Pdots with PSMA blended PFBT-SPB Pdots under the same experimental conditions. As displayed in Figure 3A,B, the fluorescence of PSMA blended PFBT-SPB Pdots could be recovered up to ∼98% of their original intensity at the first

and compared the photoswitching efficiency of polymer-SP Pdots and polymer-Py Pdots. As shown in Figure 1A in which

Figure 1. (A) Fluorescence spectra of PFBT-SP Pdots (left panel) and PFBT-Py Pdots (right panel) in water before (green line) and after (red line) 365 nm UV illumination. (B) UV−visible spectra of PPE-SP Pdots (solid blue line), PFBT-SP Pdots (solid green line), and PFBTDBT-SP Pdots (solid red line) in water before (left panel) and after (right panel) 365 nm UV irradiation. Their corresponding emission spectra are shown with dash lines. (C) Photographs of PPESP Pdots, PFBT-SP Pdots, and PFBTDBT-SP Pdots (from left to right) dispersed in water under daylight (left images) and under 405 nm light (right images). The upper and bottom panels represent the photographs before and after 365 nm UV irradiation, respectively. (D) The left panel shows the photographs of PFBT-SP Pdots, PPE-SP Pdots, and PFBTDBT-SP Pdots (from left to right) in PVA films under daylight (left image) and under 405 nm light (right image). The right panel exhibits their corresponding photographs after 365 nm UV irradiation with a flower-shaped photomask.

we took PFBT polymer as an example, the peak fluorescence of PFBT-SP Pdots was quenched by 99% with the accompaniment of a peak emergence at ∼680 nm after UV irradiation. This phenomenon could be attributed to the energy transfer from PFBT polymers to the light-induced open-ring merocyanine. In contrast, the fluorescence intensity of PFBTPy Pdots decreased by only 45% under the same experimental conditions. These results suggest that covalently linked spiropyran could lead to highly efficient energy transfer from semiconducting polymers to UV light-triggered merocyanine, probably in part due to the much shorter separation between polymers and merocyanine as compared to the physically blended one (i.e., PFBT-Py Pdots). Even in PFBT-SP Pdots, the spiropyran dyes were essentially conjugated to PSMA polymers and then physically blended with the semiconducting polymer PFBT. The distance between spiropyran and PFBT would still be much shorter as compared to that between PyBips and PFBT, considering the strong hydrophobic interactions of PSMA−PFBT and the hydrophilic properties of PyBips. Moreover, strong covalent bonds could avoid potential C

DOI: 10.1021/acsami.7b08993 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 2. Photoinduced switching cycles of (A) PFBT-SP Pdots, (B) PFBT-SPB Pdots, (C) PSMA blended PFBT-SP Pdots, and (D) PSMA blended PFBT-SPB Pdots. The black solid squares show the fluorescence intensity of Pdots after visible light irradiation, and the open squares exhibit the fluorescence intensity of Pdots after 365 nm light illumination. The data were recorded at 540 nm with excitation at 450 nm.

switching cycle, while the fluorescence of PSMA blended PFBT-SP Pdots could be recovered at less than 80%. At their tenth switching cycle, more than 80% of the emission intensity could be retrieved for PSMA blended PFBT-SPB Pdots but only less than 40% of the emission intensity could be regained for PFBT-SP Pdots (Figure 3C,D). We also plotted their normalized emission intensity as a function of irradiation time. As shown in Figure 3E, we found that PSMA-blended PFBTSPB Pdots revealed a slightly faster recovery rate as compared to PFBT-SP Pdots at their first switching cycle. Interestingly, the difference of the conversion rates became prominent after 10-cycles of photoswitching as shown in Figure 3F. These results again demonstrate that steric hindrance plays an important role in both fatigue-resistance and photoswitching efficiency of the resulting Pdots. On the basis of the above results, we thus employed PSMA-doped polymer-SPB Pdots for our following studies due to their enhanced fatigue-resistance and faster photoswitching rate. Full-Colored Patterning of Pdots for Anticounterfeiting Applications. To have practical use of these photoswitchable Pdots for anticounterfeiting purposes, we loaded Pdot solutions into pens and then wrote on a piece of white paper as shown in Figure 4A. The word “Chemistry” was handwritten with three types of Pdots (Chem: PFBTDBT; is: PFBT; try: PPE). The word was slightly visible under room light while their strong fluorescence could be observed under 405 nm light. After UV light irradiation, the half-latent word became distinct under room light due to the color mixture with dark-purple merocyanine. At the same time, their fluorescence was highly quenched by merocyanine, consistent with their spectral results (Figure 1). These results indicated that we should be able to create full-color patterns by mixing three types of Pdots at different proportions. One of the easiest ways to realize full-color patterns even at higher levels of complexity is through computerized inkjet printing. To make sure that these Pdots were suitable for modern high-throughput inkjet printing, we filled a standard RGB ink cartridge with the

Figure 3. Fluorescence spectra of (A) PSMA blended PFBT-SPB Pdots and (B) PFBT-SP Pdots as a function of UV−vis irradiation time at their first photoswitching cycle. Fluorescence spectra of (C) PSMA blended PFBT-SPB Pdots and (D) PFBT-SP Pdots as a function of UV−vis irradiation time at their tenth photoswitching cycle. Emission response of bare PFBT-SPB Pdots (black triangles) and PSMA blended PFBT-SP Pdots (red circles) upon irradiation with 365 nm UV light for 10 s and then 520 nm visible light for another 40 s (E) for the first switching cycle and (F) for the tenth switching cycle.

corresponding Pdots (R: PFBTDBT; G: PFBT; B: PPE) in a commercially available desktop inkjet printer (Canon MP258). We first printed a full-color wheel in which primary, secondary, and tertiary colors were achieved as displayed in Figure 4B. This suggested that the mixing of RGB colors could allow for the manipulation and coordination of color on patterns with full spectra. It should be noted that the Pdot concentration used in fountain pens and inkjet printing was about 5 times higher than that of the as-prepared Pdots. Besides, appropriate amounts of glycerol and ethanol were also added during ink preparation, in which glycerol was used to increase the viscosity and ethanol could help the evaporation of the Pdot inks. Too much glycerol, however, would easily clog the ink cartridge. We found that the Pdot inks could be stored at room temperature in the dark for more than 6 months without observation of any colloidal aggregation and optical alteration. Besides, their D

DOI: 10.1021/acsami.7b08993 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 5. (A) Photographs of a personal bank check with a butterfly maker printed on the upper-left corner. The back and the front panels of each photograph represent images under 405 nm light and room light, respectively. (B) Proof-of-concept proposed scheme of anticounterfeiting protection, in this example of Taiwan currencies. The upper panel represents the photograph under room light, and the bottom panel shows the fluorescent image under 405 nm light excitation.

as a security marker, allowing users or bankers to easily verify checks. We also proposed a scheme of anticounterfeiting protection for Taiwan currencies as shown in Figure 5B. In this proposed scheme, three types of Pdots doped in PVA films (PFBT, PPE, and PFBTDBT from top to bottom) were attached onto the banknote and then irradiated by UV light with a musical-note shaped photomask on top of the films. In these two samples, the color changes of the security markers after UV light illumination provide a method for immediate colorimetric authentication, and the simultaneous fluorescence modulation of Pdots offers a secondary security level. We therefore believe that this Pdot-based reagent will have a widespread adoption in anticounterfeiting devices.

Figure 4. (A) Photographs of a handwritten word “Chemistry” on paper by fountain pens loaded with Pdot inks. The upper and the bottom panels of each photograph represent images under room light and 405 nm light, respectively. (B) Full-color wheel printed by use of RGB Pdots with primary, secondary, and tertiary colors. The upper panels represent the photographs under room light and the bottom panels show the fluorescent images under 405 nm light excitation. (C) Full-color personal photographs printed by a computer-controlled inkjet printer. The upper and the bottom panels of each photograph represent images under room light and 405 nm light, respectively. (D) Photographs of a multicolor 2D barcodes generated by using Pdot inks via inkjet printing. The left and the right panels of each photograph represent images under room light and 405 nm light, respectively.

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CONCLUSIONS In summary, we have successfully developed spiropyran-based photoswitchable Pdots for anticounterfeiting applications. We found that the steric hindrance could effectively enhance the fatigue-resistant property and increase the switching rate of these Pdots. On the basis of our optimal design, full-color patterning on flexible PVA films by using three-color fluorescent Pdots was realized. This indicates that these nanomaterials can be potentially fabricated onto flexible photonic and optoelectronic devices. Moreover, security patterns/signatures could be directly handwritten on paper by Pdot-filled fountain pens, while more complicated or delicate full-color features could be created by an inkjet printer. Due to the unique photoswitchable characteristic of these Pdots, this strategy provides dual colorimetric and fluorescent authentication for both retailers and customers. We believe that this new class of photoswtichable Pdots will find broad use in future anticounterfeiting technologies associated with fraud deterrence and document authentication.

photoswtiching efficiency remains unchanged, suggesting the robust colloidal and optical stability of the Pdot-based security inks. We further printed out more complicated patterns by use of Pdots. As shown in Figure 4C, a 1 × 1.1 in. full-color personal photo (Miss Liao, one of the coauthors) was printed and its photoswitchable ability was demonstrated (the photoswitching behavior was recorded as Video S1). Strong optical contrast of both the absorption and fluorescence from Pdots after UV−vis illumination could be seen. The spatial resolution of the printed image depends on the printer’s setting and model. A 2D barcode was also printed as shown in Figure 4D, and its encoded information could be quickly and accurately retrieved by scanning through a smartphone. The capability of fabrication of barcodes with photoswitchable Pdots ensures their widespread adoption for product manufacturers and retailers to prevent counterfeiting. It is worth mentioning that the handwritten or printed patterns are waterproof and their photoswitching property remains unchanged upon exposure to water (see Video S1), suggesting the excellent stability of these Pdots. Application of Pdot-Based Security Markers for Real Samples. To apply these photoswtichable Pdots in real samples, we proposed two realistic examples to demonstrate their practical applications in anticounterfeiting systems. For the first sample, the full-color pattern was printed onto the upper-left corner of a personal bank check (Figure 5A) to serve

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EXPERIMENTAL SECTION

Materials. The chemicals used in the experiments were purchased from Alfa Aesar, Sigma-Aldrich, TCI, and Acros. All chemicals were used as received unless specified otherwise. All 1HNMR spectra were obtained on a Bruker AV300 spectrometer (300 MHz). PPE, PFBT (Mn ∼ 17 000−23 000), and PSMA (Mn ∼ 1900, 25% maleic anhydride) polymers were purchased from Sigma-Aldrich while PFBTDBT (Mn ∼ 12 000) was synthesized according to our E

DOI: 10.1021/acsami.7b08993 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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tetrahydrofuran), 20−40 μL of PSMA-SP(B) (2 mg/mL in tetrahydrofuran), and 10 mg of sodium dodecyl sulfate were mixed well in 5 mL of THF. The mixtures were injected into 10 mL of H2O under sonication. After that, tetrahydrofuran was evaporated by purging N2 gas on a 100 °C hot plate for 60 min. The Pdot solutions were further filtered by a 0.2 μm poly(ether sulfone) syringe filter to obtain the polymer-SP(B) solutions. The content of spiropyran in Pdots is 5−8% (w/w) based on the preparation protocol. Preparation of Pdot-Py Pdots. In a typical experiment, 200 μL of copolymer PPE or PFBT or PFBTDBT solution (1 mg/mL in tetrahydrofuran), 200 μL of Py-Bips (1 mg/mL in DMSO), and 10 mg of hexadecyltrimethylammonium bromide were were added into 5 mL of tetrahydrofuran. The mixtures were injected into 10 mL of H2O under sonication. After that, tetrahydrofuran was evaporated by purging N2 gas on a 75 °C hot plate for 80 min. The Pdot solutions were further filtered by a 0.2 μm poly(ether sulfone) syringe filter to obtain the Pdot-Py solutions. Ink Preparation and Inkjet Printing by Pdots. The aqueous Pdot solutions were concentrated to ∼1000 ppm by use of an Amicon Ultra-4 centrifugal filter at a speed of 1800 rpm for 7 min. 250 μL of glycerol, 75 μL of ethanol, and 2 μL (0.3 mg/mL) of sodium dodecyl sulfate were mixed with 1 mL of concentrated Pdot solution, and the resulting mixtures were ready for fountain pens as well as ink cartridges (CL-811XL, Canon MP258). Photographs and fluorescent images were obtained with a Nikon D5500 digital camera under room light or 405 nm light excitation.

previously published work.28 Spiropyran derivatives were synthesized according to previously published reports,34,36,37 and thus, their synthetic procedures were not described herein. Other compounds were synthesized with modified procedures, and the appropriate literature was cited for each compound. All water used is deionized (18.2 MΩ·cm). The light power of both UV and visible light used in these experiments is 4 W. Synthesis of (S)-3,4,10,10a-Tetrahydro-10,10,10a-trimethyl2H-[1,3]oxazino[3,2-a]indol, 1.36 1.5 mL (9.34 mmol) of 2,3,3trimethyl-3H-indole was added into 10 mL of acetonitrile in a flask, and then, 0.5 mL (10.4 mmol) of 3-iodo-1-propanol was added dropwise and reacted for 16 h at 90 °C under nitrogen atmosphere. After the reaction, the mixture was cooled down and the solvent was removed by rotary evaporator to leave ∼2 mL of acetonitrile. After that, 4.1 g (7.28 mmol) of NaOH aqueous solution was added into the mixture and then stirred for 2 h. After the reaction, 20 mL of CH2Cl2 was added and stirred for 30 min. The reaction mixture was then washed with water, and the organic layer was separated. After the extraction, CH2Cl2 was dried by magnesium sulfate and then CH2Cl2 was removed by rotary evaporator. The obtained product was further purified on a silica-gel column with hexane/ethyl acetate (4:1) as eluent to yield 0.99 g (49%) of compound 1 as dark red oil. 1H NMR (300 MHz, CDCl3, δ): 7.10 (dd, J = 17.6, 8.0 Hz, 2H), 6.80 (t, J = 7.7 Hz, 1H), 6.59 (d, J = 7.8 Hz, 1H), 4.13−4.02 (m, 1H), 3.75−3.61 (m, 2H), 3.58−3.47 (m, 1H), 1.96 (m, 8.0 Hz, 1H), 1.55 (s, 3H), 1.29 (s, 3H), 1.19 (d, J = 13.2 Hz, 1H), 1.06 (s, 3H). Synthesis of 3-(3′,3′-Dimethyl-6-nitrospiro[chromene-2,2′indolin]-1′-yl)propan-1-ol, Spiropyran.36 To a flask was added 1.33 g (7.96 mmol) of 2-hydroxy-5-nitrobenzaldehyde in 30 mL of ethanol at 70 °C, and 3.0 g (13.8 mmol) of compound 1 was added dropwise. After that, the mixture was heated to 90 °C for 16 h and then cooled down to room temperature and then 4 °C for 1 h. The precipitated solid was filtered, and the obtained residue was dissolved in CH2Cl2 to extract with brine. The CH2Cl2 layer was separated and dried by magnesium sulfate. Then, CH2Cl2 was removed by rotary evaporator. The obtained product was then reprecipitated by CH2Cl2/ methanol (1:10) to afford 2.07 g (71%) of compound spiropyran as a red solid. 1H NMR (300 MHz, CDCl3, δ): 8.04−7.97 (m, 2H), 7.16− 7.19 (m, 1.2 Hz, 1H), 7.09 (dd, J = 7.2, 0.9 Hz, 1H), 6.93−6.85 (m, 2H), 6.75 (d, J = 8.7 Hz, 1H), 6.65 (d, J = 7.8 Hz, 1H), 5.88 (d, J = 10.4 Hz, 1H), 3.71 (t, J = 6.0 Hz, 2H), 3.37 (dt, J = 15.0, 7.5 Hz, 1H), 3.25 (dd, J = 14.7, 7.3, 5.5 Hz, 1H), 2.00−1.88 (m, 1H), 1.85−1.74 (m, 1H), 1.28 (s, 3H), 1.18 (s, 3H). Conjugation of Spiropyran with PSMA, PSMA-SP(B). For the preparation of PSMA-SP, 8 mg of PSMA (Mn ∼ 1900, 75 wt % styrene), 9 mg of 1-ethyl-3-(3-(dimethylamino)propyl)carbodiimide (EDC), 2 mg of 4-dimethylaminopyridine (4-DMAP), and 4 mg of spiropyran were dissolved in CH2Cl2 and then stirred for 6 h. After the reaction, CH2Cl2 was removed and then 4 mL methanol was added to the obtained crude PSMA-SP. The unreacted spiropyran or PSMA was cleaned thoroughly by methanol until the filtrate became clear. The purified solid PSMA-SP was dried in an oven (60 °C) and then stored at 4 °C. For the preparation of PSMA-SP(B), 2 μL of 2-butyl-1octanol was added in the reaction. Synthesis of 1-(3-Hydroxypropyl)-3,3,5′,6′tetramethylspiro[indoline-2,2′-pyrano[3,2-b]pyridin]-5′-ium, Py-Bips.34,37 The detailed synthesis of Py-Bips has been reported previously.2,3 Briefly, 0.1 g (0.29 mmol) of 3-(3,3,6′-trimethylspiro[indoline-2,2′-pyrano[3,2-b]pyridin]-1-yl)propan-1-ol was dissolved in 5 mL of dry THF, and then, 0.2 mL (3.2 mmol) of iodomethane was added dropwise. The reaction mixture was heated to 100 °C for 24 h and then cooled down to room temperature. The yellow product was precipitated and then filtered out, followed by washing with copious THF to afford 35 mg (34%) of compound Py-Bips as a brown-yellow solid. 1H NMR (300 MHz, CDCl3, δ): 7.88−7.79 (m, 3H), 7.16 (dd, J = 10.7, 7.5 Hz, 2H), 6.86 (t, J = 7.4 Hz, 1H), 6.77−6.70 (m, 2H), 4.44 (s, 3H), 3.57 (s, 2H), 3.49−3.28 (m, 2H), 2.89 (s, 3H), 1.89−1.73 (m, 2H), 1.33 (s, 3H), 1.22 (s, 3H). Preparation of Pdot-SP/Pdot-SPB Pdots. Typically, 200 μL of copolymer PPE or PFBT or PFBTDBT solution (1 mg/mL in

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b08993. Experimental section, NMR spectra, and additional experimental data (PDF) Photoswitching performance and waterproof test of printed Pdot paper (AVI)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yang-Hsiang Chan: 0000-0002-9007-3910 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful for the support from the Ministry of Science and Technology (105-2113-M-110-012-MY3) and National Sun Yat-sen University.

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REFERENCES

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DOI: 10.1021/acsami.7b08993 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acsami.7b08993 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX