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Ultrabright Fluorescent Polymer Dots with Thermochromic Characteristics for Full-Color Security Marking Jyun-Chi Yang, Yu-Chieh Ho, and Yang-Hsiang Chan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b10393 • Publication Date (Web): 18 Jul 2019 Downloaded from pubs.acs.org on July 19, 2019

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ACS Applied Materials & Interfaces

Ultrabright Fluorescent Polymer Dots with Thermochromic Characteristics for Full-Color Security Marking Jyun-Chi Yang,a Yu-Chieh Ho,a and Yang-Hsiang Chan*abc a

Department of Applied Chemistry, National Chiao Tung University, Hsinchu, Taiwan 30010 Center for Emergent Functional Matter Science, National Chiao Tung University, Hsinchu 30010, Taiwan

b c

Department of Medicinal and Applied Chemistry, Kaohsiung Medical University, Kaohsiung, Taiwan 80708

ABSTRACT: Innovative and scalable security technologies are highly demanded to deter increasing counterfeiting in modern society. Here we report the first example of thermochromic-fluorescent ink based on semiconducting polymer dots (Pdots) by taking advantage of the unique optical properties of Pdots. We designed and synthesized two types thermochromic molecules and then incorporated them with multicolor fluorescent Pdots. The resulting Pdots exhibited colorimetric and fluorescent dual-readout abilities in response to different temperatures which greatly increase the security level for anticounterfeiting applications. These multifunctional Pdots can be easily doped into flexible substrates or prepared as inks. These full-color inks can be further loaded into marker pens for handwriting or cartridges for inkjet printing with excellent signal-to-background contrast. Moreover, complex and delicate full-color images can be printed on security documents or currency for practical use. We anticipate this first example of thermoresponsive dual-readout methodology based on Pdots to find broad use in advanced security marking technologies. Keywords: semiconducting polymer dots, thermoresponsive polymers, security inks, anticounterfeiting, dual-mode readouts

INTRODUCTION

inkjet printing is still lacking. The aforementioned challenges motivate us to design a stimuli-responsive material which can be widely adopted in daily use without the need of sophisticated instruments or laboratory environment. Here we aim to create a new type of fluorescent ink with thermochromic properties in which the printed patterns can exhibit reversible color and/or fluorescence changes in response to thermal stimulus. For universal and practical applications, ideally the thermal reaction can be triggered by a modest temperature discrepancy such as the human body temperature or a hair dryer.

Fast-growing counterfeiting and adulteration of in currency, banknotes, pharmaceuticals, branded/generic products, and food industry has become an important issue in the economy and in our society. However, traditional anticounterfeiting techniques such as watermarks, barcoding, holograms, security inks, or taggants are relatively easy to duplicate and forge because of the fast growth of modern high-tech computerized devices in these years. Therefore, constant and continuing efforts have been exerted to explore creative and extremely confidential anticounterfeiting technologies to protect consumers and manufacturers.

In this study, we employed highly fluorescent semiconducting polymer dots (Pdots) as the fluorescent ink and endowed Pdots with thermochromic characteristics for full-color printing. Pdots have been demonstrated to be a promising class of fluorescent probe for biomedical, analytical detection, and material science.30-43 As compared to conventional small organic dyes, there are several superior advantages of Pdots to be utilized as fluorescent inks: (i) extraordinary fluorescence brightness, that is 2-5 orders of magnitude higher than that of conventional organic fluorophores or inorganic quantum dots;30-31, 44 (ii) high colloidal stability, biocompatibility, and photostability in pure water solution which is safe and environmentally friendly; (iii) high compatibility with several harmless solvents such as ethanol, glycerol, or (poly)ethylene glycol, which is very suitable to serve as inks; and (iv) multicolor emissions from various kinds of Pdots under single excitation source, which allows for full-color fluorescent patterning. By taking the advantages of Pdots, we then integrated thermochromic fluoran derivatives into Pdots to make them thermoswitchable. The resulting device presents a thermo-stimulating dualresponsive behavior based on fluorescence intensities and

Recently, several innovative anticounterfeiting techniques have been exploited to guarantee higher security reliability.1-9 Among these anticounterfeiting technologies, stimuliresponsive materials provide even enhanced security level because the encrypted information can only be disclosed under the external stimulus.10-14 These stimuli-responsive materials usually refer to the compounds which possess stimuli-chromic characteristics including photochromism,15-17 thermochromism,18-19 mechanochromism,20-21 solvatochromism,22-23 ionochromism,24 acidochromism,25 piezochromism,26 and electrochromism.27-28 In anticounterfeiting and encryption applications, photochromic and solvatochromic materials are particularly of interest due to their fast response upon stimulation and high fatigue resistance.29 For practical use in daily life, however, it is less convenient to carry UV/visible lamps or even organic solvents in order to evaluate the authenticity of goods. As for mechanochromic materials, an elastic substrate or matrix is often required in an effort to recover back to the original arrangement after release. Due to this limitation, the fabrication of mechanochromic materials for high-throughput

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J = 2.5 Hz, 1H), 6.03 (dd, J = 9.2, 2.5 Hz, 1H), 3.36 (q, J = 7.1 Hz, 4H), 1.17 (t, J = 7.1 Hz, 6H).

color shades. We further fabricated the thermo-responsive Pdots as fluorescent inks for security marking and inkjet printing. To the best of our knowledge, this is the first example of thermochromic-fluorescent inks that can be facilely used for creating complicated and delicated images. To further increase the security level and the complexicity of this anticounterfeiting platform, we incorporated a second thermochromic molecule into the device to achieve dual thermoresponsive (double temperature responsive) properties. To demonstrate their practical applications in documents, we also applied them on copy papers, bank checks, and currency. The applications could be further extended to other substrates (e.g., plastics and ceramics) because we have successfully embedded these thermo-responsive Pdots into elastic polymer films. Overall, there exist numerous unique advantages of this thermochromic fluorescent Pdot-based anticounterfeiting system as compared to others: (i) convenient thermoresponsive dual-readout phenomenon which can be easily performed at home without professional training; (ii) multicolor emissions coupled with thermochromic characteristics for creating complicated features with high level of encryption; and (iii) facile preparation to be waterbased inks for high-throughput inkjet printing. This novel type of Pdot-based dual-readout materials could pave a new way for developing next generation of anticounterfeiting platforms.

Synthesis of 3-bromo-9-(diethylamino)-3'Hspiro[benzo[a]xanthene-12,1'-isobenzofuran]-3'-one, 2.47 To a single-neck flask was added 5.0 g (16.0 mmol) of compound 1 and 3.56 g (16.0 mmol) of 6-bromo-2-naphthol in 5 g of 85% concentrated H2SO4. The mixed solution was heated to 120 °C for 4 h and then cooled to ambient temperature. After that, the mixed solution was poured into ice-cold water and then neutralized with 2 M NaOH. The precipitated purple residue was obtained through filtration, rinsed by water, and then dissolved in 50 mL of toluene. 15 mL of 2M NaOH aqueous solution was added into the mixture and heated to 120 °C for 1 h. The organic solvent was then isolated and dried over MgSO4. Toluene was further evaporated through rotary evaporator to obtain the crude compound. The crude product was further purified by reprecipitation in CH2Cl2/hexane to yield 5.99 g (70 %) of compound 2 as a pink solid. 1H NMR (400 MHz, CDCl3): δ = 8.16 – 8.09 (m, 1H), 7.92 (d, J = 2.2 Hz, 1H), 7.84 – 7.77 (m, 1H), 7.65 – 7.52 (m, 2H), 7.44 (d, J = 9.0 Hz, 1H), 7.21 (dd, J = 9.3, 2.2 Hz, 1H), 7.12 – 7.05 (m, 1H), 6.98 – 6.90 (m, 1H), 6.51 (d, J = 9.0 Hz, 1H), 6.44 (d, J = 2.6 Hz, 1H), 6.38 (dd, J = 9.1, 2.6 Hz, 1H), 3.35 (q, J = 7.1 Hz, 4H), 1.17 (t, J = 7.0 Hz, 6H).

EXPERIMENTAL SECTION Synthesis of 9-(diethylamino)-3-(4-hydroxyphenyl)-3'Hspiro[benzo[a]xanthene-12,1'-isobenzofuran]-3'-one, FP.47 Compound FP was synthesized by typical Suzuki coupling. In a two-neck round-bottom flask was added 100 mg (0.2 mmol) of compound 2, 212 mg (2.0 mmol) of Na2CO3, and 30 mg (0.2 mmol) of benzeneboronic acid in 3 mL of THF/ethanol (1:1, v/v). The mixture was degassed and refilled with nitrogen gas for 3 times at -78 °C. 7 mg (0.006 mmol) of Pd(PPh3)4 was quickly added and the reactants were stirred 90 °C for 8 h. After the reaction, the mixture was diluted with CH2Cl2 and then extracted brine for 3 times. After the extraction, the organic layer was separated and dried over magnesium sulfate and then CH2Cl2 was removed by rotary evaporator. The crude product was further puriﬁed on a silicagel column with hexane/ethyl acetate (1:2, v/v) as eluent to yield 20 mg (10 %) of compound FP as a pink solid. 1H NMR (400 MHz, CDCl3): δ = 8.15 (d, J = 7.4 Hz, 1H), 7.96 – 7.89 (m, 2H), 7.64 – 7.54 (m, 2H), 7.45 (dd, J = 14.1, 8.7 Hz, 3H), 7.36 (dd, J = 9.0, 2.1 Hz, 1H), 7.11 (t, J = 8.4 Hz, 2H), 6.92 – 6.86 (m, 2H), 6.54 (d, J = 9.0 Hz, 1H), 6.45 (d, J = 2.6 Hz, 1H), 6.38 (dd, J = 9.0, 2.5 Hz, 1H), 3.36 (q, J = 7.1 Hz, 4H), 1.18 (t, J = 7.0 Hz, 6H).

Chemicals. For all of the experiments, high purity water (18.2 M•cm) was employed. For organic solvents or chemicals such as ethanol, toluene, dichloromethane (CH2Cl2), 1-tetradecanol, 1-tridecanol, urea, formaldehyde, potassium carbonate, PPE polymer (Mn ~ 17 000), sodium dodecylbenzenesulfonate (SDBS), citric acid, tetrahydrofuran (THF), and triethanolamine reagents were purchased from Sigma-Aldrich. Sulfuric acid (H2SO4), tetrakis(triphenylphosphine)palladium(0), glycerin, and Triton X-100 were obtained from Alfa Asear. PFBTDBT (Mn ~ 22 000) and PFCN (Mn ~ 18 000) polymers were synthesized based on our reported works.45 PS-PEG-COOH polymer (P15019-SEOCOOHcomb, Mn = 36.5x103, PDI=1.3) was purchased from Polymer Source, Inc. (Province of Quebec, Canada). Thermochromic fluoran derivatives, FP and FG, were synthesized based on the previously reported works with modified procedures.46-47 Synthesis of 2-(4-(diethylamino)-2hydroxybenzoyl)benzoic acid, 1.47 To a single-neck roundbottom flask was added 411 mg (4.1 mmol) of isobenzofuran1,3-dione and 466 mg (2.8 mmol) of 3-diethylaminophenol in 10 mL of toluene. The reaction mixture was then heated to 120 °C (reflux) for 8h and cooled down to 50-60 °C. 10 mL of 35% sodium hydroxide solution was slowly poured into the mixed solution and then stirred for another 12 h at 90 °C. After the reaction, the mixture was poured into 100 mL of water and titrated with 12 M HCl at 0 °C to pH ~ 7. The solution was kept stirring for 1 h and the precipitated residue was collected. The resulting product was washed by copious methanol for 2-3 times to yield 219 mg (25 %) of compound 1 as a white solid. 1H NMR (400 MHz, CDCl3): δ = 8.08 (d, J = 7.7 Hz, 1H), 7.59 (td, J = 7.5, 1.3 Hz, 1H), 7.55 – 7.48 (m, 1H), 7.34 (dd, J = 7.3, 1.4 Hz, 1H), 6.90 (d, J = 9.2 Hz, 1H), 6.13 (d,

Synthesis of 2'-amino-6'-(diethylamino)-3Hspiro[isobenzofuran-1,9'-xanthen]-3-one, 3.46 To a singleneck round-bottom flask was added 100 mg (1.0 mmol) of compound 1 and 300 mg (1.0 mmol) of 4-aminophenol in 2 g of 85% concentrated H2SO4. The mixed solution was heated to 140 °C for 4 h and then cooled to ambient temperature. After that, the mixed solution was neutralized with 1.6 M NaOH and then CH2Cl2 was added. The organic layer was extracted with brine for 3 times and separated, dried over magnesium sulfate, and evaporated by rotary evaporator. The crude product was purified by flash column chromatography on silica gel and then reprecipitated in CH2Cl2/hexane to yield 340 mg (85 %) of compound 3 as a dark green solid. 1H NMR (400 MHz, CDCl3): δ = 8.07 – 7.93 (m, 1H), 7.69 – 7.55 (m, 2H), 7.22 –

2


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7.15 (m, 1H), 7.08 (dd, J = 8.7, 1.3 Hz, 1H), 6.73 (m, 1H), 6.62 – 6.48 (m, 1H), 6.45 – 6.36 (m, 1H), 6.33 (dd, J = 8.8, 1.9 Hz, 1H), 6.03 (dd, J = 2.7, 1.3 Hz, 1H), 3.35 (q, J = 7.1 Hz, 4H), 1.16 (t, J = 7.0 Hz, 6H).

sonicated vigorously for about 1 min and rapidly injected into 10 mL of deionized H2O in one portion under sonication. THF was subsequently removed by purging N2(g) on a 75 °C hot plate for 30 min. The resulting Pdots were further concentrated 10 times by a centrifugal ultrafiltration tube (Macrosep® Advance Centrifugal Devices, MWCO: 100kDa). The concentrated Pdots possess the zeta potentials of -35 to -42 mV. The above-mentioned micelle solution was then mixed with 1.5 mL of concentrated Pdots, followed by the addition of 0.5 mL of Triton X-100 to form FP@Pdot or FG@Pdot nanoparticles ( = -26 mV). For the preparation of FP/FG@Pdot-doped PVA films, 7.5 mL of FP/FG@Pdot solution was added into 12.5 mL of PVA solution (15-20%) at 50 °C and then mixed well by gentle stirring to remove the air bubbles (nanoparticle : PVA=0.16-0.21 : 1 (w/w)). The mixture was then poured into a glass dish (8.5x8.5 cm) and left to dry naturally at room temperature. To prepare as inks, the FP/FG@Pdot nanoparticle solutions were mixed well with glycerol (Pdots:glycerol = 7:3, v/v) and were ready for fountain pen writing and inkjet printing (cartridge: PG-810XL, Canon MP258). Glycerol was added to gain the viscosity of the inks. The photographs and the fluorescent photos were all taken by a Nikon D5500 DX-format digital-SLR in daylight or UV (365 nm) portable handheld lamp/flashlight.

Synthesis of 6'-(diethylamino)-2'-((3hydroxybenzyl)amino)-3H-spiro[isobenzofuran-1,9'xanthen]-3-one, FG.46 100 mg (0.2 mmol) of compound 3 and 31.8 mg (0.26 mmol) of 3-hydroxybenzaldehyde was added into 5 mL of glacial acetic acid at 0 °C. The mixture was allowed to warm up to room temperature and stirred for 4 h. After that, the reaction was cooled to 0 °C again and then 20 mg (0.5 mmol) of NaBH4 was added by portion where the solution turned green immediately. The mixture was warmed up to room temperature and stirred for 8 h. After the reaction, the mixture was added slowly by 2M NaOH to adjust the final pH ~ 10. CH2Cl2 was added into the mixture to extract with brine for 3 times. The organic layer was separated, dried over magnesium sulfate, and evaporated by rotary evaporator to afford the crude product. The crude product was purified by column chromatography on silica gel using CH2Cl2/ethyl acetate (3:1, v/v) as eluent to yield 66 mg (50 %) of compound FG as a green solid. 1H NMR (400 MHz, CDCl3): δ = 7.99 (d, J = 7.5 Hz, 1H), 7.66 – 7.49 (m, 2H), 7.17 – 7.00 (m, 3H), 6.69 (d, J = 32.9 Hz, 4H), 6.55 (dd, J = 8.8, 6.3 Hz, 1H), 6.36 (d, J = 31.2 Hz, 2H), 6.01 – 5.80 (m, 1H), 4.03 (d, J = 5.9 Hz, 2H), 3.34 (t, J = 7.1 Hz, 4H), 1.16 (t, J = 7.0 Hz, 6H).

Characterization of FP/FG@Pdot Nanoparticles. The average hydrodynamic diameters of bare Pdots and FP/FG@Pdot nanoparticles were determined by dynamic light scattering (DLS) system (Malvern Zetasizer Nano S). TEM images of the bare Pdots and FP/FG@Pdot nanoparticles were acquired by a JEOL 2100 TEM with an acceleration voltage of 200 kV. The UV-visible spectra of Pdots and FP/FG@Pdot nanoparticles were characterized on a Halo DB-20S spectrophotometer. The fluorescence spectra were obtained from a FS5-NIR fluorescence spectrometer (Edinburgh Instruments Ltd.) with excitation at 380 nm, 387 nm, and 450 nm for PPE, PFCN, and PFBTDBT, respectively. The absolute quantum yields of PPE, PFCN, and PFBTDBT Pdots were determined by an integrating sphere unit to be 0.05, 0.17, and 0.31, respectively.

Preparation of Thermochromic Stock Solutions. 4.0 g of 1-tetradecanol or 1-tridecanol in a vial was heated to ~ 70 °C to dissolve 20 mg of FP or FG. The solution was mixed well by stirring or vigorous ultrasonication. The stock solution was allowed cooled down to room temperature and then can be kept stably at ~ 25 °C in darkness for at least 1 year. If the dyes precipitate out, reheating is required to help the dyes dissolve again. Synthesis of Urea-Formaldehyde (UF) Prepolymer. The prepolymer solutions were prepared by dissolving 480 mg of urea in 980 mg of 37% formaldehyde. After complete dissolution, triethanolamine was added to adjust the pH to 8.59 and then the solution was heated to 70-80 °C for 45 min. After the reaction, 2 mL of water was added to obtain the UF prepolymer solution. Freshly prepared UF prepolymer solutions were used for further surface coating.

RESULTS AND DISCUSSION Our aim was to design full-color (RGB) fluorescent polymer dots in which their absorption and emission can be tuned simultaneous and reversibly by the temperatures. Here we synthesized two types of thermochromic dyes with two different absorption and then incorporated them with fluorescent Pdots through miniemulsification. Their thermal switching behaviors were investigated and their switching efficiency was optimized. For full-color emission, we selected three types of Pdots, PPE (blue), PFCN (green), and PFBTDBT (red), because they have high fluorescence brightness and can be excited simultaneously by a single excitation wavelength (365-400 nm).

Preparation of Thermochromic-Fluorescent Pdots as Inks. Two vials containing 250 mg of thermochromic solution and 250 mg of SDBS in 5 mL of water were both heated to 70 °C for 10 min. Through a vigorous sonication, the thermochromic solution was quickly injected into the SDBS aqueous solution for emulsion. The mixture was sonicated by a probe-type ultrasonic homogenizer at 80 W for 5 min and then heated to 70 °C while stirring at 400 rpm for 4 h, followed by the addition of 1 mL of UF prepolymer solution. The resulting micelle solution was acidified by 0.13 M citric acid for 1-2 h to have the final pH value of 5-6 with positively charged surfaces ( = +12 mV). For the preparation of fluorescent Pdot solutions. 200 L of PFBTDBT/PFCN/PPE (1 mg/mL in THF) and 40 L of PSPEG-COOH (1 mg/mL in THF) were added together and mixed well in 5 mL of THF. PS-PEG-COOH was added to form the negatively charged surfaces. The mixed solution was

Scheme 1. Synthetic Routes of Thermochromic Dyes

1

2

FP

3 1 3


FG


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Scheme 2. Schematic Diagram Showing the Preparation of Thermoresponsive Fluorescent Pdotsa

FP or Miniemulsion

FG

Heat

FRET ON

Cool

FRET OFF

PFBTDBT

PFCN Color On/FL Off

PPE

Pen ink

Color Off/FL On

Printer ink

Thermochromic dyes (FP/FG) Urea-formaldehyde pre-polymer SDBS

Triton X-100

a

First, two types of thermochromic dyes (FP and FG) were synthesized and then prepared as nanoparticles by SDBS via miniemulsion. The SDBS-based micelles were then capped with UF polymers and the surface charge was adjusted to be positive by citric acid. Positively charged UF-capped micelles were stirred well with Pdots of negative charges to afford Pdot-capped FP/FG nanoparticles (FP/FG@Pdot). The FP/FG@Pdot nanoparticles were subsequently prepared as rewritable smart inks for fountain pen writing and inkjet printing.

substituted fluorans to obviate the requirement of bisphenol A. As shown in Scheme 1, diethylamino-substituted bromobenzofluoran 2 and aminobenzofluoran 3 were first synthesized through acid-catalyzed condensation with modest yields. One of the final thermochromic dye, FP, was synthesized by employing the typical Suzuki coupling reaction. The low yield of compound FP might be attributed to its high polarity and the possible ring-opening of fluorans during chromatographic purification. The other thermochromic compound FG was obtained from the alkylation of 3 with 3hydroxybenzaldehyde in 50% yield. Both compounds FP and FB possess phenolic moieties on their structures to replace the function of bisphenol A. These two intrinsically thermochromic molecules have two different absorption at a low temperature, resulting in two distinct colors. This offers a flexible manipulation of the color combinations to increase the security levels in anticounterfeiting applications. Furthermore, their color transition temperatures could be tuned by altering the co-solvents of different melting points (vide infra).

Design and Synthesis of Self-Thermochromic Molecules. For the commonly used thermochromic systems, three essential elements should be incorporated, including a color former (e.g., leuco, a fluoran derivative), a developer (e.g., bisphenol A, a Brønsted acid), and a low melting point cosolvent (e.g., methyl stearate or long-chain alcohols). The reversible color generation arises from temperature dependent phase change and proton transfer between the developer and the leuco dye. At an elevated temperature above the melting point of co-solvent, the fluoran stays a ring-closed conformation and the mixture appears to be colorless. At a low temperature, the developer comes in close with the fluoran, leading to the formation of cationic leuco dye. The resulting opening of the lactone ring in the leuco dye causes an intense color appearance. However, the involution of bisphenol A is highly undesirable due to its related issues toward ecosystem health. To this regard, we intended to synthesize phenol-

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line), FP@PPE (solid blue line), FP@PFCN (solid green light), and bare FP (dashed black line) doped PVA films at 50 °C. (E) The left panel exhibits the photographs of FP@PFBTDBT, FP@PPE, and FP@PFCN nanoparticles (left to right sequentially) in PVA films under ambient light (left image) and under 365 nm light (right image) at 25 °C. The right panel displays their corresponding photographs after raising the temperature of FP@Pdot-embedded PVA films to 50 °C.

Preparation of Thermochromic Pdots. The preparation procedures of Pdot-coated FP/FG nanoparticles were illustrated in Scheme 2. We used anionic sodium dodecyl

benzene sulfonate (SDBS) as the emulsifying agent to form thermochromic capsules. The negatively charged FP/FG embedded capsules were then capped with positively charged UF prepolymers. The pH value of the mixed solution was further adjusted to 5-6 to have a surface zeta potential of +12 mV. Here the UF prepolymers built up a capsule shell to stabilize encapsulated thermochromic dyes and at the same time serve as an interlayer for next Pdot adsorption ( = -35 to -42 mV). For the Pdot adsorption, we added Triton X-100, a nonionic surfactant, to aid the adhesion of Pdots on the surfaces of UF-coated thermochromic capsules via hydrogen boding and hydrophobic interaction.48 We employed three types of Pdots, PFBTDBT (red), PFCN (green), and PPE (blue) to have full-color emissions. The resulting nanoparticles performed excellent thermochromic properties and were ready for pen writing or inkjet printing. We denominated the FP-embedded and FG-embedded thermochromic capsules coated by Pdots to be FP@Pdot and FG@Pdot nanoparticles, respectively. The average hydrodynamic sizes of FP/FG@Pdot nanoparticles were determined by dynamic light scattering, ranging from 300-400 nm as shown in Figure S1, which is very suitable for loading in fountain pens and ink cartridge. It should be noted that the particle size of over 500 nm would easily clog or block the print head nozzles.

C

FP@PFBTDBT FP@PPE FP@PFCN bare FP

Emission

B

400 450 500 550 600 650 700

Wavelength (nm)

Wavelength (nm)


D

400 450 500 550 600 650 700

Characterization of Thermoswitchable Nanoparticles. Due to the better thermochromic performance of FP as compared to FG,46-47, 49 we mainly focused on FP-based nanoparticles and used FG as the supplementary reagent to increase the complexity of encryption design (see below). We used 1-tetradecanol as the co-solvent because the melting point of 1-tetradecanol is 38 °C in which we expected that we could observe the color transition by the human body temperature (~37 °C). The absorption spectra of FP@PFBTDBT, FP@PPE, FP@PFCN, and bare FP nanoparticles separately doped in transparent poly(vinyl alcohol) (PVA) films are displayed in Figure 1A. The intense absorption ranging from 500 to 600 nm is the characteristic peak of ring-opened form of compound FG at room temperature (25 °C). For FP@PFBTDBT, the broad absorption at 400-500 nm was attributed to the 2,1,3benzothiadiazole units on PFBTDBT polymers. The UVvisible spectra of bare PFBTDBT, PPE, and PPE Pdots were also shown in Figure S2 for comparison. At the temperature lower than the melting point of co-solvent, 1-tetradecanol, only FP@PFBTDBT nanoparticles represented strong emission over 600 nm (Figure 1B), while the emissions of both FP@PPE and FP@PFCN were well-quenched. This is because of the poor spectral overlap between the emission spectrum of PFBTDBT and the absorption spectrum of FP, leading to inefficient energy transfer from PFBTDBT to FP; while the opposite is true for FP@PPE and [email protected] At the temperature (50 °C in this case) higher than the melting point of 1-tetradecanol, the lactone ring on FP was regenerated with concomitant color fading, resulting in the very weak absorption in the visible regions as shown in Figure 1C. At this time, the apparent fluorescence of FP@PPE and FP@PFCN recovered to their original intensities as shown in Figure 1D. The photographs of FP@Pdot doped PVA films (FP@PFBTDBT, FP@PPE, and FP@PFCN from left to right) are displayed in Figure 1E, demonstrating that they can be used in flexible substrates and can be randomly bended without alteration of their thermoswitchable features. No thermoswitchable phenomenon was observed for bare Pdots (Figure S2). It is worth mentioning that we tried to rationalize the use of 1-tetradecanol but it was unsuccessful. It is because the human skin temperature is only 32-34 °C, which is lower than the melting point of 1-tetradecanol.


400 450 500 550 600 650 700

Emission

Absorption

A

Absorption

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



400 450 500 550 600 650 700

Wavelength (nm)

Wavelength (nm)

E 50 oC

25 oC

Thermoswitching Performance of FP@Pdots. We further investigated their switching efficiency in response to different temperature stimuli. Here we took FP@PFCN doped PVA films as a representative example for this study. As displayed in Figure 2A, the absorption of FP@PFCN decreased gradually accompanied by a slight increase of PFCN fluorescence as the temperature raised from 25 °C to 35 °C. A dramatic change for both absorption and emission could be observed as the temperature increased from 35 °C to 40 °C, which is consistent with the melting point of 1-tetradecanol (38 °C). The opposite phenomenon was seen for the

Figure 1. (A) Absorption spectra of FP@PFBTDBT (solid red line), FP@PPE (solid blue line), FP@PFCN (solid green light), and bare FP (dashed black line) doped PVA films at room temperature (25 °C). (B) Fluorescence spectra of FP@PFBTDBT (solid red line), FP@PPE (solid blue line), FP@PFCN (solid green light), and bare FP (dashed black line) doped PVA films at room temperature. (C) Absorption spectra of FP@PFBTDBT (solid red line), FP@PPE (solid blue line), FP@PFCN (solid green light), and bare FP (dashed black line) doped PVA films at 50 °C. (D) Fluorescence spectra of FP@PFBTDBT (solid red

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Absorption

25℃ 30℃ 35℃ 40℃ 45℃ 50℃ 55℃

B

400 450 500 550 600 650 700

is over 5 and 9, respectively even after their tenth switching cycle. The high on/off contrast is of importance for security marking applications. Their corresponding photographs are also displayed in Figure 2E where the film switched between maroon color at 25 °C (nonfluorescent) and nearly transparent (blue fluorescent) at 50 °C while remaining intact after 10 switching cycles, suggesting the high thermoswitching durability of this system. We then plotted the normalized absorption and fluorescence intensities as a function of temperature. As displayed in Figure 2F, the absorption of FP@PFCN nanoparticles decreased gradually from 25 °C to 35 °C and exhibited a sudden reduction from 35 °C to 40 °C at the first switching cycle, which is in agreement with the melting point of 1-tetradecanol as described above. The emission of FP@PFCN revealed the opposite trend in the meantime. Even at the tenth switching cycle, we found that over 90% of the fluorescence signal was recovered while the absorption decreased to almost the same level. All of the above results demonstrate the exceptional fatigue-resistance and thermoswitching efficiency of the FP@Pdot nanoparticles. We then used these nanoparticles for the following full-color security patterning.

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Figure 2. Absorption (A) and emission spectra (B) of FP@PFCN blended PVA film as a function of temperature. The normalized (C) absorption and (D) emission intensities at different switching cycles between 25 °C and 50 °C. (E) Their corresponding photographs under ambient light (upper panel) and under 365 nm UV light (bottom panel). Absorption (black cycles) and emission (red squares) response of FP@PFCN upon heating at different temperatures for (F) the first switching cycle and for (G) the tenth switching cycle.

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Figure 3. (A) Photographs of the word “CHEMISTRY” handwritten by a fountain pen filled with FP@Pdot inks. The upper photograph was taken at 25 °C while the bottom photograph was taken at 50 °C. (B) Full-color wheel generated by inkjet printing with primary, secondary, and intermediate colors. (C) Personal photographs produced by inkjet printing. (D) Photographs of a 2D barcode with RGB colors printed by using FP@Pdot-based inks. For (A),(B), the upper panels of each photograph exhibit images in daylight and the bottom panels of each photograph exhibit fluorescent images under 365 nm light. For (C),(D), the left panels of each photograph exhibit images in daylight and the right panels of each photograph exhibit fluorescent images under 365 nm light.

corresponding fluorescence (Figure 2B). The absorption and fluorescence remained almost unchanged as the temperature is over 50 °C, indicating that the ring-closing reaction has reached an equilibrium at this time. The same behavior was also found for FP@PPE but not for FP@PFBTDBT (Figure S3) due to the poor energy transfer as described above. We then evaluated the fatigue effect of this platform with alternate temperature of 25 °C and 50 °C for 10 cycles. The results are shown in Figure 2C,D in which the thermoconversion efficiency remained over 90% even at the tenth cycle, indicating the excellent fatigue-resistance of these nanoparticles. The on/off ratio of absorption and fluorescence

Full-Color Security Marking of Thermoresponsive Pdots for Anticounterfeiting Use. We further evaluated the practical use of these thermoswitchable nanoparticles for fullcolor patterning. We loaded three Pdot-based solutions,

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FP@PFBTDBT, FP@PPE, and FP@PFCN into three fountain pens separately and then handwrote a word “CHEMISTRY” a white paper. As exhibited in Figure 3A, the word “CHEMISTRY” is composed of three types of inks: CHEM is made of FP@PPE; IS is made of FP@PFCN; and TRY is made of FP@PFBTDBT, respectively. We could clearly observe that the word appeared to be light purple at room temperature but turned invisible immediately while heating to 50 °C due to the thermal-driven ring-closed conformation of fluoran on FP. For the fluorescence signals, the letters, CHEM and IS, were half-latent at 25 °C because the fluorescence of both PPE and PFCN was efficiently quenched by the ring opened lactone of FP. The quenching mechanism was excluded for PFBTDBT owing to the poor energy transfer (i.e., spectral overlap) between the PFBTDBT and FP. At 50 °C, the fluorescence intensities of PPE and PFCN were recovered, which is in agreement with their spectral results (Figure 2). The above results demonstrate that the FP@Pdot nanoparticles are very suitable for creating full-color patterns by adjusting the relative proportions of three Pdots. To produce more complicated patterns with higher security levels and high throughput ability, we utilized computerized inkjet printing to print out our designed images. We first filled red, green, blue, and black printer ink cartridges with FP@PFBTDBT, FP@PFCN, FP@PPE, and FP@PFBTDBT/FP@PFCN/FP@PPE mixture (v/v: 1:1:1) solutions, respectively in a desktop inkjet printer (Canon MP258). In an effort to assess the full-color capability of these FP@Pdot inks, we printed a full-color wheel with the primary, secondary, and intermediate/tertiary colors as shown in Figure 3B. At 50 °C, the emission of the wheel exhibited the distinct RGB colors with intermediate colors in between, which allows us to have flexible color manipulation and coordination in printing full-color patterns. It is worth mentioning that the sizes of our as-prepared FP@Pdot nanoparticles are less than 500 nm and thus can easily pass through print head nozzles.

through a smartphone (direct access to the website of our group page, see Video S2). We anticipate that this novel feature with highly enhanced security will be very useful in document authentication and product authenticity verification.

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Figure 4. Photographs of a personal bank check with a lab logo (upper-left corner) and routing/account/check numbers (bottom) printed on it. The name was signed by a fountain pen filled with FP@Pdot inks. The left and right panels of each photograph show images in daylight and 365 nm light, respectively.

Practical Applications of FP@Pdot-Based Security Inks. To demonstrate the practical applications of these full-color FP@Pdot inks in anticounterfeiting systems, we applied these inks in a personal bank check. As displayed in Figure 4, we printed our lab mark on the upper-left corner and also printed a set of artificial numbers including routing (78912:FP@PFBTDBT; 3456: FP@PPE), account (12378945: FP@PFCN; 6123: FP@PPE), and check numbers (0025: FP@PFBTDBT) on the bottom. Additionally, we signed the name “JYUN-CHI YANG” above the signature line (JYUN: FP@PPE; -CHI: FP@PFCN; YANG: FP@PFBTDBT) by use of FP@Pdot inks. The excellent contrast of both absorption and fluorescence between two temperatures could be clearly observed, offering customers and bankers an easy way to proceed check authentication. It is worth mentioning that the account number (12 digits) was printed to be invisible while the routing/check numbers were printed to be barely visible intentionally by lowering the concentration of FP@Pdot inks. All of the numbers can only be fluorescently visible at 50 °C, showing the very high security to conceal private information and avoid forgery. We noted that there exhibited blue halation on both sides of the check at 50 °C under excitation, probably due to the automatic aperture setting and light response of the camera CCD but not from the fluorescence of Pdots. We further proposed a strategy to enhance the anticounterfeiting characteristics of Taiwan currencies as illustrated in Figure 5A. In this proposed strategy, we designed and then printed a tiger logo on the upper-left corner of New Taiwan Dollar banknote (sample version not genuine one) by the full-color FP@Pdot inks. It should be noted that the cosolvent we used here is 1-hexadecanol with a melting point at 49.3 °C in order to create a temperature gradient with 1tetradecanol of a melting point at 38 °C (see below). Besides, we intentionally controlled the concentrations of FP@Pdot inks to make the tiger mark partly hidden and partly visible for anticounterfeiting purposes. As revealed in Figure 5A, the color changes of the security logo at the elevated temperatures (>49.3 °C) provide a facile way for prompt colorimetric verification, and at the same time the remarkable fluorescence transformation from Pdots allows for the secondary security authentication to highly enhance the security level of

If the particle size is larger than 500 nm, the pigments will be clogged in the head nozzles. In our system we loaded with concentrated (10x) FP@Pdot solutions along with 30% glycerol in the cartridges, these FP@Pdot nanoparticles still wouldn’t clog the ink cartridges. This suggests that the particle size is one of the important factors to be used as printing inks. We then used these FP@Pdotloaded ink cartridges to print out complicated images. As displayed in Figure 3C, a 0.8 x 0.6 in. full-color personal photo (Mr. Yang, the first author of this work) was printed out and its thermoswitching performance was evaluated, revealing the excellent optical contrast of both absorption and emission between the two temperatures (Video S1). The white color of the fluorescence at 50 °C under UV light is the combination of RGB colors, which corresponds to the dark red color at 25 °C under room light. The results indicate that we are able to have the liberal color collocation for complicated images to raise their security levels. The printed resolution of photos relies mainly on the setting/model of the printer. To further enhance the security features in anticounterfeiting applications, we embedded the hidden information in a 2D barcode as shown in Figure 3D. We carefully optimized the contrast of the images in which its encoded data cannot be decrypted at 25 °C but can be rapidly retrieved from its fluorescence at 50 °C (the fluorescent image at the bottom right corner) by scanning

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Theromoswitching performance in real samples.

currencies. We further increased the complexity for our platform to make the mark much more difficult to be forged by integrating an additional thermochromic dye, FG into the system. As compared to FP, FG displaye two distinct absorption bands at 450 nm and 579 nm, making the color of FG to be pale-green at room temperature (Figure S4). As shown in Figure 5B, we covered an additional FG-doped PVA film with 1-tetradecanol as the co-solvent on top of the printed tiger logo. In this scenario, the different of temperature response is ca. 12 °C between 1-hexadecanol and 1tetradecanol, providing a temperature gradient to selectively trigger the thermal response of only FG but not FP. When the banknote was heated to 40 °C, the FG film became translucent and the hidden tiger picture emerged accordingly. When the banknote was further heated to 60 °C, both the FG film and the logo turned invisible. This proof-of-concept experiment demonstrates the exciting potentials of these thermoswitchable dual-readout inks to have a widespread adoption in the new generations of anticounterfeiting platforms. A

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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENT This paper work is supported by Ministry of Science and Technology, Taiwan (grant No. 105-2113-M-110-012-MY3) and the Center for Emergent Functional Matter Science of National Chiao Tung University from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan.

REFERENCES (1) Yoon, B.; Lee, J.; Park, I. S.; Jeon, S.; Lee, J.; Kim, J.-M. Recent Functional Material Based Approaches to Prevent and Detect Counterfeiting. J. Mater. Chem. C 2013, 1, 2388-2403. (2) Bae, H. J.; Bae, S.; Park, C.; Han, S.; Kim, J.; Kim, L. N.; Kim, K.; Song, S.-H.; Park, W.; Kwon, S. Biomimetic Microfingerprints for Anti-Counterfeiting Strategies. Adv. Mater. 2015, 27, 2083-2089. (3) Hou, X.; Ke, C.; Bruns, C. J.; McGonigal, P. R.; Pettman, R. B.; Stoddart, J. F. Tunable Solid-State Fluorescent Materials for Supramolecular Encryption. Nat. Commun. 2015, 6, 6884. (4) Kumar, P.; Singh, S.; Gupta, B. K. Future Prospects of Luminescent Nanomaterial Based Security Inks: from Synthesis to Anti-Counterfeiting Applications. Nanoscale 2016, 8, 14297-14340. (5) Chen, L.; Lai, C.; Marchewka, R.; Berry, R. M.; Tam, K. C. Use of CdS Quantum Dot-Functionalized Cellulose Nanocrystal Films for Anti-Counterfeiting Applications. Nanoscale 2016, 8, 13288-13296. (6) Wu, H.; Chen, Y.; Liu, Y. Reversibly Photoswitchable Supramolecular Assembly and Its Application as a Photoerasable Fluorescent Ink. Adv. Mater. 2017, 29, 1605271. (7) Abdollahi, A.; Sahandi-Zangabad, K.; Roghani-Mamaqani, H. Rewritable Anticounterfeiting Polymer Inks Based on Functionalized Stimuli-Responsive Latex Particles Containing Spiropyran Photoswitches: Reversible Photopatterning and Security Marking. ACS Appl. Mater. Interfaces 2018, 10, 39279-39292. (8) Gao, Z.; Han, Y.; Wang, F. Cooperative Supramolecular Polymers with Anthracene-Endoperoxide Photo-Switching for Fluorescent Anti-Counterfeiting. Nat. Commun. 2018, 9, 3977. (9) Wu, C.; Zeng, S.; Wang, Z.; Wang, F.; Zhou, H.; Zhang, J.; Ci, Z.; Sun, L. Efficient Mechanoluminescent Elastomers for Dual ‐ Responsive Anticounterfeiting Device and Stretching/Strain Sensor with Multimode Sensibility. Adv. Mater. 2018, 28, 1803168. (10) Tsai, W.-K.; Lai, Y.-S.; Tseng, P.-J.; Liao, C.-H.; Chan, Y.-H. Dual Colorimetric and Fluorescent Authentication Based on Semiconducting Polymer Dots for Anticounterfeiting Applications. ACS Appl. Mater. Interfaces 2017, 9, 30918-30924. (11) Tsang, M.-K.; Bai, G.; Hao, J. Stimuli Responsive Upconversion Luminescence Nanomaterials and Films for Various Applications. Chem. Soc. Rev. 2015, 44, 1585-1607. (12) Stuart, M. A. C.; Huck, W. T. S.; Genzer, J.; Müller, M.; Ober, C.; Stamm, M.; Sukhorukov, G. B.; Szleifer, I.; Tsukruk, V. V.; Urban, M.; Winnik, F.; Zauscher, S.; Luzinov, I.; Minko, S. Emerging Applications of Stimuli-Responsive Polymer Materials. Nat. Mater. 2010, 9, 101-113. (13) Kelley, E. G.; Albert, J. N. L.; Sullivan, M. O.; Thomas H. Epps, I. Stimuli-Responsive Copolymer Solution and Surface Assemblies for Biomedical Applications. Chem. Soc. Rev. 2013, 42, 7057-7071. (14) Thévenot, J.; Oliveira, H.; Sandre, O.; Lecommandoux, S. Magnetic Responsive Polymer Composite Materials. Chem. Soc. Rev. 2013, 42, 7099-7116. (15) Wei, M.; Gao, Y.; Li, X.; Serpe, M. J. Stimuli-Responsive Polymers and Their Applications. Polym. Chem. 2017, 8, 127-143.

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Figure 5. (A) Proof-of-concept proposed scheme for protection of New Taiwan dollar banknote. The left-upper corner of banknote was printed with a tiger logo by FP@Pdot inks. 1-hexadecanol was served as the co-solvent in this case. The upper and bottom panels of each photograph exhibit images in daylight and 365 nm light, respectively. (B) Photographs of banknote with a tiger logo covered with FG-doped PVA film on the top. 1-hexadecanol and 1-tetradecanol were used as the co-solvents for the tiger logo and the FG-doped PVA film, respectively.

CONCLUSIONS In conclusion, we have successfully designed developer-free, fluoran-based thermoswitchable Pdots for full-color security marking. We synthesized two fluoran derivatives with two distinct absorption to combine with RGB fluorescent Pdots. These combinations allow us to create complicated patterns to deter counterfeiting. We also fabricated these nanomaterials into ink form for direct handwriting and computerized inkjet printing. These thermoresponsive inks can be facilely applied on both flexible substrates and commonly used papers. Owing to the peculiar thermoswitchable properties of these Pdots, this methodology offers dual colorimetric and fluorescent verification with much enhanced security to protect original design manufacturers and customers. We anticipate that this novel concept in anticounterfeiting will create a new way for the exploration of next generation of anticounterfeiting technologies.

ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at DOI: Supplementary experimental data and NMR spectra.

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