Screen-Printed Photochromic Textiles through New Inks Based on

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Screen-Printed Photochromic Textiles through New Inks Based on SiO2@naphthopyran Nanoparticles Tânia V. Pinto,† Paula Costa,† Céu M. Sousa,‡ Carlos A. D. Sousa,† Clara Pereira,*,† Carla J. S. M. Silva,§ Manuel Fernando R. Pereira,∥ Paulo J. Coelho,*,‡ and Cristina Freire*,† †

REQUIMTE/LAQV, Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade do Porto, 4169-007 Porto, Portugal ‡ Departamento de Química e CQ-VR, Universidade de Trás-os-Montes e Alto Douro, 5001-801 Vila Real, Portugal § Centro de Nanotecnologia e Materiais Técnicos, Funcionais e Inteligentes (CeNTI), 4760-034 Vila Nova de Famalicão, Portugal ∥ Laboratório de Catálise e Materiais (LCM), Laboratório Associado LSRE-LCM, Departamento de Engenharia Química, Faculdade de Engenharia, Universidade do Porto, 4200-465 Porto, Portugal S Supporting Information *

ABSTRACT: Photochromic silica nanoparticles (SiO2@ NPT), fabricated through the covalent immobilization of silylated naphthopyrans (NPTs) based on 2H-naphtho[1,2b]pyran (S1, S2) and 3H-naphtho[2,1-b]pyran (S3, S4) or through the direct adsorption of the parent naphthopyrans (1, 3) onto silica nanoparticles (SiO2 NPs), were successfully incorporated onto cotton fabrics by a screen-printing process. Two aqueous acrylic- (AC-) and polyurethane- (PU-) based inks were used as dispersing media. All textiles exhibited reversible photochromism under UV and solar irradiation, developing fast responses and intense coloration. The fabrics coated with SiO2@S1 and SiO2@S2 showed rapid color changes and high contrasts (ΔE*ab = 39−52), despite presenting slower bleaching kinetics (2−3 h to fade to the original color), whereas the textiles coated with SiO2@S3 and SiO2@S4 exhibited excellent engagement between coloration and decoloration rates (coloration and fading times of 1 and 2 min, respectively; ΔE*ab = 27−53). The PU-based fabrics showed excellent results during the washing fastness tests, whereas the ACbased textiles evidenced good results only when a protective transfer film was applied over the printed design. KEYWORDS: silica nanoparticles, naphthopyrans, photochromism, screen-printing, smart textiles

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INTRODUCTION The quest for innovative functional and smart textiles is one of the major goals of the textile and clothing industries, driven by the market and end-user requirements for fabrics with advanced features such as omniphobicity, antimicrobial activity, controlled release of oils and flavors, photo- and thermochromism, and fire retardancy, without compromising their comfort, easy care, and hygiene.1,2 In 2014, the world market value for intelligent textiles was $795 million, and it is expected to reach $4.72 billion in 2020, with a compound annual growth rate (CAGR) of 34% from 2015 to 2020.3 In particular, photochromic fabrics are a class of high-tech textiles with tremendous potential due to their ability to protect consumers from the harmful effects of UV exposure and their reversible UV-sensing properties; additionally, they impart trendy color changes to home decorations and fashion garments.1,4−7 Naphthopyrans (NPTs), a class of organic photochromic dyes, are promising candidates for the production of photoresponsive materials,8 because they are easy to prepare, show efficient coloration/ decoloration kinetics and good fatigue resistance, and are very © XXXX American Chemical Society

versatile molecules, allowing an extensive selection of colors to be obtained.8−13 Photochromic fibers and/or fabrics made from different substrates (e.g., cotton,14 polyester,15−17 nylon,15,16 acrylic,15,16 wool,18,19 and polyamide20,21) have been produced by different dyeing procedures through the incorporation of photochromic organic molecules, mostly NPT- and spirooxazine-based compounds. The dyeing of fabrics with photochromic dyes by conventional processes encounters several problems associated with the dyeing procedure (e.g., dye degradation) and with the limited interaction between dye and substrate, such as (i) low dye uptake and reduced dye diffusion into the fibers, (ii) slow coloration/decoloration kinetics, (iii) total inhibition of photochromism, (iv) constraints imposed by the hardness of the matrix, and (v) low washing and light fastness characteristics.7,17,21,22 Some of these drawbacks can be overcome by, for instance, processing dyes into pigments using Received: June 4, 2016 Accepted: October 5, 2016

A

DOI: 10.1021/acsami.6b06686 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Scheme 1. Chemical Structures of (a) Silylated NPT Derivatives S1−S4 and (b) Parent NPTs 1 and 3 Used in the Preparation of Photochromic SiO2 NPs Nanomaterials and Schematic Representation of the Corresponding Immobilization Strategya

a

Adapted with permission from ref 8. Copyright 2016 American Chemical Society.

microencapsulation processes;23 although this methodology tends to increase the stability of the photochromic compounds, it usually confers a certain harshness and stiffness on the fabric, compromising the comfort of the user.18,20 Alternatively, photoswitchable textiles have been produced by screen-printing using inks containing photochromic dyes,24−28 which prevents the problems related to dyeing and eventual incompatibility between the colorant and the substrate. However, the stability of the screen-printed textiles toward radiation still needs to be improved to satisfy consumer expectations, as the visual effects rapidly fade out with prolonged exposure to light, continuous washing, and abrasion. The immobilization of photochromic dyes onto inorganic matrixes8,18,19,29,30 before their incorporation on textiles is a potential strategy for the fabrication of photochromic fabrics with enhanced color-exchange properties, dye stability, and comfort. Silica nanoparticles (SiO2 NPs) have been incorporated into textiles to provide novel functionalities to the substrate [such as (super)hydrophobicity] and improved mechanical properties (mechanical strength, wear and abrasion resistance) while maintaining the original textile properties such as appearance, handle, and touch.1 In this work, we present a novel procedure for the generation of photochromic textiles with improved performance, photostability, and washing fastness. In a previous work,8 photochromic SiO2 NPs were prepared by the direct immobilization of NPTs based on 2H-naphtho[1,2-b]pyran (2H-NPT) and 3H-naphtho[2,1-b]pyran (3H-NPT) and through covalent postgrafting of their silylated counterparts onto 15-nm nanosilicas. In this work, knitted cotton fabrics were coated with the SiO2 NPs presenting the best photochromic performance (Scheme 1) by screen-printing. Through the nanoscale modification of the fabrics, new nanocoated textiles combining efficient color switching, higher comfort, and better photostability were produced, making them capable of being applied in a broad range of smart and functional applications,

including sensing and protection, camouflage, anticounterfeiting, and fashion. To the best of our knowledge, the fabrication of light-responsive textiles by screen-printing with photochromic SiO2 NPs has not been reported. Consequently, the fabrication of cost-effective high-tech textiles with tunable photoswitchable properties, good color contrast, high coloration/decoloration kinetic rates, durable effects, and improved washing fastness through their coating with photochromic SiO2 NPs is an innovative approach, opening new horizons to the development of more effective and stable smart materials.

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EXPERIMENTAL SECTION

Materials, Reagents, and Solvents. Knitted garments (100% cotton) were acquired from a local supplier, and no pretreatment was performed prior to the screen-printing process. The nanocoated textiles were prepared using two aqueous screen-printing pastes that were distinguished according to the nature of the binder additive: AC for the acrylic-based binder and PU for the polyurethane-based one. The additives used in the formulation of the AC printing paste (Acraconz BN, Acrafix ML 200%, Acramin MPG 01, Acramin RG, Emulsifier VA 02, and Emulsifier WN) were provided by Bayer AG (Leverkusen, Germany). The PU printing paste was a “ready-to-use” paste commercialized as Hydra Clear for Metallic by Quaglia Virus (Azzano San Paolo, Italy). The polyurethane transfer film, a thermoplastic and thermoadhesive transparent film (FAITGOM TM type), was supplied by Fait Plast S.p.A. (Badia, Italy). Fabrication of Photochromic SiO2 NPs. Details of the preparation and characterization of photochromic SiO2 NPs were provided in our previous work.8 Typically, SiO2@NPTs were prepared by postgrafting immobilization of silylated NPTs onto SiO2 NPs through a two-step procedure based on (1) the microwave-assisted silylation of 2H-naphtho[1,2-b]pyran (2H-NPT) (for S1 and S2) and 3H-naphtho[2,1-b]pyran (3H-NPT) (for S3 and S4) NPTs, followed by (2) covalent immobilization into SiO2 NPs; these photochromic nanomaterials are denoted as SiO2@NPT, where NPT = S1, S2, S3, and S4. SiO2@NPTs were also fabricated by the adsorption of the parent NPTs 1 and 3 onto SiO2 NPs; these nanomaterials are denoted using a similar notation, namely, SiO2@NPT, where NPT = 1, 3. B


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Scheme 2. Schematic Representation of the Production of the Nanocoated Photochromic Cotton Fabrics by Screen-Printing

Figure 1. SEM micrographs of (A) Ct, (B) Ct-AC-SiO2@S3, (C) Ct-AC-SiO2@S3_w, (D) Ct-PU-SiO2@S3, (E) Ct-PU-SiO2@S3_w, and (F) CtPU-SiO2@S3_T. Insets in panels A−F: EDS spectra of the orange square regions. Top inset in panel F: SEM micrograph of Ct_PU. Fabrication of Photochromic Cotton Textiles by ScreenPrinting. The production of photochromic nanocoated cotton fabrics (pristine cotton denoted as Ct) by a screen-printing process involved three steps: (1) Preparation of the photochromic aqueous screen-printing inks: AC-SiO2@NPT printing paste (where NPT = S1, S2, S3, S4, 1, and 3) was prepared with 10 wt % of SiO2@NPT dispersed in water along with the additives Acraconz BN (3 wt %; thickener), Acrafix ML 200% (0.5 wt %; cross-linking agent), Acramin MPG 01 (1.4 wt %; softener), Acramin RG (12.5 wt %; binder), Emulsifier VA 02 (0.4 wt %; dispersing agent), and Emulsifier WN (0.2 wt %; dispersing agent). All components were mixed until a homogeneous paste was obtained. PU-SiO2@NPT printing paste was obtained by the direct incorporation and homogeneous dispersion of 10 wt % of SiO2@NPT (NPT = S1 and S3) into a commercial Hydra Clear for Metallic ink. (2) Coating of the cotton textile by screen-printing: Nanocoated cotton fabrics were produced on a magnetic printing table (Johannes Zimmer), using a printing screen and a magnetic roller (pressure grade 1, 4 m min−1, six passages). (3) Drying and f ixation: The final fabrics, denoted as Ct-AC-SiO2@NPT and Ct-PU-SiO2@NPT, were dried at 150 °C for 5 min (Mathis Laboratory). The polyurethane transfer film (denoted as T) was applied to the printed design on a press table at 75 °C for 30 s. The fabrics with transfer are denoted with _T; additionally, textiles printed with inks not containing photochromic hybrid nanomaterials were also prepared as references for physicochemical characterization. The cotton fabrics printed with only AC and PU pastes and/or transfer are denoted as Ct-AC, Ct-PU,

Ct_T, Ct-AC_T, and Ct-PU_T. A schematic representation of the fabrication of the photochromic cotton textiles is represented in Scheme 2. Washing Fastness Tests. Washing fastness tests were carried out according to the ISO 105 C06 standard (Textiles − Tests for Color Fastness − Part C06: Color Fastness to Domestic and Commercial Laundering), using SDC ECE Phosphate Reference Detergent acquired from SDC Enterprises Limited. The nanocoated textiles were washed five consecutive times in a domestic washing machine at 40 °C for 90 min (program used for colored cotton) and then airdried. The washed textiles are denoted as _w. Physicochemical Characterization. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were performed on an FEI Quanta 400 FEG environmental scanning electron microscope (ESEM) coupled with an EDAX Genesis X4M energy-dispersive X-ray spectrometer, at Centro de Materiais da Universidade do Porto (Porto, Portugal). All cotton samples were coated with a thin gold−palladium film. Thermogravimetric analyses of the photochromic SiO2 NPs and Ct samples were performed in the temperature range of 20−700 °C at a heating rate of 5 °C min−1 under air (20 cm3 min−1) on a PerkinElmer Pyris 1 thermobalance. Fourier transform infrared-attenuated total reflectance (FTIR-ATR; Perkin-Elmer Spectrum 100 spectrophotometer equipped with an ATR accessory) spectroscopy was performed between 4000 and 650 cm−1, with 16 scans and a resolution of 4 cm−1. C


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ACS Applied Materials & Interfaces Table 1. Thermogravimetric Analysis Results of the Knitted Cotton-Based Fabrics under Air weight loss (%)

a

cotton-based fabric

first stagea (20−120 °C)

second stagea (120−600 °C)

third stagea (600−700 °C)

.

Ct Ct-AC Ct-AC-SiO2@S1 Ct-AC-SiO2@S2 Ct-AC-SiO2@S3 Ct-AC-SiO2@S4 Ct-AC_T Ct-AC-SiO2@S1_T Ct-AC-SiO2@S2_T Ct-AC-SiO2@S3_T Ct-AC-SiO2@S4_T Ct-PU Ct-PU-SiO2@S1 Ct-PU-SiO2@S3 Ct-PU_T Ct-PU-SiO2@S1_T Ct-PU-SiO2@S3_T

3.6 3.2 3.5 3.3 3.2 4.1 3.3 3.0 2.5 2.3 3.5 3.3 3.4 2.8 3.4 2.8 2.9

77.5 79.4 71.7 68.8 73.6 73.2 77.4 71.6 71.4 71.5 73.1 78.4 73.2 75.1 75.5 73.4 73.1

17.6 16.2 16.6 17.3 14.0 13.9 17.7 17.5 16.1 17.4 16.3 17.5 15.0 14.3 20.1 17.4 18.2

98.7 98.8 91.8 89.4 90.8 91.3 98.4 92.0 90.0 91.2 92.8 99.2 91.6 92.3 99.0 93.7 94.2

Partial weight loss (temperature ranges indicated within parentheses).

The photochromic properties of the nanocoated cotton samples were evaluated by diffuse reflectance UV−vis spectroscopy (CARY 50 Varian spectrophotometer with a diffuse reflectance accessory) using a UV lamp of λ = 365 nm at a power of 6 W. Before and after UV light exposure, the color of the nanocoated cotton fabrics was measured by colorimetry on a Konica Minolta CR400 Chroma meter. More details concerning the characterization of all fabrics are provided in the Supporting Information.

To overcome this drawback, two alternative strategies were followed. The first one was the use of a “ready-to-use” commercial printing paste composed of a PU-based binder, in which two selected hybrid nanomaterials (SiO2@S1 and SiO2@ S3) were individually dispersed. These nanomaterials were chosen to represent the two classes of NPTs: S1 for 2H-NPT and S3 for 3H-NPT. The SEM micrograph of Ct-PU-SiO2@S3 (as an example), presented in Figure 1D, shows cotton fibers uniformly embedded on a thin-film coating containing carbon and oxygen, provided by the components of the commercial printing paste. The presence of SiO2 NPs was confirmed through the brighter clusters composed of silicon, carbon, and oxygen. Generally, in the washed coated fabrics, shown in Figure 1E for Ct-PU-SiO2@S3_w, the uniformity of the film coating was preserved, without significant loss of SiO2 NPs, which indicates that the PU-based screen-printing paste presents a promising efficiency in washing fastness tests, not only preserving the polymeric coating itself but also preventing the leaching of SiO2 NPs. The second strategy consisted of the use of a protective transparent polyurethane transfer (T) on all nanocoated fabrics, which was applied over the screen-printed design. The SEM micrographs of the resulting fabrics, presented in Figure 1F for Ct-PU-SiO2@S3_T as an example, revealed that the cotton fibers were covered by a smooth and thin surface coating provided by the transfer. The texture of the coated knitted fabric, although smoother, seemed to be preserved, as can be seen by comparison with the SEM image of the Ct-PU fabric without the transfer (inset of Figure 1F), which shows the interlaced arrangement pattern of the cotton fibers. The different brightness/darkness contrast of the SEM image reveals the existence of two different surface topographies. The magnified SEM image presented in Figure S1A of the Supporting Information for Ct-PU-SiO2@S3_T shows the surface topographies with higher magnification; they consist of planar regions, where the transfer is positioned directly above the cotton fibers, and “valleys” for the depression zones within the interstices between fibers where the transfer is not completely in contact with the fibers. Moreover, small brighter

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RESULTS AND DISCUSSION Morphological and Chemical Characterization. The photochromic nanomaterials SiO2@NPT (NPT = S1, S2, S3, S4, 1, and 3) were incorporated on cotton fabrics by a screenprinting process. The SiO2@NPTs were produced through covalent immobilization (for S1, S2, S3, and S4) or direct adsorption (for 1 and 3) of the NPTs onto the SiO2 NPs. Cotton fabrics screen-printed with both AC- and PU-based inks showed softness to the touch and good handling. After the application of the transfer, the final fabrics had a plastic finish and became slightly rougher; however, none of the coatings compromised the handling and comfort of the resulting smart cotton fabrics. The morphological and chemical characterization of all cotton fabrics was performed by SEM/EDS. The SEM micrograph of the pristine Ct, presented in Figure 1A, shows smooth longitudinal fibril structures composed of only carbon and oxygen, as detected by EDS. The SEM micrographs of the AC-based nanocoated cotton textiles, presented in Figure 1B for Ct-AC-SiO2@S3 as an example, display cotton fibers coated with dense clusters. Although it is not possible to distinguish individual SiO2 NPs because of their small particle size (about 15 nm),8 the observation of dense clusters composed of silicon, oxygen, carbon, and small amounts of nitrogen confirmed the presence of the silica-based nanomaterials. After five consecutive washing cycles, the AC-nanocoated fabrics, shown in Figure 1C for Ct-AC-SiO2@S3_w, presented a substantially lower amount of SiO2 NPs, indicating significant nanomaterial leaching during the laundering process and the lack of adhesion of the NPs to the cotton surface. D


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Figure 2. FTIR-ATR spectra of cotton fabrics coated with SiO2@S3-based printing pastes in the 4000−650 cm−1 region: (A) Ct-AC-SiO2@S3 and Ct-AC-SiO2@S3_T and (B) Ct-PU-SiO2@S3 and Ct-PU-SiO2@S3_T cotton fabrics. Blue squares: characteristic vibrational bands of the screenprinting (A) AC paste with acrylic binder or (B) PU paste with polyurethane binder. Red circles: characteristic vibrational bands of the PU transfer.

as a result of the leaching of inorganic nanomaterials, as observed by SEM/EDS. All fabrics were characterized by FTIR-ATR spectroscopy to confirm the incorporation of both AC and PU printing pastes, as well as the PU transfer application onto the cotton fabric surface. The FTIR-ATR spectrum of the pristine cotton (Figure 2) displays the characteristic bands of the cellulose structure: O−H stretching (∼3315 cm−1) and bending (1640 cm−1) vibrations of physisorbed water, C−H stretching (∼2890 cm−1), C−H wagging (1428 and 1315 cm−1), C−H bending (1364 cm−1), O−H in-plane bending (1335, 1238, and 1204 cm−1), C−H deformation stretching (1280 cm−1), C−O−C asymmetric bridge (1161 and 1107 cm−1), asymmetric in-plane ring stretching (1053 cm−1), C−O stretching (1030 cm−1), and asymmetric out-of-phase ring stretching (897 cm−1) vibrations.35−37 The FTIR-ATR spectra of all Ct-AC-SiO2@NPT fabrics, presented in Figure 2A and Figure S4A (Supporting Information) for Ct-AC-SiO2@S3 and the remaining materials, respectively, show not only the characteristic bands from the cellulose structure but also the bands from the main components of the acrylic binder, which was the major additive (about 13 wt %) present in the AC printing paste (see more details in the Supporting Information).36,38−41 The presence of SiO2 NPs is evidenced by the bands in the 1250−800 cm−1 region, whose broadening might be due to the contribution of the characteristic vibrational modes from SiO2 structure such as Si−O−Si asymmetric stretching (1115 cm−1), Si−O stretching from Si−OH and Si−O− moieties (960 cm−1), and Si−O−Si symmetric stretching vibrations (804 cm−1).8,33,36,37 After the AC-nanocoated fabrics had been washed [Figure 2A for Ct-AC-SiO2@S3_w and Figure S4B (Supporting Information) for the other fabrics], the intensity of the bands from the printing paste decreased significantly, as a result of the leaching of the AC printing paste from the cotton surface, reinforcing the previous observations. In the case of the Ct-PU-SiO2@NPT-nanocoated fabrics [Figure 2B for Ct-PU-SiO2@S3 and Figure S5 (Supporting Information) for the other fabrics], the FTIR-ATR spectra exhibit the vibrational modes of the cellulose and commercial PU printing paste (a detailed assignment is provided in the Supporting Information).36,42−44 As previously mentioned, the presence of SiO2@NPTs can be detected by the broadening of the bands in the 1250−800 cm−1 region, owing to the contribution from the vibrational modes associated with the nanosilica framework. After the fabrics had been washed, no

regions can be observed containing reduced amounts of silicon and nitrogen, which confirm the presence of SiO2 NPs below the transfer. After washing, no differences were observed for the fabrics to which the transfer was applied, as shown in Figure S1B (Supporting Information) for Ct-PU-SiO2@S3_T_w, indicating that the transfer acts as a protective barrier, preventing the leaching of the printing pastes and the SiO2based hybrid nanomaterials for at least five washing cycles. The thermogravimetric results are presented in Table 1 and Table S1 (Supporting Information); the thermograms are presented in Figures S2 and S3 of the Supporting Information. In general, all cotton fabrics showed total weight losses between 89.4% and 99.2%, with the nanocoated-based textiles exhibiting the lowest values. The fabrics printed with pastes without photochromic hybrid nanomaterials presented higher total weight losses because of their higher amounts of organic content from the pristine cotton, both printing pastes, and the PU transfer. Moreover, all thermograms appeared to have similar profiles, in which it is possible to identify three regions of mass loss. In region I (20−120 °C), the initial weight loss (2.3−4.9%) is associated with the evaporation of water and volatile compounds present in the cotton fibers.31,32 The second and major weight loss (67.3−82.5%) occurs in region II (220−420 °C) and can be mainly attributed to the decomposition of noncellulosic materials and cellulosic macromolecules,31,32 decomposition of hydrocarbons present in the AC- and PU-based printing pastes, and polyurethane transfer and decomposition of the NPT moieties.8 Finally, region III (420−700 °C), with a weight loss of 10.5−20.5%, corresponds to the decomposition of macromolecules and the dihydroxylation of the SiO2 network.32−34 Generally, the cotton fabrics screen-printed with inks containing SiO2@NPTs presented lower total weight losses (89−94%), because of the presence of inorganic content, when compared with the pristine cotton and with the fabrics coated with AC- and PU-based inks not containing SiO2@NPTs (samples Ct, Ct-AC, Ct-PU, Ct-AC_T, and Ct-PU_T). In these latter samples, the total weight loss values were ∼99%, indicating that these textiles were mostly composed of hydrocarbons and that their decomposition was almost complete by 700 °C. The washed fabrics coated with PU printing paste and transfer showed only slight variations in the total weight loss values (Table S1, Supporting Information), indicating that they offer protection against NP leaching. In contrast, for the CtAC-SiO2@NPT fabrics, the weight loss increased significantly E


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Table 2. Photographs of the Nanocoated Cotton Fabrics before and after UV (λ = 365 nm) and Sunlight Irradiation for 1 min at Room Temperature

a

Before (uncolored region) and after (colored region) UV irradiation for 1 min at room temperature. bAfter sunlight exposure for 1 min at room temperature.

type of functionalization of SiO2 NPs with NPTs, either by postgrafting or by direct adsorption, did not lead to significant differences regarding the amount of hybrid nanomaterials incorporated onto the cotton fabric surface or on the amount lost during the washing fastness tests. Evaluation of Photochromic Properties. All nanocoated fabrics exhibited direct and reversible photochromic properties under UV or solar irradiation, developing fast and intense colorations, as can be seen in the photographs presented in Table 2 and Table S2 (Supporting Information). After irradiation of the fabrics with UV light (λ = 365 nm) for 1 min at room temperature, the light source was turned off, and the fading absorbance measured at the wavelength of maximum absorption (λmax) was monitored as a function of time. The kinetic parameters of the thermal bleaching of all nanocoated fabrics in the dark (constant rates, amplitudes, and half-life times) were calculated through the fitting of the bleaching curves to a biexponential decay equation

significant changes (regarding the position and the intensity of the bands) were observed in the FTIR-ATR spectra of the CtPU-SiO2@NPT fabrics (Figure 2B and Figure S5, Supporting Information), which resulted from the preservation of the PU printing paste on the cotton surface, overcoming the leaching problem of the AC paste. In the spectra of the Ct-AC-SiO2@ S3_T- and PU-SiO2@S3_T-coated fabrics (Figure 2 and Figures S5 and S6, Supporting Information), most of the bands in the 3000−2800 and 1700−700 cm−1 ranges were overlapped by the characteristic vibrational modes of the PU transfer, which has vibrational modes similar to those of cellulose and acrylic (or polyurethane) binder, but with higher intensity.33,35−44 Also, in the two spectra, it is not possible to identify the presence of SiO2 bands. The FTIR-ATR spectra of the washed T-based fabrics do not exhibit significant changes, because of the preservation of the PU film and the consequent preservation of both printing pastes, preventing the loss of the SiO2 NPs, corroborating the SEM/EDS and thermogravimetry results. In summary, the leaching of the AC printing paste and consequent loss of SiO2 NPs was successfully overcome by the use of a commercial PU-based screen-printing ink and/or by the application of a transfer film over the printed areas. Both strategies, used together or separately, showed promising results in the washing fastness tests, without compromising the touch or the handling of the final textiles. Moreover, the

A(t ) = A 0 + A1e−k1t + A 2 e−k 2t

where k1 and k2 are the bleaching constants of the transoid-cis and transoid-trans photoisomers, A1 and A2 are the initial absorbances due to the two species, and A0 is the initial absorbance of the fabric before UV irradiation (see Supporting F


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Table 3. Wavelengths of Maximum Absorption (λmax), Thermal Fading Kinetic Parameters (Constant Rates, k1 and k2; Respective Amplitudes, A1 and A2; and Half-Life Times, t1/2(1) and t1/2(2)), and Optical Densities (ΔOD) of the SiO2@NPT Nanomaterials and Nanocoated Cotton Fabrics materiala d

SiO2@S1 Ct-AC-SiO2@S1 Ct-AC-SiO2@S1_T Ct-PU-SiO2@S1 Ct-PU-SiO2@S1_T SiO2@S2d Ct-AC-SiO2@S2 Ct-AC-SiO2@S2_T SiO2@S3d Ct-AC-SiO2@S3 Ct-AC-SiO2@S3_T Ct-PU-SiO2@S3 Ct-PU-SiO2@S3_T SiO2@S4d Ct-AC-SiO2@S4 Ct-AC-SiO2@S4_T

λmax (nm) 480 470 475 475 480 530 530 532 450 440 435 435 435 490 530 500

k1b (min−1) 6.0 4.0 5.5 4.3 4.2 6.1 8.9 1.5 2.9 1.1 1.4 2.4 1.5 5.4 2.2 9.2

× × × × × × × × ×

−2

10 10−2 10−2 10−2 10−2 10−2 10−2 10−1 10−1

× 10−1 × 10−1

t1/2(1)c (min)

A1 (%) 77 32 40 44 12 54 50 73 33 57 63 74 63 54 59 48

11 17 13 16 16 11 8 5 2 1 1 0 1 1 0 1

k2b (min−1) 4.9 1.2 1.9 1.0 1.2 6.6 8.9 5.5 3.7 1.7 1.8 1.1 1.3 1.1 4.4 7.8

× × × × × × × × × × × × × × × ×

−3

10 10−3 10−3 10−3 10−3 10−3 10−2 10−3 10−3 10−2 10−2 10−1 10−2 10−2 10−2 10−3

A2 (%)

t1/2(2)c (min)

ΔOD

23 68 60 56 88 46 50 27 67 43 37 26 37 46 41 52

141 598 359 673 592 105 78 125 187 40 38 6 52 63 16 89

0.82 0.82 0.64 0.73 0.68 1.08 0.59 0.61 1.00 0.60 0.44 0.55 0.52 0.57 0.45 0.30

Material: SiO2@NPT nanomaterials or nanocoated cotton fabrics. bThermal bleaching rates and respective amplitudes determined from the fitting of the experimental fading curves to the biexponential decay function A = A0 + A1e−k1t + A2e−k2t (see Supporting Information). ct1/2(1) = ln(2)/k1 and t1/2(2) = ln(2)/k2. dValues from the literature.8 a

Information);13,45 the values are summarized in Table 3 and Table S3 (Supporting Information). The exposure of the NPTs to UV or sunlight induced the opening of the electrocyclic pyran ring, giving rise to two colored photoisomers, namely, transoid-cis (denoted as TC) and transoid-trans (denoted as TT), that exhibit extended conjugated systems, responsible for the large absorption in the visible region.10,13 In particular, the C(sp3)−O bond cleavage of the closed form is responsible for the initial formation of the thermally unstable and short-lived photoisomer TC (related to k1), which quickly returns (typically in a few seconds to minutes) to the uncolored closed form of the NPT. The photoisomer TC can also be partially converted, through lightinduced CC isomerization, into the more thermally stable and long-lived photoisomer TT (related with k2) that slowly returns (in several minutes to hours) to the TC structure and then to the original closed form (Figure S7, Supporting Information).10,13,45−47 Moreover, the significant difference in the kinetic constants k1 and k2 and the corresponding amplitudes not only shows which isomer is photogenerated in higher quantity but also indicates whether the total decoloration of the NPTs is fast or slow. Generally, all fabrics coated with SiO2@S1 and SiO2@S2 [i.e., SiO2 NPs functionalized with silylated 2H-naphtho[1,2b]pyran (2H-NPT) derivatives S1 and S2, respectively] exhibited an initial off-white color, that switched to an intense color upon UV (or sunlight) exposure: orange-red for SiO2@ S1 and purple for SiO2@S2 (Table 2). Simultaneously, in the UV−vis spectra of the fabrics nanocoated with SiO2@S1 and SiO2@S2 (presented in Figure S8 of the Supporting Information for Ct-AC-SiO2@S1_T and Ct-AC-SiO2@S2_T), an increase of the broad absorption band at λmax of ∼475 and ∼530 nm, respectively, can be observed. Upon removal of the light source, the intensity of the broad electronic band slowly decreases with time, with the textiles taking 2−3 h to revert to their original color (additional information is provided in the Supporting Information). Generally, these values are very similar to those of their

respective hybrid nanomaterials (e.g., fast coloration, good color contrast, and slow bleaching kinetics),8 meaning that the cotton/printing-paste matrix does not compromise the photochromic behavior of the NPTs. Moreover, these fabrics are useful for applications that require a prolonged color effect upon removal of the light source, such as for food packaging.7 The fabrics coated with SiO2 NPs functionalized with the silylated 3H-naphtho[2,1-b]pyran (3H-NPT) derivatives, S3 and S4, showed excellent photoswitchable properties, exhibiting fast coloration and decoloration/bleaching kinetics and good optical densities (ΔOD). The SiO2@S3-based coated fabrics also exhibited an off-white coloration that switched to an intense yellow-orange color after direct UV (or sunlight) exposure for 1 min (Table 2). This effect was confirmed by UV−vis spectroscopy, through the appearance of a broad and strong absorption band in the visible region (as shown in Figure 3 for Ct-PU-SiO2@S3), with λmax ≈ 435 nm and ΔOD in the range of 0.52−0.60, whose absorption intensity quickly faded with time upon removal of the light source. Comparing

Figure 3. UV−vis spectra of the decoloration evolution of the Ct-PUSiO2@S3-coated cotton fabric after being irradiated with UV light (λ = 365 nm) for 1 min. Inset: Fitting of the bleaching curve of Ct-PUSiO2@S3 in the dark at λmax = 435 nm to a biexponential function (black, experimental data; red, biexponential fitting). G


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with or without transfer application. Additionally, neither screen-printing paste compromised the photochromic behavior of the NPTs. The color-change properties and technical performances of the photochromic fabrics (photostability and fatigue resistance) were also evaluated using the CIELab color space. The values of ΔL*, Δa*, Δb*, and ΔE*ab are reported in Table 4 and Table S4 (Supporting Information), where L* corresponds to the lightness, a* denotes the green−red characteristics, and b* denotes the blue−yellow features.

these results with those obtained for the parent SiO2@S3, one can observe a blue shift in the λmax value and also an increase of the thermal bleaching rates, by almost 1 order of magnitude (k1 > 1.1 min−1 and k2 > 1.3 × 10−2 min−1), indicating that all fabrics presented constant rates higher than those of the parent SiO2@S3 nanomaterial (k1 = 2.9 × 10−1 min−1 and k2 = 3.7 × 10−3 min−1).8 In general, the fabrics lost 50% of their coloration in less than 2 min (relative amplitude of k1 between 55 and 74) and then underwent a slower decoloration that could take almost 1 h to be completed, mainly arising from the presence of the photoisomer TT. Moreover, the decrease of the ΔOD values of all fabrics, when compared with those of the parent SiO2 NPs, might be due not only to the dilution factor of the SiO2@NPT nanomaterial in the printing pastes (10 wt %) but also to the fast bleaching kinetics, which can compromise the measurement. Without discarding the other SiO2@S3-based coated cotton fabrics, Ct-PU-SiO2@S3 showed the best photochromic response, combining good optical contrast and high switching speeds between the two states (colored/ uncolored). The fabrics coated with the SiO2@S4 NPs presented an initial light yellow coloration (Table 2). UV (or sunlight) irradiation of the fabrics (1 min) led to the prompt appearance of a pinkish color, described by an electronic band with λmax = 500−530 nm and ΔOD between 0.30 and 0.45. The bleaching process occurred very rapidly, with faster kinetics (e.g., k1 = 2.2 min−1 and k2 = 4.4 × 10−2 min−1 for Ct-AC-SiO2@S4) than for the SiO2@S4 nanomaterial (k1 = 5.4 × 10−1 min−1 and k2 = 1.1 × 10−2 min−1). The fabrics lost most of their color in less than 5 min, leaving a slight coloration that faded more slowly (more than 1 h). These fabrics are suitable for applications that demand faster coloration/decoloration transformations, namely, responsive camouflage patterns for military protective clothing, brand protection (e.g., anticounterfeit markers), security printing, decorative textiles, children toys, fashion garments, and so on.7 As observed for the silylated NPT counterparts, the fabrics coated with SiO2 NPs containing nonsilylated NPTs, SiO2@1 and SiO2@3, displayed initial pale orange and pale yellow colorations, respectively, that were intensified under UV (or sunlight) irradiation (Table 2), leading to electronic bands with λmax values of 480 and 450 nm, respectively. Even though the λmax values of the absorption bands did not change significantly when compared to those of the corresponding silylated analogous SiO2@S1- and SiO2@S3-based coated fabrics, their intensities were weaker, leading to lower optical contrast values [Table 3 and Table S3 (Supporting Information)]. Nevertheless, both fabrics revealed the predicted photochromic response when considering the type of NPT; additionally, for both samples, the fading curves followed a biexponential decay, with the SiO2@1 sample being the slowest and the SiO2@3 sample being the fastest nanocoated fabrics to revert back to their original colors. Nevertheless, even though the SiO2@3based fabrics had the highest k1 values (e.g., k1 = 3.5 min−1 for Ct-AC-SiO2@3_T), their photochromic properties were less significant because they had very weak optical densities (e.g., ΔOD = 0.20 for Ct-AC-SiO2@3_T). Furthermore, the differences between unwashed and washed fabrics can be seen visually in the pictures of the materials in Table S2 of the Supporting Information: In the case of the CtAC-nanocoated fabrics, the transfer application onto the printed area overcame the leaching of the SiO2 NPs, whereas the Ct-PU-nanocoated fabrics preserved their printed designs,

Table 4. CIELab Color Values (ΔL*, Δa*, Δb*, and ΔE*ab) of the SiO2@NPT Nanomaterials and Nanocoated Cotton Fabrics

a

materiala

ΔL*

Δa*

Δb*

ΔE*abb

Ct-AC-SiO2@S1 Ct-AC-SiO2@S1_T Ct-PU-SiO2@S1 Ct-PU-SiO2@S1_T SiO2@S2c Ct-AC-SiO2@S2 Ct-AC-SiO2@S2_T SiO2@S3c Ct-AC-SiO2@S3 Ct-AC-SiO2@S3_T Ct-PU-SiO2@S3 Ct-PU-SiO2@S3_T SiO2@S4c Ct-AC-SiO2@S4 Ct-AC-SiO2@S4_T

−26.8 −25.0 −24.8 −24.9 −49.9 −39.2 −32.5 −17.1 −13.0 −12.4 −12.2 −9.9 −22.9 −17.5 −14.5

36.5 34.0 35.5 34.8 16.9 21.0 18.2 26.3 18.1 17.8 17.3 11.8 17.3 24.3 21.3

26.3 21.9 24.2 21.6 −8.7 −9.9 11.1 44.7 43.9 43.5 48.7 41.5 6.5 10.8 8.2

52.3 47.5 49.6 47.9 53.4 45.5 38.9 54.7 49.2 48.6 53.1 44.3 29.5 31.9 27.0

Material: SiO2@NPT nanomaterials or nanocoated cotton fabrics.

Total color difference (ΔE*ab) = (ΔL*)2 + (Δa*)2 + (Δb*)2 , with ΔL* = L*(after UV) − L*(before UV), Δa* = a*(after UV) − a*(before UV), and Δb* = b*(after UV) − b*(before UV). cLiterature values.8 b

All coated fabrics presented negative values of ΔL* because of the color change from an initial off-white (or pale) tone to a more intense and darker color upon UV irradiation. All fabrics presented positive Δa* and Δb* values (with exception of SiO2@S2-coated textiles), revealing a contribution from red and yellow components that are responsible for the development of different combinations of yellow-red intensities and/or degrees after UV irradiation. For the SiO2@S1- and SiO2@S4nanocoated fabrics, Δa* > Δb*, meaning that, for these fabrics, the red component is more influential than the yellow component, resulting in the development of an orange or pinkish color; for the SiO2@S3 fabrics, Δa* < Δb*, indicating a higher contribution of the yellow component, which is responsible for the appearance of a yellowish color after irradiation. The purple color of the SiO2@S2-coated fabrics (negative values of Δb*) results from a red−blue combination. Generally, all fabrics exhibited high total color difference values (ΔE*ab between 9.0 and 53.1), confirming that the color switching was very noticeable visually. As observed for the hybrid nanomaterials, the nanocoated fabrics containing silylated NPTs (SiO2@S1, SiO2@S2, SiO2@S3, and SiO2@ S4) exhibited higher values of ΔE*ab, in the range of 26.0−53.1, than the fabrics with nonsilylated NPTs (SiO2@1 and SiO2@ 3), for which ΔE*ab = 9.0−17.2. Regardless of the type of used printing paste (AC or PU), with or without transfer, the values of ΔE*ab were very similar for all fabric. H


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nanocoated fabrics exhibited good photoresistance, showing uniform behavior (L* values) under continuous UV irradiation. Both Ct-PU_w and Ct-PU_T_w fabrics showed similar and constant values of L* (∼91.6 and ∼92.5, respectively) during the irradiation time, meaning that the nanocoatings provided UV light fastness and did not degrade until at least 60 min. Moreover, the Ct-PU-SiO2@S1_w and Ct-PU-SiO2@S1_T_w fabrics also exhibited similar responses (L* ≈ 65.0 and 65.1, respectively), indicating that the photochromic mechanism was not affected by the PU transfer. The Ct-AC-SiO2@S2_T_w sample also exhibited constant behavior (L* ≈ 47.4), without color variations during UV irradiation. For technological purposes, the materials should exhibit high fatigue resistance upon repeated and continuous coloration and decoloration cycles. Therefore, the fatigue resistance properties of the Ct-PU-SiO2@S3_w-, Ct-AC-SiO2@S3_T_w-, and CtPU-SiO2@S3_T_w-coated fabrics were tested under alternating UV irradiation/darkening steps for 6 h (Figure 5). All coated fabrics showed high fatigue resistance, exhibiting reproducible and fast reversibility between colored and uncolored states and no significant loss in L* for 12 consecutive UV/dark cycles. In our previous study,8 we confirmed that the fatigue resistance of NPTs was enhanced by not only the nature and composition of the SiO2 matrix that does not compromise the photochromic behavior of the NPTs and presents high photostability upon long UV exposure, but also the covalent bonding between the SiO2 NPs and the dye molecules. Herein, we also confirmed that the photostability of the SiO2@NPTs was not compromised by the printing pastes (AC and PU) and PU transfer. Additionally, the SiO2 network exhibited high robustness to the incorporated NPTs, allowing the use of high temperatures during the drying and fixation steps (150 °C) of

The prolonged exposure of these NPT dye molecules to UV light can also induce their decomposition and gradual diminution of their photochromic response, limiting their application in outdoor environments or under strong UV light.10 In our previous work, we confirmed that the hybrid SiO2@NPTs were highly photostable under UV radiation, presenting enhanced resistance to fatigue and fast reversibility between coloration and decoloration processes, preserving their performance for up to 12 UV/dark cycles.8 The photostability of the washed fabrics was studied by subjecting them to a longer UV exposure time of 60 min and monitoring the variation of the CIELab color coordinate L* as a function of time (Figure 4). Similarly to the hybrid SiO2 NPs,8 all

Figure 4. Photostabilities of Ct-PU_w (pink diamonds), Ct-PUSiO2@S1_w (red circles), Ct-AC-SiO2@S2_T_w (purple squares), Ct-PU_T_w (green hexagons), and Ct-PU-SiO2@S1_T_w (blue triangles) fabrics during 1 h of UV exposure (at λ = 365 nm).

Figure 5. Responses to UV light/dark cycles of (A) Ct-PU-SiO2@S3_w, (B) Ct-AC-SiO2@S3_T_w, and (C) Ct-PU-SiO2@S3_T_w fabrics: UV exposure (λ = 365 nm) for 1 min and dark exposure for 28 min. I


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*Tel.: +351 259350284. Fax: +351 259350480. E-mail: [email protected]. *Tel.: +351 220402590. Fax: +351 220402659. E-mail: [email protected].

the screen-printing procedure, thus overcoming some of the problems of conventional printing techniques.23,27 Furthermore, the use of SiO2 NPs did not influence the physical properties (e.g., softness and handling) of the final nanocoated fabrics. All coated fabrics, especially those printed with SiO2 NPs modified with silylated 3H-NPTs, provided a remarkable compromise between coloration and bleaching kinetics, low residual color, good color contrast, photostability, and photodegradation resistance, proving to be excellent competitors over other photochromic textiles already reported on other works.14,24−26

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by Fundaçaõ para a Ciência e a Tecnologia (FCT)/MEC under FEDER under Program PT2020 (Projects UID/QUI/50006/2013-POCI/01/0145/ FEDER/007265 and UID/EQU/50020/2013-POCI-01-0145FEDER-006984) and through Project PTDC/CTM-POL/ 0813/2012 within the framework of Program COMPETE. T.V.P. (SFRH/BD/89076/2012), P.C. (under Project PTDC/ CTM-POL/0813/2012), C.M.S. (SFRH/BD/75930/2011), and C.A.D.S. (SFRH/BPD/80100/2011) thank FCT for their grants. C.P. thanks FCT for the FCT Investigator contract IF/ 01080/2015.

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CONCLUSIONS Photochromic SiO2 NPs functionalized with NPTs were successfully incorporated on knitted cotton fabrics by a screen-printing process. This method was rapid, easy to handle, cost-effective, and reproducible, without compromising the photochromism of the NPTs or the textiles’ aesthetic properties (handling and comfort). The screen-printing inks were prepared by direct adsorption or covalent postgrafting immobilization of NPTs and silylated NPTs, respectively, onto SiO2 NPs and subsequent incorporation into two aqueous-based screen-printing pastes (AC and PU). Typically, independently of the AC or PU binder, the fabrics coated with the silylated NPs, SiO2@S1−SiO2@S4, showed better photochromic performance than those coated with nonsilylated SiO2@1 and SiO2@3, confirming the importance of the covalent grafting of the NPTs onto the SiO2 NPs in the improvement of the photochromic behavior. The leaching of the AC screen-printing paste upon washing was overcome by the use of the PU printing paste and the application of a transfer film over the printed area. Although all coated fabrics showed photochromism under UV and solar irradiation, the SiO2@S3- and SiO2@S4-coated fabrics presented the best performances, with high color contrast, an excellent compromise between coloration and bleaching kinetics, low residual color, high photostability, and resistance to photodegradation, that compared well with those of already reported photochromic textiles, making them very promising smart textiles for generic applications and applications in specific areas such as brand protection, camouflage, and security printing.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b06686. Additional information about the physicochemical characterization; SEM micrographs of Ct-PU-SiO2@ S3_T and Ct-PU-SiO2@S3_T_w; TG, FTIR-ATR, UV−vis spectra, photographs and photochromic results of nanocoated cotton fabrics (PDF) Video clip showing the photochromic properties of the nanocoated cotton fabrics before and after UV and sunlight irradiation (AVI)

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*Tel.: +351 220402576. Fax: +351 220402659. E-mail: clara. [email protected]. J


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