Developing Noniridescent Structural Color on Flexible Substrates with

May 16, 2019 - Furthermore, the particles exert resistance to the motion of the tip, recorded as the friction coefficient of both (SiO2-only and SiO2â...
0 downloads 0 Views 5MB Size
Download PDF
Research Article www.acsami.org

Cite This: ACS Appl. Mater. Interfaces 2019, 11, 21159−21165

Developing Noniridescent Structural Color on Flexible Substrates with High Bending Resistance Mario Echeverri,†,∥ Anvay Patil,†,∥ Ming Xiao,†,‡ Weiyao Li,† Matthew D. Shawkey,§ and Ali Dhinojwala*,† †

Department of Polymer Science, The University of Akron, Akron, Ohio 44325, United States John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, United States § Evolution and Optics of Nanostructures Group, Biology Department, University of Ghent, 9000 Ghent, Belgium Downloaded via TRINITY COLG DUBLIN on August 28, 2019 at 08:03:58 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: Nanostructured materials producing structural colors have great potential in replacing toxic metals or organic pigments. Electrophoretic deposition (EPD) is a promising method for producing these materials on a large scale, but it requires improvements in brightness, saturation, and mechanical stability. Herein, we use EPD assembly to codeposit silica (SiO2) particles with precursors of synthetic melanin, polydopamine (PDA), to produce mechanically robust, wide-angle structurally colored coatings. We use spectrophotometry to show that PDA precursors enhance the saturation of structural colors and nanoscratch testing to demonstrate that they stabilize particles within the EPD coatings. Stabilization by PDA precursors allows us to coat flexible substrates that can sustain bending stresses, opening an avenue for electroprinting on flexible materials. KEYWORDS: polydopamine, electrophoretic deposition, structural colors, flexible coatings, direct assembly

■

to quickly flake off of surfaces. Recently, a third component (a polycationic polymer), in addition to an absorbing material, was added to improve the mechanical stability of electrophoretic-deposited (EPDed) coatings,19 but this process adds an additional step in their fabrication process. Melanin, a ubiquitous natural pigment, offers a robust method to produce saturated colors because it can absorb incoherently scattered light,22,23 provide UV protection,24,25 and bind materials.26−28 Here, we report a one-pot strategy of codepositing SiO2 particles in conjunction with precursors of polydopamine (pre-PDA) (a synthetic mimic of melanin) to fabricate self-assembled coatings with improved mechanical strength to withstand bending stresses. We have extended this approach to coat on conductive indium tin oxide (ITO)coated plastic sheets using a mixture of in situ synthesized prePDA and SiO2 particles. This methodology allows us to use a simple masking approach to develop patterns. Enhanced cohesive and adhesive strength enable these coated flexible sheets to be bent without delamination, a critical property for colored flexible materials.

INTRODUCTION Structural colors produced by nanostructures offer an intriguing approach to color production. Unlike organic dyes and pigments,1,2 structural colors are not oxidized upon exposure to sunlight.3−5 Moreover, they have many applications such as decorations,6 smart windows,7 humidityresponsive systems,8 photonic inks9 or pigments,10 and nontoxic colors for cosmetics.11 The last two decades have seen an accelerated effort to selfassemble colloidal particles into structural colors through many methods such as evaporation-based self-assembly10,12−14 or spray painting.15−18 Electrophoretic deposition (EPD) is a versatile technique with a high potential for rapid, low-cost, energy-efficient coating technology. By applying a direct current (DC) voltage, charged particles migrate to the oppositely charged electrode, creating a micron-thick layer of deposited particles. By controlling the size of the particles, a palette of colors can be produced. In addition, EPD can also be applied to curved surfaces.6 The faster deposition rate of particles in EPD enables creating semiordered colloidal arrays,6,19 typically difficult to achieve due to the strong tendency of monodispersed micron-sized particles to crystallize to photonic opals.15,20 Initial EPD experiments using silica (SiO2) or polystyrene particles21 produced washed-out structural colors due to incoherent scattering. Saturation was improved by adding absorbing materials such as carbon black.6 Although this strategy produced robust colors on a conductive substrate, the cohesion of these particles is weak and they tend © 2019 American Chemical Society

■

RESULTS AND DISCUSSION Preparation and Optical Properties of EPDed Coatings. Figure 1A illustrates the EPD process we have used to Received: March 13, 2019 Accepted: May 16, 2019 Published: May 16, 2019 21159

DOI: 10.1021/acsami.9b04560 ACS Appl. Mater. Interfaces 2019, 11, 21159−21165

Research Article

ACS Applied Materials & Interfaces

parameters, we reached an intermediate stage between an oligomer and a particle. The deposited films from a mixture of in situ synthesized pre-PDA and SiO2 particles (SiO2−prePDA) show vivid structural colors to the naked eye (Figure 1C). Their reflectance peak values are lower than SiO2-only films due to the strong absorption of incoherently scattered light across the complete visible spectrum by the pre-PDA. A comparison plot of reflectance spectra with respect to dopamine monomer concentration is shown in Figure S1A. It is observed that as the dopamine monomer concentration is increased, there is a continuous drop in the intensity, and the peak reflectance values show a red shift indicating that the coatings are getting darker (Figure S1B). Considering the length scale of the SiO2 particles employed and the thickness of the EPDed coatings formed, we hypothesize that the structural color of these EPDs is caused by multiple scattering11 between the SiO2 particles with additional absorption by pre-PDA. To evaluate the angle dependence of EPD films, we performed angle-resolved specular reflectance measurements, where the incident angle and the detection angle were simultaneously varied from 5 to 35° (with respect to the surface normal) in steps of 10° (Figure 2A). In the case of Figure 1. (A) Schematic of the substrate (ITO/glass) coated by SiO2 nanoparticles and pre-PDA using EPD technique; the voltage and time were kept constant in these experiments, being 10 V and 5 min, respectively. (B) Rightmost panel: photographs of coatings prepared via EPD with only SiO2 nanoparticles of 220 (top), 260 (middle), and 300 nm (bottom) diameters. Middle panel: optical micrographs of depositions made with only SiO2 nanoparticles; top: blue (220 nm), middle: green (260 nm), and, bottom: red (300 nm). Left: reflectance spectra of SiO2 depositions with each colored curve representing the color obtained from the EPD of SiO2 nanoparticles of a particular size (blue: 220 nm, green: 260 nm, red: 300 nm). (C) Rightmost panel: photographs of EPDs of SiO2 nanoparticles (sizes 220 (top), 260 (middle), and 300 nm (bottom) diameters) with incorporated prePDA. Middle panel: optical micrographs of depositions with the combination of SiO2−pre-PDA; top: navy blue (220 nm), middle: olive green (260 nm), and bottom: deep red (300 nm). Left: reflectance spectra of SiO2−pre-PDA depositions with each colored curve representing the color obtained from the EPD of SiO2−prePDA combination with a particular size of SiO2 nanoparticles employed (navy blue: 220 nm, olive green: 260 nm, deep red: 300 nm). Here, the concentration of dopamine in each case is kept constant at ∼0.3 mg/(1 mL of 4:1 ethanol/water mixture). Scale bars of optical micrographs, 200 μm.

Figure 2. (A) Schematic of the setup for the specular, angledependent reflectance measurements with the angle θ (between the incident light and the surface normal) varied from 5 to 35° in steps of 10°. (B) Angle-dependent reflectance spectra for the coating of SiO2 nanoparticles (260 nm diameter) on ITO/glass at incident angles from 5 to 35° (with respect to the surface normal). (C) Angledependent reflectance spectra for the coating of SiO2 (260 nm)−prePDA on ITO/glass at incident angles from 5 to 35° (with respect to the surface normal). In both plots (B) and (C), the reflectance spectra have been offset along the y-axis for clarity. (D) Plot of spectral peak positions at varying angles of incidence with respect to the surface normal for both SiO2-only (filled black squares) and SiO2−pre-PDA (filled red circles) EPDs.

deposit semiordered arrays of negatively charged silica particles on the anode using the electric field. The optical micrographs and photographs of EPDed coatings from SiO2 particles only (SiO2-only) with their corresponding reflectance curves for each SiO2 particle size are shown in Figure 1B. The SiO2-only EPDs show different structural colors by varying the size of the SiO2 particles, blue (220 nm), green (260 nm), and red (300 nm). These colors are whitish and pale due to the strong contribution of the incoherently scattered light across the entire visible spectrum. Inspired by the fact that melanin can increase the saturation of structural colors,23 we incorporated synthetic melanin (prePDA) into the SiO2 suspension for EPD. Polydopamine (PDA) polymerization and the final morphology are sensitive to the monomer concentration, pH, proportion of ethanol/ water mixture, and time.29 By manipulating the above

SiO2-only EPDs, we observed shifts in the peak reflectance values with the incident angle (Figure 2B), indicating some level of iridescence. However, angle-independent reflectance in SiO2−pre-PDA EPDs (Figure 2C) indicates that they are noniridescent. The position of the spectral peak shifts by less than 10 nm and is independent of the viewing angle for the SiO2−pre-PDA coating, while for the SiO2-only system, it shifts by around 50 nm (Figures 2D and S2). 21160

DOI: 10.1021/acsami.9b04560 ACS Appl. Mater. Interfaces 2019, 11, 21159−21165

Research Article

ACS Applied Materials & Interfaces

Figure 3. EPDs containing SiO2 nanoparticles of size 260 nm diameter, with or without pre-PDA. (A) SEM image of SiO2-only EPD (top view), (B) SEM image of SiO2−pre-PDA EPD (top view), and (C) SEM cross-sectional image of the EPD of SiO2−pre-PDA. The insets of panels (A) and (B) are the 2D Fourier power spectra. Scale bars of all of the images are 1 μm.

Mechanical Characterization of the EPDs. Figure 4A shows a schematic demonstrating the nanoscratch test (top)

Morphology of the EPDs. Figure 3A,B shows the scanning electron microscope (SEM) images of SiO2-only and SiO2−pre-PDA EPDed films, respectively. In contrast to the SiO2-only system, the EPD of pre-PDA (along with the nanoparticles) impedes the periodicity of the arrangement, inducing strong randomization in the packing of the particles, rather than forming small and dispersed crystalline domains as in the former EPDed system. Periodic arrangement (longrange order) of SiO2 nanoparticles leads to a bright reflectance with angle dependency due to the Bragg diffraction.30 A random, densely packed photonic structure exhibits shortrange order and less long-range periodicity and is characterized by circular- or ring-shaped two-dimensional (2D) fast Fourier transform (FFT) patterns, as shown in the insets of Figure 3A (distinct rings with bright spots indicative of a semiordered colloidal array) and Figure 3B (broad halo indicative of a more amorphous system). The effect of dopamine monomer concentration on the morphology of EPDed coatings can be seen in Figure S3. With increasing concentration, there is a tendency to produce more disordered structures, exhibiting a transition from semiordering to more disordering. The hallmark of amorphous-like colloidal arrays is that they produce noniridescent structural colors unlike crystalline-like packing observed for monodisperse nanoparticles.31,32 We analyzed the cross-section of EPDed coatings using SEM to characterize the thickness of films (Figure 3C). While it is generally necessary to sandwich a sample between an epoxy layer and the substrate to protect it from mechanical damage during cross-sectioning,6 here we obtained a clean cut by simply breaking the ITO/glass for SiO2−pre-PDA system. The stability of such EPDs is perceived from the cross-sectional SEM images; the breaking of ITO/glass preserved the structure. The SiO2−pre-PDA coatings on ITO/glass with three different sizes of particles 220, 260, and 300 nm, resulted in 8 ± 1, 12 ± 5, and 16 ± 6 μm thick films, respectively (Figure S4). All of the three films were deposited using the same conditions. The statistical study using Welch’s analysis of variance (ANOVA) revealed that the cross-sectional thickness varied significantly across the three treatment groups (F2,11.5 = 12.307, P = 0.001). The cross-sectional thickness of the 220 nm SiO2 treatment was significantly lower than that of the 300 nm SiO2 treatment (Games−Howell, P = 0.002). The 260 nm SiO2 treatment’s cross-sectional thickness, however, was not significantly different from that of either the 220 nm SiO2 treatment or the 300 nm SiO2 treatment (Games−Howell; 260 vs 220 nm: P = 0.103 and 260 vs 300 nm: P = 0.289). Although not dramatic, it has been observed that the particle size affects the thickness of the EPD layer for an applied voltage and operation time,33 as can be observed in this case.

Figure 4. Quantitative analysis of the forces for an EPD of SiO2 nanoparticles (260 nm diameter) with or without the incorporation of pre-PDA. (A) Top: a schematic demonstrating the nanoscratch test. Middle: the profile employed for the nanoscratch test using a normal force of 1000 μN. Bottom: the profile corresponds to the lateral displacement of the tip during the scratching test. (B) Compiled histogram of maximum lateral force against normal forces of 500 and 1000 μN for both SiO2-only (black bar) and SiO2−pre-PDA (red bar) EPDs, respectively. The error bars are the standard deviations calculated from four measurements for each sample and force. Mean friction coefficient plotted against time at normal forces of 500 μN (C) and 1000 μN (D) after averaging over four runs for both SiO2only (black curve) and SiO2−pre-PDA (red curve) EPDs.

along with the profile exhibited by the normal force (middle) and by the lateral displacement (bottom) as a function of time. When the diamond tip penetrates the layer of EPDs, the force (normal force) rapidly increases to a set value, followed by the lateral movement of the tip to push the particles (i.e., a scratch), giving the value of the lateral force. The red line indicates when the scratch is performed. A scratch is generated when the lateral force is high enough to break the bonding between particles and push particles away. The nanoscratch test, one of the most extensively used techniques to test the cohesive and adhesive properties of coatings and thin films,34 is often used to quantify the mechanical properties of the EPDed coatings and their mechanical robustness. The mean lateral force curves for the three sizes of SiO2 particles in SiO2-only and SiO2−pre-PDA EPDs are shown in Figure S5. 21161

DOI: 10.1021/acsami.9b04560 ACS Appl. Mater. Interfaces 2019, 11, 21159−21165

Research Article

ACS Applied Materials & Interfaces

Figure 5. (A) Left: schematic of cylinders of different diameters used for the bending test. Right: demonstration of bending test for both SiO2-only and SiO2−pre-PDA EPDs, each containing 260 nm SiO2 nanoparticles. The SiO2-only EPD fails instantly on bending slightly (top), while the SiO2−pre-PDA EPD is mechanically robust and invariant to bending (middle and bottom). (B) Demonstration of the patterned flexible structural color coatings via EPD process. A cold plasma treatment was used for selectively patterning the ITO/PET substrates. The letters “U” and “A” had been plasma-treated. The green and blue colors were obtained by employing 260 and 220 nm SiO2 nanoparticles, respectively. The structural color coatings were formed by the SiO2−pre-PDA EPD. The panels (C) and (D) show that the patterns were not destroyed or smudged even after bending the substrates, exhibiting the robustness of these EPDs.

Flexible and Patternable Structural Color Coatings. Structural colors are typically fabricated on rigid glass substrates coated with ITO via EPD. However, there is a demand of flexible coatings for flexible displays due to the low weight, breaking resistance, and adaptability. Here, we can electrophoretically deposit structurally colored coating onto flexible ITO/poly(ethylene terephthalate) (PET) substrates. Figure 5A shows that pre-PDA improves the structural integrity of the deposition, offering the capacity to strongly bend the coating to large extents without damaging it. We bent EPDed films around a series of cylinders of different diameters (0.5, 0.4, and 0.25 in.). The SiO2-only EPDs sloughed off its particles at even the largest cylinder size, demonstrating poor substrate−particle and particle−particle interactions. In the case of SiO2−pre-PDA coatings, we hypothesize that the prePDA forms strong bonds with the substrate and among the particles and thereby helps to dissipate the bending stresses, enhancing both cohesive and adhesive strength. Moreover, the dopamine monomer concentration also affects the mechanical properties of EPDed coatings, as shown in Figure S7. An optimum monomer concentration (∼0.3 mg/mL of 4:1 ethanol/water mixture) is essential to achieve the desired bending resistance without compromising on the optical properties (i.e., the brilliance of structural colors, Figure S1). The effective adhesion of pre-PDA to many substrates is attributed to its strong molecular adhesion through the catechol group.39 Similar behavior occurs regardless of particle size (Figure S8). Finally, we illustrate the concept of using a masking approach to create patterns on PET substrates. We used a fluorinated monomer, 1H,1H,2H-perfluoro-1-dodecene (perfluoro), to plasma-coat defined regions of the substrate to form a hydrophobic pattern.40 We hypothesize that due to weak adhesion between the particles and the fluorinated polymer, the particles are washed away during the drying process. This results in the production of the patterns as observed in Figure

Figure 4B shows that the maximum lateral forces increase when pre-PDA is incorporated during the EPDs: an increase of 20 μN when applied with a normal force of 500 μN (10% increase) and an increment of 88 μN (20% increase) for a normal force of 1000 μN. The increase in the maximum lateral force for SiO2−pre-PDA EPDs indicates stronger binding between silica particles. Furthermore, the particles exert resistance to the motion of the tip, recorded as the friction coefficient of both (SiO2-only and SiO2−pre-PDA) EPDs, as shown in Figure 4C for a normal force of 500 μN and Figure 4D for 1000 μN. These plots clearly indicate the increased resistance to the tip motion offered by the incorporation of pre-PDA in the EPDed system. To be precise, the mean friction coefficient for SiO2−pre-PDA coatings is higher than that of their counterparts. Even though some of these numbers appear to be a modest increase for SiO2−pre-PDA films, the integrity of these coatings is dramatically different during the bending test. The nanoscratch results for the other two SiO2 sizes (220 and 300 nm) can be found in the Supporting Information (SI) (Figure S6). In addition to enhancing the saturation of the structural colors of the EPDs, pre-PDA also binds the particles to the substrate and maintains the structural integrity of the EPDed coating. Extensive research has attempted to uncover the forces that dictate self- and directed assembly processes;35,36 however, little is known about how to improve the structural integrity of the resultant assembled structures, especially EPDed SiO2 nanoparticle-based coatings. The addition of small amounts of dopamine within the system is a step toward solving this problem. Although it is difficult to quantify the state of prePDA formed during the in situ reaction, it appears to bind to SiO2 particles due to its proclivity for the SiO2 surface. PDA coats SiO2 substrates37 and nanoparticles38 owing to the dehydration reaction between the silanol groups on the SiO2 surface and the catecholic moieties of PDA, resulting in a strong chemical bond. 21162

DOI: 10.1021/acsami.9b04560 ACS Appl. Mater. Interfaces 2019, 11, 21159−21165

Research Article

ACS Applied Materials & Interfaces

pre-PDA incorporated into the coating during EPD process is too small to be detected by IR in comparison to SiO2. Patterning via Cold Plasma Treatment. Patterns were created on ITO/PET sheets using stencils by placing the sheets under each pattern with the ITO side facing the deposition front. The exposed part was coated with perfluoro using radio frequency plasmaenhanced chemical vapor deposition (rf-PECVD) via a procedure developed before.41 Perfluoro or 1H,1H,2H-perfluoro-1-dodecene (C10F21−CHCH2) (97% pure) was purchased from Matrix Scientific and is an exemplar of the class of long-chain perfluoroalkyl monomers. The surface obtained by plasma-depositing perfluoro monomers has been reported to be superhydrophobic and oleophobic, having a low surface energy value of 7.5 mN/m.40 The plasma deposition was performed in an inductively coupled, cylindrical vacuum chamber, wherein the coupling was achieved through an impedance-matching network that connects the coil wound around the chamber to the rf generator operating at 13.56 MHz. The deposition chamber was connected to the roughing pump via a pressure gauge and a liquid nitrogen cold trap. The glass tube containing the perfluoro monomer was connected to the inlet of the chamber, and the control valve was used to adjust and monitor the vapor pressure inside the chamber. For this particular monomer, the plasma deposition was carried out for 10 min at a vapor pressure of 140−160 mTorr, and the chamber was operated in a continuous mode at an input power of 35 W. Prior to deposition, a cleaning cycle was done for an hour using air plasma at a vapor pressure of 180−200 mTorr and an input power of 50 W. We had used clean ITO/PET sheets and silicon wafers as reference substrates to evaluate the plasma coating by contact angle measurements. Following the patterning process via the cold plasma treatment, SiO2−pre-PDA EPDs were carried out on these patterned ITO/PET substrates at a constant voltage of 10 V and for a period of 5 min. The same suspensions were employed as described earlier. After the EPD, the substrates were immediately taken out and laid onto a leveled surface to allow the gradual evaporation of ethanol, thereby giving the desired patterns after complete drying. Characterization of EPDed Coatings. Optical images of the EPDs were taken using an Olympus BX51 microscope (Olympus Corp.). We used a white standard of high-density Teflon tape (TaegaTech) for measuring the white reference. Percentage of reflectance was measured using an Avaspec spectrometer with a xenon white source of beam size ∼5 mm (Avantes Inc., Broomfield) attached to a goniometer. Before any spectral measurements were collected, two calibration steps, using a white and black reference, were performed for ensuring all measurements were done using the same protocol. The detector and the incident light beam were positioned at 5° (from the surface normal) and increased in steps of 10° till 45° to carry out specular, angle-resolved reflectance measurements. These measurements were taken on the coatings EPDed on ITO/glass substrate. The top and cross-sectional views of the nanostructures resulting from the directed assembly were characterized using a field emission scanning electron microscope (SEM) (JEOL-7401, JEOL Ltd.). To measure the thickness of the EPDed coatings, we cut the ITO/glass underneath the deposition using a diamond tip and then break along the defect line with hands. Later, the pieces were vertically aligned along the cut edge for SEM imaging. No sputter coating was necessary for the aforementioned samples. We used the ImageJ software (http://imagej.nih.gov/ij/) to measure the thickness of the coating from the SEM images. The error bars mentioned in the thickness values are representative of the standard deviation, resulting from the analyses of eight cross-sectional SEM images of each type of coating (25 measurements performed on each SEM image). Welch’s analysis of variance (ANOVA) was used to compare the mean cross-sectional thicknesses across the three SiO2 treatments (220, 260, and 300 nm). This nonparametric analysis was used in place of a parametric ANOVA because the variances between the groups were heterogeneous (Hartley’s Fmax test; P > 0.05) and this violates one of the assumptions of parametric statistical analyses. The residuals of the data were normally distributed (Shapiro−Wilk test; P > 0.05). In the

5B. Hence, we developed a facile way of patterning structural color coatings via the EPD process using a cold plasma treatment of the conductive, flexible substrates. These experiments illustrate the advantages of using these simple approaches to fabricate large-scale patterns.

■

CONCLUSIONS We have developed a facile method to enhance the saturation of structural colors and also improve the mechanical integrity of particle coatings using EPD. We have EPDed SiO 2 nanoparticles along with pre-PDA. The use of pre-PDA not only enhances the color saturation of the films due to the absorption of incoherently scattered light but also leads to random packing of the SiO2 particles to produce angleindependent (noniridescent) structural colors. In addition, we demonstrated that pre-PDA increases the mechanical property of the films using the nanoscratch measurements and fabrication of bending-resistant flexible coatings. While designing such systems, it is essential to take into account the effect of dopamine monomer concentration for obtaining optimal optical and mechanical properties. This system can be scaled and provides a good opportunity to the flexible displays and electronics industry to employ flexible structural color coatings for aesthetics, long-lasting colors, and potential substitutes to toxic, metal-, or organic-based pigments. Furthermore, the cold plasma approach provides an easy way to pattern flexible PET substrates.

■

EXPERIMENTAL METHODS

Preparation of EPDed Coatings and Flexible Structural Colors. EPD of SiO2-only and SiO2−pre-PDA on ITO/glass and ITO/PET has been adapted from the previous literature.6,19 Both ITO/glass and ITO/PET were cleaned in ethanol via sonication for 2 h and then blow-dried with nitrogen gas. The suspension of silica nanoparticles (20 mg/mL) was prepared in a 4:1 ethanol/water mixture, in addition to a 70 μL NH4OH solution. For the preparation of SiO2−pre-PDA mixture, the same formulation was used except we added 0.3 mg/(1 mL of a 4:1 ethanol/water mixture) of dopamine monomer, which self-polymerizes to polydopamine chains during the 20 h oxidative reaction at room temperature.29 To study the effect of dopamine monomer concentration on the final EPDs, reactions were also carried out using 0.1 and 0.6 mg/(1 mL of a 4:1 ethanol/water mixture) of dopamine monomer. The suspension underwent successive color changes from white to yellow and finally to milky brown. The silica nanoparticles (three sizes, 220, 260, and 300 nm) were purchased from Superior Silica LLC. (SUPSIL PREMIUM), and ITO/glass was purchased from Nanocs Inc. with a resistance of 100 Ω/sq. To observe the formation of pre-PDA, the oxidative polymerization of dopamine monomer was carried out under the same reaction conditions without the silica particles and monitored over time via in situ IR absorption measurements for 20 h (Figure S9). In an attempt to comment on the form of the material, the prePDA thus formed at the end of 20 h was evaluated using lightscattering measurements, i.e., dynamic and static (Figure S10). The details for the characterization of pre-PDA can be found in the SI. The directed (electric-field-driven) assemblies were carried out on ITO/glass pieces of approximately 13 mm length (l) and 7 mm width (w), and for ITO/PET, the sizes of the pieces were 15 mm l × 10 mm w. For both SiO2-only and SiO2−pre-PDA EPDs, the voltage was kept constant at 10 V for the assembly run of 5 min. The current dropped during the process from 750 to 100 mA. The equipment used was a DC-regulated power supply CSII2005S. Transmission FTIR spectra (KBr protocol) of SiO2−pre-PDA and SiO2-only (control) systems were performed to detect pre-PDA signatures in the coating (methodology can be found in the SI). The two spectra (Figure S11) were almost identical to each other, denoting that the amount of 21163

DOI: 10.1021/acsami.9b04560 ACS Appl. Mater. Interfaces 2019, 11, 21159−21165

ACS Applied Materials & Interfaces

■

case of a significant whole model, a Games−Howell multiple comparison test, which does not assume a homogeneous variance between the treatment groups, was used to determine which groups were significantly different. Nanoscratch Test. The nanoscratch data were collected for the EPDed coatings on ITO/glass, using Hysitron TI Premier (Bruker Corp.). A diamond conical tip was used to have a constant scratch, to eliminate the edge effects from the Berkovich tip during scratching. The normal force profile used was a constant load scratch, and it is described in Figure 4A. It involved a fast, linear increase in the normal force, a stable step at the maximum normal force, and, finally, a rapid and linear decrease in the normal force. In this experiment, we had used two normal forces to penetrate the sample at different extents: 500 and 1000 μN. The tip simultaneously moves across the sample surface exerting a lateral force, providing a measure of the force required by the tip to overcome the resistance of the particles (friction) during lateral movement, resulting in a compressive stress built-up in front of the moving tip.42 Once the tip penetrated the coating, the scratch was carried out for 17 s, traveling 10 μm through the coating. For the sake of reproducibility, four scratches were run on the same sample at different regions using one normal force each time. Bending Test. The EPDed coatings of SiO2-only and SiO2−prePDA on ITO/PET were tested by utilizing different diameter (0.5, 0.4, and 0.25 in.) steel cylinders to evaluate the bending capacity of the EPDs. The flexible material was placed on the cylinder, then force was applied to conform to the shape of the cylinder, steadily held for 30 s, and later, released and passed onto the next cylinders.

■

ACKNOWLEDGMENTS The authors would like to thank Edward Laughlin for assisting in the design and fabrication of the EPD setup. The authors acknowledge the National Polymer Innovation Center (NPIC) (U Akron) and Zhorro Nikolov for giving them access and assisting in the nanoscratch measurements. The authors would like to extend special thanks to Kent Displays Inc. for providing the ITO/PET sheets. The authors would like to thank Austin Garner for his assistance in statistical analysis. In addition, the authors also recognize the assistance from Cheng Liu and Steven S. C. Chuang for in situ FTIR characterizations. The authors would also like to extend their gratitude to Saranshu Singla, Siddhesh Dalvi, Zhuang Xu, Liliana D’Alba, and Asritha Nallapaneni for insightful discussion and comments on the manuscript. The authors acknowledge financial support from the Air Force Office of Scientific Research under the MultiUniversity Research Initiative (MURI) grant (FA9550-18-10142) and grant FA9550-18-1-0477.

■

ABBREVIATIONS EPD, electrophoretic deposition DC, direct current UV, ultraviolet PDA, polydopamine pre-PDA, precursors of polydopamine ITO, indium tin oxide SEM, scanning electron microscopy FFT, fast Fourier transformation PET, poly(ethylene terephthalate) ATR, attenuated total reflectance FTIR, Fourier transform infrared spectroscopy DLS, dynamic light scattering SLS, static light scattering

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b04560. Nanoscratch results for the other two SiO2 sizes (220 and 300 nm), angle-dependent reflectance spectra of the two types of EPDs (SiO2-only and SiO2−pre-PDA) along with the plot for the spectral peak shifts with respect to the incident angles, cross-section SEM images of SiO2−pre-PDA EPDed coatings, and the bending test results for both SiO2-only and SiO2−pre-PDA EPDs, containing 220 and 300 nm SiO2 nanoparticles; characterization of pre-PDA via IR absorption and light-scattering study, IR absorbance spectra of SiO2 and SiO2−pre-PDA via transmission FTIR geometry, and figures supporting the effect of dopamine monomer concentration on the optical, mechanical, and morphological properties of SiO2−pre-PDA coatings (PDF)

■

Research Article

■

REFERENCES

(1) Sun, J.; Bhushan, B.; Tong, J. Structural Coloration in Nature. RSC Adv. 2013, 3, 14862−14889. (2) Booth, C. Evolutionary Significance of Ontogenetic Color Change in Animals. Biol. J. Linn. Soc. 1990, 40, 125−163. (3) Franck, J.; Livingston, R. Remarks on the Fluorescence, Phosphorescence and Photochemistry of Dyestuffs. J. Chem. Phys. 1941, 9, 184−190. (4) Fukuda, M.; Kodama, K.; Yamamoto, H.; Mito, K. Evaluation of New Organic Pigments as Laser-Active Media for a Solid-State Dye Laser. Dyes Pigm. 2004, 63, 115−125. (5) Potyrailo, R. A.; Ghiradella, H.; Vertiatchikh, A.; Dovidenko, K.; Cournoyer, J. R.; Olson, E. Morpho Butterfly Wing Scales Demonstrate Highly Selective Vapour Response. Nat. Photonics 2007, 1, 123−128. (6) Katagiri, K.; Tanaka, Y.; Uemura, K.; Inumaru, K.; Seki, T.; Takeoka, Y. Structural Color Coating Films Composed of an Amorphous Array of Colloidal Particles via Electrophoretic Deposition. NPG Asia Mater. 2017, 9, No. e355. (7) Ge, D.; Lee, E.; Yang, L.; Cho, Y.; Li, M.; Gianola, D. S.; Yang, S. A Robust Smart Window: Reversibly Switching from High Transparency to Angle-Independent Structural Color Display. Adv. Mater. 2015, 27, 2489−2495. (8) Xiao, M.; Li, Y.; Zhao, J.; Wang, Z.; Gao, M.; Gianneschi, N. C.; Dhinojwala, A.; Shawkey, M. D. Stimuli-Responsive Structurally Colored Films from Bioinspired Synthetic Melanin Nanoparticles. Chem. Mater. 2016, 28, 5516−5521. (9) Park, J. G.; Kim, S. H.; Magkiriadou, S.; Choi, T. M.; Kim, Y. S.; Manoharan, V. N. Full-Spectrum Photonic Pigments with NonIridescent Structural Colors through Colloidal Assembly. Angew. Chem., Int. Ed. 2014, 53, 2899−2903.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Weiyao Li: 0000-0002-8512-295X Ali Dhinojwala: 0000-0002-3935-7467 Author Contributions ∥

M.E. and A.P. contributed equally to this work.

Author Contributions

All authors contributed toward designing the experiments. M.E., A.P., and A.D. conducted the experiments. The analysis of the data and the writing of the manuscript were possible through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. 21164

DOI: 10.1021/acsami.9b04560 ACS Appl. Mater. Interfaces 2019, 11, 21159−21165

Research Article

ACS Applied Materials & Interfaces

Durable Antimicrobial Activity and UV Protection. Polymers 2018, 10, No. 495. (29) Jiang, X.; Wang, Y.; Li, M. Selecting Water-Alcohol Mixed Solvent for Synthesis of Polydopamine Nano-Spheres Using Solubility Parameter. Sci. Rep. 2015, 4, No. 6070. (30) Lopez-Garcia, M.; Masters, N.; O’Brien, H. E.; Lennon, J.; Atkinson, G.; Cryan, M. J.; Oulton, R.; Whitney, H. M. Light Induced Dynamic Structural Color by Intracellular 3D Photonic Crystals in Brown Algae. Sci. Adv. 2018, 4, No. eaan8917. (31) Shi, L.; Zhang, Y.; Dong, B.; Zhan, T.; Liu, X.; Zi, J. Amorphous Photonic Crystals with Only Short-Range Order. Adv. Mater. 2013, 25, 5314−5320. (32) Liu, X. Y.; Diao, Y. Y. Modeling Biomineralization and Structural Color Biomimetics by Controlled Colloidal Assembly. In Bioinspiration: From Nano to Micro Scales; Liu, X. Y., Ed.; Springer: New York, 2012; pp 221−274. (33) Mori, Y.; Nobuzane, Y.; Nishimura, K.; Yamada, K.; Tsuchiya, K. Thin Film Structure of Titania Nanoparticles Prepared by Electrophoretic Deposition. Chem. Eng. Trans. 2017, 57, 1507−1512. (34) Tomastik, J.; Ctvrtlik, R. Nanoscratch Test − A Tool for Evaluation of Cohesive and Adhesive Properties of Thin Films and Coatings. EPJ Web Conf. 2013, 48, No. 00027. (35) Turki, D.; Fatah, N. Description of Consolidation Forces on Nanometric Powders. Braz. J. Chem. Eng. 2010, 27, 555−562. (36) Min, Y.; Akbulut, M.; Kristiansen, K.; Golan, Y.; Israelachvili, J. The Role of Interparticle and External Forces in Nanoparticle Assembly. Nat. Mater. 2008, 7, 527−538. (37) Liu, J.; Li, J.; Yu, B.; Ma, B.; Zhu, Y.; Song, X.; Cao, X.; Yang, W.; Zhou, F. Tribological Properties of Self-Assembled Monolayers of Catecholic Imidazolium and the Spin-Coated Films of Ionic Liquids. Langmuir 2011, 27, 11324−11331. (38) Zhang, L.; Wu, J.; Wang, Y.; Long, Y.; Zhao, N.; Xu, J. Combination of Bioinspiration: A General Route to Superhydrophobic Particles. J. Am. Chem. Soc. 2012, 134, 9879−9881. (39) Lee, H.; Lee, B. P.; Messersmith, P. B. A Reversible Wet/Dry Adhesive Inspired by Mussels and Geckos. Nature 2007, 448, 338− 341. (40) Laguardia, L.; Ricci, D.; Vassallo, E.; Cremona, A.; Mesto, E.; Grezzi, F.; Dellera, F. Deposition of Super-Hydrophobic and Oleophobic Fluorocarbon Films in Radio Frequency Glow Discharges. Macromol. Symp. 2007, 247, 295−302. (41) Badge, I.; Bhawalkar, S. P.; Jia, L.; Dhinojwala, A. Tuning Surface Wettability Using Single Layered and Hierarchically Ordered Arrays of Spherical Colloidal Particles. Soft Matter 2013, 9, 3032− 3040. (42) Kareer, A. Nano-Scratch Hardness and the Lateral Size Effect (LSE). Ph.D. Dissertation, University of Leicester, Leicester, U.K., March 2015.

(10) Xiao, M.; Li, Y.; Allen, M. C.; Deheyn, D. D.; Yue, X.; Zhao, J.; Gianneschi, N. C.; Shawkey, M. D.; Dhinojwala, A. Bio-Inspired Structural Colors Produced via Self-Assembly of Synthetic Melanin Nanoparticles. ACS Nano 2015, 9, 5454−5460. (11) Kinoshita, S.; Yoshioka, S.; Miyazaki, J. Physics of Structural Colors. Rep. Prog. Phys. 2008, 71, No. 076401. (12) Jiang, P.; Bertone, J. F.; Hwang, K. S.; Colvin, V. L. SingleCrystal Colloidal Multilayers of Controlled Thickness. Chem. Mater. 1999, 11, 2132−2140. (13) Zakhidov, A. A.; Baughman, R. H.; Iqbal, Z.; Cui, C.; Khayrullin, I.; Dantas, O.; Marti, J.; Ralchenko, V. G. Carbon Structures of 3D Periodicity at Optical Wavelengths. Science 1998, 282, 897−901. (14) Hatton, B.; Mishchenko, L.; Davis, S.; Sandhage, K. H.; Aizenberg, J. Assembly of Large-Area, Highly Ordered, Crack-Free Inverse Opal Films. Proc. Natl. Acad. Sci. U.S.A. 2010, 107, 10354− 10359. (15) Takeoka, Y.; Yoshioka, S.; Takano, A.; Arai, S.; Nueangnoraj, K.; Nishihara, H.; Teshima, M.; Ohtsuka, Y.; Seki, T. Production of Colored Pigments with Amorphous Arrays of Black and White Colloidal Particles. Angew. Chem., Int. Ed. 2013, 52, 7261−7265. (16) Ge, D.; Yang, L.; Wu, G.; Yang, S. Angle-Independent Colours from Spray Coated Quasi-Amorphous Arrays of Nanoparticles: Combination of Constructive Interference and Rayleigh Scattering. J. Mater. Chem. C 2014, 2, 4395−4400. (17) Ge, D.; Yang, L.; Wu, G.; Yang, S. Spray Coating of Superhydrophobic and Angle-Independent Coloured Films. Chem. Commun. 2014, 50, 2469−2472. (18) Kohri, M.; Nannichi, Y.; Taniguchi, T.; Kishikawa, K. Biomimetic Non-Iridescent Structural Color Materials from Polydopamine Black Particles That Mimic Melanin Granules. J. Mater. Chem. C 2015, 3, 720−724. (19) Katagiri, K.; Uemura, K.; Uesugi, R.; Inumaru, K.; Seki, T.; Takeoka, Y. Structurally Colored Coating Films with Tunable Iridescence Fabricated via Cathodic Electrophoretic Deposition of Silica Particles. RSC Adv. 2018, 8, 10776−10784. (20) García, P. D.; Sapienza, R.; Blanco, Á .; López, C. Photonic Glass: A Novel Random Material for Light. Adv. Mater. 2007, 19, 2597−2602. (21) Rogach, A. L.; Kotov, N. A.; Koktysh, D. S.; Ostrander, J. W.; Ragoisha, G. A. Electrophoretic Deposition of Latex-Based 3D Colloidal Photonic Crystals: A Technique for Rapid Production of High-Quality Opals. Chem. Mater. 2000, 12, 2721−2726. (22) Xiao, M.; Hu, Z.; Wang, Z.; Li, Y.; Tormo, A. D.; Le Thomas, N.; Wang, B.; Gianneschi, N. C.; Shawkey, M. D.; Dhinojwala, A. Bioinspired Bright Noniridescent Photonic Melanin Supraballs. Sci. Adv. 2017, No. e1701151. (23) Shawkey, M. D.; Hill, G. E. Significance of a Basal Melanin Layer to Production of Non-Iridescent Structural Plumage Color: Evidence from an Amelanotic Steller’s Jay (Cyanocitta stelleri). J. Exp. Biol. 2006, 209, 1245−1250. (24) Kapp, F. G.; Perlin, J. R.; Hagedorn, E. J.; Gansner, J. M.; Schwarz, D. E.; O’Connell, L. A.; Johnson, N. S.; Amemiya, C.; Fisher, D. E.; Wölfle, U.; et al. Protection from UV Light Is an Evolutionarily Conserved Feature of the Haematopoietic Niche. Nature 2018, 558, 445−448. (25) Cheng, D.; He, M.; Ran, J.; Cai, G.; Wu, J.; Wang, X. In Situ Reduction of TiO2 Nanoparticles on Cotton Fabrics through Polydopamine Templates for Photocatalysis and UV Protection. Cellulose 2018, 25, 1413−1424. (26) Wu, T. F.; Hong, J. D. Dopamine-Melanin Nanofilms for Biomimetic Structural Coloration. Biomacromolecules 2015, 16, 660− 666. (27) Lee, H.; Scherer, N. F.; Messersmith, P. B. Single-Molecule Mechanics of Mussel Adhesion. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 12999−13003. (28) Ran, J.; He, M.; Li, W.; Cheng, D.; Wang, X. Growing ZnO Nanoparticles on Polydopamine-Templated Cotton Fabrics for 21165

DOI: 10.1021/acsami.9b04560 ACS Appl. Mater. Interfaces 2019, 11, 21159−21165