Stimuli-Responsive Structurally Colored Films from Bioinspired

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Stimuli-Responsive Structurally Colored Films from Bioinspired Synthetic Melanin Nanoparticles Ming Xiao, Yiwen Li, Jiuzhou Zhao, Zhao Wang, Min Gao, Nathan C. Gianneschi, Ali Dhinojwala, and Matthew D. Shawkey Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b02127 • Publication Date (Web): 11 Jul 2016 Downloaded from http://pubs.acs.org on July 12, 2016

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Chemistry of Materials

Ming Xiao,1,‡ Yiwen Li,2,‡,⊥ Jiuzhou Zhao,1 Zhao Wang,2 Min Gao,3 Nathan C. Gianneschi,2,* Ali Dhinojwala,1,* Matthew D. Shawkey4,* 1

Department of Polymer Science, The University of Akron, Akron, Ohio 44325, United States

2

Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Dr., La Jolla, California 92093, United States 3

Liquid Crystal Institute, Kent State University, Kent, Ohio 44242, United States

4

Department of Biology and Integrated Bioscience Program, The University of Akron, Akron, Ohio 44325, United States; Department of Biology, University of Ghent, Ghent 9000, Belgium *

Corresponding authors: [email protected], [email protected], [email protected]

‡

These authors contributed equally to this work

⊥

Current address, College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, P.R. China.

ABSTRACT: Nature has evolved a fantastic gallery of structural colors, offering a source of inspiration for the development of artificial, multifunctional photonic devices. Inspired by the widespread use of melanin particles to produce structural colors in bird feathers, we recently demonstrated that synthetic melanin nanoparticles (SMNPs) offer the same rare, but critical combination of high refractive index and high absorption as natural melanin. Here, we show for the first time fast, significant, and reversible changes of structural coloration in self-assembled SMNP films in response to changes in humidity. This process is driven by the hygroscopic nature of the particles, leading to changes in the thickness of the SMNP layer that alter the interference color. This mechanistic explanation is supported by water absorption measurements, an optical model, and environmental scanning electron microscopy. Humidity-induced dynamic colors arising from SMNP films offer possible routes for synthetic melanin as an important material in sensors and coatings.

Colors in nature play a variety of roles ranging from warning coloration to interspecific communication and protection from ultraviolet radiation.1-3 One critical function of colored patterns is camouflage, prevalent in nature in both predator and prey.1 In some cases camouflage is active and dynamic with changeable colors enabling some animals to disguise themselves in variable environments. Cuttlefish are a noteworthy example, wherein changes to their skin color, pattern, and texture are made to closely mimic or match their environment.4 Such color changes are facilitated by the use of structural colors arising from nanostructures, as these can be easily tuned by varying their periodic spacing. Although cuttlefish skin is likely under direct neural control,5 extrinsic factors like humidity (a significant parameter not only to living organisms, but also in chemical reactions and industrial applications), also affect spacing and thus color in organisms. Structural colors, for example, of Morpho butterfly wings,6 tree swallow feathers,7 and Hercules beetle elytra8 change with humidity. Similar to nature, researchers have developed synthetic optical materials in response to various stimuli,

such as temperature,9-11 chemistry,12 pH,13 light,14 mechanical force,15 and humidity. Humidity-responsive systems showing dynamic colors can be divided into onedimensional,16-18 two-dimensional,19 and threedimensional photonic crystals.20-22 Other geometries like supraballs and etalons have also been used.23, 24 To increase colorimetric sensitivity to humidity, most systems have employed inorganic materials to increase refractive index16-18 or water absorbing hydrogels to maximize moisture uptake.20-22 Replacing hydrogels with natural materials like silk-fibroin allows only limited color change (20 nm shift in wavelength) when relative humidity (RH) is increased from 30% to 80%.25 There is great demand for a more facile approach using a single biocompatible/biodegradable material for highly efficient dynamic colorimetric performance. Synthetic melanin nanoparticles (SMNPs) have been used as a single component to create structural coloration through thin-film interference26 or scattering,27 improving on other structurally colored systems by enhancing color saturation through light absorption28-31. Here, we show for

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the first time that self-assembled thin films of SMNPs produce sensitive and reversible color changes in response to humidity variation. We quantitatively demonstrate that water absorption by SMNPs leads to changes in film thickness and thus color. Experimental Section Preparation of SMNP films. We first synthesized SMNPs via oxidation polymerization of dopamine monomer (20 mg) in a mixture of 50 mL water, 20 mL ethanol and 1.0 mL ammonia aqueous solution (28-30%) at 25 C .26 The size and distribution of SMNPs were characterized using transmission electron microscopy (JSM1230, JEOL Ltd.) and dynamic light scattering measurement (BI-HV Brookhaven instrument with a 633 nm solid-state laser) (Figure S1). We then cleaned silicon wafers (University Wafer) using a piranha solution (a mixture of 98% H2SO4 and 30% H2O2 with a volume ratio of 7:3, very dangerous and requiring extensive caution) at 80 C . Clean wafers were held vertically in a SMNP aqueous solution at 60 C and SMNPs were deposited via evaporative selfassembly onto the wafer as the water evaporated. The deposited films were cut in half and both top view and side view scanning electron microscopy (SEM) images were obtained using the scanning electron microscope (JEOL-7401, JEOL Ltd.) without sputter coating. All chemicals were purchased from Sigma-Aldrich. Characterization of dynamic colors. A CRAIC AX10 UV visible-NIR microspectrophotometer (MSP) (CRAIC Technologies Inc.) was used to measure the normal reflectance spectra of SMNP films in a custom-built humidity chamber (setup in Figure S2). We used Teflon tape as a white reference standard. We controlled RH from 10% to 90% by adjusting the mixing ratio of dry and wet nitrogen gas, which was monitored using a traceable hygrometer with accuracy of 1.5% RH (10 Torr nitrogen environment due to a poor imaging quality. Results and Discussion We synthesized monodispersed SMNPs with a diameter of (192  10) nm (Figure 1a and S1) and assembled them into structurally colored films with blue and red colors using an evaporation-based process following our previous protocol (Figure 1b, 1c, and 1f).26 Both blue and red films without long range in-plane order were obtained, as shown by two dimensional Fast Fourier power spectra of the top view SEM images (Figure 1d and 1g).32 Blue and red films are (274  11) nm and (602  17) nm in thickness, based on cross-sectional SEM images (Figure 1e and 1h). The color of the films is caused by interference,26 which showed angle-dependent reflectance (Figure S3). This coloration mechanism is similar to that observed in feathers with single or multiple layers of melanin and keratin. 7, 33 Using a custom-built humidity control system (Figure S2), we varied the relative humidity from 10% to 90%, where these SMNP films displayed clearly visible dynamic color changes (Figure 2). The color of the blue film turned green, with the maximum peak position shifting from 475 nm to 530 nm when RH increased from 10% to 90% (Movie S1, readers are strongly recommended to watch the movie). The red film showed two peaks in the reflectance spectrum, a major peak at 638 nm and the secondary at 483 nm when RH was 10%. After RH increased to 90%, the red film turned green with the major peak shifting to 714 nm and the secondary peak shifting to 551 nm (Movie S2), which makes the secondary peak become the dominant color based on human visual perception (400-700 nm34). The maximum peak position increased as a function of the RH (Figure S4). The colors cover almost a full spectrum of visible light from blue to red in the color space (Figure 2c). Interestingly, the remarkable color


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change of the red film between RH 70% and 90% offers great benefit in sensing high humidity (Figure 2b and 2c). The reflectance spectra of a control consisting of only a bare silicon wafer stayed constant in the same humidity range (10%-90%) (Figure S5), demonstrating that changes in the SMNP layer explain dynamic colors of SMNP films. The color of SMNP films quickly changed back to their original colors when the humidity was reduced to 10% (Movie S3 and S4). The rate of the color changes can be observed in the live videos (Movie S1-S4) and is mainly determined by how fast the humidity changes. Our observations are on those times scales, since it took 20 seconds to decrease RH from 90% to 10% or increase RH from 10% to 80%. Increasing RH from 10% to 90% took more than 120 seconds with the first 20 seconds shown herein. This color change is faster than active color changes in chameleon skin (~350 seconds from red to green) and passive, humidity-induced color change in feathers (25 nm wavelength shift in ~80 seconds).7, 35 We measured the reversible response by cycling the RH eight times (longer experimental time windows led to drift in the intensity of the light source of the microspectrophotometer). The positions of the peak maxima are reversible (Figure 3) and we also found no change in the color saturation or broadening of the reflectance spectra after eight cycles (Figure S6), which is advantageous in designing humidity sensors in comparison to other previously-used approaches.18, 20 Many colorimetric humidity sensors show visible color changes and these results have been attributed to the swelling of photonic crystal structures. However, these mechanisms have been inferred from indirect evidence.16, 18, 22 Here, we quantitatively measured the mass of absorbed water as a function of the RH (Figure 4). The mass uptake of water increased linearly with increase in RH. From linear fitting, we extrapolated that SMNPs increased mass (  ) by 13% upon increasing the RH from 10% to 90%. This change in mass can be converted to a change in volume. Vwater  smnpVsmnp / water

(1)

In equation (1),  smnp ,  water , and Vsmnp are the density of SMNPs, water, and volume of SMNPs in the film, respectively. Based on these measurements, a SMNP film can absorb water of approximately 17% in volume with increasing RH from 10% to 90%. To investigate the mechanism behind the observed color change, we used a standard thin film interference model (Figure S11) and calculated the reflectance spectra for both blue and red SMNP films.33, 36, 37 By using the values of the refractive index and extinction coefficient of SMNPs reported in our previous work and assuming the volume fraction of SMNP film to be 55%,26 we calculate the reflectance spectra of SMNP films. The thickness measured using SEM is the lower limit due to high vacuum conditions and for fitting the data we start with this minimum thickness and increase the thickness until we obtain a best fit (Figure 5a and 5b): the thickness for blue and red film is 328 nm and 654 nm at RH of 10%, respec-

tively. The absorbed water may swell the SMNPs or condense to fill the pores of the film. We have excluded the possibility of the latter case based on the optical modeling in Supporting Information (Figure S12b and S13). Since SMNP films are confined onto silicon wafers without the freedom to swell laterally when exposed to humid conditions, we used a simplified model (Figure S12a) where SMNP films only swell vertically and thus the swelling ratio is equal to the volume expansion. The thickness of SMNP film has a 17% growth when RH rises from 10% to 90%. With this increase in thickness of SMNP films, we have calculated the theoretical spectra and compared them with those measured experimentally. After increasing the RH to 90%, absorption of water vapor will not only swell the film but also reduce the effective refractive index of SMNPs. Therefore we allowed the refractive index to vary and the best fitting was obtained with a value of 1.64 at 90% RH (Figure 5c and 5d). Based on water absorption, we calculated that the RI value of wet SMNPs is 1.68 at RH of 90% using the following equation.38 nsmnp-wet 2  nsmnp 2 (1  Vwater )  nwater 2Vwater

Where the RI of dry SMNPs, nsmnp is 1.74.

(2) 26

The slight

difference in the RI values obtained from these two methods might be due to the minor swelling of SMNPs laterally. We further used ESEM to directly investigate the structural response of SMNP films to humidity variation. The relative humidity was controlled by water vapor pressure that varied from high vacuum (10-5 Torr, ~ RH 0%) up to ~18 Torr (RH 81%). Our observations of cross-sectional samples showed a prompt and noticeable thickness increase (sometimes up to 20% at high humidity) with increasing humidity, which seemed to be caused mainly by the increasing particle size (Figure 6 and S6). In contrast, no distinguishable thickness increase was observed in our control experiment where the same SMNP film was exposed to dry nitrogen gas at the similar pressures (Figure S8). This result confirmed that the change in film thickness is due solely to absorption of moisture by the particles rather than change in atmospheric pressure. Top view ESEM images demonstrated SMNPs swell, with cracks in the film disappearing when humidity increased and this process is reversible when decreasing humidity (Figure S9). Compared with solid films, the cracks in the SMNP films help release strain from SMNP swelling at high humidity. This possibly enhances the reversibility of the observed color change during wetting and drying cycles. In this work, we achieved rapid and substantial color responses with humidity without any additional hygroscopic materials for water absorption or inorganic fillers to increase refractive index. Here, we can define the sensitivity of the color change to the humidity as,

S  peak shift (nm) / water uptake (wt %)

(3)

For SMNP films, only 13 wt % water absorption can lead to 77 nm color shift, therefore S  5.9 (nm) / (wt %) , which is a factor of two larger than that reported for polyelectro-



Conclusion In summary, we have developed bioinspired SMNP films and have demonstrated that these films show rapid, reversible dynamic color changes in response to humidity. By measuring water absorption, imaging the changes in diameter using ESEM, and by employing optical modeling, we have shown that color change is caused by swelling of SMNP particles, which leads to changes in the layer thickness. The fact that remarkable color change is achieved with a single biocompatible component and simple evaporative process will lead to intriguing applications of synthetic melanin into sensing and dynamic colors. This strategy is an example of biomimicry, where we have applied principles of structural coloration in natural systems to achieve dynamic coloration.

(b) RH 10%

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Figure 2. Dynamic colors for blue (a) and red (b) films at various RH. (c) The color changes during humidity change from 10% to 30%, 50%, 70%, and 90% for blue (black dots) and red (white dots) films, as presented in the CIE 1931 color space. (a) 530

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520 510 500 490 480 470 460 0

1

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Figure 3. Maximum peak positions in the spectra of blue (a) and red (b) films shift during eight times of cycling RH. In (b), black curve is for the primary peak position and red is the secondary peak position. 15 10 5 0 20 40 60 80 Relative humidity (%)

Figure 4. Water uptake in mass of SMNPs changes with RH. Solid line is the linear fitting 2 ( %  1.81  0.156RH , R  0.96 ).

(b) Water evaporation

(e)

RH 70%

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0.5 y

FIGURES (a)

(c) 0.9

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Water or moisture can dramatically increase the electric conductivity of melanins possibly through an electronic-ionic hybrid conductor mechanism.40-43 However, quantification of water absorption or taking advantage of this property to make dynamic color changes had never been explored. Our work combining hygroscopic and unique optical properties (high refractive index and high absorption) of melanins will further broaden the capabilities of SMNPs in photonics, in addition to their potential applications in bio-imaging, drug delivery, and cancer diagnosis.44, 45

(a)

Max peak position (nm)

lyte films ( 2.2 (nm) / (wt %) , 195 nm shift is achieved with 90% water uptake).39 With lower water uptake, the SMNP film is more likely to avoid structural deterioration after multiple wetting and drying cycles. In contrast to conventional multilayer structures,16, 17 the SMNP film can be facilely prepared via a one-step coating process. More importantly, the porosity of the films makes it easier to absorb/desorb water vapor, increasing the response speed of the dynamic colors. In addition, SMNP films show larger and faster dynamic colors than natural systems like butterfly scales, bird feathers, or beetle elytra.6-8

(h)

Figure 1. (a) A TEM image of SMNPs, scale bar 100 nm. (b) A scheme of film formation via evaporation induced selfassembly process. Optical images for blue (c) and red (f) SMNP films, scale bar, 100 μm . (d) and (e) Top view and side view SEM images for the blue film, where insets are 2D Fourier transform power spectra. (g) and (h) Top view and side view SEM images for the red film. Scale bars in SEM images, 1 μm .

Normalized reflectance (A.U.)

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Figure 5. (a) Measured (solid blue) and model spectra (solid black) for blue film at RH of 10%. (b) Measured (solid red) and model spectra (solid black) for red film at RH of 10%. (c) Measured (dash blue) and model spectra (dash black) for


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blue film at RH of 90%. (d) Measured (dash red) and model spectra (dash black) for red film at RH of 90%.

High Vacuum

5 Torr, H2O

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Figure 6. Cross-sectional ESEM images of a SMNP film at different water vapour pressures. Scale bar, 500 nm.

Supporting Information contains movies S1-S4, figure S1S13 and optical model supporting. This material is available free of charge via the Internet at http://pubs.acs.org.

*Corresponding authors: [email protected] [email protected] [email protected]

The manuscript was written through contributions of all authors.

We acknowledge support from AFOSR FA9550-13-1-0222, HFSP grant RGY-0083 and NSF EAR-1251895 to M.D.S., NSF DMR-1105370 to A.D., and AFOSR PECASE FA9550-11-1-0105 to N.C.G. M.G. acknowledges the Liquid Crystal Institute Characterization Facility, Kent State University to support ESEM studies. We thank J. Gillespie and E. Laughlin for their help in building the humidity control system. Thank D. Jain for the help with water absorption measurements and G. Amarpuri for the help in using the humidity chamber.

SMNPs, synthetic melanin nanoparticles; SEM, scanning electron microscopy; ESEM, environmental scanning electron microscopy.

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Stimuli-Responsive Structurally Colored Films from Bioinspired

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