Stimuli-Responsive Structurally Colored Films from BioinspiredSynthetic Melanin NanoparticlesMing Xiao,†,# Yiwen Li,‡,#,○ Jiuzhou Zhao,† Zhao Wang,‡ Min Gao,§ Nathan C. Gianneschi,*,‡

Ali Dhinojwala,*,† and Matthew D. Shawkey*,∥,⊥

†Department of Polymer Science, The University of Akron, Akron, Ohio 44325, United States‡Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093,United States§Liquid Crystal Institute, Kent State University, Kent, Ohio 44242, United States∥Department of Biology and Integrated Bioscience Program, The University of Akron, Akron, Ohio 44325, United States⊥Department of Biology, Terrestrial Ecology Group, Ledeganckstraat 35, University of Ghent, Ghent 9000, Belgium

*S Supporting Information

ABSTRACT: Nature has evolved a fantastic gallery of structuralcolors, offering a source of inspiration for the development ofartificial, multifunctional photonic devices. Inspired by thewidespread use of melanin particles to produce structural colorsin bird feathers, we recently demonstrated that synthetic melaninnanoparticles (SMNPs) offer the same rare, but criticalcombination of high refractive index and high absorption asnatural melanin. Here, we show for the first time fast, significant,and reversible changes of structural coloration in self-assembledSMNP films in response to changes in humidity. This process isdriven by the hygroscopic nature of the particles, leading tochanges in the thickness of the SMNP layer that alter theinterference color. This mechanistic explanation is supported bywater absorption measurements, an optical model, and environmental scanning electron microscopy. Humidity-induced dynamiccolors 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 fromwarning coloration to interspecific communication and

protection from ultraviolet radiation.1−3 One critical function ofcolored patterns is camouflage, prevalent in nature in bothpredator and prey.1 In some cases, camouflage is active anddynamic with changeable colors enabling some animals todisguise themselves in variable environments. Cuttlefish are anoteworthy example, wherein changes to their skin color,pattern, and texture are made to closely mimic or match theirenvironment.4 Such color changes are facilitated by the use ofstructural colors arising from nanostructures, as these can beeasily tuned by varying their periodic spacing. Althoughcuttlefish skin is likely under direct neural control,5 extrinsicfactors like humidity (a significant parameter not only to livingorganisms but also in chemical reactions and industrialapplications), also affect spacing and thus color in organisms.Structural colors, for example, of Morpho butterfly wings,6 treeswallow feathers,7 and Hercules beetle elytra8 change withhumidity. Similar to nature, researchers have developedsynthetic optical materials in response to various stimuli, suchas temperature,9−11 chemistry,12 pH,13 light,14 mechanicalforce,15 and humidity. Humidity-responsive systems showingdynamic colors can be divided into one-dimensional,16−18 two-

dimensional,19 and three-dimensional photonic crystals.20−22

Other geometries like supraballs and etalons have also beenused.23,24 To increase colorimetric sensitivity to humidity, mostsystems have employed inorganic materials to increase therefractive index16−18 or water absorbing hydrogels to maximizemoisture uptake.20−22 Replacing hydrogels with naturalmaterials like silk-fibroin allows only limited color change (20nm shift in wavelength) when relative humidity (RH) isincreased from 30% to 80%.25 There is great demand for amore facile approach using a single biocompatible/biodegrad-able material for highly efficient dynamic colorimetric perform-ance.Synthetic melanin nanoparticles (SMNPs) have been used as

a single component to create structural coloration through thin-film interference26 or scattering,27 improving on otherstructurally colored systems by enhancing color saturationthrough light absorption.28−31 Here, we show for the first timethat self-assembled thin films of SMNPs produce sensitive andreversible color changes in response to humidity variation. We

Received: May 26, 2016Revised: July 9, 2016Published: July 11, 2016

Article

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© 2016 American Chemical Society 5516 DOI: 10.1021/acs.chemmater.6b02127Chem. Mater. 2016, 28, 5516−5521

quantitatively demonstrate that water absorption by SMNPsleads to changes in film thickness and thus color.

■ EXPERIMENTAL SECTIONPreparation of SMNP Films. We first synthesized SMNPs via

oxidation polymerization of a dopamine monomer (20 mg) in amixture of 50 mL of water, 20 mL of ethanol, and 1.0 mL of ammoniaaqueous solution (28−30%) at 25 °C.26 The size and distribution ofSMNPs were characterized using transmission electron microscopy(JSM1230, JEOL Ltd.) and dynamic light scattering measurements(BI−HV Brookhaven instrument with a 633 nm solid-state laser)(Figure S1). We then cleaned silicon wafers (University Wafer) using apiranha solution (a mixture of 98% H2SO4 and 30% H2O2 with avolume ratio of 7:3, very dangerous and requiring extensive caution) at80 °C. Clean wafers were held vertically in a SMNP aqueous solutionat 60 °C, and SMNPs were deposited via evaporative self-assemblyonto the wafer as the water evaporated. The deposited films were cutin half, and both the top view and side view scanning electronmicroscopy (SEM) images were obtained using a scanning electronmicroscope (JEOL-7401, JEOL Ltd.) without sputter coating. Allchemicals were purchased from Sigma-Aldrich.Characterization of Dynamic Colors. A CRAIC AX10 UV

visible-NIR microspectrophotometer (MSP) (CRAIC TechnologiesInc.) was used to measure the normal reflectance spectra of SMNPfilms in a custom-built humidity chamber (setup in Figure S2). Weused Teflon tape as a white reference standard. We controlled RHfrom 10% to 90% by adjusting the mixing ratio of dry and wet nitrogengas, which was monitored using a traceable hygrometer with anaccuracy of 1.5% RH (<10 s response time, Model 4080, ControlCompany). At five humidity points (10%, 30%, 50%, 70%, and 90%)after 1 min of equilibrium time, we measured the reflectance of SMNPfilms three times with a 1 min interval. To investigate how fast thecolor changes were occurring, we recorded the real-time color change(both optical images and spectroscopic scans) during the wetting anddrying process. We also tested the stability of dynamic colors bymeasuring the spectra when cycling the humidity eight times. As acontrol, we measured the reflectance of a bare silicon wafer at RH of10% and 90% RH. We also measured angle-resolved reflectancespectra of SMNP films using an AvaSpec spectrometer with a xenonlight source (Avantes Inc.).Water Absorption Measurement. To quantify how much water

SMNPs can absorb, we used a CAHN 21 automatic electrobalance(CAHN/Ventron) with an accuracy of ±0.005% to measure the masschange of small amounts (2−3 mg) of SMNP powders as a function ofthe relative humidity. First, we measured the water uptake of threeempty standard aluminum DSC pans (13.65 ± 0.02 mg at RH 10%)separately at various RHs and found the changes in mass due to waterabsorption by the aluminum pans was small and consistent amongindividual pans (Figure S10). The net water uptake by the melaninparticles can be calculated by subtracting the water uptake by thealuminum pan at certain RH. We mixed dry and wet nitrogen gas toobtain specific RH and then started to record the mass every 3 minuntil equilibrium when the variations among the last five continuousreadings were smaller than 0.010 mg. We repeated the measurementsof two separate SMNP samples for both drying and wetting processes.In-Situ Investigation of SMNP Film Thickness. FEI Quanta 450

FEG environmental scanning electron microscopy (ESEM) was usedto study the microstructural dependence of SMNP films on gas (watervapor and nitrogen) pressure at room temperature. We performedboth cross-sectional and top view imaging of SMNP films withoutsputter coating at different humid conditions (various water vaporpressures). The ESEM chamber was first pumped to ∼10−5 Torrbefore water vapor was introduced. After each desired pressure wasreached, 5 min of stabilizing time was allowed before ESEMobservation. The water vapor pressure used in this study rangedbetween 5 and 18 Torr, corresponding to RH 4.5% and 81% at 24 °C.Dry nitrogen (1 to 10 Torr) was also introduced for a comparativestudy, which was not available for the >10 Torr nitrogen environmentdue to poor imaging quality.

■ RESULTS AND DISCUSSIONWe 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 anevaporation-based process following our previous protocol(Figure 1b, c, and f).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 SEMimages (Figure 1d and g).32 Blue and red films are (274 ± 11)nm and (602 ± 17) nm in thickness, based on cross-sectionalSEM images (Figure 1e and h). The color of the films is causedby interference,26 which showed angle-dependent reflectance(Figure S3). This coloration mechanism is similar to thatobserved in feathers with single or multiple layers of melaninand 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 colorchanges (Figure 2). The color of the blue film turned green,

with the maximum peak position shifting from 475 to 530 nmwhen RH increased from 10% to 90% (Movie S1; readers arestrongly recommended to watch the movie). The red filmshowed two peaks in the reflectance spectrum, a major peak at638 nm and the secondary at 483 nm when RH was 10%. AfterRH increased to 90%, the red film turned green with the majorpeak shifting to 714 nm and the secondary peak shifting to 551

Figure 1. (a) TEM image of SMNPs; scale bar, 100 nm. (b) A schemeof film formation via the evaporation induced self-assembly 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, whereinsets are 2D Fourier transform power spectra. (g and h) Top viewand side view SEM images for the red film. Scale bars in SEM images,1 μm.

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, aspresented in the CIE 1931 color space.

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nm (Movie S2), which makes the secondary peak become thedominant color based on human visual perception (400−700nm34). The maximum peak position increased as a function ofthe RH (Figure S4). The colors cover almost a full spectrum ofvisible light from blue to red in the color space (Figure 2c).Interestingly, the remarkable color change of the red filmbetween RH 70% and 90% offers great benefit in sensing highhumidity (Figure 2b and c). The reflectance spectra of a controlconsisting of only a bare silicon wafer stayed constant in thesame humidity range (10%−90%) (Figure S5), demonstratingthat changes in the SMNP layer explain dynamic colors ofSMNP films.The color of SMNP films quickly changed back to their

original colors when the humidity was reduced to 10% (MoviesS3 and S4). The rate of the color changes can be observed inthe live videos (Movies S1−S4) and is mainly determined byhow fast the humidity changes. Our observations of colorchange are on a timescale that matches the rate of decrease inRH from 90% to 10% and, in turn, the time taken to ramp RHfrom 10% to 80%. Increasing RH from 10% to 90% took morethan 120 s with the first 20 s shown herein. This color change isfaster than active color changes in chameleon skin (∼350 sfrom red to green) and passive, humidity-induced color changein feathers (25 nm wavelength shift in ∼80 s).7,35 We measuredthe reversible response by cycling the RH eight times (longerexperimental time windows led to drift in the intensity of thelight source of the microspectrophotometer). The positions ofthe peak maxima are reversible (Figure 3), and we also found

no change in the color saturation or broadening of thereflectance spectra after eight cycles (Figure S6), which isadvantageous in designing humidity sensors in comparison toother previously used approaches.18,20

Many colorimetric humidity sensors show visible colorchanges, and these results have been attributed to the swellingof photonic crystal structures. However, these mechanisms havebeen inferred from indirect evidence.16,18,22 Here, wequantitatively measured the mass of absorbed water as afunction of the RH (Figure 4). The mass uptake of waterincreased linearly with increase in RH. From linear fitting, weextrapolated that SMNPs increased mass (ω) by 13% uponincreasing the RH from 10% to 90%. This change in mass canbe converted to a change in volume.

ρ ω ρ=V V /water smnp smnp water (1)

In eq 1, ρsmnp, ρwater, and Vsmnp are the density of SMNPs, water,and volume of SMNPs in the film, respectively. On the basis ofthese measurements, a SMNP film can absorb water ofapproximately 17% in volume with increasing RH from 10%to 90%.

To investigate the mechanism behind the observed colorchange, we used a standard thin film interference model (FigureS11) and calculated the reflectance spectra for both blue andred SMNP films.33,36,37 By using the values of the refractiveindex and extinction coefficient of SMNPs reported in ourprevious work and assuming the volume fraction of SMNP filmto be 55%,26 we calculated the reflectance spectra of SMNPfilms. The thickness measured using SEM is the lower limit dueto high vacuum conditions, and for fitting the data, we startwith this minimum thickness and increase the thickness untilwe obtain a best fit (Figure 5a and b): the thickness for blue

and red films is 328 and 654 nm at RH of 10%, respectively.The absorbed water may swell the SMNPs or condense to fillthe pores of the film. We have excluded the possibility of thelatter case based on the optical modeling in SupportingInformation (Figures S12b and S13). Since SMNP films areconfined onto silicon wafers without the freedom to swelllaterally when exposed to humid conditions, we used asimplified model (Figure S12a) where SMNP films only swellvertically, and thus the swelling ratio is equal to the volumeexpansion. The thickness of the SMNP film has a 17% growthwhen RH rises from 10% to 90%. With this increase inthickness of SMNP films, we have calculated the theoreticalspectra and compared them with those measured experimen-tally. After increasing the RH to 90%, absorption of water vaporwill not only swell the film but also reduce the effectiverefractive index of SMNPs. Therefore, we allowed the refractiveindex to vary, and the best fitting was obtained with a value of1.64 at 90% RH (Figure 5c and d). On the basis of waterabsorption, we calculated that the RI value of wet SMNPs is1.68 at RH of 90% using the following equation.38

Figure 3. Maximum peak positions in the spectra of blue (a) and red(b) film shift during eight times of cycling RH. In b, the black curve isfor the primary peak position, and red is the secondary peak position.

Figure 4. Water uptake in the mass of SMNPs changes with RH. Thesolid line is the linear fitting (ω% = −1.81 + 0.156RH; R2 = 0.96).

Figure 5. (a) Measured (solid blue) and model spectra (solid black) ofthe blue film at RH of 10%. (b) Measured (solid red) and modelspectra (solid black) of the red film at RH of 10%. (c) Measured (dashblue) and model spectra (dash black) of the blue film at RH of 90%.(d) Measured (dash red) and model spectra (dash black) of the redfilm at RH of 90%.

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= − +‐n n V n V(1 )smnp wet2

smnp2

water water2

water (2)

Where the RI of dry SMNPs, nsmnp is 1.74.26 The slightdifference in the RI values obtained from these two methodsmight 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 relativehumidity was controlled by water vapor pressure that variedfrom high vacuum (10−5 Torr, ∼ RH 0%) up to ∼18 Torr (RH81%). Our observations of cross-sectional samples showed aprompt and noticeable thickness increase (sometimes up to20% at high humidity) with increasing humidity, which seemedto be caused mainly by increases in particle size (Figures 6 and

S6). In contrast, no distinguishable thickness increase wasobserved in our control experiment where the same SMNP filmwas exposed to dry nitrogen gas at the similar pressures (FigureS8). This result confirmed that the change in film thickness isdue solely to the absorption of moisture by the particles ratherthan change in atmospheric pressure. Top view ESEM imagesdemonstrated SMNP swelling, with cracks in the filmdisappearing when humidity increased, and this process isreversible when decreasing humidity (Figure S9). Comparedwith solid films, the cracks in the SMNP films help releasestrain from SMNP swelling at high humidity. This possiblyenhances the reversibility of the observed color change duringwetting and drying cycles.In this work, we achieved rapid and substantial color

responses with humidity without any additional hygroscopicmaterials for water absorption or inorganic fillers to increase therefractive index. Here, we can define the sensitivity of the colorchange to the humidity as,

=S peak shift (nm)/water uptake (wt %) (3)

For SMNP films, only 13 wt % water absorption can lead to77 nm color shift, therefore S = 5.9 (nm)/(wt %), which is afactor of 2 larger than that reported for polyelectrolyte films(2.2 (nm)/(wt %), and a 195 nm shift is achieved with 90%water uptake).39 With lower water uptake, the SMNP film ismore likely to avoid structural deterioration after multiplewetting and drying cycles. In contrast to conventionalmultilayer structures,16,17 the SMNP film can be facilelyprepared via a one-step coating process. More importantly,the porosity of the films makes it easier to absorb/desorb watervapor, increasing the response speed of the dynamic colors. Inaddition, SMNP films show larger and faster dynamic colorsthan natural systems like butterfly scales, bird feathers, or beetleelytra.6−8

Water or moisture can dramatically increase the electricconductivity of melanins possibly through an electronic-ionichybrid conductor mechanism.40−43 However, quantification ofwater absorption or taking advantage of this property to makedynamic color changes had never been explored. Our workcombining hygroscopic and unique optical properties (highrefractive index and high absorption) of melanins will furtherbroaden the capabilities of SMNPs in photonics, in addition totheir potential applications in bioimaging, drug delivery, andcancer diagnosis.44,45

■ CONCLUSIONSIn summary, we have developed bioinspired SMNP films andhave demonstrated that these films show rapid, reversibledynamic color changes in response to humidity. By measuringwater absorption, imaging the changes in diameter using ESEM,and by employing optical modeling, we have shown that colorchange is caused by swelling of SMNP particles, which leads tochanges in the layer thickness. The remarkable color change isachieved with a single biocompatible component, with a simpleevaporative process leading to potentially intriguing applica-tions in sense-response systems. This strategy is an example ofbiomimicry, where we have applied principles of structuralcoloration in natural systems to achieve dynamic coloration.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.chemma-ter.6b02127.

Figures and optical model information (PDF)Real-time video of the blue film wetting process (MPG)Real-time video of the red film wetting process (MPG)Real-time video of the blue film drying process (MPG)Real-time video of the red film drying process (MPG)

■ AUTHOR INFORMATIONCorresponding Authors*(N.C.G.) E-mail: ngianneschi@ucsd.edu.*(A.D.) E-mail: ali4@uakron.edu.*(M.D.S.) E-mail: matthew.shawkey@ugent.be.Present Address○Y.L.: College of Polymer Science and Engineering, State KeyLaboratory of Polymer Materials Engineering, SichuanUniversity, Chengdu 610065, P.R. China.Author Contributions#M.X. and Y. L. contributed equally to this work.NotesThe authors declare no competing financial interest.

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

Figure 6. Cross-sectional ESEM images of a SMNP film at differentwater vapor pressures. Scale bar, 500 nm.

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■ ABBREVIATIONS

SMNPs, synthetic melanin nanoparticles; SEM, scanningelectron microscopy; ESEM, environmental scanning electronmicroscopy

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