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A conformal silk-azobenzene composite for optically switchable diffractive structures Giovanna Palermo, Luca Barberi, Giovanni Perotto, Roberto Caputo, Luciano De Sio, Cesare Umeton, and Fiorenzo G Omenetto ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09986 • Publication Date (Web): 18 Aug 2017 Downloaded from http://pubs.acs.org on August 19, 2017

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ACS Applied Materials & Interfaces

A conformal silk-azobenzene composite for optically switchable diffractive structures Giovanna Palermo,∗,†,‡ Luca Barberi,†,k Giovanni Perotto,¶,⊥ Roberto Caputo,†,‡ Luciano De Sio,§ Cesare Umeton,†,‡ and Fiorenzo G. Omenetto¶ Department of Physics, University of Calabria, 87036 Arcavacata di Rende (CS),Italy, CNR-Nanotec, 87036, Cosenza, Italy, Silklab, Department of Biomedical Engineering, 200 Boston Avenue, Suite 4875, Tufts University, Medford, Massachusetts 02155, United States, and Beam Engineering for Advanced Measurements Company, 1300 Lee Road, Orlando, Florida 32789,USA E-mail: [email protected]

Abstract The use of biomaterials as optical components has recently attracted attention because of their ease of functionalization and fabrication, along with their potential use when integrated with biological materials. We present here an observation of the optical properties of a silk-azobenzene material (Azosilk) and demonstrate the operation of an Azosilk/PDMS composite structure that serves as a conformable and switchable optical diffractive structure. Characterization of thermal and isomeric properties of the ∗

To whom correspondence should be addressed University of Calabria ‡ CNR-Nanotec ¶ Tufts University § Beam Engineering for Advanced Measurements Company k a present address: LPTMS, CNRS, Univ. Paris-Sud, Université Paris-Saclay, 91405 Orsay, France ⊥ a present address: Smart Materials, Istituto Italiano di Tecnologia, via Marego 30, Genova †

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device, along with its overall performance, is presented in terms of diffractive characteristics and response times. The ease of manufacturing and functionalization opens a promising avenue for rapid device prototyping and interfaces of expanded utility.

Introduction An already exciting implementation of modern technology is represented by the possibility of controlling light by exploiting light itself, instead of using electric signals. 1,2 For an all-optical apparatus, the combination of photosensitivity with fast, large, and reversible changes in the optical properties of the material is needed. 3,4 Thus, materials that can meet this requirement have always been a hot topic since they would enable the fabrication of high performance all-optical systems. In the last years, materials such as photosensitive liquid crystals (PLCs) have been synthetized in order to manifacture light sensitive devices with time responses in the range of ms and refractive index contrast of the order of ≈ 0.2, at the expense of high production costs and a complete absence of biocompatibility. 5–8 Thus, as a matter of fact, the possibility of obtaining similar results with conformable platforms is still an open challenge due to the lack of light addressable biomaterials. Bombyx mori silkworm silk is an extraordinary material, composed of large proteins: fibroin and sericin. Regenerated silk fibroin is currently of fundamental and technical interest, because it exhibits an outstanding combination of mechanical properties with optical transparency, a high refractive index, and processability in different forms and formats. 9–14 Morphologically, silk fibroin is a polycrystalline material composed of β-sheet crystallites embedded an amorphous matrix. Doping silk with active molecules or nanoparticles, can enhance the functionality of this biomaterial. With respect to silk, this has been achieved by co-doping with Au-nanoparticles, 15 quantum dots, 16 laser dyes, 17 2-D materials, 18 demonstrating the optical versatility of this naturally derived biopolymer. Silk fibers have been chemically modified in various ways, 19–23 and one of the approaches relies in the synthesis of Azobenzenes moieties on the tyrosine residues in the fibroin chain. 20,24 The crucial aspect of 2 ACS Paragon Plus Environment

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the induced modification of tyrosines has already been reported and clarified by Murphy et al. 20 Azobenzenes are a wide class of molecules that share a simple core structure made of two phenyl rings linked by a N = N double bond, with different chemical functional groups extending from the phenyl rings. The azobenzene molecules may exist in two different isomers (cis-to-trans) that can switch from one to the other by means of a photoreaction. In particular, blue light may induce cis-to-trans isomerization while ultraviolet light may induce the opposite transition. The cis isomer is less stable than the trans one (for instance, it has a distorted configuration and it is less delocalized than the trans configuration), so that cis-to-trans isomerization may occur just by thermal relaxation. 9,25 Associated with the switching is a change of length and dipole momentum of the molecules, 9,11 which has been exploited to fabricate optically excitable and controllable actuators. 9,11,25–27 Among the many possible material forms, silk films are of particular interest for optics and photonics applications because of their transparency and surface smoothness, which are a direct result of the all-aqueous processing and self assembly of regenerated fibroin. 9,28,29 Thus, the possibility of accessing a light-driven isomerization has attracted attention for the generation of active silk composite films with demonstrations ranging from hologram writing, 30 to fiber infiltration. 31 Here, we report the first demonstration of a fully comformable, elastomeric, light responsive device fabricated using Azosilk film. The resulting material preserves most of the original mechanical properties of the regenerated silk fibroin and holds its photoresponsive properties enduringly: all samples stored at ambient conditions for 30 weeks exhibit the same response to the illumination.



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Figure 1: (a)Diazonium coupling reaction with tyrosine residues in silk.(b)Uv-vis absorbance spectrum of pure silk (blue curve) and Azosilk solution (red curve). Photographic inset shows the typical yellow-orange color of Azosilk solution.

Experimental methods and materials The doping of a silk fibroin solution with azobenzene molecules (azo-dye) is obtained by means of diazonium coupling chemistry (Fig. 1a), which modifies the tyrosine residue (which constitutes 5% of the amino acid of the fibroin) by creating an azo dye, according to a previously described protocol. 20 An absorption spectrum (red curve) and a photograph of the resulting Azosilk solution are reported in Fig. 1b. For comparison, in the same figure, we have included the absorption spectrum of pure silk (blue curve). Absorption spectra of pure silk and Azosilk samples were recorded by a UV-vis spectrometer (Ocean Optics, USB2000+). Noteworthy, the pure silk (Fig. 1b, blue curve) exhibits a well evident absorption band centered at 260 nm (typical absorption band of bio-molecules) while the azo-silk (Fig. 1b, red curve) shows an absorption band at 380 nm due to the absorption of the azo-dye molecules. The pump probe analysis was carried out by means of an all optical pump-probe experimental setup: the pump laser is a CW diode-laser (emitting at λ = 405 nm) while a weak UVVis light (DH-2000-BAL,Balanced Deuterium Tungsten Source, 200-900 nm) is used as the probe. In order to prevent saturation from the pump light in the detector, the laser beam impinges on the sample at a sufficiently large angle (' 50◦ ). Particular care is taken to ensure near-perfect overlap of the spots of the pump and the probe beams making the illuminated


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areas coincident (Fig. 2a). In order to check the functional qualities of the sample and verify the optical activity of the embedded azo-molecules, the pump radiation wavelength is chosen to induce a trans-cis isomerization transition (Fig. 2b) in the azo-molecule. 20 The thermal analysis was carried out by means of a thermographic camera (E40 by FLIR). As for the realization of the basic optical device, a PDMS grating has been fabricated by direct electron beam lithography on SU8 (Vistec EBPG5000, 100 kV acceleration voltage, at a dose of 5 µCb/cm2 ) and subsequent pattern transfer to PDMS via conventional cast molding. Morphological analysis of the PDMS grating was carry out by means of atomic force microscopy (AFM Catalyst by Bruker-Nano), employed for the analysis of sample substrates by using silicon tips (radius 8 nm) on a Antimony (n) doped Si lever in no contact mode, and scanning electron microscopy (SEM Quanta 400 by Fei, measurements performed at 0.9 mbar and 15 kV). The pump probe experimental setup used to estimate the diffraction efficiency of the Azosilk-PDMS composite structure is similar to the one used to estimate absorption variations but the probe, in this case, is a continuous green laser light (Verdi by Coherent, λ = 532 nm) (see Results and discussion).

Figure 2: (a)All optical pump - probe experimental setup. (b) Sketch of the trans-cis isomerization transition.



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Results and discussion A detailed characterization of the optical response of an azobenzene-silk sample is needed to assess the potential utility of this material format. The obtained azo-dye doped silk material (Azosilk) can be layered on many different substrates. Our measurements are performed on Azosilk layered on both rigid (glass) and flexible (PDMS, Polydimethylsiloxane) microstructured substrates. Study of the time-resolved light-induced response of the Azosilk composite films is carried out with a pump-probe experimental setup (see Experimental methods and materials). An azosilk sample on a glass substrate is utilized for the first experiment; it is obtained by means of drop deposition of about 500µL of Azosilk solution (shown inset in 1b). Also, we performed a control experiment (in the same experimental conditions) by using a film of pure silk. As a result, no change in the optical properties was detected. Moreover, an extensive optical characterization of pure silk has been previously reported in several publications. 32,33 Results are shown in Fig. 3a, which reveals that the presence of the pump beam, at powers density ranging from 6.2·10−2 W/cm2 to 4.4·10−1 W/cm2 , strongly influences the optical behavior of the sample with two relevant and different variations occuring in two spectral intervals, namely between ∆λ1 = (300 nm−480 nm) and ∆λ2 = (480 nm−650 nm). For increasing values of the pump power applied to the sample, a decrease in absorbance is observed over ∆λ1 , while an increase is observed when the pump beam is swithced off. The plot of the difference between the absorption curves under pumping and with the pump off (Fig. 3b) highlights two critical wavelengths that correspond to the maximum absorbance decrease (λtrans = 355 nm) and to the maximum absorbance increase (λcis = 520 nm), respectively. In both cases, variations almost linearly depend on the incident power (Fig. 3c). The observed effect can be interpreted by taking into account the decrease of population of trans-molecules and the following increase of population of the cis-isomers that takes place upon pumping the sample with the blue laser. Indeed, each AzoSilk molecule may be seen as a two level (trans-cis) system. In absence of the pump radiation, the excited (cis) state is almost empty; by switching the pump on, an increase in the population of this level takes 6 ACS Paragon Plus Environment

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place, which asymptotically will host all the molecules of the sample as the impinging pump power becomes high enough.

Figure 3: (a) Absorption spectra of the AzoSilk sample for different values of the pump laser intensity. (b)Curves depicting the difference of absorbance calculated with respect to the pump off condition. (c) The increase and decrease of absorbance observed at the critical wavelengths (λtrans = 355 nm and λcis = 520 nm).

The increase of cis population corresponds to a decrease in absorption at 355 nm (blue) accompanied by a corresponding increase at 520 nm (green), as shown in Fig. 3c. To confirm this hypothesis, and to exclude artifacts due to the effect of a local decrease of the sample thickness, caused by heating produced by the laser pump, an investigation on how temperature variations can affect the absorption spectrum was carried out. The sample was placed in a hot stage (CaLCTec S.r.l.), and several spectra were acquired while slowly increasing the sample temperature in the range from 40 to 160 ◦ C (Fig. 4a). Absorption variations have been calculated as the difference between the absorption curve detected at room temperature and the ones detected at higher temperature values (Fig. 4b). Incidentally,



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it is worth noting that the pump power P used in the pump-probe experiment (Pmax = 70.41 mW at λ = 405nm) can hardly heat the thick sample (i.e. ≈ 1 mm) up to the temperature values used in the thermal experiment (T = 160◦ C). This consideration is confirmed by a thermo-graphic analysis of the sample performed during the pump-probe experiment (Fig. 4d) which measures a temperature value of about T = 43◦ C when the sample is exposed to the highest blue laser intensity (i.e. I7 = 4.4 · 10−1 W/cm2 ).

Figure 4: (a) Absorption spectra of the AzoSilk sample for different values of external temperature; (b) Difference of absorbance calculated with respect to room-temperature condition; (c)The increase and decrease of absorbance observed at the critical wavelengths (λtherm ≈ 340 nm, related to a thermal effect; no particular trend is observed for λcis ); (d) Thermographic image of the Azosilk sample when exposed to the highest blue laser intensity (i.e. I7 = 4.4 · 10−1 W/cm2 ).

By comparing Fig. 3c and Fig. 4c, it is evident that an increase of the sample temperature also results in an absorption variation that, in the shorter wavelength spectral region ∆λ1 = (300 nm−480 nm), is comparable to the one obtained in the pump-probe experiment, 8 ACS Paragon Plus Environment

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in spite of an observed blue-shift in the maximum absorbance wavelength (λtherm ≈ 340 nm instead of λtrans ≈ 355 nm) in the thermal experiment case. The situation, however, is quite different at longer wavelengths, in the spectral region ∆λ2 = (480 nm − 650 nm) where the temperature increase of the sample causes negligible variations. As already reported in literature for similar materials, 21–23 both electromagnetic and thermal perturbations can induce an isomerization transition; however, the very limited absorbance variation observed in the green region in the thermal experiment (λtherm ≈ 510 nm, Fig. 4b and Fig.4c) indicates that the absorbance decrease observed in the spectral region ∆λ1 can be attributed to a different physical mechanism not concerning the isomerization. The silk utilized in the experiments is treated so as to have a high contend of beta-sheets. Extensive studies of silk based devices have addressed many of these aspects ranging from controlled solubility of silk films, welding, nanoimprinting, nanopattering, all predicated on the control of the protein conformation and on the physical crosslinks of the silk fibroin protein in the matrix. For example, the glass transition temperature shifts to higher values with crosslinking and consequently the thermal reflow/dehydration changes. 33 In the case presented the silk "strings" that are included in the elastomeric grating are largely independent to external parameters (such as temperature or humidity). It should be noted that this is not an absolute statement (there is some variability at higher temperatures, i.e. > 150 ◦ C and one can degrade the protein at temperatures around 200 ◦ C).



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Figure 5: (a) Scanning electron microscopy (SEM) and (b) atomic force microscopy (AFM) images of the PDMS mold grating. (c) Experimental pump-probe setup for measuring the dependence of the diffraction efficiency on the incident pump intensity and photograph of the PDMS mold sample.(d) Transmitted and first diffracted order of a simple PDMS grating.

The observed photo-tunability of the AzoSilk material opens a wide scenario of possible applications ranging from pure photonics to conformable devices; a functional demonstrator that combines the flexibility and optical features of silk with the phototunability coming from the azo-dye dopant has been therefore implemented by integrating the Azosilk on a patterned elastomeric substrate, by infiltrating a PDMS-diffraction grating with liquid AzoSilk. The PDMS mold grating, realized by cast-molding lithography, 34,35 (see Experimental methods and materials) was examined with scanning electron microscopy (SEM) and atomic force microscopy (AFM) (Fig. 5a and 5b) and shows a regular morphology (d = 625 lines/mm) with a height of the fringes (ridge-valley distance) of about 1 µm. By pouring some liquid AzoSilk on the PDMS microstructure, this material has filled the PDMS 10 ACS Paragon Plus Environment

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microchannels and the excess part solidified as a bulk base on top of the grating, yielding a PDMS/AzoSilk/PDMS periodic structure, Fig. 6a. In this way, the diffractive properties of the PDMS grating are modulated by the presence of AzoSilk in its channels, while the flexibility of both AzoSilk and PDMS makes this device conformal and flexible, in contrast to standard gratings. 36,37 Azosilk provides optical functionality to the device thanks to the modification of its absorption spectrum when driven by a laser pump at suitable wavelength (λ = 405nm), modulating the diffractive properties of the (Azosilk-PDMS) composite correspondingly. It is worth pointing out that the composition (Azosilk-PDMS) is delicate and, thus, the surface tension at the interface between PDMS and AzoSilk does not allow a strong intermolecular bonding between the two materials. However, with care, we have conformed several times the composite structure and we did not observe any variation in the morphological and optical properties. A bare grating made only of PDMS was used as a control in a pump-probe experiment (Fig. 4c) to verify that the blue pump beam has no effect on its diffractive properties (Fig. 5d). On the contrary, the use of the latter to characterize the PDMS-AzoSilk composite grating reveals a noticeable change in the intensity of transmitted (0T ) and first diffracted (1T ) orders as a function of the pump beam. An increase in the pump beam intensity results in a photo-induced change of the grating diffraction efficiency (η = I1T /(I0T + I1T )), going from ηOF F ≈ 30% to ηON ≈ 60% (Fig. 6c).



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Figure 6: (a)Azosilk-PDMS grating preparation and photograph of the resulting sample. (b) Experimental pump-probe setup. (c) Plot of the diffraction efficiency for different intensity values of the pump laser. In the inset the diffraction pattern related to I1 = 6.2 · 10−2 W/cm2 and I7 ; Kogelnik dependence of the diffraction efficiency on the argument ∆n with the related linear fit; (d) Transmitted and first order diffracted intensities at the maximum intensity of the pump blue laser (I7 = 4.4 · 10−1 W/cm2 ) .

The first-order diffraction efficiency (η) of the PDMS-AzoSilk grating can be approximated in the framework of Kogelnik’s theory: 38

η = sin2

π∆nL λcos(θ)

(1)

where the argument depends on the grating thickness L, probe wavelength λ, refractive index contrast of the grating ∆n and on the angle θ between the propagation direction of the impinging light and the normal to the sample. By keeping constant L, λ and θ, η can be tuned by changing ∆n, following a sinusoidal like behavior (see inset of Fig. 6c). In our case, we have observed a linear correlation between the diffraction efficiency and the intensity of the pump beam (Fig. 6a). This trend can be explained by considering that, due to the specific values of λ (532 nm), θ (0◦ ) (normal incidence) and L(1 µm), the diffraction efficiency is varying (increasing) along the linear part of the sinusoidal function (inset of Fig. 12 ACS Paragon Plus Environment

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6c, green line). Response times of the (AzoSilk-PDMS) device are τON ≈ 1.2s and τOF F ≈ 0.5s (Fig. 6d). In particular, τON is proportional to the intensity of the pump beam, while τOF F is dictated by the spontaneous relaxation time of the azo-silk molecules; both ON and OF F times can be varied by changing materials concentrations (e.g. AzBr concentration) or by using a pulsed pump beam. Both approaches are underway. Finally, it is worth pointing out that the same formula (1) can be used to provide a valuation of the change in the refractive index contrast that takes place in the AzoSilk-PDMS grating when acted on by the blue pump beam (at the highest intensity I7 = 4.4·10−1 W/cm2 ). By using in Eq.1 the same values reported above for grating thickness, probe wavelength and probe-beam incidence angle, we obtain for the refractive index contrast values, ∆nON = 0.151 and ∆nOF F = 0.098, for the cases of pump on and pump off respectively. By considering that the refractive index of PDMS matrix is not affected by the presence of the pump beam, we can deduce a photo-induced change in the refractive index of the Azosilk material given by nAS = ∆non − ∆nof f = 0.053.

Conclusions In conclusion, we have observed that the optical properties of an Azosilk compound can be tuned either with light, in an "all-optical" scheme, or by temperature variations. To illustrate the versatility of Azosilk, we have manufactured a demonstrator device obtained by integrating an Azosilk diffraction grating layered onto an elastomeric substrate. This device combines the functionality of azobenzene, the optical properties of silk and the flexibility of PDMS ultimately generating an optically addressable, conformable, diffraction grating. Results represent a step forward in the study and design of a class of multifunctional, flexible optical devices that need to rely on all-optical controls without resorting to wired connections.



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Acknowledgement The authors thank Dr Tiziana Ritacco for help in carrying out AFM analysis and Dr Giovanni Desiderio for SEM analysis. This work was supported by Air Force Office of Scientific Research (AFOSR), Air Force Research Laboratory (AFRL), U.S. Air Force, under grant FA9550-14-1-0050 (EOARD 2014/2015).

References (1) Ikeda, T.; Tsutsumi, O. Optical switching and image storage by means of azobenzene liquid-crystal films. Science - New York then Washington 1995, 1873. (2) Wicks, G.; Gupta, M. Handbook of Photonics. 1997. (3) Finkelmann, H.; Nishikawa, E.; Pereira, G.; Warner, M. A new Opto-Mechanical Effect in Solids. Phys. Rev. Lett. 2001, 87, 015501. (4) Ikeda, T.; Nakano, M.; Yu, Y.; Tsutsumi, O.; Kanazawa, A. Anisotropic Bending and Unbending Behavior of Azobenzene Liquid-Crystalline Gels by Light Exposure. Adv. Mater. 2003, 15, 201–205. (5) De Sio, L.; Serak, S.; Tabiryan, N.; Ferjani, S.; Veltri, A.; Umeton, C. Composite Holographic Gratings Containing Light-Responsive Liquid Crystals for Visible Bichromatic Switching. Adv. Mater. 2010, 22, 2316–2319. (6) De Sio, L.; Cuennet, J. G.; Vasdekis, A. E.; Psaltis, D. All-Optical Switching in an Optofluidic Polydimethylsiloxane: Liquid Crystal Grating defined by Cast-Molding. Appl. Phys. Lett. 2010, 96, 131112. (7) De Sio, L.; Serak, S.; Tabiryan, N.; Umeton, C. Mesogenic versus Non-Mesogenic Azo Dye Confined in a Soft-Matter Template for Realization of Optically Switchable Diffraction Gratings. J. Mater. Chem. 2011, 21, 6811–6814.


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Conformal Silk-Azobenzene Composite for ... - ACS Publications

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