Biomaterial Disk Lasers by Suppressing the Coffee Ring Effect - ACS

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Biomaterial Disk Lasers by Suppressing the Coffee Ring Effect Itir Bakis Dogru, Rustamzhon Melikov, and Sedat Nizamo#lu ACS Biomater. Sci. Eng., Just Accepted Manuscript • DOI: 10.1021/acsbiomaterials.8b00917 • Publication Date (Web): 15 Oct 2018 Downloaded from http://pubs.acs.org on October 16, 2018

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ACS Biomaterials Science & Engineering

Biomaterial Disk Lasers by Suppressing the Coffee Ring Effect I. Bakis Dogru, 1 Rustamzhon Melikov, 2 Sedat Nizamoglu 1,2,3,* Graduate School of Biomedical Sciences and Engineering, Koc University, Sariyer, Istanbul, 34450 Turkey

1

Department of Electrical and Electronics Engineering, Koc University, Sariyer, Istanbul, 34450 Turkey

2

KUYTAM Surface Science and Technology Research Center, Koc University, Sariyer, Istanbul, 34450 Turkey

3

*Corresponding author: [email protected]

Abstract Inspired by the suppression of the coffee ring effect, we developed selfassembled disk lasers that can be formed with a wide variety of biomaterials. For proof of concept, we formed the disks with the natural protein silk fibroin or the synthetic biopolymer polyvinylpyrrolidone, which created a whispering gallery mode resonator that we combined with organic dyes for laser light generation. The lasers were flexible enough to bend around surfaces, physically transient in aqueous environments, and could be directly placed on various substrates. Moreover, the characteristics of laser emission could be modified by altering the size of the disk. Our results therefore highlight a new combination of materials that can be used in the environmentally friendly production of waste-free photonic devices.

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Keywords: biopolymer, self-assembly, microcavity, disk resonator, whispering gallery mode

Introduction Lasers exhibit exceptional characteristics, including tunable color, narrow linewidth, high intensity, and long coherence length, that have been applied to revolutionize processes in fields such as material processing, medicine, biology, and telecommunications1-6. In those applications, inorganic compounds, including semiconductors and doped crystals, have generally been used as gain media7-8. In addition, the resonators have consisted of sensitive optomechanical parts or special reflectors made of metals, dielectrics, or plastics. The gain and cavity materials, are usually inelastic, unbendable, and manufactured in processes that often require toxic precursors in vacuum environments at high temperatures2, 9-11. In response to those challenges, biomaterials, given their unique properties, can be used to produce flexible and ecofriendly devices. Obtainable from both natural sources (e.g., gelatin) and chemical synthesis (e.g., polylactic acid), biomaterials are sustainable. They are also environmentally friendly—their waste does not generate harmful environmental effects—and can be biodegradable, due to their spontaneous degradation from bulk form into molecules. Further still, because biomaterials are easy to process, they have 2


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been employed as photosensitive materials

12-13.

Solid microspheres,

microdroplets made entirely of biomaterials, and fluorescent protein coffee rings have all been used as whispering gallery mode (WGM) lasers

14-17.

Furthermore, distributed feedback resonators18-20 and random lasers21-22 were reported. Although disk lasers in particular offer the beneficial properties of low thermal lensing, excellent heat dissipation, and the amplification of ultrashort pulses 23-24, disk lasers made with biomaterials have yet to be created. Inspired by the suppression of the coffee ring effect, we sought to develop selfassembled, biopolymer-based, WGM disk lasers. To that end, we used biopolymers such as silk fibroin and polyvinylpyrrolidone (PVP) as the host materials for the cavities and fluorine dyes as gain media. Ultimately, our single-step fabrication of truly circular disk lasers did not require high processing temperatures or toxic precursors, and we could control the laser emission of the disks by altering their size. Our versatile approach can moreover be used with a range of biomaterials to generate self-assembled, flexible, physically transient disk lasers.

Results Disk Production

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We fabricated disk microresonators by suppressing the coffee ring effect

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25.

Using transparent and biocompatible biopolymers of silk fibroin and PVP 26-27 as host materials to form the disks, we doped the biopolymers with the synthetic dyes rhodamine B (RB) and rhodamine 6G (R6G) to generate gain media. To fabricate the microresonators, we heated the droplets on a polydimethylsiloxane (PDMS) substrate in an aluminum foil cap with a standard incandescent lamp (42 W, OSRAM CLASSIC 230 V), as illustrated in Figure 1a. We used the lamp because it can produce heat and light simultaneously, which enables both the adjustment of the environmental temperature within the necessary range of 45–90°C and the visualization of the processes under illumination. We placed the disks inside the aluminum cap when the temperature reached 45°C and maintained a distance of 1.5 cm between the lamp and the droplets. Figure 1b depicts the disk formation of the droplets. After placing the droplets containing the silk fibroin and RB mixture onto the surface (t = 0), we observed a contact angle of 94.4 ± 1.8 and the beginning of liquid evaporation. As characteristic of the coffee ring effect, coffee ring formation commenced at the pinned edges, and the particles moved toward the pinned edges due to the capillary flow (t = 1 min). Next, the polymers began to coagulate due to evaporation, which started from the edge and continued toward the center of the droplets (t = 1–3 min). Once the droplets had completely evaporated, a disk made of biomaterial and dye formed (t = 4 min). When we used heater or oven to heat the droplets, we obtained same results due to increase of water evaporation rate and suppression 4


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of coffee ring effect. Numerous disks can be produced by using that method, and Figure 1c presents the RB- and R6G-doped silk fibroin disks. The scanning electron microscopy (SEM) image of the silk fibroin disks show that the surface was highly smooth (Figure 1d), which corroborated the surface profile measured with an atomic force microscope (AFM) as well (Figure S1a and S1b). We further analyzed the disk structure, as shown in Figure 1d. The AFM measurement taken from the side of the disk revealed that the disk had a thickness of 10 µm, and the side wall presented a smooth convex profile that allowed light oscillation via total internal reflection (Figure 2a). We also analyzed disk samples by using attenuated total reflectance (ATR) Fouriertransform infrared spectroscopy (FTIR) to understand the molecular structure of the silk fibroin in the disk (Figure 2b). In a control group, spin coating the silk solution resulted in an amorphous structure with an amide I peak at approximately 1,641 cm-1, whereas the methanol treatment of the amorphous silk film shifted the peak to approximately 1,622 cm-1 due to β-sheet crystal formation 28. The self-assembled disk thus showed an amorphous silk structure similar to the spin-coated films. Next, we investigated the effect of temperature and biomaterial concentration on disk formation (Figure 2c). We observed conventional coffee rings at a temperature of 26°C. At 45°C, the coagulated polymers on the surface of the droplet filled inside the ring, which generated a

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disk structure. At 65°C, faster evaporation limited the time for particle deposition, and the surface of the disks became deformed. Accordingly, we determined the appropriate starting temperature for disk formation to be 45°C, at which the solution’s concentration also directly affected disk formation. As the concentration decreased from 8%, at which disk formation could properly occur, the amount of polymer colloid that can fill the inside of the coffee ring decreased which resulted in regular coffee rings.

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Figure 1 (a) Schematic of the setup for disk production, which consists of an incandescent lamp covered by aluminum foil. When placed on a hydrophobic surface inside the aluminum cap, the droplets form a disk after evaporation. (b) The transformation of the RB–silk fibroin solution from a droplet to a disk structure on PDMS as a function of time. (c) Arrays of RB- and R6G-doped silk fibroin disks on PDMS under ultraviolet light. (d) The SEM image of a silk fibroin disk.

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Figure 2 (a) AFM images of a silk fibroin disk. The parts close to the edges were measured to observe the thickness of the disk. (b) ATR–FTIR of spincoated, methanol-treated silk films and the disk. (c) White light interferometric images of coffee rings formed at different temperatures and concentrations. The y-axis, representing the height profile in the z-direction, is normalized with respect to the different fabrication conditions for the maximum height of each disk, whereas the x-axis, representing the position of each height, is normalized with respect to the measured length from the edge of the disks. The legends refer to the concentration of the silk fibroin solution and the initial fabrication temperature of the disks.

WGM Disk Lasers

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We conducted laser experiments with the optical setup shown in Figure 3a. To pump the disks, we used a laser of 10 Hz Nd:YAG with a pulse width of 5–8 ns and adjusted the wavelength to 482 nm by using an optical parametric oscillator. We altered the input laser energy by using optical density filters from 1.36 nJ/µm2 to 43 nJ/µm2. We collected the emitted light from the disk by using a fiber placed to the side of the sample and measured it by using a spectrometer with a spectral resolution of approximately 1 nm. Figure 3b depicts the absorption and photoluminescence of RB inside the silk fibroin solution. Since the absorption and emission effectively intersected in the interval from approximately 550 nm to 590 nm, the laser emission was more observable at longer wavelengths on the emission spectrum, at which the absorption had less strength. Figure 3c depicts the emission of a RB-doped silk fibroin disk 517.3 µm in diameter on a PDMS layer under different pulse energies. As the pump energy increased, spectrally narrow emission peaks became observable at above 600 nm. We also observed threshold behavior; the integrated output energy increased rapidly above 10.8 nJ/µm2 (Figure 3d). The inset of Figure 3d presents photographs of a disk excited below and above the lasing threshold pump energy, respectively. Next, to assess the laser emission, we cut the same disk from the circumference and repeated the experiments. Since WGM lasers confine the light inside the cavity produced by total internal reflection (TIR) in the periphery, structural discontinuity at the edge prevents the light oscillation inside the cavity by increasing the losses, which halts mode formation and laser 9



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emission. Figure S2 presents the microscopy images and emission spectra of the disk before and after cutting. The emission of the cut disk thus shows a broad emission profile instead of narrow laser emission peaks, as well as a linear dependence as the pump energy increases instead of threshold behavior (Figure 3d), which indicates that the laser emission occurred via a WGM resonator, not a Fabry–Pérot interferometer. Figure S3 also shows the lasing behavior of an R6G-doped silk disk and an RB-doped disk. Figure 3e depicts the time-dependent laser intensity of a RB–silk fibroin disk. We pumped the disk with an energy density of 43 nJ/µm2 in 10-Hz pulses for 20 min, and after 12,000 shots, photodegradation of the RB dye was clearly observable

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Figure 3 (a) Schematic illustration of the optical setup. (b) Absorbance and photoluminescence spectrum of RB in silk fibroin solution. (c) Emission spectra of the RB–silk fibroin disk under different pump energy densities and (d) its integrated fluorescence intensity in terms of input energy before and after cutting. (e) Time-dependent lasing intensity of a RB–silk fibroin disk. The disk was pumped with 10-Hz pulses for 20 minutes at an energy level of 43 nJ/µm2.

Size Dependent Analysis of WGM Disk Lasers

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We assessed the lasers by altering the disk size. We fabricated disks with diameters of 267.4 µm, 517.3 µm, and 701.4 µm and investigated their lasing properties (Figure 4a, c, and e). In WGM cavities, the confined wave coherently added to itself when it completed each loop. As the size of the disks increased, the optical path that the light traveled also increased. Thus, the light with lower spectral overlap between emission and absorption experienced a higher gain along a longer optical path. For small disks, it was more difficult to confine light with high curvature, since possible defects on the surface could disturb TIR 29. As a result, outcoupling efficiency increased and blue shifts emerged, which prompted additional losses to lower wavelengths 30. Therefore, the laser emission shifted toward reddish wavelengths as the diameter of the disks increased. The free spectral range (FSR) decreased as diameter increased at a rate of FSR = λ2 / nπD, in which n is the refractive of the cavity and D is the diameter of the cavity. For example, for a disk with a diameter of 267.4 µm, the FSR corresponded to approximately 0.3 nm, which was lower than the spectral resolution of the spectrometer (approximately 1.0 nm). Therefore, we observed a collective emission with high spectral overlap between the multiple modes. Moreover, as the size of the disks increased, the separation of the observed lasing modes clearly became closer to each other due to the reduced FSR. The threshold was inversely proportional with the size (Figure 4b, d, and f).

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Figure 4 RB-doped silk fibroin disk emission spectra with a diameter of (a) 267.4 µm, (c) 517.3 µm, (e) 701.4 µm, and their corresponding output energy in terms of pump energy, in (b), (d) and (f), respectively.

Physically Transient WGM Disk Lasers The biopolymer of the PVP was a material generally recognized as safe, according to the U.S. Food and Drug Administration, and widely used in contact lenses, pharmaceutical tablets, and food additives. We fabricated a selfassembled disk by using PVP doped with RB and analyzed the structure of the PVP disk with a white light interferometer (WLI). Unlike with silk fibroin

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disks, we observed that PVP disks had a slightly convex surface (Figure 5a). In coffee ring formation, because particle size affected the contact line pinning, different polymer sizes, adhesion forces between the particles, and the surface energy of the molecules influenced the deposition of the particles inside the ring

31.

To reduce the size of the disk, we sprayed the RB–PVP solution on a

PDMS surface and thereby obtained small disks. Figure 5b shows the emission spectrum of a 45.8-µm RB–PVP disk. At that size, the disk accommodated WGM mode analysis, and we calculated the actual mode number according to the formula λm = 2πrn / m, in which r is the radius of the disk, n is the refractive index of the PVP cavity (1.53), and m is the mode number. (Calculations appear in the supporting data.) When compared with the calculated theoretical modes, the peak positions agreed with the mode numbers from 336 to 344 (Figure 5c). Moreover, since PVP is a water-soluble biopolymer, after we placed the fabricated disk inside an aqueous environment, the disk laser completely dissolved in less than 1 min (Figure 5d). We could also detach the disks from the surface and transfer them to other surfaces as well as onto different substrates. Figure 6a displays the image of a RB–PVP disk 841.9 µm in diameter detached from the PDMS and stuck on tape (MAS® Invisible Tape) rolled on a circular rod with a radius of approximately 1 mm (Figure 6b). The RB–PVP bent disk also generated a WGM laser as the pump energy increased (Figure 6c).

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Figure 5 (a) The WLI image of a PVP disk. (b) Emission spectrum of a RB– PVP disk and (c) calculated mode numbers. (d) Left of the dashed line: A RB– PVP disk on a PDMS surface; right of the dashed line: 20 µL water poured on the disk, which dissolved in 22 s.

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Figure 6 (a) RB–PVP disk structure on a flat and (b) rolled tape (MAS® Invisible). (c) Emission spectrum of a rolled RB–PVP disk at different pump intensities.

Conclusions We developed self-assembled disk lasers obtained by suppressing the coffee ring effect, a method that is suitable for mass producing disk lasers by using a wide variety of biomaterials. As proof of concept, we used the biomaterials silk fibroin and PVP as the construction materials of the disk cavities and organic dyes as optical gain media. For gain material, fluorescent proteins, laser dyes, biomarkers, and vitamins (e.g., riboflavin) can be used at designated 16


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wavelengths of the emission and with specific applications. By altering the size of the disks, we could control laser emission wavelengths and thresholds. Ultimately, the fabricated disks were flexible and detachable; even when bent or placed on another surface, the disks demonstrate laser emission. Given those properties, disk resonators can be placed on different surfaces and transferred as a building block in transient electronics and photonics for the waste-free production of devices.

Experimental Surface Preparation We prepared PDMS by using Sylgard® 184 Silicone Elastomer kit. Preparation of RB doped aqueous silk fibroin solution To 2 L of 0.02 M aqueous Na2CO3 solution boiled, we added 5 g Bombyx Mori silk cocoons and left the mixture to heat for 30 min, which separated the sericin from the fibroin. We rinsed extracted silk fibroin and stirred it in deionized water for 20 min twice that we changed between each process. After the silk fibroin air-dried, we dissolved it in a 9.3-M LiBr aqueous solution and heated it in an over at 60 °C for 4 h. During the subsequent dialysis of the mixture to separate LiBr and other contaminants, the mixture inside the dialysis cassette remained in deionized water for 2 days. In a final step, we centrifuged the silk 17



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fibroin solution at 9,000 rpm for 20 min at -2 °C 32, after which we measured the final concentration of the solution to be about 7–9 wt%. Accordingly, we mixed 0.13 wt% RB–water solution with the silk fibroin solution at a volume ration of 1:25. Preparation of RB-doped aqueous PVP solution To prepare RB-doped aqueous PVP solution, we mixed PVP10 (Sigma– Aldrich) with water to 7.6 wt% and, later, each 100 µL of PVP solution with 123.5 µg of RB dye. Characterization techniques We analyzed the surface morphology of the disks with an SEM (Zeiss EVO LS15) operating at 2 kV and analyzed their topography and roughness with an AFM (Dimension Icon Bruker) operating in a tapping mode in a scanning area of 1 µm × 1 µm. We recorded ATR–FTIR spectra measurements with an FTIR spectroscope (Thermo Scientific iS10) equipped with a single reflection diamond ATR and scanned each spectrum 16 times. Last, we measured WLI with a white light profilometer (Bruker Contour GT-K0 3B).

Acknowledgements

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S.N. acknowledges the Turkish Academy of Sciences and the Scientific and Technological Research Council of Turkey (TUBITAK) under projects 114F317, 115E242, 115F451, and 114E194. S.N. acknowledges the support by Marie Curie Career Integration Grant (PROTEINLED, 631679).

Associated Content The Supporting information is available free of charge on the ACS Publications website at DOI:…..

Author Information ORCID Itir Bakis Dogru: 0000-0001-8569-7625 Rustamzhon Melikov: 0000-0003-2214-7604 Sedat Nizamoglu: 0000-0003-0394-5790

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I. Bakis Dogru, 1 Rustamzhon Melikov, 2 Sedat Nizamoglu 1,2,3,*

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Biomaterial Disk Lasers by Suppressing the Coffee Ring Effect - ACS

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