Starch-Based Biological Microlasers Yanhui Wei, Xianqing Lin, Cong Wei, Wei Zhang, Yongli Yan,* and Yong Sheng Zhao* CAS Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *
ABSTRACT: Microlasers with good biocompatibility are of great significance to the detection of tiny changes in biological systems. Most current biolasers were realized through the introduction of biomaterials into various external resonators, resulting in an increase of difficulties in application. Here, we used starch as the host to build dye@starch microlasers by encapsulating guest organic laser dye into the interhelical structure of starch granules. The asprepared dye@starch system with high transparency and ultrasmooth spherical surface functions as an efficient whispering gallery mode resonator for low threshold lasing. The obtained laser signal is closely related to the structural transformation of the starch matrix. Our results would provide a deep insight into the relationship between biostructure and lasing properties, facilitating the monitoring of the structural variances in biological processes through lasing signals. KEYWORDS: organic laser, biological composite material, starch laser, organic nanophotonic material, bio-nanophotonics
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structures endow starch polymers with the ability to adsorb many functional substances, including iodine ions, metal ions, and organic dyes. These host−guest systems can be used as functional materials for optical devices.16,17 Depending on the source of starch, starch granules have the appearance of sphere, ellipsoidal, or polygonal shapes with characteristic sizes ranging from hundreds of nanometers to 150 μm.15,18 These starches, especially the natural spherical or ellipsoidal microgranule, would act as whispering gallery mode (WGM) resonators of high quality to provide sufficient optical gain for lasing.19,20 These starch-based biolasers would avoid the complexity and bulkiness resulting from the mirror-like cavities, providing an interesting complement to earlier WGM biolasers within living cells.5,9 Moreover, the starches can respond to environmental variations in terms of structural transformation, which might change the optical properties and act as a signal for the detection of tiny structural variations induced by external stimuli. In this paper, we report a type of biolaser based on the interhelical inclusion of laser dye in starch granules, which can generate distinct optical signals along with structural transformation. The abundant hydroxyl groups make it easy to dope organic dyes into the starch granules due to strong electrostatic interactions. The limited molecular motion in dye@starch (DS) plays a major role in enhancing the photoluminescence (PL) emission. With ultrasmooth surface and high transparency, the
icrolasers have attracted great research interests due to their huge applications in photonics,1−3 medicine,4 imaging,5 sensing,6,7 and so forth.8 Microlasers with good biocompatibility9 have received increasing attention due to their high resolution and sensitivity,6,10,11 which can be utilized to quantify tiny changes in biological processes. In order to elevate the biocompatibility, researchers have introduced biological materials as host matrices for their excellent compatibility with organic gain medium, resulting in a host−guest biological system. Until now, some biomaterials or biocompatible materials, such as protein and gelatin, have been utilized for the construction of biological lasers.10,12 The occurrence of diverse biological lasers enriches the types of lasers and expands the applications in biological fields. Unfortunately, most of the existing biological lasers are realized on the basis of extra external resonators, such as bulk dielectric mirrors and microfluidic devices, to provide sufficient feedback for laser oscillations. The introduction of mirror-like cavities not only decreases the biocompatibility but also increases the size of microscale biolaser to millimeter or centimeter level, which limits the microregion detection in the bioenvironment. Hence, exploring suitable biomaterials, with the ability of forming resonator and encapsulating gain medium, would be a promising way to fabricate miniaturized biolasers. Starch and its derivatives, as a kind of common biopolymers in many plants, have been widely used in various biological fields, due to the good enzyme resistance and unique helical structure.13−15 The basic unit of starch, helix, is composed of αD-glucose chains that contain rich hydroxyl groups on the outside of the helix. The abundant hydroxyl groups and helical © 2016 American Chemical Society
Received: October 8, 2016 Accepted: December 13, 2016 Published: December 13, 2016 597
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encapsulated DASP+ molecules were uniformly distributed in the whole starch granules, as verified by the fluorescence images at different depths (Figure S4). The scanning electron microscopy (SEM) images (Figure 1C,D) show that these DS granules possess perfect ellipsoidal shapes and ultrasmooth surfaces, which would advance the quality factor (Q) value of cavities for lasing.25 The enhanced fluorescence and perfect shape enable the ICT DS system to be a good candidate for high-performance microlasers. When a 400 nm femtosecond pulse laser (∼200 fs) was focused on the center of a single granule (Figure S5), bright orange light was emitted along the perimeter of the microellipsoid (Figure 2A), suggesting a strong microcavity
starch granules provide sufficient feedback for laser oscillations. These as-fabricated DS microgranules can lase at a low pumping intensity without optical damage, and the lasing behavior can be modulated through starch structural transformation. These results are beneficial for understanding the relationship between starch microstructures and the lasing signals, boosting the monitoring of structural transformation in biological processes through lasing characteristics.
RESULTS AND DISCUSSION Potato starch was selected as the model biopolymer due to the ellipsoidal shape as well as sensitive response to external stimuli.21 The natural ellipsoidal appearance can serve as WGM resonators to provide sufficient optical feedback. Potato starch granules are composed of the quasi-linear α-(1,4)-glucose polymer, amylose (AM), and the highly branched α-(1,4)glucose polymer with α-(1,6)-bonds, amylopectin (AP). These chain structures easily curl into six-folded left-handed helices with hydroxyl groups on the outside, facilitating the formation of a hydrophilic external environment along with a hydrophobic internal cavity. These cavities are beneficial for the intra- or interhelical inclusion of guest dyes, which would enhance the possibility of building organic microlasers based on starch granules (Figure S1). Herein, a cation dye, 4-[p-(dimethylamino)styryl]-1-methylpyridinium (DASPI), was chosen as optical gain material. A DASP+ molecule holding a positive charge tends to be trapped in the hydrophilic interhelical space of starch through strong electronic interaction (Figure 1A and Figure S2).22 Moreover,
Figure 2. Lasing property of a single DS granule. (A) Schematic illustration of a single DS microellipsoid excited with a focused 400 nm femtosecond pulse laser. (B) Simulated two-dimensional electric field intensity distribution (λ = 608 nm, ng = 1.60) inside the cavity of a DS granule (major axis 6 μm × minor axis 4 μm), showing a typical WGM mode. (C) PL spectra recorded at the edge of a microellipsoid as a function of pump density. Inset: PL image of the starch granule (major axis 67 μm × minor axis 45 μm) under laser excitation. Scale bar is 40 μm. (D) Plot of PL peak intensity as a function of pump density.
effect. The simulated 2D electric field intensity distribution shown in Figure 2B further verifies that the ellipsoidal particle is a typical WGM microcavity which sustains a traveling-wave optical field in the 2D plane. Figure 2C presents the PL spectra of a single starch granule with dimensions of 67 μm (major axis) × 45 μm (minor axis) under different pump densities. At low pump intensity, the selected starch granule exhibits a weak and broad spontaneous emission. With an increasing pump intensity, a number of sharp peaks emerge and the PL intensity increases rapidly, displaying clearly a threshold of about 1.26 nJ μm−2 (Figure 2D). Furthermore, the full width at halfmaximum (fwhm) at 610 nm dramatically narrows to 0.2 nm with the increase of pump density, displaying strong cavity effect and lasing effect.25 The lasing characteristics can be well tuned by altering the size of the DS granules. Figure 3A presents the fluorescence spectra and corresponding images of three typical DS granules with different dimensions. The resonantly modulated PL spectra were collected from the edge of DS granules with different sizes to investigate the cavity effects. The number of modes exhibits a conspicuous positive correlation with the
Figure 1. Design and structural characterization of DS granules. (A) Schematic illustration of interhelical inclusion of DASP+ dye in starch granules. The double helix (blue and yellow) stands for the helical structure in potato starch. (B) Fluorescence microscopy image of the DS granules excited with UV light from a mercury lamp. Scale bar is 40 μm. (C) SEM image of the DS granules. Scale bar is 40 μm. (D) Magnified SEM image of the selected area in C. Scale bar is 20 μm.
the optical properties of such molecules are highly sensitive to the microenvironment because of their typical intramolecular charge transfer (ICT) process, affording a way to detect the structural variances of the starch host.23 When the ICT dye was embedded in the starch cavities, the high confinement from the interhelical space would minimize the nonradiative loss by restricting the intramolecular torsional motion without destroying the crystal structure of starch (Figure S3).24 Therefore, the DS system emitted strong orange fluorescence under the UV light excitation from a mercury lamp (Figure 1B). Here, the 598
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starch possesses a relatively dense pack pattern (a = 2.124 nm, b = 1.172 nm, c = 1.069 nm, monoclinic) of double helices and combines eight H2O molecule in one unit cell. Comparatively, B-type starch, for example, potato starch, has a loose packing pattern (a = b = 1.85 nm, c = 1.04 nm, hexagonal), with 36 H2O molecules combined in one unit cell.29 This distinct content of crystal water will highly influence the type of packing pattern of the double helices and the phase transition. Particularly, B-type potato starch would transform to A-type phase via dehydration (Figure 4A).18 Here, freeze-drying (FD)
Figure 3. Size-dependent WGM lasing. (A) PL spectra of the DASP+-doped starch granules with three different sizes. Corresponding PL images are depicted as insets. All scale bars are 20 μm. (B) Relationship between Δλm and 1/L of the DS granules. L is the cavity length. The red line is a fitting to the function Δλm = λ2/ngL, λ = 608 nm. (C) Experimental hot Q factor versus L of different DS granules.
granule size, and the mode spacing (Δλm) between two adjacent lasing modes decreases as the size of starch granule increases. The mode spacing versus cavity length is in reasonable agreement with that given by the equation Δλm = λ2/ngL,26 where ng is the group refractive index and L is the cavity length calculated according to the formula shown in Figure S6. For this DS system, the group refractive index ng is determined to be 1.6 through the linear fitting between Δλm and 1/L at λ = 608 nm (Figure 3B), which is consistent with the intrinsic refractive index of the starch polymer (1.53),27 indicating that the optical modulation is indeed originated from the photon confinement of the WGM resonators. Such a value is high enough to produce strong cavity optical confinement. The Q value was determined to be greater than 103 (Q = λ/ Δλfwhm, Δλfwhm is the fwhm of peak above lasing threshold, Figure 3C).28 Here, the Q factor obtained is a “hot Q factor” that represents a combination of the quality of the passive cavity with further spectral narrowing originating from the lasing effect. The WGM lasing performances would be influenced not only by the resonator size but also by the change in the microenvironments because the confinement effect of the microsphere resonators is sensitive to the alteration in the surrounding medium.29 Besides, biomaterials have responses to various external stimuli, which are usually accompanied by the physical or chemical changes of biostructures and thus the modulation of optical properties.30 This structure-related optical behavior boosts the development of multifarious optical sensors with WGM resonators. As a kind of natural microsphere resonator, starch granules show distinct structural transformation in response to temperature, acid−base, enzyme, etc.18,31 By controllably inducing the structural transformation, the lasing properties would be modulated because of the structure-sensitive refractive index. These obtained structurerelated optical actions will be helpful for the development of the application in optical biodetection. Depending on the botanical source, the starch crystallites can adopt two different structures, namely, A-type and B-type. The fundamental A- and B-types are both based upon 6-fold lefthanded double helices with a pitch of 2.08 × 2.38 nm. A-type
Figure 4. Blue-shifted lasing wavelength resulting from structural transformation of DS granules. (A) Schematic illustration of structural transformation from B-type to A-type in potato starch via dehydration. (B) X-ray diffraction patterns of native B-type potato starch and the starch after FD treatment. (C) Lasing spectra of a single granule before (native DS, Δλm = 4.5 nm, black line) and after FD treatment (FD DS, Δλm = 3.2 nm, red line). (D) Plots and fitted lines of the group refractive index (ng) versus the wavelength of native DS and FD DS granules with different sizes.
was selected to initiate the structural transformation because this method has little effect on the dye content as well as the external morphology (Figure S7). After 12 h of freeze-drying under high vacuum at −50 °C, we collected the final well-defined DS products. The X-ray diffraction (XRD) pattern (Figure 4B) of FD DS contains a series of peaks at 15, 17, 18, and 23° (the XRD pattern of FD DS is in agreement with that of A-type starches), which are completely different in comparison with those (peaks at 5.6, 17, 22, and 24°) of B-type native DS.18 It is worth mentioning that the percentage of crystallinity of the FD DS is about 60% of that of the native DS according to the results calculated from the ratio of the total peak area to the total diffraction area (Figure S8). It can be attributed to the disruption of the crystalline arrangement (long-range-ordered structures) and/or the reduction in the amount of double helices (short-rangeordered structures) during the FD process.32 Solid-state NMR spectroscopy was carried out to determine whether the packing pattern of FD starches belongs to the A-type crystal form (Figure S9).33 In contrast to the distinctive B-type polymorphic signals at 110.79 and 99.72 ppm displayed in the C1 region of native starches, FD starches displayed a different spectrum with a triplet in the C1 region at 101.6, 100.4, and 99.3 ppm, which were typical peaks of the A-type double-helical conformation. These results further confirm the phase transition of the potato starches from B-type to A-type through FD treatment. This 599
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provide guidance for a better understanding of the relationship between the microcavity and lasing performance, and also help to develop microscale biolaser-based optical devices for the tracing of biological activities.
typical phase transition in potato starch provides a platform to study the structure-related optical performances. The induced structural transformation from B-type to A-type effectively decreased the interhelical cavity volume, which therefore exacerbated the limited motion of adsorbed DASP+ molecules. As a kind of microenvironment-sensitive laser dye, DASP+ would sensitively perceive and respond to the changes of surrounding environment.34,35 The wavelength of DS granules has an 8 nm blue shift after dehydration treatment, which is possibly due to the reduced polarity of FD starch induced by the loss of the crystal water. The restricted motion of ICT dye in limited space inhibits the nonradiative loss and hence strengthens the fluorescence emission of FD DS, which is very favorable for the optical gain and high efficiency WGM lasing behaviors (Figure S10). Besides the wavelength and PL intensity, the phase transition also influenced the resonant mode. For example, a single native DS granule with a cavity length L = 50 μm exhibited distinct multimode lasing under 400 nm pump laser before and after dehydration. The spacing Δλm between adjacent lasing modes is about 3.2 nm in FD DS, which showed a decrease of 1.3 nm in comparison with the mode space (4.5 nm) in native DS (Figure 4C). These diverse lasing modes in a single DS granule can be understood in terms of the perturbation of refractive index during starch phase transition because the shapes of starch granules were nearly unchanged after FD treatment (Figure S11).36,37 Here, we selected a number of native and dehydrated DS granules of different sizes randomly to explore the lasing properties. The refractive index was calculated according to the formula ng = λ2/ΔλmL. The obtained values and fitted lines of the group refractive index versus the wavelength are depicted in Figure 4D. The ng of both native DS and FD DS granules shows a slight decrease with the increase of wavelength, which is identical in all granules with different sizes.38 This tendency can be attributed to the normal dispersion.39 What is more, the DS granules displayed different values of ng before and after dehydration treatment. The value of ng ranges from 1.5 to 1.7 for native DS, and in comparison, a higher value of ng between 1.8 and 2.3 was obtained after dehydration treatment, which results from the dehydration-induced denser packing of the internal structure. It can be seen that the refractive index of the as-prepared starch resonator is very sensitive to the microstructure. As a result, the WGM lasing performance can be effectively modulated by controllably inducing the structural transformation of the starch-based optical system. Thus, these types of biological microlasers can be applied to trace the tiny changes of biostructures by using the lasing property as the detected signal. In addition, the relative refractive index changes when the starch granule is immersed in a different solution environment, such as a water-based biological environment, and the lasing properties will be influenced accordingly.
MATERIALS AND METHODS Potato starch granules were extracted from a potato purchased at a local market. The laser dye 4-p-(dimethylamino)styryl]-1-methylpyridinium (DASPI) was purchased from Aldrich Chemical Co. and used without further purification. Extraction Process of Potato Starch Granules. One potato was sliced into plates and dispersed in pure water with vigorous shaking. Potato granules were separated with a microporous sieve (d = 100 μm). The crude potato granules were further purified through centrifugation (4000 rpm for 5 min) three times by pure water. Finally, the as-prepared native potato granules were collected and dried in air. Preparation of Native Dye@Starch and Freeze-Dried DS Granules. Ten milligram starch granules were immersed in 5 mL of DASPI-saturated aqueous solution at ambient temperature for 24 h. Subsequently, the granules were filtered off, washed with pure water until no characteristic emission was observed in the filtrate upon excitation, and then dried naturally. The FD DS granules were prepared through vacuum freeze-drying (−50 °C, 12 h) of native DS. Note that the native products were frozen before FD treatment. Structural and Morphological Characterization. The morphologies of DS and FD DS starches were examined with scanning electron microscopy (NOVA Nano SEM 430, low vacuum). X-ray diffraction (Japan Rigaku D/max-2500) was measured with Cu Kα1 radiation. 13C CP/MAS NMR spectra were recorded on a Bruker AVANCE-600 NMR spectrometer. Optical Measurements. The fluorescence and diffused reflection spectra were measured with Hitachi F-7000 and Shimidazu UV-2600 spectrophotometers, respectively. The schematic illustration of the experimental setup for optical characterization is shown in Figure S5. PL images and confocal images were taken with an inverted microscope (FluoView-500, Olympus). The granules on a silica substrate were excited locally with a Ti:sapphire laser (400 nm, ∼200 fs, 1 kHz, Spectra Physics) focused down to a 12 μm diameter spot. The excitation energy density was altered with neutral density filters. The collected light was subsequently coupled to a grating spectrometer (Acton SP-2358, Princeton Instruments) and recorded with a thermal electrically cooled CCD (ProEm: 1600B, Princeton Instruments).
ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b06772. Helical structure of starch, dye adsorption performances, X-ray powder diffraction patterns, confocal images, detailed experimental setup for optical characterization, calculation of DS cavity, and differences between native and FD DS granules in SEM images, NMR spectra, XRD patterns, as well as lasing properties are given in Figures S1−S11 (PDF)
CONCLUSIONS In summary, we have developed a type of biological microlaser via the interhelical inclusion of organic laser dye in starch granules. The interhelical inclusion enhanced the confinement effect to ICT dye and elevated the emission efficiency. These DS granules with an inherent ellipsoidal shape serve as highquality WGM resonators to sustain low-threshold lasing. Moreover, the lasing action of the biosystem has a very sensitive response to the structural transformation of the starch from B-type to A-type, which can be utilized for monitoring the biological processes with high sensitivity. These results would
AUTHOR INFORMATION Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Yong Sheng Zhao: 0000-0002-4329-0103 Author Contributions
Y.W. conceived the idea and designed the experiments. Y.Y. and Y.S.Z. initiated the concepts. Y.W., X.L., and C.W. conducted 600
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The authors declare no competing financial interest.
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