âFreezingâ of NaClO3 Metastable Crystalline State by Optical Trapping

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“Freezing” of NaClO Metastable Crystalline State by Optical Trapping in Unsaturated Microdroplet Hiromasa Niinomi, Teruki Sugiyama, Katsuhiko Miyamoto, and Takashige Omatsu Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01116 • Publication Date (Web): 07 Dec 2017 Downloaded from http://pubs.acs.org on December 13, 2017

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Crystal Growth & Design

“Freezing” of NaClO3 Metastable Crystalline State by Optical Trapping in Unsaturated Microdroplet

Author list:

Hiromasa Niinomi†,*, Teruki Sugiyama‡,§, Katsuhiko Miyamoto†,ǁ, and Takashige Omatsu†,ǁ

Affiliations: †

Molecular Chirality Research Center (MCRC), Chiba University, Chiba, Japan Department of Applied Chemistry, National Chiao Tung University, Hsinchu, Taiwan § Graduate School of Materials Science, Nara Institute of Science and Technology, Nara, Japan ǁ Graduate School of Engineering, Chiba University, Japan ‡

*

Corresponding Author

Name: Hiromasa Niinomi, Telephone number: +81-43-290-3475, Fax number: +81-43-290-3490 E-mail address: [email protected]

ACS Paragon Plus Environment

Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract: We reversibly controlled phase conversion between a microdroplet of a NaClO3 unsaturated aqueous solution and a metastable single crystal, which is usually short-lived phase in spontaneous crystallization, simply by irradiating a tightly-focused visible continuous-wave (CW) laser to the microdroplet. The laser irradiation allowed the metastable crystal to generate and stably grow without a polymorphic transformation. This successful metastable phase control is attributed to the combination of the advantage of optical trapping-induced nucleation that nucleation takes place from unsaturated mother solution and the advantage of microdroplet method which suppresses additional nucleation leading to the transformation. In-situ observation shows the crystal dissolves when the laser irradiation is stopped whereas the laser irradiation stabilizes the crystal even if the size of the crystal becomes larger than that of focal spot. These observations indicate that a change in the relative magnitudes of chemical potentials between solution/crystalline phases. This change is possibly promoted via crystal growth by trapping of crystalline clusters in optical potential well formed on a crystal surfaces originating from “light propagation” through the crystal. Our results shed a light not only on polymorph control but also on a method to prepare a longer-lived achiral precursor for analysis on achiral-chiral transition by “freezing” a kinetic pathway of chiral crystallization


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1. INTRODUCTION Control of metastable phase in crystallization from a solution has significance on materials science, solid-state chemistry and pharmacology because each polymorph shows distinct functionality such as opt-electrical properties,1 catalytic properties2 and bioavailabilities.3 Not only for the difference in the functionality, the control of a metastable single crystal has been demanded for the purpose of fundamental research to precisely explore crystal structure of an unprecedented phase and mechanisms of polymorphic transformation or non-classical nucleation pathway intermediated by metastable precursors.4-6 Tremendous efforts have been made to control metastable phase, developing various method such as antisolvent crystallization,7,8 crystallization in microdroplet,9 under external field (e.g. electric, magnetic and electromagnetic field),10-12 in the presence of some additives.13 The essence and difficulty of metastable phase control in spontaneous crystallization lies on how crystallization conditions fulfill two requirements in a relationship of trade off: (1) High supersaturation state sufficient to induce nucleation of the target metastable phase (2) Low supersaturation state enough to suppress undesired nucleation of the stable phase during the growth of the target crystal. This is because a system is forced to transition towards the most thermodynamically favorable state by a polymorphic transformation once the favorable



phase nucleates in the system even if nucleation of the target metastable phase was achieved.14-16 The use of microdroplet as mother solution has been demonstrated to be effective to grow a metastable crystal while suppressing polymorphic transformation to the stable crystal.9 High surface tension of a microdroplet suppresses internal convection, which can be the perturbation to induce nucleation, and its small volume which is enough for solute molecule to diffuse towards a mother crystal suppresses local increment of concentration around the crystal,9,17 These conditions suppress undesired extra nucleation once the target metastable crystal nucleate, leading to stable growth of the target crystal. However, the microdroplet method has a limitation to achieve low supersaturation state in the period from the primary nucleation to early stage of crystal growth as long as the onset of the nucleation relies on spontaneous nucleation, such as cooling or evaporation method. This is because primary nucleation requires relatively-high supersaturation to overcome high activation energy barrier. Therefore, the method alone may be unsuitable for a metastable phase of which solubility is much higher than that of the stable phase.

To exploit a method which promotes primary

nucleation of the target phase under relatively-low supersaturation expands the possibility of the microdroplet method to control metastable phase.


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Recently,

optical

trapping-induced

crystallization

using

a

tightly-focused

continuous-wave (CW) laser has received increasing attention because the method can induce crystallization from the focal spot even from unsaturated mother solution.18-20 For instance, Rungsimanon et al. have demonstrated that irradiation of a tightly-focused CW infrared laser to air/D2O solution of glycine allows to crystallization of glycine from the focal spot from unsaturated mother solution.20 They attributed this phenomenon to the optical trapping of molecular clusters at the focal spot due to optical gradient force. This “optical trapping-induced nucleation” method has been achieved to crystallize various organic compounds from unsaturated mother solution.21,22 Therefore, it follows that unprecedented efficient control of a metastable phase can be achieved by compensating for the shortcoming of the microdroplet method, initial supersaturation of a mother solution should be high to induce nucleation of the desired phase, using the advantage of the optical trapping-induced nucleation, which can induce nucleation from an unsaturated mother solution. Here we report that switchable phase conversion between a single metastable crystal of inorganic sodium chlorate (NaClO3) and a microdroplet of its unsaturated aqueous solution was achieved by the combination of optical trapping-induced nucleation and the microdroplet method. The irradiation of a tightly-focused CW visible laser (532 nm)



into a hemisphere of the microdroplet allowed the single metastable crystal to generate and stably grow without a polymorphic transformation to stable crystal. The metastable phase has recently been discovered as a short-lived achiral precursor in NaClO3 chiral crystallization following non-classical nucleation manner intermediated by a metastable phase.4,23 The solubility of the metastable phase is so high that the metastable crystal instantaneously transforms to the stable crystal after nucleation or does not visibly appear in a spontaneous crystallization.4,8 Not only we report the achievement of stable growth of the metastable crystal, but also demonstrate that the growth and dissolution of the crystal can be controlled by simply turning on and off the laser irradiation. Furthermore, we provide observations which indicate stabilization of the metastable crystal by “light propagation” through a crystal and the suppression of stable phase nucleation due to small volume of the microdroplet enough for all solute molecule to diffuse to the crystal are the main reason to “freeze” state in the course of kinetic pathway of crystallization. 2. Materials and methods 2-1. Material We used sodium chlorate (NaClO3), which has a short-lived metastable phase, as a target material. Crystallization of NaClO3 from an aqueous solution has been known to exhibit two kinds of


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polymorphs: (1) monoclinic metastable phase with P21/a space group; (2) cubic stable phase with P213 space group [Figure 1 (a)].4,24 Whereas the metastable phase exhibits birefringence, the stable phase does not because of their crystallographic symmetry. Equilibrium shapes of the monoclinic

crystal and the cubic crystal are parallelogram and cubic shape, respectively. Figure 1 (b) shows

solubility curves of the two phases, indicating that the solubility of the metastable phase is about 1.6 times higher than that of the stable phase.8 This means that nucleation of the metastable phase

requires 60% supersaturation with respect to the stable phase at least. Because such a high

supersaturation state easily causes nucleation of the stable phase, the metastable phase immediately

transforms to the stable phase in general. Namely, the lifetime of the NaClO3 metastable phase is short in spontaneous crystallization. In practice, although several efforts have been previously made to keep the short-lived crystal,4,8 no method achieved to freely control the crystal. Thus, we

employed the compounds as a model material which has a highly-soluble metastable phase.

2-2. Sample preparation An aqueous solution of NaClO3 saturated at 22°C was prepared by dissolving commercially available NaClO3 powder (30g, >98%, Wako) to ultrapure water (25 mL) in a centrifuge tube. The mixture of water and NaClO3 powder was then heated up to 60°C in order to make the NaClO3 solution supersaturated at 22°C by leaving the solution in an incubator for 3 days. After 3 days, the solution was moved to a room in



which the temperature was controlled at 22°C and left for more than a week in order to precipitate NaClO3 solute excessively dissolved. After the precipitation of the solute, the resulting equilibrium solution saturated at 22°C (approximately 15% in mole fraction) was used as sample solution. A cover glass (120 µm thick) was cleaned by subjecting to sonication using acetone and ultrapure water. The sample equilibrium solution was sprayed to the cover glass using a commercially available spray container. This procedure allowed us to produce hemispherical microdroplets of the NaClO3 solution whose diameter ranges from 200 to 10 µm on the cover glass. We selected microdroplets the size of which ranges from 30 to 60 µm (about 60 to 450 pl) as mother solution for our experiment. The height of the microdroplets were measured to be about 25 µm (See Supporting Information, SI1). The hemispherical microdroplets were confined in an enclosed space by interleaving a silicone sheet (1 mm thick) with the cover glass which supports the microdroplets and another same cover glass (Figure 2 right). The microdroplets of NaClO3 aqueous solution confined in the space between the pair of the cover glasses were used as a mother solution for the laser-induced crystallization experiment. It should be noted that the mother solution is unsaturated state with respect to the metastable phase because the solution is just saturated to the stable phase having lower solubility than that of the metastable phase.


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2-3. Optical setup for laser-induced crystallization An optical system to irradiate a laser to a single microdroplet of NaClO3 was constructed by introducing circularly polarized green laser (532 nm) to an inverted polarized light microscope (IX71, Olympus Corp.) (Figure 2 left).25 Linearly polarized laser emitted from a light source firstly passed an adjustable attenuator constructed by a polarizing beam splitter and a half-wave plate. Beam diameter of the light was then expended from 2.3 mm to approximately 5.4 mm, which is identical to the pupil diameter of the objective lens equipped in the optical microscope, using Kepler-type beam expander constructed by two opposite plano-convex lenses. The expanded beam was converted to circular polarization using a quarter-wave plate. The circularly polarized light (CPL) was introduced into the inverted optical microscope equipped with a 60× objective lens (Olympus, UPLFN 60X, NA = 0.9). The CPL was then introduced to the objective lens through the reflection by a Notch-Dichroic half mirror (532 nm). The CPL was focused to the focal point by the objective lens. The focused CPL was irradiated to the air/NaClO3 microdroplet interface to induce nucleation. We observed crystallization behavior induced by laser irradiation in-situ using the polarized light inverted microscope and a CCD camera with notch filter (532 nm) to exclude Rayleigh scattering light.



3. Results and Discussion 3-1. Switching of crystallization/dissolution of the short-lived metastable phase by laser irradiation. Figure 3 shows the time-lapse micrographs showing the crystallization dynamics (left) and corresponding time evolution of crystal size (right) (See also SI2, movie). Crystallization took place from the focal spot 10 to 40 minutes after the onset of the laser irradiation to the air/solution interface. The crystal exhibited parallelogram shape and birefringence, suggesting that the monoclinic metastable crystal unexpectedly appeared despite the set concentration of the mother solution is significantly lower than the solubility of the crystal. [Figure 3 (left) (1)-(7)]. In practice, we confirmed that the birefringent parallelogram crystal is identical phase to the monoclinic metastable crystal obtained by spontaneous crystallization in the previous report4 by comparing Raman spectra of the two crystals (See SI3). The metastable crystal was optically trapped at the focal spot and continued to grow while keeping its idiomorph. The crystal continued to grow even after the crystal deviated from the focal spot. Namely, the crystal continued to grow regardless of whether the crystal was optically trapped [Figure 3 (left) (17)-(21)]. In addition, the crystal continued to grow even when the focal spot was moved to the inside of the microdroplet or interface between the solution and the glass


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substrate in contrast to the previous reports on “optical trapping-induced crystallization”21,22 [Figure 3 (left) (17)-(21)]. Once the crystal grew to the size in which the apexes of the parallelogram shape reaches to the air/liquid interface of the microdroplet, the size of the microdroplet started to decrease. The solution finally appears to disappear from the periphery of the crystal. After the solution apparently disappear, the crystal stopped growing. These observation facts indicate that the metastable crystal continued to grow under laser irradiation as long as the mother solution surrounds the crystal. On the other hand, the crystal started to dissolve when the laser irradiation was stopped. Upon stopping the laser irradiation, the crystal surface started to wet and solution spread on the cover glass, resulting in the situation that the mother solution surrounds the crystal again. [The reason why the solution apparently disappeared from the periphery of the crystal is unclear for now. Conceivable reasons are as follows: the solution spread over the crystal surface; the water in solution was incorporated into the crystal as crystalline water (the possibility that the crystal is hydrate). We are going to investigate in detail in the future work.]. After the spreading of the solution, the well-faceted parallelogram shape of the crystal gradually changed to a rounded shape because the apex of the parallelogram crystal preferentially dissolved by Gibbs-Thomson effect, the effect that the higher surface/volume ratio results in the



higher solubility.26 The rounded crystal continued to dissolve until the laser irradiation was started again. It should be noticed that the size of the microdroplet recovered to the same size as the initial state when the crystal size became almost the same size as the size at immediately after the nucleation [Compare Figure 3 (left) (1), (17) and (24)]. This indicates that the solvent does not evaporate. When the laser irradiation was restarted before the crystal disappears, the dissolution of the crystal turned into growth with a few second delay. This switching of growth/dissolution was repeatable by simply turning on/off the laser irradiation. Surface of the re-grown crystal is much smoother than that of the initial crystal before dissolution, indicating that crystallinity of the crystal improved by the re-growth process. 3-2. The effect of the microdroplet on the stable growth of the short-lived metastable crystal. In our crystallization experiments, highly unstable metastable phase stably grew without a polymorphic transformation to the stable phase in the microdroplet under laser irradiation whereas the metastable phase tends to promptly transform into stable phase within a few minutes in the case of spontaneous crystallization by droplet-evaporation method.4,8,23,25 Let us discuss the reason why the metastable phase stably grew in our system. Stable growth of a metastable crystal requires suppression of


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polymorphic transformation to stable phase. Generally, polymorphic transformation in solution growth is initiated by one of three processes: (i) the displacement of atoms packed in a metastable structure (solid-solid phase transformation); (ii) 3D homogeneous nucleation followed by the simultaneous dissolution and precipitation of the metastable phase and stable phase, respectively (dissolution/precipitation mechanism); or (iii) 2D heteroepitaxial nucleation of the stable phase on a surface of a metastable phase followed by dissolution/precipitation mechanism.27-30 The process (ii) and (iii) are significant higher rate (more frequent) process compared to the process (i) because the amount of structural change required is very high, and this perspective has been confirmed by our previous in-situ observation of the polymorphic transformation process in practice.23 Therefore, here we assume the processes (ii) or (iii) are responsible for most solvent-mediated polymorphic transformations. This assumption means that polymorphic transformation can be suppressed by suppressing nucleation of the stable phase. In other words, the successful growth of the metastable single crystal in our experiment is possibly the consequence of the suppression of the nucleation of the stable phase inside the microdroplet. Then, how is the nucleation of the stable phase suppressed?



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Maeki et al. have demonstrated that the use of a microdroplet as a mother solution is effective to suppress undesired nucleation and to obtain a single crystal of proteins.17,31 They pointed out that the concept of critical radius of a microdroplet is important to obtain a single crystal (Figure 4). The critical radius is the maximum distance in which a solute molecule can be transported towards the center of the microdroplet by self-diffusion, and its value is determined by diffusion coefficient of the solute molecules and the rate of consumption of the solute molecule by the crystal growth as follows:

ܴ௖ = ට

଺஽஼బ ௤

(1)

where ܴ௖ is the critical radius, D is the diffusion coefficient of the solute molecule, ‫ܥ‬଴ is the initial concentration of the solute in the droplet and q is the consumption rate of the solute in the droplet. Assuming that all the solute transportation proceeds by simple diffusion (This assumption is reasonable because high surface tension and small volume of a microdroplet is known to suppress internal convection), if the droplet size is smaller than the critical radius, all the solute molecules can be transported to a single crystal in the droplet. The solute molecules can be consumed by the crystal growth of


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the single crystal rather than nucleation because the activation energy of nucleation is much higher than that of desolvation of the solute molecules. On the other hand, if the droplet size is much larger than the critical radius, the solute molecules at outside of the critical radius are hardly consumed by the crystal growth since the molecules hardly reach to the crystal. This situation results in the undesired nucleation. Teshima et al. have applied the concept of the critical radius to polymorph control.9 They reported that a metastable crystal successfully grew in a microdroplet without polymorphic transformation thanks to the suppression of stable phase nucleation. Here let us discuss our system, NaClO3 crystallization from a 30 µm-sized microdroplet of a saturated aqueous solution. We evaluated that the simple diffusion governs the solute transportation in our system by calculating Rayleigh number and Marangoni number, which are the indicators for instability of buoyancy and Marangoni convections, respectively (See SI432-40). Then, we can apply the concept of the critical radius for the growth of a single protein crystal in a microdroplet to our system. To calculate the critical radius in the case of NaClO3 solution growth, we adopted the physical values of NaClO3 provided by Kang et al..37 Then, the critical radius was calculated to be about 3 mm by substituting the values, D = 1.5 × 10-5 (cm2·s-1), C0 = 970 (mg‧ml-1) (This value is equal to the solubility of the stable phase), q = 9.6 × 10-1



(mg‧ml-1‧s-1) (See SI5). This value is sufficiently larger than the size of the microdroplet (30 µm). Therefore, the nucleation of the stable phase was suppressed, leading to the stable growth of the metastable single crystal without polymorphic transformation. Here we neglected the possibility that the circularly polarized laser itself is responsible for the stable growth of the metastable phase even though several reports previously suggested that resulting polymorph depends on polarization state of the incident laser in adequate solution conditions.19 This is because the metastable phase is always the first phase to crystallize out from a microdroplet even when we use linear polarization instead of circular polarization and apply evaporation method without using laser (See SI6). For these experimental results, it is valid to consider polymorph selection in the primary nucleation follows the Ostwald’s law of stages, which predicts nucleation of a metastable phase crystal lower surface free energy precedes that of a stable crystal with higher surface free energy,27 rather than the effect of the polarization state itself in our system.

3-3. Crystallization mechanism: thermodynamic driving force modified by optical trapping of crystalline clusters by light propagation


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Why the metastable phase could be crystallized from the unsaturated microdroplet by irradiating tightly-focused laser? Why did the crystal grow even after leaving from the focal spot? The primary nucleation from the focal spot can be attributable to “optical trapping-induced nucleation” phenomena.18-22 Optical gradient force exerted on crystalline clusters in solution gathers the clusters to the focal spot, resulting in concentration increment at the focal spot. This concentration increment leads to the crystal nucleation from the focal spot. This optical trapping-induced nucleation mechanism can explain the metastable phase nucleation from the focal spot. Here, the possibility that evaporation caused crystal nucleation and growth was ruled out because of the following four reasons: (1) The observed microdroplet should be in equilibrium with the surrounding vapor and the other droplet in the enclosed cell; (2) The laser irradiation should not lead to the temperature elevation significant to the promotion of solvent evaporation because of the low absorption coefficient of the solution for 532 nm light (The temperature elevation in our system was estimated to be 2.4 × 10-2 (oC). See SI4); (3) The observed fact that the crystal started to dissolve when the laser irradiation was stopped; (4) The observed fact that the size of the microdroplet recovered to the initial size after the dissolution of the metastable crystal. Regarding (3) and (4), the crystal should not dissolve and the size of the microdroplet should not recover when the



laser was stopped if the solution evaporation caused the crystal growth of the metastable phase because it is hardly possible that the evaporated solvent returns to the same positions. Moreover, the recovery of the droplet size was observed even when the crystal growth cell was opened. For these reasons, the primary nucleation is possibly not attributable to the solvent evaporation but to the “optical trapping-induced nucleation” phenomenon. However, the optical trapping-induced nucleation mechanism, which relies on mass transfer to the focal spot without the change in chemical potentials of each phases, alone cannot explain the whole behavior of the crystal growth. This is because that the metastable crystal continued to grow to the size over the microdroplet despite the law of conservation of should impose the formation of unsaturated region in somewhere of the droplet in return for local concentration increment around the focal spot. Figure 5 shows schematic illustrations showing ideal concentration distributions in a microdroplet and the conceivable resulting crystal size in accordance with the law of conservation of mass. Before the laser irradiation, solution concentration should be spatially homogeneous in a value smaller than the solubility of the metastable phase [Figure 4 (i)]. According to the mass transportation caused by optical gradient force, the concentration in the region far from the focal spot should be lower than the initial concentration to compensate the concentration increment around the focal spot [Figure


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4 (ii)]. It should follow that the concentration in the region far from the focal spot forbids a metastable crystal to exist even if the concentration around the focal spot allows the crystallization. The metastable crystal should not grow over the size of the microdroplet [Figure 4 (iii)], contradicting the observation fact. In addition, although the supersaturation should be relatively higher at the focal spot the crystal did not grow preferentially to the focal spot. These contradictions between the observed facts and the law of conservation of mass indicates that mass transportation phenomena caused by optical trapping alone is insufficient to explain the whole behavior of the crystal growth. In addition, mass transportation caused by thermophoresis41 or Marangoni convection42 also cannot explain the behavior. One conceivable reason that can explain the observed facts is the change of the magnitude relationship between chemical potentials of the solution phase and crystalline phase which accompanies promotion of crystal growth by the “light propagation”-induced trapping of crystalline clusters on crystal surfaces. It has been known that the free energy of an isotropic and homogeneous dielectric particles with dielectric constant εc dispersed in a medium with dielectric constant εs becomes lowered under a static electrical field when εc > εs.43 Alexander et al. extended this reduction of the free energy by an electrical field to the enhancement of driving force for



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crystallization by considering the dielectric particles and the dielectric medium as crystalline clusters and a surrounding solvent, respectively.44 The free energy change by applying electrical field, ∆W, can be written as

∆W = −ܸܽ௖ ‫ ܧ‬ଶ

(2)

where

ܽ=

3ߝ଴ ߝ௦ ߝ௖ − ߝ௦ ൬ ൰ 2 ߝ௖ + 2ߝ௦

(3)

Vc is the volume of the crystalline cluster, E is the electric field strength, ε0 is the dielectric constant in vacuum. εc and εs is now dielectric constants of crystalline clusters and solvent. Because the dielectric constant of a NaClO3 crystal is higher than that of the aqueous solution, the chemical potential of the crystalline phase should decrease under light field. This means that the electrical field of a laser light has a potential to promote not only nucleation but also crystal growth. In practice, we observed that aspect ratio in the C face of the resulting metastable single crystal increased as input power of the incident laser decreased (Figure. 6)(See also SI7 showing experimental procedure and SI8, movie). This phenomenon cannot be explained by the driving force increment originating from evaporation because the change of the crystal shape is spatially anisotropic phenomenon and the driving force increment is isotropic. It is much reasonable to interpret the phenomenon as the consequence of the chemical


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potential modification by the electrical field intensity of the incident laser, which has anisotropy such as the direction of the propagation, the polarization state and so on. Therefore, the crystallization behavior supports the hypothesis that the electrical field of the incident laser modifies the relationship of the chemical potentials. However, the enhancement of the driving force for crystallization seems to be spatially limited only at the focal spot because the electrical field is generally considered to be spatially confined by the light focus. In contrast, recent studies on laser trapping have shown that effect of a focused light can extend to outside of the focal spot through “light propagation” phenomenon once dielectric particles dispersed in a solution were optically trapped. Kudo et al. have reported that the irradiation of a 1.4 W infrared focused laser (1064 nm) to the interface between glass and a solution containing polystyrene nanoparticles (500 nm) forms colloidal assembly larger than the focal spot because of the propagation of the trapping laser through the assembly.45-47 Crystal is no exception as a medium that allows the light propagation. Yuyama et al. have shown that optical trapping of a L-phenylalanine single crystal floated on its solution containing 1µm-sized polystyrene particles gathers the particles to the lateral edge of the crystal, which is spatially deviated from the focal spot.48 This experimental result demonstrated that the light that propagated through the crystal forms optical potential well on the crystal surface located



at outside of the focal spot. In the literature, they mentioned that crystal growth is possibly promoted via trapping crystalline cluster or molecules by the optical potential well at the lateral edge of the crystal surface. This growth mechanism is possibly applicable to our system as well. Figure 7 shows schematics showing the mechanism of the crystal growth in our system. Once the metastable crystal grew, the trapping light propagates through the crystal. This leads to the decrement of the chemical potential of crystal out of the focal spot because the electrical field of the propagation light was imposed to also the crystal out of the focal spot while forming surface potential well to trap crystalline clusters. Therefore, the metastable crystal possibly grew over the size of the focal spot and was stabilized by the light propagation. Since the electrical field of the laser stabilizes the metastable crystal, the metastable crystal should start to dissolve once the laser irradiation was stopped. This consideration is consistent with our observation fact that the metastable crystal started to dissolve when laser irradiation was stopped even in the open system. The enhancement of the thermodynamic driving force for crystallization because of the presence of the electric field which effects to whole volume of the crystal though light propagation caused the crystal growth rather than the increment of the concentration caused by mass transportation. Whereas this hypothesis can explain the crystal growth over the size of droplet and the dissolution after the laser


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off, it cannot explain the growth of the crystal deviated from the focal spot. To explain whole behavior observed, it may be necessary to consider the change of solution state such as highly-dense domain formation around the focal spot.51 Our method may reveal underlying mechanisms of the optical trapping-induced crystallization hidden by largeness in volume of solution. Further quantitative analysis is required to clarify the whole behavior of the crystal growth. 4. Conclusion We reversibly controlled phase conversion between a microdroplet of a NaClO3 unsaturated aqueous solution and a short-lived metastable single crystal, which is achiral precursor in NaClO3 chiral crystallization, by the combination of optical trapping-induced nucleation and crystal growth in a microdroplet. The irradiation of a tightly-focused 532 nm CW laser to the interface between air and hemisphere of a microdroplet of NaClO3 unsaturated solution unexpectedly induced nucleation of single crystal of the metastable phase from the focal spot by optical trapping-induced nucleation mechanism. The continuous irradiation of the laser allows the crystal to stably grow without a polymorphic transformation. The crystal growth is possibly driven by trapping of crystalline clusters or solute molecules to the surface optical potential formed by light propagation through the crystal, namely, the change of the



magnitude relationship between chemical potentials of the solution phase and the crystalline phase. The suppression of a polymorphic transformation was attributed to the suppression of the stable phase nucleation originating from the size of the droplet in which most of the solute molecules can diffuse to the crystal and are consumed by growth of the crystal rather than additional nucleation. Our method practically achieved to “freeze” a kinetic pathway of crystal formation intermediated by a metastable phase, being useful for precise analysis a short-lived precursor phase seen in non-classical nucleation process in calcium carbonate49 or protein50 and so on. In addition, this achievement provides additional freedom in strategies to probe the achiral-chiral transition in the course of NaClO3 chiral crystallization because we can, for instance, mechanically stimulate, apply an external field, observe the achiral precursor using various method after preparing a stabilized achiral precursor crystal. Our method shed a light not only to complete polymorph control including the improvement crystal quality but also for fundamental analysis on an unprecedented phase or nucleation pathway intermediated by a short-lived precursor phase and precise analysis on the chirality emergence mechanism in the course of the transition from the achiral precursor to chiral crystal in NaClO3 chiral crystallization.


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ASSOCIATED CONTENT Supporting information The Supporting Information is available free of charge on the ACS publications website at DOI: SI1 Evaluation of the height of microdroplet; SI2 Movie showing in-situ observation of nucleation and growth/dissolution of the metastable crystal controlled by the laser irradiation; SI3 Raman spectroscopy of the birefringent parallelogram crystal obtained in the laser-induced crystallization experiment and the monoclinic metastable crystal obtained by spontaneous crystallization; SI4 Evaluation of the suppression of internal convection in microdroplet: The temperature elevation caused by the laser irradiation, Rayleigh number, Marangoni number; SI5 The calculation of the consumption rate, q; SI6 The evaluation of the relationship between laser polarization and the resulting polymorph; SI7 Dependence of aspect ratio in the C face of the resulting metastable crystal on the incident laser power; SI8 Movie of in-situ observation showing the crystallization experiment for SI7.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Telephone: +81-43-290-3475. Fax: +81-43-290-3490.



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ORCID Hiromasa Niinomi: 0000-0001-7003-5434 Note The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by a Grant-in-Aid for Young Scientists (B) (No. 16K17512), a Grant-in-Aid

for

Scientific

Research

on

Innovative

Area“Nano-Material

Optical-Manipulation” (No. 16H06507) from JSPS KAKENHI and the Ministry of Science and Technology in Taiwan under Contracts MOST 106-2113-M-009-017-.

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Figure 1. Polymorphism of NaClO3 crystallization from an aqueous solution. (a) Idiomorphic crystal shapes, crystal systems, space groups of NaClO3 metastable phase and stable phase, and the crystal structure of the stable phase.4 (b) Solubility curves of the metastable phase and the stable phase. The black and red lines indicate the solubility curve of the stable phase and the metastable phase, respectively.8



Figure 2. Experimental set up for the laser-induced crystallization experiment. The optical set up is the same as ref. 25.25


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Figure 3. (left) Time-lapse micrographs showing the in-situ microscopic observation capturing crystallization/dissolution behaviors induced by the laser irradiation to a microdroplet of NaClO3 solution. The white dashed line indicates the edge of the microdroplet. The green spot indicated by a green arrow indicates the focal spot. (right) Dependence of time evolution of the crystal size with on/off of the laser irradiation. The crystal size was defined by the square root of the area of the crystal. The region highlighted by green indicates the duration of “laser on”. The red chain lines indicate the moment when the growth and the dissolution were switched. This graph shows that the duration of growth roughly corresponds to the duration of “laser on”, and that of dissolution does that of “laser off”, respectively.



Figure 4. Schematic illustrations showing the concept of the suppression of undesired additional nucleation based on the critical radius, Rc. This schematic is adopted from Ref. 17 with some modifications. (a) the size of a microdroplet < the critical radius. (b) the size of a microdroplet > the critical radius. The critical radius was determined by the diffusion coefficient, the initial concentration of solute, and the consumption rate of the solute in solution.


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Figure 5. Schematic illustrations showing the concentration distribution of a microdroplet and the conceivable resulting crystal behavior in accordance with the law of conservation of mass. The upper schematics show the conceivable concentration distribution before laser irradiation, after the irradiation and steady state. The lower schematics shows cross-sectional concentration distributions before and after the irradiation and steady state. If only mass transportation governs the crystallization, unsaturated region, in which the crystal cannot grow to, should be formed in accordance with the law of conservation of mass.



Figure 6. Dependence of aspect ratio of C face of the resulting crystal on the laser power.


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Figure 7. Schematic illustrations showing the concept of the change in the magnitude relationship between chemical potentials of the solution phase and crystalline phase which accompanies promotion of crystal growth by the light propagation. The schematics of the most upper line, second line, third line and forth line indicate the overview of the sample, cross-section of the sample, electrical field distribution along to the cross-section and the thermodynamic driving force for crystallization, which is the difference between the chemical potential of the solution phase and that of the crystalline phase, along the cross-section.



For Table of Contents Use Only

“Freezing” of NaClO3 Metastable Crystalline State by Optical Trapping in Unsaturated Microdroplet

Author list:

Hiromasa Niinomi†,*, Teruki Sugiyama‡,§, Katsuhiko Miyamoto†,ǁ, and Takashige Omatsu†,ǁ

Affiliations: †

Molecular Chirality Research Center (MCRC), Chiba University, Chiba, Japan Department of Applied Chemistry, National Chiao Tung University, Hsinchu, Taiwan § Graduate School of Materials Science, Nara Institute of Science and Technology, Nara, Japan ǁ Graduate School of Engineering, Chiba University, Japan ‡

We reversibly controlled phase conversion between a microdroplet of a NaClO3 unsaturated aqueous solution and a metastable single crystal, which is usually short-lived phase in spontaneous crystallization, while suppressing a polymorphic transformation by the combination of the advantages of optical trapping-induced crystallization and crystallization in microdrplet.


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âFreezingâ of NaClO3 Metastable Crystalline State by Optical Trapping

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