In Situ Observation of Chiral Symmetry Breaking in NaClO3 Chiral

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In-situ Observation of Chiral Symmetry Breaking in NaClO Chiral Crystallization Realized by Thermoplasmonic Micro-Stirring Hiromasa Niinomi, Teruki Sugiyama, Miho Tagawa, Shunta Harada, Toru Ujihara, Satoshi Uda, Katsuhiko Miyamoto, and Takashige Omatsu Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00420 • Publication Date (Web): 26 Jun 2018 Downloaded from http://pubs.acs.org on July 5, 2018

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Crystal Growth & Design

Cover page Title: In-situ Observation of Chiral Symmetry Breaking in NaClO3 Chiral Crystallization Realized by Thermoplasmonic Micro-Stirring Author list: Hiromasa Niinomi†,‡,#,*, Teruki Sugiyama§,//, Miho Tagawa¶, Shunta Harada¶, Toru Ujihara¶, Satoshi Uda#, Katsuhiko Miyamoto†,‡ and Takashige Omatsu†,‡

Affiliations: Graduate School of Engineering and‡Molecular Chirality Research Center (MCRC), Chiba University, Chiba, Chiba, 263-0022, Japan †

§

Department of Applied Chemistry and Center for Emergent Functional Matter Science, National Chiao Tung University, Hsinchu, 30010, Taiwan //

Graduate School of Materials Science, Nara Institute of Science and Technology, Ikoma, Nara 630-0192, Japan ¶

Institute of Materials and Systems for Sustainability (IMaSS), Nagoya University, Nagoya, Aichi, 464-8603, Japan #

Institute for Materials Research, Tohoku University, Sendai, Miyagi, 980-8577, Japan

*

Corresponding Author

Name: Hiromasa Niinomi, Telephone number: +81-22-215-2103, Fax number: +81-22-215-2103 E-mail address: [email protected], [email protected]

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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 have found that large chiral symmetry breaking in chiral crystallization can be achieved by irradiating several mW focused laser to a plasmonic nanolattice immersed in a stagnant NaClO3 saturated aqueous solution. Several hundreds of chiral crystals with the same handedness showed up in the solution after the laser irradiation in contrast to spontaneous crystallization. In-situ microscopic observation for the early stage of the crystallization in the vicinity of the focal spot revealed that microbubble generation followed by large supersaturation increment, in which supersaturation reaches to 360%, promotes several numbers of crystal nucleation in the vicinity of the bubble as “mother” crystal. The generation of the microbubble induced Marangoni convection, the velocity of which reaches to several hundreds of micrometer per second, crushing the first appeared chiral crystal into pieces by microfluidic shear. Namely, secondary nucleation caused by microfluidic shear amplified the number of “daughter” crystals with the same handedness. This spatiotemporally controllable micro-mixing experiment realized by laser irradiation gives us not only novel route bridging a light and chiral symmetry breaking but also the novel method to observe the early stage dynamics of the secondary nucleation, which was hard to observe by conventional observation technique, in real time.


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Title page Title: In-situ Observation of Chiral Symmetry Breaking in NaClO3 Chiral Crystallization Realized by Thermoplasmonic Micro-Stirring Author list: Hiromasa Niinomi†,‡,#,*, Teruki Sugiyama§,//, Miho Tagawa¶, Shunta Harada¶, Toru Ujihara¶, Katsuhiko Miyamoto†,‡ and Takashige Omatsu†,‡

Affiliations: Graduate School of Engineering and‡Molecular Chirality Research Center (MCRC), Chiba University, Chiba, Chiba, 263-0022, Japan †

§

Department of Applied Chemistry and Center for Emergent Functional Matter Science, National Chiao Tung University, Hsinchu, 30010, Taiwan //

Graduate School of Materials Science, Nara Institute of Science and Technology, Ikoma, Nara 630-0192, Japan ¶

Institute of Materials and Systems for Sustainability (IMaSS), Nagoya University, Nagoya, Aichi, 464-8603, Japan #

Institute for Materials Research, Tohoku University, Sendai, Miyagi, 980-8577, Japan

*

Corresponding Author

Name: Hiromasa Niinomi, Telephone number: +81-22-215-2103, Fax number: +81-22-215-2103 E-mail address: [email protected], [email protected]



Abstract We have found that large chiral symmetry breaking in chiral crystallization can be achieved by irradiating several mW focused laser to a plasmonic nanolattice immersed in a stagnant NaClO3 saturated aqueous solution. Several hundreds of chiral crystals with the same handedness showed up in the solution after the laser irradiation in contrast to spontaneous crystallization. In-situ microscopic observation for the early stage of the crystallization in the vicinity of the focal spot revealed that microbubble generation followed by large supersaturation increment, in which supersaturation reaches to 360%, promotes several numbers of crystal nucleation in the vicinity of the bubble as “mother” crystal. The generation of the microbubble induced Marangoni convection, the velocity of which reaches to several hundreds of micrometer per second, crushing the first appeared chiral crystal into pieces by microfluidic shear. Namely, secondary nucleation caused by microfluidic shear amplified the number of “daughter” crystals with the same handedness. This spatiotemporally controllable micro-mixing experiment realized by laser irradiation gives us not only novel route bridging a light and chiral symmetry breaking but also the novel method to observe the early stage dynamics of the secondary nucleation, which was hard to observe by conventional observation technique, in real time.


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Body of paper Chirality is a fundamental property underlying in various hierarchies in nature. Also, homochirality in biosphere directly impacts on our lives through enantiomer-dependent bioactivity of chiral compounds. Thus, chiral symmetry breaking in spite of the equal thermodynamic stability between a pair of enantiomers of chiral matter has been fascinated many researchers in various field such as earth science, chemistry, physics, biology and so on. To explore perturbations leading to homochiral state and to analyze the three processes necessary to achieve homochiral state proposed by Frank,1 i.e. (i) generation process of chiral asymmetry, (ii) amplification process of the same handedness and (iii) inhibition process of the opposite handedness, facilitates not only the understanding of the biohomochirality but also discovery of an efficient method for optical resolution of chiral pharmaceuticals. Complete chiral symmetry breaking in chiral crystallization is fascinating phenomenon for studies on homochirality2 because transition from achiral state to homochiral state can be reproduced by simple experimental procedures. Since Kondepudi et al. has first reported that complete chiral symmetry breaking occurs in the population of sodium chlorate (NaClO3) enantiomorphs crystallized from a stirred aqueous solution,3 various attempts to induce chiral symmetry breaking by applying abiotic perturbations, such as


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ultrasonic,4 boiling of a solution,5,6 cavitation7 has been considered in a system of chiral crystallization from a supersaturated aqueous solution. Besides such far-from equilibrium systems, the chiral symmetry breaking achieved in near-equilibrium systems has been discovered as represented by Viedma ripening8 or deracemization through attrition-enhanced Ostwald ripening9. In far-from equilibrium systems,3-7,

10

secondary nucleation scenario1, 11 has been widely accepted as the process that leads to chiral symmetry breaking. The secondary nucleation scenario ascribes the Frank’s three processes to (i) a primary nucleation of a single “mother” crystal with single handedness, (ii) high rate secondary nucleation and the generation of “daughter” crystals which inherited the same handedness of the mother crystal and (iii) suppression of additional primary nucleation of crystal with the opposite handedness, which accompanies with decrement of solution concentration by the secondary nucleation, respectively. Although secondary nucleation should be the key process to lead chiral symmetry breaking among the three processes, the mechanism that duplicates the same chirality is still under debate. Instances of the mechanism include mechanical fragmentation of the “mother” crystal by collision with stir bar,12, 13 detachment of tiny needle-like crystal from the surface of the mother crystal by fluidic shear,14,15 breeding of pre-critical nuclei of single handedness on the surface of the mother crystal (embryo coagulation



secondary nucleation),16 polymorphic transformation from achiral precursor induced by the contact of chiral stable crystal.17,18 One reason why the mechanism is still unclear is because it is difficult to investigate the early stage of the secondary nucleation process in such a perturbed solution. Although in-situ microscopic observation method is indispensable to directly reveal the actual process of the early stage, the method demands the target phenomena to be confined to a limited space enough to be covered by the microscopic field. Thus, the method have difficulty to trace a set of processes from the primary nucleation of the “mother” crystal to the following secondary nucleation, which are spatiotemporally random phenomena. This difficulty hinders us to explore the actual process. Laser irradiation is a conceivable option to spatiotemporally control crystallization from a stagnant mother solution.19-24 Non-photochemical laser-induced nucleation (NPLIN) technique has been developed for the purpose of spatiotemporal control and promotion of primary nucleation. Several mechanisms of NPLIN have been demonstrated such as optical Kerr effect,19 cavitation bubble formation20 by pulsed laser or photon pressure-induced crystallization22 by continuous-wave (CW) laser. On the other hand, complete chiral symmetry breaking has not been reported in crystallization induced by NPLIN. Because the secondary nucleation scenario and previous


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experimental works on chiral symmetry breaking have shown that a continuously agitated (not stagnant) solution are necessary to achieve complete chiral symmetry breaking,14,16,

25, 26

the transience of the pulsed laser and the moderateness of the

perturbation of CW laser are inadequate to induce complete chiral symmetry breaking. A perturbation having intensiveness of pulsed laser and continuousness of CW laser is necessary to achieve spatiotemporal control of the set of the primary nucleation and the secondary nucleation. In this communication, we report the discovery that complete chiral symmetry breaking in NaClO3 chiral crystallization was unexpectedly achieved from an initially saturated stagnant solution by irradiating a few tens mW focused CW laser to plasmonic gold (Au) nanolattice immersed in its aqueous solution. This method allowed us to directly visualize the set of process of the primary nucleation and the secondary nucleation in-situ because of the spatiotemporal control of continuous agitation leading to complete chiral symmetry breaking. We show that the microbubble generation by thermoplasmonic effect followed by microfluidic shear causes secondary nucleation by directly visualizing the early stage of the dynamics using in-situ microscopic observation, which was realized by the focused laser.



NaClO3 was used as target compound. NaClO3 aqueous solution consists of spherical sodium cations and pyramidal chlorate anions having C3v symmetry.27 Thus, the solution is achiral. NaClO3 crystallizes from an aqueous solution in two kinds of polymorphic forms: (1) cubic phase with P213 symmetry as the stable phase;28 (2) monoclinic phase with P21/a symmetry as a metastable phase.17 The cubic stable phase is chiral because of chiral space group without improper symmetry. Thus. the NaClO3 crystallization is a system of chiral crystallization.29 On the other hand, the metastable monoclinic phase is achiral phase and the phase is known to act as a precursor of the chiral crystal in spontaneous crystallization induced by droplet evaporation because of Ostwald’s rule of stages.17,18 The solubility of the metastable phase is reported to be 1.6 times higher than that of the stable phase.30 (See Supporting Information, SI1). NaClO3 aqueous solution saturated at 22oC was prepared by the same procedure as our previous study31 as shown in Supporting Information SI2. The saturated solution was used as sample solution. Plasmonic Au nanolattice was fabricated on a glass substrate by lift-off process of electron beam lithography technique [Figure 1 (a)] (See Supporting Information, SI3). Cross structure was employed as unit structure. The cross periodic nanolattice was designed so that the dimension of one cross, the interval between each crosses, its periodicity and the area occupied the whole array become 480


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nm × 480 nm, 96 nm, 576 nm and 625 µm × 625 µm, respectively. 60 nm Au layer with 5 nm Cr adhesion layer was deposited on the glass substrate. A laser irradiating system was constructed by introducing linearly polarized laser into an inverted polarized-light microscope (Olympus, IX71). Figure 1 (b) shows a schematic illustration of the optical setup for the system. Linearly polarized CW green laser (λ = 532 nm, Spectra Physics, Excelsior-532-200-CDRH) was used as light source. The laser emitted from the light source firstly passes 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 using Kepler-type beam expander constructed by two opposite plano-convex lenses. The light was introduced into the inverted optical microscope equipped with 60× objective lens (Olympus, UPLFN 60X, NA = 0.9) from a backport. The light was then introduced to the objective lens through the reflection by a Notch-Dichroic half mirror (λ = 532 nm). The objective lens concentrates the laser into the focal point. The NaClO3 saturated solution (10 µL) was placed on the gold nanolattice supported by the cover glass using a micropippet. The liquid droplet on the cover glass was used as mother solution. The cover glass supporting the mother solution was placed on the stage of the inverted polarized-light microscope. Then, focused linearly polarized laser, the intensity of which is 30 mW



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before passing though the objective lens, was irradiated to Au nanolattice for 3 minutes. The solution was then left for 3 minutes after the laser irradiation to grow the crystals to the size observable by the microscope equipped with 4-powered objective lens.

All

crystallization experiments were performed at 22oC. The polarized light microscope allows the identification of the handedness of a NaClO3 chiral cubic crystal. The method of identification of handedness relies on the rotating analyzer method as follows;32 Polarized-light microscope is constructed by setting two polarizers are orthogonally oriented, NaClO3 cubic crystals exhibit a slightly bright color and turn darker in color upon tilting one of the polarizers to clockwise or counterclockwise direction, depending on the handedness of the crystal. Dextrorotatory(d)-crystal and levorotatory(l)-crystal were identified and counted using the method described above. The degree of chiral asymmetry was quantified using crystal enantiomeric excess (CEE) defined by the following equation:3

CEE = ×

(1)

where Nd is the number of d-crystal and Nl is the number of l-crystal. The number of each enantiomorphs was counted in each crystallization experiment, and CEE were


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evaluated. The set of crystallization experiment and CEE evaluation was repeated 10 times. To investigate the early stage of the crystallization and the dynamics in the vicinity of the focal spot, in-situ microscopic observation was conducted as well. Figure 1 (b) shows the schematic illustration of the setup for the in-situ observation. The polarized-light microscope allows us to distinguish chiral stable phase and achiral metastable phase by detecting the presence of birefringence since stable cubic phase is non-birefringent whereas the metastable monoclinic phase is birefringent. The laser can be introduced from the backport while observing polarized-light microscopic image. This system allows us to observe the dynamics which takes place in the vicinity of the focal spot in-situ. The microscopic dynamics was recorded by a CCD camera (UI-3180CP-C-HQ Rev.2, IDS Imaging Development Systems) with a notch filter to exclude 532 nm light. Figure 2 is micrographs showing chiral crystals which appeared after the laser irradiation to the plasmonic Au nanolattice. The yellow colored region indicates the area on which the Au nanolattice was fabricated, and the region surrounded by the blue dashed line indicates the droplet of the NaClO3 mother solution. Several hundreds of well-faceted chiral crystals were observed in the droplet. The size of crystals distributed from several tens to hundreds micron. The spatial distribution of the crystals was



beyond the area of the Au nanolattice. These situations hardly occur in spontaneous nucleation driven by evaporation of the droplet because crystal tends to appear in a few numbers or as dendritic shape. This is because the large activation energy barrier for nucleation hinders the generation of large amount of individual crystals (See Supporting Information, SI4). Figure 2 (c), (d) shows a polarized-light microscopic image, (c) was captured using the optical setting in which the analyzer was tilted to counter-clockwise direction, and (d) is in the clockwise direction [(e) and (f) are magnified images.]. Almost all of the chiral crystals in the figure exhibited darker contrast in the optical setting with clockwise tilting of the analyzer relative to the setting with counter-clockwise. This indicates that almost all chiral crystals are the same handedness of dextrorotatory. Figure 3 (a) shows the number of the obtained enantiomorphs and the resulting CEE in each crystallization experiment. Figure 3 (b) is the graph showing the CEE value in each crystallization experiments. Strong chiral symmetry breaking was found to occur in 9 crystallizations out of 10 crystallizations, i.e., almost all crystallization experiments. Why hundreds of chiral crystal with the same handedness were obtained in the laser irradiation experiment? To explore the early stage of the crystallization process, we carried out in-situ microscopic observation in the vicinity of the laser focal spot.


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Figure 4 is time-lapse micrographs of the in-situ observation for crystallization dynamics (See also Supporting Information Movie SI5). The Au nanolattice exists in the entire region of the micrographs, and the green spot arrowed indicates the focal spot. Upon the laser irradiation, microbubble, the size of which ranges from tens to hundreds of micron, generated at the focal spot [Figure 4 (ii)]. The generation of the microbubble is possibly due to water vaporization by plasmonic heating.33 Plasmonic heating is temperature elevation caused by energy dissipation of excited localized surface plasmon resonance (LSPR).34, LSPR is the collective oscillation of free electrons on the surface of metal nanoparticles accompanying with the electrical field oscillation of incident light. Heat generates through non-radiative relaxation such as electron-electron, electron-phonon,

phonon-phonon

scattering35.

We

estimated

the

temperature

distribution in the vicinity of the focal spot by following the method reported by Bendix et al. (See Supporting Information, SI6).36 Our measurement shows that the region above 100oC extends to 5.4 µm away from the focal spot. Water in the solution may vaporize within the region 5.4 µm from the center of the focal spot. On the other hand, Guillaume et al. have reported that the local temperature required to trigger bubble generation by plasmonic heating is much larger than 100oC.37 They advocated that superheating of water reaches 220oC in a similar experimental system. Our



measurement shows that the region above 220oC extends to 2.4 µm away from the focal spot. The temperature distribution can rationalize that the plasmonic heating effect resulted in the microbubble generation. After the generation of the microbubble, NaClO3 achiral metastable crystals, which can act as the precursor of the chiral crystal in crystallization induced by droplet evaporation method,17,18 first appeared from the microbubble[Figure 4 (iv)-(vi)]. The achiral crystals flew towards outside of the bubble, and they dissolved with flowing farther away from the microbubble. Taking the experimental fact that the solubility of the metastable crystal is 1.6 times higher than that of the stable phase into account, these behaviors of the metastable crystal indicate that supersaturation value is higher than 60% at least in the vicinity of the microbubble locally. To evaluate how much the supersaturation in the vicinity of the microbubble was increased by laser irradiation, we measured growth rate of a stable cubic crystal located at about 20 µm away from a microbubble. Figure 5 (a) indicates time-lapse micrograph showing the crystal growth behavior of a chiral stable phase under the laser irradiation (See also Supporting Information, Movie SI7), and Figure 5 (b) indicates the time evolution of the crystal size when a half of the square root of the area occupied by the crystal was regarded as the crystal size. Whereas the crystal should not grow without laser irradiation since the


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mother solution is saturated with respect to the stable phase, the crystal rapidly grew upon the laser irradiation [Figure 5 (a) (i)-(vi)]. The growth rate reached about 12.3 µm/sec. According to Bennema et al. and Hosoya et al., crystal growth rate shows linear dependence on supersaturation as expected by Wilson-Frenkel's formula in the range more than 0.1% of supersaturation.38,39 This linear relationship allows us to estimate ambient supersaturation from the crystal growth rate. Hosoya et al. reported that the linear relationship in (100) of NaClO3 stable phase is given by30,39

R = 0.0339σ − 0.0635

(2)

where R is the growth rate (µm/sec) and σ (%) is the supersaturation for the stable phase. Therefore, the local supersaturation value during the laser irradiation is calculated to be about 365%. This supersaturation value is hard to be attained by spontaneous crystallization. This value can rationalize the massive nucleation of the achiral metastable crystal. The experimental condition was found to locally achieve extremely high supersaturation in the vicinity of microbubble. This fact is consistent with the previous studies on thermoplasmonic phenomenon.40-42 Kang et al. have demonstrated that metal nanoparticles dispersed in a solution of Raman active



molecules can be gathered and trapped at the interface between a substrate supporting Au nanoparticles and a microbubble generated by laser irradiation by confirming the increment of Raman intensity by plasmon-ehnacement at the vicinity of the microbubble.40 Setoura et al. have shown that fluorescent nanoparticles can be accumulated at interface between a substrate and a generated microbubble.41 Furthermore, Uwada et al. have demonstrated condensation of solute molecules in the vicinity of the microbubble by observing formation of dense liquid phase and crystallization in glycine-water system. They attributed the dense liquid formation to the accumulation of glycine molecules carried by convection at the interface.42 Because of the extremely high supersaturation in our system, massive nucleation of chiral cubic crystals followed the dissolution of the achiral crystals [Figure 4 (viii)-(x)]. The massive chiral nanoparticles spread from the vicinity of the microbubble to outward as if they are injected from the microbubble while growing to larger size. Although crystal enantiomeric excess at the massive nucleation of chiral crystals has great importance, it was impossible to identify the handedness of each crystals because crystals were not thick enough to show detectable optical rotation. Homochiral state is estimated to be achieved at the massive nucleation taking the fact that complete chiral symmetry breaking occurred in our experiment. However, after the massive nucleation of chiral


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crystal, the chiral crystals assembled and coagulated with each other while growing to lager size [Figure 4 (xii)]. This should result in the generation of a few numbers of chiral crystals on the plasmonic nanolattice rather than the generation of several hundreds of chiral crystals spatially beyond the region of the nanolattice if the laser irradiation is stopped. Nevertheless, why our crystallization experiment generated several hundreds of chiral crystals with the same handedness in the droplet? To clarify the reason why the chiral crystals nucleated in the vicinity of the microbubble spread over the entire volume of the solution droplet beyond the area occupied by the Au nanolattice, we further conducted in-situ observations for the dynamics that takes place around the microbubble after the massive nucleation of chiral crystals. Figure 6 is time-lapse micrographs showing the dynamics (See also Supporting Information, Movie SI8). Figure 6 (ii)-(vi) show that laser irradiation to the interface between the substrate and the microbubble caused the convection that flows radially from the focal spot towards outside of the bubble. The convective flow to the outside came back to the focal spot so as to depict a circle, resulting in circulation flow. The chiral crystals were agitated by the convection. The movement of the crystals allowed us to estimate the flow rate. The flow rate was estimated to be several hundreds micrometer per second. This convection is possibly Marangoni convection. Because the



plasmonic heating establishes a large temperature difference between the bubble interface closer to the heat source and the interface far from the heat source, the surface tension possibly has large difference. This large difference in surface tension should cause Marangoni force from the region with lower surface tension towards the region with higher one on the bubble/solution interface. This Marangoni force results in the Marangoni convection by friction drags to surrounding solution. Similar phenomena have already been reported previously.41,43-45 Especially, Namura et al. have precisely analyzed Marangoni convection caused by microbubble generation accompanying with plasmonic heating of Au nano-island thin film by 785 nm focused-laser irradiation.44 They suggested that mode of the Marangoni convection varies into three modes depending on the distance between the center of the microbubble and the laser focal spot: (i) Vertical mode (ii) Hybrid mode and (iii) Horizontal Mode. Vertical mode can be achieved by setting the focal spot to the center of the microbubble. The mode produces symmetric convection mainly towards the vertical direction to the substrate with the bubble as the center. The Hybrid mode can be achieved by setting the focal spot to away from the bubble center but within the region where the air/substrate interface exists. This mode produces uni-directional flow from the bubble. The horizontal mode can be achieved by setting the focal spot to the vicinity of the triple


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line. This mode produces asymmetric circular flow, and generates a pair of vortex flow the rotation axis of which is vertical to the substrate. The convection seen in our experiment can be clearly assigned to the Horizontal mode. Namura et al. mentioned that velocity of the vortex flow is more than 1000 µm/sec. This flow velocity is comparable to our case. Because of this high flow velocity, hydrodynamic shear stress becomes large enough to cause crystal fragmentation. Figure 7 shows time-lapse micrographs

showing

crystal

fragmentation

caused

by

hydrodynamic

shear

accompanying with the thermoplasonic Marangoni convection (See also Supporting Information, Movie SI9). The “mother” crystal, which indicated by the red arrow, locates in the vicinity of the triple line in the initial stage [Figure 7 (i)]. Immediately after the Horizontal mode Marangoni convection occurred, the “mother” crystal was broken off to the huge numbers of fragments by the shear stress [Figure 7 (ii), (iii)]. The “daughter” crystals generated by the fragmentation of the “mother” crystal rapidly spread to outside of the bubble [Figure 7 (iv)]. This observation proves that microfluidic shear stress caused by the thermoplasmonic Marangoni convection is large enough to fragment the “mother” crystal into huge numbers of “daughter” crystals with the same handedness. Numerical simulations by Cartwright et al. has shown that flow rate of solution advection is important for the amplification of the same handedness for the



Kondepudi’s experiment of chiral symmetry breaking because fluidic shear stress has the possibility to detach tiny needle-like crystals attached to the surface of “mother” crystal.14 Our in-situ observation directly visualized the fragmentation of the “mother” crystal by microfluidic shear in practice, showing the process of amplification of the same handedness. Namura et al. have mentioned that the Marangoni convection around a microbubble induced by the thermoplasmonic effect can be utilized for a rapid microfluid mixing.45 Therefore, one may be able to interpret the complete chiral symmetry breaking in our experiment as the result of “micro-scale Kondepudi’s experiment” realized by the microfluid stirrer. The spatial confinement allowed us to microscopically observe the set of the processes in-situ. From the in-situ observations for the early stage of the crystallization, we propose the mechanism leading to the chiral symmetry breaking observed in the current experiment as follows. Figure 8 shows the schematic illustration explaining the scenario for chiral symmetry breaking. Laser irradiation to the plasmonic nanolattice excites LSPR. The energy of the LSPR dissipates as heat by non-radiative relaxation (plasmonic heating). The resulting temperature elevation at the focal spot generates the microbubble. Since the steep temperature gradient results in the gradient of surface tension of the microbubble, Marangoni convection was induced in the vicinity of the microbubble.


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Since the flow direction of the Marangoni convection is toward the interface between the bubble surface and the glass substrate, solute molecules or clusters are entrapped at the interface. This results in the high supersaturation state in the vicinity of the microbubble. This high supersaturation state induces the crystallization of the achiral metastable crystals. After the appearance of the achiral metastable crystals, the metastable crystals transform into chiral crystal as “mother” crystal by a polymorphic transformation. One may consider that chiral symmetry does not break because each achiral crystal should transform into both enantiomorphs in equal probability. However, it should be emphasized that the mechanism of the polymorphic transformation includes not only solid-state transformation but also solution-mediated phase transformation. Once a stable chiral crystal appeared by nucleation or solid-state transformation in a system where several metastable achiral crystals exist, the metastable crystals should dissolve and consumed by the growth of the stable crystal through solution-solution mediated phase transformation as suggested in our previous study.18 This process possibly leads to single handedness after the polymorphic transformation. After the transformation, the mother crystal was fragmented by the microfluidic shear stress of the explosive Marangoni convection, generating numerous numbers of “daughter” crystals with the same handedness as the “mother” crystal. The “daughter” crystals are



spread over the entire volume of the droplet by the Marangoni convection toward the outside of the microbubble. During these process, additional nucleation of the enantiomorph with opposite handedness to the “mother” crystal is suppressed because the mother solution is originally saturated state. The spread “daughter” crystal then grew as the droplet of the solution evaporates. These processes lead to the chiral symmetry breaking observed in our crystallization experiment. In this scenario, Frank’s three processes required to achieve homochiral state, i.e., (i) generation of chiral asymmetry, (ii) amplification of the same handedness and (iii) inhibition of the opposite handedness, are corresponds to (i) the polymorphic transformation of the achiral crystal to the chiral crystals within the locally-confined high supersaturated region in the vicinity of the microbubble, (ii) fragmentation of the transformed chiral crystals by microfluidic shear induced by thermoplasmonic Marangoni convection and (iii) impossibility of the saturated mother solution to cause the additional nucleation. These process can explain the chiral symmetry breaking in our system. Our crystallization experiment demonstrated that a perturbed solution condition which achieves chiral symmetry breaking can be spatiotemporally controlled simply by irradiating a low power laser light. Furthermore, the optical system can directly visualize the microscopic crystallization phenomena in stirred solution in-situ. We are


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going to investigate microscopic process of chiral amplification by secondary nucleation, possibility of handedness direction by circular polarization of the incident laser or symmetry of the plasmonic nanolattice, threshold of the laser intensity to achieve complete chiral symmetry breaking in near future. In conclusion, we have found that chiral symmetry breaking occurs when NaClO3 chiral crystallization is induced by irradiating focused visible laser (λ = 532 nm) to plasmonic Au nanolattice immersed in a droplet of NaClO3 saturated stagnant solution. In-situ observation focusing on the early stage of crystallization dynamics which takes place in the vicinity of the focal spot revealed that plasmonic heating generates a microbubble at the focal spot. The generation of microbubble at the focal spot establishes steep gradient of the surface tension on the bubble surface. The Marangoni convection towards the bubble/substrate interface concentrate solute molecules or clusters, resulting in the extremely high supersaturation state locally confined in the vicinity of the bubble (The supersaturation value was found to reach 365%.). The high supersaturation causes nucleation of the achiral metastable crystals followed by a polymorphic transformation to chiral stable crystals as the “mother” solution. The “mother” crystal was found to be fragmented by the microfluidic shear stress originating from the Marangoni convection at the bubble/substrate interface. The



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fragmentation produces numerous numbers of “daughter” crystals with the same handedness. Marangoni convection carries the “daughter” crystals to the entire volume of the droplet of the mother solution. The saturated state of the mother solution suppresses the additional nucleation of the enantiomorph with the opposite handedness. These microscopic phenomena induced by thermoplasmonic effects led to chiral symmetry breaking in originally stagnant solution. Furthermore, our method might have possibility

to

precisely

control

crystallization

including

polymorphs

or

enantiomorphism through adequate design of plasmonic field by setting the condition of the incident laser or shape of the nanolattice. We are going to investigate the phenomena not only from the viewpoint of thermoplasmonics but also from the viewpoint of light-matter interaction. • REFERENCES [1] Frank, F. C. “On spontaneous asymmetric synthesis” Biochim. Biophys. Acta 1953, 11, 459. [2] Havinga, E.; Biochim. Biophys. Acta “Spontaneous formation of optically active aubstances” 1954, 13, 171−174. [3] Kondepudi, D. K.; Kaufman, R. J.; Singh, N. “Chiral symmetry breaking in sodium chlorate crystallization” Science 1990, 250, 975−976.


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[10] Viedma, C. “Experimental evidence of chiral symmetry breaking in crystallization from primary nucleation” J. Cryst. Growth 2004, 261, 118−121. [11] Kondepudi, D. K.; Sabanayagam, C. “Secondary nucleation that leads to chiral symmetry breaking in stirred crystallization” Chem. Phys. Lett. 1994, 217, 364−368. [12] Martin, B.; Tharrington, A.; Wu X-l. “Chiral Symmetry Breaking in Crystal Growth: Is Hydrodynamic Convection Relevant?” Phys. Rev. Lett. 1996, 77, 2826−2829. [13] McBride, J. M.; Carter, R. L. Angew. Chem. Int. Ed. “Spontaneous Resolution by Stirred Crystallization” 1991, 30, 293-295. [14] Cartwright, J. H. E.; García-Ruiz, J. M.; Piro, O.; Sainz-Díaz, C. I.; Tuval, I. “Chiral Symmetry Breaking during Crystallization: An Advection-Mediated Nonlinear Autocatalytic Process” Phys. Rev. Lett. 2004, 93, 035502. [15] Buhse, T,; Durand, D.; Kondepudi, D.K.; Laudadio, J.; Spilker, S. “Chiral Symmetry Breaking in Crystallization: The Role of Convection” Phys. Rev. Lett. 2000, 84, 4405−4408. [16] Qian, R.Y.; Botsaris, G.D. “Nuclei breeding from a chiral crystal seed of NaClO3” Chem. Eng. Sci. 1998, 53, 1745−1756.


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[17] Niinomi, H.; Yamazaki, T.; Harada, S.; Ujihara, T.; Miura, H.; Kimura, K.; Kuribayashi, T.; Uwaha, M.; Tsukamoto, K. “Achiral Metastable Crystals of Sodium Chlorate Forming Prior to Chiral Crystals in Solution Growth” Cryst. Growth Des. 2013, 13, 5188−5192. [18] Niinomi, H.; Miura, H.; Kimura, Y.; Uwaha, M.; Katsuno, H.; Harada, S.; Ujihara, T.; Tsukamoto, K. “Emergence and Amplification of Chirality via Achiral–Chiral Polymorphic Transformation in Sodium Chlorate Solution Growth” Cryst. Growth Des. 2014, 14, 3596−3602. [19] Garetz, B. A.; Aber, J. E.; Goddard, N. L.; Young, R. G.; Myerson, A. S. “Nonphotochemical,

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[29] Matsuura, T.; Koshima, H. J. “Introduction to chiral crystallization of achiral organic compounds: Spontaneous generation of chirality” Photchem. Photobiol. C-Photochem. Rev. 2005, 6, 7−24. [30] Niinomi, H.; Horio, A.; Harada, S.; Ujihara, T.; Miura, H.; Kimura, Y.; Tsukamoto, K. “Solubility measurement of a metastable achiral crystal of sodium chlorate in solution growth” J. Cryst. Growth 2014, 394, 106−111. [31] Niinomi, H.; Sugiyama, T.; Miyamoto, K.; Omatsu, T. ““Freezing” of NaClO3 Metastable Crystalline State by Optical Trapping in Unsaturated Microdroplet” Cryst. Growth Des. 2018, 18, 734−741. [32] Niinomi, H.; Sugiyama, T.; Tagawa, M.; Murayama, K.; Harada, S.; Ujihara, T. “Enantioselective amplification on circularly polarized laser-induced chiral nucleation from a NaClO3 solution containing Ag nanoparticles” CrystEngComm 2016, 18, 7441−7448. [33] Govorov, A. O.; Richardson, H. H. “Generating heat with metal nanoparticles” nanotoday 2007, 2, 30−38. [34] Hutter, E.; Fendler, J. H. “Exploitation of Localized Surface Plasmon Resonance” Adv. Mater. 2004, 16, 1685−1706.



[35] Aioub, M.; El-Sayed, M. A. “A Real-Time Surface Enhanced Raman Spectroscopy Study of Plasmonic Photothermal Cell Death Using Targeted Gold Nanoparticles” J. Am. Chem. Soc. 2016, 138, 1258–1264. [36] Bendix, P. M.; Reihani, S. N. S.; Oddershede, L. B. ACS Nano “Direct Measurements of Heating by Electromagnetically Trapped Gold Nanoparticles on Supported Lipid Bilayers” 2010, 4, 2256-2262. [37] Baffou, G.; Polleux, J.; Rigneault, H.; Monneret, S. J. Phys. Chem. C “Super-Heating and Micro-Bubble Generation around Plasmonic Nanoparticles under cw Illumination” 2014, 118, 4890-4898. [38] Bennema, P. “Interpretation of the relation between the rate of crystal growth from solution and the relative supersaturation at low supersaturation” J. Cryst. Growth 1967, 1, 287–292. [39] Hosoya, S.; Kitamura M. “Apparatus designed for in-situ observations of growth of crystals from aqueous solutions under well-controlled temperature conditions” Miner. J. 1978, 9, 73–90. [40] Kang, Z.; Chen, J.; Ho, H.-P. “Surface-enhanced Raman scattering via entrapment of colloidal plasmonic nanocrystals by laser generated microbubbles on random gold nano-islands” Nanoscale 2016, 8, 10266−10272.


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[41] Setoura, K.; Ito S.; Miyasaka, H. “Stationary bubble formation and Marangoni convection induced by CW laser heating of a single gold nanoparticle” Nanoscale, 2017, 9, 719–730. [42] Uwada, T.; Fujii, S.; Sugiyama, T.; Usman, A.; Miura, A.; Masuhara, H.; Kanoizaka, K.; Haga, M. “Glycine Crystallization in Solution by CW Laser-Induced Microbubble on Gold Thin Film Surface” ACS Appl. Mater. Interfaces 2012, 4, 1158−1163. [43] Lin, L.; Peng, X.; Mao, Z.; Li, W.; Yogeesh, M. N.; Rajeeva, B. B.; Perillo, E. P.; Dunn, A. K.; Akinwande, D.; Zheng, Y. “Bubble-Pen Lithography” Nano Lett. 2016, 16, 701–708. [44] Namura, K.; Nakajima, K.; Kimura, K. Suzuki, M. “Sheathless particle focusing in a microfluidic chamber by using the thermoplasmonic Marangoni effect” Appl. Phys. Lett. 2016, 108, 071603. [45] Namura, K.; Nakajima, K,; Suzuki, M. “Quasi-stokeslet induced by thermoplasmonic Marangoni effect around a water vapor microbubble” Sci. Rep. 2017, 7, 45776. • Acknowledgement



This work was supported by Grant-in-Aid for JSPS Fellows Grant Number 15J11361, JSPS KAKENHI Grant-in-Aid for Young Scientists (B) Grant Number 16K17512, JSPS KAKENHI Grant Number JP 16H06507 in Scientific Research on Innovative Areas “Nano-Material Optical-Manipulation”, JSPS KAKENHI Challenging Research (Exploratory) Grant Number JP 17K19070, JSPS KAKENHI Grant-in-Aid for Scientific Research (A) Grant Number JP 18H03884, the joint usage/research program of the Institute of Materials and Systems for Sustainability (IMaSS), Nagoya University and the Ministry of Science and Technology in Taiwan under Contracts MOST106-2113-M-009-017-. •Author Information Present Address #H.N.: Institute for Materials Research, Tohoku University, Sendai, Miyagi, 980-8577, Japan • Associated Content Ⓢ Supporting Information The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. The contents of the Supporting Information include “SI1. Polymorphism of NaClO3 crystallization from an aqueous solution”, “SI2. Preparation of the NaClO3 aqueous solution saturated at 27oC” “SI3. Procedure of the lift-off process to fabricate the plasmonic Au nanolattice” “SI4. Micrographs showing the spontaneous crystallization driven by solvent evaporation” “SI5. Movie of in-situ observation of the crystallization process induced by microbubble generation”, “SI6. The estimation of temperature distribution caused by plasmonic heating”, “SI7. Movie of in-situ observation of crystal growth of a chiral crystal in the vicinity of the


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microbubble under laser irradiation”, “SI8. Movie of in-situ observation showing chiral crystals stirred by the thermoplasmonic Marangoni convection”, “S9. Movie of in-situ observation showing the fragmentation of the “mother” crystal by microfluidic shear originating from thermoplasmonic Marangoni convection”. SI5, SI7, SI8 and SI9 are video files.



For Table of Contents Use Only

Title: In-situ Observation of Chiral Symmetry Breaking in NaClO3 Chiral Crystallization Realized by Thermoplasmonic Micro-Stirring Author list: Hiromasa Niinomi†,‡,#,*, Teruki Sugiyama§,//, Miho Tagawa¶, Shunta Harada¶, Toru Ujihara¶, Satoshi Uda#, Katsuhiko Miyamoto†,‡ and Takashige Omatsu†,‡

Synopsis: We found that chiral symmetry breaking in NaClO3 chiral crystallization can be achieved by irradiating several mW focused laser to a gold plasmonic nanolattice immersed in a stagnant NaClO3 saturated aqueous solution. In-situ microscopic observation revealed that strong Marangoni convection caused by plasmonic heating-induced microbubble generation reproduces Kondepudi’s stirring experiment of chiral symmetry breaking in microscale.


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(a) Schematic illustration showing the geometry of the experiment of the laser irradiation to a plasmonic Au nanolattice and AFM image of the Au nanolattice. (b) Schematic illustration of the optical setup for in-situ observation for the early stage of the crystallization dynamics. 84x168mm (300 x 300 DPI)



Figure 2. (a), (b) Optical micrograph showing NaClO3 chiral crystals which appeared in the solution droplet after the laser irradiation. The region surrounded by blue dashed line is the droplet of NaClO3 aqueous solution. The region squared by yellow dotted line is the region where the gold nanolattice was fabricated on. (c), (d) Polarized-light micrographs of the chiral crystals captured in the condition where the analyzer was tilted from the crossed-Nicols. The arrows depicted at upper right indicates the geometric relationship between the analyzer and the polarizer. All of the chiral crystals in the micrographs shows the same sign of the handedness. (e),(f) Magnified polarized-light micrographs of the obtained chiral crystals. 177x118mm (300 x 300 DPI)


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Figure 3. (a) Table showing the numbers of each enantiomorphs and the resulting CEE value in each crystallization experiments. (b) CEE value in each crystallization experiments. 177x61mm (300 x 300 DPI)



Figure 4. Time-lapse polarized-light micrographs showing the early stage of the crystallization dynamics which takes place in the vicinity of the focal spot. The green spot indicates the position of the focal spot. The nanolattice lies the entire view field other than the region with low contrast [indicated by dashed line in the inset in (i))]. The region with low contrast is the region where the nanolattice was destroyed by the laser irradiation. (i)-(iii) Microbubble generation from the focal spot. (iv)-(vi) Achiral metastable crystal generation at the primary nucleation process. Birefringent achiral crystal massively appeared from the microbubble. The inset picture is the magnified image. (vii)-(xii) Chiral stable crystal generation by a polymorphic transformation. After the dissolution of the achiral metastable crystals, massive chiral crystals generated from the microbubble. After the crystal generation, the crystals grew and coalesced. 177x105mm (300 x 300 DPI)


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Figure 5. (a) Time-lapse micrographs showing the crystal growth of a chiral crystal in the vicinity of the microbubble under laser irradiation. The black region lined by white dot line indicates a microbubble generated by laser irradiation. The crystal magnified in the inset is the target crystal for growth rate measurement [The crystal corresponds to the crystal circled by the red line in (iii).]. (b) Time-evolution of the size of the chiral crystal. The half of the square root of the area occupied by the chiral crystal was regarded as the crystal size. The growth rate was estimated from the slope of a linear function fitted for the obtained graph. 177x76mm (300 x 300 DPI)



Figure 6. Time-lapse optical micrographs showing chiral crystals stirred by the Marangoni convection. (i) A optical micrograph capture the state without laser irradiation. The black region surrounded by the white dashed line is a microbubble generated by laser irradiation. The region surrounded by the yellow dashed line is the gold nanolattice. A few hundreds of chiral crystals exist in the vicinity of the microbubble. (ii)-(vi) Time-lapse micrographs showing the Marangoni convection driven by the laser irradiation. The green spot indicates the position of the focal spot. The black dot lines indicate the direction of the solution flow. 177x71mm (300 x 300 DPI)


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Figure 7. Time-lapse micrographs showing the fragmentation of the “mother” crystal by microfluidic shear stress originating from thermoplasmonic Marangoni convection. The green dot indicates the position of the focal spot. The red allow indicates a single crystal of the chiral crystal. (i) A micrograph showing single chiral crystal located between the bubble surface and the substrate. (ii)-(iii) Fragmentation of the single chiral crystal by microfluidic shear originating from the thermoplasmonic Marangoni convection. (iv) The fragmented crystal carried by the convection. 83x66mm (300 x 300 DPI)



Figure 8. Schematic illustrations describing the scenario leading to chiral symmetry breaking. (i) Starting NaClO3 saturated aqueous solution. (ii) Laser irradiation to the Au nanolattice (AuNL) causes plasmonic heating at the focal spot. The resulting microbubble generation induces Marangoni convection. (iii) Solute molecules or clusters carried by the Marangoni convection are entrapped at the substrate/bubble interface, increasing local supersaturation value. (iv) Achiral precursor crystallize at the interface. (v) Polymorphic transformation to chiral crystal. (vi) Fragmentation of the chiral crystal by microfluidic shear originating from the thermoplasmonic Marangoni convection. (vii) The fragment spreads over the solution by the convection. (vii) Chiral symmetry breaking is achieved. 177x55mm (300 x 300 DPI)


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In Situ Observation of Chiral Symmetry Breaking in NaClO3 Chiral

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