Crystal Growth and Dissolution Dynamics of L-Phenylalanine

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Crystal Growth and Dissolution Dynamics of LPhenylalanine Controlled by Solution Surface Laser Trapping Ken-ichi Yuyama, Ding-Shiang Chiu, Yen-En Liu, Teruki Sugiyama, and Hiroshi Masuhara Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01233 • Publication Date (Web): 02 Oct 2018 Downloaded from http://pubs.acs.org on October 8, 2018

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Crystal Growth & Design

Crystal Growth and Dissolution Dynamics of L-Phenylalanine Controlled by Solution Surface Laser Trapping

Ken-ichi Yuyama,†‡ Ding-Shiang Chiu,‡ Yen-En Liu,‡ Teruki Sugiyama,*,‡§∥ and Hiroshi Masuhara*,‡§

†

Research Institute for Electronic Science, Hokkaido University, Sapporo, Hokkaido 001-0020,

Japan ‡

Department of Applied Chemistry, National Chiao Tung University, Hsinchu 30010, Taiwan

§

Center for Emergent Functional Matter Science, National Chiao Tung University, Hsinchu

30010, Taiwan ∥

Division of Materials Science, Graduate School of Science and Technology, Nara Institute of

Science and Technology, Ikoma, Nara 630-0192, Japan

ACS Paragon Plus Environment

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ABSTRACT We present sequential behavior of nucleation, growth, and dissolution of L-phenylalanine platelike crystal which is induced by a focused continuous-wave near-infrared laser beam in unsaturated solution. Upon the laser irradiation into the air/solution interface, the single crystal is generated from the focus and continuously grows two-dimensionally while being trapped by the laser. The crystal growth is stopped when the laser power is decreased. The crystal size is kept constant for a certain time period, and then the crystal starts dissolution. The dissolution is also induced by moving the crystal from the focus at the air/solution interface. When the crystal is shifted far from the original position where the crystallization is induced, the crystal starts dissolution at a certain distant position. Based on the demonstrations of the crystal size change during the laser manipulation, we conclude that the L-phenylalanine crystal is surrounded by a highly concentrated domain of a few hundreds µm consisting of L-phenylalanine liquid-like clusters.


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1. INTRODUCTION Laser trapping with the use of a tightly focused laser beam was firstly demonstrated for dielectric particles by Ashkin in 1986.1 Ever since, it has played innovative roles as optical tweezers to manipulate single micrometer-sized objects, living cells, viruses, and bacteria without mechanical contact.2–10 For the past two decades, laser trapping study in chemistry progressed with size reduction and diversification of trapping targets. Representative examples are plasmonic nanoparticle11–14, micelle15, carbon nanotube16, DNA17, quantum dot18,19, polymer20–22, supramolecule23, protein24,25, and amino acid26. The trapping targets are sequentially confined in the focal volume where optical potential reflecting the distribution of light intensity is generated, leading to the formation of their single assembly. Usually the size of the prepared assembly is comparable to that of the focus, however, in some cases, it grows toward the outside of the focus through strong intermolecular interactions.27–29 Interestingly, the protein assembly formed by laser trapping evolves to a crystal after stopping the trapping.30,31 In 2007, we demonstrated that crystallization is induced from the focal spot during laser trapping upon irradiating a continuous-wave (CW) near-infrared (NIR) laser beam into an air/solution interface of a supersaturated D2O solution of glycine.32 After that, we systematically examined this laser trapping-induced crystallization by changing solution and irradiation conditions as well as solute molecules.29, 33–36 It has been confirmed indispensable to irradiate the trapping laser into the solution surface for laser trapping-induced crystallization. The position dependent trapping behavior is reported for glycine. Crystallization is induced under the trapping at an air/solution interface.32–34 On the other hand, a particle-like molecular assembly is formed inside the solution, and a single millimeter-scale dense droplet is prepared at a glass/solution interface of a solution thin film.26,27 Considering these results, we proposed that the


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crystallization is induced through cooperative phenomena of local concentration increase at and around the trapping focus and molecular re-ordering characteristic of solution surface. To date, laser-induced crystallization has been extensively developed using different light sources, such as nanosecond pulsed laser causing optical Kerr effects (NPLIN)37–41, femtosecond pulsed laser generating cavitation bubbles42–44, UV laser triggering photochemical reactions45,46 in addition to CW laser inducing optical trapping. One notable advantage in laser trappinginduced crystallization is that single crystal is prepared in a spatio-temporally controlled manner32–36; therefore we can directly observe dynamic growth behavior of the single crystal under different solution and laser irradiation conditions. By taking this advantage, we found that a rate at which the single L-phenylalanine (L-Phe) crystal of the plate-shaped anhydrous form grows two-dimensionally can be arbitrarily controlled even in its unsaturated solutions by changing the power of the trapping laser.36 Furthermore, we suggested that a dense domain consisting of liquid-like clusters surrounds the prepared L-Phe crystal and how the crystal growth rate is determined depending on this domain. In this paper, we have studied on laser trapping-induced crystallization of L-Phe for investigating formation and dissolution dynamics of the dense cluster domain surrounding one single crystal. The designed experiments are based on direct observation of crystal dissolution as well as nucleation and crystal growth, which is due to unique laser trapping nature of possible crystallization even in an unsaturated solution. The sequential behavior of nucleation, growth, and dissolution of single L-Phe crystal of plate-like shape is examined by measuring the temporal change in its size under various trapping conditions. When the power of the trapping laser is lowered to 5% of the original one after the crystallization, crystal growth is almost completed, the crystal size is kept constant for a certain time period, and then the crystal starts


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dissolution. The dissolution is also observed by manipulating the crystal at the solution surface. We discuss this dynamic behavior of single L-Phe crystal by considering formation, growth, and dissolution of a highly concentrated cluster domain of a few hundreds µm around the crystal.

2. EXPERIMENTS An unsaturated aqueous solution of L-Phe was prepared by dissolving L-Phe (>98.5%, anhydrous form, Sigma) of 17.5 mg in deionized water of 1.0 g. The mixture was heated at 60 degrees Celsius (°C) with vigorous shaking for several hours and subsequently cooled down to 25 °C slowly. The saturation degree (SS) of the prepared solution was estimated at 0.58.47 In this paper, SS is defined as the ratio (C/Cs) of the concentration of the solute in the prepared solution (C) to that in the saturated solution at 25 °C (Cs). A thin film (120–160 µm) of the solution was prepared by placing the solution of 15 µL into a glass chamber with a highly hydrophilic surface of the wall and bottom. The sample chamber was tightly closed to avoid solvent evaporation and set on the computer-controllable motorized XY stage (Ludl Electronic Products, Ltd.) of an inverted microscope (Olympus, IX71). Figure 1 shows an optical setup used in this study. The setup was constructed by introducing a CW NIR laser (Coherent Inc., MATRIX 1064-10-CW) of the wavelength of 1064 nm as a trapping light source to the inverted microscope. The NIR laser was focused into a surface layer of the solution thin film through an objective lens (×60 magnification, numerical aperture; 0.90). A green laser was also introduced to the microscope through an optical path same as that of the NIR laser. Before the NIR laser was turned on, we used this green laser to adjust the focal position. For inducing crystallization, the initial power of the NIR laser passing through the objective lens was tuned at 1.1 W. Subsequently, the power was lowered to 0.06 W


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for experiments on crystal dissolution. The power was changed by rotating a half-wave plate placed in front of a polarization beam splitter. The half-wave plate was equipped in a computercontrollable motorized holder, so that its angle was tuned by operating a computer remotely. In experiments for manipulating the crystal, the sample stage was moved at a rate of 100 µm/sec after single L-Phe crystal was formed by the 1.1-W irradiation. The crystallization and crystal dissolution behavior was observed with a charge-coupled device (CCD) camera under halogen lamp illumination.

3. RESULTS and DISCUSSION 3-1. Nucleation, Growth, and Dissolution of Single L-Phe Crystal L-Phe is known to have two stable pseudopolymorphs of needle-like monohydrate and plate-like anhydrous forms. Their transition point is 37 °C in aqueous solution. The former and the latter are obtained at temperature below and above the transition point of 37 °C, respectively. 48

We already demonstrated that the latter crystal can be formed in a spatial-temporally

controlled manner by laser trapping.35,36 One single plate-like anhydrous crystal was prepared from the focus when the NIR laser of 1.1 W was irradiated at a surface layer of the L-Phe H2O solutions with SS of 0.67–0.92. We explained this crystallization is made possible by the concentration increase due to laser trapping and local temperature elevation due to laser heating. Here, we show the similar crystallization behavior induced in the solution with further lower SS of 0.58 where laser trapping-controlled crystal growth and dissolution are observed more clearly. In the early stage of the 1.1-W irradiation, a bright point ascribed to light reflection at the air/solution interface was observed in a CCD camera image (panel (i) of Fig. 2a). We could see a


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1064-nm laser beam at the interface as a greenish blue spot because a CCD camera used has the weak sensitivity in the near-infrared region. A small crystal of a few µm became recognized in a CCD image at about 400 sec. The crystal grew continuously while being stably trapped at the focus (panels (ii)–(iii) of Fig. 2a). In this unsaturated solution, most L-Phe molecules are considered to exist as individual zwitterions. Stable trapping of individual zwitterions is difficult due to their small size. On the other hand, their diffusion at the focus area should be suppressed by gradient force of a focused laser beam, as reported as “biased diffusion”.49,50 The suppression of molecular diffusion increases the staying time of zwitterions inside the focal volume. Consequently, they encounter with each other, a transient dimer and/or oligomer are prepared, and stronger trapping force is loaded on them, eventually leading to formation of liquid-like clusters, in which solute and solvent molecules weakly interact with each other. Subsequently, the clusters grow to a large size through interactions with further clusters and zwitterions. Since the trapping force becomes stronger with the increase in effective volume of a target object, the staying time of the larger clusters formed becomes longer. We consider that, through this dynamic formation of liquid-like clusters and subsequent their trapping, local concentration at the focus is increased nonlinearly with irradiation time and the supersaturated condition is attained locally and transiently in the present unsaturated solution during the laser irradiation. In addition, this local high concentration expands to the outside of the focus even in the unsaturated solution, giving a larger domain of the clusters. Under the present irradiation conditions, temperature elevation over the transition point is induced concurrently with laser trapping. The temperature elevation at the focal spot was estimated to be 24−27 °C, which occurs through light absorption at second overtone of OH


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vibrational mode mainly of H2O molecules.51 Through this local laser heating, the temperature in the focal volume should become around 50 °C. Consequently, cooperative phenomena of laser trapping and laser heating lead to the formation of the supersaturated region that has the temperature above the transition point, where the crystallization of a plate-like anhydrous form is induced at the focus. The laser heating also causes the change in surface tension, and its contribution to crystal growth is discussed later. Interestingly, further irradiation into a central part of the generated crystal led to continuous crystal growth although initial solution is unsaturated. We already proposed optical mechanism for this growth as follows. The trapping laser propagates inside the crystal from the focus to the crystal edge and forms the optical potential at the crystal edge. The extended optical potential attracts L-Phe molecules/clusters in the surrounding to the crystal.35 For such extension of the trapping potential during the trapping, we introduce our recent result on dielectric particles in solution.52 The trapping of 500-nm polystyrene nanoparticles at a glass/solution interface formed a large-size, disk-like assembly sticking out a chain-like structure where nanoparticles were confined one-dimensionally. The formation of this large assembly with the chain-like structure is possibly ascribed to outward light scattering and/or propagation through the assembly. Indeed, in the crystal growth of L-Phe,35 a framework-like structure was similarly formed from the focus toward the crystal edge, so that we infer that the laser guided to the edge generates optical forces resulting in continuous crystal growth. After the crystallization, the laser power was lowered from 1.1 to 0.06 W with a time-lag of several seconds that corresponds to time for rotating a half-wave plate by the computer. During the 0.06-W irradiation, the crystal was still trapped stably at the focus, but its growth was stopped. The crystal size was kept constant for 30 s (panels (v)–(vi) of Fig. 2a), and then crystal


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started dissolution. The dissolution proceeded from corners of the crystal, and the crystal gradually became ellipsoidal with accompanying the decrease in its size (panels (vii)–(viii) of Fig. 2a). The entire process of Fig. 2a can be seen in Supporting Information. For quantitative analysis, we measured the plate area of the formed single L-Phe crystal. Figure 2b shows the temporal size change of the sample in Fig. 2a. During crystal growth at 1.1 W, the crystal plate area continuously became large almost linearly with irradiation time. Its temporal change could be fitted well with a linear function. We deemed the slope as twodimensional (2D) growth rate, which was estimated to be 2.2 µm2/s. At 0.06 W, the crystal plate area was kept at 360 µm2 for 30 s and then decreased gradually, showing crystal dissolution. The decrease in the crystal plate area also proceeded linearly with time and could be fitted with a linear function. Its negative slope was defined as 2D dissolution rate and calculated to be -6.7 µm2/s. We repeated the same experiment for 22 samples. The similar behavior was observed in all samples. Namely, the 1.1-W irradiation caused the formation of a plate-like crystal and its continuous growth, and the 0.06-W irradiation kept the crystal plate area constant before dissolution. Figure 2c shows the schematic illustration of the temporal change in the crystal plate area. In all 22 samples, we calculated five parameters; time necessary for crystallization (Tcrystallization), growth rate (Rgrowth), stationary crystal size (Sstationary crystal), time period for giving stationary crystal size (Tstationary crystal), and dissolution rate (Rdissolution) by analyzing the results in the following way. The temporal change in the crystal plate area was separated into three sections; (1) crystal growth, (2) stationary state keeping constant crystal size, and (3) crystal dissolution, as shown in Fig. 2b. The temporal change in the crystal plate area during crystal growth and dissolution was fitted with a linear function of y = ax+b, where y and x are the


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crystal plate area and the irradiation time, respectively. The slopes of respective fitted linear functions are Rgrowth and Rdissolution. The crossover point between the x-axis and the fitted linear function for the crystal growth was defined as Tcrystallization. In the time period for giving stationary crystal size, the function of y = Sstationary crystal was used for fitting the measured scatter in the crystal plate area. We estimated Tstationary crystal by using crossover points of three fitted functions. The relations among these parameters are summarized in Figure 3. Tcrystallization was much different among samples. This result is probably ascribed to the stochastic nature of crystal nucleation since we analyze one single crystal formation upon each trapping experiment. A sample with long Tcrystallization showed a tendency to have fast Rgrowth (Fig. 3a). This result was also observed in the L-Phe H2O solutions with higher SS, as we reported in the previous paper.36 Tstationary crystal also became long with the increase in Tcrystallization (Fig. 3b). When Tcrystallization was long, crystal dissolution proceeded with slow Rdissolution (Fig. 3c). Although their relations had large fluctuations, the crystallization and crystal dissolution behavior can be summarized as follows. In the case that it takes long irradiation time for crystal nucleation, the crystal grows with a fast rate under the laser irradiation at 1.1 W. After the laser power is lowered to 0.06 W, the crystal size is kept for long time, and the subsequent crystal dissolution proceeds with a slow rate. Actually, the crystallization event giving fast Rgrowth showed long Tstationary crystal (Fig. 3d), leading to slow Rdissolution (Fig. 3e). Incidentally, it is likely that Tstationary crystal is less related to Sstationary crystal as shown in Fig. 3f. It is reasonable to consider that the crystal dissolution is ascribed to lowering of solution concentration around the crystal edge. Namely, the crystal is surrounded with homogeneous solution but with a local dense domain consisting of the liquid-like clusters.36 The Rgrowth,


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Tstationary

crystal,

and Rdissolution should be correlated with size of the cluster domain and

concentration of the clusters in the domain, which will be discussed in the section 3-4.

3-2. Crystal Dissolution by Optical Manipulation Optical manipulation experiments of the formed crystal allowed us to estimate the size of the dense cluster domain surrounding one single L-Phe crystal. The experimental procedure is schematically illustrated in Fig. 4a. First, an L-Phe crystal was prepared and grown to be size of several hundreds µm2 under the trapping at 1.1 W. Subsequently, the sample stage was moved at the rate of 100 µm/s. The prepared crystal was shifted away from the position where the crystallization was induced. We measured a crystal plate area during the manipulation as the function of distance from the original position. The temporal change in the crystal plate area during the crystallization and subsequent manipulation is shown in the panel (i) of Fig. 4b. An L-Phe crystal was formed at 654 s and continuously became large at a rate of 38 µm2/s. We started the optical manipulation of the crystal at 668 s. In the manipulation process, the center of the crystal was always trapped by the laser without migration away from the focus. Interestingly, the crystal suddenly started dissolving when it passed a certain point. The change in the crystal plate area during the manipulation is shown in the panel (ii) of Fig. 4b where the x-axis was the distance from the original position. Snapshots of the crystal at specific positions are included in the graph. No apparent change in crystal size and shape was observed within the distance of 200 µm. The crystal became smaller upon shifting to farther distant positions. The crystal dissolution indicates that the crystal was exposed to unsaturated condition. We consider that the crystal size is kept


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constant inside the dense cluster domain and the crystal dissolution is induced outside the domain, namely, crystal dissolution starts near the edge of the domain. Assuming that the dense domain surrounding the crystal does not move together with the crystal during its manipulation, the radius of the domain in the sample of Fig. 4b is estimated to be 200 µm which is much larger than the focal volume. Incidentally, the crystal dissolution behavior depends on the manipulation conditions. When the moving distance of the stage is short, no crystal dissolution is induced. This is probably because the crystal after the manipulation is still inside the dense cluster domain. Similarly, no crystal dissolution occurs at the slow moving speed. We infer that, at the slow manipulation, the cluster domain surrounding the crystal moves together with the crystal. We repeated the same experiment as summarized in Fig. 4c. Another sample showed Tcrystallization of 333 s and Rgrowth of 19 µm2/s which were nearly half compared to the sample of Fig. 4b. During the manipulation, the size was kept constant within the distance of 100 µm, and crystal dissolution was observed at the farther positions. This result indicates that the crystal is surrounded with a dense cluster domain with the radius of 100 µm half of that of Fig. 4b. It is likely that a dense cluster domain of the larger size surrounds the crystal formed by longer laser irradiation, because the crystal showed a faster growth rate and started dissolution at the farther point from the original focal spot.

3-3. Repetitive Behavior of Growth and Dissolution of Single L-Phe Crystal For investigating formation dynamics of a dense cluster domain, the irradiation experiments in more details were carried out by changing the laser power in an alternate manner. We firstly observed crystal growth and dissolution through the 1.1-W and the following 0.06-W irradiations


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in the procedure same as the section 3-1. After that, the laser power was again increased to 1.1 W and subsequently decreased to 0.06 W. Figure 5a presents the temporal change in a crystal plate area during the laser irradiation. The formed crystal became large continuously at 790–845 s while being trapped by the laser. At 0.06 W, the crystal plate area was kept constant at 850–980 s. Subsequently, the crystal decreased in its size through dissolution at 990–1110 s. We consider that the crystal at 850–980 s was enclosed with a dense domain whereas the dissolving crystal at 990–1110 s was not surrounded with the domain and exposed to the solution keeping initial unsaturated condition. After the laser power was returned at 1.1 W, the crystal grew at 1120–1275 s. The crystal plate area was kept constant at 1280–1460 s at 0.06 W. Subsequently, the crystal size became small through dissolution. It is notable that steady state keeping the crystal size constant was again observed at the early stage of the second 0.06-W irradiation. This result means that the crystal at 1280 s was surrounded with a dense cluster domain. Since the domain firstly surrounding the crystal is once dissolved at 980 s in the first 0.06-W irradiation, it is reasonable to consider that the domain is formed again during the crystal growth under the second irradiation with 1.1 W. Thus, a dense domain is enlarged around the crystal during its growth although the focal volume is completely occupied by the growing crystal. The domain formation can be confirmed by examining the change in the crystal growth rate. Figures 5b and 5c show the temporal change in a crystal plate area during crystal growth under the first and second 1.1-W irradiation, respectively. In the former case, the crystal constantly became large with a rate of 14 µm2/s (Fig. 5b). In the latter case, the growth behavior was different between early and late stages. The crystal growth rates at respective stages were estimated at 4.2 and 7.9 µm2/s. The acceleration of the growth rate is due to concentration


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increase around the crystal during the second irradiation with 1.1 W, which should be ascribed to re-generation of the dense cluster domain. This kind of behavior of increased growth rate was not observed in the first 1.1-W irradiation. We consider that, in the first crystal growth, the crystal is already surrounded with a large-size dense cluster domain formed before crystallization.

3-4. Dynamics of Formation, Growth, and Dissolution of a Dense Cluster Domain Figure 6 shows a schematic illustration for possible dynamics of laser trapping of L-Phe at the air/solution interface on the basis of all the above results. Upon the 1.1-W irradiation (panel (i) of Fig. 6a), L-Phe liquid-like clusters are formed, and the resultant clusters are trapped in the focal volume. Eventually, a stable aggregate consisting of L-Phe and H2O molecules is generated, resulting in the formation of a cluster domain within the laser focus. The small domain formed in the focal volume is considered to be in a metastable state and grown up to the size in the scale of several hundreds µm. A dense domain likely increases in its size and concentration with the irradiation time. We consider that the cluster domain outside the focal volume is formed not only through cluster concentration increase but also through specific orientation and mutual interactions of L-Phe molecules induced at the focus by intense irradiation of the trapping laser. We infer that the domain size is also dependent on the initial solution concentration and grows up to the size in a millimeter-scale at high concentration as we reported previously.27 We also consider the decrease in surface tension of the center part of solution due to local temperature elevation by laser heating.53 The inhomogeneously distributed surface tension generates convection flow near the solution surface that is toward the peripheral part.54 On the other hand, the convection flow in a central part of the solution induces mass transfer toward the solution surface from the bottom, supporting the growth of the dense cluster domain. We consider that


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strong intermolecular interactions have critical role for the formation of a dense cluster domain. Neutral amino acids including L-phenylalanine form zwitterions in H2O and D2O, and their zwitterions have strong electrostatic interactions with solute molecules as well as solvent molecules. We infer that such molecules have a potential to form a dense cluster domain through the intermolecular interactions under the optical trapping conditions in a neutral polar solvent. Crystal nucleation is stochastically induced by continuous laser irradiation into the generated cluster domain at the focus (panel (ii) of Fig. 6a). As explained above, it is also considered that light propagation through the formed crystal leads to collection of clusters at the edge of the crystal; therefore the clusters in a few hundreds µm-sized dense domain are optically trapped at the crystal edge, and the crystal grows continuously. We consider that laser irradiation into the center part of the crystal also leads to the continuous growth of the dense cluster domain. Although solute molecules in the cluster domain are continuously consumed for the crystal growth, the concentration decrease in the domain is compensated by the molecules supplied through the growing domain during crystal growth (panel (iii) of Fig. 6a). When the laser power is decreased to 0.06 W, the clusters cannot be trapped at the crystal edge. The crystal growth is paused, and the crystal plate area is kept nearly constant (panel (i) of Fig. 6b). Even for this time, the surrounding dense cluster domain is gradually dissolved through the outward diffusion of constituent clusters (panel (ii) of Fig. 6b). When the concentration around the crystal comes back to that of initial unsaturated one, the crystal starts dissolution (panel (iii) of Fig. 6b). The dissolution rate is likely determined by the local concentration of clusters around the dissolving crystal, which should depend on the initial size and concentration of the domain. The formation, growth, and dissolution of a dense cluster domain can be


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repetitively induced by tuning the laser power even when the focal spot is completely occupied by the crystal. In the case of pulsed-laser irradiation, a starting sample is a supersaturated solution in which large-sized liquid-like clusters exist. However, the clusters are not gathered under the irradiation of laser pulses, unlike laser-trapping induced crystallization. The rearrangement of solute molecules inside the clusters existing in the initial supersaturated solution is induced through optical Kerr effects under the strong electric field of a nanosecond laser pulse or through the generation and collapse of a cavitation bubble under irradiation of the femtosecond laser pulse. As the result, the energy barrier for crystal nucleation is overcome, and crystallization occurs.

CONCLUSION and PERSPECTIVES Sequential behavior of nucleation, growth, and dissolution of single plate-like crystal of LPhe was examined by measuring the temporal change in the crystal size under various trapping conditions. The trapping provided one single L-Phe crystal at the focus, and the formed crystal grows under the irradiation with time. When the laser power was decreased, the crystal dissolution started after a certain time period keeping the crystal size constant. We explained this dynamic behavior of the L-Phe crystal from the viewpoint of a dense cluster domain of a few hundreads µm surrounding the crystal. The dense cluster domain is formed before the crystal is nucleated, and its size enlarges and its concentration increases with the irradiation time. The domain may grow also during crystal growth by the irradiation to the crystal central part. We consider that the size of the domain and the cluster concentration in it determine the crystal growth and dissolution rates as well as the stationary time keeping the size constant.


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At the present stage, we have never succeeded in directly observing the dense cluster domain under a microscope.36,55 One of the reasons is that the domain size is much larger compared to the field of view determined by magnification of the objective lens and a sensor area of the CCD camera. New imaging or spectroscopic system is indispensable to examine a dense cluster domain under laser trapping conditions. Laser trapping experiments require a high numerical aperture objective lens of large magnification for tightly focusing the laser, whereas an objective lens of small magnification is suitable to enlarge an observation area. The optical system equipping two objective lenses that can be separately operated for laser trapping and observation will enable us to investigate the characteristics of the cluster domain under laser trapping conditions.

ACKNOWLEDGMENTS This work is financially supported by the Center for Emergent Functional Matter Science of National Chiao Tung University from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan. Thanks are also due to the Ministry of Science and Technology (MOST) in Taiwan (MOST 106-2113-M-009-017- to T.S. and MOST 107-2113-M-009-026- and 107-M-2113-M009-013- to H.M.) and JSPS KAKENHI Grant Number JP16H06507 in Scientific Research on Innovative Areas “Nano-Material Optical-Manipulation” to T.S. and JP17K14427 for Young Scientists (B) to K.Y.

SUPPORTING INFORMATION


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This material is available free of charge via the Internet at http://pubs.acs.org. Video showing the sequential process of nucleation, growth, and dissolution at 20 times speed (mpeg).

AUTHOR INFORMATION Corresponding Authors *Tel.: +886 3 5712121. Fax: +886 3 5723764. E-mail: [email protected] *Tel.: +886 3 5712121. Fax: +886 3 5723764. E-mail: [email protected]

ORCID Ken-ichi Yuyama: 0000-0002-6998-6942 Teruki Sugiyama: 0000-0001-9571-4388 Hiroshi Masuhara: 0000-0002-4183-5835

Notes The authors declare no competing financial interest.


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Figure 1: Schematic illustration of an optical setup for laser trapping experiments. HWP and PBS denote a half-wave plate and a polarizing beam splitter, respectively.


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Figure 2: (a) CCD images around the focal spot during laser trapping. Elapsed time from the beginning of the irradiation and the laser power are shown in each image. The size of images is 40 µm in width and 30 µm in height. Images except for (i) were obtained with the use of an optical filter to cut off the NIR laser. (b) The time evolution of a plate area of an L-Phe crystal generated under laser trapping. The sample is same as that in (a), and the laser power is presented in the graph. (c) Schematic illustration of the temporal change in an L-Phe crystal plate area.


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Figure 3: Relations among Tcrystallization, Rgrowth, Tstationary crystal, Sstationary crystal, and Rdissolution. The examined total sample number is 22.


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Figure 4: (a) Schematic illustration for the experiment procedure of optical manipulation of an L-Phe crystal. (b, c) (i) The temporal change in the crystal plate area during the crystallization and the subsequent optical manipulation. (ii) The change in the crystal plate area as the function of distance from the original position during the manipulation process and snapshots of the crystal at specific positions. The size of images is 40 µm in width and 30 µm in height.


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Figure 5: (a) The time evolution of a crystal plate area upon the alternate change in the laser power between 1.1 and 0.06 W. The temporal change in a crystal plate area during the (b) first and (c) second crystal growth under the 1.1-W irradiation.


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Figure 6: Schematic illustration of laser trapping dynamics of L-Phe at the solution surface at (a) 1.1 W and (b) 0.06 W.


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For Table of Contents Use Only

Crystal Growth and Dissolution Dynamics of L-Phenylalanine Controlled by Solution Surface Laser Trapping Ken-ichi Yuyama, Ding-Shiang Chiu, Yen-En Liu, Teruki Sugiyama,* and Hiroshi Masuhara*

TOC graphic

Synopsis Sequential behavior of nucleation, growth, and dissolution of a single plate-like anhydrous crystal of L-phenylalanine is examined under various laser trapping conditions. A highly concentrated domain of a few hundreds µm consisting of L-phenylalanine liquid-like clusters surrounds the L-phenylalanine crystal, and the size of the domain and the cluster concentration there determine the growth and dissolution behavior of the single crystal.


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Crystal Growth and Dissolution Dynamics of L-Phenylalanine

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