Two-Dimensional Growth Rate Control of l

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Two-Dimensional Growth Rate Control of L-Phenylalanine Crystal by Laser Trapping in Unsaturated Aqueous Solution Ken-ichi Yuyama, Jino George, K George Thomas, Teruki Sugiyama, and Hiroshi Masuhara Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.5b01505 • Publication Date (Web): 04 Jan 2016 Downloaded from http://pubs.acs.org on January 11, 2016

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Crystal Growth & Design

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Two-Dimensional Growth Rate Control of L-Phenylalanine Crystal by Laser Trapping in Unsaturated Aqueous Solution Ken-ichi Yuyama,† Jino George,‡ K. George Thomas,‡ Teruki Sugiyama,*,† and Hiroshi Masuhara*,† †

Department of Applied Chemistry and Institute of Molecular Science, National Chiao Tung University, Hsinchu

30010, Taiwan ‡

School of Chemistry, Indian Institute of Science Education and Research, Thiruvananthapuram, 695016, India

The growth rate control of single L-phenylalanine plate-like anhydrous crystal is successfully demonstrated by laser trapping at an air/solution interface of the unsaturated aqueous solution. Focusing a continuous-wave near-infrared laser beam into the solution surface generates one Lphenylalanine crystal at the focal spot. Subsequently, the formed crystal grows twodimensionally at a constant rate under unsaturated condition while being trapped by the laser. When the laser power is decreased after the crystallization, the growth rate is slowed down accordingly. Thus, the two-dimensional growth rate is controllable by tuning the power of the trapping laser after the crystallization. As the critical phenomenon underlying the growth rate control, we propose the formation of a dense domain of the liquid-like clusters induced prior to the crystallization.

*Corresponding authors, current address: Prof. Teruki SUGIYAMA, Department of Applied Chemistry and Institute of Molecular Science, National Chiao Tung University, Hsinchu 30010, Taiwan, Tel.: +886 3 5712121; Fax: +886 3 5723764, E-mail address: [email protected] Prof. Hiroshi MASUHARA, Department of Applied Chemistry and Institute of Molecular Science, National Chiao Tung University, Hsinchu 30010, Taiwan, Tel.: +886 3 5712121; Fax: +886 3 5723764, E-mail address: [email protected]

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Two-Dimensional Growth Rate Control of L-Phenylalanine Crystal by Laser Trapping in Unsaturated Aqueous Solution

Ken-ichi Yuyama,† Jino George,‡ K. George Thomas,‡ Teruki Sugiyama,*,† and Hiroshi Masuhara*,†

†

Department of Applied Chemistry and Institute of Molecular Science, National Chiao Tung

University, Hsinchu 30010, Taiwan ‡

School of Chemistry, Indian Institute of Science Education and Research, Thiruvananthapuram,

695016, India


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KEYWORDS Growth rate control, Laser trapping, L-phenylalanine, Unsaturated solution, Air/solution interface

ABSTRACT The growth rate control of single L-phenylalanine plate-like anhydrous crystal is successfully demonstrated by laser trapping at an air/solution interface of the unsaturated aqueous solution. Focusing a continuous-wave near-infrared laser beam into the interface generates single L-phenylalanine crystal at the focal spot even under unsaturated condition. Subsequently, the plane area of the generated crystal becomes larger linearly with time under continued laser irradiation into the crystal central part. Two-dimensional crystal growth rate defined as a slope of the temporal change in the crystal plane area strongly depends on initial solution concentration as well as irradiation time till single crystal formation is confirmed by eye under a microscope. When the laser power is decreased after the crystallization, the growth rate is slowed down accordingly. Thus, the two-dimensional growth rate is arbitrarily controlled by tuning the laser power. As the critical phenomenon underlying the crystal growth, we propose that a dense domain consisting of a large number of the liquid-like clusters is formed prior to the crystallization. The dynamics and mechanism of the two-dimensional crystal growth is discussed by considering the supply of the solutes to the crystal edge from the cluster domain dependent on the laser power.


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1. Introduction Laser-induced crystallization receives much attention, because it has a lot of advantages such as non-mechanical-contact process, spatial-temporal controllability, polymorph selectivity, single crystal formation, and enhancement of nucleation probability.1–22 Crystallization can be divided into two major steps of nucleation and subsequent crystal growth, and the research on laser-induced crystallization has been developed aiming to trigger crystal nucleation by laser irradiation. The pioneering studies on laser-induced nucleation was conducted by Garetz, Myerson, and their co-workers from 1996.6–9 They succeeded in triggering nucleation of urea, amino acids, and proteins by irradiating nanosecond laser pulses directly into their supersaturated solutions, which contain their liquid-like clusters consisting of solutes and solvents weakly linked with intermolecular interactions. Alexander et al. also demonstrated nucleation of some alkali halides by the similar experimental procedures.10–12 In 2002, one of the present authors (HM) and Mori et al. proposed another nucleation technique using femtosecond laser pulses.13 They directly irradiated the laser pulses into supersaturated solutions of small organic molecules and proteins of several kinds and successfully demonstrated their crystal nucleation.14–18 The laser irradiation creates a cavitation bubble through multiphoton excitation of their solutions, the bubble generates new surface giving local concentration increase, and eventually their nucleation is realized. Nanosecond laser pulses also led to crystallization through the similar mechanism involving cavitation bubble formation under certain irradiation conditions.19 In those experiments, nanosecond and femtosecond laser pulses contribute to overcome an energy barrier leading to crystal nucleation, and the resultant crystals spontaneously grow due to supersaturation of the initial solution.


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In relation to laser-induced crystal growth, we point out some papers reporting on crystal growth of small organic molecules controlled photophysically20 or photochemically21. When a needle-like urea crystal in the supersaturated solution was irradiated by single femtosecond laser pulse, a new urea crystal was generated and grown at the laser focal spot. By repeating this spatially controlled crystal growth, patterning of urea crystals was successfully demonstrated. The mechanism of this crystal growth was explained by assuming that a small seed crystal was prepared photophysically through multiphoton ablation of the mother crystal by femtosecond laser irradiation. On the other hand, Okutsu et al. demonstrated nucleation and crystal growth of benzopinacol induced photochemically by UV-laser irradiation into benzophenone in ethanol/water mixed solvent. The laser irradiation caused photochemical reaction of the initial solutes, and the photoproducts were spontaneously crystalized due to their low solubility. The increase in the fluence of the UV-laser accelerated the photoreaction and resulted in the variation of the crystal morphology through elevation of the supersaturation degree. In 2007, for the first time we applied a laser trapping technique to trigger crystal nucleation and succeeded in inducing crystallization of glycine from the focal spot.22 This technique is based on gradient force of a tightly focused continuous-wave (CW) laser beam and has been widely employed as optical tweezers for trapping and manipulating small objects ranging from micrometer to nanometer in solution.23–26 Upon the laser irradiation into solution surface, local concentration is increased by laser trapping of liquid-like clusters at the focal spot. Solute molecules at solution surface are considered to be easily rearranged into a relatively ordered structure spontaneously through their intermolecular interactions, because their diffusion is suppressed two-dimensionally and the hydrophilic and hydrophobic parts of molecules interact with water and air, respectively. This spontaneous rearrangement is coupled with local


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concentration increase caused by laser trapping, and eventually nucleation takes place at the focal spot. We call this phenomenon “laser trapping crystallization”. Since the first demonstration using glycine, we have expanded this crystallization to other amino acids under various experimental conditions.27,28 For laser trapping in solution, Tsuboi et al. successfully demonstrated laser trapping of various amino acids giving their particle-like assembly.29 Most recently, we reported the crystallization of L-phenylalanine (hereafter, abbreviated as L-Phe) in the unsaturated aqueous solution.28 One of the notable findings there was that continuous crystal growth was induced even in unsaturated solution by irradiating the laser into a central part of the formed crystal. We found that the crystal growth was ascribed to the trapping phenomenon at the crystal edge. The trapping dynamics and mechanism was explained by assuming that the incident laser is propagated up to the crystal edge and accordingly optical potential was extended to the outside of the laser focus. Based on this finding, we conceived the idea that a laser trapping technique can be used not only for inducing crystal nucleation of L-Phe but also for controlling its crystal growth. In this paper, we prepared single L-Phe crystal by laser trapping at an air/solution interface of the unsaturated aqueous solution, investigated its two-dimensional (2D) growth rate under various experimental conditions, and eventually controlled the growth rate by tuning the laser power. The dynamics and mechanism of this crystal growth is explained by considering the laser power dependent trapping of L-Phe liquid-like clusters at the crystal edge and by proposing that a dense domain consisting of the clusters is formed prior to the crystallization.


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2. Experiments We prepared L-Phe unsaturated aqueous solutions with four different concentrations. L-Phe (>98.5%, anhydrous form, Sigma; 20.0, 22.5, 25.0, and 27.5 mg) was dissolved into 1.0 g of deionized water. The mixtures were vigorously shaken for several hours at 60 degrees Celsius (°C), and then slowly cooled down to room temperature (25 °C). This sample preparation was carried out automatically with a shaking machine (TAITEC, BR-21UM MR). The supersaturation value (SS) was defined as C/CS (C; concentration of L-Phe, CS; saturation concentration of L-Phe at 25 °C). The SS of respective solutions were estimated to be 0.67, 0.75, 0.83, and 0.92, based on reported data.30,31 Within a few hours after the sample preparation, a small amount (15 µL) of the solution was poured into a handmade sample glass bottle whose bottom and side wall were made highly hydrophilic. The solution was outspread on the hydrophilic bottom glass, and eventually a solution thin film with 120–160 µm thickness was prepared. After the sample bottle was completely sealed with a spigot to suppress solvent evaporation, it was set on the stage of an inverted microscope (Olympus, IX71) for further laser trapping experiments. Figure 1 shows an optical setup for laser trapping experiments constructed on the basis of an inverted microscope. A CW near-infrared (NIR) laser beam of 1064 nm from an Nd3+:YVO4 laser (Coherent. Inc., MATRIX 1064-10-CW) was used as a light source for laser trapping. The NIR laser was introduced into an objective lens (60× magnification, NA 0.90) incorporated in the inverted microscope. A green laser (Laserglow technologies, LRS-0532-TFH-01, λ = 532 nm) was also guided to the microscope in the same optical path as that of the trapping laser. This green laser was used to adjust the focal position before starting irradiation of the NIR laser. After confirming that the green laser was focused at an air/solution interface, the green laser was


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turned off, and the NIR laser was switched on. The initial power of the NIR laser throughout the objective lens was set at 1.1 W for inducing the crystallization. The power density was calculated to 0.39 GW/cm2 by assuming that the diameter of the focal spot corresponds to the full width at half maximum of the light intensity distribution. After the crystallization, the laser power was changed between 1.1 and 0.06 W for experiments on crystal growth rate control using an operation program (Coherent, Inc., Matrix consumer software). Formation and growth behavior of an L-Phe crystal was observed by a video camera (WATEC, WAT-231S2) with a chargecoupled device (CCD) under halogen lamp illumination.


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3. Results and Discussion 3-1. Formation of Single L-Phe Plate-like Anhydrous Crystal in Unsaturated Solution Spontaneous nucleation of L-Phe in aqueous solution is reported to generate two stable pseudopolymorphs of monohydrate needle-like and anhydrous plate-like forms.32,33 The needlelike monohydrate form is the most thermodynamically stable state at room temperature, whereas the plate-like anhydrous form is precipitated at temperature above a transition point of 37 °C. Actually, we checked that many needle-like crystals were generated in the solutions with four different concentrations used in this study through slow solvent evaporation when the samples were kept at room temperature (25 °C) without a spigot. On the other hand, 1.1-W laser irradiation into an air/solution interface of the unsaturated solution generated single plate-like crystal at the focal spot, independent of the initial solution concentration. As typical behavior of this crystallization, Figure 2 represents a series of CCD images during the laser irradiation in the solution with SS of 0.67 (see video 1 in Supporting Information). At the beginning of the irradiation, a CCD image showed only one bright spot ascribed to weak reflection of the trapping laser at the interface (Figure 2a). At 538 sec, a small crystal with a size of a few micrometers was identified in a CCD image. After the crystallization, we kept irradiating the trapping laser into a central part of the generated crystal. The crystal under the irradiation continued to grow while being trapped at the focal spot (Figure 2b–2c). The crystal plane area attained to 45 × 23 µm2 at 588 sec (Figure 2d), and the crystal eventually became a size larger than the field of view (80 × 60 µm2) under the microscope. The same experiments were carried out for 12 samples at each solution concentration, and all samples showed the similar crystallization behavior giving single plate-like crystal formation and


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subsequent 2D growth. The formed crystals usually had a parallelogram shape. The c-a plane of the crystal always grew along solution surface, but the ratio between lengths of c-axis and a-axis was much different among samples (See Figure S1 of Supporting Information). In order to measure crystal thickness along b-axis, we attempted to observe light reflection from the upper and lower crystal faces by moving the objective lens manually in a vertical direction. However, it was impossible to distinguish the two crystal faces well with each other. This result implies that the crystal thickness is thinner than the focal depth of a few micrometers. In order to understand the dynamics and mechanism of the above crystallization in unsaturated solution, it is indispensable to consider the formation of L-Phe liquid-like clusters and their local concentration increase under laser trapping condition. We have explained the results on laser trapping crystallization of other amino acids, like glycine and L-alanine, from the same viewpoint of local concentration increase by laser trapping of their liquid-like clusters.22,27,28 The cluster consists of solute and solvent molecules weakly linked by intermolecular interactions such as hydrogen bonding.34 Laser trapping of the clusters should be responsible for the current crystallization, because individual molecules are too small to be trapped in the focal volume under the present irradiation conditions. In unsaturated solution, large and stable L-Phe clusters are hardly expected, and almost all of the molecules are considered to exist as individual zwitterions. Although it is quite difficult to stably trap individual zwitterions, their diffusion should be slightly suppressed by gradient force of the trapping laser. For this viewpoint, we should refer to the results on suppression of molecular diffusion of organic dye in a focal volume by gradient force, reported as “biased diffusion” by Osborne et al. and Chirico et al.35,36 The suppression of molecular diffusion under laser trapping condition causes the increase in the staying time at the focal spot. As the result, L-Phe


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zwitterions evolve to liquid-like clusters through their collision and subsequently grow to a larger size. The resultant larger clusters are trapped at the focal spot for longer time, and then their assembling is started. Once the clusters begin to assemble, their effective volume and polarizability become larger, and their local concentration is increased nonlinearly with irradiation time. In this study, under the irradiation conditions, laser trapping should accompany temperature elevation over the transition point at the focal spot. Actually, the local temperature elevation was estimated to be 24−27 °C, which is ascribed to photon absorption by third-order overtone of OH vibrational mode mainly of solvent H2O molecules.37 Upon temperature elevation, supersaturation degree should be decreased. Nevertheless, the laser trapping of L-Phe induces local concentration increase overcoming the decrease by the temperature elevation. Eventually, a surrounding area of the focal spot in the unsaturated solution becomes locally supersaturated enough to trigger the crystal nucleation, and temperature is elevated above the transition, so that a plate-like anhydrous crystal of L-Phe is generated at the focus. As we reported, optical trapping of molecules/clusters is possible at the edge of the formed L-Phe plate-like crystal by continued laser irradiation into the crystal central part.28 We infer that the incident laser beam is propagated up to the edge of the crystal and optical potential is extended from the focal spot to its outside. LPhe molecules/clusters are attracted at the crystal surface possibly giving optical potential, and the crystal is continuously grown in unsaturated solution. Thus, laser trapping of L-Phe at solution surface enables us to fabricate one plate-like crystal with a size much larger than the focal volume even under unsaturated condition.


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3-2. Crystal Growth Rate Depending on Crystallization Time and Solution Concentration Laser trapping crystallization of L-Phe in aqueous solution is always initiated at the focal spot. This spatial controllability enabled us to investigate necessary time required for the crystallization through direct observation using a CCD video camera. Here, we defined the necessary time as crystallization time, at which a small crystal of a few micrometers is identified in a CCD image (See S2 of Supporting Information). Figure 3 shows crystallization time for 12 samples at each solution concentration. The crystallization time at each concentration had wide distribution, possibly ascribed to the stochastic nature of crystal nucleation. On the other hand, averaged crystallization time showed a tendency to slightly decrease against solution concentration. We infer that initial cluster formation and subsequent laser trapping are induced more efficiently with the increase in initial concentration. In the solution of a higher initial concentration, the local formation of supersaturated condition in unsaturated solution is realized more easily, and eventually the crystallization time may be slightly decreased. We found that 2D crystal growth rate along solution surface strongly depended on crystallization time as well as initial solution concentration. As representative crystal growth behavior, Figure 4a shows the time evolution of the crystal plane area observed in three different solutions with SS of 0.83. Each sample had different crystallization time, while the temporal evolution of the growing plane area was plotted as a function of irradiation time from each crystallization time as shown in the Figure 4a. In all three samples, the plane area was enlarged linearly with the irradiation time, so that the slope can be deemed as 2D crystal growth rate. The samples with crystallization time of 466, 358, and 203 sec gave growth rate of 185, 52, and 12 µm2/sec, respectively. Thus, the crystallization induced by the longer laser irradiation gives the faster 2D growth rate.


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The similar crystallization time dependence of 2D growth rate was observed at all concentrations. In Figure 4b, the 2D growth rate and the crystallization time measured in the same crystallization event were plotted for all samples. A dashed straight line in Figure 4b shows a possible correlation at each concentration. The data separately plotted at each concentration are shown in Figure S3 of Supporting Information. The 2D growth tended to proceed at faster rate in the sample with longer crystallization time at all concentrations. In addition, the slope of the dashed straight line became larger with the increase in solution concentration. This result indicates that a solution with a higher initial concentration gives a faster 2D growth rate in the case that crystallization time is the same. We consider that 2D crystal growth rate dependent on crystallization time and initial solution concentration is ascribed to concentration variation of the solution surrounding a growing crystal, because the continuous crystal growth is realized through optical trapping at the crystal edge. In order to experimentally confirm our consideration, we carried out a more detailed irradiation experiment on the crystal growth and its pause in the solution with SS of 0.83, which was based on our previous paper.28 Figure 5a shows the time evolution of the plane area when laser power was alternately changed between 1.1 and 0.06 W. Initially, single L-Phe crystal was prepared by irradiating the trapping laser into an air/solution interface at 1.1 W. The formed crystal grew continuously at a rate of 6.7 µm2/sec under the 1.1-W irradiation. After that, the laser power was decreased to 0.06 W through a time-lag of several seconds as indicated as an unshaded area in Figure 5a. This time-lag is unavoidable since it takes several seconds to complete the power change by the operation program. During the 0.06-W irradiation for 30 s, the crystal was still trapped stably at the focal spot, but its growth was stopped while keeping the crystal size. In other words, 0.06-W irradiation into the crystal is enough to keep crystal size but


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too weak to attract L-Phe molecules/clusters in unsaturated solution. Such a phenomenon can be observed only in laser trapping crystallization, because other laser-induced nucleation methods require a supersaturated solution where spontaneous crystal growth takes place. When the power was increased to 1.1 W again, the crystal immediately started growing at a rate of 6.3 µm2/sec, almost the same as the initial one. The crystal growth at 1.1 W was invariably started with quick response to the power change, whereas the initial crystallization was induced stochastically at various crystallization time. Thus, the 2D growth rate was kept under the 1.1-W irradiation even when the growth was stopped for a few tens of seconds by the 0.06-W irradiation. This result implies that the concentration of the clusters around the crystal during the late 1.1-W irradiation is almost the same as that during the initial irradiation. On the other hand, the prolonged 0.06-W irradiation caused the decrease in 2D crystal growth rate under the second 1.1-W irradiation. Figure 5b shows the time evolution of the plane area in a crystal growth event where the 0.06-W irradiation was given for 500 sec between the 1.1-W irradiations. The initial growth rate was estimated to be 13 µm2/sec, which is faster than that of the sample in Figure 5a with earlier crystallization time. During the subsequent 0.06-W irradiation, the crystal size was kept with no crystal growth. When the laser power was increased again to 1.1 W, the crystal started growing at a rate of 1.7 µm2/sec for 30 sec. Surprisingly, the growth rate was one order of magnitude smaller compared to that of the initial rate of 13 µm2/sec. It is reasonable to consider that the decrease in the growth rate is ascribed to the lowering of solution concentration around the crystal edge. This result can be explained from the viewpoint that the crystal is surrounded not with homogeneous solution but with a local dense domain consisting of the liquid-like clusters. During the long irradiation at 0.06 W, the clusters in the domain surrounding the crystal is gradually diffused out, and the concentration around the crystal


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edge is possibly decreased. The dynamics and mechanism of the formation of a dense cluster domain surrounding the crystal is discussed in the section 3-4.


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3-3. 2D Crystal Growth Rate Controlled by Laser Power The crystal growth proceeds through optical trapping of L-Phe clusters at the crystal edge in the domain, and their amount should be controllable by tuning the laser power. Based on this idea, we investigated laser power dependence of the 2D crystal growth rate by lowering the power after the crystallization in the solution with SS of 0.83. Figure 6a shows the time evolution of the crystal plane area when the laser power was decreased from 1.1 to 0.2 W at 0.3 W intervals in a step-by-step manner. The crystallization in this sample was 282 sec, and the generated crystal was continuously exposed to the laser at 1.1 W for 5 sec. During this period, the crystal grew steadily at a rate of 45 µm2/sec. After that, the laser power was reduced to 0.8 W. During the 0.8 W-irradiation for 15 sec, the crystal also became larger linearly with irradiation time, but the growth rate was decreased to 27 µm2/sec. Subsequently, the power was reduced to 0.5 and 0.2 W in a step-by-step manner, and the corresponding growth rates were estimated to be 9.9 and 3.9 µm2/sec, respectively. Here, it should be noted that the growth rate at 1.1 W in Figure 6a was much higher than that in Figure 5b, in spite that their crystallization time was nearly identical. The growth rate surely has the tendency to be faster with the increase in the crystallization time, but it still depends on samples as shown in Figure S3 (Supporting Information). This result strongly supports that a dense cluster domain is stochastically formed by laser trapping. We carried out the same experiment using another sample with crystallization time of 338 sec, as summarized in Figure 6b. The initial growth rate at 1.1 W was estimated to be 53 µm2/sec, which was faster than that in Figure 6a due to the longer crystallization time. When the laser power was decreased from 1.1 to 0.2 W at 0.3 W intervals, the growth rate was slowed down to 4.2 µm2/sec in a step-by-step manner. At respective laser power, absolute values of the growth


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rates in Figure 6b were larger than those in Figure 6a. We compared these growth rate changes against the laser power by calculating relative rates to the initial values at 1.1 W, as shown in Figure 6c. The normalized growth rates were changed against laser power along the similar decay curve. The same experiment and analysis were carried out using 3 samples for each initial concentration. Figure 6d shows the change in growth rates against the laser power normalized by the initial rate of the corresponding crystallization event. In Figure 6d, a symbol and a bar at each laser power indicates the averaged growth rate and the difference between the maximum and minimum values, respectively. The crystal growth behavior controlled in the solution with SS of 0.67 can be seen in video 2 in Supporting Information. Interestingly, the normalized growth rates in all samples showed the similar decay against laser power, independent of solution concentration. This result indicates that the growth rate is controllable by tuning the laser power, although crystallization time is uncontrollable due to the stochastic nature of crystal nucleation. Laser trapping crystallization of L-Phe enables us not only to fabricate single plate-like crystal but also to control its 2D growth rate precisely. This result will open a new door to arbitrarily engineer only one crystal in solution using light without mechanical contact.


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3-4. Dynamics and Mechanism of L-Phe Crystal Growth by Laser Trapping We here summarize dynamics and mechanism of the 2D growth for an L-Phe plate-like crystal by laser trapping. One of critical phenomena underlying the growth control is the formation of a dense cluster domain surrounding the crystal. Firstly, we discuss timing of the appearance of a cluster domain under laser irradiation. The crystal growth behavior immediately after the crystallization gave us crucial information on the domain formation. As described above, enlargement of the crystal plane area in all samples proceeded at a constant rate under the 1.1-W irradiation. This result implies that the domain should be formed prior to the crystallization because the concentration change around the crystal induces the variation of its growth rate. The generated crystal grows in the preformed domain, and its growth rate attains a constant value. Figure 7 shows a schematic illustration for dynamics and mechanism of the crystal growth. Upon the laser irradiation into solution surface at 1.1 W, L-Phe liquid-like clusters are formed through suppression of molecular diffusion by gradient force of the trapping laser. The resultant clusters are trapped in the focal volume, and local concentration is increased nonlinearly with time. Eventually, a certain stable aggregation structure of solute and solvent molecules is realized, and the energy barrier leading to the domain formation is overcome. The small domain formed at the focal spot is extended to the surrounding solution due to intermolecular/cluster interactions, heat transfer, and convection flow, as shown in Figure 7a. The concentration of a dense domain likely becomes higher with the increase in irradiation time and initial solution concentration. During laser irradiation into the dense domain, crystal nucleation is stochastically triggered at the focal spot (Figure 7b). After the crystal nucleation, the crystal grows continuously under 1.1-W laser irradiation in a dense domain through optical trapping of the clusters at the crystal edge (Figure 7c). Since the


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anhydrous crystal is surrounded with the cluster domain consisting of the liquid-like clusters, the crystal growth should be caused through dehydration of the clusters and subsequent molecular re-orientation at the crystal surface. When the power of the trapping laser is decreased, the trapping efficiency of the clusters at the crystal edge becomes lower, and the 2D growth rate is slowed down (Figure 7d). We infer that the growth rate is determined by the cluster concentration in the domain and the trapping efficiency at the crystal edge. It is likely that the former is dependent on crystallization time and initial solution concentration and the latter is determined by laser power adjustable arbitrarily. The crystal size is kept without crystal growth under the 0.06-W irradiation, although the clusters in a dense domain are gradually diffused out (Figure 7e). Finally, we discuss the size of a dense domain. We consider that a dense domain surrounding a crystal is much larger compared to the growing crystal of several tens micrometers. Due to the large size, the concentration decrease of the clusters in a dense domain accompanied with crystal growth is compensated soon, and the cluster concentration is kept to a nearly initial one during crystal growth. Furthermore, dissolution of the large dense domain through the diffusion of the clusters will be very slow, so that crystal size is kept for several hundred seconds under the 0.06-W irradiation. In laser trapping crystallization of L-Phe in D2O, a lot of needlelike monohydrate crystals were observed in an area within 1.5 mm from the focal spot.38 We explained that the crystallization takes place in a millimeter-sized dense domain prepared by laser trapping. Based on this result, we infer that the size of a dense domain in this study is also millimeter-scale. Under the present laser irradiation conditions, local temperature elevated by laser heating decreases surface tension in the central part of the solution. The inhomogeneous distribution of surface tension causes convection flow toward the peripheral part, which may


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suppress the supply of the liquid-like clusters to the crystal edge from the dense domain. The formation of a large dense domain of liquid-like clusters was reported also in the laser trapping experiments of other amino acids and protein.39-41 We consider that the domain formation by laser trapping is common in molecular system which easily forms hydrogen bonding network.


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Conclusion and Perspectives In this study, we succeeded in demonstrating control of the 2D growth rate of single platelike anhydrous crystal of L-Phe utilizing a laser trapping technique. The laser irradiation provided always single L-Phe crystal at the focal spot, and the generated crystal became larger with time by continued laser irradiation to the center of the crystal. We explained that a dense domain consisting of L-Phe liquid-like clusters is formed prior to the crystal nucleation and the crystal growth proceeds in the cluster domain. The further crystal growth to the outside of the focal spot is ascribed to optical trapping of the clusters constituting the domain at the crystal edge. The 2D growth rate of the crystal is considered to be determined by the cluster concentration in the domain and the trapping efficiency at the crystal edge. The former depends on crystallization time as well as initial solution concentration, whereas the latter is variable by the input laser power. Therefore, the 2D growth rate can be arbitrarily controlled by tuning the laser power. We consider that the laser trapping technique is generally applicable and suitable for control crystal growth for many plate-like crystals. Actually, we are demonstrating similar crystal growth behavior by laser trapping for a plate-like serine crystal. One of our present trials is three-dimensional control of crystal growth by laser trapping. We are now developing reflection microspectroscopy method measuring the time evolution of the crystal thickness during the growth. The method and the obtained results will give some critical hints to elongate the vertical direction of plate-like crystals during crystal growth. In future, we will be able to design crystals suitable for X-ray crystallographic analysis from ultrathin crystals utilizing a laser trapping technique.


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ACKNOWLEDGMENTS The present work is partly supported by the MOE-ATU Project (National Chiao Tung University) of the Ministry of Science and Technology, Taiwan, to H.M., the National Science Council of Taiwan to K.Y. (MOST 103-2113-M-009-022-MY2), to T.S. (MOST 104-2113-M009-021-), and to H.M. (MOST 103-2113-M-009-003).

SUPPORTING INFORMATION Videos showing crystallization and crystal growth, crystals formed in the different concentration solutions, CCD images around crystallization time, and the correlation between 2D growth rate and crystallization time. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 1: Schematic illustration of an optical setup for laser trapping experiments.


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Figure 2: CCD images during the formation of single plate-like L-Phe crystal and its subsequent growth induced by 1.1-W laser irradiation into an air/solution interface of the unsaturated solution (SS = 0.67). Elapsed time from the beginning of the irradiation is shown in each image.


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Figure 3: Crystallization time measured for 12 samples at each concentration. Averaged crystallization time at each SS is shown as a filled circle.


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Figure 4: (a) The time evolution of the plane area for three samples with different crystallization time in the solution with SS of 0.83. Their 2D growth rates and crystallization time are indicated in the graph. (b) The correlation between 2D growth rate and crystallization time for all samples. Dashed straight lines are shown in the graph as a possible relation between 2D growth rate and crystallization time.


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Figure 5: The time evolution of the plane area when the lase power was alternately changed between 1.1 and 0.06 W in the solution with SS of 0.83. Crystallization time and crystal growth rates are shown in each graph.


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Figure 6: (a), (b) Time evolution of the plane area upon decreasing laser power from 1.1 to 0.2 W at 0.3 W intervals in a step-by-step manner in the solution with SS of 0.83. Crystallization time of respective samples is 282 and 338 sec, which correspond to the irradiation time of 0 sec in the horizontal axis. The input laser power and the corresponding growth rate are shown in the shaded areas of the graphs. (c) Normalized growth rates for samples of Figure 6a and 6b. (d) Laser power dependence of the normalized growth rates examined for 3 samples at each concentration. A symbol represents the growth rate averaged over 3 samples. A bar indicates the difference between the maximum and minimum values.


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Figure 7: The schematic illustration for the dynamics and mechanism of 2D growth of single LPhe plate-like crystal prepared by laser trapping at solution surface.


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For Table of Contents Use Only

Two-Dimensional Growth Rate Control of L-Phenylalanine Crystal by Laser Trapping in Unsaturated Aqueous Solution

Ken-ichi Yuyama, Jino George, K. George Thomas, Teruki Sugiyama, and Hiroshi Masuhara

Short synopsis The growth rate control of single L-phenylalanine crystal is demonstrated by laser trapping at an air/solution interface of the unsaturated aqueous solution. Focusing a continuous-wave nearinfrared laser beam into the solution surface generates single plate-like crystal at the focus. The crystal grows two-dimensionally while being trapped by the laser, and its growth rate is controllable by tuning the laser power.


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Two-Dimensional Growth Rate Control of l

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