In this study, we conducted successful experiments on ethylenediamine sulfate (EDS), an organic compound, to investigate its enantioselectivity in chiral crystallization. We employed optical trapping with circularly polarized laser beams, using a continuous wave laser at 1064 nm. By focusing the laser at the air–solution interface of a heavy water-saturated EDS solution, the formation of sub-micrometer-sized chiral EDS crystals was verified. Two generated enantiomorphs (d-crystal and l-crystal) were identified by the rotating analyzer method. The enantioselectivity in the chiral crystallization of EDS was assessed through 30 to 60 times experiments conducted under various conditions of laser powers and polarization modes, utilizing the count of generated crystals for each enantiomorph in the evaluation. Circularly polarized lasers at a specific power created an imbalance in the generation probability of the enantiomorphs, resulting in crystal enantiomeric excess values of 23% and −30%. The enantioselectivity mechanism was explored from two perspectives: refractive index differences of two enantiomorphs and 3D helical optical forces. Study of the thermodynamic mechanism was insufficient to explain the outcomes. Conversely, the 3D helical optical force mechanism revealed that the forces acting on EDS clusters in solution induced helical fluid motion, driving EDS nucleation, with the helicity of fluid motion determining the crystal’s chirality. This approach will present new insights into chirality in industrial and research fields, with potential applications in regard to improving optical resolution and addressing the origin of homochirality.

Optical trapping is an experimental technique using laser light to trap and manipulate tiny objects without mechanical contact. This technique is particularly used in combination with optical microscopes and is now applied in various fields, including biology, physics, and chemistry.1–5 Optical trapping, first reported in 1970 by Ashkin, established the theoretical framework for manipulating small objects using optical methods.6 Subsequently, in 1986, the authors of the work of Ashkin et al. succeeded in the three-dimensional trapping of dielectric micro-particles under a microscope using a single focused laser beam.7 Our research group has expanded this optical trapping method into the field of crystal science, successfully producing crystals of various compounds, such as amino acids,8,9 proteins,10,11 and inorganic compounds.12 Furthermore, we also achieved control of their crystal growth rate10,13 and polymorphs9,14 using this method. We have called this method “optical trapping-induced crystallization (OTIC).” In particular, the study of polymorph control using the OTIC method is highly intriguing. By optimizing experimental conditions, such as laser power, laser polarization modes, and initial solution concentration, it is possible to control crystal polymorph selectively. Polymorph control is not only industrially important for improving drug solubility15,16 and bioavailability17,18 but also scientifically intriguing from the standpoint of light–matter interactions in molecular aggregation dynamics.

On the other hand, chirality is associated with a wide range of chemical and physical phenomena in nature, and its understanding and control represent a core challenge in science. It plays a crucial role in various fields, such as drug design,19–21 pesticide development,22 and the design of new materials.23,24 For example, the interactions and reactions of chiral molecules depend on their stereochemical properties, which can significantly impact the efficacy and side effects of drugs, the selectivity of catalytic reactions, and the properties of materials.25 The chirality of crystals is a concept of great academic and industrial importance, just like the chirality of molecules. Chiral crystallization refers to the phenomenon in which achiral molecules in a solution undergo a phase transition to form crystals with chiral space groups.26,27 The resulting two enantiomorphs (d- and l-crystals) have mirror-image relationships in their molecular arrangements inside the crystals. These two enantiomorphs share almost all physical properties except for their optical rotation, yet chiral drugs can exhibit significant differences in pharmacological effects and side effects based on their chirality.28–30 Therefore, there is no doubt about the importance of developing control methods of enantioselectivity in chiral crystallization.

This study aims to achieve enantioselective control in the chiral crystallization of organic compounds using the OTIC method. Some researchers, including ourselves, have used circularly polarized lasers and studied enantioselective control in the chiral crystallization of sodium chlorate (NaClO3), which has been used as a standard sample.31–35 However, the chiral crystallization of NaClO3 involves complex polymorphic transitions, complicating the discussion of the mechanisms of enantioselectivity. Furthermore, considering that most discussions on the origin of homochirality primarily focus on amino acids and sugars,36,37 it makes sense to experiment with organic compounds that exhibit chiral crystallization without complex polymorphic transitions. This study focuses on ethylenediamine sulfate (EDS), an organic compound. EDS crystals have been widely used as a standard sample for chiral studies due to their optical properties, similar to NaClO3 crystals.38–41 We prepare heavy water (D2O)-saturated solutions of EDS and apply optical trapping with left- and right-handed circularly polarized lasers to induce chiral crystallization and generate an imbalance in the generation probability of the two enantiomers of EDS. Furthermore, based on all the obtained results, we discuss the comprehensive dynamics and mechanisms of enantioselectivity in EDS chiral crystallization.

In this study, we chose ethylenediamine sulfate (abbreviated as EDS) as the target compound for enantioselective chiral crystallization using the OTIC method. EDS is an achiral molecule in solution, and the absolute configuration of its chiral structure has been determined.39–43 The space group for l-crystal and d-crystal are assigned to P41212 and P43212, respectively. The molecular arrangements in the two enantiomorphs (l- and d-crystals) of EDS are mirror images of each other, each exhibiting a right-handed and left-handed helical structure.

For the OTIC experiment, we used a saturated heavy water (D2O) solution of EDS. The reason for using D2O as the solvent instead of H2O is to suppress the temperature increase at the laser focus. The authors of the work of Ito et al. estimated the temperature elevation coefficient of D2O and H2O at the laser focus in various solvents to be ∼2 and 22–24 K/W, respectively.44,45 When calculating based on the results of their research papers, the temperature elevation coefficient at the focus of EDS/D2O saturated solution was estimated to be 7.7 K/W (details are provided in supplementary material 1).

The EDS/D2O saturated solution of the sample was prepared following the procedure described below. First, 2.0 g of EDS was added to 2.0 g of D2O, and this mixed solution was heated to 60 °C while vigorously stirring for 2 h until the solute was completely dissolved. The solution was maintained in this state for an additional 2 h. Subsequently, the solution was gradually cooled to room temperature (25 °C) at a rate of 5 °C per hour for 7 h. After cooling, EDS crystals precipitated at the bottom of the sample bottle, confirming that it reached equilibrium. The solution was left undisturbed at room temperature for 3 days. The supernatant was carefully filtered through a 0.22 μm Millex-GV syringe filter unit to obtain the sample saturated solution. A volume of 15 μl of the saturated solution was carefully pipetted into a custom-made container (supplementary material 2) placed on the stage of an inverted microscope and sealed with another cover glass for the following optical trapping experiments.

Figure S3 shows the optical setup used for the OTIC experiment. A 1064 nm continuous wave (CW) Gaussian laser beam (Laser Quantum, Ventus 1064) was selected as the light source. To match the diameter of the laser beam with the pupil diameter of a 60× objective lens [numerical aperture (NA) = 0.90], a collimator consisting of two convex lenses (f1 = 30 mm and f2 = 50 mm) was used. This laser beam was introduced into an inverted microscope (Olympus, IX-71) and focused on the air–solution interface of the EDS/D2O saturated solution using the objective lens. It should be noted that no crystallization was observed when the focus was adjusted to positions other than the air–solution interface. The laser power was adjusted from 0.4 to 1.0 W using a polarizing beam splitter (PBS) and a half-wave plate (HWP) and was verified using a power meter (Newport, 842-PE) placed after the objective lens. Left- or right-handed circular polarization was adjusted by incorporating a quarter-wave plate (QWP) by adjusting the angle of its fast axis just before entering the microscope. The crystallization behavior was monitored in real time while illuminated with a halogen lamp using a CCD camera (Dahua, A5201MU150E). All experiments were conducted at room temperature.

EDS chiral crystal exhibits an optical rotation of 15.5 deg/mm along its optic axis (wavelength: 589 nm).39 To identify the enantiomorph of EDS crystals created by the OTIC method, we adopted the rotating analyzer method (supplementary material 4). In this method, the polarizer and analyzer are initially set up orthogonally [Fig. S4(a)]. Subsequently, EDS crystals generated by the OTIC method are placed between the polarizer and analyzer. The direction of optical rotation of the EDS crystals, i.e., the polarization plane that passes through the chiral crystal, is investigated by adjusting the angle of the analyzer clockwise and counterclockwise by a few degrees [Fig. S4(b)].

There are two important considerations in this experiment. The first concern is the potential misjudgment of crystal chirality due to excessive optical rotation. However, as described above, EDS crystals exhibit an optical rotation of ∼15.5 deg/mm at 589 nm, and EDS crystals produced by the OTIC method are on a microscale. Therefore, since the angle of optical rotation will not exceed 180°, there is no need to account for excessive optical rotation in this experiment. The second concern relates to the birefringence displayed by EDS crystals, which interferes with the differentiation of subtle differences in transmitted light intensity caused by optical rotation. To overcome this issue, aligning the optic axis of the generated EDS crystals almost perfectly with the propagation direction of the light is necessary. EDS crystals are uniaxial crystals, and their optic axis is perpendicular to the crystal faces of precipitated EDS crystals. In practice, it is possible to eliminate birefringence interference by carefully rotating EDS crystals to the appropriate angle using sharp tools like needles, ensuring that the optic axis and the propagation direction of the light are nearly aligned. This allows identification of the EDS enantiomorph using the rotating analyzer method.

When a small particle with a size much smaller than the wavelength of the trapping light source is exposed to an incident light field, the particle is considered a Rayleigh particle and scatters light in all directions. The scattered light changes momentum, leading to an optical force acting on the small particle. Assuming that the particle has no absorption at the wavelength of the incident light, the time-averaged optical force (⟨Fopt⟩) can be expressed as shown in Eq. (1),46 
Fopt=14Re{α}E2+σ12Re1cE×H*.
(1)
The first and second terms in Eq. (1) represent the gradient and scattering forces, respectively. In the equation, E represents the electric field and H represents the magnetic flux density. σ denotes the scattering cross section of the particle and ∇ represents the gradient of the spatial coordinates. The polarizability α is defined as
α=4πε0εmr3(np/nm)21(np/nm)2+2,
(2)
where r is the particle's radius, ε0 is the permittivity of the vacuum, εm is the relative permittivity of the surrounding medium, and np and nm are the refractive indices of the particle and surrounding medium, respectively.
When the trapping laser beam is tightly focused using an objective lens with high NA, the gradient force becomes dominant over the scattering force, resulting in the optical force being nearly equal to the gradient force, as shown in the following equation:
Fopt=14Re{α}E2.
(3)

Optical trapping generally enables capturing microscale objects in three dimensions, primarily through the gradient force. According to Eqs. (2) and (3), the gradient force increases with the target particle’s size. Therefore, achieving stable trapping for small objects like molecular clusters can be extremely challenging. However, in this experiment, the laser beam is focused at the air–solution interface, which restricts particle movement in the Z-axis direction. As a result, the scattering force with the propagation of the Z-axis direction also contributes to particle trapping at the laser focus, potentially leading to a more efficient increase in the concentration of target clusters.

When the circularly polarized CW laser beam (wavelength 1064 nm, laser power 0.7 W) was tightly focused on the air–solution interface of the EDS/D2O saturated solution, a single nearly square EDS crystal was generated from the laser focus. The time-dependent transmission images of the typical crystallization behavior are shown in Fig. 1. Immediately after the laser irradiation was started, the reflected light of the laser source was observed due to differences in their refractive indices between air and solution [Fig. 1(a)]. The irradiation time required for crystal generation varied slightly depending on the laser power and the sample, but the generation of sub-micrometer EDS crystals was almost always confirmed within ∼10 min of irradiation [Fig. 1(b)]. Subsequently, by further laser irradiation into the generated crystal, the generated crystals grew continuously, reaching sizes of several tens of micrometers within a few tens of seconds after crystal generation [Fig. 1(c)]. Notably, during the growth of these crystals, if the laser irradiation is stopped, the crystal growth almost stops. This phenomenon indicates that laser irradiation promotes crystal growth. A similar phenomenon was observed in another of our experiments (L-phenylalanine crystal: L-Phe).13 In that report, it was explained that laser light may propagate through the plate-like L-Phe crystal, achieving crystal growth by attracting surrounding L-Phe clusters at the crystal edges. A similar phenomenon may occur with EDS crystals.

FIG. 1.

Time-lapse transmission images of the EDS crystallization behavior under optical trapping conditions.

FIG. 1.

Time-lapse transmission images of the EDS crystallization behavior under optical trapping conditions.

Close modal

On the other hand, an extremely important experimental result is that a single EDS crystal is consistently generated, and the crystal can always be attributed to one of the enantiomorphs. This allows us to easily accumulate the experimental results and discuss the enantioselectivity of the generated chiral crystals. Additionally, it should be noted that the chirality of the generated EDS crystals is not altered during continuous laser irradiation into the generated EDS crystals.

Herein, we discuss the results of the main objective of this study: enantioselectivity in EDS chiral crystallization using the OTIC method with a focused circularly polarized laser beam. As shown in Figs. 1(b) and 1(c), continuous laser irradiation allowed the EDS crystals to grow to at least ∼50 µm in size. This size is necessary to identify the enantiomorph of the generated EDS crystals accurately. Subsequently, the generated crystals were transferred to a polarizing microscope equipped with a 10× objective lens, and the enantiomorph was determined using the rotating analyzer method (see Secs. II–III). Figure 2 shows a typical example of CCD transmission images using the rotating analyzer method for the case when the generated EDS crystal is l-crystal. The difference in transmitted light intensity of the EDS crystals can be observed by rotating the analyzer a few degrees clockwise or counterclockwise. This difference in intensity allows for easy determination of the enantiomorph. This is why EDS crystals have been used as one of the standard samples for studies on chiral crystallization. As described above, the optic axis of the EDS crystals generated by the OTIC method may not always align perfectly with the observation axis of the microscope. Even in such cases, appropriate rotation of the optic axis of the crystal allows for determining the enantiomorph. However, rarely, crystal faces perpendicular to the optic axis of EDS crystals may not grow sufficiently, making it difficult to stabilize the generated crystals on the substrate and properly identify the enantiomorph.

FIG. 2.

CCD transmission images using the rotating analyzer method for l-crystal EDS chiral crystals.

FIG. 2.

CCD transmission images using the rotating analyzer method for l-crystal EDS chiral crystals.

Close modal

To confirm the imbalance in generation probabilities of two enantiomorphs, experiments were repeated 30 or 60 times for each laser irradiation condition. The laser power was adjusted to 0.4, 0.7, or 1.0 W after the objective lens, and linear polarization (LP), left-handed circular polarization (l-CP), and right-handed circular polarization (r-CP) were used as the laser polarization modes. The imbalance in generation probabilities of two enantiomorphs was also evaluated using crystal enantiomeric excess (CEE). In this experiment, the CEE value is calculated as follows: CEE = (number of d-crystal generation − number of l-crystal generation)/(number of trials for each experimental condition) × 100%. A high CEE value indicates a significant difference in the generation probabilities of two enantiomorphs, implying a strong chiral bias in chiral crystallization by CP laser irradiation.

Table I shows the generation number of two enantiomorphs and the calculated CEE values for different laser powers and polarization modes. For the case of 0.7 W laser power, a reversal in the dominant enantiomorph was observed in r-CP and l-CP laser irradiation, with CEE values of 23% and −30%, respectively. A calculated p-value of ∼0.35% means that the difference in generation probabilities is statistically significant. The p-value has been widely used to determine whether the null hypothesis (equal generation probabilities) holds, and a low p-value provides strong evidence against the null hypothesis. Typically, if the p-value is below the significance level (usually 5%), the null hypothesis is rejected, and the alternative hypothesis (significant difference in the generation probability) is accepted. Furthermore, when using the LP laser beam, the CEE value was zero, suggesting that LP laser irradiation introduces no chiral bias in the generation probabilities of two enantiomorphs.

TABLE I.

Generation number of two enantiomorphs and estimated CEE values for various laser powers and polarizations.

Laser conditions0.4 W0.7 W1.0 W
r-CPl-CPLPr-CPl-CPLPr-CPl-CPLP
d-crystal 31 31 14 37 21 15 28 26 13 
l-crystal 29 29 16 23 39 15 32 34 17 
Total trial 60 60 30 60 60 30 60 60 30 
CEE (%) −7 23 −30 −7 −13 −13 
Laser conditions0.4 W0.7 W1.0 W
r-CPl-CPLPr-CPl-CPLPr-CPl-CPLP
d-crystal 31 31 14 37 21 15 28 26 13 
l-crystal 29 29 16 23 39 15 32 34 17 
Total trial 60 60 30 60 60 30 60 60 30 
CEE (%) −7 23 −30 −7 −13 −13 

Conversely, no significant differences were observed in the generation probabilities of two enantiomorphs, regardless of the polarization mode, when the laser power was set at 0.4 or 1.0 W. Therefore, using the OTIC method, it became evident that optimal laser power is required to achieve enantioselectivity in EDS chiral crystallization. The mechanism of enantioselectivity dependent on laser power is explained in Sec. III C.

In this section, we discuss the dynamics and mechanisms of enantioselectivity in EDS chiral crystallization by the OTIC method using circularly polarized laser beams from two perspectives: the refractive index difference between two enantiomorphs under the electric field created by the focused laser and the three-dimensional (3D) helical optical forces.

The first possible mechanism is discussed based on the difference in refractive index between two enantiomorphs under the electric field caused by focused laser beam. The authors of the work of Alexander et al. have conducted a thermodynamic analysis of nucleation for dielectric molecules under the electric field of light based on classical nucleation theory and proposed changes in Gibbs free energy leading to light-induced nucleation (supplementary material 5).47 Many experimental results have been discussed in the context of this concept regarding nucleation under the influence of electric field.48–51 To further this discussion, confirming the refractive index differences between two enantiomorphs under r-CP and l-CP light at 1064 nm is necessary. As described above, EDS crystal exhibits an optical rotation of ∼15.5 deg/mm at a wavelength of 589 nm. Since EDS crystal shows no absorption in the visible and near-infrared regions, the optical rotation at 1064 nm is expected to be smaller than 15.5 deg/mm with the same sign. To confirm this expectation, we created approximately one 5 mm thick EDS crystal via the spontaneous evaporation method, securely fixed the resulting crystal to the sample stage, and estimated the optical rotation at 1064 nm by carefully adjusting the angle of the optic axis of the EDS crystal. As a result, an optical rotation of ∼0.6 deg/mm at 1064 nm was estimated, with the expected sign. This optical rotation is attributed to the difference in propagation velocities of r-CP and l-CP light in the medium. As indicated in Eq. (S6), the refractive index shows an inverse correlation with the velocity of light in the medium. Therefore, under l-CP light irradiation, the refractive index of d-crystals is greater than that of l-crystals. Consequently, based on Eq. (S7), d-crystals can be considered to be thermodynamically more stable than l-crystals under l-CP light irradiation. Although this explanation seems plausible at first glance, it contradicts the results presented in Table II, particularly those at 0.7 W. Additionally, since the optical rotation signs of d- or l-clusters are expected to match those of their corresponding enantiomorphs in solution, the possibility of enantioselective trapping occurring for their clusters with varying refractive indices in a circularly polarized light field is low. Moreover, based on the above-mentioned experimental results of optical rotation, the refractive index difference (nlnr) of EDS crystal for l-CP and r-CP light at 1064 nm is estimated using the following formula:52 
nlnr=λθ180D,
where nl and nr are the refractive indices of the EDS crystal for l-CP and r-CP light, respectively, λ is the wavelength of the light source, θ is the optical rotation in degrees, and D represents the thickness of the EDS crystal. Substituting the experimental results described above into this formula, nlnr is estimated to be 7.0 × 10−4 (m−1). Thus, the refractive index difference of EDS crystal for l-CP and r-CP light is extremely small, supporting the idea that the difference in refractive index of EDS clusters is not a significant factor in enantioselectivity.
TABLE II.

Temperature dependence of the number of each enantiomorph generated by CP laser irradiation at 0.7 W and the evaluated CCE values.

Temp.25 °C35 °C
Polarizationr-CPl-CPr-CPl-CP
d-crystal 37 21 10 10 
l-crystal 23 39 10 10 
Total trial 60 60 20 20 
CEE (%) 23 −30 
Temp.25 °C35 °C
Polarizationr-CPl-CPr-CPl-CP
d-crystal 37 21 10 10 
l-crystal 23 39 10 10 
Total trial 60 60 20 20 
CEE (%) 23 −30 

Next, we discuss another possible mechanism. Herein, we consider that 3D helical optical force indirectly controls the 3D helical structure of EDS molecules, leading to enantioselectivity. First, we describe 3D helical force generation due to angular momentum (AM) conversion from spin angular momentum (SAM) to orbital angular momentum (OAM) for a focused laser beam used in this experiment. Recently, such 3D helical forces have been proposed as a possible mechanism for achieving enantioselectivity in chiral crystallization,31 chiral polymorphic transition,53 and selective nucleation of conglomerate.54 This AM conversion was initially proposed in the work of Allen et al. in 1992,55 and the authors of the work of Bliokh et al. subsequently demonstrated and discussed it from the viewpoint of interactions between nonparallel light and matter56,57 (supplementary material 7). As clearly evident from Fig. S5(b), the AM conversion increases with the angle of incident light. Considering that the maximum aperture angle of the objective lens used in this experiment is approximately π/4, the estimated average AM conversion is around 10%–15%. The OAM generated through the AM conversion provides two-dimensional (2D) optical torque to EDS clusters in solution, which have a higher refractive index than the surrounding solvent [Fig. 3(a)]. Furthermore, EDS clusters receive scattering forces along the propagation direction of the laser light, resulting in one-dimensional (1D) scattering forces in the z-direction [Fig. 3(b)]. Consequently, EDS clusters experience 3D helical optical forces due to the net forces of 2D optical torque and 1D scattering forces [Fig. 3(c)].

FIG. 3.

Schematic illustration of (a) 2D optical force, (b) 1D optical force, and (c) 3D helical optical force.

FIG. 3.

Schematic illustration of (a) 2D optical force, (b) 1D optical force, and (c) 3D helical optical force.

Close modal

Here, we tried to assess the 2D optical torque and 1D scattering forces working on EDS clusters in solution prior to crystallization. We anticipate that this assessment will furnish valuable insights for future discussions regarding intricate mechanisms. Determination of the EDS cluster size at the laser focus where optical forces work is necessary for this calculation. However, it should be noted that the size of EDS clusters at the focus could increase with laser irradiation, consequently leading to an increase in the optical force. In essence, the 3D helical optical force is projected to escalate with laser irradiation rather than remaining constant. Therefore, in this estimation of optical force, we estimate the 2D optical torque and 1D scattering forces with the cluster size fixed at 100 nm. Numerical analysis was conducted using the finite element method (FEM, COMSOL Multiphysics) with the experimental conditions specified as laser power 0.7 W, r-CP, NA = 0.9, which yielded the highest CEE among the conditions presented in Table I. The optical forces in the x-y plane (2D torque) are illustrated by Poynting vector diagrams in supplementary material 6. Notably, the incident light’s spin angular momentum aligns with the direction of the Poynting vector. Based on the results of our calculations, the optical torque in the x-y plane (2D optical torque) is estimated to be a maximum of 0.9 pN nm, and the z component of the scattering force (1D scattering force) is estimated to be a maximum of 20 fN. It should be noted that the optical force acting on the clusters predicted here may be underestimated, considering the continuous increase in optical force with the growth of cluster size under laser irradiation.

It is essential to consider important experimental results related to enantioselectivity to understand the mechanism based on 3D helical optical forces exerting on EDS clusters. Thus far, many reports have shown that helical fluid motion resulting from macroscopic rotations, such as a rotary evaporator or a magnetic stirrer, achieves enantioselectivity in the chiral assembly of supramolecular, polymeric, or phthalocyanine materials.58–60 Furthermore, the direction of this helical fluid motion aligns with the direction of helical structures inside the resulting assembly. When revisiting this study, for example, and focusing l-CP light, EDS clusters experience right-handed (P-type) 3D helical optical forces, leading to the induction of P-type helical fluid motion in alignment with the optical force direction [see Fig. 4(a)]. Here, this rotation direction is defined as the rotation direction of the Poynting vector in the XY plane when viewed from the receiver. Under P-type helical fluid motion conditions, nucleation predominates the enantiomorph composed of P-type helical structures corresponding to l-crystal [Fig. 4(b)]. These findings support the results presented in Table II at 0.7 W and provide a compelling explanation for the observed experimental outcomes.

FIG. 4.

Schematic illustration of 3D helical force determining crystal chirality by 3D micro-convection.

FIG. 4.

Schematic illustration of 3D helical force determining crystal chirality by 3D micro-convection.

Close modal

Finally, we discuss why there was no significant difference in the generation probability of two enantiomorphs when the laser power was lower (0.4 W) or higher (1.0 W) than 0.7 W, as shown in Table I. First, when the laser power was set at 0.4 W, it is possible that the 3D helical optical forces acting on EDS clusters were too weak to cause a significant difference in the generation probability. On the other hand, when a high laser power (1.0 W) was used, a temperature increase at the laser focus might be considered one of the reasons. To test our hypothesis, we set the laser power to 0.7 W and conducted a series of experiments with the sample prepared by increasing the concentration to achieve saturation at 35 °C, maintaining different temperatures but the same degree of saturation. As a result, there was no longer a significant difference in the generation probability of two enantiomorphs for both r-CP and l-CP laser irradiation, as shown in Table II. This suggests that temperature increase might lead to the prevalence of phenomena like Marangoni convection, potentially disrupting the helical fluid motion. However, as shown in supplementary material 1, the difference in temperature increase at the laser focus between 1.0 and 0.7 W was estimated to be only around 2–3 K. Whether such a minimal temperature difference truly affects the generation probability would require further detailed investigation.

In this study, we successfully conducted experiments on the enantioselectivity in chiral crystallization of ethylenediamine sulfate (EDS) using optical trapping with circularly polarized laser beams. A continuous wave laser beam with a wavelength of 1064 nm was utilized as the light source for optical trapping. The formation of sub-micrometer-sized chiral EDS crystals was confirmed by focusing the laser at the air–solution interface of a D2O-saturated EDS solution. Further laser irradiation facilitated crystal growth, and the enantiomorph was identified for grown crystals using the rotating analyzer method.

Enantioselectivity in EDS chiral crystallization was evaluated through 30–60 repeated experiments at varying laser powers and polarization modes. As a result, circularly polarized laser irradiation with a specific power produced an imbalance in the generation probability of two enantiomorphs, resulting in crystal enantiomeric excess (CEE) values of about 30%. In contrast, no significant enantioselectivity was observed under linear polarization. These experimental results underscore the need to optimize laser irradiation conditions to achieve enantioselectivity in EDS chiral crystallization.

Furthermore, the mechanism behind this enantioselectivity was discussed from two perspectives: one based on refractive index differences and the other on 3D helical optical forces. The thermodynamic mechanism was inadequate in explaining our experimental results. On the other hand, the mechanism based on 3D helical optical forces revealed that these forces acting on EDS clusters induce helical fluid motion, under which EDS nucleation occurs, and the helicity of this fluid motion determines the dominant enantiomorph. This mechanism provided a satisfactory explanation for the obtained experimental results.

This method offers new insights into the research field of chirality and can potentially have significant applications in the optical resolution of pharmaceutical compounds and elucidate the origin of homochirality, which remains one of science's greatest unsolved mysteries.

Supplementary material 1 includes an estimation of the local temperature elevation at laser focus; supplementary material 2 includes details on custom-made container preparation; supplementary material 3 includes an illustration of the optical setup; supplementary material 4 includes an illustration of the rotating analyzer method to identify enantiomorphs; supplementary material 5 includes details on the stabilization of dielectric particles under an electric field; supplementary material 6 includes details on the finite element method simulation for focused Gaussian laser beam; and supplementary material 7 includes details on angular momentum conversion by focused laser beams.

This work was supported by the National Science and Technology Council (NSTC) in Taiwan (Grant Nos. MOST 110–2113-M-A49–012-MY3 and NSTC 111-2634-F-A49-007 to T.S.) and JSPS KAKENHI (Grant Nos. JP16H06506, JP18H03882, JP18H05205, JP21H04657, and JP23H05464 to K.S.), and KAKENHI Grant-in-Aid (No. JP22H05138 to T.S.) for Transformative Research Areas (A) “Revolution of Chiral Materials Science using Helical Light Fields” from the Japan Society for the Promotion of Science (JSPS). The authors are also grateful for the support of the Center for Emergent Functional Matter Science of National Yang Ming 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 and the Research Program of “Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials” in “Network Joint Research Center for Materials and Devices.”

The authors have no conflicts to disclose.

Teruki Sugiyama: Conceptualization (equal); Funding acquisition (equal); Methodology (equal); Project administration (equal); Resources (equal); Supervision (equal); Writing – original draft (equal). Tung-Ming Lin: Data curation (lead); Formal analysis (lead); Investigation (lead); Methodology (equal); Software (equal); Visualization (equal); Writing – original draft (equal). Hao-Tse Su: Visualization (equal). An-Chieh Cheng: Investigation (equal); Writing – review & editing (equal). Keiji Sasaki: Conceptualization (equal); Writing – review & editing (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.

1.
A.
Favre-Bulle
,
A. B.
Stilgoe
,
E. K.
Scott
, and
H.
Rubinsztein-Dunlop
, “
Optical trapping in vivo: Theory, practice, and applications
,”
Nanophotonics
8
,
1023
1040
(
2019
).
2.
C. J.
Bustamante
,
Y. R.
Chemla
,
S.
Liu
, and
M. D.
Wang
, “
Optical tweezers in single-molecule biophysics
,”
Nat. Rev. Methods Primers
1
,
25
(
2021
).
3.
H.
Masuhara
and
K.
Yuyama
, “
Optical force-induced chemistry at solution surfaces
,”
Annu. Rev. Phys. Chem.
72
,
565
589
(
2021
).
4.
A.
Kritzinger
,
A.
Forbes
, and
P. B. C.
Forbes
, “
Optical trapping and fluorescence control with vectorial structured light
,”
Sci. Rep.
12
,
17690
(
2022
).
5.
H. M.
Rivy
,
S. A.
Aljunid
,
E.
Lassalle
,
N. I.
Zheludev
, and
D.
Wilkowski
, “
Single atom in a superoscillatory optical trap
,”
Commun. Phys.
6
,
155
(
2023
).
6.
A.
Ashkin
, “
Acceleration and trapping of particles by radiation pressure
,”
Phys. Rev. Lett.
24
,
156
159
(
1970
).
7.
A.
Ashkin
,
J. M.
Dziedzic
,
J. E.
Bjorkholm
, and
S.
Chu
, “
Observation of a single-beam gradient force optical trap for dielectric particles
,”
Opt. Lett.
11
,
288
290
(
1986
).
8.
T.
Sugiyama
,
T.
Adachi
, and
H.
Masuhara
, “
Crystallization of glycine by photon pressure of a focused CW laser beam
,”
Chem. Lett.
36
,
1480
1481
(
2007
).
9.
T.
Sugiyama
,
K. I.
Yuyama
, and
H.
Masuhara
, “
Laser trapping chemistry: From polymer assembly to amino acid crystallization
,”
Acc. Chem. Res.
45
,
1946
1954
(
2012
).
10.
J.-R.
Tu
,
A.
Miura
,
K. I.
Yuyama
,
H.
Masuhara
, and
T.
Sugiyama
, “
Crystal growth of lysozyme controlled by laser trapping
,”
Cryst. Growth Des.
14
,
15
22
(
2014
).
11.
Y.
Tsuboi
,
T.
Shoji
, and
N.
Kitamura
, “
Optical trapping of amino acids in aqueous solutions
,”
J. Phys. Chem. C
114
,
5589
5593
(
2010
).
12.
A.-C.
Cheng
,
H.
Masuhara
, and
T.
Sugiyama
, “
Evolving crystal morphology of potassium chloride controlled by optical trapping
,”
J. Phys. Chem. C
124
,
6913
6921
(
2020
).
13.
K. I.
Yuyama
,
D.-S.
Chiu
,
Y.-E.
Liu
,
T.
Sugiyama
, and
H.
Masuhara
, “
Crystal growth and dissolution dynamics of L-phenylalanine controlled by solution surface laser trapping
,”
Cryst. Growth Des.
18
,
7079
7087
(
2018
).
14.
T.
Rungsimanon
,
K. I.
Yuyama
,
T.
Sugiyama
,
H.
Masuhara
,
N.
Tohnai
, and
M.
Miyata
, “
Control of crystal polymorph of glycine by photon pressure of a focused continuous wave near-infrared laser beam
,”
J. Phys. Chem. Lett.
1
,
599
603
(
2010
).
15.
J.
Cruz-Cabeza
,
N.
Feeder
, and
R. J.
Davey
, “
Open questions in organic crystal polymorphism
,”
Commun. Chem.
3
,
142
(
2020
).
16.
Q.
Shi
,
H.
Chen
,
Y.
Wang
,
J.
Xu
,
Z.
Liu
, and
C.
Zhang
, “
Recent advances in drug polymorphs: Aspects of pharmaceutical properties and selective crystallization
,”
Int. J. Pharm.
611
,
121320
(
2022
).
17.
R.
Censi
and
P.
DiMartino
, “
Polymorph impact on the bioavailability and stability of poorly soluble drugs
,”
Molecules
20
,
18759
18776
(
2015
).
18.
J. T.
Sertório
,
R.
Lacchini
,
L. M.
Amaral
,
A. C. T.
Palei
,
R. C.
Cavalli
,
V. C.
Sandrim
,
G.
Duarte
, and
J. E.
Tanus-Santos
, “
Haptoglobin polymorphism affects nitric oxide bioavailability in preeclampsia
,”
J. Hum. Hypertens.
27
,
349
354
(
2013
).
19.
J.
Ceramella
,
D.
Iacopetta
,
A.
Franchini
,
M.
De Luca
,
C.
Saturnino
,
I.
Andreu
,
M. S.
Sinicropi
, and
A.
Catalano
, “
A look at the importance of chirality in drug activity: Some significative examples
,”
Appl. Sci.
12
,
10909
(
2022
).
20.
P.
Silvestri
and
P. J. J.
Colbon
, “
The growing importance of chirality in 3D chemical space exploration and modern drug discovery approaches for hit-ID
,”
ACS Med. Chem. Lett.
12
,
1220
1229
(
2021
).
21.
W. H.
Brooks
,
W. C.
Guida
, and
K. G.
Daniel
, “
The significance of chirality in drug design and development
,”
Curr. Top. Med. Chem.
11
,
760
770
(
2011
).
22.
D. M.
Whitacre
,
Reviews of Environmental Contamination and Toxicology
(
Springer
,
2012
), Vol.
217
.
23.
Y.
Liu
,
J.
Xiao
,
J.
Koo
, and
B.
Yan
, “
Chirality-driven topological electronic structure of DNA-like materials
,”
Nat. Mater.
20
,
638
644
(
2021
).
24.
H.
Kuang
,
C.
Xu
, and
Z.
Tang
, “
Emerging chiral materials
,”
Adv. Mater.
32
,
2005110
(
2020
).
25.
P. H.
Chuong
,
L. A.
Nguyen
, and
H.
He
, “
Chiral drugs: An overview
,”
Int. J. Biomed. Sci.
2
,
85
100
(
2006
).
26.
I.
Putman
and
D. W.
Armstrong
, “
Recent advances in the field of chiral crystallization
,”
Chirality
34
,
1338
1354
(
2022
).
27.
T.
Matsuura
and
H.
Koshima
, “
Introduction to chiral crystallization of achiral organic compounds: Spontaneous generation of chirality
,”
J. Photochem. Photobiol., C
6
,
7
24
(
2005
).
28.
A. K.
Scott
, “
Stereoisomers and drug toxicity
,”
Drug Saf.
8
,
149
159
(
1993
).
29.
E.
Dorey
, “
Chiral drugs viable, despite failure
,”
Nat. Biotechnol.
18
,
1239
1240
(
2000
).
30.
W.
Jung
,
J.
Kwon
,
W.
Cho
, and
J.
Yeom
, “
Chiral biomaterials for nanomedicines: From molecules to supraparticles
,”
Pharmaceutics
14
,
1951
(
2022
).
31.
K.
Toyoda
,
H.-T.
Su
,
K.
Miyamoto
,
T.
Sugiyama
, and
T.
Omatsu
, “
Chiral crystallization manipulated by orbital angular momentum of light
,”
Optica
10
,
332
(
2023
).
32.
A.-C.
Cheng
,
H.
Niinomi
,
T.
Omatsu
,
S.
Ishida
,
K.
Sasaki
, and
T.
Sugiyama
, “
Plasmonic manipulation-controlled chiral crystallization of sodium chlorate
,”
J. Phys. Chem. Lett.
11
,
4422
4426
(
2020
).
33.
H.
Niinomi
,
T.
Sugiyama
,
M.
Tagawa
,
K.
Murayama
,
S.
Harada
, and
T.
Ujihara
, “
Enantioselective amplification on circularly polarized laser-induced chiral nucleation from a NaClO3 solution containing Ag nanoparticles
,”
CrystEngComm
18
,
7441
7448
(
2016
).
34.
R.
Ward
,
G. W.
Copeland
, and
A. J.
Alexander
, “
Chiral hide-and-seek: Retention of enantiomorphism in laser-induced nucleation of molten sodium chlorate
,”
J. Chem. Phys.
135
,
114508
(
2011
).
35.
E. R.
Barber
,
N. L. H.
Kinney
, and
A. J.
Alexander
, “
Pulsed laser-induced nucleation of sodium chlorate at high energy densities
,”
Cryst. Growth Des.
19
,
7106
7111
(
2019
).
36.
G. H.
Wagnière
,
On Chirality and the Universal Asymmetry: Reflections on Image and Mirror
(
Wiley VCH
,
Weinheim
,
2007
).
37.
A.
Guijarro
and
M.
Yus
,
The Origin of Chirality in the Molecules of Life
(
RSC Publishing
,
Cambridge
,
2009
).
38.
M.
Briard
,
C.
Brandel
, and
V.
Dupray
, “
Strong enhancement of nucleation efficiency of aqueous ethylenediamine sulfate solutions by nonphotochemical laser-induced nucleation: Investigations on the role of solid impurities in the mechanism
,”
Cryst. Growth Des.
23
,
7169
7178
(
2023
).
39.
L. A.
Cuccia
,
L.
Koby
,
J. B.
Ningappa
, and
M.
Dakessian
, “
Chiral crystallization of ethylenediamine sulfate
,”
J. Chem. Educ.
82
,
1043
(
2005
).
40.
A.
Matsumoto
,
T.
Ide
,
Y.
Kaimori
,
S.
Fujiwara
, and
K.
Soai
, “
Asymmetric autocatalysis triggered by chiral crystal of achiral ethylenediamine sulfate
,”
Chem. Lett.
44
,
688
690
(
2015
).
41.
T. P. T.
Nguyen
,
P. S. M.
Cheung
,
L.
Werber
,
J.
Gagnon
,
R.
Sivakumar
,
C.
Lennox
,
A.
Sossin
,
Y.
Mastai
, and
L. A.
Cuccia
, “
Directing the Viedma ripening of ethylenediammonium sulfate using "Tailor-made" chiral additives
,”
Chem. Commun.
52
,
12626
12629
(
2016
).
42.
K.
Jayaraman
,
A.
Choudhury
, and
C. N. R.
Rao
, “
Sulfates of organic diamines: Hydrogen-bonded structures and properties
,”
Solid State Sci.
4
,
413
422
(
2002
).
43.
K.
Sakurai
, “
A direct determination of the crystal structure of ethylenediammonium sulphate
,”
J. Phys. Soc. Jpn.
16
,
1205
1213
(
1961
).
44.
K.
Setoura
,
K.
Fujita
,
S.
Ito
, and
H.
Miyasaka
, “
Temperature elevation and fluid convection under optical trapping condition as revealed by fluorescence correlation spectroscopy
,”
J. Nanophotonics
13
,
012504
(
2018
).
45.
S.
Ito
,
T.
Sugiyama
,
N.
Toitani
,
G.
Katayama
, and
H.
Miyasaka
, “
Application of fluorescence correlation spectroscopy to the measurement of local temperature in solutions under optical trapping condition
,”
J. Phys. Chem. B
111
,
2365
2371
(
2007
).
46.
S.
Albaladejo
,
M. I.
Marqués
,
M.
Laroche
, and
J. J.
Sáenz
, “
Scattering forces from the curl of the spin angular momentum of a light field
,”
Phys. Rev. Lett.
102
,
113602
(
2009
).
47.
A. J.
Alexander
and
P. J.
Camp
, “
Non-photochemical laser-induced nucleation
,”
J. Chem. Phys.
150
,
040901
(
2019
).
48.
T.
Sugiyama
and
S.-F.
Wang
, “
Manipulation of nucleation and polymorphism by laser irradiation
,”
J. Photochem. Photobiol., C
52
,
100530
(
2022
).
49.
R.
Ward
and
A. J.
Alexander
, “
Nonphotochemical laser-induced nucleation of potassium halides: Effects of wavelength and temperature
,”
Cryst. Growth Des.
12
,
4554
4561
(
2012
).
50.
L. F.
Alexander
and
N.
Radacsi
, “
Application of electric fields for controlling crystallization
,”
CrystEngComm
21
,
5014
5031
(
2019
).
51.
H.
Niinomi
,
T.
Sugiyama
,
A.-C.
Cheng
,
M.
Tagawa
,
T.
Ujihara
,
H. Y.
Yoshikawa
,
R.
Kawamura
,
J.
Nozawa
,
J. T.
Okada
, and
S.
Uda
, “
Chiral optical force generated by a superchiral near-field of a plasmonic triangle trimer as origin of giant bias in chiral nucleation: A simulation study
,”
J. Phys. Chem. C
125
,
6209
6221
(
2021
).
52.
H.
Eyring
,
H.-C.
Liu
, and
D.
Caldwell
, “
Optical rotatory dispersion and circular dichroism
,”
Chem. Rev.
68
,
525
540
(
1968
).
53.
S.-F.
Wang
and
T.
Sugiyama
, “
Femtosecond laser-driven enantioselectivity on achiral–chiral polymorphic transition
,”
Cell Rep. Phys. Sci.
4
,
101323
(
2023
).
54.
M.
Sakamoto
,
N.
Uemura
,
R.
Saito
,
H.
Shimobayashi
,
Y.
Yoshida
,
T.
Mino
, and
T.
Omatsu
, “
Chirogenesis and amplification of molecular chirality using optical vortices
,”
Angew. Chem., Int. Ed.
60
,
12819
12823
(
2021
).
55.
L.
Allen
,
M. W.
Beijersbergen
,
R. J. C.
Spreeuw
, and
J. P.
Woerdman
, “
Orbital angular momentum of light and the transformation of Laguerre–Gaussian laser modes
,”
Phys. Rev. A
45
,
8185
8189
(
1992
).
56.
K. Y.
Bliokh
,
E. A.
Ostrovskaya
,
M. A.
Alonso
,
O. G.
Rodríguez-Herrera
,
D.
Lara
, and
C.
Dainty
, “
Spin-to-orbital angular momentum conversion in focusing, scattering, and imaging systems
,”
Opt. Express
19
,
26132
26149
(
2011
).
57.
K. Y.
Bliokh
,
F. J.
Rodríguez-Fortuño
,
F.
Nori
, and
A. V.
Zayats
, “
Spin–orbit interactions of light
,”
Nat. Photonics
9
,
796
808
(
2015
).
58.
M.
Kuroha
,
S.
Nambu
,
S.
Hattori
,
Y.
Kitagawa
,
K.
Niimura
,
Y.
Mizuno
,
F.
Hamba
, and
K.
Ishii
, “
Chiral supramolecular nanoarchitectures from macroscopic mechanical rotations: Effects on enantioselective aggregation behavior of phthalocyanines
,”
Angew. Chem., Int. Ed.
58
,
18454
18459
(
2019
).
59.
J. M.
Ribó
,
J.
Crusats
,
F.
Sagués
,
J.
Claret
, and
R.
Rubires
, “
Chiral sign induction by vortices during the formation of mesophases in stirred solutions
,”
Science
292
,
2063
2066
(
2001
).
60.
O.
Ohno
,
Y.
Kaizu
, and
H.
Kobayashi
, “
J‐aggregate formation of a water‐soluble porphyrin in acidic aqueous media
,”
J. Chem. Phys.
99
,
4128
4139
(
1993
).

Supplementary Material