Plasmonic Trapping-Induced Crystallization of Acetaminophen

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Plasmonic Trapping-Induced Crystallization of Acetaminophen Hiromasa Niinomi, Teruki Sugiyama, Satoshi Uda, Miho Tagawa, Toru Ujihara, Katsuhiko Miyamoto, and Takashige Omatsu Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01361 • Publication Date (Web): 10 Jan 2019 Downloaded from http://pubs.acs.org on January 13, 2019

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Crystal Growth & Design

Title page Title: Plasmonic Trapping-Induced Crystallization of Acetaminophen Author list: Hiromasa Niinomi,*,†,‡,¶ Teruki Sugiyama,*,§,#,// Satoshi Uda,¶ Miho Tagawa,┴ Toru Ujihara,┴ Katsuhiko Miyamoto†,‡ and Takashige Omatsu†,‡

Affiliations: †Graduate School of Engineering, Chiba University, Chiba, Chiba, 2638522, Japan ‡Molecular Chirality Research Center (MCRC), Chiba University, Chiba, Chiba, 263-8522, Japan ¶Institute for Materials Research, Tohoku University, Sendai, Miyagi, 9808577, 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, 6300192, Japan ┴Institute of Materials and Systems for Sustainability (IMaSS), Nagoya University, Nagoya, Aichi, 464-8603, Japan

*Corresponding

Authors

*Hiromasa Niinomi, E-mail: [email protected]; [email protected], Telephone: +81-22-215-2103, Fax: +81-22-2152101 *Teruki Sugiyama, E-mail: [email protected]; [email protected], Telephone: +886-3-5712121

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

Abstract: We demonstrate that plasmonic trapping can control crystallization of acetaminophen from its aqueous solution. Irradiation of a focused continuous-wave (CW) near-infrared laser to a plasmonic Au nanolattice supporting a thin film of a saturated solution allowed acetaminophen molecules to crystallize in annular distribution with the center of the focal spot. The annularly distributed crystals can be spatially manipulated by changing the position of the laser focal spot. The annular pattern is rationalized by competition between electrical field gradient force as an attractive force to the focal spot and thermophoretic force as a repulsive force. It is also found that, upon stopping the laser irradiation, the crystals first transformed to highly-concentrated droplets rather than directly dissolving to the solution. Relaxation of the droplets by self-diffusion to the solution followed to the crystal/droplet transformation. These two-step dissociation dynamics indicate that not only plasmonic trapping of the molecules but also the enhanced electrical field by surface plasmon contributes to drive the crystallization, and have a possibility to provide an implication for two-step nucleation model. Our demonstration highlights the possibility that plasmonic trapping by designed near-field and temperature distribution can manipulate not only molecular assembly but also creation of functional crystalline materials in nanoscale.


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Body of paper Complete control of crystallization (nucleation and growth) from a solution impacts on tremendous variety of scientific and industrial fields such as materials science,1 solidstate chemistry,2 pharmaceutical industry3-5. This is because functionalities of the resulting crystalline materials are inevitably influenced by crystallization processes, and detailed exploration for fundamental mechanism of the process often demands spatiotemporally predictable nucleation, which intrinsically has stochastic nature.6 Since Garetz and co-workers have first demonstrated non-photochemical laser-induced nucleation (NPLIN) by shooting nanosecond pulsed near-infrared laser to supersaturated aqueous solution of urea,7 electromagnetic field of laser light has attracted increasing attention as a possible external field to spatiotemporally control crystal nucleation because they attributed the mechanism of NPLIN to optical Kerr effect, in which randomly oriented molecules in subcritical clusters align along with the direction of electrical field oscillation.7-9 Amid successive research efforts to spatiotemporally control crystal nucleation with laser light,10-12 Sugiyama and co-workers have recently developed “laser trapping crystallization”.13-15 The crystallization technique has been developed by the concept that laser trapping technique is applicable to crystal nucleation from a solution. In conventional laser trapping of microscopic (micron to nano) objects dispersed in a



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solution, electrical field gradient established by tightly-focused laser exerts field gradient force towards the focal spot on the objects. Consequently, the objects are trapped or assembled at the focal spot.16-19 For a target particle with the diameter significantly smaller than the wavelength of the incident laser (Rayleigh particle), the electrical field gradient force as the trapping force, Fgrad, can be expressed as:20 1 𝐹𝑔𝑟𝑎𝑑 = ― 𝛼∇〈𝐸2〉 2 𝜀𝑝 ― 𝜀𝑚 α = 3V 𝜀𝑝 ― 2𝜀𝑚

(1) (2)

where 〈𝐸2〉 is the time-averaged square of the electrical field of the incident laser, α is the polarizability of the target particle, V(= 4π𝑎3/3) is the volume of the target particle, 𝜀𝑝 and 𝜀𝑚 are the dielectric constants of the particles and the surrounding medium, respectively. Based on the idea that laser trapping-induced assembly of molecules or crystalline clusters in a solution leads to crystallization at the focus, they actually demonstrated forced nucleation of various organic compounds from an unsaturated mother solution by irradiating tightly focused continuous-wave (CW) near-infrared laser to air/D2O solution of the compounds.15 Successive researches have revealed not only the spatiotemporal controllability of nucleation15 but also the polymorph selectivity depending on laser polarization,14,21,22 controllable phase conversion between crystalline/solution phase by on/off of laser irradiation23 and formation of dense liquid


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precursor.24 These discoveries defied the conventional thought that laser-induced crystallization and phase transformation is irreversible process under constant temperature and pressure, and provided us a novel freedom to design crystallization. On the other hand, recent progress of nanoscience, which includes fundamental research for nucleation dynamics6,25-28, demands to precisely manipulate nanoscopic objects.29 Optical manipulation technique as represented by the laser trapping is recognized as a possible candidate to manipulate such a tiny object. However, since trapping force of conventional laser trapping relies on the field gradient force established by focusing a propagating laser beam with an optical lens, the diffraction limit intrinsically limits the strength of the trapping force. This limitation hampers laser trapping to manipulate tiny objects such as nanoparticles, clusters, molecules at will. To overcome this limitation, researchers have recently proposed to utilize near-field generated by the excitation of surface plasmon resonances.30,31 Surface plasmon resonance, which is collective oscillation of free electrons on surfaces of metal nanoparticles, occurs when light is irradiated to metal nanoparticles because the electrons oscillate while accompanying with electrical field oscillation of the electromagnetic field of the incident light.32 Because of dipole radiation caused by large charge oscillation, strongly enhanced and nanoscopically localized near-field generates in the vicinity of the nanoparticles. Namely, the utilization



of surface plasmon resonance allows us to focus light beyond the diffraction limit33.34. Thus, trapping force enhanced by surface plasmon resonance has a possibility to manipulate nanoscopic object much more precisely than conventional laser trapping. The concept, so-called plasmonic trapping, has been experimentally demonstrated for various systems31, e.g., nanoparticles35, polymer36, DNA37 and so on using two-dimensional array of metal nanostructures fabricated on a substrate with plasmon excitation by focused laser although contributions of molecular transportations caused by plasmonic heating, heat generation by energy dissipation of surface plasmon resonance, cannot be neglected. For instance, Toshimitsu and co-workers have reported that aggregation of polymer chains assembled in annular pattern with the excitation spot as a center, and that control of molecular trap/release can be controlled by on/off of the plasmon excitation successfully.36 Plasmonic trapping is nowadays recognized as the up-to-date advanced optical trapping technique, and receives increasing attention as an alternative of laser trapping to achieve complete manipulation of nanoscopic objects. The advent of the plasmonic trapping technique stimulates the idea that plasmonic trapping is applicable to control the crystallization. Realization of so-called plasmonic trapping-induced crystallization should provide novel insight crystallization control in analogy with the development of the plasmonic trapping. Here we show plasmonic


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trapping-induced crystallization of acetaminophen, which is a typical pharmaceutical compound, can be achieved by irradiating a focused near-infrared laser to a gold 2D nanolatiice supporting thin film of acetaminophen aqueous solution. We will present that crystallization takes place in annular pattern by exciting the surface plasmon, and the crystals transform to highly-concentrated droplet upon the stop of the plasmon excitation rather than directly dissolving to the solution. We show the position of the crystals can be manipulated by changing the focal spot position. These results have profound implications for novel approach to design crystallization from a solution.

Figure 1 shows schematic illustrations of experimental setup. The details of the experimental setup are described in the Supporting Information SI1. Plasmonic Au nanolattice was fabricated on a cover glass (120 m thick) by lift-off process of electron beam lithography technique [Figure 1 (a)]. Gammadion (left-handed) structure was employed as unit structure in this study. A droplet of acetaminophen aqueous solution saturated at room temperature (5 L, 24°C) was dropped on the nanolattice so that the edge of the droplet lies across the nanolattice, and the substrate was shaken to form solution thin film supported by the gaps of the nanolattice. Shaking the substrate forces the triple line of the pipetted droplet to move, followed by the formation of solution thin



film by going back of the moved triple line to the initial position. Existence of the solution thin film was confirmed by microscopic observation of response for plasmon excitation and crystallization experiments (See Supporting Information SI2 containing Movie SI22). Thickness of the thin film was estimated to be below 450 nm at least (See SI2-3). A CW near-infrared circularly polarized laser ( = 1064 nm, 20 mW) was focused to the solution thin film by a 60× objective lens (Olympus, UPLFN 60X, NA = 0.9) equipped with an inverted polarized light microscope (IX71, Olympus Co., Ltd.). The microscopic image in the vicinity of the focal spot was recorded by a CCD camera (UI-3180CP-C-HQ Rev.2, IDS) in-situ. Figure 2 shows time-lapse micrographs showing dynamics of the crystallization induced by the laser irradiation (See also Movie SI3, SI4). We found that crystallization takes place when we continuously change the in-plane position of the focal spot [Figure 2 (i)(iii)] (Although it should be possible that a stationary focal spot can induce crystallization as will in principle, it seems that continuous changing of the in-plane focal position can induce crystallization within a reasonable time for observation.). After crystal nucleation, polycrystals grew and spatially distributed so as to form annular shape with the focal spot as center [Figure 2 (ii), (iii)]. Radius and width of the annular pattern were approximately 19 m and 1-2 m, respectively. This radius is significantly larger than the radius of the


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laser focal spot (about 1 m), suggesting that the crystallization is not due to the optical trapping by the focused laser itself. This implies that phenomena caused by the excitation of surface plasmon resonance contributes to the annular patterned crystallization because surface plasmon resonance may propagates along the plane of the Au nanolattice from the excitation spot via Bloch-surface plasmon polariton (Bloch-SPP).38 To estimate feasibility of plasmon propagation reaches to 19 m away from the excitation spot, which is the distance between the spot and the precipitated crystals, we conducted a measurement of plasmon propagation length in such a way that aggregation of fluorescent nanoparticles deposited on the Au nanolattice as a marker to detect propagated plasmon was excited by propagated surface plasmon excited with 532 nm focused laser (Details are Supporting Information SI5). As the results of the measurement, the propagation length at 532 nm was found to be about 12 m. Taking into account that plasmon propagation length in smooth Au thin film at 1064 nm is much larger than that at 532 nm because of the smaller plasmon loss39, it is highly possible that the plasmon propagation length in our experiment exceeds 19 m. The sizes of each crystal are about 1 to 2 m. When the position of the focal spot moved after the crystallization, the position of the annular pattern also moved while following the movement of the focal spot [Figure 2 (iii)(v)]. This change of position is possibly progressed by simultaneous dissolution and



recrystallization. Namely, the crystals deviated from the annular position dissolved to the solution thin film, and solute entered to the annular position crystallized. We found, moreover, that the polycrystals transformed to microdroplets simultaneously with stop of the laser irradiation [Figure 2 (viii)-(xv)]. Since the microdroplet is the product of phase change from crystals, the microdroplet is possibly highly-concentrated. Namely, the crystals were found to once transform into highly-concentrated microdroplet rather than dissolving to the solution directly. After the formation of the concentrated droplet, the droplet gradually disappeared by diffusion into the thin film of the solution. Toshimitsu and co-workers have demonstrated that surface plasmon excitation of Au nanoantenna structure in a solution containing organic polymer results in the formation of annular patterned assembly with the excitation spot as the center.36 When a plasmonic substrate is excited by focused laser, the polymer chains are simultaneously subject to electrical field gradient force towards the excitation spot and thermophoretic force generated by a large temperature gradient due to plasmonic heating phenomenon. Thus, they attributed the annular pattern to competition between electrical field gradient force as an attractive force towards the focal spot and thermophoretic force as a repulsive force. It is highly possible that annular patterned precipitation observed in our experiment can also be rationalized by the competition between the electrical gradient force and the


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thermophoretic force. If thermophoresis does not occur, crystallization should take place in circular pattern rather than the annular because electrical field gradient force attracting molecules to the focal spot should be larger as approaching to the center. However, crystallization was limited to the annular region. This contradiction can be rationalized by taking the thermophoresis which repels the molecules from the focal spot into account. One may consider the possibility that the concentration increment by the thermophoresis is responsible to drive the annular crystallization.40 However, the thermophoresis alone cannot explain the response of the crystals for the changes of the laser irradiation such as (i) the behavior that the crystals spatially followed the change of the focal spot position and (ii) the phase transformation from crystalline state to highly-concentrated droplet simultaneous with stopping the laser irradiation. Regarding (i), because the thermophoresis repels molecules from the focal spot, depletion zone of the molecules in the vicinity of the focal spot should be established. Thus, if we do not take electrical field gradient force into account, the position of the crystals should not move to inward of the depletion zone. However, the in-situ observation shows that the crystal moved towards the depletion zone. This behavior proves the contribution of electrical field gradient force on the crystallization because the behavior cannot be explained without an attractive force towards the focal spot. Regarding (ii), if the crystallization took place merely by



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concentration increment by the thermophoresis, the crystal should gradually dissolve to the depletion zone with the timescale of spontaneous diffusion when the laser irradiation is stopped. However, the crystals turned to the concentrated microdroplets almost simultaneously with stopping the laser irradiation, and droplet diffusion followed the transformation. This prompt response of the transformation possibly comes from release of the molecules from the electrical field of the surface plasmon because the response is prompter than the relaxation of the concentrated microdroplet governed by spontaneous diffusion. Therefore, the observed crystallization behavior is possibly achieved with the contribution of enhanced electrical gradient force by plasmonic near-field. Toshimitsu and co-workers discussed the balance of these two forces by roughly estimating electrical field gradient force, Fgrad, and the thermoporetic force, Fth, from the following equation derived by the theory of the Ludwing-Soret effect: 𝐹𝑡ℎ = ― 𝑆𝑇𝑘𝑇∇𝑇

(3)

where ST is the Soret coefficient, the sign of which determines the direction of thermophoretic force, k is the Boltzman constant, T is temperature and ∇𝑇 is the temperature gradient.36 Thus, we roughly estimated Fgrad to the direction perpendicular to substrate, Fgrad,z, with the aid of numerical simulations based on finite-difference time-


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domain (FDTD) method and Fth by estimating temperature distribution by following the method proposed by Bendix et al.41 First, to evaluate electrical field intensity enhancement in the plasmonic near-field, we performed a numerical analysis by electromagnetic field simulation based on the FDTD method using Poynting for Optics (Fujitsu, Japan), which is a commercial software of three-dimensional FDTD program. We mimicked the experimental condition by setting the situation in which a left-handed circularly polarized plane wave propagating along z axis ( = 1064 nm, Electrical field intensity is 1 V/m) illuminates 2 by 2 array of Au gammadion nanostructures placed on SiO2 substrate in the virtual space in the program. The computational details are described in the Supporting Information (See SI6). Figure 3 shows a steady state electromagnetic field distribution in the vicinity of the Au nanolattice [Figure 3 (i)] and distributions of absolute Fgrad,z values along z-axis perpendicular to the substrate in the region 19 m away from the focal spot [Figure 3 (ii) and (iii)]. The Fgrad,z values are calculated by assuming that (i) the focal spot size of the incident laser is about 1 m, (ii) electric permittivity of acetaminophen and solution are 3.38642 and 1.753543, respectively (See SI8), (iii) the volume of a molecule is 125 Å3 (iv) the plasmon propagation length is 12 m, which is the value for 532 nm excitation wavelength. Figure 3 (i) shows that strong field enhancement can be seen at nanogaps



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between the Au nanostructures. About 2 - 8 fold enhancement is achieved within 100 nm above the nanogaps.

Figure 3 (ii) shows Fgrad,z values for x = 52.8 nm and x = 4.8 nm

depicted in (i), respectively [(iii) is one with different Fgrad,z value scale]. Figure 3 (i) and (ii) shows that Fgrad,z values ranges from the order of 1 × 10-18 N to the order of 1 × 10-17 N between z = 150 nm and z = 0. Second, regarding the estimation of Fth value, we followed the method proposed by Bendix et al.41 In their study, temperature distribution around an excited Au nanoparticle supported by a lipid bilayer was estimated by measuring the size of the region where the bilayer with known phase transition temperature transforms to disordered phase. They assumed point heat source and the temperature distribution can be expressed by the following equation: ∆𝑇(𝐷) =

C𝐼 𝐷

(4)

where I is the laser power incident on plasmonic particle, D is the distance from the heat source and C is a constant including all the physical parameters which are difficult to determine correctly. The constant C can be experimentally determined by investigating the dependency of the size of the disordered phase on input laser power. Here we employed acetaminophen form II crystal, whose melting point is 159°C,44 instead of lipid bilayer (See supporting information SI7). It should be noted that here phase


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transformation from crystal to melt of acetaminophen form II crystal was employed as a well-defined physical transformation acting the role of temperature indicator, and the use of the crystal does not have a direct connection to the crystallization from the solution in the main experiment. The polymorph was characterized by observing the crystalline morphology. Figure 4 shows the estimated temperature distribution and comparison with the micrograph of the annular patterned crystal. The estimated temperature distribution indicates that temperature in the region outside from about 12 m away is below 25oC, namely, almost equivalent to the room temperature (24oC). Since the crystals located in the region 19 m away from the center, the crystallization possibly took place under the room temperature. This estimation supports the idea that solvent evaporation is not responsible for main driving force for the crystallization. Although one may consider that such high temperature at the focal spot will generate bubbles, no bubble generation was observed in our experiment. This may be because of superheating of water and the boiling point elevation due to the dissolved acetaminophen. Baffou et al. investigated the temperature giving the bubble generation when CW focused laser is irradiated to a plasmonic substrate immersed in water. The investigation shows that temperature can easily reach 220oC without bubble generation. This super-heating phenomenon may occur in our experiment as well.45 From the estimated temperature distribution,



distribution of Fth value along x-axis can be calculated. Figure 5 shows the calculated Fth distributions. Since ST, which is unknown value for acetaminophen molecules in water, scales the values, Fth distributions for ST = 5 × 10-4, 1 × 10-3, 5 × 10-3 and 1 × 10-4 K-1 were depicted. This is because ST values of low weight organic molecules is often the order of 10-3 K-1.46 If we pay attention to the Fth distributions for ST =1 × 10-3 and 5 × 10-3, the Fth value tends to be much higher than the order of Fgrad,z within the region several m away from the focal spot. On the other hand, the steep decay of the value with increasing the distance from the focal spot results in the situation that Fth values become comparable to the values of Fgrad,z in the region several tens m away from the focal spot. If we regard the Fgrad,z values from the substrate to 100 nm above from it as representative values (x = 52.8 nm), the Fgrad,z ranges from about 5 × 10-18 N to about 20 × 10-18 N. Then, the region where the Fth and Fgrad,z can balance can be indicated by the yellow colored region in the Figure 5. In addition, the region where the line balanced with the minimum value of the representative Fgrad,z values (the line indicated by a blue arrow in Figure 4) and the line balanced with the maximum (the line indicated by red arrow) are overlapped can be recognized as the most probable region which gives the largest volume of trapped molecules because whole range of the representative Fgrad,z values is possibly balanced with the probable range of Fth. The overlapped region was highlighted by orange color.


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This indicates that highly probable distance which gives crystal precipitation ranges from about 16 to 17 m away from the focal spot. This estimated distance is comparable with the distance in which the crystallization took place in our experiment, 19 m. Therefore, the annular patterned crystallization may be rationalized by the competition between the electrical field gradient force due to plasmonic near-field and the thermophoretic force due to plasmonic heating, supporting the hypothesis that the crystallization was induced by plasmonic trapping. The radius of the annular patterned crystallization was possibly rationalized by the position where the electrical field gradient force and the thermophoretic force are balanced, namely, mechanism of plasmonic trapping. However, it is difficult for the plasmonic trapping-induced molecular condensation alone to explain the phase transformation from crystal to highly concentrated microdroplet upon turning off the laser irradiation. This is because the response of the phase transformation to turning off the laser is much prompter than that of molecular diffusion, and that the crystal should directly dissolve to the solution thin film without the phase transformation if the crystallization was driven by plasmonic trapping-induced condensation alone. One possibility to explain the phase transformation is thermodynamic driving force modification by applied electrical field. Previous studies on crystallization under



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electrical field have shown that electrical field can promote crystallization by reducing chemical potential of a crystalline phase which possesses larger dielectric constant than solution phase.47-50 Alexander et al. formulated the Gibbs free energy change on forming a spherical cluster of radius r in applied electric field based on classical nucleation theory, ∆𝐺(𝑟,𝐸), as follows: 4 ∆𝐺(𝑟,𝐸) = 4𝜋𝑟2𝛾 ― 𝜋𝑟3(A𝑙𝑛𝑆 + 𝑎𝐸2) 3

(5)

(

(6)

𝑎=

3𝜀0𝜀𝑠 𝜀𝑐 ― 𝜀𝑠 2

)

𝜀𝑐 + 2𝜀𝑠

where γ is the solution/crystal interfacial tension, A = RT/M, where  is the mass density, M is the molar mass of the solid, T is absolute temperature, S is supersaturation, E is electrical field intensity, 0 is the dielectric constant of vacuum, c is dielectric constant of crystalline phase and s is dielectric constant of solution phase.50 The terms of 𝐴𝑙𝑛𝑆 and a𝐸2 contribute to drive crystallization, respectively. The former is the contribution of supersaturation (solution concentration) and the latter is the contribution of the applied electric field as additional term on the formulation of classical nucleation theory under zero field. Taking the fact that thermodynamic driving force for crystallization under zero field can be expressed by kBTlnS = (solution) - (crystal), where kB is Boltzmann’s constant and  is chemical potential of a phase, here we express the thermodynamic driving force originating from solution concentration, c, the


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thermodynamic driving force originating from applied electrical field, E, the total thermodynamic driving force, , as follows:50

Δ𝜇 = Δ𝜇𝑐 + ∆𝜇𝐸

(7)

∆𝜇𝑐 = 𝑘𝐵𝑇𝑙𝑛𝑆

(8)

∆𝜇𝐸 =

𝑘𝐵𝑇 A

𝑎𝐸2

(9)

Namely, the driving force for crystallization comprises of the term governed by concentration and the term governed by electrostatic energy. Taking the fact that response time of electronic polarization of a dielectric material is on the order of 10-15 sec into account, the time taken to change E by imposing electrical field should be significantly shorter than that to change Δ𝜇𝑐which involves time for molecules to spontaneously diffuse. Therefore, it is reasonable that the prompt phase transformation was followed by the relaxation of the microdroplet if the E significantly contributes to the crystallization. Indeed, the E value obtained from calculated electrical field strength in the plasmonic near-field in our system is equivalent to the Δ𝜇𝑐 value when approximately 1 % supersaturation was imposed on the system (See supporting information SI8). These



relaxation dynamics of the crystals indicates that the electrical field of plasmonic nearfield indeed contributes to drive crystallization. Figure 6 is a schematic illustration showing the dynamics of the plasmonic trappinginduced crystallization observed in our experiment. Before laser irradiation, the thermodynamic driving force for crystallization is zero because the solution concentration is equilibrium concentration. After onset of laser irradiation, the incident light excites surface plasmon of the Au nanolattice, which is accompanied by generation of plasmonic near-field. The generation of the near-field may be not limited in the area of the focal spot because surface plasmon may propagate to the lateral direction. Because electrical field strength exponentially decays as being away from the excitation source, field gradient force towards the focal spot and the Au nanolattice should be exerted on the molecules. (This electrical field gradient force may be interpreted as a consequence of gradient of Δ 𝜇𝐸.) In addition, since spatially confined temperature elevation by plasmonic heating effect follows the excitation of surface plasmon, thermophoretic force is also exerted on the molecules outward from the focal spot. This results in molecular diffusion towards the annularly distributed position where the attractive electrical field gradient force and the repulsive thermoforetic force are balanced as previously reported in the system of plasmonic trapping-induced assembly of organic molecules36. Then, Δ𝜇𝑐is locally


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increased by local concentration increment in the annular equilibrium position. This results in the annular patterned crystallization. Upon stopping the laser irradiation, the driving force for crystallization should lose the contribution of electrical field, E, first. The loss possibly led to the phase transformation from the crystal to the concentrated microdroplet. Since the molecules is released from the two forces, the isotropic spontaneous diffusion of the droplet then take place, and the state gradually relaxes to the initial state by decreasing the Δ𝜇𝑐. These phase conversion processes have actually shown that plasmonic near-field enables us to manipulate molecules and their state in 1000 nm scale in our prototypical system. This suggests the possibility that further precise control of crystal nucleation or phase conversion in nano-scale may be achievable by using designed plasmonic near-field and temperature distribution. In conclusion, we found that acetaminophen polycrystals precipitate in annular pattern on the plasmonic Au nanolattice by irradiating focused near-infrared laser to the nanolattice supporting a thin film of acetaminophen aqueous solution. The position of the annular patterned crystals was found to be manipulated by changing the position of the laser focal spot. The crystals were found to disappear upon turning off the laser irradiation. These crystallization behaviors strongly suggest that the crystallization was driven by plasmonic trapping of acetaminophen molecules. The annular precipitation pattern was



rationalized by the balance between the plasmon enhanced electrical field gradient force as attractive force towards the focal spot and the thermophoretic force established by plasmonic heating as repulsive force from the focal spot with the support by the estimation of electrical field gradient force by FDTD numerical simulation and the estimation of thermophoretic force. This highlights the possibility that plasmon enhanced electrical field gradient force contributes to drive the crystallization. We also found that the polycrystals transform into highly-concentrated microdroplets first upon stopping the laser irradiation, and the droplets was then found to be relaxed by diffusion into the solution thin film. This difference in the response time between the phase conversion and the relaxation of the droplet indicates that the crystallization is driven not only by concentration increment by plasmonic trapping of the molecules but also the contribution of the electrical field on the thermodynamic driving force for crystallization. These results show not only the possibility of plasmonic trapping to precise control and design crystallization but also the possibility to manipulate molecules and its ordering in nanoscale by optimized design of the plasmonic near-field and temperature distribution caused by plasmonic heating effect.


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• Associated Content Ⓢ Supporting Information The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. The contents of the Supporting Information include “SI1. Experimental setup and procedures (SI1-1. Compound and Sample Preparation, SI1-2. Fabrication of Plasmonic Au nanolattice, SI1-3. UV-Vis absorption spectra of the plasmonic Au nanolattice, SI1-4. Optical Setup)”, “SI2 Confirmation of existence of solution thin film supported by the Au nanolattice (SI2-1. Reversible color change of Au nanolattice by laser irradiation, SI2-2. Movie showing the in-situ observation of the reversible color change of the Au nanolattice by laser irradiation, SI2-3. Crystallization in a solution thin film supported by the Au nanolattice)” “SI3. Movie showing the in-situ observation capturing the dynamics of the crystallization and the dissociation of acetaminophen induced by plasmon excitation” “SI4. Movie showing the in-situ observation capturing the dynamics of the crystallization and the dissociation of acetaminophen induced by plasmon excitation 2”, “SI5 Measurement of plasmon propagation length of surface plasmon resonance of the Au nanolattice excited by 532 nm laser” “SI6 Numerical analysis of electromagnetic field in the vicinity of the Au nanolattice based on FDTD method” “SI7. Estimation of temperature distribution”, “SI8.



Estimation of thermodynamic driving force modified by plasmonic near-field”. SI2-2, SI3 and SI4 are video files.

• Author Information Corresponding Authors *Hiromasa Niinomi E-mail:[email protected], [email protected] Telephone: +81-22-215-2103. Fax: +81-22-215-2101 *Teruki Sugiyama E-mail: [email protected], [email protected] Telephone: +886-3-5712121

• Acknowledgement This work was supported by Grant-in-Aid for JSPS Fellows Grant Number 15J11361, JSPS KAKENHI Grant-in-Aid for Young Scientists (B) Grant Number 16K17512, JSPS KAKENHI Grant-in-Aid for Early-Career Scientists Grant Number 18K14177, JSPS KAKENHI Grant-in-Aid for Scientific Research on Innovative Areas “Nano-Material Manipulation and Structural Order Control with Optical Forces” Grant Number JP


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16H06507, JSPS KAKENHI Challenging Research (Exploratory) Grant Number JP 17K19070, JSPS KAKENHI Grant-in-Aid for Scientific Research (A) Grant Number JP18H03884, the joint usage/research program of the Institute of　Materials and Systems for Sustainability (IMaSS), Nagoya University and the Ministry of Science and Technology in Taiwan under Contracts MOST106-2113-M-009-017-. We thank Mr. Kazuki Okano, Saitama University, Japan and Mr. Yoichiro Mori for giving us fruitful discussion regarding crystallization of acetaminophen. We are grateful for Dr. Daiki Oshima, Nagoya University, Japan, for giving us technical supports for the usage of electron beam lithography.

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For Table of Contents Use Only Title: Plasmonic Trapping-Induced Crystallization of Acetaminophen Authors list: Hiromasa Niinomi,*,†,‡,¶ Teruki Sugiyama,*,§,#,// Satoshi Uda,¶ Miho Tagawa,┴ Toru Ujihara,┴ Katsuhiko Miyamoto†,‡ and Takashige Omatsu†,‡ Affiliations: †Graduate School of Engineering, Chiba University, Chiba, Chiba, 263-8522, Japan ‡Molecular Chirality Research Center (MCRC), Chiba University, Chiba, Chiba, 2638522, Japan ¶Institute for Materials Research, Tohoku University, Sendai, Miyagi, 980-8577, 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 ┴Institute of Materials and Systems for Sustainability (IMaSS), Nagoya University, Nagoya, Aichi, 464-8603, Japan

We achieved to crystallize acetaminophen by plasmonic trapping of molecules in an aqueous solution. The plasmonic trapping-induced crystallization allows us to control crystallization, to manipulate precipitation position and to induce unprecedented phenomena regarding phase transformation, being a novel method to precisely design a crystallization in nanoscale.


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Figure 1. Schematic illustration showing the setup of our crystallization experiment. (a) An atomic force microscopic image of the plasmonic Au nanolattice employed. (b) A schematic illustration of the nanolattice. (c) A schematic illustration showing the geometry of our crystallization experiment. (d) Imagined schematic illustration showing the position of the laser focal spot magnified for the vicinity of the spot. The laser was focused at the position about 30 μm away from the fringe of the solution droplet. 84x116mm (300 x 300 DPI)



Figure 2. Time-lapse micrographs showing the crystallization and dissociation dynamics of acetaminophen induced by plasmonic trapping. (i)-(vii) The crystallization dynamics induced by the laser irradiation. Laser irradiation to the plasmonic Au nanolattice caused crystallization distributed in annular pattern [(i)-(iii)]. Upon changing the position of the focal spot, the annular distribution followed the movement of the focal spot [(iii)-(iv)]. The red allow depicted in (iii) indicates the direction to change the position of the focal spot. White doted circle depicted in (iv) indicated the position where the crystals precipitated in the micrograph (iii), and the red dashed circle indicates the focal spot position in the micrograph (iii). (viii)-(xv) The dissociation dynamics of the crystals accompanying with the stop of the laser irradiation. The crystals first transform to highly-concentrated microdroplets [(vii)-(xi)]. The concentrated microdrplets diffused to the solution thin film [(xi)-(xv)]. 177x107mm (300 x 300 DPI)


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Figure 3. Electromagnetic field enhancement by the excitation of surface plasmon resonance of the Au nanolattice simulated by FDTD electromagnetic numerical analysis and spatial distribution of electrical field gradient force calculated from the FDTD simulation with the 4 assumptions described in the main text. (i) Electrical field distribution of a plane perpendicular the substrate supporting the Au nanolattice (left) and an image magnified for a nanogap of the Au nanolattice, in which electric field intensity is strongly enhanced, (right). The electrical field strength of the incident light was set to be 1 (V/m) (ii) The spatial distribution of electrical field gradient force towards the substrate calculated from the FDTD numerical analysis for the coordination x = 52.8 nm (left) and x = 4.8 nm (right) in the magnified image of (i). x = 52.8 nm and x = 4.8 nm corresponds to the position of the center of the nanogap and the position of the edge of the Au nanostructure, respectively. (iii) The graph of (ii) with the smaller scale of the value of electrical field gradient force. 84x85mm (300 x 300 DPI)



Figure 4. Temperature distribution in the vicinity of the focal spot estimated by the method proposed by Bendix et al.. The notation of “Crystal” in the graph indicates the position where the acetaminophen crystals precipitated. The inset shows an actual micrograph with the lines indicating the position at 100oC and 25oC 82x51mm (300 x 300 DPI)


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Figure 5. Thermophoretic force distribution calculated from temperature distribution assessed by the method proposed by Bendix et al. with the variation of Soret coefficient, ST. The region colored by light yellow indicates the representative value range of electrical gradient force estimated from the FDTD numerical analysis. The region colored by yellow is the possible region in which thermophoretic force and electrical gradient force are balanced with the assumption (i) representative values of electrical field gradient force range from 5 × 10-18 (N) to about 20 × 10-18 (N) and (ii) The ST value of acetaminophen molecule lies between 1 × 10-3 and 5 × 10-3 (K-1). The orange colored region indicates the highly probable region which gives the largest volume of trapped molecules. The green arrow indicates the position where the crystallization actually took place. 84x58mm (300 x 300 DPI)



Figure 6. Schematic illustrations explaining simplified successive dynamics of the crystallization and the dissociation triggered by ON and OFF of the plasmon excitation. The notations “∆μ”, “Δμc” and “∆μE” indicates total thermodynamic driving force for crystallization at the position where the crystals was precipitated, thermodynamic driving forces caused by solution concentration and electrical field at the position, respectively. “0” means the value of the driving forces at the initial state. (i) The initial state. (ii) Immediately after the laser irradiation (before the onset of molecular diffusion). (iii) After the molecular diffusion. (iv) At the moment when crystallization took place. (v) Immediately after the stop of the laser irradiation (before the onset of molecular diffusion.) (vi) After the onset of molecular diffusion. 177x75mm (300 x 300 DPI)


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Plasmonic Trapping-Induced Crystallization of Acetaminophen

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