Plasmonic Heating-Assisted Laser-Induced Crystallization from a

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Plasmonic Heating-Assisted Laser-Induced Crystallization from a NaClO Unsaturated Mother Solution 3

Hiromasa Niinomi, Teruki Sugiyama, Miho Tagawa, Mihoko Maruyama, Toru Ujihara, Takashige Omatsu, and Yusuke Mori Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b01657 • Publication Date (Web): 13 Dec 2016 Downloaded from http://pubs.acs.org on December 16, 2016

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Crystal Growth & Design

Cover page Title: Plasmonic Heating-Assisted Laser-Induced Crystallization from a NaClO3 Unsaturated Mother Solution

Author list:

Hiromasa Niinomi†,‡,*, Teruki Sugiyama§, Miho Tagawa//, Mihoko Maruyama†, Toru Ujihara//, Takashige Omatsu‡ and Yusuke Mori†

Affiliations: †

Department of Electrical, Electronic and Information Engineering, Osaka University, Suita, Osaka, 565-0871, Japan

‡

Molecular Chirality Research Center (MCRC), Graduate School of Advanced Integration Science, Chiba University,

Chiba, Chiba, 263-8522, Japan §

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

30010, Taiwan //

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

Author information note: the present address of the corresponding author differs from that at which this work was done. The present address is listed below. *

Corresponding Author

Name: Hiromasa Niinomi, §H. N.: Present Affiliation: Molecular Chirality Research Center, Graduate School of Advanced Integration Science, Chiba University, 1-33 Yayoi-Cho, Inage-ku, Chiba, 263-8522, Japan Telephone number: +81-43-290-3471, Fax number: +81-43-290-3490 E-mail address: [email protected]

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Abstract: We provide a novel laser-induced crystallization mechanism which explains crystallization induced

by visible laser trapping of silver nanoparticles (AgNPs) at air/unsaturated mother solution interface

from the focal spot [Niinomi et al. CrystEngComm, 2016, 18, 7441 - 7448.]. Simultaneous in-situ

microscopic observation of Raman scattering and polarized-light image revealed that the optical

trapping of nanoparticles that exhibit surface-enhanced Raman scattering (SERS) triggers the

crystallization, showing the excitation of localized surface plasmon resonance (LSPR) significantly

promotes the crystallization. Numerical analysis of temperature distribution based on the

combination of finite-difference time-domain electromagnetic and finite-difference heat transfer calculations shows that temperature reaches 390 oC at the focal spot because of plasmonic heating,

the energy dissipation of the plasmon-enhanced electromagnetic field as heat. Conceivable

mechanism of the crystallization is local increment of supersaturation caused by local solvent

evaporation via the plasmonic heating. This plasmonic heating assisted laser-induced nucleation

process has a possibility to provide not only a novel approach for spatiotemporal control of

crystallization but also novel nucleation field based on nonlinear light-matter interaction originating

from the plasmon-enhanced electromagnetic near field through heterogeneous nucleation on the

surface of plasmonic particles.


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Title page Title: Plasmonic Heating-Assisted Laser-Induced Crystallization from a NaClO3 Unsaturated Mother Solution

Author list:


Affiliations: †


‡




30010, Taiwan //


Author information note: the present address of the corresponding author differs from that at which this work was done. The present address is listed below. *

Corresponding Author

Name: Hiromasa Niinomi, §H. N.: Present Affiliation: Molecular Chirality Research Center, Graduate School of Advanced Integration Science, Chiba University, 1-33 Yayoi-Cho, Inage-ku, Chiba, 263-8522, Japan Telephone number: +81-43-290-3475, Fax number: +81-43-290-3490 E-mail address: [email protected]



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Abstract We provide a novel laser-induced crystallization mechanism which explains crystallization induced

by visible laser trapping of silver nanoparticles (AgNPs) at air/unsaturated mother solution interface

from the focal spot [Niinomi et al. CrystEngComm, 2016, 18, 7441 - 7448.]. Simultaneous in-situ microscopic observation of Raman scattering and polarized-light image revealed that the optical

trapping of nanoparticles that exhibit surface-enhanced Raman scattering (SERS) triggers the

crystallization, showing the excitation of localized surface plasmon resonance (LSPR) significantly

promotes the crystallization. Numerical analysis of temperature distribution based on the

combination of finite-difference time-domain electromagnetic and finite-difference heat transfer calculations shows that temperature reaches 390 oC at the focal spot because of plasmonic heating,

the energy dissipation of the plasmon-enhanced electromagnetic field as heat. Conceivable

mechanism of the crystallization is local increment of supersaturation caused by local solvent

evaporation via the plasmonic heating. This plasmonic heating assisted laser-induced nucleation

process has a possibility to provide not only a novel approach for spatiotemporal control of

crystallization but also novel nucleation field based on nonlinear light-matter interaction originating

from the plasmon-enhanced electromagnetic near field through heterogeneous nucleation on the

surface of plasmonic particles.


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Body of paper 1. INTRODUCTION

Crystal nucleation from its solution is one of the most intriguing phenomena in solid-state chemistry,1-3 pharmaceutical industry,4-6 material science7 because the process is inevitable to create functional crystalline solids, and strongly influences the underlying characteristics for the functionality of final products such as shape, polymorphs, and size distribution of crystals. Therefore, the control of crystal nucleation is essential for efficient production of desired materials. Over the past decades, Non-photochemical laser-induced nucleation (NPLIN) has attracted increasing attention from the view point of nucleation control since Garetz et al. have first demonstrated that shooting infrared pulsed laser into supersaturated aqueous solution of urea can promote its crystallization efficiently than spontaneous nucleation8. Since the first demonstration of NPLIN using pulsed laser, the method have been successfully applied to promote crystallization of inorganic salt,9,10 biological molecules which are hard to crystallize relative to salts such as proteins11 and membrane proteins12 from supersaturated solutions. Moreover, it has been reported that the resulting polymorph of Glycine crystal can be switched by changing the polarization of the incident laser light under a certain appropriate supersaturation condition, indicating the possibility of



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NPLIN using pulsed laser for polymorph control.13 Various mechanisms of NPLIN have been proposed depending on the experimental situations.8,14,15 The mechanism of the NPLIN using nanosecond pulsed laser have been attributed to optical Kerr effect, which aligns the randomly-oriented molecules in subcritical clusters into relatively ordered orientation so as to minimize the interaction energy between the induced dipole moment of the solute and the electric field of the incident light, leading to the nucleation and the polymorph control.8,3,16-18 On the other hand, in the case of the NPLIN using femtosecond pulsed laser, the shockwave originating from cavitation bubble formation is considered to trigger nucleation.11,14,19-21 Contrary to the NPLIN using pulsed laser, which induces nucleation from a supersaturated solution, it has been also reported that the continuous irradiation of a tightly focused continuous-wave (CW) infrared laser onto the solution/air interface leads to crystallization from the focal spot even if the mother solution is unsaturated condition.15, 22-25 The mechanism of this nucleation has been explained by the concentration increment accompanying with molecular assembly driven by the photon pressure at the focal spot. This “photon pressure-induced crystallization” has also achieved to control polymorphs in glycine crystallization by changing the polarization of incident light under appropriate conditions.25 As mentioned


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above, NPLINs is now recognized as one powerful method to control nucleation and the mechanisms has been subdivided. Recently, Niinomi et al. have demonstrated sodium chlorate (NaClO3) chiral crystallization can be efficiently induced by the irradiation of a tightly-focused visible circularly polarized CW laser into air/unsaturated aqueous solution containing Ag nanoparticles (AgNPs) from the focal spot, i.e., optical trapping of AgNPs induced crystallization. They found also that the crystallization method yields non-equivalent probability of the nucleation of both enantiomorphs.26 In the study, they pointed out that the localized surface plasmon resonance (LSPR), which is the collective oscillation of free electron on the surface of AgNPs,27 possibly contributes to the efficient crystallization and the chiral bias in the resulting enantiomorphs. This route of crystallization may provide novel nucleation field because the enhanced localized field generated by LSPR is known to exhibit extraordinary optical phenomenon, such as nonlinear optical effects,28 breakdown of selection-rule of molecular photoexcitation29 and so on. However, whether LSPR contributes to the crystallization is still unclear because of the lack of the experimental evidences. Although the idea that the LSPR contributes to the nucleation is based on the observational fact that the nucleation occurred



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simultaneously with the optical trapping of aggregates of AgNPs, it is not sufficient to prove the contribution of the LSPR because the observation could not spatially follow LSPR-active particles during the crystallization process. In this study, we employed in-situ microscopic observation of Raman scattering to spatially follow the motion of AgNPs that exhibit surface-enhanced Raman scattering (SERS),30 which proves the excitation of LSPR, in the course of the crystallization. We show the direct evidence proving the contribution of the LSPR excitation on the crystallization by visualizing the correspondence between the motion of SERS-active particles and onset of the crystallization. We provide a novel mechanism of laser-induced crystallization assisted by the excitation of LSPR.

2. MATERIALS AND METHODS 2.1 Compound and Sample Preparation NaClO3 was used as the target compound for the laser-induced crystallization experiment. NaClO3 crystallizes from an aqueous solution in two kinds of polymorphic forms: (1) cubic phase with P213 symmetry as the stable phase; (2) monoclinic phase with P21/a symmetry as a metastable phase.31 The solubility of the metastable phase is reported to be 1.6 times higher than that of the stable phase32 (Supporting Information,


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S1). We prepared a sample solution by the same procedure as our previous study26 as following. Supersaturated aqueous solution of NaClO3 was prepared at 22°C by dissolving 30 g of NaClO3 powder (>98 %, Wako) to 25 ml of ultrapure water fabricated with Direct-Q 3UV (Millipore) in a 100 ml glass beaker. The solution was then heated up to 50°C while being stirred using hotplate magnetic stirrer to dissolve the NaClO3 powder completely. After assuring the complete dissolution of the NaClO3 powder, portion of the resulting solution was transferred to a 25 mL centrifuge tube with a screw cap and then hermetically closed. The centrifuge tube was left for a week at 22°C to precipitate the solute excessively dissolving, leading the solution to equilibrium state at 22°C. The equilibrium solution is a mixture of NaClO3 saturated aqueous solution and NaClO3 crystalline powder. The mixture was then treated in a centrifuge to separate the saturated solution from the crystalline powder.

The

supernatant of the mixture was used as a NaClO3 aqueous solution saturated at 22°C. The NaClO3 saturated solution (6.25 µL) and 2.5 µL AgNP dispersion (10 nm, 0.02 mg/mL, Sigma Aldrich) containing 2 mM sodium citrate buffer were placed in a custom-designed cell. The cell was constructed by enclosing a silicone sheet (1 mm thick) between a pair of cover glasses (120 µm thick). The surface of the cover glasses was preliminarily subjected to hydrophilic treatment using a UV irradiator (Bioforce,



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Nanoscience). The liquid mixture becomes a thin film on the surface because of the hydrophilic surface. The liquid mixture in the cell was used for the laser-induced crystallization experiments. 2.2 Optical Setup for Laser-Trapping System A laser trapping system was constructed using an inverted optical microscope (Olympus, IX71). The configuration of the optical setup is the same as our previous study26 (Supporting Information S2.). Figure S2 shows a schematic illustration of the optical setup for the laser trapping system. Linearly polarized continuous-wave (CW) green laser (λ = 532 nm, Spectra Physics, Millenia eV) was used as light source. The laser emitted from the light source firstly passes an adjustable attenuator constructed by a polarizing beam splitter and a half-wave plate. Beam diameter of the light was then expanded from 2.3 mm to approximately 5.4 mm, which is identical to the pupil diameter of the objective lens equipped in the optical microscope, using Kepler-type beam expander constructed by two opposite plano-convex lenses. The expanded beam was converted to circular polarization using a quarter-wave plate. The circularly polarized light (CPL) was introduced into the inverted optical microscope equipped with 60× objective lens (Olympus, UPLFN 60X, NA = 0.9). The CPL was then


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introduced to the objective lens through the reflection by a Notch-Dichroic half mirror (λ = 532 nm). The objective lens concentrates the CPL laser at the focal point. 2.3 Laser-Induced Crystallization Experiment The sample cell (See sec 2.1) was placed on a hand-made temperature control stage. The temperature of the stage was controlled at 16±1°C throughout crystallization experiments by Peltier devices connected to a feedback type Peltier controller (Netsudenshi) to reproduce the experimental condition in ref. 26. Assuming that (i) AgNP dispersion can be regarded as pure water (ii) the temperature of the liquid mixture is completely controlled at 16°C, supersaturation of the liquid mixture is -24 %. Namely, the sample liquid is undersaturated state, meaning that spontaneous crystallization does not occur. The laser whose intensity before passing through the objective lens is 940±5 mW was focused into the air/solution interface in the cell, leading to the induction of crystal nucleation from the focal spot. 2.4 Observational Setup -Simultaneous In-Situ Microscopic Observation for Raman Scattering and Polarized-Light Image during the Crystallization ProcessFigure 1 shows the schematic illustration of the setup for the in-situ observation. Polarized-light microscope was constructed by setting two polarizers at upper and lower side in the optical path of the inverted optical microscope with a halogen lamp of



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illumination light source. Polarized-light microscope allows us to identify the polymorph of the NaClO3 crystal by detecting the presence birefringence of the crystals. The metastable monoclinic crystal and the stable cubic crystal exhibits brilliant bright color originating from the birefringence and dark color under polarized-light microscope, respectively. Microscopic image was captured by a CCD video camera (ELMO, CN43H). The CCD camera is equipped with a notch filter which terminates 532 nm light to eliminate the incident laser light undesired for imaging. The captured video was recorded using a digital video capture (IODATA GV-D4HVR). To observe Raman scattering from the periphery of the focal spot during the crystallization, we observed the crystallization process using the inverted optical microscope without the illumination light and the CCD camera with the notch filter so as to collect only the Raman scattered light. In addition, the simultaneous observation of Raman scattering and polarized-light image was realized by slightly turning on the illumination light. 2.5 UV/Vis Spectroscopy Analysis of the Solution Used in the Crystallization Experiments UV/Vis absorption spectra of the AgNP dispersion and its mixture with the NaClO3 aqueous solution was measured in the range from 300 nm to 700 nm with UV-Vis


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spectrometer (V-630BIO, JASCO, Japan) at 10mm path length using 10mm rectangular quartz cell (JASCO) as a sample container. The measurement was carried out at 18 °C. The UV/Vis spectra of the mixtures were measured at 0.16, 0.23, 0.45, 0.65, 0.85, 1.04, 1.22, 1.40 M of NaClO3 concentration, respectively. 2.6 Particle Size Distribution Analysis of Mixture of AgNP Dispersion and NaClO3 Aqueous Solution Particle size distributions of AgNPs suspended in the mixture of the AgNP dispersion and NaClO3 aqueous solutions were measured by means of dynamic light scattering (DLS) method using Nanotracwave with one-drop measurement unit (UT251, Nikkiso, λ = 780 nm). It was assumed that refractivity and viscosity of the solvent suspending the AgNPs is identical to those of pure water (n = 1.33, η = 1.007 at 20°C, 0.797 at 30°C.). 40 µL of the AgNP dispersion liquid without NaClO3 aqueous solution was firstly measured, followed by the measurement of the mixture. The measurement of the mixture was carried out by adding an appropriate volume of droplet of NaClO3 saturated aqueous solution to the pure AgNP dispersion so that the resulting molar concentration corresponds to the molar concentrations in which the UV/Vis spectroscopy analysis was carried out. 3.

RESULTS AND DISCUSSION



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According to the framework of the conventional Raman scattering, intensity of Raman scattering is proportional to the intensity of the incident light.33 Therefore, it follows that the maximum intensity of Raman scattering should be at the focal spot. However, our observation showed that the maximum intensity is the periphery of the focal spot. Figure 2 (i)–(vi) shows time-lapse micrographs showing Raman scattered light and the corresponding intensity distribution profiles, which are represented by 256 gradation, before nucleation event. Indeed, Raman intensity at the focal spot, indicated by a green arrow, constantly exhibits high intensity relative to that of background. On the other hand, several sharp peaks of intensity can be seen at the peripheries of the focal spot. The intensities of these peaks were often greater than that of the focal spot and sometimes reached several times larger value (Supporting Information, S3). These facts contradict to the picture of the conventional Raman scattering. This contradiction can be explained by Raman scattering enhanced by LSPR of the AgNPs dispersed in the solution. The LSPR is the resonance of collective oscillation of free electrons on the surface of metal NPs with electromagnetic oscillation of the incident light.27 The oscillation of the localized surface plasmon strongly enhances electrical field near the surface of the metal NPs because of the induction of electrical dipole irradiation. The enhancement of the strength of electrical field, defined as 〈| |〉/ where 〈| |〉 is


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the average of the square of the static field over the NP surface and is the square of the field of incident light, is ordinarily a few hundred for near-field of a single NP.34 Moreover, enormously large enhancement, up to 106~107 enhancement,35 can be obtained at interparticle nanogap of NPs. This nanogap is called as plasmonic “hotspot”.36 The electrical field enhanced by the surface plasmon enhances intensity of Raman scattering. Such an enhanced Raman scattering is called as surface-enhanced Raman scattering (SERS).37 Intensity enhancement of the SERS is proportional to the fourth power of electrical field intensity because that Raman scattering is two-photon process, and both photons of the two-photon process receive the enhancement. Therefore, intensity of SERS is so remarkable that several groups have reported 1014 ~ 1015 of the SERS enhancement factor, which is defined by forth power of the ratio between the intensity of scattering light in the presence of the contribution of surface plasmon and that in the absence of it, at plasmonic hotspot of Ag colloids.35,38,39 The strong Raman intensity detected at the peripheries of the focal spot is possibly due to the SERS which comes from an adsorbate on the surface of AgNPs, although it is unclear which species show the Raman scattering at this moment, at the hotspot. Indeed, no strong Raman intensity comparable to the observation was observed at the periphery of the focal spot when non-metallic SiO2NPs were used instead of AgNPs (Supporting



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Information, S4.) This fact supports that the strong Raman intensities originates from LSPR excitation of AgNPs. It is generally known that monodisperse 10nm AgNP, which we used, predominantly resonate with the light whose wavelength is near 400 nm40 and the monodisperse AgNP should be fairly off-resonant for the wavelength of the laser we used (532 nm). UV/Vis extinction (absorption and scattering) spectroscopy can prove the wavelength of the plasmonic resonance of metal NP since surface plasmon energetically couples with incident light at resonant wavelength. Indeed, UV/Vis extinction spectrum of the monodispersion of 10 nm AgNP exhibits sharp peak at the wavelength of 400 nm and the spectrum vanishingly exhibits the extinction at the wavelength of 532 nm [Figure 3 (a) black]. Nevertheless, our observation of Raman scattering showed SERS. To confirm that the AgNPs in the NaClO3 solution resonate with the 532 nm light, we measured UV/Vis extinction spectra of AgNP dispersions containing NaClO3 aqueous solution as a function of concentration of NaClO3. Figure 3 (a) shows the UV/Vis extinction as a function of NaClO3 concentration. Gradation of the Figure indicates the NaClO3 concentration; brighter gradation indicates higher NaClO3 concentration out of 0, 0.16, 0.23, 0.45, 0.65, 0.85, 1.04, 1.22, 1.40 M. It can be seen that the sharp peak at 400 nm broadened and the absorbance at 532 nm became stronger as the NaClO3


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concentration increases. Therefore, the presence of NaClO3 allows AgNPs to resonate with the laser light we used. This means that AgNPs in our experiment are plasmon-active. In addition to UV/Vis spectroscopy measurement, we also performed particle size distribution measurement which relies on the DLS method41 for AgNPs dispersed in each NaClO3 aqueous solutions whose concentrations are the same as those of the solutions we measured in the UV/Vis-measurement to clarify the cause of the appearance of absorbance at 532 nm. Figure 3 (b) shows the results of the particle size distribution. The results indicate that the particle size distribution shifts to larger side as the NaClO3 concentration increases. Comparing the UV/Vis spectrum with the particle size distribution, the broadening of the plasmonic peak at 400 nm and the appearance of the absorbance at 532 nm probably correlates with the increment of the particle size accompanying with the increment of the NaClO3 concentration. This correlation is attributable to red-shift of plasmonic absorption peak caused by salt-induced aggregation of individual AgNPs.42 The addition of salt induces the aggregation of metal NPs in dispersion liquid because the anion of the additive salt neutralizes the surface charge of individual NPs responsible for interparticle Coulomb repulsion force. The salt-induced aggregation allows the surface plasmon of individual NPs to couple each other, generating additional plasmon absorption bands in longer wavelength side



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than the intrinsic absorption band (The additional bands is known as “gap mode plasmon resonance”).43, 44 Taking these reports into account, in our system, the addition of ClO3- ion originating from the NaClO3 salt neutralizes the surface charge on the 10 nm AgNPs, leading to the aggregation of the AgNPs (Supporting Information, S5). Since the aggregation causes the red-shift of plasmonic absorption peak, some AgNP aggregates probably becomes surface plasmon-active at the wavelength of 532 nm. Additionally, the broadening the plasmonic peak at 400 nm is may be due to the coexistence of various sized AgNP aggregates. Taking the formation of salt-induced AgNP aggregations into account, it is reasonable to consider that the enhanced SERS peaks observed in our observations originate from plasmonic hotspots in aggregation of AgNPs because the scattering intensity several times larger than that of the focal spot was observed in spite of the position where Raman intensity is undetectable in the absence of AgNPs (Supporting Information, S6). This idea can be supported by the fact that the earlier studies on single-molecule SERS, whose mechanism strongly relies on the formation of plasmonic hotspot, have been developed on the basis of use of salt-induced aggregation of AgNPs.45 Figure 4 (i)-(vi) show time-lapse micrographs captured by the simultaneous observation of Raman scattering and transmission polarized light microscopic image


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during the laser-induced crystallization experiment, which performed in order to investigate the mechanism of the nucleation from the focal spot in detail. The figures are snapshots from a half of a second before the moment of crystal nucleation to the moment of the nucleation. 0.63 seconds before the nucleation [Figure 4 (i)] (See also Supporting Information, S7), SERS-active AgNPs (red arrow) appeared in the periphery of the focal spot. Afterwards, the SERS-active particles were attracted to the focal spot by optical gradient force [Figure 4 (ii)-(iv)]. It should be noticed that a metastable crystal appeared from the focal spot almost simultaneously with the optical trapping of the SERS-active particle (0.07 sec after the trapping) [Figure 4 (v)-(vi)] (Crystals which appeared in such a way disappeared within about 10 seconds if the laser is switched off.). This observation clearly shows that the optical trapping of the SERS-active particle is one of triggers for crystal nucleation, suggesting that the excitation of LSPR possibly contributes to crystal nucleation in some way. Conceivable plasmonic phenomena contributing to crystal nucleation may include: (i) electromagnetic field enhanced by the surface plasmon resonance; (ii) plasmonic heating. First, we discuss the contribution of the electromagnetic field enhanced by the surface plasmon resonance. The electrical field enhanced by the surface plasmon is known to be applied to optical nanotweezers which manipulate nano-sized objects. Such



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nanotweezers is called as plasmonic tweezers. The mechanism of plasmonic tweezers relies on extremely high gradient force originating from the strong electrical field spatially confined beyond the diffraction limit.46-50Especially, the optical force generated by the enhanced electrical field at plasmonic “hotspot” probably gathers the nano-sized objects nearby.51 In our system, nano-sized objects dispersed in the solution may be not only the AgNPs but also subcritical crystalline clusters of NaClO3 molecules because a numbers of studies on homogeneous nucleation from an aqueous solution have suggested the presence of subcritical clusters as precursor of solid phase in unsaturated aqueous solution of various systems even for inorganic salt.52-57 Our system may be not exception of the thought of the presence of subcritical cluster. Kimura et al. have actually shown the presence of nano-sized subcritical clusters in undersaturated NaClO3-ionic liquid solution by in-situ transmission electron microscopy.58 Taking the presence of subcritical cluster into account, once surface plasmon of the AgNP aggregates, which contains numerous sites having the potential for plasmonic hotspot, was excited by the laser irradiation, subcritical cluster in the solution may be intensively gathered to the hotspot by the surface-enhanced optical force. This process may locally increase the concentration around the hotspot, promoting crystal nucleation at the focal spot. Sugiyama et al. have reported that laser


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trapping of the clusters in the undersaturated solution of organic compounds can induce crystal nucleation from the focal spot.15,24,25 These reports imply the possibility of the crystal nucleation induced by the surface plasmon-enhanced optical force. Although the plasmon-enhanced optical gradient force may contribute to the initiation of the nucleation process, this effect alone cannot explain the growth of the crystal to the size of micrometer because the gradient force ranges only nanometer scale.47,59 Second, we discuss the contribution of the plsmonic heating.29 Plasmonic heating is the locally confined temperature rise originating from the dissipation of the energy which metal NPs obtained by surface plasmon resonance. Bendix et al. have shown that the temperature rise by optical trapping of an AuNP can reach 150 K at the focal spot in the case that a 80 nm sized AuNP was optically trapped by 400 mW CW laser (λ = 1064 nm) by measuring the size of the region where a lipid bilayer transforms to disordered phase by the plasmonic heating.60 Since the temperature rising easily exceeds the boiling temperature of water, such a plasmonic photothermal effect could be significant to our system. Taking the fact that the laser power is 940 mW and the formation of plasmonic hotspots at the air/solution interface into account the plasmonic heating is possibly significant to the evaporation of the solvent. In practice, the optical trapping of AgNP aggregates floated on the interface between NaClO3 aqueous solution



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and liquid paraffin, which is used to confine the vaporized solvent at the interface, sometimes forms bubbles from the focal spot (Supporting Information,S8). The bubble formation proves intensive vaporization and evaporation of the solvent from the focal spot. It is easily anticipated that intensive evaporation by plasmonic heating locally increases the concentration around the focal spot as well. In addition, we performed a numerical analysis of temperature distribution around the focal spot in the presence of AgNPs by means of a heat transfer analysis combined with electromagnetic simulation based on finite-difference time-domain (FDTD) method to discuss the temperature increment by surface plasmon excitation. A commercial software of three-dimensional FDTD program (Poynting for Optics, Fujitsu, Japan) was used to analyze electric field intensity in the vicinity of the AgNPs placed on air/NaClO3 solution interface. The simulation box size was 200 nm × 200 nm ×200 nm in x, y and z axis. We simulate the situation that a circularly polarized plane wave at 532 nm with Gaussian intensity distribution (beam size is 720 nm, based on the Rayleigh limit) illuminates about 100 Ag spheres (diameter is 10 nm) randomly-dispersed on the x-y plane (z = 0) to mimic our experimental condition of the optical trapping of AgNP aggregate [Figure 5 (a)] (The Ag spheres are fixed at given coordinates.). The perfect matching layer boundary condition was adopted for the end faces perpendicular to the z axis, and the periodic


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boundary condition was adopted for the end faces perpendicular to the x and y axis to express the 2 dimensional spreading of AgNPs.

Filed intensity of the incident plane

was set to 1 (V/m)2. Figure 5 (b) shows the steady state field intensity distribution calculated by the FDTD simulation. Electrical fields is enhanced by more than 50 times in the vicinity of AgNPs in places because of surface plasmon resonance. Using data of the FDTD analysis, heat transfer analysis based on finite-difference method (FDM) was carried out by setting the FDTD simulation box as heat source by converting the electrical field intensity to power of the electrical field using equations S9-1 and S9-2 (Supporting Information, S9). We modeled a simulation box mimicking NaClO3 aqueous solution by constructing a 2000 nm × 2000 nm × 1000 nm box of a dielectric medium of which refractive index, relative permittivity, specific heat, heat conductivity and electric conductivity are 1.33, 1, 4186 (J/kg·K), 0.618 (W/mK) and 11.81 (S/m), respectively. These values correspond to that of water except for electric conductivity, and the value of electric conductivity is a value of NaClO3 aqueous solution referred from Ref.60. Robin condition was adopted as the boundary condition of the upper end face perpendicular to the z axis. The heat transfer coefficient of the boundary condition was set to be 10 (W/m2K) to express air. Dirichlet condition was adopted to the other faces, in which the temperature was set to be 16oC. We simulated



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the situation mimicking our experimental condition by inputting the energy obtained when 940 mW laser (λ = 532 nm) was focused by an objective lens with 0.9 of NA. Figure 5 (c) and (d) show time evolution of simulated temperature distribution. At 1.15 ns after laser irradiation, the temperature around the AgNPs, indicated by black dashed square, instantaneously increased. 102 ns after laser irradiation, the maximum temperature reaches to 390oC, and the region above 100oC radially spread to the points at 150 nm from the heat source. This temperature elevation allows the solvent to evaporate intensively from the AgNPs trapped at the focal spot, supporting the hypothesis that plasmonic heating promotes the evaporation of the solvent at the focal spot. Figure 6 (a) and (b) show time-lapse micrographs of the in-situ polarized light observation of metastable phase crystallization induced by optical trapping of AgNPs and the corresponding time evolution of crystal size during the crystallization process, respectively (See also Supporting Information, S10, movie). The figure that the timings of crystallization onset and disappearance of the crystal are synchronized with the timings of optical trapping [(ii), (iv), (vi)-(xv)] and release [(iii), (v), (xvi)], which occurs simultaneously with the crystal formation even despite the continuous laser irradiation, of the AgNPs-aggregate which is seen as black dot in the micrograph (i),


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respectively. The synchronism also indicates LSPR excitation contributes to the crystallization. After the AgNPs was stably trapped, the metastable crystal appeared from the focal spot concentrically grew to the size of several tens of micrometers in a rounded shape rather than a faceted shape [Figure 6 (a) (vi)-(xiv) and (b) 0.9-2 sec]. Upon the continuous irradiation of the laser, the crystal kept their size with fluctuation ranging several micrometers [Figure 6 (a) (xv) and (b) 2-3 sec]. These behaviors of crystal supports the hypothesis that the solvent evaporation caused by plasmonic heating contributes to the increment of supersaturation for crystallization as follows. When an optically trapped nanoparticle is considered as a point heat source at the air/solution, the maximum temperature at the focal spot concentrically decays while following the inverse proportion manner as the position disengages from the focal spot in steady state.62 The decay results in the concentric temperature distribution with the trapped nanoparticle as a center.60 Taking temperature elevation by plasmonic heating promotes the solvent evaporation into account,

the concentric temperature decay possibly results

in two situations depending on the distance from the center: (1) evaporation rate of solvent is fast enough relative to the diffusion rate of solvent molecule; (2) the evaporation rate is slow relative to the diffusion rate. The former situation, which occurs relatively near the center, leads to supersaturated state and the latter situation, which



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occurs relatively far from the center, results in unsaturated state because local concentration increment is probably governed by the competition between the outflux of the evaporated solvent to the air and the influx of solvent by diffusion from the bulk solution. Once the crystal appears from the supersaturated region, the crystal grows idiomorphically, namely, with faceted shape in the supersaturated region. On the other hand, once the crystal grew and reached to the unsaturated region, the crystal will start to dissolve preferentially from the apex of the idiomorphic crystal shape because of Gibbs-Thomson effect, resulting in rounded shape of crystal. Moreover, it is highly possible that the generation and the annihilation of the supersaturated state continuously repeats at the boundary between the supersaturated region and the unsaturated region because of the competition of the in/outflux of the solvent. This probably leads to the fluctuation of the crystal size seen in the in-situ observation. Therefore, the fluctuated round shape of the crystal supports the hypothesis of supersaturated state induced by the plasmonic heating.

We advocate that the solvent evaporation by plasmonic heating is the main factor to promote the crystallization from the focal spot. Figure 7 is schematics showing a scenario of the plasmonic heating-assisted laser-induced crystallization we proposed.


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Irradiation of focused laser to the air/unsaturated NaClO3 aqueous solution interface gathers AgNPs or its aggregate formed by the presence of ClO3- ion to the focal spot because of optical gradient force. Some of the aggregates are plasmon-active. The aggregates possibly generate several plasmonic “hotspots”, leading to the plasmonic heating significant to solvent evaporation. The solvent evaporation by plasmonic heating probably contributes to the locally confined increment of NaClO3 concentration. This probably allows the crystallization intensively from the focal spot. Although there are possibilities that (1) plasmon-enhanced optical trapping of the crystalline clusters or (2) nanoscale bubble formation triggers crystal nucleation (note that crystallization process includes “nucleation” and “crystal growth”), the solvent evaporation induced by plasmonic heating should be the main factor to produce a micrometer-scaled crystal from the focal spot because: (1) effective range of plasmon-enhanced optical gradient force cannot reach several tens of micrometer; crystal would dissolve even if the bubble formation triggers crystal nucleation in unsaturated solution.

Recalling the fact that

the solubility of the metastable crystal is 1.6 times higher than that of the stable crystal,32 it can be seen that this plasmonic heating-assisted laser-induced crystallization method can realize instantaneous locally-confined high supersaturated state enough to exceed 60% with respect to the stable crystal around the focal spot even in the



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unsaturated bulk solution. This can lead to the efficient spatiotemporal control of crystallization. Moreover, recent studies on plasmonics shows the nonlinear light-matter interaction in the near-field of the surface-enhanced electromagnetic field, which cannot be realized by the conventional far-field optics.28 Because the activation energy to overcome for the heterogeneous nucleation on the surface of the nanoparticles is lower than that of the homogeneous nucleation,63 the nucleation probably takes place on the surface of the nanoparticles through the heterogeneous nucleation, namely, under the plasmonic near-field. Therefore, our method has a potential to create a novel nucleation field that never been achieved by conventional NPLIN.

4. CONCLUSIONS To clarify the contribution of the excitation of the LSPR on the crystallization induced by visible laser trapping of silver nanoparticles (AgNPs) at air/unsaturated mother solution interface,26 we have performed in-situ simultaneous observations of Raman scattering and polarized-light image during the process of the crystallization induced by the optical trapping of Ag nanoaggregate at the air/NaClO3 solution interface The observation revealed that the optical trapping of SERS-active particles triggers the crystallization of the NaClO3 metastable crystal from the focal spot, highlighting the


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contribution of the excitation of the LSPR on the crystallization. Moreover, in-situ observation of the crystal under continuous laser irradiation shows that the crystal concentrically grew to the size of a several tens of micrometers and kept their size with fluctuations ranging several micrometers. This behavior is possibly attributable to concentric supersaturation distribution caused by the solvent evaporation via plasmonic heating. The conceivable mechanism of the crystallization is as follows: (1) The aggregation of Ag nanoparticles dispersed in the solution was induced by the presence of ClO3-, i.e., the counter ion, and the optical trapping; (2) plasmonic “hotspot” forms at the interparticle nanogaps of the Ag nanoaggregates; (2) The formation of the plasmonic “hotspot” leads to plasmonic heating at the focal spot; (3) The solvent evaporation caused by the plasmonic heating leads to the high supersaturation state concentrically distributed around the focal spot; (4) The concentric supersaturation distribution allows to take place nucleation and growth of NaClO3 metastable crystal through the heterogeneous nucleation via the surface of the optically-trapped Ag nanoparticles. Therefore, it was concluded that the excitation of LSPR at the focal spot contributes to crystallization as the nano-sized point heat source that evaporates the solvent. This plasmonic heating-assisted laser-induced crystallization has a potential to provide not only a novel approach for spatiotemporal control of the crystal nucleation from a



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solution but also novel nucleation field based on nonlinear light-matter interaction originating

from

the

plasmon-enhanced

electromagnetic

near-field

through

heterogeneous nucleation on the surface of a plasmonic particle.

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Acknowledgements: This work was supported by Grant-in-Aid for JSPS Fellows Grant Number 15J11361. This work was also supported by JSPS KAKENHI Grant-in-Aid for Young Scientists (B) Grant Number 16K17512 and JSPS KAKENHI Grant Number JP 16H06507 in Scientific Research on Innovative Areas “Nano-Material Optical-Manipulation”. The authors thank W. Odajima of Fujitsu Ltd. For his advice about optical simulations.

Author information note: The present address of the corresponding author, HN, is listed below. §H. N.: Present Affiliation: Molecular Chirality Research Center, Graduate School of Advanced Integration Science, Chiba University, 1-33 Yayoi-Cho, Inage-ku, Chiba, 263-8522, Japan Telephone number: +81-43-290-3475, Fax number: +81-43-290-3490 E-mail address: [email protected] Competing financial interests statement: The authors declare no competing financial interest.


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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 “S1.Polymorphism in NaClO3 crystallization from an aqueous solution”, “S2.Optical Setup for Laser-Trapping system”, “S3.Intense Raman scattering at the periphery of the focal spot”, “S4.Comparison of spatial intensity distribution of Raman scattering between the case of SiO2NP and the case of AgNP”, “S5.A micrograph showing a salt-induced aggregation of AgNPs”, “S6.Time-lapse spatial intensity distribution of Raman scattering from NaClO3 aqueous solution without AgNPs”, “S7.Movie of In-situ simultaneous observation of Raman scattering and polarized light image during the laser-induced crystallization.”, “S8.Bubble formation induced by plasmonic heating from an optically-trapped AgNPs at liquid paraffin/NaClO3 aqueous solution containing AgNPs”, “S9.Conversion of the electrical field intensity analyzed by FDTD calculation to power as heat source in FDM simulation.” and “S10.Movie of In-situ simultaneous observation of metastable phase crystallization synchronized with the optical trapping of an aggregate of AgNPs.”. S7 and S10 are video files.



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Figure 1. A schematic illustration showing the optical setup for the simultaneous in-situ microscopic observation of Raman scattering and polarize-light image. Raman scattering/polarized-light image can be observed by turning on/off the halogen illumination light, respectively. To slightly turn on the lamp allows the simultaneous observation of Raman scattering and polarized-light image.


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Figure 2. Time-lapse intensity distribution of Raman scattering during laser irradiation. The insets in each image are time-lapse micrographs captured by in-situ observation of Raman scattering and (i)-(vi) are the corresponding images showing the spatial intensity distribution of Raman scattering represented by 256 gradation. The Raman scattering from the focal spot is indicated by a green arrow and the green circle dashed line. The red circle dashed line indicates Raman intensities significantly higher than the background. Raman intensities higher than that of focal spot were observed around the focal in places.



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Figure 3. (a) Dependence of UV/Vis spectra of AgNP dispersion on NaClO3 concentration. Blue and black spectrum is for NaClO3 aqueous solution and monodispersed 10 nm AgNP colloidal solution, respectively. The gradation from brown to yellow indicates NaClO3 concentration in the AgNP colloidal solution. In order from darker gradation, the gradations represent 0.16, 0.23, 0.45, 0.65, 0.85, 1.04, 1.22, 1.20 M of NaClO3. (b) Dependence of particle size distributions of AgNP dispersion on NaClO3 concentration. The gradations correspond to the gradations in (a).


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Figure 4. Time-lapse micrographs captured by in-situ simultaneous observation of Raman scattering and polarized light image during the laser-induced crystallization. The green arrow indicates the focal spot and the red arrow indicates the Raman scattering from SERS-active particles. (i) 0.63 seconds before the nucleation. SERS at the periphery of the focal spot can be observed. The moment when this image was captured is set to be 0 sec. (ii),(iii) attraction of SERS-active particles towards the focal spot. (iv),(v) optical trapping of the SERS-active particles. Insets are magnified images of the focal spot and Raman scattering light from the SERS-active particles. (vi) The appearance of the metastable crystal. It should be noted that the crystal appeared within 0.07 seconds after the optical trapping of the SERS-active particles. (See also Supporting Information, S7)



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Figure 5. Analysis of temperature distribution in the vicinity of optically-trapped AgNPs based on the combination of FDTD electromagnetic simulation and FDM heat transfer simulation. (a) The model of FDTD electromagnetic simulation. Green spheres represent AgNPs with the diameter of 10 nm, and the face colored by pink is excitation source of light. (b) The spatial distribution of electromagnetic field intensity simulated FDTD method. (c) The temperature distribution calculated by FDM in the vicinity of the AgNPs excited by focused laser 1.15 ns after the laser irradiation. Green box is a dielectric medium mimicking NaClO3 aqueous solution. The FDTD simulation box was used as heat source. The inset boxed by black dashed line is a magnified image of the vicinity of the heat source. (d) The temperature distribution at 102 ns after the laser irradiation.


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Figure 6. (a) Time-lapse micrographs showing the fluctuation in the size of the metastable crystal appeared from the focal spot. The arrowed green spot is the focal spot. The black dot seen in micrographs is an aggregate of AgNPs. (b) Time evolution of the size of the metastable crystal appeared from the focal spot. The time corresponds to that of (a). The length of the line segment passing the focal spot and drawn parallel to the lower side of the micrographs [the broken arrow depicted in the inset] was adopted as the crystal size. The blue colored region indicates the duration in which the AgNPs-aggregate was released from the optical trapping. The red colored region indicates the duration in which the AgNPs-aggregate was optically trapped at the focal spot. (See also Supporting Information, S10, movie)



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Figure 7. The schematic illustrations describing the conceivable scenario of plasmonic heating-assisted laser-induced nucleation mechenism. (i) Starting NaClO3 undersaturated aqueous solution contains aggregates of AgNPs and monodisperse AgNPs. (ii) The AgNPs are gathered to the focal spot of tightly-focused laser by optical gradient force. (iii) Electromagnetic field of the incident light is largely enhanced by the formation of the plasmonic “hotspot” at the interparticle nanogaps of the aggregates. (iv) The solvent evaporates from the focal spot because of temperature rise originating from the plasmonic heating (v) The plasmonic heating forms a concentric supersaturation distribution with the focal spot as center at the air/solution interface since the temperature distribution is concentric in the steady state. (vi) The heterogeneous nucleation of the metastable crystal takes place via the surface of the optically-trapped Ag nanoparticles. The growth of the crystal is limited to the region where supersaturation state is achieved (The region indicated by A-A’ line for the case of the schematics).


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For Table of Contents Use Only

Title: Plasmonic Heating-Assisted Laser-Induced Crystallization from a NaClO3 Unsaturated Mother Solution

Author list:


Affiliations: †


‡




30010, Taiwan //


Table of Contents Graphic and Synopsis

We revealed a novel laser-induced crystallization mechanism in which locally confined evaporation of solution caused by plasmonic heating via laser trapping of Ag nanoaggregate at air/solution interface promotes crystallization from the focal spot even if the mother solution is unsaturated state. The mechanism can provide a novel nucleation field based on plasmonic near-field which never achieved by far field optics.



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Figure1 254x190mm (300 x 300 DPI)


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Figure2 338x190mm (300 x 300 DPI)



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Figure3 254x190mm (300 x 300 DPI)


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Figure 4 254x190mm (300 x 300 DPI)



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Figure5 338x190mm (300 x 300 DPI)


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Figure6 190x338mm (300 x 300 DPI)



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Figure7 408x190mm (300 x 300 DPI)


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Plasmonic Heating-Assisted Laser-Induced Crystallization from a

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