Yiqing Chen,Xin Huang,2,*,Suping Ding,Yaoguang Feng,Na Wang,2,Hongxun Hao,2,3,*

1 National Engineering Research Center of Industrial Crystallization Technology,School of Chemical Engineering and Technology,Tianjin University,Tianjin 300072,China

2 Collaborative Innovation Center of Chemical Science and Engineering (Tianjin),Tianjin 300072,China

3 School of Chemical Engineering and Technology,Hainan University,Haikou 570228,China

Keywords:Polymorph Functional nanoparticles Surface interactions Heterogeneous nucleation

ABSTRACT The precise control of active pharmaceutical ingredient (API) crystal nucleation and polymorphism is a key consideration in pharmaceutical manufacturing.In this study,tunable nanoparticles were developed to regulate the nucleation process of coumarin.Magnetic silica nanoparticles with four different functional groups (-NH2,-COOH,-SH,-NCO) were prepared and coated on the substrate for inducing the crystallization of coumarin.Confined melt crystallization and microspacing sublimation crystallization methods were used to investigate the regulation mechanism.The results indicated that three metastable forms of coumarin can be obtained as pure components based on the combined influence of crystallization methods and functionalized nanoparticles.Form II could be selectively obtained by microspacing sublimation crystallization on Fe3O4@SiO2-SH substrates,and Form IV could be obtained by confined melt crystallization on Fe3O4@SiO2-NCO substrates.Form III could be obtained by further heating Form IV crystals to 52 °C on Fe3O4@SiO2-NCO substrates.Moreover,the polarized light microscopy results also indicated that the introduction of nanoparticles could also increase the stability of the metastable crystalline forms of coumarin.Finally,the diffusion and surface dynamics during nanoparticle induced crystallization were comparatively investigated and the corresponding polymorphic selectivity mechanism was proposed.

1.Introduction

Crystallization is the basis of many natural and industrial processes [1,2].In the pharmaceutical industry,the crystallization process is particularly important as it is often the last step in the manufacture and isolation of active pharmaceutical ingredients[3].Crystal nucleation is the beginning of crystallization process.It is highly sensitive to process conditions such as temperature[4] and contamination [5,6],and can affect crystal polymorphism[7-9],crystal morphology [10-12] and crystal size distribution[13].The nucleation process can be divided into homogeneous nucleation and heterogeneous nucleation.In order to achieve control over the nucleation process,more and more studies have focused on heterogeneous nucleation because it tends to be faster than homogeneous nucleation and is less affected by fluctuations in other experimental conditions [14,15].However,our understanding of heterogeneous nucleation is still limited.

The introduction of nanomaterials into the crystal nucleation process is a promising way to investigate the mechanism of heterogeneous nucleation.The heterogeneous interface introduced by nanomaterials can significantly reduce the free-energy barrier,allowing nucleation under metastable conditions [16].Controlled nanomaterial preparation techniques can also help to customize the metastable conditions for crystal nucleation.In recent years,many studies have been reported on the application of nanomaterials to induce the crystallization of different polymorphs of small molecule drugs.Delmaset al.prepared colloidal templates using 220 nm silica nanoparticles and obtained acetaminophen Form II crystals at lower supersaturation and faster cooling rates[17].Matthew Boyeset al.discovered the preferential crystallization of metastable α-glycine polymorph on graphene surface and proposed the importance of H-bonding interactions in promoting polymorph selectivity [18].Ouyanget al.applied phenylfunctionalized silica with pores of 50 nm to crystallize carbamazepine in solution and obtained its metastable Form II [19].Thus,controlled nanomaterials have unique advantages in inducing metastable crystalline nucleation.

Coumarin is an organic small molecule with a wide range of pharmaceutical applications because of its pharmacological activities,like anti-HIV,antimicrobial,antiinflammatory,anticancer,anti-TB properties [20-23].The polymorphic forms of coumarin have been reported for a long time,but it is only recently that five polymorphic forms of coumarin have been identified.Shtukenberget al.obtained five polycrystalline forms of coumarins by crystallizing coumarin from the melt in porous poly(cyclohexylethylene)and porous glass bead (diameter of pores 7.5-55 nm) media and four new forms were obtained [24].However,all the new crystals were crystallized in polycrystalline form from the melt.The separation of products from porous materials is difficult and the nucleation and growth mechanisms of metastable forms are ambiguous.It remains a challenge to develop a more reliable and reproducible strategy for the preparation of coumarin metastable polycrystalline forms,especially Form II,III and IV.

In this study,Fe3O4@SiO2nanoparticles with specific external functional groups were synthesized and coated on the substrate for selective crystallization of coumarin.The presence of the magnetic core allows the product crystallized from the melt to be easily separated from the added heterogeneous substrate.Two crystallization methods (confined melt crystallization and microspacing sublimation crystallization) and five nanoparticles (Fe3O4@SiO2,Fe3O4@SiO2-NH2,Fe3O4@SiO2-COOH,Fe3O4@SiO2-NCO,Fe3O4@-SiO2-SH) were investigated to obtain pure component products of three metastable crystalline forms of coumarin.Polarized light optical microscope,powder X-ray diffraction,Raman spectra and hot stage optical microscope were adopted to investigate the polymorphic selectivity of coumarins.Finally,a potential mechanism for the effect of wetting properties on the diffusion and surface dynamics of twisted crystal growth was proposed to explain the polymorphic selectivity.

2.Materials and Methods

2.1.Materials

Coumarin (C9H6O2,CAS: 91-64-5) was provided by Shandong Yingyang Flavors &Fragrance Co.,Ltd.(China),and was further purified in the laboratory with a final purity of 99.8% (the purity of coumarin were obtained using liquid chromatographic characterization on Waters Alliance 2695 system and were processed by area integration method,with reference to the National Standards of the People’s Republic of China GB 5009.284-2021).FeCl3·6H2O(99%)was purchased from Shanghai Aladdin Biochemical Technology Co.,Ltd.(China).FeSO4·7H2O (98%),(3-aminopropyl)triethoxysilane(99%),(3-mercaptopropyl)triethoxysi lane(99%),and(3-isocyanatopropyl)triethoxysilane(99%),succinic anhydride(98%),polyethylene glycoltert-octylphenyl ether(triton X-100,90%) and oleic acid (90%) were acquired from Tianjin HEOWNS Biochemical Technology Co.,Ltd.(China),Canada balsam was supplied by Beijing Coolaber Science &Technology Co.,Ltd.(China).The cyclohexane,ethanol,n-hexanol,tetraethyl orthosilicate (TEOS,98%),aqueous ammonia (25%-28%),triethylamine,N,N-dimethylformamide (DMF) were of analytical grade and used without further purification.Distilled-deionized water (resistivity 18.2 MΩ·cm) was prepared in the lab.

2.2.Preparation of nanoparticles

2.2.1.Fe3O4 nanoparticles

Fe3O4NPs were prepared by coprecipitation according to the literature [25,26].In a typical synthetic procedure,5 mmol FeCl3-·6H2O (1.3515 g) was dissolved in 40 ml of H2O.The dissolved oxygen in the solution was removed by nitrogen purge.Then the temperature was increased to 80 °C in 30 min under the stirring condition of 400 r·min-1.When the temperature reaches 60 °C,2.5 mmol FeSO4·7H2O (0.7000 g) and 0.75 ml of oleic acid were added to the solution,and 5 ml of 25%(vol)of ammonium hydroxide was added at 70°C.The solution was naturally cooled to room temperature,and the particles were collected by centrifugation at 7000 r·min-1and magnetic separation.The particles were washed three times with ethanol and water to remove the residual oleic acid,Fe2+,and Fe3+cations.Finally,Fe3O4NPs were dispersed in hexane.

2.2.2.Fe3O4@SiO2 nanoparticles

Fe3O4@SiO2core-shell NPs were prepared by a reverse microemulsion method according to the literature [27].Typically,1.5 ml of Fe3O4solution (6.5 mg·ml-1in cyclohexane) (i.e.,10 mg Fe3O4),10 ml of triton X-100 was dispersed in 50 ml of cyclohexane and the mixture was sonicated for 15 min.Then,a mixture of 7.5 mln-hexanol,2.5 ml deionized water and 1.5 ml of 25%(vol) of ammonium hydroxide was added with 300 r·min-1stirring.TEOS was added by equivalent fractional drop method,adding 100 μl every 12 h for a total of two additions (i.e.,200 μl TEOS).Then 10 ml of ethanol was added to break the emulsion.The obtained Fe3O4@SiO2core-shell NPs were separated by centrifugation and magnetic methods,washed three times with deionized water and ethanol,and then were dispersed in ethanol.

Fig.1.Schematic diagram of the preparation path of different functionalized magnetic silica nanoparticles.

2.2.3.Fe3O4@SiO2-x

The silica nanoparticles were modified with amino,mercapto,and isocyanate functional groups using organosilanes,including(3-aminopropyl)triethoxysilane,(3-mercaptopropyl)triethoxysi lane,and (3-isocyanatopropyl)triethoxysilane,respectively.The synthesis method of functionalized silica can be found in many literatures[28-32].In a typical group modification process,10 mg of Fe3O4@SiO2NPs were firstly dissolved in 10 ml of anhydrous ethanol.Then 100 μl of organosilanes were added and the solution was stirred at 70 °C for 12 h.Acid-functionalized magnetic particles were obtained on the basis of amino-functionalized particles.Fe3-O4@SiO2-NH2NPs were dissolved in anhydrous DMF,treated with succinic anhydride and triethylamine,and refluxed at 120 °C for 12 h.The resulting nanoparticles were separated by centrifugation and magnetic methods.The preparation paths of all particles are shown in Fig.1.

2.3.Preparation of functional substrates

Microscope cover glasses(Sigma,1.8 cm×1.8 cm)were used as substrates.The glass substrates were first washed with ethanol.Then the functionalized substrates were prepared as shown in Fig.2.Firstly,a dilute solution (2%-5% (mass)) of the prepared functionalized particles in ethanol was added dropwise to the edge of the coverslip,which is tilted and the droplet moves toward the other end by gravity.A blast of dry hot air was provided in the opposite direction of the gradient to slow down the process of droplet movement and accelerate the evaporation process at the same time(Fig.2(a)).After the droplet moved to other side,the tilt direction and the blast direction were changed so that the droplet continued to move back (Fig.2(b)).Repeat this process until the droplets disappear completely.The volume of each droplets introduced at the edge of the coverslip was 25 μl and repeat the above functionalization process for a total of six drops.Finally,functional substrates for the following crystallization research were obtained(Fig.2(c)).

Fig.2.Schematic diagram of deposited nanoparticles for the preparation of functionalized substrates.

2.4.Induced crystallization of coumarin

In all experiments,coumarin was mixed with 25% (mass) of Canada balsam to stabilize the metastable polymorphs that might be obtained [24].Nanoparticle-induced crystallization of coumarins was obtained by two methods,confined melt crystallization and microspacing sublimation crystallization,as shown in Fig.3.In the first way,the coumarin samples were heated to 75 °C for 10 min to eliminate the thermal history and then cooled between two coverslips,where only the upper surface of the lower plate was coated with nanoparticles (Fig.3(a)).In the second way,coumarin crystals were obtained by sublimation at 75°C and condensation on the upper surface,where only the lower surface of the upper plate was coated with nanoparticles.Besides,the two plates were padded by a glass spacer and set at an interval of 0.2 mm for microspacing sublimation crystallization (Fig.3(b)) [33].The amount of coumarin used per unit area of the substrate was in the range of 2.0-4.0 mg·cm-2.

2.5.Characterization

Transmission electron microscope (TEM) imaging was performed using Tecnai G2 F20/F30 (FEI,USA) instrument.Samples were prepared on ultrathin carbon films.Scanning electron microscopy (SEM) imaging was performed using Apreo S LoVac (FEI,Czech) instrument.SEM samples were prepared on the silicon wafer by drying the dilute solution(2%-5%(mass))of the prepared functionalized particles in ethanol for 6 h.Functional groups on nanoparticles were characterized by Alpha FTIR-ATR (Bruker,Germany) instrument with 4 cm-1resolution,32 scans per spectrum and a 4000-400 cm-1range.The magnetic properties were performed on a superconducting quantum interference device(MPMS SQUID XL,Quantum Design,USA) at 27 °C.Water contact angle was measured by the sessile drop method using an optical contact angle apparatus (OCA20,Dataphysics,Germany).

The morphology of the crystals was observed by a polarized light optical microscope (BX53,Olympus,Japan).Coumarin polymorphs were identified by powder X-ray diffraction (PXRD,D/MAX-2500,Rigaku,Japan)using Cu Kα radiation(0.15405 nm)with an electric current of 150 mA and a voltage of 40 kV.The scanning angle (2θ) covered a range between 2° and 40° at a rate of 8 (°)·min-1.Raman spectra were collected with a DXR (Thermo Scientific,USA) Raman microscope (laser wavelength 532 nm,laser power 4 mW) from nanoparticle-induced crystalline films on slides.

3.Results and Discussion

3.1.Synthesis of materials and templates

In the coprecipitation reaction,the solution system turned black after the addition of ammonia,indicating the formation of Fe3O4particles.Besides,the color of the particles became lighter after separation and drying,indicating successful coating of silica on the Fe3O4particles by the reverse microemulsion method [26].Fig.4(a),(b) shows TEM images of Fe3O4NPs and Fe3O4@SiO2NPs.The enlarged TEM image of an individual nanoparticle demonstrates a spherical core/shell structure with a single core of Fe3O4nanoparticle.The size of magnetic core and the total particle are around 20 and 65 nm,respectively (Fig.4(c)).Fig.4(d) shows the SEM image of nearly monodispersed Fe3O4@SiO2NPs on the substrate,demonstrating that the Fe3O4@SiO2NPs prepared by the reverse microemulsion method have a uniform size distribution.The SEM images of Fe3O4@SiO2NPs and other nanoparticles(Fig.S1 in Supplementary Material) coated on the substrate also show that the coating method used in the experiment can disperse the particles uniformly and can obtain an approximate monolayer dispersion in the middle of the substrate.

Fig.3.Schematic diagram of the experimental setup for nanoparticle-induced crystallization of coumarin: (a) confined melt crystallization;(b) microspacing sublimation crystallization.

Fig.4.TEM images of (a) Fe3O4 NPs and (b) Fe3O4@SiO2 NPs,(c) enlarged TEM image of Fe3O4@SiO2 NPs,(d) SEM image of Fe3O4@SiO2 NPs on the substrate.

The magnetic properties of Fe3O4NPs and Fe3O4@SiO2NPs were investigated and the magnetic hysteresis curves are plotted in Fig.5.No significant hysteresis loop can be observed on the hysteresis curves.The magnification shows that the nanoparticles before and after coating with silica have the same coercive field of about 1.27 kA·m-1,indicating that the prepared nanoparticles are superparamagnetic.Besides,the saturation magnetization is greatly reduced by the introduction of the silica shell,as the values of Fe3O4NPs and Fe3O4@SiO2NPs are 53.6 and 3.27 A·m2·kg-1,respectively.However,the nanoparticles coated with a thick silica shell still have significant magnetic properties and can be separated by simple magnetic separation methods.

Fig.5.(a) Magnetic hysteresis curves of Fe3O4 NPs and Fe3O4@SiO2 NPs and (b) their local magnification at -20 to 20 kA·m-1.

Fig.6.FTIR spectra of Fe3O4 NPs,Fe3O4@SiO2 NPs and functionalized Fe3O4@SiO2 NPs.

FT-IR spectra was used to confirm the alteration of nanocomponents (surface functional group),as can be seen from Fig.6.The bands of oleate-capped Fe3O4NPs at 2919 cm-1and 2851 cm-1can be ascribed to the asymmetric and symmetric stretch vibration modes of -CH2,respectively,and the band at 1710 cm-1can be assigned to the stretch modes of C=O.Fe3O4@SiO2NPs show the antisymmetrical stretching vibration band,symmetrical stretching vibration band and bend vibration band of Si-O-Si groups at 1070,796 and 447 cm-1,respectively,indicating successful silica coating.Fe3O4@SiO2-NH2,Fe3O4@SiO2-COOH,Fe3O4@SiO2-NCO and Fe3O4@SiO2-SH all show the above characteristic peaks of nanosilica and the absorption band of the methylene groups(-CH2-) at 2933,2938,2987 and 2933 cm-1,respectively.Compared to Fe3O4@SiO2,Fe3O4@SiO2-NH2shows the absorption peaks of the amino groups (-NH2) at 1587 cm-1,Fe3O4@SiO2--COOH shows the absorption peaks of the carboxyl groups(-COOH) at 1694 cm-1and the absorption peaks of amide groups(-CONH-) at 1539 cm-1,Fe3O4@SiO2-NCO shows the absorption peaks of the isocyanate groups(-NCO)at 2350 cm-1,Fe3O4@SiO2--SH shows the absorption peaks of the sulfydryl groups (-SH) at 2552 cm-1.The results demonstrated that that the functionalized nanoparticles were prepared successfully.

The surface-wetting behaviors of the prepared nanoparticle substrates were determined by contact angle tests (Fig.7).The average contact angles of deionized water on Fe3O4@SiO2substrates,Fe3O4@SiO2-NH2substrates,Fe3O4@SiO2-COOH substrates,Fe3O4@SiO2-NCO substrates and Fe3O4@SiO2-SH substrates were obtained as 19.8°,42.0°,19.4°,63.5° and 76.0°,respectively.These results indicated that all the prepared nanoparticles are hydrophilic as their contact angles are less than 90°.The hydrophilic order of the five nanoparticles was Fe3O4@SiO2-COOH ＞Fe3O4@SiO2＞Fe3O4@SiO2-NH2＞Fe3O4@Si O2-NCO ＞Fe3O4@SiO2-SH.

3.2.Preparation and characterization of coumarin polymorphs

In our work,four crystalline forms of coumarin were observed during confined melt crystallization and microspacing sublimation crystallization methods.PXRD was used to confirm and identify the resulting polycrystalline forms,as shown in Fig.8.The dark colored lines are the PXRD of coumarin polymorphs obtained experimentally,and the light colored lines are the XRD curves obtained by murcury simulation for the corresponding crystalline forms recorded in CCDC (Form I: 1,130,209,Form II: 1,542,946,Form III:154,294,Form IV:1,542,950).Comparing the positions and relative positions of the absorption peaks in the PXRD spectra of the experimentally obtained crystals with those of the PXRD profiles obtained from simulations of single-crystal data in the literature,it can be determined that the four experimentally obtained crystalline forms are the same as the previously reported crystalline forms.The peak positions in the experimentally obtained XRD profiles have a left shift of 0.2°-0.4°relative to the simulated XRD profiles.The XRD peak shift is speculated to be the result of a combination of two factors.Firstly,this may be attributed to the doping of Canada balsam in the experiment,which enlarged the cell parameters of the resulting crystals.The second factor is the expansion of the crystal lattice of the bent crystals compared to the original crystal structure [34].

Coumarin polymorphs were characterized using micro-Raman as shown in Fig.9.The results are consistent with the Raman spectra that have been reported [24].The Raman spectra are generally similar to each other.However,the amplification results from 1000 to 1300 cm-1and from 1500 to 1800 cm-1can distinguish the different crystalline forms.Form I has characteristic peak absorption peaks at 1174.8,1226.4 and 1561.5 cm-1.The corresponding characteristic peaks shift to 1166.9,1221.4 and 1556.7 cm-1in the spectrum of Form II,1169.5,1220.2 and 1554.8 cm-1in Form III,and 1178.2,1227.9 and 1562.5 cm-1in Form IV.The peak shapes and peak heights in the range of 1700-1750 cm-1for the four polymorphs also show significant differences.Therefore,Raman results can be used to identify small crystals that are not suitable for XRD characterization.

Fig.7.Contact angle images of different nanoparticle substrates: (a) Fe3O4@SiO2;(b) Fe3O4@SiO2-NH2;(c) Fe3O4@SiO2-COOH;(d) Fe3O4@SiO2-NCO;(e) Fe3O4@SiO2-SH.

Fig.8.PXRD patterns of coumarin polymorphs.

Typical polarized light optical microscope was adopted to characterize the morphologies of the four polymorphs and the results are shown in Fig.10.Crystals with needle-like and fragmented morphology are all stable Form I(Fig.10(a),(b)).Due to the difference in molecular arrangement and corresponding interactions,Form II is two-dimensional plastic and twistable[35],and its morphology is that of a bent needle-like crystal(Fig.10(c)).Significant Maltese crosses can be observed in the polarized light micrographs of Form III (Fig.10(e)) and Form IV (Fig.10(f)).Form III can sometimes appear as rod-like or needle-like crystals(Fig.10(d)).Form IV appears as banded spherulite (Fig.10(f)).

3.3.Effect of functionalized nanoparticles on the crystallization of coumarin

3.3.1.Functionalized nanoparticles induced confined melt crystallization of coumarin

Fig.11 shows the typical polarized light optical images of induced confined melt crystallization of coumarin on different functional nanoparticle substrates by natural cooling.On the Fe3-O4@SiO2substrates,flaky Form I crystals were obtained (Fig.11(a)).On the Fe3O4@SiO2-NH2substrates,Form I,Form II and Form III crystallized from the melt in polycrystalline form (Fig.11(b)).Among them,Form III was more inclined to be obtained from small droplets.Form I also accounted for the majority of the resulting crystals on the Fe3O4@SiO2-COOH substrates,but the difference was that a large number of needle-like morphologies of Form I can be observed (Fig.11(c)).Ring-banded spherulite of Form IV was formed on the Fe3O4@SiO2-NCO substrates,accompanied by a small amount of Form I and Form II crystals(Fig.11(d)).Distorted Form II crystals could be clearly observed on the Fe3O4@SiO2-SH substrates.The flaky or fragmented crystals of Form I were interspersed between the gaps of the distorted crystals (Fig.11(e)).The PXRD patterns can also verify the above polymorphic results(Fig.11(f)).

Three different cooling rates (-5,-10,-15 °C·min-1) were applied to examine the effect of different functionalized nanoparticle substrates on the nucleation temperature of the crystallization between two plates,as shown in Fig.12.The nucleation temperature ranking of four functionalized nanoparticles is related to the cooling rate.At a cooling rate of 5 °C·min-1,the nucleation temperature ranking is2-NCO ≈,and at a cooling rate of 15 °C·min-1,the nucleation temperature ranking ise3O4@SiO2-COOH.The deviation of the nucleation temperature differs at different cooling rates on different functionalized substrates.This may be related to the different relaxation behaviors of different functional groups under variable temperature conditions [36].The crystals obtained at different cooling rates show almost no difference in polymorphic species,and the results are summarized in Table 1.

The above results exhibit the polymorphic selectivity of the Fe3-O4@SiO2-NCO substrates for Form IV.By uniformly dispersing coumarin fine particles over the entire Fe3O4@SiO2-NCO substrate,a more uniform thin liquid film could be obtained after melting between two plates.Then the pure component product of Form IV could be obtained at the cooling rate of 15°C·min-1.The results of the repeated experiments on three Fe3O4@SiO2-NCO substrates are shown in Fig.13.Form IV was first generated at the edge of the molten droplet,and then radiatively grown to the entire crystal film.The twist periods of the banded spherulites obtained were measured to be 24.2,24.2 and 24.1 μm,respectively,exhibiting good reproducibility.The twist periods and nucleation temperatures obtained in our experiments are consistent with the previously reported patterns [24].Furthermore,no polycrystallization or cross nucleation was observed except for the growth starting zone of Form IV in all three experiments.The introduction of the heterogeneous substrate raises the nucleation temperature required for the formation of the first nucleus.The higher temperature avoids the appearances of a large number of nucleation events and thus inhibits the nucleation of other crystalline forms.

Fig.9.(a) Raman spectra of coumarin polymorphs and their local magnification at (b) 1050-1350 cm-1 and (c) 1500-1800 cm-1.

Fig.10.Typical polarized light optical microscope pictures of coumarin Form I-IV obtained from our experiments:(a)needle-like and(b)fragmented Form I;(c)Form II;(d)short rod-like and (e) needle-like Form III;(f) Form IV.

According to reports,Form IV can be transcrystallized to III during the heating process[24].Fig.14 shows the transcrystallization process from Form IV to Form III on the substrates of Fe3O4@SiO2-NCO NPs.The transcrystallization process took place at 45.9 °C (Fig.14(a)).In order to accelerate the transcrystallization,the actual experimental procedure kept the temperature at 52°C.In contrast to the‘‘prismatic crystal forms that are elongated parallel to the elongation of fibers in the original spherulite”reported in the literature,Form III grew slowly into long needlelike crystals along the radial direction of the spherical crystals on Fe3O4@SiO2-NCO substrates (Fig.14(b)-(d)).Fig.14(e)-(f) show the high degree of consistency in crystal orientation before and after transcrystallization.It is demonstrated that high quality crystals of pure component Form III can be obtained by this transcrystallization method.

Fig.11.Typical polarized light optical micrographs of confined melt crystallization of coumarin on the substrate of (a) Fe3O4@SiO2 NPs,(b) Fe3O4@SiO2-NH2 NPs,(c)Fe3O4@SiO2-COOH NPs,(d) Fe3O4@SiO2-NCO NPs and (e) Fe3O4@SiO2-SH NPs,(f) PXRD patterns of the products.

Fig.12.Nucleation temperature profiles of confined melt crystallization induced by different functionalized silica nanoparticles.

3.3.2.Functionalized nanoparticles induced microspacing sublimation crystallization of coumarin

Fig.15 shows the typical polarized light optical images of induced microspacing sublimation crystallization of coumarin on different functional nanoparticle substrates.On the Fe3O4@SiO2substrates,Form I crystallized as dense crystals from small droplets (Fig.15(a)).On the Fe3O4@SiO2-NH2substrates,both Form I and Form II could be obtained(Fig.15(b)).A greater degree of epitaxial growth of the needle-like Form I was observed on the Fe3-O4@SiO2-COOH substrates (Fig. 15(c)). Sublimation crystallization products on the Fe3O4@SiO2-NCO substrate were predominantly Form II (Fig.15(d)).On the Fe3O4@SiO2-SH substrate,almost all crystals formed were twisted Form II (Fig.15(e)).The PXRD patterns can also verify the above polymorphic results (Fig.15(f)).The crystals obtained at different spacing distance show no difference in polymorphic species,and the results are summarized in Table 2.

Table 1 Table of polycrystalline types of crystals obtained at different cooling rates by confined melt crystallization.

Table 2 Table of polycrystalline types of crystals obtained at different spacing distance by microspacing sublimation crystallization.

The selectivity of the Fe3O4@SiO2-SH substrates for Form II is significant.Pure component Form II products of better quality were obtained under a lower sublimation rate by reducing the heating temperature to 65 °C.Most of the obtained Form II crystals appeared as feathery crystal clusters of about 100 μm in the growth direction(Fig.16).With the increase of coumarin addition,pendant molten droplets of larger size were formed on the upper surface.However,this did not lead to the appearance of richer‘‘feathers”.The length of single fibers remained at about 100 μm.New clusters continue to grow from the end of the previous cluster,or stack on top of each other.This phenomenon is similar to the previously reported distortion of the spherical crystal lamellae caused by diffusion limitation [37].

3.4.Stability of polycrystalline products induced by functionalized nanoparticles

Form II and Form III were peeled off from the substrate,and simple magnet attraction was performed to separate the possibly attached nanoparticles.Polarized light microscopy characterization of the separated crystals with the crystals still attached to the substrate was employed to examine the stability of the crystals within 60 days.The polarized microscopy images of the crystalsattached to the nanoparticle substrates showed different degrees of color diminution after 60 days: some of the curved crystals of Form II became darker (Fig.17(a)),and the edge part of Form III crystals almost lost their polarizability (Fig.17(c)).However,the main part of both crystals had not yet turned to stable Form I or undergone lattice collapse.Form II and Form III crystals separated from the substrate lost their polarizability completely (Fig.17(b),(d)).Their crystal outline can be seen by increasing the exposure time(300 ms)when taking microscope pictures.Thus,the binding of metastable crystals with nanoparticles may have a stabilizing effect on the stacking of molecules on the crystal surface,which would prolong the storage time of the metastable crystals.

Fig.13.Polarized light optical micrographs of pure Form IV obtained on Fe3O4@SiO2-NCO substrates.(a)Form IV obtained on Fe3O4@SiO2-NCO substrate-1 at 35.5 °C and(d)partial enlarged image of its spherulite stripes;(b)Form IV obtained on Fe3O4@SiO2-NCO substrate-2 at 35.3°C and(e)partial enlarged image of its spherulite stripes;(c)Form IV obtained on Fe3O4@SiO2-NCO substrate-3 at 34.8 °C and (f) partial enlarged image of its spherulite stripes.

Fig.14.Polarized light optical micrographs of the transformation process from Form IV to Form III.(a)The start of the transcrystallization process,t=0 s,45.9°C;(b)t=75 s,50 °C;(c) t=240 s,52 °C;(d) The end of the transcrystallization process, t=600 s,52 °C;Polarized light optical microscope images before (e) and after (f) the transcrystallization process.

3.5.Potential mechanism of selective crystallization induced by nanoparticle substrates

Fig.15.Typical polarized light optical micrographs of microspacing sublimation crystallization of coumarin on the substrate of(a)Fe3O4@SiO2 NPs,(b)Fe3O4@SiO2-NH2 NPs,(c) Fe3O4@SiO2-COOH NPs,(d) Fe3O4@SiO2-NCO NPs and (e) Fe3O4@SiO2-SH NPs,(f) PXRD patterns of the products.

Fig.16.Polarized light micrographs of pure Form II obtained on Fe3O4@SiO2-SH substrates.(a),(b),and (c) are the cases of increasing coumarin addition in sequence.

The introduction of nanoparticles provided a large number of heterogeneous nucleation sites,which lowered the nucleation temperature and facilitated the formation of metastable crystalline forms.Although the cooling rate as well as nucleation temperature would influence the formation of metastable crystallites slightly,the polymorphic selectivity we obtained is more attributable to the influence of the functionalized nanoparticle substrates.Based on the above investigation,the mechanism of the selectivity crystallization induced by nanoparticles was proposed concerning the diffusion and interface kinetics in the twisted growth of crystals[38,39].Specifically,it is regulated by different surface wetting properties.

Fig.17.Polarized light micrographs of metastable crystals after lying at room temperature for 60 days.(a)Form II on the substrate;(b)Form II peeled off from the substrate;(c) Form III on the substrate;(d) Form III peeled off from the substrate.

Fig.18.Schematic diagram of selectively crystallization by confined melt crystallization and microspacing sublimation methods.

Combining the polymorphic results of confined melt crystallization with the sequential of nanoparticle hydrophilicity,the contact angles can be roughly classified into three levels.At the first level,the nanoparticle substrates have the best hydrophilicity with the contact angle of less than 40° (Fig.18(a)).In this case,more molecules in the molten droplets would be adsorbed and accumulated at the solid-liquid interface due to hydrophilic interaction [40].Meanwhile,due to the small contact angle,the edge part of the liquid film could be in greater contact with air,thus providing greater relief from internal stresses and avoiding twisted growth due to stress effects.So non-twisted Form I is the main type obtained under such conditions.At the second level,the nanoparticle substrates have a slightly weaker hydrophilicity with the contact angle between 40° and 65° (Fig.18(b)).In this case,the internal stresses cannot be well released,which favors the nucleation and growth of distorted crystals.In our experiments,these crystals preferentially nucleated at the edges,subsequently grew radially with a distorted period of 20-30 μm and eventually forming ring-banded spherical crystals.The periodically heterogeneous surface environment provided by monolayer spherical nanoparticles may also be a favorable condition for the growth of ring-banded Form IV spherical crystals [41].At the third level,the nanoparticle substrates have worse hydrophilicity with the contact angle of larger than 65°(Fig.18(c)).More molecules are available at the edges of the crystals at low contact angles in the vertical direction,which makes the thicker twisted fibers of Form II replace Form IV as the main crystal type obtained when the substrate is less hydrophilic.

With respect to the induced microspacing sublimation crystallization,the open space could allow the relaxation of internal stresses during crystal growth and the growth of crystals would not be limited within the boundary of the liquid film.So Form IV would be hardly obtained by this crystallization method.The open-grown Form I will be obtained on the best hydrophilic substrate (Fig.18(d))and open-grown crystal Form II on the worse hydrophilic substrates (Fig.18(f)).The molten droplets produced by the sublimation crystallization method were suspended on the surface of the substrate.For substrates with contact angles less than 90°,the actual contact angle during the sublimation process is greater because pendant drops adapt contact angles that are closer to 90° than their sessile analogues[42].This‘‘pendant droplet effect”may also explain why the sublimation crystallization on the Fe3-O4@SiO2-NCO substrate would be more likely to obtain Form II,even if the substrate used has a better hydrophilicity (Fig.18(e)).

4.Conclusions

Here,we report a method for selective crystallization of metastable coumarin Form II、III and IV using templates coated with specific functional Fe3O4@SiO2nanoparticles.Unique selectivities of Fe3O4@SiO2-SH nanoparticle substrates for Form II,Fe3O4@-SiO2-NCO nanoparticle substrates for Form IV,and Fe3O4@SiO2--COOH nanoparticle substrates for needle-like habits of Form I were observed.By selecting specific crystallization methods,functionalized nanoparticle substrate types and crystallization conditions,pure component products of coumarin Form II,Form III and Form IV were obtained.Moreover,polarized light microscopy results indicated that the introduction of nanoparticles could also improve the stability of the resulting metastable crystalline forms.Finally,a reasonable polymorphic selectivity mechanism was proposed concerning the diffusion and interface kinetics in the twisted growth of crystals.The polymorph selectivity comes from the difference in the diffusion and surface dynamics due to the hydrophilic properties.Different forms of molecular distribution and internal stress profiles at the edge lead to three patterns of induced confined melt crystallization of coumarins.The open space effect and ‘‘pendant droplet effect” of microspacing sublimation crystallization result in the absence of Form IV as well as the easier obtainment to Form II crystals.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors are very grateful for financial support from the National Natural Science Foundation of China (21908159) and the Tianjin Natural Science Foundation (18JCZDJC38100).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2022.05.008.