Ultrasonics Sonochemistry 72 (2021) 105425 Contents lists available at ScienceDirect Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson Solvent-tuned ultrasonic synthesis of 2D coordination polymer nanostructures and flakes Belén Pepió a, Noemí Contreras-Pereda a, Salvio Suárez-García a, Payam Hayati a, Samia Benmansour b, Pascal Retailleau c, Ali Morsali d, *, Daniel Ruiz-Molina a, * a Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, 08193 Barcelona, Spain b Instituto de Ciencia Molecular, Parque Científico, Universidad de Valencia, José Beltrán 2, 46980 Paterna (Valencia), Spain c Institut de Chimie des Substances Naturelles, CNRS UPR 2301, Univ. Paris-Sud, Université Paris-Saclay, 1, av. de la Terrasse, 91198 Gif-sur-Yvette, France d Chemistry, Faculty of Sciences, Tarbiat Modares University, P.O. Box 14115-4838, Tehran, Islamic Republic of Iran A R T I C L E I N F O A B S T R A C T Keywords: Herein, a new 2-dimensional coordination polymer based on copper (II), {Cu2(L)(DMF)2}n, where L stands for 2D MOF 1,2,4,5-benzenetetracarboxylate (complex 1) is synthesized. Interestingly, we demonstrate that both solvent and Coordination polymer sonication are relevant in the top-down fabrication of nanostructures. Water molecules are intercalated in sus­ Nanomaterials pended crystals of complex 1 modifying not only the coordination sphere of Cu(II) ions but also the final Delamination Ultrasound chemical formula and crystalline structure obtaining {[Cu(L)(H2O)3]⋅H2O}n (complex 2). On the other hand, ultrasound is required to induce the nanostructuration. Remarkably, different morphologies are obtained using different solvents and interconversion from one morphology to another seems to occur upon solvent exchange. Both complexes 1 and 2, as well as the corresponding nanostructures, have been fully characterized by different means such as infrared spectroscopy, x-ray diffraction and microscopy. 1. Introduction years towards their miniaturization, offering improved colloidal dis­ persions, increased surface areas and optimized responses, opened novel Nowadays much effort is directly or indirectly related to two- prospects for these materials [11]. dimensional (2D) materials owing to their singular characteristics and The fast performance, low cost and environment friendliness of their atomic thickness that makes them promising for unimagined sonochemistry makes it one of the most suitable approaches for the properties [1]. For this reason, 2D materials have been proposed for a miniaturization of CPs [12] with reproducible morphologies and prop­ wide range of applications such as drug delivery [2] imaging agents [3] erties at the micro-/nanoscale [13]. Representative examples are, catalysis [4] energy storage [5] and electronics [6] among others. In among others the preparation of: I) Pb(II)-based 2D nanostructures as fact, much of the success of 2D materials corresponds to graphene due to precursors of lead(II) oxide/bromide [14] II) nickel(II) and cobalt(II) its outstanding properties such as ultra-high carrier mobility, large complexes with luminescence, electrochemical and photocatalytic specific surface area, optical transparency and high thermal conduc­ properties [15] and III) CPs that induce the photocatalytic degradation tivity [7–9]. Nevertheless, graphene still exhibits some limitations due of methylene blue [16] or Congo Red adsorption [17]. Another to the lack of chemical flexibility and its susceptibility to oxidative en­ emerging area where sonochemistry is pushing the limits of miniaturi­ vironments that can hinder its applications. Thus, in the past recent zation is that of metal–organic framework nanosheet (MONs). Two- years, different alternatives have been sought to overcome such limi­ dimensional CPs, commonly known as flakes [19] with enhanced tations. One of such families is that of coordination polymers (CPs), charge transfer between electrolyte and catalysts interface [18–19] whose rational selection of metal ions and directional bonds with among other applications [20–22] can be obtained following the top- organic ligands can be used to control their chemical topology, aiming at down liquid-phase exfoliation (LPE) approach. This approach consists finding novel and/or improved optical, electrical and/or magnetic on the delamination of the bulk material in the presence of specific properties [10]. This, together with advances accomplished over the last solvents, which intercalate between the layers [23–24] usually assisted * Corresponding authors. E-mail addresses: morsali_a@modares.ac.ir (A. Morsali), dani.ruiz@icn2.cat (D. Ruiz-Molina). https://doi.org/10.1016/j.ultsonch.2020.105425 Received 22 November 2020; Received in revised form 6 December 2020; Accepted 8 December 2020 Available online 24 December 2020 1350-4177/© 2020 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). B. Pepió et al. Ultrasonics Sonochemistry 72 (2021) 105425 with ultrasound (US) [25]. For this, the selection of the solvent is seen in Fig. 2c, bulk crystals present a greenish color with an elongated important as suitable polarities will provide stable colloidal suspensions, hexagonal shape. SEM images (Fig. 2d) of the bulk crystals were treated while some solvents can also decompose or fragment the bulk material, with ImageJ software, obtaining a size distribution of 18.8 ± 3.1 μm (N without obtaining layers [26]. Therefore, specific studies have been = 178) and a width of 6.6 ± 2.3 μm (N = 56), proving a good mono­ devoted to determining the role of solvent and US along the delamina­ dispersity (Fig. 2e and 2f). Good quality crystals were then selected for tion process of 2D-CPs [19,27]. However, there is still a lot of effort to be single-crystal X-ray diffraction (SXRD) and the results analyzed using done given the incipient nature of the field. Mercury software. Complex 1 is a layered 2D-CP complex with chemical Herein we go one step further and demonstrate how the solvent- formula {Cu2(L)(DMF)2}n. It crystallizes in the monoclinic space group sonication combination can not only favour the delamination process I2/m. The lattice parameters of the structure are a = 6.2356(10) (Å), b = but also to trigger new reactions and the obtaining of novel chemical 8.6574(12) (Å), c = 17.934(4) (Å), α = 90◦ , β = 92.365(18)◦ and γ = nanostructures, otherwise difficult to achieve. A schematic representa­ 90◦ . The asymmetric unit of 1 showed in Fig. 3a consists of one Cu(II) tion of this novel hint is schematically shown in Fig. 1. In brief, crystals ion, one DMF molecule coordinated to the metallic center and one- of the novel CP {Cu2(L)(DMF)2}n (1), where L stands for 1,2,4,5-benze­ quarter of the ligand. netetracarboxylate, have been obtained and later on exfoliated into thin As seen in Fig. 3b, the metallic center of the compound consists of flakes under US in an ethanolic dispersion. Further resuspensions such two coordinated Cu(II) ions forming a dimer in a paddlewheel structure. flakes in water results in a chemical transformation into rod-shaped Cu(II) ions are coordinated to the carboxylate groups of the ligand in the nanocrystals of complex {[Cu(L)(H2O)3]⋅H2O}n (2). Similar results equatorial positions in a tetradentate mode, each oxygen from a were obtained if we first disperse bulk crystals of complex 1 in water, carboxylate being coordinated to one Cu(II) of the dimer. In order to which are transformed into crystals of complex 2, and then we sonicate complete the paddlewheel geometry, DMF molecules are coordinated in them. Worth to mention, nanospheres of complex 2 can also be isolated the remaining axial positions. Regarding the crystalline structure, along this process. Fig. 3c-d show both the layers from the top and side view. The structure showed in Fig. 3d undergoes extended coordination leading to the for­ 2. Results and discussion mation of a 2D-sheet CP. These layers pack themselves through supra­ molecular interactions, mainly van der Waals interactions to form the 2.1. Synthesis and characterization of complex 1 bulk layered crystal, as seen in Fig. 3c, where a single layer is high­ lighted. Layers remain along (0 0 1) planes with a distance between them Crystals of complex 1 were synthesized by combining the pyro­ of 8.959 Å. More crystallographic data can be found in Table S1 and S2 mellitic acid as a ligand with four carboxylic groups in 1, 2, 4 and 5 (Supporting Information). Besides, powder X-ray diffraction (PXRD), positions and a copper salt (Fig. 2a). The synthesis was performed using Fourier Transformed Infrared (FT-IR) and energy dispersive x-ray (EDX) a mixture of DMF:DMA (1:1), where the ligand and the metal are soluble spectroscopies were performed in order to corroborate the powder (Fig. 2b). Here, the solvent induces the extent deprotonation of the crystalline pattern, the formation of the CP and the presence of C, N, O carboxylate ligand (L), consequently facilitating the coordination to Cu and Cu in the bulk crystals of 1 (Figure S1, Supporting Information). (II) ions. The morphology and size of crystals 1 were evaluated with both optical microscopy (OM) and scanning electron microscopy (SEM). As Fig. 1. Schematic representation of crystalline phase, color and morphology changes using different solvents and sonication times. Scanning electron microscopy (SEM) and macroscopic images of the sample at various stages are also provided. 2 B. Pepió et al. Ultrasonics Sonochemistry 72 (2021) 105425 Fig. 2. (Top) Synthesis of bulk crystals of complex 1. a) Chemical structure of the ligand (L) and b) strategy followed for the synthesis of the crystals. (Bottom) Morphology characterization of the crystals: (c) OM image, (d) SEM image, (e) width distribution and (f) length distribution both obtained with the ImageJ software from (d). 2.2. Solvent effect irreversible fragmentation into smaller granular crystals, as confirmed by SEM (see Fig. 4c-d). On top of that, a simple qualitative observation Bulk crystals of complex 1 were resuspended for 24 h in ethanol already revealed that the crystals become bluish immediately after (EtOH), acetone, water and a mixture of EtOH:H2O (1:3) and analyzed suspension in water or EtOH:H2O (1:3) mixture. Finally, the irrevers­ by PXRD and FT-IR spectroscopy (see Fig. 4a-b). ibility of the interconversion was tested by immersing back the crystals As can be seen there, results for EtOH and acetone reproduced the in DMF up to two months; no significant changes were detected in the patterns for the as-synthesized crystals. However, significant differences corresponding FT-IR and PXRD spectra (see Figure S2). were found for those suspended in water and the EtOH:H2O (1:3) mixture. In this last case, FT-IR analysis revealed the presence of O–H 2.3. Synthesis of complex 2 bonds (characteristic peak within the range of 3600 – 2700 cm− 1 highlighted in blue) and a displacement of the peaks to lower wave­ To fully determine the influence of water in complex 1, crystals were numbers (area highlighted in yellow), tentatively attributed to an ex­ then immersed for a longer period of one month. Interestingly, the seeds change of solvent molecules coordinated to Cu ions. In fact, PXRD shown in Fig. 4 grew more to form hexagonal bluish crystals, suitable for evidenced changes in the crystalline phase after 24 h along with SXRD (Figures S3, S4 and S5, Supporting Information). Such crystals 3 B. Pepió et al. Ultrasonics Sonochemistry 72 (2021) 105425 Fig. 3. (a) Asymmetric unit of 1, (b) coordination mode of the metallic centers, (c) layers of 1 from side view, where a single layer is highlighted in yellow, and (d) single layer of 1 from a top view. Color labels: Cu, green; C, grey; O, red; N, blue; H, white. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) corresponded to the new complex 2, which was also a 2D-CP complex dispersion of crystals of complex 1 in water induced a chemical trans­ with a layered structure of chemical formula {[Cu(L)(H2O)3]⋅H2O}n that formation over time (already detectable at 24 h) that ends in the for­ crystallizes in the monoclinic space group I2/a. The lattice parameters of mation of crystals of considerable dimensions with time. Next step was the structure are a = 9.4869(3) Å, b = 18.0136(6) Å, c = 12.0060(4) Å, the combination of water effect with US. For this, US were applied to an α = 90◦ , β = 112.734(4)◦ and γ = 90◦ . Fig. 5a shows the asymmetric unit aqueous dispersion of 1 with a 550 W powered microtip in pulses of 2 of the complex made of a metallic Cu(II) center coordinated to three min followed by 30 s of rest between steps at 25% tip amplitude energy water molecules and a half of the ligand. There are also present two for 2 h (this was the optimized time in terms of polydispersion with no interstitial water molecules. Furthermore, the metallic center could be significant changes in morphology, see Table S3 Supporting Informa­ found in the structure with a deformed octahedral coordination sphere tion). US microtip provides large powered US generated at a small (Fig. 5b). The Cu(II) center is coordinated to two L through the source (the end of the tip) resulting in large power intensities per surface carboxylate groups both in a monodentate and bidentate mode in the area of ~ 1700 W⋅cm− 2 even at 25% of its amplitude. Generating this equatorial positions. Thus, to complete the coordination sphere, the high intensity in-situ the suspension induces large-yielded nano­ remaining equatorial position is occupied by a molecule of water as well structuration. It is also worth to mention that large US power density of as the axial positions and expanded through a plane forming a 2D-sheet ~ 34000 W⋅L-1 average is obtained in this system as small volumes are of 2 (Fig. 5c). The layers pack themselves both through van der Waals used for the suspensions despite a decreasing gradient of US power interactions and hydrogen bonds, allowing the growth of the bulk density is expected upon moving away from the tip. To ensure the ho­ layered crystal. Hydrogen bonds are formed between adjacent layers in a mogenization of the sample, the crystals were maintained under mag­ zig-zag way conferring higher stability to the bulk form of 2. The netic stirring and with a water–ice bath to avoid thermal decomposition. aforementioned layers remain within (10–1) planes, with a distance Dynamic light scattering (DLS) measurements revealed the formation of between them of 4.894 Å (Fig. 5d). After a detailed search, we found that colloidal suspensions with average dimensions of 62.2 ± 7.2 nm and a complex 2 was already described in the literature following a direct polydispersity index (PDI) value of 0.331 ± 0.017 revealing the synthetic approach [28]. importance of large US energies to obtain nanometered structures of CPs. The relatively high PDI value was attributed to the inhomogeneous sonication through the solution from the tip as aforementioned. Similar 2.4. Tip-assisted sonication of complex 1 in water results were obtained using now a sonication bath (where US power is applied more homogenously over all the crystal suspension) only In the previous sections 2.2 and 2.3, we have shown how the 4 B. Pepió et al. Ultrasonics Sonochemistry 72 (2021) 105425 Fig. 4. (Top) Characterization of 2D-CP crystals in different solvents. (a) PXRD and (b) FT-IR spectra. (Bottom) SEM images of (c) bulk 2D-CP immersed in water and (d) bulk 2D-CP resuspended in DMF:DMA (1:1) after being 24 h in EtOH-H2O (3:1). Fig. 5. (a) Asymmetric unit of 2; (b) Coordination mode of the Cu(II) metallic centers to L; (c) Top view of a single layer of 2 and (d) side view of packed layers, where a single layer is highlighted in yellow. Color labels: Cu, green; C, grey; O, red; H, white. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 5 B. Pepió et al. Ultrasonics Sonochemistry 72 (2021) 105425 improved after exposition for longer periods of time (see Table S4 and 2.5. Tip-assisted sonication of complex 1 in EtOH Figure S6, Supporting Information) due to the low power intensity and density of this set-up. In a US bath, US are generated along the large Crystals of complex 1 dispersed in EtOH for 24 h already showed an surface of the bath resulting in intensities 0.1 W⋅cm− 2 with the equip­ incipient process of delamination as can be seen in Fig. 8 b-c. Though, in ment used in this work. Further, despite having a more uniform order to optimize the exfoliation process with the ultimate goal of gradient, US propagate over the voluminous water bath leading to weak reaching very-thin flakes, an US microtip was immersed in the colloidal power densities (33 W⋅L-1).To further improve the PDI, the remaining dispersion for 2 h as large energetic source to increase the exfoliation non-exfoliated material can be removed by additional centrifuge opti­ rate (Fig. 8a). Crystal dispersions were maintained under magnetic mization after sonication with the microtip. Specifically, colloidal sus­ stirring for the homogenization of the sample and kept at room tem­ pensions sonicated for 2 h were centrifuged at 13300 rpm and 4 ◦ C at perature using a water–ice bath to avoid thermal decomposition. US different times of 2 min, 5 min, 7 min, 10 min, 15 min and 20 min. SEM were applied in pulses of 2 min, followed by 30 s of rest between steps at images (see Figure S7, Supporting Information) pointed out to 10 min as 25% tip amplitude energy as in section 2.4. SEM images showed the an optimized centrifugation time. As shown in Fig. 6a-b, sonication for 2 obtaining of flakes (Fig. 8d) with an average size of 101.5 ± 4.5 nm with h and use of the optimized centrifugation conditions, resulted in nano­ a PDI of 0.254 ± 0.011 determined by DLS measurements. Once more, spheres of mean size 79.8 ± 20.5 nm (N = 106) (see Figure S8, Sup­ sonication with a US bath resulted in similar flakes and PDI values only porting Information for size statistics). Interestingly, the colloidal after long sonication times to enlarge the US energy provided to the suspension evolves after 24 h to form larger nanospheres that finally crystals (Table S4, Supporting Information). Finally, the flakes showed tended to form less stable rods that ultimately precipitate (Fig. 6c-d). excellent colloidal stability in EtOH (Fig. 8e) up to one month after the Top view and tilted SEM images of the nanorods shown in Fig. 7 exfoliation (157 ± 74 nm and PDI of 0.232 ± 0.001). revealed a mean length of 377.0 ± 99.0 nm (N = 95) and a mean Finally, the flakes shown in 8b were re-suspended in water and PBS thickness of 44.0 ± 11.0 nm (N = 66). Regarding FT-IR analysis (in order to mimic physiological conditions). For this, a combination of (Figure S9a, Supporting Information) most of the peaks at lower wave­ equivalent volumes of the flake dispersion in EtOH and water/PBS was number were maintained, indicating chemical stability during the US rotaevaporated until EtOH was removed. DLS measurements of the process. On the other hand, comparing the peaks of the Grazing angle X- resulting dispersions showed values of 153.7 ± 4.0 nm (PDI: 0.366 ± Ray Diffraction (GXRD) in Figure S9b, the pattern from the nano­ 0.029) and 122.9 ± 28.8 nm (PDI: 0.365 ± 0.043) for water and PBS, structures was similar to that of complex 2, with a main diffraction peak respectively. Morphology was only analyzed with SEM for flakes attributed to the plane along the 2D layers of 2 (2θ = 9.8◦ ). In fact, resuspended in water (Fig. 9) due to the presence of saline microcrystals immersion in water for longer periods of time (three months) of the in PBS. The change of morphology was evident as the flakes adopt a precipitate obtained after exfoliation leads to the formation of iso­ flattened sphere shape (Fig. 9a), similar to the nanospheres obtained structural hexagonal crystals (Figure S10, Supporting Information) after applying US to crystals of 1 in water seen in section 2.4. After five complex 2, as confirmed by SXRD. days in water, flakes reorganize in a hexagonal way (Fig. 9b) reminding of the shape of crystals of 2 after several months. SEM image presented Fig. 6. SEM images of the colloidal suspensions sonicated for 2 h (a) after sonication, (b) and (c) after 24 h and (d) precipitated obtained in the colloidal suspensions after 24 h. 6 B. Pepió et al. Ultrasonics Sonochemistry 72 (2021) 105425 Fig. 7. SEM images of (a) top and (b) tilted view of the nanostructures deposited onto a surface by spin coating (5000 rpm, 30 s). Size distribution for (c) length and (d) thickness of the obtained nanostructures. in Figure S11 corresponds to the resuspension before EtOH evaporation. complex 1 in water. These novel morphologies tend to aggregate over It shows a mixture of the sheet-shaped flakes typically found in EtOH days in hexagonal-shaped microstructures reminding to the shape of and flattened sphere-shaped flakes when resuspending in water. Thus, crystals of 2. These results demonstrate that the combination of a solvent change in morphology was attributed to the presence of water. of choice with US open new venues for the obtaining of novel CPs FT-IR spectra were obtained for both resuspensions in water and PBS. nanostructures with a high chemical/morphological control. Fig. 9c compares the FT-IR spectrum from the flakes exfoliated in EtOH and the resuspended ones. It seems that chemical structure was not 4. Materials and methods retained when redispersing in water and PBS as O–H bonds (3000–3500 cm− 1) in the structure appear to arise as happened when immersing the 4.1. Chemicals and materials bulk crystals in water. Changes at lower wavenumbers were also appreciated, although lack of resolution of the peaks did not allow us to Solvents and starting materials were purchased from Scharlau fully note the shift in the peaks, which could be an indicator of novel (Scharlab S.L., Barcelona, Spain) and Sigma − Aldrich (Merck, Madrid, coordination sphere. Spain). All the materials were used as received, without further purification. 3. Conclusions 4.2. Synthesis of complex {Cu2(L)(DMF)2}n (1) A novel 2D Cu(II) CP of chemical formula {Cu2(L)(DMF)2}n (1) has been synthesized and characterized. The structure obtained by SXRD Crystals of CP 1 were obtained by dissolving pyromellitic acid (LH4) revealed coordination of copper ions in a paddlewheel geometry in 2D (101.7 mg, 0.4 mol) and Cu(NO3)2⋅2.5H2O (24.4 mg, 0.1 mol) in 10 mL layers, including coordinated DMF molecules. Stability of the bulk of a N,N-dimethylformamide (DMF): Dimethylacetamide (DMA) crystals in different solvents was studied. Water caused irreversible mixture (1:1) and stirring the solution magnetically in a vial for 30 min crystalline changes into the complex to form complex 2, a 2D-CP with a at room temperature (RT). Afterwards, the mixing was heated up to layered structure of chemical formula {[Cu(L)(H2O)3]⋅H2O}n previously 100 ◦ C for 72 h. The synthetic methodology used is schematically reported in the literature [28] where coordinated DMF molecules got depicted in Fig. 2b. Crystals of 1 were then slowly cooled down to RT, replaced by water coordinated to the copper metallic center. Combina­ cleaned and resuspended in the same solvent used for the synthesis. The tion of water with US resulting in the formation of nanoparticles after 2 cleaning process was performed by repeating centrifugation cycles h that evolve with time into rods that precipitate after 24 h. Exposition (13300 rpm, 5 min) at 4 ◦ C. In between cycles, the supernatant was of these nanostructures to water for longer periods (several weeks) removed and replaced for new DMF:DMA (1:1) solutions. Final crystals resulted in the formation of large blue crystals of complex 2. On the were collected and dried for further characterization and sonication. FT- contrary, sonication of 1 in ethanol where no chemical conversion is IR (KBr, pellet, cm− 1): 3435 (m), 3013 (w), 2933 (m), 2813 (w), 1663 detected, yielded sheet-like 2D flakes. Remarkably, dispersion of such (s), 1639 (s), 1527 (w), 1492 (w), 1446 (m), 1442 (w), 1393 (s), 1335 flakes in water leads to the chemical interconversion to complex 2 and (m), 1284 (w), 1253 (w), 1139 (m), 1108 (m), 1062 (w), 1019 (w), 905 the formation of rods, similar to those obtained by direct sonication of (w), 871 (w), 821 (m), 797 (m), 762 (w), 724 (w), 680 (m), 560 (m), 517 7 B. Pepió et al. Ultrasonics Sonochemistry 72 (2021) 105425 Fig. 8. (a) Scheme of the LPE process applied for the delamination of bulk 2D-CP crystals using US; (b) and (c) SEM images of the layered 2D-CP bulk crystals exposed to EtOH for 24 h, where a preliminary exfoliation can be appreciated; (d) SEM images of flakes of complex 1 upon sonication in EtOH; (e) Tyndall effect observed in colloidal suspensions of flakes obtained with the tip in different solvents compared to water solvent without flakes (from left to right: EtOH, water suspensions and water solvent). (m). Elemental analysis calculated for [Cu2(L)DMF2]n: C, 36.72%, H, 4.3. Synthesis of {[Cu(L)(H2O)3]⋅H2O}n (2) 3.08%, N, 5.35%. Found: C, 37.20%, H, 3.14%, N, 5.37%. Crystals of CP forming complex 2 were synthesized by immersing clean crystals of 1 in water in a closed vial. After one month, crystals of 1 8 B. Pepió et al. Ultrasonics Sonochemistry 72 (2021) 105425 Fig. 9. (a) and (b) SEM images of flakes previously exfoliated resuspended in water after EtOH evaporation. (c) FT-IR spectra of flakes resuspended in different solvents. have regrouped forming hexagonal bluish bigger crystals. Crystals of 2 2 and 3 h in order to optimize the yield and size of the nanostructures. were isolated and dried at vacuum for single x-ray diffraction (SXRD). Aliquots were taken for further analysis. After the sonication process, FT-IR (KBr, pellet, cm− 1): 3443 (m), 3263 (m), 3201 (m), 1626 (s), 1583 suspensions of non-exfoliated bulk material coexisted with the nano­ (s), 1494 (m), 1418 (s), 1371 (s), 1323 (m), 1260 (w), 1186 (w), 1139 structures. Thus, a centrifugation step was performed in order to sepa­ (m), 1053 (w), 935 (w), 870 (m), 820 (m), 763 (w), 699 (m), 572 (m), rate the colloidal suspensions (supernatant) from the non-exfoliated 539 (m), 463 (w), 439 (w). material (pellet). Sorvall Legend Micro 17R centrifuge (Thermo Scien­ tific) was used. Aliquots from the suspensions were centrifuged varying parameters like time, temperature and rpm. After sonication and 4.4. Ultrasonication centrifugation, final colloidal suspensions were kept under agitation in IKA HS 260 basic shaker. Studies of ultrasonication with the US bath Ultrasonication of crystals of complex 1 was performed by were performed with equipment Elma S40H Elmasonic (Elma Schmid­ immersing an US microtip into a vial containing a dispersion of crystals bauer GmbH, Singen, Germany). This equipment has an effective power of 1 (4.6 mg) (final concentration 1 mg/mL). US were applied to the of 140 W. The bath inner dimension in width, depth and high are 240 mentioned dispersions. The equipment used was Branson Digital Soni­ mm, 137 mm and 150 mm respectively. The total surface generating the fier SFX 550 sonicator (Emerson, St. Louis, MO, USA) which has an ultrasound is 1460 cm2. Hence, the power intensity with the bath is 140/ effective power of 550 W with a tip diameter is 1/8 in.≈0.3175 cm. The 1460 = 0.096 W⋅cm− 2. The bath, with a power of 140 W, was filled to its total surface then generating the ultrasound is π(0.3175/2)2 = 0.0791 maximum which is 4.25L. This corresponds to a power density of 140/ cm2. Therefore, the power intensity in this case is 550*0.25/0.0791 = 4.25 = 32.9 W⋅L-1.Temperature was kept at RT in order to avoid thermal 1737 W⋅cm− 2. The tip was used with 25% amplitude of its 550 W power decomposition of the crystals. and the sonicated volume was the one corresponding to the crystal suspension (4 mL). Therefore, the power density in this case is 550*0.25/0.004 = 34375 W⋅L-1. Crystal dispersions were maintained 4.5. Physicochemical characterization under magnetic stirring for the homogenization of the sample and kept at RT by a water–ice bath in order to avoid thermal decomposition. US Optical images (OM) from bulk crystals of the complexes were ob­ were applied in pulses of 2 min, followed by 30 s of rest between steps at tained with a Nikon Eclipse LV100 (Nikon Instruments Inc. Melville, NY, 25% amplitude energy of the tip. The samples were sonicated for 1, 1.5, USA) microscope. Samples from all complexes were deposited on a glass 9 B. Pepió et al. Ultrasonics Sonochemistry 72 (2021) 105425 substrate by drop casting. Morphology was evaluated from this Mercury software. Powder X-Ray Diffraction (PXRD) of the bulk crystals technique. were obtained with PANalytical X’Pert PRO MRD (Multipurpose Scanning Electron Microscopy (SEM) images were collected in order Diffractometer) (Malvern PANalytical, Dusseldorf, Germany) equipped to study morphology as well as a population statistics analysis of the with a CuKα radiation source (λ = 1.54184 Å). Grazing angle X-Ray bulk crystals and the nanostructures. The images were obtained in a FEI Diffraction (GXRD) patterns of the nanostructures were obtained by Quanta 650 FEG SEM (Thermo Fisher Scientific, Eindhoven, The using grazing angle mode with PANalytical X’pet Pro MPD (Materials Netherlands) using secondary electrons mode with a beam voltage of 20 Research Diffractometer) equipment with a wavelength of CuKα anode kV and a chamber pressure of 10-5 Pa. The working distance was set at (λ = 1.54184 Å) and incident angle of 1.55◦ . Samples were prepared by 10 mm. Samples were prepared by two methods. On the one hand, by drop casting using silicon substrates. Measurement conditions were 2θ drop casting depositing a drop of the colloidal suspensions on range (10 − 45◦ ); step size = 0.03◦ ; counting time/time per step = 14 s. aluminium stubs. On the other hand, by spin coating on glass substrates Dynamic Light Scattering (DLS) measurements were performed to avoid large aggregation or solvent effects using a WS-650MZ-23NPP using a Zetasizer Nano ZS 3600 (Malvern Instruments, UK). All mea­ Spin Coater (Laurell Technologies Corporation, North Wales, PA, USA). surements were done by introducing the dispersions in a glass cuvette These glass substrates were then deposited onto carbon film stubs. Af­ for size measurements. Data reported are values coming from the mean terwards, samples were coated with 5 nm thickness of platinum using of measurements for each sample which were measured per triplicated. Leica EM ACE600 (Wetzlar, Germany) metal pulveriser. Cross-section The data was collected with the Zetasizer 7.04 software. images were taken tilting the sample 70◦ . Subsequently, SEM images were treated with ImageJ software (NIH, USA) for statistical analysis of Declaration of Competing Interest the crystal population. Morphology and size of the nanostructures after sonication and isolation were also analysed by SEM images. Images of The authors declare that they have no known competing financial the suspensions sonicated at different times and with different centri­ interests or personal relationships that could have appeared to influence fugation parameters were taken at different amplifications in order to the work reported in this paper. evaluate the optimum sonication time and isolation process. Infrared spectroscopy (FT-IR) of the bulk crystals was performed for Acknowledgements ensuring the chemical stability since infrared absorption promoting one vibrational mode to a higher energy one is characteristic of each ma­ This work was supported by grant RTI2018-098027-B-C21 from the terial. FT-IR spectra of the crystals were obtained on a Tensor 27 FTIR Spanish Government funds and by the European Regional Development spectrometer (Bruker Optik, GmbH, Berlin, Germany). Tensor 27 is Fund (ERDF). The ICN2 is funded by the CERCA programme / Gen­ equipped with a RT detector and a mid-IR source (4000 to 400 cm− 1). eralitat de Catalunya. The ICN2 is supported by the Severo Ochoa Measurements were made from a KBr matrix containing the crystals. A Centres of Excellence program, funded by the Spanish Research Agency prior step was needed, which consisted on pressing a mix of KBr and the (AEI, grant no. SEV-2017-0706). Noemí Contreras Pereda’s project that bulk crystal. A background spectrum was also necessary before sample gave rise to these results received the support of a fellowship from measurements. On the other hand, FT-IR spectra of the nanostructures “laCaixa” Foundation (ID 100010434). The fellowship code is LCF/BQ/ were recorded on a Hyperion 2000 FT-IR microscope (Bruker Optik, ES17/11600012. GmbH, Ettlingen, Germany). Hyperion 2000 in reflection mode is equipped with a nitrogen-cooled mercury-cadmium-telluride (MCT) Appendix A. 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