Energy Landscape in Supramolecular Coassembly ... - ACS Publications

Article Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Energy Landscape in Supramolecular Coassembly of Platinum(II) Complexes and Polymers: Morphological Diversity, Transformation, and Dilution Stability of Nanostructures Kaka Zhang, Margaret Ching-Lam Yeung, Sammual Yu-Lut Leung, and Vivian Wing-Wah Yam* Institute of Molecular Functional Materials (Areas of Excellence Scheme, University Grants Committee (Hong Kong)) and Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, P. R. China

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ABSTRACT: Establishment of energy landscape has emerged as an efficient pathway for improved understanding and manipulation of both thermodynamic and kinetic behaviors of complicated supramolecular systems. Herein, we report the establishment of energy landscapes of supramolecular coassembly of platinum(II) complexes and polymers, as well as the fabrication of nanostructures with enhanced complexity and intriguing properties from the coassembly systems. In the energy landscape, coassembly at room temperature has been found to only allow the longitudinal growth of platinum(II) complexes and block copolymers into core−shell nanofibers that are the kinetically trapped products. Thermal annealing can switch on the transverse growth of platinum(II) complexes and block copolymers to produce core−shell nanobelts that are the thermodynamically stable nanostructures. The extents of the transverse growth are found to increase with thermal annealing temperatures, leading to nanobelts with larger widths. Besides, rapid quenching of a hot coassembly mixture to room temperature can capture intermediate nanobelt-block-nanofiber nanostructures that are metastable and capable of converting to nanobelts upon further incubation at room temperature. Moreover, sonication treatment has been found to couple with the energy landscape of the coassembly system and open a unique energy-driven pathway to activate the kinetically forbidden nanofiber-to-nanobelt morphological transformation. Furthermore, based on the established energy landscapes, nanosphere-block-nanobelt nanostructures with distinct segmented architectures have been fabricated by thermal annealing of the ternary mixture of platinum(II) complexes, block copolymers, and polymer brushes in a one-pot and single-step procedure. Finally, the nanobelts and nanosphere-block-nanobelt nanostructures are found to possess intriguing morphological stability against acid and dilution, exhibiting characteristics that are important for promising biomedical applications.

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molecular architectures,4 liquid crystals,5 metallogels,6 and nanostructures7 in single-component assembly systems of platinum(II) complexes. In these single-component assembly systems, although Pt(II) nanostructures with various morphologies such as spheres, fibers, rods, vesicles, sheets, tubes, and rings have been obtained,7 methods for the rational fabrication of Pt(II) nanostructures with controlled dimensions and sizes have been less studied and reported.8 Besides, there has been tremendous interest in establishing a morphology−function relationship for nanostructures.7a−g,9 Despite some interesting findings in the correlation between spectroscopic and morphological properties of Pt(II) nanostructures,7a−g other properties or functions linked to the Pt(II) morphologies remain rather unexplored. Energy landscapes consist of several metastable states of local energy minima and a thermodynamic favored state of

INTRODUCTION Platinum(II) polypyridine complexes of d8 electronic configuration and square-planar geometry have been shown to display intriguing spectroscopic and luminescence properties, as well as interesting self-assembly behaviors driven by noncovalent metal−metal and π−π interactions.1 The platinum(II) complexes have been reported to self-assemble into highly ordered extended linear chains or oligomeric structures and exhibit rich polymorphism in the solid state.1,2 Polyelectrolytes have been reported to induce the aggregation of platinum(II) complexes driven by electrostatic attractions and metal−metal and π−π interactions, leading to drastic absorption and luminescence changes.3 These Pt(II)−polyelectrolyte systems have been demonstrated to show excellent chemical and biological sensing properties,3 but no welldefined nanostructure has been observed in these systems. The metal−metal and π−π stacking interactions and other noncovalent interactions have been reported to direct the formation of Pt(II) supramolecular assemblies that include © XXXX American Chemical Society

Received: May 7, 2018

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DOI: 10.1021/jacs.8b04779 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Journal of the American Chemical Society

converted to more stable nanobelts upon further incubation at room temperature. Sonication treatment has been found to be able to couple with the energy landscape of the coassembly system and open a unique energy-driven pathway to activate the kinetically forbidden nanofiber-to-nanobelt transformation at room temperature. Owing to the established energy landscapes, nanosphere-block-nanobelt nanostructures with distinct segmented architectures have been achieved by thermal annealing of the ternary mixture of platinum(II) complexes, PEG-b-PAA and PAA brushes in a one-pot and single-step procedure. Interestingly, the nanobelts and nanosphere-block-nanobelt nanostructures are found to possess intriguing morphological stability against acid and dilution, exhibiting characteristics that are important for promising biomedical applications. In short, diverse platinum(II) nanostructures and their interconversion, which have not been observed previously, are reported in the present work.

global energy minimum, as well as kinetic barriers and kinetic pathways. Establishment of energy landscape has emerged as an efficient way to achieve improved understanding and manipulation of both thermodynamic and kinetic behaviors of supramolecular systems.10 For example, the modulation of the energy landscape of supramolecular assembly of peptide amphiphiles has been reported to produce long nanofibers with enhanced cell spreading and proliferation properties when compared to their short counterparts.9c Rational selection of kinetic pathway from the energy landscape of supramolecular system has been found to exhibit intriguing temporal properties of spectroscopic and morphological transformations, which allows for the construction of supramolecular assemblies with enhanced complexity that are inaccessible by conventional thermodynamic processes.10a−c Modulating the kinetic barriers for the supramolecular polymerization of metastable monomers in the energy landscape has been demonstrated to be of critical importance in devising chain-growth supramolecular polymerizations for the fabrication of supramolecular polymers with controlled chain lengths, narrow length distributions, and diverse chain architectures.10d,e Most of the reported studies of energy landscape establishment focus on single-component supramolecular systems,10 with exceptionally few examples of two-component supramolecular systems of structurally similar components.10d To the best of our knowledge, the establishment of relatively complicated energy landscape in two- or multicomponent supramolecular systems has not been reported. Besides, despite escalating interest in fuel-driven and energy-driven supramolecular assembly systems to enrich their properties and functions,11 coupling of external energy or mechanical force to the energy landscape of two- or multicomponent supramolecular systems has been less explored. Recent studies in our laboratory showed the fabrication of core−shell nanofibers with crystalline nanostructures by twocomponent supramolecular coassembly of platinum(II) complexes and block copolymers, as well as an unexpected morphological transformation in the same two-component system from core−shell nanofibers through short patchy nanofibers to core−shell nanobelts upon the sonication treatment of the core−shell nanofibers followed by subsequent room-temperature incubation.8a The nanofiber-to-nanobelt morphological transformation suggests the existence of local energy minimum and interesting kinetic pathways in the coassembly system. Herein, we report the establishment of the energy landscapes of the two-component supramolecular coassembly of platinum(II) complexes and poly(ethylene glycol)-b-poly(acrylic acid) (PEG-b-PAA) block copolymers. In the energy landscape, core−shell nanobelts with crystalline nanostructures are found to be the thermodynamically stable nanostructures, while core−shell nanofibers are the kinetically trapped assemblies in the coassembly system. Room-temperature incubation of the coassembly mixture only allows the longitudinal growth of the platinum(II) complexes and block copolymers into core−shell nanofibers. Thermal annealing can switch on the transverse growth of platinum(II) complexes and block copolymers into core−shell nanobelts, and the extents of the transverse growth increase with thermal annealing temperatures. Rapid quenching of a hot mixture of platinum(II) complexes and block copolymers to room temperature can capture intermediate nanobelt-block-nanofiber nanostructures, and these intermediate nanostructures are found to be

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RESULTS AND DISCUSSION Supramolecular Coassembly by Thermal Treatment. Thermal annealing of a mixture of complex 1 (Scheme 1, 0.15 Scheme 1. Chemical Structures of the Platinum(II) Complexes and the Polymers

mM) and PEG45-b-PAA69 ([carboxylic acid] = 1 mM) in a mixed solvent of acetonitrile−methanol−water (1:1:8, v/v/v) at 70 °C was performed, during which the color of the mixture changes from yellow at 70 °C to brown at 25 °C, suggesting the formation of metal−metal and π−π interactions between the platinum(II) complexes.3a,8a The pH value of the annealed mixture at 25 °C is at pH 5, where the carboxylic acid groups on the PEG-b-PAA block copolymers are partially deprotonated given the pKa of PAA is 4.5.12 Dynamic light scattering (DLS) study of the annealed mixture at 25 °C gives an average hydrodynamic radius ⟨Rh⟩ of 527 nm and a polydispersity index of 1.00. The annealed mixture at 25 °C shows the emergence of a low-energy absorption band at 605 nm in the UV−vis absorption spectrum (Figure S1a), typical of metal− metal-to-ligand charge transfer (MMLCT) transition arising from metal−metal and π−π interactions of the platinum(II) complexes.3a,8a It is found that the UV−vis spectrum of the annealed mixture appears different from that of the unannealed mixture (Figure S1a), probably arising from the enhanced assembly as well as strong light scattering effects of the largersized aggregates. On the contrary, the emission at 785 nm in B


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Figure 1. (a,b) TEM, (c) SEM, and (d) AFM images of the nanobelts formed by thermal annealing of the mixture of complex 1 (0.15 mM) and PEG45-b-PAA69 ([carboxylic acid] = 1 mM) in a mixed solvent of acetonitrile−methanol−water (1:1:8, v/v/v) at 70 °C. (e) PXRD pattern of a dried film of the nanobelts. Numerical values in (e) indicate d-spacings (nanometers). (f) SAED pattern of a single nanobelt.

techniques, the folding of the nanobelts and the overlaps of the nanobelts, as well as TEM observation of freeze-dried samples, confirm that the nanobelts are formed in the solutions. Powder X-ray diffraction (PXRD) pattern of a dried film of the nanobelts shows a series of Bragg peaks in the region of 4° < 2θ < 14° that can be indexed as (110), (200), (210), (120), and (220) reflections, characteristic of a rectangular columnar phase (Figures 1e and S4).13 Selected area electron diffraction (SAED) of a single nanobelt exhibits a pair of diffraction arcs that corresponds to a d-spacing of 0.34 nm (Figure 1f). This characteristic d-spacing of 0.34 nm confirms the formation of noncovalent metal−metal and π−π interactions between the platinum(II) complexes along the long axis of the nanobelts. Attempts to identify the packing geometry of platinum(II) complexes in the cross-section of the nanobelt are unsuccessful, probably due to the small dimension of each column of the platinum(II) complexes and the instability of the lattices under an electron beam. Figure 2 illustrates the proposed structure of the core−shell nanobelts. Platinum(II) complexes are believed to be stacked into rectangularly packed

the steady-state emission spectrum of the annealed mixture at 25 °C arising from the triplet MMLCT state shows insignificant changes with respect to the unannealed mixture at 25 °C (Figure S1b), indicating that the chemical structures of complex 1 and PEG45-b-PAA69 are kept intact during the thermal annealing process. TEM images of the samples prepared by drop casting of the annealed mixture exhibit well-defined nanobelts with smooth profiles and some with folding of the nanobelts (Figure 1a,b). TEM observation of freeze-dried samples also shows the formation of nanobelts (Figure S2). SEM observation gives morphologies of the nanobelts similar to that observed by TEM with the nanobelts overlapping with each other (Figure 1c). Statistical analysis of the TEM and SEM images shows that the nanobelts have a width of 442 ± 82 nm and a length of several micrometers. The nanobelts have been found to possess a uniform thickness of 9 nm by AFM measurement (Figure 1d), and cross-sectional TEM observation shows the thickness of the nanobelts to be about 10 nm (Figure S3). The similar morphologies observed using different characterization C


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the structure of the nanobelts, while the PEG blocks stretch and reach out toward the solvent and provide the nanobelts with excellent solubility and protection. The formation of the core−shell nanobelts in the thermally annealed mixture would involve both the longitudinal growth and transverse growth of complex 1 and PEG45-b-PAA69. In contrast to room-temperature incubation of a mixture of complex 1 (0.15 mM) and PEG45-b-PAA69 ([carboxylic acid] = 1 mM) in a mixed solvent of acetonitrile−methanol−water (1:1:8, v/v/v), which only allows the longitudinal growth of complex 1 and PEG45-b-PAA69, leading to the core−shell nanofibers as reported in our previous study,8a thermal annealing has provided the coassembly systems with enhanced dynamics and thus switches on the transverse growth of complex 1 and PEG45-b-PAA69. Thermal annealing of a mixture of complex 1 (0.15 mM) and PEG 45 -b-PAA 69 ([carboxylic acid] = 1 mM) at different temperatures has also been performed. It is found that all the supramolecular assemblies formed in the annealed mixtures possess belt-like morphologies (Figure 3), and the average widths of the nanobelts obtained increase with thermal annealing temperatures (Table 1). Given that the nanobelts possess crystalline structures, the number of seed crystals in the coassembly

Figure 2. Schematic illustration of the proposed structure of the core−shell nanobelts.

molecular columns along the nanobelt long axis with intracolumnar separation of 0.34 nm. The PAA blocks of the PEG-b-PAA block copolymers bind to the platinum(II) complexes via electrostatic attractions, providing a template for the assembly of the platinum(II) complexes that can be further stabilized by metal−metal and π−π interactions to give

Figure 3. TEM images of the nanobelts formed by thermal annealing of the mixture of complex 1 (0.15 mM) and PEG45-b-PAA69 ([carboxylic acid] = 1 mM) in a mixed solvent of acetonitrile−methanol−water (1:1:8, v/v/v) at different temperatures (a, 30 °C; b, 40 °C; c, d, 55 °C). D


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nanofiber segments in an end-to-end manner (Figure 4a). The nanobelt segments possess a similar morphology as that of the nanobelts formed by thermal annealing of an identical mixture. Besides, the average diameter of the nanofiber segments (9.8 ± 2.2 nm) is close to that of the nanofibers obtained by incubation of an identical mixture at room temperature (11.1 ± 1.2 nm).8a These observations suggest that the nanobelt segments would be formed at the early stage of the cooling process when the temperature is higher than room temperature, and upon cooling down of the mixture to room temperature, the nanobelt segments would act as seeds to direct the subsequent supramolecular assembly of complex 1 and PEG45-b-PAA69 into nanofiber segments, leading to the formation of the nanobelt-block-nanofiber nanostructures.15 Interestingly, upon further incubation at room temperature for 1 week, TEM observation shows the formation of nanobelts with the disappearance of the nanobelt-block-nanofiber nanostructures (Figure 4b). The nanobelts formed have a width of 148 ± 33 nm, close to that of the nanobelt segments in the nanobelt-block-nanofiber nanostructures (124 ± 18 nm). Moreover, the nanobelts formed have been found to possess an average length close to the nanobelt-block-nanofiber nanostructures, being much longer than the nanobelt segments in the nanobelt-block-nanofiber nanostructures. These observations suggest that during the room-temperature incubation process, the nanobelt-block-nanofiber nanostructures convert to the nanobelts by side−side fusion of the neighboring nanofiber segments at the end of the nanobelt-block-nanofiber nanostructures (Figure 4c).

Table 1. Controlled Fabrication of Nanostructures by Thermal Annealing of the Coassembly Mixtures of Complex 1 and PEG-b-PAA entry

block copolymer

temperature (°C)

morphology

width (nm)

1a 1b 1c 1d 1e 2a 2b 3a 3b

PEG45-b-PAA69 PEG45-b-PAA69 PEG45-b-PAA69 PEG45-b-PAA69 PEG45-b-PAA69 PEG113-b-PAA51 PEG113-b-PAA51 PEG113-b-PAA65 PEG113-b-PAA65

25 30 40 55 70 25 70 25 70

nanofiber nanobelt nanobelt nanobelt nanobelt nanofiber nanofiber nanofiber nanofiber

11.1 ± 1.2 21.1 ± 6.5 61 ± 16 113 ± 34 442 ± 82 9.9 ± 1.0 9.2 ± 0.9 11.0 ± 1.1 11.2 ± 1.3

mixture for nanobelt growth decreases with temperature.14 In the coassembly mixture at a higher thermal annealing temperature, less crystal seeds would survive, leading to the formation of the nanobelts with larger average widths. These observations indicate that the extent of transverse growth of the coassembly systems can be controlled and increased by the thermal annealing temperature. The influence of cooling rates for the thermal treatments of the coassembly mixtures has also been investigated. Rapid quenching of the mixture of complex 1 (0.15 mM) and PEG45b-PAA69 ([carboxylic acid] = 1 mM) in a mixed solvent of acetonitrile−methanol−water (1:1:8, v/v/v) has been found to lead to the formation of nanobelt-block-nanofiber nanostructures where the nanobelt segment connects with several

Figure 4. (a) TEM images of the nanobelt-block-nanofiber nanostructures captured by rapid quenching of the mixture of complex 1 (0.15 mM) and PEG45-b-PAA69 ([carboxylic acid] = 1 mM) in a mixed solvent of acetonitrile−methanol−water (1:1:8, v/v/v) (heating to 70 °C, fast cooling to room temperature). (b) TEM image of the nanobelts formed by room-temperature incubation of the nanobelt-block-nanofiber nanostructures for 1 week. (c) Schematic illustration of the formation of the nanobelt-block-nanofiber nanostructures and their subsequent fusion into nanobelts. E


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Figure 5. Schematic illustration of the energy landscape of the supramolecular coassembly of complex 1 and PEG45-b-PAA69 block copolymers. In the energy landscape, coassembly by room-temperature incubation only allows the longitudinal growth of complex 1 and PEG45-b-PAA69 into core−shell nanofibers that are the kinetically trapped products (pathway a). Thermal annealing switches on the transverse growth of complex 1 and PEG45-b-PAA69 to produce core−shell nanobelts that are the thermodynamically stable nanostructures (pathway b). The transformation from the nanofibers to the nanobelts at room temperature are prohibited by the large kinetic barriers that are derived from the protection and steric repulsion of PEG shell-forming chains on the supramolecular assemblies. Rapid quenching of the coassembly mixtures can capture the nanobeltblock-nanofiber nanostructures, and then the nanobelt-block-nanofiber nanostructure would be converted to the nanobelts through a small kinetic barrier upon further incubation at room temperature (pathway c). Sonication treatment may couple with the energy landscape of the coassembly system and open a unique energy-driven pathway to activate the kinetically forbidden nanofiber-to-nanobelt morphological transformation.

Energy Landscape of the Supramolecular Coassembly System. The above observations show that the thermal annealing experiments at different temperatures, as well as the rapid quenching experiment followed by room-temperature incubation, all lead to the formation of nanobelt morphology. These indicate that the nanobelts are the thermodynamically stable products and are located at the deepest well in the energy landscape of the supramolecular coassembly of complex 1 and PEG45-b-PAA69 (Figure 5). The core−shell nanofibers obtained by room-temperature incubation of the identical mixture of complex 1 and PEG45-b-PAA69 (Figure S5),8a which cannot fuse into nanobelts by room-temperature incubation and exhibit morphological stability at room temperature, are the kinetically trapped assemblies in the energy landscape of the coassembly system (Figure 5). The formation of nanobelts by thermal annealing involves both longitudinal and transverse growth of platinum(II) complexes and block copolymers, whereas the formation of nanofibers by room-temperature incubation only involves the longitudinal growth of platinum(II) complexes and block copolymers. These, together with the observations that the extents of the transverse growth increase with the thermal annealing temperatures (Table 1), indicate the presence of kinetic barriers in the energy landscape that can inhibit the transverse growth of platinum(II) complexes and block copolymers. Further understanding of the kinetic barriers for the transverse growth has been accumulated from investigation of the supramolecular assembly behaviors of a system involving PEG-b-PAA block copolymers with longer PEG chains. Both thermal annealing and room-temperature incubation of the mixture of complex 1 (0.15 mM) and PEG113-b-PAA51 ([carboxylic acid] = 1 mM) in a mixed solvent of acetonitrile−methanol−water (1:1:8, v/v/v) have been found to lead to the formation of well-defined nanofibers with crystalline nanostructures under TEM observation, AFM

measurement, and SAED studies (Figure 6 and Table 1). It has also been found that both thermal annealing and roomtemperature incubation of the coassembly mixture of PEG113b-PAA65 and complex 1 lead to the formation of well-defined nanofibers (Figure S6 and Table 1), similar to the case of PEG 113 -b-PAA 51. These observations indicate that the complexity of the energy landscape can be modulated by the steric repulsion of the PEG shell-forming chains on the supramolecular assemblies. In the case of PEG45-b-PAA69 where the PEG steric repulsion is moderate, the energy landscape of the supramolecular system is relatively complicated because of the involvements of growth of platinum(II) complexes and block copolymers in the transverse direction. In contrast, the energy landscape is relatively simple because the transverse growth of the platinum(II) complexes in the system of complex 1 and PEG113-b-PAA51 is strictly prohibited. Therefore, the steric repulsion of PEG shell-forming chains on the supramolecular assemblies is responsible for the kinetic barriers of the transverse growth of platinum(II) complexes and block copolymers. In the case of rapid quenching (Figure 4), the supramolecular coassembly system exhibits intriguing temporal behaviors of morphological transformation, that is, the formation of the nanobelt-block-nanofiber nanostructures and the subsequent fusion of the nanofiber segments into nanobelts upon room-temperature incubation. The nanobelt-block-nanofiber nanostructures simultaneously possess both the thermodynamically stable nanobelt segment and the kinetically favored nanofiber segments on one single nanostructure (Figure 5). Since the nanofiber segments preorganized by the nanobelt segment are parallel to each other and are in close proximity, the probability of the contact and fusion between the neighboring nanofiber segments largely increases. In contrast, in the mixture of complex 1 and PEG45-b-PAA69 by room-temperature incubation, the core−shell nanofibers F


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Figure 6. (a) TEM image and (b) SAED pattern of the nanofibers obtained by room-temperature incubation of the mixture of complex 1 (0.15 mM) and PEG113-b-PAA51 ([carboxylic acid] = 1 mM) in a mixed solvent of acetonitrile−methanol−water (1:1:8, v/v/v). SAED pattern of a bundle of aligned nanofibers exhibits a pair of diffraction arcs that correspond to a d-spacing of 0.34 nm, which indicates the noncovalent metal− metal and π−π interactions between the platinum(II) complexes along the fiber long axis. (c) TEM image and (d) AFM image of the nanofibers obtained by thermal annealing (heating to 70 °C, slowly cooling to room temperature) of the mixture of complex 1 (0.15 mM) and PEG113-bPAA51 ([carboxylic acid] = 1 mM) in a mixed solvent of acetonitrile−methanol−water (1:1:8, v/v/v).

block and thus have a less negative free energy than the precursor core−shell nanofibers (Figure 5). The kinetic barriers for the nanofiber-to-nanobelt transformation would be reduced due to the decreased PEG steric protection on the patchy nanofibers in both their ends and sides. In the present study, vigorous magnetic stirring that can introduce strong mechanical shearing forces has been applied to treat the core− shell nanofibers, and it is found that the core−shell nanofibers can be broken into shorter ones by the vigorous magnetic stirring (Figure S7). However, further incubation of the short nanofibers shows insignificant change in the morphology of the nanofibers (Figure S7). Recent studies show that the coupling of chemical fuels or energy to supramolecular assembly systems plays an essential role in the design of fuel-driven and energy-driven far-from-equilibrium supramolecular systems.11 However, the coupling of chemical fuels or energy to multicomponent supramolecular systems with relatively complicated energy landscape has been rarely explored. The above observations indicate that the sonication treatment can cleave the noncovalent interactions between PEG-b-PAA and

formed cannot fuse into nanobelts by further room-temperature incubation due to the presence of large kinetic barriers caused by PEG steric repulsion on the nanofibers (Figure 5). In the system involving the rapid quenching experiment, the preorganization effect appears to provide a means to reduce the kinetic barriers for the fusion of nanofiber segments into nanobelts (Figure 5). During the fusion process, the PEG shell of the nanofiber segments would adjust their conformation and redistribute to allow the contact of PAA/Pt cores between neighboring nanofiber segments.16 With the establishment of the energy landscape, it reveals that the sonication-driven morphological transformation from core−shell nanofibers through short patchy nanofibers to core−shell nanobelts as reported in our previous study8a represents an interesting energy-driven pathway that can overcome the large kinetic barriers between nanofibers and nanobelts (Figure 5). Since sonication treatment can remove part of noncovalently connected PEG45-b-PAA69 chains from the precursor nanofibers,8a the short patchy nanofibers possess less electrostatic attractions between complex 1 and the PAA G


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Figure 7. (a) TEM image and (b) AFM image of the nanosphere-block-nanobelt nanostructures prepared by thermal annealing of the mixture of complex 1 (0.15 mM), PEG45-b-PAA69 ([carboxylic acid] = 0.85 mM), and PAA66 brushes ([carboxylic acid] = 0.15 mM) in a mixed solvent of acetonitrile−methanol−water (1:1:8, v/v/v). Inset in (a) shows an individually dispersed nanosphere-block-nanobelt nanostructure. (c) TEM image of the nanospheres produced by thermal annealing of the mixture of complex 1 (0.15 mM) and PAA66 brushes ([carboxylic acid] = 0.15 mM) in a mixed solvent of acetonitrile−methanol−water (1:1:8, v/v/v). (d) Schematic illustration of the formation of the nanosphere-blocknanobelt nanostructures.

pH 1 (Figure S10a). These observations confirm that the electrostatic attractions between platinum(II) complexes and PAA blocks are essential for the formation of nanofibers in the supramolecular systems. Surprisingly, at pH 10 where the PAA blocks are fully deprotonated, the color of the mixture is brown, but it is found that incubation of the mixture only leads to the formation of spherical nanoparticles with size of ∼10 nm by TEM observation (Figure S10b). This suggests that, despite the presence of significant metal−metal and π−π interactions, the strong electrostatic repulsion between PAA blocks can inhibit the stacking and growth of platinum(II) complexes into anisotropic nanostructures. The UV−vis absorption spectra of the mixtures show that the MMLCT absorption at 605 nm decreases with NaCl concentrations in the coassembly mixtures (Figure S11). TEM observations show that room-temperature incubation of the coassembly mixture at NaCl concentration from 0 to 50 mM can produce nanofibers (Figure S12a−c), whereas the coassembly mixture at 150 mM NaCl can only produce spherical nanoparticles (Figure S12d); the pH of all the coassembly mixtures has been fixed at pH 5.3 by 10 mM sodium acetate−acetic acid buffer. In aqueous solutions, NaCl can screen electrostatic interactions. These results are further indications that the electrostatic attractions between platinum(II) complexes and PAA blocks are crucial for nanofiber formation. Supramolecular coassembly in pure organic solvent has also been investigated. The mixture of complex 1 (0.15 mM) and

platinum(II) complexes and couple to the energy landscape of the coassembly system and provide a unique energy-driven pathway for the kinetically forbidden morphological transformation from nanofibers to nanobelts. The observation of sonication-driven morphological transformation from core− shell nanofibers through short patchy nanofibers to core−shell nanobelts further confirms that the nanobelts are thermodynamically stable products. Noncovalent Interactions of the Supramolecular Coassembly System. The influence of pH, salt concentration, and solvent composition for the supramolecular coassembly has been investigated by UV−vis measurements and TEM observations. Since the thermally annealed mixtures show light scattering and would complicate the UV−vis measurements (Figure S1), coassembly mixtures obtained by room-temperature incubation are selected to study the influence of the various factors. TEM observations show that when the pH of the medium is at pH 4 to 6, nanofibers can be obtained by room-temperature incubation of the mixtures of complex 1 (0.15 mM) and PEG45-b-PAA69 ([carboxylic acid] = 1 mM) in a mixed solvent of acetonitrile−methanol−water (1:1:8, v/v/v) (Figure S8). The UV−vis absorption spectra of the mixtures show that the MMLCT absorption at 605 nm increases with the pH values of the medium (Figure S9). At pH 1 where the carboxylic acid groups on the PAA blocks are fully protonated, the color of the mixture is yellow, indicating the absence of metal−metal and π−π interactions. TEM observation shows no nanofiber formation in the mixture at H


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Figure 8. (a−c) TEM images of the nanosphere-block-nanobelt nanostructures at (a) 0, (b) 25, and (c) 100 mM HOAc. (d,e) TEM images of the nanobelts at (d) 25 and (e) 100 mM HOAc. (f) TEM images of the nanofibers at 25 mM HOAc.

platinum(II) complexes driven by the metal−metal and π−π stacking interactions and other noncovalent interactions,4−7 where Pt(II) nanostructures with relatively simple morphologies such as spheres, fibers, rods, vesicles, sheets, tubes, and rings can be produced.7 The two-component supramolecular coassembly developed by our group recently allows the fabrication of Pt(II) nanostructures with controlled dimensions and diverse compositions,8a as well as the construction of 1D crystalline nanostructures of higher sophistication; for example, with heterojunctions of larger structural differences and large lattice mismatch.8b With the establishment of the energy landscape and the understanding of the driving forces in the coassembly system, we envisage that further incorporation of functional building blocks into the two-component supramolecular systems may lead to the formation of nanostructures with enhanced complexity and intriguing properties. Toward this end, studies on multicomponent supramolecular systems have been performed. Brush polyelectrolytes are selected to be incorporated into the coassembly system of the platinum(II) complexes and block copolymers. Brush polyelectrolytes have been demonstrated both by theoretical studies and experimental analysis to possess higher charge density and stronger counterion correlation when compared to their linear counterparts.17 Thermal annealing at 70 °C of a mixture of complex 1 (0.15 mM), PEG45-b-PAA69 ([carboxylic acid] = 0.85 mM), and PAA66 brushes ([carboxylic acid] = 0.15 mM) in a mixed solvent of acetonitrile−methanol−water (1:1:8, v/v/v) has been found to lead to the unprecedented formation of nanosphere-block-nanobelt nanostructures, where the nanosphere segments with a higher electron contrast connect to the nanobelt segments with a lower electron contrast in the TEM image (Figure 7a). The pH of the medium is at pH 5. AFM

PEG45-b-PAA69 ([carboxylic acid] = 1 mM) in a mixed solvent of acetonitrile−methanol (1:1, v/v) are yellow in color, which indicates that the platinum(II) complexes and the block copolymers are molecularly dispersed in the mixture. TEM observation of the mixture in the mixed solvent of acetonitrile−methanol (1:1, v/v) also shows no nanofiber formation (Figure S13). These suggest that, due to its excellent solubility in pure organic solvent, complex 1 is well dispersed and does not show signs of molecular association. This lack of metal−metal and π−π interaction-directed assembly would lead to the absence of anisotropic nanostructures. When a hydrophilic platinum(II) complex 2 (Scheme 1) is used in the place of complex 1, TEM observation shows that room-temperature incubation of the mixture of complex 2 (0.15 mM) and PEG45-b-PAA69 ([carboxylic acid] = 1 mM) in a mixed solvent of acetonitrile−methanol−water (1:1:8, v/v/ v) only leads to the formation of spherical nanoparticles rather than anisotropic nanostructures (Figure S14). This suggests that the hydrophobic interactions between complex 1 are also important for the supramolecular coassembly. The above observations show that the supramolecular coassembly in the present study involves diverse noncovalent interactions, namely, the metal−metal, π−π and hydrophobic interactions between the platinum(II) complexes, the electrostatic attractions between the platinum(II) complexes and PAA blocks, the PEG steric repulsion and the electrostatic repulsion between PAA blocks. The delicate balance between these noncovalent interactions gives rise to the formation of welldefined nanostructures. Nanostructures with Enhanced Complexity from Multicomponent Supramolecular Assembly. Most literature reported studies of supramolecular assembly of platinum(II) complexes focus on the single-component self-assembly of I


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have a higher charge density bind platinum(II) complexes to form the nanosphere segments; (ii) the as-formed nanosphere segments serve as seeds to direct the growth of complex 1, PEG45-b-PAA69, and a small amount of PAA66 brushes into the nanobelt segments. Morphological Stability of the Nanostructures against Acid and Dilution. As described above, the nanobelts (Figure 8d) and the segmented nanostructures (Figure 8b) exhibit morphological stability at 25 mM HOAc (pH = 3.2). This morphological stability is unexpected. Thermal annealing experiments of the coassembly mixtures with the presence of 25 mM HOAc have been performed. It turns out that neither nanobelts are formed by thermal annealing of the mixture of complex 1 and PEG45-b-PAA69 with the presence of 25 mM HOAc (Figure S16a) nor segmented nanostructures are observed by annealing the ternary mixture of complex 1, PEG45-b-PAA69, and PAA66 brushes at 25 mM HOAc (Figure S16b). It is known that, besides the metal−metal and π−π interactions and hydrophobic interactions between complex 1, the electrostatic attractions between complex 1 and PAA are essential driving forces for nanostructure formation and can stabilize the nanostructures formed (Figures S8−S12). At 25 mM HOAc (pH = 3.2), most of the carboxylic acid groups of PAA are in a protonated state, and the electrostatic attractions between complex 1 and PAA would be very weak. Therefore, the morphological stability of the nanobelts and the segmented nanostructures at low pH cannot be explained from the thermodynamic viewpoints of the coassembly system. The morphological stability of the nanostructures in the present study may arise from the highly ordered packing structures or crystalline structures of the supramolecular assemblies that lead to insignificant dynamic behaviors and hence the morphological stability, as have been seen in other literature reported supramolecular systems.18,19a The morphological stability of the nanostructures against HOAc motivates us to investigate the stability of the nanostructures under large dilution. Dilution stability of nanostructures is considered as an important property of drug carriers during intravenous injection since intravenous injection leads to a very large dilution of the drug carriers and may result in an undesirable burst release within the first hour after the injection.19 To investigate their dilution stability, the nanostructures obtained in the present study are diluted 100fold to 1.5 μM complex 1 and 10 μM carboxylic acid followed by incubation for 1 h; this concentration would be within the concentration range of most drug delivery systems after intravenous injection into human blood.19 The free complex 1 in the diluted nanostructures has been separated by high-speed centrifugation, and the free complex 1 in the supernatant was analyzed by ICP-MS. The concentrations of free complex 1 in the supernatants of the nanofibers (Figure S5), the nanobelts (Figure 1), and the segmented nanostructures (Figure 7) are determined to be 0.32 μM (21% of total Pt), 0.18 μM (12% of total Pt), and 0.19 μM (13% of total Pt), respectively; and the concentration of total Pt in the diluted nanostructures is fixed at 1.5 μM. In contrast, in the diluted nanofibers and the diluted nanobelts, the concentration of free complex 1 has been estimated from the binding constant (Kb = 6.5 × 104 M−1) to be 0.93 μM (62% of total Pt) when the diluted coassembly systems achieve equilibrium. These results indicate that the nanobelts and the segmented nanostructures possess excellent stability against large dilution, whereas the dilution stability of

study shows that the morphology of the nanostructures is similar to that observed by TEM (Figure 7b). The nanobelt segments have a height of 10 nm, which is close to that of the nanobelts formed by thermal annealing (Figure 1d). The height of the nanosphere segments is much larger than that of the nanobelt segments. The nanosphere-block-nanobelt nanostructures have a hydrodynamic radius ⟨Rh⟩ of 637 nm and a polydispersity index of 1.00 as determined by DLS measurements. The UV−vis and emission spectra of these nanostructures obtained from the multicomponent systems show MMLCT absorption bands at 600 nm and 3MMLCT emission bands at 785 nm (Figure S15), similar to those of the nanofibers and nanobelts formed by complex 1 and PEG45-bPAA69 (Figure S1). In the control experiments, thermal annealing of the mixture of complex 1 (0.15 mM) and PAA66 brushes ([carboxylic acid] = 0.15 mM) has been found to produce nanospheres with a hydrodynamic radius ⟨Rh⟩ of 177 nm and a polydispersity index of 0.13 (Figure 7c). The nanospheres have a similar morphology and size to the nanosphere segments in the nanosphere-block-nanobelt nanostructures (Figure 7a). To acquire more structural information, nanostructure dissociation experiments have been performed by mixing the nanosphere-b-nanobelt segmented nanostructures with acetic acid (HOAc) that can reduce the electrostatic attractions between PAA and platinum(II) complexes. It is found that the segmented nanostructures retain their morphology at 25 mM HOAc (pH = 3.2) (Figure 8a,b). At 100 mM HOAc (pH = 2.9), the segmented nanostructures are found to dissociate into nanosphere fragments and nanobelt fragments (Figure 8c). The nanosphere fragments have a similar size and electron contrast to those of the nanosphere segments in the segmented nanostructures. The nanobelt fragments are much shorter than the nanobelt segments in the segmented nanostructures (Figure 8a,c), and some nanobelt fragments as indicated by arrows in Figure 8c show a tendency to dissociate into smaller fibrous fragments. In the control experiments, the nanobelts formed by 70 °C thermal annealing of complex 1 and PEG45-bPAA69 are found to exhibit morphological stability at 25 mM HOAc (Figure 8d), whereas TEM observation shows the disappearance of the nanobelts at 100 mM HOAc (Figure 8e). Besides, the nanofibers formed by room-temperature incubation of complex 1 and PEG45-b-PAA69 are found to dissociate into short nanofiber fragments at 25 mM HOAc (Figure 8f). The morphological similarity between the nanosphere segments in the segmented nanostructures (Figure 7a) and the nanospheres obtained by thermal annealing of complex 1 and PAA66 brushes (Figure 7c), as well as the morphological stability of the nanosphere fragments at 100 mM HOAc (Figure 8c), indicate that the nanosphere segments in the segmented nanostructures mainly consist of PAA66 brushes and complex 1. The enhanced stability of the nanobelt segments at 100 mM HOAc (Figure 8c) when compared to the nanobelts of complex 1 and PEG45-b-PAA69 (Figure 8e) suggests that, besides complex 1 and PEG45-b-PAA69, a small amount of PAA66 brushes that can provide stronger electrostatic attractions between complex 1 and PAA participate in the formation of the nanobelt segments in the segmented nanostructures. Based on the above morphology and structure analysis, together with the living characteristic of the coassembly system,8b it is proposed that the nanosphereblock-nanobelt segmented nanostructures are formed by two consecutive processes (Figure 7d): (i) the PAA66 brushes that J


Article

Journal of the American Chemical Society the nanofibers is less significant. The dilution stability of the coassembly mixtures involving hydrophilic platinum(II) complexes has also been investigated. Nanoaggregates were prepared by thermal annealing of the ternary mixture of hydrophilic platinum(II) complex 2 (0.15 mM), PEG45-bPAA69 ([carboxylic acid] = 0.85 mM), and PAA66 brushes ([carboxylic acid] = 0.15 mM) (Figure S17). The free complex 2 in the supernatants of the diluted nanoaggregates after highspeed centrifugation was determined by ICP-MS to be 1.0 μM (67% of the total Pt). Based on the above observations, together with the literature reported studies on organic supramolecular systems,18,19a it is proposed that the excellent stability of the current metalcontaining nanostructures against HOAc and dilution is originated from their crystalline structures. The nanobelts and nanosphere-block-nanobelt segmented nanostructures prepared from the thermally annealed mixtures would possess higher crystallinity and show higher stability against HOAc and dilution, when compared to the nanofibers obtained by roomtemperature incubation. On the contrary, due to the more hydrophilic nature of complex 2, the nanoaggregates of complex 2, PEG45-b-PAA69, and PAA66 brushes are not able to form crystalline structures and thus accounts for their ease of dissociation upon large dilution. Given that the platinum(II) complexes and other metal complexes can be used as bioimaging agents and anticancer drugs, the present study shows the possibility of promising applications in the area of drug delivery and bioimaging. Further research study would be made in these areas by using coassembly systems with bioimaging agents and anticancer drugs.

of platinum(II) complexes, block copolymers, and brush polyelectrolytes in a simple one-pot single-step procedure. Interestingly, the nanobelts and nanosphere-block-nanobelt nanostructures are found to possess intriguing morphological stability against acid and dilution, exhibiting characteristics that are important for promising biomedical applications. In summary, the present work has presented the assembly of diverse platinum(II) nanostructures and their interconversion, which has not been observed previously. The present study also represents an example for the rational design and fabrication of nanostructures with enhanced complexity and sophistication and intriguing properties based on the improved understanding of both the thermodynamic and kinetic behaviors of the supramolecular assembly systems. Further study will focus on the potential application of the nanostructures obtained from the supramolecular coassembly systems.

CONCLUSION The energy landscape of the supramolecular coassembly of platinum(II) complexes and block copolymers has been established with core−shell nanobelts being the thermodynamically stable nanostructures and core−shell nanofibers being the kinetically trapped assemblies of the coassembly system. The energy landscape consists of several interesting kinetic pathways: (i) thermal annealing switches on the transverse growth of platinum(II) complexes and block copolymers into nanobelts, and the extents of the transverse growth increase with the thermal annealing temperatures, leading to the formation of nanobelts with controlled widths; (ii) rapid quenching of the hot coassembly mixture to room temperature exhibit intriguing temporal properties of morphological transformation, that is, the formation of the intermediate nanobelt-block-nanofiber nanostructures and their subsequent fusion into nanobelts upon further incubation at room temperature; (iii) sonication treatment is found to couple with the energy landscape of the coassembly system and open a unique energy-driven pathway for the kinetically forbidden morphological transformation from nanofibers to nanobelts. In addition, the supramolecular coassembly has been found to involve diverse noncovalent interactions, namely, the metal−metal, π−π, and hydrophobic interactions between the platinum(II) complexes, the electrostatic attractions between the platinum(II) complexes and PAA blocks, and the PEG steric protection and the electrostatic repulsion between PAA blocks. With the establishment of the energy landscape and the understanding of the driving forces of the coassembly system, nanosphere-block-nanobelt nanostructures with distinct segmented morphologies have been fabricated by thermal annealing of the multicomponent system

Notes

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b04779. Experimental section and supporting figures (PDF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

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Vivian Wing-Wah Yam: 0000-0001-8349-4429 The authors declare no competing financial interest.

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ACKNOWLEDGMENTS V.W.-W.Y. acknowledges support from The University of Hong Kong under the University Research Committee (URC) Strategically Oriented Research Theme (SORT) on Functional Materials for Molecular Electronics. This work has been supported by the University Grants Committee Areas of Excellence (AoE) Scheme (AoE/P-03/08) and a General Research Fund (GRF) grant from the Research Grants Council of Hong Kong Special Administrative Region, P. R. China (HKU17334216). We also thank the Electron Microscope Unit at The University of Hong Kong for technical support.

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Energy Landscape in Supramolecular Coassembly ... - ACS Publications

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