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Article Cite This: Chem. Mater. XXXX, XXX, XXX−XXX

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Self-Assembly of Nano- to Macroscopic Metal−Phenolic Materials Gyeongwon Yun,†,‡ Quinn A. Besford,†,‡ Stuart T. Johnston,§,⊥ Joseph J. Richardson,† Shuaijun Pan,† Matthew Biviano,∥ and Frank Caruso*,†

Chem. Mater. Downloaded from pubs.acs.org by UNIV OF SUSSEX on 08/10/18. For personal use only.

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ARC Centre of Excellence in Convergent Bio-Nano Science and Technology, and the Department of Chemical Engineering, The University of Melbourne, Parkville, Victoria 3010, Australia § Systems Biology Laboratory, School of Mathematics and Statistics, and the Department of Biomedical Engineering, The University of Melbourne, Parkville, Victoria 3010, Australia ⊥ ARC Centre of Excellence in Convergent Bio-Nano Science and Technology, Melbourne School of Engineering, The University of Melbourne, Parkville, Victoria 3010, Australia ∥ Department of Chemical Engineering, The University of Melbourne, Parkville, Victoria 3010, Australia S Supporting Information *

ABSTRACT: The self-assembly of molecular building blocks into well-defined macroscopic materials is desirable for developing emergent functional materials. However, the selfassembly of molecules into macroscopic materials remains challenging, in part because of limitations in controlling the growth and robustness of the materials. Herein, we report the molecular self-assembly of nano- to macroscopic free-standing materials through the coordination of metals with natural phenolic molecules. Our method involves a simple and scalable solution-based template dipping process in precomplexed metal−phenolic solutions, enabling the fabrication of free-standing macroscopic materials of customized architectures (2D and 3D geometries), thickness (about 10 nm to 5 μm), and chemical composition (different metals and phenolic ligands). Our macroscopic free-standing materials can be physically folded and unfolded like origami, yet are selectively degradable. Furthermore, metal nanoparticles can be grown in the macroscopic free-standing films, indicating their potential for future applications in biotechnology and catalysis.

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INTRODUCTION In nature, the hierarchical self-assembly of simple molecular building blocks can lead to sophisticated architectures across a range of length scales, such as proteins, viruses, cells, multicellular organisms, and exoskeletons.1 Molecular selfassembly has proven to be a powerful method for the construction of functional materials with applications in drug delivery, electronics, energy, imaging, and catalysis.2−13 For example, self-assembled DNA nanostructures enable precise patterning at the nanoscale and can be used to create programmable molecular machines and arrays of functional materials.14 The self-assembly of DNA nanostructures requires precisely designed base pairs and optimized synthetic conditions to prevent nonspecific termination or uncontrolled growth.14,15 Other molecularly self-assembled materials that exhibit geometrical complexity on the nanoscale3−7 are typically challenging to extend to the macroscale, due to difficulties in controlling the growth (e.g., rapid termination) and achieving material robustness due to weakly interacting (e.g., hydrogen bonding) components.14−16 Therefore, the bottom-up molecular assembly of strong artificial macroscopic structures with a control that is akin to nature (i.e., welldefined macroscopic size and shape) remains challenging.3,16,17 © XXXX American Chemical Society

Polyphenols are widely present in plants, where they are correlated with diverse biological functions including chemical defense, pigmentation, structural support, prevention of radiation damage, and metal sequestration.18 The catechol and galloyl groups of polyphenols provide multivalent chelating sites that can coordinate with a variety of metals where the coordination is controlled by pH.19−24 In 2013, we reported the self-assembly of FeIII ions and a natural polyphenol, tannic acid (TA) (Figure 1a), into metal−phenolic network films on substrates of different size, shape, and/or surface charge.25 This self-assembly process has been investigated for different metal ions and phenolic compounds, under different conditions (e.g., ionic strength), and using a variety of assembly methods such as discrete assembly (TA and FeIII), layer-by-layer assembly (cyclic, alternating deposition of TA and FeIII), continuous rust-mediated assembly (solid-state iron precursors immersed in TA solution), and electro-triggered assembly (conductive substrates that reduce FeII to FeIII in a TA solution).25−29 The discrete assembly Received: June 20, 2018 Revised: July 25, 2018

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DOI: 10.1021/acs.chemmater.8b02616 Chem. Mater. XXXX, XXX, XXX−XXX

Article

Chemistry of Materials

Figure 1. Film formation. (a) Structure of tannic acid. (b) Schematic of FeIII−TA film deposition using a simple solution-based template dipping process. (c) Digital photograph of FeIII−TA films formed on quartz substrates at different immersion times. (d) UV−visible absorption spectrum of a representative FeIII−TA film showing the LMCT band of the substrate after 114 h of immersion in the FeIII−TA solution. (e) Film growth kinetics monitored by UV−visible absorption at 565 nm (LMCT band) (red circles) and thickness profile of an FeIII−TA film on a glass substrate, as measured by AFM (blue triangles). (f) Representative AFM height image and corresponding height profile of a free-standing macroscopic FeIII− TA film after 162 h immersion. All data are for systems containing final concentrations of 11.8 mM TA and 18.5 mM FeIII.

method allows for control over film thickness, mostly in increments of ∼10 nm,25 although films can be prepared with thicknesses of the order of 8027 and 250 nm using other methods.29 However, the fabrication of free-standing macroscopic metal−phenolic network materials has remained challenging, partially because of constraints in preparing sufficiently thick films without the need of laborious processing. Herein, we report the molecular self-assembly of nano- to macroscopic metal−phenolic materials using a simple solutionbased template dipping process into precomplexed FeIII and TA solutions, where the thickness of the resulting films can be controlled by varying the precursor concentration and substrate immersion time. This method is applicable to different metal ions and phenolic molecules, allowing for further control over functionality. The key feature of our strategy is the use of phenolic molecules and coordinating metal ion solutions at concentrations greater than 1 mM. We can control the growth of these materials by varying the template immersion time and/or the concentration of the molecular components, where experimental and theoretical studies suggest this growth process is diffusion-driven. The simplicity and versatility of our method enables the fabrication of robust (Young’s modulus of ∼3 GPa) free-standing macroscopic materials of customized shape, size, and thickness, by mixing commercially available molecular precursors at ambient synthetic conditions.

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concentration of the precursors remained approximately the same as the initial concentration. Planar glass substrates were immersed in the purified (free from precipitates) FeIII−TA solution in a 50 mL tube. The film-coated substrates were washed with water thrice to remove excess FeIII and TA. The film-coated substrates were subsequently immersed in 3-(N-morpholino)propanesulfonic acid (MOPS) buffer (40 mL, 10 mM, pH 8) in a 50 mL tube to strongly cross-link the metal ions with phenolic molecules. Monitoring the FeIII−TA Film Formation. (1) Monitoring the growth of metal−phenolic complexes in solution: All solutions were freshly prepared for immediate use. Aliquots (5 mL) of FeCl3·6H2O (37 mM) and TA (23.5 mM) in water were added to water (10 mL), and the solution was vigorously mixed by a vortex mixer for 10 s. The FeIII−TA solution was centrifuged at 2000g for 5 min and then filtered with a syringe filter with a 0.22 μm pore size to remove precipitates from the solution. Then, the product was isolated at time intervals of 1 min and 2, 8, and 72 h and characterized by transmission electron microscopy (TEM). (2) Monitoring the deposition of metal−phenolic complexes on substrates by optical waveguide lightmode spectroscopy (OWLS): Silica sensor chips were washed extensively with water, followed by ethanol, and dried under a nitrogen stream. The dried chips were subsequently cleaned with Piranha solution, washed with water, dried under nitrogen, and then plasma cleaned. The clean chips were then mounted in the instrument, and dry-state optical properties were determined. Prior to the experiment, water was flushed through the system and allowed to equilibrate with the sensor chip for 30 min. For the experiment, a solution of FeCl3·6H2O (37 mM) and TA (23.5 mM) in water was prepared. The solution was vigorously mixed by a vortex mixer for 10 s, centrifuged at 2000g for 5 min, filtered with a syringe filter with a 0.22 μm pore size, injected into the instrument at the required flow rate (50 or 500 μL h−1), and allowed to flush through the system for a further 30 min. After a change in adsorbed mass was recorded, the flow was continued until all complexes had washed over the sensor and the signal stabilized (i.e., dm/dt < 10−8, where m and t denote mass and time, respectively). (3) Substrate withdrawal rate effect on FeIII−TA film formation: All solutions were freshly prepared for immediate use. Aliquots (5

EXPERIMENTAL SECTION

Film Formation on Planar Substrate. All solutions were freshly prepared for immediate use. Aliquots (10 mL) of FeCl3·6H2O (37 mM) and TA in water (23.5 mM) were vigorously mixed by a vortex mixer for 10 s. The FeIII−TA solution was centrifuged at 2000g for 5 min and then filtered with a syringe filter with a 0.22 μm pore size to remove precipitates from the solution. After the filtering step, the B


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Chemistry of Materials mL) of FeCl3·6H2O in water (37 mM) and TA in water (23.5 mM) were added to water (10 mL), and the solution was vigorously mixed by a vortex mixer for 10 s. In the purification step, the FeIII−TA solution was centrifuged at 2000g for 5 min and then filtered with a syringe filter with a 0.22 μm pore size to remove precipitates from the solution. Planar glass substrates were immersed in the purified FeIII− TA solution in a 50 mL tube. A syringe pump (New Era Pump System Inc., NE-1000 series), equipped with software, was used to control the withdrawal speed of the substrates in the purified FeIII− TA solution between 1.24 and 124 mm h−1 for film fabrication. The film-coated substrates were washed with water thrice to remove excess FeIII and TA. The film-coated substrates were subsequently immersed in MOPS buffer (40 mL, 10 mM, pH 8) in a 50 mL tube to strongly cross-link the metal ions with phenolic molecules. Details of Theoretical Study on FeIII−TA Film Formation. As TA molecules and FeIII ions attempt to move to one of their six nearest neighbor lattice sites, they do so in a random fashion. To account for the size disparity between FeIII and TA, at most one TA molecule may occupy a particular site, whereas there is no restriction on the number of FeIII ions that can occupy a particular site. This implies that FeIII ions are small relative to the size of the lattice site and hence are small relative to the size of TA molecules. If a TA molecule attempts to move to an occupied site, the movement attempt is aborted. To describe the film formation, any TA molecules that reach the left boundary of the lattice sites are bound, which represents TA binding to the template. These molecules are no longer able to move. We assume that each TA molecule has six binding sites available for iron ions, and these binding sites exist at each of the six sides of the hexagonal lattice site. If an FeIII ion arrives at a site occupied by a TA molecule that has available binding sites, then the FeIII ion will bind to a randomly selected available binding site with probability PB. TA molecules are able to bind together when a TA molecule moves to a site where at least one of the nearest neighbor sites is occupied by a TA molecule that is bound to the film. At the boundary between the two occupied sites, corresponding to the location of a binding site on each TA molecule, we impose the restriction that binding between TA molecules can only occur if exactly one of these binding sites contains an FeIII ion. To account for the possibility that two TA molecules can be bound together through more than one binding site, we impose a probability PR that a molecule, after binding to the film, is not available for further binding events. Initially, both iron ions and tannic acid molecules are distributed randomly throughout the domain of M by N lattice sites. The template is defined as the left boundary, and the width of the domain, N, is chosen such that free tannic acid molecules and iron ions do not deplete throughout the simulation. The boundaries in the vertical direction are impermeable; however, as the simulation is, on average, independent of the vertical direction, this choice is unimportant. The simulation is repeated 12 times, and the average thickness of the film is calculated at 10 time points. Fabrication of Free-Standing Macroscopic Metal−Phenolic Materials. All solutions were freshly prepared for immediate use. Aliquots (10 mL) of FeCl3·6H2O (37 mM) and TA (23.5 mM) in water were vigorously mixed by a vortex mixer for 10 s. The FeIII−TA solution was centrifuged at 2000g for 5 min and then filtered with a syringe filter with a 0.22 μm pore size to remove precipitates from the solution. Ethanol-cleaned planar glass substrates were immersed in polystyrene (PS) solution (20 mL, 50 mg mL−1 in tetrahydrofuran (THF)) for 20 s and dried in air. This process was repeated twice. The resulting PS-coated substrates were immersed in the purified FeIII−TA solution in a 50 mL tube for 65 h. The coated substrates were washed with water thrice to remove excess FeIII and TA. The coated substrates were subsequently immersed in MOPS buffer (40 mL, 10 mM, pH 8) in a 50 mL tube to strongly cross-link the metal ions with phenolic molecules. To obtain free-standing materials, the PS was removed by washing with THF four times and finally dispersed in the desired solvent. In each THF washing step, the final materials were then kept in THF for 30 min. Metal Nanoparticle-Functionalized Metal−Phenolic Materials. All solutions were freshly prepared for immediate use. Aliquots

(10 mL) of FeCl3·6H2O (37 mM) and TA (23.5 mM) in water were vigorously mixed by a vortex mixer for 10 s. The FeIII−TA solution was centrifuged at 2000g for 5 min and then filtered with a syringe filter with a 0.22 μm pore size to remove precipitates from the solution. Ethanol-cleaned planar glass substrates were immersed in PS solution (20 mL, 50 mg mL−1 in THF) for 20 s and dried in air. This process was repeated twice. PS-coated substrates were then immersed in the purified FeIII−TA solution in a 50 mL tube for 65 h. The coated substrates were washed with water thrice to remove excess FeIII and TA. The coated substrates were subsequently immersed in MOPS buffer (40 mL, 10 mM, pH 8) in a 50 mL tube to strongly cross-link the metal ions with phenolic molecules. The metal− phenolic material on PS-coated glass was immersed in 5 mL of fresh metal in water (37.5 mM of AgNO3 or Pd(NO3)2·2H2O solution) and incubated for 1 day. The metal−phenolic materials were rinsed with water thrice to remove excess AgNO3 and Pd(NO3)2·2H2O, respectively. To obtain free-standing silver nanoparticle films, the PS template was removed by washing with THF four times. In each THF washing step, the final product was kept in THF for 30 min.

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RESULTS AND DISCUSSION Simple immersion of a substrate into an FeIII−TA solution (pH ≈ 1.7) resulted in the spontaneous formation of FeIII−TA films on the substrate (Figure 1b). The substrate gradually darkened as the immersion time increased (Figure 1c), as seen by eye and monitoring of the characteristic ligand-to-metal charge transfer (LMCT) band of the FeIII−TA complexes at ∼565 nm22 (Figure 1d) by UV−visible spectroscopy. The intensity of the LMCT band increased as immersion time increased (Figure 1e), suggesting progressive deposition of metal−phenolic complexes on the substrate surface. Energydispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy analysis revealed the presence and distribution of FeIII in the films,30 indicating the formation of FeIII−TA complexes on the substrate surface (Figure S1) and corroborating the UV−visible spectroscopy results. The deposited material thickness increased up to 0.56 μm as the immersion time was increased to 89 h (Figure 1e, Figure S2), as monitored by atomic force microscopy (AFM). The kinetics of film growth observed by AFM was consistent with the increase in intensity of the LMCT band observed by UV− visible spectroscopy. These films are formed by the coordination bonding between FeIII and TA, which allows for rapid film disassembly in 1 M HCl solution (pH < 1) (Figure S3), similar to a previous report on metal−phenolic network films.25 In addition, the presence of TA molecules in the FeIII−TA film was probed by matrix-assisted laser desorption ionization−time-of-flight mass spectroscopy (Figure S4), which highlighted negligible chemical changes to TA even after 4 months of film growth. This indicates that any significant polymerization of the TA can be ruled out, providing further support that film growth occurs via complexation (and not polymerization). This assembly method is applicable to different metal ions and phenolic ligands: europium (EuIII), gallium (GaIII), and indium (InIII) and gallic acid (GA), pyrocatechol (PC), and pyrogallol (PG), respectively. Each system produced metal− phenolic materials on planar glass substrates (Figures S5 and S6), where the film thickness and surface wettability could be controlled by the immersion time (Figure S7) and choice of phenolic molecules (Figure S8), respectively. The surface wettability was largely independent of film thickness, as FeIII− TA films with different film thicknesses showed similar wettability (contact angle: 7−21°). When TA was replaced with different phenolic molecules, the film wettability C


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Figure 2. Experimental and theoretical data of FeIII−TA film formation. (a−d) Time-dependent growth of FeIII−TA complexes in solution. TEM images at each stage: 1 min (a), 2 h (b), 8 h (c), and 72 h (d). (e) Representative snapshots of hexagonal lattice-based random walk model of FeIII−TA film formation. Red species indicate “bound” complexes, and blue species indicate bulk complexes. The model demonstrates timedependent thickness evolution of the FeIII−TA film. Calculated (f, g) and experimental (h, i) data for the effects of concentration (ratio constant at TA/FeIII of 1:1.6) and TA/FeIII molar ratio (concentration constant at 5.9 mM TA) on film formation. The calculated data fit well with the experimental data, which showed that higher TA concentrations produced thicker films and that the thickest film could be achieved at a TA/FeIII ratio of 1:3. The experimental data in (i) were obtained for capsules prepared with a film deposition time of 30 min. The modeling results in (g) were obtained at the final modeled time in (f).

of substrate withdrawal rate (1.24−124 mm h−1) on film formation. When the withdrawal rate was increased from 1.24 to 124 mm h−1, the average film thickness decreased from 122 ± 22 to 3 ± 1 nm (Figure S10). This corroborated the OWLS data and confirmed that film growth strongly depends on the interaction time between the substrate and FeIII−TA complexes in solution. To better understand the film formation process, we modeled the formation of FeIII−TA films by a lattice-based random walk model. Monomers that represent FeIII and TA in solution were populated in random coordinates across the lattice. At each time step, these monomers moved in a random direction to one of their six nearest neighbor sites, where exclusion effects were imposed to prevent double site occupation. If a TA monomer interacted with the left boundary of the lattice, it was presumed to be bound to the surface, whereby it could no longer move. For two TA sites to coordinate to each other, representing film growth, at least one site between the two TA sites must contain FeIII. Our calculations revealed that TA molecules, or FeIII−TA complexes, first assemble into FeIII−TA layers on the left boundary of the lattice, where free components can further bind to the surface. This leads to a steady deposition of FeIII−

decreased. Specifically, the water contact angle of FeIII−GA, FeIII−PC, and FeIII−PG films were 39, 52, and 54°, respectively, possibly due to the lower capability of the films to form hydrogen bonds with water molecules. However, no metal ion effect was observed on the film wettability; the films showed comparable wettability (10−13°). These experiments highlight that the inherent modularity of metal−phenolic networks is maintained even when preparing thick films using the reported method. To examine the mechanistic details of film growth, we monitored the formation of metal−phenolic complexes in solution and on substrates by TEM and OWLS. Rod-shaped metal−phenolic patches emerged at t = 1 min, which grew further in both the longitudinal and transverse directions, and at t = 2 h tripled in size and thickness, and eventually attaining dense globular structures at t = 72 h, as observed by TEM (Figure 2a−d). OWLS measurements revealed differences in the deposition behavior as a function of flow rate of the FeIII− TA solution over the substrate, whereby a greater mass deposited at slower flow rates (Figure S9), consistent with thicker films obtained from longer immersion times. To further examine the relationship between the FeIII−TA complex− substrate interaction and film formation, we explored the effect D


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Figure 3. Effect of pH on FeIII−TA film formation. (a) pH-Dependent transition of dominant FeIII−TA complexation state. (b−g) AFM height images and corresponding height profiles of a scratched zone of FeIII−TA deposits/films prepared on a glass substrate at different pHs: 4 (d, g). The TA/FeIII molar ratio is constant (1:1.6). All data were obatained after immersion of the substrates in precomplexed FeIII−TA solution containing 5.9 mM TA and 9.3 mM FeIII for 20 h.

initial solution pH was approximately 2 after mixing FeIII and TA; this pH (transition region between the mono and bis coordination states) resulted in time-dependent film growth. No film was observed at pH < 1.7 (dominant monocomplex) and pH > 4 (dominant bis-complex), where “particle” and “patchy”-shaped metal−phenolic complexes were observed on the planar substrates, respectively (Figures 3 and S16). These results suggest that continuous film growth requires a critical coordination condition such as metastable mono- and biscomplexes to avoid termination of growth in solution (precipitation) and termination at the interface (completed film growth). Other factors, i.e., substrate type, solvent, and temperature, had little influence on the resulting film thickness (Figures S17−S19). Therefore, the concentration and molecular ratio of the components along with the solution pH and immersion time are important and inter-related factors for forming thick metal−phenolic films. The ability to engineer thick metal−phenolic films led us to investigate the formation of free-standing metal−phenolic materials (Figure 4). Free-standing macroscale materials were fabricated using templates coated with sacrificial polystyrene (PS) layers (Figure 4a). After the FeIII−TA films were grown on the PS-coated substrate, the sacrificial PS layer was

TA complexes on the substrate as a function of time until the consumption of the monomers or saturation of TA sites terminates film growth (Figure 2e). We also performed these calculations for systems with different concentrations and ratios of TA to FeIII. The calculated data fit well with the experimental data, showing that a higher TA concentration produced thicker films, and that the thickest film could be achieved at a TA:FeIII molar ratio of 1:3 (Figure 2f−i, Figures S11−S14). Together, these results suggest that the selfassembly of FeIII and TA on a substrate in a FeIII−TA solution is a diffusion-driven process, where the film grows thicker as a function of time and concentration of the molecular components. In addition, we explored various other parameters that influence the thickness and morphology of the metal− phenolic films, including higher concentrations of components (i.e., greater than 11.8 mM TA), pH, substrate type, temperature, and solvent (Section S4 in Supporting Information for details). When the TA concentration was greater than 29.4 mM (while keeping the TA/FeIII ratio constant (1:1.6)), poor-quality films consisting of large aggregates formed (Figure S15), suggesting that there is a critical maximum concentration for the formation of homogeneous metal−phenolic films. In terms of pH, the E


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Figure 4. Free-standing macroscopic FeIII−TA materials. (a) Schematic of the fabrication of macroscopic FeIII−TA materials using a sacrificial PS layer. (b−g) Digital photographs of the free-standing macroscopic FeIII−TA materials: square (b), triangle (c), 3D cup (d), unimelb (in cursive English) (e), love (in Hangul/Korean) (f), and heart (symbol) (g). The free-standing FeIII−TA materials were obtained after immersion of the PScoated substrates in precomplexed FeIII−TA solution containing 11.8 mM TA and 18.5 mM FeIII for 65 h and dissolution of the PS coating.

dissolved with THF, resulting in the FeIII−TA film “lifting” from the substrate. This PS-templating approach allowed for the fabrication of free-standing macroscopic (i.e., cm × cm) structures including squares, triangles, and 3D cups, demonstrating the robustness of the assembly method and that the metal−phenolic networks are well connected (Figure 4b−d). We further investigated the use of FeIII−TA solutions for forming films on substrates coated with PS designs using dippen lithography (Figure 4a); customized free-standing materials such as words and symbols were obtained (Figure 4e−g). Moreover, thick and hollow microcapsules and FeIII− TA replica particles were prepared by coating PS particles and sacrificial nanoporous calcium carbonate (CaCO3) templates in the FeIII−TA solution, followed by subsequent template removal (Figures S20−S22). The hollow microcapsules are stable in various organic solvents including acetonitrile, ethanol, dimethylformamide, and dimethyl sulfoxide (Figure S23). As the thickness of the FeIII−TA films could be controlled by immersion time, macroscopic free-standing FeIII−TA materials were engineered with thicknesses up to 5 μm (∼0.65 μm after 65 h, up to 5 μm after 162 h immersion,

Figures 1f and S24). The Young’s modulus (EY) of the macroscopic FeIII−TA materials was estimated to be 2.9 ± 1.0 GPa by AFM force measurements (Figure 5a,b); this EY value is ∼3 times greater than that of the thin FeIII−TA materials prepared previously (EY of 1.0 ± 0.2 GPa),25 where films are assembled by discrete deposition steps of TA and FeIII onto templates rather than deposition of precomplexed FeIII−TA onto templates in our current method. The robustness of the free-standing FeIII−TA materials enabled them to be physically manipulated and folded like origami and subsequently unfolded (Figure 5c). Additionally, FeIII−TA capsules are highly permeable to high molecular weight dextran (∼80% permeability to 2000 kDa fluorescein isothiocyanate-dextran) (Figure S25), whereas capsules prepared from the same components (FeIII and TA), however, by discrete deposition steps are impermeable to this dextran size.25 These results demonstrate that our strategy of using precomplexed FeIII−TA solutions generates metal−phenolic materials with different mechanical and permeability properties than the thinner metal−phenolic materials obtained by discrete assembly, thus broadening the potential use of metal−phenolic materials for various applications. F


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Figure 5. AFM analysis and shape manipulation of free-standing FeIII−TA materials. (a) Representative force−deformation curve for the small deformation regime of the FeIII−TA material. The indentation response of glass (black dashed line) was included as a control. There is hysteresis between the approach and retract curves, which is common to all force curves seen in this experiment, indicating that plastic deformation occurs during indentation. A minimal amount of adhesion is seen in this study. (b) Histogram of the Young’s modulus of the FeIII−TA material. The FeIII−TA material exhibits a Young’s modulus of 2.9 ± 1.0 GPa, as determined from analysis of 300 measurements. (c) Digital photographs showing the physical shape manipulation of FeIII−TA materials. The free standing FeIII−TA materials (c) of 1 μm thickness were obtained after immersion of the PS-coated substrates in precomplexed FeIII−TA solution containing 11.8 mM TA and 18.5 mM FeIII for 65 h and dissolution of the PS coating.

terials, thereby providing opportunities to engineer metal− phenolic materials with diverse chemical, optical, and physical properties.

The assembly of NPs into structures with precise shapes and sizes is important in fields such as photonics, metamaterials, and biotechnology.31,32 This inspired us to examine the application of our macroscopic metal−phenolic materials as a scaffold for forming free-standing structures of NPs. The macroscopic FeIII−TA materials were readily functionalized with silver nanoparticles (AgNPs) by dipping the macroscopic films into a AgI salt solution. The TA within the FeIII−TA materials could interact with AgI ions through metal−organic coordination and subsequently reduce the AgI ions to produce stabilized AgNPs on the macroscopic films (Figure 6a,b).33 TEM and EDX mapping confirmed that AgNPs with a size distribution of 34 ± 4 nm covered the surface of the metal− phenolic materials, thereby leading to a free-standing macroscopic AgNP film (Figures 6d−f, S26, and S27). The size of the NPs on the material surface could be controlled by varying the immersion time in the AgI salt solution (Figure S28). Additionally, free-standing palladium nanoparticle (PdNP) films with an average NP size of 2.4 ± 0.4 nm were fabricated by soaking the FeIII−TA materials in a PdII salt solution (Figures 6c and S29). These results are in agreement with our previous work on studying NP formation on metal−phenolic network-coated materials.33 The size, shape, uniformity, and heterogeneity of the NPs could potentially be controlled by other parameters including reaction time and temperature.34 These results indicate that the macroscopic metal−phenolic materials could readily be functionalized with other nanoma-

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CONCLUSIONS

In summary, we have demonstrated a simple and scalable method for preparing nano- to macroscopic materials through the molecular self-assembly of metals with natural phenolic ligands. The key feature of our strategy is the incubation of templates in precomplexed metal−phenolic solutions, prepared by mixing commercially available precursors at ambient synthesis conditions, resulting in a continuous, controllable growth of metal−phenolic materials. The experimental and theoretical studies suggest that this growth is a diffusion-driven process, with the material thickness determined by the immersion time and concentration of the molecular components. The simplicity and versatility of our method allows for the generation of free-standing macroscopic materials of customized architecture, size, composition, and thickness. The materials can be physically manipulated by folding like origami and used as scaffolds for generating functional, free-standing NP films. We anticipate that this molecular self-assembly method can be extended to a suite of other metals and phenolic ligands to generate a broad range of functional materials, paving the way for their potential application in diverse areas. G


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Figure 6. Surface modification of free-standing FeIII−TA materials. (a−c) Digital photographs of free-standing FeIII−TA-based NP films: AgNPs (a, b) and PdNPs (c). (d, e) TEM images of AgNP-functionalized macroscopic FeIII−TA materials. (f) Size histogram of AgNPs; a size distribution of 34 ± 4 nm was determined from analysis of 100 NPs. The NP-functionalized materials were obtained after immersion of 1 μm thick FeIII−TA films in metal salt solutions for 24 h.

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ASSOCIATED CONTENT

at The University of Melbourne and the Victorian Node of the Australian National Fabrication Facility (ANFF).

S Supporting Information *

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b02616. Experimental details and characterization data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Quinn A. Besford: 0000-0002-1779-9176 Frank Caruso: 0000-0002-0197-497X Author Contributions ‡

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank R. R. Dagastine, M. A. Rahim, M. Faria, A. J. Christofferson, Y. Ju, J. Li, and J. Guo for useful discussions. This research was conducted and funded by the Australian Research Council Centre of Excellence in Convergent BioNano Science and Technology (project number CE140100036) and by the Australian Research Council through the Discovery Project (DP170103331) scheme. F.C. acknowledges the award of a National Health and Medical Research Council Senior Principal Research Fellowship (APP1135806). This work was performed in part at the Materials Characterization and Fabrication Platform (MCFP) H


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