Nanoscale Polydopamine (PDA) Meets ÏâÏ Interactions: An Interface

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Nanoscale Polydopamine (PDA) Meets #-# Interactions: An InterfaceDirected Coassembly Approach for Mesoporous Nanoparticles Feng Chen, Yuxin Xing, Zhenqiang Wang, Xianying Zheng, Jixi Zhang, and Kaiyong Cai Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b03294 • Publication Date (Web): 02 Nov 2016 Downloaded from http://pubs.acs.org on November 8, 2016

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Nanoscale Polydopamine (PDA)

Meets

Π−Π

Interactions: An Interface-Directed Coassembly Approach for Mesoporous Nanoparticles Feng Chen, a Yuxin Xing, a Zhenqiang Wang, a Xianying Zheng, a Jixi Zhang,*, a Kaiyong Cai a a

Key Laboratory of Biorheological Science and Technology, Ministry of Education, College of

Bioengineering, Chongqing University, No. 174 Shazheng Road, Chongqing 400044, China. Email: [email protected]

ABSTRACT: Well known for the adhesive property, mussel-inspired polydopamine (PDA) has been shown to enhance performance in a wide range of adsorption-based applications. However, imparting porous nano-structures to PDA materials for enhanced loading capacities has not been demonstrated even when surfactants were present in the synthesis. Herein, we report on the preparation of mesoporous PDA particles (MPDA) based on the assembly of primary PDA particles and Pluronic F127 stabilized emulsion droplets on water/1,3,5-trimethylbenzene (TMB) interfaces. The key to the formation of this new type of the MPDA structure is the full utilization of the π–π stacking interactions between PDA structures and the π-electron rich TMB molecules. Remarkably, this method presents a facile approach for MPDA particles with an average diameter of ~90 nm, slit-like pores with a peak size of ~5.0 nm, as well as hollow cavities. When used as the adsorbent for a model dye RhB, the MPDA particles achieved an ultra-high RhB

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adsorption capacity of 1100 µg mg-1, which is significantly higher than that for the PDA-reactive dyes with Eschenmoser structure. Moreover, it was demonstrated that the cavity space in MPDA can facilitate high volumetric uptake in a capillary filling/stacking manner via the π–π interactions. These developments pave a new avenue on the mechanism and the designed synthesis of functional PDA materials by organic-organic composite assembly for advanced adsorption applications.

KEYWORDS:

mesoporous polydopamine,

π−π interactions, organic-organic self-

assembly, template synthesis, adsorption 1. INTRODUCTION Research extending from examinations of natural and biological phenomena can lead to the development of advanced materials with outstanding properties, and a greater understanding of the underlying physicochemical interactions. Mussel-inspired polydopamine (PDA) and its copolymers have attracted growing attention in the past decade because they combine biocompatibility and unique adhesive properties together.1,

2,

3

Benefiting from the

supramolecular architecture of three levels of structural organization, PDA has attracted strong interests for diverse applications including surface coating, adsorption, drug delivery, catalysis, etc.4,

5, 6

For example, material-chemistry-independent PDA coatings are facilitated by the

synergistic interactions of the ample amino and catechol groups in its structure with other organic/inorganic substrates.2, 3 PDA also allows control of the surface properties and confers new functionalities to the materials by immobilizing functional molecules.7, 8 Moreover, plentiful aromatic rings in PDA makes it possible to load chemical drugs or dyes on their surface via π−π stacking and/or hydrophobic-hydrophobic interactions.9, 10 For these applications, PDA structures


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with enlarged surface areas will greatly improve their performance in enhanced loading capacities. To date, PDA-functionalized hybrid nanomaterials have been developed as novel nanostructures. Representative examples include PDA-based composites with graphene,11 graphene oxide,8, 12 metal oxide nanoparticles,13, 14 silica,15, 16 metal particles,17 etc. Despite the success in preparing different nanocomposites, the great potential of adhesive PDA was limited by the simple morphologies (nanoscale spheres or sheets) of the hosting materials. It is of great significance and fundamental importance to prepare PDA nanomaterials by circumventing the dependence/assistance on the other materials incorporated with them. In the attempts to prepare all-PDA materials, micro-scale capsules and nano-scale nonporous particles were taken into considerations in the past, as evidenced by the related research efforts.9, 18, 19, 20, 21, 22 Colloidal PDA nanoparticles with porous morphologies which can impart PDA with high surface-tovolume ratio, however, have not yet been reported to our knowledge. In previous referable work on the preparation of mesoporous polymers (mainly resins),23, 24, 25, 26

the organic-organic self-assembly with the assistance of triblock copolymers has been proven

to be a reliable strategy.25, 26 In general, reasonably strong template-organic (polymer) interaction is required to generate porous polymer. This is challenging in PDA systems due to the typically highly cohesive property stemming from the π-electron rich backbones of units stacked along the z direction with a graphite-like stacking spacing.27 Nonporous PDA materials were found even when copolymer surfactants were used in the synthesis.28, 29 Fortunately, attention was paid to the utilization of the supramolecular nanostructure of PDA for the controllable construction of new PDA nanostructures in a few recent studies.30,

31, 32

It was demonstrated that small aromatic

molecules like folic acid could lead to the stacking of protomolecules of PDA by π-π interactions


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between PDA and the molecules.30, 31 Therefore, it is conceivable that the combination of the supramolecular nanostructure of PDA and π-π interactions in the organic-organic self-assembly might address the challenges mentioned above. Herein, we report a facile synthesis of mesoporous PDA nanoparticles (MPDA) in an aqueous tris(hydroxymethyl)aminomethane (TRIS) solution with triblock copolymer Pluronic F127 (EO106PO70EO106) and 1,3,5-trimethylbenzene (TMB) as the organic templates. Our emphasis was put on the contribution of π-electron rich TMB to the organic-organic self-assembly, as well as the morphology and the structures of the PDA particles. MPDA particles with an average diameter of ~90 nm and a peak pore size of 5 nm were synthesized with the aid of π-π interactions. Furthermore, the great potential of MPDA for loading guest molecules via π-π interactions was demonstrated by high adsorption capacities of organic dyes. 2. EXPERIMENGTAL SECTION 2.1 Materials. Unless otherwise noted, all reagent-grade chemicals were used as received, and distilled water was used for the preparation of all aqueous solutions. Ethanol (AR) and acetone (AR) were purchased from Fluka. Pluronic® F-127 was purchased from Sigma. Dopamine hydrochloride

(98%,

AR),

tris(hydroxymethyl)aminomethane

(TRIS,

99.9%),

1,3,5-

Trimethylbenzene (TMB, AR, 97%), hexane (AR), decane (AR) were purchased from Aladdin Industrial Inc. Rhodamine B (RhB, 99%)，methylene blue (MB，82%), coomassie brilliant blue (CBB), malachite green oxalate (MGO，BS) and neutral red (NR, BS, 98%) were purchased from Adamas.


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2.2 Synthesis of mesoporous polydopamine nanoparticles (MPDA). The MPDA particles were prepared by a one-pot synthesis method. In a typical experiment, 0.36 g of F127 and 0.36 g TMB were firstly dissolved in a mixture of H2O (65 mL) and ethanol (60 mL). After 30 minutes of stirring, a solution of 90 mg TRIS dissolved in 10 mL of H2O was introduced to the mixture, followed by the addition of 60 mg dopamine hydrochloride. The molar ratio used in the synthesis was 38577 EtOH: 156067 H2O: 28 TRIS: 1 F127: 112 TMB: 12 dopamine. The reaction mixture was stirred at room temperature for 24 h, then the product particles were separated by centrifugation. Ethanol and acetone were used to wash the centrifuged particles. The template removal was performed by extraction, where the samples were treated in a mixed solution of ethanol and acetone (2:1 v/v) with sonication (three times, 30 min every time). The final product was suspended in ethanol for further use. For comparison, additional samples were prepared without TMB, with the gradual increase of the TMB/F127 weight ratios from 0 to 1.5, or with different dopamine weights of 20 mg and 100 mg, while keeping the other synthesis parameters the same as MPDA. To investigate the formation kinetics of MPDA particles, a UV-Vis spectrophotometer (NanoDrop 2000c, Thermo) was first used to monitor the change in absorbance (400 nm) in the reaction solution at different time intervals. At each reaction time point, 20 µL of the reaction solution was taken and the absorbance was measured. Additionally, the reaction solution was also dipped on carbon-coated Cu grids for the investigation of morphology and structure changes by TEM. 2.3 Adsorption Experiments. For the adsorption kinetic study, 1 mg of MPDA and 2 mL of RhB solution (1 mg mL-1) was mixed by sonication. At different time intervals, the suspension was centrifuged and 3 µL of the supernatant was withdrawn for UV-vis determination of the


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adsorbed amount. For the determination of adsorption isotherm of rhodamine B (RhB), 1 mg particles was ultrasonically dispersed in 2 mL of the RhB aqueous solution with various concentrations (0.1-1 mg mL-1). The mixture was stirred at room temperature for 72 h and then centrifuged (11000 rpm, 5 min). The amount of dye loaded into PDA-MSNs was calculated by subtracting the mass of dye in the supernatant from the total mass in the initial solution. The concentrations of RhB in the solutions were analyzed with a UV-Vis Spectrophotometer (NanoDrop 2000c, Thermo). For the comparison of adsorption capacities for varying dyes, five typical dyes, namely rhodamine B (RhB), methylene blue (MB), methyl violet (MV), malachite green oxalate (MGO) and neutral red (NR) were chosen for the adsorption tests. The initial concentration of the dyes was fixed at a high value of 1 mg mL-1. 3. RESULTS AND DISCUSSION 3.1 Synthesis and characterizations of the MPDA particles. MPDA particles were prepared in an aqueous solution with F127 and TMB as the organic templates. Ethanol and TRIS was employed as the co-solvent and the catalyst for the polymerization of dopamine. The proposed mechanism shown in Figure 1 will be discussed later in the next section. Typical SEM images of the as-prepared MPDA particles at a molar ratio of 38577 EtOH: 156067 H2O: 28 TRIS: 1 F127: 112 TMB: 12 dopamine are displayed in Figure 2. The sample consisted of monodispersed spherical particles with an average diameter of ~90 nm (Figure 2a). Particles with a rough and bumpy surface were clearly observed at higher magnification (Figure 2b). A close examination of the surface topography suggests that there are irregular caves with diameters below 10 nm, implying the possible generation of porous structures in the particles.


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Figure 1. Schematic representation on the proposed mechanism for the preparation of MPDA (mesoporous PDA, a) and NPDA (nonporous mesoporous PDA, b) with and without the presence of TMB, respectively. The bonding environment of functional groups in the dried MPDA particles was investigated using X-ray photoelectron spectroscopy (XPS). The N/C ratio in MPDA is 0.12 based on XPS survey scan (Figure 2c). The C 1s high-resolution spectrum (Figure 2d) could be fitted into three components: CHx or C═C (284.8 eV), C–N (286.1 eV) and C═O (287.5 eV). The N 1s region (Figure 2e) was fitted with three components: primary (R-NH2, 401.8 eV), secondary (R1-NH-R2, 399.9 eV) and tertiary/aromatic (C═N-R, 398.6 eV) amine functionalities.33, 34, 35 The observed amines stem from the supramolecular structure of polydopamine consisting of dopamine and tautomeric species of the intermediate species 5,6-dihydroxyindole and 5,6indolequinone.6, 36, 37 The Raman spectrum of MPDA (Figure S1 in the Supporting Information)


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shows broad and strong peaks at 1000–1800 cm–1. The peak centered at 1580 cm–1 can be assigned to ν(C=C) aromatic coupled with pyrrole ring stretching vibration or indole ring vibration. Another peak at 1350 cm–1 is due to the aromatic ν(C–N) stretching mixed with indole ring stretching.38, 39

Figure 2. SEM images (a, b), XPS survey (c), as well as the high resolution scans of C 1s (d) and N 1s peak (d) of the XPS spectrum for the as-prepared MPDA particles.


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Transmission electron microscopy (TEM) images of PDA particles synthesized with varying conditions are shown in Figure 3. The effect of TMB on facilitating the generation of MPDA particles can be clearly seen by the increasingly visible pore structures in the particles, through the gradual increase of the TMB/F127 weight ratios from 0 to 1.5 (Figure 3 a-d). The synthesis with no or less TMB (0.5 TMB/F127) generated nonporous particles with rough surfaces. This indicates that F127 itself cannot facilitate the formation of mesoporous PDA by organic-organic self-assembly, possibly due to the weak interactions between them.28,

40

However, higher

TMB/F127 weight ratios at 1 and 1.5 led to the formation of MPDA with irregular pore structures resembling slit-like or spherical pores with pore diameters below 20 nm. The average pore wall thickness is around 25 nm in both cases. Moreover, a clear trend of reduction in the particle diameters (from 300 nm to 250 nm, 90 nm, and 100 nm) was observed by raising the ratio step by step, indicating another interesting effect of TMB addition on controlling the particle size. Although the utilization of the highest TMB amount can result in larger mesopores in the range of several tens of nanometers (Figure 3d), severe particle interconnections also took place. Moreover, some particles possess an opened hollow shape where mesoporous structures located around spherical cavities (Figure S2 in the Supporting Information).


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Figure 3. Representative TEM images of PDA particles prepared with varying TMB/F127 weight ratios of 0 (a), 0.5 (b), 1 (c), 1.5 (d), as well as particles obtained by changing dopamine weight from 60 mg to 20 mg (e) and 100 mg (f) in the synthesis solutions. Inset of c shows a magnified TEM image of an individual MPDA particle. The arrows in d show the opened hollow MPDA particles. The effect of dopamine amount on the particle structures was shown in Figure 3 e and f. Particles were prepared with the fixed TMB/F127 weight ratio at 1. Interestingly, neither decrease (from 60 mg to 20 mg) nor increase (from 60 mg to 100 mg) in the dopamine amount could influence the particle sizes, as evidenced by the average diameters in the range of 90-100 nm. The obvious differences in the porous structures are pore enlargement (to an average size of


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~25 nm) at the lower dopamine amount or pore diminishing in the case of the higher dopamine amount. The results above apparently indicate that only the joint incorporation of F127 and TMB at the proper weight ratios can lead to the formation of MPDA particles. The phenomenon of the size restriction will be discussed later in the section of particle formation mechanisms. Next, nitrogen sorption was used to determine the textual properties of MPDA particles. As shown in Figure 4, PDA particles in the absence of TMB show a type II isotherm typical of nonporous solids. The nonporous structure is in consistent with the TEM results above. In comparison, the nitrogen adsorption–desorption plot of MPDA shows a type-IV isotherm according to the IUPAC nomenclature. The MPDA particles exhibit a specific surface area of 45 m2 g-1 which is significantly larger than its nonporous counterpart (NPDA) synthesized in the absence of TMB, as well as a total pore volumes of 0.17 cm3 g-1. A wide capillary condensation step with a type-H4 hysteresis loop occur in the P/ P0 range of 0.4-0.8, corresponding to a relatively broad pore size distribution in the range of 3-30 nm (Figure 4b). The broad distribution and a peak at 5.0 nm should be attributed to the irregular mesopores observed in the TEM images of MPDA. Type H4 loops feature parallel and low-slope branches and their occurrence has been generally attributed to adsorption−desorption in narrow slit-like pores.41 However, hollow spheres with walls composed of mesoporous structures also exhibited hysteresis behavior of the H4 type.42, 43 Interestingly, the hysteresis loop did no close at high relative pressures. Moreover, a steep type-H3 loop, which does not exhibit any limiting adsorption, can be observed in the P/ P0 range of 0.9-1.0. These results imply that there might be hollow cavities included in MPDA particles. Hence, another pore size distribution peak at 32 nm in the range of 25-50 nm corresponds to these hollow cavities. The actual pore volumes of the


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MPDA should be even higher because nitrogen adsorption-desorption measurements cannot completely probe the volumes of the interiors in hollow nano-structures.33

Figure 4. Nitrogen sorption isotherm (a) and the corresponding pore size distribution (b) deduced from the desorption branch of the isotherm for MPDA particles. Nonporous PDA particles (NPDA) prepared without the introduction of TMB was used as a reference. 3.2 Particle formation mechanisms: the effect of π-π stacking. To understand the mechanisms for generating the MPDA structures, particle formation kinetics were studied by insitu monitoring the evolution of the absorption spectra of the reaction solution as a function of reaction time (Figure 5a). According to the previous studies, the self-polymerization of dopamine under basic conditions leads to a pronounced adsorption in the visible wavelength range (400−700 nm) and its intensity increases with time.5 It can be seen from the spectra


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evolution that the general absorption increases over a wide range of wavelengths including the instinct peak of catechols at 280 nm, owing to the formation of colloidal polydopamine.44 The intensity increase in the absorption at 400 nm with time was generally utilized to evaluate the polymerization rate of dopamine.14, 45 Figure 5b indicates that the absorbance increased slightly in the first 3-4 h, for the reaction solutions of MPDA and NPDA. After an inflection point at 4 h, the absorbance started to increase significantly in both cases. Subsequently, the most remarkable discrepancy for the profiles was emerged in the following 7 h. While the increasing profile for NPDA consisted of a fast stage up to 6 h and a slow following step, the curve for MPDA showed a slow step first and an inflection to a steep increase at 8 h. The retarded increase in the case of MPDA indicates that there might be some interaction factors introduced at the inflection point by the presence of TMB in the synthesis. Subsequently, the evolution of hydrodynamic diameters of the reaction solution was monitored in-situ. As shown in Figure 5c, the starting solution consisted of 10 nm-sized micelles and emulsion droplets with a peak size of 200 nm. After the addition of dopamine, the sizes of emulsion droplets decreased gradually to ~110 nm in the first 4 h. The micelle peak at 10 nm was replaced by two smaller ones at 3 h, which should be resulted from the generation of nucleated PDA nanoparticles.27 The single distribution peak at 4 h was further shifted to 60 nm at 4.5 h, accompanied by a quite small PDI of 0.06 and a following process of shifting to large sizes. The surfactant used in our synthesis, i.e. F127, has both the hydrophilic poly(ethylene oxide) (PEO) chains and hydrophobic poly(propylene oxide) (PPO) chains, which drives the formation of micelles with PPO as the core and PEO chains as the corona in aqueous solutions. The hydrophobic TMB can diffuse into the hydrophobic core of the surfactant micelles, which could result in pore expansion. At the high TMB amounts used in our study, a large amount of


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TMB molecules diffuse into the hydrophobic core of the surfactant micelles can form an F127 stabilized O/W emulsion.46 The results above imply that the TMB rich emulsion droplets might account for the assembly/interaction in the reaction solution of MPDA.

Figure 5. (a) UV−vis spectra of the reaction solution during the formation of MPDA. (b) Temporal evolution of the absorbance at 400 nm, for the particles synthesized with or without the presence of TMB (NPDA and MPDA). (c) Time resolved dynamic light scattering (DLS) monitoring the evolution of hydrodynamic diameter distributions of the reaction suspension at different reaction times. The XRD spectrum of MPDA (Figure 6a) shows a broad peak centered at 2θ = 22°, corresponding to a d-spacing of approximately 4 Å. Such spacing was consistent with the πstacked structures in polydopamine.27, 30, 47 It is generally accepted that the dopamine oligomers assemble in an orderly manner through π-π stacking to form a supramolecular architecture of three levels of structural organization from nano-aggregates of approximately 2 nm.27,

48, 49


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Besides, the initial supramolecular structures made up of four to five planar sheets of four-toeight 5,6-dihydroxyindole units each stacked along the z direction with a graphite-like stacking. The planar sheets, which have abundant aromatic π-electrons, are favorable for the interactions toward π-electron rich molecules.30, 31 PDA has an isoelectric point close to 4, as well as two average pKa values of 6.0 ± 0.3 (quinone-imine groups) and 9.0 ± 0.3 (catechol groups), respectively.50 At the synthesis pH of 8.5 in our study, the deprotonation of the functional groups of PDA took place to large degrees, which may induce the decrease in the solubility of primary particles and the increase in PDA’s inherent affinity toward interfaces.51 Hence, it is highly possible that they would interact with TMB rich emulsion droplets at a certain degree of organization level. The evolution of particle morphologies and structures was investigated by TEM images of the reaction suspensions at different time periods. As shown in Figure 6b-f, PDA nanoparticles with diameters in the range of 2-5 nm were observed for the product after 2 h of reaction, supportive of the small peaks in the hydrodynamic diameter distributions. They seemed to interconnect with each other to form larger ones in the case of 4 h. No emulsion droplets could be observed by TEM due to the evaporation of TEM during the drying process for TEM sampling. After 8 h of reaction, the primary PDA nanoparticles grew to be as large as 20-30 nm. Surprisingly, larger spherical particles with sizes of 50-60 nm can be clearly seen at the same time, indicative of MPDA formation. The proportion of MPDA particles kept increasing afterwards (8-11 h), accompanied by the further enlargement in particle size. At the final stage, all primary particles were diminished, and the sizes of MPDA reached around ~90 nm which is in line with the SEM and TEM results. By combining the results from time resolved DLS and TEM, a conclusion can


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be drawn that nano-aggregates of PDA with certain sizes interacted with emulsion droplets at a large scale.

Figure 6. (a) XRD pattern of the prepared MPDA sample; TEM of MPDA synthesis solutions captured at different reaction times from 2 h (b) to 4 h (c), 8 h (d), 11 h (e), and 24 h (f). To understand the nature of the interaction between primary PDA particles and TMB in the beginning stage of self-assembly, pristine PDA nanoparticles were synthesized from the standard recipe of MPDA except for the removal of F127 and TMB from the reactants. After a reaction time of 4 h, the product particles (Figure 7a, Figure S3 in the Supporting Information) were retrieved by centrifugation and added to a TMB/water biphasic system. As shown, the PDA nanoparticles were distributed into both phases, as evidenced by the black color in the upper


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TMB phase and the dark grey color in the lower water phase. Moreover, the interface in the water/TMB mixture system is indistinct. An optical microscope image (Figure 7b) of the water phase shows the existence of 4-40 µm-sized emulsion droplets. The zeta potential value of the emulsion droplets (-23 mV, Figure S4 in the Supporting Information) was much more negative than that of the F127/TMB emulsion (-2 mV), supportive of the attachment of PDA nanoparticles on the oil/water interface. No emulsion formation was observed when TMB was replaced by aliphatic hydrocarbons (hexane and decane). Besides, the PDA particles were all distributed in the water phase.

Figure 7. Digital photographs of mixtures showing the suspension of PDA nanoparticles in varying solvents (a): the 4 h-reaction solution of PDA synthesized by the same reaction solution of MPDA except for the absence of TMB and F127 (A), a TMB/water biphasic mixture (B), the biphasic mixture with the addition of PDA nanoparticles from A (C), a hexane/water biphasic


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mixture with the addition of PDA nanoparticles (D), and a decane/water biphasic mixture with the addition of PDA nanoparticles (E). Optical microscope image of the lower water phase from mixture C (b). All these results above suggest that the primary PDA nanoparticles in the MPDA synthesis have high affinity toward π-electron rich TMB for the emulsion-templated self-assembly in our synthesis. Nanoparticles could be used to effectively stabilize emulsions (Pickering emulsion) by adsorbing at the interface between the continuous and dispersed phases.52 Consequently, the adsorbed PDA nanoparticles, together with the stabilizing agent F127, form a relatively more stable layer that protected TMB emulsions against droplet coalescence. This was supported by the reduction in hydrodynamic diameters at 8 h of reaction in MPDA system, as well as the particle size restriction in the synthesis with the presence of TMB (Figure 3). By interacting with TMB emulsion droplets, the accessibility of newly-generated PDA oligomers to the primary particles was restricted due to the interfacial location of PDA in the co-stabilized droplets. This led to the retarded polymerization of dopamine in the MPDA synthesis (Figure 5b). Notably, the effectiveness of nanoparticles in stabilizing emulsions depended on particle size, particle shape, particle concentration, particle wettability and the interactions between particles. Obviously, larger particles having a larger area contacting oil and water show up a larger adsorption free energy.

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This may explain why the self-assembly of primary PDA particles took place after

certain time of reaction period in MPDA synthesis. Furthermore, the PDA particles adsorbed with TMB may penetrate into the interior of the TMB emulsion, as some of the PDA particles could disperse in TMB (C of Figure 7a). Subsequent particle organization and growth via the ππ stacking interactions led to the formation of the MPDA structure (Figure 3a). In comparison, there was no TMB in the system for the NPDA particles, which in turn resulted in the absence of


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the orgnic-organic self-assembly (Figure 3b). Consequently, the particle formation follows a typical nanoparticle’s nucleation-growth pattern with no retarded increase in the absorbance of polydopamine (Figure 5b). In light of the droplet-templating mechanism, the high TMB/F127 weight ratio of 1.5 (Figure 3d) would weaken the sterical-stabilization effect of the primary particles on the emulsion droplets. Firstly, this may lead to the gradual escape of TMB during the continuous solidification and shrinkage of the PDA network on the interface via PDA polymerization,53 which in turn generated opened hollow cavities (Figure 3d). Second, consequent flocculation of the droplets would happen,54, 55 as evidenced by the interparticle connections in Figure 3d. Moreover, the number of the composite droplets was restricted at the fixed TMB/F127 ratio, as evidenced by the fact that the dopamine amount could not influence the particle diameters (Figure 3e, f). It is thus assumed that only a small amount of primary PDA particles were required to generate the F127/PDA co-stabilized TMB droplets. As a consequence, the change in dopamine amount from 20-100 mg can only lead to the discrepancy in the number of PDA particles self-assembled on the droplets, the packing density of these particles on the oil-water interface, as well as the final porosity of the MPDA particles (Figure 3e, f). To further confirm the π-π stacking interaction mechanism and its unique role in our synthesis, we conducted four control experiments. When keeping the other synthesis parameters the same as MPDA, 40 mL and 80 mL ethanol was employed in control experiment I and II, while hexane and decane was used to replace TMB (with the same weight of 0.36 g) in control experiment III and IV. As a typical low-polarity solvent and a radical-trapping agent, ethanol is found to significantly slow down the polymerization rate of dopamine and make the PDA formation more controllable.14 The utilization of less ethanol led to a poorly-defined coral-like morphology and


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pore structure (Figure S5 in the Supporting Information), possibly due to the uncontrolled PDA formation and self-assembly toward TMB. Moreover, it is anticipated that the employment of a higher amount of ethanol would result in the reduction in the solution polarity and in turn the weakening of the π-π stacking. In all the other three experiments where the π-π stacking interaction is destroyed, nonporous PDA particles with diameters between 200 nm and 250 nm were obtained as evidenced by TEM (Figure S5 in the Supporting Information), suggesting that the interaction is crucial in the preparation of MPDA. 3.3 Dye adsorption on MPDA via π-π stacking. To demonstrate the application potential of MPDA, rhodamine B (RhB) was used as a model dye to investigate its adsorption onto MPDA particles. The adsorption kinetics of RhB was examined by using a RhB concentration of 1 mg mL-1 in water, and the supernatant solutions were collected at different time intervals up to 72 h. As shown in Figure 8a and Figure S6, the RhB loading amount for MPDA underwent a remarkably fast increase to 850 µg mg-1 with the initial 7 h, and then this increment slowed down sharply. After 48 h, the changes of the RhB concentration in the supernatant became negligible, suggesting the dye adsorption had reached an equilibrium (~1000 µg mg-1). In order to examine the controlling mechanism of adsorption processes, several kinetic models including the pseudofirst-order, the pseudo-second-order,11 and the intraparticle diffusion (Webber and Morris) model56 were used to fit the kinetic curve are used to test experimental data. As shown in Figure S7 and Table S1, the Webber and Morris kinetic model provides a better correlation in contrast to the pseudo-first-order and the pseudo-second-order model for adsorption of RhB on MPDA, implying that the intraparticle diffusion stemming from the porous structure of MPDA, is the rate-controlling step.


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Figure 8. Adsorption behaviors of RhB on MPDA particles: (a) adsorption kinetic plot prepared with 1 mg of MPDA and 2 mL of RhB solution (1 mg mL-1); (b) adsorption isotherm; (c) TEM image of RhB adsorbed MPDA; (d) comparison of UV-vis absorption spectra for a solution of RhB (1 mg mL-1, in water with 10 vol% acetone) before and after incubation with MPDA (1 mg); (e) comparison of MPDA’s loading capacities toward varying dyes: malachite green oxalate (MGO), neutral red (NR), methyl violet (MV), methylene blue (MB), and RhB. (f) Effects of initial pH on the adsorption of RhB onto MPDA. RhB adsorption isotherm of MPDA is shown in Figure 8b. The adsorption amount of MPDA progressively increased with increasing concentrations of RhB, and finally reached the saturation


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state. The loading capacity of MPDA was as high as 1100 µg mg-1, which is more than ten times higher than previous achievements of PDA materials for RhB adsorption.11 It is of great significance to get further information on the occupation of the pore space by RhB adsorption. An estimation of the mesoporous volume in MPDA is correlated to the volume that the adsorbed amount of RhB molecules can occupy. Then this amount is compared with the adsorbed amount of RhB after its adsorption. Considering the density of RhB (1.3 g cm−3), 1 mg of MPDA sample, having a free mesoporous volume of 0.17 cm3 g−1, can incorporate a maximum RhB amount of 0.22 g (only 20% of the actual loading capacity of RhB). Therefore, it is proposed that the adsorption might involve the RhB filling of the interior reservoir space of MPDA up to the total cavity volume, which can not detected by nitrogen sorption. TEM image of the RhB loaded sample (Figure 8c) shows a clear increase in the contrast between the interior space (~ 30 nm in size) and the porous PDA exterior region. The mechanism for RhB diffusing/transporting into the interior of MPDA is likely related to the hydrophobic adhesion, the π-π stacking interaction,9, 19 and the capillary uptake phenomena arising from the hollow porous structuring of MPDA.33 To prove this, acetone (10 vol%) was used as a π-π stacking breaking solvent in the adsorption medium.57, 58 The absorption spectrum of the RhB solution did not change noticeably after the incubation with MPDA (Figure 8d), indicating the absence of the RhB adsorption. Further evidence is from the adsorption of various dyes including malachite green oxalate (MGO), neutral red (NR), methyl violet (MV), and methylene blue (MB). These dyes contain an Eschenmoser structure (Figure S8) which was found to assist 1,4-Michael addition reaction toward PDA.8 However, the adsorption capacity values in the range of 100-200 µg mg-1 (Figure 8e) are much lower than that for RhB, indicating the disruption of close dye packing in the porous space of MPDA by the reaction. Besides, the


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nonporous counterpart (NPDA) possesses lower adsorption capacities for all dyes investigated, further supporting the great potential of MPDA in adsorption. The evaluation on the adsorption of RhB at varying pHs was also carried out. The obtained data was shown in Figure 8f. The adsorption amount decreased significantly from 1000 µg mg-1 to 500~860 µg mg-1 at acidic pH values ranging from 2 to 4, while there was no remarkable difference for the adsorption amount at higher pHs (5.5, 7.2, 8.5). The point of zero charge (PZC) for PDA materials is reported to be in the range of 3-4.59, 60 When the solution pH is below 4, the MPDA particles gradually acquire a positive surface charge due to the protonation of the amino groups. The consequent electrostatic repulsion between the positively charged active sites on MPDA surfaces and the cationic dye RhB led to a decrease in the adsorption capacity. However, when the solution pH value is from 2 to 4, the adsorption capacity can still reach 500 µg mg-1. The reason may be attributed to the π–π stacking interactions between MPDA and RhB. 4. CONCLUSION In summary, we have successfully developed a type of mesoporous polydopamine nanoparticles (MPDA) by combining the molecular nature of PDA with organic-organic self-assembly via the π-π stacking interactions. The 90 nm-sized particles possess a broad pore size distribution with a peak at 5.0 nm, as well as a hollow cavity with an average size of ~30 nm. A new understanding of the assembly behavior of TMB emulsion/F127 composites elucidates the central role of the TMB-rich emulsion droplets as the template for the coassembly of primary PDA particles. Demonstrated by time-resolved TEM and DLS, the primary PDA particles with certain size (~20-30 nm) could co-stabilize the emulsion and diffuse into the TMB phase. The subsequent


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packing and interconnections of the PDA particles resulted in the formation of slit-like mesopores in our MPDA structure. Our MPDA particles are used to promote the ultra-high level adsorption of a model dye, i.e. RhB (~1100 µg mg-1), via π-π interactions. These findings redefine the potential of PDA materials as new high performance adsorbents in the form of mesoporous particles for advanced applications. Furthermore, our new mechanistic understanding of the π-π stacking mediated assembly can also be applied to the synthesis of the wider family of PDA based materials. Supporting Information. Particle characterization methods, Raman spectra of MPDA particles, TEM images of MPDA particles prepared with other conditions, zeta potential distributions of the emulsion droplets, time resolved UV-vis absorption spectra in the adsorption experiments, kinetic fitting of the adsorption curve, as well as the molecular structures of the involved organic dyes. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *J. Zhang. Tel.: +86 2365102507. E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported in part by the National Natural Science Foundation of China (NSFC, Grant No. 51502027, 21274169), Basic Advanced Research Project of Chongqing (Grant No. cstc2015jcyjA10051), and 100 Talents Program of Chongqing University (J. Z.). National


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Engineering Research Center for Nanotechnology (Shanghai) is greatly acknowledged for the help for TEM characterization. REFERENCES (1) Mrówczyński, R.; Markiewicz, R.; Liebscher, J. Chemistry of Polydopamine Analogues. Polym. Int. 2016, 7, 1529-1544. (2) Lim, C.; Huang, J.; Kim, S.; Lee, H.; Zeng, H.; Hwang, D. S. Nanomechanics of Poly(Catecholamine) Coatings in Aqueous Solutions. Angew. Chem., Int. Ed. 2016, 55, 3342-3346. (3) Maier, G. P.; Rapp, M. V.; Waite, J. H.; Israelachvili, J. N.; Butler, A. Adaptive Synergy between Catechol and Lysine Promotes Wet Adhesion by Surface Salt Displacement. Science 2015, 349, 628-632. (4) Liu, Y.; Ai, K.; Lu, L. Polydopamine and Its Derivative Materials: Synthesis and Promising Applications in Energy, Environmental, and Biomedical Fields. Chem. Rev. 2014, 114, 5057-5115. (5) Dreyer, D. R.; Miller, D. J.; Freeman, B. D.; Paul, D. R.; Bielawski, C. W. Perspectives on Poly(Dopamine). Chem. Sci. 2013, 4, 3796-3802. (6) Yang, J.; Cohen Stuart, M. A.; Kamperman, M. Jack of All Trades: Versatile Catechol Crosslinking Mechanisms. Chem. Soc. Rev. 2014, 43, 8271-8298.


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Table of Contents Graphic and Synopsis


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Figure 1. Schematic representation on the proposed mechanism for the preparation of MPDA (mesoporous PDA, a) and NPDA (nonporous mesoporous PDA, b) with and without the presence of TMB, respectively. 48x34mm (300 x 300 DPI)


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Figure 2. SEM images (a, b), XPS survey (c), as well as the high resolution scans of C 1s (d) and N 1s peak (d) of the XPS spectrum for the as-prepared MPDA particles. 63x74mm (300 x 300 DPI)


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Figure 3. Representative TEM images of PDA particles prepared with varying TMB/F127 weight ratios of 0 (a), 0.5 (b), 1 (c), 1.5 (d), as well as particles obtained by changing dopamine weight from 60 mg to 20 mg (e) and 100 mg (f) in the synthesis solutions. Inset of c shows a magnified TEM image of an individual MPDA particle. The arrows in d show the opened hollow MPDA particles. 127x192mm (300 x 300 DPI)


Langmuir

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Figure 4. Nitrogen sorption isotherm (a) and the corresponding pore size distribution (b) deduced from the desorption branch of the isotherm for MPDA particles. Nonporous PDA particles (NPDA) prepared without the introduction of TMB was used as a reference. 78x116mm (300 x 300 DPI)


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Langmuir

Figure 5. (a) UV−vis spectra of the reaction solution during the formation of MPDA. (b) Temporal evolution of the absorbance at 400 nm, for the particles synthesized with or without the presence of TMB (NPDA and MPDA). (c) Time resolved dynamic light scattering (DLS) monitoring the evolution of hydrodynamic diameter distributions of the reaction suspension at different reaction times. 55x42mm (300 x 300 DPI)


Langmuir

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Figure 6. (a) XRD pattern of the prepared MPDA sample; TEM of MPDA synthesis solutions captured at different reaction times from 2 h (b) to 4 h (c), 8 h (d), 11 h (e), and 24 h (f). 64x93mm (300 x 300 DPI)


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Langmuir

Figure 7. Digital photographs of mixtures showing the suspension of PDA nanoparticles in varying solvents (a): the 4 h-reaction solution of PDA synthesized by the same reaction solution of MPDA except for the absence of TMB and F127 (A), a TMB/water biphasic mixture (B), the biphasic mixture with the addition of PDA nanoparticles from A (C), a hexane/water biphasic mixture with the addition of PDA nanoparticles (D), and a decane/water biphasic mixture with the addition of PDA nanoparticles (E). Optical microscope image of the lower water phase from the mixture C (b). 65x75mm (300 x 300 DPI)


Langmuir

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Figure 8. Adsorption behaviors of RhB on MPDA particles: (a) adsorption kinetic plot prepared with 1 mg of MPDA and 2 mL of RhB solution (1 mg mL-1); (b) adsorption isotherm; (c) TEM image of RhB adsorbed MPDA; (d) comparison of UV-vis absorption spectra for a solution of RhB (1 mg mL-1, in water with 10 vol% acetone) before and after incubation with MPDA (1 mg); (e) comparison of MPDA’s loading capacities toward varying dyes: malachite green oxalate (MGO), neutral red (NR), methyl violet (MV), methylene blue (MB), and RhB. (f) Effects of initial pH on the adsorption of RhB onto MPDA. 73x84mm (300 x 300 DPI)


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Nanoscale Polydopamine (PDA) Meets ÏâÏ Interactions: An Interface

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