Submicrometer-Encapsulation of NaBH4 by Dopamine End

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Submicrometer-Encapsulation of NaBH4 by Dopamine EndFunctionalized Polystyrene: Gas Generation at Oil-Water Interfaces Mohammad Vatankhah-Varnoosfaderani, Matthew H. Everhart, Aleksandr Zhushma, Maria Ina, and Sergei S Sheiko Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b04624 • Publication Date (Web): 08 Apr 2016 Downloaded from http://pubs.acs.org on April 11, 2016

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Chemistry of Materials

Submicrometer-Encapsulation of NaBH4 by Dopamine EndFunctionalized Polystyrene: Gas Generation at Oil-Water Interfaces Mohammad Vatankhah-Varnoosfaderani, Matthew H. Everhart, Aleksandr Zhushma, Maria Ina, Sergei S. Sheiko* Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill 27599-3290, USA ABSTRACT: We present a single-step, grafting-to synthetic method for the encapsulation of particulate NaBH4 by dopamine endfunctionalized polymer chains. Metal-catechol coordination chemistry is used to produce core-shell capsules, which generate H2 gas exclusively upon adsorption to an oil-water interface. Significantly, the synthetic process enables facile control of core diameter, shell thickness, and the chemistry of both shell and core. The interfacial reactivity of these stimuli-responsive capsules may be engineered for various applications such as medical diagnostics, therapeutics, and subsurface imaging. In addition to their triggered reactivity, the capsules react in a manner independent of pressure – and are thus well-suited for high pressure subsurface environments.

1. INTRODUCTION Development of submicrometer acoustic agents with targeted reactivity is vital to advancing the application of acoustic tools in the fields of medical diagnostics, therapeutics, and subsurface imaging.1-7 Specifically, miniaturization and selectivity of contrast agents will augment both spatial and material resolution. Of course, it is also desirable to augment signal strength in the imaging contexts. From this perspective, gaseous agents are most suitable given their distinct acoustic impedance. Controlling the action of bare gas, however, is a difficult proposition. We report stimuli-responsive submicrometer capsules that generate gas upon adsorption to an oil-water interface – thus achieving both targeting and miniaturization, while also leveraging the benefits of bare gas via in-situ production. In addition to acoustic applications, the chemistry developed herein bears relevance to self-healing materials,8,9 nutrient preservation,10,11 fragrance release,12-15 and drug delivery.16-19 Of the techniques developed over the past decade to produce smart capsules for these applications,20-42 the synthetic process of grafting-to stands out due to its single-step and low-cost nature. To

exemplify this grafting-to advantage, we have developed encapsulation chemistry driven by complexation of metal atoms in the capsule cores with catechol groups of the dopamine end-functionalized capsule shells. This metal-catechol complexation is viable with a wide range of metals including Fe, Ti, B, and Al.43-48 Thus, the synthetic platform is highly modular with respect to the core and so extends well beyond the gas-generating hydrides (NaBH4, NaAlH4, LiBH4, etc.) relevant for contrast agent applications. Additionally, the metal-catechol handle enables manipulation of the physicochemical properties of the shell, via control of both shell thickness and chemical composition. This allows for tuning permeability of fluids through the shells and subsequent gas generation triggered at specific solvent-solvent interfaces. 2. EXPERIMENTAL SECTION 2.1. Materials Sodium borohydride powder, PMDETA, CuBr (99%), 2bromoisobutyryl bromide (BIBB, 98%), 2,2’-bipyridyl (bpy, 99%), dopamine hydrochloride, imidazole (98%), SDBS, and dry solvents were purchased from Sigma-Aldrich and used as received. Tri-

NaBH4

NaBH4

Figure 1. Synthetic route toward submicrometer NaBH4 capsules. (a) Dopamine chloride is protected and then linked to 2-bromo-2methylpropionyl bromide (BIBB) to yield protected DOPA initiator (1). (b) ATRP of styrene using protected DOPA initiator and deprotection to yield DOPA polystyrene (2). (c) Bare NaBH4 cores (3) in THF chelate with (2) to yield submicrometer NaBH4 capsules (4), followed by dispersion in decane and precipitation under reduced pressure.

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Chemistry of Materials ethylsilyl chloride (TESCl, 97%) and tetra-n-butylammonium bromide (TBAB) were purchased from Gelest Inc. and used as received. SB115-4 pH10 buffer was purchased from Fisher Scientific and used as received. Styrene monomer (98%) was passed through activated, basic aluminum oxide (Brockmann I) before polymerization. 2.2. Synthesis 2.2.1. Protected DOPA Initiator (1) A modified procedure from Li et al. was followed.49 TESCl (50.0 mmol) was added via dropping funnel into a solution consisting of dopamine hydrochloride (24.0 mmol) and imidazole (50.0 mmol) in dry purified CH2Cl2 (150 mL), with stirring for 4 hrs at room temperature. The resulting solution was cooled in an ice bath and BIBB (46.0 mmol diluted with CH2Cl2) was added dropwise into the reaction mixture at 0 °C using a syringe pump. The pH of the solution was maintained at 6-7 with addition of imidazole (128.0 mmol). The reaction mixture was then stirred for 12 hrs at room temperature under anhydrous conditions. Next, the solvent was evaporated under reduced pressure yielding a yellowish liquid. The crude product was further purified by silica gel column chromatography (6% ethyl acetate in hexane) to yield (1), a colorless liquid (16.7 g, 31.5 mmol, 74.7% yield). 1H-NMR (400 MHz, CDCl3): δ 6.84-6.64 (aromatic, 3H), 3.50 (q, 2H), 2.75 (t, 2H), 1.94 (s, 6H), 0.95 (t, 18H), 0.53 (q, 12h). (Supporting Information Figure S1). 2.2.2. DOPA Polystyrene (2) Polymerization of styrene monomer with (1) was carried out in a round-bottom flask under nitrogen using various molar feed ratios

of styrene monomer, (1), CuBr, and PMDETA (e.g., 43.6 : 0.05 : 0.436 : 0.436 mmol respectively). Chain length was controlled by varying the ratio between monomer and (1) so as to achieve chains with DPs of 25, 50, and 100. Each given feed was mixed in anisole (5 mL) and degassed with three freeze-pump-thaw cycles. After degassing, CuBr was added under nitrogen and polymerization was initiated by placing the flask in a 90 °C oil bath. Samples were taken periodically (using an N2-purged syringe) and analyzed by 1HNMR to determine the degree of polymerization (Supporting Information Figure S2). The reaction was stopped after approximately 80 percent monomer conversion by opening the vessel to air. The reaction mixture was then passed through a column of neutral aluminum oxide to remove the oxidized catalyst and the polymer was purified by precipitation into methanol. After drying under vacuum, the protected DOPA polystyrene was obtained as a light brownish powder. Before use, the triethylsilyl groups were removed. A solution of 2N HCl was added dropwise to a solution of protected DOPA polystyrene in THF (0.15 g/mL polymer concentration) and stirred overnight to achieve complete deprotection. Again, samples were taken periodically and analyzed by 1H-NMR to determine the extent of reaction (Supporting Information Figure S3). THF was evaporated to obtain a suspension, the suspension was centrifuged, and the supernatant was removed. The remaining colorless solid was dissolved in THF and precipitated in hexane twice before drying under reduced pressure to yield (2), a white solid (80% yield). 1H-NMR (400 MHz, CDCl3): δ 6.84-6.63 (aromatic, 3H), 3.50 (q, 2H), 2.75 (t, 2H), 1.94 (s, 6H).

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0.0 10 100 1000 10000 -100 -50 0 50 100 Diameter [nm] ppm Figure 2. TEM of (a) bare and (b) coated NaBH4 particles indicating diameter ranges of 50-150 and 50-400 nm respectively. (c) STEM-EDX of coated NaBH4 particles. Sodium (NaBH4) is localized to the cores, while carbon (polystyrene) highlights the shells. (d) DLS showing the same relative increase of average particle size upon encapsulation as seen in TEM (100-150 nm bare and 150-300nm coated) (e) 11B-NMR spectra for bare and coated NaBH4 particles. Peaks A, B, and C correspond respectively to boron in the bare particles, to the mono complex of boron with the catechol functionality of the shell material, and to the bis-complex of two catechol groups with one boron.43,44,47 1


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2.2.3. Sodium Borohydride Cores (3) A modified anti-solvent precipitation procedure from Christian et al was followed.50,51 A saturated solution of NaBH4 was prepared by dissolving NaBH4 (500 mg) in dry diglyme (10 mL) at 50 °C. TBAB (0.5 g) was added to enhance solubility and act as a stabilizer. The solution was then filtered to remove any undissolved material and added dropwise to dry pentane (50 mL) at 5 °C with vigorous stirring. The resulting precipitate was separated by centrifugation, washed in THF and dried under vacuum at room temperature to yield submicrometer NaBH4 stabilized by TBAB at a concentration of 22 mass percent as determined by TGA (0.58 g, 90% yield with respect to NaBH4). 2.2.4. Capsules (4) A solution of (2) in dried THF (5 mL, 0.02-0.2 g/mL) was added dropwise to a suspension of (3) in dried THF (5 mL, 0.02 g/mL), with stirring at room temperature for 2 hrs. This suspension was centrifuged and washed twice with THF to remove unreacted reagents. The particles were separated from the solution and redispersed in THF with stirring for 1 hr. The new suspension was then added dropwise to dried decane (100 mL) with vigorous stirring. THF was removed under reduced pressure to precipitate the final capsules (4) which were separated by centrifugation, dried, and stored under nitrogen. The supernatant from the previous step was reduced under vacuum and dried at 50 °C for 2 hrs to isolate the DOPA polystyrene that did not deposit on (3). Thus, percent conversion with respect to (2) was calculated (75%). The overall yield of the encapsulation process was determined from 3a to be 85%. 2.3. Characterization 1

H and 11B-NMR spectra were recorded in deuterated chloroform (CDCl3) with a Bruker AM 400 spectrometer (400 MHz). TEM measurements were performed on a Philips CM120 BioTWIN operated at 120 kV. TEM samples (2 µL) were placed onto formvar-coated copper grids and dried under N2.

Macro photography was performed using a digital camera (Canon Rebel T4i). Solvent/capsule systems were prepared in small glass vials and illuminated with high intensity light (LED flashlight) from the bottom of the vial. Images and video were taken from the side of the vial such that the bottom phase, the top phase, and the interface were all visible. Microscopy was performed on an Axioskop 2 MAT (Zeiss) fitted with an AxioCam HR. Specifically, a clean glass coverslip was placed onto a clean glass microscope slide and a drop of toluene with capsules was positioned on the edge of the coverslip, slowly filling part of the free volume under the coverslip. While the oil phase spread toward the center of the coverslip, a drop of aqueous phase (NaCl 10 w/w% in water) was added to the opposite edge of the coverslip such that it too flowed toward the center. Images of the solvent fronts meeting and the subsequent activation of the capsules were captured (Fig. 3a). High pressure experiments were performed using an ISCO 260D syringe pump connected to a custom-built high pressure cell (10 mL). The cell was filled with a 4:1 solution of SB115-4 pH10 buffer (potassium carbonate potassium borate, and potassium hydroxide) and SDBS (1 w/w% in water) while the pump was loaded with 25 mL of solution to yield a total system volume of 35 mL. Capsules (1 g) were added to the cell, the system was closed, and the reaction was monitored for between 1 and 24 hrs at various constant pressures. Reaction progress was tracked as a function of volume change in the pump reservoir. 3. RESULTS AND DISCUSSION An initial series of TEM, STEM-EDX, DLS, and NMR experiments were performed in order to demonstrate the successful encapsulation of the NaBH4 cores by DOPA polystyrene (Fig. 2). As shown in Figure 2a, the uncoated NaBH4 cores are particulate in nature with a range of diameters of 50-150 nm. Upon encapsulation, the particulate morphology was maintained, whereas the range of diameters increased to a range of 50-400 nm (Fig. 2b). This relative increase in size resulted from the formation of DOPA polystyrene shells around the NaBH4 cores (STEM-EDX, Fig. 2c) and was confirmed by DLS measurements (Fig. 2d). Next, the

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Figure 3. (a) Bare and coated NaBH4 particles were exposed to ambient conditions for 24 hours and monitored by optical microscopy and concurrent mass measurements. The bare particles immediately began adsorbing atmospheric moisture, dissolving, and coalescing. The coated particles, however, were observed neither to adsorb water nor react. Again, bare and coated NaBH4 particles (with equal weight percent NaBH4) were exposed to ambient conditions for 24 hours. Percent mass increase over time corresponded to adsorption of and reaction with atmospheric moisture, and thus indicated relative stability. Steady mass increase complete after ~12 hours was observed in the bare case. In the coated case, an initial increase was observed (corresponding to residual NaBH4) but the total increase was an order of magnitude below that reached with the bare particles. (b) XRD spectra of fresh capsules and capsules exposed for 24 hours were obtained. The nearly identical nature of the spectra indicated negligible reactivity of the capsules.


Chemistry of Materials metal-catechol coordination chemistry driving the encapsulation process was characterized with 11B-NMR. Adding a solution of DOPA polystyrene to a suspension of NaBH4 cores produced two characteristic peaks in the NMR spectrum. A strong peak at -12 ppm corresponds to a single boron molecule complexed with a single catechol group. The dominant presence of this mono complex indicates that the reaction of DOPA polystyrene with intact cores is the favored process. The second peak observed at -6 ppm corresponds to the bis complex of two catechol groups with one boron, suggesting that some of the core material is solubilized during the NMR experiment.

before reaching a plateau corresponding to the complete reaction of the cores with atmospheric moisture (Fig. 3a, upper line). The 220% mass increase is ascribed to water adsorption by NaBH4 and the subsequent hydrolysis thereof. In contrast, the encapsulated particles did not exhibit any visible changes in their aggregation state and displayed only 30% mass increase (Fig. 3a, lower line indicating effective protection of the cores by the DOPA polystyrene shells). The initial increase in mass suggests that some fraction of the NaBH4 remained unprotected post-encapsulation, in line with the above conclusion via 11B-NMR that some fraction of the cores were solubilized during the NMR experiment. XRD was performed to further verify lifetime of the protected NaBH4 cores (Fig. 3b). Specifically, spectra were obtained for fresh capsules and capsules left in humid air for 24 hours. In both cases, peaks characteristic of the cubic crystalline structure of pure NaBH4 were observed, with negligible difference between the two overall spectra. This once again confirmed that the DOPA polystyrene shells afford long-term protection of the hygroscopic material from a surround-

After verifying the encapsulation process, the reactivity of the capsules was characterized (Figure 3). Bare and encapsulated NaBH4 were kept under ambient conditions for 24 hours, while visual appearance and mass variation were monitored in both cases. In the case of the bare NaBH4, powdered samples converted into liquid droplets while mass increased steadily for the first 6 hours

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0.0 10 15 20 25 30 0 60 120 180 240 300 Time [hours] Time [minutes] Figure 4. (a) Schematic of the proposed acoustic contrast mechanism in an oilfield setting. (b) Mechanism of gas generation by capsules at an oil-aqueous interface. The capsules are adsorbed to the interface, swollen by the oil phase, and then activated via penetration of water through the shells. (c) Optical microscopy demonstration of targeted gas generation at an actual interface using encapsulated NaBH4 particles. (d) The reaction of capsules in an oil/aqueous mixture at pressures up to 300 bar. Moles of gas produced at pressure over time were measured, demonstrating the pressure-independent nature of the reaction. (e) Capsules in an aqueous solution at 10 bar with delayed addition of THF at 1, 2, and 4 hours, demonstrating triggered reactivity.

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ing water-containing medium.

AUTHOR INFORMATION

Permeability of the polymer shells can be enhanced by swelling in an organic solvent (oil), providing an interesting opportunity for targeted gas generation at an oil-water interface: where oil plasticizes the shell material and thus facilitates water permeation into the core (Fig. 4a-c). To demonstrate this specific interfacial reactivity of the microencapsulated NaBH4, the capsules were first dispersed in a neat aqueous phase (brine) followed after several minutes by addition of an oil phase (toluene). In the initial aqueous dispersions, only minimal gas generation was observed (resulting from residual NaBH4 as in the 11B-NMR and mass experiments). However, upon addition of oil, vigorous gas generation took place and did so entirely at the oil-aqueous interface (Fig. 4c and Supporting Information Movie). It should be noted that the bubbles generated were of the micron scale, and thus capable of movement through microporous subsurface media for example. On the other hand, when capsules were dispersed in a neat oil phase, no reaction was observed. These visual experiments confirm that the capsules react specifically at the oil-aqueous interface, while remaining stable both in the neat aqueous and oil phases.

Corresponding Author

The gas-generation reaction was investigated further at elevated pressures to demonstrate the viability of the capsules in subsurface oilfield applications (Fig. 4d,e). First, high pressure experiments demonstrated that the number of moles of hydrogen generated by the capsules was independent of pressure (tested up to 300 bar) and that the length of reaction could be stretched to the order of days (note that the kinetic plot in Figure 4d shows only partial conversion). Both attributes are vital for subsurface imaging given that borehole engineers strive to maximize the duration and degree of contrast. Second, it was demonstrated that reactivity of the pressurized capsules can be triggered on demand. As shown in Figure 4e, encapsulated NaBH4 does not exhibit significant gas release when dispersed in water and pressurized up to 10 bar, though transmission of solvent through the shells does lead to minimal release. When THF is injected into the system, however, immediate and substantial gas generation is observed – suggesting that THF serves to swell the shells and allow penetration of water to the NaBH4 cores. Thus, the shells provide significant protection prior to the application of an external trigger. In addition to such chemicaltriggers, similar results could be achieved via changes in pH or temperature for example.

Notes

* E-mail: [email protected]

Author Contributions The manuscript was written through contributions of all authors. M.V.V. developed the encapsulation chemistry and performed NMR experiments and analysis as well as the mass experiments and analysis. M.H.E. performed the high pressure experiments and wrote the manuscript. A.Z. performed the reactivity experiments and the related imaging. M.I. performed DLS, TEM, and contributed to the synthetic effort. S.S.S. was the primary investigator. All authors have given approval to the final version of the manuscript. The authors would also like to acknowledge Amar S. Kumbhar for his work on the STEM-EDX experiments.

Funding Sources This work was supported by the Advanced Energy Consortium (BEG10-06). We gratefully acknowledge funding from the National Science Foundation DMR 1436201.

The authors declare no competing financial interest.

4. CONCLUSIONS In summary, we present gas-generating submicrometer capsules that are engineered to react selectively at oil-water interfaces under high pressure – i.e., conditions relevant to oilfield acoustic contrast. Their design highlights a modular synthetic platform and exemplifies one of a wide range of possible applications for the chemistry. Significantly, the generation is tunable in that the physicochemical properties of the polymer shells may be varied as necessary. These variations will effect changes in hydrophobicity, swellability, solvent diffusion, triggering mechanisms, stability, and reaction rate so as to broaden the scope of the platform in future work.

ASSOCIATED CONTENT 1

H-NMR spectra of synthetic intermediates/products, STEM-EDX of the final capsules, 11B-NMR background spectrum, and video of interfacial gas generation by the capsules. This material is available free of charge via the Internet at http://pubs.acs.org.



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Insert Table of Contents artwork here

𝑁𝑎𝐵𝐻4 + 𝐻2 𝑂 → 4𝐻2 + 𝑁𝑎𝐵 𝑂𝐻

4

Oil

Brine


9

Submicrometer-Encapsulation of NaBH4 by Dopamine End

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