The synthesis of polydopamine nano- and microspheres in microdroplets

Abstract

Here we developed a microdroplet-based strategy for the rapid synthesis of uniform polydopamine nano- and microspheres. Polydopamine spheres with controllable sizes were generated within hundreds of microseconds by simply spraying water solutions of dopamine into microdroplets. Mass spectrometry revealed that dopamine was primarily oxidized into aminochrome, acting as the major building block for polydopamine. We anticipate that microdroplet chemistry will be rich in opportunities for the synthesis of functional nano- and micromaterials.

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Dopamine (DA) can undergo oxidative polymerization on the surfaces of various materials in a near-neutral environment, forming the polydopamine (PDA) material.^1,2 PDA has attracted extensive attention in various fields due to its excellent adhesive properties,³ biocompatibility,⁴ antioxidant properties,⁵ corrosion resistance,⁶ and photothermal properties.⁷ The synthesis of PDA involves complicated oxidation reactions and molecular interactions, the poor understanding of which has created challenges in the precise control of the material morphology. There were even debates over the critical intermediates during the DA oxidation process.^1,8–14 In addition, the synthesis of PDA typically takes hours to days,¹⁵ and such a long time imposes limitations on its practical applications. Hence, studies that employ templating agents, oxidants, or surfactants to achieve controlled polymerization of DA emerged.^16–18

In recent years, water microdroplets have been demonstrated to possess powerful ability for initiating or accelerating various reactions and for synthesizing nanomaterials,^19–21 which has attracted tremendous attention. Their distinctive physical and chemical properties, such as the large air–water interface,²² the spontaneously generated ultrahigh electric field,^23,24 plentiful hydroxyl radicals (OH) and electrons,^25,26 partial solvation,²⁷ ordered molecular orientation,²⁸ and unusual acidity,^29,30 make microdroplets exceptional platforms for green synthesis. Therefore, using microdroplets to assist PDA synthesis could enable: (1) the ultrafast oxidation and polymerization processes, (2) the control of PDA morphology without the need of templating agents, and (3) the fast characterization of reaction intermediates using mass spectrometry.

Herein, PDA nano- and microspheres of controllable sizes were obtained by simply spraying DA aqueous solutions into microdroplets without adding any oxidants, surfactants or templating agents. The oxidation and polymerization reactions on the air–water interface of microdroplets were completed within hundreds of microseconds, significantly accelerated in comparison with conventional bulk reactions. Mass spectrometry analysis confirmed that aminochrome was the main oxidation product of DA and an essential building block for constructing the PDA structure.

The experimental setup is presented in Fig. 1a. Detailed experimental methods are provided in the ESI.† DA aqueous solutions with different concentrations were extruded from a silica capillary by a syringe pump. The capillary was located within a larger coaxial capillary, and high-pressure nitrogen sheath gas flowed through the capillary, dispersing the solutions into microdroplets. During the flight of microdroplets, DA molecules on the air–water interface of microdroplets could be oxidized by oxygen in air or by OH on microdroplets, which further triggered polymerization. PDA samples prepared with different DA concentrations were collected and analyzed by transmission electron microscopy (TEM). At a low concentration of 0.5 mM, a mixture of PDA films and nanoparticles was obtained (Fig. 1b). Some ultra-small PDA nanoparticles were scattered on the films, while others tended to aggregate, reflecting the competition between the formation of films and nanoparticles.^18,31 This competition can lead to decreased stability and uniformity of PDA, which is unfavorable for high-quality polymers. As the concentration of DA solution increased, the film structure gradually disappeared, and only PDA nano- or microspheres were formed, with their sizes increasing with the concentration of DA (Fig. 1c–e). At a high concentration of 20 mM (Fig. 1f), uniform PDA microspheres (∼1.5 μm) were obtained. TEM images with higher magnification are presented in Fig. S1 (ESI†). There was always some degree of aggregation between PDA spheres. TEM results successfully demonstrate that PDA spheres with controllable sizes can be conveniently and rapidly synthesized using microdroplets. Compared to the polymerization reaction in the bulk phase that often takes hours to days, the reaction time in microdroplets was on the microsecond scale. The oxidation and polymerization processes were greatly accelerated in microdroplets.

Fig. 1 (a) A schematic illustration of the experimental setup for intermediate characterization and product collection. TEM images of PDA nano- and microspheres prepared with (b) 0.5, (c) 1, (d) 5, (e) 10, and (f) 20 mM DA solutions in microdroplets.

We next discuss the oxidation and polymerization mechanism of DA in microdroplets. A typical mass spectrum obtained by directly spraying a solution of DA into the mass spectrometer is shown in Fig. 2a. The peaks of the reactants, protonated DA and DA dimer were identified at m/z 154 and 307. The oxidation products at m/z 150 and 303 were also observed. The species at m/z 150 could be protonated 5,6-dihydroxyindole (DHI), and the m/z 303 peak could be the protonated dimer of DA and DHI. DHI is a previously well-known oxidation product of DA, and has long been considered to be the dominating building block for PDA.^1,9,10,12 However, our tandem mass spectrometric analysis (MSⁿ) results of the peaks at m/z 150 and 303 challenged this traditional viewpoint. In the MS² spectrum of the peak at m/z 303 (Fig. 2b), peaks at m/z 150 and 154 were simultaneously observed, confirming that the peak at m/z 303 corresponded to a dimer of DA and an oxidation product. Fig. S2 (ESI†) further confirms that the m/z 154 fragment in Fig. 2b was indeed DA. Besides, minor peaks at m/z 137 and 286 in Fig. 2b were identified as the losses of NH₃ fragments (m-17). In the MS³ and MS⁴ spectra (Fig. 2c and d), the fragment ion at m/z 150 successively dissociated into fragment ions at m/z 122 and 94, indicating the consecutive losses of two CO groups (m-28). This dissociation pattern belonged to an isomer of DHI, aminochrome (AC), which had two CO moieties in its structure (embedded in Fig. 2c).^32,33 The same result was obtained in the MS² and MS³ spectra of the parent ion at m/z 150 (Fig. S3, ESI†). Therefore, the product at m/z 150 should be assigned to protonated AC but not DHI, and the peak at m/z 303 was the protonated DA + AC dimer. Previous works have demonstrated that PDA is mainly composed of a large number of oligomers capable of further self-assembly.^8,11 Our findings suggest that, AC, rather than DHI, is the dominating oxidation product and building block of PDA synthesized with microdroplets. In addition, some kinetic and thermodynamic information can also be deduced from the mass spectra. The intensity of the protonated AC at m/z 150 was very low at all times compared to the protonated dimer of DA + AC at m/z 303 (Fig. 2a), suggesting that once an AC molecule was formed, it would immediately combine with a DA molecule due to the high binding energy between these two molecules.

Fig. 2 (a) A typical mass spectrum showing DA and its oxidation product. The (b) MS², (c) MS³, and (d) MS⁴ spectra of the peak at m/z 303.

After a freshly-made DA solution was aged for 15 min and then sprayed into microdroplets, the relative intensity of the product peak at m/z 303 increased, and that of the reactant peak at m/z 154 decreased (Fig. 3a). It can be reasonably speculated that oxygen, both from the air and water, contributed to the oxidation of DA. Meanwhile, the MSⁿ spectra of peaks at m/z 150 and 303 did not change after the solution was aged (Fig. S4, ESI†), suggesting that the oxidation products were the same both in the bulk and in microdroplets. Combining all the aforementioned mass spectrometric analysis results, Fig. 3b illustrates the possible oxidation and polymerization mechanism of DA in microdroplets. DA is first oxidized to dopamine quinone (DAQ) and then further oxidized to AC, which forms dimers with DA through hydrogen bonding and π–π interactions.^8,11 Based on the very high signal of the protonated DA + AC dimer but relatively low signal of protonated AC monomer in the mass spectrum, it is reasonable to suggest that a large number of DA + AC dimers self-assemble to form PDA.

Fig. 3 (a) A typical mass spectrum showing DA and its oxidation product after aging for 15 min. (b) Proposed oxidation and polymerization mechanism of DA in microdroplets.

To further investigate the kinetics of DA oxidation, various factors influencing the productivity of AC were investigated. The product percentage was defined as {I[(AC + H)⁺] + I[(DA + AC + H)⁺]}/{I[(AC + H)⁺] + 2I[(DA + AC + H)⁺] + I[(DA + H)⁺] + 2I[(2DA + H)⁺]}, where I denoted the peak intensity. The reaction distance was defined as the distance between the sprayer and the mass spectrometer inlet. As the reaction distance increased from 10 to 30 mm, equivalent to a reaction time of 90 to 270 μs, the product percentage significantly improved (Fig. 4a). This suggests that the oxidation of DA occurs at the air–water interface of microdroplets rather than in the gas phase inside the mass spectrometer.²⁴ We also investigated the concentration-dependence of the reaction. As the concentration of DA increased from 0.1 to 5 mM, the product percentage dramatically decreased (Fig. 4b). This is because at low concentrations a higher proportion of the reactants had a chance to occupy the air–water interface,³⁴ making oxidation easier to occur. At high concentrations, only a small portion of the reactants could occupy the air–water interface, which consequently lowered the product yield. These results emphasize the significance of air–water interface in the DA oxidation. Interestingly, increasing the sheath gas pressure resulted in a decrease in product percentage (Fig. S5, ESI†), which seemed counterintuitive since higher sheath gas pressure could reduce the microdroplet size and increase their surface area-to-volume ratio.³⁵ However, higher sheath gas (N₂) pressure prevented good contact between the microdroplets and ambient O₂. Hence, this phenomenon could be a consequence of the competition of these two factors, with the latter factor prevailing.

Fig. 4 (a) Product percentage as a function of reaction distance when the DA concentration was 0.1 mM and the sheath gas pressure was 100 psi. (b) Product percentage as a function of DA concentration when the reaction distance was 10 mm and the sheath gas pressure was 100 psi. All error bars indicated the standard deviations of at least three independent measurements.

In summary, we have achieved controllable and rapid synthesis of PDA nano- and microspheres in microdroplets at the microsecond time scale. Mass spectrometry results revealed that aminochrome was the primary product of dopamine oxidation in microdroplets and served as the principal building block for the formation of PDA, deviating from the conventional belief of 5,6-dihydroxyindole. This study substantiates the potential of microdroplet chemistry in the field of nanomaterial synthesis, providing a convenient and widely applicable pathway for both innovative material design and in situ characterization of intermediates.

X. Z. acknowledges the National Key R&D Program of China (2023YFE0124200), the National Natural Science Foundation of China (22325402&22174073), the NSF of Tianjin City (21JCJQJC00010), the NCC Fund (NCC2022PY05), the Haihe Laboratory of Sustainable Chemical Transformations, and the Frontiers Science Center for New Organic Matter at Nankai University (63181206). S. T. acknowledges the Science and Technology Research Project of Henan Province (242102320184).

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4cc03017f

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