Natural Nanoparticle complexes at water-water interfaces

doi:10.21203/rs.3.rs-5157773/v1

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Natural Nanoparticle complexes at water-water interfaces

https://doi.org/10.21203/rs.3.rs-5157773/v1

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Aqueous two-phase systems (ATPS) with multifunctional attributes have significant promise as biomimetic materials, but current approaches do not harness their full potential. We show that ATPSs can be stabilized to form microcapsules by the interfacial assembly of chitin nanofibers (ChNF) and rod-like cellulose nanocrystals (CNC). The high structural stability of the ChNF/CNC complex integrates permeability and transport across the membrane at the liquid/liquid interface. Driven by density and osmotic stress gradients, the microcapsules show switchable motility, including cyclic meniscus-climbing and subsurface transport. These observations demonstrate a self-regulating system with potential for cargo transfer, cell biomimicry, as well as ATPS-based microreactors and microrobots.

Physical sciences/Materials science/Nanoscale materials/Nanoparticles

Biological sciences/Biotechnology/Biomaterials/Bioinspired materials

aqueous two-phase systems

cellulose nanocrystal

chitin nanofiber

interfacial complexation

microcapsules

meniscus-climbing

Reconfigurable all-liquid systems have been used to control the structural and chemical characteristics of soft matter.¹ Most efforts in the field have harnessed solvent-water interactions to achieve polarity-based spatial compartmentalization.^2–4 More recent efforts have focused on aqueous two-phase systems (ATPSs), produced by the phase separation of mixed aqueous solutions containing incompatible polymers.^5–9 ATPSs offer a natural platform for all-aqueous biomimicry,^10,11 e.g., based on aqueous dextran (DEX) and polyethylene glycol (PEG) solutions.¹² However, due to the ultralow interfacial tension between the two aqueous phases, typical ATPSs lack structural stability^13,14, resulting in irreversible collapse of the liquid phases. ATPS stabilization has been achieved using polymers or colloidal particles;^15–17 however, component formulation and stability is not yet optimized.

Structured liquids, generated by complexing two oppositely charged polyelectrolytes (PE) at the interface of two contiguous liquid phases,¹⁸ afford a route to address this challenge.^19–21 This approach capitalizes on the inherent mobility and transport characteristics of each phase, ease of access to the interface to enable reaction or interaction between two immiscible components, and the spatial and dimensional confinement of the interface itself.²² The complexation of the components at the interface can lock in the shapes of the liquid phases while not impeding their response to environmental stimuli, such as light,²³ electric or magnetic fields.^20,24 Previous studies have demonstrated the ability of polyelectrolyte (PE) pairs, a polycation and a polyanion, to form complexes at the water/water interface, preventing coalescence and locking-in non-equilibrium shapes to the two aqueous phases.²⁵ By adjusting the flux of oppositely charged substances to the interface so as to optimize interfacial complexation, stabilization of the phases can be achieved. Charged nanoparticles (NPs) have also been used to produce PE/NP complexes that encapsulate the aqueous phases.^26,27 Up to now, though, interfacial complexes have enabled the generation of tubular all-aqueous shapes.²⁸ Endowing such constructs with additional function offers significant potential for cargo encapsulation and release.²⁹

In principle, PE/PE systems form deformable membranes, while PE/NP complexes are thinner and more rigid.³⁰ A recent study on the complexation of positively charged diallyl dimethylammonium chloride and negatively charged rodlike cellulose nanocrystals (CNC) has shown that the thickness of the layer at the interface increased with time,³¹ enhancing the mechanical properties of the interfacial complex. These composite complexes can be used as biomimetic constructs,³² such as artificial tissues from functional ATPSs. However, for the structured ATPSs from either PE/PE or PE/NP complexes, integrating multifunctional attributes into a single all-aqueous system has been little studied. We introduce all-aqueous constructs based on NP/NP complexes with oppositely charged biological nanoparticles; specifically, rodlike anionic CNCs and cationic chitin nanofibers (ChNF).^33,34 The nanocrystalline structure of CNC is designed to provide structural integrity for the interfacial assembly, whereas the disordered domains in ChNF facilitate the flexibility properties (i.e., enabling bending and twisting behaviors).^35,36 Along with an uneven distribution of surface charges,³⁷ the ChNF/CNC complexes can integrate the characteristics of both PE/PE and PE/NP systems.

Here, ChNF/CNC complexes are formed at water-water interfaces to produce membranes with a tunable mechanical strength and permeability, and allow for the formation of all-aqueous microcapsules with the ability to compartmentalize. The ChNF/CNC complex layer can be thickened by liquid flow across the interface, for example, under an imbalanced osmotic stress. In concert with other features of the ChNF/CNC complexes, e.g., deformability and permeability, the resultant microcapsules show autonomous, switchable sub-surface mobility, tuned by the deformation of the meniscus at the surface. Such biological nanoparticle-structured ATPSs open possibilities for autonomous transport and exchange in artificial cells.

Nanoparticle complexation at water-water interfaces

As a proof-of-concept, biomimetic underwater microcapsules were fabricated in ATPSs³¹ inspired by permeable human cell membranes and water-walking striders (Fig. 1). Water-water interfaces can be generated with aqueous solutions containing DEX and PEG and stabilized in situ by the self-assembled complexes formed by oppositely charged biological nanoparticles, ChNF and CNC (Figure S1), suspended in the polymer solutions (Fig. 1c). The interfacial complexation of ChNF and CNC is driven by electrostatic interactions at the interface between the phases. The electrostatic interactions between the cationic amine groups of ChNF and the anionic half sulfate ester groups of CNCs are enhanced by release of counterions and water (entropic gain, $\:\varDelta\:S>0$).³⁸ Considering the ultralow interfacial tension at the water-water interface (limited enthalpic contributions) and the structural features of the oppositely charged nanoparticles, the spontaneous interfacial electrostatic interactions are significantly enhanced by the entropic gain.

Compared with those formed by polymer/polymer and polymer/nanoparticle (Fig. 1c) interactions, the ChNF/CNC complexes bear the characteristics of both, i.e., viscoelasticity and permeability, offering interesting opportunities, given the possibility to tailor the morphology and transport properties by varying pH.

Pendant drop tensiometry was used to investigate the interfacial complexation, where a 10 wt% DEX solution (pH = 3, $\:\rho\:$ = 1.0471) was slowly introduced into 10 wt% PEG solution (pH = 3, $\:\rho\:$ = 1.0208) (Table S1). Complexation and pendant droplet stabilization were found for the nanoparticle complexes (ChNF in DEX and CNC in PEG) (Fig. 2a). No complexation was evidenced absent the nanoparticles (Video S1), where the DEX droplet fell due to the ultralow interfacial tension. At higher pH values, the amine groups on the surface of ChNFs are protonated (Fig. 2b),³⁹ giving rise to weaker complexes with CNCs and failing to stabilize the interface (illustration of Fig. 2b). This sensitivity to pH changes is expected to be affected by the non-uniform distribution of amine groups on the surface of ChNFs, due to the random deacetylation process used in their preparation.⁴⁰

We further investigated the microstructure of ChNF/CNC complexes by transmission electron microscopy (TEM), which showed an interconnected, multi-layered network with voids (Fig. 2c). Considering the structure of the interfacial layer, the permeability of ChNF/CNC complexes was confirmed by exchange of labelled PEG (FITC-PEG, $\:{M}_{w}$ = 10000) across the microcapsules formed by the ChNF/CNC complex (DEX-in-PEG, Figure S3). By contrast, the chitosan (CS)/sodium alginate (SA) complex prevented FITC-PEG exchange (Figure S4), indicating limited permeability, as previously discussed.³¹ However, the ChNF/CNC complex layer was effective in blocking particles (50-nm polystyrene spheres, Figure S5), implying size exclusion effects due to the characteristic cells in the network formed at the interface.

As a reference, we evaluated the changes of the DEX pendant droplet in a PEG solution with each phase containing oppositely charged CS and SA. The CS/SA complex underwent rapid shrinkage and surface wrinkling when the pendant DEX droplet was withdrawn, indicative of the elasticity of the interfacial film (Figure S2a). In another referencing combination (PE/NP system), the CS/CNC complexation maintained the droplet shape (no surface deformation or wrinkling) (Figure S2b). Under the same condition, during the retraction of the droplet encapsulated with ChNF/CNC complexes fragmented, the interfacial films crumpled in response to the compressive force as the interfacial area decreased (Fig. 2a). The results indicate that ChNF/CNC complexes display the characteristics of both the PE/PE and PE/NP systems.

In-situ interfacial shear rheology was performed by using a magnetic probe to quantitatively assess the mechanical properties of the interfacial layer (Fig. 2d and Video S2). Figure S6, as a control, shows a good correlation between the probe and the trap position during the test, validating the methodology reliability. The results shown in Fig. 2e correspond to the time evolution of the storage (G’) and loss (G’’) moduli for the interfacial complexes (ChNF/CNC and CS/CNC) at the water-water interface (Fig. 2e). The CS/SA interfacial complex was not studied due to the rapid and dense association between CS and SA. Both the ChNF/CNC and CS/CNC complexes had a high G’, e.g., solid-like behavior. Notably, although ChNF/CNC complexes had a slightly lower G’, its loss factor value was significantly higher than that of the CS/CNC complex. This indicates that the ChNF/CNC system formed a tough, yet flexible interfacial layer. The observed moduli differences can be explained by the interaction and arrangement of rod-like, rigid CNCs in the complex.^41,42 In the CS/CNC complex, the CS chains facilitated strong binding and orderly arrangement with CNCs in the complex, endowing rather the rigidity features. The high aspect ratio (Figure S1) and random distribution of surface charges of ChNF led to much less ordered complex, contributing to the formation of a flexible film. TEM also supports the loose ordering of the nanoparticles in the complex layer (Fig. 2c). Overall, complexation of ChNFs and CNCs at the interface stabilized the shapes of the ATPSs and the permeable complexes with rigidity and deformability, while maintaining the structure when the external environment changed, e.g., osmotic stress.

Osmotic stress balance between the two aqueous phases

Osmotic stress can modulate the direction of water flow across the water-water interface in ATPSs (Fig. 3a), a simple route to adjust the properties of all-aqueous constructs.³¹ As such, water transport occurs between the interior of the pendant droplet and the external phase, under the influence of osmotic pressure gradients. A microcapsule and a pendant drop encapsulated with the ChNF/CNC complexes are subjected to three different regimes of osmotic stress, $\:\varPi\:$, between the internal (DEX) and external (PEG) phases (Fig. 3 and Table S1), namely $\:{\varPi\:}_{PEG}$ $\:<$ $\:{\varPi\:}_{DEX}$, $\:{\varPi\:}_{PEG}$ $\:=$ $\:{\varPi\:}_{DEX}$, and $\:{\varPi\:}_{PEG}$ $\:>$ $\:{\varPi\:}_{DEX}$. The osmotic pressure of each phase was adjusted by the concentrations of DEX and PEG, while keeping the nanoparticle loading the same (1.0 wt% in each phase). Initially, ChNF in DEX and CNC in PEG met at the interface and formed an interfacial layer by electrostatic interactions. No apparent wrinkling was observed on the droplet surface (Fig. 3b). With time, the interfacial complex equilibrated, affecting both convective flow and permeability (Fig. 3b).

At $\:{\varPi\:}_{PEG}$ $\:<$ $\:{\varPi\:}_{DEX}$, a pendant DEX droplet gradually expanded due to continuous influx of water from the PEG solution (Fig. 3a and 3b). Fluorescence and optical microscopy (Fig. 3c and Figure S7 in the case of a microcapsule) clearly showed the formation of small PEG-in-DEX emulsion droplets inside the larger droplet stabilized by the ChNF/CNC complex layer. The mechanical resistance of the latter accommodated slow deformation due to the water flow. However, the encapsulated components in the droplet gradually leaked out, e.g., following failure of the ChNF/CNC complex layer (see dashed circle in Fig. 3b, upper panel). Similar to the PE/PE system, a CS/SA complex layer expanded (flexibility), with no droplet breakage (Figure S8), while the CS/CNC complex layer burst soon after fluid influx (Figure S9). Consequently, the ChNF/CNC system bears characteristics of the other two complexes.

The droplet was stable when $\:{\varPi\:}_{PEG}$ $\:=$ $\:{\varPi\:}_{DEX}$ (see contour in Fig. 3b, middle panel). Although a slight degree of phase separation occurred, no leakage or thickening of the complex layer was observed under this condition (Fig. 3c and Figure S10), suggesting long-term stability. For $\:{\varPi\:}_{PEG}$ $\:>$ $\:{\varPi\:}_{DEX}$, the droplet retained its original shape with a layered hierarchical structure forming on the outer edge of the interface (Fig. 3a and 3b), which was further confirmed by fluorescence and optical microscopy observations of the microcapsules (Fig. 3c and Figure S11). This was caused by a thickening of the ChNF/CNC complex layer, similar to destabilized emulsions or plasmolysis in plant cells. Water transport occurred from the DEX phase to the PEG phase due to the osmotic stress imbalance (Fig. 3d), providing a driving force for ChNF diffusion that led to its detachment. Meanwhile, as the volume of DEX phase was gradually reduced, PEG penetrated the droplet, forcing CNC to diffuse to the interface. As a result, more ChNF and CNC engaged in electrostatic interactions, increasing the thickness of the complex layer, while maintaining a high permeability. From these results, it is evident that the ChNF/CNC complex can be adjusted to tailor mass exchange (Fig. 1d).

The assembly of CNC and ChNF at the microcapsule surface contributed to the rigidity of the complex layer to balance of osmotic stress. For all osmotic gradients, extinction was observed parallel to and perpendicular to the polarization direction using polarized optical microscopy for the ChNF/CNC complex, which indicates that the ChNFs and CNCs were oriented parallel to the interface (Fig. 3c). Layer thickening at $\:{\varPi\:}_{PEG}$ $\:>$ $\:{\varPi\:}_{DEX}$ strengthened the complex assembly (Fig. 3d); however, owing to the morphology of ChNF and CNC, a different ordering behavior may occur than that found for the interfacial polymer/CNC complex (Figure S9c). The assembly of ChNF and CNC at the interface was further explored by changing their distribution in ATPS (Figure S12). A droplet was stabilized when suspending ChNF (1 wt%) in PEG and CNC (1 wt%) in DEX (Figure S12a), where osmotic stress transferred water from the internal DEX phase to the external PEG phase, gradually reducing the droplet volume. However, thickening of the complex did not occur, and the CNC/ChNF complex layer wrinkled (Figure S12b). The extinctions parallel to and perpendicular to the polar was not observed (Figure S12c), suggesting a strong partitioning of CNC to the DEX phase.⁴³ The density of the DEX phase increased with a reduction in the water content and, due to the permeability and poor mechanical strength of the ChNF/CNC complex layer, eventually led to a phase separation at the bottom of the droplet (Figure S12a).

Selective transfer across ChNF/CNC complex layer

Ionic fluorescent probes were added to different phases across the complex layer⁴⁴ (Fig. 4a). Negatively charged fluorescein (sodium salt, FSS) in the PEG phase immediately diffused into a microcapsule (DEX phase) and interacted with the ChNF/CNC complex by electrostatic interactions between FSS and ChNF (Figure S13). Similarly, positively charged Nile blue A (NBA) in the DEX phase was excluded from the microcapsule and the thickened ChNF/CNC complex layer (Figure S14), due to the negatively charged CNC in the external PEG phase and the complex. The permeability of the ChNF/CNC complex layer and the asymmetric diffusion of oppositely charged molecules suggest the possibility for separation, purification, and compartmentalized serial reaction systems. Separation of the mixed ionic dyes took place when dissolving FSS and NBA in either PEG or DEX solutions, soon after forming the microcapsule; no effect was noted for the initial location of the mixture in the PEG (Fig. 4b) and DEX (Figure S15) phases. At equilibrium, the NBA was preferentially located in the PEG phase, while FSS was present in the DEX phase. This was demonstrated by the time-dependent FSS and NBA fluorescence intensity in the given phases and at a given position (dashed lines in Fig. 4b) (Fig. 4c). FSS diffused across the boundaries when a microcapsule containing FSS was placed next to two unloaded microcapsules (Fig. 4d). This transfer was preferential to the neighboring microcapsules but not to the surrounding PEG solution, which was confirmed by time-dependent fluorescent intensity measurements of FSS at given positions (dashed line in the Fig. 4d) (Fig. 4e). Furthermore, no bridging between the microcapsules was observed upon contact (Video S3), implying that the FSS transferred across the non-coalescent complex layers. Moreover, thickening of the complex continuously occurred, in contrast to the observation for polymer/nanoparticle complexes.³¹

Magnetic nanoparticles whose movement is controlled by an external field can be used to enable dynamic transport by producing a gateway for transport. We demonstrated movement of the microcapsule under a magnetic field by incorporating 1 wt% Fe₃O₄ nanoparticles, that was used to control the contact and separation distance between microcapsule (Video S4). Since control over the transport is achieved by adding foreign, synthetic nanoparticles, exploration of self-driven locomotion for the microcapsule would better benefit its development on multifunctional applications.

Autonomous mobility of microcapsules

Insect mobility using capillary forces has been shown for climbing or descending along a meniscus formed between water and a solid.⁴⁵ An upward (positive) meniscus forms and distorts the liquid surface with a contact angle ($\:\alpha\:$), where a lateral force between the floating object positioned at a distance ($\:x$) from a wall can be expressed by:⁴⁶

$$\:F\left(x\right)=-{F}_{\text{V}}cot\theta\:{\text{e}}^{-\frac{x}{{l}_{c}}}=-\gamma\:C\text{c}\text{o}\text{s}\alpha\:\:\text{c}\text{o}\text{t}\theta\:{\text{e}}^{-\frac{x}{{l}_{c}}}$$

where $\:{F}_{\text{V}}$ represents the vertical net force, $\:\gamma\:$ is the surface tension, $\:C$ is the contact line length, $\:\theta\:$ is the contact angle, and $\:{l}_{c}$ is the capillary length. As shown in Fig. 5a,⁴⁷ a positive meniscus ($\:{F}_{\text{V}}$ $\:>$ 0 or $\:\text{c}\text{o}\text{s}\alpha\:\:\text{c}\text{o}\text{t}\theta\:$ $\:>$ 0) draws a floating object towards the wall, up the meniscus, provided $\:F\left(x\right)$ is sufficiently large. For a negative meniscus ($\:{F}_{\text{V}}$ $\:<$ 0 or $\:\text{c}\text{o}\text{s}\alpha\:\:\text{c}\text{o}\text{t}\theta\:$ $\:<$ 0), the object is repelled from the wall. This was tested for ChNF/CNC microcapsules subjected to imbalances of osmotic pressure, an effective strategy for achieving directional migration (for example, by manipulating $\:\alpha\:$ of liquid surface deformation by the density of the microcapsule and the external phase). By leveraging density-gradient to drive migration (Table S1), a hanging microcapsule experienced a repulsive force from the container wall and migrated toward the center when 10 wt% DEX (1.0 wt% ChNF, $\:\rho\:$ = 1.0520) was dropped into 10 wt% PEG (1.0 wt% CNC, $\:\rho\:$ = 1.0372) (Fig. 5b, upper panel). Meanwhile, a floating microcapsule climbed a meniscus toward the container when 10 wt% PEG (1.0 wt% CNC) was dropped into 10 wt% DEX (1.0 wt% ChNF) (Fig. 5b, bottom panel). The effect of lateral forces on microcapsules surrounded by the four walls of a rectangular container were observed visually (Figure S16a and Video S5) and by optical microscopy (Figure S16b and Video S6): two microcapsules that were placed at arbitrary positions across the liquid surface migrated toward the center of the container. This one-way microcapsule migration can be used for microreactors, when combined with the selective cross-membrane transfer shown in the previous section.

Enhanced material transport can further leverage microcapsule permeability, along with mass transport of the ChNF/CNC complex. To demonstrate autonomous shuttle/transport, microcapsules consisting of 10 wt% DEX (1 wt% ChNF and 0.001 wt% FSS) and 25 wt% DEX (1 wt% ChNF) were placed in a container (20 × 20 × 20 mm³) filled with 35 wt% PEG (0.5 wt% CNC) solution. As shown in Table S1, compared to the 35 wt% PEG (0.5 wt% CNC, $\:\rho\:$ = 1.0746), the 25 wt% DEX (1 wt% ChNF) droplet of higher density ($\:\rho\:$ = 1.1010) generated a microcapsule located in the center of the container. By contrast, the 10 wt% DEX (1 wt% ChNF) droplet with a lower density ($\:\rho\:$ = 1.0520) initially formed a microcapsule that migrated toward the inner wall of the container. As shown in Fig. 5d and Video S7, S8, the microcapsule climbed the meniscus and landed at the container’s inner wall within 2 min. Thereafter, it remained attached to the wall for 3 min. After this time, the microcapsule moved down the meniscus, back to the center of the container, and eventually contacted with the hanging microcapsule. A mechanism that explains the autonomous microcapsule movement and material shuttle/transport is described in Fig. 5c. The one-way migration is driven by the difference of initial densities between the two phases, making the microcapsule climb up the meniscus. After landing at the wall, with the osmotic pressure gradient between the two phases and the permeability of the ChNF/CNC complex layer, PEG and water flow to and from the interior of the microcapsule, leading to a balanced density between the two phases. With the gradually increased density of the DEX phase, the microcapsule returns to the center of the container. Finally, transfer across the interfacial layer of the initial microcapsule takes place. Using the inherent properties of the ChNF/CNC complex, the diffusion-induced cyclic and autonomous microcapsule migration is associated with dynamic transfer without the need for external stimuli.

We investigated the switchable nature of the meniscus-climbing microcapsule. The cyclic movement of microcapsules formed by 10 wt% DEX and different ChNF concentrations, in the presence of 35 wt% PEG and varying CNC concentrations, was followed in rectangular containers (10 × 10 × 45 mm³ and 20 × 20 × 20 mm³). At 1 wt% ChNF and varied CNC concentrations, the microcapsules successfully landed on the container wall (10 × 10 × 45 mm³), but no return was observed upon increasing the CNC concentration over 1.4 wt% (Figure S17). For the larger container (20 × 20 × 20 mm³), the microcapsules could only return to the center when the CNC concentration was 0.5 wt% (Figure S18). At a CNC concentration over 0.5 wt%, the microcapsules could only move close to the center or remain stagnant at the landing site. However, for a CNC concentration of 0.5 wt%, the microcapsules formed by 0.5 wt% ChNF sank during return (for both containers). In the small container, other microcapsules completed the entire cycle (Figure S19), and at 1.0 wt% ChNF, the microcapsule could return to the center of the large container (Figure S20).

The microcapsules that were able to return to the center of the container relied on the differences of density (Fig. 5e₁ and 5e₂). The meniscus climbing was driven by the initial difference in densities between the DEX and PEG phases; however, the return process was predominantly ruled by exchange across the interface induced by the osmotic pressure difference, which was affected by the difference in initial densities (Table S1). The system viscosity increased with increasing CNC concentration in the PEG phase (Figure S21), which hindered microcapsule movement. At an increased ChNF concentration in the microcapsule, the difference of density and osmotic pressure between the two phases were both reduced (Table S1), where only a small amount of exchange was required to enable transition from a floating microcapsule to a hanging one. As a result, the driving force during the return process was reduced, halting microcapsule movement at some distance from the container center, particularly for the larger container (Figure S22).

The most crucial prerequisite for microcapsule movement is the effective formation of the ChNF/CNC complex at the interface (Figure S25), which prevents rupture and resists deformation caused by shearing and mass exchange. The complex formed by interfacial assembly with different nanoparticle concentrations was verified by injecting DEX solution into the PEG solution (Fig. 2b, S23 and S24). When the concentration of both the CNC and ChNF were 0.5 wt%, the nanoparticles failed to form a strong complex layer. So, as the internal density of the microcapsule gradually increased, the complex held the entire structure causing the sinking (Figure S19 and S20). We further verified this observation by dropping 10 wt% DEX (1 wt% CNC) into 35 wt% PEG (0.5 wt% ChNF). The cyclic movement of the microcapsule was confirmed in small and large containers. As expected, both microcapsules completed the movement to the center of the container (Figure S26 and S27). There was a reduction of microcapsule volume by mass exchange during the return, which decreased the buoyancy. Overall, the microcapsule that is encapsulated by ChNF/CNC complexes is easily tailored for self-driven motility.

In this study, ChNF/CNC complexes were assembled in structured DEX-in-PEG ATPS. The ChNF/CNC complexes formed a rigid interfacial layer surrounding the droplets, endowing them with structural stability and permeability. The complexes formed under different environmental stresses by osmotic-driven diffusion of the PEG phase into the microcapsules containing the DEX phase. Such a system resulted in layer thickening, which was tunable by the osmotic stress balance between the phases. By taking advantage of ChNF/CNC complex and microcapsule characteristics, subsurface shutting/transport was demonstrated. A cyclic microcapsule movement was shown to strongly depend on the density differences. Control of the direction of microcapsule motion was achieved by altering the density difference between the two aqueous phases or setting external osmotic pressure gradients, reversing the contact angle and enabling capillary forces to drive motion. The biobased nanoparticle-structured microcapsule extends the application of ATPS for separation and cargo transfer, relevant to cell biomimicry, microreactors, and microrobots.

Materials

Chitin nanofibers (ChNF) were produced by mechanical treatment of deacetylated -chitin. Cellulose nanocrystals (CNC) were prepared using bleached wood fibers. All chemicals were used without any further purification. Detailed information for materials and experimental procedures were included in the Supplementary Information.

Complexation at the interfaces

Complex formation at the interfaces involved injection of a droplet of DEX solution containing cationic ChNF (or chitosan) into PEG solution containing anionic CNC (or sodium alginate) via an optical tensiometer (Attention Theta Flex, Biolin Scientific, Espoo, Finland) equipped with an automatic pipette (0.5 mm inner diameter of the tip) and a high-speed camera. To visualize the complex formation, the liquid in the nascent droplet was extracted at a volumetric rate of 2 μL/s, 3 min from the onset of formation. To observe the behavior of the complex under osmotic stress gradients between the two phases, the DEX and PEG solution concentrations were adjusted by dilution with Milli-Q water. The formation of microcapsules encapsulated with the complex was analogous to that in complex-stabilized droplet. In this procedure, one phase was pipetted into a container containing the other phase. Similarly, the osmotic pressure and density of the two phases were regulated by adjusting the concentration of DEX and PEG in the respective solution.

Complex rheology

The structural integrity of the interfacial complex was assessed by using an interfacial shear rheometer (KSV NIMA ISR Flip, Biolin Scientific, Espoo, Finland) equipped with a magnetic trap (Figure 2d). The preparation of the complex for testing involved dropwise addition of 1 wt% ChNF suspension or 1 wt% chitosan (CS) solution into 1 wt% CNC suspension. Subsequently, a magnetic probe (ISR1220) was placed on the complex, and the magnetic trap above it was made to oscillate horizontally at a frequency of 1 Hz. The viscoelasticity of the complex was calculated from the motion phase difference between the magnetic probe and the magnetic trap.

Microcapsule demonstrators

Static mass transfer

Two fluorescent labelled ionic substances, anionic fluorescein sodium salt (FSS) and cationic Nile blue A (NBA), were co-mixed (0.001 wt%) into PEG or DEX phases prior to microcapsule preparation. The fluorescence of the microcapsule was observed under a fluorescent optical microscope (Axio Imager A2, ZEISS, Germany) equipped with a 5× objective lens. The excitation/emission spectra used in this study corresponded to a wavelength of 460/515 nm for FSS and 633/660 nm for NBA. The fluorescence intensity (yellow and blue) at the same position in the images and merged images were processed using ImageJ.

Autonomous microcapsule movement

Microcapsule migration toward the center of the container used a drop of 10 wt% aqueous DEX solution (1 wt% ChNF) pipetted into 20 × 20 × 20 mm³ container containing 10 wt% aqueous PEG solution (1 wt% CNC) from a 5-cm height using a pipette with a needle (inner diameter of 0.5 mm). By reversing the droplet deposition, migration of microcapsule toward the edge of the container was achieved. All the movement was recorded with a high-speed camera in the optical tensiometer.

The switchable movement of microcapsules (between hanging microcapsule and container wall) was conducted by pipetting a drop of 10 wt% and 25 wt% DEX solutions (1 wt% ChNF) respectively into 35 wt% PEG (0.5 wt% CNC) solution in the center of container (20 × 20 × 20 mm³). The floating (moveable) microcapsule was labeled with 0.001 wt% FSS. The process was recorded by a camera positioned on the top and side view under ultraviolet light, wavelength of 365 nm. To investigate the factors influencing the switchable movement of the microcapsule, drops of 10 wt% DEX were introduced in 35 wt% PEG contained in a container (10 × 10 × 45 mm³ or 20 × 20 × 20 mm³). Cyclic migration was confirmed by changing the concentrations (0.5 to 1.6 wt%) of nanoparticles or suspending the nanoparticles reversely in the polymer solutions. The migration trajectory was characterized by using ImageJ.

Dater availability

All key data that support the findings of this study are included in the article and its Supplementary Information. Additional datasets and raw measurements are available from the corresponding authors upon reasonable request.

Acknowledgements

This work was partially supported by the National Natural Science Foundation of China (32201483 and 3233005) and the China Postdoctoral Science Foundation (2021M700734). O.J.R. and Y.L. acknowledge the financial support from the Canada Excellence Research Chair Program (CERC-2018-00006) and the Canada Foundation for Innovation (Project number 38623). T.P.R. was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division under Contract No. DE-AC02-05-CH11231 within the Adaptive Interfacial Assemblies Towards Structuring Liquids program (KCTR16). The authors sincerely thank Wei Li and Peng Liu from Biolin Scientific AB for providing technical assistance with interfacial shear rheometer. This paper is dedicated to the late Prof. Long Bai (1988.04.16-2024.08.05).

Author contributions

H.W. and Y.L. contributed equally to this work and performed most of the experiments. H.W., L.B., and S.Q.H. conceived the project. O.J.R., T.P.R., C.J.C. and S.Q.H. proposed the idea and supervised this project. H.W., M.Q.Z., and X.Y.L. analyzed the data and contributed to the understanding mechanism. Z.G.L., Y.L., W.L., and S.X.L. prepared and characterized the raw materials. H.W., L.B., Y.L., O.J.R., and T.P.R. mainly wrote the manuscript, and all members contributed to writing this manuscript.

Competing interests

The authors declare no competing interests.

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There is NO Competing Interest.

VideoS1.mp4
Video S1: No jamming occurred when 10 wt% DEX was injected into 10 wt% PEG, related to Figure 2
VideoS2.mp4
Video S2: Interfacial shear rheology measurement of the complexes, related to Figure 2
VideoS3.mp4
Video S3: Contacting and separation of the microcapsules under action of magnetism wherein one microcapsule contains 1.0 wt% Fe₃O₄ (optical microscope), related to Figure 4
VideoS4.mp4
Video S4: Contacting and separation of the microcapsules under action of magnetism wherein one microcapsule contains 1.0 wt% Fe₃O₄(camera), related to Figure 4
VideoS5.mp4
Video S5: Autonomous movement (top view) of the hanging microcapsules towards the center of the container wherein one microcapsule was stained with FSS, related to Figure 5
VideoS6.mp4
Video S6: Autonomous movement (top view) of the hanging microcapsules towards the center of the container, related to Figure 5
VideoS7.mp4
Video S7: Density-difference-driven material shuttling and transporting system (top view) of the microcapsule in a quartz cuvette wherein one microcapsule was stained with FSS, related to Figure 5
VideoS8.mp4
Video S8: Density-difference-driven material shuttling and transporting system (side view) of the microcapsule in a quartz cuvette wherein one microcapsule was stained with FSS, related to Figure 5
Supplementaryinformation.docx

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Natural Nanoparticle complexes at water-water interfaces

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Abstract

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Introduction

Results

Nanoparticle complexation at water-water interfaces

Osmotic stress balance between the two aqueous phases

Selective transfer across ChNF/CNC complex layer

Autonomous mobility of microcapsules

Discussion

Methods

Declarations

References

Additional Declarations

Supplementary Files

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