Natural nanoparticle complexes at water-water interfaces

Wang, Han; Lu, Yi; Bai, Long; Zhu, Mengqi; Liu, Xiyue; Li, Zhiguo; Liu, Yang; Li, Wei; Liu, Shouxin; Huan, Siqi; Chen, Chaoji; Russell, Thomas P.; Rojas, Orlando J.

doi:10.1038/s41467-025-60587-7

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Published: 01 July 2025

Natural nanoparticle complexes at water-water interfaces

Nature Communications volume 16, Article number: 5468 (2025) Cite this article

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Abstract

Aqueous two-phase systems (ATPSs) with multifunctional attributes have significant promise as biomimetic materials, but current approaches do not harness their full potential. Here, 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 complexes 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 ATPSs-based microreactors and microrobots.

Introduction

Reconfigurable all-liquid systems have been used to control the structural and chemical characteristics of soft matter¹. Most efforts in the field have harnessed solvent-water interactions to achieve polarity-based spatial compartmentalization^2,3,4. More recent efforts have focused on aqueous two-phase systems (ATPSs), produced by the phase separation of mixed aqueous solutions containing incompatible polymers^5,6,7,8,9. ATPSs offer a natural platform for all-aqueous biomimicry^10,11, e.g., based on aqueous dextran (DEX) and polyethylene glycol (PEG) solutions¹². 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. ATPSs stabilization has been achieved using polyelectrolytes or colloidal particles^15,16,17; however, component formulation and stability are not yet optimized.

Structured liquids, generated by complexing two oppositely charged polyelectrolytes (PE) at the interface of two contiguous liquid phases¹⁸, afford a route to address this challenge^19,20,21. 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 light²³, 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 phases²⁵. By adjusting the flux of oppositely charged substances to the interface so as to optimize interfacial complexation, and accordingly, 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, interfacial complexes have enabled the generation of non-equilibrium all-aqueous structures, such as tubular shapes^5,28, as well as complex multi-compartmentalized microcapsule structures²⁹. Endowing such constructs with additional functions offer significant potential for cargo encapsulation and release³⁰.

In principle, PE/PE systems form deformable membranes, while PE/NP complexes are thicker and more rigid³¹. 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 time³², enhancing the mechanical properties of the interfacial complexes. These composite complexes can be used as biomimetic constructs³³, 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 (ChNFs)^34,35. 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)^36,37,38. The ChNF/CNC complexes possess both excellent deformability and rigidity, comparable to those of PE/PE and PE/NP systems, respectively. Moreover, their rod-on-rod porous assembly structure results in enhanced permeability, distinguishing them from conventional PE-based complexes.

In this work, ChNF/CNC complexes are formed at water-water interface 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.

Results

Nanoparticle complexation at water-water interfaces

As a proof-of-concept, biomimetic underwater microcapsules were fabricated in ATPSs³² 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 (Supplementary Fig. 1), 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, $\varDelta S > 0$)³⁹. 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. Fourier transform infrared spectroscopy (FTIR) results confirmed that no covalent bonds were formed during the interfacial complexation process, indicating that the assembly was driven purely by physical interactions (Supplementary Fig. 2). Meanwhile, a slight shift in the 1550 cm⁻¹ (N-H bending) peak suggests electrostatic interactions between the amino groups (-NH₂) of ChNF and the sulfate groups (-SO₃H) of CNC, similar to those observed in ChNF/SA complexes⁴⁰.

**Fig. 1: Biomimetic underwater microcapsules formed by interfacial complexation of oppositely charged nanoparticles in aqueous two-phase systems (ATPSs).**

Compared to interfacial complexes formed through polyelectrolyte/polyelectrolyte (PE/PE) and polyelectrolyte/nanoparticle (PE/NP) interactions (Fig. 1d), the ChNF/CNC complexes distinctively integrate the key features of all systems: rigidity, deformability, and permeability. This combination enables applications beyond the capabilities of traditional PE-based systems. While PE/PE systems have shown enhanced performance in autonomous underwater transport, controlled delivery, and microreactor applications, our nanoparticle/nanoparticle (NP/NP) system provides a broad and versatile platform for such functionalities. Nevertheless, we note that there is no intent to compare ChNF/CNC-based microcapsules to the same level of performance as biological cell membranes or the bristle structures on water strider legs. Instead, the latter serves as a natural inspiration to the development of all-aqueous, biomass-based interfacial assemblies that share similar fundamental properties using a NP/NP approach.

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) (Supplementary Table 1). Complexation and pendant droplet stabilization were found for the nanoparticle complexes (ChNF in DEX and CNC in PEG) (Fig. 2a). No complexation was evidenced in the absence of the nanoparticles (Supplementary Movie 1), where the DEX droplet fell due to the ultralow interfacial tension. At higher pH values, the amine groups on the surface of ChNFs are deprotonated (Fig. 2b)⁴¹, 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 preparation⁴².

**Fig. 2: Formation, morphology, properties of chitin nanofiber/cellulose nanocrystal (ChNF/CNC) interfacial complexes.**

We further investigated the microstructure of ChNF/CNC complexes by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and atomic force microscopy (AFM). These data showed an interconnected, multi-layered network with voids, consisting of both rigid CNC clusters and flexible ChNF networks, contributing to a balance of mechanical strength and adaptability (Fig. 2c and Supplementary Fig. 3). The permeability of ChNF/CNC interfacial complexes was evaluated using fluorescein isothiocyanate, FITC-labeled PEG (Mw = 10,000 Da). Microcapsules formed by the ChNF/CNC complexes allowed rapid diffusion of PEG within ~5 min, attributed to the porous rod-on-rod structure of the interface (Supplementary Fig. 4). In comparison, chitosan (CS)/sodium alginate (SA) microcapsules exhibited much slower PEG diffusion, requiring around 60 min for similar exchange (Supplementary Fig. 5). The slower transport in CS/SA capsules reflects the presence of a denser polymeric network, which restricts molecular permeation^5,32,43. However, the ChNF/CNC complex layer was effective in blocking particles (50 nm polystyrene spheres, Supplementary Fig. 6), implying size exclusion effects due to the characteristic cells in the network formed at the interface.

To investigate the deformability of the ChNF/CNC interfacial complex, we measured the internal pressure of microcapsules during deformation using a microsyringe pump system equipped with a vacuum pressure gauge (Supplementary Fig. 7a–c). As a reference, the deformability of CS/CNC interfacial complexes under the same matrix conditions was also measured (Supplementary Fig. 7d). PE/PE complexes are typically considered highly deformable due to their soft polymeric nature^5,32. The results indicate that the CS/CNC interfacial complexes required higher negative pressure to achieve the same deformation as the ChNF/CNC complexes, supporting the hypothesis that ChNF/CNC complexes exhibit greater deformability compared to CS/CNC complexes (Supplementary Fig. 8). More importantly, we found that the mechanical properties of ChNF/CNC interfacial complexes can be tuned by adjusting the surface charge density of ChNF. Specifically, the low-charge-density chitin nanofiber (ChNF-L)/CNC complexes deformed more easily under lower negative pressure, which can be attributed to the lower relative content of rigid CNC crystals within the interfacial complexes. However, the high-charge-density chitin nanofiber (ChNF-H)/CNC interfacial complexes exhibited enhanced rigidity (Supplementary Fig. 8). In subsequent experiments investigating the thickening behavior of the interfacial complexes, under identical osmotic pressure gradients (${\varPi }_{10{wt}\%{PEG}} > {\varPi }_{10{wt}\%{DEX}}$), ChNF-L/CNC complexes exhibited wrinkling behavior similar to PE/PE interfacial complexes, indicating higher deformability (Supplementary Fig. 17). These findings demonstrate that we can precisely regulate interfacial complexes mechanics by modifying the surface charge density of ChNF, enabling their adaptation to various applications requiring different mechanical performance.

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 Supplementary Movie 2). Supplementary Fig. 9, as a control, shows a good correlation between the probe and the trap position during the test, validating the reliability of the method. The results shown in Fig. 2e and Supplementary Fig. 10 correspond to the time and frequency evolution of the storage (G’) and loss (G”) moduli for the interfacial complexes (ChNF/CNC and CS/CNC) at the water-water interface. The CS/SA interfacial complexes were 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’, their loss factor value was significantly higher than that of the CS/CNC complexes. This indicates that the ChNF/CNC system formed a tough, yet flexible interfacial layer. To further validate these findings, we conducted additional interfacial shear rheology measurements using a double-walled Couette flow-cell setup, obtaining consistent results (Supplementary Fig. 11). The agreement between these independent methods shows that both interfacial complexes possess mechanical rigidity, with CS/CNC forming a stiffer interfacial structure, while ChNF/CNC complexes exhibit a balance of toughness and flexibility. The observed moduli differences can be explained by the interaction and arrangement of rod-like, rigid CNCs in the complexes^44,45. In the CS/CNC complexes, the CS chains facilitated strong binding and orderly arrangement with CNCs imparting the rigidity features. The high aspect ratio (Supplementary Fig. 1) and random distribution of surface charges of ChNF resulted in a less ordered complex, contributing to the formation of a flexible interfacial film. SEM and AFM further support the coexistence of randomly arranged nanofibers and ordered nanocrystalline clusters within the ChNF/CNC complex layer (Supplementary Fig. 3). 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 constructs³². 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 Supplementary Table 1), 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 complexes equilibrated, affecting both convective flow and permeability (Fig. 3b).

**Fig. 3: Osmotic stress-induced shape evolution of pendant droplet and suspended microcapsule.**

At ${\varPi }_{{PEG}}$ $ < $ ${\varPi }_{{DEX}}$, a pendant DEX droplet gradually expanded due to continuous influx of water from the PEG solution (Fig. 3a, b). Fluorescence and optical microscopy images (Fig. 3c and Supplementary Fig. 12 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 (Supplementary Fig. 13), while the CS/CNC complex layer burst soon after fluid influx (Supplementary Fig. 14). 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 Supplementary Fig. 15), 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, b), which was further confirmed by fluorescence and optical microscopy observations of the microcapsules (Fig. 3c and Supplementary Fig. 16). 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. Under this osmotic pressure gradient (${\varPi }_{{PEG}}$ $ > $ ${\varPi }_{{DEX}}$), the interfacial membrane thickness increased over time, initially undergoing a rapid thickening before gradually stabilizing (Supplementary Fig. 17). Notably, the final interfacial complexes thickness increased with higher ChNF charge density, which can be attributed to stronger electrostatic attraction between ChNF-H and CNC, leading to enhanced CNC accumulation at the interface. This also demonstrates that the presence of free nanoparticles, not involved in the interfacial assembly, is inevitable, which does not affect the preparation, performance, and application of microcapsules. From these results, it is evident that the ChNF/CNC complexes 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 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 complexes, which indicates that the ChNFs and CNCs were oriented parallel to the interface (Fig. 3c). However, owing to the morphology of ChNF and CNC, a different ordering behavior may occur than that found for the polyelectrolyte/CNC interfacial complexes (Supplementary Fig. 14c). The assembly of ChNF and CNC at the interface was further explored by changing their distribution in ATPSs (Supplementary Fig. 18). A droplet was stabilized when suspending ChNF (1 wt%) in PEG and CNC (1 wt%) in DEX (Supplementary Fig. 18a), where osmotic stress transferred water from the internal DEX phase to the external PEG phase, gradually reducing the droplet volume. However, thickening of the complexes did not occur, and the CNC/ChNF complex layer wrinkled (Supplementary Fig. 18b). Extinction along the crossed polarization was not observed (Supplementary Fig. 18c), suggesting a strong partitioning of CNC to the DEX phase⁴⁶. 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 (Supplementary Fig. 18a).

Selective transfer across ChNF/CNC complex layer

Ionic fluorescent probes were added to different phases across the complex layer⁴⁷ (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 complexes by electrostatic interactions between FSS and ChNF (Supplementary Fig. 19). Similarly, positively charged Nile blue A (NBA) in the DEX phase was excluded from the microcapsule and the thickened ChNF/CNC complex layer (Supplementary Fig. 20), due to the negatively charged CNC in the external PEG phase and the complexes. The permeability of the ChNF/CNC complex layer and the asymmetric diffusion of oppositely charged molecules suggest possible applications 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 (Supplementary Fig. 21) 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 fluorescence 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 (Supplementary Movie 3), implying that the FSS transferred across the non-coalescent complex layers. Moreover, thickening of the complexes continuously occurred, in contrast to the observation for PE/NP complexes³².

**Fig. 4: Selective transfer of ionic species across the chitin nanofiber/cellulose nanocrystal (ChNF/CNC) complex layer.**

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 (Supplementary Movie 4). 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 for 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 solid⁴⁸. 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 by⁴⁹:

$$F\left(x\right)=-{F}_{{{{\rm{V}}}}}\cot \theta {{{{\rm{e}}}}}^{-\frac{x}{{l}_{c}}}=-\gamma C\cos \alpha \,\cot \theta {{{{\rm{e}}}}}^{-\frac{x}{{l}_{c}}}$$

(1)

where ${F}_{V}$ 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. 5a⁵⁰, a positive meniscus (${F}_{{{{\rm{V}}}}}$ $ > $ 0 or $\cos \alpha \,\cot \theta$ $ > $ 0) draws a floating object towards the wall, up the meniscus, provided $F(x)$ is sufficiently large. For a negative meniscus (${F}_{{{{\rm{V}}}}}$ $ < $ 0 or $\cos \alpha \,\cot \theta$ $ < $ 0), the object is repelled from the wall. This was tested for ChNF/CNC-assembled 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 (Supplementary Table 1), 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 (Supplementary Fig. 22a and Supplementary Movie 5) and by optical microscopy (Supplementary Fig. 22b and Supplementary Movie 6): 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. Furthermore, this assembly strategy enables large-scale production of microcapsules (Supplementary Fig. 23), demonstrating its rapid, robust, and scalable nature, which holds promise for applications in microfluidics and related fields.

**Fig. 5: Autonomous microcapsule mobility and material shuttle/transport effects.**

Enhanced material transport can further leverage microcapsule permeability, along with mass transport of the ChNF/CNC complexes. 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 Supplementary Table 1, 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. 5d₁ and Supplementary Movies 7 and 8, 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 complexes, the diffusion-induced cyclic and autonomous microcapsule migration is associated with dynamic transfer without the need for external stimuli. For PE/PE systems⁵⁰, similar underwater microcapsule migration has been reported, but ionic crosslinking is typically required to form hydrogels for structural stability in catalytic applications involving gas release. However, CS/CNC complexes exhibit low deformability and strong interfacial thickening, which prevents the system from undergoing the continuous structural adaptation required for switchable mobility (Supplementary Fig. 24).

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% (Supplementary Fig. 25). For the larger container (20 × 20 × 20 mm³), the microcapsules could only return to the center when the CNC concentration was 0.5 wt% (Supplementary Fig. 26). 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 (Supplementary Fig. 27), and at 1.0 wt% ChNF, the microcapsule could return to the center of the large container (Supplementary Fig. 28). 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 (Supplementary Figs. 29 and 30).

The microcapsules that were able to return to the center of the container relied on the differences of density (Fig. 5d₂ and Supplementary Fig. 31a). 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 (Supplementary Table 1). The system viscosity increased with increasing CNC concentration in the PEG phase (Supplementary Fig. 32), 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 (Supplementary Table 1), 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 (Supplementary Fig. 31). Overall, the microcapsule that is encapsulated by ChNF/CNC complexes is easily tailored for self-driven motility.

However, when the ATPSs in a microcapsule system capable of autonomous, switchable sub-surface mobility was either completely removed or when either component (DEX or PEG) was absent, this mobility could no longer be achieved. In the absence of PEG, the droplet phase had a significantly higher density than the matrix phase (Supplementary Table 1), causing the droplet to fail to suspend beneath the interface and rapidly sink to the bottom of the container (Supplementary Fig. 33a). In the absence of DEX, the microcapsule exhibited only one-way movement, as the lack of an internal DEX phase prevented further density adjustments driven by osmotic exchange, restricting the return motion (Supplementary Fig. 33b). When the ATPSs were entirely removed, the minimal density difference between the two phases made even one-way movement difficult to achieve (Supplementary Fig. 33c). Therefore, absent additional external components, ATPSs components are essential for programming microcapsule motion by precisely tuning the density and osmotic pressure gradients, which serve as the driving forces for controlled and reversible movement.

In addition to utilizing the intrinsic density and osmotic pressure differences of the two-phase system as the driving force for autonomous and switchable underwater movement, applying buoyancy to the microcapsules presents another effective strategy^51,52. Here, we demonstrate that NP/NP-assembled microcapsules can serve as microreactors for the catalytic decomposition of hydrogen peroxide (H₂O₂) by manganese dioxide (MnO₂), enabling density-driven, switchable motion through controlled oxygen generation (Fig. 5e and Supplementary Movie 9). Due to the initially higher density of the DEX phase, two microcapsules—one containing 0.5 wt% MnO₂ (blue circle) and the other 1 wt% H₂O₂ (yellow circle)—formed hanging states and migrated toward the center of the container. Upon contact, H₂O₂ diffused through the permeable NP/NP interfacial complexes into the MnO₂-containing microcapsule, triggering catalytic decomposition and oxygen generation. As oxygen accumulated, the microcapsule’s internal density decreased, causing a transition from a hanging to a floating state. This shift led to migration toward the container wall, where the microcapsules separated, terminating the reaction. The process was successfully repeated by introducing additional MnO₂-containing microcapsules, demonstrating continuous reaction cycling and controlled transport within the system (Supplementary Fig. 34). Unlike PE/PE-assembled microcapsules, which require pre-crosslinking to enhance structural stability⁵⁰, NP/NP interfacial assemblies inherently exhibit tunable mechanical properties and high permeability, making them better suited for applications in underwater autonomous movement, substance transport, and microreactors.

The most crucial prerequisite for microcapsule movement is the effective formation of the ChNF/CNC complexes at the interface (Supplementary Fig. 37), which prevents rupture and resists deformation caused by shearing and mass exchange. The complexes formed by interfacial assembly with different nanoparticle concentrations were verified by injecting DEX solution into the PEG solution (Fig. 2b, Supplementary Fig. 35 and 36). 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 complexes held the entire structure, causing the sinking (Supplementary Figs. 27 and 28). Furthermore, the ChNF/CNC interfacial complexes prevented aqueous tubules from breaking due to Plateau-Rayleigh instabilities, enabling the fabrication of stable all-aqueous 3D architectures by 3D printer (Supplementary Fig. 38, Supplementary Movies 10 and 11). These architectures exhibit excellent biocompatibility and non-toxicity, making them promising candidates for applications in drug encapsulation and delivery, cell culture, and all-liquid fluidic devices.

Discussion

In this study, ChNF/CNC complexes were assembled in structured ATPSs. 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 complexes 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 ATPSs for separation and cargo transfer, relevant to cell biomimicry, microreactors, and microrobots.

Methods

Materials

Chitin nanofibers (ChNF) were produced by mechanical treatment of deacetylated $\alpha$-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 was included in the Supplementary Information.

Complexation at the interfaces

Interfacial complexes 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 complexes 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 complexes 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 complexes was analogous to that in complexes-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.

Interfacial rheology

We used a magnetic needle (ISR1220) with a radius of 200 μm, a mass of 5 mg, and a length of 12.2 mm, conducting the experiments in a 20 mm wide channel. Before measuring the rheological properties of the interfacial complexes, we first measured the background signal of the bulk phase (1 wt% CNC) to quantify its influence on the probe motion. The preparation of the complexes for testing involved the dropwise addition of a 1 wt% ChNF suspension or a 1 wt% chitosan (CS) solution into a 1 wt% CNC suspension. Subsequently, a magnetic probe (ISR1220) was placed on the complexes, and the magnetic trap above it was oscillated horizontally at a frequency of 1 Hz (Supplementary Movie 2). The viscoelasticity of the complexes was calculated from the motion phase difference between the magnetic probe and the magnetic trap.

The viscoelastic properties of complexes were also measured using a stress-controlled discovery hybrid rheometer (DHR-3) from TA Instruments (USA) equipped with a Du Nouy ring and the double-walled Couette flow-cell geometry. The latter is made of a glass external cylinder and a Teflon internal cylinder. The rheological study was conducted using the following protocol: (1) time sweep for 2000 s at a frequency ω = 6.28 rad s⁻¹ and strain γ = 0.2 %; (2) frequency sweep at strain γ = 0.2 %, exploring the frequency range from 0.01 to 100 rad s⁻¹.

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 (movable) 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.

Statistics and reproducibility

All experiments were repeated independently with similar results at least three times.

Data availability

All data that support the findings of this study are included in the article and its Supplementary Information. The data that support the findings of this study are available from the corresponding author upon request. Source data are provided with this paper.

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Acknowledgements

This work was partially supported by the National Natural Science Foundation of China (32301513 to S.Q.H. and 32201483 to L.B.). 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). Y.L. acknowledge the “Hundred Talents Program” of the Chinese Academy of Sciences. The authors thank Prof. Baobin Wang (Qilu University of Technology), Dr. Xin Shu (UBC), Wei Li and Dr. Peng Liu (Biolin Scientific AB) for technical assistance with the interfacial shear rheometer. The authors also thank Yuxin Zhang from Scientific Compass (www.shiyanjia.com) for providing assistance with freezing point osmometer. We dedicate this work to Dr. Long Bai (1988–2024), whose foundational contributions were vital to this research. A passionate scientist and the principal driving force of this project, he remained fully engaged until his final days. His intellect, generosity, and enthusiasm continue to inspire us.

Author information

These authors contributed equally: Han Wang, Yi Lu.
Deceased: Long Bai.

Authors and Affiliations

State Key Laboratory of Utilization of Woody Oil Resource, Northeast Forestry University, Harbin, China
Han Wang, Wei Li & Siqi Huan
Key Laboratory of Bio-based Material Science & Technology (Ministry of Education), Northeast Forestry University, Harbin, China
Han Wang, Long Bai, Mengqi Zhu, Xiyue Liu, Zhiguo Li, Yang Liu, Wei Li, Shouxin Liu & Siqi Huan
Bioproducts Institute, Department of Chemical and Biological Engineering, University of British Columbia, Vancouver, BC, Canada
Yi Lu, Long Bai, Siqi Huan & Orlando J. Rojas
Key laboratory of Hubei Biomass-Resource Chemistry and Environmental Biotechnology, School of Resource and Environmental Sciences, Wuhan University, Wuhan, China
Chaoji Chen
Department of Polymer Science and Engineering, University of Massachusetts Amherst, Amherst, MA, USA
Thomas P. Russell
Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA
Thomas P. Russell
Advanced Institute for Materials Research, Tohoku University, Aoba, Sendai, Japan
Thomas P. Russell
Department of Chemistry, University of British Columbia, Vancouver, BC, Canada
Orlando J. Rojas
Department of Wood Science, University of British Columbia, Vancouver, BC, Canada
Orlando J. Rojas

Authors

Han Wang
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Yi Lu
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Long Bai
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Mengqi Zhu
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Xiyue Liu
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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. supervised the experimental strategy and critically reviewed the manuscript. 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.

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Correspondence to Siqi Huan, Chaoji Chen, Thomas P. Russell or Orlando J. Rojas.

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Wang, H., Lu, Y., Bai, L. et al. Natural nanoparticle complexes at water-water interfaces. Nat Commun 16, 5468 (2025). https://doi.org/10.1038/s41467-025-60587-7

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Received: 26 September 2024
Accepted: 27 May 2025
Published: 01 July 2025
DOI: https://doi.org/10.1038/s41467-025-60587-7