Pickering emulsion‑driven MXene/silk fibroin hydrogels with programmable functional networks for EMI shielding and solar evaporation

In the era of rapid advancements in flexible electronics and sustainable water treatment, the demand for multifunctional materials that combine electromagnetic interference (EMI) shielding and efficient solar-driven evaporation has never been higher. Traditional hydrogels, however, struggle with controllable functional network design—often suffering from nanofiller agglomeration, poor structural stability, or limited performance tunability. Now, a team of researchers from Beijing University of Chemical Technology has developed a groundbreaking solution: surfactant-free Pickering emulsion-derived MXene/silk fibroin (SF) hydrogels with programmable hierarchical structures. Published in Nano-Micro Letters, this technology delivers exceptional EMI shielding and solar evaporation performance, opening new avenues for flexible electronics and water sustainability.

Why These Hydrogels Stand Out

The core innovation lies in leveraging the synergistic effects of amphiphilic silk fibroin and MXene nanosheets to create stable Pickering emulsions—eliminating surfactants while enabling precise control over hydrogel microstructures. This design addresses key limitations of conventional hydrogels and unlocks dual functionality:

• Surfactant-Free Stability & Programmable Structures: The amphipathy of SF (with hydrophilic amino acids like serine and hydrophobic segments like glycine-alanine repeats) and the reinforcement of MXene nanosheets form stable Pickering emulsions without any surfactants. Solvent exchange-induced microphase separation then converts these emulsions into hydrogels with tunable closed/open-cell structures—controlled by the oil phase volume fraction (φ). For example, φ=0.2 yields dense closed-cell networks (ideal for EMI shielding), while φ=0.33 forms open-celled pores (optimized for solar evaporation).
• Superior EMI Shielding Performance: The ordered conductive network of MXene (modified with PDDA for dispersion) and water polarization effect work in synergy to attenuate electromagnetic waves. The hydrogels achieve an impressive EMI shielding effectiveness (SE) of ~64 dB at 3 mm thickness—far exceeding commercial requirements for electronic devices (~20 dB). This performance is 2–3 times higher than non-emulsified MXene/SF hydrogels, thanks to reduced nanofiller agglomeration and enhanced interfacial polarization.
• Efficient Solar-Driven Evaporation: The hierarchical porous structure (large open pores for vapor escape, small pores for capillary water transport) and low evaporation enthalpy (1.4 kJ g^-1 vs. 2.3 kJ g^-1 for bulk water) enable a high evaporation rate of ~3.5 kg m^-2 h^-1 under 1-sun irradiation. Additionally, the hydrogels exhibit excellent salt tolerance—maintaining stable evaporation (over 2.5 kg m^-2 h^-1) even in 26 wt% NaCl brine for 60 hours.

Key Design, Fabrication, and Performance Details

1. Pickering Emulsion Mechanism: SF-MXene Synergy

The stability of the surfactant-free emulsion relies on the complementary roles of SF and MXene:

• Amphiphilic SF: Silk fibroin’s alternating hydrophilic/hydrophobic segments adsorb at the oil-water interface (toluene as oil phase), forming a metastable emulsion. Molecular dynamics (MD) simulations confirm SF’s strong interactions with both water (binding energy: -123 kJ mol^-1) and toluene (-41 kJ mol^-1), enabling interface stabilization.
• MXene Reinforcement: 2D MXene nanosheets (Ti₃C₂T_x) act as “nanoscale barriers” at the emulsion interface, preventing droplet coalescence. Their polar surface groups (-O, -OH, -F) form strong hydrogen bonds with SF, converting the metastable SF emulsion into a robust Pickering system. Rheological tests show the emulsion has moduli (G′, G′′) an order of magnitude higher than the SF/MXene solution—confirming enhanced structural stability.

2. Fabrication Process: From Emulsion to Multifunctional Hydrogel

The scalable fabrication process involves three key steps:

1. Emulsion Preparation: MXene (PDDA-modified for positive charge) and carbon nanotubes (CNTs, optional for enhanced conductivity) are dispersed in SF/LiCl-formic acid solution. Toluene (oil phase) is added and homogenized (3500 rpm) to form SF/MXene (SM) or SF/CNTs/MXene (SCM) Pickering emulsions.
2. Solvent Exchange: Emulsions are cast into molds and soaked in water for 24 hours. Water induces SF’s conformational transition from random coils to β-sheets (confirmed by FTIR, with β-sheet content increasing from 3.7% to 61.6%), driving microphase separation and self-cross-linking to form hydrogels.
3. Structure Tuning: Adjusting the oil phase volume fraction (φ=0.17–0.4) controls pore morphology: φ=0.2 forms closed-cell structures (for EMI shielding), while φ>0.33 creates open-cell networks (for solar evaporation). Freeze-drying further converts hydrogels into aerogels for specialized applications.

3. Dual Functional Performance

EMI Shielding: Conductive Networks + Water Polarization

• Conductivity Optimization: The emulsion-templated closed-cell structure reduces MXene agglomeration, boosting electrical conductivity to 0.58 S m^-1 (1.6 vol% MXene)—4.8 times higher than non-emulsified hydrogels.
• Shielding Mechanism: EMI SE is enhanced by two effects: (1) the continuous MXene network reflects and absorbs electromagnetic waves; (2) water in the hydrogel polarizes under EMWs, amplifying interfacial loss. Finite element simulations show the hydrogel reduces electric field intensity by 90% compared to MXene aerogels.
• Comparative Advantage: At 3 mm thickness, the hydrogel’s EMI SE (~64 dB) outperforms most polymer-based composites (e.g., MXene/polyvinyl alcohol hydrogels, ~45 dB) and meets strict shielding requirements for aerospace and consumer electronics.

Solar Evaporation: Hierarchical Pores + Low Enthalpy

• Mass/Heat Transfer Efficiency: Open-cell structures (φ=0.33) feature large pores (10–100 μm) for rapid vapor escape and small pores (~1 μm) for capillary water transport. MXene and CNTs absorb >90% of solar light, raising the hydrogel surface temperature to 38.6 °C under 1-sun irradiation.
• Low Evaporation Enthalpy: SF’s amino acids and MXene’s surface terminations reduce water hydrogen bonding (MD simulations show 30% fewer H-bonds than bulk water), lowering evaporation enthalpy to 1.4 kJ g^-1. This enables an evaporation rate of ~3.5 kg m^-2 h^-1—2.5 times higher than non-emulsified hydrogels.
• Salt Tolerance: SF’s amphiphilic chains interact with Na⁺/Cl⁻ ions to inhibit crystallization, while hierarchical pores facilitate salt diffusion. The hydrogel maintains stable evaporation in 14 wt% NaCl brine for 60 hours, with no pore blockage from salt deposits.

Future Outlook & Application Potential

This Pickering emulsion strategy offers a general platform for designing multifunctional hydrogels—extensible to other polymers (e.g., gelatin, chitosan) and nanofillers (e.g., graphene, BNNS). Key future directions include:

1. Flexible Electronics Integration: The hydrogel’s bendability (tensile strain up to 180%) and EMI shielding make it ideal for wearable devices (e.g., smartwatches, medical sensors) to protect against electromagnetic radiation.
2. Desalination Scalability: Optimizing the open-cell structure for large-area roll-to-roll production could enable low-cost solar desalination systems for arid regions.
3. Multifunctional Expansion: Incorporating thermoresponsive polymers or catalytic nanoparticles could add temperature-dependent shielding or water purification capabilities (e.g., heavy metal removal).

By bridging materials science and sustainability, these MXene/SF hydrogels demonstrate how programmable structures can unlock dual functionality—addressing critical needs in electronics and water treatment. Stay tuned for further innovations from this team as they advance toward commercialization!