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Silk fibroin as a surfactant for water-based nanofabrication

Nature Nanotechnology (2024)Cite this article

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Abstract

Water-based processing plays a crucial role in high technology, especially in electronics, material sciences and life sciences, with important implications in the development of high-quality reliable devices, fabrication efficiency, safety and sustainability. At the micro- and nanoscale, water is uniquely enabling as a bridge between biological and technological systems. However, new approaches are needed to overcome fundamental challenges that arise from the high surface tension of water, which hinders wetting and, thus, fabrication at the bio–nano interface. Here we report the use of silk fibroin as a surfactant to enable water-based processing of nanoscale devices. Even in minute quantities (for example, 0.01 w/v%), silk fibroin considerably enhances surface coverage and outperforms commercial surfactants in precisely controlling interfacial energy between water-based solutions and hydrophobic surfaces. This effect is ascribed to the amphiphilic nature of the silk molecule and its adaptive adsorption onto substrates with diverse surface energy, facilitating intermolecular interactions between unlikely pairs of materials. The approach’s versatility is highlighted by manufacturing water-processed nanodevices, ranging from transistors to photovoltaic cells. Its performance is found to be equivalent to analogous vacuum-processed devices, underscoring the utility and versatility of this approach for water-based nanofabrication.

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Fig. 1: SF as a natural surfactant.
Fig. 2: Efficient interfacial energy control using silk surfactants.
Fig. 3: Adaptive adsorption of silk surfactants at the film–solid interface.
Fig. 4: Water-enabled nanodevices with silk surfactants.

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Data availability

The main data that support the findings of this study are available in this Article and its Supplementary Information. Source data are provided with this paper. Any other relevant data are available from the corresponding authors upon request.

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Acknowledgements

This work was performed in part at the Harvard University Center for Nanoscale Systems (CNS), a member of the National Nanotechnology Coordinated Infrastructure Network (NNCI), which is supported by the National Science Foundation under NSF award no. ECCS-2025158. The authors acknowledge support of ONR grant N00014-19-1-2399 for this work.

Author information

Authors and Affiliations

  1. Silklab, Department of Biomedical Engineering, Tufts University, Medford, MA, USA

    Taehoon Kim, Beom Joon Kim, Nicholas A. Ostrovsky-Snider & Fiorenzo G. Omenetto

  2. Istituto Italiano di Tecnologia, Milan, Italy

    Giorgio E. Bonacchini

  3. Department of Physics, Tufts University, Medford, MA, USA

    Fiorenzo G. Omenetto

  4. Department of Electrical and Computer Engineering, Tufts University, Medford, MA, USA

    Fiorenzo G. Omenetto

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  1. Taehoon Kim
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Contributions

T.K. and B.J.K conceived the idea. T.K., B.J.K. and G.E.B. designed the experiments. T.K., B.J.K. and N.A.O.-S. performed the experiments. T.K., G.E.B., N.A.O.-S. and F.G.O. analysed the data and wrote the manuscript. F.G.O. supervised the activities.

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Correspondence to Fiorenzo G. Omenetto.

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There is a potential competing interest. Intellectual property based on the methods and approaches of the paper has been filed by Tufts University.

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Nature Nanotechnology thanks Jingliang Li and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Comparison of wetting efficiency between SF and commercial surfactants-added metal precursors.

a, Aqueous metal precursor solution containing commercially available surfactants or 30-minute boiled silk surfactant (SF30) at 0.01 w/v% concentration was deposited over hydrophobic substrates. While the metal precursor with the commercial surfactants showed less than 50% surface coverage, the solution with the silk surfactant exhibited uniform wetting across the entire surface. The molecular weight of each surfactant is shown in parentheses. b, Surface coverage comparison was made between Pluronic F127, a commercially available amphiphilic surfactant with a molecular weight of 12.5 kDa, and a silk surfactant (4 hour boiled, SF240) of similar molecular weight, both at 0.05 w/v% concentration. While Pluronic F127-added metal precursor showed partial wetting, SF-doped solution showed full surface coverage.

Extended Data Fig. 2 XPS depth profiling of pure SF film.

A 25 nm-thick silk film was coated on 300 nm wet thermal oxide substrate. Ion bombardment was conducted for 30 s before XPS depth profiling to get rid of adventitious carbon. Strong carbon (C1s) and nitrogen (N1s) signals were detected between the top surface and the wafer interface. Oxygen signal (O1s) was detected throughout the entire depth profile. On the other hand, silicon (Si2p) signal was only detected below the interface.

Extended Data Fig. 3 N1s signal mapping in indium oxide films made of aqueous metal precursor with varying silk concentrations on substrates with different surface energies.

All substrates were coated with 0.3 M indium precursor solutions to produce 25 nm-thick films, with the interface demarcated by a white dot line. The film coated with 0.3 M indium precursor containing 0.1 w/v% silk (1.1% SF to indium by weight) exhibited no discernible amine signal. However, as the silk content in the solution increased, a noticeable augmentation in the intensity of the N1s signal was observed. Solutions containing 0.3 w/v% and 1.0 w/v% silk (silk-to-indium weight ratios for 3.3% and 13.1%, respectively), showed N1s signals near the interface. On non-polar surfaces, SF was segregated to the interfacial boundary region, evident through a strong N1s signal in the vicinity of the interface. However, on bare or polar surfaces, SF was still detectable at the interface, but it tended to distributed more uniformly within the film. These results underpin the adaptive adsorption behavior of silk surfactants depending on the surface energy profile.

Extended Data Fig. 4 N1s signal mapping in indium oxide films fabricated by coating FOTS-treated hydrophobic surfaces with aqueous metal precursor containing silk surfactants with varying molecular weights at concentrations of 0.3 w/v%.

The trend observed in XPS depth profiling reaffirms the decrease in adsorption amount with decreasing SF MW, as demonstrated in Fig. 1 and consistent with theories explaining polymer adsorption isotherms.

Extended Data Fig. 5 AFM micrographs of the solid SF aggregates formed at the interface.

Indium oxide films were selectively etched, exposing the buried interface of SF with substrates of different surface energies, and at increasing SF concentrations. As the surface energy increased (left to right), the morphologies of SF aggregates changed from a flat-lying shape to a vertically grown island shape. Concentration increased the averaged height and horizontal density of the surface patterns but did not qualitatively affect the overall morphology. Height and deflection AFM maps of (a) buried SF layer over varying subtrates, and (b) reference substrate.

Extended Data Fig. 6 Adaptive adsorption behavior of silk surfactants.

The adsorption behavior of silk surfactant exhibited a dependence on the surface energy of the underly substrate. On low-energy non-polar surfaces, the hydrophobic segments of the silk chain adsorbed to the substrate, creating flat agglomerates along the surface. In contrast, on high-energy polar surfaces, the hydrophilic terminal ends of the silk chain were more inclined to interact with the surface compared to the internal hydrophobic domains. This interaction leads to the formation of vertically agglomerated structures.

Extended Data Fig. 7 Comparison of SiO2-gated IGZO transitor fabricated with silk and commercial surfactant-added aqueous precursor solution.

a, b, Digital (left) and microscope images (middle and right) of fabricated IGZO device using SDS or SF30 at the concentration of 0.01 w/v% (for a) and 0.1 w/v% (for b). Middle microscope image shows low magnification top view (x5 objective lens), and right microscope image shows enlarged IGZO channel morphology (x20 objective lens, scale bar is 200 μm). c, Schematic illustrating device structure and characterization setup. d, e, transfer curve of fabricated IGZO transitor with 0.01 w/v% (for d) or 0.1 w/v% (for e) of surfactant. Data represented as only mean (n = 3).

Source data

Extended Data Fig. 8 Universality of silk surfactant-assisted wetting for various water-enabled nanodevices.

The wetting mechanism proposed in this study can be applied to almost any aqueous metal precursor solution. As examples, the digital images show the coating results on a hydrophobic substrate after adding 0.01 w/v% of SF to a total of 9 different metal precursor aqueous solutions.

Extended Data Fig. 9 Another technical opportunity of silk surfactant-assisted universal wetting.

a, This technology can be applied to multi-layer stacking by sequential coating over universal substrates without surface treatment, and the coated film can be transferred to the desired substrate through a release strategy. For examples, b, device structure of an Al2O3-gated IGZO transistor prepared by sequential coating of each metal precursor and c, its transfer curve. d, Transfer-printed halide perovskite (MAPbI3) film over a transparent PET film and e, its optical properties.

Extended Data Fig. 10 Silk surfactant-assisted universal wetting over various substrates.

a, Digital image of PbNO3 and MAPbI3 film created by coating a PDMS surface with an SF-added PbNO3 solution, and followed by MAI treatment for MAPbI3. b, Digital images of the bended or stretched MAPbI3/PDMS block. c, Digital image of ZrO2 film formed by coating a Zn foil with an SF-added ZrOCl2 solution.

Supplementary information

Supplementary Information

Supplementary Notes 1–6 and Figs. 1–5.

Supplementary Data 1

Compressed source data for Supplementary Figs. 1–5.

Source data

Source Data Fig. 1

Source data for Fig. 1.

Source Data Fig. 2

Source data for Fig. 2.

Source Data Fig. 3

Source data for Fig. 3.

Source Data Fig. 4

Source data for Fig. 4.

Source Data Extended Data Fig./Table 7

Source data for Extended Data Fig. 7d,e.

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Kim, T., Kim, B.J., Bonacchini, G.E. et al. Silk fibroin as a surfactant for water-based nanofabrication. Nat. Nanotechnol. (2024). https://doi.org/10.1038/s41565-024-01720-3

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