Research ArticleChemistry

Guiding kinetic trajectories between jammed and unjammed states in 2D colloidal nanocrystal-polymer assemblies with zwitterionic ligands

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Science Advances  03 Aug 2018:
Vol. 4, no. 8, eaap8045
DOI: 10.1126/sciadv.aap8045
  • Fig. 1 Competitive binding by Lewis basic zwitterionic ligands and polymer constituents to Lewis acidic naked NC surfaces dictates the propensity and rate at which NCPSs form at a liquid-liquid interface.

    It also dictates important aspects of buckling behavior of the interfacial film when solidified on interfacial compression. At sufficiently high concentrations of the zwitterionic ligand, it becomes possible to reconfigure the solid-like state of a buckled interfacial film of NCPSs back into a liquid-like state (that is, where the wrinkles are no longer observed). The time frame for this dissipative process is also dependent on ligand concentration.

  • Fig. 2 Nonreconfigurable, jammed 2D assemblies of colloidal NCPSs.

    (A) Lewis acid–Lewis base interactions between cationic naked NCs and amine-terminated PDMS polymers allow the formation of NCPSs at the interface between DMF and PDMS oil, which are immiscible. (B) The liquid-like 2D assembly of NCPSs can be solidified by compressing the liquid-liquid interface, shown here as the DMF droplet is retracted into the syringe. This buckled state of the 2D interfacial film exhibits wrinkles, which are persistent indefinitely.

  • Scheme 1 Chemical synthesis of zwitterionic NC ligand 1.
  • Fig. 3 Competitive ligand-mediated reconfiguring of jammed 2D assemblies of colloidal NCPSs at a liquid-liquid interface.

    (A) The recruitment of 3K-PDMS-NH2 and naked Fe3O4 NCs to the interface occurs on different time scales and depends on the relative concentration of each. These processes can be observed by monitoring the interfacial tension as a function of time for a pendant droplet of DMF suspended in a bath of PDMS/dodecane for systems configured with either 3K-PDMS-NH2 (black squares), 3K-PDMS-NH2 + Fe3O4 NCs (blue circles), or 3K-PDMS-NH2 + Fe3O4 NCs + competitive ligand 1 (red triangles). [3K-PDMS-NH2] = 5% w/w; [Fe3O4 NCs] = 0.5 mg ml−1; [1] = 1 mg ml−1. (B) The solid-like character of a buckled, 2D film of colloidal NCPSs is evidenced by wrinkling for droplets whose interface has been compressed upon droplet retraction. Only in the instance where competitive ligand 1 is present do the wrinkles in the film return to a smooth state, consistent with a dissipative process whereby the ligands promote the ejection of NCs from the film back into the bulk. Snapshots of such a reconfigurable droplet are shown at steady state, before retraction (top); immediately after droplet retraction (middle), where wrinkles appear; and after 20 min, whereupon the system has relaxed to an unwrinkled state (bottom). (C) Interfacial tension as a function of time for a pendant droplet of DMF suspended in a bath of PDMS/dodecane for systems configured with 3K-PDMS-NH2 (5% w/w), Fe3O4 NCs (0.5 mg ml−1), and varying concentrations of zwitterionic ligand 1 (0.5 to 5.0 mg ml−1).

  • Fig. 4 The time scale of ligand-mediated reconfiguring of jammed, solid-like colloidal NCPS assemblies into a liquid-like state at a liquid-liquid interface depends on ligand concentration.

    (A) Snapshots of each droplet’s morphology upon NCPS formation at the liquid-liquid interface. (B) Snapshots of each droplet’s morphology after the initial film compression and after the reconfiguring film of NCPSs had reached steady state. (C) Schematic representations of the influence of ligand concentration on interface activity and binding ability of colloidal naked NCs at low (I), moderate (II), and high (III) ligand concentrations.

  • Fig. 5 Distinguishing the stiffness of NCPS monolayers in solid- and liquid-like states at a liquid-liquid interface using in situ AFM.

    (A) Schematic diagram of the in situ AFM experimental setup. (B) Force (F) curves measured for NCPS films at the DMF–silicone oil interface in the absence and in the presence of zwitterionic ligand 1 confirm that solid-like NCPS films are stiffer than liquid-like films. The distinctive mechanical properties observed for solid- and liquid-like states were concomitant with differentiated morphologies for NCPS films at the DMF–silicone oil interface in the absence (C) and in the presence (D) of zwitterionic ligand 1. Scale bars, 200 nm; z range color scale, 100 nm.

  • Fig. 6 Pendant droplet responses to magnetic fields.

    (A) When the pendant droplet was set in the middle of two magnets with the opposite polarity, the 2D NCPS assembly was displaced in space and the encapsulated fluid was expanded along the axis of the magnetic field and was compressed along the axis normal to the field. (B) At 2.5 mg ml−1 concentration of zwitterionic ligand 1, the interfacial tension of the droplet increased from γpolar/nonpolar = 4.0 to 5.6 mN m−1 when the magnetic field was applied. The droplet recovered both its equilibrium shape and interfacial tension once the field was removed.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/8/eaap8045/DC1

    Fig. S1. Interfacial tension as a function of time for a system configured with both Fe3O4 NCs and 3K-PDMS-NH2.

    Fig. S2. Interfacial tension as a function of time for a system without NCPSs.

    Fig. S3. Interfacial tension as a function of time for a system configured with only 3K-PDMS-NH2.

    Fig. S4. Interfacial tension as a function of time for a system configured with only zwitterionic ligand 1.

    Fig. S5. Interfacial tension as a function of time for a system configured with zwitterionic ligand 1 and 3K-PDMS-NH2.

    Fig. S6. Zwitterionic ligand binding to naked NC surfaces.

    Fig. S7. Steady-state interfacial tension as a function of zwitterionic ligand concentration.

  • Supplementary Materials

    This PDF file includes:

    • Fig. S1. Interfacial tension as a function of time for a system configured with both Fe3O4 NCs and 3K-PDMS-NH2.
    • Fig. S2. Interfacial tension as a function of time for a system without NCPSs.
    • Fig. S3. Interfacial tension as a function of time for a system configured with only 3K-PDMS-NH2.
    • Fig. S4. Interfacial tension as a function of time for a system configured with only zwitterionic ligand 1.
    • Fig. S5. Interfacial tension as a function of time for a system configured with zwitterionic ligand 1 and 3K-PDMS-NH2.
    • Fig. S6. Zwitterionic ligand binding to naked NC surfaces.
    • Fig. S7. Steady-state interfacial tension as a function of zwitterionic ligand concentration.

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