Research ArticleCOLLOIDS

Assembling oppositely charged lock and key responsive colloids: A mesoscale analog of adaptive chemistry

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Science Advances  15 Sep 2017:
Vol. 3, no. 9, e1700321
DOI: 10.1126/sciadv.1700321
  • Fig. 1 Responsive lock-and-key microgel particle characterization.

    (A) CLSM micrographs of bowl-shaped composite microgels fluorescently labeled in red (rhodamine dye) used as a lock particle. (B) Key particles consisting of cross-linked PNIPAM microgel particles labeled in green (fluorescein), as imaged by CLSM. Scale bars, 1 μm. (C and D) Hydrodynamic radius and electrophoretic mobility measured as a function of temperature: bowl-shaped lock composite microgel (solid circles) and spherical key microgel particle (solid squares). The measurements cross three regions delimited by the VPT temperature of the lock-and-key particles, TVPT,L and TVPT,K. In region 1 (T < TVPT,K), both particles are swollen. In region 2 (TVPT,KTTVPT,L), lock particles are swollen and key particles are collapsed. In region 3 (TVPT,L < T), both particles are collapsed.

  • Fig. 2 Calculations of the electrostatically driven self-assembly of oppositely charged lock and key particles.

    (A) Schematic representation of the oppositely charged lock and key particles used for the modeling of electrostatic interactions. (B) Minimum electrostatic energy in the lock-and-key (LK) configuration as a function of the size ratio, RK/RL, calculated for different ionic strengths using RL = 500 nm, ZL = −1000e, and ZK = +100e. (C) Same calculations as in (B) but in the LK¯ configuration. (D) Energy difference between the LK¯ and LK configurations at different ionic strengths (see text for more details).

  • Fig. 3 Minimum electrostatic energy of the specific and unspecific self-assembly between lock and key particles.

    (A) Minimum electrostatic energy determined at csalt = 10−5 M from the calculations in the LK (empty circles) and LK¯ (solid circles) assemblies schematically depicted by the insets. (B) Van der Waals interaction contribution determined at a separation distance of 10 nm in three limiting cases illustrated by the corresponding schematic representations: RLRK in LK configuration reducing to a plane-plane approximation (solid circles); RK << RL in LK configuration, where the interaction simplifies to a sphere-plane approximation (solid squares); and lock and key particles in LK¯ configuration corresponding to a sphere-sphere approximation (empty circles). (C) Entropy contribution to the free energy at different distances from the ideal contact (see text and the Supplementary Materials). The schematics represent the relative size of the lock and key particles at different temperatures pointed by the arrows.

  • Fig. 4 Influence of the ionic strength on the lock-and-key self-assembly.

    (A to C) 2D CLSM micrographs of the lock and key particles (cL = 1 wt %, NL/NK ≈ 1) recorded at 20°C at various ionic strengths [(A) csalt = 10−5 M, (B) csalt = 10−4 M, and (C) csalt = 2.5 × 10−3 M]. (D) By slowly rising the temperature from 20° to 48°C at an ionic strength of 2.5 × 10−3 M, the key microgel particles become unstable above their TVPT,key and form a network onto which the lock particles unspecifically adsorb for TTVPT,lock. Scale bars, 2 μm.

  • Fig. 5 Influence of the number ratio NL/NK on the lock-and-key self-assembly.

    (A to C) 2D CLSM micrographs recorded at 20°C for dispersions prepared at low ionic strength (csalt ≈ 10−5 M), with cL = 1 wt % and different NL/NK values [(A) NL/NK ≈ 1, (B) NL/NK ≈ 2, and (C) NL/NK ≈ 6] showing the transition from dense aggregate to colloidal molecules.

  • Fig. 6 Influence of the temperature on the lock-and-key self-assembly into colloidal molecules with adjustable valency.

    (A) 2D CLSM micrographs of a dispersion (NL/V ≈ 6.8 × 10−2 μm−3, NL/NK ≈ 6) and schematic representations illustrating the specific self-assembly in colloidal molecules with a valency of ≈4 at 20°C (methane, CH4; top, configuration). Increasing the temperature to 40°C, the key particles exhibit a collapsed state, and the valency of the assembly decreases to ≈2 (carbon dioxide, CO2; middle, configuration). When both the lock and key particles are in their collapsed state at 48°C, key microgels are confined between two lock particles (dihydrogen, H2; bottom, configuration). (B) Yield in colloidal molecules relative to the number of key particles determined from various time series recorded at different temperatures. The statistics are supported by typical configurations observed in dispersion together with their relative fractions. Top left: Methane configuration with NLK = 4. Top right: Tetramer with an unspecific contact and NLK = 3. Middle left: Carbon dioxide configuration. Middle right: Trimer with an unspecific contact and NLK = 2. Bottom: Dihydrogen configuration. Scale bars, 1 μm.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/9/e1700321/DC1

    Determination of the entropic contribution for hard lock and key particles

    Evaluation of the binding specificity

    fig. S1. Geometrical configuration of a particle in a lock-and-key assembly.

    fig. S2. Entropic contribution for hard lock and key particles.

    fig. S3. Binding specificity dependence of the mixing ratio between lock and key particles.

    fig. S4. Binding specificity dependence of the temperature at a constant mixing ratio.

    movie S1. Colloidal molecules with tunable valence via lock-and-key self-assembly.

  • Supplementary Materials

    This PDF file includes:

    • Determination of the entropic contribution for hard lock and key particles
    • Evaluation of the binding specificity
    • fig. S1. Geometrical configuration of a particle in a lock-and-key assembly.
    • fig. S2. Entropic contribution for hard lock and key particles.
    • fig. S3. Binding specificity dependence of the mixing ratio between lock and key particles.
    • fig. S4. Binding specificity dependence of the temperature at a constant mixing ratio.

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    Other Supplementary Material for this manuscript includes the following:

    • movie S1. (.mp4 format). Colloidal molecules with tunable valence via lock-and-key self-assembly.

    Files in this Data Supplement:

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