Lock and key colloids

Journal name:
Nature
Volume:
464,
Pages:
575–578
Date published:
DOI:
doi:10.1038/nature08906
Received
Accepted

New functional materials can in principle be created using colloids that self-assemble into a desired structure by means of a programmable recognition and binding scheme. This idea has been explored by attaching ‘programmed’ DNA strands to nanometre-1, 2, 3 and micrometre-4, 5 sized particles and then using DNA hybridization to direct the placement of the particles in the final assembly. Here we demonstrate an alternative recognition mechanism for directing the assembly of composite structures, based on particles with complementary shapes. Our system, which uses Fischer’s lock-and-key principle6, employs colloidal spheres as keys and monodisperse colloidal particles with a spherical cavity as locks that bind spontaneously and reversibly via the depletion interaction. The lock-and-key binding is specific because it is controlled by how closely the size of a spherical colloidal key particle matches the radius of the spherical cavity of the lock particle. The strength of the binding can be further tuned by adjusting the solution composition or temperature. The composite assemblies have the unique feature of having flexible bonds, allowing us to produce flexible dimeric, trimeric and tetrameric colloidal molecules as well as more complex colloidal polymers. We expect that this lock-and-key recognition mechanism will find wider use as a means of programming and directing colloidal self-assembly.

At a glance

Figures

  1. Fabrication of lock particles.
    Figure 1: Fabrication of lock particles.

    a, Diagram showing the synthetic steps involved in the preparation of particles with well-defined spherical cavities. Monodisperse silicon oil droplets are (1) nucleated from a homogeneous solution of hydrolysed 3-methacryloxypropyl trimethoxysilane monomer, and (2) encapsulated into cross-linked polymer shells. The liquid core (3) contracts when polymerized and (4) drives a controlled shell buckling that forms spherical cavities. b, This last step is easily followed by optical microscopy. After polymerization, cavities are visible as darker spots on the particles surfaces. c, The complementary fit between the locks and the spherical keys (here silica), is clearly visible in this transmission electron microscope image.

  2. Lock-key interactions.
    Figure 2: Lock–key interactions.

    a, The depletion attraction potential between lock and key is proportional to the overlapping excluded volume ΔV, which attains a maximum ΔVmax for the configuration in which the key particle, by virtue of its size and position, precisely fits into the spherical cavity of a lock particle. For all other configurations, ΔV<ΔVmax. Because the depletion interaction is also proportional to the density np of the polymer depletant in solution, the interaction can be tuned by adjusting np so that it is sufficiently strong to bind two particles only for the lock-and-key configuration. b, c, Snapshots from a movie showing an example of depletion-driven self-assembly of lock and key particles. The site-specificity of the interactions is captured in sequence b in which arrows indicate examples of successful (green) and unsuccessful (red) lock–key binding. Scale bars, 2μm.

  3. Selectivity of the lock-key reversible binding.
    Figure 3: Selectivity of the lock–key reversible binding.

    a, Experimentally measured lock–key association curves for different key particles along with fits to the model described in the text (see Supplementary Materials for more detail). Each point represents the ratio nLK/nL of the number of bound lock–key complexes to the number of lock particles in the sample. The values of nLK and nL were obtained by counting the number of locks and lock–key complexes in different areas of the samples, with nL103 for a typical data point. The error bars reflect the statistical error (nLK)0.5/nL associated with the finite data sample. The assembly is driven by depletion attractions and the lock–key selectivity is provided by the degree of their excluded volume overlap. 1.57-μm spherical keys maximize the overlap and assemble into lock cavities at the lowest depletant concentration (dashed yellow line). For a poor match the assembly occurs at higher depletant concentration; however, while the overlapping volume of large (2.47μm) keys in contact with cavity rims is still sufficient to give specific lock–key binding, smaller keys (1μm) do not bind at all. Insets show the different sizes of keys. b, Two different sets of monodisperse keys (2R = 1.54μm and 2.47μm; see insets), each mixed with a polydisperse population of locks (blue), selectively bind to their best-matching complementary particles. As a result, the lock particle size distribution in the assembled lock–key complexes (green and red) peaks at two different values. Scale bars, 1μm.

  4. Temperature-controlled lock-key self-assembly and electric field manipulation.
    Figure 4: Temperature–controlled lock-key self-assembly and electric field manipulation.

    a, Observed dependence of lock–key binding on depletant concentration, with error bars reflecting the statistical error (nLK)0.5/nL associated with the finite data sample. When the concentration of salt is increased to the point that it is necessary to add a steric layer (Pluronic F108, average relative molecular weight 14,600) to the particles to prevent aggregation, a significantly lower depletant concentration is needed to drive the assembly, resulting in a shifted and sharper lock–key binding transition. b, Thermosensitive microgel particles can be used as a depletant to implement a simple temperature switch for the lock–key binding reaction (bound; see left inset). At the transition temperature Tc the depletant shrinks to a size too small to give an effective lock–key attraction and the key particles are released (unbound; see right inset). c, The anisotropic shape of the lock particles allows the use of an external electric field E to control the orientation of the lock cavities and to assemble lock particles in ‘daisy-chain’-like structures.

  5. Flexibility of the lock-key junctions in self-assembled colloidal molecules and polymers.
    Figure 5: Flexibility of the lock–key junctions in self-assembled colloidal molecules and polymers.

    Time-lapse optical microscopy images (left three columns), and schematics (rightmost column), show the flexibility of lock–key bonds in various assemblies (ad), which are confined to two dimensions by being placed on a glass microscope slide. The absence of irreversible chemical bonds between the building blocks allows these ball-in-socket joints to move freely. Scale bars, 2μm.

Author information

Affiliations

  1. Department of Physics, New York University, 4 Washington Place, New York, New York 10003, USA

    • S. Sacanna,
    • W. T. M. Irvine,
    • P. M. Chaikin &
    • D. J. Pine

Competing financial interests

The authors declare no competing financial interests.

Corresponding authors

Correspondence to:

Author details

Supplementary information

PDF files

  1. Supplementary Information (878K)

    This file contains Supplementary Data including Figure 1 and legend, Supplementary References and Supplementary Figures S1 and legend.

Movies

  1. Supplementary Movie 1 (2.8M)

    This movie file shows the sequential self-assembly of a flexible colloidal molecule via our novel recognition mechanism. A lock-key complex forms only when a matching key particle (here a 1.57μm silica sphere) docks at the lock cavity site (green arrow). All other configurations result in repulsive interactions (red arrow). A key particle can accommodate more than one lock. The movie was acquired at 1 fr/s and it is displayed at 8.7 fr/s.

  2. Supplementary Movie 2 (2.1M)

    This movie file shows the simultaneous unbinding of two lock-key complexes controlled by temperature. When the temperature is raised the depletant shrinks (pNIPAM microgel particles not visible in the movie) and the attractive depletion potential falls below that required for binding. As a result, the locks release their bound keys. The movie was acquired at 10.7 fr/s and it is displayed at 12.8 fr/s.

  3. Supplementary Movie 3 (395K)

    This movie file shows self-assembled colloidal molecules with flexible bonds between their constituent particles. The flexibility is given by ball-and-socket joints held together by the depletion force. Movies were acquired at 1 fr/s and they are displayed at 10 fr/s

  4. Supplementary Movie 4 (1M)

    This movie file shows self-assembled colloidal molecules with flexible bonds between their constituent particles. The flexibility is given by ball-and-socket joints held together by the depletion force. Movies were acquired at 1 fr/s and they are displayed at 10 fr/s

  5. Supplementary Movie 5 (1.6M)

    This movie file shows a freely diffusing flexible colloidal polymer consisting of interconnected locks. The movie was acquired at 10.7 fr/s and it is displayed at 23.5 fr/s.

  6. Supplementary Movie 6 (4.8M)

    This movie file is showing a typical, randomly selected "full field of view" of our sample containing ~60× lock-key assemblies.

Additional data