The nanotechnology of life-inspired systems

Molecular Engineering of Robustness and Resilience in Enzymatic Reaction Networks

Living systems rely on complex networks of chemical reactions to control the concentrations of molecules in space and time. Despite the enormous complexity in biological networks, it is possible to identify network motifs that lead to functional outputs such as bistability or oscillations. One of the greatest challenges in chemistry is the creation of such functionality from chemical reactions. A key limitation is our lack of understanding of how molecular structure impacts on the dynamics of chemical reaction networks, preventing the design of networks that are robust (i.e., function in a large parameter space) and resilient (i.e., reach their out-of-equilibrium function rapidly). Here we demonstrate that reaction rates of individual reactions in the network can control the dynamics by which the system reaches limit cycle oscillations, thereby gaining information on the key parameters that govern the dynamics of these networks. We envision that these principles will be incorporated into the design of network motifs, enabling chemists to develop "molecular software" to create functional behavior in chemical systems.

Autocatalytic Time-Dependent Evolution of Metastable Two-Component Supramolecular Assemblies to Self-Sorted or Coassembled State

Despite substantial effort devoted in the history of supramolecular chemistry, synthetic supramolecular systems still lag behind biomolecular systems in terms of complexity and functionality. This is because biomolecular systems function in a multicomponent molecular network under out-of-equilibrium conditions. Here we report two-component supramolecular assemblies that are metastable and thus show time-dependent evolution. We found that the systems undergo either self-sorting or coassembly in time depending on the combination of components. Interestingly, this outcome, which had been previously achievable only under specific conditions, emerged from the two-component systems as a result of synergistic or reciprocal interplay between the coupled equilibria. We believe that this study sheds light on the similarity between synthetic and biomolecular systems and promotes better understanding of their intricate kinetic behaviors.

The interfacial structure of water droplets in a hydrophobic liquid

Nanoscopic and microscopic water droplets and ice crystals embedded in liquid hydrophobic surroundings are key components of aerosols, rocks, oil fields and the human body. The chemical properties of such droplets critically depend on the interfacial structure of the water droplet. Here we report the surface structure of 200 nm-sized water droplets in mixtures of hydrophobic oils and surfactants as obtained from vibrational sum frequency scattering measurements. The interface of a water droplet shows significantly stronger hydrogen bonds than the air/water or hexane/water interface and previously reported planar liquid hydrophobic/water interfaces at room temperature. The observed spectral difference is similar to that of a planar air/water surface at a temperature that is ∼50 K lower. Supercooling the droplets to 263 K does not change the surface structure. Below the homogeneous ice nucleation temperature, a single vibrational mode is present with a similar mean hydrogen-bond strength as for a planar ice/air interface.

Dynamic and programmable self-assembly of micro-rafts at the air-water interface

Dynamic self-assembled material systems constantly consume energy to maintain their spatiotemporal structures and functions. Programmable self-assembly translates information from individual parts to the collective whole. Combining dynamic and programmable self-assembly in a single platform opens up the possibilities to investigate both types of self-assembly simultaneously and to explore their synergy. This task is challenging because of the difficulty in finding suitable interactions that are both dissipative and programmable. We present a dynamic and programmable self-assembling material system consisting of spinning at the air-water interface circular magnetic micro-rafts of radius 50 μm and with cosinusoidal edge-height profiles. The cosinusoidal edge-height profiles not only create a net dissipative capillary repulsion that is sustained by continuous torque input but also enable directional assembly of micro-rafts. We uncover the layered arrangement of micro-rafts in the patterns formed by dynamic self-assembly and offer mechanistic insights through a physical model and geometric analysis. Furthermore, we demonstrate programmable self-assembly and show that a 4-fold rotational symmetry encoded in individual micro-rafts translates into 90° bending angles and square-based tiling in the assembled structures of micro-rafts. We anticipate that our dynamic and programmable material system will serve as a model system for studying nonequilibrium dynamics and statistical mechanics in the future.

Simple, Direct Routes to Polymer Brush Traps and Nanostructures for Studies of Diffusional Transport in Supported Lipid Bilayers

Patterned poly(oligo ethylene glycol) methyl ether methacrylate (POEGMEMA) brush structures may be formed by using a combination of atom-transfer radical polymerization (ATRP) and UV photopatterning. UV photolysis is used to selectively dechlorinate films of 4-(chloromethyl)phenyltrichlorosilane (CMPTS) adsorbed on silica surfaces, by exposure either through a mask or using a two-beam interferometer. Exposure through a mask yields patterns of carboxylic acid-terminated adsorbates. POEGMEMA may be grown from intact Cl initiators that were masked during exposure. Corrals, traps, and other structures formed in this way enable the patterning of proteins, vesicles, and, following vesicle rupture, supported lipid bilayers (SLBs). Bilayers adsorbed on the carboxylic acid-terminated surfaces formed by C-Cl bond photolysis in CMPTS exhibit high mobility. SLBs do not form on POEGMEMA. Using traps consisting of carboxylic acid-functionalized regions enclosed by POEGMEMA structures, electrophoresis may be observed in lipid bilayers containing a small amount of a fluorescent dye. Segregation of dye at one end of the traps was measured by fluorescence microscopy. The increase in the fluorescence intensity was found to be proportional to the trap length, while the time taken to reach the maximum value was inversely proportional to the trap length, indicating uniform, rapid diffusion in all of the traps. Nanostructured materials were formed using interferometric lithography. Channels were defined by exposure of CMPTS films to maxima in the interferogram, and POEGMEMA walls were formed by ATRP. As for the micrometer-scale patterns, bilayers did not form on the POEGMEMA structures, and high lipid mobilities were measured in the polymer-free regions of the channels.

Cell Guidance on Nanostructured Metal Based Surfaces

Metal surface nanostructuring to guide cell behavior is an attractive strategy to improve parts of medical implants, lab-on-a-chip, soft robotics, self-assembled microdevices, and bionic devices. Here, we discus important parameters, relevant trends, and specific examples of metal surface nanostructuring to guide cell behavior on metal-based hybrid surfaces. Surface nanostructuring allows precise control of cell morphology, adhesion, internal organization, and function. Pre-organized metal nanostructuring and dynamic stimuli-responsive surfaces are used to study various cell behaviors. For cells dynamics control, the oscillating stimuli-responsive layer-by-layer (LbL) polyelectrolyte assemblies are discussed to control drug delivery, coating thickness, and stiffness. LbL films can be switched "on demand" to change their thickness, stiffness, and permeability in the dynamic real-time processes. Potential applications of metal-based hybrids in biotechnology and selected examples are discussed.

Transient self-assembly of molecular nanostructures driven by chemical fuels

Over the past decades, chemists have mastered the art of assembling small molecules into complex nanostructures using non-covalent interactions. The driving force for self-assembly is thermodynamics: the self-assembled structure is more stable than the separate components. However, biological self-assembly processes are often energetically uphill and require the consumption of chemical energy. This allows nature to control the activation and duration of chemical functions associated with the assembled state. Synthetic chemical systems that operate in the same way are essential for creating the next generation of intelligent, adaptive materials, nanomachines and delivery systems. This review focuses on synthetic molecular nanostructures which self-assemble under dissipative conditions. The chemical function associated with the transient assemblies is operational as long as chemical fuel is present.