SignificanceWhile widely known as the molecule of life, DNA is also an amazing building block at the nanoscale, since it allows us to design and program the structure and dynamics of functional nanomaterials. We exploit the programmability of DNA to achieve control over the rheology of self-assembled hydrogels, which have elastic or viscous behavior (similar to that of slime) that is finely regulated by temperature. Using microrheology to investigate the mechanical properties of DNA hydrogels at the microlength scale, we map the viscoelastic response over a broad range of frequencies and temperatures. The deep understanding in the fundamental physics provides a way to design DNA-based materials with precise control over the structure stability and rigidity at molecular level.

Aqueous colloidal suspensions, both man-made and natural, are part of our everyday life. The applicability of colloidal suspensions, however, is limited by the range of conditions over which they are stable. Here we report a novel type of highly monodisperse raspberry-like colloids, which are prepared in a single-step synthesis that relies on simultaneous dispersion and emulsion polymerisation. The resulting raspberry colloids behave almost like hard spheres. In aqueous solutions, such prepared raspberries show unexpected stability against aggregation over large variations of added salt concentrations without addition of stabilisers. We present simple Derjaguin–Landau–Verwey–Overbeek (DLVO) calculations performed on raspberry-like and smooth colloids showing that this stability results from our raspberries’ unique morphology, which extends our understanding of colloidal stability against salting. Further, the raspberries’ stability facilitates the formation of superspheres and thin films in which the raspberry colloids self-assemble into hexagonally close-packed photonic crystals with exquisite reproducibility.

Controlled adsorption and phase behavior of colloids at liquid interface.

Three-dimensional DNA networks, composed of tri-or higher valent nanostars with sticky, single-stranded DNA overhangs, have been previously studied in the context of designing thermally responsive, viscoelastic hydrogels. In this work, we use linker-mediated gels, where the sticky ends of two trivalent nanostars are connected through the complementary sticky ends of a linear DNA duplex. We can design this connection to be either rigid or flexible by introducing flexible, non-binding bases. The additional flexiblity provided by these non-binding bases influences the effective elasticity of the percolating gel formed at low temperatures. Here we show that by choosing the right length of the linear duplex and non-binding flexible joints, we obtain a completely different phase behaviour to that observed for rigid linkers. In particular, we use dynamic light scattering as microrheological tool to monitor the self-assembly of DNA nanostars with linear linkers as a function of temperature. While we observe classical gelation when using rigid linkers, the presence of flexible joints leads to a cluster fluid with reduced viscosity. Using both the oxDNA model and a coarse-grained simulation to investigate the nanostar-linker topology, we hypothesise on the possible structure formed by the DNA clusters.

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One important aspect of the complete physical characterization of novel viscoelastic materials is the assessment of their response on short timescales. Optical tweezers, equipped with a fast quadrant photodiode, aid in fulfilling this task by providing high-frequency viscoelastic information about the sample. In passive microrheology, this is normally achieved by extracting rheological information from the thermal motion of an optically trapped bead embedded in a test fluid. Here we present the calibration and use of optical tweezers to study the formation of thermally reversible DNA hydrogels. We complement our results with rheological data from dynamic light scattering, video microscopy and conventional bulk rheology. Merging experimental data from different techniques allows us to study the viscoelastic behavior of these DNA networks over a wide frequency-band and the scaling of the complex viscoelastic modulus at the two frequency extremes. By analyzing the high-frequency behavior of our transient network, we prove the semi-flexible polymer nature of DNA and provide an estimate of its persistence length.

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Many industrial soft materials include oil-in-water (O/W) emulsions at the core of their formulations. By using tuneable interface stabilizing agents, such emulsions can self-assemble into complex structures. DNA has been used for decades as a thermoresponsive, highly specific binding agent between hard and, recently, soft colloids. Up until now, emulsion droplets functionalized with DNA had relatively low coating densities and were expensive to scale up. Here, a general O/W DNA-coating method using functional nonionic amphiphilic block copolymers, both diblock and triblock, is presented. The hydrophilic poly(ethylene glycol) ends of the surfactants are functionalized with azides, allowing for efficient, dense, and controlled coupling of dibenzocyclooctane-functionalized DNA to the polymers through a strain-promoted alkyne-azide click reaction. The protocol is readily scalable due to the triblock's commercial availability. Different production methods (ultrasonication, microfluidics, and membrane emulsification) are used with different oils (hexadecane and silicone oil) to produce functional droplets in various size ranges (submicron, ∼20 and >50 μm), showcasing the generality of the protocol. Thermoreversible submicron emulsion gels, hierarchical "raspberry" droplets, and controlled droplet release from a flat DNA-coated surface are demonstrated. The emulsion stability and polydispersity is evaluated using dynamic light scattering and optical microscopy. The generality and simplicity of the method opens up new applications in soft matter, biotechnological research, and industrial advances.