Polymer-Grafted, Gold Nanoparticle-Based Nano-Capsules as Reversible Colorimetric Tensile Strain Sensors.

Colloidal colorimetric microsensors enable the in-situ detection of mechanical strains within materials. Enhancing the sensitivity of these sensors to small scale deformation while enabling reversibility of the sensing capability would expand their utility in applications including biosensing and chemical sensing. In this study, we introduce the synthesis of colloidal colorimetric nano-sensors using a simple and readily scalable fabrication method. Colloidal nano sensors are prepared by emulsion-templated assembly of polymer-grafted gold nanoparticles (AuNP). To direct the adsorption of AuNP to the oil-water interface of emulsion droplets, AuNP (≈11nm) are functionalized with thiol-terminated polystyrene (PS, Mn  = 11k). These PS-grafted gold nanoparticles are suspended in toluene and subsequently emulsified to form droplets with a diameter of ≈30µm. By evaporating the solvent of the oil-inwater emulsion, we form nanocapsules (AuNC) (diameter < 1µm) decorated by PS-grafted AuNP. To test mechanical sensing, the AuNC are embedded in an elastomer matrix. The addition of a plasticizer reduces the glass transition temperature of the PS brushes, and in turn imparts reversible deformability to the AuNC. The plasmonic peak of the AuNC shifts towards lower wavelengths upon application of uniaxial tensile tension, indicating increased inter-nanoparticle distance, and reverts back as the tension is released.

Plasmonic Elastic Capsules as Colorimetric Reversible pH-Microsensors.

There is a crucial need for effective and easily dispersible colloidal microsensors able to detect local pH changes before irreversible damages caused by demineralization, corrosion, or biofilms occur. One class of such microsensors is based on molecular dyes encapsulated or dispersed either in polymer matrices or in liquid systems exhibiting different colors upon pH variations. They are efficient but often rely on sophisticated and costly syntheses, and present significant risks of leakage and photobleaching damages, which is detrimental for mainstream applications. Another approach consists of exploiting the distance-dependent plasmonic properties of metallic nanoparticles. Still, assembling nanoparticles into dispersible colloidal pH-sensitive sensors remains a challenge. Here, it is shown how to combine optically active plasmonic gold nanoparticles and pH-responsive thin shells into "plasmocapsules." Upon pH change, plasmocapsules swell or shrink. Concomitantly, the distance between the gold nanoparticles embedded in the polymeric matrix varies, resulting in an unambiguous color change. Billions of micron-size sensors can thus be easily fabricated. They are nonintrusive, reusable, and sense local pH changes. Each plasmocapsule is an independent reversible microsensor over a large pH range. Finally, their potential use for the detection of bacterial growth is demonstrated, thus proving that plasmocapsules are a new class of sensing materials.

Effect of geometry on the dewetting of granular chains by evaporation.

Understanding evaporation or drying in granular media still remains complex despite recent advancements. Evaporation depends on liquid transport across a connected film network from the bulk to the surface. In this study, we investigate the stability of film networks as a function of the geometry of granular chains of spherical grains. Using a controlled experimental approach, we vary the grain arrangement or packing and measure the height of the liquid film network during evaporation as packing shifts from loose-packed to close-packed arrangement. This height can be calculated from an equilibrium between hydrostatic pressure and the capillary pressure difference in the vertical film network. Following a simulation approach using Surface Evolver, we evaluate the pressure variation due to dewetting of the meniscus volume in the grains in both the percolating front and evaporating front within the two-phase zone of air/water mixture. Results show good agreement between model and experiment. We find that above a "critical" packing angle, the liquid continuity is broken and films connections fragment into separate, isolated capillary bridges.

Plasmonic-Based Mechanochromic Microcapsules as Strain Sensors.

Efficiently detecting mechanical deformations within materials is critical in a wide range of devices, from micro-electromechanical systems to larger structures in the aerospace industry. This communication reports the fabrication of new mechanochromic micrometer-size capsules enabling the detection of strains. These microcapsules are synthesized using an emulsification approach. They are made of densely packed gold nanoparticles embedded in a spherical silica crust. Billions of these composite spherical microcapsules are fabricated in a single batch. Each microcapsule is an opto-mechanosensor by itself, and can easily be recovered and incorporated into polymer films. When the films are stretched, the microcapsules are deformed into elongated ellipsoidal shapes and the distance between the Au NPs embedded in their shells concomitantly increases. As the extinction of Au NPs depends on the separation between the Au NPs, microcapsules exhibit different colors when they are elongated. These novel sensitive microcapsules can be used to detect and measure strain in polymer films by outputting color information.

Self-replication of information-bearing nanoscale patterns.

DNA molecules provide what is probably the most iconic example of self-replication--the ability of a system to replicate, or make copies of, itself. In living cells the process is mediated by enzymes and occurs autonomously, with the number of replicas increasing exponentially over time without the need for external manipulation. Self-replication has also been implemented with synthetic systems, including RNA enzymes designed to undergo self-sustained exponential amplification. An exciting next step would be to use self-replication in materials fabrication, which requires robust and general systems capable of copying and amplifying functional materials or structures. Here we report a first development in this direction, using DNA tile motifs that can recognize and bind complementary tiles in a pre-programmed fashion. We first design tile motifs so they form a seven-tile seed sequence; then use the seeds to instruct the formation of a first generation of complementary seven-tile daughter sequences; and finally use the daughters to instruct the formation of seven-tile granddaughter sequences that are identical to the initial seed sequences. Considering that DNA is a functional material that can organize itself and other molecules into useful structures, our findings raise the tantalizing prospect that we may one day be able to realize self-replicating materials with various patterns or useful functions.

Emergent Hyperuniformity in Periodically Driven Emulsions.

We report the self-organization of microfluidic emulsions into anomalously homogeneous structures. Upon periodic driving confined emulsions undergo a first-order transition from a reversible to an irreversible dynamics. We evidence that this dynamical transition is accompanied by structural changes at all scales yielding macroscopic yet finite hyperuniform structures. Numerical simulations are performed to single out the very ingredients responsible for the suppression of density fluctuations. We show that, as opposed to equilibrium systems, the long-range nature of the hydrodynamic interactions are not required for the formation of hyperuniform patterns, thereby suggesting a robust relation between reversibility and hyperuniformity which should hold in a broad class of periodically driven materials.

Polygamous particles.

DNA is increasingly used as an important tool in programming the self-assembly of micrometer- and nanometer-scale particles. This is largely due to the highly specific thermoreversible interaction of cDNA strands, which, when placed on different particles, have been used to bind precise pairs in aggregates and crystals. However, DNA functionalized particles will only reach their true potential for particle assembly when each particle can address and bind to many different kinds of particles. Indeed, specifying all bonds can force a particular designed structure. In this paper, we present the design rules for multiflavored particles and show that a single particle, DNA functionalized with many different "flavors," can recognize and bind specifically to many different partners. We investigate the cost of increasing the number of flavors in terms of the reduction in binding energy and melting temperature. We find that a single 2-µm colloidal particle can bind to 40 different types of particles in an easily accessible time and temperature regime. The practical limit of ∼100 is set by entropic costs for particles to align complementary pairs and, surprisingly, by the limited number of distinct "useful" DNA sequences that prohibit subunits with nonspecific binding. For our 11 base "sticky ends," the limit is 73 distinct sequences with no unwanted overlaps of 5 bp or more. As an example of phenomena enabled by polygamous particles, we demonstrate a three-particle system that forms a fluid of isolated clusters when cooled slowly and an elastic gel network when quenched.

Microscopic artificial swimmers.

Microorganisms such as bacteria and many eukaryotic cells propel themselves with hair-like structures known as flagella, which can exhibit a variety of structures and movement patterns. For example, bacterial flagella are helically shaped and driven at their bases by a reversible rotary engine, which rotates the attached flagellum to give a motion similar to that of a corkscrew. In contrast, eukaryotic cells use flagella that resemble elastic rods and exhibit a beating motion: internally generated stresses give rise to a series of bends that propagate towards the tip. In contrast to this variety of swimming strategies encountered in nature, a controlled swimming motion of artificial micrometre-sized structures has not yet been realized. Here we show that a linear chain of colloidal magnetic particles linked by DNA and attached to a red blood cell can act as a flexible artificial flagellum. The filament aligns with an external uniform magnetic field and is readily actuated by oscillating a transverse field. We find that the actuation induces a beating pattern that propels the structure, and that the external fields can be adjusted to control the velocity and the direction of motion.

Physical mechanisms driving the reversible aggregation of Staphylococcus aureus and response to antimicrobials.

Formation of non-sessile, auto-aggregated cells of Staphylococcus aureus contributes to surface colonization and biofilm formation, hence play a major role in the early establishment of infection and in tolerance to antimicrobials. Understanding the mechanism of aggregation and the impact of aggregation on the activity of antimicrobials is crucial in achieving a better control of this important pathogen. Previously linked to biological phenomena, physical interactions leading to S. aureus cellular aggregation and its protective features against antimicrobials remain unraveled. Herein, in-vitro experiments coupled with XDLVO simulations reveal that suspensions of S. aureus cells exhibit rapid, reversible aggregation (> 70%) in part controlled by the interplay between cellular hydrophobicity, surface potential and extracellular proteins. Changing pH and salt concentration in the extracellular media modulated the cellular surface potential but not the hydrophobicity which remained consistent despite these variations. A decrease in net cellular negative surface potential achieved by decreasing pH or increasing salt concentrations, caused attractive forces such as the hydrophobic and cell-protein interactions to prevail, favoring immediate aggregation. The aggregation significantly increased the tolerance of S. aureus cells to quaternary ammonium compounds (QAC). The well-dispersed cell population was completely inactivated within 30 s whereas its aggregated counterpart required more than 10 min.

Quantitative study of the association thermodynamics and kinetics of DNA-coated particles for different functionalization schemes.

Surface functionalization with complementary single-stranded DNA sticky ends is increasingly used for guiding the self-assembly of nano- and micrometer-sized particles into larger scale ordered structures. Here we present measurements, formulas, and graphs that allow one to quantitatively predict the association behavior of DNA-coated particles from readily available Web-based data. From experiments it appears that the suspension behavior is very sensitive to the grafting details, such as the length and flexibility of the tether constructs and the particles' surface coverage. Thus, if one wants to control the interactions and assembly processes, insight is needed into the structural and dynamical features of the DNA coatings. We demonstrate how a straightforward measurement of the particles' association-dissociation kinetics during selected temperature cycles, combined with a simple quantitative model, can reveal the relevant properties. We used this method in a systematic study where we varied the temperature cycle, the bead concentration, the particles' surface coverage, and the DNA construct. Among other things, we find that the backbone that tethers the sticky ends to the surface can have a significant impact on the particles' dissociation properties, as it affects the total number of interparticle bonds and the configurational entropy cost associated with these bonds. We further find that, independent of the tether backbone, self-complementary "palindromic" sticky ends readily form intraparticle hairpins and loops, which greatly affect the particles' association behavior. Such secondary structure formation is increasingly important in faster temperature quenches, at lower particle concentration, and at lower surface coverage. The latter observations are especially useful for the design of so-called self-protected DNA-mediated interactions, which we pioneered recently and for which we expect to find an increasing use, as they enable more versatile assembly schemes.

Switchable self-protected attractions in DNA-functionalized colloids.

Surface functionalization with DNA is a powerful tool for guiding the self-assembly of nanometre- and micrometre-sized particles. Complementary 'sticky ends' form specific inter-particle links and reproducibly bind at low temperature and unbind at high temperature. Surprisingly, the ability of single-stranded DNA to form folded secondary structures has not been explored for controlling (nano) colloidal assembly processes, despite its frequent use in DNA nanotechnology. Here, we show how loop and hairpin formation in the DNA coatings of micrometre-sized particles gives us in situ control over the inter-particle binding strength and association kinetics. We can finely tune and even switch off the attractions between particles, rendering them inert unless they are heated or held together--like a nano-contact glue. The novel kinetic control offered by the switchable self-protected attractions is explained with a simple quantitative model that emphasizes the competition between intra- and inter-particle hybridization, and the practical utility is demonstrated by the assembly of designer clusters in concentrated suspensions. With self-protection, both the suspension and assembly product are stable, whereas conventional attractive colloids would quickly aggregate. This remarkable functionality makes our self-protected colloids a novel material that greatly extends the utility of DNA-functionalized systems, enabling more versatile, multi-stage assembly approaches.

Highly conductive and transparent coatings from flow-aligned silver nanowires with large electrical and optical anisotropy.

Conductive and transparent coatings consisting of silver nanowires (AgNWs) are promising candidates for emerging flexible electronics applications. Coatings of aligned AgNWs offer unusual electronic and optical anisotropies, with potential for use in micro-circuits, antennas, and polarization sensors. Here we explore a microfluidics setup and flow-induced alignment mechanisms to create centimeter-scale highly conductive coatings of aligned AgNWs with order parameters reaching 0.84, leading to large electrical and optical anisotropies. By varying flow rates, we establish the relationship between the shear rate and the alignment and investigate possible alignment mechanisms. The angle-dependent sheet resistance of the aligned AgNW networks exhibits an electronic transport anisotropy of ∼10× while maintaining low resistivity (<50 Ω sq-1) in all directions. When illuminated, the aligned AgNW coatings exhibit angle- and polarization-dependent colors, and the polarized reflection anisotropy can be as large as 25. This large optical anisotropy is due to a combination of alignment, polarization response, and angle-dependent scattering of the aligned AgNWs.

Flow visualization and flow cytometry with holographic video microscopy.

The video stream captured by an in-line holographic microscope can be analyzed on a frame-by-frame basis to track individual colloidal particles' three-dimensional motions with nanometer resolution, and simultaneously to measure their sizes and refractive indexes. Through a combination of hardware acceleration and software optimization, this analysis can be carried out in near real time with off-the-shelf instrumentation. An efficient particle identification algorithm automates initial position estimation with sufficient accuracy to enable unattended holographic tracking and characterization. This technique's resolution for particle size is fine enough to detect molecular-scale coatings on the surfaces of colloidal spheres, without requiring staining or fluorescent labeling. We demonstrate this approach to label-free holographic flow cytometry by detecting the binding of avidin to biotinylated polystyrene spheres.

Preferential Root Tropisms in 2D Wet Granular Media with Structural Inhomogeneities.

We investigate certain aspects of the physical mechanisms of root growth in a granular medium and how these roots adapt to changes in water distribution induced by the presence of structural inhomogeneities in the form of solid intrusions. Physical intrusions such as a square rod added into the 2D granular medium maintain robust capillary action, pumping water from the more saturated areas at the bottom of the cell towards the less saturated areas near the top of the cell while the rest of the medium is slowly devoid of water via evaporation. The intrusion induces "preferential tropism" of roots by first generating a humidity gradient that attracts the root to grow towards it. Then it guides the roots and permits them to grow deeper into more saturated regions in the soil. This further allows more efficient access to available water in the deeper sections of the medium thereby resulting to increased plant lifetime.

Experimental investigation of water distribution in a two-phase zone during gravity-dominated evaporation.

We characterize the water repartition within the partially saturated (two-phase) zone (PSZ) during evaporation from mixed wettable porous media by controlling the wettability of glass beads, their sizes, and as well the surrounding relative humidity. Here, capillary numbers are low and under these conditions, the percolating front is stabilized by gravity. Using experimental and numerical analyses, we find that the PSZ saturation decreases with the Bond number, where packing of smaller particles have higher saturation values than packing made of larger particles. Results also reveal that the extent (height) of the PSZ, as well as water saturation in the PSZ, both increase with wettability. We also numerically calculate the saturation exclusively contained in connected liquid films and results show that values are less than the expected PSZ saturation. These results strongly reflect that the two-phase zone is not solely made up of connected capillary networks but also made of disconnected water clusters or pockets. Moreover, we also find that global saturation (PSZ + full wet zone) decreases with wettability, confirming that greater quantity of water is lost via evaporation with increasing hydrophilicity. These results show that connected liquid films are favored in more-hydrophilic systems while disconnected water pockets are favored in less-hydrophilic systems.

Diagnosing hyperuniformity in two-dimensional, disordered, jammed packings of soft spheres.

Hyperuniformity characterizes a state of matter for which (scaled) density fluctuations diminish towards zero at the largest length scales. However, the task of determining whether or not an image of an experimental system is hyperuniform is experimentally challenging due to finite-resolution, noise, and sample-size effects that influence characterization measurements. Here we explore these issues, employing video optical microscopy to study hyperuniformity phenomena in disordered two-dimensional jammed packings of soft spheres. Using a combination of experiment and simulation we characterize the possible adverse effects of particle polydispersity, image noise, and finite-size effects on the assignment of hyperuniformity, and we develop a methodology that permits improved diagnosis of hyperuniformity from real-space measurements. The key to this improvement is a simple packing reconstruction algorithm that incorporates particle polydispersity to minimize the free volume. In addition, simulations show that hyperuniformity in finite-sized samples can be ascertained more accurately in direct space than in reciprocal space. Finally, our experimental colloidal packings of soft polymeric spheres are shown to be effectively hyperuniform.

Kinetics of gravity-driven water channels under steady rainfall.

We investigate the formation of fingered flow in dry granular media under simulated rainfall using a quasi-two-dimensional experimental setup composed of a random close packing of monodisperse glass beads. Using controlled experiments, we analyze the finger instabilities that develop from the wetting front as a function of fundamental granular (particle size) and fluid properties (rainfall, viscosity). These finger instabilities act as precursors for water channels, which serve as outlets for water drainage. We look into the characteristics of the homogeneous wetting front and channel size as well as estimate relevant time scales involved in the instability formation and the velocity of the channel fingertip. We compare our experimental results with that of the well-known prediction developed by Parlange and Hill [D. E. Hill and J. Y. Parlange, Soil Sci. Soc. Am. Proc. 36, 697 (1972)]. This model is based on linear stability analysis of the growth of perturbations arising at the interface between two immiscible fluids. Results show that, in terms of morphology, experiments agree with the proposed model. However, in terms of kinetics we nevertheless account for another term that describes the homogenization of the wetting front. This result shows that the manner we introduce the fluid to a porous medium can also influence the formation of finger instabilities. The results also help us to calculate the ideal flow rate needed for homogeneous distribution of water in the soil and minimization of runoff, given the grain size, fluid density, and fluid viscosity. This could have applications in optimizing use of irrigation water.

Simple analytical model of evapotranspiration in the presence of roots.

Evaporation of water out of a soil involves complicated and well-debated mechanisms. When plant roots are added into the soil, water transfer between the soil and the outside environment is even more complicated. Indeed, plants provide an additional process of water transfer. Water is pumped by the roots, channeled to the leaf surface, and released into the surrounding air by a process called transpiration. Prediction of the evapotranspiration of water over time in the presence of roots helps keep track of the amount of water that remains in the soil. Using a controlled visual setup of a two-dimensional model soil consisting of monodisperse glass beads, we perform experiments on actual roots grown under different relative humidity conditions. We record the total water mass loss in the medium and the position of the evaporating front that forms within the medium. We then develop a simple analytical model that predicts the position of the evaporating front as a function of time as well as the total amount of water that is lost from the medium due to the combined effects of evaporation and transpiration. The model is based on fundamental principles of evaporation fluxes and includes empirical assumptions on the quantity of open stomata in the leaves, where water transpiration occurs. Comparison between the model and experimental results shows excellent prediction of the position of the evaporating front as well as the total mass loss from evapotranspiration in the presence of roots. The model also provides a way to predict the lifetime of a plant.

DNA patchy particles.

A simple and effective way to make DNA patchy particles is reported. A small patch of DNA strands is "stamped" from a gold surface onto colloidal particles of different sizes by streptavidin-biotin bonds. These DNA patchy particles provide direction-selective and thermoreversible interactions, and hence can lead to unique assembly protocols and structures controlled by temperature.