Plasma-based post-processing of colloidal nanocrystals for applications in heterogeneous catalysis.

This review summarizes the work on the use of plasmas to post-process nanostructures, in particular colloidal nanocrystals, as promising candidates for applications of heterogeneous catalysis. Using plasma to clean or modify the surface of nanostructures is a more precisely controlled method compared to other conventional methods, which is preferable when strict requirements for nanostructure morphology or chemical composition are necessary. The ability of plasma post-processing to create mesoporous materials with high surface areas and controlled microstructure, surfaces, and interfaces has transformational potential in catalysis and other applications that leverage surface/interface processes.

Evidence for root adaptation to a spatially discontinuous water availability in the absence of external water potential gradients.

We hereby show that root systems adapt to a spatially discontinuous pattern of water availability even when the gradients of water potential across them are vanishingly small. A paper microfluidic approach allowed us to expose the entire root system of Brassica rapa plants to a square array of water sources, separated by dry areas. Gradients in the concentration of water vapor across the root system were as small as 10-4⋅mM⋅m-1 (∼4 orders of magnitude smaller than in conventional hydrotropism assays). Despite such minuscule gradients (which greatly limit the possible influence of the well-understood gradient-driven hydrotropic response), our results show that 1) individual roots as well as the root system as a whole adapt to the pattern of water availability to maximize access to water, and that 2) this adaptation increases as water sources become more rare. These results suggest that either plant roots are more sensitive to water gradients than humanmade water sensors by 3-5 orders of magnitude, or they might have developed, like other organisms, mechanisms for water foraging that allow them to find water in the absence of an external gradient in water potential.

Growth of Colloidal Nanocrystals by Liquid-Like Coalescence*.

We here describe, model, and predict the growth kinetics of amine-capped PbS colloidal nanoparticles in the absence of supersaturation. The particles grow by coalescence rather than by Ostwald ripening. A comparison of different models indicates that the effective activation energy of coalescence (67.65 kJ mol-1 ) is associated with two terms: a term proportional to the contact area between the ligand shells of two colliding particles, and a constant term. Our Brownian dynamics simulations show (i) how the remarkably low activation energy (or large rate constants) are most likely due to the large difference in size between the particles and their mean free path of diffusion, and (ii) how the low polydispersity is the likely result of the suppression of collision rates between rare populations due to crowding. The model successfully predicts the growth kinetics of nanoparticles, therefore enabling the precise control of the average particle size without the need of supersaturation.

Suppressing Evaporative Loss in Slippery Liquid-Infused Porous Surfaces (SLIPS) with Self-Suspended Perfluorinated Nanoparticles.

This article describes an approach to resolving the issue of evaporative loss from slippery liquid-infused porous surfaces (SLIPS). Hydrophobic and oleophobic fluids with significantly reduced evaporative loss rates at temperatures of up to 90 °C were obtained by the one-step mixing of commercially available perfluorinated lubricants with colloidal nanoparticles to form self-suspended nanoparticle fluids (i.e., suspensions nearly devoid of solvent). No evaporative loss was detected at temperatures of as high as 50 °C for over 3 months. Furthermore, the approach allows us to combine the function of the nanoparticles with the slippery characteristic of SLIPS.

Self-Limiting Processes in the Flame-Based Fabrication of Superhydrophobic Surfaces from Silicones.

Outdoor applications of superhydrophobic coatings require synthetic approaches that allow their simple, fast, scalable, and environmentally benign deployment on large, heterogeneous surfaces and their rapid regeneration in situ. We recently showed that the thermal degradation of silicones by flames fulfills these characteristics by spontaneously structuring silicone surfaces into a hierarchical, textured structure that provides wear-resistant, healable superhydrophobicity. This paper elucidates how flame processing-a simple, rapid, and out-of-equilibrium process-can be so counterintuitively reliable and robust in producing such a complex structure. A comprehensive study of the effect of the processing speed and flame temperature on the chemical and physical properties of the coatings yielded three surprising results. (i) Three thermal degradation mechanisms drive the surface texturing: depolymerization (in the O2-rich conditions of the surface), decomposition (in the O2-poor conditions found a few micrometers from the surface), and pyrolysis at excessive temperatures. (ii) The operational condition is delimited by the onset of the depolymerization at low temperatures and the onset of pyrolysis at high temperatures. (iii) The remarkably wide operational conditions and robustness of this approach result from self-limiting growth and oxidation of the silicone particles that are responsible for the surface texturing and in the extent of their deposition. As a result of this analysis we show that superhydrophobic surfaces can be produced or regenerated with this approach at a speed of 15 cm s-1 (i.e., the length of an airport runway in ∼4.5 h).

Hydrogel-based transparent soils for root phenotyping in vivo.

Root phenotypes are increasingly explored as predictors of crop performance but are still challenging to characterize. Media that mimic field conditions (e.g., soil, sand) are opaque to most forms of radiation, while transparent media do not provide field-relevant growing conditions and phenotypes. We describe here a "transparent soil" formed by the spherification of hydrogels of biopolymers. It is specifically designed to support root growth in the presence of air, water, and nutrients, and allows the time-resolved phenotyping of roots in vivo by both photography and microscopy. The roots developed by soybean plants in this medium are significantly more similar to those developed in real soil than those developed in hydroponic conditions and do not show signs of hypoxia. Lastly, we show that the granular nature and tunable properties of these hydrogel beads can be leveraged to investigate the response of roots to gradients in water availability and soil stiffness.

Stress response to CO2 deprivation by Arabidopsis thaliana in plant cultures.

After being the standard plant propagation protocol for decades, cultures of Arabidopsis thaliana sealed with Parafilm remain common today out of practicality, habit, or necessity (as in co-cultures with microorganisms). Regardless of concerns over the aeration of these cultures, no investigation has explored the CO2 transport inside these cultures and its effect on the plants. Thereby, it was impossible to assess whether Parafilm-seals used today or in thousands of older papers in the literature constitute a treatment, and whether this treatment could potentially affect the study of other treatments.For the first time we report the CO2 concentrations in Parafilm-sealed cultures of A. thaliana with a 1 minute temporal resolution, and the transcriptome comparison with aerated cultures. The data show significant CO2 deprivation to the plants, a drastic suppression of photosynthesis, respiration, starch accumulation, chlorophyll biosynthesis, and an increased accumulation of reactive oxygen species. Most importantly, CO2 deprivation occurs as soon as the cotyledons emerge. Gene expression analysis indicates a significant alteration of 35% of the pathways when compared to aerated cultures, especially in stress response and secondary metabolism processes. On the other hand, the observed increase in the production of glucosinolates and flavonoids suggests intriguing possibilities for CO2 deprivation as an organic biofortification treatment in high-value crops.

On the kinetics of the removal of ligands from films of colloidal nanocrystals by plasmas.

This paper describes the kinetic limitations of etching ligands from colloidal nanocrystal assemblies (CNAs) by plasma processing. We measured the etching kinetics of ligands from a CNA model system (spherical ZrO2 nanocrystals, 2.5-3.5 nm diameter, capped with trioctylphosphine oxide) with inductively coupled plasmas (He and O2 feed gases, powers ranging from 7 to 30 W, at pressures ranging from 100 to 2000 mTorr and exposure times ranging between 6 and 168 h). The etching rate slows down by about one order of magnitude in the first minutes of etching, after which the rate of carbon removal becomes proportional to the third power of the carbon concentration in the CNA. Pressure oscillations in the plasma chamber significantly accelerate the overall rate of etching. These results indicate that the rate of etching is mostly affected by two main factors: (i) the crosslinking of the ligands in the first stage of plasma exposure, and (ii) the formation of a boundary layer at the surface of the CNA. Optimized conditions of plasma processing allow for a 60-fold improvement in etching rates compared to the previous state of the art and make the timeframes of plasma processing comparable to those of calcination.

Sustainable scalable synthesis of sulfide nanocrystals at low cost with an ionic liquid sulfur precursor.

Increasing the sustainability of nanocrystals is crucial to their application and the protection of the environment. Sulfur precursors for their synthesis are commonly obtained through multiple steps from H2S, only to be converted back to H2S during the synthesis of the nanocrystals. This convoluted process requires energy, reduces yields, increases waste and auxiliaries, and complicates recycling. Using H2S directly could drastically improve sustainability, but is prevented by toxicity and handling. We here show that H2S is stabilized by reaction with oleylamine (the most common and versatile ligand in nanoparticle synthesis) to form an ionic liquid precursor that addresses all major principles of green chemistry: it is made in one exothermic step, it leaves the reaction yielding a safer product and allowing the separate recycling of the precursors, and it produces high quality nanocrystals with high yields (sulfur yield > 70%) and concentrations (90 g L-1) in ambient conditions.

Large-Scale Synthesis of Colloidal Si Nanocrystals and Their Helium Plasma Processing into Spin-On, Carbon-Free Nanocrystalline Si Films.

This paper describes a simple approach to the large-scale synthesis of colloidal Si nanocrystals and their processing into spin-on carbon-free nanocrystalline Si films. The synthesized silicon nanoparticles are capped with decene, dispersed in hexane, and deposited on silicon substrates. The deposited films are exposed to nonoxidizing room-temperature He plasma to remove the organic ligands without adversely affecting the silicon nanoparticles to form crack-free thin films. We further show that the reactive ion etching rate in these films is 1.87 times faster than that for single-crystalline Si, consistent with a simple geometric argument that accounts for the nanoscale roughness caused by the nanoparticle shape.

From Petri Dishes to Model Ecosystems.

Model ecosystems could provide significant insight into the evolution and behavior of real ecosystems. We discuss the advantages and limitations of common approaches like mesocosms. In this context, we highlight recent breakthroughs that allow for the creation of networks of organisms with independently controlled environments and rates of chemical exchange.

HOMEs for plants and microbes - a phenotyping approach with quantitative control of signaling between organisms and their individual environments.

We describe a simple, scalable, modular, and frugal approach to create model ecosystems as millifluidic networks of interconnected habitats (hosting microbes or plants), which offers (i) quantitative and dynamic control over the exchange of chemicals between habitats, and (ii) independent control over their environment. Oscillatory laminar flows produce regions of vortex mixing around obstacles. When these overlap, rapid mass transport by dispersion occurs, which is quantitatively describable as diffusion, but is directional and tunable in rate over 3 orders of magnitude. This acceleration in the rate of diffusion is equivalent to reducing the distance between the habitats, and therefore, the organisms, down to the length scales characteristic of signaling in soil (<2 mm).

Calcination does not remove all carbon from colloidal nanocrystal assemblies.

Removing organics from hybrid nanostructures is a crucial step in many bottom-up materials fabrication approaches. It is usually assumed that calcination is an effective solution to this problem, especially for thin films. This assumption has led to its application in thousands of papers. We here show that this general assumption is incorrect by using a relevant and highly controlled model system consisting of thin films of ligand-capped ZrO2 nanocrystals. After calcination at 800 °C for 12 h, while Raman spectroscopy fails to detect the ligands after calcination, elastic backscattering spectrometry characterization demonstrates that ~18% of the original carbon atoms are still present in the film. By comparison plasma processing successfully removes the ligands. Our growth kinetic analysis shows that the calcined materials have significantly different interfacial properties than the plasma-processed counterparts. Calcination is not a reliable strategy for the production of single-phase all-inorganic materials from colloidal nanoparticles.

Simplicity as a Route to Impact in Materials Research.

Materials scientists and engineers desire to have an impact. In this Progress Report we postulate a close correlation between impact - whether academic, technological, or scientific - and simple solutions, here defined as solutions that are inexpensive, reliable, predictable, highly performing, "stackable" (i.e., they can be combined and compounded with little increase in complexity), and "hackable" (i.e., they can be easily modified and optimized). In light of examples and our own experience, we propose how impact can be pursued systematically in materials research through a simplicity-driven approach to discovery-driven or problem-driven research.

Building Materials from Colloidal Nanocrystal Arrays: Evolution of Structure, Composition, and Mechanical Properties upon the Removal of Ligands by O2 Plasma.

The mechanical properties of colloidal nanocrystal superlattices can be tailored through exposure to low-pressure plasma. The elastic modulus and hardness of the ligand-free 3.7 nm ZrO2 superlattice are found to be similar to bulk yttria-stabilized tetragonal polycrystals of the same relative density but without any doping.

Plant Growth Environments with Programmable Relative Humidity and Homogeneous Nutrient Availability.

We describe the design, characterization, and use of "programmable", sterile growth environments for individual (or small sets of) plants. The specific relative humidities and nutrient availability experienced by the plant is established (RH between 15% and 95%; nutrient concentration as desired) during the setup of the growth environment, which takes about 5 minutes and <1$ in disposable cost. These systems maintain these environmental parameters constant for at least 14 days with minimal intervention (one minute every two days). The design is composed entirely of off-the-shelf components (e.g., LEGO® bricks) and is characterized by (i) a separation of root and shoot environment (which is physiologically relevant and facilitates imposing specific conditions on the root system, e.g., darkness), (ii) the development of the root system on a flat surface, where the root enjoys constant contact with nutrient solution and air, (iii) a compatibility with root phenotyping. We demonstrate phenotyping by characterizing root systems of Brassica rapa plants growing in different relative humidities (55%, 75%, and 95%). While most phenotypes were found to be sensitive to these environmental changes, a phenotype tightly associated with root system topology-the size distribution of the areas encircled by roots-appeared to be remarkably and counterintuitively insensitive to humidity changes. These setups combine many of the advantages of hydroponics conditions (e.g., root phenotyping, complete control over nutrient composition, scalability) and soil conditions (e.g., aeration of roots, shading of roots), while being comparable in cost and setup time to Magenta® boxes.

Building Materials from Colloidal Nanocrystal Arrays: Preventing Crack Formation during Ligand Removal by Controlling Structure and Solvation.

Crack-free, ligand-free, phase-pure nanostructured solids, using colloidal nanocrystals as precursors, are fabricated by a scalable and facile approach. Films produced by this approach have conductivities comparable to those of bulk crystals over more than 1 cm (1.370 S cm-1 for PbS films).

Thermal Processing of Silicones for Green, Scalable, and Healable Superhydrophobic Coatings.

The thermal degradation of silicones is exploited and engineered to make super-hydrophobic coatings that are scalable, healable, and ecofriendly for various outdoor applications. The coatings can be generated and regenerated at the rate of 1 m(2) min(-1) using premixed flames, adhere to a variety of substrates, and tolerate foot traffic (>1000 steps) after moderate wear and healing.

LEGO® bricks as building blocks for centimeter-scale biological environments: the case of plants.

LEGO bricks are commercially available interlocking pieces of plastic that are conventionally used as toys. We describe their use to build engineered environments for cm-scale biological systems, in particular plant roots. Specifically, we take advantage of the unique modularity of these building blocks to create inexpensive, transparent, reconfigurable, and highly scalable environments for plant growth in which structural obstacles and chemical gradients can be precisely engineered to mimic soil.