Temperature-Responsive Self-Assembly of Nanoparticles Grafted with UCST Polymer Ligands

Temperature-responsive self-assembly (TRSA) of polymer-stabilized nanoparticles is a promising method that is useful for many applications. Currently, polymers ligands with a lower critical solution temperature are used for TRSA, which requires the use of specific polymer–solvent couples. We report a comprehensive study of TRSA of nanoparticles grafted with polymer ligands with an upper critical solution temperature (UCST). Upon cooling the nanoparticle solution below the transition temperature, the nanoparticles assembled in clusters, while upon heating these clusters dissociated into individual nanoparticles. The TRSA was reversible and reproducible. In the heating and cooling steps, the dimensions of nanoparticle clusters were controlled by the superposition of temperature and incubation time. The transition to TRSA was governed by the solvent quality for the polymer ligands and was tuned by varying solvent composition. The utilization of UCST polymer ligands offers an effective method for the preparation of assemblies of polymer-tethered nanoparticles, broadens the range of polymers used for TRSA, and enables control of the degree and temperature of nanoparticle assembly

Peclet Number Dependence of Mass Transfer in Microscale Segmented Gas–Liquid Flow

A detailed understanding of the scaling behavior associated with the fluid flow and the transport of gas molecules from a train of elongated gas plugs into neighboring liquid segments is of great importance for a broad range of microscale applications. The indirect dependence of the parameters affecting the <i>Capillary</i> and <i>Peclet</i> numbers and thereby scaling behavior (i.e., the velocity and length of the gas plugs, and the length of the liquid segments) on the directly adjustable experimental inputs (i.e., flow rate or pressure of each phase) has hindered the systematic investigation of scaling behavior in microscale gas–liquid flows. Here, we take advantage of an image-based feedback strategy that allows us to directly impose Capillary and Peclet numbers. We custom fabricated a long, straight microchannel (width 300 μm, length-to-width ratio 700) in a gas impermeable silicon–glass substrate. We automatically determined the length reduction of initially uniformly sized gas plugs at different positions along the microchannel and elucidated the gas concentration within adjacent liquid segments. In accordance with penetration theory, we analytically estimated the gas–liquid mass transfer time to scale with the Peclet number, <i>Pe</i>, to the power of −0.5. The experimentally measured scaling exponent −0.55 ± 0.5 for carbon dioxide dissolution in methanol and ethanol at <i>Pe</i> = 2060–16500 compared favorably with the analytical prediction and provides a guideline for predicting physical transport for a wide range microscale gas–liquid flow processes

Self-assembled plasmonic nanostructures

<p>Self-assembly of plasmonic nanoparticles offers a labour- and cost-efficient strategy for the expansion of the library of plasmonic nanostructures with highly tunable, coupled optical properties. This review covers recent advances in solution-based self-assembly of plasmonic nanoparticles, modelling of the self-assembly process and of the optical properties of the resulting nanostructures, and potential applications of self-assembled plasmonic nanostructures.</p

Multiple Shape Transformations of Composite Hydrogel Sheets

Soft materials undergoing shape transformations in response to changes in ambient environment have potential applications in tissue engineering, robotics and biosensing. Generally, stimulus-responsive materials acquire two stable shapes corresponding to the “on” and “off” states of the external trigger. Here, we report a simple, yet versatile approach to induce multiple shape transformations of a planar hydrogel sheet, each triggered by a particular, well-defined external stimulus. The approach is based on the integration of small-scale multiple polymer components with distinct compositions in the composite gel sheet. In response to different stimuli, the structural components undergo differential swelling or shrinkage, which creates internal stresses within the composite hydrogel sheet and transforms its shape in a specific manner

Controlling the Degree of Polymerization, Bond Lengths, and Bond Angles of Plasmonic Polymers

Plasmonic polymers present an interesting concept that builds on the analogy between molecular polymers and linear chains of strongly interacting metal nanoparticles. Ensemble-averaged optical properties of plasmonic polymers are strongly influenced by their structure. In the present work, we formed plasmonic polymers by using solution-based assembly of gold nanorods (NRs) end-tethered with photoactive macromolecular tethers. By using postassembly ligand photo-cross-linking, we established a method to arrest NR polymer growth after a particular self-assembly time, and in this manner, using kinetics of step-growth polymerization, we achieved control over the average degree of polymerization of plasmonic polymers. Photo-cross-linking of ligands also enabled control over the internanorod distance and resulted in the increased rigidity of NR chains. These results, along with a higher structural integrity of NR chains, can be utilized in plasmonic nanostructure engineering and facilitate advanced applications of plasmonic polymers in sensing and optoelectronics

From Structure to Properties of Composite Films Derived from Cellulose Nanocrystals

Many natural materials exhibit a multilayer structure in which adjacent layers rotate in a helicoidal manner. The remarkable optical and mechanical properties of these materials have motivated research and development of man-made materials with similar morphology. Among them, composite materials by cellulose nanocrystals (CNCs) and polymers have attracted great interest; however, the relationship between the cholesteric structure and the material properties is not well understood. We used the composite CNC–polymer latex films with random, stratified, and cholesteric morphologies, all with the same compositions, to explore the effect of structure on the optical and mechanical properties of the composite films. Films with a cholesteric structure exhibited strong extinction, circular dichroism, and high stiffness; however, they had lower toughness than the films with the cholesteric stratified morphology. Films with disordered morphologies exhibited the highest toughness and the lowest stiffness. These trends were attributed to the confinement effects and the difference in polymer distribution in the films. These results provide guidance for the preparation of biomimetic cholesteric films with targeted optical and mechanical properties

Structural Transitions in Nanoparticle Assemblies Governed by Competing Nanoscale Forces

Assembly of nanoscale materials from nanoparticle (NP) building blocks relies on our understanding of multiple nanoscale forces acting between NPs. These forces may compete with each other and yield distinct stimuli-responsive self-assembled nanostructures. Here, we report structural transitions between linear chains and globular assemblies of charged, polymer-stabilized gold NPs, which are governed by the competition of repulsive electrostatic forces and attractive poor solvency/hydrophobic forces. We propose a simple quantitative model and show that these transitions can be controlled by the quality of solvent, addition of a salt, and variation of the molecular weight of the polymer ligands

Large-Scale Synthesis of Metal Nanocrystals in Aqueous Suspensions

Fundamental studies and practical use of metal nanoparticles (NPs) frequently depend on the ability to reproducibly synthesize large quantities of shape-specific NPs. For this reason, facile synthetic procedures are desired that will lead to large quantities of uniformly sized metal NPs exhibiting specific shapes. Here, we report a general approach to the large-scale synthesis of noble metal nanocrystals having well-defined shapes and a narrow size distribution. This method utilizes seed-mediated NP growth in aqueous suspensions of cationic surfactants and metal salts. It leads to a ∼60-fold increase in NP volumetric production capacity, compared to the most widely used solution-based synthetic methods. In addition, it uses up to 100 times less cationic surfactant than conventional solution-based methods. The applicability of the method is demonstrated in the synthesis of Pd nanocubes, rhombic dodecahedra, and polyhedrons with low index facets; branched Pd nanocrystals; alloy Pt/Pd nanocubes; Ag nanocubes. The advantages and limitations of the approach are discussed, including accessible shapes, growth kinetics, and the capability to scale up the synthetic procedure

Switchable Water: Microfluidic Investigation of Liquid–Liquid Phase Separation Mediated by Carbon Dioxide

Increase in the ionic strength of water that is mediated by the reaction of carbon dioxide (CO₂) with nitrogenous bases is a promising approach toward phase separation in mixtures of water with organic solvents and potentially water purification. Conventional macroscale studies of this complicated process are challenging, due to its occurrence via several consecutive and concurrent steps, mass transfer limitation, and lack of control over gas–liquid interfaces. We report a new microfluidic strategy for fundamental studies of liquid–liquid phase separation mediated by CO₂ as well as screening of the efficiency of nitrogenous agents. A single set of microfluidic experiments provided qualitative and quantitative information on the kinetics and completeness of water–tetrahydrofuran phase separation, the minimum amount of CO₂ required to complete phase separation, the total CO₂ uptake, and the rate of CO₂ consumption by the liquid mixture. The efficiency of tertiary diamines with different lengths of alkyl chain was examined in a time- and labor-efficient manner and characterized with the proposed efficiency parameter. A wealth of information obtained using the MF methodology can facilitate the development of new additives for switchable solvents in green chemistry applications

Chiral Plasmonic Films Formed by Gold Nanorods and Cellulose Nanocrystals

Chiral plasmonic films have been prepared by incorporating gold nanorods (NRs) in a macroscopic cholesteric film formed by self-assembled cellulose nanocrystals (CNCs). Composite NR-CNC films revealed strong plasmonic chiroptical activity, dependent on the photonic properties of the CNC host and plasmonic properties of the NRs. The plasmonic chiroptical properties of the composite films were tuned by changing the conditions of film preparation. The strategy presented herein paves the way for the scalable and cost-efficient preparation of plasmonic chiral materials