Polymer‐Based Stimuli‐Responsive Recyclable Catalytic Systems for Organic Synthesis

The introduction of stimuli‐responsive polymers into the study of organic catalysis leads to the generation of a new kind of polymer‐based stimuli‐responsive recyclable catalytic system. Owing to their reversible switching properties in response to external stimuli, these systems are capable of improving the mass transports of reactants/products in aqueous solution, modulating the chemical reaction rates, and switching the catalytic process on and off. Furthermore, their stimuli‐responsive properties facilitate the separation and recovery of the active catalysts from the reaction mixtures. As a fascinating approach of the controllable catalysis, these stimuli‐responsive catalytic systems including thermoresponsive, pH‐responsive, chemo‐mechano‐chemical, ionic strength‐responsive, and dual‐responsive, are reviewed in terms of their nanoreactors and mechanisms.     

Hierarchical Hydrogen Bonds Directed Multi‐Functional Carbon Nanotube‐Based Supramolecular Hydrogels

Supramolecular hydrogels (SMHs) are three‐dimensional networks filled with a large amount of water. The crosslinking force in the 3D network is always constructed by relatively weak and dynamic non‐covalent interactions, and thus SMHs usually possess extremely high susceptibility to external environment and can show extraordinary stimuli‐responsive, self‐healing or other attractive properties. However, the overall crosslinking force in hydrogel networks is difficult to flexibly modulate, and this leads to limited functions of the SMHs. In this regard, hierarchical hydrogen bonds, that is, the mixture of relatively strong and relatively weak hydrogen bonds, are used herein as crosslinking force for the hydrogel preparation. The ratio of strong and weak hydrogen bonds can be finely tuned to tailor the properties of resultant gels. Thus, by delicate manipulation of the overall crosslinking force in the system, a hydrogel with multiple (thermal, pH and NIR light) responsiveness, autonomous self‐healing property and interesting temperature dependent, reversible adhesion behavior is obtained. This kind of hierarchical hydrogen bond manipulation is proved to be a general method for multiple‐functionality hydrogel preparation, and the resultant material shows potential for a range of applications.     

Laser Heating Tunability by Off‐Resonant Irradiation of Gold Nanoparticles

Temperature changes in the vicinity of a single absorptive nanostructure caused by local heating have strong implications in technologies such as integrated electronics or biomedicine. Herein, the temperature changes in the vicinity of a single optically trapped spherical Au nanoparticle encapsulated in a thermo‐responsive poly(N‐isopropylacrylamide) shell (Au@pNIPAM) are studied in detail. Individual beads are trapped in a counter‐propagating optical tweezers setup at various laser powers, which allows the overall particle size to be tuned through the phase transition of the thermo‐responsive shell. The experimentally obtained sizes measured at different irradiation powers are compared with average size values obtained by dynamic light scattering (DLS) from an ensemble of beads at different temperatures. The size range and the tendency to shrink upon increasing the laser power in the optical trap or by increasing the temperature for DLS agree with reasonable accuracy for both approaches. Discrepancies are evaluated by means of simple models accounting for variations in the thermal conductivity of the polymer, the viscosity of the aqueous solution and the absorption cross section of the coated Au nanoparticle. These results show that these parameters must be taken into account when considering local laser heating experiments in aqueous solution at the nanoscale. Analysis of the stability of the Au@pNIPAM particles in the trap is also theoretically carried out for different particle sizes.     

Engineering the pH‐Responsive Catalytic Behavior of AuNPs by DNA

Noble metal nanoparticles have attracted much interest in the heterogeneous catalysis. Particularly, efficient manipulation of the responsive catalytic properties of the metal nanoparticles is an interesting topic. In this work, a simple and efficient strategy is developed to regulate the pH‐responsive catalytic activities of glucose oxidase (GOx)‐mimicking gold nanoparticles (AuNPs). Four DNA strands (regulating strands) that differ slightly in sequences are used to interact non‐covalently with citrate‐capped AuNPs, resulting in markedly distinct pH‐dependent catalytic behavior of AuNPs. This is ascribed to the characteristic pH‐induced conformational change of the DNA strands that leads to the different adsorption capability to the NPs surface, as demonstrated by pH‐CD profiles of the respective DNA molecules. The pH‐dependent catalysis of AuNPs is also encoded with structural information of the double‐stranded DNA (including regulating strands and their complementary strands) that has conformation resistant or responsive to pH change. As a result, the catalysis can be programmed into an AND gate, a XNOR gate or a NOT gate, using pH and complementary strand as the inputs, the nanoparticle activity as the output and the regulating strands as the programs. This work can be expanded by engineering the catalytic behavior of noble metal nanoparticles to respond smartly to a variety of environmental stimuli, such as metal ions or light wavelengths. These results may provide insight into understanding ligand‐regulated nanometallic catalysis.     

Ultrastrong Anchoring Yet Barrier‐Free Adsorption of Composite Microgels at Liquid Interfaces

Microgel particles display an interesting duality with properties attributed typically both to polymeric and colloidal systems. When adsorbed at a liquid‐liquid interface, this duality becomes particularly apparent as the various phenomena at play are governed by different aspects of these soft and responsive particles. The introduction of a solid, fluorescently labeled, polystyrene core into the microgels allows direct and accurate visualization without the necessity of potential perturbing sample preparation techniques. By combining in‐situ imaging and tensiometry, we determine that composite microgels at a wide variety of oil‐water interfaces anchor strongly, with adsorption energies of approximately 10⁶ k_BT, typical for particle adsorption, yet accumulate at the interface spontaneously without any energy barrier, which is more typical for polymers. The high adsorption energies allow the particle to spontaneously form very dense crystalline packings at the liquid interface in which the microgels are significantly compressed with respect to their swollen state in bulk solutions. Finally, we demonstrate the unique nature of these particles by producing highly stable and monodisperse microgel‐stabilized droplets using microfluidics.     

Shape‐Shifting 3D Protein Microstructures with Programmable Directionality via Quantitative Nanoscale Stiffness Modulation

The ability to shape‐shift in response to a stimulus increases an organism's survivability in nature. Similarly, man‐made dynamic and responsive “smart” microtechnology is crucial for the advancement of human technology. Here, 10–30 μm shape‐changing 3D BSA protein hydrogel microstructures are fabricated with dynamic, quantitative, directional, and angle‐resolved bending via two‐photon photolithography. The controlled directional responsiveness is achieved by spatially controlling the cross‐linking density of BSA at a nanometer lengthscale. Atomic force microscopy measurements of Young's moduli of structures indicate that increasing the laser writing distance at the z‐axis from 100–500 nm decreases the modulus of the structure. Hence, through nanoscale modulation of the laser writing z‐layer distance at the nanoscale, control over the cross‐linking density is possible, allowing for the swelling extent of the microstructures to be quantified and controlled with high precision. This method of segmented moduli is applied within a single microstructure for the design of shape‐shifting microstructures that exhibit stimulus‐induced chirality, as well as for the fabrication of a free‐standing 3D microtrap which is able to open and close in response to a pH change.     

Theranostic USPIO‐Loaded Microbubbles for Mediating and Monitoring Blood‐Brain Barrier Permeation

Efficient and safe drug delivery across the blood‐brain barrier (BBB) remains one of the major challenges of biomedical and (nano‐) pharmaceutical research. Here, it is demonstrated that poly(butyl cyanoacrylate)‐based microbubbles (MB), carrying ultrasmall superparamagnetic iron oxide (USPIO) nanoparticles within their shell, can be used to mediate and monitor BBB permeation. Upon exposure to transcranial ultrasound pulses, USPIO‐MB are destroyed, resulting in acoustic forces inducing vessel permeability. At the same time, USPIO are released from the MB shell, they extravasate across the permeabilized BBB and they accumulate in extravascular brain tissue, thereby providing non‐invasive R ₂*‐based magnetic resonance imaging information on the extent of BBB opening. Quantitative changes in R ₂* relaxometry are in good agreement with 2D and 3D microscopy results on the extravascular deposition of the macromolecular model drug fluorescein isothiocyanate (FITC)‐dextran into the brain. Such theranostic materials and methods are considered to be useful for mediating and monitoring drug delivery across the BBB and for enabling safe and efficient treatment of CNS disorders.