Super Stretchable and Compressible Hydrogels Inspired by Hook-and-Loop Fasteners.

Inspired by hook-and-loop fasteners, we designed a hydrogel network containing α-zirconium phosphate (ZrP) two-dimensional nanosheets with a high density of surface hydroxyl groups serving as nanopatches with numerous "hooks," while polymer chains with plentiful amine functional groups serve as "loops." Our multiscale molecular simulations confirm that both the high density of hydroxyl groups on nanosheets and the large number of amine functional groups on polymer chains are essential to achieve reversible interactions at the molecular scale, functioning as nano hook-and-loop fasteners to dissipate energy. As a result, the synthesized hydrogel possesses superior stretchability (>2100% strain), resilience to compression (>90% strain), and durability. Remarkably, the hydrogel can sustain >5000 cycles of compression with torsion in a solution mimicking synovial fluid, thus promising for potential biomedical applications such as artificial articular cartilage. This hook-and-loop model can be adopted and generalized to design a wide range of multifunctional materials with exceptional mechanical properties.

Gold nanoparticles with externally controlled, reversible shifts of local surface plasmon resonance bands.

We have achieved reversible tunability of local surface plasmon resonance in conjugated polymer functionalized gold nanoparticles. This property was facilitated by the preparation of 3,4-ethylenedioxythiophene (EDOT) containing polynorbornene brushes on gold nanoparticles via surface-initiated ring-opening metathesis polymerization. Reversible tuning of the surface plasmon band was achieved by electrochemically switching the EDOT polymer between its reduced and oxidized states.

Poly(allylamine)-encapsulated water-soluble CdSe nanocrystals.

Water-soluble CdSe nanocrystal/poly(allylamine) clusters with sizes ranging between 50 and 200 nm were prepared using 3-amino-1-propanol as a compatibilizing agent. Photoluminescence (PL) quantum yields (QY) up to 20% were achieved in water without the need to clad these CdSe nanocrystals (NCs) with higher band gap inorganic layers. The polymer-to-nanocrystal ratio plays an important role in the internal structure and stability of these polymer/NC clusters, as determined by static and dynamic light scattering in conjunction with PL studies. These results were modeled by using an effective-mass approximation and perturbation theory on the change in dielectric constant of the immediate NC environment. The time evolution of the average cluster radius of gyration and hydrodynamic radius revealed that a higher polymer-to-NC ratio leads to increased PL stability and QY. This is a result of a denser cluster configuration, which affords improved NC passivation. Increasing the ionic strength results in greater nanocluster compaction and higher PL QYs. Decreasing the pH value below 12 resulted in dramatic reduction in PL brightness, despite cluster densification, due to partial ionization and dissolution of the amine-based NC surface-capping agents.

Nanoparticle agglutination: acceleration of aggregation rates and broadening of the analyte concentration range using mixtures of various-sized nanoparticles.

SiO(2) particles of various sizes were prepared and surface modified with biotin-chain-end-functionalized poly(ethylene glycol). Dispersions of these particles were prepared, and their aggregation was induced upon the addition of avidin. The aggregate size and growth rate were monitored by DLS analysis, and SEM and TEM images of freeze-dried samples of the aggregate solutions were used to confirm the DLS data and to image the aggregate size and dimension. A linear correspondence between apparent diameter and time was observed, and both the 20 and 300 nm particles aggregated at slower rates than for the 40 nm particles. These observations were attributed to differences in the average functionality of the systems and the different initial concentrations of particles and avidin. The observed aggregation rates of binary combinations of the three particle sizes (i.e., 20 + 40 nm or 40 + 300 nm) were faster than those of the single-sized mixtures. These results were attributed to the faster flux of smaller particles toward larger particles in the mixture solutions as well as to the potentially larger number of productive collisions possible between mixtures of small particles and large particles versus only similarly sized particles. Combinations of the three sizes of particles were studied to find an empirical optimum formulation for generating large aggregates on a short time scale and over a wide range of analyte concentrations.

Quantitative determination of the chemical composition of silica-poly(norbornene) nanocomposites.

Nanoparticle hybrid materials consisting of a silica core surrounded by a poly(norbornene) brush have been prepared by ring opening metathesis polymerization (ROMP). A quantitative determination of each stage of composite formation has been accomplished, including a determination of the density of surface-bound functional groups, catalyst molecules, and polymer chains. This analysis has enabled the determination of the reaction efficiency between the catalyst and the surface-bound functional groups as well as the determination of the fraction of metal-mediating species that initiate a polymer chain. Control of the chain density was demonstrated by two methods: the use of controlled reaction times between the catalyst and the surface, and the variation of the surface functional group density. Polymer chain densities resulting from composites prepared with different tether structures will also be reported. The resulting brush densities were found to span a wide range, including those previously reported for polymer layers formed by adsorption, grafting of preformed polymer chains, and surface-initiated polymerization (SIP).