Polymer N-Heterocyclic Carbene (NHC) Ligands for Silver Nanoparticles.

Polymer N-heterocyclic carbenes (NHCs) are a class of robust surface ligands to provide superior colloidal stability for metal nanoparticles (NPs) under various harsh conditions. We report a general method to prepare polymeric NHCs and demonstrate that these polymer NHC-AgNPs are stable against oxidative etching and show high peroxidase activity. We prepared three imidazolium-terminated poly(methyl methacrylate) (PMMA), polystyrene (PS), and poly(2-(2-methoxyethoxy)ethyl methacrylate) (PMEO2MA) through atom-transfer radical polymerization with an imidazole-containing initiator. The imidazolium end group was further converted to NHC-Ag(I) in the presence of Ag2O at room temperature. Polymer NHC-Ag(I) can transmetalate to AgNPs through ligand exchange at the interface of oil/water within 2 min. All the three polymers can modify metal NPs, such as AgNPs, Ag nanowires, and AuNPs, providing excellent thermal, oxidative, and chemical stabilities for AgNPs. As an example, in the presence of hydrogen peroxide, AgNPs modified by polymer NHCs were resistant against oxidative etching with a rate of ∼700 times slower than those grafted with thiolates. AgNPs modified by polymer NHCs also showed higher peroxidase activity, 4 times more active than those capped by citrate and polyvinylpyrrolidone (PVP) and 2 times more active than those with polymer thiolate. Our studies demonstrate a great potential of using polymer NHCs to stabilize metallic NPs for various applications.

Adaptable Eu-containing polymeric films with dynamic control of mechanical properties in response to moisture.

Self-healing polymers often have a trade-off between healing efficiency and mechanical stiffness. Stiff polymers that sacrifice their chain mobility are slow to repair upon mechanical failure. We herein report adaptable polymer films with dynamically moisture-controlled mechanical and optical properties, therefore having tunable self-healing efficiency. The design of the polymer film is based on the coordination of europium (Eu) with dipicolylamine (DPA)-containing random copolymers of poly(n-butyl acrylate-co-2-hydroxy-3-dipicolylamino methacrylate) (P(nBA-co-GMADPA)). The Eu-DPA complexation results in the formation of mechanically robust polymer films. The coordination of Eu-DPA has proven to be moisture-switchable given the preferential coordination of lanthanide metals to O over N, using nuclear magnetic resonance and fluorescence spectroscopy. Water competing with DPA to bind Eu3+ ions can weaken the cross-linking networks formed by Eu-DPA coordination, leading to the increase of chain mobility. The in situ dynamic mechanical analysis and ex situ rheological studies confirm that the viscofluid and the elastic solid states of Eu-polymers are switchable by moisture. Water speeds up the self-healing of the polymer film by roughly 100 times; while it can be removed after healing to recover the original mechanical stiffness of polymers.

A Polymer Solution To Prevent Nanoclustering and Improve the Selectivity of Metal Nanoparticles for Electrocatalytic CO2 Reduction.

The stability of metal nanocatalysts for electrocatalytic CO2 reduction is of key importance for practical application. We report the use of two polymeric N-heterocyclic carbenes (NHC) (polydentate and monodentate) to stabilize metal nanocatalysts (Au and Pd) for efficient CO2 electroreduction. Compared with other conventional ligands including thiols and amines, metal-carbene bonds that are stable under reductive potentials prevent the nanoclustering of nanoparticles. Au nanocatalysts modified by polymeric NHC ligands show an activity retention of 86 % after CO2 reduction at -0.9 V for 11 h, while it is less than 10 % for unmodified Au. We demonstrate that the hydrophobicity of polymer ligands and the enriched surface electron density of metal NPs through σ-donation of NHCs substantially improve the selectivity for CO2 reduction over proton.

Synthetic Polymers To Promote Cooperative Cu Activity for O2 Activation: Poly vs Mono.

We report polymer-promoted cooperative catalysis of Cu for oxygen activation. A series of random copolymers containing dipicolylamine as binding motifs are designed to coordinate type-3 Cu sites. The Cu-copolymers show a 6-8-fold activity enhancement, compared to the molecular complex of Cu with an identical coordination site. Michaelis-Menten analysis demonstrates that the kinetic enhancement results from flexible polymer-promoted cooperative catalysis among multi-Cu sites despite the imposed thermodynamic barrier. These observations provide guidance for the bioinspired design of metallopolymers as soluble catalysts with high activity.

Controllable Self-Assembly of Amphiphilic Tadpole-Shaped Polymer Single-Chain Nanoparticles Prepared through Intrachain Photo-cross-linking.

We report the use of intramolecular cross-linking chemistry as a tool to control the self-assembly of amphiphilic diblock copolymers (di-BCPs). Two amphiphilic di-BCPs of poly( N, N'-dimethylacrylamide)- block-polystyrene (PDMA- b-PS) with photo-cross-linkable cinnamoyl groups in either hydrophobic or hydrophilic blocks were prepared using reversible addition-fragmentation chain transfer polymerization. Intramolecular photo-cross-linking of cinnamoyl groups led to the formation of tadpole-shaped polymer single-chain nanoparticles (SCNPs) consisting of a self-collapsed block as the "head" and an un-cross-linked block as the "tail". When intramolecular photo-cross-linking was carried out in hydrophobic PS blocks, a clear morphological transition from branched cylindrical micelles (for the linear di-BCP) to completely spherical micelles at a dimerization degree of ∼63% was observed. A pattern of morphological transitions from cylindrical micelles to spherical micelles is observed through stepwise downsizing the length of cylindrical micelles when increasing the self-collapse degree of PS blocks, whereas, in case of photo-cross-linking carried out in hydrophilic PDMA blocks, the size of micelles showed a dramatic increase due to the shift of hydrophobic-to-hydrophilic balance. When the cross-linking degree of PDMA blocks reached >60%, tadpole-shaped SCNPs assembled into nonconventional aggregates with a nonsmooth surface. Our results illustrate the impact of chain topologies on the self-assembly outcomes of amphiphilic di-BCPs, which likely opens a door to control the micellar morphologies from just one parent linear di-BCP, rather than resynthesizing BPCs with different volume fractions of the two blocks.

Synthesis and assembly of colloidal cuboids with tunable shape biaxiality.

The design and assembly of monodisperse colloidal particles not only advances the development of functional materials, but also provides colloidal model systems for understanding phase behaviors of molecules. This communication describes the gram-scale synthesis of highly uniform colloidal cuboids with tunable dimension and shape biaxiality and their molecular mesogen-like assembly into various mesophasic structures in pristine purity. The synthesis relies on the nanoemulsion-guided generation of ammonium sulfate crystals that template the subsequent silica coating. The shape of the cuboidal particles can be tuned from square platelike, to biaxial boardlike, and to rodlike by independently controlling the length, width and thickness of the particles. We demonstrated the assembly of the cuboidal colloids into highly pure mesoscopic liquid crystal phases, including smectic A, biaxial smectic A, crystal B, discotic, and columnar phases, as well as established a correlation between mesophasic formation and colloidal biaxiality in experiments.

Dynamic Coordination of Eu-Iminodiacetate to Control Fluorochromic Response of Polymer Hydrogels to Multistimuli.

New fluorochromic materials that reversibly change their emission properties in response to their environment are of interest for the development of sensors and light-emitting materials. A new design of Eu-containing polymer hydrogels showing fast self-healing and tunable fluorochromic properties in response to five different stimuli, including pH, temperature, metal ions, sonication, and force, is reported. The polymer hydrogels are fabricated using Eu-iminodiacetate (IDA) coordination in a hydrophilic poly(N,N-dimethylacrylamide) matrix. Dynamic metal-ligand coordination allows reversible formation and disruption of hydrogel networks under various stimuli which makes hydrogels self-healable and injectable. Such hydrogels show interesting switchable ON/OFF luminescence along with the sol-gel transition through the reversible formation and dissociation of Eu-IDA complexes upon various stimuli. It is demonstrated that Eu-containing hydrogels display fast and reversible mechanochromic response as well in hydrogels having interpenetrating polymer network. Those multistimuli responsive fluorochromic hydrogels illustrate a new pathway to make smart optical materials, particularly for biological sensors where multistimuli response is required.

Synthesis of Mesoporous CoS2 and NixCo1-xS2 with Superior Supercapacitive Performance Using a Facile Solid-Phase Sulfurization.

Synthesis of nanostructured transition metal sulfides is of particular interest in providing new methods to control their porosity and improve their surface area because those sulfides hold promising applications in high-energy density devices. Significant challenges remain currently to prepare metal sulfides having three-dimensional (3-D) continuous mesoporous structures, known to be critical for increasing their active surface sites and enhancing ion transport. We herein present a facile solid-phase sulfurization method to synthesize 3-D continuous mesoporous CoS2, NiS2, and their binary sulfides in a two-step nanocasting using bicontinuous KIT-6 as hard templates. The solid-phase sulfurization taking place at 400 °C yields mesoporous sulfides with highly crystalline frameworks and a stoichiometric ratio of metal-to-sulfur, 1:2 (mol), within 30 min. Elemental sulfur as an inexpensive sulfur source can be directly used for the solid-phase sulfurization of mesoporous oxides of Co3O4, NiO, and their binary oxides. This facile synthetic method is highly efficient to prepare mesoporous sulfides in the gram-scale production at a very low cost. Mesoporous sulfides are demonstrated to be superior electrode materials for pseudo-supercapacitors, given their high surface area and accessible bicontinuous mesopores, the suitable crystalline sizes, and the enhanced ion transport capability. The use of binary mesoporous sulfides presents interesting synergetic effect where the doping of metal ions can significantly enhance the capacitive performance of single-component sulfides. The binary sulfides of mNi0.32Co0.68S2 show a specific capacitance up to 1698 F g-1 at a current density of 2 A g-1. The supercapacitor device of mNi0.32Co0.68S2 has a high energy density of 37 Wh kg-1 at a power density of 800 W kg-1. We believe that the reported solid-phase synthesis offers a universal method toward the conversion of mesoporous oxides materials into various useful and functional forms for energy conversion and storage applications.

Cross-linking of COOH-containing polymers using Ag(i)-catalyzed oxidative decarboxylation in aqueous solution.

Cross-linking that defines the three-dimensional networks in hydrogels has a significant impact on their physiochemical properties. The cross-linking of hydrophilic polymers via post-polymerization reactions is an ideal way to manufacture hydrogels with high reproducibility and without monomer residuals. We herein report the use of Ag(i)-catalyzed oxidative decarboxylation to cross-link poly(acrylic acid) (PAA) and a family of COOH-containing hydrophilic polymers. Our method is based on the radical-mediated elimination reaction to remove COOH group(s) and generate alkyl radical(s) simultaneously, in the presence of AgNO3 and persulfates. The further intermolecular radical coupling is demonstrated to be very effective in inducing cross-linking and gelation of COOH-containing hydrophilic polymers. The cross-linking reaction can be readily achieved by simply mixing a small amount of AgNO3 (as low as 0.03 wt%) and persulfates with polymers at room temperature in air. Rheological measurements show that the gelation occurs in 20-30 min. The applications of oxidative decarboxylation in the preparation of hydrogels of COOH-containing hydrophilic copolymers and their interpenetrating polymer network (IPN) hydrogels are further validated. Finally, the residual Ag(i) ions in hydrogels are discussed in terms of how Ag(i) ions further change the mechanical and optical properties of hydrogels by photoreduction of Ag(i) to Ag nanoparticles. We expect that this Ag(i)-catalyzed oxidative decarboxylation chemistry can not only serve as a facile and general strategy to produce hydrogels through post-polymerization, but also enrich the toolbox of cross-linking chemistries of COOH-containing polymers in all forms (e.g. films, colloids and dispersions).

Surface Engineering of Spherical Metal Nanoparticles with Polymers toward Selective Asymmetric Synthesis of Nanobowls and Janus-Type Dimers.

New synthetic methods capable of controlling structural and compositional complexities of asymmetric nanoparticles (NPs) are very challenging but highly desired. A simple and general synthetic approach to designing sophisticated asymmetric NPs by anisotropically patterning the surface of isotropic metallic NPs with amphiphilic block copolymers (BCPs) is reported. The selective galvanic replacement and seed-mediated growth of a second metal can be achieved on the exposed surface of metal NPs, resulting in the formation of nanobowls and Janus-type metal-metal dimers, respectively. Using Ag and Au NPs tethered with amphiphilic block copolymers of poly(ethylene oxide)-block-polystyrene (PEO-b-PS), anisotropic surface patterning of metallic NPs (e.g., Ag and Au) is shown to be driven by thermodynamical phase segregation of BCP ligands on isotropic metal NPs. Two proof-of-concept experiments are given on, i) synthesis of Au nanobowls by a selective galvanic replacement reaction on Janus-type patched Ag/polymer NPs; and ii) preparation of Au-Pd heterodimers and Au-Au homodimers by a seed-mediated growth on Janus-type patched Au/polymer NPs. The method shows remarkable versatility; and it can be easily handled in aqueous solution. This synthetic strategy stands out as the new methodology to design and synthesis asymmetric metal NPs with sophisticated topologies.