Conductive Nanowires Templated by Molecular Brushes

In this paper, we report the fabrication of conductive nanowires using polymer bottle brushes as templates. In our approach, we synthesized poly­(2-dimethylamino)­ethyl methacrylate methyl iodide quaternary salt brushes by two-step atom transfer radical polymerization, loaded them with palladium salt, and reduced them in order to form metallic nanowires with average lengths and widths of 300 and 20 nm, respectively. The obtained nanowires were deposited between conductive gold pads and were connected to them by sputtering of additional pads to form an electric circuit. We connected the nanowires in an electric circuit and demonstrated that the conductivity of these nanowires is around 100 S·m^–1

Hollow Microgel Based Ultrathin Thermoresponsive Membranes for Separation, Synthesis, and Catalytic Applications

Thermoresponsive core–shell microgels with degradable core are synthesized via surfactant based free radical polymerization using N,N′-(1,2-dihydroxy-ethylene)­bis­(acrylamide) (DHEA) as a cross-linker for core preparation. The 1,2-glycol bond present in DHEA is susceptible to NaIO₄ solution, and thus, the structure can be cleaved off resulting in hollow microgel. Ultrathin membranes are prepared by suction filtration of a dilute suspension of core–shell microgels over a sacrificial layer of Cd­(OH)₂ nanostrand coated on track etched membrane. After removal of the degraded cores from microgels, the membranes are cross-linked with glutaraldehyde and the nanostrands are removed by passing a 10 mM HCl solution. The prepared membranes are thoroughly characterized using scanning electron microscopy (SEM), atomic force microscopy (AFM), dynamic light scattering (DLS), and dynamic contact angle for morphology, thermoresponsive, and hydrophilic properties, respectively. The prepared membranes showed thermoresponsive permeation behavior and remarkable separation performance for low molecular weight dyes and lysozyme protein. These membranes are also used to synthesize gold nanoparticles and immobilize lactate dehydrogenase enzyme for catalytic and biocatalytic application. The results for water permeation, solute rejection, and ability to immobilize gold nanoparticles and enzymes showed its wide range of applicability. Furthermore, the synthesis of hollow microgel is simple and environmentally friendly, and the membrane preparation is easy, scalable, and other microgel systems can also be used. These responsive membranes constitute a significant contribution to advanced separation technology

Swelling and Surface Interactions of End-Grafted Poly(2-vinylpyridine) Layers in Acidic Solution: Influence of Grafting Density and Salt Concentration

In previous studies, the authors found that end-grafted layers of the weak polybase poly­(2-vinylpyridine) (P2VP) in aqueous solutions do not only swell and collapse if the pH value and salt concentration are varied but also exhibit a pH- and salinity-dependent adhesion to microsized silica spheres. For a better understanding of these effects, in situ force measurements using the AFM colloidal probe technique were applied to end-grafted P2VP layers of different grafting densities in NaCl solutions at pH 2.5. Although a mushroom-to-brush transition could be seen in the dry state, the layers were in the brush regime in aqueous solutions at all NaCl concentrations and grafting densities. We observed an increase of the brush height with increasing grafting density and a salinity-dependent collapse and reswelling of the brushes. The adhesion between the P2VP layer and a silica sphere depended on both grafting density and salinity. At low salt concentrations, the adhesion reached its highest value at the intermediate grafting density and disappeared with denser brushes. Maximum adhesion was obtained for high NaCl concentrations and the lowest grafting density. From a detailed analysis of the experiments, we gained insight into chain stretching and density profiles under complex ionic conditions and into the mechanism of adhesion of polyelectrolytes to solid surfaces

pH and Salt Response of Mixed Brushes Made of Oppositely Charged Polyelectrolytes Studied by in Situ AFM Force Measurements and Imaging

The response of mixed brushes made of poly­(acrylic acid) and poly­(2-vinyl pyridine) with a mixing ratio of about 60:40 was studied using atomic force microscopy (AFM) force measurements with colloidal probes and AFM imaging with a sharp tip in the pH range between 2.5 and 8 and at varying KCl concentrations up to 1 M. It was found that under all conditions a dense polyelectrolyte complex layer coexists with excess polyelectrolyte chains in varying swelling states depending on pH and salt concentration. The mixed brush thus combines typical features of polyelectrolyte brushes and complexes. So, the increase of the salt concentration not only led to a transition from osmotic to salted brush regime but also to salt-induced softening or partial decomposition of the complex layer. Attractive forces at high salt concentrations indicated the presence of P2VP chains in the swollen layer even at high pH values

Enhanced Activity of Acetyl CoA Synthetase Adsorbed on Smart Microgel: an Implication for Precursor Biosynthesis

Acetyl coenzyme A (acetyl CoA) is an essential precursor molecule for synthesis of metabolites such as the polyketide-based drugs (tetracycline, mitharamycin, Zocor, etc.) fats, lipids, and cholesterol. Acetyl CoA synthetase (Acs) is one of the enzymes that catalyzes acetyl CoA synthesis, and this enzyme is essentially employed for continuous supply of the acetyl CoA for the production of these metabolites. To achieve reusable and a more robust entity of the enzyme, we carried out the immobilization of Acs on poly­(<i>N</i>-isopropylacrylamide)-poly­(ethylenimine) (PNIPAm-PEI) microgels via adsorption. Cationic PNIPAm-PEI microgel was synthesized by one-step graft copolymerization of NIPAm and <i>N</i>,<i>N</i>-methylene bis-acrylamide (MBA) from PEI. Adsorption studies of Acs on microgel indicated high binding of enzymes, with a maximum binding capacity of 286 μg/mg of microgel for Acs was achieved. The immobilized enzymes showed improved biocatalytic efficiency over free enzymes, beside this, the reaction parameters and circular dichroism (CD) spectroscopy studies indicated no significant changes in the enzyme structure after immobilization. This thoroughly characterized enzyme bioconjugate was further immobilized on an ultrathin membrane to assess the same reaction in flow through condition. Bioconjugate was covalently immobilized on a thin layer of preformed microgel support upon polyethylene terephthalate (PET) track etched membrane. The prepared membrane was used in a dead end filtration device to monitor the bioconversion efficiency and operational stability of cross-linked bioconjugate. The membrane reactor showed consistent operational stability and maintained >70% of initial activity after 7 consecutive operation cycles

Bienzymatic Sequential Reaction on Microgel Particles and Their Cofactor Dependent Applications

We report, the preparation and characterization of bioconjugates, wherein enzymes pyruvate kinase (Pk) and l-lactic dehydrogenase (Ldh) were covalently bound to poly­(<i>N</i>-isopropylacrylamide)-poly­(ethylenimine) (PNIPAm-PEI) microgel support using glutaraldehyde (GA) as the cross-linker. The effects of different arrangements of enzymes on the microgels were investigated for the enzymatic behavior and to obtain maximum Pk-Ldh sequential reaction. The dual enzyme bioconjugates prepared by simultaneous addition of both the enzymes immobilized on the same microgel particles (PL), and PiLi, that is, dual enzyme bioconjugate obtained by combining single-enzyme bioconjugates (immobilized pyruvate kinase (Pi) and immobilized lactate dehydrogenase (Li)), were used to study the effect of the assembly of dual enzymes systems on the microgels. The kinetic parameters (<i>K</i>_m, <i>k</i>_cat), reaction parameters (temperature, pH), stability (thermal and storage), and cofactor dependent applications were studied for the dual enzymes conjugates. The kinetic results indicated an improved turn over number (<i>k</i>_cat) for PL, while the <i>k</i>_cat and catalytic efficiency was significantly decreased in case of PiLi. For cofactor dependent application, in which the ability of ADP monitoring and ATP synthesis by the conjugates were studied, the activity of PL was found to be nearly 2-fold better than that of PiLi. These results indicated that the influence of spacing between the enzymes is an important factor in optimization of multienzyme immobilization on the support

Facile Approach to Grafting of Poly(2-oxazoline) Brushes on Macroscopic Surfaces and Applications Thereof

This study reports on a facile and versatile approach for modification of macroscopic surface via grafting of multifunctional poly­(2-oxazoline) molecules in brush-like conformation. For this purpose, carboxyl-terminated poly­(2-isopropyl-2-oxazoline) molecules have been synthesized by ring-opening cationic polymerization and subsequently grafted on underlined substrates by exploiting the ”grafting to” approach. A systematic variation in thickness of the grafted poly (2-isopropyl-2-oxazoline) brushes has been demonstrated. Polymer-modified surfaces have been characterized by means of a number of analytical tools including ellipsometry, X-ray photoelectron spectroscopy, ultraviolate spectroscopy, attenuated total reflection infrared spectroscopy and atomic force microscopy. Interestingly, poly­(2-isopropyl-2-oxazoline) molecules have been found to retain their physical properties even after grafting on macroscopic surfaces. Finally, fabricated polymer brushes have been used as platform for stabilization of inorganic nanoparticles on macroscopic surfaces

Evaluation and Control of the Orientation of Small Molecules for Strongly Absorbing Organic Thin Films

In the photoactive film of organic solar cells, the orientation of the absorber molecules is one of the key parameters to achieve high absorption and high photocurrents as well as efficient exciton and charge transport. However, most organic absorber small molecules, such as zinc-phthalocyanine (ZnPc) or diindenoperylene (DIP) grow more or less upright standing in crystalline thin films. Considering absorption, this molecular alignment is unfavorable. In this work we control the orientation of ZnPc and DIP in crystalline absorber films by varying the substrate or organic underlayer appropriately. For this purpose, a precise evaluation of the molecular orientation and packing is important. We find that a combination of the methods variable angle spectroscopic ellipsometry (VASE) and grazing incidence X-ray diffraction (GIXRD) can fulfill this requirement. The combination of these complementary methods shows that the growth of DIP and ZnPc is nearly upright standing on weakly interacting substrates, like glass or amorphous charge transport films. In contrast, on strongly interacting metal sublayers and PTCDA templating layers, both molecules arrange in a strongly tilted orientation (mean tilt angle 54°-71° with respect to the substrate normal), inducing a significant enhancement of absorption (maximum extinction coefficient from 0.72 to 1.3 for ZnPc and 0.14 to 0.4 for DIP). However, even when deposited on metal or PTCDA sublayers, not all ZnPc and DIP molecules in the film are oriented in the desired flat-lying fashion. This highlights that classifying organic films into either solely flat lying structures or solely upright standing structures, as often made in literature, is a too simplified picture

Effect of the Cross-Linking Density on the Thermoresponsive Behavior of Hollow PNIPAM Microgels

We report on the fabrication of thermally responsive hollow pNIPAM particles through the oxidation of the metal core in an Au@pNIPAM system. The selective oxidation of the Au core is achieved by addition of AuCl₄^– to an aqueous dispersion of Au@pNIPAM particles in the presence of cetyltrimethylammonium bromide (CTAB). We fabricate hollow pNIPAM particles with three cross-linking densities (<i>N,N</i>′-methylenebis­(acrylamide), BA, at 5%, 10%, and 17.5%). The study of the effect of the amount of BA within the microgel network was performed by dynamic light scattering (DLS), transmission electron microscopy (TEM), and atomic force microscopy (AFM), showing its key role in determining the final hollow structure and thermal response. While the thermal responsiveness is largely achieved at low cross-linking densities, the hollow structure only remains at larger cross-linking densities. This was further confirmed by cryo-TEM analysis of hollow pNIPAM particles below and above the volume phase transition temperature (VPTT). Thus, it clearly shows (i) the shrinking of particle size with the temperature at low cross-linking density and (ii) the dependence of particle size on the amount of cross-linker for the final hollow pNIPAM structure. Observed differences in the hollow pNIPAM structure are attributed to different elastic contributions (Π_elas), showing higher elasticity for microgels synthesized at lower amount of BA

Tunable Fluorescence of a Semiconducting Polythiophene Positioned on DNA Origami

A novel approach for the integration of π-conjugated polymers (CPs) into DNA-based nanostructures is presented. Using the controlled Kumada catalyst-transfer polycondensation, well-defined thiophene-based polymers with controllable molecular weight, specific end groups, and water-soluble oligoethylene glycol-based side chains were synthesized. The end groups were used for the easy but highly efficient click chemistry-based attachment of end-functionalized oligodeoxynucleotides (ODNs) with predesigned sequences. As demonstrated by surface plasmon resonance spectroscopy, the prepared block copolymers (BCPs), P3­(EO)₃T-<i>b</i>-ODN, comprising different ODN lengths and specific or repetitive sequences, undergo specific hybridization with complementary, thiol-functionalized ODNs immobilized on a gold surface. Furthermore, the site-specific attachment of the BCPs to DNA origami structures is studied. We demonstrate that a nanoscale object, that is, a single BCP with a single ODN handle, can be directed and bound to the DNA origami with reasonable yield, site-specificity, and high spatial density. On the basis of these results, we are able to demonstrate for the first time that optical properties of CP molecules densely immobilized on DNA origami can be locally fine-tuned by controlling the attractive π–π-stacking interactions between the CPs. In particular, we show that the fluorescence of the immobilized CP molecules can be significantly enhanced by surfactant-induced breakup of π–π-stacking interactions between the CP’s backbones. Such molecular control over the emission intensity of the CPs can be valuable for the construction of sophisticated switchable nanophotonic devices and nanoscale biosensors