Degradable Anti-Biofouling Polyester Coatings with Controllable Lifetimes.

To achieve degradable, anti-biofouling coatings with longer lifetimes and better mechanical properties, we synthesized a series of degradable co-polyesters composed of cyclic ketene acetals, di-(ethylene glycol) methyl ether methacrylate, and a photoactive curing agent, 4-benzoylphenyl methacrylate, using a radical ring-opening polymerization. The precursor co-polyesters were spin-coated on a benzophenone-functionalized silicon wafer to form ca. 60 nm films and drop-casted on glass to form ∼32 µm films. The copolymers were cross-linked via UV irradiation at 365 nm. The degradation of films was studied by immersing the specimens in aqueous buffers of different pH values. The results show that both the pH of buffer solutions and gel fractions of networks affect the degradation rate. The coatings show good bovine serum albumin resistance capability. By adjusting the fractions of monomers, the degradation rate and degree of hydration (e.g., swelling ratio) are controllable.

Dynamic Surfaces-Degradable Polyester Networks that Resist Protein Adsorption.

We synthesized a series of novel degradable alternating copolyesters composed of diglycolic anhydride (DGA) and two epoxides, epoxymethoxytriethylene glycol (ETEG) and a photoactive crosslinking agent epoxy benzophenone (EBP). After UV crosslinking, soaking the films in a good solvent (tetrahydrofuran) removed uncrosslinked material, and the resulting film gel fractions were calculated. These network films were then degraded in buffer solutions of varying pH values. The degradation of networks with lower gel fraction (fewer crosslinks) was faster and followed first-order kinetics. In contrast, the denser network degraded slower and followed zeroth-order kinetics. The lower gel fraction networks possess a higher swelling ratio and resist bovine serum albumin (BSA) adsorption better by entropic shielding and faster degradation. In comparison, higher gel fraction networks with higher EBP mole fractions adsorb more BSA due to hydrophobic interactions and slower degradation.

Amphiphilic phospholipid-iodinated polymer conjugates for bioimaging.

Amphiphilic phospholipid-iodinated polymer conjugates were designed and synthesized as new macromolecular probes for a highly radiopaque and biocompatible imaging technology. Bioconjugation of PEG 2000-phospholipids and iodinated polyesters by click chemistry created amphiphilic moieties with hydrophobic polyesters and hydrophilic PEG units, which allowed their self-assemblies into vesicles or spiked vesicles. More importantly, the conjugates exhibited high radiopacity and biocompatibility in in vitro X-ray and cell viability measurements. This new type of bioimaging contrast agent with a Mn value of 11 289 g mol-1 was found to have a significant X-ray signal at 3.13 mg mL-1 of iodine equivalent than baseline and no cytotoxicity after 48 hours incubation of with HEK and 3T3 cells at 20 µM (20 picomoles) concentration of conjugates per well. The potential of adopting the described macromolecular probes for bioimaging was demonstrated, which could further promote the development of a field-friendly and highly sensitive bioimaging contrast agent for point-of-care diagnostic applications.

Synthesis of 5,12-Diazapentacenes and Their Properties.

An efficient synthesis via a precursor route to a new class of linear dialkyldiaminoazapentacenes is reported. The synthetic route involves the coupling of 4-substituted aniline derivatives to 2,5-dibromoterephthalonitrile via Buchwald-Hartwig amination followed by an acid-mediated cyclization to furnish the diazapentacenes. These reactions occur under short reaction times (<2 h), provide high yields (77-99%), and do not require column chromatography for purification. The electrochemical and optical properties of the diazapentacenes were evaluated, and the values indicate that these molecules are promising n-type materials for organic electronic devices. The photostability of these molecules was significantly greater than unsubstituted acenes. Their method of degradation via endoperoxide generation was confirmed by X-ray crystallography and mass spectrometry.

DendriPeps: Expanding Dendrimer Functionality by Hybridizing Poly(amidoamine) (PAMAM) Scaffolds with Peptide Segments.

In this work, the first synthesis of poly(amidoamine) (PAMAM) dendrimers whose branches are hybridized with peptide segments (DendriPeps) is reported. The intercalation of amino acids within the branches of PAMAMs provides supplementary internal functionalities to the coronal groups. Four DendriPep prototypes are synthesized with lysine or glutamic acid as "guest" amino acids, displaying, respectively, a primary amine or a carboxyl group, on generation (G)2 and G3 PAMAMs as host scaffolds. The precise control over the number, type, and topological placement of functional groups expands the functional behavior of DendriPeps beyond current PAMAM dendrimers toward new frontiers or colloids, drug delivery vectors, and catalysis.

Liquid Metal Nanoparticles as Initiators for Radical Polymerization of Vinyl Monomers.

Sonication of gallium or gallium-based liquid metals in an aqueous solution of vinyl monomers leads to rapid free radical polymerization (FRP), without the need for conventional molecular initiators. Under ambient conditions, a passivating native oxide separates these metals from solution and renders the metal effectively inert. However, sonication generates liquid metal nanoparticles (LMNPs) of ∼100 nm diameter and thereby increases the surface area of the metal. The exposed metal initiates polymerization, which proceeds via a FRP mechanism and yields high molecular weight polymers that can form physical gels. Spin trapping EPR reveals the generation of free radicals. Time-of-flight secondary ion mass spectrometry measurements confirm direct polymer bonding to gallium, verifying the formation of surface-anchored polymer grafts. The grafted polymers can modify the interfacial properties, that is, the preference of the metal particles to disperse in aqueous versus organic phases. The polymer can also be degrafted and isolated from the particles using strong acid or base. The concept of physically disrupting passivated metal surfaces offers new routes for surface-initiated polymerization and has implications for surface modification, reduction reactions, and fabrication of mechanically responsive materials.

Amidation of Polyesters Is Slow in Nonaqueous Solvents: Efficient Amidation of Poly(ethylene terephthalate) with 3-Aminopropyltriethoxysilane in Water for Generating Multifunctional Surfaces.

This paper describes surface functionalization of poly(ethylene terephthalate) (PET) films by transamidation of the ester groups with primary amines. The use of water as a solvent improves tremendously the reaction rate and yield compared to conventionally used alcohols. In this study, PET films were exposed to an aqueous solution of 3-aminopropyltriethoxysilane (APTES), which resulted in ester-to-amide reactions on the surface of the film. Hydrolysis of the resulting ethoxy moieties in APTES creates hydroxyl groups that can be used as anchoring points for further modification of PET films. This scheme offers an alternative approach to modify polyesters using water as the solvent.

Resisting protein adsorption on biodegradable polyester brushes.

The protein adsorption and degradation behaviors of poly(lactic acid), poly(glycolic acid) (PGA) and poly(ε-caprolactone) (PCL) brushes and their co-polymer brushes with oligo(ethylene glycol) (OEG) were studied. Both brush structure and relative amount of OEG and polyester were found to be important to the protein resistance of the brushes. A protein-resisting surface can be fabricated either by using OEG as the top layer of a copolymer brush or by increasing the amount of OEG relative to polyester when using a hydroxyl terminated OEG (OEG-OH) and a methoxy terminated OEG (OEG-OMe) mixture as the substrate layer. The degradation of single polyester brushes and their co-polymer brushes using OEG-OH as a substrate layer or using OEG as a top layer was hindered. This phenomenon was rationalized by the inhibition of the proposed back-biting process as the hydroxy end groups of polyester were blocked by OEG molecules. Among these brushes tested, PGA co-polymer brushes using the methoxy/hydroxyl OEG mixture as the substrate layer proved to be both protein-resistant and degradable due to the relatively large amount of OEG moieties and the good biodegradability of PGA.

Standing up versus looping over: controlling the geometry of self-assembled monolayers of α,ω-diynes on gold.

It is shown that self-assembled monolayers (SAMs) composed of α,ω-diynes on gold have different structures depending on the concentration of molecules used to make the SAM. Evidence for both hairpinned and standing-up molecules is provided. This behavior is in contrast to SAMs of α,ω-dithiols on gold, which generally form SAMs with only the straight conformation. The looped SAMs composed of α,ω-diynes offer a less densely packed and thus somewhat accessible surface that may be useful when the underlying surface is used as an electrode. Furthermore, biasing the structure of the molecules in the SAM between looped and standing-up may be useful in the design of dynamic surfaces.

Overcoming challenges in the palladium-catalyzed synthesis of electron deficient ortho-substituted aryl acetonitriles.

Highly electron deficient monoaryl, di-aryl and bis-diaryl acetonitriles were effectively synthesized using either a nucleophilic aromatic substitution (NAS) or a palladium-mediated coupling pathway. Synthesis of di-aryl acetonitriles most conveniently proceeded via NAS--palladium-mediated coupling was not required. This reaction, however, results in a product that is more acidic than the reactants. Facile deprotonation of the product prevents efficient formation of the bis-diaryl acetonitrile through a NAS pathway. Thus, palladium-mediated coupling is required to prepare the bis-diaryl acetonitrile efficiently. In the palladium-catalyzed coupling, choice of base and solvent (and thus the counter cation for the benzylic anion nucleophile) is important. Also, choice of the supporting ligand is important, indicating the sensitivity of the reaction to steric and ligand electronic effects.

Terminal alkynes as an ink or background SAM in replacement lithography: adventitious versus directed replacement.

Self-assembled monolayers (SAMs) comprised from n-alkanethiols and terminal alkynes were subjected to solutions containing ferrocene-terminated thiol, thioacetate, and terminal alkyne. The rate and extent of chemical exchange were monitored by scanning tunneling microscopy (STM). In several cases, a rate constant for exchange could be obtained by fitting to a model for exchange. In each case where this could be accomplished, a different rate model gave the best fit to the data, suggesting that the mechanism of exchange depended on either or both the original SAM and the incoming molecule. In scenarios where the rate of exchange was slow, directed exchange was accomplished via STM tip-induced lithographic patterning (replacement lithography). The extent of exchange was independent of the incoming molecule, suggesting that tip-induced desorption was the limiting factor in this process.

Dendritically encapsulated, water-soluble fe(4)s(4): synthesis and electrochemical properties.

Amphiphilic, Fe(4)S(4) cluster core dendrimers can be prepared via ligand exchange with dendrons containing carboxylic acid peripheral groups and a thiol focal group. These amphiphilic dendrons are more susceptible to oxidative disulfide formation than their non-amphiphilic analogues reported previously. Thus, an in situ deprotection of an aromatic thioacetate was necessary to prepare the dendrimers. These molecules showed the expected decrease in rate with increasing generation. A slower rate of heterogeneous electron transfer was found when these molecules were compared with non-amphiphilic analogues. This behavior correlated with their larger size and thus a larger effective distance of electron transfer. Voltammetry in DMSO with added water makes the dendrimers easier to reduce, but the change in redox potential is much smaller for all dendrimers when compared to a non-dendritic analogue. This behavior is consistent with the idea that the dendrimers encapsulate the cluster to some degree, creating a hydrophobic microenvironment around the cluster.

Cascade cyclization to produce a series of fused, aromatic molecules.

An efficient synthesis of fully aromatic, fused-ring, monoaza-acenes through a nucleophile initiated cascading cyclization is illustrated. Photophysical properties of the resulting molecules are reported.

Reversible addition-fragmentation chain transfer polymerization in DNA biosensing.

Reversible addition-fragmentation chain transfer polymerization is employed here to allow detector-free visualization of specific DNA sequences for which dynamic polymer growth is used in signal amplification. In particular, surface-initiated polymer growth was regulated by the immobilization of chain transfer agents on the Au surface where DNA hybridization occurred. A linear polymer growth was observed as a function of the reaction time, characteristic of "living" polymer reactions. Significant improvement in assay sensitivity was realized in comparison to the previously reported polymerization-based sensing method by enhancing polymer growth rate and reducing background noises caused by nonspecific adsorption. Direct visualization of fewer than 2,000 copies of a short oligonucleotide sequence was demonstrated in a detector-free fashion.

Effect of dendrimer generation on electron self-exchange kinetics between metal tris(bipyridine) core dendrimers.

Here we report the first measurement of homogeneous electron transfer between oxidized and reduced metal tris(bipyridine) core dendrimers by NMR line-broadening; the results indicated that, as the generation of the dendrimer increased, the rate of self-exchange decreased.

Scanning tunneling microscopy-based replacement lithography on self-assembled monolayers: comparison of gold, palladium, and platinum substrates.

Self-assembled monolayers of dodecanethiol on gold, palladium, and platinum can be locally replaced with a different thiol under the action of a scanning tunneling microscopy tip at elevated bias. This process is characterized and the bias dependence of the degree of replacement for each metal substrate is illustrated. Replacement on gold substrates occurs at lower applied potentials than on platinum substrates, provided care has been taken to remove surface oxides.

Real-time conductivity analysis through single-molecule electrical junctions.

Conductance through single-molecule junctions, consisting of nanoparticle/molecule/nanoparticle units between nanoscale planar electrodes, was monitored in real time during several process sequences, including dielectrophoretic directed self-assembly and post-assembly modification. Assembly faults are directly detected in real time when non-ideal assembly conditions result in molecular junction failure and nanoparticle fusion in the junction. The real-time conductivity measured through the junction was sensitive to ambient conditions, and changes persisted over several days of exposure. Atomic layer deposition of Al(2)O(3) was used to encapsulate and isolate the molecular junctions, and the effect of the deposition process sequence on current through the junction was evaluated in real time. Results indicate that the current measured during atomic layer deposition is sensitive to the chemical oxidation and reduction reactions proceeding in the 1-2 nm confined region between assembled nanoparticles.

Conduction mechanisms and stability of single molecule nanoparticle/molecule/nanoparticle junctions.

Nanoparticle/molecule/nanoparticle dimer assemblies have been successfully trapped by dielectrophoresis across nanogap electrodes, enabling temperature dependent charge transport measurements through an oligomeric phenylene ethynylene molecule, and transition from direct tunnelling to Fowler-Nordheim tunnelling is observed at approximately 1.5 V. Samples formed by dielectrophoresis show better contact stability than those formed by receding meniscus. The junction shows stable operation over several weeks in a vacuum, but current increases with time upon exposure to air, possibly due to the adsorbed water molecules near the molecule/gold nanoparticle contacts.

Attenuating electron-transfer rates via dendrimer encapsulation: the case of metal tris(bipyridine) core dendrimers.

Heterogeneous electron-transfer rates in metal tris(bipyridine) core dendrimers were measured using Osteryoung square-wave voltammetry. Rates decreased with generation, and this decrease could be correlated with the molecular weight increase. These results indicate that the coordination number around the redox center did not play any special role in sterically encapsulating the redox center.