Contact-free conductivity probing of metal nanowire films using THz reflection spectroscopy.

We utilize time-domain Terahertz (THz) reflectivity measurements for characterizing the surface conductivity of Polyethylene-terephthalate coated with nanowire (NW) films to form novel transparent electrodes (TE). We find good correspondence between the film conductivity and the THz-field reflectivity that provide uniquely desirable means for non-destructive, contactless conductivity measurements of large area NW-based-TEs. We demonstrate the robustness of THz reflectivity measurements to deviations invoked on NW film composition and film uniformity. The dependence of THz reflectivity on area NW coverage follows an anisotropic effective medium model for the dielectric constant.

Reversible Dimerization of Polymeric Amphiphiles Acts as a Molecular Switch of Enzymatic Degradability.

Enzyme-responsive polymeric micelles have great potential as drug delivery systems due to the high selectivity and overexpression of disease-associated enzymes, which could be utilized to trigger the release of active drugs only at the target site. We previously demonstrated that enzymatic degradation rates of amphiphilic PEG-dendron hybrids could be precisely tuned by gradually increasing the hydrophobic to hydrophilic ratio. However, with the increase in hydrophobicity, the micelles rapidly became too stable and could not be degraded, as often encountered for many other amphiphilic assemblies. Here we address the challenge to balance between stability and reactivity of enzymatically degradable assemblies by utilizing reversible dimerization of diblock polymeric amphiphiles to yield jemini amphiphiles. This molecular transformation serves as a tool to control the critical micelle concentration of the amphiphiles in order to tune their micellar stability and enzymatic degradability. To demonstrate this approach, we show that simple dimerization of two polymeric amphiphiles through a single reversible disulfide bond significantly increased the stability of their micellar assemblies toward enzymatic degradation, although the hydrophilic to hydrophobic ratio was not changed. Reduction of the disulfide bond led to dedimerization of the polymeric hybrids and allowed their degradation by the activating enzyme. The generality of the approach is demonstrated by designing both esterase- and amidase-responsive micellar systems. This new molecular design can serve as a simple tool to increase the stability of polymeric micelles without impairing their enzymatic degradability.

Modular Synthetic Approach for Adjusting the Disassembly Rates of Enzyme-Responsive Polymeric Micelles.

Self-assembled nanostructures and their stimuli-responsive degradation have been recently explored to meet the increasing need for advanced biocompatible and biodegradable materials for various biomedical applications. Incorporation of enzymes as triggers that can stimulate the degradation and disassembly of polymeric assemblies may be highly advantageous owing to their high selectivity and natural abundance in all living organisms. One of the key factors to consider when designing enzyme-responsive polymers is the ability to fine-tune the sensitivity of the platform toward its target enzyme in order to control the disassembly rate. In this work, a series of enzyme-responsive amphiphilic PEG-dendron hybrids with increasing number of hydrophobic cleavable end-groups was synthesized, characterized, and compared. These hybrids were shown to self-assemble in aqueous media into nanosized polymeric micelles, which could encapsulate small hydrophobic guests in their cores and release them upon enzymatic stimulus. Utilization of dendritic scaffolds as the responsive blocks granted ultimate control over the number of enzymatically cleavable end-groups. Remarkably, as we increased the number of end-groups, the micellar stability increased significantly and the range of enzymatic sensitivity spanned from highly responsive micelles to practically nondegradable ones. The reported results highlight the remarkable role of hydrophobicity in determining the micellar stability toward enzymatic degradation and its great sensitivity to small structural changes of the hydrophobic block, which govern the accessibility of the cleavable hydrophobic groups to the activating enzyme.

Patterning Metal Nanowire-Based Transparent Electrodes by Seed Particle Printing.

This article describes a unique combination of inkjet printing of functional materials with an intricate self-assembly process. Gold-silver nanowire (NW) mesh films were produced by a sequential deposition process, in which small metal seed nanoparticle film was deposited at desired areas by inkjet printing, followed by coating with a thin film of NW growth solution. Two different types of NW growth solutions were used: the first, based on benzylhexadecyldimethylammonium chloride, exhibited a bulk solution growth mode and was thus suitable for coverage of large uniform areas. The second type was based on hexadecyltrimethylammonium bromide, which induced NW growth confined to the substrate-solution interface and thus enabled patterning of small transparent electrode features, which have the same dimensions as the deposited seed droplets. A selective silver plating bath was used to thicken the ultrathin NWs, stabilize them, and reduce the sheet resistance, resulting in films with sheet resistance in the range of 20-300 Ω/sq, 86-95% light transmission, and a relatively low haze. This simple patterning method of the NW film works at ambient conditions on many different types of substrates and has the potential to replace the conventional photolithography used for indium tin oxide patterning for applications such as touch sensors and flexible/stretchable electronics.

Molecular Precision and Enzymatic Degradation: From Readily to Undegradable Polymeric Micelles by Minor Structural Changes.

Studying the enzymatic degradation of synthetic polymers is crucial for the design of suitable materials for biomedical applications ranging from advanced drug delivery systems to tissue engineering. One of the key parameters that governs enzymatic activity is the limited accessibility of the enzyme to its substrates that may be collapsed inside hydrophobic domains. PEG-dendron amphiphiles can serve as powerful tools for the study of enzymatic hydrolysis of polymeric amphiphiles due to the monodispersity and symmetry of the hydrophobic dendritic block, which significantly simplifies kinetic analyses. Using these hybrids, we demonstrate how precise, minor changes in the hydrophobic block are manifested into tremendous changes in the stability of the assembled micelles toward enzymatic degradation. The obtained results emphasize the extreme sensitivity of self-assembly and its great importance in regulating the accessibility of enzymes to their substrates. Furthermore, the demonstration that the structural differences between readily degradable and undegradable micelles are rather minor, points to the critical roles that self-assembly and polydispersity play in designing biodegradable materials.

The effect of photoisomerization on the enzymatic hydrolysis of polymeric micelles bearing photo-responsive azobenzene groups at their cores.

The design of stable polymeric micelles that can respond to specific stimuli is crucial for the development of smart micellar nanocarriers that can release their active cargo selectively at the target site, thus diminishing the therapeutic limitations due to non-selective damage to healthy tissues. Here we report the design and synthesis of photo- and enzyme-responsive amphiphilic PEG-dendron hybrids bearing one, two or four enzymatically cleavable azobenzene end-groups. These dual-responsive hybrids can respond to light through the reversible isomerization of the azobenzene end-groups from the non-polar trans isomer to the highly polar cis isomer and vice versa, upon UV and visible irradiation, respectively. The high structural precision of these hybrids, which emerges from the dendritic architecture, enabled a detailed study of the photoisomerization of the azobenzene end-groups with high molecular resolution. Remarkably, although the transition from trans-to-cis led to a significant increase in the polarity of the micellar cores, the micelles remained stable. Our kinetic studies show that although the trans isomer is a better substrate for the activating enzyme, the UV induced formation of the cis azobenzene end-groups led to significant acceleration of the enzymatic hydrolysis of the end-groups. These results provide strong indication that the enzyme cannot reach the core of the micelles and instead the end-groups have to leave the hydrophobic core in order to be exposed on the micelle's surface or even leave the micelle in order to allow their cleavage by the activating enzymes.

Supramolecular Translation of Enzymatically Triggered Disassembly of Micelles into Tunable Fluorescent Responses.

The need for advanced fluorescent imaging and delivery platforms has motivated the development of smart probes that change their fluorescence in response to external stimuli. Here a new molecular design of fluorescently labeled PEG-dendron hybrids that self-assemble into enzyme-responsive micelles with tunable fluorescent responses is reported. In the assembled state, the fluorescence of the dyes is quenched or shifted due to intermolecular interactions. Upon enzymatic cleavage of the hydrophobic end-groups, the labeled polymeric hybrids become hydrophilic, and the micelles disassemble. This supramolecular change is translated into a spectral response as the dye-dye interactions are eliminated and the intrinsic fluorescence is regained. We demonstrate the utilization of this molecular design to generate both Turn-On and spectral shift responses by adjusting the type of the labeling dye. This approach enables transformation of non-responsive labeling dyes into smart fluorescent probes.

Encapsulation and covalent binding of molecular payload in enzymatically activated micellar nanocarriers.

The high selectivity and often-observed overexpression of specific disease-associated enzymes make them extremely attractive for triggering the release of hydrophobic drug or probe molecules from stimuli-responsive micellar nanocarriers. Here we utilized highly modular amphiphilic polymeric hybrids, composed of a linear hydrophilic polyethylene glycol (PEG) and an esterase-responsive hydrophobic dendron, to prepare and study two diverse strategies for loading of enzyme-responsive micelles. In the first type of micelles, hydrophobic coumarin-derived dyes were encapsulated noncovalently inside the hydrophobic core of the micelle, which was composed of lipophilic enzyme-responsive dendrons. In the second type of micellar nanocarrier the hydrophobic molecular cargo was covalently linked to the end-groups of the dendron through enzyme-cleavable bonds. These amphiphilic hybrids self-assembled into micellar nanocarriers with their cargo covalently encapsulated within the hydrophobic core. Both types of micelles were highly responsive toward the activating enzyme and released their molecular cargo upon enzymatic stimulus. Importantly, while faster release was observed with noncovalent encapsulation, higher loading capacity and slower release rate were achieved with covalent encapsulation. Our results clearly indicate the great potential of enzyme-responsive micellar delivery platforms due to the ability to tune their payload capacities and release rates by adjusting the loading strategy.

Enzyme-responsive amphiphilic PEG-dendron hybrids and their assembly into smart micellar nanocarriers.

Enzyme-responsive micelles have great potential as drug delivery platforms due to the high selectivity of the activating enzymes. Here we report a highly modular design for the efficient and simple synthesis of amphiphilic block copolymers based on a linear hydrophilic polyethyleneglycol (PEG) and an enzyme-responsive hydrophobic dendron. These amphiphilic hybrids self-assemble in water into micellar nanocontainers that can disassemble and release encapsulated molecular cargo upon enzymatic activation. The utilization of monodisperse dendrons as the stimuli-responsive block enabled a detailed kinetic study of the molecular mechanism of the enzymatically triggered disassembly. The modularity of these PEG-dendron hybrids allows control over the disassembly rate of the formed micelles by simply tuning the PEG length. Such smart amphiphilic hybrids could potentially be applied for the fabrication of nanocarriers with adjustable release rates for delivery applications.

Direct monitoring of opto-mechanical switching of self-assembled monolayer films containing the azobenzene group.

The potential for manipulation and control inherent in molecule-based motors holds great scientific and technological promise. Molecules containing the azobenzene group have been heavily studied in this context. While the effects of the cis-trans isomerization of the azo group in such molecules have been examined macroscopically by a number of techniques, modulations of the elastic modulus upon isomerization in self-assembled films were not yet measured directly. Here, we examine the mechanical response upon optical switching of bis[(1,1'-biphenyl)-4-yl]diazene organized in a self-assembled film on Au islands, using atomic force microscopy. Analysis of higher harmonics by means of a torsional harmonic cantilever allowed real-time extraction of mechanical data. Quantitative analysis of elastic modulus maps obtained simultaneously with topographic images show that the modulus of the cis-form is approximately twice that of the trans-isomer. Quantum mechanical and molecular dynamics studies show good agreement with this experimental result, and indicate that the stiffer response in the cis-form comprises contributions both from the individual molecular bonds and from intermolecular interactions in the film. These results demonstrate the power and insights gained from cutting-edge AFM technologies, and advanced computational methods.

Formation of gold-silver nanowires in thin surfactant solution films.

Thin, long gold/silver nanowires were grown on substrates in thin surfactant solution films. This growth process occurred exclusively in thinning aqueous films as the water evaporated, and elongated surfactant template structures were formed. The nanowire growth depended on the presence of a relatively high concentration of silver ions (typical Ag:Au mole ratio of 1:1). Tuning the pH value to about 5 in the growth solution was crucial for the nanowire growth. Further development of this process may lead to a simple wet chemical technique for the fabrication of relatively uniform arrays of metal nanowires on surfaces.