Multi-Redox Responsive Behavior in a Mixed-Valence Semiconducting Framework Based on Bis-[1,2,5]-thiadiazolo-tetracyanoquinodimethane.

The two-dimensional (2-D) framework, [Cu(BTDAT)(MeOH)] {BTDAT = bis-[1,2,5]-thiadiazolo-tetracyanoquinodimethane}, possesses remarkable multi-step redox properties, with electrochemical studies revealing six quasi-stable redox states in the solid state. In situ electron paramagnetic resonance and visible-near infrared spectroelectrochemistry elucidated the mechanism for these multi-step redox processes, as well as the optical and electrochromic behavior of the BTDAT ligand and framework. In studying the structural, spectroscopic, and electronic properties of [Cu(BTDAT)(MeOH)], the as-synthesized framework was found to exist in a mixed-valence state with thermally-activated semiconducting behavior. In addition to pressed pellet conductivity measurements, single-crystal conductivity measurements using a pre-patterned polydimethylsiloxane layer on a silicon substrate provide important insights into the anisotropic conduction pathways. As an avenue to further understand the electronic state of [Cu(BTDAT)(MeOH)], computational band structure calculations predicted delocalized electronic transport in the framework. On the balance of probabilities, we propose that [Cu(BTDAT)(MeOH)] is a Mott insulator (i.e., electron correlations cause a metal-insulator transition). This implies that the conductivity is incoherent. However, we are unable to distinguish between activated transport due to Coulombically bound electron-hole pairs and a hopping mechanism. The combined electrochemical, electronic, and optical properties of [Cu(BTDAT)(MeOH)] shine a new light on the experimental and theoretical challenges for electroactive framework materials, which are implicated as the basis of advanced optoelectronic and electrochromic devices.

Mixed valency in a neutral 1D Fe-chloranilate coordination polymer.

The syntheses and structures of a pair of neutral one-dimensional (1D) Fe-anilate based coordination polymers, Fe(Fan)(4,4'-bipy)2 (Fann- = deprotonated 3,6-difluoro-2,5-dihydroxy-1,4-benzoquinone; 4,4'-bipy = 4,4'-bipyridine) and Fe(Clan)(OPPh3)2 (Clann- = deprotonated 3,6-dichloro-2,5-dihydroxy-1,4-benzoquinone; OPPh3 = triphenylphosphine oxide), are reported. In the case of Fe(Fan)(4,4'-bipy)2, the Fe centre is in the +2 oxidation state and the Fan ligand is present in its quinoidal, dianionic form. In contrast, the structurally similar Fe(Clan)(OPPh3)2 chain contains Fe centres and chloranilate ligands in oxidation states close to +3 and -3 respectively at low temperature. It is suggested that intrachain π-π interactions aid electron transfer from the Fe centres to the bridging ligands.

Capsosomes as Long-Term Delivery Vehicles for Protein Therapeutics.

We report the preparation of polymer capsules containing liposomal subcompartments, termed capsosomes, and their ability for the sustained delivery of protein therapeutics. Capsosomes were formed through the layer-by-layer (LbL) assembly of polymers and protein-loaded liposomes, followed by the formation of a capsule membrane based on disulfide cross-linked poly(methacrylic acid). The loading capacities of a model cargo (lysozyme) and brain-derived neurotrophic factor (BDNF), an important neurotrophin that has significant physiological functions on the nervous system, were determined, and the long-term release kinetics of the proteins was investigated in simulated physiological conditions. The capsosomes exhibited protein loading and release behavior that can be tuned by the lipid composition of the liposomal compartments, where inclusion of anionic lipids resulted in enhanced protein loading and slower release over the course of 80 days. These findings highlight the potential of capsosomes for the long-term delivery of protein therapeutics.

Engineering fluorescent poly(dopamine) capsules.

The recent development of poly(dopamine) (PDA) capsules provides new opportunities for their application in biology and medicine. To advance the biomedical application of PDA capsules, strategies that enable the preparation of fluorescently labeled PDA (F-PDA) capsules are required, as this will allow evaluation of their cellular interactions using a range of fluorescence-based techniques. Herein, we report a facile approach for the fabrication of F-PDA capsules via the polymerization of dopamine (DA) on sacrificial templates in the presence of hydrogen peroxide (H2O2). F-PDA capsules with well-defined sizes are prepared by templating different organic and inorganic particles. The resulting F-PDA capsules show negligible cytotoxicity in HeLa cells after incubation for 48 h. We also demonstrate visualization of the F-PDA capsules following internalization by HeLa cells using conventional fluorescence microscopy, en route toward detailed investigations on their biological interactions.

Emerging methods for the fabrication of polymer capsules.

Hollow polymer capsules are attracting increasing research interest due to their potential application as drug delivery vectors, sensors, biomimetic nano- or multi-compartment reactors and catalysts. Thus, significant effort has been directed toward tuning their size, composition, morphology, and functionality to further their application. In this review, we provide an overview of emerging techniques for the fabrication of polymer capsules, encompassing: self-assembly, layer-by-layer assembly, single-step polymer adsorption, bio-inspired assembly, surface polymerization, and ultrasound assembly. These techniques can be applied to prepare polymer capsules with diverse functionality and physicochemical properties, which may fulfill specific requirements in various areas. In addition, we critically evaluate the challenges associated with the application of polymer capsules in drug delivery systems.

One-step assembly of coordination complexes for versatile film and particle engineering.

The development of facile and versatile strategies for thin-film and particle engineering is of immense scientific interest. However, few methods can conformally coat substrates of different composition, size, shape, and structure. We report the one-step coating of various interfaces using coordination complexes of natural polyphenols and Fe(III) ions. Film formation is initiated by the adsorption of the polyphenol and directed by pH-dependent, multivalent coordination bonding. Aqueous deposition is performed on a range of planar as well as inorganic, organic, and biological particle templates, demonstrating an extremely rapid technique for producing structurally diverse, thin films and capsules that can disassemble. The ease, low cost, and scalability of the assembly process, combined with pH responsiveness and negligible cytotoxicity, makes these films potential candidates for biomedical and environmental applications.

Phototriggered, Metal-Free Continuous Assembly of Polymers for the Fabrication of Ultrathin Films.

A facile and metal-free thin film fabrication technology based on the photoinduced continuous assembly of polymers (photoCAP) is described. The efficiency and versatility of this method is demonstrated by the formation of crosslinked and surface-confined nanoengineered thin films, in the form of surface coatings and hollow polymer capsules.

Polymersome-loaded capsules for controlled release of DNA.

The formation of a novel drug-delivery carrier for the controlled release of plasmid DNA that comprises layer-by-layer polymer capsules subcompartmentalized with pH-sensitive nanometer-sized polymersomes is reported. The amphiphilic diblock copolymer poly(oligoethylene glycol methacrylate)-block-poly(2-(diisopropylamino)ethyl methacrylate) forms polymersomes at physiological pH, but transitions to unimeric polymer chains upon acidification to cellular endocytic pH. These polymersomes can thus release an encapsulated payload in response to a change in pH from physiological to endocytic conditions. Multicomponent layer-by-layer capsules are formed by exploiting the ability of tannic acid to act as an efficient hydrogen-bond donor for both the polymersomes and poly(N-vinyl pyrrolidone) at physiological pH. These capsules show release of a plasmid DNA payload encapsulated within the polymersome subcompartments in response to changes in pH between physiological and endocytic conditions.

Biodegradable click capsules with engineered drug-loaded multilayers.

We report the modular assembly of a polymer-drug conjugate into covalently stabilized, responsive, biodegradable, and drug-loaded capsules with control over drug dose and position in the multilayer film. The cancer therapeutic, doxorubicin hydrochloride (DOX), was conjugated to alkyne-functionalized poly(l-glutamic acid) (PGA(Alk)) via amide bond formation. PGA(Alk) and PGA(Alk+DOX) were assembled via hydrogen bonding with poly(N-vinyl pyrrolidone) (PVPON) on planar and colloidal silica templates. The films were subsequently covalently stabilized using diazide cross-linkers, and PVPON was released from the multilayers by altering the solution pH to disrupt hydrogen bonding. After removal of the sacrificial template, single-component PGA(Alk) capsules were obtained and analyzed by optical microscopy, transmission electron microscopy, and atomic force microscopy. The PGA(Alk) capsules were stable in the pH range between 2 and 11 and exhibited reversible swelling/shrinking behavior. PGA(Alk+DOX) was assembled to form drug-loaded polymer capsules with control over drug dose and position in the multilayer system (e.g., DOX in every layer or exclusively in layer 3). The drug-loaded capsules could be degraded enzymatically, resulting in the sustained release of active DOX over approximately 2 h. Cellular uptake studies demonstrate that the viability of cells incubated with DOX-loaded PGA(Alk) capsules significantly decreased. The general applicability of this modular approach, in terms of incorporation of polymer-drug conjugates in other click multilayer systems, was also demonstrated. Biodegradable click capsules with drug-loaded multilayers are promising candidates as carrier systems for biomedical applications.