Strategies and Molecular Design Criteria for 3D Printable Hydrogels

Covalent polymers connected by non-covalent interactions constitute a fascinating set of materials known as supramolecular polymer networks (SPNs). A key feature of SPNs is that the underlying covalent polymers endow the resulting self-assembled materials with features, such as structural and mechanical integrity, good processability, recyclability, stimuli-responsiveness, self-healing, and shape memory, that are not recapitulated in the case of classic covalent polymer systems. The unique nature of SPNs derives from the controlled marriage of traditional covalent polymers and macrocycle-based host-guest interactions. As a consequence, supramolecular polymeric networks have played important roles in a number of diverse fields, including polymer science, supramolecular chemistry, materials science, biomedical materials, and information storage technology. In this Review, we summarize advances made in the area of functional SPNs, with a focus on original literature reports appearing in the past five years. The treatment is organized according to the key macrocycle-based host-guest interactions used to produce various SPNs. The role of the underlying polymer backbones is also discussed.

Functional Supramolecular Polymeric Networks: The Marriage of Covalent Polymers and Macrocycle-Based Host-Guest Interactions

Supramolecular polymer networks are materials crosslinked by reversible supramolecular interactions, such as hydrogen bonding or electrostatic interactions. Supramolecular materials show very interesting and useful properties resulting from their dynamic nature, such as self-healing, stimuli-responsiveness and adaptability. Here we will discuss recent progress in polymer-based supramolecular networks for the formation of hydrogels and bulk materials.

Supramolecular polymer networks: hydrogels and bulk materials

Progress in quantum computing and quantum cryptography requires efficient, electrically triggered, single-photon sources at room temperature in the telecom wavelengths. It has been long known that semiconducting single-wall carbon nanotubes (SWCNTs) display strong excitonic binding and emit light over a broad range of wavelengths, but their use has been hampered by a low quantum yield and a high sensitivity to spectral diffusion and blinking. In this Perspective, we discuss recent advances in the mastering of SWCNT optical properties by chemistry, electrical contacting and resonator coupling towards advancing their use as quantum light sources. We describe the latest results in terms of single-photon purity, generation efficiency and indistinguishability. Finally, we consider the main fundamental challenges stemming from the unique properties of SWCNTs and the most promising roads for SWCNT-based chip integrated quantum photonic sources. This Perspective describes the recent advances in understanding and controlling the properties of single-wall carbon nanotubes as well as the progress towards the fabrication of new electrically driven single-photon sources.

Carbon nanotubes as emerging quantum-light sources

Artificially reducing the effective dimensionality of carbon nanotubes from one to zero dimensions increases the luminescence quantum yield of excitons confined in zero-dimensional-like states to ∼18%, which is over one order of magnitude larger than that of intrinsic one-dimensional excitons (∼1%). This finding will help realize future nanoscale photonic devices.

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Brightening of excitons in carbon nanotubes on dimensionality modification

Photoluminescence quantum yields and nonradiative decay of the excitonic S(1) state in length fractionated (6,5) single-wall carbon nanotubes (SWNTs) are studied by continuous wave and time-resolved fluorescence spectroscopy. The experimental data are modeled by diffusion limited contact quenching of excitons at stationary quenching sites including tube ends. A combined analysis of the time-resolved photoluminescence decay and the length dependence of photoluminescence quantum yields (PL QYs) from SWNTs in sodium cholate suspensions allows to determine the exciton diffusion coefficient D = 10.7 ± 0.4 cm(2)s(-1) and lifetime τ(PL) for long tubes of 20 ± 1 ps. PL quantum yields Φ(PL) are found to scale with the inverse diffusion coefficient and the square of the mean quenching site distance, here l(d) = 120 ± 25 nm. The results suggest that low PL QYs of SWNTs are due to the combination of high-diffusive exciton mobility with the presence of only a few quenching sites.

Diffusion limited photoluminescence quantum yields in 1-D semiconductors: single-wall carbon nanotubes.

In this edge article we report the preparation of a supramolecular carbohydrate hydrogel containing cyclodextrin vesicles as 3D junctions. A cellulose polymer is randomly modified with hydrophobic side groups that act as guests for the cyclodextrin hosts on the surface of the vesicles. Hence, the vesicles interconnect the polymer chains into a three-dimensional network and act as multivalent linkages. The resulting gel shows significant shear-thinning and self-healing properties, which make it highly suitable for applications that require injectability. Furthermore, SAXS and cryo-TEM measurements indicate that intact vesicles are present in the gel matrix.

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A carbohydrate-based hydrogel containing vesicles as responsive non-covalent cross-linkers

Cyclodextrin vesicles (CDVs) consist of a bilayer of amphiphilic cyclodextrins (CDs). CDVs exhibit CD cavities at their surface that are able to recognize and bind hydrophobic guest molecules via size-selective inclusion. In this study, the permeability of α- and β-CDVs is investigated by pulsed field gradient-stimulated echo (PFG-STE) nuclear magnetic resonance. Diffusion experiments with water and two types of water-soluble polymers, polyethylene glycol (PEG) and polypropylene glycol (PPG), revealed three main factors that influence the exchange rate and permeability of CDVs. First, the length of the hydrophobic chain of the CD amphiphile plays a crucial role. Reasonably, vesicles consisting of amphiphiles with a longer aliphatic chain are less permeable since both membrane thickness and melting temperature Tm increase. Second, the exchange rate through the bilayer membrane depends on the molecular weight of the polymer and decreases with increasing weight of the polymer. Most interestingly, a size-sele...

Size-Selective Permeation of Water-Soluble Polymers through the Bilayer Membrane of Cyclodextrin Vesicles Investigated by PFG-NMR

A supramolecular carbohydrate sensor was prepared by self-assembly of unilamellar bilayer vesicles of amphiphilic β-cyclodextrin with a boronic acid–adamantane conjugate (PBA-AD), which binds strongly to β-cyclodextrin through host–guest interactions (Ka≈4×104 m−1), so the vesicle surface exhibits multiple boronic acid receptors for carbohydrates. The binding of diols to the functionalized vesicles was tested with alizarin red S (ARS) as a reporter dye by using fluorescence and UV/Vis spectroscopy. Analysis of the competitive binding of monosaccharides revealed pH-dependent (pH 7.4–10.1) binding constants in the range of 100–3000 m−1 for d-fructose and 5–400 m−1 for d-glucose. Interestingly, the self-assembled sensor showed fluorescence intensity enhanced by about fivefold and a binding affinity to the reporter dye increased by about eightfold in comparison to PBA-AD and ARS without vesicles. This is attributed to increased local concentration of ARS and PBA-AD on the surface of the vesicles. Detection of d-fructose and d-glucose was possible in the physiologically relevant range in dilute aqueous solution.

Sabine Himmelein

Papers

Diffusion limited photoluminescence quantum yields in 1-D semiconductors: single-wall carbon nanotubes.

A carbohydrate-based hydrogel containing vesicles as responsive non-covalent cross-linkers

Size-Selective Permeation of Water-Soluble Polymers through the Bilayer Membrane of Cyclodextrin Vesicles Investigated by PFG-NMR

A Self-Assembled Sensor for Carbohydrates on the Surface of Cyclodextrin Vesicles

Biomimetic Amphiphiles and Vesicles