Supramolecular or molecular gels are attractive for various applications, including diagnostics, tissue scaffolding and targeted drug release. Gelators derived from natural products are of particular interest for biomedical purposes, as they are generally biocompatible and stimuli-responsive. The building blocks of nucleic acids (i.e. nucleobases, nucleosides, and nucleotides) are desirable candidates for supramolecular gelation as they readily engage in reversible, noncovalent interactions. In this review, we describe a number of organo- and hydrogels formed through the assembly of nucleosides, nucleotides, and their derivatives. While natural nucleosides and nucleotides generally require derivatization to induce gelation, guanosine and its corresponding nucleotides are well known gelators. This unique gelating ability is due to propensity of the guanine nucleobase to self-associate into stable higher-order assemblies, such as G-ribbons, G4-quartets, and G-quadruplexes.

Supramolecular gels made from nucleobase, nucleoside and nucleotide analogs

One of the most difficult challenges in the biomedical field is bacterial infection, which causes tremendous harm to human health. In this work, an injectable hydrogel is synthesized through rapid assembly of dopamine (DA) and folic acid (FA) cross-linked by transition metal ions (TMIs, i.e., Zn2+ ), which was named as DFT-hydrogel. Both the two carboxyl groups in the FA molecule and catechol in polydopamine (PDA) easily chelates Zn2+ to form metal-ligand coordination, thereby allowing this injectable hydrogel to match the shapes of wounds. In addition, PDA in the hydrogel coated around carbon quantum dot-decorated ZnO (C/ZnO) nanoparticles (NPs) to rapidly generate reactive oxygen species (ROS) and heat under illumination with 660 and 808 nm light, endows this hybrid hydrogel with great antibacterial efficacy against Staphylococcus aureus (S. aureus, typical Gram-positive bacteria) and Escherichia coli (E. coli, typical Gram-negative bacteria). The antibacterial efficacy of the prepared DFT-C/ZnO-hydrogel against S. aureus and E. coli under dual-light irradiation is 99.9%. Importantly, the hydrogels release zinc ions over 12 days, resulting in a sustained antimicrobial effect and promoted fibroblast growth. Thus, this hybrid hydrogel exhibits great potential for the reconstruction of bacteria-infected tissues, especially exposed wounds.

Rapid and Superior Bacteria Killing of Carbon Quantum Dots/ZnO Decorated Injectable Folic Acid-Conjugated PDA Hydrogel through Dual-Light Triggered ROS and Membrane Permeability.

Through a good/poor solvent strategy, native folic acid (FA) which behaves as a super-gelator in DMSO–water system can be successfully employed to construct supramolecular gels. The system exhibited morphological evolution with the increase of FA concentration; various phases such as vesicles, fiber/vesicles, fiber/nanoparticles, nanoparticles were probed. In the self-assembly process, L-glutamic acid moiety induced the formation of helical 1-dimensional (1-D) fibers which further self-assembled into a gel. Stimuli like heat, stress, pH and light which affect the molecular structure of FA or solubility in the mixed solvents had a pronounced influence on the properties of the gels, such as mechanical properties or bulk phases. A time-dependent oscillatory stress scan indicated that the supramolecular gel had a self-healing property. Without tedious modification routes and addition of alkali metal ions, native FA which served as an efficient building block and super-gelator to build up multi-responsive and self-recovery material was investigated for the first time.

Supramolecular gel from folic acid with multiple responsiveness, rapid self-recovery and orthogonal self-assemblies

Rational engineering of one-dimensional (1D) self-assembled aggregates to produce desired materials for versatile functions remains a challenge. In this work, we report the noncovalent modulation of 1D aggregates at the micro/nanoscale using a coassembly protocol. Aromatic amino acids were employed as the model building blocks, and melamine (Mm) behaves as a modulator to form coassembly arrays with aromatic amino acids selectively. The selective self-assembly behavior between aromatic amino acids and Mm allows distinguishing and detecting Mm and aromatic amino acids from their analogues in macroscopic and microscopic scales. Dimensions and sizes of fibrous aggregates prepared from different amino acids show two opposite pathways from pristine assemblies to coassemblies induced by the addition of Mm. This pathway complexity could be controlled by the molecular conformation determined by α-positioned substituents. The developed hypothesis presents an excellent expansibility to other substrates, which may gu...

Understanding Pathway Complexity of Organic Micro/Nanofiber Growth in Hydrogen-Bonded Coassembly of Aromatic Amino Acids

In this work, we developed a strategy to fabricate self-assembled aggregates encapsulating graphene from direct exfoliation of graphite. Naphthalene-anhydride-appended glutamic acid (NG) and pyrene-appended glutamic acid (PG) were utilized to exfoliate graphite to generate few-layer graphene in basic aqueous media. Acidification-triggered self-assembly of PG could encapsulate graphene sheets to give graphene@microsheet hybrids. Melamine (Mm) was introduced to induce the morphological transformation of PG from microsheets to twisted fibers with columnar packing. In the presence of graphene, this transformation allowed the fabrication of twisted-fiber-encapsulated graphene (graphene@twist fiber). Tricomponent hybrids were also built up via in situ preparation of gold nanoparticles (AuNPs) on the negatively charged surface of few-layer graphene during the self-assembly process. This study presents an approach to construct directly exfoliated graphene-based hybrids through self-assembly.

Self-Assembly of Organic Building Blocks with Directly Exfoliated Graphene to Fabricate Di- and Tricomponent Hybrids

The controllable self-growth of a supramolecular hydrogel of folic acid (FA) was developed based on the conduction of transition metal ions. The growth behavior of the gel could be flexibly controlled by adjusting the ambient environmental factors such as gelator concentration, temperature and external chemical stimuli. The obtained gel was characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic force microscopy (AFM) and rheological measurements. Differential scanning calorimetry (DSC) showed that the gel possessed excellent thermal stability. A mechanism for the fibrous formation of the gel was suggested based on the experimental results of Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance spectroscopy (NMR) and UV-Vis spectroscopy. The gel exhibited multiple stimuli-responsive properties to changes in ligand and pH. Furthermore, the gel can be incorporated into multi-layer hydrogels in both artificial and spontaneous ways, showing the advantages of self-growth and flexible control of the gel system. This novel hydrogel and the preparation strategy may provide a new route to rationally design advanced materials for biomedical applications.

Controllable self-growth of a hydrogel with multiple membranes

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Controllable self-growth of a hydrogel with multiple membranes