Diversity at the nanoscale: laser-oxidation of single-layer graphene affects Fmoc-phenylalanine surface-mediated self-assembly

We report the effects of a laser-oxidized single layer graphene (SLG) surface on the self-assembly of amphiphilic gelator N-fluorenylmethoxycarbonyl-L-phenylalanine (Fmoc-Phe) towards an gel-SLG interface. Laser oxidation modulates the levels of hydrophobicity/hydrophilicity on the SLG surface. Atomic force, scanning electron, helium ion and scattering scanning nearfield optical microscopies (AFM, SEM, HIM, s-SNOM) were employed to assess the effects of surface properties on the secondary and tertiary organization of the formed Fmoc-Phe fibres at the SLG-gel interface. S-SNOM shows sheet-like secondary structures on both hydrophobic/hydrophilic areas of SLG and helical or disordered structures mainly on the hydrophilic oxidized surface. The gel network heterogeneity on pristine graphene was observed at the scale of single fibres by s-SNOM, demonstrating its power as a unique tool to study supramolecular assemblies and interfaces at nanoscale. Our findings underline the sensitivity of assembled structures to surface properties, while our characterization approach is a step forward in assessing surface-gel interfaces for the development of bionic devices.

Surface Self-Assembly of Dipeptides on Porous CaCO3 Particles Promoting Cell Internalization

The self-assembling behavior of peptides and derivatives is crucial in the natural process to construct various architectures and achieve specific functions. However, the surface or interfacial self-assembly, in particular, on the surface of micro- or nanoparticles is even less systematically investigated. Here, uniform porous CaCO₃ microparticles were prepared with different charged, hydrophobic and hydrophilic surfaces to assess the self-assembling behavior of dipeptides composed of various sequences. Experimental results indicate that dipeptides with a negative charge in an aqueous solution preferred to self-assemble on the hydrophobic and positively charged surface of CaCO₃ particles, which can be ascribed to the electrostatic and hydrophobic interaction between dipeptides and CaCO₃ particles. Meanwhile, the Log p (lipid-water partition coefficient) of dipeptides has a significant effect on the self-assembling behavior of dipeptides on the surface of porous CaCO₃; dipeptides with high Log p preferred to self-assemble on the surface of CaCO₃ particles, resulting in the improved cell internalization efficiency of particles with low cytotoxicity. After loading with a model drug (doxorubicin), the particles show obvious antitumor activity in animal experiments and can reduce Dox side effects effectively.

Personal Perspective on Understanding Low Molecular Weight Gels

Low molecular weight gels are formed by the self-assembly of small molecules into anisotropic structures that form a network capable of immobilizing the solvent. Such gels are common, with a huge number of different examples existing, and they have many applications. However, there are still significant gaps in our understanding of these systems and challenges that need to be addressed if we are to be able to fully design such systems. Here, a number of these challenges are discussed.

Surface-controlled spatially heterogeneous physical properties of a supramolecular gel with homogeneous chemical composition

Controlling supramolecular self-assembly across multiple length scales to prepare gels with localised properties is challenging. Most strategies concentrate on fabricating gels with heterogeneous components, where localised properties are generated by the stimuli-responsive component. Here, as an alternative approach, we use a spiropyran-modified surface that can be patterned with light. We show that light-induced differences in surface chemistry can direct the bulk assembly of a low molecular weight gelator, 2-NapAV, meaning that mechanical gel properties can be controlled by the surface on which the gel is grown. Using grazing incidence X-ray diffraction and grazing incidence small angle X-ray scattering, we demonstrate that the origin of the different gel properties relates to differences in the architectures of the gels. This provides a new method to prepare a single domain (i.e., chemically homogeneous) hydrogel with locally controlled (i.e., mechanically heterogeneous) properties.

Understanding gel-to-crystal transitions in supramolecular gels

Most supramolecular gels are stable or assumed to be stable over time, and aging effects are often not studied. However, some gels do show clear changes on aging, and a small number of systems exhibit gel-to-crystal transitions. In these cases, crystals form over time, typically at the expense of the network underpinning the gel; this leads to the gel falling apart. These systems are rare, and little is known about how these gel-to-crystal transitions occur. Here, we use a range of techniques to understand in detail a gel-to-crystal transition for a specific functionalised dipeptide based gelator. We show that the gel-to-crystal transition depends on the final pH of the medium which we control by varying the amount of glucon-δ-lactone (GdL) added. In the gel phase, at low concentrations of GdL, and at early time points with high concentrations of GdL, we are able to show the nanometre scale dimensions of the self-assembled fibre using SAXS; however there is no evidence of molecular ordering of the gel fibres in the WAXS. At low concentrations of GdL, these self-assembled fibres stiffen with time but do not crystallise over the timescale of the SAXS experiment. At high concentrations of GdL, the fibres are already stiffened, and then, as the pH drops further, give way to the presence of crystals which appear to grow preferentially along the direction of the fibre axis. We definitively show therefore that the gel and crystal phase are not the same. Our work shows that many assumptions in the literature are incorrect. Finally, we also show that the sample holder geometry is an important parameter for these experiments, with the rate of crystallisation depending on the holder in which the experiment is carried out.

High-throughput characterisation of supramolecular gelation processes using a combination of optical density, fluorescence and UV-Vis absorption measurements

Herein, we showcase the use of high-throughput microplate reader methodologies for the characterisation of supramolecular gels. We demonstrate how UV-Vis absorption, optical density and fluorescence measurements can selectively define gel fibre assembly/disassembly processes, casting a new light on the construction of these materials.

Selective Discrimination of Toxic Polycyclic Aromatic Hydrocarbons in Water by Targeting π-Stacking Interactions

The development of highly sensitive and selective devices for rapid screening of polycyclic aromatic hydrocarbons (PAHs) in water is nowadays a crucial challenge owing to their alarming abundance in the environment and adverse health effects. Herein, inspired by the unique π-stacking interactions taking place between identical small aromatic molecules, a novel, generic, and straightforward methodology to electrochemically determine and discriminate such pollutants is described. Such a method is focused on covalently anchoring different PAHs on an indium tin oxide electrode surface by means of self-assembled monolayers. The surface-anchored PAHs act as recognition units to selectivity interact with a specific PAH target of the same nature. By tailoring the recognition platform with four different model PAH molecules (naphthalene, anthracene, pyrene, and fluoranthene) and carrying out an electronic tongue approximation, the selective discrimination and quantification of the selected PAHs in aqueous samples at ultralow concentrations were achieved impedimetrically, which were also validated using a certified reference PAH mixture.

Self-Assembly of Metallo-Nucleoside Hydrogels for Injectable Materials That Promote Wound Closure

Injectable hydrogels are increasingly being used as scaffolds for in situ tissue engineering and wound healing. Most of these injectable hydrogels are made from polymers, and there are fewer examples of such soft materials made via self-assembly of low-molecular weight gelators. We report the room-temperature synthesis of a functional hydrogel formed by mixing cytidine (C) with 0.5 equiv each of B(OH)₃ and AgNO₃. The structural basis for this supramolecular hydrogel (C-B-C·Ag⁺) involves orthogonal formation of cytidine borate diesters (C-B-C) and Ag⁺-stabilized C-C base pairs, namely, the C·Ag⁺·C dimer. The C-B-C·Ag⁺ hydrogels, which can have high water content (at least 99.6%), are stable (no degradation after 1 year in the light), stimuli-responsive, and self-supporting, with elastic moduli of up to 10⁴ Pa. Incorporation of Ag⁺ ions into the gel matrix endows the C-B-C·Ag⁺ hydrogel with significant antibacterial capability. Importantly, the rapid switching between the sol and gel states for this supramolecular hydrogel, as a response to shear stress, enables 3D printing of a flexible medical patch made from the C-B-C·Ag⁺ hydrogel. The C-B-C·Ag⁺ hydrogel was used to promote the closure of burn wounds in a mouse model.

Controlling the Assembly and Properties of Low-Molecular-Weight Hydrogelators

Low-molecular-weight gels are formed by the self-assembly of small molecules into fibrous networks that can immobilize a significant amount of solvent. Here, we focus on our work with a specific class of gelator, the functionalized dipeptide. We discuss the current state of the art in the area, focusing on how these materials can be controlled. We also highlight interesting and unusual observations and unanswered questions in the field.

Controlled Fabrication of Micropatterned Supramolecular Gels by Directed Self-Assembly of Small Molecular Gelators

Herein, the micropatterning of supramolecular gels with oriented growth direction and controllable spatial dimensions by directing the self-assembly of small molecular gelators is reported. This process is associated with an acid-catalyzed formation of gelators from two soluble precursor molecules. To control the localized formation and self-assembly of gelators, micropatterned poly(acrylic acid) (PAA) brushes are employed to create a local and controllable acidic environment. The results show that the gel formation can be well confined in the catalytic surface plane with dimensions ranging from micro- to centimeter. Furthermore, the gels show a preferential growth along the normal direction of the catalytic surface, and the thickness of the resultant gel patterns can be easily controlled by tuning the grafting density of PAA brushes. This work shows an effective "bottom-up" strategy toward control over the spatial organization of materials and is expected to find promising applications in, e.g., microelectronics, tissue engineering, and biomedicine.

Surface-Mediated Supramolecular Self-Assembly of Protein, Peptide, and Nucleoside Derivatives: From Surface Design to the Underlying Mechanism and Tailored Functions

Among the many parameters that have been explored to exercise control over self-assembly processes, the influence of surface properties on self-assembly has been recognized as important but has received considerably less attention than other factors. This is particularly true for biomolecule-derived self-assembling molecules such as protein, peptide, and nucleobase derivatives. Because of their relevance to biomaterial and drug delivery applications, interest in these materials is increasing. As the formation of supramolecular structures from these biomolecule derivatives inevitably brings them into contact with the surfaces of surrounding materials, understanding and controlling the impact of the properties of these surfaces on the self-assembly process are important. In this feature article, we present an overview of the different surface parameters that have been used and studied for the direction of the self-assembly of protein, peptide, and nucleoside-based molecules. The current mechanistic understanding of these processes will be discussed, and potential applications of surface-mediated self-assembly will be outlined.

Hydrophobicity of surface-immobilised molecules influences architectures formed via interfacial self-assembly of nucleoside-based gelators

Surface-mediated self-assembly has potential in biomaterial development but underlying rules governing surface-gelator interactions are poorly understood. Here, we correlate surface properties with structural characterization data of nucleoside-based gels obtained by GISAXS and GIWAXS and find that hydrophobicity descriptors (log P, polar surface area, aromaticity) are key predictors for the gel structures formed.

Supramolecular Nucleoside-Based Gel: Molecular Dynamics Simulation and Characterization of Its Nanoarchitecture and Self-Assembly Mechanism

Among the diversity of existing supramolecular hydrogels, nucleic acid-based hydrogels are of particular interest for potential drug delivery and tissue engineering applications because of their inherent biocompatibility. Hydrogel performance is directly related to the nanostructure and the self-assembly mechanism of the material, an aspect that is not well-understood for nucleic acid-based hydrogels in general and has not yet been explored for cytosine-based hydrogels in particular. Herein, we use a broad range of experimental characterization techniques along with molecular dynamics (MD) simulation to demonstrate the complementarity and applicability of both approaches for nucleic acid-based gelators in general and propose the self-assembly mechanism for a novel supramolecular gelator, N4-octanoyl-2'-deoxycytidine. The experimental data and the MD simulation are in complete agreement with each other and demonstrate the formation of a hydrophobic core within the fibrillar structures of these mainly water-containing materials. The characterization of the distinct duality of environments in this cytidine-based gel will form the basis for further encapsulation of both small hydrophobic drugs and biopharmaceuticals (proteins and nucleic acids) for drug delivery and tissue engineering applications.

Does Drying Affect Gel Networks?

The properties of low molecular weight gels are determined by the underlying, self-assembled network. To access information on the network, it is common for techniques to be used that require the gel to be dried, such as transmission electron microscopy or scanning electron microscopy. The implicit assumption is that this drying has no bearing on the data collected. Here, we discuss the validity of this assumption.

Surface-Assisted Self-Assembly Strategies Leading to Supramolecular Hydrogels

Localized molecular self-assembly processes leading to the growth of nanostructures exclusively from the surface of a material is one of the great challenges in surface chemistry. In the last decade, several works have been reported on the ability of modified or unmodified surfaces to manage the self-assembly of low-molecular-weight hydrogelators (LMWH) resulting in localized supramolecular hydrogel coatings mainly based on nanofiber architectures. This Minireview highlights all strategies that have emerged recently to initiate and localize LMWH supramolecular hydrogel formation, their related fundamental issues and applications.

Supramolecular gels made from nucleobase, nucleoside and nucleotide analogs

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.