Supramolecular Gelation of Europium and Calcium Cholates through the Nucleation‐Elongation Growth Mechanism

Abstract

A detailed understanding of gelation mechanism can enable the properties of gels to be tuned for various applications, and may possibly help in understanding the aggregation of different biomolecules. We report a detailed study of the morphological and physio‐chemical changes, dynamics (of a probe), and kinetics during the gelation of europium and calcium cholate hydrogels, leading to the development of a growth model. AFM images showed the transition of aggregated particles (100–150 nm) in the sol phase growing to a fibrous network in the gel through the entanglement of fibres, and not by dendritic growth (height analysis). The dynamic changes during this phase transformation were studied using a fluorescence probe (change in intensity and lifetime). We have been able to delineate the growth mechanism by using a combination of Eu(III) luminescence and a polarity sensitive fluorescence probe. The growth was found to follow the nucleation–elongation model, and these two phases responded in distinctly different fashions in rheological and luminescence measurements.

Insights into the co-assemblies formed by different aromatic short-peptide amphiphiles

“…To gain more information on the kinetics of the gelation, we analysed the rheological data based on Avrami's equation. 80 Since we can consider that gelation occurred via a nucleation process in which a sol phase was progressively transformed in a more stable gel phase, we could assume that the following relation held during the gelation process:…”

“…K is a temperature dependent parameter and n is the Avrami exponent, which indicates the type of growth (see scheme in Fig. 10), and that can be expressed as: 80 n…”

“…where n | gives the nucleation rate and takes the values 0 for pre-existing nuclei, <1 for decreasing nucleation rate, 1 for con-stant nucleation rate, and >1 for increasing nucleation rate; d gives the dimension of the growth; and n G the growth index, which takes the value 0.5 for controlled-diffusion growth and 1 for interface-controlled growth. 80 We obtained the n exponent as the fitting factor for the three regions: sol, transition, and gel (Table S1 †). Roughly, we observe low n values for the sol region, which can be justified by pre-existing nuclei (n | = 0), in agreement with TEM observations prior to the addition of δ-gluconolactone (Fig.…”

On the Mechanism of Self-Assembly by a Hydrogel-Forming Peptide

“…A better understanding of the mechanism of self-assembly is of great interest, as it would provide insights into the kinetic and thermodynamic determinants of fibril formation, which govern the gel material properties. ,,, Previous studies have elucidated interactions and reaction intermediates that are crucial to the self-assembly processes of various hydrogel-forming peptides using techniques such as mass spectrometry, X-ray diffraction, and 1 H NMR spectroscopy. − In particular, these studies demonstrated the importance of hydrophobic interactions and π–π stacking in disparate peptide systems, indicating the general involvement of these interactions in the self-assembly of hydrogel-forming peptides. ,− The self-assembly of hydrogel-forming peptides is moreover often dependent on both pH and ionic strength, indicating the importance of limited electrostatic repulsion. − Several previous studies of the kinetics of hydrogel fibril formation have observed the presence of a long lag phase during which the extent of self-assembly is below the signal-to-noise ratio of most techniques, , followed by a growth phase in which the fibril mass concentration increases rapidly; this led to the conclusion that self-assembly in these systems proceeds through a nucleated reaction. ,,− Furthermore, several studies have highlighted a set of possible mechanistic steps, including monomer conformational changes ,,, and hierarchical self-assembly steps in which one-dimensional aggregates associate laterally to form various complex, higher-order structures. ,, The opposite chemical kinetics approach, which takes as a starting point a set of experimental data and finds a minimal set of composite microscopic steps underlying the formation of fibrillar assemblies, which can explain all data, has to our knowledge not been reported for hydrogel-forming systems.…”

Energy transfer in FRET pairs in a supramolecular hydrogel template

“…Such interactions between the gelators or aggregates of the gelator lead to the formation of fibrils, which in turn form bundles and eventually entangle to form a gel network immobilizing solvent molecules using capillary and surface forces. 2,3 The donor and acceptor chromophores can be integrated into the gel fibres during the gelation, and when properly chosen, energy transfer between them may be achieved. This approach has been employed by some groups, especially with organogels.…”